Abstract:
A method for producing a membrane device by providing a substrate ( 24 ), adding a liquid ( 16 ) thereon, and covering the liquid ( 16 ) and at least one substrate portion bearing the liquid ( 16 ) with a homogenous continuous thin film ( 18 ) by means of a low-pressure deposition process. The liquid is made of a plastic material and forms a membrane.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
       [0001]    The present invention relates to the field of membrane devices. It relates more particularly to a method for producing plastic membrane micro-structures, as well as devices obtained by this method. 
         [0002]    The invention is particularly, but not exclusively, interesting for applications in the microtechnic field, for example in manufacturing microlenses, microchannels, wave guides. 
       SUMMARY OF THE INVENTION 
       [0003]    More precisely, the invention relates to a method for producing a membrane device, characterized in that it consists in: 
         [0004]    providing a substrate, 
         [0005]    adding a liquid thereon, and 
         [0006]    covering said liquid and at least one substrate portion bearing the liquid by a homogenous continuous thin film, which is made of a plastic material and forms a membrane, by means of a low-pressure deposition process. 
         [0007]    The invention also relates to various embodiments of a device obtained by the above method in which the substrate forms, with the liquid and the membrane, either an optical lens, a wave guide, an actuator, or a fluid channel. 
         [0008]    The invention lastly relates to a method for producing a membrane, characterized in that it consists in: 
         [0009]    providing a substrate, 
         [0010]    adding a liquid thereon, 
         [0011]    covering said liquid by a homogenous continuous thin film, which is made of a plastic material and forms a membrane, by means of a low-pressure deposition process, then 
         [0012]    detaching this membrane from the liquid. 
         [0013]    Other characteristics will be shown more clearly upon reading the description which follows, provided in reference to the annexed drawings, in which  FIGS. 1 through 8  present different membrane devices obtained thanks to the method according to the invention, as well as their embodiment. 
         [0014]    As already mentioned, the invention relates, in substance, to the deposition of a thin film on a liquid and, possibly, on at least one neighboring portion of its substrate. This film is advantageously made up of a substance known for its coating properties, such as parylene. 
         [0015]    The method of depositing the parylene, also called polyparaxylylene, is well known by those skilled in the art. Indeed, parylene is a polymer belonging to the plastics family, used as a coating for multiple applications in the electronic, medical, optical, spatial and micro-mechanic fields, among others. Deposited at low pressure (7 Pa) by pyrolyzing the diparaxylylene, it polymerizes at room temperature on all types of substrates, forming a uniform, homogenous, colorless layer perfectly fitting any excrescences and cavities, and having a thickness from approximately one hundred nanometers to sixty microns. 
         [0016]    Parylene effectively protects various materials such as metal, textile, paper, glass, plastic and others from moisture, corrosion and etching with chemical products, acids and solvents. It forms an electrically insulating, thermally stable and biocompatible barrier. Its deposition may advantageously be done using equipment by the company Comelec (La Chaux-de-Fonds, CH). 
         [0017]    Although widely spread in many industries, for various applications, parylene coating has, until now, been limited to solid materials. 
         [0018]    The present invention opens a whole new field of investigation and offers new and original possibilities by extending parylene deposition to some liquid materials which meet specific conditions. 
         [0019]    According to the invention, parylene deposition on the liquid-substrate combination is done under standard conditions. Pyrolysis of diparaxylylene is done at 680° C., the paraxylylene monomer polymerizing to the surface of the sample being kept at room temperature. The pressure in the deposition chamber being at 7 Pa, the saturation vapor pressure of the liquid must be less than this pressure, in order to avoid being vaporized during deposition. Ideally, the saturation vapor pressure of the liquid is less than or equal to one tenth of the deposit pressure, or 0.7 Pa. However, for some very specific applications, the saturation vapor pressure of the liquid must be in the vicinity of the deposit pressure, as explained later. 
         [0020]    Moreover, the liquid used must be chemically non-reactive with parylene. Various oils, for example, have been successfully tested, including turbo-molecular pump oil, optical oils, silicone oils, glycerin, etc. . . . A non-exhaustive list of the different liquids able to be used for parylene deposition, as well as their properties, is provided in the following table. Of course, any other liquid meeting the aforementioned conditions may be used for parylene deposition. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 Vapor 
                   
