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Houston, we have a problem
CHAPTER
Houston, we have a problem How to fix it when it all goes wrong CHAPTER OUTLINE
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6.1 Introduction ................................................................................................................................. 146 6.2 Integration ................................................................................................................................... 147 6.2.1 Issue: Poor-quality electrode under PZT films...............................................................147 6.2.1.1 Circumstances.......................................................................................................147 6.2.1.2 Investigation ..........................................................................................................147 6.2.1.3 Solution .................................................................................................................149 6.2.2 Issue: Catastrophic loss of film and substrate ...............................................................149 6.2.2.1 Circumstances.......................................................................................................149 6.2.2.2 Investigation ..........................................................................................................149 6.2.2.3 Solution .................................................................................................................150 6.3 Cracking and Surface Finish ......................................................................................................... 151 6.3.1 Issue: Cracking of thick film .......................................................................................151 6.3.1.1 Circumstances.......................................................................................................151 6.3.1.2 Investigation ..........................................................................................................151 6.3.1.3 Solution .................................................................................................................152 6.3.2 Issue: Uneven surface finish on dip-coated porous lanthanum strontium manganite (LSMO) films.........................................................................................................................152 6.3.2.1 Circumstances.......................................................................................................152 6.3.2.2 Investigation ..........................................................................................................152 6.3.2.3 Solution .................................................................................................................153 6.3.3 Issue: Uneven surface finish on a screen-printed film....................................................153 6.3.3.1 Circumstances.......................................................................................................153 6.3.3.2 Investigation ..........................................................................................................153 6.3.3.3 Solution .................................................................................................................154 6.3.4 Issue: Overly porous spray-deposited PZT film ..............................................................154 6.3.4.1 Circumstances.......................................................................................................154 6.3.4.2 Investigation ..........................................................................................................154 6.3.4.3 Solution .................................................................................................................155 6.4 Poor Properties ............................................................................................................................ 155 6.4.1 Issue: Poor electrical connections ...............................................................................155 6.4.1.1 Circumstances.......................................................................................................155 6.4.1.2 Investigation ..........................................................................................................155 6.4.1.3 Solution .................................................................................................................156 6.4.2 Issue: Electrical shorting on insulating thick film..........................................................156 6.4.2.1 Circumstances.......................................................................................................156 6.4.2.2 Investigation ..........................................................................................................156 Robert Dorey: Ceramic Thick Films for MEMS and Microdevices. DOI: 10.1016/B978-1-4377-7817-5.00006-7 Copyright Ó 2012 Elsevier Inc. All rights reserved.
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6.4.2.3 Solution .................................................................................................................157 6.4.3 Issue: Poor piezoelectric properties .............................................................................158 6.4.3.1 Circumstances.......................................................................................................158 6.4.3.2 Investigation ..........................................................................................................158 6.4.3.3 Solution .................................................................................................................159 6.5 Patterning.................................................................................................................................... 159 6.5.1 Issue: Poor-quality micro-molded thick-film features.....................................................159 6.5.1.1 Circumstances.......................................................................................................159 6.5.1.2 Investigation ..........................................................................................................159 6.5.1.3 Solution .................................................................................................................160 6.5.2 Issue: Poor-quality inkjet-printed features ....................................................................161 6.5.2.1 Circumstances.......................................................................................................161 6.5.2.2 Investigation ..........................................................................................................161 6.5.2.3 Solution .................................................................................................................162 6.5.3 Issue: Poor quality of edge on cantilever device............................................................162 6.5.3.1 Circumstances.......................................................................................................162 6.5.3.2 Investigation ..........................................................................................................162 6.5.3.3 Solution .................................................................................................................163 6.5.4 Issue: Unsmooth edges on etched features ..................................................................163 6.5.4.1 Circumstances.......................................................................................................163 6.5.4.2 Investigation ..........................................................................................................164 6.5.4.3 Solution .................................................................................................................164 6.5.5 Issue: Excess material attached to edges of micro-molded features ................................165 6.5.5.1 Circumstances.......................................................................................................165 6.5.5.2 Investigation ..........................................................................................................165 6.5.5.3 Solution .................................................................................................................165 References ......................................................................................................................................... 166
6.1 INTRODUCTION The previous chapters have described the challenges in integrating ceramic thick films with different types of materials and how to overcome these issues in order to create thick film structures for microdevices. However, life is not perfect and, despite your best efforts, the films that you now have in front of you are not quite what you had hoped for. Do not be disheartened; life rarely goes to plan and this chapter is here to help you overcome some of those real-life problems that inevitably occur. This chapter provides a collection of little ‘mishaps’ that I have encountered in my work. They have been grouped into common themes, allowing you to find the relevant ones more easily: Integration: The materials just will not stay together. Cracking and surface finish: The materials stay together, but the films are of poor quality.
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Poor properties: The films look great, but the properties are poor. Patterning problems: The films looked great, but the microscale structures do not. Each case study presents the problem and circumstances surrounding it. The evidence is then collected and analyzed before a conclusion is drawn. Finally, suggestions on how to overcome the issues are provided. In some cases, the solutions appear in already published work – in these cases a reference to the supporting paper is also given.
6.2 INTEGRATION The key to integration is that the film remains attached to the substrate. This does not always occur due to interactions, at the interface, between the film and its substrate.
6.2.1 Issue: Poor-quality electrode under PZT films 6.2.1.1 Circumstances It was found that the electrode below the PZT thick film was a poor surface finish and was not robust enough to allow electrical connections to be made. The film was deposited, using a composite sol–gel route, onto a Ti/Pt-coated silicon wafer by spin coating. The maximum processing temperature was 720 C and the sample was held at temperature for 15 min. After this, the film was masked and etched to reveal the underlying back electrode. When a mechanical probe was placed on the back, electrode parts of the electrode would delaminate.
6.2.1.2 Investigation The SEM micrograph (Figure 6.1) shows that the film is also cracked; however, the same degradation in the back electrode was also observed for uncracked films so it could be concluded that the cracking was not the cause of this behavior. From the SEM micrograph, it can be seen that there are unusual features on the substrate where the film has been removed. These features appeared as raised structures on the otherwise smooth surface. Elemental analysis showed no difference between the raised features and the surrounding material, indicating that no contaminants
FIGURE 6.1
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had been incorporated into the structure. On examination of a fracture cross-section of the film, defects were observed in the layer below the back electrode, indicating that some form of reaction had occurred. As no foreign elements were noted, it was reasonable to assume that the effect was as a result of interaction between the materials present. Analysis of the material next to the void showed there to be a high level of lead and silicon to be present, leading to the conclusion that the reaction phase was a lead silicate material. Lead silicate is known to be liquid above 714 C; so, it is not unreasonable to assume that a liquid phase was formed at the processing temperature used. The question of why a single void across the whole film was not formed can be answered by two possible scenarios. In the first scenario, there are a finite number of pinhole defects in the Ti/Pt electrode which allow fast diffusion through the electrode so that the Pb and Si can react together. This would give rise to localized reaction islands at the site of each pinhole. Where individual reaction sites overlap, the resultant shape would be noncircular. If a liquid phase were present, this could lead to the separation of the electrode from the substrate, especially if there were a large volume reduction on formation of the liquid phase. In the second scenario, diffusion occurs throughout the Ti/Pt film and the localized reaction zones are a result of the liquid phase forming across the whole wafer and coalescing into a network of islands due to surface tension. In this scenario, the solid material on either side of the void is the frozen liquid phase. When the PZT film was processed for longer, to allow the reaction to proceed further, the size and distribution of these voids did not appear to change, indicating that scenario two was more likely as the reaction zones in scenario one would have been expected to grow as more material was added. In scenario two, the size and shape are governed to a larger extent by the surface tension of the liquid formed. The second question of why there is no evidence of a feature protruding into the film in the SEM micrograph of the cross-section can be answered by considering the effect of the film. When the film is present, it is able to restrain the Ti/Pt electrode, but once removed there is nothing to prevent the Ti/Pt electrode from bowing outward due to the compressive stress that it is under. Where it is still bonded to the substrate, this is not possible and the electrode remains flat. Delamination occurs because the 100-nm-thick back electrode is damaged easily by the mechanical FIGURE 6.2 probe.
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6.2.1.3 Solution In this example, the degradation of the electrode was traced back to localized delamination caused by the formation of a liquid phase under the electrode during processing. This can be corrected by preventing the liquid-phase reaction product from being formed, which can be done by: Decreasing the sintering temperature so that liquid phase does not form, which may affect the processing of the PZT film and result in a lower density, or Incorporating a diffusion barrier (e.g. ZrO2 or TiO2) to prevent the diffusion of Pb into the underlying silicon. For further information see Duval et al. (2003).
6.2.2 Issue: Catastrophic loss of film and substrate 6.2.2.1 Circumstances A 10-mm-thick PZT film was deposited onto a glassy carbon substrate and processed at 720 C for 20 min. On removing the sample from the furnace, it was found that the film had separated from the substrate (Figure 6.3). Furthermore, in cases where the film had not separated from the substrate, it was found that the film could be simply lifted off the substrate by bringing an electrostatically charged object near to the film. Evidently, even when the film was still in place, there was no bonding between the substrate and film.
