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Tensile Film Stress of Parylene Deposited on Liquid Nguyen Binh-Khiem,* Kiyoshi Matsumoto, and Isao Shimoyama* Department of Mechano-Informatics, Graduate School of Information Science and Technology, the University of Tokyo, 7-3-1 Hongo Bunkyo-ku, Tokyo 113-8656, Japan Received July 12, 2010. Revised Manuscript Received October 4, 2010 We found that liquid droplets encapsulated by Parylene deposited directly on a liquid surface deformed toward spherical shapes during Parylene deposition. This deformation suggested that the film stress was tensile. We calculated the film stress of such Parylene films by studying the surface mean curvature of the droplet shape and found the film stress measured about 0.7-0.9 MPa tensile. This film stress is of opposite type to that of as-deposited Parylene films deposited on solid substrates, which was compressive. This difference might indicate a profound change of the Parylene polymer due to the use of liquid surface as deposition substrate. The tensile film stress and its effect on the droplet shape also have implications in the fabrication and operation of Parylene microdevices that have encapsulated liquid structures such as microlens or micropumps.
1. Introduction Parylene, namely, poly(para-xylylene) and its derivatives, has been used as a coating material for medical devices, electronics, and automobile parts since Szwarc discovered the material (1947) and Gorham proposed an efficient route of synthesis (1966).1,2 Parylene is conventionally applied only on solid surfaces. The deposition is called the Gorham process, which is a unique and highly efficient polymerization of Parylene from its monomer gas performed in a vacuum ambient. After a patent was given to a method of directly depositing Parylene on liquid using Gorham process in 2006,3 a paper with details on this process was published in 2010.4 Our group was also involved in the deposition process independently, which we named “Parylene on liquid deposition (POLD)”, reporting on a varifocal lens using direct deposition of Parylene on liquid.5,6 A paper in Nature also reported a direct deposition of metal on liquid in a vacuum ambient for space telescopes.7 The Parylene deposition opens new possibilities to liquid encapsulating microdevices, for which some interesting examples such as electro-driven varifocal microliquid lenses and microfluidic pumps have already been demonstrated *To whom correspondence should be addressed. E-mail: khiem@ leopard.t.u-tokyo.ac.jp. Telephone: þ81-3-5841-6318. Fax: þ81-3-3818-0835. E-mail:
[email protected]. (1) Szwarc, M. Discuss. Faraday Soc. 1947, 2, 46–49. (2) Gorham, W. F. J. Polym. Sci., Polym. Chem. 1966, 4, 3027–3039. (3) Keppner, H.; Benkhaira, M. Patent WO 2006063955-A1, 2006. (4) Charmet, J.; Banakh, O.; Laux, E.; Graf, B.; Dias, F.; Dunand, A.; Keppner, H.; Gorodyska, G.; Textor, M.; Noell, W.; de Rooij, N. F.; Neels, A.; Dadras, M.; Dommann, A.; Knapp, H.; Borter, C.; Benkhaira, M. Thin Solid Films 2010, 518(18), 5061–5065. (5) Binh-Khiem, N.; Iwase, E.; Matsumoto, K.; Shimoyama, I. Proceedings of the 20th IEEE International Conference on Micro Electro Mechanical Systems, Kobe, Japan, 2007; Bu, J. U., Konishi, S., Eds.; IEEE: Kobe, Japan, 2007; pp 305-308. (6) Binh-Khiem, N.; Matsumoto, K.; Shimoyama, I. Appl. Phys. Lett. 2008, 93, 124101–3. (7) Borra, E. F.; Seddiki, O.; Angel, R.; Eisenstein, D.; Hickson, P.; Seddon, K. R.; Worden, S. P. Nature 2007, 447(7147), 979–981. (8) Sadeghi, M.; Kim, H.; Najafi, K. Proceedings of the 23rd IEEE International Conference on Micro Electro Mechanical Systems, Hong Kong, China, 2010; Suzuki, Y., Wong, M., Eds.; IEEE: Hong Kong, China, 2010; pp 15-18. (9) Yoshihata, Y.; Takei, A.; Binh-Khiem, N.; Kan, T.; Iwase, E.; Matsumoto, K.; Shimoyama, I. Proceedings of the 22nd IEEE International Conference on Micro Electro Mechanical Systems, Sorrento, Italy, 2009; Hierold, C., Sarro, L. M., Eds.; IEEE: Sorrento, Italy, 2009; pp 967-970. (10) Yoshihata, Y.; Binh-Khiem, N.; Takei, A.; Iwase, E.; Matsumoto, K.; Shimoyama, I. Proceedings of the 21st IEEE International Conference on Micro Electro Mechanical Systems, Tucson, AZ, USA, 2008; Brand, O., Zohar, Y., Eds.; IEEE: Tucson, AZ, 2008; pp 770-773.
