U.S. patent number RE30,244 [Application Number 05/946,843] was granted by the patent office on 1980-04-01 for radial flow reactor including glow discharge limitting shield.
This patent grant is currently assigned to Bell Telephone Laboratories, Incorporated. Invention is credited to Frank B. Alexander, Jr., Cesar D. Capio, Victor E. Hauser, Jr., Hyman J. Levinstein, Cyril J. Mogab, Ashok K. Sinha, Richard S. Wagner.
United States Patent |
RE30,244 |
Alexander, Jr. , et
al. |
April 1, 1980 |
**Please see images for:
( Certificate of Correction ) ** |
Radial flow reactor including glow discharge limitting shield
Abstract
An improved radio frequency (rf) powered radial flow cylindrical
reactor utilizes a gas shield which substantially limits the glow
plasma discharge reaction to a section of the reactor over the
semiconductor substrates which are to be coated. The gas shield
permits the use of higher rf input power which contributes to the
formation of protective films that have desirable physical and
electrical characteristics.
Inventors: |
Alexander, Jr.; Frank B.
(Paterson, NJ), Capio; Cesar D. (Fords, NJ), Hauser, Jr.;
Victor E. (Palmerton, PA), Levinstein; Hyman J.
(Berkeley Heights, NJ), Mogab; Cyril J. (Berkeley Heights,
NJ), Sinha; Ashok K. (New Providence, NJ), Wagner;
Richard S. (Bernardsville, NJ) |
Assignee: |
Bell Telephone Laboratories,
Incorporated (Murray Hill, NJ)
|
Family
ID: |
27096091 |
Appl.
No.: |
05/946,843 |
Filed: |
September 28, 1978 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
651555 |
Jan 22, 1976 |
04033287 |
Jul 5, 1977 |
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Current U.S.
Class: |
118/723E; 216/67;
313/619 |
Current CPC
Class: |
C23C
16/5096 (20130101) |
Current International
Class: |
C23C
16/50 (20060101); C23C 16/509 (20060101); C23C
013/08 () |
Field of
Search: |
;118/49-49.1 ;427/39-42
;313/210,220,516 ;156/643 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kendall; Ralph
Attorney, Agent or Firm: Ostroff; Irwin
Claims
What is claimed is:
1. .[.Coating apparatus.]. .Iadd.Apparatus .Iaddend.comprising:
an evacuable chamber;
a member located in said chamber having a support region adapted to
support at least one object and a central region which defines an
aperture therethrough;
means for facilitating a radio frequency discharge within the
chamber adjacent the object to form a .[.flow.]. .Iadd.glow
.Iaddend.discharge plasma from reactant gases introduced into the
chamber;
exhaust means in communication with the centrally disposed aperture
in the member; and
.[.coating.]. gas-vapor feed means located adjacent the support
region of the member such as that gas-vapor feed is induced to flow
across the support region of the member in a radial flow and out
through the exhaust means, the .[.coating.]. gas-vapor feed means
being adapted to substantially inhibit any glow plasma discharge
reaction from occurring until reactant gases introduced into
.[.he.]. .Iadd.the .Iaddend.apparatus through the .[.coating.].
gas-vapor .Iadd.feed .Iaddend.means reach the support region of the
member.
2. Apparatus for coating a substrate comprising:
an evacuable chamber;
a member located in said chamber having a top region which is
adapted to hold the substrate and a central region defining an
aperture therethrough;
means for generating a radio frequency discharge within the chamber
adjacent the substrate to form a glow discharge plasma from
reactant gases introduced into said chamber;
means for establishing a radial flow from an outer portion of the
member which is adapted to hold the substrate toward and out of the
aperture of the reactant gases suitable for forming the plasma and
coating on the substrate;
means in communication with the aperture for exhausting the gases;
and
means for substantially confining said plasma glow discharge to a
zone which encompasses the top region of the member and extends
substantially vertically upward therefrom.
3. The apparatus of claim 2 further comprising heating means
contained within the chamber for heating the substrate to a
preselected temperature.
