NL2019031B1

NL2019031B1 - Wafer gripper assembly, system and use thereof

Info

Publication number: NL2019031B1
Application number: NL2019031A
Authority: NL
Inventors: Roelf Venema Peter; Ronald Renes Marten; Henrica Maria Van Dijk Ronald; Reinder Marc Luchies Johannes
Original assignee: Tempress Ip B V; Solaytec B V
Priority date: 2017-06-07
Filing date: 2017-06-08
Publication date: 2018-12-17

Abstract

The wafer gripper assembly provided With at least one wafer gripper (50)comprising a base (51) and a bridging element (52); vacuum means comprising a hose (53) for applying underpressure and at least one suction area (54), configured for holding a wafer (10) by means of the applied underpressure; a frame (60) coupled to the bridging element (52). The frame (60) is provided With positioning means (57) configured for contact to a wafer boat, so as to position the wafer at a predefined distance from the wafer boat. An elastic coupling (56) is present between the frame (60) and either the bridging element (52) or a holding plate (70) comprising the suction devices (54) if any such holding plate is present.

Description

WAFER GRIPPER ASSEMBLY, SYSTEM AND USE THEREOF

FIELD OF THE INVENTION

The invention relates to a wafer gripper assembly provided with at least one wafer gripper that comprises a base and an element suspended therefrom, wherein vacuum means comprising a hose for applying under-pressure and at least one suction device are present for holding a wafer by means of the applied under-pressure, said suction devices being coupled to said suspended element.

The invention further relates to a system of such a wafer gripper assembly and a wafer boat.

The invention also relates to the use of such a wafer gripper assembly for loading of wafers on a wafer boat.

The invention also relates to a method of manufacturing a solar cell comprising the steps of: Loading at least one wafer on a wafer boat comprising mutually separated electrodes Transporting the wafer boat with the at least one wafer into a deposition chamber of a plasma-enhanced chemical vapour deposition (PECVD) apparatus;

Applying at least one layer on the at least one wafer on the wafer boat by means of plasma-enhanced chemical vapour deposition (PECVD);

Transporting the wafer boat out of the deposition chamber, and Unloading the at least one wafer from the wafer boat.

BACKGROUND OF THE INVENTION

Plasma Enhanced Chemical Vapor Deposition (PECVD) apparatus are for instance used for deposition of front side antireflective coating (ARC) on solar cell wafers. Such an apparatus makes use of a wafer boat for transporting of a plurality of wafers into and out of a deposition chamber of the apparatus. The wafer boat for plasma-enhanced CVD is characterized in that it comprises mutually isolated electrodes, through which the plasma is generated in the deposition chamber. These electrodes are typically present as plates in a boat. In use in the deposition chamber, the plates are connected to an alternating current source with a frequency of typically 20 - 100kHz. In view thereof, the plates of a PECVD boat comprises electrically conductive material. The typical material is graphite. Ceramic spacers separating these plates are also present. The plasma facing side of the wafer receives a deposition. The other side of the water that is mounted to the plate, does not receive a deposition.

When loading wafers to a wafer boat, wafers are mounted to the graphite plates. Typically, the wafer boat is thereto provided with holding elements protruding from the graphite plates. In one known embodiment, the holding elements are pins, with are provided with a tip portion having a larger diameter than the main portion of the pin. This allows mounting the wafers behind said tip portions. Since the PECVD boat is very fragile and not very sturdy, the positioning of the wafers it is essential to position them carefully. If this is done in-accurately, wafers may drop and can be broken, or may be damaged by scratching the wafer surface. This invention intends to reduce or even eliminate damage to the wafers.

More particularly, to put the wafers behind the pins and close to the plates, the wafers need to be handled carefully, as the pins do not protrude far from the plates of the wafer boats. In one embodiment, the pins extend in the range from 0.25 to 0.7mm, while the thickness of the wafers is typically in the range from 0.1 to 0.18mm. Especially, care should be taken not to cause scratching al the rear side, the non-deposited side, of the wafer.

The protection of the rear side of the wafers is especially a concern for more advanced solar cells, such as for instance PERC cells, N-PERT cells or IBC cells. In standard p-type aluminium Back Surface Field cells, scratches at the rear side are typically not of a concern, since these scratches are covered by an aluminium rear side metallization. In more advanced solar cells, where the rear side is partially open, or where an additional rear side nitride may be deposited, scratches caused by rear side deposition might affect appearance and/or performance of the resulting solar cells. This may lead to poor visual appearance, or worse, may lead to lower cell efficiency, of the solar cell. Therefore it is important to prevent scratching of wafers in advanced solar cell PECVD deposition.

One such boat is for instance known from US4799451 (ASM International 1989). The graphite boat consists of multiple plates that can hold multiple wafers per plate. Wafers are positioned on both sides of the plate, and kept in position by several small pins. The total number of wafers in a boat is the total wafers per plate, which is the number of columns limes the number of plates (rows) times two. Since the outside plates can held a wafer on the internally facing side of the plate, the total number of wafers is equal to (number of columns) times (number of rows minus one) times two.

