Doc Code: PA...
`PTOIAIA/B2A (07-19)
`PTOVAIAIB2A
`alt
`(07-13)
`Document Description: Power of Aitorney
`Approved for use through 11/30/2014, OMB 0851-0051
`US. Patent and Trademark Office; U.S. DEPARTMENT OF COMMERCE
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`TRANSMITTAL FOR POWER OF ATTORNEY TO ONE OR MORE
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`directed, the Power of Attorney will not be recognized in the application.
`
`Application Number
`
`Filing Date
`
`02-Oct-2014
`
`First Named Inventor
`
`Peter John COUSINS
`
`Title
`
`FRONT CONTACT SOLAR CELL WITH FORMED EMITTER
`
`Art Unit
`
`Examiner Name
`
`10031.004211
`
`Attorney Docket Number
`SIGNATURE of Applicant or Patent Practitioner
`[Patrick D. Benedicto/
`Signature
`
`Name
`
`PATRICK D. BENEDICTO
`
`Title (f Applicant is a Attomey of Recard
`furistic entity)
`
`Date (Optional)
`
`Registration
`Number
`
`| 40909
`
`Applicant Name (if Applicant is a juristic entity)
`SunPower Corporation
`NOTE: This forra must be signed in accordance with 37 CFR 1.33. See 37 CFR 1.4(q} for signature requirements and certifications. If
`more than one applicant, use muliiple forms.
`“Total of
`forms are submitted.
`
`4
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`This callection of information is required by 37 CFR 1.131, 1.32, and 1.33. The information is required to obtain or retain a benefit by
`the public which is to file (and by the USPTO to process) an application. Confidentiality is governed by 35 U.S.C. 122 and 37 CFR
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`ff you need assistance in completing the form, call 1-G00-PTO-9199 and select aption 2.
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`Doc Code: PA..
`PTOAAIAB2B (07-13)
`Document Description: Power of Attorney
`Approved for ase thiough 11/30/2014, OMB 0051-0057
`US, Patent and Trademark Giice: U.S. DEPARTIVERT OF COMMERGE
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`POWER OF ATTORNEY BY APPLICANT
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`{ hereby appoint the Patent PractLoner(s} associated with the foligwing Customer Number as my/our attomey(s} or agent(s), and
`io transact ail busmess im the Umiad States Patent and Trademark Office connected therawstin for tre appicaton referenced m
`ihe attached transimutial ietter (form PTO/AIA/B2A) oF identihed above:
`74254
`oR
`[| i hereby appoint Practitioner(s) named in the attached list (form PTO/AIA/82C) as iny/our allamey(s} or agent(s). and to transact
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`;
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`|
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`ail business in the United States Patent and Trademark Office connected therewith for the patent application referenced in the
`attached transmittal latter orm PTOIAIA/S2A) or denified above,
`(Note. Camplete fosn PTOKAIAIG2C.}
`Please recognize or change the correspondence address for the application identified in the attached transmittal
`jatter or the boxes above to:
`‘The address associated with the abave-mentionad Customer Number
`OR
`[| The address associated with Customer Number:
`OR
`Firm.or
`individual Name
`
`Address
`City
`| Country
`| Felephone
`i Email
`tam the Applicant (if the Applicant is a juristic entity, jist the Applicant-name in the box):
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`i State
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`|
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`i Zip
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`SUNPOWER CORPORATION
`Py Inventor or Joint inventor {tile nat required below)
`[| Legal Representative of a Deceased or Lagaily incapacitated inventor (fitle nol requirad below}
`Assignee of Person fo Whom the Inventor is Under an Obligation to Assign (provide signer's title if appticant is a juristic entity)
`Person Who Otherwise Shows Sufficient Proprietary interest (e.g., a petition under 3? CFR 1.46(b)(2) was granted in the
`application of is concurrently being fled with this documents (provide siqne’s file ff applicant is a furistic entity)
`of
`nt for Patent
`SIGHATURE
`Apptican
`Hhorized to act
`of theay
`
`—
`nT bai Gotionaly | ete 7g aot
`
`hers
`
`fu
`
`y}
`
`rsigned (vehose title
`
`ae:aeee
`
`Signature
`Name
`
`.
`
`ta
`
`:
`
`DAVID PETRIAI
`HEAD IP GOUNSEL, SUNPOWER CORPORATION
`Title
`NOTE: Signature - This form must be signed by the applicant in accordance with’ 37 CFR 1.33. See 37 CFR 1.4 for signature requirements
`and certifications, If more than one applicant, use multiple forms.
`[vlrotat of
`forms. are siibiviitied.
`1
`vasesThs callestion of nformanan ie required py 3)
`> PARE FAY ane 4.33 fhe mtormatonis required to obtain er retaen a neneft by ihe punto which 45 to fi
`overned by ASUS C 129 ard BP OR
`a
`USPTO to process} an apolination Comfidady
`tec fo take 2 mt nutes to
`the ind aval case Any care
`ag. anid sutunding
`the amicunt
`mpdavedapp!
`PE DRISESS
`ermark CHfce US
`tare to com
`5 farm andr
`of time
`a
`Kemanies
`bepasment of Lommene PO Box “400, Alexanana, WA ddd iJ-i450 ire ROT
`BS OR COMPLETED §FORMS"G THIS AGOREGS. SEND TO: Commissioner
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`Af you nooo asssiance in completing the form. calf t 800 PTO-d189 and seiset option 2
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`

