Abstract:
The resistivity of a silicon boule may vary along its length, thereby making a uniform ion implantation process sub-optimal. A system and method for measuring a resistivity of a substrate, and processing the substrate based on that measured resistivity is disclosed. The system includes a resistivity measurement system, a controller and an ion implanting system, where the controller configures the ion implantation process based on the measured resistivity of the substrate.

Description:
FIELD 
       [0001]    This application relates to a method of determining the inherent resistivity of a substrate, and adjusting the subsequent processing of the substrate. 
       BACKGROUND 
       [0002]    Solar cells typically utilize a p-n junction to separate electron-hole pairs that are created by photons penetrating the substrate. This may be achieved by disposing a p-type region adjacent to an n-type region. Traditionally, one of these regions, such as the n-type region, may be provided through the use of a previously doped bulk material. For example, the bulk silicon used to create a solar cell may be n-type silicon. Methods of creating bulk silicon with n-type or p-type dopants incorporated therein are well known in the art. One surface of this bulk n-type silicon is then doped with p-type ions to create a p-type region, adjacent to the remainder of the n-type bulk silicon. 
         [0003]    Variations in the resistivity of the bulk silicon may have an adverse impact on the efficiency of the solar cell. For example, high resistivity of the bulk silicon may decrease lateral carrier mobility, which negatively impacts series resistance and therefore fill factor. While the resistivity of the bulk material may impact solar cell performance, few techniques are currently employed to determine and counteract the affect of high resistivity of the bulk material. 
         [0004]    Therefore, an improved method of processing a substrate, using information related to the bulk material&#39;s resistivity, is needed. 
       SUMMARY 
       [0005]    The resistivity of a silicon boule may vary along its length, thereby making a uniform ion implantation process sub-optimal. A system and method for measuring a resistivity of a substrate, and processing the substrate based on that measured resistivity is disclosed. The system includes a resistivity measurement system, a controller and an ion implanting system, where the controller configures the ion implantation process based on the measured resistivity of the substrate. 
         [0006]    In one embodiment, a method of processing substrates is disclosed, which comprises measuring a resistivity of a first substrate; and processing a second substrate based on said measured resistivity to achieve a desired sheet resistance. In a further embodiment, the first and second substrates may be the same substrate, such that the processing is performed on the same substrate that was measured. 
         [0007]    In another embodiment, an apparatus for processing substrates is disclosed, which comprises a measurement station to measure resistivity of the substrates; a controller to determine a dose based on the measured resistivity; and an ion implanter to implant the dose in the substrates. In a further embodiment, the controller is in communication with a database, wherein the controller stores the measured resistivity of the substrates and an indication of the substrates in the database, wherein the controller identifies a substrate in the ion implanter based on the indication, and wherein the dose for the identified substrate is based on the measured resistivity of the substrate. 
         [0008]    In another embodiment, a method of processing a substrate to form a solar cell is disclosed, which comprises measuring a resistivity of the substrate; storing the measured resistivity in a database; selecting a dose to be used in the creation of a back surface field (BSF) based on the stored measured resistivity; and implanting ions into the substrate using the selected dose to create the BSF. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0009]    For a better understanding of the present invention, reference is made to the accompanying drawings, which are incorporated herein by reference and in which: 
           [0010]      FIG. 1  shows a system which may be used in accordance with one embodiment; 
           [0011]      FIGS. 2A-2C  show several embodiments of BSF dosing; 
           [0012]      FIG. 3  shows a flowchart in accordance with one embodiment; and 
           [0013]      FIG. 4  shows a flowchart in accordance with a second embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    As described above, the bulk silicon may have varying resistivity. For example, phosphorus doped bulk silicon created using the Czochralski process may have resistivity variability along the boule. This may be due to the liquid/solid segregation of phosphorus. Because of this, one end of the boule may have a resistivity that is noticeably different than the opposite end. 
         [0015]    High resistivity of the bulk silicon may degrade the performance of a solar cell. However, for passivated emitter and rear total (PERT) solar cells, this high bulk resistivity may be counteracted by the introduction of a highly dosed back surface field (BSF). The BSF may provide a more conductive lateral path for majority carriers. Thus, in high resistivity substrates, a highly dosed BSF may decrease sheet resistance. However, in substrates having lower bulk resistivity, reduced doses for the BSF may be advantageous. Thus, a single uniform dose for the BSF, which is independent of the bulk resistivity, may compromise the performance of all solar cells. 
         [0016]    In other words, in some embodiments, it may be desirable that all processed substrates have a series resistance within a certain range. Differences in the underlying resistivity of the substrate require variations in the BSF sheet resistance to ensure consistent series resistance. The variations in the BSF sheet resistance may require changes in the processing of the substrates. Thus, by measuring the resistivity and processing the substrate based on that resistivity, the processed substrates can achieve the desired sheet resistance. 
