Ferroelectric thin and thick films for microsystems

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Materials Science in Semiconductor Processing 5 (2003) 65–76

Ferroelectric thin and thick films for microsystems R.W. Whatmore*, Q. Zhang, Z. Huang, R.A. Dorey School of Industrial and Manufacturing Sciences, Cranfield University, Cranfield, Bedfordshire, MK43 0AL, UK

Abstract Ferroelectric oxide thin and thick films are interesting materials for microsystem devices because of their wide range of useful properties, particularly the pyroelectric and piezoelectric effects. The ability to grow these films at relatively low temperatures onto a wide range of substrates, including silicon, is especially important. This paper discusses the use of CSD processes to grow high-quality ferroelectric PZT30/70 thin films onto platinised silicon substrates at low temperatures (from 4001C to 5751C), with particular reference to their use in pyroelectric infra-red detector arrays and other MEMS devices. The various factors important to the use of sol–gel processes are discussed, including mechanisms for sol ageing and for perovskite nucleation and growth. The latter is interesting, involving the formation of a transient Pt3Pb phase that acts as a nucleation layer for the templated growth of the PZT layer. The effects of Mn doping on the resulting materials properties are discussed. It leads to a strong asymmetry in the ferroelectric hysteresis behaviour as well as improved pyroelectric performance. Techniques for increasing the thickness of sol–gel layers, and the reasons for the appearance of nanoporosity, are reviewed. Finally, the use of sol–gel techniques for the fabrication of composite piezoelectric ceramic thick films (10–20 mm thick) at low temperatures (7101C) are discussed. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Ferroelectrics; Thin films; Thick films; Pyroelectricity; Piezoelectricity MEMS; Microsystems

1. Introduction Microsystems, or micro-electro-mechanical systems (MEMS), which combine semiconductor integrated circuits microassembled with mechanical, optical or other functional components [1], have received considerable attention because of their wide potential range of applications. These include such devices as accelerometers and gyroscopes for automotive air-bag sensors, automatic suspension and navigation systems, ‘‘smart’’ pills and catheters incorporating pressure, temperature, chemical and acoustic sensors for diagnostic use and a host of others. Ferroelectric materials offer a wide range of properties that make them useful for MEMS [2]. These include particularly strong pyroelectric and piezoelectric effects that can be applied in *Corresponding author. Tel.: +44-1234-750759; fax: +441234-751346. E-mail address: r.w.whatmore@cranfield.ac.uk (R.W. Whatmore). URL: http://www.nanotek.org/.

sub-systems such as uncooled infra-red thermal imaging arrays [3], 2D ultrasound arrays for 3D real-time medical imaging [4] and RF filters for mobile communications [5]. In many of these, the ability to integrate the active ferroelectric material in close association with a silicon signal processing array is central to the successful operation of the system. For example, in uncooled pyroelectric arrays [3], each sensitive element is a thin piece of ferroelectric material that generates charge via the pyroelectric effect when it absorbs infrared radiation. Each element is linked to a field effect transistor amplifier, which is in turn connected to an output via a multiplexer switch array. Many hundreds or even thousands of these need to be connected together to form a complete array and the only practical way to do this is to integrate the ferroelectric material directly onto a silicon chip that contains the FET amplifiers and switches. Many systems in manufacture use a thin ferroelectric ceramic wafer bonded to a 2D array of amplifiers and multiplexer switches integrated on an application specific silicon integrated circuit (ASIC). Companies such as BAE SYSTEMS Infra-red

1369-8001/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S1369-8001(02)00085-9

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Ltd. in the UK have developed arrays with 128
256 and 384
288 elements [3], while Raytheon in the USA have developed an array with 320
240 elements [6]. There is also a need for arrays with a lower resolution capability to address areas such as: spatially resolved people sensing (e.g. for monitoring people movements, environmental control, etc.); spatially resolved flame and fire sensing; security, automotive sensing, traffic monitoring and low-resolution imaging radiometry [7]. In all of these applications, the ability to integrate the ferroelectric material—and particularly oxide ferroelectrics such as those based on the lead zirconate titanate (PZT) solid solution series—as a thin film directly onto the silicon wafer bearing the signal processing would give major benefits in cost (because no costly ceramic processing and bonding steps would be required) and performance (because the thin films would be more thermally sensitive). However, there are many issues to address before such fully integrated structures can become a reality. Firstly, the film must be an appropriate thickness for the application. For pyroelectric applications, the ideal thickness is between 0.1 and 1 mm in thickness, depending upon the details of the physical design of the array (e.g. IR absorption mechanism, thermal isolation). Similar thicknesses are suitable for many piezoelectric sensing and some resonant devices. However, for piezoelectric actuators, the mechanical power that can be produced for a given applied field (usually limited by electrical breakdown or depoling effects) is proportional to the volume of piezoelectric. Hence, thicker films are desirable for integrated actuators and these should be typically 10–50 mm thick. Secondly, the temperature of film growth must be sufficiently low for the silicon metallisation to survive. This is typically o5751C for an array with active silicon components, although it is strongly dependent upon atmosphere [8], and can be considerably lower in air. It can be higher if an inert atmosphere is used, or a wafer without active circuitry. Thirdly, while the structure (crystallite orientation, porosity) of a film is not necessarily an issue in its own right, it is usually intimately related to the final electro-mechanical properties, which are critically important. Finally, film ‘‘processability’’ is very important in determining the ability to fabricate a device. This includes the conformality (or otherwise) of the coating method, the chemical compatibility of the layer with the underlying substrate and electrodes, the techniques available for patterning the film and its general industrial acceptability. This paper reviews the use of different processes for fabricating ferroelectric thin and thick films on silicon for MEMS applications, focussing on the use of chemical solution deposition (or CSD) for the growth of oxide ferroelectrics, especially PZT and related compositions for pyroelectric and piezoelectric devices,

