A PIEZOELECTRIC THIN FILM ELEMENT

Abstract

There is disclosed a piezoelectric thin film element comprising a first electrode, a second electrode and one or more piezoelectric thin films there between characterised in that the thin film element has at least two of: an electrode arrangement in which electrodes are arranged with the one or more piezoelectric thin films so that an electric field applied to a piezoelectric thin film or a portion of a piezoelectric thin film adjacent to the first electrode is lower than an electric field applied to a piezoelectric thin film or a portion of a piezoelectric thin film further from the first electrode when the piezoelectric thin film element actuated; a piezoelectric thin film adjacent to the first electrode in which a layer of the piezoelectric thin film near to the first electrode has a piezoelectric displacement constant which is lower than that of a layer of the piezoelectric thin film further from the first electrode; and a piezoelectric thin film adjacent to the first electrode in which a layer of the piezoelectric thin film near to the first electrode has an elastic modulus which is lower than that of a layer of the piezoelectric thin film further from the first electrode.

Claims

1. A piezoelectric thin film element comprising a first electrode, a second electrode and one or more piezoelectric thin films characterised in that the thin film element has at least two of: a piezoelectric thin film adjacent to the first electrode in which a layer of the piezoelectric thin film near to the first electrode has a piezoelectric displacement constant which is lower than that of a layer of the piezoelectric thin film further from the first electrode; a piezoelectric thin film adjacent to the first electrode in which a layer of the piezoelectric thin film near to the first electrode has an elastic modulus which is lower than that of a layer of the piezoelectric thin film further from the first electrode; and an electrode arrangement in which electrodes are arranged with the one or more piezoelectric thin films so that an electric field applied to a piezoelectric thin film or a portion of a piezoelectric thin film adjacent to the first electrode is lower than an electric field applied to a piezoelectric thin film or a portion of a piezoelectric thin film further from the first electrode when the piezoelectric thin film element is actuated.

2. A piezoelectric thin film element according to claim 1, wherein the element comprises said electrode arrangement and a piezoelectric thin film adjacent to the first electrode in which a layer of the piezoelectric thin film near to the first electrode has a piezoelectric displacement constant and/or an elastic modulus which are lower than those of a layer of the piezoelectric thin film further from the first electrode.

3. A piezoelectric thin film element according to claim 1, wherein the electrode arrangement comprises one or more additional electrodes.

4. (canceled)

5. (canceled)

6. A piezoelectric thin film element according to claim 2, wherein the piezoelectric thin films have different thicknesses and the thickness of the piezoelectric thin film adjacent the first electrode is greater than that of a piezoelectric film adjacent a neighbouring electrode.

7. (canceled)

8. (canceled)

9. A piezoelectric thin film element according to claim 1, wherein the electrode arrangement comprises an interdigitated first electrode and the second electrode on a surface of a piezoelectric thin film.

10. A piezoelectric thin film element according to claim 1, wherein the piezoelectric thin film adjacent the first electrode includes a plurality of thin film layers which together define a gradient in piezoelectric displacement constant across at least a part of the thin film in its thickness direction.

11. A piezoelectric thin film element according to claim 1, wherein the piezoelectric thin film adjacent the first electrode includes a plurality of thin film layers which together define a gradient in elastic modulus across at least a part of the thin film in its thickness direction.

12. A piezoelectric thin film element according to claim 1, wherein the piezoelectric thin film adjacent the first electrode includes a plurality of thin film layers which together define a gradient in elastic modulus across at least a part of the thin film in its thickness direction and which are doped by a dopant.

13. A piezoelectric thin film element according to claim 1, wherein the piezoelectric thin film adjacent the first electrode includes a plurality of thin film layers which together define a gradient in elastic modulus across at least a part of the thin film in its thickness direction wherein the thin film layers are doped and define a gradient in at least one of the dopant concentration across the thin film in its thickness direction.

14. (canceled)

15. A piezoelectric thin film element according to claim 13, the piezoelectric thin film adjacent the first electrode includes a plurality of thin film layers which together define a gradient in elastic modulus across at least a part of the thin film in its thickness direction wherein the thin film layers are doped and define a gradient in at least one dopant concentration across the thin film in its thickness direction and in which the thin film layer near to the first electrode is undoped.

