JPH0343254A - Printing head of thermal ink jet - Google Patents

Abstract

PURPOSE: To keep the operation temp. of a printhead constant during printing operation by applying energy pulses of a degree not generating steam so as not to eject ink liquid droplets to a heating member to apply complementary heat value to the printhead. CONSTITUTION: A thermal ink jet printhead 10 of a system having an ink supply manifold 24 and a plurality of parallel ink channels equipped with nozzles 27 and heating members 34 is held to constant operation temp. to be improved. One end openings of elongated V-shaped groove recessed parts 20 form the nozzles 27 and the other ends of the V-shaped groove recessed parts 20 are held to a closed state by end parts 21. The end parts 21 of the channels 20 forming each of sets by the alignment and bonding of wafers are directly positioned on the elongated recessed parts in the insulating layer 18 of a thick film to permit the flow of ink into the channels from the manifold 24 as shown by an arrow 23. The temp. of the improved printing head is kept substantially constant even in such a case that liquid droplets are not allowed to emit at the moving limit position of a carriage.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to thermal inkjet printing devices, and more particularly to an improved thermal inkjet printing system for inkjet printing at a constant operating temperature so that the droplet size or pixel size does not vary with temperature. Regarding print heads.

PRIOR ART AND PROBLEM SOLVED BY THE INVENTION Thermal inkjet printing generally uses thermal energy to generate vapor bubbles within an ink-filled channel through which the droplets exit. -on-demand) format inkjet printing. A thermal energy generator or heating element, usually a resistor, is placed in the channel near the nozzle at a predetermined distance from the nozzle. The resistors are adapted to individually apply electrical pulses that instantaneously evaporate the ink to form a bubble that causes the ink droplet to flow out.

As the bubble grows, the ink bulges out of the nozzle and is held in a meniscus or convex shape by surface tension. When such a bubble collapses, the ink located in the channel between the nozzle and the bubble starting point starts moving toward the collapsed bubble, causing volumetric contraction of the ink in the nozzle. The swollen part of the ink is separated into droplets. The acceleration of the ink exiting the nozzle during bubble growth imparts substantially linear momentum and velocity to the droplet toward a recording medium, such as paper.

Thermal inkjet devices therefore operate by pulsing a heated portion in contact with the ink, causing air bubbles to coalesce and eject ink droplets onto the paper. Print tests have found that print quality deteriorates as the device heats up. In particular, if the device is heated to too high a temperature (e.g., during long periods of high-density printing), the supply may be compromised and one or more ink channels in the printhead may become It shows a tendency to stop the droplet outflow. Although not a fatal drawback, one of the things that reduces print quality is that the print spot or pixel size increases as a function of device temperature. Through the study of this phenomenon, it was found that both the mass and velocity of the droplet are increased by the device temperature, and that both the mass and velocity are responsible for increasing the vixel dimensions on the paper. It was done. For carriage-based inkjet printers with sufficiently large printing densities, the spot size increases as the carriage moves across the paper. It then stops at the end of its travel and cools down slightly as it starts moving in the opposite direction, so that in the next line, i.e. the return stroke, the print traces will have more pixels in the opposite direction. The dimensions become thicker. This results in the formation of light and dark bands, which are most accentuated at the paper edge.

Similarly, other densely printed patterns will be degraded by the increase in pixel size with device temperature.

Many prior art devices include heat sinks with sufficiently large thermal capacities and sufficiently low thermal resistances. The temperature of such devices does not rise excessively. For one example of a thermal inkjet printhead with a heat sink, see U.S. Pat. No. 4,831,390 to Buschbamb et al. This method eliminated the fatal print defect mode. However, in order to ensure that the thermal resistance to the heat sink is low enough that there is no appreciable increase in device temperature over time for carriage movement across the paper in one direction, it is necessary to take a packaged approach. This approach either increases cost or otherwise forces undesirable printer designs. This temperature increase must be maintained to such an extent that negligible quality degradation occurs due to thermally induced spot size non-uniformity.

U.S. Pat. No. 4.326,2, issued to Raschke.
No. 06 discloses a method for minimizing interaction (ie, crosstalk) between piezoelectric elements driven by pulse emitting devices in an inkjet array. The drive pulse for the device is optimized by selecting a preferred pulse width.

