Characterization of polymer layers for silicon micromachined bilayer chemical sensors using white light interferometry

Sensors and Actuators B 111–112 (2005) 549–554

Characterization of polymer layers for silicon micromachined bilayer chemical sensors using white light interferometry D. Goustouridis a , K. Manoli b , S. Chatzandroulis a,∗ , M. Sanopoulou b , I. Raptis a b

a NCSR “Demokritos”, Institute of Microelectronics, 15310 Agia Paraskevi, Athens, Greece NCSR “Demokritos”, Institute of Physical Chemistry 15310 Ag. Paraskevi Attikis, Athens, Greece

Available online 17 May 2005

Abstract The swelling behavior of polymer layers used in silicon micromachined bilayer structures is studied using white light interferometry. The study focuses on poly-hydroxy-ethyl-methacrylate (PHEMA) films and their behavior in terms of film expansion, response time and sensitivity upon exposure to methanol and ethanol vapors. Thin films of PHEMA exhibit higher relative swelling than thicker films, in the presence of methanol, as well as lower diffusion coefficients corresponding to higher response times. © 2005 Published by Elsevier B.V. Keywords: Chemical sensors; Bilayer structure; Interferometry; Polymer kinetics

1. Introduction The swelling behavior of polymer layers used in silicon micromachined bilayer structures is studied using white light interferometry. Bilayer chemical sensors rely on the expansion of a selective polymer layer, upon exposure to analyte vapors, to inflict a chemical induced stress on the underlying silicon structure. In silicon microcantilever structures this results in bending which is then monitored with optical methods [1], resonance frequency shift [2], capacitance change between the structure and the substrate [3,4] or in the case of piezoresistive detection a change in resistance [5]. Up to date, optical methods using reflectrometry on thin polymeric films [6] and ellipsometry on porous silicon [7] have proved to be useful in the study of polymer systems and fabrication of optical chemical sensors for the determination of the concentration of various analytes. In the present study the behavior of the poly-hydroxy-ethyl-methacrylate (PHEMA) is investigated. PHEMA has been known for its hydrophilic properties and suitability for this kind of sensors [8]. Furthermore, it is insensitive to visible spectrum ∗

Corresponding author. E-mail address: [email protected] (S. Chatzandroulis). URL: http://www.imel.demokritos.gr. 0925-4005/$ – see front matter © 2005 Published by Elsevier B.V. doi:10.1016/j.snb.2005.03.074

and can only be patterned via deep UV irradiation. Film expansion, absorption/desorption kinetics and selectivity in the presence of two volatile organic compounds (methanol and ethanol) as a function of the thickness of the polymer layer is examined.

2. Experimental setup The experimental setup, depicted in Fig. 1, is built around a chamber where controlled concentrations of volatile organic compounds may be introduced via a dry nitrogen flux going through a bubbler with the respective compound. A splitter optical fiber is connected to a VIS-NIR light source (500–900 nm) and is equally split to two beams: one directed to the slave channel of a PC driven double spectrophotometer (Ocean Optics USB SD2000) and another connected to a bifurcated optical fiber. The bifurcated optical fiber guides then the white light onto an appropriate reflective substrate spin coated with a thin polymer layer. At the same time it collects the reflected beam, directing it to the master channel of the spectrophotometer. Each channel of the spectrophotometer is sensitive in the VIS-NIR spectrum with a resolution of 0.4 nm approximately. The temperature of the system is controlled to within 0.5 ◦ C.

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Fig. 1. Measuring apparatus. Dry N2 flow is split in a carrier bubbling through the volatile organic compound and is afterwards diluted with dry N2 at the desirable concentration.

The substrate should be totally reflective, at the spectrum used. Therefore, standard silicon wafers were a reasonable choice. The accuracy of the method depends on the number of interference fringes in the recorded spectrum, as the existence of these extremums allows accurate fitting of the recorded spectrum and thus accurate calculation of the polymeric film thickness. In order to increase the number of fringes, a SiO2 layer is grown on the Si substrate prior to spin coating thus increasing the optical path of the traveling light. In Fig. 2 the effect of the grown thermal oxide on the recorded spectra is demonstrated. In this case, 142 nm polymer films were applied on a bare silicon sample and on a thermally oxidized one, and the corresponding spectra were obtained. The thickness of the oxide was 1080 nm. In the case of the bare Si substrate no extremums could be detected

Fig. 2. Reflectance spectrum for a 150 nm thick PHEMA film over Si and SiO2 /Si substrate.

while in the case of SiO2 /Si substrate several maxima and minima are observed. For considerably thicker films, e.g. 1000 nm the intermediate layer is not required since the polymer film thickness provides long enough optical path which results in a sufficient number of spectrum extremums. In Fig. 3 the principle of the method is depicted. At each wavelength interference takes place, due to the light traveling through the polymeric film and the transparent (for the spectrum used) SiO2 layer. A complete spectrum, an average of 60 measurements, is recorded every 2 s on the PC. Film

Fig. 3. White light interferometry principle. The use of the intermediate SiO2 layer provides a substantial number of interference fringes and enhances the accuracy of the calculated results.

