Aerosol-Jet Printed Quasi-Optical Terahertz Filters

2017 IEEE 67th Electronic Components and Technology Conference Aerosol-Jet Printed Quasi-Optical Terahertz Filters Christopher Oakley, Amanpreet Kaur, Jennifer A. Byford, and Premjeet Chahal Michigan State University Department of Electrical and Computer Engineering East Lansing, MI chahal@msu.edu Abstract This paper presents the design, simulation, and characterization of metamaterial-inspired terahertz filters, fabricated by aerosol-jet printing. Filters are designed for operation at 230, 245, and 510 GHz, for both band-pass and bandstop operation. Operation of each printed filter is compared to structures fabricated from copper metal using a photolithographic process. Each of the aerosol jet printed filters are found to have performance comparable to those fabricated using lithographic techniques, demonstrating the applicability of aerosol-jet printing to the fabrication of components operating in the terahertz regime. Keywords-aerosol printing; terahertz; additive manufacturing I. INTRODUCTION Over the past several decades, systems operating in the spectral region from 100 GHz to 10 THz, commonly referred to as the terahertz (THz) regime, have found a wide range of applications. Some of these applications include biological and medical sciences, communication systems, security, and material characterization and detection [1] [3]. Despite the potential applications of THz systems, wide spread adoption has been limited due to the cost of fabrication of the necessary system components such as mixers, filters, absorbers and lenses. Additive manufacturing of these components can provide a path towards cost reduction of components, as well as reduce fabrication time. Integration of these high-frequency systems in a compact environment can lead to unwanted signals and noise coupling into the signal path, necessitating the use of filters and absorbers to counteract these undesired effects. Metamaterial filters have been of interest due to their performance at high angles of incidence, insensitivity to incident wave polarization, and compact size [4]. Such structures have been previously demonstrated through fabrication on organic substrate materials using lithographic techniques [5]. Using these structures, individual frequency bands or multiple bands can be selected to pass or to be stopped by tailoring the dimensions of the metal structures, allowing for great flexibility in desired system operation. Additive manufacturing processes are currently being explored as a method to reduce the cost of THz system components. Many techniques currently exist for fabrication of THz components through subtractive processes, including wet and dry etching, LIGA, and micromachining [6]. These processes can be used to fabricate very well defined structures; however, they can be costly, require a trained technician to produce reliable results, and may produce hazardous waste as a by-product. The feasibility of 3D printing components such as waveguides, probes and lenses for the THz regime has previously been demonstrated [7], [8]. These parts can be fabricated in a matter of hours, can take complex shapes which would be difficult, if not impossible, to achieve with substractive fabrication processes and can be inexpensively made in single quantities. Deposition of metal material has also been demonstrated through both inkjet printing of conductive polymers [9], inkjet printed metals [10], and electrohydrodynamic printing of metals [11]. Aerosol-jet printing of both metallic materials, as well as polymers, provides another method by which structures can be produced quickly, at low cost, with minimal waste material by-products. The aerosol-jet printing process can be used to deposit metal layers on materials fabricated by other additive manufacturing techniques [12], as well as fabricate passive multilayer circuitry capable of operation through 40 GHz [13], and passive single-layer transmission lines for operation through 160 GHz [14]. Due to the noncontact nature of the aerosol printing process, by rotating an object about multiple axes, both metallic and polymer materials can be deposited on non-planar surfaces, enabling fabrication of complex structures which would be difficult and costly to fabricate using other processes [15]. The aerosol