Zone-Plate-Array Lithography (ZPAL)

 

Project Staff: Dr. David J. D. Carter, Dario Gil, Rajesh Menon, and Professor Henry I. Smith

Sponsors: Defense Advanced Research Projects Agency; MDA972-97-1-000 MIT Lincoln Laboratory under Air Force contract F19628-00-C-0002.


In semiconductor lithography, glass masks are illuminated with deep UV laser light and their image is reduced through a lens onto the substrate to define circuitry. As feature sizes are pushed toward 100 nm, lithography is becoming increasingly costly and difficult, and may soon limit the industry juggernaut.

At the MIT Nanostructures Laboratory, a considerably simpler approach is showing great promise. The new scheme, called zone-plate-array lithography (ZPAL) is made possible by inexpensive, high-speed computation and micromechanics. ZPAL replaces the "printing press" of traditional lithography with a technology more akin to that of a laser printer.

Instead of a single, massive lens, an array of hundreds of microfabricated Fresnel-zone-plate lenses is used, each focusing a beam of light onto the substrate. A computer-controlled array of micromechanical mirrors turns the light to each lens on or off as the stage is scanned under the array, thereby printing the desired pattern in a dot-matrix fashion. No mask is required, enabling designers to rapidly change circuit designs. A schematic of ZPAL is shown in Figure 19.

Figure 19: Schematic of zone-plate-array lithography (ZPAL). An array of Fresnel zone plates focuses radiation beamlets onto a substrate. The individual beamlets are turned on and off by upstream micromechanics as the substrate is scanned under the array. In this way, patterns of arbitrary geometry can be created in a dot-matrix fashion. The minimum linewidth is equal to the minimum width of the outermost zone of the zone plates.

ZPAL leverages advances in nanofabrication, micromechanics, laser-controlled stages, and high-speed, low-cost computation to create a new form of lithography. We are developing ZPAL at UV, deep UV (DUV), EUV and x-ray wavelengths. Our experimental UV ZPAL system is currently operated at a wavelength of 442 nm. The system presently prints with an array of zone plates, fabricated at MIT, in conjunction with a micromirror array made by Texas Instruments.

Lithography Results

Figure 20: Scanning-electron micrograph of nine different patterns exposed in parallel with a UV ZPAL system. A micromirror array, manufactured by Texas Instruments, was used to multiplex the laser light to each zone plate. As the wafer stage was scanned, the zone-plate illumination pattern was changed to write the patterns in a dot-matrix fashion.

Figure 20 shows an array of nine pattern exposed in parallel with this system. Figure 21 shows a closer view of a nested L pattern. The image quality is very good, showing dense 350 nm lines and spaces. Future research will push to shorter wavelength and therefore finer feature size. For a DUV ZPAL system operating at l=157 nm, we expect to be able to produce 90 nm feature sizes.

For most applications of lithography, it is desirable to control the linewidth to a fraction of the minimum feature size. Figure 22 shows how this is done with grayscaling in ZPAL exposures. In this case, 330 nm-wide pixels were exposed on a 110 nm grid. In the left-most micrograph, a single column of pixels was exposed. To widen the line, a second column was exposed at increasing doses Then a third column was exposed (as shown in subsequent micrographs).

Figure 21: Scanning-electron micrograph of nested-L patterns produced at ZPAL. The minimum feature size of an optical projection system can be described as . In this case the linewidth is 350 nm, the numerical aperture (NA) of the zone plates was 0.67, corresponding to a factor of 0.55. With modest improvements to NA and , and using DUV radiation (l=157nm), we expect to be able to print sub-100 nm features.

Figure 22: Lines of varying width printed with grayscaling. Pixels (~330 nm in size) were exposed on a 110 nm grid. To widen the lines, a second column of pixels was exposed at increasing doses, then a third line was added. TOP: Scanning-electron micrographs of lines. CENTER: Schematic of exposure conditions for each line. BOTTOM: Plot of linewidth vs. line number.
Because zone plates are diffractive optical elements, ZPAL can operate at EUV wavelengths (13.4 nm) or even in the soft x-ray regime (l~1.5 nm). EUV or soft x-ray ZPAL should enable us to achieve feature sizes of about 20 nanometers at relatively low cost. We are developing a soft x-ray ZPAL system (l=4.5 nm) to demonstrate the extendibility of ZPAL. Figure 23 shows a simulated pattern produced with such a system.

Figure 23: Simulated patterns assuming l=4.5 nm and an outer zone width of 25 nm. The slight proximity effect shown could be eliminated with a narrower-bandwidth source and/or by using an order-sorting aperture.

Microscopy Results

An array of zone plates can also be used as a massively parallel confocal microscope, allowing imaging over a large field of view. In addition, since zone plates can be inexpensively fabricated to work at deep-UV wavelengths, low-cost, high-resolution imaging is possible with zone-plate-array scanning-confocal microscopy (ZPAM).

ZPAM is similar to conventional scanning confocal microscopy (SCM), but with a few differences, as shown in Figure 24. First, the objective lens in the traditional SCM is replaced by an array of zone-plate objective lenses. Second, the detector (in this case a CCD array) is placed at the image plane of the zone-plate array, allowing the reflected light from each zone plate to be analyzed independently. In order to accomplish this, the confocal aperture must be somewhat larger than in conventional SCM, to pass enough diffracted orders to properly reconstruct the zone-plate array.

Figure 25 shows an image obtained with ZPAM. The sample is a silicon substrate with etched grating lines. The rough patterns in the center of the image were present in the sample. A close-up of the ZPAM image of a region of the sample is compared with a conventional optical micrograph of the same region. Note the large filed-of-view at very high (~620 nm) resolution. Higher-NA zone plates and deep-UV illumination should allow sub-100 nm resolution, while a larger zone-plate array would allow a much larger field of view. This technology can be used for level-to-level alignment and placing the substrate at proper focus in ZPAL. ZPAM may also be suitable as a stand-alone technology for mask or wafer inspection.

Figure 24: Schematic of zone-plate-array scanning-confocal microscopy (ZPAM). The zone plates are used as an array of objective lenses. The CCD detector is placed at the image plane of the zone-plate array, after the confocal aperture, allowing the reflected light from each zone plate to be analyzed independently. ZPAM can be used for placing the substrate at the zone-plate focal plane and for imaging the substrate.

Figure 25: Images obtained with ZPAM. Such images could be used for level-to-level lithographic alignment in ZPAL. TOP LEFT: Image of etched silicon sample. Rough patterns in center were present on the sample. Dark lines indicate zone-plate unit cells ~0.5 x 0.5 mm field-of-view, taken at ~620 nm resolution. TOP RIGHT: Close-up of ZPAM image of a scratch in the sample. BOTTOM RIGHT: Conventional optical micrograph of the same region for comparison.