Wafer debonding using long-wavelength infrared radiation ablation

Methods are provided for handling a device wafer. For example, a method includes providing a stack structure having a device wafer, a handler wafer, and a bonding structure disposed between the device wafer and handler wafer, and irradiating the bonding structure with long-wavelength infrared energy to ablate the bonding structure.

TECHNICAL FIELD

The field generally relates to wafer handling techniques and, in particular, to structures and methods for temporarily bonding handler wafers to device wafers using bonding structures that include one or more releasable layers that absorb infrared radiation to achieve wafer debonding by infrared radiation ablation.

BACKGROUND

In the field of semiconductor wafer processing, increasing demands for large-scale integration, high density silicon packages has resulted in making semiconductor dies very thin. For example, for some applications, silicon (Si) wafers are backside grinded and polished down to a thickness of 50 μm or thinner. Although single crystal Si has very high mechanical strength, Si wafers and/or chips can become fragile as they are thinned. Defects can also be introduced by processing steps such as through-silicon via (TSV) processing, polishing, and dicing, which further reduces the mechanical strength of a thinned wafer or chip. Therefore, handling thinned Si wafers presents a significant challenge to most automation equipment.

In order to facilitate the processing of a device wafer, a mechanical handler wafer (or carrier wafer) is usually attached to the device wafer to enhance the mechanical integrity of the device wafer during processing. When processing of the device wafer is complete, the handler wafer needs to be released from the device wafer. The most common approach to handling a device wafer is to laminate the handler wafer with the device wafer using specially developed adhesives. Depending on factors such as the processing steps, the product requirements, and the type of the adhesive, various techniques have been used or proposed to debond or separate a thinned device wafer from a mechanical handler wafer, including thermal release, chemical dissolving, and laser ablation techniques.

A typical laser-assisted debonding process uses a polymeric adhesive (which is capable of sufficient absorption of energy in the UV (ultra violet) spectrum) to bond a device wafer to a UV transparent glass handler wafer. A laser ablation process is performed to ablate the polymeric adhesive and achieve debonding between the glass handler wafer and the device wafer. The use of a glass handler in the UV laser ablation process has several drawback including poor thermal conductivity, incompatibility with certain semiconductor processing equipment, as well as high cost. Although the use of Si wafer handlers can potentially overcome these drawbacks, silicon is not transparent to UV spectrum and therefore is not compatible with previously developed UV laser release technology.

SUMMARY

In general, embodiments of the invention include structures and methods for temporarily bonding handler wafers to device wafers using bonding structures which include one or more releasable layers that absorb infrared radiation to achieve wafer debonding by infrared radiation ablation.

In one embodiment of the invention, a stack structure includes a device wafer, a handler wafer, and a bonding structure disposed between the device wafer and the handler wafer to bond the device and handler wafers together. The bonding structure includes an adhesive layer, and a metallic layer. The metallic layer serves as a releasable layer of the bonding structure by infrared ablation of the metallic layer.

In another embodiment of the invention, a stack structure includes a device wafer, a handler wafer, and a bonding structure disposed between the device wafer and the handler wafer to bond the device and handler wafers together. The bonding structure comprises an adhesive layer having infrared energy absorbing nanoparticles. The adhesive layer serves as a releasable layer by infrared ablation of the adhesive layer.

In another embodiment of the invention, a method is provided for handling a device wafer. The method includes providing a stack structure having a device wafer, a handler wafer, and a bonding structure disposed between the device wafer and handler wafer, and irradiating the bonding structure with long-wavelength infrared energy to ablate the bonding structure.

These and other embodiments of the invention will be described or become apparent from the following detailed description of embodiments, which is to be read in conjunction with the accompanying drawings.

