Patent Application
20140342471
This application relates to a method of determining the properties of an unprocessed substrate, and adjusting the subsequent processing of the substrate based on that property.
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 p-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 p-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 p-type silicon is then doped with n-type ions to create a n-type region, or emitter, adjacent to the remainder of the p-type bulk silicon.
Variations in the sheet resistance of the emitter region, may have an adverse impact on the efficiency of the solar cell. In some instances, the sheet resistance of the emitter region may vary across the surface of the substrate. In other words, the sheet resistance of the emitter may be noticeable different in one portion of the substrate. This non-uniformity may be caused by variations in the texture of the underlying substrate and may have a deleterious effect on the performance of a solar cell produced using such a substrate.
Therefore, an improved method of processing a substrate, using information related to the underlying substrate's properties, is needed.
A system and method for determining the edge or region where a saw first enters a silicon brick, and using this information to process this region differently is disclosed. This region, referred to as the saw entry region, may be thinner, or have a rougher texture than the rest of the substrate. This difference may impact the substrate's ultimate performance. For example, if the substrate is processed as a solar cell, the performance of the saw entry region may be suboptimal.
In one embodiment, a method of processing a substrate is disclosed, which comprises determining which region of the substrate was first entered by a saw when the substrate was separated from a silicon brick, the edge defined as a saw entry region; and processing the saw entry region of a surface of the substrate differently than a remainder of the surface of the substrate. For example, the dose of ions implanted may be altered based on this determination.
In another embodiment, a method of processing a substrate to form a solar cell is disclosed. This method comprises determining a region of the substrate that was first entered by a saw when the substrate was separated from a silicon brick, the region defined as a saw entry region, wherein the determining step is based on a measurement of at least one of conductivity, thickness or texture; rotating the substrate such that the saw entry region has a predetermined orientation; transferring the substrate with the predetermined orientation into an ion implanter; and implanting a first dose of ions into the saw entry region of a surface of the substrate, greater than a second dose implanted into other portions of the surface of the substrate, to compensate for characteristics of the saw entry region.
In another embodiment, an apparatus is disclosed, comprising a detection station, configured to detect a region of a substrate that was first entered by a saw when the substrate was separated from a silicon brick, the region defined as a saw entry region; an ion implanter; a substrate handling system, comprising a rotating robot, to move the substrate from the saw entry region detection station to the ion implanter; and a controller configured to rotate the substrate using the rotating robot such that the substrate enters the ion implanter with the saw entry region in a predetermined orientation.
For a better understanding of the present invention, reference is made to the accompanying drawings, which are incorporated herein by reference and in which:
Multi-crystalline silicon (mc-Si) is typically grown as large blocks, known as bricks. A saw is then used to cut thin substrates from the larger brick. The saw may be a wire saw that enters the brick along one edge and proceeds through the entirety of the brick. Acidic texture is typically used to improve the reflectance of mc-Si solar cells. Acid is applied to an unprocessed substrate after it has been separated from the brick using the saw. The damage caused by the saw creates the initial texture pattern on the substrate. The acid then further textures the substrate.
After texturing, ion implantation may be performed to create the emitter region. After ion implantation, the emitter region of a silicon substrate may have non-uniform sheet resistance across its surface. For example, one edge may have noticeable higher sheet resistance than the rest of the substrate.
It has been discovered that this non-uniformity of sheet resistance may be due to non-uniform texturing. This non-uniformity in the texturing has been found to be correlated with the wafer thickness. Further investigation reveals that the damage caused by the saw is not uniform across the substrate. Specifically, the edge or end of the brick where the saw first enters the silicon may cause the substrates that are cut to be thinner in that region than the rest of the substrate. Throughout this disclosure, the term “saw entry region” is used to describe the portion of the substrate where the saw first entered the brick. In other words, when the saw enters and passes through the brick, it creates substrates, where these substrates may have a region that has different characteristics than the rest of the substrate. This region of the substrate correlates to the edge where the saw first entered the brick. When the saw actually enters the brick, it is recognized that the saw creates the unevenness in the cut substrates. Thus, the edge where the saw first enters the brick corresponds to a region of the substrate referred to as the “saw entry region”. The saw entry region may also include the region proximate to the edge where the saw first entered, and include portions where the thickness or texture of the substrate is different from the rest of the substrate. For example, in
Therefore, by determining which edge of the substrate corresponds to the edge of the brick that was first entered by the saw, it is possible to compensate for these effects.
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 in a processing chamber where plasma is generated.
The system 100 also includes a saw entry region detection station 130, which is used to determine the edge first entered by the saw. For example, the saw entry region detection station 130 may measure a property of the substrate to detect the saw entry region. In one embodiment, the saw entry region detection station 130 may measure the thickness of the substrate at various locations across its surface to determine the saw entry region. Thickness may be measured in a variety of ways. For example, this may be performed by optical measurement using a CCD camera to determine substrate thickness. In another embodiment, thickness is determined by determining the distribution of mass across the substrate. For example, a comparison of the center of gravity to the geometric center may be used to determine the saw entry region, which is lighter than the other edges.
In another embodiment, an eddy current detector is used. In this embodiment, a coil carrying current is disposed near the substrate, so as to induce eddy current in the substrate. One or more probes are then used to measure this eddy current at different points along the substrate. Based on these measurements, the least conductive portion of the substrate can be determined. This least conductive portion may be determined to be the saw entry region.
