Patent Publication Number: US-8540349-B2

Title: Printhead having isolated heater

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to micro heaters and their formation and, more particularly, to micro heaters used in ink jet devices and other liquid drop ejectors. 
     BACKGROUND OF THE INVENTION 
     Drop-on-demand (DOD) liquid emission devices have been used as ink printing devices in ink jet printing systems for many years. Early devices were based on piezoelectric actuators. A currently popular form of ink jet printing, thermal ink jet (or “thermal bubble jet”) devices use electrically resistive heaters to generate vapor bubbles which cause drop emission. 
     The printhead used in a thermal inkjet system includes a nozzle plate having an array of ink jet nozzles above ink chambers. At the bottom of an ink chamber opposite the corresponding nozzle is an electrically resistive heater. 
     In response to an electrical pulse of sufficient energy, the heater causes vaporization of the ink, generating a bubble that rapidly expands and ejects a drop. 
     There is a minimum threshold energy required to be applied to the heater in order to achieve bubble formation sufficient to reliably eject a drop. To eject a drop, the heater must supply sufficient heat to raise the ink at the heater-ink interface to a temperature above a critical bubble nucleation temperature, approximately 280 C for water-based inks. This minimum threshold energy depends on the volume of drop ejected and the printhead design such as the electrically resistive heater geometry. 
     Printhead designs of the prior art form the heater on an insulating thermal barrier layer, typically silicon dioxide, formed on the substrate. A protective passivation layer is formed over the electrically resistive heater for protection from the ink. When the heater is energized heat is transferred both to the ink and to the substrate. The heater in the prior art is inefficient because only about half of the energy generated by the heater goes into heating the ink. The rest flows into the substrate causing a temperature rise of the substrate. This temperature rise of the substrate is a disadvantage for high speed printing since if the substrate gets too hot, printing must be stopped to let the printhead cool down. 
     One mechanism for cooling the printhead is removal of heat by the ejecting drop. The amount of heat removed is proportional to the temperature and volume of the ejected drop. In fact for large drop volumes greater than 6 picoliters, printheads of the prior art can achieve a situation that for a 20-30 C temperature rise of the printhead, the energy required to eject a drop is equal to the heat energy removed by the ejected drop. In this case a steady state operating temperature can be achieved. 
     However, state of the art printers typically use drop sizes &lt;3 pL. The efficiency of prior art heaters is too low for these lower volume drops to carry substantial heat energy away without the printer temperature becoming too hot. These small drops are also typically printed at a higher frequency exacerbating the problem. 
     Furthermore the size of the electrical drivers for the electrically resistive heaters is in part determined by the energy needed. The inefficiency of the electrically resistive heaters require larger drivers resulting in increased chip size. It is therefore desirable to increase the efficiency of the electrically resistive heater by minimizing the amount of heat that goes into the substrate. 
     One method to increase the efficiency of the electrically resistive heater is to provide a thermal barrier positioned between the substrate and the electrically resistive heater such as a cavity. Typically, the electrically resistive heater is formed at the end of wafer processing after the controlling circuitry has been formed. It is important therefore to design a process for forming a cavity that is compatible with low temperature backend processing. 
     After ejection of the ink drop it is also important that the heater cool down sufficiently so that when ink refills the chamber the temperature at the ink heater interface is insufficient to vaporize the refilling ink. Such vaporization would limit the operating frequency of the printhead. Note that while the timescale of the initial bubble vaporization is 1-2 μsec the ink refill takes place at a later time of 6-10 μsec. Therefore it is useful to provide a thermal path that can reduce the heater temperature sufficiently for this longer time cycle while at the same time not reducing the efficiency of the initial bubble formation. It is also important that this thermal path distribute the heat into the ink rather than into the substrate. 
     For printheads used in printing systems the energy applied to the electrically resistive heater in use is greater (typically 15-20%) than the threshold energy. This extra energy is used to account for resistance variations in the electrically resistive heaters and changes in threshold energy over the life of the heater. Because of the variations in heater resistances, this extra energy can cause variations in the drop ejection. It would therefore be useful to remove this excess heat rather than have it contribute to the vapor bubble formation. 
