WO2011053153A1

WO2011053153A1 - Device for wafer handling

Info

Publication number: WO2011053153A1
Application number: PCT/NO2010/000385
Authority: WO
Inventors: Josef Filtvedt
Original assignee: Dynatec Engineering As
Priority date: 2009-10-28
Filing date: 2010-10-28
Publication date: 2011-05-05
Also published as: NO20093232A1

Abstract

A device for separating the outer slice of silicon or another material from a grouping of one or more outer slices, wherein a layer of fluid is present between the outer slice and the one or 5 more outer slices. The device is characterized in that it comprises a heating device which is placed and dimensioned in such a way that it during operation causes heating to vaporization of at least a part of the said layer of fluid. Thus, the adhesive forces caused by the layer of fluid are eliminated in such a way that the outer slice and succeeding outer slices more easily can be separated and transported to the next step in the production of a finished product. A system and a method employing said device.

Description

Device for wafer handling

Field of invention

The present invention concerns the handling of objects, in particular layered objects which are stacked together with a thin layer of fluid between the layers. In particular, the invention concerns a device for separating the outermost slice or wafer or thin plate of silicon or another material from a grouping or stack of one or more other slices, wherein a layer of fluid is present between the outermost slice and the one or more outer slices, a system with such device and a method for such separation.

Background of the invention

Some materials in industrial processes, such as silicon and in part other semiconductor materials for use in solar cells, sensors and electronics, are cast into ingots or blocks which are cut into units of the appropriate size according to the finished product. In particular, for solar cell applications, but also for some other applications, the blocks are sliced into thin wafers (also called slices, thin plates or objects) in a liquid-cooled cutting process. Wafers exit the cutting process as blocks of parallel wafers held together by the coolant from the preceding cutting process. In subsequent treatment, every single wafer should be treated individually as separate units, which, however, is difficult due to strong forces that hold the wafers together. The thin layer of fluid between the wafers is the primary cause of the forces holding the wafers together, and the forces are assumed to be a combination of cohesive, adhesive, viscous and mechanical forces (adhesive and cohesive forces constitute so-called capillary forces).

Separation of wafers from the stack or the grouping has, until now, been done manually, whereby an operator separates the first wafer from the stack by hand, and manually brings it to the next step in the process. The process is time-consuming and involves great problems for the operator. After being cut into slices, the stacked wafers are held together by several forces related to the viscous properties of the residual coolant which can be found between the wafers after the cutting process. The process is further complicated by the rough surface of the wafers as well as particles between the surfaces of the wafers after the cutting process. Manual handling leads to big losses, as a great number of wafers are damaged during the separation process. In the solar cell industry there is a need for thinner wafers, of 50 - 200 μηι, which demands even more careful processing than what is available today.

In US Patent Application No. 2008/0146003 Al, a method is described, in which the wafers are arranged in a vertical stack and submerged in a water-filled vessel. The stack of wafers is submerged in water in order to reduce the viscous forces between the wafers, and water is used to separate the wafers before separation of wafer number 1 from the stack. A transport device with suction devices adheres to side number 1 of wafer number 1, i.e. the outer side which is available and faces outwards from the stack, and separates by adhering to the one side of the wafer, pulling it up vertically from the stack. Wafer number 2, 3 etc. in the stack are held in place by means of a mechanical blocking device which should prevent the adjacent wafer or wafers from moving simultaneously when wafer number 1 is being removed from the stack. The method has several disadvantages which may lead to breakage and reduced capacity during operation. It is difficult to perfectly adjust several suction devices, and unwanted forces may occur in the wafer when vacuum is applied. Immersion in water reduces the adherence between the wafers and increases the risk that more than one wafer join during separation. The way the invention is described in the publication mentioned above, the capacity is limited.

Publication WO 2004/051735 describes a device for separating horizontal wafers arranged in a vertical stack. The wafers are lying in a horizontal plane and are moved in the same plane. A transport device lifts the top wafer away from the stack while a fluid is blown in between wafer 1 and wafer 2 in the stack in order to separate wafer number 1 from wafer number 2. There are several disadvantages of this method. A horizontal arrangement of the wafers increases the risk that several wafers move simultaneously during separation. Separation by applying a fluid between wafer 1 and wafer 2 is difficult to control in such a way that the pressure on the wafer surface is completely even. Uneven pressure over the surface exposes the wafer to forces which may damage it and result in increased loss during separation.

