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Solid on liquid deposition
Thin Solid Films 518 (2010) 5061–5065
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Solid on liquid deposition J. Charmet a,⁎, O. Banakh a, E. Laux a, B. Graf a, F. Dias a, A. Dunand a, H. Keppner a, G. Gorodyska b, M. Textor b, W. Noell c, N.F. de Rooij c, A. Neels d, M. Dadras d, A. Dommann d, H. Knapp d, Ch. Borter e, M. Benkhaira e a
Institut des Microtechnologies Appliquées ARC, HES-SO Arc, Eplatures-Grise17, 2300 La Chaux-de-Fonds, Switzerland BioInterface group, ETHZ, Wolfgang-Pauli-Strasse 10, ETH Hönggerberg HCI H 525 8093 Zürich, Switzerland Ecole Polytechnique Fédérale de Lausanne, Institute of Microengineering, Sensors, Actuators and Microsystems laboratory, Rue Jaquet Droz 1, 2000 Neuchâtel, Switzerland d Centre Suisse d'Electronique et de Microtechnique SA, Rue Jacquet-Droz 1, 2002 Neuchâtel, Switzerland e COMELEC SA, Rue de la Paix 129, 2300 La Chaux-de-Fonds, Switzerland b c
a r t i c l e
i n f o
Article history: Received 3 July 2009 Received in revised form 2 February 2010 Accepted 12 February 2010 Available online 19 February 2010 Keywords: Solid on liquid deposition Solid–liquid interface Chemical vapour deposition Polymers Parylene Conformal coating Packaging
a b s t r a c t A process for the deposition of a solid layer onto a liquid is presented. The polymer poly-di-chloro-paraxylylene, also known as Parylene C, was grown on low vapour pressure liquids using the conventional low pressure chemical vapour deposition process. A reactor was built and a process developed to enable the deposition of Parylene C at atmospheric pressure over high vapour pressure liquids. It was used to deposit Parylene C over water among others. In all cases Parylene C encapsulated the liquid without influencing its initial shape. The results presented here show also that the Parylene C properties are not affected by its growth on liquid templates and the roughness of the Parylene C surface in contact with the liquid during the deposition is extremely low. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Generations of scientists and engineers have established the theoretical and practical background for the manipulation of liquids whose shapes depend on their surface tension and can be influenced by a multitude of external factors. The natural shape of liquids has been utilised since men were able to control those factors. High quality lenses that rely on melted glass have been reported as early as the ancient Greece. Nowadays micro-lenses whose shape relies on the melting of photoresist are fabricated routinely. Furthermore, recent advances in the field of surface functionalisation have facilitated the shaping of liquid structures [1,2]. The possibility to stabilise a liquid of any given shape under a transparent, thin, non-deforming membrane opens up exciting opportunities. The so-called SOLID (solid on liquid deposition) process, pioneered by the authors [3] was used to encapsulate liquids under a thin membrane of poly-di-chloro-para-xylylene, a polymer commonly called Parylene C (see Fig. 1). Even though Parylene C is not the only packaging material that can be used with the SOLID process [4], its intrinsic properties, such as its conformal, stress-free deposition make it an ideal candidate to encapsulate liquids without
influencing their shape. The resulting configuration is stable and offers an interesting alternative to existing microfabrication processes used in the MEMS (Micro Electro Mechanical Systems) industry. Nevertheless, the nucleation and growth of a polymer layer on a liquid surface raise questions about its nature, property, composition and morphology and its relationship to conventional Parylene layers deposited on solid substrates. This paper outlines the deposition process and studies carried out on the Parylene C membranes grown on different liquids. It shows that Parylene C perfectly replicates and encapsulates the overgrown liquid and produces stable structures that can be used for further processing. This opens up interesting perspectives in the study of liquid–solid interaction. In fact all the polymers from the Parylene (poly (p-xylylene)) family deposited by a chemical vapour deposition process enable a stable encapsulation and replication of liquid structures. Parylene C was chosen for the purpose of this study as it is the most widely used polymer of the Parylene family due its biocompatibility and its excellent barrier properties and manufacturing advantages. 2. Experimental methods 2.1. Sample preparation
⁎ Corresponding author. E-mail address: [email protected] (J. Charmet). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.02.039
For the purpose of this paper, the SOLID encapsulation process consisted in the deposition of Parylene C. It was carried out on low
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Fig. 1. Schematical sketch of the mechanism of solid on liquid deposition so called the SOLID process. In this particular case, the hexagons represent the paraxylylene precursors during the condensation and polymerisation over a liquid.
