Enhancement of the Lithographic Process using Supercritical Carbon Dioxide in the Development Step

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ABSTRACT

ZWEBER, AM Y ELIZABETH. Enhancement of the Lithographic Process Using Supercritical Carbon Dioxide in the Development Step. (Under the direction of Dr. Ruben G. Carbonell.)

As microelectronic features reach the 45 nm- node and beyond, extreme ultraviolet (EUV)

lithography and other techniques are being explored as the next generation lithographic

process. The current and continuing challenges faced by these practices are the reduction of

image collapse and line width roughness (LWR). This dissertation covers two techniques

utilizing CO2 in the development step with the means to reduce these challenges: a CO2

drying method to remove the development rinse solution and a carbon dioxide compatible

salt / supercritical carbon dioxide (CCS / scCO2) direct development.

The CO2 drying method uses scCO2 to reduce the surface tension of the water rinse

solution after development. This method has potential to reduce image collapse but not in a

timely manner due to the water solubility in CO2 being too low for chemical removal and yet

too high for mechanical removal.

On the other hand, the CCS / scCO2 direct development of standard EUV photoresists

achieves reduction of both line width roughness and image collapse in high aspect ratio

features. The CCS / scCO2 one step development takes advantage of the scCO2 low surface

tension to help prevent image collapse and the plasticizing properties of CO2 in polymers to

assist in reduction of line width roughness. The CCS, a fluorinated ammonium salt,

associates with the photoresist Brönsted acid groups in the unexposed regions promoting the

photoresist dissolution into the scCO2 rich phase, which results in a reverse development.

A simplified rate model and quartz crystal microbalance (QCM) rate experiments were

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the high pressure CCS / scCO2 solution. At 5mM CCS, the zero order photoresist removal

confirmed that the photoresist phase transfer, photoresist mass transfer, or both were the rate

limiting steps which was the premise used for the rate equation. Increasing temperature

(35ºC-50ºC) at a density of 0.896 g/ml was found to increase the removal rate due to phase

transfer limitations and followed an Arrhenius behavio r (Ea = 79.0 kJ/mol). Increasing

pressure (4000-5000 psig) at 40ºC also increased the removal rate due to an increasing CO2

solubility parameter and phase transfer coefficient, but at 50ºC pressure had little effect on

the removal rate where phase transfer limitations were no longer present. When the CCS

concentration was in global excess of Brönsted acid groups 2400:1 at 5mM, the CCS / scCO2

developer removed the photoresist linearly with time. At lower CCS concentrations but still

in global excess of Brönsted acid groups, the photoresist removal slowed (0.5mM CCS,

~240:1) or was prevented (0.03mM CCS, ~15:1) due to partitioning of the CCS between the

CO2 rich phase and the film.

The fundamentals of CO2 and CCS adsorption onto the SiO2 substrate and CO2

absorption into the photoresist film were also investigated using the high pressure QCM at

35°C and 50°C. Adsorption studies of scCO2 were comparable to other flat plate geometry

studies. The adsorption of CCS was found to begin at 8.0 MPa with a temperature of 35°C

and at 9.4 MPa at a temperature of 50°C, where the adsorption of the CCS was driven by

entropy. The absorption of CO2 into the glassy photoresist resin was also measured with

QCM and found comparable to CO2 absorption in glassy polystyrene for 35°C and 50°C.

The diffusion behavior during CO2 absorption was found to be comparable to fluorescent

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ii

DEDICATION

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BIOGRAPHY

Amy Zweber, formerly Amy Geissler, was born and raised by Irvin and Colleen Geissler

in Bloomington, MN. During her time as an undergraduate at the College of Saint Benedict

in St. Joseph, MN, she spent four months studying abroad in Salzburg, Austria. In 2002, she

graduated with a major in chemistry and minor in mathematics from the College of Saint

Benedic t receiving a Bachelor of Arts degree. In aspirations of a higher degree and warmer

weather, she moved to North Carolina to attend graduate school at North Carolina State

University. In 2004, she married Jeffrey Zweber and received a Master of Science from

North Carolina State University in the Department of Chemical Engineering. In 2007, she

defended this dissertation and received a Doctorate of Philosophy also in the Department of

Chemical Engineering at North Carolina State University. Following graduation, Amy

Zweber will begin her professional career at IBM in Burlington, VT as a Mask Development

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iv

ACKNOWLEDGMENTS

The material is based upon work supported by the STC Program of the National Science

Foundation under Agreement No. CHE-9876674. This work was also performed in part at

the Triangle National Lithography Center (a member of the National Nanotechnology

Infrastructure Network) which is supported by the National Science Foundation under

Cooperative Agreement ECS-0335765.

I would like to thank my advisor, Dr. Ruben Carbonell, for the numerous opportunities to

attend and present at conferences and also for guiding me through my research and the PhD

program. I would also like to thank Micell Technologies, Inc. for engaging me in the CCS /

scCO2 development project, and especially Dr. Mark Wagner for his valuable and ongoing

help and discussion. I appreciate the time and effort of the people in the Triangle National

Lithography Center, especially Dave Vellenga, Myrick Peacock, Dr. Ginger Yu, and Joan

O'Sullivan, and also the people in the Analytical Instrumentation Facility, especially Dr. Dale

Batchelor, Chuck Mooney, and Roberto Garcia. I would like to thank the graduate students

in the NSF STC and in the Kenan CO2 center, especially Ke Wang, Yazan Hussain, Mary

Kate Boggiano, Dave Frankowski, Dawei Xu, Alan Chang, and Shaun Tanner, for the ir help

in the laboratory. For their help and support in my core courses, I thank Victoria Wagner and

Alison Julson. I am grateful for the help, encouragement and relief from work provided by

the other graduate students, especially Kristen Comfort, Don Comfort, Julie Willoughby,

Paul Chin, Michael Weiger, Sabrina Tachdjian, Alex Marchet, Josh Michel, Brian Prevo, and

many more. I am indebted and thankful to my husband's family for taking me in as a family

member when I needed them most and to my own family for keeping me involved with the

happenings in Minnesota when I wasn't around and giving me unconditional support and

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TABLE OF CONTENTS

Page

LIST OF TABLES ...viii

LIST OF FIGURES ...ix

1. LITHOGRAPHIC PROCESS BACKGROUND AND CHALLENGES ...1

1. 1 Motivation...1

1.2 Lithographic Process Overview...3

1.2.1 Spin Coat...4

1.2.2 Post Applied Bake...5

1.2.3 Exposure...6

1.2.4 Post Exposure Bake ...7

1.2.5 Development ...8

1.2.6 Hard Bake ...8

1.2.7 Etch ...8

1.2.8 Strip ...9

1.2.9 Photoresists ...9

1.3 Current Lithographic Challenge s and Literature Review ...15

1.3.1 Image Collapse...15

1.3.2 Line Width Roughness...19

1.4 Supercritical CO2 in Microelectronics ...21

1.4.1 Properties and Benefits of Supercritical CO2...21

1.4.2 Methods of a scCO2-Modified Development to Reduce Image Collapse and LWR...22

1.5 Conclusions ...27

References ...28

2. CCS / scCO2 DIRECT DEVELOPMENT PROCESS AND CHEMISTRY...33

2.1 Introduction...33

2.2 CCS / scCO2 Development Chemistry ...34

2.3 CCS / scCO2 Development Procedure with QCM as a Dissolution Rate Monitor ...36

2.3.1 Synthesis and Purification of CCS Components ...37

2.3.2 CCS Solution Preparation...37

2.3.3 QCM Apparatus and Measurements ...38

2.3.4 CCS Solution Flush...40

2.3.5 Conversion of Frequency Changes to Mass Changes ...41

2.4 Efficacy of CCS / scCO2 Solution Flush to Displace CO2...42

2.5 Typical Photoresist Removal Experimental Results ...44

2.6 Photoresist Dissolution Reproducibility ...50

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vi 2.8 CO2 Drying Step and Adhesive Coating Effect on Photoresist

