Showing papers in "Journal of Vacuum Science & Technology B in 1995"

Micromachining applications of a high resolution ultrathick photoresist

Abstract: This article describes a new negative‐tone photoresist, SU‐8, for ultrathick layer applications. An aspect ratio of 10:1 has been achieved using near‐ultraviolet lithography in a 200‐μm‐thick layer. The use of this resist for building tall micromechanical structures by deep silicon reactive‐ion etching and electroplating is demonstrated. Using SU‐8 stencils, etched depths of ≳200 μm in Si and electroplated 130‐μm‐thick Au structures with near‐vertical sidewalls have been achieved.

Oxidation behavior of aluminum nanopowders

Abstract: Rutherford backscattering spectrometry, thermogravimetric analysis, and high‐resolution transmission electron microscopy were used to investigate the oxidation behavior of ultrafine grain aluminum powder. Fractional change in mass of Al powder samples were obtained as a function of temperature and exposure time for samples with different particle size distributions. As expected from surface energy considerations, the activation energy for oxidation of ultrafine grain particles is less than that for nominally flat surfaces. Activation energy for oxidation of powder samples with average particle diameters from 240 to 650 A was determined to be 0.5 eV, which is smaller than the value of 1.7 eV known for flat Al samples.

Fabrication of Photonic Band-Gap Crystals

Abstract: We describe the fabrication of three-dimensional photonic crystals using a reproducible and reliable procedure consisting of electron beam lithography followed by a sequence of dry etching steps. Careful fabrication has enabled us to define photonic crystals with 280 nm holes defined with 350 nm center to center spacings in GaAsP and GaAs epilayers. We construct these photonic crystals by transferring a submicron pattern of holes from 70-nm-thick polymethylmethacrylate resist layers into 300-nm-thick silicon dioxide ion etch masks, and then anisotropically angle etching the III-V semiconductor material using this mask. Here, we show the procedure used to generate photonic crystals with up to four lattice periods depth.

Molecular beam epitaxy growth and properties of GaN films on GaN/SiC substrates

Abstract: Two types of nitrogen plasma sources, an electron cyclotron resonance (ECR) plasma source and a radio frequency (rf) plasma source, were used for the growth of GaN by molecular beam epitaxy (MBE). GaN film quality was correlated with the optical emission characteristics of each type of nitrogen plasma source employed. The best quality GaN films were those grown using the rf nitrogen plasma source. This source was found to emit a much larger fraction of atomic nitrogen and 1st‐positive series excited molecular nitrogen in contrast to the ECR plasma source which mainly produced 2nd‐positive series excited molecular nitrogen and nitrogen molecular ions when operated under the same conditions. The benefit of homoepitaxial growth of GaN, using metalorganic vapor phase epitaxy grown GaN layers on basal plane 6H‐SiC, was seen by the observation of surface reconstructions before, during, and after GaN film growth by MBE. In addition, the MBE‐grown GaN films exhibited remarkably intense photoluminescence dominated by a sharp band‐edge peak at 3.409 eV having a full‐width at half‐maximum (FWHM) of 29.7 meV at room temperature. At lower temperatures, splitting of the near‐edge excition peaks was observed. Double‐crystal x‐ray rocking curve measurements of selected MBE‐grown GaN films yielded (0002) diffraction peaks having FWHMs as narrow as 156 arcsec.

Achromatic interferometric lithography for 100‐nm‐period gratings and grids

Abstract: For the fabrication of periodic structures with spatial periods of 100 nm or less, achromatic interferometric lithography is preferred over other lithographic techniques. We report on processes we have developed, using achromatic interferometric lithography, to fabricate large‐area coherent gratings and grids with spatial periods of 100 nm.

