Abstract:
Methods, systems, and apparatus, for drop ejection, specifically, for driving drop ejectors using n-type double-diffused metal oxide semiconductor (NDMOS) transistors with sputtered piezoelectric transducers. In general, in one aspect, an apparatus includes a n-type double-diffused metal oxide semiconductor transistor. The apparatus also includes a piezoelectric transducer. A first surface of the piezoelectric transducer is coupled to the n-type double-diffused metal oxide semiconductor transistor. The apparatus also includes a first waveform generator configured to generate an ejector waveform to apply to a second surface of the piezoelectric transducer. The ejector waveform includes at least a positive pulse and a negative pulse. The apparatus also includes a second waveform generator configured to generate a control waveform to apply to the n-type double-diffused metal oxide semiconductor transistor to selectively actuate the piezoelectric transducer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the national stage of International Application Number PCT/US2009/042556, filed on May 1, 2009, which is based on and claims the benefit of the filing date of U.S. Provisional Application No. 61/055,108, filed on May 21, 2008, both of which as filed are incorporated herein by reference in their entireties. 
    
    
     BACKGROUND 
     This specification relates to drop ejection. 
     Some types of conventional inkjet printers use piezoelectrically-based drop ejectors to controllably eject drops of ink onto a medium (e.g., a sheet). Some drop ejector drivers use p-type double-diffused metal oxide semiconductor (PDMOS) transistors or analog switches to selectively drive the drop ejectors. In some types of devices, the size of the transistors and the analog switches cause the drop ejector drivers to be too large to integrate into the available footprint for the drop ejectors. Thus, it can be desirable to find alternative ways to drive these types of devices. 
     SUMMARY 
     This specification describes technologies relating to drop ejection, and in specific implementations, to driving drop ejectors using n-type double-diffused metal oxide semiconductor (NDMOS) transistors with sputtered piezoelectric transducers. 
     In general, in one aspect, an apparatus includes a n-type double-diffused metal oxide semiconductor transistor. The apparatus also includes a piezoelectric transducer. A first surface of the piezoelectric transducer is coupled to the n-type double-diffused metal oxide semiconductor transistor. The apparatus also includes a first waveform generator configured to generate an ejector waveform to apply to a second surface of the piezoelectric transducer. The ejector waveform includes at least a positive pulse and a negative pulse. The apparatus also includes a second waveform generator configured to generate a control waveform to apply to the n-type double-diffused metal oxide semiconductor transistor to selectively actuate the piezoelectric transducer. Other implementations of this aspect include corresponding systems, methods, and computer program products. 
     These and other implementations can optionally include one or more of the following features. The first waveform generator is coupled to the second surface of the piezoelectric transducer, and the second waveform generator is coupled to the gate of the NDMOS transistor. The amplitude of the negative pulse is small enough to avoid ink discharge from a drop ejector driven by the piezoelectric transducer. The first surface of the piezoelectric transducer has a positive polarization. The gate of the NDMOS transistor is coupled to the second waveform generator. The drain of the NDMOS transistor is coupled to the first surface of the piezoelectric transducer. The source of the NDMOS transistor is coupled to ground. The NDMOS transistor includes an intrinsic diode. The anode of the intrinsic diode is coupled to the source of the NDMOS transistor, and the cathode of the intrinsic diode is coupled to the drain of the NDMOS transistor. The piezoelectric transducer has a d 31  coefficient having an absolute value greater than or equal to 200 pico-meters per volt. 
     In general, in another aspect, an apparatus includes a n-type double-diffused metal oxide semiconductor transistor. The apparatus also includes a piezoelectric transducer. A first surface of the piezoelectric transducer is coupled to the n-type double-diffused metal oxide semiconductor transistor and a second surface of the piezoelectric transducer is coupled to ground. The apparatus also includes a first waveform generator configured to generate an ejector waveform to apply to the n-type double-diffused metal oxide semiconductor transistor. The ejector waveform includes at least a positive pulse and a negative pulse. The apparatus also includes a second waveform generator configured to generate a control waveform to apply to the n-type double-diffused metal oxide semiconductor transistor to selectively actuate the piezoelectric transducer. Other implementations of this aspect include corresponding systems, methods, and computer program products. 
