Semiconductor device including capacitive sensor and ion-sensitive transistor for determining level and ion-concentration of fluid

In an example implementation, a method of operating a fluid sensing device includes enabling a fluid level sensing circuit on a printhead to determine a fluid level by sharing an applied charge between a capacitive sensor and a reference capacitor to determine a capacitance value of the capacitive sensor. The method includes enabling a fluid property sensing circuit on the printhead to determine a fluid property by measuring a transistor voltage that indicates a concentration of ions gathered on the capacitive sensor.

BACKGROUND

Inkjet printers are used around the world to provide fast, high quality, and affordable printing in both small scale and large scale printing formats. Inkjet printheads typically comprise thermal inkjet (TIJ) or piezoelectric inkjet (PIJ) semiconductor devices that are digitally controlled to dispense small droplets of fluid quickly and accurately by creating pulses within ink-filled firing chambers. Within inkjet printing systems, sensing the levels and properties of ink in ink supply reservoirs is desirable for various reasons. Accurately sensing and reporting the correct level of ink in an ink cartridge, for example, enables printer users to prepare to replace finished ink cartridges, helps users to avoid wasting ink, and enables printing systems to trigger actions that help prevent low quality prints due to inadequate ink levels. Sensing different fluid properties can be useful, for example, to determine the health and age of ink, to differentiate between different types of ink, to determine whether the ink has been properly mixed, and so on.

DETAILED DESCRIPTION

As noted above, sensing the levels and properties of fluids in a system, such as ink in an inkjet printing system, is useful for a number of reasons. In general, such sensing creates value for both customers and manufacturers by reducing the cost of ink and improving the quality of printed output from inkjet printers. While additional sensing and reporting of fluid parameters is beneficial to both customers and manufacturers, the increased sensing comes at a cost. Currently, each sensing function involves the use of a different sensor. In addition, each sensing function usually involves the placement of a number sensors on a printhead die. As a result, with each additional sensing function added to a printhead, a considerable amount of space is consumed on the printhead die. This ultimately can reduce the number of printhead die available from each silicon wafer and result in an increased cost for each printhead.

Accordingly, example devices described herein provide for sensing both fluid properties and fluid levels using the same sensor component. That is, the sensor portion of two different sensor circuits performing two different sensing functions is shared between the two sensing functions. While the two sensing circuits are different, and the purpose of the two sensing circuits is different, a single sensor component can be used in common with both circuits. Both sensing circuits and the shared sensor component are integrated onto a printhead die. Because the sensor portion is the largest component within both of the sensing circuits, sharing the sensor component between the two circuits reduces the amount of space used on the printhead die by a significant amount. The sensing circuits can be alternately enabled by a shifting circuit so that one sensing function is performed at a time.

In one example, a device for sensing a property and level of a fluid includes a capacitive sensor that has a metal element, a switching layer positioned on the metal element, a metal sensing plate positioned on the switching layer, and a fluid in contact with the metal sensing plate. The device includes a first circuit to determine a capacitive value of the capacitive sensor by putting a charge on the capacitive sensor, where the capacitive value is to indicate a level of the fluid. The device includes a second circuit to determine a gate-to-source voltage of an ion-sensitive transistor, where the voltage is to indicate a concentration of ions within the fluid and thereby a property of the fluid.

In another example, a fluid sensing device, includes a capacitive sensor in contact with a fluid, a first sensing circuit to determine a level of the fluid based on a capacitive value of the capacitive sensor, a second sensing circuit to determine a property of the fluid based on a charge concentration of ions within the fluid, and a shifting circuit to switch the device between the first sensing circuit and the second sensing circuit.

In another example, a method of operating a fluid sensing device includes enabling a fluid level sensing circuit on a printhead, and determining a fluid level by sharing an applied charge between a capacitive sensor and a reference capacitor to determine a capacitance value of the capacitive sensor. The method also includes enabling a fluid property sensing circuit on the printhead, and determining a fluid property by measuring a transistor voltage that indicates a concentration of ions gathered on the capacitive sensor.

FIG. 1illustrates an example of an inkjet printing system100suitable for implementing devices for sensing both a property of a fluid and a level of a fluid. The example inkjet printing system100includes an inkjet printhead assembly102, an ink supply assembly104, a mounting assembly106, a media transport assembly108, an electronic printer controller110, and at least one power supply112that provides power to the various electrical components of inkjet printing system100. In theFIG. 1example, printhead assembly102includes a single printhead114, while in other examples, printhead assembly102includes an array of printheads114. Printhead114includes an on-chip fluid level sensor circuit115and fluid property sensor circuit117. Common to both the fluid level sensor circuit115and fluid property sensor circuit117is a shared capacitive sensor119. The shared capacitive sensor119is described in detail below with reference toFIGS. 4 and 5.