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expansion, as analyte molecules are absorbed in the polymer layer, is recorded as a change (shift of the extremum position) in the final spectrum. The total energy E that incidents on the master channel of the spectrometer, for each wavelength, could be approximated by the interference equation [9] as

 4πn1 4πn2 E = A + B cos d1 + C cos d2 λ λ
 4π(n2 d2 + n1 + d1 ) + D cos (1) λ where 2

2

2

2 2 2 2 2 2 + r12 (1 − r01 ) + r23 (1 − r01 ) (1 − r12 ) , A = r01 2

2 B = 2r01 r12 (1 − r01 ) ,

2

2 2 C = 2r12 r23 (1 − r01 )(1 − r12 ) , 2

2 2 D = 2r01 r23 (1 − r01 )(1 − r12 )

and r01 , r12 , r23 are the relative refractive indices between adjacent layers (0 is considered as air, 1 the polymeric film, 2 the SiO2 layer and 3 the Si substrate), n1 , n2 the refractive indices of the polymeric film and the silicon oxide, respectively, d1 , d2 the thickness of the polymeric film and the silicon oxide and λ is the corresponding wavelength. sBy applying the interference equation for all wavelengths in the spectrum, the film thickness and refractive index may be approximated for each recorded spectrum. Application of this method for every measured spectrum yields the evolution of film thickness in time. At the same time the spectrum of the light source recorded at the slave channel of the spectrophotometer, is used as a reference (multiplier in the interference equation) to adapt the theoretical approximation to the experimental data. In Fig. 4, the measured spectrums for a 258 nm thick poly-hydroxyl-ethyl-methacrylate (PHEMA) film on 1080 nm SiO2 /Si at two instances (dry nitrogen, 10,000 ppm methanol) are plotted together with the theoretical approximations from Eq. (1). The second instance

Fig. 4. Fitting of spectrums for 10,000 ppm MeOH and pure N2 for a 258 nm thick PHEMA layer. The two sets are intentionally shifted for better illustration.

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and its theoretical calculation are intentionally shifted in the vertical axis for better visualization of the results.

3. Results In the present study chemically sensitive polymeric materials with the potential of easy patterning via photolithography are investigated. PHEMA is well known for its hydrophilic properties, deep-UV irradiation patterning potential and visible spectrum insensitivity [10]. Thin PHEMA films were spin coated at thicknesses between 50 and 600 nm on oxidized silicon wafer samples. PHEMA solutions (4–8%) in ethyl-actate were prepared and films were obtained after spin coating and post-apply bake at 120 ◦ C for 30 min in an oven to insure solvent evaporation. The expansion/contraction kinetics and equilibria of PHEMA coatings when exposed to methanol or ethanol vapor, were studied in a series of absorption/desorption cycles at successively higher vapor concentrations up to 20,000 ppm. An example of the results for a 142 nm PHEMA film, exposed to four different methanol vapor concentrations is shown in Fig. 5. In Fig. 6a and b, the expansion over time of PHEMA films for four different thicknesses under exposure of 10,000 ppm of methanol and ethanol is depicted. The resulting absorption times, measured as the time needed for the film to rise from 10% of the total expansion at equilibrium to 90% of that figure, are summarized in Table 1. Data suggest a relative higher response time for the thinner film, in the case of methanol. From the swelling data of PHEMA films of different thicknesses at equilibrium, the expansion, normalized to initial film thickness, was calculated and found to increase with increasing vapor concentration (Fig. 7). No discernible effect of film thickness on equilibrium swelling was observed in the case of methanol or ethanol for thick films and for the same

Fig. 5. A thin film of (142 nm) PHEMA is successively exposed to four different concentration levels of methanol vapors.

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Fig. 7. Film expansion of four different thicknesses of PHEMA films as a function of methanol concentration.

Fig. 6. (a) Expansion vs. time for four different thicknesses of PHEMA films in 10,000 ppm of methanol vapors; (b) expansion vs. time for four different thicknesses of PHEMA films in 10,000 ppm of ethanol vapors.

film, no material difference between absorption and desorption is observed and there is a small dependence of D on concentration. On the other hand, the diffusion coefficients for the thicker film tend to be higher than those of the thinner one. This effect was found to be more pronounced in the case of methanol. The results for absorption are shown versus methanol vapor concentration in Fig. 10. As in the case of ethanol (Fig. 9) the diffusion coefficient D for all films exhibits a small trend to increase with methanol concentration. Moreover, the thickness effect observed in the case of ethanol, is more intense here. This effect is also demonstrated in Fig. 11, where the D values for absorption as well as for desorption at 20,000 ppm are plotted versus film thickness. A drastic decrease of D with decreasing film thickness has also been reported in other studies employing very thin films (<1 ␮m), as for example in the

vapor concentration relative swelling was higher in the case of ethanol (Fig. 8). The thinner 55 nm film however exhibited a considerably higher relative swelling. The kinetic data of the expansion/contraction plots of the type shown in Fig. 5 were used to estimate the diffusion coefficients D of methanol and ethanol in the PHEMA films. D values deduced from two films of different thicknesses are plotted versus ethanol vapor concentration in Fig. 9. For each Table 1 Absorption time for 10,000 ppm of methanol and ethanol vapors for four thicknesses of PHEMA Film thickness

Concentration and spinning speed

Absorption time (min)

%

rpm

Methanol

Ethanol

55 142 258 600

4 6 6 8

5000 4500 1200 1000

1.7 1.6 3.9 5.1

5.3 22.9 74.6 168.9

Absorption time (min)–10,000 ppm (10–90% of the change).

