CAPITOLO
1
ABSTRACT
The main goal of this PhD was to understand the effects of the Pulsed Nanoimprint Lithography (Pulsed-NIL) technique on thermoplastic mate-rials, by performing the imprinting process on a range of different materials and using different stamps and characterization techniques to assess how this process affects chemically and physically the plastic materials. This study has been conducted comparing the imprinting results by the standard Thermal Nano Imprint Lithography technique (standard T-NIL) and by the Pulsed Thermal NIL (Pulsed-NIL).
The study on the material was necessary because, even if both techniques heat up the material at a temperature higher than the glass temperature and, with the application of the pressure, there is a replica of the pattern from the stamp to the sample, the parameters (in particularly, time and temperature of imprint) are very different.
The standard technique has a large diffusion in the research and fab-rication in micro and nanotechnology field. The Pulsed-NIL represents an innovation but it was important to demonstrate the capability to realize the imprint maintaining a quality of the structures same as the standard T-NIL and an absence or a limited existence of degradation of the materials due to the high temperature of the imprint.
At the “Istituto Officina dei Materiali” of CNR (IOM-CNR) laboratories at Elettra - Area Science Park (Basovizza, Trieste), January 2014, a first prototype (Thunder 1.0), for Pulsed- NIL of small areas (40∗40 mm2), based
The early months of the PhD were used for the design and executive drawings process, and also completed the assembling process of the machine used for the experimental part. The drawings are not in this thesis because they are not significant for the researching field.
To better understand the innovation introduced by ThunderNIL, it is important to have, at first, an overview of the standard process. In chapter 4 the technique is presented together with the intrinsic disadvantages.
Chapter 5 enters more in detail with the innovative technique.
Two chapters (6 and 7) are dedicated to the fabrication of the stamps for the imprint with both techniques. In particular the first is an introduction, whereas the second is a step by step logbook to better explain the processes involved, the work flow and time consuming behind the stamp. At the end of that chapter also the main parameters of two examples of imprint (one for standard, the other for Pulsed-NIL) were presented.
The second part is dedicated to the analysis of the selected material and focuses on the quality of the imprint, in particular with the use of AFM (Chapter 8) that is here introduced.
CHAPTER
2
RIASSUNTO
L’obiettivo principale di questo dottorato è la comprensione degli effetti della tecnica della Pulsed Nanoimprint Lithography sui materiali termoplastici, realizzando dei processi di imprinting su differenti materiali e impiegando stampi e tecniche di caratterizzazione diversi per valutarne gli effetti fisici e chimici su materiali plastici. Questo studio è stato condotto confrontando i risultati dei processi di imprinting ottenuti con la tecnica del NIL termico standard (standard T-NIL) e con la modalità ad impulsi di corrente (Pulsed-NIL).
Lo studio, si rendeva necessario perché, pur essendo vero che entrambe le tecniche portano il materiale a una temperatura superiore a quella di tran-sizione vetrosa e, attraverso l’applicazione di una certa pressione, si effettua una replica del pattern, dallo stampo al campione, con una inversione di to-no, i parametri di imprint (in particolare tempo e temperatura di imprint) sono completamente differenti.
La tecnica standard è, a oggi, molto diffusa sia nella ricerca che nella fabbricazione nel campo delle micro e nano tecnologie. Il pulsed NIL rappre-senta una innovazione ma è necessario dimostrare la capacità di realizzare l’imprint mantenendo una qualità delle strutture pari a quelle delle tecnica standard e la mancanza o l’esistenza limitata di degradazione del materiale a causa delle alte temperature di imprint.
A gennaio 2014, presso i laboratori dell’ “Istituto Officina dei Materiali” del “Consiglio Nazionale delle Ricerche” (IOM-CNR) presso Elettra - Area Science Park (Basovizza, Trieste), era operativo un primo prototipo (Thun-der 1.0), per l’imprint di piccole aree (40 ∗ 40 mm2), basato sul brevetto di
l’im-print di micro e nano strutture su aree più grandi (4” wafer), era verso la fine della progettazione.
I primi mesi del dottorato sono stati quindi impegnati nella progettazione, nella realizzazione dei disegni esecutivi e nell’assemblaggio della macchina usata per la parte sperimentale. I disegni non sono contenuti in questa tesi in quanto valutati non significativi per il campo della ricerca.
Per capire meglio l’innovazione introdotta da ThunderNIL è importan-te, innanzitutto fare una panoramica sul processo standard di imprint. Nel capitolo 4, questa tecnica, viene presentata assieme ai principali svantaggi intrinseci.
Il capitolo 5 presenta i dettagli della nuova tecnica.
Due capitoli (6 e 7) sono dedicati alla fabbricazione degli stampi per l’imprint con entrambe le tecniche. In particolare il primo è una sorta di introduzione mentre il secondo spiega la produzione di uno stampo passo a passo per far capire meglio i processi utilizzati, il flusso di lavoro e il tempo necessario alla produzione di uno stampo. Alla fine del capitolo anche i principali parametri per l’imprint di due campioni (uno con lo standard T-NIL e uno con il Pulsed) verranno riportati.
La seconda parte della tesi è dedicata all’analisi del materiale e della qualitá dell’imprint, in particolare con l’uso dell’AFM (capitolo 8) di cui viene fornita una breve introduzione.
Il capitolo 9 è completamente dedicato al confronto tra le strutture sot-toposte a imprint standard e Pulsed-NIL.
