115)
`United States Patent
`5,376,580
`Kish et al.
`[45] Date of Patent:
`Dec. 27, 1994
`
`US005376580A,
`{11] Patent Number:
`
`[54] WAFER BONDING OF LIGHT EMITTING
`DIODE LAYERS
`
`[75]
`
`Inventors: Fred A, Kish; Frank M. Steranka,
`both of San Jose; Dennis C.
`DeFevere, Palo Alto; Virginia M.
`Robbins, Los Gatos; John Uebbing,
`Palo Alto, all of Calif.
`[73] Assignee: Hewlett-Packard Company, Palo
`Alto, Calif.
`
`[21] Appl. No.: 36,532
`[22] Filed:
`Mar. 19, 1993
`
`Tint, CLS os eecsssssssceeenesseresseeseees HO1L 21/20
`[S52]
`[52] US. Ch. weeeecsccssecssesasesecesesees 437/127; 437/129;
`437/130; 437/117
`[58] Field of Search............... 437/127, 129, 130, 905,
`437/974, 117, 229; 148/DIG. 135
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`4,771,016 9/1988 Bajoretal............... 148/DIG. 135
`4,773,645 10/1988 Kurata et al. on...
`eesssseseees 437/905
`4,846,931
`7/1989 Gnmitter et al.
`..
`- 156/633
`* 4,864,369 9/1989 Snyder et al. ou...
`eee 357/17
`
`9/1989 Steranka 0...eecseesseeeeenees 357/17
`4,864,371
`
`..
`4,883,561 11/1989 Gmitter et al.
`- 156/633
`4,902,356 2/1990 Noguchi et al.
`.
`e 437/127
`
`4,921,817 5/1990 Noguchi ........csessescoessseeeee 437/127
`4,971,925 11/1990 Alexanderetal.
`. 148/DIG. 135
`4,992,837 2/1991 Sakai et ab. oe ssesesecsees 357/17
`
`5,008,718 4/1991 Fletcher et al.
`oc.
`scsssseeee 357/17
`5,087,585 2/1992 Hayashi ............
`w 437/974
`
`5/1992 Sarma...........
`437/974
`5,110,748
`
`8/1992 Albergoetal. ......
`-» 437/229
`5,135,877
`5,153,889 10/1992 Sugawara et al... 372/45
`5,244,817 9/1993 Hawkinsetal........... 148/DIG.135
`
`OTHER PUBLICATIONS
`Dudley, J. J. , et al., “144° C. operation of 1.3 pm In-
`GaAspPvertical cavity lasers on GaAssubstrates”, Appi.
`Phys. Lett., 61 (26), Dec. 28, 1992, pp. 3095-3097.
`Ishiguro, Hisanori et al., “High efficient GaAJAslight-
`
`~emitting diodes of 660 nm with a double heterostruc-
`ture on a GaAlAssubstrates”, Appl. Phys. Lett., 43 (11),
`Dec. 1, 1983, pp. 1034-1036.
`Pollentier, I. et al., “Epitaxial Lift-off GaAs LEDsto Si
`for Fabrication of Opto-Electronic Integrated Cir-
`cuits”, Electronics Letters, vol. 36, No. 3, Feb. 1, 1990,
`pp. 193-194,
`Schnitzer, I. et al., “Ultrahigh spontaneous emission
`quantum efficiency, 99.7% internally and 72% exter-
`nally, from AlGaAs/GaAs/AlGaAs double heteros-
`tructures”, Appl. Phys. Lett., 63 (3), Jan. 11, 1993, pp.
`131-133.
`.
`Sugawara, H.et al., “High-efficiency InGaAIP/GaAs
`visible light-emitting diodes”, Appl. Phys. Lett., 58 (10),
`Mar. 11, 1991, pp. 1010-1012.
`
`Primary Examiner~Tom Thomas
`Assistant Examiner-—Kevin M. Picardat
`
`[57]
`
`ABSTRACT
`
`A method of forming a light emitting diode (LED)
`includes providing a temporary growth substrate that is
`selected for compatibility with fabricating LED layers
`having desired mechanical characteristics. For exam-
`ple,
`lattice matching is an important consideration.
`LEDlayers are then grown on the temporary growth
`substrate. High crystal quality is thereby achieved,
`whereafter the temporary growth substrate can be re-
`moved. A second substrate is bonded to the LED layers
`utilizing a wafer bonding technique. The second sub-
`strate is selected for optical properties, rather than me-
`chanical properties. Preferably, the second substrate is
`optically transparent and electrically conductive and
`. the wafer bonding techniqueis carried out to achieve a
`low resistance interface between the second substrate -
`and the LED layers. Wafer bonding can also be carried
`out to provide passivation or light-reflection or to de-
`fine current flow.
`
`29 Claims, 13 Drawing Sheets
`
`44
`
`38
`36
`
`34
`
`32
`
`42
`
`46
`
`46
`
`Cree Ex. 1010
`
`Page 1
`
`Cree Ex. 1010
`
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`
`

