Information
-
Patent Grant
-
6297067
-
Patent Number
6,297,067
-
Date Filed
Tuesday, October 5, 199924 years ago
-
Date Issued
Tuesday, October 2, 200122 years ago
-
Inventors
-
Original Assignees
-
Examiners
- Bowers; Charles
- Pert; Evan
Agents
- Ostrolenk, Faber, Gerb & Soffen, LLP
-
CPC
-
US Classifications
Field of Search
-
International Classifications
-
Abstract
A gate film made of conductive material is formed on a substrate, and a resist pattern having an opening with a predetermined shape is formed on the gate film. The resist pattern is reflowed to make the opening have a tapered shape. By using the resist pattern having the taper shaped opening, the gate film and substrate are anisotropically etched to form a taper shaped opening through the gate film and in the substrate to some depth. After the left resist pattern is etched and removed, the first sacrificial film is formed covering the gate film and substrate with the opening to thereby form an emitter film of conductive material on the first sacrificial film. Unnecessary portions are etched and removed to expose the emitter and gate film and complete a field emission element.
Description
This application is based on Japanese patent application No. 10-285661 filed on Oct. 7, 1998, the entire contents of which are incorporated herein by reference as non-essential subject matter.
BACKGROUND OF THE INVENTION
a) Field of the Invention
The present invention relates to a field emission element, and more particularly to manufacturing technologies for a field emission element having a field emission cathode from the tip of which electrons are emitted.
b) Description of the Related Art
A field emission element emits electrons from a sharp tip of an emitter (electron emission cathode) by utilizing electric field concentration. For example, a flat panel display can be structured by using a field emitter array (FEA) having a number of emitters disposed on the array. Each emitter controls the luminance of a corresponding pixel of the display.
In order to apply an electric field to the emitter tip of a field emission element and emit electrons therefrom, the gate electrode biased to a positive potential relative to the emitter (cathode) is disposed near the emitter electrode.
An increase in an emission current from the emitter (i.e., a lowered threshold voltage between the gate and emitter), a high speed drive, and the like are requisites for a field emission element. In order to provide these requisites, it is necessary to devise the structure and shape of a field emission element. It is also necessary to devise a manufacture method in order to manufacture such a field emission element stably and reliably.
A low threshold voltage between the gate and emitter can be achieved by making the distance between the emitter and gate electrode surfaces as gate hole of a gate electrode
100
has a rectangular cross section. There is a danger of a short circuit between the gate and emitter electrodes if a distance between the surface of the emitter electrode
110
and the upper edge of the gate electrode
100
is made short. From this reason, the gate electrode
100
cannot be disposed too near the emitter electrode
110
.
If a thickness t of the gate electrode
100
is made large, an electric field at the tip of the emitter electrode
110
becomes high so that the threshold voltage between the gate and emitter can be lowered without reducing an emission current. In other words, a larger emission current can be obtained without raising a voltage between the gate and emitter. Further, if the thickness t of the gate electrode
100
is made large, the wiring resistance of the gate electrode
100
becomes low so that a high speed drive becomes possible. However, if the thickness t of the gate electrode
100
having a rectangular cross section of the gate hole such as shown in
FIG. 16
is made large, the danger of the short circuit at the upper edge of the gate electrode
100
increases. In order to avoid this, it is necessary to set a longer distance between the tip surface of the emitter electrode
110
and the surface of the gate electrode
100
by reducing the thickness t. It is therefore difficult to lower the threshold voltage.
Another requisite for a field emission element is a sharp tip of the emitter electrode
110
. Namely, if an apex angle of the tip of the emitter electrode
110
is made small, the electric field at the tip of the emitter electrode
110
becomes high. It is therefore possible to lower a threshold voltage between the gate and emitter without reducing an emission current, and hence obtain a large emission current without raising a voltage between the gate and emitter. From this reason, it is an important issue to adopt a manufacture method capable of easily sharpening the tip of an emitter electrode
110
having a desired shape.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a manufacture method for a field emission element having a structure capable of increasing an emission current from the emitter (i.e., lowering a threshold voltage between the gate and emitter) and executing a high speed drive.
It is another object of the invention to provide a manufacturing method for a field emission element having a structure capable of disposing the emitter and gate electrodes as near as possible.
It is another object of the invention to provide a manufacture method for a field emission element capably of sharpening the tip of the emitter electrodes to a large extent.
According to one aspect of the present invention, there is provided a method of manufacturing a field emission element, comprising the steps of: (a) forming a surface layer including a gate film made of a conductive material on a substrate; (b) forming a resist pattern on the surface layer by photolithography, the resist pattern having an opening with a predetermined shape; (c) reflowing the resist pattern to make the opening have an inner diameter gradually reducing toward the substrate; (d) anisotropically etching the gate film by using the reflowed resist pattern as a mask to form an opening through the gate film, the opening having an inner diameter reducing gradually toward the substrate; (e) forming a first sacrificial film covering the gate film having the opening; (f) forming an emitter film made of a conductive material on the first sacrificial film; and (g) removing a portion or a whole of the substrate and a portion of the first sacrificial film to expose the emitter film and the gate film.
According to another aspect of the present invention, there is provided a method of manufacturing a field emission element, comprising the steps of: (a) forming a surface layer including a gate film made of a conductive material on a substrate; (b) forming a low melting point film on the surface layer by photolithography, the low melting point film having a melting point lower than a melting point of the surface layer and an opening with a predetermined shape; (c) reflowing the low melting point film to make the opening have an inner diameter gradually reducing toward the substrate; (d) anisotropically etching the gate film by using the reflowed low melting point film as a mask to form an opening through the gate film, the opening having an inner diameter reducing gradually toward the substrate; (e) forming a first sacrificial film covering the gate film having the opening and/or the low melting point film; (f) forming an emitter film made of a conductive material on the first sacrificial film; and (g) removing a portion or a whole of the substrate and a portion of the first sacrificial film to expose the emitter film and the gate film.
The opening is formed through the gate film, the opening having an inner diameter gradually reducing toward the substrate. This opening allows the surfaces of the gate and emitter electrodes to become near each other in a broad area.
Since such a gently tapered opening is formed by reflowing the resist pattern or low melting point film, the very gentle curve of the opening can be formed with good reproductivity.
By shortening the distance between the surfaces of the emitter and gate electrodes as much as possible without any danger of a short circuit, a low threshold voltage can be obtained. Furthermore, if the thickness of the gate electrode is made large, an electric field at the tip of the emitter electrode can be made strong so that the threshold voltage between the gate and emitter electrodes can be lowered without reducing the emission current. In other words, a large emission current can be obtained without raising the voltage between the gate and emitter electrodes. If the gate electrode is made thick, the gate wiring resistance lowers so that a high speed drive becomes possible.
If the etching conditions are selected, the shape of the tip of the emitter electrode can be controlled and the tip can be made sharp. Namely, by making small the apex angle of the tip of the emitter electrode, the electric field at the tip can be made strong so that the threshold voltage between the gate and emitter electrodes can be lowered without reducing the emission current, and that a large emission current can be obtained without raising the voltage between the gate and emitter electrodes.
Since the tip of the emitter electrode can be made sharp and the skirt thereof can be made gentle, the process of filling emitter material becomes easy.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A
to
1
L are cross sectional views of a substrate illustrating the manufacture processes for a two-electrode field emission element according to a first embodiment of the invention.
FIGS. 2A
to
2
F are cross sectional views of a substrate illustrating the manufacture processes for a field emission element (two-electrode element) according to a second embodiment of the invention.
FIGS. 3A
to
3
C are cross sectional views of the substrates of field emission elements according to a modification of the first embodiment.
FIG. 4
is a cross sectional view of the substrate of a field emission element according to a modification of the second embodiment.
FIGS. 5A
to
5
H are cross sectional views of a substrate illustrating the manufacture processes for a field emission element according to a third embodiment of the invention.
FIGS. 6A and 6B
are cross sectional views of the substrates of field emission elements according to a modification of the third embodiment.
FIGS. 7A
to
7
L are cross sectional views of a substrate illustrating the manufacturing processes for a field emission element according to a fourth embodiment of the invention.
FIGS. 8A
to
8
F are cross sectional views of a substrate illustrating the manufacture processes for a field emission element according to a fifth embodiment of the invention.
FIGS. 9A
to
9
C are cross sectional views of the substrates of field emission elements according to a modification of the fourth embodiment.
FIG. 10
is a cross sectional view of the substrates of a field emission element according to a modification of the fourth embodiment.
FIGS. 11A
to
11
H are cross sectional views of a substrate illustrating the manufacturing processes for a field emission element according to a sixth embodiment of the invention.
FIGS. 12A and 12B
are cross sectional views of the substrates of field emission elements according to a modification of the sixth embodiment.
