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SOPRA: an in-line monitor for process optimization and verification

The SOPRA multilayer monitor (MLM) is an automatic, clean-room compatible, high throughput, optical monitoring instrument based on multi-channel spectroscopic ellipsometry. The machine is capable of measuring, non-destructively, up to 5 layers simultaneously on patterned product wafers, providing information on thickness, crystallinity, composition, refractive index and roughness, together with uniformity mapping.

With the trend to larger wafer sizes and single-wafer processing, it is becoming more important to reduce the reliance on monitor wafers, which are not always representative of the product wafers. In-line monitoring, which can be applied at various process stages on actual product wafers, must be automatic, reliable, reproducible, non-contaminating, and have high throughput. If this can be achieved, metrology will be perceived to add value and provide a return on investment, since expensive test wafers and wasted product and processing time can be eliminated. The Semiconductor Equipment Assessment (SEA) Project 'IMPROVE' (In-Line Monitor for Process Optimization and Verification) is evaluating the performance of the SOPRA MLM. The MLM has robot wafer handling and pattern recognition software with a small spot (about 100 mm) to allow spectroscopic ellipsometry measurements to be made in scribe lines or test pads on patterned product wafers. Representative key complementary metal oxide semiconductor(CMOS)/bipolar composites supplied by the integrated circuit (IC) manufacturer partners, were assessed in both unpatterned and product wafer forms. Trials were conducted to assess throughput, stability, sensitivity and accuracy by comparison with corroborative techniques.

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