Patent Application
20110192993
The present invention relates generally to the field of solar cells and other large substrate implant applications. More particularly, the present invention relates to solar cell devices and methods of their formation, including the issue of selective implantation.
In one aspect of the present invention, an adjustable shadow mask implantation system comprises: an ion source configured to provide ions; and an shadow mask assembly configured to selectively allow ions from the ion source to pass therethrough to a substrate where they are implanted, wherein the shadow mask assembly is configured to adjust between a first position and a second position, wherein the shadow mask assembly enables ion implantation of multiple substantially parallel lines absent any lines with an intersecting orientation with respect to the multiple substantially parallel lines when set in the first position, and wherein the shadow mask assembly enables ion implantation of multiple substantially parallel lines and a line with an intersecting orientation with respect to the multiple substantially parallel lines when set in the second position. In some embodiments, the multiple parallel lines are substantially perpendicular to the line with the intersecting orientation.
In some embodiments, the shadow mask assembly comprises: a first exposure region having multiple elongated openings substantially parallel to a first axis; and a second exposure region having an elongated opening substantially parallel to a second axis, wherein the second axis is substantially perpendicular to the first axis, wherein the shadow mask assembly is configured to adjust between the first position and the second position by adjusting the distance between the first exposure region and the second exposure region. In some embodiments, the first exposure region comprises: a first occlusion mask having a first set of elongated openings substantially parallel to a first axis; and a second occlusion mask having a second set of elongated openings substantially parallel to the first axis, wherein the first occlusion mask and the second occlusion mask are configured such that the first set of elongated openings overlap with, but are offset from, the second set of elongated openings to form the multiple elongated openings of the first exposure region, the multiple elongated openings of the first exposure region being smaller than each one of the elongated openings in the first set of the first occlusion mask and the second set of the second occlusion mask. In some embodiments, the shadow mask assembly further comprises a homogeneous exposure region configured to enable a blanket homogeneous implantation of the substrate from the ions.
In some embodiments, the system further comprises: a moveable substrate holder configured to move the substrate at a velocity through the path of the ions passing through the shadow mask assembly; and a controller operationally coupled to the moveable substrate holder, wherein the controller is configured to adjust the velocity to a first level for the first position and to a second level for the second position. In some embodiments, the second level is a lower velocity than the first level. In some embodiments, the moveable substrate holder is configured to rotate the substrate through the path of the ions passing through the shadow mask assembly. In some embodiments, the shadow mask assembly comprises: a first exposure member having multiple elongated openings substantially parallel to a first axis, wherein the length of the multiple elongated openings is greater the farther away they are from the point of rotation about which the substrate is rotated; and a second exposure member having an elongated opening configured in an intersecting orientation relative to the first axis, wherein the elongated opening gets wider as it extends away from the point of rotation about which the substrate is rotated, wherein the shadow mask assembly is configured to adjust between the first position and the second position by adjusting the distance between the first exposure member and the second exposure member. In some embodiments, the multiple elongated openings are curved.
In another aspect of the present invention, a method of ion implantation comprises: flowing ions through a shadow mask assembly to a substrate; adjusting the shadow mask assembly to a first position, wherein the substrate is selectively implanted with multiple substantially parallel lines of ions absent any lines of ions with an intersecting orientation with respect to the multiple substantially parallel lines; and adjusting the shadow mask assembly to a second position, wherein the substrate is selectively implanted with multiple substantially parallel lines of ions and a line of ions with an intersecting orientation with respect to the multiple substantially parallel lines. In some embodiments, the multiple parallel lines are substantially perpendicular to the line with the intersecting orientation.
