Diam Wafers Research Articles

Wafer Level Surface Activated Bonding Tool for MEMS Packaging

A wafer level surface activated bonding (SAB) tool has been developed for microelectromechanical systems (MEMS) packaging at low temperature. The tool accommodates 8 in. diam wafers. The principle features of the tool are the automatic parallel adjustment for 8 in. wafers to a margin of error within ±1 μm and the X, Y, and θ axis alignments with an accuracy of ±0.5 μm. We have approached a new integration technique for the integration of ionic crystals with transparent and nontransparent thin intermediate layers using this tool. Various sizes of patterned and bare silicon, Al silicate glass, and quartz wafers cleaned by a low energy argon ion source in a vacuum have been successfully bonded by this technique at low temperature. Radioisotope fine leak and vacuum seal tests of sealed silicon cavities show leak rates of and Pa/m3 s, respectively, which are lower than the American military standard encapsulation requirements for MEMS devices in harsh environments. Void-free interfaces with bonding strengths comparable to bulk materials are found. Low adhesion between SAB-processed ionic crystals without adhesive layers is believed to be due to radiation-induced discontinuous polarization. © 2004 The Electrochemical Society. All rights reserved.

Submicrometer Platinum Electrodes by Through-Mask Plating

A through-mask electroplating process is described for forming Pt electrode structures with the vertical sidewalls and submicrometer dimensions desired for dynamic random access memory applications. The plating process used an aqueous KOH-based solution of “Pt A Salt” (Engelhard) and a sputter-deposited Pt plating base. Plating conditions were first optimized for blanket films on 200 mm diam wafers by examining the characteristics of the Pt deposits as a function of plating bath temperature and current density. Electrode features were then formed on similar wafers by plating through a patterned mask. Integration issues discussed include potential reactions of the dielectric mask with the underlying Pt plating base, surface modification of the exposed Pt plating base prior to plating, and erosion of the dielectric mask by the plating solution. © 2001 The Electrochemical Society. All rights reserved.

Low‐Cost p − / p − Epitaxial Silicon Wafers for Densely Packed Metal‐Oxide‐Semiconductor Devices

Improvements of gettering actions in the prototype thin‐film silicon epitaxial wafers (1 μm thick film) have been investigated in conjunction with avoiding failures of crystal‐originated “particles” which degraded isolation leakage between memory cells of densely packed metal‐oxide‐semiconductor (MOS) devices. In order to compensate for the appearance of a kink in current-voltage curves in MOS capacitors caused by undesirable impurities, an optimized gettering and evaluation method are proposed, leading to a low‐cost Czochralski‐grown substrate. We pinpointed the importance of the epitaxial thickness, proposing an improved type of epitaxial wafer with a 3–5 μm thick film that is cheaper than current epitaxial wafers. This epitaxial wafer meets the requirements for future miniaturized devices formed on large diameter wafers (e.g., 300 mm diam wafers). © 2000 The Electrochemical Society. All rights reserved.

Gravitational Stress‐Induced Dislocations in Large‐Diameter Silicon Wafers Studied by X‐Ray Topography and Computer Simulation

Dislocations in slip bands introduced under gravitational stress in 200 mm diam Czochralski‐grown silicon wafers were characterized by X‐ray topography, and slip band initiation was estimated by computer simulation for 300 mm diam wafers. Surface scratching on the wafer back by contact with the jig forms an amorphous region, resulting in a collective motion of dislocations during annealing at 1473 K under gravitational stress. The occurrence of slip bands along the 〈110〉 directions was observed with a length of several centimeters from the contact area of the wafer‐supporting jigs. Burgers vectors and slip planes of the gravitational stress‐induced dislocations were determined to extend along the [1¯10] direction in the (111) or (1¯1¯1) plane, terminating at the surface with screw‐type in character. Three types of half loop of dislocations were identified with Burgers vectors of [011] in (1¯1¯1), [101] in (1¯1¯1), and [011] in (111¯) planes. By the reaction , Lomer‐type edge dislocations were found to be created. The gravitational stress in a 300 mm diam wafer was computed by a finite element method. By comparing the gravitational stress with the critical stress to multiply slip dislocations previously obtained, conditions for suppressing slip bands are predicted for wafers larger than 300 mm diam.