vapour pressure liquids, between 1 and 7 Pa pressure using the conventional Parylene LPCVD (low pressure chemical vapour deposition) process also known as the Gorham process [5,6], while an APCVD (atmospheric pressure chemical vapour deposition) process, developed for the occasion, was also assessed. Parylene C layers ranging from 40 nm up to 20 μm were deposited. Various inert liquids were tested to point out the fact that the Parylene is not influenced by the liquid template on which it is deposited. For the LPCVD process: glycerol, 1,1,2-cyclohexanedicarboxylic acid diisononyl ester, bis(3,5,5-trimethylhexyl) phthalate and bis(2-ethylhexyl) adipate) were used. For the APCVD process both DI (de-ionised) water and glycerol were encapsulated. For most experiments, the liquids were dispensed on glass substrates. Clean microscope glass slides were washed 2 min in an ultrasonic acetone bath, rinsed thoroughly with DI water and dried under nitrogen flux. Then the liquids were loaded at the centre of the individual glass slides with pipettes. Care was taken not to cover the entire surface with the liquid so that the properties of Parylene C grown on a solid and on a liquid, in the same conditions, could be compared directly. Sheets of Nitto Pro Techno® 224P (blue tape) supplied by Powatec GmbH, a low surface energy polymer, were used for the contact angle measurements. The polymer sheets that had not been exposed to air previously were applied on clean microscope slides prior to the liquid dispensing. For both LPCVD and APCVD, the dimer, dichloro[2.2]- paracyclophane, obtained from Galentis Srl, is first sublimated (120 °C) and then downstream-transported through a pyrolysis stage where it is cracked into a monomer at 650 °C. The reactive monomer finally condenses and polymerises at room temperature on the samples in the deposition chamber. The reactor used for the LPCVD process was a COMELEC reactor model 1010. A home-built reactor comprising the sublimation, pyrolysis and deposition zone was used for the APCVD process. The main process difference resides in the use of an inert carrier gas, in our case nitrogen, for the transport of the dimer and monomer. The use of a carrier gas induces however heat transfer from the 650 °C hot pyrolysis station. Therefore, special care has to be taken to avoid liquid evaporation during deposition. This issue was addressed using a cooled substrate-holder. For both processes, the final film thickness depends on the amount of dimer put in the sublimation zone.
consisted in one precision syringe, one camera to take a side view of the drop and a computer with a dedicated software that calculates the contact angle by fitting the shape of the drop to the Young–Laplace equation. All measurements were made in static mode to assess the influence of the Parylene deposition on the drop shape. Parylene C layers grown simultaneously on a glass substrate and on a drop of bis(3,5,5-trimethylhexyl) phthalate were analyzed by XRD (X-ray diffraction) in Bragg–Brentano conditions using CuKα1 radiation on a Stoe STADIP high resolution diffractometer. The measurements were performed directly on the Parylene coated substrates. The Parylene C thickness used for the experiments was 2 μm. FTIR (Fourier Transform Infra Red) spectrometry measurements were performed on the films grown on glycerol and on glass substrates. The Parylene C layers were peeled off the substrate and rinsed thoroughly in ethanol prior to the measurements performed in transmission mode. The spectra were recorded on a Digilab Varian (FTS 2000). 50 scans were accommodated at a resolution of 4 cm− 1. Measurements of the Parylene C thickness and determination of optical parameters were performed by a spectroscopic phasemodulated ellipsometer HR460 (Jobin Yvon – Horiba) with measurement range from 270 to 1700 nm (0.75–4.50 eV) and an angle of incidence 70° with respect to normal. In order to find an appropriate dispersion model for refractive index of Parylene C, thick coatings (thickness about 1 micron) deposited on Si and glass were first measured. Then, by keeping the optical parameters (n and k) of Parylene C fixed, the thickness of the thinner films was calculated by fitting the experimental curves of psi and delta. In some cases, the optical dispersion model was slightly modified in order to better fit to the experimental ellipsometric data. It was observed that for thinner films (used dimer mass lower than 0.2 g), the refractive index n (measured at 633 nm) was about 1.65, while for thicker films (dimer mass more than 0.4 g) its value was 1.69–1.70. This might indicate the densification of the film with increased thickness. Two sets of samples coated with Parylene C were measured. The first set of samples has a Parylene film of a variable thickness deposited directly onto microscopic glass slides. The second set has the same Parylene film covering a bis(3,5,5-trimethylhexyl) phthalate drop put on the microscopic glass slide. Two ellipsometric sample structure models were adopted, respectively. Model 1: Semi-infinite substrate: air // layer 1: glass (thickness 1 mm) // layer 2: Parylene and Model 2: Semi-infinite substrate: liquid// layer 1: Parylene. The appropriateness of the second model is questionable, because we do not know much about the thickness of the bis(3,5,5-trimethylhexyl) phthalate drop, nor about its optical constants. Supposing that the light will be attenuated or diffused inside the oil drop and no light will be reflected back at the interface between the oil and glass, we assumed the oil to be a semi-infinite medium. Although this model should be taken with precautions, it gives reasonable values of thickness and optical parameters of Parylene C. Atomic force microscopy (AFM) images were obtained by scanning the samples with a MultiMode instrument (Digital Instruments, Santa Barbara, CA) operating in the tapping mode. Silicon tips (OMCLAC160TS from Olympus Corporation, Japan) with a radius less than 7 nm, spring constant of 42 N/m, and resonance frequency of 300 kHz were used. Parylene layers grown on a glycerol drop were peeled and rinsed thoroughly to remove all the glycerol. The area of investigation was the Parylene surface grown directly over the glycerol drop.
