METHOD OF DEPOSITING OPTICAL OXIDE COATINGS AT ENHANCED RATES

Abstract: A sputtering method is provided in which a substantially transparent target oxide film with predetermined optical properties is prepared having substantially the same degree of absorption per unit thickness as were the process practiced with pure oxygen gas as sole reactant, yet with a rate that is enhanced with respect to the method when oxygen gas is sole reactant. In practicing the inventive method, a target substrate is sputtered in a chamber while providing a source of reactive oxygen. The source of reactive oxygen has a reactivity in forming target oxide that is enhanced with respect to pure oxygen gas and includes nitrous oxide. Target oxide deposited on the substrate has an absorption per unit thickness that is substantially the same absorption per unit thickness as when pure oxygen gas is sole reactant. The deposition rate is preferably controlled by a value derived from plasma emission lines of oxygen atoms and target atoms.

Description: METHOD OF DEPOSITING OPTICAL OXIDE COATINGS AT ENHANCED RATES Field of the Invention The present invention relates to the deposi¬ tion of optical oxide coatings, and more particularly to a method of reactively sputtering in which a substan¬ tially transparent target oxide film with predetermined optical properties is deposited at a rapid rate. This is a continuation-in-part of Serial Number 390,589, filed August 7, 1989. Background of the Invention Deposition of anti-reflection coatings, heads- up displays, enhanced reflectants and other transparent optical coatings requires the utilization of transparent oxide (typically metal oxide) layers. These thin films can be used in known fashion to provide particular mechanical, chemical and optical properties. In certain applications, it is desirable to provide a film of a substantially pure oxide such as a titanium oxide, a silicon oxide, a tin oxide or an aluminum oxide. Such substantially pure oxide films offer good resistance to mechanical abrasion, resist degradation by moisture in the environment and generally can provide films with indices of refraction ranging from 1.45 (Si0 2 ) to 2.40 (Ti0 2 ) . Accordingly, a substantially pure oxide film may be utilized as an exposed surface of an optical element. For example, a window pane may be provided with a multilayer coating incorporating layers of titanium oxide and layers of silicon oxide in an alternating arrangement to provide transmission and reflection ' of light at particular wavelengths. " Pure oxide films may be deposited by a process known as reactive sputtering. In sputtering, the item to be coated, or "substrate", is placed adjacent to a source of material to be deposited, commonly referred to as a "target". The target is exposed to a plasma or ionized gas and the target is placed at a negative electrical potential with respect to the plasma. Under these conditions, ions from the plasma strike the target with considerable energy and dislodge material from the target. The dislodged material deposits on the substrate to form the coating. Ordinarily, the entire sputtering process is conducted within a chamber maintained at a low, subatmospheric pressure. In reactive sputtering, the plasma contains a reactive constituent which combines with the material of the target so that the deposited coating includes both the material from the target and the reactive constituent from the plasma. Thus, oxide coatings can be formed by reactive sputtering utilizing a target containing a metalloid such as silicon or a metal such as titanium, tin or aluminum together with a plasma containing oxygen. The oxygen in the plasma combines with the metal or metalloid dislodged from the target, and the coating deposited on the substrate is an oxide of the metal or metalloid. The preferred deposition technology for large area production of uniform films is DC magnetron reactive sputtering. Traditionally, such a reactive sputtering process has been done by sputtering metallic targets in pure oxygen. However, this typically results in a deposition rate that is up to ten times slower than the metallic rate. This slow deposition rate is believed due to coverage of the target surface with reactive compounds. The target surface can become completely reacted to form an oxidized surface, which leads to a drastic sputtering rate drop. Coatings other than oxide are known and used in a variety of applications. For example, U.S. Patent No. 3,962,062, issued June 8, 1976, inventor Ingray, discloses sputtered thin films, particularly of tantalum and niobium, prepared in the presence of oxygen gas and nitrogen gas, the flow rate ratios of which are varied during the sputtering. The gas ratio variations are taught as desirable