Method of fabricating ink jet printheads

Abstract: An ink jet printhead fabrication process produces a plurality of printheads from two mated substrates (31,36) by two dicing operations. One dicing operation produces the nozzle face (29) for each of a plurality of printheads and optionally produces the nozzles (27). This dicing blade, together with specific operating parameters, prevent the nozzles from chipping and the nozzle faces from scratches and abrasions. A second dicing operation with a dicing blade severs the mated substrates into separate printheads. The dicing operation which produces the nozzle face is preferably conducted in two steps. A first cut makes the nozzle face, but does not sever the two mated substrates. A second dicing cut severs the two substrates, but does so in a manner that prevents contact by the dicing blade with the nozzle face.

Description: This invention relates to thermal ink jet printing, and more particularly, to a method of fabricating a thermal ink jet printhead by a dicing operation capable of finishing the nozzle-containing surface. Thermal ink jet printing is a type of drop-on-demand ink jet system, wherein an ink jet printhead expels ink droplets on demand by the selective application of a current pulse to a thermal energy generator, usually a resistor, located in ink-filled, parallel channels a predetermined distance upstream from the channel nozzles or orifices. The channel end opposite the nozzles is in communication with a small ink reservoir to which a larger external ink supply is connected. US reissue-A- 32,572 discloses a thermal ink jet printhead and several fabricating processes therefor. Each printhead is composed of two parts aligned and bonded together. One part is a substantially flat substrate which contains on the surface thereof a linear array of heating elements and addressing electrodes, and the second part is a substrate having at least one recess anisotropically etched therein to serve as an ink supply manifold when the two parts are bonded together. A linear array of parallel grooves is also formed in the second part, so that one end of the grooves communicate with the manifold recess and the other ends are open for use as ink droplet expelling nozzles. Many printheads can be made simultaneously by producing a plurality of sets of heating element arrays with their addressing electrodes on a silicon wafer and by placing alignment marks thereon at predetermined locations. A corresponding plurality of sets of channel grooves and associated manifolds are produced in a second silicon wafer. In one embodiment, alignment openings are etched in the second silicon wafer at predetermined locations. The two wafers are aligned via the alignment openings and alignment marks, then bonded together and diced into many separate printheads. US-A-4,638,337 discloses a similar thermal ink jet printhead , but has each of its heating elements located in a recess. The recess walls containing the heating elements prevent the lateral movement of the bubbles through the nozzle and therefore the sudden release of vaporized ink to the atmosphere, known as blow-out, which causes ingestion of air and interrupts the printhead operation whenever this event occurs. In this patent a thick film organic structure of a material such as Riston® or Vacrel® is interposed between the heater plate and the channel plate. The purpose of this layer is to have recesses formed therein directly above the heating elements to contain the bubble which is formed over the heating elements, thus enabling an increase in the droplet velocity without the occurrence of vapor blow-out and concomitant air ingestion. US-A-4,097,310 discloses a method of forming silicon solar cells by forming an electron-generating junction on a single crystal wafer followed by a diamond blade cut through the wafer. Rotation speeds for the diamond blade fall into the range of 5,000 to 20,000 rpm, with a corresponding cutting speed of 1.25 to 250mm per second. US-A-4,564,000 discloses a method of cutting ferrite materials using a resin-bonded diamond sawblade at a cutting speed of 54 meters per second. The blade itself has a diameter of 56.25mm. Coolant is applied to the blade to prevent overheating of the blade and the ferrite. US-A-4,633,847 discloses a multiple-blade internal hole sawing method for cutting crystal-line rods into thin wafers while maintaining a connection between adjacent cut wafers. The wafers can be cut into thicknesses ranging from 0.1mm to 1.0 mm. Connecting agents, such as wax, are used to connect the cut wafers mechanically but can easily be removed by solvent or heating. JP-A-59-10182, (Laid-Open No. 60-154639) discloses a method and apparatus for dicing wafers using rotary blades of different widths to cut grooves into the wafers. The cut with a first rotary saw is deep enough to remove patterns but the through-wafer dicing is done by another rotary saw of smaller width. In all ink jet printing systems, the nozzle or orifice size, shape, and