S. R. Manalis

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.

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.

Analysis and design of an interdigital cantilever as a displacement sensor

conventional cantilever used in the atomic force microscope ~AFM!. In this paper we present a detailed analysis of the interdigital cantilever and its use as a sensor for the AFM. In this study, we combine finite element analysis with diffraction theory to simulate the mechanically induced optical response of the ID. This model is used to compare this system with the optical lever detector as used in conventional instruments by analyzing the ratio of signal to noise and overall performance. We find that optical detection of the cantilever motion with interdigital fingers has two advantages. When used in conjunction with arrays of cantilevers it is far easier to align. More importantly, it is immune to laser pointing noise and thermally excited mechanical vibrations and this improves the sensitivity as compared to the optical lever.

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.

Two-dimensional micromechanical bimorph arrays for detection of thermal radiation

We demonstrate that two-dimensional arrays of micromechanical bimorphs can be used as thermal sensors to image infrared (IR) radiation. A density of 100 pixels per mm2 is achieved by coiling a bimorph beam into the shape of a flat spiral. Temperature variations of a given spiral are converted to modulations of visible light by illuminating the spiral array with a visible source. The optical properties of the spiral resemble a Fresnel zone plate when light reflected off neighboring rings of the spiral is focused. When a spiral is heated through the absorption of IR radiation, thermally induced bending of the bimorph degrades the focusing efficiency by distorting the spiral. This reduces the optical intensity at the focal point. Arrays of spirals can be monitored with a commercial CCD camera. At 40 Hz, the temperature resolution and noise equivalent power of a 75 μm diam spiral are 50 μK/√Hz and 20 nW/√Hz, respectively, and the thermal response time is 270 μs.

Independent parallel lithography using the atomic force microscope

The scanning probe microscope ~SPM! has demonstrated itself to be a versatile and effective tool for patterning surfaces at the nanometer scale. Two common methods for modifying surfaces using probe microscopes are direct physical patterning and electric field assisted patterning. While both methods of surface modification are quite different, they both require that a sharp tip interacts with the surface to be patterned. Physical patterning consists of scribing or indenting a sample using the tip of the SPM. Jung 1 has used this process to scribe patterns into polymer surfaces, and Mamin 2 has used the physical indentation process in conjunction with laser heating to store 100 nm bits at 100 kHz in a polymer surface. This approach has the advantage that the sample is typically much softer and generally unreactive with the tip, thereby reducing tip wear. The literature on electrical modification of surfaces with probe microscopes is much more extensive. Scanning probe lithography was pioneered by Dagata, 3 who patterned ^111& silicon with the scanning tunneling microscope ~STM!, and Lyding 4 has used this same technique in ultrahigh vacuum ~UHV! to pattern features less than a few nanometers. Snow and Campbell have modified this technique and patterned Si 5 and GaAs 6 with the atomic

Contact imaging in the atomic force microscope using a higher order flexural mode combined with a new sensor

Using an atomic force microscope (AFM) with a silicon cantilever partially covered with a layer of zinc oxide (ZnO), we have imaged in the constant force mode by employing the ZnO as both a sensor and actuator. The cantilever deflection is determined by driving the ZnO at the second mechanical resonance while the tip is in contact with the sample. As the tip‐sample force varies, the mechanical boundary condition of the oscillating cantilever is altered, and the ZnO electrical admittance is changed. Constant force is obtained by offsetting the ZnO drive so that the admittance remains constant. We have also used the ZnO as an actuator and sensor for imaging in the intermittent contact mode. In both modes, images produced by using the ZnO as a sensor are compared to images acquired with a piezoresistive sensor.