bluuebunny/crossref_metadata_2025 huggingface dataset:
Dry etched molybdenum silicide photomasks for submicron integrated circuit fabrication
Authors: Pierrat C. , Tarascon R. G. , Peabody M. L. , Harriott L. R. , Vaidya S.
Published: July 2002

The aim of this paper is to demonstrate a manufacturable molybdenum silicide mask process. Each step of the fabrication of a mask was addressed: lithography, dry etching, cleaning, inspection, and repair. SAL601 resist (Shipley) was chosen for the etch mask as dry etch resistance and almost vertical resist profiles are required [J. W. Thackeray, G. W. Orsula, E. Pavelcheck, D. Canistro, L. E. Bogan, A. K. Berry, and K. A. Graziano, Proc. SPIE 1086 (1989)]. Etching experiments of the molybdenum silicide were performed using different fluorinated gas mixtures. Two of them, CF4+O2, SF6+O2, were chosen and optimized for selectivity (ratio of molybdenum silicide etch rate versus resist etch rate) using a statistical experimental design approach. Results indicate a higher selectivity for the CF4+O2 mixture but lower loading effects for SF6+O2 leading to a better uniformity in the latter case. In both cases, the pressure was chosen low enough (below 200 mTorr to get anisotropic etching conditions even if the selectivity is reduced. Inspection has been performed using standard optical equipment from KLA Instrument Corporation. None of the defects found were related to lithography and dry etching processes. Repair feasibility has been demonstrated with both laser beam and focused ion beam (FIB). Thanks to its fine grain structure, very smooth edges have been obtained, after FIB repair, for molybdenum silicide as compared to chrome.
0.1 μm x-ray mask replication
Authors: Gentili M. , Kumar R. , Luciani L. , Grella L. , Plumb D. , Leonard Q.
Published: July 2002

In a series of experiments, we investigate issues of absorber thickness increase (contrast amplification) and resolution capabilities of an x-ray mask copying process utilizing x-ray lithography. Two types of high resolution master masks were patterned using e-beam lithography and gold electroplating deposition and then replicated using x-ray lithography. The print quality and feature linewidths of each replica were then evaluated using a scanning electron microscope. The production of ULSI masks for x-ray lithography can be simplified if the necessary step of electron beam patterning can be performed in a thin resist layer, reducing the complexity of subsequent processing procedures and enhancing e-beam capabilities. When working with a thin resist, however, plated absorber thicknesses are small and the resulting mask contrast is limited. For x-ray lithography, Au absorber thicknesses on the order of 0.6 μm are required to ensure sufficient mask contrast when using synchrotron radiation (SR) sources (λ=0.1–1.2 nm). This implies than an aspect ratio of 3:1 is needed to print 0.2 μm features, and a ratio greater than 6:1 is necessary for sub-100 nm features.

A promising approach to achieving greater absorber thickness in x-ray masks is based on a mask replication process utilizing x-ray lithography. The large depth of focus and wide process latitude offered by x-ray lithography (XRL) permit exposure into thicker resist layers, and consequently, greater absorber thicknesses can be produced via direct metallization of the printed image. This technique also has the advantage of reducing e-beam time requirements, since many x-ray mask replicas can be produced from a single e-beam master mask. Two types of master x-ray masks were used in our evaluation. The first was a 2-μm-thick SiNx membrane with sub-100-nm Au features, and the second was a 2-μm-thick Si membrane with Au features down to 250 nm. Using the SiNx master mask, sub-100-nm resist features were successfully printed in 600 nm of resist on base plated wafers, using x-ray lithography. Subsequent electroplating of these samples yielded sub-100 nm features with Au absorber thicknesses of 450 nm. These results demonstrate successful mask contrast amplification (from 6 dB for the master to 15 dB for the copy), and show that high resolution, high contrast masks can be fabricated using this replication process.
Small source/drain metal–oxide-semiconductor field effect transistor using a thermally stable local TiSi2 interconnection
Authors: Yoshida Takehito , Ogawa Shin-ichi
Published: July 2002

A s_mall s_ource/d_rain c_ontact (SSDC) formation technology has been developed. In the SSDC structure, self-aligned contacts to small source/drain (S/D) regions can be formed without any contact holes because a local TiSi2 interconnect which extends over isolation fields is directly combined with the whole S/D surface area. It is essential in the SSDC process that the local TiSi2 interconnects are formed from a solid phase reaction of the amorphous-Si (sputter deposited)/Ti (sputter deposited) bilayer containing a low oxygen concentration. The local TiSi2 interconnect is thermally stable up to 900 °C furnace annealing. This thermal stability allows one to use the TiSi2 layer as a solid phase diffusion source for the S/D formation. Consequently submicron n-channel metal–oxide–semiconductor field effect transistors with the SSDC structure were fabricated and tested, with a physical gate size of 0.42 μm(length)×2.0 μm(width), and the S/D area was shrunk down to 0.4 μm(length)×2.0 μm(width). The maximum linear transconductance was ∼31 mS/mm.
New electron and hole spectroscopies based on ballistic electron emission microscopy
Authors: Bell L. D. , Kaiser W. J. , Hecht M. H. , Davis L. C.
Published: July 2002

