DROPLET EJECTOR ASSEMBLY STRUCTURE AND METHODS

Abstract

A droplet ejector assembly for a printhead comprises a substrate, the substrate comprising a CMOS control circuit, a plurality of layers on the first surface of the substrate, a fluid chamber having a droplet ejection outlet, and a piezoelectric actuator element formed by one or more said layers and comprising first and second electrodes in contact with a piezoelectric body. The piezoelectric actuator element defines part of the fluid chamber. At least one said electrode electrically is connected to the CMOS control circuit. The droplet ejector comprises a fluid chamber having a droplet ejection outlet. The piezoelectric actuator element is separate to the droplet ejection outlet and the piezoelectric body is formed of one or more piezoelectric materials processable at a temperature below 450° C. Thus, a CMOS control circuit is integrated with a droplet ejector assembly. The CMOS control circuit may receive both an analogue actuator ejection pulse and serial digital controls signals and use the serial digital control signals to determine which piezoelectric actuator elements are connected to and driven by individual actuator ejection pulses.

Claims

1. A droplet ejector assembly for a printhead, the droplet ejector assembly comprising: a substrate having a first surface and an opposite second surface, the substrate comprising a CMOS control circuit, a plurality of layers on the first surface of the substrate, a fluid chamber having a droplet ejection outlet, and a piezoelectric actuator element formed by one or more said layers and comprising a piezoelectric body and first and second electrodes in contact with the piezoelectric body, the piezoelectric actuator element defining part of the fluid chamber, at least one said electrode electrically connected to the CMOS control circuit, the droplet ejector comprising a fluid chamber having a droplet ejection outlet, wherein the piezoelectric actuator element is separate to the droplet ejection outlet and the piezoelectric body is formed of one or more piezoelectric materials processable at a temperature below 450° C.

2. A droplet ejector assembly according to claim 1, wherein the piezoelectric body comprises one or more non-ferroelectric piezoelectric materials and the CMOS control circuit is configured to actuate the piezoelectric body by applying an electrical potential gradient to the piezoelectric body in a first direction to cause the piezoelectric body to flex in a first sense and then to apply an electrical potential gradient to the piezoelectric body in the opposite direction to cause it to deform in an opposite second sense.

3. A droplet ejector assembly according to claim 1 or claim 2, wherein the piezoelectric body has a relative permittivity, ε.sub.r, of less than 100.

4. A droplet ejector assembly according to any one preceding claim, wherein the piezoelectric body has a breakdown voltage of greater than 100 V/.Math.m and the CMOS control circuit is configured to apply a potential gradient of greater than 100 V/.Math.m within the piezoelectric body.

5. A droplet ejector assembly according to any one preceding claim, wherein the CMOS control circuit comprises one or more of: (a) a digital register, (b) a nozzle trimming calculation circuit and/or register, (c) a temperature measurement circuit, (d) a fluid chamber fill detection circuit.

6. A droplet ejector assembly according to any one preceding claim, wherein the CMOS control circuit comprises an ejection transistor.

7. A droplet ejector assembly according to any one preceding claim, comprising an electrical input for receiving actuator drive pulses, and wherein the CMOS control circuits are configured to switchedly connect or disconnect at least one electrode of the or each piezoelectric actuator to the received actuator drive pulses to thereby selectively actuate the piezoelectric actuators.

8. A droplet ejector assembly according to any one preceding claim, wherein the CMOS control circuit is configured to individually and selectively actuate at least three said piezoelectric actuator elements formed by one or more said layers on the same substrate and defining part of different respective fluid chambers and droplet ejection outlets, optionally wherein actuators in the said at least three actuator elements are configured for ejecting fluid of different colours or compositions.

9. A droplet ejector assembly according to claim 8, wherein the said at least three actuator elements are located on the substrate and the CMOS control circuit is connected to a flexible printhead cable having one or more electrical signal conductors, wherein the CMOS control circuit is configured to individually and selectively actuate the actuator elements of the at least three actuator elements responsive to actuation commands received through the same signal conductor.