Surface‐science aspects of vacuum microelectronics

Abstract: Vacuum microelectronics (VME) concerns the design and manufacture of vacuum devices and components built to sizes and tolerances similar to those found in solid state microelectronic devices. This technology is well suited for use in electron devices of commercial importance, particularly flat‐panel displays. The flat‐panel display industry is projected to have annual revenues measured in the tens of billions of dollars by the turn of the century. As opposed to conventional electron tubes that utilize cathodes based on thermionic emission, vacuum microelectronic devices principally utilize cold cathodes based on field‐electron emission. The introduction of field‐electron emission cathodes can yield unique device performance; however, attaining the level of reliability required for commercial applications requires addressing a host of challenging surface physics and chemistry problems. In this review we outline the surface‐science‐related issues of importance in VME. These issues cover a large realm of sur...

Fabrication and characterization of platinum nanocrystalline material grown by electron-beam induced deposition

Abstract: The technique of electron‐beam induced deposition allows three‐dimensional structures to be generated on the nanometer scale. This is achieved in a scanning electron microscope equipped with a lithography attachment that enables separate position and time control for every pixel. By decomposing adsorbed molecules with the electron beam, structures are created on arbitrarily chosen substrates with nm precision under computer control. Deposits containing metallic nanocrystallites can be produced using organometallic precursor materials. The decomposition of cyclopentadienylplatinum (IV)‐trimethyl (CpPtMe3) results in platinum single crystals with a 2 nm diameter embedded in a carbon‐containing amorphous matrix. The metal content of deposits can be adjusted by choosing an appropriate acceleration voltage and beam current for deposition. The growth rate of deposits from CpPtMe3 is superior to that of frequently used organogold compounds. Tips can be deposited with growth rates up to 150 nm/s. This property is...

Metrology of subwavelength photoresist gratings using optical scatterometry

Abstract: The widths and overall profiles of dielectric grating lines can be determined by measuring the intensity of diffracted laser light from the sample over a specified range of incident beam angles. This technique, known as 2‐Θ scatterometry, is able to accurately and precisely measure photoresist structures in the subhalf micron regime. Moreover, a 2‐Θ scatterometer is capable of making measurements in a rapid and nondestructive manner. To test this technique we measured five identically processed wafers with nominal 0.5 μm line/0.5 μm space grating patterns. Each wafer comprised gratings in a Shipley 89131 negative photoresist exposed in a matrix of incremental exposure doses and focus settings. The scatterometry results were consistent with cross‐sectional and top‐down scanning electron microscopy (SEM) measurements of the same structures. The average deviation of 11 scatterometer linewidth measurements from top‐down SEM measurements, over a broad exposure range, is 14.5 nm. In addition, the repeatability (1σ) of the 2‐Θ scatterometer is shown to be excellent: 0.5 nm for consecutive measurements and 0.8 nm for day‐to‐day measurements.

Conducting atomic force microscopy study of silicon dioxide breakdown

Abstract: A summary is given of an experimental method used to obtain the local dielectric strength of thin insulators using atomic force microscopy with conducting tips. This technique is applied to 7–15 nm thick SiO2 films grown on either crystalline silicon or polysilicon substrates. The dielectric breakdown of the oxides over small areas (∼5×10−16 m2) follows that observed in the intrinsic breakdown of conventional metal–oxide–semiconductor structures, with a maximum breakdown field of 13.2±0.8 MV/cm. On the polycrystalline samples variation in dielectric strength between individual grains can be observed, with the oxide over some grains breaking down entirely. A difficulty when working in air is that sample or tip contamination and induced growth of material under the tip lead to changes in either the effective barrier height or local material thickness which are not related to the SiO2 film. These effects are discussed in detail.

