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Porosification behaviour of LTCC substrates with potassium hydroxide
Journal of the European Ceramic Society 38 (2018) 2369–2377
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Original Article
Porosification behaviour of LTCC substrates with potassium hydroxide a,⁎
b
a
T
a
Ali Hajian , Michael Stöger-Pollach , Michael Schneider , Doruk Müftüoglu , Frank K. Crunwellc, Ulrich Schmida a b c
Institute of Sensor and Actuator Systems, TU Wien, Gusshausstrasse 27–29, 1040 Vienna, Austria University Service Centre for Transmission Electron Microscopy, TU Wien, Wiedner Hauptstrasse 8–10, 1040 Vienna, Austria CM Solutions (Pty) Ltd, 89J Victoria Drive, London SW19 6PT, UK
A R T I C L E I N F O
A B S T R A C T
Keywords: LTCC Wet-chemical etching Permittivity reduction Porosification Glass-ceramics
The demand to meet advanced substrate requirements in terms of electrical, mechanical, thermal, and dielectric properties has led to an increasing interest in low temperature co-fired ceramics (LTCC). However, LTCC materials suffer from high permittivity. We recently showed that the wet-chemical porosification under acidic conditions allows the reduction of the permittivity of LTCCs in the as-fired state. In the present study, potassium hydroxide solution was employed as an alternative etchant which features a suitable bearing plane for further metallization lines. Various characterization techniques, including scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction analysis, and electron energy loss spectroscopy were used for investigation of the morphology and chemical composition of the substrates. Three-dimensional information of the surface topography was acquired by means of MeX® Alicona software and the obtained roughness parameters confirmed the advantage of the proposed approach over acid treatment when targeting an enhanced surface quality.
1. Introduction Low temperature co-fired ceramics (LTCC) is an advanced substrate technology for the robust assembly and packaging of electronic components, enabling even the realization of gas-proof housings. Due to a multilayer approach based on glass-ceramics sheets this technology offers the possibility to integrate into the LTCC body passive electrical components and conductor lines typically manufactured in thick film technology [1–7]. Compactness, light weight, high integration level and the good compatibility with radio frequency microelectromechanical systems (RF-MEMS) and to monolithic microwave integrated circuits (MMICs), have made LTCC a very attractive technology for a wide application range such as in wireless communication or in automotive radar systems [8–11]. In detail, these applications comprise mobile telecommunication devices (0.8–2 GHz), wireless local networks such as Bluetooth (2.4 GHz) to in-car radars (50–140 GHz, and 76 GHz) [12,13]. In order to decrease the losses of radiating elements such as patch antennas, a locally adjustable permittivity within a single-layer of an LTCC substrate would be highly beneficial where the low permittivity areas enhance both the bandwidth and the efficiency of the active components. In parallel, the high permittivity areas allow a compact
⁎
feeding circuit design [14–16]. As demonstrated recently, the application of wet-chemical porosification of the LTCC surface allows the local reduction of permittivity in their as-fired state. Therefore, with a proper masking, different regions of interest can be microfabricated on the surface of the LTCC [17]. This method was first reported for surface modification of DuPont 951 using orthophosphoric acid [18]. Due to the channel-like open pores which are created by this acid treatment, some air is locally embedded below the surface of the LTCC and thereby the overall effective permittivity decreases. The advantage of this approach is the possibility of decreasing permittivity without needing to alter the LTCC tape composition or the fabrication process. The surface quality of the porosified LTCC is a key parameter for the evaluation of the wet chemical etching process since the long-term objective is to apply metallization on the porous areas for the realization of e.g. patch antenna elements where a high-quality surface and relatively low surface roughness could offer a bearing plane with regularly allocated openings for air embedment. Further investigations of glass ceramic composite (GCC) reported the concentrated orthophosphoric acid (85% by mass) at temperatures above 100 °C as the most effective etching solution for GCC systems. These investigations show that the distribution of the pores directly depends on the distribution of the embedded alumina grains and as a direct consequence on the
Corresponding author. E-mail address: [email protected] (A. Hajian).
https://doi.org/10.1016/j.jeurceramsoc.2018.01.017 Received 30 November 2017; Received in revised form 8 January 2018; Accepted 14 January 2018 Available online 17 January 2018 0955-2219/ © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).
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ol, after which they were dried in atmospheric air. Surfaces and fracture planes of the LTCC samples were evaluated using a Hitachi SU8030 scanning electron microscope (SEM) applying the secondary electron (SE) detector. The micrographs of the samples were taken without any previous metal coating at an operating voltage of 2 kV in the charge suppression scanning mode. Three-dimensional information of the surface topography was calculated by means of automatic image correlation with the evaluation software MeX® (Alicona Imaging, Graz, Austria) [33]. Due to the facility of analysing the structural function of the surface, areal (3D) surface roughness parameters have a clear advantage over common profilometry (2D) parameters for describing the real situation. Moreover, applying the 3D parameters results in a reduction in the influence of erroneous features, and the results are statistically more meaningful. Therefore, the area analysis mode available in the MeX® software was used to calculate areal roughness parameters [34–36]. Using the above-mentioned conventional SEM, the same surface area of the sample was imaged twice with eucentrical tilting (i.e. in the focus plane). The stereo-pair images were imported together with information regarding the total titling angle, the pixel size of the images, and the working distance (WD) in the microscope. The area Analysis Mode in the MeX® software calculates the primary outline, roughness and waviness parameters of the surface structure from its digital elevation model (DEM) [37,38]. While the waviness represents the lowfrequency fraction of a given surface characteristics, the roughness indicates the higher frequency portion of a real surface. The resulting images were evaluated using the window-roughness method, whereby values above a certain cut-off wavelength, λc, are identified as waviness and are not included in the surface roughness calculation. Detailed characterization of the surface roughness can be given through a set of parameters which are defined in the international standard ISO 25178. The X-ray diffraction (XRD) experiments on the as-fired as well as the porosified LTCC sheets were performed on a XPertPro MPD goniometer. A filtered X-ray beam from a Cu anode was used. A soller collimator in the primary and secondary beam path provided a 0.04 rad divergence of the beam. The scans were taken from 5° to 90° with a step size of 0.02° in a continuous mode utilizing an X’Celerator detector with an opening of 2.1°. To detect possible X-ray amorphous parts in the diffraction diagram, each point was measured with a scan speed of up to 4 s/step. The phase analysis based on the X-ray diffraction diagrams was performed using the High Score Plus software. In order to achieve very thin foils of the as-fired and porosified LTCC with a thickness in the range of few nanometer, the ‘lift-out’ technique in a dual beam focused ion beam (DBFIB) FEI Quanta 200 3D system was used. Next, these thin foils were analyzed using an FEI Tecnai F20 (scanning) transmission electron microscope ((S)TEM) employing the high angle annular dark field (HAADF) detector. This technique is known to be sensitive to the chemical composition as the HAADF detector collects Rutherford scattered electrons. The amount of Rutherford scattering depends on the square of the mean atomic number and thus on the chemical composition. Crystalline and amorphous phases were also identified using selected area electron diffraction (SAED) patterns. Local chemical analyses were performed on the foils using energy-dispersive X-ray spectroscopy (EDX). Electron energy loss spectroscopy (EELS) was carried out using a Gatan GIF Tridiem ER energy filter which is brought into the irradiation beam. The peaks in the EDX and EELS spectra were used for the compositional ratio of Si, Al, and Ca, respectively, while ignoring the obvious presence of oxygen. The collective application of these methods allowed the determination of most of the containing materials, their distribution as well as their state of crystallinity.