and the more-successful ways in which some of the above issues have been addressed.

2. CSD of ferroelectric thin films for MEMS A wide range of CSD processes has been developed for ferroelectric thin film fabrication. Sol–gel methods, in which the metal cations are taken up into solution as alkoxides (for Zr and Ti) or acetates (for Pb and many dopant ions such as Mn) are usually based on chelating solvents such as 2-methoxyethanol (2ME) [9,10], acetic acid with alcohols and stabilisers such as acetyl acetone and ethylene glycol (EG) [11,12] and diols [13]. There has been a move to eliminate the use of 2ME, because of its carcinogenic and teratogenic properties, leading to a greater emphasis on the use of simple alcohol solvents such as ethanol or methanol [14]. Metal-organic decomposition processes that use metal carboxylates (e.g. Pb-2-etyhylhexanoate, Ti-di-methoxy-di-neodecanoate) in solvents such as butanol or xylene have also been explored with success [15], but the sol–gel processes are far more popular. A literature search conducted over the years 1981–2002 revealed 690 papers on sol–gel PZT, while only 49 were found on MOD. The popularity of the sol–gel processes is probably because the amount of carbon in the films is much lower than the MOD processes. On the other hand, while sol–gel solutions are certainly stable enough to be used for weeks, sol ageing during storage is an issue that needs to be addressed, certainly from the aspect of industrial acceptability. 2.1. Sol ageing The issue of sol ageing has been examined in detail by Zhang et al. [16,17]. They have shown (by a combination of photon correlation spectroscopy (PCS) and small angle X-ray scattering (SAXS)) that the ageing of PZT 30/70 sols based upon metal alkoxides in acetic acid/ methanol/EG solutions is due to the polycondensation of mixed metal alkoxide-acetate derivatives (Fig. 1). These form long-chain particles that are typically 0.4 nm in diameter and ca. 4.8 nm long initially [16] as measured by SAXS or ca 5–6 nm by PCS (see Fig. 2). Over a long period of time, the particles grow as indicated in Fig. 2 and eventually the solutions gel. This happens even in sols that are nominally ‘‘unhydrolysed’’, but clearly happens much more rapidly if water is added. Obviously, there is a question as to how long the sols can be used before the quality of the films becomes unuseable. Huang et al. [18] have modelled the growth of the particles in the sols as a function of time and found that there were three regimes. Initially, the particles grow by aggregation of ‘‘monomers’’ onto the particles (Fig. 1) so that the equivalent spherical diameter d (equal to the square root of the chain length)

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Fig. 1. Illustrating the growth of polymeric particles through polycondensation of a mixed metal alkoxide acetate derivative [18].

Fig. 2. Particle growth with time in PZT sols containing different amounts of acetic acid [18].

grows as d ¼ a þ bt1=2 (where t is time and a; b are constants). This is because we expect the chain length to grow linearly with time for doD0 ; where D0 is a diameter above which particle–particle aggregation takes over from direct growth of monomers onto the particles. In this stage of growth d ¼ d0 eat (where d0 is a constant—of similar magnitude to D0 —and a is a constant defining growth rate, which is in turn proportional to the H2O concentration in the sol). At very large particle sizes, one might expect that the growth rate will become limited by the diffusion of particles. In this diffusion limited aggregation (DLA) regime, the dBtb

Fig. 3. Particle sizes as functions of ageing time for hydrolysed sols (H2O/EG=1.5) at 81C, 171C and 231C. The solid, dotted and dashed lines are the theoretical curves to fit particle growth profiles of 81C, 171C and 231C, respectively. The marked decrease of particle growth rate at the end of the experiment (just before gellation) can be explained by DLA, characterised by a power-law growth, as indicated in the inset [18].

where b is a constant. A typical graph of particle size as a function of ageing time is shown in Fig. 3. The constants derived from these curves can be used to predict the time taken for growth of the particles to a