In some embodiments, the shadow mask assembly comprises: a first exposure region having multiple elongated openings substantially parallel to a first axis; and a second exposure region having an elongated opening substantially parallel to a second axis, wherein the second axis is substantially perpendicular to the first axis, wherein the shadow mask assembly is configured to adjust between the first position and the second position by adjusting the distance between the first exposure region and the second exposure region. In some embodiments, the first exposure region comprises: a first occlusion mask having a first set of elongated openings substantially parallel to a first axis; and a second occlusion mask having a second set of elongated openings substantially parallel to the first axis, wherein the first occlusion mask and the second occlusion mask are configured such that the first set of elongated openings overlap with, but are offset from, the second set of elongated openings to form the multiple elongated openings of the first exposure region, the multiple elongated openings of the first exposure region being smaller than each one of the elongated openings in the first set of the first occlusion mask and the second set of the second occlusion mask. In some embodiments, the method further comprises flowing ions through a homogeneous exposure region of the shadow mask assembly, thereby providing a blanket homogeneous implantation of the substrate from the ions.
In some embodiments, the method further comprises: a substrate holder moving the substrate at a velocity through the path of the ions passing through the shadow mask assembly;
and a controller adjusting the velocity to a first level for the first position and to a second level for the second position. In some embodiments, the second level is a lower velocity than the first level. In some embodiments, the moveable substrate holder rotates the substrate through the path of the ions passing through the shadow mask assembly. In some embodiments, the shadow mask assembly comprises: a first exposure member having multiple elongated openings substantially parallel to a first axis, wherein the length of the multiple elongated openings is greater the farther away they are from the point of rotation about which the substrate is rotated; and a second exposure member having an elongated opening configured in an intersecting orientation relative to the first axis, wherein the elongated opening gets wider as it extends away from the point of rotation about which the substrate is rotated, wherein the shadow mask assembly is configured to adjust between the first position and the second position by adjusting the distance between the first exposure member and the second exposure member. In some embodiments, the multiple elongated openings are curved.
In some embodiments, the ions are flown or accelerated through the shadow mask assembly to the substrate from an ion source, the ion source comprising at least two different ion species having different masses.
In some embodiments, the method further comprises applying a voltage selectively to spaced apart regions on a side of the substrate opposite the shadow mask assembly, wherein the selective application of voltage promotes the selective implantation of ions.
In some embodiments, the shadow mask assembly is part of a first grid assembly, the first grid assembly comprising a first grid plate and a second grid plate, each grid plate of the first grid assembly having a plurality of apertures configured to allow ions to pass therethrough, the first grid plate of the first grid assembly being positively biased by a power supply, the second grid plate of the first grid assembly being negatively biased by the power supply, the method comprising the ions flowing through the first grid assembly for selective implantation of the substrate. In some embodiments, a second grid plate is coupled to the first grid assembly via a voltage divider, the second grid assembly comprising a first grid plate and a second grid plate, each grid plate of the second grid assembly having a plurality of apertures configured to allow ions to pass therethrough, the first grid plate of the second grid assembly being positively biased by the power supply, the second grid plate of the second grid assembly being negatively biased by the power supply, the method comprising the ions flowing through the second grid assembly for implantation of the substrate.
In yet another aspect of the present invention, a shadow mask implantation system comprises: an ion source configured to provide ions; a first occlusion mask having a first set of elongated openings substantially parallel to a first axis; and a second occlusion mask having a second set of elongated openings substantially parallel to the first axis, wherein the first occlusion mask and the second occlusion mask are configured such that the first set of elongated openings overlap with, but are offset from, the second set of elongated openings to form a resulting set of elongated openings through which ions from the ion source are selectively allowed to pass therethrough to a substrate where they are implanted, each elongated opening of the resulting set being smaller than each elongated opening of the first and second sets.
In some embodiments, the thickness of each elongated opening of the resulting set is equal to or less than half the thickness of each elongated opening of the first and second sets. In some embodiments, the thickness of each elongated opening of the resulting set is equal to or less than 50 microns.