     These and other implementations can optionally include one or more of the following features. The first waveform generator is coupled to the source of the NDMOS transistor, and the second waveform generator is coupled to the gate of the NDMOS transistor. The amplitude of the negative pulse is small enough to avoid ink discharge from a drop ejector driven by the piezoelectric transducer. The first surface of the piezoelectric transducer has a positive polarization. The gate of the NDMOS transistor is coupled to the second waveform generator. The drain of the NDMOS transistor is coupled to the first surface of the piezoelectric transducer. The source of the NDMOS transistor is coupled to the first waveform generator. The NDMOS transistor includes an intrinsic diode. The anode of the intrinsic diode is coupled to the source of the NDMOS transistor, and the cathode of the intrinsic diode is coupled to the drain of the NDMOS transistor. The piezoelectric transducer has a d 31  coefficient having an absolute value greater than or equal to 200 pico-meters per volt. 
     In general, in another aspect, a method includes selectively actuating the piezoelectric transducer of the apparatuses above, using the ejector waveform and the control waveform to drive a drop ejector. Other implementations of this aspect include corresponding systems, apparatus, and computer program products. 
     In general, in another aspect, an apparatus includes a circuit. The circuit includes a n-type double-diffused metal oxide semiconductor transistor and a piezoelectric transducer. A first surface of the piezoelectric transducer is coupled to the n-type double-diffused metal oxide semiconductor transistor. The apparatus also includes a first waveform generator configured to generate a control waveform to apply to the n-type double-diffused metal oxide semiconductor transistor to selectively actuate the piezoelectric transducer. The apparatus also includes a second waveform generator configured to apply an ejector waveform across the circuit. The ejector waveform includes at least a positive pulse and a negative pulse. Other implementations of this aspect include corresponding systems, methods, and computer program products. 
     These and other implementations can optionally include one or more of the following features. The amplitude of the negative pulse is small enough to avoid ink discharge from a drop ejector driven by the piezoelectric transducer. The first surface of the piezoelectric transducer has a positive polarization. The gate of the NDMOS transistor is coupled to the first waveform generator. The drain of the NDMOS transistor is coupled to the first surface of the piezoelectric transducer. The source of the NDMOS transistor is coupled to ground. The gate of the NDMOS transistor is coupled to the first waveform generator. The drain of the NDMOS transistor is coupled to the first surface of the piezoelectric transducer. The source of the NDMOS transistor is coupled to the second waveform generator. The NDMOS transistor includes an intrinsic diode. The anode of the intrinsic diode is coupled to the source of the NDMOS transistor, and the cathode of the intrinsic diode is coupled to the drain of the NDMOS transistor. The piezoelectric transducer has a d 31  coefficient having an absolute value greater than or equal to 200 pico-meters per volt. 
     Particular embodiments of the subject matter described in this specification can be implemented to realize none, one or more of the following advantages. Using NDMOS transistors with sputtered piezoelectric material to drive the transducers can allow the size of drop ejector drivers to be reduced. By reducing the size of the drivers, the drivers can be integrated into the drop ejectors. In particular, NDMOS transistors are smaller than some other types of switching devices that are used in drop ejector drivers. NDMOS transistors can use less silicon than the conventional switching devices (e.g., about half as much silicon as conventional PDMOS transistors, and less than half as much silicon as conventional analog switches). In addition, using thin film piezoelectric layers can allow the size of the drop ejector drivers to be reduced. Reducing the size of the drop ejector drivers can also reduce the costs (e.g., manufacturing cost) of the drop ejector drivers. 
     Furthermore, using bipolar pulses to actuate the sputtered piezoelectric material can reduce the required peak voltage across the NDMOS transistors in the drop ejector drivers. Bipolar pulses may also require a lower quiescent voltage and improve the frequency response of waveforms used to actuate the sputtered piezoelectric material, thereby improving the long term stability of the drop ejectors (e.g., because the uniformity of the velocity generated by the drop ejectors is improved). 
     The details of one or more embodiments of the subject matter described in this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an exemplary droplet ejector unit in a fluid ejector. 
         FIG. 2A  is a schematic diagram of an exemplary circuit for driving a droplet ejector unit of a printhead module. 
         FIG. 2B  is a schematic diagram that includes an exemplary printhead module driver. 
