high-power high-linearity flip-chip bonded modified uni...

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

*[email protected]

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

1. K. J. Williams, L. T. Nichols, and R. D. Esman, “Photodetector nonlinearity limitations on a high-dynamic range

3 GHz fiber optic link,” J. Lightwave Technol. 16(2), 192–199 (1998).

2. P.-L. Liu, K. J. Williams, M. Y. Frankel, and R. D. Esman, “Saturation characteristics of fast photodetectors,”

IEEE Trans. Microw. Theory Tech. 47(7), 1297–1303 (1999).

3. T. Ishibashi and N. Shimizu, “Uni-traveling-carrier photodiodes,” in Ultrafast Electron. Optoelectron. ’97 Conf.,

Incline Village, NV (1997).

4. Z. Li, H. Pan, H. Chen, A. Beling, and J. C. Campbell, “High-saturation-current modified uni-traveling-carrier

photodiode with cliff layer,” IEEE J. Quantum Electron. 46(5), 626–632 (2010).

5. F.-M. Kuo, M.-Z. Chou, and J.-W. Shi, “Linear-cascaded near-ballistic unitraveling-carrier photodiodes with an

extremely high saturation current-bandwidth product,” J. Lightwave Technol. 29(4), 432–438 (2011).

6. S. Itakura, K. Sakai, T. Nagatsuka, E. Ishimura, M. Nakaji, H. Otsuka, K. Mori, and Y. Hirano, “High-current

backside-illuminated photodiode array module for optical analog links,” J. Lightwave Technol. 28(6), 965–971

(2010).

7. J. Christofferson, K. Maize, Y. Ezzahri, J. Shabani, X. Wang, and A. Shakouri, “Microscale and nanoscale

thermal characterization techniques,” J. Electron. Packag. 130(4), 041101 (2008).

8. H. Ito, S. Kodama, Y. Muramoto, T. Furuta, T. Nagatsuma, and T. Ishibashi, “High-speed and high-output InP-

InGaAs unitraveling-carrier photodiodes,” IEEE J. Sel. Top. Quantum Electron. 10(4), 709–727 (2004).

9. H. Pan, Z. Li, A. Beling, and J. C. Campbell, “Characterization of high-linearity modified uni-traveling carrier

photodiodes using three-tone and bias modulation techniques,” J. Lightwave Technol. 28(9), 1316–1322 (2010).

10. A. Beling, H. Pan, H. Chen, J. C. Campbell, A. Hastings, D. A. Tulchinsky, and K. J. Williams, “Impact of

voltage-dependent responsivity on photodiode non-linearity,” in Proc. LEOS 2008, Newport Beach, CA, Nov.

2008, pp. 157–158.

11. Y. Fu, H. Pan, Z. Li, A. Beling, and J. C. Campbell, “Characterizing and modeling nonlinear intermodulation

distortions in modified uni-travelling carrier photodiodes,” IEEE J. Quantum Electron. 47(10), 1312–1319

(2011).

12. H. Pan, A. Beling, H. Chen, J. C. Campbell, and P. D. Yoder, “The influence of nonlinear capacitance on the

linearity of a modified unitraveling carrier photodiode,” in Proc. MWP 2008, Gold Coast, Australia, Oct. 2008,

pp. 82–85.

13. D. A. Tulchinsky, J. B. Boos, D. Park, P. G. Goetz, W. S. Rabinovich, and K. J. Williams, “High-current

photodetectors as efficient, linear, and high-power RF output stages,” J. Lightwave Technol. 26(4), 408–416

(2008).

14. T. Ohno, H. Fukano, Y. Muramoto, T. Ishibashi, T. Yoshimatsu, and Y. Doi, “Measurement of intermodulation

distortion in a uni-traveling carrier refracting-facet photodiode and a p-i-n refracting-facet photodiode,” IEEE

Photon. Technol. Lett. 14(3), 375–377 (2002).

15. M. Chtioui, A. Enard, D. Carpentier, S. Bernard, B. Rousseau, F. Lelarge, F. Pommereau, and M. Achouche,

“High-power high-linearity uni-traveling-carrier photodiodes for analog photonic links,” IEEE Photon. Technol.

Lett. 20(3), 202–204 (2008).

#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


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%.


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


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].


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.


high-power high-linearity flip-chip bonded modified uni...

Documents