coatings Article
Magnetron Sputtered AlN Layers on LTCC Multilayer and Silicon Substrates Heike Bartsch 1, * 1
2 3 4
*
ID
, Rolf Grieseler 1,2
ID
, Jose Mánuel 3,4
ID
, Jörg Pezoldt 1 and Jens Müller 1
Institute for Micro- and Nanotechnologies MacroNano® , Technische Universität Ilmenau, Gustav-Kirchhoff-Str. 7, 98693 Ilmenau, Germany;
[email protected] (R.G.);
[email protected] (J.P.);
[email protected] (J.M.) Physics Department, Pontificia Universidad Católica del Perú, San Miguel, Lima 15088, Peru IMEYMAT Institute of Research on Electron Microscopy and Materials, Universidad de Cádiz, 11002 Cádiz, Spain;
[email protected] Department of Condensed Matter Physics, Facultad de Ciencias, Universidad de Cádiz, 11510 Puerto Real, Spain Correspondence:
[email protected]; Tel.: +49-3677-69-3452
Received: 20 June 2018; Accepted: 25 July 2018; Published: 18 August 2018
Abstract: This work compares the deposition of aluminum nitride by magnetron sputtering on silicon to multilayer ceramic substrates. The variation of sputter parameters in a wide range following a fractional factorial experimental design generates diverse crystallographic properties of the layers. Crystal growth, composition, and stress are distinguished because of substrate morphology and thermal conditions. The best c-axis orientation of aluminum nitride emerges on ceramic substrates at a heater temperature of 150 ◦ C and sputter power of 400 W. Layers deposited on ceramic show stronger c-axis texture than those deposited on silicon due to higher surface temperature. The nucleation differs significantly dependent on the substrate. It is demonstrated that a ceramic substrate material with an adapted coefficient of thermal expansion to aluminum nitride allows reducing the layer stress considerably, independent on process temperature. Layers sputtered on silicon partly peeled off, while they adhere well on ceramic without crack formation. Direct deposition on ceramic enables thus the development of optimized layers, avoiding restrictions by stress compensating needs affecting functional properties. Keywords: low temperature cofired ceramics; LTCC; aluminum nitride; high-rate magnetron sputtering; stress reduction; substrate influence on nucleation; cubic aluminum nitride
1. Introduction Sputtered aluminum nitride (AlN), depending on composition and microstructure, can fulfill several functional demands such as optical functionalization [1,2], high-k dielectric [3,4], piezoelectric element [5,6], microelectromechanical systems (MEMS) resonators [6] or heat spreader [7]. It is therefore widely used in electronic circuits and MEMS devices. To employ the pyro- and piezoelectric properties in microelectromechanical and electronic devices, c-axis oriented AlN is required [8,9]. The same holds also for acousto-electronic devices [10,11], nonlinear optics and optomechanical devices, [12] as well as phononic crystals [13]. Usually, silicon is the substrate of choice for the device processing. Since the coefficient of thermal expansion (CTE) of silicon and AlN differs, thermal mismatch causes significant residual stress dependent on process temperature [14]. Strategies exist, such as variation of the N2 /Ar ratio during the process [15], which compensate thermal stress by intrinsic stress components. However, this method entails an increase of lattice failures and thus, limitations of desired functional properties must be accepted. The use of substrates with adapted CTE would eliminate these disadvantages. Coatings 2018, 8, 289; doi:10.3390/coatings8080289
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Low temperature co-fired ceramic (LTCC) stands for a technology, which uses green ceramic foils consisting of ceramic and glass powder, embedded in an organic matrix, for the fabrication of multilayer circuit boards with printed electronic components [16,17]. High reliability and temperature stability completed by a wide freedom in design and the possible integration of low resistivity conductor paths made of gold or silver boosts their application as radio frequency (RF) devices [18–21] and for smart system integration [22–24]. The composite character of the fired ceramic allows the variation of functional properties, such as permittivity, thermal expansion, or insulation resistance [25,26]. This flexibility, which entails the possibility of using a substrate with adapted CTE, was the first motivation for this study. A second motivation originates from packaging aspects: Multiple device chips and passives are assembled on package carriers. Connections between chips, carrier, and passives require space and reduce the performance due to parasitic losses. The monolithic integration of functional units on the package contributes to cost reduction and system miniaturization. It requires the direct deposition of functional thin film components on LTCC ceramics [27]. Sputtering of AlN on LTCC substrates with high c-axis orientation would open new integration aspects. First works on this topic focused on the application of AlN as a heat spreader [7,28]. The used LTCC ceramic in this study has still a CTE mismatch and the layer was sputtered on the pristine and therefore rough ceramic surface. The roughness influences the grain structure and c-axis orientation is disturbed. In the present study, a CTE adapted LTCC ceramic is used and the surface is polished in order to achieve the best conditions for textured layer growth. Morphological properties and layer stress of AlN layers sputtered on (100) silicon and LTCC ceramic are compared. The setup of the sputter coater supports high deposition rates, which allows the deposition of reasonable film thickness in acceptable process time. The experimental design covers a wide parameter range in order to assess a wide process window. The current investigation is a first step towards the deposition of well-textured AlN layers on polished LTCC surfaces with minimized stress. The parallel coating of ceramic and silicon substrates provides an insight into the growth process depending on the substrate. 2. Experimental Procedure 2.1. Sample Preparation The CTE of sputtered AlN layers with (002) texture amounts to 5.3 × 10−6 K−1 along the c-axis and 4.2 × 10−6 K−1 in the basal plane according to the literature [29]. A commercially available LTCC green foil (DuPontTM, GreenTapeTM 9k7, DuPont Nemours, Bristol, UK) serves as substrate in the current work. Pursuant to the manufacturer data sheet, the sintered LTCC ceramic substrates have a CTE of 4.4 × 10−6 K−1 and a thermal conductivity of 3.3 W m−1 K−1 . The resulting thermal mismatch should therefore be low. The green foils were stacked, laminated, and sintered at a peak temperature of 850 ◦ C, which is held for 20 min. The fired ceramic is diced in pieces of 35 × 35 mm2 in order to obtain the ceramic substrates for the sputter process. Literature reports that the surface roughness influences c-axis orientation significantly [7,28]. Therefore, the LTCC substrates were polished. The surface roughness is studied in Appendix A. Pores, formed during the sintering process, are opened during the polishing process [30]. The area roughness Sq including pores is 175 nm and the porosity is 23%. The evaluation of line scans revealed mean rms value of 9 nm beside the pores. The evaluation of atomic force microscopy (AFM) measurements confirms these values. After polishing, the ceramic substrates were cleaned in three cycles in an ultrasonic assisted acetone bath for 20 min, one cycle in an ultrasonic assisted isopropanol bath, subsequently rinsed in deionized water for 5 min and finally dried for 10 min at 80 ◦ C in a convection oven. The silicon samples were diced in pieces of 10 × 10 mm and treated with a solution of BakerClean® JTB-111 FEOL Cleaner (Avantor Performance Materials, Inc., Center Valley, PA, USA):H2 O2 :H2 O of 1:0.2:5 for 20 min
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at 70 ◦ C. Rinsing in deionized water for 5 min and drying for 10 min in a convection oven finishes the protocol. Both ceramic and silicon samples were clamped on the same substrate holder. Before sputtering, the samples were exposed to an argon plasma sputter process at 200 W (Ardenne LA 440 S, von Ardenne GmbH, Dresden, Germany) and an argon flow of 180 sccm for 3 min. After this step, the plasma was initiated and the chamber was conditioned for 5 min before opening the shutter and starting the deposition. The experiments were carried out using a RF magnetron sputtering reactor and an aluminum nitride target at a base pressure of approximately 3 × 10−7 mbar and a working pressure of approximately 1 × 10−3 mbar. The distance between target and substrates in this reactor is 20 mm. This setup allows high deposition rates. The substrate heater is situated at the backside of the substrate holder and its temperature is controlled by a thermocouple placed at the heater front side. 2.2. Experimental Design, Statistical Method, and Test Realization Even though interactions between the influences of the single settings are expected to occur, a fractional factorial design was used to get as much information on the deposition process as possible with a minimum experimental effort. Heater temperature (tH ), RF power, targeted thickness (tth ) and ratio between nitrogen and argon gas flow (N2 /Ar ratio) were varied in three levels each. Table A1 in Appendix B gives an overview of the experimental design. The tH varies between room temperature and 300 ◦ C and the RF power between 300 and 500 W. In a preliminary experiment, the deposition rate was determined using different values of applied RF power for a short deposition time of 5 min at room temperature and pure argon gas flow. These rates were the base for the calculation of the tth in the range between 0.3 µm to 1.7 mm. The N2 /Ar ratio in this study varies between 0% and 10%, whereby the argon gas flow in the chamber has a constant value of 80 sccm. All layers on LTCC and silicon samples were analyzed with X-ray diffraction (XRD) (D5000, Siemens/Bruker, Karlsruhe, Germany) in Bragg–Bretano mode. The curves of a reference measurement of the uncoated substrates were subtracted in order to obtain only the information on the deposited film. All patterns are provided in Figure S1 (on ceramics) and Figure S2 (on silicon), supplementary material. The relative texture coefficient (RTC) of the respective (hkl) orientation was calculated from the intensity of the measured curves using Equation (1) [31,32]: RTC(hkl ) =
