Thermoplastic Green Machining for Textured Dielectric Substrate for Broadband Miniature Antenna Young-Hag Koh* ,w and John W. Halloran* Materials Science and Engineering Department, University of Michigan, Ann Arbor, Michigan 48109-2136 Gullu Kiziltas, Dimitris Psychoudakis, and John Volakis Electrical Engineering and Computer Science Department, University of Michigan, Ann Arbor, Michigan 48109-2121 A textured dielectric substrate for a broadband miniature an- tenna was fabricated using a thermoplastic green machining. This substrate comprised of three steps with optimized distribu- tion of a dielectric ceramic (Bi–Ba–Nd–Titania; BBNT) and an epoxy. At first, a thermoplastic compound, consisting of BBNT particles and thermoplastic binders, was machined precisely us- ing a mini-computer numeric controlled machine to make a three-dimensional void in the green BBNT body. After appro- priate binder removal and sintering, the void in the dense BBNT block was filled with an epoxy to fabricate a textured dielectric substrate. The BBNT sample was characterized by several anal- yzing tools in terms of machinability, microstructural evolutions, chemical compositions, crystalline phase, and dielectric proper- ties. Bandwidth and gain of the patch antenna with the textured dielectric substrate were measured and compared with the sim- ulated results. I. Introduction P ATCH antennas are commonly used in wireless communica- tion because they are easy to fabricate and are planar. 1–3 They are commonly printed on simple dielectric substrates with uniform dielectric constants. The resonance pattern is deter- mined by the patch, ground plane, and dielectric constant of the substrate. More and more, the miniaturizations of patch anten- nas were necessary to enhance the performances. The physical size can be reduced by the inverse square root of dielectric con- stant; however, this material suffers from narrow bandwidth, low gain, and degradation of radiation efficiency. 4 Recently, material optimization procedures have been pur- sued in order to provide a smaller physical size without degrad- ing pre-specified bandwidth and gain performance. 5–7 These new materials are composed of more than two dielectric materials with unique textures, named ‘‘textured dielectric substrate’’. Here, ‘‘texture’’ means that the dielectric constant varies at po- sition in electromagnetic field, consequently offering advanced properties due to a particular design. There are several fabrication processes that can control the local composition and microstructure in two or three dimen- sions with sub-millimeter spatial resolution, including extru- sion, 8 solid freeform fabrication (SFF) 9,10 and computer numeric- controlled (CNC)-machining of ceramic green body. 11–14 Among these processes, the CNC machining has been regarded as a cost-effective process. Furthermore, a thermoplastic com- pound, consisting of 52 vol% ceramic and 48 vol% thermoplas- tic binder, can be machined precisely without generating any defect due to the material removal in a non-brittle manner dur- ing machining. 11 Therefore, in this article, we fabricated a textured dielectric substrate using the thermoplastic green machining. This tex- tured dielectric substrate comprised of three steps with opti- mized distribution of a dielectric ceramic and an epoxy. A commercial low-temperature co-fired ceramics (Bi–Ba–Nd–Ti- tania, denoted as BBNT) was used, since such low-temperature co-fired ceramics (LTCC), comprised of multiphase glass ce- ramics or crystallizable glass, 15–18 can be co-fired with similar silver-alloy metallization with good microwave dielectric prop- erties. A thermoplastic BBNT block was CNC-machined in green stage to make a three-dimensional void. The BBNT ma- terial was characterized, including microstructural evolutions, chemical compositions, and crystalline phases, and dielectric properties after sintering at 9801C for various dwelling times. After sintering, the void in the dense BBNT material was filled with an epoxy to fabricate a textured dielectric substrate. Band- width and gain of the patch antenna with the textured dielectric substrate were measured to evaluate the antenna performance. II. Experimental Procedure An experimental procedure to fabricate a textured dielectric substrate using a thermoplastic green machining is shown in Fig. 1. 297 J ournal J. Am. Ceram. Soc., 88 [2] 297–302 (2005) DOI: 10.1111/j.1551-2916.2005.00089.x Thermoplastic Compound Preparation CNC-Green Machining Heat-treatment (Binder Removal & Sintering) Material Characterization Antenna Performance Test Dielectric Epoxy Filling Solid Block Fabrication Fig. 1. An experimental procedure to fabricate a textured dielectric substrate using a thermoplastic green machining. S. C. Danforth—contributing editor Supported by Defense Advanced Research Project under contract No. N00173-01-1- G910. *Member, American Ceramic Society. w Author to whom correspondence should be addressed. e-mail: kohyh@snu.ac.kr Manuscript No. 10698. Received November 21, 2003; approved October 13, 2004.