Abstract:
A cavitation structure for a print head has a first dielectric layer overlying at least a first portion of a substrate. A second dielectric layer has a first portion overlying at least a second portion of the substrate and a second portion, different from the first portion of the second dielectric layer, overlying at least a portion of the first dielectric layer. A cavitation layer has a first portion in contact with the first dielectric layer and a second portion in lateral contact with the second portion of the second dielectric layer. A third dielectric layer is disposed on only the first portion of the second dielectric layer.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a divisional application of application Ser. No. 10/620,666, titled “FLUID-EJECTION DEVICES AND A DEPOSITION METHOD FOR LAYERS THEREOF,” filed Jul. 16, 2003 now U.S. Pat. No. 7,025,894, which is a Continuation-in-Part of U.S. application Ser. No. 09/978,985 filed Oct. 16, 2001 (abandoned), which applications are incorporated herein by reference. 
    
    
     BACKGROUND 
     Commercial products having imaging capability, such as computer printers, graphics plotters, facsimile machines, etc., have been implemented with fluid-ejection devices producing printed media. In many cases, such devices utilize inkjet technology whereby an inkjet image is created when a precise pattern of dots is formed on a printing medium from ejected ink droplets. Typically, an inkjet print head is supported on a movable carriage that traverses over the surface of the print medium and is controlled to eject drops of ink at appropriate times pursuant to commands of a microcomputer or other controller, wherein the timing of the application of the ink drops is intended to correspond to a pattern of pixels of the image being printed. A typical inkjet print head includes an array of precisely formed nozzles in an orifice plate. The plate is attached to a thin-film substrate that implements ink firing heater resistors and apparatus for enabling the resistors. The thin-film substrate is generally comprised of several thin layers of insulating, conducting, or semiconductor material that are deposited successively on a supporting substrate, or die, in precise patterns to form collectively, all or part of an integrated circuit. 
     The thin-film substrate or die is typically comprised of a layer, such as silicon, on which are formed various thin-film layers that form thin-film ink firing resistors, apparatus for enabling the resistors, and interconnections to bonding pads that are provided for external electrical connections to the print head. Ongoing improvements in the design of fluid-ejection devices have resulted in more efficient print-head components, such as resistors barrier layers, and passivation layers. In some cases, barrier layers and passivation layers deposited by physical vapor deposition or chemical vapor deposition methods have been utilized to improve performance. In other cases; sputtering techniques have been used to form barrier layers and passivation layers. While these techniques have some utility, it is desirable to have an improved barrier layers and passivation layers capable of improving performance and increasing resistor life. 
     Of course, energy expenditure is necessary for operation of fluid-ejection devices. In this regard, the term “turn-on energy” relates to the energy required to form a vapor bubble of a size sufficient to eject a predetermined amount of ink volume through a print head nozzle. With ever increasing usage of electrically driven devices, conservation becomes an important consideration. With respect to fluid-ejection devices, a reduction in “turn-on energy” would be desirable, especially if such reduction produced improved print head performance and prolonged print head life. 
     SUMMARY 
     One embodiment of the present invention provides a method of forming a cavitation layer of a print head. The method includes utilizing an atomic layer deposition process. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a portion of a fluid-ejection device according to an embodiment of the present invention. 
         FIG. 2  is a cross-sectional view of a portion of a fluid-ejection device according to another embodiment of the present invention. 
         FIG. 3  illustrates a passivation layer according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description of the present embodiments, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that process, electrical or mechanical changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims and equivalents thereof. 
     Embodiments of the present invention involve forming layers of fluid-ejection devices, such as print heads, using atomic layer deposition (ALD). ALD involves depositing a selected composition on crystalline or amorphous substrates or layers one molecular layer at a time. Unlike atomic layer epitaxy (ALE) processes that involve growing a single crystalline layer on a crystalline substrate or layer that mimics the substrate or layer, ALD does not require a crystalline substrate or layer, as does ALE. ALE operates in a ultra-high-to-high vacuum, e.g., corresponding to absolute pressures from about 10 −10  to about 10 −7  Torr, whereas ALD operates in medium-to-low vacuum, e.g., corresponding to absolute pressures from about 10 −3  to about 1 (one) Torr. 
     For one embodiment, a passivation layer is formed on a surface of a substrate. The passivation layer generally protects exposed elements of the fluid-ejection device from environmental contaminants, e.g., fluids, such as ink, thus ensuring electrical stability of the fluid-ejection device. For another embodiment, the passivation layer is a thin dielectric layer. The passivation layer is deposited using the ALD process referred to herein as, for example, an ALD dielectric or an ALD passivation layer, as appropriate. In other embodiments, a cavitation layer of a firing chamber of the fluid-ejection device is formed using ALD and is referred to herein as an ALD cavitation layer, for example. 
