bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural defects in B-doped Si substrates for x-ray masks
Authors: Uzoh C. , Maldonado J. R. , Angilello J.
Published: July 2002

This paper describes sources of structural defects that may be present in B-doped Si membranes utilized as x-ray lithography masks. These defects may affect the mechanical properties of the membranes and cause premature breakage, undesirable distortion and/or patterning defects (in the case of pin holes). Structural defects originally present in the starting material (Si wafers) and structural defects that may develop during the fabrication of B-doped Si membranes will be described. Several material characterization techniques were used, including SIMS, scattering of He–Ne laser light transmitted through the membranes, x-ray diffraction from conventional and storage ring sources, and metallographic techniques to label the defects. The data indicate that the bulk of the strain in the B-doped membranes is due not only to lattice mismatch (between B and Si atoms), but also to strain fields caused by defects and the presence of a B rich phase.
Optical lithographic tools: Current status and future potential
Authors: Wilczynski Janusz S.
Published: July 2002

The tools used to manufacture silicon chips fall into two categories: The full-wafer scanning projection printers pioneered by Perkin–Elmer (PE) and optical steppers. The resolution of these tools is limited by the wavelength and numerical aperture (NA). Since the resolution is proportional to the wavelength divided by the NA, steppers have an advantage. The extensibility of either technology is becoming increasingly difficult and is limited by the complexity of lenses and attainable alignment. The technology for obtaining 0.5 μm resolution is well established. In the future, x-ray optics, when fully developed, may provide printers capable of dimensions ten times as fine.
Fabrication and phonon transport studies in nanometer scale free-standing wires
Authors: Smith C. G. , Ahmed H. , Wybourne M. N.
Published: July 2002

We have fabricated free-standing structures of nanometer-scale dimensions by combining high-resolution electron beam lithography, layer growth and dry etching techniques. AuPd lines having a minimum width of 40 nm, a thickness of 10 to 40 nm, and lengths from 8 to 120 μm have been made free standing. The resistance of fine wires on a substrate as well as free-standing wires has been measured from 0.35 to 20 K. Both sets of wires show quantum corrections to the resistance. For wires on a substrate the resistance rise fits well to one-dimensional electron–electron interactions, while for the free-standing wires the resistance rise takes a new form that has not yet been explained. Below 1 K the lattice component of the thermal conductivity is expected to change as the phonon dimensionality is reduced from three to one. This contribution to the thermal conductivity can be estimated from self-heating measurements, and it is found to be three orders of magnitude smaller than predicted. In addition we have estimated the tensile strength to be an order of magnitude larger than bulk values.
The percolation approach to the development of pulsed laser exposed positive photoresists
Authors: Bogdanov A. L. , Valiev K. A. , Velikov L. V. , Zaroslov D. Yu.
Published: July 2002

Results of photolithography experiments utilizing the XeCl excimer laser and positive photoresists are reported. Abrupt reciprocity loss resulting in the growth of photoresist contrast and sensitivity was found when laser pulse energy density exceeded a threshold value of about 50 mJ/cm2. Percolation theory is used to derive a model of resist thickness remaining versus dose for development of pulse exposed photoresist. The model accurately describes the change in dissolution of AZ2400 photoresist when the energy is delivered in multiple versus single pulse exposures.
Transmission electron microscope study of lattice damage and polymer coating formed after reactive ion etching of SiO2
Authors: Cerva H. , Mohr E.-G. , Oppolzer H.
Published: July 2002

Reactive ion etching (RIE) of SiO2 in a CHF3/O2 plasma provides both high anisotropy and selectivity to the Si substrate. Cross-sectional transmission electron microscopy was used to study the polymer film and the lattice damage induced by this RIE process as well as the effect of post-treatment and annealing conditions. The lattice damage consists mostly of defect clusters which increase both in density and depth with overetching. Post-treatment in an NF3/Ar plasma removes the polymer film as well as the defect zone, but produces surface roughness (up to 5 nm). Annealing at 450 °C (30 min) had only a minor effect on the lattice damage, whereas annealing at 900 °C (30 min) removed all defects. No correlation was found between RIE damage and residual ion implantation damage.
Scanning tunneling microscope liquid-metal ion source for microfabrication
Authors: Bell A. E. , Rao K. , Swanson L. W.
Published: July 2002

The use of a liquid-metal ion source (LMIS) in a scanning tunneling microscope (STM) embodiment is described. Operation of the LMIS at emitter/target spacings of the order of 100 nm or less is discussed as are also attempts to create microfabrication features on GaAs and Si substrates. The dependence of the threshold voltage for LMIS operation on spacing follows a square root relationship as would be expected from the Taylor theory. Very stable operation of the LMIS was realized at spacings corresponding to threshold voltages <200 V. The use of a proximity focused LMIS allows an exceedingly high current density to be achieved. For example, at 0.1 μm spacing of the emitter from the target and assuming an emission half-angle of 20° a target current density of ∼2300 A/cm2 can be achieved for a Ga LMIS operating at 0.1 μA total current. At closer spacings target current densities are correspondingly greater. Combining the LMIS with an STM embodiment makes possible nanometer fabrication carried out at speeds limited only by the mechanical deflection schemes.
Broad ion beam modeling for extraction optics optimization and etching process simulation
Authors: Korzec D. , Schmitz K. , Engemann J.
Published: July 2002

