1. E-MRS SPRING MEETING 2006
SnO2:Sb – A new material for high-temperature MEMS heater applications – performance and limitations
A. Helwig, J. Spannhake, G. Müller
Corporate Research Center – EADS Deutschland GmbH
T.Wassner, M.Eickhoff
Walter Schottky Institute, Technical University of Munich, Germany
G.Sberveglieri, G.Faglia
C.N.R. – INFM & Università di Brescia
29th of May 2006, Nice, France
A. Helwig, CRC / LG-ME, Phone

2. Microsystems & Electronics CRC Germany
SnO2:Sb – A new material for high-temperature MEMS heater applications – performance and limitations
Outline
IR emitter device – a high-temperature heater application
Employing SnO2:Sb as high-stability heater material
Performance & Limitations of SnO2:Sb metallisation
Liftetime estimation of SnO2:Sb based MEMS heater devices
Design & concept of IR emitter device  standard application as HT MEMS heater device  Realising
A. Helwig, CRC / LG-ME, Phone

3. Design & concept of IR emitter device
T-Sensor
Hotplate design parameters:
Chip surface 5 x 5 mm2
Suspended membrane 1.5 x 1.5 mm2
Membrane thickness 6 µm
Length of suspensions 350 µm
Width of suspensions 150 µm
Emissivity 0.8 – 0.9
Operating temperatures 800 – 1200 °C
Heating power 0.8 – 1.4 W
operating emitter
at ~T=1000 °C
bonded emitter chip
on TO8-carrier
Heater
Near Infrared
A. Helwig, CRC / LG-ME, Phone

4. Fabrication of thermal IR emitter device
State of the art: SOI-based Hotplate Technology Pt
Front side: Dry etching (STS) Pt
Platinum is standard heater metallisation. Si
SiO2
Pt heater lifetime at
T = 600°C: ~10 years
Backside: Wet etching (TMAH)
 What is the performance of Pt at higher T ?
A. Helwig, CRC / LG-ME, Phone

5. High-temperature test of emitter hotplates
Long-term stability at T ~ 850°C limited by Pt heater degradation Pt
Thermal expansion mismatch
Electro-migration due to the high electric current + high temperature.
 Alternative heater materials?
A. Helwig, CRC / LG-ME, Phone

6. SnO2:Sb metallisation:
High-stability semiconductor heater material: SnO2:Sb
Doped tin oxide SnO2:Sb
Resistivity of SnO2:Sb as a function of Ta and Sb concentration.
good substrate adherence
an oxide cannot suffer from oxidation
high-temperature stable
(Ta ~ 1050°C)
very little electro-migration
patterning by IBE and lift-off
Advantages:
SnO2:Sb metallisation:
E beam evaporation of SnO2 powder with an admixture of antimony
best conductivity at Ta~1050°C
Deposited SnO2:Sb patterned
by lift-off
Fig. 4 Conductivity of SnO2 as a function of annealing temperature Ta and Sb dopant concentration in the evaporation source.
A. Helwig, CRC / LG-ME, Phone

7. Thermal Activation of Dopant Sb
XRD measurements of deposited & annealed SnO2/Sb layers
SnO2:Sb
SnO2
 best resistivity at 1050°C
Fig. 5: XRD measurements of a Sb doped SnOx thin film layer revealing the different morphology: a) after deposition at RT and b) after annealing at 1200°C.
Fig.6: Optical absorption spectra of SnO2 thin film layers after different temperature treatments: a) SnOx layer deposited at room temperature and annealed at different temperatures Ta; b) SnO:Sb(5%) layer deposited at room temperature and annealed at different temperatures Ta.
Optical absorption spectra of annealed SnO2 and SnO2/Sb layers
A. Helwig, CRC / LG-ME, Phone

