U.S. patent application number 13/649573 was filed with the patent office on 2014-04-17 for advanced handler wafer bonding and debonding.
This patent application is currently assigned to INTERNATIONAL BUSINESS MACHINES CORPORATION. The applicant listed for this patent is INTERNATIONAL BUSINESS MACHINES CORPORATION. Invention is credited to Paul S. Andry, Russell A. Budd, John U. Knickerbocker, Douglas C. La Tulipe, JR., Robert E. Trzcinski.
Application Number | 20140103499 13/649573 |
Document ID | / |
Family ID | 50474644 |
Filed Date | 2014-04-17 |
United States Patent
Application |
20140103499 |
Kind Code |
A1 |
Andry; Paul S. ; et
al. |
April 17, 2014 |
ADVANCED HANDLER WAFER BONDING AND DEBONDING
Abstract
A method for processing a semiconductor wafer includes applying
a release layer to a transparent handler. An adhesive layer, that
is distinct from the release layer, is applied between a
semiconductor wafer and the transparent handler having the release
layer applied thereon. The semiconductor wafer is bonded to the
transparent handler using the adhesive layer. The semiconductor
wafer is processed while it is bonded to the transparent handler.
The release layer is ablated by irradiating the release layer
through the transparent handler with a laser. The semiconductor
wafer is removed from the transparent handler.
Inventors: |
Andry; Paul S.; (Yorktown
Heights, NY) ; Budd; Russell A.; (Yorktown Heights,
NY) ; Knickerbocker; John U.; (Yorktown Heights,
NY) ; Trzcinski; Robert E.; (Yorktown Heights,
NY) ; La Tulipe, JR.; Douglas C.; (Yorktown Heights,
NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
INTERNATIONAL BUSINESS MACHINES CORPORATION |
Armonk |
NY |
US |
|
|
Assignee: |
INTERNATIONAL BUSINESS MACHINES
CORPORATION
Armonk
NY
|
Family ID: |
50474644 |
Appl. No.: |
13/649573 |
Filed: |
October 11, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13649458 |
Oct 11, 2012 |
|
|
|
13649573 |
|
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Current U.S.
Class: |
257/644 ;
257/774 |
Current CPC
Class: |
H01L 21/2007 20130101;
H01L 21/6835 20130101; H01L 2221/68381 20130101; H01L 2221/68318
20130101; H01L 2924/0002 20130101; H01L 2924/00 20130101; H01L
2221/68327 20130101; H01L 23/49827 20130101; H01L 29/00 20130101;
H01L 2924/0002 20130101; H01L 21/02 20130101; H01L 2221/6834
20130101 |
Class at
Publication: |
257/644 ;
257/774 |
International
Class: |
H01L 23/498 20060101
H01L023/498; H01L 29/00 20060101 H01L029/00 |
Claims
1. A bonded semiconductor wafer, comprising: a transparent handler;
a device wafer bonded to the transparent handler; a release layer,
vulnerable to ablation by ultraviolet laser radiation and
transparent to visible light, provided directly on the transparent
handler, between the transparent handler and the device wafer; and
an adhesive layer interposed between the transparent handler and
the device wafer.
2. The bonded semiconductor wafer of claim 1, wherein the
transparent handler comprises Borofloat glass.
3. The bonded semiconductor wafer of claim 1, wherein the
transparent handler is substantially transparent to ultraviolet and
visible light.
4. The bonded semiconductor wafer of claim 1, wherein the
transparent handler is approximately 650 .mu.m thick.
5. The bonded semiconductor wafer of claim 1, wherein the device
wafer includes integrated circuit elements.
6. The bonded semiconductor wafer of claim 1, wherein the device
wafer includes one or more through-silicon via (TSV).
7. The bonded semiconductor wafer of claim 1, wherein the device
wafer is a layer for a 3D integrated circuit or 3D package.
8. The bonded semiconductor wafer of claim 1, wherein the adhesive
layer is TOK A0206.
9. The bonded semiconductor wafer of claim 1, wherein the release
layer comprises an adhesive.
10. The bonded semiconductor wafer of claim 1, wherein the release
layer comprises HD3007.
11. The bonded semiconductor wafer of claim 1, wherein the release
layer comprises cyclohexanone.
12. The bonded semiconductor wafer of claim 1, wherein the release
layer is approximately 6 .mu.m thick.
13. The bonded semiconductor wafer of claim 1, wherein the release
layer strongly absorbs a frequency of light radiated from an
ablating laser.
