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Photoresist is one of eight building blocks in the semiconductor industry. According to the most recent information from the global semiconductor industry association (SEMI), photoresist contributes 5 per cent of all semiconductor wafer manufacturing materials, followed by photoresist supporting materials in the amount of 7 per cent. They total 12 % of semiconductor material value combined, and hence photoresist and its supporting materials are the fourth-largest semiconductor materials market after silicon wafers, electronic specialty gases, and photomasks.
Photolithography is the most important step in the production of fine circuit patterning because it's the part that decides the minimum feature size for a chip. As for semiconductor chips 10-50 photolithography steps are required, which makes up 40%-50% of chip manufacturing time and 30% of chip manufacturing price. Photoresist is the most critical consumable of the photolithography process, and the quality and performance of photoresist determines the yield, performance and reliability of electronic devices.
Photoresist is a photosensitive liquid paste made of three main components (photosensitive resin, photo-initiator, and solvent), plus other additives. It becomes insoluble under the action of ultraviolet light, deep ultraviolet light, electron beams, ion beams, X-rays or other radiation, and hence is immune to etching. The photosensitive resin is the initial structure of the photoresist, and determines the photoresist's basic properties in post-exposure (hardness, pliability, adhesion, and solvability in pre- and post-exposure solvents). It is the photo-initiator which makes a difference for the light sensitivity and resolution of the photoresist. The solvent dissolves the ingredients of the photoresist and is used as the solvent for the chemical reaction. Colorants, curing agents, dispersants, and so on, all make it possible to modify the photoresist's performance in a specific manner.
Photoresists come in various types, mainly divided into positive and negative photoresists based on their chemical reaction mechanisms and development principles. Positive photoresists react under specific light exposure, turning into a solvent, allowing the exposed parts to be removed. In contrast, negative photoresists cross-link short-chain polymer molecules into long-chain molecules under light exposure, hardening the exposed parts, which remain on the substrate, while the unexposed photoresist is removed.
Comparison of Positive and Negative Photoresists
| Photoresist Characteristics | Positive Resist | Negative Resist |
|---|---|---|
| Adhesion to Wafer | Moderate | Good |
| Sensitivity | Lower | High |
| Contrast | High | Low |
| Cost | More Expensive | Cheaper |
| Developer | Water-soluble | Organic Solvent |
| Effect of Oxygen in Environment | None | Affected |
| Minimum Resolvable Feature Size | <0.5 µm | Around 2 µm |
| Etch Resistance | High | Low |
| Residual Resist | Possible only in images less than 1 µm | More common |
| Step Coverage on Wafer Surface | Good | Poor |
| Post-development Swelling | None | Present |
| Thermal Stability | Good | Moderate |
The most popular photoresists in the market can be divided into 5 categories depending on the wavelength of exposure: G-line, I-line, KrF, ArF and EUV. The shorter the exposure wavelength, the finer the process will be. To meet the increasing demand for smaller integrated circuit line widths, photoresist wavelengths continue to shorten. In terms of technical difficulty, G-line is the least advanced, while EUV is the most advanced.
Photolithography Technology Evolution
| Generation | Product Type | Exposure Wavelength | Equipment | Application in IC Processes | Applicable Wafer Size |
|---|---|---|---|---|---|
| 1st Generation | G-Line Photoresist | 436 nm | Near-field Photolithography | >0.5 µm | 6-inch / 8-inch |
| 2nd Generation | I-Line Photoresist | 365 nm | Near-field Photolithography | 0.5-0.25 µm | 6-inch / 8-inch / 12-inch |
| 3rd Generation | KrF Photoresist | 248 nm | Scanning Projection Lithography | 0.5-0.13 µm | 8-inch / 12-inch |
| 4th Generation | ArF Photoresist (Dry Process) | 193 nm | Step-and-Repeat Projection Lithography | 130-65 nm | 12-inch |
| ArF Photoresist (Wet Process) | 193 nm | Immersion Lithography | 65-14 nm, with double and multiple exposure techniques can reach 7 nm | 12-inch | |
| 5th Generation | EUV Photoresist | 13.5 nm | Extreme Ultraviolet Lithography | <7 nm | 12-inch |
In the field of photoresist applications, improving performance through core technologies drives overall industry progress. The core technologies of photoresist include formulation technology, quality control technology, raw material technology, and preparation process technology. Raw materials form the foundation of photoresist performance, while formulation technology is the core to achieving the desired performance for downstream applications. Quality control technology ensures the stability of photoresist performance. The level and scope of these technologies directly impact the photolithography results and the quality of the final product.
Formulation Technology
Due to the diverse needs of downstream users, such as IC chip manufacturers and FPD panel makers, photoresist applications vary greatly. Even within the same customer base, different photolithography requirements exist. An integrated circuit (a semiconductor chip) is usually processed by 10 to 50 photolithographic operations. Because of substrates, resolution, and etching techniques, every photolithography process presents a different set of requirements to the photoresist. To meet these varied applications, there are many different types of photoresists, with these differences primarily achieved by adjusting the formulation of the photoresist.
Quality Control Technology
Given the high demands for photoresist stability and consistency, including consistency between different batches, users expect high levels of consistency in parameters like photosensitivity and film thickness. To meet these expectations, photoresist manufacturers must not only equip themselves with comprehensive testing instruments but also establish a rigorous Operational Assurance (OA) system to ensure product quality and stability.
Raw Material Technology
Photoresist is a complex and precisely designed formulation product, made from various raw materials such as film-forming agents, photosensitizers, solvents, and additives. These materials are combined in different ways and processed through complex, precise procedures. Therefore, the quality of the raw materials plays a critical role in determining the overall quality of the photoresist.
Preparation Process Technology
The preparation process of photoresist includes several steps, such as synthesis, purification, drying, and formulation. The control of the preparation process is crucial to ensuring the quality and performance of the photoresist.
The upstream of the photoresist industry chain includes raw materials such as base resins, monomers, photo-initiators, and solvents. The midstream involves the production and synthesis of photoresists based on specific formulas, and the downstream primarily involves various chip application processes. Since photoresist is fundamentally a formula-based discipline and has a significant impact on the precision and yield of the photolithography process, there are high barriers in all three stages of the photoresist industry chain.
References
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