surface conditions affect the characteristics and trajectory of the ink droplet emitted from the nozzle. Some ink jet printers have a separate nozzle plate which is independently fabricated to obtain the desired dimensional tolerances and surface conditions. This nozzle plate is then aligned and bonded to the droplet generator or printhead. In addition to the dimensional precision, attachment is sometimes difficult and de-bonding is a constant concern. Also, if an adhesive is used, it may flow into the nozzle or ink paths, thereby restricting or impairing the droplet ejection and/or trajectory. Other known ink jet printers have printheads fabricated from two substrates, in which one substrate contains the heating elements and the other contains ink recesses, which, when the substrates are aligned and bonded together, serve as ink passageways. The open ends of the passageways terminate about 0.025 to 0.125 mm in front of the heating elements and create the nozzles. The front face of the printhead containing the nozzles has a major impact on the droplet characteristics and trajectories. One discontinuity at the nozzle is made by the adhesive bond and has a tendency to cause weeping of the ink therefrom. Generally, a polishing operation removes the bonding discontinuity as well as other surface defects. In these and other fabrication methods, the printhead nozzle plate with nozzle-containing surface must be fabricated separately and bonded to the printheads, photolithographically produced separately by complex photo-processing, or the nozzle-containing surface must be polished. All of these methods of fabrication do not lend themselves well to making economical or technically-sound devices. From US-A-4 786 357 there is further known a printhead fabrication method wherein a dicing blade is used to obtain the individual printheads. However in this disclosure neither the dicing blade nor its operating parameters are defined. This invention overcomes the disadvantages of known fabrication methods and enables an acceptable nozzle and nozzle-containing surface finish to be concurrently formed in one process step. Use of an optimized dicing blade and dicing process, with predetermined operating parameters, enables the concurrent sectioning of the mated wafers into individual printheads and forming of the nozzle-containing surface. This surface has a finish sufficient to prevent deleterious effect on the droplet or its trajectory without the need of further processing. Accordingly, the present invention provides a printhead fabricating process which is as claimed in the appended claims. In the present invention, a plurality of thermal ink jet printheads is obtained from a structure consisting of an electrically insulative or semi-conductive substrate having a plurality of sets of passivated heating elements and addressing electrodes formed thereon, which is aligned and bonded to a silicon substrate having a corresponding plurality of sets of etched ink channels and associated manifold with ink inlet, and a polymer layer such as, for example, of Vacrel®, sandwiched therebetween with etched pits which are aligned to expose each of the heating elements. The individual printheads are produced by a sectioning operation, generally accomplished through dicing by a dicing blade. The final layer making up the sectionable structure is an adhesive layer comprising a two-part thermosetting epoxy approximately 0.5 to 1.0 µm thick which bonds the silicon substrate with the recesses to the polymer layer covering the insulative or semi-conductive layer having the passivated heating elements and electrodes. This adhesive layer and its method of application are described more fully in US-A-4,678,529. For forming a nozzle face, a two-step dicing operation is the preferred sectioning procedure, though a single dicing step is sufficient. When using the two-step procedure, the first step is performed by a circular resinoid dicing blade having a predetermined thickness, diameter, and angular speed to produce the nozzle face with the appropriate surface finish. The first step cuts through the silicon substrate, adhesive layer, and polymer layer, but only partially enters the insulative or semi-conductive surface. Thus, after the first step, the bonded structure remains together by the relatively thin silicon material at the bottom of relatively deep grooves defining the rows of individual printheads. The second dicing step uses a second blade which may have a smaller thickness. The second blade is directed along the grooves formed by the first steps to separate the bonded substrate into individual rows of printheads, and is then used to cut the rows into individual printheads. A more complete understanding of the present invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings, wherein like parts have like index numerals, and of which: Figure 1 is an enlarged isometric view of the channel wafer bonded to the heating element wafer, after the excess channel wafer material has