1/28
MEMS : an overview
- What ? – why ? – how ?
- Magnetic MEMS
Micro-magnets for MEMS
- Candidate Hard Magnetic Materials
- Preparation routes
-Micro-fabrication
-Beyond magnets
Nora M. Dempsey
Institut Néel, CNRS/UJF, Grenoble, France
ESM2007 - September 9-18th 2007, Cluj-Napoca, Romania
What are MEMS ?
2/28
MEMS : “Micro-Electro-Mechanical Systems”
“Microsystems Technology” (MST)
“Mecatronics”
machines which range in size from µm to mm
e.g. actuators, motors, generators, switches, sensors….
electrostatic, thermal, piezoelectric, piezoresistive, capacitive, magnetic…
Common examples
- inkjet printer heads (ink ejection)
- accelerometers ( airbag deployment…)
- gyroscopes ( trigger dynamic stability control…)
- pressure sensors (car tires, blood…)
- displays (DLP video projectors…)
- optical switching technology (telecommunications)
ESM2007 - September 9-18th 2007, Cluj-Napoca, Romania
3/28
Domains of application
Telecommunications
µ-switch for mobile phones
commutators for optic fibre networks
Data storage
µ-positionner for HDD heads
motorisation for HDD
Bio-technologies
less-invasive surgery, µ-injections,
lab-on-chip, ophtalmology…
Automotive
Aeronautics (Glass-cockpit…)
Space (1 kg = 20 k$!)
Consumer electronics
media players, gaming devices, footpods..
ESM2007 - September 9-18th 2007, Cluj-Napoca, Romania
4/28
Why are MEMS of interest ?
SIZE : small and light !
portable applications - mobile phones, aerospace devices …
limited space applications - implantable devices, micro-surgery…..
COST : batch processing
⇒ cheap
ENERGY EFFICIENCY : they use little power themselves,
+ they can be used to improve efficiency in bigger systems (cars, houses..).
INTELLIGENT SYSTEMS: MEMS (eyes + arms + legs) + ELECTRONICS (brain)
MEMS sensor
Energy
Electronics
Communication
Signal processing
MEMS actuator
ESM2007 - September 9-18th 2007, Cluj-Napoca, Romania
5/28
Why size matters…..
effects not exploitable at the macro-scale can
become of interest at the micron-scale
Surfaces effects dominate at small scales
ESM2007 - September 9-18th 2007, Cluj-Napoca, Romania
6/28
Scaling laws
Gravitational force
(weight)
Electrostatic force
W
W
∝ L3
∝ L2
ESM2007 - September 9-18th 2007, Cluj-Napoca, Romania
Scaling laws for different forces
Weight
∝ L3
Capillary force
Van der Waals force
Electrostatic force
∝ L2
Length
For MEMS, weight is negligible,
surface forces may cause movement or deformation:
electrostatic force can be used for actuation (controlable)
capillary force can lead to MEMS failure (during fabrication or use)
Van der Waals force can lead to stiction
ESM2007 - September 9-18th 2007, Cluj-Napoca, Romania
7/28
8/28
Scaling laws
increases
-resonance frequency
∝ 1/L
size reduction
-response time
decreases
(e.g. thermal exchange)
-stiffness
-power consumption
ESM2007 - September 9-18th 2007, Cluj-Napoca, Romania
9/28
How are MEMS made ?
MEMS are made with techniques originally
developed for the microelectronics industry
Building block for MEMS fabrication :
I – Deposition
II - Lithography
III – Etching
ESM2007 - September 9-18th 2007, Cluj-Napoca, Romania
How are MEMS made ?
MEMS are made with techniques originally
developed for the microelectronics industry
Building block for MEMS fabrication :
I – Deposition
II - Lithography
III – Etching
ESM2007 - September 9-18th 2007, Cluj-Napoca, Romania
10/28
Deposition
chemical reaction:
- Chemical Vapor Deposition
(CVD)
- Electrodeposition
- Thermal oxidation
physical reaction:
- Physical Vapor Deposition
(PVD)
- Casting
ESM2007 - September 9-18th 2007, Cluj-Napoca, Romania
Chemical deposition
Chemical Vapor Deposition
-Low Pressure CVD (LPCVD)
deposition on both sides of wafer
-Plasma Enhanced CVD (PECVD)
deposition on one side of wafer
!!! hazardous byproducts !!!
