CA2055849C - Thin-film transducer ink jet head - Google Patents

Abstract

In the particular embodiments described in the specification, a thin-film transducer ink jet head is prepared by oxidizing one surface of a silicon wafer (10) to provide a dielectric layer (11), forming elec-trodes (17) on the layer by photoresist processing techniques, depositing one or more layers of PZT mate-rial to provide a thin-film piezoelectric layer (18) having a thickness in the range of 1-25 microns, form-ing another pattern of electrodes (24) on the surface of the PZT layer by photoresist techniques, and selec-tively etching the silicon substrate in the region of the electrodes to provide an ink chamber (30). There-after, an orifice plate is affixed to the substrate to enclose the ink chambers and provide an ink orifice for each of the chambers. An ink jet head having chambers 3.34mm long by 0.17mm wide by 0.15mm deep and orifices spaced by 0.305mm is provided. (Fig. 1f)

Description

DescriDtion Thin-Film Transducer Ink Jet Head ~echnical Field This invention relates to ink jet heads having piezoelectric transducers for use in ink jet systems and, more particularly, to a new and improved ink jet head having a thin-film piezoelectric transducer.

Background Art In certain ink jet systems, the ink jet head 0 contains ink chambers in which one wall or wall por-tion is provided by a plate-like piezoelectric element which moves laterally so as to expand or contract the volume of the chamber in response to electrical sig-nals. Heretofore, such plate-like piezoelectric transducers have consisted of a continuous sheet of piezoelectric material forming the transducers for a series of adjacent ink jet chambers, as described, for example, in the Fischbeck et al. Patent No. 4,584,590, or of individual plate-like piezoelectric elements disposed adjacent to each ink jet chamber, as dis-closed, for example, in the Cruz-Uribe et al. Patent No. 4,680,595. Moreover, as described in the Cruz-Uribe et al. patent, the individual transducers may, for example, be formed by etching to remove material from a single continuous sheet of piezoelectric mate-rial, leaving separate discrete transducers. Such conventional sheet-form piezoelectric materials are made, for example, by shaping green material into sheet form and firing, and they have a minimum thick-3~ ness of about 3-5 mils (75-125 microns).
Because the extent of bending of a piezoe'ectric sheet material for a given applied voltage application is inversely proportional to the thickness of the sheet, the use of transducers having a minimum thick-*

ness of about 5 mils (125 microns) requires an inkchamber with a relatively large piezoelectric wall area in order to eject an ink drop of specific size, such as 80 picoliters. As a result of the large cham-er wall area requirement, correspondingly large cham-Der size and orifice spacing, as well as ink jet head slze, are requlred.
Sheet piezoelectric materials have further innate disadvantages in manufacturability. The materials ~0 tend to be fragile, which makes processing expensive.
In addition, the sheet material must be bonded to at ,east one other part, which is generally a demanding process.

