Position-sensitive Photodiode Research Articles

The Story behind the First Automatic Atomic Force Microscope

Abstract:Over the past three decades, atomic force microscopy (AFM) has undergone a series of design changes to attain a flat XY scan and stable non-contact mode in ambient atmosphere. AFM has evolved into an ideal method for non-destructive scanning of samples with longer probe tip life, high accuracy, repeatability, and automation. Together with self-optimizing algorithms for scan parameters in the non-contact mode, AFM can become as widely adopted as other microscopies such as optical or scanning electron microscopy (SEM). Even full automation of AFM probe tip exchange is now possible with probe type recognition software and laser beam alignment on the cantilever and position-sensitive photo diode (PSPD). By separating the optics stage from the Z stage, an AFM system can be made with low mechanical noise and improved optical vision.

Carrier-envelope offset frequency stabilization in a femtosecond optical parametric oscillator without nonlinear interferometry.

By exploiting the correlation between changes in the wavelength and the carrier-envelope offset frequency (f(CEO)) of the signal pulses in a synchronously pumped optical parametric oscillator, we show that f(CEO) can be stabilized indefinitely to a few megahertz in a 333 MHz repetition-rate system. Based on a position-sensitive photodiode, the technique is easily implemented, requires no nonlinear interferometry, has a wide capture range, and is compatible with feed-forward techniques that can enable f(CEO) stabilization at loop bandwidths far exceeding those currently available to OPO combs.

A photointegrator of the molecular condensation nuclei detector

The problem of calculating the threshold sensitivity of an integrating photoreceiver (photocurrent integrator) is investigated. A noise model of a photointegrator is used to develop equivalent noise circuits and to calculate the RMS value of the voltage across the output of the photoreceiver. Among all the models of the real photointegrator it is conceivable that the approximation by an ideal photointegrator may be used. The threshold sensitivity of a photointegrator is defined as the power at the input of a photointegrator at which the root mean squared (RMS) voltage across its output is equal to the RMS voltage of total noise i.e. the signal-to-noise ratio is equal to one. A photomultiplier tube (PMT) can be used to increase the sensitivity. Formulas for calculating the sensitivity of a PMT-based photointegrator are given. The increase in sensitivity due to the use of PMT can be up to 18–30 times.An experimental study of a photometer of a molecular condensation nuclei (MCN) detector that forms a base of highly sensitive MCN gas analyzers was conducted. The measured sensitivity differed from the calculated by 10%. However, at femtowatt power levels it is very difficult to get rid of parasitic optical signals that are responsible for a small decrease in sensitivity compared to the theoretical prediction. In many practical applications, for example, for the X-ray absorption method of mineral extraction using position-sensitive photodiode X- ray receivers, an approximation of a real photointegrator must be used.

Precision absolute measurement and alignment of laser beam direction and position.

For the construction of high-precision optical assemblies, direction and position measurement and control of the involved laser beams are essential. While optical components such as beamsplitters and mirrors can be positioned and oriented accurately using coordinate measuring machines (CMMs), the position and direction control of laser beams is a much more intriguing task since the beams cannot be physically contacted. We present an easy-to-implement method to both align and measure the direction and position of a laser beam using a CMM in conjunction with a position-sensitive quadrant photodiode. By comparing our results to calibrated angular and positional measurements we can conclude that with the proposed method, a laser beam can be both measured and aligned to the desired direction and position with 10 μrad angular and 3 μm positional accuracy.

Development of precision stage with magnified displacement system using multi reflection of optical sensor

For high-resolution positioning of a precision stage, conventional positioning systems require laser interferometers or capacitance displacement sensors to measure nanoscale displacements. These can be expensive and complicated. In this study, a new measuring system was developed using multi laser reflection and mechanical magnification of its displacement. The displacement of the stage was measured using a position-sensitive photodiode (PSPD), a laser diode (LD), a hinge-connected rotating mirror plate, and two reflection mirror plates. A beam from the laser diode was focused on the center of the rotating mirror. The beam was reflected onto one of the reflection mirrors, and underwent several reflections. The position variation of the reflected beam from the reflection mirrors was then measured by the PSPD. Using this magnified displacement system a measuring resolution of less than 5 nm was achieved.