in giving films having different refractive indices. Similarly, U.S. Patent No. 4,125,446, issued November 14, 1978, inventors Hartsough et al., discloses a method for depositing aluminum layers having a predetermined reflectance or resistivity. The reactive gases used as minor constituents of the sputtering gas are nitrogen, hydrogen, oxygen, and water vapor. The major gas is argon or other ionizable inert gas. Aluminum nitride films appear to be formed. U.S. Patent No. 4,428,811, issued January 31, 1984, inventors Sproul et al., discloses use of a metal as target in a reactive deposition process where an inert gas is first ionized and then a reactive gas such as nitrogen is introduced. The films deposited were titanium and the like metallic nitrides. U.S. Patent No.4,769,291, issued September 6, 1988, inventors Belkind et al., describes a sputtering method for sputtering an alloy of aluminum and silicon with oxygen, nitrogen, compounds of oxygen and compounds of nitrogen as preferred reactive gases. The inventors found that when the sputtering gas contained nitrogen, then the coating contained a mixture of aluminum and silicon nitrides or oxynitrides. U.S. Patant No. 4,384,933, issued May 24, 1983, inventor Takasaki discloses a reactive sputtering method and exemplifies the method by preparing a film of SiN, said to be close to stoichiometrically stable Si 3 N 4 , for nitrogen gas activated by a microwave oscillator and a sputtered Si target which film is grown on the surface of a semiconductor substrate. The patent suggests use of other gases as reactive gas, use of other target materials, and the formation of compounds such as oxides or nitrides in using the reactive sputtering method. However, for applications where there must be substantially the same average film absorption per unit film thickness compared to oxide films prepared in pure oxygen, the prior nitride and oxynitride films are unsatisfactory because they absorb at visible wavelengths. Summary of the Invention It is an object of the present invention to provide a sputtering method in which a substantially transparent target oxide film with predetermined optical properties is prepared that has substantially the same degree of absorption per unit thickness as were the process practiced with pure oxygen gas a sole reactant, yet with a rate that is enhanced with respect to the method when oxygen gas is sole reactant. Another object of the present invention is to provide a means for controlling the sputtering method so that a desired (enhanced) deposition rate can be achieved. In one aspect of the present invention, a method of maximizing a substrate deposition rate of substantially transparent target oxide film comprises providing a target substrate in a chamber, introducing a source of reactive oxygen into the chamber, with the source of reactive oxygen having a reactivity in forming target oxide that is enhanced with respect to pure oxygen gas, and flowing the source of reactive oxygen while controlling an absorption per unit thickness of target oxide deposited on the substrate at substantially the same absorption per unit thickness as when pure oxygen gas is sole reactant. Practice of the invention provides, for example, deposition rates of target oxide that can be on the order of over about 50% faster than when pure oxygen gas is sole reactant. A preferred process control includes maintaining an emission ratio that is derived from emission spectra within the chamber. The source of reactive oxygen must include nitrous oxide, and may also include oxygen gas, nitrogen gas or a mixture of oxygen and nitrogen gases. Although nitrous oxide is usually the primary source of reactive oxygen, the method is practiced to avoid formation of oxynitrides on a desired substrate because the reactant gas is selected to provide an enhanced deposition rate while maintaining a desired optical property. Thus, substantially no nitrogen is incorporated into the deposited films so that there are little or no changes in optical constants of the films when compared to pure oxygen deposition. Other objects and advantages of the present invention will become apparent to persons skilled in the art upon reading the following description of the invention. Brief Description of the Drawings In the drawings: Fig. 1 is a graph illustrating emission hysteresis data during the sputtering of titanium in the presence of three different gases or gas mixtures where the vertical axis is the level of Ti detected in the plasma by an emission spectrometer and the horizontal axis is the gas flow rate in standard cubic centimeters per minute (SCCM) ; and. Fig. 2 is a graph illustrating deposition rate as a function of an emission ratio (determined for oxygen at 777 nm wavelength