360
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 7, NO. 4> APRIL 1995
GaAs MQW Modulators Integrated
with Silicon CMOS
K. W. Goossen, Member, IEEE, J. A. Walker, L. A. D’Asaro, Lije Senior Member, IEEE, S. P. Hui,
B. Tseng, R. Leibenguth, D. Kossives, D. D. Bacon, D. Dahringer, L. M. F. Chirovsky,
A. L. Lentine, Member, IEEE, and D. A. B. Miller, Fellow, IEEE
Abstract—We
demonstrate integration of GaAs–AIGaAs multi-
ple quantum well modulators to silicon CMOS circuitry via flip-
chip solder-bonding followed by substrate removal. We obtain
$J.5~0 device yield for 32 x 32 arrays of devices with 15 micron
solder pads. We show operation of a simple circuit composed of
a modulator and a CMOS transistor.
I. INTRODUCTION
F
OR many years now a much desired goal of those working
on optical interconnects and optical computing has been
the integration of high density silicon electronics with high
performance GaAs-based optoelectronics. In particular, the
possibility of direct optical communication to logic chips
has stimulated much work on photonic switching [1]. The
most desirable product is one where the silicon circuitry is
state-of-the-art, and unaffected by the integration with the
optoelectronics. For this reason flip-chip solder bonding to
finished silicon chips has been pursued [2]. Furthermore,
modulators, which can be fabricated in densities of thousand
per chip [3], are the preferred optoelectronic component in
many
systems such as in [1]. Finally, GaAs–AIGaAs multiple
quantum well modulators operating at 850 nm offer the high-
est performance compared to longer wavelength modulators
[4], [5].
In [6], we demonstrated that the GaAs substrate could
be removed after flip-chip bonding, allowing operation at
850 nm. This procedure of bonding, followed by substrate
removal, has been explored in detail by us, and here we
present its application to silicon CMOS, thus fulfilling the
above-stated goal. We demonstrate here a 99.99?0 bond yield
with a steadily improving
%~o device yield. Furthermore, all
aspects of this procedure appear to fit within a manufacturable
scheme, with no thin-film handling required as in epitaxial
lift-off [7]. We have even demonstrated that completed chips
can be sawed without damage, allowing batch fabrication of
many chips
at once. In [6], the devices operated at high optical
intensity (80 kW/cm2), a huge thermal flux and electrical
current density, showing excellent heat-sinking and ohmic
contact. The device was thermally cycled from 300 C to
Manuscript received November 1, 1994; revised December 9, 1994.
K. W. Goossen, J. A. Walker, and D. A. B. Miller are with AT&T Bell
Laboratories, Holmdel, NJ 07733 USA.
L. A, D’Asaro, S. P. Hui, B. Tseng, R. Leibenguth, D. Kossives,
D. Dahringer, and L. M. F. Cbirovsky are with AT&T Bell Laboratories,
Murray Hill, NJ 07974 USA.
A. L. Lentine is with AT&T Bell Laboratories, Naperville, IL 60566 USA.
IEEE Log Number 9409250.
‘.
,,
Pb/Sn+
h
& Al:Ti/Pt/Au
Fig. 1. Three-step hybridization process: 1) Fabrication, aligning, and bond-
ing of modulator chip on silicon chip, 2) Flowing etch-protestant between
chips, which is allowed to harden. 3) Removal of GaAs substrate using
jet etcher, and deposition of AR coating. The epoxy can be removed after
substrate removal, as desired.
1000 C over a hundred times, and it showed no degradation,
showing the practicality of the technique.
The fabrication procedure is outlined in Fig. 1. Modulators
are produced in the GaAs chip whose n and p contacts are
coplanar. In [6] this was accomplished by depositing thick
gold over the bottom contact. Here we employ implantation
[8]. Lead-tin is deposited on these for a solder using pho-
tolithography. The silicon chips are obtained from the MOSIS
foundry facility. The chip have 1.2 micron linerules. Mating
aluminum pads from the modulators are designed on those
chips, and a Ti–Pt–Au layer is deposited on them (in our lab)
to provide a solder-wettable surface. A precision bonder made
by Research Devices in Piscataway, NJ was employed to bond
the chips together. Two-micron accuracy is routine.