One example of a wafer gripper assembly is known from US8556315B2. This known wafer gripper assembly comprises at least one vacuum pad that is flexibly connected to an element suspended from the base and referred to as a vacuum gripper finger plate. In the disclosed assembly, the vacuum pads are present inside said finger plate. A hose is connected to the vacuum pad. Furthermore, a stabilization structure is present and adapted to stabilize the at least one vacuum pad in a generally flat orientation relatively to the finger plate gripper plate, when the gripper is not gripping a wafer. More specifically, the stabilizing structure is a leaf spring that extends from the finger plate to be in contact with the vacuum pad with a point of contact that represents a two-dimensional plane surface to stabilize the vacuum pad in relation to the finger plate. As apparent from the figures, the vacuum pads are arranged in apertures within said plate. Il is attached by means of four tension springs to the finger plate. The said leaf spring is coupled to the rear side of the plate facing away from the wafer. The construction of this known wafer gripper assembly however does not contribute to avoiding scratching of the rear side of a wafer during loading and/or unloading thereof from the wafer boat.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide an improved wafer gripper assembly that overcomes the disadvantages of the prior art and more particularly allows to reduce or prevent scratching at the rear side of a wafer during loading into and/or unloading from a wafer boat, it is another object to provide a system of a wafer gripper assembly and a wafer boat.

It is a further object to provide use of a wafer gripper assembly with reduced scratching or even without scratching.

It is again a further object to provide a method of manufacturing of a solar cell of the type mentioned in the opening paragraph with reduced scratching or even without scratching of the rear side. According to a first aspect, the invention provides a wafer gripper assembly provided with at least one wafer gripper that comprises a base and a bridging element suspended from said base. The gripper further comprises a frame coupled to the bridging element and provided with positioning means configured for contact to a wafer boat, so as to position the frame at a predefined distance from the wafer boat. The gripper also comprises vacuum means comprising a hose for applying underpressure and at least one suction device, configured for holding a wafer by means of the applied underpressure. According to the invention, an elastic coupling is present between either the frame and the bringing element or between the frame and the at least one suction device.

According to a second aspect, a system of a wafer gripper assembly of the invention and a wafer boat is provided.

According to a third aspect, use of the wafer gripper assembly of the invention for loading of a wafer onto a wafer boat and/or for unloading the wafer from the wafer boat is provided. More particularly, the wafer gripper assembly is used as part of a method of providing a layer by means of PECVD, and especially as part of a method of manufacturing a solar cell as specified in the opening paragraph.

The invention is based on the insight, that the scratching of the rear-side of the wafer cannot just be prevented by means of improved robotics, but requires an improved gripper assembly to facilitate the positioning. Due to spread in dimensions of wafer boats, the loading and unloading process has very limited tolerance. Today’s PECVD boats are substantial in size with plate number ranging from 19 to 27, and number of columns ranging from 6 to 8. As manufacturing of these boats also comes with some spread in dimensions of the individual parts of the boat, a quite large spread in boat dimensions may arise, leading to deviations from the expected wafer location that can exceed easily the 0.7mm range. As such, it was understood by the inventors that positioning the wafer gripper assembly relative to the wafer boat is needed.

Thereto, in accordance with the invention, the wafer gripper comprises a frame provided with positioning means that are to be positioned against the wafer boat and more particularly the relevant plate thereof. This frame is provided with an elastic coupling to either the bridging element - corresponding to the finger plate of the prior art - or to a holding plate for the suction devices, if such holding plate is present. By means of this elastic coupling the distance of the wafer to the wafer boat prior to insertion between holding pins on the wafer boat can be regulated, and/or a counterforce to movements from the wafer gripper towards the wafer boat can be provided. It has been found in preliminary experiments with wafer grippers according to the invention that the rate of failure descreased dramatically. Whereas traditionally failure rates of 2-40% are common, further dependent on the type of wafer boat, the length of the holding pins and the precision of the movement of the wafer gripper assembly, failure rates of well below 1% were obtained. In comparative experiments the decrease in failure rate was a factor 10 or the like.

In a first, preferred implementation, the elastic coupling is present between the frame and the bridging element. This has the advantage that the distance between the wafer and the wafer boat can be regulated before starting a rotating movement needed for the insertion of the wafer between the holding pins on the wafer boat. Furthermore, the springs further provide a resistance against strong forces generated in the base of the wafer gripper and transmitted towards the wafer. The resistance might prevent shocks that otherwise could lead to damage on the wafer.

In one specific embodiment hereon, the elastic coupling may be provided by means of springs that allow deformation in two opposed directions: away from the wafer boat and towards the wafer boat. As will be understood, a variety of springs may be configured to be deformed in this manner starting from a reference position (that could be prestretched so as to allow both compaction and elongation). The deformation in two opposed directions is effective to overcome variations in the shape of the wafer boat or part thereof such as a plate that may be either forwards or backwards relatively to the gripper.

In a second implementation, the elastic coupling is present between the frame and a holding plate for the suction devices. In this implementation, the distance between the bridging element and the wafer boat is fixed. Any deformation of the wafer boat or part thereof can be compensated by means of the movement of the holding plate with the suction devices and a wafer attached thereto relative to the bridging element. Such movement will however be counteracted by an increasing spring force of the spring.

In one important embodiment, the positioning means are arranged in a location adjacent to a space configured for a wafer. This may be achieved by means of the extension of the frame, more particularly a first element thereof. Hence, in use with a wafer loaded on the gripper, the positioning means are visible, when viewing from the exposed side of the wafer that is to be mounted on the wafer boat so as to face the said plate, in this way, the positioning means may also face and contact a surface of the wafer boat.

In one further embodiment, the positioning means are embodied as at least one protrusion extending from the frame. In a specific implementation, the protrusion protrudes beyond the wafer, so that there is no risk that the wafer would touch the plate of the wafer boat, in a further implementation, the protrusion comprises a contact layer for contact to the wafer boat. Such a contact layer may contain a different material than that of the frame and can be provided, locally, onto said frame, for instance by deposition. The material of the contact layer is suitably chosen so as to prevent any damage to the water boat, i.e. to prevent generation of cracks or microcracks. A suitable contact layer is silicon nitride.

