BACKGROUND

[0001] A memory system may be used to store data. In some examples, imaging devices, such as printheads may include memory to store information relating to printer cartridge identification, security information, and authentication information, among other types of information.

BRIEF DESCRIPTION OF THE DRAWINGS

[0002] The accompanying drawings illustrate various examples of the principles described herein and are a part of the specification. The illustrated examples do not limit the scope of the claims.

[0003] Fig. 1 is a diagram of a printing system according to one example of the principles described herein.

[0004] Fig. 2A is a diagram of a printer cartridge that uses a printhead with memristors having different structures according to one example of the principles described herein.

[0005] Fig. 2B is a cross sectional diagram of a printer cartridge that uses a printhead with memristors having different structures according to one example of the principles described herein.

[0006] Fig. 3 is a block diagram of a printer cartridge that uses a printhead with memristors having different structures according to one example of the principles described herein.

[0007] Fig. 4A is a cross-sectional diagram of a memristor having a first structure according to one example of the principles described herein. [0008] Fig. 4B is a cross-sectional diagram of a memristor having a second structure according to one example of the principles described herein.

[0009] Fig. 5 is a diagram of a memory bank that includes memristors having different structures according to one example of the principles described herein.

[0010] Figs. 6A and 6B are diagrams of memory banks that include memristors with similar structures according to one example of the principles described herein.

[0011] Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.

DETAILED DESCRIPTION

[0012] Memory devices are used to store information for a printer cartridge. Printer cartridges include memory to store information related to the operation of the printhead. For example, a printhead may include memory to store information related 1) to the printhead; 2) to fluid, such as ink, used by the printhead; or 3) to the use and maintenance of the printhead. Other examples of information that may be stored on a printhead include information relating to 1) a fluid supply, 2) fluid identification information, 3) fluid characterization information, and 4) fluid usage data, among other types of fluid or imaging device related data. More examples of information that may be stored include identification information, serial numbers, security information, feature information, Anti-Counterfeiting (ACF) information, among other types of information. While memory usage on printheads is desirable, changing circumstances may reduce their efficacy in storing information.

[0013] For example, an increasing trend in counterfeiting may lead to current memory devices being too small to contain sufficient anti-counterfeiting information and security and authentication information. Additionally, with loyalty customer reward programs, new business models and other customer relation management programs through cloud-printing and other printing architectures, additional market data, customer appreciation value information, encryption information, and other types of information on the rise, a

manufacturer may desire to store more information on a memory device.

[0014] Moreover, as new technologies develop, circuit space is becoming more valuable. Accordingly, it may be desirable for the greater amounts of data storage to occupy less space within a device. Memristors may be used due to their non-volatility, low operational power consumption characteristics, and their compact size. A memristor selectively stores data based on a resistance state of the memristor. For example, a memristor may be in a low resistance state indicated by a "1 ," or a high resistance state indicated by a "0." Memristors may form a string of ones and zeroes that will store the aforementioned data. If an analog memristor is used, there may be many different resistance states.

[0015] A memristor may switch between a low resistance state and a high resistance state during a switching event in which a voltage is passed to the memristor. Each memristor has a switching voltage that refers to a voltage used to switch the state of the memristors. When the supplied voltage is greater than the memristor switching voltage, the memristor switches state. The switching voltage is largely based on a number of characteristics including the size, thickness, and material composition of the memristor. For example, a thicker memristor may use a larger voltage to execute a switching event. While memristors may be beneficial as memory storage devices, their use presents a number of complications.

[0016] For example, as two logic states are represented by a particular memristor, a memory array of two-state memristors may be hijacked by counterfeiters. According, the present specification describes a printhead and printer cartridge having memristors with different structures. Specifically, a first number of memristors may have a first structure and a second number of memristors may have a second structure, the first and second structures being different from one another. The different structures produce different resistance levels in the memristor array. Accordingly, multiple groups of memristors with different resistance levels in a single array increases the total number of resistance levels of the memristor array. As memristors use resistance levels to indicate logic values, a greater number of resistance levels indicate a greater number of logic values that may be represented by the memristor array.

[0017] More specifically, the present disclosure describes a printhead with a number of memristors having different structures. The printhead includes a number of nozzles to deposit an amount of fluid onto a print medium. Each nozzle includes a firing chamber to hold the amount of fluid, an opening to dispense the amount of fluid onto the print medium, and an ejector to eject the amount of fluid through the opening. The printhead also includes a first number of memristors having a first structure and a second number of memristors having a second structure. The second structure is different than the first structure.

[0018] The present disclosure describes a printer cartridge with a number of memristors having different structures. The printer cartridge includes a fluid supply and a printhead to deposit fluid from the fluid supply onto a print medium. The printhead includes a first number of memristors having a first structure and a second number of memristors having a second structure. The second structure is different than the first structure.

