WAFERS

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

BACKGROUND OF THE INVENTION

[0002] In preparing silicon 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 length dimension of several centimeters, and thickness ranging from 100 μm to 300 μm. Such a silicon wafer is fragile. Therefore, in order to avoid breaking the wafer, any force applied to remove the wafer from the stack should be kept to a minimum.

[0003] Adhesion forces between wafers in the stack prevent easy separation of an individual wafer from the stack. The adhesion force between the wafers may be increased by presence of slurry 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, may increase the adhesive force between the wafers.

[0004] Due to the fragility of a wafer and the bonding forces between wafers, 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.

[0005] Another factor that renders automation difficult is the non-uniformity of the adhesion force between different pairs of adjacent wafers. For example, the force holding the first wafer of a stack to the second wafer may be greater than the force holding the second to the third. In this case, an attempt to remove the first wafer from the stack will likely remove the first and second wafers together from the stack.

[0006] Often, mechanical means are used to hold the other wafers in the stack when one is being removed. However, relying on mechanical means alone risks applying an excessive or misdirected force on a wafer that may cause it to break.

[0007] Other techniques attempt to weaken the force that holds the first wafer that is to be removed to the second wafer. In general, the force holding the wafers together decreases as the clearance of the space between the wafers increases. However, as stated above, mechanical means alone that are applied to increase the separation between wafers may cause a wafer to break.

[0008] Other described techniques weaken the adhesion force between the wafers by decreasing the viscosity of the fluid between the wafers. Gibbel in US6558109 describes a powerful jet of water that is directed between the first and second wafers. This technique replaces the high-viscosity slurry with low-viscosity water, as well as increasing the clearance between the wafers. However, it is difficult to aim a jet precisely at the space between the first and second wafers. If the aim is not precise, the forces holding other pairs of wafers together, such as the second wafer to the third, are also weakened. The result is, then, that two or more wafers are loosened, so that automatic removal of a single wafer remains difficult. Another difficulty with this technique is that given the high aspect ratio (the ratio of length to clearance) of the space between the wafers into which the water jet is to be directed (typical order of magnitude around 1000), the jets are likely to affect only a small portion of space. Applying a force to slightly pull on the first wafer to enlarge the clearance between the first two wafers prior to applying the water jet, as suggested by Tsuchiya et al. in US7364616, again risks breakage of the wafer.

[0009] Another suggested method described by Matsuda in US7284548 is to insert a mechanical spacer during the wire saw cutting process. Such a spacer mechanically enlarges the clearance between the wafers. However, since a spacer is inserted between every pair of wafers, the force holding the first wafer to the second is not less than the force holding the second to the third. [0010] Kimura et al. describe in US5976954 immersing a wafer stack in a warm bath in order to soften the adhesive holding the wafers together. Again, the warm bath weakens the adhesive force between all pairs of wafers equally, so that mechanical means are necessary for holding the remaining wafers of the stack while one is being moved. [0011] It is an object of the present invention to provide a means of substantially reducing the force that resists the motion of a wafer at the end of a stack, relative to the forces resisting the motion of other wafers in the stack.

[0012] 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

[0013] 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 with internal gaps between the wafers. The apparatus comprises a chuck for gripping the front wafer, coupled to a mechanical arm and a drive for moving the arm to bring the chuck to the front wafer and to separate the wafer from the stack of wafers. The apparatus further comprises a heater for heating the front wafer so as to raise the temperature within the internal gap between the front wafer and the adjacent wafer of the stack of wafers above the temperature of more distant internal gaps within the stack. [0014] Furthermore, in accordance with some embodiments of the present invention, the heater is incorporated into the chuck.

[0015] Furthermore, in accordance with some embodiments of the present invention, the heater comprises a hot jet generator for generating hot jets.

[0016] Furthermore, in accordance with some embodiments of the present invention, the hot jet generator comprises a hot jet generator for generating hot jets of water.

[0017] Furthermore, in accordance with some embodiments of the present invention, the hot jet generator comprises a hot jet generator for generating hot jets of steam.

