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Redistribution layers (RDLs) for 2.5D/3D IC integration

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Redistribution layer (RDL) is an integral part of 3D IC integration, especially for 2.5D IC integration with a passive interposer. The RDL allows for fans out of the circuitries and allows the lateral communication between the chips attached to the interposer. There are at least two ways to fabricate the RDL, namely (a) polymers to make the passivation and Cu-plating to make the metal layer, and (b) semiconductor back-end-of-line Cu damascene. In this study, the materials and processes of these methods are presented. Emphasis is placed on the Cu damascene method.
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Journal of Microelectronics and Electronic Packaging (2014) 11, 16-24
Copyright ©International Microelectronics Assembly and Packaging Society
ISSN: 1551-4897
Redistribution Layers (RDLs) for 2.5D/3D IC Integration
J. Lau,
1,
* P. Tzeng,
1
C. Lee,
1
C. Zhan,
1
M. Li,
2
J. Cline,
2
K. Saito,
2
Y. Hsin,
1
P. Chang,
1
Y. Chang,
1
J. Chen,
1
S. Chen,
1
C. Wu,
1
H. Chang,
1
C. Chien,
1
C. Lin,
1
T. Ku,
1
R. Lo,
1
and M. Kao
1
Abstract
Redistribution layer (RDL) is an integral part of
3D IC integration, especially for 2.5D IC integration with a
passive interposer. The RDL allows for circuitry fan-outs of and
allows for lateral communication between the chips attached to
the interposer. There are at least two ways to fabricate the RDL,
namely (1) polymers to make the passivation and Cu-plating
to make the metal layer, and (2) semiconductor back-end-of-line
Cu damascene. In this study, the materials and processes of
these methods are presented. Emphasis is placed on the Cu
damascene method.
Keywords
3D IC integration, through-silicon via (TSV),
redistribution layer (RDL), polymer, Cu damascene
INTRODUCTION
One of the potential applications of 3D IC integration is
wide I/O interfaces [1], which consists of a piece of
device-less silicon with through-silicon vias (TSVs) and high-
performance, high-density IC chips without TSV. This piece of
device-less silicon (also called a passive interposer) is used to
support the chips and has RDLs (mainly) for lateral communi-
cation between the chips (Fig. 1).
As mentioned in [2-5], one of the key reasons to have the
intermediate substrate (passive interposer) is created by the ever
increasing IC density and pin-out, and shrinking IC pad-pitch
and size, the conventional package substrate cannot support these
IC requirements. A couple of examples are shown in Fig. 2
(Xilinx/TSMC) [6-8] and Fig. 3 (Altera/TSMC) [9, 10] (where
CoWoS stands for chip on (interposer) wafer on (package)
substrate). In these cases, it can be seen that even with
12 buildup layers (6-2-6) on the package substrate, it is still
not enough to support the FPGA (field-programmable gate
array) chips. In addition, a passive TSV interposer with
four top RDLs (three Cu damascene layers and one aluminum
layer) at a 0.4 mm pitch is needed. The minimum thickness
of the RDLs and passivation is <1mm. Also, it can be seen
that, except for niche applications, passive TSV/RDL inter-
posers are for very high-I/O, high-performance, and high-
density applications.
The RDL is the focus of this study. There are at least
two ways to fabricate RDLs. The first method is by using
polymers, such as polyimide (PI) PWDC 1000 (Dow Corning),
benzocyclobutene (BCB) cyclotene 4024-40 (Dow Chemical),
polybenzo-bisoxazole (PBO) HD-8930 (HD Micro Systems), and
the fluorinated aromatic AL-X 2010 (Asahi Glass Corporation)
to make the passivation layer and electroplating (such as Cu)
to make the metal layers. This method has been used by the
OSAT (outsourced semiconductor assembly and test) to fab-
ricate RDLs (without using semiconductor equipment) for
wafer-level (fan-in) chip scale package [11, 12], embedded
wafer-level (fan-out) ball grid array package [13], and (fan-out)
redistribution chip package [14]. The second method is the
Cu damascene method, which is primarily modified from the
conventional semiconductor back-end-of-line to make the Cu
metal RDLs such as those shown in Figs. 2 and 3. In general,
much thinner structures (both dielectric layers and Cu RDLs),
finer pitches, smaller line-widths, and spacing can be obtained
with the Cu damascene method, which will be the emphasis
in this study. The polymer/Cu-plating method will be men-
tioned first. Also, the fabrication of TSV and Cu reveal will
be presented.