Development ...54

2.8.1 Photoresist Removal Monitored by QCM ...56

2.8.2 Photoresist Coated Wafer Development ...61

2.10 Conclus ions ...65

References ...66

3. THERMODYNAMICS AND KINETICS OF CCS / scCO2 DEVELOPMENT...69

Abstract ...69

3.2 Theory...71

3.3 Materials and Methods...76

3.3.1 CCS Cloud Point Curve Measurement ...76

3.3.2 High Pressure QCM Experiments...77

3.3.3 Determining Photoresist Dissolution Pressure...77

3.4 Results and Discussion ...78

3.4.1 Cloud Point Curve of CCS in CO2 and Photoresist Dissolution Curve in CCS/CO2...78

3.4.2 Temperature Effect on Photoresist Removal Rate ...79

3.4.3 Mass Transfer Effect on Removal Rate ...82

3.4.4 Pressure Effect on Removal Rate...84

3.4.5 CCS Concentration Effect on Removal Rate ...85

3.4.6 Polymer Type Effect on Removal Rate ...91

References ...94

4. SORPTION OF CO2 AND A CO2 COMPATIBLE SALT ONTO A BARE SIO2 SUBSTRATE AND INTO A PHOTORESIST FILM ...95

Abstract ...95

4.2 Materials and Methods...98

4.3 Results and Discussion ...99

4.3.1 CO2 and CO2 / CCS Adsorption onto a Bare SiO2 Substrate ...99

4.3.2 CO2 Absorption into the Photoresist Film ...103

4.3.3 CO2 Diffusion into the Photoresist Film ...105

References ...111

5. CO2 DRYING AFTER A TRADITIONAL DEVELOPMENT...114

5.2 Removing Water Using Dissolution or Mechanical Means ...115

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5.2.2 Shear Force Water Removal Model...125

5.3 CO2 Drying Procedure ...128

5.3.1 Processing Lines with Imminent Image Collapse...128

5.3.2 Drying Apparatus ...132

5.3.3 Wafer Processing ...133

5.3.4 Drying Procedure ...135

5.3.5 SEM Analysis ...136

5.4 CO2Drying Results ...137

5.4.1 Image Collapse Reproducibility...137

5.4.2 CO2 Dry and N2 Dry ...141

5.4.3 CO2 Dry and N2 Dry With and Without Helium Displacement ...144

References ...149

6. CONCLUSIONS...151

APPENDICES ...154

Appendix A...155

Appendix B...158

Appendix C ...160

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viii

LIST OF TABLES

Page

Table 2.1 Removal rate reproducibility ...51

Table 3.1 Estimated mass transfer coefficients...73

Table 3.2 Effect of temperature at constant CO2 density on photoresist removal rate...81

Table 3.3 Effect of pressure on photoresist removal rate at 40°C and 50°C ...84

Table 3.4 Effect of CCS concentration on photoresist removal rate ...86

Table 3.5 CCS partition coefficient ...89

Table 3.6 Estimates of the ratio of CCS molecules in the film to photoresist reactive groups ...90

Table 5.1 Peng-Robinson equation of state parameters for carbon dioxide and water...117

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LIST OF FIGURES

Page

Figure 1.1 Moore's Law of DRAM ½ pitch features over time as reported by

the ITRS 2005 ...1

Figure 1.2 The lithographic process ...4

Figure 1.3 Effect of spin speed and solvent content on film thickness ...5

Figure 1.4 Geometry pertaining to the numerical aperture of the focusing length ...7

Figure 1.5 Novolac structure ...10

Figure 1.6 DNQ light driven reactio n into an indenecarboxylic acid ...11

Figure 1.7 Difference in dissolution rates of exposed and unexposed novolac photoresist with and without DNQ allows for photoresist contrast...11

Figure 1.8 Acid production from the PAG after exposure followed by acid catalyzed photoresist deprotection during the post exposure bake ...12

Figure 1.9 Examples of photoresists for 248 nm lithography and 193 nm lithography ...13

Figure 1.10 EUV lithography tool prototype ...15

Figure 1.11 Profile and top view examples of image collapse with the EPIC-V41 193nm photoresist...16

Figure 1.12 Schematic of residual rinse solution between photoresist features...17

Figure 1.13 Scanning electron microscope image demonstrating line width roughness in the EPIC-V41 193 nm photoresist ...19

Figure 1.14 CO2 phase diagram...21

Figure 1.15 Methods for using scCO2 in the development step ...23

Figure 1.16 SEM images comparing EUV photoresist patterned with a 130 nm half-pitch...25

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x

Figure 2.1 CCS structure ...34

Figure 2.2 CCS reaction with the unexposed photoresist as seen on the top portion allows for photoresist dissolution, and the exposed photoresist as seen on the bottom portion prohibits photoresist dissolution ...35

Figure 2.3 QCM apparatus ...38

Figure 2.4 Mass fraction photoresist removed plotted with the CSTR model...44

Figure 2.5 Example of typical QCM photoresist removal results at 5mM CCS, 50°C, and 34.5 MPa ...45

Figure 2.6 Resistance-Frequency plots...49

Figure 2.7 Removal of photoresist film coated with dip and spin coating methods ...52

Figure 2.8 Removal of photoresist films coated using dip coating ...53

Figure 2.9 Optical microscope images and AFM images showing the bare QCM SiO2 substrate, a dip coated film, and a spin coated film ...54

Figure 2.10 AFM images of a developed QCM substrate with CO2 drying step and a developed pre-coated wafer with HMDS adhesive coating ...56

Figure 2.11 Sample holders used to position the QCM substrate and the silicon wafer vertically in the development cell during the experiment ...56

Figure 2.12 Removal of photoresist with time at 50°C and 34.5 MPa...58

Figure 2.13 AFM images of the photoresist film with varying process conditions of a CO2 drying step and HMDS adhesive coating...62

Figure 2.14 Contour AFM image plots with a 0° pitch and 315° rotation...64

Figure 3.1 Diagram of photoresist development ...76

Figure 3.2 Phase equilibrium analyzer diagram...77

Figure 3.3 CCS in CO2 cloud point and photoresist dissolution point for temperatures 35°C - 50°C ...79

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Figure 3.5 Arrhenius plot at constant density (0.896 g/ml)...81

Figure 3.6 Effect of mass transfer on photoresist removal...83

Figure 3.7 Effect of pressure on photoresist removal rate...85

Figure 3.8 Effect of CCS concentration at 50°C and 34.5 MPa...87

Figure 3.9 Example plots at 50°C and 34.5 MPa of low CCS concentration experiments ...88

Figure 3.10 Van't Hoff plot of partition coefficient temperature dependence ...89

Figure 3.11 Photoresist dissolution dependence on CCS concentration within the film...90

Figure 3.12 Comparison of the CCS / scCO2 development of various commercial EUV photoresists ...91

Figure 3.13 Removal of commercial photoresist compared to poly(hydroxystyene) and poly(acrylic acid)...92

Figure 4.1 Schematic depicting sorption experiments in this chapter...97

Figure 4.2 QCM apparatus ...99

Figure 4.3 CO2 adsorption onto the bare SiO2 substrate measured using QCM at 35°C and 50°C ...100

Figure 4.4 Weight gain on the QCM SiO2 surface due to CO2 and CO2 / CCS adsorption at 35°C and 50°C...102

