Hamood Ur Rahman

Cantilever beam designs for RF MEMS switches

This paper presents the design, modeling, fabrication and measurements of two novel radio frequency microelectromechanical systems (RF MEMS) switches. Two new non-standard cantilever beams have been designed in order to validate the mathematical model developed for the calculation of the spring constant and to verify the fabrication parameters. The mechanical modeling is based on the Euler?Bernoulli beam equations. The fabricated switches using these cantilever beams exhibit 19 V and 23 V actuation voltages. The fabrication consists of a six-mask all-metal process. A unique dry-release technique is also described. The RF simulations and measurements of these switches are finally presented. The isolation is better than 23 dB and 27 dB and the return loss is better than 19 dB and 18 dB, from 0 to 40 GHz. The insertion loss is 1.15 dB and 1.3 dB for the two designs, respectively, for all frequency bands of interest.

Dry release of MEMS structures using reactive Ion etching technique

Paper presents a dry release method for beam structures of radio frequency microelectromechanical systems (RF MEMS) switches. In this release process a combination of wet and dry etching has been used. The wet etching is done to remove the sacrificial layer under the beam structure. The dry release of the cantilever beam structures was done by removing the supporting layer of photoresist using the reactive ion etching technique (RIE). The power and pressure factors were varied for anisotropic and isotropic etching during the RIE process thus getting the free standing MEMS structures. The release process can effectively be used in the fabrication of suspended beams and membrane structures for RF MEMS switches. The process is not only fully compatible with standard MEMS processing equipments but could also lead to the long term storage of the MEMS devices.

Low actuation voltage RF MEMS series switch with novel beam design

Although radio frequency micro-electro-mechanical-systems (RF MEMS) switches have experienced intensive research in last decade. Major problems like high actuation voltage and low yield due to residual stress generated during the fabrication process still exist. In order to reduce the actuation voltage, we have designed and tested several configurations with different beam designs and actuation voltage as low as 6.39 V has been achieved. This paper presents an RF MEMS metal to metal contact series switch with novel low spring constant beam design. The problem of low yield due to residual stress has been addressed by introducing a dimple under the front tip of beam. The proposed design can easily be integrated in large multi-port structures. The fabrication of the switch has been implemented using a seven mask metal based process. The numerical and experimental results illustrate the novel cantilever beam switch with excellent RF performance. The cantilever beam switch shows a calculated actuation voltage of 6.39 V with 0.37 dB insertion loss and less than 22 dB return loss. The switch also exhibits an isolation of 23.5 dB for all frequency bands of interest.

Characterization and optimisation of PECVD Silicon Nitride as dielectric layer for RF MEMS using reflectance measurements

Synthesis of Silicon Nitride thin films is important in the semiconductor industry. The properties of the films make them valuable for oxidation masks, protection and passivation barrier layers, etch stop layer and inter level insulators. In the present study, we prepared silicon nitride films with different refractive index. We used various conditions of PECVD atmosphere with the purpose of obtaining high quality near stoichiometric films. Different deposition routines were employed, including variable flow ratio of silane and ammonia on to a silicon substrate. The quality and stoichiometry of the film was investigated using reflectance measurements of the films. Measured process parameters were the deposition rate, film thickness, refractive index, breakdown voltage, dielectric constant, surface morphology and film composition. The thickness of the film was measured by using AFM, DEKTAK and ellipsometer measurements. Fitting of reflectance spectra with WVASE ellipsometric software provided us with optical constants for silicon nitride and allowed analysis of the films. A Bruggeman effective medium approximation was utilized to model the refractive index of the films. Reflective measurements were carried out in the range 210 nm-2500 nm.

