WO2010037346A1

WO2010037346A1 - Methods and systems of manufacturing photovoltaic devices

Info

Publication number: WO2010037346A1
Application number: PCT/CN2009/074224
Authority: WO
Inventors: Jun Zhu
Original assignee: Changzhou Lasfocus Laser Equipment Co., Ltd.
Priority date: 2008-10-05
Filing date: 2009-09-25
Publication date: 2010-04-08
Also published as: KR20110083641A; CN102113137A

Abstract

A system (200) for fabricating a photovoltaic device including a semiconductor wafer (211) comprises a laser device (202) configured to generate a laser beam (230) and a laser scanning device configured to scan the laser beam (230) on the wafer (211) to locally heat a surface of the wafer (211). The laser scanning device comprises a beam splitting module (205) configured to split the laser beam into at least two sub-beams of equal amount of laser power; at least two scanners (208) configured to steer the sub-beams (232) scanning along a predetermined scan path defined on the wafer (211); and at least two lenses (209) configured to focus the steered sub-beams (232) at a focal point that coincides with the predetermined scan path.

Description

METHODS AND SYSTEMS OF MANUFACTURING PHOTOVOLTAIC DEVICES

TECHNICAL FIELD

[0001] The present application generally relates to the field of photovoltaic devices manufacture, more particularly to methods and systems for manufacturing photovoltaic devices.

TECHNICAL BACKGROUND

[0002] The majority of present day photovoltaic modules are made from silicon wafer-based photovoltaic devices (e.g., solar cells). Cost reduction via increasing the silicon cell efficiency and via using thinner wafers is of primary importance. One of steps in the process of manufacturing silicon wafer-based photovoltaic devices is to metalize contacts by the method of screen-printing. However, as photovoltaic devices manufacturers look to lower cost by improving cell efficiency and thining wafers, the screen-printing process is becoming a limitation. For example, the screen-printed contact technology leads to at least the following disadvantages: (a) high shading loss due to a wide line width (100 - 200μm), which leads to less efficient solar cells; and (b) mechanical pressure applied during screen-printing, which causes an increased yield loss for thin wafers (less than 200μm thickness).

[0003] Further, the photovoltaic devices production is a high throughput process at about 1400 wafers per hour for 125xl25mm wafers. Any new process, system, or method has to meet the high-throughput requirements for photovoltaic devices manufacturing.

SUMMARY

[0004] In one aspect, provided is a system for fabricating a photovoltaic device which includes a semiconductor wafer. The system comprises a laser device configured to generate a laser beam and a laser scanning device configured to scan the laser beam on the wafer to locally heat a surface of the wafer.

[0005] In another aspect, provided is a method for fabricating a photovoltaic device. The photovoltaic device includes a semiconductor wafer having a semiconductor substrate, an emitter layer formed on a surface of the semiconductor substrate, and a dielectric layer covered on a surface of the emitter layer. The method comprises the steps of forming a layer of dopant source material on a surface of the dielectric layer; locally heating a surface of the wafer by a laser beam to define a plurality of openings through the dielectric layer and melt a surface of the substrate underlying the dielectric layer, such that a dopant i contained in the layer of dopant source material diffuses into the melted substrate through the openings so as to form heavily doped zones; and depositing a conductor over the heavily doped zones.

[0006] In further another aspect, provided is a method for processing a wafer by laser scanning. The method comprises the steps of dividing the wafer into at least two regions of equal size; and scanning the regions separately by one or more laser beams.

[0007] In yet another aspect, provided is a method for processing a wafer by laser scanning. The method comprises the steps of moving the wafer in one direction at a constant speed; and steering a laser beam along a scan path in bow-tie pattern to create straight-line patterns on the wafer, wherein a scan range of the laser beam in the wafer moving direction is limited to the pitch of lines.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Fig. 1 is a flow diagram illustrating a method for fabricating photovoltaic devices according to one embodiment of the present application;

[0009] Figs. 2A-2D are schematic views illustrating steps for fabricating photovoltaic devices according to one embodiment of the present application;

[0010] Fig. 3 illustrates a range of interactions between laser beam intensity and surface material of a wafer being processed;

[0011] Fig. 4 is a schematic view illustrating a system for fabricating photovoltaic devices according to a specific embodiment of the present application;

[0012] Fig. 5 is a schematic view illustrating a system for fabricating photovoltaic devices according to another specific embodiment of the present application;

