Thermal inkjet printhead, and printing assembly and printing device comprising the same

Abstract

A thermal inkjet printhead, and a printing assembly and a printing device including the same, are provided. The thermal inkjet printhead includes a substrate, a nozzle layer including a plurality of nozzles formed through the nozzle layer, a plurality of ink ejection chambers corresponding to the plurality of nozzles, a plurality of heater resistors formed on the substrate and corresponding to the plurality of ink ejection chambers, each of the heater resistors being located in a different one of the ink ejection chambers such that heating by one of the heater resistors located in a corresponding ink ejection chamber causes ejection of an ink drop through the respective nozzle, a plurality of separate air pocket islands formed on and corresponding to the plurality of heater resistors, each of the air pocket islands covering a different one of the heater resistors, and a dielectric layer interposed between the heater resistors and the air pocket islands. Use of the present invention can help to enhance and significantly improve printhead reliability, in turn increasing manufacturing process yield.

Description

TECHNICAL FIELD

[0001] The present invention relates to the field of thermal inkjet printing and in particular to a thermal inkjet printhead. BACKGROUND

[0002] Thermal inkjet printing technology has been relatively well developed. Various thermal inkjet printheads exist. For example, US6123419A discloses a thermal inkjet printhead which employs a segmented heater resistor of relatively high resistance value in order to overcome inefficient power dissipation in parasitic resistances. US6582062B1 discloses a large array inkjet printhead which employs a multiplexing arrangement to reduce parasitic resistances and the number of access leads.

[0003] In a thermal inkjet printhead, ejection of an ink drop through a nozzle is achieved by rapid heating of a volume of ink residing in an ink chamber, and the heating of the ink is achieved by a short current pulse applied to a heater resistor located within the ink chamber. The heating of the ink causes an ink vapor bubble to form and rapidly expand, forcing liquid ink through the nozzle. Once the pulse is over and the ink drop is ejected, the ink chamber is refilled with ink through an ink channel. The heater resistor is made of a resistive film, and the thermal inkjet printhead comprises a plurality of such heater resistors as a resistor array. The heater resistors are electrically connected to associated logic circuitry and power supply circuitry by conductive traces and / or pads, so that the individual heater resistors can be suitably controlled. In implementing the logic circuitry and power supply circuitry, metal lines are used.

[0004] In prior art thermal inkjet printhead arrangements, typically all heater resistors are covered by a continuous protective layer, which prevents the underlying resistive film from being damaged during printhead operation due to the burst of an ink vapor bubble. For this purpose, some refractory metal, such as tantalum, is used for the protective layer, which shows both great mechanical strength and good thermal conductivity. Such a tantalum film is typically deposited continuously in the entire resistor area, spanning the entire resistor array. Due to the electrical conductivity of tantalum, large areas of the arrangement are consequently covered by a continuous tantalum conductive film. On the one hand, due to the fact that the voltage level in the metal lines on the arrangement does vary over time, this tantalum conductive film can capacitively couple to adjacent metal lines underneath it, and thus can cause some problems with the logic circuitry. On the other hand, small holes or discontinuities in the dielectric layer inserted between the tantalum layer and the underlying metal lines can cause parasitic electrical shorting paths, the effects of which can lead to electrical shortcomings and electrochemical effects through the ink. US6441838B1 discloses such an inkjet printhead which comprises a tantalum passivation layer to provide mechanical passivation for the inkjet resistors by absorbing the cavitation pressure of a burst drive bubble, wherein the tantalum passivation layer is arranged over the heater resistors, extending beyond the ink chamber and over the associated ink channel. SUMMARY

[0005] An object of the present application is to provide a solution that can alleviate or solve at least some of the above-mentioned problems in the prior art. The problems mentioned are solved by the subject matter of the independent claims. Further preferred embodiments are defined in the dependent claims.

[0006] According to an aspect of the present application, there is provided a thermal inkjet printhead, comprising:

[0007] a substrate;

[0008] a nozzle layer comprising a plurality of nozzles formed therethrough;

[0009] a plurality of ink ejection chambers corresponding to the plurality of nozzles;

[0010] a plurality of heater resistors formed on the substrate and corresponding to the plurality of ink ejection chambers, the heater resistors each being located in a different one of the ink ejection chambers, such that heating by one of the heater resistors located in a corresponding ink ejection chamber causes ejection of an ink droplet through the respective nozzle;

[0011] a plurality of separate air pocket islands formed on and corresponding to the plurality of heater resistors, the air pocket islands each covering a different one of the heater resistors; and

[0012] a dielectric layer interposed between the heater resistors and the air pocket islands, wherein the dielectric layer is a composite film made of silicon nitride and silicon carbide and having a thickness in the range of about 0.4 pm to about 0.65 pm.