[0019] As used in the present specification and in the appended claims, the term "printer cartridge" may refer to a device used in the ejection of ink, or other fluid, onto a print medium. In general, a printer cartridge may be a fluidic ejection device that dispenses fluid such as ink, wax, polymers or other fluids. A printer cartridge may include a printhead. In some examples, a printhead may be used in printers, graphic plotters, copiers, and facsimile machines. In these examples, a printhead may eject ink, or another fluid, onto a medium such as paper to form a desired image or a desired three-dimensional geometry.

[0020] Accordingly, as used in the present specification and in the appended claims, the term "printer" is meant to be understood broadly as any device capable of selectively placing a fluid onto a print medium. In one example the printer is an inkjet printer. In another example, the printer is a three-dimensional printer. In yet another example, the printer is a digital titration device. [0021] Still further, as used in the present specification and in the appended claims, the term "fluid" is meant to be understood broadly as any substance that continually deforms under an applied shear stress. In one example, a fluid may be a pharmaceutical. In another example, the fluid may be an ink. In another example, the fluid may be a liquid.

[0022] Still further, as used in the present specification and in the appended claims, the term "print medium" is meant to be understood broadly as any surface onto which a fluid ejected from a nozzle of a printer cartridge may be deposited. In one example, the print medium may be paper. In another example, the print medium may be an edible substrate. In yet one more example, the print medium may be a medicinal pill.

[0023] Still further, as used in the present specification and in the appended claims, the term "different" is meant to be understood broadly as indicating that at least one value of a set of values is different from others. For example, a second memristor with a set of resistance levels that differ from resistance levels of a first memristor is meant to be understood broadly as indicating that the second memristor has different resistance values that indicate particular logic values as compared to the first memristor.

[0024] Still further, as used in the present specification and in the appended claims, the term "structure" may refer to the physical structure of the memristor. While structures may vary between memristors, the layers of the memristor may remain constant. For example, a second memristor with a second structure that is different from a first memristor with a first structure may indicate that the second memristor has the same layers (i.e., a bottom electrode, a switching oxide, and a top electrode) as the first memristor made of the same material, but with different parameters for the layers, i.e., thickness, disposition on an underlying layer, etc.

[0025] Even further, as used in the present specification and in the appended claims, the term "memristor" may refer to a passive two-terminal circuit element that maintains a functional relationship between the time integral of current, and the time integral of voltage. [0026] Yet further, as used in the present specification and in the appended claims, the term "a number of" or similar language may include any positive number including 1 to infinity; zero not being a number, but the absence of a number.

[0027] In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present systems and methods. It will be apparent, however, to one skilled in the art that the present apparatus, systems, and methods may be practiced without these specific details. Reference in the specification to "an example" or similar language means that a particular feature, structure, or characteristic described is included in at least that one example, but not necessarily in other examples.

[0028] Turning now to the figures, Fig. 1 is a diagram of a printing system (100) according to one example of the principles described herein. In some examples, the printing system (100) may be included on a printer. The system (100) includes an interface with a computing device (102). The interface enables the system (100), and specifically the processor (108), to interface with various hardware elements, such as the computing device (102), external and internal to the system (100). Other examples of external devices include external storage devices, network devices such as servers, switches, routers, and client devices among other types of external devices.

[0029] In general, the computing device (102) may be any source from which the system (100) may receive data describing a job to be executed by the controller (106) in order to eject fluid onto the print medium (126). For example, via the interface, the controller (106) receives data from the computing device (102) and temporarily stores the data in the data storage device (1 10). Data may be sent to the controller (106) along an electronic, infrared, optical, or other information transfer path. The data may represent a document and/or file to be printed. As such, data forms a job and includes job commands and/or command parameters.

[0030] A controller (106) includes a processor (108), a data storage device (1 10), firmware, software, and other electronics for communicating with and controlling the printhead (1 16). The controller (106) receives data from the computing device (102) and temporarily stores data in the data storage device (1 10).

[0031] The controller (106) controls the printhead (1 16) in ejecting fluid from the nozzles (124). For example, the controller (106) defines a pattern of ejected fluid drops that form characters, symbols, and/or other graphics or images on the print medium (126). The pattern of ejected fluid drops is determined by the print job commands and command parameters received from the computing device (102). The controller (106) may be an application specific integrated circuit (ASIC), on a printer for example, which determines the level of fluid in the printhead (116) based on resistance values of memristors integrated on the printhead (116). The ASIC may include a current source and an analog to digital converter (ADC). The ASIC converts a voltage present at the current source to determine a resistance of a memristor, and then determines a corresponding digital resistance value through the ADC. Computer readable program code, executed through executable instructions enables the resistance determination and the subsequent digital conversion through the ADC.