14. The bonded semiconductor wafer of claim 13, wherein the
frequency of light radiated from the ablating laser is
approximately 350 to 360 nm.
15. The bonded semiconductor wafer of claim 13, wherein the power
of light radiated from the ablating laser is approximately 5 to 30
Watts.
16. The bonded semiconductor wafer of claim 1, wherein the release
layer is vulnerable to ablation by ultraviolet laser radiation.
17. The bonded semiconductor wafer of claim 16, wherein the release
layer is vulnerable to ablation by ultraviolet laser radiation of a
power within the range of approximately 5 to 30 Watts.
18. The bonded semiconductor wafer of claim 1, wherein the
transparent handler, the adhesive layer, and the release layer are
configured to permit inspection of the device wafer
therethrough.
19. A bonded semiconductor structure, comprising: a transparent
substrate; a semiconductor wafer bonded to the transparent
substrate; a first adhesive layer interposed between the
transparent substrate and the semiconductor substrate; and a second
adhesive layer, vulnerable to destruction by ultraviolet laser
radiation and transparent to visible light, provided directly on
the transparent substrate and between the semiconductor wafer and
the transparent substrate.
20. The bonded semiconductor structure of claim 19, wherein the
second adhesive layer comprises HD3007 or cyclohexanone.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Continuation of co-pending U.S. patent
application Ser. No. 13/649,458, filed Oct. 11, 2012, the entire
contents of which is incorporated herein by reference.
TECHNICAL FIELD
[0002] The present disclosure relates to wafer debonding and, more
specifically, to advanced methods for handler wafer debonding.
DISCUSSION OF THE RELATED ART
[0003] Three-dimensional (3D) chip technologies include 3D
integrated circuits (IC) and 3D packaging. 3D chip technologies are
gaining widespread importance as they allow for greater integration
of more complex circuitry with shorter circuit paths allowing for
faster performance and reduced energy consumption. In 3D ICs,
multiple thin silicon wafer layers are stacked and interconnected
vertically to create a single integrated circuit of the entire
stack. In 3D packaging, multiple discrete ICs are stacked,
interconnected, and packaged together.
[0004] Modern techniques for 3D chip technologies, including both
3D ICs and 3D packaging, may utilize through-silicon vias (TSV). A
TSV is a vertical interconnect access (VIA) in which a connection
passes entirely through a silicon wafer or die. By using TSVs, 3D
ICs and 3D packaged ICs may be more tightly integrated as edge
wiring and interposer layers are not required.
[0005] Temporary wafer bonding/debonding is an important technology
for implementing TSVs and 3D silicon structures in general. Bonding
is the act of attaching a silicon device wafer, which is to become
a layer in a 3D stack, to a substrate or handling wafer so that it
can be processed, for example, with wiring, pads, and joining
metallurgy, while allowing the wafer to be thinned, for example, to
expose the TSV metal of blind vias etched from the top surface.
[0006] Debonding is the act of removing the processed silicon
device wafer from the substrate or handling wafer so that the
processed silicon device wafer may be added to a 3D stack.
[0007] Many existing approaches for temporary wafer
bonding/debonding involve the use of an adhesive layer placed
directly between the silicon device wafer and the handling wafer.
When the processing of the silicon device wafer is complete, the
silicon device wafer may be released from the handling wafer by
various techniques such as by exposing the wafer pair to chemical
solvents delivered by perforations in the handler, by mechanical
peeling from an edge initiation point or by heating the adhesive so
that it may loosen to the point where the silicon device wafer may
be removed by sheering.
[0008] 3M has developed an approach which relies on
light-to-heat-conversion (LTHC) whereby bonding is performed using
an adhesive layer and a LTHC layer. Debonding is then performed by
using an infrared laser to heat up the LTHC layer and thereby
loosening or "detackifying" the adhesive to the point where the
silicon device wafer may be removed. However, the LTHC layer is
dark colored and highly opaque making it difficult to inspect the
underlying circuitry prior to removing the silicon device wafer
from the handling wafer, which is generally transparent. Moreover,
the LTHC approach employs a YAG laser operating at the infrared
(IR) wavelength of 1064 nm, which while effective at generating
heat in the LTHC layer and greatly diminishing the bonding strength
of the adhesive, is not sufficient to fully and completely ablate
the interface resulting in effectively zero adhesion.
