For printed electronics with digital additive printing
Advanced Metallization with Highly Conductive Ag, Au, and Pt Metal Complex Inks
Problem:
As the world of electronics continues to change shape - literally - and products become wearable, flexible, foldable and capable of processing data at the same time, the demand for the tiny circuitry making it all possible has hit a tipping point creating a need for new solutions. In addition, the drive for innovation and a secure supply chain in semiconductor packaging and biomedical devices is of core importance today. One of the most fundamental components and an emerging lever for innovation in additive manufacturing is conductive inks. Currently metal inks, such as silver (Ag), copper (Cu), nickel (Ni), platinum (Pt) and gold (Au), are widely used for circuits, gas, thermal and biological sensors due to their high electrical conductivity, catalytic activity and high corrosion resistance. However, traditional metallic inks are based on metal nanoparticles (NPs): these inks contain colloidal NPs suspension captured by ligands to prevent agglomeration. Performance of these inks is degraded by their high electrical resistivity and short shelf life. Additionally, making NPs based Ag, Pt and Au inks is expensive and not environmentally friendly.
Solution:
Particle-free Cu, Ni, Ag, Au and Pt conductive inks from Electroninks, Inc (Austin TX). Electroninks has developed particle-free metallo-organic -based inks that have several performance and reliability benefits over nanoparticle-based inks. Particle-free inks consisting of metal-organic precursors typically have better performance because they reduce metal films more cleanly, often at lower temperature. Because the final films do not have organic surfactants, etc., they tend to survive stress/ adhesion tests, and reliability to MSL and aerospace standards.
II. BACKGROUND
Never has there been greater pressure to pack more electronics - in all shapes, sizes and levels of power - into a wider range of consumer and industrial products than there is today. From foldable displays in new 5G flip phones to medical sensors in athletic wear to RFID tags on every item in every container entering every port around the world, powerful tiny circuitry is making it possible.
While Moore’s Law is a major enabler in fueling this trend, equally critical is advances in additive manufacturing. These advancements are creating a wealth of possibilities for low cost and highly functional microelectronics. One of the most fundamental components and an emerging lever for innovation in additive manufacturing is conductive inks.
This white paper details new formulations for advanced, high- performance conductive metal inks that Electroninks is developing to expand its proprietary line of particle-free conductive inks.
Currently, silver inks are being effectively used to make electrodes. However, due to electromigration at relatively high temperatures and humidity conditions, silver inks cannot be used for all applications. Other noble metal inks, such as those based on platinum (Pt) and gold (Au), are widely used for protective circuits, gas, thermal and biological sensors due to their catalytic activity and high corrosion resistance.
But the More Conventional Approaches to Metallization – technique of coating metal on the surface of objects – have limitations. These traditional approaches that have been used for years do not perform as well at certain temperatures, can be highly toxic, have low viscosity and have other tooling issues making them impractical for high volume manufacturing.
For example, traditional gold and platinum inks are based on metal nanoparticles (NPs): these inks contain colloidal NPs suspension captured by ligands to prevent agglomeration. Performance of these inks is degraded by their high electrical resistivity and short shelf life. Additionally, making NPs based Pt and Au inks is expensive and not environmentally friendly.
Customer Benefits:
● Multi-hour printing with fine features can be attained with particle-free inks
● Higher performance using less precious metals allows for more sustainable, cost effective products
Enter New Gold (Au) Particle-Free Inks From Electroninks: Now, there’s another way: metallo- organic precursors. This useful material is gaining attention for preparing particle-free conductive ink formulations. But what are they? Metallo-organic precursors are a chemical compound with a metal atom and one or more organic ligands that are connected to the metal atom through various functional groups.
Metallo-organic compounds can be designed to decompose at various temperature ranges by changing the strength of the complex consisting of the metal and organic compound. Still, gold inks have been used for printing conductive features. Most of them use conventional nanoparticle-based formulations. Electroninks has developed particle-free metallo- organic based precursors that have several benefits over nanoparticle-based inks. Particle-free inks consisting of metal-organic precursors decompose more cleanly and often at lower temperature. Multi-hour printing with fine features can be attained with particle-free inks.
Metallo-organic inks also have higher shelf life than nanoparticle-based inks. In this white paper, we describe how gold(I)amine-based particle-free ink formulations are being used to produce highly conductive fine gold lines by aerosol jet printing technology.
New Platinum (Pt) Particle-Free INks From Electroninks: Also gaining attention is the deposition of metallic platinum thin film. It is being used in a wide range of applications in electronics, including forming contacts in microelectronic devices, high temperature electrochemical, and catalytic applications. Compared to more common noble metals such as gold and silver, application of platinum is challenging and related reports are sparse. The common deposition method of platinum includes gas-phase deposition, electrochemical, and electroless deposition methods. These conventional processes are often impractical and difficult for large scale production.