and determined for titanium at 391 nm wavelength) . Detailed Description of the Preferred Embodiments Practice of the invention preferably begins by selecting a reactant,gas that will provide an enhanced deposition rate of an oxide film with respect to sputtering in oxygen while permitting the maintenance of a desired optical property with respect to a baseline value. The baseline value, or values, are established from target in either pure oxygen or a mixture of oxygen and an inert carrier gas at a constant gas pressure between about one 1x10 " Torr to about 5x10 " Tora? to establish baseline values for the deposition rate of target oxide and at least one target oxide optical property. Suitable inert carrier gases are generally the noble gases, preferably argon due to low cost and efficiency. When an inert carrier gas is present, then it will preferably be in amounts less than about 20 flow % (or volume %) with respect to oxygen, more preferably less than about 10 flow % with respect to oxygen. . The reactant gas selected to provide an enhanced deposition rate of oxide film while permitting the maintenance of a desired optical property must include nitrous oxide, and will be entirely or partially nitrous oxide. - For convenience, the at least one target oxide optical property will be exemplified as absorption/A thickness, bujt other target oxide optical properties, such as * the index of -refraction or the like, can be utilized. By absorption/A thickness, as used herein to exemplify the maintenance of at least one target oxide optical property, is meant as follows. Absorption is calculated from equation (1) : (l) Transmission + Reflection + Absorption = 100% where Absorption (A) = Afilm + Asubstrate. Transmission as a function of wavelength for light at normal incidence upon the coated substrate is measured by a spectrophotometer over the visible wavelength range of 380-720 nm. Reflection as a function of wavelength is measured for near normal angle of incidence (<10° angle of incidence) upon the coated substrate over the same wavelength range. Typically, an average value of reflectance and transmission for the visible wavelength region is determined from the following equation (2) : (2) Luminous Illuminant C Reflection or Transmission (R or T) y(λ) = values representing the relative sensitivity of the human eye to various wavelengths of light Pc(λ) = the relative intensity of the illuminating light source for various wavelengths of light T(λ) or R(λ) ~ = the measured reflectance or transmission of the coated substrate at various wavelengths of light R or T = the average reflectance or transmission of a coated substrate ' as perceived by the human eye under particular illuminating conditions. The weighted average is typically performed at 10 nm intervals between 380-720 nm. The values for y(λ) and Pc(λ) [representing illumination similar to normal daylight] are set as internationally accepted standards by CIE. This method of calculation is internationally accepted and the calculations aie-e routinely performed by computers incorporated into the spectrophotometers — in this case, a Pacific Scientific Spectra-Gard Spectro Colorimeter. Physical thickness of the film is typically measured with a stylus profilometer, such as a DEKTAK IIA. This instrument measures the step height between a coated and uncoated region of the substrate by traversing a diamond tip stylus across the step and measuring the vertical deflection of the stylus. The average absorption is calculated from equation (1) . The absorption in the film is calculated from the total absorption by subtracting the absorption of the uncoated substrate. This value is divided by the physical thickness in angstroms to arrive at values of absorption/unit thickness. The target to be so sputtered is one that is sputterable and capable of forming a transparent, oxide film. The target material may be a pure element, typicall a metal, or may be an alloy or mixture of elements. Host (but not all) suitable target materials will form dielectric films. Exemplary target materials are titanium, tin, zinc, tantalum, aluminum, zirconium, indium, bismuth, silicon and tungsten. An exemplary alloy is of indium-tin. Particularly preferred targets are titanium, tin, zinc, tantalum, aluminum and zirconium. The baseline values of deposition rate and optical property are established because different targets will have varied reactivities in forming the oxide compounds, and thus there will be different optimal gas ratios of nitrous oxide, with or without nitrogen and/or oxygen in producing a film at an enhanced rate, but with the desired properties. As will be understood, the particular operating power for the cathode(s) , the voltage, current and discharge type used