Silicon Needle-like Surfaces for Room Temperature Silicon-Silicon
Bonding Applications
Albert-Ludwigs-Universität Freiburg
Shervin Keshavarzi
Dissertation zur Erlangung des
Doktorgrades der Technischen Fakultät
15 kV X750 10 µm 00000
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Silicon Needle-like Surfaces for Room Temperature Si-Si Bonding Applications
Dissertation zur Erlangung des Doktorgrades der Technischen Fakultät
der Albert-Ludwigs-Universität Freiburg im Breisgau
vorgelegt von
Shervin Keshavarzi
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Autor: Shervin Keshavarzi
kesh@hs-furtwangen.de
Adresse: Albert-Ludwigs-Universität Freiburg
Technische Fakultät
Institut für Mikrosystemtechnik (IMTEK) Gorges-Köhler-Allee 102
79110 Freiburg Deutschland
Dekan: Prof. Dr. Rolf Backofen
Erstgutachter: Prof. Dr. Holger Reinecke
Zweitgutachter: Prof. Dr. Ulrich Mescheder
Drittgutachter: Prof. Dr. Jürgen Wilde
Vorsitzender: Prof. Dr. Jürgen Rühe
Beisitzer: Prof. Dr. Alfons Dehé
Tag der Disputation: 05. Dezember 2019
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“Your success and happiness lie in you”
- Helen Keller
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Abstract
This work presents and demonstrates a room temperature silicon-silicon bonding technique using porous silicon-based needle-like surfaces. Silicon (Si) surfaces are functionalized using anodic etching of silicon wafers in a hydrofluoric acid (HF) based electrolyte to create self-organized needle-like surfaces. Such self-organized needle- like surfaces allow repeated bonding and debonding of the same surfaces with adequate bond strengths at room temperature analogous to Velcro tapes. In the course of this thesis, a simple Si-based technology is described to generate Si needle- like surfaces through anodic etching (anodization) of lowly doped p-type Si wafers in an aqueous HF solution in the transition region (where pore formation and electro- polishing compete for control over the surface morphology). Impacts of anodization parameters, wafer resistivity ranges and crystal growth (fabrication) methods, and surface drying methods on morphology of needle-like surfaces are studied and discussed. An optimal process condition to generate such surfaces using 10 - 20 Ωm p-type Si wafers is obtained and presented. In addition, formation mechanisms of needles during anodic etching of 12 - 17 Ωcm p-type Si wafers in a 7.2 wt.% aqueous HF solution in the transition region is investigated through SEM surface images taken after different etch times. Finally, a simple model based on pore formation models is presented to describe formation of needles during this specific condition. The bonding mechanism between two similar Si needle-like surfaces at room temperature considering deformation and interaction mechanisms of needles is also investigated and mathematically modelled. Deformation mechanism of needles during the bonding is described by the cantilever beam approach, and Van der Waals forces and capillary forces are considered as responsible intermolecular forces in the interaction mechanism of needles. The bond strength between two similar needle-like surfaces due to each intermolecular forces is calculated based on the uncoupled multi-asperity approach by taking both tip and side interactions of needles into the account. At the end, capability of the bonding technique and validity of the proposed bonding model are demonstrated by comparison with experimental results through four different Si needle-like surfaces with different morphologies.
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Zusammenfassung
Diese Arbeit präsentiert und demonstriert eine Raumtemperatur-Silizium-Silizium-
Verbindungstechnik unter Verwendung von porösen nadelförmigen Oberflächen auf Siliziumbasis. Silizium (Si)-Oberflächen werden durch anodisches Ätzen von Siliziumwafern in einem Elektrolyte auf Basis von Flusssäure (HF) funktionalisiert, um selbstorganisierte, nadelförmige Oberflächen zu erzeugen. Solche selbstorganisierten nadelförmigen Oberflächen ermöglichen ein wiederholtes Bonden und Lösen derselben Oberflächen mit ausreichender Bondkraft bei Raumtemperatur vergeleichbar zu Klettverschluss Systemen. Im Rahmen dieser Arbeit wird eine neuartige Si-basierte Technologie zur Erzeugung von Si- nadelartigen Oberflächen durch selbstorganisierende Prozesse unter Verwendung von anodischem Ätzen (Anodisieren) von niedrig dotierten p-leitenden Si-Wafern in einer wässrigen HF-Lösung im Übergangsbereich (wo Porenbildung und Elektropolieren um die Kontrolle über die Oberflächenmorphologie konkurrieren) beschrieben. Auswirkungen von Anodisierungsparametern, Wafer Widerstandsbereichen und Methoden des Kristallwachstums (Herstellung) sowie Oberflächentrocknungsverfahren auf die Morphologie von Nadeloberflächen werden untersucht und diskutiert. Eine optimale Prozessbedingung zur Erzeugung solcher Oberflächen mit 10 - 20 Ωm p-Typ Si-Wafern wird erhalten und vorgestellt.
Darüber hinaus werden die Entstehungsmechanismen von Nadeln beim anodischen Ätzen von 12 - 17 Ωcm p-Typ Si-Wafern in einer 7,2 Gew.-%igen wässrigen HF-Lösung im Übergangsbereich durch REM-Oberflächenbilder nach unterschiedlichen Anodisierungszeiten untersucht. Schließlich wird ein einfaches Modell vorgestellt, das auf Porenbildungsmodellen basiert, um die Bildung von Nadeln während dieser spezifischen Bedingung zu beschreiben. Der Bondmechanismus zwischen zwei ähnlichen Si- Nadeloberflächen bei Raumtemperatur unter Berücksichtigung von Verformungs- und Wechselwirkungsmechanismen der Nadeln wird ebenfalls untersucht und mathematisch modelliert. Der Deformationsmechanismus der Nadeln während der Bindung wird mittels einem einseitig eingespannter Biegebalken (cantilever-beam) beschrieben, und Van-der- Waals-Kräfte und Kapillarkräfte werden als verantwortliche intermolekulare Kräfte im Wechselwirkungsmechanismus der Nadeln betrachtet. Die Bondfestigkeit zwischen zwei ähnlichen nadelförmigen Oberflächen aufgrund der jeweiligen intermolekularen Kräfte wird basierend auf dem entkoppelten Multiasperitätsansatz (uncoupled multi-asperity approach) berechnet, indem sowohl die Spitzen- als auch die Seitenwechselwirkungen der Nadeln berücksichtigt werden. Am Ende werden die Leistungsfähigkeit der Bondtechnik und die Validität des vorgeschlagenen Bonding-Modells durch den Vergleich mit experimentellen
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Ergebnissen an vier verschiedenen Si-Nadeloberflächen mit unterschiedlichen Morphologien demonstriert
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Acknowledgment
In my almost past seven years as a graduate student, I have been fortunate to be associated with several persons without whose valuable supports and inputs, this thesis would not have been possible. It is my great pleasure to express my gratitude towards them in my acknowledgement.
Foremost, I express my sincere thanks towards my advisor, Prof. Dr. Ulrich Mescheder for his valuable guidance, advices, and encouragements throughout the course of my research. I have benefited immensely from Prof. Mescheder mentorship.
He has introduced me to scientific research world and helped me a lot to grow as a scientist. For that, I am forever thankful to him.
