Patent Application
20140227527
This invention relates to inorganic materials, methods and apparatus for making same, and uses thereof; and is particularly, although not exclusively, applicable to making ceramic materials such as III-V semiconductors.
In the following, reference is made to making III-V semiconductors, in particular, but not exclusively, nitrides, but it will be clear to the person skilled in the art that the methods and apparatus disclosed may be used for the production of other materials, and as an example, manufacture of diamond is exemplified.
III-V semiconductors are materials based on binary III-V compounds in which a Group III element is matched with a Group V element.
While GaAs has been known for a long time, current interest is in the nitride III-V compounds GaN, InN, and AlN and ternary and quaternary nitrides of Ga, Al, and In. Binary alloys (ternary compounds) such as that between AlN and GaN or InN and GaN are conventionally known respectively as AlGaN and InGaN. These conventional names AlGaN and InGaN are not chemically correct formulae and in the following the term (Al,Ga,In)N is used.
“(Al,Ga,In)N” is meant to be broadly construed as meaning the nitrides of Al, Ga, and In (AlN, GaN, InN), as well as alloys thereof, and encompasses materials of composition (Ali,GajInk)N (where i+j+k=1) together with non-stoichiometric compositions (where i+j+k≠1) and doped compositions.
GaN, AlN and InN come in both hexagonal (wurtzite) and cubic (zinc blende) crystal structures. The hexagonal form is the stable form at room temperature and pressure, although the cubic form may be made by epitaxial growth of thin films on suitable substrates. Both hexagonal and cubic GaN and AlN are known as substrates for semiconductor devices. The wurtzitic polytypes of GaN, AlN and InN form a continuous alloy system with band gaps ranging from 0.78 eV for InN, through 3.42 eV for GaN to 6.2 eV for AlN thus potentially permitting the manufacture of optical devices active at wavelengths ranging from the red into the ultraviolet.
III-V semiconductors or doped III-V semiconductors have been proposed for or are in use for many applications, including:—
Electronic devices, for example and not limited to: high electron mobility transistors (HEMT) also known as heterostructure FET (HFET) or modulation-doped FET (MODFET); metal semiconductor field effect transistors (MESFET); bipolar (junction) transistors (BJT); heterojunction bipolar transistors (HBT); Schottky diodes; thyristors; rectifiers; RF/microwave transmitters; high voltage power electronics; gas sensors; actuators;
Opto-electronic devices, for example and not limited to: light emitting diodes (LED); laser diodes (LD); deep ultraviolet light sources (http://en.wikipedia.org/wiki/Deep_ultraviolet); photodetectors; dielectrics in optical storage media;
Piezoelectric devices, for example high temperature piezo electric devices that can operate in excess of 400° C.
Electro-acoustic devices, for example and not limited to: surface acoustic wave (SAW) devices (http://en.wikipedia.org/wiki/Surface_acoustic_wave);
MEMS devices, for example and not limited to: thin film bulk acoustic resonators (FBAR or TFBAR) (http://en.wikipedia.org/wiki/Thin_film_bulk_acoustic_resonator);
Spintronic devices (http://en.wikipedia.org/wiki/Spintronics)
Device substrates and packaging, particularly where the high thermal conductivity is of importance.
Reviews of applications and current manufacturing technologies can be found in a variety of places, including: Handbook of Nitride Semiconductors and Devices, ed. Hadis Morkoç, 2008, Wiley-VCH, ISBN 978-23-527-40837-5; Substrates for GaN-Based Devices: Performance Comparisons and Market Assessment, Strategies Unlimited report SC-27, 2006 and follow up report SC-29, 2009.
Materials for use as substrates in semiconductor applications require high purities and low defect levels. Defects can come in many sorts, and a common source of defect for material grown on a substrate is lattice mismatch.
Light emitting diodes may contain layers of GaN and alloys thereof with InN and AlN (commonly referred to as InGaN and AlGaN) grown on a substrate. Commonly used substrates include but are not limited to silicon, sapphire (Al2O3); silicon carbide; gallium nitride; and aluminium nitride. Sapphire and silicon have the advantage of being inexpensive (sapphire about twice as expensive as silicon), but have a large lattice mismatch with GaN. Lattice mismatch is important because it leads to strain and defects in material grown on the substrate. To overcome the strain it is common for interlayers to be used to provide for a progressive change but this adds manufacturing complexity and cost.
Single-crystal silicon carbide has a better lattice match to GaN, and has a high thermal conductivity which is useful for high power applications, but materials suitable for use as semiconductor substrates cost more than 10 times the cost of sapphire substrates. What industry is seeking is good quality GaN and AlN for use as substrates. Although substrates are available the quality is variable and the cost very high (of the order of 10 or more times the cost of silicon carbide substrates).
GaN is the preferred material to manufacture blue LED and laser diodes because of the high band gap. AlN is the preferred material for the manufacture of ultraviolet laser diodes. AlN is also a preferred material for very high power high frequency devices because of its high thermal conductivity and low dielectric constant.
Over time, various methods have been proposed for the manufacture of III-V semiconductors.
Bulk aluminium nitride powders are generally made by methods such as carbothermal reduction of alumina under a nitrogen atmosphere [e.g. U.S. Pat. No. 4,618,592]. However this produces powders that require sintering aids to produce dense bodies, and have too many impurities to permit use as substrates for semiconductors.
Directly nitriding aluminium powder has been proposed as a means to produce AlN for subsequent sintering.
JP63095103 [according to an English abstract (from espacenet)] relates to a process in which aluminium metal powder and NH3/H2 are passed through an N2/Ar plasma and the resulting powder collected downstream.
JP63095103 [according to an English abstract (from espacenet)] relates to a similar process in which gas containing N2/NH3/H2 and inert gas is excited to form a plasma, a tail part of which impinges on metallic Al to melt and evaporate it, and thereby form ultrafine AlN particles.
Physical vapour deposition (PVD) is a process in which vapour is produced by physical means and coats a substrate impinged upon the vapour. The vapour is generally produced by either an evaporative process [through heating] or by sputtering. Gas may be introduced to react with the vapour at the substrate surface in a process known as reactive deposition. Sometimes electron beams may be used to bombard the depositing surface and assist with deposition.
U.S. Pat. No. 4,997,673 relates to a physical vapour deposition method [reactive evaporation] of forming aluminium nitride films by placing a substrate in a vacuum, and subjecting the substrate to aluminium and to nitrogen ion irradiation. The aluminium was evaporated using electron beam heating of the aluminium, with the electron beam being provided from an electron gun housed within the vacuum chamber. The pressure in the chamber was 10−5 to 10−7 Torr [1.33 mPa to 13.3 μPa].
U.S. Pat. No. 7,494,546 relates to a reactive evaporation process for production of nitride thin films and coatings in a system operating at 10−1 to 10−8 Torr [13.3 Pa to 1.33 μPa] with best growth rates for GaN at 10−4 Torr [13.3 mPa] of only 3-4 Å·s−1 [1.08-1.44 μm/hour].
Reactive evaporation processes have not been demonstrated with deposition rates above ˜10 μm/hour and so have been used for the production of films rather than manufacture of bulk materials. Low deposition rates not only have the drawback of speed and energy cost of manufacture, they also risk entrapment of impurities in the growing material.
U.S. Pat. No. 5,534,314 discusses drawbacks of normal PVD processes [high vacuum requirement and low deposition rates] and relates to a process for vapour depositing an evaporant onto a substrate comprising: presenting the substrate to a deposition chamber, wherein said deposition chamber has an operating pressure of from 0.001 Torr [133 mPa] to atmospheric pressure and has coupled thereto a means for providing a carrier gas stream and a means for providing an electron beam at the operating pressure and further contains an evaporant source; impinging the evaporant source with the electron beam to generate the evaporant; entraining the evaporant in the carrier gas stream; and coating the substrate with the carrier gas stream which contains the entrained evaporant.
