Manufacturing aa6061 micro molds by hot embossing for production of polymeric microfluidic devices

72 Chapter 4 Hot embossing with silicon master for fabrication of AA6061 micro-mold and its use for hot embossing polymeric micro-channels .... Manufacturing AA6061 micro-molds by hot em

MAUFACTURIG AA6061 MICRO-MOLDS BY HOT EMBOSSIG FOR PRODUCTIO OF

POLYMERIC MICROFLUIDIC DEVICES

TRA HAT KHOA

MAUFACTURIG SYSTEMS AD

TECHOLOGY SIGAPORE-MIT ALLIACE

AYAG TECHOLOGICAL UIVERSITY

2012

MAUFACTURIG AA6061 MICRO-MOLDS BY HOT EMBOSSIG FOR PRODUCTIO OF

POLYMERIC MICROFLUIDIC DEVICES

TRA HAT KHOA

(B Eng, Ho Chi Minh City University of Technology)

A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF

PHILOSOPHY I MAUFACTURIG SYSTEMS

AD TECHOLOGY (MST)

SIGAPORE-MIT ALLIACE

AYAG TECHOLOGICAL UIVERSITY

2012

DECLARATIO

I hereby declare that this thesis is my original work and it has been written by me and its entirety I have duly acknowledged all the sources of information

which have been used in the thesis

This thesis has also not been submitted for any degree

in any university previously

Tran hat Khoa

02 July 2012

Acknowledgements

This work could not have been done without contributions of many great people during my PhD study What I have learnt in this four year will be very helpful not only in my future career but also in my life I sincerely thank the following people for their kind help

First and foremost, I would like to give the deepest gratitude to my thesis advisor, Professor Lam Yee Cheong, for his tremendous support and careful guidance over the last four years I have achieved a lot through discussions with him about research as well as many other aspects of life With me, he is always one of the best advisors

I would like to thank Professor Lallit Anand at MIT for accepting me

as his student I especially thank him for his guidance and supporting during

my stay at MIT

I would also like to take this opportunity to express my profound gratitude to Professor Yue Chee Yoon and Professor Tan Ming Jen for their valuable comments and suggestions

I am deeply indebted to Professor Tran Doan Son, Professor Pham Ngoc Tuan, and Professor Nguyen Huu Loc at Ho Chi Minh University of Technology in Vietnam, who have introduced and supported me to obtain SMA scholarship

I would like to thank my lab-mates Rajeeb, Roy, Saha, Jeffry, Hendra, Mohana, Lip Pin, and Phuong for their kind support when needed I would also like to thank my lab-mates Shawn, David, Kaspar, Claudio for their

support during my stay at MIT I would particularly like to thank Shawn for sharing graciously his knowledge as well as many fruitful discussions

I would also like to thank all lab technicians from the School of Mechanical and Aerospace Engineering, Nanyang Technological University, who have supported me during my research

Last but not least, I would like to thank my family for their understanding and patience Their emotional support is a great source of my strength My partner-to-be, Lan Anh, shared with me many wonderful experiences during my long journey and I would like to thank for her love and support

Table of Contents

Acknowledgements i

Table of Contents iii

Summary vii

List of Tables ix

List of Figures x

List of Abbreviations xix

Chapter 1 Introduction 1

1.1 Research Motivation 1

1.2 Research Objectives 3

1.3 List of publication related to this thesis 4

1.3.1 Journal papers 4

1.3.2 Conference and symposium papers 4

1.4 Organization of the thesis 5

Chapter 2 Current concept on deformation mechanism of polycrystalline materials and methods to fabricate mold for microfluidic devices 7

2.1 Current concept on deformation mechanism in forming process of polycrystalline materials 7

2.2 Current methods in mold fabrication for making polymeric microstructures 9

2.2.1 High precision micromilling 9

2.2.2 Micro Electrical Discharge machining 14

2.2.3 Laser micromachining 20

2.2.4 Laser microcutting combined with laser microwelding 23

2.2.5 Electrochemical micromachining 26

2.2.6 Micro powder injection molding 29

2.2.7 LIGA process 32

2.2.8 Photolithography and Deep Reactive Ion Etching (DRIE) 35

2.2.9 Soft lithography 38

2.2.10 Cold embossing 43

2.2.11 Superplastic embossing 47

2.2.12 Hot embossing on metals 51

2.2.13 Hot embossing on polymer 58

2.2.14 Hot embossing on bulk metallic glass 61

2.2.15 Comparison of different mold-making methods 63

Chapter 3 Deformation phenomenon for micro-hot-formability of polycrystalline materials 66

3.1 Experimental preparation 66

3.1.1 AA6061-T6 specimens 66

3.1.2 Manufacturing silicon master by photolithography and DRIE 68

3.2 Etching AA6061 specimen for grain size determination 71

3.3 Deformation mechanism of AA6061 in micro hot embossing experiments 72

Chapter 4 Hot embossing with silicon master for fabrication of AA6061 micro-mold and its use for hot embossing polymeric micro-channels 77