DETAILED DESCRIPTION

Embodiments of the invention will now be discussed in further detail with regard to structures and methods for temporarily bonding silicon handler wafers to device wafers using bonding structures that include one or more releasable layers that absorb infrared radiation to achieve wafer debonding by infrared radiation ablation. For example,FIG. 1is flow diagram that illustrates a method for processing and handling a semiconductor wafer according to an embodiment of the invention. Referring toFIG. 1, the method includes performing a wafer bonding process by bonding a handler wafer (or handler substrate) to a device wafer (or chip) using a bonding structure that comprises an adhesive layer and a thin metallic layer (step10). In one embodiment of the invention, the handler wafer is a Si handler wafer (or substrate) which is bonded to a Si device wafer, as the use of a mechanical Si handler wafer enables compatibility with standard CMOS silicon wafer processing technologies. In other embodiments of the invention, the handler wafer can be formed of other suitable materials that are transparent or semi-transparent (e.g., 50% transparent) to certain wavelength in the infrared (IR) spectrums that are used for IR laser ablation.

Moreover, bonding structures according to embodiments of the invention utilize one or more adhesive layers and thin metallic layers that serve as releasable layers that are ablated using IR radiation to debond the device and handler wafers. In particular, in one embodiment, a bonding structure comprises one or more thin metallic layers that are configured to strongly absorb IR energy emitted from a pulsed IR laser and improve the ablation efficiency, and reduce an ablation energy threshold for bonding structures. Indeed, with these bonding structures, an ultra-short pulse of IR energy from the IR laser can be readily absorbed by the thin metallic layers (constrained in a very shallow depth within the bonding structure) to thereby quickly and efficiently vaporize at least a portion of the thin metallic layer and at least a portion of the adhesive layer at an interface between the adhesive layer and the thin metallic layer, and release the device wafer from the handler wafer. Various bonding structures according to alternative embodiments of the invention will be described in further detail below with reference toFIGS. 2-9.

Referring again toFIG. 1, once the wafer bonding process is complete, standard wafer processing steps can be performed with the handler wafer attached to the device wafer (step11). For instance, in one embodiment of the invention, the handler wafer is bonded to a BEOL (back-end-of-line) structure formed on an active surface of the device wafer. In this instance, standard wafer processing steps such as grinding/polishing the backside (inactive) surface of the device wafer to thin the device wafer can be performed. Other wafer processing steps include forming through-silicon-vias through the backside of the device wafer to the integrated circuits formed on the active side of the device wafer. In other embodiments, the device wafer may be subject to a wafer dicing process with the handler wafer attached such that an individual die, or multiple dies, can be held by the temporary handler wafer for die assembly or other processes where the dies are assembled to a substrate or another full thickness die, and then released in subsequent operations such as post assembly or post underfill. During these processing steps, the handler wafer will impart some structural strength and stability to the device wafer, as is readily understood by those of ordinary skill in the art.

A next step in the illustrative process ofFIG. 1involves performing a laser ablation wafer debonding process to release the device wafer from the handler wafer (step12). In one embodiment, this process involves irradiating the bonding structure through the handler wafer using long-wavelength IR energy to laser ablate the bonding structure and release the device wafer. More specifically, in one embodiment, the process involves directing a pulsed IR laser beam at the handler wafer, and scanning the pulsed IR laser beam across at least a portion of the stack structure to laser ablate at least a portion of the bonding structure. As noted above, ablation of the bonding structure comprise vaporizing at least a portion of the thin metallic layer and/or vaporizing at least a portion of the thin metallic layer and adhesive layer at an interface between the adhesive layer and the thin metallic layer, which enables release of the device wafer from the handler wafer. Various embodiments of an IR laser ablation process will be described in further detail below with reference toFIGS. 2-10.

Once the IR laser ablation process is complete and the device wafer is released from the handler wafer, a post debonding cleaning process can be performed to remove any remaining adhesive material or other residue (resulting from the ablation of the bonding structure) from the device wafer (step13). For example, cleaning process can be implemented using a chemical cleaning process to remove any polymer based adhesive material, or other known cleaning methods to remove residue of the ablated bonding structure.