[0013] Figs. 6A-6D are schematic views illustrating a method for processing wafers by laser scanning according to further another embodiment of the present application;

[0014] Fig. 6E is a schematic view illustrating a method for processing wafers by laser scanning according to yet another embodiment of the present application;

[0015] Fig. 6F is a schematic view illustrating a method for processing wafers by laser scanning according to again another embodiment of the present application;

[0016] Figs. 7A-7C are schematic views showing a stitching method for processing wafers by laser scanning according to an alternative embodiment of the present application; and

[0017] Figs. 8A-8B are schematic views showing a method for processing wafers by laser scanning according to another alternative embodiment of the present application. DETAILED DESCRIPTION

[0018] Referring to Fig. 1 and Figs. 2A-2D, in accordance with one aspect of the present application, a method for fabricating photovoltaic devices is proposed. According to this method, a semiconductor (e.g. silicon) wafer having a substrate 100 with an emitter layer 105 is provided, wherein emitter layer 105 is formed by lightly diffusing a dopant into the substrate 100. The polarity of the dopant is opposite to that of a dopant used in the substrate 100 so as to form a p-n junction between the emitter layer 105 and the substrate 100. The upper surface of the emitter layer 105 is subsequently formed with a dielectric layer 101 which may be a passivation layer made of silicon oxide and/or silicon nitride, and may act as an antireflection coating (ARC). The process for forming the emitter layer 105 and the dielectric layer 101 may be carried out by using any available known techniques.

[0019] Subsequently, a layer of dopant source material 104 is formed on the upper surface of the dielectric layer 101 (block 20 in Fig. 1). The polarity of the dopant contained in layer 104 is the same as that of the emitter layer 105. Thereafter, as shown in Fig. 2B, the surface of the wafer is processed by a laser beam to melt the dielectric layer 101 and the substrate in localized regions where metal contacts are to be formed. Specifically, the dielectric layer 101 is opened up to define a plurality of openings so that the dopant contained in layer 104 is able to diffuse into the substrate through the openings. The dopant is trapped in the substrate as the melt substrate cools and recrystallizes. Therefore, a plurality of heavily doped zones is formed in the substrate.

[0020] After the laser-processing step, the layer of dopant source material 104 is removed such as by rinsing the wafer in a solution which is able to dissolve only the layer of dopant source material without damaging the dielectric layer 101, as shown in Fig. 2C. Thereafter, according to Fig. 2D, a metallization step to form metal-to-semiconductor contacts is performed by depositing a conductor 107 to the exposed and heavily doped zones of the substrate, for example, depositing silver, nickel, and/or copper, etc, by methods such as electrolytic or electroless plating, or using a metal-containing paste followed by firing the paste to form the metal contacts (block 40 in Fig. 1). In the metallization step, the dielectric layer 101 may function as a self-aligned mask for metallization of contacts by using the openings of localized melted zones where metal contacts are to be formed.

[0021] Those skilled in the art can understand of needing to maintain the wafer as clean as possible before and after the formation of the layer of dopant source material 104 because contaminants, especially those contaminants larger than several microns in size can scatter and absorb the laser energy, reduce the effectiveness of laser-induced material melting and cause higher resistance at the interface between the metal and the substrate. To this end, it is suggested to minimize the time gap between the layer of dopant source material forming step and the laser-processing step to prevent contaminants from landing on the wafer.

[0022] According to one embodiment of the present application, the intensity of a laser beam focused on the wafer surface should be maintained as high as enabling causing the surface of the substrate melted with little vaporization. The melting of the surface of the substrate leaves very little or no added defects on the wafer. Compared with the conventional laser groove buried contacts (LGBC) method, the laser processing according to the present embodiment does not form significant grooves on the wafer surface. According to a specific embodiment, a surface doping concentration of the heavily doped zones formed by the method of the present application may be 10¹⁹ cm^"3 or higher without substantially adding new defects to the wafer.