FIGURE 6.3
6.2.2.2 Investigation In an effort to improve the bonding between the film and the substrate, a TiO2 adhesion layer was deposited. To verify the effectiveness of the adhesion layer, only half the sample was coated with the TiO2 layer so that all other experimental variables would be the same. On examination, the PZT remained adhered to the half of the sample coated with the TiO2 layer, while on the untreated half the PZT delaminated as before. However, on closer inspection a significant step was observed between the treated and untreated halves of the sample – much larger than could be accounted for by the 10-mm-thick PZT film. An examination of the cross-section (see Figure 6.4) showed that the material loss came from the glassy carbon substrate and was not due to a pre-existing flaw as it coincided with the edge of the thin TiO2 adhesion layer. As the loss of material was far in excess of the volume of material in the PZT film above the substrate (which was still present – albeit no longer attached to the substrate), it was unlikely that the effect was the result of the formation of a high-density (i.e. low volume) product as
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a consequence of a reaction between the PZT and glassy carbon. The evidence presented more like a catalytic reaction where the PZT took part in the reaction but was not itself consumed by the reaction. Where the TiO2 was present, no loss of material was exhibited as the PZT was unable to interact with the substrate (i.e. TiO2 would also act as a diffusion barrier). Equally, where no PZT was present, no loss of glassy carbon was observed (Figure 6.5). The cause of the loss of glassy carbon material was traced to the reaction between the lead oxide in the PZT and carbon. When lead oxide is mixed with carbon and heated, the lead oxide is reduced to metallic lead and the carbon is oxidized to carbon dioxide – this is at the heart of lead production. Under normal circumstances that would have been the end of the reaction – the small quantity of PbO present in the film would have reacted with an equally small quantity of glassy carbon to produce metallic lead. However in this case, because of the high surface area of the film, the metallic lead was able to re-oxidize (as there was plenty of oxygen in the atmosphere) at which point the PbO could once again react with the underlying glassy carbon, thus leading to a runaway reaction which consumed the carbon substrate but left the PZT film intact.
FIGURE 6.4
FIGURE 6.5
6.2.2.3 Solution An apparent case of poor interfacial strength turned out to be the result of a runaway interfacial reaction. To improve the adhesion, the runaway reaction needs to be prevented, either by: Placing a diffusion barrier between the PZT and carbon substrate (e.g. TiO2) or
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Removing the oxygen from the furnace atmosphere to prevent the metallic Pb from re-oxidizing and perpetuating the reaction. However, the presence of the metallic lead at the interface may alter the electrical properties of the film.
6.3 CRACKING AND SURFACE FINISH Cracks in a thick film or poor-quality surface finish tend to occur across the whole body of the sample. These cracks and defects may not be visible to the naked eye but can greatly affect the properties of the thick film. SEM micrographs can prove invaluable in determining how such problems arose.
6.3.1 Issue: Cracking of thick film 6.3.1.1 Circumstances PZT thick film was deposited by spin coating using a multilayer approach. During the drying stage of the fourth layer, the film cracked and partially delaminated (Figure 6.6). Drying was conducted at 450 C for 30 s for all layers.
6.3.1.2 Investigation The first thing that can be noted is the fact that only the last layer deposited resulted in the catastrophic cracking observed. There is evidence that the previous layer may also have begun to show signs of cracking (see Figure 6.7); however, the degree of cracking is much less. As the top layer was able to crack independently of the lower layers, this indicates that the bonding between the layers had not yet developed fully, suggesting that the film had begun to crack in the early stages of drying when the binder was not able to hold the two layers together. An examination of
FIGURE 6.6
FIGURE 6.7
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the cross-section of the film shows that the layers become progressively thicker as the number of deposited layers increases. Such increases in thickness can occur when the viscosity of the ink increases due to a loss of carrier fluid into the underlying layers. In spin coating, the thickness of the film obtained increases as the viscosity of the ink increases. Each successive film deposited will be exposed to a larger sink for carrier fluid to be adsorbed into, resulting in the ink viscosity (and film thickness) increasing for each layer deposited. Increases in surface roughness can also result in a thicker film being deposited as more material is retained due to the increased resistance to material ejection. Thicker films can exhibit high surface roughness (especially as the critical thickness is approached); so, each layer added would be thicker than its predecessor and would also have a worse surface finish.
6.3.1.3 Solution In this case, the cracking of the higher number layers of PZT can be reduced or eliminated by considering the ink and the deposition process, either by: Decreasing the thickness of individual layers, which will increase the number of layers that need to be deposited before the critical thickness is attained. This can be done by using a higher spin speed during deposition. The surface finish of the first layer may also be enhanced so that this degradation mechanism does not start to occur until much later (if at all); or Reducing the powder loading of the ink which will decrease the viscosity of the ink that will, in turn, result in thinner films being created. This will also provide more carrier fluid so any losses will have less of an effect thus reducing subsequent increases in film thickness.
6.3.2 Issue: Uneven surface finish on dip-coated porous lanthanum strontium manganite (LSMO) films 6.3.2.1 Circumstances Porous LSMO films were deposited onto a high-density substrate by dip coating. The LSMO powder was dispersed in a propanol-based carrier fluid. The resultant films exhibited a very high roughness and nonuniform coating.
6.3.2.2 Investigation When clusters of particles are deposited on to a substrate (rather than a continuous film) when using dip coating, one possible explanation
FIGURE 6.8
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is that the ink was not very stable and that the particles had agglomerated. The same phenomenon is observed when using spreading and directed deposition techniques, but is less common in immersion techniques because such large clusters tend to precipitate and fall away from the sample. The ink in question did not show any signs of precipitated clusters after deposition had been completed; so, the poor-quality films are unlikely to have been due to this effect. An alternative explanation is poor particle packing occurring as the substrate is withdrawn. This can be due to the ink containing insufficient powder or containing excess powder, but, more commonly, it is due to the ink drying too quickly. A quick drying ink does not allow sufficient time for particle re-orientation and a high-quality layer to be assembled. As with many deposition techniques, once a poor-quality film begins to form, the degradation accelerates as material is preferentially deposited at sites with existing defects. The high drying rate is dependent on the solvent used in the ink. In this case, propanol (with a boiling point of 97 C and fast evaporation rate) was used, indicating that this may have been the cause of the poor surface finish.
6.3.2.3 Solution The rough surface of this particular dip-coated film is likely to be due to the fast evaporation rate of the solvent used within the ink. The evaporation rate can be retarded either by: Changing the carrier fluid to one that evaporates at a lower rate, e.g. butanol or water, or Adding a humectant (e.g. ethylene glycol) to retard the evaporation rate.
6.3.3 Issue: Uneven surface finish on a screen-printed film 6.3.3.1 Circumstances A ceramic thick film was deposited using the screen-printing technique using an ink with 32 vol% PZT suspended in a butanol-based solvent (Figure 6.9).
6.3.3.2 Investigation It is clear from the cross-section (Figure 6.10) that there is a significant height variation across the sample; in fact, this degree of variation makes this film unusable. Looking at the surface of the film, it is clear to see that the undulations in the film are FIGURE 6.9 very periodic. They also happen to coincide with the periodicity of the weave in the screen mesh. This pattern in the film clearly points toward the ink not reflowing after the squeegee has passed and the mesh retracted. This would most likely have been caused by a high ink viscosity resulting
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from excessive evaporation, powder loading, or poor dispersion. A dispersant was used in the ink and there is no evidence of particle clumping in the cross-sectional SEM micrograph (Figure 6.10), indicating that this is not the cause. The powder loading of 32 vol% is within acceptable range, but the small particle size of the powder (~500 nm) would have made the system more viscous. Butanol, the solvent used in this ink, has a point temperature around 117 C which is a little low in comparison to traditional solvents such as Terpineol FIGURE 6.10 which as a boiling point of 219 C. This points to the actual cause of the problem – the solvent is evaporating too quickly resulting in a rapid increase in viscosity, thus preventing the printed ink from reflowing once the screen has lifted away.
6.3.3.3 Solution The patterned surface of this film is again likely to be due to the evaporation of the solvent used in the ink. In this case, the evaporation rate can be retarded either by: Selecting a solvent with slower evaporation rate; or If the solvent cannot be changed, increasing the quantity of the solvent within the ink to increase time before the critical viscosity is reached.
6.3.4 Issue: Overly porous spray-deposited PZT film 6.3.4.1 Circumstances A PZT ceramic ink was spray coated onto a flat substrate using EHDA spray coating. The PZT ink used a propanol-based carrier fluid.