FIG. 2schematically depicts a stack structure comprising a bonding structure for temporarily bonding a device wafer to a handler wafer, according to an embodiment of the invention. More specifically,FIG. 2is a schematic side view of a stack structure20comprising a silicon device wafer21, a silicon handler wafer22, and a bonding structure23. The bonding structure23comprises an adhesive layer24and a thin metallic layer25.FIG. 2further illustrates an IR laser14that emits an IR laser beam at the handler wafer22to irradiate a portion of the bonding structure23resulting in a laser-ablated region16.

In one embodiment of the invention, IR laser14emits a pulsed infrared laser beam to laser ablate the bonding structure23, wherein the IR laser14emits a long wavelength infrared laser beam with an output wavelength that is greater than about 5 μm. In one alternative embodiment, the IR laser14is a far infrared (FIR) laser having an output wavelength in a far IR portion of the electromagnetic spectrum between about 5 μm and 30 μm. The silicon handler wafer22is approximately 50% transparent at these frequencies so that the laser beam will penetrate the handler wafer22and irradiate the bonding structure23.

In one embodiment, the adhesive layer24may be formed of any suitable polymer adhesive material that may or may not be capable of sufficiently absorbing the IR energy output from the IR laser14. Irrespective of the IR absorption ability of the adhesive layer24, in one embodiment of the invention, the thin metallic layer25is configured (in material composition and thickness) to intensely absorb the IR energy and serve as a primary releasable layer of the bonding structure23, which is ablated by the IR laser energy. The thin metallic layer25improves the laser ablation efficiency and thus, reduces the ablation threshold of the bonding structure23(as compared to a bonding structure that uses an adhesive layer alone). In one embodiment of the invention, the bonding structure23is irradiated with infrared energy sufficient to fully vaporize (ablate) at least a portion of the thin metallic layer25that is exposed to the IR energy.

Moreover, in an alternate embodiment of the invention, the bonding structure23is irradiated with infrared energy sufficient to fully vaporize (ablate) at least a portion of the thin metallic layer25that is exposed to the IR energy, as well as vaporize, denature, carbonize, or otherwise ablate and at least a portion of the adhesive layer24at an interface between the adhesive layer24and the portion of the thin metallic layer25that is irradiated and ablated. In other words, in the bonding structure23shown inFIG. 2, the portion of the thin metallic layer25that is irradiated by the IR laser14is heated and vaporized, and this heating and ablation of the thin metallic layer25results in heating of the surrounding material of the adhesive layer24(at the interface between the irradiated thin metallic layer25and adhesive layer24), which causes ablation of the adhesive layer. In addition, depending on the IR absorption properties of the material used to form the adhesive layer24, ablation of the adhesive layer24is further achieved by any additional heating that is due to absorption of the IR energy by the adhesive layer24.

In one embodiment of the invention, the thin metallic layer25is formed of a metallic material having properties such as being reactive (not inert), soft, and having a relatively low melting point. For example, the thin metallic layer25may be formed of materials such as aluminum (Al), tin (Sn) or zinc (Zn). Moreover, in one embodiment of the invention, the thin metallic layer25is formed with a thickness in a range of about 5 nanometers to about 100 nanometers. The thin metallic layer25is formed on the handler wafer22using one of various standard techniques such as chemical vapor deposition (CVD), physical vapor deposition (PVD), or atomic layer deposition (ALD). The ablation threshold of IR laser irradiation (level of exposure and time of exposure) will vary depending on the thickness and type of metallic material used to form the thin metallic layer25. In all instances, the thin metallic layer25is configured to substantially absorb (and not reflect) the IR laser energy, so that ablation of the thin metallic layer25occurs.

The adhesive layer24can be formed using known materials and deposition techniques. For instance, the adhesive layer24can be formed of any suitable polymeric adhesive material, and the adhesive material can be spin-coated either on the thin metal layer25, or on a surface of the Si device wafer21. Thereafter, a standard bonding process is implemented to bond the device and handler wafers21and22.