0 Ihkl /Ihkl 0 ∑81 Ihkl /Ihkl
× 100 [%]
(1)
Ihkl is the intensity of the measured peak determined from the XRD patterns and I0 hkl the theoretical intensity of the respective peak. In total, eight peaks are considered leading to a RTC value of 12.5% for a poly-crystalline material. Each crystalline orientation with a RTC higher than 12.5% can be considered as preferred orientation. The bar charts for all RTC calculations are given in Figure S3 for ceramic and Figure S4 for silicon in the supplementary material. The RTC(002) values for all experiments are summarized in Table A1, Appendix B. They served as criterion for the DoE evaluation. Two samples have amorphous character, for those a RTC(002) value of 0.5 was set in order to perform the analysis of means (ANOM) fulfilling the valid parameter range. The analysis was carried out for each substrate type separately using the software Minitab (Minitab 18.1, Minitab GmbH, Munich, Germany). The target value in this study is a maximum RTC(002) as index for pronounced c-axis orientation. The mean plots of RTC(002) resulting from ANOM are depicted in Figure 1. They allow the assessment of single parameters: If a factor change leads to a strong shift of the target value, it influences the process target strongly. The ANOM allows thus a factor weighting. Further, the best settings for the factors can be derived. A low shift of the target value can be attributed to a no-significant influence of the parameter. A supporting measure, the analysis of variances (ANOVA) allows a reliable assertion that the factor has significant influence. It correlates the variances of single factor levels with the total process variance and compares the result with the fisher-distribution (F-test). It allows thus the judgement of the factor
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significance on a chosen confidence level. In the present study, the analysis of variances (ANOVA) was carried out at confidence level of 99% and 99.95% to evaluate the significance of parameter influences. Coatings 2018, 8, x FOR PEER REVIEW 4 of 19
Figure 1. Analysis of Means (ANOM) plot with relative texture coefficient (RTC) (002) as evaluation Figure 1. Analysis of Means (ANOM) plot with relative texture coefficient (RTC)(002) as evaluation criterion (maximum optimization): (a) for silicon substrates; (b) for ceramic substrates. criterion (maximum optimization): (a) for silicon substrates; (b) for ceramic substrates.
The mean size of ordered domains d, which is related to grain size, was calculated for the The meanstrongest size of orientation ordered domains d, which is related to grain size, was calculated for the respective using Scherrer method, see Equation (2) [33]:
respective strongest orientation using Scherrer method, 𝐾λ see Equation (2) [33]: 𝑑=
β cos θ
(2)
Kλ d= In this equation, λ is the wavelength of copper β is the line broadening at half the (2) β cosradiation, θ
maximum intensity, θ is the Bragg’s angle, and K a dimensionless shape factor, which was set to 1.
These arewavelength also summarized in Tableradiation, B1, Appendix In thiscalculation equation,results λ is the of copper β isB.the line broadening at half the real thickness all layers was determined evaluating field emission scanning electron maximumThe intensity, θ is theofBragg’s angle, and K a dimensionless shape factor, which was set to 1. microscopy (FE-SEM) images, at a constant working distance of 8 mm, and acceleration voltages of These calculation results are also summarized in Table A1, Appendix B. 1.5 kV in all cases. The thickness results and respective real deposition rates are given in Table B1, The real thickness of all layers was determined evaluating field emission scanning electron Appendix B. microscopy (FE-SEM) images, at a constant working distance of 8 mm, and acceleration voltages of 1.5 kV3. in all cases. The thickness results and respective real deposition rates are given in Table A1, Results and Discussion Appendix B. 3.1. Influence of Process Parameters on Texture
3. Results and Discussion
Since a huge process range is investigated using a fractional factorial screening design, it is not possible to Process predict all influences. only evident trends will be highlighted in the following. 3.1. Influence of Parameters onThus, Texture The process mean for the RTC value of the (002) orientation is 23.79% for layers deposited on Since a huge process is RTC investigated using a fractional factorial it is not ceramic substrates andrange 25.38% (002) for layers deposited on silicon. Thescreening variance design, of the RTC evaluation amounts to 3%. ANOM results on RTC (002) calculations are depicted in Figure 1a for possible to predict all influences. Thus, onlybased evident trends will be highlighted in the following. silicon substrates and 1b for ceramic substrates. Both charts significantly each on The process mean forFigure the RTC value of the (002) orientation is differ 23.79% for layersfrom deposited other. While the c-axis texture of the layers deposited on silicon increases strongly with increasing ceramic substrates and 25.38% RTC(002) for layers deposited on silicon. The variance of the RTC substrate temperature and thickness, the layers deposited on LTCC ceramic do not exhibit evaluation amounts to 3%. ANOM results based on RTC(002) calculations are depicted in Figure 1a for comparable behavior. silicon substrates and Figure 1b for ceramic substrates. Both charts differ significantly from each other. The ANOM plot reveals that, regardless of the substrate material, N 2/Ar ratio has the strongest Whileinfluence the c-axis texture of the layers deposited on silicon increases strongly with increasing substrate on the formation of a (002) texture, followed by tth. A clear increasing trend for these both temperature andwas thickness, thefor layers deposited LTCC ceramic exhibit comparable behavior. parameters observed layers depositedonon silicon, while do thenot means of RTC (002) for layers The ANOM reveals that, regardless of the substrate material, ratio hastothe strongest 2 /Ar deposited on plot ceramic show rising and falling tendencies as well. This N fact is assumed have its influence theparticular formation of a conditions (002) texture, followed by LTCC tth . Aceramic clear substrates increasingattrend originon in the process which occur using N 2/Ar for ratiothese of 5%. Although temperature anddeposited applied RFon power havewhile significant influence, which both parameters washeater observed for layers silicon, the means of RTC for layers (002)results from ANOVA, their effect on textured (002) growth is minor. Since nitrogen content in its the deposited on ceramic show rising and falling tendencies as well. This fact is assumed to have origin a strong influence on arrival and LTCC incident energysubstrates of the adatoms, affects theof 5%. in theatmosphere particular has process conditions which occurrate using ceramic at N2it/Ar ratio deposition rate significantly [34,35]. Although heater temperature and applied RF power have significant influence, which results from The real thickness differs significantly from the theoretical values, which are based on a rough ANOVA, their effect on textured (002) growth is minor. Since nitrogen content in the atmosphere has estimation of the deposition rate, determined in preliminary attempts. These were carried out a strong influence on arrival rate and incident energy of the adatoms, it affects the deposition rate without substrate heating in pure argon atmosphere for 5 min. The real deposition rate changes significantly [34,35]. significantly depending on RF power and N2/Ar ratio. These parameters effect an opposing trend: A The real thickness differs the significantly from thedeposition theoreticalrate values, which based ontoa10 rough nitrogen increase decreases process mean of the from 30 nm/s are (pure argon)
estimation of the deposition rate, determined in preliminary attempts. These were carried out without
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substrate heating in pure argon atmosphere for 5 min. The real deposition rate changes significantly depending on RF power and N2 /Ar ratio. These parameters effect an opposing trend: A nitrogen increase decreases the process mean of the deposition rate from 30 nm/s (pure argon) to 10 nm/s (10% N2 /Ar), Coatings 8, x FOR whereas an 2018, increase ofPEER the REVIEW RF power increases the deposition rate from 10 nm/s (300 W) to5 of 3019nm/s (500 W). A significant influence of heat temperature on the deposition rate could not be proven. nm/s (10% N2/Ar), whereas an increase of the RF power increases the deposition rate from 10 nm/s Summarizing all experiment results in Table A1, Appendix B, the best (002) texture was achieved (300 W) to 30 nm/s (500 W). A significant influence of heat temperature on the deposition rate could in experiment #4 on ceramics with a N2 /Ar ratio of 10%, heater temperature of 150 ◦ C, RF power of not be proven. 300 W, and a theoretical 1 µm. pronounced (002) orientation in the state is Summarizing all thickness experimentof results in Since Table B1, Appendix B, the best (002) texture wasinitial achieved apparent in experiment #8 with with onlya670 nmratio thickness, was temperature repeated with prolonged deposition in experiment #4 on ceramics N2/Ar of 10%, it heater of 150 °C, RF power of time in to achieve thickthickness layers with (002)pronounced orientation(002) (#10). Although experiment 300order W, and a theoretical of 1 better µ m. Since orientation in this the initial state isleads apparent experiment #8 with onlylower 670 nm thickness, was repeated with prolonged deposition with to a high (002)intexture, the RTC is still than that initexperiment #4, which was performed in order to achieve thick layers with orientation this experiment experiment and lowertime RF power and heater temperature. Thebetter lower(002) deposition rate(#10). of 13 Although nm/s in this leads to a high (002) the effect. RTC is still lower than that in experiment #4, which was performed plasma interactions can texture, cause this with lower RF power and heater temperature. The lower deposition rate of 13 nm/s in this experiment
