Patent Application
20070169806
This invention relates to the conversion of light irradiation to electrical energy, more particularly, to methods and tools for producing photovoltaic devices (solar cells) that convert solar energy to electrical energy.
Solar cells are typically photovoltaic devices that convert sunlight directly into electricity. Solar cells typically include a semiconductor (e.g., silicon) that absorbs light irradiation (e.g., sunlight) in a way that creates free electrons, which in turn are caused to flow in the presence of a built-in field to create direct current (DC) power. The DC power generated by several PV cells may be collected on a grid placed on the cell. Current from multiple PV cells is then combined by series and parallel combinations into higher currents and voltages. The DC power thus collected may then be sent over wires, often many dozens or even hundreds of wires.
The state of the art for metallizing silicon solar cells for terrestrial deployment is screen printing. Screen printing has been used for decades, but as cell manufacturers look to improve cell efficiency and lower cost by going to thinner wafers, the screen printing process is becoming a limitation. The screen printers run at a rate of about 1800 wafers per hour and the screens last about 5000 wafers. The failure mode often involves screen and wafer breakage. This means that the tools go down every couple of hours, and require frequent operator intervention. Moreover, the printed features are limited to about 100 microns, and the material set is limited largely to silver and aluminum metallizations.
The desired but largely unavailable features in a wafer-processing tool for making solar cells are as follows: (a) never breaks a wafer—e.g. non contact; (b) one second processing time (i.e., 3600 wafers/hour); (c) large process window; and (d) 24/7 operation other than scheduled maintenance less than one time per week. The desired but largely unavailable features in a low-cost metal semiconductor contact for solar cells are as follows: (a) Minimal contact area—to avoid surface recombination; (b) Shallow contact depth—to avoid shunting or otherwise damaging the cell's pn junction; (c) Low contact resistance to lightly doped silicon; and (d) High aspect metal features (for front contacts to avoid grid shading while providing low resistance to current flow).
Given the above set of desired features, the tool set for the next generation solar cell processing line is expected to look very different from screen printing. Since screen printing is an inherently low resolution contact method, it is unlikely to satisfy all of the criteria listed above. Solar cell fabrication is an inherently simple process with tremendous cost constraints. All of the printing that is done on most solar cells is directed at contacting and metallizing the emitter and base portions of the cell. The metallization process can be described in three steps, (1) opening a contact through the surface passivation, (2) making an electrical contact to the underlying silicon along with a robust mechanical contact to the solar cell and (3) providing a conducting path away from the contact.
Currently, the silver pastes used by the solar industry consist of a mixture of silver particles and a glass frit in an organic vehicle. Upon heating, the organic vehicle decomposes and the glass frit softens and then dissolves the surface passivation layer creating a pathway for silicon to reach the silver. The surface passivation, which may also serve as an anti-reflection coating, is an essential part of the cell that needs to cover the cell in all but the electrical contact areas. The glass frit approach to opening contacts has the advantage that no separate process step is needed to open the passivation. The paste mixture is screened onto the wafer, and when the wafer is fired, a multitude of random point contacts are made under the silver pattern. Moreover, the upper portions of the paste densify into a metal thick film that carries current from the cell. These films form the gridlines on the wafer's front-side, and the base contact on the wafer's backside. The silver is also a surface to which the tabs that connect to adjacent cells can be soldered. A disadvantage of the frit paste approach is that the emitter (sun-exposed surface) must be heavily doped otherwise the silver cannot make good electrical contact to the silicon. The heavy doping kills the minority carrier lifetime in the top portion of the cell. This limits the blue response of the cell as well as its overall efficiency.
In the conventional screen printing approach to metallizing solar cells, a squeegee presses a paste through a mesh with an emulsion pattern that is held over the wafer. Feature placement accuracy is limited by factors such as screen warpage and stretching. The feature size is limited by the feature sizes of the screen and the rheology of the paste. Feature sizes below 100 microns are difficult to achieve, and as wafers become larger, accurate feature placement and registration becomes more difficult. Because it is difficult to precisely register one screen printed pattern with another screen printed pattern, most solar cell processes avoid registering multiple process steps through methods like the one described above in which contacts are both opened and metallized as the glass frit in the silver paste dissolves the nitride passivation. This method has numerous drawbacks however. Already mentioned is the heavy doping required for the emitter. Another problem is a narrow process window. The thermal cycle that fires the gridline must also burn through the silicon nitride to provide electrical contact between the silicon and the silver without allowing the silver to shunt or otherwise damage the junction. This severely limits the process time and the temperature window to a temperature band on the order of 10 degrees C. about a set point of 850 C and a process time of on the order of 30 seconds. However, if one can form a contact opening and register metallization of the desired type, a lower contact resistance can be achieved with a wider process margin.