Mechanisms of surface anodization produced by scanning probe microscopes

Abstract: The direct modification of silicon and other semiconductor and metal surfaces by the process of anodization using the electric field from a scanning probe microscope in conjunction with the absorbed water from the atmosphere as the electrolyte is one promising method of accomplishing direct write lithography for the electron device fabrication using scanning probe microscopes. Both scanning tunneling microscopes and conductive‐tip atomic force microscopes have been used for anodization with the work reported here primarily accomplished with a conductive‐tip atomic force microscope. We have found that the terminating thickness of the scanning probe microscope induced oxide is governed by the diffusion limited electric field at the surface (which in this case is a function of the scanning probe microscope tip potential), with many similarities to liquid electrolyte anodization process. In particular, when using atmospheric water as the electrolyte on a silicon substrate and a conductive‐tip atomic force microscope with a −7 to −10 V tip potential, the terminating electric field that is reached as the silicon dioxide thickness increases is to its final value of 80 A is ∼1×107 V/cm. This is consistent with the diffusion limited electric field that is observed in many other anodization processes and the native oxidation of silicon (Mott–Cabrera process).

Field emission from silicon spikes with diamond coatings

Abstract: Diamond‐coated silicon field emitters were fabricated and investigated. Emission currents of few μA per tip at voltages of several hundred volts were obtained from very blunt tips with curvature radii up to 3 μm. The values of the effective work function calculated from Fowler–Nordheim plot were in the range from 0.3 to 1.2 eV. Two models for an explanation of the experimental data are proposed.

Optically matched trilevel resist process for nanostructure fabrication

Abstract: A novel trilevel resist process has been developed which enables high‐contrast imaging of periodic structures with spatial periods down to 200 nm in thick resist on highly reflective substrates, using λ=351.1 nm argon‐ion laser exposure. The process utilizes a 200‐nm‐thick, high‐contrast, imaging resist layer, a thin (∼15‐nm) evaporated dielectric interlayer, and a 300–600‐nm‐thick bottom antireflection coating (ARC) which suppresses reflections from the substrate. Our trilevel resist scheme has been implemented in a manufacturing process which utilizes a high‐contrast interferometric lithography system for the formation of large‐area, 200–1000 nm period grating and dot array images. The choice of interlayer is the most critical feature of this process. This material must have good deposition and adhesion properties, must be optically matched to the resist and ARC, must etch quickly during the reactive‐ion etching (RIE) pattern transfer from the resist into the interlayer, must display very high selectivity to the ARC during the RIE pattern transfer into the bottom layer, and must be easily stripped after the trilevel resist structure has served its purpose. We also report on computer modeling which elucidates the factors influencing standing wave formation and present results of tests with several interlayer materials which display good optical matching and selectivities of up to 240:1 during RIE of the ARC.

Simulation of atomic force microscope tip–sample/sample–tip reconstruction

Abstract: The image obtained with the atomic force microscope is a convolution of the tip and sample. A numerical algorithm which has been previously reported enables the removal of the tip geometry, hence exposing a more accurate picture of the sample. The efficacy of such a scheme is explored with the use of simulations of the tip–sample interaction using simple geometric considerations. Two examples that illustrate the limitations of image analysis by such procedures are presented.

Fabrication of double‐gated Si field emitter arrays for focused electron beam generation

Abstract: Double‐gated Si field emitter arrays (FEAs) capable of generating focused electron beams were fabricated and experimentally evaluated. The present field emitter array has a vertical triode structure consisting of a conical Si tip and two gate openings (upper and lower) surrounding the tip. The lower gate with a 2‐μm‐diam opening acts as an extraction electrode controlling the emission current, and the upper one with a 3‐μm‐diam opening acts as an electrostatic lens focusing the electron trajectories. The focusing property was evaluated by observing the spot size of a phosphor (ZnO:Zn) screen located about 20 mm apart from the field emitter array and biased to 1 kV. It was found from experimental results that decreasing the upper gate voltage (VF) down to a few volts was quite effective to generate focused electron beams. At VF of about 4 V, the electrons emitted from the tip were well collimated and a beam current of about 0.1 nA/tip was obtained.