FIG. 3schematically depicts a stack structure comprising a bonding structure for temporarily bonding a device wafer to a handler wafer, according to another embodiment of the invention. In particular,FIG. 3is a schematic side view of a stack structure30which is similar to the stack structure20ofFIG. 2, except that a bonding structure33shown inFIG. 3comprises a first adhesive layer34, a second adhesive layer36, and a thin metallic layer35disposed between the first and second adhesive layers34,36. In the embodiment ofFIG. 3, the bonding structure33further reduces an ablation threshold by having two metal-adhesive material interfaces which increases the IR absorption and heating of the bonding structure33and, thus, increases the efficiency of the ablation process.

The first and second adhesive layers34and36, and the thin metallic layer35may be formed of the same or similar materials as discussed above with reference toFIG. 2. In the embodiment ofFIG. 3, the second adhesive layer36can be spin-coated onto the surface of the handler wafer22and then cured using a known curing process. The curing process results in formation of a polymer passivation layer upon which the thin metallic film35may be deposited using metallic materials and methods as discussed above. The first adhesive layer34can be spin-coated onto the thin metal layer35or onto the surface of the device wafer21. Thereafter, the device and handler wafers21and22are bonded together using known bonding techniques.

FIG. 4schematically depicts a stack structure comprising a bonding structure for temporarily bonding a device wafer to a handler wafer, according to another embodiment of the invention. In particular,FIG. 4is a schematic side view of a stack structure40which is similar to the stack structure20ofFIG. 2, except that a bonding structure43shown inFIG. 4comprises an adhesive layer44in contact with a thin metallic layer45having a roughed, non-planar surface (as depicted illustratively, by the cross-hatching of the layer45). The adhesive layer44and the thin metallic layer45may be formed of the same or similar materials as discussed above with reference toFIG. 2.

In the embodiment ofFIG. 4, the roughed surface topography of the thin metallic layer45serves to increase the contact area of the interface between the adhesive layer44and the thin metallic layer45. The increased contact area reduces the ablation threshold by enabling more heat transfer from the thin metal layer45to the surrounding material of the adhesive layer44as the thin metallic layer45is heated and ablated by IR irradiation. In one embodiment of the invention, the thin metallic layer45with a rough surface topography can be formed by first etching (dry etch or wet etch) the surface of the handler wafer22to roughen the silicon surface of the handler wafer22. A metallic material is then conformally deposited on the roughened surface of the Si wafer handler22(using suitable metallic materials and deposition methods as discussed above). This deposition process naturally forms a rough-surface thin metallic material45as the deposition of the metallic material conformally follows the topography of the roughened surface of the handler wafer22.

FIG. 5schematically depicts a stack structure comprising a bonding structure for temporarily bonding a device wafer to a handler wafer, according to another embodiment of the invention. In particular,FIG. 5is a schematic side view of a stack structure50which is similar to the stack structure30ofFIG. 3, except that a bonding structure53shown inFIG. 5comprises a rough surface thin metallic layer55disposed between a first adhesive layer54and a second adhesive layer56. The adhesive layers54,56and the roughened surface thin metallic layer55can be formed of the same or similar materials as discussed above. In the embodiment ofFIG. 5, the roughened surface of the thin metallic layer55serves to increase the contact area of the interface between the first adhesive layer54and the thin metallic layer55, as well as increase the contact area of the interface between the second adhesive layer56and the thin metallic layer55. This bonding structure53further reduces the ablation threshold by enabling more heat transfer from the thin metal layer55to the surrounding materials of the first and second adhesive layers54,56, thereby enhancing the ablation efficiency of the irradiated materials in the laser-ablated region16.

The stack structure50ofFIG. 5can be fabricated by spin coating a polymeric adhesive material onto the handler wafer22, followed by an adhesive cure process to form the second adhesive layer56. The second adhesive layer56is then etched using a dry etch process (e.g., plasma etch) to roughen the surface topography of the adhesive layer56. A metallic material is then conformally deposited on the roughened surface of the first adhesive layer56(using suitable metallic materials and deposition methods as discussed above). This deposition process naturally forms a rough-surface thin metallic material55as the deposition of the metallic material conformally follows the topography of the roughened surface of the etched adhesive layer56. The first adhesive layer54can be spin-coated onto the thin metallic layer55or onto the surface of the device wafer21using known techniques, followed by a bonding process to bond the device and handler wafers21and22together.

