Showing papers on "Electron-beam lithography published in 1990"

CW operation of semiconductor ring lasers

Abstract: Semiconductor ring lasers have been fabricated in single quantum well material using electron-beam lithography and SiCl4 dry etching. CW operation has been achieved in 84 μm diameter rings at a threshold current of 24 mA. This low value makes the structure very suitable for monolithic integration in optoelectronic circuits.

Laser beam lithographed micro-Fresnel lenses

Abstract: Laser beam lithography for micro-Fresnel lenses (MFLs) with blazed grooves is proposed and demonstrated including the system configuration and characteristics of the resulting lenses. The resolution is even better than that of electron-beam lithography in forming 1-μm deep relief gratings in resist. A laser beam lithographed MFL with a diameter as large as 9.6 mm is described as well as a compact MFL (N.A. 0.21) butt coupled to an optical waveguide. In these two distinct MFLs, nearly diffraction-limited spot sizes have been obtained with diffraction efficiencies of 50% or more. A specific MFL array used for an integrated optic laser Doppler velocimeter is also presented.

Coulomb interactions in particle beams

Abstract: The physical aspects of an analytical model for the different manifestations of Coulomb interactions between charged particles in low and medium density beams, e.g. as found in electron beam lithography instruments and scanning electron microscopes, are reviewed. The theory is found to be in close agreement with the results of numerical Monte Carlo simulations. A comparison is given with other existing theories on the subject.

Investigation of RF SQUIDs made from epitaxial YBCO films

Abstract: RF SQUIDs have been made from epitaxial YBCO films. The films were patterned by standard photolithography and wet chemical etching. The SQUIDs used S-N-S 'monolithic' weak link produced by oxygen ion irradiation, through a slit in a PMMA mask across the microbridge made by electron beam lithography. Changing the slit dimensions and irradiation dose, it was possible to observe proper SQUID function up to temperatures as high as 80 K.

Microfabrication below 10 nm

Abstract: We describe a new method of producing ultrasmall structures on thick substrates with electron beam lithography. Using an innovative exposure technique, we obtain features with lateral sizes smaller than the incident beam diameter. These patterns are transferred into GaAs/AlGaAs quantum well heterostructures using chemically assisted ion beam etching, and uniform arrays of structures with lateral dimensions below 10 nm are produced. We employ reflection electron microscopy measurements to correlate the structure size with the exposure and development conditions for this fabrication scheme.

Time-resolved investigations of sidewall recombination in dry-etched GaAs wires

Abstract: We have investigated the sidewall recombination in dry‐etched GaAs/GaAlAs wires with widths between 12 μm and 300 nm using picosecond spectroscopy. The wires were fabricated with electron beam lithography and different reactive ion etching processes. The excitonic lifetimes decrease strongly with decreasing wire widths due to sidewall recombination. Using a model calculation to fit the experimentally observed width dependence of the lifetimes the surface recombination velocity is determined to be S=2×106 cm/s at 50 K. S increases with temperature and is independent of the etching process.

Novel electron‐beam lithography for in situ patterning of GaAs using an oxidized surface thin layer as a resist

Abstract: The first demonstration of in situ electron‐beam (EB) lithography is reported, where a photo‐oxidized surface thin layer of GaAs is used for a resist. The in situ EB lithography sequence consists of five processes, i.e., preparation of a clean GaAs surface, photo‐oxidation for a resist film formation, direct patterning of the oxide resist by EB‐induced Cl2 etching, Cl2 gas etching of GaAs surface for pattern transfer, and thermal treatment in an arsenic ambient for resist removal and surface cleaning. The GaAs wafer is never exposed to air throughout all of the above processes to avoid an unintentional surface contamination. The minimum electron dose required for patterning of the GaAs oxide resist is about 5×1016 cm−2. An overgrown layer on the patterned GaAs surface shows a good surface morphology, which strongly indicates that this technology makes it possible to repeat crystal growth and surface patterning.

E-Beam Direct-Write in a Dry-Etched Recess Gate HEMT Process for GaAs/AlGaAs Circuits

Abstract: We developed a technology to fabricate enhancement and depletion high electron mobility transistors (E- and D-HEMTs) with sub-0.5 µm gatelengths using e-beam direct-write lithography. The gates are recessed by dry etching. The recess is stopped on 30 A AlGaAs layers for E- and D-field effect transistors (FETs) respectively. We have extensively investigated the double layer resist technique for direct-write to improve the reliability of 0.3 µm electron-beam (E-beam) lithography. Ring oscillators using direct coupled FET logic (DCFL) have been measured indicating delay times of 16 ps per stage.

