Low pH Chemical Etch Route for Smooth H-Terminated Si(100) and Study of Subsequent Chemical Stability
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the role of sulfate ions in pH buffered solutions, the air stability of these H-terminated Si(100) surfaces and, the role of thermal treatments of sacrifice oxides relating to subsequent Si(100) Hterminated smoothness. Experimental To form H-passivated Si(100) surfaces various sacrifice oxides were grown and etched with low pH solutions of interest. To form sacrifice oxide, two inch p-type Si(100) wafers were cleaned by conventional hot RCA clean and either i) chemical oxide removed with 1:10 dilute HF and IOOOA thick sacrificial wet thermal oxide grown at 950'C, ii) chemical oxide from RCA clean subjected to Rapid Thermal Anneal (RTA) under 5% 02 at I atm for I min. or, iii) direct use of chemical oxide from RCA clean. For low pH solutions 1-20ml of concentrate H2 SO4 (98wt%) or HCl (38wt%) are added to 155 ml of 30:1 H2 0:HF(49wt%). pH is calculated from literature equilibrium constants [6]. For pH=0.89 buffered solutions, 1-50 ml NH 4 HSO 4 buffer (0.81M H2 SO 4 ,
0.89M NH 3), 5ml HF(49wt%) and 2ml H2 SO 4 (98wt%) were diluted to a total volume of 156m]. Ellipsometric data was collected from 206-450 nm in a previously described system and converted to form.[7] Care was taken to maintain prepared surface under flowing dry N2 during measurement. Auger analysis was performed with a Physical Electronics Model 10-150. Atomic Force Microscopy (AFM) was performed on a Digital Instrument Dimension 3000, using Si tip in tapping mode with a vertical resolution of 0.5A. Results and Discussion Spectral ellipsometry is a highly surface sensitive technique which can detect both surface roughness and chemically different overlayers with submonolayer thickness. With single crystal Si surfaces the pseudo dielectric loss function (e") at 4.25 eV is particularly sensitive. For atomically flat Si(1 11) the ideal value of £" (4.25 eV) is 48.3 [8] with this value decreasing approximately 9.2 per nm roughness or 6.4 per nm of Si0 2 overgrowth. Shown in Figure 1. is F" (4.25 eV) as a function of pH of the HF/H 2SO 4 etch solution. As can be seen, there is a minimum roughness at an optimal H2 SO4 concentration (lower pH), however exceeding the optimal amount of H2 SO4 results in further roughening. Interestingly the best s" values are obtained after rinsing the sample with MeOH while under a N 2 purge to remove absorbed hydrocarbons. To examine the reactivity, F"at 4.25eV is shown as a function of ambient exposure in Figure 2. The 45 spikes in the data are immediately after MeOH rinse showing the removal of hydrocarbons. c" With methanol (4.25eV) decreases about 1 unit (-1.6A 44 rinse overlayer) over 40 minutes from organic exposure. However, the highest values of F" t-J43 (4.25 eV) after MeOH rinses are systematically decreasing indicating an irreversible reaction, 42 presumably from the formation of native oxide. Figure 3. shows Auger analysis of the Si(100) 41 surface after HF/H 2 S0 4 etch and 3 min. X-Without methanol rinse exposure to ambient during transfer. 40L Hydrocarbons and oxygen are readily absorbed -0.5 0 0.5 pH 1 1.5 2
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