A model for calculating the ion–current density distribution of a broad-beam ion source at the target level is proposed. Three model parameters related to a single beamlet such as the beam divergence, the maximum ion–current density in the beamlet center, and a possible misalignment between the different grids are needed to describe the overall extraction performance. These parameters were determined partially with the axcel simulation code or experimentally via etch tests. In this work the model parameters have been determined for a capacitively coupled filamentless 13.56 MHz rf source operated with argon at low power levels ranging from 20–100 W and low acceleration voltages (100–1000 V). However, the model proposed is applicable also for other types of ion sources, for different extraction grid sets with varying geometry and a wide range of working conditions. Intensity profiles predicted by the above model show good agreement with experimental results.
Fabrication of quantum wires in GaAs/AlGaAs heterolayers
Authors: Thoms S. , McIntyre I. , Beaumont S. P. , Al-Mudares M. , Cheung R. , Wilkinson C. D. W.
Published: July 2002

Conducting wires of widths down to 0.1 μm have been fabricated in modulation-doped GaAs/AlGaAs material using SiCl4 reactive ion etching with a negative resist mask. The resist used was high-resolution negative (HNR) and its applicability to microstructure fabrication is discussed. A novel dry etch for GaAs, CH4/H2 reactive ion etching, produces low damage and shows much promise for wire fabrication.
Defects induced by focused ion beam implantation in GaAs
Authors: Miyake Hideto , Yuba Yoshihiko , Gamo Kenji , Namba Susumu , Shiokawa Takao
Published: July 2002

The characteristics of defects induced by Si and Ga focused ion beam (FIB) implantation in n-GaAs have been investigated by means of deep-level transient spectroscopy (DLTS), C–V carrier profiling, and resistance measurements. The DLTS spectra of Si and Ga FIB implanted samples annealed at temperatures up to 500 °C are apparently identical to one another and show three different electron traps with an activation energy between 0.25 and 0.6 eV. The resistance increases by more than five orders of magnitude by Si and Ga FIB implantation due to the induced defects. However, it is restored to initial values after annealing at 600 °C, except for a sample of Ga implantation with a dose higher than 1014 /cm2 . For annealing of induced defects, there are no intrinsic problems for FIB implantation with a dose lower than 1013 /cm2 .
Acoustic charge transport in an (Al,Ga)As/GaAs heterojunction structure grown by molecular-beam epitaxy
Authors: Sacks R. N. , Tanski W. J. , Merritt S. W. , Cullen D. E. , Branciforte E. J. , Eschrich T. C.
Published: July 2002

The demonstration of charge transport in GaAs epitaxial layers by surface acoustic waves in 1982 has led to a great deal of interest in using acoustic charge transport (ACT) technology for high-speed analog signal processors. Successful implementation of ACT effects requires confining the moving charge packets to a buried layer. This was originally accomplished by biasing a thick, lightly doped n-GaAs layer (about 4 μm low-1015/cm3) with electrodes at the epilayer surface and at the back of the semi-insulating substrate, leading to problems in device fabrication and reproducibility, as well as making monolithic integration with field effect transistors (FET’s) difficult. The logical solution to these problems seemed to be using the charge confining capability inherent in heterojunctions. We report here the first observation of acoustic charge transport in a heterojunction structure. The heterojunction acoustic charge transport (HACT) epitaxial structures were grown by molecular-beam epitaxy and are comprised of a GaAs quantum well with (Al,Ga)As barriers. The top (Al,Ga)As layer is doped to satisfy surface states and to facilitate formation of Ohmic contacts. These structures are similar to quantum well or double heterojunction high electron mobility transistor (HEMT) [modulation-doped FET (MODFET)] epilayers, but with somewhat different material requirements for good device operation. As examples, acoustic charge transport devices operate at low current levels (about 100 μA) and thus require Schottky contacts with very low leakage. But since they operate at relatively low frequencies (hundreds of MHz) they do not need extremely high mobility two-dimensional electron gases. Initial results indicate that these devices function best with a low sheet carrier concentration, also in contrast to HEMT’s. Further details of materials and device fabrication will be presented.
Low-stress tantalum absorbers deposited by sputtering for x-ray masks
Authors: Iimura Yukio , Miyashita Hiroyuki , Sano Hisatake
Published: July 2002

A method for fabricating stable and low-stress tantalum films, suitable for absorbers of x-ray masks, by dc-bias, rf magnetron sputtering is presented. The superiority of Xe over Ar as a working gas is demonstrated. The stress-vs-bias-voltage curve has a plateau, where the film stresses are low and constant over the bias voltage ranging from −15 to 20 V. The plateau stresses vary linearly with both the working Xe pressure and the substrate temperature with slopes of 1.2×1010 dyn/cm2 Pa and 8.6×106 dyn/cm2 deg, respectively. By control of the pressure and the temperature, stresses of (3±2)×108 dyn/cm2 are achieved. The Ta film which is stable under a heat treatment up to 300 °C is obtainable when it is deposited at 220 °C and then annealed at 220 °C for 1 h. The density of the films is (15.0±0.5) g/cm3. By x-ray diffraction the films are found to be in a mixture of the α-Ta phase and the β-Ta phase. Ta patterns of 0.15 μm in width were fabricated.
Very thin silicon epitaxial layers grown using rapid thermal vapor phase epitaxy
Authors: Campbell S. A. , Leighton J. D. , Case G. H. , Knutson K. L.
Published: July 2002

