Abrasive particles for surface polishing

Polishing compositions are described that are appropriate for fine polishing to very low tolerances. The polishing compositions include particles with small diameters with very narrow distributions in size and effectively no particles with diameters several times larger than the average diameter. Furthermore, the particles generally have very high uniformity with respect to having a single crystalline phase. Preferred particles have an average diameter less than about 200 nm. Laser pyrolysis processes are described for the production of the appropriate particles including metal oxides, metal carbides, metal sulfides, SiO.sub.2 and SiC.

FIELD OF THE INVENTION
 The invention relates to abrasive compositions useful for surface
 polishing, especially mechanochemical polishing, and methods for producing
 abrasive particles.
 BACKGROUND OF THE INVENTION
 Technological advances have raised the demand for improved material
 processing with strict tolerances on processing parameters. In particular,
 smooth surfaces are required in a variety of applications in electronics,
 tool production and many other industries. The substrates requiring
 polishing can involve hard materials such as ceramics, glass and metal. As
 miniaturization continues even further, even more precise polishing will
 be required. Current submicron technology requires polishing accuracy on a
 nanometer scale. Precise polishing technology can employ mechanochemical
 polishing involving a polishing composition that acts by way of a chemical
 interaction of the substrate with the polishing agents as well as an
 abrasive effective for mechanical smoothing of the surface.
 SUMMARY OF THE INVENTION
 Improved polishing compositions are described for smoothing surfaces to
 very low tolerances. The polishing compositions are based on small
 particles with a very narrow distribution of particle diameters to provide
 for more control over the polishing process. Furthermore, a collection of
 preferred particles have effectively no particles with significantly
 larger diameters. In addition, the preferred particles have a very high
 level of purity with respect to a single crystalline phase. Laser
 pyrolysis provides for the production of preferred particles. Laser
 pyrolysis not only provides for the production of particles with preferred
 properties for abrasive applications but also for efficient and controlled
 production of the particles. These features provide for cost effective
 commercialization of the improved abrasive compositions.
 In a first aspect, the invention features a composition comprising a
 dispersion of particles, the particles including metal compounds and
 having an average particle diameter from about 5 nm to about 200 nm and a
 distribution of diameters such that at least about 95 percent of the
 particles have a diameter greater than about 60 percent of the average
 diameter and less than about 140 percent of the average diameter. The
 particles can be dispersed in an aqueous or nonaqueous solution. The
 particles preferably comprise a composition selected from the group
 consisting of SiO.sub.2, SiC, TiO.sub.2, Fe.sub.3 C, Fe.sub.7 C.sub.3,
 Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, MoS.sub.2, MoO.sub.2, WC, WO.sub.3 and
 WS.sub.2. The particles preferably have an average diameter less than
 about 100 nm.
 In another aspect, the invention features a composition comprising a
 dispersion of particles, the particles including metal compounds with an
 average particle diameter from about 5 nm to about 200 nm and a single
 crystalline phase with a uniformity of at least about 90 percent by
 weight. The particles can be dispersed in an aqueous or nonaqueous
 solution. The particles preferably have a single phase uniformity of at
 least about 95 percent by weight, more preferably at least about 99
 percent by weight and even more preferably at least about 99.9 percent by
 weight.
 In another aspect, the invention features a composition comprising a
 dispersion of particles, the particles including metal carbides or metal
 sulfides and having an average particle diameter from about 5 nm to about
 200 nm.
 In another aspect, the invention features a method of smoothing a surface
 comprising the step of polishing the surface with a composition of the
 invention, as summarized above and further described below. The polishing
 can be performed with a polishing pad and can involve a motorized
 polisher.
 In another aspect, the invention features a method of producing SiO.sub.2
 particles including the step of pyrolyzing a molecular stream comprising a
 silicon compound precursor, an oxidizing agent and a radiation absorbing
 gas in a reaction chamber, where the pyrolysis is driven by heat absorbed
 from a laser beam. The silicon compound precursor can include a compound
 that is selected from the group consisting of CH.sub.3 SiCl.sub.3. The
 laser beam preferably is supplied by a CO.sub.2 laser. The molecular
 stream preferably is generated by a nozzle elongated in one dimension.
 In another aspect, the invention features a method of producing iron oxide
 particles comprising the step of pyrolyzing a molecular stream comprising
 a iron compound precursor, an oxidizing agent and a radiation absorbing
 gas in a reaction chamber, where the pyrolysis is driven by heat absorbed
 from a laser beam. The iron precursor can comprise Fe(CO).sub.5.