Table 1: Comparison of NPs based Ag, Au and Pt inks with Electroninks particle-free Ag, Au and Pt inks
III.AEROSOL JET PRINTING PROCESS
Aerosol jet printing is one of the non-contact deposition methods. The schematic of this printing process involves jetting out an ink mist produced by ultrasonic or pneumatic atomization. Electroninks Au and Pt inks are specifically designed for ultrasonic atomization which can attain higher printing resolution compared with the pneumatic atomization process. The process of printing circuits, sensors, or interconnects with an aerosol jet printer (OPTOMEC system or IDS nanojet system) using Electroninks particle- free Au or Pt ink is mostly decided by customers and substrates. However, a typical process is described below:
Preparation – substrate preparation and cleaning are influential for less defects and good adhesion of the printed patterns to the substrate surface. Take a flat surface without any surface mounted substrate as an example: the substrates will be cleaned with a common solvent to remove any residues or dust from the surface. The substrate will thengothroughadehydrationbakesteptoevaporatethesolvent. Other cleaning or preparations can also be applied in this step such as UV ozone and plasma.
Mapping Out and Size Correction – the substrate will be placed on the printer platen. The alignment camera in the printer will precisely measure the surface dimension which will be used to map out the surface through AutoCAD®. The customer usually has the CAD design of the substrates, but this step is still necessary to align the CAD design and the real substrate size. Size modification/ correction is necessary if offset is detected from the CAD file and real measurement.
Design – The patterns will be designed based on the mapped out/ modified surface in AutoCAD. The pads can be printed by both serpentine and perimeter fill.
Printing – Electroninks particle-free Au/Pt inks are aerosol jetted through the ultrasonic atomizer. Due to the novel ink formulation design the volatile solvent will evaporate through the journey from atomizer to the printing heads and the chemical reaction creates a thin Au/Pt precursor film on the substrate surface. The Au/Pt precursor film is tacky and gel-like, so the printing can achieve a good printing quality even at a RT platen. The ink stream width can be controlled by a combination of printing tip, sheath gas, carrier gas, and printing speed. Multiple layers are applied to achieve the desired film thickness/OPS without any wait time between layers.
Electroninks Pt ink can be UV cured and achieve comparable or better electrical properties.
IV. IDS AEROSOL JET SYSTEM
Integrated Deposition Solution (IDS) has developed the next generation aerosol jetting print head technology. An image of the desktop printer and the NanoJet print head are shown in Figure 1. This jetting process, trademarked NanoJet, relies on both hydrodynamic and aerodynamic focusing to be able to collimate and focus a broad aerosol droplet size distribution resulting in printed lines with exceptional printed edge quality. The NanoJet technology relies on the use of multiple aerodynamic lenses to be able to focus the finer aerosol droplets in an aerosol stream that would have historically led to overspray at the printed line edges. Some examples of NanoJet aerosol printed lines using the Electroninks inks will be presented in part V.
Figure 1: Photograph showing (a) the NanoJet desktop printer and the next generation (b) Nanojet aerosol print head developed at IOS.
V. AEROSOL JET PRINTED GOLD AND SILVER STRUCTURES
Gold printing through an IDS aerosol jet system: Electroninks particle-free Au ink has around 4-10 wt.% solid fraction, but the mass output is competitive with NPs based inks. The ink is concentrated in the atomization and high vapor pressure solvents will be removed though the journey from the ultrasonic atomizer to the printing head to ensure a decent mass output. The deposited ink is visually translucent and tacky on the substrate as shown in Figure 2(a). Due to this novel ink formulation design, Electroninks particle-free Au ink can be printed at a room temperature platen with a relatively high mass output. This property is critical when printing on conformal surfaces. Figure 2(b) shows a 4-point resistivity structure printed by Electroninks particle-free Au ink through the aerosol jet system. The printed structure is thermal cured at 300° C for 1 hour to achieve the best electrical conductivity.
The SEM of above gold samples are shown in Figure 3. Although necking formation and fusing gold clusters are observed from the top side SEM in Figure 3(a), the average particle size is estimated at 100nm. The cured gold film is packed densely from the cross-section SEM in Figure 3(b), final thickness is measured at 900nm.
Figure 2: Au ink printing on glass (a) before thermal curing; (b) after curing at 300C for 1h
Figure 3: (a) Top side; (b) cross section SEM images of Au sample per conditions in chart
Overspray and satellite spots are inevitable in aerosol jet printing. The overspray and satellite spots are caused by less weight droplets with insufficient inertia to be focused by the centra sheath gas and deposited along the edge of designed lines. Electroninks particle-free Au ink has excellent atomization and exhibits limited overspray and satellite spots in fine features printing by IDS system. Figure 4 shows an optical image of fine-line printed through an IDS NanoJet system with approximately 60μm linewidth. This improved line edge definition and overall line quality makes Electroninks particle-free Au ink suitable for dense interconnects printing in compact electronics.
