The Basic Content of Photolithography

Photolithography plays a big part in the manufacture of semiconductors, so let's learn the basics of photolithography.

There are two types of photoresists, one positive photoresist, and one negative photoresist, and the result is shown in the picture above.

① positive photoresist: 

• Subject to light, part of the degradation reaction can be dissolved by the developing solution.

• The graphics of the non-exposed part left behind are consistent with the mask version. 

• The positive resist has the advantages of high resolution, insensitivity to standing wave effect, large exposure tolerance, low pinhole density, and non-toxicity, which is suitable for producing highly integrated devices.

②Negative photoresist:

• The exposed part becomes insoluble due to a cross-chain reaction.

• The developer dissolves the non-exposed part.

• The obtained graphics are complementary to the mask version graphics.

• Negative resist has strong adhesion, high sensitivity, and less stringent development conditions and is suitable for producing low-integration devices.

Composition of photoresist: Resin, the binder of different materials in the photoresist, gives the mechanical and chemical properties of photoresist (such as adhesion, film thickness, thermal stability, etc.); Photoreceptor, photoreceptor to light can occur photochemical reaction; Solvent, maintain the liquid state of photoresist, so that it has good fluidity; Additive, used to change some characteristics of photoresist, such as improving photoresist Additive, used to change some properties of photoresist, such as improving photoresist reflection and adding coloring agent, etc.

Negative photoresist. The resin is polyisoprene, a natural rubber; the solvent is xylene; the sensitizer is a photosensitizer that releases nitrogen after exposure. The resulting free radicals form crosslinks between the rubber molecules. Thus, it becomes insoluble in the developing solution. The negative photoresist is caused by the solvent bubble in the exposure area; the photoresist easily reacts with nitrogen during exposure and inhibits crosslinking.

Positive photoresist. The resin is phenolic formaldehyde called linear phenolic resin, which provides adhesion, and chemical resistance to the photoresist and dissolves in the developer when no dissolution inhibitor is present; the sensitizer is PAC, Photo Active Compound, most commonly DNQ, which is a strong dissolution inhibitor before exposure, reducing the dissolution rate. After UV exposure, DNQ chemically breaks down in the photoresist and becomes a solubility enhancer, dramatically increasing the solubility factor in the developer to 100 or higher. This exposure reaction produces carboxylic acid in DNQ, which is highly soluble in the developer. Positive photoresists have good contrast, so the resulting graphics have a good resolution.

ArF resin is mainly propylene glycol methyl ether acetate, which accounts for only 5-10% of the total cost of photoresist raw materials, although the resin mass is not high.

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