in establishing baseline values will vary depending upon the particular coater selected for use in practicing the invention. It is believed that one may use a variety of power sources to practice the invention (DC, RF, microwave) . A variety of suitable coaters for practicing the invention are well- known and commercially available. Illustrative suitable coaters include those commercially available from Airco, Inc. as the "C-series" and "G-series", and conventional magnetrons (such as those in a planar or cylindrical configuration) may be used to increase the efficiency of the sputtering. Regardless of the particular coater chosen, the coating chamber is preferably evacuated to a pressure below about lxio " Torr before beginning the sputtering, and the pressure during sputtering is preferably between about 1x10 " to about 5xlθ " Torr, more preferably is at about 3 microns (i.e., 3xlθ " Torr) . The desired operating power for the cathode is typically the maximum production power (and depends upon the size of the cathode and the melting point of the target) . The sputtering in pure oxygen or oxygen and inert carrier to establish baseline values preferably is continued for a:-sufficient period of time to obtain relatively thick films (about 400 A to about 1000 A) . Mass flow gas controllers are preferably used to regulate gas flows and to maintain a constant total pressure when partial gas flow rates of various gases (as subsequent reactant gases) are adjusted. During the sputtering in oxygen or oxygen and inert gas, the target oxide deposition rate is monitored and after sputtering, the film evaluated for one or more optical properties. For example, as .will be exemplified, film absorptions may be measured by means of spectrocolorimeters. The deposition rate is determinable by equation (3): P (cathode power)n (3) d (film thickness - A) = (rate constant) x SC P = Power in W n = Number of cathodes x number of passes under cathodes S ** Line speed (mm/sec) C •= Cathode race track circumference (mm) P Gives power density to allow comparison C among different size cathodes R = Rate constant of material and process with units as A-mm 2/joule The target is then sputtered in a first reactant gas that includes nitrous oxide, and the values for target oxide deposition rate and the least one target oxide optical property are measured. This may be acco plishέti by using nitrous oxide as the master control gas. The flow rate of the nitrous oxide is preferably controlled by feedback signals from an ion gauge pressure readout, in order to maintain a constant total pressure. Mass flow gas controllers preferably are also used to regulate the nitrogen and oxygen gas flows (when added to modify the reactant gas) so as to maintain a desired ratio of flow of these additional gases and proportional to the nitrous oxide flow. Then, as the nitrous oxide flow rate is adjusted to maintain the desired tdtal pressure constant, the nitrogen and/or V. **s_ oxygen flow rates are automatically maintained as a constant ratio to the nitrous oxide flow. An initial gas ratio flow, usually of nitrous oxide to nitrogen, is set. While measuring the target oxide deposition rate values, one typically increases the amount of nitrogen with respect to nitrous oxide from the initial gas ratio while measuring deposition rate, film absorption/unit thickness, and other optical constants. The addition of nitrogen to the nitrous oxide flow will increase the sputtered metal flux from the target. The optimal rate is the maximum deposition rate which can be maintained without incorporating nitrogen into the film and thus increasing film absorption. Occasionally, no low absorption films can be made with any nitrous oxide/nitrogen mix. This indicates that for a particular target there is insufficient oxygen available to react fully as an oxide. Therefore, additional oxygen relative to nitrogen is added. That is, the first reactant gas is modified to form a second, or subsequent, reactant gas. The addition of oxygen is done by controlling the nitrous oxide/oxygen ratio in a similar manner as has been described for the addition of nitrogen. The addition of oxygen will tend to suppress metal deposition rates. Therefore, the minimal level of oxygen is added that will oxidize the film. Turning to Fig. 1, the data shows that the transition to a fully reacted target surface (which is shown by a low level of Ti in the reactive plasma) occurs at lower reactive gas flow rates with nitrous oxide or with a mixture of nitrous oxide and nitrogen (where the N 2 was a constant 20 SCCM) than is true with just oxygen. This indicates that use of the nitrous oxide or nitrous oxide and nitrogen as the reactant gas is a more reactive gas mix than that of oxygen. 