U.S. Pat. No. 5,534,314 alleges deposition rates of >100 μm/minute in contrast to the ˜10 μm/hour to be expected of PVD processes, and nitrides are stated to be materials that can be evaporated using the electron beam. However, U.S. Pat. No. 5,534,314 does not envisage reaction of the evaporant with the carrier gas stream and is not a reactive evaporation process. This process is not known by the applicants to be in use for the production of III-V semiconductors.
Sputtering is generally thought to provide higher deposition rates than evaporative PVD. In sputtering, a target is bombarded with ions, generally from a plasma, and atoms from the target travel to a substrate.
U.S. Pat. No. 6,692,568 and related U.S. Pat. No. 6,784,085 (among others) disclose a method alleged to produce single crystal MIIIN by a sputtering process in which an Ar/N2 mixture is used in combination with a magnetron arrangement and hollow cathode plasma injectors to form a plasma over a target, and atoms sputtered from the target are used to reactively coat a substrate. In this arrangement the target and plasma injectors function as cathodes and the substrate as an anode. In one of the embodiments of this process an annular containment shield and magnet are provided to focus sputtered material onto a substrate.
A major problem associated with using the sputtering process for reactive deposition is the difficulty of controlling the proportion of reactants. For example, when producing AlN the ratio of N2 in the Ar would need to be carefully controlled. Too little N2 in the gas mixture means that the proportion of Al atoms reaching the substrate would be too high, and so the deposited material would be a mixture of AlN and Al metal. Too much N2 causes the Al target to become coated with a thin film of AlN causing electrical arcing on the target surface. This electrical arcing can cause the target surface to explode in tiny droplets of Al which cannot fully react with ionized nitrogen, again forming a mixture of AlN and Al metal.
U.S. Pat. No. 6,692,568 discloses insertion of gas and generation of plasma for sputtering close to the sputtering target.
As a result of their defects, neither evaporative nor sputtering PVD processes are known by the inventors to be currently in commercial use for the manufacture of bulk III-V semiconductors for microelectronics; although PVD processes such as molecular beam epitaxy (MBE) and others are used for the production of nitride films and layers on substrates.
WO2009/066286 is directed to producing amorphous III-V materials attached to a substrate by a reactive evaporation method.
Current commercial methods for making bulk high purity III-V semiconductor substrates appear to concentrate on other techniques, for example.
Chemical vapour deposition is a process in which vapour phase reactants combine to produce a deposit on a substrate.
A method used commercially is known as hydride vapour phase epitaxy in which carrier gases dissociate to form the deposit and fugitive gases such as hydrogen chloride. Typically, ammonia is used as a carrier for nitrogen, and chloride species as carriers for Group III elements, although organo metallic materials may be used.
Such chemical processes are liable to result in contamination [e.g. oxygen, halide and carbon] from the compound material sources or from the containment vessels used in the process. Oxygen is a particularly prevalent contaminant and typical purity levels for these materials are “3 nines” (99.9%) pure [ignoring any oxygen] with oxygen contamination at 1-5 percent.
Oxygen contamination greatly reduces the thermal conductivity of AlN, and introduces defects into the material.
U.S. Pat. No. 7,655,197 relates to “microelectronic device quality” (Al,Ga,In)N single crystal articles claimed to have a top surface defect density of less than 107 defects cm−2 and allegedly defect densities of less than 104.cm−2 are achievable by the method disclosed [although no examples showing such defect levels are disclosed]. These articles were made by a high growth rate vapour phase epitaxy [VPE] method using group III precursors such as halides, hydrides, and metalorganics.
Related U.S. Pat. No. 6,596,079 relates to manufacture of (Al,Ga,In)N boules made by growing on an (Al,Ga,In)N seed crystal by vapour phase epitaxy.
U.S. Pat. No. 7,576,372 relates to free-standing hexagonal AlGaN wafers made by a VPE method.
U.S. Pat. No. 7,075,111 relates to nitride semiconductor substrates made by hydride vapour phase epitaxy (HVPE).
U.S. Pat. No. 6,616,757 relates to a method of making GaN single crystal boules by a VPE method. Related U.S. Pat. No. 6,936,357 relates to an AlGaN crystal made in similar manner.
US2006/280668 relates to a freestanding AlN single crystal, wherein the freestanding AlN single crystal is crack-free, at least 5 millimetres thick, at least 2 inches in diameter, and has a defect density of less than 107 cm−2. A hydride vapour phase epitaxy method is mentioned.
US2009/0092815 relates to a freestanding AlN single crystal wafer that is crack-free, has a thickness of at least 100 μm, a diameter of at least 2 inches (˜5 cm) and a thermal conductivity greater than 3.2 W/K cm. A hydride vapour phase epitaxy method is mentioned.
Physical vapour transport (PVT) in which a source of material [e.g. AlN] is held at a hotter part of a thermal gradient to sublime the source material which then condenses to form single crystals in a cooler part of the gradient. The condensation can be either seedless condensation, or on a seed of the material being made.
US2007/0169689 relates to aluminium nitride crystals having defect densities of 104.cm−2 or less, but made through a high temperature vapour phase method that would involve significant oxygen contamination [at least 0.1%].
U.S. Pat. No. 6,296,956 relates to single crystal AlN having impurity levels below 450 ppm and produced by a high temperature vapour phase method.
US2007/0101932 relates to aluminium nitride crystals having defect densities of 104.cm−2 or less, again made by a high temperature vapour phase method.
WO2007/062250 relates to aluminium nitride crystals having dislocation densities of 104.cm−2 through a high temperature vapour phase method involving Al and N2 vapour in which efforts are made to keep the oxygen and hydrogen concentration low [less than 300 ppm and 0.5% respectively] as these gases have been shown to create microvoids in the AlN material.
GaN crystals are commercially available from companies such as TopGaN made by a process in which Ga or GaN powder is used as a source of gallium to react with N2 or nitrogen derived from NH3 at elevated temperatures and pressures (believed to be in the range of about 75,000 psi to 900,000 psi at temperatures believed to be in the range 600° C. to 2400° C.).
U.S. Pat. No. 7,642,122 relates to a method of making nitride crystals in a supercritical fluid at temperatures higher than about 550° C. and pressures higher than about 2 kbar.
Various companies adopt an “ammonothermal” process for growing hexagonal GaN on seeds. In this process Ga or GaN in one part of a sealed apparatus provides a feedstock which is dissolved in ammonia and is driven by a temperature gradient to another part of the apparatus where it crystallises on seeds. Typical conditions are 500° C.-700° C. at 0.1-0.5 GPa.
Ammonothermal processes are also used for the production of AlN.
GaN has also been made by the sodium flux method, which involves the growth of GaN in gallium-sodium melts exposed to nitrogen gas under pressure.
GaN has also been grown by transporting Ga vapour from decomposing GaN powder to the seed under an NH3 atmosphere at about 1155° C.
Other patents that discuss semiconductor substrates made from III-V semiconductors include EP1743961, U.S. Pat. No. 5,993,699, U.S. Pat. No. 5,954,874, U.S. Pat. No. 5,858,086, U.S. Pat. No. 5,972,109 and U.S. Pat. No. 6,066,205.
New methods of making (Al,Ga,In)N materials with low contaminant [e.g. oxygen] content and/or low defect densities would be useful.
