Pd-Based Integrated Optical Hydrogen Sensor on a Silicon-On-Insulator Platform

1428 OPTICS LETTERS / Vol. 38, No. 9 / May 1, 2013

Pd-based integrated optical hydrogen sensor on a silicon-on-insulator platform

M. Z. Alam, Nicholas Carriere,* Farshid Bahrami, Mo Mojahedi, and J. Stewart Aitchison Edward S. Rogers Sr. Department of Electrical and Computer Engineering, University of Toronto, 10 King’s College Road, Toronto, Ontario M5S 3G4 Canada *Corresponding author: [email protected] Received February 20, 2013; accepted March 12, 2013; posted April 1, 2013 (Doc. ID 185610); published April 24, 2013 We have experimentally demonstrated a compact, integrated optical hydrogen sensor on a silicon-on-insulator plat- form. The sensor consists of silicon waveguide coated with a thin palladium film. The sensitivity and response time of the sensor was tested for volumetric hydrogen concentrations ranging from 0% to 4%. The proposed hydrogen sensor shows great potential as a building block for an optical nose capable of simultaneous detection of multiple gases as well as environmental effects such as temperature and humidity. © 2013 Optical Society of America OCIS codes: (130.6010) Sensors; (130.3120) Integrated optics devices; (280.4788) Optical sensing and sensors. http://dx.doi.org/10.1364/OL.38.001428

Hydrogen gas has numerous applications in many indus- The sensor was tested with hydrogen concentrations vary- tries including oil and gas refineries, semiconductor, ing from 0% to 4%. The temporal response of the sensor aerospace, and fuel cells. Being a very small molecule, was obtained and the sensitivity and response time of hydrogen can easily leak through fissures and micro- the device were measured as a function of hydrogen cracks. In addition, hydrogen is colorless, odorless, and concentration. forms an explosive mixture with air for a wide range of Figure 1(a) shows the schematic of the proposed hy- concentrations. Hydrogen leaks therefore pose a serious drogen sensor. It consists of a straight ridge waveguide safety challenge for many applications and there is an fabricated on an SOI wafer. Part of the waveguide is immediate need for a fast and reliable hydrogen sensor coated with a thin palladium (Pd) film. When the sensor for safe storage, handling, and use of hydrogen. is exposed to hydrogen, molecular hydrogen (H2) disso- Currently, many different technologies for hydrogen ciates into atomic hydrogen (H) and diffuses into the Pd detection are available including catalytic, electrochemi- film, converting it to palladium hydride (PdHx)[5]. When cal sensors and semiconductor hydrogen sensors [1]. palladium is hydrated, it expands and the volume density Unfortunately, these sensors have a number of disadvan- of free electrons is decreased. This changes both the real tages. For example, catalytic bead sensors, the most and imaginary parts of the complex permittivity of the ε ˆ h ε widely used hydrogen sensors, are prone to false alarms material [5], which can be modeled as PdH × Pd in the presence of various hydrocarbons. Electrochemical where h is a nonlinear function whose value depends sensors are bulky, expensive, and require the presence of on several factors, including hydrogen concentration, oxygen for their operation. Metal-oxide semiconductor temperature, and humidity. Therefore, upon exposure based hydrogen sensors are saturated at very low hydro- gen concentration and are not suitable for use as safety sensors. Silicon Optical hydrogen sensing offers a number of advan- L Palladium tages over competing technologies including compact d size, increased safety, and immunity from electromag- W Silica netic interference. Silicon-on-insulator (SOI) is an excel- T lent platform for implementation of optical sensors as H it is easy to mass fabricate using CMOS fabrication technology, it is very compact, and it has the potential (a) to integrate electronics and photonics on the same platform. Moreover, by using an integrated SOI platform, the number of sensing elements on a single chip could be increased to create an optical nose. Despite these advan- tages there have been very few reports on SOI based op- tical hydrogen sensors [2–4]. In this Letter an integrated optical hydrogen sensor on SOI with a thin palladium (Pd) overlay is proposed and developed. The presence of hydrogen and its concentration is measured by monitor- (b) ing the change of light intensity at a single wavelength, and unlike [4], where the change in transmission spectrum is measured, the intensity measurement at a single wave- Fig. 1. (a) Schematic of the proposed hydrogen sensor. length does not require the use of a spectrum analyzer. (b) SEM image of the fabricated sample showing a number The proposed sensor is therefore simpler to implement. of silicon waveguides with Pd overlays. 0146-9592/13/091428-03$15.00/0 © 2013 Optical Society of America May 1, 2013 / Vol. 38, No. 9 / OPTICS LETTERS 1429

4% 4% to hydrogen, the intensity of the light travelling through 4% H H 2 20 H 2 the waveguide changes. This intensity modulation forms 2 the basis of the sensor. The sensor in use was fabricated on an SOI wafer with 15 a 220 nm thick top silicon layer separated from the silicon substrate by a 3 μm thick buried oxide layer. Reactive 10 ion etching using CHF3 and O2 was used to define ridge N N waveguides. The final waveguide dimensions were 5 N 2 2 2 W ˆ 2 μm, d ˆ 70 nm. A double layer lift-off process, de- N 2