SUMMARY
[0009] A method for processing a semiconductor wafer includes
applying a release layer to a transparent handler. An adhesive
layer, that is distinct from the release layer, is applied between
a semiconductor wafer and the transparent handler having the
release layer applied thereon. The semiconductor wafer is bonded to
the transparent handler using the adhesive layer. The semiconductor
wafer is processed while it is bonded to the transparent handler.
The release layer is ablated by irradiating the release layer
through the transparent handler with a laser. The semiconductor
wafer is removed from the transparent handler.
[0010] The release layer may be strongly absorbs a frequency of
light radiated from the laser. Light may be radiated from the laser
is ultraviolet light. The light radiated from the laser may have a
wavelength of approximately 350 to 360 nm. The laser used for
ablating the release layer may be a YAG laser or a XeF excimer
laser. The adhesive layer may be applied to the semiconductor
wafer. The release layer may be cured prior to bonding the
semiconductor wafer to the transparent handler with the release
layer applied thereto. The adhesive layer may be applied to the
release layer. The release layer may be cured prior to applying the
adhesive layer.
[0011] The laser used for ablating the release layer may be a
diode-pumped solid-state (DPSS) laser. The laser used for ablating
the release layer may be an excimer laser. The laser used for
ablating the release layer may be a relatively low power laser
compared to an excimer laser. The relatively low power may be in
the range from approximately 5 Watts to 30 Watts. Processing the
semiconductor wafer while it is bonded to the transparent handler
may include thinning the semiconductor wafer. Processing the
semiconductor wafer while it is bonded to the transparent handler
may include creating one or more through-silicon via (TSV).
[0012] The method may additionally include inspecting the
semiconductor wafer through the transparent hander and the release
layer after the processing of the semiconductor wafer and prior to
ablating the release layer. Repairs may be performed on the
semiconductor wafer prior to ablating the release layer when the
inspection reveals a correctable defect. The semiconductor wafer
may be added to a 3D stack after removing the semiconductor wafer
from the transparent handler.
[0013] The release layer may be substantially transparent to
visible light.
[0014] A method for processing a semiconductor wafer includes
applying a release layer strongly absorbent of ultraviolet light to
a transparent handler and substantially transparent to visible
light. An adhesive layer is applied between the release layer and a
semiconductor wafer. The semiconductor wafer is bonded to the
transparent handler using the adhesive layer. The semiconductor
wafer is processed while it is bonded to the transparent handler.
The release layer is ablated by irradiating the release layer
through the transparent handler with ultraviolet light. The
semiconductor wafer is removed from the transparent handler.
[0015] The method may additionally include inspecting the
semiconductor wafer through the transparent hander and the release
layer after the processing of the semiconductor wafer and prior to
ablating the release layer and performing repairs on the
semiconductor wafer prior to ablating the release layer when the
inspection reveals a correctable defect.
[0016] A bonded semiconductor wafer includes a transparent handler.
A device wafer is bonded to the transparent handler. A release
layer, vulnerable to ablation by ultraviolet laser radiation and
transparent to visible light, is provided directly on the
transparent handler, between the transparent handler and the device
wafer. An adhesive layer is interposed between the transparent
handler and the device wafer.
[0017] The transparent handler may include Borofloat glass. The
transparent handler may be substantially transparent to ultraviolet
and visible light. The transparent handler may be approximately 650
.mu.m thick. The device wafer may include integrated circuit
elements. The device wafer may include one or more through-silicon
via (TSV). The device wafer may be a layer for a 3D integrated
circuit or 3D package.
[0018] The adhesive layer may be TOK A0206. The release layer may
include an adhesive. The release layer may include HD3007. The
release layer may include cyclohexanone. The release layer may be
approximately 6 .mu.m thick. The release layer may strongly absorbs
a frequency of light radiated from an ablating laser. The frequency
of light radiated from the ablating laser may be approximately 350
to 360 nm.
[0019] The power of light radiated from the ablating laser may be
approximately 5 to 30 Watts.
[0020] The release layer may be vulnerable to ablation by
ultraviolet laser radiation. The release layer may be vulnerable to
ablation by ultraviolet laser radiation of a power within the range
of approximately 5 to 30 Watts.
[0021] The transparent handler, the adhesive layer, and the release
layer may be configured to permit inspection of the device wafer
therethrough.
