Master of Technologyhome.iitk.ac.in/~zainul/documentsopen/Zainul_MTech...My stay at IIT Kanpur make a lot of friends who shared with me everything, Chatting, Orkuting, and a lot of

Nano Electrode Fabrication Using Focused

Ion Beam Suitable For Organic Devices

A Thesis Submitted

in Partial Fulfillment of the Requirements

for the Degree of

Master of Technology

By

Zainul Aabdin (Y5112013)

Under the supervision

of

Prof. Y. N. Mohapatra and Prof. V. N. Kulkarni

to the

Materials Science Programme

Indian Institute of Technology Kanpur

Jan, 2008

CERTIFICATE

This is to certify that Zainul Aabdin, a student of M.Tech. in the Material Science

Programme of this Institute has worked on the topic “Nano Electrode Fabrication using

FIB Suitable for Organic Devices” under our guidance. The present thesis work is an

outcome of his sincere research efforts and the work has not been submitted elsewhere for a

degree.

Jan 11, 2008

(Prof. Y.N.Mohapatra)

Head, Department of Physics

Indian Institute of Technology

Kanpur, India

(Prof. V.N.Kulkarni)

Professor, Department of Physics

Indian Institute of Technology

Kanpur, India

i

Abstract

Focused Ion Beam (FIB) provides unique capabilities to fabricate nanostructure based

devices. Fabrication of metal contact with separation in nanometer and micrometer regime is

an integral part of most fabrication tasks in this direction. In this thesis we optimize

parameters and procedures to fabricate nanometer to micrometer interdigitated electrodes

using patterning capabilities of FIB on gold and aluminum thin films.

Such interdigitated electrodes form the central requirement in being able to fabricate

Organic Thin Film Transistors (OTFT) with channel with submicron channel length.

However, the efficacy of using FIB milled electrodes in such applications need to be

evaluated. Towards this aim, the thesis explores the study of I-V characteristics of simples

devices fabricated by depositing Alq3 and pentacene layers on the interdigitated electrodes

with nano and micrometer separation fabricated using FIB. We show that it is possible to

obtain space charge limited current in these materials using these contacts. Though the

processes controlling the quality of the organic material grown in constrained spaces between

these electrodes during high vacuum thermal deposition is not fully understood, the current

carrying capabilities are observed to be limited probably due to defects and low mobility in

such materials. These studies would aid the development of fully fledged organic thin film

transistors with nanometer channel lengths based on FIB-fabricated interdigitated electrodes.

ii

Acknowledgement

I would like to thank my sincere gratitude to my thesis supervisor Prof. Y. N.

Mohapatra and Prof. V. N. Kulkarni for his advice, guidance, patience and tremendous help

during the project. His constant encouragement, and freedom to work with anything and at

any time made my life much easier and it helped me to work more efficiently. It is a great

honor for me to have been a part of his lab. The successful completion of the work has been

only possible due to their excellent guidance, meticulous observation and critical analysis.

I would also like to thank Prof. S.K.Iyer, Prof. Monika Katiyar, Prof. Depak Gupta,

Prof. Jitendra Narayan and other Samtel staff for their useful suggestions and guidance and

timely help as and when required by me.

I would also like to thank Prof Aashtosh Sharma to permit me to use his lab without

any conditions. I also acknowledge the help from Mr. Prabhat, Mr. Manish and Mr. Dinesh

Deva for providing me access to Maskless lithography facility. I also acknowledge the help

from Mr. Masood, Mr. Rajput, Sudhansu, Servesh and Prashant.

My sincere thanks to SCDT lab, ACMS lab, Nuclear lab, Chemical engineering lab

and Central Workshop staffs to help me during the course of this work.

I sincerely appreciate the help offered by my friends especially Dheerendra, Durgesh,

Awanish, Rao, Anand Biswas, Ashish, Arun Tej, Vinod, Vibhor, Dharmendra, Anoop,

Dinesh and Ramnath for intellectual, moral and emotional support.

I would also like to greatly acknowledge the financial support from the Council of

Scientific and Industrial Research (New Delhi, India) and Ministry of Human Resource and

Development.

My stay at IIT Kanpur make a lot of friends who shared with me everything,

Chatting, Orkuting, and a lot of discussion on cricket and movie. List is very long, in no

particular, Durgesh Rai, Santosh, Mayank, Waquar, Richa, Arpu, Prashant, Chinnu, Anand

Prakash, Alka Gupta, Vijay Raj Singh, Virendra Kumar Verma, Ranbir Singh, Mo.Waseem

Akhtar and many more (sorry if I miss someone’s name). I also want to put a name here who

made my initial days at IIT Kanpur really enjoyable and cheerful and that name is Rajni

Tanwar.

iii

Finally I would like to have an exclusive thanks to my family and especially my

mother and my younger brothers (Minhaj and Jafar) for their love, support and care

throughout the course. May God bless you two; my mom and dad, as I could never ever

thought of any two persons sacrificing and dedicating themselves to my success so

trustworthy and humbly and without any expectation whatsoever. I wish I could return all

these kindness. My brother and sister, thank you very much for your love, company and

support throughout the years.

I still have to thank to the rest of my friends, whom I can't name all, but I am sure

know I owe them a lot for all the good times together.

Jan, 2008 Zainul Aabdin

iv

TABLE OF CONTENTS

ABSTRACT……………………………………………………………..……………………..i

ACKNOWLEDGMENT…………………………………………..……….………………….ii

TABLE OF CONTENTS………………………….………………………………………….iv

LIST OF TABLES………………………………………….………………………………...vi

LIST OF DATA BOXES………………..…………………………………….….…………..vi

LIST OF FIGURES………………………………...………………………………….…….vii

Chapter 1: Introduction & Literature Survey

1.1. Introduction………………………………………………….………………..….……2

1.2. Organic Semiconductor Devices……………………………..………………..………2

1.3. Focused Ion Beam (FIB)……………………………….………………………..…….6

1.4. Technology, Functions and Applications of FIB……,…….……………………….....6

Technology…………………………………..………………………………..…....…7

1.4.1. The Column…......................................................................................................7

