SIMS Measurements of Oxygen in Heavily-Doped Silicon
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SIMS MEASUREMENTS OF OXYGEN IN HEAVILY-DOPED SILICON
R. J. BLEILER*, AND R. S. HOCKETT*, P. CHU**, AND E. STRATHMAN*-* *Monsanto Electronic Materials Company, St. Louis, MO. 63167 **Charles Evans & Associates, San Mateo, CA 94402
ABSTRACT Oxygen precipitation in CZ silicon is known to provide beneficial yield improvements in integrated circuit processing if the location and amount of precipitation can be properly controlled. The concentration of oxygen in the unprocessed silicon substrate is one of the most important variables to control for achieving these improvements. Fourier Transform Infrared Spectroscopy (FTIR) has successfully been used to measure [0] in silicon when the silicon resistivity is greater than about 0.1 0-cm. At lower resistivities typical of p+ and n+ substrates used for epi-wafers as free carrier absorption interferes with the FTIR measurement of bulk [0]. This work will focus on how to quantitatively measure oxygen in heavily-doped silicon by Secondary Ion Mass Spectrometry (SIMS) with a high sample thruput, low background signal, and tight o/x distribution. SIMS calibration is performed against FTIR-calibrated substrates with resistivity higher than 0.10cm. Typical background signals as measured in FZ are a factor of 20 below signals in CZ, and the 160- signal in CZ is over 105 count/sec. resulting in an excellent signal-to-noise ratio for each single measurement. Typical thruput is 18 samples per day where each sample is analyzed four to five times to obtain a a/x of 3% for an oxygen level of 15 ppma (ASTM F121-80).
INTRODUCTION Since silicon single crystals grown by the Czochralski (CZ) method are known to incorporate impurities such as oxygen during the growth process, the measurement of the oxygen concentration within the silicon is extremely important. Other impurities such as dopants are also added to the melt and are distributed between the melt and the crystal based on their segregation coefficient. The oxygen, which can be present in the crystal in different forms such as interstitials, complexes or precipitates, is continuously supplied to the melt from the quartz crucible walls. The electrical and mechanical properties of the silicon substrate are affected by the presence of oxygen [1]. By controlling the thermal history of the silicon, the oxygen can be made to precipitate. The stress created by the oxygen precipitates results in effective intrinsic gettering sinks for impurities which could be incorporated during subsequent device processing. In order to derive the benefits of the intrinsic gettering sites, the location of oxygen in the unprocessed silicon substrates is one of the most important controllable variables to achieve reproducible oxygen precipitation. The oxygen impurity concentration can be affected by the doping level and the rate of evaporation from the melt. Shimura et al. [2] investigated the behavior of oxygen in n+(Sb) CZ silicon crystals. Comparison of the oxygen concentration of the n+ CZ silicon crystal to the expected oxygen levels in lightly doped n-ty
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