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

Section 2.5 Systems: 2.5.5 Gaussian Spot Mask Writers

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
2.5.8 Other E-Beam System Research
2.5.9 Electron Beam Fabrication Services
Table of Contents

While both of these systems are promoted for mask making, their basic technologies could be adapted for direct writing on wafers. However, their relatively low throughput compared to photolithography systems has kept them firmly rooted in the maskmaking market.

FIGURE 2.21 Gaussian-spot raster-scan writing strategy. The stage is moved continuously while the beam is rastered perpendicular to the stage motion. This technique, used by the Etec MEBES tools, is one of the most common for mask generation. Etec MEBES systems

The most popular and well established mask writing tool is the MEBES from Etec Systems Inc. [57] The MEBES uses a focused ("Gaussian") spot, writing a pattern in stripes while moving the stage continuously. The beam deflection is primarily in one direction, perpendicular to the motion of the stage (Fig. 2.21). Of course, some small deflections are needed in the direction of stage travel, to compensate for stage placement errors. These correction values are provided by the feedback system of the laser-controlled stage. The 10 kV TFE electron gun provides a current density [58] at the mask plate of 400 A/cm2.

The MEBES is designed for high-throughput mask making, with minimum feature size 0.25m. Figure 2.22 shows the MEBES IV-TFE column design, with three beam crossovers -- compared to one crossover in the Lepton column. A 160 MHz transmission line beam blanker is located at the third crossover. Since Etec Systems has implemented a full range of error compensation techniques, including a glancing-angle height sensor, dynamic focus corrections, periodic drift compensation, and substrate temperature control. Real-time correction of focus, gain, and rotation provide stitching errors (3) of 50nm. [59] The MEBES 4500 can be used as a metrology tool to characterize its own stitching and linearity. However, when errors appear in both the writing and the reading process (as would be caused by interferometer mirror defects) then a machine cannot measure its own distortions. In this case, two or more MEBES machines can be used to check for consistency.

FIGURE 2.22 Schematic of the MEBES IV TFE column (Etec Systems Inc.) The source optics include the extractor (Vx), focus (VL) and suppressor (Vs). The high-speed beam blanker assembly is a U-shaped transmission line designed to deflect the beam twice with one blanking pulse. 57 (Courtesy of Etec Systems Inc.)

As with any Gaussian beam system, throughput decreases as resolution (density of the pixel writing grid) increases. One way to increase the resolution without sacrificing speed is to implement a "graybeam" strategy, where the pixels on edges of features have dwell times and placements modulated on a per-pixel basis. This allows the bulk of a pattern to be written on a fast, coarse grid while edges are written with a finer resolution. [60] Lepton EBES4

The EBES4 mask writer from Lepton Inc. [61] also uses a Gaussian spot, with a patterning strategy similar to that of the high resolution machines. In this system the coarse/fine DAC beam placement is augmented with an extra (third) deflection stage, and the mask plate is moved continuously, using the laser stage controller to provide continuous correction to the stage position. Unlike the high resolution JEOL machines, each stage of deflection has a separate telecentric deflector (instead of simply a separate set of DACs) for high speed operation. Patterns are separated into stripes (similar to writing fields) 256 m wide (see Fig. 2.23). These stripes are separated into 32 m subfields ("cells") which are further subdivided into 2 um sub-subfields ("microfigures"). A spot of 0.125 m diameter fills in the microfigure with a raster pattern.

FIGURE 2.23 Writing strategy of the Lepton EBES4 mask writing tool: pattern data is cut into stripes 256 um wide. The stripes are fractured into smaller cells containing macrofigures. The macrofigures are split into even smaller microfigures which are finally written as a set of pixels. [62] (Courtesy of Lepton Inc.)

The entire EBES tool has been designed for high speed, with a current of 250 nA delivered in a 0.125 m spot for a current density at the sample of 1600 A/cm2. The EBES4 column uses a TFE electron gun operating at 20 kV and a single beam crossover at the center of a high-speed beam blanker. [63] The pattern generator operates at up to 500 MHz, and the high overall throughput allows production of a 16 Mbit DRAM mask in 30 min. [64]

A robot arm is used to load mask plates from a magazine module to the alignment and temperature equilibration chambers, and later to the exposure chamber. The internal mask carrier is made from the glass ceramic ZerodurTM, which minimizes substrate temperature variations during exposure. The EBES4 automatically loads each mask plate into the carrier, establishes electrical contact to the substrate, and verifies the contact resistance.

The EBES4 mask writer has a spot size of 0.12 m, uniformity to 50 nm (3), stitching error of 40 nm, and repeatability (overlay accuracy) of 30 nm over a 6 in. reticle.

Table 2.3 Comparison of Gaussian spot, raster scan mask making systems.

Lepton Inc. Etec Systems Inc.
Model EBES4 MEBES 4500
Resolution 0.125 um spot 0.25 um features
Alignment automated, optional direct write on wafers automated, mask writing only
Field 256 um x 32 um stripes, continuous motion 1.1 mm maximum stripe length, continuous motion
Energy 20 kV 10 kV
Current Density 1600 A/cm2 400 A/cm2
Speed 500 MHz 160 MHz
Samples 6 inch plates 8 inch plates
Stage laser controlled, 5 nm resolution controller, 146 mm travel laser controlled, 6.6 nm resolution controller, 6 inch travel
Contact USA: 908-771-9490 USA: 510-783-9210 France: 33-42-58-68-94 Japan: 81-425-27-8381

Next Sub-Section: 2.5.6 Shaped Spot and Cell Projection Systems

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