Patent Application: US-81720806-A

Abstract:
an apparatus and process for fast epitaxial deposition of compound semiconductor layers includes a low - energy , high - density plasma generating apparatus for plasma enhanced vapor phase epitaxy . the process provides in one step , combining one or more metal vapors with gases of non - metallic elements in a deposition chamber . then highly activating the gases in the presence of a dense , low - energy plasma . concurrently reacting the metal vapor with the highly activated gases and depositing the reaction product on a heated substrate in communication with a support immersed in the plasma , to form a semiconductor layer on the substrate . the process is carbon - free and especially suited for epitaxial growth of nitride semiconductors at growth rates up to 10 nm / s and substrate temperatures below 1000 ° c . on large - area silicon substrates . the process requires neither carbon - containing gases nor gases releasing hydrogen , and in the absence of toxic carrier or reagent gases , is environment friendly .

Description:
the present invention is a system including an apparatus and process for the epitaxial growth of iii - v semiconductors , especially group iii - nitrides , such as gan , gaaln , and gainn . the apparatus provides a low - energy , high - density plasma for plasma enhanced vapor phase epitaxy of semiconductor layers on to a semiconductor support . the present system allows for the economical fabrication of heterostructures suitable for high - frequency power amplifiers , violet , blue and white leds ( lighting ), and blue and ultra - violet semiconductor lasers . referring now to fig1 , the apparatus 10 includes a vacuum deposition chamber 20 having a chamber interior 21 communicating with a vacuum pumping system ( not shown ), such as a turbomolecular pump , attached to exhaust line 24 . the deposition chamber 20 and the pumping system are chosen such as to being compatible with ultra - clean processing of semiconductors . for example a system allowing for ultra - high vacuum in the absence of process gases has been found to be adequate . inert and normally non - reactive gases , such as argon and nitrogen , and any additional gases suitable for processing , are supplied to the deposition chamber 20 by means of gas inlets 22 . nitrogen in the form of n 2 is normally a non - reactive gas . however , when exposed to the field of the present apparatus , the n 2 nitrogen is converted to its atomic form n and becomes highly activated and reactive . the deposition chamber 20 is equipped with a dielectric window 28 through which radio frequency waves are coupled into the chamber interior 21 by means of a spiral coil assembly 30 . the spiral coil assembly 30 communicates with an impedance matching network 32 and a radio frequency generator 34 . radio frequency waves emanating from the spiral coils excite a dense , low - energy plasma within the interior 21 of the chamber 20 . for example , the inductively coupled plasma source icp - p 200 from je plasmaconsult , gmbh in wuppertal , germany , has been shown to yield argon and nitrogen ion energies below 20 ev when operated in the pressure range between 10 − 4 and 10 − 2 mbar and powers up to 1 kw . a deposition assembly 50 is electrically insulated from deposition chamber 20 by means of insulators 26 . one or more substrate supports 54 are heated from the back by a heating means 52 , such as a resistive heater or by lamp heaters . the substrate support 54 is spaced several skin depths ( typically 5 - 20 ) away from the location of highest plasma density close to the dielectric window 28 . the skin depth is on the order of 1 cm for the typical operating pressures used according to the invention . the deposition assembly 50 can either be grounded or left electrically floating . alternatively , the assembly 50 can be connected to a dc bias power supply , or it can be coupled through an impedance matching network 56 to rf generator 58 , giving rise to a dc self - bias . these measures are taken in order to control the electrical potential of substrates 54 with respect to that of the plasma . in this way the electric field component perpendicular to the surface of substrates 54 can be controlled independently from the parameters controlling the plasma 36 . the energy of ions impinging on the substrates can thus be adjusted for optimum epitaxial growth conditions . in addition , the deposition chamber 20 is equipped with one or more metal vapor emitters 40 ( effusion cells in the embodiment illustrated ) from which metals , such as ga , in and al , can be vaporized and the vapors injected into the chamber interior 21 . for these metals , the temperature of standard effusion cells used in molecular beam epitaxy ( mbe ) can easily be adjusted such as to allow much higher evaporation rates than those customary in that technique . for example an increase of gallium cell temperature by 200 ° c . was found to be adequate for a 100 - fold increase of the gaas growth rate of 1 monolayer / sec typical in mbe . similar to mbe , fast - action shutters 42 are controllable to interrupted completely the fluxes from the vapor emitters 40 . during epitaxial deposition , the radio frequency power applied to the induction coils 30 and the gas pressures in the chamber 20 are chosen such that the heated substrates 54 are fully exposed to a low - energy plasma . typically , gas pressures in chamber 20 range between 10 − 4 mbar to 1 . 