and plasma interactions can cause this effect. 3.2. Layer Composition
Energy 3.2. Layerdispersive CompositionX-ray spectroscopy (EDX) (EDAX Element detector with Team-Software, 2 25 mm , SiEnergy AMETEK Materials Analysis Division, AMETEK, Inc., Berwyn, PA, USA) was 3 N4 window, dispersive X-ray spectroscopy (EDX) (EDAX Element detector with Team-Software, 25 carried out at 15 kV acceleration order to determine the chemical composition. mm², Si3N4 window, AMETEK voltage MaterialsinAnalysis Division, AMETEK, Inc., Berwyn, PA, USA) Samples was coated in experiment #5, #7, and #10 were investigated as exemplary for the respective nitrogen content. carried out at 15 kV acceleration voltage in order to determine the chemical composition. Samples Crosscoated sections of the samples by grindingasand coated for with 20 nm carbon layer in in experiment #5, #7,were and prepared #10 were investigated exemplary thea respective nitrogen Cross sections conducting of the samples were prepared by grinding and coated with awere 20 nm carbon on ordercontent. to achieve sufficient behavior. Aluminum and nitrogen content measured layer distributed in order to achieve conducting behavior. Aluminum nitrogen content were six points over thesufficient layer. The ratio of the detected elementand concentration was evaluated. measured on six points distributed over the layer. The ratio of the detected element concentration Mean value and standard deviation are depicted in Figure 2. The nitrogen content of the layer is was evaluated. Mean value and standard deviation are depicted in Figure 2. The nitrogen content of distinguished significantly dependent on the substrate in #5 (0% N2 ). The nitrogen concentration of the layer is distinguished significantly dependent on the substrate in #5 (0% N 2). The nitrogen the layer deposited on the silicon is significantly lower than that deposited on the ceramic. Both layer concentration of the layer deposited on the silicon is significantly lower than that deposited on the present a clear substoichiometry between the Al and N. At higher nitrogen concentration (5% and 10%), ceramic. Both layer present a clear substoichiometry between the Al and N. At higher nitrogen the ratio trends towards relation of 1:1. Thetowards deviation of measurements at 5% nitrogen content was concentration (5% anda10%), the ratio trends a relation of 1:1. The deviation of measurements very high. EDX content spectrum ofvery all samples revealed the presence oxygen. The oxygen incorporation at 5% The nitrogen was high. The EDX spectrum of all of samples revealed the presence of in sputtered aluminum nitride is a known effectaluminum [2,6]. Its influence functional properties must be oxygen. The oxygen incorporation in sputtered nitride is a on known effect [2,6]. Its influence on functional must be studied in detail in further works. studied in detail inproperties further works.
Figure 2. Layer composition derived from energy dispersive X-ray spectroscopy (EDX) measurements.
Figure 2. Layer composition derived from energy dispersive X-ray spectroscopy (EDX) measurements. 3.3. Influence of Nitrogen to Argon Ratio on Morphology
3.3. Influence of Nitrogen to Argon Ratio on Morphology 3.3.1. Nitrogen to Argon Ratio 0%
3.3.1. Nitrogen to Argon Ratio 0%
All experiments carried out in poor nitrogen atmosphere produced layers with poor state of
organization. The carried XRD patterns shown in Figure 3. The thinnest layer deposited at room All experiments out in are poor nitrogen atmosphere produced layers with poor state of temperature amorphous character. The3.others form cubic layers with organization. The (#1) XRDhas patterns are shown in Figure The thinnest layer polycrystalline deposited at room temperature
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(#1) has amorphous character. The others form cubic polycrystalline layers with hexagonal inclusions. The metastable cubic phase aluminum nitride be stabilized by can none-equilibrium hexagonal inclusions. Theofmetastable cubic phasecan of aluminum nitride be stabilized by synthesis noneCoatings 2018, 8, x FOR PEER REVIEW 6 of 19 conditions [36,37],synthesis structural replication [38], and high stresses [38], (pressures) equilibrium conditions [36,37], structural replication and high[39]. stresses (pressures) [39]. hexagonal inclusions. The metastable cubic phase of aluminum nitride can be stabilized by noneequilibrium synthesis conditions [36,37], structural replication [38], and high stresses (pressures) [39].
Figure 3. X-ray diffraction (XRD) patterns of layers deposited in pure argon atmosphere on low
Figure 3. X-ray diffraction (XRD) patterns of layers deposited in pure argon atmosphere on low temperature co-fired ceramic (LTCC). temperature co-fired ceramic (LTCC). Figure diffraction (XRD) patternsstructures. of layers deposited in show pure argon atmosphere on deposited low Figure3.4X-ray depicts typical morphologic Figure 4a,b amorphous layers, temperature co-fired ceramic (LTCC). Figure 4 depicts typical morphologic structures. Figure 4a,b show amorphous layers, deposited
on silicon and ceramic. The thickness of the layer on ceramic is larger than that on silicon. On both
substrates the layers densely with vermicular structures onthan top, that originated fromOn theboth on silicon and ceramic. Theappear thickness of the layer on ceramic is larger on silicon. Figure 4 depicts typical morphologic structures. Figure 4a,b show amorphous layers, deposited aluminum excess during deposition. These features are of sub-stoichiometric aluminum rich substrates the layers appearThe densely withofvermicular top,than originated from the on silicon and ceramic. thickness the layer onstructures ceramic is on larger that on silicon. Onaluminum both composition [40]. TheirThese size differs andare theyofare larger when layersaluminum are deposited oncomposition LTCC. Figure [40]. excess during deposition. features sub-stoichiometric rich substrates the layers appear densely with vermicular structures on top, originated from the 4c shows a pronounced example on ceramic. Comparing the layers in Figure 4d,e deposited in #9, the Theiraluminum size differsexcess and they are larger when layers deposited LTCC. Figure 4c shows a pronounced during deposition. These are features are ofonsub-stoichiometric aluminum rich structure roots on ceramic start at a thickness of 3 µ m approximately, while on silicon the initial dense composition [40].Comparing Their size differs and they are larger layersinare on LTCC. example on ceramic. the layers in Figure 4d,ewhen deposited #9,deposited the structure roots Figure on ceramic layer is significantly thinner. Reference [40] confirms the relation between evident aluminum excess 4c ashows a pronounced example on ceramic. Comparing the indense Figurelayer 4d,e deposited in #9, the start at ofof 3 µm approximately, while on silicon thelayers initial is significantly thinner. andthickness occurrence vermicular structures. Obtained layer are characterized by a high light absorption structure roots on ceramic start at abetween thicknessevident of 3 µ m approximately, whileand on silicon the initial dense Reference [40] confirms the relation aluminum excess occurrence of vermicular in the UV region. The relation between AlN stoichiometry and optical properties is studied in. An layer isObtained significantly thinner. Reference [40]by confirms the relation between excess structures. layer are characterized a highindex light absorption inevident the UValuminum region. The relation aluminum excess significantly affects refractive and extinction coefficient. Accordingly, and occurrence of vermicular structures. Obtained layer are characterized by a high light absorption between AlN stoichiometry and optical properties isin studied An aluminum aluminum rich AlN can be used as absorbance layer optical in. applications [2,41]. excess significantly in the UV region. The relation between AlN stoichiometry and optical properties is studied in. An affects refractive index and extinction coefficient. Accordingly, aluminum rich AlN can be used as aluminum excess significantly affects refractive index and extinction coefficient. Accordingly, absorbance layer inAlN optical applications [2,41]. layer in optical applications [2,41]. aluminum rich can be used as absorbance
Figure 4. Scanning electron microscope (SEM) micrographs of sputtered aluminum nitride (AlN) layer cross sections (N2:Ar = 0); (a) #1 on silicon; (b) #1 on ceramic; (c) #5 on ceramic; (d) #9 on silicon and (e) #9 on ceramic. Figure 4. Scanning electron microscope (SEM) micrographs of sputtered aluminum nitride (AlN) Figure 4. Scanning electron microscope (SEM) micrographs of sputtered aluminum nitride (AlN) layer layer cross sections (N2:Ar = 0); (a) #1 on silicon; (b) #1 on ceramic; (c) #5 on ceramic; (d) #9 on silicon crossand sections (N :Ar = 0); (a) #1 on silicon; (b) #1 on ceramic; (c) #5 on ceramic; (d) #9 on silicon and (e) #9 on 2ceramic.