1.4.2. Liquid Metal Ion Source (LMIS)…….................................................................8

1.4.3. Lens System………………………….................................................................8

1.4.4. Stage, Detector and Gas Injection………………………….….………..…..…..8

1.4.5. Image Generation………………………….……………………………….…...9

1.4.6. Milling……………………………………………………………..……….…...9

Functions of the FIB……………….…………………...…………………………...10

1.4.7. Imaging…………………………………………………………………….…..11

1.4.8. Simple Ion Milling and Ion Milling with Enhanced Etch….……………….....11

1.4.9. Material Deposition with the FIB………………………………………….….12

Applications of the FIB……………………………………………………………..12

Chapter 2: Sample Preparation

2.1. Introduction……………………………………………………………………………...14

2.2. Substrate Cleaning…………………………………...……………….………………….14

2.2.1. Ultrasonic Cleaning ……………………………….……….………………….14

2.2.2. RCA Cleaning………………………………………………………….….......14

2.3. Electrodes………………………………….…………………………….….……..…….15

v

2.3.1. Aluminium and Gold Deposition……………………………….……………..16

2.3.2. Maskless Lithography……………..………………………………………......17

2.3.3. Etching……………………………………………………..………………….19

2.3.4. Removal of Photo Resist…………………………………………..….………20

2.5. Summary……………………………………………..…………..……………………...22

Chapter 3: Nano Electrode & Device Fabrication Using FIB & its Characterization

3.1. Introduction……………………………………………………………………………...24

3.2. Interdigitated Nano Electrode Fabrication Using……………………………..………..24

3.2.1. Interdigitated Nano Electrode of Aluminium Using FIB….…………………..25

3.2.2. Interdigitated Nano Electrode of Gold Using FIB…….....................................27

3.3. Characterization of Nano Electrode……………………………………………………..29

3.4. Suitability of Electrode with Organic Materials………...……………………………….29

3.4.1. Aluminium Electrodes........................................................................................30

3.4.2. Gold Electrodes……………..…………..…………………………………......32

3.5. Summary……………………..………………………………………………………….32

Chapter 4: Electrical Characterization: Result and Discussion

4.1. Fabrication of Nano Electrode Using FIB……………………………...…….……...…..34

4.2. I-V Characteristics of Al and Gold Nano Electrode……………………..........................34

4.2.1. I-V Characteristics of Aluminium Nano Electrode…………………...…….…34

4.2.2. I-V Characteristics of Gold Nano Electrode…………………………….….....36

4.3. Device Made on Aluminium Electrode……………………………….…………..…......36

4.3.1. Aluminium Nano Electrode Devices ……………………….……..………......37

4.3.2. Aluminium Micro Electrode Devices..………….…………….…..………...…38

4.4. Device Made with Gold Electrode………………………...…………………………….40

4.4.1. Gold Nano Electrode Devices……………………………………..…………..40

4.4.2. Gold Micro Electrode Devices..………………………...………….…...……..42

4.5. Peaks Due to Charging …………………………..…..……………………………...…..45

Chapter 5: Summary & Conclusions

Summary & Conclusions………………....………………………………………………….47

References…………….………..…………………………………………………………...48

vi

LIST OF TABLES

Table No. Title Page

1.1 Summary of channel length reduction year wise. 5

1.2 Milling spot size corresponding to different beam current. 9

2.1 Typical amount of different reagent for Al etchant. 19

LIST OF DATA BOXES

Data Box No. Title Page

2.1 Optimized data used in maskless lithography. . 17

3.1 Milling parameters to make Aluminium nano electrodes. 25

3.2 Milling parameters to make Gold nano electrodes. 27

3.3 Deposition parameters to deposit Alq3 on aluminium nano electrodes. 30

3.4 Deposition parameters to deposit LiF on Aluminium nano electrodes. 30

3.5 Typical deposition parameters of Au on glass substrate. 32

4.1 Milling parameters for aluminium nano electrodes. 34

4.2 Milling parameters for gold nano electrodes. 34

vii

LIST OF FIGURES

Figure No. Title Page No.

1.1

1.2

1.3

1.4

1.5

1.6

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

2.10

3.1

3.2

Typical performance of polymer OTFT devices with channel lengths 1000 nm,

200 nm, and 70 nm.

Output current–voltage and transfer characteristics of a 500 nm & 30 nm channel

length OTFT.

I-V characteristic for 10 nm OTFT.

Schematic of FIB column.

Representation of scan mechanism in FIB.

Mechanism of simple milling and image generation.

Schematics of Al and Au coated glass substrate.

Mask used for Al and gold thin film deposition.

Schematics shows strip of chrome gold coated on glass substrate.

Camera image of actual sample.

Bit map images used in maskless lithography.

Sample after PR coating, exposure & developing but before etching (1) at 50 times

magnification, (2) at 100 times magnification.

Sample after etching but before removal of PR, (1) at 50 times magnification & (2)

at 100 times magnification.

Setup used for chemical etching.

Optical microscopic image of the sample after removal of PR, (1) at 50 times

magnification & (2) at 100 times magnification.

Schematic that has been generate using maskless lithography; (1) interdigitated

electrodes separated by 90 µm, (2) interdigitated electrodes having different

channel length ranges from 15 to 45 µm on the same sample, (3) thin strips ranges

from 15 to 45 µm of Al connected with large pads (4) electrodes made of gold thin

film having channel length of 1 µm.

Basic pattern that’s plan to be created.

Bit map image used for milling the interdigitated electrodes.

3

4

5

7

10

11

15

16

16

17

18

19

20

20

21

21

24

25

viii

3.3

3.4

3.5

3.6

3.7

3.8

3.9

4.1

4.2

4.3

4.4

4.5

4.6

4.7

4.8

4.9

4.10

4.11

4.12

4.13

4.14

4.15

4.16

SEM images of the interdigitated electrodes.

Nano electrodes having narrow gap of 16 nm.

Nano electrodes having narrow gap of 41 nm.

Nano electrodes pattern on gold.

Melting of gold film due to excess of beam current.

Basic pattern of an organic device.

Steps that are followed in making all devices.

I-V characteristics of Al nano electrodes having channel separation 29 nm.

I-V characteristics curve for Al nano electrodes having channel separation 16 nm.

I-V characteristics cure for gold nano electrodes having channel separation 100 nm.

I-V characteristics of Alq3 with Al nano electrodes 16 nm apart.

I-V characteristics of Alq3 with Al nano electrodes at 41 nm separation.

I-V characteristics of Alq3 with Al micro electrodes separated by 60 µm.

I-V characteristics of Alq3 with Al micro electrodes separated by 90 µm.

Device geometry used in the fabrication of micro-electrodes.

I-V characteristics of pentacene with Au nano electrodes separated by 100 nm.

I-V characteristics of pentacene with Au nano electrodes separated by 100 nm.

I-V characteristics of pentacene with Au micro electrodes separated by 1µm.

I-V characteristics of pentacene with Au micro electrodes separated by 1 µm.



I-V up to 10 volts.

I-V of Alq3 with aluminium nano electrodes having channel separation 16 nm,

showing some interesting peaks.

26

26

27

28

28

29

31

35

35

36

37

38

39

39

40

41

41

42

42

43

44

45

45

Chapter 1

Literature Survey and

Introduction to FIB (Development in the field of OTFT & basics of FIB)

“There is a theory which states that if ever

anyone discovers exactly what the universe

is for, and why it is here, it will instantly

disappear and be replaced by something

even more bizarrely inexplicable.

There is another theory which states that

this has already happened…”

Hitchhikers Guide to the Galaxy, the

BBC radio series, Douglas Adams.

Chapter 1: Literature survey and introduction to FIB

2

1.1. Introduction

Organic electronic devices, consisting of organic materials as semiconductors, have

shown an impressive progress over the last two decades. Owing to their peculiar material

properties, organic semiconductors have several advantages over their inorganic counterparts.

The first and foremost advantage is in the processing techniques, where organic devices

can be fabricated on large area and at much lower temperatures than the conventional

silicon devices. This allows fabrication as well as integration of the organic electronic

devices on low-cost, large-area, flexible and low-weight substrates, which opens up a

whole new area of novel applications, such as active-matrix flat panel displays, smart

cards, electronic papers, identification tags and large-area sensor arrays.

One of the important components of the organic electronics is organic thin film

transistor (OTFT). Their possible application ranges from displays, smart cards, sensors to

electronic textiles. However, current applied research in OTFTs is typically focused towards

two main applications: electronic bar codes or radio frequency identification tags

(RFID); and backplane driving circuits for active-matrix organic displays.

Although, OTFT technology is on the verge of commercialization, it still faces a

number of challenges such as low mobility, high voltage operation etc. that require

fundamental understanding at the fabrication, materials and device physics level, to

make significant progress. These discrepancies can be resolved by making device of very

narrow channel length and by understanding the effect of structure of the organic

semiconductor film on carrier mobility and charge injection from metal. In this thesis, a

concentrated effort is made to make an interdigitated nano electrode of channel width

approaching to the dimension of a molecule. Further an experimental study of nano channels

are made by putting an organic material on this nano electrode as an active material.

1.2. Organic Semiconductor Devices

Organic materials were mainly known as insulating materials, until in late 70s

when it was shown that a class of organic materials could show intrinsic conducting

properties. This discovery opened up a new exciting field of organic conductors and

was later recognized in the form of Nobel Prize in chemistry for 2000. Over the last

few decades, organic semiconductors have generated considerable interest in the fields of

electronic and photonic devices due to wide range of applications, such as thin-film


3

transistors (TFTs), radio frequency (RF) identification tags, and organic light-emitting diode

devices. Among these, OTFTs have potential advantages of realizing low-cost and large-area

applications.

