. United States Patent [191
`Edmond
`
`[II] Patent Number:
`[45] Date of Patent:
`
`4,966,862
`Oct. 30, 1990
`
`[54] METHOD OF PRODUCITON OF LIGHT
`EMI'ITING DIODES
`Inventor:
`John A. Edmond, Apex, N.C.
`[75]
`[73] Assignee: Cree Research, Inc., Durham, N.C.
`[21] Appl. No.: 400,279
`[22] Filed:
`Aug. 28, 1989
`Int. a.s .......................................... H01L 31/0312
`[51]
`[521 u.s. a ..................................... 437/100; 437/226;
`437/181; 437/905; 437/906
`[58] Field of Search ............... 437/100, 226, 227, 905,
`437/906
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`3,611,064 10/1971 Hall .
`3,805,376 4/1974 D'Asaro eta! ..
`3,871,016 3/1975 Debesis .
`3,889,286 10/1975 Debesis .
`3,909,929 10/1975 Debesis .
`3,964,157 6/1976 Kuhn eta! ..
`3,991,339 11/1976 Lockwood eta! ..
`4,122,486 10/1978 Ono eta! ..
`4,396,929 8/1983 Ohki eta! ..
`4,476,620 10/1984 Ohki et a! ..
`4,531,142 7/1985 Weyrich eta! ..
`4,604,161 9/1986 Araghi ................................ 437/226
`4,610,079 9/1986 Abe ..................................... 437/227
`4,814,296 3/1989 Jedlicka ............................... 437/226
`4,822,755 4/1989 Hawkins ............................. 437/227
`
`FOREIGN PATENT DOCUMENTS
`0019384 2/1979 Japan ................................... 437/906
`0216338 9/1986 Japan ................................... 437/226
`
`OTHER PUBLICATIONS
`T. Nakata, K. Koga, Y. Matsushita, Y. Ueda and T.
`Niina; Single Crystal Growth of 6H-SiC by a Vacuum
`Sublimation Method, and Blue LEDs; Semiconductor
`Res. Cent., Sanyo Elec. Co., Ltd., 1-18-13 Hashiridani,
`Hirakata, Osaka 573 J.
`G. Ziegler, P. Lanig, D. Theis and C. Weyrich; Single
`Crystal Growth of SiC Substrate Material for Blue
`Light Emitting Diodes; IEEE Trans. on Elec. Dev.,
`vol. Ed-30, No.4, Apr. 1983.
`V. Dmitriev, L. Kogan, Ya. Morozenko, I. Popov, V.
`
`Rodkin and V. Cheinokov; Blue-Emitting Displays
`from Silicon Carbide Grown by Containerless Liquid-(cid:173)
`Phase Epitaxy; Sov. Tech. Phys. Lett. 12(4), Apr. 1986.
`W. MUnch and W. Kiirzinger; Silicon Carbide Blue-E-
`(List continued on next page.)
`
`Primary Examiner-Brian E. Hearn
`Assistant Examiner-Linda J. Fleck
`Attorney, Agent, or Firm-Bell, Seltzer, Park & Gibson
`[57]
`ABSTRACT
`The invention is a method for preparing a plurality of
`light emitting diodes on a single substrate of a semicon(cid:173)
`ductor material. The method is used for structures
`where the substrate includes an epitaxial layer of the
`same semiconductor material that in turn comprises
`layers of p-type and n-type material that define a p-n
`junction therebetween. The epitaxial layer and the sub(cid:173)
`strate are etched in a predetermined pattern to define
`individual diode precursors, and deeply enough to form
`mesas in the epitaxial layer that delineate the p-n junc(cid:173)
`tions in each diode precursor from one another. The
`substrate is then grooved from the side of the epitaxial
`layer and between the mesas to a predetermined depth
`to defme side portions of diode precursors in the sub(cid:173)
`strate while retaining enough of the substrate beneath
`the grooves to maintain its mechanical stability. Ohmic
`contacts are added to the epitaxial layer and to the
`substrate and a layer of insulating material is formed on
`the diode precursor. The insulating layer covers the
`portions of the epitaxial layer that are not covered by
`the ohmic contact, any portions of the one surface of
`the substrate adjacent the mesas, and the side portions
`of the substrate. As a result, the junction and the side
`portions of the substrate of each diode are insulated
`from electrical contact other than through the ohmic
`contacts. When the diodes are separated they can be
`conventionally mounted with the junction side down in
`a conductive epoxy without concern that the epoxy will
`short circuit the resulting diode.
