Budapest University of Technology and Economics

Faculty of Electrical Engineering and Informatics

Hungarian Academy of Sciences, Centre for Energy

Research, Institute for Technical Physics and Materials

Science (MFA), Microtechnology Department

Wafer bonding for three dimensional

microstructures

Ph.D. thesis booklet

author: Kárpáti Tamás

advisor: Dr. Pap Andrea Edit

advisor: Dr. Mizsei János

2015

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Place and address of doctoral work preparation:

Hungarian Academy of Sciences - Centre for Energy Research

Institute for Technical Physics and Materials Science (MFA)

Microtechnology Department

Konkoly Thege str. 29-33., H-1121 Budapest, Hungary

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Introduction The core activity of the Microtechnology Laboratory of MTA EK

MFA is research and development of integrated MEMS/NEMS (MEMS -

Micro-Electro-Mechanical System / NEMS - Nano-Electro-Mechanical

System) sensors, actuators, and microfluidics systems. Wafer bonding

is one of the wide-ranging spectrum of different micro-technological

processes and equipment of the process line. Wafer bonding is a key

technology that enables development of three-dimensional

microstructures. Bulk and surface machining extended with bonding

allows the design of complex sensor geometries, even the

implementation of microfluidic channel networks.

The focus of my research was therefore wafer bonding

techniques for emerging MEMS devices, partly to tackle challenges in

specific applications, and partly to develop and implement novel device

and technology solutions.

In the Thesis – among others – I introduce a new hybrid

bonding technology, where anodic and metal-diffusion wafer bonding

is applied simultaneously, establishing mechanical and electrical bond

between two suitable wafers. The application of the hybrid bonding is

demonstrated by the fabrication of a 3D force sensor, including

thorough analyses of its electrical, mechanical, and thermal properties

and the advantageous effects of the proposed hybrid method on those.

I have developed a continuous measurement system for in-situ

monitoring of the processes during the wafer bonding operations. This

method provides real-time information during the bonding process on

the magnitude of electrostatic forces, which enables faster, more

efficient and more accurate process optimization.

During my studies I realized that micro-plasma could have

advantageous applications in microfluidics systems. This motivation

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brought about the development of a micro-plasma generator, suitable

for integration in microfluidic channels, and fabricated such devices

using wafer bonding technology. I also present two possible

applications: the plasma assisted local surface modification technique

in submillimeter range, and a microfluidic reactor device applicable in

molecular emission spectroscopy.

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New scientific achievements Thesis 1. Development and evaluation of an electrostatic force

assisted aluminium metal fusion bonding process

I developed and optimized the first electrostatic force assisted

aluminium metal fusion wafer-bonding process. I proved that a

mechanically and electrically reliable bonding can be formed

between a silicon MEMS device and a glass substrate by applying

it. I showed that using the optimized parameters of 450 °C curing

temperature, 60 min bonding period, and 1000 V electrical bias

the electrostatic force between the aluminum surfaces

participating in the bonding process impairs the native Al2O3

layer, ensuring the conditions for metal diffusion. [T1, T2]

Thesis 1.1.

With structural studies and functional measurements I

revealed the formation and the resistive nature of the

contact between the Al-Al bonding layers. I proved the

occurrence of diffusion between the Al layers, and the

formation of dominantly <111> orientation crystal

structure at their interface. The long-term reliability of the

fabricated contacts was established by cyclic heat

treatment.

Thesis 1.2.

I verified the applicability of the method by fabrication of

pressure gauge and 3D force micro-sensors. I proved the

thermo-mechanical stress reduction effect of the process

by measurements of the resting state non-linear

temperature-dependent output signals.

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Figure 1. Result of metal diffusion bonding between Al layers (TEM).

Figure 2. Hybrid bonded 3D force sensor with glass substrate (SEM).

Figure 3. 3D force sensors with glass substrate.

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Thesis 2. Development of the in-situ electrostatic force measurement

between wafers during anodic bonding

I developed an in-situ measurement method that can be

integrated in industrial wafer bonding equipment to monitor the

electrostatic force during the anodic bonding process. To this

end, I designed a testing tool formed on a special Si-borosilicate

wafer pair, which enables the measurement of the electrostatic

force during the wafer bonding process in-situ. I verified that

measuring the variation of the electric potential between the

electrodes of the measurement structure, operating on a

capacitive principle, the magnitude of the electrostatic force can

be determined in real time. [T3]

Thesis 2.1.

Applying the measurement method at various bonding

parameters (200 °C / 400 to 1000 V, 300 °C / 800 V, 400 °C

/ 800 V), and based on force values calculated from the

measured electrical characteristics, I experimentally

verified the validity of force estimations based on

theoretical literature.