CONTENTS
1 Abstract 5 2 Riassunto 7 Table of contents 8 List of figures 15 3 Introduction 174 Standard Thermal NIL 19
4.1 Introduction . . . 19
4.2 The Standard Thermal NIL process . . . 22
4.2.1 The configuration . . . 22
4.2.2 The cycle . . . 22
4.3 The equipment . . . 26
4.4 Limitations . . . 27
4.4.1 Duration of the process cycle . . . 27
4.4.2 Energy considerations . . . 29
5 Pulsed Thermal NIL 31 5.1 Introduction . . . 31
5.2 The Pulsed NIL process . . . 32
5.2.1 The configuration . . . 32
5.2.2 The cycle . . . 32
5.3 Advantages of the Pulsed-NIL process . . . 32
5.3.2 Energy consumption . . . 34
5.4 The equipment . . . 34
6 Fabrication I 37 6.1 Introduction . . . 37
6.2 Substrate . . . 37
6.3 Spin-coating and baking . . . 38
6.4 UV Photo Lithography . . . 39 6.4.1 Two examples . . . 42 6.5 Etching . . . 43 6.5.1 Dry etching . . . 43 6.5.2 Wet etching . . . 44 6.5.3 Oxidation . . . 45 7 Fabrication II 47 7.1 Introduction . . . 47
7.2 From the wafer to the chip . . . 47
7.3 Pattern definition in the resist . . . 50
7.3.1 UV- Photolithography . . . 50
7.3.2 T-NIL . . . 51
7.4 Pattern definition in the resist . . . 51
7.5 Electrical contacts . . . 51
7.6 Anti sticking layer . . . 52
7.7 Imprint . . . 53
7.7.1 Standard Thermal NIL . . . 53
7.7.2 Pulsed Thermal NIL . . . 53
8 Atomic Force Microscopy 55 8.1 Introduction . . . 55
8.2 Configuration . . . 55
8.3 Samples analysis by AFM - Icon Scan . . . 57
8.4 Selection of the tips . . . 58
CONTENTS
9.6.1 The skin layer . . . 72 9.7 EBL . . . 73 9.8 Young’s Modulus of skin and bulk PMMA . . . 78
10 Discussion of the results 79
Appendices 83
A Curriculum PhD 85
B Acknowledgments 89
LIST OF FIGURES
4.1 a) SEM image of holes with a diameter D = 25 nm and a period L = 120 nm imprinted into PMMA. b) SEM image of
dots fabricated by lift-off. . . 21
4.2 (Left) Heating schemes for standard Thermal NIL configura-tion where the heat comes from the hotplates. (Right) Sum-mary of the imprinting time with standard cycle. . . 22
4.3 Time vs Force (blue) and position (red) . . . 24
4.4 Time vs Temperature . . . 25
4.5 Internal and external aspect of the Jenoptik HEX03 . . . 26
4.6 Simulation of the imprint time at different temperatures, con-sidering the residual thickness as imprint parameter . . . 28
4.7 Extrapolation of the values of viscosity at high temperature from the experimental data until 260◦C. . . 29
5.1 First row: (Left) Heating scheme for Pulsed-NIL configuration where the heat comes directly from the stamp. (Right) Sum-mary of the imprinting time with pulsed cycle. Second row: The heating scheme and the cycle of standard T-NIL is here reported for a immediate comparison . . . 33
5.2 External (left) and internal (right) aspect of ULISS equipment. 35 5.3 Imprint of full 4” inch wafer area . . . 36
6.1 Spin-coating process flow; left) deposition of the resist, cen-ter) spinning and right) bake. . . 39
6.3 Film after the development. With positive tone the developer removes the exposed resist, vice versa in the negative tone . . 40 6.4 Influence of development time and dose in a positive tone
photo-resist . . . 41 6.5 Influence of developing time and dose in a negative tone
photo-resist . . . 42 6.6 Spin Speed Curve for Dow SPR220 Products . . . 43 6.7 A substrate is covered with a structured polymeric layer. The
polymer is used as a mask for the etching step to transfer the pattern into the substrate. Finally, resist stripping leads to the final patterned surface. . . 44 6.8 Examples of etching with KOH; left) a non complete etch
from a mask with square, center) scheme of cross section with the underline of the angle of 54.7◦, right) an example of
complete etch with the formation of perfect pyramids. . . 45 6.9 One-dimensional planar growth of amorphous SiO2 . . . 46
8.1 Typical configuration of an AFM. (1): Cantilever, (2): Sup-port for cantilever, (3): Piezoelectric element(to oscillate can-tilever at its resonance frequency.), (4): Tip (Fixed to open end of a cantilever, acts as the probe), (5): Detector of deflec-tion and modeflec-tion of the cantilever, (6): Sample to be measured by AFM, (7): xyz drive, (moves sample (6) and stage (8) in x, y, and z directions with respect to a tip apex (4)), and (8): Stage. . . 56 8.2 Bruker Dimension Icon Atomic Force Microscope System with
ScanAsyst . . . 57 8.3 Operating principle of a Bruker Icon scan with PeakForce
QNM mode scanning the surface in tapping mode with a low tapping frequency of 2 kHz while typical line speeds are in the range of 1 to 10 µm/s. The force - distance curve at each tap-ping pixel is defined by: surface approach (1 and 2), point of deepest indentation (3), force release and surface restoration (4), point of largest adhesion (5) and released cantilever (6). . 58 8.4 Tips selection by expected Young’s modulus . . . 59 9.1 Profiles of a line imprinted with standard NIL (red) and
Pulsed-NIL (blue) using the same master . . . 63 9.2 3D AFM topography of line structure having 4 µm width, 8 µm
LIST OF FIGURES
9.3 a) AFM topography, b) AFM elastic modulus map of the PS reference sample surface. . . 66 9.4 Histograms of the elastic modulus of the polystyrene reference
sample showing the data for the first scan (blue peak) and after 15 scans (red peak). . . 67 9.5 Top) 512 x 512 pixel map of the Young’s modulus of a PMMA
film imprinted by Pulsed-NIL. The “roughness” i.e., the lo-cal variation of the Young’s modulus across the surface, is relatively small. The lines correspond to artifacts (AFM tip crosses steep sidewall) due to the fact that the force applied by the tip is non-perpendicular to the surface as it is possible to understand from the topography on the bottom image. . . . 68 9.6 Relative Young’s modulus of PMMA vs polystyrene of samples
that did and did not undergo an imprinting process. The two broad peaks on the right side are representing the normalized Young’s modulus for Pulsed-NIL and for standard T-NIL for PMMA while the group of peaks on the left side represents the pristine PMMA without imprinting as well as the PS reference measurements before and at the end of the measurements. . . 69 9.7 Modulus of PMMA annealed. The peaks represent the
nor-malized Young’s modulus for flat/structured samples annealed for different times. . . 71 9.8 Cross section of a line imprinted with the standard T-NIL.
The image underlines the different aspect of the skin and the bulk of PMMA . . . 74 9.9 Cross section of a line imprinted with the Pulsed-NIL. The
skin is thinner in this case. . . 75 9.10 Hierarchical structures by multiple imprints, like this
nano-pyramids (second imprint) on top of truncated micro-nano-pyramids, without (re-) melting the micro- structures obtained with the first imprint. . . 76 9.11 Development rate of the imprinted structures exposed to
dif-ferent doses . . . 77 9.12 For every dose a 512 x 512 points AFM scan was collected.
From the distribution graph, the maximum was taken and represented in this graph. It’s possible to see a difference be-tween the left (doses 10−170 µC/cm2, skin layer) and the right
CHAPTER
3
INTRODUCTION
Thermal Nanoimprint Lithography (T-NIL) is a replication process able to pattern micro- and nano- structures in a range of materials. Crucial for its implementation is selecting thermoplastic resists with suitable thermo -mechanical properties. This allows performing thermocycles, by which the polymer can be molded in its viscous state and demolded in its hard (glassy) state [2] without damages in the stamps. Ideal thermoplastic molding relies on purely viscous processes and volume conservation for a given tempera-ture, i.e., for both, the solid and the viscous state. In room temperature imprint processes it seems to be possible to compress polymers without sig-nificant flow, resulting in a compaction or to have a strain hardening [3], [4]. Less known is the effect which the NIL process has on the properties of the imprinted polymer, particularly, if imprint is performed at very high temperatures and high shear rates, as in the case of Pulsed-NIL processes [5], [6].
In particular, the effect of T-NIL on the elastic modulus of the top sur-face of spin-coated poly(methyl methacrylate) (PMMA) is examined. By using the PeakForce QNMTM(Quantitative Nanomechanical property
Map-ping) mode of a Bruker atomic force microscope (AFM), the Young’s modulus of as-coated resists and of imprinted and patterned resist were investigated. The PeakForce QNMTMmethod recently received increasing interest in
CHAPTER
4
STANDARD THERMAL NIL
4.1
Introduction
In the last three decades, a number of fabrication methods broadly referred to“nanotechnology” have been developed with the purpose of fabricating nanostructures in a variety of materials with control down to the nanometre scale and often to the atomic scale. These techniques are required to meet both the stringent nanofabrication specifications, ease of manufacturability and costs considerations of high-tech industries (e.g. electronic industry) as well as the flexibility and accessibility needed in experiments and inves-tigations in materials science, organic optoelectronics, nano-optics and life sciences.
Often techniques that offer striking performances on one specific aspect, are weak considered from other points of view. For instance, the scanning probe techniques (AFM, STM, MFM,...), which are employed in many re-search areas mainly for morphological studies but also as lithographic tools, are a clear example of the difficulty to combine high resolutions with a high throughput, moderate costs and user-friendly operation.
A brief history of nanoimprint lithography
Different varieties of micro-molding of thermoplastics, such as hot embossing or injection molding, which had been used for more than 30 years in in-dustry [13], inspired the first article about thermal nanoimprint lithography published by Stephen Y. Chou et al. [14] in 1995. It outlined the essential el-ements of this technique, such as stamp manufacturing, process parameters, polymer flow, polymer adhesion and pattern transfer.
