Extreme Ultraviolet Resists Research Articles

Stochastic exposure kinetics of extreme ultraviolet photoresists: simulation study

The stochastic nature of extreme ultraviolet (EUV) resist exposure leads to variations in the resulting acid concentration, which leads to line-edge roughness (LER) of the resulting features. Using a stochastic resist simulator, we predicted the mean and standard deviation of the acid concentration for an open-frame exposure and fit the results to analytical expressions. The EUV resist exposure mechanism of the PROLTIH Stochastic Resist Simulator is first order, and an analytical expression for the exposure rate constant C allows prediction of the mean acid concentration of an open-frame exposure to about 1% accuracy over a wide range of parameter values. A second analytical expression for the standard deviation of the acid concentration also matched the output of the simulator to within about 1%. Given the assumptions of the PROLTIH Stochastic Resist Simulator, it is possible to use the results of this paper to predict the stochastic uncertainty in acid concentration for EUV resists, thus allowing optimization of resist processing and formulations and contributing to a comprehensive LER model.

Effect of Acid Generator Decomposition during Exposure on Acid Image Quality of Chemically Amplified Extreme Ultraviolet Resists

In chemically amplified extreme ultraviolet (EUV) resists, acid generators are mainly sensitized through the reaction with thermalized electrons. The resolution blur (the migration range of thermalized electrons) and quantum efficiency depend on the acid generator concentration. Therefore, the decomposition of acid generators during exposure affects both resolution blur and quantum efficiency. In this study, the effect of acid generator decomposition on the acid image quality was investigated by Monte Carlo simulation. It was found that the degradation of acid image contrast caused by acid generator decomposition increases with a reduction in feature sizes. Independently of the contrast in the EUV aerial image, the acid image contrast was similarly degraded with the increase of exposure dose. The effect of flare on the additional contrast degradation caused by acid generator decomposition during exposure was also estimated. For the suppression of this flare effect, it is important to increase the acid generator concentration.

Assessment and Extendibility of Chemically Amplified Resists for Extreme Ultraviolet Lithography: Consideration of Nanolithography beyond 22 nm Half-Pitch

The major resist properties, namely, resolution, line edge roughness (LER), and sensitivity have trade-off relationships. The relationships among them are determined by the pattern formation efficiency. Because of these trade-off relationships, the assessment of resist performance has been a difficult task. The extraction of parameters associated with pattern formation efficiency is important for the proper assessment of resist materials. In this study, we improved the resist model and analysis procedure for the parameter extraction and assessed state-of-the-art extreme ultraviolet (EUV) resists. Using extracted parameters, we evaluated the expected resist performance upon exposure to a next-generation exposure tool with high numerical aperture (NA). Furthermore, assuming the technical limits of resist materials, the extendibility of chemically amplified resists was examined. It was found that the resist requirements (10 mJ cm-2 sensitivity and 1 nm LER) for the 16 nm node are achievable at NA=0.35, although they were comparable to the assumed technical limit.

Effect of Acid Generator Decomposition during Exposure on Acid Image Quality of Chemically Amplified Extreme Ultraviolet Resists

Effect of Acid Generator Decomposition during Exposure on Acid Image Quality of Chemically Amplified Extreme Ultraviolet Resists

A New Method for Line Width Roughness Mitigation

Demands of the continuous downscaling of device size impose stringent requirements on the RLS trade-off (R: resolution, L: line width roughness and S: sensitivity). Among the above, line width roughness (LWR) has to be controlled below 10% of critical dimensions (CD). While significant improvement in R&S was achieved, LWR has been relatively high for extreme ultra-violet lithography resists. Herein, we provide a new method for LWR mitigation that overcomes several shortcomings of previously known methods. Our new method is based on a double development method wherein, a polymeric solution in developer is coated on to the patterned surface. The wafer is then subjected to the standard development step during which LWR improvement is affected. In this paper we discuss the hypothesis of our method and provide relevant data to understand the conditions under which LWR improvement is observed. Effect of polymer/developer concentration ratio, solid content and bake temperatures will be discussed. Also, our method will be compared with currently available methods for LWR improvement on pattern transfer.