[0013] According to another aspect of the present application, there is provided a printing assembly comprising the above-mentioned thermal inkjet printhead.

[0014] According to yet another aspect of the present application, there is provided a printing device, such as a printer, comprising the above-mentioned thermal inkjet printhead.

[0015] With the solution of the present application, the overlap of each air pocket island with its adjacent circuitry can be reduced, thus the possibility of generating parasitic capacitive coupling between the air pocket layer and its adjacent circuitry is significantly reduced compared to the prior art. Moreover, due to the relatively small surface area of the individual air pocket islands, it is less likely that an air pocket island overlaps a possible defect in the thin dielectric film underneath, i.e. the possibility of a defect in the dielectric film being exactly underneath a certain air pocket island and thus causing some electrical short circuit is reduced. Thus, due to the specific composition and thickness of the dielectric layer (which is much thinner than in the prior art), the air pocket islands providing an "electrical" insulation are clearly advantageous. Thus, the present application provides an optimized heat transfer with a reduced risk of having small holes with undesired conductive bridges between different layers. Thus, using the present application can help to significantly increase the print head reliability, which in turn increases the yield of the manufacturing process. BRIEF DESCRIPTION OF DRAWINGS

[0016] Non-limiting and non-exhaustive embodiments of the present application are described by way of example with reference to the following drawings, in which:

[0017] Figure 1 is a schematic diagram illustrating an exemplary layout of a thermal inkjet printhead according to an embodiment of the present application;

[0018] Figure 2 is a schematic diagram illustrating an exemplary wafer before dicing;

[0019] Figure 3 is a perspective view schematically illustrating an exemplary printing assembly comprising a thermal inkjet printhead of the present application;

[0020] Figure 4 is a perspective view schematically illustrating a portion of an exemplary microfluidic circuit;

[0021] Figure 5 is a cross-sectional view schematically illustrating a portion of the microfluidic circuit in Figure 4 ;

[0022] Figure 6 is a cross-sectional view schematically illustrating a portion of Figure 5 ;

[0023] Figure 7 is a perspective view schematically illustrating a portion of a thermal inkjet printhead in Figure 1 ;

[0024] Figure 8 is a perspective view schematically illustrating a portion of a thermal inkjet printhead of the prior art;

[0025] Figure 9a , Figure 9b and Figure 9c illustrate a portion of Figure 8 , respectively.a possible scenario of a thermal inkjet printhead, a possible equivalent circuit corresponding to the scenario, and a modified version of the equivalent circuit; and

[0026] Figure 10a Figure 10b and Figure 10c show, respectively, another possible scenario of a thermal inkjet printhead, a possible equivalent circuit corresponding to the scenario, and another possible equivalent circuit corresponding to the scenario, partially shown in Figure 8 DETAILED DESCRIPTION

[0027] In order to make the above and other features and advantages of the application more compelling, the application is further described below with reference to the drawing. It is to be understood that the specific embodiments given herein are for purposes of illustration only and are not intended to be limiting, except as provided in the appended claims.

[0028] Figure 1 An exemplary layout of a thermal inkjet printhead according to an embodiment of the application is schematically shown. Figure 1 The thermal inkjet printhead in Figure 2 may be manufactured in a single silicon wafer 5, which is subsequently diced into individual chips using appropriate semiconductor technology, including thin film deposition, photolithography, wet and dry etching techniques, ion implantation, oxidation, etc. The columns of heater resistors 2 can be positioned in close proximity to a through slot 4 formed in an inner portion of the printhead chip to allow for ink refill. Each heater resistor 2 can be made of a resistive film and can be in contact with a respective (one or more) conductive trace. In a peripheral region of the printhead, there can be a set of contact pads 6, which are typically bonded to a flexible printed circuit using a TAB (Tape Automated Bonding) process. Each heater resistor can be electrically connected to the flexible printed circuit via the respective (one or more) conductive trace and the respective (one or more) contact pad 6. In the active portion 10 of the substrate 1, there can be an array of MOS transistors 11 for addressing the resistors, one or more logic circuits 12, one or more programmable memories 13, and other possible components, particularly when the electronic layout associated with the heater resistors becomes relatively complex as the number of heater resistors increases. In addition to the resistive film forming the heater resistors, the thermal inkjet printhead of the present application can include other layers / films to be described later.