4.1 Fabrication of AA6061 micro-mold via hot embossing with silicon master 77

4.2 Hot embossing on TOPAS 8007 using an AA6061 mold 80

4.3 Evaluation of hardness, roughness and strength of AA6061 micro-mold 83

4.3.1 Tempering process 83

4.3.3 Hardness measurements 87

4.3.4 Tensile strength measurements 89

4.3.5 Accelerated testing 91

4.4 Fabrication of different complex micro-features on AA6061 94

Chapter 5 Analyses of aluminum alloy 6061 micro-mold fabrication 101

5.1 Effect of AA6061 orientation (rolling direction) on filling of silicon master during hot embossing 101

5.2 Large-deformation theory of isotropic elastic-viscoplastic materials 105 5.2.1 Introduction 105

5.2.2 Finite-deformation theory of isotropic elastic-viscoplastic solids (This is adopted from Anand [58-59] ) 105

5.2.3 Material parameters for hot forming model of AA6061 127

5.2.4 Validation experiments and simulation 130

5.3 Comparison of numerical results of hot embossing process with corresponding experiments 135

Chapter 6 Conclusions 140

6.1 Conclusions 140

6.2 Recommendations for future work 143

Bibliography 145

Appendix A Grain size calculation of AA6061 151

A.1 Sample and reagent preparation 151

A.2 Etching aluminum alloy 6061 151

A.3 Grain size calculation 162

Manufacturing AA6061 micro-molds by hot embossing for production of polymeric

microfluidic devices

by Tran Nhat Khoa Submitted to the School of Mechanical and Aerospace Engineering

on 1st July, 2011, in partial fulfillment of the requirements for the degree of

Doctor of Philosophy

Abstract

In the manufacturing of microfluidic devices, the micro-molds will not only affect the overall manufacturing cost but also determine the quality of the molded micro-parts Thus, the focus of this research is to investigate if an aluminum alloy micro-mold could be fabricated by hot-embossing using silicon (Si) master with acceptable cost, quality, and life span By employing the appropriate processing parameters, experiments conducted indicate conclusively that the deformation mechanism of aluminum alloy 6061 (AA6061) in micro hot embossing is the plastic deformation of the grains themselves As such, contradictory to conventional wisdom, this investigation shows that grain size is not a determining factor on the smallest feature that could be reproduced for a polycrystalline material Using the processing methodology developed, AA6061 micro-molds were successfully fabricated and the effectiveness of these molds was examined by hot embossing on TOPAS 8007 substrates Finally, Anand’s large deformation theory for isotropic plastic solids has been demonstrated to be adequate in predicting the forming process of AA6061 in micro hot embossing

Thesis Advisors:

1 Prof Lam Yee Cheong, SMA Fellow, NTU

2 Prof Lallit Anand, SMA Fellow, MIT

Summary

The main focus of this thesis is the fabrication of AA6061 mold for the manufacturing of polymeric microfluidic devices In the forming process of polycrystalline metallic material, grain size is believed to be the limiting factor that determines the minimum feature size fabricated This investigation showed conclusively that the deformation mechanism of AA6061 near its solidus temperature is plastic deformation of grains themselves As such, grain size ceases to be the limiting factor on the minimum feature size With a proper choice of a set of embossing parameters, it was demonstrated that micro-features much smaller than the grain size can be fabricated on AA6061 substrate AA6061 molds containing fine features were fabricated with excellent fidelity by hot embossing using a Si master The ability of embossing different features with complex geometry on AA6061 has also been demonstrated through the successful replication of T-shaped, I-shaped or micro-mixer shaped features

Subsequently, the AA6061 mold was employed to emboss on TOPAS

8007 substrates to illustrate that a mold so fabricated could be used for mass production

Temperature cycling during the hot embossing step in AA6061 mold manufacturing process reduces significantly the original tensile strength and hardness of the mold, which is not desirable As such, in this study, a tempering process was carried out to recover the tensile strength and hardness

of the embossed mold Surface roughness, tensile strength, and hardness values were measured in each stage: (i) before hot embossing, (ii) after hot

embossing and (iii) after T4 tempering and T6 tempering The results obtained demonstrate that the original strengths and hardness can be fully recovered by

a post-tempering process after hot embossing, but with an increase in surface roughness

Accelerated testing was carried out to evaluate the changes in hardness and roughness of AA6061-T4 and T6 molds under the typical hot embossing temperature cycles of manufacturing polymeric devices The results obtained indicate that these temperature cycles have only a minor effect on the roughness of both T4 and T6 molds and will increase the hardness of T4 molds to T6 temper, and have a negligible effect on the hardness of a T6 temper mold

This study shows that when the embossing temperature is near the solidus temperature, AA6061 behaves as an isotropic material, i.e the forming ability is the same in all directions As such, it is appropriate to employ Anand’s constitutive model for hot deformation of isotropic materials to predict the filling process of AA6061 in micro hot embossing The material parameters for the model were obtained from constant true strain-rate compression experiments The predictive capability of this constitutive model was first validated by comparing predictions against macro-scale experimental results such as plane-strain cruciform forging and axi-symmetry forging Subsequently, obtained results for micro-hot embossing demonstrated that Anand’s constitutive model predict well the form process of micron-scale features near the solidus temperature of AA6061

