Shiro Kusumoto

Material design for immersion lithography with high refractive index fluid (HIF)

ArF immersion lithography is considered as the most promising next generation technology which enables to a 45 nm node device manufacturing and below. Not only depth of focus enlargement, immersion lithography enables to use hyper numerical aperture (NA) larger than 1.0 and achieve higher resolution capability. For 193nm lithography, water is an ideal immersion fluid, providing suitable refractive index and transmission properties. Furthermore the higher refractive index fluid is expected to provide a potential extension of optical lithography to the 32 nm node. This paper describes the material design for immersion lithography with high refractive index fluid. We have developed promising high refractive index fluids which satisfy the requirement for immersion fluid by screening wide variety of organic compounds. The physical and chemical properties of this high refractive index fluid are discussed in detail. Also the topcoat material which has good matching with high refractive index fluid is developed. While this topcoat material is soluble into aqueous TMAH developer, it does not dissolve into water or high refractive index fluid and gives suitable contact angle for immersion scan exposure. Immersion exposure experiments using high refractive index fluid with and w/o topcoat material was carried out and its lithographic performance is presented in this paper.

Understanding quencher mechanisms by considering photoacid-dissociation equilibrium in chemically amplified resists

The quencher mechanisms in Chemically-Amplified (CA) resists have been investigated. To explain the acid distribution with a variety of acid strengths in the presence of quencher, a new full Acid-Equilibrium-Quencher model (AEQ model) is proposed and examined in solid-model-CA-resist systems. To observe the reactions in the CA resists, real-time Fourier-Transform-Infrared Spectroscopy (FTIR) is employed during post-exposure bake (PEB). The FTIR peaks of the protection groups are detected to measure the reaction kinetics during PEB. The solid-model-CA resists used in this work consist of both a KrF-acetal-type resist with a diazomethane Photo-Acid Generator (PAG) (weaker-photoacid system) and an ArF-ester-type resist with a sulfonium-salt PAG (stronger-photoacid system). The obtained FTIR results are analyzed using conventional Full-Dissociation-Quencher model (FDQ model) and the new AEQ model. The kinetic analysis of the model resists was performed for different quencher loadings. For the weaker-photoacid system, the AEQ model much more accurately predicts the deprotection-reaction kinetics than the FDQ model with the change of quencher content. This suggests the necessity of introduction of the acid-dissociation concept in the case of the weaker photoacid. For the stronger-photoacid system, both the AEQ and conventional FDQ models adequately predict the kinetic results. This shows that the conventional FDQ model is accurate enough to simulate the super-strong photoacid system. Finally, the new AEQ model is introduced in the UC Berkeley STORM resist simulator. Some simulation examples are shown in the paper.

Various factors of the image blur in chemically amplified resist

In the current optical lithography, the resolution is being pushed for 45 nm half-pitch, and the chemically amplified resist will be used for wide variety of applications including immersion lithography. So far the chemical amplification has brought high performance for lithography. In the future, for the ArF lithography beyond 45nm half-pith, it will be important to control pattern size. On the other hand, chemically amplified resist which utilized acid catalyzed deprotecting reaction is sensitive to physical and chemical factor. Thus, there are various factors in the each process (Resist coating, Pre bake, Exposure, Post exposure bake, Development and Rinse) to cause the resist blur. For example, its acid diffusion on PEB. The influence of these factors for the resist blur is a significant issue for lithography beyond 45 nm half-pitch. Therefore the need to reduce these factors on the resist blur becomes higher in order to extend the ArF lithography beyond 45 nm half-pith. In this paper, acid diffusion coefficient (D) and resist blur with changing anion size of PAG, size of protecting group in typical ArF resist was reported. The relationship between acid diffusion coefficient and resist blur was discussed on the basis of their difference in structure and characteristics.