 Other features and advantages are evident from the detailed description of
 the invention and claims presented below.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Methods have been developed for producing nanoscale particles with a small
 distribution of particle diameters. These particles generally have a
 single crystalline phase and a high level of uniformity. These particles
 are useful as abrasives, especially for polishing hard surfaces that have
 restrictive tolerance requirements with respect to smoothness. The small
 diameters together with the narrow spread in distribution of diameters
 provide for polishing to generate a surface with reduced degree of surface
 roughness. The particles can be used in polishing compositions containing
 other polishing agents that supplement the abrasive properties of the
 particles alone. The polishing compositions can be used for manual
 polishing or for polishing with a motorized polisher.
 Suitable abrasive particles generally are ceramics, although not
 necessarily electrical insulators, and can include a variety of metal
 oxides, sulfides and carbides as well as silica compounds. Preferred
 particles are relatively hard. The particle can include, for example, one
 of the following compounds: SiO.sub.2, SiC, TiO.sub.2, Fe.sub.2 O.sub.3,
 Fe.sub.3 O.sub.4, Fe.sub.3 C, Fe.sub.7 C.sub.3, MoS.sub.2, MoO.sub.2, WC,
 WO.sub.3, and WS.sub.2. A mixture of abrasive particles of different
 chemical composition can be used to form a polishing formulation. The
 appropriate composition of the abrasive particles depends on the
 composition of the substrate to be polished.
 Laser pyrolysis, as described below, is an excellent process for
 efficiently producing suitable ceramic particles with a narrow
 distribution of average particle diameters. A basic feature of successful
 application of laser pyrolysis for the production of appropriate small
 scale particles is production of a molecular stream containing a precursor
 compound, a radiation absorber and a reactant serving as an oxygen, sulfur
 or carbon source. The molecular stream is pyrolyzed by an intense laser
 beam. As the molecular stream leaves the laser beam, the particles are
 rapidly quenched.
 Laser pyrolysis provides for formation of phases of metal compounds that
 may be difficult to form under thermodynamic equilibrium conditions. The
 particles produced by laser pyrolysis also are suitable for optional
 further processing to alter and/or improve the properties of the
 particles.
 A. Particle Production
 Laser pyrolysis has been discovered to be a valuable tool for the
 production of nanoscale silicon dioxide, silicon carbide and metal oxide,
 metal carbide and metal sulfide particles of interest. In addition, the
 particles produced by laser pyrolysis are a convenient material for
 further processing to expand the pathways for the production of desirable
 metal compound particles. Thus, using laser pyrolysis alone or in
 combination with additional processes, a wide variety of silicon dioxide,
 silicon carbide and metal oxide, metal carbide and metal sulfide particles
 can be produced. In some cases, alternative production pathways can be
 followed to produce comparable particles.
 The reaction conditions determine the qualities of the particles produced
 by laser pyrolysis. The reaction conditions for laser pyrolysis can be
 controlled relatively precisely in order to produce particles with desired
 properties. The appropriate reaction conditions to produce a certain type
 of particles generally depend on the design of the particular apparatus.
 Nevertheless, some general observations on the relationship between
 reaction conditions and the resulting particles can be made.
 Increasing the laser power results in increased reaction temperatures in
 the reaction region as well as a faster quenching rate. A rapid quenching
 rate tends to favor production of high energy phases. Similarly,
 increasing the chamber pressure also tends to favor the production of
 higher energy structures. Also, increasing the concentration of the
 reactant serving as the oxygen, carbon or sulfur source in the reactant
 stream favors the production of metal oxides, metal carbides or metal
 sulfides with increased amounts of oxygen, carbon or sulfur.
 Reactant gas flow rate and velocity of the reactant gas stream are
 inversely related to particle size so that increasing the reactant gas
 flow rate or velocity tends to result in smaller particle size. Also, the
 growth dynamics of the particles have a significant influence on the size
 of the resulting particles. In other words, different crystal forms of a
 product compound have a tendency to form different size particles from
 other crystal forms under relatively similar conditions. Laser power also
 influences particle size with increased laser power favoring larger
 particle formation for lower melting materials and smaller particle
 formation for higher melting materials.
 Appropriate precursor compounds generally include metal or silicon
 compounds with reasonable vapor pressures, i.e., vapor pressures
 sufficient to get desired amounts of precursor vapor in the reactant
 stream. The vessel holding the precursor compounds can be heated to
 increase the vapor pressure of the metal (silicon) compound precursor, if
 desired. Preferred silicon precursors include, for example, CH.sub.3
 SiCl.sub.3. Preferred iron precursors include, for example, Fe(CO).sub.5.
 Preferred reactants serving as oxygen sources include, for example,
 O.sub.2, CO, CO.sub.2, O.sub.3 and mixtures thereof. Preferred reactants
 serving as carbon sources include, for example, C.sub.2 H.sub.4 and
 C.sub.6 H.sub.6. Preferred reactants serving as sulfur sources include,
 for example, H.sub.2 S. The reactant compound from the oxygen, carbon or
 sulfur source should not react significantly with the metal compound
 precursor prior to entering the reaction zone since this generally would
 result in the formation of large particles.
