Apoorva Murarka

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,

Electrically tunable organic vertical-cavity surface-emitting laser

An electrically tunable organic vertical-cavity surface-emitting laser (VCSEL) is demonstrated and characterized. A lasing wavelength tunability of Δλ = 10 nm with 6 V actuation is shown for a red laser emission tuned between λ = 637 nm and λ = 628 nm. Wavelength tuning of the VCSEL structure is enabled by electrostatic deflection of a reflective flexible membrane that is suspended over an air gap and a dielectric mirror, forming a 3λ lasing cavity. The lasing gain medium consists of an evaporated organic thin film coated on a reflective membrane, which is then additively placed over a patterned substrate containing the dielectric mirror to fabricate an array of air-gap-VCSEL structures, each 100 μm in diameter. Beyond the electrostatic actuation of these tunable lasers, the VCSEL array geometry also has the potential to be used as pressure sensors with an all-optical remote excitation and readout and a pressure sensitivity of 64 Pa/nm in the demonstrated configuration.

Transfer-printed composite membranes for electrically-tunable organic optical microcavities

We demonstrate a method for fabricating organic optical microcavities which can be electrostatically actuated to dynamically tune their optical output spectra. Fabrication of an integrated organic micro-opto-electro-mechanical system (MOEMS) cavity is enabled by the solvent-free additive transfer of a composite membrane. Electrical actuation and optical characterization of a completed cavity show resonance tuning greater than 20 nm for membrane deflections of over 200 nm at 50 V.

Printed MEMS membranes on silicon

We report a new method for additive fabrication of thin (125±15 nm thick) gold membranes on patterned silicon dioxide (SiO2) substrates for acoustic MEMS. The deflection of these membranes, suspended over cavities in a SiO2 dielectric layer atop a conducting electrode, can be used to produce sounds or monitor pressure. This process uses a novel technique of dissolving an underlying organic film using acetone to transfer membranes onto SiO2 substrates. The process avoids fabrication of MEMS diaphragms via wet or deep reactiveion etching, which in turn removes the need for etch-stops and wafer bonding. Membranes up to 0.78 mm2 in area are fabricated and their deflection is measured using optical interferometry. The membranes have a maximum deflection of about 150 nm across 28 μm diameter cavities. Youngs modulus of these films is shown to be 74±17 GPa, and their potential sound pressure generation at 15 V is calculated.

Micro-contact printed MEMS

We report a new method for fabricating thin (140 nm thick) suspended metal films in MEMS. Our MEMS fabrication process employs micro-contact printing. It avoids the use of solvents and etchants, obviating the need for deep reactive-ion etching and other harsh chemical treatments. Solvent absence during fabrication also avoids the deleterious effects of MEMS stiction that can result during wet processing. Elevated temperature processing is also avoided to enable MEMS fabrication on flexible polymeric substrates. Thin films up to 0.78 mm2 in area are fabricated and the deflection of 25 µm diameter films is demonstrated. These films can be utilized in pressure sensors, microphones, deformable mirrors, tunable optical cavities, and large-area arrays of MEMS sensors.

Printed membrane electrostatic MEMS microspeakers

We report the fabrication and broadband actuation of capacitive electrostatic MEMS microspeakers formed by the contact-transfer printing of suspended 125-nm-thick gold membranes over cavities in a 430-nm-thick silicon dioxide (SiO2) spacer layer on a conducting substrate. The membranes deflect repeatedly to produce sound upon electrostatic actuation with a time-varying signal. The microspeaker has a flat acoustic frequency response that is devoid of any resonance peaks. Its output sound pressure level (SPL) increases at ~40 dB/decade with increasing frequency, in the 3 highest octaves of the human audio range, for constant excitation voltage autospectral density. The microspeaker consumes 24 μW of real electric power under broadband actuation in free field. It outputs 35 dB (SPL/VoltRMS) of acoustic pressure at 20 kHz drive. The microspeaker operates at a 10VDC bias, thus enabling the use of electrostatic sound sources in portable audio applications such as hearing aids and earphones. The diaphragm-forcing VBIASVRMS product is reduced to ~19 V2. The total thickness of the microspeakers is dominated by the silicon wafer substrate, with an active device thickness of less than 700 nm.

Electrically tunable organic vertical cavity surface emitting laser

Using solvent-free composite membrane transfer, we demonstrate an electrically tunable organic visible light-emitting laser with reversible tuning range of 10 nm under 6 V actuation. Large-area scalability of utilized fabrication methods suggests potential use in all-optical pressure-sensing surfaces.