Self-aligned gate field emitter device and methods for producing the same

A field emitter and its fabrication method is described in which a gate electrode is formed around and substantially encloses the emitter. The emitter is formed on a silicon substrate and is in the form of a pyramid structure. The surface of the pyramid includes an oxide layer on it. The whole device is baked until the photoresist is drawn, by surface tension, towards the base of the pyramid to expose the metal layer. Etching of the metal layer and the oxide layer produces the finished device which may suitably be employed as a switch in an electronic circuit.

The present invention relates to self-aligned gate field emitter devices 
and to methods of producing the same and has particular, although not 
exclusive, relevance to such devices as may be employed as switches in 
electronic circuits. 
The concepts of field emission, i.e. the presence of a very thin barrier 
potential at a surface from which electrons may migrate, are well known. 
Numerous devices exist which exhibit field emission. One such example is a 
sharply pointed substrate such as disclosed in "Atomically Sharp Silicon 
and Metal Field Emitters", IEEE Transactions on Electron Devices, Vol. 38, 
No. 10, Oct. 1991. This literature describes a method for producing an 
atomically sharp silicon tip of less than 10.degree.-15.degree. 
half-angle. It is known that such sharp tips provide the very thin barrier 
potential necessary for field emission. 
However during the fabrication of a device such as described above, great 
care needs to be taken to ensure that no damage occurs to the field 
emitter. This problem will be appreciated because such structures are 
generally microengineered. This term will be understood by those skilled 
in the art as meaning that fabrication is conducted on scales of around 
1.times.10.sup.-6 m. 
Furthermore if such a device is fabricated with a gate structure, as will 
generally be the case when the device is to be employed in electronic 
circuitry, then accurately positioning the gate with respect to the field 
emitter is an arduous task when the device is microengineered. 
It is thus an object of the present invention to at least alleviate the 
aforementioned problem. 
According to a first aspect of the present invention there is provided a 
self-aligned gate field emitter device comprising: a substrate carrying a 
tapered protrusion; the tapered protrusion carrying on electrically 
insulative layer at least partially covering the protrusion, the 
electrically insulative material extending along the flanks of the tapered 
protrusion from the base adjacent the substrate towards the tip of the 
protrusion remote from the substrate; electrically conductive material 
formed on the electrically insulative layer and extending further towards 
the tip of the protrusion than the insulative layer and spaced from the 
protrusion, the tapered protrusion forming the emitter of the device and 
the electrically conductive material forming the gate of the device, which 
gate, in operation of the device, provides control for the level of field 
emission from the emitter, characterized in that electrically conductive 
material is partially covered by thermoplastic material substantially 
around the base of the protrusion for supporting the electrically 
conductive material. 
Because the electrically conductive material overlies the electrically 
insulative material by some way, then a more rigid device is formed by 
provision of the thermoplastic material around the electrically conductive 
material. 
Advantageously the electrically insulative material is formed by oxidation 
of the tapered protrusion. This then obviates the need for a separate 
coating of insulative material. Alternatively it is possible for the 
insulative material to be an oxide coating formed on the protrusion. 
Additionally, the protrusion may be formed from the substrate material 
itself. 
According to a further aspect of the present invention there is provided a 
method of producing a self-aligned gate field emitter device comprising: 
providing a substrate of material from which the field emitter is to be 
produced and forming a tapered protrusion thereon; forming, on the surface 
of the protrusion, electrically insulative material; coating the 
electrically insulative material with electrically conductive material; at 
least partially coating the electrically conductive material with 
thermoplastic material; planarizing the device such that the thermoplastic 
material remains around the base of the protrusion substantially remote 
from the tip thereof to at least partially expose the electrically 
conductive material; selectively removing at least part of the 
electrically conductive material and the electrically insulative material 
thereby to define a portion of the device substantially surrounding and 
enclosing the protrusion characterized in that the planarizing comprises 
heating the thermoplastic material so that it flows and settles around the 
base of the protrusion. 
The gate is thus actually formed around the emitter and uses the emitter 
shape as a basis for its formation. Furthermore, because of the geometries 
employed, it will be apparent that such a technique requires no separate 
masking to be employed. It will also be apparent that this method allows 
exposure of the emitter to be prevented until the final steps of the 
fabrication thus reducing the tendency for it to be damaged. 