[0018] 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 with internal gaps between the wafers. The method comprises providing a chuck for gripping the front wafer, coupled to a mechanical arm and a drive for moving the arm to bring the chuck to the front wafer and to separate the wafer from the stack of wafers. The method further comprises: providing a heater and heating the front wafer so as to raise the temperature within the internal gap between the front wafer and the adjacent wafer of the stack of wafers above the temperature of more distant internal gaps within the stack.

[0019] Furthermore, in accordance with some embodiments of the present invention, the step of providing a heater comprises providing a heater which is incorporated into the chuck. [0020] Furthermore, in accordance with some embodiments of the present invention, the step of heating comprises generating hot jets.

[0021] Furthermore, in accordance with some embodiments of the present invention, the hot jets comprise hot jets of water.

[0022] Furthermore, in accordance with some embodiments of the present invention, the hot jets comprise hot jets of steam.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] 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.

[0024] Fig. 1 is a schematic illustration of a system for facilitating wafer separation, in accordance with embodiments of the present invention. [0025] Fig. 2 shows the exterior of a chuck adapted to producing a stream of warm fluid, in accordance with embodiments of the present invention.

[0026] Fig. 3 is a cutaway view showing the interior of the chuck shown in Fig. 2, viewed from the opposite side. DETAILED DESCRIPTION OF EMBODIMENTS

[0027] Embodiments of the present invention provide for facilitating the removal of a single wafer from the end of a stack of wafers by means of establishing a temperature gradient in the stack. 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. [0028] Generally, adhesive forces are present between the wafers of the stack that resist the removal of a single wafer from the stack. Often, the adhesive forces are temperature dependent. Introducing a suitable temperature gradient into the stack then may weaken the adhesive force between a wafer at one end of the stack and the adjacent wafer, relative the adhesive forces between other pairs of wafers in the stack. Weakening the adhesive force between the pair of wafers at the end of the stack may facilitate removal of the wafer.

[0029] For example, in the case of a stack of semiconductor wafers sliced from a single ingot, the presence of viscous fluid between wafers of the stack increases the adhesive forces between pairs of wafers. In general, the viscosity of a fluid decreases as its temperature increase. Heat is applied to one end of the stack of wafers in order to create a temperature gradient within the stack. Such a temperature gradient may then reduce the viscosity of the fluid between the pair of wafers at the warmest end of the stack, relative to the viscosity of the fluid between other pairs of wafers in the stack. Such a relative reduction in viscosity may facilitate removing a wafer from the heated end of the stack without disturbing the positions of the other wafers in the stack.

[0030] After an ingot of semiconductor material, such as silicon, is sliced into wafers, the clearance of a internal gap between wafers in the stack will typically be of the same order of magnitude as the thickness of the wafer, typically on the order of 0.1 mm. The internal gap may be filled with slurry or another fluid or material. A typical slurry includes a liquid such as polyethylene glycol 300 (PEG 300) mixed with particles of abrasive material. The viscosity of a typical slurry decreases as its temperature increases. For example, the viscosity of PEG 300 decreases by a factor of 10 when heated from 25°C to 100°C. Typically, the stack of wafers will be immersed in a bath of fluid. The fluid in the bath may be cooled by any active or passive cooling means known in the art in order to maintain the fluid at a substantially constant average temperature.

[0031] In general, applying heat-producing energy to one end of the stack for a limited period of time will heat that end of the stack. The end of the stack to which heat- producing is applied may be referred to as the front end of the stack. The remainder of the stack may be heated by means of transfer of heat at a finite rate to the remainder of the stack from its heated end. Therefore, a temperature gradient may be established within the stack.

[0032] Heat may be applied to the front end of the stack by means of a directed stream of heated fluid such as hot water or steam, directed radiation or waves, contact with a heating element, or by any other means know in the art for heating an end of a wafer stack. Depending on the type of heating means, the heating means may be incorporated in either a moveable or a stationary element. In the event that the stack is immersed in a fluid bath, depending on the type of heating means, the heating may be located either within or outside the bath. For example, the heating means may be incorporated within an element, such as a movable chuck, that is capable of removing the front wafer from the front end of the stack.