Study of large area high density magnetic dot arrays fabricated using synchrotron radiation based x‐ray lithography

Abstract: Large area arrays of dots have been patterned in Au/Co/Au(111) sandwiches with ultrathin Co layers (0.6 to 2 nm) and a perpendicular easy magnetization axis. Dot dimensions down to 0.2 μm have been achieved using x‐ray lithography, with either positive resist and direct ion beam etching or a lift‐off process with aluminum mask. Both processes have been tested against the damages they induce to the fragile structure of the samples. The magneto‐optical effects and magnetization reversal processes in the arrays have been characterized versus Co thickness, dot dimension, and lattice aspect ratio. For high quality samples, the domain walls propagation mechanism that drives magnetization reversal in as‐grown films is drastically modified in dot arrays, leading to a large increase of the coercive field with dot diameter reduction, together with changes in the shape of the hysteresis loops.

Atomic force microscope lithography using amorphous silicon as a resist and advances in parallel operation

Abstract: Lithography on (100) single‐crystal silicon and amorphous silicon is performed by electric‐field‐enhanced local oxidation of silicon using an atomic force microscope (AFM). Amorphous silicon is used as a negative resist to pattern silicon oxide, silicon nitride, and selected metals. Amorphous silicon is used in conjunction with chromium to create a robust etch mask, and with titanium to create a positive AFM resist. All lithographies presented here were patterned in parallel by arrays of two piezoresistive silicon or two silicon‐nitride cantilevers. Parallel arrays of five piezoresistive cantilevers were fabricated and used in imaging and lithographic applications. A 400 μm×100 μm parallel image is obtained in the time it would normally take to obtain a 100 μm×100 μm image. In our method of parallel operation, it is only possible to image and lithograph in modes that do not require feedback. In imaging, this limits the possible applications of the parallel AFM. During parallel lithography, discrepancies a...

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

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

Physical and magnetic properties of submicron lithographically patterned magnetic islands

Abstract: We have developed a procedure for high‐resolution patterning of magnetic thin films using direct‐write electron beam lithography and a multistep sputter etching process [New et al., J. Vac. Sci. Technol. B 12, 3196 (1994)]. Using this patterning procedure we have defined large arrays of 0.2 by 0.4 μm magnetic islands out of a 200‐A‐thick polycrystalline film of cobalt. The physical and magnetic structure of these islands has been examined using atomic and magnetic force microscopy, as well as transmission electron microscopy. Hysteresis loop measurements have been taken from large arrays of weakly interacting islands. The particles are not single domain and show considerable nonuniformity of magnetization when they are completely demagnetized. However, after application and removal of a large field along the long axes of the islands, most of the particles do relax into an almost uniform magnetization state.

Effect of tip shape on surface roughness measurements from atomic force microscopy images of thin films

Abstract: The distortion of images by the finite radius of the tip is a significant artifact in atomic force microscopy (AFM) imaging of thin films. This article examines the effect of the finite radius of the AFM tip on the measurement of five surface roughness parameters: Ra and Rrms roughness, horizontal roughness, bearing ratio, and power spectral density. The effect of the finite size of the AFM tip on the measurement of these roughness parameters was studied using simulations. We found that the vertical roughness measures (Ra and Rrms) were the least affected by the tip induced distortion. For a spherical tip, the error in Ra and Rrms roughness was less than 15% as long as the radius of the major features in the AFM image (RAFM) was greater than twice the radius of the tip (Rtip). The horizontal roughness of the surface was sensitive to the distortion due to the tip. For distorted AFM images (RAFM/Rtip=5) the bearing ratio curve was similar in shape to the curve for the original surface. The bearing ratio cur...

Field emission from p‐type polycrystalline diamond films

Abstract: Vapor‐deposited‐diamond field emitters are fabricated using diamond film technology compatible with integrated circuit processing. The field emission data from p‐type polycrystalline diamond films are presented. Field emission current in the range of 15 μA was detected at an electric field intensity of <20 MV/m at a pressure of 10−7 Torr. A current of approximately 50 nA was observed at a pressure up to 10−2 Torr in H2 environment.