12 The absorption per unit thickness of target oxide deposited when practicing the present method can be maintained at less than about 0.002 per A, yet with a substantially increased deposition rate with respect to sputtering in pure oxygen. Particularly preferred films deposited in accordance with the inventive method have a thickness less than about 1 micron, although thicker films can be prepared, if desired. We have found that the plasma emission spectrum can be used to control the deposition rate of the inventive process, as opposed to the -flow rate ratios used during the process. The invention will now be specifically exemplified by several embodiments. All depositions were on glass as illustrative substrate, and substrates were neither heated nor cooled during the depositing. EXAMPLE I A baseline coating sample and then an optimized sample in practice of the invention were deposited ±n an Airco, Inc. "C-series" coater with "HRC- 4500" cathodes (four) at a line speed of 100 inches/ minute. The baseline coating sample was obtained with a mixture of oxygen gas as the reactant gas that was carried by argon. The target was zinc. The optimized, subsequent reactant gas in practice of the invention was a mixture of nitrous oxide and oxygen in a flow rate ratio of 1:0.5. The baseline sample parameters and inventive method parameters are set out below. Baseline Sample: Power/cathode = 40 kW Pressure = 1.8 x 10 " Torr 0 2 flow = 2.7 standard liter/min (SLPM) Ar flow = 0.1 SLPM Ar/0 2 ratio = .04/1.0 Film thickness = 1136 A Film absorption/A = .0013 Deposition rate = 1351 A-m /joule Inventive Method (Rate Enhanced) : Power/cathode = 40 kW Pressure = 1.2 x 10 " Torr N 2 0 flow = 2.3 standard liter/min (SLPM) 0 2 flow = 1.15 SLPM N 2 0/0 2 ratio = 1.0/0.5 Film thickness = 2348 A Film absorption/A = .0017 Deposition rate = 2790 A-mm /joule As can be seen from the data of Example I, the deposition rate for the baseline sample was 1351 A-m /joule whereas the rate enhanced, inventive method was practiced with a deposition rate of 2790 A-mm /joule. This is over an 100% increase in deposition rate. However, the film absorption of the rate enhanced deposition was maintained at less than 0.002 per A and showed an increase of only about 0.0004 film absorption over the baseline sample. EXAMPLE II Zinc was again used as target, but with the sputtering performed in another coater. This other coater was manufactured by Airco, Inc. as the "G-series" and equipped with "HRC-3000" cathodes (one) . The operating parameters, deposition rates and film absorption values for both the baseline sample and the rate enhanced, inventive method were as follows: Baseline Sample: Power/cathode = 25 kW V = 425 I = 59 Pressure = 3 x 10 " Torr 0 2 flow = 319 standard cubic cm/min (SCCM) Film thickness = 455 A Film absorption/A = .0015 Depόsitipn rate = 1212 A-mm/joule Inventive Method (Rate Enhanced) : Power/cathode = 25 kW V = 371 I = 60 Pressure = 4.5 x 10 " Torr N 2 0 flow = 140 standard cubic cm/min (SCCM) N^ flow = 72 SCCM 0 2 -flow ^ *= 219 SCCM s z O/N 2 /o 2 ratio = 1/0.5/1.5 film thickness = 1205 A Film absorption/A = .0007 Deposition rate = 2140 A-mm/joule As can be seen from comparison of the baseline sample and the rate enhanced sample in Example II, there was a deposition rate enhancement of about 75% and the absorption value for the zinc oxide film actually was less than when pure oxygen gas was the sole reactant. Example II also illustrates that N 2 0 need not be the major constituent of the reactive gas in practicing the invention. EXAMPLE III Another baseline coating sample and then two optimized samples in practice of the invention were deposited in an Airco, Inc. coater with HRC-4500 cathode (one) at a line speed of 100 IPM, one pass. The target was titanium. The parameters are set out below. Baseline Sample: Power/cathode = 73 kW V = 383 1 = 189 Pressure = 3 x 10 " Torr 0 2 flow = 3.24 standard liter/min (SLPM) Film thickness = 197 A Film absorption/A = .0019 Deposition rate = 103 A-m /joule (A) Inventive Method (Rate Enhanced) : Power/cathode = 73 kW V = 496 I = 146 Pressure = 3 x lθ "3 Torr N 2 0 flow = 1.38 standard liter/min (SLPM) N 2 flow = 1.37 SLPM 0 2 flow = 0 SLPM N 2 0/N 2 ratio = 1/1 Film thickness = 341 A Film absorption/A = .0009 Deposition rate = 179 A-m /joule (B) Inventive Method (Rate Enhanced) : Power/cathode = 73 kW V = 486 1 = 149 Pressure = 3 x 10 " Torr N 2 0 flow = 1.56 SLPM N 2 flow = 0.92 SLPM 0 2 flow = 0. 15 SLPM N 2 0/N 2 /0 2 ratio=l . 0/0. 6/0. 