Pre-develop bake for end point stabilization with photo-BCB polymers

The develop end point for films of photosensitive BCB (Cyclotene 4000 series) resins drifts to longer times as the time delay between soft bake and develop increases. Unless properly compensated for, changes in the develop end point will lead to variation in the extent of overdevelopment from one processing lot to the next. This in turn can lead to variation in film thickness, resolution, and film quality. Methods for maintaining a uniform degree of overdevelopment from lot to lot include management of time delays and monitoring of develop end points. Both of these procedures are undesirable from a manufacturing standpoint, and thus another means of managing the develop end point was desired. It has been found that a mild hot plate bake immediately prior to develop resets the develop end point to its initial value, making it independent of the delay time. This resetting of the develop end point can be explained in terms of free volume in the film. Aging at room temperature causes a decrease in free volume and hence a decrease in dissolution rate; a bake before develop restores the initial free volume state. The photo-BCB process sequence is described in detail, with emphasis on the develop step and on the use of a pre-develop bake to eliminating the time dependence on this step. Optimum bake temperatures for different film thicknesses are presented in this work. The use of a pre-develop bake simplifies photo-BCB processing eliminating the process variation due to time delays between the processing steps, and improving its manufacturability.