Vapor Sensors Based on Optical Interferometry from Oxidized

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Vapor Sensors Based on Optical Interferometry from Oxidized Microporous Silicon Films Jun Gao, Ting Gao, Yang Yang Li, and Michael J. Sailor* Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093-0358 Received September 7, 2001. In Final Form: December 5, 2001 Vapor sensors using thin porous silicon (PS) Fabry-Pe´rot films were prepared and characterized. The detection method used in this work involves measurement of the intensity of reflected light from a microporous Si film as a function of analyte concentration. Analyte adsorption within the pores of the film causes the Fabry-Pe´rot fringes to shift to higher wavelengths as a result of an increase in the average refractive index of the PS layer. Two transduction methodologies are employed: measurement of the intensity of reflected light using a low-power red diode laser source, and measurement of the spectrum of reflected light in the wavelength range 400-1000 nm, using a white light (tungsten) source. The effect of PS film thickness and porosity on sensitivity are systematically studied. A detection limit of 250 ppb for the analyte ethanol in a nitrogen gas carrier stream has been demonstrated. Experimental results suggest that capillary condensation is in part responsible for the high sensitivity of these vapor sensors.

Introduction The high surface area and unique optical properties of porous silicon (PS) films have been exploited for a variety of gas or vapor sensing applications.1-22 Prepared by an * Corresponding author. Chemistry and Biochemistry, University of CaliforniasSan Diego, 9500 Gilman Drive, m/c 0358 La Jolla, CA 92093-0358. Phone: (858) 534-8188. Fax: (858) 534-5383. E-mail: [email protected]. (1) Anderson, R. C.; Muller, R. S.; Tobias, C. W. Sens. Actuators 1990, A21-A23, 835-839. (2) Barret, S.; Gaspard, F.; He´rino, R.; Ligeon, M.; Muller, F.; Ronga, I. Sens. Actuators, A 1992, 33, 19-24. (3) Schechter, I.; Ben-Chorin, M.; Kux, A. Anal. Chem. 1995, 67, 3727-3732. (4) Motohashi, A.; Kawakami, M.; Aoyagi, H.; Kinoshita, A.; Satou, A. Jpn. J. Appl. Phys. 1995, 34, 5840-5843. (5) Motohashi, A.; Ruike, M.; Kawakami, M.; Aoyagi, H.; Kinoshita, A.; Satou, A. Jpn. J. Appl. Phys. 1996, 35, 4253-4256. (6) Watanabe, K.; Okada, T.; Choe, I.; Sato, Y. Sens. Actuators, B 1996, 33, 194-197. (7) Taliercio, T.; Dilhan, M.; Massone, E.; Gue, A. M.; Fraisse, B.; Foucaran, A. Thin Solid Films 1995, 255, 310-312. (8) Bjorklund, R. B.; Zangooie, S.; Arwin, H. Appl. Phys. Lett. 1996, 69, 3001-3. (9) Le´tant, S.; Sailor, M. J. Adv. Mater. 2000, 12, 355-359. (10) Zangooie, S.; Bjorklund, R.; Arwin, H. Sens. Actuators, B 1997, 43, 168-174. (11) Content, S.; Trogler, W. C.; Sailor, M. J. Chem. Eur. J. 2000, 6, 2205-2213. (12) Snow, P. A.; Squire, E. K.; Russell, P. S. J.; Canham, L. T. J. Appl. Phys. 1999, 86, 1781-1784. (13) Le´tant, S. E.; Content, S.; Tan, T. T.; Zenhausern, F.; Sailor, M. J. Sens. Actuators, B 2000, 69, 193-198. (14) Gao, J.; Gao, T.; Sailor, M. J. Appl. Phys. Lett. 2000, 77, 901-3. (15) Ben-Chorin, M.; Kux, A.; Schechter, I. Appl. Phys. Lett. 1994, 64, 481-483. (16) Sweryda-Krawiek, B.; Chandler-Henderson, R. R.; Coffer, J. L.; Rho, Y. G.; Pinizzotto, R. F. J. Phys. Chem. 1996, 100, 13776-13780. (17) Sailor, M. J. Sensor Applications of Porous Silicon. In Properties of Porous Silicon; Canham, L., Ed.; Short Run Press Ltd.: London, 1997; Vol. 18, pp 364-370. (18) Lauerhaas, J. M.; Credo, G. M.; Heinrich, J. L.; Sailor, M. J. J. Am. Chem. Soc. 1992, 114, 1911-1912. (19) van Noort, D.; Welin-Klintstrom, S.; Arwin, H.; Zangooie, S.; Lundstrom, I.; Mandenius, C.-F. Biosens. Bioelectron. 1998, 13, 43949. (20) Zangooie, S.; Bjorklund, R.; Arwin, H. Thin Solid Films 1998, 313-314, 825-830. (21) Arwin, H.; Gavutis, M.; Gustafsson, J.; Schultzberg, M.; Zangooie, S.; Tengvall, P. Phys. Status Solidi A 2000, 182, 515-20. (22) Zangooie, S.; Jansson, R.; Arwin, H. J. Appl. Phys. 1999, 86, 850-8.