3. Results and discussions 2.2. Characterisation 3.1. Observation of the phenomenon Static contact angle measurements were performed within one minute of substrate preparation, to reduce risks of surface contamination, and after the Parylene deposition. The equipment, KSV 100,
The SOLID process enables the encapsulation of liquids to yield stable naturally driven structures as can be seen in Fig. 2a–d.
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Fig. 2. Microscopic a),b) and macroscopic c),d) Parylene-based SOLID encapsulated liquid structures. a) scanning electron microscopy image of SOLID encapsulated micro-droplets. b) Micrograph of a SOLID encapsulated micro-structure obtained by dipping the end of a fibre optic in a liquid before the SOLID encapsulation. c) Photography of a SOLID based optical lens in front of a text. The fabrication steps of the lens consisted in the SOLID encapsulation of a liquid drop on a transparent substrate. d) Photography of a macroscopic SOLID system. A liquid was dispensed over a large area on a flexible substrate. On the figure the structure is bent resulting in wrinkles on the Parylene layer.
3.2. Evidence of Parylene C on liquid The properties of the polymer grown over liquids and on solid substrate were measured and compared to assess the limits of the SOLID process for further MEMS processing. The XRD analysis has confirmed that the layer grown on the glass substrate and on the bis(3,5,5-trimethylhexyl) phthalate show the same characteristic peaks. For both measurements, one very strong reflection is found at about 14.05 ° in 2Theta corresponding to a d-spacing of 6.30 Å. According to the unit cell information given for Parylene C in the literature (a=5.92 Å, b=12.69 Å, c=6.60 Å, β=135.2 °, V=349 Å3), the first strong peak corresponds to the 020 reflection. The weaker peaks at 3.15 Å and 2.10 Å correspond to reflections of higher order such as 040 and 060, respectively. The fibre texture has been confirmed for both samples. In Fig. 3 the overlay plot compares the results of the X-ray measurement of the glass substrate and Parylene C grown on glass and on liquid. FTIR spectra of Parylene C films grown using the conventional LPCVD process under identical conditions on Glycerol and glass substrates show no major differences (see Fig. 4). It confirms that the layer deposited onto liquids is indeed Parylene C. Moreover Parylene was deposited on DI water and glycerol using the APCVD reactor described above. FTIR measurement confirmed that the material deposited is identical to that deposited using the conventional LPCVD process, even though the deposition rate measured for the APCVD process is reduced by half when compared to the deposition rate using the LPCVD process. This exciting result paves the way for the development of drug delivery systems and reservoirs for biomedical applications. 3.3. Mechanical solid–liquid interaction To point out the impact of the Parylene C growth on the shape of the liquid template, the contact angles of drops of various liquids
dispensed on a clean blue tape polymer substrate were measured before and after deposition. For thickness up to 2 μm, indeed, no changes in contact angle can be observed for drops of bis(3,5,5trimethylhexyl) phthalate prior and after deposition (see Fig. 5a). The lower contact angle presented for the 0.2 µm Parylene thickness is due to a higher surface energy contaminated substrate. This result shows that SOLID encapsulation has no influence on the contact angle prior and after deposition for different substrate conditions. For thickness of 12 μm and more, a slight systematic reduction of the contact angle can be noted. This is assumed to be due to the smoothing effects at the solid–liquid–gas interface where the liquid
Fig. 3. X-ray diffraction patterns of Parylene. The Parylene layer grown on glass (blue curve) and the Parylene on the bis(3,5,5-trimethylhexyl) phthalate (black curve) show the same diffraction peaks in their X-ray diffraction patterns (the diffractogram of the glass substrate is shown in red).
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3.4. Optical properties
Fig. 4. FTIR spectra of Parylene layers grown on Glycerol (blue curve) and on a glass substrate (red curve).
and the solid form a corner that offers preferential condensation site. In Fig. 5b the same measurements have been carried out on different liquids. The possibility to encapsulate drops with contact angles of up to 150 ° was also demonstrated. Those results prove that Parylene polymerization does not exert any stress on the liquids as the contact angle prior and after deposition, hence the initial shape of the liquid is not affected. In combination with appropriate physical and/or chemical patterning of liquid structures, this property has been used to fabricate complex SOLID structures [7].