Figure 4.5 Mass of CCS adsorbed as a function of pressure for 35°C and 50°C ...103

Figure 4.6 Comparison of CO2 absorption into polystyrene from the literature to CO2 absorption data into the standard EUV photoresist from this work at 35°C and 50°C ...105

Figure 4.7 Example experimental data used for calculating the diffusion coefficient of CO2 in the photoresist film at 50°C ...106

Figure 4.8 Example plot of M versus −1 t from the data in Figure 4.7 ...107

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xii Figure 4.10 Example plot of experimental data from Figure 4.7 and calculated

values of M/M_∞ versus t ...108

Figure 4.11 Diffusion coefficients in CO2 swollen polystyrene ...109

Figure 5.1 Surface tension of water in the presence of CO2 at temperatures 30-60oC ...115

Figure 5.2 Amount of water in CO2 plotted as mole fraction and water concentration...116

Figure 5.3 Flat plate geometry used to determine flux of water and IPA into fluid phase by convective force ...118

Figure 5.4 Initial geometry of CO2 flowing over water ...119

Figure 5.5 Effect of pressure on the flux of water/IPA into CO2 (T=40oC and CO2 velocity=0.0211 m/s) ...121

Figure 5.6 Effect of temperature on the flux of water/IPA into CO2 (P=9.7 MPa and CO2 velocity=0.0211 m/s)...122

Figure 5.7 Effect of CO2 velocity on the flux of water/IPA into CO2 (T=40oC and P=9.7 MPa) ...122

Figure 5.8 Convective Diffusion Model with mass transfer coefficients corresponding to laminar, turbulent, and jet flow...123

Figure 5.9 Jet removal of the water/IPA mixture from the high pressure cell using CO2...124

Figure 5.10 Schematic of CO2-rich phase flowing over liquid phase ...125

Figure 5.11 Shear force water removal model at 40oC and 9.7 MPa ...127

Figure 5.12 Test mask layout ...129

Figure 5.13 The critical aspect ratio for which the features will collapse...130

Figure 5.14 The critical aspect ratio is plotted for equal line and space features (l = d = 80nm) and for overexposed features (d = 80nm, l = 100nm) ...131

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Figure 5.16 Drying apparatus ...133

Figure 5.17 Wafer matrix for scanner exposure ...134

Figure 5.18 Sample holder ...135

Figure 5.19 Flowchart for conceptual picture of image collapse experiment program...137

Figure 5.20 ICE images show inconsistency in positions on wafer...138

Figure 5.21 Line and space strip and SEM image box size comparison...139

Figure 5.22 SEM images of the positions analyzed for a CO2 dried sample at 110 nm half-pitch...140

Figure 5.23 Percent image collapse as a function of position for three methods of drying 110 nm half-pitch lines and spaces...140

Figure 5.24 Drying with CO2 and N2...142

Figure 5.25 Concentration of water in CO2 and N2 phases at equilibrium for 323K ...144

Figure 5.26 Drying with CO2 and N2 with and without helium displacement ...146

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1

C

HAPTER

1 Lithographic Process Background and Challenges

1. 1 Motivation

Integrated circuits, or microchips, are used for many of today’s advanced electronic and

communications technology, including computers and cell phones. The integrated circuit

consists of a web of features that are created by a process called lithography, which assemble

and interconnect up to and beyond a million electronic components per chip. Consumer

demand for faster, higher storage devices drives the need to fabricate smaller integrated

circuit features. The precipitous reduction of feature size with time in the microelectronics

industry is often described by Moore’s Law (see Figure 1.1), which predicts that the typical

DRAM 1/2 pitch will decrease from 70 nm today to 45 nm by 2010. This dramatic feature

size reduction has been made possible by continuous improvements in the lithographic

process.

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Chapter 1: Lithographic Process Background and Literature Review

Two important issues facing litho graphy users today are the reduction of image collapse

and line width roughness (LWR). These issues will become increasingly problematic for

smaller features, where more stringent tolerances are essential. For example in regards to

LWR, the necessary critical dimension (CD) control for a 70 nm DRAM ½ pitch is 7.4 nm,

while the necessary CD control for a 32 nm DRAM ½ pitch is 3.3 nm. According to the

ITRS 2005 edition, the manufacturable solution for 3.3 nm CD control is not yet known.1

Image collapse and LWR adversely affect device performance and production yield, and thus

solving these problems is of utmost importance to the integrated circuit industry.

Beyond the technical issues of image collapse and LWR, lithography also consumes a

large quantity of solvents and chemicals. According to a study performed in 2002, the

production of a 2-gram 32MB DRAM microchip consumes 1.7 kg of chemicals and fossil

fuels, 32 kg of water, and 0.7 kg of elemental gases.2 In reaction to this concern, the

environmental, health, and safety section of the ITRS 2005 edition suggests reducing waste

from "wet" processes in lithography by further optimization of the current processes.1

The overarching goal of this project was to reduce the challenges resulting from

decreasing feature sizes and excessive material usage by converting the lithographic process

to a "dry" process. Previous work has focused on the synthesis of novel photoresists that are

soluble in both liquid and supercritical CO2, which allows the photoresist to be spin coated

with liquid CO2 and developed with supercritical CO2.3-5 The research for this dissertation

focused on the search for a "dry" process which would eliminate the need for new photoresist

materials and be more easily integrated into the current lithographic process. The objective

was to study the potential use of supercritical carbon dioxide (scCO2) in the development

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3 This chapter contains an introduction to the lithographic process, a description of how the

current lithographic challenges are being addressed in literature, and a section focusing on

using scCO2 as a means to reduce some of these challenges. The remaining chapters delve

into the results for using scCO2 in the development step using two different techniques. The

first technique is the direct development of a standard photoresist with a scCO2 and CO2

compatible salt (CCS) solution, and the second technique involves developing the photoresist

with a standard solution with subsequent drying using scCO2.

1.2 Lithographic Process Overview

The objective of lithography is to pattern a substrate (i.e., silica) with micro- and

nanostructures, where 25 or more lithographic process repetitions results in complete

fabrication of an integrated circuit.3 Lithography itself is a multi-step process: spin coat, post

applied bake, exposure, post exposure bake, development, hard bake, etch, and strip. (see

Figure 1.2). In this section, each step will be explained to provide a basic understanding of

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Figure 1.2. The lithographic process: left, negative tone processing; right, positive tone processing.

1.2.1 Spin Coat

In the first step, a uniform layer of a photosensitive polymer (photoresist) is deposited by

spin coating the material onto the substrate to be patterned. The thickness of the resulting

photoresist film is dependent on the spin speed and the solution viscosity giving the

lithographer control of the film thickness as demonstrated by Figure 1.3. As the spin speed

increases, the film thickness decreases due to centrifugal forces removing photoresist from

the surface before the film dries. As the viscosity decreases, the solution is less restrained

from spreading and the film thickness decreases. In Figure 1.3B, the effect of solution

viscosity on film thickness is represented by diluting the photoresist solution with solvent.

The more dilute solution is less viscous, which results in a thinner film. Typically,

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5

Figure 1.3. Effect of spin speed and solvent content on film thickness. Graph (A) shows the effect of spin speed on film thickness with the as received propylene glycol monomethyl ether acetate (PGMEA) solvent concentration. Graph (B) shows increasing concentrations of PGMEA or reduced viscosities of the photoresist solution and the film thickness corresponding with those concentrations.

The spin speed was 1300 rpm for 60 seconds. Both studies used thick film AZ50XT photoresist and were post applied baked at 95°C for 90 seconds before measuring the thickness with profilometry.