`

`U.S. Patent
`
`Dec, 27, 1994
`
`Sheet 1 of 13
`
`5,376,580
`
`
`
`
`
`AlGaAsin)
`
`
`
`
`AlGaAs(p) GaAs(p)
`
`
`
`12
`
`10
`
`14
`
`16
`
`FIG.
`
`7 (PRIOR ART)
`
` AlGaAs(p)
`
`
`AlGaAsin)
`
`AiGaAs(p)
`
`
`
`GaAsi(n)
`
`FIG. 2 (PRIOR ART)
`
`Cree Ex. 1010
`
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`

`

`U.S. Patent
`
`Dec. 27, 1994
`
`Sheet 2 of 13
`
`5,376,580
`
`AlGaAs(p)
`
`AiGaAs(p)
`
`
`ee
`
`AlGaAsin) 24
`
`38
`36
`
`34
`
`32
`
`30
`
`FIG. 3 (PRIOR ART)
`
`|
`
`FIG. 4
`
`Cree Ex. 1010
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`

`

`U.S. Patent
`
`Dec. 27, 1994
`
`Sheet 3 of 13
`
`5,376,580
`
`38
`36
`34
`32
`
`38
`36
`34
`
`32
`
`38
`36
`34
`
`32
`
`FIG. 5
`
`FIG. 6
`
`44
`
`40f
`
`40
`
`42
`
`40
`
`42
`
`“6
`
`FIG. 7
`
`46
`
`Cree Ex. 1010
`
`Page 4
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`Cree Ex. 1010
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`

`

`U.S. Patent
`
`~ Dec. 27, 1994
`
`Sheet 4 of 13
`
`5,376,580
`
`3—~ts—“‘C;!;*;™O™OC~C~™S
`of
`36 TNONr....
`
`oo
`32 po -
`
`FIG. 8
`
`38 -
`36
`
`34
`
`32
`
`50
`
`30
`
`FIG. 9
`
`Cree Ex. 1010
`
`Page 5
`
`Cree Ex. 1010
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`

`

`U.S. Patent
`
`Dec. 27, 1994
`
`Sheet 5 of 13
`
`5,376,580
`
`58
`52
`
`38
`36
`34
`
`32
`
`58
`52
`
`54
`
`40
`
`60
`
`62
`
`FIG. 10
`
`FIG.
`
`171
`
`Cree Ex. 1010
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`

`

`U.S. Patent
`
`Dec. 27, 1994
`
`Sheet 6 of 13
`
`5,376,580
`
`
`13
`
`FIG. 12
`
`126
`
`FIG.
`
`Cree Ex. 1010
`
`Page 7
`
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`

`

`U.S. Patent
`
`Dec. 27, 1994
`
`Sheet 7 of 13
`
`5,376,580
`
`
`146
`
`FIG. 14
`
`FIG. 15
`
`Cree Ex. 1010
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`_
`
`Page 8
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`

`

`Dec. 27, 1994
`
`Sheet 8 of 13
`
`U.S. Patent
`
`5,376,580
`
`176
`
`178nh
`
`176
`174
`
`180
`
`Cree Ex. 1010
`
`Page 9
`
`FIG. 17
`
`Cree Ex. 1010
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`

`

`U.S. Patent
`
`Dec. 27, 1994
`
`Sheet 9 of 13
`
`5,376,580
`
`
`
`Cree Ex. 1010
`
`Page 10
`
`Cree Ex. 1010
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`
`

`

`U.S. Patent
`
`Dec. 27, 1994
`
`Sheet 10 of 13
`
`5,376,580 .
`
`
`
`
`
`dWVY.MOTS.(9)
`
`64Old
`
`
`
`(ul)SWIL
`
`
`
`
`
`dWVY.LSVs.(0)
`
`
`
`SAIWdOYdSYNLVYSAdW3L
`
`
`
`QNIGNOEYASVM
`
`J3LV1S-GIN0s
`OBLOZL09
`
`TEMPERATURE (°C)
`
`Cree Ex. 1010
`
`Page 11
`
`Cree Ex. 1010
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`
`