U.S. Patent No. 4.712 to Marno et al.
, 930 discloses a progressive thermal printhead and a progressive thermal transfer printing apparatus that uses energy control means to vary the voltage or pulse width of a signal pulse voltage applied to the thermal printhead. . The printing apparatus further includes a power supply for the progressive thermal printhead and energy control means for controlling the width of voltage pulses applied to the thermal printhead in response to the recording signal.

U.S. Pat. No. 4,499,479 to Chi-Hsuen Li et al. discloses an inkjet printing device that includes a piezoelectric element having a plurality of separately actuatable portions. The strength of the drive signal is varied to control the drop volume. Further finer improvements have been made by varying not only the strength of the drive signal, but also the pulse width of the drive signal.

It is an object of the present invention to provide an improved inkjet printhead that is capable of maintaining itself at a substantially constant operating temperature during printing operations.

It is another object of the present invention to provide a supplemental amount of heat to the printhead by providing at least a number of heating elements with energy pulses of a magnitude that does not cause the generation of vapor so as not to expel ink droplets. temperature, thereby maintaining a constant operating temperature of the printhead during printing operations.

SUMMARY OF THE INVENTION In accordance with the present invention, a thermal ink jet printhead of the type having an ink supply manifold and a plurality of parallel ink channels each having a nozzle and a heating member is provided. This is achieved by maintaining the operating temperature at a substantially constant operating temperature. In print mode, the printhead temporarily energizes the heating element by selectively energizing the heating element with a pulse of electrical energy powerful enough to instantaneously evaporate ink that contacts the energized heating element. The ink droplets are ejected on demand by allowing the shaped vapor bubbles to eject the ink droplets. This improvement involves energizing the predetermined heating element from time to time with energy pulses of insufficient intensity to vaporize the ink, while the heating element is not energized for ink droplet ejection. Supplemental heating is provided to maintain the printhead at a substantially constant operating temperature. Alternatively, this supplemental heat may be provided by energizing one or more additional heating devices in the printhead. These heating devices are provided to provide heat only and are not used to evaporate the ink to a droplet ejection condition.

A more complete understanding of the invention may be obtained by considering the following description in conjunction with the accompanying drawings. Identical parts are designated by the same reference numerals in the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS An enlarged schematic perspective view of the front side of the improved inkjet printhead 10 of the present invention is shown in FIG. 1 showing the array of droplet exit nozzles 27. Referring also to FIG. 2, the lower electrically insulating substrate, heater plate 28, described below, includes a multilayer thermal element 36. This thermal element 36 comprises a heating member 34 and address electrodes 33 patterned on the surface 3o. On the other hand, the upper base, that is, the channel plate 31
are formed with parallel grooves or channels 20 that extend in one direction and pass through the front edge 29 of the upper substrate. The other ends of these grooves are inclined wall surfaces 21
It terminates at. The inner recess 24 is used as an ink supply manifold for the capillary filled ink channel 20 and has an open bottom 25 for use as a sump hole. The grooved surface of the channel plate is aligned and coupled to the heater plate 28. This causes each one of the heating elements 34 to be located within a channel defined by the groove and the lower substrate or heater plate. The ink enters the manifold formed by the recess 24 and the lower substrate 28 through the filling opening 25 and flows by capillary action through the elongated recess 38 formed in the thick film insulation layer 18, filling the channels 2o. be.

At each nozzle location, the ink forms a convex surface or meniscus whose surface tension cooperates with the slight negative pressure in the ink supply to prevent ink from flowing therefrom. Address electrodes 33 on the lower substrate or channel plate 28 terminate at terminals 32. The upper substrate or channel plate 31 is smaller than the lower substrate and exposes the electrode terminals 32 to connect to the daughter board.
The coupling wire 15 can be attached to the electrode 14 on the electrode 19.

The print head 10 is permanently fixed onto this slave board 19. Layer 18 is a thick film passivated layer, as described below, that is sandwiched between the upper and lower substrates. This layer is etched to expose the heating element, thereby locating the heating element within the pit 26, and is etched to form an elongated recess 38 to allow ink to enter the manifold 24. The ink can flow between the ink channel 20 and the ink channel 20. Additionally, the thick film insulation layer is etched to expose the electrode terminals.

