The invention will be described in more detail below in connection with the accompanying drawings, in which
As has already been mentioned above, shortage of raw material for solar cell manufacturers requires new solutions to increase the supply. In addition, a process has to be enabled which allows obtaining raw panels for solar cells on a cost competitive level. Furthermore, the new process, using semiconductor scrap wafers and dies, must assure that the required specification limits for solar panels are achieved.
The present invention provides a recycling process that is able to prevent disposal of scrap parts and related costs. The process can be used with 300 mm wafers as well as 200 mm wafers and, in addition, with all types of wafer dies. Thus, the inventive process is able to use scrapped wafers as well as sufficient large scrapped dies for photovoltaic solar element production.
According to the invention, wafer scrap from 200 mm as well as 300 mm semiconductor manufacturing can be used to make square panels for solar cell production. As is shown in
Wafers are scrapped at several process steps throughout a wafer production line. Reasons for this are, e.g., physical damages, contamination, no functionality and yield criteria. Those wafers can be used in general for solar panel production unless they are broken. However, it is necessary that the scrap wafers will fulfill the specification criteria that are the prerequisites in order to be used as solar panels. These criteria are:
Wafer is not broken into pieces
Wafer is thicker than 300 μm
Wafer is 200 or 300 mm form factor
Test or product wafer
In a first step (200) wafers are identified as scrap fulfilling the above mentioned criteria. Wafers are scrapped in a semiconductor line according to existing rules and criteria, concerning yield and performance. These criteria are checked using measurement equipment for thickness, flatness and electrical performance. This can be done throughout the whole wafer production process. Normally, the scrap wafers typically go to a specific box and area for Non-Compliant Material (NCM). No specific handling is applied, the wafers might be physically damaged (broken). According to the invention, the scrap wafers are sorted into a wafer recycle box, whereby a better handling of the wafers is assured and physical damages are prevented. The wafer recycle box assures that the wafers are not stored with their surfaces attached to each other. The box can be similar to a Front Opening Shipping Box (FOSB) or to a Front Opening Unified Pad (FOUP), but does not require sealing to prevent contamination. This means that the wafers are kept in individual slots in order to keep them apart. The box material can be plastic or metal. No specific cleaning has to be applied to the boxes before reuse. The box may also be a simple box wherein the wafers are stored on top of each other with separating paper sheets between them. After the box is filled with scrap wafers, it is closed and may be marked with a bar code for traceability purposes. The box used for the scrap wafers should be able to be handled outside the manufacturing line, meaning that the box should be sealed, like a FOUP, to prevent the contents from additional particular and organic contamination. The sorted scrap wafers are moved out of the line within FOUPs, and then they are replaced into the recycle box mentioned above. The spacing between the wafers in the recycle boxes can be tighter as compared to conventional FOUP or FOSB boxes because the wafers will again be treated afterwards.
In a second step (202) the scrap wafers are cleaned with standard wet bench cleaning, and the remaining structures (integrated circuits) are taken off by etching of the structured surface. Etching can be performed by conventional methods like selective wet etch using an etch bath to remove the metallic layers deposited onto the wafer surface.
Next (Step 204), the wafers are grinded down to a thickness of 300 μm using conventional grinding machines to grind the silicon surface with high roughness, because no smooth surface is required. The wafer is placed on the grinder surface, fixed by a frame fitting the wafer size. The grinding is performed using a calibrated removal rate vs. time. Alternatively, the original thickness of 700 μm (the standard thickness for semiconductor wafers) is kept and only a front side contacting is applied, which has the advantage of using single contacting only instead of double side contacts. Thickness measurements are performed (Step 206) until the desired thickness of 300 μm has ben achieved. In case the required thickness has not yet been achieved, step 204 is repeated. The etching and grinding (lapping) processes are generating chemical waste for disposal (Step 208). The chemical waste contains certain metals which require a special treatment.
In step 210 the wafers are now laser cut into square panels for solar cell manufacturing. Scrap parts from cutting can be fed back to the wafer bulk material manufacturer.