Low‐voltage electron beam lithography with a scanning tunneling microscope

Abstract: Studies of a polydiacetylene negative electron beam resist have been made in a scanning tunneling microscope operated in vacuum at pressures in the 10−8 Torr range. The resist can be imaged if it is applied as a thin film to a conductive flat substrate and the tip bias voltage is chosen appropriately. An exposure threshold energy close to 8 eV has been observed for the formation of raised features in the resist. A minimum feature size of about 20 nm has been measured when written at an energy just above the exposure threshold. Details of the necessary substrate preparation are described together with the operation of the scanning tunneling microscope during imaging and exposure. The exposure dose is applied by raising the bias voltage for a specific time while keeping the tip‐sample current constant which has permitted insights into the mechanisms of the resist exposure.

Multilevel phase holograms manufactured by electron-beam lithography

Abstract: Ten-level transmission phase holograms (kinoforms) manufactured in one resist layer by electron-beam lithography are reported for the first time to our knowledge. The measured hologram diffraction efficiencies were 70% for the two resist materials used. This corresponds to 82% of the maximum theoretical value for these holograms and is, to our knowledge, the highest reported to date.

Lateral quantization induced emission energy shift of buried GaAs/AlGaAs quantum wires

Abstract: Buried GaAs/AlGaAs quantum wires were prepared by Al0.2Ga0.8As overgrowth of deep etched wires defined by high‐resolution electron beam lithography and dry etching. The overgrown wires show a dramatic decrease of the optically inactive sidewall layer compared to open wires. For wires with a lateral dimension Lx=50 nm we observe a spectral shift to higher energies of the excitonic emission, in good agreement with calculations of the two‐dimensional confinement effects.

Polyimide optical waveguides fabricated with electron beam lithography.

Abstract: Fabrication of PMMA clad polyimide waveguides by electron beam lithography creates very smooth sidewalls allowing production of narrow low loss waveguides on planar substrates.

Resist heating effects in 25 and 50 kV e‐beam lithography on glass masks

Abstract: Electron sensitive resists have been exposed on optical masks in a variably shaped spot electron beam lithography system, EL3, at 25 and 50 kV and beam current densities up to 50 A/cm2 to evaluate the effects of resist heating during electron exposure Resist heating effects are primarily observed in dense patterns at high electron exposure dose For the purpose of comparison resist heating effects are quantified in terms of the development rate change that is caused by the temperature rise in the resist during electron beam exposure From an energy density point of view the maximum temperature rise in the resist during exposure is very similar for both 25 and 50 kV exposures The area on the mask over which the temperature rise in the resist can change resist properties is significantly larger at 50 kV: up to 20 μm at 50 kV compared to ∼5 μm at 25 kV Using sensitive electron beam resists requiring 1–5 μC/cm2 exposure dose no resist heating effects are observed at beam current densities up to 50 A/cm2

Electron-emitting device, and electron beam lithography machine and image display apparatus making use of it

Abstract: An electron-emitting device comprises a substrate, an electrode provided on said substrate, an insulating layer laminated on the electrode, and a second electrode having an opening and laminated on the insulating layer in such a manner that the insulating layer is uncovered at the opening and electrons are emitted from the opening of the second electrode as a result of application of an voltage between the electrodes. An image display apparatus comprises the electron-emitting device, a modulating electrode capable of modulating an electron beam emitted from the electron-emitting device, in accordance with an information signal, and an image forming member capable of forming an image as a result of irradiation with the electron beam, these of which are successively disposed. An image forming apparatus comprises the electron-emitting device, and a means for modulating an electron beam emitted from said electron-emitting device, in accordance with an information signal.

High resolution germanium zone plates and apertures for soft x-ray focalometry

Abstract: Circular Fresnel zone plates, apodized linear zone plates, and a variety of apertures have been fabricated in smooth germanium films deposited on boron doped silicon membranes. Zone widths as small as 60 nm have been fabricated in absorbing layers from 0.25 to 0.5 μm thick. These results were achieved using electron beam lithography in a trilevel resist followed by reactive ion etching to transfer the patterns. These optical elements are key components in a soft x‐ray ‘‘focalometer’’ currently being tested at the NSLS at Brookhaven National Laboratories. The focalometer is a system which can be used to determine the imaging properties of an optical element. In this work it has been used to evaluate the imaging properties of a convergent mirror both before and after a multilayer reflective coating was applied. Results of knife edge scans of images formed by a spherical mirror using 14 nm radiation are reported. The multilayer coating applied to the mirror is found to have only a small effect on these scans.

