FIELD OF THE INVENTION [0001] The present invention relates to sliced wafers. More particularly, the present invention relates to a method and device for separating sliced wafers.

BACKGROUND OF THE INVENTION

[0002] In preparing wafers for the semiconductor industry, for example for manufacturing into solar cells, an ingot of semiconductor material, such as silicon, is sliced into a stack of individual wafers. In order to maximize the number of wafers produced from a single ingot, manufacturers are motivated to reduce the thickness of the wafers. For example, a typical wafer may be in the form of a disk or square with diameter or side dimension of several centimeters, and thickness in the range 100 μm to 300 μm. Such a wafer is fragile, and a bending stress applied to the wafer may cause the wafer to break.

[0003] The wafer is typically further processed to produce a usable product. For example, the wafer may be cut to a square shape and manufactured into a solar cell (typically with length of side about 160 mm). In order to process a sliced silicon wafer into a usable product, the wafer must be separated from the stack. Various forces acting between wafers in the stack prevent easy separation of a wafer from the stack. In addition, the bonding force between the wafers is increased by the slurry found between and on the edges of the wafers. The slurry, consisting of a fluid such as polyethylene glycol 300 (PEG 300) mixed with abrasive material that is used in sawing the ingot into thin wafer slices, increases the bonding force between the wafers. In order not to stress, and possibly break, the wafer, the force that is applied to separate a wafer from a stack must be applied in a lateral shear direction, perpendicular to the edge of the wafer. For this reason, the separation process has been difficult to automate. Therefore, wafers are often separated manually from the stack. In this relatively slow and low-yield process, a human worker manually separates a wafer and transfers it to the next process. [0004] Some described devices for automatic separation of wafers, for example that described by Gibbel in US6558109, employ a chuck fixture hold the wafer and to apply the required lateral force. In these devices, the orientation of the chuck is fixed. Since, in practice, the orientation of a wafer in a stack of wafers is likely to have a random component, and its face will not, in general, be exactly parallel to the face of the chuck. Therefore, mechanical force is applied to the wafer in order to force the orientation of the wafer to conform to the orientation of the chuck. Such applied mechanical forces increase the risk of breakage of the removed wafer.

[0005] It is an object of the present invention to provide a means of removing a single wafer from a stack of wafers, without applying bending stress to any of the wafers.

[0006] Other aims and advantages of the present invention will become apparent after reading the present invention and reviewing the accompanying drawings.

SUMMARY OF THE INVENTION [0007] There is thus provided, in accordance with some embodiments of the present invention, an apparatus for separating a front wafer from a stack of wafers. The apparatus comprises: a chuck for gripping the front wafer, coupled to a mechanical arm and rotatable about at least one axis with respect to the arm, so as to allow the chuck to align substantially parallel to the front wafer, provided with a fixing mechanism for fixing the chuck in a desired orientation. The apparatus further comprises a drive for moving the arm in at least in two directions, one direction to bring the chuck to the front wafer and in another direction substantially parallel to the fixed orientation of the chuck to separate the wafer from the stack of wafers. The apparatus further comprises a stopper mechanism for preventing other wafers from separating from the stack while the front wafer is being separated.

[0008] Furthermore, in accordance with some embodiments of the present invention, the chuck comprises a vacuum chuck.

[0009] Furthermore, in accordance with some embodiments of the present invention, the stopper mechanism comprises a stopper bar which is adapted to move into position adjacent to the stack of wafers, a distal end of the stopper bar stopped at a desired position relative to the chuck when fixed in the desired orientation. [0010] Furthermore, in accordance with some embodiments of the present invention, the apparatus is provided with a drive mechanism for moving the stopper bar and a sensor for sensing a position of the stopper bar relative to a wafer of the stack of wafers, the drive mechanism being configured to move the stopper bar on the basis of the sensed position.

[0011] Furthermore, in accordance with some embodiments of the present invention, the apparatus comprises a brake for stopping motion of the stopper bar.

[0012] Furthermore, in accordance with some embodiments of the present invention, the apparatus is provided with a sensor for sensing the orientation of the chuck when substantially parallel to the front wafer.