         FIG. 2C  is a schematic diagram that includes another exemplary printhead module driver. 
         FIG. 3  is a diagram of exemplary ejector waveforms and an exemplary control waveform. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic diagram of an exemplary droplet ejector unit in a fluid ejector (e.g., a printhead module  100 ). The fluid to be ejected can be an ink, but other fluids, e.g., electroluminescent materials, biological compounds, or materials for formation of electrical circuits, can be jetted from the fluid ejector. 
     An inkjet printer (not shown) can include printhead units (not shown), and the printhead unit can include multiple droplet ejector units (only one is illustrated in  FIG. 1 ). The printhead units can include printhead modules  100  that can controllably eject drops of ink on a sheet, or a portion of the sheet, to print an image. For example, the image can be printed by selectively jetting ink from the printhead units as the inkjet printhead and the sheet move relative to one another. 
     The printhead module  100  includes a module substrate  110  and a piezoelectric actuator structure  120  (e.g., an actuator including lead zirconium titanate (“PZT”) or another piezoelectric material, and electrodes). The module substrate  110  can be a monolithic semiconductor body such as a silicon substrate. The module substrate  110  can include an actuator membrane  122 . In some implementations, the actuator membrane  122  can also be a part of the piezoelectric actuator structure  120 . For each droplet ejector unit, passages through the silicon substrate define a flow path for the fluid to be ejected, e.g., ink. Each flow path can include an ink inlet  112 , a pumping chamber  114 , and a nozzle  118 . A piezoelectric actuator structure  120  is positioned over the pumping chamber  114 . Ink flows through the ink inlet  112  to the pumping chamber  114 , where, when a voltage pulse is applied across a piezoelectric material in the piezoelectric actuator structure  120 , the ink is pressurized such that it is directed to a descender  116  and out of the nozzle  118 . These etched features can be configured in a variety of ways. 
     The piezoelectric actuator structure  120  includes a ground electrode layer  124 , a piezoelectric layer  126 , and a drive electrode layer  128 . The piezoelectric layer  126  is a thin film of piezoelectric material. For example, the thin film can range in thickness from 1 to 25 microns. The piezoelectric layer  126  can be composed of a piezoelectric material that has desirable properties such as high density, low voids, and high piezoelectric coefficients. The actuator membrane can be formed from silicon. 
     In some implementations, the thin film of piezoelectric material is deposited by sputtering. Types of sputter deposition can include magnetron sputter deposition (e.g., RF sputtering), ion-beam sputtering, reactive sputtering, ion-assisted deposition, high-target-utilization sputtering, and high power impulse magnetron sputtering. Sputtered piezoelectric material (e.g., piezoelectric thin film) can have a large as-deposited polarization. Some types of chambers that are used for sputtering piezoelectric material apply a direct current (DC) field during sputtering. The DC field causes the piezoelectric material to be polarized in the orientation that results when a negative voltage is applied to the exposed side of the piezoelectric material. 
     Piezoelectric coefficients can be used as a measure of polarization. Piezoelectric coefficients (e.g., “d” coefficients) of thin films relate to the strain of a piezoelectric material produced by an applied electrical field. In particular, a transverse piezoelectric coefficient d 31  relates the strain of the piezoelectric material perpendicular to the polarization axis to the electric field that is applied to the piezoelectric material parallel to the polarization axis. A sputtered piezoelectric material can have a d 31  absolute value greater than or equal to about 100 pico-meters per volt, e.g., greater than or equal to about 200 pico-meters per volt. For example, a sputtered piezoelectric material can have a d 31  absolute value of about 225 to 250 pico-meters per volt. 
     The piezoelectric layer  126  with the ground electrode layer  124  on one side is fixed to the actuator membrane  122 . The actuator membrane  122  isolates the ground electrode layer  124  and the piezoelectric layer  126  from ink in the pumping chamber  114 . The actuator membrane  122  can be silicon and has a compliance selected so that actuation of the piezoelectric layer  126  causes flexing of the actuator membrane  122  that is sufficient to pressurize fluid in the pumping chamber  114 . 