[0022] A bonded semiconductor structure includes a transparent
substrate. A semiconductor wafer is bonded to the transparent
substrate. A first adhesive layer is interposed between the
transparent substrate and the semiconductor substrate. A second
adhesive layer, vulnerable to destruction by ultraviolet laser
radiation and transparent to visible light, is provided directly on
the transparent substrate and between the semiconductor wafer and
the transparent substrate.
[0023] The second adhesive layer may include HD3007 or
cyclohexanone.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] A more complete appreciation of the present disclosure and
many of the attendant aspects thereof will be readily obtained as
the same becomes better understood by reference to the following
detailed description when considered in connection with the
accompanying drawings, wherein:
[0025] FIG. 1 is a flow chart illustrating an approach for
performing handler wafer bonding and debonding in accordance with
exemplary embodiments of the present invention;
[0026] FIG. 2 is a schematic diagram illustrating bonding and
debonding of a device wafer to a handler in accordance with
exemplary embodiments of the present invention;
[0027] FIGS. 3A and 3B are schematic diagrams illustrating patterns
of applying the laser light to a top surface of the handler in
accordance with exemplary embodiments of the present invention;
and
[0028] FIG. 4 is a schematic diagram illustrating a scanning laser
debonding system in accordance with exemplary embodiments of the
present invention.
DETAILED DESCRIPTION OF THE DRAWINGS
[0029] In describing exemplary embodiments of the present
disclosure illustrated in the drawings, specific terminology is
employed for sake of clarity. However, the present disclosure is
not intended to be limited to the specific terminology so selected,
and it is to be understood that each specific element includes all
technical equivalents which operate in a similar manner.
[0030] Exemplary embodiments of the present invention provide
various approaches for the temporary bonding and debonding of a
silicon device wafer to a handling wafer or other substrate that
utilize an adhesive layer and a distinct release layer. The release
layer may be transparent so that the underlying circuitry of the
silicon device wafer may be optically inspected prior to debonding.
Debonding may be performed by ablating the release layer using a
laser. The laser used may be an ultraviolet (UV) laser, for
example, a 355 nm laser. This wavelength is particularly attractive
due to the availability of robust and relatively inexpensive
diode-pumped solid-state (DPSS) lasers.
[0031] Because the bonding of the silicon device wafer to the
handling wafer includes the use of both an adhesive layer and a
distinct release layer, the bonding process may be referred to
herein as hybrid bonding. According to one approach for hybrid
bonding, the release layer may be an ultraviolet (UV) ablation
layer and it may be applied to the handling wafer, which may be a
glass handler. The UV ablation layer may then be cured. The bonding
adhesive that forms the adhesive layer may then be applied to
either the glass handler or the silicon device wafer. The UV
ablation layer may include a material that is highly absorbing at
the wavelength of the laser used in debonding. The material may
also be optically transparent in the visible spectrum to allow for
inspection of the adhesive bonded interface. Both the UV ablation
layer as well as the bonding adhesive are chemically and thermally
stable so that they can fully withstand semiconductor processes
including heated vacuum depositions including PECVD and metal
sputtering, thermal bake steps as well as exposure to wet
chemistries including solvents, acids and bases (at the edge bead
regions of the bonded wafer interface).
[0032] An exemplary glass preparation process may begin with the UV
ablation material being applied e.g. by spin coating onto the glass
handler. The glass handler with UV ablation material spin-coated
thereon may then be soft-baked to remove any solvent. Spin coating
parameters may depend on the viscosity of the UV ablation layer,
but may fall in the range from approximately 500 rpm to
approximately 3000 rpm. The soft-bake may fall in the range from
approximately 80.degree. C. to approximately 120.degree. C. The
temperature of the final cure may fall in the range from
200.degree. C. to 400.degree. C. Higher cure temperatures may be
more effective at ensuring thermal stability of the UV ablation
layer during standard CMOS BEOL processing which may take place
between 350.degree. C. and 400.degree. C. For strongly UV-absorbing
or UV-sensitive materials, very thin final layers on the order of
approximately 1000 .ANG. to approximately 2000 .ANG. thick may be
sufficient to act as release layers. One such material is Shin Etsu
ODL 38 which may be spin applied to glass and cured in a nitrogen
environment at 350.degree. C. for approximately 1 hour to produce a
film on the order of 1000 .ANG. thick. Such a film may be optically
transparent throughout the visible spectrum, but strongly sensitive
to decomposition in the UV wavelength range below .about.360 nm,
and may be fully and cleanly ablated using common UV laser sources
such as an excimer laser operating at 308 nm (e.g. XeCl) or 351 nm
(e.g. XeF) or a diode-pumped tripled YAG laser operating at 355
nm.