         [0017]      FIG. 1  shows a system which may be used in accordance with one embodiment. The system  100  includes an ion implanter  110 , which is used to introduce ions into the substrate. The ion implanter  110  may include an ion beam generator  112 , and a platen  111 . The ion beam generator  112  is configured to generate the ion beam  113  and direct it towards a front surface of the substrate  160 . The ion beam generator  112  may include many components known to those skilled in the art, such as an indirectly heated cathode ion source, an RF ion source, an extraction assembly positioned proximate an extraction aperture of the ion source, a mass analyzer, acceleration/deceleration lenses, etc. to provide the ion beam  113  having desired characteristics, such as beam current, uniformity, and energy levels. The ion implanter  110  has been described as a beam line or flood ion implanter but a plasma doping implanter may also be utilized to treat the substrate. Those skilled in the art will recognize a plasma doping implanter positions the substrate  160  in a processing chamber where plasma is generated. 
         [0018]    The system  100  also includes a measurement station  130 , which is used to measure an electrical property of the substrate  160 . For example, the measurement station  130  may measure the resistivity of the substrate  160  by disposing a plurality of probes (not shown) in electrical contact with the surface of the substrate  160 . A voltage may be applied to one of these probes and the current is measured at the other probes. Alternatively, a current may be applied to one probe and the voltage is measured at the other probes. This measurement technique allows the calculation of resistance of the substrate  160 . Of course, other mechanisms may be used to measure the resistivity of the substrate  160 . 
         [0019]    Alternatively the measurement station  130  may use a magnetic field to induce eddy currents in the substrate  160 . The magnetic field may be applied and then removed and the decay of the induced eddy currents monitored to determine the resistivity, or an AC magnetic field may be applied and the phase angle between the applied magnetic field and the induced eddy currents may be monitored to determine the resistivity of the substrate  160 . 
         [0020]    The system  100  may also include automated substrate handling equipment, such as a conveyance system  150 , for transferring substrates  160  between the measurement station  130  and the platen  111 . The conveyance system  150  may be one or more robots, conveyor belt, or other mechanisms known to those skilled in the art. In some embodiments, the substrate  160  enters the measurement station  130  prior to entering the ion implanter  110 , although the opposite order may also be used. 
         [0021]    The system  100  also includes a controller  120  in communication with the measurement station  130  and the ion implanter  110 . The controller  120  can be or may include a general-purpose computer or network of general-purpose computers that may be programmed to perform desired input/output functions. The controller  120  can also include other electronic circuitry or components, such as application specific integrated circuits, other hardwired or programmable electronic devices, discrete element circuits, etc. The controller  120  may also include communication devices, data storage devices, and software. The controller  120  is in communication with a non-transitory medium, such as a storage element  125 . This storage element  125  contains instructions, which when executed by the controller  120 , perform the steps and operations described herein. The controller  120  may receive input signals from a variety of systems and components such as the ion implanter  110 , the ion beam generator  112 , conveyance system  150 , or the measurement station  130  and provide output signals to each to control the same. The controller  120  may also be in communication with a database  140 . This database  140  may be disposed in the storage element  125 , or may be disposed in a different storage element, which may be locally or remotely located. 
         [0022]    In operation, the substrate  160  is transferred to the measurement station  130 , such as by the substrate handling equipment. A conveyance system  150  may be used to move the substrates  160  between the measurement station  130  and the ion implanter  110 . Thereafter, the controller  120  performs a resistivity measurement, using, for example, a plurality of probes. Based on the measurements obtained, a resistivity value of the particular substrate  160  can be determined. This value is then stored by the controller  120  in database  140 . In addition, an indication of the identity of the substrate  160  may be stored with the measured resistivity value. This indication may be a scribe mark. In this embodiment, an optical system (not shown) may be utilized to detect the scribe mark at the ion implanter  110  in order to identify the substrate  160 . In other embodiments, the indication of the identity of a substrate  160  may be based solely on the position of the substrate  160  in the processing order. For example, the controller  120  may be aware of the number of substrates disposed between the measurement station  130  and the ion implanter  110 , and based on this, can correlate the substrate  160  in the ion implanter  110  to its information in the database  140 . 
         [0023]    Later, the substrate  160  is transferred by the handling equipment or conveyance system  150  to the ion implanter  110 . The controller  120  identifies the substrate  160 , and uses the database  140  to retrieve the resistivity measurement for this substrate. Based on its resistivity measurement, the control  120  modifies the operation of the ion implanter  110 . In this way, the substrate  160  is processed in accordance with its measured resistivity value. 