jet printing process allows for line widths as narrow as 10 μm, and as wide as 1 mm, with individual layer thickness ranging from 300 nm to approximately 2 μm. The printing process is conducted under atmospheric conditions, and avoids the use of cleanroom facilities which are necessary for lithographic fabrication processes. Aerosol printing begins by atomizing a solution of nanoparticle material suspended in solvent, either by pneumatic means or by ultrasonic excitation. Nitrogen gas carries the now atomized ink to the print head, where a secondary sheath gas is used to focus the particle stream through a nozzle, to the suface to be metallized. Figure 1 depicts both the aerosolization concept, as well as a representation of the aerosol jet print head. The goal of this work is to demonstrate the application of 2377-5726/17 $31.00 2017 IEEE DOI 10.1109/ECTC.2017.233 248

Carrier Gas In Aerosol Stream Out Aerosolized Ink Nanoparticle Ink Aerosol Stream In Sheath Gas Dimension (μm) 230 GHz Band-Pass 240 GHz W 40 50 40 L 510 440 220 G 40 40 40 Table I SIMULATED FILTER LINE DIMENSIONS. 510 GHz Figure 1. Aerosol system concept Print Nozzle aerosol jet printing technology for low the cost fabrication of THz passive components. Three quasi-optical filters are demonstrated: a 240 GHz band-stop filter, a 510 GHz band-stop filter, and a 230 GHz band-pass filter. Each of these designs are fabricated both by traditional lithographic techniques with copper metal, as well as by aerosol printing of silver nanoparticle ink. These filters are fabricated on a 25.4 μm thick sheet of commercially available liquid crystal polymer () material to minimize unwanted insertion loss due to the substrate s dielectric properties. II. DESIGN AND SIMULATION Each of these filters are formed from a two-dimensionally periodic array of crosses. Design of these structures is constrained by minimum dimensions realizable by low-cost lithographic techniques. Due to manufacturing tolerances of the shadow mask required for photolithography, no dimensions below 40 μm are analyzed. Additionally, to reduce reradiation effects of undesired signals, metal layer thickness of at least 5 skin-depths (δ), as calculated using Equation 1 [16] is optimal. For copper metal, this corresponds to a minimum metal thickness at 510 GHz of 462 nm, and 689nm at 230 GHz. For silver nanoparticle metal layers, assuming a final metal conductivity of 10 7 S/m, the minimum metal thickness are increased to 1.11 μm at 510 GHz, and 1.66 μm at 230 GHz. δ = 1 πfμσ (1) Using these constraints, all copper metal layers are designed and simulated with a thickness of 1 μm, while silver metal layers are maintained at 2 μm thick. Simulations are performed using ANSYS High Frequency Structure Simulator (HFSS). Figure 2 shows the geometry of both simulated band-pass and band-stop filters. Each filter is simulated as a unit cell, approximating an infinite array in two dimensions using the Floquet theorem. A 230 GHz band-pass filter has been designed with line width W = 40 μm, line length L = 510 μm, and unit cell spacing G = 40 μm. A 240 GHz bandstop filter has line widthw=50μm,line length L = 440 μm, and unit cell spacing G = 40 μm. Finally, a 510 GHz bandstop filter has been designed with line width W = 40 μm, line length L = 220 μm, and unit cell spacing G=40μm. These dimensions are summarized in Table I. W G L Figure 2. Geometry of simulated filters. Left: Band-pass configuration. Right: Band-stop configuration Simulation results of these filters can be seen in Figures 3, 4, and 5. Simulated results of the 230 GHz band-pass filter show minimum pass-band insertion loss of approximately 0.28 db at 230 GHz for copper metal, and 0.77 db at 225 GHz for silver metal. Simulation results of the 240 GHz band-stop filter show a maximum insertion loss of approximately 40 db at 241 GHz for copper metal, and approximately 35 db at 242 GHz for silver metal. Simulation of the 510 GHz band-stop filter reveals maximum insertion loss of approximately 40 db at 510 GHz for copper metal, and approximately 35 db at 512 GHz for silver metal. III. FABRICATION Fabrication of each of the three filter designs has been carried out utilizing both copper metal, and silver nanoparticle ink. A commerically available substrate material, Rogers Ultralam 3850HT, 25.4 μm thick, has been selected as the substrate for each filter design. A. Copper Filters Fabrication of each filter with copper metallization begins with removal of the existing copper material. The material, as provided, has 9 μm of copper metal which has been electrodeposited on the substrate. The thickness of this metal can result in difficulty fabricating lines with sharply defined edges and controlled spacing. This existing copper is chemically stripped away in a bath of sodium persulfate. A 60 nm thick layer of titanium is deposited by sputtering 249

