KR20090067164A - Mems bubble generator for large stable vapor bubbles - Google Patents

Abstract

A MEMS vapour bubble generator that uses a heater in thermal contact with a liquid to generate a bubble. The heater is energized by an electrical pulse that is shaped to have a relatively low power, sub-nucleating portion and a high power portion that nucleates the bubble. The thermal energy transferred to the liquid by the sub-nucleating portion speeds up the nucleation of the bubble across the surface of the heater during the nucleating portion. This produces larger, more stable bubble having a regular shape.

Description

MEMS BUBBLE GENERATOR FOR LARGE STABLE VAPOR BUBBLES

FIELD OF THE INVENTION The present invention relates to MEMS devices, and more particularly to MEMS devices for evaporating liquid to produce vapor bubbles during operation.

Cross Reference to Related Applications

Various methods, systems, and apparatus relating to the present invention are disclosed in the following US patent / patent applications filed by the applicant or assignee of the present invention.

Each application is listed under its respective document number. This document number will be replaced when the application number is known. The disclosures of these applications and patents are incorporated herein by reference.

Some micromechanical systems (MEMS) devices process or use liquids to operate. As a class of devices containing such liquids, resistive heaters are used to heat the liquid to the superheat limit of the liquid and cause the formation of rapidly expanding vapor bubbles. Impact due to bubble expansion can be used as a mechanism to allow liquid to move through the device. This is the case for thermal inkjet printheads with each nozzle having a heater that generates bubbles for ejecting ink droplets onto the print media. In light of the widespread use of inkjet printers, the present invention will be described in this application with particular reference to its use. However, the present invention is not limited to inkjet printheads, but it will be appreciated that the vapor bubbles formed by the resistive heater are equally suitable for other devices used to move liquid through the device. 'Lab-on-a-chip' device).

The time scale for heating the liquid to the superheat limit determines how much thermal energy is stored in the liquid when the liquid reaches the superheat limit: this is how much steam is produced and how Determine the impact (shocks defined as integrated pressure over time and time). Heating at a longer time rate results in a larger amount of heated liquid, and the more stored energy, the greater the amount of vapor and large bubble shock. This allows some control of the bubbles produced by the MEMS heater. Control of the time rate for heating up to the overheat limit is simply a matter of controlling the power supplied to the heater during the nucleation process. Low power results in longer nucleation time and greater bubble impact at the expense of increased energy requirements (extra energy stored in the liquid must be supplied by the heater). Regulating power is through a reduced voltage across the heater, or through pulse width modulation of the voltage to obtain a lower time averaged power.

While this effect is useful, for example, in adjusting the flow rate of a MEMS bubble pump or the force supplied to a clogged nozzle in an inkjet printer (temporarily pending patent application, referred to as document number PUA011US at the same time), the designer of such a system Care must be taken to ensure bubble stability. If the time ratio for heating is much longer than 1 microsecond, a typical heater that heats a water-based liquid will create unstable, non-repeatable bubbles (Fig. 1). This non-repeatability may damage the operation of the device or severely limit the range of bubble shocks available to the designer.

Accordingly, the present invention is directed to a chamber for holding a liquid;

A heater located in the chamber for thermal contact with the liquid; And,

A MEMS vapor bubble generator comprising a drive circuit for providing an electrical pulse to the heater such that the heater generates vapor bubbles in the liquid,

The pulse has a first portion having insufficient power to nucleate the vapor bubble and a second portion having sufficient power to nucleate the vapor bubble following the first portion.

If the heating pulse is formed to increase the heating rate before the end of the pulse, the bubble stability can be significantly improved and a situation can be approached where large, repeatable bubbles can be produced by small heaters.

Preferably the first part of the pulse is not a nucleation of the vapor bubble, but is a pre-heating zone for heating the liquid and the second part is for nucleation of the vapor bubble. Trigger area. In a more preferred form, the preheat zone has a longer duration than the trigger zone. Preferably the preheating zone is at least 2 microseconds. In a more preferred form, the trigger zone is shorter than 1 microsecond.

Preferably, the drive circuit forms pulses using pulse width modulation. In this embodiment, the pre-heat section is a series of sub-nucleating pulses. Optionally, the drive circuit uses voltage modulation to form a pulse.

