U.S. patent application number 09/352063 was filed with the patent office on 2002-04-25 for field emission from bias-grown diamond thin films in a microwave plasma.
Invention is credited to AUCIELLO, ORLANDO, DING, M. Q., GRUEN, DIETER M., KRAUSS, ALAN R..
Application Number | 20020048638 09/352063 |
Document ID | / |
Family ID | 23383636 |
Filed Date | 2002-04-25 |
United States Patent
Application |
20020048638 |
Kind Code |
A1 |
GRUEN, DIETER M. ; et
al. |
April 25, 2002 |
FIELD EMISSION FROM BIAS-GROWN DIAMOND THIN FILMS IN A MICROWAVE
PLASMA
Abstract
A method of producing diamond or diamond like films in which a
negative bias is established on a substrate with an electrically
conductive surface in a microwave plasma chemical vapor deposition
system. The atmosphere that is subjected to microwave energy
includes a source of carbon, nitrogen and hydrogen. The negative
bias is maintained on the substrate through both the nucleation and
growth phase of the film until the film is continuous. Biases
between -100V and -200 are preferred. Carbon sources may be one or
more of CH.sub.4, C.sub.2H.sup.2 other hydrocarbons and
fullerenes.
Inventors: |
GRUEN, DIETER M.; (DOWNERS
GROVE, IL) ; KRAUSS, ALAN R.; (NAPERVILLE, IL)
; DING, M. Q.; (BEIJING, CN) ; AUCIELLO,
ORLANDO; (BOLINGBROOK, IL) |
Correspondence
Address: |
HARRY M LEVY ESQ
EMRICH & DITHMAR
300 S WACKER DRIVE STE 3000
CHICAGO
IL
60606
|
Family ID: |
23383636 |
Appl. No.: |
09/352063 |
Filed: |
July 14, 1999 |
Current U.S.
Class: |
427/577 ;
427/249.12; 427/249.7; 427/249.8; 427/902 |
Current CPC
Class: |
Y10S 427/104 20130101;
C23C 16/279 20130101; C23C 16/26 20130101; C23C 16/274 20130101;
H01J 9/025 20130101 |
Class at
Publication: |
427/577 ;
427/249.7; 427/249.8; 427/249.12; 427/902 |
International
Class: |
H05H 001/24; C23C
016/27 |
Goverment Interests
[0001] The United States Government has rights in this invention
pursuant to Contract No. W-31-109-ENG-38 between the U.S.
Department of Energy and The University of Chicago representing
Argonne National Laboratory.
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. A method of producing diamond or diamond like films comprising
establishing a negative bias on a substrate with an electrically
conductive surface in a microwave plasma chemical vapor deposition
system with the atmosphere subjected to the microwave energy
including a source of carbon, nitrogen and hydrogen, maintaining
the negative bias on the substrate through both the nucleation and
growth phase of the film until the film is continuous.
2. The method of claim 1, wherein the negative bias is maintained
at not less than about -100v during the growth phase of the
film.
3. The method of claim 1, wherein the negative bias is maintained
in the range of from about -100v to about -200v during the growth
phase of the film.
4. The method of claim 1, wherein the negative bias is maintained
in the range of from about -100v to about -200v during the
nucleation and the entire growth phase of the film.
5. The method of claim 1, wherein the microwave energy is about 600
watts.
6. The method of claim 1, wherein the nitrogen concentration is not
less than about 1 volume percent.
7. The method of claim 1, wherein the nitrogen concentration is in
the range of from about 1 volume percent to about 5 volume
percent.
8. The method of claim 1, wherein the source of carbon is present
at a concentration in the range of from about 1 volume percent to
about 20 volume percent, based on CH.sub.4.
9. The method of claim 8, wherein the carbon source is one or more
of CH.sub.4, C.sub.2H.sub.2, other hydrocarbons, a fullerene or
mixtures thereof.
10. The method of claim 9, wherein the source of carbon is present
at a concentration in the range of from about 5 volume percent to
about 20 volume percent, based on CH.sub.4.
11. The method of claim 1, wherein the film is at least 2000 .ANG.
thick.
12. The method of claim 1, wherein the film is about 6000 .ANG.
thick.
Description
TECHNICAL FIELD
[0002] This invention relates to a novel method of depositing
diamond or diamond-like substances in a microwave plasma chemical
vapor deposition system to produce a film having superior field
emission properties.
