Low temperature co-fired ceramics (LTCC) technology has been successfully used in microelectronics, automotive, and telecommunication applications. However, their generally high permittivity is unfavorable for micromachined devices operated at high frequencies.
Microporous and Mesoporous Materials 288 (2019) 109593 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso Wet-chemical porosification of LTCC substrates: Dissolution mechanism and mechanical properties T Ali Hajiana,*, Martin Brehlb, Thomas Kochc, Christopher Zellnera, Sabine Schwarzd, Thomas Koneggere, Dominique de Lignyc, Ulrich Schmida a Institute of Sensor and Actuator Systems, TU Wien, Gusshausstrasse 27-29, 1040, Vienna, Austria Institute of Glass and Ceramics, University of Erlangen-Nuremberg, 91058, Erlangen, Germany c Institute of Materials Science and Technology, TU Wien, Getreidemarkt 9/308, 1060, Vienna, Austria d University Service Centre for Transmission Electron Microscopy, TU Wien, Wiedner Hauptstrasse 8-10, 1040, Vienna, Austria e Institute of Chemical Technologies and Analytics, TU Wien, Getreidemarkt 9/164-CT, 1060, Vienna, Austria b A B S T R A C T Low temperature co-fired ceramics (LTCC) technology has been successfully used in microelectronics, automotive, and telecommunication applications However, their generally high permittivity is unfavorable for micromachined devices operated at high frequencies To overcome this drawback, we have established a wetchemical etching process as an effective approach which can be applied to LTCC substrates in their as-fired state and allows for a local permittivity reduction in regions of interest Understanding the etching mechanism is essential for the selection of appropriate etching conditions to control the degree of porosification Therefore, in the present work, we report on an effective approach to achieve a tailored porosification of LTCC substrates Different characterization techniques such as scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction analysis, and Raman Spectroscopy were used for investigation of the morphology and chemical composition of the substrates and thereby studying the etching mechanism Furthermore, using dynamic-mechanical analysis at temperatures up to 550 °C, the stiffness behavior of the LTCC substrates after wet-chemical etching was investigated, and promising results for the applicability of such modified modules were obtained, even when operated at elevated temperatures up to 550 °C Finally, a practical correlation between the mechanical properties and the relative porosification depth is presented, which is independent of etching conditions and the substrate thickness, and is valuable for optimization of the suitable depth of porosification for securing the desired mechanical properties Introduction In recent years, Low-Temperature Co-fired Ceramics (LTCC) substrates which are advanced composites of glass and ceramics sintered at temperatures below 1000 °C, have become an attractive technology for the robust assembly and packaging of electronic components and microelectromechanical systems (MEMS) [1–3] The low sintering temperature allows for co-firing of the LTCC with metals offering a high electrical conductivity, such as Au, Ag, Cu, and their alloys (such as Ag–Pt, Au–Pt, and Ag–Pd) which all have low melting points close to 1000 °C Moreover, the applicability of the multilayer approach based on glass-ceramics sheets not only facilitates their fabrication in the “green state”, but also allows for the realization of compact 3D structures with highly scalable manufacturing methods (e.g., microfluidic channels) as well as embedding passive electrical components such as capacitors, resistors, inductors, and conductor lines into the LTCC body [1,3–6] Therefore, this technology is very attractive for a wide range of applications including pressure [7,8], temperature-pressure [9,10], robust flow sensors [11,12], temperature-pressure-humidity [13], pH * [14,15], gas [16,17], and electrochemical [18,19] sensors as well as Pirani micro gauges [20], microfluidic systems [21–24] and lab-on-chip (LOC) devices [25,26] Due to its hermeticity, mechanical durability, attractive thermomechanical and dielectric properties, and also compatibility with thick film hybrid technology, LTCC technology has attracted the most attention in wireless communication, and electronic control units Furthermore, there is particularly a major industrial interest in using this technology for fabricating high-density multi-layer packages suitable for microwave applications and automotive electronics [27–30] However, an integration of patch antennas and accurate design of