Pulsed laser with uniform power output,particularly useful in the manufacture of solar photo-voltaic cells

Description

חוק הפטנטים׳ תשכ״ז-1967PATENT LAW, 5727- 1967For Office UseApplication for Patent7 / 46 / 3I (Name and address of applicant, and in case of body corporate-place of incorporation)BERNARD B. KATZ,a citizen of the U.S.A.,of 946 Havenhurst Drive, La Jolla,California 92037, United States of AmericaNumber61655 / 47.XII.1980 DateAnte / Post-dated02308• cn...... assignment.. dated........28..^1:2..?,.7.?....נןל אםצאה פבזן....ה<1ברה״..מ&ארי.ב...״of which is                                                                            Owner, by virtue ofof an invention the title(Hebrew)(English)EOR THE MANUFACTURE OF SOLAR PHOTO-VOLTAIC CELLSMETHODPULSED LASER WITH UNIFORM POWER OUTPUTWITH Ahereby apply for a patent to be granted to me In respect thereof.• בקשת חלוקה — Application of Division ^*גקשת פטנט מוסף - Application (or P^ielflAddltlon • דרישה דין קדימה Priority Claim מבקשת פטנט from Application No........v...........™ dated^<''”^        מיום • לבקשה / לפ^מב to Patent / Appl. No........................................פס׳ dated..........................מיום מספר / סימן Number / Mark תאריך Date מדינת האגוד Convention Country 100,025 3 .XII.1979 United States of America • יפוי כח: כאלו P.O. A.: general / Individual-attached / to be filed later- filed in case............. ax..״..........הוגש בענק המען למסירת מסמכים בישראל Address for Service in Israel א.«...י* גי.<.15..ר..ל.........................&A...E......MU.LF.QR I.:.?,•״............................4 4 5..,. P . 0 . Box ירושלים             Jerusalem חתימת המבקש Signature of Applicant א. י. מ ל פ ו ר ד היום..............?״.״בחודש.......י ״.??.״.״שנת............... of the year       ,     of                This December י לשימוש הלשכה For Office Use A.E. MULFORDATTORNEYS FOR APPLICANTSרזפרסום: ® 3 0 Wtf .w*־— -----.•BabJieatios daceThis from. Impressed with the Seal of the Patent Office and Indicating the number and . date of filing, certifies the filing of the application the particulars of which are set our above.Delete whatever is inapplicable ־ —— יי י • מחק את המיותרMETHOD FOR THE MANUFACTURE OF SOLAR PHOTO-VOLTAIC CELLS WITH A PULSED LASER WITHUNIFORM POWER OUTPUTSPECIFICATIONBACKGROUND OF THE INVENTIONField of the the InventionThe present invention relates to the manufacture of solar photo-voltaic cells. An excimer laser generating device for producing a pulsed output at a high repetition rate and with uniform power output across the beam aperture, for use in annealing solar photo-voltaic cells, is disclosed in Applicant's divisional Israel Patent Application No.6 9830.Description of the Prior ArtIn the manufacture of solar photo-voltaic cells, a semiconductor junction must be produced which will respond to incident solar radiation by producing a flow of electrons, and a counterflow of "holes". The element silicon, when suitably doped to form a P-N junction, responds in this fashion. Typical dopant materials include boron, phosphorous and arsenic. The electrical conversion efficiency of conventional solar־ voltaic cells is from about 10 to 16 percent. That is, 10 to 16 percent of incident radiant solar energy is converted to electrical energy using doped silicon wafers as solar voltaic cells. Other photosensitive semiconductor substrate materials may be substituted for silicon, for example gallium arsenide, germanium, gallium phosphide, indium phosphide cadmium telluride, aluminum antimonide, cadmium sulphide, copper oxide, and others.The principal problem that arises in the manufacture of solar voltaic cells according to the present state of the art is that in order to implant the dopant material, the underlying silicon crystalline structure is damaged during the ion implantation process. That is, the implantation process removes atoms from an orderly crystalline lattice-work at the implantation site, and creates partial and dis-oriented latticework regions. This damage is repaired, according to the current state of the art, by extended heating of the doped silicon wafers following implantation of the dopeant material for several hours at temperatures typically above 200° C. Furthermore, with the present state of the technology, only monocrystalline silicon is useful in the manufacture of solar voltaic cells, since a single crystalline lattice work is necessary in order to achieve a directed current flow, and hence a current which can be applied to external circuitry. The production of monocrystalline silicon is far more costly than polycrystalline or amorphous silicon, and much less readily available. As a consequence, the high cost of manufacture has thus far precluded the use of solar voltaic cells as a source of electrical power for any but the most exotic applications. To date, the use of solar voltaic cells as a source of electrical energy has been commercially significant only in supplying power to vehicles and instruments used beyond the earth's atmosphere, and instruments which must necessarily be used in remote, unattended locations.There are two general type of conventional dopant implantatitechniques used in the manufacture of solar voltaic cells. In the ion implantation technique the dopant is a high energy ion beam of a number of kilovolts. The impurity ions of arsenic, phosphorous or boron are rammed into the lattice5 structure of the silicon wafer with this high energy beam.This damages the crystal lattice structure of the monocrys-tailine silicon wafer, which necessitates subsequent annealing. In the other commercially signficiant technique of dopant implantation, the dopant material is applied to the surface 10 of a monocrystalline silicon wafer and thereafter thermally diffuses into the wafer. The thermal diffusion process complicates the manufacture of the cells, and adds to the manufacturing expense the same as high temperature thermal annealing.Various attempts have been made to utilize laser beams for the purpose of annealing doped silicon wafers following implantation of the dopant material. This annealing has been attempted both to reform monocry stall.ine silicon structure, and also to transform amorphous or polycrystalline silicon into a monocrystalline structure following dopant ion implantati - '-.