[0013] Furthermore, in accordance with some embodiments of the present invention, there is provided a method for separating a front wafer from a stack of wafers. The method comprises providing a chuck for gripping the front wafer, coupled to a mechanical arm and rotatable about at least one axis with respect to the arm, so as to allow the chuck to align substantially parallel to the front wafer, provided with a fixing mechanism for fixing the chuck in a desired orientation. The method further comprises providing a drive for moving the arm in at least in two directions, one direction to bring the chuck to the front wafer and in another direction substantially parallel to the fixed orientation of the chuck to separate the wafer from the stack of wafers. The method further comprises: moving the chuck towards the front wafer, aligning the chuck substantially parallel to the front wafer, gripping the front wafer, fixing the orientation of the chuck, and separating the front wafer from the stack of wafers by moving the chuck substantially parallel to the front wafer.

[0014] Furthermore, in accordance with some embodiments of the present invention, the step of aligning the chuck substantially parallel to the front wafer comprises contact between the chuck and the front wafer.

[0015] Furthermore, in accordance with some embodiments of the present invention, the step of aligning the chuck substantially parallel to the front wafer comprises no contact between the chuck and the front wafer. [0016] Furthermore, in accordance with some embodiments of the present invention, the method comprises preventing other wafers from separating from the stack while the front wafer is being separated using a stopper mechanism.

[0017] Furthermore, in accordance with some embodiments of the present invention, the step of preventing other wafers from separating from the stack comprises providing a stopper bar which is adapted to move into position adjacent to the stack of wafers, a distal end of the stopper bar stopped at a desired position relative to the chuck when fixed in the desired orientation.

[0018] Furthermore, in accordance with some embodiments of the present invention, the method comprises sensing a position of the stopper bar relative to a sensed surface of a wafer of the stack of wafers, and moving a distal end of the stopper bar to a desired position relative to the sensed wafer surface.

[0019] Furthermore, in accordance with some embodiments of the present invention, the method comprises applying a brake to the stopper bar when the stopper bar is at the desired position.

[0020] Furthermore, in accordance with some embodiments of the present invention, the method comprises providing a sensor for sensing the orientation of the chuck when substantially parallel to the front wafer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to better understand the present invention, and appreciate its practical applications, the following Figures are provided and referenced hereafter. It should be noted that the Figures are given as examples only and in no way limit the scope of the invention. Like components are denoted by like reference numerals. [0022] Fig. 1 shows a system for wafer separation in accordance with embodiments of the present invention.

[0023] Fig. 2 shows a chuck for wafer separation in accordance with embodiments of the present invention.

[0024] Fig. 3 shows the exterior of a chuck in accordance with embodiments of the present invention. [0025] Fig. 4 is a cutaway view showing the interior of the chuck in Fig. 3, viewed from the opposite side.

[0026] Fig. 5 is an enlarged view of a portion of the chuck shown in Fig. 4.

[0027] Fig. 6 is a schematic illustration of a chuck for separating a wafer from a stack of wafers, in accordance with embodiments of the present invention.

[0028] Fig. 7 is a flowchart of a method for separation of a wafer from a stack of wafers, in accordance with embodiments of the present invention.

[0029] Fig. 8 schematically shows a wafer restraining device in accordance with other embodiments of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

[0030] An apparatus and method are described for automatically removing an object, such as a wafer, from a surface to which it adheres, such as stack of similar objects. The object is removed in such a manner as to minimize bending stress on the object. In this description, the term "wafer" refers to a stackable thin object of any shape fabricated out of any material. In applying embodiments of the present invention to the semiconductor industry, the stack may typically include wafers of semiconductor material, such as silicon, that were diced, cut, sliced, or sawed from a single ingot. Typically, after slicing, a wafer may be disk or square shaped, with typical side dimension hundreds to thousands times greater than its thickness. Adhesive forces generally resist relative motion between wafers in the stack, thus resisting the removal of a single wafer from the end of the stack. The presence of viscous fluids, such as slurry, between, and around the edges of, the wafers in the stack may further increase the adhesive forces.

