Summer00 analysis of phosphorous

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Analysis of Phosphorous Auto-doping in P-Type Silicon using Corona Oxide Silicon Techniques By Brian Letherer and Greg Horner, KLA-Tencor Corporation

Semiconductor fabrication facilities rely on the integrity of the silicon to manufacture submicron devices. Cross contamination of P-type silicon to N-type carriers or vice versa in the near surface region of the silicon can be detrimental to device performance. Semiconductor processing typically includes numerous diffusion and pre-clean steps, any one of which might auto-dope a P-type silicon substrate with phosphorous. In-line monitoring of these near-surface doping effects enhances the ability to diagnose auto-doping problems.

A non-contact Corona Oxide Silicon (COS) measurement technique has the ability to detect cross-contaminated P-type silicon with phosphorous from wet clean benches and diffusion furnaces. Results show COS flatband (Vfb) and oxide total charge (Qtot) measurements are sensitive to various levels of intentional phosphorous contamination implanted into the silicon at pre-oxidation. Phosphorous at the silicon/oxide interface can pile up and create an electrically active thin “N” skin. Phosphorous from this thin “N” skin is shown to change the electrical characteristics of near surface region of the silicon. The detection of unwanted phosphorus with the use of COS in-line monitoring can greatly reduce the response time when auto-doping problems occur. Contamination control has long been an integral part of semiconductor manufacturing. Yield loss due to small amounts of contamination in silicon can cause catastrophic loss of product due to slight changes in electrical behavior of silicon based devices. The source of contamination can be widespread, as well; there are a variety of potential sources in a silicon manufacturing facility. Isolating and eliminating these sources of contamination 74

Summer 2000

Yield Management Solutions

can also be difficult and tedious. Phosphorous, in particular, is used in many areas in a fab as a dopant and can cause significant problems to the near-surface silicon region, the primary region where a device operates. Detection of contamination problems can be time consuming and timely feedback of detection is also desired to reduce the amount of product at risk. An in-line metrology tool to monitor contamination is essential in a manufacturing environment. The COS, which is commercially available, has the ability to monitor contamination and give timely feedback to ensure the risk of contamination is minimal. The primary sources of phosphorous are generally POCL3 doping of polysilicon and phosphorous implant. Bare silicon wafers, with high phosphorous content, (test or monitor) processed in wet sinks or high temperature diffusion can readily out-diffuse and will auto-dope wafers with P-type silicon in the same process step or in subsequent process steps. The source of the phosphorous contamination tends to be very localized, as only certain sections of a diffusion furnace or wet-cleaning processes will be contaminated. Quartzware in diffusion furnaces and wafers that are run continually in the diffusion furnace for thermal mass may retain phosphorous. On subsequent runs phosphorous will out-diffuse at temperatures above 850°C and diffuse into exposed silicon substrates of


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Summer00 analysis of phosphorous by KLA Corporation - Issuu