 Laser pyrolysis can be performed with a variety of optical laser
 frequencies. Preferred lasers operate in the infrared portion of the
 electromagnetic spectrum. CO.sub.2 lasers are particularly preferred
 sources of laser light. Infrared absorbers for inclusion in the molecular
 stream include, for example, C.sub.2 H.sub.4, NH.sub.3, SF.sub.6,
 SiH.sub.4 and O.sub.3. O.sub.3 can act as both an infrared absorber and as
 an oxygen source. Similarly, C.sub.2 H.sub.4 can act as both a infrared
 absorber and as a carbon source. The radiation absorber, such as the
 infrared absorber, absorbs energy from the radiation beam and distributes
 the energy to the other reactants to drive the pyrolysis.
 Preferably, the energy absorbed from the radiation beam increases the
 temperature at a tremendous rate, many times the rate that energy
 generally would be produced even by strongly exothermic reactions under
 controlled condition. While the process generally involves nonequilibrium
 conditions, the temperature can be described approximately based on the
 energy in the absorbing region. The laser pyrolysis process is
 qualitatively different from the process in a combustion reactor where an
 energy source initiates a reaction, but the reaction is driven by energy
 given off by an exothermic reaction.
 An inert shielding gas can be used to reduce the amount of reactant and
 product molecules contacting the reactant chamber components. Appropriate
 shielding gases include, for example, Ar, He and N.sub.2.
 The production of iron carbides by laser pyrolysis has been described in Bi
 et al., "Nanocrystalline .alpha.-Fe, Fe.sub.3 C, and Fe.sub.7 C.sub.3
 produced by CO.sub.2 laser pyrolysis," J. Mater. Res. 8:1666-1674 (1993),
 incorporated herein by reference.
 An appropriate laser pyrolysis apparatus generally includes a reaction
 chamber isolated from the ambient environment. A reactant inlet connected
 to a reactant supply system produces a molecular stream through the
 reaction chamber. A laser beam path intersects the molecular stream at a
 reaction zone. The molecular stream continues after the reaction zone to
 an outlet, where the molecular stream exits the reaction chamber and
 passes into a collection system. Generally, the laser is located external
 to the reaction chamber, and the laser beam enters the reaction chamber
 through an appropriate window.
 Referring to FIG. 1, a particular embodiment 100 of a pyrolysis apparatus
 involves a reactant supply system 102, reaction chamber 104, collection
 system 106 and laser 108. Reactant supply system 102 includes a source 120
 of precursor compound. For liquid precursors, a carrier gas from carrier
 gas source 122 can be introduced into precursor source 120, containing
 liquid precursor to facilitate delivery of the precursor. The carrier gas
 from source 122 preferably is either an infrared absorber or an inert gas
 and is preferably bubbled through the liquid, precursor compound. The
 quantity of precursor vapor in the reaction zone is roughly proportional
 to the flow rate of the carrier gas.
 Alternatively, carrier gas can be supplied directly from infrared absorber
 source 124 or inert gas source 126, as appropriate. The reactant providing
 the oxygen, carbon or sulfur is supplied from reactant source 128, which
 can be a gas cylinder or other suitable container. The gases from the
 precursor source 120 are mixed with gases from reactant source 128,
 infrared absorber source 124 and inert gas source 126 by combining the
 gases in a single portion of tubing 130. The gases are combined a
 sufficient distance from reaction chamber 104 such that the gases become
 well mixed prior to their entrance into reaction chamber 104. The combined
 gas in tube 130 passes through a duct 132 into rectangular channel 134,
 which forms part of an injection nozzle for directing reactants into the
 reaction chamber.
 Flow from sources 122, 124, 126 and 128 are preferably independently
 controlled by mass flow controllers 136. Mass flow controllers 136
 preferably provide a controlled flow rate from each respective source.
 Suitable mass flow controllers include, for example, Edwards Mass Flow
 Controller, Model 825 series, from Edwards High Vacuum International,
 Wilmington, Mass.
 Inert gas source 138 is connected to an inert gas duct 140, which flows
 into annular channel 142. A mass flow controller 144 regulates the flow of
 inert gas into inert gas duct 140. Inert gas source 126 can also function
 as the inert gas source for duct 140, if desired.
 The reaction chamber 104 includes a main chamber 200. Reactant supply
 system 102 connects to the main chamber 200 at injection nozzle 202. The
 end of injection nozzle 202 has an annular opening 204 for the passage of
 inert shielding gas, and a rectangular slit 206 for the passage of
 reactant gases to form a molecular stream in the reaction chamber. Annular
 opening 204 has, for example, a diameter of about 1.5 inches and a width
 along the radial direction of about 1/16 in. The flow of shielding gas
 through annular opening 204 helps to prevent the spread of the reactant
 gases and product particles throughout reaction chamber 104.