     It is also necessary for printheads to have a long lifetime. Any non-uniformities of the heater can cause poor nucleation of the vapor bubble as well as localized damage to the heater thereby reducing the lifetime of the printhead. It is therefore important that the heater surface be uniform in order to maintain the lifetime requirements of the printhead. 
     Damage to the heater also limits the lifetime of the printhead. Collapsing bubbles can create localized damage in the heater passivation layers. This localized damage in the passivation layers eventually reaches the heater layer, which causes a catastrophic failure of the heater. It is therefore important to limit this cavitation damage to a heater. 
     There is therefore a need for a printhead that has a long lifetime and provides high quality prints throughout its life. This printhead should also be capable of ejecting small drops at high frequencies with heater efficiencies adequate to prevent overheating of the printhead. 
     SUMMARY OF THE INVENTION 
     According to a feature of the present invention, a liquid ejector includes a substrate, a heating element, a dielectric material layer, and a chamber. The substrate includes a first surface. The heating element is located over the first surface of the substrate such that a cavity exists between the heating element and the first surface of the substrate. The dielectric material layer is located between the heating element and the cavity such that the cavity is laterally bounded by the dielectric material layer. The chamber, including a nozzle, is located over the heating element. The chamber is shaped to receive a liquid with the cavity being isolated from the liquid. 
     According to another feature of the present invention, a method of actuating a liquid ejector includes providing a liquid ejector including: a substrate including a first surface; a heating element located over the first surface of the substrate such that a cavity exists between the heating element and the first surface of the substrate; a dielectric material layer located between the heating element and the cavity such that the cavity is laterally bounded by the dielectric material layer; and a chamber including a nozzle located over the heating element, the chamber being shaped to receive a liquid, the cavity being isolated from the liquid; introducing liquid into the chamber of the liquid ejector; and causing the heating element and the dielectric material layer to deform into the cavity by forming a vapor bubble over the heating element. 
     According to another feature of the present invention, a method of forming a thermally isolated heating element for a liquid ejector includes providing a substrate including a first surface; depositing a sacrificial material layer over the first surface; patterning the sacrificial material layer; depositing a dielectric material layer over the patterned sacrificial material layer; forming a heating element over the dielectric material layer; and removing the patterned sacrificial material layer to create a cavity between the dielectric material layer and the first surface of the substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the detailed description of the preferred embodiments of the invention presented below, reference is made to the accompanying drawings, in which: 
         FIG. 1  is a schematic cross sectional view of a prior art liquid ejector; 
         FIG. 2  is a schematic cross sectional view of a liquid ejector made in accordance with the present invention; 
         FIGS. 3   a - 10  show one method of forming an isolated heater in the liquid ejector of  FIG. 2 ; 
         FIG. 11   a  is a schematic top view of an alternative method of patterning the sacrificial layer used in forming the isolated heater in the liquid ejector of  FIG. 2 ; 
         FIG. 11   b  is a schematic top view of two isolated heaters formed using the alternative method of patterning the sacrificial layer in  FIG. 11   a.    
         FIG. 11   c  is a schematic cross sectional view taken along line B-B′ of  FIG. 11   b.    
         FIG. 12   a  is a schematic top view of another alternative method of patterning the sacrificial layer used in forming the isolated heater in the liquid ejector of  FIG. 2 ; 
         FIG. 12   b  is a schematic top view of another alternative method of patterning the sacrificial layer used in forming the isolated heater in the liquid ejector of  FIG. 2 ; 
         FIG. 13   a  is a schematic cross sectional drawing of one isolated heater of the present invention in an open pool of ink when a current pulse is just applied; 
         FIG. 13   b  is a schematic cross sectional drawing of one isolated heater of the present invention in an open pool of ink when a bubble has nucleated; 
         FIG. 13   c  is a schematic cross sectional drawing of one isolated heater of the present invention in an open pool of ink when a bubble has further expanded; and 
         FIG. 13   d  is a schematic cross sectional drawing of one isolated heater of the present invention in an open pool of ink showing the bubble collapsing on the heater. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present description will be directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art. In the following description, identical reference numerals have been used, where possible, to designate identical elements. 
