Besides the traditional chrome on glass mask (COG) there are various types of photomasks which enhance the optical resolution of the structures. The central issue of COG masks is the diffraction of the light on edges. Thus the light will not only impact in perpendicular direction but will be deflected into areas which must not be exposed.
Spectral intensity of a COG mask
With different means which are described in the following, the intensity of the diffracted light should be reduced.
Attenuated Phase Shift Mask (AttPSM)
The attenuated phase shift mask (AttPSM; also half tone mask) uses a patterned layer of molybdenum silicide (MoSi) which represents the structures of the circuit. The molybdenum silicide has a thickness which causes a phase shift of the transmitted light of 180°. Thus the phase shifted light and the radiation which transmits through glass only interfere destructively. In addition the molybdenum silicide is dense (6 % or 18 % @ 193 nm wavelength). On the one hand the light is attenuated and on the other hand the light waves which are in opposite phase erase each other almost completely, this results in a higher contrast. A chrome layer can be added to areas which are not used for exposure to mask unused regions. This photomasks are named tritone masks.
Spectral intensity of a Attenuated Phase Shift Mask
Phase shift through molybdenum silicide
Chrome less Phase shift mask
Chrome less phase shift masks don't use opaque films. The phase shift is achieved by trenches which are directly etched into the glass substrate. The manufacturing of these masks is difficult, since the etch approach has to be stopped in the middle of the glass. In contrast to etch processes where one layer is completely etched till the layer beneath is reached - which causes changes in the etch plasma, so that one knows when the process is finished -, there is no indication when the exact depth in the substrate is reached.
Alternating Phase Shift Mask (AltPSM)
The alternating phase shift mask also uses trenches which are etched into the glass substrate alternating to non-etched areas. In addition there are areas which are covered with a chrome layer to decrease the intensity of radiation in this regions.
Spectral intensity of a Alternating Phase Shift Mask
However, there are regions with an undefined phase shift, so that one has to exposure twice with different masks. One mask contains the structures which run in x-direction, while the second mask contains the patterns which are orientated in y-direction.
Desired structures on the wafer and conflict on the mask