Post Exposure Bake Conditions Research Articles

7.4 nm linewidth Pt nanowires by electron-beam lithography using non-chemically amplified positive-tone resist and post-exposure bake

Abstract 7.4 nm linewidth Pt nanowires were demonstrated on SiO2/Si substrates via electron-beam lithography using a non-chemically amplified positive resist ZEP520A and post-exposure bake (PEB) treatment. The effect of the PEB treatment conditions on the nanowires’ characteristics was investigated. As the PEB temperature and time increased, a decrease in the mean linewidth and an improvement of the line-width (line-edge) roughness of the nanowires were observed. Pt nanowires with an ultrafine linewidth of 7.4 nm were successfully fabricated using the optimal condition of 100 °C for 2 min, verifying the effectiveness of PEB for fabricating sub-10 nm linewidth robust metal nanowires.

Negative tone photolithography with photo-sensitised polymethyl methacrylate (PMMA)

We describe the science and technology of negative tone photolithography using polymethyl methacrylate (PMMA), photo-sensitised with the addition of the commercial photoinitiator – Irgacure 651. This versatile two-active-ingredient resist system is shown to enable negative tone photolithography which can be customised to application requirements by adjusting the ratio of the two ingredients. This study describes the complete lithographic process – from resist preparation to pattern transfer – in considerable detail. Furthermore, we also elucidate the mechanism of resist action through exploration of the effects of such parameters as the molecular weight of PMMA, exposure wavelength, post-exposure bake conditions and resist development temperature, among others. This work also demonstrates that photo-cross-linkable resists can be made resistant to dry etching by a post-development blanket UV exposure step. Finally, we also show example structures fabricated using both additive and subtractive techniques, with the use of this resist system.

Effects of temperature on mechanical properties of SU-8 photoresist material

A representative fabrication processing of SU-8 photoresist, Ultraviolet (UV) lithography is usually composed of spin coat, soft bake, UV exposure, post exposure bake (PEB), development and optional hard bake, etc. The exposed region of SU-8 is crosslinked during the PEB process and its physical properties highly depend on UV exposure and PEB condition. This work was initiated to investigate if thermal baking after fabrication can affect the mechanical properties of SU-8 photoresist material because SU-8 is trying to be used as a structural material for MEMS operated at high temperature. Since a temperature of 95°C is normally recommended for PEB process, elevated temperatures up to 200°C were considered for the optional hard bake process. The viscoelastic material properties were measured by dynamic mechanical analyses (DMA). Also, pulling tests were performed to obtain Young’s modulus and Poisson’s ratio as a function of strain rate in a wide temperature range. From this study, the effects of temperature on the elastic and viscoelastic material properties of SU-8 were obtained.

Time Dependent Behavior of Chemically Amplified Resist Characterized under Sub-millisecond Post Exposure Bake

Chemically amplified photoresists require a post exposure bake (PEB) to catalytically deprotect the polymer backbone. However, excessive diffusion of the photogenerated acid during PEB results in resolution loss and line edge roughness. As both deprotection and diffusion are thermally activated processes, understanding their reaction kinetics as a function of temperature and time is potentially key to optimizing these two competing processes. In this work, we characterize the resist deprotection kinetics under laser PEB (lPEB), utilizing continuous wave CO2 laser thermal processing to shift the duration into sub-millisecond time frames at temperatures up to 500°C. At these temperatures and times, the extent of deprotection under l-PEB is comparable to that observed under conventional hot plate PEB conditions in the second time frames. Results show that, under isothermal conditions, deprotection has a power law dependence as a function of PEB duration, exhibiting secondorder or higher order kinetics under second time frames but a first-order kinetics under lPEB in sub-millisecond times.

