TWI380910B - Mems bubble generator for large stable vapor bubbles - Google Patents

Description

Micromechanical system (MEMS) bubble generator for large stable bubbles

This invention relates to MEMS devices and, in particular, to MEMS devices that vaporize liquids to create bubbles during operation.

Some micromechanical systems (MEMS) devices process or use liquids to operate. In one classification of these liquid containing devices, a resistive heater is used to heat the liquid to the superheat limit of the liquid, resulting in the formation of rapidly expanding bubbles. The impulse provided by the expansion of the bubble can be used as a mechanism to push the liquid through the device. This is the case in thermal inkjet printheads, each having a heater that produces a bubble to eject an ink droplet onto the print medium. In view of the widespread use of ink jet printers, the invention will be described with particular reference to its use in this regard. However, it should be understood that the present invention is not limited to ink jet print heads and is equally applicable to the formation of bubbles by a heater that are used to move liquid through the device (eg, some 'on-wafer experiments) On other devices of the Lab-on-a-chip device.

The length of time required to heat a liquid to its superheat limit determines how much thermal energy is stored in the liquid when it reaches this superheat limit: this determines how much steam is produced and the impulse of the expanded bubble (impulse) It is defined as the sum of the pressures over the entire area over time. The longer the heating time, the larger the volume of the heated liquid, so the more energy is stored, the larger the amount of steam and the larger the bubble impulse. This results in some degree of tunability of the bubbles generated by the MEMS heater. Controls the length of time required to heat a liquid to its superheat limit, simply controlling the power supplied to the heater during nucleation: lower power will result in longer nucleation times and larger bubble impulses At the expense of a large energy demand (the extra energy stored in the liquid must be provided by the heater). Controlling power can be achieved by reducing the voltage across the heater or by pulse width modulation of the voltage to achieve a lower time averaged power.

Although this effect is useful in control, such as the flow of a MEMS bubble pump or the force applied to a blocked nozzle in an inkjet printer (which is a patent application in the co-genus of the temporary document number PUA011US) The invention of the case), but the designer of this system needs to worry about the stability of the bubble. A typical heater that heats a water-based liquid will produce unstable, non-repeatable bubbles if the length of time used for heating is longer than 1 microsecond (see Figure 1). This non-reproducibility will involve the operation of the device or severely limit the range of bubble impulses available to the designer.

Accordingly, the present invention provides a MEMS bubble generator comprising: a chamber for containing a liquid; a heater disposed in the chamber in thermal contact with the liquid; and a drive circuit for providing the heater Pulse causing the heater to generate a bubble in the liquid; wherein the pulse has a first portion having a power insufficient to nucleate the bubble and a second portion subsequent to the first portion having sufficient to cause the bubble to Nuclear power.

If the heating pulse is shaped to increase the heating rate before the end of the pulse, the stability of the bubble can be greatly enhanced to the extent that a small heater can be used to produce large and repeatable bubbles.

Preferably, the first portion of the pulse is a preheating section for heating the liquid but does not nucleate the bubbles and the second portion is a trigger section for nucleating the bubbles. In a preferred form, the preheating section lasts longer than the duration of the initiating section. Preferably, the preheating section is at least 2 microseconds long. In a more preferred form, the initiating segment is less than 1 microsecond.

Preferably, the drive circuit shapes the pulse using pulse width modulation. In this embodiment, the preheating section is a series of sub-nucleating pulses. Alternatively, the driver circuit uses voltage modulation to shape the pulse.

In some embodiments, the time averaged power in the preheating section is fixed and the time averaged power in the initiating section is fixed. In a particularly preferred embodiment, the MEMS bubble generator is used in an ink jet printhead to eject liquid from a nozzle in fluid communication with the chamber.

Using a low power for a long length of time (typically >> 1 microsecond) to store a large amount of thermal energy in the liquid surrounding the heater without crossing the nucleation temperature, then switching to a high Power is used to cross the nucleation temperature for a short length of time (typically <1 microsecond), which can initiate nucleation and release stored energy.

Selecting the upper portion, the first portion of the pulse is a preheating section for heating the liquid but does not nucleate the bubble and the second portion is an initiation section for heating a portion of the liquid to overheating The bubble is nucleated.

The upper zone is selected and the duration of the preheat zone is longer than the duration of the initiation zone.