Resolution, Line-Edge Roughness, Sensitivity Tradeoff, and Quantum Yield of High Photo Acid Generator Resists for Extreme Ultraviolet Lithography

Ultrahigh loadings of photoacid generators (PAGs) in phenolic extreme ultraviolet (EUV) resists have generated the highest known film quantum yields (FQYs). We evaluate the performance of these resists in terms of resolution, line-edge roughness (LER), and sensitivity and collectively evaluate these three parameters (known as RLS) in terms of KLUP and Z-Parameter figures of merit. An analytical model describing the kinetics of photodecomposition was developed to explain the relationship between film quantum yield and PAG concentration. Resists were prepared using a broad range of concentrations of iodonium (DTBPI-PFBS), sulfonium (TPS-PFBS), and non-ionic (NDI-PFBS) PAGs. The model fits the experimental data (correlation coefficient R2 = 0.998, 0.994, and 0.995) and compares the rate at which electrons react with PAGs or recombine with holes. Resists prepared with 15–20 wt % of iodonium nonaflate PAG exhibit both high quantum yields and the best RLS performance as determined using both KLUP and Z-Parameter methodologies. The improvement in RLS performance correlates with the increase in FQY at higher PAG concentrations.

Resolution, Line-Edge Roughness, Sensitivity Tradeoff, and Quantum Yield of High Photo Acid Generator Resists for Extreme Ultraviolet Lithography

Ultrahigh loadings of photoacid generators (PAGs) in phenolic extreme ultraviolet (EUV) resists have generated the highest known film quantum yields (FQYs). We evaluate the performance of these resists in terms of resolution, line-edge roughness (LER), and sensitivity and collectively evaluate these three parameters (known as RLS) in terms of KLUP and Z-Parameter figures of merit. An analytical model describing the kinetics of photodecomposition was developed to explain the relationship between film quantum yield and PAG concentration. Resists were prepared using a broad range of concentrations of iodonium (DTBPI-PFBS), sulfonium (TPS-PFBS), and non-ionic (NDI-PFBS) PAGs. The model fits the experimental data (correlation coefficient R2 = 0.998, 0.994, and 0.995) and compares the rate at which electrons react with PAGs or recombine with holes. Resists prepared with 15–20 wt % of iodonium nonaflate PAG exhibit both high quantum yields and the best RLS performance as determined using both KLUP and Z-Parameter methodologies. The improvement in RLS performance correlates with the increase in FQY at higher PAG concentrations.

Relationship of Electron Diffusion Length to Line Edge Roughness in Chemically Amplified Extreme Ultraviolet Resists

In chemically amplified resists, secondary electrons sensitize acid generators upon exposure to extreme ultraviolet (EUV) radiation. The migration of secondary electrons degrades the original EUV aerial image and increases the line edge roughness (LER). However, the longer the migration of secondary electrons is, the more acid generators are decomposed. Namely, the resist sensitivity increases with the migration of secondary electrons. Furthermore, the relationship of LER to the sensitivity is a trade-off one. Therefore, the relationship of the migration of secondary electrons to LER is complex. In this study, the effect of the migration of secondary electrons on the image quality was defined using the electron diffusion length specified to line-and-space patterns. The dependence of electron diffusion length and quantum efficiency on material properties was calculated on the basis of the sensitization mechanisms of chemically amplified EUV resists to investigate the relationship of the migration of secondary electrons to LER. Among material properties, the effect of thermalization distance on LER becomes significant below the 22 nm node. The reduction in thermalization distance leads to the reductions in both electron diffusion length and quantum efficiency. Although the reduction in thermalization distance is required to suppress electron diffusion length, the short thermalization distance does not necessarily lead to a low LER. It was found that the thermalization distance of ∼4 nm is sufficient in the resist design for the 16 nm node.

Relationship of Electron Diffusion Length to Line Edge Roughness in Chemically Amplified Extreme Ultraviolet Resists

In chemically amplified resists, secondary electrons sensitize acid generators upon exposure to extreme ultraviolet (EUV) radiation. The migration of secondary electrons degrades the original EUV aerial image and increases the line edge roughness (LER). However, the longer the migration of secondary electrons is, the more acid generators are decomposed. Namely, the resist sensitivity increases with the migration of secondary electrons. Furthermore, the relationship of LER to the sensitivity is a trade-off one. Therefore, the relationship of the migration of secondary electrons to LER is complex. In this study, the effect of the migration of secondary electrons on the image quality was defined using the electron diffusion length specified to line-and-space patterns. The dependence of electron diffusion length and quantum efficiency on material properties was calculated on the basis of the sensitization mechanisms of chemically amplified EUV resists to investigate the relationship of the migration of secondary electrons to LER. Among material properties, the effect of thermalization distance on LER becomes significant below the 22 nm node. The reduction in thermalization distance leads to the reductions in both electron diffusion length and quantum efficiency. Although the reduction in thermalization distance is required to suppress electron diffusion length, the short thermalization distance does not necessarily lead to a low LER. It was found that the thermalization distance of ∼4 nm is sufficient in the resist design for the 16 nm node.