-For-example, U.S. Patent No. 4,151,008 describes a pulsedlaser annealing process which utilizes a neodynium-yag־ laser -־’ beam to effectuate annealing. Such a laser produces a beam which does not directly produce the ultraviolet wavelength so readily absorbed by silicon. Also, the pulse repetition rate achieved with such a laser is relatively low (0-20 pulses per second) limiting the average laser output power, and hence limiting the procesing capability of this type of system. Because of the low repetition rate and infrared wavelength of the yag laser, the annealing process using such a device requires an excessively large input power applied over a prolonged period of time. To achieve an ultra-violet output, an ultraviolet flash lamp was employed in place of the laser. However, the light output produced by the ultraviolet flash lamp is quite difficult to focus in order to achieve the required energy density to effectuate semiconductor annealing. In such an ultraviolet flash lamp system, the energy per pulse achieved in the beam and beam uniformity are both poor. While interesting as a laboratory tool, such a system is not feasible for use in the mass commercial manufacture of solar voltaic cells.U.S. Patent 4,154,625 also deals with the use of alaser in annealing semiconductor devices, and the fabrication of polycrystalline solar cells in particular. This patent suggests lasers in the optical range, but a ruby laser was utilized. The wavelength of a ruby laser is primarily in the red visible and infrared regions, not the ultraviolet where silicon absorbs energy well. Lasers such as this which project beams in the .infrared and visible range are capable of: high peak-power.) but their beam uniformity "has ’’.Jr historically been poor; The energy level obtained with the ruby laser is quite large, but with the poor beam uniformity achieved Large localized temperature variations in the silicon exist. The silicon therefore is melted and reforms to a polycrystalline or monocrystalline structure at some implantation sites while the energy applied is inadequate to induce epitaxialregrowth at adjacent sites. U.S. Patent 4,147,563 utilizesa similar ruby laser to implant impurities in silicon in the manufacture of solar cells, rather than to anneal the silicon following implantation.U.S. Patent 4,059,461 suggests the use of a continuous wave Nd:YAG laser for purposes of annealing silicon in the fabrication of solar voltaic cells. The laser suggested operates at only 6 or 7 watts of power, however, although lasers of larger power output, such as CO or COj having a power output of 100 watts are contemplated. However, such lasers operate primarily in the infrared region. Because of the lower absorption of silicon in this region, a higher average power is necessary to achieve annealing. This excessive power produces excessive heating of the entire silicon wafer, which can distort or damage the entire wafer. Laser scanning is applied to polycrystalline semiconductor material in annealing. However, the laser systems employed according to this patent do not provide sufficient power and do not obtain an acceptable throughput rate. A complete heating and cooling cycle of doped silicon took approximately 10         .־minutes.'^־In continuous wave scanning the beam must be focused to a־ very הtiny point־. This necessitates intricate mechanical•" apparatus to control a scan pattern. Also, in continuous wave annealing epitaxial regrowth is induced in the solid phase, not the liquid phase as in pulsed annealing processes. Consequently, the quality of repair to the damaged lattice sites is not as good in continuous wave annealing as contrastedwith pulsed annealing. Accordingly, such a device could not feasibly be scaled for the commercial manufacture of solar voltaic cells.SUMMARY OF THE INVENTIONAccording to the present invention, there is provided a method of producing solar voltaic cells comprising implanting a dopant material into a photo sensitive semi-conductor substrate to create a P-N junction, and annealing said substrate with a pulsed laser beam having a wavelength predominantly in the ultraviolet region, a beam energy of at least two joules per pulse, a pulse repetition rate of at least 100 pulses per second and a uniformity of beam pulse output that varies by no more than five percent across the area of beam output.The invention may be described with greater clarity and particularly by reference to the accompanying drawings.DESCRIPTION OF THE DRAWINGSFIG. 1 is a plan view diagramatlcally illustrating the manufacture of solar voltaic cells according to the invention.FIG. 2 is an elevational view diagramatlcally illustrating the manufacture of solar voltaic cells according to the invention.FIG. 3 is a :.