Method for depositing high quality silicon dioxide by PECVD

A method for depositing high quality silicon dioxide in a plasma enhanced chemical vapor deposition tool is described. The reactant gases are introduced into the tool together with a large amount of an inert carrier gas. A plasma discharge is established in tool by using a high RF power density thereby depositing high quality silicon dioxide at very high deposition rates. In a single wafer tool, the RF power density is in the range of 1-4 W/cm.sup.2 and the deposition rate is from 600-1500 angstroms per minute for depositing high quality SiO.sub.2 films.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention is directed to the deposition of silicon dioxide films by 
plasma enhanced chemical vapor deposition (PECVD) and more particularly, 
to an improved method for depositing high quality SiO.sub.2 films at high 
deposition rates. 
2. Background of the Invention 
Current trends in metal oxide semiconductor (MOS) technology are towards 
higher chip packing density, more complex devices with more process 
levels, and larger substrates. To accommodate these advances, MOS device 
dimensions must be scaled down and high temperature processing steps must 
be minimized. Future generations of MOS devices will require very high 
quality silicon dioxide (SiO.sub.2) gate dielectrics and low temperature 
processing. The deposition of dielectrics or insulators at low 
temperatures also has applications in a number of other semiconductor 
device technologies. For example, as secondary passivation layers, 
interlayer isolation, and lithographic masks in integrated circuits and 
also as primary gate dielectrics for thin film transistor (TFT) 
applications. 
In addition, the modern trend is towards the use of single wafer deposition 
tools which necessitates very high deposition rates (at least 600-1000 
angstroms per minute) for the tools to be commercially viable. 
The conventional method for forming SiO.sub.2 gate dielectrics is to 
thermally grow the oxide films at temperatures from about 
800.degree.-100.degree. C. The thermal oxide films have excellent 
electronic and mechanical qualities that have made this process the most 
widely used for conventional semiconductor transistor applications. 
However, in view of the above described new direction in manufacturing, 
there is a need for a method to deposit SiO.sub.2 films at low 
temperatures that have both electronic and mechanical qualities comparable 
to thermal oxides. 
Plasma enhanced chemical vapor deposition (PECVD) is a technique which is 
used to deposit electronic materials at high rates and/or at low 
temperatures. Historically, however, oxide deposited at low temperatures 
has been far from electronic grade and although various properties of 
PECVD oxide have been reported in the literature, Adams, Solid State 
Technology, 26, 135 (1983), and Hollahan, J. Electro Chemical Society, 
126, 933 (1979), there have been no reports of oxides with electrical 
characteristics approaching those deposited by conventional high 
temperature techniques. Thus, due to the fact that the electronic and 
physical characteristics of SiO.sub.2 films deposited by conventional 
PECVD are relatively poor, applications have been limited to those areas 
where film quality is relatively unimportant. 
Recently, it has been shown that by modifying the PECVD technique, thin 
films of SiO.sub.2 of exceptionally high quality can be deposited at very 
low substrate temperatures (350.degree. C. or less). See, Batey et al., J. 
Appl. Phys., 60, 3136 (1986) and Batey et al., IEEE Electron Dev. Lett. 
EDL-8, 148 (1987). The Batey et al. technique combines very low flows of 
reactive gases with massive amounts of helium dilution and a low radio 
frequency (RF) power density. As a result, the deposition proceeds at a 
much reduced rate and film quality is much improved. Typical process 
parameters for the Batey et al. technique are: 2% SiH.sub.4 in He with a 
flow rate of about 20 sccm, N.sub.2 O with a flow rate of about 50 sccm, 
He with a flow rate of about 2,000 sccm or greater, pressure of about 1 
Torr and an RF power density of about 0.02 W/cm.sup.2. The large amount of 
helium dilution insures uniformity and high quality. However, it was 
determined that the film properties depend strongly on the deposition 
rate, which had to be kept below a critical value of about 80 angstroms 
per minute or less. While good quality films were obtained, the deposition 
rate is too low for many applications, a problem which is magnified by the 
trends in the industry towards single wafer processing. 