FABRICATION OF TSV
The fabrication process of TSVs is shown in Fig. 4. The
process starts with an SiN
x
/SiO
x
insulation layer by either
thermal oxidation or PECVD (plasma enhanced chemical
vapor deposition) as shown in Fig. 4. After photoresist and
TSV lithography, the TSV is etched into the Si substrate by
Bosch-type DRIE (deep reactive ion etch) [15] to form a
high aspect ratio (10.5) via structure. The etched TSV struc-
ture is then processed with an SiO
x
liner by SACVD
(subatmosphere chemical vapor deposition), a Ta barrier
layer, and a Cu seed layer by PVD (physical vapor deposition)
[16]. Cu electroplating is used to fill the TSV structure. The
final blind TSV has a top opening of approximately 10 mmin
diameter and a depth of about 105 mm, which gives an aspect
ratio of 10.5. In such a high aspect ratio via structure, a
bottom-up plating mechanism is applied to ensure a seamless
TSV with a reasonably low Cu thickness in the field. The
SEM (scanning-electron microscopy) cross-sectional images
are shown in Fig. 5. It can be seen that the diameter of
the TSV is slightly decreased at the bottom, which is
expected from the etching process. The Cu thickness at the
field is <5mm. The postplating anneal is at 400C for 30 min.
Received on October 10, 2013; revision received on December 22, 2013;
accepted on December 23, 2013
The original version of this paper was presented at IMAPS 46th International
Symposium on Microelectronics (IMAPS 2013), September 30-October 3, 2013,
Orlando, FL.
1
Electronics and Optoelectronics Research Laboratory, Industrial Technology
Research Institute (ITRI), Hsinchu, Taiwan
2
Rambus Inc., 1050 Enterprise Way, Suite 700, Sunnyvale, California 94089
*Corresponding author; email: johnlau@itri.org.tw
doi:10.4071/imaps.406 1551-4897 ©2014 International Microelectronics Assembly and Packaging Society
To complete the TSV process, excess Cu in the field is
removed by CMP (chemical-mechanical polishing) [17].
FABRICATION OF RDL BY POLYMERS
Continuing with the wafer from Fig. 4, the fabrication pro-
cess for the RDLs using polymers is shown in Fig. 6 and also
listed as follows. UBM (under bump metallurgy) is included.
Step 1. Spin the polymers such as PI or BCB on the wafer
and cure for 1 h. This will form a 4-7-mm-thick layer.
Step 2. Apply photoresist and mask, then use photolithography
techniques (align and expose) to open vias on the PI
or BCB.
Step 3. Etch the PI or BCB.
Step 4. Strip off the photoresist.
Step 5. Sputter Ti and Cu over the entire wafer.
Step 6. Apply a photoresist and mask, and then use photoli-
thography techniques to open the redistribution-
trace locations.
Step 7. Electroplate Cu in photoresist openings.
Step 8. Strip off the photoresist.
Step 9. Etch off the Ti/Cu and RDL1 is completed.
Step 10. Repeat steps 1-9 for RDL2, and so forth.
Step 11. Repeat step 1 (for UBM).
Step 12. Apply photoresist and mask, and then use photolithog-
raphy techniques (align and expose) to open vias on
the PI or BCB for the desired bump pads and to cover
the redistribution traces.
Step 13. Etch the desired vias on the PI or BCB.
