R. A. Ghanbari

Achromatic holographic configuration for 100-nm-period lithography.

For the fabrication of large-area, spatially coherent gratings with periods of 100 nm or less, a grating interferometer is preferred over a conventional holographic configuration because of the limited coherence of available sources. Using a configuration that employs two matched fused silica phase gratings and an ArF excimer laser, we obtain high-quality 100-nm gratings in polymethyl methacrylate. We analyze the conditions for achieving high-contrast fringes with such an achromatic holographic configuration and show that the depth of focus depends only on the spatial coherence of the source. We also describe a highly accurate method for calculating the diffraction efficiency of the phase gratings as a function of polarization, incidence angle, and grating structure.

Magneto‐optical absorption in a two‐dimensional electron grid

We have fabricated a two‐dimensional (2D) electron grid in the high mobility GaAs/Al0.35Ga0.65As heterostructure and observed, in the magneto‐optical absorption, new resonances and an anomalous dispersion, similar to that recently reported by Kern et al.1 in antidots in a high density InGaAs/AlInAs heterostructure. We have studied their dependencies on the 2D electron density and demonstrate their plasma origin. The anomaly in magnetic field dependence is explained by the mixing of cyclotron resonance and edge magnetoplasma modes in this 2D electron microstructure.

Far‐infrared emission from hot quasi‐one‐dimensional quantum wires in GaAs

We have observed far‐infrared radiation from an array of hot quasi‐one‐dimensional wires in GaAs. The wires have 6000 A period with 3000 A lithographic width. Spectroscopy of the infrared emission with a bolometer detector filtered by a magnetic field‐tunable InSb cyclotron resonance wedge reveals: an intersubband plasmon resonance at 45 cm−1, and a measure of the resonance linewidth and electron temperature as a function of pulse current. The total intensity of the infrared signal yields a confirming measure of the temperature of the electrons, ranging from 6.0 to 12.1 K for currents of 40 nA to 2 μA per wire.

X-ray masks with large-area 100mm-period gratings for quantum-effect device applications

Abstract We report the fabrication and replication of x-ray masks with large-area (∼50 mm 2 ) 100nm-period gratings. Achromatic holographic lithography was used to generate 100nm-period surface gratings in PMMA resist. Subsequent dry processing formed high-aspect-ratio grating lines down to the base of the resist. The x-ray absorber was defined by either: (i) reactive-ion etching low-stress sputter-deposited tungsten, using the resist lines as an etch mask, or (ii) electroplating gold using the resist lines as a mold. The absorber patterns were fabricated on silicon substrates coated with 1 μm-thick polyimide as the membrane material. X-ray masks were formed by etching away the silicon substrate, leaving the x-ray absorber pattern supported by the polyimide membrane. Results of x-ray exposures of PMMA, using the C K line ( λ =4.5 nm ), are presented.

Fabrication of 100 nm-period gratings using achromatic holographic lithography

Abstract We have fabricated large area, 100nm-period gratings using achromatic holographic lithography. Previously, we reported fabrication of relatively small area gratings with periods of 270nm and 125nm using an achromatic configuration that incorporated feedback to stabilize the fringes during exposure. In the present scheme, the need for a feedback system has been eliminated by physically clamping together the configuration, thereby achieving mechanical stability. Back reflection from the substrate was eliminated using an anti-reflective coating between the resist (PMMA) and the substrate, resulting in grating lines of high contrast. The area of the grating (currently ≈ 1 cm2) is limited only by the size of the fused silica optical flats that contain the beam splitter and recombiner gratings.

Comparative mobility degradation in modulation‐doped GaAs devices after e‐beam and x‐ray irradiation

We report on measured Hall mobility versus temperature for high‐quality modulation‐doped AlGaAs/GaAs samples after exposure by electrons and x rays at doses and energies typically used in lithography. We find that bare samples exposed by 50 keV electrons suffered significant mobility degradation over the temperature range of 4.2–300 K (as much as a factor of 30). X‐ray‐exposed samples did not show any mobility degradation. Two‐dimensional electron densities were not dramatically affected by either exposure technique, although e‐beam exposed samples did show a slight decrease in carrier density. Our results are consistent with previous reports of mobility degradation in some e‐beam evaporators.

