with an array of holes, exposing the GaAs in the shape of fractal stars. Field emission

microscope images showed that the lithography was successful in creating an array of

fractal stars in the resist. It is important to be aware of the potential for overexposure or

underexposure with electron-beam lithography as both situations influence the final

pattern. If overexposed, the features become less defined around the edges.

Underexposure results in incomplete features or possibly no features at all because the

resist cannot be removed when developed. In both cases the dosage can be adjusted to

alleviate the problem; decreased if overexposed, increased if underexposed. The typical

appropriate dose for these patterns was around 190,000 tC/cm2.

 Gold is then deposited onto the wafer filling in the areas where the resist was

removed to form gold stars. This was done using a metal evaporator to achieve a uniform

layer thickness (Figure 13). The gold is heated by a current until it begins to evaporate

and deposit onto the wafer held above. A layer thickness of 100A was deposited at a rate

of lA/s at 8x 10-6Torr. To remove the remaining resist, the wafer is soaked patterned side

up, in an acetone bath for 30 minutes or until the PMMA resist has lifted off. It is then

rinsed with isopropanol to prevent contamination from particulates in the acetone. Left

behind is an array of gold fractal stars approximately 100A thick with 100tm between

each star on the GaAs wafer substrate. The array of gold stars on GaAs was observed

with an optical microscope (Figure 14).

 During gold deposition, there is the potential for sample contamination. The

contamination most likely comes from the other materials that have been evaporated in

the chamber, deposited on the chamberwalls, and either flaked or re-evaporated off the

walls onto the sample. To reduce this type of contamination, it may be desirable to do a