(e) #9 on ceramic.
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3.3.2. 3.3.2. Nitrogen Nitrogen to to Argon Argon Ratio Ratio 5% 5% Consulting assumed that that at at this this deposition deposition atmosphere atmosphere the the nitrogen nitrogen saturation saturation Consulting Figure Figure 22 it it can can be be assumed point necessary for stoichiometric composition is reached. It is known from the literature point necessary for stoichiometric composition is reached. It is known from the literature that that small small changes in the atmosphere affect deposition rate, layer structure, and morphology [35]. The deposition changes in the atmosphere affect deposition rate, layer structure, and morphology [35]. The rate values rate in Table A1 in have an B1 unsteady deposition values Table have ancharacteristic. unsteady characteristic. Layer crystallinity is studied on the basis of #2 #2 and and #7, #7, because because #6 #6produced produced an anamorphous amorphouslayer. layer. Layer crystallinity is studied on the basis of Figure Figure 55 depicts depicts SEM SEM micrographs micrographs of of the the polycrystalline polycrystalline layers. layers. They They demonstrate demonstrate the the characteristic characteristic appearance of layers formed in the transition zone with densely packed fibrous grains, described by by appearance of layers formed in the transition zone with densely packed fibrous grains, described Thornton Modell [42]. Figure 5a,b compare samples deposited under the same conditions on silicon Thornton Modell [42]. Figure 5a,b compare samples deposited under the same conditions on silicon and ceramic. Both Bothshow showaafine-grained fine-grainedcolumn column structure, but columns silicon substrate and ceramic. structure, but thethe columns on on thethe silicon substrate are are inclined and the layer has a significantly reduced thickness. The same inclination is observed inclined and the layer has a significantly reduced thickness. The same inclination is observed in in Figure 5c. This layer is deposited on ceramics at a significantly higher substrate temperature and Figure 5c. This layer is deposited on ceramics at a significantly higher substrate temperature and atata alower lowerdeposition depositionrate. rate.The Thetop topview viewininFigure Figure5d 5dreveals revealsthat thatthe thegrains grains on on top top of of the the layer layer have have triangular shape and the size is in the range of 200 nm. triangular shape and the size is in the range of 200 nm.
Figure 5. SEM micrographs, samples sputtered with N2/Ar ratio of 5%: (a) #2 on silicon; (b) #2 on Figure 5. SEM micrographs, samples sputtered with N2 /Ar ratio of 5%: (a) #2 on silicon; (b) #2 on ceramic; (c) #7 on ceramic; (d) #7 top view on AlN (substrate ceramic). ceramic; (c) #7 on ceramic; (d) #7 top view on AlN (substrate ceramic).
The XRD patterns revealed several peaks. The peak intensity is depicted in the form of a bar XRDcalculated patterns revealed several peaks. The peakclarity intensity is depicted in the formFigure of a bar chartThe of the RTC values to provide a better of the texture formation. 6 chart of the calculated RTC values to provide a better clarity of the texture formation. Figure 6 compares experiment #2 and #7. The RTC distribution of the layer deposited in #2 on silicon, Figure compares experiment #2 and #7. The RTC Figure distribution of the layer deposited in #2 on Figure 6a 6a resembles this deposited on ceramic, 6b. With decreasing deposition ratesilicon, and increasing resembles this deposited on ceramic, Figure 6b. With decreasing deposition rate and increasing heater heater temperature #7, the deposition on silicon (Figure 6c) evolve (002) texture while those on temperature #7, the deposition on silicon (Figure 6c) evolve (002) texture while those on ceramic ceramic (Figure 6d) are not significantly influenced by the process parameters. (Figure 6d) are not significantly by the Despite the different heaterinfluenced temperature, RFprocess power,parameters. deposition rate, and layer thickness in #2 Despite the different heater temperature, RF power, deposition rate,(Figure and layer thickness in #2 and and #7, similar crystal morphology arises on the different substrates 5a,c). Distribution of #7, similar crystal morphology arises on the different substrates (Figure 5a,c). Distribution of RTC RTC (Figure 6a,d) and mean size of ordered domains (Table B1) are comparable as well, indicating (Figure 6a,d) and mean size of ordered domainssurface (Table are A1)similar. are comparable as well, indicating that that thermodynamic conditions at the substrate thermodynamic conditions at the substrate surface are similar.
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Figure 6. Comparison of RTC distribution for layers sputtered with N2 /Ar ratio of 5% on ceramic and silicon: (a) #2 (silicon); (b) #2 (ceramic); (c) #7 (silicon); (d) #7 (ceramic). The blue line indicates the RTC value that would represent a polycrystalline material of the respective measurement.
3.3.3. Nitrogen to Argon Ratio 10% The N2 /Ar ratio of 10% provides the best deposition conditions for (002) textured layers in the investigated process range for both substrates. The deposition rate correlates with RF power. Figure 7 depicts the XRD patterns of layers deposited on ceramic. It reveals that the most pronounced (002) texture is achieved in #4 at a heater temperature of 150 ◦ C and 400 W. Figure 8 depicts cross sections of layers deposited on ceramic, the preparation of silicon samples failed because of the poor adherence. On all micrographs, an initial smooth layer at the substrate interface merges with a columnar structure. The thickness of this smooth transition layer varies. In Figure 8a (#3) the transition point starts at a thickness of 2 µm and in Figure 8b (#4) below 1 µm. In accordance with the RTC(002) values in Table A1, the columnar structure can be related to (002) texture. Figure 8c and d compare #8 and #10: The layer was deposited under same the condition, except the process time. The transition in Figure 8c starts at the 400 nm layer thickness, while in Figure 8d this zone is shifted to a thickness of almost 4 µm. Recrystallization can be accountable for this effect, supported by the fact that the mean size of the ordered domains increases from 14 nm to 24 nm. Along with this, the deposition rate rises. However, the desired increase of RTC(002) was not received. A more detailed experimental setup considering factor interactions will be carried out at this operation point in further works. 1
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Figure XRD plots of layers deposited on ceramic at an /Ar Figure7.7. 7.XRD XRDplots plotsof oflayers layersdeposited depositedon onceramic ceramicat atan anNN N222/Ar /Ar ratio Figure ratio of of 10%. 10%.
Figure 8. micrographs of layers deposited with N ratio of on (a) (b) #4; (c) #8; Figure 8. 8. SEM SEM with N22/Ar /Ar/Ar ratioratio of 10% 10% on LTCC: LTCC: (a) #3; #3;(a) (b)#3; #4;(b) (c) #4; #8; Figure SEMmicrographs micrographsofoflayers layersdeposited deposited with N of 10% on LTCC: 2 (d) #10. (d)#8; #10. (c) (d) #10.
3.4. Substrate Influence 3.4. Substrate Influence 3.4.1. Nucleation Nucleation 3.4.1. Experiment #6 a view onon thethe starting grain growth on ceramic and and silicon. The ◦ C)allows Experiment #6 (t (tHHH150 150°C) allows a view starting grain growth on ceramic silicon. XRD pattern indicates an amorphous layer. On SEM images and corresponding AFM maps (Figure The XRD pattern indicates an amorphous layer. On SEM images and corresponding AFM maps 9), commencing grain formation is visible.isNuclei size and distribution differ significantly depending (Figure 9), commencing grain formation visible. Nuclei size and distribution differ significantly on the substrate A dense A distribution of fine seeds visible on ceramic (Figure 9a). 9a). On depending on the material. substrate material. dense distribution of fineisseeds is visible on ceramic (Figure silicon, the nuclei are larger and their distribution is sparse (Figure 9b). Experiment #1 (t H 20 °C) gives ◦ On silicon, the nuclei are larger and their distribution is sparse (Figure 9b). Experiment H#1 (tH 20 C) another example for the nucleus formation. Crystallinity is stillisnot with with XRD gives another example forbeginning the beginning nucleus formation. Crystallinity stilldetectable not detectable diffraction. Figure 10 depicts a top view on the layer surfaces; the difference between the seed XRD diffraction. Figure 10 depicts a top view on the layer surfaces; the difference between the seed structure is evident. While on silicon smaller nuclei are distributed with high density, on ceramic structure is evident. While on silicon smaller nuclei are distributed with high density, on ceramic they theylarger are larger and less dense. grow to vermicular structures (compare Figure 4c). are and less dense. These These nuclei nuclei grow later tolater vermicular structures (compare Figure 4c). Despite Despite of surface influence of the substrate, the thermal conditions undoubtedly play an important of surface influence of the substrate, the thermal conditions undoubtedly play an important role. role.
