Abstract:
A method of making an inkjet printhead comprising providing a substrate having first and second opposite surfaces, providing a support member, bonding the second surface of the substrate to the support member, and, after the substrate and support member are bonded together, forming a plurality of ink ejection elements on the first surface of the substrate, the method further including forming communicating ink supply slots passing respectively through the substrate and support member to provide fluid communication between an ink supply and the ink ejection elements.

Description:
TECHNICAL FIELD 
   This invention relates to inkjet printheads and to a method of fabricating such printheads. 
   BACKGROUND ART 
   Inkjet printers operate by ejecting small droplets of ink from individual orifices in an array of such orifices provided on a nozzle plate of a printhead. The printhead forms part of a print cartridge which can be moved relative to a sheet of paper and the timed ejection of droplets from particular orifices as the printhead and paper are relatively moved enables characters, images and other graphical material to be printed on the paper. 
   A typical conventional printhead is fabricated from a silicon substrate having thin film resistors and associated circuitry deposited on a front surface of the substrate. The resistors are arranged in an array relative to one or more ink supply slots in the substrate, and a barrier material is formed on the substrate around the resistors to isolate each resistor inside a thermal ejection chamber. The barrier material is shaped both to form the thermal ejection chambers, and to provide fluid communication between the chambers and the ink supply slot. In this way, the thermal ejection chambers are filled by capillary action with ink from the ink supply slot, which itself is supplied with ink from an ink reservoir in the print cartridge of which the printhead forms part. 
   The composite assembly described above is typically capped by a metallic nozzle plate having an array of drilled orifices which correspond to and overlie the ejection chambers. The printhead is thus sealed by the nozzle plate, but permits ink flow from the print cartridge via the orifices in the nozzle plate. 
   The printhead operates under the control of printer control circuitry which is configured to energise individual resistors according to the desired pattern to be printed. When a resistor is energised it quickly heats up and superheats a small amount of the adjacent ink in the thermal ejection chamber. The superheated volume of ink expands due to explosive evaporation and this causes a droplet of ink above the expanding superheated ink to be ejected from the chamber via the associated orifice in the nozzle plate. 
   Many variations on this basic construction will be well known to the skilled person. For example, a number of arrays of orifices and chambers may be provided on a given printhead, each array being in communication with a different coloured ink reservoir. The configurations of the ink supply slots, printed circuitry, barrier material and nozzle plate are open to many variations, as are the materials from which they are made and the manner of their manufacture. 
   The typical printhead as described above is normally manufactured simultaneously with many similar such printheads on a large area silicon wafer which is only divided up into the individual printheads at a late stage in the manufacture. The silicon wafer is typically several hundred microns (μm) in depth, for example 675 μm, which is necessary to allow robust handling. This leads to the following disadvantage. 
   The ink supply slots are usually cut using laser milling. This is a slow process and typically removes material 50 μm wide by 50 μm deep at a rate of 1.5 mm/sec. A typical ink supply slot 675 μm deep by 100 μm wide by several millimeters long may require 28 milling passes. To cut the ink supply slots in an entire wafer using a two-head laser slotting machine takes about 6 hours. 
   It is an object of the invention to provide a new construction of inkjet printhead, and a method of making such a printhead, in which this disadvantage is avoided or mitigated. 
   DISCLOSURE OF THE INVENTION 
   According to one aspect of the invention there is provided a method of making an inkjet printhead comprising providing a substrate having first and second opposite surfaces, providing a support member, bonding the second surface of the substrate to the support member, and, after the substrate and support member are bonded together, forming a plurality of ink ejection elements on the first surface of the substrate, the method further including forming communicating ink supply slots passing respectively through the substrate and support member to provide fluid communication between an ink supply and the ink ejection elements. 
   The invention further provides a print cartridge comprising a cartridge body having an aperture for supplying ink from an ink reservoir to a printhead, and a printhead as specified above mounted on the cartridge body with said aperture in fluid communication with said ink supply slot. 
   The invention further provides an inkjet printer including a print cartridge according to the preceding paragraph. 
   A further disadvantage with the conventional construction of printhead results from the trend towards printheads with smaller geometries (i.e. higher nozzle densities) to provide higher resolution and operating frequencies. This entails, inter alia, the use of very narrow ink supply slots, for example, 30 μm wide. However, the depth of the conventional silicon wafer (675 μm) provides a significant resistance to ink flow in the case of narrow ink supply slots, placing a limit on the speed at which ink can be supplied to the thermal ejection chamber and correspondingly limiting the speed of operation of the printhead. 
   Accordingly, in a preferred embodiment of the invention, the ink supply slot comprises individual ink supply slots extending through the substrate and support member respectively, the ink supply slot in the support member being in register with but of greater width than the ink supply slot in the substrate. 
