Contact-printed microelectromechanical systems.

2010 WILEY-VCH Verlag Gmb Standard photolithography-based methods for fabricating microelectromechanical systems (MEMS) present several drawbacks including incompatibility with flexible substrates and limitations to wafer-sized device arrays. In addition, it is difficult to translate the favorable economic scaling seen in the capital equipment-intensive microelectronics industry to the manufacture of MEMS since additional specialized processes are required and wafer volume is comparatively small. Herein we describe a new method for rapid fabrication of metallic MEMS that breaks the paradigm of lithographic processing using an economically and dimensionally scalable, large-area microcontact printing method to define 3D electromechanical structures. This technique relies on an organic molecular release film to aid in the transfer of a metal membrane via kinetically controlled adhesion to a viscoelastic stamp. We demonstrate the fabrication of MEMS bridge structures and characterize their performance as variable capacitors. Flexible, paper-thin device arrays produced by this method may enable such applications as pressure sensing skins for aerodynamics, phased array detectors for acoustic imaging, and novel adaptive-texture display applications. The methods and tools used in the mature field of microelectronics fabrication have enabled fabrication of today’s MEMS structures with micrometer-scale features of submicrometer precision, using process sequences that can readily integrate MEMS with measurement and control circuits. However, together with the benefits of using the established processing technologies, MEMS fabricated within the existing silicon microelectronics-based framework also inherit the limitations of the present techniques including expensive per-chip processing costs of MEMS devices, limited maximum size and form-factor, and a materials set restricted to the conventional microelectronic materials. These standard processing techniques impede integration of MEMS technologies in applications that go beyond single chip or single sensor use and are particularly restrictive when one considers expanding the use of MEMS into large area or flexible substrate applications. No established market for large area MEMS has yet developed; however, promising applications include sensor skins for humans and vehicles, phased array pressure sensors, adaptive-texture surfaces, and incorporation of arrayed MEMS devices with other large area electronics. In such applications, compatibility of the MEMS technology with flexible substrates is highly desirable. If MEMS are fabricated directly on the flexible sheets, such as polymeric substrates, the elevated-temperature processing (as is typical for thermal growth of oxides and the deposition of polysilicon in conventional MEMS processing) must be avoided to prevent substrate damage. An alternative, low-temperature approach in which structures fabricated on silicon wafers are bonded to a flexible sheet and then released from the silicon by fracturing small supports or by etching a sacrificial layer, has been demonstrated for silicon electronics, but has not been applied to MEMS fabrication. The technological push to move to flexible, large-area applications while avoiding the drawbacks of conventional MEMS processing motivates development of new MEMS fabrication techniques which do not rely on photolithography or other solvent-processing, and can be performed at near room temperature, to avoid mechanical stresses and substrate damage. We demonstrate in this study a newMEMS fabrication technique using microcontact printing in atmospheric conditions to transfer continuous metal films over a relief structure, forming suspended metal membranes of sub-micrometer thickness that serve as mobile mechanical elements in capacitive MEMS devices. Our technology has the ability to form metallic MEMS structures without requiring elevated-temperature processing, high pressure, or wet chemical or aggressive plasma release etches. Simplicity and scalability of the demonstrated technique can create a paradigm shift in the design and fabrication of integrated MEMS devices. Compatibility of the technique with low temperature processing on flexible polymeric or metal foil substrates enables us to envision a complete method for rapid, near-room-temperature fabrication of flexible, large-area, integrated microor optoelectronic/MEMS circuits. TheMEMS structures are formed by the contact lift-off transfer (Contact-Transfer) technique, which enables us to pick up a thin metallic membrane from a donor transfer pad when the membrane is contacted by a viscoelastic stamp, such as polydimethylsiloxane (PDMS). The metallic membranes are first prepared by evaporating a thin metal film onto a donor transfer pad, which has been pre-coated with an organic molecular release layer prior to metal deposition. The surface of the PDMS stamp is placed in contact with the planar metallic membrane then rapidly peeled off, picking up the metal film (Fig. 1). During the rapid removal of the viscoelastic PDMS stamp, the weak adhesion energy to the metal is increased sufficiently to effect pick up, due to the kinetically controlled adhesion characteristic of elastomers. The PDMS stamp is molded with 20-mm-scale ridges using a siliconmaster grating, so that only some of the stamp area adheres to the metal film when the two are brought in contact. However, when the stamp and the donor pad are separated,