IEEE TRANSACTIONS ON DEVICE AND MATERIALS RELIABILITY, VOL. 12, NO. 1, MARCH 2012 15 Domed and Released Thin-Film Construct—An Approach for Material Characterization and Compliant Interconnects Gregory T. Ostrowicki, Nathan T. Fritz, Raphael I. Okereke, Paul A. Kohl, Member, IEEE, and Suresh K. Sitaraman Abstract—An approach for microfabricating precisely defined spherical 3-D domes through a simple low-temperature polymer reflow process was developed. Release of a thin metal film pat- terned over the domed structure was accomplished by the removal of the underlying polymer using two different methods: dry ther- mal decomposition and wet supercritical release. The domed shape impacted the effect of stiction during the release step and assisted in the release of large millimeter-size film geometries. The dome and release procedures were used to fabricate an experimental test specimen that functions as either a tensile or interfacial fracture test for thin films on rigid substrates. Other potential applications of the domed metal structures such as compliant electrical inter- connects are discussed. Index Terms—Compliant interconnects, delamination, dome, MEMS, reflow, stiction, tensile test, thin film. I. I NTRODUCTION M ICROELECTRONIC and MEMS fabrication typically involves a set of serially processed 2-D layers, result- ing in a final structure that is either flat or mildly terraced. More complicated 3-D shapes, such as angular pyramids and valleys, usually require selective etching along crystal planes in single crystalline substrates. These wet or dry chemical etching processes can be fabrication intensive with resulting geometries that are highly sensitive to process parameters. Thus, there is a continuing need to develop microfabrication techniques that enable the formation of 3-D geometries for next-generation MEMS and microelectronic devices. A particularly simple and useful shape for MEMS designs is the spherical dome or cylindrical strip. Several schemes for fabricating these shapes have been developed. Some dome- shaped features have been cast from prefabricated molds with concave cavities. For example, PMMA microlenses were cast from an isotropically etched silicon mold [1]. Machined metal spheres or reflowed solder lines have also been used as stamps Manuscript received 6, 2011; revised August 24, 2011; accepted October 7, 2011. Date of publication December 6, 2011; date of current version March 7, 2012. This work was supported by the National Science Foundation under Grants CMMI-0800037 and ECCS-0901679. G. T. Ostrowicki, R. I. Okereke, and S. K. Sitaraman are with the George W. Woodruff School of Mechanical Engineering, Georgia Institute of Tech- nology, Atlanta, GA 30332 USA (e-mail: gtostrowicki@gatech.edu; suresh. sitaraman@me.gatech.edu). N. T. Fritz and P. A. Kohl are with the School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332 USA. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TDMR.2011.2175927 to create domed depressions into soft materials [2], [3]. In addi- tion, these cavities can be coated with structural material which upon etching of the mold results in a dome-shaped membrane [2], [4], [5]. These fabrication techniques however can be rather complex and may not be appropriate for applications requiring a simple dome shape on a substrate surface. A promising method for such a bottom–up approach to dome formation relies on thermal reflow of patterned flat materials. While reflow of solder is well known to result in spherical profiles, recent work in the reflow of polymer photoresists allows for simple low-temperature dome fabrication [6], [7]. These photoresist domes are ideal sacrificial materials as they are easily both patterned and removed. However, the thermal reflow process often requires several bake steps to prevent bubble formation and acetone vapor pretreatment to aid in reflow uniformity, and the final geometry is dependent on several factors, including reflow time, acetone vapor concentration, and aspect ratio of the original photoresist feature [8]. An alternative to the traditional photoresist dome is the fabrication of air-filled domes through the thermal decomposition of a photodefinable polymer. These thermally developed photoresists decompose into gas when heated and diffuse through the encapsulating membrane, and can result in air-filled domes under the right conditions [9]. In this paper, we present a straightforward methodology of fabricating spherical domed structures using a thermally developed polymer and a one-step reflow process. This tech- nique has several advantages over conventional polymer dome formation, including the following: no vapor pretreatment, one low-temperature bake step, fully formed domes in minutes, pre- cisely defined footprint and curvature, and robust processing. Furthermore, the reflowable polymer can be spin coated, is photodefinable, is developed by a dry thermal process, and can be removed via either thermal decomposition or wet solvent etch. The use of these polymer domes as a sacrificial supporting structure was demonstrated by the fabrication of test specimens for a novel thin-film material characterization experiment. The test specimens featured an electrodeposited and patterned metal film over the dome which was released after the removal of the reflowed photoresist. There was significant learning through the iterative fabrication development process of the proposed thin- film test, particularly regarding strategies for avoiding stiction when releasing large millimeter-sized metal films over reflowed polymer domes. In the following sections of this paper, we describe the procedure for fabricating precisely defined domes, extend the fabrication to create a compliant metal structure for a thin-film characterization application, cover dry and wet 1530-4388/$26.00 © 2011 IEEE