Single-crystal Silicon Thin Films Research Articles

Temperature dependent evolution of wrinkled single-crystal silicon ribbons on shape memory polymers.

Shape memory polymers (SMPs) can remember two or more distinct shapes, and thus can have a lot of potential applications. This paper presents combined experimental and theoretical studies on the wrinkling of single-crystal Si ribbons on SMPs and the temperature dependent evolution. Using the shape memory effect of heat responsive SMPs, this study provides a method to build wavy forms of single-crystal silicon thin films on top of SMP substrates. Silicon ribbons obtained from a Si-on-insulator (SOI) wafer are released and transferred onto the surface of programmed SMPs. Then such bilayer systems are recovered at different temperatures, yielding well-defined, wavy profiles of Si ribbons. The wavy profiles are shown to evolve with time, and the evolution behavior strongly depends on the recovery temperature. At relatively low recovery temperatures, both wrinkle wavelength and amplitude increase with time as evolution progresses. Finite element analysis (FEA) accounting for the thermomechanical behavior of SMPs is conducted to study the wrinkling of Si ribbons on SMPs, which shows good agreement with experiment. Merging of wrinkles is observed in FEA, which could explain the increase of wrinkle wavelength observed in the experiment. This study can have important implications for smart stretchable electronics, wrinkling mechanics, stimuli-responsive surface engineering, and advanced manufacturing.

Higher order Cauchy–Born rule based multiscale cohesive zone model and prediction of fracture toughness of silicon thin films

In this work, we extend the multiscale cohesive zone model (MCZM) (Zeng and Li in Comput Methods Appl Mech Eng 199:547–556, 2010), in which interatomic potential is embedded into constitutive relation to express cohesive law in fracture process zone, to include the hierarchical Cauchy–Born rule in the process zone and to simulate three dimensional fracture in silicon thin films. The model has been applied to simulate fracture stress and fracture toughness of single-crystal silicon thin film by using the Tersoff potential. In this study, a new approach has been developed to capture inhomogeneous deformation inside the cohesive zone. For this purpose, we introduce higher order Cauchy–Born rules to construct constitutive relations for corresponding higher order process zone elements, and we introduce a sigmoidal function supported bubble mode in finite element shape function of those higher order cohesive zone elements to capture the nonlinear inhomogeneous deformation inside the cohesive zone elements. Benchmark tests with simple 3D models have confirmed that the present method can predict the fracture toughness of silicon thin films. Interestingly, this is accomplished without increasing of computational cost, because the present model does not require quadratic elements to represent heterogeneous deformation, which is the inherent weakness of the previous MCZM model. Quantitative comparisons with experimental results are performed by computing crack propagation in non-notched and initially notched silicon thin films, and it is found that our model can reproduce essential material properties, such as Young’s modulus, fracture stress, and fracture toughness of single-crystal silicon thin films.

Distribution of Electromagnetic Field and Energy Flux in the Thin Film Solar Cell with Silver Nano-Disk Array

We simulate and calculate numerically the electromagnetic field and energy flux in single crystal silicon thin film solar cell coated with silver nano-disk square array by using the finite-difference time-domain (FDTD) method. Because of the surface plasmon resonance (SPR) of silver nano array, the electromagnetic field is redistributed and enhanced in the solar cell. The simulation results show that the electromagnetic field distribution and corresponding energy flux component depend on the nano array and the structure of absorbed layer in solar cell. The wavelength of the incident light relative to the nano array determine the profile of the electric field around the nano array. The electromagnetic field distribution in thin film is determined by the internal structure of solar cell. For different incident wavelengths, the electromagnetic field distribution in solar cell will changes. The energy flux named as Poynting vector also changes with the incident wavelength. To investigate the absorption of the solar cell, the normalized absorbed power at different wavelengths is calculated. Based on the SPR effect, the solar cell exhibts absorption enhancement sharply at a certain wavelength.

