INK JET PRINTER NOZZLE WITH COATING FOR ENHANCED WEAR PROPERTIES

Abstract: The present invention relates to an ink jet nozzle for use in an ink jet printer of the type which comprises a body (12) of silicon or glass having an opening (16, 18) forming the nozzle, and a coating for the opening. A nozzle according to the invention is characterised in that the coating comprises a first layer (20) of silicon nitride on the silicon or glass body (12), an intermediate layer (22) formed from a mixture of silicon nitride and aluminium nitride, and a third layer (24) formed from aluminium nitride.

Description: This invention relates to silicon or glass nozzles for use in an ink jet printer of the type having a coating thereon for enhanced wear properties and other properties. Ink jet printing is well known as a technique for providing nonimpact printing of a high quality in applications such as computer output terminals. In ink jet printing, a sequence of individual ink droplets strikes the record medium (paper) to produce a desired pattern thereon. In some printers, single ink nozzles are utilised which pass the record medium at high speed while continually emitting a stream of ink that breaks up into ink droplets. Single nozzle arrangements of this type have been proposed for "drop-on-demand" systems wherein ink droplets are generated only when the nozzle approaches a specific portion of the record medium. Other ink jet systems, termed continuous systems, have been proposed wherein a single nozzle continuously generates ink droplets that are either directed to a specific portion of the record medium, or guttered to a recirculating system for reuse. In addition to the single ink droplet nozzle, the technology has utilised an array or plurality of ink droplet nozzles spaced across a record medium for generating a number of streams of ink recording droplets. For example, stationary nozzles can be arranged so that each of the nozzles directs a droplet stream toward the record medium along an initial path parallel to the plurality of ink jet trajectories. Subsequent to the production of the droplets, droplet charging and deflecting mechanisms downstream from the droplet generator interact with the droplets, changing their trajectories and thereby directing the droplets to desired portions of the record medium. In these systems, the ink droplets must be properly charged so that they can be deflected in a proper manner to the desired locations. A typical ink jet system includes a device for exciting or perturbing the ink as it is squirted from the nozzle, in order to induce droplet production which is in appropriate relation to a charging electrode. Many different nozzles and nozzle arrays have been proposed for use, including many different materials. Common materials include semiconductors such as silicon, glass, and metals. In particular, silicon nozzle arrays appear to be the most advantageous, since semiconductor processing techniques can be used to uniformly etch a series of nozzle openings in a single crystal silicon wafer. This type of technique, and others, is described in US-A-4,047,184. US-A-4,007,464, describes preferred shapes for the entrance and exit orifices of the nozzle. Fabrication of the nozzle, and its properties, is extremely critical for the provision of high quality, high speed printing using ink jets. In particular, it is known in this art that the nozzle should have very low wettability for inks in order that the inks should not dry and become coated in or around the nozzle openings. It is also known that the nozzles should have good wear-resistance and be chemically inert, so as to avoid corrosion by aqueous and non-aqueous inks. Still further, the nozzles should have good hardness and mechanical strength, so as to be durable. In order to provide mechanical strength, wear resistance, and low wettability, various materials have been proposed as coatings on nozzles. These coatings include SiO₂, polymers such as Teflon (a trademark of E.I. DuPont deNemours), and Si₃N₄. These coatings have been described in the aforementioned U.S. patents. Additionally, US-A-4,389,654 describes a metal nozzle having a coating of gold or nickel thereon to protect against corrosive attack by the ink. In all of the above-cited examples, the coatings, although advantageous, do not provide all of the features which must be provided by a superior coating. For example, oxides such as SiO₂ have high wettability and change their surface porosity during use. Polymers such as Teflon exhibit very low wettability for inks, but also exhibit poor wear resistance. Additionally, some polymers can pick up moisture and otherwise degrade in their chemical and mechanical properties. Silicon nitride (Si₃N₄) has primarily been used as a dielectric but does not exhibit as high a contact angle for aqueous and nonaqueous inks as is desired. This relatively low contact angle means that silicon nitride films can be