Micromachined Silicon Cantilever Research Articles

The qPlus sensor, a powerful core for the atomic force microscope.

Atomic force microscopy (AFM) was introduced in 1986 and has since made its way into surface science, nanoscience, chemistry, biology, and material science as an imaging and manipulating tool with a rising number of applications. AFM can be employed in ambient and liquid environments as well as in vacuum and at low and ultralow temperatures. The technique is an offspring of scanning tunneling microscopy (STM), where the tunneling tip of the STM is replaced by using a force sensor with an attached tip. Measuring the tiny chemical forces that act between the tip and the sample is more difficult than measuring the tunneling current in STM. Therefore, even 30 years after the introduction of AFM, progress in instrumentation is substantial. Here, we focus on the core of the AFM, the force sensor with its tip and detection mechanism. Initially, force sensors were mainly micro-machined silicon cantilevers, mainly using optical methods to detect their deflection. The qPlus sensor, originally based on a quartz tuning fork and now custom built from quartz, is self-sensing by utilizing the piezoelectricity of quartz. The qPlus sensor allows us to perform STM and AFM in parallel, and the spatial resolution of its AFM channel has reached the subatomic level, exceeding the resolution of STM. Frequency modulation AFM (FM-AFM), where the frequency of an oscillating cantilever is altered by the gradient of the force that acts between the tip and the sample, has emerged over the years as the method that provides atomic and subatomic spatial resolution as well as force spectroscopy with sub-piconewton sensitivity. FM-AFM is precise; because of all physical observables, time and frequency can be measured by far with the greatest accuracy. By design, FM-AFM clearly separates conservative and dissipative interactions where conservative forces induce a frequency shift and dissipative interactions alter the power needed to maintain a constant oscillation amplitude of the cantilever. As it operates in a noncontact mode, it enables simultaneous AFM and STM measurements. The frequency stability of quartz and the small oscillation amplitudes that are possible with stiff quartz sensors optimize the signal to noise ratio. Here, we discuss the operating principles, the assembly of qPlus sensors, amplifiers, limiting factors, and applications. Applications encompass unprecedented subatomic spatial resolution, the measurement of forces that act in atomic manipulation, imaging and spectroscopy of spin-dependent forces, and atomic resolution of organic molecules, graphite, graphene, and oxides.

High resolution wide range pressure sensor using hexagonal ring and micromachined cantilever tips on 2D silicon photonic crystal

Abstract Design of a pressure sensor workable from megapascal (MPa) to gigapascal (GPa) pressure range is proposed on a two dimensional triangular/hexagonal lattice silicon (Si) photonic crystal (PhC) arrangement. In order to ensure high resolution sensing over this wide pressure range, a hexagonal shaped ring of hexagonal unit cells and two micromachined silicon cantilever tips are considered on the photonic crystal arrangement. The concept of using micromachined cantilever tip in a PhC arrangement for optical sensing of pressure is novel. Moreover, use of hexagonal ring with hexagonal unit cells make the structure more sensitivity than other types of ring arrangement. It is shown that the two silicon cantilever tips can be used to detect applied pressure from 1 MPa to 10 MPa with resolution of 1 MPa and 10 MPa to 100 MPa with a resolution of 10 MPa, respectively. Detection of applied pressure from 20 MPa to 10 GPa with the resolution of 20 MPa can be done using the hexagonal ring of hexagonal unit cells. The entire design is done using the MPB (MIT Photonic Bands) and MEEP (MIT Electromagnetic Equation Propagation) simulators freely available from MIT, USA.

Custom-Designed Glassy Carbon Tips for Atomic Force Microscopy.

