Single Crystal Silicon Specimens Research Articles

From macro fracture energy to micro bond breaking mechanisms – Shorter is tougher

We show a novel fracture behavior and properties of brittle materials not previously explored. These were made possible when merging macro to micro in fracture. Our findings are based on macro-scale fracture cleavage experiments of dynamic cracks propagating in brittle single-crystal silicon specimens, focusing on the crack’s energy-speed relationships, and the fine details of the crack front. From the micro-scale, we developed an atomistic model for bond-breaking mechanisms. These mechanisms are in the form of low-energy kink advance (migration) and high-energy kink formation (nucleation) along the crack front. The energy release rate (ERR) at crack initiation, G0, and its derivative, dG0/da≡Θ, in particular, play a major role in the novel behavior and properties. G0 and Θ are influenced by the precrack length, a0. The macro-scale experiments, the micro-scale atomistic model, and the gradient of the ERR, Θ, enabled the conclusions portrayed in this work.We identified that the cleavage energy is not a constant but bounded by the Griffith Barrier as the lower bound while the upper bound (apparently the lattice-trapping barrier) may reach 3 times the lower bound. We further suggest that a brittle material can be envisaged as comprising a pseudo-R-Curve behavior mechanism typical of metallic materials. A new and essential fracture mechanism was identified, which we term ‘quasi-propagation’, a transitional mechanism between initiation and propagation. During this mechanism, the sequence of the bond-breaking mechanisms is varying, causing an increase in the macroscale cleavage energy.The evaluated cleavage energy shows another novel behavior, that of ‘shorter is tougher’, namely, shorter cracks require higher energy to propagate, and, therefore, specimens with shorter cracks are stronger than that predicted by Griffith’s theory.

A novel high-shear and low-pressure grinding method using specially developed abrasive tools

In this work, a novel grinding method using a special abrasive tool was first proposed to achieve high tangential grinding force and low normal grinding force. The abrasive tool was developed with flexible composites to remove workpiece materials under “high-shear and low-pressure” grinding mode. The concept of the high-shear and low-pressure grinding method was introduced in detail. Grinding tests were carried out on a precision grinder with the developed abrasive tool for single crystal silicon specimens. The results showed that the grinding force ratio between tangential force and normal force for the proposed grinding method was ranged between 0.9 and 1.3, which was three times larger than that of the conventional grinding method. It was verified that the developed abrasive tool possessed the grinding characteristics of high tangential grinding force and low normal grinding force. After grinding of silicon specimens for 120 min, the value of surface roughness decreased from 429.20 to 32.91 nm under the selected grinding conditions. The surface quality of the silicon specimens was greatly improved after grinding.

Tensile test of a silicon microstructure fully coated with submicrometer-thick diamond like carbon film using plasma enhanced chemical vapor deposition method

This paper reports the tensile properties of single-crystal silicon (SCS) microstructures fully coated with sub-micrometer thick diamond like carbon (DLC) film using plasma enhanced chemical vapor deposition (PECVD). To minimize the deformations or damages caused by non-uniform coating of DLC, which has high compression residual stress, released SCS specimens with the dimensions of 120 µm long, 4 µm wide, and 5 µm thick were coated from the top and bottom side simultaneously. The thickness of DLC coating is around 150 nm and three different bias voltages were used for deposition. The tensile strength improved from 13.4 to 53.5% with the increasing of negative bias voltage. In addition, the deviation in strength also reduced significantly compared to bare SCS sample.

Crystal orientation-dependent fatigue characteristics in micrometer-sized single-crystal silicon.

Repetitive bending fatigue tests were performed using five types of single-crystal silicon specimens with different crystal orientations fabricated from {100} and {110} wafers. Fatigue lifetimes in a wide range between 100 and 1010 were obtained using fan-shaped resonator test devices. Fracture surface observation via scanning electron microscope (SEM) revealed that the {111} plane was the primary fracture plane. The crack propagation exponent n was estimated to be 27, which was independent of the crystal orientation and dopant concentration; however, it was dependent on the surface conditions of the etched sidewall. The fatigue strengths relative to the deflection angle were orientation dependent, and the ratios of the factors obtained ranged from 0.86 to 1.25. The strength factors were compared with those obtained from finite element method stress analyses. The calculated stress distributions showed strong orientation dependence, which was well-explained by the elastic anisotropy. The comparison of the strength factors suggested that the first principal stress was a good criterion for fatigue fracture. We include comparisons with specimens tested in our previous report and address the tensile strength, initial crack length, volume effect, and effects of surface roughness such as scallops.