FIG. 6schematically depicts a stack structure comprising a bonding structure for temporarily bonding a device wafer to a handler wafer, according to another embodiment of the invention. More specifically,FIG. 6is a schematic side view of a stack structure60comprising a silicon device wafer21, a silicon handler wafer22, and a bonding structure63. The bonding structure63comprises a protective metal layer61and an adhesive layer64. In the embodiment ofFIG. 6, the protective metal layer61is disposed between the adhesive layer64(of the bonding structure63) and the device wafer21to protect the device wafer21from being irradiated with the infrared energy emitted from the IR laser14during a laser ablation process.

In the embodiment ofFIG. 6, the protective metal layer61is configured (in material composition and thickness) to reflect incident IR laser energy away from the device layer21back into the adhesive layer64. In this embodiment, although a thin metallic layer is not used in the bonding structure63as a primary releasable layer for IR laser ablation, the reflection of the IR laser energy from the protective metal layer61back into the adhesive layer64increases the IR absorption (and thus heat generation) in the irradiated portion of the adhesive layer64, which enhances the ablation efficiency of the irradiated adhesive material in the laser-ablated region16. The protective metallic layer61may be formed using an inert metallic material such as titanium, gold or copper, with a thickness that is sufficient to reflect the IR energy (thicker than a skin depth of the protective metal layer61at the given IR laser wavelength).

FIG. 7schematically depicts a stack structure comprising a bonding structure for temporarily bonding a device wafer to a handler wafer, according to another embodiment of the invention. More specifically,FIG. 7is a schematic side view of a stack structure70which is similar to the stack structures20(ofFIG. 2) and 60(ofFIG. 6) wherein a bonding structure73shown inFIG. 7comprises a combination of a protective metal layer61, an adhesive layer74and a thin metallic layer75that serves as the primary releasable layer for IR laser ablation. The adhesive layer74and the thin metallic layer75may be formed of the same or similar materials as discussed above with reference toFIG. 2, and the protective metal layer61may be formed of the same materials discussed above with reference toFIG. 6. In the embodiment ofFIG. 7, the ablation efficiency of the irradiated adhesive material and metallic layer75in the laser-ablated region16is further enhanced by the additional IR irradiation reflected back from the protective metal layer61.

FIG. 8schematically depicts a stack structure comprising a bonding structure for temporarily bonding a device wafer to a handler wafer, according to another embodiment of the invention. In particular,FIG. 8is a schematic side view of a stack structure80which is similar to the stack structure20ofFIG. 2, except that a bonding structure83shown inFIG. 8comprises an adhesive layer84, and a thin metallic layer85, wherein the adhesive layer84comprises infrared energy absorbing nanoparticles (schematically illustrated by the dotted fill of layer84). The IR energy absorbing nanoparticles enhances the IR energy absorption of the adhesive layer84and, thus, reduces the overall ablation threshold of the bonding structure83.

In one embodiment of the invention, the adhesive layer84is formed of a polymer adhesive material that is premixed with metallic nanoparticles that improve the IR absorption of the adhesive material. For example, the nanoparticles may be formed of Sn, Zn, Al, carbon nanotubes or graphene, or a combination thereof. The adhesive layer84may be formed by spin coating the polymer adhesive material with the premixed with metallic nanoparticles onto the surface of the thin metallic layer85or onto the surface of the device wafer21.

FIG. 9schematically depicts a stack structure comprising a bonding structure for temporarily bonding a device wafer to a handler wafer, according to another embodiment of the invention. In particular,FIG. 9is a schematic side view of a stack structure90which is similar to the stack structure60ofFIG. 6, except that a bonding structure93shown inFIG. 9comprises an adhesive layer94which comprises infrared energy absorbing nanoparticles (schematically illustrated by the dotted fill of layer94), to enhance IR energy absorption of the adhesive layer94and reduce the ablation threshold of the bonding structure93. The reflection of the IR laser energy from the protective metal layer61back into the nanoparticle adhesive layer94further increases the IR absorption and heat generation in the irradiated portion of the nanoparticle adhesive layer94to thereby even further enhance the ablation efficiency of the irradiated material in the laser-ablated region16of the bonding structure93. The protective metal layer61and nanoparticle adhesive layer94may be formed of the same or similar materials discussed above.

