Lithographic methods of imaging in resist can be extended with the addition of immersion fluid.
The higher index of refraction fluid can be used to print smaller features by increasing the numerical
aperture beyond the limits of dry lithography. Alternately, an immersion optical system can achieve
a larger depth of focus at the same numerical aperture as the equivalent dry lithography system.
When numerical apertures are significantly greater than 1.0, polarization effects start to impact
resolution seriously. Special illumination conditions will be used to extend resolution limits.
Additional factors that affect imaging in resist need to be included if we are to achieve new
resolution limits using high index of refraction materials to increase numerical apertures. In addition
to material inhomogeneities, birefringence and optical surface effects, material absorption, coatings
and index differences at boundaries will have a larger impact on image resolution as ray angles in
the imaging system continue to increase with numerical aperture. Aerial and resist imaging effects that material characteristics have on polarization, uniformity
and aberrations in the lens pupil will be studied.
Lithographic methods of imaging in resist can be extended with the addition of immersion fluid. The higher index of refraction fluid can be used to print smaller features by increasing the numerical aperture beyond the limits of dry lithography. Alternately, an immersion optical system can achieve a larger depth of focus at the same numerical aperture as the equivalent dry lithography system.
When numerical apertures are significantly greater than 1.0, polarization effects start to impact resolution seriously. Special illumination conditions will be used to extend resolution limits. Additional factors that affect imaging in resist need to be included if we are to achieve new resolution limits using high index of refraction materials to increase numerical apertures. In addition to material inhomogeneities, birefringence and optical surface effects, material absorption, coatings and index differences at boundaries will have a larger impact on image resolution as ray angles in the imaging system continue to increase with numerical aperture.
Aerial and resist imaging effects that material characteristics have on polarization, uniformity and aberrations in the lens pupil will be studied.
Early manufacture and use of 157nm high NA lenses has presented significant challenges including: intrinsic birefringence correction, control of optical surface contamination, and the use of relatively unproven materials, coatings, and metrology. Many of these issues were addressed during the manufacture and use of International SEMATECH’s 0.85NA lens. Most significantly, we were the first to employ 157nm phase measurement interferometry (PMI) and birefringence modeling software for lens optimization. These efforts yielded significant wavefront improvement and produced one of the best wavefront-corrected 157nm lenses to date. After applying the best practices to the manufacture of the lens, we still had to overcome the difficulties of integrating the lens into the tool platform at International SEMATECH instead of at the supplier facility. After lens integration, alignment, and field optimization were complete, conventional lithography and phase ring aberration extraction techniques were used to characterize system performance. These techniques suggested a wavefront error of approximately 0.05 waves RMS--much larger than the 0.03 waves RMS predicted by 157nm PMI. In-situ wavefront correction was planned for in the early stages of this project to mitigate risks introduced by the use of development materials and techniques and field integration of the lens. In this publication, we document the development and use of a phase ring aberration extraction method for characterizing imaging performance and a technique for correcting aberrations with the addition of an optical compensation plate. Imaging results before and after the lens correction are presented and differences between actual and predicted results are discussed.
International SEMATECH(ISMT) has operated a 157nm Resist Test center since June of 2000. During this time, we have processed hundreds of 157nm photoresist samples from major resist suppliers and research organizations. Almost all of these of these early 157nm resists have demonstrated unusual susceptibility to airborne molecular contamination (AMC). Tests were completed at ISMT comparing post exposure delay stability of 157nm fluoropolymer resists to production level 193 and 248 nm resists. The 157nm resist samples were approximately 5X more sensitive to AMC. We have implemented extensive AMC control measures including the introduction of mini-environments and filtered wafer carriers to mitigate the AMC sensitivity of these resists. The effectiveness of the control measures was measured using a real time airborne base monitor, grab samples, and resist delay studies. 157nm resists were severely affected by relatively low airborne base concentrations of approximately 1 ppbv NH<sub>3</sub>. Hold time stability was significantly improved when the airborne base concentrations was reduced to < 0.5 ppbv using carbon filtration. Small variations in of +/- 0.2 ppbv NH<sub>3</sub> appear to be influencing resist profiles and delay response.
This report is the second series of 157nm alternating phase shifting mask done at ISMT. In this report, we present a comprehensive study of balancing aerial image through various feature sizes and pitches. New resutls of resist imagse are analyzed from a 157-nm alternating PSM with a 0.85 NA lens. The mask is made by dual trench technique with a phase-etch of 115nm and an isotropic under-etch of 90nm based on optimized simulation results. With this dual trenched mask, the wafer printing images show tremendous improvement on 'line paring' phenomena. We also investigate some abnormal CD variation across line array observed during this study. The results from this work give an initial assessment of 157-nm capability of alternating PSM and 157-nm resist imaging quality.