Publication DE 102005016518 describes a device for separating wafers from a carrier device, and individual output from a horizontal stack. The method is principally developed for thicker wafers (300-400μιη) which are used in the electronics industry, and is not very relevant to thin wafers for solar cells.

The method has until now principally been based on increasing the thickness of the layer of fluid between the wafers, or manual work. There is a need for devices, systems and methods with advantageous effects compared to prior technique, and the objective of the present invention is to meet said demands.

Summary of the invention In accordance with the present invention, a device is provided for separation of the outermost wafer of silicon or another material from a stack of one or more further wafers (also termed slices, thin plates, thin objects), wherein a layer of liquid (sometimes termed fluid) is present between the wafers, particularly the outermost wafer and the other wafers. The device is distinctive in that it comprises a heating means (also termed a heating device) which is placed and dimensioned so that it, during operation, causes heating to vaporization of at least a part of the liquid layer between the outermost wafer and the other wafers.

With the device of the invention, the adhesive forces caused by the layer of liquid are eliminated, in such a way that the outer slice (wafer), and the subsequent outer slices, more easily can be separated and transported to the next step in the production of a finished product. It is generally sufficient to vaporize only a part of the layer of liquid, because the vapor or gas expands massively compared to the liquid and possible residual liquid is displaced from the layer by the expanding gas, which contributes to the separation of the outermost slice from the remaining slices in the stack or the grouping of slices. The expansion of volume by the phase transition from liquid to vapor, typically about 1700 times, not only results in physical separation of the outermost wafer from the rest of the wafers and displacement of any remaining liquid acting to hold the outermost wafer to the rest of the stack, but also provides insulation whereby the liquid between the second outermost and third outermost wafers do not evaporate. The result is that the other or remaining wafers are held firmly together by the liquid between said wafers whilst the outermost wafer is easily separated from the remaining stack and can be transported away.

Most advantageous, the device is placed and dimensioned in such a way that it heats the total area of the slice or a number of points spread over the area of the slice, since this causes increased separation speed and the possibility of more careful handling of slices or wafers.

Any functionally suitable heating means may be applicable, for example chosen among heating light sources pointing towards the surface of the slice, laser sources pointing towards the surface of the slice, flat resistance heaters with contact face matched to the area of the slice, heat chambers which enclose the grouping of slices, radiant heaters, induction heaters, resistance heaters, heaters that heat fluid applied between the slices to a temperature close to the boiling point and other heating means. The device is advantageously dimensioned and arranged in such a way that heating to sufficient vaporization of the layer of liquid between the outer slice and the grouping of slices is performed sufficiently fast for the layer of liquid between the second outermost and the third outermost slice not to vaporize, whereby increased separation speed without adhesion to the second outermost slice or other slices can be achieved.

The invention also provides a system for separation and handling of wafers (slices, thin plates) of silicon or another material from a stack of wafers, wherein a layer of liquid is present between the wafers, wherein the system comprises a holder which holds the stack of wafers, distinctive in that the system further comprises: a heating means which is placed and dimensioned in such a way that it, during operation, causes heating to vaporization of at least a part of the liquid layer between the outermost wafer and the remaining wafers of the stack whilst the remaining wafers are held firmly together as a stack by the liquid layers between the remaining wafers; and a transport device which can transport the outermost wafer away from the remaining wafers to a conveying belt or a wanted location for further processing.

