Tsuneo Yamashita

Novel main-chain-fluorinated polymers for 157-nm photoresists

Main-chain-fluorinated base-resins, including tetrafluoroethylene and norbornene derivatives, were synthesized and their fundamental properties, such as transparency at 157 nm and solubility in a standard alkaline developer, were characterized. A high transparency, i.e., absorbance of less then 0.5 μm-1, was achieved by optimizing the polymerization conditions with a variety of counter monomers. It was found that the polymerization conditions could also control the dissolution rates of the fluoropolymers and increased the dissolution rate of unprotected fluoropolymers by about three orders of magnitude, which was sufficient for the alkaline developability. Positive-working resists based on fluororesins were developed and showed good transparency of less than 1 μm-1 at 157 nm, and good solubility in a standard alkaline solution of 0.26-N tetramethylammonium (without any swelling behavior). And an acceptable etching rate as resistant as ArF resists was obtained and 65-nm dense lines could be delineated by the exposure at 157-nm wavelength.

Synthesis of novel fluorinated norbornene derivatives for 157-nm application

We have synthesized various main-chain fluorinated polymers and studied their transparency and solubility. The main-chain fluorinated polymers were synthesized by co- or ter-polmerization of tetrafluoeoethylene (TFE) with cyclic monomers, especially TFE with newly synthesized norbornene derivatives. Transparency of the main-chain fluorinated polymers tended to be higher with higher fluorine contents. But exact absorbance of the main-chain fluorinated polymers by modifying the STUPID calculation. Solubility of the main-chain fluorinated polymers functionalized by hydroxyfluoroalkyl groups was also studied. We have developed a model to predict pKa of hydroxyfluoroalkyl groups incorporated in the norbornene derivatives, and studied correlation between pKa(OH) and solubility of the co-polymers of the hydroxyfluoroalkyl-functionalized norbornene derivatives with TFE. pKa Of the hydroxyfluoroalkyl groups were lower with higher fluorine contents, and solubility of the co-polymers tended to be higher with lower pKa of the hydroxyalkyl groups.

The dissolution behavior of tetrafluoroethylene-based fluoropolymers for 157-nm resist materials

Main-chain-fluorinated base-resins, using the copolymer of tetrafluoroethylene and functional (hexafluoroisopropanol (HFA) group) norbornene, were synthesized. Partial protection of its hydroxyl group as ethoxymethyl group was achieved by two methods, by copolymerization (Method A) or by polymer reaction (Method B). The partial protection by copolymerization was conducted by copolymerizing TFE with the mixture of protected and unprotected monomers (Method A, copolymerization). The partial protection was also carried out by reacting hydroxyl group of the polymer, which is composed of TFE and unprotected monomers with ethoxymethyl chloride in the presence of an amine (Method B). In the polymer reaction, only exo position of the norbornene unit was protected. Their fundamental properties, such as transparency at 157 nm and solubility in a standard alkaline developer, were characterized and studied. A high transparency, i.e., absorbance of less than 0.4 μm-1, was achieved in both methods. However, the polymer prepared by the polymer reaction (Method B) was deprotected more quickly. And this polymer had a higher dissolution rate and development contrast than the polymer prepared by copolymerization (Method A). The Positive-working resists based on this fluororesins were developed and 55 nm dense lines could be delineated by the exposure at 157 nm wavelength with alternating phase shift mask on a 0.9 NA 157 nm exposure tool.

Characterization of TFE/norbornene-based fluoropolymer resist for 157-nm lithography

Fluoropolymers are key materials in the single-layer resists used in 157-nm lithography. We have been studying fluoropolymers to determine their potential use as base resins. These polymers are main-chain fluorinated polymers synthesized by co-polymerizing tetrafluoroethylene (TFE) and functional norbornene. We developed a new polymer that is highly transparent and has high dry-etching resistance by attaching a PG-F protecting group, which has high dry-etching resistance, to a TFE/norbornene-based fluorinated polymer. The dry-etching rate for the 15 % blocked polymer was 1.50 times that of a KrF resist and its absorption coefficient at a 157-nm-exposure wavelength was 1.06 /μm. We introduced various photoacid generators (PAGs) to the polymer, and compared lithographic performance. As a result, we found polymer with a triphenylsulfonium-salts-based PAG had a good pattern profile, and polymer with a high-acidity PAG resolved a fine pattern. In particular, polymer with a triphenylsulfonium perfluorooctane sulfonate PAG was able to resolve a 60-nm line and space pattern. We then added various quenchers to the polymer and the PAG, and compared pattern profiles. We found that the use of a high-basicity quencher improved the resolution of the resist and line edge roughness. Consequently, that the polymer with the triphenylsulfonium perfluorooctane sulfonate PAG and tributylamine quencher could resolve a 55-nm line and space pattern. These results provided guidelines for choosing the PAG and quencher for this polymer.

High-sensitivity EUV resists based on fluorinated polymers

There is a growing interest in the fluorinization of resist materials in improving pattern formation efficiency for extreme ultraviolet (EUV) lithography. The increased polymer absorption coefficient obtained through this resist platform is expected to enhance acid production and in effect improve pattern formation efficiency. Our work over the past several years has shown that the main-chain fluorinated base resins realized by the co-polymerization of tetrafluoroethylene (TFE) and norbornene derivatives offer high dissolution rates. Based on this, a EUV resist which was prepared using the by the fluorinated polymers was investigated. Imaging evaluations, using the small field exposure tool (SFET by Canon / EUVA) with annular (σouter 0.7 / σinner 0.3) illumination conditions were performed. Relatively high sensitivity of 6.3mJ•cm-2 for half-pitch (hp) 45nm and satisfactory resolution limit of hp 40nm was achieved. At present, line width roughness (LWR) was measured at comparatively large values of more than 8.4nm at hp 45nm. This shows that further material and process optimizations may be necessary to improve its present lithographic capability. However, these initial results have shown the potential of fluorinated-polymer based platform as a possible solution for high sensitivity, high resolution and low LWR EUV resists. In this paper, we report recent results of high sensitivity of 5.1mJ•cm-2 for half-pitch (hp) 40nm, optimization of protecting groups and photo acid generators