Printhead114ejects drops of fluid ink through a plurality of orifices or nozzles116toward a print medium118so as to print onto print media118. Print media118can be any type of suitable sheet or roll material, such as paper, card stock, transparencies, polyester, plywood, foam board, fabric, canvas, and the like. Each nozzle116includes a microelectromechanical system (MEMS) fluidics chamber, which, as shown inFIG. 3, includes several thin layers that define a fluid chamber204, a drop generator302(an example of which is a thermal inkjet firing resistor), and a bore or drop exit121. Nozzles116are typically arranged in one or more columns or arrays such that properly sequenced ejection of ink from nozzles116causes characters, symbols, and/or other graphics or images to be printed on print media118as inkjet printhead assembly102and print media118are moved relative to each other.

As shown inFIG. 1, ink supply assembly104supplies fluid ink to printhead assembly102and includes a reservoir120for storing ink. In different examples, the reservoir120of the ink supply assembly104may be removed, replaced, and/or refilled. Ink flows from reservoir120to inkjet printhead assembly102. Ink supply assembly104and inkjet printhead assembly102can form either a one-way ink delivery system or a recirculating ink delivery system. In a one-way ink delivery system, substantially all of the ink supplied to inkjet printhead assembly102is consumed during printing. In a recirculating ink delivery system, however, only a portion of the ink supplied to printhead assembly102is consumed during printing. Ink not consumed during printing is returned to ink supply assembly104.

In some examples, ink supply assembly104supplies ink under positive pressure through an ink conditioning assembly105to inkjet printhead assembly102via an interface connection, such as a supply tube. Ink supply assembly104includes, for example, a reservoir, pumps and pressure regulators. Conditioning in the ink conditioning assembly105may include filtering, pre-heating, pressure surge absorption, and degassing. Ink is drawn under negative pressure from the printhead assembly102to the ink supply assembly104. The pressure difference between the inlet and outlet to the printhead assembly102is selected to achieve the correct backpressure at the nozzles116. A suitable backpressure at the nozzles116may be a negative pressure ranging from between −1 inches of water and −10 inches of water.

As shown inFIG. 1, inkjet printing system100also includes mounting assembly106. Mounting assembly106positions inkjet printhead assembly102relative to media transport assembly108, and media transport assembly108positions print media118relative to inkjet printhead assembly102. Thus, a print zone122is defined adjacent to nozzles116in an area between inkjet printhead assembly102and print media118. In some examples, inkjet printhead assembly102is a scanning type printhead assembly. As such, mounting assembly106includes a carriage for moving inkjet printhead assembly102relative to media transport assembly108to scan print media118. In other examples, inkjet printhead assembly102is a non-scanning type printhead assembly. As such, mounting assembly106fixes inkjet printhead assembly102at a prescribed position relative to media transport assembly108. Thus, media transport assembly108positions print media118relative to inkjet printhead assembly102.

Inkjet printing system100also includes electronic printer controller110. Electronic printer controller110typically includes a processor (CPU)107, firmware and/or software such as executable instructions109, one or more memory components111including volatile and non-volatile memory components, and other printer electronics for communicating with and controlling inkjet printhead assembly102, mounting assembly106, and media transport assembly108. The components of memory111comprise non-transitory, machine-readable (e.g., computer/processor-readable) media that provide for the storage of machine-readable coded program instructions, data structures, program instruction modules, JDF (job definition format), and other data for the printing system100, such as instructions109and module128. The program instructions, data structures, and modules stored in memory111may be part of an installation package that can be executed by a processor (CPU)107to implement various examples, such as examples discussed herein. Thus, memory111may be a portable medium such as a CD, DVD, or flash drive, or a memory maintained by a server from which the installation package can be downloaded and installed. In another example, the program instructions, data structures, and modules stored in memory111may be part of an application or applications already installed, in which case memory111may include integrated memory such as a hard drive.

Electronic controller110receives data124from a host system, such as a computer, and temporarily stores data124in a memory111. Typically, data124is sent to inkjet printing system100along an electronic, infrared, optical, or other information transfer path. Data124represents, for example, a document and/or file to be printed. As such, data124forms a print job for inkjet printing system100and includes one or more print job commands and/or command parameters. The electronic printer controller110can control inkjet printhead assembly102for ejection of ink drops from nozzles116. For example, the electronic controller110can define a pattern of ejected ink drops that form characters, symbols, and/or other graphics or images on print media118. The pattern of ejected ink drops is determined by the print job commands and/or command parameters from data124.

In some examples, the electronic controller110may include a printer application specific integrated circuit (ASIC)126and a resistance-sense firmware module128which includes computer readable instructions executable on ASIC126or controller110. The printer ASIC126may include a current source130and an analog to digital converter (ADC)132. ASIC126can convert the voltage present at current source130to determine a resistance, and then determine a corresponding digital resistance value through the ADC132. Computer readable instructions implemented by the resistance-sense module128enable the resistance determination and the subsequent digital conversion through the ADC132.

