摘要:

A method for the optimization of electrostatic lenses is presented. It is applied to optimize several three‐electrode lens designs for particular applications. The free parameters of the optimization are the physical quantities describing the shape of the lens: the electrode thicknesses, the gaps between the electrodes, and the electrode radii at a number of axial points for each electrode. With these parameters a cubic spline approximation to the axial potential is found by solving a system of linear equations. The method has been named the second‐order electrode method. Constraints for the optimization are the working distance on object and image side, a breakdown limit on off‐axis field strength, a limit to electrode potentials and minimum and maximum values for all parameters. No constraints are used to keep the object and image at their specified position. Before each merit function evaluation the middle electrode excitation is adjusted in order to satisfy the specified first‐order optical properties, so this construction keeps the first‐order properties out of the optimization. The figure of merit is application dependent and is a function of the chromatic and spherical aberration coefficients. The optimization method is based on a nonlinear optimization algorithm.

ISSN:0734-211X    

DOI:10.1116/1.584622

出版商:American Vacuum Society    

年代:1989

数据来源: AIP

摘要:

The new method for enhancing the focus latitude in optical lithography, named FLEX (focus latitude enhancement exposure) has been investigated. In FLEX the imaging characteristics are greatly affected by several key parameters such as distance between adjacent focal planes and illuminator coherence factor. The effect of these parameters on optical image characteristics have been examined using computer simulations. The results of the simulations have been verified by applying FLEX to imaging of sub‐half‐micron patterns using a high numerical aperturei‐line stepper. This has confirmed the validity of FLEX in delineating feature sizes in the order of 0.3∼0.5 μm. In particular, the depth of focus for hole patterns is found to be increased more than ten times, if necessary, with no degradation in resolution. However, the high‐contrast resist process is necessary in applying FLEX to patterns likeL/S. FLEX will be a great help to future optical lithography with high NA and/or short wavelength optical systems.

ISSN:0734-211X    

DOI:10.1116/1.584623

出版商:American Vacuum Society    

年代:1989

数据来源: AIP

摘要:

The ever increasing demand for sub‐half‐micron lithography for the production of gate structures for high electron mobility transistor (HEMT) and metal–semiconductor field effect transistor (MESFET) devices is greatly stimulating development of production‐worthy technologies to realize this goal. When the sub‐half‐micron lithography regime is approached the tendency is to assume that optical lithography is no longer a viable procedure and that electron beam lithography (EBL) or x‐ray lithography are the only alternatives. We have demonstrated a portable conformable mask process using poly(dimethylgutarimide) (PMGI) and Shipley 1400‐27 resist to define 0.25 μm wide lines, using optical contact lithography for the definition of 1 μm lines, followed by deep UV flood exposure. This process is then used to define and lift narrow gates defined with metal layers up to 1 μm thick. Model diffraction calculations are compared to the resultant structures.

ISSN:0734-211X    

DOI:10.1116/1.584624

出版商:American Vacuum Society    

年代:1989

数据来源: AIP

摘要:

This work demonstrates that UV‐ozone cleaning of substrates prior to molecular‐beam epitaxial growth, which has been shown by other workers to remove carbon contamination, is effective in suppressing sidegating problems otherwise encountered in GaAs metal–semiconductor field effect transistors fabricated on the epitaxial layers.

ISSN:0734-211X    

DOI:10.1116/1.584625

出版商:American Vacuum Society    

年代:1989

数据来源: AIP

ISSN:0734-211X    

DOI:10.1116/1.584626

出版商:American Vacuum Society    

年代:1989

数据来源: AIP

摘要:

A microscopic model of epitaxial growth of GaAs on Si(100) is proposed, which explains how double‐layer steps on the Si(100) surface act as nucleation sites in the initial stages of growth. On the flat terraces of Si(100), there is a strong tendency towards the formation of mixed layers of Ga and As, which inhibit growth of zincblende GaAs. The bonding topology of the surface steps supresses this mixing, and drives the growth of ideally bonded GaAs in a three‐dimensional mode with islands forming along the steps edges. Total‐energy calculations indicate that the proposed model for nucleation of GaAs at double‐layer steps on the surface is energetically stable.

