Patent Publication Number: US-2010123758-A1

Title: Micro-fluid ejection device with on-chip self-managed thermal control system

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This patent application is related to the following copending U.S. patent applications assigned to the assignee of the present invention: (1) Ser. No. 11/427,174, filed Jun. 28, 2006, entitled “Actuator Chip For Inkjet Printhead With Temperature Sense Resistors Having Current Single-Point Output”; (2) Ser. No. 11/517,931, filed Sep. 8, 2006, entitled “Actuator Chip For Micro-Fluid Ejection Device With Temperature Sensing And Control Per Chip Zone”; and (3) Ser. No. 11/834,177, filed Aug. 6, 2007, entitled “Inkjet Printheads With Warming Circuits”. The disclosures of these applications are hereby incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates generally to a micro-fluid ejection devices, such as an inkjet printhead, and, more particularly, to a micro-fluid ejection device with an on-chip thermal control system. 
     2. Description of the Related Art 
     The art of printing images with inkjet technology is well known. In general, an image is produced by jetting ink drops from a printhead at precise moments so they impact a print medium at a desired location. The quality and consistency of the printing, however, is dependent on a number of factors, one such factor being ink temperature. 
     Print quality and ink jetting reliability of an inkjet printhead are dependent on the temperature of an inkjet integrated circuit in the form of an electronic heater or actuator chip of the inkjet printhead. In a thermal inkjet printhead actuator chip, ink drops are ejected by heating ink at intense heat for very short duration to form a bubble that is ejected onto a target medium. The heat is generated from electrical activation and deactivation of spaced apart heater elements of the actuator chip, which is formed by layers or films of semi-conductor and other materials typically deposited on a substrate of silicon or another suitable material. This process is repeated thousands of times per second for each nozzle on the printhead that overlies one of the heater elements of the actuator chip resulting in an accumulation of heat in the regions of the printhead surrounding the heater elements that raises the temperature of the ink. Variations in ink temperature affect the size, shape, and velocity of ejected drops resulting in variations of printed density that are perceivable to the eye. To alleviate this problem printhead thermal control systems have been developed. 
     Thus, thermal control systems are well known for inkjet printheads. However, typically, various components of prior art thermal control systems have been positioned at different locations in the printer. Some components are provided on the printhead itself while others, such as a controller and the like, are provided on the printer console and from there have to communicate with the components on the printhead via conventional communication lines. 
     Improvements of these thermal control systems are disclosed in the patent applications cross-referenced above. A common approach in many of these patent applications is to maintain a more consistent temperature on a printhead by employing temperature sense elements or resistors in the printhead actuator chip. However, a drawback seen in some embodiments of these improved thermal control systems is an increased processing overhead incurred on a printhead supervisor chip and a continued need to communicate with a controller located on the printer separate from the printhead. For instance, in the case of at least some embodiments of the thermal control system of the first cross-referenced patent application, a supervisor chip tracks each thermal zone&#39;s heat status and adjusts heat accordingly via communication with a controller of the inkjet printer. The delays associated with managing multiple zones can lead to larger fluctuations in control temperature. 
     Thus, there remains a need for innovation in thermal control systems that will overcome some or all of the above-referenced issues. 
     SUMMARY OF THE INVENTION 
     The present invention addresses at least some of the foregoing issues by providing a self-managed printhead thermal control system on the printhead itself. Embodiments of the present invention provide an “on-chip” thermal control system on a substrate of the inkjet printhead that is compact and has a control loop that resembles that of a type of analog-to-digital converter called a delta modulator. In some embodiments, a loop filter is physically built-in as the thermal heating time constant of the substrate. 
     Accordingly, in an aspect of the present invention, a micro-fluid ejection device includes a substrate, an actuator chip on the substrate having a plurality of actuator elements for receiving the micro-fluid and being electrically-activatable such that the actuator elements can be repetitively subjected to electrical activation and deactivation causing cyclical heating and cooling thereof and resulting in repetitive ejection of drops of micro-fluid by the actuator elements, and a thermal control system in the actuator chip being self-managed by operation of a control loop defined by the thermal control system internally of the actuator chip and the substrate for sensing and limiting the variation of the temperature of the substrate during cyclical operation of the actuator elements of the actuator chip. 