A new technique has been developed for the growth of epitaxial silicon layers thin enough to be used as active regions of highly scaled transistors. The method, referred to as rapid thermal vapor phase epitaxy, is an extension of standard dichlorosilane vapor phase epitaxy (VPE) to an appropriate time/temperature regime. To do this unique wafer heating and cooling arrangements have been developed and combined with a high-vacuum/low-temperature growth environment. The films grown with this technique are selective without the addition of HCl for thicknesses below 1500 Å. X-ray and electron channeling results indicate that they are single crystal.
Image-projection ion-beam lithography
Authors: Miller Paul A.
Published: July 2002

Image-projection ion-beam lithography is an attractive alternative for submicron patterning because it may provide high throughput; it uses demagnification to gain advantages in reticle fabrication, inspection, and lifetime; and it enjoys the precise deposition characteristics of ions which cause essentially no collateral damage. This lithographic option involves extracting low-mass ions (e.g., He+ ) from a plasma source, transmitting the ions at low voltage through a stencil reticle, and then accelerating and focusing the ions electrostatically onto a resist-coated wafer. While the advantages of this technology have been demonstrated experimentally by the work of IMS (Austria), many difficulties still impede extension of the technology to the high-volume production of microelectronic devices. We report a computational study of a lithography system designed to address problem areas in field size, telecentricity, and chromatic and geometric aberration. We present a novel ion-column-design approach and conceptual ion-source and column designs which address these issues. We find that image-projection ion-beam technology should in principle meet high-volume-production requirements. The technical success of our present relatively compact-column design requires that a glow-discharge-based ion source (or equivalent cold source) be developed and that moderate further improvement in geometric aberration levels be obtained. Our system requires that image predistortion be employed during reticle fabrication to overcome distortion due to residual image nonlinearity and space-charge forces. This constitutes a software data preparation step, as do correcting for distortions in electron lithography columns and performing proximity-effect corrections. Areas needing further fundamental work are identified.
Improvement of defocus tolerance in a half-micron optical lithography by the focus latitude enhancement exposure method: Simulation and experiment
Authors: Fukuda Hiroshi , Hasegawa Norio , Okazaki Shinji
Published: July 2002

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 aperture i-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 like L/S. FLEX will be a great help to future optical lithography with high NA and/or short wavelength optical systems.
Photoluminescence and photoluminescence excitation studies on GaAs/AlAs short period superlattices near the direct/indirect crossover
Authors: Smith G. W. , Skolnick M. S. , Pitt A. D. , Spain I. L. , Whitehouse C. R. , Herbert D. C.
Published: July 2002

A photoluminescence (PL) and PL excitation (PLE) investigation of GaAs/AlAs superlattices near the direct to indirect gap, type I to type II superlattice crossover is reported. The nature of the phonon coupling, the magnitude of the Stokes’ shifts between PL and PLE, and the dependence of the PL spectra on hydrostatic pressure are studied. From these results a change of the GaAs width (d) by just 1 monolayer (ML) from 12 to 11 ML is shown to cover the range from near coincidence of the GaAs (Γ) and AlAs (X) conduction band minima, but with type I, direct gap character dominant at d=12 ML, to clearly indirect gap, type II behavior at d=11 ML.
Apparent temperature oscillations during molecular-beam epitaxy: A useful interferometric effect
Authors: Wright S. L. , Jackson T. N. , Marks R. F.
Published: July 2002

Using a narrow-bandwidth, short-wavelength (0.94 μm) pyrometer, we have observed apparent temperature oscillations during molecular-beam epitaxy (MBE) growth of III–V heterostructures, due to optical interference between different heterostructure layers. These observations confirm the results of SpringThorpe (Refs. 1 and 2) for GaAs and AlAs growth, which indicate that simple interferometric techniques can be used as an alternative to reflection electron diffraction (RED) oscillation techniques for accurate, in situ measurements of growth rate. We have also measured temperature changes in the growth of lattice-mismatched (In,Ga)As and InAs, for which RED oscillation techniques are more difficult. For In0.2Ga0.8As growth on GaAs at 500 °C, we observe strong oscillations, with a gradual increase in the average temperature. For InAs growth on GaAs, the indicated temperature rises approximately 60 °C during the first 1000 Å of growth and shows only small, strongly-damped temperature oscillations. Using the InAs ‘‘congruent evaporation’’ temperature as a reference, we find that most of the rise in indicated temperature is real, even though a proximate back side thermocouple shows only a small temperature change and the substrate heater power is held nearly constant.
Observation of intensity oscillations in reflection high-energy electron diffraction during molecular-beam epitaxial growth of GaAs on (111) B GaAs substrates
Authors: Yen M. Y. , Haas T. W.
Published: July 2002

