Charles E. Hunt

Formation of silicon tips with <1 nm radius

Electron emitters in vacuum microelectronic devices need sharp tips in order to permit electron emission at moderate voltages. A method has been found for preparing uniform silicon tips with a radius of curvature less than 1 nm. These tips are formed by oxidation of 5‐μm‐high silicon cones through exploitation of a known oxidation inhibition of silicon at regions of high curvature.

Fabrication of Y3Al5O12:Eu thin films and powders for field emission display applications

Abstract Thin films of Y 3 Al 5 O 12 :Eu (YAG) were spin coated on different substrates from sols. The precursor solution was made from organic precursors by combining stoichiometric ratios of Y(III) iso-propoxide, Al(tri Sec-butoxide) and Eu(III) acetyl acetonate. Sol-gel derived thin films and powders are non-crystalline. Powder samples calcined at 650 °C for 5 h show the characteristic X-ray diffraction pattern for the YAG phase. Similarly, the thin film samples annealed at 600 °C for 5 h also show YAG phase in the XRD. The emission spectra were measured for both the powder and thin-film samples. Scanning electron microscopy for the powder samples show that the particles are reasonably crystallized with a particle size of 5 μm. The spectral response and out gassing characteristic of the new, low voltage YAG thin films mainly used for application in the field emission flat panel displays are also measured. The cathodoluminescent materials were tested with electron beam excitation at currents of up to 50 μA within the 3000–8000 V range (medium voltage range). The spectral coordinates with minimal optimization as compared with industrially manufactured P22 phosphors at low voltage operation are reasonable.

Silicon on insulator material by wafer bonding

Thermal bonding of oxidized silicon wafers is used to obtain high-quality silicon on insulator (SOI) starting material for electronics and sensor applications. An overview of the technology is followed by a detailed description of the bonding technique and the ensuing wafer thinning processes for making SOI of various film thicknesses. Bonded pairs of wafers can be reproducibly produced free of contact voids. Thick-film SOI is produced using a simple bond and grind/polish technique. Thin-film SOI, suitable for CMOS applications, is produced using the bond and etch back (BE-SOI) process. A comparison of selective etch-back chemistry with different etchstop fabrication techniques is presented. These methods produce inexpensive, low-defect SOI, for integrated circuit applications, using materials and equipment common to silicon integrated circuit process lines.

Beam focusing for field-emission flat-panel displays

A combination of finite element and finite difference techniques have been used to simulate the performance of micro-fabricated gated field emitters for flat-panel display applications. The computer model has been verified against both analytic models and experimental data for unfocused devices and then applied to the study of focused structures for which sufficient models and data are not yet available. Quantitative results include electrode current-voltage characteristics and electron beam widths as a function of distance from the cathode. Practical issues such as visual image quality, electrical stress and fabrication complexity are considered to identify a practical design for use in conjunction with existing high-efficiency cathode ray tube phosphors. It is found that the addition of an integrated aperture electrode to focus the emitted electrons increases the cathode-gate drive voltage by about 30% over the case of unfocused emitters. A concentric electrode design results in only 15% increase and promises simpler fabrication. Both approaches demonstrate electron beam widths of tens of microns at anode distances of several millimeters, allowing for full-color resolution in excess of 100 lines per inch with proven color phosphors. >

Direct bonding of micromachined silicon wafers for laser diode heat exchanger applications

A novel application of silicon direct-thermal-wafer bonding is demonstrated using micromachined wafers. Two (110) Si wafers are patterned and micromachined using wet chemistry. The prepared wafers are cleaned, leaving only native chemical oxides on the Si surfaces. After the cleaning steps, the wafers are thermally bonded without electrostatic pressure. The points of bonded contact are 410 mu m*25 mu m. Hydraulic pressure testing on the bonded wafers has substantiated a bond strength comparable to contiguous SiO2. The final structure is intended as a micromachined heat exchanger for cooling laser diodes.

Phosphor challenge for field-emission flat-panel displays

The requirements of low-energy excitation combined with practical constraints of commercial supply and other issues, mandate the use of readily available commercial CRT phosphors, such as ZnS and Y2O3-based P22, for first-generation field-emission flat-panel displays. The use of these phosphors at low (e.g., ⩽2–4 kV) excitation energies places considerable problems with brightness, efficacy, spectral response, long-term reliability, screen manufacture and materials synthesis, surface conditioning and outgassing protection, and low-cost manufacturing. The tradeoffs imposed by using phosphors designed for optimum performance in the 15–30 kV range at the low voltages employed by field-emission displays are presented and discussed.

Structure and electrical characteristics of silicon field emission microelectronic devices

Field emission current was measured from arrays of wet chemically etched silicon cold-cathode diodes. Two types of cathode tips were measured both as-etched and after sharpening by low-temperature oxidation. The field enhancement increase resulting from tip sharpening is less than expected from simulation. The currents measured follow a Fowler-Nordheim characteristic and are temperature insensitive from 130 to 360 K. Turn-on voltage is near 4 V, a value much less than measured from most other field emission sources. With a 920-nm anode-cathode spacing, a minimum 0.2- mu A current per cathode was found. Telegraph noise of about 1% at 20 V was observed. These sharpened silicon tips are a viable cold cathode for vacuum microelectronics and other electron device applications. >

Phosphor selection constraints in application of gated field-emission microcathodes to flat panel displays

The major issues and tradeoffs surrounding phosphor selection for field‐emission flat panel displays are identified. The two main classes of commercially available phosphors applicable to flat panel displays are contrasted, and the major physical, electrical, chemical and optical factors effecting phosphor selection are discussed. The implications of screen layering designs and cathode materials are described as they relate to phosphor characteristics. Resolution requirements for displays severely limits the maximum anode voltage, which in turn forces specific phosphor choices. Possible solutions to these limitations are explored.

Fabrication of gated silicon field‐emission cathodes for vacuum microelectronics and electron‐beam applications

Two types of silicon gated field‐emission cathodes have been fabricated. The first is fabricated using a phosphosilicate glass insulation layer and a non‐self‐aligned etched polysilicon gate. The second type is formed by evaporating silicon dioxide and chrome onto a field‐emission point capped by its etch mask. The self‐aligned gate is then patterned by lifting‐off the cap. Cathode current of the etched‐gate structure was not measured until 150 V, at which value the device also failed. Current was measured, however, between the gate and the anode of this structure. This gate‐anode current is attributed to a nonplanar gate structure and a gate‐anode spacing of ∼0.5 μm. Gate‐anode currents follow a Fowler–Nordheim characteristic, emitting 0.16 μA/gate at 4 V. Current from the lift‐off structure was measured between the cathode and the gate. Currents averaging to 0.2 μA/tip at 23 V on the gate have been measured. These devices are intended for application to vacuum microelectronics, flat‐panel displays, and ...

The Effects of Process‐Induced Defects on the Chemical Selectivity of Highly Doped Boron Etch Stops in Silicon

Chemical etch selectivity using KOH/isopropanol and water was determined for solid-source diffused, ion implanted, and in situ, epitaxially-grown boron-doped etch stops formed in single-crystal, polycrystalline, and low-percentage germanium-alloyed silicon. Defects in the etch stops were investigated using defect etching and Nomarski optical interference microscopy, scanning electron microscopy with energy dispersive x-ray, and transmission electron microscopy. Defect density and type were found for all samples. Experimental results show that etch selectivity is not only a function of boron concentration but is also a function of the defect density, and to a lesser degree, defect type