List of Tables

Table 1: Embossing parameters for Silicon and PC molds, PC, PMMA and

COP substrates [47] 60

Table 2: Comparison of different methods of making micro-molds 65

Table 3: Composition of AA6061 67

Table 4: Lubricant table for compression tests of AA6061 128

List of Figures

Fig 1: Illustration of micro-formability of coarse polycrystalline, ultra-fine grained and amorphous metals [9] 9Fig 2: (a) Machined aluminum wafer containing negative channel relief features created by CAD program (b) Acrylic mold created from aluminum wafer (c) PDMS channel profile at T-section (d) PDMS channel geometry [13] 11Fig 3: SEM of micro structures milled in brass (a) High aspect ratio wall of 20

µm wide and 400 µm tall (20:1) (b) Cross structure finished with a 100 µm radius milling bit (c) Cross structure finished with a 25 µm radius milling bit [12] 12Fig 4: (a) Micro-milled mold master (b) Sidewall of mold (c) Molded PMMA substrate (d) Sidewall of PMMA [12] 12Fig 5: SEM image of as- µEDMed Ta insert of 12 regular protrusions with length, width and height of ∼9,500 µm, ∼170 µm, and ∼400 µm respectively and ∼750 µm center to center spacing [14] 14Fig 6: (a) Embossed feature on Al (b) Close-up view of one typical channel [14] 16Fig 7: (a) Embossed feature on Cu (b) Close-up view of one typical channel [14] 16Fig 8: SEM images of (a) Ni electrode and (b) as-µEDMed Ta blank [15] 17Fig 9: SEM images of (a) Ni electrode with an array of micro gears with teeth

on external diameter and (b) as-µEDMed Ta blank [15] 17Fig 10: SEM images of (a) Ni electrode with an array of micro gears with teeth on both external & internal diameters and (b) as-µEDMed Ta blank [15] 18Fig 11: Holes machined in SD plates of different diamond particle sizes [16] 19

Fig 12: Different stages of prototype fabrication: initial design by CAD module; filling strategy for each of 40 layers (CAM module); laser patterned

steel substrate; demolded PMMA part [20] 21

Fig 13: CAD model (left) and top-view of micro-tool (right) [21] 22

Fig 14: Mold insert made of PI (left) and molded part made of PMMA (right) [20] 23

Fig 15: Micro-mold fabrication process by laser microcutting and laser microwelding [23] 25

Fig 16: Experiment setup of simple Y channel mixer, 75 µm width and 50 µm height [23] 25

Fig 17: Micro channels fabricated by ECM die-sinking method [24] 27

Fig 18: Five ribs manufactured by electrochemical milling on stainless steel [19] 28

Fig 19: (a) Debound φ100 x 200 µm microstructures using mold temperature of 120 ºC and (b) Sintered φ100 x 200 µm height microstructures at 1300ºC [27] 30

Fig 20: (a) φ60×191 µm height molded microstructures using mold temperature of 120°C (ICP) and (b) broken φ60×191 µm height microstructures (ICP) [27] 30

Fig 21: SEM micrographs of freestanding micro-parts made by (a) ZrO2 and (b) Al2O3 (b) [28] 31

Fig 22: Process steps of LIGA [30] 33

Fig 23: (a) Mask used for synchrotron radiation lithography, (b) PMMA resist structure, (c) separation nozzle made of nickel, and (d) secondary plastic template [30] 34

Fig 24: (a) PMMA square box and (b) nickel electroplated box [31] 35

Fig 25: Fabrication process of silicon mold [32] 36

Fig 26: DRIE process [32] 36

Fig 27: Before TMAH etching (left) and after TMAH etching (right) [32] 37

Fig 28: (a) Silicon comb-drive actuator mold and (b) PMMA substrate [32].38Fig 29: Schematic diagram illustrating fabrication planar (left) and orthogonal 3D (right) embossing tool [36] 40Fig 30: SEM images of channels of various dimensions 5 µm deep, 40 µm wide and 90 µm center-to-center (a), 90µm deep, 300 µm wide and 500 µm center-to-center (b) and 250 µm deep, 600 µm wide and (c) 1mm center-to-center [36] 41Fig 31: (a) Orthogonal 3D PDMS mold (b) Embossed orthogonal 3D micro-channels in PMMA substrate [36] 42Fig 32: Deformation of PDMS: pairing (left) and sagging (right) [38] 43Fig 33: (a) Silicon die-straight channel structure and (b) straight channel structure in 99.5% pure aluminum [39] 44Fig 34: (a) An intact a-Si:N coated silicon insert after one molding run (b) the corresponding replicated aluminum structure Images of insets are of higher magnification images of corresponding structures [40] 45Fig 35: Silicon mold fabrication process with e-beam lithography and dry etching [41] 46Fig 36: (a) Nanostructure on silicon mold (b) Embossed pattern on Al thin film and (c) AFM analysis of imprinting depth of embossed pattern [41] 47Fig 37: (a) Silicon die-complex structure (b) Superplastic embossed Zn78Al22 (c) Molding precision in superplastic embossed Zn78Al22 (cross section) [39] 48Fig 38: Microstructure of fine-grained Zn-22Al eutectic alloy [42] 49Fig 39: Typical curve of force vs stroke for straight bevel gear in a closed-die forging process The forging steps of straight bevel gear were also illustrated [42] 50Fig 40: SEM microphotograph of mini spur gear made from fine-grained Zn-22Al eutectic alloy [42] 51Fig 41: Ti-C:H coated micro-scale Ta insert used to mold Al [44] 52

