EE235 Presentation I CNT Force Sensor Ting-Ta YEN Feb Y. Takei, K. Matsumoto, I. Shimoyama “Force Sensor Using Carbon Nanotubes Directly Synthesized on Micro Structure”, MEMS 2007 Graduate School of Information Science and Technology, The University of Tokyo, Japan

Chiral Vector  Chiral vector (n,m): The way the graphite sheet is wrapped: (n, m) denote the number of unit vectors along 2 directions in the crystal lattice. m = 0 → zigzag n = m → armchair Otherwise → chiral  For a given (n,m) nanotube: n + m = 3q → metallic otherwise → semiconductor

Electrical Characteristics  Most idea one-dimensional structure in real world.  High current density (~10 9 A/cm 2 ).  High conductance.  Electrical resistance of the CNT increases when its length changes from its original length.  Bundle-CNT: with metallic properties

Schematic of Force Sensor  Controls of the growing position, direction and amount are important.  Position control: Pattern the catalyst on the silicon wafer.  Amount control: Parallel electric field.  Density control: Changing flow rate during CVD. Directly synthesized CNT force sensor

Fabrication Process  Fabricate on SOI wafer: 3/2/450 μm SOI wafer  Etched the top Si layer by DRIE.  Etched middle SiO 2 layer by HF.  Beam structure is done.

Fabrication Process  Carry out the thermal oxidation to form SiO 2 layer on the surface.  Metal acetate solution: Iron and cobalt acetate dissolved in ethanol  Dip the wafer in the catalyst solution.

Fabrication Process  Dry the wafer in a 400 ℃ oven.  A catalyst layer was formed on the surface.  After CVD process, bridging CNTs were synthesized.

Fabrication Process  Wire the wafer to measure the resistance.  Infiltrate and cure the PDMS on the wafer.  PDMS between the 2μm gap to limit the deformation of the bridging CNTs only in the shrinking direction.

CNT Growing Mechanism  CNTs are synthesized from the catalytic metals.  Need SiO 2 substrate to fix catalysts.  Use thermal oxidation to form the SiO 2 layer.

SEM Images

Experimental Setup & Result  The beam bend down by the force and bridging CNTs shrink.  bridging CNTs electrical resistance changes when loading and unloading the weight on the beam.

The END Thank you!