 Tubular sections 208, 210 are located on either side of injection nozzle
 202. Tubular sections 208, 210 include ZnSe windows 212, 214,
 respectively. Windows 212, 214 are about 1 inch in diameter. Windows 212,
 214 are preferably plane-focusing lenses with a focal length equal to the
 distance between the center of the chamber to the surface of the lens to
 focus the beam to a point just below the center of the nozzle opening.
 Windows 212, 214 preferably have an antireflective coating. Appropriate
 ZnSe lenses are available from Janos Technology, Townshend, Vt. Tubular
 sections 208, 210 provide for the displacement of windows 212, 214 away
 from main chamber 200 such that windows 212, 214 are less likely to be
 contaminated by reactants or products. Window 212, 214 are displaced, for
 example, about 3 cm from the edge of the main chamber 200.
 Windows 212, 214 are sealed with a rubber o-ring to tubular sections 208,
 210 to prevent the flow of ambient air into reaction chamber 104. Tubular
 inlets 216, 218 provide for the flow of shielding gas into tubular
 sections 208, 210 to reduce the contamination of windows 212, 214. Tubular
 inlets 216, 218 are connected to inert gas source 138 or to a separate
 inert gas source. In either case, flow to inlets 216, 218 preferably is
 controlled by a mass flow controller 220.
 Laser 108 is aligned to generate a laser beam 222 that enters window 212
 and exits window 214. Windows 212, 214 define a laser light path through
 main chamber 200 intersecting the flow of reactants at reaction zone 224.
 After exiting window 214, laser beam 222 strikes power meter 226, which
 also acts as a beam dump. An appropriate power meter is available from
 Coherent Inc., Santa Clara, Calif. Laser 108 can be replaced with an
 intense conventional light source such as an arc lamp. Preferably, laser
 108 is an infrared laser, especially a CW CO.sub.2 laser such as an 1800
 watt maximum power output laser available from PRC Corp., Landing, N.J. or
 a Coherent.RTM. model 525 (Coherent, Inc., Santa Clara, Calif.) with a
 maximum power output of 375 watts.
 Reactants passing through slit 206 in injection nozzle 202 initiate a
 molecular stream. The molecular stream passes through reaction zone 224,
 where reaction involving the precursor compound takes place. Heating of
 the gases in reaction zone 224 is extremely rapid, roughly on the order of
 10.sup.5.degree. C./sec depending on the specific conditions. The reaction
 is rapidly quenched upon leaving reaction zone 224, and particles 228 are
 formed in the molecular stream. The nonequilibrium nature of the process
 allows for the production of particles with a highly uniform size
 distribution and structural homogeneity.
 The path of the molecular stream continues to collection nozzle 230.
 Collection nozzle 230 is spaced about 2 cm from injection nozzle 202. The
 small spacing between injection nozzle 202 and collection nozzle 230 helps
 reduce the contamination of reaction chamber 104 with reactants and
 products. Collection nozzle 230 has a circular opening 232. Circular
 opening 232 feeds into collection system 106.
 The chamber pressure is monitored with a pressure gauge attached to the
 main chamber. The preferred chamber pressure for the production of the
 desired oxides, sulfides and carbides generally ranges from about 80 Torr
 to about 500 Torr.
 Reaction chamber 104 has two additional tubular sections not shown. One of
 the additional tubular sections projects into the plane of the sectional
 view in FIG. 1, and the second additional tubular section projects out of
 the plane of the sectional view in FIG. 1. When viewed from above, the
 four tubular sections are distributed roughly, symmetrically around the
 center of the chamber. These additional tubular sections have windows for
 observing the inside of the chamber. In this configuration of the
 apparatus, the two additional tubular sections are not used to facilitate
 production of particles.
 Collection system 106 can include a curved channel 250 leading from
 collection nozzle 230. Because of the small size of the particles, the
 product particles follow the flow of the gas around curves. Collection
 system 106 includes a filter 252 within the gas flow to collect the
 product particles. A variety of materials such as teflon, glass fibers and
 the like can be used for the filter as long as the material is inert and
 has a fine enough mesh to trap the particles. Preferred materials for the
 filter include, for example, a glass fiber filter from ACE Glass Inc.,
 Vineland, N.J.
 Pump 254 is used to maintain collection system 106 at a selected pressure.
 A variety of different pumps can be used. Appropriate pumps for use as
 pump 254 include, for example, Busch Model B0024 pump from Busch, Inc.,
 Virginia Beach, Va. with a pumping capacity of about 25 cubic feet per
 minute (cfm) and Leybold Model SV300 pump from Leybold Vacuum Products,
 Export, Pa. with a pumping capacity of about 195 cfm. It may be desirable
 to flow the exhaust of the pump through a scrubber 256 to remove any
 remaining reactive chemicals before venting into the atmosphere. The
 entire apparatus 100 can be placed in a fume hood for ventilation purposes
 and for safety considerations. Generally, the laser remains outside of the
 fume hood because of its large size.
