Patent Application: US-34076603-A

Abstract:
ohmic contact formation on p - type silicon carbide is disclosed . the formed contact includes an initial amorphous carbon film layer converted to graphitic sp 2 carbon during an elevated temperature annealing sequence . decreased annealing sequence temperature , reduced silicon carbide doping concentration and reduced specific resistivity in the formed ohmic contact are achieved with respect to a conventional p - type silicon carbide ohmic contact . addition of a boron carbide layer covering the p - type silicon carbide along with the sp 2 carbon is also disclosed . ohmic contact improvement with increased annealing temperature up to an optimum temperature near 1000 ° c . is included . addition of several metals including aluminum , the optimum metal identified , over the carbon layer is also included ; many other of the identified metals provide schottky rather than the desired ohmic contacts , however .

Description:
the present invention is believed fully disclosed by way of the series of example ohmic contact formations following . three p - type sic wafers from cree inc . of durham , north carolina , usa are used in examples 1 - 4 and other examples of the invention . the sic wafers are cut into 8 × 8 mm squares , and cleaned by the rca method , a method generally involving the steps of : a . immersion in nh 4 oh : h 2 o 2 : h 2 o ( 1 : 1 : 2 ) at 85 ° c . for 5 minutes ; b . immersion in hcl : h 2 o 2 : h 2 o ( 1 : 1 : 2 ) at 85 ° c . for 5 minutes ; c . immersion in hf : h 2 o ( 20 % solution ) for 5 minutes ; d . immersion in hno 3 ( 50 %) at 90 ° c . for 5 minutes ; f . immersion in ch 3 oh before metal thin film deposition . the three wafers used may be identified and characterized as follows : ( a ) sic substrate , p - type , si face , 8 ° off axis , and with the resistivity of 0 . 61 ωm ( b ) sic substrate , p - type , si face , 8 ° off axis , and with the resistivity of 1 . 12 ωm . ( c ) sic substrate , p - type , si face , 8 ° off axis , and with the doping concentration of 7 . 5 × 10 17 . the manner of this characterization , wherein two initial wafer resistivity values are recited for wafers “ a ” and “ b ” but a doping concentration is recited for wafer “ c ”, is believed typical of commercial practices observed with respect to p - type silicon carbide semiconductor material . in such semiconductor material , wherein aluminum is a commonly used dopant material , it is found that a doping concentration may exist in the 10 18 and 10 19 range of atoms per cubic centimeter in the sic wafers “ a ” and “ b ” respectively . tlm ( transfer length method ) structures from the wafers “ a ”, “ b ” and “ c ” may be photolithographically prepared . the size of the rectangular pads in these structures may be 50 × 100 μm , and the space between pads 25 , 50 , 75 , 100 , 125 , and 150 μm , respectively . the ti / al / c / sic structures can be prepared by a dc sputtering system ( dsm - 300a sputter , denton vacuum , inc .). in this structure the initial carbon film thickness is preferably 5 . 0 nm , unless otherwise specified , the al film thickness 150 nm , and the ti film thickness 50 nm . 500v may be applied for the carbon deposition , and 450v applied for the al and ti depositions . the resulting samples may be placed in a graphite furnace ( oxy - gon industries , inc ., epsom , n . h . ), and annealed in high purity ar ( o 2 & lt ; 1 ppm ) at various temperatures for 120 minutes , unless otherwise specified . current - voltage characterization of the achieved samples may be performed at room temperature using a micromanipulator probe station ( model 1800kgs ). a keithley model 220 programmable current source may be used for sample energization , and the drop voltage measured by a keithley model 195 multimeter . fig1 in the drawings and the first row of table 1 in appendix 2 herein show the achieved specific resistivities for the ti / al / c / 4h — sic structures of examples 1 - 4 , examples originating with the wafer “ a ” of 0 . 61 ωm original resistivity . the fig1 and table 1 data is obtained after annealing at various temperatures in ar for two hours . as shown particularly in table 1 where both example and wafer identities appear , after annealing at 700 ° c ., ohmic contact is formed with a specific resistivity of 1 . 