electrochemical etch of single crystal silicon, the pore morphology, film thickness, and porosity can be easily controlled by appropriate adjustment of the preparation conditions.23 In addition, PS can be chemically modified or permeated with other materials to modify its physical, chemical, and electronic properties.24-32 When a uniform PS layer with nanometer-scale pores and low scattering loss is prepared, thin film interference is observed from the air/PS and PS/silicon interfaces. This leads to a reflection spectrum with well-resolved FabryPe´rot fringes.33 Upon adsorption of gas or vapor species in the inner pore walls, the average refractive index of the PS layer changes, resulting in a shift of the Fabry-Pe´rot fringes. More elaborate optical structures such as Bragg mirrors,34 Fabry-Pe´rot microcavities35-37 and waveguides38,39 have also been constructed from PS. The application of these architectures to vapor sensing has been exploited by measuring shifts of the Bragg wave(23) Smith, R. L.; Collins, S. D. J. Appl. Phys. 1992, 71, R1-R22. (24) Chazalviel, J.-N. J. Electroanal. Chem. 1987, 233, 37-48. (25) Dubin, V. M.; Vieillard, C.; Ozanam, F.; Chazalviel, J.-N. Phys. Status Solidi B 1995, 190, 47-52. (26) Dubois, T.; Ozanam, F.; Chazalviel, J.-N. Proc. Electrochem. Soc. 1997, 97, 296-311. (27) Vieillard, C.; Warntjes, M.; Ozanam, F.; Chazalviel, J.-N. Proc. Electrochem. Soc. 1996, 95, 250-258. (28) Buriak, J. M. Adv. Mater. 1999, 11, 265-267. (29) Canham, L. T.; Stewart, M. P.; Buriak, J. M.; Reeves, C. L.; Anderson, M.; Squire, E. K.; Allcock, P.; Snow, P. A. Phys. Status Solidi A 2000, 182, 521-5. (30) Stewart, M. P.; Buriak, J. M. Angew. Chem., Int. Ed. Engl. 1998, 37, 3257-3260. (31) Gurtner, C.; Wun, A. W.; Sailor, M. J. Angew. Chem., Int. Ed. 1999, 38, 1966-1968. (32) Sailor, M. J.; Lee, E. J. Adv. Mater. 1997, 9, 783-793. (33) Curtis, C. L.; Doan, V. V.; Credo, G. M.; Sailor, M. J. J. Electrochem. Soc. 1993, 140, 3492-3494. (34) Vincent, G. Appl. Phys. Lett. 1994, 64, 2367-9. (35) Mazzoleni, C.; Pavesi, L. Appl. Phys. Lett. 1995, 67, 2983-5. (36) Pellegrini, V.; Tredicucci, A.; Mazzoleni, C.; Pavesi, L. Phys. Rev. B (Condens. Matter) 1995, 52, R14328-31. (37) Setzu, S.; Solsona, P.; Le´tant, S.; Romestain, R.; Vial, J. C. Eur. Phys. J., Appl. Phys. 1999, 7, 59-63. (38) Loni, A.; Canham, L. T.; Berger, M. G.; Arens-Fischer, R.; Munder, H.; Luth, H.; Arrand, H. F.; Benson, T. M. Thin Solid Films 1996, 276, 143-6. (39) Arrand, H. F.; Benson, T. M.; Sewell, P.; Loni, A.; Bozeat, R. J.; Arenz-Fisher, R.; Kruger, M.; Luth, H. IEEE Journal of Selected Topics in Quantum Electronics 1998, 4, 975-982.