The cross-section of FIG. 1 is shown in FIG. 2 as a view of line 2-2 through one channel, with ink passing from manifold 24 around end 21 of groove 20 as indicated by arrow 23. It shows a state of flow. As disclosed in U.S. Pat. A pattern is formed on the polished surface. Print head electrodes 3 forming multiple sets
3. The polishing surface of the wafer is coated with a subbing layer 39, such as silicon dioxide, to a thickness of about 2 pm before patterning the resistive material 34, which acts as a heating element, and the common return circuit or return 35. This resistive material is deposited by chemical vapor deposition (
Doped polycrystalline silicon or other zirconium boride (Zr13z) deposited by CVD
It can be used as a well-known resistance material such as .

The common return and address electrodes are typically aluminum leads and are deposited over the edges of the heating element over the base coat.

The common return ends or terminals 37 and address electrodes 32 are placed in predetermined positions and after the channel plate 31 is installed to form the print head.
A gap is formed for coupling the child board 19 to the electrode 14 by a coupling line. This common return is 35 and the address electrode 33 is 0.5 to 3p.
The preferred thickness is 1.5 m.
It is pm.

In the preferred embodiment, the lower substrate or heater plate 28 is silicon and thermal oxide.
A subbing layer 39 is provided, which may be an insulating layer such as silicon dioxide or other suitable silicon dioxide. A polysilicon heating element 34 is formed and another overcoat layer 13 is deposited over the basecoat layer and the heating element thereon. This overcoat layer 13 can be either silicon dioxide, thermal oxide, or reflowed polysilicon glass (PSG). A layer of thermal oxide is typically grown to a thickness of 0.5 to lpm to protect and insulate the heating element from the conductive ink.

Reflowed polysilicon glass (PSG)
) is typically approximately 2 pm thick. The overcoat layer is masked and etched to form a pathway near the edge of the heating element, thereby forming an electrical interface between the aluminum (Aa) address electrode 33 and the aluminum (Ae) common return electrode 35. Furthermore, the overcoat layer 13 in the bubble generation region of the heating element 34 is removed at the same time.If other resistive materials such as hafnium boride or zirconium boride are used as the heating element, For example, other suitable well-known insulating materials can be used.

The next step in fabricating the thermal element is to deposit a pyrolytic silicon nitride layer 17 directly over the exposed polysilicon heating element. Following this, 1
A tantalum layer 12 with a thickness of 1 m is deposited to provide protection against the cracking stress of the thermally decomposing silicon nitride layer 17.

Pyrolytic silicon nitride serves two very useful functions. First of all, it has very good thermal conductivity and forms a thermally effective resistive structure when deposited directly in contact with a resistor. Second
In addition, it is one of the few materials that resist Ta etching.

The structure of the multilayer thermal element is 4% by weight chemical vapor deposition (C
VD) either polysilicon glass (PSG) or preferably plasma nitride lead passivated metal. Any of these materials can be selectively etched to remove bond pads and resistive areas.

For passivation of the electrodes, a 2 pm thick phosphorus-containing doped chemical vapor deposited (CVD) silicon dioxide film 16 is attached to the heating member plate, i.e. the wafer containing the heating member and address electrodes in sets. Silicon is deposited over the entire surface. This passivation film 1
6 forms an ion barrier. This ion barrier protects the exposed electrodes from the ink. Other ion barriers such as polyimide, plasma nitride,
can be used as well as the phosphorous-doped silicon dioxide described above. Alternatively, any combination can be used.

An effective ion barrier layer is achieved with a thickness between 1000 angstroms and 10 pm, preferably 1 pm thick. The passivation film or layer 16 is removed by etching the terminal ends of the common return and address electrodes in preparation for subsequent line bonding to the daughter board electrodes.

This etching treatment of the silicon dioxide film can be carried out either by a wet method or by a dry method. Alternatively, electrode passivation can be achieved with plasma deposited silicon nitride (si3:N, ).

