Patent Grant
6905196
1. Field of Invention
This invention relates generally to the mechanical and electrical structure of fluid drop ejectors.
2. Description of Related Art
A conventional thermal inkjet transducer array is essentially a large bank of thin-film resistive heaters electrically connected in parallel. In particular, a thermal inkjet printer comprises an array of drop ejectors. Each drop ejector has an ink channel having an inlet end and a nozzle end and contains a resistive heater. The nozzle end of each resistive heater in the array of drop ejectors is connected to a common electrical bus, which in turn is connected to an electrical power supply providing a printer operating voltage. Each individual drop ejector is driven to eject a droplet of ink by grounding an inlet end of the resistive heater through an individually-addressable driver transistor.
Often, fluid ejection systems, such as inkjet printers, include an array of thin-film drop ejectors electrically connected in parallel. A drop ejector includes a channel into which fluid flows, a resistive heater to vaporize a portion of the fluid to form a bubble in the chamber, and a nozzle through which fluid downstream of the vapor bubble ejects from the chamber to form a drop projected towards a receiving medium. Vaporizing the fluid creates pressure in the channel forcing the fluid collected downstream of the heater out of the nozzle.
Each drop ejector in the array connects to a common electrical bus communicating with an electrical power supply. Each ejector is controlled by grounding an electrical supply end through an individually-addressable driver transistor. Design optimization encourages a narrow electrical bus for minimal nozzle length, and thick cross-section structures to minimize electrical resistance. Such a thick bus, underneath the channel (˜6-7 μm high), can present an obstacle over which the fluid must flow. Such a flow restriction can induce lateral forces to the fluid being ejected, creating potential directional biases and/or variations in the drop and/or satellite trajectories, thereby degrading ejection quality.
The common electrical bus should be narrow, so that the length of the ink nozzle can be kept as short as possible. This tends to increase drop ejection energy efficiency. To reduce the electrical series resistance of the common bus, it is desirable to make the common bus relatively thick. Often, the common bus will have two or more layers of metal and/or polysilicon. However, this thick bus structure presents a “bump”-shaped obstacle in the nozzle that misdirects the ejected main drop and/or associated satellite droplets that are ejected with the main drop. The misdirected satellite drops tend to limit the print quality achievable with drop ejectors having this bump-shaped obstacle. Unfortunately, no reasonable alternative to these drop ejectors was previously available.
A fluid ejector having a low topography formed by rerouting the electrical conductors from underneath to adjacent to the chamber is disclosed in U.S. Pat. No. 6,227,657 (the 657 patent), which is incorporated herein by reference in its entirety. This low topography fluid drop ejector provides for an electrical contact structure to the resistive heater that avoids placing relatively thick electrical contact layers in a fluid drop ejection path between the resistive heater and the ejector nozzle.
However, manufacturing and operational considerations for fluid ejector designs have revealed limitations to the approach described in the 657 patent. The low topography fluid ejector disclosed in the 657 patent incorporates electrically conductive metal layers separated by a concatenation of insulating layers along selected regions. Unavoidable production flaws along the interface edges of these layers can reduce the operational longevity of the circuit and/or adversely affect production yield. Over-etching on one or more of these layers can exacerbate the variation in the electrical resistance far beyond the allowable design value tolerances.
While the layered electrical and structural configuration disclosed in the 657 patent diminishes the flow obstructions in the fluid ejection channel, the added complexity in depositing patterned layers makes it difficult to control quality and/or obtain commercially useful yields. Further, the connections between the metal layer and the heavily doped polysilicon layer penetrate the protective layer.
Because of the cross-layer interface, the edge junctions between these layers facilitate leak paths through which the fluid and/or fluid vapors can percolate, thus degrading performance and reliability. Additionally, such interfaces at these layer edges require additional patterned insulating layers, thickening the overall structure.
This invention provides a fluid channel having a low-topography using a relatively simple internal structure.
This invention separately provides a low-topography fluid channel that reduces the number of leak paths into the fluid channel.
This invention separately provides a low-topography fluid channel that can be manufactured at higher yield rates.
This invention separately provides a low-topography fluid channel having increased reliability.
This invention separately provides a low-topography fluid channel having a fluid channel having a relatively simple internal structure.
This invention separately provides a method for forming a tantalum-silicide layer in a fluid ejector.
By eliminating interface connections between metal and semiconductor layers, the cross-sectional structure may be further flattened relative to the low-topography fluid ejector device disclosed in the 657 patent. This can further improve droplet trajectory control. Additionally, enhanced reliability can result by reducing potential failure modes associated with over-etching along these interfaces.
