Ink jet printhead having a palladium cavitation barrier and interconnect layer

Abstract: A printhead for an inkjet printer includes a substrate (201) with a film resistor area (209) and an electrically insulating layer (213) deposited on one surface of the substrate (201). A cavitation barrier and electrical interconnect layer (203) composed of palladium is selectively deposited on the electrically insulating layer (213) over the resistor (205) for cavitation protection and on a portion of the electrically insulating layer (213) for coupling between the resistor (205) and other electrical components. The single layer of palladium (303) reduces the number of steps required in the manufacture of such an inkjet printhead.

Description: The present invention generally relates to a printhead for a thermal inkjet printer print cartridge and more particularly to a thermal inkjet cartridge printhead and associated interconnect and method for making the same which has fewer process steps required in the construction of the printhead substrate since one layer of palladium forms both the cavitation barrier layer and the interconnect layer. Thermal inkjet printing has become one of the standard techniques of transferring computer generated images or text onto tangible media such as paper or transparency film. A printhead cartridge typically consists of one or more encased reservoirs of ink fluidically coupled to a small laminated structure (a printhead) having many interconnected ink channels and chambers which terminate in very small orifices in the printhead outer layer (the orifice plate). Generally, the small orifices are arranged in such a fashion in the orifice plate that the expulsion of a droplet of ink from a determined number of orifices relative to a particular position of the medium results in the production of a portion of a desired character or image. Controlled repositioning of the substrate or the medium and another expulsion of ink droplets continues the production of more portion of the desired character or image. Expulsion of the ink droplet in a conventional thermal inkjet printer is a result of rapid thermal heating of the ink to a temperature which exceeds the boiling point of the ink solvent and creates a gas phase bubble of ink. Each orifice is coupled to a small unique chamber filled with ink and having an individually addressable heating element in thermal contact with the ink. As the bubble nucleates and expands, it displaces a volume of ink which is forced out of the orifice and deposited on the medium. The bubble then collapses and the displaced volume of ink is replenished from the larger ink reservoir. A common failure mode of the inkjet printhead is that of erosion of the heating element and its associated protective layer found in each of the ink firing chambers by the thermal stresses created by the rapid activation of the heating element, the mechanical abrasion of the surface by the collapsing ink bubble after expulsion of ink, and the chemical attack by aggressive ink components. Conventional methods of combating the erosion are well explained and include such inventions as those disclosed in US Patent Nos. 4,513,298; 4,535,343; 4,965,611; and 5,112,812. Generally, the heating element is overlain with a passivation layer of silicon nitride and carbide designed to protect the heating element form the corrosive action of the heated ink. Overlying the passivation layer is a cavitation barrier layer to protect the structure from direct mechanical damage to the passivation layers and heating element. Typically, this layer is constructed of tantalum but tungsten or molybdenum have been suggested as alternative materials. Other reference have suggested a broad range of materials for cavitation barrier purposes, giving as desirable properties tenacity, excellent mechanical strength, excellent heat conductivity, liquid resistance, and liquid penetration prevention. From the periodic table, group VIII elements iron, cobalt, and nickel are given as appropriate materials for use. See US Patent No. 4,596,994. The foregoing solutions have proven adequate for their purposes but do not address more recent needs of increased long term reliability and reduced rate of failure at a lower material and manufacturing cost. The present invention will now be described in more detail with reference to the accompanying drawings, in which:- JP-A-01087272 describes an electrothermal recording head for an electrothermal printer. The recording head comprises a plurality of electrode elements comprising a platinum group metal, which may be palladium. JP-A-01037894 also describes an electrothermal recording head. The heating resistor is formed from a multilayer structure comprising a substrate, a lower conductive layer, a thin insulating layer, and an upper conductive layer. The conductive layers may be formed