Effect of process parameters on the surface morphology and mechanical performance of silicon structures after deep reactive ion etching (DRIE)

This paper reports the results of a study on the optimization of the etching profile, which is an important factor in deep-reactive-ion etching (DRIE), i.e., dry etching. Dry etching is the key processing step necessary for the development of the Internet of Things (IoT) and various microelectromechanical sensors (MEMS). Large-area etching (open area > 20%) under a high-frequency (HF) condition with nonoptimized processing parameters results in damage to the etched sidewall. Therefore, in this study, optimization was performed under a low-frequency (LF) condition. The HF method, which is typically used for through-silicon via (TSV) technology, applies a high etch rate and cannot be easily adapted to processes sensitive to sidewall damage. The optimal etching profile was determined by controlling various parameters for the DRIE of a large Si wafer area (open area > 20%). The optimal processing condition was derived after establishing the correlations of etch rate, uniformity, and sidewall damage on a 6-in Si wafer to the parameters of coil power, run pressure, platen power for passivation etching, and SF6 gas flow rate. The processing-parameter-dependent results of the experiments performed for optimization of the etching profile in terms of etch rate, uniformity, and sidewall damage in the case of large Si area etching can be summarized as follows. When LF is applied, the platen power, coil power, and SF6 should be low, whereas the run pressure has little effect on the etching performance. Under the optimal LF condition of 380 Hz, the platen power, coil power, and SF6 were set at 115W, 3500W, and 700 sccm, respectively. In addition, the aforementioned standard recipe was applied as follows: run pressure of 4 Pa, C4F8 content of 400 sccm, and a gas exchange interval of SF6/C4F8 = 2 s/3 s.

MoS2 ultra-thin layers are synthesized using a chemical vapor deposition method based on the sulfurization of molybdenum trioxide (MoO3). The ultra-thin layers are characterized by X-ray diffraction (XRD), photoluminescence (PL) spectroscopy and atomic force microscope (AFM). Based on our experimental results, all the processing parameters, such as the tilt angle of substrate, applied voltage, heating time and the weight of source materials have effect on the microstructures of the layers. In this paper, the effects of such processing parameters on the crystal structures and morphologies of the as-grown layers are studied. It is found that the film obtained with the tilt angle of 0.06° is more uniform. A larger applied voltage is preferred to the growth of MoS2 thin films at a certain heating time. In order to obtain the ultra-thin layers of MoS2, the weight of 0.003 g of source materials is preferred. Under our optimal experimental conditions, the surface of the film is smooth and composed of many uniformly distributed and aggregated particles, and the ultra-thin MoS2 atomic layers (1∼10 layers) covers an area of more than 2 mm×2 mm.

The notching and stiction problem, which widely exists in silicon on insulator (SOI) microstructure fabrication, were resolved in this study. In this paper, a new plasma trench technique that is based on the deep reactive ion etching (DRIE) process is proposed. In this modified process, the deep reactive ion etching (RIE) was divided into several steps, where conformal plasma enhanced chemical vapor deposition (PECVD) oxide coating, and directional oxide etch back were employed to prevent the notching effect and the reactive ion etching (RIE) lag effect is also improved. Therefore, the microstructures regardless of the feature sizes could be realized. The stiction problem is eliminated by using dry chemical release replacing wet release in this approach, where the notching effect is used. The notching or footing effect was exploited for attaining the lateral etch following the deployment of the anisotropic plasma etching of the inductively coupled plasma (ICP). This method was proven useful for both the uniform and non-uniform feature designs. With this novel process, the high aspect ratio beams can be obtained. The thickness of the silicon layer is 75 μm, while the depth of the beams is 70 μum where the 5 μm silicon was etched to suspend the movable beams. The aspect ratio is as high as 35. Trenches with very different widths of 2.5 μm and 35 μm are also achieved at the same time.

Part 1 of this paper (Eason et al 2004) investigates the manufacturing of a variety of microchannels, produced by wet and dry etching in silicon, as well as precision mechanical sawing in silicon and thermoset plastic. This paper describes the experimental equipment and methods used to measure the pressure flow characteristics of the manufactured channels. A custom designed test system has been built in order to test each sample using the same inlet and outlet manifolds, pressure tappings, pumping system and instrumentation. The pressure drop across each set of channels was measured using an inductive pressure transducer. The mass flow rate through the system is measured by weighing the flow from the system in a given time. The measured pressure flow behaviour was compared with theoretical values as calculated from macro scale theory. Channel dimensions used for this calculation are as measured in part 1 of this paper. Error analysis was then carried out in order to determine the overall accuracy of the experimental work and determine whether any deviation from theoretical values is of experimental significance. This step is essential in any attempt to determine whether microchannel flows are indeed different to macro scale flows in a fundamental way. The deep reactive ion etched (DRIE) channels show the most significant lack of correlation with theoretical predictions. Compensation must be introduced to deal with the difference in cross section between the perfectly rectangular channels used for the theoretical prediction and the actual cross section of the channels.