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given size at a given temperature [18], and hence the useful life of the sol in storage. 2.2. Control of crystallite orientation The orientation of the crystallites within a PZT thin film has been shown to have a major effect on the properties of the resulting film. Shorrocks et al. [19] showed that the best orientation for pyroelectric PZT30/ 70 films is (1 1 1), while Seifert et al. [20] have shown that the best piezoelectric properties in rhombohedral PZT films with compositions close to the morphotropic phase boundary (MPB) are produced if the film possesses a (1 0 0) orientation. Yoshimura et al. [21] also showed this with Pb(Yb1/2Nb1/2)O3–PbTiO3 (50/50) films grown epitaxially using pulsed laser deposition onto a (0 0 1) LaAlO3 substrate. Different techniques have been used to grow oriented perovskite crystallites within the films. In the case of CSD processing, the perovskite phase forms by crystallisation from a nanocrystalline phase which is a type of pyrochlore, having a disordered fluorite structure. This is accompanied by a major reduction in volume, which can lead to porosity in the films [22]. Most of the oxide ferroelectric films grown onto silicon substrates are deposited onto an underlying Pt electrode (which is in turn deposited onto a Ti adhesion layer on underlying SiO2 or Si3N4). The Pt almost always shows a high degree of (1 1 1) texture. Muralt [23] has reported that thin films of TiO2 deposited by sputtering onto the Pt prior to PZT growth can induce a strong (1 1 1) texture, while pre-depositing a thin (1 0 0) oriented layer of PbTiO3 can induce a strong (1 0 0) texture in the PZT film. Work, first by Chen and Chen [24,25] on MOD films and later by Huang et al. [26] on sol–gel films showed that the presence of residual carbon in the PZT film can lead to the formation of a lead platinum intermetallic through the reduction of the lead oxide in the film. This was shown to be Pt3Pb (lattice parameter a0 ¼ 0:405 nm) [26], which grows epitaxially on the underlying (1 1 1) Pt and acts as a buffer layer between the Pt (lattice parameter a0 ¼ 0:3923 nm) and the PZT (a0 ¼ 0:4035 nm). The Pt3Pb phase is a transient phase and the Pb within it is oxidised back into the PZT during the course of the PZT perovskite phase crystallisation. Fig. 4 shows a crosssectional TEM photograph, which illustrates a layer of perovskite PZT nucleated on top of a Pt3Pb layer, itself grown on the top of the underlying Pt electrode. An advantage of this process is that the nucleation is very uniform over the full area of the electrode and so the advancing phase front proceeds uniformly through the layer thickness. Fig. 5 shows a set of multiple layers grown to produce a composite layer about 1 mm in thickness. The kinetics of this process have been studied in detail [27,28] and it has been shown that there must be sufficient Pt3Pb formed, so that when the PZT nucleates,

Fig. 4. TEM cross section of a PZT film as in Fig. 4, in this case showing the nucleation of perovskite PZT on top of the Pt3Pb intermetallic layer after the film has been dried at 2001C and annealed at 4401C for 600 s [27].

Fig. 5. TEM cross section of a multiple layer (1 1 1) oriented PZT film.

it has a template to nucleate onto. This means that drying the films at too high a temperature (e.g. above 3001C) or for too long will drive-off too much carbon so that there is insufficient Pt3Pb formed at the perovskite phase formation temperature, leading to a more-random orientation. Perovskite can be nucleated at a very low temperature (4201C, and possibly below [28]). However, the nucleation and growth of the phase is very slow and under these conditions the Pt3Pb can be reoxidised before the perovskite phase is properly established. In this case, the PZT also tends to be more-randomly orientated, and we can see this from the increase in the relative intensity of the (1 0 0) to (1 1 1) XRD reflections (Fig. 6).

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The fabrication of Mn-doped pyroelectric PZT films is a good example of how CSD can be used for the growth of ferroelectric thin films for MEMS applications [31]. In the particular process used, Pb(OAc)2  3H2O and Mn(OAc)2 (1 mol %) were dissolved in a methanol/methanolamine (CH3OH/MEA) mixture. Zr(OnPr)4.nPrOH and Ti(OnBu)4 were mixed separately and CH3CH2OH and a stoichometric amount

of acetic acid were added. The Pb solution was added to the Zr/Ti solution and acetic acid was added to adjust the pH and the PZT concentration to 0.4 M. This solution was spun onto Pt(1 1 1)/Ti/SiO2/Si(1 0 0) substrates (thickness Pt/Ti/SiO2=100/5/500 nm) at 3000 prm for 30 s. Each layer was pre-fired on a hot plate set at 2001C for 30 s and then further annealed on another hot plate set at 5301C for 3B5 min in air. The thickness of a single layer was about 70 nm, so to obtain thicker films more layers were put down by repeating the above procedure. Au/Cr top electrodes were used. Access to the Pt bottom electrode was obtained by wet-etching a corner of PMZT film. The PMZT thin film was poled by applying an electric field at 901C. Fig. 7 shows the hysteresis loops of a PMZT thin film (700 nm thick, poled at 20 V, 10 min at 901C). Asymmetric hysteresis loops were observed. The shift-direction of the P2E loop depends on the polling direction. Although some hysteresis loop asymmetry was observed in the PZT thin film due to the use of a pair of asymmetric electrodes (Au/Cr as top and Pt as bottom), it was observed more obviously in the Mn-doped PZT thin film. If the sample was subjected to an applied electric field with the positive end connecting to the top electrode of the thin film, the P2E loop shifted to the negative direction and vice versa, as observed in Fig. 7. Table 1 lists the dielectric constant and loss tangents for PMZT films annealed at 5301C and 5601C. The

