Silicon Grass Research Articles

Ni/Al multilayer reactions on nanostructured silicon substrates

Fast energy release, which is a fundamental property of reactive multilayer systems, can be used in a wide field of applications. For most applications, a self-propagating reaction and adhesion between the multilayers and substrate are necessary. In this work, a distinct approach for achieving self-propagating reactions and adhesion between deposited Ni/Al reactive multilayers and silicon substrate is demonstrated. The silicon surface consists of random structures, referred to as silicon grass, which were created by deep reactive ion etching. Using the etching process, structure units of heights between 8 and 13 µm and density between 0.5 and 3.5 structures per µm2 were formed. Ni and Al layers were alternatingly deposited in the stoichiometric ratio of 1:1 using sputtering, to achieve a total thickness of 5 µm. The analysis of the reaction and phase transformation was done with high-speed camera, high-speed pyrometer, and X-ray diffractometer. Cross-sectional analysis showed that the multilayers grew only on top of the silicon grass in the form of inversed cones, which enabled adhesion between the silicon grass and the reacted multilayers. A self-propagating reaction on silicon grass was achieved, due to the thermally isolating air pockets present around these multilayer cones. The velocity and temperature of the reaction varied according to the structure morphology. The reaction parameters decreased with increasing height and decreasing density of the structures. To analyze the exact influence of the morphology, further investigations are needed.

A multi-step etch method for fabricating slightly tapered through-silicon vias based on modified Bosch process

In this paper, a multi-step etching method based on Bosch process was investigated to fabricate a slightly tapered via. The diameter of vias was scaled from 40 to 100 µm. Isotropic etching step was added into Bosch process to control the angle of the tapered vias. The slope angle could be adjusted by changing the time settings of isotropic etching step. The influence of the platen temperature was also studied. The passivation and etching steps are extremely sensitive to temperature. Silicon grass could be formed at low temperature. The two different processes of isotropic SF6 etching and Cl2/HBr etching were also compared. The wrinkles and cracks were observed on the surface after treatment with isotropic SF6 etching. The Cl2/HBr etching method is much better for removing the scallops.

Process flow to integrate nanostructures on silicon grass in surface micromachined systems

The process flow to integrate metallic nanostructures in surface micromachining processes is presented. The nanostructures are generated by evaporation of microstructured silicon grass with metal. The process flow is based on the lift-off of a thin amorphous silicon layer deposited using a CVD process. All steps feature a low temperature load beneath 120 °C and high compatibility with many materials as only well-established chemicals are used. As a result metallic nanostructures usable for optical applications can be generated as part of multilayered microsystems fabricated in surface micromachining.

Si Etching for TSV Formation

Through silicon via (TSV) technology is being considered as a promising technology in three-dimensional packaging, enabling massive and short interconnections between stacked chips, increasing performance and data bandwidth, and reducing signal delay and the power consumption. Currently, TSVs with diameters ranging from one hundred to ten micrometers are mainly fabricated by deep reactive ion etching (DRIE) technology. The Bosch process is the most widely utilized DRIE process for producing high-aspect ratio TSVs. The primary steps in the Bosch process are silicon isotropic etching and wall passivation in sequential cycles. SF6 is used as the main etching gas for the high density of F+ radicals; C4F8 is used in wall passivation as it polymerizes to deposits on walls to form an etch barrier that is sufficiently impervious to side scattered F+ ions but not to direct ions at the bottom of the via. Wall scalloping occurs primarily near the top of the via where scattered ions have wide trajectories and less at greater depths where ion trajectories are more restricted. Controlling the scallop is key good insulation and metal coverage. In this study, TSVs with scallops smaller than 100nm was obtained by adjusting the flow ratio of SF6 to C4F8 and the cycle time ratio between SF6 and C4F8. Unfortunately this combination leads to the formation of silicon grass at the bottom of the TSV. The addition of O2 during the SF6 etch cycle was found to eliminate the occurrence of Si grass. Finally, TSVs with low scallop were consistently obtained without Si grass by using SF6/O2 during the etch cycle and C4F8 during the passivation cycle.