I gratefully thank Prof. Dr. Holger Reinecke for taking time to serve on my thesis as supervisor and for his valuable suggestions and comments. His insightful comments have aided me to make this thesis better, and his appreciations have encouraged me in course of my research.
My thanks to past and present members of the Institute for Microsystems Technology (iMST) of Furtwangen University, Dr. Andras Kovacs, Xenia Seng, Alexander Filbert, Dr. Alexey Ivanov, Frederico Lima, Rui Zhu, and Andreas Weisshaar for their discussions and supports in the lab on countless occasions.
I would like also to thank the Ministry of Science Research and Art at the state of Baden Württemberg, Furtwangen University, and Freiburg University for the financial and material supports on the framework program entitled as “Corporative Doctoral Program for Generation Mechanisms of Microstructures I (GENMIK I)”.
Finally, I am deeply grateful to my mother, sister, brother-in-law, and fiancé (Kassy, Shadi, Koroush, and Banafsheh) for their unwavering love, for providing me the best educational opportunities, and their financial supports during these years.I am also grateful to my friends who have supported me along the way.
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Publications
Peer reviewed journals
I. S. Keshavarzi, U. Mescheder, and H. Reinecke. “Room temperature Si-Si direct bonding technique using Velcro-Like surfaces”. IEEE Journal of Microelectromechanical Systems (IEEE J-MEMS), vol. 25, no. 2, pp. 371-379, 2016.
DOI: 10.1109/JMEMS.2016.2519823
II. S. Keshavarzi, U. Mescheder, and H. Reinecke. “Effect of capillary forces in the room temperature Si-Si dircet bonding technique using Velcro-like surfaces“.
IEEE Journal of Microelectromechanical Systems (IEEE J-MEMS), vol. 26, no.
2, pp. 385-395, 2017.
DOI: 10.1109/JMEMS.2016.2646759
III. S. Keshavarzi, U. Mescheder, and H. Reinecke. “Formation mechanisms of self-organized needles in porous silicon based needle-like surfaces”. Journal of the Electrochemical Society (JES), vol. 165, no. 3, pp. E108-E114, 2018.
DOI: 10.1149/2.0501803jes
Conference proceedings
I. Sh. Keshavarzi, U. Mescheder, H. Reinecke, and A. Kovacs. “Contact mechanics and needle-like surfaces for micro-nano integration”. 23rd micromechanics and Microsystems Europe Workshop (MME). Proceeding, pp.
28, sec. D15. Sept. 9th-12th 2012, Illmenau, Germany.
ISBN: 978-3-938843-71-0
II. Sh. Keshavarzi, U. Mescheder, and H. Reinecke. “Modeling the adhesion between bonded nanoneedle surfaces based on Velcro® or Geko Principles”.
Mikrosystemtechnik Kongress (MST). Proceeding, pp. 864-867. Oct. 14th-16th 2013. Aachen, Germany.
ISBN: 978-3-8007-3555-6
III. S. Keshavarzi, U. Mescheder, and H. Reinecke. “Characterization and simulation of low temperature Si-Si direct bonding through Velcro-like surfaces based on porous silicon”. 27th International Conference on Micro Electro- Mechanical Systems (IEEE MEMS). Proceeding, pp.1119-1122. Jan. 26th-30th 2014. San Francisco, USA.
DOI: 10.1109/MEMSYS.2014.6765842
IV. S. Keshavarzi, U. Mescheder, and H. Reinecke. “Bonding mechanism in the Velcro concept Si-Si low temperature direct bonding technique”. 28th International Conference on Micro Electro-Mechanical Systems (IEEE MEMS).
Proceeding pp. 413-416. Jan. 18th-22nd 2015. Estoril, Portugal.
DOI: 10.1109/MEMSYS.2015.7050977
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V. S. Keshavarzi, U. Mescheder, and H. Reinecke. “Formation mechanisms of needles in porous silicon-based needle-like surfaces”. The 232nd Electrochemical Society Meeting (ESC), Abstract vol. MA2017-02 no. 21 1017 Oct. 1st-5th, 2017. National Harbor, USA.
ISSN: 2151-2043
Publications outside of the framework of this dissertation
I. S. Keshavarzi, W. Kronast, F. Lima, and U. Mescheder. “Controlled energy release based on explosive porous silicon”. 30th International Conference on Micro Electro-Mechanical Systems (IEEE MEMS). Proceeding pp. 712-715.
Jan. 22th-26nd, 2017. Las Vages, USA.
DOI: 10.1109/MEMSYS.2017.7863507
II. S. Keshavarzi, F. Lima, W. Kronast, and U. Mescheder. “Sequential nanoexplosion using patterned porous silicon”. IEEE Journal of Microelectromechanical Systems (IEEE J-MEMS). vol. 27, no. 2, pp. 250-258, 2018.
DOI: 10.1109/JMEMS.2018.2797306
III. A. Kovacs, S. Keshavarzi, A. O. Perez, S. Palzer, and U. Mescheder. “Tunable light source for photoacoustic sensing applications”. 2018 IEEE Sensors Conference. Proceeding. Oct. 28th -31st, 2018. New Dehli, India.
DOI: 10.1109/ICSENS.2018.8589589
IV. U. Mescheder, S. Keshavarzi, and S. Chauhan. “New concepts for miniaturized MEMS based power systems”. International IEEE Conference and Workshop in Óbuda on Electrical and Power Engineering. Proceeding. Nov. 20th -21st, 2018. Budapest, Hungary.