4. The apparatus of claim 2 wherein the means for generating a glow
discharge includes substantially parallel electrodes within the
chamber.
5. The apparatus of claim 4 wherein the chamber is electrically
coupled to one of the electrodes. .Iadd. 6. Apparatus
comprising:
an evacuable chamber having an input port and an output port;
a member located in said chamber having a support region adapted to
support at least an object;
means for facilitating a radio frequency discharge within the
chamber adjacent the support region of the member located therein;
and
gas feed means in communication with the input port which includes
shielding means disposed so as to substantially confine the radio
frequency discharge to a portion of the chamber adjacent to the
support region of the member. .Iaddend..Iadd. 7. A radial flow
reactor comprising a chamber adapted to be evacuated, there being
disposed within said chamber:
a first electrode;
a second electrode including a supporting region in spaced relation
with said first electrode and adapted, in cooperation with said
first electrode, to create a plasma discharge between said
electrodes; and
gas feed means for flowing a gas suitable for use in a plasma
discharge reaction process in radial directions across the
supporting region of said second electrode; and
CHARACTERIZED IN THAT
the gas feed means includes shielding means disposed around
portions of the second electrode for substantially confining the
plasma flow discharge at the second electrode to the supporting
region thereof. .Iaddend..Iadd. 8. A reactor as in claim 7 wherein
said gas is introduced into said chamber adjacent to the side of
said second electrode opposite to the side thereof facing said
first electrode, said second electrode having an aperture through a
central portion of said supporting region, and including exhausting
means connected to said aperture and being effective for exhausting
the space immediately surrounding said supporting region; and
CHARACTERIZED IN THAT
said shielding means defines a path for the flow of gas from where
it is introduced into said chamber to immediately adjacent said
supporting region, said shielding means shielding said path from
said first electrode for preventing a plasma glow discharge along
said path. .Iaddend..Iadd. 9. A reactor as in claim 8 further
CHARACTERIZED BY heating elements included within the second
electrode. .Iaddend.
Description
BACKGROUND OF THE INVENTION
This invention relates to apparatus for depositing coatings on
suitable substrates and, more particularly, to an improved radial
flow reactor for coating semiconductor substrates utilizing laminar
flow of reactant gases in a radial direction.
The invention will be described specifically with reference to use
in depositing silicon-nitrogen films on silicon substrates although
it should be evident that it is not limited thereto. The methods of
depositing such films is described more fully and claimed in
copending application of .[.Houser.]. .Iadd.Hauser .Iaddend.et al.,
Ser. No. 651,556 and application of .[.Levenstein.].
.Iadd.Levinstein .Iaddend.et al., Ser. No. 651,557, each filed
concurrently with this application and having the same assignee as
the instant application.
The reliability of semiconductor devices, particularly metal oxide
insulator semiconductor device (MOS) is largely a function of the
manner in which they are passivated and how the completed devices
are isolated from the environment. U.S. Pat. No. 3,757,733,
Reinberg, illustrates and teaches the use of an rf powered radial
flow cylindrical reactor for coating a plurality of semiconductor
substrates with an inorganic film by a low-temperature plasma
deposition technique. One of the problems with this reactor is that
the glow discharge reaction tends to occur prematurely in a portion
of the chamber which is below the semiconductor substrates. This
undesirable reaction tends to deplete the gases that eventually
flow over the semiconductor substrates that are to be coated. This
results in a somewhat nonuniform coating of the substrates. In
addition, this premature reaction limits the amount of power that
can be supplied by the rf source and results in films which tend to
have a relatively high tensile stress and relatively low density.
Both these characteristics have been found to contribute to
cracking of the films.
It would be desirable to have an improved radial flow reactor in
which a plasma discharge reaction occurs that is substantially
limited to the area above and near the semiconductor substrates to
be coated. Such an improved chamber would facilitate the formation
of protective films which are both uniform in coverage and have
relatively high resistance to cracking.