[0031] In accordance with embodiments of the present invention, a stack of wafers may be held in a carrier. The carrier may be immersed in a fluid bath. The fluid in the bath may be of such a composition as to assure a general low viscosity as compared with the slurry. In addition, the bath may include nozzles that cause the fluid to flow within the bath in order to perform coarse cleaning of the wafers in the stack. The fluid in the bath may be pumped through a filter in order to maintain the cleanliness of the fluid. [0032] A chuck is provided whose motion is controlled by means of a driver and control system. The chuck is inserted into the bath and moved toward the wafer stack. The chuck is capable of gripping a wafer that is at the end of the stack by mechanical or other means. For example, in the case that the chuck is a vacuum chuck, the face of the chuck that comes in contact with the wafer may be provided with a pattern of openings. Fluid pumped out through though the openings creates suction that grips the wafer. The chuck may also be capable of blowing fluid outward through the openings. Blowing fluid outward may create a pressure field that may be useful in aligning the chuck.

[0033] The chuck is mounted on a mechanical arm in such a manner that when the chuck comes in contact with, or otherwise begins to apply a force to, the wafer that it is to grip, the orientation of the chuck is adjusted to match that of the wafer. For example, the chuck may be mounted on perpendicular axes about which it is free to rotate. Contact with, or applying a force to, the wafer to be gripped causes a torque to be applied to the chuck, causing the chuck to orient itself parallel to the face of the wafer.

[0034] The chuck control determines when the chuck is oriented correctly. For example, sensors may be provided that detect when there is good contact between the chuck and the wafer. Good contact indicates that the face of the chuck is flush with that of the wafer. For example, a sensor may be provided that measures the level of a vacuum that grips the wafer by means of suction. Attainment of a predetermined level of vacuum may then indicate that the wafer is blocking suction openings on the chuck and is therefore being gripped by the suction. Alternatively, the face of the chuck may be provided with pressure-sensitive sensors that detect whether the wafer is being held with sufficient force to indicate good contact. Alternatively, a sensor may be provided that measures the torque applied to a rotation axis about which the chuck is free to rotate. Torque above a predetermined level may indicate that the chuck face is not yet parallel to the wafer face, and is still being rotated about the axis. Upon attaining its final orientation parallel to the wafer, the torque applied to the axis is reduced to a value close to zero. Alternatively, the mechanical arm on which the chuck is mounted, or a drive system adapted to moving the mechanical arm, may be provided with a sensor that senses the force required to move the arm.

[0035] Once it is determined that the chuck grips the wafer with the chuck face oriented parallel to the wafer face, the orientation of the chuck is fixed. For example, a braking, gripping, or other orientation-locking mechanism may be applied to rotation axes of the chuck to prevent further rotation. The braking mechanism is applied radially to the axis so as not to apply any torque to the axis that may possibly alter the orientation of the chuck. Alternatively, any other fixing means known in the art that are appropriate to the mounting means of chuck may be applied to fix the orientation of the chuck. For example, a braking element may be applied radially to a ball of a ball-and-socket mount. As another example, a flexible mounting element may be stiffened in such a manner as to not alter the shape of the element.

[0036] The chuck is provided with sensors that are capable of determining the orientation of the chuck. For example, an axis about which the chuck is free to rotate may be provided with an angle encoder that indicates the orientation angle of the chuck about that axis. A processor associated with the chuck control receives the sensor information regarding the orientation of the chuck. The processor then calculates the direction in which the chuck may move such that the motion of the chuck remains parallel to the face of the chuck. [0037] The chuck moves in the calculated direction. In this manner, the chuck moves the gripped wafer in a lateral direction parallel to the plane of the wafer, minimizing the bending stress applied to the wafer. During the motion of the chuck, one or more restraining fingers or stops may apply a counteracting restraining force on one or more wafers of the stack that are adjacent to the gripped wafer. The restraining stops block motion of the adjacent wafers, ensuring that the chuck moves only the gripped wafer.

[0038] At the time that the chuck is moving the gripped wafer, one or more means known in the art may be applied to reduce the force that holds the gripped wafer to the adjacent wafer in the stack. Such means may include directing a high-pressure jet of water or air, or a beam of ultrasonic waves, on the area between the gripped wafer and the adjacent wafer of the stack.

