Demonstration of using HF-etchant mixed into CO2 to lift-off thin-film GaAs layers by supercritical fluid technology

The epitaxial lift-off (ELO) process based on selectively etching a thin sacrificial AlAs layer from GaAs substrate was performed using high-concentrated aqueous hydrofluoric (HF) etchant. However, because of using the wet etching method, the traditional ELO process has many drawbacks and limitations. Supercritical fluids (SCFs) naturally have the characteristics of low viscosity, high diffusivity, and zero surface tension. Therefore, the development of a gas-phase-like dry etching method based on mixing HF into CO2 and operating the mixture of HF/CO2 in SCFs condition as etchant is hereby proposed to overcome those bottlenecks existing in traditional wet ELO processes. However, there are no available experimental results for etching AlAs layers by HF in SCFs yet. Therefore, a HF-compatible corrosion-resistant high-pressure system was designed and built up to perform the idea. The capabilities of etching sample in supercritical CO2 (scCO2) had been systemically investigated under various pressures (2000–3000 psi) and temperatures (40–60[Formula: see text]C). Besides, the etching performances separately conducted by using aqueous-HF and anhydrous HF/Pyridine as the source etchant and mixing with scCO2 at a fixed temperature, pressure and etching time were also examined and compared under different equivalent HF concentrations. An evaluation of using acetone as the co-solvent mixed with HF/scCO2 mixture for enhancing the etch rate in different volume ratio of HF/co-solvent was further investigated and discussed. With this system, we demonstrate releasing a size of [Formula: see text] (width × length) and 3 [Formula: see text]m-thick free-standing GaAs sheet from a 150 nm AlAs sacrificial layer by the etching sample in HF/scCO2 mixture. The released GaAs sheet was also successfully transferred to a flexible PET substrate by using a PDMS stamp and an adhesive layer of NOA61.

High-performance plasmonic lab-on-fiber sensing system constructed by universal polymer assisted transfer technique

Plasmonic lab-on-fiber (LOF) system has become an emerging sensing platform for the realization of miniaturized and portable plasmonic sensors. Herein, a facile and efficient polymer assisted transfer technique was reported for the preparation of plasmonic LOF systems. The proposed plasmonic LOF system was constructed through transferring plasmonic arrays to the end surface of optical fibers using polylactic acid (PLA) as the sacrificial layer. The morphology of the transferred plasmonic arrays maintains excellent consistency with the original arrays. Importantly, the as-prepared plasmonic LOF system also possesses outstanding sensing performance in refractive index sensing and quantitative label-free biosensing applications. Additionally, the proposed polymer assisted transfer technique shows broad universality for various plasmonic arrays. Together with the above features, it is believed that the polymer assisted transfer technique will show great potential for the application of future plasmonic LOF systems.

Optimization of Sacrificial Layer Etching in Single-Crystal Silicon Nano-Films Transfer Printing for Heterogeneous Integration Application

As one of the important technologies in the field of heterogeneous integration, transfer technology has broad application prospects and unique technical advantages. This transfer technology includes the wet chemical etching of a sacrificial layer, such that silicon nano-film devices are released from the donor substrate and can be transferred. However, in the process of wet etching the SiO2 sacrificial layer present underneath the single-crystal silicon nano-film by using the transfer technology, the etching is often incomplete, which seriously affects the efficiency and quality of the transfer and makes the device preparation impossible. This article analyzes the principle of incomplete etching, and compares the four factors that affect the etching process, including the size of Si nano-film on top of the sacrificial layer, the location of the anchor point, the shape of Si nano-film on top of the sacrificial layer, and the thickness of the sacrificial layer. Finally, the etching conditions are obtained to avoid the phenomenon of incomplete etching of the sacrificial layer, so that the transfer technology can be better applied in the field of heterogeneous integration. Additionally, Si MOSFETs (Metal-Oxide-Semiconductor Field Effect Transistors) on sapphire substrate were fabricated by using the optimized transfer technology.

Fabrication and Resonance Characteristics of PZT Cantilevers

Thin films of lead zirconate titanate Pb(Zr0,52Ti0,48)O3 (PZT) with thickness of 1.4 μm were prepared on Si/SiO2/TiO2/Pt substrates by chemical solutions deposition. Based on the obtained films, the structures of PZT cantilevers were formed, with a length from 500 to 1000 /um and wide from 100 to 500 μm. Platinum (100 nm) as the bottom and top electrode, has been deposited by magnetron sputtering. PZT cantilevers werereleased by etching the sacrificial layer in SF6. The resonance characteristics of the PZT cantilevers were determined by the light lever method using a special optical measuring stand. Output characteristics of the PZT cantilevers, can be used in MEMS devices, specially, in MEMS resonators.

