FIELD Embodiments of the present disclosure relate to a system and methods for improving the performance of solar cells, and more particularly, methods of reducing the amount of boron that diffuses from the workpiece during the annealing process. BACKGROUND

Semiconductor workpieces are often implanted with dopant species to create a desired conductivity. For example, solar cells may be implanted with a dopant species to create an emitter region. This implant may be done using a variety of different mechanisms. The creation of an emitter region allows the formation of a p-n junction in the solar cell. As light strikes the solar cells, electrons are energized, creating electron-hole pairs. The minority carriers, which are created by the energy from incident light, are swept across the p-n junction in the solar cell. This creates a current, which can be used to power an external load.

In some embodiments, boron is used to create the p-doped region in the solar cell. For example, in an n-type PERL (passivated emitter, rear localized) solar cell, boron is implanted in the front surface. However, when the cell is subjecting to annealing during manufacturing, boron has a tendency to diffuse out of the cells. When solar cells are annealed, the solar cells are typically disposed in a carrier, such that the front surface of one solar cell is adjacent to the rear surface of the next solar cell. During the annealing of implanted boron, the boron at or near the front surface can outdiffuse at elevated temperatures if it is not bound and driven into the workpiece effectively. This outdiffusion of boron from the front surface of the solar cell then leads to the contamination of the rear surface of that solar cell or ad acent solar cells, and causes severe degradation of the surface passivation, which results in lower cell performance. This outdiffusion of boron also reduces the doping concentration in the p-doped region.

Consequently, protective layers are often deposited on the surfaces of the solar cell prior to anneal to reduce the outdiffusion of boron from the front surface and its diffusion into the rear surface of adjacent solar cells. However, the deposition and subsequent removal of these protective layers add processes that add time and cost to the solar cell manufacturing process .

Therefore, an apparatus and method that improves the manufacturing process associated with solar cells, and particularly reduces contamination associated with boron outdiffusion, would be beneficial.

SUMMARY

An apparatus and method of processing a solar cell is disclosed, where a short thermal treatment is performed on the workpiece after it has been implanted with boron. This short thermal treatment may be performed before the workpiece is placed in a carrier. The short thermal treatment may be performed using a laser, heat lamp or LEDs . In some embodiments, the heat source is disposed in a load lock, and is actuated after the workpiece has been processed. In other embodiments, the heat source is disposed above a conveyor belt that is used to move the processed workpiece from the load lock to a load/unload station.

According to one embodiment, a method of processing a workpiece is disclosed. The method comprises implanting boron into a first surface of the workpiece; exposing the workpiece to a short thermal treatment while the workpiece is being returned to a carrier following the implanting; and subjecting the workpiece to an anneal process after the exposing. In certain embodiments, oxygen is supplied to an ambient environment during the exposing. In certain embodiments, oxygen and at least one inert gas are supplied to an ambient environment during the exposing. In some embodiments, the short thermal treatment is performed using a laser. In certain embodiments, the short thermal treatment is performed using one or more heat lamps. In certain embodiments, the short thermal treatment is performed using one or more LEDs . In certain embodiments, the method comprises implanting oxygen into the first surface of the workpiece before the exposing. In some further embodiments, oxygen is implanted simultaneously with boron. In certain embodiments, the short thermal treatment heats the workpiece to a temperature of between 850°C and 1450°C.

According to a second embodiment, an apparatus for processing a workpiece is disclosed. The apparatus comprises a load lock; a chamber housing an implant system in communication with the load lock; and a heat source disposed in the load lock to heat the workpiece after the workpiece has been processed by the implant system. In certain embodiments, oxygen is supplied to the load lock while the heat source is actuated. In certain embodiments, the heat source comprises a heat, a laser or LEDs .

According to a third embodiment, an apparatus for processing a workpiece is disclosed. The apparatus comprises a load/unload station where a workpiece is removed from a carrier; a load lock; a conveyor belt to move the workpiece between the load/unload station and the load lock; a chamber housing an implant system in communication with the load lock; and a heat source disposed above the conveyor belt to heat the workpiece after the workpiece has been processed by the implant system as the workpiece is being returned to the load/unload station. In certain embodiments, heat source comprises a heat lamp, a laser or LEDs. In certain embodiments, a length of a beam directed toward the workpiece is greater than a first dimension of the workpiece .