`

`Dec. 27, 1994
`
`U.S. Patent
`
`OQ=
`
`Sheet 11 of 13
`
`5,376,580
`
` WN
`
`FIG. 20
`
`Cree Ex. 1010
`
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`Cree Ex. 1010
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`

`

`U.S. Patent
`
`|
`
`Dec. 27, 1994
`
`Sheet 12 of 13
`
`5,376,580
`
`FIG.21
`
`Cree Ex. 1010
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`Cree Ex. 1010
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`

`

`U.S. Patent
`
`Dec. 27, 1994
`
`Sheet 13 of 13
`
`5,376,580
`
`
`
`FIG.22
`
`Cree Ex. 1010
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`

`

`1
`
`5,376,580
`
`2
`and the absorbing substrate is removed. Alternatively,
`the thinner layers 26 and 28 may be grown before the
`thicker transparent “substrate” 24.
`The above-described techniques of fabricating LEDs
`having transparent substrates suffer from inherent dis-
`advantages. Firstly, epitaxially growing a “thick” opti-
`cally transparent, electrically conductive “substrate”
`maynotbepractical, or even possible, when employing
`some growth techniques for certain semiconductor
`materials. Secondly, even when possible, a “thick”epi-
`taxial layer requires a long growth time, limiting the
`manufacturing throughput of such LEDs. Thirdly, fol-
`lowing removal of the absorbing substrate, the resulting
`LEDlayer is relatively thin, e.g. approximately 3-6
`mils. The thin wafers are difficult to handle without
`breaking, rendering fabrication more difficult. More-
`over, thin wafers create difficulties during mounting the
`wafers in an LED package. Silver-loaded epoxyis typi-
`cally utilized for mounting and contacting the bottom of
`the device. The epoxy tends to flow over the edges of
`thin wafers, causing the short circuiting of the diode
`(LED). Also, thin wafers are not as mechanically robust
`as the devices of FIGS. 1 and 2, which are grown on
`“thick” substrates of at least 16- mils. Such “thin” LEDs
`may exhibit
`increased device-failure problems when
`mounted in epoxy lamps. Thus, there are contradictory
`thickness problems when this second technique is em-
`ployed, since the transparent layer may be “too thick”
`for practical crystal growth processes and “too thin”
`for device applications.
`Consequently, there may be a tradeoff associated
`with selection of an absorbing substrate or a transparent
`substrate. Depending upon the growth and fabrication
`techniques, an LED having an absorbing substrate may
`possess mechanical characteristics that are superior to a
`transparent substrate LED, but the absorbing substrate
`LEDis generally less efficient. Increased efficiency is
`possible using a transparent substrate; however,lattice
`mismatch may create difficulties when the epitaxial
`layers are grown on a transparent substrate having a
`different lattice constant. In addition, the contradictory
`thickness problems may be encountered whena “thick”
`transparent “substrate” is epitaxially grown.
`The effect of an absorbing layer or substrate can be
`minimized by growing a Bragg reflector between the
`standard LED epitaxial layers and the absorbing sub-
`strate. An increase in efficiency is achieved, since the
`Braggreflector will reflect light that is emitted or inter-
`nally reflected in the direction of the absorbing sub-
`strate. However, the improvementis limited compared
`to transparent substrate techniques, because the Bragg
`reflector only reflects light that is of near normal inci-
`dence. Light that differs from a normal incidence by a
`significant amount is not reflected and passes to the
`substrate, where it is absorbed. Moreover, LEDs hav-
`ing Bragg reflectors are more difficult to manufacture,
`since they require the repeated growth of many thin
`epitaxial layers, typically on the order of 100 angstroms
`in thickness.
`It is an object of the present invention to provide a
`method of forming an LED having the desirable me-
`chanical characteristics of a “thick” substrate ofat least
`8 mils and the desirable optical characteristics of a trans-
`parent-substrate LED.
`
`WAFER BONDING OF LIGHT EMITTING DIODE
`LAYERS
`
`TECHNICAL FIELD
`
`The present invention relates generally to light emit-
`ting diodes and more particularly to methodsof fabri-
`cating light emitting diodes.
`BACKGROUND ART
`
`10
`
`Light emitting diodes (LEDs) are employedin a wide
`variety of applications. For example, in optical data
`transmission, LEDs are used to launch data signals
`along a fiberoptic cable.
`Unlike lasers, LEDs do not generate well-focused
`beams of light. Rather, an LED radiates light in all
`directions. That is, the light emission is isotropic. The
`layers of many conventional LEDs are grown on an
`optically absorbing substrate having an energy gap less
`than the emission energy of the active region of the