The transport device advantageously comprises an endless belt with integrated suction devices, which results in the highest possible speed, particularly if several belts or bands are arranged in parallel. With advantage, particularly for smaller industrial units due to lower investment costs, the transport device comprises a robotic arm with an integrated suction device. Of course, manually manipulated suction surfaces, or other devices, may be employed. The invention also provides a method for separating the outermost wafer of silicon or another material from a stack of wafers, wherein a layer of liquid is present between the wafers, distinctive by heating, with a heating means, to vaporization of at least a part of the liquid layer between the outermost wafer and the remaining wafers of the stack. With advantage, heating to vaporization of the layer of liquid between the outer slice and the grouping of slices is performed sufficiently fast for the layer of fluid between the second outermost slice and the third outermost slice not to vaporize, which causes increased separation speed. The slices are advantageously held together by a force which results in a thin layer of liquid between the slices in such a way that capillary forces or other forces contribute to hold the slices together, possibly except from those moments when the outer slice is being removed from the stack of grouped slices. This means that the stack of slices, the grouping, is pressed together by a physical force caused by a holder device and/or a transport device and/or other devices, which surprisingly facilitates, as proven in practice, the separation of the outer slice. It seems that the magnitude of the force may vary significantly, however, typical values are approximately 100 - 1000 N/m² (100 - 1000 Pa).

Figures

The invention is illustrated by means of five figures, of which:

Figure 1 illustrates a device in accordance to the invention, as an arrangement drawing.

Figure 2 is a sectional drawing which illustrates the separation layer between wafer 1 and wafer 2 as well as details concerning the device, the system and the method according to the invention.

Figure 3 shows a simplified embodiment of a system according to the invention.

Figure 4 illustrates an embodiment in which a robotic arm constitutes a part of the transport device of a.system according to the invention.

Figure 5 illustrates an alternative embodiment in which a robotic arm constitutes a part of the transport device of a system according to the invention.

Detailed description

Wafers (slices) which are separated by the device, the system and the method according to the invention are, for example, silicon wafers for production of solar cells. Such wafers may be thin (50 μηι - 200 μηι), and the handling of them is particularly critical to avoid damage to the wafers.

The present fluid is water; water with additives which lower the boiling point and/or improve lubrication and cutting, or other liquid fluids, like alcohol, or mixtures of liquids. The illustrated implementations are examples among many possibilities. The invention comprises the device, the system/plant and the method, and use of the device and use of the system, including any features as here described or illustrated in any operative implementation and combination, which combinations are a part of the present invention.

Fig. 1 illustrates a device (100) and a system according to the invention for separation of silicon wafers from an inclined stack of wafers (13) which are placed in a heated chamber (14b) without fluid filling of the chamber. By an inclined stack is meant that the wafers are arranged in a stack on an inclined plane (14a) sloping in such a way that gravity breaks into one force component acting perpendicularly from wafer number 1 on wafer number 2 in the stack, and a second force component perpendicular to the inclined plane (14a) which supports the wafers from the bottom side. An inclined stack with a layer of fluid between the wafers results in a stable stack which is kept in place by gravity, viscous forces between the wafers in the stack and mechanical friction caused by rough surfaces and residual particles from the cutting process.

In the chamber, one or more nozzles are arranged, which can be fixed or adjusted in all directions, whose task is to maintain the layer of fluid between the layers in such a way that they are held in a stable, joint stack. The chamber comprises an inclined mechanism for feeding wafer number 1, the outermost wafer or slice, into the correct position at the fetch position. The mechanism refers to the fetch position and not to the wafer thickness, which can vary after the preceding cutting process. Feeding the front side (la) of wafer number 1 into a fixed position at the fetch position ensures the correct function for the transport device, albeit the wafer thickness may vary.

The layer of fluid is particularly important between the wafers adjacent to wafer number 1 in order to ensure that the wafers are kept in the stack in such a way that only wafer number 1 adheres to the transport device. The fluid from the nozzles flows over the wafers in a finely divided mist which does not affect the viscous forces between the wafers. The system (100) comprises a transport unit (17) which in this implementation of the invention comprises an endless belt with suction areas matched to the size and shape of the wafers, comprising vacuum holes arranged as succeeding patterns on the belt. The vacuum holes are arranged within the area of the wafers in such a way that the belt between the wafers (17a) has no vacuum holes. Referring to Figure 2, the transport unit can adhere to the outside (la) of wafer number 1 in the stack and transport wafer 1 parallel to the surface (2a) of wafer number 2 until wafer 1 is separated from wafer 2, and by means of the same transport unit to the next step in the process. The wafer is held on a plane base which remains plane during the whole

