Gated field-emitters with integrated planar lenses

The present invention is a device for producing collimated electron beams. The device comprises a gated field emission array having at least one emission tip and a grid electrode having a grid opening disposed above the emission tip in a first direction. The device also comprises an integrated planar lens electrode for producing a focusing effect on electron beams emitted by the emission tip. The planar lens electrode has a lens edge disposed aside at a distance from the grid opening in a second direction perpendicular to the first direction. Preferably, the planar lens electrode is an integrated layer with the gated field emission array on a substrate. The grid electrode and the lens electrode can be on the same layer and separated by a gap of vacuum. The planar lens electrode can be above the grid electrode, separated by an insulative material. Similarly, the planar lens electrode can be below the grid electrode, and separated by an insulator material. Sometimes, the base electrode on which the tips are formed can act as a lens.

FIELD OF THE INVENTION 
The present invention is related in general to electrical devices. More 
specifically, the present invention is related to a lens construction for 
gated field emission arrays. 
BACKGROUND OF THE INVENTION 
Gated cathodes in the form of field emitting arrays (FEAs) were proposed by 
C. A. Spindt, I. Brodie, L. Humphrey and E. R. Westerberg (C. A. Spindt, 
I. Brodie, L. Humphrey and E. R. Westerberg, "Physical Properties of 
Thin-Film Field Emission Cathodes with Molybdenum Cones", J. Appl. Phys. 
47, 5248 (1976)). Emission is turned on or off by varying the voltage of 
the grid electrode. Fabrication using a variety of techniques has been 
demonstrated. For example, FEAs can now be fabricated on silicon wafers 
using lithographic techniques. 
The electrons emerging from an emission tip of a gated FEA typically have a 
significant angular spread, as shown in FIG. 1. If an anode at 5500 volts 
were placed at a distance of 525 .mu.m from the grid of such an emitting 
tip, the radius of the final electron spot would be about 55 .mu.m. Such 
an anode-to-grid configuration is typical of flat panel display 
applications using high-voltage phosphors. In order to prevent cross-talk 
between the pixels of a flat panel display using FEAs, each FEA would have 
to be smaller than its corresponding pixel by a boundary strip 55 .mu.m 
wide. For many applications requiring high resolution (and thus small 
pixel size), such a restriction would leave little, if any, room for the 
FEA. 
Many simulations (C. M. Tang, A. C. Ting and T. Swyden, "Field-Emission 
Arrays--A Potentially Bright Source", Nucl. Instr. and Meth. A318, 353 
(1992); W. B. Herrmannsfeldt, R. Becker, I. Brodie, A. Rosengreen and C. 
A. Spindt, Nucl. Instr. and Meth. A298, 39 (1990); and R. M. Mobley and J. 
E. Boers, "Computer simulation of Micro-Triode Performance", IEEE Trans. 
on Electron Devices, 38, 2383 (1991)) showing nearly collimated beams from 
FEAs were performed, using a thin lens-electrode configuration, where the 
voltage on the lens was lower than that of the grid and the lens opening 
was approximately equal to the grid opening. However, a large percentage 
of the emitted electrons per tip did not emerge from the lens opening. 
Besides striking the lens and/or the grid, they struck the insulator. The 
intercepted electrons were shown in (R. M. Mobley and J. E. Boers, 
"Computer simulation of Micro-Triode Performance", IEEE Trans. on Electron 
Devices, 38, 2383 (1991)), but were not shown in the figures of C. M. 
Tang, A. C. Ting and T. Swyden, "Field-Emission Arrays--A Potentially 
Bright Source", Nucl. Instr. and Meth. A318, 353 (1992) and W. B. 
Herrmannsfeldt, R. Becker, I. Brodie, A. Rosengreen and C. A. Spindt, 
Nucl. Instr. and Meth. A298, 39 (1990). Designs based on a single thin 
lower voltage lens are, therefore, not practical for many applications 
because: 
i. There is significant reduction of the extracted beam current from the 
emitting tip. 
ii. The charging of the insulator can cause breakdown. 
iii. Coating of the insulator by a conducting material has been proposed as 
a solution to drain the charge accumulation. However, the conducting 
coating also provides a leakage path between the gate and the emitter. The 
leakage of current produces power loss. 