     As described below, the present invention describes a micro heater that can be used in a liquid drop ejector, a method of actuating a liquid ejector, and a method of forming a micro heater stack for use in a liquid drop ejector. The most familiar of such devices are used as printheads in ink jet printing systems. Although the terms ink jet and liquid are used herein interchangeably, many other applications are emerging which make use of micro-heaters or heaters in systems, similar to inkjet printheads, which emit or eject other types of liquid in the form of drops. Examples of these applications include the delivery of polymers, conductive inks, and pharmaceutical drugs. These systems also have a need for the efficient heater stack of the present invention. 
     In current thermal inkjet printheads, the electrothermal heater includes a heater stack formed on the surface of a silicon chip containing control devices.  FIG. 1  illustrates a cross-section of a single inkjet ejector  2  of the prior art with a heater stack  6  that is formed on a silicon substrate  4 . On the substrate is a dielectric thermal barrier layer  10 , typically 1-3 μm thick. This dielectric thermal barrier  10  is typically made from interlayer dielectrics formed when fabricating the electrical circuitry in other areas of the chip (not shown) that controls activation of the heater area  14  of the electrically resistive heater layer  8 . An electrically conductive layer  12  is deposited on top of the electrically resistive heater layer  8  and is patterned and etched to form conductive traces that connect to the control circuitry (not shown) and also define the heater area  14 . 
     Two layers are typically added to the heater stack  6  to increase heater lifetime by protecting it from the ink. An insulating passivation layer  16  is deposited. This insulating passivation layer  16  can be formed from silicon nitride, silicon oxide, silicon carbide, or any combination of these materials. On top of the insulating passivation layer  16  is deposited a protection layer  18 . The protection layer  18  is typically formed with tantalum and protects the electrically resistive heater layer  8  from impact stresses resulting from bubble collapse. 
     Above the heater there is an ink chamber  20  with a nozzle plate  22  forming the roof of the chamber. Located above the heater a nozzle  24  is formed in the nozzle plate  22 . Not shown is the ink feed for the chamber. 
     To eject a drop an electrical pulse, typically &lt;1 μsec, is applied to the heater through the electrically conductive layer  12 . Electrical energy applied to the heater produces thermal energy that is transferred to the ink at the ink-heater interface. At nucleation threshold a sufficient amount of heat energy is transferred to raise the temperature of the ink to cause vapor bubble formation. For water-based inks, the temperature for bubble nucleation is approximately 280 C. The arrows  26   a ,  26   b , and  26   c  in  FIG. 1  represent the heat flux due to the electrical pulse. Roughly equal amounts of heat flow to the ink in the ink chamber, represented by arrow  26   a  and to the substrate, represented by arrow  26   b . A small fraction will diffuse laterally along the heater stack represented by arrows  26   c . Only the heat flux represented by arrow  26   a  will contribute directly to bubble formation. The heat represented by arrows  26   b  and  26   c  is wasted and must be removed from the ejector, either by a heat sink or by transfer to the ink that is then ejected. 
       FIG. 2  shows a cross-section of an embodiment of a single inkjet ejector  30  with an isolated heater region  34  of the present invention. As in the prior art there is an oxide thermal barrier layer  10  deposited on the substrate  4  made from interlayer dielectrics, formed when fabricating the electrical circuitry in other areas of the chip. Formed in the isolated heater region  34  above the oxide thermal barrier layer  10  and below a lower dielectric protective layer  38  is an isolating cavity  36 . The isolating cavity  36  is laterally bounded by dielectric protective layer  38 . Isolating cavity  36  is sealed on all sides and contains a gas at a pressure less than atmospheric pressure. The lower dielectric protective layer  38  protects the heater layer from attack during the cavity formation process. 