`
`16 aaims, 2 Drawing Sheets
`
`25
`
`Vizio EX1010 Page 0001
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`

`

`4,966,862
`
`Page 2
`
`OTHER PUBLICATIONS
`mitting Diodes Produced by Liquid-Phase Epitaxy;
`Solid-State Electronics vol. 21, pp. 1129-1132; 1978.
`L. Hoffmann, G. Ziegler, D. Theis, C. Weyrich; Silicon
`Carbide Blue Light Emitting Diodes with Improved
`External Quantum Efficiency; J. Appl. Phys. 53(10),
`Oct. 1982.
`Siemens; LDB5410; Blue T1 ! LED Lamp; Preliminary
`Data Sheet.
`
`B. Vishnevskaya, V. Dmitriev, I. Kovalenko, L. Kogan,
`Ya. Morozenko, V. Rodkin, A. Syrkin, B. Tsarenkov,
`V. Cheinokov; Silicon Carbide (6H) Diodes Emitting
`Blue Light; Sov. Phys. Semicond. 22(4), Apr. 1988.
`E. Violin and Yu. Tairov (1933); Light-Emitting De(cid:173)
`vices Based on Silicon Carbide.
`W. Munch; Silicon Carbide Technology for Blue-Emit(cid:173)
`ting Diodes; Journal of Electronic Materials, vol. 6, No.
`4, 1977.
`
`Vizio EX1010 Page 0002
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`

`

`U.S. Patent Oct. 30, 1990
`
`Sheet 1 of2
`
`4,966,862
`
`Fl G.1 .
`
`......
`.... .....
`.......
`....
`.......
`..... .....
`.........
`....
`.................... ................. ...
`... '-.. ............................................. ~'---
`-- ~
`.............
`............
`...........
`
`,
`
`............
`.....
`.........
`..............
`...... ................................................ ...
`.....
`...
`......
`........ .....
`......
`.....
`......
`....
`.....
`.....
`.......
`' .............................
`....
`..........
`............
`.......................................................................
`...........
`...... ......................
`...............
`.........
`
`FIG.2.
`
`FIG.3.
`
`FIG.4.
`
`10
`
`10
`
`10
`
`Vizio EX1010 Page 0003
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`

`

`U.S. Patent Oct. 30, 1990
`
`Sheet 2 of2
`
`4,966,862
`
`30--z._
`
`10
`
`Fl G.5.
`
`25
`
`FIG.6.
`
`10
`
`FIG.7
`
`FIG.8.
`
`Vizio EX1010 Page 0004
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`

`

`METHOD OF PRODUCI'ION OF LIGHT
`EMITTING DIODES
`
`FIELD OF THE INVENTION
`The present invention relates to a method of produc(cid:173)
`ing light emitting diodes and in particular relates to a
`method of producing a plurality of blue light emitting
`diodes from a single substrate or wafer of silicon car(cid:173)
`bide, and to the light emitting diodes which result. This
`application is related to co-pending application Ser. No.
`07/284,293, Filed Dec. 14, 1988 to Edmond for "Blue
`Light Emitting Diode Formed In Silicon Carbide,"
`which is incorporated entirely herein by reference.