DOI: 10.1109/CANDO-EPE.2018.8601151
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Table of content
Abstract ... i
Zusammenfassung ... iii
Acknowledgment... v
Publications ... vii
Table of content... ix
List of Figures ... xiii
List of Tables ... xix
List of Acronyms/Abbreviations ... xxi
List of Symbols ... xxv
1 | Introduction ... 1
1.1 Motivation ... 1
1.2 Objective ... 2
1.3 Summary ... 4
2 | State of the art ... 5
2.1 Silicon-Silicon bonding ... 5
2.2 Fabrication of silicon needle-like surfaces ... 17
2.3 Adhesion models based on intermolecular forces ... 22
2.4 Summary and comparison to the goals of the thesis ... 32
3 | Theoretical backgrounds, methods, and concepts ... 35
3.1 Anodic etching of silicon ... 35
3.2 Intermolecular forces ... 45
3.3 Summary ... 51
4 | Generation of silicon needle-like surfaces by anodic etching of silicon . 53 4.1 Deriving of required anodic etching parameters ... 53
4.2 Experimental investigations ... 59
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4.2.1 Formation of self-organized needles in silicon needle-like surfaces ... 59
4.2.2 Impact of wafer resistivity on morphology of needle-like surfaces ... 63
4.2.3 Impact of Si wafer fabrication method on morphology of needle-like surfaces………..………66
4.2.4 Effect of current density on morphology of needle-like surfaces ... 68
4.2.5 Effect of anodization time on morphology of needle-like surfaces ... 72
4.2.6 Effect of drying process on clustering behavior of needles ... 73
4.2.7 Impact of electrolyte additive on morphology of needle-like surfaces .... 74
4.3 Formation mechanisms of needles in the transition region in respect to pore formation models ... 76
4.4 Summary ... 78
5 | Room temperature Si-Si bonding technique using silicon needle-like surfaces ... 83
5.1 Experimental results ... 83
5.1.1 Samples preparation ... 83
5.1.2 Bond strength and rebondability ... 86
5.1.3 Bond interface and interlacing behavior of needles ... 96
5.1.4 Bond interface width ... 102
5.2 Summary ... 104
6 | Modeling and simulation of the room temperature Si-Si bonding using silicon needle-like surfaces ... 107
6.1 Modeling of the bonding between silicon needle-like surfaces ... 107
6.1.1 Deformation mechanisms of needles ... 109
6.1.1.1 Compression of needles ... 112
6.1.1.2 Lateral bending and breaking of needles ... 114
6.1.2 Interaction mechanisms of needles ... 117
6.1.2.1 Van der Waals force approach ... 118
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6.1.2.2 Contact mechanics approach ... 121
6.1.2.3 Capillary force approach ... 123
6.2 Mathematical representation of a needle-like substrate ... 127
6.3 Simulation of the bonding between silicon needle-like surfaces ... 128
6.4 Comparison between measurement and simulation results ... 135
6.5 Summary ... 143
7 | Conclusion and outlook ... 145
Appendix ... 151
A.1 Anodic current density-voltage curve ... 151
A.2 Bond interface and interlacing of needles ... 152
A.3 Extracting bond interface widths ... 154
Bibliography ... 157
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List of Figures
Figure 1. Overview of wafer bonding techniques categorized into the direct bonding and the bonding with intermediate materials (Retrieved from [37]). ... 6 Figure 2. Typical electrochemical etching cells for anodic etching of Si. a) single tank
cell arrangement and b) double tank cell arrangement. ... 36 Figure 3. The current density-voltage (J-V) curve of a p-type silicon wafer in a HF
based solution and positions of current density peaks.(Retrieved from [181]). .. 38 Figure 4. Left: equilibrium (V = 0), field and diffusion currents across the SCR on a
macro-pore tip and wall regions in a p-type Si. Right: field and diffusion currents under forward bias (V > 0). Note that due to geometric field enhancement around the pore tip, tip currents are always larger than the pore walls currents (retrieved from [186]). ... 41 Figure 5. Current variation and coverage of silicon oxide on surface of a pore bottom
(retrieved from [169]). ... 43 Figure 6. Formation of pores due to correlation of current bursts in time. Time is
increasing from left to right, black dot: active current bursts and hallow circles:
inactive current bursts, which dissolve oxide bumps (retrieved from [178]). ... 44 Figure 7. A liquid droplet on a solid surface to illustrate the Young-Dupré equation
(retrieved from [200]). ... 48 Figure 8. Schematic molecular structure of a liquid-vapor interface (retrieved from
[195]). ... 49 Figure 9. Formation of silicon islets: a) top view of randomly distributed macropores
with different sizes and b) top view of overlapped and widened macropores where the remaining bulk Si between them results in islets. ... 54 Figure 10. Critical current density as a function of HF concentrations for lowly doped
p-type Si in aqueous electrolytes. Parameters described by Lehmann and by van den Meerakker. (retrieved from [176]). ... 56 Figure 11. Measured J-V curve of a quarter of 4-inch CZ <100> 12-17 Ωcm p-type Si
wafer with a circular anodization aperture with an area of 5.7 cm² in a 7.2 wt.%
aqueous HF solution. ... 58 Figure 12. SEM images of surfaces anodized with different constant current densities
in a 7.2 wt.% aqueous HF solution for a duration of 40 minutes using quarters of a 4-inch <100> CZ 12 - 17 Ωcm p-type Si wafer with a circular anodization aperture with an area of 5.7 cm². ... 59 Figure 13. Top-view SEM surface images showing formation mechanisms of needles
during anodic etching of quarters of 4-inch <100> CZ 12 - 17 Ωcm p-type Si wafers with circular anodization aperture areas of 5.7 cm² at different etching times in a 7.2 wt.% aqueous HF solution with a constant current density of 50 mA/cm². ... 61
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Figure 14. Measured height of formed structures (asperities, irregular islets, pyramid- shape islets, and clustered needles) and its first derivate in respect to the time (dh/dt) as a function of anodization time. ... 62 Figure 15. Average dissolution valence number and dissolved Si mass as a function
of anodization time for quarters of CZ <100> 12-17 Ωcm p-type Si wafers with anodization aperture areas of 5.7 cm² anodized in a 7.2 wt.% aqueous HF solution with a constant current density of 50 mA/cm². ... 63 Figure 16. SEM images of surfaces obtained from anodization of CZ <100> p-type
silicon wafers with different doping concentrations in the lowly doped region in a 7.2 wt.% aqueous solution with a constant current density of 50 mA/cm² for 40 minutes. ... 65 Figure 17. Impact of silicon wafer fabrication method on morphology of needle-like
surfaces generated by anodic etching of <100> 10-15 Ωcm p-type Si wafers in a 7.2 wt.% aqueous HF solution with a constant current density of 68 mA/cm² for 70 minutes. Side view: 45° tilted in respect to y-axis. ... 68 Figure 18. Top-view SEM images of needle-like surfaces generated by anodization of
CZ <100> p-type silicon wafers in a 7.2 wt.% aqueous solution with various constant current densities in the transition region for 40 minutes. Surfaces a, b, and c with 10 - 20 Ωcm wafers and surfaces d, e, and f with 10 - 15 Ωcm wafers.
... 70 Figure 19. Impact of current density on geometry of needles generated by anodic
etching of CZ <100> 10 - 20 Ωcm p-type Si wafers in a 7.2 wt.% aqueous HF solution for a duration of 40 minutes. ... 71 Figure 20. Impact of current density on geometry of needles generated by anodic
etching of CZ <100> 10 - 15 Ωcm p-type Si wafers in a 7.2 wt.% aqueous HF solution for a duration of 40 minutes. ... 71 Figure 21. Impact of anodization time on geometry of needles produced by anodic
etching of 4-inch <100> CZ 10 - 15 Ωcm p-type Si wafers in a 7.2 wt.% aqueous HF solution with a constant current density of 68 mA/cm² minutes. ... 72 Figure 22.SEM images showing impact of drying process on clustering behavior of
needles. Side view (45° tilted in respect to y-axis). ... 74 Figure 23.SEM images of surfaces generated by anodic etching of <100> CZ 10 - 20
Ωcm p-type Si wafers in a 7.2 wt.% aqueous HF solution containing 5 wt.% ethanol with various constant current densities for a duration of 40 minutes. ... 76 Figure 24. Cross-sectional view of two formed PSi layers, a transition micro- or meso- PSi layer on top of a macro-PSi layer (retrieved from [234]). ... 77 Figure 25. a single needle with a diameter or width of ~ 2.4 µm (≈ 2 × 𝑆𝐶𝑅𝐿, 1.87 - 2.6
µm for 10-20 Ωcm p-type Si)) and a length of ~ 25 µm bent, stretched, and attached to other needles from top. Anodization parameters: current density = 70
xv mA/cm², substrate = 10 - 20 Ωcm <100> CZ p-type Si, electrolyte = 7.2 wt.%
aqueous HF, and anodization time = 40 minutes. ... 78 Figure 26. SEM images of generated needle-like surfaces with different applied
constant current densities and different wafer resistivity ranges in a 7.2 wt.%
aqueous HF solution for a constant duration of 40 minutes. Side views (45° tilted with respect to y-axis). ... 85 Figure 27. 2D schematic of the designed chip bonding apparatus and its components.
