JET PRINTING USING LASER-PRODUCED DRY AEROSOL

Abstract

A method of mask-free printing of dry nanoparticles, the method comprising generating a dry nanoparticle stream from a feedstock material in an atmospheric gas flow using a laser ablation system at atmospheric pressure, the dry nanoparticle stream uncontaminated by a fluidic carrier medium, wherein the dry nanoparticles uncontaminated by a fluidic carrier medium are directed to a substrate through a nozzle by the gas flow in a dry state and adhere to the substrate.

Claims

1. A method of mask-free printing of dry nanoparticles, the method comprising generating a dry nanoparticle stream from a feedstock material in an atmospheric gas flow using a laser ablation system at atmospheric pressure, the dry nanoparticle stream uncontaminated by a fluidic carrier medium, wherein the dry nanoparticles uncontaminated by a fluidic carrier medium are directed to a substrate through a nozzle by the gas flow in a dry state and adhere to the substrate.

2. The method of claim 1, further comprising the step of heating the atmospheric gas flow prior to printing on the substrate.

3. The method of claim 1 or claim 2, further comprising the step of sintering the particles after printing.

4. The method of claim 3, wherein the step of sintering the particles within the gas flow or following particle printing is by heating, laser irradiation, or plasma treatment.

5. The method of claim 4, wherein heating the particles is performed in an oven or a furnace.

6. The method of any one of the preceding claims, wherein the feedstock material is selected from a metal, a non-metal, or a combination thereof.

7. The method according to claim 6, wherein the metal is selected from copper, silver, gold, platinum, gallium, aluminium, and alloys thereof.

8. The method according to claim 6 or claim 7, wherein the non-metal is selected from carbon, graphite, graphene, single-walled carbon nanotubes, multi-wall carbon nanotubes, boron nitride, a ceramic, a polymer, a non-metal insulator, a non-metal semiconductor, superconductor, a metal oxide, and combinations thereof.

9. The method of any one of the preceding claims, wherein the gas is an inert gas selected from argon, nitrogen, helium, neon, krypton, xenon, or a combination thereof.

10. The method of any one of the preceding claims, wherein the distance between the nozzle tip and the substrate is between about 0.01 mm and about 15 mm.

11. The method of any one of the preceding claims, wherein the velocity of the gas flow within the ablation cell, leaving the nozzle, or both, is between about 0.05 m/s to about 1000 m/s.

12. The method of any one of the preceding claims, wherein the particles are printed on the substrate as a line, a coating, or a sheet.

13. The method of claim 12, wherein the particles are printed on the substrate in a line having a width of between about 5 μm and about 1 mm.

14. The method of claim 12 or claim 13, wherein the particles are printed on the substrate in a line having a width of between about 5 μm and about 400 μm, or between about 10 μm and about 400 μm, or between about 50 μm and about 400 μm.

15. The method of any one of the preceding claims, wherein the laser is a pulsed laser with a pulse duration in the range of about 500 nanoseconds (500×10.sup.−9 s) to about 5 femtoseconds (5×10.sup.−15 s).

16. The method of any one of the preceding claims, wherein the laser has a wavelength in the range from ultra-violet (150 nm) to far infra-red (20 μm).

17. The method of any one of the preceding claims, wherein the substrate is made from glass, carbon, ceramic, silicon, metal, a polymer, or combinations thereof.

18. The method of any one of the preceding claims, wherein the feedstock material is a conductive or non-conductive material selected from a metal, a ceramic, an insulator, a polymer, or a super-conductor, or combinations thereof.

19. A dry aerosol jet printing apparatus (1) comprising an ablation cell (2) and a print head (3); the ablation cell (2) comprising a housing (4) defining a chamber (5) adapted to accommodate a feedstock material (10), a transparent window (6), a gas inlet (7), and an aerosol outlet (8); the print head (3) comprising a nozzle (3a) with a tip (3b); wherein the nozzle (3a) comprises an inner cylindrical channel (18) in fluid communication with an outer converging channel (19) that joins the channel (18) at an angle of between about 90° relative to the inner cylindrical channel (18), and a jet aperture (22).

20. The apparatus (1) of claim 19, further comprising a lens (12) located between a laser source and the ablation cell (2), and adjacent the transparent window (6).

21. The apparatus (1) of claim 19 or claim 20, wherein the nozzle (3a) comprises an outer wall (3c), an inner wall (3d), a distal end (3e) and a proximal end (3f).

