Enhancing retarding potential analyzer energy measurements with micro-aligned electrodes

141 Pages·2014·18 MB·English

Enhancing Retarding Potential Analyzer Energy
Measurements with Micro-Aligned Electrodes
by
Eric Vincent Heubel
Submitted to the Department of Mechanical Engineering
in partial fulfillment of the requirements for the degree of
MASSACHUSETTS INSTITUTE,
OF TECHNOLOGY
Doctor of Philosophy in Mechanical Engineering
AUG 15 2014
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY LI BRARiE2
June 2014
@ Massachusetts Institute of Technology 2014. All rights reserved.
Signature redacted
A u th or ..............................................................
Department of Mechanical Engineering
May 2, 2014
Signature redacted ......
Certified by .....
Luis Fernando Velaisquez-Garcia
Principal Research Scientist of the Microsystems Technology
Laboratories
-Thesis Supervisor
Signa.t.u..r..e.. ..r.e..d..a..c ted..
Certified by.......
Anastasios John Hart
Ass te Professor of Mechanical Engineering
Signature redacted e~ts Committee Chair
A ccepted by ............. . ..........................................
David E. Hardt
Chairman, Department Committee on Graduate Theses
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Enhancing Retarding Potential Analyzer Energy
Measurements with Micro-Aligned Electrodes
by
Eric Vincent Heubel
Submitted to the Department of Mechanical Engineering
on May 2, 2014, in partial fulfillment of the
requirements for the degree of
Doctor of Philosophy in Mechanical Engineering
Abstract
Plasmas are ionized gases, and constitute a large fraction of the known universe.
For example, solar wind is a plasma that emanates from the sun reaching the Earth's
magnetosphere. At times these ionized species cause beautiful auroral displays over
the planet's magnetic poles. Moreover, when a hypersonic object enters the atmo-
sphere, the shock wave that is generated induces a plasma sheath that surrounds the
object. The resulting plasma is hot and dense and may cause material ablation from
the surface of the object. Other plasmas of similar or greater density exist in fusion
reactors, and in silicon processing chambers.
A Retarding Potential Analyzer (RPA) is a sensor that measures the ion energy
distribution of a plasma. The ion energy influences the ablation of surfaces, or plasma
etching, as well as the deposition processes. Integrated circuit foundries could greatly
benefit from a diagnostic tool such as an RPA in plasma chambers. Measuring particle
energy during reactive ion etching, ion implantation, ion milling, plasma enhanced
chemical vapor deposition, etc, in situ would close the control loop to improve the
uniformity and repeatability of numerous processes.
In order to measure the ion energy of a plasma, an RPA utilizes a system of grids
with holes smaller than a few Debye length - a characteristic length proportional to
the square root of the electron temperature divided by the electron number density.
Thus, cold, dense plasmas have the smallest associated Debye lengths and require
smaller grid openings than can be achieved using conventional machining.
In this thesis an improved RPA design is proposed that utilizes the following
three key concepts: (i) the aperture size and inter-electrode spacing required by
dense plasmas are defined using micro electromechanical systems (MEMS) processing
techniques; (ii) aperture alignment across successive grids is mechanically enforced
to improve the transmission of ion species through the device; (iii) densely packing
apertures in each RPA grid multiplexes the signal onto the collector.
A MEMS RPA is built with apertures as small as 100 pm in diameter having
an inter-grid spacing of only 200 pm. These are the narrowest aperture and gap
dimensions for an RPA with enforced electrode alignment to date. The new RPA
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design is benchmarked against the present state of the art downstream of an ion
source from a mass spectrometry (MS) system. An ion source is chosen because of
the fine control it offers over the ion energies, as a low energy with little variability
increases mass resolution in MS systems. Through enforced alignment, the MEMS
RPA shows an order of magnitude increase in signal strength over a conventional RPA.
In improving the transmission of ions through the sensor, the artificial broadening
of RPA ion energy distribution measurements is mitigated, resulting in a threefold
improvement in sensor energy resolution. This is characterized by a reduction in the
full width half maximum (FWHM) value from 2.5 V for the conventional device down
to 0.85 V for the MEMS RPA.
The various RPAs are then tested in a helicon plasma, capable of replicating
many dense plasmas in the range of 1 x 1016 m-1 to 1 x 1018 m-3. Langmuir probe
measurements provide estimates of the electron temperature and plasma density, from
which the Debye length is derived. In these experiments, only the new RPA designs
were able to effectively trap the plasma down to a Debye length of approximately
50 pm and obtain ion energy distributions. The range of application for RPAs is thus
expanded through the use of microfabrication techniques.