In some embodiments, the time averaged power in the preheat zone is constant, and the power of the average time in the trigger zone is constant. In a particularly preferred embodiment, the MEMS vapor bubble generator is used in an inkjet printhead to inject printing fluid from a nozzle in fluid communication with the chamber.

Low power is used for a long time ratio (generally >> 1 kW) to store a large amount of thermal energy in the liquid surrounding the heater without exceeding the nucleation temperature, followed by a short time ratio (generally < Switch to high power to exceed the nucleation temperature in 1 ms) to initiate nucleation and release of the stored energy.

Optionally, the first portion of the pulse is not a nucleation of the vapor bubble, but is a preheating zone for heating the liquid, and the second portion is for overheating a portion of the liquid to nucleate the vapor bubble. Trigger section for

Optionally, the preheat zone has a longer duration than the trigger zone.

Optionally, the preheating zone is at least 2 micro seconds.

Optionally, the trigger zone is shorter than 1 microsecond.

Optionally, the drive circuit forms pulses using pulse width modulation.

Optionally, the preheat zone is a series of nucleating pulses.

Optionally, the drive circuit uses voltage modulation to form a pulse.

The power of the average time in the preheat zone is constant, and the power of the average time in the trigger zone is constant.

In another aspect, the present invention provides a MEMS vapor bubble generator for use in an inkjet printhead to eject printing fluid from a nozzle in fluid communication with the chamber.

Optionally, the heater is suspended in the chamber to be immersed in printing fluid.

Optionally, the pulse is generated to recover a nozzle clogged by a dry or excessively viscous printing fluid.

Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

1A-1E illustrate water vapor bubbles produced at different heating rates.

2A and 2B illustrate two alternatives for forming the pulse into a preheating region and a trigger region.

3 shows the hottest place in the heater and the colder place in the heater for two different pulse shapes.

4A shows water vapor bubbles generated using traditional square-shaped pulses.

4B illustrates bubbles generated using pulses formed by pulse width modulation.

4C and 4D show bubbles generated using voltage modulated pulses.

5 shows a MEMS bubble generator in use in an inkjet printhead.

In MEMS fluid pumps, large, stable and repeatable bubbles are preferred for efficient and reliable operation. In order to analyze the mechanisms affecting bubble nucleation and growth, it is necessary to take into account the spatial uniformity of the temperature profile of the heater and then to consider the time evolution of the profile.

A finite element thermal model of the heater in the liquid showed that the heating rate of the heater strongly influences the spatial uniformity of temperature throughout the heater.

This is because different parts of the heater are heat-sink to a different degree (the sides of the heater will be colder due to the improved cooling by the liquid and the ends of the heaters due to the improved cooling by contact) Will be cooler). At low power, the time rate for heating up to the overheat limit is large relative to the thermal time rate of the cooling mechanism, so the temperature profile of the heater will be severely distorted by cooling at the boundary of the heater. Ideally, the temperature profile is "top-hat," which is a uniform temperature throughout the heater, but in the case of low heating rates, the edge of the temperature profile will be pulled down. .

The top-hat temperature profile is ideal for maximizing the efficiency of the heater, as only these parts of the heater above the overheat limit significantly contribute to the bubble impact. The nucleation rate is an exponential function of very strong temperature near the overheat limit. Portions of the heater just below a few degrees of the overheat limit will exhibit a much lower nucleation rate than these parts above the overheat limit.

By being thermally insulated by bubbles expanding from hotter parts of the heater, those parts of the heater will contribute much less to the bubble impact. In other words, if the temperature profile across the heater is not uniform, there may be a race condition between bubble nucleation in the colder portion of the heater and bubbles expanding from the hotter portion of the heater. This race condition can lead to nonrepeatability of bubbles formed at low heating rates.

The term "low heating rate" is a relative term depending on the geometry of the heater and its contact and the thermal properties of all materials in thermal contact with the heater. All of these will affect the time rate of the cooling mechanism. If the time ratio for nucleation exceeds 1 ms, a general heater material of general construction applicable to ink jet printers will begin to exhibit the race condition. If the heating rate is low enough, the exact limit is not critical since any heater is subject to competition and consequent bubble instability. This will limit the range of bubble shocks available to the designer.