CD uniformity improvement for double-patterning lithography (litho-litho-etch) using freezing process

After an analysis of the factors that causes critical dimension (CD) variation in the lithography process of the LLE (Litho-Litho-Etch) double-patterning technology that employs the freezing process, an optimum process for freezing the resist patterns to reduce the CD variation, which occurs after the 2nd litho process, was achieved. By optimizing the track parameters of freezing process, CD variation is likely to be reduced not only in the 1st resist pattern but also in the 2nd resist pattern. The optimum conditions were adopted to form patterns of 40 nm resist lines and spaces in the evaluations conducted in this paper. The formation result showed improvement of 3 sigma of the within-wafer CD uniformity of both the 1st resist pattern and the 2nd resist pattern, by about 13% and 46% respectively.

High-performance 193nm photoresists based on fluorosulfonamide

The combination of immersion lithography and reticle enhancement techniques (RETs) has extended 193nm lithography into the 45nm node and possibly beyond. In order to fulfill the tight pitch and small critical dimension requirements of these future technology nodes, the performance of 193nm resist materials needs to further improve. In this paper, a high performance 193nm photoresist system based on fluorosulfonamide (FSM) is designed and developed. The FSM group has good transparency at 193nm. Compared to the commonly used hexafluoroalcohol (HFA) group, the trifluoromethyl sulfonamide (TFSM) functionality has a lower pKa value and contains less fluorine atoms. Polymers containing the TFSM functionality have exhibited improved dissolution properties and better etch resistance than their HFA counterparts. Resists based on the FSM-containing polymers have shown superior lithographic performance for line, trench and contact hole levels under the 45nm node exposure conditions. In addition, FSM resists have also demonstrated excellent bright field and dark field compatibility and thereby make it possible to use one resist for both bright field and dark field level applications. The structure, property and lithographic performance of the FSM resist system are reported.

Contact hole shrink process with novel chemical shrink materials

Contact hole shrink process is becoming more important option for 45nm node design rules. In general, lithography of contact hole has been harder than that of line and space application due to the low contrast of incident light. The contact hole size for 45nm node device will be around 60nm and this hole size will be the limit of 193nm lithography. High NA exposure tool for 193nm lithography achieves 60nm contact hole resolution, but both under dose margin and depth of focus will be limited. This fact results in the insufficient process window of 193nm lithography. Thus some supporting process should be necessary and a chemical shrink process is one of the possible approach to resolve 60nm contact hole with appropriate process margin. The general chemical shrink process is as follows. Chemical Shrink Material (CSM) is coated on patterned photoresist, and following bake process controls chemical cross-linking reaction and forming a layer insoluble into the developer. As a result pattern size is reduced to desired CD. However current CSM has several issues: i.e. inferior etching durability of CSM than that of 193nm resist and pattern profile degradation after the process. This will be the critical problem for pattern transfer process using CSM. From this point of view, we developed a novel CSM which has good etching durability compared with 193nm resist and does not have a pattern profile degradation. This material consists of aromatic moiety to satisfy good etching durability. Also, the shrink rate and amount are not pitch dependent.

Advanced plating photoresist development for semiconductor packages

In recent years, novel electronic products like mobile phones, tablets, and personal computer have shrunk dramatically and become highly functionalized. These market trends of smaller thinner devices, have put pressure on the complexity of packaging structures for semiconductors. To satisfy these requirements, packaging technologies such as 3D-TSV, 2.5D, PoP and Flip-chip wafer bumping are being used. We have developed negative tone photoresists (ELPAC THB series) for re-distribution layer, C4 and micro bumps. The THB series is negative tone resist which incorporates an acrylate cross-linker system with photo radical initiator. This formulation shows excellent chemical resistance to various plating solutions such as Cu, Ni, Sn/Ag and Au, with good stripability. The key technology of plating photoresists for advanced packages is high resolution, allowing for high aspect ratio patterning capability that covers a wide range of film thicknesses. An example of such applications is in advanced PoP package design which require a thick film material that has high resolution capability, along with high aspect ratio performance. In TSV technology, via filling and high dissolution rate are key for micro-bump formation. Additionally, high resolution re-distribution layer photoresists can be used to create fine pitch conditions which are necessary for package downsizing. Our THB series shows good patterning performance from below 10um to above 100um film thickness conditions. In this paper, material design and key properties of novel plating resist with high resolution and high aspect ratio performance with a wide range film thickness ranges are discussed.