Another prior art method of depositing SiO.sub.2 films in a PECVD system is 
disclosed in U.S. Pat. No. 4,223,048 to Engle, Jr. Engle, Jr. is directed 
to a batch processing system which utilizes interleaved electrodes and 
vertical positioning of the wafers in order to improve the uniformity of 
the films. Engle, Jr. teaches a very high flow rate of N.sub.2 O (1,000 
liters per minute), a flow rate of SiH.sub.4 of about 50 sccm and a flow 
rate of O.sub.2 of about 10-20 sccm. The plasma discharge is established 
by a low RF power (20 watts) and a deposition rate of approximately 500 
angstroms per minute is achieved. While there is no mention of the 
electronic quality of the films, it is likely the electronic quality would 
be poor at such a high deposition rate. Moreover, the deposition rate 
still remains below that necessary for single wafer processing. 
SUMMARY OF THE INVENTION 
The present invention improves upon the Batey et al. technique by 
depositing high quality SiO.sub.2 films in a PECVD system at high 
deposition rates. The method of the present invention utilizes large He 
dilution with high reactant gas flow rates and a very high RF power 
density. In a single wafer tool, with an RF power density on the order of 
1 to 4 W/cm.sup.2, SiO.sub.2 films were deposited by the inventive method 
at rates of from 600 to 1500 angstroms per minute with excellent 
electronic and mechanical properties. MOS capacitor structures formed with 
gate insulators deposited by the present method exhibited such 
characteristics as: essentially zero current leakage, very low interface 
state density, no premature current injection, and a breakdown field 
similar to that obtained in thermal oxide structures. Excellent mechanical 
properties were also exhibited such as, an etch rate of less than two 
times that of thermal oxides, very low pinhole density, excellent 
uniformity and density, and stoichiometry very close to SiO.sub.2 with 
almost no chemical impurities.

DETAILED DESCRIPTION OF THE INVENTION 
The method of the present invention provides for the deposition of high 
quality silicon dioxide on a substrate in a PECVD deposition tool. In the 
first step of the method, flows of reactant gases containing oxygen and 
silicon, and a very large flow of an inert carrier gas are introduced into 
the reaction chamber of the tool. Typically, the silicon containing 
reactant gas is diluted in an inert carrier gas, with the silicon 
containing reactant gas being from 1.4% to 2% of the mixture. The flow 
rate of the silicon containing reactant gas must be 150 sccm or greater, 
with the preferred range being from 200 sccm to 400 sccm for a five inch 
single wafer tool. The flow rate for the oxygen containing reactant gas 
must be 200 sccm or greater, with the preferred range being from 280 sccm 
to 560 sccm for a five inch single wafer tool. Moreover, the ratio of the 
oxygen containing reactant gas to the diluted silicon containing reactant 
gas must be large enough to insure deposition of films close to the 
stoichiometry of SiO.sub.2, this ratio being typically greater than 1. The 
conventional reactant gases of N.sub.2 O and SiH.sub.4 are used; however, 
any suitable oxygen containing reactant gas and silicon containing 
reactant gas may be utilized. In addition, the inventive method may be 
used in larger single wafer tools and in multi-wafer tools, it being 
understood by those skilled in the art that the flow rates will be 
adjusted based on the size of the reaction chamber. 
As stated above, in addition to the oxygen and silicon containing gas 
flows, a large flow of inert carrier gas is also introduced into the 
reaction chamber. The flow rate of the inert carrier gas must be 1000 sccm 
or greater with the preferred flow rate being about 2000 sccm for a single 
wafer tool. The preferred carrier gas is helium because of its light 
weight; however, other carrier gases may also be used such as argon, neon 
and zenon. 
A PECVD system is a low temperature system and is typically operated with 
temperatures in the range of about 250.degree. C. to 600.degree. C. The 
pressure in the system is often used to tailor certain film properties and 
is typically 5 Torr or less. 
In a second step of the method of the invention, a plasma discharge is then 
established in the reaction chamber of the tool by applying sufficient RF 
power to provide a power density of about 1 W/cm.sup.2 or greater with the 
preferred range being between 1 and 4 W/cm.sup.2. The present method 
results in the deposition of high quality silicon dioxide films at 
deposition rates from about 600 to 1500 angstroms per minute. 
In accordance with one preferred embodiment of the present invention, a 
PECVD single wafer tool was used with a flow rate ratio of N.sub.2 O to 
SiH.sub.4 in He of 560/400 sccm and a RF power density in the range from 1 
W/cm.sup.2 to 4 W/cm.sup.2, which resulted in high quality SiO.sub.2 films 
being deposited at rates from about 1100 angstroms per minute to about 
1500 angstroms per minute. Thus, device quality silicon dioxide films can 
now be deposited in a single wafer PECVD tool at commercially acceptable 
deposition rates. 