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by our group and others.5,6,8-11 POLD provides an effective and straightforward method for encapsulating liquid droplets of various shapes and sizes, from some millimeters down to some micrometers. However, the film stress of Parylene deposited on such liquid surfaces has not been measured, even though this physical property of the film plays an important role in determining the characteristics of these Parylene microdevices, for example, the voltage at which the device can be operated, or the specific shape of the lens surface. Moreover, the substitution of solid surfaces by liquid surfaces as the substrate for Parylene deposition might have a large impact on the characteristics of the polymer itself. It is therefore expected that measurement of film stress of Parylene films deposited on liquid surfaces will provide necessary data to improve microdevices fabricated by POLD. The film stress of Parylene-N films as-deposited has been reported by Bachmann and Beach.12,13 They found that the film stress measured 18-20 MPa compressive. The dependence on temperature of film stress of Parylene films deposited on silicon wafer and the effect of thermal treatment have been reported by Dabral and co-workers.14 They measured the film stress of Parylene films deposited on silicon wafer by the radius of curvature method and found that the as-deposited film stress was 20 MPa compressive for Parylene-N films and 6 MPa compressive for Parylene-C films. A similar study was also done by Harder and co-workers.15 They assumed the as-deposited film stress of 10 MPa compressive and studied the change of film stress of Parylene-C films caused by thermal cycles by methods such as rotation tips and load-defection membranes and found results consistent with those reported by other authors. In our previous work on the POLD lens,6 we also attempted to observe the (11) Binh-Khiem, N.; Matsumoto, K.; Shimoyama, I. American Chemical Society 235th National Meeting, New Orleans, LA, USA, 2008; Kumar, R., Greiner, A., Eds.; American Chemical Society: Washington, DC, 2008; 79-POLY. (12) Bachmann, B. J. Proceedings of the 1st International SAMPE Electronics Conference, Santa Clara, CA, USA, 1987; Kordsmeier, N. H., Jr., Harper, C. A., Lee, S. M., Eds.; Society for the Advancement of Material and Process Engineering: Covina, CA, 1987; pp 431-440. (13) Beach, W. F.; Austin, T. M. Proceedings of the 2nd International SAMPE Electronics Confence, Seattle, WA, USA, 1988; Hoggatt, J. T., Ed.; Society for the Advancement of Material and Process Engineering: Covina, CA, 1988; pp 25-35. (14) Dabral, S.; Van Etten, J.; Zhang, X.; Apblett, C.; Yang, G.; Ficalora, P.; McDonald, J. J. Electron. Mater. 1992, 21, 989–994. (15) Harder, T. A.; Yao, T.-J.; He, Q.; Shih, C.-Y.; Tai, Y.-C. Proceedings of the 15th IEEE International Conference on Micro Electro Mechanical Systems, Las Vegas, NV, USA, 2002; pp 435-438.
Published on Web 11/16/2010
DOI: 10.1021/la102790w
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Figure 1. (a) A liquid droplet changes its shape during Parylene deposition. (b) A series of photographs of the side of a liquid droplet taken during a Parylene deposition. Horizontal scale bar = 5 mm. Vertical scale bar = 2 mm.
