Mask patterning capability continues to be a key enabler for wafer patterning. Mask writer performance is critical to meet reticle resolution, critical dimension uniformity, registration, and throughput requirements. Technology trends indicate that mask requirements will require higher dose resists with more complex designs producing write time growth that significantly exceeds Moore’s law estimates. Sub 10 nm technology node requirements may exceed what is practically or economically achievable using conventional single beam writers. This is driving the need to explore alternative e-beam mask writer architectures for future nodes.
Several equipment suppliers are proposing new architectures for mask patterning. These approaches share the characteristic of some level of parallelism to solve the throughput challenge caused by increasing mask pattern complexity. Although parallelism is a proven approach in laser mask writers, it has not been integrated into an e-beam platform. All of the approaches for multibeam e-beam architectures have unique technical difficulties. In some cases, suppliers have produced proof of concept results to demonstrate the feasibility of their approach and address key technical risks. Although these results are encouraging, it is clear that they need more time and industry assistance to produce a commercially worthy mask writer.
Key drivers will be considered. Proposed evolutionary extensions of the current architecture will be evaluated. The need for revolutionary architectures to satisfy future mask patterning will be explored.
Aggressive 193nm optical lithography solutions have in turn led to increasingly complex model-based OPC methodologies. This complexity married with the inevitable march of Moore's Law has produced a figure count explosion at the mask writer level. Variable shaped beam equipment manufacturers have tried to mollify the impact of this figure count explosion on the write time by the introduction of new technologies such as increased beam current density, faster DAC amplifiers and more efficient stage algorithms. Despite these efforts, mask manufacturers continue to explore ways of increasing writer throughput and available capacity. This study models the impact of further improvements in beam current density and settling times. Furthermore, this model will be used to prescribe the necessary improvement rates needed to keep pace with the shot count trends extending beyond the 45nm node.
SLM-based DUV laser writers are gaining acceptance for 2nd level PSM and binary mask patterning. These writers can use an e-beam compatible resist enabling tool and process sharing. For binary mask patterning, critical metrics include: critical dimension uniformity (CDU), CD targeting, mask registration, defect performance and inspectability. For PSM applications, pattern fidelity matching to 1st level and PSM overlay are also important. A Sigma7300 is being integrated into 65nm and 45nm production. Binary and PSM mask performance data will be presented. Tool self metrics to characterize SLM health will also be presented. Data conversion, data preparation and production write times will be discussed.
Phase shift mask (PSM) applications are becoming essential for addressing the lithography requirements of the 65 nm technology node and beyond. Many mask writer properties must be under control to expose the second level of advanced PSM: second level alignment system accuracy, resolution, pattern fidelity, critical dimension (CD) uniformity and registration. Optical mask writers have the advantage of process simplicity for this application, as they do not require a discharge layer. This paper discusses how the mask writer properties affect the error budget for printing the second level. A deep ultraviolet (DUV) mask writer with a spatial light modulator (SLM) is used in the experimental part of the paper. Partially coherent imaging optics at the 248 nm wavelength provide improved resolution over previous systems, and pattern fidelity is optimized by a real-time corner enhancement function. Lithographic performance is compared to the requirements for second level exposure of advanced PSM. The results indicate sufficient capability and stability for 2nd level alternating PSM patterning at the 65 nm and 45 nm nodes.
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.
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.
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.
As device dimensions shrink, a detailed understanding of the exposure and development of masks is necessary to optimize electron-beam lithography. Because of proximity effects and dose distributions within the resist, achieving small- pattern fidelity is one of the most challenging tasks in maskmaking. The research discussed in this paper examines the exposure and process parameters that influence the fidelity of features on a photomask, with a focus on critical dimension (CD) uniformity, CD linearity, small- feature resolution, and long-term system performance. In accordance with operating recommendations for the MEBESTM 5500 systems, all experiments are performed with ZEP 7000 resist, 10 (mu) C/cm2 dose, ZED 750 developer, and dry etch. Some experiments employ GHOST proximity effect correction (FastPEC). These results are instructive for improved 130 nm node lithography and 180 nm node productivity.
Recent developments in electron-beam (e-beam) systems and mask-writing strategies facilitate pattern generation for the 130-nm IC generation. The MEBESR 5500 pattern generation system incorporates a high-dose electron optical system and a high-throughput writing strategy, Multipass Gray-II (MPG-II). We evaluate the effectiveness of these innovations by three criteria: improved resolution, improved critical dimension (CD) control, and increased throughput. The conclusions of this paper are based on results from extensive modeling, test masks, and factory acceptance masks. Mask resist choice and processing have been optimized for the MEBES 5500 system. A consequence of these improvements is greater productivity for 150 nm devices and early development of 130 nm devices. The MEBES 5500 system uses a high-dose gun and electron optical system. The maximum current density that can be delivered to the mask is 800 A/cm2, twice the value of previous MEBES systems. Without loss of throughput, it is possible to increase the dose deposited in the resist, while using smaller e-beam sizes. These capabilities are exploited to improve printing of submicrometer features, including 200 nm-scale optical proximity correction (OPC) patterns. At small data addresses (<17.1 nm), the MPG-II writing strategy provides twice the throughput of the existing multipass gray (MPG) strategy with the same instrument, and 16X the throughput of traditional single-pass printing (SPP) with the MEBES 4500 system. The fundamentals of the MPG-II strategy are described, as well as throughput and lithographic results.