A compact and fast nano-stylus profiling head for optical instruments

The current study developed a compact and high-speed stylus-profiling head with overall dimensions of 34.3 × 32.5 × 33.7 mm3. A simple linear flexure leaf spring was used as a precision guide method for the profiling head along a vertical axis and for the assembly of an infrared light-emitting diode. A position-sensitive photo-diode was used for as a simple sensing method. The static and dynamic forces, which can be controlled electronically using a vertical solenoid, were in the range of 10−2 N to 10−5 N. Displacement was in the range of 80 nm to 87.2 μm, making it adequate for the precise surface measurements of hard materials. The sensor resolution was less than 10 nm. The designed profiling head had a moving mass of 1 mg and speed within 100 Hz.

Parametric noise squeezing and parametric resonance of microcantilevers in air and liquid environments

In this work, parametric noise squeezing and parametric resonance are realized through the use of an electronic feedback circuit to excite a microcantilever with a signal proportional to the product of the microcantilever's displacement and a harmonic signal. The cantilever's displacement is monitored using an optical lever technique. By adjusting the gain of an amplifier in the feedback circuit, regimes of parametric noise squeezing/amplification and the principal and secondary parametric resonances of fundamental and higher order eigenmodes can be easily accessed. The exceptionally symmetric amplitude response of the microcantilever in the narrow frequency bandwidth is traced to a nonlinear parametric excitation term that arises due to the cubic nonlinearity in the output of the position-sensitive photodiode. The feedback circuit, working in both the regimes of parametric resonance and noise squeezing, allows an enhancement of the microcantilever's effective quality-factor (Q-factor) by two orders of magnitude under ambient conditions, extending the mass sensing capabilities of a conventional microcantilever into the sub-picogram regime. Likewise, experiments designed to parametrically oscillate a microcantilever in water using electronic feedback also show an increase in the microcantilever's effective Q-factor by two orders of magnitude, opening the field to high-sensitivity mass sensing in liquid environments.

Abdominal stabilization of vertical image velocity during tethered flight

Abdominal stabilization of vertical image velocity during tethered flight

Optical force measurement system with mirror probe for nanoprobing inside a scanning electron microscope

In nanoprobing measurements the quality of the electrical contact strongly depends on the contact force. Probing semiconductors such as silicon requires applying very high and stable forces to establish an ohmic contact between the probe tip and the structure under study. Therefore, a compact force control system is needed for nanoprobing measurements inside a high-resolution scanning electron microscopy (SEM) system. In this work we have developed an optical force measurement system (OFMS) for nanoprobing based on the reflection of a laser beam from a mirror of the probe towards a position-sensitive photodiode detector (PSD). The system principle is similar to the so-called optical-lever principle used in atomic force microscopy (AFM). However, geometrical restrictions of the SEM chamber and adjacent probes require the incident and reflected laser beams to be on the same plane as the probe cantilever. This is achieved by placing a small mirror element perpendicular onto the cantilever surface. The use of high-precision nanomanipulators with OFMS allows to position probe tips on a selected structure on a macroscopic-scale sample with nanometer precision. The slim design of the nanomanipulator and the OFMS system allows to place several force controlled tips on the same nanostructure.

Calibration of dynamic holographic optical tweezers for force measurements on biomaterials

Holographic optical tweezers (HOTs) enable the manipulation of multiple traps independently in three dimensions in real time. Application of this technique to force measurements requires calibration of trap stiffness and its position dependence. Here, we determine the trap stiffness of HOTs as they are steered in two dimensions. To do this, we trap a single particle in a multiple-trap configuration and analyze the power spectrum of the laser deflection on a position-sensitive photodiode. With this method, the relative trap strengths can be determined independent of exact particle size, and high stiffnesses can be probed because of the high bandwidth of the photodiode. We find a trap stiffness for each of three HOT traps of kappa approximately 26 pN/microm per 100 mW of laser power. Importantly, we find that this stiffness remains constant within +/- 4% over 20 microm displacements of a trap. We also investigate the minimum step size achievable when steering a trap with HOTs, and find that traps can be stepped and detected within approximately 2 nm in our instrument, although there is an underlying position modulation of the traps of comparable scale that arises from SLM addressing. The independence of trap stiffness on steering angle over wide ranges and the nanometer positioning accuracy of HOTs demonstrate the applicability of this technique to quantitative study of force response of extended biomaterials such as cells or elastomeric protein networks.