Fig. 6. Relative intensity of Ið1 0 0Þ=Ið1 1 1Þ XRD profiles vs. annealing time at different temperatures for sol–gel PZT thin films. It can be seen that a higher annealing temperature leads to a lower (1 0 0) relative intensity. At 4601C, the film is completely (1 1 1) orientated.

Fig. 7. Hysteresis loops taken from unpoled and poled PMZT thin films (700 nm thick).

Another technique for controlling the nucleation and growth of the perovskite crystallites has been described by Wilson et al. [29]. In this work, silver and gold have been used as alloying elements to expand the lattice parameter of the Pt electrode so that it more closely matches that of the PZT film. Here again, the improved lattice parameter match improves the degree of film orientation. Wilson [30] has also shown that Pt3Pb can be formed by the evaporation of Pb onto the underlying Pt electrode prior to the PZT growth and subsequent thermal annealing in vacuo. In this case of ex situ Pt3Pb formation, a similar degree of film orientation improvement can be achieved as with the in situ case as described above, but there is no longer the necessity to rely upon the presence of carbon to form free Pb, removing one of the constraints on the sol–gel film growth—that of drying temperature. 2.3. Pyroelectric PMZT thin films

Table 1 Dielectric and pyroeclectric properties of a PMZT thin film Sample

e (100 Hz)

tan d (100 Hz)

e (1 kHz)

tan d (1 kHz)

Resistivity (O cm)

P (C K1m2)

FD (Pa0.5) (100 Hz)

PMZT-530 PMZT-560

260 253

0.006 0.006

257 253

0.0067 0.006

7:24E þ 10 7:50E þ 10

3.0
104 3.5
104

3.00
105 3.85
105

The film was poled at 20–25 V, 901C. PMZT-530 and PMZT-560 were annealed at 5301C and 5601C, respectively. e=dielectric constant, tan d=dielectric loss, p=pyroelectric coefficient.

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dielectric constant and tangent loss of the film annealed at 5301C were 260 and 0.006 at 100 Hz and 257 and 0.0067 at 1 kHz, respectively. Under similar processing conditions, PZT thin films exhibit a dielectric constant of B360 and tangent loss of 0.01 [31]. In comparison with PZT thin film with similar thickness, Mn doped PZT thin film has a lower dielectric constant and a lower or equivalent tangent loss, which is of great significance in enhancing the performance of an infrared detector. The pyroelectric coefficient of this PMZT thin film is also given in Table 1. Usually, a PZT thin film has a pyroelectric coefficient of 1B2
104 C K1 m2. Doping PZT with 1 mol% Mn increases significantly the pyroelectric coefficient of the films, leading almost to a factor of three improvement in the figure-of-merit FD [3]. This improvement is due to the significant reductions in dielectric constant and loss and the improvements in the pyroelectric coefficient. It seems likely that both of these improvements can be ascribed to the fact that the Mn acts as a ‘hardening’ dopant in the PZT lattice, creating oxygen vacancies and pinning the residual domains. It is likely that the internal bias present in the film due to the addition of Mn acts to stablise the internal polarisation, accounting for the increase in the pyroelectric coefficient. The pyroelectric coefficient can be further improved by increasing the film annealing temperature from 5301C to 5601C. At 5601C, the dielectric constant and loss of the film almost keep constant as those at 5301C but the pyroelectric

coefficient increased to 3.52
104 C K1 m2. Figureof-merit of the film has therefore also been improved and it increased to 3.85
105 Pa0.5. 2.4. Direct patterning of PZT films The fabrication of MEMS devices using ferroelectric thin films depends upon the ability to pattern the films. While it is perfectly possible to pattern the films using conventional photolithography and etching, it would be highly desirable if the films could be patterned by direct exposure under UV of the unannealed gels. Fig. 8 shows the procedure for the preparation of gel films from chemically modified alkoxides and UV irradiation. Pb(OAc)2  3H2O was dissolved in methanol with heating. Acetic acid has been widely used to synthesise PZT sols either as an additive or as a solvent because it can suppress the hydrolysis and polymerisation reactions of highly reactive metal alkoxides by replacing one or more alkoxide groups so that the sol–gel process can be carried out in an ambient atmosphere. Organic acids which contain double bonds, such as acrylic acid, can be used as organically modified agents for directly contact printing of sol–gel films. These double bonds in modified sol–gel materials are able to absorb UV energy. Therefore, the subsequent sol–gel material acts as a negative photoresist, i.e. the parts that are exposed remain as the sol–gel material is developed in wet washing. The molar ratio of metal alkoxide/organic acid was 1:1. The molar

Fig. 8. Flow chart showing the procedure for preparing photosensitive PZT sols.