FIGS. 13A
to
13
C are cross sectional views of substrates illustrating a taper shape changed with a difference of an etching rate between resist and a gate electrode film.
FIG. 14
is a perspective view of a field emission element according to an embodiment of the invention.
FIG. 15
is a cross sectional view of a flat panel display using field emission elements.
FIG. 16
is a cross sectional view of a field emission element.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIGS. 1A
to
1
L are cross sectional views of a substrate illustrating the manufacture processes for a field emission element according to the first embodiment of the invention. In the following, the manufacture processes for a two-electrode element having an emitter (field emission cathode) and a gate will be described. The two-electrode element has two electrodes, an emitter electrode for emitting electrons and a gate electrode for controlling an electric field.
Referring to
FIG. 1A
, a gate electrode film
11
made of first conductive material is formed on a single layer substrate
10
made of, for example, glass, quartz or the like or on a substrate
10
made of a lamination of an Si layer and a silicon oxide film formed thereon. The first material of the gate electrode film
11
is silicon doped with P (phosphorous) or B (boron) and deposited to a thickness of about 1 μm.
For example, the gate electrode film
11
made of silicon is formed under the conditions of a source gas of SiH
4
diluted with He and a substrate temperature of 625° C. In order to lower the resistance value of the gate electrode film
11
, P or B ions are diffused or implanted into the gate electrode film
11
.
As shown in
FIG. 1B
, resist material is coated on the gate electrode film
11
and etched through photolithography to form a resist pattern
12
having an opening
13
of about 0.45 μm in diameter. The opening
13
is cylindrical having a rectangular cross section and a circle plan view having the diameter of about 0.45 μm. This resist pattern is formed by the following processes.
For example, photoresist material of the resist pattern
12
is i-line (365 nm) resist THMR-iP3100 manufactured by Tokyo Ohka Kogyo Co. Ltd. and coated to a thickness of about 0.1 μm. In order to improve the size precision of the pattern, it is preferable to coat AZ-AQUATAR manufactured by Clariant (Japan) K.K. to a thickness of about 0.064 μm on the resist material as an upper coated antireflection film.
The resist is exposed to light by using an i-line stepper NSR2005i11D manufactured by Nikon Corp. and a reticle having a predetermined pattern under the conditions of NA=0.57 and sigma=0.4. Thereafter, the resist is developed for 60 seconds by using NMD-3 (TMAH: tetra methyl ammonium hydroxide) manufactured by Tokyo Ohka Kogyo Co., Ltd., and thereafter rinsed with pure water.
Next, the resist pattern
12
is baked (reflowed), for example, by a hot plate (not shown) at a temperature of 150° C. for about 90 seconds to form a resist pattern
12
a
having a gentle slope upper corner as shown in
FIG. 1C. A
recess
13
a
has a taper shape in cross section having an inter diameter large at the higher level and gradually making small toward a lower level. The resist pattern
12
may be exposed to ultraviolet rays before it is reflowed. When it is exposed to ultraviolet rays, the temperature for reflowing the resist pattern
12
can be lowered by about 10° C. to 20° C.
Next, by using the reflowed resist pattern
12
a
as a mask, the gate electrode film
11
made of the first conductive material is etched to form a recess (gate hole)
13
b
having a gentle tapered side wall. The recess
13
b
has a gently curved cross section gradually making its inner diameter small toward the substrate
10
(lower portion).
This etching is an over-etch to etch the substrate
10
by about 0.1 μm in depth from the surface of the substrate
10
to form a substrate
10
a
having a recess. This depth influences the length of an emitter electrode to be later formed. The diameter of the recess
13
b
of the gate electrode film
11
a
is about 0.5 μm at the bottom and about 1.5 μm at the upper level, and the depth is about 1 μm.
For example, this etching is performed by using a reactive ion etching (RIE) system under the conditions of an etching gas of CO
2
+CHF
3
+Ar and a reaction chamber pressure of 125 mTorr. Etching rate of the gate electrode film
11
is about 1.2 μm/min and etching rate of the resist pattern
12
a
is about 0.8 μm/min.
Next, as shown in
FIG. 1E
, the left resist
12
b
is removed. For example, in order to remove the resist
12
b
, H
2
SO
4
+H
2
O
2
heated to 140° C. is used. Ashing by oxygen plasma or remover such as N-methyl-2-pyrrolidone may also be used.
Next as shown in
FIG. 1F
, a first sacrificial film (insulating film)
14
of silicon oxide is isotropically deposited on the whole substrate surface to a thickness of 0.3 μm by atmospheric pressure chemical vapor deposition (CVD). For example, the first sacrificial film
14
is formed under the conditions of a source gas of O
3
and TEOS and a substrate temperature of 400° C. Under these conditions, the first sacrificial film
14
is deposited inheriting (conformal to) the side wall surface shape of the recess
13
b.
Next as shown in
FIG. 1G
, a first emitter electrode film
15
of second conductive material such as TiN
x
is isotropically deposited on the first.sacrificial film
14
to a thickness of 0.05 μm by reactive sputtering. This reactive sputtering is performed by using a DC sputtering system and Ti as a target while a gas of N
2
and Ar is introduced. The first emitter electrode film
15
is therefore formed inheriting (conformal to) the surface shape of the first sacrificial film
14
.
The emitter electrode
15
has a two-stage shape. The first stage depends upon the tapered side wall of the gate electrode
11
a
and the second stage depends upon the upper corner of the gate electrode
11
.
Next, as shown in
FIG. 1H
, a second emitter electrode film
16
made of third conductive material is deposited on the first emitter electrode film
15
to a thickness of 0.2 μm, the second electrode film
16
serving as a blanket film. For example, the third conductive material is W and is deposited by CVD under the condition of a gas of WF
6
+H
2
+N
2
+Ar, a pressure of 80 Torr and a temperature of 450° C.
Next, as shown in
FIG. 1I
, the second emitter electrode film
16
is etched back by 0.2 μm in thickness to leave only a blanket film
16
a
. The blanket film
16
a
fills a recess of the first emitter electrode film
15
. The etch-back is performed by using an RIE system under the conditions of an etching gas of SF
6
+Ar+He and a reaction chamber pressure of 280 mTorr.
It is known that in order to sufficiently stabilize an emission current from a field emission cathode (emitter), a resistor layer is connected serially to the emitter. As shown in
FIG. 1J
, therefore, a resistor layer
17
of silicon is deposited to a thickness of about 0.2 μm on the first emitter electrode
15
and blanket film
16
a
by sputtering.
The silicon film (resistor layer)
17
is sputtered by using a DC sputtering system and Si as a target while Ar gas is introduced. Instead of an Ar gas, a gas of N
2
+Ar, O
2
+Ar, or N
2
+O
2
+Ar may be used with reactive sputtering to form a resistor layer
17
of SiN
x
, SiO
x
or SiO
x
N
y
having an increased resistance value. Instead of sputtering, vapor deposition or plasma CVD may also be used for forming the resistor layer.
Next as shown in
FIG. 1K
, an emitter wiring layer
18
made of fourth conductive material such as Al is deposited on the resistor layer
17
to a thickness of about 0.5 μm by sputtering. For example, this sputtering is performed by using a DC sputtering system and Al as a target while Ar gas is introduced.
Lastly as shown in
FIG. 1L
, the substrate
10
a
and a portion of the first sacrificial film
14
are etched and removed to expose the gate electrode
11
a
and first emitter electrode
15
to complete a two-electrode element. For etching Si such as Si substrate, HF+HNO
3
+CH
3
COOH is used, and for etching a silicon oxide film or the like, HF+NH
4
F is used.
The emitter electrode
15
having a two-stage shape has a sharp tip and can easily emit electrons. Since the skirt portion of the emitter electrode can be made to have a gentle slope, a process of filing emitter material becomes easy.
According to the first embodiment, similar to the emitter electrode the gate electrode
11
a
has a tapered side wall so that a distance between the gate electrode
11
a
and emitter electrode
15
can be made short. It is therefore possible to raise an electric field intensity at the tip of the emitter electrode and manufacture a field emission element having a low threshold voltage.
Also according to the first embodiment, the position in height (position in the vertical direction) of the emitter electrode is determined by the over-etch depth of the substrate
10
shown in FIG.
1
D and the thickness of the first sacrificial film
14
shown in
FIG. 1F. A
distance between the emitter electrode
15
and gate electrode
11
a
is determined by the thickness of the first sacrificial film
14
. The shape of the emitter electrode
15
and the tapered shape of the gate hole of the gate electrode
11
a
are determined by the reflow process shown in FIG.
1
C and the etching process shown in FIG.
