CA1303903C - Barrier layer and orifice plate for thermal ink jet print head assemblyand method of manufacture - Google Patents

Abstract

ABSTRACT

This application discloses a thermal ink jet print head and method of manufacture featuring an improved all-metal orifice plate and barrier layer assembly. This assembly includes constricted ink flow ports to reduce cavi-tation damage and smooth contoured convergent ink ejection orifices to prevent "gulping" of air during an ink ejection process. Both of these features extend the maximum operating frequency, fmax, of the printhead. The nickle barrier layer and the underlying thin film resistor sub-strate are gold plated and then soldered together to form a good strong solder bond at the substrate - barrier layer interface.

Description

?3~3 IMPROVED BARRIER LAYER AND ORIFICE PLATE FOR THERMAL
INK JET PRINT HEAD ASSEMBLY AND METHOD OF MANUFACTURE

Technical Field This invention relates generally to ~hermal ink 10jet printing and more particularly to an ink jet print head barrier lay~r an~ orifice plate o~ improved geometry for extending the print head lifetime. This invention is also directed to a-novel method o~ fabricating this barrier layer and orifice plate.

Back~round Art In the art of thermal ink jet printing, it is known to provide controlled and localized heat transfer to a defined volume of ink which i~ located adjacent to an ink 2~ jet orifice. This heat transfer is sufficen~ to vaporize the ink in such volume and causa it to expand, thereby ejecting ink from the orifice during the printins of charac~
ters on a print medium. The above predefined volume of ink is customarily provided in a so-called barrier layer which is constructed ~o have a plurality of ink reservoirs therein. These reservoirs are located between a corresponding plurality of heater resistor elements and a corresponding plurality of orifice segments for ejecting ink 30 therefrom.
one purpose of these reservoirs is to contain the expanding ink bubble and pressure wave and make ink ejection more efficient. Addi~ionally, the reservoir wall is used ~o slow down cavita~ion produced by tha collapsing ink bubble.
35 For a further discussion of this pressure wave phenom~na, reference may be made to a book by F. G. Hammltt entitled Cavita~ion and Multiphase FlQw_Phenomena, ~cGraw Hill 1980, page 167 et seq, The useful li~e of these prior art ink jet print head assemblies has been limited by the cavitation-produced B

~l3~3~

wear from the pressure wave created in the assembly when an ink bubble collapse~ upon ejec~ion from an orifice. This 5 pressure wave produces a significant and repeated force at the individual heater resistor elemants and thus produces wear and ultimate failure of one or more of these resistor elPments after a repeated number of ink jet operations. In addition to the above problem of resistor wear and failure, 10 prior art ink jet head assemblies of the above type have been constructed using polymer materials, such as those known in the art by the trade names RISTON and VACREL. .CP4 Whereas these polymer materials have proven satisfactory in many respects, they have on occasion exhibited unacceptably 15 high failure rates when subjected to substantial wear pro-duced by pressure waves from the collapsing ink bubbles during ink jet printing operations. Additionally, in some printing applications wherein the printer is exposed to extreme environments and/or wear, these polymer materials 20 have been known to swell and lift from the underlying sub-strate support and thereby render the print head assembly inoperative.

~isclosure of Invention The general purpose of this invention is to increase the useful lifetime of these types o~ ink jet printhead assemblies. This purpose is accomplished by redu~ing the intensity of the pressure wave created by collapsing ink 3 bubbles, while ~imultaneously improving the structural inte grity of the barrier layer and orifice plate and strength of materials comprising aame. Additionally, the novel smoothly contoured geometry of the exit orific2 increases the maximum achievable frequency of operation, fmax.
The reduction in pressure wave intensity, the increase in barrier layer strength and integrity, and the increase of fmax are provided by a novel barrier layer and orifice plate geometry which includes a discontinuous layer of metal having a plurality of distinct sections. These sections are con~oured to de~ine a corresponding plurality of central cavity region~ which are axially aligned with .
.
.
.
.. , : .