Improvement of watermark defect in immersion lithography: mechanism of watermark defect formation and its reduction by using alkaline-soluble immersion topcoat

ArF (193nm) immersion lithography is considered as the most promising next generation technology and significant effort to establish the immersion process for semiconductor device HVM is currently focused on the tool, material and process development. One of the serious issues in the immersion process for the commercial semiconductor production is the immersion-specific defects. Typical immersion-specific defects are nanobubble, watermark (W/M) defect, and degradation of pattern profile caused by resist components leaching. The nanobubbles, which exist in the immersion medium such as water, deform the optical image, and then cause the pattern profile degradation. Small water droplet left on the resist film after scanning exposure causes W/M defect. Leaching of resist component induces insufficient de-protection reaction at the resist surface region, then cause the T-top pattern profile or bridge type defect. Among these immersion-specific defects, the effective countermeasure against W/M defect has not been established yet, because the mechanism of W/M defect formation is not fully figured out. From the model experimental result, we have found that W/M defect formation depends on the characteristics of photoresist and topcoat materials. Then we have developed the new immersion topcoat which is soluble into aqueous TMAH developer, and this material provides practical solution for W/M defect reduction. In this paper, we will report the mechanism of W/M defect formation which is related to the characteristics of photoresist and topcoat material. Also W/M defect reduction process by using alkaline soluble immersion topcoat will be discussed in detail.

Advanced plating photoresist development for advanced IC packages

The advanced IC packaging technologies such as 3D-TSV, 2.5D interposer, PoP and Flip-Chip wafer micro-bumping are being implemented for consumer electronic products like as mobile phones, tablets, and so on. The form factor of consumer product trends toward small and thin which driving IC packaging toward smaller, thinner and more complicated. A negative tone photoresists has been developed for re-distribution metal layers, C4 and micro bumps by alloy solder plating used in 3D-TSV, 2.5D interposer, PoP and Flip-Chip packages. The key technology of plating photoresists for advanced packages is high resolution allowing for high aspect ratio patterning capability that covers a wide range of film thicknesses. An example of such applications is in advanced PoP packaging designs which require a thick film material that has high resolution capability, along with high aspect ratio performance. The THB series is an acrylate cross-linker system negative tone photoresists with photo radical initiator. The formulation shows excellent chemical resistance with good stripability to various plating solutions such as Cu, Ni, Sn/Ag and Au. In TSV process, via filling and high dissolution rate are key processes to form micro-bump. The high resolution photoresists can be used to create fine pitch conditions for package downsizing. The THB series shows good patterning performance from below 10um to above 100um film thickness conditions. The material design and key properties of plating resist with high resolution and high aspect ratio performance with a wide range film thickness ranges are studied and discussed.

Lift-off photoresists for advanced IC packaging metal paternning

3D-TSV, 2.5D interposer, PoP and Flip-Chip wafer micro-bumping are being implemented for consumer electronic products such as mobile phones, tablets, and so on. The consumer product market trends toward smaller and thinner, and cause the IC packaging becoming more complexity. The lift-off method of the photoresist for IC packaging metal patterning has been widely used in the variety of electronic device fabrication processes such as MEMS, and LED manufacturing. The big advantages of using lift-off method are the cost saving and the process simplification. However there is a challenge that the morphology of the deposited metal pattern is difficult to be controlled. In order to achieve desired metal patterning, there are two types of novel lift-off photoresist were developed which are single-layer negative tone photoresist and double-layer positive tone photoresist. Both the photoresists show unique and well-controlled “undercut” profile which enables to form a targeted metal configuration after exposure, development and stripping process. This paper reports the key parameter of photoresist and shows how to control the undercut profile.