Glassy carbon is a graphenic form of elemental carbon obtained from pyrolysis of carbon-rich precursor polymers that can be patterned using various lithographic techniques. It is electrically and thermally conductive, mechanically strong, light, corrosion resistant and easy to functionalize. These properties render it very suitable for Carbon-microelectromechanical systems (Carbon-MEMS) and nanoelectromechanical systems (Carbon-NEMS) applications. Here we report on the fabrication and characterization of fully operational, microfabricated glassy carbon nano-tips for Atomic Force Microscopy (AFM). These tips are 3D-printed on to micro-machined silicon cantilevers by Two-Photon Polymerization (2PP) of acrylate-based photopolymers (commercially known as IP-series resists), followed by their carbonization employing controlled pyrolysis, which shrinks the patterned structure by ≥98% in volume. Tip performance and robustness during contact and dynamic AFM modes are validated by morphology and wear tests. The design and pyrolysis process optimization performed for this work indicate which parameters require special attention when IP-series polymers are used for the fabrication of Carbon-MEMS and NEMS. Microstructural characterization of the resulting material confirms that it features a frozen percolated network of graphene sheets accompanied by disordered carbon and voids, similar to typical glassy carbons. The presented facile fabrication method can be employed for obtaining a variety of 3D glassy carbon nanostructures starting from the stereolithographic designs provided by the user.

A Bulk Micromachined Lead Zinconate Titanate Cantilever Energy Harvester with Inter-Digital IrO<SUB><I>x</I></SUB> Electrodes

A piezoelectric vibration energy harvester with inter-digital IrO(x) electrode was developed by using silicon bulk micromachining technology. Most PZT cantilever based energy harvesters have utilized platinum electrode material. However, the PZT fatigue characteristics and adhesion/delamination problems caused by the platinum electrode might be serious problem in reliability of energy harvester. To address these problems, the iridium oxide was newly applied. The proposed energy harvester was comprised of bulk micromachined silicon cantilever with 800 x 1000 x 20 microm3, which having a silicon supporting membrane, sol-gel-spin coated Pb(Zr52, Ti48)O3 thin film, and sputtered inter-digitally shaped IrO(x) electrodes, and silicon inertial mass with 1000 x 1000 x 500 microm3 to adjust its resonant frequency. The fabricated energy harvester generated 1 microW of electrical power to 470 komega of load resistance and 1.4 V(peak-to-peak) from a vibration of 0.4 g at 1.475 kHz. The corresponding power density was 6.25 mW x cm(-3) x g(-2). As expected, its electrical failure was significantly improved.

Cantilever anemometer based on a superconducting micro-resonator: Application to superfluid turbulence

We present a new type of cryogenic local velocity probe that operates in liquid helium (1 K < T < 4.2 K) and achieves a spatial resolution of ≈ 0.1 mm. The operating principle is based on the deflection of a micro-machined silicon cantilever which reflects the local fluid velocity. Deflection is probed using a superconducting niobium micro-resonator sputtered on the sensor and used as a strain gauge. We present the working principle and the design of the probe, as well as calibration measurements and velocity spectra obtained in a turbulent helium flow above and below the superfluid transition.

Different types of ferrite thin films as magnetic cantilever coating for magnetic force microscopy

Different types of ferrites are employed in the form of thin films as magnetic coating on cantilevers for magnetic force microscopy (MFM) use. This is especially needed for cantilevers employed in high-frequency MFM (HF-MFM), where stray fields of hard disk recording heads are investigated. Our experiments show that we can operate HF-MFM successfully at carrier frequencies up 2 GHz using such ferrite-coated cantilevers. Thin films of two ferrites, NiZnFe 2O 4 spinel ferrite and Co 2 Z-type hexaferrite (Ba 3Co 2Fe 24O 41, BCFO) were prepared by RF sputtering. As a basis for these probes, we employ commercial micromachined silicon cantilevers. Additionally, films on Si (1 0 0) and Si (1 1 1)-oriented substrates with a thickness up to 100 nm were prepared for analysis purposes, enabling the optimization of the sputter process. For a high spatial resolution of MFM, however, thinner magnetic coatings are required. Therefore, the third type, c, was prepared by laser-ablation with a thickness of 30 nm, also directly onto the Si without additional buffer layer.

