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Optimizing high frequency ultrasound cleaning in the semiconductor industry. Steven Brems. Outline. Introduction to particle removal Improving state-of-the-art megasonic cleaning Acoustic pulsing Oversaturated liquids Traveling waves - PowerPoint PPT Presentation
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© IMEC 2010 / CONFIDENTIAL
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
2
© IMEC 2010 / CONFIDENTIAL
Introduction: Particle 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
Particle attached to
wafer surface
Lift-off from surface:
repulsive forces(electrostatic: z)
Breaking of the Van der Waals
forces(under)etching
Transport away fromsurface: diffusion,
convection
Mechanism of particle removal by pure chemical cleaning
vF
3
200 nm
20 nm
© 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
4
© IMEC 2010 / CONFIDENTIAL
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)
J. Lee et al., JACS 127, 16810 (2005)
Pulse on time Pulse off time period Pulseon time Pulse (DC) CycleDuty
5
© IMEC 2010 / CONFIDENTIAL
In-situ measuring micro-bubble activity
oscilloscope
amplifierHydrophone
Wafer Transduce
r
Example of cavitation noise spectra
▸ 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 cavitation6
© IMEC 2010 / CONFIDENTIAL
0 450 900 1350 1800
-80
-75
-70
-65
-60
7/2 ultraharmonic 9/2 ultraharmonic
Ultr
ahar
mon
ic s
igna
l (dB
V)
Pulse off time (ms)0 300 600 900 1200
-80
-75
-70
-65
-60
0 200 400 600
-80
-75
-70
-65
-600 50 100 150 200 0 100 200 300 400
Pulse on time (ms)0 200 400 600
Cavitation noise spectra: Influence of pulses
▸ Experimental details- 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!7
-8dB=40%
DC 10% DC 25% DC 50%
© IMEC 2010 / CONFIDENTIAL
Understanding of optimal pulse off time
Dissolved oxygen
concentration 120%
~ resonant bubble size
Dissolution time resonant
bubble
The dissolution time of a resonant bubble lies very
close to the optimal experimental determined
pulse off time
Bubble size
‘reservoir’
Lost bubbles
Dissolution during pulse-off time
Growing to active size during pulse-
on time
Production of new bubbles (transient collapse, shape
instabilities)
Bubble size distribution centered around resonance radius
Inactive bubbles that continue to grow or active bubbles that grow
out of resonance
8
© IMEC 2010 / CONFIDENTIAL
Cavitation Activity: Role of On-Time
0 200 400 600 800 1000
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Ultr
ahar
mon
ic c
avita
tion
sign
al (a
.u.)
Pulse-off time [ms]
10 ms 50 ms 250 ms
0 500 10000.0
0.1
0.2
0.3
0.4
0.5
Pulse-off time [ms]
Cavitation noise data
9
Pulse on times
▸ A simple bubble model based on bubble growth, bubble loss and bubble creation mechanisms can model the pulse on time variation.- A maximum bubble activity is
reached with a pulse on time of ~50 ms
0 200 400 600 800 1000
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Ultr
ahar
mon
ic c
avita
tion
sign
al (a
.u.)
Half integer harmonics Fit
Pulse-On time [ms]
tgrow= 8.6 ms
teff =1.1 s
Pulse on time variation at constant pulse off time (150 ms) and 105 %
dissolved gas
Bubble size
Reservoir Lost bubbles
© IMEC 2010 / CONFIDENTIAL
Continuous 125 ms 150 ms 175 ms
0.42 W/cm2
0.25 W/cm2
Influence of pulse off time
10
PRE maps for variable pulse off times, a fixed pulse on time (50 ms) and a dissolved oxygen concentration of 105%
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. Experiment Simulation
Acoustic pulsing noticeably improves particle removal without changing
acoustic power densities
PRE
(%)
0
100
50
© 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
11
© IMEC 2010 / CONFIDENTIAL
Maximazing bubble formation
12
Bubble formation is limiting the megasonic cleaning efficiency.▸ An increased dissolved gas concentration facilitates the nucleation of
bubbles
90% 100% 110% 120% 125% 130%
Impossible to nucleate bubbles
Bubbles do not dissolve anymore
PRE as function of dissolved oxygen concentration
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%)
PRE
(%)
0
100
50
© IMEC 2010 / CONFIDENTIAL
Bubble dissolution or growth in the absence of an acoustic field is given by
100 120 140 160 180
0
10
20
30
Bub
ble
radi
us (m
m)
Dissolved oxygen gas (%)
Dissolution
Upper limit dissolved gas concentration
13
Bubble resonance size
00
1
000
0
00
34111
PP
CC
RPDtR
RTCDR
dtdR gig
This term determines bubble growth or dissolution
Growth
© IMEC 2010 / CONFIDENTIAL
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
14
© IMEC 2010 / CONFIDENTIAL
▸ Standing wave field- Bubbles experience an acoustic radiation force (Bjerkness force):
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.
Increasing PRE: transport of bubbles towards the wafer surface
15
pVF
95 96 97 98 99 1000
1
2
Radial oscillation
R(t)
/ R
0
time [Ac. Cyc.]
0.265
0.270
0.275
0.280
0.285 z-position
Pos
ition
[mm
]
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.
© IMEC 2010 / CONFIDENTIAL
Influence of a traveling wave on particle removal efficiency
Wafer
Damping material
Transducer
▸ 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
16
© 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
17
© IMEC 2010 / CONFIDENTIAL
Large particles
18
Small particles
200 nm
100 nm
Boundary layer thickness >> 100 nm
Although the removal force increases for larger particles, it gets easier to remove large particles because drag force scales with radius and velocity
30 nm100 nm
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.
Particle cleaning with liquid motion
© IMEC 2010 / CONFIDENTIAL
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
19
© IMEC 2010 / CONFIDENTIAL
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)
20ANTOINE PACCO