In this paper, we present a process of balancing the aerial image and analyzing the results of resist images of 157-nm alternating PSM with a 0.85 NA lens. The mask is made by dual trench technique with a phase-etch of 115nm and an isotropic under-etch of 90nm as predicted by simulations. With this dual trenched mask, the wafer printing images show tremendous improvement on “line walking” or “line paring” phenomena. The ultimate resolution is 60nm dense line. The focus latitude is around 0.1 to 0.15 um. We also used a 157-nm AIMS tool to check intensity balance. The results supports balanced intensity of this mask. For mask quality characterization, etch depth is measured by AFM and mask CD is measured by CD SEM. The uniformity of etching depth and mask CD are all within specifications. We also present some abnormal CD variation across line array observed during this investigation. The results from this work give a good groundwork of 157-nm capability of alternating PSM and 157-nm resist imaging quality.
Aberrations, aberrations, here there everywhere but how do we collect useful data that can be incorporated into our simulators? Over the past year there have no less than 18 papers published in the literature discussing how to measure aberrations to answering the question if Zernikes are really enough. The ability to accurately measure a Zernike coefficient in a timely cost effective manner can be priceless to device manufacturers. Exposure tool and lens manufacturers are reluctant to provide this information for a host of reasons, however, device manufacturers can use this data to better utilize each tool depending on the level and the type of semiconductors they produce. Dirksen et al. first discussed the ring test as an effective method of determining lens aberrations in a step and repeat system, later in a scanning system. The method is based on two elements; the linear response to the ring test to aberrations and the use of multiple imaging conditions. The authors have been working to further enhance the capability on the test on the first small field 157 nm exposure system at International SEMATECH. This data was generated and analyzed through previously discussed methods for Z5 through Z25 and correlated back to PMI data. Since no 157nm interferemetric systems exist the lens system PMI data was collected at 248nm. Correlation studies have isolated the possible existence of birefringence in the lens systems via the 3-foil aberration which was not seen at 248nm. Imaging experiments have been conducted for various geometry's and structures for critical dimensions ranging from 0.13micrometers down to 0.10micrometers with binary and 0.07micrometers with alternating phase shift mask. The authors will review the results of these experiments and the correlation to imaging data and PMI data.
Significant improvement in 157nm optical components lifetime is required for successful implementation of pilot and production scale 157nm lithography. To date, most of the 157nm optics lifetime data has been collected in controlled laboratory conditions by introducing predetermined concentrations of contaminants and monitoring degradation in terms of transmission loss. This publication compliments prior work by documenting field experience with the 157nm Exitech Microstepper currently in operation at International SEMATECH. Failure mechanisms of various optical components are presented and molecular contamination levels in purge gas, tool enclosure, and clean room are documented. Finally the impacts of contaminant deposition and degradation of components on imaging performance is discussed.
As resist feature sizes have decreased and the performance demands on chemically amplified photoresists have increased the role of the photoacid generator (PAG) in determining overall resist performance has become increasingly apparent. Over the past 20 years a variety of different types of PAG's have been introduced as researchers have sought to optimize properties such as acid strength, acid volatility, diffusion length, wavelength response, solubility etc. PAGs that produce very strong organic acids are widely used, in part because of requirements for high photospeed resists. Most of these acid generators are based on perfluoroalkyl sulfonic acid based onium salts. In an effort to identify and characterize alternative PAGs we have investigated the performance of a variety of photoacid generators that are not based on sulfonic acids. In this report we will describe the relative reactivities of these PAGs under a variety of exposure wavelengths and processing conditions including acid diffusion proprieties and photospeed measurements.
We have surveyed the commercial resist market with the dual purpose of identifying diazoquinone/novolac based resists that have potential for use as e-beam mask making resists and baselining these resists for comparison against future mask making resist candidates. For completeness, this survey would require that each resist be compared with an optimized developer and development process. To accomplish this task in an acceptable time period, e-beam lithography modeling was employed to quickly identify the resist and developer combinations that lead to superior resist performance. We describe the verification of a method to quickly screen commercial i-line resists with different developers, by determining modeling parameters for i-line resists from e-beam exposures, modeling the resist performance, and comparing predicted performance versus actual performance. We determined the lithographic performance of several DNQ/novolac resists whose modeled performance suggests that sensitivities of less than 40 (mu) C/cm<SUP>2</SUP> coupled with less than 10-nm CD change per percent change in dose are possible for target 600-nm features. This was accomplished by performing a series of statistically designed experiments on the leading resists candidates to optimize processing variables, followed by comparing experimentally determined resist sensitivities, latitudes, and profiles of the DNQ/novolac resists a their optimized process.
