Patent Publication Number: US-2006003100-A1

Title: CVD process to deposit aluminum oxide coatings

Description:
BACKGROUND OF THE INVENTION  
      Chemical vapor deposition (CVD) of aluminum oxide is used conventionally in various applications in view of the various advantageous properties of Al 2 O 3 , including hardness; wear resistance, electrical insulating properties and chemical resistance towards oxidizing atmosphere. Natural aluminum oxide or corundum (α-phase) is thermodynamically the stable phase at typical CVD depositions temperatures in the vicinity of 1050° C. In addition to the stable α-phase, aluminum oxide exhibits several metastable allotropic modifications, such as γ, δ, η, θ, κ and χ.  
      With regard to the cutting tool industry, CVD aluminum oxide-coated cemented carbide cutting tools have been commercially available for more than two decades. Such cutting tools are often used for turning, milling and drilling applications. However, because of the compatibility problems, aluminum oxide typically is not deposited directly onto cemented carbide substrates. Interfacial coatings, based on TiC, Ti(C,N), TIN, Al 2 O 3 , HfN, etc. sublayers, have been developed in order to enhance adhesion of aluminum oxide to cement substrates, as well as enhance other characteristics such as wear and toughness.  
      Commercially, CVD aluminum oxide coatings are deposited using the AlCl 3 —CO 2 —H 2  system. Process parameters typically used are a temperature range between 1000-1050° and a pressure range between 50-100 Torr. Chemical reactions for the formation of Al 2 O 3  by the hydrolysis method are: 
 
Source: 2Al+3Cl 2 →2AlCl 3   (1) 
 
Water-gas-shift: CO 2 +H 2 →H 2 O+CO  (2) 
 
Deposition: 2AlCl 3 3+3H 2 O→Al 2 O 3 +6HCl  (3) 
 
      It has been established that the water-gas formation rate at a fixed temperature depends on the concentration of both H 2  and CO 2  and a maximum water-gas formation rate is obtained at a CO 2 /H 2  molar ratio of 2:1. It has been demonstrated that the AlCl 3 /H 2 O process is a fast reaction, and AlCl 3 /O 2  is a very slow reaction process, whereas aluminum oxide deposition from AlCl 3 /H 2 /CO 2  gas mixture is a medium rate process.  
      It is well established that the water-gas shift reaction is the critical rate-limiting step for Al 2 O 3  formation, and to a great extent, controls the minimum temperature at which Al 2 O 3  can be deposited. Extensive work has been done to attempt to deposit CVD Al 2 O 3  coatings at lower temperatures. In addition, several CVD Al 2 O 3  coatings using other than the AlCl 3 —CO 2 —H 2  system have been investigated, including AlCl 3 /C 2 H 5 OH, AlCl 3 /N 20 /H 2 , AlCl 3 /NH 3 /CO 2 , AlCl 3 /O 2 /H 2 O, AlCl 3 /O 2 /Ar, AlX 3 /CO 2 /H 2  (where X is Cl, Br, I), AlBr 3 /NO/H 2 /N 2  and AlBr 3 /NO/H 2 N 2 . However, none of these systems has been commercially successful, To provide a CVD process for depositing aluminum oxide coatings at temperatures below those previously found necessary for effective deposition on a commercial scale is therefor highly desirable.  
     SUMMARY OF THE INVENTION  
      The problems of the prior art have been overcome by the present invention, which provides a process for chemical vapor deposition (CVD) of aluminum oxide (Al 2 O 3 ). Specifically, the process of the present invention achieves effective deposition of aluminum oxide at significantly lower temperatures than previously thought possible on a commercial level. In the present invention, these temperatures are sometimes described as “medium temperatures” or “MT-Alumina”.  
      Thus the present invention is directed to a method of depositing Al 2 O 3  on a substrate, comprising (a) providing a source of AlCl 3 : (b) forming water-gas by reacting hydrogen with an oxygen donor having a vapor pressure sufficient to form water-gas at a temperature below about 950° C.; (c) reacting said AlCl 3  with said water-gas to form Al 2 O 3 ; and (d) depositing the Al 2 O 3  on the substrate. Preferably, the temperature of water-gas formation and Al 2 O 3  deposition is below about 900° C. Depending upon the substrate being coated, it may be preferable to deposit Al 2 O 3  where the temperature of water-gas formation is below about 850° C., or below about 800° C. In general, a suitable temperature range, useful for a wide variety of substrates has been found to be from about 700° C. to about 950° C.  
      For cutting tool bodies comprising TiC and/or Ti(C,N) coatings, effective deposition in accordance with the present invention has been achieved at temperatures in the range of about 8000-950° C., which is 100-250° lower than conventional deposition temperatures. The process involves the formation of water gas by mechanisms other than the rate-limiting CO 2 —H 2  reaction. Instead, water gas is formed using oxygen donors with sufficient vapor pressures to form water gas at temperatures between about 800° C. and 950°.  
      The chemical vapor deposition process to deposit aluminum oxide in accordance with the present invention is based upon altering the CO 2 —H 2  water-gas shift reaction. Water-gas can be generated using H 2 —N 2 /O 2  based species or fatty acids so as to remove the temperature imitations imposed by the CO 2 —H 2  water-gas shift reaction, and thus produce Al 2 O 3  at lower deposition temperatures. Thus, in the present invention, alternative sources of oxygen donors are used to form water-gas at desired levels and rates, and at lower temperatures.  
      Suitable oxygen donors are compounds with vapor pressures sufficient to form water gas. Exemplary compounds include NO 2 , H 2 O 2  (introduced with a carrier gas) and formic acid, or compounds with vapor pressure similar to formic acid. Compounds with vapor pressures similar to formic acid include nitromethane, trichloracetylaldehyde, trichloroethyloxysilane, dichloroethoxy-methylsilane, 2-propanol, butyric acid, tigaldehyde, ethyl acrylate, methyl methacrylate, ethyl propionate, propyl acetate, isopropyl acetate, methyl butyrate, methyl isobutyrate, isobutyl formate, sec-butyl formate and 1,2-diethoxyethane. Nitric oxide (NO) has also been studied. To date, formic acid has been particularly preferred.  
      Although the present inventor does not wish to be limited thereby, the following reactions are believed to be operating for these systems:  
      AlCl 3 —NO 2 —H 2  System: 
 
