Grid for plasma ion implant

Abstract

A grid for minimizing effects of ion divergence in plasma ion implant. The plasma grid is made of a flat plate having a plurality of holes, wherein the holes are arranged in a plurality of rows and a plurality of columns thereby forming beamlets of ions that diverge in one direction. A mask is used to form the implanted shapes on the wafer, wherein the holes in the mask are oriented orthogonally to the direction of beamlet divergence.

Claims

1. A plasma ion implant system for implanting ions into a substrate so as to form lines for interdigitated back-contact solar cell, comprising: a processing chamber; a grid assembly placed in the plasma chamber and dividing the processing chamber into a plasma section and ion implant section, the grid assembly configured to form beamlets of ions having divergence in a direction parallel to the lines; a transport mechanism for transporting substrates in a travel direction and positioning the substrate under the grid assembly; a plurality of masks, each mask placed on one substrate and comprising a plurality of elongated holes forming line segments arranged in parallel rows that are parallel to the lines for interdigitated back-contact solar cell, wherein line segments in each row are separated by bridges that block ions impinging thereupon and the line segments in each row correspond to one implanted line forming the lines for interdigitated back-contact solar cell; and, wherein the divergence in the beamlets is configured to be in the direction parallel to the line segments of the mask so as to provide a trajectory for ions to be implanted under the bridges.

2. The implant system of claim 1, wherein the grid assembly is configured to cause beamlets divergence of ions up to 6 in a direction parallel to the lines.

3. The implant system of claim 1, further configured to form a gap between the mask and the substrate, the gap set at between zero to 2 mm.

4. The implant system of claim 1, wherein the grid assembly comprises a flat plate having a plurality of elongated holes dispersed over its surface, each of the elongated holes having a cross-sectional shape of an elongated oval having a major axis and minor axis, and having a defined depth extending through the flat plate, wherein a major axis of each of the elongated holes is configured to be aligned in a direction perpendicular to a long axis of features to be implanted on a substrate when the grid is installed inside a plasma ion implant system.

5. The implant system of claim 4, wherein the plurality of elongated holes are arranged in a plurality of rows and a plurality of columns, such that each of the holes in each column are aligned with the holes in the preceding and following column, such that a line passing through the major axis of holes in the same row would intersect at a right angle a line passing through the minor axis of holes aligned in the same column.

6. The implant system of claim 4, wherein the plurality of elongated holes are arranged in a plurality of rows and a plurality of columns, such that each of the holes in each column is shifted from alignment with the holes in the preceding and following column, such that a line passing through the center of all of the holes in a selected column forms an acute angle with a line passing through the major axis of holes in the same row.

7. The implant system of claim 4, wherein the plurality of elongated holes are arranged in a plurality of rows and a plurality of columns enclosed within a rectangular area, such that each of the holes in each column is shifted from alignment with the holes in the preceding and following column, such that a line passing through the center of all of the holes in a selected row forms an acute angle with sides of the rectangular area.

8. The implant system of claim 4, wherein the plurality of elongated holes are arranged in a plurality of rows and a plurality of columns thereby forming dead zones in between the holes, such that each of the holes in each column is shifted from alignment with the holes in the preceding and following column, such that a line passing through the center of all of the dead zones in a selected row forms an acute angle with a direction of travel of a substrate to be implanted.

9. The implant system of claim 1, wherein the grid assembly comprises a flat plate having a plurality of round holes, wherein the holes are arranged in a plurality of rows and a plurality of columns within a rectangular area and wherein each of the holes in each column is shifted from alignment with the holes in the preceding and following column, such that a line passing through the center of all of the holes in a selected row forms an acute angle with sides of the rectangular area.

10. The implant system of claim 1, wherein the grid assembly comprises a flat plate having a plurality of round holes, wherein the holes are arranged in a plurality of rows and a plurality of columns and wherein each of the holes in each column is shifted from alignment with the holes in the preceding and following column, such that a line passing through the center of all of the holes in a selected row forms an acute angle with the direction of travel of a substrate to be implanted.

