Acid Diffusion Length Research Articles

Relationship between blurring factors and interfacial effects in chemically amplified resist processes in photomask fabrication

Chemically amplified resists are an indispensable technology for photomask fabrication. The formation of latent images near interfaces in resist films is strongly affected by low-energy electron dynamics at the interfaces. On the other hand, the latent images are blurred by low-energy electron migration and acid diffusion in latent image formation. In this study, the relationship between blurring factors and interfacial effects was investigated by simulating the latent image formation on the basis of the sensitization and reaction mechanisms of chemically amplified electron beam resists. For low-energy electron dynamics, the interfacial effects on the protected unit distribution was reduced and the chemical gradient slightly increased with decreasing thermalization distance. For acid diffusion, the decrease in exposure pattern width significantly reduced the interfacial effects on the protected unit distribution in the depth direction because the acid diffusion length can be increased without sacrificing resolution.

The Measurement of the Refractive Index <i>n</i> and <i>k</i> Value of the EUV Resist by EUV Reflectivity Measurement Method

Up to now, we have been researching methods for measuring the simulation parameters of EUV resist. These parameters include the development parameter, the Dill C parameter, the diffusion length of acid generated from PAG, and the deprotection reaction parameter. By using these parameters, we have attempted to simulate EUV resist. As a result, we could investigate the conditions for reducing LER and for enhancing resolution. We hereby report on the methods of calculating the refractive index n and k values of photoresist with EUV light (13.5 nm), which has been difficult to measure until now, and the Dill B parameter, which is an absorption parameter. The three types of photoresists we investigated are the main chain scission type resist, chemically amplified resist, and metal resist.

Novel EUV Photoresist for Sub-7 nm Node

Extreme ultraviolet (EUV) lithography has been recognized as a promising candidate for the manufacturing of semiconductor devices as LS/CH pattern for 7 nm node and beyond. EUV lithography is ready for high volume manufacturing stage. For the high-volume manufacturing of semiconductor devices, significant improvement of sensitivity and line edge roughness (LWR) and Local CD Uniformity (LCDU) is required for EUV resist. It is well-known that the key challenge for EUV resist is the simultaneous requirement of ultrahigh resolution (R), low line edge roughness (L) and faster sensitivity (S). Especially sensitivity and good roughness is important for EUV lithography high volume manufacturing. We are trying to improve sensitivity and LWR/LCDU from many directions. From material side, we found that both sensitivity and LWR/LCDU are simultaneously improved by controlling acid diffusion length and efficiency of acid generation using novel resin and PAG. And optimizing EUV integration is one of the good solution to improve sensitivity and LWR/LCDU. We are challenging to develop new multi-layer materials to improve sensitivity and LWR/LCDU. Our new multi-layer materials are designed for best performance in EUV lithography system. From process side, we found that sensitivity was substantially improved maintaining LWR applying novel type of chemical amplified resist (CAR) and process. EUV lithography evaluation results obtained for new CAR EUV interference lithography. And, metal containing resist is one possibility to break through sensitivity and LWR trade off. In this paper, we will report the recent progress of sensitivity and LWR/LCDU improvement of JSR novel EUV resist and process.

Novel EUV Resist Development for Sub-14nm Half Pitch

Extreme ultraviolet (EUV) lithography has emerged as a promising candidate for the manufacturing of semiconductor devices at the sub-14nm half pitch lines and spaces (LS) pattern for 7 nm node and beyond. The success of EUV lithography for the high volume manufacturing of semiconductor devices depends on the availability of suitable resist with high resolution and sensitivity. It is well-known that the key challenge for EUV resist is the simultaneous requirement of ultrahigh resolution (R), low line edge roughness (L) and high sensitivity (S). In this paper, we investigated and developed new chemically amplified resist (CAR) materials to achieve sub-14 nm hp resolution. We found that both resolution and sensitivity were improved simultaneously by controlling acid diffusion length and efficien-cy of acid generation using novel PAG and sensitizer. EUV lithography evaluation results obtained for new CAR on Micro Exposure Tool (MET) and NXE3300 system are described and the fundamentals are discussed.