Ballistic electron emission microscopy (BEEM) is shown to be one of a class of carrier transport nanospectroscopies which measure both elastic and inelastic transport of electrons and holes in thin film structures. In BEEM, electrons are injected into a metal film from a tunnel tip and are collected in the substrate after ballistic transport through the film. An analogous spectroscopy can be performed using hole transport into a p-type semiconductor substrate. In the present work, it is shown that inelastic scattering in metal films results in electron-hole pair creation which can be detected by similar techniques. A theory for this spectroscopy successfully explains the experimental results.
First-principle simulation of scanning tunneling microscopy/spectroscopy with cluster models of W, Pt, TiC, and impurity adsorbed tips
Authors: Isshiki Nobuyuki , Kobayashi Katsuyoshi , Tsukada Masaru
Published: July 2002

Tunneling current and conductance in scanning tunneling microscopy/spectroscopy (STM/STS) are calculated for various kind of tips using first-principle local density approximation (LDA) method with the tunneling-Hamiltonian formalism. As models of tip, tungsten cluster, platinum cluster, carbon adsorbed tungsten cluster, and titanium carbide cluster are used. Graphite surface is used for the simulation. STM image and qualitative feature of STS spectrum mainly depend on the arrangement of atoms at the tip apex and not so much affected by the atomic compositions of the tip because the most of tunneling current comes from a single or a few atom at the tip apex.
Fabrication of electroplated T gates with 60 nm gate length for pseudomorphic high electron mobility transistor devices
Authors: Marten A. , Schneider H. , Schweizer H. , Nickel H. , Schlapp W. , Lösch R. , Dämbkes H. , Marschall P.
Published: July 2002

We have fabricated and electrically characterized mm-wave high electron mobility transistors (HEMTs) on pseudomorphic heterostructure GaAs/InGaAs samples. The T- and Γ-shaped gates were produced using electroplating and a conventional single resist-layer lift-off process. High frequency measurements of the same device before and after plating demonstrate the reduction in gate resistance. Direct current and high frequency properties of the HEMTs depend strongly on gate length and gate recess depth. Characterization of parallel conducting layers, low field mobility, and sheet carrier concentration depth profiles were obtained with gated Hall measurements. Best HEMT performance was obtained at a gate length of 60 nm, giving an extrinsic (intrinsic) transconductance of 620 mS/mm (840 mS/mm) and a cutoff frequency ft of 135 GHz.
Mechanism of ion impact photoemission change of Si and Al during focused ion beam milling of LSI
Authors: Itoh F. , Shimase A. , Haraichi S. , Takahashi T.
Published: July 2002

Focused ion beam milling is an important tool for LSI modification and failure analysis. For both of these applications, the depth control in milling is a major requirement. The FIB machine we used was equipped with an ion impact photoemission detector for use with both Si and Al. We studied the photoemission change mechanism during LSI milling by altering the acquisition area of the photoemission, and by chemical analysis on the side walls of the milled holes. Si emission varies with the surface area of SiO2. Al emission originates in the edge lines of Al and SiO2 at the bottom of the hole, and from the Al redeposited on SiO2 of the side walls. On the edge lines and the side walls, ion mixing of SiO2 and Al enhances the Al photoemission. We demonstrated that an almost 100% modifying yield in both Al wire cutting and contact hole milling is possible by observing the variance of both emissions.
Arsenic coverage dependence of the angular distribution of secondary ions desorbed from the GaAs{001}(2×4) surface
Authors: Caffey K. , Blumenthal R. , Burnham J. , Furman E. , Winograd N.
Published: July 2002

In this paper, we present a study of structural changes which may occur as a function of surface composition within a single surface reconstruction. We examine in detail the GaAs{001}(2×4) surface by measuring the ion beam desorbed Ga+ ion angular distributions as a function of As coverage. The distributions of these ions, initiated by 3-keV Ar+ ion bombardment at normal incidence, are measured as a function of azimuthal and polar angles of ejection from the molecular-beam-epitaxy-grown (2×4) surface. The distributions display extreme anisotropy, indicative of a specific collision sequence which results from the covalent nature of the GaAs crystal. More subtle blocking and channeling effects are also found in the angular pattern which allow qualitative analysis of the (2×4) surface structure. These results agree with the currently accepted model of missing As dimers for this reconstruction, which appear as open channels parallel to the 〈011̄〉 direction on the surface. The As coverage of the (2×4) surface was modified by molecular beam deposition of either As or Ga. These results indicate structural variations within the range of As and Ga coverages which are considered to yield the (2×4) reconstruction. The excess Ga atoms appear to adsorb in the fourfold hollow sites on top of the As dimers and at least 0.25 monolayer of Ga can be deposited before the surface reconstructs to a (3×1) symmetry. It is proposed that this adsorbed Ga may destabilize the (2×4) surface by opening the dimers, and thus initiating the reconstruction. Excess As atoms appear to adsorb in the fourfold hollow sites of the open channels on the (2×4) surface. The As coverage of the (2×4) surface was also modified by ultrahigh vacuum annealing. Angle-resolved secondary ion distributions and reflection high-energy electron diffraction patterns indicate that the surface reconstructs. It is concluded that the use of molecular beam deposition to vary the surface As coverage may be more effective than the thermal desorption of As2 from a prepared surface.
Electronic structure of ideal and nonideal metal/GaAs contacts
Authors: van Schilfgaarde Mark , Newman N.
Published: July 2002