Step 14. Strip off the photoresist.
Step 15. Sputter Ti and Cu over the entire wafer.
Step 16. Apply photoresist and mask, and then use photolithog-
raphy techniques to open the vias on the bump pads
to expose the areas with UBM.
Step 17. Electroplate the Cu core.
Step 18. Strip off the photoresist.
Step 19. Etch off the Ti/Cu.
Step 20. Electroless Ni and immersion Au. UBM is completed.
A typical cross section of the RDLs with polymers (e.g.,
BCB) as passivation and Cu plating as metal layers is shown
in Fig. 7 (IZMs, 2 RDLs with BCB1, and BCB2 [18]). It can be
Fig. 2. Xilinx/TSMC’s CoWoS.
Fig. 3. Altera/TSMC’s CoWoS.
Fig. 1. TSV/RDL passive interposer on substrate.
Lau et al.: Redistribution Layers (RDLs) for 2.5D/3D IC Integration 17
seen that the thickness of the passivation layers, BCB1 and
BCB2, is about 6-7 mm, and the RDL is about 4 mm. It should
be noted that photolithography can be directly applied on PI or
BCB for larger dimensions. In that case the first set of photo-
resist is not needed.
RDL FABRICATED BY CUDAMASCENE
Another method of RDL fabrication is by a Cu damascene
process. If starting with the wafer from Fig. 4, the fabrication
process of RDLs with a Cu damascene technique is primarily
based on the semiconductor back-end-of-line process. The
details are shown in Fig. 8 and listed below.
Step 1. SiO
2
layer by PECVD.
Step 2. Apply photoresist and mask, then use photolithogra-
phy techniques (align and expose) to open vias on
the SiO
2
.
Step 3. RIE (reactive ion etch) of SiO
2
.
Step 4. Strip off the photoresist.
Step 5. Sputter Ti and Cu and electroplate Cu over the
entire wafer.
Step 6. CMP the Cu and Ti/Cu. V01 (the via connecting the
TSV to RDL1) is completed.
Step 7. Repeat step 1.
Step 8. Apply photoresist and mask, and then use photoli-
thography techniques to open the redistribution
trace locations.
Step 9. Repeat step 3.
Step 10. Repeat step 4.
Fig. 4. TSV fabrication process flow.
Fig. 5. SEM images of TSV cross sections.
18 Journal of Microelectronics and Electronic Packaging, Vol. 11, No. 1, 1st Qtr 2014
Step 11. Repeat step 5.
Step 12. CMP the Cu and Ti/Cu. RDL1 is completed.
Step 13. Repeat step 1 through step 6 to complete V12 (the via
connecting the RDL1 to RDL2).
Step 14. Repeat step 7 through step 12 to complete RDL2 and
any additional layers.
Step 15. (For UBM) Repeat step 1.
Step 16. Apply photoresist and mask, and then use photolithog-
raphy techniques (align and expose) to open vias on
the SiO
2
for the desired bump pads and cover the
redistribution traces.
Step 17. Etch the desired vias on the SiO
2
.
Step 18. Strip off the photoresist.
Step 19. Sputter Ti and Cu over the entire wafer.
Step 20. Apply photoresist and mask, and then use photolithog-
raphy techniques to open the vias on the bump pads
to expose the areas with UBM.
Step 21. Electroplate Cu core.
Step 22. Strip off the photoresist.
Step 23. Etch off the Ti/Cu.
Step 24. Electroless Ni and immersion Au. UBM is completed.
It should be noted that the RDLs can also be fabricated by
the dual Cu damascene method as shown in Fig. 9. SEM images
of the RDL cross sections fabricated by a Cu damascene tech-
nique are shown in Figs. 10 and 11. The minimum RDL line
width is 3 mm. The thickness of RDL1 and RDL2 is 2.6 mm and
of RDL3 is 1.3 mm. The passivation thickness between RDLs
is 1 mm.