Online monitoring of the passivation breakthrough during deep reactive ion etching of silicon using optical plasma emission spectroscopy

We present optical emission spectroscopy (OES) as a technique for process optimization of the etch step during deep reactive ion etching of silicon. For specific process steps, the spectrum of optical plasma emission is investigated. Two specific wavelengths are identified (fluorine at 703.8 nm and CS compounds at 257.6 nm), which significantly change intensity during the etch step. Their intensity drop is used for the recognition of the passivation layer breakthrough. Thus, the net silicon etch time can be measured. This time can be used for process optimization. A structural analysis of the passivation layer shows its fragmentation during its breakthrough. The plasma–surface interaction and their correlation with the plasma emission are described. Within an application example, the passivation breakthrough is investigated in detail. For different process regimes, the residues of the fragmented passivation layer are analyzed by scanning electron microscopy. Residue densities of 14–38 µm−2 are fabricated. For silicon grass generation, the OES technique offers a versatile tool for the process optimization of the mask generating process within the first cycles.

A capillary pumping device utilizing super-hydrophobic silicon grass

In this study, we show that a compact silicon grass surface can be generated by utilizing the induced coupled plasma method with suitably chosen fabrication parameters. This super-hydrophobic structure suspends deionized water on top of the grass and keeps the contact angle at around 153°. The silicon grass is used to improve the driving efficiency of a capillary pumping micro-duct (without sidewalls), which is completely defined by a bottom hydrophilic stripe (adjacent to a Teflon substrate) and a fully top-covered hydrophobic Teflon surface which is coated on a glass substrate. The channel has a height of 3 µm and a width of 100 µm. In this work, the Teflon substrate is replaced with the silicon grass surface. When the fluid is flowing through the micro-duct on the stripe, the interface between the silicon grass and the hydrophilic stripe forms a stable air cushion barrier to the fluid, thus effectively reducing the frictional force. By changing only the interface with this replacement, we demonstrate that the average measured velocities of the new design show improvements of 21% and 17% in the driving efficiency over the original design for transporting deionized water and human blood, respectively. It is also shown that the measured data of the present design are closer to the values predicted by a theoretical analysis which relates the flow velocity to the contact angles, surface tension and fluid viscosity.

Formation of Ordered Silicon Nano-Textures Suitable for Field Emission Applications

We report on the use of silicon nano-grass and nano-textured structures for the field emission applications. A triode device, which can be used as a field emission transistor with gate, emitter (cathode) and a collector (anode), has been realized. Anode in this structure is a silicon substrate placed opposite to the silicon grass tips acting as the emitter. The silicon nano grass formation is based on the hydrogen assisted reactive ion etching process where a mixture of O2/H2 and SF6 gases are used in a sequential manner. The results of electrical measurements for this investigation show a sharp rise in the emission current.

Formation of silicon grass: Nanomasking by carbon clusters in cyclic deep reactive ion etching

Initial cluster formation on silicon surfaces in cyclic deep reactive ion etching (c-DRIE) using c-C4F8/SF6 plasma is investigated. These clusters act as a nanomask for the fabrication of nanostructured surfaces such as silicon grass. Different wafer preconditioning regimes and subsequent x-ray photoelectron spectroscopy show that no wafer or process contaminations are the reason for nanomasking in c-DRIE. Furthermore, no Si-containing compounds, such as SiFxOy, SiOx, or SiC, are detected. The clusters consist of residues of the fluorinated carbon layer deposited in c-DRIE. Experimental process analysis using design of experiments shows the dependence of nanomask morphology on passivation time and power. The results indicate that the properties of the nanomask, in particular, density, are determined during passivation.

Parameter study for silicon grass formation in Bosch process

Bosch process is one type of silicon etching technology for high-aspect ratio where silicon grass is one of the by-products. Previously, most of the studies have been focused on eliminating these silicon grasses. However, in this study, the optimized condition for obtaining silicon grasses was studied using inductively coupled plasma reactive ion etching for its potential application as silicon wires in the field of lithium ion batteries as electrodes and texturing of Si solar cells. It was found that the diameter for silicon grass was found to be proportional to the pattern size. The diameter and length of silicon grass were found to be dependent on the chuck power, duty cycle, SF6 pressure, etch/passivation time ratio, and ratio of flow rate. As chuck power decreases, the diameter and density of silicon grass increase. Low duty cycle and high chamber pressure are favorable for obtaining high density of silicon grass formation. Moreover, it was found that the relative time ratio of SF6 to C4F8 being 7:2 seems to be the optimal condition for sub-micrometer-sized grass formation. Finally, the flow rate of SF6 was found to be inversely proportional to the grass formation; the flow ratio of SF6 and C4F8 being 2:1 is the optimum condition to obtain high density of silicon grass. Overall, various parameters can be controlled in Bosch process to control the diameter and length of silicon grasses.

