BEAMETR

BEAMETR is a technique to automatically measure electron-beam size. Performance of any e-beam system greatly depends on beam size. Frequent monitoring of beam size is necessary. Commonly used “knife edge” technique is time consuming, not accurate, and is operator dependent. BEAMETR enables beam measurement to be automated. The product includes a piece of silicon with fabricated nano-pattern and software. The method uses spatial frequency analysis of a pattern with known (designed) characteristics.

Using BEAMETR technique, the operator scans the pattern; the software reads the image and determines the beam size. While the math is fairly complex to make this method work, the procedure is ultimately simple for end users. A specially designed and fabricated test pattern uses lines and spaces of variable pitch. Their spatial frequency spectrum is known. The SEM image of the pattern has changed spectra depending on the beam used to make the image. The comparison of designed pattern and SEM image of the pattern allows for the extraction of beam size.

The central part of the pattern is shown in the Figure below. The minimum feature size is 35 nm.

Pattern

In e-beam systems like defect inspection or electron beam lithography the pattern can be installed on the stage permanently, while wafers can go in and out. This makes beam size monitoring available at any time when needed.

The analysis uses an advanced mathematical model to examine signal spectra. The output of the software is the beam sizes in x and y. The task itself is an ill-posed mathematical problem. Tikhonov regularization is involved to solve the problem. The algorithm also involves

Automatic pattern recognition
Calibration of magnification
Can work with distorted images
Noise and jitter reduction technique

Examples of images at various beam size:


Beam size 4 nm	Beam size 31 nm