Improved debonding equipment and methods for debonding temporary bonded wafers

Cross Reference to related Co-Pending Applications

0001This application claims the benefit of <patcit id="pcit0001" dnum="US61324888A" dnum-type="L"><text>U.S. provisional application Serial No. 61/324,888 filed April 16, 2010</text></patcit> and entitled "IMPROVED DEBONDING EQUIPMENT AND METHODS FOR DEBONDING TEMPORARY BONDED WAFERS".

Field of the Invention

0002The present invention relates to improved debonding equipment and methods for debonding temporary bonded wafers, and more particularly to industrial-scale debonding equipment based on thermal slide or mechanical separation.

Background of the Invention

0003Several semiconductor wafer processes include wafer thinning steps. In some applications the wafers are thinned down to a thickness of less than 100 micrometers for the fabrication of integrated circuit (IC) devices. Thin wafers have the advantages of improved heat removal and better electrical operation of the fabricated IC devices. In one example, GaAs wafers are thinned down to 25 micrometers to fabricate power CMOS devices with improved heat removal. Wafer thinning also contributes to a reduction of the device capacitance and to an increase of its impedance, both of which result in an overall size reduction of the fabricated device. In other applications, wafer thinning is used for 3D-Integration bonding and for fabricating through wafer vias.

0004Wafer thinning is usually performed via back-grinding and/or chemical mechanical polishing (CMP). CMP involves bringing the wafer surface into contact with a hard and flat rotating horizontal platter in the presence of a liquid slurry. The slurry usually contains abrasive powders, such as diamond or silicon carbide, along with chemical etchants such as ammonia, fluoride, or combinations thereof. The abrasives cause substrate thinning, while the etchants polish the substrate surface at the submicron level. The wafer is maintained in contact with the abrasives until a certain amount of substrate has been removed in order to achieve a targeted thickness.

0005For wafer thicknesses of over 200 micrometers, the wafer is usually held in place with a fixture that utilizes a vacuum chuck or some other means of mechanical attachment. However, for wafer thicknesses of less than 200 micrometer and especially for wafers of less than 100 micrometers, it becomes increasingly difficult to mechanically hold the wafers and to maintain control of the planarity and integrity of the wafers during thinning. In these cases, it is actually common for wafers to develop microfractures and to break during CMP.

0006An alternative to mechanical holding of the wafers during thinning involves attaching a first surface of the device wafer (i.e., wafer processed into a device) onto a carrier wafer and thinning down the exposed opposite device wafer surface. The bond between the carrier wafer and the device wafer is temporary and is removed upon completion of the thinning and any other processing steps.

0007Several debonding methods and equipment have been suggested. However, in many cases the thinned wafers break during the debonding process. Accordingly, there is a need for a controlled debonding process in order to avoid breakage of the thinned wafers.

0008<patcit id="pcit0002" dnum="US2002106869A1"><text>US 2002/106869 A1</text></patcit> discloses a debonder apparatus for debonding a temporary bonded wafer pair comprising a first wafer bonded to a second wafer at a bonding interface. The debonder apparatus comprises an upper chuck and a lower chuck having an upper surface oriented parallel and opposite to the lower surface of the upper chuck; means for holding an unbonded surface of the first wafer of the temporary bonded wafer pair onto the lower surface of the upper chuck; means for initiating a separation front at a point of the bonding interface of the temporary bonded wafer pair; and means for holding the unbonded surface of the separated second wafer onto the upper surface of the lower chuck.

0009<patcit id="pcit0003" dnum="US20080302481A1"><text>US 2008/0302481 A1</text></patcit> shows a debonder apparatus for debonding a temporary bonded wafer pair comprising vaccum pickers for lifting the upper wafer off the lower wafer.

0010<patcit id="pcit0004" dnum="US20080308231A1"><text>US 2008/0308231 A1</text></patcit> discloses a single chamber debonder apparatus for debonding a temporary bonded wafer pair.

0011<patcit id="pcit0005" dnum="US5783022A"><text>US 5 783 022 A</text></patcit> shows a device for manually debonding a temporary bonded wafer pair, wherein the device comprises a stand with two surfaces that are angled with respect to each other at a fixed predetermined angle.

0012<patcit id="pcit0006" dnum="JPH0964152A"><text>JP H09 64152 A</text></patcit> discloses an apparatus for separating a wafer of a wafer laminate with suction cups and a central punch. Fluid nozzles directed at the wafer laminate aid the separation process.