[0023] Fig. 3 illustrates interactions between the intensity of a laser beam and the material being processed, provided that the substrate material is silicon. The melting point of silicon is about 1414 ⁰C and its vaporization temperature is about 3217 ⁰C. The threshold of melting and threshold of vaporization of silicon divide the laser beam intensity profile into ranges 112, 113 and 114. Range 112 is a heating range, where the wafer surface is irradiated to a temperature lower than the threshold of melting. Thus, silicon remains in its solid state and the dielectric layer 101 is partially damaged and undesirable heat-affected zones (HAZ) on the wafer surface are formed. Range 113 is a melting range, where the wafer surface is heated to a temperature higher than the threshold of melting but lower than the threshold of vaporization. In range 113, the surface of silicon becomes liquid locally, as indicated by arrow 110. Range 114 is a vaporization range, where the wafer surface is heated to a temperature higher than the threshold of vaporization and the surface of silicon vaporizes locally, as indicated by arrow 111. Vaporization may decrease the surface doping concentration of the substrate, create defects on the wafer surface, and punch through the doped layer and therefore cause an electrical shunting between metal contacts and the substrate. Therefore, the intensity of the laser beam focused on the wafer surface should be maintained to heat the wafer surface to a temperature higher than the threshold of melting of silicon but lower than the threshold of vaporization of silicon. Fig. 3 shows a laser beam intensity profile, wherein the tails of the laser beam intensity profile may be truncated to minimize the HAZ effect and the top of the laser beam intensity profile may be flattened to avoid the vaporization of silicon.

[0024] Further, as the laser beam is scanned to heat the wafer surface, a spot size (D) of the laser beam focused on the wafer surface determines a size of each opening in the dielectric layer. To obtain a small spot size, the laser beam may be selected to be a TEMOO mode. The size of the opening therefore realized is typically in the range of 10 to 25 microns. These dimensions are well below the sizes typically achievable by the conventional screen-printing method.

[0025] Although the method is described above with specific reference to the production of frontside contacts, the methods described herein may also be used to produce backside contacts in a highly efficient manner.

[0026] In accordance with another aspect of the present application, a system for fabricating a photovoltaic device includes a laser device configured to generate a laser beam and a laser scanning device configured to scan the laser beam over a surface of a target object (e.g., a semiconductor wafer). In accordance with one embodiment of the present application, the system may be configured to fabricate photovoltaic devices by performing the method described above.

[0027] As shown in Fig. 4, a system 200 is provided for fabricating photovoltaic devices, including a laser device 202 configured to generate a laser beam 230 and a laser scanning device comprising a beam splitting module 205, scanners 208 and lenses 209. The laser beam 230 generated from the laser device 202 is split by the beam splitting module 205 into multiple sub-beams with substantially the same intensity. Fig. 4 exemplifies two sub-beams 232 for the sake of simplifying the illustration. Each of the sub-beams may be steered by a corresponding scanner 208 to scan along a predetermined scan path defined on the wafer. A set of lens 209 may be provided to focus the sub-beam to the surface of the wafer. According to the system as shown in Fig. 4, two or more wafers can be processed in parallel by the multiple sub-beams 233 to achieve high system throughput.

[0028] The laser device 202 may be a quasi-continuous wave (QCW) laser whose pulse repetition rate may be about 1 MHz or higher. According to one example of the present application, the QCW laser is a picosecond laser. In a specific example, a pulse width of the picosecond laser is about 15 picosecond. The laser device 202 may be a continuous wave (CW) laser. In an exemplary example, the laser device 202 generates a laser beam with (a) a wavelength of about 532nm or less; (b) a beam mode of TEM00; (c) a beam quality factor M² of about 1.3 or less; and (d) a beam output diameter of at least about lmm and preferably

2mm or more. The laser device 202 may be an UV laser or a Green laser.

[0029] The scanner 208 may be a two-dimensional (2-D) XY or a 3-D XYZ galvo scanner and may scan the laser beam at a speed of about V = 2000 to 3000 mm/s at the wafer surface. A 3-D XYZ scanner may not only scan a laser in XY plane but also control focus height Z in a real-time manner at the wafer surface. The lens 209 may be an f-theta or telecentric scan lens. A diameter of the laser beam 232 at an entrance pupil of the scanner 208 may be about 8mm to 16mm. For a pulsed quasi-CW laser, the scanner scans the laser beam to cause the overlap (η) of pulses in an amount of about 75 to 99.99%. Generally, a f V repetition rate (^{J rep} ) should satisfy the relationship of /^ > , where D is the laser

beam spot size focused on the wafer surface. In an exemplary example, the beam spot size (Z)) is between about lOμm and about 50μm, and an average laser power density at the wafer surface is between about 2MW/cm and about 20MW/cm .