1A to 1E show a stroboscopic picture of steam bubbles 12 generated at different heating rates by varying the voltage of the drive pulse. Using strobe for 0.3 microseconds, the image captures and shows when the bubble is most expanded. The heater 10 is 30 μm × 4 μm in open pool of water at an angle of 15 degrees from the surface of the support wafer surface. The double bubble shape is due to the image of the bubble projected onto the surface of the wafer.

In FIG. 1A, the drive voltage is 5 volts and the bubble 12 expands maximally in 1 microsecond. The bubbles are relatively small, but have a regular shape along the length of the heater. In Figure 1B, the drive voltage is reduced to 4.1 volts and the time to maximum bubble growth is increased to 2 microseconds. As a result, the bubble 12 is larger but bubble irregularity 14 begins to occur. In Figures 1C, 1D and 1E, the pulse voltage gradually decreases (3.75V, 3.45V and 2.95V, respectively). As the voltage decreases, the heating rate also decreases, thereby increasing the time rate for reaching the liquid superheat limit. This allows more time for heat to leak into the liquid, resulting in the production of greater amounts of stored thermal energy and more vapor when bubble aggregation occurs. In other words, the size of the bubble 12 is increased. Therefore, lower voltages cause the bubbles to grow larger, resulting in larger bubble shocks. Unfortunately, the bubble imbalance 12 also increases. Therefore, the bubble is potentially unstable and non-repetitive when the time rate of heating up to the overheat limit exceeds 1 microsecond. In Figures 1A-1E, the times at which bubble sizes are maximized are 1, 2, 3, 5 and 10 microseconds, respectively.

The present invention provides a method of avoiding the instability created by the race condition to allow designers to use low heating rates to create large bubble shocks in the heater at fixed geometry and thermal properties. 2A and 2B show two possibilities for the adjustment of the heater to produce large and stable bubbles. In FIG. 2A, the drive circuit uses amplitude modulation to reduce the power of the preheat zone 16 relative to the trigger zone 18. In FIG. 2B, pulse width modulation of the voltage (creating a series of fast auxiliary emission pulses) can be used to reduce the power of the preheat phase 16 compared to the trigger zone 18.

Those skilled in the art will appreciate that the pulse shapes that meet the standards of the relatively low powered preheating zone and the subsequent trigger zone of nucleating the bubble are infinitely varied.

Forming the pulse may be made of pulse width modulation, voltage modulation or a combination of both. However, pulse width modulation is a more preferred method for shaping the pulse and is more suitable for CMOS circuit design. It should also be noted that the pulse is not limited to the preheat zone and the trigger zone. Additional pulse zones may also be included for other purposes without denying the benefit of the present invention. Moreover, the zones do not need to maintain a constant power level. A constant average time power is more preferred for the preheat zone and the trigger zone, in the simplest case of theoretical and experimental handling.

Bubble nucleation can be achieved in the competition by switching to a higher heating rate after the preheating step because the time delay between different zones of the heater leading to the overheat limit is reduced. 3 illustrates the concept. Even if the spatial temperature uniformity is weak (an unavoidable side effect of a low heating rate in the preheating step), the time delay 32 between the hotter and colder regions of the heater leading to the overheating limit is the preheating. Later by switching to a higher heating rate 36. In this way, the cooler zones reach this overheating limit before they are insulated by bubble expansion from the hotter zones. Most of the heater surface reaches the overheat limit 34 before significant bubble expansion occurs. Thus, the heater zone will be more useful and consistently used for bubble formation.

4A-4D illustrate the usefulness of shaped pulses when producing large, stable bubbles. The bubble size can be greatly increased by using a shaping pulse without leaving the imbalance shown in FIGS. 1A to 1E. Circuit designers can choose either voltage modulation or pulse width modulation of the heating signal to make the shaped pulses, but it is generally believed that pulse width modulation is more suitable for integration of, for example, CMOS driver circuits. By way of example, such a circuit can be used to generate a maintenance pulse in the inkjet printhead, where as part of the printer maintenance cycle, the nozzle with the increased bubble impact is better You can recover. This is discussed in the ongoing application (temporarily referred to as document PUA011US), the contents of which are hereby incorporated by reference.

5 shows the MEMS bubble generator of the present invention applied to an inkjet printhead. Some details of Applicant's thermal printhead IC configuration and operation are provided in USSN 11 / 097,308 and USSN 11 / 246,687. For simplicity, the contents of these documents are incorporated herein by reference.