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Figure 9. SEM image and AFM-Plot of the sample surface in experiment #6: (a) silicon; (b) ceramic. Figure 9. SEM image and AFM-Plot of the sample surface in experiment #6: (a) silicon; (b) ceramic. Figure 9. SEM image and AFM-Plot of the sample surface in experiment #6: (a) silicon; (b) ceramic.
Figure 10. SEM micrographs of initial nuclei in #1: (a) silicon; (b) ceramic. Figure 10. SEM micrographs of initial nuclei in #1: (a) silicon; (b) ceramic. Figure 10. SEM micrographs of initial nuclei in #1: (a) silicon; (b) ceramic.
3.4.2. Consideration of the Thermal Conditions 3.4.2. Consideration of the Thermal Conditions 3.4.2.Thermal Consideration of the Conditions conditions onThermal top of both substrates can be judged based on a rough calculation, using Thermalphysical conditions on topofofheat bothtransport substrates canstorage. be judged based on of a rough using well-known relations and Since a part the RFcalculation, power produces Thermal conditions on top of both substrates can be judged based on a rough calculation, using well-known physical relations of heat transport and storage. Since a part of the RF power produces heat at the substrate surface, the substrate temperature rises during the process. The total amount is well-known physical relations of heat transport and storage. Since a part of the RF power produces heat at the substrate surface, the substrate risesto during the process. The total amount is not known, but the heat transport from the temperature substrate surface the substrate holder can be estimated. heat at the substrate surface, the substrate temperature rises during the process. The total amount is not but the heat transport from thebesubstrate to the holder can beshows, estimated. Theknown, rough calculation and the results can found insurface Appendix C. substrate This rough estimation that not known, but the heat transport from the substrate surface to the substrate holder can be estimated. The rough calculation and the results can be found in Appendix C. This rough estimation shows, that the achievement of stable deposition conditions on ceramic substrates need a considerable longer The rough calculation and the results can be found in Appendix C. This rough estimation shows, the stable Furthermore, deposition conditions on ceramic substrates need a considerable longer timeachievement than those onofsilicon. the final temperature on LTCC ceramic surfaces is certainly that the achievement of stable deposition conditions on ceramic substrates need a considerable longer time than those on silicon. Furthermore, the final temperature on LTCC ceramic surfaces is certainly higher than that at the silicon surface as a consequence of the higher thermal resistance. This leads to time than those on silicon. Furthermore, the final temperature on LTCC ceramic surfaces is certainly higher than that at the silicon surface as a consequence of the higher This leads to an apparent gradient of layer properties, which manifests in thethermal grain resistance. structure change with higher than that at the silicon surface as a consequence of the higher thermal resistance. This leads to an apparent gradient of layer properties, manifests in the grain structure change increasing film thickness, see Figure 8a–d.which Experiment #8 (Figure 8c) and #10 (Figure 8d)with are an apparent gradient of layer properties, which manifests in the grain structure change with increasing increasing film thickness, see Figure 8a–d. Experiment #8 (Figure 8c) and #10 (Figure 8d) are deposited under the same conditions varying the process time. The beginning of the columnar film thickness, see Figure 8a–d. Experiment #8 (Figure 8c) and #10 (Figure 8d) are deposited under the deposited under the sametowards conditions varyinglayers the process time. The beginning ofsize the of columnar structures is visibly shifted the thicker in experiment #10 and the main ordered same conditions varying the process time. The beginning of the columnar structures is visibly shifted structures visibly shifted in experiment and the main size ofsubstrate ordered domains dis increases fromtowards 14 nm the (#8)thicker to 24 layers nm (#10). A further#10 indication of higher towards the thicker layers in experiment #10 and the main size of ordered domains d increases from domains d increases 14 nm to 24 nm (#10). further observed indicationinofexperiment higher substrate temperature on LTCCfrom ceramic is (#8) the pronounced (002)A texture #4 in 14 nm (#8) to 24 nm (#10). A further indication of higher substrate temperature on LTCC ceramic is the temperature LTCC ceramic is the pronounced (002) texture observed in experiment #4 in comparison toon silicon. pronounced (002) texture observed in experiment #4 in comparison to silicon. comparison to silicon. 3.4.3. 3.4.3. Layer Layer Stress Stress 3.4.3. Layer Stress Visual inspection and and SEM SEM image image analysis analysis were were the the base base of of the the adhesion’s adhesion’s assessment. assessment. Peeled Visual inspection Peeled Visual inspection and SEM image analysis were the base of the adhesion’s assessment. Peeled off layers were assessed with “−“, layers with cracks and chipping with “−“, well adherent layers with off layers were assessed with “−“, layers with cracks and chipping with “−“, well adherent layers off layers were assessed with “−“, layers with without cracks and chipping with “−“, well adherent layers with cracks with “+” and excellent adhered layers cracks with “++”. The results are listed in Table with cracks with “+” and excellent adhered layers without cracks with “++”. The results are listed in cracks with “+”B.and adhered without cracksin with The results arewere listeddetected, in Table B1, Appendix Theexcellent layer onlayers ceramic is excellent, the “++”. sense thatsense no cracks Table A1, Appendix B. Theadhesion layer adhesion on ceramic is excellent, in the that no cracks were B1, Appendix B. The layer adhesion on ceramic is excellent, in the sense that no cracks were detected, there were no areas in which the top layer was peeled off the ceramic, and SEM indicated a there were no interface areas in without which the topInlayer was peeled off the is ceramic, indicated a substrate/layer voids. comparison, the adhesion partiallyand poorSEM on silicon. Here, substrate/layer interface without voids. In comparison, the adhesion is partially poor on silicon. Here,
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detected, there were no areas in which the top layer was peeled off the ceramic, and SEM indicated a substrate/layer interface without voids. In comparison, the adhesion is partially poor on silicon. Here, only the thin layer adheres well, while the thicker layer possess cracks at the interface between the substrate and layer, or chipping. Particularly those layers deposited under high nitrogen content showed poor adhesion. In the case of experiment #10, the layer is fully peeled off from the silicon substrate, Coatingsindicating 2018, 8, x FORhigh PEERstress. REVIEW 11 of 19 The layer stress can be estimated from the deviation between measured crystal parameters only the thinXRD layerpattern) adheres well, the thicker layerThe possess cracks at the interface between the (extracted from and while theoretical values. lattice expansion εx , εy in the respective substrate and layer, or chipping. Particularly those layers deposited under high nitrogen content direction in plane was calculated using Equation (3): showed poor adhesion. In the case of experiment #10, the layer is fully peeled off from the silicon substrate, indicating high stress. a − a0 (3) ε x = εy = The layer stress can be estimated from the deviation between measured crystal parameters a0 (extracted from XRD pattern) and theoretical values. The lattice expansion ε x, εy in the respective Here,direction a0 is theintheoretical a non-deformed plane was constant calculatedof using Equation (3): layer and a constant of the real layer, extracted
from the XRD pattern. The value for a0 is 3.112 Å [43]. 𝑎 −The 𝑎0 expansion εz perpendicular to the plane ε𝑥 = ε𝑦 = (3) was calculated using Equation (4): 𝑎0 2C13 a − a0 ×layer and a constant of the real layer, extracted (4) Here, a0 is the theoretical constant of a εnon-deformed z =− C33 a0 from the XRD pattern. The value for a0 is 3.112 Å [43]. The expansion εz perpendicular to the plane
Here,was cij stands for using the elastic constants in the respective direction relating to substrate surface, where calculated Equation (4): i, j = 1,2 or 3, for x, y or z directions, respectively. respective tensions σ were calculated using 2𝐶13The 𝑎− 𝑎0 ε = − × (4) 𝑧 Equation (5) for tension in plane: 𝐶33 𝑎0 " constants in the respective direction relating # Here, cij stands for the elastic 2 + C2 C − C C2 to substrate surface, where C2 C33 − C12 C13 12 13 13 11 i, j = 1,2 or 3, for x,σxx y or The respective tensions σ were =z directions, C11 + 12 respectively. × εxx calculated using 2 −C C C 33 11 Equation (5) for tension in plane: 13 2
2
2
2
𝐶12 𝐶33 to −𝐶 12 𝐶plane: 13 + 𝐶13 𝐶11 − 𝐶12 𝐶13 and Equation (6) for tensions the σ𝑥𝑥perpendicular = [𝐶11 + ] × ε𝑥𝑥 2 𝐶13
− 𝐶11 𝐶33
"
(5)
#
2 C33the−plane: 2C13 and Equation (6) for tensions perpendicular to
σzz =
(5)
C𝐶11 − + 2𝐶 C122
σ𝑧𝑧 = [
33
13
𝐶11 + 𝐶12
× εzz
] × ε𝑧𝑧
(6) (6)
The variation of the stress values was estimated on the basis of three independent measurements The variation of the stress values was estimated on the basis of three independent measurements on equal layers and amounts to 3%. Amorphous layers or such with only weak orientation cannot on equal layers and amounts to 3%. Amorphous layers or such with only weak orientation cannot be be evaluated. This concerns experiment #1, #6, and #8. Table A1 summarizes all other values and evaluated. This concerns experiment #1, #6, and #8. Table B1 summarizes all other values and Figure Figure 11 compares them graphically. 11 compares them graphically.