According to another aspect of the present invention there is provided a 
method of producing a self-aligned gate field emitter device in accordance 
with the first aspect of the invention comprising: providing a substrate 
of material from which the field emitter is to be produced and forming a 
tapered protrusion thereon; forming, on the tip of the protrusion a cap of 
the electrically insulative material and further forming on the surface of 
the protrusion, electrically insulative material; rotating the device 
about an axis through the tip of the protrusion and substantially 
perpendicular to the base thereof; coating the electrically insulative 
material, off-axis, whilst the device is rotating with electrically 
conductive material; selectively removing at least part of the 
electrically conductive material and the electrically insulative material, 
including the cap, thereby to define a portion of the device substantially 
surrounding and enclosing the protrusion. Thus by coating the protrusion 
with the electrically conductive material using off-axis rotation coating, 
it is possible to form the conductive material substantially along the 
flanks of the protrusion without the need to employ a separate mask. 
Preferably the formation of electrically insulative material on the 
protrusion is achieved by oxidation of the surface of the protrusion. This 
then obviates the need for a separate coating of insulative material. 
Alternatively it is possible for the insulative material to be an oxide 
coating formed on the protrusion. Additionally the protrusion may be 
formed from the substrate itself. 
In accordance with any of the aspects of the present invention, the 
protrusion may be formed from a semiconductor and this may be at least 
partially n-type doped. Alternatively, the semiconductor may be n-type 
doped at the tip and base regions of the protrusion, and p-type doped 
therebetween.

By reference firstly to FIG. 1, the basic structure from which a device in 
accordance with the present invention is fabricated will be seen. 
The structure consists of a substrate material 2 which may be chosen to be 
a semiconductor such as silicon, and supported by the silicon substrate 2 
is a tapered protrusion such as the pyramid 4. The pyramid 4 ultimately 
forms the emitter of the device as will become apparent hereafter. The 
pyramid 4 may be formed on the silicon substrate 2 by any of several ways, 
each of which will be readily apparent to those skilled in the art, yet 
such are not germane to the present invention. For example, the pyramid 
may be a polished single crystal silicon disc cut on the 100 axis and 
either formed on, or formed from the silicon substrate. The size of the 
pyramid 4 from its base to its tip is of the order 8.times.10.sup.-6 m, 
although pyramids 4 of any size may be employed. 
FIG. 2 illustrates the next stage in the fabrication of the device. The 
pyramid 4 has formed thereon an electrically insulative material such as 
an oxide layer 6. The oxide layer 6 may be formed either by oxidation of 
the surface of the pyramid 4 or by coating the pyramid 4 with an oxide. 
Either of these techniques is equally efficacious and are both well known 
to those skilled in the art. However, if the oxide layer 6 is formed by 
oxidation of the pyramid 4, then, as will be seen from FIG. 2, the tip of 
the pyramid 4 of silicon, per se, will become sharpened by this oxidation. 
This is advantageous as a separate sharpening of the pyramid 4 tip is then 
obviated. 
Reference now also to FIG. 3 shows that the oxide layer 6 is coated with 
electrically conductive material such as metal layer 8. This coating may 
be applied by any suitable technique, such as sputtering or evaporation. 
FIG. 4 illustrates that the metal layer 8 is coated with plastics material 
such a polymer of photoresist 10. In the example of FIG. 4, this 
photoresist 10 coating covers the metal layer 8 entirely and the 
photoresist 10 is deposited by any suitable technique, such as spinning. 
The next stage of the fabrication of the device as shown in FIG. 5 is to 
bake the entire device until the photoresist 10 is drawn down the pyramid 
4 towards its base by surface tension sufficiently to expose the metal 
layer 8. The degree to which the metal layer 10 needs to be exposed will 
depend, as will become apparent, upon the spacing ultimately required 
between the emitter and the gate of the device. In the present example, a 
temperature of around 140.degree. C. is sufficient to melt a typical 
positive photoresist material so that the desired effect is achieved. 
FIG. 6 illustrates the next stage of fabrication of the device in which 
both the metal layer 8 and the oxide layer 6 are selectively removed to an 
extent by, for example, etching away. Such removal techniques will be 
readily apparent to those skilled in the art and hence will not be 
referred to herein. 