The most common photovoltaic device cell design in production today is the front surface contact cell, which includes a set of gridlines on the front surface of the substrate that make contact with the underlying cell's emitter. Ever since the first silicon solar cell was fabricated over 50 years ago, it has been a popular sport to estimate the highest achievable conversion efficiency of such a cell. At one terrestrial sun, this so-called limit efficiency is now firmly established at about 29% (see Richard M. Swanson, “APPROACHING THE 29% LIMIT EFFICIENCY OF SILICON SOLAR CELLS” 31s IEEE Photovoltaic Specialists Conference 2005). Laboratory cells have reached 25%. Only recently have commercial cells achieved a level of 20% efficiency. One successful approach to making photovoltaic devices with greater than 20% efficiency has been the development of backside contact cells. Backside contact cells utilize localized contacts that are distributed throughout p and n regions formed on the backside surface of the device wafer (i.e., the side facing away from the sun) to collect current from the cell. Small contact openings finely distributed on the wafer not only limit recombination but also reduce resistive losses by serving to limit the distance carriers must travel in the relatively less conductive semiconductor in order to reach the better conducting metal lines.
One route to further improvement is to reduce the effect of carrier recombination at the metal semiconductor interface in the localized contacts. This can be achieved by limiting the metal-semiconductor contact area to only that which is needed to extract current. Unfortunately, the contact sizes that are readily produced by low-cost manufacturing methods, such a screen printing, are larger than needed. Screen printing is capable of producing features that are on the order of 100 microns in size. However, features on the order of 10 microns or smaller can suffice for extracting current. For a given density of holes, such size reduction will reduce the total metal-semiconductor interface area, and its associated carrier recombination, by a factor of 100.
The continual drive to lower the manufacturing cost of solar power makes it preferable to eliminate as many processing steps as possible from the cell fabrication sequence. As described in US Published Application No. US20040200520 A1 by SunPower Corporation, typically, the current openings are formed by first depositing a resist mask onto the wafer, dipping the wafer into an etchant, such a hydrofluoric acid to etch through the oxide passivation on the wafer, rinsing the wafer, drying the wafer, stripping off the resist mask, rinsing the wafer and drying the wafer.
What is needed is a method and processing system for producing photovoltaic devices (solar cells) that overcomes the deficiencies of the conventional approach described above by both reducing the manufacturing costs and complexity, and improving the operating efficiency of the resulting photovoltaic devices.
The present invention is directed to methods and systems (tools) for processing semiconductor wafers in the production of photovoltaic devices (i.e., solar cells) in which a non-contact patterning apparatus (e.g., a laser-based or particle beam patterning system) is utilized to define contact openings through a blanket passivation layer to expose doped portions of the underlying wafer, and then a direct-write metallization apparatus (e.g., an inkjet-type printing apparatus or an extrusion-type deposition apparatus) is utilized to immediately after patterning to deposit contact material and optional metallization into each of the contact openings. By utilizing a non-contact patterning apparatus to define the contact openings, the present invention facilitates the formation of smaller openings with higher precision, thus enabling the production of an improved metal semiconductor contact structure with lower contact resistance and a more optimal distribution of contacts. By utilizing a direct-write metallization apparatus to immediately print contact structures into the contact openings and, optionally, conductive lines on the passivation layer that join the contact structures to form the device's metallization (current carrying conductive lines), the present invention provides a highly efficient and accurate method for performing the metallization process in a way that minimizes wafer oxidation. This invention thus both streamlines and improves the manufacturing process, thereby reducing the overall manufacturing cost and improving the operating efficiency of the resulting photovoltaic devices.