ISSN:0734-211X    

DOI:10.1116/1.584627

出版商:American Vacuum Society    

年代:1989

数据来源: AIP

摘要:

We report on the initial stages of GaAs and AlAs growth on Si substrates, focusing on that done using atomic‐layer epitaxy (ALE). ALE was performed on 3° off (100) substrates at 500 °C using an arsine and metalorganic gas source. Thin layers (5–350 monolayers) were examined using Auger electron spectroscopy, Raman scattering, transmission electron microscopy (TEM), and several other techniques. Measurements showed that ALE on Si substrates starts from three‐dimensional growth but changes to layer by layer growth at an early stage. Coverage is remarkably improved by initiating the growth from AlAs instead of GaAs. TEM images for AlAs indicate a relatively smooth surface and regular lattice arrangement on Si substrates. The coverage improvement is attributed to the selective‐adsorption and self‐limiting effects of ALE and proper arrangement of Al atoms on Si.

ISSN:0734-211X    

DOI:10.1116/1.584628

出版商:American Vacuum Society    

年代:1989

数据来源: AIP

摘要:

A systematic knowledge of the influence of molecular‐beam epitaxy growth parameters on the properties of AlGaAs/GaAs quantum wells grown under realistic growth conditions is important in order to obtain optimal performance of modern electronic and optoelectronic devices. Photoluminescence (PL) was used to investigate the influence of the Ga‐controlled growth rate in the range below standard growth rates of 1 μm/h down to 0.1 μm/h, and of the As:Ga beam equivalent pressure ratio in the range of 10 to 60, on the growth kinetics, the interface quality, and the impurity incorporation, at a substrate temperatureTs=620 °C. As compared toinsitureflection high‐energy electron diffraction (RHEED) measurements, where no sample rotation is possible, PL has the advantage that realistic growth conditions can be used. A careful line shape analysis, together with infrared and time‐resolved PL measurements gives information on the interface roughness, the impurity incorporation, and the deep trap concentration. Non‐negligible desorption of Ga during growth is found for the range of conditions under study. The desorption is found to increase upon a decrease of As:Ga ratio. The interface roughness as well as the impurity and trap incorporation are found to decrease with decreasing growth rate, an optimum interface quality being obtained below 0.5 μm/h. At this optimal growth rate, increasing the As4:Ga ratio leads to a decrease of shallow impurity concentration and thus to a narrower line width, but to a simultaneous increase of defect generation. Optimal growth conditions are found at a beam equivalent pressure ratio of 15. The observed desorption kinetics and interface properties can be explained in accordance with existing theoretical simulations. Finally, growth interruption was found to lead to optimal formation of flat growth islands when the overall growth rate is lowered.

ISSN:0734-211X    

DOI:10.1116/1.584629

出版商:American Vacuum Society    

年代:1989

数据来源: AIP

摘要:

GaAs layers grown by molecular‐beam epitaxy (MBE) at very low substrate temperatures have gained considerable interest as buffer layers for GaAs metal–semiconductor field effect transistors (MESFET’s) due to high resistivity and excellent device isolation. However, the structure and the electronic properties of such layers have not yet been investigated in detail. We have studied unannealed low temperature (LT) MBE layers grown at 200 °C using transmission electron microscopy (TEM), analytical TEM, x‐ray diffraction, the Hall effect, and electron paramagnetic resonance (EPR) techniques. TEM data indicated large arsenic‐rich deviations from stoichiometry of ∼1–1.5 at. %. X‐ray rocking curves showed a uniform increase of 0.1% in all directions of lattice parameters compared to semi‐insulating GaAs substrate. The Hall effect and thermally induced changes of photo‐EPR measurements revealed the presence of an acceptor level at an energy of ∼0.3 eV above the valence band. This acceptor level has been tentatively attributed to a gallium vacancy defect.

ISSN:0734-211X    

DOI:10.1116/1.584630

出版商:American Vacuum Society    

年代:1989

数据来源: AIP

摘要:

A specially designed molecular‐beam epitaxy (MBE) system with scanning microprobe reflection high‐energy electron diffraction (RHEED) has been developed. The incident electron beam is designed to focus down to 10 nm in diameter and scan the surface of the substrate under MBE growth conditions. Using the RHEED intensity variation produced by the scanning of the incident beam, we obtain a microscopic image, i.e., which is the scanning reflection electron microscope (SREM) image, highly sensitive to the surface structure. The SREM images of GaAs(001) using the specular beam spot revealed granular features over the entire surfaces of MBE‐grown GaAs layers which come from undulations of the surface. The undulation wavelength was typically 0.1–0.5 μm and developed along [1̄10] direction during MBE growth at 570 °C. At higher substrate temperatures, little development of the undulation was observed. As an overall feature of MBE growth, two‐dimensional layer‐by‐layer growth was observed over the whole surface having undulations.

ISSN:0734-211X    

DOI:10.1116/1.584631

出版商:American Vacuum Society    

年代:1989

数据来源: AIP