selectively solvable feldspar phases, which are formed during the sintering process in this part of the glass matrix partly enveloping the alumina grains [19]. However, etching of the GCC samples with orthophosphoric acid leads to a rather rough surface which retains the metallization problems [20,21]. Therefore, it is the objective of this study to introduce an alternative approach for porosification of the LTCC surface targeting a better surface quality which features a suitable bearing plane for further metallization lines. According to standard literature and data sheets of commercial LTCCs, almost all are based on aluminum oxide and/or aluminum oxide containing compounds as the ceramic filler material [17,22–25]. Aluminum oxide is an amphoteric compound which can be dissolved by both acidic and alkaline etchants according to the following reactions [26–29]:
Al2O3 + 6H+ → 2Al+3 + 3H2 O
Al2O3 + 2OH− + 3H2 O →
2Al(OH)−4
(1) (2)
Therefore, in this work, a relatively low concentration of alkaline etchant was used at temperatures below 100 °C to porosify the commercial Ceramtape GC LTCC as a representative of an anorthite forming LTCC. Anorthite is the calcium endmember of plagioclase feldspar with the chemical formula of Ca[Al2Si2O8]. Feldspars can be represented by the general formula of XAl(Al,Si)Si2O8, where X is potassium, sodium or calcium. As indicated in this general formula some of the Al occupy tetrahedral sites. The feldspars consists of double chains of SiO−4 and AlO54−tetrahedra with alkali and alkaline earth metals balancing the charge between these tetrahedral [30,31]. In general, in the dissolution of an alkali feldspar it is believed that the first step is the relatively fast removal of alkali and alkaline earth metals from the mineral structure. This removal leads to a depleted surface layer and the bridging AleOeSi bonds are subsequently hydrolyzed to release Al to the solution. This leaves the surface of the alkali feldspar enriched in silica in both acidic and basic conditions. In the final step, the SieOeSi bonds from the silica enriched phase become hydrolyzed [32]. 2. Experimental details In the present study, commercially available LTCC substrates (Ceramtape GC) were used for investigating the porosification process. The etchant solutions of 3 mol L−1 potassium hydroxide (KOH) were freshly prepared by dissolving the desired amount of KOH pellets (≥99.97% from Sigma-Aldrich) in deionized water. The blank LTCC samples were laminated at a pressure of 20 MPa and fired at a peak temperature of about 850 °C for 30 min. The permittivity of commercial Ceramtape GC is in the range of 7.3–8.5 and it possesses a loss tangent of about 0.002. Local porosification of such a substrate with relatively high k allows to locally lower the permittivity and to arrange areas of different permittivity in one single layer. More detailed information about the material properties of the Ceramtape GC and the individual fabrication processes can be found in the corresponding data sheet. After firing, a compound material is generated consisting of a glass matrix with different crystalline and chemical phases in which Al2O3 particles with a typical size in the μm-range are embedded. The analyzed LTCC samples were square in shape, with an edge length of approximately 15 mm and a thickness of about 450 μm. The wet-chemical etching experiments were carried out in 3 mol L−1 aqueous KOH solutions at constant temperatures ranging from 70 to 100 °C. A capped beaker was used to inhibit water evaporation and thus an increase of the KOH concentration. The LTCC samples were flushed in propan-2-ol prior to treatment and were held in position by a fixture made of polytetrafluoroethylene (PTFE). During etching, the solution was continuously stirred at 120 rpm to support the exchange of the reactants and products. By the end of the etching process, the LTCC samples were immediately and thoroughly rinsed with deionized water and propan-2-
3. Results and discussion In general, there are several factors which influence the wet chemical etching process of LTCC. With respect to the etchant, the most 2370
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Fig. 1. SEM micrographs in the top view of Ceramtape GC LTCC a) as-fired, and etched for 2 h with KOH 3 mol L−1 at b) 25 °C, c) 55 °C, d) 70 °C, e) 80 °C, and 90 °C.
Fig. 1d shows that some parts have been removed from the surface and near-surface regions of the LTCC, but its surface characteristics is not significantly modified. A further increase of the temperature to 80 °C and 90 °C results in a more noticeable amount of porosification on the LTCC surface and also an increase in penetration depth of the etchant. These results indicate that the etching experiments should be carried out at temperatures above 70 °C. However, due to massive bubble formation which distorts the treatment, the temperature of the aqueous KOH solution cannot be raised above a maximum temperature of 100 °C. Next, the impact of etching time, as another important parameter for the porosification process was studied. The SEM images in Fig. 2 illustrates the Ceramtec GC samples etched with the KOH 3M at a constant temperature of 80 °C for different exposure times. A noticeable etching on the LTCC surface starts after 1 h of exposure to the KOH
important parameters are its chemical composition, its concentration and the temperature. Another important parameter to achieve a certain porosification depth and surface quality is the exposure time. However, our primary investigations showed that “bath temperature” is a most essential parameter since the etching rate of the GC LTCC at relatively low temperatures is too slow for practical use. Fig. 1a and b shows the as-fired GC LTCC and the sample which was exposed to KOH 3 mol L−1 for 2 h at room temperature. It is clear that there is no noticeable difference. Increasing the temperature to 55 °C caused some small etch-related features to be introduced into the surface (see Fig. 1c). This change in the surface morphology demonstrates the partial etching of the LTCC surface. However, contacting the LTCC sample with the same bath solution at a more elevated temperature of 70 °C resulted in a noticeable amount of etching as illustrated in Fig. 1d so that a larger porosification depth of the GC LTCC surface is expected. 2371
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Fig. 2. SEM micrographs in top view of Ceramtape GC LTCC etched with KOH 3 mol L−1 at 80 °C for a) 60 min, b) 220 min, c) 330 min, and d) 930 min.
