A. Yen

Negative transconductance and negative differential resistance in a grid‐gate modulation‐doped field‐effect transistor

We report on transport measurements in grid‐gate lateral‐surface‐superlattice (LSSL) field‐effect transistors on a modulation‐doped GaAs/AlGaAs heterostructure. The LSSL is created by a 0.2 μm period Ti/Au grid on top of the AlGaAs layer, which presents a tunable, two‐dimensional periodic potential modulation to the electrons traveling from source to drain. Current measurements at 4.2 K as a function of gate bias exhibit negative transconductance at a fixed drain bias below 15 mV, providing evidence of a superlattice effect (i.e., coherent back‐diffraction). In addition, negative differential resistance is observed at a fixed gate bias and a drain bias around 100 mV, which could be a manifestation of sequential resonant tunneling.

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.

An anti-reflection coating for use with PMMA at 193 nm

An antireflection coating (ARC) for use with poly(methyl methacrylate) (PMMA) resist for ArF excimer laser lithography (193 nm) was formulated. It consists of PMMA and a bis-azide, 4.4-prime-diazidodiphenyl sulfone (DDS) which crosslinks the film after deep UV (260 nm) irradiation and subsequent annealing. The reacted DDS then serves as the absorber for the 193 nm radiation and also prevents mixing of the ARC and PMMA during PMMA spin-coating and development. The effectiveness of the ARC was demonstrated by exposing, in PMMA, using achromatic holographic lithography, gratings of 100 nm period (about 50 nm linewidth) that are almost entirely free of an orthogonal standing wave.

Large‐area, free‐standing gratings for atom interferometry produced using holographic lithography

Interferometers based on matter waves promise orders‐of‐magnitude improvements in metrology over laser‐based systems by virtue of the fact that the de Broglie wavelengths of atoms are about 104 times shorter. To date, the required matched set of four aligned gratings for such atom interferometers has been made using electron beam lithography and, as a result, such gratings suffer from a lack of spatial‐phase coherence. We report on processes we have developed for fabricating free‐standing gratings over large areas using conventional holographic lithography and achromatic holographic lithography to achieve spatial periods of 200 and 100 nm, respectively (i.e., nominal linewidths of 100 and 50 nm, respectively).

Proposed method for fabricating 50-nm-period gratings by achromatic holographic lithography

A method for producing large-area, 50-nm-period gratings by using a grating-type interferometer and undulator radiation at 14 nm is described.

Sub‐100‐nm x‐ray mask technology using focused‐ion‐beam lithography

In the past, nearly all x‐ray nanolithography (i.e., sub‐100‐nm linewidths) employed the CK x‐ray line at 4.5 nm. This, in turn, necessitated near‐zero gaps (to avoid diffraction) and carbonaceous masks (e.g., polyimide, which is subject to distortion). In order to use x‐ray replication in the fabrication of multilevel devices and circuits that cover large areas (∼a few cm2) and have feature sizes well below 100 nm, we have turned to the CuL line at 1.3 nm. Masks consist of 1–1.5 μm thick Si or Si3N4 membranes and Au absorber patterns, 200 nm thick, which provide 10 db contrast. Focused‐ion‐beam‐lithography (FIBL) with Be++ ions at 280 keV was used to produce quantum‐effect‐device patterns with minimum linewidths of ∼50 nm. These were replicated using the CuL line, indicating that photoelectrons are not a serious problem. The FIBL process [exposure of 300 nm‐thick polymethylmethacrylate (PMMA), followed by Au electroplating] is high yield and much simpler than a trilevel electron‐beam‐lithography process ...

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.

Resonant tunneling across and mobility modulation along surface‐structured quantum wells

We present results of fabrication and transport measurements on surface‐structured quantum wells. The structures are fabricated on GaAs/AlGaAs modulation‐doped layers. Three different devices are examined: the grid‐gate lateral‐surface‐superlattice, the planar‐resonant‐tunneling field‐effect transistor, and the multiple parallel quantum wires. In the first two structures, transport is perpendicular to the field‐induced potential barriers. At 4.2 K, we observed evidence for resonant tunneling in both types of devices. In the third type of structure, transport is through isolated quantum wires parallel to the barriers. The presence of one‐dimensional energy subbands, and mobility modulation, above and below the two‐dimensional value, were observed.

Surface superlattices and quasi-one-dimensional conductors in GaAs HEMTs: how best to compare theory and experiment?

Summary form only given. The authors discuss evidence of (1) electron standing waves forming underneath a 2000-AA period Ti/Au Schottky grid or grating incorporated as a gate into a GaAs HEMT (high-electron-mobility transistor) structure, and (2) modulation of the scattering time due to intersubband scattering in an array of approximately 100 parallel wires of 400-AA width formed by laterally patterning a GaAs/GaAlAs heterostructure. The authors show how knowledge of the electron group velocity along with a scattering rate derived from Fermis golden rule can explain the observed effects. They compare the mobility modulation from intersubband scattering to the well-known prediction of Sakaki for increased mobility in quasi-one-dimensional wire due to the finite range of the scatterers. By first solving for the density of states and conductivity in a one-dimensional periodic potential, the authors show how to incorporate energy level broadening due to elastic and inelastic scattering, temperature broadening, and an increase in the dimensionality of the device by convolving with various known functions. >