micromachined structures operating at high frequencies require separate regions of tailored permittivities for optimized radiation While areas with low permittivity enhance both the bandwidth and the efficiency of the active components, high permittivity areas allow a compact feeding circuit design To achieve this goal, a possible strategy could be combining polymer and LTCC substrates [31,32] This approach entails inherent disadvantages associated with bond wires such as parasitic inductances, expensive and complicated manufacturing Corresponding author E-mail address: ali.hajian@tuwien.ac.at (A Hajian) https://doi.org/10.1016/j.micromeso.2019.109593 Received 25 April 2019; Received in revised form 20 June 2019; Accepted July 2019 Available online 04 July 2019 1387-1811/ © 2019 The Authors Published by Elsevier Inc This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/) Microporous and Mesoporous Materials 288 (2019) 109593 A Hajian, et al Material properties and detailed fabrication process of the Ferro L8 can be found in the corresponding datasheet [45] Phosphoric acid (H3PO4 85 wt%) from Sigma-Aldrich served as etching solution directly or after dilution with deionized water Sodium hydroxide (NaOH) pellets (99% from VWR) were used for the preparation of alkaline etchants All the etching experiments were performed at constant temperatures in a capped beaker with a polytetrafluoroethylene (PTFE) fixture for holding the substrates in position Through magnetic stirring of the etchant at 120 rpm during the etching reaction, the effective exchange of reactants and etching products was ensured The etched LTCC substrates were washed with water and propane-2-ol and then dried at 110 °C before further analyses The phase composition of the as-fired LTCC substrate was explored by Inductively Coupled Plasma Optical Emission Spectroscopy (ICPOES) measurements using a Spectro Genesis FES Also, to acquire information about the dissolved phases and thereby of the etching mechanism, Raman spectra were recorded with a Thermo Scientific Nicolet™ Almega spectrometer coupled with a high-quality Olympus microscope and high-resolution grating Laser excitation was done at a wavelength of 532 nm, and the spot size was approximately μm (using a 50 × objective lens with 0.75 NA) Weight loss of the samples due to the etching process was used as a primary criterion for the degree of porosification The gravimetrical studies were carried out by using a Sartorius R200D microbalance with a standard deviation of less than 0.02 mg The depth of porosification for the samples etched at different conditions was measured by applying the open software ImageJ, which is widely used in scientific research [46,47], for cross-sectional images of the fracture planes obtained using a Hitachi SU8030 scanning electron microscope (SEM) The SEM micrographs were acquired at an operating voltage of kV in the charge suppression scanning mode and without any pre-metal coating of the specimens To evaluate the stiffness behavior of the Ferro L8 substrates due to the etching process, dynamic-mechanical analysis (DMA) was carried in bending mode in the temperature range from 25° to 550 °C using a TA Instruments DMA Q800 The span width was 10 mm, and the dynamic amplitude was 20 μm in the case of the specimens with a thickness of 180 μm and μm for the 520 μm thick samples A frequency of Hz and a ratio of static to dynamic force of 1.25 were applied The temperature was increased stepwise, starting at 25 °C, then ramped to 50 °C and from then the step size was 50 °C After a dwell time of 10 at each temperature step, the specimen was dynamically loaded for 30 s, and the storage modulus (E′) was determined E′ is the stress-to-strain ratio in a system having sinusoidal loading and represents the energy storage capacity of a system and other relevant properties of the elastic portion Basically, the quantity E’ is a measure of the stiffness of the material [48] By using the ‘lift-out’ technique in a dual beam focused ion beam (DBFIB) FEI Quanta 200 3D system, LTCC foils were fabricated and used for the transmission electron microscope (TEM) imaging through an FEI Tecnai F20 field-emission TEM (FE-TEM) operating at 200 kV Selected area electron diffraction (SAED) was used for analyzing crystalline and amorphous phases in the LTCC processes, as well as different coefficients of thermal expansion (CTE) which result in local strain generation and thereby may lead to a reduced lifetime Another approach for reducing the permittivity is introducing air, which possesses a very low permittivity (εr = 1), into the material Different pore formation methods, including freeze-dry