X‐Ray Photoelectron Spectroscopy Analyses of Oxide‐Masked Polycrystalline SiGe Features Etched in a High‐Density Plasma Source

The chemical distribution of oxide‐masked polycrystalline structures etched using gas mixtures of , HBr, and has been investigated by x‐ray photoelectron spectroscopy (XPS). The 200 mm diam wafers were etched in a low‐pressure, high‐density plasma helicon source. The chemical constituents present on the tops, sidewalls, and bottoms of the etched features were determined by XPS. As for polysilicon, a silicon oxide‐like film is formed on the sidewalls of the features when etching with and plasmas, whereas there is no germanium oxide‐like formation. Using XPS, the silicon oxide‐like layer was estimated to be 11 Å thick on the sidewalls of the poly with both mixtures. The thinner oxide‐like layer measured on the sidewalls of polycrystalline (11 Å) compared to that on the sidewalls of polycrystalline Si (30 Å) demonstrates that the oxide‐like layer thickness depends on the germanium concentration in the alloy. XPS also shows that by using the gas mixture, the germanium of the polycrystalline SiGe sidewalls spontaneously reacts with oxygen to form volatile etching species.

Fifty centimeter ion beam sourcea)

A 50-cm-diam ion source and its industrial applications are described. This is the largest commercial ion source presently available for ion beam etching and deposition over a full 20 in. diam. It has been used to generate beams of argon ions at 300–900 eV of ion energy and beam currents of 1–5 A with standard filament-type cathodes. Operation at even higher current levels is possible with high emissivity hollow cathodes. A presentation of general performance characteristics of the ion source is given and some of the special considerations required for reliable and cost-effective operation of large scale ion beam systems are discussed. To illustrate the application of this source, a brief description of a fully automatic production ion beam etching system developed around this ion source is given along with a description of the etch results. This system can simultaneously etch a batch of five 6-in.-diam wafers with etch rates of nickel–iron exceeding 500 A/min. The etch uniformity is excellent.

Fifty centimeter ion beam source (abstract)a)

A 50-cm-diam ion source and its industrial applications are described. This is the largest commercial ion source presently available for ion beam etching and deposition over a full 20 in. diam. It has been used to generate beams of argon ions at 300–900 eV of ion energy and beam currents of 1–5 A with standard filament-type cathodes. Operation at even higher current levels is possible with high emissivity hollow cathodes. A presentation of general performance characteristics of the ion source is given and some of the special considerations required for reliable and cost-effective operation of large scale ion beam systems are discussed. To illustrate the application of this source, a brief description of a fully automatic production ion beam etching system developed around this ion source is given along with a description of the etch results. This system can simultaneously etch a batch of five 6-in.-diam wafers with etch rates of nickel–iron exceeding 500 A/min. The etch uniformity is excellent.

Observation of deep holes using the BEASTLI (backscattered electrons assisting LSI inspection) method

In scanning electron microscopy, we have discovered that the bottom of deep holes can be observed clearly when a high-energy electron beam is irradiated. To utilize this phenomena, we developed an instrument that generates a scanning electron probe of 50 to 200 keV with a TV scan rate that enables us to inspect a maximum of 8-in.-diam wafers. In the past, deep holes were inspected by observing the crosssectional view of a hole after breaking the wafer into small pieces. Using a high-energy electron beam, we can easily inspect such holes without destroying the samples. We named this inspection method BEASTLI ( b ackscattered e lectrons as sis t ing L SI i nspection). This observation does not depend on specimen conditions, such as the aspect ratio of holes and the specimen charging that often occurs in such cases of insulator observation. This instrument enables us to observe inside the structure of semiconductor devices. Irradiation damage on semiconductor devices caused by unintentional irradiation of unspecified areas, such as threshold voltage and subthreshold coefficient deviation of metal-oxide semiconductor (MOS) transistors, can be recovered by means of hydrogen annealing for 30 mm with a temperature of 450°C.