[0039] The chuck continues to move the gripped wafer until the wafer is clear of the stack. The chuck then moves the wafer, perhaps lifting it out of the bath, to the location at which the wafer is to be released. When the wafer arrives at the location at which it is to be released, the chuck releases the wafer. For example, if the chuck grips the wafer by means of suction, operation of the vacuum generation system is interrupted so that suction is not longer applied to the wafer. [0040] We now describe embodiments of the present invention with reference to the accompanying Figures. Fig. 1 shows a system for wafer separation in accordance with embodiments of the present invention. Fig. 2 shows a chuck for wafer separation in accordance with embodiments of the present invention. Wafer stack 42 of sliced wafers is held by carrier 52. Wafer stack 42 and carrier 52 may be immersed in fluid bath 44. Carrier 52 may be moveable, capable of transporting wafer stack 42 within a wafer processing facility. Thus, a single carrier may be used to transport wafer stack within a processing facility and to hold the wafer stack during several different processes. Using the single carrier may reduce the risk of breaking a fragile wafer when moving the wafer stack from process to process, for example, from a pre-separation cleaning station to a wafer separation station.

[0041] Chuck 10 is mounted on arm 21 by means of brackets 13. Arm 21 is moved by chuck drive assembly 23. Controller 54 controls the chuck drive assembly 23, as well as any gripping and rotation fixing mechanisms associated with chuck 10. Chuck 10 is capable of gripping and moving a wafer from wafer stack 42, such as gripped wafer 40. A restraining stopper is provided for restraining the motion of wafer 41 and other wafers of wafer stack 42 that may be adjacent to wafer 41. The restraining stopper may be in the form of restraining stopper bar 46, or in the form of a beam, block, finger, projection, ring, or any other shape that is suitable for restraining motion of an appropriate section of stack 42. Spacers 32 on chuck 10 ensure that restraining stopper bar 46 is positioned at a precise distance from chuck 10 so as not to impede the motion of gripped wafer 40 while prevention motion of wafer 41.

[0042] Fig. 3 shows a chuck in accordance with embodiments of the present invention.

Fig. 4 is a cutaway view showing the interior of the chuck in Fig. 3, viewed from the opposite side. Surface 1 1 of chuck 10 is adapted to have a flat object, for example a thin silicon wafer, held against it. For example, chuck 10 may be a vacuum chuck, adapted to gripping an object by applying suction to the object surface. Openings 12 on surface

1 1 connect to an internal manifold 16. Internal manifold 16 connects to inlet hose 14.

Inlet hose 14 connects to vacuum generating device 58 (Fig. 1) via tube 56 (Fig. 1). Tube 56 may partially pass within or adjacent to arm 21. Vacuum generating device 58 may include a venture vacuum generator, or any other suitable device capable of producing a vacuum or suction. Thus, by means of inlet hose 14 and internal manifold 16, suction may be applied through openings 12. The applied suction may hold an object, such as a wafer, fast against surface 11. Vacuum generating device 58 may be reversible, enabling either an outward flow of fluid through openings 12, or an inward suction of fluid through openings 12.

[0043] The orientation of chuck 10 is free to rotate in response to an applied torque. Such a torque may be applied, for example, when part of chuck 10 contacts an object toward which chuck 10 is being moved. An example of such an object is a wafer that is located at the front end of a stack of wafers that were sliced from a silicon ingot. [0044] In embodiments of the present invention, the variation in orientation of chuck 10 in response to an applied torque is achieved by providing two perpendicular rotation axes about which chuck 10 is free to rotate. Alternatively, chuck 10 may be mounted by means of a ball-and-socket mechanism, on a flexible rod, or by any other mounting means that allow at least limited free variation in orientation. [0045] In embodiments of the present invention, chuck 10 is free to rotate about a vertical or rotation axis 20, and about a horizontal or elevation axis 22. When chuck 10 is brought near to a flat object such as the front silicon wafer on a stack, the flat object applies a torque to surface 11. The origin of the torque may be a mechanical contact with the front wafer. Alternatively, the origin of the torque may be a pressure gradient that is formed across surface 1 1. The gradient is due to the dependence on the width of the gap between the front wafer and surface 1 1 of the pressure caused by an outward flow of fluid through openings 12. Thus, if surface 1 1 is not parallel to the surface of the front wafer, the fluid pressure at some points on the surface will be greater than at others, resulting in an applied torque. The applied torque causes chuck 10 to rotate about vertical axis 20 and horizontal axis 22. The rotation tends to align surface 11 parallel to the surface of the flat object. When surface 1 1 is parallel to the surface of the flat object, the net torque applied by the surface of the flat object is reduced to close to zero, and chuck 10 no longer rotates. At this point, surface 1 1 is parallel to, the surface of the flat object. Chuck 10 may be provided with one or more sensors (not shown) that measure the torque applied to vertical axis 20 or to horizontal axis 22. [0046] In some embodiments of the present invention, fluid may be caused to flow outward through openings 12 as surface 1 1 approaches the front wafer. When surface 1 1 is close to the front wafer, perhaps to within a few millimeters, the pressure in the gap between surface and the front wafer begins to increase. The gradient in pressure described above causes surface 1 1 to align parallel to the surface of the front wafer. As surface 11 continues to approach the surface of the front wafer, a sharp increase in fluid pressure may occur that resists continued motion of surface 11 toward the front wafer. Mechanical arm 21 (Fig. 1), chuck drive assembly 23 (Fig. 1), vacuum generating device 58 (Fig. 1), or any other suitable part of the system may be provided with a sensor that senses the increase in pressure or the increased resistance to motion. At this point, the flow of fluid through openings 12 may be reversed, and suction applied to openings 12.