6.3.4.2 Investigation The film exhibited a very high degree of porosity – far higher than would be expected from a poorly sintered ceramic film. A closer examination of the microstructure (Figure 6.11) showed the film to be composed of clusters of particles that were interconnected in a very open network. This indicates that the cause of porosity was not due to poor sintering. A fact confirmed when the green-body film was examined and a pillar-like structure was observed. The porous structure was, therefore, considered to have originated as a result of deposition process. Under normal spray-coating conditions, the ink droplets arrive at the sample surface where they spread, forming a uniform coating. As the clusters dry the carrier, fluid is removed forming a porous
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green-body film. This spreading does not appear to have occurred in this case (as evinced by the 3-D structures produced) signifying that the droplets were already dry on impact. If this were the case then the droplets would be arriving as solid clusters of particles that would be deposited randomly across the surface. If they were unable to rearrange themselves to achieve better packing, a very open 3-D network of clusters would be obtained – just as observed. FIGURE 6.11
6.3.4.3 Solution The highly porous structure of the film was due to the droplets containing too little liquid when they encountered the substrate. In this case, the loss of liquid can be overcome by: Increasing the proportion of carrier fluid in the ink; Increasing the size of the droplets (changing the spray parameters to form bigger droplets which evaporate more slowly); or Moving the spray head closer to the substrate. In the case above, moving the print head 1 cm closer to the substrate enabled high-quality films to be obtained. For further information see Wang, Edirisinghe and Dorey (2008).
6.4 POOR PROPERTIES The degradation of a film’s properties (functional or mechanical) can occur across the whole film or in localized spots. The location where this degradation occurs will often provide valuable information for establishing what the cause of the poor performance is.
6.4.1 Issue: Poor electrical connections 6.4.1.1 Circumstances A high rate of device failure was noted where the devices failed to respond or only partially responded. The devices were piezoelectric resonators with Ti/Pt front and back electrodes.
6.4.1.2 Investigation Electrical characterization of the devices indicated that the electrode structures were smaller than they should be.
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When the switch was made from Cr/Au to Ti/Pt top electrodes, the electrodes were deposited prior to sintering of the ceramic film. The SEM micrograph of the top electrode does not show a blanket electrode as expected. Instead, the top electrode appears as a network of connected islands (the PZT granular structure can be seen through the gaps). Such a structure would greatly increase the resistance of the electrode and may even cause it to fail. In addition, there is evidence of FIGURE 6.12 degradation of the electrodes (shown by the dark mark near the center of the micrograph). Neither such degradation, nor the formation of islands, was noted on the Cr/Au electrode devices. The presence of a reaction product indicates that the sample was heated at some stage during processing after the top electrode had been applied. The formation of the islands could also be accounted for by a high-temperature treatment that would have allowed the metal electrode to diffuse, forming small (lower energy) clusters. Subsequent investigation confirmed that when the electrode materials were changed to Ti/Pt, the sequence of processing was also altered so that the electrodes were deposited prior to sintering the ceramic which would account for the high-temperature treatment and the degradation of the electrode.
6.4.1.3 Solution The electrode degradation due to high-temperature treatment can be overcome by: Depositing the top electrode after the ceramic has been processed.
6.4.2 Issue: Electrical shorting on insulating thick film 6.4.2.1 Circumstances During testing of devices with composite thick-film structures, a significant number of devices failed due to electrical shorting between the top and bottom electrodes. The top electrodes were Cr/ Au and were applied by evaporation. The film in question was a PZT thick film deposited using a spray-deposition process and subsequently infiltrated with sol–gel material prior to sintering at 720 C.
6.4.2.2 Investigation An electrical short circuit between the top and bottom electrode requires a conduction path between the two electrodes. The film had not been subjected to a high-voltage treatment prior to measurement;
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so, this conduction path is unlikely to have been caused by electrical breakdown. The film also exhibited a high electrical resistivity away from the electrodes, suggesting that the film was contaminated with a conducting phase. The most likely explanation is, therefore, that it is the electrodes themselves that have resulted in the short circuit. It is known that when an evaporation process (or any other physical vapor deposition process) is used, it is possible to coat not only the top surface of the material but also the edges. The same effect is observed in cracks where the atomic vapor is able to enter the crack and deposit on the sidewalls of the crack. If the cracks span the entire thickness of the film, it is possible for a conduction path between the two faces of the film to be created. An examination of the microstructure (Figure 6.13) shows the presence of through-thickness cracks. These cracks were not present prior to the sol infiltration process; hence, it is reasonable to surmise that they arose as a consequence of this infiltration process. It can be seen that the cracks themselves appear to be confined to the sol–gel phase. Cracks are likely to occur when there are significant tensile stresses. These could have arisen as a result of the shrinkage of the sol on drying and pyrolysis. In this case, discrete islands will have FIGURE 6.13 formed of well-held-together (more particle–particle contacts and higher capillary forces) clusters of powder particles with crack running through the intercluster material. This could have led to the observed crack patterns. Through-thickness cracks were generated because the complete film was infiltrated in a single step. As such, the critical conditions for cracking would have been reduced.
6.4.2.3 Solution The key to achieving uncracked films is to reduce the degree of shrinkage, use a multilayer process, or encourage stress relaxation. This can occur by: Increasing the concentration of sol so that shrinkage is reduced. This may make infiltration of sol less effective which can result in the formation of interlayers; Using a multilayer deposition process instead of spray depositing the whole film and then infiltrating; or Introducing intermediate drying stages to allow the sol to dry and pyrolyze in a more controllable manner that allows a degree of stress relaxation.
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6.4.3 Issue: Poor piezoelectric properties 6.4.3.1 Circumstances The thickness piezoelectric coefficient (d33, f) for a PZT film was measured and found to be 50 pC/N. This was lower than expected; expected values were around 80 pC/N, based on previous measurements of identical films. The PZT film was 10 mm thick and poled at 180 C using contact poling with a field of 10 V/mm.
6.4.3.2 Investigation In a study of the evolution of the piezoelectric coefficient as a function of poling conditions, it was found that only when a high temperature and high field were used was the piezoelectric coefficient degraded (Figure 6.14). Conventionally, using a high field provides more energy to re-orient the domains within the structure, thus proving a higher piezoelectric response. Indeed, this was observed in the poling study when lower temperatures were used (130 C and 150 C). Increasing the temperature is supposed to make it easier to re-orientate domains, thus allowing a greater piezoelectric response to be obtained for a given applied field. This again was shown when lower fields are used. The results obtained indicate that the cause of the degraded properties of the PZT film was the combined effect of high field and high temperature during poling. When the electrical current was measured for the different poling conditions, it was found that the current was significantly higher in the cases where poor properties were observed. The increased current would have meant that it was
FIGURE 6.14
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more difficult (if not impossible) to maintain a voltage across the film with the result that domain reordination would have been significantly reduced.
6.4.3.3 Solution The combined effect of high voltage and high temperature led to increased conductivity and, possibly, even electrical breakdown. To maintain the high levels of poling, this needs to be avoided by: Reducing the temperature; Reducing the poling field; or Using pulsed fields to apply electric field for short periods of time. This is effective if it takes some period of time for the maximum current to build up.
6.5 PATTERNING The final types of problems that may arise are related to the inability to shape the functional thick film correctly. While the film may have the correct structure and properties, the poor shape definition reduces the performance of the microdevice.
6.5.1 Issue: Poor-quality micro-molded thick-film features 6.5.1.1 Circumstances PZT thick-film structures were created using a micro-molding method where the PZT was deposited as a spray to fill the molds. The molds were created using a photoresist polymer. The resultant features were smaller than the molds and also exhibited a significant sidewall angle.
6.5.1.2 Investigation Apart from the fact that the features were smaller than anticipated and possessed sidewalls which sloped significantly (Figure 6.15), the produced features were of high quality and exhibited a reasonably high density. Constrained sintering theory predicts that the sidewalls of isolated features can develop a slope due to enhanced sintering of the material near to the upper edges of the feature. The theory also predicts that the material near the bottom edge of the feature should sinter less and that
FIGURE 6.15
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the material in the body of the film should be of uniform density. However, an examination of the cross-sectional SEM micrographs (Figure 6.16) revealed no such variation in density. Furthermore, features of different sizes all exhibited the same degree of sidewall sloping and the features were all smaller than the mold feature by the same amount. This behavior cannot be explained by the sintering theory, which would predict different sizes of features to exhibit different absolute shrinkage and different sidewall angles. InterFIGURE 6.16 estingly, the material at the bottom of the edges did not taper smoothly to the substrate, but instead ended in an abrupt ‘ski jump’ feature which again could not be explained by sintering theory. The clue to the reasons behind these observed effects can be identified when the smallest features were examined, where it was shown that they were not only smaller than the molds and exhibited the low sidewall angle, but also not as high as the other features. This indicated that something was preventing the mold from filling effectively. An examination of a filled mold prior to sintering (Figure 6.16) showed that the real cause of the deformed features was the mold itself. During deposition, the mold deformed in shape (probably due to intermittent drying stages), resulting in a shadowing effect that prevented material from being deposited directly below it. As the processing progressed, this shadowing increased, leading to the formation of the sloping sidewall (Figure 6.16). In addition, it appeared that the mold materials attracted some of the material being deposited which contributed to the increasing shadowing.