Large-area electron-beam source

A new large-area electron-beam source which can operate continuously, stably, and indefinitely in a poor vacuum environment is described. This novel electron source produces a near monoenergetic electron beam which can uniformly expose large area substrates (i.e., 200 mm diam wafers and larger flat panel displays). The electron gun incorporates a cold cathode which is impervious to solvents and outgassing from irradiated polymer coatings. Unlike glow discharge electron sources, the accelerating voltage can be controlled independently of the emission current from the electron gun with a small bias voltage to a grid anode. Accelerating voltages of 1–60 keV and higher are possible. The principles of operation and performance characteristics of this new source are described, including its ability to expose insulating samples without requiring a conductive overcoat. This new electron source has enabled a number of new process innovations in photoresist stabilization, interlayer dielectric curing, lift-off processing, and pattern lithography.

The effects of gas phase convection on mass transfer in spin coating

The thickness uniformity of photoresist films deposited by spin coating critically influences the resolution of photolithography. This thickness uniformity depends on uniform evaporation from the film during drying. Simple scaling arguments demonstrate that, if the mass transfer coefficient at the surface of the wafer does not vary with radial position, then the dry coated resist film thickness will be independent of radial position. A model is presented for the compressible, laminar, steady-state, axisymmetric air flow in a spin coating apparatus for 6-in.-diam wafers. Flow fields computed by a finite-element–Newton method are used to evaluate the radial profile of the mass transfer coefficient at the surface of the rotating wafer, and to calculate the trajectories of particles that are generated as photoresist is flung from the edge of the spinning wafer. At a spin speed of 2000 revolutions/min and exhaust flow rate of 100 ℓ/min through the coater, the calculations predict that the mass transfer coefficient should be independent of radius. Comparison with film contours measured from experiments at these conditions indicates radial nonuniformities in the film thickness and suggests the importance of hydrodynamic instabilities in the gas on the uniformity of the coating.

Poly‐Si etching using electron cyclotron resonance microwave plasma sources with multipole confinement

In this study, mirror field and single coil 2.45 GHz electron cyclotron resonance microwave plasma sources were coupled to a multipole, multicusp plasma confinement system and used to produce chlorine plasmas for poly‐Si etching. Scanning Langmuir probes were used to study the effect of process parameters on plasma potentials, plasma density, plasma density uniformity, and electron temperature, while poly‐Si etching experiments on 150 mm diam wafers were used to relate process parameters and Langmuir probe results to etch rate, SiO2 selectivity, photoresist selectivity, and etch uniformity. Radio‐frequency substrate bias at 13.56 MHz was used for ion energy control, and the addition of a third coil below the substrate plane was found useful for fine‐tuning the radial plasma uniformity. Undoped poly‐Si etch rates ≳3000 A/min, Si/SiO2 etch selectivities ≳25 and 150 mm diam etch uniformities <2% at 1 σ were obtained, but not simultaneously. Tradeoffs in choosing operating conditions for optimum poly‐Si etch ...

Reactive Ion Etching Techniques for Silicon Sidewall Angle Control in Microengineering

This paper describes reactive ion etching (RIE) techniques with silicon substrates for initial processing of semiconductor field emitter array cathodes for vacuum microelectronics. A secondary focus is process research for optical reflecting gratings for opto‐ and micromechanical devices. These applications each require control of the sidewall angle and apex radius. In this paper, we describe micromachining control of trenches with remainder sidewall angles varying from 15–60° and resulting apex ridge radii of as small as 40 nm. A fluorine‐based chemistry with oblique angles for the incident beam (tilted substrates) and overetching is used. The use of both deep UV‐hardened photoresist and aluminum as RIE shadowmasks is compared. Unannealed aluminum as a shadowmask for RIE micromachining has the advantage of lower etch/sputtering rates and higher process temperature tolerance compared to photoresist in the RIE system. Etch environments in the pressure range 30–80 mTorr with flow rates of 20/2 seem, RF power 100–200 W, and etch duration 30 min are described. Both tilted and untilted substrates mountings were studied. Under these conditions, the surface erosion is primarily a combination of ion milling and chemical etch mechanisms. Both P‐type and N‐type substrates of 100 nm diam wafers were used. Relatively sharp single‐ and double‐ridged silicon structures were obtained using the photoresist shadowmask on untilted substrates. The photoresist shadowmask provides an advantage over Al only for the sidewall orientation where we were able to specify a condition for obtaining a smooth optically reflective surface. The more vertical sidewall angles obtainable with the unannealed Al shadowmask should permit fabrication of field emitter cone or ridge cathodes on pedestals with higher height‐to‐width ratios. A field cathode pedestal with a more vertical sidewall results in a higher electric field at the electron emission tip or ridge.