[0047] Alternatively, chuck 10 may be provided with a sensor to detect the level of fluid pressure, or vacuum, within internal manifold 16. As long as surface 1 1 is not in maximum contact with the surface of the front wafer, suction is not applied maximally to the object by means of openings 12. Rather, the suction sucks fluid at least partially into at least part of openings 12. After the torque applied by contact with the front wafer brings surface 1 1 into maximum contact with the object surface, openings 12 are maximally blocked by the object surface. At this point, a vacuum is formed within internal manifold 16 and suction causes chuck 10 to grip the flat object. A pressure sensor may detect the formation of the vacuum.

[0048] In order to minimize any torque applied to chuck 10 other than the flat object to be gripped, inlet hose 14 may connect to internal manifold 16 via a hollow passageway 18 that is open to manifold 16 and is incorporated into a rotation axis such as horizontal axis 22.

[0049] Movement of chuck 10 toward a flat object to be gripped applies torque to chuck 10 and causes it to rotate until surface 11 completely contacts the surface of the flat object. At this point, suction applied via openings 12 causes chuck 10 to grip the flat object, such as a wafer. Once a state is attained where chuck 10 grips the flat object, further motion of chuck 10 toward the flat object is no longer desirable. Furthermore, in this state, rotation of chuck 10 about vertical axis 20 and horizontal axis 22 is no longer desirable. The attainment of this state may be determined, for example, by means of a sensor that detects the reduction in torque applied to chuck 10, by means of a sensor that detects the increase in vacuum level that results the object blocking openings 12, by means of a mechanical, acoustic, electromagnetic, optical, or other sensor that detects full contact between surface 1 1 and the flat object, by means of a sensor that measures the force holding the object to surface 11, by means of a sensor that measures flow through openings 12, internal manifold 16, inlet hose 14, or through another portion of the suction-creating means, or by any other means capable of determining that chuck 10 grips an object. [0050] When it has been determined that chuck 10 grips an object such as a wafer, the orientation of chuck 10 is locked or fixed. Fig. 5 is an enlarged view of a portion of the chuck shown in Fig. 4. According to embodiments of the present invention, an actuating force, such as compressed air forced into chamber 30, exerts pressure on pistons 26, pushing them outward. The outward force on pistons 26 pushes braking elements 24 radially against vertical axis 20 and horizontal axis 22, preventing the rotation of the axes. Alternatively, braking elements 24 may be actuated by mechanical, electromagnetic, or hydraulic means, or by any other means suitable for causing a brake to press against an axis. Alternatively, braking elements 24 may be replaced with clamps or any other mechanical, electromagnetic, or other means known in the art for immobilizing chuck 10 without applying a torque that may change its orientation.

[0051] The orientation of chuck 10 at the time that it is immobilized is measured by one or more sensors. For example, the sensors may include an encoder 17 (Fig. 2) that senses rotation of horizontal axis 22. Alternatively, a rotation transmission mechanism may be connected to the axis, the transmission mechanism transmitting the rotation motion to an encoder at a different location. Alternatively, orientation may be determined by a level sensor, by any other type of sensor known in the art that employs mechanical, optical, electromagnetic, or other means to determine orientation.

