(12) INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT)
`
`,
`(19) World Intellectual Property Organization a | I
`International Bureau
`
`p
`
`2) AITOMTTAAA
`
` (10) International Publication Number
`
`(43) International Publication Date
`30 July 2009 (30.07.2009)
`
`(51) International Patent Classification:
`HOLL 31/042 (2006.01)
`
`(21) International Application Number:
`PCT/US2009/03 1874
`
`(22) International Filing Date: 23 January 2009 (23.01.2009)
`
`(25) Filing Language:
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`(26) Publication Language:
`
`English
`
`English
`
`(30) Priority Data:
`61/023,328
`
`24 January 2008 (24.01.2008)
`
`US
`
`(71) Applicant (for ali designated States except US): AP-
`PLIED MATERIALS, INC.
`[US/US]; 3050 Bowers
`Avenue, Santa Clara, CA 95054 (US).
`
`(72) Inventor: BORDEN,Peter; 118 Seville Way, San Mateo,
`CA 94402 (US).
`
`WO 2009/094570 A2
`
`(81) Designated States (unless otherwise indicated, for every
`kind of national protection available): AE, AG, AL, AM,
`AO, AT, AU, AZ, BA, BB, BG, BH, BR, BW, BY, BZ, CA,
`CH, CN, CO, CR, CU, CZ, DE, DK, DM,DO,DZ, EC, EE,
`EG,ES, FI, GB, GD, GE, GH, GM, GT, HN, HR, HU,ID,
`IL, IN,IS, JP, KE, KG, KM, KN, KP, KR, KZ, LA, LC, LK,
`LR, LS, LT, LU, LY, MA, MD, ME, MG, MK, MN, Mw,
`Mx, MY, MZ, NA, NG,NI, NO, NZ, OM,PG,PH,PL, PT,
`RO, RS, RU, SC, SD, SE, SG, SK, SL, SM, ST, SV, SY. TJ,
`TM,TN, TR, TY, TZ, UA, UG, US, UZ, VC, VN, ZA, ZM,
`ZW.
`
`(84) Designated States (unless otherwise indicated, for every
`kind of regional protection available): ARIPO (BW, GH,
`GM, KE, LS, MW, MZ, NA, SD, SL, SZ, TZ. UG, ZM,
`ZW), Eurasian (AM, AZ, BY, KG, KZ, MD, RU, TJ, TM),
`European (AT, BE, BG, CH, CY, CZ, DE, DK, EE, ES, FI,
`FR, GB, GR, HR, HU,IE, IS, IT, LT, LU, LV, MC, MK,
`MT, NL, NO,PL,PT, RO, SE, SI, SK, TR), OAPI (BE BJ,
`CE, CG, CI, CM, GA, GN, GQ, GW, ML, MR, NE, SN,
`TD, TG).
`
`Published:
`
`(74) Agent: JAKOPIN, David, A.; Pillsbury Winthrop Shaw
`Pittman LLP, P.o.box 10500, Mclean, VA 22102 (US).
`
`without international search report and to be republished
`upon receipt of that report
`
`(54) Title: PLATING THROUGH TUNNEL DIELECTRICS FOR SOLAR CELL CONTACT FORMATION
`
`Tunnel dielectric 204 Plated metal 202
`
`Dielectric 206
`
`n-type layer 208
`
`p-type substrate 210
`FIG,2
`
`(57) Abstract: In general, the present invention relates to forming electrical contacts in a semiconductor device, including contact
`regions in solar cells. According to certain aspects, the invention provides methods and apparatuses for forming plated contacts in
`the presenceof a thin tunnel oxide. Preferably, the tunnel oxide dielectric layer is thin enough to sustain a tunnel current. Plating over
`the tunnel dielectric is then performed. The benefits of the invention include that no annealing is required to form the metal-silicide
`contact. Moreover, there is no requirement for special metals for n- or p-type contacts. Another advantageis that shallow contacts
`according to the invention avoid punching througha shallow junction, thereby enabling the use of shallower emitters with improved
`blue response. Still further, there is no need to control the amountof suicide metal plated in order to prevent driving the suicide alloy
`through the junction.
