CONTROLLING DROPLET FORMATION

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to U.S. Application No. 62/843,712, filed May 6, 2019 and titled APPARATUS FOR AND METHOD OF CONTROLLING DROPLET

FORMATION, and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present application relates to extreme ultraviolet (“EUV”) light sources and their methods of operation. These light sources provide EUV light by creating plasma from a source material. In one application, the EUV light may be collected and used in a photolithography process to produce semiconductor integrated circuits.

BACKGROUND

[0003] A patterned beam of EUV light can be used to expose a resist coated substrate, such as a silicon wafer, to produce extremely small features in the substrate. Extreme ultraviolet light (also sometimes referred to as soft x-rays) is generally defined as electromagnetic radiation having wavelengths in the range of about 5-100 nm. One particular wavelength of interest for photolithography is 13.5 nm.

[0004] Methods to produce EUV light include, but are not necessarily limited to, converting a source material into a plasma state that has a chemical element with an emission line in the EUV range. These elements can include, but are not limited to, xenon, lithium and tin.

[0005] In one such method, often termed laser produced plasma (“LPP”), the desired plasma can be produced by irradiating a source material, for example, in the form of a droplet, stream, or wire, with a laser beam. In another method, often termed discharge produced plasma (“DPP”), the required plasma can be generated by positioning source material having an appropriate emission line between a pair of electrodes and causing an electrical discharge to occur between the electrodes.

[0006] One technique for generating droplets involves melting a source material such as tin and then forcing it under high pressure through an orifice, such as an orifice having a diameter of about 0.5 mih to about 30 mih, to produce a stream resulting in droplets having droplet velocities in the range of about 30 m/s to about 150 m/s. Under most conditions, in a process called

Rayleigh breakup, naturally occurring instabilities, e.g. noise, in the stream exiting the orifice, will cause the stream to break up into the droplets. These droplets may have varying velocities and may combine with each other in flight to coalesce into larger droplets.

[0007] In the EUV generation processes under consideration here, it is desirable to control the break up / coalescence process. For example, in order to synchronize the droplets with the optical pulses of an LPP drive laser, a repetitive disturbance with an amplitude exceeding that of the random noise may be applied to the continuous stream. By applying a disturbance at the same frequency as the repetition rate of the pulsed laser, or its higher harmonics, the droplets can be synchronized with the laser pulses. For example, the disturbance may be applied to the stream by coupling an electro- actuatable element (such as a piezoelectric material) to the stream and driving the electro- actuatable element with a periodic waveform. In one embodiment, the electro- actuatable element will contract and expand in diameter (on the order of nanometers). This change in dimension is mechanically coupled to a capillary that undergoes a corresponding contraction and expansion of diameter. The column of liquid source material e.g., molten tin, inside the capillary also contracts and expands in diameter (and expands and contracts in length) to induce a velocity perturbation in the stream at the nozzle exit.

[0008] As used herein, the term“electro-actuatable element” and its derivatives, means a material or structure which undergoes a dimensional change when subjected to a voltage, electric field, magnetic field, or combinations thereof and includes, but is not limited to, piezoelectric materials, electrostrictive materials and magnetostrictive materials. Apparatus for and methods of using an electro-actuatable element to control a droplet stream are disclosed, for example, in U.S. Patent Application Publication No. 2009/0014668 Al, titled“Faser Produced Plasma EUV Fight Source Having a Droplet Stream Produced Using a Modulated Disturbance Wave” and published January 15, 2009, and U.S. Patent No. 8,513,629, titled“Droplet Generator with Actuator Induced Nozzle Cleaning” and issued August 20, 2013, both of which are hereby incorporated by reference in their entirety. SUMMARY

[0009] The following presents a simplified summary of one or more embodiments in order to provide a basic understanding of the embodiments. This summary is not an extensive overview of all contemplated embodiments, and is not intended to identify key or critical elements of all embodiments nor delineate the scope of any or all embodiments. Its sole purpose is to present some concepts of one or more embodiments in a simplified form as a prelude to the more detailed description that is presented later.