Fig 42: Morphologies of Al structures molded at 360 °C (a) typical molded coupon (b) cross-sectional view of parallel array of micro-channels (c) corresponding top view of micro-channel array and (d) typical micro channel side wall [44] 54Fig 43: (a) A series of parallel, rectangular micro-channel in Cu (b) Close-up view of the corner of one typical micro-channel [45] 54Fig 44: (a) A series of parallel, rectangular micro-channel in Ni (b) Close-up view of the corner of one typical micro-channel [45] 55Fig 45: (a) Silicon mold produced by ICP-DRIE (b) Microstructured silver plate after forming at 400 °C with 300 MPa pressure and mold removal by KOH [46] 56Fig 46: Top view of silver plate after forming in silicon as in Fig 45b Large grains of 10 -100 µm are visible in the nearly undeformed metal on left side of picture In contrast, the severely deformed material forming pillars (top inset) and lines (bottom inset) presents a submicron grain size [46] 57Fig 47: (a) Submicron silver structure reproduced from silicon mold (insets are enlarged view of pillar with diameter between 250−500 nm) (b) Enlarged view of 250 nm wide “L”-line of Fig.43a [46] 58Fig 48: Production of working tool by hot embossing [47] 59Fig 49: SEM micrograph of Y-shaped junction in 50 µm wide by 10 µm deep channels (a) on silicon master (b) on PC working tool (c) On PMMA substrate (d) On COP substrate [47] 61Fig 50: (a) SEM image of silicon tool (b) Close-up of side-wall of channel in silicon tool showing scalloping produced by DRIE (c) SEM image of embossed channel in Vitrelooy-1 (d) Close-up of side-wall of channel in embossed BMG [48] 62Fig 51: (a) Comparison between cross-sections of silicon and BMG (b) Three-dimensional image of a channel in silicon (c) Corresponding image of the embossed channel in BMG 63Fig 52: DSC test of AA6061-T6 68

Fig 53: Instron machine for hot embossing 70Fig 54: Schematic of hot embossing test 71Fig 55: Grain boundary of AA6061-T6 72Fig 56: Microstructure of AA6061 prior to hot embossing experiment The image was colored using a X1 inverse-pole-figure color map, where X1 is the horizontal direction 72Fig 57: (a) SEM image of straight channels on silicon master; (b) SEM image

of embossed straight channels on AA6061-T6; (c) Superimposing channel profiles of silicon master and AA6061-T6 substrate; (d), (e), and (f) SEM images of X-, Y-, and O-shaped micro-features with vertical sidewall embossed on AA6061-T6 74Fig 58: Grain boundary on surface of AA6061 channel after hot embossing.75Fig 59: (a) SEM image of straight channels on silicon master (b) SEM image

of embossed straight channels on AA6061-T6 (c) Close-up image of one channel on silicon master (d) Close-up image of one embossed channel on AA6061 76Fig 60: (a) Schematic of plane-strain tool (not to scale), and (b) SEM micrograph of a portion of silicon tool 79Fig 61: (a) SEM image of silicon master (b) Close-up of channel side-wall in silicon tool showing micron-level scalloping produced by DRIE (c) SEM image of embossed ridges in AA6061 (d) Close-up of ridge side-wall in embossed AA6061 showing micron-level scalloping produced by hot-embossing 79Fig 62: (a) Comparison of cross-sections of silicon mold and embossed feature in AA6061 (b) Three-dimensional image of channels in silicon mold (c) Corresponding image of embossed ridges in AA6061 substrate 80Fig 63: DSC graph of TOPAS 8007 81Fig 64: (a) Comparison of cross-sections of AA6061 mold and embossed feature in TOPAS 8007 (b) Three-dimensional image of ridges in AA6061

mold (c) Corresponding image of embossed micro-channels in TOPAS 8007 substrate 82Fig 65: SEM micrographs of (a) Embossed micro-channels in TOPAS 8007 (b) Micro-channels in silicon master 83Fig 66: Surface roughness measurement locations on (a) AA6061-T6 specimen before hot embossing (b) AA6061 specimen after hot embossing and after tempering Channel length and width are 2 mm and 200 µm respectively 85Fig 67: Average roughness and standard deviation of all locations on top surface and at bottom of valleys of micro-channels of 3 specimens before and after hot embossing, after T4 and T6 tempering 86Fig 68: Surface roughness on top surface of AA6061 mold (a) after tempering to T4 and (b) after tempering to T6, measured by PLµ Confocal Imaging Profiler 87Fig 69: Hardness measurement locations on (a) AA6061-T6 specimen before hot embossing (b) AA6061-T6 specimen after hot embossing and after tempering Channel length and width are 2 mm and 200 µm respectively 87Fig 70: Average and standard deviation of hardness of 3 specimens at 9 locations before hot embossing, after hot embossing, after T4 and T6 tempering 89Fig 71: Tensile tests of AA6061 in stages: before hot embossing, after hot embossing, tempering to T4 and tempering to T6 90Fig 72: Evolution of hardness of AA6061-T4 and T6 specimens during accelerated testing 93Fig 73: Evolution of roughness of AA6061-T4 specimens during accelerated testing 94Fig 74: Evolution of roughness of AA6061-T6 specimens during accelerated testing 94Fig 75: SEM micrographs of (a) micro micro-mixer shaped features on silicon master and (b) corresponding embossed features on AA6061 substrate (right)