Referring now to the drawings, FIG. 1 is a simplified schematic diagram of 
a single wafer PECVD tool having a reaction chamber 2 with a gas inlet 4 
and a gas exit 6. The chamber is provided with a heater 8 and a power 
electrode 10 having cable 12 connected thereto which leads to an RF power 
source, not shown. Four flow controllers are provided for the introduction 
of the necessary gases. Controller 14 introduces the large flow of inert 
carrier gas (He), controller 16 introduces the oxygen containing reactant 
gas (N.sub.2 O), controller 18 introduces the silicon containing reactant 
gas (SiH.sub.4 in He) and controller 20 introduces N.sub.2, which is used 
as a purge gas. A substrate 22 is positioned on the heater 8 and the 
system is ready for the deposition of an SiO.sub.2 layer on the substrate. 
The substrate may be a semiconductor material for forming a metal oxide 
semiconductor (MOS) device or a metal for forming a metal oxide metal 
(MOM) device. The preferred substrate is silicon, however, the present 
invention is not limited to silicon substrates, as high quality SiO.sub.2 
may be deposited on a broad range of substrate materials, including 
metals, non-silicon semiconductors, glasses and other materials. 
In operation, the gas flow controllers 14, 16 and 18 are activated to 
permit the required flow rates of N.sub.2 O, SiH.sub.4 and He to enter the 
chamber. The RF power electrode 10 is then activated to provide the 
required power density. A plasma discharge 24 is thereby established in 
the chamber 2 causing a silicon dioxide layer 26 to be deposited onto the 
substrate 22. The present invention may be used to deposit a wide range of 
oxide film thickness, from very thin to thick film, all at device grade 
quality. 
The following examples are included merely to aid in the understanding of 
the invention and variations may be made by one skilled in the art without 
departing from the spirit and scope of the invention. 
EXAMPLE 1 
In this example, the following conditions were kept constant: 
N.sub.2 O=16 sccm 
1.4% SiH.sub.4 in He=12 sccm 
He=2000 sccm 
pressure=5 Torr 
reaction temperature=350.degree. C. 
Films were deposited at RF power densities of: 0.1, 0.2, 0.5, 1, 2, and 
3.25, W/cm.sup.2. Results of Example 1 are shown in Table I below: 
TABLE I 
______________________________________ 
RF 
Power Refrac- Deposition 
Density tive Rate Etch Stress 
Film No. 
(W/cm.sup.2) 
Index (A/min) Rate* (Dynes/cm.sup.2) 
______________________________________ 
1 0.1 1.469 55 1.34 3.10 
2 0.2 1.469 56 1.26 3.05 
3 0.5 1.468 50 1.25 2.76 
4 1 1.467 43 1.27 2.34 
5 2 1.465 31 1.23 1.93 
6 3.25 1.464 26 1.35 1.95 
______________________________________ 
*Etch rate is the factor greater than the rate for thermal oxide in (7:1) 
BHF at 23.degree. C. 
EXAMPLE 2 
In this example, the pressure, temperature and helium flow rate are 
identical to that of Example 1. In this example, however, the RF power 
density was kept constant at 2 W/cm.sup.2 but the flow rates of the 
reactant gases were varied at the following ratios of N.sub.2 O/1.4% 
SiH.sub.4 in He: 16/12, 35/25, 70/50, 140/100, 280/200, and 560/400 sccm. 
