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					Fabrication of Suspended
  Nanowire Structures


           Jason Mast & Xuan Gao




Summer 2009 REU Program at Case Western Reserve University
                   Introduction
 Nano    == 1 / billion
 Nanometer == 1 meter / billion




   Nanowire == wire with diameter between 1-100 nm

Suspended Nanowire == A nanowire suspended over a
gap.


                       Top – Nanowire on substrate
                       Bottom – Nanowire suspended over gap

                       Black == Metal
                       Green == Nanowire
                       Gray == Si/SiO2 substrate
              Applications
                                      Electrical Circuits
Biology & Medicine
Cell                    Device




                     Sensors



                                 ω'
       ω
                  Goals


1. Fabricate suspended nanowire devices
        - Simple but not easy.

2. Test the devices and look at the I-V
curves.
         - Should be different because the
substrate (read heat sink) is not in contact
with the wire.
                     Method I
Basic Idea
   1. Carve trenches out of top SiO2 layer.
   2. Drop nanowires over the trenches.
   3. Evaporate metallic contact pads.

Step 1 - Start




   Blue == 100 nm SiO2   Gray == Silicon
Step 2 – Spin Coat Photoresist

Photoresist layers act like sacrificial layers.
We use them to mold our devices.




Purple == LOR-3A   Yellow == S 1805
   Step 3 – Carve out Trenches
i. Place wafer under photomask, and expose the sample to UV light.




                                                       Top View


ii. Submerge in CD-26.            Iiii. Submerge in HF for 60 sec.




                                Rate ~ 1 nm / sec
     Step 4 – Drop nanowires
This can be done using either wet or dry transfer.
          Step 5 – Contact Pads
i. Expose 2nd pattern to UV light.




ii. Submerge in CD-26 for 60 sec.
                    Step 5 - cont.
Iiii. Evaporate metal (5 nm Ti & then 50 nm Al).

                                       Different shades of gray
                                       represent different
                                       elevations of metal.




iv. Let the wafer sit in Remover PG overnight.
                        Method II
Basic Ideas
    1. Embed nanowires between layers of photoresist.
    2. Create metallic contacts.

 Step 1 – Get Started




    Blue == 100 nm SiO2    Gray == Silicon
Step 2 – Spin coat photoresist
                      Yellow == Photoresist




Step 3 – Drop nanowires
                       Green == Nanowires
Step 4 – Spin coat photoresist
Nanowires are now embedded between two layers of photoresist.
         Step 5 – Contact Pads
i. Place under photomask, and expose to UV light.




ii. Submerge in CD-26 for 60 sec.
Step 5 - cont
Iiii. Evaporate metal (5 nm Ti & 300 nm Al).




vi. Submerge in Remover PG overnight.
           Results – Method I

   I was able to perform alignment.

However, alignment was far too complicated
to implement.
        Results – Method II

I was able to get suspended nanowire
structures.

I was able to get a few I-V curves, and a
resistance measurement.
            Acknowledgments
Special Thanks To:
 Case Western Reserve University Physics Department

 National Science Foundation

 Bob

 Mike McDonald

 Professor Kash

 Reza Sharghi-Moshtaghin




 NSF REU grant DMR-0850037

				
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posted:3/24/2013
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