been removed to expose the addressing electrode terminals, and showing the dicing blade as it forms the nozzle-containing surfaces of the printheads; Figure 2 is an enlarged partially shown cross-sectional view of Figure 1 taken along a plane including the dicing blade axis of rotation and radius located normal to the wafers; Figure 3 is the same view as Figure 2 but subsequent in time, showing the second dicing operation which sections the bonded wafers into rows of printheads; Figure 4 is a partial, cross-sectional isometric view of Figure 1 showing the formation of the nozzle-containing surface; Figure 5 is an enlarged isometric view of the portion of Figure 4 enclosed by dashed lines and identified as "A"; Figure 6 is a view similar to Figure 5, but showing the nozzle and nozzle-containing surface defects caused by a known dicing operation; Figure 7 is a view similar to Figure 5, but showing the nozzle and nozzle-containing surface defects caused by still another known dicing operation, and Figure 8 is a cross-sectional view of the dicing blade in a blade holder or flange. According to US-A-4,638,337 and US reissue 32,572 thermal ink jet printheads may be mass produced by sectioning at least two mated planar substrates containing on confronting surfaces thereof respective matched sets of linear arrays of heating elements with addressing electrodes and linear arrays of parallel elongated grooves, each set of grooves being interconnected with a common recess having an opening through the opposite substrate surface. The elongated grooves serve as ink channels, and the common recess serves as an ink reservoir or manifold. The recess opening is the ink inlet to which an ink supply is connected. Each ink channel contains a heating element, and the sectioning operation, generally a dicing operation, opens the ends of the ink channels opposite the ends connecting with the manifold, if not already open, and forms the nozzle-containing surface. After the sectioning operation, the heating elements are located at a predetermined location upstream from the nozzles. The main difference between the above identified patents is that the former discloses an intermediate thick film polymer layer sandwiched between the mated substrates. The thick film layer is etched to expose the heating elements, which effectively places the heating elements in a pit whose vertical walls inhibit vapor bubble growth in the direction parallel to the heating element surface. This prevents vapor blow-out and the resultant ingestion of air which produces a rapid printhead failure mode. This invention relates to a dicing method for sectioning the multiple printhead containing substrates into separate printheads. Though it is known to use dicing as a means of sectioning mated wafers, it is not known to define a dicing blade and its operating parameters which will provide a substantially defect-free nozzle and nozzle face (i.e., a nozzle-bearing surface). Thus, all known diced printheads require subsequent treatment of their nozzle faces by such finishing operations as grinding, lapping, or polishing. In addition, when multi-layered structures, which have different grinding, lapping, and polishing characteristics, are processed to provide a desired surface finish, it is very difficult to obtain a surface which is planar within the desired flatness tolerance. Referring to Figure 1, a resin-based dicing blade 10 is shown cutting across an edge of the channel plates 31 in the direction of arrow 13 to form the nozzles 27 and nozzle face 29 (see Figures 3 and 4) for the plurality of printheads 12. Prior to this dicing operation, excess silicon material is milled from the silicon channel wafer or upper substrate to form the individual channel plates 31 and uncover the addressing electrode terminals 32. Etched grooves and recesses (not shown) around the periphery of the channel plates greatly assist in preventing damage to the addressing electrodes 33 and their terminals 32 during this silicon removal step, by spacing the channel wafer therefrom. In the preferred embodiment, the thick film polymer layer 58 between the channel plates 31 and the heating element substrate 36 is etched to remove it from the distal ends of the electrodes 33 and their contact pads or terminals 32. Each channel plate has an inlet 25 which may be separately etched in the channel plates, or they may be simply formed by etching the manifold 22 (see Figures 2 and 3) completely through the channel wafer from which the channel plates 31 are formed. The dicing blade 10 is rotated about axis 11. For a dicing blade having a diameter of approximately 50mm and a thickness of about 0.1 to 0.25 mm, the speed may range from 32,000 to 45,000 rpm. The blade has a relatively hard, dense resin-bonded base material with substantially no abrasive fillers and contains a 60 to 90% concentration of natural or synthetic diamonds having a nominal diamond size range of 2 to 6 micrometers. The density reference is based on 72 carats per cubic