processing [33], extrusion [34], and use of pore-forming agents [35] have been reported in previous studies Modification with oxides of multivalent metals is another method which affects the porous structure of ceramics due to the filling of macropores [36] However, all the mentioned methods require alteration of the LTCC tape composition and hence, cannot be straightforwardly integrated into well-established LTCC substrate fabrication processes These problems could be overcome by the development of a porosification technique which allows for a local permittivity reduction of LTCC substrates This method, which is based on replacing some of high-k LTCC constituents with air through a wet-chemical etching process, was first reported in Ref [37] By applying this method for the commercial 951 LTCC tape from DuPont, a maximum porosification depth (dp) of about 40 μm was achieved Important information on the porosification mechanism was obtained through further studies on the wet-chemical etching of several LTCC tapes at different conditions [38–41] Since this technique can be applied to the LTCC substrate in the asfired state, no alteration in the tape chemical composition or firing profile is necessary and therefore it is considered as a very cost-effective and straightforward method for permittivity reduction Moreover, through precise masking of the substrate, areas of the different permittivity can be realized in one layer The degree of permittivity reduction is directly related to the degree of porosification because by increasing the latter parameter, more air is embedded into the LTCC substrate, and therefore, the overall permittivity will be further decreased Typically, the degree of porosification can be increased either through the axial or lateral pore growth — the later most likely results in wider pore openings and hence, a degraded surface quality However, a high-quality surface is crucial for high-frequency applications because with increasing frequency in the GHz range the skin depth, derived for ideally smooth conductor surfaces, decreases to the order of the surface roughness, thus causing a nearly linear increase in conductor loss [42–44] On the other hand, axial growth of the pores through deep penetration of the etching solution while preserving the surface quality would meet both requirements with respect to the significant air embedment and the high-quality metallization However, the porosification typically gives rise to channel-like, statistically distributed, and interconnected open meso-to macropores, which deteriorate the mechanical strength of the LTCC Therefore, deep porosification of the LTCC could result in a reduction of the mechanical substrate stability and consequently in a reduced lifetime The present work aims towards a comprehensive study on the etching process of a commercially available LTCC tape with a phosphoric acid solution to realize a tailored porosification For this purpose, L8 LTCC tape from Ferro Corporation (Ferro L8) was chosen Due to its dielectric constant εr = 7.3 ± 0.2 and loss tangent of < 0.18% at GHz Ferro L8 is suitable for low-to mid-frequency telecommunications, automotive, and medical modules and sensors as well as higher frequency aerospace and satellite applications [45] Furthermore, the impact of the etching process on the stiffness behavior of the substrates was investigated in the range from room temperature up to 550 °C Results and discussion Orthophosphoric acid solutions with two concentrations of 50 and 85 wt%, labeled as P50 and P85, were used as etchants for the porosification experiments Mass removal of the substrates due to the etching process was calculated for both P50 and P85 treatments at varied bath temperatures (Tb) and for different etching times (t) The mass removal percentages due to the etching process were normed to the initial weight of the corresponding as-fired substrates The lower boiling point of P50 in comparison with P85 gives rise to a massive bubble formation close to the boiling temperature, leading to a harshly attacked LTCC surface Therefore, for the P50 solution, bath Experimental details Commercially available Ferro L8 LTCC substrates with the dimensions of 10 mm × 10 mm × 180 μm were used for the etching experiments and for mechanical tests two different dimensions of 25 mm × mm × 180 μm, and 25 mm × mm × 520 μm were used Microporous and Mesoporous Materials 288 (2019) 109593 A Hajian, et al Fig Mass removal of Ferro L8 as a function of time at varying bath temperatures, due to the etching with a) P50, and c) P85, and two representative cross-sectional SEM micrographs of Ferro L8 LTCC etched at 90 °C for 120 