The concept of obtaining nearly collimated beams from collimating grid FEAs 
is presented in the patent disclosure, "Integrated Collimating-Grid 
Field-Emission Arrays", by Cha-Mei Tang, Antonio C. Ting and T. A. Swyden 
(C. M. Tang, A. C. Ting and T. A. Swyden, "Collimating Grid Field-Emission 
Arrays", Navy Case No. -75,216). The opening of the grid electrode acts as 
the primary focusing lens for all electrons emitted from the tip, and the 
low voltage third electrode (in a three-electrode system) provides further 
collimation. 
The concept of obtaining collimated beams using a thick sidewall lens is 
presented in the patent disclosure, "Gated Field-Emitters with Integrated 
Focusing Sidewall Lenses", by Cha-Mei Tang and T. A. Swyden (C. M. Tang 
and T. A. Swyden, "Gated Field-Emitters with Integrated Focusing Sidewall 
Lenses", Navy Case No. -75,312). 
The primary purpose of the present invention is to produce a focusing 
effect along the edges of gated field-emission arrays (FEAs) by 
introducing a simple, low voltage lens anode around, and on the same 
substrate as, the FEA. Electrons emitted near the edge of an FEA having 
such a "planar" lens will be deflected away from the edge, resulting in a 
better collimated beam. These arrays will have applications in flat panel 
displays, as cathodes for radiation sources, as cathodes for accelerators, 
etc. 
SUMMARY OF THE INVENTION 
The present invention is a device for producing collimated electron beams. 
The device comprises a gated field emission array having at least one 
emission tip and a grid electrode having a grid opening disposed above the 
emission tip in a first direction. 
The device also comprises an integrated planar lens electrode for producing 
a focusing effect on electron beams emitted by the emission tip. The 
planar lens electrode has a lens edge disposed aside at a distance from 
the grid opening in a second direction perpendicular to the first 
direction. 
Preferably, the planar lens electrode is an integrated layer with the gated 
field emission array on a substrate. 
The grid electrode and the lens electrode can be on the same layer and 
separated by a gap of vacuum. The planar lens electrode can be above the 
grid electrode, separated by an insulative material. Similarly, the planar 
lens electrode can be below the grid electrode, and separated by an 
insulator material. Sometimes, the base electrode on which the tips are 
formed can act as a lens.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings wherein like reference numerals refer to 
similar or identical parts throughout the several views, and more 
specifically to FIGS. 2a-2c thereof, there is shown a device 10 for 
producing collimated electron beams. The device 10 comprises a gated field 
emission array 12 having at least one emission tip 14 and a grid electrode 
16 having a grid opening 18 disposed above the emission tip 14 in a first 
direction 20. 
The device 10 also comprises an integrated planar lens electrode 24 for 
producing a focusing effect on electron beams emitted by the emission tip 
14. The planar lens electrode has a lens edge 26 disposed aside at a 
distance from the grid opening 18 in a second direction 22 perpendicular 
to the first direction 20. 
Preferably, the planar lens electrode 24 is an integrated layer with the 
gated field emission array 12 on a substrate 28. The grid electrode can 
have a grid edge 30 disposed aside in a spaced relation to the grid 
opening 18 in the second direction 22. The lens edge 26 is disposed 
adjacent to the grid edge 30. 
In one embodiment, as shown in FIG. 2a, the planar lens electrode 24 and 
the grid electrode 16 are disposed on the same layer such that a space 32 
is formed between the grid edge 30 and the lens edge 26. The layer can 
comprise an insulator layer 34. 
In another embodiment, and as shown in FIG. 2b, the planar lens electrode 
24 is disposed above the grid electrode 16 relative to the first direction 
20. Preferably, the planar lens electrode 24 is separated from the grid 
electrode 16 by an insulative layer 36. 