On the basis of explicit first-principles electronic structure calculations of a number of ideal and nonideal metal/GaAs interfaces, we show that nonidealities and imperfections play a major role in determining the Schottky barrier height. For the ideal interfaces, the calculated pinning positions ranged from 0 to 1 eV above the valence-band maximum, in contradiction to experiment. As one example of a defect, we incorporate an As antisite defect near the interface, and show that it significantly alters the electronic structure and Schottky barrier height.
Surface reconstructions and surface energies of monolayer-coverage cation-terminated Ga0.5In0.5P(001) surfaces
Authors: Froyen Sverre , Zunger Alex
Published: July 2002

Using the first-principles pseudopotential method we have studied the fully covered cation-terminated (001) surfaces of Ga0.5In0.5P alloys. We find that among several possible Ga and In surface patterns (within a 2×2 unit cell), the one corresponding to CuPt-like bulk ordering is stabilized by ∼100 meV per surface atom. This structure has been observed experimentally in thick films, yet is known to be bulk unstable. The stability of a CuPt-like surface layer is related to electronically driven surface reconstructions—dimerization, buckling, and tilting—which are discussed in detail.
Impact of different x-ray mask substrate materials on optical alignment
Authors: Fuentes R. I. , Progler C. , Bukofsky S. , Kimmel K.
Published: July 2002

In this article, results will be presented on how different x-ray mask substrate materials affect the quality of the alignment signal acquired through them. By affecting the signal quality, the different materials ultimately have a significant impact on the capability of the alignment system to discern the position of the alignment marks precisely. Data will also be presented on position measurement repeatability as a function of mark depth for the different mask materials. In the experiments, in-house grown SiC and diamond films were used. Vendor SiC and SiN materials were also used. For comparison and reference, signals were acquired through polyimide and B-doped Si as well. The alignment was done with a lab-based Suss ALX100 system. Since the primary alignment effect of membrane materials is to reduce signal-to-noise ratio, mark recognition repeatability and signal contrast was used as the comparative performance criteria. Experiments were done in both bright field and nonscanning dark field modes from alignment marks with depths between 5 and 150 nm. The effects of the substrate material became more evident when it was attempted to align using shallow marks. The vendor SiC and diamond exhibited poor recognition repeatability when marks of less than 45 nm in depth were used in bright field or 75 nm in dark field. Antireflective coatings seem useful in improving the performance of some of these materials, especially in bright field alignment. With a one sigma repeatability budget of 10 nm, current alignment systems may have difficulties in reproducibly locating shallow marks (below 20 nm), which may be found on some of the very demanding levels, when aligning through any of the candidate materials. The need may exist for careful processing of the marks and/or improvement in the aligners ability to see through rough or highly absorbing mask materials.
Deformation of x-ray lithography masks during tool chucking
Authors: Chen Alek C. , Maldonado Juan R.
Published: July 2002

Finite element modeling is used in this article to describe the elastic deformation of x-ray lithography masks due to chucking in the electron-beam writer or the lithographic tool. The total overlay budget for the lithography of 0.25 μm ground rule devices is less than 0.1 μm (3 σ) including all possible sources of error. Chuck induced random deformation of x-ray masks can contribute a significant amount to this overlay error [A. Chen et al., J. Vac. Sci. Technol. B 10, XXXX (1992)]. In addition, errors could arise also if different chucking techniques are used in the writing and exposure tools [A. Chen, S. Lalapet and J. R. Malonado, J. Vac. Technol. B 9, 3306 (1991)]. Based on the state of the art of the precision and accuracy of mask aligners and e-beam writers, it is required that the mask deformation due to chucking should be kept below 10 nm (3 σ). Several finite element models were constructed to simulate x-ray lithography masks under different chucking methods: (a) an idealized kinematic chucking, (b) full surface chucking, e.g., chucked by a vacuum ring, and (c) three point chucking. The maximum in-plane deformation of the x-ray mask membrane was normalized to the ratio of the modulus of elasticity of the mask support structure (Pyrex; trademark of Cornia Glass) and the chuck material. The calculated in-plane deformations of the x-ray mask membrane were compared among the various chucking techniques mentioned above. The results are helpful in the design of aligner and exposure tools from the point of view of selecting proper chuck materials and methods to minimize mask membrane deformations.
Installation and early operating experience with the Helios compact synchrotron x-ray source
Authors: Archie C. N. , Granlund J. I. , Hill R. W. , Kukkonen K. W. , Leavey J. A. , Lesoine L. G. , Oberschmidt J. M. , Palumbo A. E. , Wasik C. , Barton M. Q. , Silverman J. P. , Warlaumont J. M. , Wilson A. D. , Anderson R. J. , Crosland N. C. , Jorden A. R. , Kempson V. C. , Schouten J. , Smith A. I. C. , Townsend M. C. , Uythoven J. , Wilson M. C. , Wilson M. N. , Andrews D. E. , Palmer R. , Webber R. , Weger A. J.
Published: July 2002

