Direct Comparison of FTIR and SIMS Calibrations for [O] in Silicon
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DIRECT COMPARISON OF FTIR AND SIMS CALIBRATIONS FOR [0] IN SILICON
P.K. CHU CHARLES EVANS & ASSOCIATES San Mateo, CA 94402 R.S. HOCKETT MONSANTO ELECTRONIC MATERIALS COMPANY St. Louis, MO 63167 R.G. WILSON HUGHES RESEARCH LABORATORIES Malibu, CA 90265
ABSTRACT Fourier Transform Infra-Red spectroscopy (FTIR) is commonly used to measure interstitial oxygen concentrations in silicon. The absorption coefficient a derived from the IR transmittance curve is converted to a concentration value through a multiplicative calibration factor y . Presently used calibration factors include 2.45 x 017 cm-' (ASTM F121-88, These have and 4.81 x 017 cm-2 (ASTM F121-76). 3.03 x 10 17cm-2 (JEIDA), been obtained from different analytical methods, including charged particle activation, helium carrier gas fusion-gas chromatography, vacuum fusion gas analysis, and photon activation combined with inert gas fusion. An alternative analytical method by secondary ion mass spectrometry (SIMS) is reported here. Oxygen ions of a known fluence are implanted into silicon samples of which oxygen content has been measured by FTIR. SIMS profiles of oxygen are performed using a cesium primary ion beam on a CAMECA IMS-3f. The result is a profile of the superposition of the implanted oxygen and the constant,
bulk
CZ
oxygen.
Integration
of
the
implant
signal
followed
by
subtraction of the bulk signal can be used to calibrate the implant concentration from the implanted oxygen fluence, so that, in the same sample and during the same measurement, the SIMS calibration and FTIR calibration at the CZ bulk level can be directly compared. The y obtained from this method is 3.45 x 10 17cm-2 and is closest to Re JEIDA value of 17 -2 3.03 x 10 cm INTRODUCTION Fourier Transform Infra-Red (FTIR) spectroscopy is one of the standard techniques to determine the oxygen concentration in Czochralski (CZ) grown silicon crystal by measuring the Si-O-Si vibration at 9 pm due to interstitial oxygen [1]. However, owing to the high absorption from free carriers, the determination of the oxygen content in heavily doped silicon materials has traditionally been carried out by analytical techniques such as 3 1 8 charged particle activation analysis [160 ( He, p) F reaction] [2], photon activation analysis [160 (y, n) 150, half-life 2.03 min] combined with inert gas fusion [3, 4], helium carrier gas fusion-gas chromatography, vacuum fusion gas analysis, and secondary ion mass spectrometry (SIMS). Of these five techniques, SIMS is the most widely used because of the simplicity and speed of the measurement. However, there have been some controversies in correlating results obtained independently by FTIR and SIMS. An important factor is that the oxygen background in some SIMS instruments is quite high and sometimes approaches the oxygen concentration in CZ grown silicon crystals. In addition, it is well known that SIMS secondary ion yields vary
Mat. Res. Soc. Symp. Proc. Vol. 59. 1986 Materials Research Society
68
quite drastically for different elements and for the same
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