`LED.Thesubstrate absorbs someofthe light generated
`within the active region, thereby reducing the effi-
`ciency of the device. An example of a prior art alumi-
`num gallium arsenide (AlGaAs) LED ofthe single
`heterojunction type is shown in FIG. 1. Anepitaxial
`layer 10 of p-doped AlGaAsandan epitaxial layer 12 of
`n-doped AlGaAs are grown on a surface of a p-doped
`gallium arsenide (GaAs) substrate 14. The conduction
`of current through the junction ofthe epitaxial layers 10
`and 12 will generate light. However, since the energy
`gap of the absorbing substrate 14 is less than the emis-
`sion energy,light that is emitted or internally reflected
`downwardly toward the substrate 14 will be absorbed.
`FIG.2 is a double heterojunction AlGaAs LED on
`an absorbing substrate 16. An epitaxial layer 18 of n-
`doped AlGaAs and two layers 20 and 22 of p-doped
`AlGaAs are grown on the absorbing substrate 16. The
`bandgaps of the epitaxial layers 18-22 are chosen to
`cause light to be generated in the active layer 20 and to
`travel through the epitaxial layers 18 and 22 without
`being absorbed. However, absorption of light does
`occur at the substrate 16.
`Improved performance can be achieved by employ-
`ing a transparent substrate that has an energy gap
`greater than the emission energy of the LED active
`region. The effect of the transparent substrate is to
`prevent the downwardly emitted or directed light from
`being absorbed. Rather, the light passes through the
`transparent substrate and is reflected from a bottom
`metal adhesive and reflecting cup. Thereflectedlightis
`then emitted from the top or the edges of the chip to
`substantially improve theefficiency of the LED.
`There are several techniques for fabricating LEDs
`having transparent substrates. A first technique is to
`epitaxially grow the p-n junction on a transparent sub-
`strate. However, a problem with this technique is that
`acceptable lattice matching may bedifficult to achieve,
`depending uponthelattice constant of the LED epitax-
`ial layers. A second technique is to grow the LED
`epitaxial layers on an absorbing substrate that is later
`removed. For example, in FIG. 3 the n-doped transpar-
`ent substrate 24 and the p-doped epitaxial layers 26 and
`28 may be epitaxially grown on an absorbing substrate,
`not shown. The transparent “substrate” 24 is fabricated
`by growing a thick, greater than 75 ym,optically trans-
`parent and electrically conductive epitaxial layer on the
`lattice-matched absorbing substrate. The other layers 26
`and 28 are then grown on the epitaxial “substrate” 24
`
`20
`
`25
`
`30
`
`35
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`40
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`45
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`50
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`3
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`5,376,580
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`SUMMARY OF THE INVENTION
`The above object has been met by a method that
`utilizes a temporary growth substrate optimized for the
`growth of LED layers, but then provides a perfor-
`mance-enhancing substrate without requiring the epi-
`taxial growth of this substrate. In a preferred embodi-
`ment, the performance-enhancing substrate is a trans-
`parent memberthat is joined to the LED layers using
`wafer bonding techniques. Because the transparent
`layer is not bonded to the LEDlayers until completion
`of epitaxial growth of the LEDlayers, lattice matching
`of the transparent substrate and epitaxial layers is not a
`concern.
`
`4
`the second substrate, rather than epitaxially growing
`the second substrate, permits attachment of a thick
`substrate, e.g. 8 mils or more. This second substrate may
`be transparent and act as a performance-enhancing
`layer for optical extraction and current spreading and-
`/or act only as a means for obtaining improved mechan-
`ical stability during the steps of removing the growth
`substrate and performing a second wafer bonding of a
`transparent substrate at the side of the LED structure
`from which the growth substrate was removed.If only
`mechanical stability is desired,
`this second substrate
`- may be subsequently removed after the second wafer
`bonding step is performed.
`While the clearest use of the wafer bonding technique
`The temporary growth substrate is made of a material
`is one in which an optically absorbing substrate is re-
`compatible with fabricating LED layers having desired
`moved and replaced with an optically transparent sub-
`mechanical characteristics. For example, in order to
`Strate, this is not critical. The temporary growth sub-
`achieve high crystal quality growth and to optimize
`strate may be a transparent substrate having a low elec-