               
               
                   
                 Chemical 
                 Pressure 
               
               
                   
                 Formula 
                 (Pa, 25° C.) 
                 Appearance 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Liquid 
                   
                   
                   
               
               
                 But-2-ene-1,4- 
                 C 4 H 8 O 2   
                 0.63 
                 Pale, 
               
               
                 diol 
                   
                   
                 yellowish 
               
               
                 Propanetriol 
                 C 9 H 14 O 6   
                 0.33 
                 Oily liquid 
               
               
                 triacetate 
               
               
                 Tributyl 
                 C 12 H 27 O 4 P 
                 0.53 
                 Clear, 
               
               
                 Phosphate 
                   
                   
                 colorless 
               
               
                 Sulfuric acid 
                 H 2 SO 4   
                 7.9 × 10 −3   
                 Oily, 
               
               
                   
                   
                   
                 colorless 
               
               
                 Organic acids 
               
               
                 Silicone oils 
                   
                 Very low 
                 transparent 
               
               
                 Vacuum pump oil 
                   
                 &lt;10 −3    
                 transparent 
               
               
                 Optical oils 
                   
                 Very low 
                 Various 
               
               
                   
                   
                   
                 signs 
               
               
                 Nonylphenol 
                 C 6 H 14 (OH)C 9 H 19   
                 0.3  
                 Yellowish 
               
               
                 1,2 
                 CH 3 —CO—CH 2 — 
                 Very low 
                 Yellowish 
               
               
                 Diacetoxypropane 
                 CH(O—CO—CH 3 )— 
               
               
                   
                 CH 3   
               
               
                 Ionic liquids 
                   
                 Very low 
               
               
                 Cyhalothrin 
                 C 23 H 19 ClF 3 NO 3   
                 10 −6   
                 Clear, 
               
               
                   
                   
                   
                 viscous 
               
               
                   
               
             
          
         
       
     