This paper presents an experimental study of capabilities to fabricate high aspect-ratio microstructures in glass. The fabrication was done by molding borosilicate glass in deep silicon trenches created by deep reactive ion etching in silicon and releasing the structures by potassium hydroxide (KOH) and Xenon difluoride (XeF2) etching of the Si mold. Subsequent over-etching of the glass can also be used to increase the aspect ratio of the glass structures. By using this molding method, glass structures have been successfully fabricated achieving an aspect ratio of 42:1, a height of 212 $\mu \text{m}$ , and vertical sidewall angle up to 89.7°. We also investigated the effect of molding parameters (heat treatment time, temperature, and pressure) on the process capabilities and limitations for defect free structures. This work demonstrates the capabilities to fabricate high aspect-ratio glass structures that cannot be achieved by conventional etching techniques to date, opening new perspectives for the use of glass in MEMS and microfluidics.

Aspect ratio dependent etching and microloading effects are two mechanisms leading to non-uniformities in the etching of silicon using deep reactive ion etching technology. This paper focuses on the apparent presence of a critical aspect radio when etching three-dimensional MEMS structures. A mask designed to separate the effects of microloading from aspect radio dependent etching was made, and various sizes of features (3 to 1000 /spl mu/m) are etched simultaneously for different etching times (10 to 180 min) using a Bosch etching process. Experimental data exhibiting three distinct regimes for the etch rate, the corresponding critical aspect ratios, and the dependence on microloading effects are presented. Possible mechanisms governing these regimes are also postulated.

Hybrid Wafer-Level Packaging for RF-MEMS and Optoelectronic Applications

Switchable wetting and flexible SiC thin film with nanostructures for microfluidic surface-enhanced Raman scattering sensors

Light-absorption enhancement is one of the key research areas related to the development of high-efficiency crystalline silicon (c-Si) solar cells. Surface structuring which can reduce not only the surface reflection but also increase the optical path length is an efficient way to increase light absorption in a broadband wavelength range. Among the surface structures, vertically aligned silicon microwires (MWs) have been extensively investigated as a means for developing highly efficient c-Si solar cells because of the outstanding broadband antireflection and radial junction effect which enables efficient charge collection. The incident light is absorbed along the long MW axis, while photo-induced carriers can be collected along the short radial direction. To realize the highly efficient radial junction c-Si solar cells, we have developed novel technologies such as fabrication process of MWs, high conductive and transmittance top electrode, shape-controlled MWs, and high purity doping process. The high-aspect-ratio MWs (> 10:1) were successfully fabricated through both optimized metal-assisted chemical etching (MACE) and deep reactive ion etching (DRIE) processes. To achieve the high efficiency of MW radial junction solar cells, we developed the high conductive and transparent top electrode to replace the conventional bus-finger electrode which has a significant shading loss. We devised a novel micro-grid top electrode which shows superior transmittance (over 97%) and low sheet resistance (less than 30 Ω/□). By applying the micro-grid electrode on the top surface, the MWs solar cells showed outstanding fill factor (81.2%) and improved efficiency (16.5%). Although our MWs radial junction solar cell showed improved efficiency with the micro-grid electrode, it needs to increase the light absorption capability to maximize the efficiency. As an efficient way to decrease the flat-top-surface reflection of the MWs and increase the light absorption property of the radial junction solar cells, a tapered-MW structure was employed using a simple wet-etching process. When a c-Si wafer with MWs is dipped in a silicon etchant (RSE-100, transene), the top part of the MWs that has a shorter diffusion path compared to the bottom part is etched more quickly because of the different chemical diffusion path lengths leading to the formation of tapered MWs. Since the diameter of the tapered MWs gradually increased from the top to the bottom, the tapered MWs can act as a buffer layer to compensate for the mismatch between the refractive indexes of air (1) and the silicon substrate (4). Thus, the surface reflection of the tapered MWs was observed to be less than 2% at a wavelength of 550 nm. The tapered MW based radial junction solar cells exhibit improved efficiency up to 18.9% thanks to the enhanced light absorption property. As the last step for optimizing the device structure of the MWs solar cells, we developed high purity doping process using acid dopant sources that showed improved minority carrier lifetime (from 79.29 µs to 272.24 µs). Accordingly, we achieved high efficiency (20.2%) MWs radial junction solar cell by applying all of the developed our technologies such as the micro-grid electrode, tapered MWs, and high purity doping process. At present, we are aiming at developing an ideal passivation layer to achieve the more than 25% efficiency of the radial junction solar cells. Therefore, we believe the MW structures with the suggested technologies become a foundational technology for the highly efficient radial junction solar cells.