Spectroscopic ellipsometry was used to determine the thickness and the optical constants of solid layers deposited on bis(3,5,5trimethylhexyl) phthalate using the standard LPCVD process test liquid. The results were compared to Parylene layers grown on the glass solid substrate next to the drop. In Fig. 6 the results of a thickness series as a function of the amount of dimer put in the reactor are represented; it was attempted to reduce the thickness down to the onset of complete hermetic coverage. The thickness values, obtained on the films deposited on a bis(3,5,5-trimethylhexyl) phthalate drop, are lower than that obtained on films deposited on glass. This difference was assumed to be due to monomers diffusing inside the liquid and creating a denser surface on which the polymerisation can take place. This was confirmed by the ellipsometric model that gave the best results using a liquid with a refractive index of 1,66 close to that of Parylene C. The thickness of the Parylene films deposited directly on glass is measurable down to 49 nm (corresponding to 0.1 g of dimer mass). The thickness of the same Parylene film deposited on oil cannot be calculated using the ellipsometric sample structure model as those used for the thicker films. The fitting indicates a very high error. However, the presence of the Parylene layer on liquid is still identifiable by looking at the effective values of optical constant bnN and bkN, which are different from those of the bis(3,5,5trimethylhexyl) phthalate without any film. Indeed, the effective optical parameters bnN and bkN progressively increase with increasing the dimer mass for all samples (films deposited on glass substrates and liquid drops). In the run of these experiments, Parylene C layers as thin as 39.1 nm could be prepared and measured directly on the liquid. Moreover, we observed that for thinner films, the Parylene C refractive index n (taken at 633 nm) is about 1.65, while for thicker films its value is between 1.67 and 1.69. This indicates the densification of the film with increasing thickness.
3.5. Morphology of Parylene C on low-vapour pressure liquids Outstanding step-coverage and conformity, lack of stress, together with the natural tendency of liquids to minimise their surfaces, make Parylene-based SOLID structures, interesting candidate for the fabrication of perfectly smooth surfaces. This assumption is confirmed by the AFM (atomic force microscopy) measurement represented in Fig. 7, whereby a RMS (root mean square) roughness of about 1 nm was found on an area of 2 × 2 μm. This value indicates that the liquid is the driver of the overgrowing polymer layer.
Fig. 5. a: The graph shows the contact angle of drops of bis(3,5,5-trimethylhexyl) phthalate prior and after deposition for different Parylene thickness. b: The graph shows the contact angle of various liquids prior and after a 2 μm Parylene deposition. The liquids used were: Liquid 1: 1,2-cyclohexanedicarboxylic acid diisononyl ester, liquid 2: bis(3,5,5trimethylhexyl) phthalate and liquid 3: Bis(2-ethylhexyl) adipate.
Fig. 6. The thickness of Parylene films, deposited directly on glass substrate (squares) and onto a bis(3,5,5-trimethylhexyl) phthalate liquid drop (triangles), measured by ellipsometry as a function of the dimer mass.
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of Parylene are not influenced by the substrate on which it grows, as confirmed by FTIR and XRD measurements. Furthermore, it was confirmed that Parylene layers ranging from nanometer to tens of microns in thickness grow over the liquids without influencing their shapes. A further study by AFM measurements, confirmed that the perfectly smooth liquid surface drives the polymer growth down to a RMS roughness of 1 nm on a 2 × 2 μm area. Moreover, the possibility to deposit Parylene on “high vapour pressure” liquids such as water, opens up interesting perspectives in the field of biomedical devices. Overall, the results presented in this paper pave the way to the development of high quality devices and components that rely upon the naturally driven shape of liquids.
Fig. 7. AFM micrograph (2× 2 mkm) of the Parylene interface grown directly on the liquid substrate and corresponding z-axis scale bar. The average RMS roughness is 1.28± 0.26 nm.
4. Conclusions The possibility to encapsulate and stabilise in a one step process, naturally driven liquid structures opens up interesting perspectives for the development of low cost high quality devices. The SOLID process proposes an alternative to conventional three-dimensional microfabrication processes. It could as a consequence impact the next generation devices in the field of microfluidics, membranes technology, optics, sensors and actuators and could stand at the beginning of a next generation of medical devices. Even though other materials exist, the most promising SOLID encapsulating candidate is Parylene as it hermetically seals liquids under a stable transparent, non-deforming thin film. The SOLID encapsulated structures are suitable for subsequent applications as components or devices as the bulk properties
Acknowledgements We would like to thank L. Aeschimann, M. Höland, L. Steinmann, D. Joss, W. Lam and H. Haquette for valuable technical assistance. Swiss Innovation Promotion Agency under contract Nr. 8241.1 DCS-NM and the European project MULTIPOL (FP6-NMP4-STREP 033201) for the funding of this work.
