Fundamental Selective Etching Characteristics of HF + H 2 O 2 + H 2 O Mixtures for GaAs

Abstract

Selective etching characteristics of HF + H=O~ + H=O mixtures have been investigated for GaAs (lll)A, (11 1)B, (001), and (110) surfaces. Mixtures with excess H~O2 have shown excellent selective etching characteristics controlled by the H~O and H=O= contents for extensive practical applications. The lowest etching rate in directions other than [111]A has been achieved with the present mixture system. Etching profiles strongly reflecting crystallographic anisotropy have been produced for low H~O and high H~O~ concentrations, while those for high H~O and low H~O~ concentrations have shown similar isotropie features irrespective of the substrate orientation. A continuous and extensive control of the intersection angle between the side wall and the substrate surface has been achieved successfully on all substrates. The etching profile and its dependence on the mixture composition for the four substrates have been discussed in detail in connection with the degree of anisotropy of the etching rate and the contrast that the (III)A plane sharply intersects with other planes while the (iii)B plane has a round intersection corner. The etching characteristics of the mixture have been discussed based on the special chemical properties of HF and H~O and compared with those of other mixture systems. Among several semiconductor technologies, chemical etching is a fundamental technique indispensable for basic material research and device application. Gallium arsenide (GaAs), one of the III-V compounds, has been studied widely as a basic material for high-speed electronic devices and high-efficiency optical devices. Many reports have been published on selective chemical etching systems for GaAs. They include H~SO4 + H202 + H20,1'2'12 NH4OH + H202 + H20, 3'4'12 H3PO4 + H~O2 + H20, ~'12 Br~ +CH3OH, 6'7'~2 HF + HNO3 + H20, s'12 buffered HF + H202 + H20, 9'I~ and C3H4(OH)(COOH)3 + H202. ~ Almost all the reports focus primarily on etching behaviors on the (00 i) surface because this surface traditionally has been used for device applications. In recent years, however, there has been an increasing number of research work on crystal growth, microstructure formation, and device fabrication on substrates with orien-tations other than (001) ~3-16 and substrates composed of various surface orientations (namely, patterned sub-strates). 17-19 The aim of these works is to understand and utilize efficiently the electronic, optical, lattice, and impurity incorporation properties and the crystallographic symmetries different from those of the (001) substrate. Some reports describe etching characteristics in orientations other than (001) I'~'6'11 but provide no adequate information on the control of etching rate and profile. Excellent selective etching characteristics of HF + HeO2 + H~O mixtures which can cover wide applications are demonstrated here for the first time on the basis of systematic experiments using various mixture compositions on substrates with different orientations. This type of mixture system previously has been used only for revealing disloca-tion etch-pits9 '~I We paid special attention to orientations of sidewall surfaces produced by selective etching, which have been ignored in previous papers, because their control is essential in recent applications. Experiment Silicon-doped n-type HB GaAs substrates with orienta-tions of (001), (110), (~]-I~B, and (111)A and carrier concentrations of 1 to 3 • 1018 cm-3 were used for etching experiments. After degreasing in organic solvents, wafers were spin-coated with an AZ1350J resist and baked at 85~ for 30 rain in a flow of N2 gas. The resultant resist thickness was ca. 1.9 ~m. Stripes of 100-p,m width running in the [ii0] direction common to the four substrates and in the directions perpendicular to them were photolithographi-cally patterned. The wafers were then scribed into I0 • 6 mm pieces as etching samples. Commercial 46 weight a Present address: Solid-State Physics and Devices Research Division , Science and Technical Research Laboratories of NHK, Seta-gaya-ku, Tokyo 157, Japan. percent (w/o) HF and 35 w/o H202 of semiconductor grade were used throughout together with deionized water of 17 M~ cm for dilution. To investigate selective etching characteristics systematically, mixtures with various HF to H202 mole ratios, c~ = 0.05/0.34, 0.05/0.17, 0.05/0.09, 0.05/ 0.04, 0.25/0.11, and 0.69/0.02 were examined under wide-range dilution by H20 from 1.2 to 56.8 mol. Here, the H20 content involved ir~ the HF and H202 also was taken into account in evaluating the H20 concentration in the mixture. b The resist on the wafers was hardened by baking at 120~ for 30 rain in a flow of N2 gas for etching protection. Etching was done at 25~ under room light, and with continuous sample stirring for uniform etching. Etching also was done at i0 and 40~ for selected mixture compositions to investigate the temperature dependence of the etching characteristics. After etching, the sample was thoroughly rinsed in deionized water and the resist was removed in acetone. Then, the sample was subjected to etching depth measurements with a profilometer for determination of the etching rate, R, and observation of the top and cross-sectional views of the etching profiles by scanning electron microscopy (SEM). Good etching uniformity and reproducibility were confirmed. Results and Discussion Since the etching depth was directly proportional to the etching time, R could be defined for all the compositions examined. Etching anisotropy regarding the crystallographic orientation was represented by the ratio, W, of R in the [I 1 liB direction to that in the [Ill]A direction. ~'~'6 We hereafter refer to an expression of R in the [imn]X direction as R [imn]X. H202-excess mixtures (~ = 0.05/0.34, 0.05/0.17, 0.05/0.09, and O.05/O.04).-Excel]ent selective etching characteristics and their controllability have been obtained for H~O2-excess mixtures as described below. Etching rate.-Figure 1 shows variations of R and ~ with the H20 content, [H20], for the mixtures with ~ = 0.05/0.17 and 0.05/0.34. R and ~ for both mixtures decrease as [H20] increases. R [lll]A is the lowest of all and the three other Rs show similar values. This is a feature commonly observed for all the mixtures examined except for a = 0.05/ 0.04 and also for other etching systems. 1-35'~'n There is a difference among the magnitudes of R[001], R [l l 0] ,-and b 3 The conversion of units from cm-to tool was made according to the following equations [HF] (mol) = 0.0245 • [46 w/o HF] (cm-3) [H202] (mol) = 0.0107 • [35 w/o H2Q] (cm 3) [H20] (tool) = 0.0319 x [46 w/o HF] (cm-3) + 0.0374 x [35 w/o H202] (cm-3) + 0.0556 • [H20] (cm-~).