In accordance with an embodiment of the present invention, a laser-based ablation device is utilized to pattern the passivation layer. The laser-based ablation device generates laser pulses that have sufficient energy to ablate (remove) portions of the passivation layer in a way that forms contact openings without the need for cleaning (e.g., rising and drying) the passivation surface or other processing prior to metallization, thus increasing production through-put and yields by avoiding the need for wafer handling between patterning and metallization. The contact openings generated by laser-based ablation devices are substantially smaller than the minimum openings produced by conventional screen printing processes. The laser-based ablation device also facilitates removal of the passivation without significantly altering the thickness or doping profile of the underlying silicon layer. In a specific embodiment, the laser-based ablation device is a femtosecond laser, which facilitates shallow ablation with a minimum of debris. A particular advantage of femtosecond laser pulses is that the power density can be sufficiently high that the electric field of the optical pulse becomes comparable to the inter-atomic fields of the atoms in the material. This becomes important in the present application because it is desired to ablate the passivation without disturbing the underlying semiconductor. The passivation is typically a nitride or oxide layer and as such has a large band gap and it typically transparent. Ordinarily, light would pass through the passivation and become adsorbed by the underlying semiconductor. With sufficiently high power density, the interaction of light with matter alters such that even ordinarily transparent materials become adsorbing. Multiple photons can be adsorbed on a site in the material before the excited electronic states can relax. By adsorbing energy in the dielectric passivation, that surface layer can be selectively ablated. For a photovoltaic device with a shallow layer of dopants, this selective surface ablation is advantageous. The n-type emitter of a typical screen printed solar cell for example is only about 200 to 300 nm thick. If an ablated contact opening in the passivation were to extend through the emitter, then the metallization could form a shunt to the p-type material below the emitter, ruining the device.
In a specific embodiment, a front surface contact cell-type device is produced using a laser-based ablation device such that the laser pulses are directed across the passivation using a rotating mirror-type scanning apparatus. In this embodiment, the predetermined scan pattern defined by a main scanning direction of the rotating mirror is perpendicular to the subsequently formed grid lines of the front surface contact cell device, thereby maximizing the contact opening placement accuracy. The precise control of the timing of the laser pulses is used to place the ablated contacts at the desired locations.
In accordance with another embodiment of the present invention, an inkjet-type printing apparatus is utilized to deposit contact material and/or conductive material into each of the contact openings. Inkjet-type printing apparatus provide a highly accurate and efficient mechanism for performing the required deposition, and also provides an advantage over conventional methods by allowing the accurate deposition of two or more materials into each contact opening. In one embodiment, the contact material is a silicide-forming metal (e.g., nickel) that facilitates both low resistance contact to the underlying silicon, and also minimizes diffusion into the silicon, thus enabling lighter wafer doping than is possible using conventional silver-frit-based pastes. After the contact material is deposited into the contact openings, a highly conductive metal (e.g., copper) is printed on top of the contact material and over the passivation material, thereby forming highly conductive current-carrying metal lines that are coupled to the underlying silicon wafer by way of the low resistance contact portions.
In accordance with another embodiment of the present invention, an extrusion-type dispensing apparatus is utilized to deposit the contact material and/or conductive (metal line) material into the contact openings or over the passivation surface. In one embodiment, grid lines for a front surface contact cell-type device include a high aspect extruded metal line supported on each side by a co-extruded transparent material. In another embodiment, one or more contact materials are co-extruded below the metal line material. In another embodiment, a solder wetting material is also co-extruded over the metal line material.
In accordance with another embodiment of the present invention, two or more direct-write metallization apparatus are utilized in sequence to provide a multilayer metallization structure. In one embodiment, an inkjet-type printing apparatus is utilized to print relatively thin contact material portions into each contact opening, and an extrusion-type dispensing apparatus is utilized to print relatively thick metal lines on the passivation surface between selected contact openings. This approach greatly increases production throughput.
In accordance with another embodiment of the present invention, a contact/seedlayer is printed onto the wafer using an inkjet-type printing apparatus, and a subsequent plating process is utilized to form a highly conductive metal layer, which is self-aligned to the contact/seedlayer. This approach improves throughput by minimizing the printing time (i.e., because only a thin contact/seedlayer is required), and by utilizing electroless plating, which can be performed on several wafers simultaneously, to form the thick metal lines.
In accordance with another embodiment of the present invention, a processing system for producing a photovoltaic device includes a fixed base, at least one non-contact patterning apparatus fixedly connected to the base, at least one direct-write metallization apparatus also fixedly connected to the base, and a conveyor mechanism for supporting the photovoltaic device wafer during processing by both the non-contact patterning apparatus and the direct-write metallization apparatus, and for conveying the wafer between the non-contact patterning apparatus and the direct-write metallization apparatus. In a preferred embodiment, the wafer is held on the conveyor by a vacuum chuck. In one embodiment, processing apparatus and conveyor mechanism transport and process the device wafers in a “hard tooled” feature registration such that the device wafers remain attached to the conveyor mechanism, and the metallization deposited by the direct-write metallization apparatus is automatically aligned with the contact holes patterned by the non-contact patterning apparatus (i.e., without the need for an intermediate alignment or calibration process). In another embodiment, a sensor is positioned between the non-contact patterning apparatus (or between two non-contact patterning apparatus) and the direct-write metallization apparatus to facilitate a highly accurate metallization process. This approach provides the flexibility of using inkjet-type printing apparatus and/or paste dispensing nozzles with relatively imprecise print element placement.
