Solid State Electronics

(Japanese Journal of Applied Physics,39(1):244-247)Valence band properties of relaxed ternary group IV semiconductor alloys

Microsystems for Radio Frequency (RF) passive elements, known as RF-MEMS, have been attracting the attention of academic and industrial research since their first discussion, thanks to the remarkable performance they can trigger. Despite the flattering premises, RF-MEMS technology did not score consistent spread into mass-market applications, yet, as technical issues still needed to be managed, but also because consumer products did not use to really need such pronounced characteristics. Nowadays, the application scenarios of 5G (i.e. 5th generation of mobile communications and networks) and of the Internet of Things (IoT), highlight a growing need for cutting edge performance that RF-MEMS are capable of addressing. Given such a context, this short communication discusses an RF-MEMS series ohmic micro-switch, electrostatically driven and fabricated in a surface micromachining process, exhibiting good characteristics up to 110 GHz. In brief details, the micro-relay shows isolation (when OPEN) better than −15 dB and loss (when CLOSE) better than −1 dB up to 40 GHz. The actuation voltage is around 50 V, although it can be lowered acting on the release step temperature. Despite the design concept admits margins for improvement, the characteristics of the micro-switch reported in the following are already quite interesting in the discussion of next generation of RF passive components for 5G and IoT.

RF-MEMS technology as an enabler of 5G: Low-loss ohmic switch tested up to 110 GHz

Studies have been carried out on a RF MEMS shunt switch to analyze the effect of residual stress on its electromechanical characteristics. This paper presents the simulated results as well as theoretically calculated results of a shunt switch due to the presence of residual stress gradient in respect of resonant frequency, pull down voltage and switching characteristics. The effect of introduction of holes in the beam is also studied. The calculated results, corresponding to the switch (without holes) at zero residual stress, of resonant frequency, pull-down voltage and switch on and off time are 28.14 kHz, 28.2 V, 16.35 μsec and 8.6 μsec respectively. Modal analysis of the both the structures (with and without holes) are carried out for different values of residual stress gradients. Modal analysis predicted that higher values of tensile stress gradient are not favorable for switching action. The pull-down voltages and switch on and off times are simulated at different stress gradients. With the increase in compressive stress gradient, the pull-down voltage is found to increase, whereas, switch on and off times is decreased. Corresponding to −20 MPa/μm residual stress gradient, the resonant frequency, pull-down voltage and switch on and off times are found to be 74.5 kHz, 63.5 V, 7.5 μsec and 3.36 μsec respectively. Introduction holes in the structure modified these values to 63.77 kHz, 53.1 V, 8.7 μsec, 3.92 μsec respectively.

Effect of residual stress on RF MEMS switch

Radio frequency micro electro mechanical systems (RF MEMS) have enabled a new generation of devices that bring many advantages due to their very high performances. There are many incentives for the integration of the RF MEMS switches and electronic devices on the same chip. However, the high actuation voltage of RF MEMS switches compared to electronic devices poses a major problem. By reducing the actuation voltage of the RF MEMS switch, it is possible to integrate it into current electronic devices. Lowering the actuation voltage will have an impact on RF parameters of the RF MEMS switches. This investigation focuses on recent progress in reducing the actuation voltage with an emphasis on a modular approach that gives acceptable design parameters. A number of rules that should be considered in design and fabrication of low actuation RF MEMS switches are suggested.

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Review of low actuation voltage RF MEMS electrostatic switches based on metallic and carbon alloys

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.

https://iopscience.iop.org/article/10.1088/0960-1317/20/7/075042/pdf

Cantilever beam designs for RF MEMS switches

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.

Low actuation voltage RF MEMS series switch with novel beam design

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 337N/m and 331N/m respectively

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

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.

Dry release of MEMS structures using reactive Ion etching technique

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.

Hamood Ur Rahman

Papers

Cantilever beam designs for RF MEMS switches

Low actuation voltage RF MEMS series switch with novel beam design

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

Dry release of MEMS structures using reactive Ion etching technique

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