Patent Publication Number: US-8968527-B2

Title: Micro-fluid ejection devices, methods for making micro-fluid ejection heads, and micro-fluid ejection head having high resistance thin film heaters

Description:
This is a divisional application of U.S. patent application Ser. No. 11/683,572 filed Mar. 8, 2007, now U.S. Pat. No. 7,673,972, entitled “MICRO-FLUID EJECTION DEVICES, METHODS FOR MAKING MICRO-FLUID EJECTION HEADS, AND MICRO-FLUID EJECTION HEADS HAVING HIGH RESISTANCE THIN FILM HEATERS.” 
    
    
     TECHNICAL FIELD 
     The disclosure relates to micro-fluid ejection heads and, in a particular exemplary embodiment, to thin film heater resistors having high resistance. 
     BACKGROUND AND SUMMARY 
     Micro-fluid ejection devices such as ink jet printers continue to experience wide acceptance as economical replacements for laser printers. Micro-fluid ejection devices also are finding wide application in other fields such as in the medical, chemical, and mechanical fields. As the capabilities of micro-fluid ejection devices are increased to provide higher ejection rates, the ejection heads, which are the primary components of micro-fluid devices, continue to evolve and become more complex. 
     For example, higher resistance heater materials may be required in order to provide relatively large micro-fluid ejection head substrates so as to achieve faster fluid ejection speeds while using lower driving currents and lower nucleation energies. Higher resistance heater materials may also be required in order to provide ejection heads having less sensitivity to parasitic resistance thereby providing more consistent ejection droplet amounts. Accordingly, there continues to be a need for improved heater materials for micro-fluid ejection heads that can provide higher resistance, especially those that have greater bulk resistance and sheet resistance uniformity and a lower temperature coefficient of resistivity (TCR). 
     In one of the disclosed exemplary embodiments, a micro-fluid ejection head is provided that has relatively high resistance thin film heaters. The thin film heaters are made of silicon, metal and carbon (SiMC wherein M is a metal, such as chromium). The thin film heaters have a sheet resistance ranging from about 100 to about 600 ohms per square, and the SiMC thin film material of the heaters has a thickness ranging from about 100 to about 800 Angstroms. 
     In another exemplary embodiment, there is provided a method for making a micro-fluid ejection head, such as one for a micro-fluid ejection device. The method includes depositing a thin film material comprising silicon, metal and carbon (SiMC wherein M is a metal such as chromium) adjacent to a surface of a substrate to form a thin film resistive layer. The thin film resistive layer has a sheet resistance ranging from about 100 to about 600 ohms per square, a thickness ranging from about 100 to about 800 Angstroms, and a bulk resistivity ranging from about 300 to about 4000 μohm·cm. Anode and cathode conductors are defined adjacent to the thin film resistive layer to provide thin film heaters. 
     An advantage of the exemplary embodiments is that they may provide improved micro-fluid ejection heads having thermal ejection heaters that have lower energy requirements and less sensitivity to parasitic resistance. The heater resistors described herein may also have increased resistance which, for example, enables the resistors to be driven with smaller drive transistors, thereby reducing the substrate area required for active devices to drive the heater resistors. A reduction in the area required for active devices to drive the heaters enables the use of a smaller substrate, thereby reducing the cost of the ejection heads. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages of the exemplary embodiments may become apparent by reference to the detailed description of the exemplary embodiments when considered in conjunction with the following drawings illustrating one or more non-limiting aspects of thereof, wherein like reference characters designate like or similar elements throughout the several drawings as follows: 
         FIG. 1  is a micro-fluid ejection device cartridge, not to scale, containing a micro-fluid ejection head according to an exemplary embodiment; 
         FIG. 2  is a perspective view of an ink jet printer and ink cartridge containing a micro-fluid ejection head according to an exemplary embodiment; 
         FIG. 3  is a cross-sectional view, not to scale of a portion of a micro-fluid ejection head according to an exemplary embodiment; 
         FIG. 4  is a plan view not to scale of a typical layout on a substrate for a micro-fluid ejection head according to an exemplary embodiment; 
         FIG. 5  is a plan view, not to scale of a portion of an active area of a micro-fluid ejection head according to an exemplary embodiment; 
         FIG. 6  is a graphical representation of an estimated electromigration response of heaters made according to an exemplary embodiment; and 
         FIG. 7  is a cross-sectional view of a heater stack area of a micro-fluid ejection head according to an exemplary embodiment. 
     
