Solid State System Produces 193 nm Output
By: Jim Jacob, Photonics Automation Products Inc.
Contents
System design
CLBO and BBO
Performance
The semiconductor industry's drive towards deep ultraviolet (DUV) lithography has fostered a need for short wavelength test and inspection equipment, for stepper developers as well as for chip manufacturers. The choice of coherent sources is limited in this spectral region. At 193 nm, for instance, the market is dominated by low-power argon fluoride (ArF) excimer lasers, but these bulky, expensive systems have complicated power and gas replenishment requirements, making them undesirable for laboratory test applications. The PAP 2193 (Photonics Automation Products; Soquel, CA), a solid state laser system based on harmonic generation provides a compact, simple, and economical alternative (see Figure 1).
Solid state technology provided a natural alternative to excimer lasers. Such systems generate narrowband radiation at moderate energies in compact packages, without requiring any special gas replenishment facilities, and are environmentally more desirable. Several solid-state schemes to generate 193 nm output have been implemented to date. One approach consists of five non-linear stages: a periodically poled lithium niobate (PPLN) diode-pumped optical parametric oscillator (OPO) generates infrared output, which is up-shifted to DUV wavelengths by sum-frequency-mixing with the fifth harmonic of the pump laser.1
Another method to generate single frequency output uses an alexandrite laser that is frequency quadrupled by mixing its third-harmonic output with the fundamental wavelength of the laser.2 Finally, a high-power scheme for generating 196 nm was recently reported in which two lasers, a Nd:YLF and a titanium sapphire, are mixed in cesium lithium borate (CLBO).3
System design
The PAP 2193 system architecture (patent pending) incorporates four non-linear stages that generate up to 500 µJ of 193 nm energy in a bandwidth of less than 10 pm. This is accomplished by sum frequency mixing in beta barium borate (BBO) (see Figure 2). The output of an optical parametric oscillator (OPO) running at 703 nm is mixed with the fourth harmonic of the Nd:YAG pump laser that is quadrupled in cesium lithium borate (CLBO). The energy density sufficiently replicates ArF excimer laser so as to be a useful tool for exposure and damage testing.
We start with an OEM laser from <%=company%> (San Jose, CA) designated the Tempest-10. This Nd:YAG laser operates at 10 Hz with a Gaussian mirror output coupler that provides a low divergence beam of 5 mm dia. The beam is free of diffraction rings and hotspots for optimum OPO pumping.
The 1064 nm laser output of 200 mJ is frequency doubled to 100 mJ in a type II potassium titanate (KTP) crystal. The width of the 532-nm pulses is approximately 3.5 ns. This beam is next split (M2) into two components, one for pumping the OPO and one for quadrupling the output to 266 nm.
The polarization vectors from the pump laser throughout the optical train are arranged such that the resulting 193 nm beam is p-polarized as it passes through the final Pellin Broca dispersion prism. The OPO utilizes a type II (o-wave pump and idler, e-wave signal) KTP crystal. The interaction occurs in the x-z crystalline plane with the critical angle q set to approximately 50°.
The back mirror M6 of the OPO transmits the pump energy and totally reflects the signal wavelength of 703 nm. The output coupler M7 is 100% reflecting for the 532 nm pump and partially reflecting (30%) for the signal. The OPO is a flat-flat cavity 3 cm in length and a total energy (signal plus idler) of 25 mJ is attained.
CLBO and BBO
The CLBO crystal used for quadrupling is 8 mm long and is cut for type I 532 nm doubling at a phasematching angle of 62°. CLBO is a relatively new non-linear crystal (see CLBO Generates Higher Order Harmonics in High-Power System; ASSL '99 Postdeadline: CLBO-based System Produces 1 W of Output at 196.3 nm) that has the advantage of a much larger acceptance angle than BBO for quadrupling the Nd:YAG laser (540 µrad per cm for CLBO versus 190 µrad per cm for BBO).
In this system 15 mJ of 266 nm energy was generated or 50% of the available 532 nm energy. Mirror M4 in Figure 1 is a Nd:YAG fourth harmonic high reflector which directs the 266 nm energy to the combining mirror M5 (another fourth harmonic reflector), which also is highly transmitting for 703 nm light. The desired 193 nm radiation is generated at a phasematching angle of 76.7°. The non-linear coefficient deff for BBO at this angle is 0.58 pm/V and the acceptance angle is 240 µrad/cm. A type I BBO crystal 4 mm long cut at a phasematching angle of 76° is used for the final DUV mixer. All of the optical components after M2 are enclosed in a sealed nitrogen purged chamber to prevent any crystal degradation due to water or dust.
Performance
An <%=company%> (Acton, MA) SpectraPro 150 fitted with a 1200 line/mm grating was used to first set the OPO operating wavelength at 703 nm and then used to detect and verify radiation at 193 nm from the mixing stage. The multi-spectral output radiation from the mixer (703, 266, and 193 nm) was dispersed with a fused silica (Suprasil) Pellin-Broca prism. Mirrors M5 and M4 were adjusted for the best overlap of the quadrupled beam with the OPO signal beam inside the mixer. The 193 nm output is directed out of the enclosure by mirror M7, a special DUV high reflector.
A resulting energy of 600 µJ was measured with a pyroelectric detector. The resulting linewidth is assumed to be less than 10 pm, based on the theoretical bandwidth of one cm-1 per cm of interaction length in the BBO sum frequency generator. The 2 mm DUV beam diameter provided an energy density of 15 mJ/cm2.
Actually substantially more energy could be derived from this system. Using a two-dimensional mixing model in the SNLO non-linear optics program,5 we calculated a value of 1.5 mJ as the theoretical 193 nm energy given optimum experimental conditions (i.e. no phase mismatch and no linear absorption). The reduction in energy by a factor of three is due to a combination of the divergent OPO signal beam mismatching the narrow acceptance angle of the BBO mixer in addition to the linear absorption at the DUV wavelength. By introducing a DK of 0.7 per mm to account for the phase mismatch caused by the divergence of the signal input optical field and adding a linear loss term of 0.1 per mm at the 193 nm DUV wavelength, a value of 633 µJ is calculated which is close to the value measured.
Acknowlegements
The author would like to thank Dr. K. Kato for the initial CLBO crystals, Alpine Research Optics for the DUV mirror and New Wave Research for modifying its Nd:YAG laser mode for OPO pumping.
References
1. R. Mead, Proceedings of 3rd Intl. 193-nm Conf., Japan, 1997.
2. W-B Yan, T.F. Steckroat, R.A. Frost, J.C. Walling, and D.F. Heller , Conference on lasers and Electro-Optics, paper CFF5, OSA Technical Digest Series 17, 485 (1997).
3. J. Sakuma, A. Finch, Y. Ohsako, K. Deki, M. Horiguchi, T. Yokota, Advanced Solid State Lasers Conf., Paper MF5 (1999).
4. K. Kato, IEEE Journal of Quantum Electronics, 22, pp. 1013-1014 (1986).
5. SNLO nonlinear optics code available from A. V. Smith, Sandia National Laboratories, Albuquerque, NM 87185-1423.
About the author…
JIM Jacob is president of Photonics Automation Products Inc., 2521 South Rodeo Gulch Rd., Soquel, CA 95073. Phone: 831-479-0837; fax: 831-479-0857; e-mail: jjj@photons.com.