KrF Resists

With the rapid development of the modern microelectronics industry, the line width of integrated circuits is gradually getting smaller and reaching the nanometer level. A new generation of deep ultraviolet (DUV) photoresists has emerged, which mainly refers to KrF resists with exposure wavelengths of 248 nm and ArF resists with exposure wavelengths of 193 nm. DUV photoresists mainly use chemical amplification technology. (The concept of chemical amplification was proposed in 1982, mainly to circumvent the sensitivity limitations of inherent photoresist materials and increase resist sensitivity ^[1].) KrF resist refers to the photoresist using a KrF excimer laser as the exposure light source, and is the earliest chemically amplified (CA) photoresist used.

Composition

KrF resist is composed of resin, photoacid generator (PAG), photosensitizer, and solvent. The first two components play an important role in KrF resists. Resin gives photoresist mechanical and chemical properties due to its unique structure, while PAG is an additional photoactive compound added to the polymer matrix and photosensitizer, which can be decomposed to produce strong acids. The following mainly discusses the composition of resin and PAG in KrF resists.

• Resin

It is worth noting that the novolac resin-diazonaphthoquinone (DNQ) system in G-line and I-line resists is not suitable for deep ultraviolet bands. This is because the power of the light source of KrF is lower compared with that of G-line and I-line, and the film-forming resins of KrF resists require high optical transparency at 248 nm. But in the novolac-DNQ system, the novolak resin absorbs too much light, so that the bottom of the photoresist film cannot absorb enough photons for photoreaction, resulting in the structure not being able to be further developed. In addition, the novolac resin-DNQ system is not suitable for DUV wavelengths due to its low sensitivity and long exposure time.

Studies have shown that poly(p-hydroxystyrene) (PHS) has good light transmission at 248 nm, whose optical density is 0.22 μm^-1. The PHS protected by t-Boc is more transparent at 248 nm, with an optical density of 0.1 μm^-1. Therefore, this series of PHS compounds has gradually developed into the mainstream film-forming resins of KrF resists. Fig. 1 shows the basic structure of PHS developed for DUV lithography applications ^[2].

Fig. 1 Basic structure of DUV resist resin with acid unstable groups ^[2].

• PAG

KrF resist is a kind of CA photoresist that uses chemical amplification technology, and the PAG is the most important component except for the film-forming resin, which is directly related to KrF resist performance. For KrF resists, PAGs include sulfonium salts, iodonium salts, sulfonate nonionic compounds, organic halides, and diazo compounds. Among them, aryl sulfonium salt and aryl iodonium salt are the most widely used. Some representative PAG structures used in KrF resists are shown below ^[3].

Fig 2. Representative 248-nm PAGs ^[3].

Fig. 3 shows the involvement of PAGs in the imaging chemistry of CA resists. PAG undergoes photochemical decomposition upon light exposure to generate a small amount of acid. These generated acids act as catalysts in the post exposure bake (PEB) process, catalyzing the decomposition (for positive photoresist) or crosslinking (for negative photoresist) of the polymer in the exposed area, changing resist solubility, and thereby making the exposed area soluble in an alkaline developer. During the reaction, these acids are rarely consumed and are reusable, increasing exposure sensitivity and quantum efficiency.

Fig 3. Involvement of PAGs in the imaging chemistry of CA resists ^[1].

Alfa Chemistry has been focusing on the research of photoresists and their applications for many years. As a premium supplier, we provide high-quality KrF resists with exposure wavelengths of 248 nm in different resolutions and film thicknesses. Please click on the link at the top of this page to view our products. If you have any questions, please feel free to contact us.

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