with b) P50, and d) P85 e and f) Comparison of dissolution rates when etching Ferro L8 with different solutions For comparison, the results of NaOH etching experiments are also inserted (empty triangles) viscosity of the etchant is a crucial parameter for the etching process, and consequently, individual etching bath parameters influencing this fluid parameter need to be taken into account The viscosity of liquids decreases with increasing bath temperature, and subsequently, the diffusion of the etchant into the depth of the LTCC is facilitated, and thereby the weight loss increases when rising the temperature This effect can be observed in the mass loss trends for both P50 and P85 at temperature above 105 °C was avoided, while for the P85 with a nominal boiling temperature of 154 °C [49], a maximum bath temperature of 120 °C was applied The calculated mass removal as a function of etching time at different bath temperatures for both P50 and P85 etchants is depicted in Fig Since a suitable etching process particularly requires the penetration of the etchant into the pores and openings of the LTCC, the Microporous and Mesoporous Materials 288 (2019) 109593 A Hajian, et al corresponding activation energies continuously decrease when increasing the etching time, the calculated activated energies with values ranging from 0.63 eV for long etching times up to 1.89 eV for etching are well above the limit of the diffusion-controlled regime, i.e., 0.2 eV Therefore, it can be concluded that for the etching reaction with P50 solution, the process is reaction-controlled Also, it should be mentioned that since at low bath temperatures the etching reaction is significantly slower Therefore, dp values are reduced, and the impact of potential error sources is more pronounced, resulting in a larger deviation of the fitted straight lines from the data points By increasing the etching time, the depth of etchant penetration increases and the diffusion into the deep pores of the LTCC becomes more and more dominating but does not reach the pure diffusion-controlled regime For the P85 solution, however, as can be seen in Fig 2d, there is not a clear trend in the slopes of the fitted lines with etching time, which might be due to the enhanced temperature dependence of the P85 viscosity compared to that of P50, thus strongly affecting the diffusion ability Nonetheless, the calculated activation energies for all etching times stay significantly above 0.2 eV (between 1.05 and 1.59 eV) suggesting a dominating reaction-controlled mechanism also for this etchant To acquire detailed information about the microstructure of the substrate, Raman-based investigations were conducted to correlate the change in LTCC constituents due to the etching process The investigated samples were LTCC substrates etched with P50 solution at 90 °C at different etching times of 5, 15, 30, 60, 120, and 180 Since the etching rate at low temperatures is very slow and at high temperatures difficult to control, Tb of 75 and 90 °C were chosen as representative etching temperatures for further analyses of the porosified LTCC substrates For reasons of comparison, the as-fired LTCC was also subjected to the Raman measurements The corresponding Raman spectra are shown in Fig Each spectrum in this figure is an average of 180 spectra which have been obtained from the cross-section of the LTCC substrates with a step size of μm In the Raman spectra, major peaks corresponding to rutile (TiO2), corundum (Al2O3), and celsian (BaAl2Si2O8) phases are detected These minerals were identified based on RRUFF Database [51] and also due to the prior knowledge about the tape composition via ICP-OES measurements where the composition of as-fired Ferro L8 was identified as 42.97% Al2O3, 30.45% SiO2, 16.37 BaO, 6.52 B2O3, 2.67 CaO, and 1.02 TiO2 (wt%) For the as-fired sample, sharp peaks observed at Raman shifts below 200 cm−1 as well as small peaks observed at 509 cm−1 and 358 cm−1 are related to the crystalline celsian phase By increasing the etching time, the intensity of these peaks decreases, and finally they disappear, which represents the celsian removal due to the etching process The peaks at 600, 450, and 220 cm−1 which are attributed to the rutile phase, and also the peaks at 380, 420, and 640 cm−1 remain almost unaltered within the measurement accuracy Based on the well-known acidic dissolution reaction of anorthite (CaAl2SiO8) [52,53], the following reaction is proposed for the celsian dissolution in LTCC: different bath temperatures Similarly, direct comparison of the weight loss values for P50 with those for P85 shows that for the less concentrated solution (i.e., P50), the