Thin Single-crystalline Silicon Research Articles

Critical bending radius of thin single-crystalline silicon with dome and pyramid surface texturing

Four-point bending tests are performed on 50-μm-thick single-crystalline silicon (Si) wafers with dome- and pyramid-shaped surface patterns, which are used as flexible Si solar cells. Surface patterns, which act as stress concentrators, reduce the flexural strengths, leading to larger critical bending radius. The critical bending radii of surface-textured Si are much smaller than the calculated values for a single-notch geometry. The finite element analysis shows that the stress concentrations at the tips of the surface patterns effectively disperse in fine and periodic dome and irregular pyramid patterns.

Effects of mirror polishing brick surface on process yield of multi-wire slicing for thin single-crystalline silicon solar cells

Thin silicon (Si) wafers with thickness of 100–140μm were obtained by using a multi-wire slicing process with a SiC slurry. We investigated the process yields of wafer slicing as well as physical/electrical properties of the sliced wafers with various thicknesses. As the wafer thickness decreases, the process yield abruptly decreased due to wafer breakage during the slicing process, and conventional polishing the brick surface was not enough to gain a considerable process yield. However, elimination of defects on the brick surface by mirror polishing resulted in an 83.1% enhancement of yield even for wafers with a thickness of 100μm. The number of wafers obtained was even higher than that of conventional 180μm-thick wafers (479 vs. 415 wafers in this research). Investigation of the microstructure of brick surfaces revealed that surface defects on bricks were main parameter to determine the yield of slicing process. Surface defects containing the micro-cracks introduced residual stress, which decreased the slicing process yield especially for thinner wafers. From measurements of physical and electrical properties, it was revealed that the relative total thickness variations (TTVs) and bowings increased and the characteristic fracture strength of sliced wafer and conversion efficiencies decreased as the wafer thickness decreased. For the wafer with thickness of 100μm, the relative TTV and bowing were 14.1% and 22.5mm, respectively. The conversion efficiency of a solar cell using this wafer was 17.6%, while that of a conventional Si solar cell using a 180μm-thick wafer was 18.4%.

Generation and detection of gigahertz acoustic oscillations in thin membranes

Single crystalline membranes are a perfect model system for the study of coherent acoustic phonon generation and decay in the time domain. Coherent acoustical modes are excited and detected in thin single-crystalline silicon and gallium arsenide membranes with femtosecond pulses in the ultraviolet and infrared wavelength region using the asynchronous optical sampling technique. The measured acoustic spectra are compared with each other and are discussed in terms of different generation and detection mechanisms. A clear dependence of the generated spectra on the absorption length of the pump and probe pulses is observed. It is shown that a short absorption length for the pump pulse leads to the generation of coherent high frequency phonons up to several 100GHz frequencies. Membranes are demonstrated to be useful as broadband acoustic cavities and can help to disentangle details of high frequency phonon dynamics. Two-layer membrane systems offer additional insight into energy transfer in the GHz frequency range and adhesion properties.

Wet-texturing process for a thin crystalline silicon solar cell at low cost with high efficiency

In the wet-texturing process, a smaller etch depth of 8 µm was established on each side of the thin silicon wafer. Formation of a very small size (1–3 µm) pyramidal structure was confirmed on a thin c-Si surface through the use of SEM micrographs. For this silicon surface with small pyramidal structures, a surface reflectance of 15.14% was observed. Saw damage removal was employed for the second and the third sets of silicon wafers in a NaOH (8 wt%) solution, and the wafers were then textured in a mixture of NaOH (1.5 wt%), de-ionized water and Isopropyl alcohol (IPA, 6–8 wt%) at 80 °C for 18 min and 24 min, respectively. In this process, the scraped depths were 16 and 26 µm on each side of the second and the third sets of silicon wafers, and the pyramidal sizes were 4–9 µm and 9–12 µm respectively. We found that the reflectances of these sets of wafers were 14.98% and 14.78%, respectively. Our novel approach of ultrasonic cleaning, hot de-ionized water treatment, and texturing for thin c-Si wafers yielded small-sized (1–3 µm) uniform pyramidal structure that were stable and suitable for metalized screen printing. Even though the reflectance was high for the first set of wafers the illuminated current-voltage (LIV) measurements confirmed the wafers’ superiority over the conventional textured c-Si solar cells in terms of the open-circuit voltage and the fill factor. The small-size pyramidal-structured thin single-crystalline silicon solar cells showed a 17.9% efficiency. Since no NaOH solution was used for saw damage removal in the first set of silicon wafers, the production cost for commercial production of large-area single-crystalline silicon solar cells was reduced by nearly 20%.