[0029] Reference is made to Figure 3 ​​which shows a print assembly comprising the thermal inkjet printhead of the present application, a flexible printed circuit 7 is attached to the printhead cartridge body 8 and the thermal inkjet printhead of the present application can be mounted and connected to the printhead cartridge body 8. The flexible printed circuit 7 is provided with larger contact pads 9 to exchange electrical signals with the printer in which the thermal inkjet printhead is used. The thermal inkjet printhead, for example Figure 1 the thermal inkjet printhead shown in

[0030] Reference is made to Figure 4 and Figure 5 As schematically represented at area 14, on the substrate surface where the stack of resistive, conductive and dielectric films of the thermal inkjet printhead of the present application has been deposited and patterned, a microfluidic circuit can be deposited and implemented so that ink can flow in the deposited microfluidic circuit through a suitable channel 15 and reach an ink ejection chamber 16, where the walls of the ink ejection chamber 16 surround the respective heater resistor 2. The channel 15 is in fluid communication with a through slot 4, where the through slot 4 can open to an ink reservoir (not shown). The microfluidic circuit is typically patterned in a suitable polymer layer 17, called a barrier layer. Above the barrier layer a nozzle layer 18, for example in the form of a plate, is provided. A plurality of nozzles 19, each aligned with the underlying heater resistor, can be formed through the nozzle plate 18 and ink droplets 20 are ejected from the nozzles. During operation of the thermal inkjet printhead, if a heater resistor 2 needs to be activated, a short current pulse is applied to heat the resistor, which in turn causes the thin layer of ink just above the resistor to evaporate, forming a vapor bubble 21. The sudden increase in pressure in the evaporated layer causes a portion of the overlying liquid ink to be ejected from the respective nozzle above the activated resistor. The ink droplet travels towards a medium (for example, paper), creating an ink dot on the surface of the medium. Thereafter, new ink is drawn into the ink ejection chamber 16 to replace the ejected droplet until a steady state is reached.

[0031] In order to optimize the energy transfer from the heater resistor 2 (heated by the current pulse through the Joule effect) to the ink, the resistor needs to be thermally insulated from the substrate so that the heat flow is preferably directed towards the overlying ink, which in turn is separated from the resistive film layer by a thin dielectric film to avoid electrical leakage. The substrate can be made of silicon, which has a considerable thermal conductivity, in which case an insulating layer of sufficient thickness needs to be inserted between the substrate and the resistor: in other words, the resistor should be deposited on a suitable insulating layer grown or deposited on the substrate. Both thermal grown silicon oxide and BPSG (boron phosphorus silicon glass), produced with high temperature processes, are suitable materials for the thermal insulation of the resistor and can be used alone or in combination. Since the temperatures used for the growth or deposition and / or annealing of these materials are higher than the operating temperature of the heater resistor in the printhead, these materials will remain stable during the normal operation of the printhead.

[0032] The resistive film, which undergoes rapid and large temperature variations during the operation of the print head, should have stable properties and good resistance to thermo-mechanical stresses. Typically, the heater resistor 2 has a resistance value of a few tens of ohms; although different shapes and different resistance values can be adopted, a square heater resistor with a resistance of about 30 ohms is generally adopted. The widespread and lasting choice of the heater resistor is a composite film made of tantalum-aluminum alloy: a film thickness of about 900 angstroms gives a sheet resistance of 30 ohms per square, i.e. a square resistor made of such a film has a resistance of 30 ohms. According to a preferred embodiment of the present application, the heater resistor is a U-shaped heater resistor, which means that there is a gap between the proximate conductors biased at different voltages.

[0033] Various known solutions are available for addressing and driving the plurality of heater resistors. If the number of nozzles in the print head is relatively small (up to a few tens), each heater resistor can be directly connected to a respective contact pad by an electrical track, while the return of the current can be jointly collected by one or a few ground pads. As the number of nozzles increases, the direct individual driving of the resistors with a large number of contact pads for addressing them is difficult to implement: in fact, the pads are usually distributed along the outer border of the print head chip and the number of pads cannot be increased without any limit. A more practical solution is to adopt an addressing matrix, which allows a reduced number of contact pads to be used for driving a large number of resistors. The addressing matrix is preferably implemented with a plurality of MOS transistors, each in electrical communication with a determined heater resistor. Each heater resistor can be connected to the electrodes of the transistor matrix in a suitable manner, so that the heater resistor can be activated as needed, causing the ejection of a droplet of ink from the print head.