... 87 Figure 28. Chip bonder unit and its components and arrangement of chips. ... 87 Figure 29. 3D schematic of the bond strength measurement unit and its components.
... 89 Figure 30. Example of a pull-off force as a function of time measured by the Burster
8523 series force sensor. ... 89 Figure 31. First attempt bond strength results as a function of applied bonding load for
the needle-like surfaces 1 - 4: a) surface 1, and b) surfaces 2 - 4. Needle-like surfaces 1 and 4 with active bonding area of 0.25 cm² and needle-like surfaces 2 and 3 with active bonding area of 0.36 cm². ... 92 Figure 32. Normalized bond strength (first attempt bond strength) as a function of
applied bonding load for the needle-like surfaces 1 - 4. Needle-like surfaces 1 and 4 with active bonding area of 0.25 cm² and needle-like surfaces 2 and 3 with active bonding area of 0.36 cm². ... 93 Figure 33. Rebondability and bond strength results obtained from bonding of chips
from the needle-like surface 1 at constant applied bonding loads. Average of 3 bond attempts for every applied bonding load is shown. Inset: normalized bond strength in respect to maximum bond strength. ... 94 Figure 34. Rebondability and bond strength results obtained from bonding of chips
from the needle-like surface 2 at constant applied bonding loads. Average of 2 attempts for every applied bonding load is shown. Inset: normalized bond strength in respect to maximum bond strength. ... 95 Figure 35. Rebondability and bond strength results obtained from bonding chips from
the needle-like surface 3 at constant applied bonding loads. Average of 2 bond attempts for every applied bonding load is shown. Inset: normalized bond strength in respect to maximum bond strength. ... 95 Figure 36. Rebondability and bond strength results obtained from bonding of chips
from the needle-like surface 4 at constant applied bonding loads. Average of 3 bond attempts for every applied bonding load is shown. Inset: normalized bond strength in respect to maximum bond strength. ... 96 Figure 37. Bond interface between attached chips from the needle-like surface 1 due
to different applied bonding loads. The left-side surface is slightly shifted backwards in respect to the right-side surface. ... 98
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Figure 38. Bond interface between attached chips from the needle-like surface 3 due to different applied bonding loads. The right-side surface is slightly shifted backwards in respect to the left side surface. ... 99 Figure 39. The needle-like surfaces from the needle-like surface 1 after first
detachment and separation process. Left side surfaces of Figure 37. ... 100 Figure 40. The needle-like surfaces from the needle-like surface 3 after first
detachment and separation process. Right side surfaces of Figure 38. ... 101 Figure 41. Bond interface width measurement setup and position of reference points
on the upper chip holder. ... 103 Figure 42. The bond interface width of two bonded needle-like surfaces in respect to
applied bonding loads for the first bond attempt for needle-like surfaces 1 - 4. 104 Figure 43. 2D schematic of a cross secetional view of a bond interface between two
needle-like surfaces and interactive components of a needle in the interaction mechanisms. ... 108 Figure 44. 2D schematic of an ideal needle and its equivalent elements. ... 109 Figure 45. Deformation mechanisms of needles during bonding of two needle-like
surfaces. ... 111 Figure 46. An ideal needle under a normal point force and its equivalent models. The
deformation state is emphasized by solid lines. ... 112 Figure 47. A needle (cantilever beam) under a shear point force (retrieved from [239]).
The deformation state is emphasized by solid lines... 115 Figure 48. A needle (cylindrical cantilever beam) under various forces (retrieved from
[244]). ... 117 Figure 49. Geometrical parameters of a needle in the VdW needle-substrate
interaction. ... 118 Figure 50. Geometrical parameters of two interlaced needles in the VdW needle- needle interaction. ... 121 Figure 51. 2D schematic of the DMT sphere-plane interaction (retrieved from [237]).
... 122 Figure 52. 2D schematic of the hemisphere-plane capillary interaction and its
geometry parameters (Retrieved from[126]). ... 124 Figure 53. Geometrical parameters of interlaced needles in the capillary needle-needle
interaction. ... 126 Figure 54. Representing a needle-like substrate as a square binary matrix in which 1
represents a needle and 0 represents a gap between needles. ... 128
xvii Figure 55. Simulated and measured bond strengths of the bonded chips from the needle-like surface 1. Active bonding area: 0.25 cm². ... 133 Figure 56. Simulated and measured bond strengths of the bonded chips from the
needle-like surface 2. Active bonding area: 0.36 cm². ... 133 Figure 57. Simulated and measured bond strengths of the bonded chips from the
needle-like surface 3. Active bonding area: 0.36 cm². ... 134 Figure 58. Simulated and measured bond strengths of the bonded chips from the
needle-like surface 4. Active bonding area: 0.25 cm². ... 134 Figure 59. Impact of mismeasuring of the surface and the bond properties on
simulated bond strengths based on the VdW force model for the generated needle-like surface 1: a) height of clustered needles, b) bond interface width, c) distance between clustered needles, d) diameter of clustered needles, and e) clustered needle density. Original data with CDA = 0.3 nm means simulation results based on measured data and assuming 0.3 nm as closest distance of approach between interacting bodies. ... 138 Figure 60. Impact of mismeasuring of the surface and the bond properties on
simulated bond strengths based on the contact mechanics model for the generated needle-like surface 1: a) height of clustered needles, b) bond interface width, c) distance between clustered needles, d) diameter of clustered needles, and e) clustered needle density. Original data with uT = 0.1 means simulation results based on measured data and assuming 0.1 as Tabor parameter for contacting bodies. ... 139 Figure 61. Impact of mismeasuring of the surface and the bond properties on
simulated bond strengths based on the capillary force model for the needle-like surface 1: a) height of clustered needles, b) bond interface width, c) distance between clustered needles, d) diameter of clustered needles, and e) clustered needle density. Original data with β = 0.2 means simulation results based on measured data and assuming 0.2° as filling angle for the enclosed liquid between interacting bodies... 140 Figure 62. Impact of assumed parameters in the proposed adhesion models on
simulated bond strengths for the generated needle-like surface 1: a) closest distance of approach (CDA) for the VdW force model, b) Tabor parameter (uT) for the contact mechanics model, and c) filling angle (β) for the capillary force model.
