E SC 521 Pattern Generation at the Nanoscale Wook Jun Nam · 2018. 5. 6. · NEB31 dot patterns on Si wafer. The dimension of the dot is 100nm in diameter ... resulting in random

E SC 521

Pattern Generation at the Nanoscale

Wook Jun Nam

Unit 2

Advanced Lithography

Techniques: Direct Writing

Lecture 1C

Electron Beam Lithography III

Outline

• Ebeam Lithography (EBL) Operation

• Ebeam Resist

Preferred Conditions for High Resolution Ebeam

Lithography

• High electron energy

• Small scanning field

• Low beam current

• Low-sensitivity resist

• Thin resist layer

• Optimized resist process

• Light and conductive substrate material

• Stable environment

Copyright 2014 by Wook Jun Nam

pattern size

pattern density

required dose

required dose

resist thickness required dose

acceleration voltage required dose

resist sensitivity required dose

Summary of Dose Dependencies


• Resist sensitivity

– Dose needs to be high enough to expose resist, but low

enough that scattered electrons don’t wash out the

pattern

– Critical dose is very geometry-dependent; can vary by

more than an order of magnitude

• Throughput

– Lower dose = higher throughput, obviously

– Use a more sensitive resist to speed up your write, if

needed

– Using bigger spot size of the beam could be an option

(e.g., 100nm size beam for 100nm features)Copyright 2014 by Wook Jun

Nam

More Consideration for Dose

Dose Array

• Write a test pattern using different dose

Low doses

High doses dropped cell


Dose Array (continued)

250 uC/cm2

350 uC/cm2 450 uC/cm2


Typical Optical Lithography Process Steps

Define

Function

Partition

Design

Design

Simulation

Mask/Reticle

Fabrication

Exposure/

irradiation

Wafer

Processing

Layout

design

Process steps

for making

photomasks

Process steps for

pattern transfer


Pattern Fracturing

Complete cell

layout

More complex cells…………. ………….

Sub cells Sub cells Sub cells

unit cells unit cells unit cells

………….

………….

………….

………….


Cell Hierarchy

Complete cell

layout (Top cell)


• Turns your CAD files (e.g., gds format) into EBL tool (e.g.,

EBPG-5200) readable format files (“fracturing”).

• Divides pattern into mainfield and subfields

• Performs (optional) proximity effect correction

• Miscellaneous other functions (e.g., resolution, step size,

curved or circular shape adjustment).

• Softwares: CATS, LayoutBeamer

Pattern Fracturing


Example of Pattern Fracture


Layout Design

Draw

patterns

Generate design file format (e.g., GDSII

(graphic data system), OASIS (open

artwork system interchange standard),

CIF (caltech interchange format), DXF

(drawing exchange format)).

Fracture the gds file

(separate patterns) & create

files which a writing tool

(e.g., ebeam, laser writer)

can understand- (e.g.,

CATS, Layout Beamer)

Create job file &

exposure


Job File

• We have discussed what information we need to write a

pattern using EBL.

• The job file is a summary of the conditions converted to the

form EBL tool can understand.

• The conditions included in the file are:

– Holder

– Location of the origin

– Dose

– Beam current (spot size)

– Fractured pattern (resolution, field, step size)


Job File (continued)

• When you have more than one layers to write, you can

submit multiple job files (old systems) or a combined job file

(recent version).

• Identification labels (e.g., dose, name of pattern) can be

added, which are not included in your fractured pattern.

• Additional alignment markers can be added.


Outline

• Ebeam Lithography (EBL) Operation

• Ebeam Resist

Ebeam Resist

• Positive Resist:

– PMMA

– ZEP

• Negative Resist:

– NEB31

– HSQ

• Other Resist:

– PMMA co-polymer,

MMA/MAA

Nanofabrication: Principles, Capabilities, and Limits, Zheng Cui, Springer (2008)


Poly (methyl methacrylate): PMMA

• The most common positive ebeam resist.

• High resolution

• Good chemical stability (long shelf life/ easy for handling)

• Wide process latitude.

• Relatively high dose.

• Poor resistance in dry etching


PMMA/LOR for Undercut

• Developers:

– PMMA: Methyl isobutyl

ketone (MIBK)

– LOR: CD 26

PMMA is inert to CD 26 !!

http://www.intechopen.com/books/lithography/high-energy-electron-beam-lithography-for-nanoscale-fabricationCopyright 2014 by Wook Jun Nam

ZEP 520A

• High resolution

• Good chemical stability (long

shelf life/ easy for handling)


• Needs relatively low dose

(about 3 times lower than

that of PMMA).

• Good resistance in dry

etching.

• Form a natural undercut.

• Poor adhesionZep520A dot patterns on LOR2a coated

Si wafer. The dimension of the dot is

100nm in diameter and 250nm in height.

(100nm of spot size and 320uC/cm2 of

dose)


NEB 31

• Negative resist

• High resolution

• Good chemical stability (long

shelf life/ easy for handling)


• Needs relatively low dose

(about >10 times lower than

that of PMMA) sensitive to

white light !


etching.

• Poor adhesion

Due to the poor adhesion

NEB31 dot patterns on Si wafer. The

dimension of the dot is 100nm in diameter

and 250nm in height. (10nm of spot size

and 70uC/cm2 of dose)


HSQ (XR-1541)

• Hydrogen silsesquioxane (HSQ)

• The structure of these siloxanes

resembles a cage (a), but the

chemical reactions that produce

HSQ resins do not fully form these

cages, resulting in random

structures of various sizes (b).

• HSQ exhibits good solubility in

most hydrocarbon and siloxane

solvents, with the exception of

alcohols and water, which can

promote gelation.

HSQ polymers are three-dimensional

molecules whose structure resembles a)

a cage; however, formation of HSQ resins

results in b), random structures

http://www.electroiq.com/articles/sst/print/volume-41/issue-5/features/deposition/understanding-hydrogen-silsesquioxane-based-

dielectric-film-processing.html Copyright 2014 by Wook Jun Nam

HSQ (XR-1541)- continued

• Fourier transform infrared (FTIR)

spectroscopic analysis of HSQ-

based films deposited on silicon

clearly identifies the H-Si-O

molecular bonding network.

• Successful processing schemes

for HSQ films control the reactions

that result in dissociation of the Si-

H bond and subsequent

molecular rearrangement and

formation of SiO2 bonds. The

reactions are promoted by

thermal/UV decomposition and

oxidation.

http://www.electroiq.com/articles/sst/print/volume-41/issue-5/features/deposition/understanding-hydrogen-silsesquioxane-based-

dielectric-film-processing.html

A typical IR spectrum of a HSQ-deposited

oxide film.


HSQ (XR-1541) - continued

• Negative resist

• High resolution


etching.

• Poor adhesion

• Poor chemical stability

(short shelf life/ very

sensitive for handling)

• Relatively high dose (similar

or higher than that of

PMMA).HSQ dot patterns on Si wafer. The

dimension of the dot is 100nm in diameter

and 170nm in height. (100nm of spot size

and 750uC/cm2 of dose)


Summary

• Dose array is a process for exposing a pattern using wide range of dose conditions.

• The results of does array reflect all the experimental valuables (e.g., proximity effects, resist contrast), and provide the dose condition needed for the actual pattern writing.

• There are two types of resists, negative and positive, for ebeam lithography.

• The chemically amplified resist needs small dose, but causes more line edge roughness (LER).

Documents

E SC 521 Pattern Generation at the Nanoscale Wook Jun Nam · 2018. 5. 6. · NEB31 dot patterns on Si wafer. The dimension of the dot is 100nm in diameter ... resulting in random