 The apparatus is controlled by a computer. Generally, the computer controls
 the laser and monitors the pressure in the reaction chamber. The computer
 can be used to control the flow of reactants and/or the shielding gas. The
 pumping rate is controlled by either a manual needle valve or an automatic
 throttle valve inserted between pump 254 and filter 252. As the chamber
 pressure increases due to the accumulation of particles on filter 252, the
 manual valve or the throttle valve can be adjusted to maintain the pumping
 rate and the corresponding chamber pressure.
 The reaction can be continued until sufficient particles are collected on
 filter 252 such that the pump can no longer maintain the desired pressure
 in the reaction chamber 104 against the resistance through filter 252.
 When the pressure in reaction chamber 104 can no longer be maintained at
 the desired value, the reaction is stopped, and the filter 252 is removed.
 With this embodiment, about 3-75 grams of particles can be collected in a
 single run before the chamber pressure can no longer be maintained. A
 single run generally can last from about 10 minutes to about 3 hours
 depending on the type of particle being produced and the particular
 filter. Therefore, it is straightforward to produce a macroscopic quantity
 of particles, i.e., a quantity visible with the naked eye.
 The reaction conditions can be controlled relatively precisely. The mass
 flow controllers are quite accurate. The laser generally has about 0.5
 percent power stability. With either a manual control or a throttle valve,
 the chamber pressure can be controlled to within about 1 percent.
 The configuration of the reactant supply system 102 and the collection
 system 106 can be reversed. In this alternative configuration, the
 reactants are supplied from the bottom of the reaction chamber, and the
 product particles are collected from the top of the chamber. This
 alternative configuration can result in a slightly higher collection of
 product for particles that are buoyant in the surrounding gases. In this
 configuration, it is preferable to include a curved section in the
 collection system so that the collection filter is not mounted directly
 above the reaction chamber.
 A apparatus similar to laser pyrolysis embodiment 100 has been used to
 produce a variety of vanadium oxide nanoparticles in different oxidation
 states. These are described in commonly assigned U.S. patent application
 Ser. No. 08/897,778, filed Jul. 21, 1997, now U.S. Pat. No. 6,106,778,
 incorporated herein by reference.
 An alternative design of a laser pyrolysis apparatus has been described.
 See, commonly assigned U.S. patent application Ser. No. 08/808,850, now
 U.S. Pat. No. 5,958,348, entitled "Efficient Production of Particles by
 Chemical Reaction," incorporated herein by reference. This alternative
 design is intended to facilitate production of commercial quantities of
 particles by laser pyrolysis. A variety of configurations are described
 for injecting the reactant materials into the reaction chamber.
 The alternative apparatus includes a reaction chamber designed to minimize
 contamination of the walls of the chamber with particles, to increase the
 production capacity and to make efficient use of resources. To accomplish
 these objectives, the reaction chamber conforms generally to the shape of
 an elongated reactant inlet, decreasing the dead volume outside of the
 molecular stream. Gases can accumulate in the dead volume, increasing the
 amount of wasted radiation through scattering or absorption by nonreacting
 molecules. Also, due to reduced gas flow in the dead volume, particles can
 accumulate in the dead volume causing chamber contamination.
 The design of the improved reaction chamber 300 is schematically shown in
 FIGS. 2 and 3. A reactant gas channel 302 is located within block 304.
 Facets 306 of block 304 form a portion of conduits 308. Another portion of
 conduits 308 join at edge 310 with an inner surface of main chamber 312.
 Conduits 308 terminate at shielding gas inlets 314. Block 304 can be
 repositioned or replaced, depending on the reaction and desired
 conditions, to vary the relationship between the elongated reactant inlet
 316 and shielding gas inlets 314. The shielding gases from shielding gas
 inlets 314 form blankets around the molecular stream originating from
 reactant inlet 316.
 The dimensions of elongated reactant inlet 316 preferably are designed for
 high efficiency particle production. Reasonable dimensions for the
 reactant inlet for the production of the relevant oxide, sulfide and
 carbide particles, when used with a 1800 watt CO.sub.2 laser, are from
 about 5 mm to about 1 meter.
 Main chamber 312 conforms generally to the shape of elongated reactant
 inlet 316. Main chamber 312 includes an outlet 318 along the molecular
 stream for removal of particulate products, any unreacted gases and inert
 gases. Tubular sections 320, 322 extend from the main chamber 312. Tubular
 sections 320, 322 hold windows 324, 326 to define a laser beam path 328
 through the reaction chamber 300. Tubular sections 320, 322 can include
 shielding gas inlets 330, 332 for the introduction of shielding gas into
 tubular sections 320, 322.