`lattice matching, standard absorbing substrate materials
`trical conductivity that limits its current spreading abil-
`may be utilized. LED layers are then grownusing one
`ity. Such a substrate would ultimately limit the effi-
`or more of a variety of methods, including liquid phase
`ciency of the LED. Thus, removal of the temporary
`epitaxy, vapor phase epitaxy, metalorganic chemical
`transparent growth substrate for replacement by a
`vapor deposition and/or molecular beam epitaxy. The
`transparent substrate having a higherelectrical conduc-
`LEDlayers that form an LED structure may consist of
`tivity would improve the performance of the device.
`a light-emitting active layer, upper and lower confining
`Similarly, one absorbing layerhaving a low electrical
`layers, current spreading andlight extraction layers and
`conductivity may be replaced with an absorbing layer
`one or more buffer layers, but this is not critical.
`having a higherelectrical conductivity.
`Following the growth of the LED structure, the
`The above-described method forms a light emitting
`temporary absorbing growth substrate has completed
`semiconductor device having a wafer-bond layer. A
`its purpose of allowing the formation of high-quality
`“wafer bond layer” is defined herein as a layer or sub-
`epitaxial layers. The growth substrate is preferably re-
`strate that exhibits the properties that are characteristic
`moved, since the absorbing growth substrate has an
`of a layer that has undergone wafer bonding.It is be-
`lieved that one such characteristic is a different nature
`energy gap that is less than or equal to the emission
`of misfit dislocations formed at the wafer bonded inter-
`energy of the LED structure. Such a relationship be-
`tween the energy gap and the emission energy of the
`face, compared to an epitaxially grown mismatched
`device would significantly limit the efficiency of the
`heterointerface. An interface that has undergone wafer
`device. While the method of removing the temporary
`bonding has been observed to exhibit misfit dislocations
`growth substrate is not critical, alternatives include
`which primarily consist of “edge dislocations,” ie. dis-
`chemical etching, lapping/polishing, reactive ion etch-
`locations whose Burgers vectorlies in the plane of the
`ing, and ion milling. Removing the growth substrate
`wafer bondedinterface. These properties are in contrast
`may further include removal of a portion orall of the
`to an epitaxially grown mismatched interface, which
`layer that contacts the absorbing substrate.
`typically exhibits a much higher density of “threading
`A secondsubstrate is then wafer bonded to the LED
`dislocations,”i.e. dislocations which are not confined to
`structure. In a preferred embodiment the second wafer
`the plane of the mismatched interface and tend to prop-
`is electrically conductive and is optically transparent.
`agate perpendicular to the interface.
`As comparedto the absorbing substrate, the transparent
`In another embodiment of the present invention, lay-
`“ substrate is a performance-enhancing layer. Wafer
`ers are epitaxially grown onafirst growth substrate that
`bonding can occur at the uppermost or lowermostlayer
`is not necessarily removed at a later time. Many Al-
`of the LED structure. Conventionally, the LED device
`bearing ITI-V semiconductors are unstable in moist
`includes electrodes at opposed endsfor properly biasing
`ambients and will undergo degradation by hydrolysis.
`the p-n junction of the device, so that minimizing the
`Such degradation may cause reliability problems for
`resistivity at the interface of the transparent substrate
`LEDscontaining Al-bearing III-V epitaxial layers of
`and the grown layers is important. Utilizing compounds
`significant thickness. For example, an AlGaAs LED
`that include indium has been shownto aid in achieving
`such as the one shownin FIG. 3 will undergo signifi-
`desired ohmic characteristics. In addition to In-bearing
`cant degradation as a result of oxidation of the high
`compounds, other compounds with a high surface mo-
`Al-composition layer 28 during wet high-temperature
`bility, high diffusivity, and/or superior mass transport
`reliability testing. This degradation may potentially be
`properties (e.g., Hg-bearing, Cd-bearing and Zn-bear-
`reduced by employing wafer bonding. For example, the
`ing compounds) may provide advantages when used in
`majority of the thickness of the high Al-composition
`solid-state wafer bonding applications.
`layer 28 may be replaced bya thick optically transpar-
`One concern with employing the above-described
`ent, electrically conductive wafer-bond layer which
`method is that following removal of the temporary
`does not contain a high composition of Al. In like man-
