Takahiro Namazu

Evaluation of size effect on mechanical properties of single crystal silicon by nanoscale bending test using AFM

This paper describes a nanometer-scale bending test for a single crystal silicon (Si) fixed beam using an atomic force microscope (AFM). This research focuses on revealing the size effect on the mechanical property of Si beams ranging from a nano- to millimeter scale. Nanometer-scale Si beams, with widths from 200 to 800 nm and a thickness of 255 nm, were fabricated on an Si diaphragm by means of field-enhanced anodization using AFM and anisotropic wet etching. The efficient condition of the field-enhanced anodization could be obtained by changing the bias voltage and the scanning speed of the cantilever. Bending tests for micro- and millimeter-scale Si beams fabricated by a photolithography technique were also carried out using an ultraprecision hardness tester and scratch tester, respectively. Comparisons of Youngs modulus and bending strength, of Si among the nano-, micro-, and millimeter scales showed that the specimen size did not have an influence on the Youngs modulus in the [110] direction, whereas it produced a large effect on the bending strength. Observations of the fractured surface and calculations of the clack length from Griffiths theory made it clear that the maximum peak-to-valley distance of specimen surface caused the size effect on the bending strength.

Plastic deformation of nanometric single crystal silicon wire in AFM bending test at intermediate temperatures

This paper focuses on revealing specimen size and temperature effects on mechanical properties of nanometric single crystal silicon (SCS) wires at intermediate temperatures. Mechanical properties of the nanometric SCS wires were characterized by bending tests with an atomic force microscope (AFM). The fixed-fixed SCS wires with widths from 200 nm to 800 nm and a thickness of 255 nm were fabricated on a silicon diaphragm by means of field-enhanced anodization with AFM and anisotropic wet etching. The AFM bending tests of the SCS wires were carried out at temperatures ranging from 295 K to 573 K in high vacuum. Elastic moduli of the SCS wires showed temperature dependence, but had no size effect. However, plastic properties such as critical resolved shear stress and plastic deformation range showed a clear dependence on both of specimen size and temperature. The critical resolved shear stress ranged from 4.2 GPa to 7.2 GPa, which was 10 times higher than that in a millimeter scale specimen. Force-displacement curves and AFM observations of the slip line also showed that plastic flow in the nanometric SCS wires was induced at 373 K, which was very close to room temperature. A dislocation model is proposed on the basis of the AFM observation, which was able to rationalize that the nanometric specimen had a large influence on the activation Gibbs free energy in the thermal activated process.

Mechanical property measurements of nanoscale structures using an atomic force microscope.

This paper describes nanometer-scale bending tests of fixed single-crystal silicon (Si) and silicon dioxide (SiO2) nanobeams using an atomic force microscope (AFM). The technique is used to evaluate elastic modulus of the beam materials and bending strength of the beams. Nanometer-scale Si beams with widths ranging from 200 to 800 nm were fabricated on a Si diaphragm using field-enhanced anodization using an AFM followed by anisotropic wet etching. Subsequent thermal oxidation of Si beams was carried out to create SiO2 beams. Results from the bending tests indicate that elastic modulus values are comparable to bulk values. However, the bending strength appears to be higher for these nanoscale structures than for large-scale specimens. Observations of the fracture surface and calculations of the crack length from Griffiths theory appear to indicate that the maximum peak-to-valley distance on the beam top surfaces influence the values of the observed bending strengths.

Development of AFM tensile test technique for evaluating mechanical properties of sub-micron thick DLC films

This paper describes mechanical properties of submicron thick diamond-like carbon (DLC) films used for surface modification in MEMS devices. A new compact tensile tester operating under an atomic force microscope (AFM) is developed to measure Youngs modulus, Poissons ratio and fracture strength of single crystal silicon (SCS) and DLC coated SCS (DLC/SCS) specimens. DLC films with a thickness ranging from 0.11 /spl mu/m to 0.58 /spl mu/m are deposited on 19-/spl mu/m-thick SCS substrate by plasma-enhanced chemical vapor deposition using a hot cathode penning ionization gauge discharge. Youngs moduli of the DLC films deposited at bias voltages of -100 V and -300 V are found to be constant at 102 GPa and 121 GPa, respectively, regardless of film thickness. Poissons ratio of DLC film is also independent of film thickness, whereas fracture strength of DLC/SCS specimens is inversely proportional to thickness. Raman spectroscopy analyses are performed to examine the effect of hydrogen content in DLC films on elastic properties. Raman spectra reveal that a reduction in hydrogen content in the films leads to better elastic properties. Finally, the proposed evaluation techniques are shown to be applicable to sub-micron thick DLC films by finite element analyses.