     The piezoelectric layer  126  changes geometry, or bends, in response to an applied voltage (e.g., a voltage applied at the drive electrode layer  128 ). The bending of the piezoelectric layer  126  pressurizes fluid in the pumping chamber  114  to controllably force ink through the descender  116  and eject drops of ink out of the nozzle  118 . 
       FIG. 2A  is a schematic diagram of an exemplary circuit  200  for driving a droplet ejector unit of a printhead module (e.g., the printhead module  100 ). In some implementations, the circuit is external to the printhead module. In some implementations, the circuit is integrated into the printhead module. In particular, the circuit  200  is a circuit schematic of a NDMOS transistor. The circuit  200  includes a transistor  210  coupled to a diode  220  (e.g., an intrinsic diode of the NDMOS transistor). The anode of the diode  220  is coupled to the source of the transistor  210 , and the cathode of the diode  220  is coupled to the drain of the transistor  210 . The source, gate, and drain of the transistor  210 , correspond to the source, gate, and drain, respectively, of the NDMOS transistor. 
     In some implementations, one or more instances of circuit  200  can be fabricated on an integrated circuit element, e.g., one per fluid ejector unit to be controlled by the integrated circuit element. For example, the integrated circuit element can be attached to a die that includes greater than or equal to 1,024 nozzles. The die can be, for example, in the shape of a parallelogram that includes sides with dimensions of about 15 millimeters and 45 millimeters. In some alternative implementations, because the size of the circuit  200  can be reduced, the circuit  200  can be integrated directly onto the die. 
     Because the current between the drain and source of a transistor is limited by the voltage at the gate of the transistor, the transistor can be used as a switch. In particular, the circuit  200  is used as a switch to controllably actuate a piezoelectric actuator structure to drive a printhead module. In addition, the voltage applied to the gate of the transistor  210  can also be used to limit the current through the drain of the transistor  210 . 
       FIG. 2B  is a schematic diagram that includes an exemplary printhead module driver  280 . The printhead module driver  280  includes the circuit  200  and a piezoelectric actuator structure  285  (e.g., a PZT). In some implementations, the drain of the transistor  210  is coupled to the piezoelectric actuator structure  285  (e.g., at the drive electrode layer  128  of the piezoelectric actuator structure  120  as shown in  FIG. 1 ). The drain of the transistor  210  can be coupled to the electrode on a surface of the piezoelectric actuator structure  285  that had a negative voltage applied to it during poling (e.g., the surface of the piezoelectric transducer that has a positive polarization). This coupling facilitates over current detection. The other electrode of the piezoelectric actuator structure  285  (e.g., the ground electrode  124 ) is further coupled to a waveform generator  230  configured to generate an ejector waveform. The gate of the transistor  210  is coupled to a waveform generator  240  configured to generate a control waveform. The source of the transistor  210  is coupled to ground. 
       FIG. 2C  is a schematic diagram that includes another exemplary printhead module driver  290 . The printhead module driver  290  includes the circuit  200  and a piezoelectric actuator structure  285 . In some implementations, the drain of the transistor  210  is coupled to one electrode of the piezoelectric actuator structure  285  (e.g., at the drive electrode layer  128  of the piezoelectric actuator structure  120 ). The other electrode of the piezoelectric actuator structure  285  is further coupled to ground (e.g., at the ground electrode layer  124  of the piezoelectric actuator structure  120 ). The gate of the transistor  210  is coupled to the waveform generator  240  configured to generate a control waveform. The source of the transistor  210  is coupled to the waveform generator  230  configured to generate an ejector waveform. 
     Thus, in  FIGS. 2B and 2C , droplet ejection from different nozzles can be individually controlled by applying different control waveforms to the individual circuits  200  for each fluid ejector unit. However, the same ejection waveform can be applied to each fluid ejector unit. 