deformation of the droplet due to the formation of the Parylene film. The diameter of the lens was of some millimeters, so small that the initial liquid surface was practically spherical regardless of the existence of gravity. Tensile film stress in the Parylene film would only have made the droplet more spherical, and thus practically have no effect on this already spherical droplet shape, while a compressive film stress would have created wrinkles on the surface. Because we found no wrinkles, we concluded that the film stress of Parylene films deposited on liquid surface was either compressive with very small magnitude or tensile. In this paper, we present an observation (Figure 1) and measurement of film stress of Parylene films deposited on liquid surfaces, which indicates unambiguously that this film stress is tensile. This result opposes what has been reported on film stress of Parylene films deposited on solid surfaces. We studied the film tension, which is the product of film stress and film thickness, of Parylene-C films of 1 μm thick deposited on a liquid surface and found that the tension was tensile and measured about 700-900 μN/mm. Such a Parylene film of 1 μm thick would have possessed a compressive film tension measured about 6-10 mN/mm if calculated with the reported 6-10 MPa compressive film stress of as-deposited Parylene-C films.14,15 Our measured film tension totally differed from such estimation in both magnitude and type (compressive or tensile). We deposited Parylene on large droplets of liquid and found that the POLD process always modified the profiles of the encapsulated droplet toward more spherical shapes. This deformation of the droplet was a ready consequence of the tensile stress in the POLD film as-deposited. This result showed that the effect of Parylene film stress on liquid microdevices encapsulated by POLD, especially on their droplet shape, was larger than what we have assumed previously.6 The Parylene film of POLD did not simply polymerize on the liquid surface to create a “soft skin” for the liquid inside with a shape resembling exactly that of the initial droplet, but it deformed the whole droplet concurrently with its formation. We measured the tension of Parylene-C films deposited directly on circular droplets of 30 mm in diameter of silicone fluid 1,3,5trimethyl-1,1,3,5,5-pentaphenyl-trisiloxane and liquid paraffin. The method we used included extracting the droplet surface profile by image processing of the droplet photographs taken before, during, and after POLD process, calculating the mean 18772 DOI: 10.1021/la102790w
curvature of the surface with the data of its profile, and fitting both the data of surface mean curvature and surface profile in the equation describing force equilibrium on the surface to find the force acting on the surface, which was the tension of the Parylene film. Because our target was to measure the film tension during the film formation inside the deposition vacuum chamber, many methods of direct measurement were inapplicable. Our method was an indirect one but it was able to capture the change of Parylene film tension during POLD process. Furthermore, because in this method the whole droplet shape was measured, we could monitor the deformation of the whole surface instead of observing only local change of the film. This paper provides a plot of the film tension of a Parylene film deposited on a silicone fluid droplet in a POLD process and a table summarizing the measurement results of a number of samples including a sample deposited on liquid paraffin. We also include a discussion on the measured film stress as to how Parylene films deposited on liquid can possess a tensile stress.
2. Experimental Section Fabrication of Liquid Droplets Encapsulated by Parylene Films. Figure 2a shows the process flow for the fabrication of POLD droplets. This process consisted of three steps. In step 1, a liquid solution of CYTOP, a hydrophobic amorphous fluorocarbon polymer, was spin-coated and cured to create a thin hydrophobic coating layer of less than 1 μm in thickness on a glass wafer. The coating layer was patterned by oxygen plasma etching to remove CYTOP in circular domains of 30 mm in diameter. By this removal of CYTOP, the underneath glass surface in these domains was exposed to oxygen plasma and therefore rendered highly hydrophilic. In step 2, the liquid in use (silicone fluid or liquid paraffin) was deposited on these domains by dropping. The liquid spread on the highly hydrophilic glass surface until it came to contact with the CYTOP surface and stopped spreading because this surface was hydrophobic. Consequently, the liquid was shaped into a circular droplet of 30 mm in diameter. In step 3, Parylene-C was deposited on the wafer to form a thin film encapsulating the liquid droplet. We deposited Parylene using the Gorham process as follows: dimer of Parylene-C was sublimed at 175 °C and pyrolyzed at 690 °C to create monomer gas which was then fed into a deposition chamber kept at room temperature (25 °C) and at low pressure (25 mTorr) where the monomer could efficiently polymerize on any surface exposed to the monomer gas. Langmuir 2010, 26(24), 18771–18775
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Figure 2. (a) Process flow for the fabrication of POLD droplets. (b) Setup for observation of the inside of deposition chamber during Parylene deposition.
Figure 3. (a) Schematic showing the forces that act on the droplet surface. (b) Photograph of a silicone fluid droplet of 30 mm in diameter fabricated on a glass wafer and encapsulated by a Parylene film of 1 μm in thickness. (c) Plot of the surface force of a silicone fluid droplet during a process of Parylene deposition in respect of deposition time.
