SPIE Handbook of Microlithography, Micromachining and Microfabrication, Volume 1: Microlithography
Section 2.5 Systems: 2.5.3 Commercial SEM Conversion Systems
220.127.116.11 Nanometer Pattern Generation System (NPGS)
The SEM conversion kit sold by J.C. Nabity Lithography Systems  is built around a Windows-based PC-compatible with an ISA bus. A 16 bit multifunction board from Data Translation  is used to generate the X and Y beam deflections and to program a second board which provides the signals for blanking control. The beam is deflected from shape to shape in a writing field ("vector scan" mode), with the unique feature that the raster for filling arbitrary polygons can be defined by the user. Arbitrary polygons can be designed with up to 200 vertices and the user can specify the raster to be parallel to any side of the polygon. A unique feature of the NPGS is that the user has control over the exposure spot spacing in X and Y, allowing the critical dimension (e.g. perpendicular to grating lines) to be filled with greater accuracy (see Sect. 2.5.2). Circles and circular arcs are swept using a "polar coordinate" approach, with user control of the exposure spot spacing in r and . As with any ISA system, the data throughput is limited to around 100 kHz; and like most pattern generators, exposure points filling the features can be spaced by multiples of the DAC resolution (216) while still allowing full resolution for feature placement.
To provide for lower doses at reasonable currents, the Nabity system strobes the blanker at each exposure point.  For systems without a beam blanker, the Nabity Pattern Generation System (NPGS) can be programmed to "dump" the beam at user-defined locations within the writing field; however, this imposes significant limitations on the exposure spot spacing or on the lowest deliverable dose for a given beam current (refer to discussion above).
Mark alignment on the NPGS is performed by calculating the correlation between the measured mark image and the user-defined mark pattern. Signal processing such as averaging and edge enhancement can be executed before the alignment correlation, allowing the use of low contrast or rough marks. If the user supplies precisely defined marks (usually printed with a mask made on a commercial maskmaking tool) then NPGS can be used to correct for global rotation, scaling, and nonorthogonality. NPGS can control motorized stages, providing fully automated sample movement and pattern alignment. However, SEM stages are typically orders of magnitude slower than those of dedicated e-beam tools, and do not provide feedback to the deflection system (see Sect. 2.5.4).
Angled lines, polygons, and arbitrarily shaped features are all supported, and data can be imported in common e-beam formats: GDSII (Stream), CIF, and a subset of DXF (AutoCAD.)
18.104.22.168 Raith pattern generators
The Proxy-Writer SEM conversion kit is Raith's low end PC-based pattern generator. Like the Nabity system, the Proxy-Writer is a vector-scan system. Unlike the Nabity NPGS, the Proxy-Writer has only manual alignment, and patterns are limited to single writing fields. Corrections for rotation, shift, and orthogonality are applied to single fields (with single patterns); these corrections are not applied globally to correct the workpiece rotation and stage nonorthogonality. The unusual feature of this simple system is its support for exposure simulation and semiautomatic proximity effect correction. Pattern data can be generated with the simple CAD program included or imported from a DXF (AutoCAD) file.
The higher end Raith system, known as Elphy-Plus, supports the full range of e-beam operations, including control of a laser-controlled stage and corrections for workpiece rotation, gain, and orthogonality. The laser stage, also manufactured by Raith, allows field stitching to better than 0.1 m. While the primary control is still a PC-compatible computer, the limitations of the ISA bus are circumvented by using a separate computer and integrated DAC as the pattern generator. In this way, the PC transmits only the coordinates of the corners of a shape, and the patterning hardware generates all of the internal points for exposure. Data throughput is thereby increased to 2.6 MHz (0.4 s/point minimum); however, many SEM deflection systems will be limited to less than 1 MHz due to the inductance of the coils and low pass filters in the imaging system. The Elphy-Plus system supports fully automated mark detection and field stitching. All standard e-beam data formats are supported.
Useful features of the Raith Elphy-Plus system include support of data representation in polar coordinates (greatly reducing the data required to represent circles), bit-mapped pattern exposure, and a "path writing" mode. In the path writing mode, the beam is steered in a circular pattern (defining the width of a line) while the stage is moved over the length of the line or curve. This is a relatively slow way of writing a long line but avoids spatially localized stitching errors. Instead, the placement and drift errors are averaged over the length of the feature. The Raith Elphy-Plus is not only available for SEM conversions but is also used as the pattern generator for Leica's LION-LV1 e-beam system (see below.)
Even the most expensive SEM conversion kit will be limited by the SEM's slow magnetic deflection, large distortion, and small stage. Next, we look at fully integrated commercial systems.
22.214.171.124 Leica EBL Nanowriter
|FIGURE 2.16 Leica EBL-100, shown here with a 100 kV LaB6 electron source and a conventional SEM stage. The system is also available with a TFE source and laser-controlled stage. (Courtesy of Leica Lithography Systems Ltd.) |
Somewhere between a converted SEM and a full featured e-beam system is the Leica EBL Nanowriter (Fig. 2.16). This system takes its electron gun and upper column from the Leica EBPG e-beam system, its deflection and imaging systems from the Leica 400 SEM series,  and adds custom pattern generation hardware. The pattern generator uses 16-bit DACs and has a deflection rate up to 1 MHz for vector scan operation. With an optional laser stage (5.3 nm resolution) this system costs substantially less than large e-beam systems and competes more directly with the high end Raith Elphy-Plus. Without a laser stage, the EBL will suffer from the same limitations as SEM conversions, namely, lack of stage flatness and the need for alignment marks for calibration. The system is available with a LaB6 or Schottky thermal field emitter (TFE), and acceleration up to 100 kV. The system is unusual in offering such high voltage and a TFE emitter in a low cost system.
Next Sub-Section: 2.5.4 Gaussian vector scan systems
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