BRIEF DESCRIPTION OF THE FIGURES

For a better understanding of the present disclosure, reference is made to the accompanying drawings, which are incorporated herein by reference and in which:

FIG. 1 shows a representative manufacturing flow for an n- type PERL solar cell according to one embodiment;

FIG. 2 is a representative manufacturing flow for an n-type PERL solar cell according to a second embodiment;

FIGs. 3A-3C show thermal profiles that may be used during the short thermal treatment;

FIG. 4 shows a first embodiment of an apparatus that may be used to achieve the manufacturing flows shown in FIGs. 1-2; and FIG. 5 shows a second embodiment of an apparatus that may be used to achieve the manufacturing flows shown in FIGs. 1-2.

DE TAILED DESCRIPTION

Implanted solar cells are very sensitive to surface conditions and processing sequences. For example, implanted boron may outdiffuse from the front surface of the solar cell during high temperature annealing. As mentioned above, this reduces the concentration of p-type dopant in the front surface. In addition, the diffused boron may later diffuse into the rear surface, which may be n-doped, or not doped at all. One way to prevent the contamination of the rear surface of solar cell with unwanted boron is to remove the boron from the surface of the workpiece prior to the anneal process. In some embodiments, this may be achieved using a short thermal treatment, such as a rapid thermal process (RTP) , flash anneal or a laser anneal. This short thermal treatment (STT) is intended to remove boron that is disposed at the surface of the solar cell, and may or may not lead to the formation of an emitter. In certain embodiments, the rate of removal of boron may be altered by controlling the composition of the ambient gas. For example, the short thermal treatment may be performed in an ambient environment that comprises a gas, such as oxygen, to control the duration of surface boron removal.

FIG. 1 shows a representative manufacturing process that may be used to reduce boron outdiffusion from the front surface and/or reduce diffusion of boron into the rear surface. First, shown in Process 100, the workpiece may be textured. Texturing may increase the surface area of the front surface. In some embodiments, the workpiece may be n-type silicon. A p-type dopant, such as boron, is then implanted into the front surface of the workpiece, as shown in Process 110. Similarly, a n-type dopant, such as phosphorus, is implanted into the rear surface of the workpiece, as shown in Process 130. While FIG. 1 shows the boron being implanted into the front surface, it is understood that, in other embodiments, the boron may be implanted into the rear surface. Furthermore, while FIG. 1 shows that phosphorus is implanted into the rear surface, the disclosure is limited to this embodiment. For example, other dopants may be implanted in the surface opposite the boron- implanted surface. In other embodiments, the surface opposite the boron-implanted surface may not be implanted at all. The process described herein may be applied to any manufacturing process that includes implanting boron into at least one surface of the workpiece. One or both of the implants may be blanket implants, where the entire surface is implanted without the use of a mask. Alternatively, one or both of these implants may be patterned implants, where a mask is used to allow only a portion of the surface to be implanted with the dopant ions.

In addition, the boron ion implant (Process 110) may be performed so that the front surface is amorphized. However, in other embodiments, the energy and duration of the boron ion implant may not completely amorphize the front surface. The boron ion implantation may utilize various ion species, including but not limited to B, BF, BF ₂ , BF ₃ , or B ₂ F ₄ . Traditionally, due to the outdiffusion of boron from the front surface, a protective layer has been applied to the front surface and/or rear surface of the solar cell prior to an annealing process. While the protective surface does reduce outdiffusion, it is costly in terms of number of processes. Specifically, the protective coating is first deposited on the front and/or rear surface of the solar cell. After the anneal process is completed, these protective layers are then removed. In the process shown in FIG. 1, the protective layer is not deposited. Rather, a short thermal treatment (shown in Process 120) is performed after the boron ion implant (Process 110) . This short thermal treatment may be 10 seconds or less in certain embodiments, and may be performed using a laser anneal, a flash anneal or a rapid thermal process. The short thermal treatment is designed to cause the intentional outdiffusion of boron from the surface of the workpiece. In certain embodiments, the short thermal treatment is performed while the workpieces are disposed on their rear surfaces. For example, a laser beam, in the form of a pulse or a continuous wave, may be directed toward the front surface of each workpiece, after it has been implanted with boron. The short thermal treatment will cause the boron located near the surface of the workpiece to diffuse out of the workpiece. However, since the workpieces may be disposed on their rear surfaces, little contamination of the rear surfaces may occur during the short thermal treatment. Thus, the short thermal treatment may occur after the workpiece has been implanted and before it is returned to the carrier that is typically used to hold a plurality of workpieces.