3isclosure of Invention Accordingly, it is an object of the present in-vention to provide a new and improved ink jet head which overcomes the above-mentioned disadvantages of the prior art.
Another object of the invention is to provide an nk iet head having a piezoelectric transducer which is capable of larger deflection for a given voltage than prior art transducers.
A further object of the invention is to provide an ink jet head having a plurality of ink jet chambers in a closely-spaced array and corresponding orifices with smaller spacing than conventional ink jet heads.
Still another object of the invention is to pro-vide an ink jet head having a piezoelectric transducer of reduced thickness so as to provide increased bend-ing for a given voltage application.
Yet another object of the invention is to providean ink jet head having a chamber-forming semiconductor transducer substrate which enables integration of electronic components for operation of the ink ,et :~ead.
An additional object of the invention is to pro-~ide a new and improved method for making an ink jet :~ead in a simple and convenient manner to provide improved characteristics.
These and other objects of the invention are attained by forming one or more electrodes on a sub-strate, forming a thin film of piezoelectric material on the electrode, and forming one or more electrodes on the opposite surface of the thin film of piezoelec-tric material. Preferably, the substrate is an etch-able material and a portion of the substrate is re-moved by etching to produce an ink jet chamber forwhich the electroded piezoelectric thin-film material forms one wall portion. In a preferred embodiment, an array of adjacent ink jet chambers is formed in a semiconductor substrate containing integrated circuit components and the thin film of piezoelectric material provides the transducers for all of the ink jet cham-bers, an orifice plate being affixed to the opposite side of the substrate to provide an orifice for each ink jet chamber.
Preferably, the etchable substrate is a silicon substrate of the type used in preparing integrated circuit chips, and the circuitry and components used to actuate the piezoelectric elements, such as drive pulse switches and memory elements, are formed on the surface of the substrate in accordance with the usual semiconductor integrated circuit processing tech-niques. Similarly, the electrodes for both sides of the thin-film piezoelectric layer are preferably ap-plied in accordance with semiconductor integrated circuit technology using, for example, a photoresist material to define the electrode patterns for opposite surfaces of the transducer prior to and after deposi-tion of the thin-film piezoelectric material.
In order to provide a thin-film layer of piezo-electric material having sufficient strength to e,ectink in response to application of the desired poten-tial while avoiding cracking of the film during prepa-ration or subsequent thereto, the film is preferably formed by depositing one or more layers of piezoelec-tric material using conventional thin-film techniques, such as sol-gel, sputtering or vapor deposition. In order to create a desirable small, uniform grain structure in the piezoelectric layer, the film is preferably fired and annealed with a rapid thermal annealing technique.
Further objects and advantages of the invention will be apparent from a readinq of the following de-scription in conjunction with the accompanying draw-ings in which:

3rief DescriPtion of Drawinqs Figs. l(a)-l(f) are schematic cross-sectional illustrations showing the successive stages in a typi-cal process for preparing a thin-film piezoelectric transducer and ink jet chamber in accordance with one embodiment or the present invention;
Fig. 2 is a schematic diagram showing a represen-tative circuit arrangement for controlling the opera-tion of an ink jet head and containing electrodesformed on one surface of a semiconductor substrate for a thin-film piezoelectric transducer; and Fig. 3 is an enlarged cross-sectional view show-ing an ink jet chamber with a thin-film piezoelectric transducer in accordance with another embodiment of the invention.

