Micromachined two-dimensional array droplet ejectors

[0003] This application claims priority to provisional application serial No. 60/184,691 filed Feb. 24, 2000.

[0005] This invention relates generally to fluid drop ejectors and method of operation, and more particularly an array of fluid drop ejectors wherein the drop size, number of drops, speed of ejected drops and ejection rate are controllable.

[0006] Fluid drop ejectors have been developed for inkjet printing. Nozzles which allow the formation and control of small ink droplets permit high resolution printing resulting in sharp character and improved tonal resolution. Drop-on-demand inkjet printing heads are generally used for high resolution printers.

[0007] In general, drop-on-demand technology uses some type of pulse generator to form and eject drops. In one example, a chamber having a nozzle is fitted with a piezoelectric wall which is deformed when a voltage is applied. As a result, the fluid is forced out of the nozzle orifice and impinges directly on the associated printing surface. Another type of printer uses bubbles formed by heat pulses to force fluid out of the nozzle. The drops are separated from the ink supply when the bubbles collapse. In U.S. Pat. No. 5,828,394 there is described a fluid drop ejector which includes one wall having a thin elastic membrane with an orifice defining a nozzle and transducer elements responsive to electrical signals for deflecting the membrane to eject drops of fluid from the nozzle. The disclosed fluid drop ejector includes a matrix of closely spaced membranes and elements to provide for the ejection of a pattern of droplets. An improvement employing piezoelectric actuating transducers is disclosed in co-pending application Ser. No. 09/098,011 filed Jun. 15, 1998. The teaching of the '394 patent and of the co-pending application are incorporated herein in their entirety by reference. In order to obtain high resolution, many closely spaced ejector elements are required. For high resolution, the elastic membranes are in the order of 100 microns in diameter. We have found that, due to the small size of the elastic membranes, the displacement of the membranes is, in some cases, insufficient to eject certain fluids and solid particles.

[0008] It is an object of the present invention to provide an improved droplet ejector.

[0009] It is another object of the present invention to provide an improved two-dimensional array droplet ejector.

[0010] The foregoing and other objects of the invention are achieved by a material ejector which includes a cylindrical reservoir with an elastic membrane closing one end, and bulk actuation for resonating the material in said reservoir to eject the material through an orifice in said membrane. The injector may include an array of membranes and a single bulk actuator or an array of bulk actuators. The membrane may include individual actuators.

[0011] The invention will be more fully understood from the following description when read in conjunction with the accompanying drawings, wherein:

[0012] FIG. 1 is a cross-sectional view of a typical micromachined two-dimensional array droplet ejector in accordance with the present invention taken along the line 1—1 of FIG. 2.

[0013] FIG. 2 is a view taken along the line 2—2 of FIG. 1, showing the elastic membranes and piezoelectric actuator.

[0014] FIG. 3 is sectional view taken along the line 3—3 of FIG. 1, showing the wells which retain the fluid or particulate matter to be ejected.

[0015] FIG. 4 is a cross-sectional view of a micromachined two-dimensional array droplet ejector illustrating another type of bulk flextensional transducer.

[0016] FIG. 5 is a sectional view of a micromachined two-dimensional array droplet ejector with pneumatic bulk actuation.

[0017] FIGS. 6a-6bschematically show electrical excitation signals applied for bulk and elemental actuation.

[0018] FIGS. 7a-7bschematically show excitation signals applied in another method of bulk and elemental actuation.

[0019] FIG. 8 is a cross-sectional view of a droplet ejector in accordance with another embodiment of the present invention.

[0020] Referring to FIGS. 1-3, a micromachined two-dimensional array droplet ejector is shown. The ejector comprises a body of silicon 11 in which a plurality of cylindrical fluid reservoirs or wells 12 with substantially perpendicular walls 13 are formed as for example by masking and selectively etching the silicon body 11. The etching may be deep reactive ion etching. The one end of each well is closed by a flextensional ejector element (elastic membrane) 14 which may comprise a silicon or a thin silicon nitride membrane. The silicon nitride membrane can be formed by growing a thin silicon nitride layer on the bulk silicon prior to etching the wells. The thickness is preferably as thin as 0.25 microns in thickness. The flextensional ejector elements 14 may include transducers or actuators for deflecting or displacing the elements responsive to an electrical control signal. In the example of FIGS. 1-3, the membranes are deflected by annular piezoelectric actuators 15. A more detailed description of piezoelectrically actuated ejector elements is provided in said co-pending application Ser. No. 09/098,011. The piezoelectric actuators have conductive layers on both faces which are connected to leads 16 and 17 which form a matrix. One or more of the piezoelectric actuators 14 can be selectively actuated by applying electrical pulses to selected lines 16 and 17. Actuation of the piezoelectric actuators causes the corresponding membrane to deflect. Thus, there is provided means for deflecting the individual membrane of the array elements much in the same manner as described in U.S. Pat. No. 5,828,394, which is incorporated in its entirety herein by reference.