J032013 単結晶シリコン薄膜の破壊挙動に及ぼす水と温度の影響の評価

J032013 単結晶シリコン薄膜の破壊挙動に及ぼす水と温度の影響の評価

OS1909 加工条件の異なる(110)<111>単結晶シリコン薄膜の引張試験

We conducted uni-axial tensile testing using electrostatic-force grip on single crystal silicon thin film of <111> direction. The specimens were fabricated on (110) SOI wafer (length: 120 μm, width: 5 μm, thickness: 5 μgm) and went through 3 different conditions of patterning process composed of Bosch process and treatment of surface residue. As a result, improvement of surface morphology brought increase of average tensile strength from 1.9 GPa to 3.6 GPa. SEM observation of fractured specimens shows that, regardless of processing conditions, the relationship between the tensile strength and the size of damage on the top corners can be expressed with a common equation. This result suggests that the damage on the top corners was dominant for the fracture of this specimen.

Single Crystal Silicon Thin Film on Polymer Substrate by Double Layer Transfer Method

We report discovery of new method to transfer a single crystal silicon thin film onto a bendable polymer substrate by using layer transfer process. The method includes creation of mechanically weakened layer 100 nm - 600 nm below the Si wafer surface using boron and hydrogen ion implantations. Silicon mother wafer is then pre-bonded to glass and exfoliated. Exfoliation divides the glass-silicon assembly into weakly bound Si thin film on glass and leftover mother Si wafer. Then the silicon thin film was transferred from glass substrate into a polymer substrate.

Single Crystal Silicon Thin Film on Polymer Substrate by Double Layer Transfer Method

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Resonant Bending Fatigue Tests on Thin Films

The fatigue properties of thin-fi lm materials are extremely important in the design of durable and reliable micro-electromechanical systems (MEMS). However, it is rather diffi cult to apply the conventional fatigue testing method for bulk materials to thin fi lms as the specimen size is extremely small. Therefore, a fatigue testing method suitable for thin-fi lm materials is required. We have developed a fatigue testing method that uses the resonance of a cantilever-type specimen prepared from thin fi lms. Cantilever beam specimens with dimensions of 3(L) × 1(W) × 0.01(t) mm3 were prepared from Ni-P amorphous alloy thin fi lms and gold foils. In addition, cantilever beam specimens with dimensions of 3(L) × 0.3(W) × 0.005(t) mm3 were prepared from single-crystal silicon thin fi lms. These specimens were fi xed to a holder that was connected to an audio speaker (actuator) and were resonated in the bending mode. The Young’s moduli measured from the resonant frequencies of Ni-P and gold foil were 116 and 72 GPa, respectively. These values were comparable to those measured by other techniques, indicating that the specimens resonated in a theoretically predictable manner and that our method is valid. Resonant fatigue tests were carried out for these specimens by changing the amplitude range of resonance, and S-N curves were successfully obtained for Ni-P amorphous alloy thin fi lms, gold foils, and single-crystal silicon thin fi lms.

T0302-1-3 新規疲労試験手法を用いた単結晶および多結晶シリコンの疲労特性の評価([T0302-1]高信頼マイクロ・ナノデバイスのための設計・計測技術(1):シリコンの破壊と疲労)

Static strength tests and dynamic fatigue tests were conducted on single crystal silicon thin films under inert environment and compared with the results of polycrystalline silicon. A novel "ramping fatigue test" was utilized in this study, where the amplitude of cyclic stress applied to the specimens was gradually increased with respect to the number of cycles. It was found that single crystal silicon suffers fatigue damage also under inert environment of N_2 gas, and the strengthening due to cyclic loading found with polycrystalline silicon was not significant for the case of single crystalline silicon.

Influence of sub‐micrometer notches on the fracture of single crystal silicon thin films

ABSTRACTThe influence of notches on the fracture of single crystal silicon thin films was investigated. The tests were conducted on notched and smooth tensile specimens micromachined on a silicon wafer. The specimen geometry was 100 μm long, 50 μm wide and 5 μm thick. For the notched specimen, a V‐shaped sub‐micrometer notch was introduced on one edge of it by using a focused ion beam (FIB) process. The notch lengths ranged from 0.07 to 1.3 μm. Four types of specimens with different surfaces and tensile orientations were tested. The smooth specimens showed scattered fracture strengths and ‘collapsed’ fractures. For the restrictive‐shaped notches, the critical length was 0.5 μm. The short‐notched (&lt;0.5 μm) specimens also showed ‘collapsed’ fractures, and the stress concentrations on notch tips decreased their fracture strengths. For the long‐notched (&gt;0.5 μm) specimens, the notch was equivalent to a crack in the Griffith model and the crack mainly propagated on {111} cleaved planes.