SUMMARY OF THE INVENTION
The present invention is directed to a radio frequency (rf) powered
radial flow reactor which comprises a top plate, a bottom plate,
and cylindrical side walls all connected in a sealing relationship
.[.therwith.]. .Iadd.therewith .Iaddend.to define an evacuable
chamber. First and second electrodes, which are both typically
parallel cylindrical plate-like members, are contained within the
reactor. The first electrode is electrically coupled to an rf power
source, and the second electrode is electrically coupled to a
reference potential which is typically ground potential. The second
electrode has a central aperture therein. Heater elements are
coupled to the second electrode. A first sheath or tube
communicates through the bottom plate in a sealing relationship and
extends through to the top surface of the second electrode so as to
be in open communication with the aperture in the second electrode.
The other end of the first sheath is in open communication with a
vacuum pump.
A cylindrical gas shield surrounds and is closely spaced with all
but the top surface of the second electrode. Input gases to the
reactor are introduced into a gas ring which exits in the cavity
between the gas shield and second electrode and then pass over the
semiconductor substrates and are exhausted through the aperture
therein.
Semiconductor substrates on which it is desired to deposit
protective films are placed on a top surface of the second
electrode.
When the rf source is activated and appropriate gases are
introduced into the reactor chamber, a glow discharge reaction
occurs in the space between the two .[.electrode.].
.Iadd.electrodes.Iaddend.. The gas shield is typically spaced from
1/8 inch to 1/4 inch from the second electrode. This limits the
glow discharge reaction to essentially just the region between the
two electrodes and substantially inhibits the forming thereof
elsewhere in the reactor. The gas shield serves to intensify the
glow discharge reaction immediately above the substrates. Thus
higher rf power than would otherwise be practical can be
effectively used to increase the intensity of the rf glow discharge
reaction.
These and other features and advantages of the invention will be
better understood from a consideration of the following description
taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 and 2 illustrate one embodiment of a radial flow reactor in
accordance with one embodiment of the invention;
FIG. 3 illustrates a flow diagram of gases that may be used with
the reactor of FIGS. 1 and 2; and
FIGS. 4, 5, 6, 7 and 8 each illustrate a separate graph which has
as the abscissa axis one of the variables of a method for the
deposition of films on semiconductor substrates, and as the
ordinate axis corresponding characteristics of the deposited
films.
DETAILED DESCRIPTION
Referring now to FIGS. 1 and 2, there is illustrated in a
cross-section and a top view a cylindrical .[.readial.].
.Iadd.radial .Iaddend.flow radio frequency (rf) powered reactor 10.
Reactor 10 comprises a top plate section 12, a bottom plate section
14, and cylindrical side wall 16. Side wall 16 is connected to the
top and bottom of plates 12 and 14 in a sealing relationship to
define an evacuable chamber 24.
A first electrode 18, which is typically a circular metallic
member, is coupled to an rf source 22 through an impedance matching
network 20. Electrode 18 .Iadd.is .Iaddend.illustrated as
electrically isolated from top plate 12. A second electrode 26,
which is typically a circular metallic member, comprises a top
surface 28, which is adapted to support semiconductor substrates
30, a bottom portion 32, and an end portion 34. Heaters 36, which
are typically contained with electrode 26, are utilized to heat the
semiconductor substrates .[.3.]. .Iadd.30 .Iaddend.to a preselected
temperature.
A gas flow shield 38 is closely spaced to electrode 26 and
essentially surrounds electrode 26 except for the portion of the
top surface 28 thereof on which the semiconductor substrates
.[.wafers.]. 30 are placed. A bottom portion 40 of shield 38 is
essentially parallel to bottom portion 32 of electrode 26. A
U-shaped end portion 42 of shield 38 surrounds the end portion 34
.[.os.]. .Iadd.of .Iaddend.electrode 26.
A .[.Plurality.]. .Iadd.plurality .Iaddend.of sheaths or tubes 44
communicate with the internal portion of chamber 24 extending
through the bottom plate 14 and bottom portion 40 of shield 38 in a
sealing relationship. Sheaths 44 are coupled at first ends thereof
to a gas ring 46 which has a plurality of essentially equally
spaced small apertures 48 therethrough. Gas ring 46 exists in the
cavity between the bottom portion 32 of electrode 26 and the bottom
portion 44 of gas shield 38. Sheaths 44 are connected by second
ends thereof to a common sheath (tube) 50 which has a control valve
52 connected in series therewith.