[0010] According to one aspect a source material dispenser comprises a capillary and an electro- actuatable element mechanically coupled to and arranged coaxially with the capillary, with at least one dimension of at least one of the capillary and the electro- actuatable element being selected so that the source material dispenser has a resonant mode at a frequency component of a periodic control signal. The resonant mode may be a thickness mode of the piezo and capillary mechanical stack. The capillary and the electro-actuatable element may be dimensioned such that each resonates in a thickness direction at a common frequency. The resonant mode may be an elastic length mode. A length of the electro-actuatable element may be selected such that a resonant frequency and wavelength of the electro-actuatable element is substantially the same as a frequency and wavelength of an acoustic harmonic of source material when source material fills the capillary. The electro-actuatable element may be a piezoelectric element.

[0011] According to another aspect an apparatus comprises a source material dispenser comprising a capillary and an electro-actuatable element mechanically coupled to and arranged coaxially with the capillary to form a capillary electro-actuatable element system, and a signal generator electrically coupled to the electro-actuatable element for supplying a control signal, the control signal comprising a periodic signal with a frequency component matched to at least one resonance mode of the capillary electro-actuatable element system. The periodic control signal may have a sine wave component with a frequency matched to a thickness mode of the capillary electro-actuatable element system. The periodic control signal may have a coherent ultrasonic component matched in frequency and wavelength to an elastic length mode of the source material dispenser. The electro-actuatable element may be a piezoelectric element. The control signal may cause the electro-actuatable element to generate a longitudinal wave in the capillary having a wave frequency substantially the same as the frequency of the wave component, the longitudinal wave propagating along a length of the capillary to displace a tip of the capillary in an axial direction to facilitate Rayleigh breakup of a source material jet leaving a nozzle orifice in the capillary tip.

[0012] According to another aspect a method comprises the steps of providing a source material dispenser comprising a capillary with a nozzle and an electro-actuatable element mechanically coupled to and arranged coaxially with the capillary, supplying a control signal to the source material dispenser, the control signal having a component with a frequency substantially equal to a frequency of a resonance mode of the source material dispenser, and supplying liquid source material to the source material dispenser, the liquid source material being discharged from the nozzle in a stream, at least one of breakup of the stream into droplets and coalescence of the droplets being controlled by the control signal. The resonance mode may be a thickness mode. The resonance mode may be an elastic length mode. The control signal may have at least one sine wave component with a frequency matched to a thickness mode of a system comprising the capillary and the electro-actuatable element. The control signal may have a coherent ultrasonic component matched in frequency and wavelength to an elastic length mode of the source material dispenser. The electro-actuatable element may be a piezoelectric element. The control signal may cause the electro-actuatable element to generate a longitudinal wave in the capillary having a wave frequency substantially the same as the frequency of the wave component, the longitudinal wave propagating along a length of the capillary to displace a tip of the capillary in an axial direction to facilitate Rayleigh breakup of a tin jet leaving a nozzle orifice in the capillary tip.

[0013] Further embodiments, features, and advantages of the present invention, as well as the structure and operation of the various embodiments are described in detail below with reference to accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the methods and systems of embodiments of the invention by way of example, and not by way of limitation. Together with the detailed description, the drawings further serve to explain the principles of and to enable a person skilled in the relevant art(s) to make and use the methods and systems presented herein. In the drawings, like reference numbers indicate identical or functionally similar elements. [0015] FIG. 1 is a schematic, not-to-scale view of an overall broad conception for a laser- produced plasma EUV radiation source system according to an aspect of an embodiment.

[0016] FIG. 2 is a diagram of an arrangement for dispensing source material according to an aspect of an embodiment.

[0017] FIGS 3A - 3C are diagrams of thickness mode resonances in an arrangement for dispensing source material according to an aspect of an embodiment.

[0018] FIG. 4 is a diagram of an arrangement for dispensing source material according to an aspect of an embodiment.

[0019] FIG. 5 is a diagram of an arrangement for dispensing source material according to an aspect of an embodiment.

[0020] FIG. 6 is a flowchart of a method of dispensing source material according to an aspect of an embodiment.

[0021] Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only.

Additional embodiments will be apparent to persons skilled in the relevant art based on the teachings contained herein.

DETAILED DESCRIPTION

[0022] Various embodiments are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to promote a thorough understanding of one or more embodiments. It may be evident in some or all instances, however, that any embodiment described below can be practiced without adopting the specific design details described below. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate description of one or more embodiments. The following presents a simplified summary of one or more embodiments in order to provide a basic understanding of the embodiments. This summary is not an extensive overview of all contemplated embodiments, and is not intended to identify key or critical elements of all embodiments nor delineate the scope of any or all embodiments. [0023] Before describing such embodiments in more detail, however, it is instructive to present an example environment in which embodiments of the present invention may be implemented. In the description that follows and in the claims the terms“up,”“down,”“top,”“bottom,”

“vertical,”“horizontal,” and like terms may be employed. These terms are intended to show relative orientation only and not any orientation with respect to gravity.