[0030] As mentioned above, to melt the wafer surface, the intensity of the laser beam focused on the wafer surface should be maintained to heat the wafer surface to a temperature higher than the threshold of melting of the substrate material. That is, the intensity of the laser beam should higher than an intensity threshold P_nrahoU ■ However, since the laser beam 230 is split into a plurality of sub-beams 232, a laser power starvation for the system 200 as shown in Fig. 4 may occur. It is known that the relationship between the laser beam spot size (D) focused on the wafer surface and a focal length of an lens is D <x / . Therefore the laser intensity threshold P_nreshotd at the wafer surface is characterized by ^_Threshold ^∞ D^{2 or} ^_Threshold ^∞ f² ■ Accordingly, it is suggested to keep the focal length of the lens 209 short or P_nreshold low to avoid the laser power starvation situation. However, as discussed above, to heat the wafer surface locally to the melting temperature of the substrate material, the threshold P_nreshM should not be low. Therefore, in order to avoid laser power starvation situation, it is suggested to keep the focal length of lens 209 short. On the other hand, if the focal length is too short, the optical aberration of the system may become severe. In the circumstances, the quality of scan generally degrades as the laser beam moves away from a center area of a scan field of the system. Therefore, the uniformity of the processing may degrade due to the practical hardware limitations. There is a balance between keeping the focal length short and still achieving uniform processing results. According to one embodiment of the present application, the focal length of the lens is between about 160mm and about 300mm. According to a specific example of the present application, the focal length of the lens is 250mm.

[0031] Fig. 5 illustrates a system for fabricating photovoltaic devices according to another specific embodiment of the present application. As shown in Fig. 5, besides the components as shown in Fig. 4, the system 200 may further comprise a beam expander 204 configured to expand the laser beam and an attenuator 206 configured to attenuate the power of the laser beam to a desired power level. The laser beam 230 may be expanded by the beam expander 204 before being split by the beam splitting module 205 and may be attenuated by the attenuator 206 after passing through the beam splitting module 205. The system 200 may also comprise a safety shutter 203 to selectively shut off the laser beam and/or a laser power meter 207 for monitoring the power of the laser beam.

[0032] Still referring to Fig.5, the system 200 may comprise a laser mask 210 configured to limit a scanning range of the laser beam. The laser mask 210 is able to prevent the laser beam from hitting the regions on the wafer where should not be melted along the laser beam scanning path, such as a periphery region of the wafer along a turnaround path of the laser beam when the laser beam is steered from a scanning line to a next scanning line. The laser mask 210 may be of a simple rectangle opening or more complex designs. According to one exemplary example, the mask is mounted sufficiently close to the wafer to clearly define ends of scan lines. Vertical distance between the mask and the wafer may be less than 5mm. The wafers 211 may be loaded on a stage 220 with vacuum chucks to substantially flatten the wafers 211 for laser processing. The stage 220 may be a movable stage such as an X-stage and/or a Y-stage and/or a Z-stage and/or a rotary stage. Although it is not shown in Fig. 5, the components of the system 200 such as the laser device 202, the scanner 208, etc., excluding the laser masks 210, may be mounted on the same rigid and thermally stable plate such as a MIC-6 aluminum plate or a granite plate.

[0033] For the purpose of illustration, the beam splitting module 205 and the components disposed upstream from the beam splitting module 205 in the system 200 are referred to as a main beam processing portion, and the components disposed downstream from the beam splitting module 205 in the system 200 are referred to as a sub-beam processing portion. In another configuration of the system of Fig. 5, the expander 204 may be included in the sub-beam processing portion instead of the main beam processing portion. In another configuration of the system of Fig. 5, the expanding of the laser beam 230 may be achieved in multiple stages. For example, more than one beam expanders of same or different magnifications may be used. In another configuration of the system of Fig. 5, if a two-stage expander is used, one or both of the expander stages may be included in the sub-beam processing portion instead of the main beam processing portion. In another configuration of the system of Fig. 5, the laser power meter 207 may be disposed in different locations. For example, the laser power meter 207 may be disposed between the lens 209 and the wafer 211. In another configuration of the system of Fig. 5, the sub-beam processing portion may include only one laser power meter 207. Alternatively, the laser power meter 207 may be moved out of optical paths during wafer processing. In another configuration of the system of Fig. 5, the attenuator 6 may be included in the main beam processing portion instead of the sub-beam processing portion. Additional optical components such as turning mirrors may be used to change the direction of the laser beam. [0034] In accordance with a specific embodiment of the present application, the laser beam may be split into three or more sub-beams of about equal laser power to achieve higher throughput.