     Similar to the configuration of the heater stack in the prior art, the isolated heater stack  32  of the present invention contains an electrically resistive heater layer  8 , and an electrically conductive layer  12 . Again two protective layers are formed on the isolated heater region; an insulating passivation layer  16 , and a protection layer  18 . In this case the thickness of these layers, when compared to the prior art, is reduced so as to increase the energy efficiency of the heater. 
     When the electrical pulse, typically &lt;1 μsec, is applied to the electrically resistive heater layer  8  contained in the isolated heater stack  32  of the present invention the heat flux flows primarily into the ink in the ink chamber as represented by arrow  40 . There is very little heat flux into the substrate due to the presence of the isolating cavity  36 . As a result, the efficiency of heater stack  32  is increased when compared to the prior art. 
     There is still a lateral heat flux, represented by arrows  42 , but, when compared to the prior arts the lateral heat flux is reduced due to heater stack  32  having a lower cross-sectional area which is at least partially created by the presence of isolating cavity  36 . 
       FIGS. 3   a - 10  illustrates a fabrication method of the present invention for forming a printhead containing multiple single inkjet ejectors  30  with an isolating cavity formed in the isolating heater stack. The figures show a section of the printhead illustrating the process with two of the ejectors. 
       FIG. 3   a  shows, in cross-section along the heater length, a silicon substrate  4  on which has been fabricated electronic circuitry, for example, CMOS control circuitry and LDMOS drivers (not shown), the processing of which is well known in the art. This circuitry controls the firing of the heaters in an array of drop ejectors. The dielectric thermal barrier layer  10  is comprised of interlayer dielectric layers of the CMOS device. Contained within the interlayer dielectric layers are metal leads  44 , which originate from one of the metal layers of the CMOS device circuitry and connect to the drive transistors (not shown).  FIG. 3   b  shows a top down view with three metal leads  44 ; two leads  44   a  to drive the two ejectors and a shared common line  44   b.    
     As shown in cross-section along the heater length in  FIG. 4   a  and in top down view in  FIG. 4   b , a sacrificial layer  46  is deposited and patterned. In the preferred embodiment this layer is made from amorphous silicon deposited by physical vapor deposition. Other materials such as polyimide or aluminum can be used. The sacrificial layer  46  is deposited in a thickness range 100-2000 Angstroms. A thinner sacrificial layer results in shallower cavity thereby providing increased structural support for the suspended heater. Thinner sacrificial layers however are harder to remove and are more susceptible to stiction during both fabrication and operation. In the preferred embodiment the thickness is in the range 500-1000 Angstroms.  FIG. 4   b  shows a top plan view of a printhead illustrating the process for two ejectors of an ejector array. The sacrificial layer  46  is rectangular in shape and contains small protrusions  48  positioned on each side. 
     As shown in cross-section along the heater length in  FIG. 5 , a lower dielectric protective layer  38  is deposited. In the preferred embodiment this layer is made by plasma enhanced chemical vapor deposition (PECVD) of silicon nitride, silicon oxide, or a combination of the two materials. The lower dielectric protective layer is deposited in a thickness range 500-4000 Angstroms. A thinner layer requires less energy to heat and therefore is more thermally efficient but provides less mechanical support. In the preferred embodiment the thickness is in the range 500-2000 Angstroms. 
     As shown in cross-section along the heater length in  FIG. 6   a , and in top down view in  FIG. 6   b , vias  50 ,  50   a ,  50   b  to the metal leads are etched followed by deposition and patterning of the electrically resistive heater layer  8  and electrically conductive layer  12  to form the heater region  34  which will subsequently become the isolated heater region of the present invention. The electrically resistive heater layer  8  is deposited in a thickness range 300-1000 Angstroms. The thinner the heater layer the less energy is needed to raise the heater temperature. However in practice the uniformity of very thin layers is difficult to control. In the preferred embodiment the thickness of heater layer  8  is in the range 400-600 Angstroms. The heater material is a ternary alloy containing tantalum, silicon, and nitride. Other ternary or quaternary alloys can be used. The electrically conductive layer  12  is deposited in a thickness range 2000-6000 Angstroms. In the preferred embodiment the material is aluminum or an aluminum alloy. As shown in  FIGS. 6   a  and  6   b , the electrically conductive layer does not extend over the region containing the sacrificial layer. 