    
    
     DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
     With reference to  FIG. 1 , a fluid cartridge  10  for a micro-fluid ejection device is illustrated. The cartridge  10  includes a cartridge body  12  for supplying a fluid to a micro-fluid ejection head  14 . The fluid may be contained in a storage area in the cartridge body  12  or may be supplied from a remote source to the cartridge body. 
     The exemplary micro-fluid ejection head  14  includes a substrate  16  and a nozzle plate  18  containing nozzles  20 . The cartridge  10  may be removably attached to a micro-fluid ejection device such as an ink jet printer  22  ( FIG. 2 ). Accordingly, electrical contacts  24  are provided on a flexible circuit  26  for electrically connecting the cartridge  10  to the micro-fluid ejection device  22 . The flexible circuit  26  includes electrical traces  28  that are connected to the substrate  16  of the micro-fluid ejection head  14 . 
     An enlarged cross-section view, not to scale, of a portion of the micro-fluid ejection head  14  is illustrated in  FIG. 3 . The micro-fluid ejection head  14  has a thermal heating element  30 , referred to hereinafter as heater  30 , as a fluid ejection actuator for heating the fluid in a fluid chamber  32  between the substrate  16  and a nozzle hole  20 . The heaters  30 , described in more detail below, are thin film heater resistors which, according to an exemplary embodiment, are comprised of silicon, metal, and carbon. 
     Fluid can be provided to the fluid chamber  32  through an opening  34  in the substrate  16  and through a fluid channel  36  connecting the opening  34  with the fluid chamber  32 . The nozzle plate  18  can be adhesively attached to the substrate  16  as by adhesive layer  38 . As depicted in  FIG. 3 , the flow features including the fluid chamber  32  and fluid channel  36  may be formed in the nozzle plate  18 . However, the flow features may be provided in a separate thick film layer and a nozzle plate containing only nozzles may be attached to the thick film layer. Other configurations, such as where the flow features are partially formed in both a nozzle plate and a thick film layer, for example, are also contemplated. In an exemplary embodiment, the micro-fluid ejection head  14  is a thermal ink jet printhead. However, the exemplary embodiments described herein are not intended to be limited to ink jet printheads as other fluids, other than ink, may be ejected with a micro-fluid ejection device according to the exemplary embodiments. 
     Referring again to  FIG. 2 , the fluid ejection device may be an ink jet printer  22 . One such printer  22  includes a carriage  40  for holding one or more cartridge  10  and for moving the cartridges  10  over a medium  42  such as paper for depositing a fluid from the cartridges  10  onto the medium  42 . The contacts  24  on the cartridge mate with contacts on the carriage  40  for providing electrical connection between the printer  22  and the cartridge  10 . Microcontrollers in the printer  22  control the movement of the carriage  40  across the media  42  and can be used to convert analog and/or digital inputs from an external device such as a computer for controlling the operation of the printer  22 . Ejection of fluid from the micro-fluid head  14  can be controlled by a logic circuit on the fluid ejection head  14  in conjunction with the controller in the printer  22 . 
     A plan view, not to scale of the micro-fluid ejection head  14  is illustrated in  FIG. 4 . The illustrated micro-fluid ejection head  14  includes a substrate  16  and a nozzle plate  18  attached to the substrate  16 . A layout of device areas of the substrate  16  is shown providing predetermined locations for logic circuitry  44 , driver transistors  46 , and heaters  30 . As shown in  FIG. 4 , the substrate  16  may include a single opening  34  for providing fluid such as ink to the heaters  30 , which are depicted as being disposed on both sides of the opening  34 . However, other exemplary embodiments may include, for example, multiple openings and/or heaters disposed on only one side of the opening(s). In another exemplary embodiment, fluid may flow around edges of the substrate  16  to the heaters rather than flowing through an opening(s)  34  in the substrate. In a further exemplary embodiment, the substrate  16  may include multiple openings, one each for one or more heaters, rather than a single opening  34  in the substrate  16 . Meanwhile, the nozzle plate  18 , can be made of an ink resistant material such as polyimide, and can be attached to the substrate  16 . 
     An active area  48  of the substrate  16  (e.g., including the driver transistors  46 ) is illustrated in detail in a plan view of the active area  48  in  FIG. 5 . This figure represents a portion of a typical heater array and active area  48 . A ground bus  50  and a power bus  52  are provided to provide power to the devices in the active area  46  and/or to the heaters  30 . 
     Reducing the driver transistor  46  active area width, indicated by (W), may, for example, reduce the size of the substrate  16  required for the micro-fluid ejection head  14 . In an exemplary embodiment, the active area  48  of the substrate  16  may have a width dimension W ranging from about 100 to about 400 microns and an overall length dimension D ranging from about 6,300 microns to about 26,000 microns. The driver transistors  46  may be provided at a pitch P ranging from about 10 microns to about 84 microns, for example. 
