Abstract:
A method for assembling a micro-fluidic device better preserves the integrity of a filter in a filter layer and simplifies the bonding of the filter layer to the channel layers on each side of the filter layer. The method includes aligning a polymer layer having a plurality of filter elements and a plurality of fluid passages arranged between the filter elements between two substrates of a micro-fluidic device, and bonding the polymer layer between the two substrates to seal an area between the filter elements and the fluid passages to enable fluid flow through the filter elements to be segregated from fluid flow through the fluid passages.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates generally to micro-fluidic devices that eject fluid from a liquid supply in the device and, more particularly, to printheads that eject ink onto imaging substrates. 
       BACKGROUND 
       [0002]    Many small scale liquid dispensing devices, sometimes called micro-fluidic devices, are known. These devices include micro-electromechanical system (MEMS) devices, electrical semiconductor devices, and others. These devices are small, typically in the range of 500 microns down to as small as 1 micron or even smaller. These devices are important in a wide range of applications that include drug delivery, analytical chemistry, microchemical reactors and synthesis, genetic engineering, and marking technologies including a range of ink jet technologies, such as thermal ink jet and piezoelectric ink jet. Many of these devices have one or more layers that filter fluid flowing through the devices. These filters help keep nozzles and channels free of clogs caused by particle contaminants and air bubbles carried into the printhead from upstream liquid sources. 
         [0003]    In some of these micro-fluidic devices, the filter layers are fabricated with polymer films and in others, the filter layer is made from a thin metal layer. Examples of polymer films useful for filter layers include polyimides, such as Kapton™ or Upilex™, polyester, polysulfone, polyetheretherketone, polyphenelyene sulfide, and polyethersulfone. Metal filters may be made from stainless steel, nickel electroformed screens, or woven mesh screens. The filter layer may be laser ablated or chemically etched to produce the filter pores. These pores are required to be smaller in diameter than the final aperture through which the fluid passes so they block the passage of contaminants that might block the final aperture. Ancillary structure may also be provided to redirect fluid flow to another portion of the filter in the event that a portion of the filter becomes clogged. In some micro-fluidic devices, the final aperture may be approximately 20-50 microns. Typically, the filter pores are 5-10 microns smaller than the final opening. Care must be taken in the pore production process to ensure the placement and sizing of the pores are within these relatively tight tolerance ranges. 
         [0004]    After a filter layer is produced, it is mounted in a micro-fluidic device between two substrates, which are typically made of stainless steel or silicon. A number of methods are frequently used for the mounting of the filter. For example, a filter may be brazed, ultrasonically bonded, or anodically bonded with the lack of adhesive between the substrates. Alignment of the filter with an inlet in a substrate on one side of the filter and with an outlet in a substrate on the other side of the filter must be accomplished with some precision. Otherwise, fluid flow through the filter may be impeded. 
         [0005]    A filter layer may alternatively be mounted between substrates by applying adhesive to both surfaces of the filter layer before aligning the filter layer between two substrates. Application of the adhesive requires attention as the adhesive may clog pores in the filter if the adhesive directly contacts the filter pores. Additionally, the adhesive is typically applied to one surface of the filter layer or the mating substrate, and then the filter layer is pressed against a substrate. After the adhesive is cured, adhesive is then applied to the other filter surface or other substrate, the other substrate is pressed against the filter layer surface, and the adhesive cured. Thus, assembling a micro-fluidic device with a filter layer requires separate adhesives, assembly steps, and curing steps for each interface. 
         [0006]    While the above-described processes are effective for producing and mounting filter layers in micro-fluidic devices, they do require a number of distinct steps and careful control. Accordingly, development of more robust processes for making and mounting filters in micro-fluidic devices is desirable. 
       SUMMARY 
       [0007]    A method for assembling a micro-fluidic device better preserves the integrity of a filter in a filter layer and simplifies the bonding of the filter layer to the substrates on each side of the filter layer. The method includes aligning a polymer layer having a plurality of filter elements and a plurality of fluid passages arranged between the filter elements between two substrates of a micro-fluidic device, and bonding the polymer layer between the two substrates to seal an area between the filter elements and the fluid passages to enable fluid flow through the filter elements to be segregated from fluid flow through the fluid passages. 
