Comparisonc of remote vemote versus direct PECVD silicon nitride

Presented at the 2nd World Conference on Photovoltaic Solar Energy Conversion, Vienna, 1998.1COMPARISON OF REMOTE VERSUS DIRECT PECVD SILICON NITRIDEPASSIVATION OF PHOSPHORUS-DIFFUSED EMITTERS OF SILICON SOLAR CELLS

Beate Lenkeit1, Thomas Lauinger2, Armin G. Aberle1, and Rudolf Hezel11Institut für Solarenergieforschung Hameln/Emmerthal (ISFH), Am Ohrberg1, D-31860 Emmerthal, Germany

Tel.: (+49)5151-999-304, Fax: (+49)5151-999-400, Email: lenkeit@isfh.uni-hannover.de2ASE GmbH, Industriestrasse 13, D-63755 Alzenau, Germany

ABSTRACT: At present, plasma-enhanced chemical vapour deposited (PECVD) silicon nitride (SiN) filmsact as the most efficient low-temperature surface passivation and antireflection (AR) coating for Si solarcells. This work presents a comprehensive experimental comparison of the surface passivation properties ofn+-diffused silicon surfaces passivated by either remote or direct PECVD SiN films. Carrier lifetimemeasurements are performed on phosphorus-diffused SiN/n+/p/n+/SiN test structures to determine theemitter saturation current density J0E and the surface passivation quality. Our experimental study includes:(i) determination of the influence of the SiN deposition parameters by means of design-of-experimentsmethods, (ii) variation of the emitter profile, (iii) UV stability testing, (iv) thermal stability testing at 500°Cin a forming gas ambient and at 800°C in air, and (v) the determination of the impact of the front metal gridon J0E. Remote-plasma SiN and high-frequency direct-plasma SiN films both are found to give very goodsurface passivation on phosphorus-diffused n+-emitters, provided that the refractive index is above 2.0. Thisemitter surface passivation is stable under UV illumination and during a 60-min post-deposition anneal informing gas at 500°C. In contrast, low-frequency direct-plasma SiN films give much poorer surfacepassivation.Keywords: Silicon-Nitride – 1: Passivation – 2: c-Si - 3

1. INTRODUCTIONPresently, in all commercial applications of PECVDSiN films on Si solar cells, the direct plasma excitationapproach is used where the silicon wafer is directlyexposed to the plasma during deposition. Usually, theexcitation frequency is in the 25 - 500 kHz range (i.e. wellbelow the plasma frequency of about 4 MHz) and hence aconsiderable ion bombardment of the silicon surfaceoccurs during the SiN deposition. The bombardment canbe significantly reduced using an excitation frequency wellabove the plasma frequency. The standard frequency inthis frequency range is 13.56 MHz. In contrast to directPECVD, in the remote plasma technique the excitation ofthe plasma occurs outside the deposition chamber andhence the samples are not in direct contact with theplasma during deposition. Recently, we have shown in ourlaboratory that the remote plasma deposition of SiN filmson p-Si surfaces results in greatly improved surfacepassivation properties compared to (i) the widely usedlow-frequency direct plasma deposition technique and(ii) thermally grown SiO2 [1]. The remote techniquefurthermore has many technical advantages over directPECVD, offering significant potential for reductions of thecosts of the SiN deposition process in solar cell productionlines [2]. For example, the refractive index of remote-plasma SiN films can technologically be varied over awide range, so that these SiN films can be optimised assurface-passivating AR coatings on non-encapsulated aswell as on glass-encapsulated silicon solar cells [3,4].In this study, the deposition of the SiN films is per-formed in a commercial PECVD system (Oxford PlasmaTechnology), allowing the application of three differentdeposition techniques: (i) direct PECVD with a lowexcitation frequency of 100 kHz (LF), (ii) direct PECVDwith a high excitation frequency of 13.56 MHz (HF), and(iii) remote PECVD using microwave excitation(2.45 GHz). The process gases used are ammonia andsilane. While in the direct plasma method both gases areexcited, in the remote process only ammonia is excitedwhile silane is directly injected into the depositionchamber.

2. SAMPLE PREPARATIONThe silicon samples used in this study are 1.5-Ωcm,(100)-oriented float-zone p-Si wafers with a thickness of300 µm. Prior to the phosphorus emitter diffusion, thewafers received a standard RCA clean. Our standardemitter (labelled ‘A’ in the following) is generated at830°C (20 min deposition and 20 min drive-in in N2) in aquartz tube using liquid POCl3 as dopant source. Thisresults in an emitter with a sheet resistance of ρs =100 Ω/o, a surface dopant concentration of Ns ≈6x1019 cm-3, and an emitter depth of xj ≈ 0.4 µm, as deter-mined by the four-point probe technique and SIMS mea-surements. This homogeneous emitter has the potential forhigh-efficiency Si solar cells, as recently demonstrated atISFH by the fabrication of 21% efficient cells [5]. Toexamine the applicability of the investigated SiN passiva-tion schemes for industrially feasible screen-printed cells,a second emitter is processed with a sheet resistance ofρs = 40 Ω/o. The diffusion and oxidation furnaces were