A New Autofocus Using Image Processing Techniques in Critical Dimension Measurement SEM

Fumio KOMATSU  Hiroshi MOTOKI  Motosuke MIYOSHI  

IEICE TRANSACTIONS on Information and Systems   Vol.E81-D   No.7   pp.738-742
Publication Date: 1998/07/25
Online ISSN: 
Print ISSN: 0916-8532
Type of Manuscript: Special Section PAPER (Special Issue on Test and Diagnosis of VLSI)
Category: Beam Testing/Diagnosis
autofocus,  image processing,  pattern recognition,  Z-sensor,  CD SEM,  

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We have developed a new autofocus method using image processing techniques. This method consists of two steps. The first step is the preset of an objective lens condition with the aid of the feedback of Z-sensor. Next, a hole pattern to be measured is detected using the pattern recognition. In the second step, the E-beam is shifted to the center of a hole pattern and scanned across the axis of a pattern. The exciting current of the objective lens is changed at constant intervals, where the center position of the range is the preset value of the Z-sensor. The best focus condition is determined based on the signal profile obtained by the autofocus scan. The measurement repeatability (3σ) can be achieved within 3. 9 nm. The percentage of success of 98. 7% can be realized in the present autofocus method.