FIG. 2shows a bottom view of an example printhead114, including the on-chip fluid level sensor circuit115and fluid property sensor circuit117. Because both circuits115and117incorporate shared capacitive sensor119, they are both shown inFIG. 2as being coupled to the sensor119. As shown inFIG. 2, the orientation of the sensor circuits115and117with respect to the shared capacitive sensor119, can vary. For example, sensor circuits115and117may be oriented horizontally or vertically with respect to the shared capacitive sensor119, or they may be oriented in some other manner that facilitates an efficient use of the printhead silicon die space. The on-chip fluid level sensor circuit115implements a sample and hold technique to capture the state of the fluid (e.g., ink) level through the shared capacitive sensor119. The capacitance of the capacitive sensor119changes with the level of ink in a fluid chamber204overlying the capacitive sensor119. The operation of the fluid/ink level sensor115will be discussed further with reference toFIGS. 4-7. The on-chip fluid property sensor circuit117implements an ion-sensitive field-effect transistor (ISFET) whose sensing is based on changes in the ion concentration in the fluid and the gathering of a charge at a metal plate of the capacitive sensor119in contact with the fluid. The gathering of charge causes a shift in the transistor threshold voltage which can be measured by monitoring drain-to-source current at a particular drain-to-source voltage and used to determine properties of the fluid such as the fluid pH level. The operation of the fluid/ink property sensor117will be discussed further with reference toFIGS. 4-7.

Printhead114includes a die substrate202, which may be formed of silicon. The silicon die substrate202may be doped. An example of the doped silicon die substrate202is a p-type silicon substrate. From the bottom view of the printhead114, the die substrate202underlies a chamber layer308(FIG. 3) having the fluid chambers204formed therein and an orifice plate310having bore exits121formed therein. Both the chamber layer308and orifice plate310are described below with reference toFIGS. 3 and 4. However, the chamber layer308and orifice plate310are not shown inFIG. 2, in order to better illustrate the die substrate202. Since the fluid chambers204are formed in the chamber layer308(not shown inFIG. 2), the fluid chambers204inFIG. 2are shown in dashed lines in order to illustrate their positions with respect to a fluid slot200and the on-chip sensor circuits115and117and/or drop generators302.

The fluid slot200is an elongated slot formed in the die substrate202. The fluid slot200is in fluid communication with ink paths (not shown) that lead to the respective fluid chambers204that are positioned on both of the long sides of the fluid slot200. By “fluid communication,” it is meant that component(s) is/are configured so that a fluid can be in contact therewith. As an example, the fluid slot200may be connected to the ink paths so that fluid flows from the fluid slot200to the ink paths. As another example, a bore exit121that is in fluid communication with a chamber204may enable fluid contained within the chamber204to exit the bore121. As still another example, a fluid chamber204in fluid communication with a drop generator302and/or a capacitive sensor119may contain fluid that is capable of contacting the drop generator302and/or the capacitive sensor119.

While the example printhead114shown inFIG. 2includes a single fluid slot200, other examples are possible and contemplated in which the printhead114may include additional fluid slots200, such as two or more fluid slots200. The fluid slot200is in fluid communication with a fluid supply (not shown), such as the fluid reservoir120(shown inFIG. 1), which supplies ink to the fluid slot200and the fluid chambers204.

Each fluid chamber204is in fluid communication with the drop generator302and/or the shared capacitive sensor119. As shown in theFIG. 2example, four shared capacitive sensors119along with their corresponding fluid level sensor circuits115and fluid property sensor circuits117, may be positioned to be in fluid communication with the respective fluid chambers204located at the four corners of the ink slot200. In this example, the other fluid chambers204may be in fluid communication with respective drop generators302. Alternatively, in this example, any of the fluid chambers204may be in fluid communication with both the on-chip sensor circuits115and117, and the drop generator302(see bottom right corner of the fluid slot200inFIG. 2). Due to the small size of the shared capacitive sensor119, the capacitive sensor119and the drop generator302may be fabricated to be in fluid communication with the same fluid chamber204. In another example, shown in phantom inFIG. 2, fluid chambers204′ may be formed at the two ends E1, E2of the ink slot200, and respective on-chip shared capacitive sensors119and corresponding sensor circuits115and117may be positioned to be in fluid communication with the respective fluid chambers204′. In this example, the other fluid chambers204along the longer sides of the ink slot200may be in fluid communication with respective drop generators302.

FIG. 3shows a cross sectional view of an example of a nozzle116portion of the printhead114, including the drop generator302. The drop generator302is associated with a single fluid chamber204and bore exit121. As shown inFIG. 3, the bore exit121is formed in the orifice plate310and the fluid chamber204is formed in the chamber layer308. The bore exit121is in fluid communication with the fluid chamber204, so that ink312in the fluid chamber204can be ejected out through the bore exit121. In the printhead114, the bore exits121may be arranged in the orifice plate310along the longer sides of the fluid slot200so they are positioned to be in fluid communication with respective fluid chambers204of respective nozzles116.