         [0008]    A filter constructed for use in the method for assembling a micro-fluidic device enables filter elements in the filter layer to maintain integrity for fluid flow. The filter layer includes a polymer layer in which a plurality of filter elements have been formed, each filter element having a predetermined configuration, and at least one fluid passage formed between adjacent filter elements and the at least one fluid passage being outside a boundary of the predetermined configuration of each adjacent filter element. The filter may be made by a hybrid process in which the outline of the filter layer and large scale features in the filter layer are cut in a polymer layer, and a plurality of filter elements are formed in the polymer layer by laser ablation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The foregoing aspects and other features of an improved filter layer and how the improved filter layer facilitates micro-fluidic device assembly are explained in the following description, taken in connection with the accompanying drawings. 
           [0010]      FIG. 1  is a front plan view of a filter layer that facilitates assembly of a micro-fluidic device. 
           [0011]      FIG. 2  is an enlarged view of the array of filter elements and the array of fluid passages formed in the filter layer of  FIG. 1 . 
           [0012]      FIG. 3  is an enlarged view showing the pore structure of the filter elements in  FIG. 2 . 
           [0013]      FIG. 4  is a flow diagram of a process for making a filter layer having an array of fluid passages interspersed with an array of filter elements. 
           [0014]      FIG. 5  is a flow diagram of a process for assembling a filter layer in a micro-fluidic device. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    For a general understanding of the environment for the system and method disclosed herein as well as the details for the system and method, reference is made to the drawings. In the drawings, like reference numerals have been used throughout to designate like elements. As used herein, the word “printer” encompasses any apparatus that performs a print outputting function for any purpose, such as a digital copier, bookmaking machine, facsimile machine, a multi-function machine, etc. In the description below, reference is made in the text and the drawings to an ink jet stack; however, the discussion is applicable to other micro-fluidic devices that dispense liquid or pump fluid. Therefore, the description should not be read to limit the application of the method to ink jet stacks alone. 
         [0016]      FIG. 1  depicts a filter layer  10  having two filter areas  14  with each filter having an upper manifold  18  and a lower manifold  22  adjacent to the filter. The perimeter  26  of the filter layer  10  is cut with features, such as cutouts  30 ,  34 , and recess  38 . These features enable the filter layer  10  to follow the contours of the substrates (not shown) to which the filter layer is bonded. Additionally, a rectangular opening  40  and an elongated elliptical slot  44  are cut in the filter layer  10  to aid with the alignment and the substrates. 
         [0017]    The filter layer  10  may be cut from a thermoset polymer material, such as a polyimide or a thermoplastic polymer. Such materials include thermoplastic polyimide, polyester, polysulfone, polyetheretherketone, polyphenelyene sulfide, and polyethersulfone. Alternatively, the filter layer  10  may include a polymer core with an adhesive on each side. Examples of polymer cores include polyimide, polyester, polysulfone, polyetheretherketone, polyphenelyene sulfide, and polyethersulfone. The adhesive may be a b-staged (partially cured) adhesive, such as epoxy, acrylic, or phenolic adhesives, although other types of adhesives may be used. In another embodiment, the core may be a thermoset polyimide with a thermoplastic polyimide adhesive layer on each side. In embodiments in which each side of the thermoset polymer material has an adhesive coating, the coatings need not be the same. The filter layer  10  is formed with an adhesive coating, if one is used, before the filter pores are formed in the layer. This type of filter layer fabrication helps ensure that the adhesive does not clog or otherwise interfere with the filter pores. 
         [0018]    A portion of one of the filter areas  14  is shown in  FIG. 2 . The filter  14  includes an array of filter elements  100  and an array of fluid passages  104  that are interspersed within the filter area  14 . A filter element is a configuration of a plurality of filter pores within a boundary as described in more detail below. The fluid passages enable fluid flow through the filter layer  10  that is segregated from the fluid flow through the filter elements  100 . Thus, fluid does not migrate between a filter element and a fluid passage within the filter layer  10 . Consequently, the perimeter  108  of the filter elements  100  must be sealed to ensure that fluid does not migrate from a filter element to a fluid passage. The perimeter  108  is shown for illustration of the filter boundary, though it need not be defined as a physical structure. In previously known micro-fluidic devices, fluid passages  104  were not interspersed with a plurality of filter elements in a polymer. In some micro-fluidic devices, each filter element corresponds to one final aperture for expulsion of the fluid from the micro-fluidic device. Therefore, the height and width of each filter element are sufficient to enable adequate fluid flow through a filter element without presenting too great a resistance to the fluid flow. Alternatively, a larger filter element can replace the smaller individual filters. 