Synthesis of fluorinated materials for 193-nm immersion lithography and 157-nm lithography

Various fluorinated polymers were synthesized for application in 193-nm immersion lithography with the goal of improving 157-nm photoresist performance. Their fundamental properties were characterized, such as transparency at 193-nm and 157-nm (wavelength) and solubility in water and a standard alkaline developer. High transparency, i.e., absorbance better than 0.3 μm-1 at 193-nm wavelength, was achieved. The dissolution behaviors of them were studied by using the Quartz Crystal Microbalance (QCM) method. We find that the dissolution rate of Poly(norbornene-2-fluoro-2-hexafluoroalchol) (PNB1FVIP) in 0.065N tetramethylammonium hydroxide (TMAH) was >200 times (nm/s) faster than that of the copolymer of tetrafluoroethylene (TFE) and norbornene-2-fluoro-2-hexafluoroalchol (TFE/NB1FVIP). A resist based on TFE/NB1FVIP was able to delineate 75 nm dense lines by exposure at 193-nm (wavelength) with an alternating phase shift mask using a 0.75 NA ArF scanner. The dissolution rates of the fluoropolymers in water and a 0.262N and 0.065 TMAH can be controlled by optimizing counter monomers containing hexafluoroisopropanol (HFA) unit, carboxylic acid unit and so on. In addition, we have collect water contact angle data. This data shows that fluoropolymers can be used as resist cover materials for 193-nm immersion lithography.

Fluorinated-Polymer Based High Sensitivity Extreme Ultraviolet Resists

There is a growing interest in the fluorinization of resist materials in improving pattern formation efficiency for extreme ultraviolet (EUV) lithography. The increased polymer absorption coefficient obtained through this resist platform is expected to enhance acid production and in effect improve pattern formation efficiency. Based on this, a EUV resist which was synthesized by co-polymerizing tetrafluoroethelyne (TFE) and functional norbornene derivative was investigated. Relatively high sensitivity of 6.3 mJcm-2 for half-pitch (hp) 45 nm and satisfactory resolution limit of hp 40 nm was achieved. However, at present, line width roughness (LWR) was measured at comparatively large values of more than 8.4 nm at hp 45 nm. Further material and process optimizations may be necessary to improve its present lithographic capability. However, these initial results have shown the potential of fluorinated-polymer based platform as a possible solution for high sensitivity, high resolution and low LWR EUV resists.

Development and characterization of new 157-nm photoresists based on advanced fluorinated polymers

Fluorinated polymers show a good transparency at the 157-nm exposure wavelength for single-layer resists. We have developed fluorinated resist polymers for 157-nm lithography. These polymers are main-chain fluorinated polymers synthesized by the co-polymerization of tetrafluoroethylene (TFE) and polymers such as poly(TFE/norbornene/α-fluoroolefin) fluoropolymers (FP1). In this paper, a number of polymerization initiators were evaluated in the polymerization of PF1-type polymers in order to investigate the effect of polymer end groups on optical and dissolution properties. We found that the polymer end group greatly affects the dissolution properties of these polymers when using a standard 0.26N tetramethylammonium hydroxide (TMAH) aqueous developer solution. These end groups also affect the polymer transparencies at 157-nm, and the resulting lithographic performance. The fluorocarbon initiator named “F2” induced the lowered absorbance (~0.4μm-1) and an increase in the dissolution rate (~300 nm/sec) without noticeable amounts of swelling. These polymer-based resists can achieve a resolution of less than 60-nm using a 157-nm laser microstepper (NA=0.85) with a Levenson-type strong phase shifting mask.

Synthesis of novel α-fluoroacrylates and related polymers for immersion lithography

Immersion lithography is being actively developed toward mass production for 55nm node devices and beyond. Advances are being made toward large depths of focus and higher resolution, but the underlying problem of machine and material cost increases remains. Our work over the past few years has shown that the main-chain fluorinated base resins realized by the co-polymerization of tetrafluoroethylene (TFE) and norbornene derivatives offer high dissolution rates and moderate surface properties. However, it is difficult to synthesis these materials and their high cost is disadvantageous. Recently, we switched our attention to &agr;-fluoroacrylate and have synthesized various monomers and polymers for immersion lithography. &agr;-fluoroacrylate has a polymerization rate faster than acrylate and methacrylate, and its polymers are superior to theirs. In this paper, we will report these synthesis methods and immersion specific properties such as the dissolution rate in standard alkaline solution and water contact angle. Furthermore, we consider with relationship between dissolution rate and polymer structure by infrared method.

Dry-etch resistance of fluorine functionalized polymers

The reactive ion etch (RIE) properties of fluorine funtionalized polymers in which fluorine atoms were incorporated in the main chain were examined. There was a tendency that the etching rates of these polymers were higher as lower the fluorine contents. The existing four models such as the Ohnishi model, the Kunz model, the Ohfuji model and the Kishimura model were applied to explain the correlation between the etching rates and the polymer compositions or structures, but the errors were too large to explain the relationship. A new model has developed to explain the effect of the fluorine incorporation to the dry etch resistance. The model assumed that there would be a correlation between the number of main chain fluorine atoms and the dry etch resistance, and the main chain fluorine incorporation would increase the dry etch resistance. The model could explain the dry etch resistance of the main chain fluorine incorporated polymers with adequate accuracy.