6.5.1.3 Solution The photoresist mold and the deposition of ink onto the top of the mold combined to produce features that were smaller than the molds and had sloped sidewalls. These artifacts can be minimized by: Using a mold material with higher creep resistance so that it does not deform during deposition and heating; Removing deposited material from on top of mold structure by periodic cleaning during deposition; or Increasing the mold height to reduce shadowing. For further information see Wang, Rocks and Dorey (2009).
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6.5.2 Issue: Poor-quality inkjet-printed features 6.5.2.1 Circumstances A PZT ink was deposited by inkjet printing. While some features were obtained, many others coalesced, were deformed, or simply did not print (Figure 6.17). Where correctly printed, the features had a diameter of approximately 100 mm. The ink used contained PZT particles and used a propanolbased carrier fluid.
6.5.2.2 Investigation Issues with print quality tend to be due to either the printer or the ink used. Where there is a technical fault with the printer, the error tends to be systematic or progresFIGURE 6.17 sive (e.g. a single row is missing because the fourth print head has failed or the rows gradually tend to one side because of poor alignment). The observed faults show neither of these features, indicating that there is an issue with the ink. Variations in the properties of the ink (e.g. viscosity caused by an inhomogeneous distribution of powder) would cause differences in the landing positions of the drops but would be unlikely to cause drops (and even whole rows) to be missing. This behavior is more likely to be due to the clogging of the print-head orifice. As the print head begins to clog, ink ejection becomes more random as different amounts of ink are released, the timing of release varies, or the print head becomes partially cleared. Finally, when the print head becomes fully blocked, no more droplets are released. Print heads can become blocked for a number of reasons. Two of the most common are the ink drying at the opening of the orifice or the particles blocking the print head. An examination of the print head before (Figure 6.18) and after FIGURE 6.18 printing (Figure 6.19) showed that
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the print-head holes were partially blocked. However, there was no material on the outside face, as would be expected if the ink had dried on exposure to the air. Instead, it is clear that the particles in the ink have become lodged in the print head, preventing effective ejection of the ink droplets. The material visible at the mouth of the orifice is the residue of the binder that was dissolved in the carrier fluid.
6.5.2.3 Solution FIGURE 6.19 The deformed and missing features during inkjet printing of PZT films were due to the print-head holes being blocked by the particles in the ink, which could be addressed by:
Ensuring that the ceramic particles are at least 1/100 the size of the orifice in the print head. The print head shown here (Figure 6.19) is 20 mm wide; so, the ceramic particles used should be no larger than 200 nm. The particles used in this ink were around 300 nm in diameter and were highly likely to be the cause of the blockage; or Ensuring that the particles were stabilized in the ink to prevent flocculation and blockage of the nozzle Reducing the speed of sedimentation of the particles in the ink by reducing the size of the particles and increasing the stability of the ink. Sedimentation will cause the ink to block the outlet in the ink reservoir and is particularly important in this context, as PZT has a very high density (7.7 g/cm3) and so will exhibit a high sedimentation rate compared to other ceramic powders of comparable size.
6.5.3 Issue: Poor quality of edge on cantilever device 6.5.3.1 Circumstances A PZT cantilever structure was made consisting of a PZT film on a silicon substrate with 200-nm-thick patterned Ti/Pt electrodes and a 100-nm-thick ZrO2 blanket diffusion barrier. The PZT was etched first to expose the underlying silicon before the underlying silicon was etched by deep reactive ion etching (DRIE) from the back face of the wafer. When the devices were inspected, a ‘skirt’ of material was observed around the devices.
6.5.3.2 Investigation Analysis of the ‘skirt’ material showed it to be Zr rich, indicating that it was probably the remnants of the ZrO2 diffusion barrier. There was no evidence of silicon present on the underside of the
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cantilever, showing that the DRIE process had been effective at removing the underlying silicon. The rate of removal of ZrO2 in the Si etching process is very slow. However, it was not the intention to remove the ZrO2 in this stage. Instead, the ZrO2 was supposed to have been removed in the PZT etching stage before the DRIE treatment, indicating that there was a failure in this process. The PZT film was removed using a wet chemical etch based on HF/ HCl which should have been effective at removing the PZT as well as FIGURE 6.20 the ZrO2 material. As the ZrO2 layer remained after etching, it is clear that insufficient time was allowed for the etching, despite the thinness of the diffusion barrier layer. Clearly, the rate of etching of the ZrO2 layer is significantly lower than that of the PZT film. The high density of the ZrO2 is postulated to play a major role in this.
6.5.3.3 Solution As the primary purpose of the etching process was to achieve the best PZT etch (i.e. to obtain a PZT feature with the correct dimensions), it is impractical to extend the etch time to allow the ZrO2 to be removed. Potential solutions include: Patterning the diffusion barrier layer so that it does not extend beyond the PZT feature. In this way, there would be no material present, in the etched area, which could form a ‘skirt’ around the devices; Using an alternative diffusion barrier, which can be removed using an etchant that does not attack PZT or silicon. In this way, the ‘skirt’ can be removed by subjecting the device to a final etching process; or Removing the diffusion barrier entirely and processing the film at lower temperatures or on an alternative substrate. The barrier layer was only required to prevent reactions occurring between the film and substrate at the processing temperature.
6.5.4 Issue: Unsmooth edges on etched features 6.5.4.1 Circumstances A PZT film was deposited using a composite sol–gel approach with four infiltration steps being employed for each composite layer. The final film was sintered at 720 C of 30 min. Following wet etching, a stepped sidewall structure was observed (Figure 6.21) instead of the usual smooth sidewalled structure.
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6.5.4.2 Investigation From the micrograph (Figure 6.21) it can be seen that there are eight distinct layers present within the film. This correlates with the eight composite layers that were deposited as part of the film-forming process. When the composite layers are deposited on their own and the resultant film etched, the final features show no sign of a stepped sidewall structure. This indicates that it is the intermediate infiltration steps that are responsible for the observed effect. Interestingly, this effect only appears after four infiltration cycles FIGURE 6.21 per layer have been applied, indicating that it is at this stage that a critical transformation occurs. An examination of the surface of a layer that had been infiltrated four times (using SEM) showed that the film no longer appeared to be a powder bed, but, instead, appeared much smoother and was covered with a layer of sol–gel-derived material (Figure 6.22). This highdensity upper layer would have etched at a slower rate than the slightly porous PZT film that existed below it. The stepped structure would therefore have arisen as the throughthickness etching was momentarily slowed as each new layer was FIGURE 6.22 encountered. When less than four sol infiltration cycles had been completed, this interlay was not present and the through-thickness etching occurred at the same rate as the in-plane etching.
6.5.4.3 Solution Differences in etch rate for layers of infiltrated PZT film can be addressed by: Decreasing the number of sol infiltrations applied to each layer. A final, unifying, infiltration stage can be included to increase the overall density of the film if required.
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6.5.5 Issue: Excess material attached to edges of micro-molded features 6.5.5.1 Circumstances Ceramic thick-film structures were formed using a micro-molding process. The ceramic was deposited as a slurry using spray deposition. On removal of the molds by pyrolysis, excess material was present on the edges of the features. On occasion, this formed a considerable overhanging feature.
6.5.5.2 Investigation The presence of large overhanging material ruled out the possibility that the excess material was a result of dimensional changes during sintering. Instead, the material must have FIGURE 6.23 been deposited during the deposition process. A closer inspection of the features revealed uneven edges, indicating that some form of fracture had occurred as opposed to the features being formed by a defect in the mold. The evidence suggests that the original edge of the overhanging features had extended much further than what can be seen on the micrograph and had been damaged during processing, resulting in the fractures. This suggested that the features were formed as a result of the material inside the molds connecting with the material on top of the mold. When the mold material was removed, during heating, the join would have fractured giving rise to the observed features.
6.5.5.3 Solution The defect features are a result of the material in the mold becoming connected to the material on the mold surface during the deposition process. This would be a result of overfilling the molds or material falling into the molds and can be addressed by: Increasing the depth of molds to prevent overfilling; or Using an intermediate cleaning stage to remove excess material from the mold structures so that it does not fall into the mold cavity. In summary. Many things can go wrong during the processing of microscale functional ceramic thickfilm structures. Hopefully, this chapter has provided you with answers as to why some of this has occurred. If you have tried making some thick films on your own and found that they have not turned out as expected, Chapter 7 provides a collection of useful recipes that may help to overcome some of the issues. If you have not tried making any thick films, Chapter 7 can help you to get started more quickly.
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References Duval, F.F.C., Dorey, R.A., Haigh, R.H., Whatmore, R.W., 2003. Stable TiO2/Pt electrode structure for lead containing ferroelectric thick films on silicon MEMS structures. Thin Solid Films 444 (1–2), 235–240. Wang, D., Edirisinghe, M.J., Dorey, R.A., 2008. Formation of PZT crack-free thick films by electrohydrodynamic atomization deposition. J. Eur. Ceram. Soc. 28 (14), 2739–2745. Wang, D., Rocks, S.A., Dorey, R.A., 2009. Micromoulding of PZT film structures using electrohydrodynamic atomization mould filling. J. Eur. Ceram. Soc. 29, 1147–1155.