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length,12 peak PL wavelength,40 or transmitted power,41 respectively. While vapor sensing can be achieved based on measurements of the shift of Fabry-Pe´rot fringes or the change in optical thickness using a spectrometer, we recently reported a simpler and more sensitive technique using a monochromatic diode laser and a photodiode.14 This technique does not involve complex PS processing procedures and requires no use of expensive equipment such as an ellipsometer or a spectrophotometer, and yet still provides high sensitivity (ppb-range) and a large dynamic range (5 decades).14 It is generally assumed that capillary condensation leads to the high sensitivity of these vapor sensors; the microporous structure of the film tends to concentrate the analyte vapors in liquid nanodomains. Thus the morphology of the PS layer, which is sensitive to many factors such as the carrier type and resistivity of the substrate and the processing conditions, can affect the sensor performance greatly. In our previous report, we described the vapor-sensing characteristics of two PS samples prepared under slightly different conditions.14 In this work we report a more complete study of the PS-based laser interferometric vapor sensor, focusing in particular on the nanostructure of the films. It is found that pore size, porosity, and thickness of the PS layer play an important role in determining sensor performance. In addition, the experimental results confirm that capillary condensation is involved in the sensing process. Experimental Details Sample Preparation. Porous silicon samples were prepared by anodically etching p++ (100) silicon wafers (Boron doped, F e 1 mΩ cm, obtained from Siltronix, Inc.) as previously described.14 The etching solution consists of an equal volume of aqueous HF (49%) and ethanol. The fresh H-terminated PS samples were oxidized in a stream of ozone (8 g/hr, OZO 2HD Ozomax, Ltd.) for 30 min. Vapor Handling. The oxidized PS sample was then placed in a custom-made glass flow cell with an inlet connected to a gas mixer/mass flow controller (Cole-Parmer). Ethanol vapor was generated by bubbling a stream of pure nitrogen through liquid ethanol. The bubbler was immersed in a low-temperature bath to achieve a lower saturated vapor concentration before mixing. The gas stream was then diluted to various concentrations by mixing with pure carrier gas (ultrahigh purity nitrogen) in a flow mixer. The concentrations of ethanol vapor were calibrated by in situ measurement of the infrared absorption bands of the C-H stretching modes of ethanol at 2960-2830 cm-1 using an 8 m path length gas IR cell. The total flow rate was held at approximately 1000 mL/min for all concentrations studied. Optical Setup. Two optical configurations were used in the study. To measure the reflected light intensity at a single wavelength, the beam from a diode laser operating at 687 nm (Lasermax, Inc.) was passed through a nonpolarizing cubic beam splitter (Melles Griot), and the light reflected from the PS surface was detected with an amplified photodiode along the other arm of the beam splitter, as depicted in Figure 1. Reflection spectra from the PS samples were obtained using the same experimental arrangement shown in Figure 1, except the laser, beam splitter, and photodiode were replaced with a tungsten light source, optical microscope, and an Ocean Optics S2000 CCD spectrometer as previously described.42 The optical thickness (product of refractive index and the physical thickness) of the PS layers was obtained by Fourier transform of the (40) Mulloni, V.; Gaburro, Z.; Pavesi, L. Phys. Status Solidi A 2000, 182, 479-84. (41) Arrand, H. F.; Benson, T. M.; Loni, A.; Arens-Fischer, R.; Krueger, M. G.; Thoenissen, M.; Lueth, H.; Kershaw, S.; Vorozov, N. N. J. Lumin. 1998, 80, 119-23. (42) Dancil, K.-P. S.; Greiner, D. P.; Sailor, M. J. Mater. Res. Soc. Symp. Proc. 1999, 536, 557-562.