Design of Faster High Resolution Resists: Getting More Acid Yield from EUV Photons

This paper discusses the development of an improved EUV Resist, XP-6627A, which easily resolves sub-30nm features at a photospeed of 12mJ/cm2. This new resist is designed for increased acid yield from greater secondary electron generation. XP-6627A has comparable resolution to XP-6627G, with a concomitant photospeed improvement of >50%. Due to the high activation nature of the resist, the best postexposure bake condition is 130°C for 90s. This material also has an ultralow diffusion coefficient as derived from CD vs PEB time studies. Finally, due to the low volatility of the resist components, the resist has low outgassing rates.

Sub-50nm half-pitch imaging with a low activation energy chemically amplified photoresist

Critical lithographic dimensions are rapidly approaching the sub-50nm regime where there is a concern that image blur due to acid diffusion will impose a practical limit to the resolution of chemically amplified (CA) resists. Although recent EUV and 193- and 157nm immersion interferometric experiments have reportedly resolved line-space arrays with individual dimensions on the order of ∼40nm, smaller nested features are likely to prove problematic. Numerous reports suggest that conventional photoresist performance degrades rapidly at half-pitch dimensions in this range. New approaches to processing and materials development of photoresists will likely be required if the concept of chemical amplification is to be extended to the 32nm node and beyond. In this article we show that through materials choice and proper processing, image blur can be controlled to an extent where dense features below 40nm can routinely be resolved in CA resists. We describe our studies on high-sensitivity resists of differing activation energies and demonstrate the effect of postexposure bake conditions on the ultimate resolution. Most notably, electron-beam exposure of a CA resist that requires no postexpose bake has demonstrated half-pitch resolution down to 30nm with reasonable process latitude. Acid diffusion measurements performed on the same systems using a 193nm top surface-imaging-technique correlate with the observed resolution of the different resists and processing conditions. Additionally, this system has been probed to explore the effect of humidity on the rate of the resist deprotection reactions in order to determine the impact this might have on resolution.

Characterization of 193 nm Chemically Amplified Resist during Post Exposure Bake and Post Exposure Delay

We investigated the property change of a positive type 193 nm chemically amplified resist (CAR) during post exposure bake (PEB) and post exposure delay (PED). Upon PEB the thickness and optical property are changed. These changes depend on the exposure energy and the PEB conditions. The thickness and the imaginary refractive index changes during PEB are determined. The thickness reduction and the refractive index changes are related to the de-protection of the resist. By modeling these relationships the concentration of de-protected site of the resist and the parameters are determined. The PED effect is also considered and included in the model. The de-protection parameters needed for the 193 nm lithography are extracted. The obtained parameters are used with the lithography simulation for profile calculation, and the results are compared with the observed SEM resist profile.

Comparative study of AZPF514 and UVIII chemically amplified resists for electron beam nanolithography

Two commercial positive tone chemically amplified resists, AZPF514 and UVIII, were investigated and compared for their performance in e-beam direct write of wafers at sub-150nm resolutions. The resist sensitivity, process latitude and post exposure delay effect were studied. Both resists are of high sensitivity. They are insensitive to variation of post exposure bake conditions. The post exposure vacuum delay effect is much more pronounced in AZPF514. AZPF514 also suffers severely the ''T-topping'' effect. Feature dimension below 150nm cannot be achieved with AZPF514, while sub-50nm lines have been obtained with UVIII.

