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
  • C23C16/00—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes
  • C23C16/22—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the deposition of inorganic material, other than metallic material
  • C23C16/26—Deposition of carbon only
  • C23C16/27—Diamond only
  • C23C16/278—Diamond only doping or introduction of a secondary phase in the diamond
• • 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
  • C23C16/00—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes
  • C23C16/22—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the deposition of inorganic material, other than metallic material
  • C23C16/26—Deposition of carbon only
  • C23C16/27—Diamond only
  • C23C16/272—Diamond only using DC, AC or RF discharges
• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B25/00—Single-crystal growth by chemical reaction of reactive gases, e.g. chemical vapour-deposition growth
  • C30B25/02—Epitaxial-layer growth
• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
  • C30B29/02—Elements
  • C30B29/04—Diamond

Definitions

• This invention relates to the deposition of thin films, and, more particularly, to the deposition of thin films of diamond.
• Diamond is an allotropic crystalline form of carbon wherein the carbon atoms are covalently bonded and arranged in a diamond cubic atomic lattice.
• Naturally occurring diamond is found as polyhedral crystals and is familiar to most persons as a gemstone.
• Bulk diamond can also he manufactured synthetically, and both natural and synthetic diamonds are used in cutting tools and the like because diamond is hard and wear resistant.
• Diamond exhibits physical and chemical properties which make it potentially useful in microelectronic and . optical devices. In such applications, typically the diamond would be present as a thin layer, less ' than .001 inches thick, supported by a relatively thick substrate.
• the diamond layer might function as an active component of the device through which an electrical current or light passes, or might be a passive element such as a heat sink, depending upon which of diamond's properties are to be used.
• diamond is to find widespread use in microelectronic and optical devices, techniques must be developed to deposit thin layers of diamond onto substrates. Natural and synthetic bulk diamond cannot be used in these applications because layers less than a thousandth of an inch in thickness cannot be conveniently prepared from the bulk form and bonded to substrates. Thin layers of diamond can be deposited directly onto substrates by various techniques. For example, diamond layers can be deposited onto substrates by chemical vapor deposition, wherein a vaporous source material mixed with a carrier gas is passed over a heated substrate. With the correct source material and deposition conditions, a layer of diamond is deposited from the carbon atoms of the source material.
• the most widely used source material is methane, but carbon tetrachloride, acetone, alcohols, ethers, acetates, aldehydes, amines and other organic compounds have also been used with varying degrees of success.
• Graphite another allotropic form of carbon, is also deposited in thin layers by chemical vapor deposition. The reaction to deposit graphite competes with that to deposit diamond, and under many conditions graphite rather than diamond is deposited. Once deposited, the diamond is energetically ' favored to convert to graphite, but the reverse reaction of conversion of graphite to diamond is not thermodynamically favored.
• the deposition of a diamond layer can be interrupted by formation of graphite, which has completely different electrical, optical and physical properties and destroys the operability of the diamond layer for many applications.
• Both continuous and intermittent techniques have been developed for removing the graphite formed during the deposition of diamond, by introducing gaseous hydrogen into the chemical vapor deposition apparatus and reacting ' the hydrogen with the graphite to form methane.
• the known techniques for depositing layers of diamond have relatively slow deposition rates, typically on the order of about one micrometer per hour. Such slow deposition rates inhibit the commercial exploitation of thin-film diamond technology. It is also necessary in most cases to interrupt the growth of the diamond layer with intermittent hydrogen reaction cycles to remove any deposited graphite, or to dilute the source material gas with hydrogen for the same purpose.
• Existing processes for depositing thin layers of diamond are therefore slow and somewhat unreliable, in part because other allotropic forms of carbon can be formed in the deposition process.
• the present invention fulfills this need, and further provides related advantages.
• the present invention is embodied in a process for depositing a layer of diamond onto a substrate, which has a deposition rate greater than that of prior processes.