せん断型ひずみゲージ集積化単結晶シリコンマイクロ構造並列引張試験デバイス

A new tensile-mode parallel fatigue testing device with integrated shear strain gauge has been developed for reliability assessment of single crystal silicon (SCS) microstructures. We have previously reported a device with tensile strain gauge to test specimens a high stress and ultra-high cycles, however the test frequency was limited by the total stiffness of the testing system and the performance of piezoelectric actuator used for loading. In this report, we employed a shear strain gauge, which has higher stiffness than a tensile one. The testing device has five SCS specimens of 120 μm long, 4 μm wide and 5 μm thick, integrated with the shear strain gauge. We optimized the shear strain gauge by finite element analysis, and a gauge 15° tilted from orthogonal axis is the highest sensitivity in both dopant types. We performed parallel tensile testing with the device fabricated from n-type silicon-on-insulator wafer and confirmed output from the strain gauges. All five specimens fractured and the fracture strength of one of the specimens was measured as 1.15 GPa.

Micro-Raman spectroscopic analysis of single crystal silicon microstructures for surface stress mapping

In this paper, an experimental analysis using a micro-Raman spectroscope for surface stress distribution in single crystal silicon (SCS) microstructures is described. Specially developed tensile test equipment applies a uniaxial tensile stress on SCS specimens with a 270 nm-high, 4 µm2 convex structures in the gauge section. Raman spectra around the convex region are measured using an ultraviolet laser with an excitation line of 363.8 nm. The shape of the Raman spectrum on the flat surface is symmetrical, whereas that around the edge of the convex is asymmetrical due to the multi-stress condition. Two-curve fitting is adopted for the asymmetric spectrum obtained at the edge, and the stress distribution estimated by the two peak positions is much closer to finite element analysis (FEA) results than that obtained by the one peak position. In partial least squares (PLS) analysis that is performed at the edge of the convex section only, explanatory variables are Raman spectral parameters, such as peak position, peak intensity, and full width at half maximum, and the response variable is the FEA stress distribution. The plane stress distributions derived from PLS analyses on each component are in good agreement with that from FEA. The combination of micro-Raman spectroscopy and tensile testing enables us to directly determine the stress components as well as stress magnitudes on SCS microstructures.

Novel microstructures on the surfaces of single crystal silicon irradiated by intense pulsed ion beams

An ion beam treatment of high purity single crystal silicon specimens was performed with different shots by the irradiation of intense pulsed ion beams (IPIB), which were generated by an accelerating voltage of 350kV and with the current density of 130A/cm2. As the result of irradiation, the formation of various microstructures caused by the irradiation effect, especially the thermal effect is confirmed by SEM and XRD analysis, and the corresponding processes are described and related explanations are given.

Fracture behavior of single crystal silicon with thermal oxide layer

This paper reports on the effect of oxidation on fracture behavior of single crystal silicon (SCS). SCS specimens were fabricated from (100) silicon-on-insulator wafer with 5-μm-thick device layer and oxide layer were thermally grown. Quasi-static tensile testing of as-fabricated, oxidized and oxidized layer removed specimens was performed. The fracture origin location transited from the surface to silicon/oxide interface and inside of silicon. The transition may be caused by surface smoothing, thickening oxide layer and formation of oxide precipitation defects in silicon during oxidation. The radius of the oxide precipitation defects was estimated, which is well agreed with the fracture-initiating crack sizes.

Fracture toughness of silicon in nanometer-scale singular stress field

The purpose of this study is to examine the size effect of a singular stress field on the fracture toughness at the nanometer scale. The cracking behavior is investigated using small single-crystal silicon specimens with different precrack lengths where the size of the singular stress field, ΛK, near the crack tip is 23–58nm. The fracture toughness, KIC, is approximately 1MPam1/2 in all of the specimens, which is in good agreement with that of the macroscale (bulk) counterpart. This clearly indicates that the fracture toughness is independent of size while it was reported that the tensile strength without a crack showed strong dependence at the nanometer scale.