1
D. By properly setting these parameters, the position in height of the emitter electrode, the shapes of the gate and emitter electrodes, a distance between the emitter and gate electrodes and the like can be controlled with good reproductivity.
It has been found from the experiments made by the inventor that a slope angle θ
r
(an angle of a wall in the recess
13
a
relative to the substrate surface which angle is smaller than 90°) of the taper shaped recess
13
a
becomes smaller as the resist reflow temperature is set higher during the reflow process shown in FIG.
1
C. This tendency becomes stronger the larger the diameter of the recess
13
a
. Namely, as the diameter becomes smaller, it becomes harder for the resist to soften. The thickness of the resist also influences the slope angle. If the resist is baked at the temperature of 180° C., the resist flows completely. In the range from 100 to 140° C., the slope angle is hard to be changed. It is therefore preferable to set the reflow temperature to a range higher than 140° C. and lower than 180° C., or more preferably a range from about 150° C. to 160° C. The temperature range may change depending upon the resist material.
Experiment results of the etching process shown in
FIG. 1D
of this embodiment will be described with reference to
FIGS. 13A
to
13
C. The tapered shape of the recess
13
b
is not only determined by the shape of the resist but also can be controlled by a selection ratio (etching rate ratio) during the etching process shown in FIG.
1
D.
FIGS. 13A
to
13
D show the tapered shapes obtained by the etching processes with different selection ratios made by the inventor. Rr represents an etching rate of the resist
12
b
and Rg represents an etching rate of the gate electrode film
11
a.
The tapered shape shown in
FIG. 13A
was formed by an etching process under the condition of Rr<Rg. In this case, since a retraction (thickness reduction). of the resist during the etching process is small, the slope angle θ
g
of the gate hole cross section of the gate electrode
11
a
is large.
The tapered shape shown in
FIG. 13B
was formed by an etching process under the condition of Rr=Rg (selection ratio=1). In this case, as compared to the tapered shape shown in
FIG. 13A
, the slope angle θ
g
of the gate hole cross section of the gate electrode
11
a
becomes smaller. The tapered shape shown in
FIG. 13C
was formed by an etching process under the condition of Rr>Rg. In this case, the slope angle θ
g
of the gate hole cross section of the gate electrode
11
a
becomes much smaller and the thickness of the left resist film
12
b
reduces. The preferable condition is as follows to obtain a preferable tapered shape.
20°<θ
r
<80°
0.2<(R
r
/R
g
)<5.0
0.5<(R
r
/R
g
)<2.0(more preferable)
It is preferable that a melting point of the material of the gate electrode
11
and substrate
10
to be etched is higher than the reflow temperature of the resist. The material of the gate electrode film
11
satisfying this condition may be semiconductor such as polysilicon, metal such as W and Al and silicide such as WSi
x
and TiSi
x
. A gate electrode film
11
whose upper and lower surfaces are laminated by an insulating film such as a silicon oxide film and a silicon nitride film or an antireflection film such as TiN
x
and TiO
x
N
y
may be etched to form a tapered shape. The substrate
10
may be made of glass, quartz, silicon laminated with a silicon oxide film, aluminum laminated with an anodized aluminum film or the like.
FIGS. 2A
to
2
F are cross sectional views of a substrate illustrating the manufacture processes for a field emission element according to the second embodiment of the invention. Also in this embodiment, a two-electrode element having an emitter and gate is manufactured.
Referring to
FIG. 2A
, a gate electrode film
21
made of first conductive material is formed on a single layer substrate
20
made of, for example, glass, quartz or the like or on a substrate
10
made of a lamination of an Si layer and a silicon oxide film formed thereon. The first material of the gate electrode film
21
is silicon doped with P (phosphorous) or B (boron) and deposited to a thickness of about 0.5 μm.
For example, the gate electrode film
21
made of silicon is formed under the conditions of a source gas of SiH
4
diluted with H
e
, a substrate temperature of 625° C., and a reaction chamber pressure of 30 Pa. In order to lower the resistance value of the gate electrode film
21
, P or B ions are diffused or implanted into the gate electrode film
21
.
In order to improve the dielectric breakdown voltage between the gate and emitter electrodes and lower the capacitance therebetween, a first sacrificial film (insulating film)
22
of silicon nitride is deposited on the whole substrate surface to a thickness of 0.5 μm by low pressure CVD. For example, the low pressure CVD is performed under the conditions of a gas of NH
3
and SiH
2
Cl
2
, a substrate temperature of 750° C. and a temperature of 60 Pa. The silicon nitride film may be formed by plasma CVD or reactive sputtering.
In order to improve a size precision of a gate hole to be later formed, an antireflection film of SiN
x
, SiO
x
and TiN
x
may be used as the first sacrificial film
22
.
Instead of silicon nitride, the first sacrificial film
22
may be made of getter material such as Ti, Ta and zirconium in order to improve the vacuum degree near emitter elements of a field emission emitter array (flat panel display shown in
FIG. 15
) and suppress the attachment of molecules to the surface of the emitter electrode.
As shown in
FIG. 2B
, resist material is coated on the first sacrificial film
22
and etched through photolithography to form a resist pattern
24
having an opening
23
of about 0.45 μm in diameter at the surface level, by processes similar to those described with FIG.
1
B.
Next, the resist pattern
24
is baked (reflowed), for example, by a hot plate (not shown) at a temperature of 150° C. for about 90 seconds to form a resist pattern
24
a
having a recess
23
a
with a gentle slope upper corner as shown in FIG.
1
C. The recess
23
a
has a gentle taper shape in cross section having an inter diameter large at the higher level and gradually making small toward a lower level.
Next, by using the reflowed resist pattern
24
a
as a mask, the gate electrode film
21
made of the first conductive material and the first sacrificial film
22
are etched to form a recess (gate hole)
23
b
having a gentle tapered side wall as shown in FIG.
2
D. This etching is an over-etch to etch the substrate
20
by about 0.1 μm in depth from the surface of the substrate
20
and form a substrate
20
a
having a recess. The depth of this recess influences the length of an emitter electrode to be later formed. The diameter of the recess
23
b
of the gate electrode film
21
a
is about 0.5 μm at the bottom and about 0.7 μm at the upper level, and the depth is about 0.5 μm.
For example, this etching is performed by using a magnetron RIE system under the conditions of an etching gas of CO
2
+CHF
3
+Ar and a reaction chamber pressure of 125 mTorr. Etching rates of the gate electrode film
21
, the first sacrificial film
22
and the resist pattern
24
a
are about 1.2 μm/min, 1.2 μm/min and 0.8 μm/min, respectively.
Next, by using processes similar to those described with reference to
FIGS. 1E
to.
1
G of the first embodiment, the left resist
24
b
is removed, and thereafter as shown in
FIG. 2E
, a second sacrificial film (insulating film)
25
of silicon oxide is deposited on the whole substrate surface to a thickness of 0.15 μm by atmospheric pressure CVD, and a first emitter electrode film
26
of second conductive material such as TiN
x
is deposited on the second sacrificial film
25
to a thickness of 0.05 μm by reactive sputtering.
Next, after performing processes similar to those described with reference to
FIGS. 1H
to
1
K of the first embodiment, a blanket film
27
a
, a resistor film
28
and an emitter wiring layer
29
are sequentially laminated as shown in FIG.
2
F. Lastly, similar to the process shown in
FIG. 1L
, the substrate
20
a
and a portion of the second sacrificial film
25
are etched and removed to expose the gate electrode
21
a
and emitter electrode
26
to complete a two-electrode element.
According to the second embodiment, by using the first and second sacrificial films
22
a
and
25
a
made of insulating material are used to make the gate electrode
21
a
thinner than that of the first embodiment. Therefore, the capacitance between the gate and emitter electrodes becomes small and the dielectric breakdown voltage between the gate and emitter electrodes can be improved.
FIGS. 3A
to
3
C illustrate methods of reinforcing the emitter electrode with a support substrate according to a modification of the first embodiment.
With the method shown in
FIG. 3A
, a recess on the surface of the emitter electrode of the element subjected to the processes shown in
FIGS. 1A
to
1
G of the first embodiment is filled with a planarizing layer
19
a
made of, for example, spin-on-glass (SOG). Thereafter, the surface of the planarizing film l
9
a
is planarized by chemical mechanical polishing (CMP) or whole surface etching. Then, a support substrate
19
b
is adhered to the planarizing film
19
a
by electrostatic adhesion or by using adhesive.
Thereafter, the unnecessary portions such as substrate
10
a
are etched and removed by a process similar to the etching process shown in
FIG. 1L
, to expose the gate electrode
11
a
and emitter electrode
15
as shown in FIG.
3
A and complete a two-electrode element.