Additionally, for high pressure methods, new presses are required. High pressure high temperature (HPHT) machines have been used for a number of years to manufacture synthetic diamonds. The first successful machine was invented by H. T. Hall in 1958, [U.S. Pat. No. 2,947,610], and became known as a belt press. The belt press (and variations of it) has been in use ever since to manufacture synthetic diamonds. A second type of press was also invented by Dr. Hall. Termed a tetrahedral anvil press, this was patented as U.S. Pat. No. 2,918,699 in 1959. It was the basis for the cubic press, which may be the most widely used type of HPHT press used in diamond synthesis today. Variations of this have also been patented, with the first cubic press patented as U.S. Pat. No. 3,440,687 in 1969. Although cubic presses have smaller process chambers than belt presses, the operating costs are much lower than for belt presses. Further patents for the cubic press include U.S. Pat. No. 5,744,170.
In addition to the use of such presses in synthesis of synthetic diamonds, more recently presses have been used in clarification of natural diamonds and in the manufacture of low-defect single crystal gallium nitride. Both of these processes require much higher temperatures and longer times than are required for diamond synthesis. Diamond synthesis typically requires temperatures of about 1500° C. for times of about 40 minutes when using an iron-nickel catalyst, although the temperature required is strongly dependent on the catalyst chosen and can be below 1300° C.
In order to clarify diamonds and synthesize single crystal gallium nitride and aluminium nitride, temperatures far in excess of 1500° C. are required and this requires additional process insulation between the hot process chamber and the cemented tungsten carbide press components, more than is usual in either belt or cubic press and special designs of each are required to maintain tungsten carbide below 600° C., which is important since above this temperature the tungsten carbide strength rapidly deteriorates.
The current invention offers a new press design which is capable of achieving high temperatures and pressures and can be used in a low cost HPHT process for diamond synthesis. It is also capable of achieving the more extreme conditions required for diamond clarification and synthesis of single crystal gallium nitride and aluminium nitride and for other HPHT work.
In a first aspect the present invention provides ion beam generators as described and claimed herein.
In a second aspect the present invention provides apparatuses for manufacturing a material by vapour deposition, using said ion beam generators, as described and claimed herein.
In a third aspect, the present invention provides crucibles (which may be suitable for use in the apparatus for manufacturing a material by vapour deposition) as described and claimed herein.
In a fourth aspect, the present invention provides methods of manufacturing a composition or an article comprising a composition, using the apparatus of the second aspect, as described and claimed herein.
In a fifth aspect the present invention provides electrical, opto-electrical, optical, piezoelectric and acoustic devices comprising at least in part a composition made by the method of the fourth aspect.
In a sixth aspect the present invention provides a press for high pressure treatment of materials.
In a seventh aspect the present invention provides a method of making compositions using the press of the sixth aspect.
In an eight aspect the present invention provides methods of making compositions using the press of the sixth aspect to treat compositions formed by the method of the fourth aspect
In an ninth aspect the present invention provides electrical, opto-electrical, optical, piezoelectric and acoustic devices comprising at least in part a composition made by the methods of the seventh and eighth aspects.
In the following non-limitative description reference is made to the drawings in which:
In the following:
Relative positional terms such as “upper” “lower” and “beneath” are meant to indicate the relationships shown in the drawings and do not imply restrictions on the scope of the invention.
“Condensed phase” is to be interpreted as indicating solid, liquid or mixtures thereof.
“Ion beam” is to be taken as meaning a flow of gaseous/plasma material comprising ions but which may also contain neutral species.
The overall concept of the PVD apparatus is to provide:
Some of the vapour will traverse the plasma as neutral species; and some will be ionized while traversing the plasma adding to the plasma.
After traversing the plasma the vapour may impact a substrate to form a composition thereon.
Apparatus 100 is shown in
Beneath the aperture 104 is a vapour generator 105 (shown in more detail in
In the upper chamber 101 is a plasma generator 106 (a useful form of plasma generator 106 is shown in more detail in
Plasma generator 106 comprises a space 110 within which, in operation, a plasma 111 is generated which may extend outside the confines of the space 110. The plasma generator 106 shown is annular in form, but it can be readily appreciated that other arrangements [e.g. opposed plate cathodes, spiral cathodes—see Radio frequency hollow cathodes for the plasma processing technology, I. Bárdo{hacek over (s)}, Surface and Coatings Technology 86-87 (1996) 648-656] can be used to provide a space within which the plasma may be generated.
In WO2009/092097 reference is made to a concentric hollow cathode device in which an annular core of the hollow cathode functions as a sputtering target. The device of WO2009/092097 is not suitable for use in the present invention as it is closed at one end.
The plasma is generated from gas introduced into upper chamber 101 [for example through gas inlets 112]. An optional gas cleaner 113 may be provided within the chamber to reduce the content of oxygen and/or water vapour in the chamber. Dependent upon the gases used, the gas cleaner 113 may include a cold trap and/or an oxygen getter. Examples of cold traps include Meissner traps, in which liquid nitrogen is used to collect oxygen and/or water. [However Meissner traps are generally not useful when ammonia is a nitrogen source]. Examples of oxygen getters include, for example, a magnetron sputter source of a reactive metal which is used to getter oxygen and moisture from the process chamber. Suitable reactive metals include Ti, Zr, Hf, or Y.
The vapour generator 105 and plasma generator 106 are arranged such that when both are operating vapour 114 generated by the vapour generator 105 traverses the plasma 111.
Two rows of magnets 117 are housed between cathode backing 115 and cathode facing 116. One row of magnets has the north poles facing inward and the other row has the south poles facing inward. The resulting magnetic flux 118 is aligned parallel with the cathode facing 116 for a substantial part of the length of the space 110. A yoke [not shown] may join the ends of the magnets 117 remote from the cathode facing 116 so that the magnets 117 form part of a magnetic circuit with the yoke and the magnetic flux 118 in the space 110. It will be evident that one or more electromagnets may provide the same effect.
Cathode housing 119 is electrically isolated from the cathode backing 115 and cathode facing 116 and acts as an anode with respect to the cathode backing 115 and cathode facing 116, serving to prevent plasma forming on the outside of cathode backing 115. Conveniently cathode housing 119 is separated from cathode backing 115 to form a so-called dark space 120. “Dark space” is a narrow space small in comparison with the mean free path of electrons at the operating pressure, resulting in no plasma discharge between the housing 119 and the cathode 115. The plasma is confined only to the surface of cathode facing 116 so that there is no ionization of the gases: electrons from the cathode reach the cathode housing without exciting a discharge in the dark space. It is possible to dispense with the cathode housing and use the walls of the upper chamber 101 as an anode, but this would result in stray plasma and excessive generation of deposited material where it is not wanted, unless the walls of the chamber can be used for the dark space shield.
Such an arrangement in which the pressure in the lower chamber is lower than that in the upper chamber, has the advantage that electron beam sources work best at low pressure and maintaining a low pressure in the lower chamber prevents high voltage arcing from the filament to ground while allowing sufficient gas pressure in the upper chamber 101 to allow the hollow plasma generator to efficiently ionize nitrogen-containing gas.
A shutter can be mounted between the upper and lower chambers for selective operation if it is determined that the metal vapour is coating the surfaces of the hollow cathode with metal prior to the application of gases and plasma generation.