47 × 10 − 2 ωcm 2 . the table - identified example numbers also appear in (* x ) form in the drawings of this document where x represents an example identification number . with increasing annealing temperatures , a specific resistivity of 4 . 85 × 10 − 3 ωcm 2 is achieved at 800 ° c ., and 1 . 61 × 10 − 6 ωcm 2 is achieved after 900 ° c . in examples 1 - 4 . the specific resistivity increases however to 1 . 26 × 10 − 4 ωcm 2 after annealing at 1000 ° c . ; this occurrence suggesting the presence of an optimum annealing temperature somewhere below 1000 ° c . the lower two rows of table 1 list the specific resistivities of ti / al / c / sic achieved on lightly doped sic samples taken from wafers “ b ” and “ c ” after annealing at various temperatures in ar for two hours . one sic sample with the resistivity of 1 . 12 ωm ( achieved by doping in the lower 10 18 cm − 3 range ) and another sic sample with doping concentration of 7 . 5 × 10 17 cm − 3 are involved and the achieved ohmic contacts identified as examples 5 through 9 in table 1 . as shown , ohmic contact is formed on sic with the resistivity of 1 . 12 ωm at 800 ° c ., and is formed at 900 ° c . on the sic of 7 . 5 × 10 17 cm − 3 doping . in examples 5 through 9 the best ohmic contacts are formed on the sic samples after annealing at 900 ° c . ; this example , example 6 , provides the specific resistivity of 3 . 12 × 10 − 4 ωcm 2 . with annealing at 1000 ° c ., the specific resistivity increases slightly to 7 . 29 × 10 − 4 ωcm 2 and 1 . 03 × 10 − 3 ωcm 2 , respect examples 8 and 9 . the values of specific resistivities in these examples on 4h — sic are similar to those of ti / al film on 6h — sic [ 8 ]. however , since the band gap of 6h — sic ( 3 . 0 ev ) is smaller than 4h — sic ( 3 . 3 ev ), the addition of a carbon film according to the present invention shows an improvement in ohmic contact formation . our results with ohmic contacts for n - type c / sic for and metal / carbon on n - type sic have shown the formation of nano - graphitic flakes plays a determinative role in ohmic contact formation , and metals as graphitization catalysts accelerate the process [ 16 - 20 ]. in p - type sic , the carbon graphitization process appears to play an important role in ohmic contact formation , which is the same as with n - type sic . both al and ti are good graphitization catalysts [ 24 , 25 ]. however , al reacts with sic to form al 4 c 3 [ 21 ], and al 4 c 3 is thermally stable up to the temperature of 1370 ° c . [ 22 ]; thus the decomposition products described with respect to n - type silicon carbide in the identified companion patent document are not present in the present p - type silicon carbide instance where temperatures of 1000 ° c . and below are described . ti reacts with sic to form tic and ti silicides [ 23 ]. the high thermal stability of the al and ti catalysts decreases the effectiveness of al and ti as graphitization catalysts . this explains the fact that the improvement in ohmic contacts with the ti / al / c / sic structures for for p - type sic is not as significant as for the ni / c structures on n - type sic [ 20 ]. however as in the discussion in the next paragraph , the addition of a carbon interface layer between ti / al film and sic improves ohmic contacts , and indicates that some degree of catalytic graphitization takes place in the present p - type ti / al / c / sic structures . several of the table 1 lower temperature - annealed samples provide non - ohmic contact characteristics and are therefore not identified with example numbers herein . a pair of ti / al / sic and ti / al / c / sic samples , resulting in examples 10 and 11 herein , also are examined after annealing at 900 ° c . with the same preparation and annealing conditions as specified above in connection with examples 1 through 4 . the specific resistivities achieved with these samples are 4 . 3 × 10 − 6 ωcm 2 for the ti / al / c / sic of example 10 and 2 . 38 × 10 − 4 ωcm 2 for the example 11 ti / al / sic sample . this data clearly shows that an additional carbon layer in the sic contact interface improves ohmic contacts on p - type silicon carbide . by way of comparison in the conventional p - type ti / al / sic contact structures , ohmic contact characteristics are obtained after annealing at 800 ° c . [ 15 ] and the optimal annealing temperature for ohmic contact formation is 1000 ° c . [ 6 - 8 ]. with an additional carbon interfacial layer as provided in the present invention , ohmic contacts are formed at 700 ° c . ( example 1 in table 1 ) and the optimal annealing temperature decreases to 900 ° c . in the ti / al / c / sic structure as shown by the results of example 3 in table 1 where the excellent ohmic contact ( 1 . 