3. Exposure

3. exposure

Exposure

The exposure is performed with standard equipments for electron beam lithography and is based on the principle of direct writing or raster scan shaped beam procedures. Due to the use of electrons with very short wavelength for resist exposure, an excellent resolution of up to 2 nm can be obtained (point beam).

For mix&match procedures with resists of the AR 7000 series, in addition exposures with i- and g-line steppers or with contact exposure systems in the respective spectral UV working range can be realised.

All values for the exposure sensitivity specified in our product information are only guideline values determined for our standard processes and have to be confirmed accordingly in own experiments.

Alone the difference of sensitivity between silicon wafers and mask blanks is considerable - (PMMA mask: 15 μC/cm2 – PMMA wafer 80 μC/cm2), and also the acceleration voltage influences the sensitivity to a large degree. The higher the voltage, the more insensitive will the resist react.

The exposure dose (dose to clear) which is required to develop a large area of an e-beam resist without structures in a suitable development time (film thickness dependent, 0.5 μm: 30 – 60 s) should be increased by 10 – 20% for normal structural patterns. To obtain maximum resolution however, even higher exposure doses are required. The time required for complete development of unexposed areas (TCD) of 0.5 μm negative resist films should be in a range of 30 – 40 s. The exposure dose which produces a layer buildup of > 90 % should accordingly also be increased by 10 – 20 % for patterning purposes. If a shorter TCD is chosen (use of a stronger developer), the sensitivity is reduced since the crosslinking rate is increased at a higher exposure dose.

Coated and tempered e-beam resist films can be stored for several weeks prior to exposure without quality loss. PMMA layers are even more stable and can be stored more or less indefinitely.