Patent Publication Number: US-7716832-B2

Title: Method of manufacturing a fluid ejection device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a divisional of copending U.S. utility application entitled, “Heating Element of a Printhead Having Resistive Layer over Conductive Layer,” having Ser. No. 10/425,749, filed Apr. 29, 2003, which is a divisional of U.S. utility application entitled “Heating Element of a Printhead Having Resistive Layer over Conductive Layer,” having Ser. No. 09/846,124, filed Apr. 30, 2001, now abandoned. The aforementioned applications having Ser. Nos. 10/425,749 and 09/846,124 are both entirely incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention relates to printheads, such as those used in inkjet cartridges and the like. 
   BACKGROUND OF THE INVENTION 
   Generally, thermal actuated printheads use resistive elements or the like to achieve ink expulsion. A representative thermal inkjet printhead has a plurality of thin film resistors provided on a semiconductor substrate. A top layer defines firing 20 chambers about each of the resistors. Propagation of a current or a “fire signal” through the resistor causes ink in the corresponding firing chamber to be heated and expelled through the corresponding nozzle. 
   To form the resistors, a resistive material is deposited over an insulated substrate, and a conductive material is deposited over the resistive material. The conductive material is photomasked and wet etched to form conductor traces and a beveled surface adjacent a resistor. However, due to the difficulty in controlling the wet etching process, substantially inconsistent resistor lengths (gap in the conductor line) and beveled angles result. A dry etch is generally not used to etch the conductor traces because dry etch selectivity of typical conductor to resistor materials is poor. 
   The resistive material is photomasked and etched to form resistors. A passivation layer is deposited over the conductor traces. The passivation layer is often susceptible to pinhole defects, and wet chemistry, including those used in subsequent wet processing and inks, may travel through the defects in the passivation layer to the conductor layer. The conductor layer thereby begins to corrode. 
   SUMMARY OF THE INVENTION 
   In the present invention, a heating element of a printhead has a conductive layer deposited over a substrate, and a resistive layer deposited over and in electrical contact with the conductive layer. 
   Many of the attendant features of this invention will be more readily appreciated as the same becomes better understood by reference to the following detailed description and considered in connection with the accompanying drawings in which like reference symbols designate like parts throughout. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a perspective view of a print head cartridge of the present invention; 
       FIG. 2  illustrates a cross-sectional view of an embodiment of the printhead of  FIG. 1  shown through section  2 - 2 ; 
       FIG. 3  is a flow chart illustrating an embodiment of the process of forming the resistor over the conductor traces; 
       FIG. 4   a  illustrates a perspective view of an embodiment of the printhead formation after the conductor traces have been etched; 
       FIG. 4   b  illustrates a perspective view of an embodiment of the printhead formation after the resistors have been etched; 
       FIG. 5  illustrates a partial cross-sectional view of the formation of  FIG. 4   b  through section  5 - 5 ; 
       FIG. 6  illustrates a cross-sectional view of the formation of  FIG. 4   b  through section  6 - 6 ; 
       FIG. 7  illustrates another embodiment of the cross-sectional view of the formation of  FIG. 4  through section  6 - 6 ; 
       FIG. 8  illustrates another embodiment of the cross-sectional view of the formation of  FIG. 4  through section  6 - 6 ; 
       FIG. 9   a  illustrates a layer of photoresist over the conductive layer as part of the process of bevel definition; 
       FIG. 9   b  illustrates  FIG. 9   a  after exposing the photoresist to light through a half-tone mask; 
       FIG. 10  is a half-tone mask; and 
       FIG. 11  is another half-tone mask. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a perspective view of an inkjet cartridge  10  with a printhead  14  of the present invention.  FIG. 2  illustrates a cross-sectional view through section  2 - 2  of  FIG. 1 . In  FIG. 2 , a thin film stack is applied over a substrate  28 . A slot region  120  is shown through the thin film stack and the substrate  28 . One method of forming the drill slot is abrasive sand blasting. A blasting apparatus uses a source of pressurized gas (e.g. compressed air) to eject abrasive particles toward the substrate coated with thin film layers to form the slot. The particles contact the coated substrate, causing the formation of an opening therethrough. Abrasive particles range in size from about 10-200 microns in diameter. Abrasive particles include aluminum oxide, glass beads, silicon carbide, sodium bicarbonate, dolomite, and walnut shells. 
