Wafer Surface Charge Reversal as a Method of Simplifying Nanosphere Lithography for Reactive Ion Etch Texturing of Solar Cells

Daniel Inns,Kylie Catchpole,Patrick Campbell

doi:10.1155/2007/32707

Abstract

A simplified nanosphere lithography process has been developed which allows fast and low-waste maskings of Si surfaces for subsequent reactive ion etching (RIE) texturing. Initially, a positive surface charge is applied to a wafer surface by dipping in a solution of aluminum nitrate. Dipping the positive-coated wafer into a solution of negatively charged silica beads (nanospheres) results in the spheres becoming electrostatically attracted to the wafer surface. These nanospheres form an etch mask for RIE. After RIE texturing, the reflection of the surface is reduced as effectively as any other nanosphere lithography method, while this batch process used for masking is much faster, making it more industrially relevant.

Highlights

Multicrystalline Si wafers make up the majority of the photovoltaic market
Is outlined the process used to optimize the two solutions used for dip coating wafers, first for wafer surface charge reversal, followed by electrostatic attraction of a nanosphere lithography mask
By reversing the surface charge of a polished silicon wafer, the mask has been applied by dipping into a dilute acidic solution of nanospheres

Summary

INTRODUCTION

Multicrystalline Si wafers make up the majority of the photovoltaic market. Due to the variety of grain orientations, they cannot be textured cheaply by the anisotropic wet chemical etches used for monocrystalline silicon. In previous work with nanosphere masks, the mask has been formed by spincoating, resulting in a uniform monolayer of spheres across the surface. These particles are the etch mask for subsequent RIE [4]. The minimum reflectance is reduced around 13% by etching less than 500 nm of Si—making the process fast and high-throughput compared to other RIE textures, with or without masks. This work shows that a simple dipping process can reverse the surface charge of a wafer, and another dip can mask the surface Masking based on this idea can lead to (b) high-nanosphere surface coverage, and etching through the nanosphere masks is well controlled by RIE. The geometry of the texture features can be altered by changing RIE processing parameters (primarily pressure and gas mixtures), which controls the etch rate of the nanospheres that act as the mask

EXPERIMENTS AND DISCUSSION

Reversing the surface charge of silicon wafers

Electrostatically attaching nanosphere masks

RIE texturing results

Findings

CONCLUSION