1 Film thickness = 381 A Film absorption/A = .0014 Deposition rate = 186 A-mm/joule As can be seen from the Example III data, the deposition rate for two embodiments of the invention (A) and (B) were significantly improved over the baseline sample and the film absorption of the rate enhanced depositionswere actually less than the baseline sample. The rate enhanced run (A) illustrates practice of the invention with only N 2 0 as oxygen source. ; Example IV Two more baseline coating samples and then two optimized samples in practice of the invention were deposited in an Airco, Inc. "G-Series" coater with one and two cathodes respectively. The target was titanium. The baseline sample parameters and inventive method parameters are set out below. Baseline Samples: One Cathode Two Cathodes Power/cathode = 33 kW 33 kW V = 426 427/417 1 = 70 71/71 Pressure = 3 x 10 *3 Torr 3 x 10 *3 Torr 0 2 flow = 190 SCCM 235 SCCM Film thickness = 218 A 218 A Film absorption/A .0018 .0028 Deposition rate — 105 A-mm 2 /joule 105 A-mm 2 joule (B) Inventive Method (Rate Enhanced): Power/cathode = V = 1 = Pressure = N 2 0 flow = N 2 flow = 0 2 flow = N 2 0/N 2 /0 2 ratio= Film thickness = Film absorption/A = Deposition rate = As can be seen from the data of Example IV, the rates obtained by practicing the inventive method were more than doubled with respect to use of pure oxygen as the reactive gas. Use of two titanium cathodes benefitted by the slight addition of oxygen to the nitrous oxide and nitrogen reactive gas mix. Example V Tin was used as a target and reactive sputtering conducted in an Airco, Inc. "G-Series" coater with two tin cathodes. The baseline coating sample was coated by two passes at 36 IPM. A series of twelve samples were then obtained by the rate enhanced method with two passes at 160 IPM for each sample. A typical one of these twelve and the baseline sample had the parameters set out below. (A) Baseline Sample: Power/cathode = 15 kW ¥ = 330/355 I = 36/37 Pressure^ = 3 x 10 " Torr 0 2 flow = 530 SCCM Film thickness = 1015 A Film absorption/A = .0003 Deposition rate = 772 A-mm/joule (B) Inventive Method (Rate Enhanced) : " Power/cathode = 15 kW V = 330/346 I = 37/40 Pressure = 3 x lθ "3 Torr N 2 0 flpw = 175 SCCM N 2 flow = 175 SCCM ^) 2 --flow = 350 SCCM N 2 0/J*ϊ z /0 2 ratio = .0.5/0.5/1.0 Film thickness = 412 A Film absorption/A = .0015 Deposition rate = 1394 A-mm/joule As can be seen from a comparison of the baseline sample and the inventive sample, the rate enhancement provided by practice of the invention was by a factor approaching 2. Data for the baseline sample and two other rate enhanced runs, in addition to the just-described rate enhanced run, are tabulated in Table I. Auger analysis was performed to determine whether nitrogen had been incorporated into the tin oxide films prepared (which were sputtered to a depth of 100 A to eliminate effects of surface contamination) . As can be seen from the data of Table I, samples B, C, and D from practice of the invention illustrate the latitude at which the enhanced rate can be achieved, yet without incorporating nitrogen into the films. Analysis of the tin-to-oxygen concentrations showed that the baseline sample A (sputtered in pure oxygen) gave a film with a tin-to-oxygen ratio of 0.81, while the inventive samples B, C, and D gave tin-to-oxygen film ratios of 0.82-0.83; that is, practice of the invention with an enhanced rate can be achieved with substantially unchanged film stoichiometry with respect to reactive sputtering in pure oxygen. EXAMPLE VI Experiments were conducted to determine relationships between various system parameters, process characteristics and deposition rate. No direct relationship between deposition rate and gas flow rate ratio was observed and no relationship between deposition and current could be determined. However, investigations of an N 2 /N 2 0 plasma were conducted. Thus, the plasma ,emission spectrum was monitored as the gas ratio of nitrogen to nitrous oxide was varied from 100% nitrous oxide to 100% nitrogen, at a constant total system pressure. We found that the 391 nm titanium (Ti) line and the 777 nm oxygen (0) line noticeably changed when the gas flow ratio was varied. Accordingly, we studied the O/Ti emission ratio as a function of deposition rate by conducting a number of depositions in which the N 2 and N 2 0 gas flows were adjusted to obtain the desired O/Ti value. The results of these various depositions are illustrated by Fig. 2 where one can see a strong linear relationship between O/Ti and deposition rate. Experiments were conducted in both ILS and G Series coaters. In the ILS-1600, an oxygen to titanium plasma emission ratio of .12 was set. Deposition power was 8 kW, substrate linespeed was 21.3 inches per minute for four passes under the substrate. The N ? 