Design and imaging properties of a laser scanning microscope with a position-sensitive detector

An economical method of microscopic image formation that employs a raster-scanning laser beam focused on a sample, while a non-imaging detector receives the scattered light, is presented. The images produced by this method are analogous to the scanning electron microscopy with visible effects of shadowing and reflection. Compared to a conventional wide-field imaging system, the system allows for a greater flexibility, as the variety of optical detectors, such as PMT and position-sensitive quadrant photodiode can be used to acquire images. The system demonstrates a simple, low-cost method of achieving the resolution on the order of a micron. A further gain in terms of resolution and the depth of focus by using Bessel rather than Gaussian beams is discussed.

Microthermogravimetry using a microcantilever hot plate with integrated temperature-compensated piezoresistive strain sensors

This paper reports thermogravimetric analysis of nanogram samples of paraffin using a microcantilever hot plate. The microcantilever hot plate has an integrated temperature-controlled heater and integrated temperature-compensated strain-sensing piezoresistors. The microcantilever vibration amplitude was measured using either a laser and a position sensitive photodiode, or using the piezoresistors. The cantilever resonance was measured as the cantilever was heated, such that the analyte mass could be measured as a function of temperature. Both optical and piezoresistive methods were employed to generate thermogravimetric curves for analytes in the range of 1-3 ng, and the results of the two methods compared well.

Cantilever‐Based Chemical Sensors for Detecting Catalytically Produced Reactions and Motility Forces Generated via Electrokinetic Phenomena

This paper reports the fabrication, characterization, and modeling of a chemical sensor constructed from a microfabricated silicon cantilever, coated with gold, which is modified using photolithography techniques to contain a silver feature on the free-standing edge. When immersed in a fuel solution such as hydrogen peroxide, catalytic reactions occurring at the bimetallic silver-gold junction cause a catalytic force to act on the cantilever. The catalytic reaction is detected by measuring change in resonance frequency of the cantilever using a position-sensitive split photodiode and atomic force microscopy instrument. A model based on the Cleveland method is developed to quantify the forces produced and to study the effect of change of hydrogen peroxide concentration on the magnitude of the force. The force is observed to increase linearly for lower concentrations of hydrogen peroxide and level off at higher concentrations. The chemical sensor offers a possible method for using catalytically produced forces in microelectromechanical systems and microfluidic devices.

Probing Focused Sound Fields Using Optical-Beam Deflection Method

Optical-beam deflection was used to probe a focused sound field in water. A laser beam was deflected by ultrasonic waves radiating from a concave transducer at 2.1 MHz under conditions in which that the sound wavelength is larger than the laser beam width. The deflected signal was detected with a position-sensitive photodiode, yielding a waveform that was the time derivative of the sound pressure. Scanning the laser beam in the propagation and radial directions, we obtained a map of the focused sound field. The results were compared with those from hydrophone measurements and theoretical calculations. The comparison showed qualitative agreement, and the observed difference was explained by the optical integrated effect over a range of interaction between the laser light and ultrasound. The amplitude of the maximum pressure estimated from the deflection angle was roughly in agreement with that obtained from the hydrophone method.

A laser-based 2-dimesional angular deflection measurement system for tilting microplates

A laser-based system that is capable of measuring the angular deflection of the microplate in two-dimension is developed. The system consists of a HeNe laser, lenses, beamsplitters, objective lens, CCD camera and position sensitive photodiode. The operation of the system is based on the measurement of the displacement of a HeNe laser beam reflecting off the surface of the microplate using position sensitive photodiode. The angular resolution is estimated at 0.2° and the response time of the system is as short as 4 μs. This system can measure the static and dynamic angular response and resonance frequency of microplates, such as spatial light modulators and micromirrors, in two orthogonal directions.

A highly sensitive opto-electronic system for the measurement of movements

An opto-electronic system has been developed to measure movements of insect appendages. It is made from a mirror-lens and a linear position-sensitive photodiode. The design of the mirror-lens has been exploited to axially mount a high intensity halogen light source in front of the mirror-lens. The system monitors a reflective marker which is attached to the moving object. Upon illumination by the light source the reflected light is picked up by the optical system and is focussed on the diode. The diode provides a voltage output proportional to the distribution of the light on it's surface. Since the marker is the brightest spot in the image the output of the system corresponds to the position of the marker. At a working distance of 80 cm appendage movements with amplitudes from 10 μm to 20 mm peak-peak amplitude can be recorded. The system accurately detects movements ranging from slow positional changes to 5 kHz oscillations. Currently it used to measure the stridulatory wing movements of crickets but may be applied to a variety of movement recordings.