Though organic light emitting diode (OLED) based display technology has matured to

find application in consumer electronics, the story of organic thin film transistors

(OTFT) is still at development stage. The primary challenges are high operational voltage,

low mobility and stability. The current carrying capability is also small. One of the possible

solutions to increase the OTFT drive current per unit area is that to reduce the channel length

of the device.

Figure 1.1: Typical performance of polymer OTFT devices with channel lengths of (a) 1000 nm, (b) 200 nm, and (c) 70 nm [6]


4

Recently, there have been some efforts by different groups to reduce the channel

length to increase the current per unit area and the operating voltage. The strategy adopted to

reduce the channel length varies. Michael D. Austina and Stephen Y. Chou used the technique

of nanoimprint lithography and fabricated devices of channel length varying from 1µm to 70

nm. They showed that 1000 nm channel device show standard field effect characteristics and

the drain current density increases as the channel length is reduced while the on/off ratio

remains constant. But when the channel length is reduced further, characteristics starts

deviating from standard expectations. Below a channel length of 200nm devices are no longer

capable of standard FET saturation, and instead demonstrate a continuous growth in drain

current with drain voltage. The inability to saturate is widely observed in short-channel OTFT

devices.

Figure 1.2: Output current–voltage (ID–VD) and transfer ID 1/2

–VG characteristics of a 500 nm channel length OTFT Output current–voltage (ID–VD) characteristics of a 30 nm channel length OTFT and Fowler–Nordheim plot of the same data [7]

J. Collet, et. al. also reported similar results by making two devices one of 500 nm

and one of 30 nm channel length using electron beam lithography technique. Their typical

results are reproduced in Fig 1.2.


5

Recently Lee el. al. report pentacene based OTFT with 10 nm channel length. Their

device gets saturated upto certain voltage. After that voltage a non destructive breakdown

takes place as their results reproduces in Fig 1.3. Further, they show that devices exhibit

expected scaling trends, including an increase of on-current with a decrease in channel

length, indicating that appropriate choice of contact materials, interface structures, and device

design may enable the realization of optimized nanoscale organic devices.

Figure 1.3: I-V characteristic for 10 nm OTFT [5]

The effort made by different groups around the world toward nanoscale OTFTs is

summarized in Table 1.1.

Table 1.1

Name L

(nm)

tox

(nm)

Vg

(V)

VD

(V)

Saturated

output cure

Fabrication

method

Year

Rogers et al. 100 20 -3 -5 y Micro contact

printing

1999

Collet et al. 30 2 -15 -1.5 n e-beam

lithography

2000

Austin et al. 70 5 -3 -3 n Nano-

imprinting

2002

Zhang et al. 30 30 -4 -2 y e-beam

lithography

2003

Wang et al. 9 100 -30 -5 n e-beam

lithography

2004

Subramaniam

et al.

10 3 -0.3 -0.3 y e-beam

lithography

2005


6

1.3. Focused Ion Beam (FIB)

Focused ion beam (FIB) technique uses a focused beam of ions to scan the surface of

a specimen, analogous to the way electrons are used in a scanning electron microscope

(SEM). Application of a very high electric field onto a liquid metal ion source (LMIS)

generates ions, which are focused by electrostatic lenses. The development of LMIS is crucial

for the development of FIB. Krohn first observed the emission of large number of charged

ions along with charged droplets while performing experiments on wood metal (the eutectic

alloy of Bi, Pb, Sn and Cd) for space thruster applications. By mid 70’s the work performed

on LMIS had reached to a stage where these sources of ions could be used in a FIB.

Bombardment of the specimen surface by extracted ions from LMIS [1] results in the

generation of secondary electrons, ions and sputtered material.

These different kinds of generated species are used for different functions of the FIB.

Imaging function results due to secondary electrons and ions while fabrication function

occurs due to sputtering. Generation of secondary electrons result from closer to the surface

(10’s of nm) than in a SEM (100’s of nm) and the back scattered electron signal does not

occur as the bombarding particles are ions. The release of an appropriate gas close the surface

as Ga+ ions bombard the specimen surface leads to additional capabilities of the FIB such as

material deposition and enhanced removal of material. Different materials can be deposited

for example Pt, C and W. The primary advantage of FIB is the capability to form an image of

the specimen and then precisely mill the material away from selected areas. The major

applications of the FIB systems are failure analysis, device modification, and repair of photo

masks etc. New applications developed in this field include ion lithography and applications

related to micromachining. This project used a FEI (manufacturer’s name) 200 series type

FIB. The milling spot size of this machine can be varied from 8 nm to 500 nm, which makes

it suitable for nanofabrication.

1.4. Technology, Functions and Applications of FIB

This section has three parts, the first part discusses the FIB system technology, the

second part discusses various functions of the FIB, and the last part discusses major

applications of the FIB technology both in industry and in research.


7

Figure 1.4: Schematic of FIB

Technology

1.4.1. The Column

The column mounts on top of the specimen chamber and consists of a LMIS, two

electrostatic lenses, a set of beam blanking plates, a beam acceptance aperture, a beam

defining aperture, a steering quadrupole and an octupole deflector. Above figure shows a

schematic of the FIB system.

Application of a negative bias of 30 kV extracts Ga+ ions from the LMIS and

accelerates them towards the specimen. Two electrostatic lenses, a steering quadrupole and

an octupole deflector in the column focus the ions into a beam and scan the beam on the

specimen. The ion beam strikes the specimen and removes material by physical sputtering

process. The striking of the ions onto the specimen also generates secondary ions and

secondary electrons, which are detected to form an image of the scanned area. The scan

control system allows milling of different patterns. Injection of different gases close to the


8

specimen surface while ion milling results in deposition of different materials and enhanced

removal of material [Source: FIB Manual for FEI 200]. The following sub-sections describe

the various components of the column.

1.4.2. Liquid Metal Ion Source (LMIS)

The LMIS in the FIB used for the project consists of a conical shaped emitter; the

source material (Ga) liquefies upon heating. The source material covers the cone and when a

strong electric field is applied, the material layer deforms into a Taylor cone. The end of the

tiny cone has an end radius of approximately 2nm and ion emission occurs from this end. The

half angle of the Taylor cone is determined by the electric field force and the

counterbalancing surface tension forces [1]. Ga ion source is commercially used due to its

low melting point, low vapor pressure, very high brightness (106 Acm

-2 sr

-1), long life time

and high stability variation [4] [Source: FIB Manual for FEI 200]. There are various other

source materials including Au, Cs, In, Bi, Pb, Ga, etc.; a particular application may require a

different source material.

1.4.3. Lens System

The beam from the source first passes through a beam acceptance aperture and then

enters Lens 1. The quadrupole, located just above the beam-defining aperture (BDA) adjusts

the beam position so that the beam travels through the centre of BDA. The Lens 2 quadrupole

then aligns the beam to the optical axis of Lens 2. The octupole below the Lens 2 provides

scan and shift as well as beam astigmatism correction. The beam blanking assembly is

located between the Lens 2 steering quadrupole and the second lens assembly. It consists of

blanking plates, aperture and an electrical path for the current to be measured. Beam blanking

is useful as it protects specimens from constant milling.

1.4.4. Stage, Detector and Gas Injection

The stage is motorized and software provides control of X, Y axis and rotation and it

can be manually tilted in the XZ plane. Two different type of gases can be released above the

specimen surface at a distance of approximately 100 µm, one is used for enhanced etch, the

other for Pt deposition. During ion bombardment while milling, charged species form and


9

these are attracted to the detector, which is close to the specimen by applying appropriate

bias. The detector is a micro channel plate (MCP), which is a glass array of millions of tiny

channel electron multipliers.

1.4.5. Image Generation

The primary beam is scanned across the specimen as a raster, which consists of a

series of lines in the horizontal (X) axis, shifted slightly from one another in the vertical (Y)

axis. As the beam scans over the specimen, secondary electrons and secondary ions that are

generated by the specimen are detected and the information is stored in the computer; the

image is generated from this stored information.