In yet another embodiment, and as shown in FIG. 2c, the planar lens 
electrode 24 is disposed below the grid electrode 16 relative to the first 
direction 20. For instance, the planar lens electrode 24 separated from 
the grid electrode 16 by an insulator layer 34. 
The planar lens electrode 24 can be on one side of the grid opening 18 for 
edge focusing. The various embodiments with edge focusing showing equal 
potential lines 38 and electron trajectories 40 are shown in FIGS. 3a-3c. 
The planar lens electrode 24 can also surround the grid opening 18, as 
shown in FIGS. 4a-4c. For instance, the lens edge can be defined by an 
opening of the planar lens electrode. The opening surrounds and is larger 
than the grid opening 18. 
The gated field emission array 12 can comprise an array of field emitting 
tips 14 and an associated array of grid openings 18. There can be one lens 
per tip or a plurality of field emitting tips 14 and grid openings 
associated with each integrated planar lens 24. 
A unit of the invention consists of a gridded FEA 12, where the grid is 
surrounded by a lower voltage electrode 24. The beam confinement field 
only affects electrons emitted from tips near the edge of the grid 
electrode 16. The grid 16 of the FEA can be fabricated in any geometrical 
shape (circle, square, wedge, etc.). The planar lens electrode 24 is then 
fabricated in close proximity to (and therefore congruent in shape with) 
the FEA grid 16, but electrically isolated from it by a gap sufficient to 
withstand the voltage difference between them. In FIG. 2a, the grid 
electrode 16 and the lens electrode 24 are on the same layer and separated 
by a gap 32 of vacuum. In FIG. 2b, the planar lens electrode 24 is above 
the grid electrode 16, separated by an insulative material 36. Similarly, 
in FIG. 2c, the planar lens electrode 24 is below the grid electrode 16, 
and separated by an insulator material 34. Sometimes, the base electrode 
28 on which the tips 14 are formed can act as a lens, as shown in FIGS. 
2d, 3d and 4d. This unit of invention can be repeated either in the 
transverse or lateral dimension, or both. 
Configured in any of the above ways, the planar lens 24 modifies the 
electric field at the edges of the FEA 12, as demonstrated by the 
equipotential curve 38 and electron trajectories 40 of two configurations 
by focusing from one side as shown in FIGS. 3a-3d and focusing along two 
sides as shown in FIGS. 4a-4d. The field at the boundary between 
electrodes can provide focusing for electrons emitted near the edge of the 
array 12. The extent of the focusing field is on the order of tens of 
microns. The strength of the focusing field is dependent on the voltage 
difference between the lens electrode 24 and grid electrodes 16. 
An example of the planar lens electrode 24 on the same layer as the grid 
electrode 16 is shown in FIG. 5, where the grid electrode is 50 .mu.m wide 
and separated from neighboring grid electrodes by 30 .mu.m wide gap. The 
voltages at the grid 16, the tip 14 and the planar lens 24 are 90 V, 0 V 
and -50 V, respectively. The anode 42 is 500 .mu.m away, with an applied 
voltage of 5000 V. 
The electrons emerge from the grid opening 18 with initial angles within 
.+-.25. The spot size at the anode is 100 .mu.m with the planar lens, as 
shown in FIG. 5. Without the planar lens, the spot size is 150 .mu.m, as 
shown in FIG. 6. 
It should be appreciated that planar lenses can be used in conjunction with 
other types of electrostatic lenses to provide additional focusing. For 
example, linear sidewall lenses focus only in one direction (along the 
width). Planar lenses 24 can be used at either or both ends to provide 
focusing in the direction for which linear sidewall lenses have no 
focusing. 
The operation of the planar lens 24 requires application of appropriate 
voltages to the various electrodes of the device, fabricated according to 
design. A voltage is applied to the grid electrode 16, V.sub.g, to extract 
electrons from the emitter tip 14. This voltage is typically 30-300 volts 
above the voltage at the tip of the emitter 14, V.sub.t, which is taken to 
be ground. The invention can be operated by applying the appropriate 
voltage, V.sub.L, to the planar lens electrode 24, with V.sub.g &gt;V.sub.L. 
