Ali Fatih Sarioglu

Cantilevers with integrated sensor for time-resolved force measurement in tapping-mode atomic force microscopy

We present a micromachined cantilever with an integrated high-bandwidth resonator for direct measurement of tip-sample interaction forces in tapping-mode atomic force microscopy. Force measurements are achieved by a diffraction grating that serves as a differential displacement sensor for the tip motion relative to the cantilever body. Time-resolved tip-sample interaction force measurement is demonstrated on a silicon sample following calibration of the probe structure. By using lock-in detection, the harmonics of periodic tip-sample interaction have been utilized to obtain high-contrast, material specific images. The harmonic images of patterned silicon/silicon nitride control samples and triblock copolymers are presented.

Characterization of cross-coupling in capacitive micromachined ultrasonic transducers

This paper analyzes element-to-element and cell- to-cell cross-coupling in capacitive micromachined ultrasonic transducers (cMUTs) using an interferometer. In a 1-D linear cMUT array immersed in oil, a single element was excited, and membrane displacements were measured at different positions along the array with an interferometer. Electrical measurements of the received voltage on each array element were also performed simultaneously to verify the optical measurements. The array was then covered with a polydimethylsiloxane (PDMS) layer, and the cross-coupling measurements were repeated. The cross-coupling levels for conventional and collapsed operation of the cMUT were compared. Since the cMUTs were immersed in oil, the optical measurements were corrected for acousto-optic interaction, and the results were reviewed in time-spatial and frequency- spatial domains. The main cross-coupling mechanism was due to the dispersive guided modes supported by the membrane periodicity. In both modes of operation, cross-coupling dispersion curves predicted a gradual reduction in phase velocity at higher frequencies. At lower frequencies, this phase velocity tended to approach 1480 m/s asymptotically. Better cross-coupling suppression was observed in the collapsed (-34 dB) than the conventional operation (-23 dB). The element-to-element cross-coupling experiments showed that a 5-µm PDMS layer reduced the measured cross- coupling levels down to -39 dB in the collapsed operation. were corrected to eliminate the acousto-optic interaction due to the refractive index of the oil and the pressure created in the oil (4). The optical time domain measurements were analyzed in the wave number-frequency (k-w) domain for the multi-mode wave propagation (5). Conventional and collapsed operations of the cMUT were compared, and the influence of a 5-µm polydimethylsiloxane (PDMS) layer covering the cMUT was investigated. The main cross-coupling mechanism was due to the dispersive guided modes. Interface waves (Stoneley-Scholte) and surface waves (Rayleigh) were relatively weak in cross-coupling (3). The dispersive guided modes were determined for conventional and collapsed operations and corresponding k-w diagrams were analyzed.

Displacement Sensing With a Mechanically Tunable Photonic Crystal

We demonstrate a mechanically tunable photonic crystal (PC) designed for highly position-sensitive reflection. Our device consists of a multilayer PC membrane attached to a silicon microelectromechanical system structure for lateral actuation. Inside each of the PC holes there is a pillar, which is attached to the substrate. By moving the PC membrane with respect to the pillars we change the shape of the PC holes and thereby modulate the PC reflectivity. Our measurements show a reflectivity change of more than 80% for a 115-nm lateral displacement.

Modeling, design, and analysis of interferometric cantilevers for time-resolved force measurements in tapping-mode atomic force microscopy

Cantilevers with interferometric high bandwidth force sensors can resolve nonlinear tip-sample interaction forces in tapping-mode atomic force microscopy. In this paper, we provide a detailed analysis of time-resolved force measurements using such cantilever. We first model the probe as a coupled spring-mass system and investigate its steady state dynamics under tapping-mode imaging conditions. Next, we analyze the optical response of the interferometric force sensor: Diffraction patterns as a function of tip displacement are obtained both analytically and by simulations. Finally, the frequency response of the force sensor is calculated, and the effects of the sensor geometry variations on the sensor mechanical response are analyzed.

High-Resolution Nanomechanical Mapping Using Interferometric-Force-Sensing AFM Probes

In this paper, we demonstrate high-resolution mapping of composite surfaces based on their nanomechanical properties by employing an atomic force microscope probe that can resolve high-frequency tip-sample interaction forces in tapping-mode atomic force microscopy (AFM) (TM-AFM). Time-resolved force measurements are achieved by a small integrated interferometric high-bandwidth grating force sensor at the end of the cantilever beam. The probes are batch fabricated using optical lithography and are used in an AFM system with no further modification. The fabricated devices are characterized and grating-force-sensor signals and their measured spectra confirm that probes with integrated high-bandwidth force sensors can successfully resolve high-frequency tip-sample interaction forces. We utilize the capability of our probes to form images by recording the amplitude of the higher harmonics of the time-resolved tip-sample interaction forces with lock-in detection. These images show that our probes can successfully map, with high spatial resolution, the mechanical contrast due to different materials and structural differences in composite surfaces.

