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

gapBlack == MetalGreen == NanowireGray == Si/SiO2 substrate

Applications

Cell Device

Biology & MedicineElectrical Circuits

ω ω'

Sensors

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 IBasic 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 Trenchesi. Place wafer under photomask, and expose the sample to UV light.

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

Top View

Rate ~ 1 nm / sec

Step 4 – Drop nanowires

This can be done using either wet or dry transfer.

Step 5 – Contact Padsi. 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 IIBasic 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.

AcknowledgmentsSpecial Thanks To: Case Western Reserve University Physics Department National Science Foundation Bob Mike McDonald Professor Kash Reza Sharghi-Moshtaghin

NSF REU grant DMR-0850037