weight losses are significantly higher due to the lower viscosity Moreover, based on the obtained gravimetric results, the normalized rates of dissolution for etching with both P50 and P85 solutions were calculated, and the results are depicted as a function of bath temperatures (see Fig 1e) Both etchants show a very similar trend, in which the dissolution rate is slightly increasing up to 75 °C, whereas at higher temperatures, a higher increase is observed Nonetheless, due to the less difficult penetration into the openings and pores of the substrate, for all bath temperatures, the dissolution rate for P50 is significantly higher in comparison with P85 Moreover, the calculated dissolution rates for etching with phosphoric acid are compared with those gained with an alkaline NaOH solution having different concentrations between 0.5 and mol L−1 [50] In the latter study, it is reported that the etching is composed of two main regimes being either pure porosification or partial dissolution of the substrate A comparison of all etching results shows that the P50 solution results in the highest rate of dissolution except for the M NaOH When choosing enhanced values of key NaOH etching parameters such as concentration, etching time, and bath temperature, the etching mechanism changes from predominant porosification to the substrate dissolution regime This is not favorable for the intended purpose because the maximum porosification depth has been already reached, and after passing that critical etching time mainly complete dissolution is taking place However, due to the formation of wider pore openings, the rate of dissolution is increasing intensely Unlike for etching Ferro L8 with alkaline NaOH solution [50], no significant reduction in the substrate thickness was observed while etching with phosphoric acid, independent of the etchant concentration This indicates that the etching of Ferro L8 in phosphoric acid solution results in a pure porosification, while etching with NaOH resulted in both surface porosification and partial dissolution of the substrate Porosification depth values as a function of etching time for both P50 and P85 are plotted in Fig 2a and b For both etching solutions, at a given temperature, depth of porosification increases with etching time For P50 etching solution, the slope is steeper at short etching times and then becomes flatter showing that the porosification at the beginning of the etching process is fast, due to the facile diffusion of the etchant into the surface-near porosity Because of the more difficult exchange of the etching solution through the generated micro- and nanopores at the etch front, the dissolution rate slows down, but does not reach any saturation level On the other hand for P85, because of the less pronounced diffusion affinity, the slopes are smaller than those for P50 Increasing the etching bath temperature for a fixed etching time results in increased dp values what is due to the faster reaction kinetics and also easier penetration of the etchant into the depth of the LTCC body By further increasing the bath temperature to 105 °C, an almost linear relationship is observed between dp and t until complete porosification of the substrate is reached after 180 Please note that the reported dp values are only taken from one side of the substrates To verify the assumption on the presence of two dominating etching regimes, Arrhenius-type diagrams of dp as a function of the reciprocal bath temperature were plotted, so that the activation energy (Ea) was determined for fixed etching times through a linear regression procedure The calculated Ea values were used to acquire further information about the etching mechanism because, in a wet chemical etching process, Ea values of about 0.2 eV and below represent the domination of diffusion-controlled, while higher Ea values indicate the presence of reaction-controlled dissolution mechanisms [37,50] Time-dependent evolution of Ea for etching Ferro L8 is represented in Fig 2c and d, and it can be observed that the porosification with P50 follows the Arrhenius law over the whole temperature range up to 240 Although the slopes of the linearly fitted lines and hence, the BaAl2SiO8 + 8H+ → Ba+2 + 2Al+3 + 2H4SiO4 In order to investigate the etching behavior into the substrate depth, individual Raman spectra are shown in Fig when analyzing their cross-section and starting at the surface (i.e depth of μm) Since showing all 180 spectra will be inconvenient to distinguish the small peak changes due to the etching process, only some selected spectra which are of interest to estimate the depth characteristics are shown For the