       FIG. 1  is an unscaled cross-sectional view of a portion of a fluid-ejection device (or print head)  21  according to an embodiment of the present invention. The fluid-ejection device  21  is comprised of a plurality (or stack) of thin film layers, generally indicated by the reference numeral  26 , that are stacked atop a die  49 . Contact termination in the print head is also shown in  FIG. 1 , as described in more detail below. 
     The layers over the die  49  form thin-film ink firing resistors or heating elements, such as a resistive layer (or resistor)  48 , and an apparatus for enabling the resistors. In a particular embodiment, the die  49  (e.g., about 650 microns thick) is composed of silicon. The silicon die  49  is a semiconductor that functions as a substrate to support the overlying layers. In this regard, immediately overlying the die  49  there is formed by plasma enhanced chemical vapor deposition (PECVD) of a tetra ethyl ortho silicate (TEOS) or silane (SiH4) based oxide (e.g., about 1.0 micron thick) layer  47 . This layer insulates the overlying inkjet circuitry from the silicon die  49  and provides thermal isolation from the silicon, thereby keeping the circuitry above the layer  47  from being shorted out by the silicon below. In operation, the layer  47  functions as a standoff so that heat moves away from, rather than toward, the silicon die  49 . 
     A layer  45 , formed by plasma enhanced chemical vapor deposition (PECVD) of one embodiment, is deposited upon the layer  47 . For another embodiment, layer  45  is formed by ALD and is about 250 angstroms thick. For one embodiment, layer  45  is a layer of an amorphous material, such as silicon nitride (Si 3 N 4 ). The layer  45  chemically stabilizes the underlying TEOS-oxide layer  47  and provides thermal and chemical stabilization of resiestive layer  48 . Resistive layer  48  is patterned on layer  45  and is chemically defined by an etching process. Layer  48  is comprised of resistive materials such as tantalum, aluminum, silicon, or tantalum nitride and it functions to resistively heat the overlying structure to enable ejection of an ink droplet. 
     The overlying structure includes a passivation layer  42  that is deposited, patterned, and etched to open up contact holes at end of the resistive layer  48 . Specifically, passivation layer  42  is deposited on layer  45  and layer  48  using ALD. Passivation layer  42  is structured to create interconnects to a layer  41  (e.g., about 0.5 micron thick). In one embodiment, the layer  41  is a thin tungsten film (e.g., about 0.5 micron thick) deposited and patterned by plasma processes. Overlying the tungsten layer  41  is a TEOS-oxide layer (e.g., about 0.6 micron thick)  39  that is disposed laterally in relation to the firing chamber  24 . The layer  39  is etched to enable an overlying aluminum contact terminal  35  to contact the tungsten layer  41 . In this manner, the layer  39  functions as an interdielectric between two metals, the underlying tungsten layer  41  and the overlying aluminum contact terminal  35 . 
     In the embodiment shown in  FIG. 1 , the firing chamber  24  includes a cavitation layer  31  deposited over the stack  26  and in contact laterally with a tetra ethyl ortho silicate (TEOS)-oxide layer e.g., about 0.6 micron thick)  33 . The cavitation layer  31  provides mechanical protection to the underlying structure and, in particular, prevents chemical and impact damage to the resistor  48 . The TEOS layer  33 , on the other hand, provides insulation for the layers of the fluid-ejection device and separates the cavitation layer  31  from other structures. It will be noted that the cavitation layer  31  is isolated throughout the ejection device  21 , except where it contacts the ALD passivation layer  42 . Cavitation layer  31  can be of tantalum (Ta), titanium (Ti), molybdenum (Mo), niobium (Nb), etc. For one embodiment, cavitation layer  31  is deposited on layer  33  and layer  42  using ALD. For another embodiment cavitation layer  31  is about 500 angstroms thick. Using ALD for cavitation layer  31  results in conformal coverage over layer  33  and produces a low-stress, substantially crack-free film. 
     For some embodiments, a passivation layer  110  is disposed on layer  33  using ALD, chemical vapor deposition, or the like. For one embodiment, passivation layer  110  is a carbide layer, such as SiC silicon carbide, diamond like carbons (DLCs), e.g., fullerenes or graphite, etc. Passivation layer  110  acts to protect layer  33  against inks and other fluids. Passivation layer  110  also acts to protect against wear. 