         [0021]    In the rest of the description, the word “liquid” designates any of the liquids listed above. 
         [0022]    At the end of the method, a uniform and continuous film covers all of the liquid-substrate, thus forming an envelope fitting its contours perfectly. In particular, there is no discontinuity of the layer at the liquid-substrate interface, nor difference in quality of the layer between the portion deposited on the liquid and on the solid substrate. Of course, depending on what is needed, assuming appropriate masking, the film may be depositing only on the liquid and the portion of the substrate surrounding the liquid. 
         [0023]    This ability of the parylene to deposit itself on a liquid, thereby forming a membrane, may be exploited for numerous applications. For some of these, the liquid is eliminated after deposition and, in this case, is called sacrificial. It may also be kept under the parylene membrane and constitute an integral part of the device thus realized. 
         [0024]    In the continuation of this document, several examples of embodiments are described, without, however, constituting an exhaustive list of the possibilities related to the method according to the invention. All of the structures thus described may be realized on a flexible or rigid substrate. For a given type of substrate, spreading of the liquid and adhesion of the parylene on the substrate may be controlled and optimized either by plasma treatment, or by applying molecular layers, known as SAM (Self Assembly Monolayers). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    Other features of the invention will become apparent from the description that follows, given in conjunction with the appended drawings in which: 
           [0026]      FIGS. 1   a ,  1   b ,  1   c  and  1   d  show a selective membrane and its method of production in four steps, with the help of a sacrificial liquid; 
           [0027]      FIGS. 2   a ,  2   b ,  2   c  and  2   d  show in cross-section the manufacturing sequence of a fluid channel with the help of a sacrificial liquid, as well as two particularly advantageous variations of this; 
           [0028]      FIG. 2   e  shows in a top view the use of the fluid channel of  FIG. 2   d  in a peristaltic micropump; 
           [0029]      FIG. 2   f  shows in cross-section another possibility for manufacturing a peristaltic micropump according to the invention; 
           [0030]      FIGS. 3   a ,  3   b  and  3   c  illustrate a method of producing micro-lenses according to the invention, as well as a particularly advantageous variation of this type of lens; 
           [0031]      FIGS. 4   a ,  4   b  and  4   c  illustrate a simplified method for manufacturing a network of micro-lenses according to the invention; 
           [0032]      FIG. 5  shows, in longitudinal cross-section, a wave guide manufactured using the method according to the invention; 
           [0033]      FIG. 6  shows a hydraulic micro-actuator in a top view; 
           [0034]      FIGS. 7   a  and  7   b  illustrate a method, according to the invention, for producing a thin self-supporting film; and, 
           [0035]      FIGS. 8   a ,  8   b ,  8   c ,  8   d  and  8   e  illustrate a method of producing a contact lens according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 a    
       [0036]    A container  10  having a diameter of approximately one hundred microns to several millimeters and comprising a lower discharge pipe  12  closed by a stopper  14 , is filled with a ‘liquid’  16 . 
       FIG. 1 b    
       [0037]    The edge of the container  10  and the ‘liquid’  16  are covered with a thin film  18  of parylene, which adheres only to the container. 
       FIG. 1 c    
       [0038]    The liquid  16  is discharged from the container  10  by removing the stopper  14 . The thin film  18  remains in place, thus forming a film  20  stretched between the walls of the container  10 . 
       FIG. 1 d    
       [0039]    The film  20  is pierced with micro-holes  22  by laser ablation in order to constitute a selective membrane. It may also be made porous by controlled physico-chemical etching of the “Reactive Ion Etching” (RIE) test. The device thus realized forms a filter for gas or other fluid, simple to manufacture and inexpensive. 
       FIG. 2 a    
       [0040]    A substrate, made up of a plate  24  covered with a layer  26  of a material able to be structured, is shown in cross-section. One example of material that may be used for the layer  26  is the photosensitive resin or “blue tape” commonly used to cut silicon chips. A channel  28 , opened in the layer  26  by an etching procedure available to one skilled in the art, is filled with a ‘liquid’  16 . This liquid and the material making up the layer  26  are chosen, moreover, such that the contact angle between the ‘liquid’ and the material is greater than or equal to 20 degrees. If the ‘liquid’  16  is, for example, oil, the layer  26  is made up of an oleophobic material, meaning which repels oil. In this way, the ‘liquid’  16  minimizes its contact surface with the material making up the layer  26 , and its surface, when it is in the channel  28 , is convex. This property is also useful to evacuate the ‘liquid’  16  from the channel  28 , as described in the following. 
       FIG. 2 b    
       [0041]    A thin film  18  of parylene is deposited on the entire substrate-‘liquid’  16 , thereby closing the channel  28  containing the ‘liquid’  16 . 
       FIG. 2 c    
