This is a continuation-in-part of my earlier filed application Serial No. 350,886 filed on May 12, 1989. Technical Field

The present invention relates to thermal drop—on—demand, ink jet print heads (termed herein bubble jet print heads) and, more specifically to methods and structures for providing relatively impervious protective covering for the drop ejector components of such print heads. Background Art Typically, in bubble jet print heads a plurality of electrically resistive heater elements are deposited on a support substrate, that is formed e.g. of metal or ceramic material and has a heat control coating e.g. SiO-. Metal electrodes are formed to selectively apply voltage across the heater elements and a protective coating is provided over the heater elements and electrodes. Printing ink. is supplied between the heater elements and orifices of the print head and heater elements are selectively energized to a temperature that converts the adjacent ink to steam rapidly, so that a shock wave causes ejection of ink from the related orifice.

This ink jet printing approach is becoming increasingly useful; however, a major problem still exists in providing print heads wherein the heater elements are capable of a long operative life, particularly when used in high speed printing modes. Primarily, this is because protecting the drop ejectors from physical and chemical damage still presents a major technical problem.

- Thus, the inks that are utilized ^' can chemically attack the heater elements and effect short—circuits between their address and ground electrodes. More specifically, the resistor is an electrically energized device and the ink is an electrolyte. Any device that causes an electric current to flow through an electrolyte will cause electrolytic dissolution at the positive electrode and electrolytic plating at the negative electrode. Therefore the resistor will tend to be dissolved at the positive end, while having electrolytic material deposited at the negative end, unless the resistor is shielded from the electrolyte. For these reasons, and other reasons, e.g. protection against mechanical damage, a dielectric protective layer(s) are provided over the heater element (and usually over the electrodes).

U.S. Patents 4,450,457 and 4,577,202 describe the above and other problems and provide some exemplary listings of desired protective layers characteristics. For example, such protective layers desirably have a good resistance to heat and ink damage, having a good heat conductivity, an ink—penetration preventive property, an oxidation preventing property and a resistance to mechanical damage. To achieve such characteristics, the noted patents teach use of a two layer composite protective cover comprising a dielectric, e.g. Si0 ₂ or Si N, immediately over the heater element and a metal layer e.g. Ta or metal alloy, as the top layer. U.S. Patent 4,513,298 describes another composite protective layer construction using silicon nitride as the first overlying layer, but using silicon carbide as the top protective layer. The multilayer protective covering structures of prior art approaches are useful but difficulties still remain. For example, achieving good adherence

betweeh the different layers presents problems. Also, these approaches increase the number of different materials that must be deposited in the overall fabrication process, which presents additional set—up time and/or equipment costs. Moreover, there still remains difficulty in attaining desired imperviousness in the overall cover structures. That is, pinhole type voids often exist in the deposited ink barrier layer and can lead to crazing and/or electrolytic destruction of the overall structure.

U.S. Patent No. 4,535,343 describes one approach at avoiding pinholes in the protective coverings. In fabrication according to the '343 patent, oxides or oxynitrides of the resistive heater elements and electrodes are "grown" by anodizing those metal elements with an electrolyte such as water—soluble polyprotic acid. This approach is naturally limited as to protective film characteristics by the metals used for the heaters and electrodes.

Pisclosure of invention

One primary objective of the present invention is to provide a relatively simple and effective approach for fabricating protective cover structures for the drop ejector components of bubble jet printers. A related object is to provide bubble jet print heads having highly impervious protective cover structures for separating the resistive heater elements and their electrodes from the ink which is ejected by those drop ejection components.

Thus, in one aspect the present invention constitutes a method of fabricating a protective covering for a bubble jet drop ejector component of the kind comprising a plurality of resistive heating elements and associated electrodes disposed on a substrate. The method comprises the steps of depositing a thin first stratum of a metal selected

from the group consisting of zirconium, titanium and tantalum, over the resistive heating elements; oxidizing the first stratum; depositing a thin second stratum of such metal over the oxidized first stratum; and oxidizing the second stratum.

