1. Fabrication of Suspended Nanowire Structures Jason Mast & Xuan Gao
Summer 2009 REU Program at Case Western Reserve University

2. Introduction Nano == 1 / billion Nanometer == 1 meter / billion
Nanowire == wire with diameter between 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

3. Applications Biology & Medicine Sensors Electrical Circuits Cell
Device
Sensors ω' ω
Cell
Cell

4. 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.

5. Method I Step 1 - Start 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 SiO Gray == Silicon

6. Step 2 – Spin Coat Photoresist
Photoresist layers act like sacrificial layers. We use them to mold our devices.
Purple == LOR-3A Yellow == S 1805

7. 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

8. Step 4 – Drop nanowires
This can be done using either wet or dry transfer.

9. Step 5 – Contact Pads i. Expose 2nd pattern to UV light.
ii. Submerge in CD-26 for 60 sec.

10. 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.

11. Method II
Basic Ideas
1. Embed nanowires between layers of photoresist.
2. Create metallic contacts.
Step 1 – Get Started
Blue == 100 nm SiO Gray == Silicon

12. Step 2 – Spin coat photoresist
Yellow == Photoresist
Step 3 – Drop nanowires
Green == Nanowires

13. Step 4 – Spin coat photoresist
Nanowires are now embedded between two layers of photoresist.

14. Step 5 – Contact Pads
i. Place under photomask, and expose to UV light.
ii. Submerge in CD-26 for 60 sec.

15. Step 5 - cont Iiii. Evaporate metal (5 nm Ti & 300 nm Al).
vi. Submerge in Remover PG overnight.

16. Results – Method I I was able to perform alignment.
However, alignment was far too complicated to implement.

17. 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.

20. 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