2AlCl 3 +1.5NO 2 +3H 2 =Al 2 O 3 +0.75N 2 +6HCl  (4) 
 
 AlCl 3 —HCOOH System: 
 
2AlCl 3 +3HCOOH=Al 2 O 3 +6HCl+3CO  (5) 
 
      Suitable pressure ranges for CVD alumina deposition in accordance with the present invention are 50 to 100 Torr, with 75 Torr being particularly preferred.  
      The amount of water-gas content can be manipulated by varying the amount of CO 2  and/or H 2  addition in the reaction system. For example, in the NO 2  system, the level of water-gas formed is much higher than that of the pure CO 2  system when the ratio between CO 2  and NO 2  is varied from 5:1 to 1:5.  
      Similarly, the effect of H 2  addition to the formation of water-gas is an increase in water-gas content with increasing H 2  concentration in both the HCOOH and NO 2  systems.  
      The flow rate of the water-gas formation reactant(s) can be controlled to optimize water-gas formation. For example, excellent CVD-alumina coatings on Ti(C,N) and TiC coated tools have been achieved with a formic acid flow rate of 150% and a hydrogen flow rate of %. A commercially available low vapor pressure mass flow controller has been found to be one suitable device used to control the flow rate.  
      The substrates that be coated by the present invention include solid materials that can withstand the coating process conditions, particularly the coating temperatures. Substrates comprising high temperature heat stable metals, such as high temperature steels, super alloys, and the like are suitable for coating under the present invention. One particularly preferred class of substrates to be coated by the present invention comprises cutting tool bodies. These substrates preferably have at least one layer, and more preferably two or more layers (e.g., interfacial coatings) selected from the group consisting of carbide, carbonitride, oxynitride, oxycarbide, oxycarbonitride or nitride of aluminum, silicon, boron, or Groups IVB, VB and VIB of the Periodic Table.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIGS. 1A and 1B  are SEM micrographs showing A) the surface morphology and B) the thickness of MT-Alumina coating deposited on a TiC substrate using 75% of formic acid;  
       FIGS. 2A and 2B  are SEM micrographs showing A) the surface morphology and B) the thickness of MT-Alumina coating deposited on a Ti(C,N) substrate using 150% of formic acid;  
       FIGS. 3A and 3B  are SEM micrographs showing A) the surface morphology and B) the thickness of MT-Alumina coating deposited on a TiC substrate using 150% of formic acid and 6.0 SLM H 2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      As described above, the present invention is directed to a method of depositing Al 2 O 3  on a substrate, comprising (a) providing a source of AlCl 3 ; (b) forming water-gas by reacting hydrogen with an oxygen donor having a vapor pressure sufficient to form water-gas at a temperature below about 950° C.; (c) reacting said AlCl 3  with said water-gas to form Al 2 O 3 ; and (d) depositing the Al 2 O 3  on the substrate. Preferably, the temperature of water-gas formation and Al 2 O 3  deposition is below about 900° C. Depending upon the substrate being coated, it may be preferable to deposit Al 2 O 3  where the temperature of water-gas formation is below about 850° C., or below about 800° C. In general, a suitable temperature range, useful for a wide variety of substrates has been found to be from about 700° C. to about 950° C.  