     In one exemplary embodiment, the active area  48  of a single driver transistor  46  on substrate  16  may have an active area width (W) ranging from about 100 to less than about 400 microns and an active area  48  of less than about 15,000 μm 2 . The smaller active area  48  may be achieved by use of driver transistors  46  having gate lengths and channel lengths ranging from about 0.8 to less than about 3 microns. 
     The resistance of the driver transistor  46  is proportional to its width W. The use of smaller driver transistors  46  increases the resistance of the driver transistor  46 . The resistance of the heater  30  may be increased proportionately to maintain a constant ratio between the heater resistance and the driver transistor resistance. A benefit of a higher resistance heater  30  is that the heater requires less driving current, which can allow more heaters to be simultaneously fired. Using higher resistance heaters can also reduce sensitivity to parasitic resistance, wherein such reduced sensitivity can result in less ejection variation between different combinations of heaters being fired. In combination with other features of the heater  30 , the exemplary embodiments may provide an ejection head  14  having higher efficiency and an ejection head  14  capable of higher frequency operation. 
     One approach to providing a higher resistance heater  30  is to use a higher aspect ratio heater, that is, a heater having a length significantly greater than its width. However, such high aspect ratio design tends to trap air in the fluid chamber  32 . Another approach to providing a high resistance heater  30  is to provide a heater made from a thin film material having a higher sheet resistance. One such material is TaN. However, relatively thin TaN has inadequate aluminum barrier characteristics thereby making it less suitable than other materials for use in micro-fluid ejection devices. Aluminum barrier characteristics can be particularly important when the resistive layer is extended over and deposited in a contact area for an adjacent transistor device. Without a protective layer, for example TiW, in the contact area, the thin film TaN is insufficient to prevent diffusion between aluminum deposited as the contact metal and the underlying silicon substrate. 
     Accordingly, a suitable heater  30  may be a thin film heater made of a thin film material derived from silicon, a metal (M), and carbon, such as one having the following formula Si x M y C z . Exemplary metals that can be used in such an embodiment include, but are not limited to, chromium, hafnium, molybdenum, tantalum, titanium, tungsten, and zirconium. At least in the case where the metal (M) is chromium or tantalum, x, y, and z can be integers each ranging from about 10 to about 60 and x+y+z=100. 
     In the case of an embodiment utilizing Si x Cr y C z , the heater  30  may be formed by a sputtering process wherein a silicon/chromium/carbon target is sputtered adjacent to the substrate  16  in the presence of argon gas. The silicon/chromium/carbon target may have an exemplary composition ranging from about 20 to about 70 atomic percent silicon, from about 10 to about 50 atomic percent chromium, and from about 10 to about 40 atomic percent carbon. The resulting exemplary heater  30  may have a deposited resistive layer composition ranging from about 30 to about 60 atomic percent silicon, from about 20 to about 40 percent chromium, and from about 10 to about 30 atomic percent carbon. Exemplary bulk resistivities of such heaters  30  may range from about 300 to about 4000 micro-ohms-cm. 
     A process that may be used to sputter a silicon-chromium-carbon resistor material is described for example in U.S. Pat. No. 4,862,143, the relevant disclosure of which is incorporated herein by reference. The sputtering process can be a non-reactive sputtering process. Such a non-reactive sputtering process may result in resistor films having better film uniformity (e.g., since no reactive gas is used during the sputtering). A benefit of the use of thin, high resistance silicon/metal/carbon heaters made according to an exemplary embodiment is that such heaters should have exceptionally good uniformity for both bulk and sheet resistivity. For example, exemplary Si x Cr y C z  heaters have shown very high film uniformities, such as where the amount of heater film sheet resistance variation across multiples wafers (wafer-to-wafer non-uniformity) has been less than about ±2% (wherein it has been determined that such variations across multiple wafers should ideally be less than about ±12%). In the case of at least Si x Cr y C z , such heaters should also have a relatively low TCR (e.g., the inventors believe a TCR of less than 200 ppm/° C., such as a TCR of about −60 ppm/° C., can be achieved with Si x Cr y C z ). 
       FIG. 6  illustrates an estimated electromigration response for an exemplary Si x Cr y C z  heater, wherein x=44, y=31, and z=25, which can be used to predict heater stability for various operating temperatures and current densities. The illustrated response is based on a modified Black&#39;s equation in which parameters of the equation were generated based on experimental data based on dry fire results and a thermal finite element analysis. As shown by  FIG. 6 , with reference to point J on Curve E, for an expected operating point wherein a current density of about 105 mA/μm 2  will be used to fire a heater to achieve a peak temperature during a fire pulse of about 400° C. (wherein the thermal energy generated by heating the heater is used to supernucleate ink to cause ejection of a droplet(s) of the same), the heaters made according to such an exemplary embodiment are expected to exhibit less than a 1% change in DC resistance after 500,000,000 fires. In  FIG. 6 , Curves A-I represent different operating current densities (in milliamps per square micron) according to the following table. 
     