inch. The dicing cut to provide the nozzle-containing surface with the appropriate surface finish and optionally to open the nozzles, is made along dashed lines 18. The dashed cutting lines 23 which are perpendicular to the dashed cutting lines 18 delineate the travel of a second dicing blade to complete the sectioning of channel plates 31 and heating element wafer 36 into separate printheads 12. Figure 2 is a cross-sectional view of Figure 1, as viewed along view line 2-2, and shows that the dicing blade 10 cuts through the channel plates 31, thick film layer 58, adhesive bonding layer 15 (see Figure 5), electrode insulating underglaze layer 17, passivation layer (not shown), and partially into the heating element wafer 36 a predetermined distance. In general, the blade penetration depth versus blade width ratio falls within the range of 6:1 to 9:1. If the ratio is 10:1 or higher, the blade will wander, thus eliminating the saw cut's positional accuracy of ± 0.0025mm. A high ratio also deleteriously affects nozzle surface quality. If the ratio is lower, the removal of cut material becomes excessive and results in a stressed cut which again causes surface defects that adversely impact droplet quality and trajectory. The ink channel 21, manifolds 22, and heating elements 34 are shown in dashed line for reference to the dicing cut centerline 18. Although in the preferred embodiment the dicing blade 10 only notches or partially cuts into the heating element wafer, it could completely cut through both the channel plates 31 and heating element wafer 36. The penalty for this complete cut through is a shorter life of the resin-bonded blade. The shorter life is caused by the adhesive used to hold the heating element wafer 36 to a wafer-holding film frame (not shown). Another critical aspect of the dicing procedure is the blade exposure minus the penetration depth, referred to in Figure 2 as "X". The value "X" must fall into the range of 0.15 to 0.30 mm. At less than 0.15 mm, clearance is insufficient for coolant to get to the blade. This causes high friction resulting in a stressed cut and subsequent nozzle damage. At more than 0.30 mm clearance, the blade vibrates, damaging the nozzle and scarring the nozzle face 29. For the integrated circuit industry, the ability to process die by sawing or dicing is well known, but being able to create a nozzle face surface having an acceptable surface finish via dicing alone is totally unknown. By omitting a subsequent finishing operation, such as polishing, the fabrication of a relatively smooth surface by high-quality dicing allows higher throughput of devices, resulting in an extremely economical and cost-effective manufacturing operation. Figure 3 is the same view as Figure 2, but shown after nozzle 27 and nozzle-containing surface 29 were formed by the dicing blade 10 as it cuts along dicing centerline 18. Figure 3 shows the notches or grooves 19 in the heating element wafer 36 which were produced as a result of the cuts by dicing blade 10. Along the same dicing cut centerline 18, and also along dicing cut lines 23 perpendicular thereto, a dicing blade 20, optionally thinner than blade 10, severs the heating element wafer 36 and produces a plurality of separate print heads 12. If the dicing blade 20 is of the same thickness as or thicker than, the resin-bonded blade 10, then the wafer 36 must be slightly tilted relative to the dicing blade 20, so that the nozzle-containing surface 29 is not contacted. Figure 4 is an isometric view of the heating element wafer 36 and the channel plates 31 bonded thereto, as viewed along view line 4-4 of Figure 1. As the dicing blade 10 cuts along cutting line 18, nozzles 27 and nozzle-containing surface 29 are formed and the heating element wafer is partially cut, resulting in an elongated notch or groove 19 being produced having a floor 24. Figures 5-7 are an enlarged view of the portion of Figure 4 enclosed in dashed line and identified as area "A". Figure 5 is an enlarged partially shown view of the nozzles 27 and nozzle-containing surface 29 showing the abrasion-and scratch-free nozzles and nozzle-containing surface. This view shows a portion of the heater plate 28, produced by the dicing operation from heating element wafer 36, thick film layer 58, and channel plate 31. The triangular ink channels with interior walls 14 that were etched into the channel plate, produce the nozzles 27 when the channel plate is diced by the dicing blade 10. Other different materials are concurrently diced; namely, the electrode underglaze layer 17, electrode passivation layer (not shown), and the bonding material 15, which is generally an adhesive. Adhesive fillets 16 form along the length of the ink channel at the interface between the thick film and the channel plate. Etched-through holes 35 in the thick film layer 58 effectively place the heating elements (not shown) in a pit for inhibiting the lateral movement of the bubble growth during printhead operation, so that air ingestion is avoided. In