[0032] The processor (108) may include the hardware architecture to retrieve executable code from the data storage device (110) and execute the executable code. The executable code may, when executed by the processor (108), cause the processor (108) to implement at least the functionality of ejecting fluid onto the print medium (126). The executable code may, when executed by the processor (108), cause the processor (108) to implement the functionality of providing instructions to the power supply (130) such that the power supply (130) provides power to the components of the system (100).

[0033] The data storage device (1 10) may store data such as executable program code that is executed by the processor (108) or other processing device. The data storage device (110) may specifically store computer code representing a number of applications that the processor (108) executes to implement at least the functionality described herein.

[0034] Generally, the data storage device (110) may include a computer readable medium, a computer readable storage medium, or a non- transitory computer readable medium, among others. For example, the data storage device (1 10) may be, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. More specific examples of the computer readable storage medium may include, for example, the following: an electrical connection having a number of wires, a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing. In the context of this document, a computer readable storage medium may be any tangible medium that can contain, or store computer usable program code for use by or in connection with an instruction execution system, apparatus, or device. In another example, a computer readable storage medium may be any non-transitory medium that can contain, or store a program for use by or in connection with an instruction execution system, apparatus, or device.

[0035] The printing system (100) includes a printer cartridge (114) that includes a printhead (1 16) and a reservoir (1 12). The printer cartridge (1 14) may be removable from the printer (104) for example, as a replaceable printer cartridge (114).

[0036] The printer cartridge (1 14) includes a printhead (1 16) that ejects drops of fluid through a plurality of nozzles (124) towards a print medium (126). The print medium (126) may be any type of suitable sheet or roll material, such as paper, card stock, transparencies, polyester, plywood, foam board, fabric, canvas, and the like. In another example, the print medium (126) may be an edible substrate. In yet one more example, the print medium (126) may be a medicinal pill. As will be described below, the printhead (1 16) may include memristor arrays having memristors with different structures.

[0037] Nozzles (124) may be arranged in columns or arrays such that properly sequenced ejection of fluid from the nozzles (124) causes characters, symbols, and/or other graphics or images to be printed on the print medium (126) as the printhead (1 16) and print medium (126) are moved relative to each other. In one example, the number of nozzles (124) fired may be a number less than the total number of nozzles (124) available and defined on the printhead (1 16).

[0038] The printhead (116) may include memory elements to store information on the printhead (116). For example, the printhead (116) may include an array of memristors. Specifically, the printhead (1 16) may include a first number of memristors that have a first switching oxide. The printhead (116) may also include a second number of memristors that have a second switching oxide, which second switching oxide is distinct from the first switching oxide. Using memristors with different switching oxides may be beneficial in that it allows for a wider range of resistance levels to be used and therefore a greater number of logic values to be represented in the memory array.

[0039] The printer cartridge (1 14) also includes a fluid reservoir (112) to supply an amount of fluid to the printhead (116). In general, fluid flows between the reservoir (112) and the printhead (116). In some examples, a portion of the fluid supplied to printhead (1 16) is consumed during operation and fluid not consumed during printing is returned to the reservoir (112).

[0040] In some examples, a mounting assembly positions the printhead (116) relative to a media transport assembly, and media transport assembly positioning the print medium (126) relative to printhead (116). Thus, a print zone (128), indicated by the dashed box, is defined adjacent to the nozzles (124) in an area between the printhead (1 16) and the print medium (126). In one example, the printhead (116) is a scanning type printhead (116). As such, the mounting assembly includes a carriage for moving the printhead (1 16) relative to the media transport assembly to scan the print medium (126). In another example, the printhead (1 16) is a non-scanning type printhead (1 16). As such, the mounting assembly fixes the printhead (116) at a prescribed position relative to the media transport assembly. Thus, the media transport assembly positions the print medium (126) relative to the printhead (116).

[0041] Fig. 2A is a diagram of a printer cartridge (114) and printhead (1 16) with a number of memristors having different structures according to one example of the principles described herein. As discussed above, the printhead (1 16) may include a number of nozzles (124). In some examples, the printhead (1 16) may be broken up into a number of print dies with each die having a number of nozzles (124). The printhead (116) may be any type of printhead (1 16) including, for example, a printhead (1 16) as described in Figs. 2A and 2B. The examples shown in Figs. 2A and 2B are not meant to limit the present description. Instead, various types of printheads (1 16) may be used in conjunction with the principles described herein.

[0042] The printer cartridge (1 14) also includes a fluid reservoir (112), a flexible cable (236), conductive pads (238), and a memristor array (240). The flexible cable (236) is adhered to two sides of the printer cartridge (1 14) and contains traces that electrically connect the memristor array (240) and printhead (1 16) with the conductive pads (238).

