Aluminum scandium nitride (Al1−xScxN with x = 0–0.41) thin films were deposited by reactive pulsed-DC magnetron sputtering on Si(001) and Al2O3(0001) substrates. X-ray diffraction indicated high degree of c-axis orientation in all the films, and based on pole figure measurements, epitaxial relationship could be defined as [101¯0]AlScN//[112¯0]sapphire and (0001)AlScN//(0001)sapphire in films deposited on Al2O3. Piezoelectric coefficient increased up to d33 = 31.6 pC/N in Al0.59Sc0.41N, which is 550% higher than for AlN. The biaxial elastic modulus and the in-plane coefficient of thermal expansion (CTE) as a function of Sc concentration were determined by thermal cycling method: biaxial elastic modulus decreased from 535 GPa in pure AlN to 269 GPa in Al0.59Sc0.41N and CTE was 4.65 × 10−6 K−1 for AlN and 4.29 × 10−6 K−1 for Al0.59Sc0.41N. Additionally, we observed an increase in CTE from 4.18 × 10−6 K−1 at 65 °C to up to 6.38 × 10−6 K−1 at 375 °C for Al0.68Sc0.32N. The experimentally determined CTE and elastic modulus allow a more precise design of Al1−xScxN-based frequency filters which are used in mobile communications and are important parameters for the prediction of device performance at elevated temperatures.

Al1−xScxN has drawn a lot of attention as an attractive material for radio frequency microelectromechanical systems (RF-MEMS) after the discovery of its enhanced piezoelectric coefficient d33 = 27.6 pC/N for x = 0.43 compared with 6 pC/N for pure AlN1 and increased electromechanical coupling kt2 from 7% in AlN to 15% in Al0.7Sc0.3N.2 For MEMS device design, the mechanical properties, such as elastic modulus, and the coefficient of the thermal expansion (CTE) are important parameters.3 However, there are only a few publications that experimentally assess the elastic properties of this novel material and there is only one report on the elastic modulus of Al1−xScxN with relatively high Sc concentration.4 Moreover, to the best of our knowledge, the CTE of Al1−xScxN thin films has not been reported until now, and in addition to providing support for device design, it is also a significant parameter for the accurate determination of the pyroelectric coefficient of Al1−xScxN.5 

As reported in the literature, elastic modulus of Al1−xScxN thin films can be locally measured by nanoindentation.3,6 However, the indentation modulus can be influenced by the indentation depth, the substrate, and other factors.7 Measuring the temperature-stress relationship of thin films grown on substrates with different CTE is a non-destructive method that enables the determination of not only the elastic modulus but the CTE as well, as it was previously reported for AlN8 and other materials.9,10 The temperature-induced stress σT can be described by the following equation:11 

σT=Ef1vfT2T1(αsαf)dT,
(1)

where Ef/(1 − vf) is the biaxial elastic modulus and Ef and νf are Young’s modulus and Poisson ratio of the film, respectively. αs and αf stand for the CTE of the substrate and the film, respectively. The CTE of the film αf can also be described by the following:

αf=αs1kαs21k,
(2)

where k = (Δσs1T)/(Δσs2T) is the ratio of stress-temperature slopes, which is calculated based on temperature-induced stress as a function of temperature on substrates “s1” with CTE αs1 and “s2” with CTE αs2.

In this work, we extracted the CTE and biaxial elastic modulus of Al1−xScxN based on (1) and (2) by analyzing thin films deposited on Ø = 100 mm Si(001) and Al2O3(0001) substrates. 1 μm Al1−xScxN (x = 0–0.41) thin films were grown by reactive pulsed-DC magnetron sputtering (Evatec Radiance) in pure N2 reactive atmosphere from 100 mm Al (99.995% pure) and Sc (99.99% pure) targets. The Sc concentration in Al1−xScxN was tuned by keeping the total magnetron power PAl + PSc constant at 1000 W while adjusting the individual magnetron powers. More details about the growth process can be found elsewhere.12 The sputtering parameters are shown in Table I.