[0024] With initial reference to FIG. 1 there is shown a schematic view of an exemplary EUV radiation source, e.g., a laser produced plasma EUV radiation source 10 according to one aspect of an embodiment of the present invention. As shown, the EUV radiation source 10 may include a pulsed or continuous laser source 22, which may for example be a pulsed gas discharge CO2 laser source producing a beam 12 of radiation at a wavelength generally below 20 pm, for example, in the range of about 10.6 pm to about 0.5 pm or less. The pulsed gas discharge CO2 laser source may have DC or RF excitation operating at high power and at a high pulse repetition rate.

[0025] The EUV radiation source 10 also includes a source delivery system 24 for delivering source material in the form of liquid droplets or a continuous liquid stream. In this example, the source material is a liquid, but it could also be a solid or gas. The source material may be made up of tin or a tin compound, although other materials could be used. In the system depicted the source material delivery system 24 introduces droplets 14 of the source material into the interior of a vacuum chamber 26 to an irradiation region 28 where the source material may be irradiated to produce plasma. In some cases, an electrical charge is placed on the source material to permit the source material to be steered toward or away from the irradiation region 28. It should be noted that as used herein an irradiation region is a region where source material irradiation may occur, and is an irradiation region even at times when no irradiation is actually occurring. The EUV light source may also include a beam focusing and steering system 32 as will be explained in more detail below in conjunction with FIG. 2.

[0026] In the system shown, the components are arranged so that the droplets 14 travel substantially horizontally. The direction from the laser source 22 towards the irradiation region 28, that is, the nominal direction of propagation of the beam 12, may be taken as the Z axis. The path the droplets 14 take from the source material delivery system 24 to the irradiation region 28 may be taken as the X axis. The view of FIG. 1 is thus normal to the XZ plane. Also, while a system in which the droplets 14 travel substantially horizontally is depicted, it will be understood by one having ordinary skill in the art the other arrangements can be used in which the droplets travel vertically or at some angle with respect to gravity between and including 90 degrees (horizontal) and 0 degrees (vertical).

[0027] The EUV radiation source 10 may also include an EUV light source controller system 60, which may also include a laser firing control system 65, along with the beam steering system 32. The EUV radiation source 10 may also include a detector such as a source position detection system which may include one or more droplet imagers 70 that generate an output indicative of the absolute or relative position of a source droplet, e.g., relative to the irradiation region 28, and provide this output to a source position detection feedback system 62.

[0028] The source position detection feedback system 62 may use the output of the droplet imager 70 to compute a source position and trajectory from which a target error can be computed. The source error can be computed on a droplet-by-droplet basis, or on average, or on some other basis. The target error may then be provided as an input to the light source controller 60. In response, the light source controller 60 can generate a control signal such as a laser position, direction, or timing correction signal and provide this control signal to the laser beam steering system 32. The laser beam steering system 32 can use the control signal to change the location and/or focal power of the laser beam focal spot within the chamber 26. The laser beam steering system 32 can also use the control signal to change the geometry of the interaction of the beam 12 and the droplet 14. For example, the beam 12 can be made to strike the droplet 14 off- center or at an angle of incidence other than directly head-on.

[0029] As shown in FIG. 1, the source material delivery system 24 may include a source delivery control system 90. The source delivery control system 90 is operable in response to a signal, for example, the target error described above, or some quantity derived from the target error provided by the system controller 60, to adjust paths of the target droplets 14 through the irradiation region 28. This may be accomplished, for example, by repositioning the point at which a source delivery mechanism 92 releases the target droplets 14. The droplet release point may be repositioned, for example, by tilting the source delivery mechanism 92 or by shifting the source delivery mechanism 92. The source delivery mechanism 92 extends into the chamber 26 and is preferably externally supplied with source material and connected to a gas source to place the source material in the source delivery mechanism 92 under pressure. [0030] Continuing with FIG. 1, the radiation source 10 may also include one or more optical elements. In the following discussion, a collector 30 is used as an example of such an optical element, but the discussion applies to other optical elements as well. The collector 30 may be a normal incidence reflector, for example, implemented as a multilayer mirror (MLM) with additional thin barrier layers, for example B4C, ZrC, Si3N4or C, deposited at each interface to effectively block thermally-induced interlayer diffusion. Other substrate materials, such as aluminum (Al) or silicon (Si), can also be used. The collector 30 may be in the form of a prolate ellipsoid, with a central aperture to allow the laser radiation 12 to pass through and reach the irradiation region 28. The collector 30 may be, e.g., in the shape of a ellipsoid that has a first focus at the irradiation region 28 and a second focus at a so-called intermediate point 40 (also called the intermediate focus 40) where the EUV radiation may be output from the EUV radiation source 10 and input to, e.g., an integrated circuit lithography scanner or stepper 50 which uses the radiation, for example, to process a silicon wafer work piece 52 in a known manner using a reticle or mask 54. The mask 54 may be transmissive or reflective. For EUV applications the mask 54 is generally reflective. The silicon wafer work piece 52 is then additionally processed in a known manner to obtain an integrated circuit device.