[0043] The printer cartridge (114) may be installed into a cradle. When the printer cartridge (114) is correctly installed into a device such as a printer, the conductive pads (238) are pressed against corresponding electrical contacts in the cradle, allowing the device to communicate with, and control the electrical functions of, the printer cartridge (1 14). For example, the conductive pads (238) allow the device to access and write to the memristor array (240).

[0044] The memristor array (240) may contain a variety of information including the type of printer cartridge (1 14), the kind of fluid contained in the printer cartridge (1 14), an estimate of the amount of fluid remaining in the fluid reservoir (1 12), calibration data, error information, and other data. In one example, the memristor array (240) may include information regarding when the printer cartridge (1 14) should be maintained. The memristor array (240) may include other information as described below in connection with Fig. 3. The memristor array (240) may include memristors having different oxides and thus different resistance levels.

[0045] To eject fluid, the system (Fig. 1 , 100) moves the carriage containing the printer cartridge (114) relative to a print medium (Fig. 1 , 126). At appropriate times, the system (Fig. 1 , 100) sends electrical signals to the printer cartridge (1 14) via the electrical contacts in the cradle. The electrical signals pass through the conductive pads (238) and are routed through the flexible cable (236) to the printhead (116). The printhead (116) then ejects a small droplet of fluid from the reservoir (1 12) onto the surface of the print medium (Fig. 1 , 126).

[0046] The printhead (116) may include any number of nozzles (124). In an example where the fluid is an ink, a first subset of nozzles (124) may eject a first color of ink while a second subset of nozzles (124) may eject a second color of ink. Additional groups of nozzles (124) may be reserved for additional colors of ink.

[0047] Fig. 2B is a cross sectional diagram of a printer cartridge (114) and printhead (116) with a number of memristors having different structures according to one example of the principles described herein. The printer cartridge (1 14) may include a fluid supply (112) that supplies the fluid to the printhead (116) for deposition onto a print medium. In some examples, the fluid may be ink. For example, the printer cartridge (114) may be an inkjet printer cartridge, the printhead (116) may be an inkjet printhead, and the ink may be inkjet ink.

[0048] The printer cartridge (114) may include a printhead (116) to carry out at least a part of the functionality of depositing fluid onto a print medium (Fig. 1 , 126). For example the printhead (116) may have a number of memristors with different switching oxides to increase the storage density and security of data stored therein.

[0049] The printhead (116) may include a number of components for depositing a fluid onto a print medium (Fig. 1 , 126). For example, the printhead (1 16) may include a number of nozzles (124). For simplicity, Fig. 2B indicates a single nozzle (124); however a number of nozzles (124) are present on the printhead (116). A nozzle (124) may include an ejector (242), a firing chamber (244), and an opening (246). The opening (246) may allow fluid, such as ink, to be deposited onto a surface, such as a print medium (Fig. 1 , 126). The firing chamber (244) may include a small amount of fluid. The ejector (242) may be a mechanism for ejecting fluid through an opening (246) from a firing chamber (244), where the ejector (242) may include a firing resistor or other thermal device, a piezoelectric element, or other mechanism for ejecting fluid from the firing chamber (244).

[0050] For example, the ejector (242) may be a firing resistor. The firing resistor heats up in response to an applied voltage. As the firing resistor heats up, a portion of the fluid in the firing chamber (244) vaporizes to form a bubble. This bubble pushes liquid fluid out the opening (246) and onto the print medium (Fig. 1 , 126). As the vaporized fluid bubble pops, a vacuum pressure within the firing chamber (244) draws fluid into the firing chamber (244) from the fluid supply (1 12), and the process repeats. In this example, the printhead (1 16) may be a thermal inkjet printhead.

[0051] In another example, the ejector (242) may be a piezoelectric device. As a voltage is applied, the piezoelectric device changes shape which generates a pressure pulse in the firing chamber (244) that pushes a fluid out the opening (246) and onto the print medium (Fig. 1 , 126). In this example, the printhead (116) may be a piezoelectric inkjet printhead.

[0052] The printhead (116) and printer cartridge (114) may also include other components to carry out various functions related to fluidic ejection. For simplicity, in Figs. 2A and 2B, a number of these components and circuitry included in the printhead (116) and printer cartridge (114) are not indicated; however such components may be present in the printhead (116) and printer cartridge (1 14). In some examples, the printer cartridge (114) is removable from a printing system for example, as a disposable printer cartridge.

