Photolithography refers to a binary image transfer process that can be used to enhance many microfabrication applications.
The fundamentals of preforming photolithography consist of the following procedures:
- Surface Cleaning
- Spin Coating
- Soft Baking
- Exposure & Masking
- Post-Exposure Bake (PEB)
- Hard Bake
One of the most important steps in preforming photolithography is surface cleaning. Attaining a clean surface is essential towards creating quality microfabricated surfaces. Particulates that reside on a substrate can cause unwanted defects in the final resist pattern. Additionally, film contamination can cause poor adhesion. Often materials are simply rinsed with an alcohol solution and consequently dried. Piranha also serves as a glass cleaning solution. However, due to the presence of an acid, this method of cleaning is less desirable.
Spin coating is conducted through a spin coater with a spin coating chuck. The spin speed determines the photoresist, a light sensitive material, thickness. A photoresist consists of a resin, sensitizer, adhesion promoter, and a thinner. There are two types of photoresists, positive and negative. Positive photoresists become easier to etch when exposed to light while negative photoresists are more difficult to etch when exposed to light. Each photoresist has its own corresponding spin speed curve that can be used to determine a resulting thickness.
When depositing a photoresist onto a substrate, either static or dynamic dispense can be performed. During dynamic dispense, the substrate is in motion during dispensing. Static dispense occurs when a photoresist is applied at the center of a substrate prior to movement. The type of dispense depends on the desired application.
Following spin coating, a soft baking step serves to reduce the solvent content in order to stabilize a resist film. Soft baking is primarily used to prevent delamination by enhancing adhesion. The soft bake time determines the evaporation rate which can cause defects if preformed too quickly or too slowly. Bubbles can form if the soft baking step is done for prolonged periods of time and a sticky film can form due to under baking. Determining an adequate soft bake time is very important during the photolithography process.
Exposure & Masking
An exposure system is used to emit a specific wavelength of light needed to activate the photoactive compound in the photoresist. The time in which a substrate is subjected to exposure is used to create desired dimensions of both macro- and micro-structures. It is important to note that photoresist properties can vary with temperature. Therefore, over-exposure results in smaller resist patterns while under-exposure causes wide structures. A shorter wavelength leads to smaller features.
Masks are used to create a desired pattern onto a substrate. This can be done through contact, projection, or proximity printing. As the name suggests, contact printing occurs when a mask is directly placed onto a substrate prior to exposure. This method is used to create larger features. Projection printing involves a lens that allows for better feature resolution. Proximity printing is when a substrate is kept at a specified distance from a mask, creating a proximity gap.
A post-exposure baking step is necessary for preventing film flaking and reducing the standing wave effect. Film flaking can be reduced or prevented by adjusting the solubility of the polymer resin present within the photoresist. The presence of standing waves can cause linewidth control to become more difficult. PEB helps to equalize the max and min values in the photoresist sensitizer concentration, creating a uniform rate of development.
Development involves submerging a photoresist coated substrate into a developer solution. While in the developer, unpolymerized photoresist are washed away to create a final resist pattern. This process may need to be done a few times to ensure features are properly established onto the substrate.
The last step of a typical photolithography process is hard baking. A hard bake makes the pattern more thermally stable through cross linking. Improved adhesion of the resist to the substrate occurs through high temperatures.