         [0024]    In other embodiments, the controller  120  tracks changes in resistivity at the measurement station  130 . The resistivity of substrates in a boule may change gradually. Additionally, substrates as delivered to a customer may be arranged in roughly the order in which they were removed from the boule. In this case, it is not necessary to exactly identify a substrate with its measured resistivity. Rather, once the controller  120  identifies that the resistivity has changed, substrates  160  thereafter are processed differently. 
         [0025]    In one embodiment, shown in  FIG. 2A , the dose used to create the BSF is based on the measured resistivity value. For example, the higher the resistivity of the bulk silicon, the higher the dose used for the BSF. Thus, a proportional relationship may exist between the measured resistivity value and the dose used for the BSF. While  FIG. 2A  shows a linear relationship, other relationships may also be used. 
         [0026]    In another embodiment, a finite number of different doses are used by the ion implanter  110 . For example, in one embodiment, shown in  FIG. 2B , two different doses are used for the BSF. If the measured resistivity value exceeds a predetermined threshold, the higher dose is utilized. Otherwise, the lower dose is used. Of course, more than two dose values may be used in this embodiment. 
         [0027]    In another embodiment, shown in  FIG. 2C , a hybrid approach may be utilized. For example, the dose may be set to a first lower value for all resistivity values below a predetermined value. However, above that predetermined value, the dose may be determined based on the measured resistivity value. 
         [0028]    The dose applied to the BSF may be varied using any technique known in the art. For example, the amount of time that ions are implanted into the substrate may be modified to adjust the total dose introduced, such as by changing scan speed or pulse rate of the ion beam. In other embodiments, the ion beam energy, the ion beam current, or ion beam angle may be modified to affect the total dose applied. 
         [0029]    While the above description discloses that the ion implant process may be modified based on the resistivity measurement, other embodiments are also possible. For example, in another embodiment, the ion implant step may be identical for all substrates, regardless of resistivity value. Then, at a subsequent process step, such as the anneal step, the substrates may be processed differently based on their resistivity. For example, a higher thermal budget may be applied to substrates having a higher measured bulk resistivity. In some embodiments, a laser drive in step or an additional ion implant step may be added for high resistivity substrates. Additionally, a dopant paste may be applied to high resistivity substrates prior to the anneal process. In low resistivity substrates, an etch step can be added to remove dopant. Other process steps may also be modified according to measured resistivity. In each of these examples, the processing is varied to allow the processed substrate to have a sheet resistance within a certain range, regardless of its original measured resistivity. In other words, various process steps may be varied in order to create the desired sheet resistance. 
         [0030]      FIG. 3  shows a flowchart according to one embodiment. As shown in step  310 , a substrate enters the measurement station  130 . A resistivity measurement is then performed on the substrate, as shown in step  320 . As described above, probes may be used to contact the surface of the substrate and pass a current therethrough to determine the resistance of the substrate. 
         [0031]    The measured resistivity is then stored in a database, as shown in step  330 . This resistivity value may be associated with the identity of the substrate. For example, an indicia may be located on the substrate which uniquely identifies it. In another embodiment, the substrate is identified based on its position in the batch. 
         [0032]    Later, the substrate enters a process chamber, as shown in step  340 . This process chamber may be ion implanter  110 , an anneal station, or another process chamber. At this time, the controller  120  retrieves the resistivity measurement for this particular substrate from the database, as shown in step  350 . 
         [0033]    Finally, the substrate is processed based on this resistivity measurement, as shown in step  360 . This processing may include variable dosing of the BSF, as described above. In other embodiments, this processing may include variable anneal processes to adjust the sheet resistance of the substrate. Other process variations, such as those described above, can also be performed based on the resistivity measurement. 
         [0034]    It should be noted that steps  310 - 330  of  FIG. 3  may be performed on an unprocessed substrate. However, in some embodiments, the substrate may already have been partially processed prior to entering the measurement station  130 . For example, the substrate may be ion implanted, and then moved to the measurement station  130 . In this case, the resistivity measurement may be used to control or adjust the anneal process that will be subsequently performed. In other words, the duration or temperature of an anneal cycle may be varied based on the measured resistivity. 
         [0035]      FIG. 4  shows a flowchart according to this embodiment. First, as shown in step  400 , the substrate is ion implanted. All substrates are identically implanted in this embodiment. After the substrate has been implanted, its resistivity is measured at the measurement station  130 , as shown in step  410 . This resistivity measurement is stored in the database, as shown in step  420 . Subsequent to the resistivity measurement, the substrate enters a process chamber, as shown in step  430 . The previously measured resistivity value is retrieved from the database, as shown in step  440 . Based on this retrieved resistivity value, the substrate is processed as shown in step  450 . In some embodiments, the process chamber is an anneal chamber, and the anneal cycle is varied based on the resistivity. For example, as stated above, the duration or temperature of the anneal may be modified based on the measured resistivity value. Additionally or alternatively, other process steps, such as laser drive in, additional ion implants, or application of dopant pastes may be performed in step  450  based on this resistivity measurement. 