ANOTE ON CONTACT ALIGNER FOR CUDAMASCENE METHOD
The RDLs in this study are fabricated by the Cu damascene
method. Lithography using a contact aligner provides a low-
cost process as compared with a stepper/scanner under the
same resolution requirements. Since the minimum line width
is 3 mm in this case, the mask had to be placed very close to the
(photoresist) surface of the 300 mm wafer. In a few cases,
particles on the contact aligner mask punched holes on the
photoresist. In this case, shorts may happen such as that shown
in Fig. 12, which happened while fabricating the V12 (the via
connecting RDL1 and RDL2). This can be prevented by
cleaning the mask between exposures. Alternately, if cost is
not an issue, using a stepper/scanner is another solution.
BACKSIDE PROCESSING AND ASSEMBLY
Some of the backside processes of this wafer (shown in
Fig. 8) such as the RDLs in the backside have been reported
Fig. 6. RDL fabrication process with polymers as passivation and Cu plating as metal layers.
Fig. 7. IZM’s RDLs with BCB polymer.
Lau et al.: Redistribution Layers (RDLs) for 2.5D/3D IC Integration 19
Fig. 8. Process flow of RDLs fabricated by Cu damascene method.
Fig. 9. Process flow for fabricating RDLs by dual Cu damascene.
20 Journal of Microelectronics and Electronic Packaging, Vol. 11, No. 1, 1st Qtr 2014
in [19]. The process flow of backside and assembly is shown
in Fig. 13. It can be seen that after the fabrication of TSV,
RDLs, passivation, and UBM, the topside of the interposer
wafer is temporary bonded to a carrier by adhesive. The next
step is backgrinding the interposer wafer, Si etching, low tem-
perature passivation, and Cu revealing. Next, backside RDL
(optional), UBM, and C4 (controlled collapse chip connection)
wafer bumping are carried out. After that, the next step is
to temporary bond another carrier wafer to the backside
(with solder bumps) and de-bond the first carrier wafer. This
step is followed by chip-on-wafer bonding and underfilling.
After the whole interposer wafer is completed, the next step is
to de-bond the second carrier wafer and transfer the thin inter-
poser wafer with attached chips to a dicing tape for singulation.
Fig. 10. SEM images of cross sections of RDLs fabricated by the Cu damascene method.
Fig. 11. SEM images of cross sections of RDLs fabricated by the Cu damascene method.
Lau et al.: Redistribution Layers (RDLs) for 2.5D/3D IC Integration 21
(For thin-wafer handling without temporary bonding and
de-bonding, please see [20].) The individual TSV/RDL inter-
poser with chips is attached to the package substrate by natural
reflow and then underfilled.
Fig. 14 shows more detail on Cu revealing. Right after the
temporary bonding of the support carrier, backgrinding the
wafer to a few microns to the TSV, Si dry etching (by RIE) to
a few microns below the TSV, and low-temperature passiving
the SiN/SiO
2
are performed. Then, CMP for SiN/SiO
2
buffing
and barrier and Cu seed layers polishing are carried out. Cu
revealing is completed and shown in Fig. 15.
CONCLUSIONS
The materials and processes for the fabrication of RDLs
with two different methods, namely (1) polymer (e.g., BCB)
to make the passivation and Cu plating to make the metal
layers, and (2) semiconductor back-end-of-line Cu damascene,
have been presented. The fabrications of TSVs, UBMs, and Cu
reveal of the interposer have also been described. Some impor-
tant results and recommendations are summarized as follows.
1. The RDLs fabricated by the polymer method are usually
thicker. The thickness of the passivation is 6-7 mm and the
Cu metal layer is 3-4 mm.
2. The RDLs fabricated by the Cu damascene method are
thinner. The thickness of the passivation and the Cu metal
layer is 1mm if a contact aligner is used and in the
submicron range if a stepper/scanner is used.
