Micro-pellistor for methane detection

New Scientific Results

Ferenc Bíró

Doctoral School of Chemical Engineering and Material

Sciences, University of Pannonia

Supervisor:

István Bársony (DSc)

Budapest

2017

Introduction

Today gas-, oil- and chemical industries require many low

price, energy efficient and reliable sensors for detection of

hydrocarbon leakage and for environmental monitoring. Beside

hydrogen and LPG, detection of the highly explosive methane is a

foremost safety requirement in- and outside of facilities alike. The

first combustion type methane detector was developed in middle of

the last century. Because of the lower explosive limit (LEL) of the

above mentioned gases is in the range of 1-10 volume percent in air,

catalytic combustion type sensors are the most suitable for their

detection. These sensors have to guarantee the stability of the

response signal over one year operation, however, their drawback is

the enormous power consumption (0.5-2 W). Due to the high surface

temperature of the catalyst, sensing elements have to be integrated

into an explosion proof packaging, in order to avoid the spread of

flame when the concentration of hydrocarbon exceeds its upper

explosion limit. The aims of miniaturisation of these sensors are: to

make them more energy-efficient and integrable into wireless

networks (WSN or even IoT), furthermore to reduce the cost of

packaging.

Although in the literature catalytic type microsensors have

demonstrated many times the ability to detect methane, none of them

fulfils entirely the stability and lifetime requirements stated by the

EU standards. In my thesis I’m focusing particularly on the detection

of methane. The systematic investigation of stability problems in

mico-calorimetric devices can lead to the determination of

limitations by structural materials in different designs. An

understanding of these issues will facilitate the selection of better

material-combinations and/or new processing technologies for the

fabrication of more reliable sensors.

R&D on micro-pellistors at the Institute of Technical Physics

and Materials Science

The first micro-heater for catalytic detection of methane

was first manufactured at KFKI ATKI in the late ‘90s in the

Microtechnology Department. The size of the heated area was

merely 10000 μm2 and the cavity under the heater was formed by a

than new silicon bulk micro-machining process using the porous

silicon sacrificial layer method. The micro-heater was composed of a

suspended Si single crystal filament interconnected by Pt wires. At

temperatures above 500°C the degradation of the Si-Pt contact led to

device failure.

Later the whole filament was fabricated of Pt. The

suspended Pt heaters were embedded in low-stress non-

stoichiometric silicon-nitride layers and sensitised by an Al2O3-Pt

catalyst suspension. The devices showed acceptable sensitivity to

propane- and butane-air mixtures. The reason for lack of methane

sensing was found in the unsuitable morphology of the sintered

catalyst. Beside these drawbacks also serious thermo mechanical

problems were encountered. At temperatures above 500°C these

micro-heaters couldn’t achieve the expected lifetime of more than 1

month in DC operation mode. For the catalytic detection of methane,

catalyst requires even surface temperatures of 600°C or higher.

In order to make the micro-pellistors sensitive to methane, a

thin layer of porous alumina-oxide catalyst support was deposited on

the heated area and impregnated by hexachloroplatinic acid.

Contrary to expectations none of these devices fabricated by such a

time-consuming multistep processes showed methane sensitivity.

Based on EDS analysis, a plausible explanation for this problem was

the low dispersion of Pt catalyst. These experiments were conducted

until 2012, when I had possibility to get acquainted with this type of

sensor and its associated problems.

Scope of the Thesis

According to the literature and the results of prior research on

micro-pellistors published by co-workers of MFA, the aim of my

PhD research was determined to design, fabricate and characterise

micro-pellistoros with integrated platinum filaments sensitized by

Al2O3-Pt catalyst. Because the catalytic detection of methane

exhibits an extreme thermal load for the micro-pellistor and catalyst

alike, in my research plan I have set out the following targets:

design and realisation of micro-hotplates with integrated

platinum filament, capable of stable operation exceeding

600°Cwith respect to the structure of multilayer, the choice

of proper adhesion layer and the effect of residual

mechanical stress;

investigation of degradation mechanisms in the platinum

filament above 600°C;

development of an alternative catalyst deposition method

for the elimination of drawbacks of the commonly used

ones;

integration of the catalyst carrier should increase the surface

of the heated area by an order of magnitude, and exhibit

better adhesion to the membrane structure;

determination of catalytic properties and the processes

leading to the degradation of catalyst;

to determine the technical requirements for methane

sensitive micro-pellistors.

Experimental work

1) Design of micro-pellistors

Micro-pellistors of 1×1 mm2 chip size were fabricated on 3”

wafers by silicon bulk-micromachining. The TiO2/Pt/TiO2 filaments

of double spiral and meander type geometry were suspended on

perforated and full oxide-nitride membranes. A porous anodic

aluminium-oxide (AAO) catalyst support layer was deposited on top

of the micro-heaters. Pt catalyst was dispersed by Atomic Layer

Deposition (ALD) technique in the AAO support (AAO-ALD Pt

catalyst).