BACKGROUND OF THE INVENTION
[0003] Diamond and diamond-like carbon (DLC) thin films have
recently attracted much attention due to their potential
applications in vacuum microelectronics. Apart from their excellent
properties such as good thermal conductivity, chemical inertness
and high breakdown electric field, diamond and DLC thin films have
been found to have excellent field electron emitting properties. It
has been suggested that it is possible to use a form of amorphous
carbon known as "amorphic diamond" as a planar cold cathode in a
novel form of field emission display (FED), which is much simpler
and cheaper than the metal or Si tip arrays commonly used as large
area cold cathodes. The direct use of diamond or DLC films as
planar cathodes, however, has been hindered by poor uniformity of
field emission from these films. This has led to extensive studies
into diamond and DLC thin film processing methods and attempts to
understand field emission mechanisms of these films.
[0004] It is known that diamond nucleation density in a Carbon
Vapor Deposition (CVD) growth environment can be enhanced by a bias
induced nucleation method. This process involves applying a
negative or positive voltage to the substrate in a CH.sub.4/H.sub.2
plasma, where positive ions or electrons are attracted towards the
substrate surface. The interaction between ions or electrons and
the surface is believed to create active sites for nucleation. It
is known that a negative bias has a number of effects on the
nucleation process, such as a) acceleration of migration and
carbonization reaction, b) transition from sp.sup.2 carbon bonds to
sp.sup.3 bonds and c) sub implantation of ions.
SUMMARY OF THE INVENTION
[0005] Recently, it was reported that (001) textured diamond films
can be deposited on a (111) surface using a proper negative bias
during nucleation. However, our invention refers to films deposited
where the negative bias on the substrate is maintained through the
entire growth phase, resulting in superior field emission
characteristics.
[0006] Ion bombardment has been widely used to modify the
properties of growing films, but little is known of the effect of
ion bombardment with a negative substrate bias maintained during
the entire time of growth on field emission properties of diamond
thin films. We deposited diamond films on Si wafers under various
substrate bias conditions in a CH.sub.4-N.sub.2-H.sub.2 plasma and
determined the influence of the CH.sub.4 concentration and the bias
(+100 V to -150 V) on field emission performance. We found that
providing a negative substrate bias during the entire growth
process resulted in a film with superior field emission properties
as measured by turn-on field voltages of 1.5-2.0 v/.mu.m and low
work function.
[0007] Accordingly, it is an important object of the present
invention to provide a method of producing diamond or diamond like
films comprising establishing a negative bias on a substrate with
an electrically conductive surface in a microwave plasma chemical
vapor deposition system with the atmosphere subjected to the
microwave energy including a source of carbon, nitrogen, hydrogen
and possibly Ar, maintaining the negative bias on the substrate
through both the nucleation and growth phase of the film until the
film is continuous.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] The invention consists of certain novel features and a
combination of parts hereinafter fully described, illustrated in
the accompanying drawings, and particularly pointed out in the
appended claims, it being understood that various changes in the
details may be made without departing from the spirit, or
sacrificing any of the advantages of the present invention.
[0009] FIG. 1 is a graphical representation of a current-electric
field (I-E) curves taken from the films grown with -100 V with the
following CH4 concentrations: (a) 1%, (b) 3%, and (C) 5%;
[0010] FIG. 2 is a graphical representation of Fowler-Nordheim
plots, with data converted from one of each set of l-E curves in
FIG. 1: (a) 1% CH.sub.4, (b) 3% CH.sub.4, and (C) 5% CH.sub.4;
[0011] FIG. 3 is a graphical representation of the turn-on field
plotted as a function of CH.sub.4 concentration with the films
grown with a bias -100 V in a gas mixture of CH.sub.4, 1% N.sub.2,
and a balance of H.sub.2.
[0012] FIG. 4 is a graphical representation of a UV Raman spectum
taken from the films grown at -100 V with the following CH.sub.4
concentrations: (a) 1%, (b) 5%, (C) 10%, and (d) 20%;
[0013] FIG. 5 is a graphical representation of a current-electric
field (I-E) curves taken from the films grown in a gas mixture of
10% CH.sub.4 and 1% N.sub.2 with a balance of H.sub.2 at a bias of:
(a) +100 V, b) O V, and (C) -150 V;
[0014] FIG. 6 is a graphical representation of Fowler-Nordheim
plots with data from one of each set of I-E curves in FIG. 5: (a)
+100 V, (b) O V, and (C) -150 V;
[0015] FIG. 7 is a graphical representation of a turn-on field vs
substrate bias with the films grown in 10%
CH.sub.4-1%N.sub.2-89H.sub.2 plasma;
[0016] FIG. 8 is a graphical representation of a turn-on field vs
distance across the film grown in 10% CH.sub.4-1%N.sub.2-89%H.sub.2
plasma at a bias of -150 V; and
[0017] FIG. 9 is a graphical representation of SEM images of the
films grown in 10%CH.sub.4-1%N.sub.2-89%H.sub.2 plasma with biases;
(a) +100 V, (b) O V, and (C) -150 V.
