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CD(critical dimension) Measurement
In this process, we measure and compare the CD with customer CD specification. It is important to ensure that the actual (measured) CD value is within the acceptable range of target CD from the customer. The CD range value is typically given in terms of CD uniformity with specific statistical distribution (i.e. 3 sigma or max-min value) and it always should agree within the ranges specified by the customer.
CD could be measured either optically using Laser or using e-beam. Laser measurement utilizes the optical image created by the optical object lens and measures the distance between the right and left end of the images at a certain threshold. E-beam method utilizes the secondary electron distribution in measuring the distance at a certain threshold value. The line spacing could be measured either using the dark area (CD value over Cr or MoSiON pattern) or using the clear area (CD over Quartz).
Registration measures the accuracy of relative pattern position compared to the customer design data on the surface of the actual product photomask. The difference between the reference pattern and the actual patterns are measured in x and y direction and the difference should not exceed the registration specification. In addition, the rotation angle of the actual pattern should also fall within the customer specification.