Effects of dielectric charging on MEMS-based grating light modulator

Silicon dioxide and silicon nitride coatings are preferably used as dielectric layers for short-circuit protection in capacitive silicon MEMS based Grating Light Modulator (GLM). However, their tendency to electrostatic charging can diminish the device reliability. The influence of dielectric charges on GLM was discussed. Gas discharges and charges in the air gap of silicon cantilever GLM have been modeled and observed, resulting in surface charge accumulation on the electrode passivation of the devices. The impact of parasitic surface charges on the deflection versus voltage characteristics of bulk micromachined silicon cantilever GLM is investigated and measurements of the charge decay are shown in experimental results. Experimental results agree with the theoretical calculation well, which illustrates the influence of charging and discharging processing. One method is proposed to reduce the influence of dielectric charges.

Preparation of ferrite-coated magnetic force microscopy cantilevers

Thin films of two types of ferrites, NiZnFe2O4 spinel ferrite and Co2 Z-type hexaferrite (Ba3Co2Fe24O41) were prepared by rf sputtering on Si (1 0 0) and (1 1 1) substrates. Films with a thickness up to 100nm were prepared for analysis purposes, enabling the optimization of the sputter process. The purpose of these ferrite thin films is the preparation of magnetic force microscopy (MFM) cantilever coatings for use with a high-frequency magnetic force microscope (HF-MFM). As a basis for these probes, the authors employ commercial, micromachined silicon cantilevers. The typical thickness of the coatings was 50nm. The MFM imaging properties of both types of ferrite-coated cantilevers are discussed in detail.

Micromechanical sensor for studying heats of surface reactions, adsorption, and cluster deposition processes

We present a newly designed highly sensitive micromechanical sensor devoted to thermodynamic studies involving supported clusters. The thermally sensitive element of the sensor consists of a micromachined silicon cantilever array, onto which a thin metal film is evaporated. Due to the difference between the thermal expansion coefficients of silicon and the metal employed, thermal bending is observed when heat is exchanged with the cantilever. The sensitivity and the response time of the cantilever are studied as a function of the film material (gold or aluminum) and the thickness of the metal film. With our routinely prepared cantilevers, a minimum power of 120 nW is measurable with a submillisecond response time, corresponding to a limit of detection in the femtojoule range. The high sensitivity of the sensor is demonstrated by measuring the heat exchange which occurs during the deposition of clusters on the cantilever. Experimentally, we illustrate the 1,3-butadiene hydrogenation reaction using a cluster model catalysts created by soft-landing palladium clusters onto the cantilever surface.

Constraints on Yukawa-type deviations from Newtonian gravity at 20 microns

Recent theories of physics beyond the standard model have predicted deviations from Newtonian gravity at short distances. In order to test these theories, we have a built an apparatus that can measure attonewton-scale forces between gold masses separated by distances on the order of $25\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$. A micromachined silicon cantilever was used as the force sensor, and its displacement was measured with a fiber interferometer. We have used our measurements to set bounds on the magnitude $\ensuremath{\alpha}$ and length scale $\ensuremath{\lambda}$ of Yukawa-type deviations from Newtonian gravity; our results presented here yield the best experimental limit in the range of $\ensuremath{\lambda}=6--20\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$.

A novel piezoresistive microprobe for atomic and lateral force microscopy

In this article, we present the development of a novel probe for atomic force and lateral force microscopy, based on detailed finite-elements method (FEM) simulation. The FEM simulation is performed by means of the ANSYS 5.3 code. The developed probe consists of a micromachined silicon cantilever beam with an integrated micro-tip and piezoresistive sensors for detecting the lateral and vertical forces acting on the tip. The sensitivity for vertical displacement detection of 1.26 μV/nm (for 1 kΩ piezoresistor) and for lateral displacement detection of 0.46 μV/nm (for 0.7 kΩ resistors), by 0.6 V applied supply voltage was obtained. The principle of the probe, theoretical analysis, and experimental investigation of the probe sensor are discussed. The probe is suitable for operating in contact and noncontact atomic force microscopy (AFM) mode. The probe was utilized for imaging the surface of a chromium/quartz structure.

