Ultrasonic Sensor for Photoresist Process Monitoring
In current I-line and deep-UV lithography, in situ monitoring of photoresist processing is not required to meet design objectives. However, as feature size shrinks below a quarter micron, it is expected that the resist materials will be more sensitive to processing conditions during pre-exposure bake and development. An in situ ultrasonic sensor has been developed to monitor photoresist processing during semiconductor manufacturing. The measurement principle of the sensor involves monitoring the acoustic reflection coefficient of a high frequency longitudinal wave incident on the interface between silicon and a thin resist film. Photoresist pre-exposure bake and development were monitored for I-line and chemically-amplified resists. Results of pre-exposure bake measurements demonstrate the ability of this nondestructive sensor in measuring the in situ glass transition temperature of the resist film as well as its capability in monitoring changes in the elastic properties of the film. The glass transition temperature (Tg) is an important parameter in both the pre- and post-exposure bakes and could therefore be useful in monitoring these processes. Development monitoring was achieved by measuring the phase change of the reflection coefficient as the resist was removed. Resist thickness changes and development rates were easily determined from the phase data, providing information required for process endpoint determination. The multiple uses of this sensor make it convenient for integration into a manufacturing setting.