The cathode backing 115 (shown in
Partition 103 may be of steel or like soft magnetic material, and optionally may be connected to cathode housing 119 (shown in
The substrate mount 107 is in the form of an electrically resistance heated chuck supplied with power from a power supply 125 (for example a Eurotherm SCR 40) controlled by a power controller 126 (for example, a Eurotherm Model 1226e) connected to a thermocouple 127 which measures the temperature of the substrate mount 107. Such an arrangement of heated substrate mount, power supply, and power supply controller enables the temperature of the substrate mount, and hence the substrate, to be maintained at a desired value. Measurement of the temperature of the substrate surface [for example by pyrometry of the surface of the substrate 108] may provide greater control, particularly when the substrate is thick. The substrate 108 is held against the substrate mount by three spring loaded pins [not shown] but any other suitable means may be used.
A second RF generator 128 (for example Dressler Model Cesar) is attached to the substrate mount 107 through a second matching network 129 (for example Dressler Model Cesar)
Mass flow controllers 130 (for example Unit Instruments Model UFC 1000) controller the supply of gases 131 to gas inlets 112 (shown in
A shutter 132 is provided between the plasma generator 106 and substrate mount 107. Contaminants and vapour are inhibited or prevented from deposition on the substrate 108 by the shutter 132. The shutter also serves to inhibit deposition from the substrate onto the hollow cathode during plasma etch cleaning of the substrate prior to deposition, and inhibit deposition from the hollow cathode onto the substrate while the hollow cathode plasma reaches equilibrium.
The shutter 132 may be provided with a periscope (not shown) for internal inspection of the apparatus as described below.
Vapour generator power supply 133 supplies power to the vapour generator 105.
In the example apparatus of
In the example apparatus of
Much expertise in the handling of substrates has been developed in the field of semiconductor processing and the invention contemplates the use of any and all known technology that can be usefully applied to the present invention.
The apparatus can be operated in the following general steps, realizing that modifications can be made in the steps and procedure and still accomplish the purposes/features of the present invention. A substrate 108 may be mounted on substrate mount 107 and a condensed phase source of material [e.g. gallium or aluminium, alloys thereof, or other desired components] can be placed in crucible 135. The shutter 132 would be in place between the vapour generator 105 and substrate 108. The system would be pumped down to evacuate the upper and lower chambers 101,102. A typical pressure in the chambers at this stage would be about 10−6 Torr, though other pressures above and below this may be used.
Once down to pressure, and in either order the following can be done:—
This serves to get the substrate 108 to a suitable deposition temperature, and to clean the substrate by Ar bombardment. The bombardment can typically proceed for 5 to 10 minutes. Inert gases other than argon [e.g. Ne, Kr or Xe] may be used in the cleaning step as may mixtures of inert gases.
The argon supply would then be turned off and the RF generator 128 turned off.
Vapour generator power supply 133 would then be turned on and the electron beam current increased to start heating of the condensed phase source of material. Typically the aim is to increase the condensed phase source of material to a temperature at which it is molten, and has a usefully high vapour pressure for deposition, without splashing the material through excessive boiling or outgas sing of impurities or trapped gases. It can be useful to be able to observe the condensed phase source of material to ensure appropriate heating. For example, if the vapour generator is an electron beam device [for example as shown in
During this heat up process, surface contaminants on the condensed phase source of material may be vaporized. Contaminants and vapour are inhibited or prevented from deposition on the substrate 108 by the shutter 132.
Once the condensed phase source of material is molten, contaminants produced on the condensed phase source of material are either vaporized or sink or float to the edges of the molten material.
Argon, or any other inert gas such as Ne, Kr, Xe or others, can be injected over the material to be evaporated in the E-beam. The argon or other inert gas reduces the possibility of the reactant gas reacting with the molten surface material. Typically, without this argon blanket over the molten metal surface, AlN forms on the surface preventing the Al vapour from leaving the molten surface, and the AlN on the surface is sputtered by the E-beam causing particles of Al+AlN to be ejected from the E-beam source onto the substrate. These same procedures can be implemented with other source materials as described herein to protect the source material during melting and vaporizing.
One or more reaction gases [e.g. for nitride production N2, NH3, N2H2, or other nitrogen hydrides or nitrogen containing compounds; for oxide production, gases containing oxygen; for fluoride production fluorine containing gases; for mixed products [e.g. oxynitrides] a single gas comprising the required elements or a mixture of gases may be used] may be introduced into the upper chamber through one or more of the gas inlets 112. Some argon may also be introduced into the reaction chamber to assist in bombardment of the substrate 108, as this may improve the properties of the deposited material. A typical pressure in the upper chamber 101 would be about 10−4 to 10−2 Torr [for example 1×10−3 to 5×10−3 Torr; 1×10−3 to 2×10−3 Torr; or 3×10−3 to 5×10−3 Torr]. The pressure experienced in the lower chamber 102 would normally be less, for example about half or between a fifth and a tenth that for the upper chamber 101. For example, the pressure might be in the range 1×10−3 to 2×10−3 Torr in the upper chamber and 3×10 to 5×10−4 Torr in the lower chamber, though other pressures and pressure differences between the two chambers can be used.
The operating pressure of the vapour generator can be less than 5×10−4 Torr and the operating temperature of the plasma generator can be 1 to 2×10−3 Torr with an evacuation rate of >1000 l/min. A reactant gas feed rate (if used) can be 2-5 sccm (or above or below this range), and an inert gas feed rate to blanket the melt (source material) can for example be 10 sccm or less. These are examples and other amounts can be used for any of these ranges.
Once the upper chamber 101 is at pressure, RF generator 128 can be turned on to supply 15-25 watts resulting in an RF bias of 80-150 volts to the substrate 108; and RF generator 123 turned on to supply RF to the cathode of the plasma generator 106. A typical bias for RF generator 123 is ˜350 volts.
The RF applied to the vapour generator initiates a plasma discharge 111 in the space 110 resulting in ionization of the gases present, and the discharge may extend out of the confines of the space 110 towards either or both of the vapour generator 105 or substrate 108.
Vapour 114 from the vapour generator 105 can react with the ionized gases in plasma 111. For instance, in the case of Al as the source material and nitrogen as reactant gas, AlN starts to form on the shutter 132. On removal of shutter 132 the vapour 114 and/or plasma 111 can reach the substrate 108 and commence deposition on the substrate. Application of RF bias to the substrate results in localised plasma generation and assists in bombarding the surface of the depositing material. For amorphous deposition it is preferable that no bias is applied to the substrate.
The rate of deposition can be measured [for example by using a deposition monitor, for example an Inficon Model U200]. Deposition temperatures have been used from 400-1100° C. depending on the substrate material and deposition rates have been from 0.1 to 60 μm per hour, typically 40-60 μm per hour, although the invention is not limited to these temperatures or deposition rates.
Once the deposited material has reached a required thickness or depth the following may be done:—
The above describes a procedure with the apparatus shown. It will be evident that the apparatus can be used in different ways depending upon the nature of the material to be deposited. For example, the constitution of the gases may vary during the process, and the constitution of the vapour may be varied, for example by switching between alternative vapour sources. Control of the process can be automated, for example by computer control such as National Instruments LabVIEW.
Aluminium nitride was made by placing pure Al (5 nines purity) in the crucible of the apparatus of
Growth rate shown was >40 μm/hour and on occasion>80 μm/hour and XRD of the deposited material showed hexagonal AlN.
Following these procedures, AlN has been successfully deposited on Si, Al, sapphire, Mo, W, Nb, Ta, SiC, diamond, graphite, Cu, and Ta with no peeling or cracking in the machine. The deposited AlN films were transparent.