`

`Doc Code: FALREQ
`Document Description: Request First Action Interview
`
`PTO/SB/443C (05-11)
`Approved for use through 01/81/2013. GMB 0651-0031
`U.S. Patent and Trademark Ovtice; U.S. DEPARTMENT OF COMMERCE
`Under the Paperwork Reduction Act of 1995, no persons are required io respond to a collection of information unless it displays a valid OMB control number.
`
`REQUEST FOR FIRST ACTION INTERVIEW (FULL PILOT PROGRAM)
`
`Attorney Docket
`Number:
`
`10031.004214
`
`Application Number
`(if known):
`
`Filing date: 49799/9014
`
`First Named
`inventor
`
`Peter John COUSINS
`
`Tite: FRONT CONTACT SOLAR CELL WITH FORMED EMITTER
`
`APPLICANT HEREBY REQUESTS 4 FIRST ACTION INTERVIEW IN THE ABOVE-IDENTIFIED
`APPLICATION, See Instruction Sheet on page 2.
`The application must contain three () or fewer independent claims and twenty (20) or fewer total claims.
`
`i.
`
`2.
`
`3.
`
`The application must not contain any multiple dependent claims.
`By fling this request:
`
`Applicant is agreeing to make an election without traverse if the Office determines that the claims are nat
`obviously directed to a single invention; and
`
`Applicant is agreeing not to request for a refund of the search fee and any excess claims fee paid in the
`application after the mailing or notification of the pre-interview communication prepared by the examiner.
`
`4.
`
`Other attachments:
`
`/Patrick D. Benedicto/
`
`Signature
`
`Date October 2, 2014
`
`~
`PATRICK D. BENEDICTO
`cr
`
`Name
`Registration Number 40,110
`(Print/Typea)
`Note: This form must be signed in accordance with 37 CFR 1.33. See 37 CFR 1.4(d) for signature requirements and certifications.
`Submit multiple forms if more than one signature is required, see below".
`
`CI
`
`:
`
`“Total of
`
`1
`
`ss forms are submitted
`
`The inforrriation is required to obtain or retain a benefit by the public which is to file (and by the USPTO to grocess) an application. Confidentiality is governed by
`35 U.S.C. 122 and 37 CFR 1.11 and 1.14. This collection is estimated to take 12 hours ta complete, including gathering. preparing, and subm
`the completed
`application form to the USPTO. Time will vary depending upon the indivicual case. Any comments an the amount of time you require to campieie this form and/ar
`suggestions for reducing this burden, should be sent te the Chief information Officer, U.S. Patent and Trademark Office, US. Department of Cornmerce, P.C. Box
`1450, Alexandria, VA 22313-1450. DO NOT SEND FEES OR COMPLETED FORMS TO THIS ADDRESS.
`SEND FO: Commissioner for Patents, P.O.
`Box 1456, Alexandria, VA 22313-1450.
`ifyou need assistance in completing the form, call 1-800-PTO-9199 and seleci option 2.
`
`