• the formation of graphite is greatly reduced, increasing the certainty that the deposited layer is purely diamond without any minute patches of graphite.
• the perfection of the diamond layer and its epitaxy with the underlying substrate, important considerations for diamond layers used in microelectronic applications, are also Improved.
• a process for depositing a layer of diamond onto a substrate comprises furnishing a vapor of a hydrocarbon source material , the source material having all carbon . atoms saturated and having a ratio of hydrogen atoms linked to carbon atoms of less than 2, and depositing the vapor of the source material onto the substrate, the substrate being heated to promote decomposition of the source material to form diamond on the surface of the substrate.
• the carbon atoms in a diamond cubic crystal structure are bonded to each other in a tetrahedral configuration by covalent single bonds wherein a single pair of electrons is shared by the adjacent bonded carbon atoms.
• the angle between each of the bonds is about 109.5 degrees (or, more precisely, 109°28'), which is determined geometrically as the angle between the adjacent rays extending from the center of a regular tetrahedron to each of its vertices.
• the result of this geometry is an essentially three dimensionally bonded array of carbon atoms.
• the carbon atoms in graphite are arranged in parallel tiers about 3.4 Angstroms apart.
• the bonds between the carbon atoms in each tier re relatively strong, but the bond between the carbon atoms in adjacent tiers Is relatively weak.
• the crystals ' In each tier are hexagonal plates that can e easily separated from the crystals in adjacent tiers.
• Graphite therefore exhibits essentially a two-dimensional structure of weakly bonded plates that are arranged In a three-dimensional stack.
• the structure of the vaporous source material of the present invention Is chosen to promote the formation of the deposited diamond structure rather than the graphite structure.
• the carbon atoms in the source material are bonded to each other by tetrahedrally arranged single bonds, so that the natural tendency for each carbon atom Is to form four tetrahedrally arranged - single - bonds to adjacent carbon atoms in the deposited layer.
• the source material is chosen to have a bond structure comparable with that of the deposited diamond, minimizing the need for the breaking, relaxation, and rearrangement of carbon bonds upon deposition and the consequent energetic process.
• the ratio of the number of hydrogen atoms bonded to carbon atoms in the source material of the invention is less than 2.
• the selection of this numerical maximum value for the ratio is not arbitrary, but is closely related to the possible hydrocarbon structures that may be used as the source material in conjunction with the invention.
• the single-bond limitation together with this limitation, exclude source materials that tend to form deposited graphite.
• this limitation excludes the branched or unbranched straight-chain alkane hydrocarbons (C n H2n+2)» which have a ratio of linked hydrogen atoms to carbon atoms decreasing from 4 to 2 as x increases from 0, and the cyclic alkanes ((CH2)n)» which have a ratio of linked hydrogen atoms to carbon atoms of exactly 2.
• the source materials are hydrocarbons, formed of hydrogen and carbon atoms. Related compounds containing other atoms substituted for the carbon or in functional groups are not used, except for considerations of doping to be discussed In the following paragraph. For example, oxygen-containing compounds (e.g., ethers, hydroxyls, carbonyls) are not used, as the Included oxygen reacts with hydrogen to form water, which poisons the deposition. Similarly, nitrogen-containing compounds (e.g., amines) are not used.
• the hydrocarbon source materials can be codeposited with modified doped source materials containing small amounts of intended dopant atoms bonded to an otherwise hydrocarbon structure.
• Dopant atoms such as boron, phosphorus, or nitrogen can be incorporated into hydrocarbon molecules In a small, controlled amount to form doped source materials. These doped source materials are mixed with a larger amount of the pure hydrocarbon source material and codeposited, so that the diamond layer is continuously and uniformly doped with a small and controllable amount of the dopant as deposition proceeds. A subsequent, separate diffusional doping step is therefore not required.
• the dopant atoms are bonded into the structure that is selected to be favorable to deposition of diamond, according to the criteria set forth above.
• the more common doping approach of mixing a separate gas with the hydrocarbon as In the process of mixing a source material with a structurally different gas such as PH3 for doping with phosphorus.