[0035] In accordance with another specific embodiment of the present application, the system may have no beam splitting module.

[0036] In accordance with another specific embodiment of the present application, optical paths in the system may be enclosed and purged with clean dry air (CDA) or nitrogen gas (N₂) to keep away the ambient contaminants.

[0037] In accordance with another specific embodiment of the present application, the system may include a debris removal mechanism to keep the system clean and the optics such as lens 209 substantially free of contaminants, especially those contaminants generated from laser processing of the wafer.

[0038] In accordance with another specific embodiment of the present application, the system may include a retaining and trapping mechanism to collect and trap the excess of dopant chemicals from the wafers 211 to prevent corrosions in the system.

[0039] In accordance with another specific embodiment of the present application, the polarization of the laser beam may be controlled using an adjustable polarizer such as a quarter wave plate or a half wave plate in the optical paths.

[0040] For large scale wafers such as industry standard 125x125mm, 156x156mm or 210x210mm wafers, there are two challenges in maintaining the required process window of present invention over the entire wafer: (a) the laser beam spot size or the intensity of a laser beam at the wafer surface generally degrades as the beam moves away from the center area of the scan field of the system due to practical hardware limitations of the lens 209 and the scanner 208, hence the system has a limited "sweet spot" area in the center of the scan field that provides for best material processing results; and (b) a desired short focal length (J) of the present invention combined with practical hardware limitations such as limited scan angle of the scanner 208 may make it difficult to process a whole wafer by one scanning laser beam. The methods below are directed towards addressing above two issues so that the processing window of present invention is maintained over an entire area of large wafers. The methods described below can be combined with the system of the present application to process wafers.

[0041] Figs. 6A-6D are schematic views illustrating a method for processing wafers by laser scanning according to further another embodiment of the present application. Referring to Figs. 6A-6D, a large wafer is divided into four regions (region Rl, R2, R3 and R4) of substantially equal size. The number of regions in Figs. 6A-6D is chosen only for the purpose of illustration, and the wafer may be divided into different number of regions of substantially equal size for processing. The wafer may undergo a laser-material processing region-by-region consecutively under the "sweet spot" area of the scan field by the laser beam 233. Any necessary wafer movement may be assisted by a movable stage. The mask 210 may be employed to prevent the laser beam from hitting the regions on the wafer where should not be processed along the laser beam scanning path.

[0042] Fig. 6E is a schematic view illustrating a method for processing wafers by laser scanning according to yet another embodiment of the present application. Referring to Fig. 6E, the wafer is still divided into four regions for the purpose of illustration. However, the wafer is processed by four individual laser beams. All four regions of the wafer are processed after the wafer goes through all four individual processing stations. The wafer 211 may be divided into different number of regions of substantially equal size for processing, and the number of laser beams for wafer processing may be different as well. For example, a wafer may be divided into six regions of substantial equal size and processed by two or three or six laser beams.

[0043] Fig. 6F is a schematic view illustrating a method for processing wafers by laser scanning according to again another embodiment of the present application. Referring to Fig. 6F, each of the wafers is still divided into four regions of substantial equal size for illustrative purpose, and is transported by a rotary stage 220. The multiple regions on the wafer may be processed by same number of individual laser beams to optimize throughput. According to an example of implementation as shown in Fig. 6F, the rotary stage 220 with six positions Pl, P2, P3, P4, P5, and P6 is able to transport wafers so as to complete the laser processing of four regions Rl, R2, R3, and R4 on the wafer. The six positions Pl, P2, P3, P4, P5, and P6 may be symmetrically and circumferentially located about a central axis of the stage 220. The wafers 211 may be respectively secured on the positions via vacuum chucks. The stage 220 may rotate in an increment of 60 degrees to move wafers from one position to next and be kept stationary in each position for a period of time. A wafer 211 is processed by four individual laser beams when it is in positions P2, P3, P4, and P5. A process will be described as below by referring to the example shown in Fig. 6F:

[0044] After a 1^st wafer is loaded at the position Pl, the stage 44 rotates 60 degrees to move the 1^st wafer from the position Pl to the position P2 and stops. Then a region Rl of the 1^st wafer is processed by a laser beam while the stage 44 is stationary. In the meantime, a 2^nd wafer is loaded at the position Pl . After the region Rl of the 1^st wafer has been processed, the stage 44 rotates another 60 degrees in a same direction and advances the 1^st wafer to the position P3 from P2 and advances the 2^nd wafer to the position P2 from Pl . A region R2 of the 1^st wafer is now processed by a laser beam at the position P3 while a region Rl of the 2^nd wafer is processed by another laser beam at the position P2.