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ratio of Pb:Zr:Ti was 1.1:0.3:0.7 and the concentration of final solution was 0.4 M based on Pb. A photosensitive initiator (1 mol%) and accelerator (1 mol%) were also added to the final solution. The fine-patterning process in the present study is shown in Fig. 9. The film was exposed to UV light (wavelength was B365 nm) by a Karl Suss . MJB21 Mask Aligner and the exposed area of the sol–gel thin film became insoluble in water or methanol so that negative patterns were obtained by development with water or methanol. The patterned film was blown-dry prior to it being sintered at high temperature (5301C) in air for 10 min. A two-layer film was about 200 nm thick, as measured by a Dektak surface profilometer. The dielectric and ferroelectric properties of the film were measured by a Genrad 1689M RLC Digibridge impedance analyzer and a Radiant Technologies RT66A, respectively. Fig. 10

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shows the optical photographs of patterned PZT film on Pt/Ti/SiO2/Si substrate. The picture of fine patterning indicates that the acid-modified PZT films are patternable and the UV-irradiation made the gel films insoluble in methanol or water. Fig. 11 shows some SEM photographs of the sintered pattern, as examples. The negative pattern of mask used was precisely transferred and the thickness of the PZT film was about 0.2 mm. The minimum size of the patterns was 10 mm. There are some residues of PZT around the patterns after development. 100 nm thick Au/Cr top electrodes were deposited on the film by RF magnetron sputtering. The remanent polarisation (Pr ) and coercive field (Ec ) at 8 V were 34 mC/cm2 and 108 kV/cm, respectively. The dielectric constant and loss tangent are 330 and 0.015, respectively at 1 kHz and 340 and 0.015 respectively at 33 Hz. These values are very comparable with those of PZT thin films fabricated by conventional sol–gel processing. 2.5. Thickness increase and porosity in sol–gel PZT films

Fig. 9. Fine-patterning process based on the photolysis of chemical modified gel films.

To promote the (1 1 1) orientation of PZT film and to reduce the number of time-consuming steps in the sol– gel processing, a bi-layer firing approach was investigated. In this approach, two layers were deposited and dried successively and then the bi-layer was annealed at high temperature to transform it into perovskite. Such prepared PZT films were highly (1 1 1) oriented, but a significant number of voids appeared at the centre of each bi-layer. Fig. 12 shows a cross-section TEM image for a film, which was obtained by annealing every bilayer at 4601C for 2 h after deposition and drying each layer successively at 2001C for 3 min. Many voids can be observed at the centre of the 2nd, 3rd, 4th, and 6th

Fig. 10. Optical microscopic photograph of the patterned PZT film on Pt surface. Dark area is the PZT film.

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Fig. 11. SEM photograph of patterned PZT film on Pt surface. Bar area is PZT.

Fig. 12. TEM cross section of a PZT film which was obtained by annealing every bi-layer after two successive deposition and drying. Many voids are observed at the centre of the 2nd, 3rd, 4th, and 6th bi-layer layers. Very few voids could be seen in the first bi-layer and non in the 9th layer, which corresponds to the 5th annealing with only a single layer (annealing interfaces are marked by arrow heads). The size of the voids is the largest for the 4th bi-layer.

bi-layer layers. Very few voids could be seen in the first bi-layer and non in the 9th layer, which corresponds to the 5th annealing (annealing interfaces are marked by arrow heads) with only a single layer. Noted that the sizes of voids are the largest for the 4th bi-layer. Fig. 13 shows that voids are formed in the bi-layer which is still pyrochlore—before the phase transformation from

pyrochlore into perovskite (py–pk) taking place. These suggest that the voids started to form during the pyrochlore formation from the as-deposited materials, and grew larger during the transformation from pyrochlore to perovskite. This void formation is possibly partly due to the volume shrinkage of ca 13% that occurs on transformation from the pyrochlore to the

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Fig. 13. TEM cross section of a PZT film obtained by annealing every bi-layer after two successive deposition and drying. Voids are observed in the bi-layer which is still pyrochore. This suggests that the voids start to form during the pyrochore formation from the asdeposited materials.