interpretation of the obtained results, however, it is worth mentioning that LTCC is a composite of ceramic grains heterogeneously embedded in a glass matrix Therefore, minor differences in the spectra of a similar region even for the as-fired LTCC are reasonable, what is the reason why the averaged spectra were shown in Fig But, it is also intended to track the significant peak changes into the depth of the LTCC Therefore, the etching depth was estimated from the Microporous and Mesoporous Materials 288 (2019) 109593 A Hajian, et al Fig Porosification depth dp and corresponding Arrhenius diagrams when etching Ferro L8 with P50 (a and c), and P85 (b and d) solutions at different etching times The Ea values determined from the linearly fitted lines are inserted in the figures The R-square values for the linear fitting are also given in brackets 120 no or only weak celsian-related peaks were found at the surface However, at a certain depth (approximately 70 μm), these peaks are getting perceivable Nevertheless, for the sample etched for 240 min, which represents the maximum etching depth, no significant peak associated with the celsian phase was observed at any depth which shows that through a highly selective celsian dissolution, the whole substrate has been porosified These results are in a reasonable agreement with the dp values obtained from cross-sectional SEM images as well as with those obtained from TEM investigations, which will be discussed next For microstructural analyses of Ferro L8 after porosification, the cross-sectional image of an LTCC substrate is shown in Fig 5a On top, a platinum layer is deposited, which is applied to reduce any charging effects and to avoid structural damaging of the sample surface during FIB preparation As it can be seen, discrete alumina grains are distributed in the whole glass matrix, and the etchant penetrates via small grain-near gaps and openings into the LTCC body Therefore, it can be concluded that this portion of the matrix is very important for enabling the penetration of the etchant into the LTCC body and the realization of deeply etched samples Nonetheless, due to the depth limitations of the FIB technique, the large penetration depths which were obtained in this work cannot be fully displayed Hence, for studying the depth of porosification, cross-sectional SEM images of fracture planes of the LTCC were used In the next step, the obtained FIB foil was subjected to transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) investigations Fig 5b shows corundum grains and partially dissolved grain-near regions in a high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image The chemical composition of the LTCC was explored through an Fig Raman spectra normalized to the maximum of the band in the Q-range, for as-fired LTCC and substrates etched with P50 at Tb = 90 °C Each spectrum is an average of 180 individual measurements disappearance of the peak at 404 cm−1 which was most sensitive to the dissolution Three representative samples i.e as-fired, partially etched (120 min), and totally etched (240 min) are shown For the as-fired substrate, except for the small peak found at large Raman shifts, the characteristic spectra remain unchanged For the sample etched for Microporous and Mesoporous Materials 288 (2019) 109593 A Hajian, et al EDX line scan which is indicated by an arrow The corresponding EDX spectra and quantifications with respect to the elements Al, Si, Ba/Ti, and O are shown in Fig 5c In these spectra the counts of Si and Al change in opposite directions representing corundum grains and the glass matrix, respectively The intermediate area corresponds to the partially attacked or depleted celsian phase The celsian phase which has been crystallized from the glass matrix during liquid-phase sintering, and its dominant removal due to the etching process, are more clearly observable in the recorded bright field TEM images (BFTEM) in Fig In addition to the chemical composition and morphology of the LTCC, its crystallographic structure was explored in selected areas with Selected Area Electron Diffraction (SAED) measurements The SAED patterns for three representative positions were recorded, and the results are shown in the lower row of Fig Position represents the amorphous glassy matrix, while positions and correspond to the crystalline corundum and celsian phase, respectively Position shows a typical single crystalline diffraction pattern, while in position in addition to the diffraction pattern, which is far out from a low-indexed zone axis, concentric blunt circles are seen indicating the polycrystalline morphology Mechanical characterization — As it