Gy. Bognár 1, P. Fürjes 2, V. Székely 1, M. Rencz 3 TRANSIENT THERMAL CHARACTERISATION OF HOT...
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Transcript of Gy. Bognár 1, P. Fürjes 2, V. Székely 1, M. Rencz 3 TRANSIENT THERMAL CHARACTERISATION OF HOT...
Gy. Bognár1, P. Fürjes2, V. Székely1, M. Rencz3
TRANSIENT THERMAL CHARACTERISATION OF HOT
PLATES
&of MEMS MOEMS
200
4
3MicReD Ltd., Budapest, Hungary
1BUTE, Budapest, Hungary
2KFKI-MFA Research Institute for Technical Physics and Materials Science, Hungary
The physical structure to be characterised thermally: an integrated gas sensor• Thermally isolated heater and • sensing resistor filament (Pt)• 100m x 100m x 1m• Encapsulated by reduced
stress silicon rich silicon-nitride (LPCVD)
• Selective dissolution ofelectrochemically formedporous silicon (60-80m)
• Mechanical support under the hotplate
100m
Mechanical support
Thermal operation needs thermal characterisation
Reasons of thermal characterisation
• To check the maximal operation speed of the sensor device (strongly influenced by the thermal isolation of the membrane structure)
• To check how to reach maximal temperature elevation with minimal heating power (e.g.: for explosion-proof detection of combustible gases)
100-600C achieved with 10-25mW
• To detect the differences in the thermal behaviour of hotplates with and without mechanical support
Outline• Presentation of the following studies:
– Simulation:• Structure without mechanical support: steady-state,
transient
– Measurement – thermal transient• Structure with mechanical support• Structure without mechanical support
• Comparison by means of – Time-constant spectra– Structure functions– Simple compact model created
• Conclusions
The simulation
• Simulated by the SUNRED program (without mechanical support)
• FD model, solved by SUccessive Network REDuction
The simulation results were verified by thermal transient measurements using the T3Ster equipment and related analysis software
The model:
The simulationTransient result
Time evaluation of temperature is not to scale
The 1µs .. 1s time range was covered on a logarithmic time-scale
The simulationTransient result
The 1µs .. 1s time range was covered on a logarithmic time-scale
Max. temperature elevation is 227oC @ 8.5mW
The simulationSteady-state result (figure is not to scale)
Steady-state resultThe simulation
Uniform temperature distribution on the hotplate
Verification by measurements
• The resistor of the hotplate was used both as a heater and a temperature sensor– Sensitivity of the sensor was identified by a
calibration process• The thermal response was recorded by T3Ster
using the 4 wire method:
Idrive Isense
DUT
Umeas ~ T
Force: Sense:
Verification by measurements
Simulated8.5mW
Measured8.5mW
Structure without mechanical support
Steady state values agree well
Verification by measurements
Simulated
Measured
Structure without mechanical support
The dominant time constants are in a good agreement
2.24 ms1.10 ms
time [s]
Tem
pera
ture
[C
]
Simulated8.5mW
Measured8.5mW
Measured6.5mW
(with support)
Verification by measurements
Verification by measurements
Simulated woMeasured w
Measured wo
The dominant time constant is only slightly influenced by the mechanical support
• Foster type network model of the structure is constructed from the time constant spectra
• Equivalent Cauer type network model corresponds to the real physical structure
Structure functions
• The discrete RC model network in the Cauer canonic form now corresponds to the physical structure, but
n
iiRR
1
n
iiCC
1
• This is called cumulative structure function
• it is very hard to interpret its “meaning”
• Its graphical representation helps:
Structure functions
The cumulative structure function is the map of the heat-conduction path:
n
iiRR
1
n
iiCC
1
ambi
ent
heater
Structure functions
Structure functions
Agrees well with the volume calculated from exact geometry
hotplate
27000 K/W40 nWs/K
Structure functions
• The thermal capacitance ~ 40 nWs/K
• The thermal resistance ~ 27000 K/W
• The structure has only one dominant time constant
• The simplified thermal model constructed
hotplate
27000 K/W40 nWs/K
Summary of transient characterisation
Power level
Thermal resistance
Thermal capacitance
Time constant
measured w support 8.5mW 27000 K/W 40 nWs/K 1.10ms
measured wo support 6.5mW 26000 K/W 40 nWs/K 1.12ms
simulated wo support 8.5mW 30000 K/W 40 nWs/K 2.24ms
Identified from the structure functions
• The structures can be represented by one dominant time constant ( ~ 1.1ms)
• The time constants of the two structures are nearly the same
• The pillar support has small thermal capacitance and high resistance, so it hardly influences the thermal behavior of the hotplate
Summary of transient characterisation
Summary• The response time of the heater was investigated
by time constant analysis, and the single dominant time constant of the structure was found in the range of milliseconds
• We identified and generated a reduced order (compact) thermal model of the structure
• The thermal properties (Rth, Cth, ) of the structures with and without support were nearly identical
• Consequently the dynamic behaviour was not deteriorated significantly by the mechanical support
Acknowledgment
This work was partially supported
by the
• OTKA T033094 project of the Hungarian National Research Fund
• INFOTERM NKFP 2/018/2001 project of the Hungarian Government
and the
• SAFEGAS and the REASON FW5 Projects of the EU
Measurements: temperature calibration
0
100
200
300
400
500
600
700
800
0 10 20 30
Power loss [mW]
Tem
per
atur
e [o
C]
calculated
measured
• Surface temperature was measured by resistance calibration technique
• Rth26.5K/mW (with mechanical support)
• heat conduction in the suspending beams,• conduction and convection in the surrounding gas,• radiation from the hot surfaces
• The complex loci – Nyquist diagram – was calculated from the measured thermal impedance curves
• Slight transfer effect can be observed that is due to the heat transfer between different sections of the heating meander
Frequency domain behavior derived from measured transient curves
Measuredwithout support
Measuredwith support
Frequency domain behavior derived from measured transient curves