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Figure 1. Experimental setup used for the testing and calibration of porous silicon laser interferometric vapor sensors. reflection spectra.9 All experiments were carried out on an optical table to minimize vibration.

Results and Discussion Operation of the Optical Interferometric Sensors. The optical spectrum from a thin porous Si film is governed by the Fabry-Pe´rot relationship. The wavelength of a peak in the reflectivity spectrum is given by eq 1:

mλ ) 2nl

(1)

where m is the spectral order of the optical fringe, λ the wavelength, n the refractive index of the film, and l its thickness. The product nl is the quantity referred to as “optical thickness” (OT) in this work. Since m is an integer, the derivative of eq 1 is given by

m∆λ ) 2∆nl

(2)

Any change of the refractive index n will then induce a proportional shift of the position of the interference fringe position λ. This is a very general optical transduction method that has been applied to a variety of thin-film and fiber-optic-based sensor systems.43-46 When a porous Si interferometer is exposed to analytes in the gas phase, adsorption or capillary condensation induces an increase of its effective refractive index by replacement of a fraction of air (n ) 1) by a fraction of analyte (n > 1). Surface Preparation. As-formed PS is hydrideterminated and is moderately susceptible to air oxidation.47-50 Oxidation of the PS layer leads to drift in the sensor response due to the different refractive indices of Si and SiO2. Earlier work has shown that a variety of electrochemical, thermal, or chemical oxidation treatments can stabilize the material toward further oxidation in ambient air or aqueous environments.13,51,52 For this work we chose a room-temperature treatment with ozone. Fourier transform infrared (FTIR) spectra show that the ozone treatment removes the initial silicon hydride coverage (Si-HX with X ) 1-3 stretching bands at around 2100 cm-1) and forms silicon oxide and some silanol groups (Si-O-Si and Si-OH vibrational bands at around 1100 cm-1). This treatment stabilizes the PS and renders it (43) Brecht, A.; Gauglitz, G.; Nahm, W. Analusis 1992, 20, 135-140. (44) Brecht, A.; Gauglitz, G. Biosens. Bioelectron. 1995, 10, 923936. (45) Brecht, A.; Gauglitz, G. EXS 1997, 81 (Frontiers in Biosensorics II), 1-16. (46) Gauglitz, G.; Brecht, A.; Kraus, G.; Nahm, W. Sens. Actuators, B 1993, 11, 21-27. (47) Ogata, Y.; Niki, H.; Sakka, T.; Iwasaki, M. J. Electrochem. Soc. 1995, 142, 1595-601. (48) Kato, Y.; Ito, T.; Hiraki, A. Appl. Surf. Sci. 1989, 41/42, 614618. (49) Kato, Y.; Ito, K.; Hiraki, A. Jpn. J. Appl. Phys. 1988, 27, L1406L1409. (50) Unagami, T. Jpn. J. Appl. Phys. 1980, 19, 231-241. (51) Petrova-Koch, V.; Muschik, T.; Kux, A.; Meyer, B. K.; Koch, F.; Lehmann, V. Appl. Phys. Lett. 1992, 61, 943-945. (52) Dancil, K.-P. S.; Greiner, D. P.; Sailor, M. J. J. Am. Chem. Soc. 1999, 121, 7925-7930.