0 mbar , with pressures in the range of 10 − 2 to 10 − 1 mbar being the most typical . under such conditions , activated nitrogen and metal vapor from effusion cells 40 both move by diffusive transport in the plasma . metal atoms reacting with the nitrogen form an epitaxial nitride layer on the hot substrates 54 . referring now to fig2 , a detailed view of a growing film 55 on a substrate 54 exposed to a low - energy plasma 36 can be seen . the ion density in the plasma decreases exponentially from the dielectric window 28 to the substrate 54 . for example for the plasma source “ icp - p 200 ,” the ion density in a nitrogen plasma may still exceed 10 11 cm − 3 at a substrate located about 10 cm below dielectric window 28 , when a nitrogen pressure of 10 − 1 mbar at a gas flow of 10 sccm , and a rf - power of 1000 w are used . in order to keep the ion energy low , it may be advantageous to keep the total gas pressure fixed , for example around 10 − 1 mbar , by admitting a controlled flow of ar through gas inlets 22 to enter the vacuum chamber 20 along with nitrogen gas , when nitrogen partial pressures substantially below 10 − 1 mbar are used . as a result of the efficient activation of the reacting species in a dense plasma 36 , and intense bombardment of the substrate surface 54 by low - energy ions the substrate temperature can be significantly lowered with respect to the substrate temperatures of 1000 ° c . and more typical for mocvd . the problems of layer cracking due to different thermal expansion coefficients of typical substrates ( sapphire , silicon carbide and silicon ) are hence expected to be greatly reduced . referring now to fig3 , a detailed view of part of the vacuum chamber 20 is shown , where in order to confine the plasma 36 , and to increase its density and uniformity , the chamber is optionally equipped with coils or permanent magnets 70 . the magnetic field generated by these coils or permanent magnets helps in shaping the plasma . even weak fields of the order of 10 − 3 to 10 − 2 tesla are considered to be sufficient to have a beneficial effect . in a preferred embodiment of the invention no reactive gases are used for epitaxial nitride semiconductor growth at all . additional cells 40 a may contain those doping species which are preferably used in elemental form , such as mg , zn and similar metals acting as acceptor impurities . similarly , dopants acting as donors , such as silicon , may be provided by additional cells 40 a . these emitters 40 a ( effusion cells ) also are equipped with fast - acting shutters 42 permitting rapid and complete interruption of the dopant vapors . the preferred substrate support 54 choice is silicon in order to allow for scaling up to 300 mm wafers , and potentially beyond . however , the use of other substrates employed in state of the art techniques is equally possible in the new technique according to the invention . the combination of effusion cells for metal evaporation with a dense low - energy plasma suitable for epitaxial layer deposition has not been proposed before . we call the new process low - energy plasma enhanced vapor deposition ( lepevpe ). lepevpe is a process being operated under completely different conditions with respect to all other known processes , including lepecvd where a dc plasma discharge and reactive gas phase precursors are used . in one embodiment of the invention , the region of the vapor emitters 300 is differentially pumped ( 320 in fig6 ) in order to exclude thermal reactions with the hot metals inside and diffusive transport in the connecting tube to the deposition chamber . in a preferred embodiment of the invention more than one vapor emitter 40 & amp ; 40 a ( effusion cell ) is used per evaporated metal . each cell can be operated at a different temperature , thereby easily allowing rapid changes in growth rates or doping densities by switching from one cell to another . in another embodiment of the invention , additional gas lines 23 are used to insert doping gases into the deposition chamber for those doping elements which are preferably applied in gaseous form . the doping gases , such as silane for n - type doping , are preferably diluted in a non - reactive gas , such as argon . the dynamic range of doping can be increased by using more than one gas line per doping gas . in a preferred embodiment where vapor emitters 40 a of only the solid source type are used for doping , the process is operated hydrogen - free . this embodiment is especially desirable for p - doped gan layers since a hydrogen - free process does not need any dopant activation by thermal annealing . the process of the invention is carbon - free because it does not require any carbon - containing precursor gases . in the preferred embodiment of the invention illustrated in fig1 , the assembly of substrate supports 54 is facing up . this configuration , customarily used in semiconductor processing , facilitates wafer handling and design of the deposition assembly or substrate holder 50 . according to the invention , lepevpe is characterized by a high density low - energy plasma in direct contact with the surface of the substrate support 54 . the surface of the substrate support 54 is therefore under intense bombardment of low - energy ions , the energy of which may be adjusted by appropriate choice of the substrate bias . this is in marked contrast to plasma processing methods using remote plasma sources , which typically deliver radicals only , whereas ion densities at the substrate surface are negligibly low . heavy substrate bombardment by low - energy ions has been shown to be beneficial to epitaxial growth of device quality semiconductor layers at extremely high growth rates of more than 5 nm / s at substrate temperatures as low as 500 ° c . ( see , for example , von känel et al ., appl . phys . lett . 