In various exemplary embodiments, a thermal fluid ejector structure according to this invention includes a fluid channel having a resistive heater that terminates in a nozzle, and a common bus formed transverse to the fluid channel and extending between the resistive heater and the nozzle. The fluid ejector further includes a connection line that extends longitudinally adjacent to the fluid channel, and a connection structure that electrically connects the common bus with the resistive heater and the connection line. The connection structure includes a first set of one or more layers that electrically connects the connection line to the resistive heater and a second set of one or more layers that covers the common bus and the connection line.
In various exemplary embodiments, the first set of one or more layers can be formed on or over a field oxide layer, and can further include a heavily doped polysilicon layer formed on or over the field oxide layer and a tantalum-silicide layer formed on or over the heavily doped polysilicon layer. In various exemplary embodiments, a second set of one or more layers can be formed on or over the first set of one or more layers and can further include a nitride layer formed on or over the first set of one or more layers, and a tantalum layer formed on or over the nitride layer.
In various exemplary embodiments, a fluid channel can be formed on or over the field oxide layer and can further include a heavily doped polysilicon layer formed on or over the field oxide layer upstream of the nozzle, a tantalum-silicide layer formed on or over the heavily doped polysilicon layer, a lightly doped polysilicon layer formed on or over the field oxide layer adjacently upstream of and electrically connected with the heavily doped polysilicon layer, and a set of protective layers formed on or over the tantalum-silicide layer and the lightly doped polysilicon layer. The set of protective layers can further include a nitride layer formed on or over the tantalum-silicide layer and the lightly doped polysilicon layer, and a tantalum layer formed on or over the nitride layer.
In various exemplary embodiments, a common bus can be formed on or over the field oxide layer and can further include a first set of one or more layers formed on or over the field oxide layer, a lightly doped polysilicon layer formed on or over the field oxide layer adjacent to and electrically separated from the first set of one or more layers, an insulating layer formed on or over the first set of one or more layers and a first portion of the lightly doped polysilicon layer, and a second set of one or more layers formed on or over the insulating layer and a second portion of the lightly doped polysilicon layer. The first set of one or more layers can further include a heavily doped polysilicon layer formed on or over the field oxide layer and a tantalum-silicide layer formed on or over the first doped polysilicon layer. The second set of one or more layers can further include a nitride layer formed on or over the tantalum-silicide layer and the second doped polysilicon layer, and a tantalum layer formed on or over the nitride layer.
Various exemplary embodiments of a method to produce a low topography fluid ejector according to this invention include forming a fluid channel having a resistive heater and terminating in a nozzle, forming a common bus transverse to the fluid channel and between the resistive heater and the nozzle, forming a connection line longitudinally adjacent to the fluid channel, forming a first set of one or more layers that electrically connects the common bus with the resistive heater and the connection line, and forming a second set of one or more layers that covers the common bus and the connection line. Forming the first set of one or more layers over the field oxide layer further includes forming a first doped polysilicon layer on or over the field oxide layer, and optionally forming a tantalum-silicide layer on or over the first doped polysilicon layer. Forming the second set of one or more layers over the first set of the one or more layers further includes forming a nitride layer on or over the first set of the one or more layers, and forming a tantalum layer on or over the nitride layer.
These and other features and advantages of this invention are described in, or are apparent from, the following detailed description of various exemplary embodiments of the systems and methods according to this invention.
Various exemplary embodiments of the systems and methods of this invention will be described in detail with reference to the following figures, wherein:
The following detailed description of various exemplary embodiments of the fluid ejection systems according to this invention are directed to one specific type of fluid ejection system, an inkjet printer, for sake of clarity and familiarity. However, it should be appreciated that the principles of this invention, as outlined and/or discussed below, can be equally applied to any known or later developed fluid ejection systems, beyond the inkjet printer specifically discussed herein.
When the circuit including the polysilicon resistive heater 16 and the connection structure 18 is closed, current flows through the connection structure 18 and the polysilicon resistive heater 16, causing resistive heating. This resistive heating pumps thermal energy into the ink contained within the ink channel 20. Eventually, a portion of the ink in the ink channel 20 vaporizes, forcing ink past the bump 18 and through a nozzle 22. A top of the nozzle 22 is defined by the channel plate 12, while a bottom of the nozzle 22 is defined by the heater plate 14, and the sides of the nozzle 22 are defined by the polymer spacer layer. In particular, the nozzle 22 is on the other side of the connection structure 18 from the polysilicon resistive heater 16. Thus, the bump-shaped connection structure 18 tends to act as a flow-restriction-like member in the ink channel 20.
