05. Principles of Photolithography

The lens equation in photolithography relates the object distance, image distance, and focal length of the lens used to project the pattern onto the substrate.

The resist thickness in spin coating depends on the resist viscosity and the spinning velocity.

The substrate material used is Silicon wafer.

Solvents are applied to remove the photoresist pattern.

During the etching process, all residue photoresist is removed from the substrate.

The principal reason for using silicon and silicon compounds for most MEMS and microsystems is because these are the materials used to produce integrated circuits (ICs).

Photolithography process involves the use of an optical image and a photosensitive film to produce desired patterns on a substrate.

Pattern Development

A photomask is a template used in photolithography to transfer patterns onto a substrate using light exposure.

Etching on substrate

Silicon (Si) is typically used as the substrate material in the fabrication of 3D parts at the micro level.

Photoresist adhesion can be improved by using photoresist primers.

The optical image is originally in macro scale, but is photographically reduced to the micro-scale to be printed on the silicon substrates.

Photoresist Development by applying Solvent

The light source in photolithography provides the necessary illumination to transfer patterns from the photomask to the photoresist on the substrate.

The descumming process uses O2 plasma to remove the bulk of photoresist after development, ensuring a clean substrate surface.

Positive photoresists are those which become more soluble when exposed to light. An example is Polymethymethacrylate (PMMA), which is sensitive to UV light and developed in an alkaline solvent such as KOH. They offer better edge definition and higher resolution.

The photoresist changes solubility when exposed to light.

Post Processing - Cleaning/Baking

Light is used in photolithography to transfer geometric patterns from a photomask to a light-sensitive chemical photoresist on the substrate.

To clean the surface from dirt/impurities such as abrasive particles, fibers, residues from previous photolithography, bacteria at micron level, and oil, water, solvent residues.

Photoresist removal/stripping

The spinner in a spin coating machine is used to dispense solvents evenly across the substrate.

Common developer agents for positive photoresists are KOH (potassium hydroxide) or TMAH (tetramethylammonium hydroxide).

The usual practice for obtaining a uniform coating in spin coating involves spinning slowly at first, followed by high-speed spinning.

Surface Preparation/Cleaning

An etching process is done on the substrate.

Factors affecting photoresist adhesion include moisture content on the surface, wetting characteristics of the resist, delay in exposure after the pre-bake, resist chemistry, surface smoothness, stress from the coating process, and surface contamination.

Alignment & Exposure

Photolithography is an optical means for transferring patterns onto a wafer, used to pattern oxide/nitride film on a Si substrate. It is the most efficient and viable process for micro-level fabrication of semiconductor-based devices.

Photolithography is a process used in microfabrication to pattern parts of a thin film or the bulk of a substrate using light.

Spin coating is a process where a substrate or wafer is kept on a rotated chuck by vacuum, and resist is applied by spinning the substrate at typically 3000-6000 rpm for 15-60 seconds.

The light-transparent mask is usually made of quartz.

Magnification in photolithography refers to the ratio of the size of the image produced by the lens to the size of the object being imaged.

A common problem associated with spin coating is bead formation at the edge of the wafer.

Postbaking is used to remove the residue of solvent from the photoresist after development, typically done at 120°C for 20 minutes.

Negative photoresists are those which become more soluble under shadow. Examples include Bisazide rubber and Kodak KTFR. They are less sensitive to x-rays and optical rays but more sensitive to electron beams. The solvent used is Xylene.

Layer thickness in spin coating is controlled by the spinning time.

The mask is placed above the top-face of a silicon substrate coated with a thin film of photoresistive materials to transfer the desired patterns.

Photo-resist development - Spin Coating

Xylene is commonly used as a developer agent for negative photoresists.

UV light is used in photolithography because it has the appropriate wavelength range (300 to 500 nm) to effectively expose photoresist materials.

High pressure mercury vapor lamps are used as a light source in photolithography due to their ability to emit strong UV light.