CA1071140A - Protective coating for semiconductor substrates - Google Patents

Abstract

PROTECTIVE COATING FOR SEMICONDUCTOR SUBSTRATES

Abstract of the Disclosure A low-temperature, high-pressure, high-power process, which utilizes a radio frequency radial flow reactor, utilizes only silane and ammonia as the reactant gases for forming films on silicon substrates. The choice of appropriate operating conditions results in the deposition of essentially stoichiometric silicon nitride films having compressive stress and good crack resistance.
In addition, these films provide good step coverage, good scratch resistance, and an inert barrier to sodium and moisture.

Description

-Background of the Invention . _ . . _ .
This invention relates to methods for depositing silicon nitride films utilizing a radio frequency (rf) powered radial-flow reactor in which laminar flow of reactant gases in a radial direction over a surface is utilized.
The reliability of semiconductor devices, particularly metal oxide silicon semiconductor devices (~OS), is largely a function of the manner in which these devices are passivated and how the completed devices are isolated from the environment. Two of the main problems associated with semiconductor devices are moisture and sodium contamination. Both these elements tend to attack unprotected semiconductor devices and can lead to a failure of the device. Some passive layers are utilized to protect the surface of the semiconductor device from handling during the fabrication thereof and to provide an electrical isolation barrier. Generally, devices with this kind of passivation layer are packaged in hermetically sealed units in order to prevent sodium and moisture damage.
The relatively high cost of hermetic packaging and the testing associated therewith present a serious economic problem. Many of today's integrated circuit chips cost significantly less than the hermetic packages used to house these chips. It has become very desirable to provide a covering film which will not only protect semiconductor chips from mishandling, but will insulate the chips against moisture and undesirable impurities such as sodium.