3. Lithography using a stepper/scanner can go down to sub-
micron line width and line spacing. However, for a few
microns’ applications, a contact aligner will provide a low-
cost process under the same resolution requirements. In
order to prevent shorts (not to allow holes in the photoresist
and thus the passivation during the fabrication of the vias)
between the RDLs, all the particles must be cleaned off
from the mask.
ACKNOWLEDGMENTS
The ITRI authors would like to thank the financial support
of Rambus, USA and MOEA, Taiwan. The strong support of
the 3D IC Integration program by Dr. C. T. Liu, VP and
Fig. 12. SEM/FIB showing the short between RDL1 and RDL2. The thick-
ness of the passivation layer between RDL1 and RDL2 is <1mm.
Fig. 13. Conventional process flow for 2.5D/3D IC integration (chip on interposer wafer on package substrate).
22 Journal of Microelectronics and Electronic Packaging, Vol. 11, No. 1, 1st Qtr 2014
Director of Electronics & Optoelectronics Research Lab is
greatly appreciated.
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Chapter
The 2.5D IC integration defined by the electronic semiconductor industry as chips are supported by a passive TSV (through-silicon via) interposer.
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Silicon Interposer with Through Silicon Via (TSV) is a newly developed technology that enables multichip integration and offers great potential to improve system performance with less delay, higher wiring density, and lower power consumption. One challenge of this new technology is to maintain the PDN electrical performance. Micro bumps, TSV, interposer front and back side Re-Distribution Layer (RDL) metallization add additional interfaces and drives up complexity for an optimized PDN design in 3D IC integration. This paper reports on a R&D test vehicle that was developed for engineering evaluation of electrical and physical interfaces through TSV silicon interposer. The test vehicle consisted of a FPGA die side-by-side with its daughter die on a passive silicon interpose. The paper reports TSV loss mechanisms and its performance impacts on Power Delivery Network (PDN). Embedded MIM capacitor is implemented to increase interposer decoupling capacitance (IDC) to improve high speed PDN performance. PDN impedance characteristics are analyzed and evaluated for the interposer-based 3D system combining on-die PDN, interposer power/ground grids, TSV and package/PCB PDN components.
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A novel and reliable wafer level chip scale package (WLCSP) is investigated in this paper. It consists of a copper conductor layer and two low cost dielectric layers. The bump geometry consists of the eutectic solder, the copper core, and the under bump metallurgy. Nonlinear time-temperature-dependent finite element analyses are performed to determine the shear stress, shear creep strain, shear stress and shear creep strain hysteresis loops, and creep strain energy density of the corner solder joint. The thermal-fatigue life of the corner solder joint is then predicted by the averaged creep strain energy density range per cycle and a linear fatigue crack growth rate theory. The WLCSP solder bumps are also subjected to shear test. Finally, the WLCSP solder joints are subjected to both mechanical shear and thermal cycling tests.
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In this paper, the optimization of Cu chemical-mechanical polishing (CMP) performance (dishing) for the removal of thick Cu-plating overburden due to Cu plating for deep through silicon via (TSV) in a 300-mm wafer is investigated. Moreover, backside isolation oxide CMP for TSV Cu exposure is examined. To obtain a minimum Cu dishing on the TSV region, a proper selection of Cu slurries is proposed for the current two-step Cu-polishing process. First, a bulk of Cu is removed with the slurry of high Cu removal rate and second, the Cu surface is planarized with the slurry of high Cu passivation capability. The Cu dishing can be improved up to 97% for the 10-μm\mu{\rm m}-diameter TSVs on a 300-mm wafer. The dishing/erosion of the metal/oxide can be reduced with respect to a correspondingly optimized Cu-plating overburden for TSVs and redistribution layers. Cu metal dishing can be drastically reduced once the Cu overburdens are increased to a critical thickness. For backside isolation oxide CMP for TSV Cu exposure, the results show that the Cu studs of TSVs with a larger TSV diameter still keep in a plateau-like shape after CMP.