`

`Doc Code: FALREQ
`Document Description: Request First Action Interview
`
`PTO/SB/443C (05-11)
`Approved for use through 01/81/2013. GMB 0651-0031
`U.S. Patent and Trademark Ovtice; U.S. DEPARTMENT OF COMMERCE
`Under the Paperwork Reduction Act of 1995, no persons are required io respond to a collection of information unless it displays a valid OMB control number.
`
`REQUEST FOR FIRST ACTION INTERVIEW (FULL PILOT PROGRAM)
`
`Attorney Docket
`Number:
`
`10031.004214
`
`Application Number
`(if known):
`
`Filing date: 49799/9014
`
`First Named
`inventor
`
`Peter John COUSINS
`
`Tite: FRONT CONTACT SOLAR CELL WITH FORMED EMITTER
`
`APPLICANT HEREBY REQUESTS 4 FIRST ACTION INTERVIEW IN THE ABOVE-IDENTIFIED
`APPLICATION, See Instruction Sheet on page 2.
`The application must contain three () or fewer independent claims and twenty (20) or fewer total claims.
`
`i.
`
`2.
`
`3.
`
`The application must not contain any multiple dependent claims.
`By fling this request:
`
`Applicant is agreeing to make an election without traverse if the Office determines that the claims are nat
`obviously directed to a single invention; and
`
`Applicant is agreeing not to request for a refund of the search fee and any excess claims fee paid in the
`application after the mailing or notification of the pre-interview communication prepared by the examiner.
`
`4.
`
`Other attachments:
`
`/Patrick D. Benedicto/
`
`Signature
`
`Date October 2, 2014
`
`~
`PATRICK D. BENEDICTO
`cr
`
`Name
`Registration Number 40,110
`(Print/Typea)
`Note: This form must be signed in accordance with 37 CFR 1.33. See 37 CFR 1.4(d) for signature requirements and certifications.
`Submit multiple forms if more than one signature is required, see below".
`
`CI
`
`:
`
`“Total of
`
`1
`
`ss forms are submitted
`
`The inforrriation is required to obtain or retain a benefit by the public which is to file (and by the USPTO to grocess) an application. Confidentiality is governed by
`35 U.S.C. 122 and 37 CFR 1.11 and 1.14. This collection is estimated to take 12 hours ta complete, including gathering. preparing, and subm
`the completed
`application form to the USPTO. Time will vary depending upon the indivicual case. Any comments an the amount of time you require to campieie this form and/ar
`suggestions for reducing this burden, should be sent te the Chief information Officer, U.S. Patent and Trademark Office, US. Department of Cornmerce, P.C. Box
`1450, Alexandria, VA 22313-1450. DO NOT SEND FEES OR COMPLETED FORMS TO THIS ADDRESS.
`SEND FO: Commissioner for Patents, P.O.
`Box 1456, Alexandria, VA 22313-1450.
`ifyou need assistance in completing the form, call 1-800-PTO-9199 and seleci option 2.
`
`

`

`Attorney Docket No. 10031.004211
`
`FRONT CONTACT SOLAR CELL WITH FORMED EMITTER
`
`Inventor: Peter John Cousins
`
`5
`
`REFERENCE TO RELATED APPLICATIONS
`
`This application is a continuation of U.S. Application No. 13/495,577, filed on
`June 13, 2012, which is a divisional of U.S. Application No. 12/070,742, filed on
`
`February 20, 2008, now U.S. Patent No. 8,222,516. The just-mentioned disclosures are
`
`incorporated herein by reference in their entirety.
`
`10
`
`BACKGROUND OF THE INVENTION
`
`1.
`
`Field of the Invention
`
`The present invention relates generally to solar cells, and more particularly but
`
`not exclusively to solar cell fabrication processes and structures.
`
`15
`
`2.
`
`Description of the Background Art
`
`Solar cells are well known devices for converting solar radiation to electrical
`
`energy. They may be fabricated on a semiconductor wafer using semiconductor
`processing technology. A solar cell includes P-type and N-type diffusion regions that
`form a junction. Solar radiation impinging on the solar cell creates electrons and holes
`
`20
`
`that migrate to the diffusion regions, thereby creating voltage differentials between the
`
`diffusion regions.
`
`In a backside contact solar cell, both the diffusion regions and the
`
`metal contacts coupled to them are on the backside of the solar cell. The metal
`
`-1-
`
`