[0045] The above step repeats until the 1^st wafer has gone through the laser processing at positions P2, P3, P4, and P5 by four individual laser beams. With this wafer processing method, the wafer is fully processed after going through the positions sequentially and may be unloaded at the position P6.

[0046] According to the example shown in Fig. 6F, the number of laser beams equals to the number of regions to be processed on a wafer to optimize the throughput. In another example, only one laser beam is used to process a wafer with multiple regions. In the circumstances, additional time and movement are required to move the laser beam or the wafer in order to process all the regions of the entire wafer.

[0047] Still referring to Fig. 6F, wafer load and unload may be taking place during laser processing time, therefore no extra delay is introduced. In addition, by combining the rotary stage 220 with four individual laser-processing positions P2, P3, P4, and P5, the system enables high throughput with single wafer load and unload operation. Moreover, positions Pl and P6 may be physically next to each other, thus only one wafer handling apparatus is necessary to load and unload wafers and the required hardware is minimized.

[0048] According to another example, wafer load and unload positions Pl and P6 may be combined into one single position if the wafer handling apparatus is fast enough to exchange wafers within the laser-processing time. In that case there will be only five wafer positions on the rotary stage 220 and the rotation step size is 72 degrees instead of 60 degrees.

[0049] Still referring to Fig. 6F, each of the four individual laser beams may scan at a different region (Rl, R2, R3, and R4) of the wafer, the order of which may be different from what is shown in Fig. 6F, so long as all four regions of the wafer are fully processed after going through all wafer positions on the rotary stage 220.

[0050] Still referring to Fig. 6F, the direction of rotation of the stage 220 may be clockwise or counterclockwise.

[0051] Still referring to Fig. 6F, the wafers 211 may also be divided into different number of regions of substantially equal size for processing. The number of the laser beams for wafer processing may be different from the number of regions on the wafer.

[0052] Figs. 7A-7C are schematic views showing a stitching method for processing wafers by laser scanning according to an alternative embodiment of the present application. Referring to Figs. 7A-7C, when a wafer is divided into multiple regions for region-by-region laser processing, stitching is generally required to maintain continuity of patterns (such as metal contact patterns) generated by the laser scanning on the wafer. Referring now to Figs. 7A-7C, there is shown the frontside metal contact patterns of a wafer 211 and an exemplary example of the method of stitching. To create metal contact patterns of fingers 241 and busbars 242, the wafer 11 is divided into four regions (region Rl, R2, R3 and R4) by lines 243. The lines 243 also indicate the overlapping areas between neighboring regions, where the patterns may not exactly align due to inherent system misalignment errors. Between regions R2 and R3 the fingers 241 are stitched together in the shape of "X" according to Fig. 7B. Between regions R3 and R4 the busbars 242 are stitched together in the shape of "X" according to Fig. 7C. The dimensions of the stitching shape "X" may be minimized through the choice of system hardware, alignment, and calibration. The stitching shape is not limited to just the shape of "X", other stitching shapes may be used as long as the alignment errors of the patterns between neighboring regions are sufficiently accounted for. According to another embodiment, the stitching processing may not be needed if the neighboring patterns lines are aligned to within +/-10μm errors since the metallization process is tolerant to such small misalignment errors.

[0053] Still referring to Figs. 7A-7C, an alternative embodiment of the stitching method is to stitch the busbars 242 but not the fingers 241 in the overlapping areas since the fingers 241 have already continuous electrical paths to the busbars 242. If necessary, more than two busbars 242 should be created on the wafer 211.