A .[.sheat.]. .Iadd.sheath .Iaddend.54 communicates with the
interior of chamber 24 and extends through 14 and 38 in sealing
relationship and contacts electrode 26. Electrode 26 has a central
region generally at 56 which defines an aperture therethrough.
Sheath 54 extends to this aperture and terminates at the top
surface 28 of electrode 26. The other end of .[.sheth.].
.Iadd.sheath .Iaddend.54 is coupled to vacuum pumps 58 that are
used to evacuate the interior of chamber 24.
The reactant gases required to coat the semiconductor substrates
.[.wafers.]. 30 contained within chamber 24 are introduced into
tube 50 and flow as indicated by the arrows.
An rf glow discharge reaction is caused to occur within chamber 24
between electrodes 18 and 26 when the rf source 22 is activated and
appropriate gases are introduced into chamber 24 through 50. Gas
shield 38 is typically spaced 1/4 .Iadd.inch .Iaddend.or less from
electrode 26. This close spacing substantially inhibits the glow
discharge reaction which occurs between electrode 18 and the top
surface 28 of electrode 26 from occurring around end portion 34 and
bottom portion 32 of electrode 26. This serves to intensify the rf
glow discharge reaction immediately above semiconductor substrates
30. In addition, the gas shield 38 permits the effective use of
higher input rf power than is possible without the shield. Without
shield 38 there is a tendency for the gases introduced into the
chamber 24 to react below electrode 26 and therefore to dissipate
before reaching semiconductor substrates 30. Thus, without the
shield 38 the increasing of rf power beyond a certain point is not
particularly helpful in intensifying the glow discharge reaction
above the substrates 30 where it is important that the reaction
occur.
The vacuum pumps of FIG. 1 are selected to be compatible with a
high gas flow rate of approximately 2 liters .Iadd.per
.Iaddend.minute .[.minutes.]. at greater than 1 mm pressure. A 150
cfm Leybold-Heraeus roots blower backed with two 17 cfm mechanical
pumps running in parallel were found to be sufficient to achieve
the needed high gas flow rate. Additional pumping capacity
comprising a cryopanel and a 400 l/s vacuum pump located below an
isolation valve (not illustrated) in the reactor 10 of FIGS. 1 and
2 is utilized to initially pump the reactor 10 and 100 to a base
pressure of .about.10.sup.-6 mm.
In operation, semiconductor substrates 30 are loaded on support
surface 28. The reactor 10 is then sealed, closed, and pumped down
to 10.sup.-6 mm. The heaters connected or part of the electrodes 26
are turned on and the semiconductor substrates are heated to
approximately 275.degree. C. The .[.vacion.]. .Iadd.Vacion
.Iaddend.isolation valve is closed and reactant gases are admitted
to the reactor and the roots blower valve is opened again. A
dynamic pressure of approximately 600.mu. is established in the
reactor with the input gases flowing at the desired flow rates.
Thereafter the roots blower valve is throttled to the desired
pressure. The rf power source is now activated to the desired power
level.
A fully functional reactor very similar to the reactor 10 of FIG. 1
was constructed with sections 12, 14 and 16 all being stainless
steel and electrode 18 being aluminum. Two tubes 44 are utilized
and the gas ring utilized is a tubular member having a diameter of
5 inches. The spacing between the U-shaped section 42 of gas shield
38 and the end portion 34 of electrode 26 is approximately 1/4
.Iadd.inch.Iaddend.. The spacing between the top surface 28 of
electrode 26 and electrode 18 is approximately 1
.Iadd.inch.Iaddend..
