1 © 2015 IOP Publishing Ltd Printed in the UK

Journal of Micromechanics and Microengineering

M T Rahman et al

Printed in the UK

107002

JMMIEZ

© 2015 IOP Publishing Ltd

2015

25

J. Micromech. Microeng.

JMM

0960-1317

10.1088/0960-1317/25/10/107002

10

Journal of Micromechanics and Microengineering

1. Introduction

Aerosol Jet micro-additive manufacturing has emerged as an environmentally friendly method of fabricating 3D electronic structures at length scales down to 10 µm [1]. The addi-tive fabrication has inherent advantages since no material is

wasted and the method does not require the use of harmful chemicals as in the case for lithography based techniques. The Aerosol Jet method also allows a standoff height of up to 5 mm allowing printing on complex 3D surfaces. Electronic structures at sub-mm length scale are of great interest to be used as passives as well as antennas for wireless high-speed

Aerosol based direct-write micro-additive fabrication method for sub-mm 3D metal-dielectric structures

Taibur Rahman1, Luke Renaud2, Deuk Heo2, Michael Renn3 and Rahul Panat1

1 School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164, USA2 School of Electrical Engineering and Computer Science, Washington State University Pullman, WA 99164, USA3 Optomec Inc, St Paul, MN 55114, USA

E-mail: rahul.panat@wsu.edu

Received 19 March 2015, revised 17 July 2015Accepted for publication 6 August 2015Published 21 September 2015

AbstractThe fabrication of 3D metal-dielectric structures at sub-mm length scale is highly important in order to realize low-loss passives and GHz wavelength antennas with applications in wearable and Internet-of-Things (IoT) devices. The inherent 2D nature of lithographic processes severely limits the available manufacturing routes to fabricate 3D structures. Further, the lithographic processes are subtractive and require the use of environmentally harmful chemicals. In this letter, we demonstrate an additive manufacturing method to fabricate 3D metal-dielectric structures at sub-mm length scale. A UV curable dielectric is dispensed from an Aerosol Jet system at 10–100 µm length scale and instantaneously cured to build complex 3D shapes at a length scale <1 mm. A metal nanoparticle ink is then dispensed over the 3D dielectric using a combination of jetting action and tilted dispense head, also using the Aerosol Jet technique and at a length scale 10–100 µm, followed by the nanoparticle sintering. Simulation studies are carried out to demonstrate the feasibility of using such structures as mm-wave antennas. The manufacturing method described in this letter opens up the possibility of fabricating an entirely new class of custom-shaped 3D structures at a sub-mm length scale with potential applications in 3D antennas and passives.

Keywords: aerosol jet printing, micro-additive manufacturing, additive manufacturing, 3D antennas, chip-to-chip communication, wearable devices, Internet of Things (IoT)

(Some figures may appear in colour only in the online journal)

Technical Note

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doi:10.1088/0960-1317/25/10/107002J. Micromech. Microeng. 25 (2015) 107002 (8pp)

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data transfer for emerging applications such as wearable devices, biometric devices, on-chip electronics for chip-to-chip, and intra-chip communication. Conventional antennas and passives are typically limited to 2D surfaces due to the planar photolithography used in the conventional fabrica-tion processes. Note that the current 2D planar technologies force designers to use differential antennas to achieve desired directivity [2] that increases power consumption and reduces system efficiency either due to extra circuitry used to gener-ate the differential output signal, or the use of lossy baluns to convert single-ended amplifier outputs to differential feeds. In addition, the existing 2D electronic elements are imple-mented in the top metal layers of an integrated circuit chip, while the physical space that the radio waves propagate in is the top-metal-layer-region as well. Fabrication of 3D metal-dielectric structures away from the top metal layer in IC chips are thus necessary for efficient low-power signal transmission in antenna applications and low energy loss in the case of passives.