[0034] As mentioned above, the dielectric layer above the heater resistor provides electrical insulation for the ink: usually, a silicon nitride film alone or in combination with silicon carbide is used for the dielectric layer formed for this purpose. The insulating film used for the dielectric layer should be thin enough to allow a strong heat flow while withstanding the thermo-mechanical stresses experienced during the operation of the print head and the impact due to the bubble collapse. According to the present invention, the dielectric layer is a composite film made of silicon nitride and silicon carbide, the thickness of the dielectric layer being at least 4000 Angstrom (0.4 pm) and at most 6500 Angstrom (0.65 pm). In fact, the rapid expansion of the vapor bubble due to the heating of the heater resistor has the effect of reducing the internal pressure of the bubble to a level much lower than the external atmospheric pressure. At the maximum of the bubble expansion, the bubble results in a cavity with low pressure inside, the lower part of which is limited by the floor of the ink ejection chamber and surrounded by ink. The greater external atmospheric pressure pushes the liquid ink located above the cavity back, causing a violent impact on the chamber floor. This impact, due to the rupture of the cavity previously formed in the ink, can damage the films constituting the chamber floor, i.e. the resistive film and the insulating film above. Usually, the thin insulating film is not strong enough and an additional protective film, called air pocket layer, made for example of a refractory metal such as tantalum, is deposited above the insulating film. The tantalum film is heat conducting and maintains a strong heat flux from the resistive film towards the ink even in the presence of the additional layer. According to the present invention, a novel arrangement for the air pocket layer is proposed. The concept serves to reduce the area of the film surface of the air pocket layer without affecting its function. In particular, the air pocket layer can consist of a plurality of separate air pocket islands each patterned above a respective heater resistor of the heater resistors. A schematic representation of the region 14 in Figure 7 A further description of such air pocket layer is given.

[0035] A more detailed view can be observed in the cross-sectional view of Figure 6 Figure 5 A schematic representation of the region 14 in

[0036] Figure 7 A part of a thermal inkjet print head in Figure 1 is shown schematically. As Figure 7 ​As shown, a series of heater resistors 2 are surrounded by a barrier layer 17 such that each heater resistor 2 is contained in an ink ejection chamber defined by two vertical walls of the barrier layer 17. Ink flows from the edge 26 of the slot 4 through the channel 15 to the ink ejection chamber. In this embodiment, the slot edge is straight, but could take on an edge shape that follows the staggered placement of the heater resistors in order to equalize the refill times of all the heater resistors.

[0037] In Figure 7 the plurality of air pocket islands 33 together form an air pocket layer. Such an air pocket layer can be referred to as a split air pocket layer or a segmented air pocket layer, and each air pocket island can also be referred to as an air pocket segment. These air pocket islands 33 are separated from one another. Each air pocket island 33 corresponds to and covers a different individual heater resistor 2, and the area of the air pocket island 33 can be just larger than the area of the resistor it covers. Each air pocket island 33 can be composed of tantalum, although other suitable materials, particularly refractory conductive materials, can also be used.

[0038] In a preferred embodiment, the air pocket islands 33 can be floating, i.e., not connected to any voltage source.

[0039] Each air pocket island 33 has only a small overlap area with its adjacent circuitry 29, so the possibility of generating parasitic capacitive coupling due to the presence of the air pocket layer is significantly reduced compared to the prior art. In addition, since the total area covered by the segmented air pocket layer is relatively small, the possibility of having an undesired possible small hole or discontinuity in the dielectric layer between the air pocket layer and the underlying metal lines directly below the air pocket islands can also be significantly reduced. Furthermore, the use of the novel layout helps to increase the distance between the air pocket layer and the underlying logic circuitry, thereby reducing the possible parasitic capacitance and capacitive coupling. The use of a segmented air pocket layer as Figure 7 shown can help to enhance and significantly improve the printhead reliability, which in turn increases the yield of the manufacturing process.