spite of the dissimilar materials encountered by the dicing blade during the dicing operation, the blade 10, produced a high-quality surface finish as well as chip-and scratch-free nozzles. Thus, this method of creating a quality orifice or nozzle surface at the same time the sectioning operation occurs allows high throughput with established parts-handling techniques. Step 24 is produced by the dicing operation with the second dicing blade 20, if this second blade has a smaller thickness than the blade 10. The width of this step is 0.025mm or less, and has no impact on the printing performed by the printhead. To achieve the desired nozzle and nozzle-face surface finish, the specific operating parameters are: (a) the dicing blade must be a resin-based blade, and the composition must contain a relatively hard dense resin-bonded base material. (b) the blade must contain little or no abrasive fillers, and must have a high density fine diamond distribution, in which the diamond acts as the abrasive medium. The diamonds may be natural or synthetic, have a nominal diamond size range from 2 to 6 micrometers and a diamond concentration of 60 to 90%. The density reference is based on 72 carats per cubic inch. (c) the blade penetration depth versus blade width ratio must fall within the range of 6:1 to 9:1. Blade thickness is dependent upon the depth of the cut, for example, the preferred thickness range of the blade is 0.1 to 0.25mm for penetrations of 0.75 to 1.0mm. If the ratio is higher than 9:1, the blade will wander, thus eliminating saw cut quality and dimensional precision. If the ratio is lower, the removal of cut material becomes excessive and again affects saw cut quality. (d) the feed speed of the dicing blade into the silicon ranges from 0.275 to about 4.0mm per second, with the preferred speed being around 3.0 mm per second. (e) the final aspect of the dicing process dictates that the peripheral speed of the dicing blade be greater than 80 m per second, preferably 100 m per second or about 40,000 rpm for a 50 mm diameter saw blade. The two key factors involved in this process, are the use of resin-bonded versus metal-bonded blades, and process speeds equal to or greater than 80m per second versus speeds greater than 100 m per second or dicing blade speeds of 40,000 rpm. For the integrated circuit industry, the ability to process die by sawing is well known. However, the capability of being able to create a nozzle-quality, polished surface by using a dicing blade without the need for further surface treatments is not known. Dicing by the above operating parameters provides a 98% device yield. Assuming a printhead having 50 nozzles per printhead, with one chip caused by the sawing operation between two adjacent nozzles constituting a failure, means a nozzle dicing yield would be required of around 100%. Only the dicing operation of the present invention provides such a yield. There is a linear relationship between the peripheral speed (S v ) of the blade and its speed (F) over the substrate in inches per second; S v approximately equals F x 2.9 x 10⁴ + 2400. Peripheral speed can easily be converted to rpm for a given diameter blade. In general, the blade penetration depth versus blade width ratio falls within the range of 6:1 to 9:1. The final critical aspect of the dicing procedure is the blade exposure minus penetration depth. Refer to Figure 2 wherein this dimension is identified as "X". The blade exposure is that portion of the blade extending from the blade holder 38. This value must fall in the range of about 0.15 to 0.30mm. The procedure of dicing according to the present invention in order to obtain the appropriate nozzle-containing surface finish increases blade wear. The integrated circuit industry does not usually operate at speeds higher than 30,000 rpm or use resinoid blades for this very reason. It becomes economically inefficient. With the resinoid blades used, the wear rate is approximately 0.025 to 0.05mm per 75mm diameter wafer pair processed. Given the constraint on the exposure minus the penetration, this suggests that a flange change be required after processing three to four wafer pairs. It is estimated that the approximate flange sets of about 30 wafer pairs can be processed per blade. Assuming 300 die per wafer pair, 9,000 could be produced with a single resin-bonded blade. Also, considering process speeds discussed above, these 9,000 dice or printheads could be produced in one day. Thus, though the blade wear is much higher than would be desired, the alternative is that the nozzles and nozzle faces would require a subsequent finishing or polishing operation. Figure 6 is a similar view to that of Figure 5 but showing the effects of a nozzle and nozzle-surface created by either a metal-bonded or resin-based dicing blade produced by dicing blade speeds of 82.5m/s or less (30,000 rpm or less for a 50mm diameter blade). Numerous chips 26 of silicon material have been removed from the periphery of the nozzle rendering it