TABLE I.

Sputtering parameters for Al1−xScxN on Si(001) and Al2O3(0001).

ParameterValue
PAl + PSc 1000 W 
Sc concentration x 0 (AlN), 0.06, 0.14, 0.23, 0.32, and 0.41 
Process pressure <1.5 × 10−1 Pa 
N2 gas flow 20 SCCM 
Target-to-substrate distance 65 mm 
Heater temperature 400 °C (Al2O3), 500 °C (Si) 
ParameterValue
PAl + PSc 1000 W 
Sc concentration x 0 (AlN), 0.06, 0.14, 0.23, 0.32, and 0.41 
Process pressure <1.5 × 10−1 Pa 
N2 gas flow 20 SCCM 
Target-to-substrate distance 65 mm 
Heater temperature 400 °C (Al2O3), 500 °C (Si) 

Al1−xScxN/Al2O3 thin films have higher tensile stress compared with Al1−xScxN/Si with corresponding Sc concentration due to lattice mismatch. In order to avoid cracks in Al1−xScxN/Al2O3, the heater temperature was decreased to 400 °C, while 500 °C was used for Al1−xScxN/Si to achieve higher crystalline quality. Scanning electron microscope (Zeiss Auriga Crossbeam FIB-SEM) with energy dispersive X-ray (EDX) spectroscopy (Bruker Quantax) was used to determine the film composition; the uncertainty with EDX is in the range of x ± 0.02 in Al1−xScxN.

To investigate the crystallinity of the Al1−xScxN(0001) thin films, X-ray diffraction (XRD) 2θ/θ scans and Al1−xScxN 0002 reflection rocking curves (ω-scans) were recorded by PANalytical X’Pert Pro MRD diffractometer equipped with Ge 220 hybrid monochromator providing Cu-Kα1 radiation. All of the Al1−xScxN thin films on both Si(001) and Al2O3(0001) substrates showed only 000l (l = 2, 4, 6) reflections in 2θ/θ scans, indicating c-axis-oriented growth [Figs. 1(a) and 1(b)]. Texture analysis (XRD pole figure measurements) was done at wurtzite-type AlN 101¯1 reflection position [insets in Figs. 1(a) and 1(b)]. For all Al1−xScxN/Si samples, a closed ring is seen at the polar angle ψ = 62°, which indicates fiber textured material with no preferential orientation in-plane,8,13 and for Al1−xScxN/Al2O3, the 6-fold symmetry was observed with the rotation of 30° between the substrate and the film, typical of epitaxial growth of group-III nitrides on Al2O3 substrates,14,15 where the epitaxial relationship can be defined as [101¯0]AlScN//[112¯0]sapphire and (0001)AlScN//(0001)sapphire. Peak shift of Al1−xScxN 0002 is due to different texture on Si and Al2O3 substrates, thermal strain caused by heater temperature during the deposition, as well as Sc concentration. It is reported that higher Sc concentration may lead to a peak shift toward lower16,17 or higher 2θ angles.18,19 The full width at half maximum (FWHM) of Al1−xScxN 0002 reflection rocking curves was determined by fitting of pseudo-Voigt function [Fig. 2(a)]. In the Al1−xScxN/Si, with increasing Sc concentration, the FWHM is decreasing from 1.9° for AlN to 1.4° for Al0.59Sc0.41N, indicating improved film quality. Based on theoretical predictions, the mixing enthalpy in Al1−xScxN is increasing with increasing Sc concentration and should lead to degradation in crystallinity.20 However, in similar studies of sputtered Al1−xScxN/Si, no significant correlation between 0002 reflection rocking curve FWHM and Sc concentration was observed.18 In the Al1−xScxN/Al2O3, the FWHM increases from 0.7° for AlN to 1.6° for Al0.59Sc0.41N, indicating marginal degradation of the film quality in the Al1−xScxN/Al2O3 for high Sc concentrations.