2) Measurement and analytical methods used

a) Power dissipation of micro-hotplates

The resistance change vs. heating power characteristics of

micro-hotplates were tested at 1mPa and 1 bar air pressure in a

vacuum chamber. The comparison of the steady state power

dissipations by the filament resistance change obtained at similar

driving powers reflects the heat loss to the ambient.

b) Average surface temperature determination by the miro-

melting point method

Minute amounts of salts (approx. 50 μg) of different

melting points were placed in the centre of the hotplate. By raising

the input power the start and the completion of the melting process

of the salt was observed under the microscope via the reflection

changes, and both input power values were assigned to the melting

point temperatures, respectively.

c) “Visible pyrometry”

Just as in conventional pyrometry, we assume that the

emissivity of the thin film covered Pt filaments does not depend on

the wavelength (gray body approximation) and on the temperature.

By using appropriate tables the red/green intensity ratio of every

pixel in a photograph is transformed to an interpolated temperature.

Visual pyrometry could thus be applied to determine the variation of

temperature along the spiral filament in order to localize the position

of developing critical temperature gradients in all samples during

operation until failure.

d) Determination of the rate of reaction in catalytic oxidation

When chemical reaction is present on the active area, the

difference between the heating powers of the reference and the active

element is just the chemical power. The rate of reaction is calculated

by dividing the chemical power by the standard enthalpy of the

known hydrocarbon.

e) Functional testing

The sensors were mounted on DIL type sockets without dif-

fuser caps and characterised in a flow-through type test-camber fed

by 6 mass-flow controlled gas lines. Gases were mixed in synthetic

air and the flow rate of gas mixtures was set to 50 cm3/min. The

response to gas exposure of the individually heated sensors powered

by constant current was measured in a Wheatstone bridge

arrangement.

f) Lifetime measurements

The filaments deterioration was analysed by using self-

heating for the determination of characteristic lifetimes and the

locations of failure. A sufficient number of cantilever type double

spiral heaters were tested in the power range of 35–45 mW by a

lifetime tester system (PLTT-10 of Weszta-T Ltd.). To verify the

results, a meander type heater was also investigated.

g) Further surface analytical methods applied:

Scanning Electron Microscopy (SEM)

Transmission Electron Microscopy (TEM, XTEM, EBD)

Energy Dispersive Spectroscopy (EDS)

Makyoh topography.

New Scientific Results

1.)

I have experimentally proven that the cantilever type micro-

hotplate (ES type) with integrated double spiral platinum

filament regarding its power dissipation is more favourable,

because the nominal heated area/power dissipation ratio is only

55% higher than needed to achieve the same average surface

temperature on the bridge type micro-heates used before. (To

achieve the same average surface temperature on the twice

larger area requires for the latter merely 17% higher heating

power.) [S3]

a) I have identified for perforated membranes that 90% of the

power dissipation under atmospheric conditions is caused by

convective and conductive losses over the gaseous medium. The

power loss across the suspensions amounts to 24-30% and 5-7%

in case of full and perforated membranes, respectively.

b) By the analysis of optical micrographs taken from the glowing

micro-heaters I have shown that the temperature inhomogeneity

of the heated area drastically increases due to the thermal

conductivity in the gas phase with increasing pressure.

2.)

I provided experimental proof for the degradation of cantilever

type micro-heaters with integrated double spiral filament above

600°C, which takes place in four distinct stages, and can be

ascribed to the cumulative impact of three parallel physical

phenomena: i.e. the recrystallisation of the TiO2 adhesion layer,

thermomigration and electromigration of Pt [S4].

1st Stage

Recrystallisation of the TiO2 adhesion layer and protrusion of the

TiO2 grains into the Pt filament with inhomogeneous particle

distribution over the heated area reflecting the surface temperature

distribution. These TiO2 agglomerates decrease the cross section of

the Pt filament thereby leading to a fast increase of resistance.

2nd

Stage

Grain growth of TiO2 crystallites further reduces the cross section of

the Pt filament, by which its resistance is moderately increased.

3rd

Stage

Due to Pt mass-transport effects driven by potential- and thermal

gradients Pt discontinuities are formed along the filament.

4th

Stage

In the final phase electromigration is killing the device by leading to

rupture or breakage of the wire at the weakest point.

Based on the temperature dependent rate of resistances I have

determined the activation energies of the individual effects, which

was found to 1.7 (+0.1;-0.3) eV and 2.1 (+0.1;-0.2) eV for TiO2

grain growth and electro-, thermomigration, respectively.

3.)

I have proven experimentally the correlation between the

position of the rapture sites and thermal gradients along the

filament on cantilever type micro-heaters with integrated double

spiral Pt filament above 600°C where the direction of electro-

and thermomigration coincide [S3].

a) I determined the positions of the maximum thermal

gradients along the filament by using “visible pyrometry”.

b) In the section of the filament where the direction of both

gradients (thermal and potential) is opposite, (sign of both

gradients are opposite) the resulting Pt mass transport is

lower, therefore, breakdown would occur later than in

sections where they coincide.

4.)

I prepared the first time Pt nano-catalyts (AAO-ALD Pt) in high

surface area anodic alumina (AAO) thin films by Atomic Layer

Deposition (ALD) technique using cantilever type micro-heaters

with integrated double spiral Pt filaments for catalytic detection

of methane, and characterised the size- distribution of the

catalytic Pt isles by XTEM analysis (approx. 1.8 nm) [S2, S5, S6].