Summary of the Invention

0013The present invention relates to improved debonding equipment and methods for debonding temporary bonded wafers, and more particularly to industrial-scale debonding equipment based mechanical separation.

0014In general, in an aspect, the invention features a debonder apparatus for debonding a temporary bonded wafer pair that includes a first wafer bonded to a second wafer at a bonding interface. The debonder apparatus includes a clam-shell type reactor, an upper chuck and a lower chuck. The reactor includes first and second isolated chambers. The upper chuck is contained within the first chamber and has a lower surface protruding into the second chamber and an edge configured to be held in fixed position via clamping means. The lower chuck is contained within the second chamber and has an upper surface oriented parallel and opposite to the lower surface of the upper chuck. The debonder apparatus also includes vacuum means for holding an unbonded surface of the first wafer of the temporary bonded wafer pair onto the lower surface of the upper chuck, and means for pressurizing the first chamber. The first chamber pressurizing means applies pressure onto an upper surface of the upper chuck while the upper chuck edge is held in fixed position via the clamping means, and thereby causes the lower surface of the upper chuck and the attached wafer pair to bow downward. The debonder apparatus also includes means for initiating a separation front at a point of the bonding interface of the temporary bonded wafer pair. The debonder apparatus also includes means for pressurizing the second chamber while the first chamber pressure is reduced thereby causing removal of the downward bowing of the upper chuck and the attached wafer pair and propagation of the separation front through the entire bond interface. The debonder apparatus also includes vacuum means for attaching the unbonded surface of the separated second wafer onto the upper surface of the lower chuck.

0015Implementations of the invention may include one or more of the following features. The separation front is initiated at an edge of the wafer pair. The means for initiating separation comprises an air knife, or means for pushing sidewise the first or the second wafer. The temporary bonded wafer is attached to a frame and the means for initiating separation comprises means for pushing up the frame. The upper chuck comprises a circular edge. The clamping means comprise mechanical clamps or vacuum clamps. The means for pressurizing the first and second chambers comprises means for evacuating the first and second chambers and means for filling the first and second chamber with a pressurized gas. The pressurized gas may be nitrogen. The first wafer is stacked upon and is temporarily bonded to the second wafer via an adhesive layer and a release layer. The upper and lower chucks comprise porous ceramic material through which vacuum is drawn for attaching the unbonded surfaces of the first and second wafers, respectively.

0016In general, in another aspect, the invention features a method for debonding a temporary bonded wafer pair including the following steps. First, providing a debonder apparatus wherein the apparatus comprises a clam-shell type reactor comprising first and second isolated chambers, an upper chuck contained within the first chamber and comprising a lower surface protruding into the second chamber and an edge configured to be held in fixed position via clamping means, and a lower chuck contained within the second chamber and having an upper surface oriented parallel and opposite to the lower surface of the upper chuck. Next, attaching an unbonded surface of the first wafer of the temporary bonded wafer pair onto the lower surface of the upper chuck via vacuum means. Next, pressurizing the first chamber and thereby applying pressure onto an upper surface of the upper chuck while the upper chuck edge is held in fixed position via the clamping means, and thereby causing the lower surface of the upper chuck and the attached wafer pair to bow downward. Next, initiating a separation front at a point of the bonding interface of the temporary bonded wafer pair, and then pressurizing the second chamber while the first chamber pressure is reduced thereby causing removal of the downward bowing of the upper chuck and the attached wafer pair and propagation of the separation front through the entire bond interface. Finally, attaching the unbonded surface of the separated second wafer onto the upper surface of the lower chuck via vacuum means.

Brief Description of the Drawings

0017Referring to the figures, wherein like numerals represent like parts throughout the several views: <ul id="ul0001" list-style="none"><li><figref idref="f0001">FIG. 1</figref> is an overview schematic diagram of the improved temporary wafer bonder system according to this invention;</li><li><figref idref="f0002">FIG. 1A</figref> is a schematic diagram of temporary wafer bonding process A and debonding process A performed in bonder module A and debonder A of <figref idref="f0001">FIG. 1</figref>, respectively;</li><li><figref idref="f0003">FIG. 1B</figref> depicts a schematic cross-sectional view of the bonder module A of <figref idref="f0001">FIG. 1</figref> and a list of the process steps for performing the temporary wafer bonding process A of <figref idref="f0002">FIG. 1A</figref>;</li><li><figref idref="f0004">FIG. 2</figref> depicts a first embodiment of a new debonder apparatus;</li><li><figref idref="f0005 f0006 f0007">FIG. 3A - 3E</figref> depict the operational steps of the new debonder apparatus of <figref idref="f0004">FIG. 2</figref>.</li></ul>