61 × 10 − 6 ωcm 2 ) is obtained after annealing at 900 ° c . therefore , we conclude that a carbon interfacial film and the ensuing post anneal graphitic layer on p - type sic provides an improvement with respect to ohmic contact formation . fig2 in the drawings herein shows the specific resistivities resulting from ti / al / c / sic structures having various initial carbon thickness after annealing at 800 ° c . in ar for two hours . the fig2 data points are identified with example numbers . the initial thickness of the fig2 carbon films are 1 . 0 , 2 . 0 , 5 . 0 , 10 . 0 , and 25 . 0 nm , respectively for examples 12 through 16 . these examples originate in the same wafer “ a ” described above in connection with examples 1 through 4 . as shown in fig2 the achieved specific resistivities range from 5 × 10 − 3 to 9 × 10 − 3 ωcm 2 . since the al based film has coarse morphology after thermal annealing accurate resistivity measurement is somewhat difficult , however we conclude from the results shown that the initial thickness of the interfacial carbon film has no significant effects on ohmic contact formation . fig3 in the drawings shows the specific resistivities of ti / al / c / sic structures obtained with various annealing times after annealing at 800 ° c . in ar for two hours . the annealing times represented are 30 , 60 , 120 , and 240 minutes and these times are identified as examples 17 , 18 , 19 and 20 in fig3 . as shown , the specific resistivities achieved with these different annealing times are all in the order of 10 − 3 ωcm 2 . the specific resistivity with the annealing time of 30 minutes is slightly larger than those with longer annealing times . different portions of the above identified wafers “ a ” and “ b ”, p - type sic wafers from cree inc . are used for these examples . these wafer portions can also be cut into 8 × 8 mm 2 size samples , and cleaned by the rca method as described above . the detailed information for each of the two wafers , “ a ” and “ b ”, is as shown above : ( a ) sic substrate , p - type , si face , 80 off axis , and with the resistivity of 0 . 61 ωm ( b ) sic substrate , p - type , si face , 8 ° off axis , and with the resistivity of 1 . 12 ωm . the tlm ( transfer length method ) structures for the present examples can be photolithographically prepared from these wafer portions . the size of the rectangular pads may be 50 × 100 μm , and the space between pads made 25 , 50 , 75 , 100 , 125 , and 150 μm , respectively . b 4 c / sic , m / b 4 c / sic , and m / c / b 4 c / sic structures may be prepared by using a dc sputtering system ( dsm - 300a sputter , denton vacuum , inc .) to add b 4 c film to the wafer “ a ” and “ b ” portions . in the b 4 c / sic structures of example 21 , the b 4 c film thickness is 50 . 0 nm , and the sic wafer “ b ” material is used . in the m / b 4 c / sic and m / c / b 4 c / sic structures , the b 4 c and carbon film thickness are 5 . 0 nm each , the metal film thickness is 200 nm , and the sic wafer “ b ” material is used . eight metals are added to the b 4 c film as are identified in table 2 ; these metals include al , co , cr , mo , ni , ti , ta , and w . a potential of 500 volts is applied for each carbon and b 4 c deposition , and 450 volts is applied for each metal deposition . the samples are placed in a graphite furnace ( oxy - gon industries , inc ., epsom , n . h . ), and annealed in high vacuum ( 10 − 5 - 10 − 6 torr ) at 900 and 1000 ° c . temperatures for 60 minutes , unless specified . current - voltage characterization of the samples are performed at room temperature using a micromanipulator probe station ( model 1800kgs ). a keithley 220 supply is used as current source , and the drop voltage measured by a keithley 195 instrument . b 4 c film on p - type sic shows ohmic contact characteristics after thermal annealing . for the wafer “ a ” p - type sic material with resistivity of 0 . 61 ωm , ohmic contact is formed after annealing at 1350 ° c . in ar for 30 minutes . the specific resistivity achieved for this example 21 material is greater than desired , at 2 . 