SIMS Measurements of Oxygen in Heavily-Doped Silicon

ABSTRACTOxygen 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 Ω-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 σ/x distribution. SIMS calibration is performed against FTIR-calibrated substrates with resistivity higher than 0.1Ωcm. 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 σ/x of 3% for an oxygen level of 15 ppma (ASTM F121−80).