Abstract:
A micro-fluid ejection device for ultra-small droplet ejection and method of making a micro-fluid ejection device. The micro-fluid ejection device includes a semiconductor substrate containing a plurality of thermal ejection actuators disposed thereon. Each of the thermal ejection actuators includes a resistive layer and a protective layer for protecting a surface of the resistive layer. The resistive layer and the protective layer together define an actuator stack thickness. The actuator stack thickness ranges from about 500 to about 2000 Angstroms and provides an ejection energy per unit volume of from about 10 to about 20 gigajoules per cubic meter. A nozzle plate is attached to the semiconductor substrate to provide the micro-fluid ejection device.

Description:
FIELD OF THE DISCLOSURE 
     The disclosure relates to micro-fluid ejection devices and in particular to ultra-low energy devices for ejecting ultra-small liquid droplets. 
     BACKGROUND AND SUMMARY 
     Since the inception of thermal fluid ejection devices, the size of droplets ejected by the devices has continually decreased. For the production of printed images by the ejection of inks, the droplet size need not be decreased below about 10 femtoliters (0.01 picoliters) as the spot size provided by such droplet is about 3 microns in diameters. Human vision measurements have shown that spot sizes of 42 microns are easily detectable, whereas spot sizes of less than 28 microns were substantially undetectable. Only about 0.07% of people can detect a spot size of about 20 microns, and less than 1 person per million can see a 3 micron spot. Nevertheless, fluid droplets of 10 femtoliters or less may be suitable for other non-printing applications including, but not limited to, pharmaceutical applications, electronics fabrication, and other applications where visual detection of spots of fluid on a media are not required. 
     One of the challenges for producing micro-fluid ejection devices for ultra-small droplets is the ability to provide high frequency droplet ejection without a substantial increase in wasted heat energy. For example, an ejection head containing 9000 nozzles operating at a frequency of 200 KHz and requiring 0.08 microjoules of energy per activation may require 144 watts of precisely regulated power resulting in about 0.125 picloliters per microjoule of energy. Such a power requirement results in a significant amount of wasted heat energy. 
     In order to reduce the amount of wasted heat energy for micro-fluid ejection devices for ultra-small fluid ejection, unique ejection devices and manufacturing techniques are needed. 
     With regard to the above, embodiments of the disclosure provides a micro-fluid ejection device for ultra-small droplet ejection and method of making a micro-fluid ejection device. The micro-fluid ejection device includes a semiconductor substrate containing a plurality of thermal ejection actuators disposed thereon. Each of the thermal ejection actuators includes a resistive layer and a protective layer for protecting a surface of the resistive layer. The resistive layer and the protective layer together define an actuator stack thickness. The actuator stack thickness ranges from about 500 to about 2000 Angstroms and provides an ejection energy per unit volume of from about 10 to about 20 gigajoules per cubic meter. A nozzle plate is attached to the semiconductor substrate to provide the micro-fluid ejection device. 
     In another embodiment there is provided a method of ejecting ultra-small fluid droplets on demand. The method includes providing a micro-fluid ejection device containing a resistive layer and a protective layer on the resistive layer. In combination, the resistive layer and protective layer define a thermal actuator stack. The thermal actuator stack has a thickness ranging from about 1000 to about 2500 Angstroms and a thermal actuator stack volume ranging from about 1 cubic micron to about 5.4 cubic microns. An electrical energy is applied to the thermal actuator stack sufficient to eject less than about 10 femtoliters of fluid from the micro-fluid ejection device with a pumping effectiveness of greater than about 125 femtoliters per microjoule to provide a fluid spot size ranging from about 1 up to about 3 microns on a substantially non-porous surface. 