Next, in the form of a thick film, for example R15ton (
R registered trademark), Vacrel (registered trademark), Probi
An insulating layer 18, such as mer 52 (registered trademark) or polyimide, has a thickness of 1011 m to 1100 p,
Formed over the passivation layer 16, preferably with a thickness in the range 25 pm to 5011 m. This insulating layer 18 is photolithographically processed and etched to form respective portions (forming recesses 26) over each heating element, elongated recesses 38 forming ink passageways from manifold 24 to ink channels 20; The portion of layer 18 above each electrode terminal 32, 37 is then allowed to be removed. The elongated recesses 38 are formed by removing those portions of the thick film layer 18.

In the thick film layer 18, the bits 26 are in the form of elongated recesses 38 that open the ink channels to the manifold and the walls exposing each of the bubble generating regions of the multilayered thermal elements 36. It is formed with a wall surface. The recess walls 42 inhibit lateral movement of each of the bubbles generated by the pulse-energized heating element located at the bottom of the recess 26, thereby promoting bubble growth in the vertical direction. It is. Therefore,
As disclosed in U.S. Pat. It is.

The passivated address electrode is exposed to ink along most of its length, and any pinholes formed in the normal electrode passivation layer 16 expose the electrode 33 to electrolytic conditions. It turns out.

This will ultimately lead to operational failure of the heating element addressed by that electrode.

Additional protection for the address electrodes is therefore provided by the thick film layer 18. This is because the electrodes are passivated by two superimposed layers: passivation layer 16 and thick film layer 18.

As disclosed in U.S. Reissue Pat. An upper base body 31 is created. Heating member plate 28 may also be formed from a single wafer or from a wafer-sized structure (not shown) containing multiple wafers. Relatively large square through-hole recesses and sets of equally spaced parallel square groove recesses are etched into one side of a wafer (not shown). These recesses ultimately become the ink manifold 24 and ink channels 20 of the printhead. The channel plate containing the wafer and the heating member plate are aligned and bonded together and then processed by a die to form a plurality of individual printheads. One side formed by the die cutting forms an end face 29, and the opening at one end of the elongated square groove recess 20 forms a nozzle 27. ■The other end of the groove recess 20 is the end 2
1, it remains closed. however,
By aligning and bonding the wafers as described above, the ends 21 of each set of channels 20 are positioned directly over the elongated recesses 38 in the thick film insulating layer 18, as shown in FIG. This allows ink to flow from manifold 24 into the channels as shown by arrow 23.

The temperature of the improved printhead remains substantially constant even when the carriage is at the extremes of travel and is not discharging droplets. U.S. Patent Specification Drawing 4, issued to Rezanka and also shown in FIG.
No. 571,599, a multicolor thermal inkjet printer 11 is shown that includes several disposable ink supply cartridges 22. Each ink supply cartridge is integrally attached to the printhead 10 of the present invention. This cartridge and printhead combination is removably attached to a movable cartridge 40. In print mode, the cartridge is marked e.g. by arrow 4.
As shown at 5, the recording medium is reciprocated back and forth on a guide rail 43 parallel to the recording medium. The recording medium, e.g. paper, is held stationary while the cartridge is moved in one direction, and the paper is moved stepwise in the direction of arrow 46 before the cartridge is moved in the opposite direction. It will be done. This step distance is equal to the height of the swath of data printed by printhead 10 while traversing the paper in one direction. Ink droplets are ejected or ejected from nozzles 27 on the front face 29 of the printhead along a track 47 onto the paper on demand. The front of the print head is zero from the paper.
．． 254 mm~2.54 mm (0.01~0.1
separated by a distance of inches). 0.508 mm
(0.02 inches). Paper stepping tolerances and printhead linear deflection tolerances are maintained within acceptable limits to allow continuous swath widths of information to be printed without gaps or overlaps.

Each carriage 40 is equipped with a different color of ink, ie, one black cartridge and selected additional cartridges of one to three different colors.

The cartridge and printhead combination is removed and discarded after the ink supply to the cartridge is lost. In this embodiment, some nozzles do not eject any drops during one complete lateral movement of the cartridge, and some nozzles do not eject any drops at all when the printhead moves past the paper edge. They don't do it. At this end of the lateral movement of the carriage, a short pause is allowed during which the paper is moved stepwise in the direction of arrow 46 by the height of the swath width. Therefore, as discussed above, prior art printheads are cooled. However, the printhead of the present invention requires constant operation by applying electrical or energy pulses of insufficient strength to evaporate the ink to the heating member so as not to cause droplets to escape. temperature is maintained. This supplemental heat maintains a constant operating temperature of the printhead. Number of unused heating elements, pulse width, and! Alternatively, the output of the supplemental pulses controls the operating temperature of the printhead in print mode.