^perspective view diagramatlcally illustrating the manufacture of solar voltaic cells according to the invention.The application of the laser beam generating device 10 disclosed in said divisional application to the manufacture of solar voltaic cells is illustrated in FIGS. 1, 2 and 3.As described in said divisional application, the pulsed laser outputs occur at a frequency of at least 100 hertz, and more typically one kilohertz. As a result; several hundred watts of average power (from 200 to 500 watts) are transmitted by the laser beam 15. Moreover, because of the uniform triggering at the voltage VT, the amplitude variation of the beam output is. extremely small. Because of the fast rising voltage pulse produced by the׳ rail spark gap 28, and because of the uniform volume pre-ionization induced by the multiple preionization arcs from the electrode pins 24 to the cathode 22, the uniformity of the beam across the beam window 13, varies by only between about 2% and 5%.Referring now to the drawings of the present application, the application of the laser beam generating device 10 disclosed in said •divisional application to the manufacture of solar.voltaic cells is illustrated in FIGS. 1, 2 and 3. In this system, conveyor belt 120 is operated in stepped movement at specific intervals to transport disk-shaped wafers 122 of silicon past an annealing station 'at which the laser generating device 10 scans the wafers 122 with the-laser beam 15. The wafers 122 may be typically about 8 centimeters in diameter and of variable thickness -. Prior to arriving at the annealing station at which they are treated by the laser beam 15 of the laser            .generating device 10, the wafers 122 are first implanted with a suitable dopant, typically boron, phosphorus or arsenic. The dopant may be deposited by one of several conventional techniques. Typical doping techniques are described and referred to in U.S. Patent 4,147,563.FIGS. 1 and 3 illustrate a conventional dopant implantation unit 124 which is used to deposit boron or some other dopant material on the wafers 122 as the conveyor belt 120 moves from left to right as indicated. Following implantation of the dopant material, the silicon wafers 122 move into alignment with the laser beam 15. The laser beam 15 is projected through a cylindrical lens 128 which reshapes the beam from the cross section of the window 13 in the laser generating device 48 to a swath 130 which is approxi-mately four milimeters in thickness by about 8 centimeters in length aligned with the direction of movement of the conveyor belt 120, as illustrated in FIG. 1 . The laser generating device 10 is typically located to the side of the conveyor belt 120 and transmits the beam 15 through the cylindrical lens 128 as the reshaped beam 130 laterally to a tilted mirror 132 which reflects the beam 130 downward onto tne wafers 122 as they pass beneath on the conveyor belt 120־ As illustrated in FIG. 2 , the mirror 132 may be tilted alternatively to scan the rectangular swath 130 across the entire wafer 122 located therebeneath.The beam 15 delivers energy at 1 to 2 joules per square centimeter at a laser repetition rate of 100 hertz. Twoblsquare centimeters of the wafer are therefore treated with each laser pulse so that it requires 50 pulses from the laser generating device 10 to anneal an entire wafer. 122. This requires approximately 1 / 2 second at a laser repetition 5        rate of 100 pulses per second. Consequently, at this repeti irate approximately 2 wafers can be annealed each second,A throughput of approximately 7,000 wafers per hour is there-fore easily achieved. This throughput can be increased by a magnitude of 10 by merely operating the laser generating 1q device 10 at a repetition rate of 1 kilohertz, a rate easily achieved with the device of the invention.'  ;      .           . '                                                  . זי'     . 'As previously noted, wafers 122 formed of monocrystallin silicon may be annealed in this fashion. Also, polycrystalli silicon wafers 122 can each be regrown into monocrystalline !5 structure by annealing with the laser generating device?10. Furthermore, and also as previoulsy noted, amorphous silicon can be deposited as silicon tetrachloride through vapor deposition on saphire or graphite. With the laser generating device 10 of the invention, monocrystallihe silico 2q can be grown upon a disk-shaped saphire or graphite base through the annealing process described herein.Preferably, in treating the silicon wafers 122 to• a P-N junction for use as solar voltaic cells, includecreateannealingthe silicon is annealed with a pulse laser beam havinga wave-length predominantly in the ultraviolet region, a beamenergyof at least 2 joules per puise, a pulse repetition rate of atleast about 100 pulses per second and a uniformity of beam pulse output that varies.by no more than 5 percent across the area of beam output. Preferably, dopant implantation and annealingaccording to the invention using the laser beam generatingdevice 10 are carried out at atmospheric pressure in air. Preferablyalso, the laser beam energy per pulse is from about 2 to about-5 joules and the average power achieved is several hundred watts, typically from about 200 to 500 watts. Of course, as a pulserepetition rate increases from 100 to 1000 pulses per second,theenergy per pulse reduces below 2 joules to as little as 0.2 joules for a 200 watt laser operating at 1000 hertz. Laseruniformity of better than five percent may be achieved with thelaser beam generating device 10 of the invention, and a uniformityof from 2 to 5 percent is typical.The scope of the present invention is not limited to thespecific embodiment and utilizations described and illustratedherein but rather is defined in the claims appended hereto..