In the proof of principle, as stamp was employed a silicon substrate with a silicon dioxide layer patterned by electron beam lithography (EBL) and reac-tive ion etching (RIE). The pattern was an array of pillars with a diameter of 25 nm and a period of 120 nm. The stamp was imprinted at a pressure of 131 bar and a temperature of 200 C into a 55 nm thick poly-methylmethacrylate (PMMA) layer deposited on silicon. After system cooling below the glass transition temperature (Tg), the stamp and substrate were separated. The patterned PMMA was exposed to an oxygen plasma in RIE in order to re-move the polymer residual layer in the holes imprinted by the pillars of the stamp uncovering the silicon beneath. 5 nm of titanium and 15 nm of gold was subsequently deposited on the surface followed by the lift off in acetone of the polymer. The resulting pattern was a uniform array of Ti-Au dots on the silicon substrate. The dots had the same dimension and period of the pillars array on the stamp as shown in figure 4.1
After the first publications by Chou and collaborators others followed by several research groups coming from Wupperthal University (1998) [15], Wurzburg University (1999) [16] and Micro Resist Technology (1999) [17] in Germany, CNRS in France (1999) [18], Seoul University (1999) in Korea [19], PSI in Switzerland (1999) [20] and Lund University in Sweden (1999) [21].
In 2003, the International Technology Roadmap for Semiconductor (ITRS) introduced NIL into semiconductor industry roadmap as “next generation lithography” candidate for the 32 nm node and beyond, scheduled for in-dustrial manufacturing in 2013. The importance of nanoimprint has been recognized by the EU, that financed and coordinated the research in Europe in this field through projects as SPINUP project, Nanotech (Development of Nanoimprinting technique suitable for large area mass production of nm-scale patterns), which ran 1997-1999, CHANIL (CHances for NanoImprint Lithography), which ended in 2002 and NAPA (Emerging Nanopatterning Methods), which ran until 2008, followed by NaPanil, a project on nanopat-terning, production and applications based on nanoimprinting lithography.
4.1. Introduction
Figure 4.1: a) SEM image of holes with a diameter D = 25 nm and a period L = 120 nm imprinted into PMMA. b) SEM image of dots fabricated by lift-off.
Imprints and Nanonex in USA, Jenoptik and Micro Resist Technology in Germany, NIL Technology in Denmark and others.
In the last decade the process of imprint has become always more used in the industries in a large field of objects of common use.
To understand the imprint technique used, it is important to know that typically the thermal NIL (Nano Imprinting Lithography), is performed by a hydraulic-press equipped with hot plates. From the analysis of the process it is easy to understand problems and limits of the standard technique. For this reason, this chapter is divided in several parts:
• Process description: to understand the different phases of the im-print, focusing on the parameters (pressure, time and temperature) and the behavior of the plastic materials.
• Equipment: a presentation of the equipment used for the experimen-tal part.
4.2
The Standard Thermal NIL process
4.2.1
The configuration
The standard configuration for the Thermal NIL is a heating plates/press system. Heating plates, typically, are made of steel to guarantee the appro-priate rigidity which enables them not to be deformed during the applying of the pressure. Figure 4.2 shows the basic scheme of the standard T-NIL. Here it is possible to see the sandwich of substrate with the resist and stamp. In this scheme the aspect of the resist at the end stage of the imprint is shown.
Figure 4.2: (Left) Heating schemes for standard Thermal NIL configuration where the heat comes from the hotplates. (Right) Summary of the imprinting time with standard cycle.
4.2.2
The cycle
The cycle of the standard thermal NIL can be deduced from the logbook of the equipment that will be presented in the next section. In particular is important to consider the graphs time/force and time/temperature. The first is shown in figure 4.3.
4.2. The Standard Thermal NIL process
temperature, the machine returns up and starts to apply the force (20 kN in this case).
At this point, it is important to underline that the pressure is the process parameter. For this reason it is important to know what is the needed surface of the imprint (the minimum surface in the sandwich stamp/resist/substrate) to calculate the force which needs to be applied in order to obtain the correct value of pressure. The force and the position remain constant until the end of the cooling phase.
In figure 4.4 it is possible to see the trend of the temperature. The equipment is set to start at a temperature of 80◦C. The temperature of
imprint is 180◦C and it is achieved after 200 s. In the next 100 seconds
there is a stabilization of the temperature and, after the applying of the pressure, there is the real imprint (10 minutes in this case). At the end of the cooling phase (both plates at 65◦C), there is the moving up of the system
hot plates/press.
To summarize it is possible to divide the standard imprint process in three phases:
1. Heating: This time is strictly dependent on the power of the equip-ment, on the mass of the hot plates, on the glass temperature that define the temperature of the imprint.
2. Imprint: Time of this step depends of size and shape of the structures, material viscosity etc. (typically 5-10 minutes).
0 200 400 600 800 1.000 1.200 1.400 1.600 −0,5 0 0,5 1 1,5 2 2,5 3 ·104 s N −1 −0,5 0 0,5 1 1,5 2 2,5 3 ·106 mm
4.2. The Standard Thermal NIL process 0 200 400 600 800 1.000 1.200 1.400 1.600 60 80 100 120 140 160 180 200 s ◦ C
4.3
The equipment
An important part of the experiments was done at Paul Scherrer Institute, in Villigen Switzerland. The Laboratory of Micro and Nanotechnology - LMN has in its clean room, two Jenoptik HEX03 [figure 4.5] imprinting systems for Thermal NIL (T-NIL) and UV-NIL and micro-embossing. This kind of equipment is able to:
• Manage substrates up to 150 mm; • Temperature up to 320◦C;
• Pressing force up to 200 kN (force/position controlled); • Embossing under vacuum.
At the beginning of the process the software provides the operator with the full control of the every single parameter of the imprint. Later, the machine is completely automatic starting from the initial status check of the safety doors up to the NIL. The monitoring of eight parameters (time, force, position and temperature of tool, substrate, oil, top and bottom surface) gives the operator the storyboard of the process and, with a look at the logbook, it is possible to understand the characteristics of the standard Thermal NIL process.
To enter more in detail (the parameters in Bold can be under the direct control of the operator),
• Time: this parameter plays an important role in the imprint. Usually, the time required to thin the residual layer down to a suitable level for the pattern transfer process, one has to wait a time that diverges
4.4. Limitations
approaching residual layer zero. From the analysis of the needed time to accomplish a process, often several minutes, it clearly emerges one of the disadvantages of the standard technique (section 4.4).
• Force: in Newton. The equipment uses the operator’s setting to apply the force and, therefore, the pressure of the imprint. This parameter controls also the applying rate.
• Position: in millimeters. This parameter shows the position of the press during the imprint.
• Temperature of
oil: it is an internal control parameter. The equipment has a tem-perature sensor in the oil-dynamic component, used to control the cor-rect operation of the water cooling circuit that preserves the electronics and the mechanical parts from the heat that come from the hot plates. substrate: also this temperature is an inner control parameter. In this case the sensor is in the proximity of the hot plates close to the cooling circuit.
top surface, bottom surface: these two temperatures are regis-tered near the sample, one in the master side, the other in the substrate side and both can be considered as the temperature at which the im-print process takes place.
4.4
Limitations
4.4.1
Duration of the process cycle
Heating and cooling times depend on the equipment, and are in the following range: between 1 and 10 minutes. The main problem, is that one has to heat up bulky and heavy hotplates with a mass in the order of 10 kilograms, while what is strictly necessary to heat is the resist, which is in the order of few milligrams.