In accordance with another embodiment of the present invention, a front surface contact-type photovoltaic device includes grid lines formed in the manner described above to include a high aspect central metal line, and transparent support portions formed on each side of the central metal line. An advantage of this arrangement is that conduction through the grid lines is maximized while interruption of light passing into the cell is minimized.
These and other features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings, where:
FIGS. 14(A) and 14(B) are cross-sectional side views showing grid lines formed on a photovoltaic device according to alternative embodiments of the present invention;
The present invention relates to an improvement in photovoltaic devices (e.g., solar cells) that can be used, for example, to convert solar power into electrical energy. The following description is presented to enable one of ordinary skill in the art to make and use the invention as provided in the context of a particular application and its requirements. As used herein, directional terms such as “upper”, “lower”, “side”, “front”, “rear”, are intended to provide relative positions for purposes of description, and are not intended to designate an absolute frame of reference. Various modifications to the preferred embodiment will be apparent to those with skill in the art, and the general principles defined herein may be applied to other embodiments. Therefore, the present invention is not intended to be limited to the particular embodiments shown and described, but is to be accorded the widest scope consistent with the principles and novel features herein disclosed.
Referring to
After initial treatment, device 211T1 is transferred to an optional loading mechanism 220 of a processing system (tool) 230, which loads device 211T1 onto a conveyor 235. In accordance with the present invention, processing system 230 includes at least one non-contact patterning device 240 and at least one direct-write metallization device 250 that are sequentially arranged in the conveying direction of conveyor 235 (e.g., to the right in
Conveyor 235 is depicted in
In accordance with a first aspect of the present invention, as indicated in block 120 in
In accordance with a specific embodiment shown in
In accordance with a specific embodiment, laser 310 is a Coherent Inc. model AVIA 266-300 Q-switched Nd-YAG operating at a pulse repetition rate on the order of 100 KHz. The fluence needed to ablate the surface passivation is on the order of 1 Joule/cm2. The pulse length of the laser is on the order of tens of nanoseconds. The wavelength can be on the order of 266 nm. The short pulse and wavelength of such lasers ensure that the energy is deposited near the surface and any melting in the silicon wafer 212 is short lived. This minimizes any change to the doping profile of the diffusion regions. The energy of a 266 nm photon is 4.66 electron Volts. Although the bandgap of silicon nitride passivation layer 215 varies over a wide range, this photon energy is comparable to the band gap of silicon nitride in its most transparent forms (see “Optical properties of silicon nitride films deposited by hot filament chemical vapor deposition”, Sadanand V. Deshpande and Erdogan Gulari, J, Appl. Phys. 77 (12), 15 Jun. 1995). These highly energetic photons are absorbed in the surface passivation and/or in the topmost nanometers of the underlying silicon. A lightly doped emitter will have a phosphorous diffusion depth of about 200 nm, a sheet resistance on the order of 100 Ohms/sq., and a non-degenerate level of dopant at the physical surface. Silicon is a good thermal conductor causing rapid quenching of the silicon melt formed below the surface of the passivation. Suitable control of the process conditions allows removal of the silicon nitride passivation without significantly altering the thickness or doping profile of the underlying silicon layer.
In an alternative embodiment of the invention, laser-based non-contact patterning apparatus 240-1 includes a femtosecond laser. The advantage of using a femtosecond laser is that the laser energy can be deposited in a timeframe that is faster than the time required for the material to reach thermal equilibrium. Thus, passivation material can be ablated with less debris.
Returning to
Conventional wisdom suggest that, upon forming openings 217 through passivation layer 215, metallization would then proceed using essentially the same silver metallization that is used in nearly all of today's solar cells. Silver, however, diffuses rapidly in silicon and would not make a good metal contact to a lightly doped emitter because of the risk of the silver shunting through to the far side of the junction. The silver contact also requires heavy emitter doping. Silver is also expensive in comparison to other metals such as copper and tin.