Fatigue Life Prediction Criterion for Micro–Nanoscale Single-Crystal Silicon Structures

This paper describes fatigue damage evaluation for micro-nanoscale single-crystal silicon (SCS) structures toward the reliable design of microelectromechanical systems subjected to fluctuating stresses. The fatigue tests, by using atomic force microscope (AFM), nanoindentation tester, and specially developed uniaxial tensile tester, have been conducted under tensile and bending deformation modes for investigating the effects of specimen size, frequency, temperature, and deformation mode on the fatigue life of SCS specimens. Regardless of frequency and temperature, the fatigue life has correlated with specimen size. For example, nanoscale SCS specimens with 200 nm in width and 255 nm in thickness have showed a larger number of cycles to failure, by a factor of 105, at the same stress level, as compared to microscale specimens with 48 ¿m in width and 19 ¿m in thickness. Deformation mode has also affected the lifetime; however, no frequency and temperature dependences have been observed unambiguously in the S-N curves. The stress ratio parameter corresponding to the ratio of peak stress to average fracture strength has enabled us to estimate the lifetime for each deformation mode. To predict the fatigue life of SCS structures regardless of deformation mode and specimen size, we have proposed an empirical parameter that includes the resolved shear stress. The mechanism of fatigue failure of SCS structures is discussed from the viewpoint of dislocation slip, crack nucleation, growth, and failure through observations using AFM and scanning electron microscope.

Thermoresistive properties of p-type 3C–SiC nanoscale thin films for high-temperature MEMS thermal-based sensors

We report for the first time the thermoresistive property of p-type single crystalline 3C–SiC (p-3C–SiC), which was epitaxially grown on a silicon (Si) wafer, and then transferred to a glass substrate using a Focused Ion Beam (FIB) technique. A negative and relatively large temperature coefficient of resistance (TCR) up to −5500 ppm K−1 was observed. This TCR is attributed to two activation energy thresholds of 45 meV and 52 meV, corresponding to temperatures below and above 450 K, respectively, and a small reduction of hole mobility with increasing temperature. The large TCR indicates the suitability of p-3C–SiC for thermal-based sensors working in high-temperature environments.

Nano-scale bending test of Si beam for MEMS

We carried out a nanometer scale bending test for a single crystal silicon (Si) beam using an atomic force microscope (AFM). Nanometer scale Si beams with widths from 200 nm to 800 nm and a thickness of 255 nm were fabricated on an Si diaphragm by means of the field-enhanced anodization using AFM and the anisotropic wet etching. Bending tests for a micro- and millimeter scale beam were also carried out using an ultra-precision hardness tester and scratch tester, respectively. The mechanical property of Si beams on a nanometer scale was compared with that measured on a micro- and millimeter scale. SEM observations of the fracture surface were performed in order to reveal the size effect on the bending strength.

Piezoresistive effect in p-type 3C-SiC at high temperatures characterized using Joule heating.

Cubic silicon carbide is a promising material for Micro Electro Mechanical Systems (MEMS) applications in harsh environ-ments and bioapplications thanks to its large band gap, chemical inertness, excellent corrosion tolerance and capability of growth on a Si substrate. This paper reports the piezoresistive effect of p-type single crystalline 3C-SiC characterized at high temperatures, using an in situ measurement method. The experimental results show that the highly doped p-type 3C-SiC possesses a relatively stable gauge factor of approximately 25 to 28 at temperatures varying from 300 K to 573 K. The in situ method proposed in this study also demonstrated that, the combination of the piezoresistive and thermoresistive effects can increase the gauge factor of p-type 3C-SiC to approximately 20% at 573 K. The increase in gauge factor based on the combination of these phenomena could enhance the sensitivity of SiC based MEMS mechanical sensors.

Piezoresistive effect of p-type silicon nanowires fabricated by a top-down process using FIB implantation and wet etching

The piezoresistive effect in silicon nanowires (SiNWs) has attracted a great deal of interest for NEMS devices. Most of the piezoresistive SiNWs reported in the literature were fabricated using the bottom up method or top down processes such as electron beam lithography (EBL). Focused ion beam (FIB), on the other hand, is more compatible with CMOS integration than the bottom up method, and is simpler and more capable of fabricating very narrow Si nanostructures compared to EBL and photolithography. Taking the advantages of FIB, this paper presents for the first time the piezoresistive effect of p-type SiNWs fabricated using focused ion beam implantation and wet etching. The SiNWs were locally amorphized by Ga+ ion implantation, selectively wet-etched, and thermally annealed at 700 °C. A relatively large gauge factor of approximately 47 was found in the annealed SiNWs, indicating the potential of using the piezoresistive effect in top-down fabricated SiNWs for developing NEMS sensors.

High-cycle fatigue damage evaluation for micro-nanoscale single crystal silicon under bending and tensile stressing

This paper proposes a new high-cycle fatigue parameter for correlating fatigue lives of micro/nanoscale single crystal silicon (SCS) structures under bending and tensile stressing. The authors have reported the bending fatigue lives of nanoscale fixed-fixed SCS beams at MEMS 2003, and discussed the effects of specimen size, temperature and frequency on the fatigue lives. This research carried out fatigue tests of micro/nanoscale SCS structures under bending/tensile stressing for revealing the influence of deformation mode on SCS fatigue lives. Consequently, we enabled to propose an effective fatigue damage parameter for predicting fatigue lives of SCS structures ranging from micro-to nanoscale, regardless of deformation mode and specimen size. The fatigue damage parameter can be used for design of MEMS components subjected to fluctuating stressing.