Semiconductor chips utilizing such a protective film could be placed in relatively inexpensive packages (i.e., ~- nonhermetic plastic packages). This would slgnificantly reduce the total cost of the packaged chip./
One commonly used passivation layer on aluminum metallized semiconductor chips is a phosphosilicate glass film which acts as a getter for sodium. One of the problems with this film is that it has a tendency to react with moisture and form phosphoric acid as a result thereof. This tends to corrode the aluminum metallization. One other problem is that the metallized films deposited subsequent to the film's deposition do not tend to adhere well to the protective film and, therefore, both moisture and sodium damage can occur in the areas of poor adherence. These films are typically deposited by a chemical vapor deposition (CVD) process. Physical steps occurring on the semiconductor chip surface can be poorly covered by these films and, as a result, some areas of the semiconductor surface have little or no protective covering.
Contamination of the semiconductor chips at or near these uncovered areas is very likely.
It is known that silicon nitride films provide not only an electrical barrier but also a barrier to sodium as well as moisture. One of the problems of using a CVD
process to deposit silicon nitride films is that the temperature range used is generally 700 to 900 degrees C.
This poses a problem because aluminum metallization has a melting point of approximately 660 degrees C. Still further, CVD-deposited films have relatively high tensile stresses and, consequently, they tend to crack if made thicker than a few thousand angstroms.
Various publications indicate methods of depositing silicon nitride at temperatures below 450 degrees C, using 1071~40 -an rf plasma to provide some of the activation energy used for the reaction of silane and ammonia or silane and nitrogen, or a combination bf silane, ammonia and nitrogen.
Other publications show the use of a quartz tube, and inductively coupled plasma, and gases flowing at a pressure of approximately 100 microns. These conditions generally result in poor uniformity of film layer thickness, from semiconductor wafer to semiconductor wafer, and of inadequate step coverage.
The IBM Technical Disclosure Bulletin of July, 1967, Vol. 10, No. 2, p. 100, discloses the use of an rf _ reactive sputtering technique for depositing a layer of silicon nitride by utilizing a silicon cathode and a gaseous mixture of ammonia and argon. The publication points out the advantage of using ammonia over pure nitrogen. However, the resulting silicon nitride film tends to have poor step coverage and the relatively high power utilized in conjunction with the low pressure required for sputtering can result in X-rays which tend to damage the semiconductor devices. The difficulties of this kind of system are clearly pointed out in U.S. Patent 3,565,674, lines 6-11.
U.S. Patent 3,757,733, Reinberg, describes a radial flow reactor which is utilized to deposit silicon-nitrogen films on semiconductor wafers. The apparatus described represents an improvement over prior art apparatus and illustrates that contrary to prior thinking, rf plasma deposition of silicon-nitrogen films can be achieved without undue complexity and expense. The gases described for use in Reinberg's apparatus are silane and nitrogen contained in the carrier gas argon. One of the problems we have experienced with this system is that the films produced have 107~140 relatively low densities and high tensile stresses. This combination of low density and high tensile stress leads to a tendency for the films to crack during subsequent relatively high temperature processing steps as are normally required during the attaching of the semiconductor chip to a lead bearing package. These characteristics tend to limit the useful thickness of the film to a few thousand angstroms in order to prevent excessive cracking and it is often desirable to have thicker films for good coverage over the steps in the semiconductor chip.
It has been found that the use of ammonia with silane and nitrogen tends to improve the quality in the resulting silicon-nitrogen film; however, film cracking still is a serious problem.
It would be desirable to be able to produce a silicon-nitride film on the surface of semiconductor devices which provides protection against handling, which has good electrical isolation, step coverage and good resistance to cracking upon heating, and which can be deposited to thicknesses of approximately 1 micron without any resulting cracking.
Sumr.lary of the Invention To this end, the present invention is a method for coating substrates with silicon nitride by plasma deposition in a radio frequency (rf) powered nonsputtering type of radial flow reactor. The reactant gases utilized are silane and ammonia with an inert carrier gas, such as argon.
Careful precautions, such as the use of exclusively stainless steel interconnections, are utilized to limit the presence of gases such as N2 and 2 i~ the reactor during the deposition process. A relatively high gas flow rate of .
typically 2320 sccm and a relatively high dynamic pressure of typically 950 microns are utilized.
/The protective films deposited by use of the above-described method have relatively low compressive stress and therefore tend to be very resistant to cracking.
The films tend to be essentially stoichiometric silicon nitride (Si3N4) and tend to be essentially free of other organic combinations and/or inert carrier gas incorporation.
These films have electrical characteristics very similar to those of silicon nitride films which are produced by a chemical vapor deposition (CVD) processes. One significant - difference is that films produced by CVD processes have a relatively high tensile stress, while those produced by the method of the present invention have relatively low compressive stress. It is well recognized that films with compress-ive stress tend to have much greater resistance to cracking than those with tensile stress. These crack resistant films of the present invention protect substrates and thus eliminate the need to mount the substrates in hermetically sealed packages and permit the packaging thereof in much less expensive packages. The methods of the present invention are particularly advantageous for use with semiconductor wafers. The films deposited on semiconductor wafers provide high-crack resistance, good step coverage and scratch resistance, and an inert barrier to sodium and moisture.
Semiconductor wafers having aluminum metallization are heated to a temperature of typically 330 degrees C
during the deposition process. Semiconductor wafers having -`` 1071140 gold metallization with titanium, platinum or palladium e~ and gold beam leads, are heated to a temperature of t~pically 275 degrees C during the deposition process.
The silane and ammonia mixture allows for considerable variation of such process parameters as input rf power, substrate temperature, dynamic pressure and gas flow rate.
In accordance with an aspect of the present invention there is provided a method for depositing a silicon-nitrogen film having compressive stress on substrates by plasma deposition in a radio frequency powered nonsputtering-type of reactor comprising the steps of:
evacuating the reactor which contains the substrates to a relatively high vacuum; introducing a flow of silane, ammonia and an inert carrier gas into the reactor and over the substrates while limiting the presence of other gases in the reactor to insignificant amounts; establishing a relatively high flow rate of 1500 SCCM or greater of the gases through the reactor and a relatively high dynamic pressure within the reactor of 800~ or greater; the ratio of silane to ammonia being 0.5 ~ silane/ammonia ~ 1.0;
heating the substrates to a temperature within the approximate range of 250 degrees C ~ T ~ 400 degrees C;
forming a plasma-glow discharge reaction in the reactor adjacent the substrates contained therein to cause a film to be deposited on the substrates; and adjusting the radio frequency power into the reactor to a power level priorly determined to result in the deposition of a film of a silicon-nitrogen compound having compressive stress and having an approximate ratio of silicon to nitrogen of three to four.
These and othee features and advantages of the ~ - 6 -~07~40 invention will be better understood from a consideration of the following detailed description taken in conjunction with the accompanying drawings.
Brief Description of the Drawin~
FIGS. 1 and 2 illustrate a radial flow reactor useful in one embodiment of the method 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 wafers, and as the ordinate axis, corresponding characteristics of the deposited film.
Detailed Descrlption Referring now to FIGS. 1 and 2, there is illustrated in a cross section and a top view a cylindrical radial 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 - 6a ~