As shown inFIG. 3, a passivation layer306may be formed between the ejection element303and the fluid chamber204to protect the ejection element303from ink312in the chamber204, and to act as a mechanical passivation or protective cavitation barrier structure to absorb the shock of collapsing vapor bubbles. Examples of materials making up the passivation layer306include SiC, Si3N4, or layers of these materials, such as a layer of Si3N4 followed by a layer of SiC.

The chamber layer307has walls308that define the fluid chambers204, and that separate the die substrate202(and the various layers and elements formed thereon) from the orifice plate310. An example of a material used to form the chamber layer307includes an epoxy-based negative photoresist (e.g., SU-8, IJ5000 from 3M, etc.).

During a thermal inkjet printing operation, a fluid drop is ejected from the chamber204through its corresponding bore exit121. Ink312then refills the chamber204with fluid from the fluid slot200. The fluid drop is ejected as a result of electric current being passed through the ejection element303, which rapidly heats the element303. As a result of this heating, a thin layer of the ink312adjacent to the passivation layer306in contact with the ejection element303is superheated and vaporizes. This creates a vapor bubble in the corresponding chamber204. The rapidly expanding vapor bubble forces a fluid drop out of the corresponding bore exit121. When the heated ejection element303cools, the vapor bubble quickly collapses, which draws more fluid from the fluid slot200into the chamber204in preparation for ejecting another drop from the nozzle116.

Referring again toFIGS. 1 and 2, in addition to the drop generators302, the printhead114also includes on-chip fluid level sensor circuits115and fluid property sensor circuits117. As noted above, each fluid level sensor circuit115shares a capacitive sensor119with a fluid property sensor circuit117. The fluid level sensor circuits115may or may not also include a clearing resistor214. Since the ejection element303fires ink312directly, the clearing resistor214may be excluded. The components of both the fluid level sensor circuit115and fluid property sensor circuit117are integrated on the printhead114. The sensor circuits115and117may additionally be electrically connected to off-chip components (i.e., components that are not integrated on the printhead114), such as the current source130and the ADC132of the printer ASIC126(shown inFIG. 1). The off-chip components may be located on a printer carriage or the electronic controller110of the inkjet printing system100.

As shown inFIG. 2, the components of the on-chip fluid level sensor circuits115and fluid property sensor circuits117may be located on the die substrate202along the ink slot200in any position where a drop generator302may be located. Various suitable positions for the on-chip sensor(s)115and117are described above, and include, for example, at the four corners of the ink slot200.

FIG. 4shows a portion of an example printhead114in cross section and top down views, including a fluid level sensor circuit115, a fluid property sensor circuit117, and a capacitive sensor119that is part of both circuits115and117, and is shared between them.FIG. 5shows an example circuit diagram depicting examples of a fluid level sensor circuit115, a fluid property sensor circuit117, and a capacitive sensor119that is shared between the two circuits115and117.FIG. 5additionally shows an example of a shift register circuit500, illustrated as a first shift register502and a second shift register504. Shift registers502and504operate to switch between the two sensor circuits115and117, in order to enable sharing of the capacitive sensor119to achieve multiple sensing functions including fluid level sensing from sensor circuit115and fluid property sensing from sensor circuit117.

Referring primarily now toFIGS. 4 and 5, the fluid property sensor circuit117will be discussed. It is noted that the fluid property sensor circuit117shown inFIGS. 4 and 5may include additional components, and that some of the components described herein may be removed and/or modified without departing from the scope of the fluid property sensor circuit117. As shown inFIGS. 4 and 5, the fluid property sensor circuit117is depicted as including an ion-sensitive field effect transistor (ISFET)400formed in a substrate402. The substrate402may correspond to a portion of the substrate202of the printhead114and may be formed of silicon. Alternatively, the substrate402may be a different substrate. The ISFET400is depicted as including a gate (G)404formed on a gate oxide layer406. The gate404may be formed of a polysilicon material. The ISFET400is also depicted as including a source (S)408and a drain (D)410, which are in contact with the gate oxide layer406, and may form respective diffusion regions. In an example, field oxide is not used to isolate transistors. Rather, polysilicon is pattered and used as a mask to selectively diffuse regions in the substrate402. Hence, a transistor may include a polysilicon ring separating one diffusion region from another.

Such a structure is one example, but other examples are possible and may include substrates having other field oxide separating diffusion regions. In one example, the fluid property sensor circuit117is implemented using N-type metal-oxide semiconductor (NMOS) logic such that the substrate402includes a P-type substrate and the diffusion regions corresponding to the source408and the drain410include N+ doped regions. NMOS logic may be used for implementing the fluid property sensor circuit117. However, in other examples the fluid property sensor circuit117may be implemented using P-type metal-oxide semiconductor (PMOS) logic or complementary metal oxide semiconductor (CMOS) logic. In the case of PMOS logic, the substrate402may include N-type silicon and the diffusion regions corresponding to the source408and the drain410may include P+ doped regions. The configuration for N-wells in N-well CMOS logic are similar to the PMOS configuration, and the configuration for P-wells in P-well CMOS logic are similar to the NMOS configuration.