[0052] The orientation data is input to a processor associated with chuck drive 23 (Fig.

1). The processor calculates the direction in which the chuck drive must move chuck 10 such that the direction of chuck motion remains parallel to surface 11 (Fig. 3). Motion parallel to surface 1 1 moves a gripped flat object gripped by chuck 10 in a direction tangential to the flat surface of the gripped object, minimizing bending stress on the object. Until the gripped object is free from any forces that may impede the motion of the object (such as a stack of wafers that exerts friction, adhesive, or viscous forces on the gripped object), motion may limited to a direction along, or parallel to, the vertical axis of the chuck mount (axis 20 in Fig. 4). Limiting motion to a direction parallel to the vertical axis may obviate the need for an orientation sensor about the vertical axis.

[0053] Fig. 6 is a schematic illustration of a chuck removing a wafer from a stack of wafers, in accordance with embodiments of the present invention. Fig. 7 is a flowchart of a method for separation of a wafer from a stack of wafers, in accordance with embodiments of the present invention. It should be understood that the organization into discrete steps, as well as the order of the steps, indicated in the flowchart is for illustrative purposes only. The organization into discrete steps, as well as the order of execution of some of the steps, may be altered without affecting the results of the method. All such variations should be considered to be within the scope of the present invention. Chuck 10 is moved toward the front end of wafer stack 42 (step 60). During this motion, chuck 10 is free to rotate in response to a torque exerted on chuck 10 by wafer stack 42. Motion continues until it is determined that chuck 10 is parallel to the front surface of wafer stack 42, or that chuck 10 is at a desired distance from the front surface (step 62). At this point, motion of chuck 10 is stopped (step 64). Chuck 10 grips front wafer 40 of wafer stack 42 (step 66). In some embodiments of the present invention, chuck 10 may be gripping front wafer 40 prior to stopping motion of chuck 10. The orientation of chuck 10 is fixed so that rotation of chuck 10 is no longer enabled (step 68). The orientation of chuck 10 is sensed (step 70).

[0054] Stopper bar 46 is positioned above wafer stack 42 (step 72). The stopper bar 46 is maintained at the desired distance from chuck 10 with the aid of spacers 32. Chuck 10 moves gripped wafer 40 in the direction indicated by the arrow, parallel to the sensed orientation of chuck 10 and the surface of gripped wafer 40 (step 74). Motion of gripped wafer 40 in the direction of the arrow slides gripped wafer 40 across the surface of adjacent wafer 41 of wafer stack 42. Friction, viscous, or other forces may resist relative motion between gripped wafer 40 and adjacent wafer 41. The resisting force may tend to drag adjacent wafer 41, and perhaps other wafers of wafer stack 42, in the direction of motion of gripped wafer 40. Restraining stopper bar 46 prevents this dragging. Spacers 32 ensure that restraining stopper bar 46 does not impede the motion of gripped wafer 40.

[0055] Means may be utilized to reduce any friction, viscous, or other forces that resist relative motion between gripped wafer 40 and adjacent wafer 41. For example, high pressure jet 48 of low viscosity fluid, such as water, may be directed at wafer stack 42 in order to assist in reducing the viscosity of the fluid in the gaps between adjacent wafers. Beam 50 of ultrasonic waves may be directed at wafer stack 42 in order to reduce the adhesion force between gripped wafer 40 and adjacent wafer 41. [0056] Chuck 10 continues to move gripped wafer 40 in the direction of the arrow until gripped wafer 40 is free of wafer stack 42. At this point, the chuck drive may move chuck 10 to the location, either inside or outside of bath 44, to where further processing is to be performed on wafer 40. When gripped wafer 40 is brought to the desired location, chuck 10 releases wafer 40. For example, the vacuum generating device that applies the suction that holds wafer 40 to chuck 10 may be turned off.

[0057] The orientation of chuck 10 may remain fixed until it is again moved near to the next wafer of wafer stack 42 to be gripped and moved. This ensures that the initial orientation of chuck 10 is approximately parallel to the surface of the next wafer of the stack. Alternatively, means may be provided to actively orient chuck 10 in the last sensed orientation prior to moving chuck 10 near the next wafer to be gripped.