[0053] Fig. 3 is a block diagram of a printer cartridge (114) that uses a printhead (116) with a number of memristors (348-1 , 348-2) having different structures according to one example of the principles described herein. In some examples, the printer cartridge (114) includes a printhead (116) that carries out at least a part of the functionality of the printer cartridge (114). For example, the printhead (116) may include a number of nozzles (Fig. 1 , 124). The printhead (1 16) ejects drops of fluid from the nozzles (Fig. 1 , 124) onto a print medium (Fig. 1 , 126) in accordance with a received print job. The printhead (116) may also include other circuitry to carry out various functions related to printing. In some examples, the printhead (116) is part of a larger system such as an integrated printhead (IPH). The printhead (1 16) may be of varying types. For example, the printhead (116) may be a thermal inkjet (TIJ) printhead or a piezoelectric inkjet (PIJ) printhead, among other types of printhead (116).

[0054] The printhead (116) includes a memristor array (240) to store information relating to at least one of the printer cartridge (114) and the printhead (116). In some examples, the memristor array (240) includes a number of memristors (348) formed in the printhead (116). To store information, a memristor (348) may be set to a particular resistance state. As memristors (348) are non-volatile, this resistance state is retained even when power is removed from the printhead (1 16).

[0055] A memristor (348) has a metal-insulator-metal layered structure. More specifically, the memristor (348) may include a bottom electrode (metal), a switching oxide (insulator), and a top electrode (metal). A memristor (348) may be classified as an anion device which includes an oxide insulator. Examples of such oxide insulators include transition metal oxides, complex oxides, and large band gap dielectrics in addition to other non-oxide materials. In this example, a tantalum oxide may be an example of a switching oxide in an anion device. In an anionic device, the switching mechanism is the formation of oxygen vacancies in the oxide that are positively charged and therefore lead to the formation of conducting channels in the switching oxide. By comparison, in a cation device the conducting channel is formed from an electrochemically active metal such as copper or silver. In some examples, a memristor (348) may be both an anionic device and a cationic device. For example, an aluminum-copper-silicon alloy oxide based memristor (348) could be an anionic device when the copper concentration is low or a cationic device when the copper concentration is high.

[0056] As will be described in more detail below, different memristors (348) in the memristor array (240) may have different structures. For example, the structure of a first memristor (348-1) may be such that a switching oxide is disposed on just a top surface of a bottom electrode and a top electrode is disposed on just a top surface of the switching oxide. The second memristor (348-2) may be such that the switching oxide is disposed on a top surface and side surfaces of the bottom electrode and the top electrode may be disposed on a top surface and side surfaces of the switching oxide.

[0057] The number of memristors (348) are grouped together into a memristor array (240). In some examples, the memristor array (240) may be a cross bar array. In this example, each memristor (348) may be formed at an intersection of a first set of elements and a second set of elements, the elements forming a grid of intersecting nodes, each node defining a memristor (348). In another example, the memristor array (240) may include a number of memristors (348) that form a one-to-one structure with a number of transistors. For example, an integrated circuit may include a number of addressing units. Each addressing unit may include a number of components that allow for multiplexing and logic operations. The memristor (348) may be designed to be individually addressed by a distinct addressing unit. In some examples, the addressing units may be transistors. In this example, the memristor (348) may share a one transistor-one memristor (1T1 M) addressing structure with the addressing units of the integrated circuit.

[0058] The memristor array (240) may be used to store any type of data. Examples of data that may be stored in the memristor array (240) include fluid supply specific data and/or fluid identification data, fluid characterization data, fluid usage data, printhead (116) specific data, printhead (116) identification data, warranty data, printhead (116) characterization data, printhead (116) usage data, authentication data, security data, Anti- Counterfeiting data (ACF), fluid drop weight, firing frequency, initial printing position, acceleration information, and gyro information, among other forms of data. In a number of examples, the memristor array (240) is written at the time of manufacturing and/or during the operation of the printer cartridge (114).

[0059] In some examples, the printer cartridge (1 14) may be coupled to a controller (106). The controller (106) receives a control signal from an external computing device (Fig. 1 , 102). The controller (106) may be an Application-Specific Integrated Circuit (ASIC), for example, a printer ASIC. A computing device (Fig. 1 , 102) may send a job to the printer cartridge (1 14), the job being made up of text, images, or combinations thereof to be deposited onto a print medium (Fig. 1 , 126). The controller (106) may facilitate storing information to the memristor array (240). Specifically, the controller (106) may pass at least one control signal to the number of memristors (348). For example, the controller (106) may be coupled to the printhead (116), via a control line such as an identification line. Via the identification line, the controller (106) may change the resistance state of a number of memristors (348) in the memristor array (240) to effectively store information to a memristor array (240). For example, the controller (106) may send data such as authentication data, security data, and job data, in addition to other types of data to the printhead (116) to be stored on the memristor array (240).

[0060] While specific reference is made to an identification line, the controller (106) may share a number of lines of communication with the printhead (116), such as data lines, clock lines, and fire lines. For simplicity, in Fig. 3 the different communication lines are indicated by a single arrow.