NREL/CP-520-36747, presented at the 14th Workshop on Crystalline Silicon Solar Cells and Modules, Winter Park, Colorado, Aug. 8-11, 2004). The ink or paste bearing the silicide forming metal may optionally contain a dopant such as phosphorous or boron to provide additional doping of the contact region during the thermal processing steps applied to the deposited metal.
As depicted in
A preferred source of Ni is ink composed on suspended particles of nanophase Ni.
It will be appreciated that the immediate execution of metallization following the formation of contact openings 217 provides the additional advantage of limiting the air-exposure of exposed portions 213A. This short-duration exposure prevents the formation of an oxidized silicon layer that can otherwise interfere with the formation of the subsequently formed silicide (discussed below). Subsequent heating of the device to drive off volatile components of the ink or paste and a temperature cycle of the device, optionally in a reducing ambient such as hydrogen or forming gas, completes the contact.
In accordance with another aspect of the present invention, the one or more metallization materials are deposited onto the patterned semiconductor wafer using one of an inkjet-type printhead and an extrusion-type dispensing nozzle, as described in the following exemplary embodiments. By arranging such non-contact, direct-write metallization apparatus immediately downstream of the laser-based non-contact patterning apparatus (described above), the present invention enables the precise placement of metallization over the just-formed contact openings without an expensive and time-consuming alignment step.
Print assembly 450 includes a print head 430 and an optional camera 470 (having high magnification capabilities) mounted in a rigid mount 460. Print head 430 includes one or more ejectors 440 mounted in an ejector base 431. Ejectors 440 are configured to dispense droplets of the appropriate metallization material in a fluid or paste form onto wafer 211T2 in the manner described above.
Control circuit 490 is configured in accordance with the approaches described below to provide appropriate control signals to printing support structure 480. Data source 491 can comprise any source of data, including input from an in-line sensor (as described below), a networked computer, a pattern database connected via a local area network (LAN) or wide area network (WAN), or even a CD-ROM or other removable storage media. The control signals provided by computer/workstation 490 control the motion and printing action of print head 430 as it is translated relative to wafer 211T2.
Note that the printing action can be provided by printing support structure 480, by conveyor 235, or by both in combination. Computer/workstation 490 is optionally coupled to receive and process imaging data from camera 470. In one embodiment, camera 470 provides both manual and automated calibration capabilities for printing apparatus 250-1.
By properly calibrating and registering printing apparatus 250-1 with respect to wafer 211T2, the metallization pattern (e.g., contact portions 218 and metal portions 219L and 219U, described above with reference to
In one embodiment, the metallization applied over the contact openings by the direct write metallization devices described above (i.e., inkjet-type printing apparatus 250-1 and/or extrusion-type dispensing apparatus 250-2) may, after subsequent thermal processing, serve as the complete cell metallization in preparation for tabbing and stringing the cells for module assembly. Alternatives to tabbing may also be applicable, for example the adhesive bonding of the cells to a flexible backplane (see “Fast and easy single step module assembly for back-contacted C-Si solar cells with conductive adhesives”, Bultman, J. H., Eikelboom, D. W. K., Kinderman, R., Tip, A. C., Tool, C. J. J., Weeber, A. W. (ECN, Petten (Netherlands) Nieuwenhof, M. A. C. J. van den (TNO, Eindhoven (Netherlands)), Schoofs, C., Schuurmans, F. M. (Shell Solar Energy BV, Helmond (Netherlands)) ECN-RX--03-019 (May 2003)).
As set forth in the following exemplary embodiments, the processing methods described above may be modified to optimize the production of both front surface contact cell-type photovoltaic devices and backside contact cell-type photovoltaic devices.
Referring to
In an alternative embodiment, continuous trenches (not shown) are formed (instead of linearly arranged contact openings 217-1) by laser pulses LP that are used to provide contact between the grid lines and the N-type diffusion region.
In accordance with another alternative embodiment, extrusion-type dispensing apparatus 250-2 (described above with reference to
In accordance with an exemplary embodiment depicted in
In accordance with another embodiment of the present invention, the contact material (i.e., the material disposed at the substrate-gridline interface) contains compounds that adhere to the silicon nitride (i.e., the preferred passivation material). In conventional silver pastes the glass frit promotes adhesion between the gridline and the substrate. In a preferred embodiment, the frit employed has the novel distinction from conventional pastes in that it is designed to not burn through the silicon nitride, but only to stick to the nitride in order to promote adhesion. It is also of sufficiently low density to permit silicide formation in the contact openings. In another preferred embodiment, the emitter doping of front surface contact cell-type photovoltaic devices formed in accordance with the present invention is such that the emitter sheet resistance is on the order of 100 ohms/square or higher, and the surface concentration of the emitter dopant species is non-degenerate. The light emitter and surface doping improves the conversion efficiency and blue response of the solar cell.