11. The implant system of claim 1, wherein the grid assembly comprises a flat plate having a plurality of round holes, wherein the holes are arranged in a plurality of rows and a plurality of columns thereby forming dead zones in between the holes, such that each of the holes in each column is shifted from alignment with the holes in the preceding and following column, such that a line passing through the center of all of the dead zones in a selected row forms an acute angle with a direction of travel of a substrate to be implanted.

12. The implant system of claim 1, wherein the grid assembly comprises a combination of exit grid and implant mask for a plasma-based ion implant system, wherein: the exit grid comprises a flat plate having a plurality of elongated holes dispersed over its surface, each of the elongated holes having a cross-sectional shape of an elongated oval having a major axis and minor axis, and having a defined depth extending through the flat plate to enable ions to pass through the elongated holes; the implant mask comprises a flat plate having a plurality of linear holes dispersed over its surface, each of the linear holes having a cross-sectional shape of an elongated oval having a long axis and short axis, and having a defined depth extending through the flat plate to enable ions to pass through the linear holes, wherein the linear holes are arranged in a plurality of parallel rows and linear holes in each row are separated from each other by ion blocking bridges; and, wherein a major axis of each of the elongated holes is aligned in a direction perpendicular to a long axis of the linear holes.

13. The implant system of claim 12, wherein the plurality of elongated holes of the exit grid are arranged in a plurality of rows and a plurality of columns, such that each of the holes in each column are aligned with the holes in the preceding and following column, such that a line passing through the major axis of holes in the same row would intersect at a right angle a line passing through the minor axis of holes aligned in the same column.

14. The implant system of claim 12, wherein the plurality of elongated holes of the exit grid are arranged in a plurality of rows and a plurality of columns, such that each of the holes in each column is shifted from alignment with the holes in the preceding and following column, such that a line passing through the center of all of the holes in a selected column forms an acute angle with a line passing through the major axis of holes in the same row.

15. A method for implanting ions into a substrate so as to form lines for interdigitated back-contact solar cell, comprising: igniting plasma in a plasma processing chamber and extracting ions from the plasma through a grid assembly so as to form beamlets of ions having divergence in a direction parallel to the lines; placing masks on substrates to be implanted, the mask comprising a plurality of elongated holes forming line segments arranged in parallel rows, wherein line segments in each row are separated by bridges that block ions impinging thereupon and the line segments in each row correspond to one implanted line forming the lines for interdigitated back-contact solar cell; placing the substrates on a conveyor so as to be transported to implant zone; and, directing the beamlets to pass through the mask and implant into the substrate to thereby form the parallel implanted lines for the interdigitated back-contact solar cell.

16. The method of claim 15, further comprising forming a gap between the mask and the substrate, the gap being set at between zero to 2 mm.

17. The method of claim 16, wherein the predetermined gap is calculated to enable diverging beamlets to implant ions at locations on the substrate that are positioned directly below the bridges, to thereby implant the plurality implanted lines, wherein each line has length corresponding to the sum of all the line segments and bridges on one row of the mask.

18. The method of claim 15, wherein extracting the ions further comprises forming beamlets that have no divergence in a direction perpendicular to the lines.

19. The method of claim 15, wherein extracting the ions further comprises forming beamlets that have no divergence in the direction of width of the lines for the interdigitated back-contact solar cell.

20. The method of claim 15, further comprising forming the mask with elongated holes having width of the same width as the lines to be implanted.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other aspects and features of the invention would be apparent from the detailed description, which is made with reference to the following drawings. It should be appreciated that the detailed description and the drawings provides various non-limiting examples of various embodiments of the invention, which is defined by the appended claims.

(2) The accompanying drawings, which are incorporated in and constitute a part of this specification, exemplify the embodiments of the present invention and, together with the description, serve to explain and illustrate principles of the invention. The drawings are intended to illustrate major features of the exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of actual embodiments nor relative dimensions of the depicted elements, and are not drawn to scale.