16. A piezoelectric thin film element according to claim 1, wherein the piezoelectric thin film element has an end surface which is beveled or filleted.

17. A method for manufacturing a piezoelectric thin film element having a first electrode, a second electrode and one or more piezoelectric thin films between the electrodes, characterised in that the method comprises at least two of: forming a piezoelectric thin film adjacent to the first electrode so that a layer of the piezoelectric thin film near to the first electrode has a piezoelectric displacement constant which is lower than that of a layer of a piezoelectric thin film further from the first electrode; forming a piezoelectric thin film adjacent to the first electrode so that a layer of the piezoelectric thin film near to the first electrode has an elastic modulus which is lower than that of a layer of the piezoelectric thin film further from the first electrode; and arranging electrodes with the one or more piezoelectric thin films so that an electric field applied to a piezoelectric thin film or a portion of a piezoelectric thin film adjacent to the first electrode is lower than an electric field applied to a piezoelectric thin film or a portion of the piezoelectric thin film adjacent to the second electrode when the piezoelectric thin film element is driven by one or more predetermined voltages.

18. (canceled)

19. (canceled)

20. (canceled)

21. (canceled)

22. (canceled)

23. A method according to claim 17, comprising arranging the first and second electrodes with one or more additional electrodes and a plurality of piezoelectric thin films so that they interpose and alternate with the plurality of the piezoelectric thin films wherein the piezoelectric thin films have different thicknesses from one another, and the thickness of the piezoelectric thin film adjacent the first electrode is greater than that of a piezoelectric thin film adjacent a neighbouring electrode, with the electrodes so that the thin film adjacent the first electrode has thickness greater than the thin film adjacent a neighbouring electrode and the first and second electrodes are separately addressed with a respective additional electrode by two predetermined voltages.

24. (canceled)

25. (canceled)

26. (canceled)

27. A method according to claim 17, comprising forming a piezoelectric thin film adjacent to the first electrodes having a plurality of thin film layers which together define a gradient in piezoelectric displacement constant across at least a part of the thin film in its thickness direction.

28. A method according to claim 17, comprising forming a piezoelectric thin film adjacent to the first electrode having a plurality of thin film layers which together define a gradient in elastic modulus across at least a part of the thin film in its thickness direction.

29. A method according to claim 17, wherein the piezoelectric thin film adjacent to the first electrode has a plurality of thin film layers that are doped and that together define a gradient in elastic modulus across at least a part of the thin film in its thickness direction that are doped by at least a dopant.

30. A method according to claim 17, wherein the piezoelectric thin film adjacent to the first electrode has a plurality of thin film layers that together define a gradient in elastic modulus across at least a part of the thin film in its thickness direction that are doped and define a gradient in at least one dopant concentration across at least a part of the thin film in its thickness direction.

31. A method according to claim 17, wherein the piezoelectric thin film adjacent to the first electrode has a plurality of doped thin film layers that together define a gradient in elastic modulus across at least a part of the thin film in its thickness direction and wherein thin film layer near to the first electrode is undoped.

32. A method according to claim 17, comprising forming the piezoelectric thin film element so that it has an end surface which is beveled or filleted.

33. (canceled)

34. A printhead for an inkjet printer comprising a piezoelectric actuator comprising a piezoelectric thin film element comprising a first electrode, a second electrode and one or more piezoelectric thin films characterised in that the thin film element has at least two of: a piezoelectric thin film adjacent to the first electrode in which a layer of the piezoelectric thin film near to the first electrode has a piezoelectric displacement constant which is lower than that of a layer of the piezoelectric thin film further from the first electrode; a piezoelectric thin film adjacent to the first electrode in which a layer of the piezoelectric thin film near to the first electrode has an elastic modulus which is lower than that of a layer of the piezoelectric thin film further from the first electrode; and an electrode arrangement in which electrodes are arranged with the one or more piezoelectric thin films so that an electric field applied to a piezoelectric thin film or a portion of a piezoelectric thin film adjacent to the first electrode is lower than an electric field applied to a piezoelectric thin film or a portion of a piezoelectric thin film further from the first electrode when the piezoelectric thin film element is actuated.

