Solar cell and fabrication thereof using microwaves

• US 4,273,950 A
• Filed: 05/29/1979
• Issued: 06/16/1981
• Est. Priority Date: 05/29/1979
• Status: Expired due to Term

• Current Assignee: Photowatt International
• Original Assignee: Photowatt International
• Inventors: Chitre, Sanjiv R.
• Primary Examiner(s): Rutledge, L. Dewayne
• Assistant Examiner(s): Saba, W. G.
• Application Number:
  US06/043,149
• Time in Patent Office: 749 Days
• Field of Search: 148/1.5, 148/174, 148/175, 148/188, 29/572, 136/89 TF, 136/89 CC, 136/255, 136/256, 136/258, 136/261, 427/45, 427/46, 219/10.55 M, 219/10.55 R, 34/4, 34/18, 357/30, 357/59, 357/65
• US Class Current: 136/255
• CPC Class Codes: H01L 21/2254 from or through or into an applied layer, e.g. photoresist, nitrides
• CPC Class Codes: H01L 21/2255 the applied layer comprising oxides only, e.g. P2O5, PSG, H3BO3, doped oxides
• CPC Class Codes: H01L 21/324 Thermal treatment for modifying the properties of semiconductor bodies, e.g. annealing, sintering H01L21/20 - H01L21/288 and H01L21/302 - H01L21/322 take precedence
• CPC Class Codes: H01L 31/03682 including only elements of Group IV of the Periodic Table
• CPC Class Codes: H01L 31/1804 comprising only elements of Group IV of the Periodic Table
• CPC Class Codes: H01L 31/186 Particular post-treatment for the devices, e.g. annealing, impurity gettering, short-circuit elimination, recrystallisation
• CPC Class Codes: Y02E 10/546 Polycrystalline silicon PV cells
• CPC Class Codes: Y02E 10/547 Monocrystalline silicon PV cells
• CPC Class Codes: Y02P 70/50 Manufacturing or production processes characterised by the final manufactured product
• CPC Class Codes: Y10S 148/153 Solar cells-implantations-laser beam

Abstract

Solar cells are fabricated by spraying a dopant coating onto a semiconductor wafer and heating the surface of the wafer using unipolar microwaves. The resultant controlled heating drives dopant atoms from the coating into the wafer to produce a shallow junction at a selectable depth. Advantageously, metallic conductors are predeposited atop the dopant coating and then sintered to the semiconductor by the same unipolar microwave field concurrently with dopant drive-in. Efficient solar cells can be made with this process using polycrystalline silicon, since with unipolar microwave surface heating the grain boundaries do not become so deeply doped as to short circuit the junctions formed in the individual grains. Unipolar microwave heating also may be used to anneal ion implanted semiconductor devices.

Claims

1. A process for making solar cells or like junction devices comprising:

providing a dopant adjacent the surface of a polycrystalline silicon semiconductor substrate, and

exposing said dopant and substrate to a unipolar microwave field so as to drive atoms from said dopant into said substrate to a selected depth, while controlling one or more of the microwave field parameters of frequency, power, duty cycle and on/off time so as to obtain said selected depth, and wherein;

said parameters are selected so that said dopant atoms are driven into grains of said substrate to a depth sufficient to form a shallow junction therein, concurrent drive-in of dopant atoms into grain boundaries of said substrate being insufficient to convert said boundaries to conductive short circuits for said shallow junction, the dopant level of said grain boundaries being lower than that of said grains.

2. A solar cell or like junction drive formed by the process of claim 1.

6. A process according to claim 1 wherein said dopant is sprayed onto said semiconductor substrate surface.

7. A process according to claim 1 further comprising:

providing a second dopant adjacent the obverse surface of said semiconductor substrate, and

also exposing said second dopant to a unipolar

microwave field so as to drive atoms from said second dopant into said substrate from said obverse surface to a second selected depth.

3. A process for making solar cells or like junction devices comprising:

providing a dopant adjacent the surface of a semiconductor substrate,

exposing said dopant and substrate to a unipolar microwave field so as to drive atoms from said dopant into said substrate to a selected depth, and

disposing one or more metal conductors on said substrate atop said dopant, said step of exposing including simultaneously or sequentially also exposing said metal conductors to said unipolar microwave field so as to sinter said metal conductors to said substrate to form electrical connection to the portion of said substrate into which said dopant atoms are driven.

4. A process for making a solar cell, comprising:

spraying a dopant coating onto one surface of a semiconductor substrate,

depositing at least one metallic conductor atop said dopant coating, and

exposing said dopant coating, said conductor and said semiconductor one surface to a unipolar microwave field, said field driving dopant atoms from said coating into said substrate so as to form a shallow junction while concurrently sintering said metallic conductor to said substrate thereby to provide an electrical connection to said junction.

5. A process for making solar cells or like junction devices comprising:

providing a dopant adjacent the surface of a semiconductor substrate,

exposing said dopant and said substrate to a pulsed unipolar microwave field so as to incrementally heat progressively deeper layers of said substrate, the initial microwave pulse primarily heating and hence changing the dielectric constant of only a thin top surface layer of said substrate, subsequent unipolar microwave pulses then progressively heating and changing the dielectric constant of incremental, sequentially progressively deeper layers of said substrate, atoms from said dopant being driven by said pulsed unipolar microwave heating into said progressively deeper heated layers of said substrate,

said exposing step being carried out with the magnetic field component of said unipolar microwave field oriented parallel to the surface of said substrate and the electric field component of said microwave field oriented perpendicular to said substrate, and

controlling the unipolar microwave field parameters of frequency, power, pulse duty cycle and on/off time so as to obtain incremental layer heating and concomitant dopant depth of penetration and concentration in fine, progressively deeper steps.

8. A solar cell comprising:

a polycrystalline silicon substrate,

a shallow junction formed in a substantial percentage of the individual single crystal silicon grains of said substrate, the grain boundaries in said substrate being doped to a level that is lower than that of the single crystal grains dopant level and insufficient to short circuit said junctions, and

electrical conductors on a portion of the surface of said substrate, said conductors being sintered to a surface adjacent portion of said substrate and providing electrical contact to said junctions,

said shallow junctions being formed by coating said substrate with a dopant and driving atoms of said dopant into said substrate using a unipolar microwave field, the dielectric constant of said grain boundaries being sufficiently different from that ofsaid silicon grains so that said field will cause sufficient drive-in of said atoms into said grains to produce a shallow junction therein while said field will cause insufficient heating of said grain boundaries to cause substantial doping thereof by said atoms, said field concurrently sintering said conductors to said surface adjacent portion.