As shown in FIGS. 2d, 3d and 4d, a focusing effect can be induced simply by 
having an edge adjacent to the grid opening 18. The edge 30 can be formed 
by the grid electrode 16. 
FIG. 4d illustrates the beam confinement concept for emitters inside a 
narrow strip of grid electrode 16. The grid electrode 16 is 20 .mu.m wide. 
The voltage of the grid electrode is 150 V. On either side of the grid 
electrode, the potential is 0 V. The anode, located 100 .mu.m from the 
emitters, is held at 156 V. The electron trajectories at the anode with 
the focusing effect of the substrate acting as a planar lens is shown. 
FIG. 3d illustrates beam confinement along the edge of a grid electrode 16. 
An emitter is located 10 .mu.m to the right of the edge of a grid 
electrode 16. The voltage of the grid electrode is 150 V. To the left of 
the grid electrode 16, the potential of the substrate is 0 V. 
Thus, as shown in FIGS. 2d, 3d and 4d, the present invention is also a 
gated field emission device 50 comprising a base electrode 28 and at least 
one field emission tip 14 disposed on the base layer 28. There is a grid 
electrode 16 having an opening 18 disposed over the emission tip 14 in a 
first direction 20. The grid electrode 16 has at least one edge 30 
disposed aside in a spaced relation to the grid opening 18 in a second 
direction 22 perpendicular to the first direction 20 which by influence of 
voltage on the base layer 28 induces a focusing effect on electrons 
emitted by the emission tip 14. 
Preferably, there is an insulator 34 integrally disposed between the grid 
electrode 16 and the base electrode 28. 
The potential difference along the edge 30 affects the electric field and 
causes the electrons to deflect away from the edge 30 of the grid 
electrode 16. This edge effect focusing of FEAs was discovered after 
experimenting with 1.times.100 and 2.times.100 arrays. The FEAs used in 
the experiment are fabricated by MCNC. These FEAs are processed from 
silicon wafers, with the emitting tips on pyramids supported by 
mini-columns, similar to those reported in H. H. Busta, D. W. Jenkins, B. 
J. Zimmerman and J. E. Pogemiller, Technical Digest of the 1991 IEEE IEDM 
Meeting, pp. 213-215. The grid opening diameters are.about.1.1 .mu.m. The 
smaller grid opening diameter allows a higher packing density than was 
possible with the previous 2 .mu.m diameter MCNC silicon FEAs (G. W. 
Jones, C. T. Sume and H. F. Gray, IEEE Trans. on Components, hybrids and 
Manufacturing Tech., vol. 15, pp. 1051-1055; H. F. Gray, G. J. Campisi and 
R. F. Greene, Technical Digest of the 1986 IEEE IEDM, pp. 776-779). 
In the experiment, each chip contained 5 devices: two single tips, two 
1.times.100 arrays and one 2.times.100 array. The grid thickness was 
between 0.5-0.7 .mu.m. Emitter patterns from two different mask designs 
were tested. (i) Mask set 1: The tip-to-tip separation is 8 .mu.m. The 
total width of the grid electrode was 10 .mu.m. The emitter array was 
located at the center of the grid electrode. The grid electrodes of the 
2.times.100 arrays are separated by a gap of 10 .mu.m. (ii) Mask set 2: 
For the 1.times.100 arrays, the tip-to-tip separation was 20 .mu.m and the 
total width of the gate electrode was 40 .mu.m. For the 2.times.100 array, 
the tip-to-tip separation was 20 .mu.m in both directions and the total 
width of the grid electrode is 60 .mu.m. The edge of the grid electrode 
was 20 .mu.m to either side of the emitter arrays. 
Testing was performed using an electron gun in an UHV chamber that projects 
electrons emitted from the FEA onto a distant phosphor screen, shown in 
FIG. 7. The details are given below. 
