Phase-shift mask

Phase-shift masks are photomasks that take advantage of the interference generated by phase differences to improve image resolution in photolithography.

[2] A phase-shift mask relies on the fact that light passing through a transparent media will undergo a phase change as a function of its optical thickness.

A conventional photomask is a transparent plate with the same thickness everywhere, parts of which are covered with non-transmitting material in order to create a pattern on the semiconductor wafer when illuminated.

Attenuated phase-shift masks are already extensively used, due to their simpler construction and operation, particularly in combination with optimized illumination for memory patterns.

For example, the alternating phase-shift mask technique is being used by Intel to print gates for their 65 nm and subsequent node transistors.

Attenuated phase-shift masks also improve the image log-slope without requiring a very high exposure dose with a widened dark feature.

A schematic illustration of various types of masks: (a) a conventional (binary) mask; (b) an alternating phase-shift mask; (c) an attenuated phase-shift mask.
Left: the real part of a plane wave traveling downward. Right: the effect of introducing in the path of the wave a transparent mask with a 180° phase-shifting region. (The illustration on the right ignores the effect of diffraction which increases in significance as the wave propagates.)
Phase-Shift Mask Types: (1) Binary mask, (2) Phase Shift mask, (3) Etched Quartz mask (Levenson mask), (4) Half-tone mask. (Top) Mask, (Red) Light Energy/Phase on Mask, (Blue) Light Energy/Phase on Wafer, (Green) Light Power on Wafer, (Bottom) Resist on Silicon Wafer
Plot of scattered light (normalized to incident light) as a function of the phase of a phase edge.