New approach in scanning electron microscope resolution evaluation

Since the most advanced circuits on the market are drawn using 0.35 micron design rules the Scanning Electron Microscopes (SEM) is currently being used for the microelectronics field. SEM take the place of optical microscopes both for observation and for Critical Dimension Measurements (CD). The images, or signals, given by SEM are not so reliable with time as those given by optical means, and frequent settings of SEM are necessary to avoid bad image quality. A semistandardized procedure recommended by SEM manufacturers is recommended to measure the resolution: A carbon sample is covered by gold particles or by Latex Spheres to give the maximum signal contrast. The transition quality between gold and carbon evaluated by measuring the gap between two isolated particles is called the resolution. The previous method cannot give a good result in terms of both precision and repeatability. In order to overcome the drawbacks, the operator's eye is replaced by image densitometry analysis and the measurement accuracy is improved by averaging the signal: The image Fourier transform provides two dimensional spatial frequency information. The results are robust and objective and lead to a quantitative resolution and astigmatism measurement. This method is fast (30 seconds), non operator dependent, automatic and the SEM images can be monitored in line.