(3) FIG. 1 is a schematic of a plasma ion implant system utilizing any of the grids and mask arrangements according to embodiments of the invention.

(4) FIG. 2 is a schematic perspective view of a plasma ion implant grid according to one embodiment.

(5) FIG. 3 is a schematic top view of a plasma ion implant grid according to anther embodiment.

(6) FIG. 4 is a schematic top view of a plasma ion implant grid according to anther embodiment.

(7) FIG. 5 is a schematic top view of a plasma ion implant grid according to anther embodiment.

(8) FIG. 6 is a schematic of a plasma ion implant system according to one embodiment.

(9) FIG. 6A illustrates an example of a shadow mask positioned over a substrate with a defined gap between the shadow mask and the substrate.

(10) FIG. 7 is a schematic of a plasma ion implant system according to another embodiment.

DETAILED DESCRIPTION

(11) Various embodiments disclosed herein enable high throughput of substrates in a plasma or any type of ion implant system, while providing accurate implant characteristics for selective implantation. The high throughput enabled by these embodiments is particularly suitable for solar cell fabrication, although it may be used for fabrication of other items, especially when there's a need for selective ion implant with the use of a mask. Certain embodiments utilize setback of an implant mask from the wafer, although the mask may travel with the wafer. The embodiments are particularly applicable to plasma implantation system that utilizes a plasma chamber in which grids are used to extract ions from the plasma and accelerate the ions towards the wafer.

(12) FIG. 1 schematically illustrates a cross-section of a plasma ion implant chamber 100. Grid assembly 110 is used to extract ion beamlets 115 from plasma 105. The grids can be biased with respect to the plasma so as to extract the ions and accelerate the ion in the ion beamlets towards the wafer 120, so as to be implanted in the wafer 120. In one example, three grids are used to form grid assembly 110: a top grid that is positively biased to extract ions, a middle grid that is negatively biased to repel electrons, and a bottom grid which is grounded. The substrate may also be grounded or it may be biased. When the bottom grid and wafer are biased, a drift zone is created between the bottom grid and wafer, such that the implant energy is determined by the extraction potential. Other grid arrangements may be used, but regardless of the arrangement, the bottom-most grid would be referred to herein as the exit grid, i.e., the last grid the ions traverse when they exit the grid assembly 110. Various embodiments for the structure of the grids will be described below, in conjunction with the design of the implant mask.

(13) A top view of an exit grid of grid assembly 110 is shown in the top callout of FIG. 1. In this example, the exit grid is in the form of a plate 135 having a plurality of circular holes 137. When the implant should be selective, i.e., over only selected areas of the wafer, such as, e.g., for selective emitter or IBC, a mask 125 may be placed over the wafer, such that only ions passing through the holes in the mask will be implanted in the wafer.

(14) As illustrated in FIG. 1, when ions exit the holes 137 of the exit grid 135, mutual repulsion causes the ions to travel at a divergent trajectory. Thus, while some ions enter the wafer at 90 to the surface of the wafer, divergent ions enter the wafer at an angle that is off the perpendicular to the surface of the wafer. This beam divergence is particularly problematic when a mask is used during the implant, such as, for example, for creating selective emitters or IBC.

(15) The dashed-line callout in FIG. 1 is an enlargement showing a cross-section of part of the mask 125 and wafer 120, and the ions 115 passing through the holes in the mask 125 and hitting the wafer 120. As shown, although the aspect ratio (i.e., hole diameter over hole height) is made so as to allow mostly ion beams traveling perpendicularly to the wafer's surface, when the ions emerge from the holes of the mask they still diverge again. This necessitates placing the mask very close to the wafer in order to control the dimensions of the implanted area. Moreover, even if the mask is placed very close to the wafer, some divergence still occurs, which adversely affects the accuracy of the implant pattern, especially when the implanted features have very small dimension such as, for example, line width and pitch of selective emitters or IBC.