10. A droplet ejector assembly according to claim 8 or claim 9, wherein the CMOS control circuit is configured to individually and selectively actuate at least double the number of piezoelectric actuator elements than signal conductors through which the CMOS control circuit receives actuation control signals.

11. A droplet ejector assembly according to any one of claims 8 to 11, further comprising a fluid supply block in contact with one or more of the said layers and defining at least three separate fluid supply manifolds for supplying fluid of different colours or compositions of liquid to different said fluid chambers.

12. A droplet ejector assembly according to claim 11, wherein the fluid supply manifolds comprise a fluid conduit which is connected to each of a plurality of fluid chambers, to supply fluid of the same composition to each of the plurality of fluid chambers, wherein the piezoelectric actuator elements which define part of each of the plurality of fluid chambers are actuated by the CMOS control circuit, optionally responsive to actuation commands received through the same signal conductor.

13. A droplet ejector assembly according to any one preceding claim wherein the CMOS control circuit is configured to switchedly connect one or more of ground and a single fixed non-zero voltage line, or multiple fixed voltage lines of different voltages, one or more of which may be ground, to one or more both electrodes of a piezoelectric actuator to cause droplet ejection.

14. A droplet ejector according to any one preceding claim, wherein the CMOS control circuit is configured to modify the voltage pulses applied to one or more electrodes of one or more piezoelectric actuators responsive to data stored by the CMOS control circuit or measurements from one or more sensors, which are typically within the droplet ejector assembly.

15. An inkjet printer comprising a controller and one or more droplet ejector assemblies according to claim 7 in electronic communication with and controlled by the controller, wherein the controller further comprises a pulse generator configured to generate a sequence of actuator drive pulses and the electrical input of the droplet ejector assembly receives actuator drive pulses through an electrical connection to the controller, and wherein the CMOS control circuit of the one or more droplet ejector assemblies is configured to switchedly connect or disconnect at least one electrode of the or each of a plurality of piezoelectric actuators to the received actuator drive pulses to thereby selectively actuate the piezoelectric actuators.

16. An inkjet printer according to claim 15, comprising a plurality of droplet ejector assemblies, wherein pulses from the pulse generator are conducted to a plurality of control circuits which are part of a plurality of droplet ejector assemblies, wherein the controller is further configured to generate digital control signals which are conducted to the droplet ejector assemblies and which are processed in the CMOS control circuits of the droplet ejector assemblies to determine which actuator drive pulses are conducted to at least one electrode of the piezeoelectric actuators of the one or more droplet ejector assemblies to cause droplet ejection.

17. A method of manufacturing a droplet ejector assembly for a droplet ejector according to any one preceding claim, the method comprising: providing a substrate having a first surface, forming the CMOS control circuit on the first surface, forming the plurality of layers on the first surface, the plurality of layers comprising the piezoelectric actuator element comprising the first and second electrodes and the piezoelectric body.

18. A method of operating a droplet ejector assembly according to any one of claims 1 to 14, or an inkjet printer according to claim 15 or 16, wherein the CMOS control circuit receives digital actuation control signals and processes the digital actuation control signals to selectively actuate the piezoelectric actuator element to cause droplet ejection.

19. A method according to claim 18, comprising the step of generating actuator drive pulses and conducting them to the droplet ejector assembly through an electrical connection, and switchedly connecting or disconnecting at least one electrode of the or each of a plurality of piezoelectric actuators to the received actuator drive pulses to thereby selectively actuate the piezoelectric actuators.

20. A method according to claim 18 or claim 19 comprising generating a plurality of different sequences of actuator drive pulses and conducting them to the droplet ejector assembly through separate electrical connections, and switchedly connecting or disconnecting at least one electrode of the or each of a plurality of piezoelectric actuators to one or more received actuator drive pulses received from a variable one of the plurality of different sequences of actuator drive pulses.