     An advantage of embodiments of the disclosure is that apparatus for delivery of ultra-small volumes of liquids may be provided for use in electrical fabrication, pharmaceutical delivery, biotechnology research applications, and the like. Another advantage of the embodiments is that the methods may provide ultra-small volume delivery devices that may be fabricated in existing micro-fluid ejection device fabrication facilities. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages of the embodiments will become apparent by reference to the detailed description of preferred embodiments when considered in conjunction with the drawings, wherein like reference characters designate like or similar elements throughout the several drawings as follows: 
         FIG. 1  is a cross-sectional view, not to scale, of a portion of a prior art micro-fluid ejection head; 
         FIG. 2  is a graphical representation of jetting energy versus protective layer thickness for micro-fluid ejection heads; 
         FIG. 3  is a graphical representation of estimated substrate temperature rise versus input power for ejection head pumping effectiveness; 
         FIG. 4  is a cross-sectional view, not to scale, of a portion of a micro-fluid ejection head according to an embodiment of the disclosure; 
         FIG. 5  is a perspective view of a fluid cartridge containing a micro-fluid ejection head according to the disclosure; and 
         FIG. 6  is a schematic drawing of a control device for controlling a micro-fluid ejection head according to the disclosure. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In accordance with embodiments described herein, micro-fluid ejection actuators for micro-fluid ejection devices having improved operating characteristics for ultra-small drop volumes will now be described. 
     For the purposes of this disclosure, the term “ultra-small” is intended to include fluid droplets of less than about 10 femtoliters. The terms “heater stack”, “ejector stack”, and “actuator stack” are intended to refer to an ejection actuator having a combined layer thickness of a resistive material layer and passivation or protection material layer. The passivation or protection material layer is applied to a surface of the resistive material layer to protect the actuator from chemical or mechanical corrosion or erosion effects of fluids ejected by the micro-fluid ejection device. 
     With reference to  FIG. 1 , a cross-sectional view, not to scale, of a portion of a prior art micro-fluid ejection head  10  is illustrated. The view of  FIG. 1  shows one of many fluid ejection actuators  12 . The fluid ejection actuators  12  are formed on a semiconductor silicon substrate  14  containing a thermal insulating layer  16  between the silicon substrate  14  and the ejection actuators  12 . The fluid ejection actuators  12  may be formed from an electrically resistive material layer  18 , such as TaAl, Ta 2 N, TaAl(O,N), TaAlSi, TaSiC, Ti(N,O), Wsi(O,N), TaAlN, and TaAl/Ta. The thickness of the resistive material layer  18  may range from about 500 to about 1000 Angstroms. 
     The thermal insulation layer  16  may be formed from a thin layer of silicon dioxide and/or doped silicon glass overlying the relatively thick silicon substrate  14 . The total thickness of the thermal insulation layer  16  is preferably from about 1 to about 3 microns thick. The underlying silicon substrate  14  may have a thickness ranging from about 0.5 to about 0.8 millimeters thick. 
     A protective layer  20  overlies the fluid ejection actuators  12 . The protective layer  20  may be a single material layer or a combination of several material layers. In the illustration in  FIG. 1 , the protective layer  20  includes a first passivation layer  22 , a second passivation layer  24 , and a cavitation layer  26 . The protective layer  20  is effective to prevent the fluid or other contaminants from adversely affecting the operation and electrical properties of the fluid ejection actuators  12  and provides protection from mechanical abrasion or shock from fluid bubble collapse. 
     The first passivation layer  22  may be formed from a dielectric material, such as silicon nitride, or silicon doped diamond-like carbon (Si-DLC) having a thickness of from about 1000 to about 3200 Angstroms thick. The second passivation layer  24  may also be formed from a dielectric material, such as silicon carbide, silicon nitride, or silicon-doped diamond-like carbon (Si-DLC) having a thickness preferably from about 500 to about 1500 Angstroms thick. The combined thickness of the first and second passivation layers  22  and  24  typically ranges from about 1500 to about 5000 Angstroms. 
     The cavitation layer  26  is typically formed from tantalum having a thickness greater than about 500 Angstroms thick. The cavitation layer  26  may also be made of TaB, Ti, TiW, TiN, WSi, or any other material with a similar thermal capacitance and relatively high hardness. The maximum thickness of the cavitation layer  26  is such that the total thickness of protective layer  20  is less than about 7200 Angstroms thick. The total thickness of the protective layer  20  is defined as a distance from a top surface  28  of the resistive material layer  18  to an outermost surface  30  of the protective layer  20 . An ejector stack thickness  32  is defined as the combined thickness of layers  18  and  20 . 