The FEA chips were mounted using conductive silver epoxy onto 8-pin TO-5 
canisters. A TO-5 canister was then inserted into a test fixture 
consisting of a machined ceramic socket and an electrostatic Einzel lens. 
The lens was fixed at a distance of approximately half an inch from the 
centered chip and maintained at 300 Volts. The fixture was mounted in one 
port of a small UHV chamber, facing a phosphor screen held at 900 Volts. 
Data was collected using a non-transmitting phosphor screen placed at a 
45.degree. angle to the electron beam. Later the vacuum chamber was 
reconfigured to use a transparent 3 inch sapphire/phosphor/aluminum screen 
from Raytheon, Inc., placed perpendicular to the electron beam trajectory, 
shown in FIG. 7. The new screen did not have the image distortion of the 
non-transmitting screen, which resulted from the 45.degree. observation 
angle. With the phosphor screen, it was able to detect trajectory 
information in addition to voltage-current data. All chips are tested at 
around 1.times.10.sup.-8 Torr after two to three hours of baking at 
100.degree. C. 
Current to the screen was monitored by measuring the voltage drop across a 
load resistance between the screen and voltage source (with one leg of the 
voltage tied to ground). The total currents going to the tips, as well as 
grid current, were monitored using load resistances. 
To test a device on a chip, the gate to that device was connected to 
ground, while a variable negative voltage was applied to the chip 
substrate. In order to prevent other devices not being tested from 
emitting, their gates were shorted to the chip substrate 28 so that there 
will be no potential difference. 
Initially, emission patterns from 1.times.100 and 2.times.100 on screen 
were expected to be close to circular. However, elongated emission 
patterns were observed. At low emission currents, one or two thin small, 
slightly twisted images appear on the phosphor screen. Typically, the 
length of these filaments were more than 10 times their width, as shown in 
FIG. 8. We attribute each of these images to a single emitting tip. As 
current increases, the images become brighter and develops into football 
shapes, as shown in FIG. 9. As more tips emit and current increases, the 
images from the 1.times.100 arrays merge into a single large football 
shape with increasing brightness, as shown in FIG. 10. In the 2.times.100 
arrays of mask set 1, two distinguishable football images were observed, 
each attributed to an individual 1.times.100 array, as shown in FIG. 11. 
FIGS. 9 and 10 were obtained using the transmitting phosphor screen, while 
FIGS. 8 and 11 were obtained with the reflecting phosphor screen. The 
football shaped images from 1.times.100 arrays of both mask sets are 
roughly the same. 
The 2.times.100 array began to emit at a tip voltage of -66.9 V with a 
"second turn-on" voltage of -40 V. The maximum screen current obtained was 
70 .mu.A at tip voltage of -100.2 V. The voltage-current data for this 
array is shown in FIG. 12. At total current below 10 .mu.A, the image on 
screen was a roughly rectangular patch approximately 1 inch long, with two 
bright, straight, parallel line segments forming the sides, and many 
fainter filamentary structures visible inside, which we interpret as 
images of individual tips emitting. At higher currents (50-70 .mu.A), the 
images became two football shaped patches, overlapping each other, but 
slightly offset, as shown in FIG. 11. 
These experiments showed that electron beam trajectories far from field 
emitters are influenced not only by electric field formed between emitter 
and grid, but by other fields in proximity to the emission site. It was 
discovered that the field formed along the edge of the grid electrode due 
to an adjacent lower voltage potential can act as a focusing lens. This 
situation is exactly that of the narrow strip gate electrode of 
1.times.100 array surrounded by the lower voltage base electrode. The 
equipotential lines and the electron trajectories simulated by EGUN2 (W. 
B. Herrmannsfeldt, EGUN--An Electron optics and Gun Design Program, SLAC 
Report 331 (1988)), shown in FIG. 13, illustrate beam confinement for 
emitters inside a narrow strip of gate electrode. The electrons are 
assumed to be emitted with half angle up to 30.degree. . The gate 
electrode is 20 .mu.m wide. The voltage of the gate electrode is 150 V. On 
either side of the gate electrode, the potential is 0 V. The anode, 
located 100 .mu.m from the emitters, is held at 156 V. The electron 
trajectories without the focusing effect is shown in FIG. 14. 