A metal or graphite or diamond sheet, or a crucible, was attached to a chuck above the electron beam hearth in the same vacuum chamber. The chuck was face down towards the electron beam hearth. The vacuum chamber was pumped down, typically <5×10−6 Torr, and backfilled with Ar to 3×10−3 Torr. An RF generator, with any range, for example 13.56 MHz, was attached to the chuck and turned on, typically at 100 W, for 10 minutes to clean the substrate. The RF generator was turned off and the vacuum chamber again pumped down to <5×10−6 Torr. The chuck temperature was increased to 800° C. The electron beam was turned on to melt the Al and bring the deposition rate to ˜0.3 nm/sec. This rate can be increased to the rates described herein by increasing the power of the electron beam. Ar was turned on at ˜3 sccm and NH3 was turned on at ˜10 sccm resulting in a vacuum pressure of ˜6×10−4 Torr as measured in the lower chamber.
A RF generator, attached to the hollow cathode was turned on, which created a plasma and ionized the N2. The RF generator attached to the chuck was also turned on which created a self bias on the chuck, typically 120-140 volts. This bias caused the Ar ions to bombard the AlN being deposited resulting in a very dense, crystalline film.
A shutter between the electron beam gun and the crucible was then opened. The shutter was opened typically for 15 minutes, resulting in ˜15 μm of AlN deposition. The shutter was closed and the electron beam turned off, the RF generators were turned off and gases turned off. The chuck heater was turned off. When the chuck reached 500° C., the vacuum chamber was back filled with N2 to atmospheric pressure. The vacuum chamber was opened and the metal or graphite or diamond sheet, or crucible was removed.
The AlN coatings produced had excellent adhesion to metals, graphite and diamond such that the coated substrates were heat cycled to 1100° C. with no cracking or peeling of the AlN. A graphite substrate with a 5 μm AlN coating that was heat cycled to 1100° C. showed no degradation of the AlN film. A 15 μm film on a diamond coated silicon substrate that was heated to 1100° C. showed no degradation of the AlN film. Such AlN coated, high thermal conductivity materials can be used in thermal management, as may other products produced by the method of the invention. Thermal management applications include heat conductors, heat spreaders, and heat sinks. The substrates for such applications may be, but are not limited to, Cu, Al, diamond, graphite, pyrolytic graphite, or silicon carbide.
Following essentially the same procedures,
Greater thicknesses are readily achievable.
In these tests adhesion was measured by a “tape peel test’, wherein the deposited AlN film was scratched with a diamond scribe with enough pressure to break through the deposited AlN. Adhesive tape was then pressed onto the scratched area. The tape was then peeled off the surface and the scratched area examined with a microscope at 100× to see if any of the deposited film had peeled off the substrate. This test has been done on AlN deposited on Si, Ta and Cu and has shown there is no peeling of the AlN using the tape peel test.
AlN has been deposited onto a refractory such as SiC with excellent adhesion and no cracking or peeling of the AlN.
With the present invention, an intermediate region can be present between the deposited material and the substrate. This intermediate region may have a different composition or structure than the deposited material or the substrate itself. The intermediate region may be a reaction product of one or more components that form the deposited material and one or more components that form the substrate.
For example, when aluminium nitride was deposited directly on silicon, and the silicon substrate subsequently dissolved away, it was seen that a blackened layer did not dissolve away and this blackened layer was different from the transparent aluminium nitride formed on top, thus confirming the presence of an intermediate layer presumed to be a reactant product between the silicon and the aluminium nitride.
This intermediate region can occur through reaction in the initial stages of deposition, with the remaining thickness of the deposited material forming on top; or it may occur through subsequent reaction of deposited material with the substrate; or indeed it may be a separately applied layer, which optionally may have a distinct difference in chemical composition from either substrate or deposited material [as exemplified, for example, in the AlN coating formed on a diamond coated silicon substrate of
As an example, without limiting the present invention, substrate 401 can be copper or sapphire or silicon, for instance, and layer 405 can be AlN, wherein intermediate region 403 would comprise a reaction product of aluminium, nitrogen, and/or aluminium nitride with one or more components that comprise the substrate, namely, the sapphire, silicon, or copper.
The present invention contemplates both intermediate layers formed through reaction with the substrate, and deliberately formed intermediate layers. Such laminates can be used as heat sinks for thermal management.
As an example, aluminium nitride as a layer that is deposited on a metal substrate or ceramic substrate or glass substrate can provide excellent thermal management properties due to the properties of aluminium nitride. For instance, the layer can have a thermal conductivity of 210 W/mK to 319 W/mK, such as from 210 to 275 W/mK or 210 to 250 W/mK.
The aluminum nitride located on the substrate can have one or more of the following additional properties:
Hardness has been measured of a 30 μm AlN coating on silicon by using a Knoop indenter and taking the average of ten indents at 100 grams load. This showed a microindentation hardness measured was 1412±143 HK 0.1, 10.
Dielectric strength has been measured for 200 nm and 500 nm thick coatings of AlN on silicon after an 800° C. anneal. The dielectric strength was found to be in the range 1-3 MV/cm.
Thermal management may be for any device, for example as an electronic device, like a CPU, light radiation emitting device (e.g. LED), phone, smart device, and the like. For instance, the laminate 400 may comprise a layer of material adhered to a substrate, with either or both the layer and substrate being of high thermal conductivity material. For example, a material such as aluminium nitride adhered to a metal substrate like copper, can be used as a heat sink in lieu of, for instance, a printed circuit (PC) board or mount (or sub mount) which typically are made from a polymer material or resin and are not good conductors of heat. Other substrates with high thermal conductivity include, for example and without limitation, other metals, graphite, and diamond.
With the present invention, one or more chips, such as integrated circuits (ICs) or computer chips, can be connected to the laminate through interconnects or bumps, and, due to the dielectric nature of the layer (for instance, aluminium nitride), the interconnects or bumps can be made out of metal without any need for separate insulation layers.
As an alternative, the laminate heat sink of the present invention can be located on a mount or PC board or be the mount or PC board itself.
In another option, the laminate heat sink of the present invention can be a heat sink for a light-emitting diode device, wherein a dye(s) is located on the heat sink and, again, due to the dielectric nature of the layer used for the laminate heat sink of the present invention, no separate insulator is needed between the dye and the heat sink.
The mount or PC board 413 can have the laminate of the present invention 415 located on top and adhered to the mount or PC board 413. The laminate 415 can comprise a substrate 416, which can be any of the substrates identified in the present invention, but for this embodiment, the substrate preferably is a good thermal conducting substrate such as copper. Located on the substrate is a layer 421, for instance, of aluminium nitride or other material that has a good thermal conductivity. The bumps 417 from the IC chip or other electronic part can be connected to the layer 421 for purposes of securing the chip 419 and providing good thermal connectivity.
In an alternative example,
Although bumps are shown in
In the apparatus of
The inventors have found that coating refractory crucibles [e.g. of tantalum, niobium, molybdenum or tungsten] with AlN or similar layers reduces or prevents the wetting of the melted material to the crucible, which in turn prevents the molten metal from flowing up the crucible wall and causing a thermal short. In addition, the non-wetting prevents the crucible from bonding to the melted material and thus the crucibles do not crack with repeated heating and cooling cycles.
Refractory materials that may be used for forming crucibles include refractory metals or carbon.
Refractory metals on one definition are those with a melting point above 1850° C. [Ti, V, Cr, Zr, Hf, Ru, Rh, Os, Ir, Nb, Mo, Ta, W, Re], and on another definition those exceptional metals with a melting point above 2200° C. [Nb, Mo, Ta, W, Re]. The present invention contemplates the use of any or all of these elements and alloys [both between these elements and/or with other elements] that have a melting point above 1850° C. or 2200° C. respectively.