We report the observation of reflection high-energy electron diffraction (RHEED) intensity oscillations during molecular-beam epitaxial growth of GaAs on (111)B GaAs substrates. It was found that RHEED intensity oscillations occur only over a limited range of growth conditions.
Optimization of variable axis immersion lens for resolution and normal landing
Authors: Sturans M. A. , Petric P. F. , Pfeiffer H. C. , Stickel W. , Gordon M. S.
Published: July 2002

This paper describes a variable axis immersion lens system capable of better than 0.1-μm resolution at 25 A/cm2 over a 10-mm2 deflection field with perpendicular landing for both electrostatic (high speed) and magnetic deflections. Computer predictions of spot acuity and landing angles are compared to actual measurements for various degrees of field compensation accuracy.
Imaging of uncoated purple membrane by scanning tunneling microscopy
Authors: Guckenberger R. , Hacker B. , Hartmann T. , Scheybani T. , Wang Z. , Wiegräbe W. , Baumeister W.
Published: July 2002

Scanning tunneling microscopy images of uncoated purple membrane are easily obtained at tunneling currents lower than 1 pA and tunneling voltages above ∼7 V. However, the images sometimes show positive, sometimes negative contrast. This effect appears to depend only on the shape of the tunneling tip. A simple model, assuming a constant voltage drop across the purple membrane, is proposed which can explain this observation.
Conformation, packing, defects, and molecular dynamics in monolayers of dialkyl-substituted benzenes
Authors: Buchholz Stefan , Rabe Jürgen P.
Published: July 2002

Monolayers of the two homologues dihexadecylbenzene and didodecylbenzene were adsorbed at the interface between highly oriented pyrolytic graphite (HOPG) and an organic solution. The derivatization of a small molecule, in this case benzene, with long alkyl sidechains is shown to be a method to immobilize various molecular entities at the interface between HOPG and an organic solution. In situ STM studies at the internal interface revealed that the first adsorbed monolayer of the dialkyl-substituted benzenes exhibits a lamellar phase with the molecules extended and oriented parallel to the basal plane of graphite. Bias-dependent experiments allowed to either image preferentially the underlying substrate (at low bias) or the molecular adsorbate (at higher bias), without disrupting the adsorbate layer. From such experiments it was concluded that the alkane sidechains tend to orient parallel to a graphite lattice axis. High resolution images revealed different types of packing within a lamella, which were separated by sharp domain boundaries. Also packing defects involving individual molecules were observed. Most interesting, fast image recording on video tape allowed to directly follow the dynamics of such individual defects on the time scale between 100 ms and several minutes, i.e. the relevant timescale for the slow dynamics in crystalline organic phases.
Photoelectron effects in x-ray mask replication
Authors: White V. , Ocola L. , Cerrina F. , Vladimirsky Y. , Maldonado J.
Published: July 2002

In this paper some experimental observations of the interference of photoelectrons in an x-ray mask replicating process are reported on. High resolution low contrast (i.e., producing absorbed dose ratios of less than 2) x-ray masks have been used to produce lines smaller than 0.1 μm in poly(methlmethacrylate) (PMMA) on a silicon substrate with an aspect ratio of about 2. When the experiment was repeated on a gold plated substrate, anomalous adhesion problems were encountered. Lines smaller than 0.2 μm, for example, had poor adhesion, and those smaller than 0.1 μm had entirely lifted off. A higher contrast mask, with a dose contrast of about 6, and with smaller features was tried on the same two substrates. Using this mask, 800 Å lines in PMMA in 0.5 μm thick resist have been demonstrated, with good adhesion. These effects are consistent with the assumption that the x rays that penetrate the mask’s absorber, and the resist, but are absorbed in the plating base, generating photoelectrons, which then propagate across the interface. These extra photoelectrons create a thin layer of increased exposure in the resist near the interface that is responsible for loss of adhesion for ultrasmall structures. A low-Z photoelectron block layer can be used to eliminate this problem. Experimental evidence in conjuncture with computer simulation were done to investigate this assumption.
Optimal design of an x-ray lithography mask
Authors: Laird D. L. , Engelstad R. L.
Published: July 2002

The 0.25 μm 1:1, masks used in x-ray lithography have a very tight overlay requirement: it is not just sufficient to produce a perfect mask, it is also necessary not to degrade its characteristics by incorrect mounting. This is particularly true for synchrotron-based x-ray lithography where the masks are patterned and used in different configurations. The computational tools necessary to model mask distortions in the nanometer range with high accuracy and reliability are developed here and used to predict an optimal mask design. The results show that the overall mask distortion resulting from gravitational loading can be accommodated within the allocated error budget.
Scanning tunneling microscopy and spectroscopy of p
n junctions formed by ion implantation
Authors: LaBrasca John V. , Chapman Richard C. , McGuire G. E. , Nemanich R. J.
Published: July 2002

Results of scanning tunneling spectroscopy measurements of 〈100〉 n- and p-type silicon are reported. Measurements were made in air on surfaces with a native oxide. Different I/V characteristics were observed for differently doped samples, indicating that the surfaces were unpinned. Images are presented of pn junctions formed by the implantation of phosphorus into boron doped Si wafers. The pn junction, or the boundary between implant and substrate regions, was observed in both constant current and constant height scans. The junction was imaged with 125 Å point-to-point resolution.
Theory of elastic tip–surface interactions in atomic force microscopy
Authors: Overney Gregor , Zhong Weiqing , Tománek David
Published: July 2002

