In this paper, two negative-tone chemically amplified resists (CAR) are evaluated. The methodology and results are compared and discussed. The resists include EN-024M from TOK, and NEB 31 from Sumitomo. Both resists show high contrast, good dry etch selectivity, and high environmental stability. EN-024M showed good coating uniformity while NEB31 showed a coating uniformity problem. This was a round “dimple” approximately one centimeter in diameter of different thickness and density at the center of the plate. We addressed the “dimple” coating problem as described in the paper. Optimum PAB and PEB temperatures and nominal to maximum doses for isolated features were determined by running a matrix of PAB and PEB temperatures along with a dose series. We evaluated the process and compared the lithographic performance in terms of dose sensitivity, dose and bake latitude, resolution, resist profile, OPC (Optical Proximity Correction) pattern fidelity, CD uniformity, environmental stability, Line Edge Roughness (LER) and etching bias and resistance.
In this paper, a process established with a positive-tone chemically amplified resist (CAR) from TOK REAP200 and Fujifilm Arch FEP171 and 50kV MEBES system is discussed. This TOK resist is developed for raster scan 50 kV e-beam systems. It has high contrast, good coating characteristics, good dry etch selectivity, and high environmental stability. In the mask industries, the most popular positive tone CAR is FEP171, which is a high activation energy type CAR. REAP (Raster E-beam Advanced Process) 200 is low activation energy type and new acetal protecting polymer. In this study, we compared to these different type resists in terms of contrast, PAB and PEB latitude, resist profile, footing, T-topping, PED stability, LER, Global CDU (Critical Dimension Uniformity) and resolution. The REAP200 Resist obtained 75nm isolated lines and spaces, 90nm dense patterns with vertical profile, and a good stability of delay time.
Electron beam (e-beam) proximity effect correction (PEC) has become a critical consideration with the reduction of feature sizes and the increasing use of optical proximity effect correction (OPC). PEC is accomplished on the 50kV MEBES eXara system with the embedded PEC (emPEC) technique, which is an automated version of GHOST.
This method equalizes the background exposure by writing a fifth pass in the reverse tone of the four primary exposure passes. This compensates for the dose differences in the background due to backscattered electrons. The result is feature size control independent of the proximity and density of surrounding features, and independent of feature size. The method is simple and does not require additional computations for raster scan systems.
A description of the method and its results for both coverage proximity effect correction (feature size deviation as a function of the surrounding exposure density) and CD linearity (deviation of different feature sizes) is presented.
Photomask complexity threatens to outpace mask pattern generator productivity, as semiconductor devices are scaled down and optical proximity correction (OPC) becomes commonplace. Raster scan architectures are well suited to the challenge of maintaining mask throughput and mask quality despite these trends. The MEBES eXara mask pattern generator combines the resolution of a finely focused 50 keV electron beam with the productivity and accuracy of Raster Graybeam writing. Features below 100 nm can be imaged, and OPC designs are produced with consistent fidelity. Write time is independent of resist sensitivity, allowing high-dose processes to be extended, and relaxing sensitivity constraints on chemically amplified resists. Data handling capability is enhanced by a new hierarchical front end and hiearchical data format, building on an underlying writing strategy that is efficient for OPC patterns. A large operating range enables the MEBES eXara system to support the production of 100 nm photomasks, and the development of 70 nm masks.
A chemically amplified resist (CAR) process has been recognized as an approach to meet the demanding critical dimension (CD) specifications of 100nm node technology and beyond. Recently, significant effort has been devoted to optimizing CAR materials, which offer the characteristics required for next generation photomask fabrication. In this paper, a process established with a positive-tone CAR from TOK and 50kV MEBES eXara system is discussed. This resist is developed for raster scan 50 kV e-beam systems. It has high contrast, good coating characteristics, good dry etch selectivity, and high environmental stability. The coating process is conducted in an environment with amine concentration less than 2 ppb. A nitrogen environment is provided during plate transfer steps. Resolution using a 60nm writing grid is 90nm line and space patterns. CD linearity is maintained down to 240nm for isolated lines or spaces by applying embedded proximity effect correction (emPEC). Optimizations of post-apply bake (PAB) and post-expose bake (PEB) time, temperature, and uniformity are completed to improve adhesion, coating uniformity, and resolution. A puddle develop process is optimized to improve line edge roughness, edge slope, and resolution. Dry etch process is optimized on a TetraT system to transfer the resist image into the chrome layer with minimum etch bias.