Also with high performing equipment, that reduces heating and cooling time, it is known that indentation time is strictly correlated to temperature. The simulation, in figure 4.6 shows the indentation of a line of 5 µm width into PMMA of 100 kDalton with a initial thickness of 200 nm [6]. On y axis there is the ratio:
Figure 4.6: Simulation of the imprint time at different temperatures, consid-ering the residual thickness as imprint parameter
The imprinting time is described by Stefan’s squeeze flow equation [9], [22], [23], [24], where h(t) is the thickness of the resist in function of the time, ho
is the original thickness, p is the pressure, η is the viscosity (which depends on the type of resist, temperature and shear rate) and w is the width of the structure, in this case a line of 5 µm [equation 4.2].
The viscosity is extrapolated from the experimental data. Figure 4.7 shows the fit of the experimental data for the viscosity of the PMMA 100 100 kDalton until a temperature of 260◦C. Using the equation shown in the
graph, it is possible to extrapolate the trend for higher temperatures (dashed line). The values are used to complete the figure 4.6 for the temperatures from 260◦Cto 600◦C.
At fixed applied pressure, a given residual layer, for instance one tenth of the initial film thickness, the imprinting time will depend strongly on temperature, as the viscosity is linked to the inverse of the temperature exponentially.
Considering the temperature of the imprint used for the example in sec-tion 4.2.2, the time is close to 1200 seconds 1 It is possible to reduce the
imprint time by increasing the temperature but there is a limitation in the
1The simulation is not correlated to the experiment that has a different morphology
4.4. Limitations
Figure 4.7: Extrapolation of the values of viscosity at high temperature from the experimental data until 260◦C
maximum value due to the equipment.
Using the Jenoptik HEX03, that arrives at 320◦C, the imprint time could
be, virtually, 0.1 s but the heating and cooling time increase to values higher than 30 minutes. For this reason, the standard imprint cycle in any case in the order of more than 15-30 minutes in total.
1 h2(t) = 1 h2 o + 2p ηw2t (4.2)
4.4.2
Energy considerations
10 min. The imprinted area is the one of the stamp used also for the Pulsed-NIL and is a square of 10 ∗ 10 cm2. The result is shown in the equation 4.3
that demonstrate a energy consumption in the order of 104J cm−2.
CHAPTER
5
PULSED THERMAL NIL
5.1
Introduction
The implementation of the Thermal Nanoimprinting process introduced by ThunderNIL S.r.l. moves from the consideration that the time used in the standard T-NIL process is long, unnecessarily long to be useful in appli-cations. The time plays a fundamental role, together with the costs, if is necessary to transfer a technique from the research into the industrial en-vironment, where the productivity is a key parameter and the rates and volumes are not comparable with those typical of R&D activities.
Other problems are introduced by the industrial production as differ-ent imprint areas (normally larger), imprint on non planar surfaces, multi-imprint, different materials, of lower purity and less controlled, or incorpo-rating dyes and pigments for coloring the material.
These considerations stimulated the rethinking of the standard process and two were the main points:
1. As described in the chapter 4 the time of the imprint cycle depends critically on the equipment (heating and cooling phases), due to the problem of heating heavy and bulky plates with associated large ther-mal capacities, and on the temperature that is attained during the imprinting process, from which depend strongly (exponentially) the in-dentation time.
the long heating and imprint time allow heat to diffuse far from the contact region and, as a result of this, already imprinted in previous steps in areas around the contact region are deleted due to polymer relaxation.
To solve the second point the idea was to heat up only the stamp thanks to Joule’s effect. The temperatures achieved with a short high voltage pulse are 400 − 500◦C, much higher than the temperature of the standard thermal
NIL.
5.2
The Pulsed NIL process
5.2.1
The configuration
In the approach developed by ThunderNIL, the heat comes directly from the stamp. A short current pulse flowing below the stamp surface heats up the resist to a temperature of about 500◦Cto have a complete indentation within
ca. 100 µs. The full imprinting, made by manual handling of the substrates requires less than 1 min. The introduction of an automatic substrate handling system is expected to reduce the imprinting cycle below 10 s.
5.2.2
The cycle
In the equipment developed at ThunderNIL, the pulse duration can be set to 35 − 70 − 105 µs and the maximum temperature at the stamp/polymer interface is reached at the end of the pulse. After the pulse the stamp surface cools down very quickly. For a pulse of 35 µs (maximum simulated tempera-ture 550◦C, the simulations show that the temperature decreases by 250◦C
in 15 µs and is reduced below the glass temperature of the resist (for PMMA is 105◦C, in a time of other 20 µs. For this reason, the
heating/imprint-ing/coaling time can be estimated approximately as twice the pulse duration (70 − 210 µs).
5.3
Advantages of the Pulsed-NIL process
5.3.1
Duration of the imprinting cycle
5.3. Advantages of the Pulsed-NIL process
Figure 5.1: First row: (Left) Heating scheme for Pulsed-NIL configuration where the heat comes directly from the stamp. (Right) Summary of the imprinting time with pulsed cycle. Second row: The heating scheme and the cycle of standard T-NIL is here reported for a immediate comparison
other and for the substrate handling. This time is in the order of 10 s but, by the experience, in any case the total cycle time is in the order of 45 − 60 s. The other part of the discussion about the process time is related to the imprinting performed using a operation mode consisting of multiple pulses. There are situation that require the use of multiple pulses, in order to achieve higher indentation depth. In this case the full indentation can be obtained by a series of small indentation. In other cases, it is also useful to deliver multiple pulses to increase the average temperature of the stamp/polymer to melt a polymer or a material with higher Tg. The equipment can complete the
charging of the capacitors within 100-500 msec (depending on the required charging voltage) an repeat the same pulse a pre-defined number of times. The operator can set different frequency but normally 1 Hz is an appropriate choice. The number of the pulses defines the total imprint time.
of the sample by the operator to the complete release of the pressure on the stamp/sample sandwich in both case).
5.3.2
Energy consumption
The energy spent in an imprinting process performed with the Pulsed-NIL is here calculated. In this case, the energy dissipated into heat can be ob-tained by the integrating the product of the voltage drop on the stamp and current passing through versus the time for the duration of the pulse. The following calculation is an order of magnitude estimate. A typical value of the resistance of a stamp is 2 ohm. At the maximum pulse voltage for the equipment, which is (8000 V), the maximum electric current is 4 kA. With a pulse duration of 105 µs this makes 3360 J. These parameters have proven to be well in excess to imprint a full square of 10∗10 cm2. The typical value used
for imprinting such an area is (4000 V), which leads to a maximum current in the order of 2 kA and total energy of 840 J. The result shown in the equation 5.1 is a total available energy per pulse is around 34 J cm−2, and 8.5 J cm−2 is
this simple estimate of the pulse energy actually needed for the imprinting. More accurately, taking into account that the pulse is not constantly provid-ing the maximum voltage and current to the stamp for the entire duration of the pulse the corrected energy dissipated per unit area is approximately 5 J cm−2, which differs by more than 3 order of magnitude in terms of energy consumption from the standard thermal NIL (3.4 × 101J cm−2, for
Pulsed-NIL, vs 1.2 × 104J cm−2, for standard T-NIL).
Emax = 4.0 × 103A ∗ 8000 V ∗ 105µs/100 cm2 = 3.36 × 101J cm−2 (5.1)
Etot = 2.0 × 103A ∗ 4000 V ∗ 105µs/100 cm2 = 8.5 J cm−2 (5.2)
5.4
The equipment
The ULISS equipment, developed by ThunderNIL, has the possibility of set the pulse with:
• Duration: 35, 70 or 105 µs • Max voltage: 8 kV
5.4. The equipment
Figure 5.2: External (left) and internal (right) aspect of ULISS equipment. ,
• Max power: 30 MW
CHAPTER
6
FABRICATION I
6.1
Introduction
It is to be considered that there is a world behind nano and micro technology. In the thermal imprint field, the fabrication of the stamp plays a fundamental role. For this reason, this topic is an essential part of the PhD work and represents 50% of it.