`

`Attorney Docket No. 10031.004211
`
`contacts allow an external electrical circuit to be coupled to and be powered by the solar
`
`cell.
`
`In a front contact solar cell, at least one of the metal contacts making an
`
`electrical connection to a diffusion region is on the front side of the solar cell. The front
`
`5
`
`side of the solar cell, which is opposite the backside, faces the sun during normal
`
`operation to collect solar radiation. While backside contact solar cells have an aesthetic
`
`advantage over front contact solar cells due to the absence of metal contacts on the
`
`front side, and are thus preferred for residential applications, aesthetics is not a major
`
`requirement for power plants and other applications where power generation is the main
`
`10
`
`concern. Disclosed herein are structures for a relatively efficient and cost-effective front
`
`contact solar cell and processes for manufacturing same.
`
`SUMMARY
`
`A bipolar solar cell includes a backside junction formed by an N-type silicon
`
`15
`
`substrate and a P-type polysilicon emitter formed on the backside of the solar cell. An
`antireflection layer may be formed on a textured front surface of the silicon substrate. A
`negative polarity metal contact on the front side of the solar cell makes an electrical
`
`connection to the substrate, while a positive polarity metal contact on the backside of
`
`the solar cell makes an electrical connection to the polysilicon emitter. An external
`
`20
`
`electrical circuit may be connected to the negative and positive metal contacts to be
`
`powered by the solar cell. The positive polarity metal contact may form an infrared
`
`reflecting layer with an underlying dielectric layer for increased solar radiation collection.
`
`-2-
`
`

`

`Attorney Docket No. 10031.004211
`
`These and other features of the present invention will be readily apparent to
`
`persons of ordinary skill in the art upon reading the entirety of this disclosure, which
`
`includes the accompanying drawings and claims.
`
`‘5
`
`DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 schematically shows a cross-section of a solar cell in accordance with an
`
`embodiment of the present invention.
`
`FIG. 2 is a plan view schematically showing the front side of the solar cell of FIG.
`
`10
`
`FIG. 3 is a plan view schematically showing the backside of the solar cell of FIG.
`
`FIG. 4, which comprises FIGS. 4A-4M, schematically illustrates the fabrication of
`
`the solar cell of FIG. 1 in accordance with an embodiment of the present invention.
`
`The use of the same reference label in different figures indicates the same or like
`
`15
`
`components. The figures are not drawn to scale.
`
`DETAILED DESCRIPTION
`
`In the present disclosure, numerous specific details are provided, such as
`
`examples of apparatus, process parameters, materials, process steps, and structures,
`
`20
`
`to provide a thorough understanding of embodiments of the invention. Persons of
`
`ordinary skill in the art will recognize, however, that the invention can be practiced
`
`-3-
`
`

`

`Attorney Docket No. 10031.004211
`
`without one or more of the specific details.
`
`In other instances, well-known details are
`
`not shown or described to avoid obscuring aspects of the invention.
`
`FIG. 1 schematically shows a cross-section of a solar cell 100 in accordance with
`
`an embodiment of the present invention. The solar cell 100 has a front side where a
`
`5
`
`metal contact 102 is located and a backside on a same side as the metal contact 110.
`
`The front side faces the sun during normal operation to collect solar radiation.
`
`In the example of FIG. 1, the solar cell 100 includes a backside junction formed
`
`by a P-type doped polysilicon emitter 108 serving as a P-type diffusion region and an N-
`type silicon substrate 101 servings as an N-type diffusion region. The N-type silicon
`
`10
`
`substrate 101 may comprise a long lifetime (e.g., 2 to 5ms) N-type silicon wafer and
`a
`may have a thickness of about 100 to 250 um as measured from the backside surface
`to a tip of the textured front side surface of the substrate. The front side surface of the
`
`substrate 101 is randomly textured (labeled as 113) and includes N-type doped regions
`
`105 and 106 formed in the substrate. The N-type doped region 105 provides low front
`
`15
`
`surface recombination and improves lateral conductivity whilst not compromising the
`
`blue response of the solar cell. The region 106, which may be a phosphorus diffusion,
`
`provides low contact resistance and minimizes contact recombination. The region 106
`
`is also referred to as an "N-dot" because, in one embodiment, it forms a dot-shape to
`
`minimize the area of heavily diffused regions on the front surface. The N-type doped
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`20
`
`region 105 may have a sheet resistance of 100 to 500 O/sq, whilst the n-type doped
`
`region 106 may have a sheet resistance of 10 to 50 O/sq.
`
`An antireflective coating (ARC) of silicon nitride layer 103 is formed on the
`textured front side surface of the substrate 101. The texture front side surface and the
`-4-
`
`