DETAILED DESCRIPTION OF THE INVENTION
[0018] Diamond thin films used in this study were grown on (100)
surfaces of n-type Silicon wafers with a resistivity of 0.01 ohm-cm
in a microwave plasma enhanced CVD system (ASTeX PDS-17). While Si
wafers were used, a variety of substrates may be employed, such as
a ceramic with an electrically conducting layer of metal, like Ti
or Mo or W. The substrate was first polished with diamond powder
with a particle size of 0.1 .mu.m and then placed in the plasma
chamber and heated to 800.degree. C. The deposition was carried out
at 600 Watts microwave power, while the chamber was maintained at
11 Torr total pressure with a gas mixture of 1% N.sub.2 and 1-20%
CH.sub.4 with a balance of H.sub.2. The substrate was biased at a
given voltage ranging from +100 V to -150 V. The film thickness was
around 0.6 .mu.m (6,000 .ANG.) as monitored by a laser
interferometer. N.sub.2 acts as a dopant in the deposited film.
CH.sub.4 was used as a carbon source by way of illustration, but
C.sub.2H.sub.2 or other hydrocarbons as well as fullerenes may be
used. Concentrations of CH.sub.4 have to be divided by 2 if
acetylene (C.sub.2H.sub.2) is used and by 60 if fullerenes are
used.
[0019] The field emission properties of the films were determined
in a field emission testing system. The anode was a stainless steel
rod 1.89 mm in diameter and was flat except for a slight rounding
at the corners to eliminate sharp edges. The gap between the anode
and the cathode (sample) was computer controlled via a
micro-stepping motor and was measured by an optical microscope
attached to a CCD camera and a TV monitor. The chacteristics of the
emission current vs applied voltage were then obtained by
increasing the applied voltage from 0 to 3 KV and then decreasing
to zero, with a series of increasing gap distances usually from 50
to 200 .mu.m. The emission current was converted to a 0-10 volt
signal by a Keithley electrometer that was typically operated to
provide a maximum output voltage for an emission current of 10
.mu.A. The measurements were carried out under a low 10.sup.-8 Torr
vacuum. The reported electric field values were calculated by
dividing the applied voltage by the anode-cathode distance,
assuming no local enhancement of the field by topographic
asperities.
[0020] Morphology and structure of the films were studied by a
Hitachi S-4500 microscope scanning electron microscope (SEM) and a
UV Raman spectroscopy system.
[0021] It is known that "pure" or high quality diamond does not
field-emit well, and that CH.sub.4 concentration in a CVD plasma is
one of the important parameters that determines the quality of the
diamond film. We first determined the effect of CH.sub.4
concentration on field emission of the films grown under a bias
condition. All the films in this set of samples were grown with a
substrate bias of -100 V in a gas mixture of 1% N.sub.2, 1-20% by
volume CH.sub.4 and a balance of H.sub.2. The current-field
measurements were made at more than 5 sites per sample, and at each
site several sets of I vs E data were collected.
[0022] FIG. 1 shows three typical sets of l-E data obtained from
the films grown at the following CH.sub.4 concentrations: (a) 1%,
(b) 3%, and (c) 5%. Note that each set of data contains four curves
(two up scans and two down scans), with the x-axis plotted as
electric field. It can be clearly seen that the l-E curves shift
towards low electric field and the current signal becomes less
noisy as the CH.sub.4 concentration increases from 1% to 5%. The
l-E curves were interpreted by Fowler-Nordheim equation given
by:
J=154 F.sup.2/.O slashed.exp(-6830 .O slashed..sup.{fraction
(3/2)}/F)
[0023] with F=.beta.V/d,
[0024] where,
[0025] J is the current density (Amps/cm2), F is the electric field
(Volts/.mu.m), .beta. is the geometric field enhancement factor, .O
slashed. is the work function (eV), and d is the distance (.mu.m)
between the anode and the cathode in a planar diode structure. One
of the four curves (the smoothest one) in each set shown in FIG. 1
was converted into Fowler-Nordheim plots, shown in FIG. 2. From
these plots, each set of data points was fitted approximately into
a straight line corresponding to tunneling electron emission. These
three lines exhibit very distinct slopes and intercepts with the
Log (I/V.sup.2) axis. Assuming the field enhancement factor to be
unity, the effective work function is calculated to be 0.027,
0.032, and 0.064 eV for the films grown at 5%, 3%, and 1% CH.sub.4,
respectively, showing that higher carbon concentrations produced
lower effective work functions.
