Robert L. Dean

100-nm OPC mask patterning using raster-scan 50-kV pattern generation technology

ABSTRACT The complexity of photomasks is rapidly increasing as semiconductor devices are scaled down and optical proximitycorrection (OPC) becomes commonplace. Raster scan architectures are well suited to the challenge of maintaining maskthroughput despite these trends. Electron-beam techniques have the resolution to support OPC requirements into theforeseeable future. The MEBES ® eXara mask pattern generator combines the resolution of a finely focused electronprobe with the productivity and accuracy of Raster Graybeam patterning. Features below 100nm can be created, andOPC designs are produced with consistent fidelity. Write time is independent of resist sensitivity, allowing high-doseprocesses to be extended, and relaxing sensitivity constraints on advanced chemically amplified resists. The system isdesigned for the production of 100nm photomasks, and w ill support the development of 70nm masks.Keywords: MEBES, electron beam, lithography, photomask, graybeam, CAR, OPC 1. INTRODUCTION

Multipass gray printing for the new MEBES 4500S mask lithography system

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.

REAP (raster e-beam advanced process) using 50-kV raster e-beam system for sub-100-nm node mask technology

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.

Raster scan patterning solution for 100- and 70-nm OPC masks

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.

Further work in optimizing PBS: is it capable of meeting specifications for 256 Mbit DRAM reticle manufacturing?

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.

Evaluation of commercial and experimental resist materials for use in MEBES mask making

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.

MEBES 4000 optimization and characterization of MEBES 4500

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.

Investigation of lithography performance using multipass gray (MPG) with MEBES 5000

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.

MEBES reticle writers for 350-nm and 250-nm design rules

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.

PBS resist profile studies for submicron mask lithography

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.