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Here we present a protocol for the fabrication of inkjet-printed multilayer sensor structures on additively manufactured substrates and foil.
A method to combine additively manufactured substrates or foils and multilayer inkjet printing for the fabrication of sensor devices is presented. First, three substrates (acrylate, ceramics, and copper) are prepared. To determine the resulting material properties of these substrates, profilometer, contact angle, scanning electron microscope (SEM), and focused ion beam (FIB) measurements are done. The achievable printing resolution and suitable drop volume for each substrate are, then, found through the drop size tests. Then, layers of insulating and conductive ink are inkjet printed alternately to fabricate the target sensor structures. After each printing step, the respective layers are individually treated by photonic curing. The parameters used for the curing of each layer are adapted depending on the printed ink, as well as on the surface properties of the respective substrate. To confirm the resulting conductivity and to determine the quality of the printed surface, four-point probe and profilometer measurements are done. Finally, a measurement set-up and results achieved by such an all-printed sensor system are shown to demonstrate the achievable quality.
Additive manufacturing (AM) is standardized as a process where materials are joined to make objects from 3D model data. This is usually done layer upon layer and, thus, contrasts with subtractive manufacturing technologies, such as semiconductor fabrication. Synonyms include 3D-printing, additive fabrication, additive process, additive techniques, additive layer manufacturing, layer manufacturing, and freeform fabrication. These synonyms are reproduced from the standardization by the American Society of Testing and Materials (ASTM)1 to provide a unique definition. In the literature, 3D-printing is referred to as the process where thickness of the printed objects is in the range of centimeters to even meters2.
More common processes, such as stereolithography3, enable the printing of polymers, but the 3D-printing of metal is also already commercially available. The AM of metals is employed in manifold areas, such as for the automotive, aerospace4, and medical5 sectors. An advantage for aerospace structures is the possibility to print lighter devices through simple structural changes (e.g., by using a honeycomb design). Consequently, materials with, for instance, greater mechanical strength, that would otherwise add a significant amount of weight (e.g., titanium instead of aluminum)6, can be employed.
While the 3D-printing of polymers is already well established, metal 3D-printing is still a vibrant research topic, and a variety of processes have been developed for the 3D-printing of metal structures. Basically, the available methods can be combined into four groups7,8, namely 1) using a laser or electron beam for cladding in a wire-fed process, 2) sintering systems using a laser or electron beam, 3) selectively melting powder using a laser or electron beam (powder bed fusion), and 4) a binder jetting process where, commonly, an inkjet print head moves over a powder substrate and dispenses binding agent.
Depending on the process, the respective manufactured samples will exhibit different surface and structural properties7. These different properties will have to be considered in further efforts to further functionalize the printed parts (e.g., by fabricating sensors on their surfaces).
In contrast to 3D-printing, the printing processes to achieve such a functionalization (e.g., screen and inkjet printing) cover only limited object heights from less than 100 nm9 up to a few micrometers and are, thus, often also referred to as 2.5D-printing. Alternatively, laser-based solutions for high-resolution patterning have also been proposed10,11. A comprehensive review of the printing processes, the thermally dependent melt temperature of nanoparticles, and the applications is given by Ko12.
Although screen printing is well established in the literature13,14, inkjet printing provides an improved upscaling ability, together with an increased resolution for the printing of smaller feature sizes. Besides that, it is a digital, noncontact printing method enabling the flexible deposition of functional materials on three-dimensional. Consequently, our work is focused on inkjet printing.
Inkjet printing technology has already been employed in the fabrication of metal (silver, gold, platinum, etc.) sensing electrodes. Application areas include temperature measurement15,16, pressure and strain sensing17,18,19, and biosensing20,21, as well as gas or vapor analysis22,23,24. The curing of such printed structures with limited height extension can be done using various techniques, based on thermal25, microwave26, electrical27, laser28, and photonic29 principles.
Photonic curing for inkjet-printed structures allows researchers to use high-energy, curable, conductive inks on substrates with a low-temperature resistance. Exploiting this circumstance, the combination of 2.5D- and 3D-printing processes can be employed to fabricate highly flexible prototypes in the area of smart packaging30,31,32 and smart sensing.
The conductivity of 3D-printed metal substrates is of interest to the aerospace sector, as well as for the medical sector. It does not just improve the mechanical stability of certain parts but is beneficial in near-field as well as capacitive sensing. A 3D-printed metal housing provides additional shielding/guarding of the sensor's front-end since it can be electrically connected.
The aim is to fabricate devices using AM technology. These devices should provide a sufficiently high resolution in the measurement they are employed for (often at micro- or nanoscale) and, at the same time, they should fulfill high standards regarding reliability and quality.
It has been shown that AM technology presents the user with enough flexibility to fabricate optimized designs33,34 which improve the overall measurement quality that can be achieved. Additionally, the combination of polymers and single-layer inkjet printing has been presented in previous research35,36,37,38.