   In one embodiment, the substrate is a monocrystalline silicon wafer. The wafer has approximately 525 microns for a four-inch diameter or approximately 625 microns for a six-inch diameter. In one embodiment, the silicon substrate is p-type, lightly doped to approximately 0.55 ohm/cm. 
   Alternatively, the starting substrate may be glass, a semiconductive material, a Metal Matrix Composite (MMC), a Ceramic Matrix Composite (CMC), a Polymer Matrix Composite (PMC) or a sandwich Si/xMc, in which the x filler material is etched out of the composite matrix post vacuum processing. The dimensions of the starting substrate may vary as determined by one skilled in the art. 
   In one embodiment, a capping layer  32  is deposited or grown over the substrate  28 . In one embodiment, the layer  32  covers and seals the substrate  28 , thereby providing a gas and liquid barrier layer. Because the capping layer is a barrier layer, fluid is substantially restricted from flowing into the substrate  28 . Capping layer  32  may be formed of a variety of different materials such as silicon dioxide, aluminum oxide, silicon carbide, silicon nitride, and glass (PSG). In one embodiment, the use of an electrically insulating dielectric material for the capping layer also serves to electrically insulate substrate  28 . In one embodiment, the capping layer  32  is a thermal barrier of the substrate from the resistor. The capping layer may be formed using any of a variety of methods known to those of skill in the art such as thermally growing the layer, sputtering, evaporation, and plasma enhanced chemical vapor deposition (PECVD). The thickness of capping layer may be any desired thickness sufficient to cover and seal the substrate. Generally, the capping layer has a thickness of up to about 1 to 2 microns. 
   In one embodiment, the layer  32  is a phosphorous-doped (n+) silicon dioxide interdielectric, insulating glass layer (PSG) deposited by PECVD techniques. Generally, the PSG layer has a thickness of up to about 1 to 2 microns. In one embodiment, this layer is approximately 0.5 micron thick and forms the remainder of the thermal inkjet heater resistor oxide underlayer. In another embodiment, the thickness range is about 0.7 to 0.9 microns. 
   In another embodiment, the capping layer  32  is field oxide (FOX) that is thermally grown on the exposed substrate  28 . The process grows the FOX into the silicon substrate as well as depositing it on top to form a total depth of approximately 1.3 microns. Because the FOX layer pulls the silicon from the substrate, a strong chemical bond is established between the FOX layer and the substrate. 
   In one embodiment, a layer  30  is deposited or grown over the capping layer  32 . In one embodiment, the layer  30  minimizes junction spiking and electromigration. In one embodiment, the layer  30  is one of titanium nitride, titanium tungsten, titanium, a titanium alloy, a metal nitride, tantalum aluminum, and aluminum silicone. 
   In one embodiment, layer  32  is deposited over or grown directly onto the substrate  28 . In another embodiment, there are layers (not shown), in addition to layer  30  and layer  32 , that are deposited over the substrate. These layers are composed of materials chosen from the layers  30  and  32  described above. 
   In one embodiment, a conductive layer  114  is formed by depositing conductive material over the layer  30 . The conductive material is formed of at least one of a variety of different materials including aluminum, aluminum with about ½% copper, copper, gold, and aluminum with ½% silicon, and may be deposited by any method, such as sputtering and evaporation. Generally, the conductive layer has a thickness of up to about 1 to 2 microns. In one embodiment, sputter deposition is used to deposit a layer of aluminum to a thickness of approximately 0.5 micron. 
