Experimental Photoresist Sensitometry And Exposure Modeling

Abstract Sensitometric properties were measured for a positive diazo-type photoresist at wavelengths of 365, 405, and 436 nm. It is shown that the exposure data at these three wavelengths may be combined according to Van Kreveld's additivity law to accurately predict both simultane­ ous and successive exposures made with combinations of these wavelengths. Exposure model­ ing using Beer's lav/ and first order photolysis kinetics, when combined with the measured sensitometric properties, supports the validity of Van Kreveld's law for simultaneous and successive exposures. The combined modeling and experimental data also support the exis­ tence of a critical inhibitor concentration that defines complete exposure and is the same for all wavelengths as well as for monochromatic simultaneous and successive exposures. Applications of characteristic curves of exposure depth as a function of exposure are outlined.IntroductionDimensional control and device yield are drastically affected by photoresist exposure. The control of exposure requires both the measurement of the sensitometric properties of photoresist at the wavelengths used for its exposure and the measurement of irradiance at these wavelengths. These two properties must then be combined to predict the exposure ac­ curately. 1' 2In this investigation, sensitometric properties were measured for a positive, diazo-type photoresist, Shipley AZ 1350J. Its chemistry is similar to that of other diazo-types of photoresist. All of these types of photoresist operate on the mechanism involving pho­ tolysis of a dissolution inhibitor. This dissolution inhibitor, in all cases, is based on the o-quinone diazide group.3 Thus, it follows that the reaction kinetics, the models involving these kinetics, and the behavior derived from these are equally applicable to other diazo-type photoresists as they are to 1350J."1"The exposure conditions investigated are monochromatic exposures, exposures to the simultaneous action of radiation of several monochromatic wavelengths, exposure to white unfiltered light, and exposures to the action of monochromatic radiation of several wavelengths applied successively.Van Kreveld's Additivity LawVan Kreveld's law1' 4 ' 5 states that the total exposure is the sum of the fractional contributions of the exposures at all of the individual wavelengths. The fractional con­ tribution of radiation of wavelength j to the total exposure is EjtjTj/Hcj where Ej is the exposing irradiance of wavelength j incident on a filter of transmittance Tj, tj is the corresponding exposure time, and Hcj is the corresponding exposure energy required to produce complete exposure at wavelength j. The irradiance EQJ at the air-resist interface is given by EQJ = EjTj. The fractional contributions sum to unity for complete exposure which is Van Kreveld's additivity lav; and is mathematically expressed as* This work was conducted as part of the Semiconductor Technologgy Program at the National Bureau of Standards and was supported by the Defense Research Projects Agency under Order 2397 and by NBS. Contribution of the National Bureau of Standards, not subject to copyright.**Certain commercial equipment, instruments, or materials are identified in this report for the purpose of providing a complete description of the work performed. The experiments reported do not constitute a complete evaluation of the performance characteristics of the products so identified. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the material or equipment identified is necessarily the best available for the purpose.t This statement is not to infer that the specific values measured for the resist investi­ gated are representative of all lots of this resist or all diazo-types of photoresists. Measured values of resist properties can vary from resist to resist and lot to lot, as well as among samples of the same resist when used under different processing conditions.