1.4.6. Milling

During ion milling the beam is un-blanked and lines, circles, rectangles, polygons

with four corners and stepped profiles can be milled using the scan control system. Following

table shows different beam currents and the corresponding milling spot sizes [Source: FIB

manual for FEI 200].

Table 1.2

Beam current(pA) Milling spot size(nA)

1 8

4 12

11 15

70 25

150 35

350 55

1000 80

2700 120

6600 270

11500 500

Dwell time is the period of time the beam stays at a particular position. Overlap is

defined as the area overlapped when the beam moves from one position to the next and is


10

calculated in terms of percentage of area. For milling the pixel size is calculated by

measuring the full length of the image observed on the work station divided by 4096, the

number 4096 comes from the use of a 12 bit analogue-to-digital card. The milling pixel

resolution and milling spot size has to be of appropriate size to define fine.

Figure 1.5: Representation of scan mechanism in FIB

Functions of the FIB

FIB has various functions, for example, imaging, simple ion milling, ion milling with

enhanced etch and Pt, C, W deposition. Both secondary electrons and secondary ions can be

used to perform imaging. In this project FIB used as a simple imaging and simple ion milling.

Release of an organo-metallic gas close to the surface while ion milling results in the

formation of a deposit. Electrical signal generated while milling through a multi-layered

structure can be used to find the end point of a layer. The following sections discuss these

different functions of the FIB.


11

1.4.7. Imaging

Scanning of the ion beam on the specimen surface results in the ejection of electrons

and ions. The primary Ga+ ions scan the surface and penetrate into the surface to depth of

10’s of nm; this penetration depth varies from material to material. During ion milling the

secondary electron yield is much higher than secondary ion yield, due to this reason FIB is

mostly used in the secondary electron mode. Secondary-ion images are of better quality at

medium to high probe currents (medium currents include 70 pA and 350 pA, while high

beam currents are 1000 pA and above). This arises due to the generation of more secondary

ions with these beam currents. At very low beam currents such as 4 pA and 1 pA the images

generated by secondary ions are of low quality due to the small amount of secondary ions

generated. Figure shows schematically the generation of secondary electrons and ions during

FIB milling.

Figure 1.6: Mechanism of simple milling and image generation

1.4.8. Simple ion Milling and Ion milling with Enhanced Etch

Ion milling is one of the most important functions of the FIB, it depends on several

factors; two very important factors are overlap of collision cascades and re-deposition of

sputtered material. Material removal is performed in the FIB by either physical sputtering or

physical sputtering combined with chemicals. The material removal rate in physical

sputtering depends on various parameters such as dwell time, overlap etc. Release of an

appropriate gas close to the surface while performing milling leads to enhanced removal of

material.


12

1.4.9. Material Deposition with the FIB

There are two types of FIB deposition: chemical assisted deposition, and direct

deposition. Chemical assisted deposition uses chemical reactions between the substrate

surface and the molecules adsorbed on the surface. On the other hand, direct deposition uses

low energy ions from the source.

In chemical assisted deposition a gas carrying the element to be deposited is delivered

through a capillary nozzle, various gases are used for this purpose. Several materials such as

W, Pt, Al, Ta, C, SiO2 can be deposited in the FIB. FIB deposition process requires that gas

molecules delivered to the surface are adsorbed in adequate numbers and their binding energy

to the surface is sufficiently large. An incoming ion produces collision cascade effect in the

target surface. If the adsorbate binding energy (BE) is less than its decomposition energy

(DE), the gas molecules on the surface will be sputtered away and no deposition will occur.

Only if the binding energy is sufficiently large compared to the decomposition energy of the

molecule that efficient deposition will take place.

In FIB direct deposition, ions are bombarded on the surface with such low

accelerating voltages that instead of sputtering the material these ions stick to the surface.

The limitation of this method is its slow deposition rate. FIB direct deposition has several

advantages over the FIB gas assisted deposition one such advantage is that the purity of the

deposited film is higher because of the high vacuum conditions maintained during the

deposition process.

Applications of the FIB

FIB is mostly used in the semiconductor industry thus major applications are related

to the semiconductor industry. Repair of lithographic mask, defect analysis and circuit re-

wiring are major industrial applications of the FIB. FIB is also used for TEM specimen

fabrication and approximately 100 nm thick sections for TEM can be prepared with this

technique.

Chapter 2

Sample Preparation (Cleaning, Deposition & Maskless Lithography)

“Scientists investigate that which already is;

Engineers create that which has never

been.'”

Albert Einstein

“The engineer's first problem in any design

situation is to discover what the problem

really is.”

“Strive for perfection in everything you do.

Take the best that exists and make it better.

When it does not exist, design it.”

Sir Henry Royce

Chapter 1: Sample preparation

14

2.1. Introduction

The first step in thin film device fabrication technology is the preparation of smooth,

clean and undamaged substrate surfaces. The task is to remove commonly encountered

contaminants like oil, fingerprints, airborne articulate matter, residues from manufacturing

and packaging, etc.

In addition, for electrical characterizing of these nano structures it is essential to

provide suitable contacts in order to connect them to the measuring devices. To accomplish

this task a special procedure was evolved and Al pads were fabricated in the vicinity of the

nano structures using mask less lithography combined with deposition and chemical etching.

This chapter describes in detail the cleaning procedure and the steps involved in making good

quality Al pads of 3mm x3mm size connected by micro strips on glass substrate.

2.2. Substrate cleaning

Borosilicate glass slides of dimension of 2.5cm x2.5cm were used in the present study

to prepare the electrodes. Standard procedures of either RCA or ultrasonic cleaning were used

for cleaning the samples. The steps involved in these two procedures are given below

Ultrasonic cleaning:

Clean the substrate with soap solution using a soft brush.

Ultrasonicate in DI water for 5 min.

Ultrasonicate in acetone for 5 min.

Ultrasonicate in trichloroethylene for 5 min.

Ultrasonicate in acetone for 5 min.


Ultrasonicate in ethyl alcohol for 5 min.

RCA Cleaning:

Clean the substrate with soap solution using a soft brush.

Rinse thoroughly in DI water.


Immerse in RCA solution for 20 minutes at 75~90 °C (RCA Solution consist DI


15

water, H2O2 and liquid NH3 in the ratio of 5:1:1).

Rinse thoroughly in DI water.


In both the cases the water droplets were removed by use of hot air blower. Finally,

the substrates were kept at 120 °C for 20 minutes in an oven. Several dozens of samples were

made using the above procedures. All the samples were kept in air tight desiccators.

2.2. Electrodes

Generally any electronic device requires two or three electrodes for making contact

separated by a finite distance. Therefore, fabrication of electrodes is the next important step

in sample preparation. Two types of organic materials were used in the present study,

namely, AlQ3 and pentacene. As mentioned in the previous chapter, the choice of these

materials was made because they are at the focus of the research activity for making organic

devices in Samtel Centre at IIT Kanpur and also around the world. The choice of the

electrodes depends on the fact that to obtain single carrier conduction with high work-

function metals as Al (Au), one needs to choose an organic semiconductor which is good

electron (hole) transport layer. Generally, Al is used as contact material for AlQ3 and Au is

used for pentacene. The electrodes were fabricated first by depositing thin film of Al or Au

on the glass substrate and by subsequent patterning using maskless lithography and finally

etching using either chemical process or Focused Ion beam Milling.

Fig 2.1: Schematics of Al or Au coated glass substrate


16

2.2.1. Aluminium and Gold Deposition

Al and Au thin films were deposited using thermal evaporation techniques on cleaned

Glass substrates through a mask shown in Fig. 2.2. The deposition was done at a rate of 0.1 to

0.7 Å/s under clean and high vacuum of the order of 5 x 10-6

mbar. For Al deposition “Glove

Box Coating Unit” was used and for Au deposition the “12-inch Coating Unit” at Samtel

Center was used. A schematic drawing of sample deposited with thin Al or Au film is

presented in Fig. 2.1.

