Optical Lever Detection Research Articles

Back action evasion in optical lever detection

The optical lever is a centuries old and widely used detection technique employed in applications ranging from consumer products and industrial sensors to precision force microscopes used in scientific research. However, despite the long history, its quantum limits have yet to be explored. In general, any precision optical measurement is accompanied by optical force induced disturbance to the measured object (termed as back action) leading to a standard quantum limit (SQL). Here, we give a simple ray optics description of how such back action can be evaded in optical lever detection. We perform a proof-of-principle experiment demonstrating the mechanism of back action evasion in the classical regime, by developing a lens system that cancels extra tilting of the reflected light off a silicon nitride membrane mechanical resonator caused by laser-pointing-noise-induced optical torques. We achieve a readout noise floor two orders of magnitude lower than the SQL, corresponding to an effective optomechanical cooperativity of 100 without the need for an optical cavity. As the state-of-the-art ultralow dissipation optomechanical systems relevant for quantum sensing are rapidly approaching the level where quantum noise dominates, simple and widely applicable back action evading protocols will be crucial for pushing beyond quantum limits.

Thermal noise in contact atomic force microscopy

As one of the fundamental sources of noise in atomic force microscopy (AFM), thermal fluctuations of the cantilever have been studied for the case of a free tip but not much for cantilevers in contact. In this paper, using the equipartition theorem, we calculated the thermal deflection amplitude for all normal modes of an elastically supported AFM cantilever, including the free cantilever as a special case. With increasing contact stiffness, the mean thermal fluctuation amplitude decreases for all cantilever modes when in the elastic contact. In addition, considering the optical lever detection scheme used in most AFMs, we calculated the corresponding output thermal noise amplitude. The experiments validated our theoretical calculations. Our investigation facilitates a more comprehensive understanding of the thermal noise in AFM. It provides guidance for thermally excited contact-resonance AFM, which is promising for quantitative viscoelastic measurements.

Fast atomic force microscopy with self-transduced, self-sensing cantilever

The conventional optical lever detection technique involves optical components and their precise mechanical alignment. An additional technical limit is the weight of the optical system in cases where a top-scanner is used with high-speed and high-precision metrology. An alternative represents the application of self-actuated atomic force microscopy (AFM) cantilevers with integrated two-dimensional electron gas (2-DEG) piezoresistive deflection sensors. A significant improvement in the performance of such cantilevers with respect to deflection sensitivity and temperature stability has been achieved by using an integrated Wheatstone bridge configuration. Due to employing effective crosstalk isolation and temperature drift compensation, the performance of these cantilevers was significantly improved. In order to enhance the speed of AFM measurements, we present an adaptive scanning speed procedure. Examples of AFM measurements with a high scanning speed (up to 200 lines/s) committed to advanced lithography process development are shown.

Flexural sensitivity of high resonant atomic force microscopy cantilever based on optical lever detection

Flexural sensitivity is a key parameter of atomic force microscopy. In order to improve the dynamic atomic force microscopy’s flexural sensitivity, a scanning method that drives the atomic force microscopy cantilever vibrating at its high resonant frequency is utilized. In this article, the operation principle of the high resonant atomic force microscopy is introduced and the factors that affect the sensitivity in the measurement of the displacement of the amplitude based on optical lever detection method are analyzed, and the flexural sensitivities of the cantilever vibrating at different resonant frequencies are compared theoretically and measured experimentally. The experimental results indicate that the flexural resolution of atomic force microscopy operated on the fundamental mode and the second mode is 0.30 and 0.13 nm, respectively. Both the theoretical and experimental results demonstrate that the flexural sensitivity of the high resonant cantilever is better than that of the fundamental mode cantilever. It is an effective method to improve the sensitivity of dynamic atomic force microscopy cantilever by working at higher resonant frequency.