Electrodeposition
restricted to electrically conductive materials
Well suited to Cu, Au, Ni
1µm to >100µm
ESM2007 - September 9-18th 2007, Cluj-Napoca, Romania
hot-wall LPCVD reactor
11/28
12/28
Physical Vapor Deposition (PVD)
far more common than CVD for metals (lower process risk, cheaper material costs)
Evaporation
heating : e-beam or resistive - material specific
Sputtering
ion bombardment (Ar+) of target
material released at much
lower T than evaporation
Well suited to alloys
ESM2007 - September 9-18th 2007, Cluj-Napoca, Romania
13/28
Casting
-material dissolved in a solvent and applied to the substrate by spraying or spinning
- used for polymers, photoresist, glass
- thicknesses: from a single monolayer of molecules to tens of µm
ESM2007 - September 9-18th 2007, Cluj-Napoca, Romania
How are MEMS made ?
MEMS are made with techniques originally
developed for the microelectronics industry
Building block for MEMS fabrication :
I – Deposition
II - Lithography
III – Etching
ESM2007 - September 9-18th 2007, Cluj-Napoca, Romania
Lithography
Resolution:
Optical
UV ≈ 0.5 µm
Deep UV ≈ 0.3 µm
E-beam < 100 nm
14/28
15/28
Pattern transfer
lift-off approach less common because resist is incompatible with most
MEMS deposition processes (high temperatures + contamination)
How are MEMS made ?
MEMS are made with techniques originally
developed for the microelectronics industry
Building block for MEMS fabrication :
I – Deposition
II - Lithography
III – Etching
ESM2007 - September 9-18th 2007, Cluj-Napoca, Romania
Dry etching
1) Sputter etching - similar to sputtering deposition
2) Reactive ion etching (RIE)
accelerated ions react at surface – have both chemical and physical etching
deep-RIE
→ hundreds of microns + almost vertical sidewalls, aspect ratios ≤ 50 : 1
"Bosch process"
alternates repeatedly between two gases:
gas # 1: deposition of a chemically inert passivation layer (polymer)
gas # 2: etches, polymer on horizontal surfaces is immediately
physically sputtered while sidewalls not sputtered
3) Vapor phase etching
material is dissolved at the surface in a chemical reaction with gas molecules
e.g. HF for etching Si02
XeF2 for etching Si
16/28
17/28
Etching
Wet etching
material is dissolved in a chemical solution
dry etching
wet etching
simple and cheap
vs
expensive
much higher resolution
High aspect ratio (≤ 50 : 1)
ESM2007 - September 9-18th 2007, Cluj-Napoca, Romania
MEMS technologies
Bulk micromachining
- defines structures by selectively etching inside a substrate,
typically Si is used and wet etched with KOH or TMAH
- relatively simple and inexpensive
Surface micromachining
- deposition / etching of different layers on a substrate
- many fabrication steps (expensive)
- can produce complicated devices
LIGA : x-ray Lithographie-Galvanoformung-Abformung
(x-ray Lithography-Electrodeposition-Moulding)
x-ray litho. of PMMA to produce template for electro-dep. (e.g. Ni)
the electrodeposited structure may be used as a mould for
replication from another material ( plastic, ceramic)
- small, but relatively high aspect ratio devices
18/28
Why is magnetism of
interest for MEMS ?
