DNQ Photoresist Chemistry

Overview

Diazonaphthoquinone (DNQ) photoresist is a well-established material in the field of photolithography, particularly for positive photoresists used in semiconductor manufacturing. The chemistry of DNQ photoresists is complex, involving several key components and reactions that allow for pattern formation on substrates.

Key Components of DNQ Photoresist:

Diazonaphthoquinone (DNQ): 

Role: Acts as a photoactive compound (PAC). DNQ is typically esterified with sulfonic acid, creating a dissolution inhibitor in the resist system.

Structure: It's a derivative of naphthoquinone with a diazo group, which is sensitive to UV light.

Novolac Resin:

Role: Serves as the film-forming base or matrix. It's a phenol-formaldehyde resin that provides mechanical stability, adhesion to the substrate, and etch resistance.

Properties: Novolac is soluble in basic developers unless inhibited by DNQ.

Solvent:

Role: Dissolves DNQ and novolac to create a uniform resist solution that can be spin-coated onto the substrate.

Common Solvents: Propylene glycol methyl ether acetate (PGMEA) is prevalent due to its low toxicity and good film-forming properties.

Mechanism of DNQ Photoresist:

Initial State: 

In the unexposed state, DNQ inhibits the dissolution of novolac in aqueous alkaline developers due to hydrogen bonding or diazocoupling, making the entire resist layer resistant to the developer.

Exposure to Light (Photoreaction):

Wolff Rearrangement: When exposed to UV light, DNQ undergoes a photochemical reaction known as the Wolff rearrangement. Here, the diazo group (-N₂) is lost, and the molecule rearranges to form a ketene intermediate.

Acid Formation: The ketene then reacts with water to form indene carboxylic acid. This acid significantly increases the solubility of the exposed areas in alkaline developers because it disrupts the hydrogen bonding network with novolac.

Development: 

After exposure, the resist is developed using an aqueous base solution, typically 0.26N tetramethylammonium hydroxide (TMAH). The developer removes the exposed areas where the DNQ has converted to carboxylic acid, leaving behind the pattern defined by the photomask.

Additional Chemical Interactions:

Hydrogen Bonding: In the unexposed regions, DNQ forms hydrogen bonds with the phenolic groups of novolac, reducing its solubility in bases.

Polarity Change: The transformation from DNQ to its acidic form leads to a significant polarity change, which is crucial for the contrast between exposed and unexposed areas.

Performance Characteristics:

Resolution: DNQ/novolac resists have been key for I-line (365 nm) and G-line (436 nm) lithography, offering good resolution for their time but becoming less suitable for shorter wavelengths due to absorption.

Contrast: The large solubility difference between exposed and unexposed areas provides high contrast, which is vital for clear pattern delineation.

Sensitivity: These resists require relatively high exposure doses compared to newer resists like chemically amplified resists.

Challenges and Limitations:

Wavelength Limitation: DNQ absorbs light below 300 nm, making it unsuitable for deep UV lithography without modifications.

Environmental Sensitivity: The performance can be affected by airborne contaminants like amines, which can neutralize the acids formed during exposure.

Thermal Stability: While generally stable, the resist's performance can degrade if subjected to high temperatures or post-exposure delays.

Conclusion

DNQ photoresist chemistry has been fundamental in advancing photolithography technology, particularly in the manufacturing of integrated circuits. Despite its limitations in modern deep-UV processes, the understanding of its chemical reactions still informs the development of newer resist technologies. The interplay between DNQ, novolac, and the solvent system creates the basis for a photoresist that can selectively pattern complex microstructures, essential for the semiconductor industry.

DNQ Synthesis Process

Diazonaphthoquinone (DNQ) Synthesis Overview

Diazonaphthoquinone (DNQ) is a key component in positive photoresists used in photolithography. The synthesis of DNQ involves several chemical steps starting from naphthol compounds, leading to the creation of a photoactive compound that is sensitive to UV light. Here's a detailed look at the synthesis process:

1. Starting Material - Naphthol

1-Naphthol or 2-Naphthol: These are common starting materials. The choice between 1-naphthol and 2-naphthol affects the final structure of the DNQ.

2. Nitration

Reaction: The naphthol is nitrated to introduce a nitro group:

Reagents: Nitric acid (HNO₃) and sulfuric acid (H₂SO₄) form a nitrating mixture.

Process: The naphthol is added to the nitrating mixture, typically at low temperatures to control the reaction, leading to the formation of nitronaphthol.

3. Reduction

Reaction: The nitro group on the nitronaphthol is reduced to an amino group:

Reagents: Common reducing agents include tin(II) chloride (SnCl₂) in hydrochloric acid or catalytic hydrogenation using palladium on carbon (Pd/C) with hydrogen gas.

Outcome: This step yields aminonaphthol.

4. Diazotization

Reaction: The amino group is converted to a diazonium salt:

Reagents: The aminonaphthol is treated with sodium nitrite (NaNO₂) in the presence of hydrochloric acid, typically at around 0-5°C to keep the diazonium salt stable.

Outcome: This produces a diazonium salt of naphthol.

5. Coupling Reaction

Reaction: The diazonium salt is coupled with a quinone to form DNQ:

Reagents: Hydroquinone or another suitable quinone derivative is used. The reaction often occurs in an alkaline medium to facilitate the coupling.

Process: The diazonium salt reacts with the quinone to form a diazocoupling product. The exact quinone used can vary, affecting the properties of the DNQ (e.g., solubility, sensitivity).

6. Esterification

Reaction: To make DNQ compatible with novolac resin in photoresists, it's often esterified:

Reagents: Sulfonyl chlorides (like p-toluenesulfonyl chloride) are used to esterify the hydroxyl group on the DNQ, creating sulfonic acid esters which enhance the dissolution inhibition properties in unexposed areas.

Outcome: This step results in DNQ sulfonic acid esters, which are the photoactive compounds used in photoresists.

7. Purification

Process: The synthesized DNQ needs to be purified to remove impurities, byproducts, and unreacted starting materials. Common methods include:

Recrystallization

Column Chromatography

Solvent Extraction

Key Considerations in DNQ Synthesis:

Yield and Purity: The synthesis process must balance yield with purity since impurities can affect photoresist performance.

Safety: Diazonium salts are unstable, and the reactions involve handling potentially hazardous chemicals, necessitating careful control of reaction conditions.

Scale-Up: Moving from lab-scale to industrial synthesis requires optimization for cost, safety, and environmental impact.

Environmental and Economic Aspects:

Waste Management: The synthesis involves chemicals that need proper disposal or recycling to minimize environmental impact.

Cost: The cost of starting materials and reagents, along with the complexity of the process, influences the economics of DNQ production.

Conclusion

The synthesis of DNQ is a multi-step chemical process that requires precision, safety measures, and an understanding of organic chemistry. Each step from nitration to esterification is critical for producing DNQ with the desired properties for use in photolithography. Advances in chemistry continue to refine these processes, aiming for higher yields, lower costs, and environmentally friendlier methods.