Behavior of Metal-Induced Negative Oxide Charges on the Surface of N-type Silicon Wafers Using Frequency-Dependent AC Surface Photovoltage Measurements

Abstract

The metal-induced negative charge on the surface of n-type silicon (Si) wafers, previously reported in the atomic bridging model as (AlOSi)- and (FeOSi)- networks, was investigated using AC surface photovoltage (SPV) as a function of the frequency ( f) of a chopped photon beam, with the aid of an AC SPV instrument developed in-house. A frequency-dependent AC SPV in Al-contaminated wafers was observed immediately after the rinsing. In the early stages of exposure to air at room temperature after rinsing, the induced AC SPV at frequencies of less than approximately 100 Hz was flat with frequency. This indicated that majority carrier conduction was the dominant mechanism, suggesting that the surface was depleted and/or weakly inverted. With the passage of time, the AC SPV in the lower frequency region increased, yielding an AC SPV vs frequency relationship approaching 1/ f. This demonstrated that the Al-induced negative charge [(AlOSi)-] increased and finally became limited by the depletion layer capacitance (strongly inverted state). The frequency-dependent AC SPV caused by Fe-induced negative charge [(FeOSi)-] was identified as being dependent on Fe concentration on the Si surface. The higher the Fe concentration, the more strongly inverted the Si surface became. The metal-induced oxide charge could be observed, together with other oxide charges such as fixed oxide charge.