Process and material properties of polydimethylsiloxane (PDMS) for Optical MEMS

Sensors and Actuators A 151 (2009) 95–99

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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna

Process and material properties of polydimethylsiloxane (PDMS) for Optical MEMS Florian Schneider ∗ , Jan Draheim, Robert Kamberger, Ulrike Wallrabe Laboratory for Microactuators, Department of Microsystems Engineering – IMTEK, University of Freiburg, Georges-Koehler-Allee 102, 79110 Freiburg, Germany

a r t i c l e

i n f o

Article history: Received 10 September 2008 Received in revised form 20 November 2008 Accepted 25 January 2009 Available online 5 March 2009 Keywords: PDMS Modulus of elasticity Refractive index Optical absorption

a b s t r a c t This article focuses on the rheological, mechanical and optical properties of polydimethylsiloxane (PDMS) relevant for microelectromechanical systems (MEMS). In view of the limited amount of published data, we analyzed the two products most commonly used in MEMS, namely RTV 615 from Bayer Silicones and Sylgard 184 from Dow Corning. As far as rheological parameters are concerned, the viscosity and spin curves were measured. With regard to mechanical properties, we focused on the measurement of the inconstant elastic modulus up to 115% strain. For the optical characterization we concentrate on the optical dispersion and the wavelength-dependent damping coefficient. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Wacker Chemie synthesized the first silicones in the 1950s. The primary field of application for polydimethylsiloxane (PDMS), a silicone elastomer, is the embedding of electronic components through casting, which prolongs chip lifespan. It acts as a dielectric isolator and protects the components from environmental influences and mechanical shock within a large temperature range (−50 to 200 ◦ C) [1]. Due to its very good contour accuracy (<10 nm), it has been increasingly used in the micro- and nanotechnologies since 1995 [2]. It is often found in fluidics (valves, pumps, fluidic circuits) [3–6], optical systems (adaptive lenses, tilting mirrors) [7–12] and sensors (acceleration sensors, tactile sensors, chemical sensors, medical sensors) [13–16]. Since the aforementioned primary use of silicones is in protection of electronic components, foremost their electronic and thermal material parameters are known. In contrast, the rheological and optical properties, which are very important in microtechnology, are barely or not at all verified. Yet, the characterization of the process properties of PDMS is essential for casting and spin coating. For the mechanical properties we continue the measurements of the strain-dependent elastic modulus up to 115% strain. A detailed characterization of the constant elastic modulus for small strains (ε < 40%) has been already published in [17]. For the specific use in Optical MEMS the dispersion and the transparency are the most important measures. Therefore, the

∗ Corresponding author. E-mail address: fl[email protected] (F. Schneider). 0924-4247/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2009.01.026

wavelength-dependent refractive index and the optical damping coefficient have also been characterized. 2. Rheological characterization 2.1. Viscosity Rheologic properties play an important role in casting and spin coating PDMS. They are decisive in preserving the accuracy of the form while casting and the thickness of the layer while spin coating. The relative movement of the fluid produces shear moments between individual particles. The shear rates dv/dx cause a shear stress . At infinitesimally small intervals of dv/dx and  the terms are proportional to one another: =

dv dx

(1)

where  is the dynamic viscosity. In the case of most pure liquids (i.e. water), the viscosity is not dependent on the shear rate. These fluids are called Newtonian materials. In dispersions and emulsions, as well as with polymer casts, the viscosity can be dependent upon the shear rate. Here, shear-thinning behavior means a reduction in viscosity with increasing shear rate [18]. A cone-plate viscometer (CVO50 of Bohlin Instruments) is used to measure the shear rate-dependent viscosity. The resolution of the viscometer is 3 × 10−9 Nm for the torque and 9 × 10−7 rad for the cone position. The material to be analyzed is placed in a small gap (g = 150 ␮m) between cone and plate. Based on the moment of inertia of the cone, which is dependent on the rotational frequency,

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Fig. 1. Viscosity as a function of the shear rate at T = 24 ◦ C. Continuous measurement with exemplarily shown error bars. Materials: RTV 615 and Sylgard 184.

the viscosity of a material is determined. The characteristics of the most commonly used two silicones, RTV 615 and Sylgard 184 has been measured at different temperatures. The liquid silicones are mixed by combining monomer and hardener in a ratio of 10:1 and subsequently degassed in the exsiccator. After 30 min of degassing, we put the samples into a refrigerator (T = 3 ◦ C) to reduce the hardening rate. For a good repeatability of the measurement the samples are taken out of the refrigerator onto the viscometer successively. Fig. 1 shows the viscosities of the two silicones for a shear rate ranging from 0.01 to 30 1/s. The diagram includes the errors of three measurements at a measurement temperature of 24 ◦ C. The measurement is done continuously which results in the slightly irregular line. The points with the error bars show exemplarily the errors obtained from three runs in a series. The two materials measured display a viscosity almost independent (curve slope: mRTV615 = 2.55 mPa s2 , mSylgard184 = 2.93 mPa s2 ) of the shear rate (dv/dx > 10 1/s), which corresponds to a Newtonian fluid. A comparison of the mean viscosity with the literature (Sylgard184 = 3900 mPa s [1], RTV615 = 4300 mPa s [19]) shows that the values for RTV 615 (RTV615,mean = 4180 mPa s) are 3% smaller