By tuning the ALD parameters both contiguous and nano-particle

coverage of Pt coating can be achieved. The nano-particles form

during the deposition (T=350°C) or under operation (T>650°C).

5.)

I determined experimentally the sensitivity of the AAO-ALD Pt

for methane- and propane-syntetic air gas mixtures as a function

of heating power and temperature versus gas concentrations up

to their lower explosive limit. By analysing the sensitivity

degradation of the device I identified the cause of sensitivity

degradation [S2, S5].

The values of response signal of AAO-ALD Pt catalyst reach or

exceed the best values published in the literature so far. The

comparison between the response signals obtained with DC

magnetron sputtered or impregnated AAO-Pt catalysts, and AAO-

ALD Pt confirmed that only the AAO-ALD Pt catalyst was sensitive

to methane.

a) Using the micro-pellistor in micro differential calorimeter

mode I conducted the measurements for both methane and

propane at 100% LEL. I have plotted the Arrhenius

diagrams of the oxidation reactions. I identified the

diffusion and surface reaction controlled regions and

determined their activation energies.

b) In case of AAO-ALD Pt catalyst, the recommended optimal

condition, i.e. the diffusion controlled can’t be achieved for

methane between 900-1200 K. The start of the diffusion

control region for propane, however, was found between

780-900 K.

c) I have proven that due to the inhomogeneous temperature

distribution in the temperature range of oxidation of

methane (900-1200 K) the degradation of catalyst was

caused by the agglomeration of Pt particles and their

thermally driven migration across the AAO surface towards

the cooler perimeter of the membranes.

Based on the temperature dependent response signals, SEM, TEM

analysis and the distribution of surface temperature measured by

“visible pyrometry” I have proven that the physical effect behind the

loss of sensitivity is the migration of the ALD Pt catalyst towards the

cold perimeter of the heater driven by temperature gradients.

Own publications used in the Thesis [S1… S7]

[S1] Ferenc Bíró, Csaba Dücső, Zoltán Hajnal, Ferenc Riesz, Andrea

Edit Pap, István Bársony, Thermo-mechanical design and

characterisation of low dissipation micro-hotplates operated above

500˚C, Microelectronics Journal, 45, 2014, 1822-1828

[S2] Ferenc Bíró, Csaba Dücső, György Z. Radnóczi, Zsófia Baji,

Máté Takács, István Bársony, ALD nano-catalyst for micro-

calorimetric detection of hydrocarbons, Sensors and Actuators B

Chemical, 247, 2017, 617-625

[S3] Ferenc Bíró, Zoltán Hajnal, Csaba Dücső, István Bársony, The

role of phase changes in TiO2/Pt/TiO2 filaments, Journal of

Electronic Materials, javított cikk bírálat alatt

[S4] Ferenc Bíró, Zoltán Hajnal, Csaba Dücső and István Bársony,

The critical impact of temperature gradients on Pt filament failure,

Microelectronics Reliability, 78, 2017, 118-125

[S5] Ferenc Bíró, Andrea Edit Pap, István Bársony, Csaba Dücső,

Micro-pellistor with integrated porous alumina catalyst support,

Procedia Engineering, 87, 2014, 200-203

[S6] Ferenc Bíró, György Z. Radnóczi, Máté Takács, Zsófia Baji,

Csaba Dücső, István Bársony, Pt deposition techniques for catalytic

activation of nano-structured materials, Procedia Engineering, 168,

2016, 1148-1151

[S7] Ferenc Bíró, Zoltán Hajnal, Andrea Edit Pap, István Bársony,

Multiphysics modelling of the fabrication and operation of a micro-

pellistor device, Thermal, mechanical and multi-physics simulation

and experiments in microelectronics and microsystems (Eurosim),

2014 15th international conference, pp:1-6, ISBN: 978-1-4799-4791-

1

Presentations [S8… S11]

[S8] Bíró Ferenc, Csutak Réka, Mesoporous TiO2 layers for gas

sensing application, Műszaki Kémiai Napok, Veszprém, 2012

[S9] Ferenc Bíró, Andrea Edit Pap, István Bársony, Csaba Dücső

Micro-pellistor with integrated porous alumina catalyst support,

Eurosensors 2014, Brescia

[S10] Ferenc Bíró, György Z. Radnóczi, Máté Takács, Zsófia Baji,

Csaba Dücső, István Bársony, Pt deposition techniques for catalytic

activation of nano-structured materials, Eurosensors 2016

[S11] F. Bíró, Gy. Z. Radnóczi, Zs. E. Horváth, Cs. Dücső, Zs. Baji,

Conformal ALD platinum coating of porous substrates for gas

sensing, 2016, Dublin

[S12] F. Bíró, Cs. Dücső, Z. Hajnal, A. E. Pap, I. Bársony,

Optimisation of low dissipation micro-hotplates - Thermo-

mechanical design and characterisation, 19th

International

Workshop on Thermal Investigations of ICs and Systems

(THERMINIC), 2013, Berlin