image of the GC LTCC sample etched with orthophosphoric acid (85% by mass) at 100 °C. It should be noted that this is a relatively mild condition for the effective porosification of the LTCC with orthophosphoric acid; and etching at higher temperatures results in an even stronger surface devastation and thereby a rougher surface. Parameters such as Sa, Sp, and Sv which have significant importance for the areal roughness analysis, are defined and listed in Table 1. The overall measure of the surface change due to the etching was evaluated by Sa, whereas peaks and valleys of the surface were evaluated by Sp and Sv, respectively. The reported values are averaged for four different areas randomly selected from the surface. The average height of a selected area represented by the Sa parameter is defined as the arithmetic mean of the absolute value of the height within a sampling area. Sa shows an increase for the etched sample compared to the as-fired LTCC. However, the increase in the maximum peak height of a selected area (Sp parameter) compared with maximum valley depth of a selected area (Sv parameter) is less significant. This kind of surface roughness is of the major advantage of the present porosification approach since a reliable metallization of a surfaces topography having sharp edges and high peak roughness is most challenging. As it was expected, the more devastated surface which was obtained by using orthophosphoric acid results in larger areal surface roughness parameters compared with the one obtained by using KOH. The XRD diagrams of the GC LTCC before and after treatment with KOH solution are shown in Fig. 4. Besides the reduced peak intensities of the porosified LTCC compared with the untreated sample it is clear that some anorthite peaks have become significantly smaller or vanished entirely, as illustrated in Fig. 4. These observations confirm that upon alkaline treatment of the Ceramtape GC LTCC, the predominantly etched phase in the LTCC is the anorthite phase. Recently, we have shown that GC LTCC consists mainly of
solution and specific surface features resulting from porosification are distinguishable in Fig. 2a. By increasing the exposure time, the etchant penetrates more into the LTCC and deeper holes are created indicated by darker areas in the SEM image (see Fig. 2b). By further increasing the exposure time to 330 min, even though the basic surface features are still preserved a lateral growth in pore size occurs what results in a wider pore opening. By long-time exposure of the sample to the KOH solution of 930 min a severe etching takes place which creates laterally disconnected grains on the surface and thereby the surface of the LTCC transforms strongly. The topography of the etched LTCC was further characterized by means of stereo-SEM images. SEM images were recorded with similar contrast and brightness settings to prevent differences in parameter values for further analyses. MeX® software was used for calculating 3Dsurface roughness parameters [39], and cut-off wavelength value of 2.8 μm was applied for both as-fired and etched samples. The stereo-pairs SEM micrographs of the as-fired and etched LTCC, reconstructed with MeX, are shown in Fig. 3. In comparison, the surface morphology is clearly altered. Due to the etching process, some parts of the LTCC are removed resulting in the generation of relatively deep valleys as well as small pores into the surface. These changes in surface topography which can be clearly observed in the stereo-SEM images lead to an increased roughness of the LTCC compared with the as-fired samples. In order to directly compare the KOH-based porosification of the GC LTCC with the one developed by our group in the past, orthophosphoric acid solution was also applied as a well-established etchant. Orthophosphoric acid treatment forms through selective etching of the anorthite phase, which is generated in the alumina-grain near portion of the glass matrix in the as-fired LTCC, the porous structure. As a result, applying orthophosphoric creates a stronger surface devastation compared to the KOH-treated sample. Fig. 3c represents the stereo-SEM 2372
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Fig. 3. Stereo-SEM images of Ceramtape GC LTCC substrates a) as-fired, and b) etched with KOH 3 mol L−1 at 75 °C, and c) etched with H3PO4 85% at 100 °C.
whole EDX line scan with respect to the elements Al, Si, and Ca are shown on the right-hand side of Fig. 5. Basically, the counts of Si and Ca change simultaneously and in opposite direction to Al. Furthermore, like the intermediate regions between corundum grains in the porosified layer which is shown in Fig. 6, an increase of Si and Ca amounts was observed parallel to a reduction in the amount of Al. This confirms the presence of discrete corundum grains surrounded by an anorthite phase. The presence of the Al, Si, Ca, and O as main constitutes of the Ceramtec GC LTCC was also confirmed with EELS. The intensities of the EELS-ionization edges are depicted for the different locations along the line which is indicated by an arrow. These intensities are dependent on chemical element, concentration, and sample thickness. In order to investigate the changes in elemental composition due to the etching process, an EDX line scan through the porosified part of the LTCC was recorded. Fig. 6 represents the EDX results of the porosified LTCC sample. As an example, the three different types of spectra are shown. Position one (pos. 1) corresponds to the corundum phase and position two (pos. 2) represents a very porous silica-rich phase formed during the etching process. In position three (pos. 3) in addition to Al
aluminum (Al), silicon (Si), calcium (Ca), and oxygen (O) elements in which alumina grains (in the form of rhombohedral corundum) are dispersed within the amorphous glass matrix [19]. The alumina grains are discrete and clearly visible in the FIB foils and their average diameter is approx. 0.7 μm. GC provides a high degree of crystallization and only a low glass fraction remains amorphous after firing due to the low amount of viscosity lowering glass modifier oxides and thereby the relatively high maximum firing temperature (900–950 °C) [40]. The TEM images of the GC Ceramtape sample etched in KOH 3 mol L−1 at 90 °C for 4 h are shown in Figs. 4–6. The overview images of the porosified LTCC show that the maximum etch depth is about 4.6 μm. In the positions lower than this depth, no further etching was detected. As indicated by the white arrow line scan, an EDX line scan parallel to the sample surface but at a depth of 5 μm, which is below the formed etched front, was recorded applying STEM. Every five nanometers an EDX spectrum was taken. As can be seen in Fig. 5 only corundum and Ca-rich aluminosilicate phases (anorthite) were identified from this line scan and no pure silica was detected. The respective EDX spectra and the quantification of the
Table 1 Roughness parameters for as-fired and etched GC LTCC. Parameter
Value for as-fired GC
Value for KOH etched GC
Value for H3PO4 etched GC
Definition
Sa Sp Sv
53.1 nm 447.7 nm 518.0 nm
91.7 nm 513.7 nm 765.7 nm
151.8 nm 745.7 nm 933.3 nm
Average roughness across selected area Maximum peak height within selected area Maximum valley depth within selected area
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GC that the generated porosity mainly consists of pores with narrow necks (i.e. small pore openings) and a relatively wide pore body. This so-called ink-bottle-type pores result in a better surface quality with only a moderate increase in the surface roughness. Besides morphology and chemical composition, it is also of interest to study the crystallographic structure of the sample in selected areas after the etching process. For this purpose, Selected Area Electron Diffraction (SAED) measurements were carried out and the results are shown in Fig. 7. The top left subfigure gives an overview of the TEM specimen. The top layer which consists of platinum is applied in order to avoid any structural damage to the sample surface during FIB preparation. The three diffraction patterns were recorded from the corresponding positions marked within the overview image which is shown in Fig. 7. The formed silica, which has a foam-like appearance, consists of a nano-crystalline or quasi-amorphous microstructure. The sharp ring in the SAED pattern of position one, however, indicates a well-defined near ordering which can be found only in single-crystalline microstructures. But due to the fact that the whole ring is visible with constant intensity, the crystallites are of arbitrary orientation and of a very confined size. Positions two and three show a typical single crystalline diffraction pattern far out from a low-indexed zone axis. The EDX investigations of the LTCC in both the near-surface porosified and the non-porosified area show that independent where the analysis was performed, the presence of Al changes in opposite direction to Si. The areas with the maximum amount of Al were attributed to the corundum phase while the intermediate regions between the grains contain an increased amount of Si and Ca. Moreover, it was found that similar to the etching with orthophosphoric acid, also the anorthite phase is etched faster and more effectively in contrast to the corundum phase. But etching with KOH the glassy phase of the LTCC is also partially dissolved. This concurrent and competitive dissolution of the LTCC components assists in avoiding the sharp and emerged edges around the corundum grains which are typically observed while using orthophosphoric acid as the etching solution. This etching behavior not only results in a less corroded surface compared with the acid-treated LTCC but also provides very scaled-down pores in the remaining part of
Fig. 4. X-ray diffraction diagrams with a subtracted background of Ceramtape GC powder samples before and after treatment with KOH 3 mol L−1 at 90 °C (an: anorthite, c: corundum). The intensity levels are shifted for comparison reasons.
and Si, calcium is detected as chemical element. Unlike the former EDX line profile (see Fig. 5), where no position could be found indicating the presence of pure silica, at the line scan recorded in the porosified segment of the LTCC, pure silica was detected within the foam-like structure. In addition, an increased amount of Ca was detected close to the pore walls of the etched LTCC, which are indeed the borders that the etchant does not penetrate further into the depth of the LTCC. The elemental analysis of Al, Si and Ca from the EDX line scan is depicted in the right lower corner. From this image, it can be concluded that the intermediate regions between corundum grains feature a reduction in the amount of Al accompanied by an increase of Si and Ca, respectively. This part which envelops the alumina grains and consists of Al, Si, Ca, and also O which is not shown here, can be referred to the anorthite phase. It can be seen in the TEM overview image of the porosified Ceramtec
Fig. 5. EDX line profile across an as-fired area of the Ceramtec GC LTCC etched in KOH 3 mol L−1 at 90 °C for 4 h.