In some embodiments, the system further comprises a shadow mask assembly having a first exposure region and a second exposure region, wherein: the first occlusion mask and the second occlusion mask form the first exposure region comprising the resulting set of elongated openings substantially parallel to the first axis; the second exposure region has an elongated opening substantially parallel to a second axis, wherein the second axis is substantially perpendicular to the first axis; the shadow mask assembly is configured to adjust between a first position and a second position by adjusting the distance between the first exposure region and the second exposure region; the shadow mask assembly enables ion implantation of multiple substantially parallel lines absent any lines with an intersecting orientation with respect to the multiple substantially parallel lines when set in the first position, the multiple substantially parallel lines corresponding to the resulting set of elongated openings; and the shadow mask assembly enables ion implantation of multiple substantially parallel lines and a line with an intersecting orientation with respect to the multiple substantially parallel lines when set in the second position, the multiple substantially parallel lines corresponding to the resulting set of elongated openings. In some embodiments, the multiple parallel lines are substantially perpendicular to the line with the intersecting orientation. In some embodiments, the system further comprises: a moveable substrate holder configured to move the substrate at a velocity through the path of the ions passing through the shadow mask assembly; and a controller operationally coupled to the moveable substrate holder, wherein the controller is configured to adjust the velocity to a first level for the first position and to a second level for the second position, the second level being a lower velocity than the first level.
In yet another aspect of the present invention, a method of ion implantation comprises: flowing or accelerating ions through a shadow mask assembly to a substrate, wherein the shadow mask assembly comprises a first occlusion mask having a first set of elongated openings substantially parallel to a first axis and a second occlusion mask having a second set of elongated openings substantially parallel to the first axis, wherein the first occlusion mask and the second occlusion mask are configured such that the first set of elongated openings overlap with, but are offset from, the second set of elongated openings to form a resulting set of elongated openings through which ions from the ion source are selectively allowed to pass therethrough to the substrate, each elongated opening of the resulting set being smaller than each elongated opening of the first and second sets; and implanting the ions into the substrate, thereby forming multiple substantially parallel lines of ion implantations corresponding to the resulting set of elongated openings.
In some embodiments, the thickness of each elongated opening of the resulting set is equal to or less than half the thickness of each elongated opening of the first and second sets. In some embodiments, wherein the thickness of each elongated opening of the resulting set is equal to or less than 50 microns.
In some embodiments, a shadow mask assembly has a first exposure region and a second exposure region, the first occlusion mask and the second occlusion mask form the first exposure region comprising the resulting set of elongated openings substantially parallel to the first axis, the second exposure region has an elongated opening substantially parallel to a second axis, wherein the second axis is substantially perpendicular to the first axis, the shadow mask assembly is configured to adjust between a first position and a second position by adjusting the distance between the first exposure region and the second exposure region, the shadow mask assembly enables ion implantation of multiple substantially parallel lines absent any lines with an intersecting orientation with respect to the multiple substantially parallel lines when set in the first position, the multiple substantially parallel lines corresponding to the resulting set of elongated openings, and the shadow mask assembly enables ion implantation of multiple substantially parallel lines and a line with an intersecting orientation with respect to the multiple substantially parallel lines when set in the second position, the multiple substantially parallel lines corresponding to the resulting set of elongated openings. In some embodiments, the multiple parallel lines are substantially perpendicular to the line with the intersecting orientation. In some embodiments, the method further comprises: a moveable substrate holder moving the substrate at a velocity through the path of the ions passing through the shadow mask assembly; and a controller adjusting the velocity to a first level for the first position and to a second level for the second position, the second level being a lower velocity than the first level.
The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the described embodiments will be readily apparent to those skilled in the art and the generic principles herein may be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein.
Furthermore, it is contemplated that any features from any embodiment can be combined with any features from any other embodiment. In this fashion, hybrid configurations of the illustrated embodiments are well within the scope of the present invention.
Various aspects of the disclosure may be described through the use of flowcharts. Often, a single instance of an aspect of the present disclosure may be shown. As is appreciated by those of ordinary skill in the art, however, the protocols, processes, and procedures described herein may be repeated continuously or as often as necessary to satisfy the needs described herein. Additionally, it is contemplated that method steps can be performed in a different order than the order illustrated in the figures, unless otherwise disclosed explicitly or implicitly.