Acoustic lens for capacitive micromachined ultrasonic transducers

Capacitive micromachined ultrasonic transducers (CMUTs) have great potential to compete with traditional piezoelectric transducers in therapeutic ultrasound applications. In this paper we have designed, fabricated and developed an acoustic lens formed on the CMUT to mechanically focus ultrasound. The acoustic lens was designed based on the paraxial theory and made of silicone rubber for acoustic impedance matching and encapsulation. The CMUT was fabricated based on the local oxidation of silicon (LOCOS) and fusion-bonding. The fabricated CMUT was verified to behave like an electromechanical resonator in air and exhibited wideband response with a center frequency of 2.2 MHz in immersion. The fabrication for the acoustic lens contained two consecutive mold castings and directly formed on the surface of the CMUT. Applied with ac burst input voltages at the center frequency, the CMUT with the acoustic lens generated an output pressure of 1.89 MPa (peak-to-peak) at the focal point with an effective focal gain of 3.43 in immersion. Compared to the same CMUT without a lens, the CMUT with the acoustic lens demonstrated the ability to successfully focus ultrasound and provided a viable solution to the miniaturization of the multi-modality forward-looking endoscopes without electrical focusing.

Singulation for imaging ring arrays of capacitive micromachined ultrasonic transducers

Singulation of MEMS is a critical step in the transition from wafer-level to die-level devices. As is the case for capacitive micromachined ultrasound transducer (CMUT) ring arrays, an ideal singulation must protect the fragile membranes from the processing environment while maintaining a ring array geometry. The singulation process presented in this paper involves bonding a trench-patterned CMUT wafer onto a support wafer, deep reactive ion etching (DRIE) of the trenches, separating the CMUT wafer from the support wafer and de-tethering the CMUT device from the CMUT wafer. The CMUT arrays fabricated and singulated in this process were ring-shaped arrays, with inner and outer diameters of 5 mm and 10 mm, respectively. The fabricated CMUT ring arrays demonstrate the ability of this method to successfully and safely singulate the ring arrays and is applicable to any arbitrary 2D shaped MEMS device with uspended microstructures, taking advantage of the inherent planar attributes of DRIE.

Time-Resolved Tapping-Mode Atomic Force Microscopy

Atomic force microscopy has unprecedented potential for quantitative mapping of material-specific surface properties on the nanoscale. Unfortunately, methods developed for local stiffness measurements suffer from low operational speeds and they require large forces to be applied to the surface, limiting resolution and precluding measurements on soft materials such as polymers and biological samples. On the other hand, tapping-mode AFM, which is well suited to soft materials due to its gentle interaction with the surface, cannot be used to recover information on the tip–sample interaction (and hence, on the material properties) due to limited mechanical bandwidth offered by the resonant AFM probe. In this chapter, a technique, called Time-resolved Tapping-mode Atomic Force Microscopy, designed for rapid quantitative material characterization on the nanoscale is described. The technique is based on time-resolved measurement of tip–sample interaction forces during tapping-mode AFM imaging by a specially designed micromachined AFM probe. The probe has an integrated high-bandwidth interferometric force sensor that is used to resolve tip–sample interaction forces with high sensitivity and temporal resolution. In the first part of the chapter, the theory, design, and fabrication of the probes are described in detail. Then quantitative force measurements with microsecond time resolution in tapping-mode imaging are presented. Finally, higher harmonic images based on the interaction force measurements are presented for various samples, demonstrating the range of applications of the technique.

Interferometric force sensing AFM probes for nanomechanical mapping of material properties

In this paper, we describe an AFM probe that measures tip-sample interaction forces in tapping-mode AFM imaging. In our probes, a high-bandwidth interferometric force sensor at the end of the cantilever is coupled to the tip motion, and it is used to resolve tip-sample interaction forces with high temporal resolution. Measurements of the sensor signal show that the tip-sample interaction during imaging can be resolved with high sensitivity and high temporal resolution. In our experiments, the harmonics of the tip-sample interaction are used to map chemical and structural properties of the materials on the nanoscale.

Large Area 1D CMUT Phased Arrays for Multi-Modality Ultrasound Imaging

We present the design and fabrication of 1D capacitive micromachined ultrasonic transducer (CMUT) arrays optimized for imaging using multiple modalities. A new variation of the fabrication process based on a thick buried oxide layer is used to build these CMUTs. In our process, the via connections for each cells electrode with the handle layer are made from the front side. This enables fabricating CMUT cells with smaller sizes, and higher resonant frequencies. Initial characterization results agree well with our design simulations.