of palladium. The ivention will now be described in more detail with reference to the accompanying drawings, in which:- The present invention provides a print head for a thermal inkjet printer and a method of manufacture thereof as designed in the accompanying claims. FIG. 1 is an isometric diagram of a portion of a printhead which may employ the present invention. FIG. 2 is a cross sectional view of a printhead. FIG. 3 is a cross sectional view of a printhead which may employ the present invention. The present invention encompasses a thermal inkjet cartridge having a printhead which employs a layer of palladium as both the cavitation barrier and the electrical interconnect layer for reduced manufacturing costs and high reliability. FIG. 1 is a view of a portion of a thermal inkjet printhead illustrating an ink firing chamber 101 and an orifice 103 associated with the ink firing chamber 101. Part of a second orifice 105 associated with another ink firing chamber is also shown. Many orifices are typically arranged in a predetermined pattern on the orifice plate so that the ink which is expelled from selected orifices creates a defined pattern of print on the medium. Generally, the medium is maintained in a position which is parallel to the external surface of the orifice plate. Ink is supplied to the firing chamber 101 via an opening 107 to replenish ink which has been expelled from orifice 103 when ink has been vaporized by localized heating from a heating structure 109. The ink firing chamber is bounded by walls created by an orifice plate 111, a layered silicon substrate 113, and walls 115, 117 created by an ink barrier layer. A cross section of a conventional inkjet firing chamber taken through the heating structure 109 is shown in FIG. 2. The features of the various layers deposited on the substrate 113 are shown in greater detail but the relative thicknesses of the layers is exaggerated for ease of viewing. In a conventional printhead construction, a p-type silicon substrate 201 is covered with a thermally grown field oxide SiO 2 as an underlayer 203 to the remainder of the structure. A layer 205 of Tantalum-Aluminum (TaAl) is conventionally deposited on the surface of the SiO 2 layer 203 and, because of its relatively high electrical resistance, forms a resistive layer. A layer 207 of aluminum (Al) is selectively deposited on the TaAl layer 205, leaving open areas (such as area 209) to the TaAl layer 205. Because of the relatively low electrical resistance of the aluminum layer 207, the high resistance of the TaAl layer 205 is effectively shorted by the aluminum layer 207 except in the open area 209. The resulting area 209 is a resistor capable of rapid electrical resistance heating. The areas above the resistor must be capable of withstanding thermal extremes, mechanical assault, and chemical attack which result from the heating of the resistor, rapid vaporization of the ink, and subsequent collapse of an ink bubble formed by the vaporized ink. Accordingly, a passivation layer 213 consisting of silicon nitride (SiN) and silicon carbide (SiC) is selectively deposited over the structure. This passivation layer 213 is masked and selective openings 219, 221 to the aluminum conductor layer 207 are created. Conventionally, a layer of tantalum 215 is selectively deposited over the passivation-protected resistor area 209 and over openings 219, 221 to the aluminum conductive layer 207 such that an electrical contact between the tantalum layer 215 and the aluminum layer 207 is established at the openings 219, 221. The selective deposition of tantalum allows interconnection between contact openings and to external circuitry such that relatively complex electrical interconnections are realized. Tantalum is a relatively hard material and resists the mechanical abrasion from the collapsing ink bubble following ink nucleation in the firing chamber. Tantalum also exhibits good adhesion to the passivation layer 213. Unfortunately, conventional bonding techniques do not work well with tantalum, however, so an additional layer of conductive material must overlay the tantalum to provide a surface for bonding or other electrical interconnection. Typically, a layer of gold 227 is selectively placed over the tantalum layer 215 to provide a surface for bonding. The gold layer is not placed or retained in the ink firing chamber (i.e., over resistor area 209) because gold does not provide adequate protection from mechanical abrasion. It is important to note that additional processing steps are required to selectively place the gold layer 227 over the tantalum layer 215. Further, a cavitation barrier 215 consisting of tantalum (Ta) is selectively deposited over the passivation layer 213 in the ink firing chamber to protect against the fluid turbulence and abrasion created by the collapsing ink bubble. The layer of tantalum is