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Borophosphosilicate glass (BPSG) films are frequently used in silicon integrated circuits for multilevel interconnection coatings and for surface passivation to provide mechanical protection and improve electrical stability and reliability. BPSG films also act as "getters" of sodium and other metal ions impurities which might otherwise drift to the oxide/silicon interface and affect the electrical properties of the device. Many of these applications require careful control of the phosphorous and boron concentration in the glass film; consequently a rapid and precise measurement of these components is needed. Infrared analysis of BPSG films provides one non-destructive method for the determination of boron and phosphorous in BPSG films deposited on silicon. The BPSG analysis is done from the transmission spectra of silicon wafers coated with the BPSG film. Since heavily doped silicon exhibits continuous plasma absorption in the region where BPSG bands are present, the BPSG film should be deposited on silicon substrates having a resistivity of greater than 10 ohm-cm. A typical transmission spectrum of a BPSG film deposited on silicon is shown in Fig. 1. The phosphorous in the film is normally detected by measuring the band centered at about 1335 cm-1 which arises from P-0 stretch in the P205 moeity in the film, and the boron is measured from the B-0 stretch band centered at about 1420 cm-1. This absorption is from the B203 moiety. Normally, when BPSG glass films are deposited on the silicon substrate, the deposited films contain the phosphorous in different chemical forms - P2O5, P203, etc. - and also contain some trapped hydroxyl groups or moisture. The moisture may react with the P205 to form phosphoric acid. When this happens, and there is some P203 present in the film, the infrared methodology may not detect all of the phosphorous in the film since only the 1335 cm-1 band due to P2O5 is observed. It has been shown [1] that annealing of the deposited film removes the free hydroxyls and stablilizes the P205 moiety. Densification of the film and resistance to water absorption result from the annealing process. There are a number of annealing processes for BPSG films, and it appears that any of these is compatible with the IR method. Because of variations in film densities and ratios of different oxide species present in films deposited by different reactors, it is necessary to calibrate the infrared method with some primary technique that will detect all of the boron and phosphorous present, regardless of chemical species in which it is bound. These methods include ion chromatography, wet chemical analysis, ICP, and SIMS.
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