S. C. Minne

Parallel atomic force microscopy using cantilevers with integrated piezoresistive sensors and integrated piezoelectric actuators

We have fabricated and operated two cantilevers in parallel in a new mode for imaging with the atomic force microscope (AFM). The cantilevers contain both an integrated piezoresistive silicon sensor and an integrated piezoelectric zinc oxide (ZnO) actuator. The integration of sensor and actuator on a single cantilever allows us to simultaneously record two independent AFM images in the constant force mode. The ZnO actuator provides over 4 μm of deflection at low frequencies (dc) and over 30 μm deflection at the first resonant frequency. The piezoresistive element is used to detect the strain and provide the feedback signal for the ZnO actuator.

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.

Terabit-per-square-inch data storage with the atomic force microscope

An areal density of 1.6 Tbits/in.2 has been achieved by anodically oxidizing titanium with the atomic force microscope (AFM). This density was made possible by (1) single-wall carbon nanotubes selectively grown on an AFM cantilever, (2) atomically flat titanium surfaces on α-Al2O3 (1012), and (3) atomic scale force and position control with the tapping-mode AFM. By combining these elements, 8 nm bits on 20 nm pitch are written at a rate of 5 kbit/s at room temperature in air.

Fabrication of 0.1 μm metal oxide semiconductor field‐effect transistors with the atomic force microscope

Using the atomic force microscope (AFM), we have fabricated a metal oxide semiconductor field‐effect transistor (MOSFET) on silicon with an effective channel length of 0.1 μm. The lithography at the gate level was performed with the scanning tip of the AFM. The gate was defined by electric‐field‐enhanced selective oxidation of the amorphous silicon gate electrode. The electrical characteristics were reasonable with a transconductance of 279 mS/mm and a threshold voltage of 0.55 V.

Near-field photolithography with a solid immersion lens

We have exposed 190 nm lines in photoresist by focusing a laser beam (λ=442 nm) in a solid immersion lens (SIL) that is mounted on a flexible cantilever and scanned by a modified commercial atomic force microscope. The scan rate was 1 cm/s, which is several orders of magnitude faster than typical reports of near-field lithography using tapered optical fibers. The enhanced speed is a result of the high optical efficiency (about 10−1) of the SIL. Once exposed with the SIL, the photoresist was developed and the pattern was transferred to the silicon substrate by plasma etching.

ATOMIC FORCE MICROSCOPY FOR HIGH SPEED IMAGING USING CANTILEVERS WITH AN INTEGRATED ACTUATOR AND SENSOR

A cantilever with an integrated ZnO piezoelectric actuator in feedback with a piezoresistive sensor is utilized in an atomic force microscope (AFM) to achieve a new high speed imaging technique. The imaging bandwidth is increased from 0.6 to 6 kHz by bending the cantilever over sample topography with the actuator rather than moving the sample with a 2 in. piezotube. Images taken in the constant force mode with a 3 mm/s tip velocity of a sample containing 2 μm vertical steps are presented. The effects of electrical coupling from the actuator were eliminated by measuring the piezoresistor sensor with a lock‐in amplifier.

Interdigital cantilevers for atomic force microscopy

We present a sensor for the atomic force microscope (AFM) where a silicon cantilever is micromachined into the shape of interdigitated fingers that form a diffraction grating. When detecting a force, alternating fingers are displaced while remaining fingers are held fixed. This creates a phase sensitive diffraction grating, allowing the cantilever displacement to be determined by measuring the intensity of diffracted modes. This cantilever can be used with a standard AFM without modification while achieving the sensitivity of the interferometer and maintaining the simplicity of the optical lever. Since optical interference occurs between alternating fingers that are fabricated on the cantilever, laser intensity rather than position can be measured by crudely positioning a photodiode. We estimate that the rms noise of this sensor in a 10 hz–1 kHz bandwidth is ∼0.02 A and present images of graphite with atomic resolution.

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.