Molecular dynamics analysis for fracture behaviour of single crystal silicon thin film with micro notch

Thin films of single crystal silicon are widely required in many applications of semiconductor devices and micro-electro-mechanical systems. In this study, molecular dynamics simulation is conducted to investigate the effect of notch depth on fracture strength. The specimen size is about 10 nm × 5 nm, and the periodic boundary condition is applied for thin films. The loading directions are [100], [110] and [111] direction. Brittle fracture occurs on the plane perpendicular to the loading direction. The stress intensity factor at the onset of the crack propagation increases with increasing notch depth. When the notch depth is longer than 2 nm, the stress intensity factor is nearly constant. At the onset of the crack propagation, the stress at the notch root becomes constant irrespective of the notch depth.

Bendable single crystal silicon thin film transistors formed by printing on plastic substrates

Bendable, high performance single crystal silicon transistors have been formed on plastic substrates using an efficient dry transfer printing technique. In these devices, free standing single silicon objects, which we refer to as microstructured silicon (μs-Si), are picked up, using a conformable rubber stamp, from the top surface of a wafer from which they are generated. The μs-Si is then transferred, to a specific location and with a controlled orientation, onto a thin plastic sheet. The efficiency of this method is demonstrated by the fabrication of an array of thin film transistors that exhibit excellent electrical properties: average device effective mobilities, evaluated in the linear regime, of ∼240cm2∕Vs, and threshold voltages near 0V. Frontward and backward bending tests demonstrate the mechanical robustness and flexibility of the devices.

Thermal conductivity of periodic microporous silicon films

This letter reports the experimental in-plane thermal conductivity of microfabricated, free-standing, single-crystal silicon thin films with periodically arranged through-film micropores. The experimental data suggest that strong size effects exist even in micro-sized porous silicon structures, particularly at low temperatures. Even at room temperature, all porous membranes show thermal conductivity values lower than expected by porosity and bulk phonon mean free path of silicon, and small-pore membranes have smaller thermal conductivity compared to the large-pore ones despite that they have close porosity values.

SOI 웨이퍼를 이용한 Top emission 방식 AMOLEDs의 스위칭 소자용 단결정 실리콘 트랜지스터

We fabricated a single crystal silicon thin film transistor for active matrix organic light emitting displays(AMOLEDs) using silicon on insulator wafer (SOI wafer). Poly crystal silicon thin film transistor(poly-Si TFT) Is actively researched and developed nowsdays for a pixel switching devices of AMOLEDs. However, poly-Si TFT has some disadvantages such as high off-state leakage currents and low field-effect mobility due to a trap of grain boundary in active channel. While single crystal silicon TFT has many advantages such as high field effect mobility, low off-state leakage currents, low power consumption because of the low threshold voltage and simultaneous integration of driving ICs on a substrate. In our experiment, we compared the property of poly-Si TFT with that of SOI TFT. Poly-Si TFT exhibited a field effect mobility of 34 <TEX>$\textrm{cm}^2$</TEX>/Vs, an off-state leakage current of about l<TEX>${\times}$</TEX>10<TEX>$\^$</TEX>-9/ A at the gate voltage of 10 V, a subthreshold slope of 0.5 V/dec and on/off ratio of 10<TEX>$\^$</TEX>-4/, a threshold voltage of 7.8 V. Otherwise, single crystal silicon TFT on SOI wafer exhibited a field effect mobility of 750 <TEX>$\textrm{cm}^2$</TEX>/Vs, an off-state leakage current of about 1<TEX>${\times}$</TEX>10<TEX>$\^$</TEX>-10/ A at the gate voltage of 10 V, a subthreshold slope of 0.59 V/dec and on/off ratio of 10<TEX>$\^$</TEX>7/, a threshold voltage of 6.75 V. So, we observed that the properties of single crystal silicon TFT using SOI wafer are better than those of Poly Si TFT. For the pixel driver in AMOLEDs, the best suitable pixel driver is single crystal silicon TFT using SOI wafer.