During 1991 the Oxford Instruments’ compact synchrotron x-ray source Helios was installed at IBM’s Advanced Lithography Facility, following factory tests in Oxford during 1990. The machine has met or exceeded all performance specifications at the East Fishkill site. Since January, 1992 it has been run routinely for lithography development work. Running and reliability statistics are being generated, and the results for the first five months are presented here.
Extending scalar aerial image calculations to higher numerical apertures
Authors: Cole Daniel C. , Barouch Eytan , Hollerbach Uwe , Orszag Steven A.
Published: July 2002

The usual formula for the scalar aerial image of an isolated object due to a projection lens system has been generalized beyond the paraxial approximation in an attempt to extend scalar diffraction theory to include numerical aperture (NA) values up to about 0.6. Beyond this regime, or certainly beyond NA=0.7, polarization effects need to be included, thereby demanding a full vector treatment and invalidating the present scalar formulation. A key point to the present scalar result without the paraxial approximation is the predicted functional dependence of the aerial image on magnification as NA increases. A second key point is that the usual scaling of λ/NA for the object dimensions and λ/NA2 for defocus become invalid for high NA systems. Numerical results of illustrative test cases are shown.
Depth of focus enhancement in optical lithography
Authors: von Bünau R. , Owen G. , Pease R. F. W.
Published: July 2002

Apodization, or pupil plane filtering, can be used to enhance the depth of focus of lithographic projection systems. However, this improvement comes at the expense of decreasing radial concentration of energy in the point spread function. To examine this tradeoff quantitatively, we first define the average encircled energy as a criterion of the energy concentration over a nonzero axial distance. We then derive and numerically solve an integral equation for the apodization function that maximizes the average encircled energy. Finally, we use the eigenfunctions of this equation to construct pupil functions producing an on-axis intensity profile that is approximately constant over a nonzero axial distance, while maintaining a large value of the average encircled energy within that distance.
Characterization of a positive resist and the application of proximity effect correction in electron-beam direct write
Authors: Tan Z. C. H. , Standiford K. , Lem H. Y. , Nurmi C.
Published: July 2002

The resolution capability of a positive resist, TNS, was characterized under optimized process conditions and proximity effect correction. The proximity function is based on a double-Gaussian scattering model. A ‘‘mesh’’ exposure pattern was used to extract the proximity parameters of the resist. Before parameter determination, the process conditions were optimized by comparing the resist image structures as a function of developer concentration, development time, and temperature. Based on the best process conditions, the proximity parameters were determined and input into the aeble proximity compensation software, where the adequacy of the parameters was investigated. Results demonstrate that both process optimization and proximity effect correction are necessary for resist characterization in fine line lithography.
Scanning probe microscopy of deposits employed to image the current density distribution of electron beams
Authors: Weber M. , Koops H. W. P. , Görtz W.
Published: July 2002

The current density distribution in the final spot of an electron beam lithography system can be judged from beam diameter measurements performed by means of a knife edge method or recording beam crossover images with conventional lithographic media. Electron beam induced deposition allows to record traces of the current density distribution of the primary beam only, with very small contributions of secondary electrons and x rays. The readout of the focus and stigmation status is obtained by measuring the three-dimensional appearance of the deposit with a scanning probe microscope. To be able to find the few deposits after transferring the sample to the scanning probe microscope, a ‘‘spotter’’ chip was developed, which supports optical coarse positioning and can be easily scanned by probe microscopes. By scanning a sharp tip structure, deposited by electron beam near the center of the chip, the shape of the probing tip is imaged. This information can be used to unfold the image from the probe tip contributions. Results of beam intensity profiles obtained with this new method using a scanning force microscope are presented.
Optical and electron beam lithography for electroless copper multilevel metallization
Authors: Shacham-Diamand Y. , Angyal M. , Dedhia A. , Nasir Q.
Published: July 2002

Copper fine lines made by electroless deposition are investigated for multilevel metallization in ultra-large-scale integration technology. The results are presented using positive patterning, i.e., depositing copper on patterned areas by electroless deposition. Two techniques have been developed: (a) a nonplanar technique that produces 100 nm wide copper lines standing over an interlevel dielectric (ILD); and (b) a fully planar method that produces 0.5 μm copper lines buried in an ILD. Both electron beam and optical lithography processes were developed and are described for the nonplanar process. First, a single layer exposure by e-beam is described and second, a portable conformal mask (PCM) process with double exposure, the first by i-line stepper and the second by deep ultraviolet flood exposure. For the fully planar process, the PCM process was modified to produce a capped PCM process. By using two resists of different etch selectivity, an overhang structure was produced which prevented the base metal from being deposited on the sidewalls of the oxide trench. A trilayer resist structure and a single layer structure with isotropic overetch were also developed. Scanning electron microscope pictures of submicron lines are presented followed by a discussion of the problems and highlights of the individual processes.
Simulation of projection optic lithography images with finite impulse response filters
Authors: White L. K. , Matey J. R.
Published: July 2002