107~40 ,,~ .
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 is 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 wafers 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 wafers _ 30 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 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 of electrode 26.
A plurality 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 40 are connected by second ends thereof to a common sheath (tube) 50 which has a control valve 52 connected in series therewith.
A sheath 54 communicates with the interior of 107~40 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 sheath 54 is coupled to vacuum pumps 58 that are used to evacuate the interior of chambex 24.
The reactant gases required to coat the semiconductor 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" 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 wafers 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 wafers 30. Thus, without the shield 38 the increasing of rf power beyond a certain point is not particularly h~lpful in intensifying the glow discharge reaction above the wafers 30 where it is important that the reaction occur.
The vacuum pumps of FIG. 1 are selected to be ~" 1071140 compatible with a high gas flow rate of approximately

2 liters per minute at greater than 1 mm pressure. A
150 cfm Leybold-Hereaus roots blower backed with two 17 cfm mechanical pumping 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 value (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 -10 6mm.
In operation, semiconductor substrates 30 are loaded on support surface 28. The reactor 10 is then sealed, closed, and pumped down to 10 6mm. The heaters connected or part of the electrodes 26 are turned on and the semiconductor substrates are heated to approximately 275 degrees C. The vacion 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~ 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.
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 (SiH4) in a carrier gas argon (Ar) 1000, ammonia (NH3) in a carrier gas argon (Ar) 1100, carbon tetrafluoride (CF4) 1200, and oxygen (2) 1300, 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 g _ 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 communication with reaction chamber 2700 through valve 2800.
The reactant gases SiH4 and NH3 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 rnore 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 2(` 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 CF4 and 2) and a new set of semiconductor wafers 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 30 essentially prevents any but the desired gases frbm entering the systems during the deposition operations. Essentially 1071~40 pure sources of SiH4, NH3, and Ar could be easily substituted for the Si~14 in Ar and ~H3 in Ar sources.
In the earlier work described in the first three sets of operating conditions, below described, there were deposited films which were nonstoichiometric and such work forms the basis of a separate Canadian copending application, Serial r~o. 269,012, filed concurrently and having the same assignee as the instant application. Moreover, in the first set of eonditions described, the reactor was essentially that shown in FIGS. 1 and 2 without the gas shield and with electrode 18 in electrical contact with tip 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 2nd Operating Condition Condition (using apparatus (using apparatus of FIGS. 1 & 2 of FIGS. 1 & 2) without gas shield) 20 Reactant gas SiH4/NH3/Ar SiH4/NH3/Ar SiH4 1.25% 1.70%
NH3 1.56% 2.39%
Ar 97.19% 95.91%

