SPIE Handbook of Microlithography, Micromachining and Microfabrication, Volume 1: Microlithography
Section 2.5 Systems: 2.5.5 Gaussian Spot Mask Writers
|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.|
The most popular and well established mask writing tool is the MEBES from Etec Systems Inc.  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  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.  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. 
The EBES4 mask writer from Lepton Inc.  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.  (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.  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. 
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.
|Lepton Inc.||Etec Systems Inc.|
|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
This material is based upon work supported by the National Science Foundation under Grant No. ECCS-1542081. Any opinions, findings, conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
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