ESM2007 - September 9-18th 2007, Cluj-Napoca, Romania
Remarkable features of magnetic interactions
Large forces
Magnetic pressure = 4 bar / Tesla² = 400 mN/mm / T
Large energy densities Magnetic: ½B²/µ => 400 000 J/m3
2
0
19/28
2
Electrostatic : ½ε0E² => 40 J/m3
@1T
@ 3 MV/m
(elect. breakdown in air)
1/ µ0range
= 8.105
=> 400 000 J/m3 @ 1 T
Long
-12 to 100 =>
Several
ε0 = 9.10
10
µm or 40
more
J/m3 @ 3 MV/m
Contactless actuation
Remote / Wireless actuation Medical implantable
Action through sealed membranes / vaccuum / skin
Suspension / Levitation
(Bi)stability
Magnets : permanent forces without power waste
Long term forces - Safety
Bidirectionality
Repulsion / Attraction
Homothetical scale reduction :
20/28
Field gradients & forces between magnets = increased
0.9 T
Torque
preserved
∆ : 0.3
T
10 mm
0.6 T
0.9 T
∆ : 0.3
T
1 mm
Gradient
xL
Force
xL
0.3 T
0T
P
H
r
cm3
1
J=1T
0.6 T
0.3 T
ZOOM
mm3
1
J=1T
0T
topology of field preserved:

→
→
∆(Field)
/
Distance
=
Gradient


→
v1
(
J1
⋅ r) →


=
10
x10
=
x10
H1( M
⋅ 3⋅
⋅ r − J1
P ) :=
Gradient
x
Moment
=
Force

4 ⋅ π ⋅ µo ⋅ r3 
r2

Magnitude and
/
Laplace/Lorentz forces
by a magnet onto a current
same current density
x same field = same force density
F= i. ℓ ∧B
B
F
i
21/28
22/28
Effects of a scale reduction 1 / L on
conductor / conductor interactions (volumic, massic)
(at constant current density)
infinite wire)
B =
H
H = i / 2πR
R
µ0H = 4π10-7 . J.s / 2πR
∝ J.d²/d
∝ 1/L
F
ℓ
(
reduced
s
i
.s
J
=
i
F = B. i.
2
⇒
ℓ
F = 2.10-7.i1.i2.
ℓ
/R
F/volume ∝ J².s.s.
ℓ
/R .d3
∝ 1/L
reduced
23/28
Effects of a scale reduction 1 / L
on magnetic interactions (volumic, massic)
(at constant current density)
Scale
reduction
1/L
magnet
current
magnet
current
iron
induction
e = -dΦ /dt
×
L
L
/L
× frequency
/L
/ L²
× frequency
×
/L
µ-coils can withstand gigantic current densities
24/28
thanks to better cooling
Heat generation RI² = volume = d3
Heat dissipation = surface = d²
=> S/V ratio = d²/d3 = 1/d : improves with scale reduction factor L
Distance between thermal sources = d
⇒
⇒
Thermal gradient = x L
Surface thermal flow
Φ/s = λ.∆T/e
heat flow through surface = x L
Cylindrical wire = S/V min
Planar conductor = S/V max
=> S/V ratio increases also
Conductor = metallic thin film
Si substrate = excellent heat conductor/absorber
Short time constants = current pulses = µs
Classical electrotechnics : 5 A/mm² DC…
Integrated µ-coils : up to 1 Million A/mm² in pulses!
25/28
Effects of a scale reduction 1 / L
on magnetic interactions (volumic, massic)
with increased current density x Li
Scale
reduction
1/L
magnet
current
iron
induction
e = -dΦ /dt
magnet
×
L
current
×
Li
×
×
Li
L²i / L
L
/L
× frequency
L²i / L
L²i / L²
× frequency
×
×
×
26/28
The importance of µ-magnets
???
100,000
magnet
10,000
Current
density
100
10
ℓ
2R
coil
coil
100 µm
ℓ
µ-coils
high-tech
electrotechnics
Coil + Fe core
10 µm
H = N.i /
Well cooled Silicon
Superconductors,
1 Tesla
coil
1000
(A/mm²)
??? < µs
(A/m)
H i = J.s = J.π R²
H = N.J.π R² / ℓ
size
1 mm
10 mm
Everyday life
A µ-magnet is much more interesting
than a µ-coil/µ-electromagnet of same size
Some examples of magnetic MEMS…..
Deformable mirror for adaptive optics
27/28
(astronomy, ophtalmology…)
Image from star distorted
by atmosphere
LEG - LAOG - LPMO - IEMN
Reflective
coating
Flexible
membrane
membrane
magnets
coils
Silicon ring
waffer
Magnets
Array of planar
micro-coils
Image corrected
by adaptive optics
Si wafer
SmCo Ø 0.85 x 0.25 mm (COMADUR)
Energy harvesting (vibrations) : µ-generator (Univ. Hong Kong)
Membrane pump for µ-fluidics (Suwon, Korea)
28/28
MAGNETIC MICRO-ACTUATOR prototypes
Collab. G2ELab + CEA/LETI (Grenoble, France)
Planar µ-motor
Bistable ultra-fast
Planar µ-turbo-generator
levitating µ-switch
fixed moving
magnet magnet
fixed
magnet
200 µm
2 x 3 phases
brushless
275,000 rpm on
magnet / air bearings
Ø 8 mm
planar
coil
Commutation 5 µm/30 µs/30 µJ/pulse
Voltages measured at 80,000 rpm
Output ~ 1 W @ 100,000 rpm
µ-machined bulk NdFeB
1st fully integrated prototype
Electrodep. Co85Pt15
8 & 15 pole pairs
Need for high quality integrated thick film magnets for MEMS !