Houston, we have a problem How to fix it when it all goes wrong CHAPTER OUTLINE
6
6.1 Introduction ................................................................................................................................. 146 6.2 Integration ................................................................................................................................... 147 6.2.1 Issue: Poor-quality electrode under PZT films...............................................................147 6.2.1.1 Circumstances.......................................................................................................147 6.2.1.2 Investigation ..........................................................................................................147 6.2.1.3 Solution .................................................................................................................149 6.2.2 Issue: Catastrophic loss of film and substrate ...............................................................149 6.2.2.1 Circumstances.......................................................................................................149 6.2.2.2 Investigation ..........................................................................................................149 6.2.2.3 Solution .................................................................................................................150 6.3 Cracking and Surface Finish ......................................................................................................... 151 6.3.1 Issue: Cracking of thick film .......................................................................................151 6.3.1.1 Circumstances.......................................................................................................151 6.3.1.2 Investigation ..........................................................................................................151 6.3.1.3 Solution .................................................................................................................152 6.3.2 Issue: Uneven surface finish on dip-coated porous lanthanum strontium manganite (LSMO) films.........................................................................................................................152 6.3.2.1 Circumstances.......................................................................................................152 6.3.2.2 Investigation ..........................................................................................................152 6.3.2.3 Solution .................................................................................................................153 6.3.3 Issue: Uneven surface finish on a screen-printed film....................................................153 6.3.3.1 Circumstances.......................................................................................................153 6.3.3.2 Investigation ..........................................................................................................153 6.3.3.3 Solution .................................................................................................................154 6.3.4 Issue: Overly porous spray-deposited PZT film ..............................................................154 6.3.4.1 Circumstances.......................................................................................................154 6.3.4.2 Investigation ..........................................................................................................154 6.3.4.3 Solution .................................................................................................................155 6.4 Poor Properties ............................................................................................................................ 155 6.4.1 Issue: Poor electrical connections ...............................................................................155 6.4.1.1 Circumstances.......................................................................................................155 6.4.1.2 Investigation ..........................................................................................................155 6.4.1.3 Solution .................................................................................................................156 6.4.2 Issue: Electrical shorting on insulating thick film..........................................................156 6.4.2.1 Circumstances.......................................................................................................156 6.4.2.2 Investigation ..........................................................................................................156 Robert Dorey: Ceramic Thick Films for MEMS and Microdevices. DOI: 10.1016/B978-1-4377-7817-5.00006-7 Copyright Ó 2012 Elsevier Inc. All rights reserved.
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6.4.2.3 Solution .................................................................................................................157 6.4.3 Issue: Poor piezoelectric properties .............................................................................158 6.4.3.1 Circumstances.......................................................................................................158 6.4.3.2 Investigation ..........................................................................................................158 6.4.3.3 Solution .................................................................................................................159 6.5 Patterning.................................................................................................................................... 159 6.5.1 Issue: Poor-quality micro-molded thick-film features.....................................................159 6.5.1.1 Circumstances.......................................................................................................159 6.5.1.2 Investigation ..........................................................................................................159 6.5.1.3 Solution .................................................................................................................160 6.5.2 Issue: Poor-quality inkjet-printed features ....................................................................161 6.5.2.1 Circumstances.......................................................................................................161 6.5.2.2 Investigation ..........................................................................................................161 6.5.2.3 Solution .................................................................................................................162 6.5.3 Issue: Poor quality of edge on cantilever device............................................................162 6.5.3.1 Circumstances.......................................................................................................162 6.5.3.2 Investigation ..........................................................................................................162 6.5.3.3 Solution .................................................................................................................163 6.5.4 Issue: Unsmooth edges on etched features ..................................................................163 6.5.4.1 Circumstances.......................................................................................................163 6.5.4.2 Investigation ..........................................................................................................164 6.5.4.3 Solution .................................................................................................................164 6.5.5 Issue: Excess material attached to edges of micro-molded features ................................165 6.5.5.1 Circumstances.......................................................................................................165 6.5.5.2 Investigation ..........................................................................................................165 6.5.5.3 Solution .................................................................................................................165 References ......................................................................................................................................... 166
6.1 INTRODUCTION The previous chapters have described the challenges in integrating ceramic thick films with different types of materials and how to overcome these issues in order to create thick film structures for microdevices. However, life is not perfect and, despite your best efforts, the films that you now have in front of you are not quite what you had hoped for. Do not be disheartened; life rarely goes to plan and this chapter is here to help you overcome some of those real-life problems that inevitably occur. This chapter provides a collection of little ‘mishaps’ that I have encountered in my work. They have been grouped into common themes, allowing you to find the relevant ones more easily: Integration: The materials just will not stay together. Cracking and surface finish: The materials stay together, but the films are of poor quality.
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Poor properties: The films look great, but the properties are poor. Patterning problems: The films looked great, but the microscale structures do not. Each case study presents the problem and circumstances surrounding it. The evidence is then collected and analyzed before a conclusion is drawn. Finally, suggestions on how to overcome the issues are provided. In some cases, the solutions appear in already published work – in these cases a reference to the supporting paper is also given.
6.2 INTEGRATION The key to integration is that the film remains attached to the substrate. This does not always occur due to interactions, at the interface, between the film and its substrate.
6.2.1 Issue: Poor-quality electrode under PZT films 6.2.1.1 Circumstances It was found that the electrode below the PZT thick film was a poor surface finish and was not robust enough to allow electrical connections to be made. The film was deposited, using a composite sol–gel route, onto a Ti/Pt-coated silicon wafer by spin coating. The maximum processing temperature was 720 C and the sample was held at temperature for 15 min. After this, the film was masked and etched to reveal the underlying back electrode. When a mechanical probe was placed on the back, electrode parts of the electrode would delaminate.
6.2.1.2 Investigation The SEM micrograph (Figure 6.1) shows that the film is also cracked; however, the same degradation in the back electrode was also observed for uncracked films so it could be concluded that the cracking was not the cause of this behavior. From the SEM micrograph, it can be seen that there are unusual features on the substrate where the film has been removed. These features appeared as raised structures on the otherwise smooth surface. Elemental analysis showed no difference between the raised features and the surrounding material, indicating that no contaminants
FIGURE 6.1
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had been incorporated into the structure. On examination of a fracture cross-section of the film, defects were observed in the layer below the back electrode, indicating that some form of reaction had occurred. As no foreign elements were noted, it was reasonable to assume that the effect was as a result of interaction between the materials present. Analysis of the material next to the void showed there to be a high level of lead and silicon to be present, leading to the conclusion that the reaction phase was a lead silicate material. Lead silicate is known to be liquid above 714 C; so, it is not unreasonable to assume that a liquid phase was formed at the processing temperature used. The question of why a single void across the whole film was not formed can be answered by two possible scenarios. In the first scenario, there are a finite number of pinhole defects in the Ti/Pt electrode which allow fast diffusion through the electrode so that the Pb and Si can react together. This would give rise to localized reaction islands at the site of each pinhole. Where individual reaction sites overlap, the resultant shape would be noncircular. If a liquid phase were present, this could lead to the separation of the electrode from the substrate, especially if there were a large volume reduction on formation of the liquid phase. In the second scenario, diffusion occurs throughout the Ti/Pt film and the localized reaction zones are a result of the liquid phase forming across the whole wafer and coalescing into a network of islands due to surface tension. In this scenario, the solid material on either side of the void is the frozen liquid phase. When the PZT film was processed for longer, to allow the reaction to proceed further, the size and distribution of these voids did not appear to change, indicating that scenario two was more likely as the reaction zones in scenario one would have been expected to grow as more material was added. In scenario two, the size and shape are governed to a larger extent by the surface tension of the liquid formed. The second question of why there is no evidence of a feature protruding into the film in the SEM micrograph of the cross-section can be answered by considering the effect of the film. When the film is present, it is able to restrain the Ti/Pt electrode, but once removed there is nothing to prevent the Ti/Pt electrode from bowing outward due to the compressive stress that it is under. Where it is still bonded to the substrate, this is not possible and the electrode remains flat. Delamination occurs because the 100-nm-thick back electrode is damaged easily by the mechanical FIGURE 6.2 probe.
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6.2.1.3 Solution In this example, the degradation of the electrode was traced back to localized delamination caused by the formation of a liquid phase under the electrode during processing. This can be corrected by preventing the liquid-phase reaction product from being formed, which can be done by: Decreasing the sintering temperature so that liquid phase does not form, which may affect the processing of the PZT film and result in a lower density, or Incorporating a diffusion barrier (e.g. ZrO2 or TiO2) to prevent the diffusion of Pb into the underlying silicon. For further information see Duval et al. (2003).
6.2.2 Issue: Catastrophic loss of film and substrate 6.2.2.1 Circumstances A 10-mm-thick PZT film was deposited onto a glassy carbon substrate and processed at 720 C for 20 min. On removing the sample from the furnace, it was found that the film had separated from the substrate (Figure 6.3). Furthermore, in cases where the film had not separated from the substrate, it was found that the film could be simply lifted off the substrate by bringing an electrostatically charged object near to the film. Evidently, even when the film was still in place, there was no bonding between the substrate and film.
