Improving global CD uniformity by optimizing post-exposure bake and develop sequences

Real-time control is the trend for photoresist processing in the advanced lithography. In this paper we develop an in-situ ellipsometer integrated with a programmable thermal heating system, which could perform real-time monitoring and control for critical dimension (CD) latent image through the entire post-exposure bake (PEB) process. The inline ellipsometry measurements are fitted into an electromagnetic wave model built on the rigorous coupled-wave analysis (RCWA) theory for CD latent image characterization. Thereafter a real-time control scheme is proposed by applying the programmable thermal bake-plate. The experimental outcome demonstrates a significant improvement on the final CD result comparing to the conventional baking method.

Recent studies on dynamic temperature profiling and lithographic performance modeling of the post-exposure bake (PEB) process have demonstrated that the rate of heating and cooling may have an important influence on resist lithographic response. Measuring the transient surface temperature during the heating or cooling process with such accuracy can only be assured if the sensors embedded in or attached to the test wafer do not affect the temperature distribution in the bare wafer. In this paper we report on an experimental and analytical study to compare the transient response of embedded platinum resistance thermometer (PRT) sensors with surface-deposited, thin-film thermocouples (TFTC). The TFTCs on silicon wafers have been developed at NIST to measure wafer temperatures in other semiconductor thermal processes. Experiments are performed on a test bed built from a commercial, fab-qualified module with hot and chill plates using wafers that have been instrumented with calibrated type-E (NiCr/CuNi) TFTCs and commercial PRTs. Time constants were determined from an energy-balance analysis fitting the temperature-time derivative to the wafer temperature during the heating and cooling processes. The time constants for instrumented wafers ranged from 4.6 s to 5.1 s on heating for both the TFTC and PRT sensors, with an average difference less than 0.1 s between the TFTCs and PRTs and slightly greater differences on cooling.

Characterization for the photomask fabrication based on a high-resolution technique with a non-chemically amplified resist and a post-exposure bake

Progress towards 65 nm next-generation lithography requires unprecedented global CD uniformity, with the actual ITRS 2002 roadmap proposing 4.2 nm 3/spl sigma/ (dense lines) for 65 nm binary masks. Since resolution requirements are satisfied only by using chemically amplified resists (CARs), exposure and post-exposure bake (PEB) are key processes to successful mask making, both introducing global CD errors. Develop and etch processes potentially contribute further global CD errors. The global CD uniformity can be improved significantly by adaptive PEB, especially for CARs showing moderate to strong PEB sensitivity, like NEB22. With the 25-zone hotplate of the APB5500 bake system, facilitated through a novel calibration mask with 25 equidistant temperature sensors within the resist plane, an appropriate temperature profile can be applied during PEB. This temperature profile is automatically calculated by an adaptive optimization algorithm, based on 2-dimensional spline fitting of a CD measurement. A CD-uniformity improvement (dense lines) from 4.8 nm 3/spl sigma/ to 3.9 nm 3/spl sigma/ (/spl cong/20%) on a state-of-the-art production mask is achieved for the chrome layer (ASI, after strip inspection).

Nanoimprint lithography (NIL) is the preferred technique for next-generation nanometer-scale patterning because of its cost effectiveness and simplicity when compared to conventional technology. The replicated pattern depends on the mold pattern, hence efficient methods for fabricating the fine mold are critically required and have recently become intensive subjects of research. This study reports a new method for the fabrication of a fine mold: this method uses low-acceleration-voltage (4kV) electron beam lithography with spin on glass (SOG), which is used as a positive-tone inorganic resist. Although our SOG process does not contain chemically amplified material, postexposure bake (PEB) was characteristically used in the authors’ process. The PEB process caused the annealing effect for the SOG and also suppressed the proximity effect. Consequently, a line-and-space pattern nanoimprint mold with a sub-50nm spacing was fabricated at 4kV using their process, and ultraviolet NIL was successfully carried out using the fabricated mold.

A method for simulating exposure, post exposure bake and development process is described and applied for analyzing the problem of reflective stepped substrate. The method is based on the numerical solver of Helmholtz equation and the rigorous boundary condition which considers the entire system consisted of projection optics, photoresist and substrate. Diffusion equation and the ray tracing model are used for simulating the post exposure bake and the development process, respectively. The developed photoresist pattern on the stepped reflective substrate topography are obtained, and it shows irregular resist profiles due to the reflection of light at the step edge.