The channel on silicon master has a width of about 200 µm and a depth of about 60 µm 96Fig 76: SEM micrographs of (a) micro micro-mixer shaped features on silicon master and (b) corresponding embossed features on AA6061 substrate (right) The channel on silicon master has a width of about 200 µm and a depth of about 60 µm 97Fig 77: SEM micrographs of (a) features with different geometry on Si master (right) and (b) corresponding embossed features on AA6061 substrates The channel on silicon master has a width of about 200 µm and a depth of about 60

µm 98Fig 78: SEM micrographs of (a) Series of micro-holes on Si master and (b) corresponding embossed features on AA6061 The micro-holes on Si master have a diameter of about 200 µm and a depth of about 60 µm 99Fig 79: SEM micrographs of (a) Micro-mixer shaped, T shaped and I shaped features on Si master and (b) Corresponding embossed micro-features on AA6061 substrate The channels on Si masters have a width of 30 µm and a depth of 30 µm; aspect ratio 1:1 100Fig 80: 8 mm × 8 mm square silicon master with channels in four different orientations 102Fig 81: 3D images of channels of different orientations on silicon master 102Fig 82: SEM images of channels of different orientation on silicon master 103Fig 83: 3D images of channels of different orientations embossed on AA6061-T6 103Fig 84: SEM images of channels of different orientations embossed on AA6061-T6 104Fig 85: Superimposed channel profiles on silicon master and AA6061-T6 104Fig 86: A typical compression specimen 127Fig 87: (a) AA6061 specimen before compression (left), and AA6061 specimen after compression to a true strain of 1.0 (right) Fit of the model to

experimental stress-strain curves for AA6061 at three temperatures: 450°C, 500°C, 550°C and three strain rates: (b) 10-3 1/s, (c) 10-2 1/s, (d) 10-1 1/s The experimental data curves are plotted as solid lines, while the fitted curves are shown as dashed lines 129Fig 88: (a) Schematic of the plane-strain cruciform forging experiment (b) Quarter-symmetry finite-element mesh for the workpiece and the rigid surface for plane strain cruciform forging simulations 131Fig 89: Comparison of numerically-predicted and experimentally-measured force-displacement curves for forging of AA6061 at 500°C to displacement of (a) 4 mm, and (b) 5 mm The experimental curves are plotted as dashed lines, while the numerical predictions are shown as solid lines 132Fig 90: Comprarison of numerically-predicted and experimentally-measured unloaded deformed shapes for cruciform forging at 500°C and a die-displacement of 4 mm (a) experimental macrograph; (b) deformed mesh; (c) outlines of simulated shapes superimposed over the experimentally-measure shapes 132Fig 91: Comprarison of numerically-predicted and experimentally-measured unloaded deformed shapes for cruciform forging at 500°C and a die-displacement of 5 mm (a) experimental macrograph; (b) deformed mesh; (c) outlines of simulated shapes superimposed over the experimentally-measure shapes 132Fig 92: Schematic of the axi-symmetric forging experiment 134Fig 93: (a) Half-symmetry finite element mesh for the workpiece and the rigid surfaces employed in the axi-symmetric spike forging simulation of AA6061 (b) Comparison of numerically-predicted and experimentally-measured force-displacement curves at 500°C 134Fig 94: Comparison of numerically-predicted and experimentally-measured unloaded deformed shapes for the axi-symmetric spike forging at 500°C after die-displacement of 5 mm (a) experimental macrograph; (b) deformed mesh; (c) outlines of simulated shapes superimposed over the experimentally-measure shapes 135

Fig 95: Finite element mesh for plane-strain simulation showing the meshed AA6061substrate and the silicon tool modeled as a rigid surface The prescribed displacement boundary conditions are u1=0 on portions AD and BC

of the mesh boundary, and u1=u2=0 on portion CD of the mesh 137Fig 96: Comparisons of SEM micrographs from micro-hot-embossing experiments on AA6061 against corresponding simulations at 500°C, at the following embossing pressures: (a) 28.91 MPa, and (b) 30.40 MPa The plane-strain simulation has been extruded and mirrored for ease of comparisons 138Fig 97: Comparison of experimentally-measured and numerically-simulated ridge cross-sections at embossing pressures of 28.91 MPa and 30.40 MPa 138Fig 98: (a) SEM image of micro-ridges hot-embossed in AA6061 at embossing pressure of 60 MPa, and (b) corresponding numerical prediction; plane-strain simulation has been extruded and mirrored for ease of comparison 139Fig 99: Comparison of experimentally-measured and numerically-simulated ridge cross-sections 139

List of Abbreviations

AA6061-T6 Aluminum alloy 6061 in T6 temper

HARMS High-aspect-ratio micron scale structures

Chapter 1 Introduction

1.1 Research Motivation

A microfluidic device can be defined as any apparatus incorporating flow passages in microns, i.e from around 1 µm to hundred of micron [1] In these devices, tiny quantities of solvents, samples, and reagents are steered through narrow channels on the chips or devices, where they are mixed and analyzed by such techniques as electrophoresis, fluorescence detection, immunoassay, or indeed almost any classical laboratory method [2]

