The search is on for more sensitive materials to create images with.

Researchers in Japan have found a way to make photopolymers up to 50 times more photosensitive.

The number of applications of photosensitive materials is increasing, and the field needs to advance. Kunihiro Ichimura, from the Tokyo University of Science, believes that a breakthrough in photosensitivity enhancement will require amplifying chemical species capable of initiating catalytic or polymerisation reactions.

Ichimura argues that boosting the concentrations of photogenerated catalysis molecules, using autocatalytic decomposition, should improve a material's photosensitivity considerably. This is a similar concept to that used in silver halide photography, in which the extraordinarily high photosensitivity of silver halide emulsions comes from the autocatalytic process used to reduce silver halide. With this in mind, the team has focused on developing 'base amplifiers'. These molecules are readily decomposed by base catalysis to liberate a base, in this case an amine, which leads to autocatalytic decomposition. This process is referred to as base proliferation.

The researchers add their base amplifier to a photopolymer system known as PGMA. In the presence of a photobase generator and upon UV irradiation, PGMA becomes insoluble because a photogenerated amine reacts to form crosslinked networks. Adding the base amplifier greatly enhances the photosensitivity of PGMA because the proliferation reaction increases the number of amine molecules.

The photosensitivity of thin films of PGMA containing 9 per cent base amplifier increased by a factor of 50. 'It is reasonable to assume that the addition of base amplifiers to other types of base-sensitive polymers will enhance their photosensitivity so that they can be used in microlithographic patterning,' notes Ichimura.

Helen Fletcher