`
`BACKGROUND OF THE INVENTION
`Light emitting diodes, commonly referred to as
`"LED's", are semiconductor devices which convert
`electrical energy into light. As is known to those famil(cid:173)
`iar with semiconducting materials, diodes formed from 20
`certain types of materials will produce energy in the
`form of light when current passes across the p-n junc(cid:173)
`tion in such a semiconducting diode. When current
`passes across a diode's junction, electronic events occur
`that are referred to as "recombinations," and in which 25
`electrons in the semiconductor combine with vacant
`energy level positions, referred to as "holes," in the
`semiconductor. These recombination events are typi(cid:173)
`cally accompanied by the movement of an electron
`from a higher energy level to a lower one in the semi- 30
`conductor material. The energy difference between the
`energy levels determines the amount of energy that is be
`given off. When the energy is given off as light (i.e. as
`a photon), the difference in energy levels results in a
`particular corresponding wavelength of light being 35
`emitted. Because the positions of various available en(cid:173)
`ergy levels are a fundamental characteristic of any par(cid:173)
`ticular element or compound, the color of light that can
`be produced by an LED is primarily determined by the
`semiconductor material in which the recombination is 40
`taking place. Additionally, the presence in the semicon(cid:173)
`ductor material of added dopant ions, which are re(cid:173)
`ferred to as either "donors" because they provide extra
`electrons, or as "acceptors" because they provide addi(cid:173)
`tional holes, results in the presence of additional energy 45
`levels in the semiconductor material between which
`electrons can move. This in turn provides different
`amounts of energy that are given off by the available
`transitions and provides other characteristic wave(cid:173)
`lengths of light energy given off by these additionally 50
`available transitions.
`Because of this relationship between energy and
`wavelength-which in the visible portion of the elec(cid:173)
`tromagnetic spectrum represents the color of the light(cid:173)
`-blue light can only be produced by a semiconductor 55
`material having a band gap larger than 2.6 electron
`volts (eV). The "band gap" refers to the basic energy
`transition in a semiconductor between a higher or "con(cid:173)
`duction" band energy level and a more regularly popu(cid:173)
`lated lower or "valence" energy band level. For exam- 60
`pie, materials such as gallium phosphide (GaP) or gal(cid:173)
`lium arsenide (GaAs) cannot produce blue light because
`the band gaps are on the order of about 2.26 e V or less.
`Instead, a blue light emitting solid state diode must be
`formed from a semiconductor with a relatively large 65
`band gap such as gallium nitride (GaN), zinc sulfide
`(ZnS), zinc selenide (ZnSe) and alpha silicon carbide
`(also characterized as "hexagonal" or "6H silicon car-
`
`1
`
`4,966,862
`
`15
`
`2
`bide," among other designations. Accordingly, anum(cid:173)
`ber of investigators have attempted to produce blue
`light emitting diodes using alpha silicon carbide.
`Nevertheless, silicon carbide has not presently
`5 reached the full commercial position in the manufacture
`of electronic devices, including light emitting diodes,
`that would be expected on the basis of its otherwise
`excellent semiconductor properties and its potential for
`producing blue LED's. For example, in addition to its
`10 wide band gap, silicon carbide has a high thermal con(cid:173)
`ductivity, a high saturated electron drift velocity, and a
`high breakdown electric field. All of these are desirable
`properties in semiconductor devices including LED's.
`The failure of silicon carbide LED's to reach commer-
`cial success appears to be the result of the difficulties
`encountered in working with silicon carbide. In particu(cid:173)
`lar, high process temperatures are required, good start(cid:173)
`ing materials are typically difficult to obtain, particular
`doping techniques have heretofore been difficult to
`accomplish, and perhaps most importantly, silicon car(cid:173)
`bide crystallizes in over 150 polytypes, many of which
`are separated by very small thermodynamic differences.
`Accordingly, the goal of controlling the growth of
`single crystals or monocrystalline thin ftlms of silicon
`carbide which are of sufficient quality to make elec(cid:173)
`tronic devices such as diodes practical, useful, and com(cid:173)
`mercially viable, has eluded researchers in spite of years
`of diligent effort, much of which is reflected in both the
`patent and nonpatent literature.
