One of the critical parameters in many semiconductor-processing steps such as plasma etching, chemical vapor deposition, and so on, is the substrate temperature. For example, the etch rate and etch profile are sensitive to the substrate temperature in the plasma etching process. However, contact measurement such as thermo-couples and fluorescent temperature sensor cannot measure the accurate temperature, especially in low pressure due to poor thermal transfer between the wafer and the probe.

We have developed the non-contact temperature measurement of substrates by using optical low-coherence interferometer. [1-6] The low-coherence interferometry is based on Michelson interferometry, and a white light such as a super-luminescent diode (SLD) was used as a low-coherence light source. SLD has the central wavelengths of 1325 nm and the full width at half maximum of 64.5 nm. The light which is reflected at the surface of Si substrate interfered with the light reflected at the scanning reference mirror. The interfered light was detected with a spectrometer and the signal which is a frequency domain spectrum was converted to a time domain signal by inverse Fourier transformation. The temperature of the Si substrate is derived from the change in the optical path length of Si substrate due to thermal expansion and changes in the refractive index.

The temperatures of Si wafer and chamber parts made of Si were successfully monitored in the SiO₂ plasma etching using capacitively coupled plasma. The performance was compared with that of fluorescent temperature sensor. From these results, we have demonstrated highly precise and rapid response measurement of the temperature of Si wafer at real time by using Fourier domain low coherence interferometer.

References

[1] K. Takeda et al., “Temperature-Measurement System Using Optical Fiber-Type Low-Coherence Interferometry for MultiLayered Substrate” Jpn. J. Appl. Phys., 43, 7737 (2004).

[2] T. Ohta et al., “Simultaneous measurement of substrate temperature and thin film-thickness on SiO2/Si wafer using optical fiber-type low-coherence interferometry”, J. Appl. Phys., 105, 013110 (2009).

[3] C. Koshimizu et al., “Low-coherence interferometry - based non-contact temperature monitoring of a silicon wafer and chamber parts during plasma etching”, Appl. Phys. Express, 3, 056201 (2010).

[4] C. Koshimizu et al., “Simultaneous In situ Measurement of Silicon Substrate Temperature and Silicon Dioxide Film Thickness during Plasma Etching of Silicon Dioxide Using Low-Coherence Interferometry”, Jpn. J. Appl. Phys., 51 046201 (2012).

[5] H. Yamamoto et al., “Feature Profiles on Plasma Etch of Organic Films by a Temporal Control of Radical Densities and Real-Time Monitoring of Substrate Temperature”, Jpn. J. Appl. Phys., 51 016202 (2012).

[6] T.Tsutsumi et al., “Rapid measurement of substrate temperatures by frequency-domain low-coherence interferometry”, Appl. Phys. Lett. 103, 182102(2013).