In accordance with yet another embodiment, the multiple layer grid line structures described above (e.g., with reference to
Although the present invention is described above with specific reference to the production of front surface contact cell-type photovoltaic devices, the methods described herein may also be used to produce backside contact cell-type photovoltaic devices in a highly efficient manner. In particular, the overall fabrication costs required to produce backside contact cell-type photovoltaic devices in accordance with the teachings of US Published Application No. US20040200520A1 may be substantially reduced by utilizing the laser patterning and direct-write metallization procedures described herein.
Referring to
In accordance with another embodiment, the laser scanning process utilized by non-contact patterning device 240D can be timed in such a way that the hard tooled registration of contact openings 217 and the subsequent deposition of contact portions 218 are achieved electronically. For example, a feedback system 750 incorporated into non-contact patterning device 240D may be utilized to determine the start of each laser scan, and the firing of laser pulses LP is timed in such a way that contact openings 217 fall in regions where the metallization elements will subsequently deposit metal. The feedback system 750 may sense the optical pulses generated by the laser, or may optionally sense an additional laser beam injected co-linearly with the optics. Such additional laser beam may operate as a continuous wave device and thereby serve as a beam spot location reference even when the ablation source is not firing. This provides the flexibility of using inkjet-type printing apparatus and/or paste dispensing nozzles with relatively imprecise print element placement. Registration is maintained through a one-time calibration.
In accordance with a specific embodiment, electronic registration of the contact openings with the metallization can be achieved using the characteristics of a femto-second laser. Typically, these lasers provide ablative pulses at a much faster repetition rate than is required to place the contact openings at their optimal 0.1 mm to 1.5 mm pitch distance. The repetition rates for these pulses can be 80 MHz, perhaps a thousand times faster than the slower rate required to place the contact openings. The slower firing rate can be achieved by counting the pulses, and only allowing the pulses to ablate the passivation layer after counting a plurality of pulses, for instance 1000 pulses. An acusto-optic modulator may be used to select the particular pulse used for ablation, refracting unused pulses out of the ablation light path. Therefore, it is an aspect of this invention that this count be adjusted dynamically. The count could be set to 990 or 1005, for instance, therefore adjusting in small increments the location in the fast direction where the laser ablates the passivation. This dynamic adjustment can be used for several purposes: The first can be to remove inherent non-linearities in the scan lens or scanning instrument, where the scan velocity may vary from a constant velocity by enough to cause the passivation openings to fall outside the region that would place them directly under the linear metallization grid. By measuring the actual velocity variation in a scan beforehand and storing the information, the velocity variation information could be used to compute the correction counts applied during a scan time to place the openings co-incident with the metallization grid. The scan would be broken into several regions, each region having an average velocity. The correction algorithm would use the piecewise linear velocity information to compute a count that would direct a pulse of laser light to create an opening when the laser is predicted to be co-incident with the metalization grid.
The second purpose is to adjust the high energy pulse firing positions to account for a polygon rotation velocity that may vary. A large enough variation in polygon speed over hundreds of scans could place the opening position outside the region required to be co-incident with the metallization grid. By dynamically measuring the true polygon scan or rotation rate during scanning, the adjustment counts could be computed and applied to stabilize the variation and accurately place the opening directly under the metallization grid.
Finally, these correction counts delivered to the acusto-optic modulator to deliver a pulse to the ablation layer could be computed simultaneously using speed variation information from both velocity variations, therefore together dynamically adjusting the passivation ablation opening position in the fast direction to compensate for polygon rotation rate variation and for laser scan velocity variation.
Although hard tooled registration is presently preferred, it is recognized that certain aspects of the present invention may be utilized in processing tools that do not utilize hard tooled registration. For example,
Although the present invention has been described with respect to certain specific embodiments, it will be clear to those skilled in the art that the inventive features of the present invention are applicable to other embodiments as well, all of which are intended to fall within the scope of the present invention. For example, although the description above is primarily limited to silicon-based photovoltaic devices, the various aspects of the present invention may also be utilized in the production of photovoltaic devices on wafers formed by amorphous silicon, CdTe, or CIGS (copper-indium-gallium-diselenide).
