Alignment marks on the substrate is used in E-beam lithography to enable consecutive patterns to be aligned with the required sub-micron or nanometer accuracy. These marks can be of various design but their position must be close to the expected position for the automatic mark detection system to find them and measure their positions.

Experimental or non-standard wafers mounted in the cassette often have considerable and unpredictable placement errors. The SEM-mode of the E-beam must then be used to manually search for and find the marks. Once found, their positions relative to the coordinate system of the E-beam tool must be measured. There are 3 problems involved in this procedure:

• The field of view in SEM-mode is very limited, typically only 0.5-1 mm, so finding the marks can be difficult.
• The electrons used for viewing will expose the photoresist and can damage the substrate.
• The rotation error of the sample mounting must be within the small range of what the E-beam can adjust for. If too large, the cassette has to be removed from the E-beam and the sample readjusted, which is a very time-consuming operation.
  

How PAMS solves these problems
After mounting the sample in the cassette, the cassette is placed on the PAMS for optical non-destructive inspection with a large field of view and with positional measurement and registration that mimics the coordinate system of the E-beam.

The actual mark position is compared with the nominal mark position and the offset extracted in a format to be entered directly into the job description files - enabling immediate exposure without further realignment.

The rotation of the workpiece is also measured to verify that it is within the permitted range. If too large mounting errors are found, the workpiece position and rotation can easily be readjusted in the PAMS. A final control measurement will guarantee success.

This saves much time compared with the very time consuming process of removing the cassette from the E-beam for readjusting the rotation of the sample.

System description
The PAMS has a 300 mm transmission microscopy stage with digital encoders, a computer interface and a computer controlled camera with a wide-range zoom. The cassette is mounted upside-down and viewed from below. The workpiece is held by gravity force to enable simple adjustment prior to locking it.

Special software compensates for differences between the E-beam stage encoders and the microscopy stage encoders. The software also uses the same terminology as the E-beam software to simplify the extraction of offset parameters for exposure job control files.

The PAMS system has been developed and used extensively in an E-beam user environment using the JEOL JBX-9300FS and JBX-5D2 and has already proven itself as an invaluable tool to reduce E-beam exposure setup time.