Claims

IN THE CLAIMS1. A method of producing solar voltaic cells comprisingimplanting a dopantmaterial into a photo sensitive semi-conductor substrateto create a P-N junction, and annealingsaid substrate witha pulsed laser beam having a wavelengthpredominantly in the ultraviolet region, a beam energy ofat least twojoules per pulse.a pulse repetition rate ofat least 100pulses per secondand a uniformity of beampulse outputthat varies by nomore than five percent acrossthe area of beam output.

2. A method according toClaim 1 further characterizedin that said substrate is silicon in monocrystalline formprior to annealing with said laser beam.

3. A method according to Claim 1 further characterized in that said substrate. is silicon in polycrystalline form prior to annealing with said laser beam..4• .. A. method according to Claim 1 .further characterized in that said substrate is silicon in amorphous form’prior• to annealing with said־laser beam.־                   — •5. A method according to Claim 1 further characterized in that said implantation and annealing of sfcid substrate are carried out at atmospheric pressure in air.

6. A method according to Claim 1 further characterized in that at least about two square centimeters of said doped substrate are subjected to each pulse of said laser beam.

7. A method according to Claim 1 further characterized in that said laser energy per pulse is from two to five joules.

8. A method according to Claim 1 further characterized in that said laser beam uniformity is from two to five percent.

9. A method according to Claim 1 further characterized in that said repetition rate of laser beam pulses is at least about one kilohertz.

10. A method according to Claim 1 further characterized in that the average power of said laser beam output is from two hundred to five hundred watts.

11. A method of producing semi-conductor devices comprising implanting a dopant material into a semi-conductor substrate to create a P-N junction, and annealing said substrate with a pulsed laser beam having a wavelength predominently in the ultraviolet region, a beam energy of at least two joules per second, a pulse repetition rate of at least 100 pulses per second and a uniformity of beam pulse output that varies by no more than five percent across the area of beam output.

12. A method according to Claim 11 in which said semi-conductor substrate is selected from the group consisting of silicon, gallium arsenide, germanium, gallium phosphide, indium phosphide, cadmium telluride, aluminum antimonide, cadmium sulphide and copper oxide.KE. Mt)LFORDAttorneys- for ApplicantsBERNARD B. KATZONE SHEET ONLYTRUE COPYFIG. 3