     Referring to  FIGS. 7   a  and  7   b , a photoresist layer  51  is next coated and exposed to expose arrays of openings  52 .  FIG. 7   a  shows a top plan view of the photoresist openings  52 . The size of the openings is in the range 0.8-2 μm. The use of small openings increases the strength of the suspended heater and also seals the isolating cavity better than larger openings. The photoresist openings are arranged to be aligned above the protrusions  48  of the sacrificial layer  46 . A dry etch is used to remove the lower dielectric protective layer  38  below these openings  52  in order to expose the sacrificial layer  46  on the protrusions  48 . In the preferred embodiment the dry etch is a plasma etch utilizing a sulfur hexafluoride gas.  FIG. 7   b  shows a cross-section taken through line A-A′ of  FIG. 7   a  after the dry etch has removed the lower dielectric protective layer  38  and exposed the sacrificial layer  46 . 
     Referring to  FIG. 8 , the patterned substrate is next put into a chamber containing xenon difluoride gas. The xenon difluoride gas selectively removes the entire sacrificial layer  46 , which is amorphous silicon in the preferred embodiment, to create an isolating cavity  36 . The patterned photoresist layer  51  is left on to protect the electrically resistive heater layer  8  from attack by the xenon difluoride gas and then removed afterward. Alternatively a thin silicon nitride layer can be deposited on top of the electrically resistive layer  8  to protect it. In that case the photoresist layer can be removed prior to this step. This xenon difluoride gas etch removes the sacrificial material as shown in cross-section in  FIG. 8  taken through line B-B′ of  FIG. 7   a , shown after the photoresist layer  51  has been removed. 
       FIG. 9   a  shows a cross-section taken through line B-B′ of  FIG. 7   a  after an insulating sealing layer  54  has been deposited. This layer seals the isolating cavity  36  under the isolated heater region  34  of the present invention by filling up the openings  52 .  FIG. 9   b  shows a cross-section taken through line A-A′ of  FIG. 7   a  after the openings  52  have been sealed. The insulating sealing layer material is silicon nitride, silicon carbide, or a combination of the two materials. The deposition in the preferred embodiment is by plasma enhanced chemical vapor deposition (PECVD). The pressure in the sealed isolating cavity will be similar to the pressure used for the PECVD deposition and is typically &lt;1 Torr. In the preferred embodiment the thickness of the insulating passivation layer is 1000-2500 Angstroms thick. The insulating sealing layer  54  also acts as the insulating passivation layer  16  and provides protection for the electrically resistive layer  8  from the ink. 
       FIG. 10  is shows a cross-section after the deposition and patterning of a heat spreading layer  55 . The heat spreading layer  55  is a good thermal conductor. In the preferred embodiment the heat spreading layer  55  is tantalum, deposited by physical vapor deposition, with a thickness of 500-2500 Angstroms. In this embodiment, the heat spreading layer  55  is a lateral extension of the protection layer  18  that protects the heater from the ink. The heat spreading layer  55  is left on throughout the ink chamber and acts as a heat transfer medium from the heater to the ink. 
     To use the device as an inkjet ejector, a chamber and nozzle plate can be fabricated as described in commonly assigned copending patent applications U.S. Ser. Nos. 11/609,375 and 11/609,365, both filed Dec. 12, 2006, the disclosures of which are incorporated by reference herein. 
