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.

A new technology developed at the NanoStructures Laboratory at M.I.T. 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, a large array of microfabricated diffractive optical elements (Fresnel-zone-plate lenses) is used, with each element focusing a beam of light onto the substrate. A computer-controlled array of micromechanical elements 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 1.

Figure 1: 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

This technology is capable of printing patterns of complex geometry as illustrated by the scanning-electron micrograph of the seal of MIT that was printed using the MIT-ZPAL system at a wavelength of 400nm. Note that features as small as 140nm was resolved.

Figure 2: Scanning-electron micrographs of the seal of MIT.

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 and deep UV (DUV) wavelengths, although extensions to EUV and X-ray wavelengths are possible. With the goal of proving both the technical merit and the potential commercial viability of ZPAL we are currently building a prototype system that will be used for quick-turn-around, maskless patterning for a number of research applications, from MEMS to microphotonics.

Figure 3 presents some of the high quality lithographic patterns that the M.I.T. ZPAL system produces. All patterns show good fidelity, low edge roughness, and the ability to pattern very dense features down to the minimum spot size. A large number of patterns were exposed with the system, including 2D photonic bandgap structures (Fig 3(d-e)), microcomb structures (Fig 3(f)), waveguides (Fig 3(g)), zone plates (Fig 3(h)), and a number of lithographic test patterns (Fig 3(a-c)).

Figure 3. Scanning electron micrographs of patterns exposed at MIT's continuous-scan UV-ZPAL system. (a) Dense nested Ls, (b) single-pixel lines with different spacing between lines, (c) small section of a 81 mm-long, 600nm-period grating, (d) 2D photonic bandgap structures with 1.29 mm period, (e) 2D photonic bandgap structures with 600 nm period, (f) microcomb structure for MEMS, (g) curved waveguides, (h) portion of a zone plate. All exposures presented in Figure 2 employed a 25 mW GaN laser diode operating at 400 nm wavelength. For these experiments no multiplexing device was used, i.e., all zone plates in the array wrote the same pattern simultaneously.

Figure 4: (a) Scanning-electron micrographs of dense line/space patterns produced with MIT's UV (l=400 nm) ZPAL system. The minimum feature size of an optical projection system can be described as Wmin=k1*l/NA. In this case the smallest linewidth is 135 nm, the numerical aperture (NA) of the zone plates was 0.85, corresponding to a k1 factor of 0.287. (b) Scanning-electron micrograph of a non-periodic pattern with smallest feature-width of 122nm. By using 193nm light under water immersion, we expect to be able to print 39 nm features.

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.