     In one embodiment, the passivation layer  42  is a dielectric film, such as silicon carbide, diamond like carbon, aluminum oxide etc. For one embodiment, passivation layer  42  has a thickness of between about 250 angstroms and 2000 angstroms. For another embodiment, passivation layer  42  has a thickness between about 250 to 500 angstroms, preferably about 300 angstroms. This thin film enables substantially reduced drive energies because of the thinness of the dielectric and, possibly, because of enhanced thermal conductivity. Dielectrics that can be deposited by the ALD technique contain refractory metals, transitional metals, and insulators, such as silicates. Other dielectrics depositable by atomic level deposition include metal oxides, nitrides, borides, and carbides. 
     Examples of oxides depositable by atomic level deposition include aluminum oxide (Al 2 O 3 ), titanium oxide (TiO 2 ), tantalum oxide (Ta 2 O 5 ), hafnium oxide (HfO 2 ), magnesium oxide (MgO), cesium oxide (CeO 2 ), niobium oxide (Nb 2 O 5 ), lanthanum oxide (La 2 O), yttrium oxide (Y 2 O 3 ), aluminum titanium oxide (Al x Ti y O z ), tantalum hafnium oxide (Ta x Hf y O z ), etc. Examples of nitrides depositable by atomic level deposition include silicon nitride (SiN), aluminum nitride (AlN), titanium nitride (TiN), tantalum nitride (TaN), niobium nitride (NbN), molybdenum nitride (MoN), tungsten nitride (WN), etc. Examples of refractory metals depositable by atomic level deposition include tantalum (Ta), titanium (Ti), tungsten (W), molybdenum (Mo), niobium (Nb), titanium nitride (TiN), tantalum nitride (TaN), niobium nitride (NbN), molybdenum nitride (MoN), tungsten nitride (WN), etc. Examples of transitional metals depositable by atomic level deposition include tantalum (Ta), titanium (Ti), tungsten (W), copper (Cu), molybdenum (Mo), hafnium (Hf), etc. Examples of borides depositable by atomic level deposition include titanium diboride (TiB 2 ), zirconium diboride (ZrB 2 ), arsenic hexaboride (AsB 6 ), etc. 
     During the ALD process, a source-material precursor and a binding precursor are employed alternately with inert purge gasses in between. The purge gasses ensure that no stray gasses, such as the source-material precursor, are present before the next gas, such as the binding precursor, is employed. The deposited source-material precursor chemically reacts on the surface with the deposited binding precursor to form a single molecular ALD layer. The single molecular ALD layers build up molecular layer-by-molecular layer using this process. As a result of the monolayer-by-monolayer build up, the final thickness of the ALD layer is well controlled 
     Examples of source-material precursors include trimethylated aluminum (Al(CH 3 ) 3 ), aluminum trichloride (AlCl 3 ), titanium tetrachloride (TiCl 4 ), tantalum pentachloride (TaCl 5 ), bis(tert-butylimido), bis(dimethylamido)tungsten ((BuN) 2 (Me 2 N) 2 W), methane (CH 4 ), etc. Examples of binding precursors include oxygen-source materials, e.g., water vapor, a nitrogen-source materials, e.g., ammonia, hydrogen, etc. 
     For one embodiment, the source-material precursors include a dopant, such as aluminum, nitrogen, carbon, oxygen, etc. For this embodiment, the ALD process is used to deposit layers that include the dopant. For another embodiment, the ALD process is used to deposit a cavitation layer  31  with a dopant. For some embodiments, the dopant, e.g., nitrogen or the like, reduces a thermal resistance of cavitation layer  31 . This acts to reduce the thermal resistance between resistive layer  48  and ink contained in firing chamber  24 , resulting in a lower turn-on energy. 
     For another embodiment, the ALD process is used to deposit a passivation layer  42  that includes a dopant, such as aluminum, boron, phosphorous, germanium, barium, calcium, strontium, etc., for reducing the thermal resistance of layer  42 , which acts to reduce the thermal resistance between resistive layer  48  and ink contained in firing chamber  24 . For other embodiments, adding a dopant to layer  42 , e.g., carbon, oxygen, etc., acts to increase the thermal resistance of layer  42 . For other embodiments, dopants such as phosphorous, oxygen, carbon, nitrogen, etc., act to increase the hardness, reduce plastic flow, etc. of the respective layer. 
     For some embodiments, a seed layer  115 , e.g., of tungsten, titanium nitride, or tantalum nitride is deposited on ALD passivation layer  42  using ALD and layer  41  is subsequently formed on seed layer  115 . For other embodiments, a seed layer  120 , e.g., of titanium nitride or tantalum, is deposited on layer  39  using ALD and aluminum contact terminal  35  is subsequently formed on seed layer  120 . For various embodiments, seed layers  115  and  120  are about 100 angstroms thick. 