The upper preheating zone is selected to be at least 2 microseconds in length.

Selecting the upper level, the initiating section is less than 1 microsecond.

Selecting the ground, the drive circuit uses pulse width modulation to shape the pulse.

The upper preheating zone is selected as a series of sub-nucleating pulses.

Alternatively, the driver circuit uses voltage modulation to shape the pulse.

Selecting the upper ground, the time averaged power in the preheating section is fixed and the time averaged power in the firing section is fixed.

Another aspect of the present invention provides a MEMS bubble generator for use in an ink jet printhead for ejecting liquid from a nozzle in fluid communication with the chamber.

The upper level is selected and the heater is suspended in the chamber for immersion in a printing fluid.

Selecting the ground, the pulse is generated to recover the nozzle that is clogged with the dried or overly viscous printing fluid.

In a MEMS fluid pump, large, stable, and repeatable bubbles are desirable for efficient and reliable operation. In order to analyze the mechanism affecting bubble nucleation and growth, it is necessary to consider the spatial consistency of the temperature profile of the heater and the time evaluation of the volume curve. The finite element thermal model of the heater in the liquid can be used to show that the heating rate of the heater strongly affects the spatial uniformity of the temperature of the entire heater. This is because different parts of the heater are dissipated to different degrees (the side of the heater will be cooler because the liquid will cool the cooling and the end of the heater will be cooler because of the contact lift cooling effect). At low power, when the length of time used to heat up to the superheat limit is longer relative to the thermal time length of the cooling mechanism, the temperature profile of the heater will be due to cooling at the boundary of the heater. Was severely distorted. Ideally, the temperature profile will be a "high hat top-hat" with consistent temperature across the heater, but in the case of a low heating rate, the edge of the temperature profile will Will be pulled down.

The temperature profile of the flat top is ideal for maximizing the effectiveness of the heater, and only those heater sections above the hot pole line have a significant effect on the bubble impulse. The nucleation rate is a very strong exponential function of the temperature close to the superheat limit. The nucleation rate drop produced by the heater a few degrees below the superheated pole line will be much lower than the nucleation rate produced by the portion above the superheated pole line. These portions of the heater contribute much less to the bubble impulse because they will be thermally isolated by the bubbles that are expanded by the hotter portion of the heater. In other words, if the temperature variation curve over the entire heater is not uniform, then there is a bubble nucleation between the cooler portion of the heater and the bubble expansion from the hotter portion of the heater. A situation of a competition. It is this race condition that causes the non-reproducibility of the bubbles formed at a low heating rate.

The term "low heating rate" is a relative term and relates to the shape of the heater and its contact with the thermal properties of all materials in thermal contact with the heater. All of this will affect the length of the cooling mechanism. A typical heater material in a configuration that can be applied to an inkjet printer will begin to reveal the race condition if the length of time for nucleation exceeds 1 microsecond. The exact threshold value is not important, because if the heating rate is low enough, any heater will encounter the race condition and subsequent bubble instability. This will limit the range of bubble impulses available to the designer.

1A to 1E are line graphs of stroboscopic observer photographs of bubbles 12 produced at different heating rates. A flash with a duration of 0.3 microseconds is used, which captures the image of the bubble to its maximum extent. The heater is 30 microns by 4 microns in size and is at an angle of 15 degrees to the surface of the support wafer in an open pool. The shape of the double lobes is due to the reflection of the bubble image from the surface of the wafer.

In Figure 1A, the drive voltage is 5 volts and the bubble 12 reaches its maximum extent at 1 microsecond. The bubble is relatively small but has a rectangular shape along the length of the heater. In Figure 1B, the drive voltage drop is 4.1 volts and the time required to reach maximum bubble growth is increased to 2 microseconds. Therefore, the bubble 12 is large but the irregularity 14 of the bubble starts to occur. The pulse voltages were gradually reduced in Figures 1C, 1D and 1E (3.75 volts, 3.45 volts and 1.95 volts, respectively). When the voltage is lowered, the heating rate is also lowered, so that the time to reach the superheat limit of the liquid becomes long. This allows for a longer period of time for heat to escape into the liquid, while there is a large amount of stored thermal energy and more steam is generated as the bubble nucleation occurs. In other words, the size of the bubble 12 becomes larger. Therefore, a low voltage can give a large bubble impulse, allowing the bubble to grow to a greater extent. Unfortunately, the irregularities of the bubbles will also increase. Therefore, when the length of time heated to the superheated electrode line exceeds 1 microsecond, the bubble may be unstable and non-repeatable. In Figs. 1A to 1E, the time to reach the maximum bubble size is 1, 2, 3, 5 and 10 microseconds, respectively.