Cross‐linked polymers for nanofabrication of high‐resolution zone plates in nickel and germanium

Abstract: A high resolution cross‐linked PMMA resist has been synthesized and optimized for the generation of zone plate patterns down to 19 nm linewidth with e‐beam lithography. This resist shows an increased resolution compared to PMMA for generating periodic structures with a line to space ratio of 1:1. Furthermore, we developed a cross‐linked copolymer based on styrene and divinylbenzene, which is used in a new trilevel reactive ion etching (RIE) process. In this process a resist pattern of low aspect ratio can be transferred into a copolymer galvanoform with high aspect ratios for the electrodeposition of nickel. The copolymer has also been used as a highly selective etching mask for zone plate pattern transfer into germanium by RIE.

Directional deposition of Cu into semiconductor trench structures using ionized magnetron sputtering

Abstract: The primary metallization technique in the semiconductor industry for the past decade has been magnetron sputtering. The general industry trend towards damascene processing requires the capability to fill trenches and vias with sub‐one‐half micron widths and aspect ratios (AR) as high as 3:1. At these dimensions and geometries, the angular distribution of magnetron sputtered atoms results in the production of voids during deposition. A new approach to this problem is directional sputter deposition, where neutrals generated from magnetron sputtering are ionized through an inductively coupled rf plasma and accelerated towards the substrate via a small dc bias on the substrate, causing a significant portion of the flux to arrive at normal incidence. The experimental parameters of this process have been explored by depositingCu on patterned Si wafers. The parameters have varying effects on the morphology of the deposited layer, which indicate relationships between the parameters and the ion‐to‐neutral ratio, the total flux, and the average ion energy. The ionized magnetron sputter deposition process has been used successfully to fill trenches of 600 nm width and 1.1 AR with Cu near room temperature, and appears to be extendable to more aggressive dimensions.

Simulations of trench‐filling profiles under ionized magnetron sputter metal deposition

Abstract: Numerical simulation results are presented for microscopic profile evolutions of deposited metal films in trench structures under ionized magnetron sputter deposition. The model used for the simulations takes account of the deposition of both ionized and neutral metal species and sputtering (i.e., etching) of the deposited film by the bombardment of metal and inert‐gas (such as argon) ions. The evolution of the surface topography is calculated numerically using the shock‐ tracking algorithm. Numerical results are also compared with experimental observations. A primary application of this metal deposition technique is interconnect metallization on semiconductors.

Exposure strategies for polymethyl methacrylate from in situ x-ray absorption near edge structure spectroscopy

Abstract: We have observed the chemical changes in PMMA irradiated by x rays in situ. The chemical changes are monitored by micro‐x‐ray absorption near edge structure spectra at the carbon absorption edge. The loss of the ester group (C=O) and formation of C=C bonds have been determined quantitatively from changes in the intensities of the respective π* resonant peaks as a function of dose. Samples prepared under different conditions were examined. From the dose dependence of bond formation, scission and linking, the performance of the resist can be predicted, so that the preparation strategy, dose, and development can be optimized.

Application of lead zirconate titanate thin film displacement sensors for the atomic force microscope

Abstract: Elimination of optical and tunneling displacement sensors from the atomic force microscope (AFM) is an important breakthrough for improved performance of the AFM. The interaction of an oscillating tip and a surface has become a popular tool for obtaining information which could not be obtained by the conventional repulsive force AFM mode. In this application, lead zirconate titanate (PZT) is one of the most promising materials with large piezoelectric constants which can be used not only for detecting distortions such as the displacement detection of an AFM cantilever, but also oscillations of the cantilever. However, incorporating PZT into the microfabrication process to make the AFM cantilever has not been easily accomplished because PZT has a delicate chemical nature. We have successfully developed an AFM with a microfabricated cantilever which had a PZT thin film applied for displacement sensing. The linearity of the output signal was sufficient for displacement sensing. Images of a compact disk were ...