Bc,t Mode for Carryina Out the Invention A typical process for preparing an ink jet lead having ink chambers with a thin-film piezoelectric transducer in accordance with the invention is illus-trated in Figs. l(a)-l(f). In Fig. l(a), an etchable semiconductor substrate 10, such as an N-type silicon substrate wafer with a [1,1,0] crystal orientation having a thickness of about 6 mils t150 microns) is first oxidized in steam at 1000C in the usual manner to form a 2500A-thick silicon oxide layer 11 which 2055~349 will act as a dielectric and an etch barrier. Eor use as an ink chamber plate in a hot melt ink jet head, silicon provides desirable mechanical, electrical and thermal properties and is a highly suitable substrate for thin-film deposition and photoresist processes.
It also permits the incorporation of suitable system control components on the same substrate by integrated circuit techniques as described hereinafter. To en-able etching of the substrate a [1,1,0] crystal orien-:0 tation is desirable.
Thereafter, a layer 12 of conductive materialabout 0.2 micron thick is applied to the silicon oxide layer. The conductive layer 12 may be a sputtered or a vacuum-evaporated aluminum, nickel, chromium or platinum layer or an indium tin oxide (ITO) layer deposited by a conventional sol gel process.
As shown in Fig. l(b), a conventional photoresist layer 13, spin-coated on the conductive layer 12, is exposed by ultraviolet rays 14 through a mask 15 and developed to harden the resist layer 12 in selected regions 16 in accordance with a conductor pattern which is to be provided on one side of the piezoelec-tric layer. The unhardened photoresist is removed, the exposed metal layer 12 is etched in the usual manner, and the photoresist is stripped of, leaving a conductive electrode pattern 17 on the layer 11, as shown in Fig. l(c).
A thin film 18 of lead zirconium titanate (PZT) piezoelectric material is applied to the electroded substrate 10 by the sol gel process described, for example, in the publication entitled "Preparation of Pb(ZrTi)03 Thin Films by Sol Gel Processing: Electri-cal, Optical, and Electro-Optic Properties" by Yi, Wu and Sayer in the Journal of Applied Physics, Vol. 64, No. 5, 1 September 1988, pp. 2717-2724. While the PZT
film strength increases with increasing thickness, the magnitude of the PZT bending in response to a given applied voltage decreases with increasing thickness, as described above. Accordingly, the film thickness should be the minimum necessary to withstand the stresses applied to the film during ink ,et operation.
For ink jet systems having orifice and ink chamber 5 sizes in the general range described herein, and using inks having operating viscosities in the range of about 1-40cps, the ?ZT .^ilm should have a thickness in the range of about 1-25 microns, preferably about 2-10 microns, and, desirably, about 3-5 microns. If the 19 f lm thickness is greater than a few microns, the film ,s preferably prepared by depositing it in several layers, each from 0.1 to 5 microns thick depending on the sol-gel solution used, to avoid cracking of the ~ilm and to assure a small perovskite grain size.
The coated substrate is then fired at about 600C
to create a solution of the PZT components, cooled, and finally annealed. Preferably, rapid thermal an-nealing is used to reduce the cycle time and to assure a small, uniform grain structure necessary for good mechanical performance. This may be accomplished by heating the coated substrate at a rate of about 100C
per second to approximately 600C and maintaining it at that temperature for about 10 seconds, after which the coated substrate is cooled to room temperature in about 30 seconds by inert gas circulation. This pro-vides a uniform, small PZT grain size of about 0.3 microns.
The PZT film 18 is then coated with another layer 19 of conductive material, such as aluminum, nickel, chromium, platinum or ITO, and, as illustrated in Fig.
l(d), a photoresist layer 20 is coated on the conduc--ive layer and then exposed to ultraviolet rays 21 through a mask 22 and developed to produce hardened !egiOnS 23. Thereafter, the unhardened photoresist is removed and the exposed portion of the conductive layer 19 is etched to provide a pattern of electrodes on the upper side of ~he PZT film 18 corresponding to ~he hardened regions 23. ?he resulting upper elec-trode pattern 24 is shown in Fig. lte). Followingformation of 'he electrode pattern 24, a protective layer 25 of polyimide material is spin-coated on the top surface of the PZT layer to protect that layer and the electrode pattern.
In certain transducer arrangements with inter-digitated electrodes, as described in the copending ~oisington et al. Application Serial No. 07/615,898, filed November 20, 1990, electrodes are required on ;0 only one surface of the piezoelectric film. In such cases, the ste? of forming electrode patterns on one side of the film may be eliminated.
In order to produce the ink chambers which are to be acted upon by the PZT layer, the opposite side of i5 the silicon substrate 10 is coated with a photoresist layer 26 and exposed to ultraviolet light rays 27 through a mask 28 and developed to provide a pattern of hardened photoresist regions 29. The unhardened photoresist is then removed and the exposed silicon is etched down to the silicon oxide layer 11 to produce a pattern of ink chamber cavities 30, as shown in Fig.
l(f).
After the ink chambers 30 have been formed, the pol~imide coating 25 on the top surface is removed by etching at locations where electrical contacts are to be made to the top electrodes, and both the polyimide layer and the PZT film are etched away in locations ~here contacts to the bottom electrodes are desired.