Characterizing acid diffusion lengths in chemically amplified resists from measurements of deprotection kinetics

The acid-catalyzed deprotection of glassy poly(4-hydroxystyrene-co-tertbutyl acrylate) films was studied with infrared absorbance spectroscopy and stochastic simulations. Experimental data were interpreted with a simple description of subdiffusive acid transport coupled to second-order acid loss. This model predicts key attributes of observed deprotection rates, such as fast reaction at short times, slow reaction at long times, and a nonlinear dependence on acid loading. Fickian diffusion is approached by increasing the postexposure bake temperature or adding plasticizing agents to the polymer resin. These findings demonstrate that acid mobility and overall deprotection kinetics are coupled to glassy matrix dynamics. To complement the analysis of bulk kinetics, acid diffusion lengths were calculated from the anomalous transport model and compared with nanopattern line widths. The consistent scaling between experiments and simulations suggests that the anomalous diffusion model could be further developed into a predictive lithography tool.

Determination of spot size and acid diffusion length in positive chemically amplified resist for e-beam lithography at 100 and 5 kV

An experimental method to determine the beam spot size and the acid diffusion length of a positive chemically amplified electron-beam resist (p-CAR) is proposed. This method is applied to a reference p-CAR with a Vistec VB6 electron beam at 100 kV and with a MAPPER tool at 5 kV. Beam spot size characterization is based on the width measurement of a single line exposure. This procedure is applied to two specific dose ranges. In the first one, the chemical mechanism occurring in the exposed resist is dominated by acid catalytic deprotection of a partially protected polymer (standard process). In the second dose range, the chemistry is governed by the cross-linking mechanism due to the intentional overdose of the p-CAR, leading to a polarity inversion. The authors assume that comparing the results obtained with the standard and the cross-linked p-CAR enables the determination of the acid diffusion length of the p-CAR process. This hypothesis was verified with measurements performed with a nonchemically amplified process. A relationship between the best exposure latitudes achieved at 5 and 100 kV and the minimum spot sizes determined with the present method is observed with very high resolution grating exposures. In this work, a suitable method is proposed for fine resist process characterization.

Mesoscale modeling: a study of particle generation and line-edge roughness

A lattice-type Monte Carlo–based mesoscale model and simulation of the lithography process have been adapted to study the insoluble particle generation that arises from statistically improbable events. These events occur when there is a connected pathway of soluble material that envelops a volume of insoluble material due to fluctuations in the deprotection profile. The simulation shows that development erodes the insoluble material into the developer stream and produces a cavity on the line edge that can be far larger than a single polymer molecule. The insoluble particles can coalesce to form aggregates that deposit on the wafer surface. The effect of the resist formulation, exposure, postexposure bake, and development variables on particle generation was analyzed in both low- and high-frequency domains. It is suggested that different mechanisms are dominant for the formation of line-edge roughness (LER) at different frequencies. The simulations were used to assess the commonly proposed measures to reduce LER such as the use of low molecular weight polymers, addition of quenchers, varying acid diffusion length, etc. The simulation can be used to help set process variables to minimize the extent of particle generation and LER.

EUV Resist Chemical Analysis by Soft X-ray Absorption Spectroscopy for High Sensitivity Achievement

Simultaneous achievement of high sensitivity and low line edge roughness (LWR) is necessary in EUV resist. The chemical reaction analysis of EUV (Extreme Ultraviolet) chemical amplified (CA) resist and acid diffusion length evaluation was carried out. In order to achieve low LWR of the CA resist, the large chemical structure of the anion of photoacid generator (PAG) is required for shortening the acid diffusion length. On the other hand, in order to increase the sensitivity, on the basis of the chemical reaction analysis using the soft x-ray absorption spectroscopy, the decomposition reaction of the large chemical structure of the PAG anion should be taken in account in addition of ionization reaction. However, if the decomposition reaction occur, the acid diffusion length will become shorter than that as expected. It is found that the Imidate-type of anion of PAG has high sensitivity and short diffusion length. The chemical reaction analysis by the soft x-ray absorption spectroscopy using the synchrotron radiation with the combination analysis of the acid diffusion are useful method for the mitigation of high sensitivity and low LWR.

