Novel Liquid Sensor Based On Porous Silicon Optical ... - IEEE Xplore

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 10, NO. 10, OCTOBER 1998

1467

Novel Liquid Sensor Based on Porous Silicon Optical Waveguides H. F. Arrand, T. M. Benson, Member, IEEE, A. Loni, R. Arens-Fischer, M. Kr¨uger, M. Th¨onissen, H. L¨uth, and S. Kershaw

Abstract—The introduction of solvents into the pores of optical waveguides formed using porous silicon is shown to dramatically reduce the interfacial scattering loss of the waveguides (by as much as 34-dB1cm01 in one example), in a reversible manner. The degree of loss reduction is dependent on the type of solvent introduced. These observations, combined with the fact that a substantial portion of the guided-mode field interacts with the solvent introduced into the pores, indicate that an enhanced sensitivity for sensor applications may be achievable across a broad range of operational wavelengths. Index Terms—Detectors, integrated optics, optical losses, optical waveguides, silicon.

I. INTRODUCTION

P

OROUS SILICON has been extensively studied as a material base for all-silicon optoelectronics due to its attractive light emission properties [1]. A key requirement for optical interconnection is to have optical waveguides to route and manipulate optical signals on-chip. By coupling the LED and waveguide base process, the advantages of reduced cost and compatible processing may be realized. A number of technologies have recently been demonstrated for the formation of optical waveguides in porous silicon material [2]–[5]. Both multilayer [2] and graded-index [3] slab waveguides can be formed from porous silicon simply by varying the current density, which determines the porosity (and, therefore, refractive index) during the electrochemical anodization process. Strip-loaded waveguides can be formed from such planar layers using CMOS-compatible etching processes [2], [3]. More recently, a very attractive and much simpler method for the fabrication of porous silicon waveguides was demonstrated, involving direct waveguide production by the localized formation of porous silicon [4]. In this method, the location and lateral geometry of the waveguide is determined, photolithographically, before anodization. These waveguides are thus referred to as being “self-aligned” [4]. Manuscript received May 12, 1998; revised June 22, 1998. This work was supported by the EPSRC under Grant GR/L35744. H. F. Arrand and T. M. Benson are with the Department of Electrical and Electronic Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, U.K. A. Loni is with the Defence Evaluation and Research Agency, Malvern WR14 3PS, U.K. R. Arens-Fischer, M. Kr¨uger, M. Th¨onissen, and H. L¨uth are with the Institut f¨ur Schicht und Ionentechnik (ISI), Forschungszentrum J¨ulich GmbH, D-52425, J¨ulich, Germany. S. Kershaw is with the British Telecom Laboratories, Martlesham Heath, Ipswich IP5 7RE, U.K. Publisher Item Identifier S 1041-1135(98)07096-7.

Fig. 1. SEM of the cross section of a self-aligned optical waveguide.

Fig. 1 shows an example of a self-aligned waveguide. The propagation losses of these waveguides are, however, high with typical values of 25 5 dB cm at 1.3 m for asprepared waveguides and 20 4 dB cm at 0.633 m for an oxidized self-aligned waveguide [6]. The high losses have been attributed to scattering at interfacial layer boundaries, which show roughness on the order of 50–100 nm. High losses have also been reported more recently by Charrier et al. [7], on similar porous silicon waveguides structures. In this letter, we show that by filling the pores of porous silicon based waveguides with a solvent (isopropan-2-ol (IPA), methanol or acetone), the optical losses are significantly reduced over a range of operating wavelengths (0.633–1.3 m). This change in attenuation is attributed to a dramatic reduction in interfacial scattering loss within the waveguide. The measured changes in loss are reversible, in that when the solvent evaporates from the material the loss returns to the original (higher) value. This offers the potential for use of porous silicon waveguides in the field of sensing, where it is required that some measurable waveguide property is dependent on the presence of a particular material. Whereas other sensor configurations (e.g., surface plasmon types) rely on an interaction through the evanescent tail only [8], porous silicon waveguides allow better sensitivity to the substance to

1041–1135/98$10.00  1998 IEEE

1468

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 10, NO. 10, OCTOBER 1998

SUMMARY

OF

TABLE I TRANSMISSION MEASUREMENTS

Fig. 2. Transmitted power through an oxidized porous silicon waveguide as a function of time after a drop of solvent is placed on the surface.

