Process Control Structures

Lines and Spaces

Theory

The lines and spaces serve two purposes. During development, the lines and spaces indicate the progress of development. As exposed photoresist develops, the lines tend to narrow and disappear while the spaces tend to enlarge. The existence and size of the lines indicate the quality of the photolithography.

The lines and spaces also monitor the etch process. All of our etches are isotropic; therefore, for each micron etched in the vertical direction, one micron is etched in the horizontal direction. The lines and spaces can be used to indicate the depth of the etch and the quality of PR development.

For example, assume that the lines and spaces are accurately depicted by the photoresist on the wafer. To completely etch a 1.5 μm thick oxide layer, we would expect to completely etch away the three-micron line, but not the four-micron line assuming that any overdevelopment was negligible. A gate designed to be 10 μm long will actually be different on your wafer due to mask-making lithography (unlikely), mask duplication, PR development, oxide etching, and lateral diffusion.

Measurement

After developing and etching each layer, note which lines and spaces are visible and intact on your wafer using the PR microscope. Also note any anomalies, for example, "the three-micron line is intact, but the four-micron line has a break in it."

Etch Control Structures

Theory

The etch control structures are similar to the lines and spaces and can also be used to monitor etches and PR development. The structures consist of adjacent boxes with spaces between the corners of 0, 1, 2, 4, 6, 8, and 10 μm. The etch control structures can be used in the same way as the lines to approximate the completeness of etch. If the corners of the boxes touch, then half of the distance between the boxes the etch has been etched.

For example, if the fourth and fifth boxes are touching, than you have etched each box laterally two microns. The fourth and fifth boxes are separated by 4 μm. Note which boxes are touching after each etch.

The aluminum etch control structure is different. The aluminum boxes overlap by 0, 1, 2, 4, 6, 8, and 10 μm since the boxes become smaller as the aluminum is etched.

Measurement

Note which boxes are touching for each level.

Surface Profilometer Structure

Theory

The surface profilometer structure is designed to be used with an stylus-type profilometer such as the alpha step in the Microelectronics Laboratory. The structure allows you to measure the oxide thickness and variations in the silicon surface. This structure will not be regularly used by ece444 students, but may be used to diagnose problems.

Questions

1. As mentioned in the notes, a 10 μm gate length designed on the mask will not result in a 10 μm gate length on your wafer. Assuming isotropic etching and the diffusion characteristics learned in lecture, what gate length would you expect on your wafer from a 10 μm gate length on the mask?
2. Assuming that one micron line widths are the minimum feature size, what is the smallest FET theoretically possible?

For additional information...

• Martin G. Buehler, "Semiconductor Measurement Technology: Microelectronic Test Pattern NBS-3 for Evaluating the Resistivity-Dopant Density Relationship of Silicon", NBS Special Publication 400-22. U.S. Department of Commerce, National Bureau of Standards, p. 33, (June 1976).
• L. B. Valdes, "Resistivity Measurements on Germanium for Transistors," Proc. IRE 42, 420-427 (1954).