Patent Publication Number: US-2009233386-A1

Title: Method for forming an ink jetting device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a printhead, and, more particularly, to a method for forming an ink jetting device. 
     2. Description of the Related Art 
     A typical ink jet printhead includes a silicon chip to which a nozzle plate fabricated from a polymer material is attached. However, during printhead assembly, significant out-gassing and thermal contraction may occur. Also, under certain conditions the polymer nozzle plate may tend to sag, thus affecting the accuracy and repeatability of ink drop placement. Other issues with current polymer nozzle plates, for example, are difficulty with adhesion of polymer to a substrate and print quality issues associated with alignment of nozzle holes after polymer cure. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method for forming an ink jetting device that uses, for example, a sacrificial polymer layer and a single deposited conformal nozzle layer. 
     The terms “first” and “second” preceding an element name, e.g., first surface, second surface, etc., are used for identification purposes to distinguish between similar or related elements, results or concepts, and are not intended to necessarily imply order, nor are the terms “first” and “second” intended to preclude the inclusion of additional similar or related elements, results or concepts, unless otherwise indicated. 
     The invention, in one form thereof, is directed to a method for forming an ink jetting device. The method includes: providing a silicon substrate having a first surface and a second surface opposite to the first surface, the first surface having formed thereon a plurality of electrical heater elements to form a first upper exposed surface; depositing a polymer over the first upper exposed surface to form a sacrificial polymer layer; patterning the sacrificial polymer layer to remove a portion of the sacrificial polymer layer to form a second exposed upper surface; depositing a conformal material over the second exposed upper surface to form a conformal nozzle layer; patterning the conformal nozzle layer to form a plurality of nozzle holes respectively located over the plurality of electrical heater elements; applying a mask layer over the second surface of the silicon substrate; patterning the mask layer to form a plurality of mask portions and an exposed region of the second surface of the silicon substrate that defines a location of a central ink via; etching the exposed region of the second surface of the silicon substrate to form the central ink via in the silicon substrate; and removing a portion of a remainder of the polymer layer to form a plurality of ink ejection chambers. 
     The invention, in another form thereof, is directed to a method for forming an ink jetting device. The method includes: forming a plurality of electrical heater elements on a first surface of a silicon substrate to form a first upper exposed surface, the silicon substrate having a second surface located opposite to the first surface; depositing a polymer over the first upper exposed surface to form a sacrificial polymer layer; patterning the sacrificial polymer layer to remove a portion of the sacrificial polymer layer to form a second exposed upper surface, the second exposed tipper surface including first sacrificial polymer layer areas over the first surface of the silicon substrate and second sacrificial polymer layer areas that cover the electrical heater elements, the first sacrificial polymer layer areas and the second sacrificial polymer layer areas defining a central channel with respect to a centerline, the second sacrificial polymer layer areas covering and extending beyond the electrical heater elements toward the centerline; depositing a conformal material over the second exposed upper surface to form a conformal nozzle layer; patterning the conformal nozzle layer to form a plurality of nozzle holes, which are respectively located over the plurality of electrical heater elements; applying a mask layer over the second surface of the silicon substrate; patterning the mask layer to form a plurality of mask portions and an exposed region of the second surface of the silicon substrate that defines a location of a central ink via; etching the exposed region of the second surface of the silicon substrate to form the central ink via in the silicon substrate; and removing the second sacrificial polymer layer areas of the sacrificial polymer layer to form a plurality of ink ejection chambers respectively adjacent to the plurality of electrical heater elements and the plurality of nozzle holes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1A  is a diagrammatic illustration of a top view of a silicon substrate having heating elements formed thereon. 
