Optimized Wafer Thickness for TLS Measurements
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In many cases of TLS (thermal laser stimulation) measurements, wafer back grinding is needed because thinning of the wafers improves the signal strength. Figure 2809 shows the percent transmission as a function of wavelength and p-doping concentration for 625 pm of Si [1]. Heavier doping increases laser absorption and thus reduces transmission. The percent transmission of infrared wavelengths is given by [1],

          percent transmission of infrared wavelengths ----------------------------- [2809]

          %T -- The percent transmission,
          r -- The reflection coefficient,
          α -- The absorption coefficient,
          d -- The thickness of Si wafer.

As indicated in Equation 2809, decreasing the thickness can dramatically increase the %T and therefore the TLS signal.

Optimized Wafer Thickness for TLS Measurements

Figure 2809. Percent transmission of laser intensity through p-doped Si in 625 pm thickness and having doping concentrations of (a) 1.5 x 1016, (b) 3.3 x 1017, (c) 1.2 x 1018, and (d) 7.3 x 1018 cm-3. The 1340 nm wavelength is marked by a vertical line. [2]




[1] S. E. Aw, H. S. Tan, and C. K. Qng, “Optical Absorption Measurements of Band-gap Shrinkage in Moderately and Heavily Doped Silicon”, J. Phys.: Condens. Mutter, 3, 1991, pp. 8213-8223.
[2] Cole, EI; Tangyunyong, P; Barton, DL, Backside localization of open and shorted IC interconnections, 36th Annual IEEE International Reliability Physics Symposium, (1998). DOI: 10.1109/RELPHY.1998.670462.




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