We present a first-principles study of elastic surface deformations, limits of atomic resolution, and of atomic-scale friction in atomic force microscopy (AFM). In the case of a Pd AFM tip interacting with a graphite surface, we find that atomic resolution can be achieved in a narrow load range near ≊10−8 N (per Pd tip atom). For these loads, we determine the microscopic friction coefficient to be μ≊10−2.
Vacuum tunneling of spin-polarized electrons detected by scanning tunneling microscopy
Authors: Wiesendanger R. , Bürgler D. , Tarrach G. , Wadas A. , Brodbeck D. , Güntherodt H.-J. , Güntherodt G. , Gambino R. J. , Ruf R.
Published: July 2002

We show that the scanning tunneling microscope (STM), operated in ultrahigh vacuum, can be made sensitive to the electron spin and therefore to magnetic structures down to the atomic scale. The experiments were performed with a ferromagnetic CrO2 tip providing electrons of high spin polarization. As sample, we used a Cr(001) single crystal which offers an ideal magnetic test structure. The experimental results obtained with the ferromagnetic CrO2 tip characteristically differ from those obtained with a nonmagnetic tungsten tip. We give an expression, relating the local electron spin polarization of the tunneling junction with quantities directly measurable with the STM. We also discuss possible contributions from magnetic dipole and exchange forces as well as the feasibility of magnetic exchange force microscopy.
100 kV Schottky electron gun
Authors: McGinn J. B. , Swanson L. W. , Martin N. A. , Gesley M. A. , McCord M. A. , Viswanathan R. , Hohn F. J. , Wilson A. D. , Naumann R. , Utlaut M.
Published: July 2002

We present a comparison between experimental results and computer calculations on a high current, high resolution single lens electrostatic 100 kV Schottky electron gun. One promising application for such an electron gun is for direct electron-beam patterning of x-ray masks. The high energy helps provide precise patterning of the thick resist, maintains vertical resist profiles, and minimizes the proximity effect. The gun was designed to operate from 25 to 100 kV, capable of focus at a distance of 145–245 mm with a magnification of 1.15. The emitter, of apex radius ∼0.6 μm operated at 1800 K in the extended Schottky regime, provides an angular intensity of 0.5 mA/sr for an extraction voltage of 5000 V and with a beam limiting aperture of 2.2 mrad, the gun delivers 7 nA of probe current. The gun consists of a replaceable high voltage optic module mounted on a precision insulator with the main acceleration occurring between the exit of the optic module and the grounded anode. A provision is made for alignment of the emitter with respect to the central optical axis of the optic module in a special alignment chamber eliminating the need for high voltage emitter alignment. Final gun alignment is achieved by X–Y motion of the grounded anode aperture. The gun is constructed to allow ease of replacement of the emitter, the beam defining aperture, and the differential pumping aperture. The beam supply has 10 ppm of ripple while lens supplies have <50 ppm of ripple. At 100 kV the power supply, cabling, connectors, insulator, and optic module draw 1 μA of ground leakage current. Pressured SF6 chambers are used for high voltage connector interfaces within the power supply and on the gun.
Reactive-ion etching of tungsten silicide using NF3 gas mixtures
Authors: Lee Ru-Liang , Terry Fred L.
Published: July 2002

We report in this paper the reactive-ion etching of WSix films using gas mixtures containing NF3. A combination of NF3/H2 or NF3/He was used to anisotropically etch WSix films as thick as 4000 Å with photoresist masks. They yielded satisfactorily high etch rates and good selectivities with respect to photoresist and III–V materials. The NF3/He mixture yielded higher etch rates for both WSix and photoresist but a lower selectivity with respect to photoresist. CHF3 was used with NF3 for thicker films and/or submicron features in order to form polymer to protect the sidewalls. Metal masks have to be used in this etch since the etch rates are very low and the selectivity with respect to photoresist is no longer high. A low percentage of NF3 and a low pressure were necessary for successful etches using the NF3/CHF3 plasma. Etched lines as narrow as 0.18 μm are shown. In addition to the optimal etch conditions, data will also be presented for the effects of variations of cathode coverage, rf power, gas pressure, and gas composition.
Optical spectroscopy of (001) GaAs and AlAs under molecular-beam epitaxy growth conditions
Authors: Wassermeier M. , Kamiya I. , Aspnes D. E. , Florez L. T. , Harbison J. P. , Petroff P. M.
Published: July 2002

Reflectance-difference (RD) studies were performed on variously reconstructed (001) GaAs and AlAs surfaces. The spectra of the (2×4) and (4×2) reconstructions on (001) GaAs show prominent features due to electronic transitions between lone-pair orbitals and dimer states, as previously identified by theoretical calculations. The spectra of the c(4×4) reconstructions on (001) GaAs and AlAs show similar features that we also interpret in terms of surface dimer excitations. These dimer features provide a capability of obtaining real-time, in situ information of dynamics on polar surfaces.
Theory for electron and hole transport in HgTe–CdTe superlattices
Authors: Meyer J. R. , Arnold D. J. , Hoffman C. A. , Bartoli F. J. , Ram-Mohan L. R.
Published: July 2002