The next chapter (cap. 7) will show the complete fabrication of a stamp, step by step.
Several techniques are used to prepare the pattern and, in the case of the Pulsed-NIL, the parts of the stamps outside the pattern too. In this chapter there are presented the main instruments used for the preparation of what is necessary for the experimental part of the PhD.
6.2
Substrate
one of the first steps in the preparation of a nanofabrication process by NIL or Pulsed-NIL is the selection of the substrate and it is necessary to distinguish between:
• choice of the material for the stamp: for the standard thermal NIL for the Pulsed thermal NIL
For the second case, several kinds of plastic materials were imprinted as PC carbonate), PET ethylene terephthalate), PMMA (poly-methyl methacrylate), NAFION (a sulfonated tetrafluoroethylene), ABS (acry-lonitrile butadiene styrene), PDMS (poly- dimethylsiloxane).
The pulsed imprint process is able to transfer the pattern directly on the material without distorting the macroscopic shape, the size, and the mor-phology as described in the previous chapters. For this reason it is possible to transfer the pattern in different objects as foils, tiles or finished plastic products (mobile phone covers, glasses, bottles, toys, etc).
The over possibility is to spin and coating a polymer film on a substrate to use it as a mask (for future fabrication passages) or as final product.
Normally, for the stamps, a silicon wafer of 4 inches is used as substrate.
6.3
Spin-coating and baking
Polymer spin-coating process is the most common way to produce uniform thin polymeric film on planar surface. The substrate for the imprint depends on the application but, for most cases of interest a <100> silicon wafer is used. Other common substrates are, for example, glass or plastic sheets or plates, metal plates, etc.
A small amount of the coating material, resist, is deposited onto the center of substrate that, previously, was locked in position on the holder thanks to a vacuum system. Particles can be removed using a nitrogen stream. Sometimes a cleaning with acetone and 2-Propanol spinned directly on the substrate can improve the cleanliness.
The rotation at high speed (between 1000 and 4000 rpm) spreads the coating material by centrifugal force and the acceleration causes the spread-ing of the resin to the edge of the substrate. Thin film thickness depends on several parameters of the resist as viscosity, drying rate, resist dilution, surface tension, etc and on the spin-coater: rotational speed, acceleration and overall spin time.
When the resin dries, the viscosity increases until the radial force of the spin process can no longer appreciably move the resin over the surface. At this point, the film thickness will not decrease significantly with increased spin time.
6.4. UV Photo Lithography
Figure 6.1: Spin-coating process flow; left) deposition of the resist, center) spinning and right) bake.
6.4
UV Photo Lithography
The next step in the fabrication normally is the definition of the pattern that has to be transferred in the silicon. Several techniques can be used as, for example, laser interference lithography, electron beam lithography or UV Photo Lithography. these technologies are available at the FNF- facility of Nano fabrication of Istituto Officina dei Materiali IOM-CNR and at LMN of PSI in Switzerland. In this work, the UV-NIL was used for all fabricated stamps and for this reason only this technique is, here, presented very shortly. UV light, that has the wavelength in the range of 250-400 nm is a a part of Photo Lithography (PL), also termed optical lithography.
UV- Lithography is an essential component of modern technologies used to produce electronic devices and, by extension, it is the preferred process to be used also for MEMS and LOC (Lab on a chip) production.
PL exploits light to transfer a 2D lay-out (pattern) from a photo-mask (a plate of Soda lime or Quartz glass, on which a 2D pattern is defined on a 100 nm thick chromium layer) to a resist deposited, as previously described, on a substrate.
The process scheme is reported in figure 6.2. The resulting structure can remain as functional ones or can be transferred to the substrate by additive (evaporation or sputtering) or subtractive (etching) processes.
Photoresists are sensitive to the light and two reactions may occur in the film:
Figure 6.3: Film after the development. With positive tone the developer removes the exposed resist, vice versa in the negative tone
1. chain scission: exposure changes the chemical structure of resist (re-ducing the mean molecular weight) so that it becomes more soluble in the developer (positive tone resist).
2. chain cross-linking: the resist becomes cross-linked/polymerized (in-crease of mean molecular weight of the polymer), and more difficult to dissolve in developer (negative Tone resist). In this case a thermal treatment after exposure is needed for the propagation of the reaction (post exposure bake).
In both case, there is a difference in the averaged molecular weight of the chains in the exposed and unexposed regions of the film. During development, thanks to the different solubility, the solvent selectively dissolves the region where the smaller chains are. The result is show in figure 6.3
Several parameters determine the quality of the lithographic pattern and an optimization is required for any combination of resist/pattern/substrate. Thickness of the film, tone of the resist, bake conditions, exposure dose, pat-tern geometry, development chemistry, time and temperature are parameters that mostly affect the pattern quality. An ideal structure has vertical walls and, for this reason, the aspect of the slope reveals the quality of the pattern. For the positive tone (figure [6.4]), two are the critical parameters that influence the final results and the aspect of the structures, the dose and the development time:
• Correct development time and dose:
Fully developed structures without or with slight positive slope. • Correct development time and higher dose:
Fully developed structures but with higher slope. • Correct development time and lower dose:
Not fully developed structures.
• Longer development time and correct dose:
6.4. UV Photo Lithography
Figure 6.4: Influence of development time and dose in a positive tone photo-resist
• Longer development time and higher dose:
Developed structures, high slope angle and eroded upper edges. • Longer development time and lower dose:
Not fully developed structures, lower slope of the walls. • Shorter development time and correct dose:
Not fully developed structures.
• Shorter development time and higher dose: Fully developed structures with low slope.
• Shorter development time and lower dose: Not developed structures.
The developing time is not so critical in a negative tone and, for this reason, only the exposure dose plays an important rule (figure [6.5]). In this case:
• Correct dose:
Fully developed structures. • Higher dose:
Fully developed structures with negative slope. • Lower dose:
Figure 6.5: Influence of developing time and dose in a negative tone photo-resist
6.4.1
Two examples
To complete this short overview on the UV lithography two examples are here reported, one for every tone of resist. For each of them the datasheet is quoted for a complete overview of the characteristics.
Positive tone
One example is the SPR 220 -1.2 photo resist1 which is used for fabricating
stamps. From the datasheet and figure 6.6 the user can obtain the principal pieces of information for the processing of the resist as the spin speed curve to determine the expected thickness and softbake (after the spin-coating) parameters (115◦C for 90 seconds).
Negative tone
Su-8 2000.52 is an example of a negative tone. Also in this case the datasheet
gives the main information for processing the resist.
The negative tone is typically used for EBL exposure that was not used for the samples of this work. For this reason the discussion about this resist is concentrated in this section.
1http://micromaterialstech.com/wp-content/dow_electronic_materials/
datasheets/SPR220_Photoresist.pdf
6.5. Etching
Figure 6.6: Spin Speed Curve for Dow SPR220 Products
6.5
Etching
In most cases of technological interest, the resist does not have the physical properties required to perform the function that the pattern is meant to. In the majority of cases, it is therefore necessary to transfer the pattern from the “sacrificial” resist layer, to the material with the suited properties. The definition of the pattern in the resist is followed by several subtractive or additive process steps where materials can be either removed from or added to a substrate.
In the field of micro and nano technologies, etching processes are widely used as subtractive techniques. In this case, a patterned structure over the substrate is transferred by chemical and/or physical removal of material from the substrate. The resist has the function of a protective masking layer and it is more resistant (different etching rates) than the substrate to the etching agents.