     Referring to  FIGS. 11   a - 11   c , another embodiment is shown.  FIG. 11   a  shows a top plan view of the patterned sacrificial layer  46  in which two holes  56  are formed in the sacrificial layer  46 . The processing is then completed as described above with reference to  FIGS. 3-10 .  FIG. 11   b  shows a top plan view of a heater of this embodiment.  FIG. 11   c  shows a cross-section taken through line B-B of  FIG. 11   b . Two support posts  58  in the isolating cavity  36  have been formed in holes  56 . 
     When the dielectric protective layer  38  is deposited over the sacrificial layer  46  (as in  FIG. 5 ), the dielectric material (e.g. silicon nitride, silicon oxide, or a combination of the two materials) fills the holes  56 . When the xenon difluoride gas removes the sacrificial layer  46 , the material that is deposited into holes  56  is not removed. As a result, supports  58  provide mechanical support for heater layer  8  over isolating cavity  36 . The diameter of the supports is in the range 0.4-1.0 μm with a preferred embodiment of 0.6-0.8 μm diameter. Two supports are shown in  FIG. 11   c  although the number of supports  58  can vary, for example, between one and ten. The number, size, shape and position of the supports  58  is determined by the structural support requirements of the heater stack and is implemented through the mask design for patterning the sacrificial layer  46 . The spacing between supports  58  can vary between one third and two thirds of the heater length. 
     Referring to  FIGS. 12   a  and  12   b , another embodiment is shown.  FIG. 12   a  shows a top plan view of the patterned sacrificial layer  46  in which a strip  60  along the heater length is formed in the sacrificial layer. Alternatively  FIG. 12   b  shows a top plan view of the patterned sacrificial layer  46  in which an opening, for example, a strip  60 , perpendicular to the heater length is formed in the sacrificial layer. In alternative embodiments there can be more than one strip or a combination of strips and other openings, such as holes, in sacrificial layer  46 , which, when filled as described above, result in corresponding support structures, for example, ridges or posts, respectively, that support heater layer  8  over isolating cavity  36 . 
     The fabrication process described herein (starting with dielectric thermal barrier layer  10  including interlayer dielectric layers of CMOS circuitry fabricated on the device) is compatible with the fabrication of drive electronics and logic on the same silicon substrate as the heaters. This is a prerequisite in order to control the large number of heaters needed on a thermal inkjet printhead able to meet current and future requirements for print speed. In contrast, the heater with an underlying cavity that is described in U.S. Pat. No. 5,751,315 uses a polysilicon heater. Such a heater material requires high temperature deposition and is not compatible with CMOS fabrication requirements in which the heater is deposited subsequent to the sintering of aluminum for the CMOS circuitry, thereby constraining heater deposition temperature not to exceed 400 C. 
     A second prerequisite of thermal inkjet printheads able to meet current and future printing resolution requirements is that heaters for adjacent drop ejectors must be closely spaced, for example at a spacing of 600 to 1200 heaters per inch. For a center to center heater spacing of about 42 microns, corresponding to 600 heaters per inch, the heater width would be approximately 30 microns or less. For a center to center heater spacing of about 21 microns, corresponding to 1200 heaters per inch, the heater width would be approximately 15 microns or less. The fabrication processes of the present invention have been demonstrated to be capable of providing heaters having a center to center distance of about 21 microns and having a heater width of less than 15 microns. Fabrication methods described in U.S. Pat. No. 5,861,902 for forming a heater having an underlying cavity for thermal isolation have difficulty providing heaters at such close spacing. In particular for the embodiment described with reference to FIG. 7 of U.S. Pat. No. 5,861,902, the sacrificial layer (90) is not bounded laterally, as layer  46  is in the present invention (see  FIG. 5 ). In the present invention, the etching of the sacrificial layer  46  proceeds until it is stopped by the laterally bounding dielectric protective layer  38  which provides a fixed lateral limit to the isolating cavity  36  (see  FIGS. 2 and 8 ). By contrast, while the laterally unbounded sacrificial layer (90) of U.S. Pat. No. 5,861,902 may provide adequate manufacturing tolerances for a heater spacing of 300 per inch and a heater width of about 50 microns, it will not provide the tight tolerance on width of the isolating cavity that is required for a heater spacing of 600 or 1200 per inch and a heater width of 30 microns or less. 