       [0042]    The ‘liquid’  16  is discharged from the channel  28  through an opening located in one of its ends. The device thus realized, by an extremely simple method, is a fluid channel  28  closed by a parylene membrane. This channel  28  may be used to transport any fluid compatible with the materials used. 
       FIG. 2 d    
       [0043]    The illustrated device is different from the device shown in  FIG. 2   c  by the addition of a layer  30  of a piezoelectric material at the bottom of the channel  28 , deposited and structured before deposition of the layer  26 . This layer  30  is, moreover, made accessible during structuring of the layer  26 , so as to connect it to an electrode  32 . An electric impulse transmitted by this electrode  32  is transformed, thanks to the properties of the material, into a physical impulse causing a shock wave within the fluid transported by the channel  28 . 
       FIG. 2 e    
       [0044]    The fluid channel  28 , shown in a top view in this figure, is equipped with a layer of a piezoelectric material structured in a plurality of rectangular elements  34  distributed along said channel. Each of the elements  34  creates, locally, a shock wave in the transported fluid, the slightly unsynchronized action of each element with regard to its neighbor enabling propagation of the fluid in the channel  28 . A peristaltic micropump is thus manufactured. 
       FIG. 2 f    
       [0045]    Another possibility for manufacturing a peristaltic micropump consists of depositing, instead and in place of the piezoelectric layer  30 , a conductive layer  36 , transparent if possible, for example in ITO (Indium Tin Oxide) and structuring it in rectangular elements  34 , as before. A second conductive layer  38  is deposited on the parylene membrane and structured in rectangular elements  34  aligned with the first. A difference of potential applied between each pair of elements  34  causes a local deformation of the parylene film  18  acting as a contraction of the channel  28 . Propagation of this contraction enables transport of the fluid present in the channel  28 . 
       FIG. 3 a    
       [0046]    As for the production of a fluid channel, a substrate made up of a plate  24  covering a layer  26  of a material able to be structured, such as a photosensitive resin or “blue tape”, is shown in cross-section. A circular hole  40 , having a diameter of between one micron and several millimeters, is opened in the layer  26 , then filled with a ‘liquid’  16 . The convexity of the drop of ‘liquid’ thus formed depends entirely on the surface tension of the ‘liquid’  16  and the free energy of the surface of the layer  26 . Thus, the geometry of the drop of ‘liquid’  16  is, on one hand, perfectly reproducible for a same material making up the layer  26  and a same ‘liquid’  16 , and, on the other hand, adaptable according to the needs of the lens to be produced. 
       FIG. 3 b    
       [0047]    A thin film  18  of parylene is deposited on the substrate-‘liquid’  16  assembly, thus closing the circular hole  40  containing the ‘liquid’  16 . The ‘liquid’  16  is, this time, captured by the parylene membrane, and the substrate-liquid-parylene assembly forms a lens having a focal distance determined by the materials used. 
       FIG. 3 c    
       [0048]    In a particularly interesting variation of the micro-lens, a resistive heater  42 , made up of a track of a transparent conductive material, is lodged under the ‘liquid’  16 . This track is produced by depositing and structuring a layer of a transparent conductive material, this deposition preceding that of the layer  26 . When the resistive heater  42  produces an increase in the temperature of the ‘liquid’  16 , this expands, thereby modifying the geometry of the lens and its optic properties. A variable focus lens is thus realized. 
       FIG. 4 a    
       [0049]    A substrate  44 , which may be rigid or flexible and which comprises a plurality of circular holes  45  going through it, is soaked in a ‘liquid’  16  contained by a container  10 . The substrate  44  and the ‘liquid’  16  are chosen, on one hand, according to the selection criteria for the ‘liquid’  16  already cited and, on the other hand, such that the surface of the substrate  44  strongly repels the ‘liquid’  16 . If the ‘liquid’  16  is an oil, the material used for the substrate  44  must be strongly oleophobic. ‘Blue tape’ combined with oil is a material compatible with this application. 
       FIG. 4 b    
       [0050]    When the substrate  44  is removed from the container  10 , drops  46  are captured by the circular holes  45 , whereas the remaining surface of the substrate is free of ‘liquid’  16 , due to the properties of the chosen materials. The convexity of the lenses is significant due to the principle of the method itself, but it may be adapted depending on what is needed. 
       FIG. 4 c    
       [0051]    A thin film  18  of parylene is deposited on both surfaces of the substrate  44 , thereby enclosing the drops  46  held in the holes  45  of the substrate  44 . The substrate  44 —drops  46 —parylene membrane assembly then forms a network of micro-lenses manufactured using a simple and inexpensive technique. 
       FIG. 5 
       [0052]    A channel  48  etched in the substrate  24  is filled with a ‘liquid’  16 . The material forming the substrate  24  and the ‘liquid’  16  are chosen such that the contact angle between the ‘liquid’ and the substrate is sufficiently large. In this way, the surface of the ‘liquid’  16  on the edge of the channel  48  is convex. Moreover, the index of the materials and optical qualities of the ‘liquid’  16  must be taken into account. 