[0061] Figs. 4A and 4B are cross-sectional diagram of memristors (348-1 , 348-2) having different structures according to one example of the principles described herein. For example, the memristors (348-1 , 348-2) in Figs. 4A and 4B have at least different switching oxides (450-1 , 450-2) structures compared to one another. In other words, the switching oxides (450- 1 , 450-2) may be of the same material, but have different thicknesses, shape, geometry, or combinations thereof. In other words, Figs. 4A and 4B depict memristors (348-1 , 348-2) having different switching behaviors and resistance levels that correspond to their respective resistance states. More specifically, Fig. 4A is a cross-sectional diagram of a first memristor (348-1) having a first switching oxide (450-1) that is as wide as, or narrower than, the bottom electrode (454). While Fig. 4A specifically depicts a first memristor (348-1) having a first switching oxide (450-1) that is narrower than the bottom electrode (454), the first switching oxide (450-1) of the first memristor (348-1) may be as wide as the bottom electrode. In the first memristor (348-1), having a first switching oxide (450-1) that is as wide as or narrower than the bottom electrode (454), the switching voltage transfers across the first switching oxide (450-1) in a vertical direction, perpendicular to the bottom electrode (454). By comparison, Fig. 4B is a cross-sectional diagram of a second memristor (348-2) having a second switching oxide (450-2) that is wider than the bottom electrode (454). In the second memristor (348-1), having a second switching oxide (450- 2) that is wider than the bottom electrode (454), the switching voltage transfers across the second switching oxide (450-2) in both a horizontal direction and a vertical direction relative to the bottom electrode (454).

[0062] As described above, a memristor (348) is a non-volatile memory element that has a metal-insulator-metal layered structure. More specifically, the memristor (348) includes a bottom electrode (454), a switching oxide (450), and a top electrode (452). The bottom electrode (454) may be an electrical connection between the memristor (348) and other components. Examples of components that may attach to the bottom electrode (454) include a ground connection, a number of connection pads, a current regulator, a capacitor, a resistor, and metal traces, among other memristor array (Fig. 2, 240) components.

[0063] A switching oxide (450) may be disposed on the bottom electrode (454). The switching oxide (450) may be an insulator between the bottom electrode (454) and the top electrode (452). For example, in a first state, the switching oxide (450) may be insulating such that current does not readily pass from the bottom electrode (454) to the top electrode (452). Then, during a switching event, the switching oxide (450) may switch to a second state, becoming conductive. In a conductive state, the switching oxide (450) allows a memristor (348) to store information by changing the memristor (348) state.

[0064] As depicted in Fig. 4A, a switching oxide (450) may be as wide as, or narrower than, the bottom electrode (454). For example, a first switching oxide (450-1) of a first memristor (348-1) may be as wide as, or narrower than, the bottom electrode (454). In this example, the narrower first switching oxide (450-1) is entirely disposed on a top surface of the bottom electrode (454).

[0065] In some examples, as depicted in Fig. 4B, the switching oxide (450) may be wider than the bottom electrode (454). For example, a second switching oxide (450-2) of a second memristor (348-2) may be wider than the bottom electrode (454). In this example, the wider second switching oxide (450- 2) may be disposed on a top surface of the bottom electrode (454) as well as a number of side surfaces of the bottom electrode (454).

[0066] In some examples the memristor (348) may include a dielectric layer (456) to electrically isolate the memristor (348) from other components such as transistors and other memristors (348). The dielectric layer (456) may define the switching oxide (450). For example, an opening in the dielectric layer (456) of Fig. 4A may be narrower than, or as narrow as, the bottom electrode (454) and define the first switching oxide (450-1) as being narrower than, or as wide as, the bottom electrode (454). Accordingly, during a formation operation, an opening may be etched in the dielectric layer (456) that is narrower than the bottom electrode (454) such that the first switching oxide (450-1) is narrower than the bottom electrode (454) and is disposed entirely on a top surface of the bottom electrode (454).

[0067] As depicted in Fig. 4B, the opening in the dielectric layer (456) may be wider than the bottom electrode (454) and may define the second switching oxide (450-2) as being wider than the bottom electrode (454).

Accordingly, during a formation operation, an opening may be etched in the dielectric layer (456) that is wider than the bottom electrode (454) such that the second switching oxide (450-2) is wider than the bottom electrode (454) and is disposed on a top surface of the bottom electrode (454) as well as a number of side surfaces of the bottom electrode (454). As will be described below, the opening in the dielectric layer (456) may be sufficiently wide such that the formation of a top electrode (452) is on a top surface of the second switching oxide (450-2) as well as a number of side surfaces of the second switching oxide (450-2) as depicted in Fig. 4B. In some examples, the side surfaces of the second switching oxide (450-2) may be thinner than a top surface of the second switching oxide (450-2). Narrower side surfaces may be beneficial by concentrating an electrical field around the side surfaces and the corners of the switching oxide (450-2). The concentrated electrical field may lead to a lower resistance value for the second memristor (348-2) as compared to the first memristor (348-1). [0068] The memristor (348) also includes a top electrode (452) disposed on a top surface of the switching oxide (450). As with the bottom electrode (454), the top electrode (452) may be an electrical connection between the memristor (348) and other components. Examples of components that may attach to the top electrode (452) include a ground connection, a number of connection pads, a current regulator, a capacitor, a resistor, and metal traces, among other memristor array (Fig. 2, 240) components.