Fig. 3. Spin curve of RTV 615 and Sylgard 184, mean across the whole wafer area.

and those for Sylgard 184 (Sylgard184,mean = 4150 mPa s) 6% larger. For investigation of the hardening process of the silicones we measure the viscosity as a function of the time at a constant shear rate of 30 1/s. Generally, the datasheets of the two components [1,19] promise a hardening time of 4 h at a temperature of 60 ◦ C. On this account we increased the temperature to 60 ◦ C, to characterize the viscosity at the beginning of the hardening process. Fig. 2 shows the measured results for a temperature of 24 and 60 ◦ C. Again the measurement is done continuously and exemplarily error bars are shown. The viscosity of Sylgard 184 at 24 ◦ C is increasing by 8% during the 15 min measurement time. At the same conditions the viscosity of RTV 615 changes by 5.5% at T = 60 ◦ C, however the diagram shows a drastically increase of the viscosity to over 50,000 mPa s. The curve of Sylgard 184 increases faster compared to RTV 615, which corresponds to a faster hardening process. The average standard deviation of 1.8% results mainly from the dispersed PDMS volume in the gap between the cone and plate of the viscometer. Thereby, small air bubbles and different PDMS menisci influence the measurement. 2.2. Spin curve

Fig. 2. Viscosity as a function of the curing time for several temperatures. Continuous measurement with exemplarily shown error bars. dv/dx = 30 1/s.

The viscosity characterized above plays a large role in the process of spin coating layers. It is especially important for the determination of the thickness and homogeneity of a spun layer. Since the spin curve (rotational frequency-dependent layer thickness) and its homogeneity are decisive in microsystems processes, they are analyzed here. Typical spin curves show a hyperbolic correlation between the rotational frequency and the thickness of the layer. This corresponds to a thicker layer through fewer rotations and a thinner layer through a higher rotational frequency. After being stirred and degassed, the silicone is applied onto a silicon wafer and spun 60 s at the corresponding rate of rotation. The hardening process is carried out in the oven for 15 min at 150 ◦ C. The layer thickness is evaluated by cutting a thin strip of PDMS from the center of the silicon wafer and measured with a white light interferometer (Zygo New View 5000). The vertical resolution of the interferometer is 0.1 nm. The spin curves for RTV 615 and Sylgard 184 are illustrated in Fig. 3. Each point is composed of 18 measurements on the wafer, determined parallel to the edge across the entire wafer. The progression of the spin curve shows a strong conformity to a typical hyperbolic curve. In this case, the layer thickness of Sylgard 184 was smaller in comparison to that of RTV 615, due to

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Table 1 Standard deviation of the layer thickness across the whole wafer area of RTV 615 and Sylgard 184. Rotations (s−1 )

1000 2000 3000 4000 5000

Standard deviation RTV 615 (␮m)

Sylgard 184 (␮m)