     There are also important differences between the design of the structural supports  58  of the present invention and the design of the thermally conductive columns described with reference to FIG. 7 of U.S. Pat. No. 5,861,902. In the present invention, the supports  58  are made by providing small holes only through the sacrificial layer  46  and then filling them with the dielectric protective layer  38 . In a preferred embodiment, dielectric layer  38  is about twice the thickness as sacrificial layer  46 . Dielectric layer  38  provides a substantially planar base for electrically resistive heater layer  8 , so that heater layer  8  is nearly planar with substantially uniform thickness, even in embodiments including supports  58 . In addition the width of the supports  58  is preferably less than or equal to 1 micron, so that very little heat is transferred through the supports to the substrate. By contrast, in order to make the thermally conductive columns described with reference to FIG. 7 of U.S. Pat. No. 5,861,902, the holes are made through two layers (sacrificial silicon dioxide layer 90 and silicon nitride dielectric layer 92). The subsequently formed dielectric layer (24) is deliberately kept thin and will not be able to provide a significant amount of planarization. As a result, resistive heating element (14) of U.S. Pat. No. 5,861,902 is not nearly planar and does not have substantially uniform thickness, as a substantial portion of resistive layer (14) forms the interior of the vertical thermally conductive columns. At each column where the resistive heating element (14) gets thicker, the heater will have an undesirable cool spot. The thermally conductive columns may be appropriate in the case of the 50 micron wide heaters contemplated in U.S. Pat. No. 5,861,902 in order to remove heat from interior regions of the heater. However, it has been found for heaters narrower than about 30 microns, such thermally conductive columns are unnecessary. Supports  58  of the present invention are made small in width providing a large thermal impedance, and do not degrade the thermal efficiency of the isolated heater. 
     Experimentally determined advantages of the design of the present invention when compared to prior art devices having no isolating cavity underlying the heater will now be described. 
     Two sets of devices were fabricated, one set with the isolated heaters of the present invention and one set using non-isolated heaters of the prior art design. Both heaters used the same material and thicknesses for the insulating passivation layer  16  and protective layer  18 . Both heaters were the same size. The lower dielectric layer  38  of the isolated heater of the present invention was 0.2 μm of silicon nitride and the isolating cavity was 0.1 μm high. Devices were measured in an open pool of ink, without the nozzle plate on. A 0.6 μsec heat pulse of increasing energy (voltage) was applied until bubble nucleation was observed using a strobed light and a camera for observation. For the isolated heater of the present invention the threshold energy for bubble nucleation was &lt;70% of the threshold energy required for the non-isolated heater of the prior art design. 
     In the course of testing heaters for lifetime another observation was made. Isolated heaters showed a much lower degradation due to cavitation. When the nucleated bubble collapses it can damage the protective layer drilling a small hole that deepens for every bubble nucleation. Eventually such damage can make it through the protective layer exposing the heater. This shortens the lifetime of the heater. It was observed during experimental open pool testing that isolated heaters do not exhibit this defect. It is believed that in the isolated heater case when the bubble collapses the isolated heater is able to absorb some of the momentum energy by converting it to elastic membrane deformation. In contrast heaters of the prior art are not suspended and are formed on a rigid surface so that the heater layers can absorb the full shock of bubble collapse. In an actual device having a nozzle plate, how much of an impact bubble collapse has on lifetime can also be a function of the chamber geometry and whether or not the bubble is vented through the nozzle during drop ejection. Still, the elastic membrane deformation that occurs for the suspended heater of the present invention can have beneficial effects for reducing the amount of cumulative damage to the heater that otherwise could occur due to many firings of the same jet. 
