Transcript Slide 1

Optimizing high frequency ultrasound
cleaning in the semiconductor industry
Steven Brems
© IMEC 2010 / CONFIDENTIAL
Outline
▸ Introduction to particle removal
▸ Improving state-of-the-art megasonic cleaning
- Acoustic pulsing
- Oversaturated liquids
- Traveling waves
▸ Future of particle removal with liquid motion in
the semiconductor industry
▸ Conclusions
© IMEC 2010 / CONFIDENTIAL
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Introduction: Particle cleaning
Particle
attached to
wafer surface
Breaking of the
Van der Waals
forces
(under)etching
Lift-off from surface:
repulsive forces
(electrostatic: z)
Transport away from
surface: diffusion,
convection
Mechanism of particle removal by pure chemical cleaning
▸ Nanoparticle removal with pure chemical cleaning is
only effective if >2 nm material is removed.
▸ A combination of physical and chemical cleaning
methods will become more important
v
200 nm
20
nm
F
© IMEC 2010 / CONFIDENTIAL
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Outline
▸ Introduction to particle removal
▸ Improving state-of-the-art megasonic cleaning
- Acoustic pulsing
- Oversaturated liquids
- Traveling waves
▸ Future of particle removal with liquid motion in
the semiconductor industry
▸ Conclusions
© IMEC 2010 / CONFIDENTIAL
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Towards a control of bubble size: Pulsing
▸ At sufficiently high gas concentration and acoustic pressures,
bubbles can grow by rectified diffusion and bubble coalescence
▸ Microbubbles (< 4 mm) will always shrink when ultrasound is
turned off and dissolved gas saturation is below 130%
- Bubbles could kept around resonance radius by turning the acoustic
field on (bubbles grow) and off (bubbles dissolve)
Pulse on time
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Pulse off time
Duty Cycle (DC) 
Pulse on time
Pulse period
J. Lee et al., JACS 127, 16810 (2005)
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In-situ measuring micro-bubble activity
Example of cavitation noise spectra
Hydrophone
amplifier
oscilloscope
Wafer
Transducer
▸ Bubble oscillation
-
Frequency distribution of the oscillating bubble motion can contain harmonics, subharmonics and ultraharmonics

The components arise from the nonlinear motion of a bubble  acoustic emission
▸ Non-integer harmonics (5f0/2, 7f0/2, 9f0/2…) :
-
Particular characteristic of non-linear (stable) bubble motion
 Can be used as an indicator for bubble activity
▸ Strong (transient) cavitation produces white noise (increase of background signal)
-
Instable cavitation = damaging cavitation
© IMEC 2010 / CONFIDENTIAL
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Cavitation noise spectra: Influence of pulses
Ultraharmonic signal (dBV)
Pulse on time (ms)
-60
0
50
100
150
200
-60
0
100
200
300
400
-60
-65
-65
-65
-70
-70
-70
-75
-75
-75
DC 10%
-80
0
450
900 1350 1800
▸ Experimental details
DC 25%
-80
0
300
600
900
0
200
600
7/2 ultraharmonic
9/2 ultraharmonic
-8dB=40%
-80
1200
400
DC 50%
0
200
400
600
Pulse off time (ms)
- Oxygen concentration: 120 %, applied power: 640 mW/cm2
- Duty Cycle is varied
▸ Optimal pulse off time (indicated with
) is independent of duty cycle variation
▸ Bubble activity decreases with increasing duty cycle
▸ However, a lower DC also means a lower effective cleaning time!
© IMEC 2010 / CONFIDENTIAL
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Understanding of optimal pulse off time
~ resonant bubble
size
Dissolution time
resonant bubble
Dissolved
oxygen
concentration
120%
The dissolution time of a
resonant bubble lies very close
to the optimal experimental
determined pulse off time
Production
of newaround
bubblesresonance
(transientradius
Bubble size distribution
centered
collapse, shape instabilities)
‘reservoir’
Growing to active size
during pulse-on time
Inactive bubbles that continue to grow or
active bubbles that grow out of resonance
Lost bubbles
Bubble size
Dissolution during pulse-off time
© IMEC 2010 / CONFIDENTIAL
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Cavitation Activity: Role of On-Time
Pulse on time variation at constant pulse off
time (150 ms) and 105 % dissolved gas
Cavitation noise data
0.4
0.3
3.0
0.2
2.5
0.1
0.0
2.0
0
500
1000
Pulse-off time [ms]
1.5
10 ms
50 ms
250 ms
1.0
Pulse on times
0.5
0.0
0
200
400
600
800
Ultraharmonic cavitation signal (a.u.)
Ultraharmonic cavitation signal (a.u.)
0.5
3.5
4.0
Half integer harmonics
Fit
3.5
3.0
tgrow= 8.6 ms
2.5
2.0
1.5
teff =1.1 s
1.0
0.5
0.0
1000
0
200
Pulse-off time [ms]
800
1000
▸ A simple bubble model based on
bubble growth, bubble loss and bubble
creation mechanisms can model the
pulse on time variation.
-
Bubble size
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600
Pulse-On time [ms]
Lost bubbles
Reservoir
400
A maximum bubble activity is reached
with a pulse on time of ~50 ms
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Influence of pulse off time
PRE maps for variable pulse off times, a fixed pulse on time (50 ms)
and a dissolved oxygen concentration of 105%
Continuous
125 ms
150 ms
175 ms
0.42 W/cm2
PRE (%)
100
0.25 W/cm2
50
0
Acoustic pulsing noticeably improves particle
removal without changing acoustic power densities
Acoustic field 145 mm from
transducer surface
▸ Non-uniform acoustic field is a
near-field (interference) effect
caused by the transducer size.
▸ Non-uniform fields result in
localized cleaning.
© IMEC 2010 / CONFIDENTIAL
Experiment
Simulation
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Outline
▸ Introduction to particle removal
▸ Improving state-of-the-art megasonic cleaning
- Acoustic pulsing
- Oversaturated liquids
- Traveling waves
▸ Future of particle removal with liquid motion in
the semiconductor industry
▸ Conclusions
© IMEC 2010 / CONFIDENTIAL
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Maximazing bubble formation
Bubble formation is limiting the megasonic cleaning efficiency.
▸ An increased dissolved gas concentration facilitates the nucleation of
bubbles
PRE as function of dissolved oxygen concentration
PRE (%)
100
90%
100%
Impossible to nucleate
bubbles
110%
120%
125%
130%
50
0
Bubbles do not dissolve
anymore
Duty cycle is 10%, pulse off time is optimized for dissolved gas concentrations
and applied power is 420 mW/cm2.
The optimal dissolved gas concentration facilitates bubble formation
( ≥ 100%) and enables bubble dissolution ( < 130%)
© IMEC 2010 / CONFIDENTIAL
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Upper limit dissolved gas concentration
Bubble dissolution or growth in the absence of an acoustic field
is given by
dR0 DRgTC0

