David A. Crewe

Sub-centimeter micromachined electron microscope

A new approach for fabricating macroscopic (∼10×10×10 mm3) structures with micrometer accuracy has been developed. This approach combines the precision of semiconductor processing and fiber optic technologies. A (100) silicon wafer is anisotropically etched to create four orthogonal v‐grooves and an aperture on each 10×12 mm die. Precision 308 μm optical fibers are sandwiched between the die to align the v‐grooves. The fiber is then anodically bonded to the die above and below it. This procedure is repeated to create thick structures and a stack of 5 or 6 die will be used to create a miniature scanning electron microscope (MSEM). Two die in the structure will have a segmented electrode to deflect the beam and correct for astigmatism. The entire structure is ultrahigh vacuum compatible. The performance of a SEM improves as its length is reduced and a sub‐cm 2 keV MSEM with a field emission source should have approximately 1 nm resolution. A low‐voltage high‐resolution MSEM would be useful for the examination of biological specimens and semiconductors with a minimum of damage. The first MSEM will be tested with existing 6 μm thermionic sources. In the future a micromachined field emission source will be used. The stacking technology presented in this paper can produce an array of MSEMs 1–30 mm in length with a 1 mm or larger period. A key question being addressed by this research is the optimum size for a low‐voltage MSEM which will be determined by the required spatial resolution, field of view, and working distance.

Initial images with a partially micromachined scanning electron microscope

The focusing properties of a microfabricated silicon electrostatic electron lens have been tested in a machine tool fabricated assembly. Images of a 200 mesh gold transmission electron microscopy wire grid at a working distance of 4 mm are being obtained in transmission. The electron source is a zirconiated tungsten thermally assisted Schottky field emitter operating at 1800 K. The electron detector is a Faraday cup. The beam is scanned over the sample using parallel plate deflectors. The silicon lens is 1.64 mm long and consists of three silicon die separated by Pyrex optical fibers. Images of the grid at magnifications >10 000 × are being obtained.

Micromachined electrostatic electron source

A microfabrication technique has been developed that combines the precision of silicon micromachining and fiber optics to allow the construction of large three‐dimensional structures with dimensional tolerances approaching 1 μm [A. D. Feinerman, S. E. Shoaf, and D. A. Crewe, Proceedings of the 180th Annual ECS Conference, Phoenix, AZ, October 1991 (unpublished)]. A miniature scanning electron microscope (MSEM) is being designed using this method. In this article we will present the electron optic calculations of a simple 1 kV MSEM consisting of a source, a three element electrostatic lens, deflectors, and a detector. The MSEM measures less than one cubic centimeter. There are many advantages of a MSEM. The performance of a SEM is improved as its length is reduced. [T. H. P. Chang, D. P. Kern, and L. P. Murray, J. Vac. Sci. Technol. B 8, 1698 (1990)]. The need for mechanical adjustments and motion feedthroughs is eliminated since the microscope components are prealigned to the optic axis. All components ar...

Microfabrication of arrays of scanning electron microscopes

Arrays of miniature scanning electron microscopes (MSEMs) can conceivably solve the low throughput rates traditionally associated with direct write lithography. An inexpensive and accurate method for fabricating arrays of electron beam columns has been proposed: horizontal surface mounted electrode sectioning (slicing). This method combines the precision of semiconductor processing and fiber optic technology to fabricate macroscopic structures consisting of charged particle sources, deflecting and focusing electrodes, and detectors. Slicing can be used to miniaturize columns operating at 10–50 kV. Voltages in this range are required to produce characteristic x rays for elemental analysis. Slicing should allow the SEM to be considerably reduced in size while preserving performance and also can be adapted to produce arrays of MSEMs. The performance of the proposed sliced columns is discussed and initial results indicate that a 15 kV sliced MSEM 8.5 mm long will have 2.2 nm resolution.

High-throughput electron-beam lithography

A new approach for fabricating arrays of electron beam columns by stacking silicon wafers with micron accuracy has been developed. This approach combines the precision of semiconductor processing and fiber optic technologies. A (100) silicon wafer is anisotropically etched to create an array of apertures on the top or bottom of the wafer and four orthogonal v- grooves on both surfaces of the wafer. Precision pyrex fibers align and bond the v-grooves on the top of one wafer to the bottom of the next wafer. This procedure is repeated to create thick structures and a stack of six wafers is used to create arrays of scanning electron microscopes (SEMs). This technique is suitable for fabricating 1 - 30 mm long electron optical columns. The optimum size is determined by the desired array size, operating voltage, resolution, field of view, and working distance. The first wafer contains an array of micromachined field emission electron sources. The next three wafers accelerate and focus the electron beams. The last two wafers in the stack have electrodes to deflect each beam and correct for astigmatism. The performance of an SEM improves as its length is reduced and a subcm 2 keV SEM with a field emission source should have approximately 7 nm resolution.

Focusing properties of a micromachined electron lens

The probe forming capability of a microfabricated silicon electrostatic electron lens is under investigation. The lens measures 7 mm by 9 mm by 1.64 mm and consists of three silicon electrodes separated by Pyrex optical fibers. A test structure was designed to house the micromachined lens and a commercially available electron emitter as well as deflectors and an electron detector. Images of a 1000 mesh gold TEM wire grid at a working distance of 4 mm are being obtained at magnifications greater than 10,000 X. Data from the images will be analyzed to estimate the quality of the electron beam.

Micromachined thermionic emitter

Arrays of sputtered tungsten thermionic emitters with different shapes have been fabricated. A single filament had an emission current of 10 nAmps and a one hour lifetime. The resistance of a filament decreased an order of magnitude with 25 minutes of annealing. After emission the filament appears to have recrystallized in the emitting area. The surface roughness of the recrystallized area strongly depends on the duration of annealing and emitting time. The variation of filament resistance during operation has been investigated. A thermionic emitter has been used as a source for a sub-cm electron optical column. An array of emitters could be used as a high resolution cathode ray tube if the supports are made from a low thermal conductivity material like glassy carbon with thermal conductivity of 10-3 watt/cm-K1.

Micromachined electron source

Abstract not available.