22. The apparatus (1) of any one of claims 19 to 21, wherein the tip (3b) tapers inwards towards a jet aperture (22).

23. The apparatus (1) of any one of claims 19 to 22, wherein the jet aperture (22) has a diameter of between about 200 to about 400 μm.

24. The apparatus of any one of claims 19 to 23, further comprising an aerodynamic lens.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0069] The invention will be more clearly understood from the following description of an embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which:

[0070] FIG. 1 illustrates a schematic drawing of the dry aerosol jet printing apparatus of the claimed invention, showing an ablation cell (particle generator), a jet printing head, and a substrate mounted on a computer-controlled translation/rotation stage.

[0071] FIG. 2A illustrates a schematic drawing of the apparatus of the claimed invention where uniform ablation is achieved by one aspect of the invention where the target is moved while the laser is kept fixed. The focused laser beam is kept fixed while a cylindrical rod of the target material is rotated and translated beneath the focused laser beam. The aerosol is carried away by gas flow across the ablation site. FIG. 2B illustrates where the laser is moved over a flat feedstock material.

[0072] FIG. 3 illustrates a schematic drawing showing an aerodynamic lens being used in the apparatus of the claimed invention to focus the aerosol jet.

[0073] FIG. 4 illustrates silver lines printed at standoff distances of 1, 2, 3, 4, and 5 mm using the apparatus and method of the claimed invention.

[0074] FIG. 5 illustrates zig-zag line 38 cm long printed on a glass slide to measure build rate using the method and apparatus of the claimed invention.

[0075] FIG. 6 is a bar chart illustrating the comparison of build rates obtained using the laser produced dry aerosol (3 W) method of the claimed invention, and the Optomec Aerosol Jet® system of the prior art.

[0076] FIG. 7 illustrates a scanning electron microscope (SEM) image of a silver track printed using the laser produced dry aerosol method and apparatus of the claimed invention.

[0077] FIGS. 8A and 8B are SEM images of the as-printed material at two different magnifications.

[0078] FIGS. 9A and 9B are SEM images of the laser sintered printed material at two different magnifications.

DETAILED DESCRIPTION OF THE DRAWINGS

[0079] In the present invention, the dry aerosol, produced by laser ablation of a solid target of the feedstock of functional material, is enclosed in an ablation cell containing a suitable flowing gas at atmospheric pressure. The laser enters the ablation cell through a suitable transparent window. Suitable gases include inert gases, such as noble gases or nitrogen, for materials subject to oxidation; or air for the ablation of materials not subject to oxidation, such as noble metals or ceramics. The range of functional feedstock materials of interest includes metals, insulators, semiconductors, polymers and ceramics. The laser ablation is done using a repetitively pulsed laser with pulse duration in the range 500 nanoseconds (500×10.sup.−9 s) to 5 femtoseconds (5×10.sup.−15 s). The laser should be significantly absorbed by the feedstock material and should have a wavelength in the range from ultra-violet (150 nanometers (50×10.sup.−9 m)) to far infra-red (20 microns (20×10.sup.−6 m)). To obtain pulsed laser ablation the laser is focused on the surface of the feedstock material to generate a temperature causing vigorous evaporation, typically above the boiling point temperature of the material. For each laser pulse, an amount of the feedstock material leaves the surface as a vapour, and in some cases as a plasma. Uniform removal of material from the surface of the feedstock material is achieved by moving the ablation spot in a suitable manner: either by scanning the laser beam while keeping the feedstock material fixed, moving the feedstock material while keeping the laser fixed, or moving both the laser and the feedstock material.

[0080] The laser ablation process produces an ablation plume of feedstock material vapour, plasma, or nanoparticles at high pressure. The ablation plume expands into the gas, and the expansion is slowed by the counter pressure and inertia of the gas and will come to a halt when the plume dimensions are 0.1 mm to 30 mm, depending on the energy of each laser pulse and the gas density, that is, the expansion is halted by the ablation material expanding against a combination of the pressure and inertia of the gas atmosphere. (T. E. Itina and A. Voloshko, Appl. Phys. B (2013) 113:473-478). The spatial confinement of the ablation plume expansion promotes the condensation of nanoparticles. The nanoparticle ablation plume is captured by the flowing gas, forming a nanoparticle aerosol, which is carried out of the ablation cell to the jet printing head. The typical gas flow velocity over the target surface is between about 0.1 m/s (subsonic) to about 1000 m/s (supersonic). As the aerosol is transported to the printing head, coagulation of the nanoparticles will occur, forming larger particles with size in the μm range.