The present invention is directed towards a series of mechanisms that enable the formation of patterned implant doping, deposition and evaporation of various elements. One benefit of the present invention is that it can achieve such implantation without the use of any additional lithographic or other external masking. The shadow-defining mask of the present invention preferably resides within, or can be translated into, a doping or deposition system. An adjustable shadow mask assembly is disclosed that is not only tailored for the manufacturing of solar cells, but that can be used for semiconductor and other surface and near-surface modification applications as well. Various embodiments of the mask can be used in an in-line system with stationary or continuously-moving wafers. It can also be used with a linear or a rotary system, thereby enabling complete scanning of the wafers with an ion beam. This mask can provide accurate and well-defined doping and deposition patterns, as well as unique atomic profile tailoring capability for solar cells, incorporating features from commonly-owned U.S. patent application Ser. No. 12/483,017, entitled “FORMATION OF SOLAR CELL-SELECTIVE EMITTER USING IMPLANT AND ANNEAL METHOD,” filed Jun. 11, 2009, and from commonly-owned U.S. Provisional Application No. 61/131,698, entitled “FORMATION OF SOLAR CELL-SELECTIVE EMITTER USING IMPLANT AND ANNEAL METHOD,” filed Jun. 11, 2008, which are both hereby incorporated by reference as if set forth herein. These include change in doping levels, resistance of contact, bus bar, fingers, contact resistance of metal-silicon interface, resistance of backside metallization, achieving the desired resistivity under the metal grid contact (preferably 10 to 30 Ohms/Sq.) and in between the fingers (preferably 80-100 Ohms/Sq.) to meet higher efficiency solar cells. To achieve these benefits, the present invention can incorporate features from commonly-owned U.S. patent application Ser. No. 12/482,980, entitled “SOLAR CELL FABRICATION USING IMPLANTATION,” filed Jun. 11, 2009, commonly-owned U.S. Provisional Application No. 61/131,687, entitled “SOLAR CELL FABRICATION USING IMPLANTATION,” filed Jun. 11, 2008, commonly-owned U.S. patent application Ser. No. 12/482,685, entitled “SOLAR CELL FABRICATION WITH FACETING AND ION IMPLANTATION,” filed Jun. 11, 2009, and commonly-owned U.S. Provisional Application No. 61/133,028, entitled “SOLAR CELL FABRICATION WITH FACETING AND ION IMPLANTATION,” filed Jun. 24, 2008, which are all hereby incorporated by reference as if set forth herein. The present invention also meets the demands of future requirements for solar cell wafer thickness, as well as contact metal width and spacing.
Moreover, the advantageous formation of simultaneous homogenous and selective patterned emitter, Back Surface Field (BSF) and metal-implanted silicide, as well as the present invention's ability to improve performance will be possible, and can incorporate features from commonly-owned U.S. patent application Ser. No. 12/482,947, entitled “APPLICATION SPECIFIC IMPLANT SYSTEM AND METHOD FOR USE IN SOLAR CELL FABRICATIONS,” filed Jun. 11, 2009, commonly-owned U.S. Provisional Application No. 61/131,688, entitled “APPLICATIONS SPECIFIC IMPLANT SYSTEM AND METHOD FOR USE IN SOLAR CELL FABRICATIONS,” filed Jun. 11, 2008, commonly-owned U.S. Provisional Application Ser. No. 61/210,545, entitled “ADVANCED HIGH EFFICIENCY CRYSTALLINE SOLAR CELL FABRICATION METHOD,” filed Mar. 20, 2009, and commonly-owned U.S. patent application Ser. No. 12/728,105, entitled “ADVANCED HIGH EFFICIENCY CRYSTALLINE SOLAR CELL FABRICATION METHOD,” filed Mar. 19, 2010, which are all hereby incorporated by reference as if set forth herein. The present invention can be applied to as-grown single or mono-crystalline, poly or multi-crystalline or electrical-grade or metallurgical-grade silicon, as well as very thin silicon wafers and very thin film deposited silicon, or other materials used for solar cell formation and other applications. The present invention can also be applied to multi junction devices, and can be extended to atomic species placement for any other material used in fabrication of junctions and metal semiconductor interface enhancements.