[0031] The arrangement of FIG. 1 may also include a temperature sensor 34, e.g., a

thermocouple positioned within the chamber 26 to measure the local temperature, i.e., temperature at the sensor, of the gas within the chamber 26. FIG. 1 shows one temperature sensor but it will be apparent that additional temperature sensors may be used. The temperature sensor 34 generates a signal indicative of the measured temperature and supplies it as an additional input to the controller 60. The controller 60 bases the control signal it supplies to the beam steering system 32 at least in part on this temperature signal.

[0032] FIG. 2 illustrates the components of a simplified droplet source 92 in schematic format. As shown there, the droplet source 92 may include a capillary 94 holding a fluid 96, e.g. molten tin, under pressure. Also shown, the capillary 94 may be formed with a nozzle 98 allowing the pressurized fluid 96 to flow through the nozzle 98 establishing a continuous stream 100 which subsequently breaks into droplets 102. The droplet source 92 shown further includes a sub system producing a disturbance in the fluid 96 having an electro-actuatable element 104 that is operably coupled with the fluid 96 and a signal generator 106 driving the electro-actuatable element 104. [0033] As described further below, waveforms having different amplitudes, frequencies, or shapes may be used to drive electro-actuatable element 104 to produce droplets for EUV output. The electro-actuatable element 104 produces a disturbance in the fluid 96 which generates droplets having differing initial velocities causing at least some adjacent droplet pairs to coalesce together prior to reaching the irradiation region. The ratio of initial droplets to coalesced droplets may be two, three or more and in some cases tens, hundreds, or more.

[0034] More specifically, when the source material 96 first exits the nozzle 98, the source material is in the form of a velocity-perturbed steady stream 100. The stream 100 breaks up into a series of microdroplets having varying velocities. The microdroplets coalesce into droplets of an intermediate size, referred to as subcoalesced droplets, having varying velocities with respect to one another. The subcoalesced droplets then coalesce into droplets 102 having the desired final size. The number of subcoalescence steps can vary. The distance from the nozzle 98 to the point at which the droplets reach their final coalesced state is the coalescence distance L. Control of the breakup / coalescence process thus involves controlling the stream and droplets such that the stream breaks up into droplets and then the droplets coalesce sufficiently before reaching the irradiation region and have a frequency corresponding to the pulse rate of the laser being used to irradiate the coalesced droplets.

[0035] Control of some aspects of droplet coalescence may be achieved through the application of a blockwave voltage signal applied to an electro-actuatable element 104 in the form of a piezoelectric actuator to stimulate droplet coalescence. The piezoelectric element may be mounted coaxially with the capillary, and the capillary made be made of a material such as a glass. In the discussion that follows a piezoelectric actuator and a glass capillary will be used as a specific example, but it will be apparent to one having ordinary skill in the art that other types of actuators may be used, and that the capillary may be made of a material other than or in addition to glass.

[0036] The blockwave may composed of multiple high frequency components. These may indirectly facilitate Rayleigh breakup and subcoalescence of droplets necessary for fully coalesced 50kHz droplets, but do not provide a means for direct control of these processes.

[0037] Sine wave excitation of the piezo actuator with a frequency matched to the thickness mode of the piezo/capillary coaxial system generates a longitudinal wave of the same frequency that propagates along the capillary length to displace the capillary nozzle in the axial direction. This directly facilitates Rayleigh breakup of the tin jet leaving the nozzle orifice. Also, coherent ultrasonic excitation, having a frequency in the range of about 100kHz to about 1MHz, can be applied to a piezo actuator arranged coaxially with the capillary and mechanically bonded to the capillary. The elastic length mode of the piezo actuator may be matched in both frequency and wavelength to the desired subcoalescence droplet frequency acoustic harmonic in the liquid source material in the capillary. This provides a means for direct control of the coalescence process.