In this work, available studies are extended, and a review about the physical properties of AM substrates, with a focus on metals,and their compatibility with multilayer inkjet printing and photonic curing is provided. An exemplary multilayer coil design is provided in Supplementary Figure 1. The results are used for providing strategies for the inkjet printing of multilayer sensor structures on AM metal substrates.
CAUTION: Before using the considered inks and adhesives, please consult the relevant Material Safety Data Sheets (MSDS). The employed nanoparticle ink and adhesives may be toxic or carcinogenic, dependent on the filler. Please use all appropriate safety practices when performing inkjet printing or the preparation of samples and make sure to wear appropriate personal protective equipment (safety glasses, gloves, lab coat, full-length pants, closed-toe shoes).
NOTE: The protocol can be paused after any step except steps 6.3 - 6.6 and steps 9.2 - 9.5.
1. Preparation of 3D-printed Substrates
2. Fabrication of Interconnects
NOTE: The fabrication of interconnects differs depending on the type (conductive/nonconductive) of substrate.
3. Preparation of the Inkjet Printing System
4. Inspection of the Surface Properties of the Respective Substrates for Printability and the Adjustment of Printer Parameters for the First Layer
5. Curing Parameter Adjustments for the First Layer
6. Inkjet Printing and Curing of the First Device Layer
7. Inspection of the Surface Properties of the Respective Substrates for Printability and the Adjustment of Printer Parameters for Subsequent Layers
NOTE: Please refer to the user manuals of the measurement equipment to perform the profilometer measurements and microscopy inspections.
8. Curing Parameter Adjustments for Subsequent Layers
9. Inkjet Printing and Curing of Subsequent Device Layers
From the SEM images shown in Figure 1, conclusions on the printability on the respective substrates can be drawn. The scale bars are different due to the different ranges of the surface roughness. In Figure 1a, the surface of the copper substrate is shown, which is by far the smoothest. Figure 1c, on the other hand, shows steel, a substrate which is not usable for inkjet printing due to the high poro...
A way to fabricate multilayer sensor structures on 3D-printed substrates and on foil is demonstrated. AM metal, as well as ceramic and acrylate type and foil substrates are shown to be suitable for multilayer inkjet printing, as the adhesion between the substrate and the different layers is sufficient, as well as the respective conductivity or insulation capability. This could be shown by printing layers of conductive structures on insulating material. Furthermore, the printing and curing processes for all layers was suc...
The authors have nothing to disclose.
This work has been supported by the COMET K1 ASSIC Austrian Smart Systems Integration Research Center. The COMET-Competence Centers for Excellent Technologies-Program is supported by BMVIT, BMWFW, and the federal provinces of Carinthia and Styria.
Name | Company | Catalog Number | Comments |
PiXDRO LP 50 | Meyer Burger AG | Inkjet-Printer with dual-head assembly. | |
SM-128 Spectra S-class | Fujifilm Dimatix | Printheads with nozzle diameter of 50 µm, 50 pL calibrated dropsize and 800 dpi maximum resolution. | |
DMC-11610/DMC-11601 | Fujifilm Dimatix | Disposable printheads with nozzle diameter 21.5 µm, 1 or 10 pL calibrated dropsize | |
Sycris I50DM-119 | PV Nanocell | Conductive silver nanoparticle ink with 50 wt.% silver loading, with an average particle size of 120 nm, in triethylene glycol monomethyl ether. | |
Solsys EMD6200 | SunChemical | Insulating, low-k dielectric ink which is a mixture of acrylate-type monomers. Viscosity is 7-9 cps. | |
Dycotec DM-IN-7002-I | Dycotec | UV curable insulator, Surface Tension: 37.4 mN/m | |
Dycotec DM-IN-7003C-I | Dycotec | UV curable insulator, Surface Tension: 29.7 mN/m | |
Dycotec DM-IN-7003-I | Dycotec | UV curable insulator, Surface Tension: 31.4 mN/m | |
Dycotec DM-IN-7004-I | Dycotec | UV curable insulator, Surface Tension: 27.9 mN/m | |
Pulseforge 1200 | Novacentrix | Photonic curing/sintering equipment. | |
DektatkXT | Bruker | Stylus Profiler with stylus tip of 12.5 µm diameter and constant force of 4 mg. | |
C4S | Cascade Microtech | Four-point-probe measurement head. | |
2000 | Keithley | Multimeter to evaluate the measurements using the four-point-probe. | |
Helios NanoLab600i | FEI | Focused Ion Beam analysis station which provides high-energy gallium ion milling. | |
SeeSystem | Advex Instruments | Water contact angle measurement device. | |
Projet 3500 HDMax | 3D Systems | Professional high-resolution polymer 3D-printer. See also (accessed Sep. 2018): https://www.3dsystems.com/sites/default/files/projet_3500_plastic_0115_usen_web.pdf | |
Polytec PU 1000 | Polytec PT | Electrically conductive adhesive based on Polyurethane, available | |
Microdispenser | Musashi | Needle for microdispensing. | |
Micro-assembly station | Finetech | Equipment for assembly of, e.g., printed circuit boards (PCBs) and placing of chemicals (e.g. solder) and SMD parts. |
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