Comparison of smooth and hairy attachment pads in insects: friction, adhesion and mechanisms for direction-dependence

James M. R. Bullock, Patrick Drechsler and Walter Federle

Purpose:

Biomemitic technologies provide a valuable opportunity to borrow methods and uses from many years of optimization through evolution.  The aim of this study was to compare the performance of smooth and fibrillar systems in insects to clarify the mechanisms of attachment and removal.  This includes determining how smooth versus hairy attachment influences frictional performance, the importance of fluid pad secretion and its role in adhesion, and if there is any mechanism for direction dependence of attachment.

 

Methods:

A force transducer was used to measure friction forces of the insects feet which employed foil strain gauges and a three dimensional DC driven stage. Optical images were obtained using a camera and LabView and Matlab were used for data analysis from the home built system.

 

Key Findings:

1. Direction dependence of adhesion was observed with dramatically decreased adhesion in the distal direction but not the proximal.

2. No load dependence was observed for friction, contact area, or shear stress

3. Smooth/Hairy comparison of attachment found nearly 2x adhesion forces for the hairy system over the smooth

4.  Fluid pad secretion was quickly depleted from the contact zone and provide hydrophobic contacts to promote adhesion.

A Si stencil mask for deep X-ray lithography fabricated

Harutaka Mekaru, Takayuki Takano, Yoshiaki Ukita, Yuichi Utsumi, Masaharu Takahashi 

Microsyst Technol (2008) 14:1335–1342 

 

Purpose:

The development of technologies to fabricate resist structures with high aspect ratios over large area is important in X-ray lithography. This paper successful demonstrates the ability to fabricate a Si stencil mask for DXL (deep x-ray lithography) using MEMs fabrication techniques.  Higher energy x-ray fabrication allows for greater aspect ratios but requires specialized stencils and masks which can withstand the photons of higher energy. In this experiment they succeeded in the X-ray exposure of PMMA sheets through their entire thicknesses of 0.5 and 1 mm.

Methods:

In this fabrication a combination of lithography and dry etching techniques were used to create the mask. Typical photolithography was implemented to create the outline for the mask.  This includes cleaning silicon wafter, spin coating photoresist (AZP4903), exposed, developed, and final processing (ashing).  The Si mask was then used in deep-RIE to pattern PMMA which is the final device. Optical micrographs and SEM images were obtained to verify the success of the end device.

 

Key Findings:

1. A 200 micron thick Si mask can be used for patterning in deep-RIE processes.

2. Optimization of Bosch process yeilds smooth and vertical sidewall.

3. X-ray irradiated PMMA has minimum line resoultion of 20 microns using the Si mask

4. Pattern sticking, removal, and feature size has been improved and optimized by specific selection of process solvents.

Low cycle fatigue of single crystal silicon thin films

Hsien-Kuang Liu, BJ Lee, Pang-Ping Liu

Sensors and Actuators A 140 (2007) 257-265

 

Purpose: The work in this study was aimed at understanding the fatigue of single crystal silicon thin films by a cantilever microbeam.  SEM was used to observe the modes of fracture and deformation for various conditions of loading.

Methods: Fatigue testing was conducted using a tungsten probe such that the probe is held firmly against the specimen and subjected to a cyclic load. SEM imaging was used to determine the failure modes of the cantilever under various strain amplitudes. EDS (Engergy Dispersive Spectrometer) was used to analyze the chemical composition of the failed sample.

Key Findings: 

1. Single crystal silicon can fail at strains near one half of their static failure strain for cyclic loading in excess of a million cycles.

2. Dominant failure mode is cleavage at a surface flaw with striations parallel to the length direction.

3. Fatigue crack initiates from the top flat surface of the beam and propagates in the {1 1 1} plane.

4. Two sub-mechanisms are suggested for failure: 1st) fracture debris at incident point inhibits crack propgation, and 2nd) moisture enhances surface reaction resulting in crack formation and eventual failure.

Preparation and Adhesion of a Dual-Component Self-Assembled Dual-Layer Film on Silicon by a Dip-Coating Nanoparticles Method

Yufei Mo and Mingwu Bai

J. Phys. Chem. C 2008, 112, 11257-11264

 

Purpose:

Mixed component patterned surfaces have gained attention for potential application in many fields including microfluidics, MEMs, and biological systems.  This paper outlines a novel microprocessing technique utilizing dip-coated nanoparticles as a mask or template to create molecular films that requires no further processing.

 

Methods:

Piranha treated silicon wafers were exposed to a 5 mM solution of 3-aminopropyltriethoxysilane (APS) for 24 hours to allow monolayer formation.  The SAM was then nanopatterned by dip-coating silver nanoparticles in hexane solution.  Chemical vapor depostion (CVD) was used to deposit a monolayer of steric acid (STA) and samples were subsequently sonicated to remove silver nanoparticles and physisorbed molecules.  Backfilling of void spaces left by silver nanoparticles with perfluorooctadecanoic acid (PFOA) was done via CVD and subsequently sonicated again.  Surfaces were characterized by contact angles, ellipsometry, XPS, and AFM (tapping and contact modes).  AFM tapping mode probes film morphology and AFM contact mode investigates the adhesion of the film.  A Si₃N₄ cantilever with a radius <10 nm and 0.4 N/m normal force constant was used.

 

Key Findings:

1. Verification of dual-layer film formation as well as nanoparticle deposition and removal.
2. AFM morphology experiments show successful completion of each step in the process by quantifying Ag nanoparticles and “pits” left upon Ag removal.  Any deviations in pit size from that of Ag nanoparticle was explained by either particle agglomeration in solution before deposition or tip-particles interactions that obscure the measurements.
3. Adhesive forces of the two-component film were reported 3.21 and 6.43 nN for STA and PFOA respectively.
4. Surface coverage of PFOA was calculated at 20% using supplied AFM software that approaches surface coverage of pits calculated from adhesive force measurements (31%).