As a result of using supplemental heat to maintain a constant printhead temperature during printing, the average device temperature is higher than it would otherwise be. However, it is advantageous if this temperature is kept below a predetermined maximum temperature at which print defects occur. This maximum temperature is approximately 70°C for inks using ethylene glycol and water as a base.
It is. However, it varies due to differences in ink composition and ink channel geometry. Below 70°C, the droplet velocity becomes more constant at higher temperatures.

At 20°C, marginally acceptable drop velocities were observed in some channels of printheads with water-based ink compositions. A rapid increase in printhead temperature increases drop velocity over a very wide range. The ideal operating point depends on the temperature of the ink and device, and in this case appears to be roughly 30'C to 50'C. Yet another advantage of operating at elevated temperatures is that ink viscosity is reduced, reducing channel refill time and allowing for increased print cycles.

FIG. 4 shows one embodiment of the invention. In this, for example, a 48 jet or channel printhead is used, with up to two channels being able to print at the same time. In this example of an energy compensated pulse scheme, the 48 channels are pulsed two at a time and channels (1, 2, 3, 24° 25, 26 and 48) are printed. It is assumed that this is done. A short pulse is applied during the compensation cycle so that the total energy dissipated for the 48 pixels within this time interval remains constant. Since the carriage moves continuously, it is necessary to finish printing all 48 jets from one pixel to the next within a short period of time. Otherwise, the pattern of dots or pixels will become excessively jagged. Operating at 2 kHz gives 500 microseconds to get from one pixel location to the next, while at 3 kHz it takes 3.
33 microseconds is obtained. By comparison, a print cycle consists of 24 time intervals of 5 microseconds (120 microseconds total) during which up to two channels are activated at once using pulses of approximately 3 microsecond duration. In other words, it is energized. The energy dissipated during the print cycle is EP=nPtP by setting up to 48 pixels. where n is the number of activated channels (0-48), P is the power per print pulse, and t is the pulse width (3 microseconds in this example). The maximum energy dissipated is E, x = NPt
It becomes P. Here, in this example, N=48. For a strictly constant energy force, m short pulses are typically applied during the F rest period (none large enough to generate bubbles), and Ep+EC=nPtP+m
Pto=Emax=NPt. Here, EC is the compensation energy and to is the pulse width or duration of the compensation pulse. For example, jc=tP/4(0,75
microseconds), then n + m/4 = 48
Therefore, 192 short pulses are required during the time (n=0) when no printing is being performed. 2
For kHz operation, the energy compensation cycle is 350 microseconds (120 microsecond print cycle and 30
(setting times in microseconds are allowed). By pulsing up to two heating elements at a time during the energy compensation cycle, as is done during the printing cycle, 96 pulse intervals are produced, with short pulses of 75 microns. It is turned on for 2.9 microseconds and turned off for 2.9 microseconds. Other cases of interest are shown in Table 1. Assuming a 120 microsecond print cycle and a 30 microsecond setup time. The selection criteria is that no bubbles are generated in between, but that the drive transistor is sufficiently fast.

Various methods and techniques have been implemented in implementing the logic for energy compensation pulses. One method is to count the number of pulses during the print cycle and decrement the counter on compensation cycles. This method does not maintain a track in which the heating element is actuated and is a simple cycle in which the heating element is not used for drop ejection until sufficient compensation pulses are energized. In a preferred embodiment, the method performs an rNOTJ operation on the print data;
This is used as data for the compensation cycle so that only the unprinted heating elements are used for energy compensation. According to this, the life of the heating member can be further predicted.

The invention is not limited to the case where Ep + EC = "max. For one, Ec is EfflaX
It will probably be slightly smaller than Ep. This is because no energy is carried away by ejecting droplets during the compensation cycle. Furthermore, it is not necessary to hold the printhead temperature precisely constant. It has been found that an upper limit of compensation even less than Emax is sufficient. The advantage of using a small energy compensation is that it is easier to maintain thermal equilibrium such that the upper operating temperature is not approached for long periods of time.