Detailed Description of the Invention

0018Referring to <figref idref="f0001">FIG. 1</figref>, an improved apparatus for temporary wafer bonding 100 includes a temporary bonder cluster 110 and a debonder cluster 120. The temporary bonder cluster 110 includes temporary bonder module A, module B, module C, and module D, 210, 310, 410 and 510 respectively. Debonder cluster 120 includes a thermal slide debonder A 150, a mechanical debonder B 250 and a radiation /mechanical debonder C 350. Bonder cluster 110 facilitates the temporary bonding processes A, 60a shown in <figref idref="f0002">FIG. 1A</figref> among others. Debonder cluster 120 facilitates the debonding process A, 60b shown in <figref idref="f0002">FIG.1A</figref>.

0019Referring to <figref idref="f0002">FIG. 1A</figref>, temporary bond process A 60a includes the following steps. First, device wafer 20 is coated with a protective coating 21 (62), the coating is then baked and chilled (63) and then the wafer is flipped (64). A carrier wafer 30 is coated with an adhesive layer 31 (65) and then the coating is baked and chilled (66). In other embodiments, a dry adhesive film is laminated onto the carrier wafer, instead of coating an adhesive layer. Next, the flipped device wafer 20 is aligned with the carrier wafer 30 so that the surface of the device wafer with the protective coating 20a is opposite to the surface of the carrier wafer with the adhesive layer 30a (67) and then the two wafers are bonded (68) in temporary bonder module A, shown in <figref idref="f0003">FIG. 1B</figref>. The bond is a temporary bond between the protective layer 21 and the adhesive layer 31. In other embodiments, no protective coating is applied onto the device wafer surface and the device wafer surface 20a is directly bonded with the adhesive layer 31. Examples of device wafers include GaAs wafers, silicon wafers, or any other semiconductor wafer that needs to be thinned down to less than 100 micrometers. These thin wafers are used in military and telecommunication applications for the fabrication of power amplifiers or other power devices where good heat removal and small power factor are desirable. The carrier wafer is usually made of a non-contaminating material that is thermally matched with the device wafer, i.e., has the same coefficient of thermal expansion (CTE). Examples of carrier wafer materials include silicon, glass, sapphire, quartz or other semiconductor materials. The diameter of the carrier wafer is usually the same as or slightly larger than the diameter of the device wafer, in order to support the device wafer edge and prevent cracking or chipping of the device wafer edge. In one example, the carrier wafer thickness is about 1000 micrometers and the total thickness variation (TTV) is 2-3 micrometers. Carrier wafers are recycled and reused after they are debonded from the device wafer. In one example, adhesive layer 31 is an organic adhesive WaferBOND™ HT-10.10, manufactured by Brewer Science, Missouri, USA. Adhesive 31 is applied via a spin-on process and has a thickness in the range of 9 to 25' micrometers. The spin speed is in the rage of 1000 to 2500 rpm and the spin time is between 3-60 second. After the spin-on application, the adhesive layer is baked for 2 min at a temperature between 100 °C to 150 °C and then cured for 1-3 minutes at a temperature between 160 °C to 220 °C. WaferBOND™ HT-10.10 layer is optically transparent and is stable up to 220 °C. Appendix A depicts the specification for WaferBOND™ HT-10.10. After the thinning of the exposed device wafer surface 20b the carrier wafer 30 is debonded via the debond process A 60b, shown in <figref idref="f0002">FIG. 1A</figref>. Debond process A 60b, includes the following steps. First heating the wafer stack 10 until the adhesive layer 31 softens and the carrier wafer 30 slides off from the thinned wafer (69). The WaferBOND™ HT-10.10 debonding time is less than 5 minutes. The thinned wafer 20 is then cleaned, any adhesive residue is stripped away (52) and the thinned wafer is placed in a dicing frame 25 (53) In some embodiments, a small rotational motion (twisting) of the carrier wafer takes place prior to the sliding translational motion.