1.2.2 Post Applied Bake

The residual spin-coating solvent is removed from the photoresist film by heating the

wafer with a hot plate or oven during the post applied bake. The post applied bake raises the

photoresist temperature near its glass transition temperature (Tg), releasing trapped residual

solvent and densifying the photoresist film. This seemingly straightforward step has a strong

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length of the post applied bake.6-11 Initially, the residual solvent is removed exponentially

and the rate is dependent on the solvent volatility, where higher temperatures remove more

solvent by increasing the solvent volatility. As the film densifies the removal becomes

dependent on solvent mass transfer due to slowing solvent diffusion through the film. In

regards to adjusting the two parameters, the length of the post applied bake only needs to be

long enough to relax the polymer and remove the bulk of the residual solvent. The post

applied bake temperature has two constraints: the temperature must be above the Tg to

maximize the solvent removal and below the degradation temperature. More detail on how

the solvent content and the post applied bake affect the resulting features for specific

photoresist types will be discussed in the photoresist section.

1.2.3 Exposure

In the next step, the photoresist is exposed to light through a mask that contains the

pattern to be transferred to the wafer. The mask pattern is projected onto the photoresist and

the light creates a chemical reaction in the exposed regions. The smallest obtainable feature

size or resolution

( )

R in lithography is directly proportional to the wavelength of exposure

light

( )

λ and inversely proportional to the numerical aperture of the focusing length

( )

NA .12

NA k

R= λ

The numerical aperture (NA) is related to the refractive index of the atmosphere that the light

propagates (n) and the angle at which the light is focused onto the wafer (θ) (see Figure 1.4

for geometry).

θ

sin

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7 As a result, the wavelength of light used in lithography is one of the variables used to reduce

feature size by decreasing over time from 436 nm wavelength to the current 193 nm

wavelength exposure. Initially, the trend of reducing the wavelength of light made 157 nm

lithography appear to be the most logical next step in reducing the smallest possible feature

size, but this proposal was met by a variety of photoresist and exposure tool complications.

Instead, it is expected that future reductions in feature size will be made by increasing the

numerical aperture (NA) using techniques such as immersion lithography or eventually

leaving optical lithography for extreme ultraviolet (EUV) (13nm) lithography.

Figure 1.4. Geometry pertaining to the numerical aperture of the focusing length.

1.2.4 Post Exposure Bake

Following exposure, the wafer is baked in the next step called the post exposure bake.

The post exposure bake temperature is set near the glass transition temperature of the

photoresist to improve diffusion in the polymer resin and to propagate the photosensitive

reaction originally initiated by exposure. Even heat distribution is important to allow for

even diffusion and reaction propagation. During this step, the chemically modified regions

can be visually seen appearing on the photoresist. The chemical reactions occurring in the

photoresist during exposure and the post exposure bake vary from one photoresist to another

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1.2.5 Development

After the post exposure bake, the exposed regions of the photoresist differ chemically

from the unexposed regions, and a developing solution, commonly 0.26 N

tetramethylammonium hydroxide (TMAH), selectively removes one region, leaving the other

behind. In negative tone development, unexposed photoresist dissolves in the developing

solution and is removed. In positive tone development, the exposed photoresist is soluble in

the aqueous TMAH solution and removed during development. A common development

method is puddle development where a fixed volume is deposited on the wafer and spun off

with a rotating table. Water is used to rinse the remaining developer and is also spun off the

surface. In the experiments presented within this dissertation using the traditional

development, the photoresist is placed in a TMAH bath followed by a water rinse bath to

remove the base from the surface.

1.2.6 Hard Bake

An optional hard bake often follows to drive off residual water from the development

process. The lithographic steps beyond the hard bake were not executed for this research.

The rest of this section briefly explains them for clarity on the entire lithographic process

typically used in integrated circuit processing.

1.2.7 Etch

With the pattern now on the photoresist, the wafer is etched. The photoresist is resistant

to the etching process, so that only the exposed areas of the substrate are etched. The two

types of etching used are the dry and wet etch. The wet etch is more cost effective, but is

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9 nanometer size features. The components and conditions of each etch vary widely to

accommodate for the variety of substrates and photoresists used in the lithographic process.

1.2.8 Strip

Finally, the remaining photoresist is stripped from the wafer with a stripping solution,

leaving the patterned wafer behind. Stripping solutions are traditio nally a sulfuric acid and

hydrogen peroxide mixture. At this point the patterned wafer is ready for application of

dopants, metals, or other layers befo re repetition of the lithographic process.

1.2.9 Photoresists

The name photoresist comes from its purpose in lithography. The photoresist resin used

must be light sensitive ('photo') and resistant to the etching step ('resist') in order to produce

the desired features in the substrate. A variety of photoresists are available commercially,

which are each tailored to meet specific applications. Some photoresists are used to build

integrated circuits through multiple lithographic steps, while some are used for packaging

purposes and need high aspect ratio holes for electrical contacts. The photoresists ha ve

different chemistries, some which dissolve after being exposed to light (positive tone) and

some dissolve without exposure to light (negative tone). Overall, photoresists need to meet a

few basic requirements. The photoresist must have a built- in chemistry available that upon

exposure to a certain wavelength of light it reacts in such a way to provide contrast upon

development. The photoresist must be transparent to that same wavelength of light, so the

light can propagate through the photoresist material. The photoresist must also be rigid

enough to prevent defects. Finally, the photoresist must have a molecular weight below the

critical entanglement molecular weight, so that dissolution in the development solution is

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In the next few paragraphs, some typical positive tone photoresists used to make

integrated circuit features in the different feature size nodes will be discussed concerning

their application, chemistry, and basic properties. This explanation of lithography and the

photoresists involved is aimed to give the reader a general understanding of the process.

There are also many other techniques to improve the process, such as addition of a quencher

to the photoresist and use of double patterning, which are not covered here for simplicity.

Micron Size Features: G-line and I-line Lithography

Used mainly for micron sized features, both G- line (436 nm wavelength) and I-line (365

nm wavelength) lithography use novolac resists (see Figure 1.5). The no volac resin by itself

is not photoactive, so a second molecule is needed to provide contrast upon exposure to light.

The photoactive compound used is diazonaphthoquinone (DNQ) (see Figure 1.6). The

contrast in development between exposed and unexposed no volac comes from a difference in

dissolution rates. DNQ acts as a dissolution inhibitor, but after exposure to light the DNQ

photoproduct acts as a dissolution promoter. Thus, the unexposed regions of novolac and

DNQ have a slow dissolution rate in TMAH, while the exposed regions of novolac and the

indenecarboxylic acid photoproduct have a much faster dissolution rate in TMAH (see Figure

1.7).

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11

Figure 1.6. DNQ light driven reaction into an indenecarboxylic acid.12

Figure 1.7. Difference in dissolution rates of exposed and unexposed novolac photoresist with DNQ allows for photoresist contrast.13

The post applied bake plays an important role in processing thick novolac photoresist

films because it controls the film residual solvent content. In the novolac photoresist, the

DNQ light activated reaction requires water to form the photoproduct carboxylic acid, which

provides the photoresist with contrast. The amount of residual solvent in the film after the

post applied bake can determine how fast the film can hydrate. With thin novolac photoresist

films, film hydration is fast, but with thick photoresist films, the necessary hydration time is

approximately 90 minutes in a cleanroom where the humidity is well monitored. If the

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form interconnects.14 If the solvent content is too low, the novolac resist cracks or

delaminates under stress15-17 and the photospeed is reduced due to little to no rehydration.14

100 Nanometer to Micron Size Features: Deep Ultraviolet Lithography

Deep ultraviolet (DUV) lithography, which includes 248 nm and 193 nm wavelengths

coming from an excimer light source, is used to make features as small as 100 nm. The light

sources are weaker causing the photoresist resin contrast to be insufficient. A new technique

using chemically amplified resists and a photoacid generator (PAG) was implemented to

increase resist contrast. The chemically amplified polymer resins have protecting groups to

make them insoluble in TMAH. Once exposed to UV light each PAG produces an acid

which reacts with the photoresist to remove the protecting group. (see Figure 1.8) The

removed protecting group then produces another acid group. Since the acid is reproduced in

this catalytic process and can unprotect another polymer segment, the photoresist is

considered chemically amplified. Contrast is possible by the unexposed photoresist

insolubility and the exposed photoresist solubility in the developing solution.