Figure 2.2: Masks used for Al and gold thin film deposition

In case of Au thin films a buffer layer (≈10 to 20 Å) of Cr was evaporated at a very

slow rate 0.1 to 0.2 Å/s for improving the adhesion. The Al samples were annealed in an oven

at 120 0C and those of Au were annealed at 80

0C for three hours. The surface exhibited a

smooth surface after annealing.

Figure 2.3: Schematics shows strip of chrome gold coated on glass substrate used in making micro electrodes.


17

Figure 2.4: Camera image of actual sample

2.2.2. Maskless Lithography

To generate the required pattern on the Al thin film, Maskless lithography technique

was used. This technique involves basically three main steps namely photo resist coating,

exposing & developing. These steps are described below:

Step 1: Photo Resist coating

Spin coating of the photo resist on the substrate.

Baking of the sample in an oven at 60 0C for 2 hrs.

Step 2: Exposing

Opening of the bit map image of the required pattern on the screen of maskless

lithography instrument.

Exposing of the sample with UV light for an optimized time.

Step 3: Developing

Just after the exposure dip the sample into Shipley developer solution for an

optimized time.

Dry it by hot blow of air.

Note that there are two kinds of photo resists (PR) namely positive photo resist (PPR)

and negative photo resist (NPR). In case of PPR the exposed area gets removed after

developing, but in case of NPR the unexposed area gets removed after developing the sample.

In this work Shipley 1818 PR has been used which is a PPR. Following table gives a typical

value of the parameters involved in the above three steps.


18

Data Box 2.1: Optimized data used in maskless lithography.

Fig. 2.5 shows the bit map images that were used in exposing the samples during

maskless lithography. Image 1 was used in optimizing the Maskless lithography and chemical

etching parameters, image 2 was used to generate the strips of size varying from 15 µm to 45

µm connected with pads of the size 3mm×3mm, image 3 was used to generate the micro

channels say of 1 µm and 2 µm and image 4 was used to generate the micro channel of

different size on the same sample.

Figure 2.5: Bit map images used in Maskless lithography

Fig. 2.6 shows the microscopic image of one of the strips that has been generated

using maskless lithography technique.

Spinning rate=5 krpm

Time allowed for spinning=50 s.

Baking temperature=60 0C

Baking Time=2 hrs.

UV Exposure Time=9 s.

Dipping time in the developer=40 s.


19

Figure 2.6: Sample after PR coating, exposure & developing but before etching, (1) at 50 times magnification & (2) at 100 times magnification

2.2.3. Etching

To remove the aluminium/gold from the exposed area of the sample to generate the

desire pattern, chemical etching was employed. Aluminium has been removed by using the

etchant having the composition shown in the following Table 2.1 and gold has been removed

using acquaregia (1 HCL: 3 HNO3). The solutions were diluted and the process was

optimized to obtain sharp etching.

Table 2.1

Reagent / Formula % Amount Actual Amount for 50 ml

Phosphoric Acid (H3PO4) 65 32.5 ml

Nitric Acid (HNO3) 15 7.5 ml

Acetic Acid (CH3COOH) 5 2.5 ml

DI Water 15 7.5 ml


20

Figure 2.7: Sample after etching but before removal of PR, (1) at 50 times magnification & (2) at 100 times magnification

Figure 2.8: Setup used for chemical etching

A set-up was designed and assembled for controlled chemical etching of aluminum

which is shown in Fig. 2.8. It consists of a beaker having etchant whose temperature is

maintained at 5 to 0 0C. The cooling is done by passing liquid nitrogen through a copper coil

wound on the beaker. The etchant is constantly stirred using a magnetic stirrer for achieving

uniform temperature of the electrolyte and for removal of the material.

2.2.4. Removal of Photo Resist

After removing Al, the photo resist has to be removed from the substrate to open up

the Al pattern on glass.


21

Figure 2.9: Optical microscopic image of the sample after removal of PR, (1) at 50 times magnification & (2) at 100 times magnification

Figure 2.10: Schematic that has been generate using maskless lithography; (1) interdigitated electrodes separated by 90 µm, (2) interdigitated electrodes having different channel length ranges from 15 to 45 µm on the same sample, (3) thin

strips ranges from 15 to 45 µm of Al connected with large pads (4) electrodes made of gold thin film having channel length of 1 µm

The photo resist can be removed by dipping the sample in formic acid (HCOOH)

solution for one hour. Alternatively the photo resist can also be removed much faster by using

acetone. In the former case the sample has to be cleaned using ethyl alcohol while in the later

case it is not necessary. A typical microscopic image of one of the sample after removal of

PR is shown in Fig. 2.9.

Using above technique different kinds of patterns of aluminum and gold were made

on glass substrates. These patterns (See Fig. 2.10-3) were then used for making nano and


22

micro level interdigitated electrodes using FIB milling technique. Interdigitated electrodes

having separations of a few microns were also fabricated using the above technique. The

images of these electrodes are shown in Fig. 2.10.

2.3. Summary

Thin film deposition, maskless lithography and chemical etching process have been

used to make interdigitated electrodes of Al and Au having micro meter separation on clean

glass substrates. It may be noted that the smallest electrode separation that can be achieved

by mask less lithography followed by chemical etching is about a few tens of micrometer. In

order to fabricate electrodes having smaller separation of about a micron to few tens of

nanometer the process of ion beam milling using a focused ion beam has to be used. This has

been discussed in the next chapter.

Chapter 3

Electrodes Fabrication Using

FIB and its Suitability with

Organic Molecules (Nano & micro electrodes fabrication using FIB, device fabrication)

“In the spirit of science, there really is no

such thing as a "failed experiment." Any

test that yields valid data is a valid test.”

Adam Savage quotes

“Physics does not change the nature of the

world it studies, and no science of behavior

can change the essential nature of man,

even though both sciences yield technologies

with a vast power to manipulate the subject

matters.”

Pope Paul VI quotes

Chapter 3: Electrode Fabrication Using FIB & its suitability with organic molecules

24

3.1. Introduction

As mentioned in the previous chapters, the main aim of this work is to make good

quality interdigitated nano electrodes of different materials on glass substrate for

characterization and device making using FIB. It may be pointed out that we have undertaken

design and fabrication of interdigitated electrodes to enhance the sensitivity of the

measurements. This chapter describes in detail the fabrication steps and characterization of

interdigitated nano electrodes fabricated using FIB. The basic pattern (Fig. 2.10.3) made by

maskless lithography and chemical removal is the starting point for the generation of

interdigitated electrodes by FIB.

3.2. Interdigitated Nano Electrode Fabrication Using

Focused Ion Beam

The beam of Ga ion source in FIB is pixel sensitive; therefore a bit-map image of

suitable pixel counts is required for any kind of pattern. A bit map image that generally has

been used in this project to make interdigitated nano electrode is show in figure (3.2).

Fig 3.1: Basic pattern that’s to be created


25

Figure 3.2: Bit map image used for milling the interdigitated electrode

This bit map image has been made by assuming 10 pixels= 10 nm and made to get the

interdigitated nano electrodes of 40 nm spacing between the electrodes, electrode of 1.5

micrometer length and 250 nm width each.

3.2.1. Interdigitated Nano Electrode of Aluminium Using FIB

The pre prepared sample of Aluminium on Glass substrate (Fig. 2.10.3) was used to

fabricate electrodes having separation of few tens of nanometers. The milling parameters are

given in below.

Data Box 3.1: Milling parameter to make Aluminium nano electrode

The electron and ion beam images of the patterns formed after FIB milling are shown

in Fig 3.2. Electrical separation between the portions A and B shown in the figure is

achieved by ion milling the lateral portion. This is shown in Fig. 3.2.4, Fig 3.5 and 3.6 shows

Milling Parameters for Al Nano Electrode:

Beam Voltage=30 kV

Beam Current=50 pA

Magnification=5221

HFW=24.5

Dwell Time= 4 ms

Scanning=Bottom to Top

Total time Taken=46 s


26

electrodes having separation of 41 nm and 16 nm respectively. This is the smallest

selectrode separation that has been achieved so far using FIB.