         [0036]    Additionally, this resistivity measurement could also provide feedback to the ion implanter to vary the dose for a subsequent substrate, based on the resistivity measured after ion implantation. For example, as described above, the resistivity of a batch of substrates may change gradually. Therefore, measuring the resistivity of a substrate after ion implantation and using that information to determine the dose of a subsequent substrate may be effective. 
         [0037]    In some embodiments, like those described above, each substrate is processed individually, and therefore, the process step performed in step  360  may be modified for the resistivity of that particular substrate. However, in some embodiments, multiple substrates may be processed simultaneously. For example, a group of substrates may enter an anneal chamber and be processed together. Similarly, a group of substrates may be ion implanted at the same time. In these embodiments, it may not be possible to tailor the processing of each substrate individually. 
         [0038]    In one embodiment, after the resistivity of each substrate is measured, the substrates are then sorted such that each group of substrates that are processed simultaneously possesses similar or nearly identical resistivity values. Thus, since all substrates in the group have similar characteristics, the processing is appropriate for all substrates in that group. 
         [0039]    In this way, substrates can be sorted such that a group of substrates that is processed simultaneously all have similar characteristics. The process can then be varied for the next group of substrates. Thus, while a first group of substrates that are processed simultaneously uses a first set of process parameters, the next group of substrates may use a different set of parameters. Thus, a batch of substrates may be divided into smaller groups, where each group uses a common set of process parameters. 
         [0040]    In another embodiment, sorting the substrates may not be desirable due to its complexity or impact on throughput. In this embodiment, the controller  120  retrieves the resistivity value for each substrate in the batch. It then performs some function on this set of resistivity values to calculate a batch resistivity value. In some embodiments, this function may be an averaging function. In other embodiments, this function may be a maximum function, where the largest resistivity value is selected as the batch resistivity value. In other embodiments, this function may be a different mathematical function. 
         [0041]    In one particular embodiment, a maximum function is employed. In this way, most of the substrates in the batch receive a higher BSF dose than would otherwise be used. However, the inefficiencies caused by a higher dosed BSF in a lower resistivity substrate may be less than the inefficiencies caused by a lower dosed BSF in a higher resistivity substrate. 
         [0042]    In other embodiments, it is assumed that the changes in resistivity of the substrates changes gradually, as described above. In this way, substrates may be sampled at the measurement station  130 , and this sampled value is then used to process some number of substrates. For example, every tenth substrate may be measured for resistivity and processing based on that measurement may be performed on a larger number of substrates. Note that the measured substrate may or may not be one of the substrates processed based on this measurement. 
         [0043]    In some embodiments, as described above, a conveyance system  150  is employed where substrates  160  move from the measurement station  130  to the ion implanter  110 . Due to the distance between these two apparatuses, there may be a number of substrates  160  that have passed the measurement station  130  but not reached the ion implanter  110 . In one embodiment, the controller  120  is aware of the number of substrates  160  between these two components and properly correlates the measurement of a substrate  160  to the ion implanting of that substrate  160  at a later time. In another embodiment, the controller  120  uses the measurement presently made at the measurement station  130  to change the dose at the ion implanter  130 . In other words, the dose for a particular substrate  160  is varied based on the resistivity measurement of a different substrate  160 . In another embodiment, substrates  160  are only sampled at the measurement station, such that a plurality of substrates  160  is processed in accordance with the resistivity measurement of a single substrate  160 . Other variations are possible. For example, as described above, the process changes do not necessarily occur at the ion implanter  110 . These process changes may be at the anneal station or at another process step. 
         [0044]    In addition, the conveyance system  150  may also be configured such that substrates enter the ion implanter  110  prior to the measurement station  130 . In this embodiment, the measured resistivity is applied to a subsequent substrate  160  entering the ion implanter  110 . 
         [0045]    Several different mechanisms which use the measured resistivity of a substrate  160  to alter the processing of that, or a different substrate, are disclosed. In each of these embodiments, the processing of the substrate  160  is varied such that the sheet resistance of the processed substrate is within a predetermined range. Otherwise, if all substrates are treated identically, the resulting sheet resistance of the substrate is correlated to its underlying resistivity. This may create undesired variation in performance, such as if the substrates  160  are used to create solar cells. By measuring the resistivity, the subsequent processing can be varied to allow a BSF to be applied, where these BSFs all have a sheet resistance within an acceptable range, regardless of the underlying resistivity. 
         [0046]    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. Further, 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.