The complexity of photomasks is rapidly increasing as semiconductor devices are scaled down and optical proximity correction (OPC) becomes commonplace. Raster scan architectures are well suited to the challenge of maintaining mask throughput despite these trends. Electron-beam techniques have the resolution to support OPC requirements into the foreseeable future. The MEBES® eXara mask pattern generator combines the resolution of a finely focused electron probe with the productivity and accuracy of Raster Graybeam patterning. Features below 100nm can be created, and OPC designs are produced with consistent fidelity. Write time is independent of resist sensitivity, allowing high-dose processes to be extended, and relaxing sensitivity constraints on advanced chemically amplified resists. The system is designed for the production of 100nm photomasks, and will support the development of 70nm masks.
Photomask complexity increases rapidly as semiconductor devices continue to shrink and as optical proximity correction becomes commonplace. This trend stresses the performance of mask pattern generators due to the increase in both primary and subresolution features. However, the next-generation MEBES raster scan architecture is well-suited to the challenge of maintaining throughput regardless of increases in pattern complexity. In addition, this new system provides an operating envelope that is sufficiently broad to expose all practical resist materials with a fixed number of writing passes. Write time is independent of material sensitivity, which has the benefits of allowing high-dose processes to be optimized, and also of supporting a wide selection of chemically amplified resist candidates for critical mask patterning. The new system shows the promise of being extendible to the 70 nm technology generation.
Gray-level printing is an efficient strategy to create small-address patterns on photomasks. This work provides a technical description of the multipass gray (MPG) raster- scan writing technique as implemented on the MEBES 4500S and MEBES 5000 electron-beam pattern generation tools. The differences between single-pass printing (SPP) and MPG are reviewed. The factors that allow increase in throughput and dose with MPG are explained. Aerial image simulations of edge placement and corner rounding verify the MPG model. Multipass writing with offset scan voting, which reduces random and systematic errors, is explained. Because MPG is a gray-level printing technique, the dose distribution across feature edges is necessarily broader than that derived from SPP writing. Simulations and experimental results indicate that, using ZEP 7000 resist and dry etch, edges can be placed without loss of accuracy, despite the width of this 'gray' profile. The spot size necessary to obtain optimal critical dimension quality is also determined by simulation and empirically. The lithographic quality of MPG writing/processing is confirmed by composite metrology test that sample the whole quality area of the mask. We conclude that MPG is a viable technique for writing advanced masks.
Leading edge technologies require continually shrinking design grids due to the demands of decreasing minimum feature size and higher resolution. Using conventional raster-scanned exposure tools to place these patterns on photomasks result in longer write times, because linear decreases in address result in exponential increases in write time. This phenomenon can be compensated for by changes in writing strategies. Multipass gray is one method of drastically improving throughput at small addresses while retaining lithographic quality.
