HF-HCl-H2O2混合物在单晶硅晶片上的结构分析

INTRODUCTION

Wet-chemical treatments of silicon surfaces are frequently used for  several different processing steps in photovoltaic, electronic and other  industries. They are used for example for saw damage removal,  surface cleaning, and polishing or texturing of wafer surfaces.1,2 Here we report on solutions containing hydrofluoric acid, hydrochloric  acid and hydrogen peroxide as novel acidic etching mixtures for the  texturing and cleaning of silicon wafers. Reactivity and cleaning  characteristics of the solutions are shown as well as surface  morphologies and reflectivity properties of the treated silicon wafers.

REACTIVITY STUDIES

Silicon etching rates up to 8 nm s-1 are observed during etching of  diamond wire sawn wafers at room temperature. The reactivity  depends on the concentrations of hydrogen peroxide and hydrochloric  acid. This can be explained by the rising formation of chlorine acting  as oxidant for silicon surface atoms.3 An increase of the concentration  of hydrofluoric acid causes a higher rate of dissolution of the oxidized  silicon species.

CLEANING PROPERTIES

After sawing there are high concentrations of metals on the surface of  diamond-wire sawn wafers. After treatment with HF-HCl-H2O2 mixtures  the concentrations are more than 10 times lower. The cleaning results  are comparable to cleaning baths used in semiconductor industries.

SURFACE MORPHOLOGIES

Pyramidal structures were found on silicon (100) wafer surfaces after  etching in mixtures with high amounts of HCl. A regularly pyramidal  structure was generated, which is so far only known from alkaline  etching solutions. Confocal laser scanning microscopy (CLSM) reveals  damage removal and roughening of the wafer surface. The maximum  height of the textured surface Sz = 4.063 μm gets smaller compared  to the as-cut surface Sz = 4.881 μm. At the same time, the root mean  square height increases from Sq = 0.266 μm before etching to Sq =  0.536 μm after etching in HF-HCl-H2O2 mixtures.

REFLECTIVITY CHARACTERISTICS

Surface properties of wafers etched by HF-HCl-H2O2 mixteres are  compared against wafers etched by KOH-IPA solutions. The reflectivity  is plotted as ∆R = Rtextured – Ras-cut at wavelength of 700 nm and  depends on the etch depth. At small etch depths the reflectivity is very  low. This can be explained by the widening of saw damages and very  small etch moulds. By the generation of bigger pyramids the reflectivity  increases. Interestingly, the results of HF-HCl-H2O2 treated wafers are  better than KOH-IPA treated wafers at relevant etch depths of about  5 μm.

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CONCLUSION 

HF-HCl-H2O2 solutions can be used for anisotropic texturization of  monocrystalline, as-cut SiC-slurry and diamond wire-sawn as well as  polished silicon wafers. In contrast to HF-HNO3 solutions, pyramidal  formations similar to typical alkaline textures are generated. Relatively  high etching rates up to 8.0 nm s-1 are observed at room temperature.  Since alkaline mixtures are usually applied at temperatures of 60 °C – 80 °C, a more energy-efficient process for texturing silicon wafers and  producing solar cells should be possible. The cleaning results are quite  similar to cleaning baths used in semiconductor industries. These NOXfree solutions are a promising alternative to common etching and  cleaning solutions without being more expensive, since H2O2 and HCl  are cheap chemicals.