Gold is then sputtered through a mask onto these loca-tions so that wire bonds or pressure contacts may beused for electrical connections and an orifice plate is bonded to the lower surface of the substrate 10 to close the ink chambers and provide an orifice for each cAamber in the usual manner. By appropriate energiza-3~ tion of the electrode patterns 17 and 24, the thin-film piezoelectric transducer layer 18 may be selec-lively deformed in each chamber 30 in the usual manner so as to eject ink from the chamber through the corre-sponding orifice.
Fig. 2 lllustra~es schematically a representative conductor pattern applied to the upper surface of a coated substrate to energize the electrode patterns 24 opposite each of the nk chambers 30. In the top plan view shown in ~ig. 2, the elongated shape of each of the ink chambers 30 in the underlying substrate is illustrated in dotted outline as are the orifices 31, :o which are centrally positioned with respect to each in~ chamber, and two ink supply apertures 32, one at each end of each ink chamber, which are connected to an ink supply ~not shown).
In the schematic representation of a typical ;5 embodiment shown in Fig. 2, selected electrodes in each of the patterns 24 are connected rhrough corre-sponding conductors 33, 34, 35 and 36 to appropriate contact regions 37 aligned adjacent to the edges of the substrate 10 and exposed to permit bonding of wires or engagement by pressure contacts. A corre-sponding conductor pattern is provided beneath the PZT
layer to supply potential to the underlying electrode patterns 17 (which are not illustrated in Fig. 2) from appropriate contact regions 37.
If the substrate 10 is a silicon wafer of the type used in semiconductor processing, various ink jet system control compor~nts may be provided on t~e same substrate using conventional semiconductor integrated circuit processing technology. Such components may include a transducer drive unit 38 containing conven-tional switches and other electronic components re-quired to supply the appropriate electrical pulses to actuate the transducer elements, a nonvolatile memory unit 39 containing semiconductor storage elements to store information relating, for example, to calibra-tion o~ the ink jet head to provide appropriate firing times and pulse amplitudes for the ink Jet system in which it is used, a temperature-sensing and control ~nit 40 and a related thin-film heating element 41 to detect and maintain the correct temperature for proper operation of the ink jet head, and a drop counter ~2 to count drops of each type of ink ejected by the ink jet head and provide a warning or shut-off signal when an ink supply is nearly depleted.
In a typical ink jet system utilizing thin-film piezoelectric transducers of the type described herein, a single silicon substrate may be formed with :0 a series of adjacent ink chambers approximately 3.34mm long, 0.17mm wide and 0.15mm deep and spaced by about 0.13mm so as 'o provide a spacing between adjacent orifices of about 0.3mm. With this arrangement, a 300-line per inch (11.8-line per mm) image can be obtained by orienting the angle of the aligned ori-fices at 33.7 to the scan direction. Moreover, a silicon substrate containing 48 ink jets with associ-ated drivers, memory and temperature-control circuitry can be provided on a single chip measuring about lOmm by 15mm.
In an alternative structure illustrated in the enlarged view of Fig. 3, a silicon substrate 10 having an orifice plate 43 affixed to the lower surface to provide an orifice 31 for each chamber 30 is coa~ed on the upper surface with a thin metal barrier layer 44 of platinum, nickel or the like about 0.2 microns thick and a dielectric layer 45 of aluminum oxide, also about 0.2 microns thick, is applied over the metal barrier layer. Thereafter, the electrode pat-terns and the PZT fiim 18 are applied in the manner described above with respect to Fig. 1. With this arrangement, the PZT film is effectively protected from attack by constituents of the ink contained in the chamber 30.
Moreover, the thin-film piezoelectric transducer described herein need not be combined with a silicon substrate which is etched to form the ink chambers.
.nstead, if desired, after the thin-film transducer and associated electrodes have been prepared in the manner described herein, the upper surface of the assembly may be affixed to another substrate having the desired ink chamber pattern and the silicon sub-strate may be etched away. With this arrangement, the thin-film PZT may be further protected by an optional intervening membrane or other flexible support member interposed between the PZT film and the new substrate containing the ink chambers. In addition, if the 0 silicon substrate is removed entirely, two thin-film PZT transducer layers may be mounted on opposite sides of a membrane, which is then mounted on another sub-strate containing the desired ink ~et chamber pattern, thereby increasing the ejection pressure available for i5 a given applied voltage. As another alternative, multiple layers of thin-film PZT transducer and asso-ciated electrode patterns may be applied in succession to the same substrate to produce increased displace-ment of the transducer for a given applied voltage.
Although the invention has been described herein with reference to specific embodiments, many modifica-tions and variations therein will readily occur to those skilled in the art. Accordingly, all such vari-ations and modifications are included within the in-2~ tended scope of the invention.