Another fully functional reactor similar to that of reactor 10 of
FIG. 1 was constructed with sections 12 and 14 made of aluminum and
sidewalls 16 made of pyrex. Since section 12 is electrically
isolated from sections 14 and 16, it is not necessary that the
electrical connection from the impedance matching network 20 to
electrode 18 be electrically isolated from top plate section 12. In
this reactor the gas ring is a cylindrical member having a diameter
of approximately 14 inches. There are approximately 120 gas outlet
apertures of 40 mils each in diameter equally spaced around this
gas ring. The spacing between the end portion 34 of electrode 26
and the U-shaped portion 42 of gas shield 38 is approximately 1/8
of an inch. The distance between surface 28 .Iadd.of
.Iaddend.electrode 26 and electrode 18 is approximately 1 inch.
Referring now to FIG. 3, there is illustrated a flow diagram of
reactant gases that may be used in the reactor of FIGS. 1 and 2.
Sources of silane (SiH.sub.4) in a carrier gas argon (Ar) 1000,
ammonia (NH.sub.3) in a carrier gas argon (Ar) 1100, carbon
tetrafluoride (CF.sub.4) 1200, and oxygen .[.(o.sub.2).].
.Iadd.(O.sub.2) .Iaddend.are connected through a separate one of
valves 1400, 1500, 1600 and 1700, respectively, to separate flow
meters 1800, 1900, 2000 and 2100, respectively, and then through
separate leak valves 2200, 2300, 2400, and 2500, respectively. The
outputs of leak valves 2400 and 2500 are both connected through a
valve 2900 to a reaction chamber 2700. Reaction chamber 2700 can be
the chamber 24 of FIGS. 1 and 2. The outputs of leak valves 2200
and 2300 are both connected to mixing chamber 2600. Mixing chamber
2600 is in communiction with reaction chamber 2700 through valve
2800.
The reactant gases SiH.sub.4 and NH.sub.3 mix in the mixing chamber
2600 and then pass through valve 2800 into reaction chamber 2700.
During the time of depositing inorganic films on semiconductor
substrates, valves 1600, 1700, 2400, 2500 and 2900 are closed and
valves 1400, 1500, 2200, 2300 and 2800 are open.
After one or more deposition runs, inorganic films form on the
electrodes 18 and 26 and on other areas in the reactor of FIGS. 1
and 2. To clean off the films, the heaters and rf source of FIG. 1
are turned on and valves 1600, 1700, 2400, 2500 and 2900 are all
opened, and valves 1400, 1500, 2200, 2300 and 2800 are all closed.
The films deposited on internal parts of the reactor are cleaned by
the resulting rf glow discharge reaction (the reactant gases being
CF.sub.4 and O.sub.2) and a new set of semiconductor substrates can
then be placed in the reactor for deposition of protective films
thereon.
Advantageously all interconnecting tubing connecting the sources of
gases illustrated in FIG. 3 to the reactor of FIGS. 1 and 2 are
made of stainless steel to insure these connections are essentially
leak-free. This essentially prevents any but the desired gases from
entering the systems during the deposition operations. Essentially
pure sources of SiH.sub.4, NH.sub.3, and Ar could be easily
substituted for the SiH.sub.4 in Ar and NH.sub.3 in Ar sources.
In the first set of operating conditions described below the
reactor was essentially as illustrated in FIGS. 1 and 2 without the
gas shield and with electrode 18 in electrical contact with top
plate 12. Side wall 16 is pyrex in this case. The following
operating conditions were utilized to deposit protective films
having the denoted characteristics on semiconductor substrates;
______________________________________ 1st Operating Condition 2nd
Operating (using apparatus Condition of FIGS. 1 & 2 (using
apparatus without gas shield) of FIGS. 1 & 2)
______________________________________ Reactant gas SiH.sub.4
/NH.sub.3 /Ar SiH.sub.4 /NH.sub.3 /Ar SiH.sub.4 1.25% 1.70%
NH.sub.3 1.56% 2.39% Ar 97.19% 95.91% Total gas 2000 2320 flow
(SCCM) Pressure in 1000 950 reactor (.mu.) Substrate 330 deg. C.
