AlN thin film, as a piezoelectric material with a wurtzite structure, has attracted much attention due to its excellent performance. However, compared with other piezoelectric materials such as Pb (ZrxTi1-x) O3 (lead zirconate titanate, PZT), pure AlN thin films exhibit poorer piezoelectric response. Doping another element in AlN is considered an effective method to increase the piezoelectric modulus of AlN. The most successful method is scandium(Sc) doping, which can even increase the piezoelectric coefficient d33 to five times that of pure AlN. The emerging device based on wurtzite scandium doped aluminum nitride (ScAlN) demonstrates the enormous potential of ScAlN in RF microelectromechanical systems (MEMS) applications.
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Generally speaking, the temperature stability of thin films determines the temperature stability of devices, which is crucial for MEMS devices. The use of Sc doping to enhance piezoelectric response will also introduce uncertainty in the temperature characteristics of ScxAl1 − xN films. Many studies have focused on the effect of temperature on AlN and ScxAl1-xN thin films, while there is little systematic research on the effect of high-temperature annealing on the properties of AlN and ScxAl1-xN thin films. Therefore, it is necessary to study the effect of high-temperature annealing on the properties of AlN and ScxAl1 − xN thin films.
Research has shown that ScxAl1-xN thin films can remain stable in vacuum or air environments below 900 ℃, while AlN thin films can remain stable in vacuum environments at 1300℃. As the Sc content increases, the stability of ScxAl1-xN will decrease. Adding Sc element can increase the d33 of AlN by five times. AlN can also maintain piezoelectricity at 1150℃. RTA can promote the crystallization of AlN along the c-axis orientation. Annealing at 1200℃ can promote AlN stress relaxation and surface smoothness. The thermal conductivity of AlN, Sc0.125Al0.875N, and Sc0.2Al0.8N thin films was tested at -173.15◦C to 176.85◦C, and the thermal conductivity of the three materials showed a positive temperature trend below 26.85◦C. High temperature annealing can significantly reduce the infrared propagation loss of insulation structure AlN. The coupling factor of Sc0.07Al0.93N deposited on Mo electrode can be increased from 6.5% to 8.5% by in-situ annealing at 500◦C for 15 minutes.
A 1um thick ScxAl1-xN (x=0, 0.09, 0.20) thin film was deposited on an 8-inch (100) Si substrate using SPTS Sigma fxP pulsed DC power supply at 200 ℃, using pure Al targets and Al SC alloy targets. Sputtering power of 5kW, bias power of 165W, Ar/N2 ratios of 20/100, 20/100, and 24/120 sccm, respectively, were used to cut 8-inch wafers into 1×1 cm2 samples. The annealing time is 1 hour, and the temperature gradually increases from 300 ℃.
1. SEM and AFM Results of AlN, Sc 0.09 Al 0.91 N, Sc 0.20 Al 0.80 N Films
The RMS roughness of AlN, Sc0.09Al0.91N, Sc0.20Al0.80N thin films are 1.7nm, 1.5nm, and 9.7nm, respectively, as shown in Fig.1. After vacuum annealing at 900 ℃ and 1100 ℃ for 1 hour, the grain shape, size, and surface roughness of the three films did not show significant changes. After vacuum annealing at 1300 ℃ for 1 hour, cracks appeared in all three films.
Fig. 1 Surface SEM and AFM images of as deposit ScxAl1-x thin films (a) AlN (b)Sc0.09Al0.81N; (c) Sc0.20Al0.80N
Fig. 2 SEM and AFM images of piezoelectric ScxAl1-x thin films annealed in vacuum at 900℃ and 1100℃ (a) AlN (b)Sc0.09Al0.81N; (c) Sc0.20Al0.80N
2. XRD Results of AlN, Sc 0.09 Al 0.91 N, Sc 0.20 Al 0.80 N Thin Films
As the vacuum annealing temperature increases, the (002) diffraction peaks of Sc0.09Al0.91N and Sc0.20Al0.80N change, especially after reaching 900 ℃, while AlN does not show significant changes. As the annealing temperature increases, the FWHM of the three films increases, indicating a deterioration in the crystal quality of the films. The surface roughness increases with the increase of annealing temperature and becomes increasingly unstable with the increase of Sc content. Peak shift is mainly caused by high temperature, and the effect of oxygen is relatively small.
Fig. 4 XRD patterns of ScxAl1-xN thin films annealed in vancuum at various temperatures: (a) AlN (b)Sc0.09Al0.81N; (c) Sc0.20Al0.80N; FWHM values of the (0002) peak(d) and oxygen content percentage (e) of these three films as a function of temperatures
Fig. 5 XRD patterns of ScxAl1-xN thin films annealed in air at various temperatures: (a) AlN (b)Sc0.09Al0.81N; (c) Sc0.20Al0.80N; FWHM values of the (0002) peak(d) and oxygen content percentage (e) of these three films as a function of temperatures
Fig. 6 Vacuum high-temperature annealing has little effect on the hardness and modulus of AlN and AlScN, but in comparison, AlN has better stability.
Fig. 7 When annealed in air at temperatures above 900℃, the hardness and modulus of the three films significantly decrease due to the significant increase in oxygen content.
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