35. (canceled)

36. (canceled)

Description

[0174] The present invention is now described in more detail with reference to the following non-limiting embodiments and the accompanying drawings in which:

[0175] FIGS. 1 to 5 show section views of piezoelectric thin film elements (and diaphragm) particularly pointing out electrode arrangements according to the present invention;

[0176] FIGS. 6 to 9 are graphs showing lateral stress in the bottom electrode and across the piezoelectric elements of FIGS. 1 and 3;

[0177] FIGS. 10 and 11 show section views of piezoelectric thin film elements (and diaphragm) according to several embodiments of the present invention;

[0178] FIGS. 12 to 14 are graphs showing lateral stresses in the piezoelectric thin film element according to several embodiments of the present invention;

[0179] FIGS. 15 and 16 show section views of piezoelectric thin film elements (and diaphragm) according to several other embodiments of the present invention;

[0180] FIGS. 17 and 18 are graphs showing lateral stresses in the piezoelectric thin film element according to several embodiments of the present invention; and

[0181] FIG. 19 shows a section view of part of a piezoelectric actuator according to one embodiment of the present invention.

[0182] FIG. 1 shows a section view of a piezoelectric thin film element 20 (and diaphragm 21) in which the electrode arrangement comprises a plurality of piezoelectric thin films F1 to F3 alternately arranged between a top electrode 22, a bottom electrode 23 and intermediate electrodes 24 and 25.

[0183] The films Fi may each comprise a plurality (n) of identical or different thin film layers Ir1 , Ir2 and Ir3 etc. but, as mentioned above, these need not be discrete.

[0184] The thickness of the piezoelectric thin film adjacent the bottom electrode F1 is greater than the thickness of the adjacent piezoelectric thin film F2and thickness of the piezoelectric thin film F2 is greater than the thickness of the adjacent piezoelectric thin film F3.

[0185] The thickness of the piezoelectric thin film F2 may, however, be similar to or less than the thickness of the adjacent piezoelectric thin film F3.

[0186] In any case, the top electrode 22 is connected with an intermediate electrode 24 separating adjacent piezoelectric thin films F2 and F1 to a voltage source V.sub.1. The bottom electrode 23 is connected with an intermediate electrode 25 separating adjacent piezoelectric thin films F2 and F3 to another voltage source V.sub.2.

[0187] The electric field strength experienced by F1 is lower than the electric field strength experienced by F2 and F3 when the piezoelectric element is driven at voltages V.sub.1 and V.sub.2, provided that V.sub.2<V.sub.1; V.sub.2 may be 0.

[0188] FIG. 2 shows a section view of a piezoelectric thin film element 20 (and diaphragm 21) of similar arrangement except that the thickness of each piezoelectric thin film Fi is similar.

[0189] The top electrode 22 and the intermediate electrode 24 separating adjacent piezoelectric thin films F2 and F1 are connected to separate voltage sources V.sub.1 and V.sub.2. The bottom electrode 23 and the intermediate electrode 25, separating adjacent piezoelectric thin films F2 and F3, are connected to another voltage source V.sub.3.

[0190] The piezoelectric thin film F1 experiences an electric field strength which is lower than the electric field strength experienced by piezoelectric thin films F2 and F3 when the piezoelectric element is driven at predetermined voltages V.sub.1 to V.sub.3, provided that V.sub.3<V.sub.2<V.sub.1.

[0191] If the bottom electrode 23 and the additional electrode 25 are separately connected to different voltages V.sub.3 and V.sub.4, the electric field strength experienced by the piezoelectric thin film adjacent the bottom electrode F1 is lower than the electric field strength experienced by the adjacent piezoelectric thin film F2 when the piezoelectric element is driven at predetermined voltages (V.sub.1 to V.sub.4) so that (V.sub.2?V.sub.3)<(V.sub.2?V.sub.4).

[0192] FIG. 3 shows a section view of a piezoelectric thin film element 20 (and diaphragm 21) similar to that shown in FIG. 1 except that the piezoelectric thin film element has end surfaces which are beveled. The end surfaces contact the diaphragm 21 at angle of 45? C. to the plane of the substrate (underlying the diaphragm; not shown).

[0193] FIG. 4 also shows a section view of a piezoelectric element 20 (and diaphragm 21) similar to that shown in FIG. 1 except that the piezoelectric thin film element has filleted end surfaces.