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Fig. 6. EDX line profile across an etched region indicating three different phases: pure Alumina at pos. 1, pure Silica at pos. 2 and Ca-rich aluminosilicate at pos. 3. Fig. 7. TEM lamella of a GC LTCC with three representative locations labeled and the corresponding SAED analyses. At position 1 pure silica shows a nano-crystalline microstructure, close to an amorphous phase, whereas position 2 and 3 feature pure alumina and Ca-rich aluminosilicate with a singlecrystalline morphology.
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Fig. 8. EELS line profile across an etched area of the Ceramtec GC LTCC etched in orthophosphoric acid (85% by mass) at 100 °C for 4 h. (White areas in the TEM image are indicating the pores).
be altered since the porosification is generated in the as-fired state of commercially available LTCC samples, before final metallization. Furthermore, the advantage of the proposed method is the generation of a surface quality for the etched LTCC which features a bearing plane suitable for further metallization, being an essential pre-requirement for the reliable operation of compact devices or modules for high-frequency applications. The surfaces treated in aqueous KOH solutions are much less corroded compared with the state of the art approach for LTCC porosification i.e. aqueous orthophosphoric acid solution treatment. Preliminary studies on the essential etching parameters i.e. bath temperature, etchant concentration, and etching time revealed that temperature is the most influential parameter since putting the sample in contact with the KOH solutions at room temperature, even for a long period of time would not result in an appropriate etching. An explicit surface porosification of the LTCC requires the temperatures more than 70 °C. Complementary investigations will be carried out on optimization of key parameters of the etching process. Moreover, the TEM analysis showed that the introduced porosity to the Ceramtec GC due to the KOH-treatment mainly consists of the favorable ink-bottle pores which are restricted to the near-surface region while ortho orthophosphoric acid-treatments results in cylindrical pore structures which are distributed in the deeper regions of the LTCC rather. The etching mechanism with the KOH solution will be further studied in our future research and also the effect of produced porosity on the physical materials properties of the LTCC will be investigated.
the LTCC surface. This is regarded as a significant achievement for the long-term objective of the porosified LTCC which is the air embedment through applying metallization on the porous areas. Also, for comparison reasons the Ceramtec GC LTCC treated with orthophosphoric acid (85% by mass) for four hours at 100 °C was cut with FIB and analyzed with TEM. As it has been shown in Fig. 8 the etching behavior of Ceramtec GC with orthophosphoric is very different in comparison with KOH. Although etching of the Ceramtec GC with KOH resulted in a localized porosity and locally modifies the surface and near-surface regions by the creation of mainly ink-bottle-type pores, a delocalized porosity is observed for the sample etched with orthophosphoric acid. Also, etching of the Ceramtec GC with orthophosphoric acid results in a more selective dissolution of the anorthite compared with KOH solution. Furthermore, in contrast to the KOH treated sample which comprises mainly ink-bottle-type type porosity, the acid treated sample contains a network of cylindrical pores which are formed due to the preferential dissolution of anorthite phase and are distributed within a much deeper region of the Ceramtec GC LTCC. Additionally, in order to investigate the elemental composition of the acid-treated sample, an EELS line scan through the porosified part of the Ceramtec GC LTCC was recorded and the results are depicted in the right side of Fig. 8. As can be observed in this figure the elements which are found in this line scan, are dominantly Al and Si and the amount of found Ca is negligible. From this observations, a strong dissolution of the anorthite phase can be concluded. This porosification behavior will strongly affect the mechanical properties of the LTCC and therefore, restricts their handling for further applications.
Acknowledgements The authors would like to acknowledge the financial support from the Austrian Science Fund (FWF), No. I 2551-N30. In addition, we thank Dr. Klaudia Hradil from the X-ray Center (XRC) of TU Wien for her support with the XRD measurements.
4. Conclusion and outlook In this study, a novel approach for wet-chemical etching of the LTCC substrate was introduced in which a tailored porosity in as-fired Ceramtape GC was generated. In this approach, an aqueous solution of KOH in the temperatures lower than 100 °C under constant stirring is used as an etching bath. The method is very simple to monitor and the degree of the porosification can be controlled with straightforward key parameters, such as the bath temperature and the etch time. By using the porosification process, LTCC fabrication procedure does not need to
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[21] F. Steinhäußer, A. Talai, G. Göltl, A. Steiger-Thirsfeld, A. Bittner, R. Weigel, A. Koelpin, U. Schmid, Concentration and temperature dependent selectivity of the LTCC porosification process with phosphoric acid, Ceram. Int. 43 (1) (2017) 714–721. [22] I.J. Induja, K.P. Surendran, M.R. Varma, M.T. Sebastian, Low κ, low loss aluminaglass composite with low CTE for LTCC microelectronic applications, Ceram. Int. 43 (1) (2017) 736–740. [23] DuPont, DuPont GreenTape™ 9K7, DuPont Microcircuit Material, New York City, 2009. [24] DuPont, 943 Low Loss Green Tape™, E.I. DuPont de Nemours and Company, 2003. [25] DuPont, 951 Green Tap™, DuPont Microcircuit Material, New York City, 2001. [26] A.F. Dresvyannikov, I.O. Grigor’eva, Kinetics of Sn(II) contact reduction in alkaline solutions with compact and dispersed aluminum, Russ. J. Appl. Chem. 74 (4) (2001) 611–615. [27] A. Hajian, A.A. Rafati, A. Afraz, M. Najafi, Electrosynthesis of high-density polythiophene nanotube arrays and their application for sensing of riboflavin, J. Mol. Liq. 199 (2014) 150–155. [28] A. Hajian, A.A. Rafati, A. Afraz, M. Najafi, Electrosynthesis of polythiophene nanowires and their application for sensing of chlorpromazine, J. Electrochem. Soc. 161 (9) (2014) B196–B200. [29] Z.L. Xiao, C.Y. Han, U. Welp, H.H. Wang, W.K. Kwok, G.A. Willing, J.M. Hiller, R.E. Cook, D.J. Miller, G.W. Crabtree, Fabrication of alumina nanotubes and nanowires by etching porous alumina membranes, Nano Lett. 2 (11) (2002) 1293–1297. [30] F.K. Crundwell, The mechanism of dissolution of minerals in acidic and alkaline solutions: part II application of a new theory to silicates, aluminosilicates and quartz, Hydrometallurgy 149 (2014) 265–275. [31] F.K. Crundwell, The mechanism of dissolution of the feldspars: part I. Dissolution at conditions far from equilibrium, Hydrometallurgy 151 (2015) 151–162. [32] E.H. Oelkers, J. Schott, J.-L. Devidal, The effect of aluminum, pH, and chemical affinity on the rates of aluminosilicate dissolution reactions, Geochim. Cosmochim. Acta 58 (9) (1994) 2011–2024. [33] Alicona Imaging GmbH, Graz, Austria Mex Software (2007). [34] J. Löberg, I. Mattisson, S. Hansson, E. Ahlberg, Characterisation of titanium dental implants I: critical assessment of surface roughness parameters, Open Biomater. J. 2 (2010) 18–35. [35] F. Glon, O. Flys, P.J. Lööf, B.G. Rosén, 3D SEM for surface topography quantification – a case study on dental surfaces, J. Phys. Conf. Ser. 483 (1) (2014) 012026. [36] A. Townsend, N. Senin, L. Blunt, R.K. Leach, J.S. Taylor, Surface texture metrology for metal additive manufacturing: a review, Precis. Eng. 46 (2016) 34–47. [37] H. Ostadi, K. Jiang, D.W.L. Hukins, A comparison of surface roughness analysis methods applied to urinary catheters, Precis. Eng. 34 (4) (2010) 798–801. [38] M. Pirisinu, V. Mazzarello, 3D profilometric characterization of the aged skin surface using a skin replica and alicona Mex software, Scanning 38 (3) (2016) 213–220. [39] S. Ghosh, J. Bowen, K. Jiang, D.M. Espino, D.E.T. Shepherd, Investigation of techniques for the measurement of articular cartilage surface roughness, Micron 44 (2013) 179–184. [40] R. Müller, R. Meszaros, B. Peplinski, S. Reinsch, M. Eberstein, W.A. Schiller, J. Deubener, Dissolution of alumina, sintering, and crystallization in glass ceramic composites for LTCC, J. Am. Ceram. Soc. 92 (8) (2009) 1703–1708.
