湿处理过程中硅介质上污染物的沉积

Abstract

The evaluation of photovoltaic man-ufacturing wet process steps shows howimpurities may be deposited on siliconmedia. Upon withdrawing a wafer, a liquid layer remains on the silicon surface.Any contaminants in the liquid will bedeposited on the silicon when the liquidevaporates.This data shows the biggestfactor affecting deposition of impuritiesis the concentration of impurities in the bath.

The introduction of contaminants onsilicon media during processing canreduce the efficiency of photovoltaic (PV)cells and shorten their lifetime. One of thesources of contamination is processbaths, where media is dipped into waterand/or chemicals for etching or cleaningThe sources of bath contaminants arenumerous. They can arise from insufficiently pure chemicals or water. Thetanks, silicon carriers and handlingequipment may be constructed fromindustrial-grade materials that desorbimpurities, thereby contaminating baths.

Alternatively, the impurities could beextracted from common plastics used inindustrial-grade plumbing components orchemical containers.

The following information discussesan examination of how a contaminatedprocess bath may potentially depositimpurities on silicon media. Upon with-drawing a silicon wafer from a bath, athin layer ofliquid is left on the surface ofthe silicon, as depicted in Figure 1. Anycontaminants in the liquid layer will bedeposited on the silicon when the liquidevaporates. This information will showthat the biggest factor affecting deposition of impurities is the concentration ofimpurities in the bath. Other factors suchas removal speed of the wafers from thebath and properties of the liquid including surface tension, density and viscosityare less influential.

Sources of Contaminationin Wet Processing

Processing of silicon wafers into photovoltaic cells involves multiple steps. Theprocess starts out with growing the silicon ingot and proceeds through various steps including wafer sawing, cleaningetching, diffusion, screen printing andfinally testing Throughout this process, itis crucial to maintain the purity of the silicon wafer, as it has been shown that con-tamination can negatively impact thecell's efficiency and effective life.

Pre-diffusion wet processes are onesource of contamination in the manufacturing process.These wet processes typically use high-purity chemicals such as hydrofluoric acid (HF) andhydrochloric acid (HCI)along with 18 megohm cm deionized (DI)water While the chemicals start out with low impurities, the chemical's exposure tomaterials used in the chemical distributionsystem such as pipes, valves and containerscan contaminate the liquids to unaccept-able levels. Extractable testing has shownthat the type of material used for wettedcomponents such as polyvinyl chloride(PVC), polypropylene (PP) or perfluoroalkoxy(PFA) has a significant impact on the level ofpresent in the fluidcontaminationmedia.

The concern over contamination iscompounded in wet benches wherechemical baths are often recirculated without the use of filtration, causing thecontamination levels to grow over time.

Once a process bath becomes con-taminated, it leads to the transfer ofthese impurities onto the surface of thesilicon wafer. This is accomplishedthrough the submersion and withdrawalof a silicon wafer from a process bath.During submersion of the wafer into aspecific liquid media, the chemical andits constituents surround the wafer.Upon removal of the wafer from the liquid media, a layer of the chemical willremain on the surface of the wafer. Thislayer of liquid will contain the desiredchemical elements but will also containcontaminants found in the liquid. As theliquid evaporates, the impurities foundwithin the liquid will remain on the surface, resulting in increased levels of contamination on the silicon wafer.