275 deg. C. temperature (degrees C.) Tuned RF 60 250 power (watts)
(reflected power = .about. 0) Thickness 1.1 1.1 of deposited layer
(.mu.) Stress in 1-2 (tension) 1-5 (tension) resulting layer
(10.sup.9 dynes/cm.sup.2) Etch rate in 175 180 BHF (Angstroms per
min.) Density (GCM.sup.-3) 2.4 2.55 Composition of 1.1 1.05
resulting layer (Si/N) Refractive Index 2.15 2.05 Cracking 400 450
resistance (deg. C. to which substrates with deposited layers could
be raised without cracking) Adhesion of Good Good deposited layer
Step Coverage of Very good Very good deposited layer Scratch
resistance Good Good Dielectric constant 6.9 6.4 Breakdown strength
3.4 3.9 (10.sup.6 V/cm) Resistivity at 5 .times. 10.sup.18 4
.times. 10.sup. 13 2 .times. 10.sup.6 V/cm (ohm/cm)
______________________________________ 3rd Operating 4th Operating
Condition Condition (using apparatus (using apparatus of FIGS. 1
& 2) of FIGS. 1 & 2) ______________________________________
Reactant gas SiH.sub.4 /NH.sub.3 /Ar SiH.sub.4 /NH.sub.3 /Ar
SiH.sub.4 1.78% 1.78% NH.sub.3 2.25% 2.25% Ar 95.97% 95.97% Total
gas 2320 2320 flow (SCCM) Pressure in 950 950 reactor (.mu.)
Substrate 275 deg. C. 275 deg. C. temperature (degrees C.) Tuned RF
300 400 power (watts) (reflected power = .about. O) Thickness
(.mu.) 1.1 1.1 Stress in 1-2 (compression) 1-2 resulting layer
(compression) (10.sup.9 dynes/cm.sup.2) Etch rate in 125 75 BHF
(Ang- stroms per min) Density (GCM.sup.- 3) 2.75 2.90 Composition
of 0.8 0.75 resulting layer (Si/N) Refractive Index 2.00 1.94
Cracking 550 550 resistance (deg. C. to which substrates with
deposited layers could be raised without cracking) Adhesion of Good
Good deposited layer Step Coverage Very good Very good of deposited
layer Scratch Good Good resistance Dielectric 6.8 5.8 constant
Breakdown 5.0 8.1 strength (10.sup.6 V/cm) Resistivity 3 .times.
10.sup.15 5 .times. 10.sup.19 at 2 .times. 10.sup. 6 V/cm (ohm/cm)
______________________________________
The tuned rf power indicated for each of the above operating
conditions was read from a meter on the rf power supply. It is to
be appreciated that the effective rf input power density between
the electrodes of a reactor is a function of the geometry of the
electrodes and the spacing therebetween. The reactors utilized with
the above operating conditions have a circular top electrode having
a .[.radius.]. .Iadd.diameter .Iaddend.of 14 inches. Electrode 18
was separated from the electrode 26 by approximately 1
.Iadd.inch.Iaddend.. A reactor with different type .[.of.].
.Iadd.or .Iaddend.size of electrodes and different spacing between
.[.electrode.]. .Iadd.electrodes .Iaddend.would require an
appropriately different input rf power in order to produce films on
semiconductor substrates with essentially the same characteristics
as described herein.
The first operating condition is useful for depositing protective
films on semiconductor substrates which utilize aluminum
metallization. The aluminum metallization can easily withstand
temperatures at and above the 330.degree. C. used. The second
through fourth operating conditions can be used with semiconductor
substrates which have aluminum or gold with titanium, palladium and
gold beam leads since the temperature utilized is below that at
which titanium and palladium and gold interact.
Cracking of the protective films allows moisture and inpurities
(i.e., sodium) to attack the surface of the semiconductor
substrates and thereby destroy the circuitry contained thereon. It
is therefore very important that protective films be as
crack-resistant as possible.
The fourth operating condition results in films which are
substantially stoichiometric silicon nitride (Si.sub.3 N.sub.4) and
which contain essentially no other organic combinations or argon
incorporation. The physical characteristic of the resulting
Si.sub.3 N.sub.4 film are superior to Si.sub.3 N.sub.4 films
produced by chemical vapor deposition (CVD) processes in that they
are much less susceptible to cracking than the CVD produced
Si.sub.3 N.sub.4. The reason for this is that the silicon nitride
films resulting from operating condition four have relatively low
compressive stress and not the relatively high tensile stress of
the CVD produced films.
