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High-power high-linearity flip-chip bonded
modified uni-traveling carrier photodiode
Zhi Li,1,*
Yang Fu,1 Molly Piels,
2 Huapu Pan,
1 Andreas Beling,
1 John E. Bowers,
2 and
Joe C. Campbell1
1Department of Electrical and Computer Engineering, University of Virginia, 351 McCormick Road, Charlottesville,
VA 22904, USA 2Department of Electrical and Computer Engineering, University of California, Santa Barbara, CA 93106, USA
Abstract: We demonstrate a flip-chip bonded modified uni-traveling carrier
(MUTC) photodiode with an RF output power of 0.75 W (28.8 dBm) at 15
GHz and OIP3 as high as 59 dBm. The photodiode has a responsivity of 0.7
A/W, 3-dB bandwidth > 15 GHz, and saturation photocurrent > 180 mA at
11 V reverse bias.
©2011 Optical Society of America
OCIS codes: (040.5160) Photodetectors; (230.5170) Photodiodes.
References and links
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#155839 - $15.00 USD Received 3 Oct 2011; revised 30 Oct 2011; accepted 1 Nov 2011; published 18 Nov 2011(C) 2011 OSA 12 December 2011 / Vol. 19, No. 26 / OPTICS EXPRESS B385
1. Introduction
High-power, high-linearity photodiodes are essential components for photonic microwave
applications since they have the potential to improve many aspects of the link performance
such as link gain, noise figure, and spurious free dynamic range. With respect to linearity, the
output third-order intercept point (OIP3) is widely accepted as a key figure of merit to
characterize nonlinear distortions in microwave and photonic devices [1]. At present the RF
output power is limited by two primary factors, saturation originating from the space-charge
effect [2] and heat-induced catastrophic failure. Uni-traveling carrier (UTC) photodiodes [3]
have mitigated the space charge effect and demonstrated improved high-power performance
relative to p-i-n photodiodes while maintaining high speed and good linearity. The advantage
of the UTC structure is that only electrons, which have higher saturation velocity than holes,
are employed as active carriers in the drift/collection region. Modified uni-traveling carrier
(MUTC) photodiodes have been developed to further enhance space charge tolerance by
incorporating a “cliff” layer to control the relative electric field strength in the absorber and
collector regions [4]. This approach achieved a maximum photocurrent of 152 mA at 6-V
reverse bias and 18-GHz bandwidth with a 40-µm-diameter backside-illuminated MUTC
photodiode. In this device heat generated in the active-region mesa is mostly dissipated into
the substrate and, to some extent, into air. The light is coupled through the substrate, which
has an anti-reflection coating on the input interface. The maximum output power achieved by
MUTC photodiodes with a cliff layer was not limited by saturation but by thermal failure.
Higher RF output power necessitates improved thermal management. Among the various
techniques to improve thermal dissipation, flip-chip bonding has achieved the best results.
Kuo et al. reported a cascaded two-diode photodetector flip-chip bonded onto AlN with an
output power of 63 mW at 95 GHz [5]. Itakura et al. have demonstrated a maximum output
power of 790 mW at 5 GHz using a flip-chip bonded 4-diode array with a monolithically
integrated Wilkinson power combiner circuit on AlN [6]; the measured output third order
intercept point at 4.95 GHz was 32.5 dBm.
In this paper, a thermal-reflectance imaging method was used to characterize the surface
temperature of conventional backside-illuminated MUTC photodiodes. These measurements
enabled calibration of simulation parameters, which were used to generate two-dimensional
plots of the temperature distribution inside the photodiode. The thermal modeling predicted
that flip-chip bonding to AlN would result in ~70% improvement in the thermal limit. This
was verified experimentally. A 40-µm-diameter photodiode achieved 0.7 A/W responsivity
(1540 nm), 15 GHz 3-dB bandwidth, and 0.75 W RF output power. The OIP3 at 330 MHz
was 59 dBm at 140 mA photocurrent and remained as high as 40 dBm at 15 GHz and 160
mA.