[0054] Figs. 8A-8B are schematic views showing a method for processing wafers by laser scanning according to another alternative embodiment of the present application. According to the method as shown in Figs. 8A-8B, the wafer is moved at a substantially constant speed during laser processing time instead of remaining stationary. In the circumstances, the laser beam 233 may be steered along scan paths 240A, 240B, 240C, and 240D in "bow-tie" pattern repeatedly to create straight-line patterns on the wafer 211. Line patterns are written on the wafer 211 when the laser beam is steered along paths 240A and 240C, and the laser beam are brought back along the path 240B or 240D to a starting position of next line. Along the path 240A or 240C, in addition to the straight line scan motion the laser beam 233 is steered slightly forward to track the wafer speed so as to leave straight lines on the wafer 211. The mask 210 may be used to prevent the laser beam from hitting the wafer along its turnaround paths 240B and 240D. As the laser beam is steered in the "bow-tie" scan pattern to create straight lines on the wafer 211, the scan range in wafer travel direction is limited to about the pitch of lines, which is an advantage for the system of limited "sweet spot" area. [0055] Further, the method as shown in Figs. 8A-8B is not restricted to form line patterns. For example, matrix of dots may be created with the help of a mask that can block or pass the laser beam at preprogrammed locations to write patterns on the wafer.

[0056] Still further, the method as shown in Figs. 8A-8B may be combined with the method of Figs. 6A-6D and Fig. 6E to process wafers. For example, a wafer is divided into a plurality of regions of substantially equal areas for processing and each region may be processed by the method of Figs. 8A-8B.

[0057] In broad embodiment, the present application provides for methods and systems for fabricating photovoltaic device at high throughput for production use. The methods and systems may apply to not only frontside but also backside metal contacts formation on wafers.

[0058] While the present invention has been illustrated by the above description and embodiments or implementations, it is not intended to restrict or in any way limit the scope of the appended claims thereto.

Claims

What is claimed is:

1. A system for fabricating a photovoltaic device, the photovoltaic device including a semiconductor wafer, the system comprising: a laser device configured to generate a laser beam; and a laser scanning device configured to scan the laser beam on the wafer to locally heat a surface of the wafer.

2. The system according to claim 1, wherein the wafer has a substrate and a dielectric layer formed on the substrate, and the laser scanning device is further configured to heat a surface of the wafer so as to define openings through the dielectric layer and melt a surface of the substrate underlying the dielectric layer.

3. The system according to claim 2, wherein the photovoltaic device further includes a layer of dopant source material formed on a surface of the dielectric layer, and the laser scanning device is further configured to heat the surface of the wafer such that a dopant contained in the layer of dopant source material diffuses into the melted substrate through the openings so as to form heavily doped zones in the substrate.

4. The system according to any one of claims 1-3, wherein the laser scanning device comprises: a beam splitting module configured to split the laser beam into at least two sub-beams of equal amount of laser power; at least two scanners configured to steer the sub-beams scanning along a predetermined scan path defined on the wafer; and at least two lenses configured to focus the steered sub-beams at a focal point that coincides with the predetermined scan path.

5. The system according to claim 4, wherein each of the scanners is configured to steer each of the sub-beams.

6. The system according to claim 4, wherein each of the lenses is configured to focus each of the sub-beams.

7. The system according to claim 1, further comprising a laser mask configured to prevent the laser beam from scanning regions of the wafer where need not to be heated.

8. The system according to claim 1, wherein the laser device is a quasi-continuous wave laser or a continuous wave laser.

9. The system according to claim 1, wherein the laser device is a quasi-continuous wave laser with a pulse repetition rate of IMHz or higher.

10. The system according to claim 1, wherein the laser device is a picosecond laser.

11. The system according to claim 1, wherein the laser device has a wavelength of about 532nm or less.

12. The system according to claim 1, wherein a beam quality factor of the laser beam is 1.3 or less.

13. The system according to claim 1, wherein a profile of the laser beam generated by the laser device is of TEMOO mode.

14. The system according to claim 1, wherein a profile of the laser beam generated by the laser device is of TEMOO mode such that a spot size of the laser beam focused on the wafer surface is between lOμm and 50μm.

15. The system according to claim 1, wherein a scanning speed of the laser beam is between 2000mm/s and 3000mm/s.

16. The system according to claim 1, wherein an average laser power density of the laser beam is between 2MW/cm² and 20MW/cm².

17. The system according to claim 4, wherein an intensity of each sub-beam focused on the wafer surface is maintained to heat the wafer surface to a temperature higher than a melting threshold of the material of the wafer and lower than a vaporization threshold of the material of the wafer.