   The conductive layer  114  is patterned and etched as described in more detail below with respect to steps  210  and  220  of  FIG. 3 . A conductor trace width  16  and a resistor length  17 , as shown in  FIG. 4   a , is defined by the etch of the conductive layer. (The resistor length is a gap or opening in the conductive line). At this point, the layer  30 , as shown in  FIG. 4   a , or possibly even layer  32 , as shown in  FIG. 5 , is exposed along the resistor length  17  (or opening) in between the traces due to etching. At opposite ends of the defined resistor length  17 , the conductive material  114  has a beveled surface  126  defined as described in more detail below. The conductor traces have a top surface  128 , two opposing side surfaces  130 , and the end beveled surface  126 . 
   After forming the conductor traces, a resistive material  115  is deposited over the etched conductive material  114 , as shown in  FIG. 2  (step  240  of  FIG. 3 ). The resistive material is etched to form resistors having the resistor length  17 , as described in more detail below with respect to steps  250  and  260  of  FIG. 3 . The width of the resistors across the conductor traces is a cap width  18 , which varies with the embodiment, as described in more detail below with regard to  FIGS. 6 ,  7  and  8 . There is also a resistor width of the gap  17  that is the same length as the cap width, in one embodiment Alternatively, the resistor width is different than the cap width. In one embodiment, the resistive material encapsulates the conductor traces. In one embodiment, sputter deposition techniques are used to deposit a resistive material layer of tantalum aluminum  115  composite across the etched conductor traces. The composite has a resistivity of approximately 30 ohms/square. Typically, the resistor layer has a thickness in the range of about 500 angstroms to 2000 angstroms. However, resistor layers with thicknesses outside this range are also within the scope of the invention. 
   A variety of suitable resistive materials are known to those of skill in the art including tantalum aluminum, nickel chromium, and titanium nitride, which may optionally be doped with suitable impurities such as oxygen, nitrogen, and carbon, to adjust the resistivity of the material. The resistive material may be deposited by any suitable method such as sputtering, and evaporation. 
   As shown in the embodiment of  FIG. 2 , an insulating passivation layer  117  is formed over the resistors and conductor traces to prevent electrical charging of the fluid or corrosion of the device, in the event that an electrically conductive fluid is used. Passivation layer  117  may be formed of any suitable material such as silicon dioxide, aluminum oxide, silicon carbide, silicon nitride, and glass, and by any suitable method such as sputtering, evaporation, and PECVD. Generally, the passivation layer has a thickness of up to about 1 to 2 microns. 
   In one embodiment, a PECVD process is used to deposit a composite silicon nitride/silicon carbide layer  117  to serve as component passivation. This passivation layer  117  has a thickness of approximately 0.75 micron. In another embodiment, the thickness is about 0.4 microns. The surface of the structure is masked and etched to create vias for metal interconnects. In one embodiment, the passivation layer places the structure under compressive stress. 
   In one embodiment, a cavitation barrier layer  119  is added over the passivation layer  117 . The cavitation barrier layer  119  helps dissipate the force of the collapsing drive bubble left in the wake of each ejected fluid drop. Generally, the cavitation barrier layer has a thickness of up to about 1 to 2 microns. In one embodiment, the cavitation barrier layer is tantalum. The tantalum layer  119  is approximately 0.6 micron thick and serves as a passivation, anti-cavitation, and adhesion layer. In one embodiment, the cavitation barrier layer absorbs energy away from the substrate during slot formation. In this embodiment, tantalum is a tough, ductile material that is deposited in the beta phase. The grain structure of the material is such that the layer also places the structure under compressive stress. The tantalum layer is sputter deposited quickly thereby holding the molecules in the layer in place. However, if the tantalum layer is annealed, the compressive stress is relieved. 
   In one embodiment, a top (or barrier) layer  124  is deposited over the cavitation barrier layer  119 . In one embodiment, the barrier layer has a thickness of up to about 20 microns. In one embodiment, the barrier layer  124  is comprised of a fast cross-linking polymer such as photoimagable epoxy (such as SU8 developed by IBM), photoimagable polymer or photosensitive silicone dielectrics, such as SINR-3010 manufactured by ShinEtsu™. 
   In another embodiment, the barrier layer  124  is made of an organic polymer plastic which is substantially inert to the corrosive action of ink. Plastic polymers suitable for this purpose include products sold under the trademarks VACREL and RISTON by E. I. DuPont de Nemours and Co. of Wilmington, Del. The barrier layer  124  has a thickness of about 20 to 30 microns. 
