Manufacturing Bits: Sept. 3

Dancing with the stars; frustrated ice chips; making 3D structures.

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Dancing With The Stars
In telescopes, the ability to see distant stars and galaxies is driven by the light-gathering area and detectors in the system.

In the last 50 years, the collecting area in large-scale telescopes has increased by only a factor of four, according to researchers from the University of California at Santa Barbara. Meanwhile, the sensitivity of CCD detectors has increased by a factor of 20, according to researchers.

Cryogenic detectors are the preferred technology in large-scale telescopes. With operating temperatures on the order of 100~mK, these devices allow astronomical observations over most of the electromagnetic spectrum. But these types of detectors are limited by the band gap of the semiconductor itself, according to researchers.

To improve detectors for astrophysics, U.C. Santa Barbara has devised a promising class of superconducting detectors. The university’s technology, dubbed Microwave Kinetic Inductance Detectors (MKIDs), is an alternative to cryogenic detector technology. “While a CCD is limited to about 0.3-1 microns, the MKIDs described are in principle sensitive from 0.1 microns in the UV to greater than 5 microns in the mid-IR, enabling observations at infrared wavelengths vital to understanding exoplanets,” according to researchers at U.C. Santa Barbara.

A summary of the detection principle of a microwave kinetic inductance detector (MKID). (a) An incident photon breaks Cooper Pairs in a superconducting film that is part of a resonant circuit (b), causing a change in the dissipation (c) and phase (d) of the resonator. Source: SPIE

The MKID makes use of a superconducting LC oscillator with a resonant frequency in the microwave region. In an MKID, photons with energy are absorbed in a superconducting film, producing a number of excitations, called quasiparticles. Then, the quasiparticles are measured in a planar resonant circuit. The amplitude and phase of an excitation signal are sent through the resonator.

Researchers recently presented the results of the MKIDs in a photon counting integral field unit (IFU). The IFU is called ARCONS (Array Camera for Optical to Near-IR Spectrophotometry). The IFU has been deployed within the Palomar 200-inch and Lick 120-inch telescopes for 24 nights of observation.

These detectors allow researchers to determine the energy and arrival time of individual photons. The technology is geared for astronomical observations in the near infrared, optical, ultraviolet, and X-ray. Application ranges from detecting earth-like planets around nearby stars to untangling the emission mechanisms of pulsars, according to researchers.

Frustrated Ice Chips
Artificial spin ice is a class of lithographically created arrays of interacting and ferromagnetic nano-islands. Discovered in the 1990s, the technology is considered a new class of “frustrated” materials that could one day be used in magnetic memories and related devices.

The University of Illinois has reported direct visualization of magnetic charge crystallization in an artificial spin ice material. Researchers also have developed a new annealing protocol for the technology.

In 2006, scientists designed the first artificial spin ice, a 2D array of magnetic nano-islands that are fabricated to interact in complex ways. The large magnetic energy scales of these nanoscale islands make it difficult to anneal artificial spin ice into desired ensembles, according to researchers.

In a breakthrough, the University of Illinois has demonstrated a method for thermalizing artificial spin ices with square and kagome lattices. This was done by heating above the Curie temperature of the constituent material.

As a result, artificial square spin ice achieved thermal ordering. Researchers observed incipient crystallization of the magnetic charges embedded in pseudo-ice, with crystallites of magnetic charges whose size can be controlled by tuning the lattice constant.

Researchers said the islands were lithographically printed onto a substrate. They were arranged in a square-lattice pattern. The north and south poles of each nanomagnet met and interacted at their four-pronged vertices. “Nanomagnets are so small that their behavior becomes relatively simple,” said Peter Schiffer, a professor at the University of Illinois, on the university’s Web site. “We can arrange the magnets in a particular lattice pattern—square or honeycomb—and they interact in a way that we can predict and control. The challenge—you have to get the nanomagnets to flip their north and south poles to show how they interact. It’s hard to force them to show the effects of interaction, since they get stuck in one particular arrangement.”

Making 3D Structures
The National Institute of Standards and Technology (NIST) has developed a new technique for manufacturing high-aspect-ratio, 3D nanostructures over large device areas.

The technique makes use of a combination of electron-beam lithography, photolithography, and resist spray coating. The new technique enables researchers to etch trenches and other high-aspect ratio structures without using masks and in only a two-process stage.

For the most part, complex 3D structures can be made using many photomask layers or expensive grayscale masks. NIST’s new technique makes use of photolithography to generate large-area grayscale structures. E-beam lithography is used to add grayscale features smaller than 200nm.

In this approach, a patterned a layer of photoresist is exposed with a focused laser beam. The modulating intensity of the light forms a grayscale gradient. As a result, a reaction in the photoresist is generated, according to NIST.

Scanning electron micrograph (SEM) showing a top-view along with an SEM (inset) showing a cross-sectional view of grayscale structures fabricated using a combination of e-beam lithography, photolithography, and resist spray coating. The superimposed schematic illustrates e-beam direct writing of nanoscale vertical staircases (SEM inset) on a substrate with microscale grayscale topography. The initial grayscale patterns were generated on a laserwriter. After reactive ion beam etching, the patterns were simultaneously written into 2 µm, 0.5 µm, and 30 µm deep features. Source: NIST

The sample is immersed in a solution. This leaves the photoresist layer with varying thicknesses, which matches the initial exposure pattern. It is then exposed to a deep reactive ion etch (DRIE), which removes substrate material. This in turn transfers the 3D photoresist pattern into the substrate to form deep grayscale microstructures, according to NIST. Then, the process is repeated, but at feature sizes 10 times smaller.

Then, an e-beam resist spray coating is applied to obtain conformal coverage of the topography. Using an e-beam, the patterned grayscale step heights are directly written on the resist. Finally, the resist is developed and the sample is exposed to DRIE.

The two-stage process results in a vertical feature size of 45 ± 6nm within a substrate structure that varies from 2μm to 30μm deep and with horizontal feature sizes of 100nm to 200nm, according to NIST.

—Mark LaPedus