Finite impulse response (FIR) filters are used to simulate projection optic lithography images. For large arbitrary arrays of imaging data, the FIR filter simulation approach has advantages that lead to dramatic improvements in computation times. The transmission cross-coefficient (TCC) transfer function is more easily computed, the forward and reverse complex Fourier transforms need not be computed, and, foremost, the application of the TCC can be greatly simplified. Comparisons between FIR and Fourier transform simulation methods for one-dimensional objects in many cases show excellent agreement to within 1% of the total illumination intensity. The computation of projection optic images for 1000×1000 pixel arrays objects in real time (≤30 ms) appears possible using massively parallel, single-instruction multiple data computers. This capability provides a powerful design, layout, and design rule checking tool for phase-shift masks.
Evaluation of the etch depth dependence of three-dimensional confinement in strain-induced quantum well dot structures
Authors: Tan I-Hsing , Chang Ying-Lan , Shi Song , Mirin Richard , Hu Evelyn , Bowers John , Merz James
Published: July 2002

Strain-induced quantum well dots (SIQWDs) 120 nm in diameter have been fabricated by laser holography and wet etching. Lateral confinement was generated in the GaAs quantum well (QW) by etching a doubly exposed grating pattern into a strained layer of InGaAs which overlies the QW. Atomic force microscopy was used to examine the surface morphology of the SIQWDs. Photoluminescence spectra of quantum wells located at different distances below the InGaAs stressor probe the etch depth dependence of the strength of the strain modulation. The extent of the strain modulation in the vertical direction falls off within a distance comparable to the lateral dimension of the dot. A maximum redshift of 12 meV is observed for the uppermost QW located 22 nm away from the InGaAs stressor, implying carrier confinement in the SIQWD region. The dots were etched for varying amounts of time, changing the distance between the etched surface and the uppermost QW from 18 to 0 nm. Although the spatial distribution of strain modulation did not change significantly, the luminescence intensities from the uppermost QW decreased by about a factor of 50. This observation suggests that extrinsic causes, such as surface states formed on the etched surfaces are the major cause of diminished luminescence in these SIQWDs, rather than intrinsic causes such as slower electron–phonon interaction rate.
Design and characterization of compact 100 nm-scale silicon metal–oxide–semiconductor field effect transistors
Authors: Reeves C. M. , Wind S. J. , Hohn F. J. , Lii Y. T. , Bucchignano J. J. , Newman T. H. , Klaus D. P. , Volant R. P. , Keller J. , Tebin B.
Published: July 2002

Ultracompact n-channel silicon metal–oxide–semiconductor field effect transistors (MOSFETs) have been fabricated using electron beam nanolithography for all critical levels leading to a first demonstration of silicon MOSFETs which meet 100 nm ground rules. The smallest devices fabricated have active areas which measure only 700 nm×150 nm and are enclosed by a narrow trench isolation scheme. Devices of this size and with physical gate lengths of 120 nm and widths of 150 nm display an extrinsic transconductance of 410 mS/mm at room temperature. This level of miniaturization and performance arises in part from direct application of conventional scaling principles and in part from implementation of a novel device configuration. This article discusses the design of these devices as well as details of their electrical characteristics.
HCl, H2, and Cl2 radical-beam ion-beam etching of AlxGa1−xAs substrates with varying Al mole fraction
Authors: Skidmore J. A. , Lishan D. G. , Young D. B. , Hu E. L. , Coldren L. A.
Published: July 2002

The possible benefits of H* radicals on dry etching characteristics of AlGaAs were explored using HCl and separately mixed H2 and Cl2 radical-beam ion-beam etching. H* radicals strongly affect the Cl*–AlGaAs surface chemistry resulting in large changes in etch rate and surface morphology. Etch rates were measured in situ by reflectance interferometry using a quarter-wavelength structure with varying AlxGa1−xAs mole fraction. The presence of H* (from HCl or H2 added to Cl2) increases the Cl* etch rate threefold. An increase in surface roughness with H* (compared to Cl* alone) is correlated with increased Al content.
Three-dimensional electron-beam resist profile simulator
Authors: Moniwa A. , Yamaguchi H. , Okazaki S.
Published: July 2002

A three-dimensional (3D) electron-beam (EB) lithography simulator SEED-3D (system for evaluating electron-beam writing and developing 3D) has been developed for resist profile simulation and analysis of proximity effects. SEED calculates an exposure intensity distribution (EID), a 3D accumulated energy distribution after pattern exposure, and a 3D profile of the developed resist. SEED can take into account proximity effects in a wider region, because SEED takes a short time to calculate the 3D accumulated energy distribution. The concept of the 3D accumulated energy calculation is that the proximity effects in the forward scattering region are estimated precisely, and the proximity effects in the backscattering region are evalulated roughly. This 3D EB lithography simulator allows us to examine resist-pattern profiles of 0.3-μm line and space patterns. This simulator also gives an acceptable value for η which can be used for dose correction.
Dynamic performance of a scanning X–Y stage for automated electron-beam inspection
Authors: Clark D. J. , McMurtry J. , Chadwick C. , Simmons R. , Meisburger W. D. , Veneklasen L. , Chitayat A. , Squires S. , Squires W. , Levine M.
Published: July 2002