`

`Attorney Docket No. 10031.004211
`
`silicon nitride layer 103 help improve solar radiation collection efficiency. A passivating
`
`oxide 124 may comprise silicon dioxide thermally grown to a thickness of about 10 to
`
`250 Angstroms on the front side surface of the substrate 101.
`
`In one embodiment, the polysilicon emitter 108 is formed on a tunnel oxide layer
`
`‘5
`
`107. The polysilicon emitter 108 may be formed by forming a layer of polysilicon using
`Chemical Vapor Deposition (CVD), such as Low Pressure CVD (LPCVD) or Plasma
`Enhanced CVD (PECVD), and thermal anneal. The polysilicon emitter 108 may have a
`sheet resistance of 100 O/sq, and a thickness of 1000 to 2000 Angstroms. The tunnel
`
`oxide layer 107 may comprise silicon dioxide thermally grown to a thickness of about 10
`to 50 Angstroms on the backside surface of the substrate 101. A metal contact 110
`
`10
`
`electrically connects to the polysilicon emitter 108 through contact holes 123 formed
`a
`silicon dioxide layer 109. The metal contact 110
`througha dielectric comprising a
`a
`provides a positive polarity terminal to allow an external electrical circuit to be coupled
`a
`
`to and be powered by the solar cell 100. The silicon dioxide layer 109 provides
`
`15
`
`electrical isolation and allows the metal contact 110 to serve as an infrared reflecting
`
`layer for increased solar radiation collection.
`
`In one embodiment, the metal contact 110
`
`a
`comprises silver having a conductance of about 5-25 mQ.cm and a thickness of about
`
`15-35ym.
`
`On the front side of the solar cell 100, the metal contact 102 electrically connects
`
`20
`
`to the region 106 through a contact hole 120 formed through the silicon nitride layer
`
`103. The metal contact 102 provides a negative polarity terminal to allow an external
`
`electrical circuit to be coupled to and be powered by the solar cell 100.
`
`In one
`
`embodiment, the metal contact 102 comprises silver having a sheet resistance of about
`-5-
`
`

`

`Attorney Docket No. 10031.004211
`
`5mQ.cm and a thickness of about 15m. The pitch between adjacent metal contacts
`
`102 may be about 1 to 4 mm.
`
`In one embodiment, the metal contacts 102 are spaced
`
`at 400 to 1000 um along each metal contact 102 (see FIG. 2).
`
`In the example ofFIG. 1, the edge isolation trench 111 is formed through the
`
`5
`
`silicon dioxide layer 109, the polysilicon emitter 108, and a portion of the substrate 101
`
`to provide edge electrical isolation.
`
`FIG. 2 is a plan view schematically showing the front side of the solar cell 100.
`
`In
`
`the example of FIG. 2, two bus bars 201 run parallel on the front side of the substrate
`
`10
`
`101. The contact holes 120, in which the metal contacts 102 are formed, may each
`A
`have a diameter of about 50 to 200 um. A plurality of metal contacts 102 is formed
`perpendicular to the bus bars 201. Each metal contact 102 may have a width of about
`
`60-120pm.
`
`FIG. 3 is a plan view schematically showing the backside of the solar cell 100.
`
`In
`
`the example of FIG. 3, two bus bars 301, which are electrically coupled to metal
`
`15
`
`contacts 110, run parallel on the backside.
`
`In practice, the bus bars 201 and 301 will be
`
`electrically connected to corresponding bus bars of adjacent solar cells to form an array
`
`of solar cells.
`
`Solar cells have gained wide acceptance among energy consumers as a viable
`
`renewable energy source. Still, to be competitive with other energy sources, a solar cell
`
`20
`
`manufacturer must be able to fabricate an efficient solar ceil at relatively low cost. With
`
`this goal in mind, a process for manufacturing the solar cell 100 is now discussed with
`reference to FIGS. 4A-4M.
`
`6
`
`