Equations Governing the Surface Profile. The surface of a droplet during the POLD process experienced forces acting on both its tangent and normal directions: in the tangent direction were forces such as liquid surface tension and film tension of the Parylene film gradually formed during the process; in the normal direction was the pressure difference across the liquid surface or the Parylene film. Figure 3a shows the schematic with σ and ΔP denoting the forces tangent and normal to the surface, respectively. At stationary conditions, the droplet surface shape was described in terms of mean surface curvature H, pressure difference ΔP, and surface force σ by a generalized form of the Young-Laplace equation derived for a surface with elasticity16
This equation was derived for an infinitesimally thin elastic film. This condition was satisfied in our samples because the Parylene films measured 1 μm thick and were therefore of negligible thickness compared to the curvature radius of the droplet surface which measured at least in some tens of millimeters. Negligible thickness of the film also meant that the bending energy of the Parylene film was negligible compared to its stretching energy. Considering the static hydraulic problem of the liquid inside the droplet, we related the pressure difference ΔP and the altitude h at an arbitrary position on the surface with the pressure difference ΔP0 and the altitude h0 at the highest point of the surface, which meant the center point of the droplet, as
ΔP ¼ 2σH
ΔP ¼ ΔP0 þ Fgðh0 - hÞ
ð1Þ
(16) Chen, T. Y.; Chiu, M. S.; Weng, C. N. J. Appl. Phys. 2006, 100, 074308–5.
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ð2Þ
with F and g denoting the density of the liquid and the acceleration of gravity, respectively. Finally, we derived the equation that DOI: 10.1021/la102790w
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expressed the effect of gravitational force on the surface shape of the droplet ΔP0 þ Fgðh0 - hÞ ¼ 2σH
ð3Þ
This equation held true for any droplet shape. However, we only considered the problem for rotationally symmetrical shapes for the sake of calculation and measurement ease. For such shapes, the altitude h of the surface profile was a function h(x) of the distance x from the center of the droplet, and the surface mean curvature H was expressed in terms of the altitude h of the surface profile as H ¼ -
h0 h00 pffiffiffiffiffiffiffiffiffiffiffiffiffiffi - pffiffiffiffiffiffiffiffiffiffiffiffiffiffi3 2x 1 þ h0 2 2 1 þ h0 2
ð4Þ
Substituting eq 4 for eq 3 gave a differential equation that governed the profile of the droplet surface. However, this equation could only be solved numerically for h(x) with specified values of surface force and, therefore, was of little use for us to calculate the surface force. Without gravitational force or with the surface force exceedingly larger than the gravitational force, the surface force tended to render the surface spherical. Surfaces too spherical could introduce large errors to the calculated values of surface tension. However, thanks to gravity, the shape of real liquid droplets was not spherical, especially those whose characteristic dimension was larger than the capillary length of the liquid λ0 =(σ/Fg)1/2. In order to limit the calculation errors due to spherical shape, we used a circular sessile droplet of 30 mm in diameter, about 15 times larger than the capillary lengths of the liquids in use, to enlarge the effect of gravity on the shape to prevent the droplet from becoming too spherical when the surface tension increased. Calculation of Film Tension. We observed the inside of the deposition chamber with the setup shown in Figure 2b and took a series of black and white photographs of the side of a liquid droplet while depositing Parylene (Figure 1b). This setup included a lighting unit and a tilted mirror to precisely photograph the side of the droplet in close-up through the glass window of the chamber. A video showing the deformation of the droplet shape is provided in the Supporting Information. The profile of the droplet surface was extracted by a homemade image processing computer program, which marked in every pixel column the pixel that had the largest change in blackness in the vertical direction. These marked pixels gave the altitude of the surface profile along a diameter direction. We obtained the function h(x) of the surface profile by compensating the distance on the diameter direction and the altitude of these pixels with the magnification of the photographs. This function of the surface profile was smoothened by approximating with a polynomial, so that we could calculate its first and second derivatives to use in the calculation of surface mean curvature. Considering the eq 3, we had the values of both the altitude and the surface mean curvature of the surface profile and two unknowns, which were ΔP0 and σ. We calculated these unknowns by fitting this eq 3 with data obtained from image processing.