The design and performance of an X–Y stage for fast electron-beam inspection of wafers and x-ray masks is described. The inspection technique involves the comparison of images that are acquired by the raster scan acquisition of long swath images recorded while the stage moves at constant velocity. Pairs of images acquired serially must remain registered to about 0.05 μm net accuracy, requiring interferometer controlled motion with very low vibration. The system design requires high-vacuum compatible, nonmagnetic construction, with provision for electron and light optical elements above the stage, and additional electron optics and substrate loading elements below it. Accordingly, an open frame stage with internal linear motors and bearings was selected. High stiffness and particular attention to smooth motion results in very low vibration with a relatively large moving mass. The stage is driven by brushless linear motors inside a 20 Hz bandwidth servo loop closed around high-resolution λ/256=2.5 nm interferometers. Smooth motion contributes to accurate short term position measurement, allowing residual errors to be corrected by beam deflection. Long range accuracy relies largely upon occasional re-registration using features on the die patterns. At the end of swaths, programmed turnaround trajectories employing position and acceleration feedback allow serpentine paths with a minimum acceleration and overhead time. Using several complementary measurement techniques, overall registration accuracy was shown to be sufficient for detection of 0.05 μm defects.
Robust subpixel alignment in lithography
Authors: Gatherer Alan , Meng Teresa H.-Y.
Published: July 2002

Lithographic alignment is the problem of estimating the position of a feature that has previously been etched onto a wafer. Ideally, the alignment algorithm should be accurate without being computationally complex. In this article we describe an algorithm that, for symmetric pulses, achieves the accuracy of correlation at a fraction of the computational expense. The algorithm will automatically adapt to the minimum error condition for an unknown mark. We show that this algorithm is more robust than simple edge detection.
Granulation of silicon surface through reactive ion etching
Authors: Tandon U. S. , Pant B. D.
Published: July 2002

A spherelike granular structure has been observed to appear on the surface of single crystal silicon as a result of a two step reactive ion etching (RIE). Fluorocarbon plasma in RIE mode has been found to create a micromasking of bare silicon. Subsequent application of silicon etch recipe produces a texture with submicron spherical granules. Energy dispersion of x ray and Auger spectra analysis reveal that the suspected deposition or adsorption of etchant and effluent species onto the final surface is insignificant. The technique has a potential for controlling the reflectivity of silicon in specific regions of the spectrum and providing a large throughput.
Diffraction effects in x-ray proximity printing
Authors: Dubner A. D. , Wagner A. , Levin J. P. , Mauer J.
Published: July 2002

The influence of diffraction on the shape and size of features printed using x-ray proximity printing with a collimated x-ray source (measured beam divergence of ∼0.2 mrad full width at half-maximum) at mask to wafer gaps of 25 μm and above is described. Three major conclusions can be drawn from the results: (1) Diffraction can distort the shape of a printed feature, and the systematic shape changes observed in resist images can be explained using simple scaling based on Fresnel diffraction; (2) The linewidth change with exposure dose is independent of feature type and size, and depends only on the square root of the mask to wafer gap; and (3) The bias of each printed feature varies when all of the feature types and sizes are printed at the same dose. Resist images of 2.0–0.25 μm contact holes printed at mask to wafer gaps ranging from 25 to 515 μm are presented. The square contact holes on the mask print diamond shaped at a Fresnel number of 2.5. Isolated lines, spaces, and line-space arrays ranging from 1.0 to 0.25 μm were printed in thin PMMA (0.1 μm thick) resist at gaps ranging from 25 to 80 μm over a series of exposure doses. A 10% increase in exposure dose results in a 16, 20, and 30 nm change in linewidth at gaps of 25, 40, 80 μm, respectively. When the 1.0–0.25 μm lines, spaces, and line-space arrays are printed with a single dose at a 40 μm gap, the bias variation among the features appears to be approximately ±20 nm. However, this bias measurement is very sensitive to the metrology technique used, and systematic errors in these absolute measurements may be a significant contributor to the observed bias variation. All the experimental results are compared to calculated aerial images. Exposures in thick PMMA (0.5 μm) show trends similar to those observed in thin resist. However, as the thickness of the resist increases, the dose at which the printed linewidth has zero bias decreases, and approaches the dose required to clear a large open area.
Internal barriers in GaAs epitaxial layers produced by delta-doped dipoles
Authors: Shen T.-H. , Ford A. C. , Elliott M. , Williams R. H. , Westwood D. I. , Woolf D. A. , Marlow J. P. , Aubrey J. E. , Hill G.
Published: July 2002

Using a closely spaced pair of δ-doped layers, one n- and one p-type, at a semiconductor homojunction with different isotype doping densities on each side of the junction, controlled internal Schottky-type barriers may be produced. We have studied such prototype GaAs diodes with Be and Si δ doping. Good agreement has been obtained between the experimentally determined barrier and that from a self-consistent Poisson calculation of the zero bias band profile. The temperature dependence of the I–V characteristics has been measured in detail. The results are consistent with the weak temperature dependence of barrier height predicted by the theoretical model. The value of the Richardson constant was also calculated. It has been found that in general the current transport behavior is close to the thermonic limit. A preliminary study of the thermal stability of such diodes has also been carried out.
Investigation of mercury interstitials in Hg1−xCdxTe alloys using resonant impact-ionization spectroscopy
Authors: Littler C. L. , Maldonado E. , Song X. N. , Yu Z. , Elkind J. L. , Seiler D. G. , Lowney J. R.
Published: July 2002