discontinuously extended to cover additional surfaces of the substrate and various layers. In areas 219 and 221, the passivation layer 213 has been selectively removed to provide exposure to the aluminum conductor layer 207. Tantalum interconnect layers are selectively deposited over the exposed aluminum conductor layer 207 and the passivation layer 213. A gold (Au) interconnect layer 227 is deposited over the tantalum interconnect layers to provide a layer to which external electrical connections can be made, by bonding, soldering, or by contact, to an oxidation free surface. Electrical connections to the inkjet printer electronics external to the inkjet print cartridge are then accomplished by bonding, soldering, or otherwise connecting to the gold interconnect layer 227, as shown at 231. A similar connection may be made to other portions of the gold interconnect layer 227 to complete the electrical circuit for energizing the resistive heater area. A preferred embodiment of a printhead employing the present invention is illustrated in the cross sectional view of FIG. 3. It is a feature of the present invention that the process-intensive method utilizing both a tantalum and a gold interconnect layer as well as a tantalum cavitation layer have been eliminated. A single material layer 303 consisting of palladium (Pd) is deposited over the passivation layer 213 in the resistor area 209 to provide the necessary cavitation barrier over the resistor. In the preferred embodiment, this palladium cavitation barrier layer 303 extends throughout the bottom surface of the ink firing chamber and terminates beneath the ink barrier layer 115 and 117. Furthermore, the palladium layer 303 is selectively deposited over the exposed aluminum conductor layer 207 at the locations 219 and 221 so that an electrical connection may be realized to the aluminum conductor layer 207. This palladium layer 303 is also selectively deposited over the passivation layer 213 so that an electrical connection to the aluminum conductor layer 207 may be extended to a point where a connection to the inkjet printer may be made by bonding, soldering, or contacting a wire or other external electrical lead, such as shown at 311. It can be appreciated that an electrical connection made at point 311 is coupled to the aluminum conductor layer 207 at area 219. From this connection electrical flow proceeds to the TaAl resistive layer 205 at area 209 (the heater resistor) where it flows through the resistor and then to the other aluminum conductor layer 207 then to the palladium layer 303 at point 221. The palladium layer 303 is then electrically connected to multiplexing or other circuitry disposed on or in the substrate 201 or coupled off the substrate. The printhead, in the preferred embodiment, is manufactured using the following steps. The p-type silicon substrate 201 (for example a monocrystalline silicon structure doped with boron) having a thickness of about 510-540 microns is conventionally thermally oxidized to form a silicon dioxide layer 203 of approximately 15,250 angstroms. The electrically resistive layer of TaAl 205 is a mixture formed, in the preferred embodiment, by cosputtering both materials such that the deposited mixture is approximately 50 atomic (at.)% of Ta and 50 at. % Al and has a thickness of between 770 and 890 angstroms. Alternatively, the resistive layer may be of phosphorus-doped polycrystalline silicon. The conductive layer 207 is applied directly to the TaAl resistive layer 205 using conventional sputtering or vapor deposition techniques to deposit a layer of between 4000 and 6000 angstroms of Al. (In the preferred embodiment, the material composition is approximately 95.5% by weight aluminum). The layered substrate, at this point in its process, is then masked and etched (preferably using known plasma etching techniques) in order to produce exposed areas of the TaAl resistive layer 205 (such as area 209) and to remove both the Al conductive layer 207 and the TaAl resistive layer 205 (such as at area 315). The passivation layer 213 is then deposited over the remaining layers to provide protection from the corrosive action of the ink used in the inkjet printer for these layers. In the preferred embodiment, a first sublayer of silicon nitride is deposited from the decomposition of silane mixed with ammonia at a pressure of about 2 torr at a temperature from 300°C to 400°C. This first sublayer is deposited as a thickness of between 4000 and 6000 angstroms. A second passivation sublayer of silicon carbide formed by plasma enhanced chemical vapor deposition (PECVD) using silane and methane at a temperature of about 300°C to 400°C. This composite passivation layer 213 is subsequently masked and etched using reactive ion etching techniques or other chemical means to provide openings to the aluminum conductor layer 207 (at areas 