A High Production System for the Deposition of Silicon Nitride

Abstract

Amorphous silicon nitride is well known as a barrier to the penetration of alkali metal contaminants into junctions of semiconductor devices and as a gate dielectric for MIS devices. This paper discusses the operating characteristics of a production system for the deposition of silicon nitride by the ammonolysis of silicon tetrachloride in a hot wall furnace, and the characteristics of the resulting films.The protective film (1700-2500A) for bipolar devices can be deposited at a rate of 120 wafers/hr. For MIS work, where film thickness directly affects device threshold voltage, better uniformity can be achieved by modifying the wafer placement and operating procedures. The nitride films are amorphous with an index of refraction of approximately 1.95; the surface-state density is in the order of 1012; and the etch rate in buffered hydrofluoric acid 1 is 11 _ 2 A/min. This paper describes the design of a production system for the deposition of silicon nitride and some of the characteristics of the films produced.Amorphous silicon nitride was introduced into the manufacture of beam lead chips to act as a contamination barrier, as has been reported in the literature (1). The system was designed to meet the following specifications for the barrier film: thickness, 1750-2500A; etch rate, 11 __+ 3 A/rain in buffered HF at 23~ sodium penetration (2), 92% removed after etching 50A. The etch rate has been specified to fall within a normally achievable range that will allow the silicon nitride to pass the sodium penetration test; i.e., when the etch rate in buffered hydrofluoric acid increases, the probability of sodium penetration increases (3). The etch rate is directly related to the amount of oxygen in the silicon nitride; and because the presence of oxygen weakens the nitride barrier to alkaline contaminants, the etch test is sufficient to provide some indication as to the quality of silicon nitride. The actual testing procedure for sodium penetration will be discussed later.Initially, silicon nitride was deposited in either a carousel or a barrel-type epitaxial reactor. However, the need for large production numbers (greater than 50 wafers/hr) suggested that an alternate system be developed due to the high cost of reactors and the relatively long cycle time. It was decided to develop a system using a standard diffusion furnace to deposit silicon nitride. Table I compares the reactor statistics with the goals of the proposed furnace system. Table I Reactor Furnace Cost (installed) $50K (min) $18K Turn-around time 60 min 25 rain Capacity/run 25 wafers 50 wafersThese figures are based on data when the study began four years ago. A capacity ratio of 3:1 (furnace wafers to reactor wafers) and a cost ratio of 1:8 for the furnace system have been obtained.In the initial furnace operation, the effort was directed toward producing a satisfactory nitride film. This was done using a streamlined quartz boat with the wafers lying flat. Several conclusions were drawn from this work: (i) the gases must be absolutely dry Key words: t...

Passivation Coatings on Silicon Devices

“…Silicon nitride can also be deposited over refractory metals (160) such as W and Mo, or over the polycrystalline silicon used in silicongate MOS devices. A considerable amount of information has been published on deposition (54,109,87,208,191,216,347,258) and properties (75,54,109,344,168,87,265,216,194,347,193) of SisN4, and on device applications (322,279,262,109,78,310,337,351,196,160,355,195,125,12).…”