Photolithography: What is Photolithography?

By Mohit Uniyal|Updated : August 26th, 2022

Photolithography is an integrated circuit fabrication process which plays an important role in the fabrication of different devices or different structures on devices. The photolithography process imprints the specific pattern on the wafer. It begins using a photoresist(Polymer) material, which is poured evenly onto the wafer while spins.

Photoresist coins its name from the fact that it is sensitive to certain frequencies of light (“photo”) and is resistant to certain chemicals that will be used later to remove portions of a layer of material (“resist”). Here, we will explain photolithography in detail, along with Photoresist and its steps.

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.

Photolithography Process

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:

  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. Alignment and Exposure.
  6. Development.
  7. Hard Bake.
  8. 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.

Soft Baking

  • Partial evaporation of photo-resist solvents.
  • Improves adhesion.
  • Improves uniformity.
  • Improves etch resistance.
  • Optimizes light absorbance.
  • Characteristics of Photoresist.

Exposure

  • Cool the wafer to room temperature before exposure to UV light.

Development

  • Developer chemicals dissolve soluble areas of photoresist.
  • Visible patterns appear on the wafer.
    -Windows
    -Islands
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FAQs on Photolithography

  • Photolithography processes can be classified according to the type of light used, such as ultraviolet (UV lithography), extreme ultraviolet, or X-rays. The wavelength of light used determines the minimum feature size that can impress the photoresist.

  • A photolithography mask is an opaque plate or film with transparent areas which allows light to shine through a defined pattern. They are commonly used in photolithography processes but are also used in many other applications by a wide range of industries and technologies, notably microfluidics.

  • A photoresist is a light-sensitive polymer. When exposed to ultraviolet light, it turns to a soluble material. Those exposed areas can then be dissolved by using a solvent, leaving behind a pattern.

  • Spin coating is the most common method for applying photoresist to a substrate surface. Other less common methods include spraying, roller coating, dip coating and extrusion coating.

  • Photolithography is commonly used to produce computer chips. When producing computer chips, the substrate material is a resist-covered wafer of silicon. This process allows hundreds of chips to be simultaneously built on a single silicon wafer.

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