In another related aspect the present invention constitutes an improved protective cover for a bubble jet print head device of the kind having a substrate with a plurality of separately addressable resistive heater portions that are formed by address and common electrode pairs that provide electrical energy flow to and way from spaced edges of such heater portions. The protective cover construction comprises a first stratum of oxidized zirconium formed on the heater portions; a second stratum of oxidized zirconium formed on the first stratum and overlying the heater portions; and a third layer stratum of oxidized zirconium formed on the second stratum overlying the first and second strata and the heater portions.

Brief Description of Drawings

The subsequent description of preferred embodiments refers to the accompanying drawings wherein: FIG. 1 ,is a cross-sectional view of one kind of prior art print head in which the present invention can be utilized;

FIG. 2 is a perspective view partially in cross—section showing another kind of prior art print head in which the present invention can be utilized;

FIG. 3 is an exploded perspective view of the FIG. 1 print head showing the top of its drop ejection component and the terminals for coupling that component to driver circuits; FIG. 4 ⁴ <-is an enlarged cross—section of a portion of a drop ejection component showing one embodiment of the present invention;

FIG. 5 is a further enlarged schematic illustration of a portion of the FIG. 4 component;

FIGS. 6A-6E are schematic illustrations of terminal portions of a drop ejection component at various stages of fabrication in accord with one preferred fabrication procedure of the invention; and

FIG. 7 is a cross-section like FIG. 5 but showing an alternative protective cover structure. Modes of Carrying Out the Invention Referring to FIG. 1 the prior art bubble jet head 10 comprises in general, a base substrate 11 formed of thermally conductive material, such as metal or glass, on which is coated a heat control layer 12 such as Si0 ₂ and a grooved top plate 13, which defines a plurality of ink supply channels 14 leading from an ink supply reservoir 15 formed by a top end cap 16. A heat sink portion 17 can be provided on the lower surface of substrate 11 if the characteristics of that substrate warrant. Located upstream from the orifices 19, formed between the grooves of top plate 13 and substrate 11, are a plurality of selectively addressable electro-thermal transducers. These transducers each comprise a discrete resistive heater portion 21, formed e.g. of ZrB ₂ , HfB ₂ , Ta Al etc. and a discrete address electrode 22 formed e.g. of aluminum or other metal conductor. A common ground electrode 23 can be coupled to the edge of each heater element opposite its address electrode. The electrodes and heater elements can be formed on the surface of layer 12 by various metal deposition techniques.

Formed over both the electrodes and heater elements is a protective layer(s), e.g. of Si0 ₂ , intended to meet the various requirements described in the background section above. Upon application of an electrical potential to the address electrodes 22, current flows through the resistive heater element 21

to the- ground electrode 23 and heat is provided to vaporize the ink proximate the heater element and eject an ink drop through orifice 19.

FIG. 2 illustrates another prior art bubble jet print head embodiment which has components similar to the FIG. 1 embodiment that are indicated by corresponding "primed" numerals. The primary difference in the FIG. 2 prior art print head is that the top plate comprises separate components 13', 13", which cooperate to provide top ejection passages 19' and an orifice plate 19" is provided over the passages 19'. Upon application of potential to address electrodes 22', current passes through heater 21' to ground electrode 23 ^■ and ink is heated to eject a drop through the related orifice of plate 19".

FIG. 3 shows the drop ejector component 30 of the FIG. 1 print head, with the print head to plate 13 and reservoir cap 16 removed. It can be seen in FIG. 3 how component 30 has terminal pads 28, 29 respectively coupled by ground and address electrodes 23 and 24 to resistive heater elements 21. A flexible connector 31 extends from the main ink jet printer control system (not shown) and has individual connection circuits 32, 33 for engagement with terminal pads 28, 29. Thus, while the protective coating 25 (FIG. 1) desirably is over the portions of the heaters and electrodes that contact ink, it is not wanted over at least pad portions 28, 29.

