Timothy R. Groves

Maskless electron beam lithography: prospects, progress, and challenges

Abstract The resolution of electron beams is unlimited, for practical purposes in lithography. The central problem of e-beam lithography remains throughput, which has been historically too slow for mainstream VLSI manufacturing. The rapidly escalating cost of masks has given rise to renewed interest in various maskless alternatives, including direct write e-beam lithography. Usable writing current in probe forming systems is limited by the Coulomb interaction in the beam path. The resulting writing speed increases roughly as the cube root of the pixel parallelism for these systems. In contrast, distributed systems are not limited in usable writing current by the Coulomb interaction. These systems scale more favorably in writing speed. The proposed DiVa (Distributed Variable shaped beam system) is projected to be capable of writing at a speed of 5.2 cm 2 /s with 1000 shaped beams. If this can be achieved in practice, it will open up the possibility of mainstream manufacturing of VLSI circuits using maskless e-beam lithography. Feasibility issues include the electron source, parallel operation, and lithography demonstration at 50 nm and below. An experimental apparatus has been constructed, and has achieved its first beam.

EL‐4, a new generation electron‐beam lithography system

The new generation electron‐beam lithography system EL‐4 is described, designed for direct wafer exposure as well as optical reticle and x‐ray mask making. The new architecture features control through workstations and local area network communication between these and the microprocessor‐controlled subsystems. The system has on‐line error checking and diagnostics. Wafers up to 200 mm diam are handled individually with a Standard Mechanical InterFace‐compatible, fully robotic system, and are electrostatically chucked to the stage. Reticles are clamped to the stage with double‐sided e/s chucks, ring‐bonded membrane masks are kinematically held in a carrier chucked to the stage. The reticle/mask maker has an internal temperature control system in addition to the clean‐room climate control for the entire mechanical hardware. The electron optics accommodate triangle as well as rectangle spot formation, and for direct write application a throughput‐enhancing third level in the deflection hierarchy. High resolut...

Recent advances with the variable axis immersion lens

The novel variable axis immersion lens (VAIL) magnetic focusing and deflection system has been shown to produce excellent images over large fields. A practical lithography tool, however, requires high throughput (writing speed) which the magnetic deflection alone cannot provide. This paper describes the incorporation of a unique high‐speed electrostatic deflection system in VAIL which maintains perpendicular landing everywhere on the target. In order to keep the beam at optimum focus when the target height or spot size is changing, a high‐speed focus coil was developed which does not produce any positional errors when excited.

EL5: One tool for advanced x-ray and chrome on glass mask making

The state-of-the-art for mask making continues to be driven by 1× x-ray masks. The IBM EL4+ e-beam mask writer at the Advanced Mask Facility in Burlington, Vermont, was originally designed for 0.35 μm ground rules (GRs) direct write at 50 kV, but delivered at 75 kV operation to achieve 0.25 μm GR performance for 1× mask making. Over the next 2 years, with optimization and improvements in each of the subsystems, its performance was enhanced beyond the 0.18 μm GR requirements. It is clear, however, that for 0.13 and 0.1 μm GR mask manufacturing, a new tool is required. It has also become apparent that because of the very high development and tool build costs, and small number of required x-ray mask makers, the same technology must be applicable for chrome on glass (COG) mask making. Based on the experience with EL4+, IBM is designing an EL5 tool which will provide the 0.13/0.1 μm GR performance for 1×, and easily convert to 4× COG exposure for 9 in. glass as well as 300 mm wafer direct write operation. As w...

Performance of IBM's EL-4 e-beam lithography system

IBMs latest electron beam mask maker, EL-4, is online at IBMs Advanced Mask Facility (AMF) in Essex Junction, Vermont. The EL-4 system is a 75KV shaped beam lithography system utilizing a Variable Axis Immersion Lens (VAIL) designed to produce 1X or NX masks for 0.25 micrometers lithography ground rules, extendable to 0.13 micrometers . It is currently producing NIST-style X-ray membrane masks with pattern sizes over 30 X 30 mm2. This paper will give a brief description of the EL-4 tool and its operating features, specific measures used to enhance tool stability and accuracy, and measurement data from masks recently produced on the tool.