41 × 10 − 2 ωcm 2 , with the thickness of b 4 c film at 50 nm . b 4 c is thermally stable in air up to 600 ° c . therefore use of b 4 c as ohmic contact material has advantages for applications of higher temperature devices . the electrical contact behavior of b 4 c / sic samples exhibit schottky contact characteristics in the annealing temperatures ranges from 1050 ° c . to 1250 ° c . ; this however converts to ohmic contact with annealing at 1350 ° c . this is similar to characteristics of c / sic samples [ 16 ]. the results recited are believed to discover b 4 c as a new material for ohmic contacts on p - type sic . in order to achieve lower resistivity ohmic contacts involving b 4 c , ohmic contact combinations of metal and b 4 c may be considered ; specifically the combinations m / b 4 c / sic and m / c / b 4 c / sic are of interest . eight metals , al , co , cr , mo , ni , ti , ta , and w , are considered here and are described in table 2 . the table 2 sic is p - type , si face , 8 ° off axis , with the resistivity of 1 . 12 ωm i . e ., material from wafer “ b ” described above . as shown in table 2 al / b 4 c / sic exhibits ohmic contacts with the specific resistivity of 8 . 13 × 10 − 4 and 1 . 89 × 10 − 4 ωcm 2 after annealing at 1000 ° c . and 1100 ° c . in vacuum for 60 minutes . ni / b 4 c / sic shows poor ohmic contact behavior . six other samples , co / b 4 c / sic , cr / b 4 c / sic , mo / b 4 c / sic , ti / b 4 c / sic , ta / b 4 c / sic , and w / b 4 c / sic achieve schottky contacts rather than ohmic contacts after annealing at 1000 ° c . and 1100 ° c . in vacuum for 60 minutes . as shown in example 25 of table 2 therefore al / c / b 4 c / sic achieves excellent ohmic contact with p - type silicon carbide after annealing at 1000 ° c . in vacuum for 60 minutes . this contact provides the specific resistivity of 1 . 56 × 10 − 5 ωcm 2 . the ni / c / b 4 c / sic of example 24 in table 2 however achieves poor ohmic contact behavior . six other samples , co / c / b 4 c / sic , cr / c / b 4 c / sic , mo / c / b 4 c / sic , ti / c / b 4 c / sic , ta / c / b 4 c / sic , and w / c / b 4 c / sic provide schottky contacts after annealing at 1000 ° c . in vacuum for 60 minutes and are thus not identified as examples herein . since the specific resistivity of ti / al / c / sic on the same wafer with 1000 ° c . annealing is 7 . 29 × 10 − 4 ωcm 2 , as shown in table 1 we conclude the addition of carbon in the al / c / b 4 c / sic combination provides the best structure for ohmic contact on p - type sic . the ti / al / c / sic structures disclosed herein therefore decrease the annealing temperature for p - type silicon carbide ohmic contact formation by about 100 - 200 ° c . in comparison with the conventional ohmic contact formation technique . the structures achieved also reduce the fabricated contact resistivity by one order of magnitude . the optimal annealing temperature for ti / al / c / sic structures is found to be 900 ° c ., while a higher temperature annealing at 1000 ° c . results in the best ohmic contact in the conventional ti / al / sic structures . excellent ohmic contact can therefore be formed on p - type sic with a one order lower doping concentration than in the conventional contact fabrication technique . a new material , b 4 c , is disclosed for use in ohmic contacts on p - type sic with metal / carbon / b 4 c / sic structures . this technology is believed usable to improve the performance of high power and high frequency silicon carbide devices , and provide more flexibility in device fabrications . the foregoing description of the preferred embodiment has been presented for purposes of illustration and description . it is not intended to be exhaustive or to limit the invention to the precise form disclosed . obvious modifications or variations are possible in light of the above teachings . the embodiments are chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly , legally and equitably entitled . r . j . trew , “ semiconductors and semimetals ”, edited by y . s . park ( academic press , san diego , calif ., 1998 ), vol . 52 , chap . 6 , p237 . g .- b . gao , j . sterner , and h . morkoc , ieee trans . electron dev ., 41 , 1092 ( 1994 ). l . m . porter , and r . f . davis , mater . sci . & amp ; 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