`
`
`
`WO2009/094570A2IMMINENTATICTMANTAMTROAC i
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`

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`WO 2009/094570
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`PCT/US2009/031874
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`PLATING THROUGH TUNNEL DIELECTRICS FOR SOLAR CELL CONTACT
`
`FORMATION
`
`CROSS REFERENCE TO RELATED APPLICATIONS
`
`(0001)
`
`The present application claimspriority to U.S. Prov. Appln. No. 61/023,328 filed
`
`January 24, 2008, the contents of which are incorporated by reference herein in their entirety.
`
`FIELD OF THE INVENTION
`
`{0002}
`
`The present invention relates to forming electrical contacts in a semiconductor
`
`device, and more particularly to methods and apparatuses for plating through tunnel oxides to
`
`metallize contact regionsin solar cells.
`
`BACKGROUND
`
`(0003]
`
`Plating is a known methodto selectively metallize contact regions, including
`
`contact regions for solar cells. FIG. 1 showsa prior art plated contact. As shown in FIG.1, ina
`
`conventional process, a dielectric layer 106 such as a nitride or oxide, is laid down onthe silicon
`
`surface. The silicon has a p-n junction, for example a shallow n-type region 108 over a low
`
`doped p-type substrate 110. Contact holes are openedin the dielectric 106 by etching, for
`
`example. A metal 102 such as nickelis then selectively plated in these contact holes. For
`
`example, a seed material is deposited by electroless plating and then the metalis plated on the
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`seed material. More particularly, the seed selectively deposits only where the semiconductoris
`
`exposed via the contact holes, and the plating only happens on a conductive surface, which is the
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`seed layer, but not on the dielectric.
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`[0004]
`
`Many problemsexist with the prior art approaches and techniques. For example,
`
`the nickel mustbe relatively thick in manycasesto act as a diffusion barrier should a subsequent
`
`layer of copper be plated over the nickel, for example. Accordingly, the wafer must then be
`
`carefully alloyed to form a nickel-silicide contact. However, care must be taken, lest the contact
`
`alloy too deep and short out the p-n junction.
`
`{0005}
`
`Accordingly, there remains a need in the art for improved plated contacts and
`
`contact regions, including for use with solarcells.
`
`SUMMARY
`
`[0006]
`
`In general, the present invention relates to forming electrical contacts in a
`
`semiconductor device, including contact regions in solar cells. According to certain aspects, the
`
`invention provides methods and apparatuses for forming plated contacts in the presence of a thin
`
`tunnel oxide. Preferably, the tunnel oxide dielectric layer is thin enough to sustain a tunnel
`
`current. Plating over the tunnel dielectric is then performed. The benefits of the invention
`
`include that no annealing is required to form the metal-silicide contact. Moreover, there is no
`
`requirement for special metals for n- or p-type contacts. Another advantageis that shallow
`
`contacts accordingto the invention avoid punching through a shallow junction, thereby enabling
`
`the use of shallower emitters with improved blue response. Still further, there is no need to
`
`control the amountofsilicide metal plated in order to prevent driving thesilicide alloy through
`
`the junction.
`
`[0007]
`
`In furtherance ofthese and other aspects, a method of forming a contact in a solar
`
`cell having a p-n junction according to embodiments of the invention includes creating one or
`
`more contact regions overthe p-n junction; forming a tunnel dielectric in the one or more contact
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`WO 2009/094570
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`regions, wherein the forming step includes forming the tunnel dielectric thin enough to sustain a
`
`tunnel current therethrough; and plating a metal over the tunnel dielectric material to form the
`
`contact.