   Even if it were practical to use thin wafers, say 50 μm thick, a high operating frequency generates more heat due to the increased resistor firing. It is necessary to dissipate this heat quickly after firing the resistor, as if it does not dissipate quickly, drive bubble collapse time is long. Drive bubble collapse time is dead-time and by reducing dead-time faster operation can be provided. However, the thin silicon substrate may not in all cases constitute an efficient heat sink, and in such circumstances this again places a limit on the frequency of operation. 
   Accordingly, in an embodiment, the support member acts as a heat sink. 
   As used herein, the terms “inkjet”, “ink supply slot” and related terms are not to be construed as limiting the invention to devices in which the liquid to be ejected is an ink. The terminology is shorthand for this general technology for printing liquids on surfaces by thermal, piezo or other ejection from a printhead, and while the primary intended application is the printing of ink, the invention will also be applicable to printheads which deposit other liquids in like manner. 
   Furthermore, the method steps as set out herein and in the claims need not necessarily be carried out in the order stated, unless implied by necessity. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic side view of a silicon wafer undergoing a reduction in thickness for use in a printhead according to an embodiment of the invention; 
       FIG. 2  is a plan view of a carrier for the thinned wafer of  FIG. 1 ; 
       FIG. 3  is cross-section through part of the carrier of  FIG. 2 ; 
       FIGS. 4 to 7  show successive steps in making a printhead according to an embodiment of the invention; 
       FIG. 8  is a cross-section of the final printhead made by the method of  FIGS. 4 to 7 ; and 
       FIG. 9  is a cross-sectional view of a print cartridge incorporating the printhead of  FIG. 8 . 
   

   In the drawings, which are not to scale, the same parts have been given the same reference numerals in the various figures. 
   DETAILED DESCRIPTION OF THE INVENTION 
   There will now be described, by way of example only, the best mode contemplated by the inventors for carrying out embodiments of the invention. 
   The left hand side of  FIG. 1  shows, in side view, a substantially circular silicon wafer  10  of the kind typically used in the manufacture of conventional inkjet printheads, the wafer  10  having a thickness of 675 μm and a diameter of 150 mm (the thickness of the wafer is greatly exaggerated in  FIG. 1 ). The wafer  10  has opposite, substantially parallel front and rear major surfaces  12  and  14  respectively, the front surface  12  being flat, highly polished and free of contaminants in order to allow ink ejection elements to be built up thereon by the selective application of various layers of materials in known manner. 
   The first step in the manufacture of a printhead according to the embodiment of the invention is to grind the rear surface  14  of the wafer by conventional techniques to reduce the thickness of the wafer  10  to 50 μm. This is shown on the right hand side of  FIG. 1 , where the front surface  12  remains undisturbed while the ground rear surface is indicated at  14 ′. The reduced thickness wafer is referenced  100 . 
   The next step is to bond the rear surface  14 ′ of the reduced thickness wafer  100  to a substantially circular support member, herein referred to as a wafer carrier  16 . The wafer carrier  16  is shown in plan view in  FIG. 2 , and it has a diameter substantially the same as that of the wafer  100 . The wafer carrier  16  is moulded using a standard injection moulding process and has a thickness of 625 μm so that the combined thickness of the carrier  16  and wafer  100  is substantially the same as the original wafer  10  so that the same wafer handling apparatus as is used for conventional wafers  10  can be used in subsequent manufacturing steps. 
   The carrier  16  is preferably made of aluminium nitride which has a high thermal conductivity and allows the carrier to act as a heat sink in the finished printhead. In the moulding process, aluminium nitride powder is mixed with a standard polymer carrier to allow moulding, after which the polymer is burned off at high temperature which also sinters the aluminium nitride particles together to give the final carrier  16 . Silicon nitride particles may be used instead of aluminium nitride. 
   As seen in  FIG. 2 , the carrier  16  has a large number of slots  18  grouped in threes, each slot  18  extending fully through the thickness of the carrier. The bottom surface (not seen in  FIG. 2 ) of the carrier  16  has grooves running vertically between each group of three slots  18  and horizontally between each row of slots  18  so that ultimately the carrier can be divided up using a conventional dicing saw into individual “dies” each containing one group of three slots  18 .  FIG. 3  is a cross-section through the carrier  16  showing one of the dies prior to separation from the carrier. The grooves  20  are the vertical grooves between adjacent groups of slots; the horizontal grooves are similar but run perpendicular to the grooves  20 . 
   The wafer  100  is bonded to the top surface of the carrier  16  (i.e. the surface not containing the grooves  20 ), using a lead borate glass frit at 390 deg C. The result is an intimately bonded composite structure in which the upper part is a 50 μm thick layer of silicon  100  and the lower part is a 625 μm thick aluminium nitride carrier  16  containing slots  18  grouped in threes and each group of three being separated from its neighbors by horizontal and vertical grooves  20 . 