transportation until the wafer is delivered to the next step in the process. The transport unit can be a continuous belt or a plane area with holes, grid, weave, lattice or another surface which is permeable to fluid and/or gas. The transport unit can fetch wafer 1 from a fixed position, or have a movable function perpendicular to the outside (la) of wafer number 1. After separation the transport unit can move in all angles and curves suitable for careful transportation and delivery of the wafer to the next step in the process. The transport device in system (100) illustrated in Fig. 1, moves by means of an edge roller (19) which moves synchronously to the band conveyor (17) in such a way that the wafers always lie on a plane surface without deformation or outer forces acting on the wafers. The swing rollers (29) fitted onto the edge roller are equipped with guiding pulleys which ensure that the transport band always moves in a straight line from the separation point to the edge roller (19). The guide bar (28) of the swing rollers (29) follows the edge roller (19) until the wafer is leveled

horizontally or nearly horizontally.

The transport device in implementation (300), illustrated in Fig. 3, comprises a solution without an edge roller (19) which is shown in implementation (100). The wafers are hanging in vacuum on the bottom side of the conveying belt, and fall down onto the next conveying belt when they are passing the vacuum chamber (23). Delivery of the wafer can also take place at a controllable vacuum chamber (24) which can provide a controlled cessation of vacuum in order to offer a careful transition for the wafer.

The vacuum system comprises one suction unit (not shown) with nearly constant vacuum which is connected to the implementation (100) through the vacuum connection (27), as well as a vacuum chamber (24) with adjustable vacuum pressure by means of a flap (25). Vacuum for the vacuum chamber (24) can be engaged and disengaged by moving the flap (25) which can close and open the vacuum connection (26) to the constant vacuum chamber (23).

The vacuum level at the front side (la) of wafer number 1 may thereby be adjusted as needed by a combination of the vacuum chamber (23) with constant vacuum and the variable volume in the vacuum box (24) with intermittent vacuum. Servo controlled movement of the flap can provide wanted pressure/vacuum in the vacuum chamber (24) as needed.

Heating of wafer number 1 for separation with vapor/gas, can be achieved by heating from the transport device and/or heating of wafer number 1 directly in fetch position. The transport device can be preheated before fetch position by a heating unit (31), by electric current through the part of the transport device which is in fetch position (32), and/or heating by a heating source (16) which is located at wafer number 1 in fetch position. The heating can be achieved by a combination of 2 or more of the following 5 steps: preheating of the wafer in heating chamber (14b) before separation, preheating of the transport device before fetch position (32), heating of the transport device in fetch position (32), heating of the wafer in fetch position, heating of the layer of fluid (3) between wafer 1 and wafer 2 in fetch position. At least 1 of the heating methods should be possible to time accurately in order to ensure a controlled and accurate separation of wafer number 1 at the wanted time. Additional and other types of heating devices are also applicable.

One or more stacks of silicon wafers (13) are arranged on an inclined plane (14a) in a heating chamber (14b) equipped with adjustable heating. The chamber contains a mechanism (14) which can feed the stack of wafers parallel to a plane (14a) in the lower part of the chamber into wanted position for wafer number 1 in fetch position. The stack of wafers are kept moist by 1 or more nozzles (15) which keep the stack of wafers moist during the process and maintain the forces that hold the wafers together in the stack.

The outside (la) of wafer number 1 are fed into fetch position (32) by the feeding mechanism (14) which pushes the complete stack of wafers along the inclined plane (14a). The feeding stops when the outside (la) of wafer number 1 touches the transport device in the wanted position and exerts a controlled pressure against the transport device. Vacuum from the vacuum chamber (23) with constant vacuum (23) are connected through the valve (25) to the vacuum chamber (24) which is directly connected to the outside (la) of wafer number 1. Heat from the transport device in fetch position (32) and/or heat from the heating source (16) in fetch position heats wafer number 1 from the outside (la) until the wafer becomes thoroughly hot, and the heat from the inside (lb) of wafer number 1 is transmitted to the water layer (3a) adjacent to the inside (lb) of wafer number 1. The layer of fluid adjacent to the inside (lb) of wafer number 1 are heated to the boiling point and sublimates into vapor/gas, and wafer number 1 is separated from wafer number 2 by a evenly distributed gas pressure that pushes wafer 1 towards the transport direction. The feeding mechanism (14) removes the pressure against the transport device by feeding the stack of wafers back in the opposite direction of the forward feeding, i.e. perpendicularly to the plane of the transport device in fetch position. The wafer is now connected to the transport device through vacuum on the total area of the wafer, and separated from wafer number 2 by a vapor/gas layer between wafer number 1 and wafer number 2. Constant heating maintains a vapor/gas layer and keeps wafer number 1 and wafer number 2 separated until the transport device has transported wafer number 1 away so that it no longer touches wafer number 2.