[0194] FIG. 5 shows a section view of a piezoelectric thin film element 20 (and diaphragm 21) comprising piezoelectric thin films F1 to F3 of similar thickness which are not separated by intermediate electrodes. Instead two interdigitated electrodes 22 and 23 are formed on the upper surface of piezoelectric thin film F3.

[0195] The interdigitated electrodes 22 and 23 are connected to different voltage sources V.sub.1 and V.sub.2 (not shown).

[0196] This electrode configuration provides that the electric field strength experienced by the piezoelectric thin film F1 is lower than the electric field strength experienced by the piezoelectric thin film F2 when the piezoelectric element is driven at a predetermined voltage or by predetermined voltages (V.sub.1 and V.sub.2).

[0197] A model study based on finite element analysis (using the commercially available software COMSOL v4.4/5.0) was used to calculate piezoelectric displacements and lateral stresses for a piezoelectric element having a single piezoelectric thin film and for the piezoelectric elements of FIGS. 1 and 2.

[0198] The study assumes PZT thin films provided on a platinum electrode, an alumina adhesive layer, a silica-silicon nitride diaphragm 21, and a silicon substrate (within conventional parameters and voltages).

[0199] The thickness of the single piezoelectric thin film was set at 1.8 ?m. The thicknesses of the piezoelectric thin films F1 to F3 was set to vary in accordance with one or other electrode arrangement within a total thickness of 1.8 ?m. The thickness of the platinum electrodes was set at 200 nm and the thickness of the bilayer diaphragm was set at 1.4 ?m (0.7 ?m for each layer).

[0200] FIG. 6 shows a graph which particularly points out the lateral stress produced in the diaphragm 21 at point (10 nm) below its upper surface by the piezoelectric element shown in FIG. 1 when it is driven; the thicknesses of the piezoelectric thin film layers F1 to F3 are respectively 0.7 ?m, 0.6 ?m, and 0.5 ?m.

[0201] The peak interface stress of about 620 MPa compares well with that found for the piezoelectric thin film element having the single film F with thickness equal to the sum of the thicknesses of the Fi piezoelectric thin films, driven at a voltage which is equal to three times the voltage applied to each of the Fi piezoelectric thin films (about 640 MPa).

[0202] The peak interface stress is, however, similar to that found for the piezoelectric thin film element of FIG. 1 when the piezoelectric thin films F1 to F3 have the same thicknesses (0.6 ?m) and are driven at the same voltage.

[0203] FIG. 7 shows a graph which particularly points out the lateral stress at the centre of the piezoelectric element shown in FIG. 1 plotted against the distance from the bottom surface of the diaphragm 21 in the thickness direction of the element when it is driven.

[0204] The lateral stress in the thin film contacting the bottom electrode F1 is about 140 MPaand compares well with that found for the piezoelectric thin film element having the single film (about 170 MPa).

[0205] It also compares well with that found for the piezoelectric thin film element of FIG. 1 when the piezoelectric thin films F1 to F3 have the same thicknesses (0.6 ?m; 160 MPa).

[0206] FIG. 8 shows a graph similar to that of FIG. 6, but related to the piezoelectric thin film element of FIG. 3 when the thicknesses of the piezoelectric thin film layers F1 to F3 are respectively 0.7 ?m, 0.6 ?m, and 0.5 ?m.

[0207] The peak interface stress is about 500 MPawhich compares well with that for a piezoelectric element having a single film and similar end surfaces (about 530 MPa).

[0208] The peak interface stress is, however, similar to that found for the piezoelectric thin film element of FIG. 1 when the piezoelectric thin films F1 to F3 have the same thicknesses (0.6 ?m).

[0209] FIG. 9 shows a graph similar to that shown in FIG. 7. The lateral stress at the centre of the piezoelectric element shown in FIG. 3 is about 140 MPawhich compares well with that obtained for a piezoelectric element having a similar film and similar end surfaces (about 170 MPa).

[0210] It also compares well with the lateral stress found for the piezoelectric thin film element of FIG. 3 when the piezoelectric thin films F1 to F3 have the same thicknesses (0.6 ?m; 170 MPa).

[0211] These and further results relating to the electrode arrangement shown in FIG. 1 are collected together in Table 1. The Table shows that peak interface stress is not particularly sensitive to film thicknesses but depends more upon end surfaces. It appears highest for those piezoelectric elements having vertical end surfaces and lower for piezoelectric elements having beveled or filleted end surfaces.