`Recently, however, a number of developments have
`been accomplished which offer the ability to grow large
`single crystals of device quality silicon carbide, to grow
`thin ftlms of device quality silicon carbide, to success(cid:173)
`fully etch silicon carbide, and to introduce dopants into
`silicon carbide, all steps that are typically required in
`the manufacture of LED's and other electronic devices.
`These developments are the subject of co-pending pa(cid:173)
`tent applications that have been assigned or exclusively
`licensed to the common assignee of the present inven(cid:173)
`tion and which are incorporated entirely herein by ref-
`erence. In addition to the application mentioned earlier,
`these include Davis et al, "Growth of Beta-SiC Thins
`Films
`and Semiconductor Devices Fabricated
`Thereon," Ser. No. 113,921, Filed Oct. 26, 1987; Davis
`et a1, "Homoepitaxial Growth of Alpha-SiC Thin Films
`and Semiconductor Devices Fabricated Thereon," Ser.
`No. 113,573, Filed Oct. 26, 1987; Davis et al, "Sublima-
`tion of Silicon Carbide to Produce Large, Device Qual(cid:173)
`ity Single Crystals of Silicon Carbide," Ser. No.
`113,565, Filed Oct. 26, 1987; Palmour, "Dry Etching of
`Silicon Carbide, Ser. No. 116,467, ftled Nov. 3, 1989;
`and Edmond et al, "Implantation and Electrical Activa-
`tion of Dopants Into Monocrystalline Silicon Carbide,"
`Ser. No. 113,561, Filed Oct. 26, 1987.
`As set forth in some detail in the Edmond '293 appli-
`cation, a number of different doping techniques and
`basic device structures are used to produce light of
`approximately 424-428 nanometers (nm), light of ap(cid:173)
`proximately 455-460 nm, and light of approximately
`465-470 nm in diodes formed from silicon carbide. Al(cid:173)
`though describing the visible colors of these wave(cid:173)
`lengths is somewhat of a generalization, the 424-428 nm
`light has a characteristic violet color, the 455-460 nm
`transition gives a more medium blue color, and the
`465-470 nm transition gives a characteristic light blue
`color.
`
`Vizio EX1010 Page 0005
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`

`

`4,966,862
`
`4
`3
`by avoiding using the substrate portion as a conductor
`As further set forth in the Edmond '293 application,
`one usual goal in producing LED's is to obtain as much
`in the diode. U.S. Pat. No. 4,531,142 discusses such a
`mechanical technique. This is an extremely difficult
`emitted light as possible. This is addressed by a number
`manufacturing technique, however, as reflected in the
`of techniques familiar to those in the art, some of which
`are: injecting as much current as possible across the p-n 5 low availability and high cost of such diodes. Another
`junction; having the greatest possible dopant population
`solution is to use a relatively large ohmic contact to the
`in the emitting layer; obtaining the greatest possible
`p+ layer so as to increase the current across the june-
`tion. The practical effect, however, is to block light
`efficiency in producing recombination events; and using
`a physical structure, including the optical characteris-
`from being emitted from the p + layer by the presence
`tics of the semiconductor material itself that enhances 10 of the ohmic contact.
`In contrast the co-pending and incorporated Edmond
`the visible light obtained from the diode. With regard to
`'293 application teaches a number of solutions to these
`this last characteristic, a transparent semiconductor
`material will often be the most desireable.
`problems and in particular discusses the successful use
`Using these considerations, one of the more desirable
`of chemical vapor deposition as set forth in the similarly
`and efficient transitions in silicon carbide is that be- 15 copending and incorporated Davis '573 application in
`order to successfully produce such diodes.
`tween an impurity band of nitrogen (donor) below the
`conduction band and an impurity band of aluminum
`Once such diodes are shown to be practical and effi-
`(acceptor) above the valence band. This transition is
`cient, however, interest and need arises for having them
`especially favorable when combined with a physical
`manufactured on a commercial scale. For example, in
`structure that encourages most of the current passing 20 one of the most effective LED's described by the Ed-
`across the junction to be p-type current; i.e. the flow of mond '293 application, the diode consists of an n-type
`substrate, an n-compensated epitaxial layer and a p +-
`holes across the junction and into the n-type material.