While FIG. 1 shows the implant of phosphorus (Process 130) occurring after the boron implant (Process 110) and the short thermal treatment (Process 120), other embodiments are also within the scope of the disclosure. For example, the implant of phosphorus (Process 130) may be performed before the implant of boron (Process 110) . In another embodiment, the implant of phosphorus (Process 130) may occur before the short thermal treatment (Process 120) . In all of these embodiments, the short thermal treatment (Process 120) occurs after the boron implant (Process 110) and before the anneal process (Process 140) . Next, as shown in Process 140, an anneal process is performed. In certain embodiments, a cleaning process may be performed prior to the anneal process. The purpose of the anneal process is to drive the implanted dopants into the workpiece, repair any damage caused by the implant and to activate the dopant. In certain embodiments, the anneal process is performed while a plurality of workpieces are disposed in a carrier, which could be made of quartz. The carrier may stack the workpieces so that the front surface of one workpiece is proximate the rear surface of an adjacent workpiece. However, since the boron outdiffused during the short thermal treatment, the rear surfaces of the workpieces may not be contaminated during the anneal process.

Next, passivation layers are formed on the front and rear surfaces of the solar cell, as shown in Process 150. An anti- reflective coating (ARC) is then applied to the front and/or rear surfaces, as shown in Process 160. This ARC may be silicon nitride (SiN) , although other materials may be used. The metal contacts are then applied using screen printing (SP) , as shown in Process 170. Metal paste is typically fritted to ensure good contact through the ARC to the solar cell. The substrate is then fired to cause the metal to bond and diffuse into the substrate, as shown in Process 180. The resulting solar cells are then tested and sorted, as shown in Process 190. While Processes 150- 190 show a particular set of processes, it is appreciated that other or different processes may be performed after the anneal process (Process 140) .

FIG. 2 shows another embodiment of a manufacturing process that may be used. In this embodiment, like processes are given the same reference designators as was used in FIG. 1. The embodiment of FIG. 1 assumed that only boron is implanted into the front surface during Process 110.

However, in the embodiment of FIG. 2, oxygen is also implanted with boron in Process 210. In certain embodiments, such as non-mass analyzed systems, the oxygen may be co- implanted with the boron. In other words, a first feed gas containing boron and a second feed gas containing oxygen may be introduced into an ion source to create first ions containing boron and second ions containing oxygen. The number of oxygen ions, relative to the number of boron ions, may be determined based on the gas flow, the power applied to the ion source, or other parameters. The oxygen ions may be in the form of 0 or O2 ions. In other embodiments, oxygen may be implanted in a separate implant. For example, the oxygen ions may be implanted at an implant energy of between 2-20 kV. In either embodiment, the concentration of oxygen implanted into the workpiece may be between 1E14 and 5E15 cm ^-2 .

The implanting of oxygen may vary the rate of diffusion of boron out of the front surface of the workpiece. FIGs. 3A-3C show various embodiments of the short thermal treatment. In these embodiments, the temperature of the short thermal treatment process reaches a plateau. At this plateau, the maximum temperature, T _max , may be between 850°C and 1450°C, although other temperature ranges are possible. The workpiece remains at this temperature plateau for a time, t2, which may be between 1 nanosecond and 10 seconds, although other durations are possible. FIG. 3A shows a first embodiment. In this embodiment, the temperature is ramped from its ambient temperature to the T _max plateau. In all of the embodiments, the temperature may be ramped at rates exceeding or close to 1450 °C/s, although other rates are possible. Ramp rate may be dependent on pulse duration and input power of the heating source.

