Laser-induced damage threshold by high pulse repetition rate ArF excimer laser radiation

Abstract

In the semiconductor industry, optical projection lithography is employed for the production of microchips. In this process ultraviolet radiation has been used to exposure of photoresists on silicon wafers. Light sources with shorter wavelength are needed to shrink the chip size due to the diffraction limit. Pulsed excimer lasers have been used since the middle of 1990s instead of mercury lamps. At first KrF lasers (248-nm) were adopted, then ArF lasers (193-nm) have been applied to satisfy tighter leading edge device requirements. Now almost 5,000 excimer lasers for lithography tools are being operated at the world-wide semiconductor fab with stable, its availability up to 99.8%. The latest ArF excimer laser can pulse 15mJ to 20mJ energy with 6-kHz repetition rate, its typical module lifetime which can be replaced is several dozen Billion pulses. The module lifetime are expected to expand to reduce the downtime to replace. Also for precise micromachining applications, ArF hybrid laser consists of all-solid-state DUV light source as a seed laser and excimer laser as amplifier is been developed. The pulse width of this laser is typically sub-nanosecond and its high peak power is another concern for laser optics. Generally, the lifespan of optical elements has been growing to reach 100 Bpls, and its evaluation takes a very long time, typically several years. Comprehensive durability evaluation becomes more efficient by creating accelerated element tests [1]. As an alternative, accelerated lifetime testing with high fluence are helpful approach to screen and select the optics to satisfy the lifetime requirement. We have been developing the accelerated test system to determinate the laser-induced damage threshold of optical surfaces. In this paper, the test system and some results of field-approved optics with 20ns pulse duration will be explained.