Electron cyclotron resonance plasma chemical vapor deposition of large area uniform silicon nitride films

Electron cyclotron resonance plasma (f=2.45 GHz, microwave power P=200–800 W) generated in a radially uniform magnetic field (B=875–1000 G) was used to produce a large area (15–20 cm diam) uniform plasma stream at 20–30 cm from the source output. Low temperature (70–300 °C) silicon nitride films with a thickness of 800–3000 Å were deposited on 5–20 cm diameter wafers with deposition rates of 100–350 Å/min. Film thickness uniformity was ±1% for 7.6–10.0 cm diam wafers, ±3% for 15 cm diam wafers, and ±9% for 20.0 cm diam wafers. It was found that the film deposition rate Wg increased linearly with the silane flow rate, while Wg increased slower than power 1/2 with the nitrogen flow rate. The film refractive index was 1.9–2.0 at a silane/nitrogen flow rate ratio of 0.40–0.6. The effects of plasma density and its profile on the film growth rate and uniformity are discussed.

Conversion of a vacuum generators model V80H molecular-beam epitaxy system from HgCdTe to fluoride deposition and from 3- to 4-in. wafer handling capabilities

We have converted a Vacuum Generators V80H molecular-beam epitaxy (MBE) system, originally designed to handle 3-in. diam wafers, to accommodate and process 4-in. diam wafers. The MBE system, which was previously used in the same laboratory for the deposition of HgCdTe films, has been subjected to a thorough cleaning procedure, and is now being used successfully to grow high quality BaMgF4 films on 4-in. silicon wafers. The modification procedure is described in this note.

Molecular-beam epitaxy of CdTe on large area Si(100)

We have grown CdTe directly on 2- and 5-in. diam Si(100) by molecular-beam epitaxy and characterized the layers by in situ reflection high-energy electron diffraction, double crystal x-ray diffraction, scanning electron microscopy, transmission electron microscopy, and low-temperature photoluminescence. The films are up to 10-μm thick and mirror-like over their entire surface. Even on 5-in. diam wafers, the structural and thickness uniformity is excellent. Two domains, oriented 90° apart, are observed in the CdTe films on oriented Si(100) substrates, whereas single-domain films are grown on Si(100) tilted 6° or 8° toward [011]. The layers on misoriented substrates have better morphology than those on oriented Si(100), and the substrate tilt also eliminates twinning in the CdTe layers. First attempts to grow HgCdTe on Si(100) with a CdTe buffer layer have produced up to 10-μm thick layers with cutoff wavelengths between 5 and 10-μm and with an average full width at half-maximum of the double-crystal x-ray diffraction peaks of 200 arc s.

X‐Ray, Photoluminescence, Stoichiometry, and Thickness Mapping of In1 − xGaxAsy P 1 − y

Maps of double heterostructures grown by hydride vapor phase epitaxy and by metalorganic chemical vapor deposition on (001) substrates have been obtained. X‐ray double‐crystal rocking curves of the (004) reflection and photoluminescence spectra at room temperature were separately obtained at 65 locations on two in. diam wafers. The x‐ray data were used to obtain maps of the lattice mismatch and of the thickness of the layer. The photoluminescence data were used to obtain maps of the bandgap, and these were combined with the mismatch data to yield maps of the Ga and As mole fractions. The thickness was found to closely correlate to both the Ga and As mole fractions for hydride vapor phase epitaxy but to weakly correlate to only the Ga mole fraction for metal‐organic chemical vapor deposition.