TABLE-US-00001 TABLE 1 Peak Interface Centre Number of piezoelectric thin Stress/ Stress/ Edge films and related thickness MPa MPa Vertical single film 1.8 ?m 640 170 3 films t.sub.1 = t.sub.2 = t.sub.3 = 0.6 ?m 620 160 3 films 620 140 t.sub.1 = 0.7 ?m, t.sub.2 = 0.6 ?m, t.sub.3 = 0.5 ?m Bevelled single film 1.8 ?m 520 170 3 films t.sub.1 = t.sub.2 = t.sub.3 = 0.6 ?m 500 165 3 films 500 140 t.sub.1 = 0.7 ?m, t.sub.2 = 0.6 ?m, t.sub.3 = 0.5 ?m Filleted single film 1.8 ?m 620 170 3 films t.sub.1 = t.sub.2 = t.sub.3 = 0.6 ?m 600 170 3 films 590 135 t.sub.1 = 0.7 ?m, t.sub.2 = 0.6 ?m, t.sub.3 = 0.5 ?m

[0212] The lateral stress at centre (viz. in most of the area of the element) depends on film thickness and not on end surfaces. It is about the same in piezoelectric thin film elements having a single film and the piezoelectric thin film elements having piezoelectric thin films of similar thicknessesbut is significantly lower for piezoelectric thin film elements having piezoelectric thin films of different thicknesses.

[0213] The model shows, therefore, that lateral stress in piezoelectric thin film elements can be managedby engineering the electric field strength through different thicknesses of the piezoelectric thin films.

[0214] FIG. 10 shows a section view of a piezoelectric thin film element according to one embodiment of the present invention. The piezoelectric thin film element 20 (and diaphragm, 21) comprises a single film which is interposed between a top electrode 22 and a bottom electrode 23.

[0215] The piezoelectric thin film comprises a plurality of piezoelectric thin film layers, for example, Ir1 to Ir5. These layers are shown as discrete layers of defined thickness and may be obtained, for example, by a sol-gel method.

[0216] However, as mentioned above, the layers need not have a defined thickness at all but simply be put down in the piezoelectric thin film by adaptation of the film forming method to provide a different material or a different processing condition at a particular time in the process.

[0217] The piezoelectric thin film layers Ir1 to Ir5 are singly doped by an acceptor dopant (or a donor dopant) at different dopant concentrations (Di). The dopant concentration is such that it gradually changes across the piezoelectric film thickness.

[0218] The thin film comprises a piezoelectric thin film layer Ir1, near to the bottom electrode 23 which has lower displacement performance compared to the layers further from the bottom electrode, so that the stress at the interface between the bottom electrode and the adjacent piezoelectric film layer is reduced. The displacement performance increases in the thickness direction either continuously or reaching a plateau.

[0219] FIG. 11 shows a section view of a piezoelectric thin film element according to another embodiment of the present invention. The piezoelectric thin film element 20 (and diaphragm 21) is similar to that shown in FIG. 10.

[0220] However, the piezoelectric thin film layer Ir3 is undoped, the piezoelectric thin film layers Ir1 and Ir2 are singly doped by a donor dopant and the piezoelectric thin film layers Ir4 and Ir5 are singly doped by an acceptor dopant.

[0221] The thin film comprises a piezoelectric thin film layer Ir1 near to the bottom electrode 23 which has lower displacement performance compared to the layers further from the bottom electrode, so that the stress at the interface between the bottom electrode and the adjacent piezoelectric film layer is reduced. The displacement performance increases in the thickness direction either continuously or reaching a plateau. A model study based on finite element analysis (using the commercially available software COMSOL v4.4/5.0) was used to calculate piezoelectric displacements and lateral stresses for piezoelectric elements similar to those shown in FIGS. 10 to 12.

[0222] The study assumes the same parameters as those mentioned in relation to FIGS. 5 to 7 but substitutes parameters for singly doped PZT and different processing condition or different composition of PZT which continuously vary (from 10 nm) in the thickness direction of the thin film.

