Gettering of Gold and Copper in Silicon during Gallium Diffusion

Abstract

The gettering of Au and Cu during the sealed tube diffusion of Ga has been studied by using neutron activation analysis and the spreading resistance microprobe technique. It was found that Au in silicon was gettered during Ga diffusion regardless of the cooling conditions. Most of the Cu in the surface layer of silicon diffused out during Ga diffusion, while Cu in the bulk silicon was gettered during the slow cooling process after diffusion. Increases in the minority carrier lifetime in p+n diodes prepared by Ga diffusion occurred with increasing the amount of Ga used as a source. It was ascertained that in Ga gettering the Ga metal sources played an important role as impurity ~inks.In silicon device processing, high temperature steps, such as oxidation and dopant diffusion, often result in the soft reverse characteristics of pn junctions. The origin of this high leakage current is believed to be the localized high electric field caused by the precipitates of metals, such as Au and Cu (1). These metals in silicon also lead to the degradation of minority carrier lifetime (2-4) which affects the forward voltage drop and other device parameters. These undesired metallic impurities can be removed by so called "gettering" processes. Up to now, various gettering techniques have been reported (5-14). Among them, gettering during diffusion of dopant impurities, such as phosphorus, boron, gallium, and so forth, is most important, because the dopant diffusion is the typical method for making pn junctions. The gettering by phosphorus and boron diffusion has been widely studied analytically and electrically (5-9), but only a few works have been reported concerning the gettering during Ga diffusion (14). Because of the relatively high diffusion coefficient of Ga in silicon, Ga diffusion is often used for the formation of p-type layers of silicon power devices. In these devices high lifetime values are often desirable. Therefore, the maintenance of a minimum lifetime during Ga diffusion is an outstanding problem in the power device processing. Lambert and KShl (14) reported that metallic Ga layers painted on the silicon surface show pronounced gettering effect. However, the precise features of Ga gettering have not been offered yet. It is the purpose of this study to investigate analytically the behavior of Au and Cu in the sealed tube diffusion of Ga and to reveal the mechanism of the gettering during Ga diffusion. ExperimentalSample preparation.--The starting materials used in this study were float-zone, n-type silicon wafers, 130-180 ~-cm, with mechanically lapped surface. The wafers were intentionally contaminated with a moderate to a large amount of Au or Cu to obtain impurity levels above the background level. Au-contaminated samples were prepared by the following procedure. First, the wafers were etched by HF/HNO~ solutions to remove the surface-damaged layer produced by mechanical lapping. Au films were deposited on one side of the silicon wafers by vacuum evaporation and then annealed at 90O~ for 30 rain in an argon a...

Phase formation of nanolaminated Mo2AuC and Mo2(Au1−xGax)2C by a substitutional reaction within Au-capped Mo2GaC and Mo2Ga2C thin films

“…The proposed mechanism is based on the fact that the relatively mobile A element in nanolaminated carbides, compared to the M element, can be attracted to out-diffuse from the structure in presence of another high affinity element/compound. In our case, a high chemical affinity between Ga and Au is supported by experimental observations on room-temperature interdiffusion at Au/Ga interfaces, 25,26 on Au gettering by elemental Ga in semiconductors, 27 and on fast dissolution of Ga from GaAs into Au contact without melting. 28,29 On the whole, the reaction of Ga layers in the nanolaminated carbides is likely initiated by attracting Ga atoms into the Au capping layer.…”