Compatibility of the alternative seed layer (ASL) process with mono-Si and poly-Si substrates patterned by laser or wet etching

Abstract

An alternative seed layer (ASL) process is proposed in order to increase the efficiency of silicon solar cells by forming a low cost, front metal contact with reduced contact resistance and increased line conductivity and aspect ratio. A nickel seed layer is deposited directly on silicon to form a low resistivity nickel silicide (NiSi) ohmic contact and this contact is thickened by light induced plating (LIP) of nickel and copper. Unlike the traditional screen printing process currently used in industry, the ARC layer must be patterned to expose the silicon surface for nickel deposition.This paper investigates the compatibility of the ASL process with two different ARC patterning methods: 1) masking & wet chemical etching, and 2) laser ablation. In addition, the ASL process is demonstrated on both mono-crystalline and polycrystalline silicon substrates with ARC layers from different sources. The nickel seed layer and resulting NiSi layer are evaluated using scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy (EDS) and focused ion beam (FIB) cross section.X-ray photoelectron spectroscopy (XPS) is used to investigate the completeness of the ARC removal step. In addition, contact resistance testing will be performed to determine the quality of the ohmic contact formed from the ASL process. The importance of chemistry optimization in the development of a robust ASL process that is compatible with mono-Si and poly-Si substrates and exposed to two different ARC patterning methods will be discussed.

Improved contact formation for large area solar cells using the alternative seed layer (ASL) process

“…The quality of the laser ablation process can vary from vendor to vendor; therefore, an activating solution has been developed in order to prepare the laser ablated cells for nickel deposition [2]. However, if the laser ablation process does not completely remove the ARC layer in the bus bar and grid line areas, the quality of the nickel plating can be compromised.…”

“…A proprietary activating solution is used to clean the laser ablated solar cells prior to LIP Ni [2]. The nickel, copper, and tin layers are deposited using Technic's LIP plating tool designed to keep the backside of large area solar cells dry during plating.…”

“…The nickel, copper, and tin layers are deposited using Technic's LIP plating tool designed to keep the backside of large area solar cells dry during plating. A proprietary nickel solution is used to deposit the nickel seed layer directly on the silicon surface [2]. The copper layer is deposited using TechniSol Cu 2440 [6] and the tin layer is deposited using a proprietary chemistry formulated specifically for LIP.…”

“…The alternative seed layer (ASL) process coupled with light induced plating (LIP) of nickel (Ni) and copper (Cu) to produce high quality, low cost front metal contacts for industrial high efficiency silicon solar cells has been presented previously [1,2]. Nickel mono-silicide (NiSi) has been discussed as the favored metal silicide for formation of ohmic contacts in silicon solar cells due to a low silicide formation temperature and low silicon consumption [3].…”

In situ PL imaging toward real-time plating process control

“…TechniSol UV-PR etch resist [9,10] was screen printed to pattern the front metallization and to protect the Al backside. This was carried out at Technic, Inc. All experiments were conducted on 2 × 2 cm 2 sections from the same wafer to ensure uniform characteristics.…”