Acid diffusion length in contact hole imaging of chemically amplified extreme ultraviolet resists

Contact holes and line-and-space patterns are the most basic elements in the fabrication of semiconductor devices. In this study, the chemical processes in contact hole imaging were investigated to clarify the optimum acid diffusion length in terms of the quality of latent images. The chemical processes were simulated using a Monte Carlo method based on the reaction mechanisms of chemically amplified extreme ultraviolet (EUV) resists. The theoretical dependence of the optimum acid diffusion length on the exposure dose and half-pitch (diameter) was clarified. At an exposure dose of 20 mJ cm−2, the optimum acid diffusion length (three-dimensional) was 10.7 nm for 28 nm contact holes (56 nm pitch). By decreasing the diameter to 16 nm (32 nm pitch), the optimum acid diffusion length (three-dimensional) was decreased to 8.9 nm, which was smaller than that of line-and-space patterns by 12%. The chemical reactions for the dissolution of resist polymers should be sufficiently induced within these acid diffusion lengths for contact hole imaging.

Acid Diffusion Length in Line-and-Space Resist Patterns Fabricated by Extreme Ultraviolet Lithography

Acids generated upon exposure to radiation induce the polarity change of the polymer through catalytic chain reactions in chemically amplified resists. With the reduction of feature size, the acid diffusion length increasingly becomes an important issue. In this study, we investigated the acid diffusion length in line-and-space patterns fabricated using a small field exposure tool for extreme ultraviolet (EUV) lithography and the EIDEC standard resist to clarify the acid diffusion length in a state-of-the-art resist. The acid diffusion length depended on the nominal line width and exposure dose. Upon exposure to EUV radiation with an exposure dose of 16 mJ cm-2, the acid diffusion length (three-dimensional) in a line-and-space pattern with 21 nm nominal line width was 9.5 nm.

(Invited) Development Status of EUV Resist toward Sub-20nmhp

In this paper, we focus on resolution improvement for 1xnm half pitch. We report development of new short acid diffusion length PAG and under layer to achieve 1xnm half pitch resolution. The patterning of sub 20 nm half pitch LS was gathered from NXE: 3100 will be discussed. Patterning sub 26 nm half pitch contacts is feasible with new resist materials. The optimization of development process and application of rinse process was found to be effective to improve the RLS performance. Furthermore, we will also present the effect of underlayer materials on RLS performance.

Theoretical Study on Acid Diffusion Length in Chemically Amplified Resists Used for Extreme Ultraviolet Lithography

Acid diffusion length has been regarded as the most important factor in the development of chemically amplified resists used for ultrafine patterning. In this study, the acid diffusion length in chemically amplified extreme ultraviolet (EUV) resists was investigated by a Monte Carlo method in the presence of quenchers. The acid diffusion length corresponding to the highest chemical gradient, which results in the lowest line edge roughness, was investigated by varying the exposure dose, the acid generator concentration, and the effective reaction radius for deprotection. Although the optimum acid diffusion length was decreased with the increase of effective reaction radius for deprotection, it did not depend on the exposure dose or acid generator concentration.

