What is Photolithography?
Photolithography originates from the words “Photo” and “Lithos”. Lithos and Graphy is the Greek word carved from a single stone. The objective of Photolithography is to create small structures or features on a silicon wafer or glass using a photoresist(thin photosensitive polymer film). By etching with UV light, Features are made out of photoresist.
Photolithography is a patterning process in which a photosensitive polymer is selectively exposed to light through a mask, leaving a latent image in the polymer that can then be selectively dissolved to provide patterned access to an underlying substrate.
The photolithography process consists of several steps to perform to complete the process. Each step is done for a certain duration to achieve the desired objective. Photoresist plays an important role during the entire process of photolithography.
Steps in Photolithography
The steps in photolithography are described below. The sequence of key processes involved in photolithography are:
- Wafer Cleaning: One should not start any process in micro-engineering before cleaning the wafer. Cleaning the wafer means first dipping the wafer into HF; what we will see is an etching of the silicon dioxide from the wafer once the silicon dioxide is etched. After the HF Dip, Clean or rinse with the help of DI(deionized water) followed by drying with Nitrogen air in addition to this, pre-baking is done to remove any moisture from the surface. Once the Cleaning is done, the wafer is ready for photolithography.
- Pre-bake and Primer coating: HMDS (hexamethyldisilazane) is one of the primers which is coated to improve the addition of Photoresist on the surface of the substrate. Priming is done to improve photoresist adhesion. High-temperature baking is done to remove moisture after the wafer cleaning process.
- Photoresist spin coating: After HMDS primer, One has to spin coat the Photoresist, and the thickness of the photoresist depends upon the rotation per minute(rpm) and time.
- Soft bake: Once the spin coating with photoresist is performed, we go for soft baking. In conventional situations, Soft bake is done at 90 degrees centigrade for one minute on the hot plate.
- Alignment and Exposure.
- Hard Bake.
- Pattern Inspection.
What is Photoresist in Photolithography?
The photoresist is Solid organic material which transfers the designed pattern to the wafer surface. Photoresists have high etch reaction and adhesion properties. Following are the real-time parameters that we fix in the fabrication lab during photolithography:
- Changes in photo solubility due to photochemical reactions exposed to UV light.
- Wafer held onto the vacuum chuck.
- Dispense ~5ml of Photoresist.
- Slow spin ~500rpm.
- Ramp up to ~1000-5000rpm.
- Quality measures - Time, speed, thickness, uniformity, particles and defects.
Types of Photoresist
There are two types of Photoresist:
- Positive PR: Exposure to UV light removes resistance.
- Negative PR: Exposure to UV light maintains resistance.
- Partial evaporation of photo-resist solvents.
- Improves adhesion.
- Improves uniformity.
- Improves etch resistance.
- Optimizes light absorbance.
- Characteristics of Photoresist.
- Cool the wafer to room temperature before exposure to UV light.
- Developer chemicals dissolve soluble areas of photoresist.
- Visible patterns appear on the wafer.
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