To date, researchers have attempted to achieve sub-mm length scale 3D metal-dielectric structures by: (a) manu-facturing metal lines in 2D over a dielectric substrate and folding them into 3D shapes similar to an origami [3–6], or (b) printing metal lines on a pre-manufactured 3D surface [7]. The Origami folding of the substrate can result in 3D shapes only if the metal traces at the origami folds are com-pliant enough to accommodate the extreme strain at or near the Origami folds. In addition, the number of 3D shapes are rather limited. Finally, the degree of folding of Origami is difficult to control due to the inherent structural instabilities and the nonlinear stress—strain response of these structures [8] leading to an unpredictable response over the operat-ing frequencies in the case of its use in antennas [3, 4]. The approach in (b) suffers from the fact that the metal lines need to be stretched well beyond the yield/ultimate tensile strength of metals to reach the final shapes, requiring ser-pentine structures and thus increasing the device resistance. Direct write processes such as inkjet printing are also avail-able for 2D fabrication of antennas structures [9–11]. Inkjet printing is cheap and allows improved layout flexibility such as post-process deposition of thick dielectric layers and a relatively improved isolation from the substrate. Cook et al [10] demonstrated inkjet fed patch antenna at 24 GHz with a gain of 7 dBi. Adams et al [11] showed electronic traces on a pre-manufactured curved surface at a length scale of up to a centimeter by the inkjet printing technology. Producing complex shapes or geometries along with traces, however, is difficult with inkjet printing at sub-mm length scale mainly because of the passive drop-on-demand nature of the inkjet process.

In this paper, we demonstrate a novel Aerosol Jet micro-additive manufacturing method to fabricate custom-shaped sub-mm length-scale metal-dielectric structures. A UV cur-able dielectric is dispensed over a substrate (Si or otherwise) [12, 13] where the dispense length scale is 10–100 µm. A sim-ulation study is also carried out to identify the antenna-like structures to be fabricated using the proposed method. The paper demonstrates the feasibility of fabricating complex 3D

printed structures at sub-mm length scale using the aerosol jet micro-additive manufacturing method.

2. Simulations

Simulations can be used to establish microstructures that can act as mm wave antennas in 3D with superior directiv-ity and gain. The Aerosol Jet micro-additive manufacturing method will be described later to demonstrate the fabri-cation of such structures. The electrical simulations were performed using the software, Ansoft HFSS. For the model, micro-strip transmission lines are used to feed two resonant quarter-wave monopole antennas printed onto dielectric support pillars (figure 1). The antennas are designed for input resonance around 60 GHz, and produce a directional gain pattern through the formation of an on-chip antenna array. The relative permittivity of the dielectric was taken as 4.023 (from Loctite 3105 data sheet, Henkel Inc, used in the fabrication section described later), while the loss tan-gent at 0.031. The resistivity of the metal lines was taken to be 9 × 10−8 Ω m. Note that the gain is a key performance figure which describes an antenna’s directivity. The design of such a system, though trivial in apparent complexity, is unavailable to designers with existing fabrication technolo-gies. The example structure proposed for use in the wireless system (figure 1) currently fits within a 4 mm by 2 mm foot-print. Figures 2(a)–(c) show the gain pattern of the antenna system in various directions. Higher gains indicate direc-tions where the signal is radiated out with higher relative power to a mathematically ideal uniform isotropic radiator. Figure 2(a) shows full radiation pattern assuming far field operation with an antenna placed at the origin. Figure 2(b) shows the gain pattern along the XY-plane corresponding to energy radiated towards other structures placed on the same plane. Figure 2(c) shows gain versus elevation angle, look-ing in the forward ( �φ = ° −y90  or   ) direction. The 0º and

Figure 1. 60 GHz 2-element monopole array used for simulations.

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180º corresponds to radiation vertically into the substrate

below the antennas and air directly above the antennas, respectively; while 90° corresponds to the desired direc-tion to another antenna. Figure  3 shows the gain in the

θ φ= ° = °90    270 (or equivalently �−y ) direction as the

input frequency is varied. The marked points at −1.5 dB show the frequencies at which the antenna’s radiated power is half that of the peak output shown at 65 GHz. As modula-tion produces a bandwidth at RF frequencies equivalent to the frequency content of a baseband signal, wide bandwidth of RF radiation structures are required to prevent distortion and a consequent increase in bit-error rate of high speed signals. Figure  3 shows that the system achieves a usable gain of roughly 1.5 dBi while providing a −3 dB bandwidth in excess of 35 GHz, targeted for 60 GHz. The simulation results indicate the benefits brought by moving the antenna off of top interconnect metal such that it is effective at RF bandwidth beyond the capabilities of the current state-of-the-art 2D antennas.