Total gas flow (SCCM) 2000 2320 Pressure in reactor (~) 1000 950 Substrate temperature 330 275 30 (degrees C) Tuned rf - 60 250 power (watts) (reflected power=~O) Thickness of deposited 1.1 1.1 layer (~) 1(~71140 Stress in 1-2 (tension) 1-5 (tension) res~lting layer (10 dynes) cm Etch rate in 175 180 BHF (Angstroms per min.) Density (GCM 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 2') 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 (106 V/cm) Resistivity at 5 x 10 4 x 10 2 x 106 V/cm (ohm/cm) 3U 3rd Operating 4th Operating Condition Condition (using apparatus (using apparatus of FIGS. 1 & 2) of FIGS. 1 & 2 Reactant gas SiH4/NH3/Ar SiH4/NH3/Ar SiH4 1.78~ 1.78%
NH3 2.25% 2.25%
Ar 95.97% 95.97%
Total Gas 2320 2320 flow (SCCM) Pressure in 950 950 reactor (~) ~071140 , ~
Substrate 275 275 temperature (degrees C) Tuned rf - 300 400 power (watts) (reflected power=-O) Thickness (~) 1.1 1.1 Stress in 1-2 (compression) 1-2 (compression) resulting layer lU (109 dynes) Cm' Etch rate in 125 75 BHF (Ang-stroms per min) Density (GCM ) 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 3r) 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 (106 V/cm) 40 Resistivity6 3 x 10 5 x 10 9 at 2 x 10 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 . 1071140 , 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 of 14 inches. Electrode 18 was separated from the electrode 26 by approximately 1". A reactor with different type or size of electrodes and different spacing between electrodes would require a corxesponding input rf power in 1(` order to produce films on semiconductor wafers with essentially the same characteristics as described herein.
The first operating condition is useful for depositing protective films on semiconductor wafers which utilize aluminum metallization. The aluminum metallization can easily withstand temperatures at and above the 330 degrees C used. The second through fourth operating conditions can be used with semiconductor wafers which have aluminum or gold with titanium, palladium and gold beam leads since the temperature utilized is below that at which 2~ titanium and palladium and gold interact.
Cracking of the protective films allows moisture and impurities (i.e., sodium) to attack the surface of the semiconductor wafer and thereby destroy the circuitry contained thereon. It is therefore very important that protective films be as crack-resistant as possible, which is in accordance with the present invention.
The fourth operating condition results in films which are substantially stoichiometric silicon nitride (Si3N4) and which contain essentially no other organic combinations or argon incorporation. The physical characteristics of the resulting Si3N4 films are superior to ~--` 107~40 Si3N4 films produced by chemical vapor deposition (CVD) processes in that they are much less susceptible to cracking than the CVD produced Si3N4. 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 (N2) or oxygen (2) in the reactor during the flow discharge reactions. It has been determined through experimentation that the addition of even small amounts of N2 (up to 2%) or 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~ 2 to the reactant gases resulted in a 7 times increase in the BHF etch rate. Accordingly, it is desirable to keep the nitrogen and oxygen levels at least below such amounts and it appears advantageous to keep the levels as low as it is economically practical.
It has been observed that utilizing the methods of the present invention had on occasion resulted in sporadic nodular growths or hillocks in the films deposited on some semiconductor wafers. A high correlation is believed to exist between contamination of the surface of semiconductor wafers by carbon (from baked-on residues of inactive photo-resist) and/or particulate matter and the occurrence of nodular growths. It is believed that the presence of contamination ori the surface of semiconductor wafers causes 1071~4(~

a gas-phase reaction in the vicinity of the contaminated area which results in the nodular growths. Such gas-phase reactions in response to surface contamination are found to ore likely to occur under conditions of temperatures in excess of 200 degrees C and flow rates of greater than 1000 sccm. It is important that surface contamination be attenuated because the methods of the present invention utilize such temperatures and flow rates.
A method of cleaning semiconductor wafers prior to the deposition of protective films thereon has been found to be particularly effective in essentially eliminating all ~ nodular growth. After the semiconductor wafers have been processed through metallization, but before any standard metallization-bake process, the following cleaning process may be utilized.
1. Car~fully strip all inactive photoresist using A30 stripper wherever practical.
2. Remove any residual carbon by boiling in a mixture of 90% H2O and 10% H2O2 for 10 minutes followed by a de-ionized H2O rinse (15 minutes or longer).