         [0019]    The filter elements  100  and the fluid passages  104  are shown in an enlarged view in  FIG. 3 . Each filter element  100  includes an array  120  of filter pores  124 . The filter pores  124  are shown as being circular, however, other shapes may be used. The hexagonal closely packed arrangement of the filter elements in the array as shown maximizes the number of pores that can be placed in a given area. Rectangular and other arrangements, however, may also be used. The arrays  120  are also shown as being configured with hexagonal perimeters  108 , although other perimeter shapes may be used. Also, the hexagonal shape of the arrays  120  in  FIG. 3  are depicted as being non-symmetrical hexagons, but symmetrical hexagons may be used as well. Likewise, the fluid passages  104  are depicted as being circular, although other shapes may be used. The filter pores are formed in the filter layer  10  using a laser ablation process. Such a process uses a lithographic mask containing the filter design including the fluid passages. This mask is imaged onto the polymer film and an excimer laser is used in an imaging mode to illuminate the mask image on the surface of the polymer. In areas where the mask is not present, the laser removes the unprotected material to produce a fluid passage through the material. In this manner, filter pores  124  that are less than 0.05 mm in diameter may be produced within each filter element. Alternatively, the pores can be made by a laser drilling process using a scanned laser system in which the pores are formed individually by a point and drill process or by scanning a small circle for each pore. 
         [0020]    To produce a filter layer  10  for a micro-fluidic device, a process, such as process  400  shown in  FIG. 4 , is performed. The starting material is either a polymer film that is self-adhesive, such as a thermoplastic material, or a polymer film having a partially cured, b-staged, adhesive, which has been deposited as a thin layer on the film. A sheet of polymer material is cut with a perimeter compatible for bonding to adjacent substrates in the device (block  404 ). This cutting may be done with a die tool or with a laser, for example, a scanned laser beam. This cutting not only forms the perimeter with a compatible shape for bonding to other substrates, but it also forms large scale features in the layer. Large scale features are structures, such as fluid directing structures, that have at least one dimension that measures at least 40 microns. Such large scale features also include the perimeter, cutouts, and recesses in the layer shown in  FIG. 1  above, but also the alignment features depicted in the same figure. The pores for the filter elements and the fluid flow structure are formed with the laser ablation process described above (block  412 ). While the process of  FIG. 4  may be performed in the order shown in  FIG. 4 , the filter elements and fluid flow structure may be formed first before the outline and large scale features are cut. Also, as noted above, the pores in the filter array may be formed with the same scanned laser that cuts the layer perimeter and other fluid flow features, although a different scanned laser may be used. 
         [0021]    After the filter layer has been fabricated with its large scale features and filter elements, it may be bonded to the adjacent substrates. A process to perform this bonding is shown in  FIG. 5 . The process  500  begins by aligning the filter layer with one of the adjacent substrates (block  504 ). This alignment includes aligning the outline of the filter layer  10  with the outline of the adjacent substrate and fitting the alignment features around protuberances or other structure on the adjacent substrate. In a similar manner, the layer is aligned with the other adjacent substrate (block  508 ). The substrates are then pressed together and the adhesive is activated (block  512 ). Activation of the adhesive may be achieved by pressure alone, heating the adhesive alone, or both. If the adhesive on each surface of the filter layer is different, then an activation method corresponding to the type of adhesive may be used, either serially or simultaneously. If a single layer polymer without adhesives is used, the sandwich of the filter layer and two adjacent substrates is heated so the filter layer reaches its glass transition temperature. The two adjacent layers are then pressed together so the filter layer conforms to the surfaces of the two adjacent substrates. Once the filter layer cools, the adjacent substrates are bonded to the filter layer. 
         [0022]    In operation, filter layers are cut from a polymer material that is either self-adhesive thermoplastic polymer or coated with thermoplastic or thermoset adhesives on both sides of the material with an appropriate outline and large scale features. This operation enables filter layers to be produced in relatively large numbers. The filter layers are also laser ablated to form the filter elements. As noted above, the order of these operations may be reversed depending upon whether adhesive is used and the properties of the adhesive. The filter layer may then be aligned between two adjacent substrates, the three layers pressed together, and the adhesive activated so the bonding of the substrates to the filter layer is completed. This bonding effectively seals the filter elements from the other fluid directing features in the filter layer. The ability to segregate fluid flow elements within a filter layer to support bidirectional fluid flow through the filter layer may be used to simply the design of a micro-fluidic device. 
         [0023]    It will be appreciated that various of the above-disclosed and other features, and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art, which are also intended to be encompassed by the following claims.