Front-side Process Research Articles

Fabrication of nanoscale single crystal InP membranes

The authors have developed a process for fabricating fully enclosed single crystal InP freestanding membranes using entirely front side processing. A multilayer lattice matched InP∕InGaAs heterosystem incorporating a 500Å membrane layer was grown by molecular beam epitaxy followed by two step surface patterning, first for the support structure and then for the membrane structure definition. Membranes with lateral dimensions ranging from 1to10μm have been fabricated using optical and electron beam lithography. The process is easily scalable, limited only by the ability of the membrane layer to act as an effective etch stop, i.e., ∼30Å.

Thermal behavior of freestanding microstructures fabricated by silicon frontside processing using porous silicon as sacrificial layer

Freestanding microstructures are essential elements in thermal and mechanical microsensors. In this paper, microbridges were fabricated by silicon surface micromachining using porous silicon as sacrificial layer. Two different approaches were considered. In first approach, n-Si was used as anodization masking material and n-Si/SiO2 as microstructure material. In the second approach, silicon nitride and SiO 2 /Si3 N4 bilayer has constituted masking and microstructure materials, respectively. In order to characterize their thermal behavior, platinum heating elements were defined on developed microbridges. Microstructures fabrication process was described, insisting specially on silicon anodization step. The process parameters (HF-electrolyte concentration, current density, and process duration) were established for both approaches. Thermal behavior of developed microbridges was studied in relation to anodization masking materials and freestanding microstructure materials. Microbridge temperature versus applied power to heating element was analyzed. Additionally, entire sample thermal behavior and microbridge dynamic thermal behavior was characterized. The obtained results suggest developing a third approach where n-Si will be used as masking material and SiO2 / Si3 N4 bilayer as freestanding microstructure material

Single-crystalline silicon micromirrors actuated by self-aligned vertical electrostatic combdrives with piston-motion and rotation capability

We present micromirrors actuated by self-aligned, high aspect ratio, vertical electrostatic combdrives with multi-level electrical isolation that allows bi-directional and dual-mode (independent rotation and piston motion) operation. The fabrication process, which requires only front-side processing, is based on self-aligned deep-reactive ion etching (DRIE) of silicon-on-insulator (SOI) wafers with two device layers. The two oxide layers between the device layers provide electrode isolation and etch stops for thickness control. Self-alignment of electrostatic combs in the two device layers is accomplished by creating masking features in the upper layer that are transferred into the lower layer. The masking features are typically removed, but in some cases they remain as an integral part of the completed device. SEM pictures show perfectly aligned vertical combteeth both with and without removal of the masking features. We demonstrate micromirrors of this type with ±9° of optical scanning and 7.5 μm of piston motion under static actuation. Micromirrors designed for resonant scanning have optical scan angles up to ±25° at 13.5 kHz.

The HARPSS process for fabrication of precision MEMS inertial sensors

The high aspect-ratio combined poly- and single-crystal silicon micromachining technology (HARPSS) and its application to fabrication of precision MEMS inertial sensors are presented. HARPSS is a single wafer, all silicon, front-side release process which is capable of producing 10–100's of microns thick, electrically isolated, 3-D poly- and single-crystalline silicon microstructures with various size air-gaps ranging from sub-micron to tens of microns. High aspect-ratio (>50:1) polysilicon structures are created by refilling 100's of microns deep trenches with polysilicon deposited over a sacrificial oxide layer. This technology provides features required for precision micromachined inertial sensors. The all-silicon feature of this technology improves long term stability and temperature sensitivity while fabrication of large area, vertical electrodes with sub-micron gap spacing will increase the sensitivity by orders of magnitude.

Vertical coupler with separated inputs and outputs fabricated using double-sided process

A novel vertical coupler used as a wavelength multiplexer with separated input and output waveguides is demonstrated. The wafer fusion technique enables both frontside and backside processing of the thin epitaxial films. A vertical coupler with a 4 mm interaction region demonstrates 20 nm wavelength spacing.