`growth substrate, the remaining LED structure may be
`ner, a wafer-bond layer of GaP may besubstituted for a
`extremely thin, e.g.,
`less than 10 pm, and therefore
`major portion of the transparent substrate 24. Thatis, it
`fragile and difficult to handle. In a second embodiment,
`is possible to employ the wafer bonding technique to
`the temporary growth substrate is removed only after
`achieve passivation.
`attachment of the second substrate to the uppermost
`In another embodiment, an electrically conductive
`layer of the LED structure. Utilizing wafer bonding of
`mirror can be wafer bonded to the LED layers that
`
`5
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
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`40
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`45
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`50
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`55
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`5,376,580
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`5
`form the LEDstructure. Light emitted in the direction
`of the mirroris then reflected back to the LED struc-
`ture so as to improvetheefficiency ofthe device.In this
`embodiment, the mirror is typically supported by a
`substrate, which may be an absorbing substrate or a
`transparent substrate, since light does not reach the
`substrateitself.
`Wafer bonding can also be used to provide increased
`mechanical and/or thermal stability, regardless of any
`optical benefits. For example, a robust III-V semicon-
`ductor wafer or a SiC wafer can be bondedto a II-VI
`LEDstructure to addstability.
`In yet another embodiment, at least one of the wafer
`surfaces to be wafer bonded is patterned in a manner to
`selectively vary the electrical and/or optical properties
`of the wafer. For example, depressions may be formed
`at selected areas prior to wafer bonding to define de-
`sired current paths to the active region of the LED.
`Possible applications include, but are not limited to,
`reducing light directed at metallized electrodes for ap-
`plying voltages to LEDsand simplifying the fabrication
`of spot emitters. Patterning may also be used to vary
`optical properties along the wafer surface, so as to redi-
`rect light in a desired manner.
`°
`Asnoted above, the wafer-bond interface is prefera-
`bly one having a low electrical resistivity and good
`mechanical strength. It has been discovered that van
`der Waals’ forces are typically ineffective in obtaining
`the desired ohmic characteristics and structural integ-
`rity. It has also been discovered that a combination of
`pressure and high temperature processing morereliably
`achieves the desired ohmic and mechanical characteris-
`tics. In addition, the application of pressure at high
`temperatures allows the wafers to conform to each
`other, minimizing any problems which mayresult from
`unevenness ofthe wafer surfaces, especially for bonding
`relatively thick layers.
`An advantage of the present invention is that the
`performance of the resulting LED is enhanced. Both
`light extraction and current spreading can be improved.
`Another advantageis that a thick substrate of 8 mils or
`more can be formed in a cost efficient manner, since
`wafer bonding is not subjected to the limitations of
`epitaxially growing a substrate. The thick substrate
`provides improved handling and device mounting char-
`acteristics.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG.1 is a side view of a prior art, single heterojunc-
`tion LED device having an absorbing substrate.
`FIG.2 is a side view of a prior art, double heterojunc-
`tion LED device having an absorbing substrate.
`FIG.3 is a side view of a prior art, double heterojunc-
`tion LED device having a transparent substrate.
`FIG.4 is a side view of a double heterojunction LED
`device having a temporary growth substrate in accor-
`dance with the present invention.
`FIG.5 is a side view of an LED structure having the
`growth substrate of FIG. 4 removed.
`FIG.6 is a side view of the LEDstructure of FIG. 5
`having a permanent substrate attached using wafer
`bonding techniques.
`FIG.7 is a side view ofthe structure of FIG. 6 having
`electrodes on opposedsides.
`FIG.8 is a side view of an alternative LED structure
`fabricated by wafer bonding.
`FIG.9 is a side view of a third embodimentof attach-
`ing a wafer-bond substrate to the device of FIG.4.
`
`6
`FIG. 10 is a side view of another embodiment of
`attaching a wafer-bond substrate to the LED structure
`of FIG.5.
`FIG.11 is a side view of the LED structure of FIG.
`5 attached to a mirror by the use of wafer bonding
`techniques.
`FIG.12 is a side view of a stacked LED device.
`FIGS. 13-17 are side views of LED devices utilizing