scribed in Table 2 of [6], was used to deposit 13 nm thick (%) transmission in Change 0 Pd films of various lengths on the silicon waveguides as shown in Fig. 1(b). All the results presented in this work 0 200 400 600 800 1000 1200 are for a silicon waveguide covered with a 10.79 μm long Time (sec) Pd film like the one shown in the lower right corner of Fig. 3. Response of the hydrogen sensor to cycles of nitrogen Fig. 1(b). and 4% hydrogen. The optical set up used for testing the sensor is shown in Fig. 2. Light from a JDS Uniphase tunable laser tuned at 1.55 μm wavelength was coupled in and out of the sen- can be seen in Fig. 3, the transmission changed more than sor using 60X objectives lenses. Since in general, TM 16% in presence of 4% hydrogen. After the system was modes supported by ridge waveguides suffer from signifi- flushed with nitrogen gas, the results were repeatable cant leakage loss [7], the experiment was done only for with a small amount of baseline drift. This drift can be the TE mode. The end fire coupling rig supporting the explained by the fact that the alignment in the end-fire sample and objective lenses were covered by a polycar- coupling rig gradually shifted over time. bonate box of dimensions 38.5 cm × 18.5 cm × 12.5 cm. The Pd–H system can exist in two different phases: To minimize the effect of noise, two SR830 lock-in ampli- the α phase and the β phase [5]. At low hydrogen concen- fiers were used for input and output signal detection. tration, the Pd lattice is in the α phase, and for very high To control the gas composition inside the box contain- hydrogen concentration the lattice completely trans- ing the sample, the gas flow from two cylinders contain- forms into β phase. For an intermediate range of concen- ing compressed nitrogen and 4% hydrogen (balanced by trations, the two phases coexist, and a sharp change 96% nitrogen) were controlled by two Brooks 5850S dig- in transmission is observed as the Pd film undergoes a ital mass flow controllers using a Brook 0154 control phase transformation. Eventually, the lattice is com- unit. The individual flow rates were computer controlled pletely transformed into the β phase with a correspond- and the outputs led to a T-junction to mix the gases to the ing decrease in sensitivity. To test the magnitude of the desired hydrogen concentration. The total gas flow rate response of the sensor to varying hydrogen concentra- throughout the experiment was two liters per minute and tions, the sensor was exposed to increasing concentra- all the measurements were carried out at room temper- tions of hydrogen (from 0% to 4% in increments of 0.5%). ature (approximately 23°C). The output of the T-junction After each change in concentration, the transmission was then led to a union connection on the box where it was allowed to stabilize before recording its value. The re- connected to a stainless steel tube that directed the gas sults of this test can be seen in Fig. 4. flow directly onto the sample. A commercial hydrogen The response time of the sensor for various hydrogen sensor from Nova Analytical Systems was used to verify concentrations was also measured. Response time is de- the hydrogen gas concentration in the gas mixture. fined as the time required for the sensor to reach 90% of At the beginning of the experiment, power trans- its average maximum transmission upon exposure to hy- mission through the sensor, i.e., Pd coated silicon wave- drogen. As seen in Fig. 5, the response time decreased guides, were monitored under a steady flow of nitrogen with increasing hydrogen concentration, as expected. gas. The transmission loss of the sensor under this The 40 s response time at 4% hydrogen concentration condition was 15.1 dB∕cm. The sensor was then exposed to repeated cycles of 4% hydrogen and flushed with nitro- 20 gen in between. Figure 3 shows the percentage change of transmission through the sensor under this condition. As 15

10

5

Change in transmission (%) Change 0 01234 Hydrogen concentration (%) Fig. 4. Sensitivity of the sensor for increasing hydrogen Fig. 2. Optical setup for testing the hydrogen sensor. concentrations. 1430 OPTICS LETTERS / Vol. 38, No. 9 / May 1, 2013

90 platform. The device is very compact, highly sensitive and can be integrated with other sensors on the same 80 chip for implementing an “optical nose” capable of 70 real-time simultaneous detection of multiple gases. We would like to acknowledge Dr. Pulin Mondal of the 60 Department of Civil Engineering of the University of Toronto for his help in developing the experimental 50 setup. We also thank NSERC for financial support

Response time (sec) through its CREATE Training Program in Nanoscience 40 and Nanotechnology. 1.0 1.5 2.0 2.5 3.0 3.5 4.0 References Hydrogen concentration (%) 1. T. Hubert, L. Boon-Brett, G. Black, and U. Banach, Sens. Fig. 5. Response time for the sensor for hydrogen concentra- Actuators B 157, 329 (2011). tions ranging from 1% to 4%. 2. N. A. Yebo, D. Taillaert, J. Roels, D. Lahem, M. Debliquy, D. Van Thourhout, and R. Baets, IEEE Photon. Technol. Lett. 21, 960 (2009). compares favorably with other recently reported inten- 3. P. K. Guha, S. Santra, J. A. Covington, F. Udrea, and J. W. sity-based sensors [8]. The response time will be even Gardner, Procedia Eng. 25, 1473 (2011). shorter for a thinner Pd film [9]. 4. http://hdl.handle.net/1854/LU‑547471. An ideal hydrogen sensor should be sensitive only 5. F. Lewis, The Palladium Hydrogen System (Academic, to change in hydrogen concentration and completely 1967). 6. P. Berini, N. Lahoud, and R. Charbonneau, J. Vac. Sci. insensitive to other factors, for example, changes in tem- 26 perature and humidity, and presence of other gases. For Technol. A , 1383 (2008). 7. M. A. Webster, R. M. Pafchek, A. Mitchell, and T. L. Koch, this reason, possible future work includes the integration IEEE Photon. Technol. Lett. 19, 429 (2007). of a reference waveguide, temperature sensor, or humid- 8. S. F. Silva, L. Coelho, O. Frazao, J. L. Santos, and F. X. ity sensor on the same platform. Malcata, IEEE Sens. J. 12, 93 (2012). In conclusion, we have demonstrated a palladium 9. J. Vilatoro and D. Monzón-Hernández, Opt. Express 13, based integrated optical hydrogen sensor on an SOI 5087 (2005).