Goksen G. Yaralioglu

Automated parallel high-speed atomic force microscopy

An expandable system has been developed to operate multiple probes for the atomic force microscope in parallel at high speeds. The combined improvements from parallelism and enhanced tip speed in this system represent an increase in throughput by over two orders of magnitude. A modular cantilever design has been replicated to produce an array of 50 cantilevers with a 200 μm pitch. This design contains a dedicated integrated sensor and integrated actuator where the cells can be repeated indefinitely. Electrical shielding within the array virtually eliminates coupling between the actuators and sensors. The reduced coupling simplifies the control electronics, facilitating the design of a computer system to automate the parallel high-speed arrays. This automated system has been applied to four cantilevers within the array of 50 cantilevers, with a 20 kHz bandwidth and a noise level of less than 50 A. For typical samples, this bandwidth allows us to scan the probes at 4 mm/s.

High-speed tapping mode imaging with active Q control for atomic force microscopy

The speed of tapping mode imaging with the atomic force microscope (AFM) has been increased by over an order of magnitude. The enhanced operation is achieved by (1) increasing the instrument’s mechanical bandwidth and (2) actively controlling the cantilever’s dynamics. The instrument’s mechanical bandwidth is increased by an order of magnitude by replacing the piezotube z-axis actuator with an integrated zinc oxide (ZnO) piezoelectric cantilever. The cantilever’s dynamics are optimized for high-speed operation by actively damping the quality factor (Q) of the cantilever. Active damping allows the amplitude of the oscillating cantilever to respond to topography changes more quickly. With these two advancements, 80μm×80 μm high-speed tapping mode images have been obtained with a scan frequency of 15 Hz. This corresponds to a tip velocity of 2.4 mm/s.

Centimeter scale atomic force microscope imaging and lithography

We present a 4 mm2 image taken with a parallel array of 10 cantilevers, an image spanning 6.4 mm taken with 32 cantilevers, and lithography over a 100 mm2 area using an array of 50 cantilevers. All of these results represent scan areas that are orders of magnitude larger than that of a typical atomic force microscope (0.01 mm2). Previously, the serial nature and limited scan size of the atomic force microscope prevented large scale imaging. Our design addresses these issues by using a modular micromachined parallel atomic force microscope array in conjunction with large displacement scanners. High-resolution microscopy and lithography over large areas are important for many applications, but especially in microelectronics, where integrated circuit chips typically have nanometer scale features distributed over square centimeter areas.

Calculation and measurement of electromechanical coupling coefficient of capacitive micromachined ultrasonic transducers

The electromechanical coupling coefficient is an important figure of merit of ultrasonic transducers. The transducer bandwidth is determined by the electromechanical coupling efficiency. The coupling coefficient is, by definition, the ratio of delivered mechanical energy to the stored total energy in the transducer. In this paper, we present the calculation and measurement of coupling coefficient for capacitive micromachined ultrasonic transducers (CMUTs). The finite element method (FEM) is used for our calculations, and the FEM results are compared with the analytical results obtained with parallel plate approximation. The effect of series and parallel capacitances in the CMUT also is investigated. The FEM calculations of the CMUT indicate that the electromechanical coupling coefficient is independent of any series capacitance that may exist in the structure. The series capacitance, however, alters the collapse voltage of the membrane. The parallel parasitic capacitance that may exist in a CMUT or is external to the transducer reduces the coupling coefficient at a given bias voltage. At the collapse, regardless of the parasitics, the coupling coefficient reaches unity. Our experimental measurements confirm a coupling coefficient of 0.85 before collapse, and measurements are in agreement with theory.

Characterization and optimization of scan speed for tapping-mode atomic force microscopy

Increasing the imaging speed of tapping mode atomic force microscopy (AFM) has important practical and scientific applications. The scan speed of tapping-mode AFMs is limited by the speed of the feedback loop that maintains a constant tapping amplitude. This article seeks to illuminate these limits to scanning speed. The limits to the feedback loop are: (1) slow transient response of probe; (2) instability limitations of high-quality factor (Q) systems; (3) feedback actuator bandwidth; (4) error signal saturation; and the (5) rms-to-dc converter. The article will also suggest solutions to mitigate these limitations. These limitations can be addressed through integrating a faster feedback actuator as well as active control of the dynamics of the cantilever.

5×5 2D AFM cantilever arrays a first step towards a Terabit storage device

Abstract In this paper we report on the microfabrication of a 5×5 2D cantilever array and its successful application to parallel imaging. The 5×5 array with integrated force sensing and tip heating has been fabricated using a recently developed, all dry, silicon backside etching process. The levers on the array have integrated piezoresistive sensing, and are placed on a constriction in the lever to improve sensitivity. The array is scanned in x and y directions using voice coil actuators. Three additional voice coil z actuators are used in a triangular arrangement to approach the sample with the array chip. The system is thus leveled in the same way as an air table. We report details of the array fabrication, the x – y scanning and approach system as well as images taken with the system. The results are encouraging for the development of large-scale VLSI-Nano EMS, allowing the fabrication and operation of large AFM cantilever arrays to achieve high-data-rate Terabit storage systems.

A new regime for operating capacitive micromachined ultrasonic transducers

We report on a new operation regime for capacitive micromachined ultrasonic transducers (cMUTs). Traditionally, cMUTs are operated at a bias voltage lower than the collapse voltage of their membranes. In the new proposed operation regime, first the cMUT is biased past the collapse voltage. Second, the bias voltage applied to the collapsed membrane is reduced without releasing the membrane. Third, the cMUT is excited with an ac signal at the bias point, keeping the total applied voltage between the collapse and snapback voltages. In this operation regime, the center of the membrane is always in contact with the substrate. Our finite element methods (FEM) calculations reveal that a cMUT operating in this new regime, between collapse and snapback voltages, possesses a coupling efficiency (k/sub T//sup 2/) higher than a cMUT operating in the conventional regime below its collapse voltage. This paper compares the simulation results of the coupling efficiencies of cMUTs operating in conventional and new operation regimes.