[0081] The stream of particulate aerosol leaving the particle generator is formed into a jet by passing it through a suitable aperture, and directed at a suitable substrate, where it is printed. The shape and dimensions of the jet are controlled by the shape and dimensions of the jet aperture. The substrate can be moved relative to the print head to form fine lines of print, or to obtain a uniform coating. The jet may be focused to smaller dimensions using an aerodynamic lens. Typically, the standoff distance between the jet aperture and the substrate is 0.1-10 mm. The substrate can be moved relative to the print head to form fine lines of deposit, or to obtain a uniform coating.

[0082] In general, the aerosol of coagulated nanoparticles will lead to the formation of a porous deposit. For most applications it will be necessary to provide some form of further processing, for example heat application, to obtain the required physical and/or electrical properties. Firstly, the sintering can be done in situ by using a laser or plasma to supply heat to the aerosol jet near the substrate, or at the printing site. The substrate may also be heated. Secondly, the sintering may be carried out in a post printing process by heating with a plasma or laser, or in an oven. The heating may be carried in a vacuum, or not, and/or in an inert atmosphere, or not, or combinations thereof.

[0083] The approach potentially allows for low temperature and conformal printing of functioning structures. These structures may be conductive lines, antennas, or sensors, for example. This allows the printing of functioning metal lines on a range of locations and configurations for different products. These metal lines can be used as conductive or signal lines. This approach can potentially greatly reduce product complexity, increase reliability and cost as these metal structures often require dedicated components (PCB's etc).

[0084] This “printed electrics” approach is considered important for the establishment of key platform technologies such as 5G, MMIC, IoT and electric power. These platform technologies will underpin consumer electronics, range finding, industrial, medical, pharmaceutical and transport markets in the future.

Apparatus

[0085] The apparatus of the claimed invention will now be described, where FIG. 1 illustrates a general embodiment of a dry aerosol jet printing apparatus of the present invention. Specifically, FIG. 1 illustrates a cross-section view of the dry aerosol jet printing apparatus of the claimed invention and is generally referred to by reference numeral 1. The apparatus 1 typically comprises an ablation cell 2 in fluid communication with a deposition or print head 3.

[0086] The ablation cell 2 comprises a housing 4 defining a chamber 5, a transparent window 6 on one side of the housing 4, a gas inlet 7 and an aerosol outlet 8. The aerosol outlet 8 is in fluid communication with the print head 3. The chamber 5 of the housing 4 accommodates a feedstock material 10.

[0087] Typically, the print head 3 is a jet printing head having a nozzle 3a with a tip 3b. The print head 3, as shown in more detail in FIG. 3, further comprises an outer wall 17 encompassing an inner cylindrical channel 18 in fluid communication with an outer converging channel 19 that joins the channel 18 at an angle of about 90° relative to the inner cylindrical channel 18. The inner cylindrical channel 18 is in fluid communication with the nozzle 3a. The nozzle 3a comprises an outer wall 3c, an inner wall 3d, a distal end 3e and a proximal end 3f. The distal end 3e comprises the tip 3b, which tapers inwards towards a jet aperture 22.

[0088] The apparatus 1 also includes a lens 12 adjacent the transparent window 6 and positioned between a laser source and the transparent window 6. The lens 12 focuses a laser beam (arrow C) before it enters the transparent window 6 to act upon the feedstock material 10 within the chamber 5 and generate ablated feedstock material.

Particle Generator/Ablation Cell

[0089] The particle generator or ablation cell 2 is described above and contains a solid mass of the feedstock material 10 to be printed. A vapour, or plasma, of the feedstock material is formed by pulsed laser ablation. The ablation plume of abated material is confined by the counter pressure and inertia of the gas, followed by condensation of the vapour to form nanoparticles in the size range of about 1 to 150 nm, preferably between about 3 to about 100 nm, and finally coagulation of the nanoparticles to form nanoparticle aggregates in the size range of about 100 nm to about 35000 nm. The nanoparticle aggregates are mixed with the gas and carried out of the ablation cell 2 as an aerosol jet flow (as per arrow B in FIG. 1). A virtual impactor may be used to increase the particulate concentration in the aerosol flow. The transparent window 6 is used to admit the laser beam. The material of the transparent window 6 is not critical, so long as it is transparent. Materials which can be used are, for example, quartz, sapphire, glass, and the like.

