Method for forming micron-sized and smaller liquid droplets

Abstract: A method for forming micron-sized or smaller drops of liquid, and the use of the method in fabricating micro electro mechanical and micro mechanical devices is disclosed. A micropipette (4) is formed having an inside diameter no larger than the size of the drops to be formed. The micropipette is connected to a system (14) capable of developing a positive and optionally negative pressure within the micropipette. The tip of the micropipette is placed in liquid. The liquid is drawn into the micropipette via capillary action or from the negative pressure developed by the system. The micropipette is then positioned to deliver liquid to an intended location on a surface. To deliver the liquid, a positive pressure is developed within the micropipette (4). The positive pressure forces a micron-sized or smaller drop of liquid out of the micropipette. The method can be used to form micron-sized or smaller drops of adhesive for fixing in place various structural members that form microdevices.

Description: This invention relates to methods for making a micro device on a support. Assembling micro mechanical devices often requires the attaching of micron-scale objects to one another. One method for creating such assembled micro mechanical devices is disclosed in Pister et al., "Microfabricated Hinges: Sensors and Actuators", vol. A33, No. 3, 1 June 1992, pages 249-256. Pister et al. is directed to a surface micromachining process for interconnecting micron-scale objects. Substrate hinges are integrally fabricated to a polysilicon structure for connecting micron-scale objects to a base structure. Scissor hinges are also created for interconnecting different micron-scale objects so as to form the assembled micro mechanical device. Although Pister et al. extends high-resolution surface lithography to three-dimensional micro mechanical structures, such a hinge-based design requires additional post-process assembly and is incompatible with typical MOS processes. To facilitate this assembly, it would be convenient to attach the micron-scale objects by gluing them together with an adhesive. Gluing such micron-scale objects, however, is problematic. When applying glue, it is preferable, if not necessary, to deliver the glue in precise amounts to precise locations, have reasonable working time and restricted flowability to other regions. Drops of glue tend, however, to assume a size that is disadvantageously typically several orders of magnitude larger than the typical micro mechanical device. Moreover, even if there was a method for delivering suitably small drops of glue, such small drops of glue tend to have a short working time because the glue's solvent rapidly evaporates due to the large surface area to volume ratio. Thus, there is a need for a method for gluing such micron-scale objects. According to this invention there is provided a method as claimed in claim 1. A micropipette, typically used for biological experiments, is used to form and apply micron-sized and smaller glue drops. The micropipette may be attached to a vacuum/pressurization system for wicking up/delivering the adhesive. Low viscosity, ultraviolet light (UV) curable epoxies are preferably used, which provide working times of several hours for the glue drops. Micromanipulator may be used to deliver the micropipette, and hence a glue drop, to the desired location. The epoxy may be cured by exposing it to UV. Such micron-sized and smaller glue drops can advantageously be used to fabricate micro mechanical and micro electro mechanical structures. Microchannels can be formed in such microstructures to deliver glue to locations having limited access. Brief Description of the Drawings Further features of the invention will become more apparent from the following detailed description of specific embodiments thereof when read in conjunction with the accompanying drawings, in which: FIG. 1 shows an arrangement suitable for delivering micron-size or smaller glue drops; FIG. 2 shows an exemplary embodiment of a vacuum/pressurizing system; FIG. 3 shows an exemplary micropipette; FIG. 4 shows microchannels for adhesive delivery; FIG. 5 is a flow diagram of a method for delivering submicron-sized glue drops; FIG. 6 shows a micro electro mechanical systems (MEMS) device that is assembled using micron-sized or smaller glue drops; and FIG. 7 shows a channel-forming membrane on a support to deliver adhesive to a hinge of the MEMS device of FIG. 6. Detailed Description A method method for forming micron-sized and smaller drops of liquid is described in the context of forming glue drops for the assembly of micro-sized structures. Glue drops for forming such structures will preferably have a size in the range from about 0.6 microns to about 20 microns. It should be understood, however, that the method is more broadly applicable. For example, the present method can be used to form micro lenses. FIG. 1 shows an exemplary arrangement 2 suitable for practicing the present method. The arrangement includes a micropipette 4, a vacuum/pressurization system 14, means 12 for interconnecting the micropipette and the vacuum/pressurization system, and an adhesive 16. In a basic implementation shown in FIG. 2, the vacuum/pressurization system 14 is a syringe 14a having plunger 18. It will be appreciated that other more sophisticated vacuum/pressurization systems 14 can be used. Means 12 can be rubber tubing 12a or the like capable of withstanding slight negative and positive pressures. Capillary effect may be sufficient to wick the adhesive 16 into the micropipette 4. In preferred embodiments, a slight negative pressure is developed through the