         FIG. 1B  is a cross section of the diagrammatic illustration of  FIG. 1A  taken along line  1 B- 1 B. 
         FIG. 2A  is a diagrammatic illustration of a top view the substrate of  FIGS. 1A ,  1 B with a sacrificial polymer layer deposited and patterned. 
         FIG. 2B  is a cross section of the diagrammatic illustration of  FIG. 2A  taken along line  2 B- 2 B. 
         FIG. 3A  is a diagrammatic illustration of a top view the silicon substrate and sacrificial polymer layer at the process stage of  FIGS. 2A ,  2 B after a conformal nozzle layer is deposited and the nozzle holes formed. 
         FIG. 3B  is a cross section of the diagrammatic illustration of  FIG. 3A  taken along line  3 B- 3 B. 
         FIG. 4A  is a diagrammatic illustration of a top view of the silicon substrate, sacrificial polymer layer, and conformal nozzle layer at the process stage of  FIGS. 3A ,  3 B after the formation of a central ink via on the back side of the silicon substrate. 
         FIG. 4B  is a cross section of the diagrammatic illustration of  FIG. 4A  taken along line  4 B- 4 B. 
         FIG. 5A  is a diagrammatic illustration of a top view of the silicon substrate and sacrificial polymer layer at the process stage of  FIGS. 4A ,  4 B after removal of the sacrificial polymer layer to form the ink ejection chambers. 
         FIG. 5B  is a cross section of the diagrammatic illustration of  FIG. 5A  taken along line  5 B- 5 B. 
         FIGS. 6A and 6B  is a flowchart of a method for forming an ink jetting device in accordance with an aspect of the present invention. 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate one embodiment of the invention, in one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings and particularly to  FIGS. 1A-5B , there is shown various fabrication stages associated with a method for forming an ink jetting device  10  (see  FIG. 5B ) in accordance with an aspect of the present invention. Those skilled in the art will recognize that the structures shown in  FIGS. 1A-5B  are exaggerated in size and shape to more clearly show fabrication aspects of the present invention. 
     The various acts associated with the method for forming an ink jetting device  10  in accordance with the present invention are summarized in the flowchart of  FIGS. 6A and 6B . 
     At act S 100 , a plurality of electrical heater elements, e.g., resistors, is formed on a silicon substrate to form a first upper exposed surface. As illustrated, for example, in  FIGS. 1A and 1B , fabrication of ink jetting device  10  begins with a silicon substrate  12 , e.g., such as portion of a silicon wafer. Silicon substrate  12  includes a first surface  12 - 1  and a second surface  12 - 2 , and edges  12 - 3 ,  12 - 4 . Second surface  12 - 2 , which may be planar, is parallel to, and on an opposite side of silicon substrate  12  from, first surface  12 - 1 , which may be planar. Silicon substrate  12  is pre-cleaned, and a plurality of metallic electrical heater elements  14 , e.g., electrical heater elements  14 - 1 ,  14 - 2 ,  14 - 3  and  14 - 4 , are formed on first surface  12 - 1  of silicon substrate  12 , such as by a metal vapor deposition process, to form a first upper exposed surface  10 - 1 . In the present example only four electrical heater elements  14  are shown, but it is to be understood that the actual number of electrical heater elements  14  may be in the hundreds or thousands. 
     At act S 102 , a polymer is deposited over the first upper exposed surface to form a sacrificial polymer layer. Referring to  FIGS. 2A and 2B , a polymer is deposited over first upper exposed surface  10 - 1 , e.g., the entirety of first surface  12 - 1  of silicon substrate  12  and the plurality of electrical heater elements  14 , to form a sacrificial polymer layer  16 . The depositing of sacrificial polymer layer  16  may be achieved, for example, by a spin-coat process. 
     The composition of the sacrificial polymer material forming sacrificial polymer layer  16  may be a standard photoresist material or a special polymer chosen for the process. Desirable characteristics of the sacrificial polymer material include being able to withstand temperatures necessary for deposition of a conformal nozzle layer (see act S 106 ), and being capable of being patterned without the forming of a re-entrant profile in the sacrificial polymer layer (i.e. without the top of the trench being smaller than bottom). One example of such polymer material suitable for use as the sacrificial polymer material is polyimide. 
     Table 1, below is a polymer reference table that includes thermal stability information for common classes of polymers. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Polymer Reference Table 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 Linear 
                 Flexural 
               