(28)

13 Poly(hydroxystyrene) is typically used in 248 nm lithography due to its transparency in

248 nm wavelength light, while the photoresist typically used for 193 nm lithography is

poly(acrylic acid). One type of protecting group used for both types of photoresists is the

tertiary-butoxycarbonyl (t-BOC) group. Figure 1.9 shows poly(acrylic acid) and polystyrene

with and without the t- BOC protecting group. Beyond these basic photoresists, variations are

used that include copolymers and blends of these and other polymers.

Figure 1.9. Examples of photoresists for 248 nm lithography and 193 nm lithography. Poly(hydroxystyrene)-type photoresist with t-BOC protecting group shown on left and poly(acrylic

acid)-type photoresist with t -BOC protecting group shown on right.

Residual solvent remaining after the post applied bake affects chemically amplified

photoresist sensitivity and contrast with more intensity than those without chemical

amplification due to acid diffusion. Looking again at the chemistry, remaining solvent in the

film acts as an acid diffusion path facilitating the acid mobility.7, 8 Increasing the post

applied bake temperature or length reduces the residual solvent content and acid diffusion,

which positively affects image contrast. At the same time, the exposure dose required to

create these features (photoresist sensitivity) increases with decreasing solvent content due to

increased photoresist density and inhibition to diffusion.7-10 A compromise must be reached

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Sub-100 Nanometer Lithography

193 nm lithography can achieve features just below 100 nm, but its ability to make

smaller features is limited. Lithographers attempted to move to 157 nm lithography to

further decrease the wavelength used and improve the possible resolution, but the challenges

met were extreme. Instead the industry has moved to increase the numerical aperture (NA).

The latest technique is 193 nm immersion lithography, where the light propagates through a

median with a higher refractive index than air (n = 1.00) which inc reases the NA. Currently,

water (n = 1.33) is one medium being explored, and exposure tools are being developed to

accommodate for this change.

Once photolithography can not be extended further, extreme ultraviolet (EUV)

lithography is expected to follow. EUV lithography uses 13 nanometer light from a xenon

gas source directed onto a mask and then onto the photoresist coated wafer. Challenges

include producing mirrors which are smooth and reflective enough to provide adequate

exposure to the wafer. Sandia National Laboratories is one location where this idea is being

explored, and Figure 1.10 shows the concept and prototype of the EUV tool being developed.

The photoresist used in EUV lithography is similar to the 248 nm lithography photoresist in

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15

Figure 1.10. EUV lithography tool prototype. Images taken from Sandia National Laboratories website.

1.3 Current Lithographic Challenges and Literature Review

The lithographic process often needs to be repeated 25 or more times on each wafer to

fabricate a complete integrated circuit.3 Integrated circuit manufacturing is both energy and

material intensive,2 as mentioned previously. In addition to health and environmental

concerns, the use of organic solvents and water poses significant defect related challenges to

the production of the next generation 35 nm features, including image collapse and line width

roughness. In this section, both image collapse and line width roughness are discussed.

1.3.1 Image Collapse

Image collapse is one of the main concerns during the development process using

aqueous solvents. As the aspect ratio of deposited photoresist patterns increases, image

collapse is promoted. Image collapse, which is the deformation of photoresist structures, is

caused by weak adhesion of photoresist to the substrate, the weak structure of the photoresist,

and the high surface tension of the aqueous rinse solution.18, 19 As the rinse solution

evaporates from feature crevices, the surface tension of the remaining rinse solution can pull

(31)

collapse, the new development process must prevent image collapse for aspect ratios greater

than three, be of relatively low cost, and also be compatible with current development times,

which are about 2-3 minutes, to be attractive to industry.

Figure 1.11. Profile and top view examples of image collapse with the EPIC-V41 193nm photores ist.

A closer look at the theory behind image collapse helps identify the aspect ratio where

the features will collapse. A beam sway model for the resist feature and the surface tension

acting on it has been used to model the forces responsible for image collapse. The

expressions below describe the aspect ratio (A_c) at which the features will begin to collapse

(see Figur e 1.12 for geometry) :18

For o

90 << θ 3 / 1 2 ) cos(

24 _

    = = θ γH d E L H A c c

For o

90

≈

θ is

3 / 1 2 ) sin(

4 _

(32)

17 The parameters used to estimate the critical aspect ratio are the resin critical tensile modulus

(E_c), the distance between the features (d), the fluid surface tension (γ ), the height of the

feature (H), and the contact angle of the fluid on the resist wall (θ ). Looking more closely

at the parameters involved, increasing the strength of photoresist material (Ec) and reducing

the surface tension of the development rinsing solution (γ ) can help prevent image collapse

for high aspect ratio features.

Figure 1.12. Schematic of residual rinse solution between photoresist features.

Enhancing Photoresist Strength

Photocrosslinking the photoresist20 and using blended polymer additives, such as

fullerenes21, are methods that have been used to increase photoresist strength and rigidity.

Tanaka, et al., photocrosslinked photoresist features underwater by flood exposure and dried

the features with nitrogen gas. Flood exposure increases the crosslink density of the

photoresist preventing image collapse. Shibata, et al., blended C60 and C70 fullerenes to

positive tone ZEP520 photoresist.21 Aspect ratios of 5.5 were achieved for a 90 nm pitch

feature with a height of 250 nm, but the approach was expensive. The added cost of

fullerenes per wafer was approximately $28 if 10 weight percent fullerenes were used as

(33)

Reducing Surface Tension

Tanaka, et al., were some of the first researchers to reach the conclusion that image

collapse occurred during the drying step of development because they observed no image

collapse while the features remained in the rinse solution.18 The authors acted on their

finding by replacing the water rinse with tert-butylalcohol and then freeze-drying to remove

the solvent.22 Freeze-drying lowers the rinse solution surface tension to near-zero, and image

collapse was prevented. Finding the best rinse solution to freeze dry posed problems because

the rinse solution should not damage the resist, the volume change during the freeze-drying

process must be small, and the melting point of the rinse solution must be around room

temperature.22 Instead of freeze-drying, Yamashita chose to use perfluorohexane to rinse

water because of its low surface tension (10 dyne/cm) compared to water (72.8 dyne/cm).23

Aspect ratios of 5.25 were achievable with a ZEP resist, but outermost lines of the pattern

still collapsed. Extra or dummy lines outside of the necessary pattern were the recommended

solution, which hindered efficiency and reduced wafer real estate.

Simply adding surfactants to the aqueous rinse solution also reduces surface tension.

This method could be easily incorporated in a fabrication plant without changing current

equipment. The necessary surfactant concentration to reduce surface tension and the

compatibility with the photoresist have been studied by a variety of groups.24-26 High

concentrations of hydrophobic surfactants can swell and damage the photoresist, while low

surfactant concentrations do not reduce the rinse solution surface tension enough.26 The

reported surfactants that proved harmless to the resist only reduced the surface tension to

(34)

19 shows that rinsing with the current tested surfactants in water will not completely reduce

image collapse anymore than a less expensive organic solvent.