Chemical recognition based on micromachined silicon cantilever array

We report on the performance of a measurement system for the recognition of individual analytes and their binary mixtures which is based on a multiarray of four micromachined silicon cantivelers actuated at their resonance frequency. The cantilevers have been functionalized by organic polymers [polydimethylsiloxane (PDMS) and polyvinylpyridine (PVP)] and amorphous nitrogen-rich carbon nitride films. We found that the sensitivity and selectivity of the cantilevers coated with CNx films change according to the layer thickness. Our results show that the selected combination of sensitive layers ensures a wide range of specific, reversible and reproducible sensor responses upon exposure to methanol, 2-propanol, water and their binary mixtures. Further, it was found that the differences in recovery times of PDMS and CNx films after exposure to the two alcohols and their mixtures could be used especially for low analyte concentrations as a second characteristic in addition to the resonance frequency shift for the identification of individual components in the mixtures.

Superhard materials in the B/C/N system and modern micro system technology: synergistic effects

Diamond-like materials from the B/C/N system (e.g. diamond, tetrahedral amorphous carbon, cubic boron nitride (c-BN)) possess a great number of outstanding properties. Accordingly, they are very promising candidates for applications as active or passive layers in modern micro or nano systems, e.g. in probes for scanning probe microscopies (SPM). On the other hand, specially designed micro systems, in general, and SPM techniques especially, may help to overcome current problems in the optimization of existing thin films from the B/C/N system and in developing new ones by providing new techniques of material characterization, in the ideal case in situ during deposition. Therefore, synergistic effects may come into play, leading to a mutual improvement of both, the thin films and their properties, and the micro or nano systems. Although this ideal case has not yet been reached for films from the B/C/N system, we will present examples from our current research, which may eventually help to reach this aim. Fabrication of all-diamond tips and cantilevers for AFM applications, and of humidity sensors including carbon nitride films show the applicability of thin B/C/N films in micro systems, while the in situ characterization of diamond nucleation and growth by STM measurements and the measurements of the stress of c-BN films by micromachined silicon cantilevers demonstrate the usefulness of material characterization by modern micro systems.

Characterization of a highly sensitive ultra-thin piezoresistive silicon cantilever probe and its application in gas flow velocity sensing

We present a highly sensitive ultra-thin micromachined silicon cantilever beam with an integrated strain gauge on its root for optimizing piezoresistive readout. The mechanical characteristics and electrical readout of the cantilever beam, such as spring constant, resonant frequencies and piezoresistive sensitivity, are theoretically given from the derived formulae or from finite element modeling. The results of characterization show reasonably good agreement between the experimental results and the theoretical values. As one of the applications, for the first time the fabricated silicon cantilever beams have been applied to measure airflow velocity distribution in a steel pipe with an inner diameter of 7.0 mm. The experimental piezoresistive sensitivity (ΔR/R)/y(0) is in the range of 0.23–2.89 × 10−6 nm−1 in the beam bending tests, and the experimental flow sensitivity (ΔR/R)/Vgas2 is in the range of 0.652–4.489 × 10−5 (m s−1)−2 in the airflow velocity tests. The experimental detectable minimum airflow velocity is 7.0 cm s−1, which is comparable to that of a hot wire anemometer.

