Tracking the Temperature of a Mo/Si Multilayer Mirror Used in Semiconductor Fabrication

By Surbhi JainReviewed by Susha Cheriyedath, M.Sc.Jul 26 2022

In an article recently published in the journal ACS Applied Electronic Materials, researchers discussed the extreme-ultraviolet (UV) lithography monitoring of a Mo/Si mirror's temperature using photoluminescence.

Study: Monitoring the Temperature of a Mo/Si Mirror with Photoluminescence in Extreme-Ultraviolet Lithography. Image Credit: asharkyu/Shutterstock.com

Background

A key technique for creating nanochips is photolithographic technology. The incoherent tin-droplet source's extreme ultraviolet (EUV) radiation is used in a fabrication facility for this purpose. The power output of a commercial EUV scanner is currently rather considerable, and as a result, the heat loads of the machine's components must be closely watched.

A Mo/Si multilayer mirror is now used as an optical unit to create nanochips in a semiconductor fabrication facility because the window type of optical element is currently impractical in the EUV 13.5 nm scanner due to an absorption issue. Under EUV operation, the Mo/Si multilayer mirror has a significant heat load due to its reflectance of around 67% at 13.5 nm.

The mirror temperature must be maintained for the EUV scanner to function properly, necessitating machine monitoring of the mirror's temperature. Materials may use photoluminescence (PL) to relax from an excited state to the ground state if they are exposed to photo-absorption after EUV excitation. The detector or sensor is a crucial component that needs to be carefully taken into account while working with EUV radiation. Diamonds and optical windows were discovered to emit potent PL when excited by EUV or beyond extreme ultraviolet (BEUV) light, and the spectral characteristics of the PL spectra appear to be substantially temperature dependent.

About the Study

In this study, the authors used a Mo/Si multilayer mirror as an optical component in a EUV lithography process at a semiconductor manufacturing facility to create nanochips. Upon activation with light at 13.5 nm, the Mo/Si multilayer mirror emitted a strong PL in the wavelength range of 200–420 nm. This property was used to directly monitor the temperature of a Mo/Si multilayer mirror in EUV lithography since the emissions of the Mo/Si multilayer mirror were dependent on its operational temperature. Due to the inherent quality of photoluminescence, this method of monitoring a mirror's temperature could track the temperature in real-time.

The team discussed the utilization of the Mo/Si multilayer mirrors in the EUV scanner to confirm the effectiveness of the EUV PL scheme. A synchrotron-based exciting source was used to capture the photoluminescence of the Mo/Si mirror at the national synchrotron radiation research center, where the EUV light was emitted from beamline 08A1. A 45° incidence angle was used to direct the scattered EUV radiation onto the Mo/Si mirror. To prevent light from the beamline's scattering, emissions were collected close to 98° with regard to the EUV beam, and a grating monochromator with a photomultiplier tube operating in photon-counting mode was used for analysis.

The researchers produced the PL spectra after being normalized with the monochromator's calibrated spectral response. With the mirror fastened to the holder, which was encased in a homemade thermal bath to control the temperature, temperature-controlled PL spectra were observed. In the 5–90 °C range, liquid water was used as a thermal medium. With an accuracy of up to 0.1 °C, a calibrated thermocouple was used to measure the temperature. The Mo/Si multilayer mirror, which had a diameter of 25.4 mm, had a reflectance greater than 65% at 13.5 nm, and its acceptance angle for the beam was 45°.

Observations

The temperature of the mirror was accounted for by deriving the ratio between the peak intensities at bands 246 and 308 nm. At 68 and 5 °C, the band at 308 nm reported intensities of 11.87 and 6.25, respectively, with a background noise level of roughly 0.001. Over the course of 112 minutes, the Mo/Si multilayer mirror's temperature changed from around 5 to 83 °C. For the emission at 246 nm, the temperature relationship of the EUV PL seemed nonlinear. At temperatures of 5.0, 15.0, 35.0, 45.0, 26.0, 56.0, and 68.0 °C, the ratios of the intensities of the PL bands from 246 to 308 nm were 1.04, 0.87, 0.65, 0.62, 0.67, 0.59, and 0.55, respectively.

Each datum's integration time was 1 second. The emission band's maximum counting rate was over 100 000 counts per second, which was significantly higher than the background noise's rate of fewer than 10 counts per second.

The band at 246 nm was substantially more intense at low temperatures; however, the band at 308 nm demonstrated a contrary tendency of the thermal profiles when compared to the thermal trends of relative intensities of bands 246 and 308 nm. In the whole UV range, the PL spectrum was visible as continuous emission, with broadband that peaked at about 308 nm and a narrow band that overlapped at 246 nm.

For bands at 246 and 308 nm, the full widths of the half-maximum (fwhm) were around 12 and 102 nm, respectively. Thus, the zero-phonon line (ZPL) and the characteristic band at 246 nm could be related, although the broadband at 308 nm was close to a phonon sideband.

Conclusions

In conclusion, this study employed a Mo/Si multilayer mirror as an optical component in a EUV scanner to create nanochips. The authors demonstrated that when stimulated with UV light at 13.5 nm, the Mo/Si multilayer mirror emitted intense PL in the wavelength range of 200–420 nm. The results showed the potential applicability of EUV PL in a semiconductor fabrication plant. The PL emission sensitively was dependent on temperature in the range of 10–86 °C.

In order to directly assess the temperature for fabricating nano semiconductor devices in EUV technology, the team proposed using a EUV PL of a Mo/Si multilayer mirror.

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