1/28
Micro-magnets for MEMS
- Candidate materials
- Integration issues
- Preparation of high performance materials
in thick film form by sputtering
-Film patterning
-Beyond magnets
Nora M. Dempsey
Institut Néel, CNRS/UJF, Grenoble, France
ESM2007 - September 9-18th 2007, Cluj-Napoca, Romania
2/28
High performance
permanent magnet materials
µ0M
↑(BH)max → ↓ magnet volume
B=µ0(H+M)
µ0Mr
MHc
(BH)max
Material
µ0MS
(T)
µ HA
(T)
(BH)max,th
kJ/m3
TC (K)
Nd2Fe14B
1.61
7.6
514
585
SmCo5
1.05
40
220
1000
Sm2Co17
1.30
6.4
333
1173
FePt
1.43
11.6
407
750
CoPt
1.00
4.9
200
840
H
RE-TM
Mr ≤ MS
Hc ≤ HA
Mr, Hc determined
by microstructure
L10
3/28
Routes to prepare thick film magnets
Top-down routes
Bottom-up routes
Micro-machining
of bulk magnets
Sputtering
10 mm
Screen
printing
LEG / MISA
CMSM
Cincinnati
Deformation
(e.g. rolling)
LLN
l
L = 10xl
µm
50
x
0
5
Pulsed Laser Deposition
(PLD)
Plasma spraying
Electro-deposition
5 cm
t = 5 µm
LETI
l = 200 µm
3/28
Routes to prepare thick film magnets
Top-down routes
Bottom-up routes
Micro-machining
of bulk magnets
Screen
printing
Sputtering
10 mm
LEG / MISA
Material CMSM
specific !
Cincinnati
Deformation
(e.g. rolling)
5 cm
l
rollingL :=L1
0 alloys
10xl
LLN
µm
50
x
0
5
Pulsed Laser Deposition
(PLD)
electro-deposition:
L10 alloys
Plasma spraying
Electro-deposition
t = 5 µm
LETI
l = 200 µm
Micro-magnets for MEMS
- Candidate materials
- Integration issues
- Preparation of high performance materials
in thick film form by sputtering
- Film patterning
ESM2007 - September 9-18th 2007, Cluj-Napoca, Romania
4/28
Influence of processing on
magnetic properties
• Sample degradation
e.g. drop in coercivity:
µ-machining,
screen printing
µ-machined
NdFeB
Bulk
T
500 µm
k
• Dilution of magnetic phase → reduction of remanent magnetisation:
screen printing (vol. fraction of binder)
plasma spray (porosity);
• Magnet texture : choice of process + process parameters
will determine ability to produce a textured magnet
For now Isotropic : screen printing, plasma spraying, rolling…..
5/28
Magnet Dimensions
thickness
1 mm
1 µm
1 nm
1 µm
1 mm
lateral dimensions
1m
5/28
Magnet Dimensions
thickness
1 mm
200 µm
1 µm
Individual magnet
φ=8 mm; t = 500 µm
individual magnet
1 nm
1 µm
1 mm
lateral dimensions
1m
5/28
Magnet Dimensions
thickness
1 mm
200 µm
1 µm
Individual magnet
φ=8 mm; t = 500 µm
Magnet array
1x1 mm2; t = 100 µm
individual magnet / magnet array (batch fab.)
1 nm
1 µm
1 mm
lateral dimensions
1m
5/28
Magnet Dimensions
zone of interest for
MEMS
thickness
1 mm
200 µm
1 µm
Individual magnet
φ=8 mm; t = 500 µm
Magnet array
1x1 mm2; t = 100 µm
individual magnet / magnet array (batch fab.)