     The fluid ejection actuator  12  is defined by depositing and etching a metal conductive layer  34  on the resistive layer  18  to provide power and ground conductors  34 A and  34 B as illustrated in  FIG. 1 . The conductive layer  34  is typically selected from conductive metals, including but not limited to, gold, aluminum, silver, copper, and the like and has a thickness ranging from about 4,000 to about 15,000 Angstroms. 
     Overlying the power and ground conductors  34 A and  34 B is another insulating layer or dielectric layer  36  typically composed of epoxy photoresist materials, polyimide materials, silicon nitride, silicon carbide, silicon dioxide, spun-on-glass (SOG), laminated polymer and the like. The insulating layer  36  and has a thickness ranging from about 5,000 to about 20,000 Angstroms and provides insulation between a second metal layer  38  and conductive layer  34 . 
     Layers  14 ,  16 ,  18 ,  20 ,  34 ,  36 , and  38  provide a semiconductor substrate  40  for use in the micro-fluid ejection head  10 . In order to complete the ejection head  10 , a nozzle plate  42  is attached, as by an adhesive  44 , to the semiconductor substrate  40 . The nozzle plate  42  contains nozzle holes  46  corresponding the plurality of fluid ejection actuators  12 . A fluid in fluid chamber  48  is heated by the fluid ejection actuators  12  to form a fluid bubble which expels fluid from the fluid chamber  48  through the nozzle holes  46 . A fluid supply channel  50  provides fluid to the fluid chamber  48 . 
     One disadvantage of the micro-fluid ejection head  10  described above is that the multiplicity of protective layers  20  within the micro-fluid ejection head  10  increases the ejection stack thickness  32 , thereby increasing an overall jetting energy required to eject a drop of fluid through the nozzle holes  46 . 
     Upon activation of the fluid ejection actuator  12 , some of the energy ends up as waste heat energy used to heat the protective layer  20  via conduction, while the remainder of the energy is used to heat the fluid adjacent the surface  30  of the cavitation layer  26 . When the surface  30  reaches a fluid superheat limit, a vapor bubble is formed. Once the vapor bubble is formed, the fluid is thermally disconnected from the surface  30 . Accordingly, the vapor bubble prevents further thermal energy transfer to the fluid. 
     It is the thermal energy transferred into the fluid, prior to bubble formation, that drives the liquid-vapor change of state of the fluid. Since thermal energy must pass through the protective layer  20  before heating the fluid, the protective layer  20  is also heated. It takes a finite amount of energy to heat the protective layer  20 . The amount of energy required to heat the protective layer  20  is directly proportional to the thickness of the protective layer  20  and the thickness of the resistive layer  18 . An illustrative example of the relationship between the protective layer  20  thickness and jetting energy requirement for a specific fluid ejection actuator  12  size is shown in  FIG. 2 . 
     Jetting energy is important because it is related to power (power being the product of energy and firing frequency of the fluid ejection actuators  12 ). The temperature rise experienced by the substrate  40  is also related to power. Adequate jetting performance and fluid characteristics, such as print quality in the case of an ink ejection device, are related to the temperature rise of the substrate  40 . 
       FIG. 3  illustrates a relationship among the temperature rise of the substrate  40 , input power to the fluid ejection actuator  12 , and droplet size. The independent axis of  FIG. 3  has units of power (or energy multiplied by frequency). In  FIG. 3  the dependent axis denotes the temperature rise of the substrate  40 . The series of curves (A–G) represent varying levels of pumping effectiveness for fluid droplet sizes (in this example, ink droplet sizes) of 1, 2, 3, 4, 5, 6, and 7 picoliters respectively. Pumping effectiveness is defined in units of picoliters per microjoule. As can be seen from  FIG. 3 , it is desirable to maximize pumping effectiveness. For the smaller droplet sizes (curves A and B), very little power input results in a rapid rise in the substrate  40  temperature. As the droplet size increases (curves C–G), the temperature rise of the substrate  40  is less dramatic. When a certain substrate temperature rise is reached, no additional energy (or power) can be sent to the ejection head  10  without negatively impacting ejection actuator  12  performance. If the maximum of allowable temperature rise of the substrate  40  is surpassed, performance and print quality, in the case of an ink ejection head, will be degraded. 
