Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
  • C23C14/0021—Reactive sputtering or evaporation
  • C23C14/0036—Reactive sputtering
  • C23C14/0057—Reactive sputtering using reactive gases other than O2, H2O, N2, NH3 or CH4
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
  • C23C14/22—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material characterised by the process of coating
  • C23C14/34—Sputtering
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
  • C23C14/06—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material characterised by the coating material
  • C23C14/08—Oxides

Definitions

• 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.
• a substantially pure oxide film may be utilized as an exposed surface of an optical element.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• SCCM standard cubic centimeters per minute
• 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) .
• 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.
• 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.
• absorption/A thickness as used herein to exemplify the maintenance of at least one target oxide optical property, is meant as follows.
• 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) :
• y( ⁇ ) values representing the relative sensitivity of the human eye to various wavelengths of light
• Pc( ⁇ ) the relative intensity of the illuminating light source for various wavelengths of light
• R( ⁇ ) ⁇ 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.
• 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.
• 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.
• 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 5xl ⁇ " Torr, more preferably is at about 3 microns (i.e., 3xl ⁇ " 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.
• 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):
• 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.
• a first reactant gas that includes nitrous oxide
• 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
• **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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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:
• Example II illustrates that N 2 0 need not be the major constituent of the reactive gas in practicing the invention.
• 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 V 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.
• the rate enhancement provided by practice of the invention was by a factor approaching 2.
• 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.
• 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
• 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.

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• Engineering & Computer Science (AREA)
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• Physical Vapour Deposition (AREA)
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• 1990
  • 1990-08-06 KR KR1019920700259A patent/KR920703870A/ko not_active Withdrawn
  • 1990-08-06 JP JP2511876A patent/JPH04507266A/ja active Pending
  • 1990-08-06 EP EP90912513A patent/EP0594568A1/de not_active Withdrawn
  • 1990-08-06 WO PCT/US1990/004382 patent/WO1991002103A1/en not_active Ceased
  • 1990-08-06 AU AU61819/90A patent/AU6181990A/en not_active Abandoned
  • 1990-08-06 CA CA002059525A patent/CA2059525A1/en not_active Abandoned
• 1994
  • 1994-05-05 AU AU61913/94A patent/AU6191394A/en not_active Abandoned

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