3~3 , -respect to the direction of ink flow e~ected ~rom a print head assembly. Each of these c~ntral cavity regions connect 5 with a pair of constricted ink flow ports having a width dimension substantially smaller than the diameter of the central cavity regions. In addition, these sections have outer walls of a scalloped configuration which serve to reduce the reflective acoustic waves in the assembly, to oreduce cros~-talk between adjacent orifice~, and to thereby increase the maximum operating frequency and the quality of print produced.
A continuous layer of metal adjoins the layer of discontinuous metal sections and includes a plurality of 15output orifices which are axially aligned with the cavities in the discontinous metal layer. These orifices have diame-ters smaller than the diameters of the cavities in the discontinuous layer and ~urther include contoured walls which defin~ a convergent output orifice and which extend to 20 the psripheries o~ the cavities. Th~s convergent output orifice geometry serves to reduce air "gulping" which inter-fers with the continuou~ smooth operation of the ink jet printhead. Gulping i5 the phenomenon of induced air bubbles during the process of bubble collapsing.
By li~iting thQ width of the ink ~low ports extending ~ro~ the cavit~e~ defined by the discontinuous metal layer, the resistance to pr~s~ure wave forces within the assembly is increasod. This feature reduces and mini-30 mizes the a~ount of "gulping" and cavitation (and thuscavitation producQd wear) upon th~ individual heater resis-tor alement~ in the assembly. Additionally, the limited width of the~e ink flow ports ~erve~ to increa~e the effi-ciency of ink e~ection and li~its the r~fiil~time for khe 35 ink reservoir~, further reducing cavitation damage.
Furthermor~, by using a layered nickel barrier structure instead of polymer materials, the overall streng~h and inte-grity of the print head assembly i~ suhstantia~l~ increased.
Accordingly, it is an object of an aspect of the present invention to increase the lifetime of thermal ink jet print head assemblied by reducing cavitation-produced wear on the ~3~3~''3 individual resistive heater elements therein.
An object of an aspect of the invention is to increase the lifetime of such assemblies by increasing the strength and integrity of the barrier layer and orifice plate portion of the ink jet print head assembly.
An object of an aspect of the invention is to increase the maximum achievable operating frequency, fmax~ f the inX jet print head assembly.
A feature of an aspect of this invention is the provision of a smoothly contoured wall extending between the individual ink reservoirs in the barrier layer and the output exit orifices of the orifice plate. This contoured wall defines a convergent orifice opening and serves to reduce the rate of ink bubble collapse and reduce the interference with the next succeeding ink jet operation.
A feature of an aspect of this invention is the provision of a economical and reliable fabrication process used in construction of the nickel barrier layer and orifice plate assembly which required a relatively small number of individual processing steps.
A feature of an aspect of this invention is the precise control of barrier layer and orifice plate thickness by use of the electroforming process described herein.
Various aspects of this invention are as follows:
In a thermal ink jet print head assembly including a plurality of resistive heater elements located on a thin film resistor structure and urther having a plur-ality of individual ink reservoirs constructed atop the plurality of resistive heater elements, respectively, for receiving thermal energy therefrom during an ink jet printing operation, the improvement comprisingo a barrier layer and orifice layer structure and geometry including a discontinuous layer of metal having a B