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Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Original Article
Porosification behaviour of LTCC substrates with potassium hydroxide a,⁎
b
a
T
a
Ali Hajian , Michael Stöger-Pollach , Michael Schneider , Doruk Müftüoglu , Frank K. Crunwellc, Ulrich Schmida a b c
Institute of Sensor and Actuator Systems, TU Wien, Gusshausstrasse 27–29, 1040 Vienna, Austria University Service Centre for Transmission Electron Microscopy, TU Wien, Wiedner Hauptstrasse 8–10, 1040 Vienna, Austria CM Solutions (Pty) Ltd, 89J Victoria Drive, London SW19 6PT, UK
A R T I C L E I N F O
A B S T R A C T
Keywords: LTCC Wet-chemical etching Permittivity reduction Porosification Glass-ceramics
The demand to meet advanced substrate requirements in terms of electrical, mechanical, thermal, and dielectric properties has led to an increasing interest in low temperature co-fired ceramics (LTCC). However, LTCC materials suffer from high permittivity. We recently showed that the wet-chemical porosification under acidic conditions allows the reduction of the permittivity of LTCCs in the as-fired state. In the present study, potassium hydroxide solution was employed as an alternative etchant which features a suitable bearing plane for further metallization lines. Various characterization techniques, including scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction analysis, and electron energy loss spectroscopy were used for investigation of the morphology and chemical composition of the substrates. Three-dimensional information of the surface topography was acquired by means of MeX® Alicona software and the obtained roughness parameters confirmed the advantage of the proposed approach over acid treatment when targeting an enhanced surface quality.
1. Introduction Low temperature co-fired ceramics (LTCC) is an advanced substrate technology for the robust assembly and packaging of electronic components, enabling even the realization of gas-proof housings. Due to a multilayer approach based on glass-ceramics sheets this technology offers the possibility to integrate into the LTCC body passive electrical components and conductor lines typically manufactured in thick film technology [1–7]. Compactness, light weight, high integration level and the good compatibility with radio frequency microelectromechanical systems (RF-MEMS) and to monolithic microwave integrated circuits (MMICs), have made LTCC a very attractive technology for a wide application range such as in wireless communication or in automotive radar systems [8–11]. In detail, these applications comprise mobile telecommunication devices (0.8–2 GHz), wireless local networks such as Bluetooth (2.4 GHz) to in-car radars (50–140 GHz, and 76 GHz) [12,13]. In order to decrease the losses of radiating elements such as patch antennas, a locally adjustable permittivity within a single-layer of an LTCC substrate would be highly beneficial where the low permittivity areas enhance both the bandwidth and the efficiency of the active components. In parallel, the high permittivity areas allow a compact
⁎
feeding circuit design [14–16]. As demonstrated recently, the application of wet-chemical porosification of the LTCC surface allows the local reduction of permittivity in their as-fired state. Therefore, with a proper masking, different regions of interest can be microfabricated on the surface of the LTCC [17]. This method was first reported for surface modification of DuPont 951 using orthophosphoric acid [18]. Due to the channel-like open pores which are created by this acid treatment, some air is locally embedded below the surface of the LTCC and thereby the overall effective permittivity decreases. The advantage of this approach is the possibility of decreasing permittivity without needing to alter the LTCC tape composition or the fabrication process. The surface quality of the porosified LTCC is a key parameter for the evaluation of the wet chemical etching process since the long-term objective is to apply metallization on the porous areas for the realization of e.g. patch antenna elements where a high-quality surface and relatively low surface roughness could offer a bearing plane with regularly allocated openings for air embedment. Further investigations of glass ceramic composite (GCC) reported the concentrated orthophosphoric acid (85% by mass) at temperatures above 100 °C as the most effective etching solution for GCC systems. These investigations show that the distribution of the pores directly depends on the distribution of the embedded alumina grains and as a direct consequence on the
Corresponding author. E-mail address: [email protected] (A. Hajian).
https://doi.org/10.1016/j.jeurceramsoc.2018.01.017 Received 30 November 2017; Received in revised form 8 January 2018; Accepted 14 January 2018 Available online 17 January 2018 0955-2219/ © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).
