|The post exposure bake PEB (performed after exposure, but before development) can be applied above the softening point of the resist without destroying the structures to be developed due to the still closed resist film. Possible reasons for a PEB (typically performed at 110°C for 1-2 min on a hotplate) are:In chemically amplified resists, the PEB catalytically performs and completes the photo reaction initiated during exposure. The AZ® and TI resists distributed by MicroChemicals® do not belong to chemically amplified resists, and therefore do not require a PEB for this purpose.A PEB performed near the softening point of the photo resist reduces mechanical stress formed during softbake and exposure of especially thick resist films due to the expanding nitrogen and therefore improves resist adhesion and reduces underetching in subsequent wet chemical etching. However, a certain delay between exposure and PEB is required to outgas N2. Otherwise, during PEB the N2 in the resist will expand and increase mechanical stress in the film!The PEB promotes the thermally activated diffusion of carboxylic acid formed during exposure from the photo active compound. This diffusion step smoothens the spatial periodic pattern of carboxylic acid having their origin in standing light waves during monochromatic exposure especially in case of highly reflective substrates. These patterns otherwise would transfer to the resist profile thus e.g. reducing the spatial resolution of the resist and the desired aspect ratio.
In many cases, for processes without high (<1 micron) resolutions required, the PEB is not necessary. However, for certain (negative tone) resists such as the AZ® 2000 nLOF family, a PEB is inevitable for the crosslinking induced by the exposure.|