FIGURE 6.3
6.2.2.2 Investigation In an effort to improve the bonding between the film and the substrate, a TiO2 adhesion layer was deposited. To verify the effectiveness of the adhesion layer, only half the sample was coated with the TiO2 layer so that all other experimental variables would be the same. On examination, the PZT remained adhered to the half of the sample coated with the TiO2 layer, while on the untreated half the PZT delaminated as before. However, on closer inspection a significant step was observed between the treated and untreated halves of the sample – much larger than could be accounted for by the 10-mm-thick PZT film. An examination of the cross-section (see Figure 6.4) showed that the material loss came from the glassy carbon substrate and was not due to a pre-existing flaw as it coincided with the edge of the thin TiO2 adhesion layer. As the loss of material was far in excess of the volume of material in the PZT film above the substrate (which was still present – albeit no longer attached to the substrate), it was unlikely that the effect was the result of the formation of a high-density (i.e. low volume) product as
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a consequence of a reaction between the PZT and glassy carbon. The evidence presented more like a catalytic reaction where the PZT took part in the reaction but was not itself consumed by the reaction. Where the TiO2 was present, no loss of material was exhibited as the PZT was unable to interact with the substrate (i.e. TiO2 would also act as a diffusion barrier). Equally, where no PZT was present, no loss of glassy carbon was observed (Figure 6.5). The cause of the loss of glassy carbon material was traced to the reaction between the lead oxide in the PZT and carbon. When lead oxide is mixed with carbon and heated, the lead oxide is reduced to metallic lead and the carbon is oxidized to carbon dioxide – this is at the heart of lead production. Under normal circumstances that would have been the end of the reaction – the small quantity of PbO present in the film would have reacted with an equally small quantity of glassy carbon to produce metallic lead. However in this case, because of the high surface area of the film, the metallic lead was able to re-oxidize (as there was plenty of oxygen in the atmosphere) at which point the PbO could once again react with the underlying glassy carbon, thus leading to a runaway reaction which consumed the carbon substrate but left the PZT film intact.
FIGURE 6.4
FIGURE 6.5
6.2.2.3 Solution An apparent case of poor interfacial strength turned out to be the result of a runaway interfacial reaction. To improve the adhesion, the runaway reaction needs to be prevented, either by: Placing a diffusion barrier between the PZT and carbon substrate (e.g. TiO2) or
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Removing the oxygen from the furnace atmosphere to prevent the metallic Pb from re-oxidizing and perpetuating the reaction. However, the presence of the metallic lead at the interface may alter the electrical properties of the film.
6.3 CRACKING AND SURFACE FINISH Cracks in a thick film or poor-quality surface finish tend to occur across the whole body of the sample. These cracks and defects may not be visible to the naked eye but can greatly affect the properties of the thick film. SEM micrographs can prove invaluable in determining how such problems arose.
6.3.1 Issue: Cracking of thick film 6.3.1.1 Circumstances PZT thick film was deposited by spin coating using a multilayer approach. During the drying stage of the fourth layer, the film cracked and partially delaminated (Figure 6.6). Drying was conducted at 450 C for 30 s for all layers.
6.3.1.2 Investigation The first thing that can be noted is the fact that only the last layer deposited resulted in the catastrophic cracking observed. There is evidence that the previous layer may also have begun to show signs of cracking (see Figure 6.7); however, the degree of cracking is much less. As the top layer was able to crack independently of the lower layers, this indicates that the bonding between the layers had not yet developed fully, suggesting that the film had begun to crack in the early stages of drying when the binder was not able to hold the two layers together. An examination of
FIGURE 6.6
FIGURE 6.7
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the cross-section of the film shows that the layers become progressively thicker as the number of deposited layers increases. Such increases in thickness can occur when the viscosity of the ink increases due to a loss of carrier fluid into the underlying layers. In spin coating, the thickness of the film obtained increases as the viscosity of the ink increases. Each successive film deposited will be exposed to a larger sink for carrier fluid to be adsorbed into, resulting in the ink viscosity (and film thickness) increasing for each layer deposited. Increases in surface roughness can also result in a thicker film being deposited as more material is retained due to the increased resistance to material ejection. Thicker films can exhibit high surface roughness (especially as the critical thickness is approached); so, each layer added would be thicker than its predecessor and would also have a worse surface finish.
6.3.1.3 Solution In this case, the cracking of the higher number layers of PZT can be reduced or eliminated by considering the ink and the deposition process, either by: Decreasing the thickness of individual layers, which will increase the number of layers that need to be deposited before the critical thickness is attained. This can be done by using a higher spin speed during deposition. The surface finish of the first layer may also be enhanced so that this degradation mechanism does not start to occur until much later (if at all); or Reducing the powder loading of the ink which will decrease the viscosity of the ink that will, in turn, result in thinner films being created. This will also provide more carrier fluid so any losses will have less of an effect thus reducing subsequent increases in film thickness.
6.3.2 Issue: Uneven surface finish on dip-coated porous lanthanum strontium manganite (LSMO) films 6.3.2.1 Circumstances Porous LSMO films were deposited onto a high-density substrate by dip coating. The LSMO powder was dispersed in a propanol-based carrier fluid. The resultant films exhibited a very high roughness and nonuniform coating.
6.3.2.2 Investigation When clusters of particles are deposited on to a substrate (rather than a continuous film) when using dip coating, one possible explanation
FIGURE 6.8
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is that the ink was not very stable and that the particles had agglomerated. The same phenomenon is observed when using spreading and directed deposition techniques, but is less common in immersion techniques because such large clusters tend to precipitate and fall away from the sample. The ink in question did not show any signs of precipitated clusters after deposition had been completed; so, the poor-quality films are unlikely to have been due to this effect. An alternative explanation is poor particle packing occurring as the substrate is withdrawn. This can be due to the ink containing insufficient powder or containing excess powder, but, more commonly, it is due to the ink drying too quickly. A quick drying ink does not allow sufficient time for particle re-orientation and a high-quality layer to be assembled. As with many deposition techniques, once a poor-quality film begins to form, the degradation accelerates as material is preferentially deposited at sites with existing defects. The high drying rate is dependent on the solvent used in the ink. In this case, propanol (with a boiling point of 97 C and fast evaporation rate) was used, indicating that this may have been the cause of the poor surface finish.
6.3.2.3 Solution The rough surface of this particular dip-coated film is likely to be due to the fast evaporation rate of the solvent used within the ink. The evaporation rate can be retarded either by: Changing the carrier fluid to one that evaporates at a lower rate, e.g. butanol or water, or Adding a humectant (e.g. ethylene glycol) to retard the evaporation rate.
6.3.3 Issue: Uneven surface finish on a screen-printed film 6.3.3.1 Circumstances A ceramic thick film was deposited using the screen-printing technique using an ink with 32 vol% PZT suspended in a butanol-based solvent (Figure 6.9).
6.3.3.2 Investigation It is clear from the cross-section (Figure 6.10) that there is a significant height variation across the sample; in fact, this degree of variation makes this film unusable. Looking at the surface of the film, it is clear to see that the undulations in the film are FIGURE 6.9 very periodic. They also happen to coincide with the periodicity of the weave in the screen mesh. This pattern in the film clearly points toward the ink not reflowing after the squeegee has passed and the mesh retracted. This would most likely have been caused by a high ink viscosity resulting
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from excessive evaporation, powder loading, or poor dispersion. A dispersant was used in the ink and there is no evidence of particle clumping in the cross-sectional SEM micrograph (Figure 6.10), indicating that this is not the cause. The powder loading of 32 vol% is within acceptable range, but the small particle size of the powder (~500 nm) would have made the system more viscous. Butanol, the solvent used in this ink, has a point temperature around 117 C which is a little low in comparison to traditional solvents such as Terpineol FIGURE 6.10 which as a boiling point of 219 C. This points to the actual cause of the problem – the solvent is evaporating too quickly resulting in a rapid increase in viscosity, thus preventing the printed ink from reflowing once the screen has lifted away.
6.3.3.3 Solution The patterned surface of this film is again likely to be due to the evaporation of the solvent used in the ink. In this case, the evaporation rate can be retarded either by: Selecting a solvent with slower evaporation rate; or If the solvent cannot be changed, increasing the quantity of the solvent within the ink to increase time before the critical viscosity is reached.
6.3.4 Issue: Overly porous spray-deposited PZT film 6.3.4.1 Circumstances A PZT ceramic ink was spray coated onto a flat substrate using EHDA spray coating. The PZT ink used a propanol-based carrier fluid.