The results of this example are shown in the following Table II below: 
TABLE II 
______________________________________ 
Flow rates 
Refrac- Deposition 
Film N.sub.2 O/SiH.sub.4 
tive Rate Etch Stress 
No. (sccm) Index (A/min) Rate* (Dynes/cm.sup.2) 
______________________________________ 
1 16/12 1.465 31 1.23 1.93 
2 35/25 1.467 63 1.28 2.41 
3 70/50 1.468 107 1.3 2.62 
4 140/100 1.469 250 1.3 3.12 
5 280/200 1.470 607 1.38 3.31 
6 560/400 1.471 1368 1.59 3.45 
______________________________________ 
*See Table I 
EXAMPLE 3 
In this example, the pressure, temperature and helium flow rate were kept 
the same as in Example 1. In this example, however, the N.sub.2 O/1.4% 
SiH.sub.4 in He flow rate ratio was kept constant at 560/400 sccm. The RF 
power density was again varied at 0.1, 0.2, 0.5, 1, 2, and 3.25, 
W/cm.sup.2. The results of this example are shown in the following Table 
III: 
TABLE III 
______________________________________ 
RF 
Power Refrac- Deposition 
Film Density tive Rate Etch Stress 
No. (W/cm.sup.2) 
Index (A/min) Rate* (Dynes/cm.sup.2) 
______________________________________ 
1 0.1 1.464 1210 2.63 2.55 
2 0.2 1.460 1415 2.63 3.10 
3 0.5 1.462 1478 2.31 3.24 
4 1 1.467 1469 1.85 3.38 
5 2 1.471 1368 1.59 3.45 
6 3.25 1.470 1138 1.53 3.31 
______________________________________ 
*See Table I 
In each of the above examples, the equipment used was an Applied Material 
Precision 5000 PECVD tool. The reaction chamber and flow controllers of 
such a system are shown in FIG. 1 in a simplified schematic form. 
In Example 1, the low reactant flow rate ratio resulted in a decrease in 
the deposition rate as the power density was increased, indicating that 
there was an insufficient amount of silane to continue to deposit 
SiO.sub.2. In Example 2, it is seen that as the reactant flow rate was 
increased, the deposition rate also increased. A significant result was 
obtained in that the quality of the layer remained good as shown by the 
relatively constant etch rate for each film. Provided the films are close 
to stoichiometric, the etch rate is a good figure of merit for physical 
quality, and it is known that an etch rate of less than 2 times that of 
thermal oxide is an indication of a good quality film. 
Example 3 shows that at high reactant flow rates the deposition rate will 
be high, independent of the power density. However, a further significant 
result was obtained in that the quality actually increased as the power 
density was increased. The quality factor is again seen by the etch rate 
which was above 2 for power densities below 0.5 W/cm.sup.2 and below 2 for 
power densities of about 1 W/cm.sup.2 or greater. Thus, it can be seen 
from the three examples, in particular, films 5 and 6 of Example 2, and 
films 4, 5 and 6 of Example 3, that for a power density of about 1-4 
W/cm.sup.2 and a flow rate ratio of the reactant gases within the range of 
about 280/200-560/400 sccm, a high quality film was deposited at rates 
from about 600 angstroms per minute to about 1500 angstroms per minutes. 
For insulators which are to be considered for primary insulation purposes, 
such as gate insulators, the most important property is the electrical 
integrity. The interfacial region is clearly an important concern, but 
equally important are the bulk properties; fixed insulator charge, 
transient charging, leakage current and stability. Many of these 
properties can be assessed from simple I-V or C-V characteristics of MOS 
or MOM structures. 
For electrical characterization, aluminum counter electrodes, typically 
500-1000 angstroms thick with an area of about 5.2.times.10.sup.-3 
cm.sup.2, were evaporated through a shadow mask on the PECVD deposited 
SiO.sub.2 layer and the back side of the wafer was etched to insure good 
contact. FIGS. 2 and 3 show I-V and C-V characteristic curves for MOS 
structures formed as above. For comparison purposes, curve A in FIGS. 2 
and 3 corresponds to film No. 1 of Example 3 and curve B of FIGS. 2 and 3 
corresponds to film No. 5 of Example 3. 
FIG. 2 shows dynamic ramp I-V curves from which much information can be 
ascertained relative to the electrical properties of the SiO.sub.2 film. 
For curve B, the onset of current injection from the silicon substrate 
occurs at F 6-7 MV/cm, as expected for Fowler-Nordheim injection through 
the 3.2 eV barrier corresponding to the Si:SiO.sub.2 conduction band 
discontinuity. The pronounced ledge in curve B is related to trapping 
events in the bulk of the film. This is a powerful technique as trap 
parameters such as the capture cross-section and capture probability can 
be determined. This capture probability is related to the current level at 
which the ledge is observed, the ramp rate and the centroid of the charge 
distribution. Curve B shows that the trapping level occurs at about 
10.sup.-8 A which is indicative of a relatively low trapping probability. 
Thus, the high current injection, the high breakdown field and the low 
trapping probability results in a characteristic that resembles thermally 
grown SiO.sub.2. 