[0090] The flow of gas (arrow A) through the ablation cell 2 carries the ablated feedstock material to the print head 3 via the aerosol jet flow (arrow B), and on to a substrate 14 where it is deposited. To obtain uniform printing, or produce a printed pattern, the substrate 14 is moved relative to the print head 3 using a computer-controlled translation/rotation stage 16. Uniform removal of material from the surface of the feedstock material 10 is achieved by moving the ablation spot on the feedstock material in a suitable manner: (i) moving the feedstock material 10 while keeping the laser fixed, (ii) scanning the laser beam (arrow C) while keeping the feedstock material 10 fixed, or (iii) moving both the laser and the feedstock material 10. FIG. 2A shows the scheme (i) whereby the focused laser beam (arrow C) is kept fixed while a cylindrical rod of the feedstock material 10 is rotated (arrow D) and translated (arrow E) beneath the focused laser beam (arrow C). For (ii), as shown in FIG. 2B, a flat feedstock material is held stationary in the ablation cell 2 and the focused laser beam (arrow C) is scanned across the feedstock material 10 using a computer-controlled laser beam scanner 50. The flow of gas (arrow A) is consistent in both FIG. 2A and FIG. 2B.

Jet Printing

Without Aerodynamic Focusing

[0091] The stream of particulate aerosol leaving the ablation cell 2 is formed into an aerosol jet stream 20 (see FIG. 3) by passing it through the jet aperture 22, and directed at the substrate 14, where it is printed. The shape and dimensions of the aerosol jet stream 20 are controlled by the shape and dimensions of the jet aperture 22. The substrate 14 can be moved relative to the print head 3 to form various deposition patterns, or a uniform coating. Alternatively, the print head 3 can be moved relative to the substrate 14, or both the substrate 14 and the print head 3 can move relative to each other.

With Aerodynamic Focusing

[0092] The aerosol jet stream 20 may be focused to smaller dimensions using an aerodynamic lens, based on a coaxial nozzle, as shown in FIG. 3. The aerosol flow (Q.sub.a) (arrow F) enters the inner cylindrical channel 17, while the sheath gas flow (Q.sub.s) (arrow G) enters the outer converging channel 19. The diameter (D.sub.j) of the aerosol jet stream 20 leaving the nozzle 3a is given by D.sub.j=D.sub.n√{square root over (Q.sub.a/Q.sub.t)} where Q.sub.t is the total gas flow, given by Q.sub.t=Q.sub.a+Q.sub.s. The sheath gas also acts to prevent printing of aerosol particulates on the inner surface of the inner wall 3d of the nozzle 3a. Typically, the standoff distance between the jet aperture 22 and the substrate 14 is 0.1-10 mm. The substrate 14 can be moved relative to the print head 3 to form various deposition patterns, including fine lines with widths down to about 5 μm. Alternatively, the print head 3 can be moved relative to the substrate 14, or both the substrate 14 and the print head 3 can move relative to each other.

Working Distance

[0093] The working distance of the laser from the substrate 14 was estimated by visually inspecting the deposition at nozzle 3a to tip 3b distances of 1, 2, 3, 4, and 5 mm. A laser power of 3 W, speed RPM of 90, and flow rates of 33 CC/min and 100 CC/min were used for the carrier and sheath gas, respectively.

Build Rates

[0094] The build rate was measured by spraying a ˜38 cm line onto a glass slide and measuring the mass difference (Δm) using the same parameters as above.

Results and Discussion

[0095] FIG. 4 shows some examples of silver lines printed with the apparatus 1 of the claimed invention. A high repetition rate fibre laser operating at a wavelength of about 1 μm (1.065 μm) and a pulse duration of about 250 ns was used. The laser was operated at a pulse repetition rate of about 5 kHz and the average power was 3 W. The laser was focused using a 20 cm lens. The lens was positioned so that the surface of the silver target was about 1 mm beyond the focus, giving a peak fluence in the laser spot of about 20 J cm.sup.2, which is sufficient to cause ablation of the feedstock material. The feedstock material was rotated at 90 RPM and translated at between about 0.005 cm s.sup.−1 and about 1 cm s.sup.−1. Argon gas at atmospheric pressure was fed into the particle generator at a rate of 33 standard cubic centimeters per minute (sccm). This gas flow carries the silver aerosol into the print head, which was equipped with an aerodynamic lens to reduce the diameter of the aerosol jet leaving the nozzle. The diameter of the nozzle was 300 microns. The sheath gas was fed into the aerodynamic lens at a rate of 100 sccm. Lines were printed on the glass substrates using a scan rate of 5 mm s.sup.−1, and various values of standoff distance. FIG. 4 shows lines printed at standoff distances of 1, 2, 3, 4, and 5 mm.