Domain structure and polarization reversal in ferroelectrics studied by atomic force microscopy

Abstract: The ferroelectric domain structure and its dynamics under applied electric field have been studied with nanoscale resolution by atomic force microscopy (AFM). Two mechanisms responsible for the contrast between opposite domains are proposed: large built‐in domains are delineated in friction mode due to the tip–sample electrostatic interaction, and small domains created by an external field are imaged in topography mode due to piezoelectric deformation of the crystal. The ability of effective control of ferroelectric domains by applying a voltage between the AFM tip and the bottom electrode is demonstrated. It is experimentally confirmed that the sidewise growth of domain proceeds through the nucleation process on the domain wall.

Field emitter array mask patterning using laser interference lithography

Abstract: We have fabricated uniform arrays of 120‐nm‐diam dot masks on 300 nm centers using laser interference lithography. Chrome, cobalt, nickel, and germanium dot arrays have been fabricated. The density of these arrays is ≳109 dots/cm2. The standard deviation of the average dot diameter is 7.4% over a 5‐cm‐diam silicon substrate. The center‐to‐center spacing of the dot mask is determined by the laser wavelength and interference angle. Some control over the dot diameter is possible by varying the angle of the substrate during the metal deposition prior to liftoff. We have used a reactive ion etch with these metal dot masks to form single crystal silicon pedestals demonstrating that these structures are suitable for self‐aligned gated field emitter array fabrication.

Approach to obtain high quality GaN on Si and SiC-on-silicon-on-insulator compliant substrate by molecular-beam epitaxy

Abstract: Crystalline SiC thin layers have been grown on 125 mm silicon‐on‐insulator (SOI) substrates as a promising and economical substrate for the growth of GaN epilayers. Through the use of an AlN/GaN strained superlattice buffer layer, high quality GaN layers as thin as 2000 A on Si(111) substrates have been achieved. X‐ray diffraction curves with a full width at half‐maximum (FWHM) as narrow as 25 arcmin were obtained. The associated low‐temperature photoluminescence (PL) spectrum showed a dominant bound‐exciton peak with a FWHM of 8 meV. We have further combined these two techniques to synthesize the first GaN on SiC on a SOI structure.

Generation of degradation defects, stacking faults, and misfit dislocations in ZnSe‐based films grown on GaAs

Abstract: Transmission electron microscopy is used to investigate the structure of the crystalline defects in degraded and undegraded ZnSe‐based films/GaAs heterostructures. In degraded areas, dark line defects (DLDs) originate from the preexisting or grown‐in defects (i.e., stacking faults and misfit dislocations) during device operation and make the laser diode fail. From our observation, the nucleation of DLDs is based on the initial emission of a mobile faulted defect from the preexisting grown‐in defects. Along the trace of the faulted defects, [100] dark line defects form. We also investigated the origins and mechanisms for the generation of stacking faults and misfit dislocations. The generation of stacking faults is strongly related to the doping concentration, substrate surface stoichiometry, and growth mode of the films. The vacancy contained Ga–Se interfacial layers are thought to be sources for the generation of Frank‐type stacking faults. In addition, Shockley partial dislocations form due to island co...

Field emission characteristics of diamond coated silicon field emitters

Abstract: Single crystal silicon field emitters have been modified by surface deposition of diamond using bias‐enhanced microwave plasma chemical vapor deposition Polycrystalline diamond with a high nucleation density (1010/cm2) and small grain size (<20 nm) was achieved on silicon field emitters Field emission from these diamond coated emitters exhibited significant enhancement both in total emission current and stability compared to pure silicon emitters A large effective emitting area comparable to the tip surface area was obtained from a Fowler–Nordheim analysis The effective work function of the polycrystalline diamond coated emitter surface was found to be larger than that of a pure silicon emitter surface