In contrast, curve A of FIG. 2 shows a ramp I-V characteristic for a PECVD 
film deposited at 0.1 W/cm.sup.2, film 1 of Example 3. The electrical 
integrity of this film is very poor. Premature current injection occurs at 
very low fields, less than 1 MV/cm and there is no well defined trapping 
ledge. In addition, it has been determined that the characteristics were 
not reproducible from run to run as the premature injection varied 
greatly. 
The data of FIG. 2 indicates the improvement in the electrical integrity of 
the SiO.sub.2 film with increasing RF power density. With a power density 
of 2 W/cm.sup.2, premature injection is reduced and the breakdown field 
increases. It has been determined that the I-V curves for films 4 and 6 of 
Example 3 and films 5 and 6 of Example 2 are similar to curve B of FIG. 2. 
The above characteristics have shown to be reproducible from run to run. 
It is clear from FIG. 2 that the leakage current for curve B is essentially 
zero for fields less than 5 MV/cm. The absence of low field breakdowns 
indicates that pinholes and growth defects should not be a problem. Curve 
A is a leaky oxide. It would require several thousand angstroms of 
material to minimize the gate leakage and gradual charging of the oxide 
would be a long term stability problem making the film useless for 
applications that require good electrical integrity. 
FIG. 3 shows C-V data for a MOS capacitor fabricated on a 2 ohm-cm silicon 
substrate for the films identified for curves A and B of FIG. 2. Curve A' 
is the high-frequency characteristic for film 5 of Example 3. These data 
were measured after a postmetallization anneal (PMA) in forming gas (10% 
H.sub.2 in N.sub.2) at 400.degree. C. for 30 min. Curves A and B are low 
frequency (quasistatic) characteristics which can be analyzed to measure 
the interface state density. The interface state density causes a 
deviation of the quasistatic curve from the high frequency curve, the 
magnitude of which can be used to determine the interface state density. 
The difference between curves A and A' at about -1 volts is caused by an 
interface state density of 3.times.10.sup.11 cm.sup.-2 eV, which is quite 
high. The difference between curve B and A' at about -1 volts is caused by 
an interface state density of 4.times.10.sup.10 cm.sup.-2 eV.sup.-1, which 
is very low. 
The refractive index measured by ellipsometry can be used to detect 
deviations from stoichiometry in good quality oxides. The known range for 
stoichiometric SiO.sub.2 deposited by PECVD is from 1.46 to 1.52. As can 
be seen in Tables I, II and III, the refractive indices for all the films 
are close to 1.465 which is the index for thermal oxide. In addition, as 
stated above, a good figure of merit is the etch rate for the oxides. The 
etch rate shown in Tables I, II and III is the factor that the etch rate 
for the sample was greater than the etch rate for thermal oxide in 7:1 
buffered hydrofluoric acid at 23.degree. C. An etch rate factor of less 
than 2 is an excellent etch rate. 
Additional tests on films 1 and 4 of Example 3 were performed to determine 
the extent of impurities. In film 1, there are significant amount of 
impurities such as nitrogen and hydrogen. In film 4, however, the only 
impurity found was a low amount of hydrogen. 
Conventional PECVD process results (i.e. low or no He dilution), are 
comparable to that of curve A for FIG. 2 and are worse than curve A of 
FIG. 3. Typical conventional PECVD process conditions are 2% SiH.sub.4 in 
He=200 sccm, N.sub.2 O=400 sccm, power density=0.1 W/cm.sup.2, pressure 
1-10 Torr, temperature=350.degree. C., and a deposition rate of 
approximately 1000 angstroms per minute. The interface state density is 
approximately 10.sup.12 cm.sup.-2 eV.sup.-1 and the I-V curves indicate 
that the films are leaky. The etch rate is 2-4 times that of the thermal 
oxide and impurities are C, N and H. 
As can be seen from the above data, the conventional PECVD process does not 
produce a high quality silicon dioxide layer at acceptable deposition 
rates for single wafer tools. In accordance with the present invention, a 
PECVD system utilizing large amounts of helium dilution and high RF power 
density produces high quality silicon dioxide films at very high 
deposition rates. As a result the low temperature PECVD system is now 
commercially useful for depositing device quality SiO.sub.2 films. 
While the invention has been particularly shown and described with respect 
to preferred embodiments thereof, it will be understood by those skilled 
in the art that the foregoing and other changes in form and details may be 
made therein without departing from the spirit and scope of the invention 
which would be limited only by the scope of the appended claims.