(16) As shown in FIG. 1, the ions that exit each grid's hole or indeed any beam (ribbon or otherwise) are generally divergent, depending on the shape of the plasma meniscus, that is determined by multiple factors such as plasma conditions, acceleration and suppression voltages applied. On the other hand, ions that exit at close proximity to the straight edges of an elongated oval hole or sides of a slot are not as divergent, that is dictated by the shape of the meniscus. This is along the long axis of the elongated slots, rendering extracted beam non-divergent in the long direction and divergent in the short direction of the beam extraction slot. This is true for any slotted extraction grids.

(17) To take advantage of this phenomenon, the embodiment of FIG. 2 utilizes an exit grid having elongated oval holes or elongated slots rather than round holes. Consequently, as shown in the solid-line callout, which illustrates the cross-section along line A-A, over the long axis there is no beam divergence except at the very end of the elongated hole. Of course, as shown in the dashed-line callout, over the short axis there would be larger beam divergence. Therefore, in this embodiment the mask is oriented such that the long axis of the elongated hole is set perpendicular to the direction of the features to be implanted, as illustrated in FIG. 2. If a mask 125 is used, then the long axis of the grid holes should be perpendicular to the long axis of the features on the hard mask. Similarly, if the implant system uses movable substrate, i.e., pass-by ion implant, the motion of the substrate should be in a direction perpendicular to the long axis of the elongated grid holes.

(18) While the grid according to the embodiment of FIG. 2 provides improved implant results, it is somewhat susceptible to degradation during the implant operation. Specifically, the plasma and ion extraction process causes heating of the grid. Using holes that are too long may lead to warping of the sidewall of the holes due to the excessive heat. This can be resolved by using a series of smaller elongated holes arranged in rows and columns, as illustrated in FIG. 3. The embodiment illustrated in FIG. 3 is less susceptible to degradation due to heat. In using the grid of FIG. 3, the elongated axis of the holes is positioned in a direction perpendicular to the long axis of the features to be implanted, perpendicular to the elongated holes on the mask, or perpendicular to the direction of travel of the substrate, if a pass-by implant is used.

(19) The embodiment of FIG. 3 provides enhanced reliability by breaking the elongated holes into a series of shorter elongated holes. The shorter elongated holes are aligned in rows and columns, such that a straight line can pass through the major axis of holes in the same row, and it would intersect at a right angle a line passing through the minor axis of the holes in the same column. However, in between elongated holes there's a dead zone where no ions exit the grid, so that no implant occurs in the substrate travel direction, particularly for a homogenous implant. The embodiment of FIG. 4 alleviates this problem by having the holes arranged along an angled line, such that the dead zones are aligned along a line that is off from the perpendicular to the direction of travel. That is, in the embodiment of FIG. 4 each successive hole is shifted a given amount from alignment to its neighbor hole. Stating it another way, while a straight line can pass through the major axis of holes arranged in the same row, no straight line can pass through the minor axis of all the holes arranged in the same column. Also, a straight line passing through the major axis of holes arranged in the same row would intersect at a non-perpendicular angle a line passing through the center of all holes arranged in the same column. Consequently, for a traveling wafer, each point of a dead zone will be projected only once, such that all of the dead zones will be averaged and each will be covered by ions emanating from other holes. Therefore, once the wafer exits the system, each part on the wafer has been implanted, regardless of the dead zones.

(20) A similar effect can be achieved by using round holes arranged on a diagonal or slanted direction to the direction of wafer travel. Such an embodiment is illustrated in FIG. 5. As illustrated in FIG. 5, a line passing through the center of each hole in all of the holes it passes is aligned at an acute angle to the direction of wafer travel. On the other hand, a line drawn parallel to the direction of travel cannot pass through the center of each hole it crosses. Consequently, every point on the wafer would be exposed to ions emanating from the center of some holes and the edges of other holes, such that the total implant is evened out.