One of the main features of the present invention is a fast beam shutter mechanism used to define the typical busbar patterns.
In some embodiments, a fast shutter mechanism is provided on the shadow mask assembly 100 and is configured to quickly block or un-block the elongated vertical opening 115, such as by adjusting the positioning of a beam shutter 105 to block the elongated vertical opening 115. The adjustment lasts for the required duration, thereby exposing the substrate 140 to the desired ion dose with the pre-determined mask pattern.
In some embodiments, a homogeneous exposure member 130 is also coupled to the adjustable shadow mask assembly 100. The homogeneous exposure member 130 comprises an opening 135 configured to allow a blanket ion implantation of the substrate 140 to provide a homogeneous doping of the substrate, as seen by the dark regions on the resulting substrate 140′.
The resulting substrate 140′ represents the typical crystalline solar cell metal lines. The adjustable shadow mask assembly 100 allows vertical and horizontal, as well as homogeneous, exposure of ion beams to a wafer. The wafer can be moved underneath the shadow mask assembly 100 in steady state or truncated movement, or it can be modified to cater for stop-exposure and move pattern. As described above, the beam shutter 105 is deployed when the vertical line exposure is not needed and is used only for the exposure time required. The horizontal openings 125 trace dopant lines underneath the metal gridlines on a solar cell as the wafer moves beneath them. The length and width of these lines can be either manually or automatically adjusted to provide the desired exposure and doping levels.
In some embodiment, the shadow mask assembly 100 is configured to quickly block or un-block the elongated vertical opening 115 by adjusting the positioning of the vertical exposure member 110 or the horizontal exposure member 120 with respect to one another to block the elongated vertical opening 115. In
In some embodiments, either the homogeneous doping or the selective patterned doping or both can be processed. In some embodiments, the timing of the wafer passage and shutter opening can be interrelated and define the necessary exposure.
In a preferred embodiment, the multiple horizontal lines are substantially parallel to one another and perpendicular to the vertical busbar lines. However, it is contemplated that other configurations are within the scope of the present invention. Furthermore, in some embodiments, instead of or in addition to elongated horizontal lines, the openings in the shadow mask can comprise other shapes as well, including, but not limited to, circular point openings or ring-shaped openings. In some embodiments where the elongated horizontal implantation lines on the substrate are replaced by rows and columns of such spaced-apart circular- or other-shaped implantations, a surface of the substrate can be blanketed with a conducting material to contact each individual contact collection region, instead of or in addition to using the busbar collection lines.
In some embodiments, the stage speed can be dynamically adjusted while processing the substrate with the energetic ion sources, either with or without the fast shutter movement. The stage speed is the velocity at which the implantation beam scans the substrate during a particular stage of the implantation, such as the during the horizontal-only implantation stage and the horizontal-and-vertical implantation stage.