[0033] Upon establishment of the temperature gradient, the front end of the stack to which heat is applied is warmest, with the temperature of the remainder of the stack decreasing as a function of distance from the end to which heat is applied. Similarly, the temperature of fluid or other material in the internal gap between the front wafer and the wafer adjacent to the front wafer is warmer than the fluid in internal gaps between other pairs of adjacent wafers in the stack. Therefore, viscosity of the fluid in the internal gaps between wafers is lowest in the internal gap between the pair of wafers that is closest to the end of the stack to which heat is applied. The viscosity of the fluid increases as a function of distance from that end. Decreasing the viscosity of the fluid in a internal gap decreases the force that resists relative sliding motion between the wafers that adjoin the internal gap. [0034] Once the temperature gradient is established, the wafer at the warmest end of the stack may be removed. Due to the reduced adhesiveness between wafers at the heated end of the stack, the force required to remove the wafer at the heated end of the stack is reduced. Therefore, suitable manual or automated means may be applied to the wafer to separate it from the remainder of the stack. For example, a mechanical chuck may grip the wafer through suction, mechanical, electrostatic, magnetic, or any other suitable means, and slide it laterally. Since the temperature gradient causes the adhesiveness in the remainder of the stack to be greater than the adhesiveness at the heated end of the stack, it may be possible to move the wafer at the end of the stack without moving the other wafers in the stack. Additional mechanical or other restraining means may be applied to the remainder of the stack to assist in holding stationary the wafers in the remainder of the stack.

[0035] We now describe embodiments of the present invention with reference to the accompanying Figures. Fig. 1 is a schematic illustration of a system for facilitating wafer separation, in accordance with embodiments of the present invention. A wafer stack 18 has been created by slicing an ingot of silicon into individual wafers, such as wafers 20a, 20b, and 20c. Viscous fluid may be present in internal gaps between adjacent individual wafers of wafer stack 18. Front wafer 20a at the front end of wafer stack 18 is to be separated from the remainder of wafer stack 18. Wafer stack 18 may be immersed in a bath 22 of a liquid fluid. Alternatively wafer stack 18 may be surrounded by a gas. Wafer stack 18 may be held by a carrier 23. Carrier 23 may be moveable, capable of transporting wafer stack 18 within a wafer processing facility. Typically, the fluid in the bath may be of such a composition so that its viscosity is less than the viscosity of fluid that may be present between the wafers in wafer stack 18. 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.

[0036] According to embodiments of the present invention, a moveable chuck 10 is capable of gripping and moving a wafer of wafer stack 18, and is mounted on arm 26. Arm 26 may be moved by drive 32 as controlled by control system 34. Chuck 10 is inserted into bath 22 and moved toward wafer stack 18. Chuck 10 includes inlet hose 14. Pump 28 pumps heated fluid, such as steam or hot water, from hot fluid source 30. The heated fluid is pumped through hose 27. Part of hose 27 may pass through, or be attached to, arm 26. Heated fluid from hose 27 is pumped into inlet hose 14. An internal manifold 16 (shown in Fig. 3) connects inlet hose 14 to openings 12 (Fig. 2). Chuck 10 expels heated fluid jet 24 in the direction of at wafer stack 18. In particular, chuck 10 directs heated fluid jet 24 at front wafer 20a at the front end of wafer stack 18.