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Figure 2. Sensor response (change in optical thickness, ∆OT) versus the current density used in the preparation of the PS films. Each line represents a different concentration of analyte (ethanol) in N2 carrier gas as indicated. Sensor response is quantified as the change in optical thickness (∆OT) in the films relative to pure nitrogen gas, and is determined from the optical reflectivity spectrum. All samples have the same nominal optical thickness of 13576 ( 515 nm when measured in pure N2. Table 1 current etch time optical porosity thickness density (mA/cm2) (min) thickness (nm) (%) (µm) 2.5 5.0 10 20 50

42 25 15 8 4

13442 13480 13425 14091 13440

79 80 82 86 94

3.6 3.7 4.2 4.4 4.9

Samples etched at the lower current densities display a greater response at a given dose of analyte, despite the fact that they have smaller porosity and thickness, as seen in Table 1. A lower porosity means less void volume in the structure, and should translate to a smaller change in n (∆OT) upon exposure to analyte. However, the data (Figure 2 and Table 1) demonstrate the opposite: on samples which all have the same values of optical thickness, those with lower porosities display larger changes in OT on exposure to a given concentration of analyte. The above observation can be explained by the phenomenon of capillary condensation, in which vapors condense more readily in smaller pores. It has been shown that the pore size distribution of PS depends strongly on the current density used in preparation of the material.55 When the current density decreases, the mean values of pore radii decrease and the pore size distribution sharpens. Thus the samples used in this study with the lowest porosities also have the smallest pores, and would be expected to display enhanced sensitivities if capillary condensation were operative. Capillary Condensation Mechanism. The phenomena of capillary condensation can be described by the Kelvin equation56 (eq 3), which relates the pore radius to the relative vapor pressure at which condensation occurs:

γV r)RT ln(P/Po)

(3)

moderately hydrophilic.52 Under the conditions of the current experiments, the ozone treatment was found to adequately stabilize the PS films such that little to no drift was observed. Effect of Pore Size on Sensitivity. To study the effect of pore size and porosity on the performance of the PS vapor sensors, PS samples were prepared using various etching current densities. The samples were all prepared to yield the same value of optical thickness and therefore the same reflection spectrum. Samples with constant optical thickness values were prepared empirically, by first obtaining the optical thickness (OT) vs etch time characteristics for a given Si wafer resistivity under a variety of etching current densities. It was found that OT increased linearly with etch time for all the current densities studied. A series of PS samples were then prepared that possessed the same optical thickness of 13576 ( 515 nm. All samples displayed a smooth reflective surface. The porosity and thickness of these samples were obtained by gravimetric measurements53 and are listed in Table 1. This result is consistent with the characteristics of the electrochemical etching process that produces porous silicon; higher current densities lead to higher porosities and larger pore structures.54 Figure 2 shows the change in optical thickness (∆OT) vs etching current density for different ethanol vapor concentrations, relative to pure nitrogen. In all cases the OT increases upon ethanol exposure due to filling of micropores in PS, and the larger the ethanol concentration, the larger the increase in OT. The most significant finding is that the value of ∆OT increases with decreasing current density used in sample preparation. These results indicate that a small etching current density should be used in PS sample preparation to maximize the sensor response.

Where r is the pore radius, γ is the surface tension of the liquid, V is the molar volume of the liquid, R is the gas constant, T is the temperature, Po is the normal vapor pressure of the liquid, and P is the observed pressure of the vapor. Therefore, the smaller the pore radius, the lower the vapor pressure at which capillary condensation can occur at a given temperature. This relationship is expected to hold for pores with dimensions of the order of several nanometers and larger.57 The experimental results suggest that the sensitivity of a PS vapor sensor increases by increasing the number of and reducing the size of pores in the material. In addition to affecting pore dimensions, the preparation conditions are expected to increase the total surface area of the porous matrix as pore dimensions decrease. An increase in surface area is expected to increase sensitivity because there will be an increase in the amount of analyte physisorbed at the silica surface. The sensitivity of the films at the low end of the analyte concentration curve is ascribed to submonolayer physisorption rather than capillary condensation (see below). Effect of Layer Thickness on Sensitivity. Another important processing parameter that can affect the performance of the vapor sensor is the PS layer thickness, which is determined by etch time if the etching current density is fixed. It can be predicted from eq 1 that a thinner film (smaller value of l) should give a smaller change in optical thickness (∆OT) upon vapor exposure if porosity is held constant. This prediction is supported by the data presented in Figure 3, which compares OT and ∆OT vs etch time for a fixed ethanol dose of 464 ppm (in N2). All samples were etched at the same current density of 2.5 mA/cm2. Representative spectra are presented in Figure 4. The thickness and porosity of the samples, determined

