Richard C. Tiberio

Self‐assembled monolayer electron beam resist on GaAs

We present results on electron beam exposure of a self‐assembled monolayer film as a self‐developing positive resist on GaAs. A 1.5 nm thick monolayer of n‐octadecanethiol (C18H37SH) deposited on a GaAs (100) substrate showed a electron beam sensitivity of about 100 μC/cm2. The monolayer resist was used as a mask for chemical etching of the GaAs. Patterns in GaAs have been created with widths approximately equal to the exposing electron beam width of 50 nm.

Self‐assembled monolayer electron‐beam resists on GaAs and SiO2

It was demonstrated that self‐assembled monolayers of n‐octadecanethiol [ODT; CH3(CH2)17SH] on GaAs and n‐octadecyltrichlorosilane [OTS; CH3(CH2)17SiCl3] on SiO2 act as self‐developing positive electron beam resists with electron‐beam sensitivities of ∼100–200 μC/cm2. For the OTS monolayer on a silicon native oxide, atomic force microscopy (AFM) images of the exposed layer before etching demonstrate the removal of all or part of the layer upon electron‐beam exposure. Features as small as 25 nm were resolvable in a 50 nm period grating. A resist contrast curve for OTS was obtained from AFM depth measurements as a function of dose. An ammonium hydroxide water etch was used to transfer patterns into the GaAs to a depth of at least 30 nm and buffered HF was used for pattern transfer into the SiO2 to a depth of at least 50 nm.

Fabrication of thin‐film metal nanobridges

Thin‐film fabrication techniques for forming three‐dimensional ‘‘point contacts’’ are presented. As‐fabricated nanobridges can be modified using electromigration to make the constriction region smaller or dirtier. Scientific applications to quantum transport studies, 1/f noise, and electromigration are discussed.

Pattern transfer of electron beam modified self‐assembled monolayers for high‐resolution lithography

Self‐assembled monolayers of octadecylsiloxane and octadecylthiol have been modified by high‐resolution electron beam lithography. Focused electron beams from 1 to 50 keV and scanning tunneling microscopy at ∼10 eV have been used as patterning tools. The patterns have been transferred into many substrates by wet, dry, and combinations of wet and dry etches. Wet etching almost always results in a positive tone, but reactive ion etching of GaAs with Cl2 at very low dc biases (<10 V) results in a negative tone. The effect of electron beam damage on the monolayers and the subsequent etching reactions has been explored through x‐ray photoelectron spectroscopy.

Fabrication of dissimilar metal electrodes with nanometer interelectrode distance for molecular electronic device characterization

We report a versatile process for the fabrication of dissimilar metal electrodes with a minimum interelectrode distance of less than 6 nm using electron beam lithography and liftoff pattern transfer. This technique provides a controllable and reproducible method for creating structures suited for the electrical characterization of asymmetric molecules for molecular electronics applications. Electrode structures employing pairs of Au electrodes and non-Au electrodes were fabricated in three different patterns. Parallel electrode structures 300 μm long with interelectrode distances as low as 10 nm, 75 nm wide electrode pairs with interelectrode distances less than 6 nm, and a multiterminal electrode structure with reproducible interelectrode distances of 8 nm were realized using this technique. The processing issues associated with the fabrication of these structures are discussed along with the intended application of these devices.

Electron beam lithography with monolayers of alkylthiols and alkylsiloxanes

Self‐assembled monolayers have been modified with focused electron beams of energy 1–50 keV and scanning tunneling microscopy (STM) based lithography with energies of ∼10 eV. Modifications ∼15 nm in size have been formed by STM and ∼25 nm in size by 50 keV beams. The fact that these materials work as self‐developing electron beam resists is demonstrated by both atomic force microscopy imaging and pattern transfer using conventional wet etchants. Patterns have been transferred to silicon substrates to a depth of ≳120 nm with a multistep wet etching process. The mechanism of electron beam modification has also been explored to better design future monolayer processes.

Bilayer, nanoimprint lithography

Nanoimprint lithography has been shown to be a viable means of patterning polymer films in the sub-100 nm range. In this work, we demonstrate the use of a bilayer resist to facilitate the metal liftoff step in imprinter fabrication. The bilayer resist technology exhibits more uniform patterns and fewer missing features than similar metal nanoparticle arrays fabricated with single layer resist. The bilayer resist relies upon the differential solubility between poly(methyl methacrylate) and poly(methyl methacrylate methacrylic acid copolymer). Evidence is presented that shows the technique has a resolution of better than 10 nm.

Application of optical filters fabricated by masked ion beam lithography

Masked ion beam lithography (MIBL) was employed to fabricate optical filters as a critical component of an energy conversion system which utilizes semiconductor photovoltaics. This article will describe the operation and novel application of these devices and the MIBL pilot production line being facilitized. The conversion concept, thermophotovoltaics (TPV), when coupled with these MIBL produced bandpass filters, is capable of converting heat to electrical power with >20% conversion efficiency. The EDTEK TPV filter is based on a high density array of slotted antenna elements patterned into a single layer of thin gold film.

Facetless Bragg reflector surface‐emitting AlGaAs/GaAs lasers fabricated by electron‐beam lithography and chemically assisted ion‐beam etching

We report the fabrication and characterization of facetless Bragg reflector surface‐emitting AlGaAs/GaAs lasers. Both first‐order (120‐nm period) and second‐order (240‐nm period) gratings were fabricated by electron‐beam lithography and chemically assisted ion‐beam etching (CAIBE). These grating pairs provide the optical feedback of the laser, eliminating the need for cleaved or etched mirror facets. Specifically, this work includes: the fabrication and testing of a variable pitch grating‐laser array which demonstrates optical emission peaks with 5‐A separation for adjacent lasers; demonstration of facetless Bragg reflector lasers with 120/240‐nm grating pairs that show lower threshold currents, higher quantum efficiencies, and improved beam width compared to conventional facetless second‐order grating lasers; and a demonstration of grating surface‐emitting diode lasers with hybrid first‐order and nonresonant, 120/307‐nm, grating pairs that produced a directed beam at 45° with respect to the substrate. Th...

Electron beam nanofabrication with self-assembled monolayers of alkylthiols and alkylsiloxanes

Self-assembled monolayers have been demonstrated to perform as high-resolution electron beam resists with minimum resolutions of <20nm for 50keV electron beams and <15nm for 10eV electrons from a scanning tunneling microscope. The resulting patterns have been transferred into many substrates by both wet and dry etching.