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ol, after which they were dried in atmospheric air. Surfaces and fracture planes of the LTCC samples were evaluated using a Hitachi SU8030 scanning electron microscope (SEM) applying the secondary electron (SE) detector. The micrographs of the samples were taken without any previous metal coating at an operating voltage of 2 kV in the charge suppression scanning mode. Three-dimensional information of the surface topography was calculated by means of automatic image correlation with the evaluation software MeX® (Alicona Imaging, Graz, Austria) [33]. Due to the facility of analysing the structural function of the surface, areal (3D) surface roughness parameters have a clear advantage over common profilometry (2D) parameters for describing the real situation. Moreover, applying the 3D parameters results in a reduction in the influence of erroneous features, and the results are statistically more meaningful. Therefore, the area analysis mode available in the MeX® software was used to calculate areal roughness parameters [34–36]. Using the above-mentioned conventional SEM, the same surface area of the sample was imaged twice with eucentrical tilting (i.e. in the focus plane). The stereo-pair images were imported together with information regarding the total titling angle, the pixel size of the images, and the working distance (WD) in the microscope. The area Analysis Mode in the MeX® software calculates the primary outline, roughness and waviness parameters of the surface structure from its digital elevation model (DEM) [37,38]. While the waviness represents the lowfrequency fraction of a given surface characteristics, the roughness indicates the higher frequency portion of a real surface. The resulting images were evaluated using the window-roughness method, whereby values above a certain cut-off wavelength, λc, are identified as waviness and are not included in the surface roughness calculation. Detailed characterization of the surface roughness can be given through a set of parameters which are defined in the international standard ISO 25178. The X-ray diffraction (XRD) experiments on the as-fired as well as the porosified LTCC sheets were performed on a XPertPro MPD goniometer. A filtered X-ray beam from a Cu anode was used. A soller collimator in the primary and secondary beam path provided a 0.04 rad divergence of the beam. The scans were taken from 5° to 90° with a step size of 0.02° in a continuous mode utilizing an X’Celerator detector with an opening of 2.1°. To detect possible X-ray amorphous parts in the diffraction diagram, each point was measured with a scan speed of up to 4 s/step. The phase analysis based on the X-ray diffraction diagrams was performed using the High Score Plus software. In order to achieve very thin foils of the as-fired and porosified LTCC with a thickness in the range of few nanometer, the ‘lift-out’ technique in a dual beam focused ion beam (DBFIB) FEI Quanta 200 3D system was used. Next, these thin foils were analyzed using an FEI Tecnai F20 (scanning) transmission electron microscope ((S)TEM) employing the high angle annular dark field (HAADF) detector. This technique is known to be sensitive to the chemical composition as the HAADF detector collects Rutherford scattered electrons. The amount of Rutherford scattering depends on the square of the mean atomic number and thus on the chemical composition. Crystalline and amorphous phases were also identified using selected area electron diffraction (SAED) patterns. Local chemical analyses were performed on the foils using energy-dispersive X-ray spectroscopy (EDX). Electron energy loss spectroscopy (EELS) was carried out using a Gatan GIF Tridiem ER energy filter which is brought into the irradiation beam. The peaks in the EDX and EELS spectra were used for the compositional ratio of Si, Al, and Ca, respectively, while ignoring the obvious presence of oxygen. The collective application of these methods allowed the determination of most of the containing materials, their distribution as well as their state of crystallinity.
selectively solvable feldspar phases, which are formed during the sintering process in this part of the glass matrix partly enveloping the alumina grains [19]. However, etching of the GCC samples with orthophosphoric acid leads to a rather rough surface which retains the metallization problems [20,21]. Therefore, it is the objective of this study to introduce an alternative approach for porosification of the LTCC surface targeting a better surface quality which features a suitable bearing plane for further metallization lines. According to standard literature and data sheets of commercial LTCCs, almost all are based on aluminum oxide and/or aluminum oxide containing compounds as the ceramic filler material [17,22–25]. Aluminum oxide is an amphoteric compound which can be dissolved by both acidic and alkaline etchants according to the following reactions [26–29]:
Al2O3 + 6H+ → 2Al+3 + 3H2 O
Al2O3 + 2OH− + 3H2 O →
2Al(OH)−4
(1) (2)
Therefore, in this work, a relatively low concentration of alkaline etchant was used at temperatures below 100 °C to porosify the commercial Ceramtape GC LTCC as a representative of an anorthite forming LTCC. Anorthite is the calcium endmember of plagioclase feldspar with the chemical formula of Ca[Al2Si2O8]. Feldspars can be represented by the general formula of XAl(Al,Si)Si2O8, where X is potassium, sodium or calcium. As indicated in this general formula some of the Al occupy tetrahedral sites. The feldspars consists of double chains of SiO−4 and AlO54−tetrahedra with alkali and alkaline earth metals balancing the charge between these tetrahedral [30,31]. In general, in the dissolution of an alkali feldspar it is believed that the first step is the relatively fast removal of alkali and alkaline earth metals from the mineral structure. This removal leads to a depleted surface layer and the bridging AleOeSi bonds are subsequently hydrolyzed to release Al to the solution. This leaves the surface of the alkali feldspar enriched in silica in both acidic and basic conditions. In the final step, the SieOeSi bonds from the silica enriched phase become hydrolyzed [32]. 2. Experimental details In the present study, commercially available LTCC substrates (Ceramtape GC) were used for investigating the porosification process. The etchant solutions of 3 mol L−1 potassium hydroxide (KOH) were freshly prepared by dissolving the desired amount of KOH pellets (≥99.97% from Sigma-Aldrich) in deionized water. The blank LTCC samples were laminated at a pressure of 20 MPa and fired at a peak temperature of about 850 °C for 30 min. The permittivity of commercial Ceramtape GC is in the range of 7.3–8.5 and it possesses a loss tangent of about 0.002. Local porosification of such a substrate with relatively high k allows to locally lower the permittivity and to arrange areas of different permittivity in one single layer. More detailed information about the material properties of the Ceramtape GC and the individual fabrication processes can be found in the corresponding data sheet. After firing, a compound material is generated consisting of a glass matrix with different crystalline and chemical phases in which Al2O3 particles with a typical size in the μm-range are embedded. The analyzed LTCC samples were square in shape, with an edge length of approximately 15 mm and a thickness of about 450 μm. The wet-chemical etching experiments were carried out in 3 mol L−1 aqueous KOH solutions at constant temperatures ranging from 70 to 100 °C. A capped beaker was used to inhibit water evaporation and thus an increase of the KOH concentration. The LTCC samples were flushed in propan-2-ol prior to treatment and were held in position by a fixture made of polytetrafluoroethylene (PTFE). During etching, the solution was continuously stirred at 120 rpm to support the exchange of the reactants and products. By the end of the etching process, the LTCC samples were immediately and thoroughly rinsed with deionized water and propan-2-
3. Results and discussion In general, there are several factors which influence the wet chemical etching process of LTCC. With respect to the etchant, the most 2370
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Fig. 1. SEM micrographs in the top view of Ceramtape GC LTCC a) as-fired, and etched for 2 h with KOH 3 mol L−1 at b) 25 °C, c) 55 °C, d) 70 °C, e) 80 °C, and 90 °C.