be measured, which itself is introduced throughout the porous layers, thus interacting with a substantially greater proportion of the guided-mode profile. II. EXPERIMENTAL PROCEDURE Self-aligned porous silicon waveguides were fabricated from a p (100) orientated silicon wafer by first opening widows 2–8 m wide in a silicon nitride layer deposited onto the surface of the silicon using standard photolithography and reactive ion etching techniques. The silicon nitride was used as a mask in the subsequent anodization process to form the porous silicon. During anodization the anodizing current density was switched from 200 to 100 mA cm and back to 200 mA cm to form a three layered structure similar to that shown in Fig. 1, with a guiding layer of thickness 2 m and (as-anodized) porosity 48%, and upper and lower cladding layers with thicknesses of 0.5 and 2 m, respectively, and (as-anodized) porosity 60%. Following anodization the porous silicon waveguides were oxidized, in air, first at 300 C for 30 min and followed by a second oxidation, again in air, at 800 C for 2 h. Waveguiding was established in a 6- m-wide waveguide at a wavelength of 0.633 m in a 4-mm-long sample. The estimated loss for this particular waveguide was measured (using a cutback method) to be of the order of 60 dB cm . From finite difference calculations using refractive indices calculated using a Bruggemann approximation [9], the 6- mwide structure is believed to support six guided modes, each with negligible evanescent electric field present at the interface between the upper PS cladding layer and air. As Fig. 2 shows, the transmission through the waveguide increases dramatically when a drop (volume approximately cm ) of IPA is placed on the waveguide surface and 10 allowed to spread across the sample and into the pores. The maximum throughput (minimum loss) was reached soon after the solvent was dropped on the sample and remained at around this level for approximately 120 s before slowly decaying to the level observed in the as-prepared state of the waveguide. Increased waveguide transmission was also observed when either acetone or methanol was placed on the sample and typical transmission characteristics for these solvents are also shown in Fig. 2. The increase in the transmission of the waveguide when exposed to each solvent was measured by recording a voltage

proportional to the output intensity, using a lock-in amplifier. The voltage reproducibly increased from an original level of 0.2 V to (a) 6.5 V for IPA, (b) 10 V for acetone, and (c) 2 V for methanol. These measurements are summarized in Table I together with the approximate decay times for the response to each solvent, the estimated reduction in the loss of the waveguide and potentially relevant properties of each solvent. While the results were reproducible, the solvent selectivity is not simply related to any one of the solvent parameters presented. Increases in transmission were also observed for this waveguide at wavelengths of 1.15 and 1.3 m for all three solvents, and also for as-prepared waveguides similar to those described in [4], though the changes were not quantified. Control experiments were performed to eliminate interactions between evanescent fields and solvent at the PS/air interface as a cause of the observed reduction in loss. In the first of these experiments similar volumes of water were dropped onto the surface of the waveguide. The high surface tension coefficient of water (72-m N/m) compared with those of the solvents used (22–23.5-m N/m) suggests that pore penetration by the water should be minimal. No noticeable increase in the waveguide transmission was observed when the water was placed on the sample surface. In the second control experiment, the three solvents and water were dropped in turn onto the surface of planar porous silicon waveguide samples fabricated by a method similar to that described by Maiello et al. [5]. These waveguides differ from the multilayer ones in that the oxide is densified (i.e., the resulting oxidized material is not porous) by high-temperature oxidation and annealing, with the expected result that any material deposited onto the sample surface will remain there (i.e., no penetration of the solvent into the sample). Thus, any interaction with the guided mode would be due to evanescent coupling only. In this experiment, no change in the waveguide transmission was observed, again ruling out surface contributions as the origin of the previously observed dramatic reduction in loss. III. DISCUSSION SEM investigations reveal that the interfaces between the guiding and cladding layers of the waveguides studied are rough, with estimated roughness amplitude of up to 50–100

ARRAND et al.: NOVEL LIQUID SENSOR BASED ON POROUS SILICON OPTICAL WAVEGUIDES

Fig. 3. Graph of estimated roughness loss as a function of amplitude roughness for an oxidized waveguide both in its oxidized state and following the addition of acetone into the pores.