While the positioning means is configured for contact onto a planar portion of the surface of the wafer boat, it is not excluded that the positioning means are configured for contact onto a non-planar portion. Such non-planar portion could be a protrusion but also a cavity relative to the surface of the wafer boat. In a specific implementation, the positioning means may be configured for contact to positioning elements on the wafer boat, wherein the positioning means and the positioning elements may have a complementary shape. Said implementation enables that the wafer gripper and the relevant plate of the wafer boat are positioned to each other in more than one dimension, i.e. not merely to define a normal distance of the gripper to the wafer boat, but also to define a position parallel to the plate of the wafer boat. The latter is particularly suitable to define a position relative to any pins on the wafer boat.

Preferably, the gripper comprises more than a single protrusion as the positioning means.

In one embodiment, wherein the positioning means comprise a first protrusion arranged at said location adjacent to said space configured for a wafer and a second protrusion arranged at a location adjacent to said space and at a side thereof opposed to the first protrusion. This implementation provides a more stable positioning of the wafer gripper relatively to the wafer boat. Moreover, in case that the wafer boat would not extend entirely parallel to the frame, typically in the vertical direction, such orientation will be communicated to the gripper.

In a further implementation a first protrusion has a smaller thickness than a second protrusion. In this further implementation, the gripper is configured for cooperation with a wafer boat on which a first positioning element is present, which is to cooperate with the first protrusion. The first protrusion herewith constitutes a reference for the positioning, whereas any further protrusions are intended for defining a distance to the wafer boat. Rather than a second protrusion with a smaller thickness, it is feasible that some or all of the pins on the wafer boat are configured for contact to the frame of the wafer gripper. Preferably, this applies only to part of the pins, as a close distance of a pin to the frame may reduce the ability of inserting the wafer behind tip portions of the pins. In such event, the pins that are to contact the frame tire suitably provided with a spacer layer, for instance of silicon nitride, aluminium nitride or the like.

In again another embodiment, which is preferably combined with the preceding embodiment, the first protrusion of the positioning means is present so as to be arranged above the wafer when in use. Any second protrusion may be suitably arranged below the wafer when in use. This arrangement is deemed preferable in combination with a positioning protocol comprising both a translation towards the wafer boat and a rotating movement to arrange the wafer in between holding elements of the wafer boat. At least part of said holding elements is then present, in use, sidewise to the wafer.

In a further embodiment, the elastic coupling means are embodied as a plurality of springs, for instance in the range of 2-8, such as 3-6. The springs may be arranged at mutually equal angles from each other, although that is not necessary. The springs or at least part thereof, are preferably embodied as leaf springs. This allows that the extension in the direction normal to the wafer boat can be tuned in a precise manner. Furthermore, leaf springs constitute a reliable implementation. It seems beneficial that at least some of the springs extend such that in use with a wafer attached to at least one suction device, said springs are oriented at least substantially in a direction towards angles of the wafer. While the leaf springs may be the only connection between the frame and the bridging element, it is not excluded that additional connections are present. Such additional connections may ensure that the overall construction of the wafer gripper is sufficiently robust to hold the wafer. Such additional connection may be embodied by means of adhesion or in a mechanical manner, while allowing movement in a direction normal to the plane through both of the bridging element and the frame. In a further implementation, the suction devices are arranged onto a holding plate coupled to the bridging element. This reduces the weight carried by the frame. This contributes again to the robustness of the wafer gripper.

The use of springs and preferably leaf springs is further deemed beneficial for the intended positioning of the wafer towards the wafer boat upon loading. Because the plurality of leaf springs, they may deformed in differential manner.

In again a further embodiment, the number of suction devices is larger than one.

Generally, three is a preferred number. The suction devices are for instance embodied as pads or vacuum devices as shown in US8556315B2. However, alternative implementations are not excluded. It appears sufficient to provide an element that can be placed on or sufficiently close to a wafer and that includes an opening to the connected vacuum hose. The implementation hereof is known per se to the skilled person in the art. While it is preferred that all suction devices present are connected to the same vacuum hose, this is not strictly necessary and more than one vacuum hose may be present if also more than one suction device is present. In one embodiment, the vacuum hose is arranged as a tube and is separate from the bridging element. It is however not excluded that the vacuum hose is connected to the bridging element, or that at least part of said vacuum hose is implemented as a channel within said bridging element.

In again a further embodiment, the wafer gripper assembly is provided with a memory and a processor that is configured for memorizing appropriate positions on the wafer boat. Suitably, a sensor is present as well. Such a sensor is for instance implemented by means of a laser or an imaging system like a camera, which allows more accurate location information to allow improved positioning of the wafers. This reduces some of the boat to boat variations and column to column variation.

The wafer gripper assembly of the invention is preferably provided with a plurality of wafer grippers, enabling simultaneous loading respectively unloading of wafers, as known per se in the art. The bases of the wafer gripper assembly are suitably coupled to each other, as is known per se in the art. Typically, the bases of the wafer grippers are fixed to each other and can be moved in its entirety. This is typically carried out by means of a drive means also known as a robot.

In another embodiment the frame and the elastic coupling between the frame and the bridging element is configured such that the frame remains in the same position while the wafer is rotated to behind the support pins on the plate of the wafer boat. Such a configuration can be achieved by suitable choice of the spring constant and dimensions of the springs, relative to the weight and the size of the frame. The ad vantage is that merely the wafer with the suction areas will rotate. This enables to carry out a more precise rotation and further reduces the risk of scratching.