Thesis 2.2.

I designed and implemented a silicon micromachined

probe structure, capable of in-situ monitoring of the

bonding process without further modifications of the

equipment.

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Figure 4. Schematic drawing of in-situ electrostatic force measuring.

Figure 5. Prepared wafer pair for measuring.

Figure 6. Detected electric field strength in sensors.

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Thesis 3. Design, fabrication, and experimental analysis of a micro-

plasma generator integrated in microfluidic systems

I designed and prepared a DBD (dielectric barrier discharge)

micro-plasma reactor fabricated by microtechnology processing

stages. I developed a glass-glass bonding technology based on an

intermediate polycrystalline silicon layer, which enables the

integration of the device in microfluidic systems and reactors. At

normal atmospheric pressure and 13 kHz driving frequency the

device is suitable for the ionization of various gases (air, N2, He,

Ar, Ne, CO2, acetone-vapor) and maintaining the forming plasma

at less than 1.07 W/mm3 power consumption. [T4, T5]

Thesis 3.1.

Using the developed technology, integrating the device as

plasma source, I constructed a vacuum-tight reactor

structure for molecular emission spectroscopic

measurements of small amounts (<0.75 mm3) of

gas/vapor. The applicability for analytical studies was

demonstrated by photoemission measurements of various

gases and vapors (N2,He, Ar, Ne, CO2, acetone-vapor).

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Figure 7. Realized micro-plasma generator.

Figure 8. Detail of integrated micro-plasma generator inside a microfluidic

channel (SEM).

Figure 9. Neon plasma inside a channel.

(1 atm Ne, Upp=542 V, f=13 kHz, photo exposure time: 15 s)

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Related publications [T1] T. Kárpáti, A.E. Pap, Gy. Radnóczi, B. Beke, I. Bársony and P. Fürjes.

Reliable aluminum contact formation by electrostatic bonding. J.

Micromech. Microeng. 25(075009):1-8, 2015

[T2] T. Kárpáti, S. Kulinyi, R. Végvári, J. Ferencz, A. Nagy, A.E. Pap and G.

Battistig. Packaging of a 3-axial Piezoresistive Force Sensor with

Backside Contacts. Microsyst. Technol. 20:1063-1068, 2014

[T3] T. Karpati, A. E. Pap, M. Adam, J. Ferencz, P. Furjes, G. Battistig and

I. Barsony. Electrostatic force detection during anodic wafer bonding.

Sensors, 2012 IEEE, pp.1-4, Oct. 28-31, 2012

[T4] T. Kárpáti, E. Holczer, J. Ferencz, A. E. Pap and P. Fürjes. In-situ

surface modification of microfluidic channels by integrated plasma

source. Eurosensors 2014 XXVIII, Procedia Engineering 87:484-487,

2014

[T5] T. Kárpáti, I. Bársony and P. Fürjes. Microplasma Chamber for

Molecular Emission Spectroscopy. Sensors, 2014 IEEE, pp.1077-1079,

2014

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Further publications [N1] T. Kárpáti, Andrea Edit Pap, Sándor Kulinyi. Prototype MEMS

Capacitive Pressure Sensor Design and Manufacturing. Periodica

Polytechnica-EE 57(1):3-7, 2013

[N2] S. Kulinyi, R. Vegvari, A. Pongracz, A. Nagy, T. Karpati, M. Adam, G.

Battistig, I. Bársony. Flexible packaging for tyre integrated shear force

sensor. Sensors, 2012 IEEE, pp.1-4, Oct. 28-31, 2012

[N3] Z. Fekete, P. Furjes, T. Karpati, G. A. B. Gal, I. Rajta. MEMS-

compatible hard coating technique of moveable 3D silicon

microstructures. Material Science Forum 659:147-152, 2010

[N4] Kárpáti Tamás. Szilícium alapú 3 dimenziós erőmérők kialakítása és

tokozása: (1. rész). Elektronet, 22(7):36-37, 2013

[N5] Kárpáti Tamás. Szilícium alapú 3 dimenziós erőmérők kialakítása és

tokozása: (2. rész). Elektronet, 22(8):28-30, 2013

[N6] Kárpáti Tamás. MEMS Kapacitív Nyomásmérő Kialakítása: (1. rész).

Elektronet, 19(5): 20-22, 2010

[N7] Kárpáti Tamás. MEMS Kapacitív Nyomásmérő Kialakítása: (2. rész).

Elektronet, 19(6):18-19, 2010