Figure 11. Layer stress depending on substrate material, ordered by gas atmosphere and heater
Figure 11. Layer stress depending on substrate material, ordered by gas atmosphere and temperature. heater temperature. The resulting stress superposes internal stress generated by layer growth (intrinsic) and thermal stress generated by thermal mismatch between substrate and layer (extrinsic). Lattice mismatch, grain boundary effects, and lattice defects cause intrinsic stress. In the current experiment, no significant influence of RF power and thickness on layer stress is apparent. A clear difference between the resulting stress of layers deposited on ceramic and silicon
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The resulting stress superposes internal stress generated by layer growth (intrinsic) and thermal stress generated by thermal mismatch between substrate and layer (extrinsic). Lattice mismatch, grain boundary effects, and lattice defects cause intrinsic stress. In the current experiment, no significant influence of RF power and thickness on layer stress is apparent. A clear difference between the resulting stress of layers deposited on ceramic and silicon is evident and due to the reduction of the extrinsic stress component for the LTCC substrate having a lower thermal mismatch to AlN. As pointed out in Figure 11, the stress σxx in plane12 for layers Coatings 2018, 8, x FOR PEER REVIEW of 19 deposited on ceramic substrates mainly depends on nitrogen content. The heater temperature has is evident and dueLayers to the reduction of the extrinsic stresscontent component for the LTCCabsolute substrate stress having lower no significant influence. deposited with nitrogen of 10% show a lower thermal mismatch to AlN. As pointed out in Figure 11, the stress σ xx in plane for layers than 60 MPa (both, tensile and compressive). The stress perpendicular to the substrate plane σzz has deposited on ceramic substrates mainly depends on nitrogen content. The heater temperature has no compressive character at low nitrogene content and turns over to tensile character at higher nitrogen significant influence. Layers deposited with nitrogen content of 10% show absolute stress lower than content. Independently from heater temperature, reaches values around MPa tensile strain when 60 MPa (both, tensile and compressive). Theitstress perpendicular to the700 substrate plane σ zz has the N2 /Ar gas flow ratio is 10%. compressive character at low nitrogene content and turns over to tensile character at higher nitrogen content. Independently from heater temperature, itdifferent reaches values around 700 MPa strain Layers deposited on silicon evince a significantly behavior. Besides thetensile nitrogen content N2/Arcomponents, gas flow ratio isσ10%. influence,when boththe stress in plane and σ perpendicular, depend strongly on heater xx zz Layers deposited on silicon evince a significantly different behavior. Besides the nitrogen temperature as well. Increasing heater temperature leads to a shift of the in plane stress σxx towards a content influence, both stress components, σxx in plane and σzz perpendicular, depend strongly on compressive character. It turns to tensile for layers deposited at high nitrogen content and reaches a heater temperature as well. Increasing heater temperature leads to a shift of the in plane stress σ xx value of 810 MPa stress. The perpendicular stress components have compressive character towards atensile compressive character. It turns to tensile for layers deposited at high nitrogen content and and their variation the of heater temperature is low. The comparison the process means in Figure 12 reacheswith a value 810 MPa tensile stress. The perpendicular stressof components have compressive character theirvalues variation with the from heater signed temperature is low. The comparison the process reveals that meanand of real (obtained values) as well as mean ofofabsolute value is means in Figure 12 reveals that mean of real values (obtained from signed values) as well as mean of and lower when the layers are deposited on LTCC substrates. This pertains to stress in the basal plane absolute value is lower when the layers are deposited on LTCC substrates. This pertains to stress in the perpendicular as well, independent from the process conditions. the basal plane and the perpendicular as well, independent from the process conditions.
Figure 12. Comparison of the process means based on real and absolute stress value on silicon and
Figure 12.ceramic Comparison of the process means based on real and absolute stress value on silicon and substrate. ceramic substrate. 4. Conclusions
4. Conclusions The deposition of aluminum nitride on silicon and LTCC ceramic was investigated over a wide parameter range using RF magnetron sputtering. The N2/Ar ratio has the most significant influence
The deposition of aluminum nitride on silicon and LTCC ceramic was investigated over a wide on all investigated characteristics, due to a variation of the layer composition. The nitrogen content parameterofrange using RF magnetron sputtering. Theratio N2 /Ar most significant influence on the layers increases strongly with rising N2/Ar fromratio 0% tohas 5%.the At 5% N2/Ar ratio, saturation all investigated dueunder to a variation of the with layer 10% composition. Theshow nitrogen content of the occurs. characteristics, Layers deposited an atmosphere N2/Ar ratio stoichiometric composition. layers increases strongly with rising N2 /Ar ratio from 0% to 5%. At 5% N2 /Ar ratio, saturation occurs. The of an process parameter change, heatershow temperature, RF power, N 2/Ar ratio Layers deposited effect under atmosphere with 10%including N2 /Ar ratio stoichiometric composition. and targeted thickness, on layer morphology depends strongly on the substrate material. Different The effect of process parameter change, including heater temperature, RF power, N2 /Ar ratio and thermal properties leading to dissimilar surface temperatures and heat-up performance are an targeted thickness, on layer morphology depends strongly on the substrate material. Different thermal essential reason for this. Stable deposition conditions on LTCC ceramic need considerable longer propertiestime. leading to dissimilar surface temperatures and heat-upprocess, performance aretemperature an essentialonreason Since a high heat amount emerges during the sputtering the surface for this. Stable deposition conditions on than LTCC ceramic need considerable longer time. Since the LTCC ceramic substrate is higher that on silicon because of a two order of magnitude highera high thermal resistance. Due to these circumstances, the nucleation differs significantly in dependence heat amount emerges during the sputtering process, the surface temperature on the LTCC on ceramic substrate material layer formation Thus, the process conditions substrate the is higher than that and on silicon becausecontinues of a twodifferently. order of magnitude higher thermalforresistance. c-axis oriented AlN layer deposition vary significantly dependent on the substrate. Best (002) texture is achieved on ceramic substrates at a heater temperature of 150°C and RF power of 300 W.