~L3~3~3 4a plurality of interrupted sections therein defining a corresponding plurality of cavity regions axially aligned with said heater elemPnts and with respect to the direction of ink flow; each of said cavity regions being connected to constricted ink flow ports having widths substantially smaller than the diameters of said cavities, and a continuous layer of metal joining said discontinuous layer and having a plurality of output orifices axially aligned with said cavities and having output openings smaller than the diameters of said cavities; said output orifices further including smooth contoured walls extending from the peripheries of said cavities to said output openings and operative to minimize the turbulance of ink flow through said cavities and exiting said output orifices and thereby increasing the maximum achievable frequency of operation.
A process for fabricating a barrier layer and orifice plate structure for a thermal ink jet printhead comprising:
a. forming a mask of a predetermined limited thickness on a selected metallic substrate;
b. electroforming a first layer of metal on said substrate and extending in a contoured surface geometry into contact with said mask and defining an ori~ice output opening;
c. forming a second mask atop said first mask and thicker than said first mask, and having vertical walls extending above the surfare of said first layer of metal;
d. electroforming a second layer of metal on said first layer and adjacent said vertical walls of said second mask so as to define an ink reservoir cavity bounded by vertical walls ext~nding from edges of said contoured surface geometry of said first metal layer; and B

3~ 3 ~,h3 4b e. removing said first and second masks and said selected metallic substrate, thereby leaving intact said first and second metal layers in a composite layered configuration where said vertical walls of said second layer define boundaries of ink reservoirs of said structure.
The process for forming an integrated orifice plate and barrier layer structure which includes the steps of:
a. forming a first mask portion having a convergently contoured external surface and a second mask poxtion having straight vertical walls, and b. electroforming a first metal layer around said first mask portion to define an orifice plate layer having one or more convergent orifices, and -electroforming a second metal layer around said second mask portion to define a barrier layer having one or more ink reservoir cavities aligned respectively with one or more of said convergent orifices in said orifice plate layer.
These and other objects and features of this invention will become more readily apparent in the following description of the accompanying drawings.

Brief Description of Drawinqs Figures lA through lH are schematic cross-sectional diagrams illustrating the sequence of process steps used in the fabrication of the barrier layer and orifice plate assembly according to the invention.
Figure 2 is an isometric view of the barrier layer and orifice plate assembly of the invention, including 0 two adjacent ink reservoir cavities and exit orifices.
Figure 3 is a sectional isometric view illustrating how the barrier layer and orifice plate assembly is mounted on a thin-film resistor structur~ of a ~3~t3~3~

thermal ink jet print head assembly.

Best Mode For CarrYin~ Out The Invention Referring now to Figure 1, there is shown in Figure lA a stainless steel substrate 10 which is typically 30 to 60 mils in thickness and has been polished on the upper surface thereof in preparation ~or the deposition of a positive photoresist layer 12 as ~hown in Figure lB. The positive photoresist layer 12 is treated using a conven-tional masking, etching and relat~d photolithographic processing steps known to those skilled in the art in order to form a photoresist mask 14 as shown in Figure lCo Us.ing a positive photoresist and conventional photolitography, the mask portion 14 i~ exposed to ultraviolet light and there-upon is polymerized to remain intact on the sur.Eace of ~he stainless steel substrate 10 as shown in Figure lC. The remaining unexposed portion~ of the photoresist layer 12 are developed using a conventional photoresi~t chemical developer.
Next, the structure of Figure lC is transferred to an electroforming metal deposition station where a fixst, continuous layer 16 of nickel is deposited as shown in Figure lD and forms smoothly contoured walls 18 which pro-ject downwardly toward what eventually becomes the output orifice 19 of the ori~ice plate. This contour 18 is achieved by the ~act that the electroform~d first nickel layer 16 overlaps the outer edges of the photoresist mask 14, and this occurs because there will be some electro-forming reaction through the outer edge~ of the photoresist mask 14. This occurs due to the small 3 micron thickness of th~ photoresist mask 14 and ~he ~act tha~ the elect~oforming process will penetrate the thin ma~k 14 at least around its outer edge and ~orm the convergent contour as shown.
Electro~orming is more commonly known as an adap-tation of electroplating. The electroplating is accomplished by placing the part to be plated in a tank (not shown) that contains ~he plating solution and an anode. The plating solution contain~ ions o2 the metal to be plated on 3~(~3 the part and the anode is a piece of that same metal. The part being plated i~ called the cathode. Direct current is 5 then applied between the anode and cathode, which causes the metal ions in the solution to move toward the cathode and deposit on it. The anode dissolves at the same rate that the metal is being deposited on the cathode. This system (also not shown) is called an electroplating cell.
At the anode, the metal atoms lose electrons and go into the plating solution as ca~ions. At the cathode, the reverse happens, the metal ions in the plating solution pick up electrons from the cathode and deposit themselves there as a metallic coating. The chemical reactions at the 15 anode and cathode, where M represents the metal being plated, are:
Anode: M M+ + e Cathode: M+ + e M