`As is known to those familiar with silicon carbide, the
`epitaxial layer. Under a forward bias, hole current in-
`donor band of nitrogen is approximately 0.075 eV
`jected from the p+ to then-compensated layer is the
`below the conduction band of silicon carbide, while the 25 predominating current in this diode. As discussed ear-
`lier and in the Edmond '293 application, the generally
`acceptor band of aluminum is approximately 0.22 eV
`above the valence band. The resulting transition is on
`higher resistivity of the top p+ layer makes it more
`the order of about 2.62 e V and emits a photon having a
`'difficult to get an appropriate amount of current spread-
`wavelength between about 465 nm and 470 nm and with
`ing, which is exhibited as a corresponding lack of uni-
`30 formity in the light generated in that layer.
`a characteristic blue color.
`Furthermore, in order to produce this transition, one
`Because the blue light generated by diodes formed of
`of the portions of the diode must be doped with both
`alpha silicon carbide is commercially desirable, how-
`donor and acceptor dopants, but with one dopant pre-
`ever, there exists the additional need for developing a
`dominating over the other to give a distinct p or n elec-
`method and structure for commercially producing and
`trical characteristic to the material This technique is 35 packaging such diodes and which will have advantages
`for other similar diodes as well. As is known to those
`referred to as "compensation," and the resulting portion
`of semiconductor material is referred to as being "com-
`familiar in the industry, in order to be useful such a
`pensated." For example, in order to use hole current to
`diode has to be mounted and it is most useful if such a
`produce blue light in a silicon carbide LED-another
`diode can be conventionally mounted using techniques
`favorable technique in particular applications-the por- 40 such a conductive epoxy and reflective cups. Similarly,
`tion of the diode which is n-type must be doped with
`the nature and position of the ohmic contacts and the
`both donor (often nitrogen) and acceptor (often alumi-
`crystal structure itself should also be designed for com-
`num) dopants, with the nitrogen predominating, to give
`mercial manufacture and the techniques used should
`an overall n-type characteristic even with the acceptor
`avoid damaging the junctions during the manufacturing
`atoms present.
`45 process. Finally, such a technique should be appropriate
`for multiple or mass production. To date, appropriate
`Certain problems arise, however, in attempting to
`form LED's that have these characteristics. For exam-
`commercial applications of this type are rarely seen in
`ple, where a p-type substrate is desirably or necessarily
`the pertinent art.
`used (depending upon the manufactUring technique
`used or the device that may be desired) it will have a 50
`rather high resistivity. This results from the well-known
`facts that the mobility of holes is one-sixth that of elec(cid:173)
`trons, and that typically less than two percent of accep(cid:173)
`tor atoms are ionized (i.e. able to act as charge carriers)
`at room temperature. These characteristics result in a 55
`higher resistance in forward bias for p-substrate diodes,
`which is a less desirable trait for a diode.
`One attempted manner of addressing the resistivity
`problem is to increase the hole concentration in the
`p-type substrate. The addition of the extra dopant re- 60
`quired to increase the hole concentration, however,
`tends to make the crystal opaque and reduces the emit(cid:173)
`ted light that can be observed. Conversely, by keeping
`the dopant concentration lower, the crystal will be
`more transparent, but at the cost of an undesirably high 65
`resistivity.
`The problems associated with high resistivity sub(cid:173)
`strates can also be addressed mechanically, for example
`
`SUMMARY OF THE INVENTION
`Accordingly, it is an object of the present inventimi
`to provide a method for preparing a plurality of light
`emitting diodes on a single substrate of a semiconductor
`material in which the resulting diodes can be separated
`and mechanically flxed to transmit from their substrate
`side rather than the junction side using otherwise con(cid:173)
`ventional mounting techniques. The method is appro(cid:173)
`priately used for structures where the substrate includes
`an epitaxial layer of the same semiconductor material on
`one surface thereof, and in which the epitaxial layer
`comprises a layer of p-type material and a layer of n(cid:173)
`type material that defme a p-n junction therebetween.
