Thin films of SiN are well suited as antireflection (AR) coatings for Si solar cells because their optical properties, such as refractive index and absorption coefficient, can be tailored during deposition to match those of Si solar cells. The SiN layers, particularly those deposited by a plasma-enhanced chemical vapor deposition (PECVD) process, can serve other functions in Si solar-cell fabrication. They can be excellent buffer layers through which the front metal contact can be fired. The PECVD nitridation also introduces H into the Si surface, which diffuses deep into the solar cell and passivates residual impurities and defects during metal-contact firing. The optimization of SiN properties and processing conditions may have conflicting demands based on its multifunctional role. To fully exploit these multiple functions, the SiN processing sequence must be optimized based on the properties of the nitride, the diffusion behavior of H, and the interactions of metal with the SiN/Si composite substrate.

INTRODUCTION

Silicon nitride (SiN) is used in the semiconductor industry for many applications, such as a diffusion mask during device processing, a dielectric for memory devices, and an insulator for isolation.1 A stoichiometric SiN, deposited at a high temperature, is typically used for these applications because it offers a high breakdown-field strength and a low fixed-interface charge density. The SiN films deposited by a plasma deposition process have also been used as antireflection (AR) coatings for Si solar cells. However, this use was quite limited because it required a high-cost, wet-etching step to open a metallization pattern in the SiN coating. The process cost was reduced by a sequence that used plasma patterning through a metal mask, followed by electroless plating of Cu on a Pd barrier. A further cost reduction was achieved by directly firing a screen-printed silver contact through the SiN layer. In spite of these advantages, the use of SiN did not become very popular in the solar-cell industry until recently when it was found that SiN can also passivate surfaces and the bulk of a solar cell. These combined features have led to a universal acceptance of SiN in the solar-cell manufacturing industry.

Solar-cell fabrication involves a number of major process steps, which include formation of an N/P junction, contact metallization, and AR coatings. Additional process steps are often necessary to accommodate auxiliary functions, such as impurity gettering, passivation of defects and impurities in the bulk, and surface passivation.2,3 Because these auxiliary functions can significantly improve device performance, separate process steps were generally used for each function. However, the additional costs incurred for each process step were considered excessive, and therefore, the processes were unwarranted. Consequently, the photovoltaic (PV) industry has sought processes that can accomplish multiple functions. For example, in a typical solarcell processing regime, impurity gettering is accomplished during junction and contact formation. Likewise, a thin layer of SiO^sub 2^ is used to produce surface passivation and is then made a part of the AR coating. The PV industry is looking aggressively for ways to further minimize the number of process steps needed for solar-cell fabrication as a method for cost reduction. Recently, it has been determined that SiN coatings deposited by plasma-enhanced chemical vapor deposition (PECVD) can accomplish the following functions: (1) act as an excellent AR coating whose refractive index can be controlled by controlling the deposition conditions, (2) serve as a buffer through which a contact metallization can be fired to make reliable ohmic contacts, (3) produce a passivation layer to reduce the surface recombination of the device, and (4) allow diffusion of H into the device to passivate defects and impurities for improved device performance.4-7

An integration of these multiple functions demands a detailed knowledge of various mechanisms that influence optical parameters, interface charge, and transport of H in Si. This paper reviews optical properties of SiN and its design as an optimum AR film, the role of SiN as a metallization buffer, process considerations for retention and subsequent diffusion of H to produce impurity defect passivation, and considerations for surface passivation by the nitride coating.

MULTIPLE ROLE OF SiN

Recently, hydrogenated silicon nitride (SiN^sub x^:H) has found an exclusive niche in Si solar-cell application because it can accomplish multiple functions, eliminating several additional process steps in the fabrication of high-efficiency solar cells, and concomitantly, lowering solar-cell fabrication cost. In a typical commercial solar-cell processing sequence, a thin layer of silicon nitride is deposited by a PECVD process on the front (facing the sun) side of an N/P junction to serve as an AR coating. Often, it is desirable to have a thin SiO2 layer under the nitride layer (discussed later in this paper). A nitridation process also produces an accumulation of positive charge at the SiN:H/Si interface that helps in surface passivation. Furthermore, because nitridation is performed in an atomic H ambient, it introduces H into a thin plasma-damaged surface layer.6 Following the nitride deposition, an Ag-based contact metallization is then screen-printed and fired through the nitride.8,9 In this step, the metal penetrates through the nitride to form a lowresistance ohmic contact, while the H diffuses into the bulk of the cell to passivate impurities and defects. The multipurpose role of the nitride demands that it be a low-absorption AR coating, serve as a barrier layer for control in metallization, and promote favorable electronic processes that can passivate the surface, as well as the bulk, of the device. It is imperative that SiN:H deposition and processing be designed carefully to optimize optical and electronic properties of the solar cell. The desirable features of a well-designed layer are the following: (1) front surface of the cell should have very low reflectance within a broad band of wavelengths, (2) low surface-recombination velocity (SRV) on the N^sup +^/P device, (3) it should interact with Ag-based paste to form a uniformly wettable flux, and (4) it should encapsulate H within the surface layer of Si and prevent its out-diffusion in subsequent processing. We briefly discuss the properties of the nitride needed to accomplish these features for a high-efficiency Si solar cell on a low-cost substrate.