(21) In most, if not all, current implant applications, such as selective emitter and IBC, the selectively implanted features are in the form of long and narrow lines and other shapes, where lines can be of varying thickness to enhance current carrying capabilities, other shapes such as holes, Halo shapes and other features such a fiducial marking etc. One method to achieve this is to use standard photoresist or other hard mask, such as deposited or preferentially grown layered, masks and form a mask on the surface of the wafer. However, such mask formation process is long and costly and after the implant is completed the mask needs to be removed. If a second set of lines of the opposite polarity needs to be implanted, the masking needs to be repeated.

(22) Therefore, it would be beneficial to use a shadow mask instead. Shadow masks are physical masks that are formed separately from the substrate and are plated on top or above the substrate during processing. However, when forming shadow mask for such application, the mask has many thin and long lines, as exemplified in FIG. 2. Since the mask is relatively thin and undergoes heating due to the impinging ions, the mask cannot maintain its original flat shape and the resulting implanted lines cannot be maintained according to the specifications. In addition the requirements for such selective implanted lines and fingers, that range from 100 to several micron opening width and a pitch, center to center for one type of dopant, of less than a 1000 micron, will make the shadow mask non-rigid, which makes it difficult for handling, in general, and not suitable for varying temperature, due to ion beam, and alignment accurately to the substrate. Such mask may not hold its fidelity for scribing dopant lines on the substrate consistently and over a long period of time. Furthermore, since this mask needs to be made out of thin materials, to avoid causing formation of Moir effect, then this problem is particularly accentuated.

(23) An example of an implant system that can be implemented for implanting long lines using hard mask is illustrated in FIG. 6. The plasma implant chamber may be of any design employing the grid plate or ribbons beam arrangement, but in this particular example the plasma chamber 600 has a conveyor 612 to convey wafers through the implant chamber 600. Chamber 600 has a grid assembly 610 or could have a long single or 3 to 5 slots as in the ribbon beam having an exit grid 635, which is shown in the upper callout. The grid assembly 610 is used to extract ions from plasma 605 and generate beamlets 615 directed towards the wafer 620. As illustrated in FIG. 6, since the major axis of the elongated holes in exit grid 635 are oriented perpendicular to the direction of travel of the wafers, the beamlets expand in a direction parallel to the direction of travel of the wafer, with negligible expansion in a direction perpendicular to the direction of travel of the wafer.

(24) The mask 625 is shown in the lower callout. It is formed of a flat plate, e.g., stainless steel plate, and, rather than having long holes or elongated slots of the length required for the implanted lines, the long holes are broken using bridges 627. These bridges allow for the ease of mask fabrication and type of materials at various thickness. Of course, the actually implanted lines cannot have such bridges, since they will cause an open circuitthe implanted lines must have continuity of electrical conductance. However, by aligning the major axis of the elongated holes in the exit grid 635 to be perpendicular to the major axis of the elongated holes in the mask 625, the resulting beam divergence enables implanting ions under the bridges 627. This can be understood by referring to the illustration of FIG. 6A. Specifically, since beamlets 615 diverge in a direction parallel to the major axis of the elongated holes of mask 625, the divergent part travels under the bridge 627 so as to result in an implanted full line, rather than the line sections of the mask. By adjusting the gap, G, between the mask 625 and the wafer 620, the amount of ion implant delivered below the bridge 627 can be controlled.

(25) Also, in the embodiment of FIG. 6, each wafer travels with its own mask. When the wafer reaches the implant zone, i.e., in alignment below the exit grid, the conveyor can be stopped or continue to travel, depending on the implant process, i.e., stationary or pass-by, and the implant process takes place. When the desired dose is achieved, or the duration of substrate passage under the beam is achieved, the ion beam is turned off or blocked and the conveyor is re-energized or continues to operate to remove the implanted wafer and introduce a new wafer underneath the beam. The mask from the implanted wafer may then be removed and cleaned for reuse.