FIG. 1.

(a) X-ray diffraction 2θ/θ scans for Al1−xScxN/Si with x = 0–0.41 and pole figure of Al0.68Sc0.32N/Si (inset); (b) X-ray diffraction 2θ/θ scans for Al1−xScxN/Al2O3 with x = 0–0.41 and pole figure Al0.68Sc0.32N/Al2O3 (inset). This figure will appear in color online.

FIG. 1.

(a) X-ray diffraction 2θ/θ scans for Al1−xScxN/Si with x = 0–0.41 and pole figure of Al0.68Sc0.32N/Si (inset); (b) X-ray diffraction 2θ/θ scans for Al1−xScxN/Al2O3 with x = 0–0.41 and pole figure Al0.68Sc0.32N/Al2O3 (inset). This figure will appear in color online.

Close modal
FIG. 2.

(a) Rocking curve FWHM of AlScN 0002 reflection rocking curves as a function of the Sc concentration in Al1−xScxN/Si (black squares) and Al1−xScxN/Al2O3 (red circles); (b) Piezoelectric coefficient d33 as a function of the Sc concentration. Lines are a guide for the eye.

FIG. 2.

(a) Rocking curve FWHM of AlScN 0002 reflection rocking curves as a function of the Sc concentration in Al1−xScxN/Si (black squares) and Al1−xScxN/Al2O3 (red circles); (b) Piezoelectric coefficient d33 as a function of the Sc concentration. Lines are a guide for the eye.

Close modal

The clamped piezoelectric coefficient d33,clamp of Al1−xScxN/Si was measured by Berlincourt method (Piezotest PM300) on diced 1 × 1 cm2 samples with sputtered Ti electrodes,12 and d33 corrected for influence of substrate stiffness21 was calculated based on the following equation:21 

d33=d33,clamp+2d31(S13+σ/Y)/(S11+S12),
(3)

where the σ and Y are the Poisson ratio and Young’s modulus of Si,22 respectively, and theoretical values for d31, S11, and S12 for Al1−xScxN are based on literature.4 The corrected d33 increases from 5.5 pC/N for pure AlN to 31.6 pC/N for Al0.59Sc0.41N [Fig. 2(b)] and concurs well with theoretical predictions,4 as well as is comparable with the published values by Akiyama et al.,1 confirming the high quality of the material.

To determine the film stress in as-deposited Al1−xScxN, first the film thickness was measured by ellipsometry (SENTECH SE800), the wafer curvature before and after the sputtering was measured by FSM 500TC laser profiler, and then the in-plane stress σ was calculated by using Stoney-equation,23 

σ=Esds261vsdf1R1R0,
(4)

where the Es/(1 − vs) is biaxial elastic modulus of the substrate and df and ds are the thicknesses of the film and the substrate, respectively. R0 and R stand for the radius of curvature before and after the film deposition. In order to determine the CTE and the biaxial elastic modulus of Al1−xScxN, the temperature-induced stress was measured under N2 atmosphere, in the same laser profiler experimental setup. First, thermal cycling for Al1−xScxN/Si and Al1−xScxN/Al2O3 samples was done between room temperature and 400 °C with heating and cooling rate of 2 K/min, where every 25 K the temperature was held constant for 5 min before the wafer curvature measurement was performed. However, Al1−xScxN/Al2O3 samples with x = 0.06 and 0.14 were prone to cracking at elevated temperatures, and thus, the maximum temperature in the thermal cycling experiments was reduced to 125 °C with 2 K/min heating and cooling rate and the wafer curvature was recorded every 10 K for improved accuracy [Fig. 3(a), red circles].

FIG. 3.