       FIG. 2  is an unscaled cross-sectional view of a fluid-ejection device  221  according to another embodiment of the present invention. The device  221  is comprised of a plurality (or stack) of thin-film layers, generally indicated by the reference numeral  226 . The device  221  utilizes ALD layers, and utilizes contact termination as described above in reference to the ejection device  21 . The fluid-ejection device  221  includes a firing chamber  224 . In addition, the fluid-ejection device  221 , like the device  21  of  FIG. 1 , is comprised of a plurality of thin-film layers stacked on a silicon die  65 . 
     The die  65  is similar in structure and function to the die  49  of  FIG. 1 . A field oxide or TEOS layer (e.g., about 1.0 micron thick)  63 , similar in structure and function to the layer  47  of  FIG. 1 , is disposed on the die  65  and a heating (or resistor) layer  57 , composed of tantalum/aluminum, or other suitable metal, is disposed on the layer  63 . An aluminum layer (e.g., about 0.5 micron thick)  55  is disposed laterally of a region  228  of firing chamber  224  and overlying the layer  57 . The aluminum layer  55  is covered by an ALD dielectric e.g., about 0.1 micron thick) film  52 . The ALD film  52  is similar to the layer  42  of  FIG. 1  and is formed according to the above-described process. For other embodiments, layer  52  is similar to and is formed as described for layer  10  of  FIG. 1 . 
     Firing chamber  224  includes a cavitation layer  51  deposited over the stack  226 . For one embodiment, cavitation layer  51  is deposited on layer  33  using ALD. Cavitation layer  51  can be of tantalum (Ta), titanium (Ti), molybdenum (Mo), niobium (Nb), etc. For another embodiment cavitation layer  31  is about 500 angstroms thick. For one embodiment, a seed layer  230 , e.g., of refractory metal, is deposited on layer  57  using ALD, and layer  55  is subsequently formed on seed layer  230 . For some embodiments, seed layer  230  is about 100 angstroms thick. 
     For some embodiments, the passivation layers of the present invention, such as passivation layers  42  and  110  of  FIG. 1  and passivation layer  52  of  FIG. 2  include multiple layers as is shown generally for a passivation layer  300  in  FIG. 3 . For one embodiment, passivation layer  300  includes layers  310   1  to  310   N . For another embodiment, each of layers  310   1  to  310   N  is formed using ALD, chemical vapor deposition (CVD) or the like and has a thickness between about 250 angstroms and about 350 angstroms. For another embodiment, some of the layers  310  are formed using ALD and others are formed using CVD, for example. For one embodiment, some of the layers  310  are of one material, such as silicon carbide and others are of another material, such as silicon nitride. For another embodiment, passivation layer includes two layers, e.g., one of silicon nitride the other of silicon carbide. 
     The present invention affords several distinct advantages. Because the ALD passivation layers are so thin, they permit reduced drive energies with consequent low turn on energy drop generation of the resistor, for example, in the resistor regions of the ejection devices  21  and  221 . This, in turn, results in faster thermal response, thereby enabling a higher frequency of operation. The present invention enables rapid print head resistor heating and cool down. As a result, a thermally more efficient print head is achieved with resulting swath size increases. Such increases, in turn, substantially improve fluid-ejection device throughput. 
     In another embodiment, the invention affords the flexibility of using very thin multiple dielectrics for custom tailoring of thermal properties. This is because the ALD process enables addition of a single molecular layer at a time, a dielectric film having a precise predetermined thickness can be achieved. 
     A possible limitation of ALD is low growth rate that may lead to potential problems in mass production. Thus, ALD may not be able to compete with other widely used thin film deposition techniques, such as chemical vapor deposition (CVD) or physical vapor deposition (PVD). 
     Advantageously, however, the films produced by the ALD technique have low stresses and are substantially free of voids, pinholes, and cracks. These attributes of ALD films act to increase resistor life and print head life. 
     Because the chemical purity is very high, resistor printing and storage life are substantially extended. The high thermal efficiency of the present invention translates into comparatively lower steady state die temperatures and enhanced resistor life. 
     It is known by those skilled in the art that electrical shorts reduce yield in some fluid-ejection devices. In the embodiments described above, high particle tolerance in passivation is achieved. Thus, the likelihood of shorts is diminished thereby raising circuit yield. 
     CONCLUSION 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiments shown. Many adaptations of the invention will be apparent to those of ordinary skill in the art. Accordingly, this application is intended to cover any adaptations or variations of the invention. It is manifestly intended that this invention be limited only by the following claims and equivalents thereof.