High-temperature tensile testing machine for investigation of brittle–ductile transition behavior of single crystal silicon microstructure

This paper reports development of a high-temperature tensile testing machine and testing of single crystal silicon (SCS) for investigation of the size effect on brittle–ductile transition temperature (BDTT). Two different-width 〈110〉 SCS specimens (120 µm long, 5 µm thick and 4 or 9 µm wide) were tested in a vacuum at the temperature range from room temperature to 600 °C. The specimens tested at 500 °C and above exhibit slip, which indicated that the micrometer-sized silicon structures have lower BDTT compared to millimeter-sized ones. By investigating the temperature ranges of the slip occurrences among the differently-sized specimens including other researchers’ reports, we found that the length along slip direction, i.e., thickness, might dominate the BDTT.

J2210304 Si単結晶におけるナノスケールの特異応力場を有するき裂の伝ぱ基準

In order to investigate the size effect of a singular stress field on the fracture toughness at the nanometer scale, crack propagation tests are carried out using small single-crystal silicon specimens with different precrack lengths where the size of the singular stress field near the crack tip is 23-57.9 nm. The fracture toughness, K_<IC>, is ebvaluated to be approximately 1 MPam^<1/2>, which is in good agreement with that of the bulk counterpart. This obviously indicates that the fracture toughness is independent of size while it has been reported that the tensile strength without a crack shows strong dependence at the nanometer scale.

A prediction scheme of the static fracture strength of MEMS structures based on the characterization of damage distribution on a processed surface

This paper presents a scheme for predicting the strength of MEMS structures patterned into arbitrary shapes by deep reactive ion etching of a silicon wafer. The scheme is based on the inhomogeneous defect distribution on the etched surfaces. Single-crystal silicon specimens with different shapes were subjected to four-point bending tests with monotonically increasing load. The distributions of the fracture strength were described using two-parameter Weibull statistics, where the two parameters are defined as functions of the etching depth representing the inhomogeneity of the damage on the etched surface in the direction perpendicular to the wafer. In order to estimate the distribution of the local strength determined by the local level of damage, the etched surfaces of specimens without a notch were tested in three different bending directions corresponding to three different stress distributions, and three different strength levels were given due to surface roughness conditions caused by the non-uniformity of the etching process. The estimated values of the parameters were used to estimate the fracture strength of four types of notched specimens with different notch tip radii. The results of comparison between the distributions of predicted strengths and experimental data showed that the fracture strength of arbitrarily shaped structures is predictable on the basis of the information obtained from specimens without notch, by taking into account the characteristics of etched surface, i.e. the inhomogeneous damage.

OS1910 単結晶シリコン破壊強度の変形モード依存性

In this article, the influences of specimen size and deformation-mode on the fracture strength of single-crystal silicon (SCS) micro-beam structures are described. We have designed and developed a new materials test device that is able to apply pure torsion, uniaxial-tension, bending, torsion-bending-combined, and torsion-tension-combined deformations to a micro-scale beam specimen. The specimen made of SCS consists of a SCS plate supported by two SCS micro-beams, and a frame, and was fabricated by deep reactive ion etching (DRIE) with the same fabrication recipe. All the SCS specimens tested in those deformation-modes were fractured in a brittle manner at a laboratory temperature. The fractured strength was calculated assuming that all the specimens had an ideal flat surface, and the data were arranged using two-parameter Weibull distribution function. Both the specimen size and deformation-mode dependencies were apparently found. To estimate the "true" strength, finite element analyses (FEAs) using the models that included scalloping sidewalls were carried out. The revised strength data that were estimated by multiplying stress concentration factors into the original data suggested that no deformation-mode dependency on strength existed.

Measurement of anisotropic fatigue life in micrometre-scale single-crystal silicon specimens

The fatigue life of micrometre-scale single-crystal silicon specimens was measured using the resonant vibration of a micromachined resonator device. Two kinds of identically shaped specimens oriented to 〈110〉 and 〈100〉 crystal directions on the (001) plane were tested. On the deflection–life relationship, the deflection amplitudes of the 〈100〉 oriented specimens were approximately 1.56 times higher than that of 〈110〉 oriented ones at the same fatigue life. Observation by scanning electron microscope showed that the fracture surfaces differed depending on the specimen orientation and deflection amplitude. The life-shortening effect of the surface roughness was also demonstrated.