[0223] FIG. 12 shows a graph similar to that shown in FIG. 7. The curves 1 to 4 show the lateral stress in the piezoelectric thin film and how it changes when the Young's modulus and/or the piezoelectric constant d.sub.31 is made to change from the bottom electrode to the top electrode by gradually changing the acceptor dopant concentration.

[0224] Curve 1 shows a stress profile for a piezoelectric thin film in which the Young's modulus and the piezoelectric constant d.sub.31 are the same for every layer of the thin film at respectively 65 GPa and ?170 pm/V. As may be seen, the interface stress in the thin film is about 165 MPa.

[0225] Curve 2 shows a stress profile for a piezoelectric thin film in which the Young's modulus changes from 65 GPa in a thin film layer near to the bottom electrode (10 nm from the start of the film) to 85 GPa in a thin film layer near to the top electrode and the piezoelectric constant d.sub.31 is the same (at ?170 pm/V) for every layer of the thin film. As may be seen, the interface stress is slightly lower that that found from Curve 1at about 155 MPa.

[0226] Curve 3 shows a stress profile for a piezoelectric thin film in which the piezoelectric constant d.sub.31 changes from ?120 pm/V in the thin film layer near to the bottom electrode to ?170 pm/V in the thin film layer near to the top electrode and the Young's modulus is the same (at 65 GPa) for every layer of thin film layer. As may be seen, the interface stress is significantly lower than that found from Curve 1 and Curve 2at about 90 MPa.

[0227] Curve 4 shows a stress profile for a piezoelectric thin film in which the Young's modulus and the piezoelectric constant d.sub.31 changes from respectively 65 GPa and ?120 pm/V in the thin film layer near to the bottom electrode to respectively 85 GPa and ?170 pm/V in the thin film layer near to the top electrode. As may be seen, the interface stress is lower than that found from Curve 3 at about 85 MPa.

[0228] FIG. 13 shows a graph similar to that shown in FIG. 7. The Curves 1 to 3 show the lateral stress in the piezoelectric thin film and how it changes when the Young's modulus and/or the piezoelectric constant d.sub.31 is made to change across the thin film by gradually changing the donor dopant concentration and/or the processing condition inside the piezoelectric thin film from the bottom electrode to the top electrode.

[0229] Curve 1 shows a stress profile for a piezoelectric thin film in which both the Young's modulus and the piezoelectric constant d.sub.31 are the same for every layer of the thin film at respectively 65 GPa and ?170 pm/V. As may be seen, the interface stress in the thin film is about 165 MPa.

[0230] Curve 2 shows a stress profile for a piezoelectric thin film in which the Young's modulus changes from 45 GPa in the thin film layer near to the bottom electrode (10 nm from the surface) to 65 GPa in a thin film layer near to the top electrode and the piezoelectric constant is the same (at ?170 pm/V) in every layer of the thin film. As may be seen, the interface stress is significantly lower than that found in Curve 1at about 105 MPa.

[0231] Curve 3 shows a stress profile for a piezoelectric thin film in which the Young's modulus and the piezoelectric constant d.sub.31 change from respectively 45 GPa and ?120 pm/V in the thin film layer near to the bottom electrode to respectively 65 GPa and ?170 pm/V in the thin film layer near to the top electrode. As may be seen, the interface stress is significantly lower compared to that found in Curves 1 and 2at about 60 MPa.

[0232] FIG. 14 shows a graph similar to that shown in FIG. 7. The Curves 1 to 3 show the lateral stress in the piezoelectric thin film and how it changes when the Young's modulus and/or the piezoelectric constant d.sub.31 are made to change across the thin film by gradually changing the donor dopant concentration, acceptor dopant concentration and/or the processing condition inside the piezoelectric thin film from the bottom electrode.

[0233] Curve 1 shows a stress profile for the piezoelectric thin film in which both the Young's modulus and the piezoelectric constant d.sub.31 are the same for every layer of the thin filmat respectively 65 GPa and ?170 pm/V. As may be seen, the interface stress in the thin film is about 165 MPa.

[0234] Curve 2 shows a stress profile for the piezoelectric thin film in which the Young's modulus changes from 45 GPa in the thin film layer near to the bottom electrode (10 nm from start) to 85 GPa in a thin film layer near the top electrode and the piezoelectric constant d.sub.31 (at ?170 pm/V) is the same for every thin film layer. As may be seen the interface stress is significantly lower than that found from Curve 1at about 100 MPa.

