MASK INSPECTION USING FAST CHANNEL

  In the mask inspection application, a circular mask used in IC fabrication is inspected for contamination and damage. The inspection apparatus contains one or more line scan cameras which are swept over the surface of the mask imaging the surface in stripes. The mask is lit in a dark field configuration so that defects scatter light into the camera resulting in bright spots in a background of black.

The optics have an optical resolution of 0.5 um. The line scan camera outputs data on 4 taps at up to 40 MHz using LVDS signaling. Up to two cameras can be provided so that the peak data rate is 320 MB/s. The masks being inspected are 200 mm in diameter, which when imaged present over 500 gigabytes of data.

Data Processing

Image data is via the following steps:

• The data is corrected for a flat field. (Offset and gain correction on a per pixel basis)
• The data is filtered with a 3x3 median filter to eliminate shot noise
• The filtered image is compared to a threshold and if the threshold is exceeded, the image is processed further
• If the threshold is not exceeded, the image data is discarded

The image is processed as it comes in so that only the data of interest is retained. Sub-images are extracted from the image data to characterize the defect. This processing occurs very infrequently and does not effect the processing bandwidth required significantly. The TM1300 can perform these tasks at an average pixel rate of 81.8 mega-pixels per second. Image data can be provided to the TM1300 processor via two routes - its PCI bus or the digital video input from the FastChannel.

Inspection Time vs. Data Rate

If two cameras are used, then 4 TM1300 processors are required to process the 320 MB/s input data rate. This data rate is too high to be transferred over the PCI bus, but can be transferred over the FastChannel at 80 MB/s. The TM1300s are able to accept data at 80 MHz, or 80 mega pixels per second, and is able to process that data slightly faster than that. At 80 MB/s, one TM1300 processor is all that is required. Below 133 MB/s the PCI bus could be used to transfer the data, but the FastChannel provides the additional benefit of isolation of this transfer from the host CPU.

The table below illustrates how lower data rates significantly increase the time needed to inspect the mask. At the highest rate, the mask can be inspected in less than 1/2 an hour. At the slower data rates it can take over 31/2 hours to inspect the mask. The cost of the higher data rate, due to additional processors and camera required, is offset by the savings obtained from the shorter mask inspection time.

Due to the fact that many masks must be inspected (sometimes over 10 masks), the high data rate allows that to be done in less than a day while the lower data rates may take almost a week, when setup and tear down time is considered.

The higher bandwidth of the FastChannel provides a significant savings over using the PCI bus in this data intensive application, even though additional CPUs and cameras are required to achieve the higher data rates.

As can be seen from the figure above, using the PCI bus for I/O limits the number of useful processors to 2. With the addition of the FastChannel, the increased I/O band-width allows the use of 4 processors, improving the performance by over a factor of two when compared to that obtained with the PCI bus. Adding an additional board with its FastChannel I/O allows an additional factor of two improvements in performance. Additional performance improvements, in this application, require the addition of cameras.

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