Electron-beam nanolithography, acid diffusion, and chemical kinetics in SAL-601

The work presents a unique investigation of the role of postexposure bake (PEB) on resist insolubility, PEB reaction kinetics, and the high resolution behavior of Microposit™ SAL-601. Patterns of 20–100 μm rectangles and single pass isolated lines were written on a JEOL JBX-5DII e-beam lithography system operated at 50 kV. The effects of PEB temperatures of 90–110 °C for periods of 1–11 min on resist insolubility and linewidth were examined. The samples were developed in MF-322 or acetone. The range of patterns allowed measurement of both the resist exposure curves and the line spread for each PEB condition. Insolubility in MF-322 can result from protection reactions and/or crosslinking reactions. Acetone insolubility, a characteristic of crosslinked resist, was observed at PEB temperatures of 100 °C and above. At 90 °C, acetone insolubility (crosslinking) was observed only after a PEB of 11 min. From the resist exposure curves for a sequence of PEB times, reaction orders for the PEB processes that led to resist insolubility were determined. At temperatures of 100–110 °C, reaction orders of 2.4 and 2.1 were obtained for reactions that led to insolubility in MF-322 and acetone, respectively. At 90 °C the order of reaction leading to MF-322 insolubility was 1.8. PEBs at 90 °C for 3 and 6 min produced the finest lines, with linewidths ⩽50 nm over an order of magnitude range in dose. An acid diffusion mth order reaction model, which allows for H+ loss to the surface, was introduced to analyze the linewidth measurements. In the 105–110 °C temperature range, the model best fit the data with diffusion coefficients of 4–8 nm2/s and acid loss rates of 0.003–0.005%/s.

Optimization of DUV Negative Resists for 0.15 .MU.m Lithography.

Recent performance improvements of a negative tone chemically amplified DUV resist consisting of a hydroxystyrene based copolymer, a melamine crosslinker, a photoacid generator (PAG) and certain amine additives are described. The general trends of formulation and process changes on the lithographic performance were investigated using a response surface method. PAG concentration variations and optimization of the prebake conditions allowed for the elimination of microbridging in the subquarter micron region. The pattern profiles were improved by the optimization of the polymer properties and the selection of a specific amine/ammonium hydroxide combination. The effects of certain polymer properties and of individual amine components are discussed in more detail and a correlation with their structures is given based on dissolution rate measurements. The optimized resist material has a resolution potential below 0.15μm (NA=0.55) combined with a large depth of focus (>1.0μm @ 0.15μm iso lines), acceptable iso-dense bias, and small sensitivity towards changes in the post exposure bake conditions.

Control in sub-100 nm lithography in SAL-601

This article focuses on e-beam nanolithography in SAL-601, a negative tone, chemically amplified resist from the Shipley Corporation. It examines the role of exposure conditions and the postexposure bake (PEB) on the ability to define features of critical dimensions of 100 nm and below. Lithography was performed with a 50 kV e beam and 140 nm thick resist, where forward scattering of the e beam is expected to be minimal. Common PEB conditions of 105 °C for 1, 3, and 10 min and 110 °C for 1 min were investigated. Although the different PEB conditions produced a factor of up to 2.4× increase in sensitivity, no change was observed in the contrast, 5.8±0.1, or the minimum linewidths, ∼60 nm. The measured line spread function forward Gaussian widths were characterized by standard deviations, σ=27–33 nm, depending on PEB conditions. These are compared to σ of 19 nm for PMMA and the expected σ of 11 nm for the incident probe (σ=10 nm) convoluted with a Monte Carlo code generated point spread function. The measured backscatter coefficient was 0.51, in agreement with our Monte Carlo simulation results. In the analysis of dose latitude to form critical dimension gaps (±10% precision) between a series of pads of differing sizes, the different PEB conditions produced no observed difference. The feature size, however, had a large effect on the ability to form the critical dimension gap. Furthermore, the dose latitude was much smaller than calculated by integration of Monte Carlo derived line spread functions. A semiempirical model is introduced in which the forward Gaussian width of a measured line spread function (LSF) in developed resist is convoluted with the Monte Carlo code LSF. The calculated exposure profiles based on the semiempirical LSF show much better agreement with the experiment than those based on the expected, σ=11 nm, forward Gaussian width.

Diagnostics of patterning mechanisms in chemically amplified resists from bake dependencies of images

Measurement and analysis of bake dependencies of resist profiles are used to establish the critical role of diffusion and acid loss in lithographic performance. Imaging experiments carried out with APEX-E with postexposure bake temperatures above and below its glass transition temperature evidence widely dissimilar feature size dependencies attributable to different acid diffusion mechanisms in the glassy and the rubbery polymer, respectively. Examination of the mechanisms determining e-beam exposed resist image stability show that, when an acid loss mechanism is present, the initial amount of acid generated the reaction rate, and the diffusion mechanisms are essential in predicting feature size dependence on postexposure bake conditions.