Manufacturing shadow masks can be difficult and costly given the challenge of achieving small exposure openings, such as the multiple horizontal exposure openings. Photolithography can be used to cut out the mask patterns, but it is very expensive. Masks are consumable and will erode after so many exposures to ion beams, thus requiring them to be changed. Another feature of the present invention is the use of offset stackable shadow masks to define the finer features, such as the horizontal exposure openings, using much larger, more easily manufacturable and cost effective patterns. Two or more stacking shadow masks can be used to achieve these finer features by offsetting the same or different patterns on two masks, where a primary mask with a certain opening is occluded by an occlusion mask of similar or varying opening. Such a mask can be either hard mask or made from disposable material for renewable usage of the mask assembly. In particular for the solar industry, it is imperative to implement the most cost effective process manufacturing method possible. This invention can drastically reduce the cost of manufacturing the desired shadow mask patterns without using more expensive manufacturing method such as lithography, etching or laser patterning to define the pattern on the shadow mask. In contrast, a more conventional and simpler mechanical cutting or wire EDM method, which tends to have poorer dimensional specifications at such precise and very small openings, can be used to manufacture the larger features on two shadow masks. The two masks can be then stacked together to effectively define much smaller patterns. Such a stacked shadow mask assembly can define any pattern scale down to zero micrometers. Indeed, any pattern or shape can be constructed. For interdigitated selective lines, one could image a series of masks that, with an automated and slight movement, could transfer the opening from one selective line to the other, as a substrate translates under varying species plasmas and varying energized grids (as will be discussed in further detail below with respect to
Furthermore, since the masks can be separated, a voltage can be applied to them in order to provide an optical means of managing the beam shape. It is contemplated that properties of charged ions can be utilized to shape and form even smaller features by applying varying voltages to the two or more stacked masks. Such independent applications of voltage can be used to change the ionized beam dimension independent of any mechanical movement described above. The spacing of the two or more masks and the masks and wafers can also be used to provide a selective pattern of exposure or a homogeneous pattern. The spacing of the shadow mask assembly from the substrate can also be utilized to provide a better geometrical shaping of the beam, using the space charge properties of the ion beam. It can reside very close to the substrate for best definition of selective doping and evaporation, or can be moved away for better homogeneity of the beam if required.
The present invention provides a wafer processing scheme for different ion implantation energies and species configurations to tailor the dopant junction profiles in the solar cells or semiconductor devices using a plasma grid implanter similar to the plasma grid implantation system disclosed in commonly-owned U.S. patent application Ser. No. 12/821,053, entitled “PLASMA GRID IMPLANT SYSTEM FOR USE IN SOLAR CELL FABRICATIONS,” filed Jun. 22, 2010, and from commonly-owned U.S. Provisional Application No. 61/219,379, entitled “PLASMA GRID IMPLANT SYSTEM FOR USE IN SOLAR CELL FABRICATIONS,” filed Jun. 23, 2009, which are both hereby incorporated by reference as if set forth herein, and the features of which can be incorporated into the present invention.
The system comprises a chamber that houses a first (top) grid plate, a second (middle) grid plate, and a third (bottom) grid plate. These grid plates 1150 can be formed from a variety of different materials, including, but not limited to, graphite, silicon carbide, and tungsten. Each grid plate comprises a plurality of apertures configured to allow ions to pass therethrough. A plasma source provides a plasma to a plasma region 1160 of the chamber. This plasma region is located above the first grid plate. In some embodiments, the chamber walls are configured to repel ions in the plasma region using an electric field. For example, in some embodiments, one or more magnets are coupled to the wall of the chamber. The magnetic field is used to push the plasma off the walls, thereby maintaining a gap between the plasma and the chamber walls, and avoiding any sputtering off of the wall material into the plasma. A target substrate 1140 is positioned on the opposite side of the grid plates 1150. The target substrate 1140 can be supported by an adjustable substrate holder, thereby allowing the target substrate to be adjusted between a homogeneous implant position and a selective implant position.
In some embodiments, plasma ions 1170 are accelerated towards the target substrate 1140 by application of a DC or pulsed potential to the first grid plate. These ions are implanted into the substrate 1140. The deleterious effect of secondary electrons resulting from the impingement of ions on the substrate 1140 and other materials is avoided through the use of the second grid plate, which is negatively-biased with respect to the first grid plate. This negatively-biased second grid plate suppresses the electrons that come off of the substrate. In some embodiments, the first grid plate is biased to 80 kV and the second grid plate is biased to −2 kV. However, it is contemplated that other biasing voltages can be employed. The third grid plate acts as a beam defining grid and is preferably grounded. It is positioned in contact with or very close to the surface of the substrate in order to provide a final definition of the implant. This third grid plate can act as a beam defining mask and provide the critical alignment required, if a selective implant is required. The third grid plate can also be replaced or supplemented with any form of beam shaping that does not require a mask, such as the pulsed beam shaping embodiment of
This grid plate assembly arrangement enables the use of DC or pulsed bias for the acceleration of ions and minimizes the back streaming electrons that has hampered plasma immersion technology by limiting the energy range and making the pulser/PSU cost prohibitive. This dramatically simplifies the power supply needed.