sputter deposited. Each of the substrates are spin coated with a positive photoresist material, and patterned under UV light. Areas of copper exposed to UV light are etched away, leaving the desired cross shaped elements. Remaining photoresist material is then removed by an acetone wash. The steps of this process are illustrated in Figure 6. Align Mask 1 6 2 Remove Copper Pattern photoresist Deposit 60nm Ti 7 3 Deposit 1um Cu Etch Cu and Ti layers Figure 3. 230 GHz Band-pass filter simulated insertion loss 4 Apply photoresist 8 Remove photoresist 5 9 Figure 6. Copper filter fabrication steps Figure 4. Figure 5. 240 GHz Band-stop filter simulated insertion loss 510 GHz Band-stop filter simulated insertion loss on one surface of the substrate, to promote adhesion of the final copper layer. Next, a 1 μm layer of copper is B. Aerosol Jet Printed Filters Each of the substrates used for aerosol printing are chemically etched to remove the existing copper layer. These pieces are affixed to a ceramic carrier to facilitate handling without inadvertent damage to the deposited metal layers during fabrication. Clariant Prelect TPS 50 G2 silver nanoparticle ink is used, which consists of silver particles of approximately 10-20 nm in diameter, dispersed in ethylene glycol. This ink is atomized ultrasonically, to provide consistent line dimensions and quality. However, this atomization method is limited to low viscosity inks, with viscosity limited to 1-5 cp. As manufactured, this nanoparticle ink has a viscosity of approximately 15 cp. To reduce ink viscosity to approximately 5 cp, 1 ml of ink is diluted into 3 ml of deionized water. By varying the print nozzle diameter, as well as the carrier and sheath gas flow rates and print velocity, both line width and layer thickness can be tailored to each specific application. Each of these filters were fabricated using a nozzle with an inner diameter of 150 μm. A carrier gas flow rate of 24 SCCM, and sheath gas flow rate of 35 SCCM, have been utilized. Lines were printed at a velocity of 1 mm/s, in 3 layers. While silver nanoparticle ink conductivity has been previously shown to increase with higher sintering temperature [17], it has been found that materials deform under such ideal temperatures. To balance these two conflicting constraints, each of these filters have been cured for 3 hours at a temperature of 160 C, under atmospheric conditions. Figure 7 shows both the aerosol jet printed 230 GHz bandpass filter, and its etched copper counterpart. The dimensions of each of these differ from the desired dimensions. The aerosol printed filter has a measured line width W = 30 μm, 250

Figure 7. 230 GHz Band-pass filters after fabrication. Left: aerosol printed filter. Right: etched copper filter 510 GHz band-stop filters. The final measured dimensions of the aerosol jet printed filter show a line width W = 20 μm, line length L = 240 μm, and cell spacing G = 30 μm. Measured dimensions of the etched copper filter show a line width W = 38 μm, line length L = 210 μm, and cell spacing G=45μm. Final measured line dimensions show a metal layer approximately 2.5 μm thick over a width of 25 μm. Additionally, an area of over-spray is observed, extending approximately 10 μm around each line. This is likely due to underlying layers of deposited ink being slightly perturbed by the incoming carrier/sheath gas combination while additional layers of ink are deposited. Tables II and III summarize measured dimensions of all fabricated filters. Dimension (μm) 230 GHz Band-Pass 240 GHz W 30 20 20 L 520 465 240 G 30 25 30 510 GHz Table II MEASURED AEROSOL PRINTED FILTER LINE DIMENSIONS. Figure 8. 240 GHz Band-stop filters after fabrication. Left: aerosol printed filter. Right: etched copper filter Dimension (μm) 230 GHz Band-Pass 240 GHz W 65 45 38 L 510 440 210 G 25 50 45 510 GHz Table III MEASURED ETCHED COPPER FILTER LINE DIMENSIONS. Figure 9. 