The gate oxide layer406may include a dielectric oxide material, such as silicon dioxide (SiO2), a high-k dielectric material, such as hafnium oxide (HfO2) or aluminum oxide (Al2O3), or the like. A polysilicon layer may be formed and patterned over the gate oxide layer406resulting in formation of a polysilicon gate404between the source408and the drain410. A metal layer may be formed and patterned over the polysilicon gate404resulting in the formation of metal elements412in electrical contact with the polysilicon gate404, source408, and the drain410.

A dielectric material414may be positioned to generally isolate the metal elements412and the polysilicon gate404from each other with the exception of the specific electrical contacts described above. The dielectric material414may be formed of, for example, silicon dioxide. A passivation layer416may be formed on the dielectric material414, such that the passivation layer416is separated from the metal elements412by a section of the dielectric material414. The passivation layer416may also be formed of a dielectric material, such as silicon nitride (Si3N4), silicon carbide (SiC), a combination thereof, or the like.

As also shown inFIG. 4, a switching layer418may be provided in electrical contact with a first metal element412. The switching layer418is also depicted as being in electrical contact with a metal sensing plate420, such that the switching layer418is sandwiched between the first metal element412and the metal sensing plate420. The metal sensing plate420is further depicted as extending through a second passivation layer422and being exposed to the fluid312contained in the fluid chamber424. Thus, the shared capacitive sensor119comprises a first metal element412, the switching layer418, the metal sensing plate420, and the fluid312contained in the fluid chamber424. The second passivation layer422can be formed, for example, of silicon nitride (Si3N4), silicon carbide (SiC), a combination thereof, or the like. According to a particular example, the metal sensing plate420is formed of TaAl.

The switching layer418may be formed of a switching oxide, such as a metallic oxide, may have a relatively small thickness, and may be formed of a high-K dielectric material (i.e., with a high dielectric constant). By way of example, the switching layer418may have a thickness in the range of between about 1 nm to about 50 nm and may have a dielectric constant (K) of at least about 6 to 80. Specific examples of suitable switching oxide materials may include silicon nitride, titanium dioxide, magnesium oxide, titanium oxide, zirconium oxide, hafnium oxide, vanadium oxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, iron oxide, cobalt oxide, copper oxide, zinc oxide, aluminum oxide, gallium oxide, silicon oxide, germanium oxide, tin dioxide, bismuth oxide, nickel oxide, yttrium oxide, gadolinium oxide, and rhenium oxide, among other oxides. In addition to the binary oxides presented above, the switching oxides may be ternary and complex oxides such as silicon oxynitride. The oxides presented may be formed using any of a number of different processes such as sputtering from an oxide target, reactive sputtering from a metal target, atomic layer deposition (ALD), oxidizing a deposited metal or alloy layer, etc. According to an example, the switching layer418may be formed directly on the surface of the first metal element412and/or the surface of the metal sensing plate420.

The resistance level of the switching layer418may be changed in response to various programming conditions, and the switching layer418is able to exhibit a memory of past electrical conditions. For example, the switching layer418may be programmed to have a first resistance state or a second resistance state and may retain the programmed resistance state following removal of a programming condition. More specifically, the resistance level of the switching layer418may be changed through application of a voltage or current (e.g., through an electrode426, discussed below), in which the voltage or current may cause mobile dopants in the switching layer418to move, which may alter the electrical operation of the switching layer418. Thus, the resistance levels of the switching layer418may correspond to different electrical fields applied to the switching layer418through application of different voltages or currents. By way of example, the switching layer418may be programmed to have a lower resistance level through application of a higher voltage or current.

After removal of the voltage or current, the locations and characteristics of the dopants in the switching layer418are to remain stable until the application of another programming or writing electrical field. That is, the switching layer418remains at the programmed resistance level following removal of the voltage or current. In addition, the resistance level of the switching layer418may be changed after the resistance level has been set or programmed, i.e., the resistance state is reversible. For instance, following the setting of the switching layer418to have a first resistance state, another voltage or current, for instance, having a reverse polarity, may be applied to the switching layer418, which may cause the mobile dopants to move in an opposite direction, thereby causing the switching layer418to have a second resistance state. In this example, the second resistance state may correspond to a higher resistance level as compared with the first resistance state. When in the first resistance state, a voltage or current may flow between the metal sensing plate420and the first metal element412through the switching layer418. When in the second resistance state, the switching layer418may prevent the flow of a voltage or reading current between the metal sensing plate420and the first metal element412. In this regard, when in the first resistance state, the switching layer418may prevent a capacitor from being formed by the first metal element412and the metal plate420. In other words, the switching layer418, when in the first resistance state, may prevent the ISFET400from being operational and may thus prevent the ISFET400from performing a sensing operation. In contrast, when in the second resistance state, the switching layer418may enable the formation of a relatively high capacitance capacitor between the first metal element412and the metal plate420. In other words, the switching layer418, when in the second resistance state, may enable the ISFET400to be operational.