The technology in use in today's mask shop may not be adequate to meet 180 nm production requirements. In particular, requirements for linewidth control, linewidth uniformity, and registration are tightening at a breakneck pace. In the past, incremental improvements to registration and linewidth have been adequate to keep pace with steadily evolving mask requirements. This paradigm of continuous incremental improvement is not longer a valid model. Mask writers are needed that can deliver a higher dose to support advanced resists for superior critical dimension (CD) control, write faster to compensate for shrinking device sizes, and incorporate advanced calibration and error control schemes to improve placement accuracy. This paper describes key electron-beam pattern generation activities necessary to meet 180 nm mask requirements. This includes testing and implementation of multipass graybeam to improve throughput at lower addresses without compromising lithography quality, a new resist and process capable of supporting dry etching, and a data path capable of supporting addressing to 10 nm. Multipass gray (MPG) writing strategy was introduced with the MEBES 4500S. The ability to deliver a 4X improvement in dose while improving throughput is a significant advantage over previous MEBES systems. Because MPG is used in conjunction with offset scan voting, improvements in registration performance and a reduction in butting of over 50 percent have been demonstrated. Some of the process improvements attributed to a high dose mask writer are also documented in this paper. CD uniformity improvements with ZEP 7000 and dry etch at a 720 nm nominal feature size and smaller are also discussed in some detail.
Etec Systems, Inc. has developed a new e-beam mask lithography system, the MEBES 4500S, featuring a higher productivity writing strategy called multipass gray and a number of mechanical and electrical improvements. This new system, based on the proven technologies introduced in the MEBES 4500 system, provides improved throughput and accuracy. The MEBES 4500S system with multipass gray supports smaller mask design addresses needed for high resolution masks, while providing higher dose for high contrast processes with low sensitivity and improved CD linearity. Improved print performance is achieved by the introduction of several system design changes that work in conjunction with the multipass gray writing mode. These changes include improved column deflection system temperature control, enhanced TFE current control, improved work chamber thermal management, and improved stage drive vibration damping. Details of these features are presented along with first performance data for the new system.
As requirements for critical dimension (CD) control tighten, continuing advances in processing techniques are necessary. Mask blanks with reduced film thickness, good control of film thickness uniformity, and resistance to pinholes are now commercially available. The reduction in the amount of forward scattering with these reduced film stacks is primarily responsible for improvements in CD uniformity and CD linearity. CD uniformity, CD linearity, and CD control (mean to target) were measured using 6' by 0.25' PBS substrates and exposing them with MERESR 4500 and 4500S machines. Both thick and thin resists were studied. The thin PBS process was modified by changing developer solvents. This was done to keep the development time in the desired range for the best CD control. Both processes use puddle develop with a reduced temperature postbake and exposure at 2.0 (mu) C/cm2. Blanks from two vendors were examined and compared. Results show a significant improvement (greater than 30%) in CD uniformity over a 132 mm2 array. In addition, an improvement in CD linearity and in the ability to maintain mean-to-target CD control were also noted. Modeling and experimental results were compared. We conclude that adoption of a thin resist process has significant advantages in commercial maskmaking. The lithographic properties of thin (200 nm) and standard (400 nm) PBS were modeled using ProBEAM/3D. ProBEAM/3D uses a Monte Carlo model to predict the energy deposited in the resist during exposure and a development rate model to predict the resist profile and CD size after development. Results from modeling were compared to experimental results. As expected, modeling confirms the hypothesis that reduced forward scatter is the dominant mechanism to improve CD performance.
Experiments were initiated that examined parameters for optimum proximity effect correction using GHOST with 400 nm of PBS and the standard PBS process. Parameters examined included the size of the correction spot and the ratio of the correction dose to the primary dose. Critical dimension CD deviations from the mean in proximity tower patterns were used to determine optimum parameters. Results show that without data bias, the minimum feature size that can be achieved with GHOST within +/- 5% of the mean CD at 10 keV and 400 nm of resist is 300 nm. The correction scheme begins to degrade rapidly with smaller features. CD control at optimum GHOST parameters can be achieved by introducing data bias of 100 nm per side of a line to account for the wet etch undercut in the PBS process. This effectively increases the minimum feature size achievable from 300 nm to 500 nm. A resist that can be dry etched, thereby eliminating the wet etch undercut, will alleviate this problem.