A technique for studying low concentrations of trap levels in narrow-gap Hg1−xCdxTe has been combined with the deliberate introduction of defects to determine the activation energies of these impurities in this ternary material. In this investigation, mercury (Hg) interstitials, believed to be responsible for dark current in metal-insulator-semiconductor devices, were deliberately introduced into samples with x≊0.22 and x≊0.24. Each sample was divided into two parts with the second part of each slice used as a control. The results from these samples provide direct evidence that Hg interstitials create trap levels near 45 and 60 meV above the valence-band edge for these x-value samples.
T-gate, Γ-gate, and air-bridge fabrication for monolithic microwave integrated circuits by mixed ion-beam, high-voltage electron-beam, and optical lithography
Authors: Woodham R. G. , Cleaver J. R. A. , Ahmed H. , Ladbrooke P. H.
Published: July 2002

Mixed lithography combining a 140 keV Be++ focused ion beam and a 60 keV electron beam has been applied to fabricate T-section and Γ-section structures for gate electrodes in GaAs metal–semiconductor field effect transistors for monolithic microwave integrated circuits. The gates are formed by lift-off metallization, following successive electron and ion beam exposures of a single resist layer and a single stage of development. Gates with lengths down to 170 nm have been fabricated. Gate resistance parameters given by reverse modeling with data from microwave measurements are consistent with direct measurements on test structures. Forward modeling shows the benefits of the Γ-gate structure for well-balanced device design. High-voltage lithography has been applied also to a novel procedure for generating air-bridges for monolithic microwave integrated circuits.
Scanning tunneling microscopy and spectroscopy for studying cross-sectioned Si(100)
Authors: Johnson M. B. , Halbout J.-M.
Published: July 2002

Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) are used to investigate a cross-sectional surface of Si(100) prepared by an ex situ cleave HF-dip technique. The agreement between the measured current–voltage spectra (I/Vs) and those calculated with an unpinned surface for both n- and p-type bulk surfaces is good, thus indicating that the prepared surface is unpinned and the model is valid. Both the experimental and calculated I/Vs show three components of current: tunneling out of valence-band states (VB), tunneling through dopant states (D), and tunneling into conduction-band states (CB). As demonstrated by experiment, in agreement with the model, the shape of the I/Vs allows the discrimination of n-type from p-type surfaces. Furthermore, the model indicates that by measuring the dopant state current D-component STM/STS is sensitive to the carrier density within the range of 1018–1021 cm−3. This suggests that this ex situ cleave sample preparation can be used to produce unpinned cross-sectional surfaces for ultra-shallow dopant profiles in Si(100). On such a surface STM/STS can be used to determine the carrier profile that results from the dopant profile.
Precise pattern delineation by refractive index matching in a trilayer resist system
Authors: Tanaka Toshihiko , Hasegawa Norio , Okazaki Shinji , Azuma Yoshie
Published: July 2002

In order to obtain precise pattern dimension control, optical indexes of the middle layer (ML) in a trilayer resist system are investigated. Photoabsorption in ML and bottom layer (BL) is important. However, optimization of photoabsorption in these layers is not sufficiently to suppress linewidth variations. From calculations of the reflection at each interface of the trilayer resist system, the matching of the refractive index between each layer [top layer (TL), ML, and BL] is found to be also important. The optimal refractive index of the ML for the novolac-type of TL and BL materials is estimated to be 1.7 at i line, which is the same value as that of novolac resin. To confirm this matching effect, an admixture material of Ti–alkoxide and silanol is used as the ML material. The mixing ratio is 50 wt %. The refractive index of this material with curing at 230 °C is 1.7. Using this material, the linewidth variation caused by the interference effect can be reduced as 0.04 μm, which is half that using spin-on glass as the ML. The material also shows a high O2 reactive ion etching resistance (1.7 nm/min) and good adhesion to the TL and BL. In the trilayer resist system, it is possible to fabricate a fine, precise pattern even on a large topography.
Improvement of heterodyne alignment technique for x-ray steppers
Authors: Koga K. , Itoh T. , Kusumoto S. , Araki K. , Yasui J. , Takeuchi H. , Aoki S.
Published: July 2002

This article describes the alignment accuracy of the x-ray stepper that was codeveloped by the SORTEC Corporation and Matsushita Electric Industrial Co., and installed in the Tsukuba Research Laboratory for the SORTEC Corporation [K. Koga et al., J. Vac. Sci. Technol. B 8, 1633 (1990)]. Although the optical heterodyne method used for the stepper has a high position detection sensitivity, this method has the drawback that the alignment accuracy is apt to be influenced by an uncontrollable interference, because it uses a coherent laser light source. The overlay accuracy of the stepper was evaluated and identified the error factor contributing to the total overlay error. It was found that the deterioration of the alignment accuracy resulting from position detection error is caused by subtle deflection in the optical alignment system. In order to solve this problem, the alignment unit of the x-ray stepper has been improved. As a result, an alignment accuracy of less than 24 nm (3σ) was obtained in a double-exposure experiment with a posiresist. In addition, an alignment accuracy of less than 38 nm (3σ) was also obtained with SiO2 etched marks.
Confocal filtering of the instantaneous image in scanned darkfield alignment
Authors: Rosenbluth A. E. , Progler C. , Fullenbaum M.
Published: July 2002