We report the first demonstration of combustion-assisted hydrogen production achieved without electrical power input in a MEMS device. The chemical reactor/heat exchanger consists of suspended tubes of low-stress silicon nitride, with integrated slabs of silicon, attached to a silicon substrate. The self-sustained combustion was accomplished after implementation of several improvements over the previous reactor design. These include the integration of composite heater metallizations with improved thermal stability relative to Ti/Pt. In addition, functional passive fluidic stop valves fabricated by deep reactive ion etching and modified with silane coupling agents are integrated in the reactor for catalyst localization. Results from simultaneous combustion (of hydrogen and butane) and ammonia cracking at temperatures up to /spl sim/930/spl deg/C are presented.

A methodology to decouple irregular small-scale roughness and regular long-range features on deep reactive ion etched (DRIE) silicon surfaces is presented. Height-height correlations of three different DRIE silicon surfaces are evaluated via atomic force microscopy height data and fit to an analytic, five-parameter model based on a phenomenological scaling function for the small-scale roughness and a Bessel function for the long-range features. The resulting roughness parameters are constant for all three surfaces at small lateral length scales, indicating self-affine roughness inherent to the DRIE process, but dependent on the etch process at large lateral length scales, increasing by a factor of five as the controlled portion of the DRIE process decreased. The results from the analysis are also compared to fracture strengths from recently introduced “theta” test samples with the same etch features as an example of the potential of the analysis in providing an unbiased assessment of the processing-struct...

Practically applicable microelectromechanical systems (MEMS) and nanomachines have been developed by applying dry processes. Deep reactive ion etching (RIE) of silicon and its applications to an electrostatically levitated rotational gyroscope, a fibre optic blood pressure sensor and in micro-actuated probes are described. High density electrical feedthrough in glass is made using deep RIE of glass and electroplating of metal. Multi-probe data storage system has been developed using the high density electrical feedthrough in glass. Chemical vapour deposition (CVD) of different materials have been developed for MEMS applications; trench-refill using SiO2 CVD, microstructures using Silicon carbide CVD for glass mold press and selective CVD of carbon nanotube for electron field emitter. Multi-column electron beam lithography system has been developed using the electron field emitter.

Development of a moving reflector type micro optical switch fabricated by deep reactive ion etching (DRIE) in silicon on insulator (SOI) substrates is presented. The device discussed is a key component in a MEMS-based safety and arming (S&A) system for use in underwater weapons. In this switch, an etched vertical sidewall reflector is electrostatically actuated in and out of the optical path between input and output optical fibers. Fabrication is performed on 100 µm thick silicon substrates with fiber alignment channels, reflectors and actuators being fabricated at the same time with a single etch step. A single pair of multimode fibers is used to transmit optical power of the order of 1000 mW at a working wavelength of 810 nm. Sources of optical loss in the system are identified and their value calculated in order to predict the overall system optical efficiency. The optical efficiency of the switch has been found to have an average value of 55% with the etched vertical sidewall mirror having an average reflectivity of 62.8%. Switching time is 10 ms from the off to the on state with a maximum operational frequency of 60 Hz. Isolation between the on and off states is 32 dB.

This paper reports on novel robust microprobe systems for simultaneous neural recording and drug delivery. The systems comprise needle-shaped silicon probes with microfluidic channels and recording electrodes integrated into a robust polyetheretherketone (PEEK) packaging. The microprobes are fabricated using deep reactive ion etching (DRIE) of silicon combined with silicon wafer bonding, wafer grinding, and thin film processing. An integrated system assembly with mechanical, fluidic and electrical interfaces is developed for easy handling and robustness during operation. This enables the microprobe systems to be used in acute as well as chronic applications. The assembly process includes a dead volume free fluidic interconnection technology which attaches polytetrafluoroethylene (PTFE) tubing to the probes by using heat shrinkable tubing. Furthermore, the novel approach of system integration is characterized with respect to its mechanical, fluidic, and electrical properties. Finally, the operation of the microprobe systems is verified in acute in-vivo experiments in which liquid delivery and recording of neural signals is successfully demonstrated.Keywordsdrug deliverysilicon microprobesneural recordingmicrofluidics

A microengineered alignment bench for a nanospray ionization system is described. The bench combines a V-groove mount for a capillary-based nano-ESI source with an extraction electrode and allows accurate axial and transverse alignment of the capillary. An input channel and plenum for nebulizer gas are also demonstrated. The structure is formed in two halves, which are assembled by stacking. Electrically conducting features are constructed by crystal plane etching, deep reactive ion etching and metallization of silicon, while the insulating base is formed in a photopatterned plastic. Low voltage (about 850 V) positive ion emission is demonstrated using commercial nanospray (15 µm ID) capillaries, with total ion currents >10 nA. The structures are robust, and can survive voltages >1 kV and immersion in common solvents. Mass spectrometry is carried out using a commercial atmospheric pressure ionization instrument, and high signal stability is demonstrated.