Fig. 14. SEM cross sectional image for a single PZT layer. Noted that the cross section was imaged at 451 tilt. The thickness of this single layer was about 0.4 mm.

perovskite phase, as has been previously discussed by Voigt et al. [32]. In their work they observed void formation around the rosette nuclei. In the work reported here, there is uniform nucleation of the perovskite across the whole of the underlying platinum and a uniformly advancing perovskite phase front, and the void formation starts before the py–pk transition occurs. It seems likely that the voids form at least partly because of loss of organic material and at the interfaces between the bi-layers because these are lines of weakness in the structure. Upon further annealing these voids start to shrink until they finally disappear completely after annealing for 12 h at 4601C. The precise evolution profile will depend on the thickness of the film, drying and final annealing temperatures, etc. The film thickness increases with increase in the viscosity of PZT sols. Addition of the polymer, polyvinylpyrrolidone [33] to the PZT precursors, with the ratio of Pb:polymer=1:1 (weight %) increase the sol viscosity and the ‘‘strength’’

of the layers, preventing cracking. The solution was then spun onto the substrate under the same conditions as in the bi-layer experiments. The film was dried at 2001C for 30 min and then heated at 4501C for 10 min, and finally annealed by rapid thermal annealing at 6001C for 60 min. Fig. 14 shows a cross section SEM image for such a single PZT layer. The film was crack-free, but contained a substantial amount of voids. This void formation is almost certainly due to the accommodation of volume shrinkage due to the loss of organic material, as well as the pyrochlore to perovskite transition. In summary, in the normal sol–gel method, it is observed that voids are formed if the layer thickness exceeded a certain limit, normally no more than 100 nm after perovskite phase transformation. Novel approaches are needed to eliminate such voids in thin films (o1 mm) apart from reducing the thickness of each annealing layer.

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3. Thick ferroelectric films by composite/CSD processing Films of up to 1 mm thick can be readily produced by conventional CSD processing as described above. These are very well suited to sensors, such as the pyroelectric infra-red detector application discussed above. However, there are many piezoelectric actuator applications for which thicker films (10’s of microns) are needed. The conventional CSD approaches are not feasible for these. Composite Film (ComFi) technology is an adaptation of conventional sol–gel film processing. It is based on a technique originally described by Barrow et al. [34]. This process was modified by Corker et al. [35] and then further adapted by Dorey et al. [36]. It enables the deposition of dense PZT films between 1 and 50 mm thick at temperatures as low as 7101C.

Once the required film thickness is attained the film is sintered at 7101C for 30 min, a very low temperature for thick film growth. 3.2. Film microstructure and properties Fig. 15 shows the microstructures resulting in films produced using 0, 2 and 4 intermediate sol infiltration/ pyrolysis steps. It can be seen that homogenous highdensity PZT films are obtained when 4 intermediate sol

3.1. Film deposition Composite films are deposited by spin coating onto a substrate a slurry made by dispersing a PZT ceramic powder into a sol. The resulting porous layer is infiltrated with sol to further increase the density of the film. In this case, the sol used in the process is produced using a 2-methoxyethanol route. Lead acetate is dehydrated by refluxing with acetic acid and then subsequently distilling the product. Titanium-isopropoxide, zirconium isopropoxide and 2-methoxyethanol are then refluxed to yield the B-site cation mix. The A and B-site cation mixes are then combined and refluxed prior to a final distillation process. The sol is diluted with 2-methoxyethanol to produce a 1.1 M stock solution. Ethylene glycol is added to the sol as a stabilising agent prior to use. The composition of the sol can be matched to that of the PZT powder by adjusting the dopant levels—typically manganese acetate, antimony ethoxide and niobium ethoxide are added to the B-site cation mix prior to initial reflux stage. The composite slurry is produced by mixing the sol with PZT powder (1.5 g/ml), a liquid phase sintering aid [37] (4.7 wt% 0.8PbO–0.2Cu2O) and a dispersant (2 wt% KR55, Kenrich Petrochemicals). The slurry mix is ball milled for 24 h under a nitrogen atmosphere. Prior to deposition of the films, the substrates are cleaned using an acetone/propan-1-ol wash and plasma ashing processes (Polaron PT7160 RF plasma barrel etcher). The substrate is then coated with the composite slurry and spun at 2000 rpm for 30 s. Following spinning the film is subjected to a heat treatment, at 2001C for 60 s and 4501C for 15 s, designed to remove the solvent and pyrolise the sol. The resultant porous film is then infiltrated with sol and subjected to a further heat treatment. By varying the number of sol infiltration/ pyrolysis treatments the level of porosity can be controlled. To increase the film thickness further composite layers, with sol infiltrations, are deposited.

Fig. 15. Scanning electron photomicrographs for a 4 layer composite film made using (a) 0, (b) 2, (c) 4 intermediate sol infiltration/pyrolysis steps.

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substrate to the film. The thin film values for the piezoelectric coefficients of the films made with 4 infiltration steps are d33;f ¼ 60 pC/N, e31;f ¼ 4:6 Cm2. These compare with values of d33;f ¼ 130 pC/N, e31;f ¼ 16 Cm2 that would be predicted for a similar bulk ceramic bonded to a rigid substrate and are definitely in the range that would be useful in MEMS devices.

4. Conclusions Fig. 16. d33;f of composite films as a function of the number of intermediate sol infiltration/pyrolysis steps employed per composite layer.