Electroforming is similar to electroplating, but in the electroforming process an object is electroplated with a metal, but the plating is then separated from the object. The plating itself is the finished product and in most case6, the object, or substxate 10 in the present process, can be reused many times. As will be seen in the following description, the removed plating retains the basic shape of the substra~e surface and masks thereon.
In the next step shown in Figure lE, a thick layer of laminated photoresist 20, typically 3 mils in thickness, is deposited on the upper surfac~ of the first layer 16 of nickel and therea~ter the coated structure is transferred to a photolithographic masking and developing sta~ion where a 35 second photoresist mask 22 is formed as shown on top of the first photoresist mask 14 and covers the contoured wall section 18 of the first stainless steel layer 16. This second photoresist mask 22 includes ver~ical side walls 24 of substantial vertical thickne~s, and these steep walls prevent any electroforming beyond ~hese vertical boundaries in the next electro~orming step illu~trated in Figure lG.

... .

~3~ 3 In the second plating or eleotroforming step shown in Figure lG, a second, discontinuous layer 26 of nickel is 5 formed as shown on the upper surface of the first nickeel layer 16, and the first and second layers 16 and 26 of nickel are approximately a combined thickness of 4 mils.
The thickness of layer 16 will be about .00~5 inches and ~he thickness of layer 26 will be about .0015 to .0020 inches.
loThe second photoresist mask 22 is shaped to provide the resultant discontinuous and scalloped layer geometry shown in Figure lH, including the arcuate cavity walls 31 and 33 extending as shown between ~he ink flow ports 35 and 37 respectively. The scalloped wall portions 30 o~ the dis-15 continuous second layer of metal 26 serve to reduce acousticreflective waves and thus reduce cross-talk between adjacent orifices 32.
A significant advantaga of using the above elec-troforming process lies in the fact that the nickel layer20 thickness may be carefully controlled to any desired measure. This feature is in contrast to the use of VACREL
and RISTON polymers which are currently available ~rom cer-tain vendors in only selectively spaced thicknesses.
onc~ the barrier layer and orifice plate-composite 25 structure 28 is completed as shown in Figure lG, the struc-ture of Figure lG iq tran~ferred to a chemical stripping station where the structure is imoersed in a suitable photo-resi~t stripper which will remove both the first and second 30 photoresist masks 22 and 24, carrying with them the stain-less steel subs~rate 10. Advantageously this substrate 10 has been used as a carrier or "handle" throughout the first and second electro~orming step~ described above and may be reused in sub~equent electrofo~ming processes. Thus, the 35 completed barrier layer and orifice plate assembly 28 is now ready for transfer to a gold plating bath where it is immersed in the bath for a time of approximately one minute in order to form a thin coating of gold over the nickel surface of about 20 micrometers in thickness.
This gold plating step per se i5 known in the art and is advantageously used to pro~ide an inert coating to ~a3~3~