Lithographic studies of an e‐beam resist in a vacuum scanning tunneling microscope

Abstract: The scanning tunneling microscope (STM) provides a confined low energy electron beam which can be exploited for e‐beam lithography. It offers the potential of overcoming the resolution degradation due to secondary and backscattered electrons produced by the high energy primary beam in conventional e‐beam lithography systems. A polydiacetylene negative e‐beam resist has been evaluated in an STM mounted in a ultrahigh vacuum (UHV) chamber. The resist can be both imaged and exposed in situ by the STM. Under e‐beam irradiation raised features are formed which can be imaged directly. A minimum feature size of 20 nm has been observed which is less than third of the minimum feature size observed following exposure in a high voltage e‐beam writer and development. The smallest features were written in the STM at an electron energy just above the exposure energy threshold which was measured to be about 8 eV.

Assessing thermal Cl2 etching and regrowth as methods for surface passivation

Abstract: The damage layer that results from ion beam processing ultimately limits the smallest size structure which will emit light and hence prevents the realization of low‐dimensional quantum size effects. Effective surface passivation of etched structures requires both layer removal and compensation of surface states. Ultimately these two processes must be made compatible. To address this problem, we have coupled an ultrahigh‐vacuum (UHV) dry‐etching system to a molecular beam epitaxy (MBE) growth chamber. Material is patterned and masked outside of the combined etching/regrowth system using electron beam lithography. Samples are then loaded into the combined UHV system and vertical structures are produced by chemically assisted ion beam etching (CAIBE). These structures are then subjected to a thermal Cl2 gas etch to remove the damage layer created by the ion beam etching. Samples are subsequently transferred directly into the MBE growth chamber, where a cladding layer of Al0.3Ga0.7As is grown over the etched ...

Energy density function determination in very‐high‐resolution electron‐beam lithography

Abstract: A proximity effect correction algorithm requires an accurate knowledge of the energy density function (EDF) deposited in the electron resist layer. In case of high‐resolution electron‐beam lithography (EBL), whenever the required resolution is below 0.5 μm, forward and large‐angle backward scattering contribution has to be carefully analyzed. In this paper, we compare Monte Carlo (MC) simulation data with experimental point exposure measurements over different substrates and with different beam energy (range 20–40 kV). The substrates we analyze are silicon and silicon nitride membranes (2 μm thick), indium phosphide, and gold. A good correlation between MC data and experiments is proved. Experiments and MC data suggest that the usual double Gaussian fit applied to the distribution is not suitable for the application of high‐resolution lithography or with high‐Z materials. These data indicate that the recently proposed triple Gaussian fit can be considered as a particular case of a more complex multi Gauss...

A study of deposited charge from electron beam lithography

Abstract: This paper presents results from a study of pattern placement errors due to substrate charging in fixed spot direct write electron beam lithography. Experimental results indicate charging‐induced errors of over 0.5 μm can occur on a grounded Si substrate. Several theoretical models of the charge distribution are presented. Beam deflection due to the monopole field of the accumulated charge predominate over dipole fields in our system and electrons deposited in the Si substrate create negligible pattern placement errors. Finally, about half of the electrons initially trapped in the resist remain there many hours after exposure.

Potentiometry for thin‐film structures using atomic force microscopy

Abstract: The compliance of the tip–sample contact region in a scanning tunneling/atomic force microscope (STM/AFM) is found to increase from ∼1 N/m to more than 200 N/m with decreasing tip‐to‐surface distance in a distance interval of ∼200 nm. When a tungsten tip and a metal sample are used, this interval includes the distance at which a tunneling current is measurable. An STM/AFM was operated at this tip‐to‐surface distance in the constant‐compliance mode. Since a tunneling contact to the tip is established at conducting sections of the sample, the electrical potential can be measured there. An array of interconnected niobium squares and an array of metal strips alternately connected to two electrodes of different voltage were examined using this method. Both structures were fabricated by electron beam lithography from Nb films on oxidized Si substrates. The topography and the value of the electrical potential were recorded. A potential resolution of the order of 1 mV was achieved.

Submicron pattern etching of GaAs by in situ electron beam lithography using a pattern generator

Abstract: Pattern etching of GaAs at submicron size is carried out by in situ electron beam (EB) lithography using a computer‐controlled pattern generator. GaAs oxide on the wafer surface is used as a resist film in EB‐induced Cl2 etching. A 0.5 μm linewidth in a 1 μm pitch line‐and‐space pattern with flat top surface is obtained. Observations of the pattern edge with a scanning electron microscope show that the boundary between the etched area and the oxide mask area is abrupt and that the undulation of the pattern edge is less than about 30 nm.

