Metal contamination is generally not considered as harmful in MEMS technology as in IC technology. However, it has been previously shown that metal contamination enhances oxygen precipitation in CZ silicon, which can be detrimental also in MEMS devices as oxide precipitates are known to affect the anisotropic etching behaviour of silicon. [1] In literature, iron contamination was found to increase both the total amount of precipitated oxygen and the density of oxygen precipitates. [2, 3] It was proposed that iron could promote oxygen precipitate nucleation. Also copper precipitates have been noticed to increase oxygen precipitation. [4] The reason is assumed to be copper precipitates and related defects that serve as nucleation sites for oxygen precipitates. In these studies, however, the contamination levels were rather high. In this paper, we investigate the effects of iron and copper contamination on the amount of precipitated oxygen and oxygen precipitation induced defects using more realistic contamination levels.
CZ silicon wafers were contaminated with iron or copper and subjected to different two-step thermal treatments. Iron concentration was measured to be about 2 × 1013 cm-3 and copper concentration about 6 × 1013 cm-3 by µPCD. The interstitial oxygen concentration was measured by FTIR before and after wafer processing. In addition, defect etching was carried out to some of the wafers to study the possible secondary defects related to oxide precipitates. We found that in iron contaminated wafers the amount of precipitated oxygen did not differ much in comparison to uncontaminated wafers that had experienced exactly the same thermal treatments. In copper contaminated wafers some indication of higher amount of precipitated oxygen as compared to uncontaminated wafers could be seen. However, defect etching showed no difference between contaminated and uncontaminated wafers in the density of precipitation induced defects.
[1] T. Müller et al., J. Electrochem. Soc. 147, 1604 (2000)
[2] B. Shen et al., Jpn. J. Appl. Phys. 35, 4187 (1996)
[3] J. Jablonski et al., Jpn. J. Appl. Phys. 35, 520 (1996)
[4] Z. Xi et al., Semicond. Sci. Technol. 19, 299 (2004) |