Actuation of silicon pillar micro-mechanical resonators by Kelvin polarization force

Pillars resonators arrays are emerging as powerful sensors for their peculiar characteristics, namely high mass sensitivity, high active areas density and operation in liquid. On the other hand, their characteristic vertical geometry and fabrication strategy hinders a direct integration within integrated circuits. So far, the examples reported in literature are based on a bulk piezoelectric actuation and optical lever detection, which are not suitable for further integration. In this paper we report on the design, fabrication and characterization of Kelvin polarization force actuated pillar resonator. This work represents the first step toward a full integration of pillars with on-chip electronic actuation and detection.

Optimization and calibration of atomic force microscopy sensitivity in terms of tip-sample interactions in high-order dynamic atomic force microscopy

The mechanism of dynamic force modes has been successfully applied to many atomic force microscopy (AFM) applications, such as tapping mode and phase imaging. The high-order flexural vibration modes are recent advancement of AFM dynamic force modes. AFM optical lever detection sensitivity plays a major role in dynamic force modes because it determines the accuracy in mapping surface morphology, distinguishing various tip-surface interactions, and measuring the strength of the tip-surface interactions. In this work, we have analyzed optimization and calibration of the optical lever detection sensitivity for an AFM cantilever-tip ensemble vibrating in high-order flexural modes and simultaneously experiencing a wide range and variety of tip-sample interactions. It is found that the optimal detection sensitivity depends on the vibration mode, the ratio of the force constant of tip-sample interactions to the cantilever stiffness, as well as the incident laser spot size and its location on the cantilever. It is also found that the optimal detection sensitivity is less dependent on the strength of tip-sample interactions for high-order flexural modes relative to the fundamental mode, i.e., tapping mode. When the force constant of tip-sample interactions significantly exceeds the cantilever stiffness, the optimal detection sensitivity occurs only when the laser spot locates at a certain distance from the cantilever-tip end. Thus, in addition to the “globally optimized detection sensitivity,” the “tip optimized detection sensitivity” is also determined. Finally, we have proposed a calibration method to determine the actual AFM detection sensitivity in high-order flexural vibration modes against the static end-load sensitivity that is obtained traditionally by measuring a force-distance curve on a hard substrate in the contact mode.

Cantilever spring constant calibration using laser Doppler vibrometry

Uncertainty in cantilever spring constants is a critical issue in atomic force microscopy (AFM) force measurements. Though numerous methods exist for calibrating cantilever spring constants, the accuracy of these methods can be limited by both the physical models themselves as well as uncertainties in their experimental implementation. Here we report the results from two of the most common calibration methods, the thermal tune method and the Sader method. These were implemented on a standard AFM system as well as using laser Doppler vibrometry (LDV). Using LDV eliminates some uncertainties associated with optical lever detection on an AFM. It also offers considerably higher signal to noise deflection measurements. We find that AFM and LDV result in similar uncertainty in the calibrated spring constants, about 5%, using either the thermal tune or Sader methods provided that certain limitations of the methods and instrumentation are observed.

Investigation on AFM-based micro/nano-CNC machining system

To solve the problems in the atomic force microscope (AFM)-based nanomachining process such as nonlinearity and low repeatability positioning accuracy of AFM scanner in a larger scale, a novel micro/nanomachining system similar to conventional CNC machine tools was presented. The system integrated AFM with a precision stage. AFM optical lever detection method was employed to apply a very light normal load on the sample surface. An AFM diamond tip was utilized as a cutting tool to scratch the sample. The precision stage was used as a worktable to move the sample. This process was different from machining with a planer. Based on the system, effects of tip geometry, the scratching direction, the normal load, the machining velocity and the feed on the machining depth were discussed. Fabricating techniques of two-dimensional and three-dimensional complex micro/nano-structures using AFM-based mechanical scratching method based on this system were presented. Complex regular structures were fabricated. Moreover, AFM-tip-induced local anodic oxidation was also carried out based on this system, which displayed higher repeatability positioning accuracy with a larger machining dimensions and a significant machining ability of this system.