[0026] To explore the effect of CH.sub.4 concentrations on field
emission properties, the turn-on field (the field required to
attain an emission current of 1.times.10.sup.-7 A) was plotted as a
function of CH.sub.4 concentration, as shown in FIG. 3. These data
were obtained by averaging measurements on at least five sites of
each sample, with the error bar representing the standard deviation
in the measurement of a sample. Apparently, the turn-on field drops
drastically with the increasing CH.sub.4 concentrations in a low
CH.sub.4 range (1-5%), whereas above 5% CH.sub.4, it approaches a
constant value of about 2 V/.mu.m. Such field emission behavior is
generally in agreement with known observations. However, the
turn-on fields with the inventive films are lower than those
previously reported. We believe that the negative bias and the
addition of nitrogen in the plasma play an important role in
promoting field emission. The deviation in the measured field
threshold becomes small as the CH.sub.4 concentration increases,
which may result from an increase in emission site density.
[0027] It is believed that the inclusion of graphite particles or
sp.sup.2 bonded carbon atoms in diamond or DLC films promotes field
emission from diamond and amorphous diamond films.
[0028] To gain some insight into the above field emission behavior,
the bonding structure of these films was studied using UV Raman
spectroscopy. FIG. 4. shows the Raman spectra taken from the films
grown with (a) 1%, (b) 5%, (c) 10%, and (d) 20% CH.sub.4. All these
spectra exhibit two major features. One peak at wave number 1332
cm.sup.-1 results from diamond or sp.sup.3 bonding. The other
feature at 1580 cm.sup.-1 is attributed to sp.sup.2 bonded carbon.
As the CH.sub.4 concentration increases, the diamond peak intensity
decreases, accompanied by an increase in the sp.sup.2 carbon peak
intensity. The initial decrease in the Sp.sup.3 carbon peak
intensity with the increase in CH.sub.4 concentration (from 1% to
5%), correlates with the decrease in the turn-on field shown in
FIG. 3. However, the sp.sup.3 peak remains constant for CH.sub.4
content 25%, and the sp.sub.2 peak continues to grow as the
CH.sub.4 content increases, while the emission threshold does not
change. This confirms that the field emission properties are
closely associated with the content of both the sp.sup.2 and
sp.sup.3 bonded carbon in diamond or DLC films. Meanwhile, the
broadening of the diamond peak with the increasing CH.sub.4
concentration is also correlated with the lowered field emission
threshold. Above 5% CH.sub.4 concentration, however, the width of
the diamond peak does not show significant change.
[0029] To examine the effect of bias during film growth on field
emission, a voltage ranging from +100 V to -150 V was applied to
the substrate, while the growth conditions were kept as follows:
substrate temperature of 800.degree. C., microwave power of 600 W,
gas mixture of 10% CH.sub.4, 1% N.sub.2 and 80% H.sub.2 at a total
pressure of 11 Torr. FIG. 5 shows three typical sets of l-E curves.
These three sets of data were obtained from the films grown under
biases of (a) +100 V, (b) O V, and (c) -150 V. Each set has four
curves (two up scans and two down scans). The onset field decreases
as the bias varies from +100 V to -150 V. The data show the lowest
turn-on field of 1.5 V/.mu.m at a bias of -150 V, and the current
has the lowest noise level. FIG. 5 displays the Fowler-Nordheim
plots of these films. From the plots, the effective work function
was calculated to be 0.019, 0.048, and 0.073 eV for the films grown
at -150 V, O V, and +100 V, respectively, assuming .beta. is unity.
Negative voltages in the range just less than zero to -200 V are
useful, but voltages in the range of from about -100 to about -200
V are preferred and in the range of from about -150V to about -200
V are most preferred.
[0030] The effect of the bias on the field emission is shown from
the turn-on field vs. substrate bias, as shown in FIG. 7. As the
bias varies from positive voltage to negative voltage, the turn-on
field drops rapidly and then slowly decreases. Again, the error
bars show a decreasing deviation in the measured turn-on field as
the absolute value of the negative bias increases, which may be a
result of an increase in the density of emission sites. These
observations show that a negative bias during growth promotes
low-field cathode electron emission. The film grown at the bias of
-150 V was tested over a length of 40 mm for 14 measurements, and
exhibits a relatively uniform turn-on field of 2.+-.0.55 V/.mu.m,
as shown in FIG. 8. The behavior was reproducible in other
films.