0.74 0.22 0.16 0.07 0.08

0.52 0.35 0.14 0.16 0.22

the difference in viscosity between the two products. Generally, thinner fluids exhibit a smaller layer thickness. The deviations in the layer thickness display a strong homogeneity (see Table 1), which is underlined by the low standard deviations ( layer /tlayer < 1.1%). The small standard deviation of the layer homogeneity can be traced back to the long spinning time of 60 s. 3. Mechanical characterization The mechanical properties of polydimethylsiloxane are important for the design of a microsystem. Therefore we measure the inconstant elastic modulus of Sylgard 184 and RTV 615 in a large stain range up to 115%. For this purpose we performed a tensile test with long but thin test bars of the dimensions of 90 mm × 4 mm × 2 mm. The bars are fixed by two clamps into a tensile testing machine of type Zwick 74 which results in a test length l0 of 80 mm. Thereby, the high length to width ratio of 20 is used to reduce the influence of the clamping. The resolution of the force sensor is 1 mN and of the position sensor 1 ␮m, respectively. The machine runs in the position-controlled mode. In this operation mode the machine pulls the test bar at one end at a constant strain rate whereby a sensor measures the resulting force. The derivative of the measured stress–strain diagram is used to compute the elastic modulus E. Classical solid state materials, such as metals, show a reversible linear strain behavior up to 1%. Silicones, however, may also show a non-linear behavior even in the elastic range. On the one hand, natural rubber behaves linearly up to 10% strain [20]. On the other hand, the group of room-temperature-vulcanized (RTV) silicones shows a larger linear behavior up to a strain of 100% [21]. Beyond the linear regime, the stress–strain dependence needs to be corrected by introducing higher orders using nonlinear material models as the Mooney-Rivlin or the Neo-Hooke model [22]. In order to obtain a representative test result, both products were characterized via five test samples. Fig. 4 shows the averaged, continuously measured stress–strain diagram including measurement errors at a strain rate of 0.1 mm/s. Both curves show a linear regime up to a strain of 45%, which corresponds to a constant elastic modulus of 1.76 MPa for Sylgard 184 and 1.54 MPa for RTV 615. Beyond the linear range, unlike to the most common silicones, the curve slope is increasing, which corresponds to an increasing elastic modulus. Due to the fact, that the nonlinear material models are based on a decrease of the slope after the linear regime, it is not possible to express our two materials with the Neo-Hooke or Mooney-Rivlin model. On this account we show the elastic modulus as a function of the strain (see Fig. 5). For small strain the diagram shows the constant elastic modulus. Afterwards, the value of Sylgard 184 is increasing up to 13.9 MPa at 97% strain. The curve for RTV 615 increases only up to 7.5 MPa at 92% strain, before is decreases again to 2.16 MPa at 115% strain. The measurement of the stress–strain curve, respectively the calculation of the elastic modulus shows an average standard deviation of 7.6%. This relatively large error is related to the mounting of the test bar on the tensile testing machine. Especially for high strains

Fig. 4. Stress–strain diagram of RTV 615 and Sylgard 184. Continuous measurement with exemplarily shown error bars.

the standard deviation is increasing due to the different clamping forces and angles of the test bar fixation. 4. Optical characterization 4.1. Dispersion For the specific use of PDMS in Optical MEMS we also determined the optical dispersion and damping coefficient measurement of the two materials. First, the refractive index is measured in the visible light spectrum. We used an Abbérefractometer, which measures the refractive index of thin liquid or solid layers using the critical angle of the total internal reflection [23]. The accuracy of reading of the pure mechanically measurement tool is 0.0001. For this purpose we prepared two spin-coated pyrex and silicon substrates (2500 rpm for 60 s) with our two silicones. Pieces with a size of 10 mm × 30 mm were cut off with the wafer saw and placed on the refractometer with the 30 ␮m thick PDMS layer pointing towards the measurement device. In order to

Fig. 5. Elastic modulus as a function of the strain for RTV 615 and Sylgard 184. Continuous measurement with exemplarily shown error bars.

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Table 2 Refractive index of RTV 615 and Sylgard 184 as a function of the wavelength. Wavelength (nm)

405 532 635

Refractive index RTV 615

Sylgard 184

1.4470 ± 0.0006 1.4335 ± 0.0004 1.4282 ± 0.0004

1.4483 ± 0.0007 1.4348 ± 0.0006 1.4295 ± 0.0006

Fig. 7. Optical damping coefficient as a function of the wavelength for RTV 615 and Sylgard 184. Continuous measurement with exemplarily shown error bars.

4.2. Damping coefficient

Fig. 6. Optical dispersion of RTV 615 and Sylgard 184.

improve the light coupling from the sample to the refractometer, an immersion oil with an refractive index of 1.64 is used. The measurement at the different wavelengths, were implemented with three lasers diodes ( = 405, 532, 635 nm) on an optical bench. Table 2 shows the measurement result of RTV 615 and Sylgard 184. The mean values of each material at all wavelengths are calculated by eight samples. Thereby, Sylgard 184 has an impalpably bigger (n ≈ 0.1%) refractive index in comparison to RTV 615. The average standard deviation of 0.0005 results on the one hand side on the accuracy of reading of the Abbé-refractometer, and on the other hand on the cross-link homogeneity of the silicone which influences the refractive index. The refractive index of both products is decreasing for increasing wavelength, which is typical for glass and polymeric materials. For the approximation of the dispersion across the whole visible light spectrum the Sellmeier dispersion model is used [23]. It describes the empirical relationship of the refractive index n() and the wavelength  (see (2)). 2

n() = 1 +

B1 2 B2 2 B3 2 + 2 + 2 2  − C1  − C2  − C3

(2)

B1 , B2 , B3 , C1 , C2 , C3 are the experimentally determined Sellmeier coefficients. Due to the limited number of measurement points (three wavelengths with eight measurements each) the second and third coefficients are set zero. Fig. 6 illustrates the good correlation of the measurement results and the Sellmeier fit. The coefficients (B1 , C1 ) of the two products are given in Table 3. Table 3 Sellmeier coefficients of RTV 615 and Sylgard 184. Constant