The electronic controller110(FIG. 1) may control operations of the fluid property sensor circuit117. In one example, the controller110may control whether the fluid property sensor circuit117is to detect the property of the fluid312by changing the resistance state of the switching layer418. That is, the controller110may set the switching layer418to be in the first resistance state, i.e., have a first resistance level, in which the switching layer418is to short a capacitor in the fluid property sensor circuit117and thus render the fluid property sensor circuit117non-operational.

In the first resistance state, the switching layer418may thus prevent the fluid property sensor circuit117from detecting the property of the fluid312. In this example, the controller110may cause a first electrical field having a sufficiently high strength to be created across the switching layer418, which may cause the switching layer418to switch from the first resistance state to the second resistance state, in which the resistance level of the switching layer418is higher than the resistance level under the first resistance state. As the resistance level of the switching layer418is increased, the capacitance between the metal plate420and the first metal element412may be increased, thereby enabling the fluid property sensor circuit117to detect the property of the fluid312.

As shown inFIG. 4, an electrode426may be positioned in an aligned and spaced relation to the metal plate420such that fluid312in the fluid chamber424may be present between the metal plate420and the electrode426. The electrode426may be formed on the orifice plate310(FIG. 3) over the ISFET400. In addition, the electrode426may be capacitively coupled to the ISFET400through fluid312in the fluid chamber424, the metal plate420, the switching layer418, and the first metal element412. In some examples, the fluid property sensor circuit117may be disposed in a fluid chamber424that does not contain a fluid ejector302(FIG. 3).

In an example, the orifice plate310is formed of metal and the electrode426is formed as a protrusion of the orifice plate310. In such case, the orifice plate310and the electrode426may include nickel (Ni) with a palladium (Pa) or Titanium (Ti) coating, for example. In another example, the orifice plate310may be formed of a polymer material and the electrode426may be embedded in the polymer material. In such case, the electrode426may be formed of TaAl, for example.

The polysilicon gate404together with the respective portions of the first metal layer412, the switching layer418, and the metal sensing plate420in electrical contact with the polysilicon gate404may form a “floating-gate” of metal-oxide field effect transistor (MOSFET) having the source408and the drain410(assuming N-MOS). Together with the dielectric layers414,416, the MOSFET comprises the ISFET400. The metal element(s)412and the metal sensing plate420may be formed of any suitable metal or metal alloy, for instance, Aluminum (Al), Aluminum copper (AlCu), Tantalum aluminum (TaAl), etc. The electrode426may also be formed of any of these types of metal or metal alloy materials.

Referring still toFIGS. 4 and 5, in operation, the source (S)408may be coupled to a reference voltage (e.g., electrical ground, GND), and the drain (D)410may be coupled to current source ID (i.e., current source130in printer ASIC126) through enabling transistor T5, discussed below. A voltage may be applied to the electrode426, causing the electrode426to essentially act as the reference gate of the ISFET400. The voltage between the electrode426and the source408is the gate-to-source voltage, referred to as Vgs. The charge distribution for the ISFET400will change according to the ion concentration in the fluid312. As the charge distribution changes, the threshold voltage of the ISFET400changes. For example, if the fluid property sensor circuit117is to measure pH, then the ISFET's threshold voltage depends on the pH of the fluid312in contact with the metal sensing plate420. A change in the threshold voltage of the ISFET400may be measured by measuring the change in ID, or drain-to-source current (Ids) for a particular drain-to-source voltage (Vds). In general, materials for the electrode426and the metal plate420may be selected such that the threshold voltage of the ISFET400changes over time in response to changes in a particular ion combination (pH described herein by way of example). Changes in the threshold voltage may be detected through measurements of ID, the drain-to-source current, given a particular drain-to-source voltage.

The operations described above may be performed when the resistance state of the switching layer418is set to cause a capacitor to be operational in the fluid property sensor circuit117. However, if the resistance state of the switching layer418is set to allow the flow of a current or a voltage from the metal sensing plate420to the first metal element412, a change in drain-to-source current (Ids) may not be measured and thus the fluid property sensor circuit117may be in the “off” condition.

The electronic controller110may control the resistance state of the switching layer418through application of a changing voltage or a changing current, or a changing voltage or a changing current having a reverse polarity, through the switching layer418as applied between the electrode426and the source408. In this example, the switching layer418may be formed such that the voltage or current level required to change the resistance state of the switching layer418(e.g., a changing voltage or a changing current) is higher than the voltage or current level used by the fluid property sensor circuit117to detect a property of the fluid312(e.g., a reading voltage or a reading current). That is, the voltage applied to the electrode426during a sensing operation of the fluid312may not generate a sufficiently strong electrical field through the switching layer418to cause the resistance state of the switching layer418to be changed.