0 flow increased from an initial value of 36 SCCM to a final value of 39. SCCM, while the N 2 flow increased from 21 to 23 SCCM. Flows were controlled to maintain the total pressure and plasma emission ratio during deposition. In G-6, a one meter substrate width system, an experiment was conducted where the O/Ti plasma emission ratio was fixed at .18. A power of 30 kW at 52 AMPS, a total system pressure of one micron and a linespeed of 200 IPM were maintained for 36 passes of the substrate under the target. A variation of less than ±.005 in the O/Ti ratio was maintained by controlling N 2 0 flow rates between 107 and 117 SCCM and N 2 flow rates between 26 and 41 SCCg. An additional deposition was conducted maintaining the O/Ti emission ratio value of 0.20, but all other system parameters were kept the same. Similar results to the previous experiment were achieved. The deposition rates for the two experiments were: Experiment (1) 178A mm 2/J ±14; Experiment (2) 190A mm2/J ±6. Due to measuring error, these two rates overlap each other. The emission spectrum ratio difference between the two depositions was greater than the emission spectrum ratio control fluctuations, and the difference in the depositions barely measurable. The depositions lasted about 20 minutes and thus illustrate that control can be maintained through use of emission spectrum data. Thus, the deposition rates achieved in practicing the inventive process while preparing titanium oxide coatings were more than double when compared to use of oxygen solely as the reactive gas, and the deposition rate was found to be closely related to the ratio of oxygen to titanium plasma emission lines at 391 nm and 777 nm respectively. Measurements determining the sample film absorption and durability properties showed equivalent optical and mechanical properties when compared to films deposited in an 0 2 plasma. The use of the plasma emission ratio of oxygen to metal lines is directly indicative of the metal flux and oxygen concentration in the plasma. Therefore, this technique can be extended to the other metal dielectric systems shown to deposit in enhanced rates with the use of nitrous oxide. The general technique involved would be to identify those metal and oxygen plasma emission lines which show the greatest change as the reactive gas ratios are varied. Once these lines are determined, the plasma emission ratio can be varied by changing the ratio of nitrous oxide and other reactive gases. The deposition rate and relevant optical properties (index of refraction or absorption/ ngstroms) can be measured as the gas ratios are varied. When the optimum ratio of gases is determined based on deposition rate and optical properties, the metal/oxygen plasma emission ratio is fixed. Control of reactive gas ratios at constant total system pressure will then control the plasma emission ratio, fixing the metal to oxygen flux ratio in the plasma at the desired combination of enhanced deposition rate and optimum optical prDperti-es. It is to be understood that while the invention has been described in conjunction with preferred specific embodiments, the foregoing description as well as the examples are intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims.

Claims: IT IS CLAIMED: 1. In a method of reactively sputtering a target in the presence of oxygen gas as reactant to form a substantially transparent film of target oxide having a determinable degree of absorption per unit thickness at a determinable rate, the improvement wherein: the reactant includes sufficient nitrous oxide to increase the film forming rate while permitting maintenance of the film being deposited at substantially the same degree of absorption per unit thickness or less as when oxygen gas is sole reactant. 2. The method as in claim 1 wherein the target includes titanium, tin, zinc, tantalum, aluminum, zirconium, indium, indium-tin alloys, bismuth, silicon, tungsten, alloys or mixtures thereof. 3. The method as in claim 1 wherein the reactant includes one or both of oxygen gas and nitrogen gas in addition to the nitrous oxide. 4. The method as in claim 1 or 3 wherein the sputtering is conducted in a chamber, and the chamber is evacuated to a pressure below about 1x10 * Torr before the sputtering. 5. The method as in claim 4 wherein the sputtering is conducted at a pressure of about lxlo " to about 5x1θ "3 Torr. 6. The method as in claim 1 wherein the target is titanium, tin, zinc, tantalum, aluminum or zirconium. 7. The method as in claim 1 wherein the film has a thickness of less than about 1 micron. 8. The method as in claim 3 wherein the target oxide is a zinc oxide and the reactant includes oxygen gas in addition to the nitrous oxide. 9. The method as in claim 3 wherein the target oxide is a titanium oxide and the reactant includes nitrogen and oxygen gases in addition to the nitrous oxide. 10. The method as in claim 3 wherein the target oxide is a zinc oxide and the reactant includes nitrogen and oxygen gases in addition to the nitrous oxide. 