[0040] Although the presence of the segmented air pocket layer can make the surface on which the barrier layer 17 is deposited slightly rough, the deposition and subsequent patterning of the barrier layer can be performed anyway, thereby providing a flat surface and good adhesion near the resistor array.

[0041] The advantages of the thermal inkjet printhead of the present application employing the segmented air pocket layer described above will become more apparent from the following description, including those advantages described above.

[0042] Figure 8 A portion of a prior art thermal inkjet printhead device is shown schematically. As Figure 8As shown, a series of heater resistors 102 is surrounded by a barrier layer 117, the vertical walls of which form the boundaries of the ink ejection chambers corresponding to the heater resistors. Ink flows from the edge 126 of the slot 104 to the chambers through the passages 115.

[0043] The front edge 127 of the continuous air pocket layer 122, represented schematically by the area filled with dots, is located at a distance from the slot edge 126 to prevent the slot formation process from damaging the layer. The same caution is taken for the dielectric layer (not shown) underneath the air pocket layer. The edges of the mentioned layers do not necessarily need to coincide: the edge of the dielectric layer can be closer to the slot edge 126 than the edge of the air pocket layer, or vice versa, without affecting the reliability of the device. The back edge 128 of the air pocket layer 122 is located just behind the resistors 102. There are several reasons for such an implementation: the air pocket layer of tantalum generally provides a good adhesion to the upper barrier layer, which is highly desirable in the areas where the air tightness around the chambers and between the adjacent chambers is crucial to ensure the correct performance of the device. The continuity of the tantalum layer surface near the ejection area of the device improves the adhesion even better, since the smooth topography without sharp edges makes the deposition and patterning of the polymer barrier layer easier.

[0044] However, as shown below, there are drawbacks caused by the large area covered by the tantalum air pocket layer 122.

[0045] The printhead device is controlled and powered by a suitable circuit 129, represented schematically by the hatched area, which is in close proximity to the ejection area, and thus is partially overlapped by the tantalum air pocket layer, although the circuit 129 and the air pocket layer are separated by intervening dielectric layers made of silicon nitride and silicon carbide.

[0046] The tantalum air pocket layer and the metal lines of the lower layer circuit, separated by a thin dielectric layer, together act as a number of capacitors, although they are not designed for this purpose. Even if these parasitic capacitors do not belong to the circuit of the device, they can have an unexpected and undesirable influence on the behavior of the device, mainly if there are complex logic circuits. The presence of parasitic capacitors throughout the device is due to the close proximity of the conductive parts, either side by side separated by small gaps, or stacked with an insulating layer between them. Since the cost requirements for the manufacturing process urge the designers to increase the surface density of the electrical components, in turn leading to a higher risk of parasitic effects, it is difficult to avoid the presence of parasitic effects in monolithic electronic devices.

[0047] Due to the large surface of the tantalum cavity layer, there are a large number of conductors belonging to the underlying layers that can be overlapped by the tantalum plate itself. Therefore, there are also a large number of parasitic capacitors with the upper tantalum plate as the upper electrode. Since the lower conductors can find themselves at a voltage level that varies dynamically over time, this can cause some capacitive coupling between different conductors at a lower level during voltage commutation, depending on the operating mode of the device.

[0048] As an example, in Figure 9a , the following situation is depicted in a cross-sectional view: There are two conductors 130 and 131, which are not necessarily close together. Both conductors are covered by a dielectric layer 123, which in turn is overlapped by a wide continuous cavity layer 122. At a certain time, as Figure 9b shows, the conductors 130 and 131 (also referred to as conductors) can be set to voltages V1 and V2, respectively. Figure 9b depicts a simplified equivalent circuit corresponding to this situation. In Figure 9b , the resistance values RT of the conduction paths through the tantalum layer 122 and the resistance values R1 and R2 of the conductors 130 and 131 are considered.

[0049] According to the Figure 9b model, if the value of voltage V1 undergoes a sudden change ΔV in the form of a stepped waveform, this change ΔV causes a sudden perturbation on the lower plate of the capacitor C2 corresponding to the conductor 131. It is easy for those skilled in the art to see that the magnitude and trend of the perturbation on the conductor 131 do indeed depend on the resistance values R1, R2, and RT and the capacitance values of the capacitors C1 and C2 compared to ΔV. Generally, since the capacitor behaves as a short circuit for sudden voltage changes, immediately after the voltage V1 changes, the sudden change ΔV is distributed across the resistors with the Figure 9b shown resistance values R1 and R2. Therefore, if R1 and RT << R2, the sudden change ΔV is first shown to be almost completely transferred to the conductor 131. Subsequently, due to the gradual charge accumulation on the capacitor plates, the system tends to reach a new stationary state after a certain period of time, at which point the magnitude of the perturbation almost drops to zero: the larger the capacitance values of the parasitic capacitors C1 and C2, the longer the duration of the perturbation.