The invention also contemplates use of materials other than refractory materials, depending on what is melted in the crucible. For instance, a stainless steel crucible can be used and coated with AlN as described herein and then used for melting material, such as Al.
In tests, a tantalum crucible was coated with AlN by using it in place of a substrate in the apparatus of
In addition to tantalum, AlN has been proven in similar experiments to have excellent adherence to many substrate materials such as Cu, Si, W, Nb, Mo, Ta, graphite and diamond. AlN has been successfully deposited onto Ta, Nb, Mo and W crucibles used in electron beam evaporation of Ag.
A W crucible coated with 10 μm of AlN was tested in an electron beam system in which high purity (4 nines) Ag was evaporated. It was found that while largely non-wetting the molten Ag had dissolved some of the AlN coating.
Au was melted and cooled a number of times in a AlN coated crucible over a period of several months with no wetting of the Au to the AlN coating.
Another W crucible coated with 10 μm of AlN was placed into a vacuum oven and heated to 1100° C. in forming gas (5% H2 in N2) at 20 mTorr for 1 hour and then in air at 20 mTorr for an additional hour. The AlN coating on the W crucible showed no cracking or peeling after this heat treatment. The heat treated AlN coated W crucible was tested by evaporating Ag in an electron beam system. There was no degradation of the AlN in contact with the molten Ag. This test was performed several more times with no reaction of the Ag with the AlN.
It is suspected, although the applicant does not wish to be bound by this hypothesis, that the AlN is at least partially oxidized in the heat treatment, perhaps to form, in part at least, an aluminium oxynitride. The present invention contemplates a process in which an applied nitride coating, particularly although not exclusively an AlN coating, is further modified through exposure at elevated temperature to an oxygen containing material. The material may be an oxygen containing gas or may result from contact with a liquid or solid oxygen-containing reactant.
The invention further contemplates oxidation being part of the process of deposition, for example by introducing oxygen-containing gases [e.g O2, H2O or nitrogen oxide] during the final stages of deposition of the AlN.
The invention further contemplates heat treatment (with or without oxygen present) to anneal the coating. Heat treatment temperatures for AlN may be at any useful temperature below the melting or deposition temperature of the substrate and may be, for example, above or below 500° C., above or below 750° C., above or below 1000° C., above or below 1250° C., above or below 1500° C., above or below 1750° C., above or below 2000° C., or within any range defined by these temperatures or outside these temperatures.
The invention still further contemplates complete oxidation of the AlN to provide an aluminium oxynitride throughout the coating.
Al was melted and cooled in an AlN coated crucible (Ta) in an E-beam system 96 times. The Al adhered to the AlN coated crucible but did not wet up the sides of the crucible. There was no evidence of the AlN coating cracking or peeling after the 96 cycles. There was also no evidence of the Ta cracking.
Reducing surface roughness before coating assists in reducing defects in the coating. For example excellent coatings with no defect have been obtained by electro polishing the crucibles prior to coating to obtain a very smooth surface prior to coating. It is believed that surface roughness does not necessarily improve adhesion but high surface roughness can cause defects in the AlN coating.
Whereas tantalum and niobium crucibles are commonly made by machining from bulk and are hence expensive, the inventors have found that these materials may be pressed from sheet material and used to form crucibles.
U.S. Pat. No. 7,056,383 discloses tantalum crucibles that are machined from 99.9% pure tantalum, carburised to form a carbide surface and then treated to produce a tantalum nitride coating over the carburised surface. The present application contemplates using pressed tantalum or niobium to make coated crucibles without the need for a carburised surface.
In the apparatus of
U.S. Pat. No. 5,534,314 discloses a process in which a carrier gas stream is provided, in effect to stream evaporated material to a substrate. Such a process could be used in combination with the present process by introducing gas at the lower end of the plasma generator 106.
U.S. Pat. No. 5,534,314 discloses a process in which no crucible is used, but a rod of material is provided for evaporation by an electron beam. Rod fed evaporation could be used in conjunction with the present apparatus. A rod could be fed into a crucible as disclosed above.
The material being evaporated may be elemental or an alloy.
The combination of vapour generator and plasma generator together constitute an ion beam gun that may have wider applicability than in the methods and apparatus described herein.
The process described above may be used generally to produce laminated articles comprising one or more deposited layers on a substrate. Although in the examples above metals, graphite, silicon carbide, silicon and diamond-coated silicon have been mentioned as potential substrates for deposition of layers, the invention has wider applicability and encompasses deposition on any substrate capable of receiving an adherent deposit without material damage to the substrate.
Typically, the substrate may have a melting or decomposition temperature of over 200° C., or over 300° C. or over 400° C. Substrates may include, for example, a metal, glass, ceramic, glass ceramic, or polymer, as long as the melting temperature of the substrate is high enough to resist damage in the process of deposition. The substrate may be crystalline, partially crystalline, or amorphous. If a polymer is used it may be thermoset or thermoplastic although thermoset polymers may be better in resisting damage in processing.
More specific examples of substrates include, but are not limited to, the following: aluminium, boron, carbon, chromium, cobalt, copper, gold, hafnium, iron, lead, magnesium, manganese, molybdenum, nickel, niobium, platinum, silicon, silver, tantalum, tin, titanium, tungsten, zinc, and zirconium, or compounds containing one or more of these elements, such as oxides thereof, nitrides thereof, carbides thereof, ceramics thereof, and the like.
More specific examples of substrate include, AlN, copper, aluminium, diamond, glass, silicon carbide, silicon, silica, graphite, pyrolytic carbon and the like. Substrates may also comprise any of these materials [or any other material] having a coating of these materials. Such products may be used in thermal management, as described above.
By adherent deposit is meant a deposit that exhibits no cracking or peeling in ordinary use of the laminated article. This can include no cracking or peeling occurring even after the substrate with the adhered deposit has been exposed to high temperatures, such as temperatures that are within 20% of the melting temperature of the substrate or layer (whichever is the lower) even after return to a lower temperature [e.g. 25° C.]. Adherence can be tested by other methods [e.g. cycling between a lower and a higher temperature].
The deposited layers adhered to the substrate(s), can have low oxygen contents when oxides are not intended. For instance, a metal or metal nitride or other non-oxygen containing layer can be deposited on a substrate(s), wherein the layer has a low oxygen content. The oxygen content for such layers can be under 300 ppm, such as from about 1 ppm to 299 ppm, or 3 ppm to 100 ppm, or 1 ppm to 100 ppm, or 1 ppm to 10 ppm, and the like. Due to the processes of the present invention, other impurities, such as gaseous impurities and/or metal impurities, can be very low. For instance, if using a source material that is vaporized that has a very high purity, such as 99.999% or 99.9999% purity, all other impurities (gaseous and/or metal or total other impurities) in the layer can be below 10 ppm, can be below 5 ppm, can be about 1 ppm or 1 ppm, such as from 1 ppm to 5 ppm.
With the present invention, the layer deposited on a substrate can have excellent coating uniformity, for example as low as ±5% throughout the entire deposited surface of the substrate. For the interior of convex shapes such as crucibles the coating uniformity tends to be less uniform, but a uniformity of ±10% has been achieved. The uniformity was assessed using an SEM to examine different areas.
With the present invention, the deposited layer on the substrate can have no observable voids or pin holes at a magnification of 300× or 500×. The deposited layer adhered to the substrate can have any desired thickness, such as from about 0.1 micron to 2 mm or more, such as 0.1 micron to 2 mm, 0.1 micron to 1 mm, 10 microns to 500 microns, 10 microns to 100 microns, and the like.