In other embodiments, the saw entry region detection station may determine the roughest portion of the substrate. For example, profilometry or reflectance techniques may be employed to determine the roughest portion of the substrate. This roughest edge may be determined to be the saw entry region.
While the saw entry region detection station 130 may be used to measure a property of the substrate that is altered by the saw entry, other embodiments are also possible. For example, an indication of the saw entry region may be created when the substrate is cut. For example, a fiducial may be placed on the saw entry region immediately after the saw cut. The saw entry region detection station 130 would then use optical means to detect the fiducial.
In another embodiment, the substrate may be imaged immediately after the saw cut. The grain pattern at the saw entry region is then stored. The saw entry region detection station 130 then compares this stored grain pattern to an optical image of the substrate to determine the saw entry region.
Other methods of determining the saw entry region of the substrate may also be employed by the saw entry region detection station 130. Having determined the saw entry region, several different subsequent steps can be performed to process this saw entry region in order to equalize the sheet resistance of the emitter for the entire substrate.
The system 100 may also include automated substrate handling equipment 150 for transferring substrates between the measurement station 130 and the platen 111, which may include robots, conveyor belts, or other systems known to those skilled in the art. The substrate enters the measurement station 130 prior to entering the ion implanter 110. The automated substrate handling equipment 150 may include a rotating robot, which may be used to orient the substrates such that the saw entry regions are all aligned consistently.
The system 100 also includes a controller 120 in communication with the saw entry region detection station 130, the automated substrate handling equipment 150 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 beam generator 112, and the measurement station 130 and provide output signals to each to control the same.
In operation, the substrate is transferred to the saw entry region detection station 130, such as by the substrate handling equipment 150. Thereafter, the controller 120 performs a saw entry region detection technique, such as any of those described above. Based on the detection technique, the saw entry region can be determined. The orientation of this saw entry region may then stored by the controller 120 in storage element 125. As the substrate is removed from the saw entry region detection station 130, the controller 120 instructs the substrate handling equipment 150 to rotate the substrate to orient the saw entry regions of all of the substrates before these substrates enter the ion implanter 110.
The ion implanter 110 can then be configured to apply additional dose to the saw entry region to compensate for its higher sheet resistance due to its relative thinness and roughness. For example, the ion implanter may utilize a scanner to move the substrate in the path of the ion beam 113. The scanner may be slowed when the saw entry region is in the path of the ion beam 113. This allows additional ions to be implanted in this saw entry region of the substrate. For example,
While changing scanning speed can be used to vary the dose at the saw entry region, other techniques can also be used. For example, in a pulsed beam architecture, the pulse rate of the ion beam can be increased under the region near the saw entry region. In another embodiment, the extraction or beam optics can be modified to increase beam current. In other embodiments, a mask may be used to cover most of the substrate while additional ions are implanted into the region near the saw entry region. In yet another embodiment, a first uniform dose can be applied to the entire substrate. A second patterned implant may be subsequently applied to the region near the saw entry region. Of course, other methods of applying a greater dose to one particular region of a substrate are also possible and are within the scope of the disclosure.
The controller then instructs the substrate handling equipment to rotate the substrate to properly orient the saw entry region, as shown in step 530.
Later, the substrate enters a process chamber, as shown in step 540. This process chamber may be ion implanter 110. At this time, the substrate is processed based on this saw entry region detection, as shown in step 550. This processing may include increased dosing of the substrate in the region near the saw entry region. Stated differently, the system processes the saw entry region differently than the rest of the substrate. For example, the rest of the substrate may be implanted with a first dose, while the saw entry region is implanted with a second dose, greater than the first dose. In one embodiment, the ion implantation is used to form an emitter region of a solar cell, where the saw entry region is implanted with a dose greater than that applied to the rest of the substrate.
The above description describes a process where each substrate is implanted individually. In some embodiments, a set of substrates, also known as a batch, are implanted simultaneously. For example, a batch of the substrates 600 may be arranged in an array, as shown in
Further, the above disclosure describes a system and method where the saw entry region is identified and, once identified, processed accordingly. This may assume that the thickness and texture of all saw entry regions are sufficiently similar so a single process to compensate for non-uniformity is applicable to all saw entry regions. However, in some embodiments, properties of saw entry regions of different substrates may differ from each other, either in terms of thickness, texture or both. Thus, a single process to compensate for non-uniformity of all saw entry regions may be inadequate. In these embodiments, several different process steps may be undertaken.
In systems where substrates are processed in the ion implanter 110 individually, the scanning speed or other mechanism to increase the dose in the region near the saw entry region may be tailored specifically to that substrate. For example, the saw entry region detection station 130 may measure the thickness of the saw entry region, and the controller 120 may calculate an optimal dose for that thickness to improve the uniformity of the emitter. This optimal dose can then be converted to a scanning speed, ion beam current, or another parameter. The substrate is then processed accordingly. For example, the saw entry region region may receive this optimal dose, while the rest of the substrate receives the nominal dose, as described above.
When substrates are processed in batches, as shown in
In other embodiments, the controller 120 stores the saw entry region properties of each substrate in its storage element 125. When the substrates are arranged in arrays, as shown in
Once the batch property of a particular column is determined, this value is converted to an optimal dose, which can be implemented by varying scanning speed, ion beam current or some other parameter. Every substrate in a particular column is then processed in accordance with this batch column value.
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.