prevent corrosion from the ink and also to provide an excel-lent bonding material for the subsequent ~hermosonic (heat 5 and ultrasonic energy) bonding to solder pads ~ormed on the underlying and supporting thin film resistor substrate.
Thus, the fact that the metal orifice plate and barrier layer may be gold plated to produce an inert coating thereon makes this structure highly compatible with the soldering lOprocess which is subsequently used to bond the barrier layer to the underlying passivation top layer of the thin film resistor substrate. That is, nickle which has not been gold plated is subject to surface oxidation which prevents the making of good strong solder bondq. Also, the use o~ poly~
15 mer barrier materials of the prior art prevents the gold plating thereof and renders it incompatible ~ith solder bonding.
Referring now to Figure 2, ther~ i5 shown an isometric view looking upward through the exit ori~ice~ of 20 the composite barrier layer and orifice plate assembly 28.
The contoured walls 18 ~xtend ~etween the output orifice opening and the second nickel layer 26 and serve to increase the maximum achievable operating frequency, fmax~ of the ink jet print head when compared to prior art ~arrier plate 25 configurations having no such contour. In addition, this nickle-nickle barrier layer and orifice plate and geometry thereof serves to prevent gulping, to reduce cavitakion, and to facilitate high yield manufacturing with excellent solder 30 bonding properties as previously dasired.
The width of the constricted ink flow port 58 will be approximately .0015 inche~, or about one-half or less than the diameter of ink reservoir 59. This diamet~r will typically range from .003 to .005 inches. The diameter of 35 the outpu~ ink ejection orifice 32 will be about .0025 inches.
Referring now to Figure 3, the composite barrier layer and oriPice plate 28 is m~unted atop a thin film resistor struc~ure 33 which includes an underlying silicon sub~trate 40 typically 20 mils in thickness and having a thin sur~ace passivation layer 42 of silicon dioxide ~3U3~

thereon. A layer of electrically resistiYe material 44 is deposited on the surface of the SiO2 layer 42, and this 5 resistive material will typically be tantalum-aluminum or tantalum nitride. Next, using known metal conductor deposi-tion and masking techniques, a conduc~ive pa~tern 46 o~
aluminum is formed as shown on top of the resis~ive layer 44 and includes, for example, a pair of openings 47 and 49 lO therein which in turn defin~ a pair of electrically active resistive heater elements (resistors) indicated as 50 and 52 in Figure 3.
An upper surface passivation layer 53 is provided atop the conductive trace pattern 46 and is preferably a 15 highly inert ma~erial such as silicon carbide, SiC, or silicon nitride, Si3N4, and thereby serv~s to provide good physical isolation between the heater resistors 50 and 52 and the ink located in the reservoixs above th~se resistors.
Next, a layer (or pads) 55 of solder is disposed 20 between the top surface of the passivation layer 53 and the bottom surface of th~ nickel barrier layer 26, and as previously indicated provides an excellent bond to the gold plated sur~aces of the underlying pa~sivation layer 53 and the overlying nickle barrier layer 26.
As i well known in the art of thermal ink jet printing, electrical pul~e3 applied to the aluminum conduc-tor 46 will provide resistance haating of the heater elements 50 and 5~ and thus provid~ a trans~er of thermal 30 en~rgy from the~e heater elements 50 a~d 52 through the sur~ace pa~sivation l~yer 53 and to the ink in the reser-voirs in the nickel layer 26.
The silicon sub~trate 40 i5 bonded to a manifold header tnot shown) u~ing conventional silicon-die bonding 35 techniques known in the art. Advantageously, thi~ header may be of a chosen pla~tic matarial which is preformed to receive the conductive leads 46 which have been previously stamped from a lead frame ~also not shown~. This lead frame is known in the art as a tape automated bond (TAB) flexible circuit of the type disclosed in U.S. Patent l~o. 4, 635, 073, issued January 6, ]987, Gary Hanson and assigned to the ~l3~3~ 3 present assignee.
In operation, heat is transmitted through the 5 passivation layer 53 and provides rapid heating of the ink sto~ed within the cavities of the barrier layer and orifice plate structure 28. When this happens, the ink stored in these cavities is rapidly heated to boiling and expands through the exit orifices 32. However, when the expandin~
10 ink bubble subsequently collapses during cavitation at the ink jet orifices 32, the contour of the convergent output orifices and the reduced width of the constricted ink flow ports 58 serve to slow down the collapse of the ink bubble and thereby reduce cavitation intensity and the damage 15 caused thereby. This latter feature results in a signifi-cant resistance to this cavitation-produced downward pres-sure toward the resistive heater elements 50 and 52.
Thus, there has been described a novel barrier layer and orifice plate assembly for thermal ink jet print 20 heads and a novel manufacturing process therefor. Various modifications may be made to these above described embodi-ments of the invention without departing from the scope of the appended claims.