We present results of the first detailed theory for electron and hole transport in HgTe-CdTe superlattices. The calculation incorporates the superlattice band structure in full generality, and also treats multi-well scattering and screening processes which have been ignored in previous theories. It is predicted that whereas the electron and hole mobilities should be nearly equal at low temperatures, the hole mobility falls far below the electron value at somewhat higher temperatures due to the extreme nonparabolicity of the valence band. This prediction is entirely consistent with experimental results reported previously. Excellent quantitative agreement with the data over a broad temperature range is achieved if interface roughness scattering is considered in addition to ionized impurity scattering, acoustic and optical phonon scattering, and electron–hole scattering. It is pointed out that low-temperature electron mobilities for a number of thin-well HgTe–CdTe superlattices follow the d6W dependence expected for the interface roughness mechanism.
Study of CdTe epitaxial growth on (211)B GaAs by molecular-beam epitaxy
Authors: Sasaki T. , Tomono M. , Oda N.
Published: July 2002

Recently, misoriented (100)GaAs or (211)GaAs substrates have been found to be favored for HgCdTe and CdTe epitaxial growths, due to low occurrence of crystalline defects. It was previously reported that both tilted (211) and (133)CdTe were grown on (211)B GaAs, like on (100)GaAs, and that (133)CdTe seemed to have better crystallinity than tilted (211)CdTe. However, these results were obtained only at substrate temperature ∼300 °C, and the occurrence of both growth orientations has not been clear yet. In this article, substrate temperature dependence of CdTe growth on (211)B GaAs substrate is studied in detail, from the view points of crystallinity and surface morphology. It turns out that these orientations can be controlled by the initial substrate temperature, i.e., tilted (211)CdTe appears at substrate temperature below 290 °C, and above 290 °C (133)CdTe can be grown. Furthermore, double-crystal rocking curve-full width at half-maximum reaches 60 arcsec for (133)CdTe/(211)B GaAs. Finally, a model for the occurrence of both tilted (211) and (133) CdTe growths was qualitatively proposed.
All-reflective phase-shifting masks for Markle–Dyson optics
Authors: Hsieh Robert L. , Grenville Andrew , Owen Geraint , Pease R. Fabian
Published: July 2002

Previous work using a Markle–Dyson projection system, operating at 248 nm wavelength, demonstrated 0.19 μm resolution with nonphase-shifting masks. The use of Levenson-type phase-shifting masks should in principle, allow sub-0.1 μm feature resolution using a k1 of 0.25. To investigate this possibility, a novel reflective phase mask was fabricated and used on the Markle–Dyson system to projection print 0.125 μm lines and spaces in photoresist. An analytical study has been carried out to determine the tolerance of alternating-phase masks to errors in linewidth and phase.
Low-voltage electron beam lithography
Authors: Peterson P. A. , Radzimski Z. J. , Schwalm S. A. , Russell P. E.
Published: July 2002

The use of low-voltage electron beam lithography to reduce proximity effects, improve throughput, and reduce substrate damage caused to underlying materials has been investigated. Various films of PMMA were exposed with a field emission scanning electron microscope adapted with blanking capability and a 16 bit resolution beam control package. The exposure voltages used were from 1 to 15 keV with probe sizes of ≤150 to ≤70 Å, respectively. The dose latitude or working dose range was determined for each voltage on film thicknesses of 0.05, 0.18, and 0.38 μm poly(methylmethacrylate). Optimum beam voltage for a particular thickness which maximizes the energy deposited within the resist has been approximated using Monte Carlo modeling and verified experimentally. Atomic force microscopy showed that at lower voltages the dose required to properly expose a feature is relatively low and as beam voltage increases, the dose required to expose a given area increases. This data also verifies the fact that if the exposing voltage is too low, the resist will not be fully exposed. Because low energy electrons have a smaller interaction volume, there is a significant decrease in the proximity effect that is evident when high energy electrons are used for exposure. Comparison of low energy and higher energy electron beam exposures in this study reveal this decrease. Results indicate that by using low energy electrons for resist exposure the dose can be lowered and thus the throughput increased; the proximity effect is significantly decreased; and because of the limited penetration depth of low energy electrons, the substrate damage is potentially minimized.
Effects of a pseudomorphic InGaAs layer on focused ion-beam modulation doped GaAs–AlGaAs quantum well structures
Authors: Li Y. J. , Hashemi M. , Wassermeier M. , Mishra U. K. , Merz J. L. , Petroff P. M.
Published: July 2002

GaAs/AlGaAs double heterostructures have been implanted with a focused Si ion beam to directly form a two-dimensional electron gas in the quantum well (QW). The effects of a strained pseudomorphic In0.3Ga0.7As layer 500 Å above the QW on the electron mobility were studied by magneto-transport and Hall measurements. The electron mobility improves with the introduction of the strained In0.3Ga0.7As layer and this is tentatively attributed to the dechanneling and localization of the implanted Si ions above the GaAs QW.
Mark detection for alignment and registration in a high-throughput projection electron lithography tool
Authors: Farrow R. C. , Liddle J. A. , Berger S. D. , Huggins H. A. , Kraus J. S. , Camarda R. M. , Jurgensen C. W. , Kola R. R. , Fetter L.
Published: July 2002