3. Fabrication process

3.1. Materials

The primary material sets used to create the 3D antenna-like structures include a metallic nanoparticle ink and a UV curable dielectric. Silver nanoparticle ink was used (Advanced Nano Particle Inc., Fremont, CA, AS1:2), while the dielectric mate-rial used was a UV curable Acrylic Urethane (Loctite 3105, Henkel Inc.). The size of the metal nanoparticles had about 50 wt % ±5% particle loading with particle size of 50–100 nm with a viscosity of about 160 cP at 0.4 rpm. The dielectric material has a viscosity of 300 cP. A minimum nanoparticle curing regimen was applied at 130 °C/2 h. The resistivity measured in these conditions is about 9 × 10−8 Ω m or about 20% of the bulk silver (Optomec data sheet). Glass and a functional Si chip were used as the substrate. The substrates were cleaned with DI water followed by isopropyl alcohol. An atmospheric plasma at 100 W for 1 min was then applied to the substrate to improve the uniformity of the printed patterns.

Figure 2. Gain pattern of the antenna system for various directions for the structure in figure 1. Higher gains indicate directions where the signal is radiated out with higher relative power to a mathematically ideal uniform isotropic radiator. (a) Full 3D radiation pattern assuming far field operation with an antenna placed at the origin, (b) gain pattern along the XY-plane (θ = °90 ) corresponding to energy radiated towards other structures placed on the same plane, and (c) gain versus elevation angle, looking in the forward (φ = °270 or equivalently

ˆ−y) direction. 0° and 180° correspond to radiation vertically into the substrate below the antennas and air directly above the antennas, while 90° corresponds to the desired direction to another antenna.

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Before printing the structures, ink material was rolled in on a roller (SCILOGEX MX-T6-S Analog Tube Roller) for 24 h to minimize particle agglomeration per the manufacturer guidelines.

3.2. Manufacturing process

The manufacturing process to fabricate the antenna-like struc-tures shown in figure 1 involved the in situ curing of a dispensed dielectric material at the micro-scale followed by direct write of metal nanoparticle inks on vertical as well as horizontal walls of the dielectric using a tilted nozzle head. Such a direct write process requires that the dielectric (at 300 cP) as well as the metal nanoparticle ink be jetted at a length scale of tens of micrometer. We used the Aerosol Jet micro-additive manufacturing machine to print materials directly onto a base substrate. The main components of an Aerosol Jet sys-tem are shown in figure 4 and includes atomizers (ultrasonic and pneumatic), a programmable XY motion stage with an accuracy of ±6 µm, two deposition heads, a laser (700 mW, 830 nm IR laser) and a UV light attachment (Panasonic Spot Curing System, Aicure UJ35). The nanoparticle ink is placed in the ultrasonic/ pneumatic atomizer, which creates a dense vapor of the ink with droplet sizes of about 1–5 µm that is transferred to the deposition head with the help of a gas flow running through the atomizer towards the deposition head. The mist stream is then focused by a secondary gas flow to form the micro-jet [14, 15]. The printed material can be sintered in a programmable oven. The UV was to instantaneously cure a