`

`Attorney Docket No. 10031.004211
`
`FIG. 4, which comprises FIGS. 4A-4M, schematically illustrates the fabrication of
`
`the solar cell 100 in accordance with an embodiment of the present invention.
`
`In FIG. 4A, an N-type silicon substrate 101 is prepared for processing into a solar
`
`cell by undergoing a damage etch step. The substrate 101 is in wafer form in this
`
`‘5
`
`example, and is thus typically received with damaged surfaces due to the sawing
`
`process used by the wafer vendor to slice the substrate 101 from its ingot. The
`
`substrate 101 may be about 100 to 200 microns thick as received from the wafer
`
`vendor.
`
`In one embodiment, the damage etch step involves removal of about 10 to 20
`
`yum from each side of the substrate 101 using a wet etch process comprising potassium
`
`10
`
`hydroxide. The damage etch step may also include cleaning of the substrate 101 to
`
`remove metal contamination.
`
`In FIG. 4B, tunnel oxides 402 and 107 are formed on the front and back surfaces,
`
`respectively, of the substrate 101. The tunnel oxides 402 and 107 may comprise silicon
`
`dioxide thermally grown to a thickness of about 10 to 50 Angstroms on the surfaces of
`the N-type silicon substrate 101. A layer of polysilicon is then formed on the tunnel
`
`15
`
`oxides 402 and 107 to form the polysilicon layer 401 and the polysilicon emitter 108,
`
`respectively. Each of the polysilicon layer 401 and the polysilicon emitter 108 may be
`formed to a thickness of about 1000 to 2000 Angstroms by CVD.
`
`20
`
`In FIG. 4C, a P-type dopant source 461 is formed on the polysilicon emitter 108.
`As its name implies, the P-type dopant source 461 provides a source of P-type dopants
`for diffusion into the polysilicon emitter 108 in a subsequent dopant drive-in step. A
`
`dielectric capping layer 462 is formed on the P-type dopant source 461 to prevent
`dopants from escaping from the backside of the solar cell during the drive-in step.
`-7-
`
`In
`
`