• the present invention Is operable with either of these gaseous dopant approaches and with conventional diffusional doping, but the preferred technique is to use a dopant bonded into a hydrocarbon that structurally favors deposition of diamond.
• the limitations of saturated tetrahedral carbon bonds and a ratio of linked hydrogen to carbon atoms of less than 2 are satisfied by the polycyclic alkanes.
• the preferred source materials of the invention are adamantane, congressane, cubane, and basketane. Each of these source materials has a sufficiently high vapor pressure in its solid or liquid form that operable amounts of vapor can be provided in a gas stream delivered to a reactor. . Each molecule of these hydrocarbons has a plurality of carbon atoms, Joined to the other carbon atoms in a three-dimensional array approximating or matching that of deposited diamond.
• Adamantane and congressane are particularly preferred, as their core structure comprises carbon atoms with the same diamond cubic arrangement and bond angle of 109.5 degrees as required for the deposited diamond film.
• bonds of cubane and basketane are oriented at 90 degrees, and must be relaxed slightly for deposition as diamond.
• a layer of diamond can be formed by stripping the hydrogen atoms from the adamantane or congressane molecule and depositing the remaining carbon atoms onto the surface without breaking or rearranging any of the carbon-carbon bonds.
• the use of the preferred hydrocarbons increases the rate of deposition significantly as compared with the use of methane. Only one carbon atom is deposited per molecule of methane, while 10 carbon atoms are deposited per molecule of adamantane, for example.
• Figure 1 is a schematic side sectional view of a chemical vapor deposition apparatus for practicing the invention
• Figure 2 is a perspective schematic view of the diamond crystal structure, with the carbon atoms of . a molecule of adamantane superimposed and highlighted for reference;
• Figure 3 is a perspective schematic view of the diamond crystal structure, with the carbon atoms of a molecule of congressane superimposed and highlighted for reference;
• Figure 4 is a perspective schematic view of the carbon atoms in a molecule of cubane.
• Figure 5 Is a perspective schematic view of the carbon atoms in a molecule of basketane.
• Diamond can be employed as a heat sink because of its high thermal conductivity, which Is about 5 times that of copper.
• the electrical properties of diamond suggest its use as an active element of devices, particularly in devices to be used at elevated temperatures or in severe radiation environments.
• the band gap of diamond is 5.2 eV. Its hole mobility Is about 1600 cmSsec' ⁇ volt -1 , while the electron mobility is about 2000 cmSsec-ivolf 1 .
• the charge carrier velocity is high, about 2.3 x 10? cm per second at 20 kv per cm.
• Diamond Is nonpolar, resulting in reduced charge carrier scattering. Diamond exhibits a resistivity of about 10 13 ohm-cm. Its breakdown voltage is 10 7 .volts per centimeter.
• the dielectric constant is about one-half that of silicon.
• Diamond also has optical and physical properties of great interest. It is transparent over a broad range of wavelengths from the infrared through the near ultraviolet, and has an index of refraction of 2.38. Diamond is the hardest natural material, making it resistant to physical damage. It is radiation hard and has a low scattering cross section. Its melting point is over. 3500°C, and It does not transform at lower temperatures. Diamond therefore has great potential for use In semiconductor and electro-optical devices, particularly those required- to-.resist high temperatures and radiation exposure. Thin films of diamond can also be expected to find use as a wear-resistant layer over softer materials, as in machine tools, drills, gems, etc.
• the present invention uses chemical vapor deposition to deposit a thin layer of diamond.
• a substrate upon which the layer is to be deposited, is heated or otherwise activated so that a reactant source gas passed over the surface decomposes and deposits atoms to the growing film.
• Figure 1 illustrates an apparatus 10 suitable for the chemical vapor deposition of a diamond layer 12 upon a substrate 14.
• the substrate is mounted in a conducting holder 16, which can be heated by conduction along a pedestal 18.