(53) Halimaoui, A. Porous Silicon Formation by Anodisation. In Properties of Porous Silicon; Canham, L., Ed.; Short Run Press Ltd.: London, 1997; Vol. 18, pp 12-22. (54) He´rino, R. Pore Size Distribution in Porous Silicon. In Properties of Porous Silicon; Canham, L., Ed.; Short Run Press Ltd.: London, 1997; Vol. 18, pp 89-96.

(55) He´rino, R.; Bomchil, G.; Barla, K.; Bertrand, C.; Ginoux, J. L. J. Electrochem. Soc. 1987, 134, 1994-2000. (56) Israelachvili, J. N. Intermolecular and Surface Forces; 2nd ed.; Academic Press London: London, 1992; p 331. (57) Adamson, A. W. Physical Chemistry of Surfaces; 5th ed.; John Wiley & Sons, Inc.: New York, 1990; pp 58-59.

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Figure 3. Effect of the duration of etch used in preparing the PS samples on the sensitivity of the sensors. Optical thickness (open circles) measured in nitrogen increases linearly as etch duration increases. The change of optical thickness (∆OT, solid circles) upon exposure to a fixed concentration (464 ppm) of ethanol is a measure of the sensitivity of the sensor, and it also increases linearly with increasing etch time. All samples were etched at 2.5 mA/cm2. The lines shown are linear least-squares fits to the data. Table 2 etch time (min)

porosity (%)

thickness (µm)

fringe fwhm (nm)

15 30 60 90 120 150

58 86 70 76 80 76

1.7 2.7 3.7 7.9 9.5 13.3

80 41 22 13 11 7

by gravimetric measurements, are listed in Table 2. Both OT and ∆OT increase linearly with etch time. The ratio ∆OT/OT, however, is nearly constant for all samples (between 0.89 and 1.02%). As a result, the shift of the Fabry-Pe´rot fringes in the reflection spectrum is the same for all samples, as expected from the interference relationship. Nevertheless, this does not imply that the thickness of the PS sample has no effect on the performance of vapor sensors. If a single wavelength source is used, the sensitivity is expected to be strongly dependent on sample thickness.

Detection Using a Laser Line Source. If a single wavelength of light is used in the optical setup as depicted in Figure 1, the intensity of reflected light is determined by the relationship of eq 1. As the PS layer thickness increases, the reflection spectrum presented as intensity vs wavelength shows more and narrower interference fringes and thus a larger slope between each maximum and minimum, as shown in Figure 4 and Table 2. Under these conditions, the slope of the reflectivity fringe determines the sensitivity. Thus, for a given shift in wavelength of a given optical fringe, the thicker sample should give rise to a larger change in reflected signal from a narrow line optical source such as a laser. Figure 5 shows the dose/response data taken from an optical setup incorporating a laser line source. The calibration curve shown in Figure 5 indicates that the PS sensor based on this sample can detect ethanol vapor at concentrations as low as 250 ppb. There is a monotonic increase in response with increasing analyte concentration up to 4000 ppm. At concentrations above 4000 ppm, however, the response curve turns over and decreases, because a reflection maximum has shifted through the detection wavelength window at 687 nm. This comprises the dynamic range of the sensor. In principle, such behavior is expected for all samples, but the analyte concentration at which the dose/response curve turns over is a function of sample thickness, the order of the spectral fringe being monitored and the detection wavelength used. Since the width of an interference fringe at a given wavelength is inversely proportional to sample thickness, using samples of different thickness can extend the dynamic range of the measurement. As shown in Figure 3, at a constant analyte concentration the magnitude of the change in optical thickness scales linearly with sample thickness. The 5 µm thick samples previously reported displayed a dynamic range of nearly 5 orders of magnitude in concentration, although the dose/response curves are not linear.14 A variety of PS vapor sensors have been reported in which the signal transduction mechanism is based on a change in refractive index of the PS layer. Depending on

Figure 4. Reflected light intensity (uncorrected) as a function of wavelength of thin film PS samples etched for the indicated periods of time. All samples were etched at a constant current density of 2.5mA/cm2. The data show an increase in the number of optical interference fringes as the film thickness increases, as expected from the Fabry-Pe´rot relationship described in the text. Wafer resistivity and etch solution composition were the same for all samples shown. The metric thickness and porosity values for each of these samples are provided in Table 2.