Resolution–linewidth roughness–sensitivity performance tradeoffs for an extreme ultraviolet polymer bound photo-acid generator resist

The goal of this work is to use a combination of experiment and calibrated resist models to understand the impact of photo-acid generator (PAG) and sensitizer loading on the performance of a polymer bound PAG resist based processes for extreme ultraviolet (EUV) lithography. This paper describes construction of a chemically amplified resist model across 248 nm, 193 nm, and EUV imaging wavelengths. Using resist absorbance input as obtained from experiment and modeling, only the acid formation kinetics are allowed to vary across imaging wavelengths. This constraining system affords very good fitting results, which provides high confidence that the extracted parameters from the model have actual physical significance. The quantum efficiency for acid formation in EUV is found to be ∼8× higher than at 248 or 193 nm, due to the excitation mechanism by secondary electrons. Most notably for the polymer bound PAG system under study the model provides an extremely low acid diffusion length (∼8 nm), suggesting an excellent inherent resolution for this material. Next, resist models are created for a series of sensitizer containing polymer bound PAG formulations, where the sensitizer loading is systematically varied. Compared to the reference polymer bound PAG resist without sensitizer the efficiency of acid formation is significantly increased, without a negative impact on either resolution or linewidth roughness. For these materials the quantum efficiency of acid formation in EUV is found to be ∼12× higher than at 248 nm. In these formulations the impact of sensitizer loading on the sizing dose is found to be rather moderate. This may suggest that even at the lowest sensitizer loading studied the energy of the secondary electrons is already efficiently transferred to the PAGs.

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.

Material Development for ArF Immersion Resist Extension

ArF immersion process is being widely applied for advanced device manufacturing. ArF immersion lithography can cover 4x nm hp lithography. In addition, double patterning process allows extendibility towards 2x nm hp lithography. From the point of view of photoresist, it is utterly important for materials to bring out the maximum performance of ArF immersion process. In this study, we focused on photoresist development and substrate optimization, demonstrating good lithographic performance of sub-40nm hp resolution with well controlled line width roughness (LWR).First of all, we studied photo acid generator (PAG) behavior focusing on shortening acid diffusion length during Post Exposure Bake (PEB). Secondly, high dissolution contrast polymers in which dissolution properties are changed by acid catalyzed reaction through exposure and PEB are being studied. Thirdly, reflectivity control from substrate through optimization of light intensity over resist film thickness has been carried out. In this report, we have achieved 36.5nm line and space (LS) resolution with LWR of less than 10% of patterned critical dimension (CD). This achievement is very useful especially when utilized in combination with process tricks such as multiple patterning for extending existing ArF immersion lithography towards sub-20nm hp lithography.

Novel Material Development for EUV Resist towards sub-20nm half pitch

Extreme Ultra-Violet (EUV) lithography is considered as one of the most promising candidates for next generation lithography (NGL) that can print sub-20nm half pitch. In order to implement EUV technology for actual device production, resist material is one of the critical items that require significant improvement in overall performance. Current most challenging technical requirement is the simultaneous improvement in resolution (R), line width roughness (L) and sensitivity (S). In order to achieve this requirement, many research groups are developing novel materials such as molecular glass[1],[2], polymer bound photo-acid generator (PAG)[3], high quantum yield PAG[4], sensitizer[5] and high absorption resin[6].In this study, we focused on innovative PAG material development through the investigation of PAG acid diffusion length and PAG anion structure. Also, novel molecular resist consisting of protected Noria has been developed and its feasibility demonstrated.As the results of this study, it was found that PAG plays key function in order to achieve simultaneous RLS improvement. Additionally, it turned out that molecular resist protected Noria has high potential towards sub-20nm resolution. This study is hoped to contribute to the EUV resist development at sub-20nm half pitch generation.

Studies of the Photo Acid Generator Material Design for Chemically Amplified Photoresists

In current optical lithography, resolution is required to reach for 45 nm half-pitch and a chemically amplified resist (CAR) is used for a wide variety of applications. For ArF lithography beyond the 45 nm half-pitch, it is important to control pattern quality. The molecular design of a photo acid generator (PAG) is very important in the study to control not only acid strength but also acid diffusion length. Various novel PAGs that have different characteristics were synthesized for resist performance improvement. Acid molecular size was determined by molecular orbital (MO) calculation, and the acid diffusion coefficients (D) of these PAGs were evaluated by a bilayer method. As a result, it was found that acid diffusion coefficient (D) could not be controlled simply by adjusting anion molecular size. It may be presumed that the molecular interaction between acid generated by the exposure and polymer matrix areas is one of the most important key factors for controlling acid diffusion.