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Due to these circumstances, the nucleation differs significantly in dependence on the substrate material and layer formation continues differently. Thus, the process conditions for c-axis oriented AlN layer Coatings 2018, 8, x FOR PEER REVIEW 19 deposition vary significantly dependent on the substrate. Best (002) texture is achieved 13 onofceramic ◦ substratesThe at ause heater temperature of 150 C and RF power of 300 W. stress reduction. Since stress of CTE adapted LTCC substrates leads to a significant The use of CTE adapted LTCC substrates leads to a significant stressproduced reduction. Since stress caused by CTE mismatch is minimized, resulting components are mainly by internal caused by CTE is minimized, are mainly produced by(tensile internal factors, factors, suchmismatch as lattice defects or grainresulting boundarycomponents influences. Very low stress of 60 MPa and such compressive as lattice defects or grain boundary influences. Very low stress of 60 MPa (tensilewith andadapted compressive as well) is achieved in the basal plane. The use of ceramic substrates coefficient of thermal expansion enables thus targeted processsubstrates development, producing layers with as well) is achieved in the basal plane. The ause of ceramic with adapted coefficient of application tailored properties whose characteristics are not restricted by stress compensating needs thermal expansion enables thus a targeted process development, producing layers with application affecting functional properties. tailored properties whose characteristics are not restricted by stress compensating needs affecting functional properties. Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Figure S1: XRD patterns of all samples, sputtered on ceramic, Figure S2: XRD patterns of all samples, sputtered on silicon, Figure
Supplementary Materials: The following are available online at http://www.mdpi.com/2079-6412/8/8/289/s1, S3: RTC calculations for all experiments, samples prepared on ceramics, Figure S4: RTC calculations for all Figure S1: XRD patterns of all samples, sputtered on ceramic, Figure S2: XRD patterns of all samples, sputtered on experiments, samples prepared on silicon. silicon, Figure S3: RTC calculations for all experiments, samples prepared on ceramics, Figure S4: RTC calculations for all experiments, samples prepared on silicon. Author Contributions: Conceptualization, Heike Bartsch and Jörg Pezoldt; Methodology, Rolf Grieseler (XRD),
Author Contributions: and J.P.; Methodology, R.G. (XRD), H.B. (DoE), (SEM, AFM); Heike Bartsch (DoE),Conceptualization, Jose Mánuel (SEM,H.B. AFM); Resources, Jens Müller; Writing—Original DraftJö.M. Preparation, Resources, Writing—Original Preparation, H.B.;Jose Writing—Review & Editing, H.B., Jö.M., J.P., R.G.; Heike Je.M.; Bartsch; Writing—Review &Draft Editing, Heike Bartsch, Mánuel, Jörg Pezoldt, Rolf Grieseler; Project Project Administration, H.B.; Funding Acquisition, Je.M. Administration, Heike Bartsch; Funding Acquisition, Jens Müller. Funding: ThisThis research was funded von Humboldt HumboldtFoundation, Foundation, through the “Humboldt Funding: research was fundedbybythe theAlexander Alexander von through the “Humboldt Research Fellowship for Postdoctoral Researchers Program” (Ref. 3.3-1 1S8421-ES-HFST-P) and from the German Research Fellowship for Postdoctoral Researchers Program” (Ref. 3.3-1 1S8421-ES-HFST-P) and from the Federal Ministry for Economic Affairs (BMWi) within the project “BiSWind, Integrated Sensor Technolgies for German Federal Ministry for Economic Affairs (BMWi) within the project “BiSWind, Integrated Sensor Wind Turbines” (Ref. 0325891B). Technolgies for Wind Turbines” (Ref. 0325891B).
Acknowledgments: The authors would like to thank Ulrike Brokmann for her support at AFM and Peter Acknowledgments: The authors would like to thank Ulrike Brokmann for her support at AFM and Peter Schaaf, Schaaf, who kindly provided the access to the analytical equipment. We acknowledge support for the Article who kindly provided access Research to the analytical equipment. for theFund Article Processing Charge by thethe German Foundation and We theacknowledge Open Accesssupport Publication ofProcessing the Technische ChargeIlmenau. by the German Research Foundation and the Open Access Publication Fund of the Technische Universität Universität Ilmenau.
Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest.
Appendix A
Appendix A
The ceramic substrates are lapped with cubic B4 C slurry for 20 min and polished for 90 min using The ceramic substrates are lapped with cubic B4C slurry for 20 min and polished for 90 min using diamond slurry with 1 µm Thefinal final substrate thickness was 0.9The mm. The roughness of diamond slurry with 1 µ mgrain grainsize. size. The substrate thickness was 0.9 mm. roughness of the the polished usinglaser laserscanning scanning microscopy (LSM) atomic microscopy polished areas areas is is measured measured using microscopy (LSM) andand atomic forceforce microscopy (AFM). Figure A1aA1a shows a LSM image A1ban anAFM AFMscan. scan. (AFM). Figure shows a LSM imageand andFigure Figure A1b
Figure Surface topographyofofa apolished polished LTCC LTCC ceramic captured by (a) scanning Figure A1. A1. Surface topography ceramicsubstrate, substrate, captured bylaser (a) laser scanning microscopy (LSM); (b) atomic force microscopy (AFM) beside pores. microscopy (LSM); (b) atomic force microscopy (AFM) beside pores.
The LSM (OLS 4100, Olympus, Hamburg, Germany) has a spot size of 200 nm. Surface scans The LSM (OLS 4100,130 Olympus, a spotThe sizeroughness of 200 nm. Surface were carried out over µ m × 130 Hamburg, µ m (field ofGermany) view) on 9 has substrates. of the wholescans weresubstrate carried out 130 µm × was 130 obtained µm (fieldfrom of view) on 9 substrates. The roughness of athe whole areaover including pores the primary data after applying a filter with cutoff length 8 µ m. Root mean square height of the Sq isdata evaluated using theadevice software substrate area of including pores was obtained from thesurface primary after applying filter with a cut-off OLS4100 (Version). The results Sq (LSM) areofdepicted in Figure Three of these LSM length of 8 µm. Root mean square height the surface Sq isA2. evaluated using thesurface devicescans software were chosen and 6-line scans along closed polished areas were evaluated to obtain root mean squarescans OLS4100 (Version). The results Sq (LSM) are depicted in Figure A2. Three of these LSM surface roughness Rrms of non-porous regions as illustrated in Figure A1a. Figure A2 presents the obtained
were chosen and 6-line scans along closed polished areas were evaluated to obtain root mean square
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roughness Rrms of non-porous regions as illustrated in Figure A1a. Figure A2 presents the obtained 2018, 8, x FOR PEER REVIEW 14 of 19 values RCoatings The cut-off length for this evaluation amounts to 2.5 µm. As a third parameter, the q (LSM). portion of pores, was calculated on the base of the LSM surface scans. The threshold was defined values Rq (LSM). The cut-off length for this evaluation amounts to 2.5 µ m. As a third parameter, the at a level of 300 nm underneath the maximum values of the topography. All regions lower than the portion of pores, was calculated on the base of the LSM surface scans. The threshold was defined at threshold wereofinterpreted as pore. obtained porosity 23% and the standard deviation a level 300 nm underneath theThe maximum values of the is topography. All regions lower than the is 5%. Additionally, AFM scans (VECCO measurements, Instruments Inc., Plainview, threshold were interpreted as pore. AFM The obtained porosity isVeeco 23% and the standard deviation is 5%.NY, USA) AFM scans measurements, Veeco Instruments Inc., Plainview, NY,without were carriedAdditionally, out on regions with an(VECCO area of AFM 1.4 µm × 1.4 µm. As indicated in Figure A1a, regions USA) were carried out on regions with an area of 1.4 µ m × 1.4 µ m. As indicated in Figure A1a, regions pores were selected. The obtained Sq values base on seven measurements, Figure A2 depicts the values in without pores were selected. The obtained Sq values base on seven measurements, Figure A2 depicts under category Sq (AFM). the values in under category Sq (AFM).
Figure Surface parameter obtained LSM andand AFM. Figure A2.A2. Surface parameter obtainedfrom from LSM AFM.
Appendix B
Appendix B
Table B1. Summary of experimental design and results. Point Measured results RTC(002) Table A1. Set Summary of experimental design and results. Heater Targeted Nitrogen RF Power Rate Thickness Temperature Thickness Gas Flow Si 9k7 RTC(002) [W] Set Point [nm min−1] Measured [nm] results [°C] [µm] [sccm] Exp. No. Heater RF Power Targeted Nitrogen Gas Rate Thickness b 1 20 300 0.3 0 8 1400 n.a. n.a. Si 9k7 ◦ 1] [W] Thickness [µm] Flow [sccm] 27 [nm min− [nm] 2Temperature 20[ C] 400 1 4 3500 12 5 3 500 1.7 8 25 53 52 20 20 300 0.3 0 8 4300 1400 n.a. n.a. 1b 2 20 150 400 4 27 2400 3500 12 5 4 300 11 8 13 54 64 3 20 150 500 1.7 8 25 6000 4300 53 52 5a 400 1.7 0 31 19 22 4 150 150 300 8 13 700 2400 54 64 6b 500 0.31 4 22 n.a. n.a. 5a 150 300 400 1.7 0 31 5200 6000 19 22 7 300 1.7 4 18 50 7.5 150 300 500 0.3 4 22 670 70020 n.a. n.a. 6b 8 400 0.3 8 17 40 7 300 300 300 4 18 4500 5200 50 7.5 9a 500 11.7 0 43 18 22 8 300 300 400 0.3 8 17 7100 670 20 40 10 400 1.7 8 23 n.a. 60 a 9 300 500 1 0 43 4500 18 22 Stress [GPa] Adhesion Mean size of Ordered 10 300 400 1.7 8 23 7100 n.a. 60 Exp. Domains d Si 9k7 No. Si 9k7 Adhesion 9k7 [nm]d σxx σzz Stress [GPa] σxx σzz Mean sizeSiof[nm] Ordered Domains Exp. No. 1 b Si n.a. 9k7 n.a. n.a. n.a. n.a. n.a. + ++ Si 9k7 (110) 2 19(200) −0.34 0.41 σ 0.21 ++ Si [nm] 9k721[nm] σxx σxx −1.03 σzz− zz 3 27 24 −0.03 0.90 −0.02 −0.69 − ++ n.a. 23 n.a. n.a. n.a. −0.71 n.a.− + ++ 1b 4 23 −0.81 1.34 n.a. 0.04 ++ (200) (110) 2 −0.34 0.41 0.21 1.39 −1.03 − ++ 19 13 b 21 12 b 5a −2.40 2.46 −1.17 − ++ 3 27 24 −0.03 0.90 −0.02 −0.69 − ++ 6b n.a. n.a. n.a. n.a. n.a. n.a. ++ ++ 4 23 23(110) −0.81 1.34 0.04 −0.71 − ++ 7 26 22 0.67 0.57 0.18 −0.37 − ++ 5a −2.40 2.46 −1.17 1.39 − ++ 13 b (112) 12 b 8 30 14 n.a. n.a. n.a. n.a. ++ ++ n.a. c n.a. c n.a. n.a. n.a. n.a. ++ ++ 6b a 9 8 13 0.10 3.35 −1.08 1.94 + ++ 7 26 0.67 0.57 0.18 −0.37 − ++ 22(110) 10 n.a. 24 n.a. n.a. −0.06 −0.64 −− ++ 8 14 n.a. n.a. n.a. n.a. ++ ++ 30(112) 9a 8a c 13 c 0.10b 3.35c −1.08 1.94 + ++ Notes: (111) cubic orientation dominant; amorphous layer; RTC(111) cubic; d (002) hexagonal if not 10 n.a. 24 n.a. n.a. −0.06 −0.64 −− ++ Exp. No.