219 and 221). The passivation layer is thereby separated into a portion covering the resistor area 209 and extending beneath the ink barrier layers 115 and 117, and a portion extending from beneath the ink barrier layer to the connection area 315. It is an important feature of the present invention that a single layer of palladium is then deposited upon selected areas of the layered substrate to provide both a cavitation layer over area 209 and an interconnect layer to which an electrical connection can be made at 311. In the preferred embodiment, a palladium target is sputtered in thin layer deposition on the layered substrate, producing a thin film layer of palladium between 4000 and 9000 angstroms thick of 99.95% wt. palladium. The palladium layer 303 is then masked and etched into electrically insulated portions such as the portion over the resistor area 209 (which portion forms the cavitation barrier) and portions, which are electrically insulated from each other, over contact areas 219 and 221. The ink barrier layer 115, 117 is selectively applied to and above the cavitation barrier of layer 303 (at area 209), portions of the interconnect portion of layer 303, and portions of the passivation layer 213, essentially as indicated in FIG. 3. This ink barrier layer 115,117 is preferably made of an organic polymer plastic, such as products sold under the names VACREL and RISTON by E. I. DuPont de Nemours and Co. of Wilmington Del., and which is both substantially inert to the corrosive effects of the ink and adheres well to palladium. In the preferred embodiment, the ink barrier layer is applied by conventional lamination techniques to a thickness of between 20 and 30 microns. Finally, a conventional orifice plate 111 (which, in the preferred embodiment is made of gold plated nickel) is applied to the ink barrier layer 115, 117 in a manner which orients each orifice over the ink firing chamber and above the heater resistor. It should be noted that additional processing steps may be included, such as those described in US Patent No. 5,159,353, to produce other semiconductor elements which provide additional functions such as signal multiplexing or printhead identification. Like gold, palladium is considered to be a noble element by the electro-chemical industry. Palladium is also a face-centered-cubic material which exhibits twelve independent planes of slippage which would lead a casual observer to conclude that palladium would be a mechanically soft material, unsuitable for use as a cavitation barrier. Furthermore, the specific heat of palladium is higher than that of tantalum, also suggesting that palladium would be inferior to tantalum in an environment requiring heat conduction. Thus, although palladium is a group VIII element, its use as a cavitation barrier material has not been apparent to those working in the field of inkjet printheads. It has been determined, however, that palladium has characteristics which are beneficial for its use in printheads. For example, the passivation layer 213 (consisting of silicon nitride and silicon carbide in the preferred embodiment) yields atomic hydrogen when exposed to high temperatures. It has become known that hydrogen can build up under a cavitation barrier due to the high temperature operation of the heater resistor. A tantalum cavitation barrier is relatively impervious to the passage of hydrogen and it is believed that one failure mechanism in thermal inkjet print heads is related to failure of the tantalum cavitation barrier due to hydrogen build-up. A similar layer of palladium, however, has the ability to absorb 300 times its volume of hydrogen during use such as inkjet printhead operation. (See "Atlas of Electromechanical Equilibrium in Aqueous Solutions" authored by Marcel Pourbaux, 1974, Houston Texas, published by National Association of Corrosion Engineers). Accordingly, hydrogen build-up is reduced when a cavitation barrier layer of palladium is used. Furthermore, adhesion between palladium and silicon nitride/silicon carbide can be tailored for strength more easily than tantalum because a relatively low temperature (at least 200°C) reaction occurs between palladium and the silicon compounds at the boundary and palladium silicide is formed. An equivalent reaction involving tantalum requires higher temperatures (at least 1387°C). Palladium has a higher oxidation temperature than tantalum so it is more resistant to oxidation and, for the range of pH expected in inks used in inkjet printers (5.0-9.0), palladium is stable and non-corroding (Pourbaux, "Atlas of Electromechanical Equilibrium in Aqueous Solutions"). However, palladium can be etched with aqua regia to easily realize desired interconnection patterns for the printhead. Although the specific heat of palladium is higher than that of tantalum, the thermal diffusivity (defined as α = k/ρCp , where α palladium = 0.2448 cm 2 /sec and α tantalum = 0.2462 cm 2 /sec, a 0.5% difference in thermal diffusivity between the elements) is essentially the same and the thermal conductivity of palladium (0.718 watts/cm °K) is better than tantalum (0.575 watts/cm °K). Thus, in fact, the two materials are nearly the same in applications involving heat transfer. Palladium also has advantages when used as the common cavitation barrier and electrical interconnect layer 303. Prior designs have used gold overlaying tantalum as the interconnect layer to reduce gold-aluminum interactions which produce both high resistance interconnects with positive volume expansion intermetallic byproducts. The conductivity of palladium (11x10 -6 ohm-cm) is sufficient for the material to be a good conductor for the current densities employed in thermal inkjet printer printheads. Unlike tantalum, palladium accepts direct thermal compression bonding of external leads so the two-element dual layer of gold overlaying tantalum for an interconnect layer becomes unnecessary. Furthermore, the price of palladium is approximately one-third to one-fifth that of gold in a sputter target configuration. Gold has been tried as a cavitation barrier but the reliability of such an assembly has been shown to be less than desirable. It appears that gold fails prematurely because, as a noble element, the adhesion of gold to the polymer ink barrier layer 115, 117 is insufficient. Palladium represents a combination of good mechanical resistance, good thermal performance, high temperature resistance to oxidation and corrosion, bondability, and high electrical conductivity. Although palladium has properties which are not superior to all properties of gold or to all properties of tantalum, it has certain properties not shared by both but which enable it alone to perform well the roles of both elements for the dual purpose of cavitation barrier layer and interconnect layer in a printhead environment. These advantageous properties have not previously been realized or exploited in a thermal inkjet printhead environment. Thus, the use of palladium for both cavitation barrier and interconnect layer reduces the number of steps required in the manufacture of such an inkjet printhead and provides improved performance at lower manufacturing costs.

Claims: A printhead for a thermal inkjet printer comprising: a substrate (201,203) having a resistive layer (205) disposed in an area on one surface of said substrate; a layer of conductive material (207) disposed on said resistive layer except in a predetermined area corresponding to a resistor an electrically insulating passivation layer (213) disposed at least on and adjacent said resistor area and a common cavitation barrier and electrical interconnect layer (303) composed only of palladium and having a first portion disposed on said electrically insulating passivation layer (213) at least in said resistor area and a second portion electrically separate from said first portion, disposed on said electrically insulating passivation layer (213), and electrically coupling said resistor area to an electrical connection. A method of manufacture of a printhead for a thermal inkjet printer comprising the steps of: selectively placing a layer of resistive material (205) on an oxidized surface (203) of a silicon substrate (201); placing a first layer of conductive material (207) at least on said layer of resistive material except in a predetermined area corresponding to a resistor; placing an electrically insulating passivation layer (213) on said first layer of conductive material (201) and said oxidized surface (203) of the silicon substrate (201) except in at least one predetermined area of said conductive material (207); placing a single electrically discontinuous second layer (303) of palladium on said passivation layer and on said at least one predetermined area of said conductive material, whereby an electrical connection to said resistor is made by a first portion of said second layer of palladium but without an electrical connection to a second portion of said second layer (303) of palladium disposed over said area corresponding to a resistor; and connecting an electrical lead (311) to said first portion of said electrically discontinuous second layer of palladium whereby a connection to external circuitry of the inkjet printer may be made. A method of manufacture in accordance with claim 2 wherein said step of placing an electrically discontinuous second layer (303) of palladium further comprises the steps of depositing a layer of palladium and selectively etching said layer of palladium (303) into said electrically separate first and second portions.

Inventors: BUONANNO MARK A, BUONANNO, MARK A.
Applicants: HEWLETT PACKARD CO, HEWLETT-PACKARD COMPANY
Application year: 1995
Main classification: B41J2
IPC codes: B41J2