`patterned layers that are wafer bonded.
`FIG.18 is an exploded view of a wafer bonding appa-
`ratus for carrying out the steps of the present invention.
`FIG.19 is a graph of temperature profiles for operat-
`ing the apparatus of FIG. 18.
`FIG.20 is a schematic view of an alternate apparatus
`for carrying out the steps of the present invention.
`FIGS.21 and 22 are different embodiments of graph-
`ite members for use with the apparatus of FIG.20.
`BEST MODE FOR CARRYING OUT THE
`INVENTION
`
`With reference to FIG.4, a first step in carrying out
`the invention is to select a substrate 30 onto which a
`plurality of LED layers will be sequentially grown. In a
`preferred embodiment, the substrate 30 is a temporary
`growth substrate which is removed subsequentto fabri-
`cation of the LED layers. In this embodimentthe elec-
`trical and optical properties of the substrate are irrele-
`vant to the operation of the LED to be fabricated, so
`that the substrate can be selected solely for properties
`which affect the growth of the LED layers. Fer exam-
`ple, lattice matching is typically an important consider-
`ation in the selection of the substrate. However, in some
`embodiments the substrate may remain, so that proper-
`ties other than growth-compatibility are important to
`these embodiments.
`An exemplary temporary growth substrate 30 is a
`GaAssubstrate within the range of 250 to 500 ym thick.
`Four LED layers 32, 34, 36 and 38 are then grown on
`the growth substrate 30. The layers 32-38 may be.
`grown using any of a variety of known methods,includ-
`ing liquid phase epitaxy, vapor phase epitaxy, metalor-
`ganic chemical vapor deposition and molecular beam
`epitaxy. The layers 32-38 form a double heterojunction
`LED,but the invention maybeutilized with any type of
`LED device.
`Thelayer 32 directly above the growth substrate 30 is
`an n-doped buffer layer. Grown abovethe buffer layer
`is a lower confining layer of n-doped AlGaInP. The
`lower confining layer 34 has an exemplary thickness of
`800 nanometers.
`An active layer 36 of AlGaInP is grown to an exem-
`plary thickness of 500 nanometers. An upper confining
`layer 38 of p-doped AlGaInP then completes the struc-
`ture of FIG. 4. The upper confining layer has an exem-
`plary thickness of 800 nanometers. Optionally, a win-
`dow layerthat is transparent and that has a higher elec-
`trical conductivity than the layers 34, 36 and 38 may be
`grown atop the upper confining layer 38 in order to
`promote current spreading, thereby enhancing the per-
`formance of the resulting structure. Such a window
`layer is described in U.S. Pat. No. 5,008,718 to Fletcher
`et al.
`Some degree of optical absorption and electrical re-
`sistivity can be tolerated in the grown layers 32-38,
`since these layers are sufficiently thin that less than
`optimal characteristics will not seriously compromise
`device performance. However, an optically absorptive
`temporary growth substrate 30 will clearly affect per-
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`7
`formance. Referring now to FIG. 5, the growth sub-
`strate has been removed, leaving the LED structure
`formed by the grown layers 32-38. Removal of the
`growth substrate can be accomplished in various ways,
`including chemical etching, lapping/polishing, reactive
`ion etching, ion milling, or any combination thereof. As
`will be described more fully below, the method ofre-
`moving the substrate is not critical so long as a clean,
`planar surface is presented following the removal. In
`addition to the growth substrate, the buffer layer 32
`may be wholly or partially removed and the lower
`confining layer may be partially removed.
`Following removal of the temporary growth sub-
`strate, a performance-enhancing substrate is bonded to
`either the lowermost layer 32 or the uppermost layer 38
`of the LED structure 40 shown in FIG. 5. The location
`of the wafer to be bonded depends upon the LEDstruc-
`ture 40 and the electrical and optical properties of the
`grown layers 32-38 and/or the substrate to be bonded.
`A wafer bonding technique is employed. Wafer bond-
`ing offers a number of advantages over other methods
`of providing a performance-enhancing substrate to an
`LED.
`FIG.6 illustrates an embodiment in which an electri-
`cally conductive, optically transparent substrate 42 has
`been wafer bonded to the buffer layer 32. The wafer
`bonding offers the advantage that a transparent sub-
`strate may be provided without requiring the growth of
`such a substrate. Preferably, the wafer bonded transpar-
`ent substrate 42 has a thickness exceeding 8 mils. Grow-
`ing a substrate having a comparable thickness would be
`difficult or impossible utilizing conventional techniques,
`and would require an extremely long time. Because
