[B._Beckhoff,_et_al.]_Handbook_of_Practical_X-Ray_(b-ok.org).pdf

The mechanisms associated with the introduction of

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The mechanisms associated with the introduction of impurities onto the silicon wafer surface during the VPD process are identical to what happens, for example, during chemical cleaning of wafers prior to gate oxidation. In these situations, metal impurities present in the etching solutions or in the UPW used in wafer processing will become readily ionized. As a result the solutions behave like an electrochemical bath in which the impurities can be deposited on the wafer surface. The deposition mechanism for a particular impurity can be affected by a variety of parameters including the amount of dissolved oxygen in or the pH value of the solution as well as the doping in the semiconductor. Important insights into the chemical reactions leading to metal deposition can be obtained by understanding the oxidation state or specia- tion of the deposited metal atoms. Fortunately, the oxidation state of the trace impurities can be obtained from X-ray Absorption Near Edge Spectroscopy (XANES) [208] in conjunction with a SR-TXRF measurement where the pho- ton energy is now scanned across the absorption edge of the element of interest. This requires the use of a silicon crystal monochromator vs. multilayers so that some sensitivity is sacrificed but the resulting MDL of 1 × 10 9 atoms/cm 2 is still adequate for most of these studies. By using XANES spectra of reference materials as fingerprints, specific details about the chemical environment can be derived. For example, Fig. 7.71 shows the Cu 1s XANES spectrum of an Si wafer dipped into Cu-contaminated (1000 ppb) 2% HF (solid dots) with a
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538 C. Streli et al. 1.2 0.8 0.6 0.4 0.2 0 1 8.96 8.97 8.98 8.99 9 9.01 9.02 9.03 9.04 Energy (keV) Absorption (a.u.) Cu-1s 2% HF + 1000 ppb Cu Fig. 7.71. Cu 1s XANES spectrum of a Si wafer dipped into Cu- contaminated (1000 ppb), 2% HF (dots) with a resulting surface concentration of 6 × 10 13 atoms/cm 2 . The solid curve is the XANES spectrum from a Cu metal foil standard resulting Cu surface concentration of 6 × 10 13 atoms/cm 2 . This is identical to the spectrum of bulk Cu metal (solid line) also shown in Fig. 7.71 except for a small but real 0.35 eV shift. The use of this simple fingerprinting method verifies that the deposited Cu exists as clusters, which are mainly metallic in character. This is also in line with electrochemical models predicting that the reaction pathway for metal deposition in a low pH solution such as HF will be reductive [209]. Such studies have also been extended to more complex solutions involving oxidative depositions in high pH solutions [204]. As mentioned in the previous section, the detection of light elements such as Al presents a challenging situation where the tunability of synchrotron radiation can be used to suppress the excitation of the substrate Si K α flu- orescence while enhancing the cross section for the Al K α . Results from a bending magnet at SSRL (BL 3-3) from a Si wafer intentionally contami- nated with 3 × 10 11 atoms/cm 2 of Al are shown in Fig. 7.72. The spectrum was taken with an excitation energy of 1720 eV at an angle of incidence of 0.1 and measured for 10000 s. The low energy peak is the Al K
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