wetted by aqueous and nonaqueous inks. Further, as newer inks are developed having acidic or basic properties, and using various additives such as colourant dyes and surface activation additives, the requirements for the nozzle coatings are likely to increase and the heretofore mentioned coatings may exhibit additional disadvantages and limitations. The object of the present invention is to provide an improved ink jet nozzle for use in an ink jet printer. The present invention relates to an ink jet nozzle, for use in an ink jet printer, of the type which comprises a body of silicon or glass having an opening forming the nozzle, and a coating for the opening. A nozzle according to the invention is characterised in that the coating comprises a first layer of silicon nitride on the silicon or glass body, an intermediate layer formed from a mixture of silicon nitride and aluminium nitride, and a third layer formed from aluminium nitride. According to a preferred embodiment of the invention the intermediate layer is a graded layer of mixed nitrides. This intermediate layer is silicon-rich at the interface of the intermediate layer and the first silicon nitride layer, and aluminium rich at the interface between the intermediate layer and the third overlying aluminium nitride layer. Thus, the intermediate layer is primarily silicon nitride in its initially deposited portion and aluminium nitride in its portion adjacent to its top surface. The first silicon nitride layer provides excellent adhesion to the glass or silicon of the nozzle body and good electrical properties in terms of its dielectric constant. The intermediate layer provides excellent adhesion between the third aluminium nitride layer and the silicon or glass nozzle body, and, in its graded composition form, eliminates any thermal expansion differences between the AlN layer and the silicon or glass body of the nozzle. The AlN layer has excellent hardness and exhibits a low degree of wettability to inks. It is superior to Si₃N₄ in this regard, and does not change its wettability characteristics after extended ink-soaking tests. Also, it is very chemically inert and has good stability and resistance to corrosion when exposed to all types of inks. It is mechanically hard and of good strength, and can be easily provided as a uniformly thick layer by a plurality of well known techniques. It has been found that the use of the multilayer coating on the nozzle opening ensures that good adhesion and all of the mechanical, chemical, and electrical properties required for successful nozzle operation are present. These properties cannot be achieved by the use of any known single layer. In order that the invention may be more readily understood an embodiment will now be described with reference to the accompany drawings, in which: FIG. 1 is a side view partially in section of an ink jet printer nozzle including an opening having a three-layer coating in accordance with the invention thereon. FIG. 2 schematically illustrates the composition of the three layers of the coating, and particularly illustrates the graded composition of the intermediate layer. In FIG. 1, a single ink jet nozzle 10 is illustrated partly in section in the region of the opening in the nozzle, although it will be understood by those of skill in the art that this nozzle can be one of many in an array of such nozzles. The body 12 of the nozzle 10 is formed from either silicon or glass and has a three-layer coating 14 thereon in the region of the nozzle opening. Openings 16 and 18 can be used for the entrance and exit of the opening of the nozzle 10, the direction of ink flow through these openings being reversible as is known in the art. Coating 14 comprises a first layer 20 formed on the body 12 of the nozzle, an intermediate layer 22, and a top or third layer 24. Layer 20 comprises silicon nitride Si₃N₄, which is typically an amorphous nitride formed on the glass or silicon body 12. In the fabrication of the nozzle, it may be that a small layer of silicon oxide is present on the surface of the body 12 prior to the formation thereon of silicon nitride layer 20. The presence of this oxide does not interfere with the adhesion of or affect the coating 14. Intermediate layer 22 is a mixed nitride of silicon and aluminium, while the third layer 24 is aluminium nitride AlN. In practice, the thicknesses of the three layers 20, 22, and 24 are not critical, the thicknesses being chosen with respect to design requirements such as dielectric strength, the provision of continuous, pinhole-free films, and thermal matching so as to avoid stresses. As a representative embodiment, when the layers 20, 22, and 24 are formed by chemical vapour deposition (CVD), the thicknesses of layers 20, 22, and 24 are about 1000 angstroms (10×10⁻⁸m), 1000-5000 angstroms (10-50×10⁻⁸m), and 1000 angstroms (10×10⁻⁸m), respectively. When using CVD, the preferred thickness of the intermediate layer 22 