Following cutting, the square panels are treated in a print process to add a doping glass layer with a phosphorous content (Step 212). However, any other deposition process capable of placing the phosphor layer on top of the panels can be used. Followed by a furnace step, the attached doping glass layer causes the phosphor content to migrate into the Si-layer by diffusion. The diffusion length is determined by the energy used (maximum temperature) as well as the duration of the furnace process. The ionization process flow is given in
Sometimes wafers are manufactured such that they show a high boron doping. This can be compensated for by using phosphorus doping to either reduce p-doping or even achieve n-doping. The doping process must be different in the case of a high p-doping on the wafer level. This requires a two-step process, namely neutralizing the high doping (˜10 Ωcm) in a first step and applying “regular” doping (0.5-2.5 Ωcm) in a second step in order to achieve the required p/n junction.
Finally, characterization of panels and supply to the customer takes place (Step 214) by final testing, packaging and shipping.
Doping is followed by front- and backside contacting, this being a conventional process used in solar panel manufacturing. Normally, this is achieved by using a printing method (Siebdruck) to place the contact pads.
In the following, the inventive solution for individual dies will be presented.
Wafer die scrap from semiconductor manufacturing can be used to make square panels for the solar cell production unless they are broken. Dies are scrapped after dicing in the wafer backend
Die is not broken into pieces
Die is thicker than 300 μm
Die has footprint of ≧100 mm2
Normally, the scrap dies, like the scrap wafers, go into a specific box and area for NCM. According to the invention, the scrap dies now go into a recycle box, thus assuring a better handling. No special care is required due to later treatment. Thus, the transport boxes can be simple plastic boxes.
The finished dies require a specific metallic processing carrier (Step 218), assuring conductivity for the die backside. The special carrier should prevent the dies from being damaged.
Next, the dies are cleaned and etched to remove the structure from the semiconductor process (Step 202; cf.
The former die front side, functional surface, now is the conducting backside of the solar unit, whereas the former die backside now is the active solar surface which has to be treated with n+-doping (Step 212, cf.
Conductive soldering between the dies and the solar panel carrier surface is done during the n+-doping furnace process step.
As to the doping process itself, reference is made to the doping of wafers.
After doping is complete, characterization of panels and supply to the customer takes place (Step 214, cf.
As with the wafers, doping is followed by front- and backside contacting, this being a conventional process used in solar panel manufacturing. Normally, this is achieved by using a printing method (Siebdruck) to place the contact pads.
The quads (wafer and die panels) are now ready for regular solar panel treatment.
In the semiconductor line, the scrapped wafers and dies are handled using vacuum tweezers. The parts are sorted into the appropriate boxes and carriers without applying a specific cleaning, and shipped to the solar cell recycling.
In the solar process, the wafers can also be simply handled with vacuum tweezers, before as well as after dicing. The dicing can be carried out using a glass cutter (manual mode), for low volume, or a laser cutter for high volume. After the wafers have been prepared (etching and grinding/polishing) and cut into solar panels, the follow-on processes are those normally used in solar cell technology.
The dies are etched and grinded/polished. This is done in a carrier frame in the case of grinding/polishing. In the case of etching, the dies are collected in an etch basket. Die sorting into the final solar frame is carried out either with vacuum tweezers, in the case of low volume, or a pick and place machine, in the case of high volume.
When using dies, the photovoltaic effectiveness can be improved using not a full panel size, but focusing the illumination into a centre area. This is outlined in
The device 18 shown there uses the effect of parabolic mirrors 20 to focus daylight or sunlight 22 on the centered solar device 24 containing only a few semiconductor dies manufactured by the inventive method, thus increasing the efficiency. The effect is that the solar cell output is increased using the higher light intensity. Also, the effect is that less solar cell surface is required to generate solar voltage. The small form factor of the individual die 24 enables any focus area size. To realize this with existing solar panels would require to cut the panel, which raises additional cost.
Number | Date | Country | Kind |
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06116972.8 | Jul 2006 | DE | national |