It is important to note that in all of applicants' operating
conditions careful precautions were taken to limit the presence of
nitrogen (N.sub.2) or oxygen (O.sub.2) in the reactor during the
glow discharge reactions, it has been determined through
experimentation that the addition of even small amounts of N.sub.2
(up to 2%) or O.sub.2 (up to 0.2%) in the reactant gas mixture can
significantly adversely affect the characteristics of the resulting
films. The addition of only 2% nitrogen to the reactant gases
resulted in an order of magnitude increase in tensile stress of the
resulting film, and an increase in the BHF etch rate of over 7
times. The addition of only 0.2%, O.sub.2 to the reactant gases
resulted in a 7-times increase in the BHF etch rate.
Using the second operating conditions as a standard, the effects of
varying the five main process parameters, namely, (A) Gas pressure,
(B) Total gas flow, (C) Pressure, (D) Substrate temperature and (E)
RF input power into the reactor, were studied. The graphs
illustrated in FIGS. 4, 5, 6, 7 and 8 each illustrate on the
abscissa one of the variables denoted above, and on the respective
ordinate axis some of the resulting characteristics of the film
deposited on semiconductor substrates.
A. Gas Composition
The graph of FIG. 4 illustrates the effect of increasing SiH.sub.4
concentration (1.4.ltoreq.% SiH.sub.4 .ltoreq.1.9;
0.5.ltoreq.SiH.sub.4 /NH.sub.3 .ltoreq.0.9) in the reacting gases.
These gas compositions were achieved by adjusting the flowmeters
for 3%SiH.sub.4 -Ar and 5%NH.sub.3 -Ar to various complementary
settings so as to keep the total flow constant.
As expected, increasing the SiH.sub.4 concentration in the gas led
to a corresponding linear increase in the Si/N ratio in the film
(from .about.1.0 to .about.1.2), and a linear increase in the
refractive index (from .about.1.9 to .about.2.2). For the lowest
SiH.sub.4 concentration used, (SiH.sub.4 /NH.sub.3 =0.52), the film
density was found to be relatively low (.about.2.3 .[.gcm-116.sup.3
.]. .Iadd.gcm.sup.-3.Iaddend.), and the BHF etch-rate was
.[.corresponding.]. .Iadd.correspondingly .Iaddend.high (250
angstroms/min.). With increasing SiH.sub.4 /NH.sub.3 ratio, the
film density .rho. showed a broad peak (.rho..apprxeq.2.55
gcm.sup.-3) for 0.58.ltoreq.SiH.sub.4 /NH.sub.3 .ltoreq.0.79. The
.rho. decreased again at SiH.sub.4 /NH.sub.3 .about.0.9; however,
this was not accompanied by corresponding increase in BHF
etch-rate, presumably because the films now had a much higher Si
content (Si/N.about.1.2). The film .sigma., which was always
tensile, showed a peak at SiH.sub.4 /NH.sub.3 .about.0.6, which is
located at a slightly lower SiH.sub.4 concentration than that for
the peak in .rho..Iadd...Iaddend.
While most of our work has involved operating conditions in which
the ratio of silane to ammonia was between 0.5 and 0.9, which is
believed the preferred range, it may be feasible to deposit useful
protective films with ratios outside this range.
B. Gas Flow
The graph of FIG. 5 illustrates the effect of increasing the total
gas flow on the other variables of the process. The total gas flow
was varied in the range 1.0 to 2.5 liters min.sup.-1, with the
SiH.sub.4 /NH.sub.3 ratio constant at 0.71(%SiH.sub.4 =.sub.1.70).
It may be seen from FIG. 5 that increasing the flow led to a higher
deposition rate (from 120 to 200 angstroms/min.Iadd.).Iaddend., a
greater refractive index, and a larger Si/N ratio in the film (from
0.8 to 1.05). For this range of film composition, the film density
seems to have a dominant effect on the BHF etch rate; a broad
maximum in .rho. corresponds to a broad minimum in the etch rate.