[0033] According to exemplary embodiments of the present invention,
the bonding adhesive may be any temporary or permanent adhesive
desired. The bonding adhesive may be applied to either the glass
(e.g., after the UV ablation layer is added) or to the device
wafer. Because the UV ablation layer controls the glass release,
the adhesive may be chosen irrespective of its UV absorption
characteristics. This vastly increases the possible choices. For
relatively low-temperature wafer applications (e.g. up to
.about.250.degree. C.) a wide variety of materials exist (e.g. TOK
TZNR-0136) which can be bonded at low pressures and temperatures
(<1 atmosphere, approximately 200.degree. C.). A typical bonding
cycle for such a material takes place in a bonding tool where the
glass and Si wafers are held in alignment but separated by a small
gap to allow a vacuum to be created between the wafer and the
handler before the two are brought into contact. The wafers are
heated to the desired bonding temperature while they are pressed
together. Bonding cycles are typically on the order of 3 to 5
minutes. For higher temperature applications (e.g. approximately
300.degree. C. to approximately 350.degree. C.) the adhesive
choices are fewer, and include BCB and polyimide-based materials
such as HD Microsystems HD3007. These are generally much less
viscous once cured, and may be bonded at higher pressures and
temperatures (>1 atmosphere, approximately 300.degree. C. to
approximately 350.degree. C.). The adhesive chosen may be spin
applied at approximately 500 to approximately 3000 rpm, soft-baked
at between approximately 80.degree. C. and approximately
120.degree. C. and then cured at between approximately 300.degree.
C. and approximately 350.degree. C. for up to an hour in nitrogen.
Bonding cycles may be on the order of approximately 20 to
approximately 40 minutes for these materials.
[0034] Laser debonding to release the glass handler at the ablation
later interface may be performed using any one of a number of UV
laser sources including excimer lasers operating at 308 nm (e.g.
XeCl) or 351 nm (e.g. XeF) as well as diode-pumped (tripled) YAG
laser operating at 355 nm or diode-pumped (quadrupled) YAG laser
operating at 266 nm. Excimer lasers may be more expensive, may
require more maintenance/support systems (e.g. toxic gas
containment) and may have generally have very large output powers
at low repetition rates (e.g. hundreds of Watts output at several
hundred Hz repetition). UV ablation thresholds in the materials
specified here may require 100-150 milliJoules per square cm
(mJ/sqcm) to effect release. Due to their large output powers,
excimer lasers can supply this energy in a relatively large area
beam having dimensions on the order of tens of mm.sup.2 area (e.g.
0.5 mm.times.50 mm line beam shape). Due to their large output
power and relatively low repetition rate, a laser debonding tool
which employs an excimer laser may consist of a movable x-y stage
with a fixed beam. Stage movement may be on the order of 10 to 50
mm per second. The wafer pair to be debonded may be placed on the
stage, and scanned back and forth until the entire surface had been
irradiated.
[0035] An alternative laser debonding system may be created using a
less expensive, more robust and lower power solid-state pumped
tripled YAG laser at 355 nm by rapidly scanning a small spot beam
across the wafer surface. The 355 nm wavelength laser may compare
favorably to the quadrupled YAG laser at 266 nm for two reasons: 1)
Output powers at 355 nm are typically 2 to 3 times larger than at
266 nm for the same sized diode laser pump power, and 2) many
common handler wafer glasses (e.g. Schott Borofloat 33) are
.about.90% or more transmissive at 355 nm but only .about.15%
transmissive at 266 nm. Since 80% of the power is absorbed in the
glass at 266 nm, starting laser powers may be .about.6.times.
higher to achieve the same ablation fluence at the release
interface, and there is risk of thermal shock in the glass handler
itself.