1.3.2 Line Width Roughness

As features become smaller, the acceptable tolerance for the edge roughness of the lines

decreases (see Figure 1.13). For example, when targeting 70 nm DRAM ½ pitch features,

the tolerated roughness (3σ) for the critical dimensions is 7.4 nm, whereas 32 nm features

have a tolerated roughness of only 3.3 nm.1 The tolerated roughness must be stringent

because it affects device performance and production yield, especially as critical dimensions

decrease.

Figure 1.13. Scanning electron microscope image demonstrating line width roughness in the EPIC-V41 193nm photoresist.

Line width roughness (LWR) and surface roughness are caused by multiple factors,

which makes reducing roughness challenging.28, 29 Reynolds, et al., have categorized the

various factors that affect total roughness assuming each contribution is independent of the

others:27

2 2

aerial t

developmen diffusion

shotnoise mask

total σ σ σ σ σ

(35)

In this approach, the square of the deviation from the mean dimension is viewed as having

contributions from mask deviations, PAG diffusion effects, development errors, and other

factors.

When Re ynolds, et al., examined experimentally the effects of exposure shot noise and

acid diffusion, they noticed that contrary to popular belief, acid diffusion plays a small role in

the overall roughness. They also found that varying post exposure bake temperature and

length had little to no effect on the overall roughness. The main contributions to LWR were

the developer concentration and mask defects. Yoshizawa and Moriya performed a similar

analysis of LWR for e-beam lithography, confirming the conclusions made by Reynolds, et

al.30 In 2002, Houle, et al., also concluded that post exposure bake temperature and length

does not control roughness.28,29 Instead, once the activation energy is met, acid catalysis

has no dependence on the post exposure bake temperature or length.

Two contributing factors absent on Re ynolds’ list are the chemical structure and

polydispersity of the photoresist. In 1993, Yoshimura, et al., found lower photoresist

molecular weight (Mw) and polydispersity (Mw/Mn) resulted in lower photoresist roughness,31

which was confirmed by Narasimhan, et al.32 Another factor Yoshimura, et al., found

contributing to LWR is photoresist rigidity. As the photoresist is developed, more pliable

resist materials are better able to flow and smooth photoresist surfaces, although reducing

photoresist rigidity can also increase image collapse. In general, the chemical structure of the

photoresist has a noticeable effect on LWR.

Overall, both image collapse and LWR are challenges that continually need to be

addressed in lithography as the feature sizes become smaller and aspect ratios become larger.

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21 or rinse solution surface tension with the goal to reduce image collapse and increase

photoresist pliability for LWR reduction. Both of those goals were achievable using

supercritical CO2 modifications to the development step of lithography.

1.4 Supercritical CO2 in Microelectronics

1.4.1 Properties and Benefits of Supercritical CO2

As a promising solvent, CO2 is currently used in a wide range of industries, such as

polymerization,33,34 dry cleaning,35,36 extractions,37 and pharmaceuticals.38 CO2 has

environmental and economical benefits because it is recyclable, non-toxic, inexpensive, and

nonflammable. When brought into the supercritical region, the density of CO2 is highly

tunable with the ability to reach values close to 1 g/ml, and the CO2 surface energy is near

zero. Beyond high density and low surface tension, supercritical CO2 (scCO2) has a

liquid-like viscosity, while maintaining a gas- liquid-like diffusivity, which can easily be accessed due to

the relatively low critical pressure and temperature (Pc = 7.375 MPa, Tc = 31°C) as seen in

the phase diagram in Figure 1.14.

(37)

In microelectronics, CO2 has been pursued in the cleaning or stripping,39 spin coating,3, 5

and development4 steps of lithography. Using CO2 specifically in microelectronics has the

valuable potential to replace traditional solvents reduc ing toxicity, the cost of the materials,

and the cost of their disposal.40, 41 Additional to these benefits the ultimate drive for utilizing

scCO2 is based on further improvements to the current microelectronic processes. For

example, the low surface tension, tunable density, and gas- like diffusivity of scCO2 were

employed to improve dirt and scum removal from small crevices between features in

photoresist stripping and cleaning. In this dissertation, these same properties were used in

the development step to reduce image collapse in the high aspect ratio photoresist features.

Reduction of LWR was realized by taking advantage of the ability of CO2 to act as a

plasticizer in the photoresist film, which improved photoresist pliability during development.

The following section focuses on the use of CO2 during the development step found in

literature, and introduces the CO2-modified development techniques used in this dissertation.

1.4.2 Methods of a scCO2-Modified Development to Reduce Image Collapse and LWR

Image collapse results from capillary forces occurring during the development step of

lithography. As explained in the current lithographic challenges section, an aqueous, basic

solution of TMAH develops the patterned features in the photoresist, a water rinse displaces

the TMAH, and the water is dried from the surface during photoresist development. The

surface tension of water tends to pull the features together as it dries within the crevices. At

a critical aspect ratio and spacing, the features will collapse from the surface tension.18, 19

Thus far, a few techniques have been implemented to reduce image collapse by reducing the

(38)

23 secondary rinse solution22-26 resulting in longer processing times and/or are limited by the

lowest value attainable for the surface tension, which is approximately 20 mN/m.

Supercritical carbon dioxide (scCO2), with properties of low surface tension, low

viscosity, and high density, has the potential to reduce image collapse and LWR in

lithography. Three approaches have been examined to use scCO2 in the deve lopment process

(see Figure 1.15). Method 1 and its variants use a CO2-soluble rinsing solution, such as

hexane, to displace water, and then subsequently remove the CO2-soluble rinse solution with

scCO2.42-45 Then, when only pure scCO2 remained in the high pressure development

chamber, the chamber was vented, and image collapse was prevented. The method

successfully reduced image collapse in the photoresist features but also added further solvent

waste, cost, and processing time. Development of only one wafer coupon took longer than

one hour, while traditional development is about 2-3 minutes for a TMAH development,

rinse with water, and spin dry.43

(39)

A second CO2 drying method was investigated which uses scCO2 to displace the rinse

water directly with the aid of a cosolvent or surfactant (see Figure 1.15, Method 2).46 A

cosolvent or surfactant is necessary for efficient water removal because water solubility in

pure CO2 is low. For example, the solubility of water is only 0.13 weight percent in CO2 at

25oC and 16.5 MPa.47 Zhang, et al., determined water removal using pure CO2 takes

approximately 62 minutes to dry water from a 12x4 mm wafer piece.46 Some of the

surfactants used for this method are fluoroether carboxylate45 and a nonionic methylated

branched hydrocarbon surfactant (TMN-6).46 The total development time when using

fluoroether carboxylate was not reported, and when using TMN-6, 26 minutes was needed.

Incorporating mechanical removal methods Zhang, et al., reduced total development time to

12 minutes by rinsing the wafer coupon with TMN-6 / scCO2, rotating the cell 45o, and

venting.46 Even though the process time was halved, the development time is still four times

longer than traditional development. It is evident that further improvements are necessary

before the semiconductor industry will consider scCO2 development modifications.

However, it is clear from their work that mechanical removal techniques should be examined

more thoroughly to speed up process throughput.

Later investigations in literature led to direct development with a scCO2-based

development solution, which was a more efficient approach to utilizing the beneficial

properties of scCO2 in lithography. Method 3 from Figure 1.15 depicts direct development

with scCO2. Previous work in this area focused on the synthesis and use of novel

photoresists that have been modified to increase their solubility in CO2, usually through

(40)

25 this area has been limited, however, since the studies used unproven resist systems and

significant improvements in the reduction of pattern collapse were not observed.

