Inadequacy of Radiation Measurements

Single Abstract Single parameter irradiance measurements commonly used for the determination of photo­ resist exposure times are incapable of giving unique values of either the broad band ir­ radiance from a mercury-arc lamp, or the photoresist exposure times. Resolution of these measurement incapabilities through a series of irradiance measurements of narrow, approxi­ mately monochromatic radiation bands is discussed. A basis for establishing photoresist sensitivity indices that can be used to calculate, unambiguous exposure times by applying Van Kreveld's additivity law to these narrow band irradiance measurements is suggested. The thickness variation of photoresist films is included in the method of calculating ex­ posure time. Conditions that may simplify the calculations of exposure times or reduce the number of irradiance measurements when a mercury lamp is used as the exposure source are discussed. Suggestions are made for the use of these indices as photoresist specifica­ tions, and for the real-time automatic monitoring and control of exposure times in wafer fabrication where optical density variation in the photomasks and thickness variations in the photoresist films are automatically incorporated into the control by a microprocessor. Inadequacy of Radiation Measurements The use of a detector sensitive to only broad-band radiation (for example, 300 to 500 nm) to measure the irradiance and to calculate the photoresist exposure times is common in the electronics industry. It is not generally recognized that this type of single param­ eter, detector photocurrent measurement is incapable of measuring a unique value of the total actinic irradiance. This may be seen from the following discussion. The irradiance E(W cm"2)** is the total radiant power per unit area incident upon the detector surface and is given by 00