1 nm
1 µm
1 mm
lateral dimensions
1m
Magnet Dimensions
5/28
zone of interest for
MEMS
thickness
1 mm
1 µm
1 nm
1 µm
1 mm
lateral dimensions
1m
Magnet Dimensions
zone of interest for
MEMS
µ-machining
1 mm
thickness
5/28
1 µm
1 nm
1 µm
1 mm
lateral dimensions
1m
Magnet Dimensions
zone of interest for
MEMS
µ-machining
Screen printing / plasma spray
1 mm
thickness
5/28
1 µm
1 nm
1 µm
1 mm
lateral dimensions
1m
Magnet Dimensions
zone of interest for
MEMS
µ-machining
Screen printing / plasma spray
Rolling
1 mm
thickness
5/28
1 µm
1 nm
1 µm
1 mm
lateral dimensions
1m
Magnet Dimensions
zone of interest for
MEMS
µ-machining
Screen printing / plasma spray
Rolling
1 mm
thickness
5/28
Sputtering
1 µm
1 nm
1 µm
1 mm
lateral dimensions
1m
Magnet Dimensions
zone of interest for
MEMS
µ-machining
Screen printing / plasma spray
Rolling
1 mm
thickness
5/28
Sputtering
Electro-deposition
1 µm
1 nm
1 µm
1 mm
lateral dimensions
1m
Magnet Dimensions
zone of interest for
MEMS
µ-machining
Screen printing / plasma spray
Rolling
1 mm
thickness
5/28
1 µm
PLD
Sputtering
Electro-deposition
1 nm
1 µm
1 mm
lateral dimensions
1m
6/28
Integration issues : magnet insertion
Simple milli-systems
Planar µ-motor
1) produce many individual magnets
2) "pick & place" magnet into µ-system
c.f. wrist-watches,
surface mounted µ-electronic circuits
Complex micro-systems
produce and insert magnet in same step
full integration, on-chip processing
Bi-stable µ-switch
200 µm
LEG/LETI
7/28
Integration issues for film magnets
• Choice of substrate / buffer / capping layers:
will determine magnetic and mechanical properties,
could influence cost of system
must consider compatibility with - other µ-system components
- microfabrication techniques
• Film patterning
e.g. Si vs MgO
pre-patterned substrates
deposition through masks
post-deposition patterning
• Thermal compatibility
for high anisotropy phases*, need elevated processing temperatures
(Nd2Fe14B > 580°C, SmCo5 > 350°C, FePt > 450°C)
-Wafer bonding allows full processing of magnet before integration of other
system components
*non-L10 Co80Pt20 may be produced by electro-deposition at low processing temp.
8/28
Integration issues for film magnets
• Mechanical compatibility
will have a build up of stress in thick films
mechanical stress
∝ difference in thermal expansion coefficients of
susbstrate /buffer layer /magnetic layer
Minimise strain :
NdFeB (50 µm) / Si
choice of substrate / buffer
reduced surface area of magnet
• Chemical compatibility
pollution of - other system components
- microtechnology equipment (clean room environment !!)
Micro-magnets for MEMS
- Candidate materials
- Integration issues
- Preparation of high performance materials
in thick film form by sputtering
-Film patterning
-Beyond magnets
ESM2007 - September 9-18th 2007, Cluj-Napoca, Romania
9/28
Preparation of thick film magnets by sputtering
Triode sputtering machine
- Base pressure : 10-6 mbar
- Sputtering pressure : 10-3
mbar
- Target ≤ 10x10 cm²
- Deposition rate ≤ 20 µm/h
- Substrate ≤ 100 mm
Tsub ≤ 750°C
-adapter for 200 mm substrates
Ar gas
Anode
Target
Substrate
filament
r
r(cm)
t (µm)
0
5
5
4
10
2
magnetic field
10/28
Film preparation parameters
• Target:
Composition
Target potential
(kinetic energy of impinging ions)
• Substrate:
distance to target (thickness+homogeneity)
material (Si, Al2O3..)
bias
temperature during deposition
• Deposition conditions
Ar pressure
• Post-deposition annealing
Film:
composition
µ-structure
crystal structure
crystal texture
11/28
NdFeB thick films
- influence of deposition temperature
{Si / Ta (100 nm) / NdFeB (5µm) / Ta (100 nm) }
2-step process: deposition at Tsub = X °C + annealed in situ at 750°C for 10 min.