A key feature of the technique for tlip-chip bonding then
substrate removal is the etching of outer mesas around the
devices into the substrate. Then, when the substrate is removed
by applying a chemical stream to it (that stops on the AIGaAs
‘stop-etch layer), isolated devices will be left. This is desirable
since, if the stop etch layer was left extending over the
whole chip, slight warpages would cause it to break, possibly
damaging the modulators. This procedure requires placing
something between the mesas so that the substrate etchant does
not attack the front faces of the chips. The substrate etchant,
100:1 H202 :NH40H, does not attack Si or Al appreciably.
1041-1 135/95$04.00 @ 1995 IEEE
GOOSSEN etal.: GaAs MQW MODULATORS INTEGRATED WITH SILICON CMOS
361
Fig. 3.
Fig. 2. Photo of integrated GaAs modulators on silicon CMOS. Results on
the transistor modulator circuit (near bottom of photo) are presented here.
Results on the complex circuits will be reported later.
However, it would attack the GaAs regions of the modulators.
To protect the front faces of the chips, a silica-filled epoxy was
flowed between the chips and allowed to harden, as shown in
the middle pictorial of Fig. 1. This was done by depositing a
bead of the epoxy on the side of the GaAs substrate using a
optical fiber manipulated by a precision stage. The epoxy then
wicked neatly between the chips. The chip is heated to 1000 C
to reduce the viscosity of the epoxy so that it flows between the
chips more easily. It is possible to meter the amount of epoxy
so that it just fills the volume between chips. The epoxy is
then cured by baking the chip at 100 “C for one hour. Epoxies
have been used previously in this manner in flip-chip bonded
assemblies to provide hermetic sealing and increase robustness
[9]. For those applications the epoxy is termed an encapsulant,
or underfill. Here we call it an interchip flowable hardener, to
express the added function of providing a surface between the
chips that is impenetrable by the substrate etch. The epoxy can
be removed after substrate removal by applying a dry plasma
etch using 5:1 02 :CF4 flow rates.
In these devices, a Ti–Au pad, place next to the n ohmic
contact, is used as an integral reflector. We have previously Fi~ A,
GaAs modulator test array on silicon.
Best section of LED {forward-biased modulator) array with 98/99
demonstrated that modulators such as these using pure Au
pads have performance equal to the best monolithic GaAs
modulators [10]. Here the Ti was added to provide better
sticking of the Au and so improve yield. Unfortunately our
Ti–Au only has about 40% reflectivity, so the modulators here
have marginal performance. We are developing schemes to use
pure Au reflectors with good adhesion.
We have fabricated CMOS chips with switching node
electronics (Fig. 2). Results on the switching nodes will be dis-
cussed in a later paper. Here we discuss device performance,
consisting of three tests: n-ohmic bond test arrays, LED device
working devices,
test arrays (forward-biased modulators), and simple circuits
(near bottom, Fig. 2; inset, Fig. 6). Our n-ohmic bond testers
consisted of daisy-chains of devices with only n-contacts. For
these we obtained 99.9470 bond yield for 15x 15 micron solder
pads (Table I). However, our LED test arrays had only 9590
device yield (Fig. 4). We have attributed this to an observable
intermetallic reaction that occurs between the solder and the
P-tYPe metal during solder reflow (melting), which is shown
in Fig. 6, This reaction is visible in about half the devices
362
Fig. 5
scope
Two bonded devices viewed through substrate with infrared mi
The “tab’-shaped metal is the pohmic contact. The device on the
mo-
Ieft
sho-ws no degradatio-n. The device on the right shows a reaction with the
solder. There is a strong correlation with this observation and a failed device.
We are examining methods of bonding without reflow to avoid this effect.
I
1
1
1
1
!
I
0.27-
0.20-
~ 0.18-
:?
%
$ 0.16-
vD~=lov
0.14-
Fig, 6, Reflectivity of modulator in inset circuit versus gate-source voltage,
showing electrical integration. Modulation is degraded compared to earlier
devices using Au as a reflector (here T1–Au is used).
with an infrared microscope. We have measured that if an
LED is dark, there is a 96% probability that it also exhibits
the intermetallic reaction. The reaction could be avoided by
not performing reflow. However, it is during reflow, which is
performed in a solder flux, when the solder oxide is removed.
We have attempted bonding without reflow, by subjecting the
chips to a plasma before bonding to remove the oxide. We have
obtained sections of arrays as large as 12 x 42 with uniform
illumination of all devices, but the results are still incomplete.