Development of interference lithography for 22 nm node and below

An extreme ultraviolet (EUV) interference lithographic exposure tool was installed at the long undulator beamline in NewSUBARU to evaluate EUV resists for 25nm node and below. The two-window transmission grating of 40 and 50nm half pitch (hp) were fabricated with techniques of spattering, electron beam lithography, dry etching and wet etching. hp patterns (20 and 25nm) of chemically amplified resist (CAR) and non-CAR were successfully replicated using the EUV interference lithographic exposure tool.

Status of EUV Resist and Process Development at IC Manufacture to Implement EUV Lithography to 2X DRAM and Beyond

Extreme ultra violet (EUV) resists have been developed to be able to print sub-30nm L/S features with EUV alpha DEMO tool (ADT), and several resists start to meet the target for 2X node DRAM application in resolution and sensitivity. However, an overall performance of EUV resists is still not comparable to that of DUV resists. At the same process condition having same process constant (k1), the imaging capability of EUV resists is poorer than that of DUV resists. The most critical issues are line width roughness (LWR) and critical dimension (CD) variation across a field. Although there are many studies to improve the LWR of EUV resist, the issue on CD variation across a field is not much explored, because the problem can be detected at full field exposure. In this paper, sources of the CD variation across a field are mainly investigated, and solutions to improve the CD uniformity are explored. Out of band (OOB) radiation and its reflectivity at REticle MAsking (REMA) unit of scanner or absorber of mask is regarded as one of the sources which aggravates imaging quality of EUV resist. In addition, the optical density of black border at EUV wavelength is also known to have an impact on this CD variation. Therefore, to improve pattern fidelity and LWR of EUV resist, the mitigation of OOB radiation impact is required. It is found that the resist sensitivity to DUV compared to EUV is important, and this property affects on CD uniformity. Furthermore, new material which can mitigate the OOB radiation impact is developed. The resolution limit of current EUV resists is also investigated to see the extendibility of it. At an exposure condition which readily resolves 16nm half pitch (HP) resolution, 20nm resolution is hardly achieved even with a champion resist. To overcome this barrier, more efforts on development of alternative resists are required from today.

Determination of Optimum Thermalization Distance Based on Trade-off Relationship between Resolution, Line Edge Roughness and Sensitivity of Chemically Amplified Extreme Ultraviolet Resists

In chemically amplified extreme ultraviolet (EUV) resists, the thermalization distance of secondary electrons is a major cause of the degradation of the original EUV aerial image and increases the line edge roughness (LER). However, the longer the thermalization distance is, the more acid generators are decomposed. As the acid concentration increases, the acid diffusion length required for the polarity change of resist polymers decreases. With the reduction in diffusion length, the resolution blur induced by acid diffusion is suppressed. In this study, we investigated the optimum thermalization distance for obtaining the best latent image quality. The optimum thermalization distance for 32 nm line-and-space patterns was 7 nm at 10 wt% acid generator (TPS-tf) concentration. The optimum thermalization distance decreased with the half-pitch of line-and-space patterns. For 11 nm half-pitch, the optimum thermalization distance was 3 nm, which is approximately the same as that in the poly(4-hydroxystyrene) matrix.

EUV Resist Testing Status and Post Lithography LWR Reduction

Extreme Ultraviolet (EUV) lithography is a leading technology option for manufacturing at the 22nm half pitch node and beyond. Implementation of the technology will require continued progress on several key supporting infrastructure challenges, including EUV photoresist materials. The main development issue regarding EUV photoresists is simultaneously achieving the high resolution, high sensitivity, and low line width roughness (LWR) required. This paper describes our strategy, the current status of EUV materials, and some integrated post-development LWR reduction efforts. Data collected utilizing Intel′s Micro-Exposure Tool (MET) is presented in order to examine the feasibility of establishing a resist process that simultaneously exhibits ≤22nm half-pitch (HP) L/S resolution at ≤11.3mJ/cm2 with ≤3nm LWR

Dissolution Kinetics in Chemically Amplified EUV Resist

Polymer structure effects on the dissolution kinetics and deprotection reaction were investigated to understand inherent extreme ultraviolet (EUV) resist characteristics. The effect of protecting groups and protecting ratio on activation energy were observed. Also, the protecting ratio effect on dissolution kinetics was observed. The speed of TMAH penetration into each resist film was different for each resist as the concept of the ″penetration zone″. This indicated that the dominant rate-determining step in resists is TMAH penetration into the resist film. Thus, the dissolution rates were affected by changing the protection ratio of polymer. Moreover, the preliminary evaluation of resist profile for EUV chemically amplified resists was performed by using PROLITH and parameters obtained with EUV exposure tool. Furthermore, the resist patterns were produced by electron beam and the SEM observation results were compared with the PROLITH simulation results. It is important for EUV lithography to select appropriate protecting group and protecting ratio.