Finite-element analysis of capacitive micromachined ultrasonic transducers

In this paper, we present the results of finite-element analysis performed to investigate capacitive micromachined ultrasonic transducers (CMUTs). Both three-dimensional (3-D) and 2-D models were developed using a commercially available finite-element modeling (FEM) software. Depending on the dimensionality of the model, the membranes were constructed using plane or shell elements. The electrostatic gap was modeled using many parallel plate transducers. An axisymmetric model for a single membrane was built; the electrical input impedance of the device then was calculated in vacuum to investigate series and parallel resonant frequencies, where the input impedance has a minimum and a maximum, respectively. A method for decomposing the membrane capacitance into parasitic and active parts was demonstrated, and it was shown that the parallel resonant frequency shifted down with increased biased voltage. Calculations then were performed for immersion transducers. Acoustic wave propagation was simulated in the immersion medium, using appropriate elements in a 3-D model. Absorbing boundaries were implemented to avoid the reflections at the end of the medium mesh. One row of an array element, modeled with appropriate boundary conditions, was used to calculate the output pressure. The results were compared with a simpler model: a single membrane in immersion, with symmetry boundary conditions on the sidewalls that cause the calculations to reflect the properties of an infinitely large array. A 2-D model then was developed to demonstrate the effect of membrane dimensions on the output pressure and bandwidth. Our calculations revealed that the small signal transmit pressure was inversely proportional to the square root of gap height. We also compared FEM results with analytical and experimental results.

50 kHz capacitive micromachined ultrasonic transducers for generation of highly directional sound with parametric arrays

In this study, we examine the use of capacitive micromachined ultrasonic transducers (CMUTs) with vacuum- sealed cavities for transmitting directional sound with parametric arrays. We used finite element modeling to design CMUTs with 40-mum- and 60-mum-thick membranes to have resonance frequencies of 46 kHz and 54 kHz, respectively. The wafer bonding approach used to fabricate the CMUTs provides good control over device properties and the capability to fabricate CMUTs with large diameter membranes and deep cavities. Each CMUT is 8 cm in diameter and consists of 284 circular membranes. Each membrane is 4 mm in diameter. Characterization of the fabricated CMUTs shows they have center frequencies of 46 kHz and 55 kHz and 3 dB bandwidths of 1.9 kHz and 5.3 kHz for the 40-mum- and 60-mum-thick membrane devices, respectively. With dc bias voltages of 380 V and 350 V and an ac excitation of 200 V peak-to-peak, the CMUTs generate average sound pressure levels, normalized to the devices surface, of 135 dB and 129 dB (re 20 muPa), respectively. When used to generate 5 kHz sound with a parametric array, we measured sound at 3 m with a 6 dB beamwidth of 8.7deg and a sound pressure level of 58 dB. To understand how detector nonlinearity (e.g., the nonlinearity of the microphone used to make the sound level measurements) affects the measured sound pressure level, we made measurements with and without an acoustic low-pass filter placed in front of the microphone; the measured sound levels agree with numerical simulations of the pressure field. The results presented in this paper demonstrate that large-area CMUTs, which produce high-intensity ultrasound, can be fabricated for transmitting directional sound with parametric arrays.

Capacitive micromachined ultrasonic transducer design for high power transmission

Capacitive micromachined ultrasonic transducers (cMUTs) were developed to meet the demands of the ultrasonic industry. To achieve maximum efficiency, the conventional operation of the cMUT requires a bias voltage close to the collapse voltage. Total acoustic output pressure is limited by the efficiency of the cMUT and the maximum-allowed pulse voltage on the membrane. In this paper, we propose the collapse-snapback operation of the cMUT: the membrane is collapsed onto the substrate in the collapsing cycle, and released in the snapback cycle. The collapse-snapback operation overcomes the above-mentioned limitations of the conventional operation. The collapse-snapback operation utilizes a larger range of membrane deflection profiles (both collapsed and released profiles) and generates higher acoustic output pressures. The static finite element calculations were performed to design cMUTs with specific collapse and snapback voltages by changing the electrode parameters (radius (r/sub e/), position (d/sub e/), and thickness (t/sub e/)). These designs were refined for optimum average displacement per cycle. An electrode radius greater than 60% of the membrane radius significantly improved the displacement per volt. Moderately thick membranes (t/sub e//spl sim/0.2 /spl mu/m) were preferred, as thicker membranes reduced the displacement per volt. Under proper bias conditions, the collapse-snapback operation, designed for high-power transmission, allowed the application of pulse voltages larger than the difference of collapse and snapback voltages. Dynamic finite element calculations of an infinite cMUT array on the substrate loaded with acoustic fluid medium were performed to determine the dynamic response of the cMUT. Commercially available FEM packages ANSYS and LSDYNA were used for static and dynamic calculations, respectively. The cMUTs were fabricated for optimal performance in the collapse-snapback operation. The transmit experiments were performed on a 2-D cMUT array using a calibrated hydrophone. Taking into account the attenuation and diffraction losses, the pressure on the cMUT surface was extracted. The cMUT generated 0.47 MPa (6 kPa/V) and 1.04 MPa (11 kPa/V) in the conventional and collapse-snapback operations, respectively. Therefore, collapse-snapback operation of the cMUTs was superior for high-power transmission.