[0038] With regard to breakup, the thickness of the piezo actuator and capillary may be sized such that they each resonate in the thickness direction at a common frequency. This is illustrated in FIGS. 3A - 3B. FIG. 3A shows a profile for a first order thickness mode for the capillary 94 at l/4. FIG. 3B shows a profile for a piezo actuator 104 having a third order thickness mode resonance at 5l/4. FIG. 3C shows a profile of an overall third order thickness mode of the capillary / piezo system 300 at 3l/2.

[0039] With the capillary and piezo being so configured, and as shown in FIG. 4, the system thickness mode 400 generates a longitudinal wave 402 in the glass capillary 94 that propagates along the capillary axis to displace the capillary nozzle 98 a distance D. These longitudinal waves thus cause an axial displacement of the nozzle 98, which provides a direct means of controlling Rayleigh breakup of the stream of source material. Through the decomposition of the system thickness mode into multiple component level thickness modes it is possible to design a system that resonates at an optimal Rayleigh breakup frequency through the selection of a frequency-tuned piezo and glass capillary thicknesses. For example, the resonant frequency may be in a range of about 4MHz to 6MHz.

[0040] An impedance scan may be performed on the system to identify a frequency fi of the thickness mode resonance. A control signal may be generated having a first component at this frequency and additional sine components to control coalescence. In other words, the additional sine components may be made up of multiple (two or more) sine waves having frequencies, amplitudes (voltages) and relative phase selected to fully coalesce droplets within a desired coalescence length. For example, the second component may be a 600kHz sine wave and 50kHz sine. The voltage amplitude of the first component, which may, for example, have a frequency of about 4.2MHz, may be selected to minimize droplet velocity jitter. [0041] Thus, according to an aspect of an embodiment, an acoustic wave is generated to propagate in an axial direction through the capillary to displace the nozzle orifice in the axial direction at a designed frequency to facilitate controlled tin jet Rayleigh breakup. High frequency Rayleigh droplets are first generated with a sine wave frequency matched to the system piezo/capillary thickness mode, then are subcoalesced at an intermediate sine wave frequency, e.g., 600kHz, and then are fully coalesced using another sine wave using, e.g., a combination of three sine waves.

[0042] As regards control of coalescence, as shown in FIG. 5, the length of a piezo 104 coaxially bonded to the capillary 94 can be designed to target a specific resonant frequency and wavelength that corresponds to the acoustic harmonic of the source material 96, e.g., tin, within the capillary 94. For example, a second order length mode resonance 500 for the piezo element 104 at 2l/2 may be used to target a tin acoustic harmonic shown by trace 502 at ((2h-1)l/4).

More specifically, and also purely as an example, if the length of the piezo element 104 is about 6mm then the second length mode of the piezo is about 500kHz with a wavelength of about 2.6mm, after accounting for coaxial capacitance fringe effects that reduce the overall effective actuation length. The (19l/4) acoustic frequency and wavelength of a system using a capillary having a length of about 20mm may be calculated to be about 516kHz and 2.6mm, respectively. By matching the frequency and wavelength of the both the piezo and liquid source material in the capillary it is possible to maximize transfer of electrical energy into an axial jet velocity perturbation at the targeted frequency. This linearizes the droplet generator droplet breakup process leading to more precise droplet spacing and also enables more deterministic control of the droplet formation process.

[0043] Thus, according to an aspect of an embodiment, coherent excitation of a capillary filled with source material using a coaxially-bonded piezo actuator with a design length selected to resonate elastically at a frequency and wavelength common to the desired acoustic harmonic in the ultrasonic (100kHz - 1MHz) frequency range is used to control the droplet coalescence process.