The bubble formed in the ink channel 20 causes a portion of the ink 24 to extend out of the nozzle 22. In particular, the force applied by the bubble on the incompressible ink 24 causes a main drop 30 to be ejected from the nozzle 22. However, due to the shape and position of the bump-shaped connection structure 18, one or both of two disadvantageous effects can occur as the main drop 30 is ejected from the ink channel 20.
First, the main drop 32 can be misdirected as it is ejected out of the inkjet nozzle 22. That is, the main drop 30 ideally exits the ink channel 20 in a direction that is perpendicular to the surface of the recording medium 40 at which the ink drop 30 is ejected. However, due to the bump-shaped connection structure 18, the main drop 30 exits the ink channel 20 at an angle to the desired direction, reducing the accuracy of ink spot placement on the recording medium 40 from the desired location.
Secondly, the bump-shaped connection structure 18 can cause disturbances in the flow of the ink as it exits the nozzle 22. When the main drop 30 is ejected from the nozzle 22, one or more small satellite drops 32 are generated which also impact the recording medium 40. This disturbance causes one or more satellite drops 32 to depart from the trajectory of the main drop 30 as the ink is ejected from the nozzle 22. In particular, the satellite drops 32 will be ejected at an angle θ divergent relative to the main drop 30.
Thus, the topography of the ink channel 20 created by the bump-shaped connection structure 18 induces one or more print defects in the images formed by the inkjet printer. As described above, these print defects are related to departures from the ideal flight path of the main drop 30 and differences in the flight paths between the main drop 30 and any satellite drops 32 that may have been ejected with the main drop 30. These defects cause the resulting printed images to be fuzzy, to have elongated spot aspect ratios, to have banding, and/or to have spot width variations. For example, if the inkjet printer forms images by printing swaths in both a forward and a return direction, the motion vector of the printhead will alternately additively or subtractively add to the flight path vectors of the satellite drops, causing the satellite drops to alternatively extend outside of, or fall within, the main drop as it lands on the recording medium 40. Thus, depending on which way the printhead carriage moves relative to the recording medium 40, the size of the spot formed by the combination of the main drop 30 and any satellite drops 32 will change.
Each of the common bus connection structures 120 forms a connection structure 124 that connects a common bus portion 130 to a drive voltage bus that is held at the drive voltage. In general, for most common thermal inkjet printers, the drive voltage is typically between 12 and 50 volts. The common bus portion 130 extends across a front portion of the heater plate 102 and connects to each of the resistive heaters 114. As shown in
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The multi-layer protective layer 230 should overlap the first insulative layer 220 by approximately 2 μm to reduce the likelihood that, outside of the ink channel, the β-phase tantalum layer 235 does not terminate on the polysilicon layers 210, described above, and 270, described below. Otherwise, if the tantalum layer 235 terminates in electrical contact with one of the polysilicon layers 210 or 270, the polysilicon becomes damaged near the edge of the tantalum layer 235 and unacceptably low polysilicon-tantalum breakdown voltages occur.
The protective layer 230 is used both to protect against the cavitation forces generated within the ink channel 20 as vapor bubbles of the ink form and collapse within the ink channel 20 to eject ink drops from the ejector structures 110, and to provide electrical isolation between the polysilicon heater structure 210, which is held at the drive voltage, and the ink 24 in contact with the tantalum layer 235.
A second insulative layer 240 is formed on or over the protective layer 230 and positioned generally vertically over the space formed between the relatively lightly-doped polysilicon layers 210. A conductive metal layer 250 is then formed on or over the second insulative layer 240. An insulative passivation layer 260 is formed on or over the conductive metal layer 250, the second insulative layer 240 and partially over the protective layer 230 to completely encapsulate the second insulation layer 240 and the conductive metal layer 250.
As mentioned above, the protective layer 230 acts as a heater protection layer providing both chemical and mechanical protection to the resistive heater 114 in the ejector structure 110. The passivation layer 260 also acts as a mechanical and chemical protection layer. Because the passivation layer 260 encapsulates the conductive metal layer 250, the passivation layer 260 also provides electrical protection.
This current flow through the relatively lightly-doped polysilicon layer 210 causes resistive heating in the relatively lightly-doped polysilicon layer 210. In particular, the relatively heavily-doped polysilicon layer 270 has a resistivity that is less than the resistivity of the relatively lightly-doped polysilicon layer 210. This tends to cause most of the resistive heating to occur in the relatively lightly-doped polysilicon layer 210, and relatively little of the resistive heating to occur in the relatively heavily-doped polysilicon layer 270.
The heat created by the resistive heating in the relatively lightly-doped polysilicon layer 210 flows through the thermally conductive protective layer 230 and heats the ink in the ink channel 20 sufficiently to cause the ink to vaporize and eject a drop through the nozzle 118.