... 141 Figure 63. Measured J-V curves of quarters of 4-inch CZ <100> p-type Si wafers with
different resistivity ranges in the lowly doped region with circular anodization apertures (with an area of 5.7 cm²) in a 7.2 wt.% aqueous HF solution. ... 151 Figure 64. Decreasing the bond interface width between two attached needle-like
surfaces due to an increase in the bonding load. The left side surface is slightly shifted backwards in respect to the right-side surface. Two sided arrows show width of a bond interface. ... 153
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Figure 65. Thickness reduction of four adhesive tapes as a function of applied bonding load. ... 154 Figure 66. Measured distance between two bonded chips and extracted bond interface
width as a function of applied load for the needle-like surface 1. ... 155 Figure 67. Measured distance between two bonded chips and extracted bond interface
width as a function of applied load for the needle-like surface 2. ... 155 Figure 68. Measured distance between two bonded chips and extracted bond interface
width as a function of applied load for the needle-like surface 3. ... 156 Figure 69. Measured distance between two bonded chips and extracted bond interface
width as a function of applied load for the needle-like surface 4. ... 156
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List of Tables
Table 1. Effect of substrate resistivity on morphology of anodized surfaces. ... 66 Table 2. Used materials and applied conditions for employed experimental
investigations. ... 81 Table 3. Surface properties and geometrical parameters of needles in generated
needle-like surfaces. ... 86 Table 4. Bond conditions for the first bond attempt for every used pair of probes for
the needle-like surfaces 1 - 4. ... 90 Table 5. The resultant stress at four arbitrary points on a needle shown in Figure 48.
... 117 Table 6. Number of needles and gaps between needles and size of square matrixes
used for simulations of the bond strength for the first bond attempt for the investigate needle-like surfaces. ... 130
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List of Acronyms/Abbreviations
AAO Anodic Aluminum Oxide AFM Atomic Force Microscopy ASE Advance Silicon Etching DCB Dual Cantilever Beam DI Deionized water
DMT Derjaguin, Müller, and Toporov CB Current Burst
CBM Current Burst Model
CDA Closest Distance of Approach CMP Chemical Mechanical Polishing CR Carbothermal Reduction
CTAC Cetyl Trimethyl Ammonium Chloride CVD Chemical Vapor Deposition
CZ Czochralski
DRIE Deep Reactive Ion Etching
FTIR Fourier Transform Infrared Microscopy
FZ Float Zone
GW Greenwood and Williamson HF Hydrogen Fluoride
HRTEM High Resolution Transmission Electron Microscopy JKR Johnson, Kendal, and Roberts
IC Integrated Circuit
ICP-RIE Inductively Coupled Plasma Reactive Ion Etching IR Infrared
KOH Potassium Hydroxide
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LA Laser Ablation
LD London-Dispersion Forces
MACE Metal Assisted Chemical Etching MBE Molecular Beam Epitaxy
MEMS Micro-Electro-Mechanical Systems MIR Mid Infrared Range
NEMS Nano-Electro-Mechanical Systems NRA Nuclear Reaction Analysis
PECVD Plasma Enhanced Chemical Vapor Deposition PSi Porous Silicon
PVC Polyvinyl Chlorine PW Pullen and Williamson RF Radio Frequency RIE Reactive Ion Etching RH Relative Humidity RMS Root Mean Square
SAM Scanning Acoustic Microscopy SCR Space Charge Region
SCRL Space Charge Region Length SEM Scanning Electron Microscopy SIMS Secondary Ion Mass Spectroscopy SOI Silicon on Insulator
TEM Transmission Electron Microscopy UHV Ultra High Vacuum
UV Ultraviolet Radiation VdW Van der Waals Forces VLS Vapor Liquid Solid
xxiii XPS X-ray Photoelectron Spectroscopy
XRR X-ray Reflection
xxiv
xxv
List of Symbols
𝑎 m Contact radius
𝐴 m² Cross sectional area
𝐴𝑐 m² Contact area
𝑎𝑙 Å Lattice constant
𝐴𝑠 m² Substrate area
𝑏 m Distance between an arbitrary point on a needle and applied forces
𝐶𝐻𝐹 wt.% HF concentration
𝐶𝑚 Jm6 Attractive interaction potential constant 𝐶𝑛 Jm12 Repulsive interaction potential constant 𝐶𝑃𝑆𝑖 A/cm² Constant
𝐶𝑆 kgf/mm² Hardness coefficient
𝑑 m Distance between a needle and the opposite substrate in bonded needle-like substrates
𝐷 m Distance between two bonded needle-like substrates (bond interface width)
𝑑′ m Distance between two interlaced needles
𝑑𝑔 m Distance between two adjacent needles on a needle-like substrate
𝑑𝑛 m Diameter of a needle
𝑑𝑝 m Pore diameter
𝑒 C Elementary charge
𝐸 MPa Young’s modulus
𝐸𝑎 eV Activation energy
𝐸𝐺 eV Band gap energy
𝐹 N Normal point force
xxvi
𝐹𝑎𝑑ℎ N Adhesion force 𝐹𝐶𝑜𝑛𝑡𝑎𝑐𝑡 N Contact force
𝐹𝐶𝑝−𝐶𝑎𝑝 N Capillary forces between a cylinder and a plane
𝐹𝐶𝑝−𝐶𝑀 N Cylinder-plane interaction force based on contact mechanics approach
𝐹𝐷𝑒𝑏𝑦𝑒 N Debye force
𝐹𝐾𝑒𝑒𝑠𝑜𝑚 N Keesom force
𝐹𝐿𝐷 N London dispersion forces 𝐹𝑛 N Point force acting on a needle 𝐹𝑁 N Needle-substrate interaction force
𝐹𝑁1 N Needle-substrate interaction force between a needle from the substrate-I and the substrate II
𝐹𝑁2 N Needle-substrate interaction force between a needle from the substrate-II and the substrate I
𝐹𝑁−𝐶𝑎𝑝 N Needle-substrate interaction force based on capillary force approach
𝐹𝑁−𝐶𝑀 N Needle-substrate interaction force based on contact mechanics approach
𝐹𝑁−𝑉𝑑𝑊 N Needle-substrate interaction force based on VdW force approach
𝐹𝑁−𝑉𝑑𝑊−𝑐𝑠 N VdW forces between cylindrical body of a needle and the opposite substrate in the needle-substrate interaction 𝐹𝑁−𝑉𝑑𝑊−ℎ𝑠 N VdW forces between hemisphere head of a needle and the opposite substrate in the needle-substrate interaction 𝐹𝑆 N Needle-needle interaction force
𝐹𝑆1,2 N Needle-needle interaction force between a needle from the substrate-I interlaced with a needle from the surface-II 𝐹𝑆−𝐶𝑎𝑝 N Needle-needle interaction force based on capillary force approach
xxvii 𝐹𝑆−𝐶𝑀 N Needle-needle interaction force based on contact mechanics approach