References [1] [2] [3] [4]
S. Morgenthaler, S. Lee, S. Zürcher, N.D. Spencer, Langmuir 19 (2003) 10459. D. Falconnet, A. Koenig, F. Assi, M. Textor, Adv. Funct. Mater. 14 (2004) 749. H. Keppner, M. Benkhaira, Patent WO/2006/06395, Dec. 16 2004. E. Laux, J. Charmet, H. Haquette, O. Banakh, L. Jeandupeux, B. Graf, H. Keppner, J. Phys. Conf. Ser. 182 (2009) 012029. [5] H.F. Mark, Encyclopedia of Polymer Science and Technology, John Wiley & Sons, New York, 2004, p. 589. [6] W.F. Gorham, J. Poly, Sci., Part A-1: Polym. Chem. 4 (1966) 3027. [7] J. Charmet, H. Haquette, E. Laux, G. Gorodyska, M. Textor, G. Spinola Durante, E. Portuondo-Campa, H. Knapp, R. Bitterli, W. Noell, H. Keppner, J. Phys. Conf. Ser. 182 (2009) 012021.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Solid on liquid deposition J. Charmet a,⁎, O. Banakh a, E. Laux a, B. Graf a, F. Dias a, A. Dunand a, H. Keppner a, G. Gorodyska b, M. Textor b, W. Noell c, N.F. de Rooij c, A. Neels d, M. Dadras d, A. Dommann d, H. Knapp d, Ch. Borter e, M. Benkhaira e a
Institut des Microtechnologies Appliquées ARC, HES-SO Arc, Eplatures-Grise17, 2300 La Chaux-de-Fonds, Switzerland BioInterface group, ETHZ, Wolfgang-Pauli-Strasse 10, ETH Hönggerberg HCI H 525 8093 Zürich, Switzerland Ecole Polytechnique Fédérale de Lausanne, Institute of Microengineering, Sensors, Actuators and Microsystems laboratory, Rue Jaquet Droz 1, 2000 Neuchâtel, Switzerland d Centre Suisse d'Electronique et de Microtechnique SA, Rue Jacquet-Droz 1, 2002 Neuchâtel, Switzerland e COMELEC SA, Rue de la Paix 129, 2300 La Chaux-de-Fonds, Switzerland b c
a r t i c l e
i n f o
Article history: Received 3 July 2009 Received in revised form 2 February 2010 Accepted 12 February 2010 Available online 19 February 2010 Keywords: Solid on liquid deposition Solid–liquid interface Chemical vapour deposition Polymers Parylene Conformal coating Packaging
a b s t r a c t A process for the deposition of a solid layer onto a liquid is presented. The polymer poly-di-chloro-paraxylylene, also known as Parylene C, was grown on low vapour pressure liquids using the conventional low pressure chemical vapour deposition process. A reactor was built and a process developed to enable the deposition of Parylene C at atmospheric pressure over high vapour pressure liquids. It was used to deposit Parylene C over water among others. In all cases Parylene C encapsulated the liquid without influencing its initial shape. The results presented here show also that the Parylene C properties are not affected by its growth on liquid templates and the roughness of the Parylene C surface in contact with the liquid during the deposition is extremely low. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Generations of scientists and engineers have established the theoretical and practical background for the manipulation of liquids whose shapes depend on their surface tension and can be influenced by a multitude of external factors. The natural shape of liquids has been utilised since men were able to control those factors. High quality lenses that rely on melted glass have been reported as early as the ancient Greece. Nowadays micro-lenses whose shape relies on the melting of photoresist are fabricated routinely. Furthermore, recent advances in the field of surface functionalisation have facilitated the shaping of liquid structures [1,2]. The possibility to stabilise a liquid of any given shape under a transparent, thin, non-deforming membrane opens up exciting opportunities. The so-called SOLID (solid on liquid deposition) process, pioneered by the authors [3] was used to encapsulate liquids under a thin membrane of poly-di-chloro-para-xylylene, a polymer commonly called Parylene C (see Fig. 1). Even though Parylene C is not the only packaging material that can be used with the SOLID process [4], its intrinsic properties, such as its conformal, stress-free deposition make it an ideal candidate to encapsulate liquids without
influencing their shape. The resulting configuration is stable and offers an interesting alternative to existing microfabrication processes used in the MEMS (Micro Electro Mechanical Systems) industry. Nevertheless, the nucleation and growth of a polymer layer on a liquid surface raise questions about its nature, property, composition and morphology and its relationship to conventional Parylene layers deposited on solid substrates. This paper outlines the deposition process and studies carried out on the Parylene C membranes grown on different liquids. It shows that Parylene C perfectly replicates and encapsulates the overgrown liquid and produces stable structures that can be used for further processing. This opens up interesting perspectives in the study of liquid–solid interaction. In fact all the polymers from the Parylene (poly (p-xylylene)) family deposited by a chemical vapour deposition process enable a stable encapsulation and replication of liquid structures. Parylene C was chosen for the purpose of this study as it is the most widely used polymer of the Parylene family due its biocompatibility and its excellent barrier properties and manufacturing advantages. 2. Experimental methods 2.1. Sample preparation
⁎ Corresponding author. E-mail address: [email protected] (J. Charmet). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.02.039
For the purpose of this paper, the SOLID encapsulation process consisted in the deposition of Parylene C. It was carried out on low
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Fig. 1. Schematical sketch of the mechanism of solid on liquid deposition so called the SOLID process. In this particular case, the hexagons represent the paraxylylene precursors during the condensation and polymerisation over a liquid.