8. Performance of SnO2:Sb as metallisation
Accelerated degradation tests of different metallisations
Pt heater
SnO2:Sb heater
- Accelerated aging tests of Pt (upper curve) and SnO2:Sb (lower curve) heater elements in air. - Increased temperature (el. input power) leads to a rapidly increasing Pt degradation.
The seeming instability at lower temperatures is due to adsorption/desorption effects at the SnO2:Sb surface.
Fig. 3 Accelerated aging tests of Pt (upper curve) and SnO2:Sb (lower curve) heater elements in air.
Increased temperature (el. input power) leads to a rapidly increasing Pt degradation, whereas SnO2:Sb emitters
can be operated up to temperatures in the 1000°C range. The seeming instability at lower temperatures is due to
adsorption/desorption effects at the SnO2:Sb surface
SnO2:Sb exhibits no degradation up to T ~1000 °C
Instability at lower T due to adsorption/desorption effects at the SnO2:Sb surface.
 Long-term stability of SnO2:Sb heater ?
A. Helwig, CRC / LG-ME, Phone

9. Long-term stability of heater metallisation
Degradation of Platinum and SnO2:Sb heaters
Pt heater after 32h at ~800°C
SnO2:Sb after 18h at ~1000°C
Clear signs of degradation visible in hotspot areas.
Transmission microscopy reveals loss of Pt in these areas.
A. Helwig, CRC / LG-ME, Phone

10. Thermal stability limitation of SnO2:Sb
Loss of conductivity due to high temperature annealing
No current flow!
XPS Results:
Loss of
Sb dopants
at T > 1050°C
Fig. 7 Variation of the dc resistivity of SnO2:Sb samples in the course of isothermal annealing at successively higher temperatures.
Fig. 8 XPS spectra of SnO2:Sb in different states of annealing: a) Sn core peaks are hardly affected by annealing;
b) the magnitude of the Sb-related core peaks is decreased by high- temperature annealing.
Stability limited by loss of antimony !!
A. Helwig, CRC / LG-ME, Phone

11. Activation energies for degradation of SnO2:Sb
Ea could not be obtained with accelerated degradation tests due to residual gas sensing effects
Thermal degradation rate depending on T
Activation
energy for
degradation
EA ~ 9.45 eV
SnO2:Sb heaters exhibited even larger activation energies.
Because of a residual gas sensing effect reliable values of in this latter case.
Rapid thermal degradation of SOI-based emitters
Resulting Ea ~9.5 eV allows to estimate lifetime of SnO2:Sb as heater metallisation
 Verification via long-term durability tests
EA of thermal
degradation
Allows to compare performance and to estimate heater lifetime
A. Helwig, CRC / LG-ME, Phone

12. Performance of heater metallisations
Activation energies for heater degradation
Pt heater
Si:B heater
SnO2:Sb performs better than metals and silicides:
Little electro-migration!
No Oxidation problems!
SnO2:Sb heater
Fig. 10 Temperature dependence of the relative resistance drift of noble metal and semiconductor heater materials
Activation energy for heater degradation for different heater materials. Si:B heaters are seen to perform considerably better at high temperature than metallic ones.
----- SnO2:Sb heaters exhibited even larger activation energies.
Because of a residual gas sensing effect reliable values of Ea could not be obtained in this latter case.
 EA allows estimating emitter life time
A. Helwig, CRC / LG-ME, Phone

13. Life time estimation for heater materials
Emitters operated with DC voltage in ambient air
Estimated lifetime at
T ~ 950°C: 10 years
-> Fehlerbalken vom SnO2:Sb  deltaT=50°C
Fig. 11 Lifetime estimates as determined from the data of Fig.10.
The red line corresponds to useful lifetime of 10 years.
The respective temperature limits for the individual materials can be read off from the abscissa.
The magnitude of the error bars shown derives from the observed magnitude of the temperature inhomogeneities on the MEMS heater devices.
Activation energy for heater degradation of different heater materials. Si:B heaters are seen to perform considerably better at high temperature than metallic ones.
----- SnO2:Sb heaters exhibited even larger activation energies.
Because of a residual gas sensing effect reliable values of Ea could not be obtained in this latter case.
Next Step  Verification of lifetime estimation!
A. Helwig, CRC / LG-ME, Phone

14. Verification of lifetime estimation
Long-term durability test of heater metallisation inside a sealed tube with ambient air.
Heater degradation test about 7 weeks
Fig. 12 Result of a direct long-term test on hotplate devices with SnO2:Sb heater resistors.
During the test the heater devices were operated at constant electrical input power as shown by the staircase-like curve.
 lifetime estimation has been approved!
A. Helwig, CRC / LG-ME, Phone