Also with the method shown in
FIG. 3B
, the emitter electrode
15
of the element subjected to the processes shown in
FIGS. 1A
to
1
G of the first embodiment is adhered to a support substrate
19
b
by using adhesive
19
c
such as low melting point glass.
Thereafter, the unnecessary portions such as substrate
10
a
are etched and removed by a process similar to the etching process shown in
FIG. 1L
, to expose the gate electrode
11
a
and emitter electrode
15
as shown in FIG.
3
B and complete a two-electrode element.
With the method shown in
FIG. 3C
, a recess on the surface of the emitter electrode of the element subjected to the processes shown in
FIGS. 1A
to
1
G of the first embodiment is filled with a planarizing layer
19
a
made of, for example, SOG. Thereafter, the surface of the planarizing film
19
a
is planarized by CMP. Then, a support substrate
19
b
is adhered to the planarizing film
19
a
and emitter electrode
15
by using adhesive
19
c
such as low melting point glass.
Thereafter, the unnecessary portions such as substrate
10
a
are etched and removed by a process similar to the etching process shown in
FIG. 1L
, to expose the gate electrode
11
a
and emitter electrode
15
as shown in FIG.
3
C and complete a two-electrode element.
FIG. 4
illustrates a method of reinforcing the emitter electrode with a support substrate according to a modification of the second embodiment.
With the method shown in
FIG. 4
, a recess on the surface of the emitter electrode
26
of the element subjected to the processes shown in
FIGS. 2A
to
2
E of the second embodiment is filled with a planarizing layer
27
f
made of, for example, SOG. Thereafter, the surface of the planarizing film
27
f
is planarized by CMP or whole surface etching. Then, a support substrate
27
s
is adhered to the emitter electrode
26
and planarizing film
27
f
by using adhesive
27
g
such as low melting glass.
Thereafter, the unnecessary portions such as substrate
20
a
are etched and removed by a process similar to the etching process shown in
FIG. 2F
, to expose the gate electrode
21
a
and emitter electrode
26
as shown in FIG.
4
and complete a two-electrode element.
FIGS. 5A
to
5
H are cross sectional views of a substrate illustrating the manufacture processes for a field emission element (three-electrode element) according to the third embodiment of the invention. The three-electrode element of the third embodiment has tree electrodes, an emitter electrode, a gate electrode and an anode electrode.
Referring to
FIG. 5A
, a substrate
30
is formed by depositing an anode electrode film
30
b
made of first conductive material to a thickness of 0.1 μm by low pressure CVD, on either a single layer substrate
30
a
made of, for example, glass, quartz or the like or on a substrate
30
a
made of a lamination of an Si layer and a silicon oxide film formed thereon, and by forming a first sacrificial film (insulating film)
30
c
made of silicon oxide on the anode electrode film
30
b
to a thickness of 0.1 μm by atmospheric pressure CVD.
In this case, the first conductive material of the anode electrode
30
b
is silicon doped with P or B in order to lower the resistance value. The anode electrode
30
b
is formed under the conditions of a source gas of SiH
4
diluted with He, a substrate temperature of 625° C. and a reaction chamber pressure of 30 Pa. The first sacrificial film
30
c
is formed under the conditions of a source gas of O
3
and TEOS. and a substrate temperature of 400° C.
On the first sacrificial film
30
c
of the substrate
30
formed as above, a gate electrode film made of second conductive material is formed. The second conductive material of the gate electrode film
31
is made of silicon doped with P (phosphorous) or B (boron) and deposited to a thickness of about 0.3 μm. For example, this gate electrode film
31
made of silicon is formed by low pressure CVD under the conditions of a source gas of SiH
4
diluted with He, a substrate temperature of 625° C. and a reaction chamber pressure of 30 Pa. In order to lower the resistance value of the gate electrode film
31
, P or B ions are diffused or implanted into the gate electrode film
31
.
Further, as shown in
FIG. 5A
, resist material is coated on the gate electrode film
31
and etched through photolithography to form a resist pattern
32
having an opening
33
of about 0.45 μm in diameter at its surface.
For example, i-line (365 nm) resist THMR-iP3100 manufactured by Tokyo Ohka Kogyo Co. Ltd. is coated to a thickness of about 0.5 μm. In order to improve the size precision of the resist pattern
32
, it is preferable to coat AZ-AQUATAR manufactured by Clariant (Japan) K.K. to a thickness of about 0.064 μm the resist material as a coated antireflection film.
The resist is exposed to light by using an i-line stepper NSR2005i11D manufactured by Nikon Corp. and a reticle having a predetermined pattern under the conditions of NA=0.57 and sigma=0.4. Thereafter, the resist is developed for 60 seconds by using NMD-3 (TMAH: tetra methyl ammonium hydroxide 2.38%) manufactured by Tokyo Ohka Kogyo Co., Ltd., and thereafter rinsed with pure water.
Next, the resist pattern
32
is baked (reflowed), for example, by a hot plate (not shown) at a temperature of 150° C. for about 90 seconds to form a resist pattern
32
a
having a recess
33
a
with a gentle slope upper corner as shown in FIG.
5
B. The recess
33
a
has a taper shape in cross section having an inter diameter large at the higher level and gradually making small toward a lower level.
Next, by using the reflowed resist pattern
32
a
as a mask, the gate electrode film
31
made of silicon and first sacrificial film
30
c
are etched to form a recess (gate hole)
33
b
having a gentle tapered side wall. It is preferable to over-etch the substrate
30
by about 0.1 μm (corresponding to a thickness of the first sacrificial film
30
c
) in depth from the surface of the substrate
30
to form a substrate
30
having a recess. The depth of the recess influences the height and length of an emitter electrode to be later formed.
For example, this etching is performed by using a magnetron RIE system under the conditions of an etching gas of CO
2
+CHF
3
+Ar and a reaction chamber pressure of 125 mTorr. Etching rates of the gate electrode film
31
and the resist pattern
32
a
are about 1.2 μm /min an 0.8 μm/min, respectively.
Next, as shown in
FIG. 5D
, the leftover resist
32
b
is removed. For example, in order to remove the resist
32
b
, H
2
SO
4
+H
2
O
2
heated to 140° C. is used. Shing by oxygen plasma or remover such as N-methyl-2-pyrrolidone may also be used
Next as shown in
FIG. 5E
, a second sacrificial film (insulating film)
34
of silicon oxide is isotropically deposited on the whole substrate surface to a thickness of 0.3 μm by atmospheric pressure CVD. For example, the second sacrificial film
34
is formed under the conditions of a source gas of O
3
and TEOS and a substrate temperature of 400° C. Under these conditions, the second sacrificial film
34
is deposited inheriting (conformal to) the side wall surface shape of the recess
33
b.
Next as shown in
FIG. 5F
, an emitter electrode film
35
of third conductive material such as TiN
x
is isotropically deposited on the second sacrificial film
34
to a thickness of 0.05 μm by reactive sputtering. This reactive sputtering is performed by using a DC sputtering system and Ti as a target while a gas of N
2
and Ar is introduced. The emitter electrode film
35
is therefore formed inheriting (conformal to) the surface shape of the second sacrificial film
34
.
Next, a resist mask (not shown) having a predetermined opening is formed on the whole surface of the emitter electrode film
35
by using ordinary photolithography. A portion of the emitter electrode
35
not used as the cathode is etched and removed through the opening to form a slit opening
36
shown in FIG.
5
G. For example, this etching is performed by using a magnetron RIE system under the conditions of a gas of Cl
2
and a reaction chamber pressure of 125 mTorr. On the second sacrificial film
34
, emitter electrodes
35
a
and
35
b
having predetermined patterns as viewed in cross section are therefore left.
Next, via the slit opening
36
, portions of the second and first sacrificial films
34
and
30
c are etched to expose the gate electrode
31
a
, emitter electrode
35
a
and anode electrode
30
b
and complete a three-electrode element as shown in FIG.
5
H. For etching the first and second sacrificial films
30
c
and
34
of silicon oxide, HF+NH
4
F is used.
The three-electrode element has the emitter electrode as a cathode and the anode electrode
30
as a plate. As a positive potential having a predetermined value is applied to the gate electrode, an electron beam is emitted from the emitter electrode
35
a
toward the anode electrode
30
b
while being converged.
FIG. 6A
shows a three-electrode element modified from a two-electrode element of the second embodiment. The manufacture method and structure of a substrate are the same as the third embodiment shown in FIG.
5
A. By using processes similar to those shown in
FIGS. 2A
to
2
E, an Si gate electrode film
21
a
, an SiN
x
second sacrificial film
22
a
, an SiO
2
third sacrificial film
25
and a TiN
x
emitter electrode
26
are formed on the substrate
30
.
