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Photoresists are one of the raw materials for the chip production, and they are employed in the wafer photolithography process. In these, the resin is the most critical part of the photoresist. Photoresist resin is not a common polymer and must be custom-produced by specialized suppliers or by photoresist manufacturers themselves. The photoresist monomer, however, is the building block of artificial resins.
In the photolithography step of wafer fabrication, the photolithographic effect is controlled on the nano-scale, and so the photoresist's consistency and stability are important. Resin — which forms the primary part of the photoresist, affects how lithographic and etch-resistant the photoresist is. Resins are synthesized from monomers, and in the synthesis process from "monomer → resin → photoresist," every step affects the quality of the final photoresist product. The performance and quality stability of the monomer determine the performance and stability of the resin, as the resin is polymerized from monomers, and its structure is similar to many long fibers. The highest quality batches of fibers contain long, medium, and short lengths, and premium resins require each fiber length to be consistent or close in terms of length and number. This is a key factor in ensuring the final photoresist's performance stability and consistency. In short, to produce high-quality photoresist, it is essential to have monomers with excellent performance and stable quality.
Semiconductor photoresists are primarily used for processing fine electronic circuit patterns. On the basis of the wavelength of exposure, semiconductor photoresists can be separated into G/I-line photoresists, KrF photoresists, ArF photoresists and EUV photoresists. In parallel, as circuit line integration gets smaller, so too does the wavelength of exposure for photoresists, which also get shorter to enable better resolution. In the same way, resolution of photoresists has also been improved with resolution enhancement technology.
Different types of photoresists correspond to different monomers. For example, traditional I-line photoresist monomers are mainly methylphenol and formaldehyde, KrF photoresist monomers are primarily styrene-based monomers, and ArF photoresist monomers come in both solid and liquid forms. The relationship between the monomers and the resins determines the resin's yield, and high-end photoresist monomers correspond to relatively lower resin production capacities per unit.
| Photoresist Name | Exposure Wavelength | Applicable Line Width | Application Area | Monomers |
|---|---|---|---|---|
| G-Line Photoresist | 436 nm | >0.5 µm | LED devices, advanced packaging | Methylphenol and formaldehyde |
| I-Line Photoresist | 365 nm | 0.5-0.25 µm | LED devices, wafer manufacturing, advanced packaging | Phenolic and hydroxyl-containing aromatic monomers |
| KrF Photoresist | 248 nm | 0.5-0.13 µm | Wafer manufacturing | Styrene-based monomers, liquid form |
| ArF Photoresist | 193 nm | 7-65 nm (Wet Process) /65-130 nm (Dry Process) | Wafer manufacturing | Methyl methacrylate-based monomers, both solid and liquid forms |
| EUV Photoresist | 13.5 nm | <7 nm | Wafer manufacturing | Special metal-organic monomers with high absorption rate and high resolution |
The synthesis of semiconductor-grade photoresist monomers has certain special characteristics, which differentiate them from general monomers in three main aspects:
First, the synthesis technology of semiconductor-grade photoresist monomers is more complex.
Second, semiconductor-grade photoresist monomers require greater stability in quality, with much lower levels of metal ion impurities. For instance, monomers used in semiconductor applications must be 99.5% pure with a metal ion below 1 ppb (parts per billion), while monomers used in panel applications, commonly made from ethylene oxide, only need to be 99.0% pure with a metal ion below 100 ppb.
Third, photoresist monomers that are semiconductor-grade cost a lot more than general monomers.
Purity, moisture, acid and metal ion values are the performance metrics of photoresist monomers. In addition, the yield of resins from different photoresist monomers also differs (yield = how much resin you get for a certain amount of monomer).
In particular, KrF monomers have a good ratio to KrF resin with 1 ton of monomer yielding 0.8-0.9 tons of resin. In contrast, the yield for ArF monomers is lower, with 1 ton of monomer producing about 0.5-0.6 tons of ArF resin.
The industrialization of photoresist monomers faces several challenges, primarily in the following aspects:
Complexity of Synthesis and Purification: Monomers of photoresist must not polymerize in the process of synthesis and purification and metal ions must be controlled effectively. So, for example, metal ions in semiconductor photoresist monomers should be below 1 ppb and purification technology must meet extreme requirements.
High Purity Requirements: Photoresist monomers have extremely high purity (up to or above 99.9%). For different monomers, there are various purity testing procedures (eg, gas chromatography (GC), high-performance liquid chromatography (HPLC) and gel permeation chromatography (GPC).
Difficulty in Process Scale-up: Monomers produced in the laboratory that meet the required standards cannot directly meet customer demands; stable large-scale production is necessary for industrial supply. The purification techniques at the lab scale are quite different from large-scale production and the metal ion balance at the large-scale scale is hard to regulate.
Variety and Diverse Synthesis Methods: There are many photoresist monomers and various monomers demand different synthesis methods of various complexity. For instance, classic I-line photoresist monomers consist of mainly methylphenol and formaldehyde, KrF photoresist monomers consist of mostly styrene, and ArF photoresist monomers consist of mostly methyl methacrylate.
Long Customer Certification Cycle: Photoresist monomer manufacturers must undergo an excruciating certification process to join the supplier network of downstream customers, and customers tend not to change suppliers easily. The customer certification process typically takes 1-3 years and involves high testing and verification costs.
High Technical Barriers: The synthesis of photoresist monomers is technically challenging, requiring high stability, purity, and costly production processes. In addition, high-end photoresist monomers are associated with relatively low resin yields per unit, and the production process must strictly control environmental temperature and cleanliness.
The industrialization of photoresist monomers is primarily challenged by the complexity of synthesis and purification technology, high purity requirements, difficulties in scaling up processes, the wide variety of monomers with diverse synthesis methods, long customer certification cycles, and high technical barriers. These factors collectively form a significant challenge for the industrialization of photoresist monomers.
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