     In another aspect of the present invention, an inkjet printhead includes a substrate, a heater chip on the substrate and a structure on the substrate for supplying ink to the heater chip so as to divide said heater chip into an intersecting matrix of regions and zones, the heater chip having a plurality of electrically-activatable spaced apart heater elements in said matrix of regions and zones of said heater chip that can be repetitively subjected to electrical activation and deactivation so as to cause cyclical heating and cooling of the heater elements and thereby of the ink in the heater chip resulting in repetitive ejection of drops of ink by the cyclical operation of the heater chip, and a plurality of thermal control systems in the heater chip each in a section of said matrix of regions and zones of said heater chip, each thermal control system being self-managed by operation of a control loop defined by the thermal control system internally of the heater chip and the substrate for sensing and limiting the variation of the temperature of the substrate during cyclical operation of the heater elements of the heater chip. 
     In another aspect of the present invention, an on-chip thermally-controlled actuator device includes a substrate, an actuator chip on the substrate for receiving a micro-fluid and being electrically-activatable such that the actuator chip can be repetitively subjected to electrical activation and deactivation causing cyclical heating and cooling thereof and resulting in repetitive ejection of drops of micro-fluid by the actuator chip, and a thermal control system in the actuator chip being self-managed by operation of a control loop defined by the thermal control system internally of the actuator chip and the substrate for sensing and limiting the variation of the temperature of the substrate during cyclical operation of the actuator chip. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein: 
         FIG. 1  is a perspective view of an inkjet printhead to which can be applied an on-chip self-managed thermal control system of the present invention. 
         FIG. 2  is an enlarged plan view of a fragmentary portion of the printhead of  FIG. 1  showing a portion of the heater chip and the substrate of the printhead. 
         FIG. 3  is a schematic diagram of an exemplary embodiment of an on-chip self-managed thermal control system of the present invention on the heater chip of the inkjet printhead. 
         FIG. 4  is a schematic diagram of a plurality of the on-chip self-managed thermal control systems of the present invention on different regions and zones of a heater chip of an inkjet printhead. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the invention are shown. Indeed, the invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numerals refer to like elements throughout the views. 
     Referring now to  FIGS. 1 and 2 , there is illustrated a micro-fluid ejection device and, more particularly, an inkjet printhead  10 . The printhead  10  includes a substrate  12 , an actuator chip, such as a heater chip  14 , supported on the substrate  12  in a suitable manner, and a nozzle member or plate  16  on the heater chip  14 . The actuator or heater chip  14  has a plurality of spaced apart electrically-activatable actuator elements, such as heater elements  18 , aligned and overlaid by nozzles  20  in the nozzle plate  16 . An ink chamber  22  is defined between each heater element  18  of the heater chip  14  and each nozzle  20  of the nozzle plate  16 . A structure in the form of one or more channels or vias  23  are defined in the substrate  12  and heater chip  14  for supplying ink to each ink chamber  22  in the heater chip  14  from a compartment  24  in a housing  25  of the printhead  10 . 
     The heater elements  18  of the heater chip  14  are formed by layers or films of semi-conductor and other suitable materials typically formed or deposited, by using known micro-electronic fabrication techniques, on the substrate  12 , which typically is silicon or another suitable material. The heater elements  18  of the heater chip  14  are electrically connected via input terminals  26  on the heater chip  14  and input/output (I/O) connectors  28  on a flexible circuit  30  on the printhead which are interconnected by electrical conductors  32  on the printhead  10 . The I/O connectors  28  are, in turn, connected to an external device, such as a printer, fax machine, copier or the like. By such means, the heater elements  18  can be repetitively subjected to momentary electrical activation and deactivation to cause cyclical operation of the heater chip  14  to generate heating and cooling of ink in the heater chip  14 . The heating and cooling of ink in the heater chip  14  results in the repetitive ejection of drops of ink from the ink chambers  22  through the nozzles  20  of the printhead  10 . Exemplary embodiments of the printhead  10  as just briefly described are the ones disclosed in more detail in U.S. Pat. Nos. 6,676,246, 6,805,431 and 6,834,941 assigned to the assignee of the present invention. The disclosures of these patents are hereby incorporated herein by reference. 