       FIGS. 13   a - 13   d  schematically illustrates this effect using a simplified schematic cross-section of an isolated heater region  34  of the present invention where the different layers are not delineated.  FIG. 13   a  shows a simplified schematic cross-section of an isolated heater of the present invention at the start of an application of a heat pulse, represented by the current arrows  62 . Ink  80  lies above the isolated heater. When the temperature at the ink heater interface reaches a critical temperature (approx. 280 C) a bubble  70  will start to nucleate. At the start of nucleation of the bubble the pressure on the heater rapidly rises to approximately 70 Atmospheres and then immediately starts to drop. Modeling has shown that due to this pressure pulse the suspended heater will be pushed down to contact the lower surface  72  as shown schematically in  FIG. 13   b.    
     One issue to resolve when designing a suspended heater is that there are fewer paths to transfer the heat away from the heater region before the bubble collapses and fresh ink flows back over the heater. If the heater temperature is greater than approximately 100 C when fresh ink flows over the heater, then there is a possibility of boiling of the refilling ink causing drop ejection instability. While the heater is in contact with the lower surface  72 , due to pressure created during bubble nucleation, some of this excess heat is removed from the heater at a point in time where it is not detrimental to the bubble formation process as illustrated by heat flow arrow  64 . 
     As the bubble expands the pressure drops, falling an order of magnitude in approximately 0.2 μsec, and the heater returns to its suspended position as shown schematically in  FIG. 13   c . After approximately 1 μsec the pressure inside the bubble has fallen below ambient pressure and the bubble begins contracting. The bubble collapses to a point with the inertia from the bubble collapse causing an impact to the heater surface at a point as shown schematically in  FIG. 13   d . At this point the suspended heater compliantly deforms from the force of the collapsing bubble impact as shown schematically in  FIG. 13   d  by the directional recoil arrow  66 . It is believed that this recoil minimizes the damage due to bubble collapse that is normally seen on heaters of the prior art. 
     Another aspect of the present invention is the heat spreading layer  55 . While the nucleation and expansion of the bubble occurs in &lt;1 μsec, the collapse of the bubble and refilling of the ink occurs on a time scales of the order of 5 μsec. The heat spreading layer  55  will carry heat away from the heater layer over this time scale and allow the ink to preferentially absorb the heat so that it can be ejected during subsequent drop ejections as depicted by heat flow arrows  68  in  FIG. 13   d . No boiling of the ink during the ink refilling process was observed during experimental testing. 
     Another aspect of the isolated heater region  34  of the present invention is the limited amount of thermal capacitance used in the isolated heater stack  32 . The total thickness of the isolated heater stack  32  is limited to &lt;0.6 μm. The small amount of energy storing capacity contained in the isolated heater region  34  limits the amount of thermal energy available to the returning ink, thus limiting the temperature rise of the ink, thus improving the thermal efficiency of the heater and decreasing the likelihood of unwanted bubble nucleation during refill. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention. 
     PARTS LIST 
     
         
           2  Prior art single inkjet ejector 
           4  Silicon substrate 
           6  Prior art heater stack 
           8  Electrically resistive heater layer 
           10  Dielectric thermal barrier layer 
           12  Electrically conductive layer 
           14  Heater area 
           16  Insulating passivation layer 
           18  Protection layer 
           20  Ink chamber 
           22  Nozzle plate 
           24  Nozzle 
           26  Arrows 
           30  Single inkjet ejector of the present invention 
           32  Isolated heater stack of the present invention 
           34  Isolated heater region of the present invention 
           36  Isolating cavity 
           38  Lower dielectric protective layer 
           40  Arrow 
           42  Lateral Arrows 
           44   a,b  Metal lead 
           46  Sacrificial layer 
           48  Protrusions 
           50   a,b  Vias 
           51  photoresist layer 
           52  openings 
           54  insulating sealing layer 
           55  heat spreading layer 
           56  holes 
           58  supports 
           60  strip along heater 
           62  current arrows 
           64  heat flow arrow 
           66  recoil arrow 
           68  heat flow arrow 
           80  ink