6.5.1
Dry etching
Figure 6.7: A substrate is covered with a structured polymeric layer. The polymer is used as a mask for the etching step to transfer the pattern into the substrate. Finally, resist stripping leads to the final patterned surface. action through the generation of reactive species at the surface, or by the combination of both.
In the case of Physical Etching, atoms are ejected from the surface by inert heavy ions bombardment, while in Chemical Etching, plasmas that contain reactive species (like ions or radicals) are able to modify the surface chemistry and leading to highly volatile compounds. Working at very low pressure, these atoms or small molecules pass into the gas phase and are removed by pumping systems.
6.5.2
Wet etching
In this case, liquid-phase (“wet”) etchants are used. The sample can be immersed in a bath of etchant, which must be agitated to achieve good process control.
The most important parameters in chemical etching are etch rate, selec-tivity, anisotropy, bias (undercut), tolerance, overetch, feature size control and loading effects.
Wet etching is usually faster than dry one.
Another consideration: wet etching requires the disposal of large amounts of toxic waste and, therefore, an inevitable increase of the disposal costs. The most used, for this work, were two, divided by type:
• Isotropic etchants as buffered oxide etch (BOE)or buffered HF, used for etching silicon dioxide (SiO2). The solution is a mixture of
ammonium fluoride (NH4F), and hydrofluoric acid (HF). BOE is used
typically to reduce and stabilize the etching rate. Infact, typical prob-lems with the HF etching are peeling of the photo-resist or a too quick etch for good quality [25].
A common buffered oxide etch solution comprises a 6:1 volume ratio of 40% NH4F in water to 49% HF in water. This solution will etch
6.5. Etching
Figure 6.8: Examples of etching with KOH; left) a non complete etch from a mask with square, center) scheme of cross section with the underline of the angle of 54.7◦, right) an example of complete etch with the formation of
perfect pyramids.
Isotropic etchants etch in all crystallographic directions at the same rate, which leads to large bias when etching thick films.
• Anisotropic wet etching (KOH) used for etching of silicon.
This etchant has different rates depending on which crystal face is ex-posed. The selectivity is 400 times higher in <100> crystal directions than in <111> directions. Etching, for example, a <100> silicon sur-face through a rectangular hole in a masking material creates a cavity with a trapezoidal cross-section with later walls <111> oriented. The angle to the main surface is 54.7◦. If the etching is continued to
com-pletion until the flat bottom disappears, the pit becomes a trench with a V-shaped cross section. If the original rectangle was a perfect square, the pit when etched to completion displays a pyramidal shape. Some examples of etching with KOH are shown in figure 6.8.
6.5.3
Oxidation
For some applications it can be necessary to grow a SiO2 layer on the silicon.
One easy system to obtain the oxide layer is to use an oven with a controlled atmosphere (air, oxygen, water vapor) to have a uniform layer on both sides of a wafer. Even a room temperature,a “native oxide” with a thickness of approx. 1 − 2 nm forms. In the “dry oxidation” there is the formation of oxide with the reaction 6.1, in the “wet” following 6.2.
Si(solid) + O2 ←→ SiO2(solid) (6.1)
Si(solid) + 2 H2O ←→ SiO2(solid) + 2 H2 (6.2)
The growing of SiO2 in not only inside of the wafer but there is a 0.46
Figure 6.9: One-dimensional planar growth of amorphous SiO2
CHAPTER
7
FABRICATION II
7.1
Introduction
The previous chapter presented the different technologies that were employed in the fabrication of the T-NIL and Pulsed-NIL stamp used in this PhD thesis. To better understand the complexities that could occur in the fabrication of the stamps, a series of examples is reported.
7.2
From the wafer to the chip
The fabrication starts from a 4” silicon wafer, which had been ion implanted by a commercial foundry using 31P ions and a dose of 2 × 1016cm−2 at
1. Growth of SiO2 The first step is to grow
a SiO2 layer (purple, in figure) of
approxi-mately 1 µm with an oven at 1000◦Cwith a
water vapor atmosphere.
2. Spin coating and baking Back and front side have to be protected and covered with resist (red in figure). The work flow is:
• Pre-wash with acetone and IPA (on both side)
• spin coating of the backside with a photoresist, usually SPR 220 1.2 fil-tered with 0.2 µm filter (2000 rpm, ac-celeration of 1000 rpm/s, spinning for 30 s).
• soft baking for 90 s at 115◦C
• spin coating of the front side with SPR 220 1.2 filtered with 0.2 µm filter (2000 rpm, acceleration of 1000 rpm/s, spinning for 30 s).
7.2. From the wafer to the chip
3. Lithography of both sides simultaneously (aligned lithography) with two masks (only one it is depicted in the top figure to sim-plify. The result is a definition of exposed lines (light red) on the resist) with the design of the intaglio carvings. Exposure time: 13 s at dose of 3.0 mW cm−2. The resist needs a
post exposure bake of 30 s at 115◦C.
4. Development for 45 s in MF-24A1 hand
stirred (the purple oxide appears).
5. BOE to remove the oxide for 15 − 20 min in the areas deprotected from the resist.
6. Piranha etch at 90◦C to remove the
re-sist. Piranha is a mixture 70:30 of sul-furic acid (H2SO4) and hydrogen peroxide
(H2O2), used to decompose organic residues,
thus cleaning the substrates [27].
7. Etching silicon in KOH 30% in weight in deionized water for 2 h at 80◦Cand 600 rpm
8. BOE to remove the oxide from the surfaces (20 min).
9. Piranha etch, acetone and IPA for gen-eral cleaning.
10. Cutting. That process defines lines in the silicon that can be used as guide for the cut-ting of stamps according the needed size. In most cases this consisted of 25 by 45 mm size (5 for every wafer).
An important consideration: a silicon wafer can easily be cut in a precise way thanks to the crystal direction simply with a diamond cutter. In case of stamps for Pulsed-NIL, the masks of front and back side, define some macroscopic pattern (in the range from 1 mm to cm scale), beyond the cutting lines. The design of these masks and these patters are confidential and can’t be presented in this thesis.
7.3
Pattern definition in the resist
After defining the previous patterns on the front and back side of the stamp we proceed to the next step of the fabrication, namely the definition of the nano/ micro pattern on the front side of the stamp.
At this point it is possible to undertake two ways: definition of the pattern with lithography and by T-NIL.
7.3.1
UV- Photolithography
1. spin coating of the front side of the chip with HDMS to promote the adhesion and increase the quality of the final structures (2000 rpm, acceleration of 1000 rpm/s, spinning for 30 s).
7.4. Pattern definition in the resist
3. spin coating with SPR 220 1.2 filtered with 0.2 µm filter (2000 rpm, acceleration of 1000 rpm/s, spinning for 30 s).
4. soft baking for 90 s at 115◦C
5. Lithography of front side with the mask with the pattern. Exposure time: 13 s at dose of 3.0 mW cm−2. Post exposure bake of 30 s at 115◦C.
6. Development for 45 s in MF-24A hand stirred.
7.3.2
T-NIL
1. spin coating of the front side of the chip with a mr-I resist (7000 or 8000 series), filtered with 0.20 PTFE (1000 rpm, acceleration of 1000 rpm/s, spinning for 30 s).
2. soft baking for 120 s at 140◦C.
3. imprint using a stamp as described in the chapter 4 (parameters de-pend on type of resist and structures)
7.4
Pattern definition in the resist
1. ICP to transfer the pattern in the silicon for 500 nm. Normally this phase is divided in:
• O2 plasma cleaning (Pressure 4 mtorr, time 20 s, 40 sccm of O2,
Platen power 10 W, Coil power 210 W, Bias 38 V).
