使用溶解在HF和HCl溶液中的臭氧清洁方法

ABSTRACT: In this work ozone dissolved in deionized water (DIO:) cleaning is investigated as a low-cost alternativemethod to the current wet-chemical cleaning in high-eficiency solar cell manufacturing. Higher effective lifetime anclower  were achieved, compared to the reference cleaning process of high-lifetime RCA, by simply adding a smalamount of HF and HCl to DIO, solution, The effective lifetime of cleaned and passivated wafers increased as theprocesing time in DIO/HF/HCl cleaning solution increased. The stability of the wafer lifetime even after some storagetime of up to an hour post cleaning makes the DIO/HF/HCl cleaning suitable for industrial application. TheDIO,/HE/HC1 cleaning shows a great potential to be a high-lifetime wet-chemical cleaning process in mass production.Keywords: wafer cleaning, dissolved ozone, high-lifetime.

INTRODUCTION

For high efficiency solar cells such as PERC, HIT andIBC cells, effective cleaning of silicon wafer surfaces isfor obtaining high lifetime and goodparamounteffect. The conventional wet-chemicalpassivationcleaning sequence used in solar cell production line andPV lab usually consumes a large amount of chemicalssuch as HSO4, HNO3, HF, HCI, NHOH and H,O2. Suchchemical solutions have finite bath lifetime, which meansthat after running through certain amount of wafers or fora certain period of time even without cleaning wafers.new bath has to be mixed to maintain effective cleaningFor high-efficiency Si solar cells, which are moresensitive to the surface condition of silicon and havehigher passivation requirements [1, 2], the RCA cleaningsequence has been a reference [3]. The RCA cleaningrequires more frequent changes of chemicals, whichgreatly increases the manufacturing cost.

Ozone is a strong oxidizer which has been widely usedfordisinfectionmedicalapplications.Forinsemiconductor and PV industry, using ozone to cleanwafer has been under investigation for several years (4].Compared to conventional wet-chemicalcleaningsequences, aqueous ozone cleaning combines excellentcleaning result with a potentially low cost [5-8]. Inprevious works, the diffusion uniformity on the waferwas significantly improved by using ozone pre-treatmentdue to better wettability on the wafers (5]. Cleaningmethods using DIO3 are known to be very efficient forremoving both organic and metallic contaminations. Theinterface state densities of ozone-cleaned wafers arecomparable to the values obtained with the standard RCAprocess followed by HF dip [6]. Better lifetime andimplied Voc were obtained on a-Si:H passivated samplesusing ozone-based cleaning, compared to samples usingHNO3/HF cleaning [7]. Diluted HCI/O3 solution'sremoval ability of organic and metallic contaminationand its effect on alkaline texturing was compared to thatofKOH and SC1 cleaning [8].

In this work, we choose another way to takeadvantages of the ozone by using DIO,/HF/HCl mixtures.The lifetime and J results are compared to the referenceRCA cleaning sequence. Different DIO/HF/HClcleaning times and storage times between cleaning andpassivation steps are evaluated using lifetime data and PIimages. The dark saturation current (Jo) and effective lifetime of boron and phosphorus diffused wafers cleanedby both methods are evaluated. Results show that theDIO3/HF/HCl is a promising alternative method totraditional wet-chemical cleaning sequences for the highefficiency solar cell process.

2 EXPERIMENT

High-lifetime FZ N-type wafers with a thickness of190 um and a resistivity of 160 ohm.cm were used . Allwafers are TMAH chemical polished to completelyremove the saw-damaged layer prior to the high lifetime cleaning.

The cleaning sequences of the reference RCA cleaningand DIO3/HF/HCl cleaning are listed in table I. Wherethe recipe ofSPM clean is H,SO4: HO,-4: 1, 120°C,10minutes. RCAl clean is NHOH: HO: HO=1: 1: 580°C,10 minutes. RCA2 clean is HCl: HO: HO=1: 1:5，80°C，10 minutes. And the DIO/HF/HCl clean isCpro3 20 ppm,Chr-2. g/l, Chc=10 g/l, liquid flowrate=l2 l/min at room temperature, the investigatedcleaning time is varied from 5 to 10 minutes to evaluatethe cleaning effect.