Grayscale lithography creates 3D film profiles with gradually varying thickness1, see Fig.1. The grayscale lithography is especially advantageous for the micro- structures where reflow patterning2 does not produce the desired result, e.g.:
Applications with high structure density, where adjacent reflow patterns could merge;
Complex shapes such as random microlense arrays;
Additional patterns on bigger lenses;
Microlenses with different heights, or patterns for diffractive optics;
Other free-form profiles used for e.g. light-beam-shaping and steering, MEMS, MOEMS. etc.
REQUIREMENTS TO PHOTORESISTS FOR GRAYSCALE TECHNOLOGY
A standard positive photoresist exhibits an inhibition at low doses, i.e. the solubility in the developer (trans-
lating into the remaining film thickness after devel- opment) increases only after the exposure to a certain minimum dose. Above this dose, a standard photores- ist shows a very steep decrease in thickness after development, the resulting thickness depending on the exposure dose in a non-linear fashion (Fig. 2).
- An ideal resist for grayscale lithography should have:
- A more linear response than a standard photoresist;
- A smaller contrast than that of standard binary resists to allow true grayscale response;
- Yet, the response curve should be sufficiently steep to avoid long exposure times;
- None or negligible initial solubility inhibition;
- Low residual absorption to enable exposure and development to the bottom even in very thick films.
Furthermore, due to the comparatively narrow pro- cess window, especially in grayscale mask litho- graphy, the resist should have a high batch-to-batch reproducibility of the response curve.
Direct laser writing often uses high laser intensity during exposure, which can cause an „explosive“ N2 release from the resist. A resist suitable for direct laser writing in grayscale mode should have a strongly reduced tendency for this kind of outgassing during exposure.
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