As noted above, in addition to the fluid property sensor circuit117,FIGS. 4 and 5also include a fluid level sensor circuit115which will now be discussed. The fluid level sensor circuit115may include additional components, and some of the components described herein may be removed and/or modified without departing from the scope of the fluid level sensor circuit115. As an example, the dashed box shown inFIG. 4is intended to indicate additional components of the sensor circuit115that are not specifically illustrated inFIG. 4. In addition, because the description above regarding the capacitive sensor119and its components applies similarly to the fluid level sensor circuit115, the discussion of the components of the capacitive sensor119(i.e., the first metal element412, the switching layer418, the metal sensing plate420, and the fluid312contained in the fluid chamber424) will not be repeated, other than to indicate their functioning within the fluid level sensor circuit115. For example, operations described above with respect to the resistance state of switching layer418that can cause the fluid property sensor circuit117to be operational or not operational, may apply in a similar manner to cause the fluid level sensor circuit115to be operational or not operational. Accordingly, discussion of the fluid level sensor circuit115can be made with primary reference to the circuit diagram ofFIG. 5and timing diagram ofFIG. 6.

FIG. 6shows an example of a partial timing diagram600having non-overlapping clock signals (S1-S3) with synchronized data and fire signals that may be used to drive an example printhead114. The clock signals in timing diagram600can also be used to drive the operation of the fluid/ink level sensor circuit115as shown inFIG. 5. Thus, referring now primarily toFIGS. 5 and 6, it is shown that the fluid level sensor circuit115employs a charge sharing mechanism to determine different levels of ink in a chamber424(FIG. 4). The sensor circuit115includes two first transistors, Ti (T1a, T1b), configured as switches. During operation of the sensor circuit115, in a first step a clock pulse S1is used to close the transistor switches T1aand T1b, coupling memory nodes M1and M2to ground and discharging the capacitive sensor119and the reference capacitor506. Reference capacitor506is the capacitance between node M2and ground. In this example, reference capacitor506is implemented as the inherent gate capacitance of evaluation transistor T4, and it is therefore illustrated using dashed lines. Reference capacitor506additionally includes associated parasitic capacitance such as gate-source overlap capacitance, but the T4gate capacitance is the dominant capacitance in reference capacitor506. Using the gate capacitance of transistor T4as a reference capacitor506reduces the number of components in sensor circuit115by avoiding a specific reference capacitor fabricated between node M2and ground. However, in other examples, adjustments to the value of reference capacitor506can be made through the inclusion of a specific capacitor fabricated from M2to ground (i.e., in addition to the inherent gate capacitance of T4).

In a second step, the S1clock pulse terminates, opening the T1aand T1bswitches. Directly after the T1switches open, an S2clock pulse is used to close transistor switch T2. Closing T2couples node M1to a pre-charge voltage, Vp (e.g., on the order of +15 volts), and a charge Q1is placed across capacitive sensor119(illustrated as Csense inFIG. 5) according to the equation, Q1=(Csense)(Vp). At this time the M2node remains at zero voltage potential since the S3clock pulse is off. In a third step, the S2clock pulse terminates, opening the T2transistor switch. Directly after the T2switch opens, the S3clock pulse closes transistor switch T3, coupling nodes M1and M2to one another and sharing the charge Q1between capacitive sensor119and reference capacitor506. The shared charge Q1between capacitive sensor119and reference capacitor506results in a reference voltage, Vg, at node M2which is also at the gate of evaluation transistor T4, according to the following equation:

Vg remains at M2until another cycle begins with a clock pulse S1grounding memory nodes M1and M2. Vg at M2turns on evaluation transistor T4, which enables a measurement at ID (the drain of transistor T4). In this example it is presumed that transistor T4is biased in the linear mode of operation, where T4acts as a resistor whose value is proportional to the gate voltage Vg (i.e., reference voltage). The T4resistance from drain to source (coupled to ground) is determined by forcing a small current at ID (i.e., a current on the order of 1 milliamp). ID is coupled to a current source, such as current source130in printer ASIC126. Upon applying the current source at ID, the voltage is measured at ID (VID). Firmware, such as Rsense module128executing on controller110or ASIC126can convert VID to a resistance Rds from drain to source of the T4transistor using the current and VID. The ADC132in printer ASIC126subsequently determines a corresponding digital value for the resistance Rds. The resistance Rds enables an inference as to the value of Vg based on the characteristics of the evaluation transistor T4. Based on a value for Vg, a capacitance value for capacitive sensor119(i.e., Csense) can be found from the equation for Vg shown above. A level of ink can then be determined based on the value of capacitive sensor119.

Once the resistance Rds is determined, there are various ways in which the fluid ink level can be found. For example, the measured Rds value can be compared to a reference value for Rds, or a table of Rds values experimentally determined to be associated with specific ink levels. With no ink (i.e., a “dry” signal), or a very low ink level, the value of the capacitive sensor119is very low. This results in a very low Vg (on the order of 1.7 volts), and the evaluation transistor T4is off or nearly off (i.e., T4is in cut off or sub-threshold operation region). Therefore, the resistance Rds from ID to ground through T4would be very high (e.g., with ID current of 1.2 mA, Rds is typically above 12 k ohm). Conversely, with a high ink level (i.e., a “wet” signal), the value of the capacitive sensor119is close to 100% of its value, resulting in a high value for Vg (on the order of 3.5 volts). Therefore, the resistance Rds is low. For example, with a high ink level Rds is below 1 k ohm, and is typically a few hundred ohms.