     Because power equals the product of energy and frequency, and the substrate  40  temperature is a function of input power, there is thus a maximum jetting frequency for operation of such micro-fluid ejection actuators  12 . Accordingly, a primary goal of modern micro-fluid ejection head technology using the micro-fluid ejection actuators described herein is to maximize the level of jetting frequency while still maintaining the substrate  40  at an optimum temperature. While the optimum temperature of the substrate  40  varies due to other design factors, it is generally desirable to limit the substrate  40  temperature to about 75° C. to prevent excessive flooding of the nozzle plate  42 , air devolution, droplet volume variation, premature nucleation, and other detrimental effects. 
     With regard to the foregoing, providing the ejection head  10  with 9000 of the fluid ejection actuators  12  operating at a firing frequency of 200 KHz and requiring an energy of 0.08 microjoules per fire would require 144 watts of precisely regulated power. Such an ejection head  10  ejecting 10 femtoliters per fire would have a pumping effectiveness of 0.125 picoliters per microjoule. It will be appreciated from  FIG. 3 , that a pumping effectiveness of 0.125 picoliters per microjoule would result in an undesirable substrate temperature rise as the resulting curve would be to the left of curve A. Thus, there is a need for reducing the energy per fire in order to reduce power costs and improve the thermal performance of the ejection head. 
     The disclosed embodiments improve upon the prior art micro-fluid ejection head structures  10  by reducing the number layer and thickness of the protective layer  20  in the micro-fluid ejection head structure, thereby reducing a total ejection actuator stack thickness for a micro-fluid ejection head. A reduction in protective layer thickness translates into less waste energy and improved ejection head performance. Since there is less waste energy, jetting energy that was used to penetrate a thicker protective layer may now be allocated to higher jetting frequency while maintaining the same energy conduction as before to an exposed surface of the protective layer. 
     With reference to  FIG. 4 , a cross sectional view, not to scale, of a portion of a micro-fluid ejection head  60  containing a semiconductor substrate  62  and nozzle plate  64  according to the disclosure is provided. In the embodiment shown in  FIG. 4 , the nozzle plate  64  has a thickness ranging from about 5 to 65 microns and is preferably made from an fluid resistant polymer such as polyimide. Flow features such as fluid chambers  66 , fluid supply channels  68  and nozzle holes  70  are formed in the nozzle plate  64  by conventional techniques such as laser ablation. However, the embodiments are not limited by the foregoing nozzle plate  64 . In an alternative, the fluid chambers  66  and the fluid supply channels  68  may be provided in a thick film layer to which a nozzle plate is attached or the flow features may be formed in both a thick film layer and a nozzle plate. 
     Unlike the ejection head  10  illustrated in  FIG. 1 , the ejection head  60  according to the disclosure contains a single protective layer  72 . The protective layer  72  may be provided by a material selected from the group consisting of diamond-like carbon (DLC), titanium, tantalum, and an oxidized metal. For the purposes of ejecting fluid in the less than 10 femtoliter range, it is desirable for the protective layer to have a thickness ranging from about 100 to about 700 Angstroms. Such a protective layer  72  thickness provides an ejection actuator stack  74  having a thickness ranging from about 600 to about 1700 Angstroms. 
     In the case of a Ta—Al resistive layer  18 , the protective layer  72  may be provided by an oxidized an upper about 100 to about 300 Angstrom portion of the Ta—Al resistive layer  18 . Hence, the protective layer  72  may be provided by oxidizing the Ta—Al resistive layer  18  either by post deposition plasma, or in-situ by adding oxygen during the final moments of a sputtering deposition process for the resistive layer  18 . A thin oxide protective layer  72  may provide all of the cavitation protection needed for the ejection of ultra-small fluid droplets through nozzle holes  70 . 