These experiments demonstrated experimentally and theoretically a simple 
method of producing focusing for arrays of field emitters without the 
fabrication of additional focusing electrodes. The focusing mechanism 
results from fringe fields formed at the edge 30 of the grid electrode 16 
due to it being at a higher potential than its surroundings. This 
principle can be applied in a variety of configurations. 
The application requirement of the FEA determines the final design. The 
following parameters are important: i) the spot size of the electrons at 
the anode, ii) the distance between the anode and grid and iii) the anode 
voltage. The design considerations are: 
i) the size and shape of the grid 16, 
ii) the size and shape of the planar lens 24, 
iii) the voltages of the grid 16 and the planar lens 24, 
iv) the best method of fabricating the planar lens 24 and the FEA, 
v) the current density desired and 
vi) the best type of emitter technology, i.e., Si emitters, metallic 
emitters, eutectic emitters, cones, edges, etc. In practice, the design 
considerations often alter the initial desirable specifications. 
Currently, the dominant potential application for FEAs is in the area of 
flat panel displays. The pixel size of a field-emission display without 
lenses would be large, as described in Section II. The advantages of the 
application of this lens to the display are the ability to: 
i. reduce pixel sizes, 
ii. increase brightness of the pixels by increasing the number of emitting 
tips for each pixel and/or 
iii. reduce the impact of electrons on spacers, which separates the 
phosphor and the emitters. 
The planar lens electrode 24 can reduce back ion bombardment on the 
emitters 14, where the ions were produced at the anode. Following the 
field lines, the ions will go to the low voltage planar lens 24. Impact of 
ions on the lens 24 is not as detrimental as on the emitter tip 14. Planar 
lenses are simple and relatively inexpensive to fabricate and are 
insensitive to variations in fabrication. The devices 10 and 50 can be 
processed with many emitters over large areas. The devices 10 and 50 can 
be processed on the same wafer or substrate. 
There is also no high voltage gradient in the devices 10 and 50. A voltage 
gradient less than 250 V/.mu.m in the insulator 34 is sufficient for 
focusing. Insulator material that can sustain this kind of voltage 
gradient over large areas is readily available. Also, there is no high 
voltage near the emitter tip 14. This reduces the chance of secondary 
electron production and back ion bombardment. 
Quite simply, the advantage of using planar lens for FEAs to produce 
collimated beams is the feasibility and simplicity of producing spatial 
and temporal modulation. 
Utilizing FEAs in conjunction with planar lenses as electron guns, 
extremely compact, low power radiation sources are possible in the 
infrared, optical and x-ray regimes. 
The concept of the invention remains the same, independent of the 
following: 
1) Variations in the shape of the planar lens 24. 
2) Variations in the height (with respect to FEA gate), thickness and 
voltage. 
3) Materials used in the fabrication of the device. 
4) Fabrication method. 
5) The layout of the emitter tips 14 pattern inside the planar lens 24. 
6) The concept can be applied to a single emitter 14 (micro planar lens) or 
to a large group of emitters 14 (macro planar lens). 
This collimation concept can be applied alone or in conjunction with other 
focusing mechanisms. For example, when the planar lens is applied alone, 
edge focusing will produce pixel sizes of a few tens of .mu.m or larger. 
For smaller pixel sizes, this concept is most useful when it is applied 
with other lens designs: 
i) to reduce the alignment tolerance requirement of the other lens, 
ii) to focus the beams at the end of linear sidewall lenses, 
iii) to simplify the processing of other lenses, etc. We will explore the 
various possibilities. 
Although the invention has been described in detail in the foregoing 
embodiments for the purpose of illustration, it is to be understood that 
such detail is solely for that purpose and that variations can be made 
therein by those skilled in the art without departing from the spirit and 
scope of the invention except as it may be described by the following 
claims.