`
`[0008]
`
`In additional furtherance of these and other aspects,a solar cell according to
`
`embodiments of the invention comprises a p-n junction; one or more contact regions formed over
`
`the p-n junction; a tunnel dielectric formedin the one or more contact regions, wherein the tunnel
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`dielectric is thin enough to sustain a tunnel current therethrough; and a metal plated over the
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`tunnel dielectric material to form a contact to the p-n junction.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`[9009]
`
`These and other aspects and features of the present invention will become
`
`apparent to those ordinarily skilled in the art upon review of the following description ofspecific
`
`embodimentsof the invention in conjunction with the accompanyingfigures, wherein:
`
`FIG. 1 showsa prior art plated contact;
`
`FIG. 2 shows an improved contact according to embodiments of the invention;
`
`]0010]
`
`[0011]
`
`and
`
`[0012]
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`FIG. 3 shows a process flow according to embodiments ofthe invention.
`
`DETAILED DESCRIPTION
`
`[0013}
`
`The present invention will now be described in detail with reference to the
`
`drawings, which are providedas illustrative examples of the invention so as to enable those
`
`skilled in the art to practice the invention. Notably, the figures and examples below are not meant
`
`to limit the scope of the present invention to a single embodiment, but other embodiments are
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`possible by wayof interchange of someorall of the described or illustrated elements. Moreover,
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`3
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`wherecertain elements of the present invention can bepartially or fully implemented using
`
`known components, only those portions of such known components that are necessary for an
`
`understanding of the present invention will be described, and detailed descriptions of other
`
`portions of such known components will be omitted so as not to obscure the invention.
`
`In the
`
`present specification, an embodiment showing a singular componentshould not be considered
`
`limiting; rather, the invention is intended to encompass other embodiments includingaplurality
`
`of the same component, and vice-versa, unless explicitly stated otherwise herein. Moreover,
`
`applicants do not intend for any term in the specification or claims to be ascribed an uncommon
`
`or special meaning unless explicitly set forth as such. Further, the present invention
`
`encompasses present and future known equivalents to the known componentsreferred to herein
`
`by wayofillustration.
`
`[0014]
`
`In general, the present invention relates to forming plated contacts in the presence
`
`of a thin tunnel oxide. According to certain aspects, the present inventors recognize that a thin
`
`oxide underneath a contact can improve contact properties and eliminate the need for alloying the
`
`contact.
`
`[0015]
`
`Accordingly, in the present invention, a thin tunnel dielectric is formed before
`
`plating. FIG. 2 shows an improved contact according to the invention. As shown in FIG. 2,
`
`similar to a conventional process,a dielectric layer 206 such as a nitride or oxide, is laid down on
`
`a substrate surface. The substrate has a p-n junction, for example a shallow n-type region 208
`
`over a p-type substrate 210. Contact holes are openedin the dielectric 206. In contrast to the
`
`prior art, a tunnel dielectric 204 is provided in the contact region, and then a plated contact 202 is
`
`formed onthe tunnel dielectric 204.
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`[0016]
`
`In embodiments, substrate 210 is comprisedofsilicon, and is low-doped with p-
`
`type impurities. Many other substrate materials can be used and this and many other methods for
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`obtaining a desired polarity concentration and type are possible, as will be appreciated by those
`
`skilled in the art. Shallow n-type region 208 is preferred because shallow emitters provide
`
`improved blue response. In such a case, the n-type region 208 may be approximately 0.3-0.5
`
`micronsthick at the surface of the substrate 210. Moreover, as will be described in more detail
`
`below, one advantage of forming the tunnel oxide according to embodimentsofthe inventionis
`
`that an alloyingstep is not necessary, whichalleviates the potential problem ofthe plated contact
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`punching throughthe shallow p-n junction. However, shallow emitters are not necessary for the
`
`invention.
`
`[0017]
`
`It should be noted that the term contact hole should be construed broadly so as to
`
`relate to many types of openings through dielectric layer 206 and manytypes ofsolar cell
`
`contacts. For example, the holes can provide for point contacts having an area of only a few
`
`square microns or millimeters (e.g. having a diameter from about 2 jum to up to 100-200 pm), or
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`they can providefor line contacts that span many centimeters or more, and having widths about 2
`
`to 100 um. Thoseskilled in the art of solar cell contacts will appreciate how the teachings of the
`
`invention can be applied to these and other various types of contacts and openings.