is about 2000 angstroms (20×10⁻⁸m). As noted previously, intermediate layer 22 provides a good thermal match between the underlying silicon or glass of the nozzle body 12 and the overlying AlN layer (24). Layer 22 comprises a mix of silicon nitride and aluminium nitride, with this layer being silicon-rich at interface 26, and aluminium-rich at interface 28. Thus, layer 22 blends the silicon nitride composition 20 to the aluminium nitride composition 24 by decreasing the amount of silicon as the thickness of layer 22 increases during deposition, while at the same time increasing the amount of aluminium. Fig. 2 is a diagrammatic cross-section of the three layer coating 14, illustrating the change in composition of the intermediate layer 22 with distance from the silicon nitride layer 20 towards the AlN layer 24. At interface 26 between layers 20 and 22, there is 100% Si, no Al. However, the amount of Si decreases to 0% at interface 28 between layers 22 and 24 while, in the same distance, the percentage of Al increases from 0% to 100% at interface 28. Thereafter, layer 24 is substantially pure AlN. Layer 22, having mixed composition, can be formed from solid solutions of silicon nitride and aluminium nitride, produced by a variety of known techniques. These techniques include sputtering, CVD, and evaporation. Multiple sources can be used to form the intermediate layer 22, while single sources are used to form the silicon nitride layer 20 and the aluminium nitride layer 24. As an example, CVD can be used to form the three layers in coating 14. To form the Si₃N₄ layer 20, a mixture of silane SiH₄ and ammonia (NH₃), together with an inert carrier gas, is passed over a hot substrate formed by the silicon or body layer 12 of the nozzle. The reaction parameters for the deposition of silicon nitride by this technique are well known in the art and will not be described in detail herein. After the required thickness of silicon nitride has been formed, the gas stream can be changed to introduce another gas. For example, the amount of silane can be reduced to reduce the amount of silicon in intermediate layer 22, while a source gas for Al can be introduced and increased in amount to provide an increasing amount of Al in layer 22. A suitable gas for this is trimethyl aluminium Al(CH₄)₃ aluminium and bromine or aluminium and chlorine such as Al Br₃ or AlCl₃, together with NH₃. Typical substrate temperatures are 700-800°C. After this, the silane gas source is completely shut off, and only AlN is deposited to form layer 24. The thickness of layer 22 is such as to provide compositional grading between layers 20 and 24 in a manner to provide minimal thermally induced stress. Compositional mixing of silicon and aluminium achieves this. This type of mixing can be achieved rapidly by a technique such as dual beam sputtering, wherein sources of silicon and aluminium are separately sputtered onto the substrate including the body 12 and the first layer 20. If dual beam sputtering is used, a layer 22 of 500 angstroms (5×10⁻⁸m) thickness or less, is suitable to provide the necessary matching and compositional grading. A reference generally describing dual ion beam techniques is J.M.E. Harper et al, Appl. Phys. Lett. 43 , (6) p.547, Sept. 15, 1983. In particular, this reference describes the formation of AlN by dual ion beam deposition. As an example, Si₃N₄ can be deposited on either silicon or quartz glass nozzle bodies using a target of silicon and an atmosphere comprising argon and nitrogen. The argon can be present in a pressure of 24 mTorr while the nitrogen is present at a pressure of 6 mTorr. The cathode can be held at approximately 400-500 volts (1000 volts peak) negative bias, while the anode can be held at about -50 volts dc. The target to substrate distance is about 3.25 inches (7.26×10⁻²m). Silicon nitride will be deposited at a rate of about 300 angstroms (3×10⁻⁸m)/minute onto the silicon or glass substrate, which is water-cooled to room temperature. The intermediate layer 22 can be formed in the same deposition apparatus by introducing and sputtering from an Al target. The same type of gaseous environment can be utilised with approximately the same gas pressures. However, the cathode voltage and/or the anode voltage will be changed to increase the sputtering from the aluminium target while decreasing the sputtering from the silicon target. This will produce the graded composition of silicon and aluminium in intermediate layer 22. A pure AlN layer 24 can also be formed in this dual beam sputtering apparatus. This is accomplished by the presence of an argon and nitrogen atmosphere in which the argon pressure and nitrogen pressure are approximately 20 mTorr. A dc cathode voltage of about 130 volts (350 volts peak) is applied, while the anode voltage is maintained at -50 volts dc. A deposition rate of approximately 25 angstroms (25×10⁻¹⁰m)/minute onto the intermediate layer 22 is provided, where the