𝐹𝑆−𝑉𝑑𝑊 N Needle-needle interaction force based on VdW force approach 𝐹𝑆−𝑉𝑑𝑊−𝑐𝑐 N VdW forces between cylindrical bodies of needles in the needle-needle interaction
𝐹𝑆−𝑉𝑑𝑊−ℎ𝑐 N VdW forces between the hemispherical head of a needle and the cylindrical body of its partner needle in the needle-needle interaction
𝐹𝑠ℎ N Shear point force 𝐹𝑉𝑑𝑊 N VdW forces
ℎ m Hight of a truncated conical needle
𝐻 m Hight of a needle
𝐻𝐶 J Hamaker constant
𝐼 A Current
𝐼𝑑𝑖𝑓𝑓 A Diffusion current 𝐼𝑓𝑖𝑒𝑙𝑑 A Field current 𝐼𝑚 kgm² Moment of inertia 𝐼𝑝 m4 Polar moment of inertia 𝐼𝑡𝑖𝑝 A Current at a pore tip 𝐼𝑤𝑎𝑙𝑙 A Current at a pore’s walls 𝐽 A/cm² Current density
𝐽𝑏 A/cm² Current density on side walls of a pore 𝐽𝑂𝑥𝑖𝑑𝑒 A/cm² Oxidation current density
𝐽𝑃𝑆𝑖 A/cm² Critical current density
𝐽𝑠 A/cm² Current density on side walls of a pore bottom 𝐽𝑡 A/cm² Current density at a pore tip
𝑘 eV/K Boltzmann constant
𝐾 F/m Dielectric constant
xxviii
𝐿 m Length of cylinder or length of cylindrical body of a needle 𝐿1 m length of cylindrical body of needle 1
𝐿2 m length of cylindrical body of needle 2
𝐿𝐶 m Overlapping or interlacing length of interlaced needles
𝑙𝑟 m Azimuthal radius
m # Number of needles on the substrate-I
𝑀 - Moment
𝑚𝑆𝑖 kg Atomic mass of silicon
n # Number of needles on the substrate-II 𝑁𝐴 atoms/cm³ Acceptor concentration
𝑁𝑑 #/cm2 Needle density
𝑁𝑒 # Total elements of 1s and 0s 𝑛𝑖 atoms/cm³ Intrinsic carrier concentration 𝑁𝑔 # Number of gaps
𝑁𝑛 # Number of needles
𝑛𝑣−𝑎𝑣𝑒 - Average dissolution valence 𝑃𝑜 Pa Saturation vapor pressure 𝑃𝑣 Pa Actual vapor pressure
𝑟 m Distance between two molecules
𝑅 m Radius of the hemispherical head and the cylindrical body of a needle
𝑅1 m Radius of the hemispherical head and the cylindrical body of the needle 1
𝑅2 m Radius of the hemispherical head and the cylindrical body of the needle 2
𝑟𝑏 m Radius of base circle of a truncated cone 𝑟𝑐 m Radius of curvature of a meniscus 𝑅𝑒𝑞 m Equivalent radius
xxix 𝑅𝑝1, 𝑅𝑝2 m Principal radii of curvatures (in two normal planes that cut the interface along two principal) of curvature sections an
interface between liquid and gas 𝑟𝑟 m Meridional radius
𝑆 kg/mm² Hardness
𝑡 s Anodization time
𝑇 K Temperature
𝑇𝑡 Nm Torque
𝑈 - Lennard Jones Potential
𝑈𝑚𝑎𝑐𝑟𝑜𝑠𝑐𝑜𝑝𝑖𝑐 J VdW interaction potential obtained using the macroscopic theory
𝑈𝑁−𝑐𝑠 J VdW interaction potential between the cylindrical body of a needle and a substrate
𝑈𝑁−ℎ𝑠 J VdW interaction potential between the hemispherical head of a needled and a substrate
𝑈𝑆 J VdW interaction potential between two interlaced needles 𝑈𝜖 Nm Strain energy
𝑉𝑎 V Applied voltage
𝑉𝑏 V Built-in voltage
𝑉𝑚 m³/mol Molar volume 𝑋 J/molK Molar gas constant 𝑌 GPa Yield strength
𝑍0 - Critical gap at which adhesive stress falls to zero
𝛽 degree Filling angle
𝛾 mN/m Surface tension
𝛾𝐿𝑉 J/m² Interfacial energy at an interface between liquid and vapor 𝛾𝑆𝑉 J/m² Interfacial energy at an interface between solid and vapor 𝛾𝑆𝐿 J/m² Interfacial energy at an interface between solid and liquid
xxx
𝛿 m Lateral displacement
𝛥𝑚 kg Dissolved mass
∆𝑃 Pa Laplace pressure
∆𝛾 J/m² Work of adhesion
𝜀 F/m Permittivity of free space 𝜁 - Depth of the potential well
𝜂 - Infinite distance at which the inter-particle potential is zero 𝜃1 degree Contact angle between the hemispherical head or cylindrical body of a needle and formed liquid meniscus
𝜃2 degree Contact angle between a substrate and formed liquid meniscus
𝜆𝐾 - Kelvin length
𝜇𝑇 - Tabor’s parameter
𝜌1 kg/m³ Molecular density of molecule 1 𝜌2 kg/m³ Molecular density of molecule 2
𝜎 Pa Normal stress
𝜎𝑎 Pa Axial stress
𝜎𝑏 Pa Bending stress
𝜎0 Pa Adhesion stress
𝜎𝑥−𝑟𝑒𝑠𝑢𝑙𝑡𝑎𝑛𝑡 Pa Resultant stress in the x-axis 𝜎𝑥−𝑚𝑎𝑥 Pa Maximum stress in the x-axis
𝜏 Pa Shear stress
𝜏𝑑 Pa Direct shear stress
𝜏𝑡 Pa Torsional shear stress
𝑣 - Poisson’s ratio
𝜑 degree Polar angle between the axis of the pore and the radius vector placed to some point at the pore surface
𝜓 degree Contact angle that liquid forms with a solid
xxxi 𝜔 m Compression length of needle
𝜔𝑎 - Critical compression parameter
𝜔𝑐 m Compression length of the cylindrical body of a needle 𝜔ℎ m Compression length of the hemispherical head of a needle 𝜚 # Number of needles from the substrate-II, which interlaced with a needle from the substrate-I
𝜖 - Strain
xxxii
1
1 | Introduction
This chapter introduces this thesis work by stating necessities of using silicon needle-like surfaces for direct Si-Si bonding applications. Additionally, it states problems or unclear phenomena, which are planned to be solved or understood during this study.
1.1 Motivation
Bonding of a silicon wafer or a chip to another silicon wafer or chip or to other substrates (e.g., Pyrex wafer) and packaging of miniaturized sensing devices play a crucial role in Micro Electro Mechanical Systems (MEMS), Nano Electro Mechanical Systems (NEMS), optoelectronics, and Integrated Circuit (IC) packaging [1]. In such devices, the package must protect sensitive internal structures against environmental influences (e.g., temperature, moisture, high pressure, and oxidizing specie) and mechanicals shocks, ensuring a long-term mechanical stability and reliability of functional elements [2]. Direct Si-Si bonding techniques along with dry or wet etching techniques can be used to realize different membranes appropriate for pressure and force sensors [3], complex 3D accelerometer [4], and microfluidic micro-pumps [5].
Numerous techniques have been so far developed to bond silicon wafers or substrates together. These techniques are generally categorized into two main approaches [6]: i) surface direct bonding and ii) bonding with intermediate materials. Although process conditions used for these approaches are different, they all follow the same three steps consisting of surface preparation, surface contacting, and annealing. Typical silicon direct bonding techniques necessitate high annealing temperatures between 700 - 1000 °C to achieve sufficient bond strengths [7]. However, high temperatures can arise gas and void formation in cavities and damage temperature-sensitive devices [8].