`The method comprises etching the epitaxial layer and
`the substrate in a predetermined pattern to deflne indi(cid:173)
`vidual diode precursors and wherein the etch is deep
`enough to form mesas in the epitaxial layer that deline(cid:173)
`ate the p-n junctions in each diode precursor from one
`another. The method further comprises grooving !he
`
`Vizio EX1010 Page 0006
`
`

`

`4,966,862
`
`5
`substrate from the side of the epitaxial layer and be(cid:173)
`tween the mesas of the diode precursors to a predeter(cid:173)
`mined depth into the substrate to defme side portions of
`a device precursors in the substrate while retaining
`enough of the substrate beneath the grooves to maintain 5
`its mechanical stability. An ohmic contact is added to
`the epitaxial layer and a layer of insulating material is
`formed on the diode precursor. The insulating layer
`covers the portions of the epitaxial layer that are not
`covered by the ohmic contact, any portions of the one 10
`surface of the substrate adjacent the mesas, and the side
`portions of the substrate. When a desired ohmic contact
`is added to the substrate, the result is a diode structure
`wherein the surface, the junction and the side portions
`of the substrate of each diode are insulated from electri- 15
`cal contact other than through the ohmic contacts.
`When the diodes are separated they can be convention(cid:173)
`ally mounted with the junction side down in a conduc(cid:173)
`tive epoxy without concern that the epoxy will short
`circuit the resulting diode.
`The foregoing and other objects, advantages and
`features of the invention, and the manner in which the
`same are accomplished, will become more readily ap(cid:173)
`parent upon consideration of the following detailed
`description of the invention taken in conjunction with 25
`the accompanying drawings, which illustrate preferred
`and exemplary embodiments, and wherein:
`
`20
`
`6
`particular, the Davis method of chemical vapor deposi(cid:173)
`tion of alpha silicon carbide on alpha silicon carbide
`(application Ser. No. 113,573) is the preferred method
`of adding the epitaxial layer to the silicon carbide sub(cid:173)
`strate 10. In the Davis method, the step of forming an
`epitaxial layer of alpha silicon carbide 11 on one surface
`of the substrate 10 comprises homoepitaxially deposit(cid:173)
`ing a film of an alpha silicon carbide polytype 11 on a
`prepared surface of the alpha silicon carbide substrate
`10 wherein the planar surface of the substrate 10 is
`inclined more than one degree towards one of the
`< 1120> directions. The particulars of this method are
`described in detail in the Davis '573 application which,
`as stated earlier, is incorporated entirely herein by refer(cid:173)
`ence and which will therefore will not otherwise be
`described in detail.
`FIG. 2 illustrates the results of the next step in the
`method of the invention, that of etching the epitaxial
`layer 11 and the substrate 10 in a predetermined pattern
`to defme individual diode precursors. In a preferred
`embodiment, the etch is deep enough to form mesas
`broadly designated at 15 in the epitaxial layer 11 that
`delineate the p-n junctions 14 'in each diode precursor
`from one another. As illustrated in FIGS. 2-5, the mesa
`portions 15 partly defme the diode precursors so that
`FIGS. 2, 3 and 4 illustrate two such diode precursors. In
`a preferred embodiment the mesa structure 15 on the
`substrate 10 is defmed by the border between the sub-
`strate 10 and the epitaxial layer 11.
`FIG. 3 illustrates that the next step is that of grooving
`the substrate 10 from the side of the epitaxial layer 11
`and between the mesas 15 of the diode precursors to a
`predetermined depth into the substrate 10. The resulting
`35 grooves are broadly designated at 16 and in tum defme
`side portions 17 of the device precursors in the substrate
`10. Enough of the substrate 10 is retained beneath the
`grooves 16 to maintain the mechanical stability of the
`substrate. As will be described later herein, the depth to
`which the substrate 10 is grooved also relates to con(cid:173)
`ventional mounting techniques which enhance the
`value of the method of the present invention and the
`diodes that result.