arrangement 2 to facilitate wicking up the adhesive 16. If the vacuum/pressurization 14 is implemented as a syringe 14a, drawing the plunger 18 partially out of the syringe 14a will develop a negative pressure suitable for wicking up the adhesive 16. To deliver the adhesive 16 from the micropipette 4, the plunger 18 is pushed into the syringe 14a thereby generating a positive pressure suitable for forcing the adhesive 16 out of the micropipette. The micropipette 4 includes a tip 6, tapered neck 8 and body 10. The inside diameter of the micropipette 4 will typically be in the range of about 500 angstroms to tens of microns, as a function of the desired size of the glue drop. Micropipette 4 suitable for use in conjunction with the present invention can be made using commercially available micropipette pullers, such as available from U.S. Narishige International, Inc., East Meadow, New York. Such a micropipette puller is suitable for producing a micropipette having a mouth less than about 0.1 micron in diameter. A pipette grinder, also available from U.S. Narishige International Inc., can be used to grind the tip 6 of the micropipette 4 back, increasing its diameter as desired. The diameter of the tip 6 of the micropipette should be no larger, and preferably slightly smaller, than the desired drop size. Adhesives 16 suitable for use in conjunction with the present invention include heat curable, and, more preferably, UV curable glues. One suitable UV-curable glue is Norland Optical Adhesive no. 81, available from Norland Products Inc. of New Jersey. Adhesive should be selected to provide an appropriate working time during which the adhesive does not solidify, dry out, or become appreciably more viscous. Norland Optical Adhesive no. 81 may be used in conjunction with the present invention for forming micron-sized lenses, as well. Additionally, the adhesive 16 should be homogeneous on a submicron scale so that no phase separation takes place as the micropipette 4 is filled with adhesive. For example, some conductive epoxies contain metal particles having a size in the range of from about 1 to 10 microns. Such particles may be larger than the internal diameter of the micropipette 4. Such an adhesive should not be used. The time required to wick-up and deliver adhesive 16 is a function of applied pressure differential and adhesive viscosity. A relatively higher adhesive viscosity results in relatively longer wick-up and delivery time. A viscosity of 300 centipoise has been found to provide suitably short wick-up/delivery times for reasonable applied pressure differences, e.g., less than about one atmosphere. With reference to FIG. 3, a further consideration in adhesive selection is the contact angle ϕ 1 between the adhesive 16 and the interior surface 20 of the micropipette 4. The contact angle ϕ 1 is determined by properties of both the adhesive 16 and the interior surface 20. In preferred embodiments, the contact angle ϕ 1 should be less than about 90°. It has been found that at contact angles greater than about 90°, unsuitably high pressure differentials are required to wick-up the adhesive 16 into the micropipette 4. In less preferred embodiments, selection of a particular micropipette inner diameter d and particular contact angle ϕ 1 may require external pressurization to wick-up the adhesive. For a contact angle ϕ 1 less than 90°, any adhesive 16 in contact with the tip 6 that is not drawn within the micropipette 4 tends to spread up the outer surface 22 of the micropipette. Thus, glue does not remain outside the micropipette 4 in the vicinity of the tip 6. As such, adhesive is not deposited during unintended surface contacts unless the micropipette 4 is positively pressurized. For a contact angle ϕ 1 less than 90°, a finite positive pressure differential ΔP between the interior of the micropipette 4 and the surroundings is required to start delivery of the adhesive 16 from within the micropipette. The positive pressure differential ΔP is required to overcome a capillary effect that tends to keep the adhesive 16 within the micropipette 4. The positive pressure differential ΔP is a function of the contact angle ϕ 1 , surface tension σ of the adhesive 16, and the inner diameter d of the micropipette 4 as follows: [1] ΔP = (4σ/d)cos ϕ 1 . It will be appreciated from expression [1] that as the contact angle ϕ 1 or inner diameter d of the micropipette decreases, the pressure differential ΔP increases. Expression [1] is based on delivering the adhesive through a regular-shaped, i.e., circular, tip 6. If the tip shape is irregular or otherwise differs from a circle, then the pressure differential required to overcome the capillary effect should decrease. As such, expression [1] should provide a conservative estimate of the required pressure differential. Moreover, it should be understood that the present description pertains to delivering the adhesive liquid to a surface, as opposed to another liquid, or into air. Selection of adhesive 16 is also influenced by characteristics of the surface upon which the adhesive is to be deposited. Those characteristics result in a particular contact angle ϕ 2 between the adhesive and that surface. The contact angle ϕ 2 should be greater than zero, otherwise the adhesive 16 will spread over the surface, rather than remaining contained as a submicron-sized drop. Moreover, ϕ 2 should be less than ϕ 1 to minimize the tendency for the adhesive 16 to spread along the outer surface 22 of the micropipette 4 in preference to remaining at the intended delivery site. A combination of adhesive 16, micropipette 4 and target surface resulting in a value of about 60° for ϕ 1 and a value of about 30° for ϕ 2 has been