   In one embodiment, the barrier layer  124  includes a firing chamber  132  from which fluid is ejected, and a nozzle orifice  122  associated with the firing chamber through which the fluid is ejected. The fluid flows through the slot  120  and into the firing chamber  132  via channels formed in the barrier layer  124 . Propagation of a current or a “fire signal” through the resistor causes fluid in the corresponding firing chamber to be heated and expelled through the corresponding nozzle  122 . In another embodiment, an orifice layer having the orifices  122  is applied over the barrier layer  124 . 
   As shown more clearly in the printhead  14  of  FIG. 1 , the nozzle orifices  122  are arranged in rows located on both sides of the slot  120 . In one embodiment, the nozzle orifices, and corresponding firing chambers are staggered from each other across the slot. In  FIG. 2 , a fining chamber in the printhead that is staggered across the slot from the firing chamber  132  is shown in dashed lines. 
   The flow chart of  FIG. 3  illustrates an embodiment of the process of forming the heating element of the printhead. After depositing the conductive material in step  200 , the conductive material is photomasked, such as by photolithography, and etched to form the conductor traces. In one embodiment, photoresist material is deposited in step  210  over the conductive material. The photoresist material is exposed to light through a mask and developed to form a pattern over the conductive material, as described in more detail below with regard to  FIGS. 9   a ,  9   b ,  10  and  11 . Conductive material that is not covered by the photoresist material is removed using a dry plasma etch in step  220 , which is a conventional gaseous etch technique. 
     FIG. 4   a  illustrates one embodiment where the formation after the conductor trace width  16  and the resistor length or gap  17  have been etched. The beveled surface  126  of the conductor trace is defined as described in the embodiments below. In another embodiment, only the resistor length or gap  17  is formed in step  220 . The trace width and cap width are then formed together in step  260  to look like the embodiment shown in  FIG. 8 . 
   The photoresist material is then stripped in step  230  before the resistive material is deposited in step  240 . Similar to step  210 , the resistive layer  114  is patterned and etched in step  250 , as shown in  FIG. 4   b . Thereby, the cap width  18  of the resistive material and the conductor terminations (not shown) are defined. In one embodiment, the photoresist material is deposited, masked, exposed and developed to the pattern over the resistive material in step  250 , as described in more detail below. The resistive layer and photoresist material is then etched in step  260 . In one embodiment, the resistive layer is dry etched. In another embodiment, the resistive layer is wet etched. The photoresist material deposited over the resistive layer is removed in step  270  before the passivation layer is deposited. 
     FIG. 5  illustrates a cross-sectional view of the resistive material  115  deposited over the opening (or resistor length  17 ) and the beveled surfaces  126  of the etched conductive layer  114 .  FIG. 6  illustrates a cross-sectional view of the width of the conductor traces with the etched resistive material  115  deposited thereover.  FIGS. 7 and 8  illustrate other embodiments as alternatives to the embodiment shown in  FIG. 6 . 
   For  FIG. 5 , the photoresist material in step  250  covers the resistor and conductor terminations (not shown). The photoresist material pattern in step  250  varies for defining the formations of  FIGS. 6 ,  7 , and  8 . For  FIGS. 6 and 7 , the photoresist material in step  250  is in a pattern that covers the conductor trace. For  FIG. 8 , the photoresist material in step  250  is in a pattern that defines the top surface  128  of the conductor trace. During the etch step  260 , the area that is not covered with the photoresist material is etched away. 
   In one embodiment, as shown in  FIG. 5 , the layer  30  is etched away in step  220  with the conductive layer in the area defining the resistor length  17 . In one embodiment, the layer  30  is conductive and electrically conducts under the opening in the conductor traces, if not removed. In another embodiment, additionally the layer  32  and/or the substrate  28  are partially etched in the gap area ( 17 ). In yet another embodiment, the layer  30  is not etched away with the conductive layer. 