Next, by using a process similar to that shown in
FIG. 5G
, a resist mask (not shown) having a predetermined opening is formed on the whole surface of the emitter electrode film
26
by using ordinary photolithography. A portion of the emitter electrode
26
a
not used as the cathode is etched and removed through the opening to form a slit opening
26
c
shown in FIG.
6
A. Next, by using a process similar to that shown in
FIG. 5H
, via the slit opening
26
c
, portions of the third and first sacrificial films
25
a
and
30
c
are etched to expose the gate electrode
21
a
, emitter electrode
26
a
and anode electrode
30
b
and complete a three-electrode element.
FIG. 6B
shows another example of a three-electrode element modified from a two-electrode element of the second embodiment. The structure different from that shown in
FIG. 6A
is that the second sacrificial film
22
a
is made of the same material SiO
2
as the material of the first and third sacrificial films
30
c
and
25
a.
By using a process similar to that shown in
FIG. 5G
, a resist mask (not shown) having a predetermined opening is formed on the whole surface of the emitter electrode film
26
by using ordinary photolithography. A portion of the emitter electrode
26
not used as the cathode is etched and removed through the opening to form a slit opening
26
c
shown in FIG.
6
B. Next, by using a process similar to that shown in
FIG. 5H
, via the slit opening
26
c
, portions of the third, second and first sacrificial films
25
a
,
22
a
and
30
c
are etched. In this case, since the third, second and first sacrificial films
25
a
,
22
a
and
30
c
are made of the same silicon oxide material, the etching rates are the same. Since the unnecessary portions are removed, the gate electrode
21
a
, emitter electrode
26
a
and anode electrode
30
b
are exposed and a three-electrode element is completed as shown in FIG.
6
B.
FIG. 7A
to
7
L are cross sectional views of a substrate illustrating the manufacture processes for a field emission element according to the fourth embodiment of the invention. In the embodiment, the manufacture processes for a two-electrode element having an emitter (field emission cathode) and a gate will be described.
In the first to third embodiments, a tapered recess is formed by heating a resist film and by using this tapered shape an emitter electrode is formed. In the fourth embodiment to be described hereinunder, instead of the resist film, a reflow film made of low melting material (material having a melting point lower than that of electrode material and other materials of layers laminated on the upper and lower surfaces of each electrode layer) is used to form a tapered recess, and an emitter electrode is formed by using this recess.
Referring to
FIG. 7A
, a gate electrode film
41
made of first conductive material is formed on a single layer substrate
40
made of, for example, quartz or the like or on a substrate
40
made of a lamination of an Si layer and a silicon oxide film formed thereon. The first conductive material of the gate electrode film
41
is silicon doped with P (phosphorous) or B (boron) and deposited to a thickness of about 0.3 μm by low pressure CVD.
For example, the gate electrode film
41
made of silicon is formed under the conditions of a source gas of SiH
4
diluted with He, a substrate temperature of 625° C. and a reaction chamber pressure of 30 Pa. In order to lower the resistance value of the gate electrode film
41
, P or B impurity ions are diffused or implanted into the gate electrode film
41
.
A low melting point film (low temperature reflow film)
42
is formed on the gate electrode film
41
. For example, the low melting point film
42
is made of phosphosilicate glass (PSG) deposited to a thickness of about 0.5 μm. Instead of PSG, other low melting glass may be used such as borophosphosilicate glass (BPSG), borosilicate glass (BSG), arsenoslilicate glass (AsSG), arsenophosphosilicate glass (AsPSG) and phosphogermanosilicate glass (PGSG), frit glass, covar, solder, Si—Ge, or low melting point metal. The low melting point film may be a single layer film or a lamination film. If a low melting point film having a lamination structure is used, the material of the uppermost layer thereof is selected to have a lowest melting point.
Next as shown in
FIG. 7B
, photoresist material is coated on the low melting point film
42
and etched through photolithography to form a resist pattern
43
having an opening
43
a
of about 0.45 μm in diameter at its surface level. This resist pattern
43
having the opening
43
a
is formed by the following processes.
For example, i-line (365 nm) resist THMR-iP3100 manufactured by Tokyo Ohka Kogyo Co. Ltd. is coated to a thickness of about 0.1 μm as the material of the resist pattern
43
. In order to improve the size precision of the resist pattern
43
, it is preferable to coat AZ-AQUATAR manufactured by Clariant (Japan) K.K. to a thickness of about 0.064 μm on the resist material as a coated antireflection film.
The resist is exposed to light by using an i-line stepper NSR2005i11D manufactured by Nikon Corp. and a reticle having a predetermined pattern under the conditions of NA=0.57 and sigma =0.4. Thereafter, the. resist is developed for 60 seconds by using NMD-3 (TMAH: tetra methyl ammonium hydroxide) manufactured by Tokyo Ohka Kogyo Co., Ltd., and thereafter rinsed with pure water.
Next, by using the resist pattern
43
as a mask, the low melting point film
42
is anisotropically etched to form a low melting point film
42
a
having an opening
42
b
as shown in FIG.
7
C. This opening
42
b
has a vertical or a generally vertical side wall reaching the gate electrode film
41
. For example, this etching is performed by using a magnetron RIE under the conditions of an etching gas of CHF
3
+CO
2
+Ar+He and a reaction chamber pressure of 50 mTorr. After the etching, the leftover resist is removed. In order to prevent the resist from being softened during the etching, it is preferable to cool the bottom surface of the substrate
40
with He.
Next, the low melting point film
42
a
is heated and reflowed to form a low melting point film pattern
42
d
having a tapered recess
42
c
with a gentle slope upper corner or shoulder, as shown in FIG.
7
D. For example, this reflow process is performed by using a furnace under the conditions of an atmosphere of N
2
and a temperature of 1000° C. for about 30 minutes. Lamp heating or laser heating may also be used. The specific conditions and the like of the reflow process will be detailed with reference to the accompanying drawings after the description of the processes of this embodiment.
Next, by using the reflowed low melting point film pattern
42
d
as a mask, the gate electrode film
41
made of the first conductive material is etched to form a recess (gate hole)
42
e
having a gentle tapered side wall, as shown in FIG.
7
E and to form a reduced thickness low melting point film pattern
42
f
. This etching is an over-etch to etch the substrate
40
by about 0.1 μm in depth from the surface of the substrate
40
to form a substrate
40
a
having a recess. The depth of this recess influences the length and height of an emitter electrode to be later formed. The diameter of the recess
42
e
of the gate electrode film
41
a
is about 0.51 μm at the bottom and about 0.7 μm at the upper level, and the depth is about 0.3 μm. The depth of this recess influences the length and height of an emitter electrode to be later formed. The diameter of the recess
42
e
of the gate electrode film
41
a
is about 0.5 μm at the bottom and about 0.7 μm at the upper level, and the depth is about 0.3 μm.
For example, this etching is performed by using a magnetron RIE system under the conditions of an etching gas of CO
2
+CHF
3
+Ar and a reaction chamber pressure of 125 mTorr. Etching rates of the gate electrode film
41
and the low melting point film pattern
42
d
are about 1.2 μm/min and 1.0 μm/min, respectively.
Next, as shown in
FIG. 7F
, a first sacrificial film (insulating film)
44
of silicon oxide is isotropically deposited on the whole substrate surface to a thickness of 0.3 μm by atmospheric pressure CVD. For example, the first sacrificial film
44
is formed under the conditions of a source gas of O
3
and TEOS and a substrate temperature of 400° C. Under these conditions, the first sacrificial film
44
is deposited inheriting (conformal to) the side wall surface shape of the recess
42
e.
Next as shown in
FIG. 7G
, a first emitter electrode film
45
of second conductive material such as TiN
x
is isotropically deposited on the first sacrificial film
44
to a thickness of 0.05 μm by reactive sputtering. This reactive sputtering is performed by using a DC sputtering system and Ti as a target while a gas of N
2
+Ar is introduced. The first emitter electrode film
45
is therefore formed inheriting (conformal to) the surface shape of the first sacrificial film
44
.
Next, as shown in
FIG. 7H
, a second emitter electrode film
46
made of third conductive material is deposited on the first emitter electrode film
45
to a thickness of 0.2 μm, the second electrode film
46
serving as a blanket film. For example, the third conductive material is W and is deposited by CVD under the condition of a gas of WF
6
+H
2
+N
2
+Ar, a pressure of 80 Torr and a temperature of 450° C.
Next, as shown in
FIG. 7I
, the second emitter electrode film
46
is etched back by 0.2 μm in thickness to leave only a blanket film
46
a
. The etch-back is performed by using an RIE system under the conditions of an etching gas of SF
6
+Ar+He and a reaction chamber pressure of 280 mTorr.