Fabrication of parallel quasi‐one‐dimensional wires using a novel conformable x‐ray mask technology

We report on the fabrication of quasi‐one‐dimensional wires on modulation‐doped GaAs/AlGaAs using a novel conformable x‐ray mask technology which allows us to expose arbitrary sized samples, including samples much smaller than the membrane area, using our laboratory’s standard 31 mm‐diam silicon‐nitride x‐ray mask. After optical alignment, the sample and mask are brought into contact electrically, and then loaded into a specially designed cartridge which allows a vacuum to be pulled between mask and substrate. The vacuum causes the x‐ray mask to conform around the sample. We find that a vacuum hold down is necessary to allow easy separation of the sample from the mask with minimal risk to both.

Mask Technology For X-Ray Nanolithography

In this report we describe a system of technologies we have developed for fabricating, patterning, and replicating x-ray masks with linewidths as fine as 50 nm (0.05 μm). This effort has evolved into a fairly routine service supporting a growing community of researchers at MIT interested in fabricating sub-micron and nanometer scale structures and devices. Strong, transparent, silicon-rich nitride (SiN) membranes bonded to optically-flat Pyrex rings and patterned with gold absorbers form the basic x-ray mask structure. We have recently completed the installation of a vertical thermal reactor (VTR) for the deposition of SiN coatings. Results indicate superb film uniformity, decreased defect counts, and increased strength over previously available nitrides. Patterning of the masks is performed at an off-site location by electron-beam lithography followed by development and gold electroplating at MIT. We describe a new fountain plating bath and mask chuck which allows us to obtain clean, uniform gold films of low stress. Once completed, the so-called “master” masks are replicated onto new x-ray mask substrates which are then processed into “daughter” masks. The resulting polarity-reversed patterns are required in order to achieve the desired pattern in positive resist when printed onto substrates. We will describe our in-house built x-ray aligner which features a high-power electron bombardment source and SiN vacuum isolation window to achieve exposures in a helium environment.

Proximity effect reduction in x‐ray mask making using thin silicon dioxide layers

A novel method is reported for reducing the proximity effect in high‐resolution electron beam patterning of high atomic number materials such as tungsten. The method involves interposing a thin (50–400 nm) layer of SiO2 between the resist and the underlying high‐Z substrate. Examples are shown in which gratings of 0.2 μm lines with a 0.5 μm period were written without proximity effect compensation. Optimal intermediate layer thickness for the best resolution of the gratings is determined to be 200 nm. A Monte Carlo model of electron scattering including inelastic processes has been implemented to interpret our experimental results. The model presented shows that having the low atomic number SiO2 layer between the resist and the tungsten prevents the fast secondary electrons being generated at the surface of the tungsten from propagating back into the resist, suggesting a mechanism for proximity effect reduction. The results presented here have important practical applications for x‐ray mask making.

Quantum Device Modeling with the Convolution Method

Recent advances in materials fabrication and nanolithography have made possible a generation of semiconducting structures whose conductance is governed by quantum mechanical phenomena. In particular, nanostructures on Si MOSFETs and GaAs MODFETs have shown modulations in their conductance versus gate voltage characteristics that have been attributed to quantum mechanical effects. In this paper, we review a modeling scheme which gives a unified way of understanding how these quantum effects are affected by temperature, mobility, voltage, and the structure of the device. This model provides not only a qualitative understanding of the various quantum phenomena, but also a basis for developing efficient computational algorithms for modeling specific devices. We have called this scheme the convolution method because most of the calculations can be written in terms of separate convolutions involving the individual phenomena of temperature, mobility, voltage, and structure.