3. Result and Discussion Figure 3c shows the change of the surface force acting on a silicone fluid droplet of 900 mm3 in volume during deposition of Parylene in respect to deposition time. Before deposition, the surface force was equal to the liquid surface tension of 33.9 μN/mm of the silicone fluid. This value was obvious because the liquid surface tension was the sole force that acted on the initial droplet surface. When monomer gas filled the deposition chamber, an event detected by the increase of chamber pressure and lasting about 15 min in this experiment, Parylene film started to form on the liquid surface. As a result, the surface force increased rapidly during the first 5 min of this period and gradually slowed down after it exceeded 700 μN/mm. It was worth noting that this surface force was more than 20 times larger than the liquid surface tension 18774 DOI: 10.1021/la102790w
Table 1. Calculation Results of the Film Tension of Parylene Films Deposited on Liquid Droplets surface force [mN/mm]
droplet volume [mm3]
0.0339a ∼0.030a
silicone fluid liquid paraffin parylene on silicone fluid (1) parylene on silicone fluid (2) parylene on silicone fluid (3) parylene on silicone fluid (4) parylene on liquid paraffin
0.88 0.91 0.71 0.81 0.83
average standard deviation a Data obtained from the vendor.
0.83 0.08
896 904 911 919 886
of the silicone fluid. The thickness of the Parylene film deposited on the portion of the glass wafer that was not covered by liquid was measured after the deposition was finished. The Parylene film was 1 μm thick, as thick as estimated by the amount of dimer used. Using this thickness, the film stress was calculated to be 0.7 MPa. As for the type of this film stress, because we found no wrinkle on the surface but only a change in the whole droplet shape toward a more spherical one, there was only one logical possibility meaning that the film stress was tensile, so that when it shrank, it forced the droplet to take a shape that had the same volume but less surface area. Table 1 gives the measured values of film tension of Parylene films of 1 μm in thickness deposited on silicone fluid and liquid paraffin (density=1.097 and 0.87 mg/mm3, respectively, obtained from the vendor). The values range from 0.7 to 0.9 mN/mm with an average of 0.83 mN/mm and a standard deviation of 0.08 mN/mm. These values are all at least more than 20 times larger than the liquid surface tensions of the liquids. The film stress calculated as 0.83 ( 0.08 MPa tensile, providing that the film thickness measured 1 μm. The differential of film tensions between samples is due mainly to the slight variation of deposition conditions, which is very difficult to eliminate. We also studied the error of our calculation procedure of film tension on droplet profiles generated by Surface Evolver, a computer program often used in simulating the geometry of liquid surfaces, and found that the error was less than 5% for surface tension less than 1.5 mN/mm. The series of photographs in Figure 1b shows a continuous change of surface profile of the droplet during Parylene deposition. The temperature of 25 °C and pressure of 25 mTorr inside the deposition chamber can have very limited effect on surface tension of the liquid in use, whose boiling temperature and vapor pressure are 243 °C and 3 10-9 mTorr, respectively. As expected, we found no change in the surface profile during the early stage of the deposition process when Parylene monomer gas had not filled the deposition chamber. We also verified that the final shape of the droplets right after Parylene deposition was retained after they were released from the deposition chamber. Considering the difference between the pressure of the vacuum of about 25 mTorr inside the deposition chamber and the atmospheric pressure of 760 Torr outside, one might argue that the change of droplet shape was a consequence of the change of pressure when the droplet was released from the deposition chamber. However, our observations led us to an unambiguous conclusion that this change of pressure was not the cause of the change of droplet shape. It was the tensile film stress of Parylene films that caused the change of the droplet shape, and this occurred during the deposition time. It was reported that Parylene deposition involved both the polymerization of the monomer from gas phase and the crystallization of Langmuir 2010, 26(24), 18771–18775
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Figure 4. SEM images of the surfaces of the Parylene films. (a) Complex surface that is in contact with the liquid phase during deposition. The close-up view shows polymer bridges connecting polymer islands. (b) Smooth and hole-free surface that is in contact with the gas phase during deposition.