RF MEMS switches: design and performance in wireless applications

A road map has been proposed for the design methodology of a RF MEMS switch. The design methodology for RF MEMS is often an interactive process which involves continuous adjustments for the specifications, structure and the fabrication steps. This helps in defining and obtaining the required RF microwave specifications. A novel RF MEMS D shape capacitive shunt switch is proposed. The switch structure is fabricated on a high resistive silicon substrate. This structure consists of coplanar waveguide transmission line, a D shape lower electrode with thin dielectric layer on top and suspended membrane bridge. The lower electrode of the switch is fabricated in D shape using high viscosity and high conductive adhesives. The switch RF path is fabricated on top of silicon dioxide using a thick coplanar waveguide transmission line. The thick transmission line metal connects to the lower electrode and the dielectric material to form the through path of a shunt switch. The suspended metal membrane spans the two coplanar ground lines. With no applied actuation voltage the residual tensile stress keeps the membrane suspended above the RF path. By applying an electrostatic field between membrane and the lower electrode an attractive force causes the floating membrane to pull down and make contact with the lower electrode and dielectric surface to form a low impedance RF path to ground. Main area to which the development aims is lowering the actuation voltage to levels compatible with mainstream IC technologies, while maintaining the RF performance. Proposed switch has shown satisfactory performance between 0-40 GHz frequency range. Paper will present simulation and theoretical results. Experimental results of conductive epoxy fabrication are also presented. This switch could have an important application in the telecommunication network like switching networks.

Three-bar novel RF MEMS switches

In this paper we report two novel RF MEMS series switches. The novel cantilever beam structures are supported by three low spring constant cantilever beam springs at the fixed end of the bridge. Design-2 has an additional extended cantilever beam structure at the front which further reduces the spring constant of the beam design. The spring constants of the cantilever beams are 3.37N/m and 3.31N/m respectively. The fabricated switches are electrostatically actuated and have achieved an actuation voltage of 19V and 23V respectively. The variation in the actuation voltage has been studied in relation to the presence of residual stress in the beams during the fabrication. The switches have shown good RF performance exhibiting isolation better than 24dB and 29dB respectively. The reported cantilever beam designs can easily be integrated into large multiport structures.

Nano fabrication analysis of RF NEMS switch

This paper presents fabrication analysis of a silicon-based RF NEMS switch. The nano structure is fabricated using surface micromachining approach. Electron beam lithography is introduced as mask-less pattern generator. During the nano fabrication, proximity effect is optimized by dose-shape correction method. Field stitching errors optimization and minimum feature adjustment are also discussed in the paper.

Investigation of residual stress effects and modeling of spring constant for RF MEMS switches

Any film which is deposited during the fabrication undergoes some form of stress. It can be either a compressive stress or tensile stress. It is important to have right value of either compressive or tensile stress for the particular application. In this paper we present an investigation of the stresses generated by the gold thin film during the fabrication process of RF MEMS switches. During the fabrication of RF MEMS series switches a small value of tensile stress is intentionally generated so that beam does not lead to stiction after final release. We are also briefly presenting a new mathematical model for calculation of the spring constant of the beam structures. The method we used is based on the Euler-Bernoulli beam equations. The spring constants calculated for two new beam designs were 3.37N/m and 3.31N/m respectively.

Deep level transient spectroscopy study of defects at Si/SiO 2 and Si/Si 3 N 4 interfaces

The properties of traps at SiO2/Si and Si3N4/Si interfaces were examined and compared using deep level transient spectroscopy (DLTS). Small pulse DLTS was used to determine the effective electron capture cross-section of the interface states. Similar trends are found for both systems with the capture cross-section decreasing rapidly towards the band edge. The effective capture cross-section and energy level is also given for the Si≡Si3 defect at the Si3N4/Si interface.

Experimental considerations for fabrication of RF MEMS switches

This paper presents analysis on parameters required for the fabrication process of RF MEMS switches. Stress analysis on gold thin film and variation of stress with different deposition conditions has been studied. Adhesion issues which occurred during the fabrication have also been high lighted. While fabricating the RF MEMS switch the suspended cantilever beam or membrane can touch the lower actuation electrode and as a result DC short circuit or arcing can occur. Silicon nitride has been used as a dielectric layer to avoid any short circuit or arcing effect while doing the actuation. I-V and C-V characterization of silicon nitride has been done to analyze the effective break down voltage of the film. A new cantilever beam design is proposed and used to verify the fabrication parameters. The front end of the cantilever beam has been curved in making small contact area and providing easy release and better isolation. Finally, a six mask all metal fabrication process that has been defined for the fabrication of metal to metal contact switch is given. SEM results of fabricated switch have been presented.