Excimer laser damage testing of optical materials

ABSTRACTPhotolithography technology is moving towards deep UV wavelengths in order to obtain smaller feature sizeson integrated circuits. Optical glasses are being replaced with fused silica because of the DUV application. Thereare concerns that the short pulse, high instantaneous power generated with excimer lasers can cause structuralchanges in the fused silica materials. The damage includes: changing absorption, observed fluorescence, andinduced stress birefringence. This paper will address the laser damage testing protocol for fused silica. We willdiscuss the setup, equipment, and procedures for laser damaging. Precision measurements of the absorption,fluorescence, and stress birefringence are required to have a good understanding of the laser damage. Thepaper will also address the equipment problems associated with these measurements and initial comparisonsbetween 193 nm and 248 nm laser damage.1.0 INTRODUCTIONThe need for sub-micron feature sizes in computer chip manufacturing is driving the light sources into the DUVwavelengths. In the past optical glasses have been used in the stepper optics. As the stepper has evolved fromMercury lamp g-line (436 nm), h-line (405 nm) and i-line (365 nm) it has pushed optical glasses to have improvedabsorption coefficients. With the advent of the excimer laser it has become the light source of choice for thephotolithographic market. Since excimer lasers generate short nano-second pulses of light the instantaneouspower is very high on the order of 10-15 MW/cm2. This allows fast development ofthe photoresist and improvedthroughput. The material needs for the stepper market include low optical absorption, low stress birefringence,high index homogeneity, laser damage resistance, and low fluorescence. These are the reasons fused silica hasbecome the material of choice for the manufacture of DUV optics. But at the same time, this gives the fused silicamanufacturer a significant challenge to develop material that will not damage during the life of the stepper system.The glass lens will be exposed to billions of pulses of excimer light. Therefore test methods must be developed tocharacterize the material and predict glass performance. This challenge extends beyond the glass manufacturerinto the equipment required to make the critical measurements to evaluate the material during and after exposureto the laser light. This paper will focus on the experimental setup and testing protocol for evaluating fused silicamaterials. Although the main goal for this development is focused towards the photolithographic industry, manyother applications will benefit from this research such as excimer optics, photoablation, UV laser optics andmedical applications.Corning was the first to manufacture fused silica, starting in 1952. Since then significant improvements havebeen made in the formation of large optics, low inclusions and improved homogeneity. One of the key facilities setup within Corning to improve the DUV material for future applications is the Excimer Laser Lab.2.0 EQUIPMENTThe laser laboratory is set up for the dual purpose of measuring fluorescence and laser damage testing offused silica. The equipment consists of four main components as shown in Fig. 1 . This equipment consists of ahigh average power excimer laser (Lumonics LTD.), a power meter to monitor laser performance (Coherent LaserGroup), a joulemeter for measuring pulse-to-pulse laser stability (Molectron Detector, Inc.), and aspectroradiometer for measuring the fluorescence response to the laser (Optronics Laboratories, Inc.).