Single-Crystalline Silicon Layer Transfer to a Flexible Substrate Using Wafer Bonding

This paper reports the successful transfer of a thin single-crystalline silicon film to a flexible, transparent polymer substrate. Thin-film silicon on polymer was realized by bonding a silicon-on-insulator (SOI) wafer to a flexible substrate using a spin-on polymer as an adhesive. The SOI wafer was thinned by a grinding operation followed by chemical mechanical polishing (CMP). The SOI was further thinned to the buried oxide using wet etchants. The residual stress in the transferred substrate was investigated by ultraviolet (UV) micro-Raman spectroscopy and numerical modeling. Both approaches showed that a low level of stress was created at the bonded interface during the layer transfer.

Study on the noise of silicon capacitive resonant mass sensors in ambient atmosphere

This article reports on the resolution of mass detection by optical and capacitive methods in ambient atmosphere using very thin single-crystalline silicon microcantilevers. The detectable minimum mass by optical sensing increases slightly with decreasing cantilever size due to temperature fluctuation noise. Using capacitive detection, a detectable minimum mass of below 1×10−14 g can be obtained using cantilevers with resonance frequencies of 78–80 kHz. From an experimental comparison, we find that the detectable minimum mass by capacitive detection is slightly smaller than that obtained by optical detection using a cantilever with resonant frequency of 78 kHz. Using theoretical modeling and experimental results relevant to gas adsorption-desorption to the sensor surface, it is found that the adsorption-desorption noise by gases with infinitesimal partial pressure in the air surrounding a cantilever significantly influences the resonance frequency fluctuation of the cantilever in the high resonance frequency region.

Microwave enhanced ion-cut silicon layer transfer

Microwave heating has been used to decrease the time required for exfoliation of thin single-crystalline silicon layers onto insulator substrates using ion-cut processing. Samples exfoliated in a 2.45GHz, 1300W cavity applicator microwave system saw a decrease in incubation times as compared to conventional anneal processes. Rutherford backscattering spectrometry, cross sectional scanning electron microscopy, cross sectional transmission electron microscopy, and selective aperture electron diffraction were used to determine the transferred layer thickness and crystalline quality. The surface quality was determined by atomic force microscopy. Hall measurements were used to determine electrical properties as a function of radiation repair anneal times. Results of physical and electrical characterizations demonstrate that the end products of microwave enhanced ion-cut processing do not appreciably differ from those using more traditional means of exfoliation.

A single-sided micromachined piezoresistive SiO2 cantilever sensor for ultra-sensitive detection of gaseous chemicals

This paper presents a novel SiO2 microcantilever sensor for high-sensitive gaseous chemical detection. A thin single-crystalline silicon piezoresistor is integrated in the SiO2 cantilever for low-noise and high-resoluble signal readout. The paper relates the MEMS formation, modeling and detecting experiments of the sensor. The micro-fabrication of the cantilever sensor is processed at a single side (front side) of the silicon wafer, combined with functionalization of a self-assembled monolayer (SAM) on the sensing cantilever for specific molecular capturing. The model of the piezoresistive SiO2 cantilever sensor is given by using both analysis and simulation, resulting in good agreement with the measuremental data. With the cantilever modified by the specific SAM, the detecting performance of the sensor is experimentally obtained. Attributed to the high sensitivity and the low electric noise of the piezoresistive SiO2 cantilever, ammonia gas of 0.1 ppm level concentration and dimethyl methylphosphonate vapor of 10 ppb concentration have been well detected. The integrated cantilevers are promising for inexpensive, highly resoluble and portable chemical/biological detection.