X-ray mask fogging by electrons backscattered beneath the membrane

Abstract

Proximity effects can be reduced when exposing x‐ray mask membranes by increasing the electron‐beam accelerating voltage. Forward scattering in the resist is reduced and exposure due to backscattered electrons is also reduced since the more energetic electrons are able to pass through the electroplating base and thin mask membrane. However, electrons that pass through the mask can strike an object down stream from the mask, backscatter, and produce a broad background exposure or ‘‘fogging’’ in the resist. Under some conditions, fogging can cause >40% dose variations over the mask, degrading the line width control severely. We investigated this fogging effect for different electron trap materials at various electron energies. Our results indicate that dose variation from fogging can be reduced to <3% by placing a carbon mat downstream from the membrane. The effect of fogging has been further attenuated to <1% by using a textured, low Z material (Aluminium Honeycomb) to trap (absorb) the electrons that are transmitted through the mask. This paper will discuss the magnitude and distribution of backscatter fogging as a function of the absorber’s composition, texture, and distance from the membrane.