80 , 2922 ( 2002 ), the content of which is incorporated herein by reference ). according to the invention very high throughputs may therefore be expected by combining lepevpe with state of the art wafer handling tools ( not shown ). according to the present invention , the apparatus 10 may be used for growing epitaxial iii - v semiconductors , especially group iii - nitrides onto specially treated single - crystal substrates 54 . possible surface treatments of substrates 54 may involve state of the art chemical pre - cleans , in situ thermal cleans or plasma cleans , followed by in situ formation of epitaxial templates , such as oxides , carbides or low - temperature nitrides , suitable for subsequent epitaxial nitride semiconductor growth . referring now to fig4 , an apparatus 10 of the resent system is shown in which the substrate support 54 on which the growing materials are deposited is mounted on a table of the substrate holder 50 in the interior chamber 21 which is now facing down . this configuration is characterized by fewer problems with particulate contamination , at the cost of a more complex wafer handling system and design of the substrate holder 50 . as noted above , the deposition chamber 20 may be equipped with optional coils or permanent magnets which may help in shaping the plasma , and is similarly equipped with effusion cells 40 , etc . referring now to fig5 , another embodiment of the invention is shown , whereby the substrate supports 54 mounted on deposition assembly 50 inside the chamber 20 are again facing down . the deposition chamber 20 may be equipped with optional coils or permanent magnets which may help in shaping the plasma ( see fig3 ). in this embodiment , the elemental metal vapors are supplied to the plasma by means of water cooled sputter sources 60 , holding sputter targets 62 . it is advisable to arrange the sputter targets 60 in the form of concentric rings or ring segments around the dielectric window 28 of the icp source . the sputter targets are connected through an impedance match box 64 to an rf power supply 66 , whereby power supply 66 provides an alternate voltage at a frequency preferably substantially different to that used by generator 34 to power the icp coils 30 . this reduces undesirable interferences between the two kinds of power sources 34 and 66 . in another embodiment of the invention , the sputter sources 60 are powered by a dc power supply . it has been shown that for typical pressures - distance products on the order of 0 . 2 × 10 − 2 mbar m the thermalization of sputtered particles reaching the substrate is nearly complete , such that electronic - grade semiconductor material can be grown by using sputter sources ( see , for example , sutter et al ., appl . phys . lett . 67 , 3954 ( 1995 ), the content of which is incorporated herein by reference ). in order to allow cleaning of sputter sources 60 prior to epitaxial layer deposition , chamber 20 may be optionally equipped with a movable shutter assembly 82 allowing the shutter blade 80 to be positioned close to and below the substrates 54 and hence avoiding any sputtered particles to reach the substrate during pre - sputtering . in a preferred embodiment of the invention no reactive gases are used for epitaxial nitride semiconductor growth at all . additional sputter targets 60 a may contain those doping species which are preferably used in elemental form , such as mg , zn and similar metals acting as acceptor impurities . similarly , dopants acting as donors , such as silicon , may be provided by additional sputter targets 60 a . in another embodiment of the invention each sputter gun 62 may be equipped with optional shutters ( not shown ) in order to avoid cross - contamination between the individual targets 60 . during epitaxial deposition , the radio frequency power applied to the induction coils 30 and the gas pressures in the chamber 20 are chosen such that the heated substrates 54 are fully exposed to a low - energy plasma . typically , gas pressures in chamber 20 range between 10 − 3 mbar to 10 − 1 mbar , with pressures in the range of 10 − 2 to 10 − 1 mbar being the most typical . under such conditions , activated nitrogen and metal vapor from sputter guns 62 both move by diffusive transport in the plasma and the process proceeds as noted above . in another embodiment of the invention sputter guns 62 may be combined with effusion cells 40 , whereby both sources are preferably arranged symmetrically around the dielectric window 28 . the combination of effusion cells and sputter guns for evaporating reactants and dopants in elemental form with a dense low - energy plasma suitable