21. A method according to any one of claims 18 to 20, comprise switching an electrode between a connection to ground and a connection to a fixed voltage or multiple fixed voltage lines of different voltages and back to ground again in order to cause a droplet ejection.

22. A droplet ejector assembly for a printhead, the droplet ejector assembly comprising: a substrate having a first surface and an opposite second surface, the substrate comprising a CMOS control circuit, a plurality of layers on the first surface of the substrate, a fluid chamber having a droplet ejection outlet, and a piezoelectric actuator element formed by one or more said layers and comprising a piezoelectric body and first and second electrodes in contact with the piezoelectric body, the piezoelectric actuator element defining part of the fluid chamber, at least one said electrode electrically connected to the CMOS control circuit, the droplet ejector comprising a fluid chamber having a droplet ejection outlet, wherein the piezoelectric body is formed of one or more piezoelectric materials processable at a temperature below 450° C.

23. A droplet ejector assembly according to claim 22, wherein the piezoelectric body has a breakdown voltage of greater than 100 V/.Math.m and the CMOS control circuit is configured to apply a potential gradient of greater than 100 V/.Math.m within the piezoelectric body.

24. A droplet ejector assembly according to claim 22 or claim 23, comprising an electrical input for receiving actuator drive pulses, and wherein the CMOS control circuits are configured to switchedly connect or disconnect at least one electrode of the or each piezoelectric actuator to the received actuator drive pulses to thereby selectively actuate the piezoelectric actuators, or wherein the CMOS control circuit is configured to switchedly connect one or more of ground and a single fixed non-zero voltage line, or multiple fixed voltage lines of different voltages, one or more of which may be ground, to one or more both electrodes of a piezoelectric actuator to cause droplet ejection.

25. An inkjet printer comprising a controller and one or more droplet ejector assemblies according to any one of claims 22 to 24 in electronic communication with and controlled by the controller, wherein the controller further comprises a pulse generator configured to generate a sequence of actuator drive pulses and the electrical input of the droplet ejector assembly receives actuator drive pulses through an electrical connection to the controller, and wherein the CMOS control circuit of the one or more droplet ejector assemblies is configured to switchedly connect or disconnect at least one electrode of the or each of a plurality of piezoelectric actuators to the received actuator drive pulses to thereby selectively actuate the piezoelectric actuators.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0116] The invention will now be described with reference to the following Figures:

[0117] FIG. 1 is a schematic cross section of a prior art droplet ejector chip;

[0118] FIG. 2 is a cross section of a droplet ejector chip showing a single actuator according to the invention;

[0119] FIG. 3 is a further schematic cross section of the monolithic droplet ejector chip substrate with CMOS and actuators of FIG. 2;

[0120] FIGS. 4 and 5 show possible droplet ejector chip configurations according to the invention;

[0121] FIGS. 6 and 7 show control printhead configurations;

[0122] FIG. 8 is a schematic diagram of print control circuitry;

[0123] FIGS. 9(a) through 9(c) show actuator control pulses; the x-axis is time and y-axis is voltage per .Math.m thickness of the piezoelectric body; and

[0124] FIGS. 10 through 12 show three alternative droplet ejector chips showing single actuator configurations.

DETAILED DESCRIPTION

[0125] With reference to FIG. 1, a known class of inkjet printheads 1 comprises piezoelectric actuator elements 2 formed as layers on a silicon printhead substrate 4. The actuator elements each form a wall of a fluid chamber 6 which is in fluid communication with an ink reservoir 12 through a conduit 8 and with a nozzle 10 having a radius in the range 6 to 25 .Math.m. The ink fluid chamber and conduit are formed in a fluid manifold layer 14 covered with a nozzle defining layer 16 having the nozzles therein. Chambers 18 behind each actuator provide space for the actuator to flex to pull ink into the respective fluid chamber and eject it from the respective nozzle. In some embodiment chambers may be vented directly into the flex cavity as shown. An external controller 20 drives the actuators via a flexible interconnect 22 which contains a chip on film (COF) having latches and/or nozzle trim data which switch individual piezoelectric actuators on. The flexible interconnect connects to the silicon through a parallel connection 24 which contains individual signal conductors for each piezoelectric actuator. Accordingly, for printheads with many actuators, the flexible interconnect has many individual signal conductors. If there are 600 nozzles per inch, for example, there are at least 600 individual connections per inch. Conductive line attachments at greater than 600 per inch are very difficult to achieve reliably. The target for a high resolution printer needs to be greater than 1000 per inch. This is very important to achieve for a stationary pagewide printhead that cannot scan in order to achieve high target resolutions. Thus, a four colour printhead at 1200 dots per inch requires 4800 connections per inch.