 The improved apparatus includes a collection system to remove the particles
 from the molecular stream. The collection system can be designed to
 collect a large quantity of particles without terminating production or,
 preferably, to run in continuous production by switching between different
 particle collectors within the collection system. The collection system
 can include curved components within the flow path similar to curved
 portion of the collection system shown in FIG. 1. The configuration of the
 reactant injection components and the collection system can be reversed
 such that the particles are collected at the top of the apparatus.
 As noted above, properties of the product particles can be modified by
 further processing. For example, oxide nanoscale particles can be heated
 in an oven in an oxidizing environment or an inert environment to alter
 the oxygen content and/or crystal structure of the metal oxide. The
 processing of metal oxide nanoscale particles in an oven is further
 discussed in commonly assigned, U.S. patent application Ser. No.
 08/897,903 now U.S. Pat. No. 5,989,574, filed Jul. 21, 1997, entitled
 "Processing of Vanadium Oxide Particles With Heat," incorporated herein by
 reference.
 In addition, the heating process can be used possibly to remove adsorbed
 compounds on the particles to increase the quality of the particles. It
 has been discovered that use of mild conditions, i.e., temperatures well
 below the melting point of the particles, results in modification of the
 stoichiometry or crystal structure of metal oxides without significantly
 sintering the particles into larger particles.
 A variety of apparatuses can be used to perform the heat processing. An
 example of an apparatus 400 to perform this processing is displayed in
 FIG. 4. Apparatus 400 includes a tube 402 into which the particles are
 placed. Tube 402 is connected to a reactant gas source 404 and inert gas
 source 406. Reactant gas, inert gas or a combination thereof to produce
 the desired atmosphere are placed within tube 402.
 Preferably, the desired gases are flowed through tube 402. Appropriate
 reactant gases to produce an oxidizing environment include, for example,
 O.sub.2, O.sub.3, CO, CO.sub.2 and combinations thereof. The reactant gas
 can be diluted with inert gases such as Ar, He and N.sub.2. The gases in
 tube 402 can be exclusively inert gases if an inert atmosphere is desired.
 The reactant gases may not result in changes to the stoichiometry of the
 particles being heated.
 Tube 402 is located within oven or furnace 408. Oven 408 maintains the
 relevant portions of the tube at a relatively constant temperature,
 although the temperature can be varied systematically through the
 processing step, if desired. Temperature in oven 408 generally is measured
 with a thermocouple 410. The silicon oxide, silicon carbide, metal oxide,
 metal sulfide or metal carbide particles can be placed in tube 402 within
 a vial 412. Vial 412 prevents loss of the particles due to gas flow. Vial
 412 generally is oriented with the open end directed toward the direction
 of the source of the gas flow.
 The precise conditions including type of oxidizing gas (if any),
 concentration of oxidizing gas, pressure or flow rate of gas, temperature
 and processing time can be selected to produce the desired type of product
 material. The temperatures generally are mild, i.e., significantly below
 the melting point of the material. The use of mild conditions avoids
 interparticle sintering resulting in larger particle sizes. Some
 controlled sintering of the particles can be performed in oven 408 at
 somewhat higher temperatures to produce slightly larger average particle
 diameters.
 For the processing of titanium dioxide, for example, the temperatures
 preferably range from about 50.degree. C. to about 1000.degree. C., and
 more preferably from about 50.degree. C. to about 500.degree. C. and even
 more preferably from about 50.degree. C. to about 200.degree. C. The
 particles preferably are heated for about 1 hour to about 100 hours. Some
 empirical adjustment may be required to produce the conditions appropriate
 for yielding a desired material.
 B. Particle Properties
 A collection of particles of interest generally has an average diameter of
 less than a micron, preferably from about 5 nm to about 500 nm, more
 preferably from about 5 nm to about 100 nm, and even more preferably from
 about 5 nm to about 50 nm. The particles usually have a roughly spherical
 gross appearance. Upon closer examination, the particles generally have
 facets corresponding to the underlying crystal lattice. Nevertheless, the
 particles tend to exhibit growth that is roughly equal in the three
 physical dimensions to give a gross spherical appearance. Diameter
 measurements on particles with asymmetries are based on an average of
 length measurements along the principle axes of the particle. The
 measurements along the principle axes preferably are each less than about
 1 micron for at least about 95 percent of the particles, and more
 preferably for at least about 98 percent of the particles.
 Because of their small size, the particles tend to form loose agglomerates
 due to van der Waals and other electromagnetic forces between nearby
 particles. Nevertheless, the nanometer scale of the particles (i.e.,
 primary particles) is clearly observable in transmission electron
 micrographs of the particles. For crystalline particles, the particle size
 generally corresponds to the crystal size. The particles generally have a
 surface area corresponding to particles on a nanometer scale as observed
 in the micrographs. Furthermore, the particles manifest unique properties
 due to their small size and large surface area per weight of material. For
 example, TiO.sub.2 nanoparticles generally exhibit altered absorption
 properties based on their small size, as described in commonly assigned
 and simultaneously filed U.S. patent application Ser. No. 08/962,515, now
 U.S. Pat. No. 6,099,798, entitled "Ultraviolet Light Block and
 Photocatalytic Materials," incorporated herein by reference.