`
`{0018}
`
`An example process flow according to embodimentsofthe invention is described
`
`more particularly in connection with FIG.3.
`
`[0019]
`
`In step $302, a substrate with a p-n junction is prepared or obtained. As set forth
`
`above, according to aspects of the invention,a silicon substrate with a shallow emitter is used.
`
`Details of its fabrication are not necessary for an understanding of the present invention. Next, in
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`step $304,a dielectric layer is formed on the substrate surface. For example,a nitride such as a
`
`silicon nitride with an index of refraction of about 2.1 and thickness of about 76 nm or a
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`stoichiometric oxide such as SiO. with thickness about 100 nm is laid down by CVD deposition
`
`for nitride or oxide, or thermal oxidation for oxide. A stack could also be used, with a 5 nm SiO,
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`formed with rapid thermal oxidation and a 70 nm SiNover the oxide (SiNx refers to a material
`
`that may not have the standard Si;N4 stochiometry ofsilicon nitride).
`
`[0020]
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`Next, in step $306, contact holes are openedin the dielectric layer. This opening
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`step may be donebyetching, laser ablation, or any other methodthat provides a suitable opening
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`the dielectric to expose the underlying substrate such as silicon. More particularly, as set forth
`
`above, the type of opening andits dimensions can depend onthe particular solar cell contact
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`application such as point contacts and other types of contacts. Those skilled in the art will be
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`able to understand how to form suitable openings for such various types of contacts using various
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`conventional and proprietary methods. Methods mightincludelaser ablation or patterning and
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`etching, or local deposition of an etchant.
`
`[0021]
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`Next, in step $308, a thin tunneldielectric is then formed, using either a wet
`
`process (step S308A)or a dry process (step $308B). A dry process can include oxidation ina
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`furnace tube or in a rapid thermal annealer. One wet process, called Chemox (developedat
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`IMEC)formsa thin oxide in an ozonated hydrogen peroxide bath. A wet process (step $308A)
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`such as Chemoxis preferred because it is a process step that immediately precedes plating, which
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`is typically another wet processing step. Therefore, both can be donein the same wettool
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`without additional wafer handling or loading. However,this is not necessary for the invention.
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`{0022}
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`In some embodiments,it is important for the dielectric to be thin enoughto carry
`
`sufficient current without creating a series impedance. Layers in the 8 to 12A range are readily
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`formed using rapid thermal oxidation. Chemoxlayers are on the order of 8A thick. Such thin
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`layers should readily pass current densities consistent with the requirementsofa solarcell
`
`operating at one sun.
`
`[0023]
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`Next, in step $310, the contact metal is plated. In the preferred embodiment,
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`nickel is used,although other metals such assilver, tungsten or copper mayalso be used. A
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`conventional process for plating including a seed material can be used as described above. Those
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`skilled in the art will understand manyalternatives, however.
`
`[0024]
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`According to aspects of the invention,in contrast to the priorart plating process,
`
`no annealing or alloying process is required. Moreover, there is no requirementfor special
`
`metals for n- or p-type contacts, such as, for direct contacts, Al contacts p-type and Ag contacts
`
`n-type. In this case, contacting to reasonably high doped (mid to high 10'° doping can be done
`
`with any metal, although some may provide better contacts that others by virtue of differing work
`
`functions. Still further, there is no need to carefully control the amountofsilicide metal plated in
`
`order to prevent driving thesilicide alloy through the junction as is required in thepriorart.
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`Accordingly, plating thicknesses down to about 2 jum can be used, for example.
`
`[0025]
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`In any event, to do the plating in step S310,it is necessary to create a potential
`
`across the tunneldielectric, so that electrons can tunnel through thedielectric to reduce metal
`
`ions. There are two ways to accomplish this, as will be appreciated by thoseskilled in theart.
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`For example, oneis to place an electrical bias between the solution and the back contact. This
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`must be donein constantcurrent mode,as it will reverse bias the junction andcarries the risk of
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`damaging the cell. Another techniqueis the well known methodoflight inducedplating. The
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`sampleis illuminated, causing a photocurrent to flow through the tunnel dielectric. Those skilled
`
`in the art will be able to understand such conventional techniquesto the overall process flow of
`
`the present invention.
