This paper reports an advanced deep reactive ion etching (DRIE) process for realizing ultra-deep (>500- $\mu \text{m}$ ) and ultra-high aspect-ratio (UHAR) silicon structures (AR > 40 for 1-mm through-trench etch, $\text {AR}\approx 80$ for 500- $\mu \text{m}$ through-trench etch, and AR > 20 for 500- $\mu \text{m}$ through-hole etch), with straight sidewalls across a wide range of feature sizes. The challenges of making such structures are overcome by continuously ramping critical parameters of the Bosch DRIE process throughout the process, including the 380-kHz bias power during etch step, the etch/passivation step duration, and the chamber pressure. The masking material capable of enduring the long DRIE process is also discussed; 10- $\mu \text{m}$ and 25- $\mu \text{m}$ wide trenches are etched to a depth of >750- $\mu \text{m}$ and >1000- $\mu \text{m}$ , respectively, in 1-mm-thick silicon wafers with straight sidewall profiles and flat trench bottoms. Deeper trenches are expected to be etched beyond a 1-mm thick wafer with thicker and/or higher selectivity masking materials. We have also demonstrated etching of circular holes of diameters as small as 25- $\mu \text{m}$ to a depth of >500- $\mu \text{m}$ , and potentially with 10–15 $\mu \text{m}$ diameter holes. This advanced DRIE process offers opportunities for applications ranging from through-silicon via in 3-D CMOS integration to emerging micro- and meso-scale microelectromechanical system applications that demand ultra-deep and UHAR DRIE. [2017–0298]

Ultra Deep Reactive Ion Etching of High Aspect-Ratio and Thick Silicon Using a Ramped-Parameter Process

Microgrippers are typical microelectromechanical systems (MEMS) that are widely used for micromanipulation and microassembly in both biological and micromanufacturing fields. This paper presents the design, modelling, fabrication and experimental testing of an electrothermal microgripper based on a ‘hot and cold arm’ actuator design that is suitable for the deformability characterisation of human red blood cells (RBCs). The analysis of the mechanical properties of human RBCs is of great interest in the field of medicine as pathological alterations in the deformability characteristics of RBCs have been linked to a number of diseases. The study of the microgripper’s steady-state performance is initially carried out by the development of a lumped analytical model, followed by a numerical model established in CoventorWare® (Coventor, Inc., Cary, NC, USA) using multiphysics finite element analysis. Both analytical and numerical models are based on an electothermomechanical analysis, and take into account the internal heat generation due to the applied potential, as well as conduction heat losses through both the anchor pads and the air gap to the substrate. The models are used to investigate key factors of the actuator’s performance including temperature distribution, deflection and stresses based on an elastic analysis of structures. Results show that analytical and numerical values for temperature and deflection are in good agreement. The analytical and computational models are then validated experimentally using a polysilicon microgripper fabricated by the standard surface micromachining process, PolyMUMPs™ (Durham, NC, USA). The microgripper’s actuation is characterised at atmospheric pressure by optical microscopy studies. Experimental results for the deflection of the microgripper arm tips are found to be in good agreement with the analytical and numerical results, with process-induced variations and the non-linear temperature dependence of the material properties accounting for the slight discrepancies observed. The microgripper is shown to actuate to a maximum opening displacement of 9 μ m at an applied voltage of 3 V, thus being in line with the design requirement of an approximate opening of 8 μ m for securing and characterising a RBC.

https://www.mdpi.com/2072-666X/9/3/108/pdf

Analytical, Numerical and Experimental Study of a Horizontal Electrothermal MEMS Microgripper for the Deformability Characterisation of Human Red Blood Cells.

As many studies show, there is a relation between the tissue’s mechanical characteristics and some specific diseases. Knowing this relationship would help early diagnosis or microsurgery. In this paper, a new method for measuring the viscoelastic properties of soft materials at the microscale is proposed. This approach is based on the adoption of a microsystem whose mechanical structure can be reduced to a compliant four bar linkage where the connecting rod is substituted by the tissue sample. A procedure to identify both stiffness and damping coefficients of the tissue is then applied to the developed hardware. Particularly, stiffness is calculated solving the static equations of the mechanism in a desired configuration, while the damping coefficient is inferred from the dynamic equations, which are written under the hypothesis that the sample tissue is excited by a variable compression force characterized by a suitable wave form. The whole procedure is implemented by making use of a control system.

https://www.mdpi.com/2072-666X/9/1/15/pdf

New MEMS Tweezers for the Viscoelastic Characterization of Soft Materials at the Microscale

In this paper, a microsystem with prescribed functional capabilities is designed and simulated. In particular, the development of a straight line path generator MEMS device is presented. A new procedure is suggested for avoiding branch or circuit problems in the kinematic synthesis problem. Then, Ball’s point detection is used to validate the obtained pseudo-rigid body model (PRBM). A compliant Micro Electro Mechanical System (MEMS) device is obtained from the PRBM through the rigid-body replacement method, by making use of Conjugate Surfaces Flexure Hinges (CSFHs). Finally, the functional capability of the device is investigated by means of FEA simulations and experimental testing at the macroscale.

Kinematic Synthesis of a D-Drive MEMS Device through the Rigid-Body Replacement Method

Effect of initial curvature in uniform flexures on position accuracy

The fabrication of a novel class of microgrippers is demonstrated by means of bulk microelectromechanical systems (MEMS) technology using silicon on insulator wafer substrates and deep reactive ion etching. Hard masking is implemented to maximize the selectivity of the bulk etching using sputtered aluminum and aluminum–titanium thin films. The micro-roughness problem related to the use of metal mask is addressed by testing different mask combinations and etching parameters. The O2 flow, SF6 pressure, wafer temperature, and bias power are examined, and the effect of each parameter on micro-masking is assessed. Sidewall damage associated with the use of a metal mask is eliminated by interposing a dielectric layer between silicon substrate and metal mask. Dedicated comb-drive anchors are implemented to etch safely both silicon sides down to the buried oxide, and to preserve the wafer integrity until the final wet release of the completed structures. A first set of complete devices is realized and tested under electrical actuation. [2017-0039]

Matteo Chiste

Papers

Fabrication of Novel MEMS Microgrippers by Deep Reactive Ion Etching With Metal Hard Mask