Microfluidics has three main advantages over a conventional scale laboratory operation First, due to the small size of the device, one test only needs a few tens or hundreds of nanoliters of reagents or solvents and, sometimes, reactions in miniature are more accurate and faster than that in macro-scale For example, capillary electrophoresis is more accurate in narrow channels because reaction heat lost through the walls is faster, eliminating the thermal effect that could otherwise impede accurate separation [2] Secondary, microfluidics technology can be automated to eliminate the need of human interference Routine assays and sample preparation can be performed in standardized chips (and an associated chip reader) with little human intervention, reducing the element of human error intrinsic to experimental

macro-“wet” biology The third advantage, and generally accepted as the most important, is integration A lab-on-a-chip can be designed with multiple “on-board” functions −for example: purification, labeling, reaction, separation, and

detection −with the sample being automatically guided from one location to another on the chip until the entire operation is complete [2]

Microfluidics is now an integral part of precision miniaturized on-a-chip” technology, a laboratory on a single chip of only millimeters to a few square centimeters in size allowing analysis and chemical manipulation of small samples Other applications for microfluidic devices include capillary electrophoresis, multicomponent reactions, bio-sensors, genetic analysis, single-cell analysis, cell migration, drug screening and long-term culture of stem cells and neurons [3] Many of these applications have utility in clinical diagnostics or in animal sciences Microfluidic devices may also be used to measure parameters such as molecular diffusion coefficients, fluid viscosity and density, pH and those related to reaction kinetics

“lab-Although there are many methods to fabricate polymeric microfluidic devices such as micro-injection molding, hot-embossing, soft lithography and micro-casting, most of them are carried out in a similar fashion: replication of

a pattern from a tool (or mold) which can be metal, silicon, polymer and other materials to a polymer substrate Micro-mold plays an important role in the manufacturing process of microfluidic devices; the quality of a mold determines the quality of the microfluidic devices because all the features on the mold will be transferred to the substrate Moreover, micro-mold can affect the manufacturing cost by its fabrication methods and materials which determine the life span of the mold Consequently, along with the development of large scale production of polymeric microfluidic devices is necessity of the development of the various methods of mold fabrication

Currently, there are many methods and materials for mold fabrication for the manufacturing of microfluidic devices Among them, hot embossing on polycrystalline a metal substrate resulting in a metallic mold has good potential However, current accepted deformation mechanism indicates that the size of micro-features fabricated on a polycrystalline material will be restricted by its grain size, i.e micro-features smaller than the grain size were envisaged not possible to be hot embossed on a polycrystalline metal If at certain process parameters, deformation mechanism other than the current accepted deformation mechanism can be shown to exist, and which do not have the grain size limitation on the fine features produced, will be a significant breakthrough It will allow the production of micro-molds with fine features on polycrystalline metals which are durable, and well suited for large scale production

1.2 Research Objectives

There are three main objectives in this investigation:

o Investigating the deformation mechanism in micro-forming of a polycrystalline material, namely aluminum alloy 6061 The possibility

of fabricating features smaller than the grain size of the metallic substrate will be explored

o Manufacturing AA6061 micro-mold by hot embossing method for fabricating polymeric microfluidic devices and evaluating the strength, roughness and hardness of AA6061 mold fabricated

o Investigating the predictive capability of Anand’s isotropic viscoplastic model for micro hot forming of aluminum alloy This can

elastic-be applied not only in the mold-making industry, but also in the field

of fabricating aluminum micro-parts for microelectromechanical systems (MEMS) or micro-systems technology

1.3 List of publication related to this thesis

Y C Lam, N K Tran, C Y Yue, M J Tan “New deformation

phenomenon for micro-formability of polycrystalline materials” Materials Science and Engineering: A, vol 528, no 3, pp 1906-1909, 2011

N K Tran, Y C Lam, C Y Yue, M J Tan “Evaluation of roughness, hardness, and strength of AA 6061 molds for manufacturing polymeric

microdevices” International Journal of Advanced Manufacturing Technology,

1.3.2 Conference and symposium papers

N K Tran, Y C Lam, C Y Yue, M J Tan “Aluminum alloy mold for micro-hot embossing of polymeric micro-devices” in Singapore MIT Alliance Symposium, Singapore, 2010

Y C Lam, N K Tran, C Y Yue, M J Tan “Surface roughness, hardness and strength of an aluminium mold fabricated by hot embossing” in Australasian Congress on Applied Mechanics, Perth, Australia, 2010

N K Tran, Y C Lam, C Y Yue, M J Tan “Aluminum alloy 7075 mold for manufacturing polymeric micro-devices” in Advances in Microfluidics and Nanofluidics (AMN) and Asian-Pacific International Symposium on Lab on Chip (APLOC), Singapore, 2011

N K Tran, Y C Lam, C Y Yue, M J Tan “Fabricating protruded micro-features on AA6061 substrates by hot embossing method”, International Conference on Mechanical, Aeronautical and Manufacturing Engineering, Tokyo, 2012

1.4 Organization of the thesis

The structure of this thesis is as follows Chapter 2 provides a detail review of the current concept about forming behavior of polycrystalline materials and methods on micro-molds fabrication for microfluidic devices In chapter 3, the deformation phenomenon for micro-formability of polycrystalline materials at high temperature is presented Chapter 4 introduces the manufacturing of an AA6061 mold by hot embossing using a silicon master The roughness, hardness and strength of fabricated AA6061 mold is also evaluated in this chapter In Chapter 5, Anand’s isotropic elastic-

viscoplastic constitutive model for the hot forming of metals was employed to predict the micro hot embossing process on AA6061 The material constants for this model are obtained from constant true strain rate compression tests The predictive capability of this constitutive model is first validated by comparing the experimental results in some inhomogeneous macro-scale experiments (plane-strain cruciform forging, axi-symmetry forging) against those from corresponding numerical simulations Chapter 6 summarizes this thesis It presents conclusions on fabricating AA6061 molds with some final remarks and a discussion on future research directions