               
                   
                   
                   
                   
                 TGA 
                 CTE 
                 Modulus 
               
               
                 Acronym 
                 Polymer 
                 T g (° C.) 
                 T m (° C.) 
                 Decomp T 
                 (° C. −1 ) 
                 (MPa) 
               
               
                   
               
               
                 ABS 
                 Acrylonitrile 
                 110-125 
                 — 
                 375 
                 65-95 
                 2070-4140 
               
               
                   
                 Butadiene styrene 
               
               
                 PMMA 
                 Polymethylmethacrylate 
                  85-110 
                 160 
                 313 
                 50-60 
                 2240-3170 
               
               
                   
                 Acrylonitrille 
                  95 
                 135 
                 — 
                 66 
                 3450-4070 
               
               
                 PTFE 
                 Polytetrafluoroethylene 
                 126 
                 327 
                 525 
                  70-120 
                  525 
               
               
                 PVDF 
                 Polyvinylidene fluoride 
                 −60-−20 
                 170-178 
                 470 
                  70-142 
                 1724-2896 
               
               
                 Nylon 6 
                 Nylon 6 
                 40-87 
                 210-220 
                 400 
                 80-83 
                 2690 
               
               
                 Nylon 6,6 
                 Nylon 6,6 
                  50 
                 255-265 
                 426 
                 80 
                 2830-3240 
               
               
                 PC 
                 Polycardonate 
                 140-150 
                 — 
                 473 
                 68 
                 2350 
               
               
                 PBT 
                 Polybutylene 
                 — 
                 220-287 
                 386 
                 60-95 
                 2280-2760 
               
               
                   
                 terephthalate 
               
               
                 PET 
                 Polyethylene 
                 73-80 
                 245-265 
                 414 
                 65 
                 2410-3100 
               
               
                   
                 terephthalate 
               
               
                 PEEK 
                 Polyetheretherketone 
                 150 
                 334 
                 575 
                  40-108 
                 3860 
               
               
                 PEI 
                 Polyetherimide 
                 215-217 
                 — 
                 — 
                 47-56 
                 3310 
               
               
                 LDPE 
                 Low Density 
                 −25 
                  98-115 
                 459 
                 100-220 
                 240-330 
               
               
                   
                 Polyethylene 
               
               
                 HDPE 
                 High Density 
                 60-80 
                 130-137 
                 469 
                  59-110 
                 1000-1550 
               
               
                   
                 Polyethylene 
               
               
                 PI 
                 Polyimide 
                 — 
                 310-365 
                 — 
                 45-56 
                 3100-3450 
               
               
                 PPO 
                 Polyphenylene Oxide 
                 100-142 
                 — 
                 400 
                 38-70 
                 2250-2760 
               
               
                 PPS 
                 Polyphenylene Sulfide 
                  88 
                 285-290 
                 508 
                 49 
                 3790 
               
               
                 PP 
                 Polypropylene 
                 −20 
                 160-175 
                 417 
                  81-100 
                 1170-1720 
               
               
                 PS 
                 Polystyrene 
                  74-109 
                 240-250 
                 351 
                 50-83 
                 2620-3380 
               
               
                 PSO 
                 Polysulfone 
                 190 
                 — 
                 510 
                 56 
                 2690 
               
               
                 PES 
                 Polyethersulfone 
                 220-230 
                 — 
                 — 
                 55 
                 2400-2620 
               
               
                 PVC 
                 Polyvinyl Chloride 
                  75-105 
                 — 
                 265 
                  50-100 
                 2070-3450 
               
               
                   
               
               
                 In Table 1, T g  is glass transition temperature; T m  is melt temperature; TGA Decomp T is decomposition temperature determined by thermogravimetric analysis; and Linear CTE is coefficient of thermal expansion. 
               