510 GHz Band-stop filters after fabrication. Left: aerosol printed filter. Right: etched copper filter line length L = 520 μm, and cell spacing G=30μm.The copper metal band-pass filter has line width W = 65 μm, line length L = 510 μm, and cell spacing G = 25 μm. Additionally, due to the etching process, the ends of the copper metal filter take on a slightly more round shape than that which was aerosol jet printed. Figure 8 shows both the aerosol jet printed and etched copper 240 GHz band-stop filters. The final measured dimensions of the aerosol jet printed filter show a line width W = 20 μm, line length L = 465 μm, and cell spacing G = 25 μm. The measured line dimensions of the etched copper 240 GHz band-stop filter show a line width W = 45 μm, line length L = 440 μm, and cell spacing G=50μm. Figure 9 shows both aerosol jet printed and etched copper IV. MEASUREMENT Each filter was measured using an Emcore PB-7200 frequency-domain THz measurement system. To minimize unwanted effects of the finite area of each filter, a9mmx 9 mm window was fabricated through a 1.524 mm thick piece of copper-clad printed circuit board material. This window has been mounted on the face of the THz detector. Each filter is fixed in place, centered on this window. Figure 10 shows the measurement system configuration. Figure 10. Frequency-domain THz measurement system 251

Both copper and silver metal 240 GHz band-stop filters have been measured. Insertion loss of these filters can be seen in Figure 11. The copper metal filter shows a maximum insertion loss of 23 db at approximately 250 GHz. Simulation of this filter with feature dimensions similar to which has been fabricated shows a maximum insertion loss of approximately 37 db at 240 GHz. The same filter fabricated by aerosol jet printing shows a maximum insertion loss of 25 db at approximately 210 GHz. Simulation of this same filter with updated feature dimensions shows maximum insertion loss of approximately 22 db at 220 GHz. Figure 12. 510 GHz Band-stop filters Figure 11. 240 GHz Band-stop filters Measured results of the fabricated 510 GHz band-stop filters can be seen in Figure 12. The filter fabricated from copper metal shows a maximum insertion loss of approximately 17 db at 540 GHz. Simulation of this filter with updated feature dimension results in insertion loss of 38 db at approximately 525 GHz. The filter fabricated from silver nanoparticle ink has a maximum insertion loss of 23 db at 465 GHz. Simulation of this filter with updated feature dimensions shows maximum insertion loss of 37 db at the same frequency as the fabricated filter. Measured performance of the fabricated 230 GHz bandpass filters can be seen in Figure 13. Filters fabricated from both copper metal and aerosol jet printed silver nanoparticle ink show minimum insertion loss of approximately 1 db at 215 GHz. Simulation of these filters with updated line dimensions result in minimum insertion of approximately 0.3 db at 220 GHz for copper metal, while silver metal results in minimum insertion loss of approximately 1 db at 215 GHz. These updated simulations correlate well with measured results. V. CONCLUSIONS In this paper, aerosol jet printed filters for operation in the THz regime have been fabricated, measured and Figure 13. 230 GHz Band-pass filters compared to equivalent structures fabricated from copper metal patterned using a photolithographic process. Both band-pass and band-stop filters have been demonstrated, with performance comparable to their copper metal counterparts. Additionally, simulation of such filters has been shown to accurately predict performance of these structures, despite the metal roughness apparent through microscopic observation. While proper operation of each of these filters is strongly dependent on line dimensions, aerosol jet printing provides great flexibility in controlling these parameters. Due to the direct-write nature of aerosol jet printing, line geometry can be quickly adjusted, allowing for fabrication of new filters within hours, significantly reducing fabrication time compared to the length of time required for fabrication by conventional lithographic techniques. 252

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