(a) Temperature-induced stress as a function of temperature in Al0.94Sc0.04N grown on Si(001) (black squares) and on Al2O3 (0001) (red circles); (b) Biaxial elastic modulus (blue circles) and average coefficient of thermal expansion (black squares) as a function of Sc concentration in Al1−xScxN. Lines are a guide for the eye.

FIG. 3.

(a) Temperature-induced stress as a function of temperature in Al0.94Sc0.04N grown on Si(001) (black squares) and on Al2O3 (0001) (red circles); (b) Biaxial elastic modulus (blue circles) and average coefficient of thermal expansion (black squares) as a function of Sc concentration in Al1−xScxN. Lines are a guide for the eye.

Close modal

Additional thermal cycling experiments under the same conditions were also performed for Al1−xScxN/Si samples, and the stress-temperature slopes did not show any significant difference from the original thermal cycling series up to 400 °C; thus, the original measurement data were used. To investigate the possible film quality degradation or structural changes before and after the thermal cycling, FWHM of Al1−xScxN 0002 reflection rocking curve was compared, showing ±0.1° difference for all the investigated samples; sample composition recorded in SEM-EDX varied only within the measurement error; surface roughness Rrms < 1.5 nm was measured by atomic force microscopy both before and after the thermal cycling, indicating that the samples did not undergo any irreversible changes in their microstructural or crystalline properties.

For calculation of the Al1−xScxN CTE and biaxial elastic modulus, we first assume the constant CTE in the temperature range of 25-400 °C, and the following literature values are used for Si(001): biaxial elastic modulus Es/(1 − vs) = 180 GPa22 and CTE α = 3.57 × 10−6 K−1,24 and for Al2O3(0001):25 biaxial elastic modulus Es/(1 − vs) = 472.6 GPa and CTE α = 5.23 × 10−6 K−1.

Due to the different substrate CTE, the Al1−xScxN/Si films become more compressive stressed and the Al1−xScxN/Al2O3 become more tensile; as an example, the temperature-induced stress curves recorded for Al0.94Sc0.06N are shown in Fig. 3(a). Using (1) and (2), the average CTE and the biaxial elastic modulus were calculated and the results are shown in Fig. 3(b); here, the error originates from scattering of the data when fitting the stress-temperature slope and increases with the Sc concentration. Based on our measurements, CTE of AlN was determined to be α = 4.65 ± 0.20 × 10−6 K−1 (black squares), while values in the literature range from 2.56 to 5.27 × 10−6 K−15,26,27 and biaxial elastic modulus of 535 GPa (blue circles) while 450-489 GPa has been reported previously.4,8 With increasing Sc concentration, the CTE of Al1−xScxN first increases and reaches the highest value of α = 4.95 ± 0.26 × 10−6 K−1 at x = 0.23 and then decreases down to α = 4.29 ± 0.36 × 10−6 K−1 for x = 0.41. The biaxial elastic modulus of Al1−xScxN as a function of Sc decreases linearly by Ef/(1 − vf) = 534.77 − x·601.36 GPa. Our findings are in good agreement with theoretically predicted and experimentally determined biaxial elastic modulus by Caro et al.4 The non-linear behavior of CTE could be explained by the non-linear evolution of lattice parameter c,1,20,28 suggesting that the shape of the unit cell is changing non-linearly as a function of Sc concentration. However, in-depth theoretical study needed to explain this behavior is outside the scope of the current work.

In addition, the temperature-dependent CTE of AlN and Al0.68Sc0.32N was calculated by using temperature-dependent stress data [Fig. 4(a)] as well as temperature-dependent Si(001) and Al2O3(0001) CTE every 50 °C.24,25 The calculation of temperature-dependent CTE helps not only in the optimization of the mechanical properties during film deposition but also in the design of the temperature-compensated devices.29,30 Figure 4(b) shows the CTE of AlN (black squares) and Al0.68Sc0.32N (blue triangles) as a function of temperature. The CTE increases with temperature from 4.21 × 10−6 K−1 at 65 °C to 5.75 × 10−6 K−1 at 400 °C for AlN and from 4.18 × 10−6 K−1 at 65 °C to 6.38 × 10−6 K−1 at 400 °C for Al0.68Sc0.32N. Similar trends in literature can be seen in previous studies of temperature-dependent CTE in AlN.24,31 Summary of experimentally determined average CTE and elastic modulus as a function of Sc concentration as well as calculated values based on literature are summarized in Table II.