It is known that in order to sufficiently stabilize an emission current from a field emission cathode (emitter), a resistor layer is connected serially to the emitter. As shown in
FIG. 7J
, therefore, a resistor layer
47
of silicon is deposited to a thickness of about 0.2 μm on the first emitter electrode
45
and blanket film
46
a
by sputtering.
The resistor layer
47
is sputtered by using a DC sputtering system and Si as a target while Ar gas is introduced. Instead of an Ar gas, a gas of N
2
+Ar, O
2
+Ar, or N
2
+O
2
+Ar may be used with reactive sputtering to form a resistor layer
47
of SiN
x
, SiO
x
or SiO
x
N
y
having an increased resistance value. Instead of sputtering, vapor deposition or plasma CVD may also be used for forming the resistor layer.
Next as shown in
FIG. 7K
, an emitter wiring layer
48
made of fourth conductive material such as Al is deposited on the resistor layer
47
to a thickness of about 0.5 μm by sputtering. For example, this sputtering is performed by using a DC sputtering system and Al as a target while Ar gas is introduced.
Lastly as shown in
FIG. 7L
, the substrate
40
a
and portions of the first sacrificial film
44
and low melting point film
42
b
are etched and removed from the bottom side to expose the gate electrode
41
a
and first emitter electrode
45
to complete a two-electrode element. For etching Si such as the Si substrate
40
a
, HF+HNO
3
+CH
3
COOH is used, and for etching a silicon oxide film, PSG (low melting point film) or the like, HF+NH
4
F is used.
The effects of the fourth embodiment are similar to those of the first to third embodiments. According to the fourth embodiment, similar to the emitter electrode
45
, the gate electrode
41
a
has a tapered side wall so that a distance between the gate electrode
41
a
and emitter electrode
45
can be made short. It is therefore possible to raise an electric field intensity at the tip of the emitter electrode and manufacture a field emission element having a low threshold voltage. Also according to the fourth embodiment, the position in height of the emitter electrode
45
is determined by the over-etch depth of the substrate
40
shown in FIG.
7
E and the thickness of the first sacrificial film
44
shown in
FIG. 7F. A
distance between the emitter electrode
45
and gate electrode
41
a
is determined by the thickness of the first sacrificial film
44
. The shape of the emitter electrode
45
and the tapered shape of the gate hole of the gate electrode
41
a
are determined by the reflow process shown in FIG.
7
D and the etching process shown in FIG.
7
E. By properly setting these parameters, the position in height of the emitter electrode, the shapes of the gate and emitter electrodes, a distance between the emitter and gate electrodes and the like can be controlled with good reproductivity.
Reflow of the low melting point film of the fourth embodiment will be described more specifically. Other usable low melting point materials include low melting point glass such as PSG, BPSG, BSG, AsSG, AsPSG, and PGSG, frit glass, covar, solder, Si—Ge, or low melting point metal. These materials can be reflowed by baking in a furnace in a temperature range from 750° C. to 940° C. The reflow temperature depends on an impurity concentration.
It is required that the gate electrode film laminated with a low melting point film and upper and lower layers formed on the gate electrode film have a melting point higher than the reflow temperature. A melting point of polysilicon and amorphous silicon is 1412° C., and the melting points of refractory metals of W and Mo are 3377° C. and 2622° C., respectively. The melting points of refractory silicide films such as WSi
2
, MoSi
2
, TiSi
2
and TaSi
2
are 1887° C., 1980° C., 1538° C., and 2200° C., respectively. The melting points of wiring metal materials of Cu, Ag, Au, Rh, Ir, Co and Ni are 1080° C., 960° C., 1063° C., 1966° C., 2450° C., 1495° C. and 1453° C., respectively. These materials have a melting point higher than the reflow temperatures of low melting point film materials such as PSG, BPSG, BSG, AsSG, AsPSG, PGSG, and Si-Ge, and can be used as the materials of films used in the embodiment.
Al or Al alloy used as the gate electrode film material has a melting point of 660° C. which is lower than the reflow temperatures of PSG, BPSG, BSG, AsSG, AsPSG, PGSG, and Si—Ge, and the like. In this case, flowable SOG can be used. For example, FOx (flowable oxide) manufactured by Dow Corning can be used for taper etching low melting point metal such as Al since Fox softens at 200° C. or higher. The low melting point material of this embodiment can therefore be used for a reflow process at a lower temperature.
FIGS. 8A
to
8
F are cross sectional views of a substrate illustrating the manufacture processes for a field emission element according to the fifth embodiment of the invention. In the embodiment, the manufacture processes for a two-electrode element having an emitter (field emission cathode) and a gate will be described. Also in this embodiment, a tapered shape is formed by reflowing a low melting point film.
Referring to
FIG. 8A
, a gate electrode film
51
made of first conductive material is formed on a single layer substrate
50
made of, for example, quartz or the like or on a substrate
50
made of a lamination of an Si layer and a silicon oxide film formed thereon. The first conductive material of the gate electrode film
51
is silicon doped with P (phosphorous) or B (boron) and deposited to a thickness of about 0.3 μm by low pressure CVD.
For example, the gate electrode film
51
made of silicon is formed under the conditions of a source gas of SiH
4
diluted with He, a substrate temperature of 625° C. and a reaction chamber pressure of 30 Pa. In order to lower the resistance value of the gate electrode film
51
, P or B ions are diffused or implanted into the gate electrode film
51
.
A low melting point film (low temperature reflow film)
52
is formed on the gate electrode film
51
. For example, the low melting point film
52
is made of PSG deposited to a thickness of about 0.5 μm. Instead of PSG, other materials such as BPSG, BSG, AsSG, AsPSG, PGSG and Si—Ge may be used.
Next, photoresist material is coated on the low melting point film
52
and etched through photolithography to form a resist pattern
53
having an opening
53
a
. The low melting point film
52
is isotropically etched via the opening
53
a
to some extent. In this case, an opening
53
b
in the low melting point film
52
expands in a lateral direction by side etching. Next, the low melting point film
52
is anisotropically etched in a vertical direction to expose the partial surface of the gate electrode film
51
.
Next, as shown in
FIG. 8B
, the resist mask
53
is removed. By using a process similar to that shown in
FIG. 7D
of the fourth embodiment, the low melting point film
52
is heated and reflowed to form a gentle tapered shape as shown in FIG.
8
C. Since the low melting point film
52
shown in
FIG. 8B
has the opening
53
b
, a low melting point film
52
a
having a tapered opening
53
c
can be formed easily.
Next, by using the reflowed low melting point film pattern
52
a
as a mask, the gate electrode film
51
made of the first conductive material and the low melting point film
52
a
(changed to a film
52
b
) are etched to form a recess (gate hole)
52
c
having a gentle tapered side wall, as shown in FIG.
8
D. This etching is preferably an over-etch to etch the substrate
50
by about 0.1 μm in depth from the surface of the substrate
50
. The depth of this over-etch influences the length and height of an emitter electrode to be later formed. The diameter of the recess
52
c
of the gate electrode film
51
a
is about 0.5 μm at the bottom and about 0.7 μm at the upper level, and the depth is about 0.3 μm.
For example, this etching is performed by using a magnetron RIE system under the conditions of an etching gas of CO
2
+CHF
3
+Ar and a reaction chamber pressure of 125 mTorr. Etching rates of the gate electrode film
51
and the low melting point film pattern
52
a
are 1.2 μm/min and 0.9 μm/min, respectively.
Next, as shown in
FIG. 8E
, a first sacrificial film (insulating film)
54
of silicon oxide is isotropically deposited on the whole substrate surface to a thickness of 0.3 μm by atmospheric pressure CVD. For example, the first sacrificial film
54
is formed under the conditions of a source gas of O
3
and TEOS and a substrate temperature of 400° C. Under these conditions, the first sacrificial film
54
is deposited inheriting (conformal to) the side wall surface shape of the recess
52
c.
Next, a first emitter electrode film
55
of second conductive material such as TiN
x
is isotropically deposited on the first sacrificial film
54
to a thickness of 0.05 μm by reactive sputtering. This reactive sputtering is performed by using a DC sputtering system and Ti as a target while a gas of N
2
+Ar is introduced. The first emitter electrode film
55
is therefore formed inheriting (conformal to) the surface shape of the first sacrificial film
54
.
Next, by using processes similar to those of the fourth embodiment shown in
FIGS. 7H
to
7
K, a second emitter electrode film
56
made of third conductive material is deposited and etched back to leave a blanket film
56
a
, and a resistor layer
57
of silicon is deposited to a thickness of about 0.2 μm on the first emitter electrode
55
and blanket film
56
a
by sputtering. An emitter wiring layer
58
made of fourth conductive material such as Al is deposited on the resistor layer
57
to a thickness of about 0.5 μm by sputtering.