Fig. 17. e31;f of composite films as a function of the number of intermediate sol infiltration/pyrolysis steps employed per composite layer.

infiltration/pyrolysis treatments are used. If no sol treatments are employed the resultant microstructure is highly porous. The density gradient through the film is caused by the sol from ‘‘higher’’ layers of the composite slurry infiltrating the ‘‘lower’’ layers. The films were poled at 1301C for 5 min using a field of 8 V/mm. The piezoelectric coefficients d33;f and e31;f of a hard doped PZT thick film have been assessed using a modified Berlincourt type piezometer (TakeControl PM25) [38]. d33;f (Fig. 16) is shown to be approximately constant (B60 pC/N) once a critical number of intermediate sol infiltration/pyrolysis treatments have been exceeded. A slightly lower value of d33;f is observed for very porous samples. The value of 60 pC/N is lower than that reported for bulk material of comparable composition. However, it should be noted that the value of d33;f measured is a ‘clamped’ piezoelectric coefficient and as such the ‘free’ value is expected to be higher. e31;f shows a marked increase as the number of intermediate sol infiltration/pyrolysis treatments are increased (Fig. 17). This is a direct result of the increase in the density of the film enabling better transfer of lateral stress from the

This paper has reviewed the use of CSD processing in the growth of thin and thick films of ferroelectrics, particularly PZT, with particular emphasis upon their potential use in MEMS applications. It has been shown that the basic sol–gel process is a versatile one, which can be used for growing a wide range of film compositions with excellent electrical and mechanical properties. The structure of the underlying substrate can be altered in various ways to control the nucleation of the perovskite PZT crystallites at the electrode/film interface, to produce either (1 1 1) or (1 0 0) orientations. These include the deposition of interface layers, or the alteration of the lattice parameter of the electrode by the creation of intermetallics or alloys. The basic sol–gel process can be altered by the inclusion of ceramic particles that allow the growth of thicker films suitable for piezoelectric actuator applications, again at relatively low temperatures.

Acknowledgements The authors would like to thank several sponsors who have made aspects of the work reported here possible. These include EPSRC through projects GR/N05970 and GR/R92448, the Defence Research Agency (Malvern), the CEC through the SEMDEFT and PARMENIDE projects and TDK (Japan). RWW would like to acknowledge the financial support of the Royal Academy of Engineering.

References [1] Bryzek J, Petersen K, McCulley W. Micromachines on the march. IEEE Spectrum, May 1994, 20–31. [2] Whatmore RW. Ferroelectrics, microsystems and nanotechnology. Ferroelectrics 1998;225:179–92. [3] Whatmore RW, Watton R. Pyroelectric materials and devices. In: Capper P, Elliott CT, editors. Infrared detectors and emitters: materials and devices (ISBN 07923-7206-9). London: Chapman & Hall, 2000. p. 99–148. [4] http://www.ifi.uio.no/Bsverre/nice.html. [5] Su Q-X, Kirby P, Komuro E, Imura M, Zhang Q, Whatmore RW. Thin film bulk acoustic resonators and

76

[6] [7] [8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

[16]

[17]

[18]

[19]

[20]

[21]

R.W. Whatmore et al. / Materials Science in Semiconductor Processing 5 (2003) 65–76 filters using ZnO and lead zirconuium titanate thin films. IEEE Trans. MMT 2001;49(4):769–78. Hanson CM, Beretan HR, Belcher JF, Udayakumar KR, Soch KL. Proc SPIE 1998;3379:60. http://www.irisys.co.uk/. Donohue PP, Todd MA, Anthony CJ, Brown AG, Harper MAC, Watton R. High temperature processing of ferroelectric thin films using interconnect wafer technology. Integ Ferroelectrics 2001;41:25–34. Gurkovitch SR, Blum JB. Crystallization of amorphous lead titanate prepared by a sol gel process. Ferroelectrics 1985;62:189–94. Budd KD, Dey SK, Payne DA. Sol–gel processing of PbTiO3, PbZrO3 PZT and PLZT thin films. Br Ceram Proc 1985;36:107–22. Yi G, Wu Z, Sayer M. Preparation of Pb(Zr,Ti)O3 thin films by sol gel processing: electrical, optical and electrooptic properties. J Appl Phys 1988;64(5):2717–24. Schwartz RW, Boyle TJ, Lockwood SJ, Sinclair MB, Dimos D, Buchheit CD. Sol–gel processing of PZT thin films: a review of the state-of-the-art and process optimisation strategies. Integ Ferroelectrics 1995;7:259–77. Phillips NJ, Milne SJ. Diol-based sol–gel system for the production of thin films of PbTiO3. J Mater Chem 1991; 1(5):893–4. Zhang Q, Whatmore RW, Vickers ME. A comparison of the nanostructure of lead zirconate, lead titanate and lead zirconate titanate sols. J Sol Gel Sci Technol 1999;15: 13–22. Klee M, Eusemann Wasser R, Brand W, van Hal H. Processing and electrical properties of Pb(ZrxTi1x)O3 (x=0.20.75) films: comparison of metallo-organic decomposition and sol–gel processes. J Appl Phys 1992; 72(4):1566–76. Zhang Q, Vickers ME, Patel A, Whatmore RW. Determination of particle size and shape during the hydrolysis of Pb(Zr0.3Ti0.7)O3 precursor solutions. J Sol Gel Sci Technol 1998;11:141–52. Zhang Q, Huang Z, Whatmore RW. Studies of lead zirconate titanate sol ageing Pt. I: Factors affecting particle growth. J. Sol Gel Sci Technol 2002;23(2): 135–44. Huang Z, Zhang Q, Whatmore RW. Studies of lead zirconate titanate sol ageing Pt. II: Particle growth mechanisms and ageing. J Sol Gel Sci Technol 2002;24(1): 49–55. Shorrocks NM, Patel A, Walker MJ, Parsons AD. Integrated thin film PZT pyroelectric detector arrays. Microelectron Eng 1995;29(1–4):59–66. Seifert A, Ledermann N, Hiboux S, Baborowski J, Muralt P, Setter N. Processing optimization of solution derived PbZr1xTixO3 thin films for piezoelectric applications. Integ Ferroelectrics 2001;35(1–4):1889–96. Yoshimura T, Trolier-McKinstry S. Transverse piezoelectric properties of epitaxial Pb(Yb1/2Nb1/2)O3–PbTiO3 (50/50) films. J Cryst Growth 2001;229(1):445–9.