 As produced, the particles preferably have a high degree of uniformity in
 size. As determined from examination of transmission electron micrographs,
 the particles generally have a distribution in sizes such that at least
 about 95 percent of the particles have a diameter greater than about 40
 percent of the average diameter and less than about 160 percent of the
 average diameter. Preferably, the particles have a distribution of
 diameters such that at least about 95 percent of the particles have a
 diameter greater than about 60 percent of the average diameter and less
 than about 140 percent of the average diameter.
 Furthermore, essentially no particles have an average diameter greater than
 about 5 times the average diameter. In other words, the particle size
 distribution effectively does not have a tail indicative of a small number
 of particles with significantly larger sizes. This is a result of the
 small reaction region and corresponding rapid quench of the particles.
 Preferably, less than about 1 particle in 10.sup.6 have a diameter greater
 than about 5 times the average diameter. The narrow size distributions and
 lack of a tail in the distributions can be exploited in a variety of
 applications, as described below.
 In addition, the particles generally have a very high uniformity with
 respect to displaying a single crystalline phase and corresponding
 stoichiometry within the phase. Also, the silicon oxide, silicon carbide,
 metal oxide, metal sulfide and metal carbide particles produced by the
 above methods generally have a purity greater than the reactant gases
 because the crystal formation process tends to exclude contaminants from
 the lattice. Furthermore, particles produced by laser pyrolysis generally
 have been found to have a high degree of crystallinity. High degrees of
 crystallinity can result in harder and/or more abrasion resistant
 particles, which may be desirable for some applications. In view of all of
 these characteristics, especially small particle size, uniformity in size,
 crystalline phase and purity, the particles described herein are
 particularly suitable for abrasive applications.
 Although under certain conditions mixed phase material can be formed, laser
 pyrolysis generally is effective to produce single phase, crystalline
 particles with a high degree of uniformity. Primary particles generally
 consist of single crystals of the material. The single phase, single
 crystal properties of the particles can be used advantageously along with
 the uniformity and narrow size distribution. Under certain conditions,
 amorphous particles are formed by laser pyrolysis. The amorphous particles
 can be useful for certain applications, and the amorphous particles
 generally can be heated under mild conditions to form crystalline
 particles.
 The attributes of some of the compositions of particular interest are
 described. Iron is known to exist in several different oxidation states.
 For example, iron oxides are known with stoichiometries of Fe.sub.2
 O.sub.3, Fe.sub.3 O.sub.4 and FeO. FeO has a cubic crystal structure
 similar to NaCl, and Fe.sub.3 O.sub.4 has a cubic, inverse spinel crystal
 structure. .alpha.-Fe.sub.2 O.sub.3 has a trigonal crystal structure while
 .gamma.-Fe.sub.2 O.sub.3 has a cubic, spinel crystal structure that
 transforms to .alpha.-Fe.sub.2 O.sub.3 above 600.degree. C. Similarly,
 iron carbides have been observed with stoichiometries of Fe.sub.3 C
 (cementite-orthorhombic), Fe.sub.7 C.sub.3 (triclinic and hexagonal,
 pseudo-hexagonal or orthorhombic), Fe.sub.5 C.sub.2 (Hagg
 carbide--monoclinic), Fe.sub.2 C (cementite, orthorhombic), Fe.sub.20
 C.sub.9, Fe.sub.4 C and .epsilon.-carbide (Fe.sub.x C, 2&lt;x&lt;3, hexagonal).
 The conditions used in laser pyrolysis generally can be altered to select
 the desired forms of the iron compounds. The conditions in a particular
 apparatus for the selective production of Fe.sub.3 C and Fe.sub.7 C.sub.3,
 have been described in the Bi et al., J. Material Res. article, supra.
 Silicon oxides can have stoichiometries of SiO (amorphous) and SiO.sub.2.
 Silicon dioxide can have a variety of crystal structures such as hexagonal
 (quartz), trigonal, monoclinic (coesite), amorphous and combinations
 thereof. Silicon carbide similarly can have a variety of crystal
 structures.
 Molybdenum and tungsten also exhibits multiple oxidation states. For
 example, molybdenum oxides can have stoichiometries of, for example,
 MoO.sub.2 (monoclinic, deformed rutile), MoO.sub.3 (triclinic), Mo.sub.3
 O.sub.8, and Mo.sub.8 O.sub.23. Similarly, Molybdenum sulfides can have
 stoichiometries of, for example, Mo.sub.2 S.sub.3, MoS.sub.2, MoS.sub.3
 and Mo.sub.2 S.sub.5. Tungsten oxides can have stoichiometries of, for
 example, WO.sub.2 (tetragonal, deformed rutile), WO.sub.3 (orthorhombic),
 W.sub.18 O.sub.49 and W.sub.20 O.sub.58. Tungsten carbides are known with
 stoichiometries of W.sub.2 C (hexagonal) and WC (.alpha.-tetragonal and
 .beta.-cubic).