FIG. 4 shows a portion of a drop ejection component such as described above, but having one protective covering embodiment according to the present invention. Thus, the drop ejection portion 40 comprises a substrate 41, a heat control layer 42, a resistive heater layer 43 and ground and address electrodes 44, 45. These parts of the ejection component 40 can be formed of the various materials known in the art for such structures. In accord with

the present invention, the protective covering 46 comprises a multi-stratum layer comprising a plurality (here three) separately deposited and treated stratum 46a, 46b and 46c. One particularly preferred construction for covering 46 comprises three separately formed stratum of zirconium dioxide (Zr0 ₂ ), each in the order of about 1.5 to 2.5 thousand Angstroms in thickness.

In general the fabrication of protective coverings, such as multi—stratum layer 46, comprises the successive deposition and oxidation of a plurality of suitable metal or semiconductor material layers over the surface of the substrate bearing the resistive heater elements and their electrodes. Referring to FIGS. 4-6, in one preferred fabrication mode, a layer 46a of zirconium is deposited, to a thickness of about 1.5 to 2.5 thousand Angstroms, by conventional sputtering techniques over the entire surface of substrate 41 which, like substrate 17 shown in FIG. 3, supports the heater portions 43, the electrode portions 44, 45 and the electrode terminal portions. After deposition of such a layer, the metal stratum is patterned (e.g. by coating and exposing a photoresist and subsequent etching) to remove the metal stratum only over the terminal pads. Thus, as shown in FIGS. 6A—6E, the photoresist coating 51 is exposed and developed to form openings 52 overlying the regions of stratum 46a that overlie terminals 48 of the electrodes 45. The stratum 46a is then etched through openings 52 to form openings 49 in the stratum 46a and expose the terminals 48. After this step of removing selected regions of the zirconium stratum 46a, the substrate bearing the patterned is placed in an oxygen atmosphere and heated e.g. at about 200 to 300°C for a period of time sufficient to completely oxidize that stratum. This converts the zirconium metal stratum to zirconium dioxide throughout its thickness.

After completion of the oxidizing step, the substrate is allowed to cool and then replaced in the sputtering chamber so that a second stratum 46b of zirconium metal can be deposited over the previously formed Zr0 ₂ stratum 46a. The second stratum is also deposited in a thickness of about 1.5 to 2.5 thousand Angstroms and then is patterned in the same manner as the first stratum to again expose terminals 48. Thereafter, the substrate is placed in the oxidizing environment and heated to convert the Zr layer 46b to Zr0 _2< After removal and cooling the sequence — Zr deposition, patterning and oxidization — is again repeated to form the third structure 46c.

One important advantage of the fabrication just described can be appreciated more fully by reference to FIG. 5. Thus, during formation of each of stratum 46a, 46b and 46c it is extremely difficult, if not impossible, to form a continuous covering without pinhole defects such as represented schematically by "D" in FIG. 5. However, FIG. 5 illustrates that when a protective cover layer is constructed as a multi—stratum components the pinhole defects D will not be aligned and the possibility of damage to the resistive heater and electrode elements of the drop ejector component is substantially negated. Another advantage of the present invention is that the multi—strata approach allows more time-efficient fabrication of relatively thicker layers for the time for the oxidation process to complete increases exponentially with stratum thickness. Thus, the present invention facilitates thicker layers where applications warrant.

The specific .example described above refers to construction of a preferred multi—strata Zr0 ₂ layer. However, other metal and semiconductor oxides can be useful. Such materials can be selected based on several important characteristics. First, the

oxide material should have a high electrical resistivity in comparison to that of the resistive

4 6 heater element, e.g. 10 to 10 greater resistivity. Second, the oxide material should be relatively chemically stable vis a vis the ink in an electrolyte environment. Although not preferred, a stable top overcoat layer (e.g. of metal) could be provided over the multi—strata layer for this purpose. Also, it is particularly desirable that the selected metal or semiconductor material have a density characteristic not greatly different from its oxide density characteristic. This minimizes the stresses created within the layers during their fabrication and thus reduces stress—caused defects. Tanatalum yielding its pentoxide (Ta ₂ 0 ₅ ) is another particularly preferred material for achieving the abovenoted characteristics in accord with the present invention. With the foregoing guidelines other materials will occur to those skilled in the art. Referring back to FIGS. 6A-6E, that illustrated embodiment of the invention provides intermediate patterning of each successive metal deposit, prior to its oxidation because patterning of Zr0 ₂ is not easily accomplished. However, in alternative modes for practicing the present invention such intermediate patterning steps can be eliminated. For example, an appropriate template can be provided during each deposition step to block deposit of material on the terminal regions of the substrate 41. In another alternative mode, the plurality of deposition-oxidation sequences can be performed to provide the FIG. 5 covering over the entire substrate. The covering can then be masked by a noble metal, except for terminal regions and the multi-stratum, and chemically etched to expose the terminals 48. Such masking metal could be used to provide additional chemical stability in certain multi—strata embodiments