`FIGS. 4 and 5 illustrate that the next steps in the
`method are the additional of an ohmic contact 20 to the
`epitaxial layer 11 and the formation of a layer of insulat(cid:173)
`ing material21 to the device precursor structure. It will
`be understood that depending upon the particular tech(cid:173)
`niques selected, either the ohmic contact 20 or the insu-
`lating material 21 can be added fu:st followed by the
`addition of the other. FIG. 4 illustrates a preferred
`order of steps in which the formation of the layer of
`insulating material 21 takes place first and shows that
`when the respective steps are completed the layer 21
`covers the portions of the epitaxial layer 11 that are not
`covered by the ohmic contact (FIG. 5), any portions of
`the one surface of the substrate 10 adjacent the mesas 15
`that are not covered by the epitaxial layer 11, and por(cid:173)
`tions of the side portions 17 of the substrate 10. As a
`result, the layer 21 insulates the one surface, the junc(cid:173)
`tion and the side portions of the substrate of each diode
`precursor from electrical contact other than through
`the ohmic contact 21.
`In the preferred embodiment of the invention, the
`grooves are formed using a diamond blade dicing saw
`and the mesas are etched using the technique described
`by Palmour in co-pending and incorporated application
`Ser. No. 116,467.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIGS. 1-5 illustrate a number of the steps of the 30
`method of forming the diode of the present invention;
`FIG. 6 illustrates the diode of the present invention in
`a conventional mounting;
`FIG. 7 is a plan view taken along lines 7-7 of FIG.
`6;and
`FIG. 8 is a plan view taken along lines 8-8 of FIG.
`
`6.
`
`40
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`As illustrated in FIGS. 1-5, the invention is a method
`for preparing a plurality of light emitting diodes on a
`single substrate of a semiconductor material, which in
`preferred embodiments is alpha silicon carbide. FIG. 1
`shows a silicon carbide substrate 10 upon which an 45
`epitaxial layer broadly designated at 11 has been
`formed. Preferably, the expitaxial layer 11 comprises
`individual epitaxial layers 12 and 13 which have oppo(cid:173)
`site conductivity types (p or n) from one another and
`thereby form a p-n junction therebetween which is 50
`schematically illustrated at 14. The substrate 10 itself
`also has a given conductivity type, either p or n for
`reasons which will become apparent throughout the
`following description. Although the invention broadly
`comprises forming the epitaxial layer 11 on the substrate 55
`10, it will be understood that the substrate 10 with the
`epitaxial layer 11 already thereon can also be considered
`the starting material for the method of the invention. In
`the preferred embodiment, the epitaxial layer 10 com(cid:173)
`prises a wafer of silicon carbide of the type that is con- 60
`ventionally used in the manufacture of multiple semi(cid:173)
`conductor devices.
`As discussed in the background portion of the specifi(cid:173)
`cation, the ability to successfully produce blue light
`emitting diodes in silicon carbide has resulted from 65
`particular foundational work described in co-pending
`applications which are either assigned or exclusively
`licensed to the assignee of the present invention. In
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`4,966,862
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`7
`In the preferred embodiment, the insulating layer 21
`is a thermally grown native oxide, and is also referred to
`as a "passivation" layer for the diode. As FIG. 5 illus(cid:173)
`trates, the step of adding the ohmic contact 21 to the
`exposed epitaxial layer 11 comprises removing the
`oxide layer 21 to expose the epitaxial layer 11 to the
`ohmic contact, and in a most preferred embodiment
`comprises selectively etching the oxide layer 21 FIG. 5
`also illustrates that another ohmic contact 24 is added to
`the substrate 10 before individual devices are separated
`from one another.