         [0053]    The substrate  24  and the ‘liquid’  16  are covered with a thin film  18  of parylene keeping the ‘liquid’  16  in its housing. The wave guide thus formed is particularly advantageous since it allows easier coupling of the light entering and exiting the wave guide, without the intermediary of a diffraction network, due to the curvature of the ‘liquid’  16  on the edge of the channel  48 . 
       FIG. 6 
       [0054]    The device shown in a top view in  FIG. 6  is a hydraulic micro-actuator. It is made up of a substrate  24 , flexible or rigid, long relative to its width, and in which is etched a channel  48 , filled with a ‘liquid’  16  verifying the conditions for compatibility with parylene deposition. A thin film of parylene covers the whole, such that the ‘liquid’  16  is enclosed in its housing. The device thus produced uses the property of incompressibility of the ‘liquid’  16  to generate a controlled micro-jog of the ‘liquid’  16  and the parylene film. Indeed, pressure exerted on the parylene membrane covering the ‘liquid’  16 , in one of the ends of the device, results in a slight movement of the ‘liquid’  16  and of the parylene membrane in another point of said device. This actuator presents the advantage of being entirely hydraulic and adaptable to microscopic movements. 
       FIG. 7 a    
       [0055]    The entire surface of a substrate  24  is covered with a ‘liquid’  16 , thanks to a cavity  50 . A thin film  18  of parylene is deposited on the assembly, perfectly fitting the excrescences of the substrate  24  and the flatness of the ‘liquid’  16 . 
       FIG. 7 b    
       [0056]    The thin film  18  is separated from the substrate  24  and from the ‘liquid’  16 , for example by cutting the thin film  18  at the boundary between the substrate  24  and the liquid  16 . One will note that the thin film  18  is separated from the ‘liquid’  16  without any difficulty, since it does not adhere to it. Its lower surface, previously in contact with the ‘liquid’  16 , presents a surface morphology comparable to that of the ‘liquid’  16 . This type of extremely flat surface may be used, for example, as a reference for an atomic force microscope. 
         [0057]    This ability of the parylene film  18  to easily detach itself from a ‘liquid’  16  by peeling may also be used to form a self-supporting film. It is sufficient to roll the thin film  18  thus constituted around itself like a ribbon of adhesive paper, then use it to envelope any object. To this end, a particularly advantageous embodiment consists of depositing the parylene on a substrate  24  having a spiral shape and coated with the ‘liquid’  16 . The parylene film  18  deposited on the two sides may be detached without difficulty and form a ribbon ready to be rolled. 
         [0058]    This type of self-supporting film may also be realized such that the properties of the two surfaces are different. Indeed, using a ‘liquid’  16  having a saturation vapor pressure close to the deposit pressure makes it possible to modify the beginning of growth of the parylene film  18 . A reaction of the molecules making up the ‘liquid’  16 , present in the gaseous phase next to the ‘liquid’  16 , with the paraxylylene monomer causes this change of the initial growth phase of the film. Depending on the choice of ‘liquid’  16 , this initial layer may have mechanical, electrical or other properties, different from the ‘pure’ parylene film. 
       FIG. 8 a    
       [0059]    A substrate  24  is covered with ‘liquid’  16 . 
       FIG. 8 b    
       [0060]    A first deposition of parylene forms a thin film  18  on the ‘liquid’  16 . 
       FIG. 8 c    
       [0061]    A drop of ‘liquid’  16  is deposited on the thin film  18 . The ‘liquid’  16  is chosen according to its optical and mechanical properties. Its surface tension must be such that the drop forms a specific angle with the parylene film  18 . 
       FIG. 8 d    
       [0062]    A second layer of parylene  52  is deposited on the assembly, sealing the drop of ‘liquid’  16  between the first parylene film  18  and the second  52 . 
       FIG. 8 e    
       [0063]    The thin film  18  is separated from the ‘liquid’  16  on which it was resting, the entire parylene membrane—drop  16 —parylene membrane forming a contact lens having an extremely simple design. 
         [0064]    This method of encapsulating a ‘liquid’  16  between two parylene membranes  18  and  52  may give rise to various applications. In the medical field, for example, capsules containing a medication may be manufactured in this way. Parylene not being able to be broken down by the human body, a system for opening the capsule is necessary to deliver the active ingredient. 
         [0065]    The above description was provided in reference to a deposition of parylene. Of course, any other substance having similar properties may be used. 
         [0066]    One will note, lastly, that the use of a cavity, such as a circular hole  40  or a channel  28 , to guide the liquid  16 , is not crucial to the realization of the devices described. A technique of depositing material by direct writing, such as inkjet printing or local dispensing, may be used in order to form the ‘liquid’ structures intended to be covered by a parylene film  18 . In this case, the ‘liquid’  16  must be strongly repelled by the substrate  24  so as not to spread on it. Another possibility is to use a substrate which strongly repels the liquid to be deposited, and to apply locally, by embossed printing or inkjet printing, an “acceptance” layer. This layer is made up of a material capable of strongly retaining the ‘liquid’. It is then sufficient to soak the substrate in the ‘liquid’  16 . Only the zones covered with the acceptance layer will be covered by the ‘liquid’ in the end.