Finally, we show here a simple CMOS-modulator circuit
(inset, Fig. 6). This circuit is shown on the bottom of the
photo in Fig. 2. By charging the gate of the transistor, the
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 7, NO. 4, APRIL 1995
TABLE I
YIELD OF ARRAYS OF GaAs DEVICES WITH Two
n-OHMIC CONTACTS SOLDER-BONDED TO SILICON
pad size a
(microns)
rraysize bond yield
transistor turns on and the modulator is biased. In Fig. 6 we
show the turn-on characteristic. The design gate threshold of
this transistor is about one volt. The turn-on of the modulator
at 200 mV is consistent with this since the modulator had only
nanowatts of optical power on it, so required only subthreshold
operation of the transistor.
II.
CONCLUSION
We have demonstrated a practical method of integrating
GaAs modulators onto silicon circuits via flip-chip bonding,
followed by substrate removal. We obtain 95% device yield,
and indicate that this can improve to
99.9~0. We have demon-
strated a simple transistor-modulator circuit to prove viability.
More complex circuits will be reported at a later date.
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
REFERENCES
A. L. Lentine, R. A. Novotny, T. J. Cloonan, L. M. F. Chirovsky,
L. A. D’Asaro, G. Livescu, S. Hui, M. W. Focht, J. M. Freund, G. D.
Guth, R. E. Leibenguth, K. G. Glogovsky, and T. K. Woodward, “4x 4
arrays of FET-SEED embedded control 2
x 1 optoelectronic switching
nodes with electrical fan-out,”
IEEE Photon. Technol. Lett., vol. 6, p.
1126, 1994.
J. Wieland, H. Melchior, M. Q. Kearley, C. Morris, A. J. Moseley,
M. G. Goodwin, and R. C. GoodfelIow, “Optical receiver array in
silicon bipolar technology with self-aligned, low parasitic 111/Vdetectors
for DC-1 Gbit/s parallel links,”
Electron. Lett., vol. 27, p. 2211, 1991.
A. L. Lentine, F. B. McCormick, R. A. Novotny, L. M. F. Chirovsky,
L, A. D’Asaro, R. F. Kopf, J. M. Kuo, and G. D. Boyd, “A two
kbit array of symmetric self-electro-optic effect devices,” IEEE Photon.
Technol.-Lett.,_ vol. 2, p. 51, 1990. “
K. W. Goossen, M. B. Santos, J. E. Cunningham, and W. Y. Jan,
“Independence of absorption coefficient-linewi~th product to material
system for multiple quantum wells with excitons from 850 nm to 1064
rim,” IEEE Photon. Technol. Lett., vol. 5,
p. 1392, 1993.
R. N. Pathak, K, W. Goossen, J. E. Cunningham, and W. Y. Jan, “ln-
GaAs–InP 17-i(MQW)-n surface normal electroabsorption modulators
exhibiting better than 8:1 contrast ratio for 1.55 pm applications grown
by gas-source MBE,” IEEE Photon. Technol. Lett., vol. 6, no 12, pp.
1439–1441, 1994.
K. W. Goossen, J. E. Cunningham, and W. Y. Jan, “GaAs 850 nm
modulators soldel.-bonded to silicon, ”
IEEE Photo=. Techml. Lett. , vol.
5, p. 776, 1993.
C. Camperi-Ginestet, M. Hargis, N. Jokerst, and M. Allen, “AlignabIe
epitaxial liftoff of GaAs materials with selective deposition using
polyimide diaphragms,” IEEE Photam Technol. Lerr., vol. 3, p. 1123,
1991.
L. A. D’Asaro, L. M. F. Chirovsky, E. J. Laskowski, S. S. Pei,
T. K. Woodward, A. L, Lentine, R. E. Leibenguth, M. W. Focht,
J, M. Frermd, G. G. Guth, and L. E. Smith, “Batch fabrication and op-
eration of GaAs–AIGaAs field-effect transistor-self-electro-optic effect
device (FET-SEED) smart pixel array s,” IEEE J. Quantum Electron.,
VOI. 29, p. 670, 1993.
G. O’Malley, J. Giesler, and S. Machuga,
“The importance of material
selection for flip chip on board assemblies,” IEEE Trans. Components,
Hybrids, A4arzufacturing Techno/., part B, vol. 17, p. 248, 1994.
K. W. Goossen,
“Integration of GaAs modulators to silicon IC’S
by monolithic and hybrid techniques,” in Prvc. 1994 IEEE Sarnoff
Symposium, section IIc.