The present invention provides a way to avoid instability due to race conditions, allowing the designer to use a low heating rate to create a large bubble impulse on a heater having a fixed shape and thermal characteristics. Figures 2A and 2B show two possibilities for driving a heater to produce large and stable bubbles. In FIG. 2A, the drive circuit uses amplitude modulation to reduce the power of the preheat section 16 relative to the firing section 18. In FIG. 2B, pulse width modulation of the voltage (generating a series of fast sub-discharge pulses) can be used to reduce the power of the preheat section 16 relative to the firing section 18.

Those skilled in the art will appreciate that there are numerous pulse shape variations that can satisfy a relatively low preheating section and a subsequent initiation section that nucleates the bubble. Forming the pulse can be accomplished with pulse width modulation, voltage modulation, or a combination of both. However, pulse width modulation is the preferred method of shaping the pulse because it is easier for CMOS circuit designers to accept. It should also be noted that the pulse is not limited to only one preheating and initiating section; additional pulsed sections may be included for other purposes without adversely affecting the advantages of the present invention. Also, all segments must maintain a constant power level. For the preheating section and the initiating section, a time-averaged power is preferably fixed because this is the simplest case for theoretical and experimental processing.

By switching to a higher heating rate after a warm-up phase, the race will win by bubble nucleation because the time delay between reaching different thermal limits between different heater zones is reduced. Figure 3 shows the concept that even if the spatial temperature uniformity is poor (the inevitable side effects of the low heating rate in the preheating phase), between the hotter and colder regions of the heater The time delay 32 up to the superheat limit can be reduced by switching to a higher heating rate 36 after warming up. In this way, the cooler regions can reach the superheat limit before these regions are thermally insulated by the bubble expansion from the hotter regions. Most heater surfaces reach the superheat limit 34 before severe bubble expansion occurs, so the heater area can be used more efficiently and consistently for bubble formation.

Figures 4A through 4D show the effectiveness of the shaped pulse on producing large and stable bubbles. The bubble size can be temporarily increased by using the shaped pulse and does not suffer from the irregularities shown in Figs. 1A to 1E. A circuit designer will have the option of using voltage modulation or pulse width modulation of the heating signal to produce a shaped pulse, but in general pulse width modulation is considered to be more suitable for integration with a CMOS driver circuit. For example, such a circuit can be used to generate a service pulse in an ink jet print head that is more capable of restoring the blocked nozzle for use as part of a printer maintenance cycle. This is discussed in the co-pending application of the present application (hereinafter referred to as the suffix number PUA001US), the contents of which are hereby incorporated by reference.

Figure 5 shows a MEMS bubble generator of the present invention used in an ink jet print head. A detailed description of the manufacturing and operation of the thermal printhead IC of the applicant is provided in U.S. Patent Application Serial No. USSN 11/097,038 and US Serial No. 11/246,687. The contents of these two documents are hereby incorporated by reference.

A single unit cell 30 is shown in Figure 5. It will be appreciated that a number of unit cells are fabricated on a support wafer substrate 28 in a well-stacked array using lithography etching and deposition techniques common in the semiconductor and/or MEMS fabrication arts. The chamber 20 contains a quantity of ink. The heater 10 is suspended within the chamber 20 such that it is in electrical contact with the CMOS drive circuit 22. The heater 10 is energized by a drive pulse generated by the drive circuit 22 to generate a bubble 12 that forces an ink droplet 24 through the nozzle 26. The use of drive circuit 22 to profile the pulse in accordance with the present invention provides the designer with a wider range of bubble impulses from a single heater and drive voltage.