Critical dimension (CD) uniformity performance has improved significantly over the years by optimizing postbaking and development steps. As we continue to improve CD uniformity with PBS, our focus has shifted to other steps in the process. The crossover step, the point in the process where both develop and rinse are introduced simultaneously, has often been thought to have little or no effect on the develop time or to influence the uniformity results. However, careful examination of this step has determined that the crossover time, spin speed, and concentration of the develop/rinse mixture can have a significant effect on CD uniformity. Reduction of the typical radial effect was noted. Several design of experiment projects were run to examine these variables. A screening experiment was first run to determine the significant variables and their influence on uniformity. An optimization run was then made on crossover conditions, which included examination of plates from several vendors. Repeatability and sensitivity of the process to other variables were also determined. Results and recommendations are presented in this paper.
This paper examines the steps needed to improve CD uniformity to meet desired goals for 250 nm optical lithography. The exploratory study of a number of different approaches included development work on both APT 9155 and Hammatech developer tools, an assessment of different developers and rinses, an examination of solvent delivery systems, and materials evaluations such as resist annealing and determination of resist thickness variations at plate edges. The goal for this project is to reach a 25 nm 3 sigma uniformity. Much of the work focused on CD uniformity in a 132 mm field. Improvements to the PBS process were made with a gravity-flow solvent-delivery system, the use of less aggressive (slower) developers, and the use of single or multiple puddles with the Hammatech tool. Improvements were also observed with higher dose, thinner resist films, and smaller spot sizes. With the initial Hammatech process, uniformity was improved with PBS at 2.0 (mu) C/cm2. The results are plotted. Progress over the initial results is predicted with improvements in materials, optimization of the Hammatech puddle process, and automated gravity-flow solvent delivery. Better performance is also expected with higher doses, smaller spot sizes at the equivalent address, optimum solvents, and thinner resist. These results show that there are still improvements to be made to PBS, which make it a potential candidate for 250 nm reticle manufacturing.
The SIA roadmap has identified CD control as a critical issue in mask making. PBS, the most popular resist used for electron-beam mask making in the U.S., may not perform at the level required for production of 250 nm devices. There is a need in the industry today for precise CD control and tight control of CD uniformity, as well as a desire to dry etch thin films on masks. These industry trends make the use of an alternative resist attractive. A project was initiated to determine if an acceptable substitute to PBS exists. A group of eleven negative and positive resists were examined. These included chemically amplified materials, two part- novolacs, and a silicon-containing resist, among others. The resists were evaluated by using design of experiments (DOE) methodology whenever possible. All masks were exposed on 10 kV MEBES writing tools. The results were tabulated and compared, using a SEMATECH criterion for acceptability. Results are presented, including optimization of some of the materials for sensitivity, process robustness, and dry etch capability. While none of the materials met all criteria, several resists performed at a level that make them candidates to replace PBS. Several options are presented that are of interest to the mask maker contemplating process changes to accommodate 250 nm and 180 nm technologies.
Performance of a MEBES tool depends in part on how well it is optimized for a particular user application. This paper examines the efforts made to optimize a MEBES 4000 at Intel to meet performance goals of 350 nm design rules. The areas of particular concern are critical dimension, resolution, and composite positional accuracy. PBS resist processes and cassette- specific corrections (CAZOC) for six cassettes are examined to meet these goals. As part of a SEMATECH development program, a MEBES 4000 system at Etec is being upgraded to a MEBES 4500. The performance of the tool is characterized at each incremental phase of the upgrade. Results show that significant advances have been made in accuracy, system calibration and control, and data path.
