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

Section 2.5 Systems: 2.5.7 SCALPEL

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

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Cell projection uses small reticle areas to avoid spherical aberration and to minimize space charge effects. A natural extension of the idea would be to separate a large pattern into many small sections, etch each section into its own area of the aperture wafer, and then select and stitch the patterns together using a set of two deflectors. There are a number of limitations to this extension of cell projection: (1) 20 um of silicon is needed to stop 50 kV electrons, [83] so the pattern must include deep holes. Because the aspect ratio of these holes is limited, lines can be no wider than ~2 um; therefore, the electron optics must demagnify the pattern by a factor of at least 20 to produce linewidths of 0.1 um. This limits the area available for cell patterns. (2) Multiply connected (e.g., doughnut shaped) patterns require complementary stencil masks, so the throughput and available pattern area is further reduced. (3) Residual stress in the stencil mask will distort the mask in a pattern-dependent way, and since stencil masks absorb most of the electron energy, the changing temperature will also cause similar pattern-dependent distortions. [84]

FIGURE 2.26 Schematic of the SCALPEL technique. [87] Electrons (1) that hit the scatterer (the patterns on the mask) are scattered, and most are filtered out by the aperture. Electrons traveling through the membrane (2,3) are demagnified through the aperture and form a high contrast image on the substrate. The mask is a pattern of tungsten supported on a low stress silicon nitride membrane. The membrane is supported on a silicon wafer, with periodic silicon support struts (not shown.) (Courtesy of Lucent Technologies Inc.)

Instead of using an absorbing mask, Koops and Grob [85] proposed and researchers at AT&T Bell Laboratories [86-88] (now known as Lucent Techologies) later implemented the idea of using a scattering mask to produce a high contrast image with a technique commonly used in transmission electron microscopy. Figure 2.26 illustrates the technique "scattering with angular limitation in projection electron beam lithography," or SCALPEL. Electrons traveling through a thin (typically 150 nm) silicon nitride membrane are focused by a lens and pass through an aperture (the "back focal plane filter"). Electrons scattered by the adsorber (typically 50 nm of Au or W) are most likely not to pass through the aperture. By choosing an optimal accelerating voltage (95 kV) for the membrane thickness (100 nm of low-stress silicon nitride) and adsorber (50 nm W), the contrast at the substrate can be as high as 95%, with a transmission of 55%. [89]

If the focal plane aperture includes an annular ring, then some of the "dark field" electrons pass through to expose the resist. The unfocused dark field image of the mask can thereby be used to provide a background dose correction to compensate for proximity effect, using a technique similar to GHOST [40] (see Sec. 2.4.3.3). Although this compensation scheme is still in the design stage, it holds the promise of proximity effect correction without any loss of throughput. [90]

As in cell projection, the mask is sequentially scanned and the image shifted and reduced onto the wafer. However, because the scattering features can be much thinner than the holes of cell projection, patterns can be fabricated at smaller dimensions and the demagnification of the mask can be decreased to 5. A much larger chip can then be fabricated, with up to 210¹⁰ pixels. [91] Massive support struts between the "cells" are not imaged onto the wafer since the patterns are shifted into place as they are illuminated. While the mask structure is similar to those used for x-ray lithography, the support struts provide greater dimensional stability, [84] and use of reduction optics makes mask fabrication simpler.

The throughput of a fully-developed SCALPEL tools (which to date has only been modeled) is expected to be comparable to that of an optical stepper, while delivering resolution on the scale of 0.1 um. However, several questions remain concerning its practical use: At energies in the 100 kV range resists are proportionally less sensitive, and the energy delivered to the substrate will be larger than in conventional e-beam systems. The effect this may have on transistor thresholds and mobility is still unknown.

Next Sub-Section: 2.5.8 Other E-Beam System Research

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