`
`{0026}
`
`Althoughthe present invention has been particularly described with reference to
`
`the preferred embodimentsthereof, it should be readily apparentto those of ordinary skill in the
`
`art that changes and modifications in the form and details may be made without departing from
`
`the spirit and scope ofthe invention. It is intended that the appended claims encompass such
`
`changes and modifications.
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`

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`WHATIS CLAIMED IS:
`
`1. A method of forming a contact in a solar cell having a p-n junction, comprising:
`
`creating one or more contact regions over the p-n junction;
`
`forminga tunnel dielectric in the one or more contact regions, wherein the formingstep
`
`includes formingthe tunnel dielectric thin enoughto sustain a tunnelcurrent therethrough; and
`
`plating a metal over the tunnel dielectric material to form the contact.
`
`2. A method according to claim 1 wherein the step of forming the tunnel dielectric includes
`
`forming an oxide layer using a Chemox process.
`
`3. A method according to claim 1 wherein the step of forming the tunnel dielectric includes
`
`forming an oxide layer using thermal oxidation.
`
`4. A method according to claim 1 wherein the metal includes nickel.
`
`5. A method according to claim 1 wherein the metal includes copper.
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`6. A method according to claim | wherein the plating step includes creating a potential across
`
`the tunnel dielectric using a light bias to induce a photocurrent.
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`7. A method according to claim 1 whereinthe plating step mcludescreating a potential across
`
`the tunnel dielectric using an electricalbias.
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`

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`WO 2009/094570
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`8. A method according to claim 1, wherein the step of forming the tunnel dielectric includes
`
`forming a nitride layer.
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`9. A method according to claim 1, wherein the p-n junction comprisesa silicon substrate doped
`
`with impurities of a first polarity, and an emitter region near a surface of the substrate with a
`
`second polarity opposite the first polarity.
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`10. A method according to claim 9, wherein the emitter region comprises a shallow emitter,
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`11. A solar cell, comprising:
`
`a p-n junction;
`
`one or more contact regions formed over the p-n junction;
`
`a tunnel dielectric formed in the one or more contact regions, wherein the tunnel
`
`dielectric is thin enough to sustain a tunnel current therethrough; and
`
`a metal plated over the tunnel dielectric material to form a contactto the p-n junction.
`
`12. A solar cell according to claim 12 wherein the tunnel dielectric comprises an oxide layer.
`
`13. A solar cell according to claim 12 wherein the metal includes one or more of nickel and
`
`copper.
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`14. A solar cell according to claim 12, wherein the tunnel dielectric comprises a nitride layer.
`
`10
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`15. A solar cell according to claim 12, wherein the p-n junction comprisesa silicon substrate
`
`doped with impurities ofa first polarity, and a shallow emitter region near a surface of the
`
`substrate with a second polarity opposite the first polarity.
`
`11
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`1/2
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`Plated metal 102
`
`Alloyed region 104
`
`.
`.
`Dielectric 106
`
`n-type layer 108
`
`p-type substrate 110
`
`FIG. 1
`(Prior Art)
`
`Tunneldielectric 204
`
`Plated metal 202
`
`p-type substrate 210
`
`Dielectric 206
`
`n-type layer 208
`
`FIG,2
`
`SUBSTITUTE SHEET (RULE 26)
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`WO 2009/094370
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`2/2
`
` Prepare substrate with
`
`
`
`shallow emitter
`5302
`
`
`
` Form dielectric layer
`
`5304
`
`
` Open contact holes
`
`
`
`$306
`
`
`
`
`
`
`
`
`
`
`Form oxide using
`rapid oxidation
`5308B
`
`Form oxide using
`Chemox process
`S308A
`
`
`
`
`
`Form plated contact
`S310
`
`
`
`
`FIG. 3
`
`SUBSTITUTE SHEET (RULE 26)
`
`