Fig. 8. Relative expansion for 600 nm PHEMA films in methanol and ethanol vapors (10,000 ppm).

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case of moisture absorption in polyelectrolyte films in the range of 3–200 nm [11], and has been attributed to thin film confinement effects. The data of Fig. 11 also indicate that the value of D approaches a limit value as the thickness of the film approaches a value close to that of a bulk film. Due to the above discussed thickness dependence of the diffusion coefficient, comparison of the relative values for methanol and ethanol should be made for films of similar thickness. On this basis, the D for methanol is distinctly higher than the corresponding values for ethanol, as expected due to the smaller molecular diameter of the former. For example, for the concentration range studied, a mean value of D for ethanol in a 386 nm film is (2.5 ± 0.4) × 10−13 cm2 /s, compares to corresponding mean values for methanol in the 600 nm and the 257 nm films, equal to (7.4 ± 1.0) × 10−11 and (1.2 ± 0.4) × 10−12 cm2 /s, respectively. Fig. 9. Absorption (open points) and desorption (full points) diffusion coefficients of ethanol in PHEMA films of d0 = 86.1 nm (circles) and 368.1 nm (squares) vs. vapor ethanol concentration.

Fig. 10. Absorption diffusion coefficients of methanol in PHEMA films of different thicknesses vs. vapor methanol concentration.

4. Conclusions A new methodology for the characterization of polymer materials based on white light interferometry has been presented. The method is particularly useful in the study of the kinetics and equilibrium of polymer coatings in the presence of analytes. PHEMA, a polymer material often used in chemical bimorph sensors, has been studied in the presence of methanol and ethanol vapors using the proposed method. Ethanol diffusion through the polymer conforms to Fick’s law. Methanol, however, exhibits peculiar behavior for thinner coatings, and polymer confinement effects are suggested as the probable cause. In particular higher relative expansion is observed for the thinner 55 nm film, accompanied with a lower diffusion coefficient. Furthermore, the data from Table 1 suggest that for a tenfold increase of thickness we get a less than a fivefold increase in absorption time in methanol further indicating polymer confinement effects. Thus, a tradeoff between response time and film expansion exist which has to be taken into account in the design of bilayer sensor systems. In future work the study will seek to further investigate the thin film effects observed for PHEMA, as well as to extend the study to other polymer–analyte systems.

References

Fig. 11. Diffusion coefficients of methanol in PHEMA films vs. film thickness. Vapor methanol concentration 20,000 ppm.

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Biographies Dimitris Goustouridis was born in 1969. He received the BS in 1992 from the department of Physics of the University of Patras. In 2002 he received the PhD degree in microelectronics from the Faculty of Applied Mathematics and Physics of National Technical University of Athens for his work on capacitive type pressure sensors. He is currently with the Institute of Microelectronics at NCSR Demokritos. His interests include silicon sensors and pressure sensors in particular, and silicon micromachining.

Stavros Chatzandroulis was born in Athens, Greece in 1967. He received the bachelor degree and the MSc in electronic automation from the Department of Physics at the University of Athens in 1990 and 1993, respectively. In 1999 he received the PhD degree from the same department for his work on integrated silicon sensors. He is currently with the Institute of Microelectronics at NCSR “Demokritos”. His interests include silicon sensors and pressure sensors in particular, and sensor electronic interfaces. Merope Sanopoulou received her BS degree in Chemistry from National University of Athens, Greece in 1977 and her PhD in physical chemistry from the same University in 1984. From 1984 she works at the Institute of Physical Chemistry NCSR “Demokritos” (Laboratory of Transport of Matter Phenomena) She has worked and published extensively on sorption and transport of micromolecular substances in polymer membranes. Her current research activities include experimental investigation and model interpretation of non-Fickian sorption kinetics of vapors and liquids in glassy polymer films, evaluation of hydrocarbon transport parameters in novel polymeric materials for natural gas hydrocarbon separation as well as controlled drug delivery systems. Ioannis Raptis received his BS degree in physics from National University of Athens in 1989 and his PhD in the field of e-beam lithography from the same University in 1996. During his PhD thesis he has worked on the development of a fast electron beam lithography simulator (LITHOS). Additionally he has worked at IESS-CNR (Rome) on the evaluation of conventional and chemically amplified resists (CARs) for very high resolution lithography (1995). From 1997 he works at the Institute of Microelectronics NCSR “Demokritos”. His current research activity is focused on the characterization of polymeric materials and the development of patterning technologies for the micro/nano-scale. He is author and co-author of more than 40 publications in international journals.