found to be suitable for practicing the present invention. To avoid limitations on adhesive selection imposed by the surface characteristics of the outer surface 22 of the micropipette 4, the outer surface 22 can be coated with a material that a results in a contact angle ϕ 3 between the adhesive and the coating on the outer surface 22. If the contact angle ϕ 3 is greater than ϕ 1 , then the aforementioned condition ϕ 2 < ϕ 1 may be relaxed, and the requirement now becomes ϕ 2 < ϕ 3 . As shown in FIG. 4, a channel-forming member 31 for forming a microchannel 30 can be incorporated into micro-sized structures. A micron-sized adhesive drop 40 delivered at site 32 will spread, via capillary action, through the microchannel 30 to site 34. In this manner, a drop 40 can be deposited a safe distance away from delicate structures and delivered to a desired location via a microchannel. Such a microchannel 30 advantageously relaxes drop size requirements and facilitates delivery of adhesive 16 to locations that are (i) inaccessible to the micropipette 4, and (ii) too close to delicate structures to risk direct delivery of adhesive from the micropipette. FIG. 5 shows a flow diagram of a method wherein, in operation block 100, an adhesive, and an arrangement suitable for wicking-up the adhesive and delivering micron-sized and smaller drops of the adhesive, is provided. As indicated in optional block 102, a negative pressure is developed in the arrangement to draw adhesive therein. Developing negative pressure is optional, in some cases, since capillary action may be sufficient to wick up the adhesive. The tip of the arrangement is positioned at the intended site for adhesive delivery as indicated in operation block 104. Micromanipulators can be used for such purpose. A positive pressure is developed in the arrangement to force the adhesive out to the intended site, as noted in block 106. The adhesive is then cured, typically by UV or heat exposure. In further embodiments, micro channels can be provided in an object for which the adhesive is intended to facilitate delivering adhesive to a desired location. FIG. 6 shows an exemplary micro electro mechanical systems (MEMS) device 50 that is fabricated using submicron-sized drops of adhesive. The MEMS device 50 shown in FIG. 6 is a hinged-plate actuator. Such an actuator, and its use in optical switches, is described in copending patent application no. 98303498.4. It will be appreciated that the present method can be applied advantageously to a variety of other MEMS structures. The MEMS device 50 is formed from a plurality of hinged plates 56, 60, 66, 70 and 76. To assemble the MEMS device 50, the various hinged plates, which are typically photo lithographically patterned in the plane of substrate 48, are rotated to an out-of-plane position such as shown in FIG. 6. Hinges 59, 62, 72 and 78 allow the plates to rotate. Hinged plate 70 receives hinged plate (frame) 60 in notch 74. Plate 66 is suspended from cross member 64 of the frame 60 in a manner that allows plate 66 to swing freely. In the embodiment of FIG. 6, hinges 68 provide that function. Plate 66 functions as a movable electrode. Hinged plate 76 receives edge 58 of hinged plate 56 in notch 80. The hinged plate 56 functions as a fixed electrode. As a voltage is applied across the hinged plates 56 and 66, an electrostatic force is developed that causes the movable hinged plate 66 to swing in a substantially horizontal path towards the hinged plate 66. The substantially horizontal displacement of the hinged plate 66 can be imparted to a linked object, via a linkage, not shown, causing the linked object to move. To prevent the movable electrode 66 and the fixed electrode 56 from shorting, dielectric stops 82 are disposed near the lower comer edges of one or both of the surfaces 57, 67 of respective electrodes 56 and 66. A single stop 82 is visible in FIG. 6. The stops 82 may be micron-sized drops of adhesive formed and delivered according to the present method. A drop of adhesive about 5 microns in diameter has been found to be suitable for forming the stops 82. Moreover, micron-sized drops of adhesive can be applied to the hinges 59, 62, 72 and 78 to aid in fixing the various hinged plates in their desired out-of-plane position. Channel-forming members, such as the exemplary channel-forming member 77, can be used to deliver adhesive to a hinge of a MEMS device, such as the hinge 59 of the MEMS device 50 shown in FIG. 6. The channel-forming member 77, in conjunction with the support 48, provides a microchannel 79. A micron-sized drop of adhesive 90 placed at an end of the microchannel 79 will be drawn in the direction of arrow 92 through the microchannel and delivered to the hinge 59 at location 94. The hinge itself functions as a microchannel, spreading the adhesive in the direction of arrow 96 across the full hinge. For clarity, a single channel-forming member 77, microchannel 79 and hinge 59 are pictured in FIG. 7. It should be understood that such microchannels can be associated with each hinge of such a MEMS device. Additionally, micron-sized drops of adhesive can be delivered to the notches 74 and 80 in respective hinged plates 70 and 76 to fix those plates to the frame 60 and fixed electrode 56. Although specific embodiments of this invention have been shown and described herein, it is to be understood that these embodiments are merely illustrative of the many possible specific arrangements that can be devised in application of the principles of the invention. Numerous and varied other arrangements can be devised in accordance with these principles by those of ordinary skill in the art without departing from the scope of the invention.