nm. Simple roughness loss calculations based on the theory of Kendall [10] suggest that scattering from the interfaces is a major contributor to the relatively large experimental losses observed in the as-prepared samples. The experimental observation that the loss in the porous silicon waveguides generally increases with an increase in the operating wavelength [6] precludes bulk scattering as the dominant loss mechanism. The introduction of solvent into the air-filled pores will reduce the refractive index contrast in the waveguide since the solvent has a higher refractive index than air and proportionally more solvent will be present in the cladding regions than the guiding region, which has a lower porosity. The theory suggests that a substantial reduction in scattering loss will result. Fig. 3 shows the predicted roughness losses for an oxidized single-mode planar porous silicon waveguide before and after the introduction of acetone into its pores. For the calculation, the as-anodized porosity of the guiding layer is assumed to be 48% and that of the cladding layers 60%. The guiding layer is assumed to have a thickness of 0.4 m and the roughness at the interface is assumed to be sinusoidal with a periodic roughness of 8 m. The refractive indices of the oxidized porous silicon, both with and without acetone, were again calculated using the Bruggemann approximation. Fig. 3 illustrates how, regardless of the roughness amplitude, a dramatic reduction in the waveguide loss is expected following the introduction of acetone into the pores. A similar trend is also predicted following the introduction of both IPA and methanol into the pores of the porous silicon and at other wavelengths. IV. CONCLUSION It has been shown that the introduction of solvents into optical waveguides formed using as-prepared and oxidized porous silicon dramatically increases the transmission through the waveguide. This appears in part to be as a result of reduced interfacial scattering loss in the waveguides. Reproducibility and solvent selectivity have been demonstrated, the

1469

latter manifesting itself in terms of the time taken for the transmission to return to its initial state. It is assumed that the decay time will be dependent on a number of factors in combination such as local vapor pressure within the pores, the degree of solvent penetration into the pores (itself related to the wettability associated with a particular solvent), and the ambient temperature around the waveguide. The number of potentially interrelated parameters make it difficult to predict the sensitivity of the waveguide sensor, although this could be determined directly using dedicated analysis equipment. Gross fluctuations due to temperature changes have not been quantified, although the measurements indicate stability at room temperature (within experimental noise limits). The technology is particularly attractive since the waveguides are produced using low-cost silicon processing technologies and offers the capability for monolithic integration with silicon electronic devices. The concept of using porous silicon optical waveguides as a sensing medium could also be exploited for vapor or gas sensing applications. Moreover the properties of the material introduced might be tailored to detect specific species [11] and since a substantial portion of the guided-mode field interacts with the material introduced into the pores one might expect enhanced sensitivity. REFERENCES [1] L. T. Canham, “Silicon quantum wire array fabricated by electrochemical and chemical dissolution of wafers,” Appl. Phys. Lett., vol. 57, pp. 1046–1048, 1990. [2] A. Loni, L. T. Canham, M. G. Berger, R. Arens-Fischer, H. Munder, H. Luth, H. F. Arrand, and T. M. Benson, “Porous silicon multilayer optical waveguides,” Thin Solid Films, vol. 276, pp. 143–146, 1996. [3] H. Arrand, T. M. Benson, T. Anada, M. Krueger, M. G. Berger, R. Arens-Fischer, H. G. Munder, H. Luth, A. Loni, and R. J. Bozeat, “Optical waveguides and components based on porous silicon,” Integrated Photon. Res., OSA Tech. Dig. Ser., vol. 6, pp. 311–314, 1996. [4] H. F. Arrand, T. M. Benson, A. Loni, M. G. Krueger, M. Thoenissen, and H. Lueth, “Self-aligned porous silicon optical waveguides,” Electron. Lett., vol. 33, pp. 1724–1725, 1997. [5] G. Maiello, S. La Monica, A. Ferrari, G. Masini, V. P. Bondarenko, A. M. Dorofeev, and N. M. Kazuchits, “Light guiding in oxidised porous silicon optical waveguides,” Thin Solid Films, vol. 297, pp. 311–313, 1996. [6] H. F. Arrand, “Optical waveguides and components based on porous silicon,” Ph.D. dissertation, Univ. of Nottingham, U.K., 1997. [7] J. Charrier, E. Le Gorju, L. Haji, and M. Guendouz, “Porous semiconductors—Science and technology,” J. Porous Mater., to be published. [8] B. Culshaw, F. Muhammad, G. Stewart, S. Murray, D. Pinchbeck, J. Norris, S. Cassidy, M. Wilkinson, D. Williams, I. Crisp, R. Van Ewyk, and A. McGhee, “Evanescent wave methane detection using optical fibers,” Electron. Lett., vol. 28, pp. 2232–2234, 1992. [9] P. A. Badoz, D. Bensahel, G. Bomchil, F. Ferrieu, A. Halimaoui, P. Perret, J. L. Regolini, I. Sagnes, and G. Vincent, “Characterization of porous silicon: Structural, optical and electrical properties,” in Proc. Symp. Materials Research Soc., 1993, vol. 283, pp. 97–108. [10] R. J. Bozeat, “Thin film optical waveguides on silicon,” Ph.D. dissertation, Univ. of Nottingham, U.K., 1993. [11] M. C. Petty, “Application of multilayer films to molecular sensors—Some examples of bioengineering at the molecular-level,” J. Biomed. Eng., vol. 12, pp. 209–214, 1991.