   This is shown in  FIG. 4  for a single die of three slots  18 , such die being shown as a separate entity in  FIG. 4  but actually still at this point forming an undivided part of the composite structure. However, from this point on, the method will be described for a single die for simplicity, but it will be understood that in practice the further steps required to complete the printhead, as described below, will be carried out at the wafer level simultaneously for all dies, and the individual printheads will be cut from the wafer along the grooves  20  after the printheads are substantially complete. 
   Next, the front surface  12  of the wafer is processed in conventional manner to lay down an array of thin film heating resistors  22  ( FIG. 8 ) which are connected via conductive traces to a series of contacts which are used to connect the traces via flex beams with corresponding traces on a flexible printhead-carrying circuit member (not shown), which in turn is mounted on a print cartridge. The flexible printhead-carrying circuit member enables printer control circuitry located within the printer to selectively energise individual resistors under the control of software in known manner. As discussed, when a resistor  22  is energised it quickly heats up and superheats a small amount of the adjacent ink which expands due to explosive evaporation. The resistors  22 , and their corresponding traces and contacts, are not shown in  FIGS. 5 to 7  due to the small scale of these figures, but methods for their fabrication are well-known. 
   After laying down the resistors  22 , a blanket barrier layer  24  of, for example, dry photoresist is applied to the entire front surface  12  of the wafer  100 ,  FIG. 5 . Then, selected regions  26  of the photoresist are removed and the remaining portions of photoresist are hard baked. Each region  26  is centered over a respective slot  18  and extends along substantially the full length thereof. In the finished printhead, the regions  26  define the lateral boundaries of a plurality of ink ejection chambers  28 ,  FIG. 8 , as will be described. Again, the formation of the barrier layer is part of the state of the art and is familiar to the skilled person. 
   Next,  FIG. 6 , slots  30  ( FIG. 7 ) are laser machined fully through the thickness of the wafer  100  using one or more narrow laser beams  32  (not all the slots  30  are necessarily machined simultaneously as suggested by the presence of beams  32  in all three slots  18  in  FIG. 6 ). In this embodiment each slot  30  is 30 μm wide and is centered over, and extends substantially the full length of, a respective slot  18  in the carrier  16 . The slots  30  could alternatively be cut by reactive ion etching. In the preferred embodiment, in either case the machining or etching is performed from below, i.e. on the rear surface  14 ′ upwardly through the slots  18 , while maintaining a greater air pressure at the front surface  12  of the wafer than at the rear surface  14 ′ to prevent contamination reaching the front surface. 
   The result is shown in  FIG. 7 . Clearly, wafer slotting time is significantly reduced compared to the conventional 675 μm thick wafer; typically processing is twenty times faster. 
   Next,  FIG. 8 , a pre-formed metallic nozzle plate  42  is applied to the top surface of the barrier layer  24  in a conventional manner, for example by bonding. The final composite carrier/wafer structure, whose cross-section is seen in  FIG. 8 , comprises a plurality of ink ejection chambers  28  disposed along each side of each slot  30  although, since  FIG. 8  is a cross-section, only one chamber  28  is seen on each side of each slot  30 . Each chamber  28  contains a respective resistor  22 , and an ink supply path  34  extends from the slot  30  to each resistor  22 . Finally, a respective ink ejection orifice  36  leads from each ink ejection chamber  28  to the exposed outer surface of the nozzle plate  42 . It will be understood that the manufacture of the structure above the wafer surface  12 , i.e. the structure containing the ink ejection chambers  28 , the ink supply paths  34  and the ink ejection orifices  36  as described above, can be entirely conventional and well known to those skilled in the art. 
   Finally,  FIG. 9 , the composite carrier/wafer processed as above is diced by cutting along the grooves  20  to separate the individual printheads and each printhead is mounted on a print cartridge body  38  having respective apertures  40  for supplying ink from differently coloured ink reservoirs (not shown) to the printhead. To this end the printhead is mounted on the cartridge body  38  with each aperture  40  in fluid communication with a respective slot  18  in the carrier  16 . 
   It will be evident that each pair of registered slots  18  and  30  together supply ink of the relevant colour to the printhead, and replace the single ink supply slot in the much thicker (675 μm) substrate used in the prior art. However, due to the small depth (50 μm) of the narrow ink supply slot  30  in the substrate  100  compared to the much wider ink supply slot  18  in the carrier  16 , the resistance to ink flow is much less and so faster operating frequencies can be achieved. Furthermore, the aluminium nitride carrier  16 , which is directly below the resistors  22  and separated therefrom only by the thin substrate  100 , has a high thermal conductivity and thus acts as a good heat sink to dissipate the heat quickly after firing the resistors  22 . 
   Although the slots  18  in each group of three slots are shown as disposed side by side, they could alternatively be disposed end to end or staggered or otherwise offset without departing from the scope of this invention. Also, in the case of a printhead which uses a single colour ink, usually black, only one ink supply slot  18 , and correspondingly only one ink supply slot  30 , will be required per printhead. 
   The invention is not limited to the embodiment described herein and may be modified or varied without departing from the scope of the invention.