substrate temperature is about 200°C. Soak tests were performed to evaluate the corrosion resistance of the AlN layer 24. In these tests, various inks were placed in contact with layer 24 and the contact angle of the liquid to the layer 24 was measured. These measurements were taken at selected intervals to see whether the liquid drops would spread. In one type of test, a drop of ink was placed on the AlN test surface and the contact angle was measured. The ink was then allowed to dry, after which it was washed from the surface under a stream of ionised water. It was then checked to see if water tended to wet the surface preferentially in those areas where the ink drop had been soaking. In another test, the AlN surface was immersed in inks for many hours, either at room temperature or at an elevated temperature (65°C). After the soak period, the surface was thoroughly cleaned, dried and vacuumed at about 65°C, and the contact angle measured. This was repeated after each soak treatment, the surfaces being examined under a scanning electron microscope to check for possible corrosion. In these tests, the AlN layer did not exhibit any change in the measured contact angles before and after the soak treatment, indicating the excellent chemical stability of these surfaces toward various inks. Further, the scanning electron microscope photographs did not indicate any significant differences in the surfaces before and after testing, indicating that the films were extremely corrosion-resistant. Adhesion of these layers to the underlying silicon or glass body of the nozzle is excellent. In contrast with organic materials which have a large difference in thermal expansion coefficients compared to silicon, the nitrides used in forming the three layer coating have more comparable coefficients of thermal expansion. This, coupled with the graded composition layer 22, provides excellent adhesion and low stress in the deposited layers. Also, the silicon nitride layer 20 can be used as a dielectric layer in a nozzle array of the type known in the art, wherein such a dielectric is used between current carrying conductors. A three layer coating for a silicon or glass ink jet nozzle has been described which has superior mechanical and chemical properties regardless of the types of inks which are used. Further, the stability and endurance of this coating exceeds that of presently known coatings. The exact composition of the intermediate layer 22 and the thicknesses of the various layers can be varied depending upon the design requirements of the nozzle. Further, it will be understood that impurities can be introduced in the different layers in order to modify their properties somewhat.

Claims: 1. An ink jet nozzle for use in an ink jet printer, comprising a body (12) of silicon or glass having an opening (16, 18) forming said nozzle, and a coating for said opening, characterised in that said coating comprises a first layer (20) of silicon nitride on said silicon or glass body (12), an intermediate layer (22) formed from a mixture of silicon nitride and aluminium nitride, and a third layer (24) formed from aluminium nitride. 2. An ink jet nozzle as claimed in claim 1 characterised in that said intermediate layer (22) is a graded composition, being silicon-rich near its interface with said first layer (20) and aluminium-rich near its interface with said third layer (24). 3. An ink jet nozzle as claimed in claim 2 characterised in that the percentage of silicon in said intermediate layer (22) decreases in a direction normal to the interface of said intermediate layer (22) and said first layer (20), measured from said interface, and the percentage of aluminium in said intermediate layer (22) increases in a direction normal to the interface of said second layer (22) and said first layer (20), measured from said interface. 4. An ink jet nozzle as claimed in claim 3 characterised in that the compositional grading in said intermediate layer (22) provides a good thermal match between said third layer (24) and said silicon or glass body (12). 5. An ink jet nozzle as claimed in any one of the preceding claims characterised in that said first layer (20) and said third layer (24) are substantially continuous layers. 6. An ink jet nozzle as claimed in any one of the preceding claims characterised in that said first and third layers (20, 24) are formed from amorphous material. 7. An ink jet nozzle as claimed in any one of the preceding claims characterised in that said first layer (20) is about 1000 angstroms (10×10⁻⁸ m) thick and said third layer (24) is about 1000 angstroms (10×10⁻⁸ m) thick. 8. An ink jet nozzle as claimed in claim 7 characterised in that said second layer (22) has a thickness in the range of about 500-5000 angstroms (5-50×10⁻⁸ m).

Inventors: CHANDRASHEKHAR, G. V., SHAFER, MERRILL WILBERT
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1986
Main classification: B41J2
IPC codes: C03C17, B41J2