Thesis Supervisor: Luis Fernando Velasquez-Garcfa
Title: Principal Research Scientist of the Microsystems Technology Laboratories
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Acknowledgments
When I set out to earn this degree I was following in the footsteps of my father
and grandfather, albeit in a slightly different field. My grandfather taught Chemistry
at the Universit6 des Sciences et Techniques de Lille in France, my father obtained his
PhD in Chemistry from Michigan State University, and I was led toward Mechanical
Engineering by a passion for "tinkering." Over the past eight years in the Boston
area, I have come to know many people throughout the Mechanical Engineering
department, across MIT, and outside. Since moving into Ashdown in 2006, I have had
the pleasure of making friends from around the world and have greatly appreciated
the time we spent sharing meals, working out, walking around the river, getting
coffee, and partaking in this journey together. I am grateful to so many people from
numerous aspects of life on and off campus here that there are simply too many to
list by name. Please forgive me if this short section fails to express how important
you all are to me.
I first wish to thank my family, my mother Nancy and father Pierre-Henri for their
love, encouragement, and prayerful support. Along with my siblings, Caroline, Alex,
and Mariette, they have been an example of what goals can be achieved through
perseverance, and I am very proud of them. And to by brother-in-law Steve, and
niece Margaux, for their encouraging words and songs.
To my many friends from MIT's Tech Catholic Community, thank you for your
support, for helping me keep sight of what is important. I have been blessed to be a
part of such a wonderfully loving and supportive group of people. With my friends
on campus, you have truly been my family away from home. Thank you Fr. Clancy
for keeping such a strong faith present in our chapel and across MIT. I am glad to
have spent these years with you.
I would like to thank Bill Butera, my boss during my internship, for encouraging
me to follow my desire to pursue this degree. My experience working in an Electrical
Engineering research lab led me to the Microsystems Technology Laboratories (MTL)
at MIT.
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Throughout these last four years, I have learned a lot about microfabrication, and
would like to thank the MTL technical staff for training me on the various machines in
the cleanrooms here. I thank my advisor, Luis Velisquez-Garcia, for the opportunity
to work on this project and for imparting his microfabrication expertise. I would like
to extend a special thanks as well to the other members of this research group for
their helpful feedback and numerous conversations.
I would like to express my gratitude to NASA, for funding part of this research un-
der Award No. NNC08CA58C with program managers Robert Manning and Thomas
Wallett. As well as to Professor Tayo Akinwande for our discussions regarding the
work and experiments performed in his laboratory. I thank MIT's Plasma Science
and Fusion Center for allowing me to use its facility to carry out measurements, and
specifically to Regina, Graham, and Professor Dennis Whyte for meeting with me to
discuss my results. I am especially thankful to my committee, Professor John Hart,
Professor Sangbae Kim, and Professor Jeff Lang, for keeping me on track with my
work. I greatly appreciated your feedback, and it was a real pleasure working with
you.
I am grateful for having had the opportunity to work alongside Professor Dave
Gossard and the rest of the 2.003 teaching staff for these past two years. I enjoyed
helping teach the course and working with the MITx team to offer 2.03x worldwide.
Thank you for the advice and insight on pursuing a career in academia.
I would also like to express a special thanks to the administrative staff. Thank
you for all you do to help keep students apprized of deadlines and progressing in their
studies, but most especially for lending a welcoming ear when panic strikes. Thanks
Leslie, Joan, Una, Carolyn and Debb.