Design rules for 250 nm devices and optical proximity correction (OPC) enhancement techniques require improved resolution. Resolution requirements for these applications extend into the 500 nm realm. The most widely used resist for e-beam generated masks is PBS. Difficulties have been reported when using PBS for features smaller than 2.0 micrometers . These have included poor resolution and CD linearity anomalies at <EQ 1.5 micrometers . Methods were investigated to improve the resolution and CD linearity of PBS resist. The investigation included a study of resist profiles. Results show that resist wall angles improve with increases in dose. At a nominal dose of 1.0 (mu) C/cm2, angles can range from 40 to 60 degrees, depending on the type of feature. A significant improvement in wall angle is noted at 2.0 (mu) C/cm2, and the angles continue to improve with higher doses until they approach 90 degrees at 4.0 (mu) C/cm2. The symmetry of resist lines improves when a puddle process is used instead of the standard spin-spray process. Fluid flowing across the resist during spin-spray processing creates asymmetries, especially with submicron features. This flow asymmetry is eliminated with a puddle process. Postdevelop baking is critical for both resolution and CD linearity. Resist baked at the standard 120 degree(s)C shows degradation of wall angles, asymmetry of resist profiles, and loss of unexposed resist at feature edges. Poor CD linearity is caused by postbaking at temperatures that are too high. A postbake of 98 degree(s) to 102 degree(s)C maintains sharp wall angles and prevents line edge roughness. Experiments show good resolution at 0.7 micrometers for all features, including contacts in both tones, with good CD linearity, CD control, and CD uniformity. The changes made in PBS processing described in this paper can result in acceptable processing of features down to 0.5 micrometers for most mask layers, without relying on the use of proximity effect correction.
New MEBES reticle writers are described that meet the production requirements of the 350- nm and 250-nm design rules required for 64 Mb and first generation 256 Mb DRAM techniques. These raster scan e-beam systems are based on the MEBES IV thermal field emission (TFE) exposure system, in production use since early 1992. The MEBES IV-TFE system exceeds its 500-nm design rule requirement and is routinely used to product reticles of first-generation 64 Mb DRAMs, prototype 256 Mb DRAMs, and phase shift masks. The success of MEBES IV-TFE is based on a close working relationship with system users, who provided input to establish the requirements of the new reticle writers. The new reticle writers are the result of a two-phase development program. The initial phase, completed in 1993, focused on productivity improvements to the base system, which proved to have excellent accuracy. These improvements ease the handling of the large pattern files, improve the use of the 160 MHz writing rate with a faster data path and more efficient writing strategy, and improve overall system utilization with in situ (maskless) beam-calibration techniques. The second phase of development, completed early in 1994, focused on the production reticle requirements of second-generation 64 Mb DRAM, including optical proximity correction features, and first-generation 256 Mb DRAM. The second development phase improves data path speed, system accuracy, and system productivity. System and subsystem performance is shown for the first and second development phases. Lithographic and write-time performance on the product is presented and discussed in the context of system requirements.
OCG895i and Tokyo Ohka OEBR2000 (both commercially available) and two experimental resists were evaluated by experimental design. The design factors investigated included developer normality, softbake temperature, and develop time with a sodium hydroxide-based developer. The design responses included optimum dose, remaining film thickness, and dose latitude (change in critical dimension per unit dose). The best results were given by AZ141C, an experimental resist from Hoechst. At 90 degree(s)C prebaking temperature, AZ141C could be imaged at 4.0 (mu) C/cm2 with good film thickness retention and dose latitude. A second set of optimization experiments was done evaluating metal ion-free developer. Finally, multiple develop processing was evaluated for improving process latitude and film thickness loss and for minimizing the dose required. A two-step process shows promise: it consists of a high initial normality develop for a short time to accomplish breakthrough of the resist surface inhibition layer, followed by a second low normality develop. Another sequence of statistically designed experiments performed to optimize this scheme and results of the optimizations are presented.
Proximity effect correction is necessary to fabricate masks with 0.25 micron design rules using electron beam lithography. The GHOST technique of proximity correction has the advantage of no pattern preprocessing and is easily implemented on a raster scan system such as MEBES. Recent results show proximity corrected features at 0.3 micron. To minimize constraints on the resist characteristics, such as the Srg ratio, global sizing of patterns has been investigated and found to provide an additional degree of freedom to control sensitivities and process latitude. Simulation and experimental results will be presented to demonstrate the use of GHOST and sizing for 1X mask making, including discussion of some of the relevant issues and tradeoffs.
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