In an further embodiment, the rotating movement of the gripper is combined with a movement towards or away from the plate of the wafer boat. Such a combination, which is for instance achieved with a corkscrew movement, gives a very precise control over the trajectory of the wafer and facilitates insertion of the wafer behind the holding pins of the wafer boat. For sake of clarity, the holding pins referred to are those that are in a location on the wafer boat corresponding with the position of the wafer.

In the system of the invention, the wafer boat typically comprises a plate and a plurality of holding elements protruding from said plate, and wherein the wafer gripper is configured for placement of a wafer between said holding elements of the wafer boat. The holding elements are for instance embodied as pins protruding from the plate. More particularly, a single boat comprises a plurality of plates. Preferably said holding elements are provided with a main portion and a tip portion, wherein said main portion extends between the plate of the wafer boat and the tip portion, wherein said tip portion has a larger diameter than said main portion. This type of holding element prevents that a wafer can fall off a wafer boat. In one implementation, the tip portion has a shape of a truncated cone. Such a shape is deemed advantageous for smoothening the processes of loading and unloading. The wafer boat particularly comprises graphite plates and spacers. The graphite plates are in use coupled to voltages in alternating order so as to create a voltage difference between adjacent plates.

For sake of clarity, it is observed that the term ‘wafer’ is herein used for a substrate, such as a semiconductor substrate, which is typically thinned back to a thickness of 300 pm or less. The semiconductor substrate is for instance a silicon substrate, which is monocrystalline, polycrystalline or amorphous, but may alternatively be any other type of suitable substrate such as an SOI (silicon-on-insulator), silicon-germanium, aluminium-nitride, aluminium nitride on silicon, gallium arsenide etcetera. The substrate may have been subject to several processing steps prior to the loading onto the wafer boat by means of the wafer gripper in accordance with the invention.

For instance, diffusion and/or implantation steps may have been carried out, as well as deposition steps, including formation of polysilicon layers, thermal oxide, other oxides, as well as generation of texture and manufacturing of via holes. The processing may have been carried out on one side or two-sided. Preferably, the processing has been carried out two-sided. The processing may have been carried out to prepare different types of devices. Most preferably, the processing has been carried out to form solar cells. The loading onto the wafer boat in accordance with the invention is particularly foreseen for the deposition of any layers by means of PECVD, such as silicon oxide, silicon nitride, silicon oxynitride, silicon carbide, amorphous silicon, polysilicon and the like. However, any alternative use is not excluded.

In the use of the wafer gripper assembly of the invention, it is particularly preferred to carry out following steps:

Mounting at least one wafer to the wafer gripper assembly and holding the at least one wafer by application of an underpressure via the at least one suction device of the water gripper; Positioning the wafer gripper assembly to the wafer boat, such that positioning means of at least one wafer gripper contact a plate of the wafer boat;

Rotating the wafer gripper assembly, preferably along an axis substantially running through said wafer, so as to move the wafer behind at least one holding element on the said plate of the wafer boat;

When the at least one wafer has been mounted to the plate of the wafer boat behind said at least holding element, removing the underpressure.

When using the wafer gripper assembly of the invention for unloading of a wafer boat, the wafer gripper assembly will first be positioned to the wafer boat, such that positioning means of at least one wafer gripper contact a plate of the wafer boat. Then the wafer gripper assembly will be moved in a manner so that the suction devices will contact the wafer. Underpressure will be applied via the suction devices so as to achieve attachment between the suction devices and the wafer. Subsequently, the wafer gripper assembly will be rotated, preferably along an axis substantially running through said wafer, so that the wafer may pass one or more of the holding pins. When the water has passed all pins, it may be moved out of the spaces between adjacent plates of the wafer boat. This typically occurs by movement of the entire wafer gripper assembly. While the above method has been described for a single wafer, it will be understood that in the context of the invention a single wafer gripper assembly typically handles a plurality of wafers simultaneously.

BRIEF INTRODUCTION OF THE FIGURES

These and other aspects of the invention will be further elucidated with reference to the figures, which are not drawn to scale and wherein equal reference numerals refer to equal or corresponding elements, wherein:

Fig. 1 shows a schematic view of a wafer boat;

Fig. 2 shows a schematic front view of a wafer gripper that is provided with a wafer, according to a first embodiment;

Fig. 3 shows a side view of the wafer gripper as shown in Fig. 2;

Fig. 4 and 5 show side views of a wafer gripper comparable to that of Fig. 3, but in modified implementations;

Fig. 6 shows a schematic front view of a wafer gripper provided with a wafer, according to a second embodiment;

Fig. 7 shows a schematic front view of a wafer gripper provided with a wafer, according to a third embodiment, and

Fig. 8 shows a schematic side view of a wafer boat plate with two wafers loaded and part of the wafer gripper positioned to load wafers

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS

Fig. 1 shows in a bird eye’s perspective a wafer boat 1 with the rods and spacers shown separately. The shown overview is known in the art. The wafer boat comprises a plurality of first plates 11 and second plates 21, which are spaced apart by means of spacers 31. The plates are provided with holes or windows 13, which alternatively may be cavities. In the present embodiment there are eight windows 13 in a row. The plates 11,21 comprise holes close to their upper sides 2 and close to their bottom sides 3. Electrically insulating rods 15 are present to pass the holes in the plates 11,21 and in the spacers 31, which are also provided with holes. The rods have a first end A and an opposed second end B. In order to fixate the rods 15, a nut 34 and a securing nut 35 are present at their ends A, B. As is known to the skilled person, the plates 11,21 are typically provided with windows. Such window is however closed to form a cavity 13, in the outermost plate 11.