[22] Voigt JA, Tuttle BA, Headley TJ, Lamppa DL. The pyrochole-to-perovskite transformation in solution-derived lead zirconate titanate thin films. Mater Res Symp Proc 1995;361:395–402. [23] Muralt P. PZT thin films for microsensors and actuators: where do we stand? IEEE Trans UFFC 2000;47(4):903–15. [24] Chen S-Y, Chen I-W. Phase transformations of oriented Pb(ZrxTi1x)O3 thin films from metalo-organic precursors. Ferroelectrics 1994;152:25–30. [25] Chen S-Y, Chen I-W. Texture development, microstructure evolution and crystallization of chemically derived PZT thin films. J Am Ceram Soc 1998;81(1):97–105. [26] Huang Z, Zhang Q, Whatmore RW. The role of an intermetallic phase on the crystallisation of lead zirconate titanate in sol gel processes. J Mater Sci Lett 1998;17: 1157–9. [27] Huang Z, Zhang Q, Whatmore RW. Low temperature crystallisation of lead zirconate titanate thin films by a sol gel method. J Appl Phys 1999;85(10):7355–61. [28] Huang Z, Zhang Q, Whatmore RW. Structural development in the early stages of annealing of sol–gel prepared lead zirconate titanate thin films. J Appl Phys 1999; 86(3):1662–9. [29] Wilson R, Huang Z, Zhang Q, Whatmore RW, Wang P. Effect of alloying platinum with silver: in relation to CSD processing of PZT thin films. Integ Ferroelectrics 2000;29: 251–71. [30] Wilson R. Nano structured electrodes for ferroelectric thin films. PhD thesis, Cranfield University, 2002. [31] Zhang Q, Whatmore RW. Sol gel PZT and Mn-doped PZT thin films for pyroelectric applications. J Phys D 2001;34:2296–301. [32] Voigt JA, Tuttle BA, Headly TJ, Lamppa DL. The Pyrochlore to Perovskite transformation in solution derived Lead Zirconate Titanate thin films. Mat Res Soc Symp Proc 1995;361:395–402. [33] Kozuka H, Kajimura M, Hirano T, Katayama K. Crackfree, thick ceramic coating films via non-repetitive dipcoating using polyvinylpyrrolidone as stress-relaxing agent. J Sol–Gel Sci Techn 2000;19:205–9. [34] Barrow DA, Petroff TE, Sayer M. Thick ceramic coatings using a sol gel based ceramic-ceramic 0–3 composite. Surf Coatings Technol 1995;76–77:113–8. [35] Corker DL, Zhang Q, Whatmore RW, Perrin C. PZT ‘composite’ ferroelectric thick films. J Eur Ceram Soc 2002; 22:383–90. [36] Dorey RA, Stringfellow SB, Whatmore RW. Effect of sintering aid and repeated sol infiltrations on the dielectric and piezoelectric properties of a PZT composite thick film. Eur Ceram Soc 2002;22:2921–6. [37] Corker DL, Whatmore RW, Ringgaard E, Wolny WW. Liquid-phase sintering of PZT ceramics. J Eur Ceram Soc 2000;20:2039–45. [38] Southin JEA, Wilson SA, Schmitt DA, Whatmore RW. e31,f determination for PZT films using a conventional ‘d33’ meter. J Phys D 2001;34:1456–60.