Fig. 1d shows that some parts have been removed from the surface and near-surface regions of the LTCC, but its surface characteristics is not significantly modified. A further increase of the temperature to 80 °C and 90 °C results in a more noticeable amount of porosification on the LTCC surface and also an increase in penetration depth of the etchant. These results indicate that the etching experiments should be carried out at temperatures above 70 °C. However, due to massive bubble formation which distorts the treatment, the temperature of the aqueous KOH solution cannot be raised above a maximum temperature of 100 °C. Next, the impact of etching time, as another important parameter for the porosification process was studied. The SEM images in Fig. 2 illustrates the Ceramtec GC samples etched with the KOH 3M at a constant temperature of 80 °C for different exposure times. A noticeable etching on the LTCC surface starts after 1 h of exposure to the KOH
important parameters are its chemical composition, its concentration and the temperature. Another important parameter to achieve a certain porosification depth and surface quality is the exposure time. However, our primary investigations showed that “bath temperature” is a most essential parameter since the etching rate of the GC LTCC at relatively low temperatures is too slow for practical use. Fig. 1a and b shows the as-fired GC LTCC and the sample which was exposed to KOH 3 mol L−1 for 2 h at room temperature. It is clear that there is no noticeable difference. Increasing the temperature to 55 °C caused some small etch-related features to be introduced into the surface (see Fig. 1c). This change in the surface morphology demonstrates the partial etching of the LTCC surface. However, contacting the LTCC sample with the same bath solution at a more elevated temperature of 70 °C resulted in a noticeable amount of etching as illustrated in Fig. 1d so that a larger porosification depth of the GC LTCC surface is expected. 2371
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Fig. 2. SEM micrographs in top view of Ceramtape GC LTCC etched with KOH 3 mol L−1 at 80 °C for a) 60 min, b) 220 min, c) 330 min, and d) 930 min.
image of the GC LTCC sample etched with orthophosphoric acid (85% by mass) at 100 °C. It should be noted that this is a relatively mild condition for the effective porosification of the LTCC with orthophosphoric acid; and etching at higher temperatures results in an even stronger surface devastation and thereby a rougher surface. Parameters such as Sa, Sp, and Sv which have significant importance for the areal roughness analysis, are defined and listed in Table 1. The overall measure of the surface change due to the etching was evaluated by Sa, whereas peaks and valleys of the surface were evaluated by Sp and Sv, respectively. The reported values are averaged for four different areas randomly selected from the surface. The average height of a selected area represented by the Sa parameter is defined as the arithmetic mean of the absolute value of the height within a sampling area. Sa shows an increase for the etched sample compared to the as-fired LTCC. However, the increase in the maximum peak height of a selected area (Sp parameter) compared with maximum valley depth of a selected area (Sv parameter) is less significant. This kind of surface roughness is of the major advantage of the present porosification approach since a reliable metallization of a surfaces topography having sharp edges and high peak roughness is most challenging. As it was expected, the more devastated surface which was obtained by using orthophosphoric acid results in larger areal surface roughness parameters compared with the one obtained by using KOH. The XRD diagrams of the GC LTCC before and after treatment with KOH solution are shown in Fig. 4. Besides the reduced peak intensities of the porosified LTCC compared with the untreated sample it is clear that some anorthite peaks have become significantly smaller or vanished entirely, as illustrated in Fig. 4. These observations confirm that upon alkaline treatment of the Ceramtape GC LTCC, the predominantly etched phase in the LTCC is the anorthite phase. Recently, we have shown that GC LTCC consists mainly of
solution and specific surface features resulting from porosification are distinguishable in Fig. 2a. By increasing the exposure time, the etchant penetrates more into the LTCC and deeper holes are created indicated by darker areas in the SEM image (see Fig. 2b). By further increasing the exposure time to 330 min, even though the basic surface features are still preserved a lateral growth in pore size occurs what results in a wider pore opening. By long-time exposure of the sample to the KOH solution of 930 min a severe etching takes place which creates laterally disconnected grains on the surface and thereby the surface of the LTCC transforms strongly. The topography of the etched LTCC was further characterized by means of stereo-SEM images. SEM images were recorded with similar contrast and brightness settings to prevent differences in parameter values for further analyses. MeX® software was used for calculating 3Dsurface roughness parameters [39], and cut-off wavelength value of 2.8 μm was applied for both as-fired and etched samples. The stereo-pairs SEM micrographs of the as-fired and etched LTCC, reconstructed with MeX, are shown in Fig. 3. In comparison, the surface morphology is clearly altered. Due to the etching process, some parts of the LTCC are removed resulting in the generation of relatively deep valleys as well as small pores into the surface. These changes in surface topography which can be clearly observed in the stereo-SEM images lead to an increased roughness of the LTCC compared with the as-fired samples. In order to directly compare the KOH-based porosification of the GC LTCC with the one developed by our group in the past, orthophosphoric acid solution was also applied as a well-established etchant. Orthophosphoric acid treatment forms through selective etching of the anorthite phase, which is generated in the alumina-grain near portion of the glass matrix in the as-fired LTCC, the porous structure. As a result, applying orthophosphoric creates a stronger surface devastation compared to the KOH-treated sample. Fig. 3c represents the stereo-SEM 2372
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Fig. 3. Stereo-SEM images of Ceramtape GC LTCC substrates a) as-fired, and b) etched with KOH 3 mol L−1 at 75 °C, and c) etched with H3PO4 85% at 100 °C.
whole EDX line scan with respect to the elements Al, Si, and Ca are shown on the right-hand side of Fig. 5. Basically, the counts of Si and Ca change simultaneously and in opposite direction to Al. Furthermore, like the intermediate regions between corundum grains in the porosified layer which is shown in Fig. 6, an increase of Si and Ca amounts was observed parallel to a reduction in the amount of Al. This confirms the presence of discrete corundum grains surrounded by an anorthite phase. The presence of the Al, Si, Ca, and O as main constitutes of the Ceramtec GC LTCC was also confirmed with EELS. The intensities of the EELS-ionization edges are depicted for the different locations along the line which is indicated by an arrow. These intensities are dependent on chemical element, concentration, and sample thickness. In order to investigate the changes in elemental composition due to the etching process, an EDX line scan through the porosified part of the LTCC was recorded. Fig. 6 represents the EDX results of the porosified LTCC sample. As an example, the three different types of spectra are shown. Position one (pos. 1) corresponds to the corundum phase and position two (pos. 2) represents a very porous silica-rich phase formed during the etching process. In position three (pos. 3) in addition to Al
aluminum (Al), silicon (Si), calcium (Ca), and oxygen (O) elements in which alumina grains (in the form of rhombohedral corundum) are dispersed within the amorphous glass matrix [19]. The alumina grains are discrete and clearly visible in the FIB foils and their average diameter is approx. 0.7 μm. GC provides a high degree of crystallization and only a low glass fraction remains amorphous after firing due to the low amount of viscosity lowering glass modifier oxides and thereby the relatively high maximum firing temperature (900–950 °C) [40]. The TEM images of the GC Ceramtape sample etched in KOH 3 mol L−1 at 90 °C for 4 h are shown in Figs. 4–6. The overview images of the porosified LTCC show that the maximum etch depth is about 4.6 μm. In the positions lower than this depth, no further etching was detected. As indicated by the white arrow line scan, an EDX line scan parallel to the sample surface but at a depth of 5 μm, which is below the formed etched front, was recorded applying STEM. Every five nanometers an EDX spectrum was taken. As can be seen in Fig. 5 only corundum and Ca-rich aluminosilicate phases (anorthite) were identified from this line scan and no pure silica was detected. The respective EDX spectra and the quantification of the
Table 1 Roughness parameters for as-fired and etched GC LTCC. Parameter
Value for as-fired GC
Value for KOH etched GC
Value for H3PO4 etched GC
Definition
Sa Sp Sv
53.1 nm 447.7 nm 518.0 nm
91.7 nm 513.7 nm 765.7 nm
151.8 nm 745.7 nm 933.3 nm
Average roughness across selected area Maximum peak height within selected area Maximum valley depth within selected area
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GC that the generated porosity mainly consists of pores with narrow necks (i.e. small pore openings) and a relatively wide pore body. This so-called ink-bottle-type pores result in a better surface quality with only a moderate increase in the surface roughness. Besides morphology and chemical composition, it is also of interest to study the crystallographic structure of the sample in selected areas after the etching process. For this purpose, Selected Area Electron Diffraction (SAED) measurements were carried out and the results are shown in Fig. 7. The top left subfigure gives an overview of the TEM specimen. The top layer which consists of platinum is applied in order to avoid any structural damage to the sample surface during FIB preparation. The three diffraction patterns were recorded from the corresponding positions marked within the overview image which is shown in Fig. 7. The formed silica, which has a foam-like appearance, consists of a nano-crystalline or quasi-amorphous microstructure. The sharp ring in the SAED pattern of position one, however, indicates a well-defined near ordering which can be found only in single-crystalline microstructures. But due to the fact that the whole ring is visible with constant intensity, the crystallites are of arbitrary orientation and of a very confined size. Positions two and three show a typical single crystalline diffraction pattern far out from a low-indexed zone axis. The EDX investigations of the LTCC in both the near-surface porosified and the non-porosified area show that independent where the analysis was performed, the presence of Al changes in opposite direction to Si. The areas with the maximum amount of Al were attributed to the corundum phase while the intermediate regions between the grains contain an increased amount of Si and Ca. Moreover, it was found that similar to the etching with orthophosphoric acid, also the anorthite phase is etched faster and more effectively in contrast to the corundum phase. But etching with KOH the glassy phase of the LTCC is also partially dissolved. This concurrent and competitive dissolution of the LTCC components assists in avoiding the sharp and emerged edges around the corundum grains which are typically observed while using orthophosphoric acid as the etching solution. This etching behavior not only results in a less corroded surface compared with the acid-treated LTCC but also provides very scaled-down pores in the remaining part of
Fig. 4. X-ray diffraction diagrams with a subtracted background of Ceramtape GC powder samples before and after treatment with KOH 3 mol L−1 at 90 °C (an: anorthite, c: corundum). The intensity levels are shifted for comparison reasons.