3. Perform the standard metallization-bake process (e.g. aluminum 450 degrees C in H2 for 1/2 hour; Ti/Pd/Au 250 degrees C in forming gas for 16 hours; Ti/Pt/Au 325 degrees C in forming gas for about 3 hours).

4. Reclean by scrubbing both sides of the semiconductor wafer in triton X
(1:20,000 dilution) followed by a mixture of 90% H2O and 10% H2O2 followed by a H2O rinse (15 minutes or longer).

10~1~40 , With the above-described cleaning process it has been found that nodular growth has been essentially eliminated on at least several hundred semiconductor wafers which were coated with protected films by the methods described herein.
After the protective coating has been deposited on the semiconductor wafers they are removed from the reactor.
Contact windows are now opened in the protective coating to the metallization below to facilitate bonding of wires to 1~ these areas or the deposition of Ti/Pd/Au beam leads into the areas. Care should be taken to insure that the side ~ walls of the contact windows are at least vertical and not at re-entrant angles. This helps insure that only the silicon areas designated to be contacted are exposed. This leaves the wafers essentially hermetically sealed.
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 characteris-tics of the film deposited on semiconductor wafers.
A. Gas Composition The graph of FI~. 4 illustrates the effect of increasing SiH4 concentration (1. 4 < % SiH4 < 1. 9;
O. 5 < SiH4/NH3 < O . 9) in the reacting gases. These gas 30 compositions were achieved by adjusting the flow-meters for 3~SiH4 in Ar and 5%NH3 in Ar to various complementary ~071~0 settings so as to keep the total flow constant.
As expected, increasing the SiH4 concentration in the gas led to a corresponding linear increase in the Si/n ratio in the film (from -1.0 to -1.2~, and a linear increase in the refractive index (from -1.9 to -2.2). For the lowest SiH4 concentration used, (SiH4/NH3 = 0.52~, the film density was found to be relatively low (-2.3 gcm 30, and the BHF
etch-rate was correspondingly high (250 angstroms/min).
With increasing SiH4/NH3 ratio, the film density p showed a broad peak (p - 2.55 gcm 3) for 0.58 < SiH4/NH3 < 0.79. The p decreased again at SiH4/NH3 ~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 (SijN-1.2). The film a, which was always tensile, showed a peak at SiH4/NH3-0.6, which is located at a slightly lower SiH4 concentration than that for the peak in P
While most of the present 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 , with the SiH4/llH3 ratio constant at 0.71 (~SiH1 = 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), a greater refractive index, and a larger Si/N ratio in the film (from 0.8 to 1.05). For this range 07~40 of film composition, the film density seems to have a dominant effect on the BHF etch rate; a broad maximum in p corresponds to a broad minimum in the etch rate. The tensile stress decreases 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 ~700 to 1000~ (+25~). 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 <750~) 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 ~ TS ~ 300 degrees C) was influenced by the desire to stay below temperatures at which Pd-Au interdiffusion (in Ti/Pd/Au me~allization) becomes excessive. As shown in FIG . 7~ TS (substrate temperature) has a pronounced effect on the BHF etch rate, which decreases almost exponentially with increasing Ts.
The decrease in BHF etch~rate is associated with a linear increase in the film density, p, and in the refractive index, . Thus, for films deposited at 200 degrees C, the BHF etch rate was 700 angstroms/min, the density was -.