Additionally, by decoupling the plasma formation from the ion acceleration, the present invention allows for independent methods to be used for the formation of the plasma above the grid plates. The grid plates can provide some degree of beam definition. For example, the extracted ion beam can be focused to a particular dimension of selective emitter applications.
In this plasma grid implant system, the chamber is configured to allow the plasma to form and expand. As previously discussed, the first grid plate is at a positive potential with respect to ground. By shaping this biased grid plate (electrode) and managing the shape of the meniscus 1180 formed above each of its openings, ions are extracted and optically shaped.
A beam of ions exiting past an aperture is divergent by its nature, which is because the typical equilibrium of plasma is convex. The ions repel each other because of their like electric charge and they have randomly orientated velocities due to thermal motion within the plasma. Therefore, careful design of the grid plate apertures and the plasma condition is necessary to control both the emittance of ions and system acceptance to the ion beam. The emittance is a measure of the beam quality. Typically, high quality beams have low emittance, which means minimal loss of ions during transmission. This has to be balanced against the system specific phase-space boundary such that the beam fits within this boundary or has good acceptance. The control of ion divergence in the system of the present invention is achieved primarily through adjusting the shape of the ensuing meniscus 1180 at the plasma boundary as it enters the first grid plate electrode. Such shaping can be controlled by adjusting the voltage difference between various electrodes, the shapes of the opening and spacing between various electrodes, the temperature of the plasma, how much plasma gas is used, the density of the plasma and the ion species and current being extracted. For the concave dome shape of meniscus 1180, the second grid plate has to have a negative potential with respect to the first grid plate, and the plasma ion density has to be less than the plasma boundary. Although
The system in
Referring back to the stacked mask assembly, it will also allow for better cooling capability of the mask that is impacted by the beam. The cooling lines can be sandwiched between masks and ensure repeatable spacing between the masks. The cooling lines can be used as insulation of the various mask for the application described above. In some embodiments, the temperature of one or more of the stackable shadow masks can be monitored. In some embodiments, cooling lines or channels are provided on the masks to help cool and regulate the temperature of the mask. The mask can also be preheated to help handle the heat of the implantation beams. The temperature of the shadow mask can be monitored and maintained at the same level, either elevated or cooled. In some embodiments, one or more of the masks are formed from a silicon material, including, but not limited to, silicon carbide. In some embodiments, one or more of the masks comprises a silicon coating, such as a silicon carbide coating.
Additionally, in some embodiments, the present invention can use the current arriving at the shadow mask as a means of monitoring the implantation system's performance. In some embodiments, the implantation system is configured to measure the beam current passing through the shadow mask. For example, the system can calculate what fraction of the total beam actually passes through the shadow mask to the substrate based on the ratio of the area of the opening(s) in the shadow mask to the area of the shadow mask. This feature can function as a simple beam current metrology tool. In some embodiments, the implantation system can be configured to make adjustments based on it's monitoring of the implantation if a certain predetermined condition arises.
