Ming Gan

An in situ platform for the investigation of Raman shift in micro-scale silicon structures as a function of mechanical stress and temperature increase

Raman spectroscopy provides an accurate approach to measure temperature and stress in semiconductors at micro-scale and nano-scale. In the present work an in situ experimentation-based approach to separate a measured room to high temperature Raman shift signal into mechanical and thermal components when a uniaxial compressive load is applied in situ is presented. In situ uniaxial compressive loads were applied on examined silicon cantilever specimens from room temperature to 150 °C. The Raman shift measurements were performed as a function of strain at constant temperature and as a function of temperature at constant strain levels. The results show that the Raman shift measured at a given temperature under a given level of applied stress can be expressed as a summation of stress-induced Raman shift signal and temperature-induced Raman shift signal measured separately. For silicon, the stress-induced Raman shift is caused by inelastic interaction between the incident laser and the vibration of crystal lattice, while the temperature-induced Raman shift is caused by the anharmonic terms in the vibrational potential energy. Analyses indicate that such separation of Raman shift signal can be used to measure localized change in thermal conductivity and mechanical stress of semiconductor structures under applied stress.

Correlating Microscale Thermal Conductivity of Heavily-Doped Silicon With Simultaneous Measurements of Stress

The functioning and performance of today’s integrated circuits and sensors are highly affected by the thermal properties of microscale silicon structures. Due to the well known size effect, the thermal properties of microscale silicon structures are not the same as those of the bulk silicon. Furthermore, stress/strain inside microscale silicon structures can significantly affect their thermal properties. This article presents the first thermal conductivity measurements of a microscale silicon structure under applied compressive stress at 350 K. Atomic force microscope (AFM) cantilevers made of doped single-crystal silicon were used as samples. A resistance temperature detector (RTD) heater attached to another RTD sensor was used as the heating module, which was mounted onto a nanoindentation test platform. This integrated system applied compressive load to the cantilever in the longitudinal direction while supplying heat. The thermal conductivity of the cantilevers was calculated using steady state heat conduction equation. The result shows that the measured thermal conductivity varies from 110 W/(m·K) to 140 W/(m·K) as compressive strain varies from 0.1% to 0.3%. Thermal conductivity is shown to increase with increase in compressive strain. These results match with the published simulation values. The measured thermal conductivity and stress values vary in the similar manner as a function of applied strain.

Surface stress variation as a function of applied compressive stress and temperature in microscale silicon

Surface stress has been shown to affect the mechanical properties of materials at or below the microscale. Surface-stress-induced dislocation activity at such length scales has been shown to be a major factor affecting the mechanical behavior of materials. Defect generation as a function of applied stress at the microscale has previously been measured experimentally and predicted using simulations. However, the change in surface stress in a material in response to externally applied stress as a function of temperature has not been explored experimentally. Such an investigation is presented in this work for the case of microscale silicon samples. In-situ nondestructive measurements of the applied compressive stress and the corresponding microscale surface stress were performed from room temperature to 100 °C. The applied stress was controlled by a nanomechanical loading system. Micro-Raman spectroscopy was used to measure the surface stress in-situ as the samples deformed under the applied uniaxial compres...

Raman Spectroscopy-Based Investigation of Thermal Conductivity of Stressed Silicon Microcantilevers

Multiscale experiments and models have repeatedly shown that thermal and mechanical properties of materials are a strong function of the length scale of measurement. In the case of silicon, a length-scale-dependent interrelationship of thermal and mechanical properties has been used to improve the thermal dissipation and performance of the microscale electronic devices. This work uses a newly established nanomechanical Raman spectroscopy approach to analyze thermal conductivity of microscale silicon cantilevers as a function of temperature and mechanical strain. The results are compared to available experimental data on silicon by other researchers, and explanations are offered regarding the effect of the length scale on the obtained results. The results show that the thermal conductivity of silicon increases from 114 to 145  W/(m·K), with the compressive strain level increasing from 0 to 0.25% at room temperature. At higher temperatures, the dependence of thermal conductivity on strain significantly incr...

Raman Thermometry Based Thermal Conductivity Measurement of Bovine Cortical Bone as a Function of Compressive Stress

Biological materials such as bone have microstructure that incorporates a presence of a signifi cant number of interfaces in a hierarchical manner that lead to a unique combination of properties such as toughness and hardness. However, studies regarding the infl uence of structural hierarchy on physical properties such as thermal conductivity and its correlation with mechanical stress of biomaterials are limited. Such studies can point out important insights regarding the role of biological structural hierarchy in infl uencing mechanophysical properties. This study presents an analytic-experimental approach to establish stress–thermal conductivity correlation in bovine cortical bone as a function of nanomechanical compressive stress using Raman thermometry. Analyses establish empirical relations between Raman shift and temperature as well as a relation between Raman shift and nanomechanical compressive stress. Analyses verify earlier reported thermal conductivity results at 0% strain and room temperature. In addition, measured trends and established thermal conductivity–stress relation indicates that the thermal conductivity values increases and then decrease as a function of increase in compressive strain.

Small Scale Thermomechanics in Si with an Account of Surface Stress Measurements

Multiscale experiments and models have repeatedly shown that thermal and mechanical properties of materials are a strong function of the length scale of measurement. This work uses a newly established nanomechanical Raman spectroscopy approach to analyze creep deformation of microscale Si cantilevers as a function of temperature and mechanical strain. This research reports in-situ creep properties of silicon micro-cantilevers in this temperature range under uniaxial compressive stress. The experimental setup consists of micro-scale mechanical loading platform and localized heating module. The results reveal that in the stress range of 50–150 MPa, the strain rate of the silicon cantilever increases linearly as a function of applied stress. The strain rate also increases a function of temperature increase. However, the strain rate increase slows down with increase in temperature. The strain rate of the microscale silicon cantilever (0.2–2.5 × 10−6 s−1) was comparable to literature values for bulk silicon reported in temperature range 1100–1300 °C but with only one tenth of the applied stress level. The relaxation of the near-surface atoms also contributes to the creep of the material. Analyses are also used to establish a surface stress relation in one dimensional nanostructures subjected to mechanical loading at high temperatures.

Role of Length Scale and Temperature in Nanoscale and Microscale Creep of Si-C-O Ceramics

This investigation presents nanoindentation and microindentation creep analyses on polymer derived Si-C-O ceramic coatings at temperatures ranging from room temperature to 500 degree-C. The properties of focus include elastic modulus, hardness, creep exponent, and creep strain rate. Analyses show that at the nanoscopic length scale the deformation mechanism is dominated by dislocation climb and diffusion. With increase in length scale to microscale the thermal activation volume increases by approximately 10 times. The increase in free volume leads to the deformation mechanism switching to volumetric densification and dislocation pile up. An important physical effect analyzed is the effect of increase in temperature on the observed deformation mechanism. At the nanoscale, with increase in temperature, both hardness and elastic moduli show an increase. At the microscale, however, hardness reduces with increase in temperature. The indentation size effect is observed at both scales. However, at the nanoscale the indentation size is linked with strain hardening. At the microscale, a strain softening behavior is observed.