[0058] An alternative to spacers 32 may be provided for determining the position of restraining stopper bar 46 relative to wafer stack 42. For example, a mechanism may be provided for precisely positioning the wafer stack, the restraining stopper, or both using information relating to sensed positions of various reference objects and surfaces. A mechanism based on sensed positions may be advantageous where the thickness of wafers in a stack, or the distances between wafers in the stack, is variable.

[0059] Fig. 8 schematically shows a wafer restraining device in accordance with some other embodiments of the present invention. Position sensor 1 1 may sense position data regarding one or more object surfaces, such as one or more surfaces of a wafer of wafer stack 42, or of other components of a wafer separation system. The sensed position data regarding an object surface may include a sensed location, orientation, and dimensions of the surface. Sensed position data with regard to one or more object surfaces may be interpreted or analyzed to yield a location, orientation, and dimensions, for example the thickness, of an object. In particular, position sensor 11 may sense position data regarding surfaces of a front wafer 40 that is to be gripped and separated from wafer stack 42, of wafer 41 that is adjacent to front wafer 40 in wafer stack 42, and of a restraining stopper bar 46. Position sensor 11 may represent a plurality of sensors, each sensor adapted to sensing position data with regard a particular object surface or set of object surfaces. Sensor 1 1 may include, for example, an imaging sensor, such as a camera sensitive to a suitable spectral range. A camera may be configured to form an image of one or more objects. Image data may be analyzed using various image processing or analysis techniques, such as are known in the art, in order to extract the position data regarding one or more object surfaces or objects. Sensor 1 1 may include mechanical sensing of one or more object positions, or a proximity or rangefinder sensor, such as, for example, a sensor based on mechanical, electromagnetic, or inductive forces, or on the reflection of on optical (e.g. laser) or ultrasound beam.

[0060] Sensor data may be input to controller 54. Controller 54 may include one or more processor units that are incorporated into, or communicate with, controller 54. Controller 54 may further process the sensed data in order to determine the locations of one or more surfaces or objects. For example, controller 54 may process data from an imaging sensor, for example, by applying image processing or analysis techniques, in order to determine the locations of one or more surfaces or objects. In particular, the locations of the surfaces of wafer 41 may be determined at least partly on the basis of the sensed data. Based on the sensed data, controller 54 may position edge 46a of restraining stopper bar 46 on or near an edge of wafer 41. Positioning of edge 46a at wafer 41 , for example, on or near an edge of wafer 41, may enable removal of front wafer 40 while preventing undesirable or unintentional movement of adjacent wafer 41.

[0061] Controller 54 may position edge 46a at wafer 41 by causing motion of restraining stopper bar 46, carrier 52, or both. Devices may be provided for causing motion of restraining stopper bar 46 or carrier 52. Such devices may include, for example, an electric motor (e.g. a stepper, servo, or direct current motor), or a pneumatic, hydraulic, or electromagnetic propelling device. For example, controller 54 may control carrier motion device 53 to move carrier 52. For example, carrier 52 may be moved such as to bring front wafer 40 into contact with a chuck or other grabbing device. Controller 54 may control stopper motion device 86 to move restraining stopper bar 46. For example, restraining stopper bar 46 may be mounted on the end of stopper rod 47. Motion of stopper rod 47 may be constrained by rod guide brackets 49. Stopper motion device 86 applies a linear force to joint 84, which in turn applies a linear force to stopper rod 47, moving restraining stopper bar 46 along an edge of wafer stack 42.

[0062] When restraining stopper bar 46 has been moved to a desired position, typically such that stopper edge 46a is positioned at an edge of wafer 41 that is adjacent to front wafer 40, motion of restraining stopper bar 46 may be stopped. For example, controller 54 may control braking device 80. When position sensor 88 detects that stopper edge 46a is at a desired position, controller 54 may cause braking device 80 to press brake 82 against stopper rod 47.

[0063] It should be clear that the description of the embodiments and attached Figures set forth in this specification serves only for a better understanding of the invention, without limiting its scope.

[0064] It should also be clear that a person skilled in the art, after reading the present specification could make adjustments or amendments to the attached Figures and above described embodiments that would still be covered by the present invention.