`FIG. 5 also illustrates that the method can further
`comprise the step of separating the diode precursors
`into individual diodes. Preferably, this step comprises
`cutting the substrate 10 from the side opposite the epi(cid:173)
`taxial layer 11 and at positions shown as the dotted lines
`22 that correspond to the grooves 16. A diamond blade
`dicing saw is also appropriately used for this step as
`well. In this manner, cutting the diode precursors into
`individual diodes proceeds in a manner that avoids me- 20
`chanica! stress upon either the epitaxial layer or the
`junction. It is understood by those familiar with such
`devices and techniques that the mechanical stress of
`cutting a wafer of device precursors into individual 25
`devices (also referred to both singularly and plurally as
`"die") can often have an deleterious effect on the crys-
`tal structure of the substrate and the epitaxial layers as
`well as the integrity of the junctions. The particular
`technique of the present invention thus advantageously 30
`provides a method of forming a plurality of diode pre(cid:173)
`cursors on a single wafer while still permitting efficient
`and careful manufacture of individual diodes from the
`wafer. FIGS. 3, 4 and 5 also show that the grooves in
`~he wafer are limited to a depth that preserves sufficient 35
`substrate material10 beneath the grooves 16 to maintain
`the mechanical integrity of the wafer for further pro(cid:173)
`cessing and handling as an integral wafer.
`In preferred embodiments of the invention, the ohmic
`contact 20 comprises aluminum or an aluminum alloy to 40
`which an overlay contact of a noble metal such as plati(cid:173)
`num, palladium, gold or silver is most preferably added
`to prevent oxidation of the ohmic contact. Similarly, a
`wire-bondable overlay contact is preferably added to
`ohmic contact 24 and is most preferably formed of 4S
`aluminum, gold, or silver.
`Following the mounting of individual diodes in the
`method to be described hereafter, the resulting overall
`structure is illustrated in FIG. 6. The light emitting
`diode is broadly designated at 23 and includes the sub- so
`strate 10, the epitaxial layer 11, illustrated as individual
`epitaxial layers 12 and 13, the ohmic contacts 20 and 24,
`the insulating layer 21 and the junction 14. FIG. 6 illus(cid:173)
`trates that in the preferred embodiment, the substrate 10
`is substantially transparent which permits the diode to
`emit light through the substrate as indicated by the
`atrows L. In the preferred embodiment, and for reasons
`thoroughly discussed the co-pending Edmond '293 ap(cid:173)
`plication, the substrate 10 comprises n-type alpha silicon
`carbide, the ftrst epitaxial layer 13 comprises n-type 60
`alpha silicon carbide and the second epitaxial layer 12
`comprises p-type alpha silicon carbide. Most preferably,
`the epitaxial layer 12 is a p-type layer of alpha silicon
`carbide and epitaxial layer 13 is a compensated n-type
`layer of alpha silicon carbide. As understood by those 65
`familiar with silicon carbide and its polytypes, alpha
`silicon carbide can be selected from the group consist(cid:173)
`ing of the 6H, 4H, and 15R polytypes.
`
`8
`As further illustrated in FIG. 6, in addition to being
`substantially transparent, the substrate 10 has a dimen(cid:173)
`sion small enough between the epitaxial layer and its
`bottom surface to permit the visible light L to be emit-
`s ted from the diode device 23 through the substrate 10
`even when the epitaxial layer 11 is substantially blocked
`from emitting light therefrom as is the case with the
`device illustrated in FIG. 6.
`. FIG. 6 also illustrates that the device 23 includes an
`10 ohmic contact 24 to the substrate 10 for completing a
`conductive path from the ohmic contact 20 through the
`epitaxial layers 12 and 13, the junction 14, and the sub(cid:173)
`strate 10 that permits the desired current flow.
`FIG. 6 also illustrates another advantage of the inven-
`15 tion, namely the ability to center both ohmic contacts
`20 and 24 relative to the substrate and the epitaxial
`layer. As stated earlier and as known to those familiar
`with the manufacture of such devices, there are a num-
`ber of theoretical or laboratory type devices which use
`rather contorted geometries to deal with the problems
`of making contact to substrates of higher or lower resis-
`tivity, but which have not found wide commercial ac(cid:173)
`ceptance. By enabling the contacts to be centered with
`respect to the entire device 23, the invention grea