A new method for the measurement of acid diffusion parameters in negative-tone e-beam chemically amplified resists and its application for proximity effect correction is presented. It is based on the measurement of the diameter of lithographic features (pillars) obtained from single-pixel e-beam exposures. Generally, long post exposure bake (PEB) times or low post apply bake (PAB) temperatures lead to wider features compared with short PEB times and high PAB temperatures, due to enhanced acid diffusion. The acid diffusion coefficient was found to depend exponentially on cross-linking density. Additionally, proximity effect correction parameters (triple Gaussian parameters) are easily calculated. These parameters are t PEB dependent and in the limit (t PEB→0) are almost equal to the corresponding values for non-amplified resists. The method has been applied to SAL-601 (Shipley), and the calculated proximity parameters were used successfully to correct the proximity effect of complicated layouts with a 0.15 µ m critical dimension.

In previous papers, we described initial evaluations of CGR 248 negative resist using a variety of exposure tools. During subsequent studies, the emphasis has been placed on optimizing material and process for Micrascan and Micrascan II pilot line and manufacturing operations. The formulation is based on polyhydroxystyrene (PHS), tetramethoxymethyl glycoluril, and a sulfonate ester of an N-hydroxy compound. We will discuss image stability as a function of delay time between post apply bake (PA) and expose and as a function of delay time between expose and the post expose bake (PEB). Further, data will show that immersion or puddle development provides a larger process window than spray development for features in the 0.30 to 0.35 micrometers range. The thermal stability of the imaged resist will be discussed as well as the shelf life which is at least 6 months at 23 degree(s) C. Finally, additional data is available concerning image uniformity and how print bias and etch bias contribute to the overall nested-isolated line offset for positive tone (APEX-M) and negative tone (CGR) resists. Data obtained from Micrascan II exposures with test reticles will demonstrate an acceptable process latitude for 0.30 and 0.35 micrometers features and a wafer to wafer image uniformity similar to that observed for APEX.

Extreme ultra-violet lithography (EUVL) has been prepared for next generation lithography for several years. We could get sub-22 nm line and space (L/S) pattern using EUVL, but there are still some problems such as roughness, sensitivity, and resolution. According to 2007 ITRS roadmap, line edge roughness (LER) has to be below 1.9 nm to get a 22 nm node, but it is too difficult to control line width roughness (LWR) because line width is determined by not only the post exposure bake (PEB) time, temperature and acid diffusion length, but also the component and size of the resist. A new method is suggested to reduce the roughness. The surface roughness can be smoothed by applying the resist reflow process (RRP) for the developed resist. We made resist profile which has surface roughness by applying exposure, PEB and development process for line and space pattern. The surface roughness is calculated by changing parameters such as the protected ratio of resin. The PEB time is also varied. We compared difference between 1:1 L/S and 1:3 L/S pattern for 22 nm. Developed resist baked above the glass transition temperature will flow and the surface will be smoothed. As a result, LER and LWR will be much smaller after RRP. The result shows that the decreasing ratio of LER due to RRP is larger when initial LER is large. We believe that current ~ 5 nm LWR can be smoothed to ~ 1 nm by using RRP after develop.

Modeling has become indespensable tool for chemically amplified resist (CAR) evaluations. It has been used extensively to study acid diffusion and its effects on resist image formation. Several commercial and academic simulators have been developed for CAR process simulation. For commercial simulators such as PROLITH (Finle Technologies) and Solid-C (Sigma-C), the user is allowed to choose between an empirical model or a concentration dependant diffusion model. The empirical model is faster but not very accurate for 2-dimension resist simulations. In this case there is a trade off between the speed of the simulator and the accuracy of the results. An academic simulator such as STORM (U.C. Berkeley) gives the user a choice of different algorithms including Fast Imaging 2nd order finite difference algorithm and Moving Boundary finite element algorithm. A user interested in simulating the volume shrinkage and polymer stress effects during post exposure bake will need the Moving Boundary algorithm whereas a user interested in the latent image formation without polymer deformations will find the Fast Imaging algorithm more appropriate. The Fast Imaging algorithm is generally faster and requires less computer memory. This choice of algorithm presents a trade off between speed and level of detail in resist profile prediction. This paper surveys the different models and simulator algorithms available in the literature. Contributions in the field of CAR modeling including contributions to characterization of CAR exposure and post exposure bake (PEB) processes for different resist systems. Several numerical algorithms and their performances will also be discussed in this paper.

