Electrochemical Wet Etching of Silver STM Tips

Presentation By:Anthony R. Davis

Senior Electrical Engineering MajorHampton University

Mentor: Dr. Wilson HoGraduate Student: Xiuwen Tu

Outline

• Background (Tips, STM)Background (Tips, STM)• Research Inspiration (Why Silver

Tips)• Process/Results/Findings• Future Applications• Acknowledgements

What are “tips”

• Used in STMs• Atomic-sized

radius of curvature

• Etched using electrochemistry

• Usually made of tungsten metal

Tips usage in STMs

Atomic Mapping: The Art of Drawing Atoms

Outline

• Background (Tips, STM)• Research Inspiration (Why Silver Research Inspiration (Why Silver

Tips)Tips)• Process/Results/Findings• Future Applications• Acknowledgements

Ho Lab Group Goals & Research Inspiration

• Nanoscale Chemical & Physical Phenomena

• Detailed Descriptions of Individual Molecules and Atoms

What’s Special About Silver Tips?

• Research suggests that Ag tips have higher photon-coupling efficiency than tungsten tips

• Enhanced Photo-Induced Tunneling yields more comprehensive data from specimens scanned

Why is Etching Silver Tips so Challenging Compared to Etching other Metal Tips? • Malleable/Soft composure of Ag

• Quick evaporation of electrolyte Solution

• Appearance of AgOH reside

• Silver tip etching is relatively new; etching methods are primitive

• Well known methods & commerical kits are available for etching other metals tips (W, Au, Pt-Ir); but not Ag

Outline

• Background (Tips, STM)• Research (Why Silver Tips)• Process/Results/FindingsProcess/Results/Findings• Future Applications• Acknowledgements

Primary Objective of Research

The challenge presented is optimizing the procedure and parameters for etching silver tips to ensure efficiency and

reproducibility.

2-Step Electrochemical Etching

1st Etch 2nd Etch

Setup of Primary Etch System(Electropolishing)

Graphite

Tip

Electrolyte

Objectives for Optimizing 1st Etch• Prevent Solution concentration for

changing– Retard electrolyte evaporation

• Increase Speed of First Etch– Obtain & interpret voltage vs. time data

• Automate tip etching to increase efficiency – Create circuitry to monitor current flow through

tip over time

Stabilize that Solution

• Began using a new recipe NH4:C2H6:H2O2 to prevent evaporation

• This solution works just as well as NH4

• Needs to be replenished after 3 hours

• Liquid Levels are consistent over time

DC Voltage Slows and Speeds Etching

Effect of Voltage on Etching Time

0

5

10

15

20

25

30

0 10 20 30 40

DC Voltage (Volts)

Tim

e (

min

ute

s)

Time Trial 1

Time Trial 2

Time Trial 3

A Need For Automation

• The electrochemical reaction is lengthy & subtle

• Visual Inspection Required• Cannot Perform both 1st and 2nd

Etches at the same Time

What’s the “Current” Trend?

Current Flow Through Silver Tips While Etching

0

1

2

3

4

5

6

0 10 20 30

Time (minutes)

Cu

rren

t (m

A)

V=24V

V=30V

V=21V

How Can we Measure It?

Measure voltage Across a Resistor using a Voltmeter

Use LabVIEW software to analyze signal

R

I

Current

Voltmeter Resistor

The New System: From Idea to Reality

Triggered Alarms

I = 0 mA

I < 2 mA

Variation in Apex Shape After First Etch

Secondary Etch Setup DiagramDC Voltage 5-12V

Tip

NH4

Second Etch: Useful Findings

Outline

• Background (Tips, STM)• Research (Why Silver Tips)• Process/Results/Findings• Future Applications• Acknowledgements

Future Possibilities/Applications:

Chemical/Optical Sensors

High Density Data Storage

Nanoelectronic Circuit Components

Bio-toxin & Pollution Removal

Quantum Computing

Innovation

Acknowledgements University of California, Irvine

National Science Foundation (NSF)

The UROP Team

Department of Engineering

Department of Physics & Astronomy

Mentor: Dr. Wilson Ho

Graduate Student: Xiuwen Tu