Tsub=300°C
Tsub = « Cold»
Tsub=450°C
Tsub=400°C
1,5
1,5
1,5
1,0
1,0
1,0
0,5
0,5
0,5
Tsub=500°C
1,5
0,0
-0,5
oop
ip
oop
-1,0
-6
-4
-2
0
µ0Hi (T)
2
4
6
0,0
-0,5
-0,5
-1,0
-1,0
-1,5
-1,5
0,5
0,0
-0,5
8
0,0
-0,5
-1,0
-1,0
-1,5
-1,5
-8
0,0
1,0
µ0M (T)
µ 0M (T)
µ 0M (T)
0,5
µ0M (T)
ip
1,0
µ0M (T)
1,5
-8
-6
-4
-2
0
2
4
6
8
-8
-6
µ0 Hi (T)
-4
-2
0
µ0Hi (T)
2
4
6
8
-8
-6
-4
-2
0
2
4
6
8
-1,5
-8
-6
µ0Hi (T)
-4
-2
0
2
4
µ0Hi (T)
(0 0 6)
Cross-section
Equiaxed grains
Columnar grains
Grain size ↓ as Tsub ↑ => ↑ of the density of nucleation sites during deposition
6
8
Mechanical issues
have mechanical problems with continuous films….
on 100 mm wafer, central area (φ ≈ 3 cm) peels off when
film annealed
Attributed to
differential thermal expansion (NdFeB vs Si)
+ volume change during crystallisation
NdFeB
Si
12
10
8
0
1 6
•
3
Si
4
∆
l
/
l
2
TTcr
cryst
0
-2
0
200
400
T, °C
600
800
B.A. Kapitanov et al., J. Magn. Magn. Mater. 127, 289 (1993).
12/28
13/28
Mechanically intact thick NdFeB films
(deposit crystallised (Tdep = 550°C) + through mask)
Steel mask, φ = 5 mm
Surface roughness !!!
0,8
20µm
1,0
5µm
0,6
0,4
1,0
0,0
-0,2
oop
ip
-0,4
-0,6
µ 0M (T)
0,2
0,0
-1,0
-1,0
-1,5
-4
-2
0
µ 0Hi (T)
2
4
6
oop
ip
oop
-1,0
-6
0,0
-0,5
-0,5
-0,8
-8
50µm
0,5
0,5
µ 0M (T)
µ 0M (T)
1,5
1,5
1,0
8
-1,5
-8
-6
-4
-2
0
2
4
6
8
-8
AFM
43µm
rms=161nm
-4
-2
0
µ 0Hi (T)
µ0Hi (T)
MEB
MEB
-6
2
4
6
8
14/28
SmCo thick films
-infleunce of deposition temperature
TsubT = =400°
400°C C
Tsub
500°
T =
= 500°
C C
sub
sub
ip
oop
ip
oop
4
2
0
500°C
1
2
1
0
0
1
1
1
0
2
0
0
2
2
0
0
1
0
0
3
6000
Si
Th2Zn17
1
1
1
1 01
0
4
M (a.u.)
1
0
2
2
TbCu7
M (a.u.)
0
Cr
-8 -6 -4 -2 0 2
µ H (T)
4
6
400°C
-8 -6 -4 -2 0 2
µ H (T)
8
3000
0
350°C
4
6
8
0
300°C
TsubT ==350°
350°C C
Cold dep
sub
Tsub
300°
= 300°
C C
T =
0
sub
ip
oop
40
60
80
ip
-8 -6 -4 -2
0
2
µ H (T)
4
6
8
1,0
0,5
0,0
200 300 400 500 600
0
Tdep (°C)
For Tsub
= 350-400°C
µ0Mr ≈ 0.8 T
M (a.u.)
µ0Hc (T)
M (a.u.)
1,5
-8 -6 -4 -2 0 2
µ H (T)
0
4
6
8
15/28
Thick hard magnetic films (5 µm) on Si
oop
NdFeB
out-of-plane texture
SmCo
in-plane texture
Tdep: 450°C + T ann : 750°C
T
= 350°C
M (a.u.)
oop
0,0
-0,5
ip
-1,0
-8
-1,5
-8
-6
-4
-2
0
2
4
6
8
-6
-4
-2
0
2
4
6
8
µ H (T)
0
µ0Hi (T)
"cold"
400°C
500°C
600°C
M (a.u.)