`only the relatively thin layers 32-38 of the LED struc-
`ture 40 need to be grown,epitaxial growth times can be
`drastically reduced,
`thereby maximizing throughput.
`Moreover, the wafer bonding process provides a thick
`device having enhanced mechanical properties, as com-
`pared to transparent substrates which are epitaxially
`grown. Because the resulting LED devices are easier to
`handle and less susceptible to breakage, fabrication is
`made easier and device yields are increased. Wafer
`bonding mayalso beutilized to displace the p-njunction
`from the bottom of the device, so as to reduce the possi-
`bility of short-circuiting the device when it is mounted
`in conductivesilver-loaded epoxy, as is conventional in
`the art.
`Referring now to FIG.7, the remainder ofthe fabri-
`cation process involves standard LED techniques. An
`electrode 44 is formed on the upper confining layer 38,
`as for example by evaporation. A typical material for
`forming the electrode is a gold-zinc alloy. A second
`electrode 46 is formed on the transparent substrate 42.
`Again, evaporation may be used,butthis is notcritical.
`A typical material is gold-germanium alloy.
`In some circumstances, it may be desirable, or even
`necessary, to modify the above-described process to
`accommodate wafer bonding. For example, in FIG. 8a
`second substrate 48 has been wafer bondedto the struc-
`ture of FIG. 4. That is, a second substrate is wafer
`bonded before removal of the temporary growth sub-
`strate 30. Preferably the second substrate 48 is a “thick”
`layer exceeding 6 mils. Wafer bonding prior to removal
`of the growth substrate 30 wouldsignificantly improve
`the mechanical stability of the device, since there would
`be no time in which the epitaxial layers 32-38 would be
`unsupported by a substrate. Optionally, a buffer layer
`may be epitaxially grown on the second substrate 48
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`prior to wafer bonding. Such an epitaxial buffer layer
`may also be utilized with a substrate that replaces the
`growth substrate 30 at the bottom ofthe buffer layer 32.
`In another embodiment, the device of FIG. 4 may be
`a conventional structure having the layers 32-38 grown
`on a transparent or absorbing substrate 30. The wafer
`bondlayer 48 of FIG. 8 would then be a thick, electri-
`cally conductive, optically transparent layer, such as
`the current-spreading window layer described above
`with reference to U.S. Pat No. 5,008,718 to Fletcher et
`al. Furthermore, after bonding the toplayer 48,it is also
`possible to removethe original growth substrate 30 and
`wafer bond another performance-enhancing substrate
`to the bottom of the remaining structure for reasons of
`improving optical extraction and/or current spreading.
`Moreover, the device of FIG. 4 may have a conven-
`tional transparent layer 30 that possesses a low electri-
`cal conductivity, limiting the current spreading ability
`of the device. In this circumstance, it would be desirable
`to wafer bond a transparent substrate having a higher
`electrical conductivity. The increase in electrical con-
`ductivity would improve the performance of the de-
`vice. The substitute transparent substrate should be
`wafer bonded to the exposed LED layers with low
`electrical conductivity. The substitute transparent sub-
`strate having the higherelectrical conductivity could be
`wafer bonded to the LED structure either before or
`after removal of the transparent layer with low electri-
`cal conductivity.
`Similarly,
`temporary growth absorbing substrates
`may be replaced with absorbing substrates having a
`higher electrical conductivity. While the use of wafer
`bonding to attach an absorbing layer is not the preferred
`embodiment, such wafer bonding would indeed im-
`prove the performance of the LED device.
`Referring now to FIG. 9, wafer bonding mayalso be
`employed in providing passivation to the structures of
`either FIG. 4 or FIG. 6. Many Al-bearing III-V semi- .
`conductors are unstable in moist ambients, since such
`semiconductors are susceptible to degradation by hy-
`drolysis. The degradation may cause reliability prob-
`lems in LEDs containing Al-bearing ITI-V epitaxial
`layers 30-38 ofsignificant thickness. For example, deg-
`radation mayresult from oxidation of the high Al-bear-
`ing confining layer 38 during wet, high-temperature
`use. The degradation can be retardedif the majority of
`the Al-bearing layer is replaced by a thick optically
`transparent, electrically conductive wafer bond layer
`that does not contain a high Al composition. For exam-
`ple, the wafer-bond layer 50 may be GaP.
`Returning to FIG.6, the desired electrical connec-
`tion between the transparent substrate 42 and the LED
`structure 40 can be ensured by utilizing a metalliza