EL3 system for quarter‐micron electron beam lithography

The introduction of the shaped beam imaging technique has greatly enhanced the exposure efficiency of electron beam (e‐beam) lithography systems. The electron beam lithography systems from IBM provide the throughput capability needed for lithography applications in semiconductor fabrication lines. The resolution of these systems has steadily improved over the past 15 years, in support of the semiconductor trend towards submicron dimensions. This paper describes the latest version EL3 system, capable of fabricating 0.25‐μm dimension features. The technical challenges of submicron e‐beam lithography are discussed. A practical solution is described, with experimental results. The outlook for the future is also highlighted.

Performance enhancements on IBM’s EL‐4 electron‐beam lithography system

IBM’s latest electron‐beam mask maker, EL‐4, is installed at IBM’s Advanced Mask Facility in Essex Junction, Vermont. The EL‐4 system is a 75 kV variable‐shaped‐beam lithography system designed to produce 1X or NX masks for 0.25 μm lithography ground rules, extendable to 0.13 μm. It is currently producing NIST‐style x‐ray membrane masks with pattern sizes up to 50×50 mm2. After a brief description of the EL‐4 tool and its operating features, the article describes the recently implemented new writing subsystem, provides an overview of the tool software structure, and presents measurement data from masks recently produced on the tool.

Shaped beam technology for nano-imprint mask lithography

the Leica SB350MW 50keV shaped-beam e-beam lithography tool was used to write large-area 1X templates applicable in Step and Flash Imprint Lithography (S-FIL). This paper describes how information from the pattern analysis can be used to define the ZEP7000 resist exposure optimization technique for the SB350 MW tool together with the Motorola template pattern transfer process to obtain final template images in the transparent template. As a result of the complete process, well-resolved trenches measuring 33 nm and contacts as small as 44nm were obtained. Further improvements in the resist patterning will be possible by an adaptation of our standard proximity corrector (currently used in the 90 nm node maskmaking) with a high resolution upgrade.

Performance of the EL‐3+ maskmaker

IBM’s new e‐beam maskmaker, designated EL‐3+, is installed and operating in the IBM Advanced Mask Facility in Burlington, Vermont. This tool represents a significant extension in the state of the art in the manufacture of masks, particularly in the areas of minimum feature size and overlay. The tool routinely operates with 0.35 μm ground rules at 70 nm (3σ) registration to grid. The tool has demonstrated the ability to work at 0.25 μm ground rules as well. The primary mission of the tool is the production of 1X x‐ray masks. Some of the tool parameters include a 50 keV electron beam operating at a current density of 20 A/cm2. The system uses a shaped spot with a maximum size of 2×2 μm2. Exposures are made over areas with dimensions of up to 80 by 80 mm with a 2.1 mm field size. Deflection within a field is done through a combination of magnetic and electric deflection in a variable axis immersion lens (VAIL) configuration. During exposure the pattern data is stored on‐line in a 1 G‐byte buffer. The pattern buffers are loaded directly from a host IBM 4381. The system automatically corrects for any field distortions to a level of 6.25 nm using a calibrated reference grid.

Electron beam lithography tool for manufacture of X-ray masks

An electron beam lithography system suitable for manufacturing X-ray masks with critical dimensions down to 0.35 µm is described. The system features a 50-kV variable shaped spot (VSS) electron column with a variable axis immersion lens (VAIL). This column is capable of maintaining 0.035-µm edge acuity of the focused spot over a 2.1-mm deflection field. These fields are stitched together over an 84 × 84-mm active pattern area via motion of an xy table. The table position is measured using a laser interferometer. The measurement data are fed back to the magnetic deflection to correct small errors. Maintaining positional accuracy of the beam relative to the writing surface relies on a strategy of measuring and correcting repeatable errors. This is described in detail. Pattern placement accuracy is 0.070 µm (3σ) and image size control is 0.025 µm (3σ), achieved over the entire 84 × 84-mm pattern area. This performance is achieved with yield better than 90%, as confirmed by routine measurements. The system is currently used to manufacture product X-ray masks with 0.35-µm critical dimensions. Typical measurement results on product masks are presented.