was already illustrated, by employing the wet chemical etching process a defined porosity can be introduced to the LTCC although the high penetration of the etchant into the depth of LTCC may also raise concerns about the mechanical robustness of the LTCC Therefore, a detailed study was carried out on the stiffness behavior of the LTCC substrates For this purpose, as-fired and porosified LTCC substrates were subjected to DMA analyses Substrates treated for different etching times, corresponding to different depths of porosification, were chosen First, the samples etched with P50 at 90 °C were investigated However, for this etching condition, the reaction rate is so high that within an etching time of 20 min, the stiffness of substrates decreases by about 90%, and after that, the samples become too fragile to handle and to measure To have a lower reaction rate and better control on the depth of porosification, samples etched at an etching temperature of 75 °C were chosen Furthermore, along with the LTCC substrates with a thickness of about 180 μm, another set of substrates with a thickness of about 520 μm were used, as they represent two typical thicknesses in LTCC substrate technology Results of room temperature analyses for both sets of 180 and 520 μm thick samples are shown in Fig Independent of substrate thickness, the stiffness of both samples sets decreases with etching time due to the mass removal and the introduction of air up to a certain depth into the LTCC body However, for the 520 μm thick samples due to the lower percentage of mass removal in comparison to the 180 μm thick samples, the decrease with respect to their corresponding as-fired samples is with about 40% lower compared to 80% This confirms that choosing thicker multilayered substrates for the etching experiments secures to a higher degree the original mechanical properties after porosification DMA scans of the as-fired and etched LTCC samples indicate almost constant values for all substrates up to measurement temperatures of 550 °C, which means that the proposed method does not limit the application of the LTCC even at such elevated temperatures (see Fig 8) After the DMA test, the samples were taken for cross-sectional SEM imaging to measure the porosification depth values and correlate the measured storage moduli with the different sample thicknesses Therefore, the measured porosification depth values were normalized to the substrates thickness, and the resulting plot is shown in Fig 9a Both sets of substrates follow a very similar trend in the decrease of stiffness However, for the thicker substrates due to the reduced percentage of the porosified area, the decrease in stiffness is also less Basically, this plot is independent of the sample thickness, etching time, and etchant temperature In order to be able to predict the correlation between storage modulus E′ and the relative porosification depth, a modification of wellestablished minimum solid area (MSA) models was employed These Fig Raman spectra normalized to the maximum of the band in the Q-range, for the as-fired LTCC and substrates etched with P50 for different times at Tb = 90 °C All spectra were normalized to their overall area Red spectrum is related to the estimated maximum depth of porosification, dp The values given next to the spectra represent the depth at which the spectrum is aquired (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Microporous and Mesoporous Materials 288 (2019) 109593 A Hajian, et al Fig a) Cross-sectional view on the microstructure of Ferro L8, and b-d) HAADF-STEM image with the corresponding EDX line profiles across a partially porosified section of the Ferro L8 LTCC etched with P50 for 30 at Tb = 90 °C Fig TEM images of a porosified LTCC substrate with three labeled locations for SAED analyses At position 1, amorphous glass can be observed, whereas position and represent crystalline corundum and celsian phase, respectively Microporous and Mesoporous Materials 288 (2019) 109593 A Hajian, et al time and bath temperature, solely by knowing the depth of porosification which can be measured straightforwardly by cross-sectional SEM imaging, assuming a homogeneous porosity within the etched layer Hence, for obtaining a given storage modulus, the substrate thickness and etching conditions can be carefully chosen to acquire the appropriate relative depth of porosification Therefore, these results confirm the applicability of the proposed etching method for obtaining porosified LTCC substrates with tailored mechanical properties by choosing the suitable depth of porosification for a substrate of the desired thickness Conclusions Porosification of