Lastly, by using a process similar to that of the fourth embodiment shown in
FIG. 7L
, the substrate
50
a
and portions of the first sacrificial film
54
and low melting point film
52
b
are etched and removed from the bottom side to expose the gate electrode
51
a
and first emitter electrode
55
to complete a two-electrode element, as shown in FIG.
8
F.
FIGS. 9A
to
9
C illustrate methods of reinforcing the emitter electrode with a support substrate according to a modification of the fourth embodiment.
With the method shown in
FIG. 9A
, a recess on the surface of the emitter electrode
45
of the element subjected to the processes shown in
FIGS. 7A
to
7
G of the fourth embodiment is filled with a planarizing layer
49
a
made of, for example, spin-on-glass (SOG). Thereafter, the surface of the planarizing film
49
a
is planarized by chemical mechanical polishing (CMP). Then, a support substrate
49
b
is adhered to the planarizing film
49
a
by electrostatic adhesion or by using adhesive.
Thereafter, by using a process similar to that shown in
FIG. 7L
, the unnecessary portions such as substrate
40
a
are etched and removed to expose the gate electrode
41
a
and emitter electrode
45
as shown in FIG.
9
A and complete a two-electrode element.
Also with the method shown in
FIG. 9B
, the emitter electrode
45
of the element subjected to the processes shown in
FIGS. 7A
to
7
G of the fourth embodiment is adhered to a support substrate
49
b
by using adhesive
49
c
such as low melting point glass. Thereafter, the unnecessary portions such as substrate
40
a
are etched and removed by a process similar to the etching process shown in
FIG. 7L
, to expose the gate electrode
41
a
and emitter electrode
45
as shown in FIG.
9
B and complete a two-electrode element.
With the method shown in
FIG. 9C
, a recess on the surface of the emitter electrode
45
a
of the element subjected to the processes shown in
FIGS. 7A
to
7
G of the fourth embodiment is filled with a planarizing layer
49
a
made of, for example, SOG. Thereafter, the surface of the planarizing film
49
a
is planarized by CMP. Then, a support substrate
49
b
is adhered to the planarizing film
49
a
and emitter electrode
45
a
by using adhesive
49
c
such as low melting point glass.
Thereafter, the unnecessary portions such as substrate
40
a
are etched and removed by a process similar to the etching process shown in
FIG. 7L
, to expose the gate electrode
41
a
and emitter electrode
45
as shown in FIG.
9
C and complete a two-electrode element.
FIG. 10
illustrates a method of reinforcing the emitter electrode with a support substrate according to another modification of the fourth embodiment. This method is the same as the method shown in
FIG. 9C
excepting that BPSG is used as the material of a low melting point film
42
h
. An etching rate of BPSG is about one fourth of that of SiO
2
made of a gas of O
3
and TEOS. Since the etching rate of the low melting point film
42
h
made o BPSG is slow, it is possible to prevent the low melting point film
42
h
from being etched completely, and a short circuit of the gate electrode
41
a
and emitter electrode
45
from being formed.
FIGS. 11A
to
11
H are cross sectional views of a substrate illustrating the manufacture processes for a field emission element (three-electrode element) according to the sixth embodiment of the invention. Also in the embodiment, a tapered shape is formed by reflowing a low melting point film.
Referring to
FIG. 11A
, a substrate
60
is formed by depositing an anode electrode fmm
60
b
made of first conductive material to a thickness of 0.1 μm by low pressure CVD, on either a single layer substrate
60
a
made of, for example, glass, quartz or the like or on a substrate
60
a
made of a lamination of an Si layer and a silicon oxide film formed thereon, and by forming a first sacrificial film (insulating film)
60
c
made of silicon oxide on the anode electrode film
60
b
to a thickness of 0.1 μm by atmospheric pressure CVD.
In this case, the first conductive material of the anode electrode
60
b
is silicon doped with P or B in order to lower the resistance value. The anode electrode
60
b
is formed under the conditions of a source gas of SiH
4
diluted with He, a substrate temperature of 625° C. and a reaction chamber pressure of 30 Pa. The first sacrificial film
60
c
is formed under the conditions of a source gas of O
3
and TEOS and a substrate temperature of 400° C.
On the first sacrificial film
60
c
of the substrate
60
formed as above, a gate electrode film
61
made of second conductive material is formed. The second conductive material of the gate electrode film
61
is made of silicon doped with P (phosphorous) or B (boron) and deposited to a thickness of about 0.3 μm by low pressure CVD. For example, this gate electrode film
61
made of silicon is formed by low pressure CVD under the conditions of a source gas of SiH
4
diluted with He, a substrate temperature of
625
° C. and a reaction chamber pressure of 30 Pa. In order to lower the resistance value of the gate electrode film
61
, P or B ions are diffused or implanted into the gate electrode film
61
.
A low melting point film (low temperature reflow film)
62
is formed on the gate electrode film
61
. For example, the low melting point film
62
is made of PSG deposited to a thickness of about 0.5 μm. Instead of PSG, other materials such as BPSG, BSG, AsSG, AsPSG, PGSG and Si—Ge may be used.
Next, by using a process similar to that of the fourth embodiment shown in
FIG. 7B
, photoresist material is coated on the low melting point film
62
and etched through photolithography to form a resist pattern
63
having an opening
63
a
about 0.41 μm in diameter at its surface level.
By using the resist pattern
63
as a mask, the low melting point film
62
is isotropically etched to form an opening
62
b
having a vertical or generally vertical side wall reaching the gate electrode film
61
, as shown in FIG.
11
B. For example, this etching is performed by using a magnetron RIE under the conditions of an etching gas of CHF
3
+CO
2
+Ar and a reaction chamber pressure of 50 mTorr. In order to prevent the resist from being softened during the etching, it is preferably to cool the bottom surface of the substrate
60
with He.
Next, by using a process similar to that of the fourth embodiment shown in
FIG. 7D
, the low melting point film
62
b
is heated and reflowed to form a low melting point film pattern
62
d
having a tapered recess
62
c
with a gentle slope upper corner or shoulder.
Next, by using a process similar to that of the fourth embodiment shown in
FIG. 7E
, the gate electrode film
61
made of the first conductive material and the low melting point pattern
62
d
are etched to form a recess (gate hole)
62
e
having a gentle tapered side wall such as shown in
FIG. 11D
, by using the reflowed low melting point film pattern
62
d
as a mask. This etching is an over-etch to etch a portion of the first sacrificial film
60
c
and expose the anode electrode
60
b
of the substrate
60
and to form a reduced thickness low melting point film pattern
62
f
. Etching rates of the gate electrode film
61
and the low melting point pattern
62
d
are about 1.2 μm/min and 0.9 μm/min, respectively.
Next, by using a process similar to that of the fourth embodiment shown in
FIG. 7F
, a second sacrificial film (insulating film)
64
of silicon oxide is isotropically deposited on the whole substrate surface to a thickness of 0.3 μm by atmospheric pressure CVD as shown in FIG.
11
E.
Next, by using a process similar to that of the fourth embodiment shown in
FIG. 7F
, an emitter electrode film
65
of third conductive material such as TiN
x
is isotropically deposited on the second sacrificial film
64
to a thickness of 0.05 μm by reactive sputtering.
Next, a resist mask (not shown) having a predetermined opening is formed on the whole surface of the emitter electrode film
65
by using ordinary photolithography. A portion of the emitter electrode film
65
not used as the cathode is etched and removed through the opening to form a slit opening
66
shown in FIG.
11
G. For example, this etching is performed by using a magnetron RIE system under the conditions of a gas of Cl
2
and a reaction chamber pressure of 125 mTorr.
Next, via the slit opening
66
, portions of the second and first sacrificial films
64
and
60
c
are etched to expose the gate electrode
61
a
, emitter electrode
65
a
and anode electrode
60
b
and complete a three-electrode element. For etching the first and second silicon oxide films and PSG of the low melting point film, HF+NH
4
F is used.
FIG. 12A
shows a modification of a three-electrode element of the sixth embodiment. BPSG is used as the material of a low melting point film
62
f
, instead of PSG. Therefore, during the last process shown in
FIG. 11H
for etching and removing the unnecessary portions, the low melting point film
62
f
is hardly removed.
FIG. 12B
shows another modification of a three-electrode element of the sixth embodiment. The different structure from the three-electrode element shown in
FIG. 12A
is that a layer corresponding to the gate electrode layer
61
a
has a lamination structure of a gate electrode film
61
b
and an antireflection film
61
c
such as SiN
x
and SiO
x
N
y
and that the etching rates of first and second sacrificial films
60
c
and
64
a
are made generally equal.