   In one embodiment, the end beveled surface  126  has an angle of about 35 to 55 degrees with the substrate, as shown in  FIG. 5 . In another embodiment, the end beveled surface has an angle of about 45 degrees with the substrate. As shown, the beveled surface  126  is substantially smooth from the dry etch. The horizontal length of the beveled surface  126  is about ½ to 3 microns. In one embodiment, the horizontal length depends upon the drop weight of the print cartridges. For higher drop weights, the more slope (or higher length) is desired. 
   In  FIGS. 6 and 8 , the side surfaces  130  are substantially vertical, so that conductor traces are able to be etched closer together, thereby increasing the die separation ratio. In one embodiment, the side surfaces  130  of the conductor traces are dry etched in the process described herein. In one embodiment, the side surfaces  130 , have an angle of about 60 to 80 degrees with the substrate. In another embodiment, the side surfaces have an angle of about 70 degrees with the substrate. The side surfaces  130  are formed as described herein. 
   In  FIG. 7 , the side surfaces  130   a  are sloped more than the side surfaces  130  shown in  FIGS. 6 and 8 . The side surfaces  130   a  have an angle of about 35 to 55 degrees, or about 45 degrees, with the substrate. In one embodiment, the angle of the side surfaces  130   a  is substantially similar to the angle of the beveled surface  126 . In another embodiment, the angle of the side surfaces  130   a  is different than the angle of the beveled surface  126 . In one embodiment, the side surfaces  130   a  are formed using the photomasking and dry etching techniques, as described herein. In another embodiment, the side surfaces are formed in a manner substantially similar to forming the end beveled surfaces  126 , as described below. 
   In  FIGS. 6 and 7 , the cap width  18  of the resistive material is greater than the width  16  of the conductor trace. In this embodiment, the resistive material encapsulates the conductor traces. In the embodiment where the layer  30  is formed of the same material as the resistive material  115 , the conductor layer  114  is substantially completed encapsulated. The resistive material encapsulating the side surfaces  130  of the conductor traces aid in protecting the traces from corrosion due to wet chemistry, including those fluids used in subsequent wet processing and inks. 
   In  FIG. 8 , the cap width  18  of the resistive material covers the top surface  128  of the conductor traces, the width  16 . The side surfaces  130  are not covered with the resistive material in this embodiment. The passivation layer  117 , when deposited, is in direct contact with the side surfaces and aid in protecting the conductor traces from corrosion. 
   In one embodiment of step  210  of  FIG. 3 , the conductor traces and the beveled surfaces  126  (and in some embodiments, the side surfaces  130   a  of  FIG. 7 ) are defined using masking techniques illustrated in  FIGS. 9   a ,  9   b , and  10 . The sloped end surfaces  126  and the substantially vertical side walls  130  are formed using a half-tone mask  136 , as shown in  FIG. 10 . In some embodiments, a half-tone mask  137  ( FIG. 11 ) that is similar to the mask  136  is used to form both the sloped end surfaces  126  and the sloped side surfaces  130   a . The masks  136  and  137  are described in more detail below. 
     FIG. 9   a  illustrates a layer of photoresist material  134  over the conductive layer  114  as part of the process of bevel definition. The photoresist material  134  is a chemical substance rendered insoluble by exposure to light. The unexposed areas are washed away. After exposing the photoresist material  134  to light through the mask  136 , the formation in cross-section is illustrated in  FIG. 9   b . The photoresist material  134  is sloped as shown in  FIG. 9   b  after step  210  is performed. The photoresist material  134  along with the conductor layer  114  of  FIG. 9   b  is then etched using a dry etch in step  220 . After etching, the beveled surfaces  126  are defined as shown in  FIG. 5 . In addition, the gap or resistor length  17 , and the side surfaces  130 , as shown in  FIGS. 6 and 8 , are defined. In some embodiments the sloped side surfaces  130   a  of  FIG. 7  are also defined using this photomask technique, but using the mask  137 . 