UV curable polymer dielectric and create 3D shapes (similar to that shown in figure 1) directly over the substrate. Note that the stand-off distance between the substrate and the nozzle tip along with the jetting from a tilted nozzle allows the printing over a curved surface—a unique capability for the Aerosol Jet direct write processes. The Aerosol Jet micro-additive printing method has been used in the manufacturing process for solar cells [16, 17], microelectronics [18, 19], and sensors [20–22]. The nozzle exit diameter for the polymer posts was 200 µm and for the printed silver lines it was 150 µm (the minimum line width is about 10 times smaller than the nozzle diameter based upon the sheath gas pressure). The nozzle was tilted approximately 45° from vertical for the sidewall printing. The dielectric printing used a pneumatic atomizer with an air pres-sure of 3 psi and a sheath gas pressure of 0.13 psi. We also used a virtual impactor that serves to concentrate the aerosol in-between the atomizer and print head. The silver nanoparticle ink used ultrasonic atomizer with a gas pressure of 0.25 psi, and a sheath gas pressure of 0.25 psi. The sheath air pressure for the silver printing is higher than that for the polymer print-ing because of the smaller nozzle diameter (150 µm versus 200 µm). The standoff distance between the chip and the tip was about 5 mm. Note that the overspray and hence the print-ing quality can depend upon (a) the standoff distance (lower overspray for lower standoff distance), (b) ratio of sheath gas pressure to the atomizing pressure for the ultrasonic atomizer, and (c) the combination of sheath gas pressure, atomizing pressure and exhaust pressure for the pneumatic atomizer. The optimized parameters such as sheath gas pressure, atom-izing pressure and exhaust pressure may vary for different inks based upon their viscosity, particle size and the solvent used. The process parameters used in the current study were optimized for the nanoparticle ink and the 3D structures to be manufactured. Note that the major advantage of the micro-additive manufacturing methods is that it does not require the use of environmentally harmful chemicals used in subtractive (e.g. lithographic or MEMS) techniques. In addition, the pro-posed method does not create any material waste.

4. Discussion

Figures 5 and 6 show the fabricated antenna-like structures directly over a substrate using the proposed method. A dielec-tric cylinder is created by dispensing the UV curable Acrylic Urethane from the deposition head followed by its instanta-neous curing using the UV light. This arrangement allowed the fabrication of dielectric substrates with extremely high aspect ratio (up to 1 : 20). A schematic is shown in figure 5(a), while a fabricated hollow dielectric cylinder structure is shown in figure 5(b). The average pillar height is about 1.2 mm, the pil-lar diameter is about 1 mm, while the wall thickness is about 58.5 ± 2.5 µm. The tilted head of the Aerosol Jet AJ300 sys-tem used to deposit the metal lines is shown in figures 5(c) (schematic), and figure  5(d) (structure). The metallic inter-connects are approximately 22 µm in width. After printing the structure, the nanoparticle traces were thermally sintered. Note that the dielectric permittivity can be easily modulated by

Figure 3. The gain in the θ = °90 φ = °270   direction as the input frequency is varied for the microstructure shown in figure 1. The marked points at −1.5 dB show the frequencies at which the antenna’s radiated power is half that of the peak output shown at 65 GHz. As modulation produces a bandwidth at RF frequencies equivalent to the frequency content of a baseband signal, wide bandwidth of RF radiation structures is required to prevent distortion and a consequent increase in bit-error rate of high speed signals.

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mixing it with high permittivity particles prior to deposition. Functionally graded dielectric materials can help control the electrical characteristics of antennas, interconnects, and other RF structures and opens up an additional dimension of con-trol to the electrical designers. Additional antenna elements were also fabricated using the same micro-additive fabrication method. An antenna-like monopole structure with high aspect ratio over a glass substrate (similar to that shown in figure 1) is shown in figure 6. The figure 6(a) shows a schematic of the Aerosol Jet micro-additive manufacturing process to create

the monopole structure, while figures 6(b) and (c) show the structures fabricated with 75 µm dielectric pillar and a 25 µm metal trace (the pad dimension in these perspective images is 400 µm). Figure 6(d) shows periodic dielectric pillars directly fabricated over a Si chip. Clearly, the novel manufacturing method can fabricate the 3D antenna-like structures at sub-mm length scale.

The metal nanoparticles printed and sintered on the die-lectric were characterized for roughness using atomic force microscopy. The surface roughness is highly important at

Figure 4. Micro-additive printing mechanism of the Aerosol Jet manufacturing method.