Errata to "High-cycle fatigue of single-crystal silicon thin films"

Errata to "High-cycle fatigue of single-crystal silicon thin films"

LPE growth of textured single crystal silicon thin film for PV applications

The fabrication of solar cells based on thin silicon film on foreign substrates is an attractive way to realise cheap and efficient photovoltaic devices on a large scale. In this work, we propose an innovative technique to obtain textured monocrystalline Si on mullite owing to the transfer of a nucleation layer and subsequent LPE growth. The nucleation layer (with the shape of a grid) is elaborated by photoelectrochemical etching. The grid pattern parameters will determine the shape of the LPE layer surface, flat or pyramidal textured for efficient light trapping.

High-cycle fatigue of single-crystal silicon thin films

When subjected to alternating stresses, most materials degrade, e.g., suffer premature failure, due to a phenomenon known as fatigue. It is generally accepted that in brittle materials, such as ceramics, can only take place in toughened solids, i.e., premature failure would not be expected in materials such as single crystal silicon. The results of this study, however, appear to be at odds with the current understanding of brittle material fatigue. Twelve thin-film (/spl sim/20 /spl mu/m thick) single crystal silicon specimens were tested to failure in a controlled air environment (30/spl plusmn/0.1/spl deg/C, 50/spl plusmn/2% relative humidity). Damage accumulation and failure of the notched cantilever beams were monitored electrically during the fatigue test. Specimen lives ranged from about 10 s to 48 days, or 1/spl times/106 to 1/spl times/1011 cycles before failure over stress amplitudes ranging from approximately 4 to 10 GPa. A variety of mechanisms are discussed in light of the life data and fracture surface evaluation.

Silicon epitaxy from pulsed supersonic jets: Growth, modeling, and simulation

Single crystal silicon thin films have been grown on Si(100) substrates using pulsed supersonic jets of a disilane (Si2H6)-hydrogen mixture. Digital epitaxy has been achieved at substrate temperatures of 400–500 °C, with a growth rate of 0.1 Å/pulse at Ts=450 °C. Sticking coefficients of up to 0.3 are observed implying an order of magnitude increase over conventional gas source molecular beam epitaxy. A fundamental model, based on an approach proposed in the literature, has been developed to explain experimental results. Incorporating cyclic adsorption and regeneration steps, the resulting growth rate expression is fit to data to extract rate constants. An activation energy of 2.22 eV is obtained for the kinetic rate constant for hydrogen desorption, the limiting step in low temperature growth. A Monte Carlo approach has been used for the first time to study the atomic scale roughness variations in supersonic jet growth. Simulation studies show that the high energy jets only affect short-range diffusion, whereas long-range diffusion, responsible for improving surface roughness, is still thermally driven. A true atomic layer epitaxy (growth process, independent of substrate temperature), is predicted for incident jet energy exceeding the hydrogen desorption activation energy.

Single crystal silicon thin film transistors fabricatedat low process temperatureson glass substrates

Thin film transistors were fabricated in single crystal silicon films bonded to glass substrates. High electron mobilities were achieved, making integration of the driving circuitry and pixel switching elements on the same glass substrate possible and allowing improved display aperture ratios. These single crystal silicon TFTS are attractive for AMLCD applications.

Defect microstructure in single crystal silicon thin films grown at 150° C-305° C by remote plasma-enhanced chemical vapor deposition

Defect microstructure in terms of defect density and impurity concentration of epitaxial Si films grown by low temperature Remote Plasma-enchanced Chemical Vapor Deposition (RPCVD) in the temperature range of 150–305° C has been investigated using Transmission Electron Microscopy (TEM), Secondary Ion Mass Spectroscopy (SIMS), Reflection High Energy Electron Diffraction (RHEED) and defect etching/Nomarski microscopy. Defect density in the epitaxial Si films is found to be a strong function of growth temperature in the temperature range under study, indicating that thermal excitation is an important source of energy, in addition to plasma excitation, for driving surface reactions in the RPCVD expitaxial process. Impurity concentrations of H, O and C in the epitaxial films have been determined by SIMS analysis. The trace amounts (∼1 ppm) of oxygen and water vapor in the reactant gas (2%SiH4/He) was identified to be an important source of oxygen in the epitaxial Si films. An oxygen concentration as low as 3 × 1018 cm-3 in the epitaxial Si film grown at 150° C has been achieved through the use of a gas purifier. The higher hydrogen concentration in the films grown at lower temperatures is believed to be due to insufficiently rapid hydrogen desorption from the surface during growth. The results of characterization using TEM and SIMS are discussed to elucidate the atomistic mechanisms of Si epitaxial growth by RPCVD.