Extensions 12, 22 extend from the first plate 11 and the second plate 21 respectively. The first plate 11 and the second plate 12 each have an extension at the upper side 2 and an extension at the bottom side 3. However, this is merely one suitable implementation. Importantly, a separation is achieved between the first and second extensions 12, 22. The first plates 11 may thus be electrically connected with each other by means of the first extensions 12 and conductive spacers 32. Similarly, the second plates 12 may be electrically connected by means of the second connections 22 and conductive spacers. In this manner, a pattern of interdigitated, mutually spaced electrodes is formed. A voltage difference may be applied between the electrodes, i.e. the first and second plates 11,21. This is deemed a particularly useful method for generating a plasma between the substrates (also known as wafers) to be positioned in the boat between a first and an adjacent second plate. The spacers 32 are connected to the first extensions 12 with first rod 16 and auxiliary rod 17. Further spacers are connected to the second extensions 22 with second rod 18 and auxiliary rod 19. In this embodiment, the first and second rod 16, 18 are electrically conductive and the auxiliary rods 17, 19 are electrically insulating. This is deemed suitable, but not essential. The boat is supported by a support element 33 that extends from said conductive spacers at the bottom side 3. Additionally, a support pin 23 is present for supporting the wafers. A wafer boat 1 of the type shown comprises typically plates 11,21 of graphite and insulating rods 15 of aluminium oxide. However, other materials or variations of the specified materials, such as blends, are not excluded.

In accordance with the invention, a wafer gripper is provided that facilitates the mounting of a wafer 10 onto a plate 11, 21 of the wafer boat 1. It is to be understood, that this mounting implies the positioning of the wafer 10 on said support pins 23. Particularly, this occurs in the spaces between adjacent first and second plates 11, 21. A plurality of wafers is provided in two rows between a single pair of first and second plates 11,21, wherein a first row is mounted to the first plate 11 and a second row is mounted to the second plate 21. The total number of wafers in a boat is the total wafers per plate, which is the number of columns times the number of plates (rows) times two. Since the outside plates can hold a wafer on the internally facing side of the plate, the total number of wafers is equal to (number of columns) x (number of rows - 1) x 2. To put the wafers behind the pins and close to the graphite plates, the wafers need to be handled carefully since the space between the graphite plate and the pin is limited in the range from 0.25 to 0.7mm, while the thickness of the wafers is in the range from 0.1 to 0.25mm. Today’s wafer boats are substantial in size with plate number ranging from 19 to 27, and number of columns ranging from 6 to 8. As manufacturing of these boats also comes with some spread in dimensions of the individual parts of the boat, a quite large spread in boat dimensions may arise, leading to deviations from the expected wafer location that can exceed easily the 0.7mm range. Especially, care should be taken not to cause scratching at the rear side, the non-deposited side, of the wafer.

Fig. 2-3 shows a wafer gripper 50 according to a first embodiment. The wafer gripper comprises a a base 51 and a bridging element 52 suspended from said base 51. In the shown embodiment, the bridging element 52 is substantially in the form of a plate. However, it is not excluded that the bridging element 52 is embodied as a shaft or as a plurality of bars. The bridging element essentially bridges a space between said base 51 and the area where a wafer can be attached and kept by means of the suction devices 54. It is observed for sake of clarity that the base 51 of the wafer gripper 50 is connected or coupled to adjacent bases of further wafer grippers of the wafer gripper assembly. Typically, a single wafer gripper assembly comprises at least 10 wafer grippers. The assembly in its entirety can be moved and rotated, so as to position the wafers held by the wafer gripper from an original place to a destination. The original place is for instance a storage or rack or first wafer boat, whereas the destination is another wafer boat, or vice versa.

Fig. 2 and 3 further illustrate the presence of vacuum means comprising a hose 53 and suction devices 54, and a vacuum connection 58. The suction devices may be provided as pads or vacuum devices as shown in US8556315B2. However, alternative implementations are not excluded. As a minimum, a suction device is an element that can be placed on or sufficiently close to a wafer and that includes an opening to the connected vacuum hose. The implementation hereof is known per se to the skilled person in the art. While the illustrated embodiment shows that all suction devices present are connected to the same vacuum hose, this is not strictly necessary and more than one vacuum hose may be present if also more than one suction area is present.

Fig. 2 and Fig. 3 further show the presence of positioning means 57 arranged on a frame 60. In the illustrated embodiment, as visible in Fig. 3, the positioning means 57 are protrusions that are extended to a surface of a plate 11 of a wafer boat. A first and a second protrusion 57 are present and arranged so as to be present at opposed sides of a space configured for attachment of a wafer 10. In the embodiment shown, the protrusions 57 have a height such that they extend beyond the wafer 10. In the illustrated embodiment, the frame 60 comprises a first element 61, a second element 62 extending parallel to the first element 61 and any further element 63 connecting the first and the second element 61, 62. The elements 61-63 are herein embodied as bars. This is however not essential and one or more of them could be embodied as plates, blocks, cylinders. Furthermore, while the first and second element 61, 62 run in parallel in the illustrated embodiment, this is not necessary. The illustrated embodiment is however beneficial so as to keep sufficient space between the first and second elements 61,62, configured for arrangement of for instance the continuing bridging element 52 and a plurality of springs 56, in this embodiment leaf springs. In this embodiment the suction devices 54 are present in or on the frame 60. In the shown embodiment, the first and the second element 61, 62 of the frame 60 extend in a first direction substantially parallel to the direction along which the bridging element 52 suspends from the base 51.