[0036] An exemplary 355 nm scanning laser debonding system may
include the following: 1) a Q-switched tripled YAG laser with an
output power of 5 to 10 Watts at 355 nm, with a repetition rate
between 50 and 100 kHz, and pulsewidth of between 10 and 20 ns. The
output beam of this laser may be expanded and directed into a
commercial 2-axis scanner, comprising mirrors mounted to x and y
galvanometer scan motors. The scanner may be mounted a fixed
distance above a fixed wafer stage, where the distance would range
from 20 cm to 100 cm depending on the working area of the wafer to
be released. A distance of 50 to 100 cm may effectively achieve a
moving spot speed on the order of 10 meters/second. An F-theta lens
may be mounted at the downward facing output of the scanner, and
the beam may be focused to spot size on the order of 100 to 500
microns. For a 6 watt output power laser at 355 nm, at 50 kHz
repetition and 12 ns pulsewidth, a scanner to wafer distance of 80
cm operating at a raster speed of 10 m/s, the optimal spot size may
be on the order of 200 microns, and the required .about.100 mJ/sqcm
ablation fluence may be delivered to the entire wafer surface twice
in .about.30 seconds (for example, using overlapping rows). The use
of overlapping rows where the overlap step distance equals half the
spot diameter (e.g., 100 microns) may ensure that no part of the
wafer is missed due to gaps between scanned rows, and that all
parts of the interface see the same total fluence.
[0037] FIG. 1 is a flow chart illustrating an approach for
performing handler wafer bonding and debonding in accordance with
exemplary embodiments of the present invention. First the release
layer and the adhesive layer may be applied. According to one
exemplary approach, the release layer may be applied to the handler
(Step S11) while the adhesive layer may be applied to the device
wafer (Step S12). However, according to other exemplary approaches,
the release layer may be applied to the handler and then the
adhesive layer may be applied to the release layer.
[0038] The release layer is always interposed between the glass and
the adhesive. Thereafter, the device wafer may be bonded to the
handler (Step S13) such that the release layer and the adhesive
layer are provided between the device wafer and the handler. The
bonding may include a physical bringing together of the device
wafer and the handler under controlled heat and pressure in a
vacuum environment such as offered in any one of a number of
commercial bonding tools.
[0039] After the device wafer has been successfully bonded to the
handler, desired processing may be performed (Step S14). Processing
may include such process steps as patterning, etching, thinning,
etc. until the device wafer has achieved its desired state.
Thereafter, the circuitry of the device wafer may be inspected
(Step S15). Inspection of the device circuitry may be performed to
ensure that the device wafer has been properly processed.
Inspection may be optically performed, for example, using a high
quality microscope or other imaging modality. Optical inspection
may be performed though the handler, which, as described above, may
be transparent. Optical inspection of the device circuitry may also
be performed through the release and adhesive layers as each of
these layers may be transparent as well.
[0040] Optical inspection may be performed after all processing has
been completed and/or at any stage during the processing of the
wafer. According to some exemplary embodiments of the present
invention, optical inspection may be performed after one or more
critical processing steps that are likely to create defects. In the
event that optical inspection results in a determination that a
defect is present in the device wafer, the device wafer may be
rejected on the spot and subsequent processing may be canceled.
Because the device wafer may be optically inspected through the
handler, removal of the device wafer from the handler is not
required to perform testing and accordingly, defects may be
detected at an earlier stage in processing than would otherwise be
possible. Additionally, waiting until the entire 3D stack has been
assembled before performing testing may result in the rejection of
the entire 3D stack thereby substantially reducing yield and adding
substantially to the cost of manufacture. Moreover, seeing the
bonded interface through the glass may be useful in that it may be
verified that processing has not generated small voids in the
bonding adhesive itself, which can lead to yield loss during
thinning and vacuum processing. Because defects such as these may
be known to exist at early stages of processing, subsequent
processing steps performed on the wafer defective may be
avoided.
[0041] This opportunity for optically inspecting the device wafer
may not be present in prior art approaches such as the 3M
light-to-heat-conversion (LTHC) approach discussed above, where the
LTHC layer is necessarily opaque in order to be able to generate
heat from the infrared laser light exposure.
[0042] After inspection and any necessary repair has been performed
to the device wafer, a laser ablation process may be performed to
sever the device wafer from the handler (Step S16). Laser ablation
may be performed by exposing the release layer to UV laser light
through the transparent handler. Upon exposure to the UV laser
light, the release layer may burn, break down or otherwise
decompose. This stands in contrast to the 3M LTHC approach
discussed above, where the LTHC layer generates heat as a result of
being exposed to the infrared laser light and the heat in turn
softens the adhesive layer to the point where the device wafer may
be peeled from the handler. Thus, the release layer according to
exemplary embodiments of the present invention comprises a material
that is broken down under the exposure of the UV laser light. As
the adhesive layer may remain hard during this process, the device
wafer, along with the adhesive layer, may be easily removed from
the handler. Where desired, the remainder of the adhesive layer may
be removed from the device wafer using various processing
techniques.