and Si, calcium is detected as chemical element. Unlike the former EDX line profile (see Fig. 5), where no position could be found indicating the presence of pure silica, at the line scan recorded in the porosified segment of the LTCC, pure silica was detected within the foam-like structure. In addition, an increased amount of Ca was detected close to the pore walls of the etched LTCC, which are indeed the borders that the etchant does not penetrate further into the depth of the LTCC. The elemental analysis of Al, Si and Ca from the EDX line scan is depicted in the right lower corner. From this image, it can be concluded that the intermediate regions between corundum grains feature a reduction in the amount of Al accompanied by an increase of Si and Ca, respectively. This part which envelops the alumina grains and consists of Al, Si, Ca, and also O which is not shown here, can be referred to the anorthite phase. It can be seen in the TEM overview image of the porosified Ceramtec
Fig. 5. EDX line profile across an as-fired area of the Ceramtec GC LTCC etched in KOH 3 mol L−1 at 90 °C for 4 h.
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Fig. 6. EDX line profile across an etched region indicating three different phases: pure Alumina at pos. 1, pure Silica at pos. 2 and Ca-rich aluminosilicate at pos. 3. Fig. 7. TEM lamella of a GC LTCC with three representative locations labeled and the corresponding SAED analyses. At position 1 pure silica shows a nano-crystalline microstructure, close to an amorphous phase, whereas position 2 and 3 feature pure alumina and Ca-rich aluminosilicate with a singlecrystalline morphology.
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Fig. 8. EELS line profile across an etched area of the Ceramtec GC LTCC etched in orthophosphoric acid (85% by mass) at 100 °C for 4 h. (White areas in the TEM image are indicating the pores).
be altered since the porosification is generated in the as-fired state of commercially available LTCC samples, before final metallization. Furthermore, the advantage of the proposed method is the generation of a surface quality for the etched LTCC which features a bearing plane suitable for further metallization, being an essential pre-requirement for the reliable operation of compact devices or modules for high-frequency applications. The surfaces treated in aqueous KOH solutions are much less corroded compared with the state of the art approach for LTCC porosification i.e. aqueous orthophosphoric acid solution treatment. Preliminary studies on the essential etching parameters i.e. bath temperature, etchant concentration, and etching time revealed that temperature is the most influential parameter since putting the sample in contact with the KOH solutions at room temperature, even for a long period of time would not result in an appropriate etching. An explicit surface porosification of the LTCC requires the temperatures more than 70 °C. Complementary investigations will be carried out on optimization of key parameters of the etching process. Moreover, the TEM analysis showed that the introduced porosity to the Ceramtec GC due to the KOH-treatment mainly consists of the favorable ink-bottle pores which are restricted to the near-surface region while ortho orthophosphoric acid-treatments results in cylindrical pore structures which are distributed in the deeper regions of the LTCC rather. The etching mechanism with the KOH solution will be further studied in our future research and also the effect of produced porosity on the physical materials properties of the LTCC will be investigated.
the LTCC surface. This is regarded as a significant achievement for the long-term objective of the porosified LTCC which is the air embedment through applying metallization on the porous areas. Also, for comparison reasons the Ceramtec GC LTCC treated with orthophosphoric acid (85% by mass) for four hours at 100 °C was cut with FIB and analyzed with TEM. As it has been shown in Fig. 8 the etching behavior of Ceramtec GC with orthophosphoric is very different in comparison with KOH. Although etching of the Ceramtec GC with KOH resulted in a localized porosity and locally modifies the surface and near-surface regions by the creation of mainly ink-bottle-type pores, a delocalized porosity is observed for the sample etched with orthophosphoric acid. Also, etching of the Ceramtec GC with orthophosphoric acid results in a more selective dissolution of the anorthite compared with KOH solution. Furthermore, in contrast to the KOH treated sample which comprises mainly ink-bottle-type type porosity, the acid treated sample contains a network of cylindrical pores which are formed due to the preferential dissolution of anorthite phase and are distributed within a much deeper region of the Ceramtec GC LTCC. Additionally, in order to investigate the elemental composition of the acid-treated sample, an EELS line scan through the porosified part of the Ceramtec GC LTCC was recorded and the results are depicted in the right side of Fig. 8. As can be observed in this figure the elements which are found in this line scan, are dominantly Al and Si and the amount of found Ca is negligible. From this observations, a strong dissolution of the anorthite phase can be concluded. This porosification behavior will strongly affect the mechanical properties of the LTCC and therefore, restricts their handling for further applications.
Acknowledgements The authors would like to acknowledge the financial support from the Austrian Science Fund (FWF), No. I 2551-N30. In addition, we thank Dr. Klaudia Hradil from the X-ray Center (XRC) of TU Wien for her support with the XRD measurements.
4. Conclusion and outlook In this study, a novel approach for wet-chemical etching of the LTCC substrate was introduced in which a tailored porosity in as-fired Ceramtape GC was generated. In this approach, an aqueous solution of KOH in the temperatures lower than 100 °C under constant stirring is used as an etching bath. The method is very simple to monitor and the degree of the porosification can be controlled with straightforward key parameters, such as the bath temperature and the etch time. By using the porosification process, LTCC fabrication procedure does not need to
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