2. Thermal imaging and simulation
To investigate the thermal characteristics of the conventional backside-illuminated photodiode
[4], the temperature profile on the surface of the Au contact area was measured by thermo-
reflectance imaging [7]. A top-view photograph and a reconstructed thermal image of a 34-
µm photodiode are shown in Figs. 1(a) and 1(b), respectively. The experimental setup
measures the change in surface reflectivity using pulses from a green LED array (λ = 530
nm), timed to coincide with the end of the device heating cycle. The reflectivity of the surface
is a function of temperature and, thus, the change in reflectivity can be used to obtain the
change in temperature. In our measurement, the diode was placed on a thermoelectric cooler
maintained at 15°C. A 2-mm-diameter hole in the center of the cooler permitted back-
illumination through the InP substrate. The optical input signal was modulated at 400 Hz with
10% duty cycle and the optical illumination was carefully adjusted to ensure a stable output
#155839 - $15.00 USD Received 3 Oct 2011; revised 30 Oct 2011; accepted 1 Nov 2011; published 18 Nov 2011(C) 2011 OSA 12 December 2011 / Vol. 19, No. 26 / OPTICS EXPRESS B386
Fig. 1. (a) The layout image and (b) thermal image of 34-µm backside-illuminated MUTC
photodiode.
photocurrent of 40 mA for each bias level. Adjusting the bias voltage varied the amount of
heat generated in the active area. Figure 2(a) shows the measured surface temperature of 34-
and 40-µm-diameter backside-illuminated photodiodes versus power dissipation. The imaging
measurements were carried out with increasing power dissipation until the devices failed;
catastrophic failure occurred when the surface temperature was ~500 K. The relatively low
surface temperature at failure can be attributed to lateral inhomogeneities, which create “hot
spots”, as shown in Fig. 1(b). A simulation model was created using the finite element
analysis tool ANSYS. The model structure follows the epitaxial structure layout of MUTC2 in
[4]. The backside of the diode was set at 15°C and the environment temperature was set at
23°C to simulate the same environmental conditions as the measurement. As shown in Fig.
2(a) good agreement with the measurements was achieved. The parameters obtained from
thermal imaging were incorporated into the two-dimensional model.
Fig. 2. (a) Measured and simulated surface temperatures of 34- and 40-µm-diameter backside-
illuminated photodiodes under different heat generation levels. (b) Simulated surface
temperature distribution of 40-µm-diameter photodiode with 680 mW heat flow.
It was found that catastrophic thermal failure occurs when the core temperature of a 40-
µm photodiode is ~470 K with dissipated power of 680 mW as shown in Fig. 3(a). For flip-
chip bonding to an AlN substrate through 8-µm thick gold vias the core temperature was
reduced to 370 K for the same operating conditions, Fig. 3(b). In order to reach the same
failure core temperature of 470 K, the dissipated power of the flip-chip device can increase to
1.15 W, which means the flip-chip bonded photodiode shows a thermal limit enhancement of
70%.
#155839 - $15.00 USD Received 3 Oct 2011; revised 30 Oct 2011; accepted 1 Nov 2011; published 18 Nov 2011(C) 2011 OSA 12 December 2011 / Vol. 19, No. 26 / OPTICS EXPRESS B387
Fig. 3. Simulated vertical cross section temperature distribution of a 40-µm MUTC
photodiode: (a) conventional back-illuminated structure, (b) flip-chip bonded on AlN substrate
with 680 mW power dissipation.
3. Photodiode structure
The epitaxial layer structure of the active diode structure was grown by MOCVD on semi-
insulating InP substrate. The detailed diode structure can be found in [4] labeled as MUTC2.
The diameter of the diode is 40 µm and the total depletion thickness is 1.18 µm. The wafer
was fabricated into back-illuminated double-mesa structures via ICP etching. A 250 nm-thick
SiO2 film was deposited on the backside as anti-reflection coating. Gold contact vias were up-
plated on p- and n- mesas to serve as electrical contacts and heat dissipation paths. The wafer
was cut into 2.8 mm × 1.2 mm chips and flip-chip bonded onto a gold contact pad circuit on
0.5-mm thick AlN. The finished flip-chip photodiode cross section and SEM picture are
shown in Figs. 4(a) and 4(b), respectively.
Fig. 4. (a) Cross-sectional schematic view of the photodiode. (b) SEM picture of the bonded
chip on probe pads.
4. Measurement results
The frequency tunable optical input was obtained from two equal power heterodyned DFB
lasers operating near 1540 nm. The modulation depth was 100% and the frequency was swept
by tuning the temperature of one of the lasers. The measured responsivity was 0.7 A/W at 1-
mA photocurrent and 5-V reverse bias. Figure 5(a) shows the frequency responses of a 40-
µm-diameter flip-chip bonded photodiode under 5 V reverse bias and various photocurrent
levels. The 3-dB bandwidth was >15 GHz when the photocurrent was greater than 40 mA.
The bandwidth increased with increasing photocurrent, which can be attributed to carrier
acceleration by the enhanced self-induced field in the gradated p-type absorber [8]. The S-
parameter S11 of the 40-µm flip-chip bonded photodiode at various bias levels is plotted in
#155839 - $15.00 USD Received 3 Oct 2011; revised 30 Oct 2011; accepted 1 Nov 2011; published 18 Nov 2011(C) 2011 OSA 12 December 2011 / Vol. 19, No. 26 / OPTICS EXPRESS B388
Fig. 5(b). The extracted capacitance of 210 fF at 5-V reverse bias is 50 fF higher than a
backside-illuminated photodiode of the same diameter. The extra capacitance is due to the
relatively large pad surface area in the bonding region that was employed for alignment
tolerance. The series resistance from S11 measurement is only 2 Ω.