The Simultaneous Determination of the Forces and Viscoelastic Properties of Adsorbed Polymer Layers

The viscoelastic properties of a solvated adsorbed film of gelatin on glass have been determined by modifying a particle forces apparatus which is a derivative of an atomic force microscope (AFM). Here a particle is attached to the end of an AFM lever. Gelatin is allowed to adsorb to the particle and to a flat glass surface. The surface is driven toward the particle using a piezoelectric ceramic and the deflection of the lever monitored using a laser beam and a position-sensitive photodiode. In the experiments reported here a relatively low frequency oscillation (20–1000 Hz) is superimposed on the linear motion. By examining the in-phase and out-of-phase responses of the particle to the applied oscillation, the viscous and elastic properties of the gap may be determined with separation. This information is obtained simultaneously with the forces of interaction between the gelatin-coated glass surfaces. The data show that a viscous response dominates until the gelatin layers interact. After contact the elastic response increases and dominates at high compression of the adsorbed gelatin layers. Problems concerning the quantitative analysis of the data in terms of viscous and elastic moduli are discussed.

Lagrangian acceleration measurements at large Reynolds numbers

We report experimental measurements of Lagrangian accelerations in a turbulent water flow between counter-rotating disks for Taylor–Reynolds numbers 900<Rλ<2000. Particle tracks were recorded by imaging tracer particles onto a position sensitive photodiode, and Lagrangian information was obtained from fits to the position versus time data. Several challenges associated with extracting Lagrangian statistical quantities from particle tracks are addressed. The acceleration variance is obtained as a function of Reynolds number and shows good agreement with Kolmogorov (1941) scaling. The Kolmogorov constant for the acceleration variance is found to be a0=7±3.

Analysis of heat-affected zone phase transformations usingin situ spatially resolved x-ray diffraction with synchrotron radiation

Spatially resolved X-ray diffraction (SRXRD) consists of producing a submillimeter size X-ray beam from an intense synchrotron radiation source to perform real-time diffraction measurements on solid materials. This technique was used in this study to investigate the crystal phases surrounding a liquid weld pool in commercial purity titanium and to determine the location of the phase boundary separating the high-temperature body-centered-cubic (bcc) β phase from the low-temperature hexagonalclose-packed (hcp) α phase. The experiments were carried out at the Stanford Synchrotron Radiation Laboratory (SSRL) using a 0.25 × 0.50 mm X-ray probe that could be positioned with 10-µm precision on the surface of a quasistationary gas tungsten arc weld (GTAW). The SRXRD patterns were collected using a position-sensitive photodiode array in a φ-2φ geometry. For this probe size, integration times of 10 s/scan at each location on the specimen were found adequate to produce high signal-to-noise (S/N) ratios and quality diffraction patterns for phase identification, thus allowing real-time diffraction measurements to be made during welding. The SRXRD results showed characteristic hcp, bcc, and liquid diffraction patterns at various points along the sample, starting from the base metal through the heat-affected zone (HAZ) and into the weld pool, respectively. Analyses of the SRXRD data show the coexistence of bcc and hcp phases in the partially transformed (outer) region of the HAZ and single-phase bcc in the fully transformed (inner) region of the HAZ. Postweld metallographic examinations of the HAZ, combined with a conduction-based thermal model of the weld, were correlated with the SRXRD results. Finally, analysis of the diffraction intensities of the hcp and bcc phases was performed on the SRXRD data to provide additional information about the microstructural conditions that may exist in the HAZ at temperature during welding. This work represents the first directin situ mapping of phase boundaries in fusion welds.

In-situ height correction for laser scanning of semiconductor wafers

An <i>in situ</i> technique for servo control of surface height during laser scanning of semiconductor wafers is described. The scheme corrects any macroscopic height changes due to tilt and bow in the wafer and rejects local and pattern-dependent changes of height and reflectivity. The waist of the scanning beam is imaged on a slit aperture placed in front of a position-sensitive photodiode, leading to an ac signal at the scanning frequency. This ac signal then undergoes synchronous detection using a reference signal at the scanning frequency. This detection scheme leads to a reduced sensitivity to low-frequency electronic and thermal drifts. Normalization circuitry provides means for excluding the effects of reflectivity which can vary over four orders of magnitude on patterned wafers. The height signal, so obtained, is used to drive a PZT stage to a nominal height position in closed loop. On patterned wafers, an rms height accuracy of better than 0.1 μm has been achieved in 20-Hz bandwidth.