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 10.06.2018 A. Helwig, CRC / LG-ME, Phone +49-89-607-28197 Email: Andreas.Helwig@eads.net

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 10.06.2018 A. Helwig, CRC / LG-ME, Phone +49-89-607-28197 Email: Andreas.Helwig@eads.net

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 10.06.2018 A. Helwig, CRC / LG-ME, Phone +49-89-607-28197 Email: Andreas.Helwig@eads.net

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 ? 10.06.2018 A. Helwig, CRC / LG-ME, Phone +49-89-607-28197 Email: Andreas.Helwig@eads.net

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? 10.06.2018 A. Helwig, CRC / LG-ME, Phone +49-89-607-28197 Email: Andreas.Helwig@eads.net

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. 10.06.2018 A. Helwig, CRC / LG-ME, Phone +49-89-607-28197 Email: Andreas.Helwig@eads.net

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 10.06.2018 A. Helwig, CRC / LG-ME, Phone +49-89-607-28197 Email: Andreas.Helwig@eads.net

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 ? 10.06.2018 A. Helwig, CRC / LG-ME, Phone +49-89-607-28197 Email: Andreas.Helwig@eads.net

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. 10.06.2018 A. Helwig, CRC / LG-ME, Phone +49-89-607-28197 Email: Andreas.Helwig@eads.net

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 !! 10.06.2018 A. Helwig, CRC / LG-ME, Phone +49-89-607-28197 Email: Andreas.Helwig@eads.net

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 10.06.2018 A. Helwig, CRC / LG-ME, Phone +49-89-607-28197 Email: Andreas.Helwig@eads.net

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 10.06.2018 A. Helwig, CRC / LG-ME, Phone +49-89-607-28197 Email: Andreas.Helwig@eads.net

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! 10.06.2018 A. Helwig, CRC / LG-ME, Phone +49-89-607-28197 Email: Andreas.Helwig@eads.net

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! 10.06.2018 A. Helwig, CRC / LG-ME, Phone +49-89-607-28197 Email: Andreas.Helwig@eads.net

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 10.06.2018 A. Helwig, CRC / LG-ME, Phone +49-89-607-28197 Email: Andreas.Helwig@eads.net

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 10.06.2018 A. Helwig, CRC / LG-ME, Phone +49-89-607-28197 Email: Andreas.Helwig@eads.net

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 10.06.2018 A. Helwig, CRC / LG-ME, Phone +49-89-607-28197 Email: Andreas.Helwig@eads.net

SnO2:Sb Based Area Heater Elements Previous design optimized for metallic heater materials. Disadvantage: High resistivity of SnO2:Sb (~4500µWcm) causes operating voltages of 100-120V 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 10.06.2018 A. Helwig, CRC / LG-ME, Phone +49-89-607-28197 Email: Andreas.Helwig@eads.net