Fig. 2, 3further show springs 56. These springs are in the shown embodiment present as leaf springs and extend between the frame 60 and the bridging element 52 of the wafer gripper 50. As shown in Fig. 3a the springs 56 will in use extend, so as to include an angle with a plane 50 running through the wafer 10 and through the frame 60. This opening of the springs 56 allows a controlled positioning of the wafer behind the pins 23 of the wafer boat.

Moreover, the springs 56 and the positioning means may overcome deviations and variations in the location of the plate 10 of the wafer boat. It is herein observed that a wafer gripper assembly typically positions a plurality of wafers in “columns”, i.e. one after the other. While the gripper assembly, such as a controller or robot therein, may adapt its positioning upon recognizing a specific wafer boat, any variations in the location of individual plates 11 of the wafer boat cannot be compensated by means of any of the known wafer gripper assemblies, but can by means of hte wafer gripper assembly according to the invention. In the event that the plate 11 of a wafer boat would be deformed towards the back (i.e. the right-hand side in the view of Fig. 3), the frame 60 will move further towards said plate 11, in order that the positioning means 57 contact said plate 11. As a consequence, the springs 56 will deform and create a distance in the direction normal to said plate 11 between the frame 60 and the bridging element. If on the other hand, the plate is deformed towards the front (i.e. to the left-hand side in the view of Fig. 3), the frame needs to move towards the bridging element 52. The deformation of the springs 56 will reduce, as compared to the view in Fig. 3, or may deform in the opposite direction, i.e. in the drawing towards the left.

In this manner, an appropriate distance is maintained between the plate 11 of the wafer boat and the wafer 10, which prevents or at least significantly reduces scratching of the wafer 10 to the plate 11 of the wafer boat. It will be understood that a similar mechanism and advantage is obtained when removing the wafer from the wafer boat.

Furthermore visible in Fig. 3 is that the pins 23 are provided with top portions having a larger diameter. In this embodiment, the top portion has the shape of a truncated cone, but other shapes are possible as well. The wafer 10 is herein positioned behind these pins 23, such that the wafer 10 cannot by accident fall off the wafer boat, but is kept between the pins 23 and the plate 11. In order to insert the wafer 10 in this position, it is desired to carry out a movement including a rotation, so as to pass the supporting pins with a tip portion having a larger diameter than a main portion of the pin 23. In presence, the loading movement is carried out in two consecutive steps, wherein the wafer gripper is first positioned relative to the plate 11 of the wafer boat by means of the positioning means 57. Thereafter, the base 51 is moved, as part of a gripper assembly, by means of a driving means conventionally referred to as a robot. This robot is suitably configured to carry out movements in six dimensions. In order that the wafer passes the tip portions of said pins 23, it a rotating movement is carried out. The springs 56 may herein be deformed, particularly differentially, so as to enable said rotation and/or said movement that is not entirely parallel to the frame 60. Suitably, the rotating movement is followed by a corkscrew movement or at least a movement such that the wafer carries out a circular, oval and/or spiral trajectory, in this manner, the wafer 10 can be moved behind all supporting pins 23 present. Typically, as visible in Fig. 1, a plurality of supporting pins is present. In fig. 1 five pins can be seen around a single hole in the first plate 11. It is observed for sake of clarity that Fig. 3 is somewhat simplified relative to Fig. 2 and does merely show a single suction device 54 and a single hose 53.

Fig. 4 shows substantially the same side view as Fig. 3, but for a modified wafer gripper 50. In fact, the shown side view corresponds with the front view of Fig. 2, just as that of Fig. 3. In the implementation shown in Fig. 4, the positioning means 57 is present on one side of the space reserved for the wafer 10 only, i.e. on the upper side in the illustration of Fig. 4. While the presence of the positioning protrusion 57 on the upper side is deemed beneficial, it is not excluded that the lower side or a side face could be chosen. Furthermore, it is not excluded that more than one positioning protrusion 57 is present on the chosen upper side. On the lower side, no protrusion is present. Rather, the corresponding pin 23 on the wafer boat is provided with a spacer layer 67, for instance embodied as a layer of silicon nitride or any other material that withstands temperatures used in PECVD deposition processes.

Fig. 5 shows again a further implementation of the wafer gripper 50, which differs from that shown in Fig. 3 and 4 and still corresponds to the view of Fig. 2. In this embodiment, the wafer boat is further provided with a boat extension pin 24 that is arranged at a lower level than a lower one of the holding pins 23. Said boat extension pin 24 may effectively be arranged below said holding pin 23, but that is not necessary. The boat extension pin 24 is provided with a spacer layer 67 and is configured for being in contact with the frame 60 of the wafer gripper assembly. These pins 24 are not intended to hold the wafer, but are merely used as stand-off for the frame (defining a gripper plane) 60 in addition of the gripper protrusions 57. The additional boat extension pins are one optional implementation, other forms are possible as well. Preferably the total number of spacers 68 and pins 23, 24 on which the frame 60 is positioned is more than three, to keep the wafer substantially parallel to the first plate 11 of the wafer boat.

Fig. 6 shows a second embodiment of a wafer gripper 50. Herein the bridging element 52 is shorter than in the first embodiment of Fig. 2-3. The bridging element 52 is herein arranged between the base 51 and the frame 60. In this embodiment, the frame 60 has the shape of a plate provided with an aperture 64. As in the first embodiment, the positioning means 57 are present on the frame 60. While the frame is coupled to the bridging element by means of a fixed connection, an elastic coupling is present between the frame 60 and a holding plate 70. This holding plate 70 comprises the suction devices 54, and is thus configured for holding a wafer 10. The elastic coupling is again embodiment by leaf springs 56. It is visible in Fig. 4 that the leaf springs 56 are attached to the side of the frame 60 and the holding plate 70 facing away from the wafer 10.