Figure 3.3: SEM images of the interdigitated electrodes

Figure 3.4: An interdigitated nano electrodes having narrow gap of 16 nm


27

Figure 3.5: An interdigitated nano electrode having narrow gap of 41 nm

3.2.2. Interdigitated Nano Electrode of Gold Using FIB

Gold nano electrodes were fabricated using similar procedure of patterning used for

aluminium. Due to large difference in the characteristics of Aluminium and Gold the milling

parameters differ a lot. The milling parameters used for Au are given below.

Data Box 3.2: Milling parameter to make Gold nano electrodes

Milling Parameters for Al Nano Electrode:

Beam Voltage=30 kV

Beam Current=20 pA

Magnification=5221

HFW=25.6

Dwell Time=1 ms


Total time Taken=32 sec


28

Figure 3.6: Nano electrodes pattern on Gold

The SEM images of Au electrodes fabricated by FIB are shown in Fig. 3.7. These

images show that electrodes with sharp boundaries and uniform separation can be made by

FIB. However the quality of the electrodes depends on the parameters used. Fig. 3.8 shows

images of the Au electrodes made under much higher ion beam current of 1 nA. The

deterioration of the electrodes is clearly seen. The images suggest partial melting and

coagulation of Au islands on the glass substrate.

Fig (3.7) shows that due to excess beam current the Gold thin film melts locally and

destroyed the nano patterns.

Figure 3.7: Melting of gold film due to excess beam current


29

3.3. Characterization of Nano Electrode

It is necessary to characterize the electrodes for Before going to fabricate any device

using the above fabricated nano electrode one should test the quality of these electrodes that

these are really cutout completely and show no short (resistance should be very high) by any

means. This is done by I-V measurement on Keithly instrument. Detail discussion about

electrode characterization can be found in Result and Discussion Chapter i.e. in Chapter 4.

3.4. Suitability of Electrode with Organic Materials

In this section we discuss the suitability of the nano electrodes made of Aluminium

and Gold for measurements of electrical properties of organic materials and for use in the

devices made up of these materials. The sketch of a typical device which employs the

electrodes fabricated by FIB milling is shown in Fig. 3.8. This device uses Alq3 as active

material with Al electrodes. Before such device is made it is necessary to check the

suitability of the electrode for the given active material. To check the suitability of Al

electrodes we have used Alq3 for measurements and for Au electrodes pentaceen has been

used. The electrode gaps were filled by these materials by thermal evaporation and I-V

characteristics have been measured to infer the suitability and effectiveness of electrodes for

these organic materials.

Figure 3.8: Basic structure of an organic device

Glass Substrate

Dielectric Layer

Alq3

Drain (Al) Source (Al)

Gate (Gold)


30

3.4.1. Aluminium Electrodes

A thin layer (100 nm) of Alq3 molecules has been deposited on the FIB made

interdigitated electrodes having separation of 16 nm and 41 nm using thermal evaporation

techniques under the high vacuum. A low evaporation rate of 0.3 to 0.5 Å/s was used.

Optimized evaporation parameters used for deposition of thin film of Alq3 small molecule

organic material are given below.

Data Box 3.3: Deposition parameter to deposit Alq3 on aluminium nano electrodes I-V measurements were performed immediately after deposition of the film since the

organic materials degrade very fast with time. I-V characteristics deviate as compared to the

traditional thin film devices and do not show saturation as we increase the voltage up to 30 V.

The current increases gradually up to about few nA. At this stage the reason is not fully

know. Enhancement in the current (about two orders of magnitude) has been observed when

the electrodes were first coated with a thin layer of LiF (about 10 A) before Alq3 deposition.

The deposition parameters for LiF deposition are given below.

Data Box 3.4: Deposition parameter to deposit LiF on aluminium nano electrodes

Deposition Parameters of Alq3:

Ultimate Vacuum=2.2×10-6

mbar

Current passing through boat= 25 amp

Maximum rate of deposition=0.5 Å/s

Thickness of deposited thin film=1080 Å

Deposition Parameters of LiF:

Ultimate Vacuum=2.2×10-6

mbar





31

Figure 3.9: Steps that are followed in making all devices


32

3.4.2. Gold Electrodes

The gold electrodes were also tested in similar fashion using pentaceen. In this case

also the characteristics remain same except that we get much higher current as compared to

Alq3. The deposition parameters for pentaceen are given below.

Data Box 3.5: Deposition parameter to deposit pentacene on gold nano electrodes 3.5. Summary

This chapter described the fabrication of interdigitated electrodes by FIB and the

preparation of samples for I-V measurements. The following diagram summarizes the steps

involved in the entire fabrication process. The discussion of I-V characteristics forms the

subject of the next chapter.

Deposition Parameters of Pentacene:

Ultimate vacuum=2.1×10-6

mbar




Chapter 4

Electrical Characterization:

Results and Discussions (I-V Characteristics of electrodes and devices)

“New opinions often appear first as jokes

and fancies, then as blasphemies and

treason, then as questions open to

discussion, and finally as established

truths.”

“The power of accurate observation is

commonly called cynicism by those who

haven't got it.”

“Progress is impossible without change,

and those who cannot change their minds

cannot change anything.”

George Bernard Shaw quotes

Chapter 4: Electrical Characterization: Results and Discussions

34

4.1. Fabrication of Nano Electrode Using FIB

In this chapter, we describe the electrical results obtained using nano electrodes

fabricated using FIB and micro electrodes using maskless lithography. Organic material such

as Alq3 and pentacene are deposited on these electrodes for the study of I-V characteristics.

The parameters for fabrication of Al and Au electrodes are given Box 4.1 and 4.2

respectively.

Data Box 4.1: Milling parameter for Aluminium nano electrodes

Data Box 4.2: Milling parameter for Gold nano electrodes 4.2. I-V Characteristics of Al and Gold Nano Electrodes

4.2.1. I-V Characteristics of Al Nano Electrodes

After fabricating the electrodes, they were checked by taking I-V measurement. Fig

4.1 and 4.2 show the typical I-V characteristics only for those electrodes which are of good

quality and can be used for device fabrication.

Milling Parameters for Al Nano Electrodes:

Beam voltage=30 kV

Beam current=50 pA

Magnification=5221

HFW=24.5

Dwell time= 4 ms


Total time taken=46 s

Milling Parameters for Au Nano Electrodes:

Beam voltage=30 kV

Beam current=20 pA

Magnification=5221

HFW=25.6

Dwell time=1 ms


Total time taken=32 s


35

0.000 0.001 0.002 0.003 0.004 0.005 0.006

0.00E+000

5.00E-008

1.00E-007

1.50E-007

2.00E-007

2.50E-007

3.00E-007

IV Characteristic of

41 nano electrodeC

UR

RE

NT

(A

mp

)

VOLATAGE (Volts)

Linear fitted Curve

Slope of the straight line=20 K Ohms

Blue: 1st

Run

Green: 2nd

Run

Black: 3rd

Run

Red: 4th Run

Figure 4.1: I-V characteristics of Al nano electrodes having channel separation 29 nm

In several of our initial trials fluctuations were observed in the current. This may have

been due to some Ga ion or similar impurities shortening the channel. However after a few

measurements they burn out, and the I-V characteristics becomes very smooth. In this case

we see the resistance between the electrodes is approximately (≈20 kΩ) and is ohmic. We

have encountered such low resistance contacts often in the early part of our effort. Hence it

was realized that the fabricated contacts need be tested thus before proceeding further to

deposit the organic material.

0.0 0.2 0.4 0.6 0.8 1.0

0.00E+000

1.00E-010

2.00E-010

3.00E-010

4.00E-010

5.00E-010

6.00E-010

7.00E-010

Cu

rre

nt (A

mp

)

Voltage (in Voltage)

Slope(R) =1.69×10^9 ohm

I-V of Electrode

without any material

Figure 4.2: I-V characteristics curve for Al nano electrodes having channel separation 16 nm


36

Analysis of the plot shown in fig 4.2 shows that resistance between is 1.69×109 ohm,

which is obviously very high, showing that the quality of electrode is very good and there is

no shorting between channels.