vapour pressure liquids, between 1 and 7 Pa pressure using the conventional Parylene LPCVD (low pressure chemical vapour deposition) process also known as the Gorham process [5,6], while an APCVD (atmospheric pressure chemical vapour deposition) process, developed for the occasion, was also assessed. Parylene C layers ranging from 40 nm up to 20 μm were deposited. Various inert liquids were tested to point out the fact that the Parylene is not influenced by the liquid template on which it is deposited. For the LPCVD process: glycerol, 1,1,2-cyclohexanedicarboxylic acid diisononyl ester, bis(3,5,5-trimethylhexyl) phthalate and bis(2-ethylhexyl) adipate) were used. For the APCVD process both DI (de-ionised) water and glycerol were encapsulated. For most experiments, the liquids were dispensed on glass substrates. Clean microscope glass slides were washed 2 min in an ultrasonic acetone bath, rinsed thoroughly with DI water and dried under nitrogen flux. Then the liquids were loaded at the centre of the individual glass slides with pipettes. Care was taken not to cover the entire surface with the liquid so that the properties of Parylene C grown on a solid and on a liquid, in the same conditions, could be compared directly. Sheets of Nitto Pro Techno® 224P (blue tape) supplied by Powatec GmbH, a low surface energy polymer, were used for the contact angle measurements. The polymer sheets that had not been exposed to air previously were applied on clean microscope slides prior to the liquid dispensing. For both LPCVD and APCVD, the dimer, dichloro[2.2]- paracyclophane, obtained from Galentis Srl, is first sublimated (120 °C) and then downstream-transported through a pyrolysis stage where it is cracked into a monomer at 650 °C. The reactive monomer finally condenses and polymerises at room temperature on the samples in the deposition chamber. The reactor used for the LPCVD process was a COMELEC reactor model 1010. A home-built reactor comprising the sublimation, pyrolysis and deposition zone was used for the APCVD process. The main process difference resides in the use of an inert carrier gas, in our case nitrogen, for the transport of the dimer and monomer. The use of a carrier gas induces however heat transfer from the 650 °C hot pyrolysis station. Therefore, special care has to be taken to avoid liquid evaporation during deposition. This issue was addressed using a cooled substrate-holder. For both processes, the final film thickness depends on the amount of dimer put in the sublimation zone.
consisted in one precision syringe, one camera to take a side view of the drop and a computer with a dedicated software that calculates the contact angle by fitting the shape of the drop to the Young–Laplace equation. All measurements were made in static mode to assess the influence of the Parylene deposition on the drop shape. Parylene C layers grown simultaneously on a glass substrate and on a drop of bis(3,5,5-trimethylhexyl) phthalate were analyzed by XRD (X-ray diffraction) in Bragg–Brentano conditions using CuKα1 radiation on a Stoe STADIP high resolution diffractometer. The measurements were performed directly on the Parylene coated substrates. The Parylene C thickness used for the experiments was 2 μm. FTIR (Fourier Transform Infra Red) spectrometry measurements were performed on the films grown on glycerol and on glass substrates. The Parylene C layers were peeled off the substrate and rinsed thoroughly in ethanol prior to the measurements performed in transmission mode. The spectra were recorded on a Digilab Varian (FTS 2000). 50 scans were accommodated at a resolution of 4 cm− 1. Measurements of the Parylene C thickness and determination of optical parameters were performed by a spectroscopic phasemodulated ellipsometer HR460 (Jobin Yvon – Horiba) with measurement range from 270 to 1700 nm (0.75–4.50 eV) and an angle of incidence 70° with respect to normal. In order to find an appropriate dispersion model for refractive index of Parylene C, thick coatings (thickness about 1 micron) deposited on Si and glass were first measured. Then, by keeping the optical parameters (n and k) of Parylene C fixed, the thickness of the thinner films was calculated by fitting the experimental curves of psi and delta. In some cases, the optical dispersion model was slightly modified in order to better fit to the experimental ellipsometric data. It was observed that for thinner films (used dimer mass lower than 0.2 g), the refractive index n (measured at 633 nm) was about 1.65, while for thicker films (dimer mass more than 0.4 g) its value was 1.69–1.70. This might indicate the densification of the film with increased thickness. Two sets of samples coated with Parylene C were measured. The first set of samples has a Parylene film of a variable thickness deposited directly onto microscopic glass slides. The second set has the same Parylene film covering a bis(3,5,5-trimethylhexyl) phthalate drop put on the microscopic glass slide. Two ellipsometric sample structure models were adopted, respectively. Model 1: Semi-infinite substrate: air // layer 1: glass (thickness 1 mm) // layer 2: Parylene and Model 2: Semi-infinite substrate: liquid// layer 1: Parylene. The appropriateness of the second model is questionable, because we do not know much about the thickness of the bis(3,5,5-trimethylhexyl) phthalate drop, nor about its optical constants. Supposing that the light will be attenuated or diffused inside the oil drop and no light will be reflected back at the interface between the oil and glass, we assumed the oil to be a semi-infinite medium. Although this model should be taken with precautions, it gives reasonable values of thickness and optical parameters of Parylene C. Atomic force microscopy (AFM) images were obtained by scanning the samples with a MultiMode instrument (Digital Instruments, Santa Barbara, CA) operating in the tapping mode. Silicon tips (OMCLAC160TS from Olympus Corporation, Japan) with a radius less than 7 nm, spring constant of 42 N/m, and resonance frequency of 300 kHz were used. Parylene layers grown on a glycerol drop were peeled and rinsed thoroughly to remove all the glycerol. The area of investigation was the Parylene surface grown directly over the glycerol drop.