Recently, a new technique for the direct development of standard EUV resists with

development times comparable to the standard TMAH process has been reported.52-54 The

process uses a proprietary CO2 compatible salt in homogenous scCO2 (CCS / scCO2) solution

to dissolve the unexposed portions of positive tone EUV photoresists resulting in reverse

image development. Figure 1.16 contains scanning electron microscopy images of a

commercial EUV photoresist developed with TMAH (A), and the same photoresist

developed using the CCS / scCO2 method (B). The photoresist features developed with

TMAH have an aspect ratio of 4.5 and exhibit image collapse, while the photoresist

developed with CCS / scCO2 has features with an aspect ratio of approximately 12 showing

no image collapse. Aspect ratio up to four times greater than TMAH developed features

have been seen using the CCS / scCO2 development method. The CCS / scCO2 technique

takes advantage of the scCO2 properties of low surface tension to reduce image collapse, low

viscosity to develop small features, and high density to dissolve the photoresist.

Figure 1.16. SEM images comparing EUV photoresist patterned with a 130 nm half-pitch. The photoresist was developed with (A) the standard TMAH process (110 nm linewidth) and with (B) CCS

(41)

In addition to reducing image collapse, the CCS / scCO2 direct development method also

tends to reduce LWR due to the ability of scCO2 to plasticize the polymeric photoresist. In

Figure 1.17, the CCS / scCO2 developed sample is noticeably smoother than the comparable

sample developed in TMAH. Measurement of LWR using a three parameter model

developed by the Institute of Microelectronics (IMEL) has shown an average 30% reduction

in RMS roughness for CCS / scCO2 developed samples versus control samples developed

with standard the TMAH process.55 Multiple factors have been proven to affect LWR,

including resist properties, development conditions, and mask deviations.27, 30 -32, 56-57

Previously proposed remedies to LWR include using surface conditioner solutions after the

development step,58 using a new photoresist with a higher glass transition temperature and

higher transparency,59 and using a new photoresist with a lower molar mass.60 However,

direct development with CCS / scCO2 is a simpler method using commercial EUV

photoresists and developing the resist in a one step process, while reducing both roughness

and image collapse. The CCS / scCO2 direct development mechanism and material properties

(42)

27

Figure 1.17. SEM images comparing EUV photoresist developed with the standard TMAH process (A, B) and with reverse image development using CCS chemistry in scCO2 (C, D) in which the unexposed

area is removed and the exposed area remains. Images taken by Micell Technologies , Inc.

1.5 Conclusions

The current lithographic process and its challenges have been discussed in this chapter.

As feature sizes decrease, the lithographic process must be reinvented to address the ensuing

challenges. This dissertation explores the use of supercritical CO2 in the development step

using both methods 2 and 3. Method 3 CCS / scCO2 development was the main thrust for

this dissertation due to its fruitful results and potential application to industry. The chemistry

of the CCS and a basic understanding of the CCS / scCO2 development are given in Chapter

2. In Chapter 3, the effects of temperature, mass transfer, pressure, CCS concentration, and

photoresist type on the photoresist removal rate are discussed. In Chapter 4, the CO2 and

CCS sorption behavior on the substrate and in the polymer film, as well as, the CO2 diffusion

into the glassy polymer are explored. Finally, Chapter 5 discusses the attempt to further

reduce the process time of method 2 CO2-modified development, where the final results

show that with the current cell design the time could not be reduced due to the limitations of

(43)

References

(1) SEMATECH. International Technology Roadmap for Semiconductors. 2005, www.itrs.net/Common/2005ITRS/Home2005.htm.

(2) Williams, E. D.; Ayres, R. U.; Heller, M. Environ. Sci. Technol. 2002, 36, 5504-5510. (3) Hoggan, E. N.; Flowers, D.; Wang, K.; DeSimone, J. M.; Carbonell, R. G. Ind. Eng. Chem. Res. 2004, 43, 2113-2122.

(4) Boggiano, M. K.; DeSimone, J. M. Abs. Amer. Chem. Soc. 2005, 229, U1115-U1116. (5) Hoggan, E. N.; Wang, K.; Flowers, D.; DeSimone, J. M.; Carbonell, R. G. IEEE Sens. J. 2004, 17, 510-516.

(6) Ishii, T.; Matsuda, T. Jap. J. Appl. Phys., Part 2. 1991, 30, L1215-L1217.

(7) Asakawa, K.; Ushirogouchi, T.; Nakase, M. J. Vac. Sci. Technol., B. 1995, 13, 833-839. (8) Itani, T.; Yoshino, H.; Hashimoto, S.; Yamana, M.; Samoto, N.; Kasama, K. Jap. J. Appl. Phys., Part 1. 1996, 35, 6501-6505.

(9) Raptis, I. Jap. J. Appl. Phys., Part 1. 1997, 36, 6562-6571. (10) Raptis, I. Jap. J. Appl. Phys., Part 1. 2001, 40, 5310-5311.

(11) Mack, C. A.; Mueller, K. E.; Gardiner, A. B.; Sagan, J. P.; Dammel, R. R.; Willson, C.

G. J. Vac. Sci. Technol., B. 1998, 16, 3779-3783.

(12) Levinson, H. J. Principles of Lithography. SPIE Press: Washington, 2001. (13) Henderson, C. L., Integrated Circuits: A Brief History;

http://dot.che.gatech.edu/henderson/Introductions/microlithography%20intro.htm.

(14) Dammel, R. R. interview. 2005, February.

(44)

29 (16) Lei, H.; Francis, L. F.; Gerberich, W. W.; Scriven, L. E. AIChE J. 2002, 48, 437-451. (17) Lei, H.; Payne, J. A.; McCormick, A. V.; Francis, L. F.; Gerberich, W. W.; Scriven, L.

E. J. Appl. Polym. Sci. 2001, 81, 1000-1013.

(18) Tanaka, T.; Morigami, M.; Atoda, N. Jap. J. Appl. Phys., Part 1. 1993, 32, 6059-6064. (19) Namatsu, H.; Kurihara, K.; Nagase, M.; Iwadate, K.; Murase, K. Appl. Phys. Lett. 1995,

66, 2655-2657.

(20) Tanaka, T.; Morigami, M.; Oizumi, H.; Ogawa, T.; Uchino, S. Jap. J. Appl. Phys., Part 2. 1994, 33, L1803-L1805.

(21) Shibata, T.; Ishii, T.; Nozawa, H.; Tamamura, T. Jap. J. Appl. Phys., Part 1. 1997, 36, 7642-7645.

(22) Tanaka, T.; Morigami, M.; Oizumi, H.; Ogawa, T. Jap. J. Appl. Phys., Part 1. 1993, 32, 5813-5814.

(23) Yamashita, Y. Jap. J. Appl. Phys., Part 1. 1996, 35, 2385-2386. (24) Shin, K. S.; Lee, G. J. Photopolym. Sci. Technol. 2003, 16, 363-367.

(25) Jung, M. H.; Kim, H. W.; Lee, S. H.; Woo, S. G.; Cho, H. K.; Han, W. S. J. Photopolym. Sci. Technol. 2003, 16, 463-466.

(26) Tanaka, K.; Yamada, Y.; Masuda, S.; Kobayashi, M.; Kim, W. K.; Anyadiegwu, C.;

Padmanaban, M.; Dammel, R. R. J. Photopolym. Sci. Technol. 2004, 17, 527-534. (27) Reynolds, G. W.; Taylor, J. W. J. Vac. Sci. Technol., B. 1999, 17, 334-344.

(28) Hinsberg, W. D.; Houle, F. A.; Sanchez, M. I.; Wallraff, G. M. IBM J. Res. Dev. 2001,

45, 667-679.

(45)

(30) Yoshizawa, M.; Moriya, S. J. Vac. Sci. Technol., B. 2000, 18, 3105-3110.