[0044] According to another aspect a method of controlling the breakup and/or the subsequent coalescence of the source material stream is disclosed. With reference to FIG. 6, in a step S10 a source material dispenser is provided wherein the source material dispenser has been

dimensioned and configured as described above to have a resonance at a frequency of a control signal to be applied. This selection may be, for example, selecting the thickness mode resonance of the dispenser to cause axial displacement of the dispenser nozzle to control the process of the stream exiting the nozzle breaking up into droplets. The selection may also be for matching a longitudinal resonance of the piezo with the acoustic harmonic of the liquid source material filling the capillary to control coalescence. In a step S20 the control signal is applied to the source material dispenser with the frequency components of the control signal being selected to match the resonance characteristics that the source material dispenser has been constructed to exhibit. In a step S30 the source material is supplied to the dispenser. It will be appreciated that depending on the specific application the step of supplying source material may be carried out before the step of supplying the control signal.

[0045] The present invention has been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed.

[0046] The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying knowledge within the skill of the art, readily modify and/or adapt for various applications such specific embodiments, without undue experimentation, without departing from the general concept of the present invention. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by the skilled artisan in light of the teachings and guidance. The breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

[0047] Other aspects of the invention are set out in the following numbered clauses.

1. A source material dispenser comprising:

a capillary; and

an electro- actuatable element mechanically coupled to and arranged coaxially with the capillary, at least one dimension of at least one of the capillary and the electro- actuatable element being selected so that the source material dispenser has a resonant mode at a frequency component of a periodic control signal.

2. A source material dispenser as in clause 1 wherein the resonant mode is a thickness mode of the capillary together with the electro- actuatable element.

3. A source material dispenser as in clause 1 or 2 wherein the capillary and the electro- actuatable element are dimensioned such that each resonates in a thickness direction at a common frequency.

4. A source material dispenser as in clause 1 wherein the resonant mode is an elastic length mode.

5. A source material dispenser as in clause 4 wherein a length of the electro- actuatable element is selected such that a resonant frequency of the electro-actuatable element is substantially the same as a frequency of an acoustic harmonic of source material when the source material fills the capillary.

6. A source material dispenser as in any one of clauses 1-5 wherein the electro-actuatable element comprises a piezoelectric element.

7. Apparatus comprising:

a source material dispenser comprising a capillary and an electro-actuatable element

mechanically coupled to and arranged coaxially with the capillary to form a capillary electro- actuatable element system; and

a signal generator electrically coupled to the electro-actuatable element for supplying a control signal, the control signal comprising a periodic signal with a frequency component matched to at least one resonance mode of the capillary electro-actuatable element system.

8. An apparatus as in clause 7 wherein the periodic control signal has a sine wave component with a frequency matched to a thickness mode of the capillary electro-actuatable element system.

9. An apparatus as in clause 7 wherein the periodic control signal has a coherent ultrasonic component matched in frequency and wavelength to an elastic length mode of the source material dispenser.

10. Apparatus as in clause 7 wherein the electro-actuatable element comprises a piezoelectric element. 11. Apparatus as in clause 7 wherein the control signal causes the electro- actuatable element to generate a longitudinal wave in the capillary having a wave frequency substantially the same as the frequency of the wave component, the longitudinal wave propagating along a length of the capillary to displace a tip of the capillary in an axial direction to facilitate Rayleigh breakup of a source material jet leaving a nozzle orifice in the capillary tip.

12. A method comprising the steps of:

providing a source material dispenser comprising a capillary with a nozzle and an electro- actuatable element mechanically coupled to and arranged coaxially with the capillary;

supplying a control signal to the source material dispenser, the control signal having a component with a frequency substantially equal to a frequency of a resonance mode of the source material dispenser; and

supplying liquid source material to the source material dispenser, the liquid source material being discharged from the nozzle in a stream, at least one of breakup of the stream into droplets and coalescence of the droplets being controlled by the control signal.

13. A method as in clause 12 wherein the resonance mode is a thickness mode of the capillary together with the electro- actuatable element.

14. A method as in clause 12 wherein the resonance mode is an elastic length mode.

15. A method as in clause 12 wherein the control signal has at least one periodic wave component with a frequency matched to a thickness mode of a system comprising the capillary and the electro-actuatable element.

16. A method as in clause 12 wherein the control signal has a coherent ultrasonic component matched in frequency and wavelength to an elastic length mode of the source material dispenser.

17. A method as in clause 12 wherein the electro-actuatable element is a piezoelectric element.

18. A method as in clause 12 wherein the control signal causes the electro-actuatable element to generate a longitudinal wave in the capillary having a wave frequency substantially the same as the frequency of the wave component, the longitudinal wave propagating along a length of the capillary to displace a tip of the capillary in an axial direction to facilitate Rayleigh breakup of a tin jet leaving a nozzle orifice in the capillary tip.