Fig. 5. (a) Frequency responses of 40-µm flip-chip bonded photodiode under various
photocurrent conditions at 5-V reverse bias. (b) S-parameter S11 of 40-µm flip-chip MUTC2
photodiode at various bias levels.
Figure 6(a) shows the measured output RF power versus average photocurrent at 15 GHz
under various reverse bias conditions. The space-charge-limited saturation effect has been
greatly mitigated through the cliff layer structure [4], and with improved thermal dissipation,
the 40-µm diameter photodiode was able to operate under a high reverse bias of 11 V and 180
mA photocurrent without saturation. The RF power from a single photodiode at 15 GHz was
0.75 W (28.8 dBm). Figure 6(b) shows the maximum output power versus frequency for
different operating conditions, 5V/130mA to 11V/180mA.
Fig. 6. (a) Measured RF power versus average photocurrent at 15 GHz, under various bias
levels. (b) Maximum output power at various bias levels versus frequency.
We define the dissipated power in the photodiode as the product (V*Iavg), where V is the
applied reverse bias voltage and Iavg is the average photocurrent. With V = 11 V and Iavg = 180
mA it follows that the power dissipated by the photodiode is 1.98 W. Recall that the
maximum power dissipated by the backside-illuminated photodiode of equal size was only
0.91 W [4].
#155839 - $15.00 USD Received 3 Oct 2011; revised 30 Oct 2011; accepted 1 Nov 2011; published 18 Nov 2011(C) 2011 OSA 12 December 2011 / Vol. 19, No. 26 / OPTICS EXPRESS B389
The OIP3 of the photodiode was measured with a 3-tone apparatus [9]. Figure 7 shows the
dependence of OIP3 on photocurrent under various bias conditions at 330 MHz and 15 GHz.
It has become standard to report OIP3 in terms of 2-tone measurements. Hence, the data in
Fig. 7 has been converted to equivalent 2-tone OIP3 [9]. Two significant OIP3 peaks were
observed with bias > 7 V. The OIP3 peaks can be explained by the third-order
intermodulation distortion products (IMD3) trough caused by the Franz-Keldysh effect [10],
which impacts the voltage-dependent responsivity [11]. OIP3 peaks higher than 59 dBm are
present at photocurrents of 55 mA and 141 mA at 9V, the higher current peak corresponds to
an output RF power of more than 400 mW at 15 GHz. It has previously been shown that at
higher frequencies the OIP3 degrades primarily as a result of the nonlinear voltage-dependent
capacitance [12]. However, since we were able to operate the photodiode at a high reverse
bias of 9 V, the minimum OIP3 at 15 GHz remained above 30 dBm up to average
photocurrents of 170 mA. The OIP3 maximum of 40 dBm near 160 mA suggests a partial
compensation of the nonlinear capacitive effect by the aforementioned Franz-Keldysh effect
and/or impact ionization. The high OIP3 values up to photocurrents of 170 mA at both low
and high frequencies compare favorably with previous reports of 39.5 dBm at 1 GHz (100
mA) [13], 40 dBm at 5.8 GHz (22 mA) [14], and 35 dBm at 20 GHz (40 mA) [15].
Fig. 7. OIP3 of 40-µm MUTC PD versus photocurrent at various bias levels measured (a) at
330 MHz and (b) at 15 GHz.
5. Conclusions
A MUTC photodiode flip-chip bonded on AlN substrate has been demonstrated. Thermal
imaging and simulation were utilized to characterize thermal failure and implement improved
heat dissipation. The responsivity was 0.7 A/W and the 3-dB bandwidth was 15 GHz.
Saturation current > 180 mA was measured for reverse bias greater than 9 V leading to an
output power of 0.75 W at 15 GHz. At 330 MHz, a high OIP3 > 59 dBm was obtained at 140
mA, while the OIP3 at 15 GHz was 40 dBm at a photocurrent of 160 mA.
Acknowledgments
This work was supported by DARPA through the TROPHY and PICO programs and the
Naval Research Laboratory. The authors thank Keith Williams for numerous enlightening
discussions.
#155839 - $15.00 USD Received 3 Oct 2011; revised 30 Oct 2011; accepted 1 Nov 2011; published 18 Nov 2011(C) 2011 OSA 12 December 2011 / Vol. 19, No. 26 / OPTICS EXPRESS B390