15. Conclusion:
SOI-based micro heaters with standard metallisation long-term-stable up to T ~ 600°C
By introducing SnO2:Sb as heater material: Long-term stable operation can be extended up to T ~ 1000°C
SnO2:Sb employed as heater metallisation  - superior performance at high temperatures - reduced electromigration effect no oxidisation problems - realized with standard technologies
Conclusions:
The results presented above show that SnO2:Sb is a useful complement to the existing silicon micromachining technology that
opens up interesting new applications in the field of high-temperature MEMS devices; The use of SnO2:Sb pushes the range of possible MEMS heater operation temperatures from 600°C,
which can currently be reached with standard Pt heaters, to about 950°C.
At this latter temperature the expected useful heater lifetime is of the order of 10 years; SnO2:Sb as an oxide semiconductor cannot suffer from oxidation during high-temperature operation in ambient air.
This property relaxes constraints with regard to inert gas or vacuum packaging
which needs to be considered in the case of competitor heater materials; SnO2:Sb as a degenerately doped semiconductor material does not exhibit a conductivity as high as that of metallic ones.
Area heater designs therefore need to be considered instead of the commonly employed meander designs. As a relatively low-conductivity material SnO2:Sb does not suffer from electro-migration degradation at extremely high operation temperatures
but rather from an ongoing bulk diffusion and evaporation of the Sb dopants through the SnO2:Sb surface.
Wide Range of Potential Applications  - using SnO2:Sb as heater metallisation
A. Helwig, CRC / LG-ME, Phone

16. Wide range of applications for high temperature MEMS
WG n°C
Wide range of applications for high temperature MEMS
Operating at T ~1000°C
Low Power supply
Long Lifetimes
thermal infrared emitter
Ionisation detector
Hydraulic Fluid Life Monitoring System
Leakage
& Fire detection
Detection of drugs & explosives
NDIR & Light scattering
Photo-Acoustic Gas Sensing
Ion Mobility Spectroscopy
Ion-Mobility-Spektrometer
A. Helwig, CRC / LG-ME, Phone

17. THANK YOU FOR YOUR ATTENTION!
Microsystems & Electronics CRC Germany
THANK YOU FOR YOUR ATTENTION!
SnO2:Sb – A new material for high-temperature MEMS heater applications – performance and limitations
A. Helwig, J. Spannhake, G. Müller
Corporate Research Center – EADS Deutschland GmbH
T.Wassner, M.Eickhoff
Walter Schottky Institute, Technical University of Munich, Germany
G.Sberveglieri, G.Faglia
C.N.R. – INFM & Università di Brescia
A. Helwig, CRC / LG-ME, Phone

18. SnO2:Sb Based Area Heater Elements
Previous design optimized for metallic heater materials.
Disadvantage:
High resistivity of SnO2:Sb (~4500µWcm)
causes operating voltages of V for 1000°C operation.
Solution: New Emitter design featuring a SnO2:Sb area heater contacted by Pt metallisations
Investigations are underway to validate the performance of the new emitter design.
The above results have clearly shown the superior performance of semiconductor over metallic heater elements and
in particular the excellent performance of heater elements based on oxide semiconductors such as SnO2:Sb.
A drawback of this latter material is its relatively high resistivity of about 4500µohmcm (see Table 1) which is about
three orders of magnitude higher than the resistivitity of Pt. As a consequence operation voltages of about 100V
were necessary to achieve operation temperatures in the vicinity of about 1300K with the EM1 emitter design,
which we commonly employed in the case of Pt heater elements.
From an application point of view such high operation voltages are undesirable. In order to attain operation voltages
of less than 24V, the EM3 emitter design was introduced (Fig.1). In this latter case SnO2:Sb strips were deposited across
the hotplates and low-resistivity Pt contacts on the much colder Si suspensions to provide a low-resistance access to
the SnO2:Sb strips (Fig.3d). With this design the heater resistance could be reduced to about 130W, which compares
very favourably with the 8kW heater resistance of the EM1 design in Fig.3a. At present investigations are underway to
validate the performance of the new emitter design [31].
New heater resistance: 130 W (Old: ~8kW)
Operating voltages < 24 V sufficient to obtain T > 1000 °C
A. Helwig, CRC / LG-ME, Phone