Influence of resist blur on resolution of hyper-NA immersion lithography beyond <inline-formula><math display="inline" overflow="scroll"><mrow><mn>45</mn><mtext>-</mtext><mi>nm</mi></mrow></math></inline-formula> half-pitch

For lithography of 45-nm half-pitch and beyond, the resist blur due to photoacid diffusion is a significant issue. On the other hand, it has been generally recognized that there is a trade-off between resist resolution and sensitivity. We study the influence of the resist blur on resolution in hypernumerical aperture ArF immersion lithography by utilizing a two-beam interferometric exposure tool. We evaluated the current photoresist performance for some of the latest commercial resists and estimated their acid diffusion lengths as 8 nm to 9 nm in sigma assuming Gaussian blur kernel. In addition, we found that the acid diffusion length, which is directly related to the resist resolution and is controllable by photoacid generator (PAG) anion size, polymer resin size, and post-exposure bake (PEB) temperature. We confirmed that there is a trade-off between resist resolution and sensitivity. Our results indicate that the resist blur is still a concern in order to extend lithography for 45 nm and beyond; however, it will not likely be a showstopper. We consider that total optimization of resists and exposure tools is important in order to achieve ultimate resolution in hyper-NA immersion lithography.

Heat Conduction to Photoresist on Top of Wafer during Post Exposure Bake Process: I. Numerical Approach

The post exposure bake (PEB) of a chemically amplified resist is one of the key processes for fabricating very small features of semiconductor devices. The use of photogenerated acid enables the de-protection of protected polymers, and this de-protection highly depends on PEB temperature and time. The diffusion length of acid is also strongly dependent on PEB temperature and time. As the line width of a device decreases, a smaller diffusion length is required to reduce the roughness of the line edge and width, and an acid diffusion length less than 20 nm is needed. One of the key factors for determining de-protection and acid diffusion is the initial temperature rise of the resist. The unpredictable temperature rise to the preset temperature mainly causes a variation in line width. In addition, in order to accurately predict the PEB temperature and time dependencies of line width, heat transfer from the hot plate to the resist on the top of a silicon wafer has to be calculated since reaction and diffusion occur inside the resist, not on the top of the bare silicon wafer. Heat transfer includes multiscale conductivity and thickness, so that we need an accurate and reliable approach. For this purpose, a novel numerical approach incorporated with analytic method is proposed to solve the heat conduction problem. Since this approach is incorporated with an analytic method, the number of unknowns can be markedly reduced. Indeed, only the interface temperatures are unknowns in this method and we can derive a system of Volterra-type integral equations for the same number of unknowns. Accordingly, this method has many advantages over other methods. Since it is not a difference method but an integral method, it is stable and robust in time step. The unknowns for temperature are located only at the interfaces between layers, so that this approach is fast and effective. The discretization in time variable is flexible enough to readily achieve the accuracy of the numerical solutions over time, i.e., in both ranges of short and long times. In this paper, we calculated the time consumed for the resist to attain the prescribed PEB temperature. Additionally, the effects of the different layer stacks and thicknesses are also investigated through our numerical approach.

High-Absorption Resist Process for Extreme Ultraviolet Lithography

The trade-off relationships among resolution, sensitivity, and line edge roughness (LER) have been the most serious problem for the realization of extreme ultraviolet (EUV) lithography. The enhancement of polymer absorption in combination with an ultrathin resist process is a solution for the trade-off problem. In this study, the relationship between aerial images of incident photons and latent images generated by polymer deprotection in high-absorption resists was investigated by a simulation based on the EUV sensitization mechanisms. Because of the restricted acid diffusion length, the side wall degradation is a problem at 22 nm fabrication. The improvement of aerial image quality significantly improves the side wall profiles. The relationship between the acid generator concentration and latent image quality was also investigated in terms of side wall angle and LER. By increasing the acid generator concentration from 10 to 30 wt %, the extent of LER can be reduced by ∼17%.