It will be seen that the oxide layer 6 is etched away further towards the 
base of the pyramid 4 than the metal layer 8. This is because, in the 
finished device, the metal layer 8 will form the gate and needs to be as 
close as possible to the emitter (formed by the tip of pyramid 4) in order 
to function effectively. Removal of at least part of the metal layer 8 and 
the oxide layer 6 also exposes the tip of the pyramid 4. This tip acts as 
the emitter of the finished device. It will be apparent that the above 
fabrication stages leave the emitter of the device covered by another 
material until the final stage of fabrication, thus offering some 
protection against accidental damage. Furthermore, it will be apparent 
that the gate region (formed by the metal layer 8) has been automatically 
formed in self-alignment with the emitter region by virtue of the above 
fabrication. 
Reference to FIG. 7 illustrates the device described above in use. As has 
been detailed herebefore, the gate region is formed by the metal layer 8 
and the emitter by the pyramid 4. A power supply 12 is arranged to be 
connected to the emitter and the gate such that the emitter is at a 
negative potential with respect to the gate, with suitable biassing, 
electrons will be emitted from the tip of the pyramid 4. The gate, in this 
example, acts as a control mechanism determining the level of emission 
current. This is altered by simple adjusting of emission current. This is 
altered by simply adjusting the difference in potential between the gate 
and the emitter. 
A second embodiment of the present invention will now be described with 
reference to FIGS. 8-11 in which parts corresponding to those shown in 
FIGS. 1-7 are correspondingly numbered. The formation of the device up to 
and including the deposition of the metal layer 8 is as described before. 
However, the photoresist 10 is deposited in less abundance than previously 
such that the pyramid 4 stands proud of the photoresist 10 and has a 
portion of the metal layer 8 exposed. On baking the device, the 
photoresist, by virtue of surface tension, moves away from the tip of the 
pyramid 4 to cover only the base region thereof, as is shown in FIG. 8. 
The metal layer 8 is once again selectively removed by, for example, 
etching and the photoresist 10 is completely removed by washing in a 
suitable solvent leaving the pyramid 4 bearing the metal layer 8 only at 
the base as is shown in FIG. 9. Next, a metal plating 14 is formed on the 
metal layer 8 and at least a part of the oxide layer 6. There are various 
methods known to those skilled in the art which may achieve this, one such 
method being utilising the metal layer 8 as the deposition electrode in a 
metal electroplating bath. Reference to FIG. 10 illustrates the effect of 
forming the metal plating 14. Finally, as before, the oxide layer 6 is 
selectively removed by, for example, etching to leave the finished device 
of FIG. 11. As with the device of FIG. 7, the metal plating 14 of FIG. 11 
helps to provide support for the metal layer 8 gate structure in regions 
where it does not overlie the oxide layer 6 and is separate from the 
emitter tip, and because the plating 14 is an electrical conductor, will 
also act as the gate in tandem with metal layer 8. 
Referring now to FIGS. 12-15, a further embodiment of the present invention 
will be described. Referring firstly to FIG. 12, the pyramid 4 has formed 
on its tip, an oxide cap 16. The way in which the cap 16 is formed on the 
pyramid is not of significance to the present invention and so will not be 
described herein. Those skilled in the art will be aware of suitable 
microengineering techniques apt to achieve this structure. Next, as 
illustrated in FIG. 13, an oxide layer 6 is formed on the surface of the 
pyramid 4 in the same manner as described above. If the oxide layer 6 is 
chosen to be formed by oxidation of the silicon, then it will be apparent 
this process will not effect the cap 16 in anyway, because cap 16 is 
already an oxide. 
FIG. 14 illustrates the next stage of fabrication in which the whole device 
is rotated about an axis formed through the pyramid 4 from its tip to a 
point substantially perpendicular to its base. Thus it will be seen that 
rotation about this axis results in a rotation of the pyramid 4 about its 
point of symmetry. As before, a metal layer 8 is coated onto the oxide 
layer 6. However, it must be noted that the coating must be performed 
off-axis, as illustrated clearly in the Figure. This is necessary to 
achieve coating of the oxide layer substantially along the flanks of the 
pyramid 4. If an on-axis coating were performed, then there would be no 
metal layer 8 deposited on the flanks of the pyramid 4. 
It will be apparent that the source of the coating to provide the metal 
layer 8 should be of sufficient distance away from the device to provide a 
substantially collimated beam of coating material. During the coating, the 
cap 16 acts as a screen to prevent a metal layer 8 being formed around the 
tip region of the pyramid 4. 
