Extending Optical Lithography to 157 nm?

An F2 excimer laser with emission at 157 nm may be the most likely successor to the 193 nm ArF laser. While 500 mW, 50 Hz F2 lasers have been around for years as a scientific tool, in the past six months Lambda Physik (Goettingen, Germany, and Ft. Lauderdale, Fla.) has demonstrated the first industrial grade laser at 20 W and 500 Hz (Fig. 1). These numbers are comparable to current lithography 248 nm KrF and 193 nm ArF lasers, which typically have powers in the 10-20 W range and frequencies of 1 KHz and 800 Hz, respectively.

2. A 20 W output power at 500Hz is demonstrated with a 157.629 nm F2 laser

The dependence of the total output and the two separated F₂ lines at 157.629 nm and 157.523 nm to the repetition rate is shown in Figure 2. A key advantage of this laser transition is the naturally narrow spectral bandwidth. A true excimer laser, an excited dimer, F₂ has a linewidth in the picometer range (exact measurements are still under investigations), 100 times smaller than that of unnarrowed KrF and ArF lasers. A narrow linewidth eliminates much of the optics and accompanying losses involved in line narrowing. Since the short wavelength and very high photon energy are strongly absorbed by oxygen absorption bands, operation in a vacuum or an inert gas ambient is required, hence the designation vacuum ultraviolet (VUV).

The identification of a laser source, however, is only the beginning. To correct for aberrations in the stepper optics, today a combination of at least two materials with different diffraction indices are needed. However, fused silica, currently used in DUV lens systems, cannot be used at 157 nm wavelengths. The most discussed candidate is calcium fluoride. Recently some experts have discussed the possibility that the natural bandwidth of F ₂ is small enough so that only one optical material is necessary, noted Dr. Heinrich Endert, national sales manager at Lambda Physik U.S. In the midst of all this speculation, it is too early to even know whether a refractive system can still be considered or if a solely reflective optical system has to be developed.

The main task in developing a viable technology for production is the search for optical materials with corresponding optical properties, laser damage resistance and long lifetime. Resist and mask development are other key challenges. Though there is still a great deal of technology development needed, the first step, the development of a laser source, originally assumed to be the main "show stopper," has achieved a major breakthrough.

• Ultratech Stepper (San Jose, Calif.) is shipping the UltraBeam V2000 electron beam lithography system to Photronics (Brookfield, Conn.) for use in the production of advanced reticles. Implementing a vector writing strategy, an alternative to raster scanning, the V2000 is able to move directly to those areas that need to be exposed, bypassing unnecessary areas and saving valuable time. Targeted for 0.25 µm and below reticle production, the system will be installed in Photronics' Allen, Texas, facility.
  
• A 300 mm DUV stepper, Canon Semiconductor's (Santa Clara, Calif.) FPA-3000EX3L, is operational at the International 300 mm Initiative (I300I) facility in Austin, Texas. Following a performance test in which the EX3L exposed more that 1500 300 mm wafers without failure, Intel (Santa Clara, Calif.) purchased the new machine for use by I300I under a one- year contract. Information collected will be shared among all member companies. The EX3L is essentially a modified EX3 production tool with a 300 mm airbearing stage. Systems are currently in use at SELETE, a 300 mm development consortium of Japanese IC manufacturers Hitachi (Tokyo, Japan) and Siemens (Munich, Germany).
  
• Photronics (Brookfield, Conn.) completed the acquisition of Motorola's (Mesa, Ariz.) internal photomask manufacturing operations. The acquired assets include advanced lithography tools, inspection systems, metrology tools and repair tools. Photronics will continue to operate the facility in place until it can be relocated to an outside facility in the Mesa area. The two companies have also entered into a long-term supply agreement where Photronics will continue to source Motorola's photomask requirements previously provided internally.