[0126] With reference to FIG. 2, a droplet ejector chip 100 (functioning as the droplet ejector assembly) according to the invention comprises a silicon substrate 102 comprising CMOS control circuit 104 on the first surface 106 of the substrate. There may, in addition, be circuit components on the opposite second surface 108. The person skilled in the art will appreciate that a CMOS circuit comprises both doped regions of the substrate and metallisation layers and interconnections formed on the first surface of the substrate. The substrate has a DRIE etched aperture 110. This aperture may also be formed using an anisotropic etch with slanted side walls. A plurality of layers shown generally as 112, 114 are formed on the first surface of the substrate. Layer 112 is the CMOS metallization layer and comprises metal conductive traces and a passivation insulator such as SiO.sub.2, SiN, SiON. All or some of these layers may (or may not) extend across the aperture 110 to form a piezoelectric actuator element 118 comprising a piezoelectric body 120 which in this examples is formed of AIN or ScAIN but may be formed of another suitable piezoelectric material which is processable at a temperature of below 450° C. The piezoelectric actuator element forms a diaphragm with layers 115 of materials such as silicon, silicon oxide, silicon nitride or derivatives thereof and has a passivation layer 113 which prevents applied electrical potentials from contacting fluid.

[0127] At least one metallisation layer 112 includes interconnects, conducting signals from the external controller 20 to the control circuit and from the control circuit to the piezoelectric actuator element, in particular to first and second electrodes (not shown in FIG. 2) arranged to apply an electrical potential difference across and thereby actuate the piezoelectric body.

[0128] The piezoelectric actuator element 118 defines a wall of a fluid chamber 122 which receives ink (in the case of an inkjet printer) or another printable fluid (for example in the case of an additive manufacturing printer) through a conduit 124 and which is in communication with a nozzle 126 for ejecting liquid. The conduit is defined by a channel defining layer 128 mounted to the layers on the surface of the substrate, which may for example be defined by DRIE etching of silicon substrates and or wafer bonding, and a nozzle defining layer 130 provides the external surface of the printhead and has apertures which define the nozzles 126. The piezoelectric actuator element 118, chamber 122 and nozzle 126 together form a droplet ejector shown generally as 101.

[0129] FIG. 3 shows more details of the CMOS/actuator substrate and electrical connections of the droplet ejector chip 100 of FIG. 2. The CMOS control circuit comprises patterned regions of doped silicon 132 and metallisation layers 134. The number of metallisation layers depends on the complexity of the CMOS control circuit but three layers should suffice for many applications. Metallization layer 112 extends from contact pads 136 where a cable 138 connects to the CMOS control circuit which are subsequently connected to first and second electrodes 140, 142 located on and in contact with opposite sides of the piezoelectric body 140. Although two electrodes are shown here, there may be two or more electrodes on either side of, or different regions of, the piezoelectric body.

[0130] With reference to FIGS. 4 and 5 which shows a printhead formed of a single droplet ejector chip 100 (functioning as the droplet ejector assembly) having multiple droplet ejectors (individual piezoelectric actuators, fluid chambers and droplet ejection outlets), flexible cable interconnect 138 with a limited number of signal conductors connects an external controller through wires 144 to a printhead assembly that comprises multiple droplet ejectors shown as 101, for ejecting inks of different colours. The droplet ejector chip with multiple droplet ejectors is typically formed from a single CMOS/actuator substrate. In these examples, as well as the main portion of CMOS control circuit 104, the CMOS control circuit includes separate circuit elements 104′ associated with each droplet ejector, which may for example comprise a latch and an ejector transistor for each piezoelectric actuator.