Claims (11)

1. In a thermal ink jet print head assembly including a plurality of resistive heater elements located on a thin film resistor structure and further having a plur-ality of individual ink reservoirs constructed atop the plurality of resistive heater elements, respectively, for receiving thermal energy therefrom during an ink jet printing operation, the improvement comprising: a barrier layer and orifice layer structure and geometry including a discontinuous layer of metal having a plurality of interrupted sections therein defining a corresponding plurality of cavity regions axially aligned with said heater elements and with respect to the direction of ink flow; each of said cavity regions being connected to constricted ink flow ports having widths substantially smaller than the diameters of said cavities, and a continuous layer of metal joining said discontinuous layer and having a plurality of output orifices axially aligned with said cavities and having output openings smaller than the diameters of said cavities; said output orifices further including smooth contoured walls extending from the peripheries of said cavities to said output openings and operative to minimize the turbulance of ink flow through said cavities and exiting said output orifices and thereby increasing the maximum achievable frequency of operation.

2. The improvement defined in Claim 1 wherein said discontinuous layer has scalloped outer walls which serve to reduce cross talk and reflective acoustic waves.

3. The improvement defined in Claim 1 wherein said continuous and discontinuous layers are electroformed of nickel.

4. The improvement defined in Claim 2 wherein said continuous and discontinuous layers are electroformed of nickel.

5. A process for fabricating a barrier layer and orifice plate structure for a thermal ink jet printhead comprising:
a. forming a mask of a predetermined limited thickness on a selected metallic substrate;
b. electroforming a first layer of metal on said substrate and extending in a contoured surface geometry into contact with said mask and defining an orifice output opening;
c. forming a second mask atop said first mask and thicker than said first mask, and having vertical walls extending above the surface of said first layer of metal;
d. electroforming a second layer of metal on said first layer and adjacent said vertical walls of said second mask so as to define an ink reservoir cavity bounded by vertical walls extending from edges of said contoured surface geometry of said first metal layer; and e. removing said first and second masks and said selected metallic substrate, thereby leaving intact said first and second metal layers in a composite layered configuration where said vertical walls of said second layer define boundaries of ink reservoirs of said structure.

6. The process defined in Claim 5 wherein said second mask is configured to have discontinuous arcuate side wall sections defining openings which function as ink flow ports for passing ink from the exterior of said second metal layer to said orifice output openings.

7. The process defined in Claim 6 wherein said first mask is of contoured geometry and provides an output orifice opening, and said second mask is configured to have a scalloped wall geometry which is replicated in the outer wall geometry of said second metal layer.

8. The process defined in Claim 6 wherein said barrier layer and orifice plate structure is aligned and mounted on a thin film resistor structure including an array of resistive heater elements, with said heater elements aligned with respect to the ink reservoirs in said barrier layer and orifice plate assembly.

9. The process defined in Claim 8 which further includes die bonding said thin film resistor structure to a header which is also functional to receive conductive leads extending from resistive heater elements in said thin film resistor structure.

10. The process for forming an integrated orifice plate and barrier layer structure which includes the steps of:
a. forming a first mask portion having a convergently contoured external surface and a second mask portion having straight vertical walls, and b. electroforming a first metal layer around said first mask portion to define an orifice plate layer having one or more convergent orifices, and electroforming a second metal layer around said second mask portion to define a barrier layer having one or more ink reservoir cavities aligned respectively with one or more of said convergent orifices in said orifice plate layer.

11. The process defined in any one of Claims 5, 6, 7, 8 or 9 in which said first and second layers are of nickel.