the newly formed polymer chains.17,18 The reconfiguration of polymer chains during crystallization could change the length of the polymer chains and cause a film stress.19 As it was reported in Parylene deposition on solid substrates at room temperature, Parylene films as-deposited had compressive film stresses, though these film stresses could be rendered tensile by thermal treatment, that is, by heating and cooling appropriately.12-15 When heated to an elevated temperature, polymer chains could become mobilized and capable of moving and folding in a configuration more stable than the initial one, which resulted in film stress changing when cooled to the initial temperature. We suggest that, in the case of Parylene films deposited at room temperature, the configuration corresponding to compressive film stresses is less stable than the tensile one. When Parylene was deposited on solid surfaces, the movement of polymer chains and monomer molecules into the bulk of the solid substrate was restricted by its solid surface, while the lateral movement was obstructed by the sticking to the surface. The polymer chains might therefore not grow into an optimal configuration, creating polymer chains of large volumes. Because these polymer chains were stuck inside the bulk solid of Parylene film, they prevented the contraction of the Parylene film during polymerization and crystallization and rendered the film stress compressive. In the contrary, when Parylene was deposited on liquid surfaces, the polymer chains and the monomer molecules could diffuse, move, and fold inside the bulk liquid as well as on the liquid surface, which allowed the contraction during polymerization and crystallization and resulted in tensile film stresses of the Parylene film. We took scanning electron microscopic (SEM) images of the surfaces of the Parylene film. Panels (a) and (b) in Figure 4 show the SEM images of the surfaces in contact with the liquid phase and gas phase during deposition, respectively. The surface in contact with the gas phase during deposition was smooth and hole-free, similar to the surface of normal Parylene films deposited on solid. The surface in contact with the liquid phase during deposition, on the other hand, had a very complex morphology, with multilayer polymer islands and bridges intertwining with a network of complex voids, which must be caused by the diffusion of monomer molecules and the polymerization of Parylene in liquid. Because the gas phase continuously transported new monomer molecules to the liquid surface, the polymer islands on this side grew larger, overlapping one another and gradually covering the liquid surface (17) Kubo, S.; Wunderlich, B. J. Polym. Sci., Part B: Polym. Phys. 1972, 10, 1949–1966. (18) Treiber, G.; Boehlke, K.; Weitz, A.; Wunderlich, B. J. Polym. Sci., Part B: Polym. Phys. 1973, 11, 1111–1116. (19) Gazicki, M.; Surendran, G.; James, W.; Yasuda, H. J. Polym. Sci., Polym. Chem. 1985, 23, 2255–2277.
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completely, forming the hole-free polymer surface on the gas phase side (Figure 4b). Once a complete polymer surface was formed, it hindered the diffusion of monomer molecules into the liquid phase, therefore terminating the formation of polymer inside the liquid phase. The film then only continued to grow on the gas phase side. The effect that the liquid substrate had on the deposited Parylene film was only remarkable near the liquid surface. When the liquid surface was completely covered by Parylene film and the growing surface of Parylene film moved far away and separated from the liquid surface by the polymer bulk, the increase of film tension slowed down. This remark is supported by Figure 3c, which shows that the film tension of Parylene film increased rapidly for only a short duration at the beginning of the formation of the film when the monomer gas had just arrived at the deposition chamber. After this duration, the film tension no longer increased remarkably even though the monomer gas still filled the deposition chamber and the film therefore continued growing.
4. Conclusions When liquid surface was used as deposition substrate for Parylene, the Parylene film stress was rendered tensile and measured about 0.7-0.9 MPa. This tensile film stress was in contrary to reported compressive film stress of Parylene films deposited on solid substrates. This tensile stress reshaped liquid droplets encapsulated by POLD toward more spherical shapes during Parylene deposition. Both the film stress and the effect on the droplet shape could have important consequences on the performance of microdevices fabricated by POLD. Moreover, we also pointed out that the liquid surface might have a profound effect on the polymerization and crystallization of Parylene, especially at the interface between the liquid and the Parylene polymer. A study on the morphology of the polymer surface in contact with the liquid and the polymer bulk up to some hundreds of nanometers away from this surface might capture valuable insights about the interaction between the liquid and the low pressure monomer gas occurring on the liquid surface during the formation of such Parylene films. Acknowledgment. The photolithography masks were made using the University of Tokyo VLSI Design and Education Center (VDEC)’s 8 in. EB writer F5112 þ VD01 donated by ADVANTEST Corporation. N.B.K. was supported by the Research Fellowship of the Japan Society for the Promotion of Science. Supporting Information Available: Video showing the deformation of the droplet shape during Parylene deposition. This material is available free of charge via the Internet at http://pubs.acs.org.
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