At an intermediate temperature, T _dwe n, which may be between

150°C and 850°C, but less than T _max , the temperature ramp is paused, so as to allow the workpiece to dwell at this temperature, T _dW eii ·

The workpiece may dwell at this temperature for a dwell period, tl, which may be between 0 and 60 seconds, although other time durations are possible. The use of an intermediate dwell temperature may minimize thermal shock and prevent thin workpieces from cracking.

While the workpiece is at T _d weii , oxygen gas may be supplied to the ambient environment. In one embodiment, oxygen is supplied during the entire dwell period. In another embodiment, the oxygen is supplied at the start of the dwell period and is turned off prior to the end of the dwell period. In another embodiment, the oxygen is supplied after the start of the dwell period and is turned off at or prior to the end of the dwell period. In yet another embodiment, the oxygen may be supplied at a plurality of intervals during the dwell period. The duration of time during which the oxygen is supplied during the dwell period may also vary. For example, the oxygen may be supplied for the entire dwell period, tl, or any portion thereof. Additionally, if the oxygen is supplied at a plurality of intervals, those intervals may or may not be of equal duration.

The oxygen may be supplied at any flow rate, up to the maximum flow rate attainable. In addition, the total amount of oxygen supplied may also vary. While FIG. 3A shows the temperature being held constant during time duration, tl, other embodiments are possible. For example, rather than dwelling at one constant temperature, the slope of the temperature ramp may be slowed, such that the temperature rises much more slowly during the dwell period than during the initial temperature ramp. For example, the initial temperature ramp may be 1450 °C/s. This rate may be slowed to a rate as low as l°C/min during the dwell period once the temperature reaches T _dW eii · After the dwell period, the temperature ramp may return to its initial rate, or may remain at a lower rate. Thus, the dwell period is defined as a period of time, at a temperature or range of temperatures less than the maximum temperature, which is used to acclimate the workpiece to the elevated temperature. As described above, this dwell period may be at a constant temperature as shown in FIG. 3A, or may be a time duration having a reduced temperature ramp. After the dwell period, the temperature may be ramped again until it reaches T _max . As before, the rate of temperature change may be close to 1450 °C/s, similar to the initial rate, although other rates are possible. The workpiece may remain at this temperature, T _max , for a duration of t2, where t2 is less than 10 seconds, in some embodiments. Oxygen may also be supplied to the ambient environment during this time period as well. As was described above with respect to the dwell period, the oxygen may be supplied for the duration of this plateau, t2, or any portion thereof. In addition, the oxygen may be supplied during one interval or during a plurality of intervals. As was the case during the dwell period, the flow rate of oxygen may be varied and the total volume may also be varied. In some embodiments, oxygen may be provided as the sole ambient gas. In other embodiments, the oxygen may be mixed with other gasses or mixture of gasses, such as, but not limited to nitrogen and argon .

After the duration of the temperature plateau has elapsed, the temperature may be ramped back to the ambient temperature, at any desired rate.

FIG. 3B shows a second embodiment, which does not have a defined dwell period during the initial temperature ramp. In this embodiment, the oxygen may be supplied to the ambient environment during the time duration, t2, where the maximum temperature, T _max , is achieved. In some embodiments, the oxygen may be supplied as soon as the workpiece reaches the maximum temperature. In other embodiments, oxygen may be supplied at a later time during this plateau. As before, the oxygen may be supplied for all or any portion of the duration of temperature plateau, t2. Additionally, the oxygen may be supplied at a plurality of intervals, which may be equal or different durations. As was true above, the flow rate of the oxygen may be varied, as can the total volume of oxygen that is introduced. In a variation of FIG. 3B, oxygen may be supplied to the ambient environment during a portion of the initial ramp period, t3. In one embodiment, the oxygen may be supplied at some point after the temperature reaches a specific temperature, for example, at least 550°C. In another embodiment, the temperature ramp may be less than the maximum attainable to allow oxygen to be supplied for an extended period of time.

As described above, in certain embodiments, oxygen may be supplied during at least a portion of the short thermal treatment. The presence of oxygen in the ambient environment may affect the rate that boron diffuses out of the workpieces.