Growth Kinetics and Annealing Studies of the “Cathodic” Plasma Oxidation of Silicon

A study of the growth kinetics for plasma‐enhanced oxidation in an electrodeless RF system based on an earlier design is presented. Oxidation growth rates and thickness uniformity variations across 75 mm diam wafers for various plasma parameters are given. The time‐dependence of the grown oxides were found to fit a power law model. Other parameters investigated include RF frequency in the range 6–21 MHz, RF power from 100–500 W, chamber pressure from 85 to 500 mtorr, and oxygen in argon concentration in the plasma from 1 to 100% oxygen. The etch rate of the plasma oxide in 10:1 buffered oxide etch was found to be comparable to that of thermally grown oxides. Defect levels (net coulombic charge) and breakdown values for postoxidation‐annealed oxides were measured initially with poly‐gated capacitors, prior and subsequent to post‐poly anneal. Defect levels and breakdown values for postoxidation annealed oxides were also obtained with aluminum‐gated capacitors, with thermal oxides used as “controls” and subjected to the same annealing treatments. It was found that those oxides which had received a postoxidation anneal in oxygen at 1000°C for 15 min were comparable to the thermal “controls” in defect level density and electric breakdown integrity.

Charged particle densities and energy distributions in a multipolar electron cyclotron resonant plasma etching source

The discharge downstream from a 9 cm i.d. multipolar electron cyclotron resonant (ECR) plasma was characterized in terms of positive ion density, electron energy distribution functions (EEDFs), ion energies incident on a biased conducting substrate, and silicon etching performance. Ion density in the process chamber was found to decrease exponentially away from the plasma source with a peak density of 6×1011 cm−3 in 1 mTorr argon with 250 W of microwave power. While EEDFs are non-Maxwellian, no evidence of high-energy ECR electrons was observed in the process chamber. The ion flux to a conducting substrate diverges by approximately 15° from normal but the ion incidence energy is easily controllable with a dc bias to the substrate. Despite the divergent ion flux from the ECR source, anisotropic etching of silicon was possible with etch rates of 300 nm/min. Uniform etching of silicon over 7.6 cm (3 in.) diam wafers was attained 8 cm below the plasma source.

Formation and properties of rapid thermally annealed TiSi2 on lightly doped and heavily implanted silicon

Detailed material and electrical characteristics of rapid thermally annealed (RTA) TiSi2 on doped silicon are presented using transmission electron microscopy, Rutherford backscattering spectrometry, secondary ion mass spectrometry (SIMS), Auger analysis, and four-point probe measurements. TiSi2 films with varying sheet resistances were formed on lightly doped and heavily arsenic and phosphorus implanted 〈100〉 silicon by rf sputtering titanium and forming the silicide using two-step flash anneals at different temperatures. It is shown that the silicide sheet resistance is a sensitive function of the silicon surface condition prior to titanium sputtering; in particular, silicide films formed on heavily implanted silicon had significantly higher sheet resistance compared to films formed under identical conditions on lightly doped prime silicon. The higher silicide sheet resistance resulted because of the surface damage created during arsenic and phosphorus implantation and higher silicon dopant concentration. The RTA silicide films showed excellent film properties across 4-in.-diam wafers with good thickness uniformity and minimal sheet resistance variations compared to furnace annealed samples. Detailed SIMS and Auger analyses showed minimal film contamination and negligible dopant redistribution for RTA silicided wafers.

Electrical activation of implanted Be, Mg, Zn, and Cd in GaAs by rapid thermal annealing

The formation of p channel (p∼3×1017 cm−3) and p+ regions (p&amp;gt;5×1018 cm−3) in GaAs by rapid annealing of Be, Mg, Zn, or Cd implants at 900–950 °C is reported. The electrical characteristics of each species were investigated by capacitance-voltage profiling, Hall-effect and microwave photoconductance measurements, and particular emphasis was placed on determining the uniformity of activation over 2-in.-diam wafers. The results demonstrate that rapid thermal annealing is well suited to GaAs integrated circuit applications.