The substrate, prior to having a layer deposited on it, can be pre-treated with any conventional techniques, such as cleaning the surface, acid treating, polishing (e.g., electro-polishing) the surface, and the like. These various cleaning or polishing steps can be done using any conventional technique associated with a substrate material. Similarly, after the layer is deposited on the substrate, the laminate which can have any shape, can be subjected to any number of post-treatments, such as, but not limited to, cleaning treatments, heat treatments at any various temperatures or pressures, passivated, or otherwise treated with liquids or gasses (nitrogen and/or oxygen, halides, and/or air at optional room and/or elevated high temperatures, and/or at optional elevated pressures).
A typical process chamber of a belt press is depicted in
The use of pyrophyllite allows the compression of the process chamber 141 and allows for electrical isolation between the punch or anvils 139a, 139b and the carbide belt 13. The pyrophyllite also allows for some thermal insulation between the hot process chamber 141 and the punch or anvils 139a, 139b and the carbide belt 140. The thermal insulation between the process chamber 141 and punch or anvils 139a, 139b and the carbide belt 140 is minimal as the space between the process chamber 141 and punch or anvils 139a, 139b and the carbide belt is limited to approximately 1-2 mm. A space larger than 1-2 mm between the process chamber 141 and punch or anvils 139a, 139b and the carbide belt 140 would allow the pyrophyllite to continue to flow as the pressure is increased, and would thereby prevent the pressure increasing to greater than 7 GPa (70 kbar) required to manufacture synthetic diamonds. Such a thin layer of pyrophyllite does not sufficiently thermally insulate the carbide members from the heat of the process chamber. The belt press can only be used at temperatures>1500° C. for very short times, such 5-10 minutes, before the carbide members reach 600° C. and begin to fail.
Conventional belt press geometries therefore do not provide sufficient thermal insulation to the press parts and cannot be modified to provide added insulation simply by increasing gasket thickness. The present invention overcomes this problem through the use of a press in which a punch is movable to enter a divergent press cavity from a narrower end of the cavity.
Accordingly, the present invention provides a press for high pressure, and optionally high temperature, treatment of materials, comprising a first press assembly having a cavity broadening from a mouth, and a punch mounted on a second press assembly such that relative motion between the first press assembly and second press assembly can move the punch into the cavity from the mouth.
The cavity may be defined by a ring having a through hole and a closure closing an end of the hole remote from the mouth.
The cavity may be a frusto-conical cavity.
The cavity may narrow after broadening from the mouth.
The cavity may be formed from two opposed cavities.
Within the cavity a thermally insulating liner may be provided having an outer surface shaped to conform to lateral surfaces of the cavity and an inner surface shaped to receive a process chamber housing material to be treated under pressure.
The process chamber may comprise an electrical resistance heating element and conductors may be provided in the cavity to permit transmission of current to the electrical resistance heating element.
The thermally insulating liner may be made from one piece, or in two or more sections of different materials, and may comprise a first section disposed towards the mouth of the cavity and a second section within the cavity adjacent the process chamber, in which the first section has a lower compressive strength at a specified temperature and lower thermal conductivity at said specified temperature than the second section. The thermally insulating liner may further comprise a third section disposed between the second section and the lateral surfaces of the cavity, the third section having a lower compressive strength at said specified temperature and lower thermal conductivity at said specified temperature than the second section. The specified temperature may be, for example 1500° C., 1750° C. or 2000° C.
The press may comprise channels for the passage of coolant.
The cavity may comprise carbide bodies mounted in a metal surround.
This invention does not require the use of a thin gasket material and therefore there is not a limitation of the space between the process chamber and the carbide components allowing for much greater thermal insulation between the process chamber and the tungsten carbide components. This allows for processing at temperatures>1500° C., or example greater than 2000° C., or greater than 2500° C., such as from 1500° C. to 3000° C. Carbide components [e.g. tungsten carbide components] are attached to water cooled steel holders allowing for the continuous removal of any heat transferred to the carbide components. This allows for processing at high temperatures for indefinite amounts of time as certain processes may require high temperature operation for days or weeks of continuous operation.
Referring to
The upper plate 308 and the piston cylinder 301 are constrained by bolts 306. The process chamber assembly 304 is contained between the upper press assembly 305 and lower press assembly 303. The process chamber assembly 304 is water cooled by a water channel 216.
The piston 302 is contained within the piston cylinder 301 and is sealed by an o-ring 309. High pressure hydraulic oil may be pumped into the space 310 between the piston 302 and the piston cylinder 301 to cause the piston 302 to rise and apply pressure on the lower press assembly 303 which in turn causes the punch or anvil 214, shown in
The appropriate pressure depends on the particular process required. Synthetic diamond production typically requires pressures in the range of 6 GPa (60 kbar) while single crystal production of GaN typically requires pressures of 5 GPa (50 kbar) using the apparatus of the present invention.
U.S. Pat. No. 7,294,198 describes forming single crystal GaN in an HPHT apparatus. However the minimum pressure is stated to be 6 GPa (60 kbar). The lower pressure required using the apparatus of the present invention is advantageous, as lower pressures are much easier to maintain when the temperatures approach or exceed 2000° C., or even 3000° C.
An electric current is passed through a graphite cylinder 239, shown in
The electrically conductive disks 221 and 222 are typically although not necessarily made of iron (other electrically conductive materials, for example as exemplified for the electrically conductive rings 218 and 220, may be used) and spread the electrical current from the anvil 214 and upper insert 212 to electrically conductive rings 220 and 218 respectively. The electrically conductive rings 218 and 220 surround insulators 223 and 219 respectively. The insulators 223 and 219 have low thermal conductivity to prevent the high temperatures in the process chamber 207 from overheating the anvil 214 and the upper insert 212
The electric current passing through process chamber 207 is insulated by an insulating cone 211 from a compression ring 213 [which is typically made of carbide, e.g. tungsten carbide] having a conical inner surface. The insulating cone also provides thermal insulation between the process chamber 207 and the compression ring 213. The compression ring 213 may have a conical outer surface. The compression ring 213 is surrounded by a water cooled ring 203, which may be of steel. A water channel 216 is contained by an outer ring 204 which may be of steel. The outer ring 204 is heat shrunk around the water cooled ring 203 which is heat shrunk around the compression ring 213. The insulating cone 211 is pressed into the compression ring 213 by the carbide insert 212. The insert 212 is pulled against the insulating cone 211 by a circular pattern of 16 bolts 209 to pre-compress the insulating cone 211. This pre-compression of the insulating cone 211 reduces the amount of travel required by the punch or anvil 214 during the compression of the process chamber 207. In effect, a two stage compression is provided: first by clamping together the upper press assembly 305 and process chamber assembly 304; and then by insertion of the punch or anvil 214.
The carbide punch or anvil 214 is made as short as possible to maximize the strength of the punch or anvil: typically a 1:1 height to diameter ratio would be desired. However, this 1:1 ratio limits the allowable length of the punch or anvil 214 and therefore limits the allowable travel of the punch or anvil 214. It has been determined that height to diameter ratio can be as high as 3:1 and still hold up under pressure. Higher ratios are not precluded. The upper press assembly 305 and lower press assembly 303 are water cooled by water channels 215,217. The upper press assembly 305 has a carbide insert 212 [typically tungsten carbide] pressed into a water cooled ring 201. An outer ring 202 is heat shrunk around the water cooled ring 201 which contains the water channel 217. The lower press assembly 303 is constructed similarly to the upper press assembly 305 with outer ring 206 heat shrunk around water cooled ring 205 which contains water channel 215. The water cooled rings 201, 205 and outer rings 202, 206 may be made, for example, of steel.