for epitaxial layer deposition has not been proposed before . in a preferred embodiment of the invention more than a single sputter gun 62 and effusion cell 40 are used per evaporated metal . each source can be operated in such a way as to deliver a different flux of metal vapors , thereby easily allowing rapid changes in growth rates or doping densities by switching from one source to another . in still another embodiment , the effusion cells 40 and sputter guns 62 may be replaced or complemented by electron beam evaporators . electron beam evaporators are especially suitable for evaporating elements with low vapor pressures , where significant fluxes are difficult to achieve with effusion cells 40 . referring now to fig6 , another embodiment of the invention is shown , in which the apparatus 10 includes a broad area plasma source 100 with an assembly of thermionic cathodes 130 , an inert gas inlet 120 , and an integrated or separate anode 110 . preferably , the voltage difference between the cathodes 130 and the anode 110 is less than 30 v , to provide that ions striking the substrate have energy less than about 20 v . the plasma source 100 in which an arc plasma 140 can be ignited is attached to a deposition chamber 200 . the deposition chamber , equipped with a load - lock 220 , is pumped for example by a turbomolecular pump 210 communicating with chamber 200 by means of valve 205 , and contains a substrate heater assembly 230 . gas lines 240 for injecting an inert gas such as nitrogen , and additional gases , such as hydrogen , are connected to the deposition chamber . the plasma density may be changed rapidly by changing the confining magnetic field produced by coils 250 . in addition , this chamber is equipped with effusion cells 300 from which metals can be vaporized , such as ga , in and al . additional cells 300 may contain those doping species which are preferably used in elemental form , such as mg , zn and similar metals acting as acceptor impurities . the effusion cells are equipped with shutters 310 permitting complete interruption of the metal vapor . the heated assembly of substrates 400 is fully exposed to the low - energy plasma generated by the arc discharge in the plasma source and expanding into the deposition chamber through the permeable anode 110 . the arc discharge is sustained by thermionic cathodes 130 in the plasma chamber 200 , and can be operated in a wide pressure range in the deposition chamber from 10 − 4 mbar to at least 10 − 1 mbar , with pressures in the range of 10 − 2 mbar being the most typical . plasma activated nitrogen flowing through the deposition chamber reacts with the metal vapor , forming an epitaxial nitride film on the substrate 400 . effusion cells are normally used for evaporating metals in ultra - high vacuum for example in a molecular beam epitaxy system . here , they serve to introduce a metal vapor into a high - density low - energy plasma generated at typical pressures of about 10 − 2 mbar at which transport is diffusive . lepevpe is hence a process being operated under completely different conditions with respect to other processes . in one embodiment of the invention , the region of the effusion cells 300 is differentially pumped 320 in order to exclude thermal reactions with the hot metals inside and diffusive transport in the connecting tube to the deposition chamber . in a preferred embodiment of the invention more than a single effusion cell 300 is used per evaporated metal . each cell can be operated at a different temperature , thereby easily allowing rapid changes in growth rates or doping densities by switching from one cell to another . in addition , changes of the plasma density , brought about by changing the magnetic field produced by the coils 250 , can further enhance the dynamic range of growth rates . in another embodiment of the invention , additional gas lines 240 a are used to insert doping gases into the deposition chamber for those doping elements which are preferably applied in gaseous form . the doping gases , such as silane for n - type doping , are preferably diluted in a non - reactive gas , such as argon . the dynamic range of doping can be increased by using more than one gas line per doping gas . the process of the invention is carbon - free because it does not require any carbon - containing precursor gases . in a preferred embodiment , it is also operated hydrogen - free . this embodiment is especially desirable for p - doped gan layers since a hydrogen - free process does not need any dopant activation by thermal annealing . since lepevpe is a plasma - activated process it can be operated at lower substrate temperatures than competing techniques where tensile stress induced by different thermal expansion coefficients of epilayer and substrate often lead to undesirable crack formation during cooling from the growth temperature . annex a — the below documents are incorporated herein by reference thereto and relied upon . j . d . brown et al ., “ algan / gan hfets fabricated on 100 - mm gan on silicon ( 111 ) substrates ”, solid - 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2924 . p . sutter et al ., “ quantum transport in sputtered , epitaxial si / si 1 - x ge x heterostructures , applied physics letters , vol . 67 , no . 26 ( 25 dec . 1995 ), pp . 3954 - 3956 .