The design considerations and performance requirements of an alignment and registration method for high-throughput projection electron lithography are discussed. We have devised a mark detection system for a lithography tool employing the SCALPEL principle. The process involves scanning the image of a mark from a mask, over a mark that is fabricated on a wafer and measuring the integrated backscattered-electron intensity. Using a mark that consists of equally spaced lines of tungsten on silicon, we have investigated the effects of spatial and rotational misalignment, and focus in a small field of view SCALPEL machine. We have modeled these and other effects and found agreement between experimental and theoretical results. The measured precision of estimating position from the experimental data is ∼10 nm. By extrapolation we believe that in a practical lithography machine this value can be improved significantly.
Atomic resolution scanning tunneling spectroscopic studies of Bi2Sr2CaCu2Oy single crystals
Authors: Ikeda Kazuto , Takamuku Kenshi , Itti Rittaporn , Koshizuka Naoki
Published: July 2002

The authors developed a scanning tunneling spectroscopy system which enables the current imaging tunneling spectroscopy (CITS) to be measured under an ultrahigh vacuum environment. The scanning tunnel microscope and CITS of Bi–O surfaces of Bi2Sr2CaCu2Oy superconductors were studied. No remarkable difference was found for the tunneling spectra at different positions in the modulated surface structures. It is concluded that the missing atom rows in the modulated structure do not originate in the local density of states, but in the real-spatial structure.
Bias-switchable dual-band HgCdTe infrared photodetector
Authors: Blazejewski E. R. , Arias J. M. , Williams G. M. , McLevige W. , Zandian M. , Pasko J.
Published: July 2002

The feasibility of an all molecular-beam epitaxially (MBE) grown, bias-switchable, dual-band HgCdTe detector is demonstrated. Detection in the midwavelength infrared (MWIR) band only or the long-wavelength band (LWIR) only is accomplished by the proper selection of detector bias in the ≥±100 mV range. The devices were all grown in situ by the MBE on CdZnTe or GaAs substrates and consisted of three intentionally doped layers in an n–p–n sequence. At 77 K the floating base two terminal devices responded to the ∼4.9–8 μm spectral band with the application of ≥−100 mV to the bottom contact and to the ∼2.1–4.9 μm band with the application of ≥0 mV. Quantum efficiency depended on bias with a maximum of 59% for LWIR and 66% for MWIR wavelengths at −150 mV and ≥0 mV, respectively. The operation of the dual-band detector is discussed. Significant design differences between the heterojunction phototransistor and the dual-band detector are noted.
Practical cage-effect model for crosslinking in a negative chemically amplified resist and its use in comparing e-beam and optical exposure
Authors: Tam N. N. , Ferguson R. A. , Neureuther A. R.
Published: July 2002

A closed-form, accurate, and easy to fit model for crosslinking of chemically amplified acid-hardened resist (Shipley SNR-248) is presented and used to compare effects of e-beam and optical exposure. In this model, the saturation of the acid-catalyzed crosslinking reaction during the postexposure bake is assumed to be caused by the ‘‘cage-effect’’ mechanism–restriction of segmental diffusion of the polymer chains as the reaction progress. [D. Seligson, S. Das, H. Gaw, and P. Pianetta, J. Vac. Sci. Technol. B 6, 2303 (1988)]. To model this cage effect, the rate coefficient in the rate equation is assumed to be a linearly decreasing function of the extent of the crosslinking with the asymptotic saturation level of the crosslinking as a parameter. According to this cage-effect model, the order of the acid catalyst in the crosslinking reaction of e-beam exposed SNR-248 resists m is 1.37, which is similar to that of the deep-ultraviolet (DUV) exposed resists (1.42). However, in e-beam exposed resists, some crosslinking is induced by the electrons during exposure. As a result, this initial crosslinking might contribute to a slightly higher activation energy (0.866 versus 0.694 eV). The equilibrium conversions of the melamine crosslinking sites are also different between the two exposure types. The power of the acid concentration n in the expression for the equilibrium conversion for e-beam exposed resists is ∼0.49 whereas in DUV exposed resists, n is 1.1. There is also a difference in the activation energies of the equilibrium constants which suggested the cage effect might have a stronger influence on the backward reaction in e-beam exposed resists.
Negative i-line photoresist for 0.5 μm and beyond
Authors: Conley Willard , Gelorme Jeffery
Published: July 2002

In this article we will discuss a new high-resolution aqueous base developable negative tone i-line photoresist system that has demonstrated subhalf-micron resolution with commerically available i-line exposure systems. The photoresist system consists of a novolak resin, an aminoplast crosslinker, triphenyl sulfonium triflate as the photoacid generator, and 9-anthracene methanol, a commercially available aromatic energy transfer compound. Using statistical experimental design, data will be presented showing the experimentation required to optimize the formulation and the process in 2.38% TMAH developer (0.263 N). We will report linewidth, dose, and focus latitude data, energy transfer compound effects on contrast, background dissolution along with linewidth tolerance to changes in postexposure bake temperature. Additionally, we will briefly discuss initial phase-shift mask work that is currently in progress.
A contact model for Poisson-based spreading resistance correction schemes incorporating Schottky barrier and pressure effects
Authors: Clarysse T. , Vandervorst W.
Published: July 2002