Low-cost thermo-chemical process of TiO2 powder purification: Study of iterative gettering effect

The titanium dioxide (TiO2) purity is very important for the TiO2-based applications making essential the impurities density reduction. In this study, we propose an efficient purification process of TiO2 powder in order to reduce impurities. The low-cost proposed approach is based on an iterative gettering (IG) process combining three main steps: (1) a porous TiO2 sacrificial layer formation (p-TiO2), (2) a rapid thermal annealing (RTA) of p-TiO2 powder in an infrared oven at 950°C under air permitting the residual impurities diffusion to the porous layer surface and (3) etching in acid solution to remove the porous layer. Effect of the proposed gettering process on purification efficiency was evaluated by different characterization techniques such as the transmission electron microscopy (TEM), the energy dispersive x-ray spectroscopy (EDX), the UV–Visible-NIR spectroscopy, the X-ray diffraction (XRD) and atomic absorption spectroscopy (AAS). The obtained results showed the efficient removal of metal impurities, such as Cu, Al, P, and Fe confirming the efficiency of the process improving the purity from 89% to 99.96%.

Design analysis and fabrication of side-drive electrostatic micromotor by UV-SLIGA

In the present work, a micro-electro-mechanical system (MEMS)-based electrostatic micromotor is designed and fabricated. Finite element analysis is done and various parameters affecting the torque are studied. Maximum torque is achieved at 120° phase angle. The effect of change in voltage, micromotor height and frequency is analysed and discussed. UV-SLIGA, a microfabrication technique, is used for the fabrication of electrostatic micromotor of height 30µm and higher. UV lithography is conducted by both positive AZ P4620 and negative (SU-8 10 and SU-8 2150) photoresists. Copper (Cu) is used as a sacrificial layer to release the rotor (the movable part) of the electrostatic micromotor. Electroformed nickel (Ni) is used for making stator, rotor and axle, whereas chromium (Cr) is used as a seed layer. The micromotor is fabricated with a stator-rotor pole having configuration ratio of 3:2. The gap between the rotor and axle is 20 µm. Wet chemical etching is used to etch the deposited metal layers (Cr, Ni and Cu). Challenges such as the adhesion between the photoresist mould and substrate, cracks, seepage and misalignment are faced during the microfabrication. These challenges are overcome by optimizing the various parameters. The fabrication of electrostatic micromotor is done successfully and the results are discussed in the article.

Microchip Transfer Process for Implantable Flexible Bioelectronic Devices

The demands on flexible implants for recording of neural signals and electrical stimulating have increased in recent years with regard to their functionality, miniaturization, and spatial resolution. These requirements can be met best by embedding powerful complementary metal oxide semiconductor (CMOS) microchips into thin biocompatible polymer substrates. So-called chip-in-foil systems thus combine mechanical properties of a polymer substrate and performance of CMOS technology. The development of a process for direct transfer of multiple CMOS microchips (edge length <400 μm) simultaneously into thin polyimide (PI) substrates is subject of this study. It allows the use of standard microelectromechanical systems (MEMS) processes for further levelled superficial layer build-up. This is achieved with the help of a silicon carrier wafer equipped with cavities for precise chip placement and a sacrificial layer to facilitate release of the chip-in-foil systems. In a post-processing step all silicon chips are thinned down to 100 μm. With this process a transfer yield of 100 % (n = 34) was achieved for the silicon chips on a die level. Chip rotational error on substrates was determined to be as low as 0.21° ± 0.10°. Die adhesion was examined by shear tests, resulting in shear strength of 58.1 MPa ± 13.7 MPa, which dropped to 15.2 MPa ± 10.5 MPa after accelerated ageing in 60 °C phosphate buffered saline solution (PBS) for 16 days (equivalent to 78 days at 37 °C). This study demonstrated a reliable microchip transfer process with low positioning error into flexible PI substrates with post-processing thinning of the dies. The use of a carrier silicon wafer allowed precise electrical interconnect fabrication with standard MEMS processing techniques and without handling of thin and fragile chips. These results are a prerequisite to meet needs of reliability and structural biocompatibility in implantable flexible bioelectronic devices.