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FIELD EMITTER ARRAYS - MORE THAN A SCIENTIFIC CURIOSITY ?
H. Gray
To cite this version:
H. Gray. FIELD EMITTER ARRAYS - MORE THAN A SCIENTIFIC CURIOSITY ?. Journal de
Physique Colloques, 1989, 50 (C8), pp.C8-67-C8-72. �10.1051/jphyscol:1989812�. �jpa-00229910�
COLLOQUE DE PHYSIQUE
Colloque C8, s u p p l h e n t au n o 11, Tome 50, novembre 1989
FIELD EMITTER ARRAYS
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MORE THAN A SCIENTIFIC CURIOSITY ?H . F . GRAY
Naval Research Laboratory, Washington, DC 20375-5000, U.S.A.
Abstract -
Field Emitter Arrays(FEAs)
have become the basis of a new microelectronics technology which we have named Vacuum Microelectronics. It combines standard solid-state fabrication and processingwithvacuum ballistic electron transport of field emitted electrons. It promises to extend the limits of todays integrated circuit technology and to address new applications such as flat paneldisplays and millimeter-wave amplification. FEAs also promise the scientific community a new class of electron sources which have the following properties: large current densities, narrow energy spread, low extraction voltages, integral focussing and deflection, optical excitation, and the possibility of multiple beams from a single chip.Solid-state microelectronics is the mainstay of the electronics industry for very good reasons, e.g. the availability of significant current densities, the attainment of high transconductance, the capability to pmvide precise doping profiles, and the ability to grow heterostructure superlattices. However, as the physical dimensions get smaller and smaller, solid-state devices begin to run into some limitations, e.g a finite saturation drift velocity, power-frequency limitations, the inability tooperate satisfactorily at high temperatures, and the lackof reliability in severe radiation environments. If one could take advantage of vacuum ballistic transport in devices which were as small as solid-state transistors, then it might be possible to address applications that are difficult, or impossible, to address with devices that are completely solid-state. These applications could take advantage of some of the properties which are intrinsic to vacuum technology, namely voltage isolation, variable electron deflection, bigh electron velocities, high temperature operation, and a possible minimization of radiation effects. However, in order to take advantage of the vacuum, one must be able to obtain current densities comparable to that found in solid-state devices. Indeed, classical electron field emission1) promises sufficient current densities. In addition, electron velocities in vacuum in microelectronic size structures may be an order of magnitude greater than that available in solids. Furthermore, if one uses metals or other high current conducting materials in field emitter based vacuum microelectronic devices, considerably higher radiation protection should be obtained Also, because of the vacuum tunneling characteristic, high temperature operation appears feasible. But the fabrication technology and known failure mecbanisms associated with electron field emitters seemed to be insolvable until the demonstration of both metal2) and semiconductor3) Field Emitter Arrays (FEAs). Consequently, the time has come when there may be specific application niches where vacuum microelectronic devices based on FEAs will have an advantage over solid-state devices.
The concept of individually gated micro-miniature field emitters was reported almost 30 years ago4). More than 10 years ago, using unique electron beam processing and fabrication techniques, it was demonstated that individual metal field emitter array elements, and arrays of these elements, could be fabricated with dimensions as small as any solid-state device2). Because the goal of this research was to develop a heaterless *coldw cathode to replace thermionic cathodes in high power RF vacuum tubes, it was demonstrated these unique metal Field Emitter Arrays (FEAs) could deliver significant macroscopic current densities ) with moderate eat=
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989812
voltages. and that these devices could operate in reasonable vacuum environments. Rut this fabrication approach has several weaknesses. First, metal field emitters have a tendency to self-destruct i.e. too much emission current from a single emitter results in "tip blowup". Second. emission uniformity from tip t o tip is difficult to obtain because of the strong field dependence on tip radius and the enormous metallic charge densities in the conduction band of metals. And third. the processing and tahrication of these structures employed unique multipiee-beam evaporation techniques in vacuum.