In yet another embodiment, shown in FIG. 3C, the temperature profile may be similar to that shown in FIG. 3B, however, the plateau at T _max might look similar to a saw tooth pattern. In this embodiment, heat may be replenished to maintain the plateau temperature (T _max ) with short pulses, rather than a constant power supply. This approach could result in lower overall power consumption.

FIG. 4 shows an exemplary apparatus that may be used to perform the sequences shown in FIGs. 1 and 2. The apparatus 400 may include a load/unload station 450. In certain embodiments, the load/unload station 450 may comprise a Front Opening Universal Pod (FOUP) . In some embodiments, a plurality of workpieces is provided in a carrier. Workpieces may be individually removed from the carrier and placed on first conveyor belt 440a. The first conveyor belt 440a may move the workpieces 10 from the load/unload station 450 to a load lock 420. The first conveyor belt 440a may move workpieces 10 at a speed of between 10-20 cm/sec, although other speeds may be used.

A load lock 420 typically comprises a sealable chamber, having a first point of access 421 and a second point of access 422. A workpiece 10 may be placed in a load lock 420 by opening the first point of access 421 and placing the workpiece 10 in the sealable chamber. The sealable chamber is then pumped down to near vacuum conditions. The second point of access 422 is then opened, and the workpiece 10 is removed, typically by a substrate handling robot disposed in a chamber containing the implant system 430. The process operates in the reverse manner for workpieces 10 leaving the chamber containing the implant system 430.

The implant system 430 is not limited by this disclosure. For example, the implant system 430 may be a beam line ion implanter. A beam line ion implanter has an ion source, which generates an ion beam. This ion beam is directed toward the workpiece. In some embodiments, the ion beam is mass analyzed so that only ions of a desired mass/charge are directed toward the workpiece. In other embodiments, the ion beam is not mass analyzed, allowing all ions to implant the workpiece. The ion beam energy may be controlled through the use of electrodes in the path of the ion beam that serve to accelerate or decelerate the ion beam, as desired. The ion beam may be in the form of a ribbon beam, where the width of the ion beam is much larger then its height. In other embodiments, the ion beam may be a spot beam or a scanned ion beam. The ion source may be a Bernas ion source, or may use inductive or capacitive coupling to generate the desired ions.

Alternatively, the implant system 430 may be a plasma chamber, where the workpiece is disposed in the same chamber where the plasma is generated. The plasma may be generated using an RF source, although other techniques are also possible. The workpiece is then biased to attract ions from within the plasma toward the workpiece, implanting the desired ions in the workpiece. Other types of apparatus may also be used to perform these ion implantation processes.

After the implant system 430 has completed the implant process, the workpiece 10 is removed from the chamber using the load lock 420. As described above, after the workpiece 10 is placed in the load lock 420, the second point of access 422 is closed and gas is introduced into the load lock 420 to return the sealable chamber to atmospheric conditions. After atmospheric conditions are reached, the first point of access 421 is opened, and the workpieces 10 may be removed. The workpieces 10 are returned to the load/unload station 450 via second conveyor belt 440b. As was the case with first conveyor belt 440a, the workpieces 10 may be moved at a speed of 10-20 cm/sec .

Disposed above the second conveyor belt 440b may be a heat source 410 and its associated optics 411. The heat source 410 may comprise a laser, operating in continuous wave or pulse mode. In other embodiments, the heat source 410 may be one or more infrared lamps. In other embodiments, the heat source 410 may be one or more LEDs . In certain embodiments, the heat source 410 and associated optics 411 produce a beam that extends and/or scans across the entirety of one dimension of the workpiece 10 as it moves on the second conveyor belt 440b. In other words, the workpiece 10 may have a first dimension, which extends perpendicular to the direction of travel of the second conveyor belt 440b (i.e. into the page in FIG. 4), and a second dimension, which extends in the direction of the movement of the second conveyor belt 440b. In certain embodiments, the heat source 410 creates a beam having a length at least as great as the first dimension of the workpiece 10. The beam created by the heat source 410 may have a width that is much smaller than the second dimension of the workpiece 10. In certain embodiments, the beam created by the heat source 410 is pulsed such that all portions of the workpiece 10 are exposed to the beam. In other embodiments, the beam may be constantly energized.