[0021] The two-dimensional array droplet ejector also includes bulk actuation means 20 for bulk actuation of the fluid within the wells to set up standing pressure waves in the fluid. For example, in FIG. 1 the bulk actuation means comprises longitudinal piezoelectric member 21 which forms the upper wall of the fluid enclosure. In one mode of operation, the bulk longitudinal piezoelectric member is excited to provide standing pressure waves in the fluid of such amplitude that the fluid forms a meniscus at each of the orifices or apertures 22 formed in the membrane 14. When the individual piezoelectric actuators are actuated, they will move the membrane and eject the fluid in the meniscus. That is, the membrane moves toward the fluid to eject a droplet. This provides an improved ejection of droplets because the droplets are partially formed by the pressure waves. In this mode of operation, the bulk actuation waves and actuation of the individual array element actuation occur in phase at the fluid/liquid interface of the orifice. The frequencies of the bulk and individual array element actuations should be the same for continuous mode ejection, e.g. one drop per cycle. However, these frequencies may be different for tone burst mode of ejection, e.g. several drops per bulk wave cycle. FIG. 6ashows the bulk actuation pulses 26, while FIG. 6bshows the in phase selected element actuation pulses 27. The amplitude of either of these pulses is selected such that in and of itself it will not eject droplets. However, the combined amplitude of the bulk pressure waves and the array element actuation pulses are sufficient to eject droplets. Referring to FIGS. 6A and 6B, it is seen that droplets are ejected at 27a,27band 27c.In essence, the individual ejector elements (membranes) act as switches, operable at relatively high frequencies to eject droplets. If the bulk actuation pulses have a long duration, the membrane may be actuated several times to eject a number of droplets for each bulk pressure wave.

[0022] In another mode of operation, the bulk actuation waves have an amplitude large enough to eject fluid droplets through the orifices of the individual array elements, one for each cycle. However, if the array elements are individually excited out of phase, they will inhibit the ejection by moving the array element membrane away from the fluid to prevent droplet ejection. That is, they act as switches which turn off droplet ejection. This is illustrated in FIG. 7, wherein 7ashows the pulse amplitude of bulk waves 28 sufficient to eject droplets, whereas the out-of-phase membrane actuation shown in FIG. 7bat 29 will stop the ejection of such droplets at 29a,29band 29c.

[0023] Thus, in either of the above events, application of a signal to the bulk actuation piezoelectric transducer sets up the pressure waves which affect the fluid at each membrane while individual excitation of the flextensional diaphragms via the piezoelectric actuators acts as a switch to turn on or off the ejection of the droplet depending upon the amplitude of the bulk pressure waves. The diaphragms or membranes therefore control the drop ejection. Thus, by applying control pulses to the lines 16 and 17, the droplet ejection pattern can be controlled.

[0024] FIG. 4 shows a droplet ejector in which the bulk excitation is by a diaphragm 31 and a piezoelectric element 32. All other parts of the fluid drop ejector array are the same as in FIG. 1 and like reference numbers have been applied. In FIG. 5, the same array includes a flexible wall 33 which is responsive to pressure, arrows 34, such as pneumatic pressure, magnetic actuation or the like, to set up the bulk pressure waves.

[0025] It is to be understood and is apparent that although a piezoelectric transducer has been described and illustrated for driving the elastic membranes, other means of driving the elastic membranes such as electrostatic deflection or magnetic deflection are means of driving the membranes. Typical drive examples are described in U.S. Pat. No. 5,828,394.

[0026] In one example, the diameter of the wells was 100 microns, the depth of the wells was 500 microns, the membrane was 0.25 microns thick, and the orifice was 4 microns. The spacing between orifices was in the order of 100 microns. It is apparent that other size orifice wells and spacing would operate in a similar manner. FIG. 8 shows a micromachined droplet ejector which does not include a membrane actuator. In this droplet ejector, the fluid reservoir becomes an acoustic cavity resonator which resonates at the resonance frequency of the bulk actuator, which is tuned to the same frequency as the resonant frequency of the membrane loaded with fluid. The cylindrical configuration increases the quality of the resonator. At resonance, the membrane vibrates flexurally, vibrating the orifice, generating fluid droplets as small as 4 microns in diameter. The bulk actuation mechanism sets up standing waves in the fluid reservoir. This is in contrast to squeezing the fluid chamber as in the prior art. In other words, the fluid reservoir behaves as an acoustic cavity resonator. Therefore, incident and reflected acoustic waves interfere constructively at the orifice plane.

[0027] Thickness mode piezoelectric transducers in either longitudinal or shear mode can be used for bulk actuation. Single or multiple (i.e. arrays of) thickness mode piezoelectric transducers can be used for the bulk actuation. The bulk actuation can be piezoelectric, piezoresistive, electrostatic, capacitive, magnetostrictive, thermal, pneumatic, etc. Piezoelectric, electrostatic, magnetic, capacitive, magnetostrictive, etc. actuation can be used for the array elements. The actuation of the original array elements can be performed by selectively activating the piezoelectric elements associated with each orifice to act as a switch to either turn on or turn off the ejection of drops. The meniscus of the orifice can always vibrate (not as much as for ejection) to decrease transient response, to decrease drying of the fluid and prevent self-assembling of the fluid ejected near the orifice. Excitation frequencies of bulk and individual array element actuations can be the same or different depending upon the application.

[0028] The devices eject fluids, small solid particles and gaseous phase materials. The droplet ejector can be used for inkjet printing, biomedicine, drug delivery, drug screening, fabrication of biochips, fuel injection and semiconductor manufacturing.

[0029] The thickness of the membrane in which the orifice is formed is small in comparison to the droplet (orifice size), which results in perfect break-up and pinch-off of the ejected droplets from the air-fluid interface. Although a silicon substrate or body having a plurality of cylindrical reservoirs has been described, it is clear that the substrate or body can be other types of semiconductive material, plastic, glass, metal or other solid material in which cylindrical reservoirs can be formed. Likewise, the apertured membrane has been described as silicon nitride or silicon. It can be of other thin, flexible material such as plastic, glass, metal or other material which is thin and not reactive with the fluid being ejected. An ejector of the type shown in FIG. 8 may form part of an array. An array of bulk actuators would be associated with the array of cylindrical reservoirs, one for each reservoir, whereby there can be selective ejection of droplets from the apertures. Although each membrane has been illustrated with a single aperture, the membranes may include multiple apertures to increase the volume of fluid which is ejected in such applications as fuel injection.