      One preferred embodiment of the present invention provides methods for the medium-temperature (MT) CVD alumina coating of substrates such as cemented carbide cutting tools. As described herein, the method of the present invention involves the formation of water gas by alternative sources of oxygen donors with sufficient vapor pressures to form water gas at desired levels and rates, and at temperatures between about 800° C. and 950°.  
      The present invention thus provides a process for the CVD of Al 2 O 3  on a substrate at so-called “medium” temperatures. In preferred embodiments, water-gas is generated using H 2 —N 2 /O 2  based species or fatty acids, which have been found to produce Al 2 O 3  at lower than conventional deposition temperatures.  
      One preferred fatty acid in the present invention is formic acid (HCOOH). The preferred HCOOH processing system utilized a commercially available low vapor pressure mass flow controller to provide precise control over the HCOOH introduction into the CVD reactor. Coatings of 1.5 μm thickness were deposited on average. Using HCOOH, alumina coatings were consistently deposited in the temperature range of 800°-875° C.  
      In order to study the medium temperature alumina coating processing conditions in greater detail, several process parameters such as temperature, pressure and gas flow velocities were varied. Table 1 shows various combinations of temperature and gas flow velocities that were investigated at a deposition pressure of 75 Torr.  
               TABLE 1                          DEPOSITION PRESSURE OF 75 TORR                             875° C.   850° C.   825° C.   825° C.                                     2.0 SLM   4.0 SLM   6.0 SLM   2.0 SLM   2.0 SLM   2.0 SLM       Hydrogen*   Hydrogen**   Hydrogen***   Hydrogen*   Hydrogen*   Hydrogen*                75%   150%    75%    75%    75%    75%       Formic Acid   Formic Acid   Formic Acid   Formic Acid   Formic Acid   Formic Acid       150%       150%   150%   150%   150%       Formic Acid       Formic Acid   Formic Acid   Formic Acid   Formic Acid       300%       300%   300%   300%   300%       Formic Acid       Formic Acid   Formic Acid   Formic Acid   Formic Acid                 All other reactant gas flows:            *Cl = 50%, Ar = 250%            **Cl = 50%, Ar = 500%            ***Cl = 50%, Ar = 750%             
 
      Chemical vapor deposition, in general, is very sensitive to chamber contamination. Contaminants that can change the nature of the deposited coating can originate from a variety of sources. In an effort to further reduce the risk of contamination, the substrates that were to be coated were each first cleaned using acetone and then methanol in an ultrasonic bath for ten minutes per solution.  
      One important factor to the CVD process is the abundance and availability of the critical gases for the reaction. In short, it is important that the reaction chamber be saturated with the gases that are critical to the reaction. For MT-Alumina, water-gas is the key compound in the reaction. As previously mentioned, water-gas is a product of the dissociation of formic acid. Hence, the amount of formic acid in the chamber had a profound effect on the Al 2 O 3  coating. As described above, a commercially available low vapor pressure mass flow controller has been found to be one suitable device used to control the flow rate of these critical gases. One especially preferred mass flow controller employed herein was the MKS 1553 available from MKS Instruments, Inc. of Andover, Mass.  
      Processing parameters derived herein included the following “standard” run:  
                                                   Temperature   Pressure   H 2  flow   Cl 2  flow   Ar flow   HCOOH flow                  875° C.   75 Torr   2.0 SLM   50%   250%   variable                  
 