The goal of this paper is to define a 'state of the art' of the lithographic performance obtained with an advanced 193 nm single layer resist process, for 150 nm technology generation specification and below. Even if the goal of the paper is not to propose a process ready to be implemented in pilot lines, the resist used should be commercially available and exhibit good performance. The Sumitomo PAR101 A4 meets both criteria and is selected for the study. In order to get the best performance from the resist, all evaluation work is completed using a BARC. Both inorganic and organic materials are first considered; their processes are optimized to obtain the best reflectivity control and chemical compatibility with the resist. Then, using G0, conventional illumination and a binary reticle, the process performance is evaluated in terms of linearity, depth of focus, energy latitude and proximity effects for 150 nm and 130 nm lines, and depth of focus and energy latitude for 170 nm contact holes. Different optical extension techniques are then compared for increasing the 130 nm lines process latitudes: off-axis illumination, and alternating phase shift masks.
The design of 193 nm photoresists with improved reactive ion etch (RIE) resistance has been a longstanding aim of both industrial and academic research and development programs. A variety of correlations between photoresist polymer structure and etch resistance have been developed, however, the universality of these approaches, and in particular, the practicality of marking comparisons across specific polymer families and specific RIE processes has recently been called in to question. In order to examine structure: RIE correlations in more detail, we have developed a new model based on the incremental structural parameters (ISP). This model makes use of a molecular fragment-based definition of polymer structure which incorporates and extends aspects of previous parameters such as the Ohnishi and Ring parameters. An initial study revealed that this model allowed quantitative correlations between polymer families and across etch processes to be made. Continuing studies which examine the use of the ISP model in integrating 193 nm photoresists in prototype production processes will be described. Various polymer families used in deep-UV and 193 nm photoresists including methacrylates, alternating copolymers, styrenes and cyclic olefins will be compared. We will present a more detailed description of the ISP a model and of the follow-on 'new' ISP method which has been developed base don insights gained from the original ISP model, and made extended comparisons between the tow ISP models.
We have surveyed the commercial resist market with the dual purpose of identifying diazoquinone/novolac based resist that have potential for use as e-beam mask making resists and baselining these resist for comparison against future mask making resist candidates. For completeness, such a survey would require that each resists be compared with an optimized developer and develop process. To accomplish this task in an acceptable time period we have chosen to perform e-beam lithography modeling to quickly identify the resist developer combinations that will lead to superior resists performance. We describe the development and verification of a method to quickly screen commercial i-line resists under e-beam exposure with different developers. This was accomplished by determining modeling parameters for i-line resist from e-beam exposures, modeling the resist performance, and comparing predicted performance versus actual performance. We evaluated whether the technique of combining e-beam resist modeling with lithography can be used to quickly and efficiently screen i-line resists for use in e-beam mask making. This was accomplished by comparing experimentally determined resists sensitivities and profiles with those predicted from ProBeam/3D lithography modeling software.
Described here is an approach to aberration tolerancing utilizing aerial image parameterization based on photoresist capability. A minimum aerial image metric is determined from either a characterized resist process or desirable resist attributes and includes definition of resist exposure, diffusion, and development properties. Minimum aerial image requirements can then be correlated to resist performance to allow for the evaluation of various feature sizes and types. This allows, for example, the prediction of lens performance through focus, across the field, and in the presence of small levels of internal flare. Results can then be compared with more conventional optical metrics such as Strehl ratio, partial coherence contrast, or image threshold CD. Results are presented for three commercial small field catadioptric 193 nm lithographic lenses. Aberration levels for each lens at several field positions and at several wavelengths has been described using 37. Zernike polynomial coefficients. Minimum aerial image requirements have been correlated to resist performance to allow the evaluation of various feature types, a unique situation when no mature 193 nm resist process existed. Additionally, the impact of modified illumination on aberrations is presented.
We have examined the reactive ion etch (RIE) resistance of two families of 193 nm photoresist candidates, poly(methacrylates) and vinyl-polymerized poly(cyclic olefins), in three RIE processes. Correlation of these measurements to polymer structure and composition using known methods (Ohnishi and Ring Parameter fits) was moderately successful in demonstrating global trends but proved generally inadequate for providing quantitative predictions. To address this shortcoming, we have developed a new empirical structural parameter which provides a much more precise model for predicting RIE rates within a given family of polymers. The model is applicable across polymer platforms, with two caveats: (1) The methacrylate and cyclic olefin families examined to date fall on essentially parallel, offset curves when examined with the new model, (2) The offset between polymer family curves is RIE tool- and process-dependent. While these caveats imply a setback to the idea of a truly `universal' model, they may in fact represent a powerful and unanticipated feature; the model appears to separate chemical RIE processes which affect individual functionalities within a polymer from predominantly polymer-family dependent processes such as global backbone degradation. In the course of conducting these studies, we have encountered several potential pitfalls in the measurement of etch rates. These illustrate the complex nature of plasma: resist interactions and highlight the careful experimental design and controls that are required if meaningful RIE rate comparisons between polymer and resist families are desired.