A study of acid diffusion in chemically amplified deep ultraviolet resist

Postexposure bake (PEB) dependence of photogenerated acid diffusion was investigated in a chemically amplified deep ultraviolet positive resist. The resist consisted of a tert-butoxycarbonyl protected polystyrene as base resin and 2,4-dimethylbenzenesulfonic acid derivative as photoacid generator. The diffusion length of photoacid increased with increasing PEB temperature or its time. Moreover, the activation energy of acid diffusion reaction within the resist film became smaller, with increased exposure dose. It is considered that hydrophilic OH sites of the base resin generated by the deprotection of hydrophobic protecting groups has a role as one of the diffusion paths in the polymer matrix. Furthermore, it was found that the diffusion coefficient under high PEB conditions was affected by the acid reduction. Based on the analysis of diffusion characteristics, clear correlation between acid diffusion in the resist film and PEB conditions was obtained. These results are useful for improving both resolution capability and pattern profiles.

Measuring system for diffusion length of photoactive compound in post‐exposure bake process

AbstractThe post‐exposure bake (PEB) characteristics of a photoresist can be examined with a method that estimates the diffusion length of a photoactive compound (PAC) in the photoresist PEB process. Diffusion lengths are estimated with development rate measurement equipment using PEB conditions at temperatures of 50°C, 100°C, 110°C, and 120°. An ultrahigh‐resolution i‐line photoresist THMR‐iP3000 is used in the research. The obtained values are input into a photolithography simulator PROLITH/2 to calculate photoresist profiles. Using a 365‐nm wavelength, 0.5 NA, and 0.6‐coherence factor conditions, resist profiles for a 0.4‐μm line space feature are calculated with varied focus. The simulated results are compared with SEM observations to evaluate the validity of the estimation method; they are found to be substantially consistent with experimental results, thus confirming the validity of the method.

Dissolution Kinetics Analysis for Chemically Amplified Deep Ultraviolet Resist

Dissolution kinetics of a chemically amplified deep-ultraviolet (DUV) positive resist, which consists of tert-butoxycarbonyl (t-BOC)-protected phenolic resin, benzenesulfonic acid derivative as a photoacid generator (PAG) and an additional dissolution inhibitor, has been investigated by focusing on the t-BOC-protected phenolic resin structure (t-BOC protection ratio/molecular weight M w) and postexposure bake (PEB) temperature. Based on the analysis of the dissolution rate curve and Arrhenius plots, it was concluded that only one mechanism, namely, the penetration of tetramethylammonium hydroxide (TMAH) developer into hydrophobic t-BOC resin, rules the dissolution kinetics. It was also found that a steep slope of the dissolution rate curve is very effective for improving resolution capability. Polymer structure, as well as the PEB condition, was optimized from the correlation between dissolution kinetics and resist performance, and good resist performance was realized.

Resist pattern fluctuation limits in extreme-ultraviolet lithography

Extreme ultraviolet (EUV) projection lithography has been proposed to achieve features as small as L=180 nm to 70 nm for 1 G to 16 G DRAMs. Application will require high-sensitivity resists and pattern fluctuation control to less than 10% of nominal linewidth. To evaluate low-dose, resist material, and resist process dependent resist roughness limits in EUV lithography, a roughness model originally by Neureuther and Wilson is extended, and a new model for chemical amplification resists is presented and applied to EUV lithography. Analyses of molecular scale simulation and EUV exposures of novolac negative chemical amplification resists complete the study. For 13 nm exposure wavelengths, 180 nm lithography with positive chain-scission resists requires at least 0.69 mJ/cm2, which scales to 54.3 mJ/cm2 for 70 nm features, accounting for both intrinsic resist polymer roughness and absorption in 100 nm PMMA. At 4.5 nm exposure, the dose minima are 15.9 mJ/cm2 and 1254 mJ/cm2, for 1 G and 16 G respectively. Novolac negative chemical amplification resists have a theoretical minimum linewidth of 115 nm, dependent on postexposure bake conditions and resist composition. Printability criteria limit the extent to which process variation can improve roughness. Existing novolac-based negative chemical amplification resists are not suitable for EUV lithography in the 16 G regime, but may be suitable for the 4 G regime under certain conditions.