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Contents
List of Symbols 15
1 Introduction 17
2 Plasma Diagnostics 23
2.1 Langmuir Probe .............................. 23
2.1.1 Single Langmuir Probe Theory ................. 24
2.1.2 Double Langmuir Probe Theory ..................... 27
2.2 Retarding Potential Analyzer . . . . . . . . . . . . . . . . . . . . . . 31
2.2.1 Theory of Operation . . . . . . . . . . . . . . . . . . . . . . . 31
2.2.2 Design Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3 Hybrid RPA 37
3.1 Hybrid RPA design ............................ 37
3.2 Fabrication ................................ 39
3.2.1 RPA Housing .......................... . 39
3.2.2 Packaging and Electrical Connections .............. 39
3.2.3 G rids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.2.4 Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.3 Hybrid RPA Ion Source Characterization . . . . . . . . . . . . . . . . 43
4 MEMS RPA 59
4.1 MEMS RPA Design ............................ 60
4.2 Fabrication ................................ 64
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4.2.1 Housing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
4.2.2 Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
4.2.3 Assembler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
4.2.4 Assembly Procedure . . . . . . . . . . . . . . . . . . . . . . . 70
4.2.5 MEMS RPA Ion Source Characterization . . . . . . . . . . . . 72
5 Device Characterization Using a High-Density Plasma 77
5.1 Helicon Plasma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
5.2 Langmuir Probe Data . . . . . . . . . . . . . . . . . . . . . . . . . . 80
5.3 Conventional RPA DIONISOS Plasma Characterization ........ 87
5.4 Hybrid RPA DIONISOS Plasma Characterization .............. 90
5.5 MEMS RPA DIONISOS Plasma Characterization .............. 92
5.6 Conventional, Hybrid, and MEMS RPA Comparison .......... 94
6 Future Work 97
7 Conclusion 101
A Detailed Microfabrication Process Flow 105
B Mask Detail 107
B.1 Hybrid RPA Grids ............................ 107
B.2 MEMS RPA Housing ................................ 108
B.3 MEMS RPA grids .................................. 108
C Engineering Drawings 121
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List of Figures
1-1 Electron number density from reentry flight experiments . . . . . . . 19
1-2 Electron temperature measurements from reentry flight experiments . 19
1-3 Debye length estimates from reentry flight experiments . . . . . . . . 20
1-4 Debye lengths for various plasmas as a function of electron temperature
and density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2-1 Typical single Langmuir probe trace . . . . . . . . . . . . . . . . . . . 24
2-2 Double Langmuir probe plasma measurement . . . . . . . . . . . . . 27
2-3 RPA schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3-1 Hybrid RPA schematic . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3-2 Hybrid RPA microfabricated electrodes . . . . . . . . . . . . . . . . . 40
3-3 Hybrid RPA assembly . . . . . . . . . . . . . . . . . . . . . . . . . . 42
3-4 Mass spectrometry ionizer energy measurement experiment . . . . . . 44
3-5 Conventional RPA construction . . . . . . . . . . . . . . . . . . . . . 45
3-6 Conventional RPA ion energy sweep . . . . . . . . . . . . . . . . . . . 46
3-7 Modified conventional RPA energy sweep . . . . . . . . . . . . . . . . 47
3-8 Conventional RPA electron emission sweep at 10V ion energy and
3 x 10-5 Torr ...... ............................... 48
3-9 Conventional RPA total collected current versus electron emission current 49
3-10 Conventional RPA electron emission sweep at 10 V ion energy and
6 x 10-7 Torr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
3-11 Simulation of ion source using CPO 2D . . . . . . . . . . . . . . . . . 51
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3-12 Hybrid RPA ion energy sweep with 0.2 mA electron emission current
and 3 x 10~5 Torr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
3-13 Comparison of the hybrid energy distribution and conventional data
for an ion source set to 10 V at 3 x 10~ Torr with 0.2mA electron
emission current. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
3-14 Hybrid RPA current trace for 10 V ion energy region and 0.2 mA elec-
tron emission current at 3 x 10-5 Torr . . . . . . . . . . . . . . . . . . 55
3-15 Simulation of single RPA aperture using CPO 2D . . . . . . . . . . . 56
3-16 Simulation of modified RPA aperture stack using CPO 2D . . . . . . 57
4-1 Stress analysis of a MEMS RPA retaining spring . . . . . . . . . . . . 62
4-2 MEMS RPA Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4-3 Misalignment simulation of a single 100 pm RPA aperture stack using
CPO 3D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
4-4 MEMS RPA housing fabrication . . . . . . . . . . . . . . . . . . . . . 65
4-5 MEMS RPA electrode fabrication . . . . . . . . . . . . . . . . . . . . 68
4-6 MEMS RPA assembly tool . . . . . . . . . . . . . . . . . . . . . . . . 69
4-7 MEMS RPA assembly . . . . . . . . . . . . . . . . . . . . . . . . . . 71
4-8 Backlit MEMS RPA assembly . . . . . . . . . . . . . . . . . . . . . . 72
4-9 MEMS RPA testplate . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
4-10 150 pm, 300 pm, 300 pm grid stack MEMS RPA ion energy sweep test
at 3 x 10-1 Torr and 0.2 mA emission . . . . . . . . . . . . . . . . . . 73
4-11 100pm, 250pm, 300pm grid stack MEMS RPA ion energy sweep test
at 3 x 10-' Torr and 0.2 mA emission . . . . . . . . . . . . . . . . . . 74
4-12 Comparison of 150 pm grid apertures to 100 pm in the MEMS RPA . 75
4-13 MEMS RPA comparison to hybrid and conventional probes at 10 V ion
energy region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
4-14 MEMS RPA normalized distribution (normalized height) comparison
with hybrid and conventional probes at 10V ion energy region . . . . 76
5-1 DIONISOS Helicon plasma chamber . . . . . . . . . . . . . . . . . . . 78
10