[0235] Curve 3 shows a stress profile in which both the Young's modulus and the piezoelectric constant d.sub.31 change respectively from 45 GPa and ?120 pm/V in the thin film layer near to the bottom electrode to respectively 85 GPa and ?170 pm/V in the thin film layer near to the top electrode. As may be seen the interface stress is significantly lower than that found from Curves 1 and 2at below 60 MPa.

[0236] Table 2 shows how the performance (the displaced area) of the piezoelectric element changes as the Young's modulus and piezoelectric constant d.sub.31 change in these studies.

[0237] The first four entries relate to the stress profiles shown by the curves in FIGS. 12 and 13. When the Young's modulus and the piezoelectric constant d.sub.31 are constant across the thin film, the displaced area of the actuator is 7.34?10.sup.?12 m.sup.2 and the interface stress is 165 MPa.

[0238] When the Young's modulus changes by changing the concentration of acceptor dopant across the thin film, the performance of the element is better but there is only a marginal improvement in interface stress.

[0239] When the piezoelectric constant d.sub.31 changes across the thin film, the interface stress is substantially lower but at the expense of performance.

TABLE-US-00002 TABLE 2 Displaced Interface Y/GPa d.sub.31/pmV.sup.?1 area/10.sup.?12 m.sup.2 stress/MPa 65 (const) ?170 (const) 7.34 165 65 .fwdarw. 85 ?170 8.02 155 65 (const) ?120 .fwdarw. ?170 6.73 90 65 .fwdarw. 85 ?120 .fwdarw. ?170 7.49 85 45 .fwdarw. 65 ?170 6.88 105 45 .fwdarw. 65 ?120 .fwdarw. ?170 6.32 60 45 .fwdarw. 85 ?170 7.70 <100 45 .fwdarw. 85 ?120 .fwdarw. ?170 7.18 <60

[0240] However, when the Young's modulus and the piezoelectric constant d.sub.31 change across the thin film, the performance of the element is better and the interface stress is substantially lower.

[0241] On the other hand, when the Young's modulus changes by changing the concentration of donor dopant across the thin film, the interface stress is substantially lower but at the expense of performance.

[0242] When the Young's modulus and the piezoelectric constant d.sub.31 change across the thin film, the interface stress is significantly lower and the performance of the element is lower.

[0243] However, when the Young's modulus changes by changing the concentration of an acceptor dopant and the concentration of a donor dopant, the interface stress is significantly lower and the performance of the element is substantially unaffected.

[0244] FIG. 15 shows a section view of a piezoelectric element according to an embodiment of the present invention in which a piezoelectric element 20 (and diaphragm, 21) similar to that shown in FIG. 1 has a thin film F1 adjacent to the bottom electrode 23 comprising piezoelectric thin film layers Ir1 to Ir5 which are singly doped by an acceptor or by a donor dopant.

[0245] The piezoelectric thin film layers Ir1 to Ir5 define an acceptor dopant concentration gradient or a donor dopant concentration gradient.

[0246] FIG. 16 shows a section view of a piezoelectric element according to still another embodiment of the present invention in which a piezoelectric element 20 (and diaphragm, 21) similar to that shown in FIG. 1 has the thin film F1 adjacent to the bottom electrode 23 comprising piezoelectric thin film layers Ir1 to Ir5 which adjacent piezoelectric film layers are singly doped with an acceptor or donor dopant and are separated by an undoped piezoelectric film layer (Ir3).

[0247] The piezoelectric thin film layer near to the bottom electrode Ir1 has a lower displacement performance than the adjacent piezoelectric thin film layer Ir2. And this latter piezoelectric thin film layer has displacement performance lower than that of the adjacent piezoelectric thin film layer Ir3 and so on.

[0248] A model study based on finite element analysis (using the commercially available software COMSOL v4.4/5.0) was used to calculate piezoelectric displacements and lateral stresses for piezoelectric elements similar to those shown in FIGS. 16 to 18.

[0249] The study assumes the same parameters as those mentioned in relation to FIGS. 5 to 7 and to FIGS. 10 to 11 and that the voltage applied to each thin film is equal to one third of the voltage applied to a piezoelectric thin film element with a single piezoelectric thin film of thickness equal to the sum of the thicknesses of F1 to F3 in order to obtain the desired displacement.