     Referring now to  FIG. 3 , there is illustrated a compact on-chip thermal control system, generally designated  36 , in the actuator or heater chip  14  of  FIG. 1  in accordance with an embodiment of the present invention. The thermal control system  36  basically is self-managed by operation of a control loop defined by the thermal control system  36  internally of the actuator chip  14  and the substrate  12  for sensing and limiting the variation of the temperature of the substrate  12  during cyclical operation of the actuator chip  14 . The control loop functions similar to a type of analog-to-digital converter called a delta modulator, but here has a loop filter  38  physically built-in as the thermal heating time constant of the silicon material of the substrate  12 . 
     More particularly, the control loop includes a set point driver  40  that inputs a desired temperature set point current input I(set) to the control loop and a temperature sensor  42  that senses and inputs an actual sensor temperature current input I(sense) to the control loop. The set point driver  40  and temperature sensor  42  combine these current inputs and produce a current output I(diff) being the difference between these current inputs. 
     The control loop defined by the thermal control system  36  also includes a quantizer  44  in the form of an inverter that receives the current output I(diff) and inverts it before inputting it to one input of an AND gate  46 . The other input of the AND gate  46  receives a sampling square wave  48 . The AND gate  46  converts the two inputs into drive pulses P(drive). The AND gate  46  may be one made of six CMOS transistors. The quantizer or inverter  44  may be one made of two CMOS transistors, interconnected between a junction  50  between the output sides of the set point driver  40  and the temperature sensor  42  and the one input  46 A of the AND gate  46 . The sampling square wave  48  is applied from a suitable source to the other input  46 B of the AND gate  46 . The source of the sample rate may be a simple square wave generator. The duty cycle of the square wave generator can be modified to control the level of quantization but a fixed 50% duty cycle square wave will work as well. The frequency of the square wave is the sample rate and it is inversely proportional to the magnitude of the temperature fluctuation or ripple. A square wave with a 1 ms period has been shown to achieve a ripple of less than 1 degree Celsius. 
     In some embodiments, as an alternative to the use of the simple square wave generator, a pulse width modulation (PWM) generator controlling the sampling pulse width of the quantizer  44  is used to make modifications/improvements to the basic control system. In fact, the square wave  48  duty cycle may be used as another input in the algorithm of the control system  36 . For example, the quantizer  44  could have a large duty cycle during the initial thermal ramp up to the set point temperature, to allow a smaller delay before the first page begins to print. After the initial thermal ramp the duty cycle could be proportional to the difference between the set point temperature and the current chip temperature. The resulting pulse width control would make the thermal system response faster and more accurate for a small additional circuit to generate pulse width values. 
     The control loop defined by the thermal control system  36  further includes a substrate heater resistor  54  in or above the substrate  12  and a switch  56 , such as a NMOS switch, interconnected between an output  46 C of the AND gate  46  and the substrate heater resistor  54 . The switch  56  receives the drive pulses P(drive) from the output  46 C of the AND gate  46  and in response thereto periodically activates the substrate heater resistor  54  to produce heat pulses P(heat) that are delivered to the substrate  12  and averaged as the heat propagates through the substrate  12 . The heat pulses P(heat) travel throughout the silicon substrate  12  which spreads out the heat and acts equivalently to the loop filter  38  which averages the pulses to an average temperature. The temperature sensor  42 , receives the average temperature and converts it to a current that is proportional with increasing temperature. The control loop servos until the current at the temperature sensor  42  is equal to the desired set current of the set point driver  40  which corresponds to the required temperature. 
       FIG. 4  shows an exemplary embodiment of a heater chip  58  of an inkjet printhead  60  having a pair of vias  62  and  64  each formed between a pair of columns of nozzle and heater units  66 ,  68  and  70 ,  72 . The vias  62 ,  64  are formed in the substrate  74  which underlies the heater chip  58  and also extend into portions of the heater chip  58 . Thus, the vias  62 ,  64  may interrupt the travel of heat pulses throughout the silicon substrate  74  should only a single thermal control system  36  be provided for the entire heater chip  58 . Since the thermal control system  36  is an on-chip, control loop, self-managed unit, a plurality of such systems  36  can be readily provided on the heater chip  58  so as to provide coverage of an intersecting matrix of regions  1 - 3 , running along an X-axis, and zones  1 - 3 , running along a Y-axis, as illustrated in  FIG. 4 . 
     The foregoing description of one or several embodiments of the invention has been presented for purposes of illustration. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be defined by the claims appended hereto.