[0050] For example, a similar situation can be found when the conductor 131 is connected to the gate of a MOS transistor. In most cases, the transistor gate in a circuit is not left floating and can be connected to ground through a pull-down resistor or a pull-up resistor with a resistance value significantly greater than the resistance value of the conductive layer; therefore, conditions R1 and RT<<R2 are met. If the disturbance on the gate electrode lasts long enough, the sudden change in voltage V1 can cause an unwanted commutation of the transistor state. This can cause malfunctions in the device, mainly when the disturbed gate is part of a logic circuit and the electrical disturbance can trigger some unwanted operations. Even, since in a print head the power supply lines that power the nozzle heater resistors are usually biased at a voltage higher than 10 volts, while the power supply of the logic circuit is usually in the range of 3 to 5 volts, even if the disturbance on the gate is attenuated with respect to AV, a sudden voltage change in the power supply line parasitically coupled to the logic transistor can have a severe impact on the logic transistor.

[0051] Increasing the thickness of the dielectric layer 123 to reduce the capacitance value of the parasitic capacitors C1 and C2, which in turn reduces the disturbance duration, is not recommended because the effectiveness of the heat transfer from the heater resistor to the ink exploits the thin dielectric layer. On the other hand, the use of two different thicknesses of the dielectric layer for the heater resistor area and for the following circuit represents a complexity of the manufacturing process, therefore more costly.

[0052] As shown in Figure 9c , where the resistance values RT' and RT" of the conductive paths from the tantalum air pocket layer to ground are reflected, a possible solution to solve the problem can be obtained by connecting the tantalum air pocket layer to ground, so as to decouple the parasitic capacitors from each other. This implementation proves to be very effective in reducing the cross-talk caused by the capacitive coupling with the air pocket layer; however, this implementation is prone to increase the likelihood of other drawbacks.

[0053] In fact, during the manufacturing of the device, many processes, such as deposition, patterning and etching, follow one another and it is usually not possible to avoid the presence of some defects in the layers of the device. For example, when residual particles remain on the surface after an etching process, these residual particles can compromise the integrity of the subsequent layer deposited immediately above. If this layer is a dielectric film, small holes or zones of missing material can appear on the entire film surface, compromising the uniformity of the insulation. If a conductive layer is deposited above the defective dielectric layer, some of the conductive material can pass through the holes on the film and, in the worst case, some of the conductive material can make some contact with the conductive track(s) located below the insulating dielectric layer itself. This can happen very likely when the upper conductive layer covers a large surface area, like a continuous air pocket layer according to the prior art: as Figure 10aThe large overlap area, as depicted, increases the likelihood of some of the vias in the tantalum cut-off dielectric film, in turn just above the conductive tracks.

[0054] Figure 10a A cross-section of the layer stack is shown, in which a defect in the middle dielectric layer 123, in particular a via, has been filled with the material of the topmost air pocket layer, thus generating a conductive bridge 132 towards the lower conductive track 130. This defect will act as a short circuit between two conductive layers, which should be electrically insulated in a defect-free device, or at least as a resistive path. Depending on whether the air pocket layer is floating or grounded, the equivalent circuit corresponding to this situation can be shown as Figure 10b or Figure 10c The conductive bridge 132 between the metallic air pocket layer and the lower metallic track 130 is represented by a resistor RB.

[0055] In the case shown in Figure 10b the entire floating air pocket layer is brought to the same potential VI as applied to the conductor 130. The parasitic capacitive coupling between the air pocket layer and the lower circuit becomes even stronger, since the voltage VI directly influences the tantalum air pocket layer, even if the voltage VI is a variable quantity. Moreover, since the ink generally exhibits a certain amount of electrical conductivity, other electrical problems can spread in the device circuit through defects in the dielectric film; moreover, electrochemical effects related to the ink can also occur, thus possibly closing the current path through the body of the silicon bare die.

