Microelectronic Engineering (0305-221)
Resist Sensitivity & Contrast
OBJECTIVE
To achieve high resolution and adequate throughput, a photoresist must possess relatively
high contrast and sensitivity to exposing radiation. The objective of the experiment is to
determine the lithographic sensitivity and contrast of positive photoresists and relate
these performance criteria to process capability.
INTRODUCTION
Photolithography is the production of a three dimensional relief image based on the
exposure with light and subsequent development of a photographic emulsion or light
sensitive polymer called a photoresist. Microlithography is the lithography technique
used to print ultra-small patterns, used primarily in the semiconductor industry. The
types of radiation, materials, and tools are important to characterize a process, but the
basic steps are generally the same as those for conventional optical lithography. Here, a
radiation sensitive polymer material, called a photoresist, is applied as a thin film to a
substrate. Image-wise exposure is then given to the photoresist, usually through a mask
of clear and opaque areas on a glass substrate. Clear areas within the mask allow
exposure of the photoresist material, which photochemically alters its composition.
Depending on the photoresist type, exposure will either increase or decrease the solubility
of exposed areas in suitable solvents, or developer. A positive photoresist material will
become more soluble in exposed regions, while a negative photoresist will become less
soluble in exposed regions. This solubility differential of exposed to unexposed areas in
a resist allows reproduction of the mask image into the photoresist. After development,
regions of the substrate (usually Si or SiO2) are no longer covered by the photoresist
film. Further subtractive or additive processing can now be performed by either etching
unprotected areas or depositing layers over exposed areas of the substrate. The
photoresist, therefore, must be capable of reproducing desired pattern images and provide
protection, or resistance, for the substrate for subsequent processes.
Photoresists are generally organic materials, polymeric in nature, with properties tailored
to specific performance criteria. Resists may be classified either as positive or negative,
depending on response to exposure. In two component systems, the resist is formulated
from a base matrix resin, which serves as a binder for the material, and a sensitizer,
which provides appropriate exposure sensitivity. In addition to these components is a
casting solvent which keeps the resist in a liquid state until application, along with dyes,
plasticizers, surfactants, or other additives to tailor resist performances. Positive
photoresist accomplishes an image-wise solubility differential upon exposure through
changes produced in its sensitizer component.
The effectiveness of a photoresist for microelectronic fabrication depends on a number of
factors. Not only must a material possess proper sensitivity and resistance properties, it
must be suited to the remainder of the fabrication process.