Exposure characteristics of high‐resolution negative resists

Abstract: Positive and negative resist systems are both essential in the microfabrication of experimental devices. While numerous positive resists have been shown to have high‐resolution, negative resists were considered to have poor contrast and resolution. In this paper, the electron‐beam exposure characteristics of several negative resist systems including epoxy novolac‐type systems [K. J. Stewart, M. Hatzakis, J. M. Shaw, and D. E. Seeger, J. Vac. Sci. Technol. B 7, 1734 (1989); M. Hatzakis, K. Stewart, J. Shaw, and S. Rishton, Microelectron. Eng. 11, 487 (1990)] and two selected vendor resist systems have been investigated. These resist exposure characteristics are related to resist resolution, image profile, edge acuity, and process latitude. The sensitivity of various resists to slight exposures which may be caused by electron scattering in electron‐beam lithography or mask diffraction in optical or x‐ray lithography varies from resist to resist. The resist system that is least sensitive to these slight expo...

Fabrication of First-Order Gratings for GaAs/AlGaAs LD's by EB Lithography

Abstract: High-precision first-order gratings, 110 nm pitch, for GaAs/AlGaAs LD's were successfully fabricated using EB lithography. The field stitching error of EB lithography caused by height difference between reference marks for calibration and the samples, was reduced to 5 nm by controlling the EB deflection voltage calibrated by the detection of the position of the marks on the samples. We also investigated the effect of field stitching error on lasing characteristics using measurement and calculation. We found that a 20 nm stitching error is sufficiently small for single-mode LD operation.

Using a neural network to proximity correct patterns written with a Cambridge electron beam microfabricator 10.5 lithography system

Abstract: This letter describes our initial results of using a theoretical determination of the proximity function and an adaptively trained neural network to proximity correct patterns written on a Cambridge electron beam lithography system. The methods described are complete and may be applied to any electron beam exposure system that can modify the dose during exposure. The patterns produced in resist show the effects of proximity correction versus noncorrected patterns.

Double 15‐nm‐wide metal gates 10 nm apart and 70 nm thick on GaAs

Abstract: A method that can fabricate two narrow but thick metal lines separated by a small gap on a bulk semiconductor substrate is described. By opening a 40‐nm‐wide trench in a single layer of 70‐nm‐thick polymethylmethacrylate resist on GaAs using high resolution electron beam lithography, and by double shadow evaporations and a lift‐off, two 15‐nm‐wide metal lines 10 nm apart and 70 nm thick were fabricated on a bulk GaAs substrate. The pitch size of the double metal lines is 25 nm. This is a factor of 2 smaller than the previous smallest pitch size on bulk semiconductors. It is found that the width and spacing of the two lines are uniform over tens of microns. It is also found that metals shadow evaporated on top of the resist can be removed successfully by a lift‐off, even though they were connected to the metals in the resist trenches. These results suggest that using this method metal lines with even finer linewidth (<15 nm) can be achieved on a bulk semiconductor substrate.

Electron focusing with multiparallel GaAs‐AlGaAs wires defined by damageless processing

Abstract: Magnetoresistance modulation, resulting from electron focusing, is investigated with multiparallel GaAs‐AlGaAs wires, which are defined by electron beam lithography and damageless wet‐chemical etching. Distinct focusing peaks in magnetoresistance are observed, although the samples have wires longer than a ballistic mean free path, which is derived from the focusing peak intensity. Specularity coefficient p for the etched region boundary is also obtained as p≊1 from the focusing peak ratio. The mechanism limiting ballistic transport in the electron focusing condition is discussed, based on the temperature and the electron density dependence of the peak intensity.

Fine Pattern Formation of Gallium Arsenide by In Situ Electron-Beam Lithography Using an Ultrathin Surface Oxide as a Resist

Abstract: Formation of fine patterns such as lines and spots on a GaAs surface by in situ electron beam (EB) lithography is demonstrated. An ultrathin surface oxide (<1 nm) of GaAs is used as a resist film, which can be patterned by EB-assisted etching. The minimum pattern size of less than 1.0 µm is obtained; it is limited by the electron beam shape in the present experiment. The EB-induced patterning of the oxide resist is not due to substrate heating or minority carrier generation, but to excitation of the sample surface and/or the chlorine atoms adsorbed on the oxide resist film.

Planarized, reusable calibration grids

Abstract: An improved structure for electron beam lithography grids and a method of fabricating such grids yields calibration grids having grid lines coplanar with the surface of a the grid body and laterally supported by grooves formed in the grid body and which can also be cleaned after contamination by outgassing resist during use by virtue of the provision of such lateral support for the grid lines. The grid exhibits improved accuracy due to the technique of fabrication of the grooves. The invention thus allows the electron beam lithography process to be conducted with less expense and at a greater speed. The improved accuracy of the calibration grid also permits integrated circuits and masks used in the fabrication of such devices to be designed more flexibly and fabricated at reduced cost and improved integration densities and manufacturing yields.