[0069] In some examples, the top electrode (452) may be disposed on just a top surface of the switching oxide (450). For example, as depicted in Fig. 4A, a first top electrode (452-1) may be disposed just on a top surface of the first switching oxide (450-1). In other examples, the top electrode (452) may be disposed on a top surface and a number of side surfaces of the switching oxide (450). For example, as depicted in Fig. 4B, a second top electrode (452-2) may be disposed on a top surface and a number of side surfaces of the second switching oxide (450-2). Accordingly, the opening in the dielectric layer (456) may be sufficiently wide such that both the second switching oxide (450-2) and the second top electrode (452-2) are disposed on side surfaces of the bottom electrode (454) and the second switching oxide (450-2), respectively.

[0070] As the different memristors (348-1 , 348-2) have different switching oxides (450-1 , 450-2), the different memristors (348-1 , 348-2) also have different resistance levels, resistance levels referring to a resistance level of a resistance state, which resistance state indicates a particular logic value. For example, the second memristor (348-2) having a second, and wider, switching oxide (450-2) that encompasses the bottom electrode (454) may have a lower switching voltage or may conduct electricity differently than the first memristor (348-2) that has a first, and narrower, switching oxide (450-1) that is entirely disposed on a top surface of the bottom electrode (454).

[0071] The memristor array (Fig. 2, 240) includes both types of memristors (348-1 , 348-2). Specifically, the memristor array (Fig. 2, 240) includes a number of first memristors (348-1) having first switching oxides (450- 1) that are narrower, or as wide as, the bottom electrode (454) and also includes a number of second memristors (348-2) having second switching oxides (450-2) that are wider than the bottom electrode (454). In other words, the memristor array (Fig. 2, 240) includes memristors (348) that have different switching oxides (450-1 , 450-2) and therefore that have different resistance states. Including memristors (348) with different switching oxides (450) in a single memristor array (Fig. 2, 240) may be beneficial by providing additional resistance levels in a single array, which allows for more logic values to be represented by the memristor array (Fig. 2, 240). In other words, the

memristors (348) with different switching oxides (450) may allow for additional density in the memristor array (Fig. 2, 240) as well as providing additional data security to the memristor array (Fig. 2, 240).

[0072] In some examples the memristors (348) with different structures may be formed using the same process. As used in the present specification and in the appended claims, the term same is meant to be understood broadly as a process that is substantially the same in most respects. More specifically, a first memristor (348-1) with a first, and narrower, switching oxide (450-1) and a second memristor (348-2) with a second, and wider, switching oxide (450-2) may be made with the same process by varying the etching width of the dielectric layer (456). Still further, a first memristor (348-2) with a narrower first switching oxide (450-1) and a second memristor (348-2) with a wider second switching oxide (450-2) may be manufactured at the same time. In other words, the first memristor (348-1) with a first, and narrower, switching oxide (450-1) and a second memristor (348-2) with a second, and wider, switching oxide (450-2) may be formed using a shared process.

[0073] More specifically, a first switching oxide (450-1) with a narrow switching oxide may be formed by etching a dielectric layer (456) with an opening that is narrower than the bottom electrode (454) and a second switching oxide (450-2) with a wider switching oxide may be formed by etching a wider opening in the dielectric layer (456). In some examples, the etching of the narrow opening and the etching of the wide opening may be performed on the same dielectric layer (456) at the same time.

[0074] For example, two instances of the bottom electrode (454) may be formed on a single substrate. A single layer of dielectric material (456) may then be deposited on top of both instances of the bottom electrode (454). Using a mask, a first opening in the dielectric layer (456) is then formed over a first instance of the bottom electrode (454), which opening is narrower than the bottom electrode (454). Either using the same mask or a different mask, a second opening in the dielectric layer (456) may be formed over a second instance of the bottom electrode (454), which second opening is wider than the bottom electrode (454). A first switching oxide (450-1) may then be formed in the first opening and a second switching oxide (450-2) formed in the second opening. The formation of the first switching oxide (450-1) and the second switching oxide (450-2) may be performed via physical vapor deposition (PVD), deposition, or other switching oxide (450) formation method.