[0043] Because the release layer burns away during the debonding,
the debonding may be substantially cleaner than conventional
techniques such as the 3M LTHC approach discussed above.
[0044] After the laser ablation has resulted in the severing of the
device wafer from the handler, the device wafer may be easily
removed from the handler, for example, by simply pulling the
handler away, and the device wafer may be cleaned to remove the
adhesive (Step S17).
[0045] FIG. 2 is a schematic diagram illustrating bonding and
debonding of a device wafer to a handler in accordance with
exemplary embodiments of the present invention. The device wafer 21
may be a silicon wafer that is to be processed, for example, to be
added to a 3D stack such as a layer in a 3D IC or an IC to be
included in a 3D package. The device wafer 21 may be processed
prior to bonding, however, prior to bonding the device wafer 21 may
be a full-thickness wafer. The device wafer 21 may be bonded to the
handler to provide structural support thereto during subsequent
processing which may include a thinning of the device wafer 21 to
the point where it may no longer poses the structural integrity
necessary to withstand certain processing steps that may have to be
performed. The device wafer need not comprise silicon and may
instead comprise an alternative semiconductor material. The device
wafer 21 may originate as a full-thickness wafer and may
subsequently be thinned down to a size of between approximately 200
um and 20 um.
[0046] The handler 22 may be a transparent substrate and may
comprise, for example, Borofloat glass. The handler may be
sufficiently thick to provide structural integrity to the device
wafer 21 bonded thereto. For example, the handler may be
approximately 650 .mu.m thick.
[0047] As described above, the adhesive layer 23 and the release
layer 24 may be provided between the device wafer 21 and the
handler 22. According to one exemplary embodiment of the present
invention, the release layer 24 is disposed directly upon the
handler 22. The release layer 24 may comprise a material that is
highly specialized to absorb strongly near the UV wavelength of
laser light used during laser ablation. As exemplary embodiments of
the present invention may employ a UV laser, for example, at or
near the wavelength 355 nm, the release layer 24 may comprise a
material highly absorbent of UV light, and in particular, light
having a 355 nm wavelength. The release layer 24 may itself
comprise an adhesive, but at least for reasons discussed below, the
release layer 24 may be an entirely distinct layer from the
adhesive layer 23.
[0048] The release layer 24 may comprise, for example, HD3007,
which is a polyimide-based adhesive which may be spin applied and
cured at 350.degree. C. The release layer may be approximately 6
.mu.m thick. The thermoplastic nature of HD3007 and similar
materials may permit the release layer 24 material to be applied in
a liquid state and to flow to fill the surface of the handler 22
during application of the release layer 24. This material may be
strong enough to withstand commonly used processing techniques that
may subsequently be performed on the device wafer 21 while bonded
to the handler 22 without the release layer 24 prematurely breaking
down. Such processes may include wafer grinding, application of
heat in excess of 260.degree. C., PECVD, CMP, metal sputtering at
200.degree. C., seed metal wet etch, resist strip, and polymer
curing at 320.degree. C.
[0049] Additionally, while HD3007 may stand up to processing steps
such as those described above, it may also strongly absorb UV light
and may be easily ablated by radiation from a 308 nm excimer
laser.
[0050] An example of a more advantageous UV release layer which is
not itself an adhesive, but rather an optically planarizing
material used as an underlayer in photolithography, is Shin Etsu
ODL-38. Very thin layers of this material on the order of 1000
.ANG. may be spin applied to glass handlers and cured at
350.degree. C. in nitrogen. This material absorbs very strongly at
UV wavelengths below .about.360 nm and decomposes rapidly, and thus
is an excellent release layer for use at the 355 nm laser
wavelength.
[0051] Regardless of the material used, the release layer 24 may
comprise a material that can be laser ablated at the UV wavelength
of choice. The release layer 24 may be generated, for example, by
spin coating or spraying the release layer material, for example,
onto the handler, and then curing the material using heat (e.g.
350.degree. C.) and/or UV light. Curing of the release layer
material may either be performed prior to bonding of the handler 22
to the device wafer 21 or at the same time.
[0052] The adhesive layer 23 may be created by applying an adhesive
material to either the device wafer 21 or to the release layer 24.
The adhesive layer 23 may comprise a distinct material from that
which is used as the release layer 24, and in particular, the
adhesive layer 23 may be an adhesive that does not strongly absorb
the light of the wavelength that is used to ablate the release
layer 24. While any number of suitable adhesives may be used for
this layer, one example of a suitable adhesive is TOK A0206. The
adhesive layer may be created, for example, by applying the
adhesive material to the device wafer 21. The adhesive layer 23 may
be cured using heat (e.g. 220.degree. C.).