The tensile stress .[.descreases.]. .Iadd.decreases .Iaddend.with
increasing flow; this is probably the result of a higher film
purity (with respect to possible nitrogen/oxygen contamination) as
the flow is increased.
C. Pressure
The graph of FIG. 6 illustrates the effect of increasing pressure
on the other variables of the process. The average pressure during
film deposition was varied from .about.700 to 1000.mu.
(.+-.25.mu.). As shown in FIG. 6, increasing the pressure also led
to a higher deposition rate, whereas the density and the BHF etch
rate did not change much. The refractive index decreased linearly
This generally (i.e., for pressures .gtoreq.750.mu.) correlates
with a decrease in the Si/N ratio in the film.
D. Substrate Temperature
The graph of FIG. 7 illustrates the effects of varying the
temperature of the semiconductor substrates on the other variables
of the process. The limited range of substrate temperatures studied
(200.degree..ltoreq.T.sub.s
.ltoreq..Badd..[.300.degree.".]..Baddend. .Iadd.300.degree.
.Iaddend.C.) was influenced by the desire to stay below
temperatures at which Pd-Au interdiffusion (in Ti/Pd/Au
metallization) becomes excessive. As shown in FIG. 7, T.sub.s
(substrate temperature) has a pronounced effect on the BHF etch
rate, which decreases almost exponentially with increasing T.sub.s.
The decrease in BHF etch-rate is associated with a linear increase
in the film density, .rho. and in the refractive index, n. Thus,
for films deposited at 200.degree. C. the BHF etch rate was 700
angstroms/min, the density was .about.2.3 gcm.sup.-3 and the
refractive index was .about.1.85. Interestingly, these films also
had a rather large Si/N ratio (.about.1.2) and a high tensile
stress (7.times.10.sup.9 dynes cm.sup.-2). With increasing T.sub.s,
both .sigma. and the Si/N ratio in the film displayed a shallow
minimum at .about.250.degree. C.; however, a higher T.sub.s of
275.degree. C. was preferred because it led to films with yet
greater density (2.55 gcm.sup.-3) and somewhat lower etch-rate
without an excessive increase in .sigma..
E. RF Input Power
The graph of FIG. 8 illustrates the effect of increasing the rf
input power. Tuned rf input powers were investigated in the range
of 100 to 350 watts (reflected power=0). For this series of
experiments, the SiH.sub.4 /NH.sub.3 ratio was kept constant at
0.8, and SiH.sub.4 at 1.81. For increasing rf power, there was
found to be a rapid and linear increase in the film .rho.
(weight-gain measurement.[...]..Iadd., .Iaddend.using
.Iadd.1.Iaddend..mu. thick films) from 2.2 gcm.sup.-3 at 100 watts
to 2.8 gcm.sup.-3 at 350 watts. Films (1.mu.) thick with lower
density had a distinct yellowish tinge to them when deposited on
Al-metallized devices, whereas those with densities .gtoreq.2.4
gcm.sup.-3 appeared to be grayish and more truly transparent. Both
the film .[..rho..]. .Iadd..sigma. .Iaddend.and BHF etch-rate
showed a bimodal behavior at .about.275 watts. Below this power
level the stresses were very low tensile (.about.0.5.times.10.sup.9
dynes cm.sup.-2) and the etch-rates were relatively high (275 to
325 angstroms/min). At rf powers .gtoreq.300 watts, the stresses,
which had been tensile, became compressive (1-2.times.10.sup.9
dynes cm.sup.-2) and the BHF etch rates were relatively low
(<150 angstroms/min). Significantly, the refractive index showed
a decrease with increasing rf power. The refractive index, film
composition, and film density have been correlated using the
Lorentz-Lorentz equation.
Finally, it should be understood that the specific embodiment of
the invention described is merely illustrative of the general
principles and various modifications thereof are feasible,
including change in dimensions, geometry, and materials. Moreover,
it should be evident that other gases may be utilized to form films
of other materials.
* * * * *