µ 0M (T)
sub
ip
oop
1,5
0,5
FePt
isotropic
Tdep: ≥ 400°C
Tdep: 350°C
or Tdep = 550°C
1,0
ip
-4 -3 -2
-1 0 1
µ H(T)
2
3
0
(006)
(BH)max=400 kJ/m3
(200)
(BH)max=140 kJ/m3
µ0Hc > 1 T
Corrosion resistive
4
Micro-magnets for MEMS
- Candidate materials
- Integration issues
- Preparation of high performance materials
in thick film form by sputtering
-Film patterning
-Beyond magnets
ESM2007 - September 9-18th 2007, Cluj-Napoca, Romania
16/28
Micro-structured Magnets
Electro-deposited
Electro-deposited Co50Pt50
in nanoporous membranes
450 µm pole pitch
2 µm Co80Pt20 pillars
PLD of FePt (L10)
Local laser annealing
disordering
→
Film thickness 40 nm
µ0Hc= 0.4 T, (BH)max= 34 kJ/m3
2002 Zana_JAP_91_7320
µ0Hc= 1.3 T
2005 Rhen_JAP_97_113908
Sputtered SmCo
Lift-off of Cu mask by wet etch
40 µm
2006 J. Buschbeck_JAP (IFW)
Sputtering of amorphous NdFeB
Local laser annealing
crystallisation
→
SmCo
sacrificial
Cu mask
SmCo
Si substrate
2002 Budde JMMM242_1146
2003 Okuda_JJAP_42_6859
17/28
Proposed routes for the
structuration of magnetic films
1) Topographic
Deposition on
prepatterned subst.
2) Magnetic
Thermomagnetic patterning
Heat
mask
M
H
+ polishing
Etching of
magnetic layer
cf: thermomagnetic writing
for recording media
irreversible
reversible
Proposed routes for the
structuration of magnetic films
1) Topographic
Deposition on
prepatterned subst.
2) Magnetic
Thermomagnetic patterning
Heat
mask
M
H
+ polishing
Etching of
magnetic layer
cf: thermomagnetic writing
for recording media
irreversible
reversible
18/28
Topographically patterned films
y
x
« cold » deposition of
Ta(100nm) / NdFeB (5 µm) /Ta (100 nm)
+ in-situ anneal (750°C/10 min)
10 µm
SiO2
Si
in-plane loop of a piece containing
3 sets of trenches
(widths 5, 10 and 20 µm)
0.06
0.04
0.02
M (a.u.)
Use of pre-patterned Si/SiO2
substrate with sets of trenches of
different lateral dimensions
0
-0.02
-0.04
-0.06
-8
-6
-4
-2
0
2
4
6
8
µ H(T)
0
Need local magnetic characterisation !
19/28
Trench filling on pre-patterned wafers
NdFeB
10µm
10µm
SmCo
2µm
10µm
10µm
20/28
Local magnetic
characterisation for MEMS
Need to measure stray fields produced by µ-magnets :
- to characterise inhomogeneities
(process optimisation)
- to calculate forces at work in the µ-system
(simulation/design optimisation)
Tools :
Scanning Hall probes (sensititvity + scan param. adapted to MEMS)
Kerr microscope + MOIF
21/28
MOIF* imaging of
topographically patterned films
MOIF
sample saturated
out-of-plane
•Magneto Optic Imaging Film
(e.g. ferrite garnet)
multipole
Theoretically calculated Bz/Br at
1, 2, 4 and 8 µm from film surface
(tfilm = 3.5 µm, p = 20 µm
h = 10 µm l = 100 µm)
unidirectional
Bz distribution reconstructed
from halftone planar MOIF
images at a distance of 4 µm
Tver State University (Russia)
uniaxial MOIF
22/28
Wet etching/planarisation of RE-TM films
Wet etching
SmCo
NdFeB
RE-TM
Si
lateral over-etching
≈ 15 µm
Films etched in amorphous state
#1: capped with Ta + annealed
#2: ion etched + capped with Ta + annealed
#3: annealed
0,5
M/M4T
Hystersis loops
of wet etched /planarised
and annealed NdFeB
1,0
0,0
-0,5
-1,0
-4
-2
0
µ 0H (T)
2
covered
# 1 with Ta
ion etching
#+2covered with Ta
# covered
3
not
4
Proposed routes for the
structuration of films
1) Topographic
Deposition on
prepatterned subst.
2) Magnetic
Thermomagnetic patterning
Heat
mask
M
H
+ polishing
Etching of
magnetic layer
cf: thermomagnetic writing
for recording media
irreversible
reversible
Thermo-Magnetic patterning
of NdFeB films
planar MOIF
23/28
Pulsed laser
NdFeB
Hc > Hext
mask
Hext
Film surface
Magnetisation modulation due to Fresnel diffraction at mask
Pitch (~12 µm) determined by λlaser and
distance from mask to sample.