[0053] According to one exemplary embodiment of the present
invention, the release layer 24 may be cured prior to performing
bonding. In this way, potential adverse interaction between the
release layer 24 material and the adhesive layer 23 material may be
minimized. Bonding may be performed in a bonder, for example, a
Suss bonder using approximately 500 mbar of applied force in a
temperature of 220.degree. C. (the curing temperature of the
adhesive layer 23 material). In bonding, the device wafer 21 may be
bonded, by the adhesive layer 23, to the handler 22 having the
release layer 24 attached thereto.
[0054] Thereafter, processing, testing, and repair may be
performed, for example, as described in detail above. Testing and
inspection may be facilitated by the use of a transparent handler
such as one made of Borofloat glass.
[0055] When the processing, testing and repair is complete, and it
is time to debond the device wafer 21 from the handler 22, a laser
25 may be used to irradiate the release layer 24. As discussed
above, the laser may be a 308 nm excimer laser or a 355 nm DPSS
laser, for example, one created by frequency tripling a diode-laser
at 1064 nm. According to one exemplary embodiment of the present
invention the laser 25 may be a HIPPO 355QW laser with a wavelength
of 355 nm, a power of 5 W at 50 kHz, a repetition rate of 15-300
kHz, and a pulse width of less than 12 ns at 50 kHz. However, other
UV lasers may be used such as a HIPPO 266QW having a 266 nm
wavelength.
[0056] The release layer 24 may be irradiated though the handler
22, which may be transparent, at least to the wavelength of the
laser 25 used. The laser 25 may produce a spot beam that is scanned
across the surface of the handler 22, for example, in a raster
pattern, or the laser 25 may produce a fan beam that is swept once
or multiple times across the handler 22. Directing of the light
radiated from the laser 25 may be handled by the use of a scanner
and lens 26, which may be, for example, an F-Theta scan lens having
an 810 mm fl. FIG. 3 is a schematic diagram illustrating pattern of
applying the laser light to a top surface 31 of the handler 22 in
accordance with exemplary embodiments of the present invention. As
seen in FIG. 3A, the laser light may be directed across the top
surface 31 of the handler 22 as a spot beam drawn to lines 32 which
move along an x-axis direction of the top surface 31 of the handler
22 with each successive line 32 being drawn lower in the y-axis
direction. Alternatively, as seen in FIG. 3B, the laser light may
be directed in a serpentine pattern 33.
[0057] As the UV wavelength of the laser 25 used may contain
relatively high energy, the light may efficiently ablate the
release layer 24. Once ablated, the device wafer 21 may be freely
removed from the handler layer 22. Thereafter, a solvent or
cleaning chemical may be used to remove any remaining elements of
the adhesive layer 23 and/or release layer 24 that may remain on
the device wafer 21. The debonded and cleaned device wafer 21 may
then be further processed, diced and applied to a 3D stack and/or
joined to a package or another 3D element.
[0058] FIG. 4 is a schematic diagram illustrating an apparatus for
performing laser debonding in accordance with exemplary embodiments
of the present invention. According to some exemplary embodiments
of the present invention, such as is shown here in FIG. 4, the
bonded handler and device wafer 41 may remain stationary, for
example, on a stage. According to other exemplary embodiments, the
stage may be movable. The laser 42 may provide a beam that may then
be sent into a beam expander 45 to provide the desired beam size.
The beam may then enter a scanner 46 where the beam can be directed
along the x and y axes. One or more control units 43 may affect
control of the laser 42, beam expander 45 and the scanner 46. Where
the stage upon which the bonded handler and wafer 41 are held is
movable, the controller 43 may control the movement of the stage as
well. In such a case the scanner 46 may be omitted. A computer
system 44 may be preprogrammed with the manner of control and these
instructions may be executed though the one or more control units
43. A scan lens 47 may adjust the beam so as to strike the bonded
handler and device wafer 41 with the desired spot
characteristics.
[0059] Exemplary embodiments described herein are illustrative, and
many variations can be introduced without departing from the spirit
of the disclosure or from the scope of the appended claims. For
example, elements and/or features of different exemplary
embodiments may be combined with each other and/or substituted for
each other within the scope of this disclosure and appended
claims.
* * * * *