Figure 4: Fine feature printed by Electroninks Au ink through an IDS system, cured per conditions in chart
Silver printing through an IDS aerosol jet system:
Electroninks particle free silver ink has 14% silver loading, and it has the same ink formulation mechanism as Electroninks gold ink to ensure a high-mass output. Most of the existing silver inks in the markets require to print at the elevated temperature to prevent spreading during printing. This limits the printing on the conformal surfaces or printing on 3D substrates. One of the highlights of Electroninks silver ink is able to print high aspect ratio structures without heated platen. Figure 5(a) shows a fine features printing processed by Electroninks silver ink at a room temperature platen. The printed features have minimized overspray and satellites spot, line width is measured at 15μm with approximate 2μm thickness as shown in Figure 5(b).
Figure 5: (a) Fine feature printed by Electroninks Ag ink; (b) profile of printed fine features
Early efforts in developing the aerosol jetting technologies were focused on printing of very fine-line features with a linewidth as small as 10μm, which is shown in Figure 5. While this capability is very useful for a variety of applications, some more recent efforts have focused on increasing the material output rate from IDS NanoJet to meet the demands of higher throughput, production applications. An example of a structure printed for RF applications is shown in Figure 6(a). These structures were printed with the Electroninks silver ink using a bed temperature of 120°C. Profilometry measurements for these printed structures are shown in Figure 6(b). For these particular structures, multiple printing passes were used to achieve the printed layer thickness. As the profilometry data shows, the printed layer thickness varies somewhat and further optimization is needed to improve feature uniformity; however, these features are useful for the proof-of-concept work requested. The average height of the printed structure is 20μm and the maximum height is 40μm. These types of printed structures are proving useful for a variety of applications. One device under development includes a flexible, implantable stimulator where thicker printed coils provide high power coupling efficiency to implanted device at a range up to 5 cm from the power source.
Figure 6: (a) NanoJet printed structures for RF performance evaluation, (b) Profilometry showing measured height of the NanoJet printed structures
Another important advantage of aerosol jet printing is the extended working distance between the print nozzle exit face and the print substrate surface. Since the IDS NanoJet aerosol stream is collimated, clean prints can be made with the spacing between the nozzle face and the print substrate ranging from 1 – 5 mm or more. This ability can be particularly useful for printing features onto non-traditional substrates.
An example of such a print is shown in Figure 7. The substrate is a 3D printed plastic dome and the Electroninks silver ink was used by an IDS system to print full-bridge, strain gauge circuit onto the dome. This demonstration was used to showcase recent advancements in the printed electronics space. By combining the next generation aerosol printing technology from IDS NanoJet with particle-free inks from Electroninks, it was able to demonstrate the ability to print an electronic circuit onto a complex shaped surface in a continuous fashion using an ink that was able to be cured in-situ during printing on a low melting temperature substrate. This demonstrator circuit was part of a larger proof-of-concept project to show the ability to embed sensors in a structure where the sensors are inactive until the sensors are interrogated using remote telemetry for powering and communicating with the sensor to intermittently measure changes in the response of the sensor.
Figure 7: Aerosol printed full-bridge strain gauge circuit on low temperature 3D printed thermoplastic substrate
VI. CONCLUSION
Advances in additive manufacturing are literally affecting the shape, size and even purpose of new products yet to be created by consumer and industrial companies. This paper presents a novel fabrication of sensors and circuits on a variety surfaces by using Electroninks particle-free metal inks along with the next generation of NanoJet provided by IDS system. The combination of Electroninks particle-free inks and IDS NanoJet provides a higher output, better printing quality, and lower cost to fabricate electronics compared with other traditional digital printing technologies. Electroninks Ag and Au inks can be processed at room temperature which are excellent candidates for printing on complex surfaces. Electroninks Ag ink provides 5-7 uohm-cm resistivity while Au can provide 6 uohm-cm resistivity, low resistivity requires fewer printing passes and saves time and efforts compared with other inks in the market. Pilot testing has shown the IDS’s next generation aerosol jetting technology using Electroninks particle-free inks can print feature size range from 20μm wide by 2μm tall to mm wide by 20μm tall on even conformal surfaces, the printed features showed improved edge quality with less satellites spots and overspray.
AUTHORS
Yuan Gu, senior scientist, Electroninks Ayan Maity, senior scientist, Electroninks Steven Brett Walker, CEO, Electroninks Dr. Marcelino Essien, president, IDS
Dr. Yun Li, IDS
Jacob Chavez, IDS
David Keicher, vice president, IDS
COMMUNICATION AUTHOR:
Melbs LeMieux, president, Electroninks