4.2.2. I-V Characteristics of Gold Nano Electrode

I-V characteristic curve for Gold nano electrode having a channel separation of about

100 nm is shown in Fig 4.3.

Figure 4.3: I-V characteristics cure for Gold nano electrodes having channel separation 100 nm

The contacts show a resistance of 2.33×1012

Ω, which is again very high, and suitable

for proceeding further in the process of fabrication.

4.3. Devices Made with Aluminium Electrode

Interdigitated Nano electrodes of aluminium of different channel length (16 nm & 41

nm) have been made using FIB to test their suitability with Alq3 organic material. The results

have been compared with micro electrodes made through Maskless Lithography Technique

and FIB Milling.


37

4.3.1. Aluminium Nano Electrode Devices

The fabricated electrodes are first checked for any short by measuring the I-V

characteristics. Then a thin layer of about 100 nm of Alq3 small molecule is deposited on the

electrode by thermal evaporation in a high vacuum chamber. Two devices having channel

length of 16 nm and 41 nm are tested to find the effect of channel length “L”.

Figure 4.4: I-V characteristics of Alq3 with Al nano electrodes 16 nm apart

Fig 4.4 is a typical plot of I-V characteristics of an Alq3 device with an electrode

separation of 16 nm. The fitting parameters are also indicated in a table as inset in the figure.

Note that the power of V is 2.3. For a single carrier device operating in the space charge

limited current regime it is expected to be 2 when the mobility is field independent.

Fig 4.5 shows similar results for a device with an electrode separation of 41 nm. The

exponent of V obtained from fittings in the two cases is 2.3 and 1.7 respectively, which is

reasonably close to an expected value of 2.

From the above results we find that:

Current in 16 nm channel device corresponding to 4 volt =1.29×10-9

Amp

Current in 41 nm channel device corresponding to 4 volt =4.73×10-11

Amp

0 1 2 3 4 5

-2.00E-010

0.00E+000

2.00E-010

4.00E-010

6.00E-010

8.00E-010

1.00E-009

1.20E-009

1.40E-009

1.60E-009

Y = 1.29789732E-9

Equation:

y = a + b*x^c

a 8.0351E-12 ±1.9891E-12

b 5.2975E-11 ±5.1861E-12

c 2.3 ±0

Cu

rre

nt (A

mp

)

Voltage (Volts)

16 nm electrode


38

The ratio of the current is approximately proportional to the inverse ratio of the

electrode separation. Hence, clearly I is proportional to 1/d3, where d is the electrode

separation. This is a proof of the fact that the observed current is truly SCL in nature.

0 2 4 6 8 10

0.00E+000

5.00E-011

1.00E-010

1.50E-010

2.00E-010

Y = 4.73989623E-11

Equation:

y = a + b*x^c

a 5.5584E-12 ±7.3381E-13

b 3.8545E-12 ±1.8299E-13

c 1.70912 ±0.02029

Cu

rre

nt (A

mp

)

Voltage (Volts)

41 nm electrode

Figure 4.5: I-V characteristics of Alq3 with Al nano electrodes at 41 nm separation

4.3.2. Aluminium Micro Electrodes Devices

To further test the dependence of the length scale micro electrodes of aluminium were

fabricated using the Maskless Lithograph Technique and FIB. A thin layer of nearly 100 nm

of Alq3 small molecule was deposited in each case and I-V characteristics were measured

using Keithley 4200. The results are shown in Fig 4.6 and 4.7.


39

-2 0 2 4 6 8 10 12 14 16

-1.00E-009

0.00E+000

1.00E-009

2.00E-009

3.00E-009

4.00E-009

5.00E-009

6.00E-009

7.00E-009

8.00E-009

Equation:

y = a + b*x^c

a 6.8734E-10 ±1.3758E-10

b 2.3111E-11 ±1.3461E-12

c 1.99 ±0

Cu

rre

nt (A

mp

)

Voltage (Volts)

Y = 2.89172583E-9

60 m Channel Device made on Aluminium electrode

Figure 4.6: I-V characteristics of Alq3 with Al micro electrode separated by 60 µm

0 2 4 6 8 10 12

-1.00E-009

0.00E+000

1.00E-009

2.00E-009

3.00E-009

4.00E-009Equation:

y = a + b*x^c

a 8.8324E-11 ±1.1241E-10

b 3.762E-12 ±3.1852E-12

c 2.74912 ±0.34176

Cu

rre

nt (A

mp

)

Voltage (Volts)

Y = 2.04437466E-9

90 m Channel Device made on Aluminium electrode

Figure 4.7: I-V characteristics of Alq3 with Al micro electrode separated by 90 µm

From these plots we see that there is no significant change in the nature of

characteristics except that there is an increase in the order of current. A comparison of current

for different channel length shown:

Current in 60 µm channel device corresponding to 10 volt =2.89×10-9

Amp


40

Current in 90 µm channel device corresponding to 10 volt =2.04×10-9

Amp

Thus we see that current is nearly 1.5 times greater in 60 µm channel device than in

90 µm device, so we can say that it nearly follows the dependence as proportional 1/d3.

Further in these devices we get the voltage exponent 1.99 and 2.7 respectively which again

very close to 2. The contact conductivity in these cases most probably is limited due to the

damages created in the process of milling at high current.

4.4. Devices Made with Gold Electrode

Since Aluminium surface is more likely to get oxidized and form a very high

resistance at the interface. We have chosen gold as the contact material as well. To obtain

single carrier conduction with a high work function metal as gold, one need to chose an

organic semiconductor which is a good hole transport layer. Hence we have chosen pentacene

as the material for the experiments. For this case also micro and nano electrodes have been

tested.

4.4.1. Gold Nano Electrodes Devices

By following the same steps as in the case of Al, nano electrodes of Gold were

fabricated, and Pentacene layer of about 180 nm was deposited using thermal evaporation

technique.

Two typical plots of I-V characteristics are shown in fig 4.9 and 4.10. The I-V

characteristics in these cases are nearly piecewise linear. The dependence on length of the

channel should distinguish the two regimes. Experiments in this direction need be carried out

in future. It is often found empirically that at low voltages

Figure 4.8: Device geometry used in the fabrication of micro-electrodes


41

0 2 4 6 8 10

0.00E+000

2.00E-012

4.00E-012

6.00E-012

8.00E-012

Cu

rre

nt (A

mp)

Voltage (Volts)

V

V'

Figure 4.9: I-V characteristics of pentacene with Au nano electrodes separated by 100 nm

0 5 10 15 20

0.00E+000

5.00E-012

1.00E-011

1.50E-011

2.00E-011

Cu

rre

nt (A

mp)

Voltage (Volts)

V

V'

Figure 4.10: I-V characteristics of pentacene with Au nano electrodes separated by 100 nm


42

4.4.2. Gold Micro Electrodes Devices

We have also fabricated devices of Gold at a micro level using FIB and by depositing

a thin layer of pentacene on them.

0 1 2 3 4 5

-2.00E-008

0.00E+000

2.00E-008

4.00E-008

6.00E-008

8.00E-008

1.00E-007

1.20E-007

1.40E-007

1.60E-007

1 m Channel Device made Gold electrode

Equation:

y = a + b*x^c

a 1.235E-9 ±7.735E-10

b 5.6371E-9 ±5.9295E-11

c 2.1 ±0

Cu

rre

nt (A

mp

)

Voltage (Volts)

Y = 1.05300382E-7

Figure 4.11: I-V characteristics of pentacene with Au micro electrodes separated by 1 µm

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0.0

1.0x10-8

2.0x10-8

3.0x10-8

4.0x10-8

5.0x10-8

6.0x10-8

Data: Data2_B

Model: Allometric2

Equation:

y = a + b*x^c

Weighting:

y No weighting

Chi^2/DoF = 2.6785E-18

R^2 = 0.99313

a -1.226E-9 ±2.0576E-10

b 6.4551E-9 ±1.7572E-10

c 2.11245 ±0.02524

Cu

rre

nt (A

mp

)

Voltage (Volts)




43

Figure 4.11 and 4.12 show typical characteristics for Au/pentacene/Au device with a

contact separation of 1 µm. At this large separation, the characteristics are parabolic

indicating space charge limited conduction in the sample. The voltage exponent is fairly close

to 2. Fig 4.13 and 4.14 show similar results for channel separation of 2 µm. Comparing the

magnitude of current for example at 4 V, is 12 times in the case of 1 µm separation that of

sample having a separation of 2 µm. This is higher than the expected ratio according to I ~ 1 /

d3. This is due to trap controlled space charge limited current.