In another aspect of the present invention, the substrate can be processed in a rotating process stage, as shown in
Similar to the in-line system in
The pattern of opening is uniquely curved and/or angled to adjust for the rotational movement of the wafers. Such arcing or wedge-shaped opening can be occluded to provide a similar pattern of exposure as the linear system described above. In other embodiments a stop-exposure move can also be employed with a similar desired effect. As described above, the substrate to be processed is introduced through the load lock chamber to the rotating processing stage. While moving near the shadow mask 805, the substrate can be exposed to the energetic ions for the homogeneous emitter application step 830. When the substrate is moved under the shadow mask 805, the selective emitter processing is carried out while the subsequent substrate is being loaded via the load lock. In this particular example, only the first half of the substrate is being exposed under the shadow mask 805. During this exposure, the subsequent substrate is being loaded through the load lock. In some embodiments, the sequencing is described as follow:
The shadow mask assembly comprises a vertical exposure member 910 having an elongated vertical opening 915 for selective exposure of the substrate to a vertically-elongated ion beam when the assembly is in the open position 900′. In the open position 900′, the vertical wall 912 of the vertical exposure member 910 is offset from the horizontal exposure member 920, thereby forming the elongated vertical opening 915. In the open position 900′, the vertical wall 912 of the vertical exposure member 910 is in contact with the horizontal exposure member 920, thereby blocking the elongated vertical opening 915. The horizontal exposure member 920 comprises multiple horizontal openings 925 for selective exposure of the substrate to multiple horizontally-elongated ion beams. In order to accommodate the rotational movement of the substrate, in some embodiments, the farther away from the point of rotation (the bottom of the shadow mask assembly), the longer the horizontal openings 925, since the portions of the substrate towards the outside move along faster and, therefore, need longer exposure. This gradual increase in length from the inside to the outside of the shadow mask assembly provides this additional exposure. Similarly, it is also contemplated that the elongated vertical opening 915 can also be wedge-shaped so that it increases in width as it extends away from the point of rotation.
At step 1010a, an adjustable shadow mask assembly, such as those discussed above, is set for selective implantation of multiple horizontal lines without any vertical busbar lines. Here, the shadow mask assembly is adjusted to the appropriate configuration with the elongated vertical opening blocked, if it is not already in this configuration. At step 1020a, the relative velocity between the wafer and the shadow mask assembly is increased, such as by speeding up the movement of the wafer. At step 1030a, multiple horizontal lines are implanted onto the wafer without any implantation of a vertical busbar line.
At step 1010b, the adjustable shadow mask assembly is set for selective implantation of multiple horizontal lines with a vertical busbar line. Here, the shadow mask assembly is adjusted to the appropriate configuration with the elongated vertical opening exposed, if it is not already in this configuration. At step 1020b, the relative velocity between the wafer and the shadow mask assembly is decreased, such as by slowing down the movement of the wafer. At step 1030b, multiple horizontal lines are implanted onto the wafer with an implantation of a vertical busbar line.
As previously mentioned, the method can alternate between selective horizontal line implantation with or without vertical busbar implantation. Additionally, homogeneous doping can be performed at step 1005, before the selective implantation, or at step 1035, after the selective implantation.
The present invention also provides a method of tailoring the dopant junction profile in semiconductor devices or solar cell devices by incorporating two or more plasma source gas feed stocks to generate ion species with significantly different masses and/or doping types. Heavier mass ions form a shallower dopant junction profile than those of lighter mass ions. This characteristic can be utilized to form a dopant junction profile of particular interest, as shown in
It is contemplated that the present invention can be configured to provide either the embodiment of
The present invention has been described in terms of specific embodiments incorporating details to facilitate the understanding of principles of construction and operation of the invention. Such reference herein to specific embodiments and details thereof is not intended to limit the scope of the claims appended hereto. It will be readily apparent to one skilled in the art that other various modifications may be made in the embodiment chosen for illustration without departing from the spirit and scope of the invention as defined by the claims.
This application claims priority to co-pending U.S. Provisional Application Ser. No. 61/302,861, filed Feb. 9, 2010, entitled “AN ADJUSTABLE SHADOW MASK ASSEMBLY FOR USE IN SOLAR CELL FABRICATIONS,” which is hereby incorporated by reference in its entirety as if set forth herein.
| Number | Date | Country | |
|---|---|---|---|
| 61302861 | Feb 2010 | US |
