Amil Patel

Immersion zone-plate-array lithography

An immersion scheme is used to improve resolution, exposure latitude, and depth-of-focus in zone-plate-array lithography (ZPAL). We believe this is the first implementation of an immersion scheme in a maskless lithography system. Replacing air with de-ionized water as the medium between the zone-plate array and the substrate effectively increases the system’s numerical aperture and consequently, enhances its patterning capabilities. The design and fabrication process of an immersion zone plate is described. Its behavior is then characterized through the experimental reconstruction of its point-spread function, and compared to the theoretical model. A wide variety of patterns were printed, demonstrating the improved lithographic performance of immersion ZPAL.

Alpha-prototype system for zone-plate-array lithography

In this article, we present lithography results from a continuous-scan zone-plate-array lithography (ZPAL) system using the grating light valve (GLV) as the multiplexing element. ZPAL is an optical-maskless-lithography technique, in which an array of diffractive lenses (e.g., zone plates) focuses incident light into an array of spots on a photoresist-coated substrate. The intensity of the light incident on each lens is controlled by the GLV. By scanning the wafer and appropriately modulating the incident light, patterns are written in a “dot-matrix” fashion. We have incorporated the elements of ZPAL into an alpha-prototype system. We describe this system and characterize its lithographic performance.

Membrane stacking: A new approach for three-dimensional nanostructure fabrication

Future applications of nanotechnology, including integrated photonics, will require new three-dimensional (3D) fabrication techniques beyond those employed by the semiconductor industry. The authors investigate membrane stacking as a potential avenue for fabricating photonic crystals and other 3D structures with high yield. They fabricated membranes in silicon nitride with freestanding photonic structures and investigated the technology for stacking them.

Elliptical-ring magnetic arrays fabricated using zone-plate-array lithography

Zone-plate-array lithography and lift-off processing were employed to fabricate large arrays of elliptical-ring thin film magnets with widths of 600nm and above. An undercut profile was created using WiDE™ antireflection coating spun underneath a PFI-88 resist layer. The process allowed for up to 60-nm-thick sputtered magnetic multilayered structures. The magnetic properties of the elliptical ring arrays clearly show that the shape anisotropy induced by the ellipticity of the ring creates different magnetization reversal depending on the applied field direction. Magnetic force microscopy shows that the rings display magnetic states characteristic of ring structures, as well as sharp transitions between them. A fabrication process to produce magnetic memory prototypes based on these elliptical rings is presented.

Coherent diffraction lithography: Periodic patterns via mask-based interference lithography

Periodic structures, such as gratings and grids, are required in a variety of applications including spectroscopy, photonic and phononic devices, and as substrates for basic studies in materials science. Interference lithography readily forms periodic patterns in photoresist, but conventional approaches, using a Lloyd’s mirror or Mach–Zehnder configuration, suffer from a number of shortcomings including difficulty in aligning patterns with respect to pre-existing structures on a substrate and difficulty in precisely repeating a given spatial period. Coherent diffraction lithography (CDL), a mask-based approach, utilizes the well-known Talbot effect to accurately replicate the one- or two-dimentional pattern on a mask by reimaging the mask pattern in photoresist. Moreover, with appropriate alignment marks on the mask, one can align the replicated pattern relative to pre-existing patterns on the substrate. The authors describe the design, construction, and utilization of a dedicated CDL apparatus that permits replication, at a well-defined mask-substrate gap, of the periodic structure of a phase mask. The system also incorporates interferometric-spatial-phase imaging for aligning the replicated pattern relative to fixed fiducials on a substrate. They obtained high quality replications of a mask pattern, consisting of a 600 nm period grating, from the 1st to the 52nd plane of reimaging, i.e., from 1.55 to 40.16 μm.

Poisson's spot with molecules

In the Poisson-spot experiment, waves emanating from a source are blocked by a circular obstacle. Due to their positive on-axis interference an image of the source (the Poisson spot) is observed within the geometrical shadow of the obstacle. In this paper we report the observation of Poissons spot using a beam of neutral deuterium molecules. The wavelength independence and the weak constraints on angular alignment and position of the circular obstacle make Poissons spot a promising candidate for applications ranging from the study of large molecule diffraction to patterning with molecules.

3D fabrication by stacking prepatterned, rigidly held membranes

The authors describe an approach to fabricating high resolution, complex 3D structures based on the stacking of thin membranes that have been patterned in advance. The membranes are attached to a rigid frame by means of tethers that are strong enough to permit normal handling but can be cleaved after bonding. The tether shape was designed using finite-element analysis to enable clean cleavage at a specific location so that fragments are avoided that would interfere with the bonding of subsequent layers. The authors used 12 × 12 mm SiNx membranes, 350 nm thick, patterned with a square array of holes at 600 nm pitch and demonstrate the stacking of three layers.

Maskless optical lithography using MEMS-based spatial light modulators

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.

Maskless Lithography Using Diffractive-Optical Arrays

An optical maskless-lithography system is described that is capable of patterning complex geometries at very high resolution, and in a cost-effective manner with potential applications in the fabrication of nanostructures for photonic devices.

Zone-plate-array lithography (ZPL): a maskless fast-turn-around system for microoptic device fabrication

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.