In other embodiments, the heat source and the associated optics produce a beam that is smaller than the first dimension of the workpiece 10. In these embodiments, the associated optics 411 may scan the beam in the first direction (i.e. in and out of the page) as the workpiece 10 moves along the second conveyor belt 440b. The scan can also be performed in the belt movement direction. The focused heat from the heat source 410 may serve to raise the temperature of the workpiece 10 to the temperatures shown in FIGs. 3A-3C.

Thus, as the workpiece 10, which has been implanted with boron, is moved from the implant system 430 back to the load/unload station 450 along the second conveyor belt 440b, the workpiece 10 is subjected to the short thermal treatment. Further, since the workpieces 10 are disposed on their rear surfaces on the second conveyor belt 440b, the heat is directed at the front surface of the workpiece 10, and the boron diffuses out of the front surface and away from the workpiece 10.

While FIG. 4 shows a first conveyor belt 440a, which brings workpieces 10 from the load/unload station 450 to the load lock 420, and a second conveyor belt 440b, which returns the workpieces 10 to the load/unload station 450, other embodiments are also possible. For example, each conveyor belt may be capable of operating in both directions. For example, first conveyor belt 440a may also be capable of returning workpieces 10 to the load/unload station 450. Further, the number of conveyor belts is not limited by this disclosure. For example, there may be one or more conveyor belts. All or any subset of these conveyor belts may be capable of returning workpieces 10 from the load lock 420 to the load/unload station 450. In certain embodiments, a respective heat source 410 and its associated optics 411 is disposed above each conveyor belt that is capable of moving workpieces 10 from the load lock 420 to the load/unload station 450. In other embodiments, a respective heat source 410 and its associated optics 411 is disposed above at least one conveyor belt that is capable of moving workpieces 10 from the load lock 420 to the load/unload station 450.

FIG. 5 shows another embodiment of an apparatus 500 which may be used. Components that are identical to those used in FIG. 4 have been given like reference designators, and will not be described again. In this embodiment, rather than being disposed above the second conveyor belt 440b, a heat source 510 is disposed within the load lock 420. This heat source 510 may be one or more heat lamps, or may be a laser or a plurality of LEDs . In operation, after processing, a workpiece 10 is placed in the load lock 420. In this embodiment, the heat source 510 may be activated as the load lock 420 is being brought back to atmospheric conditions with the implanted workpiece disposed therein. In certain embodiments, the time to pump the load lock 420 back to atmospheric conditions may be up to 10 seconds, allowing the short thermal treatment to occur during this period. In certain embodiment, oxygen is pumped into the load lock 420 as the load lock 420 is being brought back to atmospheric conditions. In other embodiments, oxygen and at least one other gas are pumped into the load lock 420 as the load lock 420 is being brought back to atmospheric conditions. In this manner, oxygen is introduced into the load lock 420 while the workpiece 10 is being subjected to the short thermal treatment.

Although the disclosure describes the use of this method as used in the manufacture of n-type PERL solar cells, the methods are applicable to a wide range of workpieces, such as n-type PERT, IBC and other high efficiency solar cells where boron is implanted into at least one of the surfaces of the workpiece.

The present apparatus and methods have many advantages. First, the present method avoids having to apply a protective coating to the surfaces of the workpiece to avoid boron outdiffusion contamination. This may save processing time, improve throughput and reduce cost. Further, the present methods may be conveniently incorporated into existing semiconductor equipment. For example, a heat source may be disposed in the load lock as the processed workpiece is removed from the chamber. Alternatively, the heat source may be disposed above the conveyor belts that return the processed workpiece to the load/unload station. Additionally, the non-equilibrium nature of these processes may also yield further benefits, such as process simplification and improvements. One aspect of boron implant and downstream processing is the removal of implant related defects. Since the STT employs relatively high processing temperatures (Tmax) , albeit for short times, the short thermal treatment might be able to eliminate boron implant related defects and lead to improved emitter performance, and subsequently, improved solar cell performance.

The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Furthermore, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein .