SPIE Handbook of Microlithography, Micromachining and Microfabrication, Volume 1: Microlithography

Section 2.5 Systems: 2.5.6 Shaped Spot and Cell Projection Systems

2.5.1 Environment
2.5.2 SEM and STEM Conversions
2.5.3 Commercial SEM Conversion Systems
2.5.4 Gaussian vector scan systems
2.5.5 Gaussian Spot Mask Writers
2.5.6 Shaped Spot and Cell Projection Systems
2.5.7 SCALPEL
2.5.8 Other E-Beam System Research
2.5.9 Electron Beam Fabrication Services

Table of Contents

 All of the e-beam tools described above focus the beam into a small spot, and shapes are formed by rastering the beam. This spot is the reduced image of the source, often referred to as the "gun crossover," which has a current intensity profile resembling a Gaussian distribution. The time needed to paint a shape can be eliminated by forming the electron beam into primitive shapes (rectangles and triangles) and then exposing large areas with single "shots" of the beam. The optics of these shaped spot systems is shown schematically in Fig. 2.24. The upper aperture typically uses a square to form two sides of a rectangle, and the overlap of the lower aperture defines its length and width. More complex shapes are fractured into rectangles and triangles before exposure.

FIGURE 2.24 Shaped spot optics. (a) The first square aperture defines two sides of a rectangle, and the second square aperture defines the other two sides. Deflectors determine the overlap and thus the length and width of the rectangle, as shown in (b). By deflecting over a corner, triangles can be formed. The shaped spot image would be further demagnified by magnetic lenses lower in the column.

Because of their increased parallelism over Gaussian raster-scan tools, shaped spot systems are much faster. However, throughput is still limited by the remaining serialism, by stage movements, and in a few cases by data transfer times. Shaped spot systems can readily be extended to 0.15 m resolution (compared to the 0.25 m resolution of the Gaussian raster beam systems). While there is no well defined standard for the comparison of throughput, we can say that the throughput of shaped spot machines remains under 10 wafers/hour -- making them superior to Gaussian systems but not competitive with optical steppers which produce, typically, 40 to 80 wafers/hour. The market for high speed shaped spot systems remains in maskmaking, direct-write prototyping, and low volume production of 0.15 m scale features.

2.5.6.1 IBM EL-4

Shaped spot systems have been pioneered, but never sold, by IBM. The latest version, EL-4, combines an extraordinarily large number of lenses [65-67] (Fig. 2.25) with a unique three-stage deflection for optimum speed. The final lens, termed a variable axis immersion lens (VAIL) provides minimized off-axis aberrations (or maximum field coverage) as well as telecentric beam positioning, with the beam landing normal to the substrate, thereby reducing stitching errors due to substrate height variance. The system runs at 75 kV with a LaB₆ emitter, providing up to 50 A/cm² at the substrate. Wafers are held on the stage by electrostatic clamping, which is claimed to provide improved flatness, superior thermal stability, and lower contamination than conventional front-surface reference wafer chucks. An advanced feature of the EL-4 is its use of redundant data registers and a cyclic redundancy code for checking the validity of the many gigabytes of data flowing into the system. Another unique feature is the use of a servo guided planar stage which slides on a base plate without guide rails, moved by push rods coupled with friction drives to servo motors outside the vacuum chamber. The stage is positioned entirely through feedback from a multi-axis laser controller. [68-69]

2.5.6.2 Etec Systems Excaliber and Leica Lithographie Systeme Jena ZBA 31/32

The Leica Jena ZBA 31/32 [70] handles plates up to 7 in and wafers up to 8 in. The "31" is a maskmaking tool, and the "32" is a direct-write instrument. Like Etec's AEBLE and Excaliber systems, the ZBA writes while the stage is moving. The ZBA delivers 20 A/cm². Its continuous stage motion and cassette-to-cassette wafer loader give it relatively high throughput when using high speed resist.

The latest generation of commercial shaped spot systems will offer resolution to 0.1 m. Under development at Etec is the "Excaliber," with a field emission source, larger stage, and higher resolution than its predecessor, the AEBLE-150. The Excaliber system incorporates a number of features from IBM's EL-4, such as telecentric deflection and the sliding chuck ("wayless") stage with yaw compensation. Unlike EL-4, the Excaliber will keep the field size below 1 mm, thereby decreasing beam settling times while the stage moves continuously.

FIGURE 2.25 Schematic of the IBM EL-4 column for shaped-beam lithography. On the right, the dashed ray trace corresponds to the source, and the solid trace to the shaped spot. [66] (Courtesy of IBM Corp.)

2.5.6.3 JEOL shaped spot systems

JEOL's JBX-8600DV [71] provides 0.1 m resolution at 30 A/cm² for direct-write applications. The system uses two stage electrostatic deflection, and handles 6 in. wafers. The JBX-7000MVII [72] has been developed as a 4 reticle making system for 256 Mbyte DRAM class devices. As with most shaped spot systems, the JEOL machines can create a map of distortion values for the deflection so that patterns can be mapped more precisely onto optically-generated features. The JBX 7000MVII handles up to 7 in. plates with a laser stage measurement unit of 0.6 nm (/1024). Overlay accuracy is 30 nm (3) and placement accuracy is 40 nm.

The attention to absolute pattern placement accuracy is always much more extensive in dedicated maskmaking tools than in direct-write machines. To control thermal expansion of the plates, temperature monitoring and stabilization is far more elaborate. Like other manufacturers, JEOL creates a map of stage nonlinearity by measuring a set of marks, turning the plate in 90 increments and measuring the set again. The resulting stage distortion map is used to reduce the runout due to imperfections in the stage mirror. In fact, each individual plate holder has its own specific distortion table, which is identified automatically by reading a bar code on the cassette.

2.5.6.4 Cell Projection

The throughput of shaped beam tools is primarily limited by the average beam current in the spot, and by the pattern density. The average beam current for cell projection is modestly larger than for variable shaped beams. Both are limited by Coulomb interaction to a few microamperes. However, by replacing the simple beam shaping aperture with a more complex pattern, a "cell projection" system can greatly increase the pattern density without sacrificing throughput.

In cell projection systems the upper deflector steers the beam into one of a number of hole patterns, or "cells." The shaped beam is deflected back to the center of the column and is demagnified by another lens, forming an image on the substrate. The shaping aperture is made of a silicon membrane, around 20 m thick, patterned with holes and coated with gold or platinum. To maintain small aberrations and high resolution, the cell is demagnified by a factor of 20 to 100, and the final cell size on the wafer is only 2 to 10 m. The wafer containing these patterns also contains a simple rectangular aperture for general purpose pattern generation in a standard shaped spot mode. While a number of cell patterns may be placed on the beam shaping wafer, it is clear that the cell projection technique is advantageous and economical only for highly repetitive designs with small unit cells, namely, memory chips. Patterns for cell projection will require proximity correction by shape modification [38-39] or through a variation of the GHOST technique [40] (see Sect. 2.4.3).

To achieve throughput comparable to that of optical steppers, cell projection tools must reduce the shot count by a factor of around 100. Current machines have achieved shot reductions on the order of a factor of 10 and have throughputs of less than 10 wafer levels/hour (for a 6 in. wafer populated with 256 Mbyte DRAMs, ~10⁹ shots/chip).

IBM, [73] Hitachi, [74-75] Toshiba, [76] Fujitsu, [77-78] and Leica have developed cell projection tools targeted for 256 Mbyte DRAM manufacture. Leica's "WePrint 200" instrument is a modified version of the ZBA-32. Hitachi also offers a cell projection/shaped spot system for sale: the HL-800D. Common features of cell projection systems include continuous stage motion [79] and resolution around 0.2 um. Hitachi's HL-800D reduces the cell reticle by a factor of 25, while Fujitsu uses a factor of 100 and Toshiba uses a factor of 40. The final demagnified cell size is kept below ~10 m to reduce aberrations. [76] Space charge effects also reduce the feature edge sharpness, but these can be compensated by using a current-dependent dynamic refocusing of the image. [75] [80-82] Cell projection has not yet achieved the throughput of optical steppers but as a transitional technology may provide the resolution needed for near-term 256 Mbyte DRAM production.

Table 2.4 Comparison of shaped spot systems.

Next Sub-Section: 2.5.7 SCALPEL

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