            
           
         
       
     
     At act S 104 , the sacrificial polymer layer is patterned to form a second exposed upper surface. For example, as illustrated in  FIGS. 2A and 2B , a portion of sacrificial polymer layer  16  is removed to form a second exposed upper surface  10 - 2  that includes first sacrificial polymer layer areas  16 - 1 ,  16 - 2 , e.g., outer areas, over the first surface  12 - 1  of silicon substrate  12 , as well as second separated sacrificial polymer layer areas  16 - 3 ,  16 - 4 ,  16 - 5  and  16 - 6  that cover electrical heater elements  14 . As shown in  FIGS. 2A and 2B , the second separated sacrificial polymer layer areas  16 - 3 ,  16 - 4 ,  16 - 5  and  16 - 6  of sacrificial polymer layer  16  cover and extend beyond electrical heater elements  14  toward a centerline  18 . After sacrificial polymer layer  16  is patterned, the first sacrificial polymer layer areas  16 - 1 ,  16 - 2  and the second separated sacrificial polymer layer areas  16 - 3 ,  16 - 4 ,  16 - 5  and  16 - 6  define a central channel  20 - 1  formed symmetrical with respect to centerline  18 , which extends down to first surface  12 - 1  of silicon substrate  12 , and trenches  20 - 2 ,  20 - 3 ,  20 - 4  and  20 - 5  are formed that respectively extend from central channel  20 - 1  around the respective separated areas  16 - 3 ,  16 - 4 ,  16 - 5  and  16 - 6  of sacrificial polymer layer  16 . Central channel  20 - 1  and trenches  20 - 2 ,  20 - 3 ,  20 - 4  and  20 - 5  will aid in forming ink paths used for channeling ink flow to respective ink ejection chambers when selected portions of sacrificial polymer layer  16  is removed. 
     At act S 106 , a conformal material is deposited over the second exposed upper surface to form a conformal nozzle layer. Referring to  FIGS. 3A and 3B , for example, a conformal material is deposited over the entirety of the second exposed upper surface  10 - 2  (see  FIGS. 2A ,  2 B), which includes the sacrificial polymer layer areas  16 - 1 ,  16 - 2 ,  16 - 3 ,  16 - 4 ,  16 - 5  and  16 - 6 ; central channel  20 - 1 ; and trenches  20 - 2 ,  20 - 3 ,  20 - 4  and  20 - 5 , to form a conformal nozzle layer  22 . 
     The composition of the conformal material used in forming conformal nozzle layer  22  is selected such that the material is capable of completely filling trenches  20 - 2 ,  20 - 3 ,  20 - 4  and  20 - 5  (see  FIG. 2A ) formed in sacrificial polymer layer  16 , since trenches  20 - 2 ,  20 - 3 ,  20 - 4  and  20 - 5  outline the walls for the ink ejection chambers. The material used in forming conformal nozzle layer  22  for the present embodiment may be a ceramic material or a metallic thin film material, such as for example, silicon oxide, silicon nitride, silicon oxynitride, polysilicon, tantalum, and gold. 
     At act S 108 , the conformal nozzle layer  22  is patterned to form a plurality of nozzle holes. For example, as illustrated in  FIGS. 3A and 3B , a portion of conformal nozzle layer  22  is removed to form a plurality of nozzle holes  24 - 1 ,  24 - 2 ,  24 - 3 , and  24 - 4 , which extend down through conformal nozzle layer  22  to sacrificial polymer layer  16 , and which are respectively located over electrical heater elements  14 - 1 ,  14 - 2 ,  14 - 3  and  14 - 4 . 
     The formation of nozzle boles  24 - 1 ,  24 - 2 ,  24 - 3 , and  24 - 4  in conformal nozzle layer  22  may be achieved by using, for example, a standard photolithography and etch processes. 
     At act S 110 , a mask layer is deposited over the second surface of the silicon substrate. Referring to  FIGS. 4A and 4B , for example, a mask layer  26  is deposited over the entirety of second surface  12 - 2  of silicon substrate  12  (see  FIG. 1B ). 
     At act S 112 , the mask layer is patterned to form an exposed region of the second surface of silicon substrate to define a location of the central ink via. Referring to  FIGS. 4A and 4B , for example, mask layer  26  is patterned to form an exposed region  26 - 1  (shown by a phantom line) that separates two ink via mask portions  26 - 2  and  26 - 3 , which are located symmetrical with respect to centerline  18 . Exposed region  26 - 1  between the ink via mask portions  26 - 2  and  26 - 3  defines a location of a central ink via  28 , as best seen in  FIG. 4B . 