as described above. A similar alternative approach is to complete the multi-stratum formation without intermediate patternings and then dry etch through a photoresist pattern to provide the terminal openings through the multi-stratum structure.

Other modifications of the specifically described fabrication method can be utilized in accord with the present invention. For example, deposition of the metal strata can be by other techniques than sputtering, e.g. chemical vapor deposition techniques. Also, other techniques, e.g. oxygen plasma exposure, can be used for oxidizing the strata.

FIG. 7 schematically illustrates another modification of the present invention. Thus, in the FIG. 7 embodiment, stratum 46b is not oxidized through its entire thickness so that a lower thickness zone 46b ¹ of metal, e.g. zirconium, remains sandwiched between the top of stratum 46a and the top, oxidized portion of stratum 46b. This modified embodiment is useful to provide an electrostatic shield layer which can function in a manner generally as described in concurrently filed U.S. Application Serial No. 350,867 entitled "Bubble Jet Print Head Having Improved Multi—Layer Protective Structure For Heater Elements", to deter electrolytic dissolution at the ink/protective cover interface. The teachings of that application are incorporated herein by reference for that teaching. Another particularly preferred material for forming multilayer protective structures in accord with the present invention is titanium, deposited in successive thin layers and oxidized to yield titanium oxide strata that are generally clear and have a refractive index greater than about 1.6. A preferred mode for forming such structures comprises sputtering approximately 99.9% pure Ti to a thickness of about

1.5 to 2.5 thousand Angstroms to form a layer similar to the layer 46a in FIG. 5. The deposited layer is then patterned by photofabrication, such as previously described, to remove the titanium over the connective pad portions. The element bearing the photofabricated layer is then heated in an oxygen atmosphere at about 300 to 400°C for a period of time sufficient to completely oxidize the first stratum.

After the completion of the first oxidizing step, the substrate is allowed to cool and then replaced in the sputtering chamber so that a second layer, similar to stratum 46b (FIG. 5) but of titanium metal, can be deposited over the previously formed titanium oxide stratum 46a. The second stratum is also deposited in a thickness of about 1.5 to 2.5 thousand Angstroms and then is patterned in the same manner as the first stratum to again expose the terminals. Thereafter, the substrate is placed in the oxidizing environment and heated as described to form another titanium oxide layer such as 46b. After removal and cooling the sequence — Ti deposition, patterning and oxidization — is again repeated to form the third titanium oxide structure similar to layer 46c of FIG. 5. Another preferred embodiment of the present invention is formed of multilayers of separately deposited and oxidized titanium, as described above, but includes an intermediate deposition of a thin layer of tantalum between the step of oxidizing the first titanium layer and the step of depositing the second titanium layer. The sandwiched tantalum layer provides electric field shielding as described in above-mentioned U.S. Application Serial No. 360,867, and also provides mechanical strength for the protective layer composite.

' Although embodiments of the invention which use oxides of preferred materials as the top protective surface afford the advantages of minimal different-material—deposition set ups, some of such oxides can be chemically reactive to certain inks. In such instances, significant advantages of the present invention can be retained by depositing a thin top covering of a non—reactive metal. For example, tantalum or silicon carbide top covering layers will obviate such problems. Industrial Applicability

The present invention affords industrial advantage by providing simplified techniques for fabricating ink jet print heads and in providing elements so fabricated that are substantially free of pinhole defects in heater protecting structures.