Using standard microlectronic processing techniques, we showed3) that it was possible t o fabricate F E k s from single c~ystal silicon wafers. The goal of this work was not t o create a "cold" cathode for high power R F tubes. but rather to develop a new type of transistor, a vacuum transistor, which would extend the operating limits of microelectronic deviws.This first semiconductor FEA is shown schematically in figure 1 where the sharp pyramids of silicon a r c crc.;itcd by opticit1 lithography and stnactard VMOS orii.ntation dependent wet etching in KOH. Figure& a scanningelectron micrographofthe first silicon PEA, has a e l l spacing of about 10 micrometers in a squarearray of point-like field emitters.
INTEGRAL GRIDDED SINGLE CRYSTAL SILICON FEA
I l l
1 1 1
! I
I+ 100 VOLTS I,
1 /
, Au CONTACTSINGLE CRYSTAL SILICON (100)
Fig.1
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A schematicdiagramofthefirstSilicon Fieldl'mitter Array. The s p a ~ i n g b e h \ ~ e ~ n rellsis ten micrometers and the designed aperture diameter is 1.5 micrometers.Fig. 2
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A Scanning Electron Micrograph of the First Silicon Field Emitter Array. Molybdenum was used for t h e g a t r metallization. and chemically deposited Sic)? was used as the insulator.Avacuum transistor can take at least two forms, namely a vertical structure similar to a permeable base transistor, or a planar structure where the source, gate, and drain are in the same plane, e.g. on the surface of the semiconductor "chipn. A schematic diagram of the vertical structure is shown in figure 3.
ULTRA FAST NO LATCH-UP PLANAR OR 3-D
OUT
I
Fig3
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A schematic diagram of a Vertical Field Emitter Array Vacuum Transistor. In concept, it is similar to a vacuum tube triode or a permeable base transistor.Note that for a spacing of one micrometer between the source (the field emitter) and drain (the electron collector), with a 100 volt bias on both the gate and drain, the total camer transit time is calculated to be about 815 ps, which corresponds to a transit time limited frequency approaching 10 Thz.
Aschematicdiagram of one possible implementation of a Planar Field Emitter Array transistor is shown in figure 4.
SILICON PLANAR FIELD EMITTER ARRAY VACUUM FET
Fig. 4
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A schyatic diagram of the first Planar Silicon Vacuum FET Transistor based on Field Emitter Arrays@%As). Note that this design allows both direct analysis by various probes as well as the ability to modify the structure by ion implantation, deposition, ktc.
Vote that rn t h ~ s particular configuration the electrons are injectedvertically into thevacuumspaceimmediately above the gate and are captured by the field produced by the collector without being intercepted by the gate, because of the non-zero momentum of the moving electrons. Note also that this particular configuration lends itself to analysis and modification by electrons, ions, neutrals, and photons. This first implementation of a planar silicon FE,A vacuum transistor. or vacuum F E T ~ ~ ) . shown in figure 5. was fabricated on a 10 ohm-cm n-type 15
< 100> silicon wafer. The width of the interdigitatcd gate and drain fingers is 20 micrometers, and the spacing hrtween them is 10 micrometers. Even with this large spacing, the calculated total translt trme 1s about S ps.
rigurc 6 is a modified Fowler-Nordheim plot of the collected current
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gate voltage characteristic of the device shown in fig. 5.INTERDIGITATED SILICON PLANAR FIELD EMllTER ARRAY VACUUM FET
GATE &f@
F"
SOURCE =SUBSTRATE
Fig. 5
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A bcanning electron iilicrogrdph of the first Planar Silicon Vacuum Transistor. ? l e width of the zate and collector fingers is 20 micrometers. and the spacing between them is 10 micrometers. Each gate finger contains 10 miniature silicon field emitters with a spacing of 10 micrometcrs.N.NPE SILICON 6 n-cm 110161srn3 PHOSPHORUS1 INTEROIGITATEO PIANAR COLLECTOR UITIP FIELD EMIHER ARRAY
Fig. 6 - A modified Fowler-Nordheim plot showing the current-voltage characteristics of the first Planar Silicon Vacuum FEl'transistor based on Field Lmitter Arrays (kbAs). Note the initiation of current saturation at high gate voltages. It is believed that this is due to velocity saturation and current crowding inside the field emitter near the emitter tip surface.
