使用 TMAH 溶液垂直蚀刻氮化铝铝薄膜

A wide bandgap, an enhanced piezoelectric coefficient, and low dielectric permittivity aresome of the outstanding properties that have made ScxAl1−xN a promising material in numerousMEMS and optoelectronics applications. One of the substantial challenges of fabricating ScxAl1−xNdevices is its difficulty in etching, specifically with higher scandium concentration. In this work, wehave developed an experimental approach with high temperature annealing followed by a wet etchingprocess using tetramethyl ammonium hydroxide (TMAH), which maintains etching uniformity acrossvarious Sc compositions. The experimental results of etching approximately 730 nm of ScxAl1−xN(x = 0.125, 0.20, 0.40) thin films show that the etch rate decreases with increasing scandium content.Nevertheless, sidewall verticality of 85◦~90◦(±0.2◦) was maintained for all Sc compositions. Basedon these experimental outcomes, it is anticipated that this etching procedure will be advantageous inthe fabrication of acoustic, photonic, and piezoelectric devices.

Group III-V materials are getting notable attraction for their diverse applications suchas microelectromechanical systems (MEMS), piezoelectric transducers, resonators, andradio frequency (RF) acoustic filter devices [1–4]. Due to some of its promising qualities and simplicity of process integration, aluminum nitride (AlN) is widely employedin piezoelectric MEMS devices [5,6]. AlN can be doped with other metals to increase itspiezoelectric properties [7,8], which advanced the success of ScxAl1−xN based optoelectronics devices [9–11]. At the earlier stage of twenty-first century, Takeuchi et al. usedfirst-principles analysis to determine that wurtzite structure of Sc-IIIA-N alloys can be fabricated [12]. Later, Akiyama et al. demonstrated that by measuring co-sputtered ScxAl1−xNfilms [13–15] a piezoelectric coefficient of 27.6 pC/N could be achieved and is more thanfive times higher compared to AlN [13]. Furthermore, according to the experiments ofWingqvist et al. [16], the electromechanical coupling coefficient value of ScxAl1−xN filmcan be improved by up to 15%, with recent studies showing coupling coefficients exceeding20%. Thus, using ScAlN thin films with increasing scandium concentration facilitates thefabrication of high-frequency and wideband acoustic devices [17–19]. However, ScxAl1−xNthin films become challenging to etch as the scandium concentration (x) increases, especiallywhen using reactive ion etching (RIE) or inductively coupled plasma (ICP) etching [20].Substantial research has been conducted on ScxAl1−xN to understand its growth and howdifferent etching techniques can be used to fabricate piezoelectric devices [21].

Like any other group III-V material, ScxAl1−xN can be etched by dry or wet etchingtechniques. One of the known dry etching approaches is ion beam etching, which can bephysical or chemical and can result in smooth etched surfaces at a suitable etch rate [22].Table 1 summarizes experiments that dry etch ScxAl1−xN and reports sidewall verticalityand etch rate. Luo et al. [23] performed ICP etching with thick S1818 photoresist (PR)as an etch mask. They demonstrated how RF power might control the sample’s plasmaetching energy, and how the energy of the Cl2/BCl3/N2 plasma enhances the sidewallangle of Sc0.06Al0.94N. James et al. demonstrated that the reactive ion beam etching (RIBE)process of ScxAl1−xN etching is superior to ion beam etching (IBE) in terms of etchingrate, selectivity, and sidewall angle (73◦) (See Table 1). The ScxAl1−xN etching rate andselectivity degrade when the identical beam parameters (See Table 1) are used without thereactive gas [24]. Wang et al. [18] presented the design, fabrication, and characterizationof Sc0.20Al0.80N thin films used for piezoelectric micromachined ultrasound transducers(PMUTs) by using RIE as an etching process (See Table 1). They claimed that the etchedlayer has good verticality and concluded that increasing the scandium concentration wouldenhance PMUT performance [18].

Fig1

During the wet etch process, we heated the 25% TMAH solution to 80 ◦C and thenimmersed the ScxAl1−xN samples in the solution. Figure 3a–d shows the wet etch resultsof the Sc0.20Al0.80N film after 3 min of etching. The 730-nm thick layer was removedin 3 min, resulting in an etch rate of 243 nm/min with 87◦~90◦ vertical sidewalls. Thisresult demonstrates that wet etching using TMAH can be used as an alternative to achievesidewall angles better than 80◦, which can be considered state-of-the-art compared tostudies in Table 2. Figure 3c,d illustrates that the same quality of etched profiles canbe obtained utilizing different patterns as well as an inverted mask. For Sc0.40Al0.60Nand Sc0.125Al0.875N, identical TMAH concentration and temperature were used, and theresults are shown in Figure 4a–d. The etching rate is found to be relatively lower forSc0.40Al0.60N (~80 nm/min) compared to Sc0.125Al0.875N (350 nm/min). This result isconsistent with other work, as shown in Tables 1 and 2. For the lower concentration, theverticality resulted to be 88.2◦ ± 0.2◦(Figure 4d), which is practically the same for what wefound for the Sc0.20Al0.80N. However, as shown in Figure 4b, the profile of the Sc0.40Al0.60Nshows a verticality of ~85◦from the provided SEM image.

As the scandium concentration increases, usually the undercut (lateral) etching increases [20,21]. Chen et al., demonstrated the reason behind the dependency of lateraletch with the AOGs [35]. The experiment demonstrated that AOGs are grains that containScAlN unit cells that have their c-axis tilted from the normal direction of the film surfaceand do not precisely nucleate from the bottom of the film [34]. The SiO2 mask is resistantto the etching process and the ScxAl1−xN film is etched laterally. The result is a suspendedSiO2 layer. Thus, we performed high-temperature annealing (thermal diffusion) in thenitrogen atmosphere to recover the surface damage by minimizing the effect of ion bombardment. Annealing is expected to increase the piezoelectric properties of the samples aswell [20,36–38]. In addition to this, AOGs generated during the growth process increasesetch resistivity, especially in wet etch processes [20,39]. In our works, we were successful inreducing the lateral etching with almost vertical sidewalls for higher Sc concentrations (SeeFigure 4a). This was possible thanks to the combination of the optimized annealing process(temperature and nitrogen concentration) and wet etching recipes (TMAH concentration,temperature, etch time).