硫酸-过氧化氢-水系统中砷化镓的化学蚀刻

1. Introduction 

Investigation of the chemical etching of (100) GaAs  in a solution consisting of sulphuric acid, hydrogen  peroxide and water is of technological and scientific  importance [1]. This solution is often used for the  preparation of a surface in the sequence of operations  in the course of the manufacture of semiconductor  devices. For this reason the solution should be optimized; on the other hand, the results obtained constitute a good starting point for the discussion of  heterogenous reaction mechanisms.

The sequential chemical reactions taking place on  the phase boundary are: the diffusion of reagents  from solution to phase boundary; the adsorption of  reagents on the solid surface; the oxidation process;  transformation of oxidation products to the ionic  form; desorption; diffusion of products into the  solution.  

Reagents in the solutions considered are H202 molecules and H + ions similar to in the H3PO4-H202-  H20 system [2]. The flow of reagents to the solid  surface depends on their concentration and on solution viscosity, i.e. indirectly on the H2SO 4 concentration. The adsorption generally takes place on the socalled active centres of crystal surface depending on  surface orientation. In some cases, etching reveals a  crystal structure.  

2. Experimental procedure  

The range of solution concentrations of the H20-  H202-H2SO4 system used in the experiments was  limited by the available concentrations of reagents.  The composition of the solutions is commonly described by volume percentage on the Gibbs' triangle,  its vertices being water, 30 wt % hydrogen peroxide  and 96 wt % sulphuric acid. High concentrations of  sulphuric acid in the solution result in partial hydrogen peroxide decomposition. To estimate the stability  range of the solutions, they were titrated with KMnO4  immediately after the components were mixed, and  then after 2 and 24h. If the results of the analyses  differed by less than 5% from the calculated compositions and were stable as the same level, the solutions  were considered to be stable. The stability line at room  temperature corresponds to 50 vol % H2SO 4 without  stirring, and a little below this value in stirred solutions, as shown in Figs 1 and 2. All solutions were  prepared by mixing proper quantities of H2 SO4 with  H20 and cooling to room temperature. H202 was then  added slowly to avoid temperature increase. For discussion of the reaction kinetics and mechanism it is  convenient to present the solution composition as  molar percentage. The range of usable compositions is  shown in Figs 3 and 4.

Fig1

3. Results and discussion  

The results of etching are presented as lines linking the  points of the same etching rates in the field of the  Gibbs' triangle and as curves showing the etching rate  dependence on one component concentration assuming the concentration of the other components to be  constant. The results obtained during etching without  stirring are presented in Figs 1.3, 5, 6 and with stirring  in Figs 2, 4, 7, 8. Moderately driven stirring enhances  the reagent diffusion to/from the phase boundary,  thus increasing the dependence on the chemical reaction. Comparison of the results obtained with and  without stirring leads to significant conclusions, presented in Figs 9 and 10, in the form of a Gibbs' triangle, in which the fields corresponding to similar  surface states and with similar etching mechanisms are  indicated.  

The shapes of the grooves aligned with [1 T 0], [1 0 0]  and [0 1 0] did not depend on solution composition.  They were V-shaped with (t 11)A limiting surfaces for  the grooves aligned with [1 TO] and U-shaped (with  round or flat bottom) for both remaining directions.  The change in shape of the [1 1 0] aligned grooves is  quite interesting, though difficult to explain. These  shapes are seen in Fig. 13. With a fixed concentration  of H2SO4 the increase of oxidant concentration is  followed by gradual decay of (1 1 1)B walls until the  bottom becomes rounded. The increase in H 2 SO 4 concentration causes the (1 1 1)A walls to disappear and  the profile also becomes round.

The diversity of groove shapes is accounted for by  anisotropy, i.e. the different etching rates of exposed  surfaces, in particular (1 00), (1 11)a and (1 1 1)~. It is  also connected with the adhesion of masking material  to the wafer surface.  

Adachi and co-workers [6-10] reported results of  wide investigations of the correlation between the  shapes of etched grooves and surface orientation,  groove direction and type of solution applied. However, he also could find no simple relation between  solution composition and groove shape. An analysis  of the masking material and the undercutting  influence on groove shape was carried in detail by  MacFadyene [3].