[0131] FIGS. 6 and 7 show the arrangement of flexible cable 144 and flexible cable interconnect 138 and droplet ejector chips 100 for a printhead having a single droplet ejector chip/substrate (FIG. 6) and for a printhead having a plurality of different droplet ejector chips having individual substrates (FIG. 7). Due to the integration of the control circuit in the substrate, the number of signal conductor may be less, and potentially much less, than the number of discrete actuators.

[0132] FIG. 8 is a block diagram of the control circuitry for a printhead according to the invention. Actuator control is distributed between a machine controller 220 and the CMOS circuit 104 within the droplet ejector chip 100. They are connected in part by conductors extending through a single or multiple flexible cable interconnects 138. Multiple actuators 120 are controlled by the application of potentials to their electrodes 140, 142. The machine controller comprises at least a processor 200, such as a microprocessor or microcontroller which has memory 202 storing relevant data and program code. A wired or wireless electronic interface 204 receives input data from an external device driver. One skilled in the art will appreciate that the machine controller may be distributed between a number of separate components or functional modules, such as one component which converts an image into a pixelated pattern for printing using a dither matrix, for example, and a separate component which converts the pixelated pattern into a print pattern for the different nozzles.

[0133] The machine controller may comprise at least one waveform generator and a voltage amplifier 208 which provides a continuous pattern of actuator control pulses (shown in FIG. 9) to the printhead through one or more drive signal conductors 210. A ground conductor 212 also extends from the machine controller to the droplet ejector chip 100. (Ground connections within printhead not shown for clarity). The processor 200 generates digital control signals 214 typically as a serial bus, and also transmit clock signals 216 to the printhead which serve to synchronise printing with movements of the printhead. The connector also provides voltage levels associated with the operational voltage of CMOS control electronics.

[0134] Within the printhead, contact pads 136 are connected to the conductors of the flexible connector and signals are routed through patterned metallised layer 112 to the CMOS control circuit 104 and from the CMOS control circuit to the electrodes 140, 142 which actuate individual piezoelectric bodies 120 within respective piezoelectric actuators. The control circuit 104 on substrate 102 comprises ejection switch circuit 220, including ejection transistors having outputs which are in direct electrical connection with the electrodes 140, 142 (i.e. without a further intervening switching semiconductor junction). The ejection switch circuit switches the actuator control pulse signals and if one of the electrodes remains connected to ground, the ejection switch circuit may be as simple as single transistor per actuator, or a single transistor per electrode to switch the signal applied to that electrode. The ejection switch circuit may be distributed around the substrate with a portion (e.g. a transistor or transistor and latch) proximate each droplet ejector, corresponding to feature 104′ of FIGS. 4 and 5.

[0135] The ejection switch circuit does not carry out power amplification. Instead it switches the actuator control pulses, determining whether each pulse is relayed to the respective actuator or not, for each pulse. Voltage amplification is carried out in the machine controller by amplifier 208.

[0136] The ejection switch circuit is controlled by latch and shift transistors 222, which receive and store digital data from a control circuit 224 which processes received data, for example converting received serial data, storing these in registers 226 and using the received data to determine which actuators are to actuate during each successive actuator firing events. The control circuit 228 also stores trim data used to customise the precise timing of voltage switching for each actuator, which is typically determined during a calibration step on set-up, and may store configuration data 230 which indicates the physical layout of nozzles, security information and or nozzle actuation count history information. The control circuit 224 also receives data from sensors 232, 234, 236, some of which are associated with individual actuators, for example nozzle fill levels sensors, and some of which sense parameters relevant to the function of the printhead as a whole, for example temperature sensors.