Energy compensation is required whenever printing is occurring or is about to occur. In particular, energy compensation must continue to a maximum extent while the cartridge is stopped at the end of its travel. It must also be done just before the start of printing. The warm-up time should not be uncomfortably long; it should be 1 to 5 seconds for a one-page-per-minute printer, with the temperature of most parts rising within three seconds. should be satisfied. The heat sink must be designed so that the device temperature rises in most of its parts within a few seconds and then slowly. Energy compensation must also be applied to obtain long-term heating effects, for example by counter-decrementing the exact number of pulses for each printed line.

Energy compensation can also be controlled by changing the pulse width, which depends on the number of channels energized during the print cycle. In one embodiment, short compensation pulses are applied only during compensation cycles. In other embodiments, short compensation pulses may be activated during the print cycle as well, with the pulse width broadened for channels where printing is desired. The minimum pulse width increment is determined by the fastest clock on the device and is typically 10-20 kHz. Another way to control energy compensation is to change the power of the pulses, but this is more difficult to implement.

Compensation energy is printed work &! Although it has been assumed that this is provided by the same heating element involved, this is not a necessary condition. One or more special heating elements (not shown), which are solely responsible for providing a compensating amount of heat, can be formed anywhere on the heating element plate 28. Preferably, they are placed in a position where they do not come into contact with the ink.

The advantages of the intended scheme of compensation pulses of the present invention are as follows. That is, 1. It can be carried out without requiring a temperature sensor or a special heating member.

2. Thermal induced spot size changes and widths are made negligible.

3. Thermal packaging becomes less important to provide a low thermal resistance path from the device to the heat sink.

4. Since the spot size increases with differential temperature similar to the print pulse condition, the peak power for bubble generation can be reduced.

5. Operation at elevated temperature is improved to make the droplet temperature uniform, improving production of good performance.

6. Operation at elevated temperatures is expected to reduce ink viscosity inside the device, improving channel refill time.

Another feature that may prove useful is a temperature sensor with absolute temperature loading. The energy compensation plan can then be modified, for example using a chart, to provide the required device temperature regardless of the ambient temperature or the length of time that printing is taking place. Such a temperature sensor (not shown) can be physically located inside the inkjet printer or on the printhead itself.

Many modifications and variations will become apparent from the foregoing description of the invention and such modifications and variations are intended to be included within the scope of the invention.

[Brief explanation of drawings]

FIG. 1 is a schematic partial perspective view of a printhead embodying the invention. FIG. 2 is a cross-sectional view taken along line 2--2 of FIG. FIG. 3 is a schematic perspective view of a carriage-type multicolor thermal inkjet printer having the printhead of FIG. 1 integrally attached to a disposable ink cartridge. FIG. 4 is a diagram illustrating a sample plot of an exemplary energy compensated pulse technique for adding the required heat to a printhead. 10...Print head, 11...Printer 16...
・Passivation film layer, 18... Film insulation layer, 1... Child board, 20... Channel or groove, 2
4... Concavity, ie, manifold, 27... Nozzle, 2
8... Heating device plate, 31... Channel plate, 32. 37... Terminal, 33... Address electrode, 34... Heating member, 35... Return electrode, 36... Thermal element, 38... Concavity, 40... Cartridge, 44... Recording medium, 47... Orbit.

Claims (1)

[Claims] (1) an ink supply manifold, a plurality of parallel ink channels filled with capillary tubes communicating with the manifold at one end and terminating in a nozzle at the other end, one ink channel disposed within each ink channel; a linear array of heating members, and selectively energizing the heating members with pulses of electrical energy powerful enough to instantaneously evaporate ink that contacts the energized heating members. an improved thermal inkjet printhead of the type that ejects ink droplets on demand by causing vapor bubbles to form and eject the ink droplets; While the heating element is not energized, the predetermined heating element is occasionally energized with energy pulses of insufficient intensity to evaporate the ink to provide supplemental heating to the printhead. A thermal inkjet printhead characterized in that the printhead is maintained at a substantially constant operating temperature while in a print mode.