 C. Polishing Compositions
 A variety of polishing compositions can advantageously incorporate the
 abrasive particles described above. In its simplest form, the polishing
 composition can just involve the abrasive particles, produced as described
 above. More preferably, the abrasive particles are dispersed in an aqueous
 or nonaqueous solution. The solution generally includes a solvent such as
 water, alcohol, acetone or the like. The abrasive particles should not be
 significantly soluble in the solvent. The polishing composition generally
 includes from about 0.05 percent to about 50 percent, and preferably from
 about 0.1 percent to about 10 percent by weight abrasive particles.
 The solvents preferably have a low level of contaminants. In particular,
 water used as a solvent should be deionized and/or distilled. Any solvent
 should be greater than about 99 percent pure, and more preferably at least
 about 99.9 percent pure. The polishing composition preferably is free from
 any contaminants, i.e., any composition not included for effectuating the
 polishing process. In particular, the polishing composition should be free
 from particulate contaminants, which are not soluble in the solvent.
 The polishing compositions can include other components to assist with the
 polishing process. For example, the polishing composition can include a
 slurry of colloidal silica. The use of colloidal silica for polishing hard
 substrates is described in U.S. Pat. No. 5,228,886, incorporated herein by
 reference. Colloidal silica has been suggested to chemically react with
 certain surfaces. Silica particles produced by laser pyrolysis are ideally
 suited for the production of colloidal silica due to all of the properties
 described above. When using colloidal silica along with additional
 abrasive particles such as those described above, the polishing
 composition preferably includes from about 0.05 to about 5 percent
 abrasive particles and more preferably from about 0.1 to about 2 percent
 by weight.
 The polishing composition can be acidic or basic to improve the polishing
 characteristics. For polishing metals an acidic pH generally is preferred,
 for example, in the range from about 3.0 to about 3.5. A variety of acids
 can be used such as glacial acetic acid. For polishing oxide surfaces a
 basic polishing composition can be used, for example, with a pH from about
 10.5 to about 11.
 Preferred abrasive particles include silicon oxide, silicon carbide, metal
 oxides, metal sulfides and metal carbides with average diameters less than
 about 100 nm and more preferably from about 5 nm to about 50 nm. Preferred
 abrasive particles include compounds such as SiO.sub.2, SiC, TiO.sub.2,
 Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Fe.sub.3 C, Fe.sub.7 C.sub.3,
 MoS.sub.2, MoO.sub.2, WC, WO.sub.3, and WS.sub.2. Also, preferred abrasive
 particles have a relatively narrow diameter distribution and an effective
 cut of particle diameters that are several times larger than the average
 diameter, as described above. The particular composition of abrasive
 particles should be selected such that the particles have an appropriate
 hardness for the surface to be polished as well as an appropriate
 distribution of diameters to obtain efficiently the desired smoothness.
 Abrasive particles that are too hard can result in undesired scratches in
 the surface while particles that are too soft may not be suitably
 abrasive.
 The composition of the abrasive particles should also provide for removal
 of the polishing compositions after completion of the polishing. One
 approach to cleaning polished surfaces involves dissolving the abrasive
 particles with a cleaning solution that does not damage the polished
 surface.
 The polishing compositions can be used for mechanical or mechanochemical
 polishing that is performed manually or using a powered polishing machine.
 In either case, the polishing composition is generally applied to a
 polishing pad or cloth to perform the polishing. Any of a variety of
 mechanical polishers can be used, for example, vibratory polishers and
 rotary polishers.
 The polishing compositions are particularly useful for the polishing of
 substrate surfaces for the production of integrated circuits. As the
 density of integrated circuits on a single surface increases, the
 tolerances for smoothness of the corresponding substrates become more
 stringent. Therefore, it is important that polishing process is able to
 remove small surface discontinuities prior to applying circuit patterns
 onto the substrate. The small size and uniformity of the abrasive
 particles disclosed herein are particularly suitable in polishing
 compositions for these applications. SiO.sub.2 particles are suitable for
 the polishing of silicon based semiconductor substrates. Similarly,
 layered structures involving patterned portions of insulating layers and
 conducting layers can be simultaneously planarized, as described in U.S.
 Pat. No. 4,956,313, incorporated herein by reference.
 The embodiments described above are intended to be representative and not
 limiting. Additional embodiments of the invention are within the claims.
 As will be understood by those skilled in the art, many changes in the
 methods and apparatus described above may be made by the skilled
 practitioner without departing from the spirit and scope of the invention,
 which should be limited only as set forward in the claims which follow.