Glucose biosensing using an enzyme-coated microcantilever

A microcantilever-based biosensor is described. The enzyme glucose oxidase was immobilized on a micromachined silicon cantilever containing a gold coating, such as those used for atomic force microscopy. Specific, quantifiable deflection of the derivatized cantilevers was observed in the presence of the appropriate analyte. An analysis of the reaction energetics and the expected thermal response of the cantilever indicates that cantilever deflection is not simply a result of reaction-generated heat. This deflection appears to result from surface induced stresses. The combination of a highly specific enzyme and the microcantilever platform provides a unique approach for quantifying enzyme substrates without the complication of sample labeling.

Characterization of a two-dimensional cantilever array with through-wafer electrical interconnects

The characterization of two-dimensional micromachined silicon cantilever arrays with integrated through-wafer electrical interconnects is presented. The approach addresses alignment and density issues associated with operating two-dimensional scanning probe arrays. The tungsten based interconnect (30 μm diameter, 1 Ω resistance) is shown not to degrade the sensitivity of the piezoresistive deflection sensor embedded on each cantilever. Operation of the array (up to 2×7) as a microscope for imaging large areas (3.8×0.45 mm2) and with vertical row stitching is demonstrated with images of samples orders of magnitude larger than images possible with standard atomic force microscope techniques.

Selectivity of chemical sensors based on micro-cantilevers coated with thin polymer films

In an effort to impart selectivity to chemical sensors based on micro-machined silicon cantilevers, thin films of polymeric chromatographic stationary phases (SP-2340 and OV-25) were applied to one side of the cantilever surface using a spin coating procedure. These coatings influenced the response of the micro-cantilever to vapor phase test analytes of varying chemical compositions. For the SP-2340 coated micro-cantilevers, the effects of both polymeric film thickness and cantilever structure thickness on response characteristics were investigated. Sensitivity improved as both film thickness and cantilever leg thickness were decreased. The selectivity, as indicated by differences in relative responses to the test analytes, were different for the two phases which differed significantly in polarity. The SP-2340 coated cantilevers exhibited response characteristics that are fairly similar to that expected for adsorption of the test analytes onto silica. Responses are shown to be proportional to analyte concentration. Response characteristics are shown to be consistent with predictions based on a gas chromatographic stationary phase classification scheme.

Microcantilever torque magnetometry of thin magnetic films

We have developed a microcantilever torque magnetometer based on a torsion-mode atomic force microscope. Thin magnetic films are deposited directly onto micromachined silicon cantilevers. We have measured hysteresis loops of iron thin films with thicknesses ranging from 1 to 40 nm and total magnetic volumes ranging from 2.2×10−11 to 8.8×10−10 cm3. The magnetic moment sensitivity is estimated to be 1.3×10−12 A m2/Hz1/2 at room temperature and ambient conditions. We expect that by operating at the cantilever torsion resonance frequency and at higher torque fields sensitivity will be improved by a factor of 100–1000.

Monitoring chemical and physical changes on sub-nanogram quantities of platinum dioxide

We have measured in real-time the chemical reaction-induced weight loss and variation in physical and mechanical properties of sub-nanogram quantities of deposits utilizing the resonance response of microcantilevers. The hydrogen reduction of sub-nanogram quantities of alpha platinum oxide deposited on micromachined silicon cantilevers and resultant weight loss, as well as the kinetics of this process, were investigated. Comparison with theory suggests that this technique could be a powerful tool to measure physical and chemical changes occurring on sub-nanogram deposits.

A system for the dynamic characterization of microstructures

This paper describes a fully automated measurement system designed to evaluate the dynamic characteristics of micromechanical structures (millimeter dimensions). To validate the system, vibration measurements have been carried on two structures-a micromachined silicon cantilever and bridge-and the results are presented. Out-of-plane measurements show that for the cantilever, both the mode shapes and resonant frequencies agree with beam theory predictions. However, for the bridge structure, tension due to boron doping causes a change from beam-like behavior and a more complex model is required. Mode-shapes natural frequencies and modal damping are determined from data obtained by vibrating the structures using a piezoelectric mounting system and deriving the transfer function between the piezodrive voltage and beam vibrational velocity.