• Si etching (Pressure 8 mtorr, time 1650 s, 60 sccm of C4F8, 30 sccm
of SF6, 10 sccm of Ar, Platen power 20 W, Coil power 400 W, Bias
100 V).
• O2 final cleaning (Pressure 20 mtorr, time 120 s, 50 sccm of O2,
Platen power 20 W, Coil power 800 W, Bias 50 V).
At this point the fabrication of the stamp for standard T-NIL is completed.
7.5
Electrical contacts
1. Sputtering To protect the pattern it is enough to cover the area with a piece of silicon or glass. The dimension of the most used stamp is 25 mm ∗ 45 mm. A mask of 25 mm ∗ 60 mm, on the top of the center of the stamp, guarantees the creation of the two pads without any metallic connection.
• Plasma cleaning with 20 sccm Ar and 5 sccm of O2. Pressure
in the chamber at the begging of the process is 1.90 × 10−5mbar.
Working pressure: 2.8 × 10−3mbar. Power in RF 0.55 W cm−2,
Bias 115 V, time 3 min.
• Chromium deposition with 25 sccm Ar. Working pressure: 1.10 × 10−3mbar. Power in DC 1.32 W cm−2, 1.1 A, time 6 min. Aspected thickness 280 nm
• Silver deposition with 25 sccm Ar. Working pressure:
3.50 × 10−3mbar. Power in DC 2.77 W cm−2, 1.75 A, time 1 min. Aspected thickness 250 nm
2. Contacts Soldering The connection with the equipment is done with a copper foil soldered on the Cr/Ag pads with the use of melted Indium. With the use of the “Master’s Contacts Soldering Station”, assembled at the beginning of the PhD, it is possible to heat up the Indium at 325◦C
with a pressure of 20 bar applied for 2 min. A cooling (same pressure, releasing temperature smaller of the melting temperature of the Indium (156.6◦C), guarantee a permanent and efficient (for a mechanic and
electric point of view) fastening of the copper foil on the pads.
7.6
Anti sticking layer
At the end the stamps are functionalized with a monolayer of alkyl - trichlorosi-lane or Octadecil Trichloro Sitrichlorosi-lane (OTS) for easy release of the samples after the imprints [28]. The silicon pieces, if this passage is done immediately af-ter sputaf-tering, is ready for the deposition under vacuum. In other case, the cohesion of the layer can be increased with a general cleaning (Acetone and IPA) and a O2 plasma at RIE (40 W, 100 V, 4.30 × 10−1mbar, 30 sccm of O2,
60 s) for the surface activation.
7.7. Imprint
An important consideration: the anti sticking layer becomes the most close to the heating layer of the Pulsed-NIL stamp and for this reason it reachs the high temperature of the process. A series of experiment, not part of this thesis, demonstrated that there is not a degradation of the monolayer even if it is submitted to a hundreds of cycles of heating (with the current pulses), cooling and release of the samples.
7.7
Imprint
The characterization of the effect of micro- and nano- imprint on the elastic modulus were concentrated on PMMA. The work is based on a comparison between the the two types of imprint (standard and pulsed) and for this reason both samples were prepared by spin-coating a 1050 nm thick PMMA film of about 121.2 kg/mol molecular weight Mw on standard silicon wafer
substrates.
7.7.1
Standard Thermal NIL
With this initial thickness, the time for a complete imprint cycle using stan-dard T-NIL is about 25 minutes (including heating/cooling), however it is known that the total indentation time is in the range of 5 s [29]. The samples for the characterization with AFM were imprinted at 150◦ with a pressure of
100 barand a indentation time of 10 min.
7.7.2
Pulsed Thermal NIL
CHAPTER
8
ATOMIC FORCE MICROSCOPY
8.1
Introduction
Atomic-force microscopy (AFM) is a technique that uses a sharp tip to probe the surface of a sample providing topographic and other physical information about a surface with a resolution on the order of a nanometer. By AFM it is possible to image, measure obtaining maps of properties of the sample such as height, profilem friction forces, work function, magnetic domains, etc.
For imaging, the reaction of the probe to the forces that the sample imposes on it can be used to form an image of three-dimensional shape (to-pography) of a sample surface at a high resolution. The instrument records, for every pixel, the height of the probe as a function of the lateral position. The signal is commonly displayed as a pseudo-color plot. Simultaneous with the acquisition of topographical images, other properties of the sample can be measured locally and displayed as an image, often with similarly high res-olution. Examples of such properties are mechanical properties like stiffness or adhesion strength and electrical properties such as conductivity or surface potential.
8.2
Configuration
Figure 8.1: Typical configu-ration of an AFM. (1): Can-tilever, (2): Support for can-tilever, (3): Piezoelectric el-ement(to oscillate cantilever at its resonance frequency.), (4): Tip (Fixed to open end of a cantilever, acts as the probe), (5): Detector of de-flection and motion of the cantilever, (6): Sample to be measured by AFM, (7): xyz drive, (moves sample (6) and stage (8) in x, y, and z di-rections with respect to a tip apex (4)), and (8): Stage.
The sample is positioned on a sample stage that can move in x,y, and z direction to follow the scan in order to maintain the tip at fixed distance from the surface (averaging on the high-frequency oscillations of the cantilever), using, as feedback, the signal of the laser, usually on a 4 quadrant photodiode sensor, measuring the deflection of the beam. Some AFM systems have the sample blocked and static while the scan head moves in x and y direction with the drive.
In the present work the AFM was used in “contact mode”. The tip is brought into contact with the sample, and the sample is raster scanned along an x-y grid. An electronic feedback loop is employed to keep the probe-sample force constant during scanning. This feedback loop has the cantilever deflection as input, and its output controls the distance along the z axis between the probe support and the sample support.
As long as the tip remains in contact with the sample, and the sample is scanned in the x-y plane, height variations in the sample will change the deflection of the cantilever. The feedback then adjusts the height of the probe support so that the deflection is restored to a user-defined value (the set-point).
A properly adjusted feedback loop adjusts the support-sample separation continuously during the scanning motion, such that the deflection remains approximately constant. In this situation, the feedback output equals the sample surface topography to within a small error[30] [31].
Sys-8.3. Samples analysis by AFM - Icon Scan
tem with ScanAsyst was used. The system uses the PeakForce QNMTM
(Quantitative Nanoscale Mechanical characterization) technique patent by the Bruker company.
8.3
Samples analysis by AFM - Icon Scan
AFM analysis with a Bruker Dimension Icon AFM gives a high resolution mechanical feedback at each tapping point providing spatially correlated to-pography and mechanical property mapping within a single scan [11], [12].
While the topography is recorded in tapping mode, the AFM tip mea-sures a force distance curve for each topography pixel (Figure 8.3). In the experiments the tapping frequency of PeakForce QNMTM was set to 2 kHz
and the line speed was chosen between 1 and 10 µm/s [Figure 8.2].
Figure 8.2: Bruker Dimension Icon Atomic Force Microscope System with ScanAsyst
The graph in figure 8.3 shows a typical force-distance curve where the modulus is fitted by the software from the dotted region using an imple-mented Hertzian model with adhesion (DMT: Derjaguin - Muller - Toporov) [32], [33]. In a zone that is ideally fully elastic, this method uses an indenta-tion in the range of 3 - 5 nm to image local modulus variaindenta-tions.
Figure 8.3: Operating principle of a Bruker Icon scan with PeakForce QNM mode scanning the surface in tapping mode with a low tapping frequency of 2 kHz while typical line speeds are in the range of 1 to 10 µm/s. The force -distance curve at each tapping pixel is defined by: surface approach (1 and 2), point of deepest indentation (3), force release and surface restoration (4), point of largest adhesion (5) and released cantilever (6).
