DOI: 10.1007/s00339-007-4032-7 Appl. Phys. A 88, 711–714 (2007) Materials Science & Processing Applied Physics A p. kumar 1 s. kanakaraju 2 d.l. devoe 1, ✉ Sacrificial etching of Al x Ga 1−x As for III–V MEMS surface micromachining 1 Department of Mechanical Engineering, University of Maryland, College Park, MD 20742, USA 2 Laboratory for Physical Sciences, 8050 Greenmead Drive, College Park, MD 20740, USA Received: 27 December 2006/Accepted: 4 April 2007 Published online: 24 May 2007 • © Springer-Verlag 2007 ABSTRACT A study of Al x Ga 1−x As as a sacrificial film for surface micromachining is presented. Al x Ga 1−x As etch rate and selectivity are measured over a range of aluminum mole fractions and HF etchant concentrations during the release of structural features up to 500 μ m in width. The etch process is found to be diffusion limited, with an inverse power law rela- tionship between etch depth and etch rate. Excellent selectivity greater than 10 5 is achieved between sacrificial AlAs and struc- tural GaAs, even for long etches up to 250 μ m in length. Com- pared with previous studies of Al x Ga 1−x As etching for epitaxial liftoff processing, measured etch rates for surface micromachin- ing are approximately an order of magnitude lower, primarily due to the longer effective etch lengths required. However, un- like epitaxial liftoff, Al x Ga 1−x As surface micromachining is compatible with higher HF concentrations which can provide comparable overall etch rates, with important implications for AlGaAs MEMS fabrication. PACS 81.05.Ea; 85.85.+j 1 Introduction Single crystal Al x Ga 1−x As is an attractive ma- terial for many MEMS applications due to its optical and electrical properties [1], and more recently the piezoelec- tric properties of Al x Ga 1−x As have been used to realize devices including microscale resonators [2, 3] and bimorph microactuators [4]. Early Al x Ga 1−x As MEMS development relied upon bulk micromachining of the underlying GaAs substrate to fabricate free-standing Al x Ga 1−x As structures with relatively slow lateral etch rates, typically well below 1 μ m/min [1]. Furthermore, many etchants developed for sac- rificial GaAs etching, such as ammonium hydroxide [2] or citric acid [6, 7] combined with hydrogen peroxide provide only moderate selectivity to Al x Ga 1−x As films, on the order of 100 : 1 for GaAs : Al 0.3 Ga 0.7 As. While alternative etch chemistries based on succinic acid [8] offer improved selec- tivity, these etchants are highly anisotropic with rapid etching of the {001} planes [9]. The resulting anisotropy is gener- ally undesirable in surface micromachining processes, since it limits the orientation of mechanical structures which may be released. ✉ Fax: 301-314-9477, E-mail: ddev@umd.edu Unlike bulk micromachining, surface micromachining al- lows a material other than the substrate to be used as the sacri- ficial layer. For releasing microstructures fabricated on GaAs substrates, a key benefit of surface micromachining is that sacrificial materials other than GaAs can offer improved etch rate, selectivity, and etch isotropy, while also providing the ability to precisely define the gap between structural and sub- strate layers. The ideal combination of etchant and sacrificial layer should exhibit perfect etch selectivity over the structural film, a fast etch rate, and fully isotropic etching behavior to en- able effective undercutting of mechanical structures, regard- less of their orientation. To this end, hydrofluoric acid (HF) etching of sacrificial AlAs or Al x Ga 1−x As films with high Al content, combined with structural GaAs or Al x Ga 1−x As films with relatively low Al content, is a particularly useful etch sys- tem. As early as 1978, it was noted by Konagai et al. that thick Al 0.7 Ga 0.3 As films exhibited rapid isotropic etching when im- mersed in an HF solution, while pure GaAs and Al 0.5 Ga 0.5 As remained unaffected by the etchant [10]. Further investiga- tions by Yablonivitch et al. revealed that HF in varying con- centrations is an effective isotropic etchant for Al x Ga 1−x As when the aluminum mole fraction is greater than x = 0.5, while exhibiting a precipitous decrease in etch rate for mole fractions below x = 0.4, with a remarkable etch selectivity greater than 10 7 [11]. Because HF etching is fully isotropic, the released structures can possess arbitrary orientation. Fur- thermore, the structural films may be grown epitaxially on a sacrificial layer with nearly perfect lattice matching between the layers, leading to low stress gradients through the thick- ness of the structural films. This etch system was initially of interest in solar cell pro- duction as a cost effective method for fully releasing large- area (> 1 cm 2 ) single crystal heterojunction multilayers from GaAs substrates. In this epitaxial lift-off (ELO) process, Kon- agai et al. [10] and Yablonovitch et al. [11,12] applied black wax to the structural film to induce a stress gradient which peeled the film back from the substrate, thereby enhancing exposure of the sacrificial layer and increasing the etch rate. In related work, Voncken et al. used an applied weight rather than black wax to peel back the structural layer in a system- atic study considering HF concentrations from 1%–10% and Al mole fractions between 0.65–1.0 [13]. Etch rates above 500 μ m/min were achieved for AlAs when applying suffi- cient peel-back force on the structural film. While high etch rates are attractive for surface micromachining applications,