In this embodiment, the positioning means 57 are in a fixed relationship to the bridging element 52. It is therefore preferred in this embodiment that the positioning means are somewhat overdimensioned, i.e. have a somewhat larger height than needed in accordance with the average positioning of the bridging element 52 relative to the first plate 11 of the wafer boat. It is to be observed that the term ‘somewhat larger height’ has to be seen in the context of the typical dimensions. With a pin height of typically 0.7 mm or less, the effective distance between the bridging element 52 and the plate 11 will be at most a few mm, and may be as small as 1-2 mm. The larger height of the positioning means is thus merely in the order of 0.1-1 mm, typically 0.1-0.5 mm or even 0.1-0.3 mm.

In this embodiment, when the positioning means 57 are in contact with the first plate 11, any movement generated by the base 51 will be transmitted via the springs 56 to the holding plate 70. The larger the extension of the spring 56, the stronger the counterforce. This creates an inherent tendency that the wafer 10 is not moved too far towards the plate 11, during the translation and particularly the rotating movement and subsequent movement to insert the wafer 10 behind the pins 23 of the wafer boat. As a consequence, the springs 56 allow a controlled movement of the water 10, which can prevent scratching.

It is observed in this context, that the controlled movement may not merely reduce scratching, but may further be used to reduce the height of the pins 23. The latter would typically increase the risk of scratching but has been found to provide a better deposition.

Fig. 7 shows a third embodiment of the wafer gripper 50 in accordance with the invention. As in the first embodiment shown in Fig. 2, the frame 60 is herein elastically coupled to the bridging element 52, which is shaped in accordance with a preferred design as an elongated plate, extending well into a cavity defined by the frame 60. The elastic coupling is again embodied by means of leaf springs 56. However, this is not to be construed as limiting and leaf springs may be replaced by any other type of spring known in the art. Different from the embodiment in Fig. 2, but comparable to that of Fig. 4, the suction devices 54 are herein arranged on or in a holding plate 70. in the illustrated embodiment, the holding plate 70 is fixed to the bridging element 52. The holding plate 70 is substantially circular in shape. This is deemed advantageous so as to enable optimal rotation of the holding plate 70 during the insertion of the wafer 10 behind the pins of the wafer boat. The circular shape of the holding plate is matched by a circular aperture around said holding plate 10. In order to leave place for said holding plate 70, the first and the second element 61, 62 of the frame are herein provided with a outward extending portion, herein with an angular design.

In the embodiment shown in Fig. 7, the positioning means 57 - not indicated in Fig. 7 -will contact the plate 11 and/or the pin 23, 24 of the wafer boat. The springs 56 provide the tolerance for any deformation of the wafer boat, either forward or backward. Due to the fact that a plurality of springs 56 is provided that are divided over the extension of the frame 60, it is furthermore feasible to tolerate more local deformations of the wafer boat. In the latter case, not all springs will deform equally. After this positioning of the positioning means 57 in combination with operation of the springs 56, the wafer 10 may be inserted behind the holding pins 23 of the wafer boat by means of the rotating movement of the base 51 and thus also the rest of the gripper. The springs 56 and the frame 60 likely will exert a counterforce on against movement towards the wafer boat. They thus dampen and provided resistance against too strong movement. Therewith the positioning means 57 and springs 56 contribute to reduces scratching by keeping the wafer at sufficient space from the wafer boat due to the positioning and/or by providing resistance against too strong movement towards the wafer boat during the insertion of the wafer behind the pins of the wafer boat - which suitably comprises a rotating movement.

Fig. 8 shows in a diagrammatical view part of a wafer boat. The plate 11 comprises windows or cavities 13. The plate 11 is further provided with a plurality of holding pins 23 arranged around each window 13. These are arranged such that a wafer 10 to the first plate 11 covers said window 13. In the figure, several situations are depicted: a first window 13 (on the right hand side) is still uncovered (no wafer placed); a wafer 10 is present before a second window 13 (in the middle), and a frame 60 of a wafer gripper is shown before a third window 13 (on the left hand side), so as to load a wafer 10 or unload a wafer 10. Corresponding to the embodiment shown in Fig. 5, not merely holding pins 23 are present but also boat extension pins 24. It is furthermore visible that protrusion means 57 are arranged at locations on the plate 11, where no pin 23, 24 is present. There is furthermore a pin 23 that is covered by the frame 60, which also contributes to the fixation of the wafer 10 on the first plate 11. However, the boat extension pin 24 is located slightly further away from the window 13. Under normal conditions, this pin 24 does not contribute or contributes merely to a limited extent to fixation of the wafer 10.

Claims

A wafer gripper assembly comprising at least one wafer gripper comprising: a base; A bridging element that hangs at the base; A frame which is coupled to the bridging element and is provided with positioning means, which are arranged to make contact with a wafer boat, in order to position the frame at a predetermined distance from the wafer boat, Vacuum means comprising a hose for applying an underpressure and at least one suction device adapted to hold a wafer by means of the applied underpressure; Where an elastic coupling is present, either between the frame and the bridging element, or between the frame and the at least one suction device.

The wafer gripper assembly according to claim 1, wherein the elastic coupling is located between the frame and the bridging element.