3D force components measurement in AFM scratching tests

The demand for enhancing the prospects in the application of AFM in nano tribology and nano machining has made it necessary to measure 3D force components in the AFM scratching tests. Based on B. Bhushan's calibration method for AFM, a modified calibration method to measure 3D force components is proposed by using the optical lever detection method and a high precision stage. The method is further verified through friction coefficient measurement between the sapphire and a diamond tip. Using a micro machining system combining AFM and the high accuracy stage, forces in scratching a single line are measured. The method for force data collecting and processing is also given. This method can be applied in the cutting force measurement using AFM cantilever to perform nano machining or nano tribology tests.

Optical lever detection in higher eigenmode dynamic atomic force microscopy

The optical lever detection scheme is widely used in atomic force microscopy for the detection of the cantilever deflection. Laser spot size as well as adjustment of the laser along the cantilever determine the zeros of the transfer function of the signal path from the tip-sample forces to the optical readout. This can cause (almost) pole-zero cancellations which lead to a significantly reduced sensitivity in the detection of higher mode vibrations of the cantilever. Physically, the light lever detection integrates over the slope of the cantilever. However, the sign of the slope of higher flexural modes varies along the cantilever. Thus, integration can lead to a significantly decreased sensitivity to higher eigenmode vibrations. Illuminating only the area between the free end and the next zero crossing of the slope of the modal shape provides a good compromise between high and low frequency sensitivity.

Calibration of optical cantilever deflection readers

Because of its ultrahigh sensitivity, the optical lever detection method similar to that used in the atomic force microscope (AFM) has been widely employed as a standard technique for measuring microcantilever deflection. Along with the increasing interest in using the microcantilever as a sensing platform, there is also a requirement for a reliable calibration technique. Many researchers have used the concept of optical lever detection to construct microcantilever deflection readout instruments for chemical, physical, and biological detection. However, without an AFM piezo z scanner, it is very difficult to precisely calibrate these instruments. Here, we present a step-by-step method to conveniently calibrate an instrument using commercially available piezoresistive cantilevers. The experimental results closely match the theoretical calculation. Following this procedure, one can easily calibrate any optical cantilever deflection detection system with high reproducibility, precision, and reliability. A detailed discussion of the optical lever readout system design has been addressed in this article.

Three-dimensional beam tracking for optical lever detection in atomic force microscopy

For the requirement to move a probe with respect to a large specimen like a silicon wafer, a three-dimensional beam tracking for optical lever force detection in atomic force microscopy has been developed. This beam tracking is not an approximate solution but an ideal solution in three-dimensional scanning. A set of tracking mirrors that are attached at the middle of a tube scanner steers a laser beam to a probe and to a detector during scanning. Tracking errors achieved were 2 and 7 nm in horizontal and vertical direction scans over 50 μm scanning. A simple theoretical error estimate revealed that less than 2 nm error is obtainable with moderate mirror mounting accuracy. To reduce the thickness of the scanner for coaxial optical microscope observation, the tube scanner was arranged sideways, resulting in 12 mm scanner thickness. A twist probe, pointed in the vertical direction to enter its tip into a narrow trench, was used for the probe. This system could obtain a semiconductor trench image successfully.

The Effect of Instrumental Uncertainties on AFM Indentation Measurements

The accuracy and precision of the quantitative measurements made with various types of scanning probe microscopes (SPMs) can be limited by instrumentation error. The piezoelectric scanners that are utilized in SPMs control either the motion of the cantilever probe with respect to a stationary sample or the motion of the sample with respect to a stationary probe. While these scanners offer many advantageous characteristics that are critical to the performance of SPMs, they also exhibit several behaviors, e.g., hysteresis and creep, that introduce uncertainties in measurements. Also, the nonlinearities associated with the photodiode used in the optical lever detection system can detract from the accuracy of measurements

High-speed atomic force microscopy using an integrated actuator and optical lever detection