MNM-3A-4 (110)単結晶Siマイクロ試験片における破壊の結晶異方性のワイブル統計解析(セッション 3A 単結晶・多結晶シリコンの疲労寿命評価とメカニズムの解明)

Uniaxial tensile testing for single crystal silicon (SCS) films was conducted to investigate the fracture mechanism of silicon. The loading axis of the SCS specimens had three main crystallographic orientations, that is <100>, <110>, and <111>, fabricated from one (110) silicon-on-insulator wafer. The dimensions of specimen test part were 10 μm wide, 5 μm thick, and 120 μm long having 1 μm-depth notch at the center. The measured average fracture force of <100>, <110>, and <111> specimens was 44.2, 53.5, and 45.8 mN, respectively. The difference in fracture force between <110> and <111> specimens was well elucidated with the normal stress for (111) cleavage plane using Weibull statistics considering the stress distribution on notch surface and existence of multiple cleavage planes in SCS.

Tensile and Tensile-Mode Fatigue Testing of Microscale Specimens in Constant Humidity Environment

A tensile and tensile-mode-fatigue tester has been developed for testing microscale specimens in high humidity environments in order to investigate the fracture mechanisms of microelectromechanical materials. A humidity control system was installed on a tensile-mode fatigue tester equipped with an electrostatic force grip. A specimen and a griping device were inserted into a small chamber and the humidity was controlled by air flow from a temperature and humidity chamber. The humidity stability was within ±2%RH for humidities in the range 25–90%RH for eight hours of testing. Fatigue tests were performed on single-crystal silicon (SCS) specimens in constant humidity environments and laboratory air for up to 106 cycles. The gauge length, width, and thickness of the SCS specimens were 100 or 500 μm, 13.0 μm, and 3.3 μm, respectively. The average tensile strength was 3.68 GPa in laboratory air; this value decreased in high humidity environments. Fatigue failure was observed during cyclic loading at stresses lower than the average strength. A reduction in the fatigue strength was observed at high relative humidities. Different fracture origins and fracture behaviors were observed in tensile tests and fatigue tests, which indicates that the water vapor in air affects the fatigue properties of SCS specimens.

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.

Damage characteristics of low-temperature BSi molecular ion implantation in silicon

This study investigates the damage characteristics of BSi molecular ions implanted into silicon at liquid nitrogen temperature (LT) and room temperature (RT). 〈100〉 single-crystal silicon specimens tilted 7° were implanted with 77keV BSi molecular ions for various ion fluences (1×1013–2×1015cm−2) and then rapid thermal annealed at 1050°C for 25s in nitrogen ambient. The SRIM Monte-Carlo computer code was adopted to calculate theoretical damage depth profiles. Raman scattering spectroscopy (RSS) was employed to experimentally characterize damage behavior. It is found that the existence of crystalline and amorphous phases in the specimens can be clearly identified by the longitudinal and transverse optical phonon Raman peaks, respectively, in terms of peak intensity, peak position, area under the peak, and full-width at half-maximum (FWHM) of the peak. The as-implanted results reveal that LT leads to a greater amount of implantation-induced damage than RT does. However, the as-annealed results show that the amount of residual damage in the LT specimen is only slightly smaller than it is in the RT specimen.

OS5-3-2 Raman Spectroscopic Analysis of Surface Stress Distribution on Single Crystal Silicon Microstructures under Uniaxial Tensile Loading

This paper describes an experimental analysis of surface stress distribution on single crystal silicon (SCS) microstructures using laser Raman spectroscope. A handmade tensile tester was employed to apply a uniaxial tensile stress to SCS specimen with a 270 nm-height and 4 μm-square SCS boss in the gauge section. In room temperature, Raman spectroscope measured surface stress, applied by the tensile tester, around the boss. The stress distribution obtained from two-curve fitting of Raman spectrum was in good agreement with that estimated by finite element analysis (FEA).

Effect of Surface Oxide Layer on Mechanical Properties of Single Crystalline Silicon

AbstractFatigue fracture under cyclic loading of single crystal silicon (SCS) is concerned and fatigue properties and mechanism are investigated widely. Surface oxide is considered to have an important role on fatigue fractures. In this paper, the tensile testing of SCS whose surface was intentionally oxidized and the effect of the oxide thickness on the mechanical properties were reported. After fabrication of SCS specimens, they were oxidized in dry oxidization at 1100 ºC. Quasi-static tensile testing of SCS specimens with no, 100-nm-oxide, and 200-nm-oxide thick oxide was performed. As the results, the deviation in fracture strain was decreased by oxidation, and the fracture origin was observed to be the inner silicon part. These results might be caused by the decrease of surface roughness and defects formation by oxygen precipitation during thermal oxidation.