<title>Microlithography Techniques Using A Microwave Powered Deep UV Source</title>

Abstract

Microlithography Techniques Using a Microwave Powered Deep UV SourceJohn C. Matthews, Michael G. Ury, Anthony D. Birch and Mitchell A. LashmanFusion Systems Corporation12140 Parklawn Drive - Rockville, Maryland 20852AbstractA number of multilevel resist processes are being developed to solve the difficult prob-lem of producing submicrometer circuit features over profiled surfaces. The applicationof a novel illumination system, comprising a microwave powered source and catadiopticoptical system, to accomplish the pattern transfer step through planarization resists bydeep UV flood exposure is characterized. The microwave powered lamp generates 114 wattsof deep UV (200 - 260 nm) with an efficiency of greater than 9%. Data showing an effec-tive bulb lifetime of 500 hours or more is presented. Optical filtering techniques aredescribed which effectively attenuate energy above 245 nm and result in excellent criticaldimension control. Enhancing areas of the deep UV spectrum by using microwave poweredbulbs with additive materials such as cadmium is discussed, and spectral data presented.Problem areas for successful use of the PCM process in production are defined and solutionsare discussed. Finally, extensions of microwave powered deep UV lamp technology to otherareas of microlithography are discussed.IntroductionMicrowave energized bulbs are efficient and reliable sources of deep UV providing nearlyconstant power and spectral output over the life of the bulb. Bulb lifetimes are warran-tied for 500 hours. They are therefore ideally suited for production applications wherehigh deep UV intensity is required, process parameters must be critically controlled, andbulb replacement costs kept low. The use of a microwave energized source in a deep UVflood exposure system for the portable conformable mask (PCM) process enables full produc-tion capability with one micron feature resolution. This advances production -based featureresolution capability by approximately two years over previous estimates.' Reliable, high -intensity deep UV sources are needed in other areas of semiconductor fabrication as well,including UV hardening of resists. UV hardened resists are more stable in high throughputplasma etch and ion implantation processes and can maintain a stable edge profile in hightemperature hard bakes.Microwave powered bulbThe bulb consists of a quartz sphere 19 mm in outer diameter with less than 1 mm wallthickness. It is attached to a quartz stem terminated with a stainless steel holder. Thebulb is filled with argon, mercury and relatively minute quantities of additives for plasmastabilization.2 Microwaves from a magnetron operated at 1400 watts output power are beamedinto a spherical cavity in which the bulb is mounted (see Figure 1). The microwaves heatthe argon, which in turn vaporizes the mercury, producing a pressure of 1 to 2 atmospheresin the bulb. Full UV output intensity is achieved within 2 -3 seconds. Approximately 85%,or 1200 watts, of the microwave energy is effectively coupled to the bulb. About 275 wattsare released as energy in the UV range (200 - 400 nm), 225 watts in the visible light range(400 - 700 nm), and the remainder as radiated and convected heat.The bulb is rotated under air jets inside the cavity, maintaining the bulb surface tem-perature between 800 °C and 900 °C. Additional cooling for the reflector cavity, magnetron,and other system components is provided by low pressure air drawn through the system. Thereflector cavity opening is covered with a fine metal mesh screen for blocking microwaveswith over 92% free area for transmission of ultraviolet and visible light.Bulb and system propertiesIn order to establish a uniform and reliable method of measurement of bulb power in thedeep UV range, spectral data are gathered usina a Jarrell -Ash 1/4 meter monochromator, aphotomultiplier, a picoammeter and a microcomputer.3 The microcomputer provides correctionfactors at each wavelength. The system is calibrated using an NBS traceable deuteriumsource. Data gathered by this method are estimated to be accurate to within 12%.The output spectrum from a typical new bulb is shown in Figure 2. The cumulativeirradiated power from 200 nm to 260 nm is over 114 watts. At an input power of 1200 watts,this demonstrates a conversion efficiency of over 9%. A typical bulb with 500 operating