       
         
           
               
               
               
             
               
                   
               
               
                   
                 Curve 
                 mA/μm 2   
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 A 
                 130 
               
               
                   
                 B 
                 125 
               
               
                   
                 C 
                 120 
               
               
                   
                 D 
                 115 
               
               
                   
                 E 
                 110 
               
               
                   
                 F 
                 105 
               
               
                   
                 G 
                 100 
               
               
                   
                 H 
                 95 
               
               
                   
                 I 
                 90 
               
               
                   
               
            
           
         
       
     
     A more detailed illustration of a portion of an ejection head  14  showing an exemplary heater stack  64  including a heater  30  made according to the above described process is illustrated in  FIG. 7 . The heater stack  64  is provided on an insulated substrate  16 . First layer  66  is thin film resistor layer made of a high bulk resistivity Si x Cr y C z , wherein x, y, and z are as set forth above, which is deposited on the substrate  16  according to the process described above. 
     After depositing the thin film resistive layer  66 , a conductive layer  68 , made of a conductive metal such as gold, aluminum, copper, and the like, is deposited on the thin film resistive layer  66 . The conductive layer  68  may have a suitable thickness known to those skilled in the art, but typically has a thickness ranging from about 0.4 to about 0.6 microns. After deposition of the conductive layer  68 , the conductive layer is etched to provide anode  68 A and cathode  68 B contacts to the resistive layer  66  and to define the heater  30  between the anode and cathode  68 A and  68 B. 
     In another embodiment, a thin barrier layer is deposited between resistive layer and conductive layer, such as to prevent electromigration and material interaction between the resistive layer and the conductive layer. Suitable barrier layer materials can include, for example, titanium, and titanium-tungsten. 
     A passivation layer  70  may be deposited on the heater  30  and anode and cathode  68 A and  68 B. The layer  70  may be comprised of a material such as diamond-like carbon, doped diamond-like carbon, silicon oxide, silicon oxynitride, silicon nitride, silicon carbide, and a combination of silicon nitride and silicon carbide. The layer  70  may have a thickness ranging from about 1000 to about 8000 Angstroms. 
     After depositing the passivation layer  70 , a cavitation layer  72  may be deposited and etched to cover the heater  30 . A particularly suitable cavitation layer  72  is one comprising tantalum and having a thickness ranging from about from about 1000 to about 6000 Angstroms. 
     It is often desirable to keep the passivation layer  70  and cavitation layer  72  as thin as possible yet provide suitable protection for the heater  30  from the corrosive and mechanical damage effects of the fluid being ejected. Thin layers  70  and  72  may reduce the overall thickness dimension of the heater stack  64  and provide reduced power requirements and increased efficiency for the heater  30 . 
     Once the cavitation layer  72  is deposited, this layer  72  and the underlying layer or layers  70  may be patterned and etched to provide protection of the heater  30 . A dielectric layer, such as one comprised of silicon dioxide, can be deposited over the heater stack  64  and other surfaces of the substrate to provide insulation between subsequent metal layers that are deposited on the substrate for contact to the heater drivers and other devices. 
     At numerous places throughout this specification, reference has been made to a number of U.S. patents and/or patent publications. The relevant portions of all such cited documents are expressly incorporated in full into this disclosure as if fully set forth herein. 
     The foregoing embodiments are susceptible to considerable variation in their practice. Accordingly, the embodiments are not intended to be limited to the specific exemplifications set forth hereinabove. Rather, the foregoing embodiments are within the spirit and scope of the appended claims, including the equivalents thereof available as a matter of law. 
     The patentees do not intend to dedicate any disclosed embodiments to the public, and to the extent any disclosed modifications or alterations may not literally fall within the scope of the claims, they are considered to be part hereof under the doctrine of equivalents.