• the pedestal may be mounted to permit its rotation, thereby encouraging uniform deposition of the layer 12.
• the holder 16 also acts as a susceptor- and is heated by the " radio frequency signal applied to an rf coil 20 that surrounds the holder 16.
• the rf signal also influences the source gas from which deposition occurs, probably by activating the gas phase molecules to promote their decomposition.
• the hydrogen atoms are stripped from the hydrocarbon gas molecules by the elevated temperature and radio frequency power of the rf coil 20.
• the substrate 14, holder 16 and support 18 are placed inside a reactor tube 22, which is typically a nonconductive glassy material such as .fused quartz or a ceramic such as silica that is resistant to chemical attack by the source gas.
• a gas flow is introduced into the interior of the reactor tube through an inlet 24, permitted to flow past the substrate 14, and removed through an outlet 26.
• a view port 28 is provided so that the deposition can be monitored visually.
• a source gas containing a hydrocarbon, from which the carbon atoms are deposited to the growing diamond layer 12, is introduced in a mixture with a carrier gas, normally hydrogen.
• the mixture of source gas and carrier gas is prepared by passing hydrogen over a solid source of the source gas or bubbling hydrogen through a liquid source of the source gas, so that a controlled amount of the source gas vaporizes under the driving force of the solid or liquid vapor pressure.
• the quantity of the source gas in the gas stream is controllable, using established techniques, by varying the flow rate of the carrier gas, and the temperature of evaporation.
• the hydrocarbon-containing source gas comprises from about 1 • to about 4 percent by volume of the- total gas stream.
• Two or more source gases can be mixed with the carrier gas to -form the gas stream that is flowed past the substrate 14.
• a second hydrocarbon source gas may be mixed with the source gas of the invention to improve deposition rate or stoichiometry of the deposited diamond layer 12.
• the second hydrocarbon source gas could be a second source gas in accordance with the invention, or could be a source gas that Is not In accordance with the invention, such as methane.
• a second source gas that Is not a pure hydrocarbon can also be mixed into the gas stream, as a means of doping the diamond layer 12.
• FIG. 2 depicts the diamond crystal structure that must be obtained in the layer 12.
• the structure of diamond can be viewed as a regular repeating three-dimensional arrangement of carbon atoms tetrahedrally bonded to neighboring carbon atoms by single bonds. In this bonding arrangement, the carbon orbltals are in the sp 3 state. The angles between the four bonds of any one carbon atom are all about 109.5 degrees, as dictated by the tetrahedral geometry.
• One preferred source material for providing the source gas is adamantane, having the gross molecular composition of CIQ H 16* Tlle structure of the carbon ato * ms
• adamantane is also illustrated in Figure 2, as the shaded atoms superimposed upon the diamond structure.
• the adamantane molecule is not linear in form, but instead is more compact with four of the carbon atoms bonded to three other carbon atoms. (The remaining six carbon atoms are each bonded to two other carbon atoms.)
• Another result of the bonding of some carbon atoms to . more than two other carbon atoms is that the number of bonded hydrogen atoms per carbon atom is, on the average, less than two.
• Adamantane is formed of 10 carbon atoms each tetrahedrally bonded by single bonds, with the angles between the carbon bonds 109.5 degrees. Those electrons of the carbon atoms not participating in the bonds to other carbon atoms help bond hydrogen atoms to the carbon atoms, saturating the carbon atoms.
• a unit of the diamond structure can be formed by removing the hydrogen atoms from a molecule of adamantane and depositing the remaining carbon atoms directly onto a growing layer of diamond. Ten carbon atoms are added to the diamond layer for each molecule so deposited. By comparison, only a single carbon atom is added for each molecule of methane deposited by conventional procedures for growing diamond layers.
• Another preferred source material is
• Some other, more complex hydrocarbon molecules are expected to have their carbon atoms all tetrahedrally bonded, In the sp 3 state with bond angles of 109.5 degrees.