Uniaxial MOIF
Demonstrates the perspective
for optical interference thermomagnetic writing
24/28
Applications of µ-magnets
RF µ-switches
Collaboration:
G2ELab : µ-system designer
LETI : µ-fab platform
Institut Néel : magnet films
Alcatel Space : end user
58 process steps
(including 9 litho.)
z
y
x
Funding: ANR “Nanomag2“
Diamagnetic levitation
LEG and CEA (Biochips Laboratory,
SMOC/LETI)
« clip » C. Pigot
H. Chetouani et al., Transducers ‘07
Contactless containment of 3µm latex
microbeads with linear magnetic traps
Micro-magnets for MEMS
- Candidate materials
- Integration issues
- Preparation of high performance materials
in thick film form by sputtering
-Film patterning
-Beyond magnets
ESM2007 - September 9-18th 2007, Cluj-Napoca, Romania
25/28
Beyond hard magnets:
switchable materials
Why go beyond hard magnets?
They are driven by currents in coils
Joule heating losses
Complex design
⇒
⇒
Switchable magnetic materials:
Reversible change of magnetic properties
(MS, HA) by an external parameter.
Thermo-switchable
E-field switchable
Strain switchable
New functionality
(e.g. laterally resolved
modifications by controlled
laser heating)
Novel concept
Unexplored potential
Magnetostrictive
Multiferroics
e.g. FePt, FeRh
magnetic shape memory
Contactless !
26/28
Thermo-switchable materials
Antiferro
ferro
on / off switching
Ferrimagnetic
Compensation
up / down switching
e.g. FeRh
GdCo3Cu2
140
M(T) at 2mT
Fe Rh
48
70
52
M(emu/g)
M (emu/g)
105
310
T(K)
320
330
= 335 K
-1
-2
300
comp
0
35
0
290
T
1
GdCo Cu
3
2
-3
300
320
340
T(K)
360
out-of-plane
90°switching
NdCo5
3
2
In-plane
380
27/28
E-field switchable materials
FePd
FePt
2
-+
-+
-+
-+
-+
δ Coercivity / %
Electrolyte
reversible modification of
magnetic properties in
itinerant electron systems
U
0
-2
-4
2 nm
4 nm
FePd
4 nm
FePt
2 nm
-1 2 0 0 -1 0 0 0 -8 0 0
-6 0 0
-4 0 0
F e P t p o te n tia l v s . P t / m V
~ 1 nm
Surface effect: High surface-to-volume ratio needed
FePt
E>0
MS
E=0
Nanocrystalline powder
FeRh
E>0
Alloy film
H
M
E=0
• Compaction
• Sintering
nanoporous
Selective dissolution of
sacrificial component
nanoporous
T
Prospect
for NEMS
28/28
Other Magnetic Materials
for MEMS
- Magnetostrictive
bimorph membranes (µ-pump)
Planar micropump
TbFe2 (λ > 0)
Si
(λ = 0)
SmFe2 (λ < 0)
- Magnetic Shape Memory Alloy
See K. Dörr (talk)
+ M. Thomas (poster)
Acknowledgments
Institut Néel :
A. Walther (PhD), C. N’dao (PhD) and D. Givord
G2Elab :
O. Cugat, J. Delamare, G. Reyne and H. Chetouani (PhD)
LETI/CEA
C. Marcoux, B. Desloges, C. Dieppedale
TVER STATE UNIVERSITY
R. Grechishkin, M. Kustov (PhD)
WEB:
Bernard Legrand (IEMN) « Ecole Nanosciences et Nanotech. – Autrans 2006 »
http://www.mems-exchange.org
http://www.memx.com
Applications of THERMO-REVERSIBLE Permanent Magnet
GdCo3Cu2 ( Tcomp is 90 oC)
indicates direction of TM magnet
thermally controlled
Actuator:
suspended TR magnet
rotates by 180° when
heated above Tcomp
suspended TR magnet
remotely interrogated temperature sensor :
fixed magnet
no bias
signal element square loop soft
magnetic tape (nc Fe81B13.5Si3.5C2) producing
detectable harmonics.
Bias with magnet → appearance of even harmonics
Grechishkin et al. APL 89, 122505 (2006); D. Mavrudieva et al, SENSOR LETTERS 5, 1 (2007)