[0250] FIG. 17 shows a graph similar to that shown in FIG. 7. The curves show the lateral stress in a piezoelectric thin film element similar to that shown in FIG. 15 and how it changes when the Young's modulus and/or the piezoelectric constant d.sub.31 of the thin film near to the bottom electrode is changed by gradually changing donor dopant concentration as described above.

[0251] Curve 1 shows a stress profile in the piezoelectric thin film element when the Young's modulus and the piezoelectric constant d.sub.31 are the same for every layer in the thin film adjacent to the bottom electrode (respectively, at 65 GPa and ?170 pm/V). As may be seen (left hand side), the interface stress is about 140 MPa.

[0252] The reduction in stress as compared to the piezoelectric thin film element comprising a single thin film is due to the lower electric field strength experienced by the thin film adjacent the bottom electrode.

[0253] Curve 2 shows a stress profile in the piezoelectric thin film element when the Young's modulus changes from 45 GPa to 65 GPa and the piezoelectric constant d.sub.31 is the same for every layer in the thin film adjacent to the bottom electrode. As may be seen, the interface stress is substantially lower than that found in Curve 1at about 90 MPa.

[0254] The displacement area for the piezoelectric thin film element is similar to that for the piezoelectric element comprising the single thin filmat 6.07?10.sup.?12 m.sup.2. This slightly lower value is due to the additional electrode layers present in this actuator.

[0255] Curve 3 shows a stress profile in the piezoelectric thin film element when the Young's modulus and the piezoelectric constant d.sub.31 change from respectively 45 GPa and ?120 pm/V to respectively 65 GPa and ?170 pm/V in the thin film layer adjacent to the bottom electrode. As may be seen, the interface stress is substantially lower at about 50 MPa.

[0256] The displacement area for the piezoelectric actuator is similar to that for the piezoelectric element comprising the single thin filmat 6.73?10.sup.?12 m.sup.2.

[0257] FIG. 18 shows a graph similar to that shown in FIG. 7. The curves show the lateral stress in a piezoelectric thin film element similar to that shown in FIG. 16 and how it changes when the Young's modulus and/or the piezoelectric constant d.sub.31 of the thin film near to the bottom electrode are changed by gradually changing donor dopant concentration and acceptor dopant concentration as described above. As may be seen, the interface stress is about 50 MPa.

[0258] The displacement area for the piezoelectric actuator is slightly higher than that for the piezoelectric element comprising the single thin filmat 7.79?10.sup.?12 m.sup.2.

[0259] FIG. 19 shows a section view of part of an inkjet printhead according to one embodiment of the present invention. The piezoelectric thin film element is similar to that shown in FIG. 16 (piezoelectric thin film layers not shown) and is provided to a diaphragm 21 comprising a bilayer on top of a pressure chamber 26, provided with a nozzle plate 27.

[0260] The pressure chamber 26 is formed in a silicon single crystal of thickness about 200 ?m and the diaphragm comprises a thin film comprising a bilayer of silicon dioxide and silicon nitride.

[0261] A buffer layer of ultra-thin titanium film or chromium film (not shown) (about 10 nm thick) may be interposed between the first electrode 23 and F1 and or underneath the first electrode 23. Other components including buffer layers, adhesion layers, seed layers may also be present.

[0262] In use, predetermined drive voltages V.sub.1 and V.sub.2 are applied to the electrodes 22 to 25 by a signal from a control circuit (not shown). The voltages cause the piezoelectric thin film element 20 to deform so deflecting the diaphragm 21 into the pressure chamber 26 and changing its volume. A sufficient increase in pressure within the pressure chamber 26 causes ink droplets to be ejected from the nozzle 30.

[0263] It will be appreciated, therefore, that the present invention provides for piezoelectric actuators having good performance and excellent reliability.

[0264] The present invention also permits tuning of piezoelectric elements to a particular requirement for performance and/or reliability depending on a particular application of the element, for example, between sensing, actuating and energy harvesting.

[0265] The present invention has been described in detail with reference to certain embodiments which are illustrated by the drawings. However, it will be understood that other embodiments not described in detail or illustrated by the drawings are also included within the scope of the present invention.