[0037] Directing heated fluid jet 24 on front wafer 20a for a finite period of time raises the temperature of front wafer 20a and of fluid in the internal gap between front wafer 20a and wafer 20b. Heat is also transferred at a finite rate from front wafer 20a to the remainder of wafer stack 18. In general, the rate of temperature increase in the remainder of wafer stack 18 is slower than the rate of temperature increase of front wafer 20a. Therefore, if heated fluid jet 24 is directed onto front wafer 20a for a finite period of time, a temperature gradient forms within wafer stack 18. Temperature within wafer stack 18 decreases as a function of distance from front wafer 20a. Therefore, fluid in the internal gap between front wafer 20a and wafer 20b is warmer than the fluid in the internal gap between wafer 20b and wafer 20c. In the event that the viscosity of the fluid decreases as a function of increasing temperature, the viscosity of the fluid between front wafer 20a and wafer 20b is less than the viscosity of the fluid between wafer 20b and wafer 20c. [0038] For example, it has been found that directing heated fluid jet 24 on front wafer 20a for a time interval on the order of one second resulted in a temperature gradient where the temperature of the fluid in the internal gap between front wafer 20a and wafer 20b was approximately 50 ⁰ C warmer than the fluid in the internal gap between wafer 20b and wafer 20c. When the fluid in the internal gap is PEG 300, this temperature difference implies that the viscosity of the fluid between front wafer 20a and wafer 20b is lower than the viscosity of the fluid in between wafer 20b and wafer 20c by a factor of about 5.

[0039] In order not to delay the operation of an automated wafer separation process, the time interval required to produce a suitable temperature gradient should preferably not be significantly longer than the length of a process cycle, or tact time, required for the step of separating a wafer from the stack and moving it to a new location. A typical tact time may be on the order of one second. Therefore, applying heat for a period on the order of a second would be suitable for such a process cycle.

[0040] When the viscosity of the fluid between front wafer 20a and wafer 20b has been reduced relative to the viscosity of the fluid between wafer 20b and wafer 20c, chuck 10 may grip front wafer 20a and slide front wafer 20a free of wafer 20b and wafer stack 18. Due to the relatively greater viscosity of the fluid between wafer 20b and wafer 20c, wafer 20b remains stationary together with the remainder of wafer stack 18. During the sliding of front wafer 20a, other means may be used assist in facilitating the movement of front wafer 20a and in ensuring that the remainder of the stack remains stationary. Such means may include mechanical devices that hold or block the motion of the remainder of the stack, directed fluid jets, ultrasound, or any other suitable means known in the art.

[0041] Fig. 2 shows the exterior of a chuck adapted to producing a stream of warm fluid, in accordance with embodiments of the present invention. Fig. 3 is a cutaway view showing the interior of the chuck shown in Fig. 1 , viewed from the opposite side. Openings 12 on the front surface of chuck 10 (the surface adapted to contacting a wafer) connect to an internal manifold 16. Internal manifold 16 connects to inlet hose 14. Inlet hose 14 connects to a source of flowing heated fluid, such as heated water. Alternatively, chuck 10 may be provided with one or more heating elements to heat fluid in internal manifold 16. When flowing heated fluid is provided to inlet hose 14, a stream of heated fluid flows out of openings 12. Alternatively, chuck 10 may be provided with one or more heating elements to heat chuck 10. When a surface of chuck 10 is brought into contact with a front end of a stack of wafers, heat is conducted from chuck 10 to the front end of the stack. [0042] In the event that chuck 10 is adapted to gripping a wafer by means of suction, openings are provided on the front surface of chuck 10 through which the suction is applied. Chuck 10 may be provided with a separate system of holes, manifold, and hose for providing the suction. Alternatively, the suction system may also be provided via openings 12, manifold 16, and inlet hose 14. In this case, a valve or other mechanism (not shown) alternately connects inlet hose 14 to a vacuum generating device or to a source of flowing fluid. Inlet hose 14 is connected to a source of flowing fluid when chuck 10 is required to direct heated fluid at a wafer stack in order to produce a temperature gradient within the stack. Inlet hose 14 is connected to a vacuum generating device when chuck 10 is required to grip a wafer using suction.

[0043] Thus, embodiments of the present invention facilitate the removal of a wafer from a stack of wafers by means of heating the wafer to establish a temperature gradient in a stack of wafers.

[0044] 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. [0045] 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.