`

`Attorney Docket No. 10031.004211
`
`one embodiment, the P-type dopant source comprises BSG (borosilicate glass)
`deposited to a thickness of about 500 to 1000 Angstroms by atmospheric pressure CVD
`
`(APCVD) and has a dopant concentration of 5 to 10% by weight, while the capping layer
`
`462 comprises undoped silicon dioxide formed to a thickness of about 2000 to 3000
`Angstroms also by APCVD.
`
`5
`
`In FIG. 4D, the edge isolation trench 111 is formed near the edge of the
`
`substrate 101 on the backside. The trench 111 is relatively shallow (e.g., 10 um deep
`
`into the substrate 101) and provides edge electrical isolation.
`
`In one embodiment, the
`
`trench 111 is formed by cutting through the capping layer 462, the P-type dopant source
`
`10
`
`461, the polysilicon emitter 108, the tunnel oxide 107, and into a shallow portion of the
`
`substrate 101 using a laser.
`
`In FIG. 4E, exposed regions on the front surface of the substrate 101 is randomly
`
`textured to form the textured surface 113.
`
`In one embodiment, the front surface of the
`
`substrate 101 is textured with random pyramids using a wet etch process comprising
`
`15
`
`potassium hydroxide and isopropyl alcohol.
`
`In FIG. 4F, an N-type dopant source 412 is formed on regions of the textured
`
`surface 113 where contact holes 120 (see FIG. 1) will be subsequently formed to allow
`subsequently formed metal contacts 102 to electrically connect to the substrate 101. As
`its name implies, the N-type dopant source 412 provides a source of N-type dopants for
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`diffusion into the front side of the substrate 101.
`
`In one embodiment, the N-type dopant
`
`source 412 is formed by inkjet printing the dopant material directly onto the substrate
`
`101.
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`Attorney Docket No. 10031.004211
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`In one embodiment, the N-type dopant source 412 comprises silicon dioxide
`doped with phosphorus. Only one N-type dopant source 412 is shown in FIG. 4F for
`
`clarity of illustration.
`
`In practice, there are several dot-shaped N-type dopant sources
`
`412, one for each region where a contact hole 120 is to be formed (see FIG. 2). This
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`‘5
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`allows formation of several dot shaped N-type doped regions 106 (see FIG. 1) after a
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`subsequently performed drive-in step now discussed with reference to FIG. 4G.
`
`In FIG. 4G, a dopant drive-in step is performed to diffuse N-type dopants from
`
`the N-type dopant source 412 into the substrate 101 to form the N-type dope region
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`106, to diffuse P-type dopants from the P-type dopant source 461 to the polysilicon
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`emitter 108, and to diffuse N-type dopants into the front side of the substrate 101 to
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`form the N-type doped region 105. Silicon dioxide layer 109 represents layers 461 and
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`462 after the drive-in step. The polysilicon emitter 108 also becomes a P-type doped
`layer after the drive-in step. The N-type doped region 105 may be formed by exposing
`
`the sample of FIG. 4G to phosphorus in a diffusion furnace, for example. The use of the
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`15
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`N-type dopant source 412 allows for a more controlled and concentrated N-type
`diffusion to the N-type doped region 106. The thin thermal silicon dioxide layer 124 may
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`be grown on the textured surface 113 during the drive-in process.
`
`The drive-in step to dope the polysilicon emitter 108 on the backside and to form
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`the N-type doped regions 105 and 106 on the front side may be formed in-situ, which in
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`the context of the present disclosure means a single manual (i.e., by fabrication
`
`personnel) loading of the substrate 101 in a furnace or other single chamber or multi-
`
`chamber processing tool.
`
`In one embodiment, the drive-in step is performed in a
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`Attorney Docket No. 10031.004211
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`diffusion furnace. The preceding sequence of steps leading to the drive-in step allows
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`for in-situ diffusion, which advantageously helps in lowering fabrication cost.
`
`It is to be noted that the step of using an N-type dopant source 412 to diffuse
`
`dopants into the N-type doped region 106 may be omitted in some applications. That is,
`in an alternative process, the formation of the N-type dopant source 412 in FIG. 4F may
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`5
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`be omitted.
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`In that case, the N-type doped regions 105 and 106 will be both doped by
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`introduction of an N-type dopant in the diffusion furnace during the drive-in step. All
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`other process steps disclosed herein remain essentially the same.
`
`In FIG. 4H, the antireflective coating of silicon nitride layer 103 is formed over the
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`textured surface 113 after removal of the N-type dopant source 412. Besides being an
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`antireflective coating, the silicon nitride layer 103 also advantageously serves as a
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`dielectric, enabling the selective contacts to be formed on the front surface to reduce
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`front surface recombination. The silicon nitride layer 103 may be formed to a thickness
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`of about 450 Angstroms by PECVD, for example.
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`15
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`In FIG. 41, a front contact mask 420 is formed on the silicon nitride layer 103 to
`
`create a pattern 421 defining the contact holes 120 (see FIG. 1). The mask 420 may
`comprise an acid resistance organic material, such as a resist, and formed using a
`a
`printing process, such as screen printing or inkjet printing.
`
`In FIG. 4J, a back contact mask 422 is formed on the silicon dioxide layer 109 to
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`20
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`create patterns 423 defining the contact holes 123 (see FIG. 1). Similar to the mask
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`420, the mask 422 may comprise an organic material formed using a printing process.
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`Attorney Docket No. 10031.004211
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`In FIG. 4K, contact holes 120 and 123 are formed by removing exposed portions
`
`of the silicon nitride layer 103 and the silicon dioxide 109 in a contact etch step.
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`In one
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`embodiment, the contact holes 120 are formed by using a selective etch process that
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`removes exposed portions of the silicon nitride layer 103 and stops on the substrate
`101. The same etch process removes exposed portions of the silicon dioxide 109 and
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`‘5
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`stops on the polysilicon emitter 108.
`BOE (buffered oxide etch).
`
`In one embodiment, the etch process comprises a
`
`In FIG. 4L, the metal contact 110 is formed on the silicon dioxide layer 109 to fill
`
`the contact holes 123 and make electrical connection to the polysilicon emitter 108.
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`The metal contact 110 may be formed using a printing process. The metal contact 110
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`may comprise silver, which, together with the silicon dioxide layer 109, makes an
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`excellent backside infrared reflector. Other metals may also be used as a metal contact
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`110, such as aluminum, for example.
`
`In FIG. 4M, the metal contact 120 is formed on the silicon nitride layer 103 to fill
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`15
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`the contact holes 120 and make electrical connection to the substrate 101. The metal
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`contact 120 may comprise silver and formed using a printing process.
`a
`
`a
`Formation of the metal contacts 110 and 102 may be followed bya firing step.
`The firing step is applicable when using screen printed silver paste as metal contacts,
`but not when using other processes or metals. The solar cell 100 may then be visually
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`inspected and tested.
`
`While specific embodiments of the present invention have been provided, it is to
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`be understood that these embodiments are for illustration purposes and not limiting.
`
`Many additional embodiments will be apparent to persons of ordinary skill in the art
`-11-
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`