Referring to
The outer cone 240 has a high thermal resistance to prevent excessive heat transfer to the compression ring 213. Typically, very dense high strength materials such as aluminium oxide also have a fairly high thermal conductivity, for example 30-38 W·m−1·K−1. If the entire insulating cone is made of aluminium oxide the high thermal conductivity will conduct too much heat from the process chamber 207 to the compression ring 213 resulting in over heating of compression ring 213 which may cause the compression ring 213 to fail under high pressure conditions.
The ideal insulating cone material should have high thermal resistance, a high melting point and low compressibility. The inner cone 241 and outer cone 240 can be formed as one piece with the same material, however two or more part construction has advantages. Constructing the insulating cone 211 of several different materials permits optimization of the entire cone. For example, the cone can successfully be constructed to withstand temperatures of 2300° C. or even up to 3000° C. or more, and pressures up to 5-6 GPa (50-60 kbar) by using SiC for the inner cone 241 and yttria stabilized zirconia or magnesium stabilised- or partially-stabilised-zirconia for the outer cone 240 and the lower section 242. Yttria stabilized zirconia has a very high compressive strength at temperatures below 2000° C. The SiC inner cone 241 protects the zirconia cone 240 from reaching 2000° C. SiC however has a high thermal conductivity of 70-120 W·m−1·K−1 and therefore for cannot be used alone at such elevated temperatures. ZrO2 has a low thermal conductivity of 2 W·m−1·K−1 and therefore prevents the heat transfer to the surrounding tungsten carbide compression ring 213.
Materials that enable a one piece cone to work at elevated temperatures include zirconia and stabilised zirconias, for example yttria stabilised zirconia, and magnesium stabilised- or partially-stabilised-zirconia, the last of which appears to perform better than yttria stabilised zirconia.
The conical arrangement of the present press will result in a slight temperature gradient within the process chamber, with the diamond seed at the cooler end and the graphite at the hotter end of the process chamber. Such an arrangement may promote conversion of the graphite to diamond.
Aluminium nitride powder was compressed and heated in a press as shown in
Large pieces (3-5 mm in diameter) of 5-6 nines purity aluminium nitride are preferred to reduce the risk of oxygen contamination through air or moisture oxidation; however in tests commercially available “high purity” aluminium nitride powder was used. This had 3-5% by weight oxygen impurity.
The powder was placed in a foil cup 272, shown in
Placement of Encapsulated AlN into the HPHT Machine:
The assemblies of cup 272 with aluminium nitride 274 and the second cup 273 with aluminium nitride 275 were placed mouth to mouth into a silicon carbide cylinder 233. A graphite disk 270 was placed over the cup 272 and a graphite disk 271 was placed under the cup 273. A disk of zirconia 232 was placed over the graphite disk 270. A disk of zirconia 237 was placed under the graphite disk 271. A zirconia disk 231 was placed over the zirconia disk 232 and a zirconia disk 238 was placed under the zirconia disk 237 closing the ends of the SiC cylinder 233. Zirconia discs 231, 232, 237, and 238 need not be of stabilized zirconia, and indeed materials other than zirconia may be used. These discs serve primarily to transfer pressure to the assemblies of cups 272 and 273 with associated aluminium nitride 274 and 275.
The assembly of cups 272 and 273, aluminium nitride 274 and 275, silicon carbide cylinder 233; and zirconia discs 231, 232, 237, and 238 was placed in a graphite tube 239 to form the process chamber 207. The process chamber 207 together with:—
The entire assembly shown in
The hydraulic piston 302 was raised causing the anvil 214 [which was a tungsten carbide anvil] to compress the process chamber 207 to approximately 460 kpsi [˜3.2 GPa]. The iron disks 221 and 222 expand due to the high pressure which seals the process chamber 207 from the atmosphere. The process chamber 207 was then heated to 2500-2800° C., causing the AlN 274 and 275 to melt. The pressure and temperature were held for approximately 30 minutes and the temperature slowly reduced at approximately 10° C. per minute.
When temperature was reduced to less than 600° C., the power was turned off and the pressure reduced to zero. The HPHT machine was then opened up. A tantalum capsule with crystallized AlN was removed from the HPHT machine.
It should be noted that the pressures indicated above were calculated from knowing the hydraulic pressure operating on the piston 302 and the mechanical advantage given by the difference in size between the piston and the substantially narrower anvil 214.
Temperatures were estimated by determining an initial calibration curve of power against temperature for the geometry used. This curve extended up to about 1200° C. By exposing various materials to the temperatures involved without application of significant pressure it was possible to extend the calibration curve to higher temperatures. For example, if alumina melted, the temperature in the process cell had to be in the region of the melting point of alumina [˜2000-2100° C.] and if zirconia melted the temperature in the process cell had to be in the region of the melting point of zirconia [˜2800° C.]. Once the calibration curve was obtained temperature could be controlled simply by varying applied power.
High purity AlN appears necessary to produce very large AlN crystals and, for the reasons stated above, large pieces (3-5 mm in diameter) of 5-6 nines purity aluminium nitride are preferred.
The control of pressure and temperature in the above examples was controlled manually; however the HPHT system in the following examples is now controlled by a computer resulting in improved pressure and temperature control.
The following example was done using commercially available AlN rod. One of the problems with commercially available AlN powder is the surface area of the powder. The surface area can absorb significant amounts of oxygen. When melting and crystallizing the AlN in the HPHT apparatus of
The next experiment was to use AlN material made by the PVD apparatus described above and further processed in the HPHT apparatus described above. As an example of such an integrated process, apparatus in accordance with
Sputter clean substrate;
The tantalum or niobium cups containing the AlN were then processed in the machine of
AlN from this process was analyzed for impurities. No impurities were found except oxygen, which had levels<5000 ppm as measured by Glow Discharge Mass Spectrometry, and <300 ppm by SIMS and XRD analysis where these figures represent detection limits for the techniques and suppliers used to make the measurements.
XRD showed the as-deposited AlN from the machine of
Material made by the processes described above may be used as seeds for the processes described above.
Material made by the processes described above may have one or more layers deposited thereon of different composition to said material.
Material made by the processes described above may be used as is, or further processed to form one or more components in a device comprising one or more: electronic components; opto-electronic components; electro-acoustic components; MEMS components; and/or spintronic components.
It should be noted that the above description is for the purpose of exemplification, and the present invention is not limited thereto.
The present invention can include any combination of these various features or embodiments above and/or below as set forth in sentences and/or paragraphs. Any combination of disclosed features herein is considered part of the present invention and no limitation is intended with respect to combinable features.
To the extent permissible under national law the entire contents of all references cited in this disclosure are incorporated herein in their entireties, by reference. Further, when an amount, concentration, or other value or parameter is given as either a range, preferred range, or a list of upper preferable values and lower preferable values, this is to be understood as specifically disclosing all ranges formed from any pair of any upper range limit or preferred value and any lower range limit or preferred value, regardless of whether ranges are separately disclosed. Where a range of numerical values is recited herein, unless otherwise stated, the range is intended to include the endpoints thereof, and all integers and fractions within the range. It is not intended that the scope of the invention be limited to the specific values recited when defining a range.
Other embodiments of the present invention will be apparent to those skilled in the art from consideration of the present specification and practice of the present invention disclosed herein. It is intended that the present description and examples be considered as exemplary only.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2012/069156 | 9/27/2012 | WO | 00 | 3/28/2014 |
| Number | Date | Country | |
|---|---|---|---|
| 61540558 | Sep 2011 | US |
