Linewidth Measurement By Low Voltage SEM

Abstract

The reduction of integrated circuit dimensions to the submicron range has necessitated the use of low voltage scanning electron microscopy (SEM) for linewidth metrology 1,2. SEM offers advantages of higher resolution and increased depth of focus relative to optical linewidth measurement systems. Low voltage SEM offers a nondestructive method to acquire precise linewidth measurements from features too small for optical systems. In SEM metrology, a primary electron beam traverses the sample surface and the interaction results in a variety of electron signals3. Imaging of wafer topography in the SEM is accomplished by collecting secondary electrons which are emitted from depths of less than about 10 nm of the sample surface. The secondary electron signal is transformed into a video signal as an intensity distribution to be displayed on a CRT. To perform a linewidth measurement, a threshold is selected to determine the distance between the edges of the video profile at that threshold. The threshold technique is critically sensitive to various SEM parameters including the primary electron energy, the beam diameter, and the defocus of the beam4. This sensitivity makes it necessary to calibrate the SEM system after most changes in operating conditions5,6,7. Thus accurate linewidth measurements require an understanding of how the profile and physical dimensions of the feature being measured relate to the video signal. Complications arise because features formed in different materials and on varying substrates result in a variety of video signal profiles. This study was undertaken for the following reasons: *Determine the correlation between low voltage SEM measured linewidth and the physical linewidth of photoresist features on different substrates. *Determine the correlation between low voltage SEM linewidth and physical linewidth for different resist profiles. *Determine the correlation of low voltage SEM linewidth with electrical linewidth measurements performed on polysilicon and aluminum features. *Determine the dependence of low voltage SEM linewidth measurements on wafer tilt and SEM defocus.