Vapor Sensors Based on Optical Interferometry

Figure 5. Dose-response curve of a PS sensor using a laser light source and the optical setup shown in Figure 1. The analyte is ethanol in air. The sample was etched at 2.5mA/cm2 for 150 min and had a porosity of about 75% and a thickness of 13.4 µm. The curve was obtained by averaging three sets of data. The error bars enclose the largest and smallest values observed at each vapor concentration.

the device structure and the experimental setup, this causes either a shift in the peak wavelength of a PS microcavity,40 a PS Bragg mirror,12 or changes in ellipsometric parameters of PS layers.10,22 In these studies, different degrees of liquid filling have been calculated by fitting the experimental results to theoretical models. The present work (in particular the results of Figure 2) supports the capillary condensation mechanism as the source of enhanced sensitivity in these microporous materials at higher analyte concentrations (above about 10 ppm). It is believed that physisorption of analyte in these materials contributes to their sensitivity in the low analyte concentration regime (2 h) etch times. Samples with poor fidelity will provide lower sensitivity and lower dynamic range because the maximum change in reflected light intensity that can be observed at a given wavelength is reduced. For the thickest samples used in this study, the corresponding resolution of the fringes is approaching the line width of the laser (approximately 3 nm). This is also not desirable because it limits the maximum change in reflected light intensity that can be observed. Thus the line width of the laser forces a practical upper limit to the sample thickness that can be used in this type of sensor. A practical test of the above sensitivity-limiting factors was performed. Sensor response vs ethanol concentration curves similar to Figure 5 were obtained from samples in which the thickness was systematically varied. The laser

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Figure 6. Sensitivity of the porous Si films as a function of sample thickness. The experimental configuration uses a singlewavelength laser source and photodiode detector as shown in Figure 1. Sensitivity is defined as the largest slope of the calibration curve (voltage measured at the photodiode vs log of analyte concentration, see Figure 5). Samples were prepared according to the procedure used for the samples in Table 2 and Figure 4.

interferometer setup shown in Figure 1 was used for these comparative studies, and the results are presented in Figure 6. The sensitivity of each sample is defined as the largest slope observed in its calibration curve, corresponding to an analyte concentration regime in which we believe capillary condensation to be the active mechanism. The data do not show large differences in sensitivity, although there is a slight increase in sensitivity with increasing sample thickness. The thickest samples (corresponding to etch times >80 min) show no significant change in sensitivity with increasing thickness. This is ascribed to the limiting effect of the laser line width described above. Conclusions A microporous silicon sensor based on laser interferometry provides high sensitivity and large dynamic range for detection of condensable analytes in air. The sensors are inexpensive and can be readily miniaturized using off-the-shelf lasers and optical components. Optimization of sample preparation conditions including film thickness, porosity and appropriate surface modification provided a detection limit of 250 ppb for ethanol. The results suggest that capillary condensation is responsible for enhanced vapor adsorption in these PS vapor sensors. Acknowledgment. The authors thank Dr. Sonia Letant for advice and experimental assistance, Professor William C. Trogler for assistance with the concentration measurements, and Dr. Robert Bedard, Dr. Wenbin Lin, and Dr. Walter G. Klemperer for helpful discussions. This project was supported by the Office of Naval Research, the National Science Foundation (DMR-97-00202), and DARPA’s Tactical Sensors Program via a Space and Naval Warfare Systems Center Contract (N66001-98-C-8514). The technical point of contact for this DARPA program is Dr. Edward Carapezza. LA015568F