[0137] FIG. 9 shows three possible drive waveforms generated by waveform generator or voltage amplification 206 in alternative embodiments. The x axis is time (in milliseconds) and the y axis is potential per .Math.m thickness of actuator. As the piezoelectric bodies are made of a non-ferroeletric material in this example the pulses may be applied in either direction. In FIG. 9(a) the signals have a default voltage of 0 and in each pulse are switch to a positive potential and back to zero after a predetermined period of time. In FIG. 9(b) the signals have a default voltage of 0 and are switched first to a positive potential (to cause the piezoelectric actuator to deform in one direction) and then to a negative potential (to cause the piezoelectric actuator to deform in the opposite direction) before returning to zero. In FIG. 9(c) the signals have a default voltage of 200 V and are switched to a voltage of -200 V (causing the electric fields in the piezoelectric body to reverse in direction) before returning to 200 V.

[0138] During operation, the processor 200 receives printing data, such as bitmaps, in digital form through interface 204 and processes this data by known means to send a sequence of printing instructions through serial connection 216 to each droplet ejector chip. These printing instructions may be as detailed as instructions for each droplet ejector chip as to whether and when to eject a droplet during printing cycles. In one embodiment, the waveform generator generates repeating voltage pulses suitable for application to the electrodes of individual piezoelectric actuators. These are periodic with a time spacing which determines the time between droplet ejection events on the printhead. Alternatively, the voltage amplification, 208, may provide and maintain a single voltage level of multiple voltage levels to the printhead assembly. The ejection transistors within the droplet ejector chip will switch these voltages according to the CMOS control circuit.

[0139] As the waveform generator or generators are not located on the printhead and is used to drive numerous piezoelectric actuators, it or they can generate a significant amount of heat without causing problems. There are not substantial substrate space limitations so it or they may be relatively complex circuits adapted to carefully control the shape of the waveform, with selected, and optionally variable, slew rates, and the power amplifier may be selected to produce the desired voltage up to the maximum possible current requirement in the event that all actuators which may be actuated simultaneously be actuated together.

[0140] The control circuit 224 on an individual printhead substrate receives the printing instructions through serial connection 216 and processes these (for example converting from serial to parallel instructions). With reference to the clock signals 214, it is determined whether each individual piezoelectric actuator should be actuated to eject a droplet during each printing cycle and this data is loaded into latches 222. At an appropriate time during each printing cycle, the latched data is passed to the ejection switch circuit which thereby either switch the received printing waveform to the electrodes of the respective actuator element, causing it to carry out a droplet ejection cycle, or to not do so in which case both electrodes of the respective actuator element remain connected to ground and the droplet ejector does not carry out a droplet ejection cycle.

[0141] Sensors 232, 234, 236 are monitored during printing. The precise timing of switching the received printing waveform to the electrodes of the respective actuator element can be varied responsive to a measure of temperature using a temperature sensitive CMOS element.

[0142] Each nozzle may have slightly different ejection characteristic behaviour (drop volume, velocity) based on variance in wafer manufacturing (on a single wafer - or between wafer lots), due to printhead assembly, due to actuation lifetime. This data can be used to alter the drive waveform for specific nozzles by the CMOS control circuit - for example - changing the actuation pulse duration or switching to a different level - or to switch certain nozzles to different drive waveforms.

[0143] The viscosity and surface tension of some inks is highly sensitive to temperature - this ultimately changes the droplet ejection characteristics. Certain print patterns will result in certain nozzles firing continuously whereas others fire sporadically. This will result in a variable heat pattern. The monitored temperature can be used by the control circuit to modify waveforms and/or feedback control information to the controller for appropriate action such as reducing print speed etc.

[0144] The shift registers move the droplet fire pattern information through to the latch registers. Thus, the shift registers interface with the serial connection, and move all print data to the latch registers in a given print cycle. The latch registers interface with the ejection registers to initiate a print command.