Claims (33)

1. A method for making an ink jet transducer comprising providing a substrate, depositing a piezoelectric film on the substrate, and firing the piezoelectric film to form a layer having a thickness between about 1 and about 25 microns, and forming at least one electrode pattern adja-cent to a surface of the piezoelectric film to provide a transducer element.

2. A method according to Claim 1 including separat-ing the transducer element from the substrate and applying the transducer element to a membrane.

3. A method according to Claim 1 including applying the transducer element to a second substrate and removing at least a part of the substrate on which the transducer element was formed.

4. A method according to Claim 1 including the step of removing a portion of the substrate to provide a chamber adjacent to a region of the transducer element containing at least one electrode.

5. A method according to Claim 4 including the step of affixing an orifice plate to the side of the substrate opposite the transducer element to enclose the chamber and provide an orifice com-municating with the chamber.

6. A method according to Claim 1 wherein the piezo-electric film is formed by depositing at least two successive layers of piezoelectric material on the substrate.

7. A method according to Claim 6 wherein each of the successive layers deposited to form the piezo-electric film has a thickness from about 0.1 to about 5 microns.

8. A method according to Claim 1 including annealing the piezoelectric film after deposition on the substrate.

9. A method according to Claim 1 wherein the sub-strate is suitable for solid state circuitry fabrication.

10. A method according to Claim 9 including forming a transducer drive circuit for the ink jet head on the substrate.

11. A method according to Claim 9 including forming a memory circuit for the ink jet head on the sub-strate.

12. A method according to Claim 9 including forming a temperature control element for the ink jet head on the substrate.

13. A method according to Claim 9 including forming a thin-film heater for the ink jet head on the substrate.

14. A method according to Claim 9 including forming a drop ejection pulse control element for the ink jet head on the substrate.

15. A method according to Claim 9 including forming a drop counter circuit for ink supply detection on the substrate.

16. A method according to Claim 9 wherein the sub-strate is silicon.

17. A method according to Claim 1 wherein the thick-ness of the piezoelectric film is in the range from about 2 to about 10 microns.

18. A method according to Claim 1 wherein the thick-ness of the piezoelectric film is in the range from about 3 to about 5 microns.

19. A method according to Claim 1 including the step of forming at least one electrode adjacent to the other surface of the piezoelectric film.

20. An ink jet head for use in an ink jet system com-prising a substrate having a plurality of open-ings providing ink chambers therein, an orifice plate on one side of the substrate containing a plurality of orifices for corresponding ink cham-bers in the substrate, and a thin-film piezoelec-tric transducer element on the opposite side of the substrate including a piezoelectric film having a thickness in the range from about 1 micron to about 25 microns and having an elec-troded portion disposed adjacent to each of the chambers for selective actuation of the corre-sponding portion of the transducer element to vary the volume of the adjacent chamber.

21. An ink jet head according to Claim 20 wherein the thickness of the piezoelectric film is between about 2 microns and about 10 microns.

22. An ink jet head according to Claim 20 wherein the thickness of the piezoelectric film is between about 3 microns and about 5 microns.

23. An ink jet head according to Claim 20 wherein the substrate is suitable for solid state circuitry fabrication.

24. An ink jet head according to Claim 23 including a transducer drive circuit for the ink jet head formed on the substrate.

25. An ink jet head according to Claim 23 including a memory circuit for the ink jet head formed on the substrate.

26. An ink jet head according to Claim 23 including a temperature control circuit formed on the sub-strate for controlling the temperature of the ink jet head.

27. An ink jet head according to Claim 23 including a thin-film heater on the substrate for heating the ink jet head.

28. An ink jet head according to Claim 23 including a drop counter circuit formed on the substrate.

29. An ink jet head according to Claim 23 wherein the substrate is silicon.

30. An ink jet head according to Claim 20 including membrane means interposed between the piezoelec-tric film and the ink chambers.

31. An ink jet head according to Claim 20 including a membrane and two piezoelectric films disposed on opposite sides of the membrane.

32. An ink jet head according to Claim 20 including a plurality of superimposed transducer elements including electroded piezoelectric films disposed on the substrate for joint operation in response to electrical signals.

33. An ink jet head according to Claim 20 wherein the electrode means comprises electrodes disposed on both surfaces of the piezoelectric film.