      This combination produced the highest quality of coatings in terms of surface morphology and thickness. In other words, the surface of the coating was the most uniform in density and grain size. The thickness of these Al 2 O 3  coatings averaged approximately 1.5 μm.  
      In further studies it was found that the following parameters produced coatings that were approximately 25% thicker on average than the standard run:  
                                                   Temperature   Pressure   H 2  flow   Cl 2  flow   Ar flow   HCOOH flow                  875° C.   75 Torr   6.0 SLM   50%   750%   variable                  
 
      A typical MT-Alumina coating grown using the standard run conditions and 75% of formic acid had a surface morphology with individual crystals of a size between 0.8-1.0 μm. These coatings had an average thickness of between 1.0-1.5 μm.  
      The typical surface morphology and thickness of an MT-Alumina coating deposited using the standard run conditions and 150% of formic acid showed a more uniform surface than those of the 75% formic acid runs. The grains were of an equiaxed shape with an average size of 0.5-1.0 μm. The average thickness for these coatings was 1.5-2.0 μm. For these experimental parameters, the water vapor content was approximately 3.08%.  
      A typical coating deposited using 150% of formic acid and a revised standard of 6.0 SLM hydrogen and 750% argon showed larger average equiaxed grain size of 0.75-1.25 μm. The average thickness for these coatings was 1.5-2.0 μm. It is important to note that the uniformity and absence of flatness in these coatings has been preserved. For these experiments, 2.16% of the reactant gas was water vapor.  
      To investigate the effect of deposition temperature on Al 2 O 3  deposition, experiments were done between 875° C. and 800° C. in 25° C. increments. All other coating parameters were as follows:  
                                                   Temperature   Pressure   H 2  flow   Cl 2  flow   Ar flow   HCOOH flow                  875° C.   75 Torr   2.0 SLM   50%   250%   150%                  
 
      These experiments showed that the coating thickness (growth rate) increased slightly with increasing temperature. At a deposition temperature of 800° C. the average coating thickness was 1.25 μm. The thickness of the coatings increased by approximately 20% between 800° C. and 825° C. to an average of 1.5 μm. Between 825° C. and 850° C. the average coating thickness remained the same. An increase of approximately 17% was noticed in coating thickness between experiments done at 850° and 875° C., with an average coating thickness of 1.75 μm.  
      Experimental results show that as the temperature increases so does that growth rate of Al 2 O 3 . This suggests that as the temperature increases so does the water vapor concentration. These results are consistent with historical information and theoretical thermodynamic calculations. No significant difference in Al 2 O 3  growth rate with temperature was noticed however. Typically, Al 2 O 3  coatings are deposited in the 5-10 μm range. The thickest coatings deposited herein were 2.0 μm. The most successful coatings were deposited using a formic acid flow rate of 150% and a hydrogen flow rate of 2000%.  
     EXAMPLES  
       FIGS. 1A and 1B  are SEM micrographs showing A) the surface morphology and B) the thickness of MT-Alumina coating deposited on a TiC substrate using 75% of formic acid;  
       FIGS. 2A and 2B  are SEM micrographs showing A) the surface morphology and B) the thickness of MT-Alumina coating deposited on a Ti(C,N) substrate using 150% of formic acid;  
       FIGS. 3A and 3B  are SEM micrographs showing A) the surface morphology and B) the thickness of MT-Alumina coating deposited on a TiC substrate using 150% of formic acid and 6.0 SLM H 2 .  
      The present invention has been described in detail, including the preferred embodiments thereof. However, it will be appreciated that those skilled in the art, upon consideration of the present disclosure, may make modifications and/or improvements on this invention and still be within the scope and spirit of this invention as set forth in the following claims.