Session: MNS-03: Micro/Nano Robotics and Functional Materials

Paper Number: 143428

143428 - Fabrication of Rotary Comb Drives With Floating Center 

Rotary comb drives have been recently adopted to operate MEMS-Technology based micro systems. In several cases it is convenient to adopt a kinematic structure based on the four-bar linkage, where the coupler link has the guiding function for a generic tool as, for example, one jaw of a gripper. In this case the grasping function is granted by two symmetric jaws that are guided by two symmetric four-bar linkage structures. Since the ordinary kinematic pairs are not suitable for most planar fabrication methods of micro machining, compliant flexures must be used, with the consequence that some flexures must be embedded in the mechanical structure. For example, a microgripper based on two symmetric four-bar linkages needs 8 flexures. With the aim of coping  with the problem of providing higher power to inflect all these flexures, this paper presents a new concept design which allows the coupler to be directly actuated by a second cooperating rotary comb drives, in addition to the fixed axis rotary comb drive that actuates the crank link. The secondly added comb drive has also the advantage that its rotation center can be adjusted during the design stage with considerable freedom of choice in the plane, including positions
that are not physically placed within the wafer.

The microgripper belonging to this class has been fabricated on 6 inches SOI wafers having a device layer thickness of 40 micron, a 2 micron thick buried silicon oxide, and a handle layer with thickness of 400 micron. The fabrication process was performed using a previously reported sequence by the Authors, using HF vapor etching for the final release in order to avoid stiction. 

Masking layers were initially deposited on both wafer sides as follows: 200 nm thick TEOS Silicon Oxide by LPCVD (E1200HT, Centrotherm), 150 nm thick magnetron-sputtered Aluminum (MRC Eclipse), and 300 nm thick PECVD Silicon Oxide (Surface Technology Systems). 

A PECVD layer is needed on top of the aluminum film to avoid resputtering during the first part of DRIE etching, when the aspect ratio of the etched silicon cavity is still low. In this initial phase, aluminum resputtering inhibits silicon etching and generates the so-called silicon grass. 

The mask layers were patterned on both sides of the wafer using a stepper photolithography process (Nikon 2205i11D) with standard photoresist (Fujifilm). The  pattern was transfererd into the multilayer mask using plasma etching (Tegal Corporation TEGAL900 for silicon oxide, TEGAL 6520 for aluminum) to expose the underlying silicon. 

The exposed silicon was then etched with deep reactive ion etching (DRIE, SPTS Technologies Alcatel SMS 200). 

Both front and back etchings proceeded down to the buried silicon oxide that is embedded in the SOI wafers and acts as an etch stop. 

After DRIE, the residual masking layers were removed using standard microelectronics wet etching solutions: aluminum etching was performed with Aluminium etchant type A and silicon oxide was removed with 
diluted HF.

Last, the devices were released by removing the buried silicon oxide with an HF vapor etching (SPTS technologies Primaxx uEtch 2021). 

The devices underwent etching in a top-down position, causing the sacrificial components to detach from the device as soon as the underlying buried oxide layer was removed.

FCRCDs are characterized by two main advantages: the center of the mobile array of fingers can be positioned 
in any point of the wafer and even outside of its area; an additional torque can be applied directly to the coupler link,
which is quite an advantage because generally this link is the one that sustains the tip of interest.

The main constructive difficulty consists in the complexity of the geometry to be transferred to the wafer, because four CSFHs (Conjugte Surface Flexure Hinges) are needed for each embedded compliant four-bar linkage.

This article demonstrates that despite the high complexity of the device's geometry, 
the DRIE process and the HF vapor releasing technique are able to produce perfectly functional prototypes.   

Presenting Author: Nicola Pio Belfiore Roma Tre University

Presenting Author Biography: Nicola P. Belfiore, Professor, IEEE Member, teaches Applied Mechanics, Functional Design and Underwater Robotics at the University of Roma Tre, Italy. After the achievement of the Ph.D. degree, completed at Sapienza University of Rome, in cooperation with the University of Maryland of College Park, he was with Sapienza University from 1996 to 2000 as research fellow, and from 2001 to 2017 as Associate Professor. In October 2017 he moved from Sapienza to Roma Tre University, where he got the position of Full Professor in 2019.
In 1993 he won the AMR Best Paper Award at the Third National Applied Mechanisms and Robotics Conference (Cincinnati, OH) and in 1997 he also won the AMR Unique Contribution Award at the same Conference. In 2009 he won the Best Research Paper Award at the 18th Int. Workshop on Robotics in Alpe-Adria-Danube Region in Brasov, Romania. Since 2008 he is also a Honorary Professor of the Obuda University, Hungary. Author of three textbooks, four patents and about one hundred scientific papers he has been the coordinators of several scientific projects, both National and European. In 2013 he was the director of the 2nd Level Vocational Master in Energy Conversion Efficiency and Renewable Energy. From 2019 to 2022 he was nominated Head of the Degree Programs of Mechanical Engineering BSc and MSc in Mechanical Engineering, MSc in Aeronautical Engineering and BSc and MSc in Marine and Ocean Engineering, while in 2022 he was appointed as Deputy Director coordinator of the teaching activities of the Department of Industrial, Electronic and Mechanical Engineering. His actual interests are Topology, Kinematics and Dynamics of Mechanisms and Robots, MEMS and NEMS Design, Functional Design, and Tribology.

Authors:

Matteo Verotti University of Genua
Alvise Bagolini Foundation Bruno Kessler
Leandro Lorenzelli Foundation Bruno Kessler
Rocco Crescenzi Sapienza University of Rome
Nicola Pio Belfiore Roma Tre University