In other embodiments of the invention, a bonding structure may include a nanoparticle adhesive layer alone, with no laser-ablated thin metallic layer or protective metal layer. In particular, a stack structure can be formed by bonding a silicon device wafer and a silicon handler wafer together with an adhesive layer having infrared energy absorbing nanoparticles, wherein the adhesive layer serves as a releasable layer by infrared ablation of the adhesive layer. In other alternative embodiments, the adhesive layers shown inFIGS. 3,4,5, and7can be formed with nanoparticle adhesive layers.

FIGS. 10A,10B and10C schematically depict an apparatus to perform a debonding process to separate a device wafer and handler wafer, according to an embodiment of the invention. In particular,FIGS. 10A,10B and10C schematically illustrate an apparatus100for processing a stack structure comprising a device wafer21, a handler wafer22, and a bonding structure123disposed between the device wafer21and the handler wafer22. The bonding structure123may be any one of the bonding structure depicted inFIG. 2,3,4,5,6,7,8or9, for example. The apparatus100comprises a vacuum system comprising a first vacuum chuck110and a second vacuum chuck120, as well as an infrared laser scan system115,117. The vacuum system applies a vacuum suction force through the first vacuum chuck110to hold the stack structure21/123/22in place with the device wafer21in contact with the first vacuum chuck110.

The infrared laser scan system115,117applies a pulsed infrared laser115at the backside of the handler wafer22to irradiate the bonding structure123with infrared energy and ablate the bonding structure123to release the handler wafer from the device wafer. A scan system117is used to scan the IR laser115back and forth across the stack structure22/123/21, wherein the infrared laser scan system115,117controls the laser ablation scan process by controlling the power (energy density beam), the scan speed, and the pulse rate, for example, in a manner that is sufficient to effectively ablate the bonding structure123, or a portion of the bonding structure123at desired target regions of the stack structure. The parameters of the IR laser scan can vary depending on the bonding structure framework.

FIG. 10Billustrates a state of the apparatus100in which the IR laser scan is complete and the entire bonding structure is sufficiently ablated to release the handler wafer22from the device wafer21. In particular,FIG. 10Bschematically illustrates a state in which a completely ablated bonding structure123A exists between the handler wafer22and the device wafer21(as schematically illustrated by cross-hatching of the layer123A shown inFIG. 10B). In other embodiments of the invention, the IR laser scan process can be controlled such as certain regions of the bonding structure are laser ablated (e.g., diced die regions), while other regions of the bonding structure are not.

After IR laser ablation of the bonding structure123, referring toFIG. 10C, the vacuum system places the second vacuum chuck120in contact with the handler wafer22, and applies a vacuum suction force through the second vacuum chuck120, and the second vacuum chuck120is lifted up with a lifting device122to pull the handler wafer22from the device wafer21.

Thereafter, the device wafer21can be transferred to a chemical station to etch or otherwise remove the residual temporary adhesive layer123A that remains on the surface of the device wafer21after the debonding process shown inFIG. 10C. Although not shown inFIGS. 10A,10B and10C, the apparatus100may further comprise an air handler, filtration/condensation system or exhaust system to remove and trap debris and exhaust excess gases that are generated during the debonding process. It is to be understood thatFIGS. 10A,10B and10C generically illustrate a high-level structural depiction of a standard wafer-processing machine that can be implemented or retrofitted for IR laser ablation and wafer debonding, as discussed herein.

Although embodiments have been described herein with reference to the accompanying drawings for purposes of illustration, it is to be understood that the present invention is not limited to those precise embodiments, and that various other changes and modifications may be affected herein by one skilled in the art without departing from the scope of the invention.