Secondly, in the remaining parts of AFM calibration one has to measure the spring constant and the tip radius. Both procedures are complicated, quite long and cannot be applied to all cantilevers. There is another source of difficulties: the tip degrades resulting in a change of its radius during a series of scans. This is important because the Young’s modulus in the DMT-model is inversely proportional to the square root of the tip radius [12]. A change of the radius could happen either by wear, deformation or by collection of contaminations from the surface.
For this study of the Young’s modulus, a “comparative method” was used. The idea behind is to scan a reference sample (polystyrene film on silicon, supplied by the Bruker. Commercial name: PSFILM-12M1), that has a
known value of Young’s modulus (near 2.7 GPa), and to normalize the results to this value.
8.4
Selection of the tips
The selection of the correct AFM probe is quite easy. The idea is to start from the nominal values of Young’s modulus of the reference and of the PMMA. From the figure 8.4, two tips were selected to cover a large range of modulus. The AFM tips used for this analysis were TAP525A with a Young’s modulus
8.4. Selection of the tips
range of 1-20 × 109Pa and RTESPA with a range of 0.2 - 2 × 109Pa [34]
(Figure 8.4).
CHAPTER
9
AFM DATA ANALYSIS
9.1
Introduction
The AFM data analysis of imprinted polymer materials can be performed in two ways: one is faster and more qualitative, the other more rigorous and quantitative.
To evaluate the quality of the structures, indeed, a series of inspection methods were used. Normally, already, a bare eye look can reveal the main quality of the structures and the uniformity of the imprint or macroscopic flaws due to particles or problems occurred during the nano imprint process. Areas, that look different, imprinted with a stamp with uniform and a peri-odic pattern indicate clearly a non complete imprint or a problem regarding the releasing of the stamp (ripped-away structures).
The next level is the inspection with optical microscopes and different focal lengths to a 2D-top vision of the structures (in case of lines, width and period can be controlled, and the general aspect of topography as the presence of voids or microscopic flaws).
SEM (Scanning Electron Microscope) offers the possibility to perform imaging at nanoscopic resolution with 2D view (from the top), tilted view (3D view) and in cross section. The software provides a series of instruments for the characterization of the dimension of the structures.
Other instruments are used for an accurate analysis of the structures and, in particular the AFM, as described in chapter 8.
The preparation of the stamp is described in chapter 7, whereas the pa-rameters of the standard imprint and Pulsed-NIL are in the chapter 4 and 5.
The samples have these characteristics:
• pattern of lines 4 µm width with a period of 8 µm and a height of the structures of 525 nm
• the resist was PMMA (with average weight of 121.2 k`/mol). The initial thickness of the spin-coated film was around 1050 nm, resulting in a residual layer, after the imprint with complete filling of the cavities, of 800 nm, important to avoid the influence of the substrate (silicon) in the measurement of the modulus due to compression of a soft material against a rigid surface.
9.2
Profile analysis
The simplest analysis was obtained by profilometry. The profilometer, with a sharp tip, scans the surface along a line. It is possible to adjust some parameters such as the force applied, the length of the scan, the speed (with a trade-off between accuracy and acquisition time) and the resolution (number of points).
9.2.1
Results
9.3. 3D thopography by AFM 0 1 2 3 4 5 6 0 100 200 300 400 500 µm nm
Figure 9.1: Profiles of a line imprinted with standard NIL (red) and Pulsed-NIL (blue) using the same master
9.3
3D thopography by AFM
(a) standard T-NIL (b) Pulsed-NIL
Figure 9.2: 3D AFM topography of line structure having 4 µm width, 8 µm period, 500 nm height and being imprinted in PMMA
9.4
Roughness
An analysis of the surface roughness was performed with the help of the software GWYDDION 2.37 for data visualization and analysis [34]. In aver-age the roughness (given by equation 9.1), was approximately 1.1 nm for the structures imprinted by standard T-NIL and 1.7 nm in the case of the pulsed-NIL imprint. The root mean squared roughness (equation 9.2), was 1.4 nm and 2.1 nm for the standard and the pulsed imprint, respectively (Table 9.1). Each of the formulas listed assumes that the roughness profile has been fil-tered from the raw profile data, subtracting the mean value and the average slope. The roughness profile contains n ordered, equally spaced points along the trace, and yi is the vertical distance from the mean line to the ith data
point. Height is assumed to be positive in the upward direction.
Ra = 1 n n ∑ i=1 | yi | (9.1) Rq = √ 1 n n ∑ i=1 y2 i (9.2)
9.5. Mechanical properties
Therefore, the mechanical properties of the imprinted microstructures with the two methods could be compared point-by-point on the correspond-ing regions of their respective maps.
Standard T- NIL Pulsed T-NIL
Bottom Top Bottom Top
Arithmetic average Ra 1.1 nm 1.0 nm 1.7 nm 1.6 nm
Root mean squared Rq 1.3 nm 1.4 nm 2.0 nm 2.1 nm
Table 9.1: Comparison of the roughness of the imprinted structures confirms that both methods result in very comparable surface patterns and allow for a good comparison of the elastic properties between them.
9.5
Mechanical properties
At the end of a scan a topographic image (Figure 9.3(a)) and the correspond-ing AFM elastic modulus image (Figure 9.3(b)) relative to the polystyrene reference are produced with the same lateral resolution. During the analysis of the histogram of the modulus (for example the blue graph in figure 9.4), it is possible to introduce the spring constant of the cantilever and the radius of curvature of the tip (within a typical range provided for a given type of tips) to reproduce the correct value for Young’s modulus, which is 2.7 GPa for the polystyrene reference sample. This procedure reduces significantly the time for calibration with respect to an absolute calibration.
Going back to the issue of degradation of the tip during a series of mea-surements, this introduces an artifact not only during the calibration but also in the measurement of the samples. This spurious effect depends on the “landing behavior” on the surface and on the indentation depth into the poly-mer, and introduces a drift in the estimated Young’s modulus that requires to be compensated. For this reason, after the series of scans on the samples it is necessary to scan again the polystyrene reference sample. Assuming a linear behavior of the calibration factor as a function of the number of scans, the drift of calibration can be compensated linearly scan-by-scan on the Young’s Modulus histograms.
(a) (b)
Figure 9.3: a) AFM topography, b) AFM elastic modulus map of the PS reference sample surface.
consequent experimental error reduces the possibility to detect and resolve small changes of the modulus and for this reason the histogram of the final scan on polystyrene appears narrower. Linearly interpolating the Young’s modulus between the initial and final reference scan allows to compensate in the data analysis for the continuous AFM tip wear.
Figure 9.5, shows a typical example of a scan on imprinted PMMA. It is possible to observe the uniformity of the average value of the modulus on the micrometer scale and the uniformity between the top (center) and bottom(lateral) of the lines. At the same time local fluctuations from the average value at the scale of a few tens of nanometers are clearly detected. The rather high number of points being analyzed (512 x 512 points) allows an accurate sampling of the various regions of the imprinted structures. The sharp step in the Young’s modulus along the lines center are related to the edge of the imprinted lines where the tip is not applying its force normally to the surface, as it is traveling across the steep gradient (525 nm high step); clearly this area needs to be disregarded from the analysis as it has been affected by measurement artifacts.
Finally, it is possible to determine the average value of the Young’s mod-ulus of the PMMA surface (Figure 9.6). In that phase, due to the deci-sion to use the relative method, all values were normalized to the value of the polystyrene reference, i.e. 2.7 GPa. Therefore the x-axis represents the Young’s modulus in units of the reference material (normalized Young’s mod-ulus E*).