   The mask  136  has three areas, area  138 , gradiated area  140 , and open area  142 . The area  138  is substantially non-transparent. In one embodiment, this area  138  is made of chrome. When this area of the mask is placed over the photoresist material  134 , and the photoresist material is exposed to light, the area under  138  is unexposed and can be washed away. The open area  142  is an opening in the mask through which the light exposing the photoresist material passes through. The photoresist material under the open area  142  substantially hardens (or is rendered insoluble) in response to the light The area  140  is gradiated. The area  140  gradually moves from being substantially non-transparent to being substantially transparent when moving away from area  138  and closer to area  142 . The photoresist material that is exposed to the light under the area  140  forms a slope as shown in  FIG. 9   b.    
   In an alternative embodiment, the photoresist material is a positive photoresist material. Opposite to the negative photoresist material described above, the positive photoresist material that is not exposed to light is rendered insoluble, while the material that is exposed to light is washed away. A mask used in this embodiment that is similar to mask  136  has, for example, areas  138  and  142  switched to render the same shape of material  134  in  FIG. 9   b . Similarly, the area  140  gradually moves from being substantially non-transparent to being substantially transparent when moving away from area  138  and closer to area  142 . 
   The mask  137  is similar to the mask  136  except that the mask  137  has a u-shaped gradiated area  140  that surrounds the open area  142 . The u-shaped gradiated area  140  is in between the open area  142  and the area  138 . The u-shaped forms photoresistive material in a substantially trapezoidal cross-section over the conductive material. After the photoresistive material is etched, the sloped end surfaces  126  and the sloped side surfaces  130   a  are formed. In one embodiment, the u-shaped area  140  is formed such that the surfaces  126  and  130   a  have different dimensions and angles. In another embodiment, the u-shaped area  140  is substantially of a uniform width and the surfaces  126  and  130   a  have substantially similar dimensions and angles. 
   In another embodiment of step  210 , the conductor traces and the beveled surfaces  126  (and in some embodiments, the side surfaces  130   a  of  FIG. 7 ) are defined using a technique of intentionally misfocused or indefinite exposure of the photoresist material  134  of  FIG. 9   a  to light. The misfocused light functions in a manner similar to the mask  136 . In one embodiment, the misfocused light is used in conjunction with a mask having the sections  138  and  142  (not shown). To form the sloped areas of the photoresist material, the light is substantially clearly focused in areas where the photoresist material is rendered insoluble, and gradually changes along the photoresist material surface to being substantially misfocused where the photoresist material is to be removed. The sloped sections of photoresist material as shown in  FIG. 9   b  are thereby formed. The beveled surfaces  126  are then defined by etching. Additionally, in another embodiment, the photoresist material is sloped over the width of the conductive material using misfocused light to form the side surfaces  130   a  of  FIG. 7 . 
   In another embodiment of step  210 , the sloped end surfaces  126  and the sloped side surfaces  130   a  shown in  FIG. 7  are formed using a pre-etch hard bake technique. In the pre-etch hard bake technique, the photoresist material  134  of  FIG. 9   a  is masked, exposed to light and developed in a pattern to form the conductor traces. Then the photoresist material is exposed to the hard bake (a high temperature) until the photoresist flows into a substantially trapezoidal cross-section. The formation is then etched in step  220  to form the sloped side surfaces  130   a  and the beveled surfaces  126 , as shown in  FIGS. 5 and 7 . In this embodiment, the surfaces  126  and  130   a  have substantially similar dimensions due to the flowing symmetry of the photoresist material. 
   The cross-sections of the substantially vertical side surfaces  130  illustrated in  FIGS. 6 and 8  are capable of being formed by the half-tone mask  136 .  FIG. 8  is also capable of being formed by either the intentionally misfocused light technique or the pre-etch hard bake technique. 
   The cross-section of the sloped side surfaces  130   a  illustrated in  FIG. 7  is capable of being formed by intentionally misfocused light on the side surfaces, the mask  137 , or the pre-etch hard bake. In one embodiment, using any of these three methods for forming the sloped side surfaces  130   a , the end surfaces  126  are able to be beveled using the same method at the same time. 
   While the present invention has been disclosed with reference to the foregoing specification and the preferred embodiment shown in the drawings and described above, it will be apparent to those skilled in the art that changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.