Note that the emitted electron current saturates at high electric fields inside the semiconductor field tip thereby providing an "electronic tip blunting" mechanism to prevent "tip blowup". The level of the current saturation can be changed by creating a specific doping profile inside the field tip. This current crowding mechanism is shown in figure 7. As the cross-sectional area of the emitter decreases as the electrons approach the tip surface, either the number of electrons in the conduction band or the electron velocity must increase in order to maintain current conservation. Interband excitation is considered to be small in this case. Consequently, we feel that the electron velocity increases until optical and/or acoustic phonon generation limits its growth causing the surface accumulated charge layer to become slightly depleted. The resultant flattening of the bands from expected equilibium values is also shown in figure 7.
Current Saturation In n-Type Semlconductors
Fig. 7
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A schematic diagram showing current crowding, velocity saturation, and "electronic tip blunting" inside a Silicon Field Emitter Array at high electric fields. This model is used to explain the current saturation shown in figure 6.COLLECTOR = 200 VOLTS 1 MICRON SILICON FIELD EMITTER 1.25 MICRON APERTURE
I
.. . - . MONTE CARLOTIME DEPENDENT ELECTRON SCAITERING HALF-SPACE SIMULATION
Fig.8
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Results of Monte Car10 electron scattering calculations whichshow arc breakdown of awedgc-typesilicon Field Emitter Array because of the close spacing between field tip and aperture edge. Smoothing of the aperture edge by etching eliminates the problem. In addition, the time required for charge accumulation is obtained from these calculations.Gate-breakdown resultingin the destructionof the field tip and gate metallization has been a problemwith metal field emitter arrays. An analysis of this problem and a determination of the details of the timedependent buildup of the charge accumulation a t the surface of silicon field emitters was studied using a Monte Carlo electron scattering program. The results of these calculations are shown schematically in figure 8. 8
Note that the close spacing between the gate edge and the flat surface of the wedge-type field emitter creates an electric field greater than that at the top of the emitter. This high field causes an arc breakdown initiated by field emission from the flat walls of the wedge. When gate edges are smoothed by etching thereby increasing that distance, gate breakdown is essentially eliminated.
This new electronics technology, .which we have named Vacuum Microelectronics, may even go beyond specific niche applications which are difficult, or impossible, with all solid-state microelectronics. These applications include the promise of superior full color flat panel displays that might be used for high definition television screens and computer monitors, the possibility of very high frequency amplifiers in the 100 Ghz range and above, perhaps the feasibility of extremely radiation hard and temperature insensitive electronics, and maybe even the possibility of superfast computers and memoly. In addition, tpese FEA based structures may provide a new type of electron source for high power microwave, millimeter, and submillimeter wave tubes; the possibility of a new class of multiple e-beam lithography machines which might challenge other lithographic methods, and perhaps even a new class of optically actuated electron sources and optical detectors. They might also find uses in scientific instrumentation such as the ST'M, SEM, STEM. and TEM, as well as making possible fundamental studies of bulk and surface energy levels and their state densities such as with a Field Emitter Array Appearance Potential Spectrometer (FEAAPS).
/1/ R.H. Fowler and L.W. Nordheim; Proc. R. Soc. London, A, (1928) 173.
C.A. Spindt, e t al; J. Appl. Phys., 42 (1976) 5248.
131 H. Gray, Proc. 29th I n t Field-Emission Symp., (1982) 111-118.
/4/ K Shoulders, Advances in Computers,
2
(1961) 135./5/ C.A. Spindt, e t al, Appl. of Surface Sci, 16, (1983) 268.
/6/ H.F. Gray, e t al, IEDM Tech. Dig., (1986) 776.
/7/ H.F. Gray, et al, M a t Res. Soc. Symy, 26 (1987) 25.
/8/ H. F. Gray and C. Moglestue, Presented at the First Conference on Ballistic Electrons in Transistors, Santa Barbara, California, March 1987; and at the 34th International Field Emission Symposium, Osaka, Japan, July 1987.