The wafer gripper assembly according to claim 2, wherein the elastic coupling is designed as at least one spring, which spring is able to deform in two opposite directions relative to the frame.

The wafer gripper assembly according to claim 2, wherein the at least one suction device is located on the frame.

The wafer gripper assembly according to claim 2, wherein the at least one suction device is present on a retaining plate connected to the frame or to the bridging element.

The wafer gripper assembly according to claim 1 or 2, wherein the at least one suction device is present on a retaining plate that is coupled to the frame, the retaining plate being elastically coupled to the frame.

The wafer gripper assembly according to any of the preceding claims, wherein the frame comprises a first and a second element which are designed as bars.

The wafer gripper assembly according to claim 7, wherein the bridging element is suspended from the base along a first, preferably vertical direction, and wherein at least one of the first and the second element extends substantially parallel to the first direction.

9. A wafer gripper assembly as claimed in any one of the preceding claims, wherein the positioning means are located at a location adjacent to a space intended for a wafer to be held by the at least one suction device.

10. A wafer gripper assembly as claimed in claim 9, wherein the positioning means are designed as at least one protrusion which in operation extends in the direction of the wafer boat.

A wafer gripper assembly according to claim 10, wherein the positioning means comprises a first protrusion and a second protrusion, which first protrusion is located at said location adjacent to said space for a wafer, and which second protrusion is located at a locale adjacent to said space but on the side opposite to that of the first protrusion.

The wafer gripper assembly according to claim 10 or 11, wherein said at least one protrusion comprises a contact layer adapted to contact the wafer boat.

13. A wafer gripper assembly as claimed in any one of the preceding claims, wherein the elastic coupling means are designed as a plurality of springs which are coupled to the frame.

The wafer gripper assembly of claim 13, wherein the springs are leaf springs, and wherein the leaf springs are preferably oriented inward of the frame.

The wafer gripper assembly according to any one of the preceding claims, wherein a plurality of suction devices are present, preferably at least three.

A wafer gripper assembly according to any one of the preceding claims, wherein the wafer gripper is adapted to perform a rotary motion, said rotary motion taking place around an axis defined in a space in which a wafer is placed during operation.

The wafer gripper assembly according to claim 16, wherein the wafer gripper is further adapted to perform a movement in accordance with a corkscrew.

A system of a wafer gripper assembly according to any one of the preceding claims and a wafer spool.

The system of claim 18, wherein the wafer boat comprises a plate and a plurality of retaining elements protruding from said plate, and wherein the wafer gripper is adapted to place a water between the retaining elements of the wafer boat.

The system of claim 19, wherein said retaining elements are provided with a primary portion and an end, said primary portion extending between the wafer boat plate and the end, and wherein said end has a larger diameter than the primary part.

The system of claim 20, wherein the end is in the form of a truncated cone.

A system according to any one of the preceding claims 18-21, wherein the wafer boat is provided with positioning means which are intended for cooperation with the positioning means of the wafer gripper assembly.

The system of any one of claims 18 to 22, wherein the wafer boat plate comprises graphite.

Use of a wafer gripper assembly according to any of the preceding claims 1-17 for loading a wafer onto a wafer boat and / or for discharging the wafer from the wafer boat, preferably while forming a system according to one of the claims 18-23.

The use of claim 24, wherein the wafer is a wafer that is processed to form a solar cell.

Use according to claim 24 or 25, comprising the steps of: Placing at least one wafer on the wafer gripper assembly and retaining the at least one wafer by applying an underpressure by means of the at least one suction device of the wafer gripper; Positioning the wafer gripper assembly on the wafer boat such that positioning means of at least one wafer gripper come into contact with a plate of the wafer boat; Rotating the wafer gripper assembly along an axis that extends substantially through said wafer to move the wafer beyond at least one retaining member on said wafer boat plate; Removing the underpressure when the at least one wafer is placed against the plate and behind said at least one retaining element.

The use according to claim 26, further comprising the step of moving the wafer gripper, preferably according to a movement according to a corkscrew, after said rotation, so as to place the wafer behind a plurality of retaining elements located on opposite sides of the wafer.

Use according to claim 24 or 25, comprising the steps of: Positioning the wafer gripper assembly at the wafer boat such that positioning means of at least one wafer gripper come into contact with a location of the wafer boat; Moving the wafer gripper assembly such that at least one suction device comes into contact with a wafer placed on the wafer and causing the wafer to adhere to said at least one suction device by applying an underpressure via said suction device; Rotating the wafer gripper assembly, preferably along an axis substantially running through said wafer, to disengage the wafer from behind the at least one retaining element on said wafer boat plate; When releasing the one or more wafers is complete, moving the wafer gripper assembly to move the one or more wafers from the wafer boat to a predetermined location, and preferably placing the one or more wafers on the predetermined location and removal of the underpressure.

A method for manufacturing a solar cell comprising the steps of: Loading at least one wafer onto a wafer boat comprising mutually separated electrodes; Moving the wafer boat with the at least one wafer into a deposition chamber of a device for plasma-stimulated chemical vapor deposition (PECVD); Applying at least one layer on the at least one wafer on the wafer boat by plasma-stimulated chemical vapor deposition (PECVD); Moving the wafer boat from the deposition chamber, and Unloading the at least one wafer from the wafer boat, wherein the wafer gripper assembly according to any of claims 1-17 is fitted for loading and unloading the at least one wafer into and from the wafer boat.

The method of claim 29, wherein the loading step comprises the use of any one of claims 26 and 27.

The method of claim 29 or 30, wherein the discharge step comprises the use of claim 28.