Hence, several low annealing temperature (120 - 400 °C) bonding techniques, such as plasma activated bonding [9], sequential plasma activated bonding [10], medium vacuum bonding [11], and ultra-high vacuum bonding [12] have been recently developed. In these approaches, an ultra-cleaning process is required prior to the bonding, and a large number of voids can be also generated during the annealing
2
process [13]. Additionally, obtained bonds are permanent in all approaches. However, further miniaturization of microsystems and microelectronic devices towards “smart dust” [14] require new bonding techniques at room temperature with adequate bond strengths and pick and place capabilities [15]. One possible approach, which does not require any ultra-cleaning and annealing process and is even capable of multiple bonding and debonding of the same substrates with adequate bond strengths (similar to gecko or Velcro adhesion concept) is the room temperature Si-Si direct bonding technique using needle-like Si surfaces [16, 17]. Reversible alignment and fixation of needle-like surfaces may enable new methods for assembly of hybrid micromechanical systems, e.g., micro-optics, sensor and actuator arrays through a simple pick and place technique. Needle-like surfaces can be simply brought into a contact using a bonding load. In this way, interlaced or interlocked partners (needles) are adhered or bonded to each other due to intermolecular forces (e.g., Van der Waals forces or capillary forces) acting between them.
1.2 Objective
Adhesive ability of geckos and their quick movements on vertical surfaces have made them a great subject to study the bonding mechanisms in rebondable surfaces.
The secret of a gecko’s adhesive strength and ability lies in the structure and function of its feet and in its adhesive toe pads borne [18]. Two front feet of a tokay gecko with a pad area of around 227 mm² can produce up to 20.1 N force parallel to an interacting surface [19], which is enough to hold its body attached to the surface. The foot of a tokay contains approximately 14400 setae (bristle-like structure) per mm² [20]. A single seta is approximately 100 microns in length and 5 microns in diameter [21] and can produce an average force of 6.2 µN, and an average shear stress of 0.090 N mm-2 [22].
One of the simplest and cost-effective approaches to implement the gecko toe pad or the Velcro concept to wafer or chip level is anodic etching of surface of a silicon wafer in a solution containing hydrogen fluoride (HF). In the transition region (between the pore formation and the electropolishing), self-organized needle-like surfaces with random profiles, quite similar to gecko toe pads, can be generated by choosing proper anodization parameters (current density, electrolyte mixture, and HF concentration)
3 and substrate material properties (type, doping level, and crystal growth method). An anodically etched needle typically has smoother sidewalls compared with a needle generated by other cost-effective techniques such as the Reactive Ion Etching (RIE) since sidewalls roughness in needles produced by the RIE technique is strongly related to SF6 or O2 content and flow of the gas and increases accordingly [23]. This may make electrochemically etched needles more suitable and effective for room temperature Si-Si direct bonding applications since they can provide larger area of contact or interaction during interlacing, and consequently higher adhering force.
Additionally, surface of needles produced by the RIE etching are covered by a thin layer of SiFxOy (about 7-13 Å) due to redeposition of used chemicals (the passivation process used in this technique) [24]. This could result in a change in the nature of the Si or SiO2 surface, and consequently the bonding mechanism (especially the interaction mechanism) between these surfaces.
Although generation of Si needle-like surfaces through the anodic etching technique is presented in [16, 25–27], formation mechanisms of needles and impact of anodization parameters and substrate properties on morphology of needle-like surfaces are still unclear. Hence, understanding of formation mechanisms of needles during anodic etching of Si in the transition region and effects of anodization parameters and substrate properties on morphology of needle-like surfaces are one of the aims of this thesis work.
Bonding of two Si needles-like surfaces at room temperature can be carried out by pressing these surfaces together through a bonding load. In this way, needles from opposite surfaces interlace and interlock, and two surfaces adhere together due to van der Waals (VdW) forces or capillary forces acting between them. Presence of these forces are highly dependent on the bonding environment. For instance, VdW forces dominate other forces when the environment is dry, the distance between involved surfaces is in the range of interatomic spacing (~ 0.3 nm), and size of objects are in micrometer range [28]. However, when the environment is humid or surfaces are hydrophilic, capillary forces contribute to the bonding with a significant influence and dominate other forces [29].
Adhesion between two rough surfaces based on intermolecular forces has been modeled by several researchers [30–34]. In these approaches, roughness has been defined in term of asperity, and an asperity has been assumed as a sphere. The
4
adhesion between two rough surfaces has been then obtained by considering interactions between contributing asperities or between asperities and a flat surface.
However, in the case of two needle-like surfaces, needles mostly interlace each other (side interaction) and at a certain condition interact with opposite substrates (tip interaction) during the bonding. Therefore, both side and tip interactions of needles must be considered in the adhesion of two needle-like surfaces. In addition, during the bonding process, needles may compress, bend, or some may break depending on amount of the applied bonding load and do not participate in the adhesion. Hence, developing a model to describe the bonding mechanism and estimating the adhesion between two needle-like surfaces are other goals of this thesis work.
1.3 Summary
In brief, the overall goals of this thesis are:
- Study the process of formation of needles in silicon needle-like surfaces fabricated by the anodization technique.
- Study the impacts of the process parameters on morphology of silicon needle- like surfaces.
- Study and model the room temperature bonding of silicon needle-like surfaces.
- Verify the validation of the proposed model by comparison to experimental results.
5
2 | State of the art
This chapter reviews the state of the art of Si-Si bonding, with emphasis on direct fusion bonding techniques; especially the room temperature bonding technique using Velcro-like and needle-like surfaces. It also reviews common fabrication techniques for creation of needle-like Si surfaces. Additionally, it outlines adhesion models based on intermolecular forces (VdW forces and capillary forces) between two interacted surfaces since the bonding between two Velcro-like or needle-like surfaces at room
temperature is mainly due to presence of these forces.
2.1 Silicon-Silicon bonding
The Si-Si bonding refers to local attachment of Si wafers or chips due to VdW forces, adhering materials, or chemical bonds when they are brought into a contact [35]. The Si-Si bonding has gradually become a key technology for materials integration in numerous areas of MEMS, optoelectronics, microelectronics, vacuum packaging, hermetic sealing, and encapsulation [36]. To bond Si wafers, various techniques have been developed. These techniques can be simply classified into two main groups [37]:
i) direct bonding techniques and ii) bonding techniques using intermediate materials.
Overview of Si wafer bonding techniques based on their classifications is shown in Figure 1 [37]. The common basic principle between all these bonding techniques is fusing of two materials by bringing them into a sufficiently close contact. Although most of these bonding techniques are widely used in MEMS and microelectronics industries, only fusion direct bonding techniques are the focus of this thesis.
6
Figure 1. Overview of wafer bonding techniques categorized into the direct bonding and the bonding with intermediate materials (Retrieved from [37]).