(26) Another example is illustrated in FIG. 7. In the example of FIG. 7 it is desired to implant wafer 705 with a plurality of long lines 745. These lines may form, e.g., contact lines for selective emitter cell or doping lines for IBC cell. In order to implant the long and narrow lines 745 using a hard mask, one would need to have a mask with long cut outs in the form of the implanted lines 745. That would make the mask rather flimsy and unstable during heating and cooling cycles and general handling. Instead, rather than making the mask in the form of long lines, in mask 725 each line is broken into several line segments 728 by bridges 727. However, using such a mask in standard implant system would lead to line segments implanted on the wafer, such that the line segments do not have electrical continuity. Conversely, using omnidirectional diverging ion beams would make it difficult, if not impossible, to properly control the width of the implanted lines 745. Therefore, the exit grid 735 is formed to have elongated holes or line segments 738, which are oriented with their major axis perpendicular to the major axis of the line segments 728 of the mask. This can be adopted as the required doping features on the substrate changes direction, where both the shadow mask and extraction grid can accommodate such change in direction. This arrangement leads to beam divergence in the direction of the minor axis of the elongated holes 738, i.e., in the direction parallel to the line segments of the mask. The beam divergence in this direction would provide a trajectory for ions to be implanted under the bridges 727, so as to make the implanted lines 745 continuous. Conversely, because the holes 738 are elongated, there would be little beam divergence in the direction of the long axis of holes 738. Since the long axis of holes 738 of exit grid 735 are oriented perpendicular to the long axis of holes 728 of mask 725, there would be little divergence in the direction of the width of the lines 728, such that the width of lines 745 can be closely controlled.

(27) In the example of FIG. 7, the beamlets flare or divergence of ions exiting the elongated holes of the exit grid may be up to about 6 in the minor axis direction, but almost zero in the major axis directionexcept at the two extreme edges of the elongated holes, where the divergence may also reach 6. The gap between the mask and the wafer may be set at between zero to about several millimeters, or more likely to be about 2 mm. At such gap the beam divergence on the wafer may be up to about 420 microns. Thus, unless the beam divergence is controlled as shown in the example of FIG. 7, ion passing through a mask opening of about 150 microns width can be implanted at line width of up to about 570 microns. Therefore, if the divergence control of FIG. 7 is not employed, in order to implant a line of 150 micron width, the holes in the mask must be made much narrower, which is more difficult and more costly to fabricate and control. Conversely, since in FIG. 7 the divergence of the beam is controlled in the direction of the line width, the width of the holes in the mask may be made of the same width as the lines to be implanted, making it simpler and cheaper to fabricate.

(28) While the elongated holes or slots of the masks in the above embodiments are shown to all have the same shape and length, this is not necessary. To the contrary, the elongated slots of the masks may be of different shapes and lengths. For example, mask 725 of FIG. 7 has elongated slots 728 of different lengths.

(29) The above described embodiment may be implemented in a method for implanting ions to fabricate solar cells. The method proceeds by introducing a wafer to be implanted into the plasma processing chamber and positioning a mask at a predetermined gap above the wafer. The mask is configured to comprise a plurality of line segments of varying shape and size, arranged in parallel rows, and line segments in each row are separated by bridges that block ions impinging thereupon. The method proceeds by igniting plasma in the plasma processing chamber and extracting ions from the plasma so as to form beamlets of ions having divergence in a direction parallel to the direction of the rows. The predetermined gap is calculated to enable diverging beamlets to implant ions at locations on the wafer that are positioned directly below the bridges, to thereby implant plurality of implanted lines each having length corresponding to the sum of all the line segments and bridges on one row of the mask.

(30) While this invention has been discussed in terms of exemplary embodiments of specific materials, and specific steps, it should be understood by those skilled in the art that variations of these specific examples may be made and/or used and that such structures and methods will follow from the understanding imparted by the practices described and illustrated as well as the discussions of operations as to facilitate modifications that may be made without departing from the scope of the invention defined by the appended claims.