Negative-tone chemically amplified resists (nCARs), like NEB22 are promising candidates for next-generation lithography, e.g. 90 nm and 65 nm technology node and next-generation lithography. For these resists, e-beam exposure and post-exposure bake (PEB) are most critical processes, since these resists show a strong sensitivity to post-exposure delay (PED) in vacuum during e-beam writing of about 0.5 nm/h, and in air while waiting for PEB. Further, such resists show a strong PEB temperature sensitivity of up to 8 nm/K. The multi-zone hotplate approach of the APB AFB 5500 bake system with its use prior temperature uniformity results in excellent global CD-uniformity already. However, all kinds of systematic large area effects of processes, e.g. blank coat/bake, exposure, PED, the PEB itself, etch loading, etc. may transfer in additional systematic CD-errors. Such systematic, repeatable errors can be reduced during PEB by superimposing an appropriate non-uniform temperature profile onto the regular, optimized uniform bake temperature profile, thereby compensating for such CD-non-uniformities. The required temperature profile can automatically be calculated from a suitable gobal CD measurement, determined in a typical process flow. The compensation of CD-errors resulting from vacuum PED and hotplate temperature characteristics is demonstrated here, by using automated temperature profile calculation. The global CD uniformity was improved significantly, the achieved results show a typical reduction of about 20-30%, from a total global range of about 9nm to about 6-7nm on leading-edge production photomasks.

Iii previous work by Sturtevant et al [1994] it was shown how the use of scatterometry data obtained in real time during the post-expose bake (PEB) step of the photolithography process can improve critical dimension (CD) control. However, it is not obvious that the ad hoc methodology utilized during these experiments conducted at IBM, is the best way to use this valuable information. This paper focuses on the development of more rigorous methods that can incorporate scatterometry data in on-line prediction of the bake time necessary to achieve the target CD. First, a brief description of the system and required tools is given. Then, a model that can relate the measured intensity and the time the wafer has been baked (referred to hereafter as the bake time) is introduced. Then, two theoretical methods that can utilize this model to calculate the bake end point are described. Finally, these two methods are applied to the analysis ofadditional data collected at IBM and operation oftbe complete end point control system is emulated to the extent that is possible. Keywords: end-point detection, process control, deep-UV lithography, scatterometry, post-exposure bake process

Physically-based photoresist models, such as those in PROLITH, have been very successful in describing photolithography from a continuum standpoint. These models allow engineers to accurately predict the final resist CD on the wafer and to analyze process robustness, such as calculation of focus-exposure process windows. However, as feature sizes continue to shrink, we are beginning to see yield-limiting phenomena that are due to the molecular nature of photoresist materials. One example of this is line-edge roughness (LER). LER is believed to be due to fluctuations during the exposure process (shot noise) and post-exposure bake (thermal diffusion and reaction). We present a model that explicitly takes into account the molecular nature of the photoresist during the exposure and post-exposure bake processes. We do this by writing a master equation that describes the probability that acid molecules are generated during exposure, and then describes the evolution of the acid, quencher, and blocking-group probability distributions during the bake process. We show how all the parameters in this model can be simply derived from the parameters in a calibrated PROLITH continuum model. Finally, we demonstrate prediction of LER from an accurately tuned PROLITH continuum model and compare the LER predictions with experimental results.

The second level exposure of Alternating Phase Shift Mask has to be accomplished by e-beam lithography. However, e-beam writing on structured open chrome layers may induce severe charging and yields to deteriorate overlay accuracy. In case of charging problems, applying of conductive top coats on resists help to reduce the distortion of the second layer. ESPACER makes the conductive layer on the e-beam resist and prevent a positioning error during e-beam writing by its shield effect. However, ESPACER required removal of its topcoat before post-exposure bake (PEB) because of the undesirable effect to the resist, due to the acid diffusion from the ESPACER film to high sensitive Chemical Amplified Resists (CARs). Our investigations were focused on the combination of the representative positive tone CAR: FUJIFILM ELECTRONIC MATERIALS CO., LTD and the new-type ESPACER called ESPACER 300F. ESPACER 300F was possible to use the PEB process before its removal, improved by exchanging a lower mobility surfactant than before. This material also had a good wettability, a low resist-thickness losses and kept good pattern shapes of the resist. Additionally, ESPACER 300F have little influence in both overlay accuracy at the variety of pattern density and sensitivity of the resist, therefore it would be the good material for making reticles for 65nm node and beyond.

Previously, a novel method for fabricating microfluidic and microelectromechanical devices with buried microchannel structures using thermally sacrificial polymers was reported. These previous methods required separate lithographic and etching sequences to pattern the sacrificial polymer. In this work, a more advanced approach in which the sacrificial material is radiation sensitive and can be patterned directly using standard lithographic techniques is explored. The lithographic performance of a new class of photosensitive polynorbornene (PNB) sacrificial materials has been characterized. The effect of soft bake and postexposure bake (PEB) on the cross-linking of photodefinable PNB has also been investigated. It was found that significant cross-linking of PNB occurs after exposure during the subsequent postexposure bake. However, this phenomenon is strongly dependent on the soft bake conditions used in preparing the sample, presumably due to varying levels of residual solvent content. This may be due to the high mass transport of the reactive species because of evaporation of residual solvent and shrinking of polymer matrix during the PEB process. No noticeable influence of residual solvent on cross-linking has been found during exposure. © 2002 The Electrochemical Society. All rights reserved.