3. Results and discussions 2.2. Characterisation 3.1. Observation of the phenomenon Static contact angle measurements were performed within one minute of substrate preparation, to reduce risks of surface contamination, and after the Parylene deposition. The equipment, KSV 100,
The SOLID process enables the encapsulation of liquids to yield stable naturally driven structures as can be seen in Fig. 2a–d.
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Fig. 2. Microscopic a),b) and macroscopic c),d) Parylene-based SOLID encapsulated liquid structures. a) scanning electron microscopy image of SOLID encapsulated micro-droplets. b) Micrograph of a SOLID encapsulated micro-structure obtained by dipping the end of a fibre optic in a liquid before the SOLID encapsulation. c) Photography of a SOLID based optical lens in front of a text. The fabrication steps of the lens consisted in the SOLID encapsulation of a liquid drop on a transparent substrate. d) Photography of a macroscopic SOLID system. A liquid was dispensed over a large area on a flexible substrate. On the figure the structure is bent resulting in wrinkles on the Parylene layer.
3.2. Evidence of Parylene C on liquid The properties of the polymer grown over liquids and on solid substrate were measured and compared to assess the limits of the SOLID process for further MEMS processing. The XRD analysis has confirmed that the layer grown on the glass substrate and on the bis(3,5,5-trimethylhexyl) phthalate show the same characteristic peaks. For both measurements, one very strong reflection is found at about 14.05 ° in 2Theta corresponding to a d-spacing of 6.30 Å. According to the unit cell information given for Parylene C in the literature (a=5.92 Å, b=12.69 Å, c=6.60 Å, β=135.2 °, V=349 Å3), the first strong peak corresponds to the 020 reflection. The weaker peaks at 3.15 Å and 2.10 Å correspond to reflections of higher order such as 040 and 060, respectively. The fibre texture has been confirmed for both samples. In Fig. 3 the overlay plot compares the results of the X-ray measurement of the glass substrate and Parylene C grown on glass and on liquid. FTIR spectra of Parylene C films grown using the conventional LPCVD process under identical conditions on Glycerol and glass substrates show no major differences (see Fig. 4). It confirms that the layer deposited onto liquids is indeed Parylene C. Moreover Parylene was deposited on DI water and glycerol using the APCVD reactor described above. FTIR measurement confirmed that the material deposited is identical to that deposited using the conventional LPCVD process, even though the deposition rate measured for the APCVD process is reduced by half when compared to the deposition rate using the LPCVD process. This exciting result paves the way for the development of drug delivery systems and reservoirs for biomedical applications. 3.3. Mechanical solid–liquid interaction To point out the impact of the Parylene C growth on the shape of the liquid template, the contact angles of drops of various liquids
dispensed on a clean blue tape polymer substrate were measured before and after deposition. For thickness up to 2 μm, indeed, no changes in contact angle can be observed for drops of bis(3,5,5trimethylhexyl) phthalate prior and after deposition (see Fig. 5a). The lower contact angle presented for the 0.2 µm Parylene thickness is due to a higher surface energy contaminated substrate. This result shows that SOLID encapsulation has no influence on the contact angle prior and after deposition for different substrate conditions. For thickness of 12 μm and more, a slight systematic reduction of the contact angle can be noted. This is assumed to be due to the smoothing effects at the solid–liquid–gas interface where the liquid
Fig. 3. X-ray diffraction patterns of Parylene. The Parylene layer grown on glass (blue curve) and the Parylene on the bis(3,5,5-trimethylhexyl) phthalate (black curve) show the same diffraction peaks in their X-ray diffraction patterns (the diffractogram of the glass substrate is shown in red).
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Fig. 4. FTIR spectra of Parylene layers grown on Glycerol (blue curve) and on a glass substrate (red curve).
and the solid form a corner that offers preferential condensation site. In Fig. 5b the same measurements have been carried out on different liquids. The possibility to encapsulate drops with contact angles of up to 150 ° was also demonstrated. Those results prove that Parylene polymerization does not exert any stress on the liquids as the contact angle prior and after deposition, hence the initial shape of the liquid is not affected. In combination with appropriate physical and/or chemical patterning of liquid structures, this property has been used to fabricate complex SOLID structures [7].