1071~40 -2.3 gcm- 3 and the refractive index was -1.85.
- Interestingly, these films also had a rather large Si/N
ratio ( 1.2) and a high tensile stress (7xlO9 dynes cm 2).
With increasing Ts, both a and the Si/N ratio in the film displayed a shallow minimum at -250 degrees Cî however, a higher TS of 275 degrees C was preferred because it led to films with yet greater density (2.55 gcm 3) and somewhat lower etch-rate without an excessive increase in a.
E. _F Input Power 10The 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 = O). For this series of experiments, the SiH4/NH3 ratio was kept constant at 0.8, and SiH4 at 1.81. For increasing rf power, there was found to be a rapid and linear increase in the film p (weight-gain measurements, using 1~ thick films) from 2.2 gcm 3 at 100 watts to 2.8 gcm 3 at-350 watts. Films (1~) thick with lower density had a distinct yellowish tinge to them when deposited on Al-metallized devices, whereas those with densities >2.4 gcm 3 appeared to be grayish and more truly transparent. Both the film a and BHF etch-rate showed a bimodal behavior at -275 watts. Below this power level the stresses were very low tensile (~0.5xlO9 dynes cm 2) and the etch-rates were relatively high (275 to 325 angstroms/min). At rf powers _300 watts, the stresses, which had been tensile, became compressive (l-2xlO9 dynes cm 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-Lorenz equation.

~-` 1071i40 It is believed that with increasing rf power, the - plasma acquires a greater electron density, which causes a more complete decomposition of the reactants SiH4 and NH3.
This condition leads to films with stoichiometric Si3N4 composition with essentially no hydrogen or argon incorporation and, therefore, excellent electrical insulating characteristics which are comparable to those of CVD-produced Si3N4.
Stoichiometric silicon-nitride films have been 10 produccd using a 0.7 ratio of silane to ammonia, 1.72%
silane, argon as the carrier gas, a flow of 2320 sccm, a pressure of 950~, a substrate temperature of 275 degrees C, and an input rf power of 400 watts. It is believed that stoichiometric silicon nitride can be produced with the proper combination of the ranges of the variables listed below: .
Flow rate (sccm) - 1500 - 2800 Pressure (~) - 800 - 1100 % SiH4 - 0.5 - 2.0 2~ SiH4/NH _ 0.5 - 1.0 rf lnput power - 200 - 500 (watt~) Substrate - 250 - 400 temperature (degrees C) It should be feasible to make changes in the reactor and in gases employed without departing from the spirit and scope of the invention.

Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for depositing a silicon-nitrogen film having compressive stress on substrates by plasma deposition in a radio frequency powered nonsputtering-type of reactor comprising the steps of:
evacuating the reactor which contains the substrates to a relatively high vacuum;
introducing a flow of silane, ammonia and an inert carrier gas into the reactor and over the substrates while limiting the presence of other gases in the reactor to insignificant amounts;
establishing a relatively high flow rate of 1500 SCCM
or greater of the gases through the reactor and a relatively high dynamic pressure within the reactor of 800µ or greater;
the ratio of silane to ammonia being 0.5 ? silane/ammonia ? 1.0;
heating the substrates to a temperature within the approximate range of 250 degrees C ? T ? 400 degrees C;
forming a plasma glow discharge reaction in the reactor adjacent the substrates contained therein to cause a film to be deposited on the substrates; and adjusting the radio frequency power into the reactor to a power level priorly determined to result in the deposition of a film of a silicon-nitrogen compound having compressive stress and having an approximate ratio of silicon to nitrogen of three to four.

2. The method of claim 1 wherein the carrier gas is argon.

3. The method of claim 2 further comprising the steps of:
removing the substrates from the reactor after the desired coating is achieved;
introducing carbon tetrafluoride and oxygen into the reactor; and causing a second glow discharge reaction in the reactor to clean any deposited films off internal areas of the reactor.

4. The method of claim 3 wherein the reactor is heated during the cleaning steps.

5. The method of claim 4 wherein the substrates are wafers of silicon having either aluminum metallization patterns and aluminum bonding pad areas, aluminum metallization patterns with titanium-platinum-gold beam leads, or gold metallization patterns with titanium-palladium-gold beam leads.