The single nozzle device 30 is shown in FIG. 5. Such an array of nozzles is preferably formed on a wafer substrate 28 supported using lithographic etching and deposition techniques common in semiconductor / MEMS construction. The chamber 20 holds a large amount of ink. The heater 10 is suspended in the chamber 20 in electrical contact with the CMOS drive circuit 22. The drive pulses generated by the drive circuit 22 heat the heater 10 to produce vapor bubbles 12 that push the ink droplets 24 through the nozzles 26. Using the drive circuit 22 to form a pulse in accordance with the present invention gives the designer a wider range of bubble shocks from a single heater and drive voltage.

4A-4D show stroboscopic images of water vapor bubbles in an open pool on a 30 μm × 4 μm heater. As shown in FIGS. 1A-1E, the bubble 12 is trapped when it is most inflated. 4A shows the prior art situation of a simple square profile pulse of 4.2V for 0.7 microseconds. In FIG. 4B, at 4.2 V, the pulses are formed by a pre-heat series with 900 nanosecond pulses divided by 150 nanoseconds followed by pulse width modulation followed by a 300 nanosecond trigger pulse. do. The bubble size in FIG. 4B is larger because of the significant amount of thermal energy delivered to the liquid prior to nucleation in the trigger pulse. 4C and 4D, the pulse is voltage modulated. The pulse of FIG. 4C has a preheat portion of 2.4V for 8 microseconds, followed by 4V for 0.1 microseconds to initiate nucleation. In contrast, the pulse of FIG. 4D has a preheat zone of 2.25 volts for 16 microseconds followed by a trigger zone of 4.2 V for 0.15 microseconds. These figures clearly illustrate that the bubbles (FIGS. 4B, 4C and 4D) generated using orthopedic pulses are larger, more uniform in shape, and repetitive.

Once the problem of imbalance or non-repeatability is eliminated, the designer has great flexibility in controlling the size of the bubble during the design phase or operation by changing the length of the preheat zone of the pulse. Care must be taken to avoid exceeding the overheat limit during the preheating zone so that nucleation does not generate until the trigger zone is reached. If the pulse is pulse width modulated, the modulation should be fast enough to provide a reasonable, approximation of temperature rise produced by a constant, reduced voltage. Care should be taken to ensure that the whole heater exceeds the overheat limit with sufficient margin to account for system changes, without ov erdriving to the extent that the heater is damaged. This consideration can be met in routine thermal modeling or experimentation of the heater in an open pool of liquid.

The present invention has been described herein by way of example. Those skilled in the art will readily recognize many variations and modifications without departing from the spirit and scope of the broad inventive concept.

Claims (12)

A chamber for holding a liquid; A heater located in the chamber for thermal contact with the liquid; And, A MEMS vapor bubble generator comprising drive circuitry for providing electrical pulses to the heater to enable the heater to create vapor bubbles in the liquid, The pulse having a first portion having insufficient power to nucleate the vapor bubble and a second portion having sufficient power to nucleate the vapor bubble following the first portion; Bubble generator. The method of claim 1, The first part of the pulse is not a nucleation of the vapor bubble, but a pre-heating section for heating the liquid, and the second part is a portion of the liquid to nucleate the vapor bubble. MEMS vapor bubble generator, characterized in that the trigger section (trigger section) to overheat. The method of claim 2, And wherein said preheating zone has a longer duration than said trigger zone. The method of claim 3, And wherein the preheating zone is at least 2 micro seconds. The method of claim 3, And the trigger zone is less than 1 microsecond. The method of claim 1, Wherein said drive circuitry forms pulses using pulse width modulation. The method of claim 6, And wherein said preheating zone is a series of sub-nucleating pulses. The method of claim 1, And said drive circuit uses pulse modulation to form pulses. The method of claim 2, MEH vapor bubble generator, characterized in that the power of the time averaged in the preheat zone is constant, the power of the average time in the trigger zone is constant. The method of claim 1, And the MEMS vapor bubble generator is used in an inkjet printhead to eject printing fluid from a nozzle in fluid communication with the chamber. The method of claim 10,  And the heater is suspended in the chamber to be immersed in a printing fluid. The method of claim 10, And wherein the pulse is generated to recover a nozzle clogged by a dry or excessively viscous printing fluid.

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