Spectroscopic ellipsometry was used to determine the thickness and the optical constants of solid layers deposited on bis(3,5,5trimethylhexyl) phthalate using the standard LPCVD process test liquid. The results were compared to Parylene layers grown on the glass solid substrate next to the drop. In Fig. 6 the results of a thickness series as a function of the amount of dimer put in the reactor are represented; it was attempted to reduce the thickness down to the onset of complete hermetic coverage. The thickness values, obtained on the films deposited on a bis(3,5,5-trimethylhexyl) phthalate drop, are lower than that obtained on films deposited on glass. This difference was assumed to be due to monomers diffusing inside the liquid and creating a denser surface on which the polymerisation can take place. This was confirmed by the ellipsometric model that gave the best results using a liquid with a refractive index of 1,66 close to that of Parylene C. The thickness of the Parylene films deposited directly on glass is measurable down to 49 nm (corresponding to 0.1 g of dimer mass). The thickness of the same Parylene film deposited on oil cannot be calculated using the ellipsometric sample structure model as those used for the thicker films. The fitting indicates a very high error. However, the presence of the Parylene layer on liquid is still identifiable by looking at the effective values of optical constant bnN and bkN, which are different from those of the bis(3,5,5trimethylhexyl) phthalate without any film. Indeed, the effective optical parameters bnN and bkN progressively increase with increasing the dimer mass for all samples (films deposited on glass substrates and liquid drops). In the run of these experiments, Parylene C layers as thin as 39.1 nm could be prepared and measured directly on the liquid. Moreover, we observed that for thinner films, the Parylene C refractive index n (taken at 633 nm) is about 1.65, while for thicker films its value is between 1.67 and 1.69. This indicates the densification of the film with increasing thickness.
3.5. Morphology of Parylene C on low-vapour pressure liquids Outstanding step-coverage and conformity, lack of stress, together with the natural tendency of liquids to minimise their surfaces, make Parylene-based SOLID structures, interesting candidate for the fabrication of perfectly smooth surfaces. This assumption is confirmed by the AFM (atomic force microscopy) measurement represented in Fig. 7, whereby a RMS (root mean square) roughness of about 1 nm was found on an area of 2 × 2 μm. This value indicates that the liquid is the driver of the overgrowing polymer layer.
Fig. 5. a: The graph shows the contact angle of drops of bis(3,5,5-trimethylhexyl) phthalate prior and after deposition for different Parylene thickness. b: The graph shows the contact angle of various liquids prior and after a 2 μm Parylene deposition. The liquids used were: Liquid 1: 1,2-cyclohexanedicarboxylic acid diisononyl ester, liquid 2: bis(3,5,5trimethylhexyl) phthalate and liquid 3: Bis(2-ethylhexyl) adipate.
Fig. 6. The thickness of Parylene films, deposited directly on glass substrate (squares) and onto a bis(3,5,5-trimethylhexyl) phthalate liquid drop (triangles), measured by ellipsometry as a function of the dimer mass.
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of Parylene are not influenced by the substrate on which it grows, as confirmed by FTIR and XRD measurements. Furthermore, it was confirmed that Parylene layers ranging from nanometer to tens of microns in thickness grow over the liquids without influencing their shapes. A further study by AFM measurements, confirmed that the perfectly smooth liquid surface drives the polymer growth down to a RMS roughness of 1 nm on a 2 × 2 μm area. Moreover, the possibility to deposit Parylene on “high vapour pressure” liquids such as water, opens up interesting perspectives in the field of biomedical devices. Overall, the results presented in this paper pave the way to the development of high quality devices and components that rely upon the naturally driven shape of liquids.
Fig. 7. AFM micrograph (2× 2 mkm) of the Parylene interface grown directly on the liquid substrate and corresponding z-axis scale bar. The average RMS roughness is 1.28± 0.26 nm.
4. Conclusions The possibility to encapsulate and stabilise in a one step process, naturally driven liquid structures opens up interesting perspectives for the development of low cost high quality devices. The SOLID process proposes an alternative to conventional three-dimensional microfabrication processes. It could as a consequence impact the next generation devices in the field of microfluidics, membranes technology, optics, sensors and actuators and could stand at the beginning of a next generation of medical devices. Even though other materials exist, the most promising SOLID encapsulating candidate is Parylene as it hermetically seals liquids under a stable transparent, non-deforming thin film. The SOLID encapsulated structures are suitable for subsequent applications as components or devices as the bulk properties
Acknowledgements We would like to thank L. Aeschimann, M. Höland, L. Steinmann, D. Joss, W. Lam and H. Haquette for valuable technical assistance. Swiss Innovation Promotion Agency under contract Nr. 8241.1 DCS-NM and the European project MULTIPOL (FP6-NMP4-STREP 033201) for the funding of this work.
References [1] [2] [3] [4]
S. Morgenthaler, S. Lee, S. Zürcher, N.D. Spencer, Langmuir 19 (2003) 10459. D. Falconnet, A. Koenig, F. Assi, M. Textor, Adv. Funct. Mater. 14 (2004) 749. H. Keppner, M. Benkhaira, Patent WO/2006/06395, Dec. 16 2004. E. Laux, J. Charmet, H. Haquette, O. Banakh, L. Jeandupeux, B. Graf, H. Keppner, J. Phys. Conf. Ser. 182 (2009) 012029. [5] H.F. Mark, Encyclopedia of Polymer Science and Technology, John Wiley & Sons, New York, 2004, p. 589. [6] W.F. Gorham, J. Poly, Sci., Part A-1: Polym. Chem. 4 (1966) 3027. [7] J. Charmet, H. Haquette, E. Laux, G. Gorodyska, M. Textor, G. Spinola Durante, E. Portuondo-Campa, H. Knapp, R. Bitterli, W. Noell, H. Keppner, J. Phys. Conf. Ser. 182 (2009) 012021.