FIG. 4.

(a) Temperature-induced stress as a function of temperature in Al0.68Sc0.32N grown on Si(001) (black squares) and on Al2O3 (0001) (red circles); (b) Temperature-dependent coefficient of thermal expansion of AlN (black squares) and Al0.68Sc0.32N (blue triangles) as a function of temperature. Lines are a guide for the eye.

FIG. 4.

(a) Temperature-induced stress as a function of temperature in Al0.68Sc0.32N grown on Si(001) (black squares) and on Al2O3 (0001) (red circles); (b) Temperature-dependent coefficient of thermal expansion of AlN (black squares) and Al0.68Sc0.32N (blue triangles) as a function of temperature. Lines are a guide for the eye.

Close modal
TABLE II.

Experimental CTE, elastic modulus, and theoretical elastic modulus of Al1−xScxN.

Sc concentration xCTE (×10−6 K−1)Elastic modulus (GPa)Elastic modulus in literaturea (GPa)
0 (AlN) 4.65 ± 0.20 535 490 
0.06 4.70 ± 0.26 492 452 
0.14 4.73 ± 0.30 456 415 
0.23 4.95 ± 0.26 389 367 
0.32 4.84 ± 0.17 371 317 
0.41 4.29 ± 0.36 270 261 
Sc concentration xCTE (×10−6 K−1)Elastic modulus (GPa)Elastic modulus in literaturea (GPa)
0 (AlN) 4.65 ± 0.20 535 490 
0.06 4.70 ± 0.26 492 452 
0.14 4.73 ± 0.30 456 415 
0.23 4.95 ± 0.26 389 367 
0.32 4.84 ± 0.17 371 317 
0.41 4.29 ± 0.36 270 261 
a

Values extracted from Caro et al.4 

In conclusion, growth of highly c-axis-oriented thin films of Al1−xScxN with up to Sc concentration x = 0.41 on Si(001) and Al2O3(0001) substrates was achieved by pulsed-DC magnetron sputtering. Epitaxial relationship [101¯0]AlScN//[112¯0]sapphire and (0001)AlScN//(0001)sapphire was determined for all Al1−xScxN/Al2O3 samples. Rocking curve FWHM of AlScN 0002 reflection was <2° for all the investigated samples, indicating a high crystalline quality suitable for device fabrication, as also confirmed by 550% higher d33 measured in Al0.59Sc0.41N/Si as compared with that in AlN. The in-plane CTE and the biaxial elastic modulus of AlN and Al1−xScxN were simultaneously determined by the thermal cycling method. The CTE of AlN was in the same range as typical literature values, and this is the first report of CTE of Al1−xScxN as a function of Sc concentration. The biaxial modulus is decreasing linearly to 270 GPa in Al0.59Sc0.41N compared with 535 GPa in AlN, matching the theoretical predictions. Interestingly, the CTE was non-linear and was highest at x = 0.23, while later decreasing again, which could be caused by anisotropic evolution of the out-of-plane lattice parameters. The measured biaxial elastic modulus and CTE could be beneficial for designing the next generation Al1−xScxN-based RF-MEMS, and the temperature-dependent CTE could be used in predicting device performance at elevated operating temperatures.

We gratefully acknowledge our colleagues S. Leone and S. Müller for the sapphire substrates, R. Iannucci for helping with EDX measurements, and T. Hugger for assistance with the laser profiler measurements. This work was supported by the FhG Internal Programs under Grant No. Attract 005-600636.

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