11. The method as in claim 3 wherein the target oxide is a tin oxide and the reactant includes nitrogen and oxygen gases in addition to the nitrous oxide. 12. The method as in claim 1 wherein the absorption per unit thickness of the film at the increased film forming rate is less than about 0.002 per A. . 13. The method as in claim 1 wherein the film forming rate * is increased by at least about 50%. 14. A method of reactively sputtering a target to deposit a substantially transparent target oxide film at a rapid rate comprising: providing a target and a substrate in a chamber evacuated to a pressure below about 1x10 " Torr, the target selected from the group consisting of titanium, tin, zinc, tantalum, aluminum, zirconium, indium, indium-tin alloys, bismuth, silicon, tungsten, and alloys or mixtures thereof; introducing a source of reactive oxygen into the chamber, the source of reactive oxygen having a reactivity in forming target oxide that is enhanced with respect to pure oxygen gas; and, flowing the source of reactive oxygen at a controlled rate into the chamber to provide a pressure of about 1x10 " to about 5xlθ "3 while maintaining an absorption per unit thickness of target oxide deposited on the substrate at substantially the same absorption per unit thickness or less as when pure oxygen gas is sole reactant. 15. The method as in claim 14 wherein the deposition rate of target oxide is at least about 50% faster than when pure oxygen gas is sole reactant. 16. The method as in claim 14 wherein the absorption per unit thickness of target oxide deposited on the substrate is maintained at less than about 0.002 per A. 17. The method as in claim 14 wherein the source of reactive oxygen includes nitrous oxide. 18. The method as in claim 17 wherein the source of reactive oxygen includes oxygen gas, nitrogen gas or a mixture of oxygen and nitrogen gases. 19. A method for selecting a reactant gas providing an enhanced deposition rate of an oxide film while maintaining a desired optical property comprising: (a) sputtering a target in pure oxygen or a mixture of oxygen and an inert gas as reactant gas at a constant gas pressure between about 1x10 Torr to about 5x10 Torr to establish baseline values for target oxide deposition rate and at least one target oxide optical property; (b) sputtering a target in a first reactant gas with at least a portion thereof being nitrous oxide, and measuring first values for target oxide deposition rate and the at least one target oxide optical property; (c) comparing the baseline values and first values and either selecting the reactant gas of step (b) or modifying the first reactant gas with additions of nitrogen gas, oxygen gas or mixtures thereof to form a second or a subsequent reactant gas; (d) repeating step (b) but with the second or subsequent reactant gas, and measuring second or subsequent values for target oxide deposition rate and the at least one target oxide optical property. 20. The method as in claim 19 wherein the first reactant gas comprises nitrous oxide and one or both of nitrogen and oxygen, and the at least one target optical property is the absorption of the target oxide deposited. 21. A method of maximizing a substrate deposition rate of substantially transparent target oxide film comprising: ** providing a target substrate in a chamber and forming a target flux by sputtering; introducing a source of reactive oxygen into the Chamber, the source of reactive oxygen including nitrous oxide and a gas selected from the group consisting of nitrogen, oxygen, and mixtures of nitrogen and oxygen, the reactant gas increasing the ratio of target flϊfc sputtered with respect to the amount of reactive oxygen until just before there is a substantial increase in the visible absorption per unit thickness of the target oxide film with respect to when pure oxygen gas is the r sole reactant gas; determining a relationship between the deposition, rate of target oxide film deposited and a value derived from the plasma emission lines of oxygen atoms and "target atoms; and, maintaining the derived value during sputtering of the target substrate. 22. She method as in claim 21 wherein the target is titanium and the derived value is a ratio of the oxygen plasma emission line at 391 nm and the titanium plasma emission line at 777 nm. 23. Φhe method as in claim 21 wherein the source of reactive oxygen is a mixture of nitrogen and nitrous oxide.

Inventors: NADEL STEVEN J, VAN SKIKE TAMZEN L, WAMBOLDT LEONARD G, WOLFE JESSE D, NADEL, STEVEN, J., VAN SKIKE, TAMZEN, L., WAMBOLDT, LEONARD, G., WOLFE, JESSE, D.
Applicants: BOC GROUP INC, THE BOC GROUP, INC.
Application year: 1990
Main classification: C23C14
IPC codes: C23C14