Acid-diffusion effect on nanofabrication in chemical amplification resist

Acid-diffusion effect on nanometer pattern fabrication in a chemical amplification resist, SAL601 (Shipley Co.), is investigated with a finely focused electron beam. During postexposure bake (PEB), acid generated by the electron beam exposure diffuses and is assumed to cause pattern size changes. A scanning electron microscope, S-900 (Hitachi), which has a beam diameter of approximately 2 nm at 5 kV, is used to make latent images of nanometer isolated lines in a resist film. After electron beam exposure, the resist films with a thickness of 20 nm are baked in different conditions before development. The measured linewidths are found to be proportional to the square root of the PEB time. According to a simple acid diffusion model, this can be explained by the diffusion of generated acid during PEB. A minimum feature size of a 20 nm isolated line is obtained by adjusting the PEB conditions. It is therefore important to control the PEB conditions to suppress the acid diffusion for the critical dimension control of nanofabrication when using chemical amplification resist systems. Fine edge roughness is also observed in the delineated patterns. This is thought to originate from the distribution of diffused acid or from the random distribution of the base polymer with finite molecular sizes.

Acid generation and deprotecting reaction by diphenyliodonium 9,10‐dimethoxyanthracene‐2‐sulfonate in a novolak positive photoresist based on chemical amplification

AbstractIn a positive photoresist composed of diphenyliodonium 9,10‐dimethoxyanthracene‐2‐sulfonate as a novel photoacid generator, bisphenol A protected with tertbutoxycarbonyl group as a dissolution inhibitor, and a novolak resist matrix, the efficiency of photo‐acid generation and deprotective reaction were investigated by means of UV‐visible and IR spectroscopies. The quantitative measurement of photogenerated acid by using the acid‐sensitive dye exhibited 0.18 as the quantum yield of acid generation in novolak resin film. The lithographic evaluation of this system as a chemically amplified resist was studied. The catalytic chain length for the acid‐catalyzed deprotection step was determined as about 100 when 10 min post‐exposure bake (PEB) at 80°C was given. The sensitivity and the resolution as a positive resist are 180 mJ/cm2 and higher than 1 μm, respectively under the PEB conditions mentioned above.

Process control with chemical amplification resists using deep ultraviolet and x-ray radiation

Resists for optical electron beam, and x-ray lithographies that operate on the principles of chemical amplification are entering into widespread use in laboratories. They offer an attractive alternative to conventional positive Novolac photoresists and we may soon see similar use in manufacturing environments. One class of resists within this family is based on acid-hardening chemistry. We have characterized one such resist (Shipley ECX-1033) for illumination by excimer laser deep ultraviolet sources and also by x-ray radiation. A matrix of postexposure bake (PEB) conditions and development conditions was used to examine resist sensitivity and contrast. For all exposure sources we found that contrast is independent of PEB processing and that sensitivity obeys an Arrhenius dependence. Contrast increases with increasing development time while exposed resist loss is minor. A simple kinetic model was developed to explain the observed variations of apparent resist sensitivity with PEB time and temperature. The generality of this model suggests that it is widely applicable to other chemical amplification resists that require a postexposure bake. Together with aerial image calculations for different light sources, the model makes it possible to predict the dependence of linewidth on PEB conditions. In other words, we can anticipate the PEB process control required to achieve a specified critical dimension control. Electrical linewidth measurements of submicron features and their temperature dependence are compared with the predictions of the model.