In contrast to 1 µm device, the voltage exponent is 3.3 and 2.99 for the two cases

shown for 2 µm separation. In space charge current regime a value more than 2 (and in this

range) is obtained when mobility is exponentially field dependent as

In pentacene, mobility is known to be field activated.


0 1 2 3 4 5-2.00E-009

0.00E+000

2.00E-009

4.00E-009

6.00E-009

8.00E-009

1.00E-008

1.20E-008

1.40E-008

1.60E-008

Y = 8.60879497E-9

Equation:

y = a + b*x^c

a -4.2296E-11 ±3.9133E-11

b 8.5052E-11 ±8.2591E-12

c 3.30555 ±0.07219

Cu

rre

nt (A

mp

)

Voltage (Volts)



44

0.0 0.5 1.0 1.5 2.0 2.5 3.0-5.00E-010

0.00E+000

5.00E-010

1.00E-009

1.50E-009

2.00E-009

2.50E-009

3.00E-009

3.50E-009

4.00E-009


Parameter Values

Fitting Equation

Device 1st of Sample 2nd C

urr

en

t (A

mp

)

Voltage (Volts)

y=a+bxc

a=-2.53*10-12

b=1.39*10-10

c=2.99

Fitted Curve

Expt. Curve


The observation of limited current carrying capability in these structures can be

attributed to lower mobilities in the materials grown in constrained spaces. The mechanism

controlling the mobility in such cases is not clear at the moment. It is known that the

microstructure of organic materials can be very different on process parameters, especially

near the electrodes [ ].

4.5. Peaks Due to Charging

In this section, we describe some unusual behavior during the study of I-V

characteristics. When the bias is scanned between -10 to 10 Volt, no hysteresis or peak are

observed in I-V characteristics. However, when the bias is increased typically beyond 15V,

peaks occur in the first quadrant only during the ramp down scan as shown in Fig 4.16. This

behavior was observed for the most of the electrodes studied in this work. Though we do not

understand the origin of occurrence of these peaks, they are most probably due to charging of

inhomogenities in the channel during positive voltage ramp. More detailed studies are

required to understand this phenomenon.


45

-10 -5 0 5 10

-2.00E-010

-1.50E-010

-1.00E-010

-5.00E-011

0.00E+000

5.00E-011

1.00E-010

1.50E-010

2.00E-010

2.50E-010

Cu

rre

nt in

Am

p

Voltage in Volts

Figure 4.15: I-V upto 10 volts

Current

Voltage

Figure 4.16: I-V of Alq3 with Aluminium nano electrodes having channel separation 16 nm, showing some interesting peak

Chapter 5

Summary and Conclusions (A summary of results obtained)

“A conclusion is simply the place where

someone got tired of thinking.”

Ambrose Bierce quotes

“Don't fear failure so much that you refuse

to try new things. The saddest summary of a

life contains three descriptions: could have,

might have, and should have.”

Thomas Carlyle quotes

“Success builds character, failure reveals

it”

Dave Checkett quotes

http://thinkexist.com/quotes/dave_checkett/

Chapter 5: Summary and conclusions

47

Summary and Conclusions

In this thesis we explore the possibility of fabrication of organic thin film based

devices using nano and micrometer interdigitated electrodes using Focused Ion Beam. The

difficulties in fabricating nanometer and micrometer separated electrodes using FIB have

been studied. Such electrodes have been successfully fabricated using gold and aluminium

thin films. The I-V characteristics of devices fabricated by depositing organic layers such as

Alq3 and pentacene have been studied. The efforts are aimed at eventually developing

organic thin film transistors with small channel length using these techniques.

Aluminum electrodes with separate of 16nm and 41nm have been fabricated, and

Alq3 has been deposited on them using high vacuum deposition. These devices are compared

with electrode separation of hundreds of nanometers fabricated using maskless lithography.

On the basis of both Voltage and distance dependence, the current is proved to be space

charge related current in these devices, though the corresponding mobility in nanometric

channels seems to be much less than that expected in the bulk.

The fabrication of gold interdigitated electorodes is found to be easier than that of aluminium.

To test the efficacy of gold interdigitated electrodes, pentacene as a hole transporting layer

has been used. Channel lengths in nanometer and micrometer range have been fabricated

from gold thin films using FIB. For nanaometric size devices, the observed I-V

characteristics with pentacene is piecewise-linear showing the evidence of ohmic and defect

related space charge conduction. The devices with a separation of 1μm, however, shows

space charge limited current with a square dependence on voltage. Devices with a separation

of 2μm showed a voltage exponent between 3 and 4, which is normally observed for field

enhanced mobility in these materials. We conclude that the quality of material and the

conduction paths in nanometer sized channel lengths is different from those grown between

micrometer separated electrodes. We also report observation of unusual peaks in the first

quadrant of I-V characteristics which are dependent on voltage scan direction at higher

voltages for both aluminium and gold electrodes. The origin of these peaks is not understood.

48

References

1

2

3

4

5

6

7

8

9

10

11

S. Matsui, and Y. Ochiai, “Focussed ion beam induced deposition of superconducting nano wires,”

Nanotechnology, vol 7, pp. 247, 1996.

P. D. Prewett and G. L. R. Mair, “Focused ion beams from Liquid metal ion source,” Chapter 1 & 6,

John Wiley & Sons, 1991.

R. A. D. Mackenzie, “Focussed ion beam application to solid state devices,” Jour. Vac. Sci. Technol.,

vol 9, pp. 2561, 1991.

L. Bischoff and J. Teichert, “Application of highly focused ion beam,” Materials Science Forum, vol.

248-249, pp. 445-450, 1997.

Josephine B. Lee, Paul C. Chang, J. Alexander Liddle and Vivek Subramaniam, “10-nm Channel

Length Pentacene Transistors,” IEEE Trans. Elect. Dev., vol 52, pp. 1874-1879, 2005.

M. D. Austin and S. Y. Chou, “Fabrication of 70 nm channel length polymer organic thin-film

transistors using nanoimprint lithography,” Appl. Phys. Lett., vol. 81, pp. 4431–4433, Dec. 2, 2002.

J. Collet, O. Tharaud, A. Chapoton, and D. Vuillaume, “Low-voltage, 30 nm channel length, organic

transistors with a self-assembled monolayer as gate insulating films,” Appl. Phys. Lett., vol. 76, pp.

1941–1943, 2000.

J. H. Schon and Z. Baoa, “Nanoscale organic transistors based on self-assembled monolayers,” Appl.

Phys. Lett., vol 80,pp. 847-849, 2002.

Yen-Yi Lin, David J. Gundlach, Shelby F. Nelson and Thomas N. Jackson,” Pentacene-Based Organic

Thin-film Transistors,” IEEE Trans. Elect. Dev., vol 44, pp. 1325-1331, 1997.

Thesis on “Nano fabrication using focused ion beam” by “Adnan Latif,” Darwin College, Cambridge.

FIB Manual for FEI 200.

Documents

Master of Technologyhome.iitk.ac.in/~zainul/documentsopen/Zainul_MTech...My stay at IIT Kanpur make a lot of friends who shared with me everything, Chatting, Orkuting, and a lot of