[0145] The droplet ejector chips are made by first forming the CMOS control circuit 104, 134 and the metal interconnect layer 112 on the substrate 102. The CMOS circuit is formed by standard CMOS processing methodologies including ion implantation on a p-type or n-type substrate and the interconnect later is also formed by standard processes such as ion implantation, chemical vapour deposition, physical vapour deposition, etching, chemical-mechanical planarization and/or electroplating.

[0146] Additional layers of material are formed on the substrate, including the electrodes 140 and 142, with an intervening piezoelectric body using successive thin film deposition techniques. Each step must avoid damage to the CMOS control circuit. The piezoelectric body is formed of a material such as AIN or ScAIN which may be deposited at a temperature below 450° C. by PVD (including low-temperature sputtering). Electrodes are formed of, for example titanium, platinum, aluminium, tungsten or alloys thereof. Fluid channels and apertures through the substrate may be formed using etching procedures such as DRIE. Channel defining layer 128 may be formed using DRIE etch and wafer bonding of silicon MEMS substrates. The nozzle defining layer can be formed of metal, silicon MEMS wafer or a plastics material by deposition on or adhesion to the channel defining later. Each droplet ejector chip is connected to the machine controller via a flexible interconnect. In contrast to prior art devices according to FIG. 1, the number of discrete conductors in the flexible interconnect is limited, for example 4 to 16 conductors.

[0147] The material from the which the piezoelectric body is formed cannot be and is not PZT due to the requirement to avoid damaging the CMOS control circuit upon which the piezoelectric actuator, including the piezoelectric body is formed. Accordingly, the piezoelectric actuator has a piezoelectric constant d.sub.31 which is much lower, usually at least one and potentially two orders of magnitude, less than PZT depending on its precise composition. On the face of it, this would make it impossible for the printhead ejector to operate properly. However, we have found that it is nevertheless possible for the printhead ejector to operate because: [0148] piezoelectric materials such as AIN, ScAIN and ZnO can have a higher breakdown voltage than PZT, and so may be operated with a higher potential gradient, allowing a corresponding force to be applied to the actuator; [0149] piezoelectric materials such as AIN, ScAIN and ZnO can have a higher Young’s modulus than PZT, increasing the force which they can exert; [0150] in some embodiments, the actuator control pulses may be generated off chip and switched by transistors with the control circuit on the substrate supporting the piezoelectric actuator, enabling relatively high voltages to be applied when required to the piezoelectric bodies; [0151] some piezoelectric materials other than PZT are non-ferroelectric and so are actuated in different directions by electric fields in opposite directions, enabling a greater change in electric field (from a negative field strength to a positive field strength or vice versa), which increases the variation in the forces applied to the actuator during a printing cycle.

[0152] The droplet ejector chips may have alternative configurations and several are shown in FIGS. 10 through 12, where features corresponding to those which have already been described are labelled with corresponding numbers. In the embodiments of FIGS. 10 and 11, there is a through silicon hole, formed through the silicon substrate 102 for example using a DRIE etch or anisotropic etch procedure. The fluid chamber 122 extends into the substrate and the head volume 110 provides a vent for air flow during actuation.

[0153] Referring back to FIGS. 4 and 5, the flexible interconnect may be mounted to an edge of a printhead and used to drive several or many individual droplet ejector chips, for example droplet ejector chips for different colours of ink (or other materials in the case of an additive printer) or droplet ejectors for different colours of ink (or other materials) may all be formed in a single continuous substrate in an individual droplet ejector chip.

[0154] In an alternative embodiment, instead of the machine controller including a waveform generator and the waveform being conducted to the droplet ejector assembly and the CMOS control circuit thereon, the CMOS control circuit actuates the piezoelectric actuators, causing droplet ejection, by switching the voltage applied to one or more of the electrodes of each piezoelectric actuator, for example between ground and a fixed voltage, or between multiple fixed voltage levels, one or more of which may be ground. In this case, the flexible connector 138 contains one or more electrical conductors carrying a fixed voltage from the machine controller to the droplet ejector chip.