specified; − − peel off; − cracks and chipping; + well adhesion, cracks at the substrate-layer-interface; Notes: a (111) cubic orientation dominant; b amorphous layer; c RTC(111) cubic; d (002) hexagonal if not specified; ++ excellent adhesion, no cracks. − − peel off; − cracks and chipping; + well adhesion, cracks at the substrate-layer-interface; ++ excellent adhesion, no cracks.
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Appendix Appendix C C The The rough rough calculation calculation of of thermal thermal conditions conditions bases bases on on the the consideration consideration of of an an area area element element on on the the respective respective substrate. substrate. Figure Figure C1 A3illustrates illustratesthe theconditions. conditions.With Withthe thebeginning beginningsputtering sputteringprocess, process, heat heat is is generated generated on the substrate surface by ion bombardment. The The generated generated heat heat is is dissipated dissipated through substrate towards towardsthe thesubstrate substrateholder. holder.Thermal Thermal resistivity and heat capacitance th through the substrate resistivity RthRth and heat capacitance CthCof of substrate characterize heat flow. thethe substrate characterize thethe heat flow.
Figure conditions and geometric parameters determining the heatthe dissipation: Tsurface— Figure C1. A3.Thermal Thermal conditions and geometric parameters determining heat dissipation: temperature at the substrate —temperature at the substrate holder; th—substrate Tsurface —temperature at the surface; substrateTholder surface; Tholder —temperature at the substrate holder; thickness. th—substrate thickness.
R Rththisiscalculated calculatedusing usingEquation Equation(C1): (A1): 𝑡ℎ 𝑅𝑡ℎ = (C1) th 𝑘 Rth = 𝑡ℎ × 𝐴 (A1) k th × A −1 −1 The presumed thermal conductivity kth of the substrates is 150 W m K for silicon [44] and 3.3 W m−1The K−1presumed for ceramicthermal [45]. Considering thekdifferent substrate thickness andand 0.9 conductivity is 150 W th m−of1 0.5 K−1mm for(silicon) silicon [44] th of the substrates − 1 − 1 −1 mm (ceramic), th of a representative area element A of 1 mm² amounts to 3.3 th KW formm silicon and 3.3 W m K Rfor ceramic [45]. Considering the different substrate thickness of 0.5 (silicon) 2 amounts −1 for(ceramic), 273 ceramics.RCthth of of athe substrates can beelement calculated (C2): andKW 0.9 mm representative area A ofusing 1 mmEquation to 3.3 KW−1 for silicon and 273 KW−1 for ceramics. Cth of the substrates can be 𝐴 calculated using Equation (A2): (C2) 𝐶𝑡ℎ = 𝑐𝑠𝑝 𝑡ℎ A Cth = csp (A2) csp is the specific thermal capacity. It can be experimentally determined or calculated using th Equation (C3): csp is the specific thermal capacity. It can be experimentally determined or calculated using 𝑘𝑡ℎ Equation (A3): 𝑐𝑠𝑝 = (C3) kαρ (A3) csp = th −1 K−1, α represents the thermal diffusivity in m2 where kth is the specific thermal conductivity in W mαρ −1 and kρ is the specific swhere density thermal in kg m−3conductivity . in W m−1 K−1 , α represents the thermal diffusivity in m2 th − 1 − 3 Reliable data csp forinthe s and ρ is the density kgLTCC m . material are not available. Since LTCC ceramics are a composite of different glasses ceramic fillers, the specific capacity is roughly estimated 1:1 mixture Reliable data cand for the LTCC material are notheat available. Since LTCC ceramics areasaacomposite of sp of alumina as frequently usedfillers, ceramic andheat glass. Reference [46] specifies the cas sp a of1:1 glasses to be different glasses and ceramic thefiller specific capacity is roughly estimated mixture of −1 850 J kg−1as K−1 with a tolerance of 10%. Other sources specify values between J kg −1toKbe alumina frequently used ceramic filler andinternet glass. Reference [46] specifies the csp of503 glasses −1 K[47]) −1 with (lead glass, anda720–800 J kg K−1 (soda glass, [48]). Values silicon vary from 850 J kg tolerance of−110%. Otherlime internet sources specify for values between 503 J 703 kg−J1 kg K−−11 − 1 − 1 −1 −1 −1 −1 −1 −1 K [49] to 741 J kg [50]. JFor sources 990 JValues kg Kfor[51] andvary 850–1050 J kgJ kgK−−11 (lead glass, [47]) and K 720–800 kg alumina, K (soda lime specify glass, [48]). silicon from 703 [52]. The calculation of csp based on thermal diffusivity (Equation (A3)) is an alternative way to obtain data. Necessary material constants are gathered in Table C1.
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K−1 [49] to 741 J kg−1 K−1 [50]. For alumina, sources specify 990 J kg−1 K−1 [51] and 850–1050 J kg−1 K−1 [52]. The calculation of csp based on thermal diffusivity (Equation (A3)) is an alternative way to obtain data. Necessary material constants are gathered in Table A2. Table A2. Thermal material data for silicon, alumina and lead oxide. Material
kth (W/mK)
α (10−6 m2 /s)
156 [53] ~150 [44] 36.96 [58] 30.5 * [59]
88.0 [54,55] 76.9 [56] 12 [58] 13.7 [56]
Silicon Alumina
ρ (103 kg/m3 ) 2.329002 [57] 3.48 * [59]
Notes: * sintered alumina at 881.7 ◦ C (k values change also with this T), open pores 10.65%, closed pores 2.35%
The calculated heat capacity based on these data for silicon and alumina results in the data range presented in Table A3. Table A3. Data calculated using Equation (A3) for silicon and alumina. Material
min
csp [J kg−1 ] average
max
Silicon
761
816
871
Alumina
640
710
781
Considering calculated values for csp and those found in the literature, a minimum-maximum assessment for Cth was carried out. These valued and resulting time constant τ and period 5τ considering the above calculated thermal resistance are presented in Table A4, whereby the time constant is calculated with Equation (A4): τ = Rth × Cth
(A4)
Table A4. Calculated values for Cth using Equation (A2), based on different csp values. Specific Heat Capacity Csp [J kg−1 K−1 ]
Cth
τ [s]
5τ [s]
5τ [min]
Si 871 703
– 1.017 0.821
– 3.4 2.7
– 17 13.7
– 0.3 0.2
2.497 1.594
681 435
3405 2174
57 36
Alumina
glass
ceramic *
990 640
800 503
895 571
Notes: * value of the composite ceramic results from the mean of Al2 O3 and glass.
Despite of huge deviations due to the uncertainty of material data, it is evident that the time constant τ of the ceramic substrate is significantly higher than that of silicon. The thermal resistance, which amounts to 3.3 KW−1 for silicon and 273 KW−1 for ceramic, has the strongest influence. As a consequence, the silicon substrate reaches the steady state in a few seconds and the heat rapidly dissipated. The ceramic substrate accumulates the heat; it can take up to an hour until the steady state is reached.
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