The transport device returns to fetch position when wafer number 1 has been transported at least one wafer width away from the fetch position, and the whole process is repeated for the next wafer in the stack. Continuous implementation of the transport device may result in considerable capacity during operation; in that a new wafer can be fetched as soon as the preceding wafer is no longer in fetch position.

The Figures 4 and 5 illustrate implementation of the system according to the invention whereby the transport device comprises a robotic arm.

Experiments with the invention have shown that the production speed can be increased several times whereas the fault rate can be reduced significantly.

Claims

1.

A device for separation of the outermost wafer of silicon or another material from a stack of one or more further wafers (slices, thin plates), wherein a layer of liquid is present between the wafers, particularly the outermost wafer and the other wafers, c h a r a c t e r i s e d i n that the device comprises a heating means (a heating device) which is placed and dimensioned so that it, during operation, causes heating to vaporization of at least a part of the liquid layer between the outermost wafer and the other wafers.

2. A device according to claim 1 , characterized in that the heating means is placed and dimensioned such that it heats the full area of the wafer or a number of points spread over the area of the wafer.

3.

A device according to claim 1, characterized in that it comprises several heating means chosen among heating light sources directed towards the surface of the wafer, laser sources pointing towards the surface of the wafer, flat resistance heaters with contact face matched to the area of the wafer, heating chambers which enclose the stack of wafers, radiant heaters, heaters heating fluid which is brought in between the wafers to a temperature close to the boiling point, etc.

4.

A device according to any one of claims 1-3, characterized in that the heating means is dimensioned and operatively arranged in such a way that heating to sufficient vaporization of the layer of fluid between the outermost wafer and the group of other wafers is performed sufficiently fast for the layer of liquid between the second outermost and the third outermost wafer not to vaporize.

5.

A system for separation and handling of wafers (slices, thin plates) of silicon or another material from a stack of wafers, wherein a layer of liquid is present between the wafers, wherein the system comprises a holder which holds the stack of wafers, c h a r a c t e r i s e d i n that the system further comprises: a heating means which is placed and dimensioned in such a way that it, during operation, causes heating to vaporization of at least a part of the liquid layer between the outermost wafer and the remaining wafers of the stack whilst the remaining wafers are held firmly together as a stack by the liquid layers between the remaining wafers; and a transport device which can transport the outermost wafer away from the remaining wafers to a conveying belt or a wanted location for further processing.

6.

A system according to claim 5, characterized in that the transport device comprises an endless belt with integrated suction devices.

7.

A system according to claim 5, characterized in that the transport device comprises a robotic arm with an integrated suction device.

8. A method for separating the outermost wafer of silicon or another material from a stack of wafers, wherein a layer of liquid is present between the wafers, c h a r a c t e r i s e d b y heating, with a heating means, to vaporization of at least a part of the liquid layer between the outermost wafer and the remaining wafers of the stack.

9. A method according to claim 8, characterized in that the heating to vaporization of the layer of fluid between the outermost wafer and the group of remaining wafers of the stack is performed sufficiently fast for the layer of fluid between the second outermost and the third outermost wafers not to vaporize.

10. A method according to claim 8, characterized by holding the wafers together by applying a mechanical force resulting in a thin layer of liquid between the wafers so that capillary forces or other forces contribute to hold the wafers together, optionally except of those moments when the outermost wafer is being removed from the stack of grouped wafers.