6.3.4.2 Investigation The film exhibited a very high degree of porosity – far higher than would be expected from a poorly sintered ceramic film. A closer examination of the microstructure (Figure 6.11) showed the film to be composed of clusters of particles that were interconnected in a very open network. This indicates that the cause of porosity was not due to poor sintering. A fact confirmed when the green-body film was examined and a pillar-like structure was observed. The porous structure was, therefore, considered to have originated as a result of deposition process. Under normal spray-coating conditions, the ink droplets arrive at the sample surface where they spread, forming a uniform coating. As the clusters dry the carrier, fluid is removed forming a porous
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green-body film. This spreading does not appear to have occurred in this case (as evinced by the 3-D structures produced) signifying that the droplets were already dry on impact. If this were the case then the droplets would be arriving as solid clusters of particles that would be deposited randomly across the surface. If they were unable to rearrange themselves to achieve better packing, a very open 3-D network of clusters would be obtained – just as observed. FIGURE 6.11
6.3.4.3 Solution The highly porous structure of the film was due to the droplets containing too little liquid when they encountered the substrate. In this case, the loss of liquid can be overcome by: Increasing the proportion of carrier fluid in the ink; Increasing the size of the droplets (changing the spray parameters to form bigger droplets which evaporate more slowly); or Moving the spray head closer to the substrate. In the case above, moving the print head 1 cm closer to the substrate enabled high-quality films to be obtained. For further information see Wang, Edirisinghe and Dorey (2008).
6.4 POOR PROPERTIES The degradation of a film’s properties (functional or mechanical) can occur across the whole film or in localized spots. The location where this degradation occurs will often provide valuable information for establishing what the cause of the poor performance is.
6.4.1 Issue: Poor electrical connections 6.4.1.1 Circumstances A high rate of device failure was noted where the devices failed to respond or only partially responded. The devices were piezoelectric resonators with Ti/Pt front and back electrodes.
6.4.1.2 Investigation Electrical characterization of the devices indicated that the electrode structures were smaller than they should be.
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When the switch was made from Cr/Au to Ti/Pt top electrodes, the electrodes were deposited prior to sintering of the ceramic film. The SEM micrograph of the top electrode does not show a blanket electrode as expected. Instead, the top electrode appears as a network of connected islands (the PZT granular structure can be seen through the gaps). Such a structure would greatly increase the resistance of the electrode and may even cause it to fail. In addition, there is evidence of FIGURE 6.12 degradation of the electrodes (shown by the dark mark near the center of the micrograph). Neither such degradation, nor the formation of islands, was noted on the Cr/Au electrode devices. The presence of a reaction product indicates that the sample was heated at some stage during processing after the top electrode had been applied. The formation of the islands could also be accounted for by a high-temperature treatment that would have allowed the metal electrode to diffuse, forming small (lower energy) clusters. Subsequent investigation confirmed that when the electrode materials were changed to Ti/Pt, the sequence of processing was also altered so that the electrodes were deposited prior to sintering the ceramic which would account for the high-temperature treatment and the degradation of the electrode.
6.4.1.3 Solution The electrode degradation due to high-temperature treatment can be overcome by: Depositing the top electrode after the ceramic has been processed.
6.4.2 Issue: Electrical shorting on insulating thick film 6.4.2.1 Circumstances During testing of devices with composite thick-film structures, a significant number of devices failed due to electrical shorting between the top and bottom electrodes. The top electrodes were Cr/ Au and were applied by evaporation. The film in question was a PZT thick film deposited using a spray-deposition process and subsequently infiltrated with sol–gel material prior to sintering at 720 C.
6.4.2.2 Investigation An electrical short circuit between the top and bottom electrode requires a conduction path between the two electrodes. The film had not been subjected to a high-voltage treatment prior to measurement;
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so, this conduction path is unlikely to have been caused by electrical breakdown. The film also exhibited a high electrical resistivity away from the electrodes, suggesting that the film was contaminated with a conducting phase. The most likely explanation is, therefore, that it is the electrodes themselves that have resulted in the short circuit. It is known that when an evaporation process (or any other physical vapor deposition process) is used, it is possible to coat not only the top surface of the material but also the edges. The same effect is observed in cracks where the atomic vapor is able to enter the crack and deposit on the sidewalls of the crack. If the cracks span the entire thickness of the film, it is possible for a conduction path between the two faces of the film to be created. An examination of the microstructure (Figure 6.13) shows the presence of through-thickness cracks. These cracks were not present prior to the sol infiltration process; hence, it is reasonable to surmise that they arose as a consequence of this infiltration process. It can be seen that the cracks themselves appear to be confined to the sol–gel phase. Cracks are likely to occur when there are significant tensile stresses. These could have arisen as a result of the shrinkage of the sol on drying and pyrolysis. In this case, discrete islands will have FIGURE 6.13 formed of well-held-together (more particle–particle contacts and higher capillary forces) clusters of powder particles with crack running through the intercluster material. This could have led to the observed crack patterns. Through-thickness cracks were generated because the complete film was infiltrated in a single step. As such, the critical conditions for cracking would have been reduced.
6.4.2.3 Solution The key to achieving uncracked films is to reduce the degree of shrinkage, use a multilayer process, or encourage stress relaxation. This can occur by: Increasing the concentration of sol so that shrinkage is reduced. This may make infiltration of sol less effective which can result in the formation of interlayers; Using a multilayer deposition process instead of spray depositing the whole film and then infiltrating; or Introducing intermediate drying stages to allow the sol to dry and pyrolyze in a more controllable manner that allows a degree of stress relaxation.
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6.4.3 Issue: Poor piezoelectric properties 6.4.3.1 Circumstances The thickness piezoelectric coefficient (d33, f) for a PZT film was measured and found to be 50 pC/N. This was lower than expected; expected values were around 80 pC/N, based on previous measurements of identical films. The PZT film was 10 mm thick and poled at 180 C using contact poling with a field of 10 V/mm.
6.4.3.2 Investigation In a study of the evolution of the piezoelectric coefficient as a function of poling conditions, it was found that only when a high temperature and high field were used was the piezoelectric coefficient degraded (Figure 6.14). Conventionally, using a high field provides more energy to re-orient the domains within the structure, thus proving a higher piezoelectric response. Indeed, this was observed in the poling study when lower temperatures were used (130 C and 150 C). Increasing the temperature is supposed to make it easier to re-orientate domains, thus allowing a greater piezoelectric response to be obtained for a given applied field. This again was shown when lower fields are used. The results obtained indicate that the cause of the degraded properties of the PZT film was the combined effect of high field and high temperature during poling. When the electrical current was measured for the different poling conditions, it was found that the current was significantly higher in the cases where poor properties were observed. The increased current would have meant that it was
FIGURE 6.14
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more difficult (if not impossible) to maintain a voltage across the film with the result that domain reordination would have been significantly reduced.
6.4.3.3 Solution The combined effect of high voltage and high temperature led to increased conductivity and, possibly, even electrical breakdown. To maintain the high levels of poling, this needs to be avoided by: Reducing the temperature; Reducing the poling field; or Using pulsed fields to apply electric field for short periods of time. This is effective if it takes some period of time for the maximum current to build up.
6.5 PATTERNING The final types of problems that may arise are related to the inability to shape the functional thick film correctly. While the film may have the correct structure and properties, the poor shape definition reduces the performance of the microdevice.
6.5.1 Issue: Poor-quality micro-molded thick-film features 6.5.1.1 Circumstances PZT thick-film structures were created using a micro-molding method where the PZT was deposited as a spray to fill the molds. The molds were created using a photoresist polymer. The resultant features were smaller than the molds and also exhibited a significant sidewall angle.
6.5.1.2 Investigation Apart from the fact that the features were smaller than anticipated and possessed sidewalls which sloped significantly (Figure 6.15), the produced features were of high quality and exhibited a reasonably high density. Constrained sintering theory predicts that the sidewalls of isolated features can develop a slope due to enhanced sintering of the material near to the upper edges of the feature. The theory also predicts that the material near the bottom edge of the feature should sinter less and that
FIGURE 6.15
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the material in the body of the film should be of uniform density. However, an examination of the cross-sectional SEM micrographs (Figure 6.16) revealed no such variation in density. Furthermore, features of different sizes all exhibited the same degree of sidewall sloping and the features were all smaller than the mold feature by the same amount. This behavior cannot be explained by the sintering theory, which would predict different sizes of features to exhibit different absolute shrinkage and different sidewall angles. InterFIGURE 6.16 estingly, the material at the bottom of the edges did not taper smoothly to the substrate, but instead ended in an abrupt ‘ski jump’ feature which again could not be explained by sintering theory. The clue to the reasons behind these observed effects can be identified when the smallest features were examined, where it was shown that they were not only smaller than the molds and exhibited the low sidewall angle, but also not as high as the other features. This indicated that something was preventing the mold from filling effectively. An examination of a filled mold prior to sintering (Figure 6.16) showed that the real cause of the deformed features was the mold itself. During deposition, the mold deformed in shape (probably due to intermittent drying stages), resulting in a shadowing effect that prevented material from being deposited directly below it. As the processing progressed, this shadowing increased, leading to the formation of the sloping sidewall (Figure 6.16). In addition, it appeared that the mold materials attracted some of the material being deposited which contributed to the increasing shadowing.