B1 C1 (nm2 )

Sellmeier coefficients RTV 615

Sylgard 184

1.0057 13,217

1.0093 13,185

Second, we want to measure the optical damping coefficient of the two silicones. Therefore five test samples with different thicknesses (t = 20, 150, 450, 750 and 1260 ␮m) were characterized with an optical spectrum analyzer (HQ-6315A of Yokogawa Electronic Corporation) and a reference white light source (AQ-4303B of Yokogawa Electronic Corporation) [24]. The resolution of the optical spectrum analyzer is 0.02 nm. Fig. 7 illustrates the extracted mean optical damping coefficient as a function of the wavelength for RTV 615 and Sylgard 184. Beside a wavelength between 560 and 700 nm RTV 615 shows a lower coefficient in comparison to Sylgard 184. Just in this wavelength range Sylgard 184 displays an absolute minimum. For higher wavelength ( > 800 nm) both products show the same spectrum progression due to the similar chemical composition. The high average standard deviation of 24% of the calculated optical damping coefficient goes back to the alignment failure and the mechanical thickness measurement of the test samples. 5. Conclusion We have investigated the processes and material properties of two different silicones (PDMS) which are most commonly used in microsystems technology, namely RTV 615 and Sylgard 184. Since there is a steady increase of using PDMS in Optical MEMS we have measured the viscosity, the spin curve, the elastic modulus, the optical dispersion and the damping coefficient. The rheological characterization of the two materials at 24 ◦ C showed a mean viscosity of 4180 mPa s for RTV 615 and 4150 mPa s for Sylgard 184. Both measurements are in a good correlation with the literature values (maximum aberration 6%). We also investigated the hardening process of the silicone with a viscosity measurement at a temperature of 60 ◦ C. It shows a drastic increase of the viscosity to over 50,000 mPa s. The curve of Sylgard 184 increases faster compared to that of RTV 615 which corresponds to a faster hardening process. The spin-coated layers display a good thickness homogeneity across the whole wafer surface. Using a tensile test machine we measured the elastic modulus as a function of the strain. On the one hand, for low strains (ε < 45%) an elastic modulus of 1.54 MPa for RTV 615, respectively 1.76 MPa for Sylgard 184 is measured. On the other hand, for high strains the

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elastic modulus increases significantly up to 7.5 MPa for RTV 615 at 92% strain and 13.9 MPa for Sylgard 184 at 97% strain. The optical characterization of the two silicones shows a typical behavior for polymers with an index of n(635 nm) = 1.4282 ± 0.0004 for RTV 615 and n(635 nm) = 1.4295 ± 0.0006 for Sylgard 184. The damping coefficient was measured as a function of the wavelength in a range of 400–1770 nm. Thereby, RTV 615 shows a lower damping in comparison to Sylgard 184 over almost the whole wavelength range.

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The authors gratefully acknowledge the financial support provided by the Landesstifung Baden-Württemberg. Also, great thanks to Thomas Fellner and Prof. Jürgen Wilde from the chair for Assembly and Packaging for allowing the use of the tensile testing machine, and to Bernd Aatz from the Chair for Microoptics (IMTEK) for the use of the optical measurement setup for the damping coefficients. Furthermore, we appreciate the spontaneous support of Uwe Hollenbach from the Forschungszentrum Karlsruhe for the allocation of the Abbé-refractometer.

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Florian Schneider studied microsystem technology at the University of Freiburg between 2000 and 2005. After the diploma thesis he did a research internship at the Simon-Fraser-University in Vancouver, Canada. Since September 2005, he is working as a PhD student at the laboratory for microactuators. He focuses on adaptive silicone membrane lenses with integrated magnetic and piezoelectric actuator.

Acknowledgments

Jan Draheim studied microsystem technology at the University of Freiburg between 2002 and 2008. He earned his diploma degree with a thesis on elastic pump actuators for adaptive optics. Since February 2008 he is working as a PhD student at the laboratory for microactuators. Robert Kamberger is studying microsystem technology at the University of Freiburg. Since August 2007 he is working as a student research assistant at the laboratory for microactuators in the field of adaptive optics. Ulrike Wallrabe studied physics at Karlsruhe University, Germany. In 1992, she received her PhD degree for mechanical engineering about microturbines and micromotors. From 1989 to 2003, she was with the Institute for Microstructure Technology (IMT) at Forschungszentrum Karlsruhe where she focused on microactuators and Optical MEMS made by the LIGA technique. Since 2003, she holds a professorship for microactuators at the Department of Microsystems Engineering – IMTEK at Freiburg University, Germany. She has published more than 85 articles in journals, books and proceedings.