• increasing the atom mass of a molecule ordinarily decreases its vapor pressure, so that It becomes increasingly difficult to introduce a sufficient amount of the vapor into the carrier gas stream to attain high deposition rates of the layer 12.
• Adamantane Is the molecule of the lowest mass presently known, which also has the carbon atoms tetrahedrally bonded with 109.5 degree bond angles and a ratio of hydrogen atoms linked to carbon atoms of less than 2.
• hydrocarbon molecules are known wherein the carbon atoms are saturated and have less than 2 hydrogen atoms per carbon atom. However, these molecules have bond angles of the carbon bonds distorted to angles other than 109.5 degrees, and the carbon-carbon bond angles must be relaxed in order to form the diamond structure. In this respect such molecules are less favored than adamantane and congressane, but In other respects are more favored. Speci ically, these other hydrocarbons have lower ratios of numbers of hydrogen atoms to carbon atoms than do adamantane and congressane, and also have higher vapor pressures because of their lower atomic weights. Consequently, they are also preferred embodiments of the invention. The selection of the exact hydrocarbon to be used depends upon the circumstances of the deposition.
• cubane whose structure is illustrated in Figure 4.
• Cubane whose gross molecular formula is CgHg, has carbon-carbon bond angles of 90 degrees.
• Each carbon atom Is bonded " by a single bond to three other carbon atoms, and has one hydrogen atom to saturate the remaining bond. It" is apparent that the carbon-carbon bonds " must be at least rearranged to produce the diamond structure having 109.5 degree bond angles. However, only single bonds are involved, and the rearrangement is small. In principle, opening two parallel edges of the cubane molecule provides the freedom for the carbon-carbon bond couples to relax from 90 degrees to 109.5 degrees.
• the ratio of hydrogen atoms to carbon atoms in cubane is one, even smaller than for adamantane.
• a variation of cubane is basketane, whose gross molecular structure is C ⁇ oHi2 « The molecular structure of basketane is illustrated in Figure 5, and is seen to be a variation of cubane wherein two carbon atoms have other carbon atoms substituted for the hydrogen atoms. In basketane, two carbon atoms are bonded to two other carbon atoms each, and eight carbon atoms are bonded to three other carbon atoms each.
• the remaining carbon orbitals bond hydrogen atoms.
• the carbon atoms have bond angles which are not 109.5 degrees.
• the rearrangement of carbon-carbon bonds Is required to deposit diamond from basketane, although, as with cubane, only single bonds are Involved, and the relaxation of the bond angles is small.
• the ratio of hydrogen atoms to carbon atoms is low, In the case of basketane 1.2.
• the orbital bond state for cubane and basketane is thought to be a hybridization of the sp 3 state. Such an intermediate hybridization.would be of a high free energy, so that it is relatively easy to "open" the molecule and relax the bond angles to the required 109.5 degree orientation and sp 3 state.
• a further reduction in the number of hydrogen atoms might yield benefits in increased vapor pressure of the source material, but small molecules tend to be geometrically limited In undesirable ways so that the carbon-carbon bond angles are very different form that of diamond.
• the hydrocarbons of lower mass contribute fewer carbon atoms per molecule deposited. Even more significantly, the lower mass hydrocarbons may strongly favor the deposition of a graphite structure in preference .to a diamond structure.
• hydrocarbons based upon the benzene structure have carbon atoms which are not tetrahedrally bonded and therefore do not meet the requirements of the present invention.
• Polyacenes which are condensation products of benzene, form a two dimensionally bonded structure resulting in a three-dimensional stack of plates characteristic of graphite. Deposition of graphite is therefore favored.

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• 1987
  • 1987-09-08 EP EP87907264A patent/EP0288526A1/de not_active Withdrawn
  • 1987-09-08 JP JP62506860A patent/JPH01501142A/ja active Pending
  • 1987-09-08 WO PCT/US1987/002251 patent/WO1988002792A1/en not_active Application Discontinuation
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