The role of lithography in the future of nanoscale science and engineering is to put high-density spatial information into nanoscale assemblies. Because information content determines the functionality of such assemblies, lithography will be a key enabler. Conventional lithographic techniques generally lack the flexibility, low cost and the resolution that research in nanoscale science and engineering requires. Although no single lithographic technique is likely to be a panacea, it is important to seek novel approaches that meet the needs of researchers, and open a path to directly manipulating nanoparticles and macromolecules. We review the various forms of lithography and focus special attention on maskless zone-plate-array lithography, assessing its impact, advantages and extendibility to the limits of the lithographic process.
Nanoscale assemblies will require control at the macromolecular level, and this has begun with research on templated self assembly. Going beyond that to the control and utilization of the information content of nanoparticles and molecules will require innovations whose origin is uncertain at this point.
Zone-Plate-Array Lithography (ZPAL) is an optical-maskless-lithography technique, in which an array of tightly focused spots is formed on the surface of a substrate by means of an array of high-numerical-aperture zone plates. The substrate is scanned while an upstream spatial-light modulator, enabling "dot-matrix" style writing, modulates the light intensity in each spot. We have built a proof-of-concept system using an array of zone plates, and the Silicon Light Machines Grating Light Valve (GLVTM) as the light modulator. We have demonstrated fully multiplexed writing, multilevel alignment and resolution corresponding to k1 < 0.3. This system currently operates at l = 400nm and utilizes well-known I-line processes. Diffractive optics such as zone plates offer significant advantages over refractive approaches since near-ideal performance is achieved on axis, reliable planar fabrication techniques are used, costs are low, and the approach can be readily scaled to shorter wavelengths. In this paper, we also developed models and analyzed the cost-of-ownership of maskless lithography (ZPAL) versus that for optical-projection lithography (OPL). In this context, we propose the use of an effective throughput to consider the photomask delivery times in the case of OPL. We believe that ZPAL has the potential to become the most practical and cost-effective method of maskless lithography, enabling circuit designers to fully exploit their creativity, unencumbered by the constraints of mask-based lithography. This may revolutionize custom circuit design as well as research in electronics, NEMS, microphotonics, nanomagnetics and nanoscale science and engineering.
The semiconductor industry has been driven by significant improvements in optical-lithographic capability. As feature sizes on the wafer shrink faster than the wavelength of the exposing illumination, increasingly complex and expensive steps such as immersion, resolution-enhancement techniques, and optical-proximity correction (OPC) are required. Traditionally, high costs have been amortized over large volumes of chips, and by progressive technological maturity. Optical lithography using MEMs-based spatial-light modulators provides an alternative means of lithography. Significantly lower costs-of-ownership coupled with throughputs acceptable for mask manufacturing, mask prototyping, and low-volume-chip manufacturing are the enabling attributes of such techniques. At MIT, we have pursued a unique version of this technology, which we call Zone-Plate-Array Lithography (ZPAL). In ZPAL, an array of high-numerical-aperture diffractive lenses (for example, zone plates) is used to create an array of tightly focused spots on the surface of a photoresist-coated substrate. Light directed to each zone plate is modulated in intensity by one pixel on an upstream spatial-light modulator. The substrate is scanned, and patterns of arbitrary geometry are written in a “dot-matrix” fashion. In this paper, we describe results from our proof-of-concept ZPAL system and its future potential. Lithography using distributed, tightly focused spots presents a different set of advantages and challenges compared to traditional optical-projection lithography. We discuss some of these issues and how they bear on practical system designs.
Ever-increasing demands of smaller feature sizes and larger throughputs have catapulted the semicondutor lithography juggernaut to develop immensely complex and expensive systems. However, it is not clear if the lithography needs for microoptic and other “botique” device fabrication are being addressed. ZPAL is a new nanolithography technique which leverages advances in micromechanics and diffractive optics technologies. We present ZPAL as the ideal system for such non-conventional lithography needs.
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