A new procedure for high-speed imaging with the atomic force microscope that combines an integrated ZnO piezoelectric actuator with an optical lever sensor has yielded an imaging bandwidth of 33 kHz. This bandwidth is primarily limited by a mechanical resonance of 77 kHz when the cantilever is placed in contact with a surface. Images scanned with a tip velocity of 1 cm/s have been obtained in the constant force mode by using the optical lever to measure the cantilever stress. This is accomplished by subtracting an unwanted deflection produced by the actuator from the net deflection measured by the photodiode using a linear correction circuit. We have verified that the tip/sample force is constant by monitoring the cantilever stress with an implanted piezoresistor.

A simple low-temperature ultrahigh-vacuum scanning tunneling microscope capable of atomic manipulation

The design of a low-temperature scanning tunneling microscope is described. The microscope can be operated in ultrahigh vacuum in the temperature range between 15 and 300 K. The main features are a scanner which is based on the Besocke ‘‘beetle’’ design principle combined with a spring suspension of the microscope and complete surrounding of the whole microscope by a 4 K radiation shield. The microscope can be extended to work as a force microscope using the optical lever detection technique for force measurement. It is demonstrated that this comparatively small and reliable setup is well suited to study the adsorption of single atoms and molecules at low temperatures. Moreover, examples of lateral and vertical manipulation of atoms and molecules are shown to demonstrate the performance of the system.

Influence of spring stiffness and anisotropy on stick-slip atomic force microscopy imaging

This paper presents a detailed analysis of high-load friction atomic force microscopy (AFM) images of layered structures in terms of a discrete stick-slip model. It turned out that based on a geometric approach, the characteristics of slip behavior can be linked to the cantilever/sample spring anisotropy. In particular, the use of polar scans is emphasized to analyze and to quantify these characteristics. The measured stiffness as derived from the slip behavior is in correspondence with the stiffness inferred from static friction. It is concluded that the combined stiffness of substrate and cantilever is constant during an AFM scan in a given direction, which supports the simple stick-slip model.

Calibration of optical lever sensitivity for atomic force microscopy

Accurate force determinations in atomic force microscopy require the precise measurement of cantilever deflections. A limiting factor in making these measurements is the calibration of the optical lever detection system, particularly when the tip cannot be pressed against a hard surface. Here we show that, for a given instrument, a fixed displacement of the photodiode detector produces a change in the detector voltage that correlates strongly with optical lever sensitivity. This provides a simple method for optical lever sensitivity calibration not requiring contact of the tip with any surface. The data also suggest that differences in the shape of the laser spot on the photodiode are a major source of variability in optical lever sensitivity.

Dual Optical Levers for Atomic Force Microscopy

Development of micro machined cantilever and optical lever detection system has greatly facilitated the operation of atomic force microscopy. However, since the detection system measures only the deflection of the cantilever at one set point where the laser beam is focused, care must be taken in implementing force control or in interpreting the acquired data. In this paper, a dual optical lever detection system is introduced, which has the potential to resolve the deformation of the cantilever with multidegree of freedom and thus detect the position of the tip end point with resolution in the 10 pm order. The detection system proved to be effective in real-time monitoring of the behavior of the tip end point while scanning, and in explaining the scanning direction dependence of the acquired images.

Comment on ‘‘Scanned-cantilever atomic force microscope’’ [Rev. Sci. Instrum. 64, 908 (1993)

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An optical detection low temperature atomic force microscope at ambient pressure for biological research

A low temperature atomic force microscope, based on the optical lever detection method and operated in liquid nitrogen vapor at ambient pressure, has been constructed. The experimental results show that atomic resolution images can be obtained on both mica and highly oriented pyrolytic graphite at temperatures down to 79 K with sufficient thermal stability. In liquid nitrogen vapor, no surface contamination was observed after many hours of continuous operation, which demonstrates that such an approach is ideal for biological applications for its lower cost and easy maintenance.