Claims: A method for making a micro device (50) on a support (48), comprising: forming plates (56, 66) having hinges (59, 68), wherein a first hinge (59) of one of said plates (56) rotatably attaches said plate (56) to a surface of the support (48), wherein said plate (56), after formation, lies on said surface of said support; forming a channel (79) that leads from a first region of said support (48) to said first hinge (59) of said one plate (56); rotating said one plate (56) away from said surface of said support (48) about said first hinge (59); delivering adhesive with a micropipette (4) to said first region of said support (48) wherein the adhesive is applied as a first micron- or smaller-sized droplet (90); and conducting said adhesive from said first region to said first hinge (59) via said channel (79). The method of claim 1., wherein said adhesive is conducted from the first region to said first hinge (59) via capillary action. The method of claim 1, wherein the step of delivering adhesive comprises: drawing said adhesive into said micropipette (4); positioning an orifice (6) of said micropipette at said first region of the support (48); and developing a positive pressure in said micropipette to force the first droplet of adhesive therefrom. The method of claim 3, wherein the operation of drawing comprises developing a negative pressure in the micropipette (4). The method of claim 3 , wherein the operation of drawing . comprises drawing an adhesive into the micropipette (4) that develops a contact angle ϕ 1 between the adhesive and an interior surface (20) of the micropipette that is less than about 90°. The method of claim5, wherein the operation of drawing comprises drawing an adhesive into the micropipette (4) that develops a contact angle ϕ 2 between the adhesive and said surface of said support (48) that is greater than zero and less than ϕ 1 . The method of claim 6 wherein ϕ 1 is about 60° and ϕ 2 is about 30°. The method of claim5, and comprising coating an outer surface (22) of the micropipette (4) with a first material, wherein a contact angle ϕ 3 between the adhesive and first material is greater than a contact angle ϕ 2 between the adhesive and said surface of said support (48). The method of claim 1 wherein: the operation of forming plates comprises: forming first (56) and second (66) electrically-conductive plates that move toward one another upon application of voltage thereto, wherein: said first electrically-conductive plate (56) is rotatably attached, via said first hinge, to said surface of said support (48); said second electrically-conductive plate (66) is rotatably attached to a third plate (60, 64) via a second hinge (68); and said third plate is rotatably attached, via a third hinge (82), to said surface of said support (48); the operation of rotating comprises: rotating said first electrically-conductive plate (56) away from said surface of said support (48) about said first hinge (59); fixing said first electrically-conductive plate (56) in its rotated position; rotating said third plate (60, 64) away from said surface of said support (48) about said third hinge (82); and fixing said third plate (60, 64) in its rotated position; the method comprising applying a second droplet (82) of adhesive having a size on the order often microns to at least one of said first (56) and second (66) electrically-conductive plates at a location thereon that is suitable for preventing contact between said first and second electrically-conductive plates when voltage is applied. The method of claim 9. wherein: the operation of forming plates comprises forming a support plate (76) that is rotatably attached, via a fourth hinge (78), to the surface of said support (48), said support plate (76) having a notch (80) for engaging said first electrically-conductive plate (56); the operation of rotating comprises rotating said support plate (76) away from the surface of the support (48) about said fourth hinge (78); and the operation of fixing said first electrically-conductive plate (56) comprises rotating said support plate (76) until it engages said first electrically-conductive plate (56) and applying a third droplet of adhesive on the order often microns to the notch. The method of claim 10 comprising exposing the first, second and third droplets of adhesive to ultraviolet light. The method of claim 1, wherein said channel (79) is formed via a channel-forming member (77) that is disposed on said surface of said support (48). The method of claim 12, wherein said channel-forming member (77) comprises a portion of said first hinge (59).

Inventors: AKSYUK VLADIMIR A, BISHOP DAVID J, DENK WINFRIED, TANK DAVID W, AKSYUK, VLADIMIR A., BISHOP, DAVID J., DENK, WINFRIED, TANK, DAVID W.
Applicants: LUCENT TECHNOLOGIES INC, LUCENT TECHNOLOGIES INC.
Application year: 1998
Main classification: B05C5
IPC codes: C09J163, B81C3, B05C11, B05C9, B05D1, B01L3, B05C5, C09J5