[0031] The morphology of the films was studied using high
resolution SEM. FIG. 9 displays the SEM images of the films grown
with biases: (a) +100 V, (b) O V, and (c) -150 V. Although all
these films are very smooth, the film grown at -150 V shows some
noticeable differences. First, the particle size of the film is
slightly smaller than the films grown at O V and +100 V biases.
Second, the particles of the film seem to coalesce together. These
differences possibly result from re-nucleation under ion
bombardment during the growth. The films grown at biases of +100 V
and O V look similar to each other.
[0032] It is well known that the interaction of ions with the
surface of a growing film may result in a number of effects, which
in turn affect the properties of the film or the surface. Negative
biases during diamond growth have been found to alter the
orientation of the diamond film. As mentioned previously, (001)
textured diamond films can be deposited on a (111) surface using a
negative bias. Improved field emission for Ar ion irradiated carbon
with tens of Kilo-volt ions has been reported. Until our invention,
it has been unclear how the bias growth affects field emission. We
believe that the increased defect density of the films grown under
the negative bias may be responsible for the enhancement in the
field emission properties.
[0033] A defect model has been proposed as a mechanism of field
emission from synthesized diamond films. In this model, it was
suggested that diamond films contain a number of structural defects
which may form bands within the diamond band gap from which
electrons can tunnel through the surface barrier into the vacuum.
Carbon ion implantation into diamond films was also reported to
induce defects resulting in a field emission enhancement. It is,
therefore, expected that the improvement in the field emission
properties of the negative bias-grown films may be associated with
additional ion induced defects during the film growth. As the
absolute value of the negative voltage increases, the defect
density also increases due to the increased kinetic energy and flux
of ions such as C.sup.+. This may explain the enhanced field
emission behavior shown in FIG. 7.
[0034] Electron bombardment has been reported to enhance nucleation
via defects created in a positive bias condition. Field emission
from the film grown at a bias of 100 V, however, is relatively
poor. One possible reason for this may be associated with a doping
effect. Based on a defect-induced stabilization model for diamond
growth, it has been suggested that it is easier to grow p-type
diamond films under electron-rich conditions, whereas it is easier
to grow n-type diamond films under positive ion bombardment. This
is because under electron (ion) bombardment, the Fermi energy of
the diamond film shifts toward the conduction (valence) band, which
leads to a lowered energy of incorporation of n-(p-) type dopants.
Nitrogen substitution in diamond is known to be n-type with a deep
level 1.7 eV below the conduction band minimum, although it has
been extremely difficult to develop a process which leads to
substitutional doping in CVD diamond. However, it seems that ion
beam damage in general results in effective n-type behavior since
even carbon sub-implantation in diamond results in n-type behavior.
Studies show that carbon ion sub-implantation and nitrogen
substitution in diamond result in more defects in the film.
[0035] From the above it is believed that ion bombardment under a
negative bias has three effects on the film: one is to increase the
density of defects (including sp.sup.2 bonded carbon) in the
diamond further is to promote N incorporation and carbon
ion-sub-implantation, and a third effect is to change the
morphology of the film. These effects may contribute to the
improvement of the field emission properties.
[0036] We have investigated field emission properties from bias
grown diamond thin films in a CH.sub.4-N.sub.2-H.sub.2 plasma.
Field emission performance of the film grown at a substrate bias of
-100 V has been found to be considerably improved as CH.sub.4
concentration increases from 1 to 5%, which is ascribed to a
decrease in sp.sup.3 carbon as verified from the Raman spectra. For
the films grown under a bias of +100 V to -150V in a gas mixture of
10% CH.sub.4, 1% N.sub.2 and a balance of H.sub.2, the turn-on
field and the deviation in the measured threshold field decreased
as the negative bias voltage increased. At a bias of -150 V, the
lowest turn-on field of .about.1.5 V/.mu.m was achieved, and 14 I-E
measurements over a length of 40 mm across the sample showed a
relatively uniform turn-on of 2.0.+-.0.55 V/.mu.m. On the other
hand, the film grown at a positive bias had relatively poor field
emission properties. We have demonstrated that the field emission
performance of the CVD diamond films can be substantially improved
by applying a negative bias to the substrate during the growth.
[0037] While there has been disclosed what is considered to be the
preferred embodiment of the present invention, it is understood
that various changes in the details may be made without departing
from the spirit, or sacrificing any of the advantages of the
present invention.
* * * * *