Darkfield (DF) alignment signals are commonly obtained by scanning targets beneath a focused probe beam while integrating the light scattered outside a pupil stop, the latter serving to block specular reflections. Additional information pertaining to target position can be obtained by refocusing the collected light to form an instantaneous DF image that moves across the detector as the target is scanned through the focused illumination probe. A simple method to achieve a significantly sharper and more selective response to the target is to apply a confocal prefiltering to the instantaneous image before integrating it to read out the signal level, i.e., successive frames of the instantaneous image are multiplied by a template aperture corresponding to the image of a centered alignment mark. A fiducial scan is used to approximately center the template, providing a reference position. In the case of a test wafer so grainy that the conventional DF signal is significantly broadened and almost overwhelmed by random scatter, the signal obtained with a confocal filter modified to trigger on target symmetry and matched edge slopes is sharper by an order of magnitude and is virtually free of background.
Characterization of chemically vapor deposited tungsten on low-pressure chemically deposited and reactively sputtered titanium nitride films
Authors: Kumar Sunil , Chopra D. R. , Smith Gregory C.
Published: July 2002

The growth of low-pressure chemical vapor deposition (LPCVD) tungsten on LPCVD-grown and reactively sputtered titanium nitride (TiN) films has been studied employing the x-ray photoelectron spectroscopy depth profiling technique. Two types of gas ambients were used to grow tungsten (W) films at a deposition temperature of 400 °C. In one case, a very thin overlayer of W was formed on both types of TiN films by exposing the samples to WF6. The W thus deposited diffuses deep into the bulk of the reactively sputtered TiN film whereas it is confined to the W/TiN interface for the LPCVD-grown TiN film. In the second case, both types of TiN films were exposed to the WF6, SiH4, and H2 ambient. No diffusion of W into any of the TiN films was observed.
Rearrangement of Au(111) surface as a result of scanning with scanning tunneling/atomic force microscopes
Authors: Wang Hong , Jing Jing , Chu H. T. , Henriksen P. N.
Published: July 2002

Rearrangements of monolayer islands on the (111) surface of Au films, grown epitaxially on mica substrates, are observed as a result of continuous scanning with either a scanning tunneling microscope (STM) or an atomic force microscope (AFM) under ambient conditions. With the STM, a contiguous monolayer as large as 150×150 nm2 can be moved from atop to the edges of underlying terrace in about an hour of continuous scanning at a rate of 2.0 μm/s. In general, islands coalesce, vacancies fill, and terrace edges become straighter as a result of the tip scanning over the Au surface. Crystal defects such as screw dislocations, however, are not modified appreciably with scanning, although rearrangements are seen in regions near the defects. Similar effects are observed with an AFM scanning, but to a lesser extent, and only on freshly grown films.
Analysis of highly ordered pyrolytic graphite step defects via scanning tunneling microscope
Authors: Noll J. D. , Cooper J. B. , Myrick M. L.
Published: July 2002

Graphite has a surface that is often used to image molecules with the scanning tunneling microscope. However, confusion of molecules like DNA with natural defects on graphite has properly called these results into question. In this article, a statistical study of graphite is reported. With this information, a model of the origin of the step or ledge defects and other features observed is produced.
Practical considerations in x-ray mask mounting methodology
Authors: Laird D. L. , Laudon M. F. , Engelstad R. L.
Published: July 2002

Overlay requirements for 1:1 masks used in x-ray lithography require robust mask mounting techniques which are relatively insensitive to uncontrollable imperfections. Small variations in mask flatness or mounting surface flatness should not make the mask unusable. The cases of 3-2-1 pinch-type kinematic mounts, stiff vacuum pad mounts, and full surface mounts are investigated here. The results show the kinematic mount to be superior in its insensitivity to the original equilibrium configuration of the mass after bonding.
Resolution limits and process latitude of x-ray mask fabrication
Authors: McCord M. A. , Wagner A. , Donohue T.
Published: July 2002

This article reports on the process latitude and resolution limits of x-ray mask fabrication by electron beam lithography for both additive (gold plated) and subtractive (etched tungsten) masks. The experiments were performed using a vector scan thermal field emission electron beam lithography machine with a maximum beam voltage of 100 kV and a minimum beam diameter of 27 nm. Features as small as 0.075 μm have been patterned on x-ray masks. Exposure latitude was studied as a function of beam diameter and acceleration voltage. The results indicate that forward scattering in the resist is a major cause of degradation in process latitude, and that going to a higher beam energy or thinner resist provides substantial benefits. For subtractive processes where a high-Z material (e.g., tungsten) underlies the resist, backscattered electrons are also a significant factor in limiting exposure latitude and resolution. For both substrate types, increasing the accelerating voltage from 50 to 100 kV can improve the exposure latitude by a factor of 2. At 100 kV the resolution limits are caused by the high aspect ratios found in the sub-tenth micron structures. As the aspect ratio exceeds 5:1 three major problems are encountered. First, the resist tends to fall over due to a lack of mechanical strength. Second, a noticable decrease in linewidth control occurs. Finally, the plating rate becomes dependent on the feature size, causing absorber thickness nonuniformity.