Similar to a process shown in
FIG. 11G
, a resist mask having a predetermined opening is formed on an emitter electrode film
65
, and a portion of the emitter electrode
65
not used as the cathode is etched and removed via the opening to form a slit opening
66
such as shown in FIG.
11
G. Then, similar to the process shown in
FIG. 11H
, portions of the second sacrificial film
64
a
, low melting point film
62
f
and first sacrificial film
60
c
are etched and removed via the slit opening
66
. In this case, since the etching rates of the first and second sacrificial films
60
c
and
64
a
are approximately equal, the unnecessary portions are removed as shown in
FIG. 12B
to expose the gate electrode
61
b
, emitter electrode
65
a
and anode electrode
60
b
and complete a three-electrode element.
FIG. 14
is a perspective view of a three-electrode element of the third embodiment shown in FIG.
5
H. The tip of the emitter electrode
35
a
is disposed within the gate hole of the gate electrode
31
a
and has a shape edge like a needle. The three-electrode element has the emitter electrode
35
a
as a cathode and the anode electrode
30
b
as a plate. As a predetermined positive potential is applied to the gate electrode
31
a
, an electron beam is emitted from the emitter electrode
35
a
toward the anode electrode
30
b
while being converged.
FIG. 15
is a cross sectional view of a flat panel display using field emission elements of the embodiment described above. Each field emission element used is a two-electrode element formed by the manufacture method of, for example, the first embodiment.
Formed on a support substrate
71
made of insulating material, are a wiring layer
72
made of A
1
, Cu or the like and a resistor layer
73
made of polysilicon or the like. The wiring layer
72
and resistor layer
73
have predetermined patterns. On the resistor layer
73
, a number of gate electrodes
75
having a gate hole (opening) and a number of emitter electrodes
74
whose tips are disposed within the gate holes are disposed to form a field emitter array (FEA). A voltage can be applied independently to each gate electrode
75
, although the detailed structure is not shown in
FIG. 15. A
voltage can also be applied independently to each emitter electrode
74
.
Facing an electron source including the emitter electrode
74
and gate electrode
75
, an opposing substrate is disposed including a transparent substrate
76
made of glass, quartz, or the like. The opposing substrate has a transparent electrode (anode electrode)
77
made of ITO or the like disposed under the transparent electrode
76
and a fluorescent member
78
disposed under the transparent electrode
77
.
The electron source and opposing substrate are joined together via a spacer
80
made of a glass substrate and coated with adhesive, with the distance between the transparent electrode
77
and emitter electrode
74
being maintained about 0.1 to 5 mm. The adhesive may be low melting point glass.
Instead of the spacer
80
of a glass substrate, a spacer
80
made of adhesive such as epoxy resin with glass beads being dispersed therein may be used.
A getter member
81
is made of Ti, Al, Mg, or the like and prevents emitted gas from attaching again to the surface of the emitter electrode
74
.
An air exhaust pipe
79
is coupled to the opposing substrate. By using this air exhaust pipe
79
, the inside of the flat display panel is evacuated to about 10
−5
to 10
−9
Torr, and then the air exhaust pipe
79
is sealed by using a burner
82
or the like. Thereafter, the anode electrode (transparent electrode)
77
, emitter electrode
74
, and gate electrode
75
are wired to complete the flat panel display.
The anode electrode (transparent electrode)
77
is always maintained at a positive potential. A display pixel is two-dimensionally selected by an emitter wiring and a gate wiring. Namely, a field emission element at a cross point between the emitter and gate wirings applied with voltages is selected.
The emitter electrode is applied with a negative potential (or ground potential) and the gate electrode is applied with a positive potential, so that electrons are emitted from the emitter electrode toward the anode electrode. When electrons are collided with the fluorescent member
78
, light is emitted from the collided area (pixel).
As another embodiment of the invention, the first sacrificial film of the first to sixth embodiments is made of an insulating film such as a silicon oxide film SiO
x
, a silicon nitride film SiN
x
and a silicon oxynitride film SiO
x
N
y
. The first sacrificial film may be made of getter material such as Ti, Ta and zirconium.
The gate electrode and emitter electrode may be made of semiconductor such as polysilicon and amorphous silicon, silicide compound such as WSi
x
TiSi
x
and MoSi
x
, or metal such as Al, Cu, W, Mo, Ni, Cr, Hf, and TiN
x
.
The present invention has been described in connection with the preferred embodiments. The invention is not limited only to the above embodiments. It is apparent that various modifications, improvements, combinations, and the like can be made by those skilled in the art.
Claims
- 1. A method of manufacturing a field emission element, comprising the steps of:(a) forming a surface layer including a gate film made of a conductive material on a substrate; (b) forming a resist pattern on the surface layer by photolithography, the resist pattern having an opening with a predetermined shape; (c) reflowing the resist pattern to make the opening have an inner diameter gradually reducing toward the substrate; (d) anisotropically etching the gate film by using the reflowed resist pattern as a mask to form an opening through the gate film, the opening having an inner diameter reducing gradually toward the substrate; (e) forming a first sacrificial film covering the gate film having the opening; (f) forming an emitter film made of a conductive material on the first sacrificial film; and (g) removing a portion or a whole of the substrate and a portion of the first sacrificial film to expose the emitter film and the gate film.
- 2. A method according to claim 1, wherein the surface layer includes only the gate film.
- 3. A method according to claim 1, wherein:said step (c) forms the resist pattern having the opening, the opening having the inner diameter gradually reducing toward the substrate, and a cross section of the opening having a gently tapered shape; and said step (d) forms the gate film having the opening, a cross section of the opening having a gently tapered shape.
- 4. A method according to claim 1, wherein said step (d) anisotropically etches the gate film and the substrate by using the reflowed resist pattern as a mask to form the opening which passes through the gate film and forms a recess in a surface layer of the substrate.
- 5. A method according to claim 1, further comprising a step of removing the resist pattern after said step (d) and before said step (e).
- 6. A method according to claim 1, wherein said step (c) reflows the resist pattern by heat treatment.
- 7. A method according to claim 1, wherein said step (c) performs the heat treatment at a temperature higher than 140° C.
- 8. A method according to claim 7, wherein said step (c) performs the heat treatment at a temperature higher than 140° C. and lower than 180° C.
- 9. A method according to claim 6, wherein said step (c) performs the heat treatment at a temperature of 150° C. or higher.
- 10. A method according to claim 9, wherein said step (c) performs the heat treatment at a temperature range from 150° C. to 160° C.
- 11. A method according to claim 6, wherein a melting point of the gate film is higher than a temperature of the heat treatment at said step (c).
- 12. A method according to claim 6, wherein melting points of the substrate and the gate film are higher than a temperature of the heat treatment at said step (c).
- 13. A method according to claim 1, wherein:the surface layer includes only the gate film; said step (d) anisotropically etches the gate film and the substrate by using the reflowed resist pattern as a mask to form the opening which passes through the gate film and forms a recess in a surface layer of the substrate; and the method further comprises a step of removing the resist pattern after said step (d) and before said step (e).
- 14. A method according to claim 1, wherein said step (g) etches and removes a portion or a whole of the substrate and a portion of the first sacrificial film.
- 15. A method according to claim 1, wherein said step (g) etches and removes a whole of the substrate and a portion of the first sacrificial film.
- 16. A method according to claim 1, wherein:the substrate includes an anode film made of a conductive material and an insulating film formed on the anode film; and said step (g) removes a portion of the first sacrificial film and a portion of the insulating film to expose the emitter film, the gate film and the anode film.
- 17. A method according to claim 1, wherein the first sacrificial film is made of an insulating material.
- 18. A method according to claim 1, further comprising a step of filling a recess on a surface of the emitter electrode with a blanket film made of a conductive material, after said step (f) and before said step (g).
- 19. A method according to claim 1, further comprising a step of adhering the emitter electrode to a support substrate, after said step (f) and before said step (g).
- 20. A method according to claim 1, wherein:the surface layer has the gate film and a second sacrificial film formed on the gate film; said step (d) forms an opening through the second sacrificial film and the gate film; and said step (e) forms the first sacrificial film covering the surface layer with the opening.
- 21. A method according to claim 20, wherein the second sacrificial film is an antireflection film.
- 22. A method according to claim 21, wherein the antireflection film is made of SiNx, SiOxNy or TiNx.
- 23. A method according to claim 20, wherein the second sacrificial film is made of a getter material.
- 24. A method according to claim 23, wherein the getter material is Ti, Ta or zirconium.
- 25. A method according to claim 20, wherein the second sacrificial film is made of an insulating material.
Priority Claims (1)
Number |
Date |
Country |
Kind |
10-285661 |
Oct 1998 |
JP |
|
US Referenced Citations (1)
Number |
Name |
Date |
Kind |
5599749 |
Hattori |
Feb 1997 |
|