Figure 5. Sub-mm length scale, custom made 3D metal-dielectric. (a) Schematic of the dielectric pillar, (b) image of instantaneously cured dielectric pillar. (c) Schematic of the process to write the metal lines, and (d) image of the vertical metal lines onto the wall of the dielectric pillar, (e) top-down image of the cylinder in (b) and (d) with scale bar.

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high frequencies to determine the losses due to ‘skin effect’ observed in microelectronics. At high frequency, the current may concentrate along the skin of the conductor leading to resistive losses [23]. The skin depth has to be much greater than the surface undulations for this effect to be negligible. The skin depth, δ, in meters, is given by,

δ ρ≈ μ f503  / ,r( ) (1)

where, µr, is the relative permeability of the medium, ρ is the resistivity in SI units, and f is the frequency. The skin depth at 60 GHz is about 1.2 µm for sintered silver nanoparticles. The observed roughness of four traces shown in figure  6(b) are shown in figure 7. Figure 7(a) shows the actual metallic trace surface profile, while figure 7(b) shows the same profiles as in figure 7(a) after the moving average has been subtracted from the profile to get a roughness estimate. Clearly, the roughness range is of the order of <300 nm (i.e. less than ±150 nm). The skin effect is thus not expected to affect the antenna perfor-mance at frequencies of interest in the present study. Note that the trace width was within about 10% of the mean for 5 printed traces, a variation comparable to the traces obtained using lithography. The variability in trace width, height and roughness can potentially affect the device performance and will be investigated as part of a future study.

The sintering bonds the nanoink particles in a coherent or organized way by the mechanism of mass transport in atomic scale [24]. The driving force for sintering of micro/nanopar-ticles of a system is the chemical potential gradient due to surface curvature. Different mass transport phenomena are active in the material movement in the sintering particles.

Figure 7. (a) Surface profile of the metallic trace shown in figure 6(b). (b) The same profiles as in figure 7(a) after the moving average is subtracted from the profile to get the roughness estimate.

Figure 6. (a) Schematic of the Aerosol Jet micro-additive manufacturing process to create the monopole structure, (b), (c) fabricated antenna micro-structures, (d) periodic dielectric pillars directly fabricated over a Si chip.

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Generally, the six dominant mass transport mechanisms are active during sintering are: (i) evaporation and condensation (EC), (ii) surface diffusion (SD), (iii) grain boundary diffusion (GDB), (iv) volume diffusion from the surface of the particle (VDS), (v) volume diffusion from the interior of the particle (VDV) and (vi) viscous flow (VF). In SD, EC and VDS fills the area between the particles. As a result of sintering, the mass center of the particles approach towards each other cre-ating a continuous film. A schematic of the mechanism and an example of the sintered Ag nanopowders are shown in figure 8 along with sintered nanoparticles obtained by the PIs using an Ag ink used to create the structures in figures 5 and 6.

5. Conclusion

We have successfully demonstrated a novel additive manu-facturing method to fabricate 3D metal-dielectric structures at a sub-mm length scale. A UV curable dielectric is instan-taneously cured upon dispense from an aerosol jet system to form the required 3D shapes at length scale down to 10–100 µm. A metal nanoparticle ink is then dispensed over the com-plex dielectric shapes, also at a length scale down to 10 µm by an Aerosol Jet followed by thermal sintering techniques. The proposed method, being additive, avoids the use of harm-ful chemicals in the fabrication process and reduces material wastage. The microstructures demonstrated in the paper have potential applications in 3D passives and mm-wave antennas. Antenna simulations are carried out on some of the fabricated geometries and the results indicate superior gain and directiv-ity for the antenna with the proposed fabrication approach. The fabrication method described in this letter opens up the possibility of environmentally conscious manufacturing of an entirely new class of custom-shaped 3D metal dielectric struc-tures at a length scales down to 10 µm.

Acknowledgment

The work was supported by the WSU startup funds for RP. We thank Mr Urs Burger and Dr Kurt Christenson of Optomec Inc for their help during the experiments.

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Figure 8. (a) Various mass transport mechanism involved in sintering process, (b) a metal trace with sintered nanoparticles.

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