Around the world, high-power CO₂ laser has been the main light source for laser cutting, welding, and surface treatment, including cross-flow CO₂ laser, axial fast-flow CO₂ laser, and radio-frequency (RF) slab CO₂ laser. RF-excited diffusion-cooled slab CO₂ laser has compact structure and high beam quality, which once could not be achieved by all gas lasers above kilowatt level. It has an important application in the field of plate cutting and welding and represents the development direction of CO₂ gas laser. However, with the rapid development of fiber laser, the CO₂ laser metal processing market has almost been replaced by fiber laser. Yet, in recent years, RF slab CO₂ laser has been the only one used for laser annealing of VLSI wafers, with no other laser to replace it.

Today, two types of annealing equipment are mainly used in the annealing of integrated circuit wafers. One is the traditional rapid thermal processing (RTP) equipment. The RTP equipment uses halogen tungsten lamps to heat a single wafer to 300 ℃-1050 ℃ within 1-30 s. In addition, it has been adopted by semiconductor manufacturing industry for more than 20 years. Another novel heat treatment method is millisecond annealing (MSA), which can heat the wafer to 1100 ℃-1350 ℃ (just below the melting point of silicon) in a few hundred microseconds to a few milliseconds.

MSA can be realized using two different methods: laser spike annealing (LSA) or flash laser annealing. LSA uses long-wavelength CO₂ laser beam to irradiate the semiconductor wafer at grazing angle to form a "line beam" on the wafer surface to scan the wafer back and forth. For the formation of USJ (ultra shallow junction) and nickel silicide, each method has its own challenges.

With the reduction of device size, the formation of nickel silicide is a new challenge in wafer manufacturing. Conventional formation of nickel silicide includes two low-temperature RTP steps and an optional etching step between the two steps. When the device size is reduced to 28 nm or even smaller, the nickel diffusion in the second RTP processing step will result in junction leakage and reduce the yield. A feasible solution is to replace the second RTP treatment step with MSA because the MSA treatment time is short, which can reduce the nickel diffusion. The special laser annealing device adopts short-wavelength diode laser, high-power RF CO₂ laser, and two linear spots, and the incident light is close to the vertical incidence. To remove the lattice defects and obtain a silicon wafer with perfect electrical characteristics, the temperature should be controlled to avoid overheating during the laser annealing and the temperature uniformity should be ensured, which sets extremely strict requirements for laser stability.

This paper introduced the development of RF slab CO₂ laser. It mainly focused on key technologies such as laser structure principle, area amplification, diffusion cooling, strip electrode, thermal effect, power extraction, output beam characteristics, unstable-waveguide hybrid cavity, beam shaping, RF transmission and impedance matching, RF discharge plasma, and uniformity of gas discharge.

For example, in the aspect of electrode cooling and diffusion, the electrode deformation seriously affected the discharge power injection and laser power extraction due to the large discharge electrode area and small discharge spacing. When the laser worked, the heat of the gas diffused to the electrode and was taken away by the circulating cooling water in the polar plate. The electrode length, width, and height of the 2.5 kW laser were about 1000, 200, and 40 mm, respectively. Matching inductors were inserted at the outer ends of the upper plate in the length direction to realize impedance matching and ensure uniform and stable discharge. According to the heat transfer theory, the finite element model of the electrode was established by using the creative serpentine water-cooling channel design, and its temperature field distribution cloud chart was obtained by loading and solving. The highest temperature of the whole electrode was 31.994 ℃, and the lowest temperature was 23.333 ℃, which achieved good diffusion cooling effect(Fig. 6).

The goals of developing high-power RF CO₂ laser were as follows: (1) to solve the problem of localization of high-power, high beam quality, and high-stability laser that restricts the laser annealing equipment of VLSI wafers in China; (2) to break through the key technologies such as RF slab electrode, diffusion cooling, optical resonator, RF discharge excitation, beam shaping, etc.; (3) to realize the engineering of high-power diffusion cooling slab CO₂ laser; (4) to improve the localization level of high-end industrial gas lasers in China; and (5) to reverse the situation that the RF slab laser needed for laser annealing of integrated circuits in China is completely dependent on imports.