Advances in Direct-Write Printing ofRF-MEMS using M 3 D A. Md N Al-Mobin, R. Shankar, W Cross, J Kellar, K W Whites and D. E Anagnostou South Dakota School of Mines and Technolo Abstract - We report an additive manufacturing process using in-house custom-made inks for the fabrication of ohmic contact RF MEMS switches. The fabrication involves multiple steps of additive printing using a conductive silver ink and a PMMA [poly(methyl methacrylate)] photoresistive polymer ink. The inks comply with the rheology requirements of the M3D material deposition system. Deposition is made at 40°C and feature sizes involve 1O-20m. The maximum temperature of the process depends on ink curing and was 250°C. A functional ohmic contact cantilever RF MEMS switch on flexible KaptonTM substrate was fabricated and tested successfully, and results are presented. Index Terms - Direct-Write, Flexible Electronics, RF MEMS. I. INTRODUCTION AND PROCESS NOVELTY  MEMS switches were first proposed more than 20 years ago in a patent by L. E. Larson [1], while a few years later their use for microwave and antenna applications was proposed [2]-[3]. MEMS fabrication typically requires a cleanroom with photolithography and microfabrication equipment. Traditionally, MEMS fabrication involves masks to deposit and patte the various layers (bias lines, electrode layer, sacrificial spacer, and cantilever structure), as well as electroplating and patteing followed by release and critical point drying [4]-[5]. While specific steps may vary for different substrate materials and switch designs, the entire process generally requires costly facilities and can be time- consuming. An entirely different fabrication methodology is considered in this paper. The methodology is additive manufacturing and is based upon the fact that most MEMS fabrication steps are additive, with only one being subtractive (the removal of the sacrificial photoresist). Therefore, a direct-write aerosol printing machine, such as the Maskless Mesoscale Materials Deposition (M 3 D) system by Optomec™ can be used to construct all the metallic parts of MEMS. The M 3 D deposits nanoink droplets with accuracy of about 10�m to 20�m, which suffices for a first proof-of-concept prototype. The main challenges to meet this goal are then: (a) the development of metallic and photoresistive inks that can be deposited with M 3 D (there are no such inks commercially available), and (b) the deposition accuracy and alignment of the different printed layers. The subtractive steps (removal of the sacrificial layer and membrane release) are the same as in traditional MEMS processes. The inks used in the proposed process are not commercially available and so they were developed in the direct-write laboratory of SDSMT, based on the requirements of the M 3 D. Using the proposed process, an ohmic contact series cantilever  MEMS switch was fabricated and tested successlly. The presented fabrication method is still in experimental stage. Advantages of the method include the redundancy of masks (they are replaced by computer CAD files), of c1eanroom facilities for prototyping (a c1eanroom may be used at a later stage for mass productivity with high reliability), deposition processes take place in an ambient environment at room temperature and using a single piece of equipment, and the ease to develop, print and experiment with customized designs. Disadvantages involve the serial printing (but can be addressed using multiple heads), as well as the various technology limitations in the accuracy and alignment of the direct-write printing. However the ease of fabrication may make it possible soon to print switching electronic devices using desktop printers at home with minimal post-processing. II. DEVELOPMENTS FOR PRINTING WITH M 3 D First, a conductive and a photoresistive ink that conform to the rheological requirements of M 3 D were developed. A. Formulation a/Conductive Ink First, a conductive silver nanoparticle ink was developed to print the conductive layers. Silver nanoparticles were chosen as the base of the ink due to their high conductivity and inertness. A modified polyol process was used to synthesize the nanoparticles that yields PVP (PolyVinyl Pyolidone) capped silver nanoparticles. In polyol synthesis, a precursor solution initially forms small nuclei or seeds which later grow into nanocrystals. We synthesized the PVP capped silver nanoparticles with a size range of �50nm to 80nm in the ethylene glycol media using a polyol process. AgN03 (99%, Sigma-Aldrich) was used as the silver precursor salt, and both PVP (molecular weight MW-55,000, Sigma-Aldrich) and Ethylene Glycol (99+%, Acros Organics) were both used as reducing media and solvent. PVP was also used as the capping agent for the nanoparticles formed during the process. The molar ratio of PVP:AgN03 was held at 5: l. Two individual solutions were prepared separately, the first one by dissolving AgN03 in ethylene glycol and the second by dissolving PVP in ethylene glycol. The PVP solution was heated at 140°C to dissolve PVP completely until it formed a clear solution. The AgN03 solution was then added dropwise into the PVP solution and the mixture was stirred for 50 minutes. The reaction was 978·1-4799-3869-8/14/$31.00 ®2014 IEEE