`Attorney Docket No. 10031.004211
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`reading this disclosure.
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`Attorney Docket No. 10031.004211
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`CLAIMS
`
`What is claimed is:
`
`1.
`
`5
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`A solar cell, comprising:
`a substrate;
`a first tunnel dielectric disposed over the substrate;
`an emitter disposed over the first tunnel dielectric;
`a front electrode disposed over a front surface of the substrate; and
`a back electrode disposed over a back surface of the substrate.
`
`10
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`2.
`
`The solar cell of claim 1, wherein the emitter is a polysilicon emitter.
`
`The solar cell of claim 1, wherein the substrate is a monocrystalline silicon
`3.
`substrate.
`
`15
`
`4.
`
`The solar cell of claim 1, wherein the first tunnel dielectric is a tunnel oxide.
`
`The solar cell of claim 1, wherein the emitter is disposed over the back surface of
`5.
`the substrate.
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`20
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`The solar cell of claim 1, further comprising a second tunnel dielectric disposed
`6.
`on an opposite side of the substrate than the first tunnel dielectric.
`
`7.
`
`The solar cell of claim 1, wherein the back electrode includes silver.
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`25
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`The solar cell of claim 1, further comprising an antireflective layer disposed over
`8.
`the front surface of the substrate.
`
`9.
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`30
`
`A solar cell, comprising:
`a substrate;
`an emitter over a back surface of the substrate;
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`

`A
`Attorney Docket No. 10031.004211
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`a first dielectric between the emitter and back surface;
`a
`a front electrode disposed on a front surface of the solar cell; and
`a back electrode disposed on a back surface of the solar cell.
`
`5
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`10.
`
`The solar cell of claim 9, wherein the emitter is a polysilicon emitter.
`
`The solar cell of claim 9, wherein the substrate is a monocrystalline silicon
`11.
`substrate.
`
`10
`
`12.
`
`The solar cell of claim 9, wherein the first dielectric is an oxide.
`
`The solar cell of claim 9, further comprising a second dielectric disposed on an
`13.
`opposite side of the substrate than the first dielectric.
`
`15
`
`14.
`
`The solar cell of claim 9, wherein the back electrode includes silver.
`
`15.
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`20
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`A method of fabricating a solar cell, the method comprising:
`a
`forming a first tunnel dielectric on a front surface of a substrate;
`forming a second tunnel dielectric on a back surface of the substrate;
`forming an emitter over the first or second tunnel dielectric;
`forming a front side electrode on a front surface of the solar cell; and
`forming a back side electrode on a back surface of the solar cell.
`
`|The method of claim 15, wherein said forming the emitter comprises forming a