Acoustic resonant imaging technique for characterization ofphotoresist properties depending on hard bake temperature

Photoresists are essential materials in semiconductor fabrication, specifically in lithography, where their physical and chemical properties influence pattern generation. However, their light-reactive make property measurements challenging. We investigated the effects of hard bake conditions on photoresists using an acoustic resonant imaging technique. This technique is based on acoustic resonance, where sound passing through a thin layer reaches extreme values in its transmission and reflection coefficients at a resonant frequency. With plotting these acoustic properties in two-dimensional, in the case of photoresists, this method allows for the identification of regions with distinct properties, facilitating optimal alignment for mask positioning and direction. This study contributes to the advancement of non-destructive characterization methods for photoresists. This study examines the impact of hard bake temperature on photoresist film properties. By transmitting ultrasonic waves into a silicon substrate with a photoresist film and analyzing the reflected echo waveforms to determine the resonant frequency and the maximum amplitude ratio. Then plotting these properties with acquired echo position, properties distribution of photoresist films are obtained. The density of photoresist film showed a trend of increasing with higher bake temperatures, likely due to the evaporation of remaining solvents in the film. Interestingly, around 160°C, the density exhibited a temporary decrease before resuming an upward trend. This anomaly is hypothesized to result from a reduction in the diazonaphthoquinone (DNQ, sensitizer in positive photoresist) concentration, leading to a decrease in density at specific temperatures. As the temperature rises further, thermal crosslinking begins, causing the density to increase again. Other studies suggest that higher bake temperatures initially reduce dark erosion—an effect where unexposed photoresist regions are eroded during development—until reaching a temperature threshold where erosion increases slightly before decreasing again. This observed trend suggests a potential inverse relationship between density and dark erosion, as higher-density films may exhibit greater resistance to developer-induced erosion. This study provides new insights into the effect of bake temperatures on photoresist density, which may help optimize fabrication parameters for improved material performance.