0020The temporary bonding (68) of the carrier wafer 30 to the device wafer 20 takes place in temporary bonder module A, 210. Referring to <figref idref="f0003">FIG. 1B</figref>, the device wafer 20 is placed in the fixture chuck 202 and the fixture chuck is loaded in the chamber 210. The carrier wafer 30 is placed with the adhesive layer facing up directly on the bottom chuck 210a and the two wafers 20, 30 are stacked and aligned. The top chuck 210b is lowered down onto the stacked wafers and a low force is applied. The chamber is evacuated and the temperature is raised to 200 °C for the formation of the bond between the protective coating layer 21 and the adhesive layer 31. Next, the chamber is cooled and the fixture is unloaded.

0021The debond process A 60b is a thermal slide debond process and includes the following steps, shown in <figref idref="f0002">FIG. 1A</figref>. The bonded wafer stack 10 is heated causing the adhesive layer 31 to become soft. The carrier wafer is then twisted around axis 169 and then slid off the wafer stack under controlled applied force and velocity (69). The separated device wafer 20 is then cleaned (52) and mounted onto a dicing frame 25 (53).

New Debonder Apparatus and Method

0022Referring to <figref idref="f0004">FIG. 2</figref>, a new debonder apparatus 700 includes an upper chuck 730 and a lower chuck 720 contained within a clam-cell type reactor having a lower portion 702 and top cover portion 704. Two separate chambers 706 and 708 are formed within the reactor 700 with lower chamber 706 containing the lower chuck 720 and upper chamber 708 containing the upper chuck 730. The lower surface 730a of the upper chuck protrudes into lower chamber 706. Upper and lower chucks 730, 720 are porous ceramics and vacuum is drawn through them making them capable to apply a force along directions 712, 714, respectively, shown in <figref idref="f0006">FIG. 3C</figref>. Forces 712, 714 are applied throughout the lower surface 730a and upper surface 720a of upper and lower chucks 730 and 720, respectively. Forces 712, 714 are used for holding substrates by the upper or lower chucks 730 and 720, respectively. Upper and lower chambers 708, 706 are configured to be filled with a pressurized gas. The pressurized gas applies pressure 711 on the upper surface 730b of the upper chuck 730 or pressure 716 the top surface 720a of the lower chuck 720. In one example, pressure 711 and 716 is about 2 bars and the pressurized gas is nitrogen. The outer edges of the upper chuck 730 are held in a fixed position by clamps 725 while pressure 711 is applied. Clamps 725 may be mechanical clamps or vacuum clamps.

0023In operation, the bonded wafer pair 10 supported on the tape frame 25 is loaded in chamber 708 and is held via vacuum force 712 in contact with the lower surface 730a of the upper chuck 730, shown in <figref idref="f0005">FIG. 3A</figref>. Vacuum force 712 is applied in the entire unbonded surface of wafer 30. Next, chamber 708 is filled with gas and pressure 711 is applied on the back surface 730b of chuck 730 by the pressurized gas while the edges of the chuck 730 are held fixed by clamps 725. This causes the lower surface 730a of chuck 730 and the attached bonded wafer pair 10 to bow downward until it contacts the upper surface 720a of the lower chuck 720 in point 721, shown in <figref idref="f0005">FIG. 3B</figref>. At this point, separation of the bonded wafer pair 10 is initiated around point 10a. The separation is initiated via mechanical means, i.e., pushing up of frame 25 or pushing of the carrier wafer 30 sidewise. In other embodiments, the separation is initiated via an air knife. Next, vacuum is drawn through the lower chuck 720 applying a vacuum force 714 on the lower surface of the wafer pair 10, shown in <figref idref="f0006">FIG. 3C</figref>. Next, the pressure 711 in chamber 708 is reduced causing gradual removal of the downward bowing of the lower chuck surface 730a and the attached wafer pair 10. Simultaneously, the pressure 716 in the lower chamber 706 is increased. The combination of these steps causes the propagation of the separation front that was initiated in point 10a, throughout the entire bond interface, as shown in <figref idref="f0006">FIG. 3D</figref>. Finally, pressure 711 is turned off, vacuum force 712 is turned off and then pressure 716 is turned off leaving the separated carrier wafer 30 and thinned device wafer 20 supported upon tape frame 25 on the lower chuck 720, where they are held by vacuum force 714, as shown in