6.5.1.3 Solution The photoresist mold and the deposition of ink onto the top of the mold combined to produce features that were smaller than the molds and had sloped sidewalls. These artifacts can be minimized by: Using a mold material with higher creep resistance so that it does not deform during deposition and heating; Removing deposited material from on top of mold structure by periodic cleaning during deposition; or Increasing the mold height to reduce shadowing. For further information see Wang, Rocks and Dorey (2009).
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6.5.2 Issue: Poor-quality inkjet-printed features 6.5.2.1 Circumstances A PZT ink was deposited by inkjet printing. While some features were obtained, many others coalesced, were deformed, or simply did not print (Figure 6.17). Where correctly printed, the features had a diameter of approximately 100 mm. The ink used contained PZT particles and used a propanolbased carrier fluid.
6.5.2.2 Investigation Issues with print quality tend to be due to either the printer or the ink used. Where there is a technical fault with the printer, the error tends to be systematic or progresFIGURE 6.17 sive (e.g. a single row is missing because the fourth print head has failed or the rows gradually tend to one side because of poor alignment). The observed faults show neither of these features, indicating that there is an issue with the ink. Variations in the properties of the ink (e.g. viscosity caused by an inhomogeneous distribution of powder) would cause differences in the landing positions of the drops but would be unlikely to cause drops (and even whole rows) to be missing. This behavior is more likely to be due to the clogging of the print-head orifice. As the print head begins to clog, ink ejection becomes more random as different amounts of ink are released, the timing of release varies, or the print head becomes partially cleared. Finally, when the print head becomes fully blocked, no more droplets are released. Print heads can become blocked for a number of reasons. Two of the most common are the ink drying at the opening of the orifice or the particles blocking the print head. An examination of the print head before (Figure 6.18) and after FIGURE 6.18 printing (Figure 6.19) showed that
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the print-head holes were partially blocked. However, there was no material on the outside face, as would be expected if the ink had dried on exposure to the air. Instead, it is clear that the particles in the ink have become lodged in the print head, preventing effective ejection of the ink droplets. The material visible at the mouth of the orifice is the residue of the binder that was dissolved in the carrier fluid.
6.5.2.3 Solution FIGURE 6.19 The deformed and missing features during inkjet printing of PZT films were due to the print-head holes being blocked by the particles in the ink, which could be addressed by:
Ensuring that the ceramic particles are at least 1/100 the size of the orifice in the print head. The print head shown here (Figure 6.19) is 20 mm wide; so, the ceramic particles used should be no larger than 200 nm. The particles used in this ink were around 300 nm in diameter and were highly likely to be the cause of the blockage; or Ensuring that the particles were stabilized in the ink to prevent flocculation and blockage of the nozzle Reducing the speed of sedimentation of the particles in the ink by reducing the size of the particles and increasing the stability of the ink. Sedimentation will cause the ink to block the outlet in the ink reservoir and is particularly important in this context, as PZT has a very high density (7.7 g/cm3) and so will exhibit a high sedimentation rate compared to other ceramic powders of comparable size.
6.5.3 Issue: Poor quality of edge on cantilever device 6.5.3.1 Circumstances A PZT cantilever structure was made consisting of a PZT film on a silicon substrate with 200-nm-thick patterned Ti/Pt electrodes and a 100-nm-thick ZrO2 blanket diffusion barrier. The PZT was etched first to expose the underlying silicon before the underlying silicon was etched by deep reactive ion etching (DRIE) from the back face of the wafer. When the devices were inspected, a ‘skirt’ of material was observed around the devices.
6.5.3.2 Investigation Analysis of the ‘skirt’ material showed it to be Zr rich, indicating that it was probably the remnants of the ZrO2 diffusion barrier. There was no evidence of silicon present on the underside of the
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cantilever, showing that the DRIE process had been effective at removing the underlying silicon. The rate of removal of ZrO2 in the Si etching process is very slow. However, it was not the intention to remove the ZrO2 in this stage. Instead, the ZrO2 was supposed to have been removed in the PZT etching stage before the DRIE treatment, indicating that there was a failure in this process. The PZT film was removed using a wet chemical etch based on HF/ HCl which should have been effective at removing the PZT as well as FIGURE 6.20 the ZrO2 material. As the ZrO2 layer remained after etching, it is clear that insufficient time was allowed for the etching, despite the thinness of the diffusion barrier layer. Clearly, the rate of etching of the ZrO2 layer is significantly lower than that of the PZT film. The high density of the ZrO2 is postulated to play a major role in this.
6.5.3.3 Solution As the primary purpose of the etching process was to achieve the best PZT etch (i.e. to obtain a PZT feature with the correct dimensions), it is impractical to extend the etch time to allow the ZrO2 to be removed. Potential solutions include: Patterning the diffusion barrier layer so that it does not extend beyond the PZT feature. In this way, there would be no material present, in the etched area, which could form a ‘skirt’ around the devices; Using an alternative diffusion barrier, which can be removed using an etchant that does not attack PZT or silicon. In this way, the ‘skirt’ can be removed by subjecting the device to a final etching process; or Removing the diffusion barrier entirely and processing the film at lower temperatures or on an alternative substrate. The barrier layer was only required to prevent reactions occurring between the film and substrate at the processing temperature.
6.5.4 Issue: Unsmooth edges on etched features 6.5.4.1 Circumstances A PZT film was deposited using a composite sol–gel approach with four infiltration steps being employed for each composite layer. The final film was sintered at 720 C of 30 min. Following wet etching, a stepped sidewall structure was observed (Figure 6.21) instead of the usual smooth sidewalled structure.
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6.5.4.2 Investigation From the micrograph (Figure 6.21) it can be seen that there are eight distinct layers present within the film. This correlates with the eight composite layers that were deposited as part of the film-forming process. When the composite layers are deposited on their own and the resultant film etched, the final features show no sign of a stepped sidewall structure. This indicates that it is the intermediate infiltration steps that are responsible for the observed effect. Interestingly, this effect only appears after four infiltration cycles FIGURE 6.21 per layer have been applied, indicating that it is at this stage that a critical transformation occurs. An examination of the surface of a layer that had been infiltrated four times (using SEM) showed that the film no longer appeared to be a powder bed, but, instead, appeared much smoother and was covered with a layer of sol–gel-derived material (Figure 6.22). This highdensity upper layer would have etched at a slower rate than the slightly porous PZT film that existed below it. The stepped structure would therefore have arisen as the throughthickness etching was momentarily slowed as each new layer was FIGURE 6.22 encountered. When less than four sol infiltration cycles had been completed, this interlay was not present and the through-thickness etching occurred at the same rate as the in-plane etching.
6.5.4.3 Solution Differences in etch rate for layers of infiltrated PZT film can be addressed by: Decreasing the number of sol infiltrations applied to each layer. A final, unifying, infiltration stage can be included to increase the overall density of the film if required.
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6.5.5 Issue: Excess material attached to edges of micro-molded features 6.5.5.1 Circumstances Ceramic thick-film structures were formed using a micro-molding process. The ceramic was deposited as a slurry using spray deposition. On removal of the molds by pyrolysis, excess material was present on the edges of the features. On occasion, this formed a considerable overhanging feature.
6.5.5.2 Investigation The presence of large overhanging material ruled out the possibility that the excess material was a result of dimensional changes during sintering. Instead, the material must have FIGURE 6.23 been deposited during the deposition process. A closer inspection of the features revealed uneven edges, indicating that some form of fracture had occurred as opposed to the features being formed by a defect in the mold. The evidence suggests that the original edge of the overhanging features had extended much further than what can be seen on the micrograph and had been damaged during processing, resulting in the fractures. This suggested that the features were formed as a result of the material inside the molds connecting with the material on top of the mold. When the mold material was removed, during heating, the join would have fractured giving rise to the observed features.
6.5.5.3 Solution The defect features are a result of the material in the mold becoming connected to the material on the mold surface during the deposition process. This would be a result of overfilling the molds or material falling into the molds and can be addressed by: Increasing the depth of molds to prevent overfilling; or Using an intermediate cleaning stage to remove excess material from the mold structures so that it does not fall into the mold cavity. In summary. Many things can go wrong during the processing of microscale functional ceramic thickfilm structures. Hopefully, this chapter has provided you with answers as to why some of this has occurred. If you have tried making some thick films on your own and found that they have not turned out as expected, Chapter 7 provides a collection of useful recipes that may help to overcome some of the issues. If you have not tried making any thick films, Chapter 7 can help you to get started more quickly.
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References Duval, F.F.C., Dorey, R.A., Haigh, R.H., Whatmore, R.W., 2003. Stable TiO2/Pt electrode structure for lead containing ferroelectric thick films on silicon MEMS structures. Thin Solid Films 444 (1–2), 235–240. Wang, D., Edirisinghe, M.J., Dorey, R.A., 2008. Formation of PZT crack-free thick films by electrohydrodynamic atomization deposition. J. Eur. Ceram. Soc. 28 (14), 2739–2745. Wang, D., Rocks, S.A., Dorey, R.A., 2009. Micromoulding of PZT film structures using electrohydrodynamic atomization mould filling. J. Eur. Ceram. Soc. 29, 1147–1155.