Photolithography in VLSI Fabrication .pptx

Photolithography is a critical process used in the fabrication of integrated circuits (ICs) and other semiconductor devices. It is a patterning technique that transfers intricate circuit designs onto a silicon wafer, forming the basis for microelectronic devices. By using light to project geometric shapes onto a photosensitive material, photolithography enables the precise control of component features, often at the nanoscale level.

Process Overview
The photolithography process begins with a clean silicon wafer, which acts as the substrate. A thin layer of light-sensitive material, known as photoresist, is applied to the surface. There are two types of photoresist: positive and negative. In positive photoresist, the exposed regions become soluble in a developer solution, while in negative photoresist, the exposed regions harden and become insoluble, allowing the unexposed regions to be washed away during development.

Once the photoresist is applied, the wafer undergoes a process called soft baking, which helps to remove any solvent residues, making the resist more durable and ready for exposure.

The next step is mask alignment and exposure. A photomask, which contains the circuit pattern in the form of opaque and transparent regions, is placed over the wafer. Light, typically ultraviolet (UV), is shone through the mask. The areas of the photoresist that are exposed to light undergo chemical changes, depending on whether a positive or negative resist is used. Modern photolithography systems use stepper or scanner machines, which project a reduced image of the mask onto the wafer to achieve high precision.

After exposure, the wafer is subjected to a development process. In this stage, the exposed photoresist is washed away, revealing the underlying areas of the wafer. The remaining photoresist forms a protective layer over regions of the wafer that are not meant to be etched. This process creates a pattern of exposed and protected regions on the surface of the wafer.

Etching and Further Steps
Once the pattern has been created, the wafer undergoes etching. This process removes material from the exposed regions of the wafer, transferring the pattern into the silicon or other underlying material layers. Dry etching methods, such as plasma etching, are commonly used for their ability to produce highly precise features. After etching, the remaining photoresist is stripped away in a process called photoresist stripping, leaving behind the desired circuit patterns.

In some cases, a process called ion implantation or doping is used after photolithography to modify the electrical properties of the exposed areas of the wafer. These doped regions become the functional parts of the transistor or other electronic components.

Resolution and Challenges
One of the key challenges in photolithography is achieving ever-smaller feature sizes as semiconductor technology advances. This challenge is addressed by using shorter wavelengths of light

1. PHOTO LITHOGRAPHY IC Fabrication Photolithography

2. Three Steps of Photolithography Coat Expose Develop

3. Surface Conditioning Boosts adhesion of the photoresist Hexamethyldisalizane or HMDS

4. Photoresist (Resist) Photo Sensitive material Resin(Polymer) Sensitizer Solvent Positive PR Negative PR

5. PHOTOLITHOGRAPHY PROCESS

6. EXPOSURE SYSTEMS Contact Exposure System Proximity Exposure System Projection Exposure System condenser lens reduction lens

7. Resolution The term critical dimension (CD) refers to a specific minimal feature size and is a measure of the resolution of a lithographic process Increase the NA Reduce the Wavelength Reduce by using PSM   Increasing the Refractive Index

8. Phase Shift Mask Conventional Mask Phase Shift Mask

9. Immersion Photolithography Incresed Refractive Index     For Air:   For Water:              

10. Future Trends Even shorter wavelength - 193 nm - 157 nm Silicate glass absorbs UV light when optical system Next Generation Lithography (NGL) Extreme UV (EUV) Electron Beam X-ray   Photolithography: temporary patterning process Most critical process steps in IC processing Requirement: high resolution, low defect density Photoresist, positive and negative Process steps: Pre-bake and Primer coating, PR spin coating, soft bake, exposure, PEB, development, hard bake, and inspection NGL: EUV and e-beam lithography SUMMARY

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