dt
R0
1

1  
4   Ci Pg 

   
1

R

1


 
0

Dt
3
P
R
P0 
0 0   C0

 
This term determines
bubble growth or
dissolution
Bubble radius (mm)
30
Bubble
resonance size
20
Growth
10
0
Dissolution
100
© IMEC 2010 / CONFIDENTIAL
120
140
160
180
Dissolved oxygen gas (%)
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Outline
▸ Introduction to particle removal
▸ Improving state-of-the-art megasonic cleaning
- Acoustic pulsing
- Oversaturated liquids
- Traveling waves
▸ Benchmarking of physical cleaning techniques
▸ Future of particle removal with liquid motion in the
semiconductor industry
▸ Conclusions
© IMEC 2010 / CONFIDENTIAL
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Increasing PRE: transport of bubbles towards
the wafer surface
▸ Standing wave field
- Bubbles experience an acoustic radiation force (Bjerkness force): F   Vp
 At moderate acoustic powers, bubbles smaller (larger) than resonance size will
travel up (down) a pressure gradient. So small bubbles go to pressure antinodes
and large bubbles go to pressure nodes.
▸ Traveling wave
- To simulate bubble motion in a traveling wave, acoustic radiation force, added mass
force (inertia) and viscous drag force need to be taken into account. As a result,
radial and translational equations are coupled.
z-position
R(t) / R0
2
0.285
0.280
0.275
1
0.270
0
95
96
97
98
99
Position [mm]
Radial oscillation
Simulation of a 2.7 mm sized bubble (radius) in an
acoustic field of 0.73 W/cm2. The average bubble
velocities is in the order of m/s.
0.265
100
time [Ac. Cyc.]
© IMEC 2010 / CONFIDENTIAL
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Influence of a traveling wave on particle
removal efficiency
Transducer
Wafer
Damping material
▸ A silicon wafer is
transparent for acoustic
waves at a specific angle
▸ With the combination of
damping material, a
traveling wave can be
formed
-
Bubbles are transported towards
the wafer surface and improve
particle removal
© IMEC 2010 / CONFIDENTIAL
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Outline
▸ Introduction to particle removal
▸ Improving state-of-the-art megasonic cleaning
- Acoustic pulsing
- Oversaturated liquids
- Traveling waves
▸ Future of particle removal with liquid motion in
the semiconductor industry
▸ Conclusions
© IMEC 2010 / CONFIDENTIAL
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Particle cleaning with liquid motion
Large particles
Small particles
Boundary layer thickness >> 100 nm
200 nm
100 nm
100 nm
30 nm
Although the removal force increases for larger
particles, it gets easier to remove large particles
because drag force scales with radius and velocity
A structure with a high aspect ratio gets
problematic, due to a strong increase in drag
force on that structure
Physical cleaning techniques based on a fluid flow are ideally
suited to remove ‘larger’ particles.
© IMEC 2010 / CONFIDENTIAL
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Conclusions
▸ System optimization
- Experimental megasonic system is optimized
 Controlling average bubble size with acoustic pulsing
 Facilitating bubble nucleation with slightly oversaturated liquid
 Transporting bubbles towards wafer surface with traveling waves
▸ Challenges
- Megasonic cleaning uniformity needs to be solved
- Cleaning of 30 nm and smaller silica particles with low damage levels is
not yet achieved
 Boundary layer and aspect ratio of structures makes current
techniques not suitable for continued scaling
© IMEC 2010 / CONFIDENTIAL
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Acknowledgements
Thanks to
▸ Marc Hauptmann, Elisabeth Camerotto, Antoine Pacco, Geert
Doumen, Stefan De Gendt, Marc Heyns, Geert Doumen and
Tae-Gon Kim (Imec)
▸ Christ Glorieux (KULeuven)
▸ Aaldert Zijlstra (University of Twente)
© IMEC 2010 / CONFIDENTIAL
ANTOINE PACCO
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