ScxAl1-xN piezoelectric film grown at room temperature

In order to overcome the shortcomings of AlN piezoelectric thin films such as low piezoelectric coefficient and low electromechanical coupling coefficientrealize, ScxAl1−xN thin films with CMOS process compatibility were prepared by pulsed DC reactive magnetron sputtering. High performance ScxAl1−xN piezoelectric thin films were prepared at room temperature by optimizing process parameters including power, gas flow ratio, substrate temperature, seed layers and so on. The phase separation phenomenon was investigated through XRD, TEM, and XPS. We found that the phase separation is related to the appearance of rocksalt ScN and led to the degeneration of film properties. The existence of ScN phase is a key factor affecting the performance of ScxAl1−xN. The results show that the piezoelectric constant d33 of ScAlN film achieved 27.5 pC/N and the full width at half maximum (FWHM) of the rocking curve is 1.9° in room temperature when the content of Sc reaches 35%.


Introduction
AlN (Aluminum nitride) is a promising piezoelectric material with its high acoustic velocity, large band gap, and strong thermal durability.However, the application of AlN is limited by its low piezoelectric property [1][2][3].Akiyama et.al has reported a new piezoelectric material ScAlN (scandium-doped aluminum nitride) which enlarged the piezoelectric constant d33 by a factor of 3 through doping Scandium (Sc) into AlN [4][5][6].Benefits from its enormous d33, ScAlN has a superior performance in film bulk acoustic resonators(FBARs) than commercially available AlN films [7,8].However, ternary semiconductor ScAlN has a potential risk of phase separation and loss of its piezoelectric response, especially when scandium concentration is above 20% while the highest piezoelectric performance is achieved around 43% [9].Therefore, most ScAlN processes require extra heating [10,11], and this leads to a higher thermal budget.Preparing ScAlN thin film with low temperature comes to an issue before its CMOS compatibility.In this article, we successfully deposited Sc0.35Al0.65Nfilm without extra heating under optimized sputter conditions.The piezoelectric constant d33 of ScAlN film achieved 27.5 pC/N.After deposition, the phase separation phenomenon was investigated through XRD, TEM, and XPS.We found the phase separation is related to the appearance of rocksalt ScN and led to the degeneration of film properties.