(31) Yoshimura, T.; Shiraishi, H.; Yamamoto, J.; Okazaki, S. Jap. J. Appl. Phys. 1993, 32, 6065-6070.

(32) Narasimhan, B.; Wanat, S. F.; Rahman, M. D. Polym. Eng. Sci. 2000, 40, 2251-2261. (33) Desimone, J. M.; Guan, Z.; Elsbernd, C. S. Science. 1992, 257, 945-947.

(34) Desimone, J. M.; Maury, E. E.; Menceloglu, Y. Z.; McClain, J. B.; Romack, T. J.;

Combes, J. R. Science. 1994, 265, 356-359.

(35) McClain, J. B.; Betts, D. E.; Canelas, D. A.; Samulski, E. T.; DeSimone, J. M.;

Londono, J. D.; Cochran, H. D.; Wignall, G. D.; ChilluraMartino, D.; Triolo, R. Science. 1996, 274, 2049-2052.

(36) DeSimone, J. M.; Romack, T. J.; Betts, D. E.; McClain, J. B. US Pat. 5,944,996. 1999. (37) McHugh, M.; Krukonis, V. Supercritical Fluid Extraction: Principles and Practice. Butterworths: Boston, 1996.

(38) Cocero, M. J.; Ferrero, S. J. Supercrit. Fluids. 2002, 22, 237-245.

(39) Supercritical Fluid Cleaning: Fundamentals, Technology and Applications. Noyes Publications: Westwood, New Jersey, 1998.

(40) Jones, C. A.; Zweber, A.; DeYoung, J. P.; McClain, J. B.; Carbonell, R.; DeSimone, J.

M. Crit. Rev. Solid State Mater. Sci. 2004, 29, 97-109.

(41) Weibel, G. L.; Ober, C. K. Microelectron. Eng. 2003, 65, 145-152.

(42) Namatsu, H.; Yamazaki, K.; Kurihara, K. Micoelectron. Eng. 1999, 46, 129-132. (43) Goldfarb, D. L.; de Pablo, J. J.; Nealey, P. F.; Simons, J. P.; Moreau, W. M.;

Angelopoulos, M. J. Vac. Sci. Technol., B. 2000, 18, 3313-3317.

(46)

31 (45) Namatsu, H. J. Vac. Sci. Technol., B. 2000, 18, 3308-3312.

(46) Zhang, X. G.; Pham, J. Q.; Ryza, N.; Green, P. F.; Johnston, K. P. J. Vac. Sci. Technol., B. 2004, 22, 818-825.

(47) Weibe, R. Chem. Rev. 1941, 29, 475.

(48) Pham, V. Q.; Rao, N.; Ober, C. K. J. Supercrit. Fluids. 2004, 31, 323-328.

(49) Sundararajan, N.; Yang, S.; Ogino, K.; Valiyaveettil, S.; Wang, J. G.; Zhou, X. Y.;

Ober, C. K.; Obendorf, S. K.; Allen, R. D. Chem. Mater. 2000, 12, 41-48.

(50) Pham, V. Q.; Weibel, G. L.; Nguyen, P. T.; Ferris, R. J.; Ober, C. K. Abs. Am. Chem. Society. 2002, 224, U469-U469.

(51) Pham, V. Q.; Ferris, R. J.; Hamad, A.; Ober, C. K. Chem. Mater. 2003, 15, 4893-4895. (52) Wagner, M.; DeYoung, J.; Harbinson, C. Proc. SPIE. 2006, 6153, 61533W.

(53) Wagner, M.; DeYoung, J.; Harbinson, C.; Miles, M. Proc. SPIE. 2006, 6153, 615346. (54) DeYoung, J.; Wagner, M.; Harbinson, C.; Miles, M.; Zweber, A. E.; Carbonell, R. G.

Proc. SPIE. 2006, 6153, 615345.

(55) Constantoudis, V.; Gogolides, E.; Patsis, G. P.; Wagner, M.; DeYoung, J.; Harbinson,

C. Proc. SPIE. 2006, 6153, 61533W-1.

(56) Eder-Kapl, S.; Loeschner, H.; Zeininger, M.; Fallmann, W.; Kirch, O.; Patsis, G. P.;

Constantoudis, V.; Gogolides, E. Microelectron. Eng. 2004, 73-74, 252-258.

(57) Ma, Y. S.; Tsvid, G.; Cerrina, F. J. Vac. Sci. Technol., B. 2003, 21, 3124-3130.

(58) Zhang, P.; Jaramillo, M., Jr.; Rao, M. B.; Ross, B.; Horvath, B.; Wong, P.; Gehoel, W.;

Sinkwitz, S. Proc. SPIE. 2005, 5753, 1018-1023.

(59) Somervell, M. H.; Fryer, D. S.; Osborn, B.; Patterson, K.; Cho, S.; Byers, J.; Wilson, C.

(47)

(60) Chang, S. W.; Yang, D.; Dai, J.; Felix, N.; Bratton, D.; Tsuchiya, K.; Kewark, Y.;

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33

C

HAPTER

2 CCS / scCO

2

Direct Development Process and Chemistry

2.1 Introduction

The most efficient way to implement low surface tension scCO2 in the development step

is to use scCO2 directly as the developer. As discussed in Chapter 1, direct development with

scCO2 requires the photoresist material to be soluble in scCO2 either before or after exposure

with acceptable contrast and sensitivity, which usually entails synthesis of a new fluorinated

photoresist material. Recently a CO2 compatible salt (CCS) has been synthesized, which

allows standard commercial EUV photoresists to be developed directly in the CCS / scCO2

solution.1-5 CCS / scCO2 direct development not only reduces image collapse and LWR but

eliminates the use of a traditional tetramethylammonium hydroxide (TMAH) developer and

is compatible with existing standard photoresists.

The development of the photoresist with the CCS / scCO2 developing solution relies on

the selective reaction of the CCS, a fluorinated ammonium salt shown in Figure 2.1, with the

unexposed photoresist to produce high contrast features. The chemistry of this process

involves the reaction of the CCS with the photoresist rendering it soluble in CO2. Without

the CCS-photoresist reaction, the photoresist is insoluble in CO2. This chapter explains the

chemistry occurring during the process and then delves into the procedure used for

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Chapter 2: CCS / scCO2 Direct Development Process and Chemistry

Figure 2.1. CCS structure.

2.2 CCS / scCO2 Development Chemistry

The CCS / scCO2 development is both a reactive and diffusive process, involving the

same general four steps as TMAH development: 1) diffusion of the CCS from the bulk

developer solution to the polymer/solution interface and into the polymer, 2) reaction of the

CCS with the photoresist acidic groups, 3) dissolution of reacted photoresist, and 4) diffusion

of the dissolved photoresist into the bulk development solution. The fundamental reactive

step in the CCS / scCO2 process is the association of the CO2 soluble cation of CCS with

unprotected Brönsted acid groups in the resist. In Figure 2.2, the simplified, unexposed

photoresist depicted on the top left consists of protected and unprotected polymer pendant

groups as well as a photoacid generator (PAG). When the photoresist is developed with the

CCS / scCO2 developing solution, the CCS interacts with the Brönsted acid groups in the

polymer backbone to modify the photoresist and enable solubility in CO2. In the regions

exposed to light shown on the bottom left of Figure 2.2, the photoacid generator cleaves the

protecting groups and leaves excess acid in the film. During the CCS / scCO2 development,

the CCS-photoresist reaction is no longer favored due to Le Chatelier's principle, which

renders the resist insoluble in CO2. The high level of contrast and reverse image

development observed in CCS / scCO2 development is believed to be due to this inhibition of

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Chapter 2: CCS / scCO2 Direct Development Process and Chemistry

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