History of fusion bonding is gone back to 1725 when Desagulier showed that pressing of two lead (Pb) spheres together resulted in a strong adhesion [38]. The Pb spheres deformed enough for an intimate contact after plastic deformation of their rough surfaces due to a large external pressure [7]. However, bonding without an external pressure (spontaneous bonding) was reported for the first time in the early of 1900's in Sweden and Germany in experiments with polished metal pieces used in distance measuring tools [7, 39]. The effect was termed as “Ansprengen” (jumping contact) and considered as an undesirable and a harmful effect. However, it was later utilized to stick optical elements such as prisms in places without any interfacial layers [40]. The effect was studied for the first time in 1936 by Lord Rayleigh in “A study of glass surfaces in optical contact” where he had determined the interaction energy between polished flat silica spheres and plates [41]. This phenomenon was further investigated in 1969 by Tabor et al. as part of a study on interaction between surfaces [42]. He attributed this phenomenon to VdW interactions between adsorbed
Wafer bonding
Direct bonding
Fusion bonding
High tmeperature fusion bonding
Low temperature fusion bonding
Chemical activation
Plasma activation
Using ultra high vacuum
Using micro-velcro principle Anodic bonding
Silicon-glass laser bonding
Bonding with intermediate materials
Thermo compression bonding
Solder bonding
Eutetic bonding
Polymer bonding
Glass-frit bonding
7 monolayers of molecules with large dipole moments. An early form of wafer fusing bonding was reported in 1975 by Antypas and Edgecumbe [43]. This involved bonding of a thin film of gallium arsenide (GaAs) to a glass wafer at an elevated temperature of 600 °C. A mirror-like polished GaAs wafer was contacted and bonded to a mirror- like polished glass wafer by VdW interactions. The sandwiched wafers were then heated above 800 °C to fuse the layers together.
The hydrophilic fusion direct wafer bonding was first introduced in 1985 when Lasky et al. [44, 45] and Shimbo et al. [46] reported on bonding of Si to Si at room temperature followed by an elevated anneal temperature. Both groups used mirror- polished surfaces of Si wafers and placed them into a contact after a chemical cleaning stage, which had covered the surfaces with OH groups. The initial contact between surfaces was resulted in formation of a weak water-mediated bond that drew wafers into an intimate contact at room temperature. The bonded wafer pairs were then annealed above 800 °C to remove the water interlayer and to form strong permanent covalent bonds. Although the first explicit fusion direct wafer bonding literature was published in 1985, its theoretical background had established much earlier in 1969 by Armistead et al. [47], who provided a description of how silica and hydroxyl (Si − OH, or OH groups) are arranged on a silicon surface. The first detailed study of fusion based direct wafer bonding was published later in 1998 by Maszara et al. [48]. This was the first paper demonstrating many features, techniques, and assertions of the bonding technique. It particularly introduced the Double Cantilever Beam (DCB) test and began to develop hypotheses on mechanics of the wafer bonding. Several wafers were prepared and hydrophilically activated by wet ammonium hydroxide. The hydrophilically activated wafers were then contacted and annealed at temperatures up to 1400 °C to study influence of temperature on bond strength. Bond strengths were measured after annealing, and results were used to develop a bonding model consisting of three distinct stages (creation of weak hydrogen bonds due to initial bond at room temperature, formation of covalent oxide bonds due to temperature increase, and formation of a “perfect” bond by filling interfacial micro-voids by the plastically deformed oxide resulting from the elevated temperature). However, the first modern three stage wafer bonding model was presented by Stengl et al. in 1989 [49]. This was based on authors observations on the contact wave velocity of Si and fused quartz wafers, initiated at various temperatures. The model describes the bonding based
8
upon the chemical interactions of OH terminated Si surfaces. In the model, a hydrophilic Si surface is considered to contain Si − O − Si and Si − OH bonds. The surface hydroxyl groups are polarized, and thereby are very reactive to water as [40]:
Si − OH … . nH2O … HO − Si (1) During annealing, OH groups of surfaces come sufficiently close to form covalent bonds between Si surfaces, and to form water [40, 45]:
Si − OH + OH − Si → Si − O − Si + H2O (2) The released water diffuses then into the silicon bulk and forms a relatively thin oxide layer and releases hydrogen [40]:
Si − Si + H2O → Si − O − Si + H2 (3) The released hydrogen does not react with Si and may form voids.
The next significant stage in the wafer bonding was realized by Tong et al. in 1994 while they were studying the low temperature wafer bonding through a long-term storage of contacted wafers [50]. This was led to observation of formation of annealing voids. They hypothesized voids as a result of released hydrogen and products of broken hydrocarbon contaminants. Two approaches were attempted to avoid formation of voids during annealing. The first approach was usage of a thermally oxidized wafer (1 µm thick oxide layer) as part of the wafer sandwich to absorb the excess hydrogen at the bond interface. It was comprehended that after annealing at a temperature > 110 °C, the excess water in the bond interface tended to oxidize the available bulk silicon or the hydrolyze oxide and resulted in release of excess hydrogen. The second approach was to anneal wafer pairs to reveal voids at 1000 °C for an hour. This permitted the trapped hydrogen to diffuse away from the bond interface causing voids to close. Their conclusions to the annealing voids were then supported and expanded later in 2010 by Vincent et al. [51]. Vincent presented a detailed study on annealing voids using the Fourier Transform Infrared Microscopy (FTIR), the X-ray Reflection (XRR), the Nuclear Reaction Analysis (NRA), and the Scanning Acoustic Microscopy (SAM). The XRR results showed that the bond interface expands by increasing annealing temperature as the excess water oxidizes the bulk silicon. The density of evolved hydrogen during annealing was directly measured by the NRA and calculated from the density of water required for increasing
9 the oxide thickness detected by the XRR. The SAM results showed that voids form during the low temperature annealing and then start dissolving as annealing temperature exceeds 900 °C. Finally, models were presented along with experimentally derived coefficients to describe formation and dissolution of voids with increasing annealing temperature.
The preliminary hydrophilic activation of Si has conventionally been achieved using a chemical dip, so called wet activation technique [40]. Without an elevated annealing temperature, this approach normally produces low bond strengths. The basis of the plasma activation was provided for the first time by Farrens et. al in 1995. [52]. Natively and thermally oxidized silicon wafers were activated for 5 - 10 seconds with oxygen- plasma created in a reactive ion etching (RIE) chamber. Activated wafers were then contacted ex-situ without any annealing step. The bond strength between contacted wafers was then measured using a stud-pull tester. The bond strength results were not directly comparable with other reported bond strengths due to the used measurement technique. However, images of detached samples showed substantial
‘pull out’ of material from the bulk silicon, indicating achievement of high strength bonds. The bond interface was studied through the Transmission Electron Microscopy (TEM) analysis and indicated no voids. However, the resolution was in the order of 10 nm, and thereby was not capable of detecting any voids created with nano scale features. The first detailed analysis of impact of the plasma activation on oxide surfaces was provided by Amirfeiz, et al. in 2000 [53]. This work retried the process done by Farrens et al. There, silicon bonding samples were prepared by an Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE) system in which the charged species (oxygen and argon) were accelerated towards samples by applying a voltage on the base plate for 10 - 240 seconds. The Atomic Force Microscopy (AFM) technique was used for topographical analysis of the surfaces. It was observed that the AFM analysis is only possible after deionized (DI) water dip or storage of samples for 24 hours, otherwise tip of the AFM sticks to surfaces. Measurements showed that surface roughness increases with the plasma power, and long activation durations (> 60 seconds) result in inferior bond strengths. Capacitance versus Voltage (CV) measurements of the bond interface (achieved by deposition of a thin aluminum layer on top of the activated surface) showed no high fixed charge density in the oxide. The FTIR in Mid Infrared Range (MIR), the ellipsometry, and the Secondary Ion Mass