non-functional. Any chip or material void larger than 2 micrometers is not acceptable. Figure 7 is a similar view to that of Figure 5 but shows the quality of nozzle and nozzle-containing surface produced by a metal-bonded blade operating at 100m/s (40,000 rpm for a 50mm diameter blade). Note the chips 26 produced by the slower rotating dicing blades in Figure 6 are also present with a higher-speed metal-bonded blade. In addition, severe scratching and abrasions 37 on the surfaces of the nozzle-containing surface and even larger chips 26, are produced at the nozzle by the metal-bonded blades. The metal-bonded blades at the higher rpm speed not only scar the surfaces produced by them, but produce saw marks 39 in the thick film layer 58. Figure 8 is a schematic cross-sectional view of a typical dicing blade holder or flange 38 which comprises inner flange 40 and outer flange 42 between which the dicing blade 10 is held on the circular shoulder 43 of inner flange 40. Flange nut 44 tightened on screw threads 43 of outer flange 42 lock the dicing blade in place. The blade holder is mounted on a rotatable shaft (not shown) which rotates the blade holder about axis 11. The exposure of the blade, identified as "Y", is that annulus of the circular dicing blade which extends from the blade holder. As discussed above, the blade exposure "Y" minus the depth of penetration of the blade into the material being cut is identified by the dimension "X" in Figure 2 and is, of course, a critical aspect of this dicing operation as discussed above. In summary, an inexpensive process for creating a high quality orifice and orifice-containing surface in ink jet printheads is enabled by the use of a high-density, fine grit diamond resin-bonded rotary saw operated at very high process speeds with an appropriate depth of penetration and blade holder clearance.

Claims: A method of fabricating a plurality of ink jet printheads from at least two substrates having confronting surfaces aligned and bonded together, wherein the confronting surface of one substrate (38) contains a plurality of sets of equally-spaced, linear arrays of heating elements and addressing electrodes having terminals for enabling the individual addressing of each heating element with current pulses, and wherein the confronting surface of the other substrate (31) contains a plurality of sets of equally-spaced, parallel grooves (21) and a recess (25), one end of each set of grooves being adapted to communicate with an ink reservoir, the fabricating method comprising the steps of: (a) removing the substrate material above the electrode terminals on the other substrate and thus producing a plurality of separate portions in which each contains one set of grooves (21) and interconnected recesses (25); (b) dicing the end portions from each separate substrate portion containing the ends of the set of grooves opposite the ends communicating with the ink supply means to form a nozzle face (29) containing open groove ends, to serve subsequently as droplet-emitting nozzles (27), the dicing being accomplished by a first circular, resin-bonded blade having a predetermined diamond particle size and density, a predetermined diameter and thickness, and operated at a peripheral speed of greater than 80m/s with a predetermined depth of cut, so that the entire region containing the grooves is cut thereby providing the nozzle face (29) with a planar, substantially defect-free surface, and (c) dicing in a direction perpendicular to the nozzle faces to separate the rows of printheads into individual printheads. The method of claim 1, wherein the first blade cuts through the entire thickness of the substrate portions containing the grooves in a first dicing cut to form the nozzle face, but the predetermined depth of cut prevents cutting entirely through the other substrate, so that it is only grooved , and wherein step (b) further comprises the step of: dicing along the groove in the other substrate with a second dicing cut done in a manner not to contact the nozzle face produced by the first dicing cut. The method of claim 1 or 2, wherein the second cut is made by a different rotary blade. The method of claim 3, wherein the second blade is thinner than the first blade. The method of any preceding claim, wherein the ratio of the depth of cut of the first blade to its thickness lies within the range of 6:1 to 9:1. The method of any preceding claim, wherein the first blade exposure is greater than the depth of cut by 0.15 to 0.3 mm. The method of any preceding claim, wherein the feed speed of the first blade is in the range from 0.275 to 4.0mm/s. The method of any preceding claim, wherein the diamond particle size is within the range of 2 to 6 micrometers, and wherein the diamond density is from 60% to 90%, with a density reference based on 72 carats per cubic inch.

Inventors: CAMPINELLI MICHAEL R, CAMPINELLI, MICHAEL R.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1989
Main classification: B41J2
IPC codes: B41J2, B23D61, H01L21