[0096] The measured build rate was compared with the typical rate obtained using the Aerosol Jet® system. (King, Bruce H., Michael J. O'Reilly, and Stephen M. Barnes. 2009. “Characterizing Aerosol Jet® Multi-Nozzle Process Parameters for Non-Contact Front Side Metallization of Silicon Solar Cells.” Conference Record of the IEEE Photovoltaic Specialists Conference, 001107-11. https://doi.org/10.1109/PVSC.2009.5411213). It should be noted that the laser power was only 3 W. Pulsed lasers with average power up to about 300-500 W are readily available, and, with average powers increasing all the time, the build rate can be correspondingly increased. FIG. 6 clearly shows that the build rate using the apparatus of the claimed invention is superior to that of the build rate using an apparatus of the prior art.

[0097] FIG. 7 shows a scanning electron microscope (SEM) picture of silver track printed using the apparatus of the claimed invention. The track width varies from 150 micron to 200 micron. According to the aerodynamic focusing equation (see above), using an aerosol flow (Q.sup.a) of 33 sccm, a sheath gas flow (Q.sub.s) of 100 sccm, and a 300 micron nozzle, the expected line width is 150 micron.

[0098] FIG. 8A and FIG. 8B show an SEM image of the as-printed material at two different magnifications. The lower magnification image reveals the highly porous nature of the print as would be expected for the assembly of aggregated metal nanoparticles. The individual nanoparticles can be seen in the higher magnification image. The larger highly spherical particles seen in this image are most likely due to liquid droplets expelled from the target during the ablation process.

Electrical Resistivity Measurements

[0099] The electrical resistance of the printed tracks was measured by printing a track between two silver paint pads separated by 15 mm and using a multi-meter to measure the resistance. For a 15 mm long line printed at a mass loading of 2.3×10−3 mg mm.sup.−1, the resistance was found to be 390 ohms (Ω), which corresponds to 26 Ωmm.sup.−1. The resistance of a 15 mm long bulk silver line at the same mass loading is 1.1Ω, which corresponds to 0.073 Ωmm.sup.−1. As expected, the resistance of the as-printed tracks have a much higher resistance than bulk silver lines with the same mass loading.

[0100] Various sintering methods were investigated to reduce the electrical resistance of the silver tracks. These were: (i) furnace annealing in air at 200° C. for 1-4 hours, (ii) irradiation with a 100 W infra-red lamp focussed through a 10 mm diameter output aperture and scanned over the silver tracks at 5 and 2.5 mm s.sup.−1, and (iii) irradiation with a 45 W continuous CO.sub.2 (10.6 μm) laser focussed to a 6 mm diameter spot and scanned over the silver tracks at 10 mm s.sup.−1. The largest reduction in resistance was obtained using laser sintering where it was found that resistance of a 15 mm long track decreased from 350Ω before sintering, to 32Ω after laser sintering. This corresponds to 2.1 Ωmm.sup.−1, which is about 30 times higher than a bulk silver line with the same mass per unit length. With this method, it is possible to achieve a resistivity between the bulk value of the material being printed and approximately 1000 times higher of the bulk value.

[0101] Visually, the as-deposited tracks appear black, but have a shinier appearance after annealing. FIG. 9A and FIG. 9B show two different magnification images of the laser sintered material.

[0102] The advantages of the described invention are, for example, that the particles are uncontaminated when ablated, unlike the particles of the prior art which are contaminated by a fluidic carrier medium. This improves the options, efficiency and ease for post-processing treatment if required. Using a laser for the ablation step, and by separating the particle generation from the particle carrier gas flow, the method significantly improves process control over the size of the particles that can be generated, without affecting the carrier flow dynamics. This allows for the creation of very small particles. Typically, these particles are very difficult to handle and are very expensive. However, the claimed invention allows for ease of handling and associated hugely reduced costs. Smaller particles are known to sinter much easier and at lower temperatures. Particle energy can be varied by changing the laser parameters depending on applications—for example, to improve adhesion or ease of sintering. The system could be combined with an additional sintering system to facilitate sintering of the printed particles within the inert gas flow environment (for example, a sintering laser focussed on the target print point).

[0103] In the specification the terms “comprise, comprises, comprised and comprising” or any variation thereof and the terms “include, includes, included and including” or any variation thereof are considered to be totally interchangeable and they should all be afforded the widest possible interpretation and vice versa.

[0104] The invention is not limited to the embodiments hereinbefore described but may be varied in both construction and detail.