As optical lithography is extended to the 130 nm generation and beyond, demanding requirements are placed on mask pattern generators to produce quartz substrate masks. This paper reports on the lithography and critical dimension (CD) performance of the MEBES 5500 mask pattern generator. Compared to previous MEBES tools, this system employs a new high-dose electron gun and column design. We summarize experiments relating lithographic quality to increased dose and the effects of spot size on lithography. Methods to reduce beam-induced pattern placement errors are reviewed. A new graybeam writing strategy, Multipass Gray-II, is described in detail. This strategy creates eight dosed gray levels and provides increased writing throughput (up to 8X compared to single-pass printing) without loss of lithographic quality. These experiments are performed with ZEP 7000 resist and dry etch process; improvements in CD control have been achieved by optimizing the process. A consequence of the improvement in CD control and throughput is improved productivity in generating 180 nm devices.
This paper describes improvements in column design and writing strategy that, together, enable mask production for the 130 nm technology node. The MEBESR 5500 system employs a new high-dose electron gun and column design. We summarize experiments relating lithographic quality to increased dose and the effects of spot size and input address on lithography. These experiments are performed with ZEP 7000 resist and dry etch. A new graybeam writing strategy, Multipass Gray-II (MPG- II), is described in detail. This strategy creates eight dosed gray levels and provides increased writing throughput (up to 8X, compared to single-pass printing) without loss of lithographic quality. Significantly, critical dimension (CD) uniformity, butting, and other important specifications are improved with MPG-II. Lithographic results and throughput data are reviewed. A consequence of the improvement in CD control and throughput is greater productivity for 180 nm devices.
Analysis of pattern placement errors has shown a pattern and exposure sequence dependent component of placement error exists that cannot be accounted for by beam and stage positioning errors alone. The interaction of the electron beam (e-beam) with the resist can cause displacement of the e-beam from its desired position. This is commonly referred to as mask charging. A special pattern was created to enhance this effect in order to study its functional dependencies, including resist thickness, resist type, and exposure sequence. These errors are noticeable when writing in multipass strategies and where there are large gradients in pattern density. Customer and acceptance test patterns are included in the matrix to determine the magnitude of the errors with more production-oriented patterns. To further characterize this placement error phenomenon, the MEBES e-beam column was modified to minimize the distance between the exit location of the electrons from the electron optics and the surface of the resist-coated mask. Preliminary test results indicate pattern placement error is reduced by approximately 25% with this 'reduced gap' design. We are currently assessing the long-term effects of this new design. Of more importance, choice of resist and process are key components in reducing the charging effect. Reduction of more than 50% in placement errors using ZEP 7000 resist is detailed in this paper.
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.
Pattern generation tools must employ improved hardware and new writing strategies to accommodate progressively smaller geometries. At the same time, the lithographic process and metrology strategy must evolve to achieve targets for minimum feature size and feature quality. The MEBES 5000 electron-beam (e-beam) system incorporates hardware and process improvements necessary for 180-nm mask production. Significantly, the system can deliver the high dose needed to pattern advanced resists in practical times. This report describes the 320-MHz data path implemented on the MEBES 5000 system. With additional improvements, including updated temperature regulation and dynamic correction of scan errors, improved throughput and critical dimension (CD) control are achieved. Multipass gray (MPG) is the recommended writing strategy for writing small-address patterns. This high-throughput writing strategy is described in some detail. The high doses that are possible with MPG support the use of high-contrast resists and dry etch. As documented here, patterns with excellent CD qualities can be produced rapidly with MPG and ZEP 7000 resist.
Optical lithography will be the dominant technique used for 180 nm generation production devices. With a reduced feature size on the wafer, 4X optical reduction, optical proximity correction (OPC), and phase shift lithography techniques, mask-related errors become even more critical to wafer yield. In addition, small feature sizes and lithography enhancement techniques require finer edge resolution. Clearly, new patten generation tools are needed for this generation of maskmaking requirements. Multipass gray (MPG) writing strategy was introduced with the MEBESR 4500S. The ability to deliver a 4X improvement in dose while improving throughput is a significant advantage over previous MEBES systems. Since MPG is used in conjunction with offset scan voting, reduction in butting of over 50% has been demonstrated with MPG. Higher doses are now possible with use of a multipass writing strategy and a brighter source. As a result, resists with higher contrast and process robustness can be used. A significant improvement in uniformity is noted with the new process, an essential step needed in meeting 180 nm requirements. Dry etch is essential to meet these new requirements and with sufficient process margin to be manufacturable. This paper describes the key electron-beam pattern generation technology necessary to meet the requirement of 180 nm masks, including a high dose field- emission gun and column capable of delivering 800 A/cm2; complete dynamic beam correction; a digital stage servo to provide stable, reproducible stage control under high acceleration conditions; a high speed data path to support 320 MHz beam blanking and a 10 nm data address. This paper also examines the improvements made to the MEBES platform and documents the resulting improvements and compares these results to the requirements for 180 nm masks.