Figures 4A through 4D show stroboscopic imagery of water vapor bubbles on a 30 micron by 4 micron heater in an open cell. Similar to Figures 1E to 1E, the bubbles 12 are captured at their maximum extent. Figure 4A shows the prior art 4.2 volt simple square profile pulse lasting 0.7 microseconds. In Figure 4B, the pulse is shaped by pulse width modulation - a preheating series with nine 100 nanosecond pulses separated by 150 nanoseconds, followed by a 300 nanosecond firing pulse, all of which are pulsed. Both are 4.2 volts. The size of the bubble in Figure 4B is greater because of the greater amount of thermal energy delivered to the liquid prior to nucleation within the initiation pulse. In Figures 4C and 4D, the pulses are voltage modulated. The pulse of Figure 4C has a preheating portion of 2.4 volts for 8 microseconds followed by 4 volts for 0.1 microseconds to initiate nucleation. In contrast, the pulse of Figure 4D has a preheating portion of 2.25 volts for 16 microseconds followed by 4.2 volts for an initiation portion of 0.15 microseconds. These figures clearly show that the bubbles generated using the shaped pulses (Figs. 4B, 4C and 4D) are large, regular and repeatable.

In the event that the problem of irregularity or non-reproducibility has been removed, the designer can have greater flexibility in controlling the size of the bubble during the design phase or during operation by varying the length of time the preheating section of the pulse. Care must be taken to avoid accidentally exceeding this superheat limit during the preheat zone so that nucleation does not occur before the initiation zone. If the pulse is modulated by the pulse width, the modulation should be fast enough to give a reasonable approximation of the temperature rise caused by a fixed, reduced voltage. Care must be taken to ensure that the firing section raises the entire heater above the superheat limit with a sufficient margin to cope with system variations and does not overdrive the heater to the extent that the heater will be damaged. These conditions can be met by the general thermal model or experiment of the heater in an open liquid bath.

The invention has been described herein by way of example. A person skilled in the art can readily appreciate that many variations and modifications can be made without departing from the spirit and scope of the invention.

12. . . bubble

14. . . Bubble irregularity

10. . . Heater

16. . . Preheating section

18. . . Trigger section

32. . . time delay

34. . . Superheat line

36. . . Higher heating rate

30. . . Nozzle device

20. . . Room

twenty two. . . Drive circuit

twenty four. . . Droplet of ink

26. . . nozzle

28. . . Support wafer substrate

Preferred embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which FIGS. Two options for the hot segment and the firing segment; Figure 3 is a graph of the hottest spot on a heater with two different pulse shapes and a colder spot on the heater; Figure 4A shows the use of a conventional Water vapor bubbles generated by square pulses; Figure 4B shows bubbles formed by a pulse width modulated pulse; Figures 4C and 4D show bubbles generated using voltage-modulated pulses; and Figure 5 shows the use of a spray The MEMS bubble generator in the ink print head.

Claims (10)

A MEMS bubble generator comprising: a chamber for containing a liquid; a heater disposed in the chamber for thermal contact with the liquid; and a driving circuit for providing the heater with an electrical pulse such that The heater generates a bubble in the liquid; wherein the pulse has a first portion having a power insufficient to nucleate the bubble and a second portion subsequent to the first portion having a power sufficient to nucleate the bubble Wherein the MEMS bubble generator is used in an ink jet printhead for ejecting liquid from a nozzle in fluid communication with the chamber; and wherein the heater is suspended within the chamber for immersion in a printing fluid . The MEMS bubble generator of claim 1, wherein the first portion of the pulse is a preheating section for heating the liquid but does not allow the bubble to nucleate and the second portion is an initiation section for A portion of the liquid is heated to superheat to nucleate the bubbles. A MEMS bubble generator according to claim 2, wherein the duration of the preheating section is longer than the duration of the initiating section. A MEMS bubble generator according to claim 3, wherein the preheating section is at least 2 microseconds in length of time. A MEMS bubble generator according to claim 3, wherein the initiation section is less than 1 microsecond. A MEMS bubble generator as claimed in claim 1, wherein the drive circuit shapes the pulse using pulse width modulation. A MEMS bubble generator according to claim 6 wherein the preheating section is a series of sub-nucleating pulses. A MEMS bubble generator as claimed in claim 1, wherein the drive circuit shapes the pulse using voltage modulation. A MEMS bubble generator according to claim 2, wherein the time averaged power in the preheating section is fixed and the time averaged power in the initiating section is fixed. A MEMS bubble generator according to claim 1 wherein the pulse is generated to recover a nozzle that is clogged with a dry or excessively viscous printing fluid.