       FIG. 3  is a diagram of exemplary ejector waveforms  300  and  310  and an exemplary control waveform  350 . In particular, the ejector waveform  300  is experienced by printhead modules that are in the on condition (e.g., printhead modules that are selected to fire). The ejector waveform  310  is experienced by printhead modules that are in the off condition (e.g., printhead modules that are not selected to fire). The ejector waveform  310  can be substantially the same as the ejector waveform  300 , except during a positive pulse  324  of the ejector waveform  300  (e.g., as represented by the dotted line of ejector waveform  310  in  FIG. 3 ). During a positive pulse  324  of the ejector waveform  300 , the ejector waveform  310  can be at a lower voltage than the ejector waveform  300 . For example, the ejector waveform  310  can be at a reference voltage (e.g., a quiescent voltage). In some implementations (e.g., the implementation of  FIG. 2B ), the ejector waveform  300  and the control waveform  350  are applied to a piezoelectric actuator structure to produce acceptable drop ejection by the printhead modules. In some implementations (e.g., the implementation of  FIG. 2C ), the ejector waveform  300  is inverted and the inverted ejector waveform and the control waveform  350  are applied to the source of an NDMOS transistor to produce acceptable drop ejection by the printhead modules. 
     The ejector waveform  300  can include one or more firing pulses  320 . In some implementations, the firing pulses  320  of the ejector waveform  300  are bipolar pulses. A bipolar pulse includes a negative pulse  322  and a positive pulse  324 . The negative pulse  322  is a pulse with voltages that are less than a reference voltage (e.g., a quiescent voltage). The positive pulse  324  is a pulse with voltages that are greater than the reference voltage. In some implementations, the reference voltage is about one third of the operating voltage of the ejector. For example, if the operating voltage of the ejector is 20V, then the reference voltage can be about 7V. The positive pulse  324  causes the piezoelectric actuator structure to contract the pumping chamber to eject ink. 
     The negative pulse  322  can be used to expand a pumping chamber so that it can be filled with ink. During the negative pulse  322 , a drive current is pulled out of an application specific integrated circuit (e.g., the circuit  200 ). The drive current creates a voltage at the piezoelectric actuator structure that expands the pumping chamber. Because the drive current conducts through the intrinsic diode of an NDMOS transistor, the printhead modules will have a negative pulse applied to the piezoelectric actuator whether or not the gate voltage is high. Therefore, the negative pulse should have an amplitude small enough to avoid causing ink discharge (e.g., drop ejection or oozing) from printhead modules that are in the off condition. 
     Optionally, the flat component between the negative pulse and the following positive pulse allows the NDMOS transistors to turn off. However, the flat component is not required if the printhead module drivers can be turned off with enough accuracy so that the voltage uncertainty in the printhead modules in the off condition is small (e.g., about 1 volt). In addition, the flat component between the positive pulse and the following negative pulse is not required. In particular, the flat component does not affect printhead modules in the on condition, and the flat component is not required for printhead modules in the off condition because the diode will turn on automatically (e.g., forward bias). 
     In some implementations, a control waveform  350  includes high components  352  and low components  354 . The high components  352  and low components  354  can be applied to the circuit  200  of  FIG. 2A . The high components  352  can turn on the printhead modules because the gates of the transistors  210  are high. The high components  352  allow the printhead modules that are not selected to fire to conduct current during the rising edge of a negative pulse  322 . Without the use of high components  352  of a control waveform  350 , a voltage would develop across the NDMOS transistor during the rising edge of a negative pulse  322  that would persist to the end of the ejector waveform  300 . The control waveform  350  can be used to discharge the voltage before a printhead module is selected to fire. 
     The high components  352  are applied to the transistor  210  to enable corresponding components of the ejector waveform  300  to actuate the piezoelectric actuator structure. The low components  354  are applied to the transistor  210  to prevent the ejector waveform  300  from actuating the piezoelectric actuator structure. In some implementations, the low components  354  can be zero volts. In some implementations, the low components  354  can be greater than zero volts if the low components  354  can cause the piezoelectric actuator structure  285  to experience negative pulses that are small enough in amplitude to avoid ink discharge from the printhead modules that are not selected to fire. 
     In particular, the control waveform  350  can be used to selectively turn on the NDMOS transistor. The control waveform  350  can prevent the diode  220  from rectifying a current and producing a net voltage across the NDMOS transistor. 
     While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any invention or of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments of particular inventions. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Particular embodiments of the subject matter described in this specification have been described. Other embodiments are within the scope of the following claims. For example, the actions recited in the claims can be performed in a different order and still achieve desirable results. As one example, the processes depicted in the accompanying figures do not necessarily require the particular order shown, or sequential order, to achieve desirable results. In certain implementations, multitasking and parallel processing may be advantageous.