Chapter 2 Current concept on

In the micro-forming of polycrystalline metallic alloys, grain size is believed to be a limiting factor on the minimum size of the geometrical features that can be produced by micro-forming [5-10] Illustration of the current concept of deformation of coarse polycrystalline, ultra-fine grained and amorphous metals in a micro-forming process is shown in Fig 1 With this perception, it appears that to fabricate metallic micro-parts with micro-scale

which have no crystal structure, are the only options Ultra-fine grained alloys can be fabricated by severe plastic deformation (SPD) techniques [11], which could reduce the alloy grains to submicron scale [7] As such, these alloys have been employed to manufacture micro-parts with features in the range of 5–50 µm [5] With the absence of grain boundaries, amorphous metals have been demonstrated to have no difficulties in filling the micro-cavity of the mold with good fidelity [7]

The production and preparation of ultra-fine grained and amorphous alloys are more involved and costly as compared to normal polycrystalline alloys This drawback poses limitations in their wide adoption for the mass production of metallic micro-parts as compared to the potential offered by normal coarse grained polycrystalline alloys As such, there is a need to re-examines the concept that the smallest feature that can be replicated in coarse polycrystalline metals is constrained by its grain size Specifically, investigation must be performed to re-visit the notion that micro-replication across grain boundaries would lead to discontinuities in the micro-feature

Fig 1: Illustration of micro-formability of coarse polycrystalline, ultra-fine

grained and amorphous metals [9]

2.2 Current methods in mold fabrication for making polymeric microstructures

2.2.1 High precision micromilling

High precision micro-milling is a simple method of metallic mold master fabrication This method does not involve lithographic steps As a result, it does not require clean room environment Thus it is accessible to those who do not have lithographic-based equipments Moreover, in this method, only three fabrication steps are required: design, CNC milling and finishing as compared to ∼15 and ∼10 steps for X-ray LIGA and UV lithography-based techniques [12]

Zhao et al [13] fabricated microstructures (64 µm wide and 17 µm deep) on aluminum substrate by micromilling First, the aluminum plate was polished down to 0.15 µm roughness and then micromilled to create the negative channel features by CNC high precision mill machine as shown in Fig 2a Once the first negative master was created in the aluminum substrate, the positive relief mold was constructed with acrylic (Fig 2b) Acrylic is a heat insulator, which has a glass transition temperature (Tg) of 105 °C and a crystalline melting temperature (Tm) of 200 °C When acrylic was heated above its Tm, only the acrylic surface close to the aluminum mask reached the set temperature As a result, this surface gradually transformed into a viscous liquid and sank into the negative features on the aluminum mask The depth of the channel created in the acrylic was strongly dependent on the heating time

It was approximately 7−20 µm after heating above Tm in 2 hours Polydimethylsiloxane (PDMS) microfluidic devices were finally obtained by casting on the acrylic negative mold (Fig 2c and Fig 2d) The entire fabrication process from concept to realization took less than 8 h and thus could be used to rapidly prototype microfluidic designs The advantage of this method was that milling the negative structures on an aluminum substrate was relatively easy and could produce smaller features than the process of milling the positive structures

Fig 2: (a) Machined aluminum wafer containing negative channel relief features created by CAD program (b) Acrylic mold created from aluminum wafer (c) PDMS channel profile at T-section (d) PDMS channel geometry

[13]

Hupert et al [12] used high-precision micromilling machine to mill a

brass plate to produce microstructure The whole milling process was monitored by an attached microscope whereas the tool length and radius were automatic detected by a laser measuring system In one fabrication cycle, high-precision micromilling was capable of producing multi-level structures with highly vertical sidewalls and aspect ratios exceeding 20:1 (Fig 3a) However, it is unable to make sharp corner due to the finite size of the milling bit, with the corner having a radius of the milling bit (Fig 3b) Using smaller milling bits can minimize the curvature of the corners (Fig 3c) but at the same time it limits the achievable height of the structure due to the useful flute length of the milling bit For example, aspect ratio of commercially available micromilling bits is typically less than 3 As such, milling bits with diameter

of 25 µm will have an useful flute lengths of 75 µm; thus, the maximum height of the microstructures is ∼75 µm The feed rates optimized for maximum speed and quality of the micro structures were 200 mm/min for a

500 µm bit, 100-150 mm/min for 200 µm bit, 50-75 mm/min for 100 µm and 10-20 mm/min for a 50 µm bit The speed of micromilling process was set at 40,000 rpm

Fig 3: SEM of micro structures milled in brass (a) High aspect ratio wall of 20

µm wide and 400 µm tall (20:1) (b) Cross structure finished with a 100 µm radius milling bit (c) Cross structure finished with a 25 µm radius milling bit

[12]

Fig 4: (a) Micro-milled mold master (b) Sidewall of mold (c) Molded

PMMA substrate (d) Sidewall of PMMA [12]