As noted above, in addition to showing examples of a fluid level sensor circuit115, a fluid property sensor circuit117, and a capacitive sensor119shared between the two circuits115and117,FIG. 5additionally shows an example of a shift register circuit500. The shift register circuit500includes shift registers502and504that enable switching between the two sensor circuits115and117. This enables sharing of the capacitive sensor119to achieve multiple sensing functions, including the fluid level sensing from sensor circuit115and the fluid property sensing from sensor circuit117.FIG. 7shows a more simplified block diagram of the sensing circuits115and117, and the shift registers502and504that function to alternately enable and disable the sensing circuits115and117. The Sout1and Sout2outputs of the shift registers are coupled as indicated inFIGS. 5 and 7to corresponding enable inputs at transistors T5and T6, respectively, of the sensing circuits115and117. This allows the shift registers502and504to control which sensing circuit115or117is enabled and which is disabled through alternately coupling and decoupling circuits115and117to a current source ID (i.e., current source130in printer ASIC126). Thus, one sensing function at a time can be enabled for operation (i.e., fluid level sensing, or fluid property sensing).

FIG. 8shows a flow diagram that illustrates an example method800of operating a fluid sensing device. Method800is associated with examples discussed above with regard toFIGS. 1-7, and details of the operations shown in method800can be found in the related discussion of such examples. The operations of method800may be embodied as programming instructions stored on a non-transitory, machine-readable (e.g., computer/processor-readable) medium, such as memory111shown inFIG. 1. In some examples, implementing the operations of method800can be achieved by a processor, such as a processor107ofFIG. 1, reading and executing the programming instructions stored in a memory111. In some examples, implementing the operations of method800can be achieved using an ASIC126and/or other hardware components alone or in combination with programming instructions executable by processor107.

Method800may include more than one implementation, and different implementations of method800may not employ every operation presented in the flow diagram ofFIG. 8. Therefore, while the operations of method800are presented in a particular order within the flow diagram, the order of their presentation is not intended to be a limitation as to the order in which the operations may actually be implemented, or as to whether all of the operations may be implemented. For example, one implementation of method800might be achieved through the performance of a number of initial operations, without performing one or more subsequent operations, while another implementation of method800might be achieved through the performance of all of the operations.

Referring now to the flow diagram ofFIG. 8, an example method800of operating a fluid sensing device begins at block802, with enabling a fluid level sensing circuit on a printhead. Enabling the fluid level sensing circuit comprises coupling a current source to an evaluation transistor with a second of two shift register outputs. The method800continues with determining a fluid level by sharing an applied charge between a capacitive sensor and a reference capacitor. Sharing the charge between the capacitive sensor and the reference capacitor enables a determination of the capacitance value of the capacitive sensor. The capacitance value can be determined by applying a pre-charge voltage Vp to the capacitive sensor to charge the capacitive sensor with a charge Q1, sharing Q1between the capacitive sensor and reference capacitor, which causes a reference voltage Vg at the gate of an evaluation transistor, and then determining a resistance from drain to source of the evaluation transistor that results from Vg. A current can then be forced at the drain of the evaluation transistor and the voltage Vid can be measured. This enables a calculation of the resistance using the current and Vid, and the resistance can be converted to a digital value for comparison. As shown at block806, method800can continue with determining a fluid level. This can be done by comparing the resistance with a group of resistances that have predetermined associated fluid levels.

The method800continues at block808with enabling a fluid property sensing circuit on the printhead. Enabling the fluid property sensing circuit comprises coupling the current source to an ion-sensitive transistor with a first shift register output. As shown at block810, the method800can continue with determining a fluid property by measuring a transistor voltage that indicates a concentration of ions gathered on the capacitive sensor. This determination can include setting a resistance state of a switching layer in the capacitive sensor to one of a first resistance state and a second resistance state. The switching layer is positioned between a metal sensing plate and a metal element that is coupled to a gate of an ion-sensitive field effect transistor (ISFET), and the metal sensing plate is positioned in an aligned and spaced relation to an electrode that is capacitively coupled to the gate of the ISFET. Determination of a fluid property can continue with applying a reading voltage to the source of the ISFET and the electrode, where the capacitive sensor is non-operational when the switching layer is set to the first resistance state and is operational when the switching layer is set to the second resistance state.

At block812of the method800, a drain-to-source current of the ISFET is measured once the reading voltage is established between the source and the drain of the ISFET. At block814, when the resistance state of the switching layer has been set to the first resistance state, the method800includes resetting the resistance state of the switching layer from the first resistance state to the second resistance state by applying an electrical field across the switching layer.