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As indicated above, the set of electrically conductive layers 450 includes the relatively heavily-doped polysilicon layer 410 and the tantalum-silicide layer 420. The set of electrically conductive layers 450 can be used, according to this invention, in place of a separately-routed layer of one or more high-conductivity materials, such as, for example, copper (Cu), and/or aluminum (Al).
It should be appreciated that, as outlined below, the resistive heater 314 can be formed by the relatively lightly-doped layer of doped polysilicon 460. However, it should also be appreciated that the resistive heater 314 can also be formed using a thin-film resistor in place of the relatively lightly-doped polysilicon layer 460 within the ink channel. It should further be appreciated that the thin-film resistor can be formed using any appropriate process, such as, for example, sputtering.
In various exemplary embodiments, by forming the common bus and interconnection structures with tantalum-silicide, rather than by simply using relatively highly-doped polysilicon, the interconnect line resistance can be reduced from 2 mΩ-cm to approximately 50 μΩ-cm. With the known highly-doped polysilicon interconnection structures, parasitic resistances prevent efficient drop ejector operation for heater resistivities less than approximately 3000 Ω/□ (ohms/square). In contrast, in various exemplary embodiments according to this invention, the silicide interconnection structures enable efficient operation with heater resistances of 300 Ω/□ or less. Additionally, in various exemplary embodiments of this invention, by replacing the known superstructure of the metal layer 250 and the accompanying insulating layers 220, 232 and 240 and the passivation layer 260, with the set of electrically conductive layers 450 and the set of protective layers 430, the cross-sectional profile through the drop ejector nozzle region may be flattened. This tends to further improve the directional flow consistency of ejected fluid droplets. Finally, replacing the complex cross-sectional structures represented in
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One exemplary embodiment of a method for forming the polysilicon feed-through heater includes forming the fluid channel 316 with a resistive heater 314, forming the common bus 330 transverse to the fluid channel 316, forming the connection line 322, forming the first set of the one or more layers 450 that electrically connects the common bus 330 with the resistive heater 314 and the connection line 322, and forming the second set of the one or more layers 440 and 430 that covers the common bus 330 and the connection line 322. In various exemplary embodiments, forming the electrically conductive first set of the one or more layers 450 includes forming the heavily-doped polysilicon layer 410 or the relatively thick polysilicon layer 470 on or over the field oxide layer 400, and optionally forming the tantalum-silicide layer 420 on or over the relatively heavily-doped polysilicon layer 410. In various exemplary embodiments, forming the protective second set of the one or more layers 430 includes forming the nitride layer 432 on or over the electrically conductive first set of the one or more layers 450, and forming the tantalum layer 435 on or over the nitride layer 432.
Another exemplary embodiment of a method for forming the fluid channel 316 includes forming the electrically-conductive first set of the one or more layers 450 having the relatively heavily-doped polysilicon layer 410 or 470 on or over the field oxide layer 400, forming the tantalum-silicide layer 420 on or over the heavily doped polysilicon layer 410, forming the relatively lightly-doped polysilicon layer 460 on or over the field oxide layer 400 so that the relatively lightly-doped polysilicon layer 460 is electrically connected to the relatively heavily-doped polysilicon layer 410 or the relatively thick polysilicon layer 470, and forming the protective second set of the one or more layers 430 on or over the tantalum-silicide layer 420 and the relatively lightly-doped polysilicon layer 460.
Yet another exemplary embodiment of a method for forming the common bus includes forming an electrically conductive first set of the one or more layers 450 on or over a first portion of the field oxide layer 400, forming the relatively lightly-doped polysilicon layer 460 on or over a second portion of the field oxide layer 400, forming the insulating layer 440 on or over the electrically conductive first set of the one or more layers 450 and on or over the relatively lightly-doped polysilicon layer 460, and forming a protective second set of the one or more layers 430 on or over the tantalum-silicide layer 420 and the relatively lightly-doped polysilicon layer 460.
While this invention has been described in conjunction with the exemplary embodiments outlined above, many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the exemplary embodiments of the invention, as set forth above, are intended to be illustrative, not limiting. Various changes can be made without departing from the spirit and scope of the invention.
This nonprovisional application claims the benefit of U.S. Provisional Application No. 60/378,398, filed May 8, 2002.
| Number | Name | Date | Kind |
|---|---|---|---|
| 6227657 | Raisanen et al. | May 2001 | B1 |
| 6315384 | Ramaswami et al. | Nov 2001 | B1 |
| 6565762 | Silverbrook | May 2003 | B1 |
| Number | Date | Country | |
|---|---|---|---|
| 20030210301 A1 | Nov 2003 | US |
| Number | Date | Country | |
|---|---|---|---|
| 60378398 | May 2002 | US |