     For example, an 800 Angstrom Ta—Al resistive layer  18  having a sheet resistance of about 28 ohms per square providing a ejection actuator  12  of about 1 square is provided. The ejection actuator  12  contains a 200 Angstrom oxidized protective layer  72  which may be effective to lower the applied current for the fluid ejection actuator  12  from about 45 milliamps to about 18 milliamps with a nucleation response similar to the nucleation response of the ejection head  10  illustrated in  FIG. 1 . In this example, the energy of the ejection actuator  12  is reduced from about 0.06 microjoules to about 0.01 microjoules, a six-fold improvement in ejection energy per fluid droplet. For an ejection actuator stack  74  having a volume ranging from about 1 cubic micron to about 6 cubic microns, the ejection energy per unit volume of the actuator stack  74  may range from about 10 to about 20 gigajoules per cubic meter. The pumping effectiveness for less than 10 femtoliter droplets may range from greater than about 125 femtoliters per microjoule to about 900 femtoliters per microjoule or more. 
     The micro-fluid ejection head  60  for ultra-small fluid droplets may be attached to a fluid supply cartridge  80  as shown in  FIG. 5 . As shown in  FIG. 5 , the ejection head  60  is attached to an ejection head portion  82  of the fluid cartridge  80 . A main body  84  of the cartridge  80  includes a fluid reservoir for supply of fluid to the micro-fluid ejection head  60 . A flexible circuit or tape automated bonding (TAB) circuit  86  containing electrical contacts  88  for connection to an ejection head control device  100  ( FIG. 6 ) is attached to the main body  84  of the cartridge  80 . Electrical tracing  102  from the electrical contacts  88  are attached to the semiconductor substrate  62  ( FIG. 4 ) to provide activation of ejection actuators  12  on the substrate  62  on demand from the control device  100  to which the fluid cartridge  80  is attached. The disclosure, however, is not limited to the fluid cartridges  80  as described above as the micro-fluid ejection head  60  according to the disclosure may be used for a wide variety of fluid cartridges, wherein the ejection head  60  may be remote from the fluid reservoir of main body  84 . 
     An illustrative control device  100  for activation of the ejection head  60  is illustrated in  FIG. 6 . For the purpose of illustration only, the control device  100  is described as an ink jet printer. However, the control device  100  may be provided by any devices or combination of devices suitable for activating the ejection head  60  on demand. 
     In  FIG. 6 , the cartridge  80  containing ejection head  60  is attached to a scanning mechanism  110  that moves the cartridge  80  and ejection head  60  across a fluid delivery media  112 . In the case of the control device  100  being an ink jet printer, indicia  114  is printed on the media  112 . 
     The control device  100  includes a digital microprocessor  116  that receive input data  118  a host computer  120 . In the case of an ink jet printer, the input data  118  is image data generated by a host computer  120  that describes the indicia  114  to be printed in a bit-map format. 
     During operation of the control device  100 , the scanning mechanism  110  moves the cartridge  80  across the media  112  in a scanning direction as indicated by arrow  122 . The scanning mechanism  110  may include a carriage that slides horizontally on one or more rails, a belt attached to the carriage, and a motor that engages the belt to cause the carriage to move along the rails. The motor is driven in response to the commands generated by the digital microprocessor  116 . 
     The control device  100  may also include a media advance mechanism  124  that moves the media  112  in the direction of arrow  126  based on input commands from the digital microprocessor  116 . Typically, the advance mechanism  124  advances the media  112  between consecutive scans of the cartridge  80  and ejection head  60 . In one embodiment, the media advance mechanism  124  is a stepper motor rotating a platen which is in contact with the media  112 . The control device  100  also includes a power supply  128  for providing a supply voltage to the ejection head  60 , scanning mechanism  110  and media advance mechanism  124 . 
     It is contemplated, and will be apparent to those skilled in the art from the preceding description and the accompanying drawings, that modifications and changes may be made in the embodiments of the disclosure. Accordingly, it is expressly intended that the foregoing description and the accompanying drawings are illustrative of preferred embodiments only, not limiting thereto, and that the true spirit and scope of the present disclosure be determined by reference to the appended claims.