Relationship between Line Edge Roughness and Fluctuation of Acid Concentration in Chemically Amplified Resist

Line edge roughness (LER) is a critical issue in the development of chemically amplified resists used for extreme ultraviolet (EUV) lithography. The main cause of LER in state-of-the-art resists is the chemical inhomogeneity intrinsic to chemical reactions. However, the fluctuation of the initial acid concentration possibly caused by the inhomogeneous distribution of resist components such as acid generators becomes a concern with the reduction of feature sizes. In this study, the effect of the fluctuation of the initial acid concentration on LER was theoretically investigated on the basis of the reaction mechanisms of chemically amplified EUV resists. The fluctuation of line edge originating from the fluctuation of the initial acid concentration was compared with LER originating from the fluctuation of chemical reactions. Even at 16 nm half pitch, the fluctuation in the initial distribution was significantly reduced through catalytic chain reaction. The smoothing effect of acid diffusion is essential to meet the requirement for fine patterning with high sensitivity.

Radiation Chemistry of Fluoronaphthalene as a Candidate for Absorption Enhancement Component of Chemically Amplified Extreme Ultraviolet Resists

With the reduction of resist thickness accompanying the miniaturization of feature sizes, increasing the absorption coefficient of extreme ultraviolet (EUV) resists is becoming increasingly important from the viewpoint of the efficient use of incident radiation. The fluorination of the resist polymer is the most effective means of increasing the absorption coefficient. However, it has been pointed out that the dissociative electron attachment of the fluorinated polymer is a problem in the application of the fluorinated polymer to chemically amplified resists used for ionizing radiation. In this study, the electron flow in fluorinated resists was examined using fluoronaphthalene as a possible candidate for suppressing the dissociative electron attachment. The molecular structure dependence of the reactivity with tetrahydrofuran-solvated electrons, the electron transfer from fluoronaphthalene radical anions to triphenylsulfonium-triflate, the dissociation of fluoronaphthalene radical anions, and the charge recombination of fluoronaphthalene radical anions with protons were clarified by the comparison of octafluoronaphthalene, 1-fluoronaphthalene, and naphthalene. The dissociation of fluoronaphthalene radical anions was negligibly slow. Also, the recombination of octafluoronaphthalene radical anions with protons was significantly delayed compared with that of naphthalene radical anions. These results suggest that the molecular structure of fluoronaphthalene is suitable for the molecular design of chemically amplified EUV resists from the viewpoint of the control of electron flow in the acid generation processes.

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 mJ·cm-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.

Relationship between Normalized Image Log Slope and Chemical Gradient in Chemically Amplified Extreme Ultraviolet Resists

In lithography, normalized image log slope (NILS) is an important metric that describes the quality of an aerial image of incident photons. The aerial image is converted to a latent image through lithographic processes in the resist. The quality of the latent image correlates with line edge roughness (LER). Chemical gradient is also an important metric that describes the quality of a latent image. In this study, we investigated the relationship between NILS and chemical gradient in chemically amplified resists for extreme ultraviolet lithography. In particular, the effect of effective reaction radius for catalytic chain reactions on the relationship between NILS and chemical gradient was clarified.

Negative-type extreme ultraviolet Resist Materials based on Water-Wheel-like Cyclic Oligomer (Noria)

The synthesis and properties of noria derivatives containing oxetanyl groups were investigated for their application as negative-type extreme ultraviolet (EUV) resist materials. The solubility, film-forming ability, and photochemical reactivity were consistent with the structures of noria derivatives. The patterning properties of the synthesized noria derivatives were also examined using an EUV system, and patterns with 45 and 50 nm half-pitch resolution were obtained.

Can Acid Amplifiers Help Beat the Resolution, Line Edge Roughness, and Sensitivity Trade-Off?

In this paper, we describe the structures of several new acid amplifiers and of extreme ultraviolet (EUV) resist formulations prepared from them. We have synthesized and lithographically evaluated eleven new compounds specifically designed for use as acid amplifiers in EUV resists. We make direct comparisions between resolution, line-edge roughness (LER) and sensitivity using the common resolution, line-edge roughness, and sensitivity (RLS) analysis technique of Z-parameter. We show that acid amplifiers are capable of simultaneously improving resolution, LER, and sensitivity.