In microfabrication, lithography is used to create detailed patterns on a substrate. It is quite often the first step in structuring the glass or silicon wafers. Lithography steps are mostly repeated several times, and thus basically form the heart of the structuring process.
To structure a substrate, a light-sensitive layer (photoresist) is applied on the entire surface. A mask ensures that parts of the photoresist remain protected and therefore are not affected by the light. When the substrate is exposed to the light source, the exposed parts of the photoresist will react, creating the pattern in the substrate. An etching step removes the silicon in these uncovered parts, thus creating the final structure of the substrate. The structuring of bulk substrate materials or thin films using a light-sensitive layer is a robust process that is almost fully automated, which leads to a stable, fast, and accurate procedure.
Light-sensitive layers are usually deposited by spin coating, a common technique that works perfectly on smooth surfaces. If the substrate already contains a significant topography on which the photoresist has to be applied, spray coating is a better option.
There are two varieties of the photolithography process:
UV contact lithography, where a one-on-one projection of the mask into the photoresist takes place. The features cannot be smaller than 1 mu.
Stepper lithography, which uses a reduced projection of a mask. Because only a small region of the substrate can be targeted, the process takes place in ‘steps’: the substrate is repeatedly moved under the light source until the pattern is transferred onto the entire surface of the wafer. Stepper lithography also allows sub-micrometer features to be imaged.
Positive and negative photoresist
In the case of a positive photoresist, the photo-sensitive material is degraded by light and only the regions that were exposed to light will dissolve, leaving behind a coating where the mask was placed. In the case of a negative photoresist, the photosensitive material is strengthened by light and only the regions that were not exposed to light will dissolve, leaving behind a coating in areas where the mask was not placed.
A positive photoresist may seem the most straightforward method, but a negative approach has its advantages. Because the sidewalls of the trenches are never at a 90-degree angle, but always slightly tapered, this has consequences for the coating that remains. In the case of a negative photoresist, which leaves negatively tapered structures on the substrate, the sidewalls are not covered with a coating, unlike positively tapered sidewalls, which are left coated. The absence of the coating can be an advantage in further processing of the substrate.
When an even more detailed result has to be achieved, Micronit has other techniques at hand, like nanopatterning. Nanoimprint lithography (NIL) is a method of patterning on the nanometer scale. In this process, a mold (stamp) is imprinted in a polymer layer. The most important item here is the master mold. Replicas of this master mold can be used several times. With NIL, a wafer can be structured in one run, up to sub 100 nm features.
Nanopatterning is used in flow cells for DNA sequencing and applications such as LED, lab-on-a-chip, integrated photonics, high-density memory, and bio-applications.
There are more processes in this field, for example, e-beam lithography.