The Use of Metallographic and SEM Analysis for Characterization of Sidewall Surfaces in MEMS Devices with DRIE Processing

The Bosch Deep Reactive Ion Etch Process is commonly used for the manufacture of MEMS and MOEMS devices that require deep high aspect ratio trenches. In many cases fully released, high aspect ratio features can be generated in one pass. However the process must be understood to avoid generating some of the defect structures that are characteristic of the process. Defects such as scalloping, silicon grass, and undercutting at the interface of a nonconductive layer can be controlled by process parameters and optimization. Measurement and characterization of the defective structures is a key element of controlling them. The use of SEM measurement techniques for characterizing the small features associated with scalloping and silicon grass is essential. No other technique is capable of providing the large depth of focus required to visualize these features. The use of metallographic techniques furthers understanding of the surface conditions on the side walls of these deep trenches.

Effect of Clamping Ring Materials and Chuck Temperature on the Formation of Silicon Nanograss in Deep RIE

In this paper, we report the effect of clamping ring materials and the chuck temperature on the formation of silicon grass in deep reactive ion etching (RIE), carried out with an Alcatel A601E etching tool. In the beginning, rectangular trenches of varying widths were etched with conventionally used aluminum clamping rings and later with ceramic rings, keeping other parameters unchanged. Scanning electron microscope images of etched trenches reveal that needle-shaped silicon grass structures were formed on the bottom of the trenches etched with an aluminum ring. Formation of silicon grass was more severe in wider trenches and near the wafer edges; however, the bottom silicon grass was eliminated when trenches were etched with a ceramic ring. Formation of silicon grass is mainly due to “micromasking” caused by sputtered aluminum particles. Due to continuous bombardment of highly reactive radicals (, , and other neutral atoms) on the wafer and surrounding aluminum ring, some aluminum atoms are physically displaced and deposited on the wafer. These micrometer-size aluminum particles deposited on the silicon wafer start acting as a mask layer and prevent the etching of the silicon area beneath them. Due to this aluminum mask, the local silicon etching rate is reduced drastically compared to other silicon areas, hence gradually vertical-grass-type silicon structures start forming. Because more aluminum atoms are deposited near the wafer edges, the amount of silicon grass is nearer the edges than in the center. As the ceramic ring is an insulator, it is not polarized by radio frequency power; hence, there is no ion-bombardment phenomenon and no sputtering of foreign particles. Due to the absence of such micromasking, the silicon grass formation is completely eliminated.

D225 高エネルギー物質のパルス加熱に関する研究

Establishing the technology on combustion of microscale energetic materials is important for realization of digitalmicrothruster, spacecraft equipped with it and other application. In this study, we inquired about the size of energetic material for solid propellant tank with a given form. And we manufactured the micro-tank using silicon wafer and grass essential for microfabrication and performed the heating examination of nitrocellulose charged.

Cell attachment on silicon nanostructures

Advances in neural probe technology are currently hindered by a lack of understanding of the cues and mechanisms responsible for rejection and isolation of probes implanted in the central nervous system. To gain additional insight into this topic, the attachment of astrocytes on nanoscale textured silicon surfaces was investigated. Silicon surfaces were textured using a reactive ion etch process designed to produce nanometer-scale columnar structures in silicon (“silicon grass”). Standard photolithographic techniques were used to pattern the surface thereby allowing selective modification of the surface texture by a wet chemical etch for silicon. The resulting surface allowed a side-by-side presentation of different surface textures to cells grown in culture. The silicon surfaces were characterized by scanning electron microscopy (SEM) and scanning Auger electron microscopy. The cell attachment and morphology were observed with laser scanning confocal microscopy and SEM. Transformed astrocytes from a continuous cell line showed a preference for wet-etched regions over grassy regions. In contrast, primary cortical astrocytes from neonatal rats showed a preference for silicon grass over the wet-etched surface. For the transformed astrocytes cells, the degree of response was continuous with the degree of wet-etch modification.