18. The system according to claim 17, wherein a focal length of the lens is configured to be short enough to ensure that the intensity of each sub-beam focused on the wafer surface is maintained to heat the wafer surface to a temperature higher than the melting threshold of the material of the wafer.

19. The system according to claim 18, wherein the focal length of the lens is between 160mm and 300mm.

20. The system according to claim 18, wherein the focal length of the lens is 250mm.

21. The system according to claim 4, wherein the wafer is divided into at least two regions of equal size, and the laser scanning device is configured to scan the regions separately by the sub-beams.

22. The system according to claim 21, wherein the wafer is divided into four regions of equal size.

23. The system according to claim 21, wherein the wafer is scanned region-by-region consecutively by one sub-beam.

24. The system according to claim 21, wherein each of the regions of one wafer is scanned by a respective sub-beam.

25. The system according to claim 21, wherein the laser scanning device is configured to steer the sub-beams to stitch misaligned scan paths in a same line of neighboring regions on one wafer.

26. The system according to claim 25, wherein the paths are stitched together in a shape of "X" at an overlapping area between the neighboring regions.

27. The system according to claim 1, wherein the wafer remains stationary when it is scanned by the laser beam.

28. The system according to claim 1, wherein the wafer is moved in one direction at a constant speed when it is scanned by the laser beam.

29. The system according to claim 28, wherein the laser scanning device is configured to steer the laser beam along a scan path in a bow-tie pattern so as to create straight-line patterns on the wafer.

30. The system according to claim 4, further comprises a stage configured to load wafers.

31. The system according to claim 30, wherein the stage is a rotary stage including a plurality of positions circumferentially disposed around a central axis of the stage, each of the positions is configured to load a wafer and the stage is configured to be rotated to move a corresponding one of the wafers from one position to next.

32. The system according to claim 31, wherein each of the wafers is divided into at least two regions of equal size, the number of regions on one wafer equals to the number of the sub-beams, each of the regions on one wafer is scanned by a respective sub-beam.

33. The system according to claim 32, wherein each wafer is loaded at a start position and unloaded at an end position, and all regions on the wafer are scanned by the sub-beams when the wafer is unloaded.

34. The system according to claim 4, further comprises a debris removal mechanism configured to keep the system free of contaminants.

35. A method for fabricating a photovoltaic device, the photovoltaic device including a semiconductor wafer having a semiconductor substrate, an emitter layer formed on a surface of the semiconductor substrate, and a dielectric layer covered on a surface of the emitter layer, the method comprising: forming a layer of dopant source material on a surface of the dielectric layer; locally heating a surface of the wafer by a laser beam to define a plurality of openings through the dielectric layer and melt a surface of the substrate underlying the dielectric layer, such that a dopant contained in the layer of dopant source material diffuses into the melted substrate through the openings so as to form heavily doped zones; and depositing a conductor over the heavily doped zones.

36. The method according to claim 35, further comprising a step of removing the layer of dopant source material after the locally heating step.

37. The method according to claim 35, wherein the dielectric layer is configured to also act as an antireflection layer.

38. The method according to claim 35, wherein the locally heating step comprises maintaining an intensity of the laser beam to heat the wafer surface to a temperature higher than a melting threshold of the material of the wafer and lower than a vaporization threshold of the material of the wafer.

39. The method according to claim 35, wherein a surface doping concentration of the heavily doped zones is 10¹⁹ cm^"3 or higher.

40. The method according to claim 35, wherein a profile of the laser beam is of TEMOO mode.

41. A method for processing a wafer by laser scanning, including: dividing the wafer into at least two regions of equal size; and scanning the regions separately by one or more laser beams.

42. The method according to claim 41, wherein the wafer is scanned region-by-region consecutively by one laser beam.

43. The method according to claim 41, wherein each of the regions of one wafer is scanned by a respective laser beam.

44. The method according to claim 41 , further including: stitching misaligned scanning patterns in a same line of neighboring regions at an overlapping area between the neighboring regions.

45. The method according to claim 44, wherein the patterns are stitched together in a shape of "X" at the overlapping area between the neighboring regions.

46. A method for processing a wafer by laser scanning, including: moving the wafer in one direction at a constant speed; and steering a laser beam along a scan path in bow-tie pattern to create straight-line patterns on the wafer, wherein a scan range of the laser beam in the wafer moving direction is limited to the pitch of lines.