Accurate determination of the electrically active dopant profile from spreading resistance measurements requires the application of a carrier spilling correction scheme based on the iterative solution of the one-dimensional Poisson–Boltzmann equation. It will be shown that the results are strongly influenced by the boundary conditions applied describing the probe–silicon contact. An exploratory and somewhat speculative new theoretical probe contact model will be presented which takes into account Schottky barrier effects, surface states on the bevel surface, band-gap narrowing and variations of the dielectric constant due to the large probe pressures applied at the metal–silicon interface. It will be shown that this model can predict adequately the detailed behavior of an experimentally measured spreading resistance low dose implant.
Selective molecular-beam epitaxy for integrated npn/pnp heterojunction bipolar transistor applications
Authors: Streit Dwight C. , Umemoto Donald K. , Velebir James R. , Kobayashi Kevin , Oki Aaron K.
Published: July 2002

We have developed a selective molecular-beam epitaxy (MBE) growth process for the production of complementary npn and pnp heterojunction bipolar transistors (HBTs) on the same GaAs substrate. The resulting devices have excellent dc and microwave characteristics, with no degradation observed due to the additional growth and processing steps required to monolithically integrate npn and pnp HBTs. We believe the ability to fabricate high-quality complementary HBT devices and circuits will greatly expand the application base of MBE-grown heterojunction bipolar transistors.
Novel scheme for the preparation of transmission electron microscopy specimens with a focused ion beam
Authors: Overwijk M. H. F. , van den Heuvel F. C. , Bulle-Lieuwma C. W. T.
Published: July 2002

A novel scheme is presented for the preparation of cross-section transmission electron microscopy (TEM) specimens, with a focused ion beam (FIB). This scheme is particularly suitable for highly structured substrates, such as integrated circuits. The specimen is made by cutting a thin slice of material from the substrate by sputtering with the FIB. The position of the specimen can be selected with submicron resolution. The specimen is subsequently removed from the substrate and transported to a standard TEM-specimen holder. A specimen, ready for TEM inspection, can be prepared within 2 hs. The samples are of excellent quality as is illustrated with cross-section TEM images of FIB-made specimens of an electrically programmable read-only memory.
X-ray mask membrane motion in narrow gap lithography: Hydrodynamic model and experiment
Authors: Yanof A. W. , Zipfel G. L. , Moon E. E.
Published: July 2002

The viscosity of the gas in the proximity gap between x-ray mask and silicon wafer causes damping of any membrane motion. As x ray is extended to future integrated circuit generations, this effect becomes increasingly significant, because the gap must be rapidly scaled down to permit resolution of finer lithographic features. Damping is much greater at a reduced gap. Damping can be beneficial in reducing unwanted fluctuations in the gap due to vibration, convection, or ambient sound. The longer settling time following gap adjustment, however, can be detrimental to the throughput of x-ray steppers. This article introduces simple hydrodynamic models, formulas, and numerical algorithms to calculate membrane response to adjustments in gap setting. It applies the analysis to a variety of mask formats, including advanced ‘‘pedestal’’ design. Time constants of several seconds are shown to result from gap setting <10 μm, which may be needed for 0.12 μm proximity x-ray lithography.
Direct aerial image measurement as a method of testing high numerical aperture microlithographic lenses
Authors: Partlo William N. , Fields Charles H. , Oldham William G.
Published: July 2002

A method for testing high numerical aperture (NA) microlithographic lenses using direct aerial image measurements has been developed. By measuring contrast versus focus for dense line/space patterns, this system can quantify the amount of astigmatism, distortion, field curvature, and spherical aberration produced by a lens under test. Experimental results show high correlation with more traditional lens testing methods such as transmissive interferometry and evaluating exposed resist patterns. The lens used in these experiments is an 0.53 NA, 248 nm objective designed for imaging 0.35 μm features over a 31 mm diam field.
Incident dose modification for proximity effect correction
Authors: Thomson M. G. R.
Published: July 2002

Electron-beam lithography (EBL) suffers from the effects of scattering of the incident electron beam. For good control of the size of the developed features, particularly when the fraction of incident electrons that is backscattered is large, incident dose modification is essential. The self-consistent correction technique equalizes the exposure of the resist within the desired features in the pattern, and this algorithm provides good correction for a wide range of feature shapes and sizes using both positive and negative resist systems. For EBL at the limits of resolution, the effects of forward scattering must be considered. The incident dose must be increased to compensate for the forward scattering effects when writing features smaller than about five times the range of the forward scattering. In order to equalize the average dose within these features, the incident doses must be increased by factors of up to 2, and greater increases are required to extend the system to even smaller features. If the backscattered fraction is η, the incident dose to define an isolated feature must be increased by as much as 1+η. In combination with a correction for forward scattering, the range of incident doses required can easily become 3:1. Curves are shown exhibiting the dose modifications required for different feature sizes and types as the resolution limit is approached, and the exposure contrast available. Sufficient dynamic range must be accommodated in the design of the electron-beam pattern generator to give sufficiently fine dose variations over a wide enough range.