Ferro L8 LTCC through wet-chemical etching process was studied in detail Phosphoric acid with different concentrations of 85 wt% and 50% were applied as etchants at different bath temperatures and treatment times The process was so fast that even for etching a considerable etching depth was reached However, the rate of the etching process and hence, the degree of porosification, and the corresponding depth of porosification can be precisely tailored by a careful selection of etching parameters including etchant concentration, bath temperature, and treatment time The high surface quality of the porosified LTCC allows for a high-quality metal deposition, which is essential for the reliable operation of high performance, high-frequency devices Therefore, the focus of our next study would be on the realization of high-quality metallic structures serving, e.g as antenna elements and on the corresponding changes in dielectric properties due to the etching process Kinetic studies were conducted through gravimetric investigations and analyses of the porosification depth, which show a higher reaction rate for the less concentrated etchant solution and a dominating reaction-controlled mechanism Further investigations prove the selective dissolution of celsian phase, which is surrounding corundum grains, thereby a very deep porosification up to the whole substrate thickness could be realized Moreover, the stiffness behavior of the substrates subjected to this wet-chemical etching process was investigated for substrates with two different thicknesses The result shows that for the applications where high mechanical strength of LTCC are desired, the desired mechanical properties are secured when choosing thicker substrates Also, the mechanical strength of the porosified LTCC substrates was investigated over a large temperature range up to 550 °C indicating a constant storage modulus within the measurement accuracy This demonstrates that the proposed method does not limit the application of porosified LTCC Fig Room temperature storage modulus of Ferro L8 substrates with two different thicknesses of 180 and 520 μm etched with P50 at 75 °C for different etching times The results for 180 μm thick LTCC etched at 90 °C are shown for comparison models are typically used to describe the porosity-dependency of Young's modulus E in ceramic materials by the relationship E = E0 e-bP Here, E0 is the modulus of the non-porous base material, P is the total porosity, and b is a fitting factor which is, in part, affected by the type of pore structure [54] Here, the model was modified by introducing a modified porosity P′ = (dp/L)∙Pl, which takes into account the relative porosification depth (dp/L), whereas L is the thickness of the substrate and Pl the porosity within the etched layer For a total porosity within the etched layer of Pl = 0.2, which was estimated by mercury intrusion porosimetry measurements of comparable specimens [55], a fit convergence as shown in Fig 9b is achieved with E0 = 100.5 ± 4.0 GPa and b = 11.5 ± 1.0, the former value is in good accordance to values of the as-fired Ferro L8 as reported in the material datasheet [45] Also, a b value of 11.5 hints towards a pore structure derived by stacking of solid spheres [54] This type of structure can be confirmed by cross-sectional SEM images showing the continuous removal of the celsian phase during etching while solid particles primarily consisting of alumina remain Consequently, these findings can be applied for a direct estimation of elastic sample properties of Ferro L8 LTCC with P50 for any etching Fig The temperature-dependent storage modulus of Ferro L8 substrates with two different thicknesses of a) 180, and b) 520 μm, 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forming tapes, J Eur Ceram Soc 35 (2015) 4465 A Talai, F Steinhäußer, U Schmid, R Weigel, A Bittner, A Koelpin, A finite 3D field simulation method for permittivity gradient implementation of a novel porosification process in LTCC, 2013 IEEE MTT-S International Microwave Symposium 10 ... macropores, which deteriorate the mechanical strength of the LTCC Therefore, deep porosification of the LTCC could result in a reduction of the mechanical substrate stability and consequently in a reduced... sinusoidal loading and represents the energy storage capacity of a system and other relevant properties of the elastic portion Basically, the quantity E’ is a measure of the stiffness of the material... amplitude was 20 μm in the case of the specimens with a thickness of 180 μm and μm for the 520 μm thick samples A frequency of Hz and a ratio of static to dynamic force of 1.25 were applied The temperature