Theoretical Study of the Mechanisms of Ethylene Polymerization with Metallocene-Type Catalysts

Three possible mechanisms of ethylene polymerization in the presence of zirconocene catalysts have been described through density functional calculations. About 20 structures (reactants, intermediates, transition states, and products) have been located on the potential energy surface. Only two mechanisms, called frontside and stepwise backside, appear to be competitive. The frontside mechanism has already been described in the literature, but we present here some important new aspects. In particular, we discuss the important role of the perpendicular form of the olefin π-complex for the frontside process. The stepwise backside mechanism is reported for the first time. The energy barriers in both mechanisms are on the order of 3 kcal/mol, which is compatible with the experimental and theoretical values available for similar processes. The energy difference between both mechanisms is certainly below the computational method accuracy, and a definitive conclusion regarding the most favorable mechanism cannot be drawn. The influence of other factors such as substituents on the catalysts or the size of the polymer chain could be fundamental.

Design of bulk micromachined suspensions

Microsuspensions are very useful mechanical structures in microelectromechanical systems. The fabrication processes of microsuspensions, including front-side etching and back-side etching processes, have been studied extensively. Due to the restriction of the undercutting process, the front-side etching approach offers only limited patterns of microsuspension. The present study intends to develop a method to predict the possibility of fabricating microsuspensions on a (100) substrate through the front-side etching process. Microsuspensions with arbitrary shapes can be designed easily according to the proposed method. It is found that the formation of the microsuspensions predicted by the proposed technique agrees well with the experimental observation. The contribution of this paper is to provide a convenient tool to design microsuspensions fabricated through the front-side etching process. The application of the front-side etching process on microsuspensions will become more attractive. Thus the problems of having a large cavity on the substrate, longer etching time and larger die size caused by the back-side etching process can be prevented.

Combined Static and Dynamic Density Functional Study of the Ti(IV) Constrained Geometry Catalyst (CpSiH2NH)TiR+. 1. Resting States and Chain Propagation

The resting state structure of the metallocene−alkyl cation, the coordination of the olefin to the preferred resting state structure, and the insertion process of the Ti-constrained geometry catalyst (CpSiH2NH)TiR+ have been studied with density functional theory. A combined static and dynamic approach has been utilized whereby “static” calculations of the stationary points on the potential surface are meshed with first principles Car−Parrinello molecular dynamics simulations. The first molecular dynamics simulation specifically addressing the structure of a metallocene−alkyl cation is presented showing rapid interconversion between γ- and β-agostic conformations. Complementary static calculations show a small energetic preference for a γ-agostic resting state. Coordination of the olefin to the Ti−alkyl resting state is likely to result in the formation of a β-agostic π-complex which is highly favored energetically over other π-complexes that may initially form. The whole propagation cycle was studied from π-complex to subsequent π-complex. The propagation barrier corresponds to the insertion process which was calculated to have a free energy barrier of ΔG⧧ = 24.3 kJ/mol at 300 K. The initial β-agostic interactions which stabilize the π-complex are replaced by α-agostic bonds which stabilize the insertion transition state. A study of the back-side insertion process reveals that it may be competitive with the front-side insertion process.

The Effect of Pressure on the Aquation of Chromium(III) Complexes

The effect of pressure on the rates of aquation of Cr(NCS)63− and Cr(NH3)2(NCS)4− has been studied over the pressure range 1 to 2043 atm, at 50 ˆC. The activation volumes for these reactions are found to be 16 ± 2 and −2.4 ± 0.8 ml/mol, respectively. The pressure dependence of the activation volumes, (∂ΔV*/∂P)T, are 15 ± 5 and 0 ± 1 ml/katm mol, respectively. These results are consistent with a dissociative interchange (Id) process for the aquation of Cr(NCS)63− and an SN2 front-side process for the aquation of Cr(NH3)2(NCS)4−.