The brass mold milled with 100 µm milling bit was employed to emboss Polymethylmethacrylate (PMMA) substrate to test the molding capability of the micro-milled brass plate The SEM images of the brass mold and the PMMA substrate are shown in Fig 4 The width and depth of the channel were 100 µm and 90 µm respectively Milling marks could be observed on the sidewall of the brass mold (Fig 4b) The imperfection of the cutting edge of the milling bit was propagated in the same direction as the milling bit movement (parallel to the mold master floor) and was a direct indicator of the quality of the milling bit As shown in Fig 4d, during hot embossing, the milling marks were transferred to the sidewall of PMMA micro-channel

hot-o Advantages hot-of micrhot-o-milling:

• Capability of highly vertical sidewalls and aspect ratios [12]

• Wide selection of materials can be used for mold fabrication [12]

• The reduction in production time and cost as compared to silicon

or glass wafers [13]

• The utilization of fairly simple instrumentation and facilities The only necessary tool is a high precision miller, which is present in most machine shops [13]

• There is no limitation on the size of the created device [13]

o Disadvantages of micro-milling:

• Unable to make sharp inside corner due to the intrinsic feature of the process itself [12]

• Higher wall roughness of micro structure as compared to lithography based technique [12]

• Difficult to achieve the fine resolution and or minimum feature size of most lithographic techniques [12]

2.2.2 Micro Electrical Discharge machining

The micro-electrical-discharge-machining (µEDM) process, derived from the conventional EDM process and adapted from micromachining, can generate high-aspect-ratio micron scale structures (HARMS) in various engineering materials Typical machining technologies for µEDM are wire EDM (WEDM), sinking EDM (SEDM), EDM drilling, EDM milling, and wire electro discharge grinding (WEDG)

Fig 5: SEM image of as- µEDMed Ta insert of 12 regular protrusions with length, width and height of ∼9,500 µm, ∼170 µm, and ∼400 µm respectively

and ∼750 µm center to center spacing [14]

Cao et al [14] reported the successful fabrication of Ta mold insert by µEDM The insert had a cross-sectional area of 9500 µm x 9500 µm, and was

2000 µm in height A SARIX high precision micro-erosion machine was

employed for the µEDM with the blade electrode made from 500 µm thick Mo sheet metal The processing parameters, namely discharge frequency, on-time width, and maximum voltage, were set at 150 kHz, 4 µs, and 100 V respectively IonoPlus/3000 was used as the dielectric medium for the machining process Subsequently, the insert was then subjected to the following steps: (a) electro chemical polishing (ECP) of as-µEDMed micro-scale Ta features, and (b) deposition of a conformal Ti-C:H coating over the micro-scale Ta features The Ta insert was then employed for the replication

of HARMS in AL and Cu by compression molding (Fig 5) During embossing, the substrate temperature and the insert temperature were ∼361°C and∼371°C respectively for embossing Al and ∼410°C and ∼423°C respectively for embossing Cu The SEM images of embossed Al and Cu are shown in Fig 6 and Fig 7 respectively The rectangular protrusions on the Ta insert are ∼160 µm in width, ∼750 µm in center-to-center spacing and ∼220

µm in depth for embossed Al or ∼350 µm for embossed Cu Fig 7a shows that the top surface between two channels was somewhat convex upward, indicating that material pile-up existed during compression molding of Cu Similar phenomenon regarding pile-up was also observed on molded Al Regardless of material pile-up, the channels were faithfully replicated onto the Al and Cu substrates The molding stress or applied pressure for embossing Al and Cu were 46 MPa and 187 MPa respectively

Fig 6: (a) Embossed feature on Al (b) Close-up view of one typical channel

array of micro-channels with two different widths (Fig 8), an array of hollow gears with teeth on the external diameter (Fig 9) and an array of hollow gears with teeth on both the external and internal diameters (Fig 10) could be replicated on the Ta or stainless steel blank The Ni based HARMS can subsequently be employed as a metal mold in the manufacturing of polymeric microfluidic devices

Fig 8: SEM images of (a) Ni electrode and (b) as-µEDMed Ta blank [15]

Fig 9: SEM images of (a) Ni electrode with an array of micro gears with teeth

on external diameter and (b) as-µEDMed Ta blank [15]

Fig 10: SEM images of (a) Ni electrode with an array of micro gears with teeth on both external & internal diameters and (b) as-µEDMed Ta blank

[15]

Other than metals, sintered diamond was employed with µEDM method Nakaoku et al [16] introduced the machining of micro holes and micro trenches on sintered diamond substrate by using µEDM The experiments were carried out on Panasonic MG-ED72 micro EDM machine The tools used in the machining process were fifty micrometers-diameter cylindrical tungsten electrodes, which were fabricated by wire electrodischarge grinding The discharge circuit was a relaxation-type pulse generator (RC circuit) and the charging resistance was 2 kΩ The residual inductance of the discharge loop was roughly estimated as 0.2 µH and the dielectric fluid used was EDM oil They employed four types of sintered diamond composing of 1-, 3-, 10-, and 20 µm diamond particles respectively (Fig 11) 50 µm through holes and 50 µm (width) × 4 mm (length) × 30 µm (depth) trenches were successfully fabricated on the sintered diamond substrate The diamond substrate could then be employed to hot-emboss polymer substrate to create microstructures