[0056] On the other hand, in the case shown in Figure 10c in which the resistance values RT' and RT" of the conductive path from the tantalum air pocket layer to ground are shown, the voltage at the air pocket layer remains at ground, which suppresses or greatly reduces the possible effects of capacitive coupling involving the tantalum film. However, if the voltage VI is different from zero (assuming a value of the ground potential), a short circuit or a low-resistivity current path will be established, with a detrimental effect on the device as a whole: in most cases, these problems can be detected during the electrical tests of the device carried out during manufacturing, which in turn leads to the rejection of the device and thus to a reduction in the yield of the manufacturing process.

[0057] In summary, the presence of a large continuous air pocket layer in a prior art thermal inkjet printhead, regardless of the electrical state of the latter, requires several critical aspects. On the other hand, it is necessary to prevent the film in the ejection area from being damaged by the rupture of the vapor bubble during the operation of the printhead.

[0058] On the contrary, for the solution of the present application that adopts the novel layout of the air pocket layer as described above, the presence of the air pocket layer is maintained only in a small area that surrounds only the heater resistors in the array of resistors, and the film surface area of the air pocket layer is significantly reduced. Due to the reduced film surface area, it is less likely for the air pocket layer to overlap with possible defects in the dielectric film underneath, i.e. the likelihood of defects in the dielectric film being located right underneath the air pocket layer and causing some electrical short is reduced. On the other hand, using the novel layout helps to increase the distance between the air pocket layer and the underlying logic circuit. The smaller area of the air pocket layer and the larger distance between the air pocket layer and the critical logic circuit help to reduce the parasitic capacitance. Therefore, the thermal inkjet printhead of the present application is more robust and less susceptible to unwanted electrical interference.

[0059] The various technical features described above can be combined in any combination. Although not all possible combinations are described, any combination of the technical features should be considered covered by the present specification, as long as such a combination does not result in a contradiction.

[0060] While the present application has been described in connection with the examples, it will be understood by those skilled in the art that the description and drawings merely illustrate the application and should not be considered restrictive. Various modifications and changes can be made thereto without departing from the spirit and scope of the application.

Claims

1. A thermal inkjet printhead, comprising: base(1); A nozzle layer (18) comprising a plurality of nozzles (19) formed through the nozzle layer (18); Multiple inkjet chambers (16) corresponding to the multiple nozzles (19); Multiple heater resistors (2) are formed on the substrate (1) and correspond to the multiple inkjet chambers (16), each heater resistor (2) being located in a different inkjet chamber in the inkjet chamber (16), such that heating of one of the heater resistors (2) located in the respective inkjet chamber (16) causes ink droplets to be ejected through each nozzle (19). Multiple separate cavitation islands (33) are formed on and correspond to the multiple heater resistors (2), each of the cavitation islands (33) covering a different heater resistor in the heater resistors (2); as well as A dielectric layer (23) is inserted between the heater resistor (2) and the cavitation island (33), wherein the dielectric layer (23) is a composite film made of silicon nitride and silicon carbide and having a thickness in the range of 0.4 μm to 0.65 μm, wherein the heater resistor (2) is a U-shaped heater resistor.

2. The thermal inkjet printhead according to claim 1, wherein, Each of the cavitation islands (33) is made of a refractory metal film.

3. The thermal inkjet printhead according to claim 2, wherein, The refractory metal film is a tantalum film.

4. The thermal inkjet printhead according to claim 1, wherein, Each of the cavitation islands (33) has a minimized surface area, while the surface area is large enough to completely cover the corresponding heater resistor in the heater resistor (2).

5. The thermal inkjet printhead according to claim 1, further comprising: A barrier layer (17) is formed above the plurality of said cavitation islands (33) and below said nozzle layer (18). The inkjet chamber (16) is defined by the barrier layer (17).

6. The thermal inkjet printhead according to claim 5, wherein, The barrier layer (17) is patterned to form a plurality of ink channels (15) corresponding to the plurality of inkjet chambers (16), each of the ink channels (15) leading to a different inkjet chamber in the inkjet chamber (16).

7. The thermal inkjet printhead according to claim 1, further comprising: An insulating layer is inserted between the substrate (1) and the heater resistor (2).

8. The thermal inkjet printhead according to claim 1, wherein, Each of the air cavitation islands (33) is suspended.

9. A printing assembly comprising a thermal inkjet printhead according to any one of claims 1 to 8.

10. A printing apparatus comprising a thermal inkjet printhead according to any one of claims 1 to 8.

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