Photolithography is a process used in micro fabrication to transfer geometric patterns to a film or substrate. Geometric shapes and patterns on a semiconductor make up the complex structures that allow the dopants, electrical properties and wires to complete a circuit and fulfil a technological purpose. Lithography comes from the Greek words lithos and graphia which directly translated would be writing on stones. Essentially, lithography is transferring a pattern onto another surface, and photolithography directly refers to semiconductor lithography.
Optical photolithography is basically a photographic process by which a light sensitive polymer called a photoresist is exposed and developed to form a 3-D relief images on the substrate.
Process Involved in Photolithography
There are multiple steps within this process in order to get to the final, usable silicon wafer product. An example of the steps involved in a typical photolithography with positive photo resist is shown here.
To begin the wafer, or substrate, needs to be chemically cleaned. Any impurities, organic, ionic or metallic, can affect the wafer negatively or affect the adhesion of the patterns. There are usually two types of cleaning done, one is RCA1 and another is RCA2. Depending on the type of impurities any type of solution can be used. After the intensive cleaning, silicon dioxide is applied to the surface of the wafer to create a barrier layer where the photoresist will be applied. The photoresist layer is applied by a spin coating method in order to be evenly thick throughout the entire wafer surface.
There are two types of photoresist and depending on the purpose of the silicon wafer one or the other is used: positive or negative photoresist. With positive photoresist, the mask contains an exact copy of the pattern which is to remain on the wafer and negative resists behaves exactly opposite of this. The negative resist will contain an inverse of the patter to be transferred. Both type of photoresist has their own advantages and disadvantages. After the photoresist is applied, a soft baking process takes place. This step is to remove all the solvents from the photoresist coating. The soft-baking process makes the photoresist layers become photosensitive or, in other words, susceptible to photo-imaging. The resist is dried in this step and adhesion is improved.
The next step is mask alignment and exposure. The pattern that needs to be transferred onto the wafer is at this point aligned with the wafer. Each mask must be aligned correctly with the previous pattern. Once the mask has been properly aligned, the photoresist is exposed through the pattern with a high intensity UV light. There are different exposure techniques: contact, proximity and projection printing. With contact, the resist is brought into physical contact with the glass photo mask. In proximity printing, there is a small distance maintained during exposure and with projection there is a larger distance maintained. Projection printing is the most common method of exposure used today because of the mask damage that can occur with the first two methods.
A photo mask is an opaque plate with holes or transparencies that allow light to shine through in a defined pattern. They are commonly used in photolithography.
Now the post exposure bake and development phases begin where first the standing waves are reduced and then the shape of the photoresist profile and line width control are determined. In these phases, the photoresist is stabilized and hardened. These steps are also important in improving the adhesion of the photoresist to the wafer surface. The patterns at this point have been transferred and then the photoresist must be stripped after the imaged wafer has been processed.
Lithography printing in semiconductor manufacturing has evolved from contact printing (in the early 1960s to projection printing.
Main application of photolithography is in IC fabrication process and computer chip manufacturing.