Referring now to FIG. 15, it will be seen that the final stage of 
fabricating the device is to selectively remove by, for example, etching, 
the oxide layer 6 and cap 16; this stage being essentially the same as the 
similar stages described with reference to FIGS. 6 and 11. 
The device and methods for fabrication of the device described above may, 
as has been detailed, be employed as a switch in an electronic circuit. 
For this employment, it may be advantageous to dope the substrate material 
in order to achieve a more efficient switch. Reference to FIGS. 16 and 17 
illustrate this. 
Referring firstly to FIG. 17, if the final device as, for example, 
illustrated in FIG. 6 is arranged to have the silicon doped to be n-type, 
either before, during or after the fabrication, then when the power supply 
12 is connected to the gate and substrate regions appropriately, the 
device may act as a field effect device such as a MOSFET. 
Because the gate 8 (formed by the metal layer 8) is, in this example, 
biassed negatively with respect to the n-type silicon, then a depletion 
region 18 is set up adjacent the flank surfaces of pyramid 4. The 
electrons emitted via the tip of pyramid 4 are thus "pinched" through the 
channel defined by the depletion region, as is standard. The gate 8 thus 
controls the electron channel. This is, depending on the relative biassing 
of the gate 8 in relation to the n-type silicon, the width of the electron 
channel surrounded by the depletion region 18 may be controlled, and hence 
the rate of efflux of electrons from the tip of the pyramid 4. With the 
example illustrated in FIG. 16, it will be apparent that an electrode 
structure 20, positively biassed, is necessary in order to attract the 
electrons emitted from the tip of pyramid 4, because the gate 8 is 
negatively biassed. 
Referring now to FIG. 16, it will be seen that the tip and base (and the 
remainder of the silicon substrate) have been doped to be n-type, whilst 
the region of the pyramid 4 therebetween has been doped p-type. The gate 8 
is biassed by power supply 12 to be positive with respect to the silicon. 
As will be understood with this MOSFET arrangement, the positive gate 
biassing causes an n-type channel 22 to be formed along the surface of the 
flanks of pyramid 4. It is along this channel 22 that the electrons are 
attracted by the attraction of gate 8 and emitted from the tip of the 
pyramid. Those skilled in the art will appreciate that the structure of 
FIG. 17 does not require a further separate electrode structure to induce 
field emission. 
By employing a device in accordance with either of FIGS. 16 or 17, an 
efficient switch is formed as compared generally with the prior art. The 
reason for this is that by forming the gate 8 substantially along the 
flanks of pyramid 4, a greater degree of control is exercisable over the 
movement of charge carriers within those regions of the pyramid 4. 
In the above examples, the device has been described by reference to a 
pyramid. it will be understood that this is merely illustrative of a 
tapered protrusion, and other structures may equally well be employed, for 
example cones, needles or the like. 
In the above examples, all coatings have been formed completely around the 
periphery of the pyramid. This is not essential to the present invention. 
A device in accordance with the present invention function equally well if 
such coatings are substantially around the periphery of the pyramid. It 
will be apparent that this still permits the appropriate physical effects 
to be achieved. Similarly, the degree to which the coatings enclose the 
pyramid is arbitrary and to be dictated solely by the performance desired 
in the final device. Thus, for example, the metal mayer may extend up to 
the tip or only half-way between the tip and the base of the pyramid. 
Whilst in the above examples, oxide and metal have been illustrative of an 
electrical insulator and conductor respectively; it will be appreciated 
that any suitable material exhibiting the requisite physical properties 
will suffice. 
Furthermore, whilst photoresist has been described as illustrative of a 
plastics material, any material exhibiting suitable plastics properties, 
i.e. under the baking action, the material is drawn towards the base of 
the pyramid by surface tension 80 as to at least partially expose its tip, 
will suffice. 
Whilst the above examples employ microengineering fabrication techniques, 
it must be appreciated that the tip of the pyramid will have a diameter in 
the range 10.sup.-9 m in order to provide an efficient field emission. 
It will be appreciated to those skilled in the art that modifications to 
the above description are possible whilst still remaining within the scope 
of the invention, for example, it may be advantageous to have a coating of 
an oxy-nitride between the oxide and metal layers.