     At act S 114 , the exposed region of the second surface of the silicon substrate between the two separated ink via mask portions is etched to form the central ink via in the silicon substrate. As best shown in  FIG. 4B , for example, the exposed region  26 - 1  of second surface  12 - 2  of silicon substrate  12  between the two separated ink via mask portions  26 - 2  and  26 - 3  is etched away to form central ink via  28 , which serves as a primary ink flow channel. The etching may he performed, for example, by a deep reactive ion etch (DRIE) process, a wet chemical etch, a mechanical blasting technique or some combination thereof. 
     At act S 116 , the two separated ink via mask portions are removed from the second surface of the silicon substrate. Referring to  FIGS. 4B and 5B , for example, the two separated ink via mask portions  26 - 2  and  26 - 3  are remove from second surface  12 - 2  of silicon substrate  12 . 
     At act S 118 , a portion of a remainder of the sacrificial polymer layer is removed to form a plurality of ink ejection chambers. As illustrated in  FIGS. 5A and 5B , with reference to  FIGS. 2A and 2B , for example, a portion (second separated sacrificial polymer layer areas  16 - 3 ,  16 - 4 ,  16 - 5  and  16 - 6 ) of the remainder of the sacrificial polymer layer  16  (sacrificial polymer layer areas  16 - 1 ,  16 - 2 ,  16 - 3 ,  16 - 4 ,  16 - 5  and  16 - 6 ) is removed to form the ink ejection chambers  30 - 1 ,  30 - 2 ,  30 - 3  and  30 - 4  of ink jetting device  10 . The ink ejection chambers  30 - 1 ,  30 - 2 ,  30 - 3  and  30 - 4  are formed respectively adjacent to electrical heater elements  14 - 1 ,  14 - 2 ,  14 - 3  and  14 - 4  and to nozzle holes  24 - 1 ,  24 - 2 ,  24 - 3 , and  24 - 4 . The removal of second separated sacrificial polymer layer areas  16 - 3 ,  16 - 4 ,  16 - 5  and  16 - 6  of sacrificial polymer layer  16  may be achieved, for example, through oxidation during a standard oxygen-plasma photoresist-ashing process. 
     By using the process described above for forming ink jetting device  10 , it is recognized that both fabrication of the chip (i.e., the portion including the silicon substrate) and the nozzle layer may be integrally formed in the wafer fabrication facility. Thus, the process may be completed in a single clean room, rather than shipping the wafer to a separate facility for nozzle plate attachment, thereby providing fewer opportunities for contamination. Also, the process provides improved alignment of the flow feature of the wafer and nozzle features in comparison to using a separate polymer nozzle plate that is attached to the silicon chip, as in the prior art. 
     By forming the conformal nozzle layer from a ceramic or metallic thin film material, the conformal nozzle layer exhibits superior rigidity over that of a polymer nozzle plate, i.e., is less likely to sag, as is commonly observed in ink jetting devices that use a polymer printhead material over the ink vias. Also, a ceramic or metallic nozzle material is more stable than a polymer film over a range of temperatures, which may reduce or eliminate out-gassing and excessive thermal contraction during processing. Furthermore, the use of a ceramic nozzle plate allows use with non-aqueous inks, if desired. 
     Also, the ceramic or metallic nozzle layer material is less permeable to moisture and gas, as compared to a polymer, thereby allowing the nozzle layer to also act as protective overcoat and reduce susceptibility to corrosion. This may allow for the elimination of the protective overcoat layer used on ink jetting chips of the prior art. 
     While this invention has been described with respect to embodiments of the invention, the present invention may be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.