Experiment and result discussion
The ScAlN film was sputtered by DC-pulsed reactive sputtering with FHR MS100X6-L magnetron sputtering system on 4 inches N<100> silicon wafers.This process allows fast deposition of ScAlN film along the c-axis direction.A 6-inches Al0.57Sc0.43alloy target was utilized for high concentration and better uniformity.Buffer layer material is important for ScAlN sputtering since it has a large lattice mismatch with the silicon substrate.Many materials have been successfully applied as the buffer layer in the growth of pure AlN such as Al, Au, Pt, Mo, and 4H-SiC [12][13][14].A commonality between them is they have a similar crystal to AlN.Furthermore, Kevin R has calculated the piezoelectric strain coefficient d33 and found it highly related to the wurtzite structure [15].In this article, AlN is chosen as the buffer layer for the growth of ScAlN since it has similar constants with ScAlN and wurtzite structure.Also, Ti/ Pt was pre-sputtered as a buffer between Si and AlN, which is classic for the growth of AlN.The thickness of Ti/Pt/AlN was sputtered for 40nm/120nm/20nm respectively under optimized sputter conditions.
In order to realize the sputtering of ScAlN under a relatively low temperature to reduce the thermal budget, no extra heating was applied to the substrate during the process.Instead, 3 parameters including sputter power, gas ratio, and total gas flow were carefully optimized for a better crystal quality.During sputtering, the ionized gas Ar+ and N+ were accelerated by the electric field and bombard atoms out from the target.The bombarded source atoms combine with active gases and then deposit on the surface of the substrate.With the residual kinetic energy, sputtered particles will move along the surface with the lowest surface energy before energy depletion.This explains why ScAlN crystals used to have a preferred orientation along the c-axis.Heating, sputter power, gas ratio, and flow respectively influence surface energy, kinetic energy, Al(Sc)-N reaction rate, and mean free path.Since no heat is applied, other parameters were adjusted to eliminate the influence of temperature.To check the crystal quality, all films were characterized using XRD (Rigaku 2500PC) and SEM (Thermofisher Apreo) after sputtering.All ScxAl1-xN grwon was done under high vacuum (pressure < 6× 10-7 mbar) because oxygen is more active compare to nitrogen and likely to combine with Al(Sc) before the latter.
As is shown in Figure 1, after optimization the 2θ pattern of ScAlN film shows an obvious (0002) diffraction peak around 2θ = 36°, which is close to AlN (0002).The full width at half maximum (FWHM) of the rocking curve is 1.9°.This confirms the crystal quality of the prepared c-axis preferred film.A test structure was designed and fabricated to further confirm the crystal composition and test the piezoelectric response.Since ScAlN films are chemically inert and hard to be patterned.Low-resistance silicon(0.02Ω•cm) was chosen as the bottom electrode.100 nm Al films were deposited on the front and back side of the wafer for electrical contact.The highest piezoelectric constant d33 is 27.5 pC/N.We also tested the d33 of other films deposited during gas flow optimization and found it's highly related to the FWHM of the rocking curve.The enormous promotion in d33 of ScAlN benefits from the softening of stiffness among the c-axis of the unit cell, which is caused by the replacement of Al by Sc in the wurtzite ScAlN cell.However, the crystal structure of pure ScN is non-polarity rock salt (same as NaCl), which has no piezoelectric response and also will cause misalignment between cells [16].This will lead to the deterioration in crystal quality.In grain boundaries, Sc atoms need more energy to diffuse than Al and are more likely to gather and form rocksalt cells.This explains why higher power and lower gas flow contribute to the growth of ScAlN.Considering target poisoning and reactive rate, our optimized sputter parameter without heating is 900W, 15 sccm N2 and 3 sccm Ar. 20nm AlN was used as seed layer and Ti/Pt was used as bottom electric materials.However, the existence of rocksalt ScN remains unconfirmed.The SEM images were carried out to further characterize the crystal structure.As is shown in Figure 2, many abnormally oriented grains (AOGs) were observed on the surface of films.At first, the atoms have no enough energy to diffuse so the grains are small and loose.With the increase of sputtering power, the grains become larger and occupy the whole surface.Finally, the AOGs were reduced after parameter optimization.The test reveals the connection between AOGs and degrades of crystal quality.To further search the induce reason of AOGs and investigate the crystal structure.The HRTEM was carried out with SAED.The sample was thinned by IBE for electrons to transmit.A square near AOGs was chosen for SAED.From the diffraction pattern, some abnormal spots were observed.Meanwhile, the pattern from normal grains is in order and uniform (The different pattern is caused by a rotation of grains).This reveals the existence of a second compensation in AOGs.Also, HRTEM reveals the irregular arrangement of atoms inside AOGs.By contrast, inside normal grains, the atoms formed regular arrays.This composites our assumption: the existence of rocksalt ScN leads to the abnormal orientation of ScAlN cells above it and induced AOGs.XRD can't be used for the search of rocksalt ScN since it's a microscopic characterize method and hard to find rock salt ScN with little contents and small grains (possibly about 5 nm).Instead, XPS was carried out to figure out the crystal content of ScAlN film.After calibration using carbon, peaks related to AlN and rocksalt ScN were found in the pattern.As is shown in Figure 4.The peak of AlN switched to lower energy, which indicates a longer bond distance.Besides, an abnormal peak near rocksalt ScN is observed.Considering a higher bond distance in the wurtzite structure.This peak indicates the appearance of the Sc-N bond in the wurtzite structure.The alloying changes the energy level of the 2 materials and forms the new structure.Therefore the peak position of bonds moves a little.However, this new grain structure is more instability the pure AlN, and few rocksalt ScN appears during deposition and generated AOGs.In this article, the sputter of ScAlN with high piezoelectric response was done under room temperature using a unique buffer layer, ScAlN film with the piezoelectric response of 27.5 pC/N is sputtered.Then we characterized the structure of AOGs, which led to the decoration of piezoelectric response.Rocksalt ScN is the main reason for AOGs and caused irregular arrangement of atoms.The existence of rocksalt ScN is then confirmed through XPS.This result can indicate later experiments of ScAlN and similar ternary compounds.

Figure 1 .
Figure 1.The XRD patterns of the sputtered ScAlN films

Figure 3 .
Figure 3.The TEM, SAED, and HRTEM pattern of AOGs and normal grains

Figure 4 .
Figure 4.The XPS test result of ScAlN film.