[0075] Corresponding top electrodes (452-1 , 452-2) are then formed on top of corresponding switching oxides (450-1 , 450-2). While a specific example of two instances of memristor (348) formation has been described, any number of memristors (348) may be formed as described herein. The method described herein may be beneficial as a first memristor (348-1) with a first, and narrow, switching oxide (450-1) and a second memristor (348-2) with a second, and wide, switching oxide (450-2) may be made on the same substrate and may be used in a similar memory bank. More detail regarding a memory bank with different types of memristors (348) is given below in connection with Fig. 5. In these and other examples, the memristors (348) having narrow and wide switching oxides (450) may include the same layers of materials.

[0076] Fig. 5 is a diagram of a memory bank (558) that includes memristors (348) having different structures according to one example of the principles described herein. The dashed-dot lines of Fig. 5 indicate regions of the memory bank (558) that identify an individual memristor (348). In some examples, the memristors (348) in a memristor array (Fig. 2, 240) may be organized into memory banks (558). In one example, as described above, memristors (348) having different structures may be formed into a single memory bank (558). For example, the memory bank (558) may include a number of first memristors (348-1) that have first switching oxides (450-1) that are narrower than the bottom electrodes (454). The previously discussed Fig. 4A is a cross-section of the first memristor (348-1) taken along the line A. In this example, a portion of the top electrodes (452) that cover the number of first switching oxides (450-1) may be entirely disposed on a top surface of the first switching oxides (450-1) as depicted in Fig. 4A.

[0077] The memory bank (558) may also include a number of second memristors (348-2) that have second switching oxides (450-2) that are wider than the bottom electrodes (454). The previously discussed Fig. 4B is a cross- section of the second memristor (348-2) taken along the line B. In this example, a portion of the top electrodes (452) that cover the number of second switching oxides (450-2) may be disposed on a top surface of the second switching oxides (450-2) and a number of side surfaces of the second switching oxide (450-2) as depicted in Fig. 4A. In Fig. 5, the dashed line indicates that the switching oxides (450) are disposed underneath the top electrodes (452) but disposed on top of the bottom electrodes (454).

[0078] A memristor bank (558) that includes first memristors (348-1) with first, and narrow, switching oxides (450-1) and second memristors (348-2) with second, and wide, switching oxides (450-2) may be beneficial in that a wider range of resistance levels, and corresponding logic values, may be obtained from the memristor bank (558). In other words, the memristor bank (558) using memristors (348) with different resistance levels increases the storage density of a memristor bank (558). The different types of memristors (348) also helps for data security within the memristor bank (558).

[0079] Figs. 6A and 6B are diagrams of memory banks (558-1 , 558-2) that include memristors (348) with similar structures according to one example of the principles described herein. As described above, the memristors (348) in a memristor array (Fig. 2, 240) may be formed into a number of memory banks (558-1 , 558-2). In some examples, each memory bank (558) within the memristor array (Fig. 2, 240) may include memristors (348) of a similar type. For example, a first memristor bank (558-1) may include first memristors (348-1) having first switching oxides (450-1) that are narrower than the bottom electrode (454). In the same memristor array (Fig. 2, 240), a second memristor bank (558-2) may include second memristors (348-2) having second switching oxides (450-2) that are wider than the bottom electrode (454). Implementing memristors (348) of a similar type in a single memristor bank (558) may be beneficial by simplifying manufacturing of memristor banks (558).

[0080] A printer cartridge (Fig. 1 , 114) and printhead (Fig. 1 , 1 16) with memristors (Fig. 3, 348) having different structures may have a number of advantages, including: (1) allowing for the construction of different types of memristors (Fig. 3, 348) in a single memory bank (Fig. 5, 558); (2) increasing the number of resistance levels (and corresponding logic values) represented in a memristor array (Fig. 2, 240); (3) improving printhead (Fig. 1 , 1 16) memory performance; and (4) reducing cost of effective memory array fabrication.

[0081] Aspects of the present system are described herein with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to examples of the principles described herein. Each block of the flowchart illustrations and block diagrams, and combinations of blocks in the flowchart illustrations and block diagrams, may be implemented by computer usable program code. The computer usable program code may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the computer usable program code, when executed via, for example, the processor (Fig. 1 , 108) of the system (Fig. 1 , 100) or other programmable data processing apparatus, implement the functions or acts specified in the flowchart and/or block diagram block or blocks. In one example, the computer usable program code may be embodied within a computer readable storage medium; the computer readable storage medium being part of the computer program product. In one example, the computer readable storage medium is a non-transitory computer readable medium.

[0082] The preceding description has been presented to illustrate and describe examples of the principles described. This description is not intended to be exhaustive or to limit these principles to any precise form disclosed. Many modifications and variations are possible in light of the above teaching.