micro-hotplate-sensors

Transcription

1 micro-hotplate-sensors Karsten Henkel, 2001 micro hotplate metal oxide semiconductor sensor sensing principle history/need fabrication setup of parts power consumption sensing layers principle of measurements example of publication others new material other sensing principles other heating principle

2 sensing principle of metal oxide semiconductor (n-type) depletion area at the surface (adsorption of atmospheric oxygen) decreasing by reducing gases (CO, H 2 ) => increasing of conductivity increasing by oxidising gases (NO 2 ) => decreasing of conductivity typical materials SnO 2, ZNO, TiO 2, WO 3 [1] typical temperatures C

3 Taguchi sensor (1970) nowadays screen print technique on ceramic substrates Taguchi-type sensor (a) Sensor Element: the heater is embedded in an alumina ceramic tube and the semiconductor material is mounted on the tube with two printed gold electrodes. The heater is needed to achieve the typical sensor working temperatures in 200±400 C range and the two electrodes allow the measurement of gas sensitive changes in electrical conductivity. (b) Packaged Sensor: the heated sensor element is mounted such that thermal contact between substrate and element is only given via the connecting wires. [1] Applications industrial domestic gas monitoring systems gas leak alarm gas leak detectors in factories ventilation control analysis equipment cooking for microwave fermentation control oven fire and toxic gas detector breath alcohol detector humidity control in tape recorders Gas sensor prepared by screen-printing on a ceramic substrate of size 6 mmx8 mm. The sensor element consists of a CO- and a NOx -sensitive layer. The platinum structures (heater, electrodes and contact pads) and the two sensitive layers are manufactured in screen-printing technology. The sensitive layers consist of specifically doped and coated tin dioxide. Their electrical resistance is measured by interdigitated platinum electrodes located below the sensitive layers and the necessary sensor working temperature of about 300 C is achieved by a heater located on the backside of the ceramic. With help of the platinum contact pads the sensor is fixed to a steel lead frame by parallel gap welding and mounted in the housing. [1]

4 Why? power consumption of screen printed devices 200mW - 1 W bad selectivity of metal oxide semiconductor sensor difficult mounting of the overall hot ceramic elements Integration combination of standard microelectronic and micromachining => micromachined metal oxide sensor battery, automobile sensor arrays thermal isolation driving electronic too hot thin membrane of low thermal conductivity (isolation between active area and substrate) Advantages power consumption mw (substrate nearly at ambient temperature) fast thermal response signal processing electronic on the same chip sensor arrays interdigitated electrodes reproducibility of preparation much better lower costs per unit Schematic of the MGS 1100 sensor from Motorola. Micromachined sensor element on the left and sensor housing on the right. The sensitive film were obtained by rheotaxial growth and thermal oxidation of tin layers (RGTO) deposited on the silicon oxide - nitride membrane. [1]

5 Schematic process flow for the formation of a closed-membrane type gas sensor. [1] In literature mostly 4-6 masks

6 closed membrane silicon oxide and/or silicon nitride (1-2 µm) nitrides porous silicon (25-30 µm) double side etching factor larger (may be plasma etching vertical walls) suspended membrane (spider type) silicon oxide and/or silicon nitride selective formation of porous silicon in p-type silicon wafer => undercut profile below implanted n-type silicon regions =>5 µm thick n-type silicon membrane hanging on suspension beams (silicon nitride) =>Silicon: good temperature uniformity excellent mechanical stability [1]

7 [1]

8 electrodes Au, Pt, Al, W Al standard IC-process metallization but bad contact properties to sensing film and limit to 500 C Ti as adhesion layer (in range of 20 nm) horizontal setup in case of Au or Pt heater in the same layer as electrodes vertical setup more freedom heater and thermometer polysilicon IC-compatible, sheet resistance adjustable by n-type doping and negative temperature coefficient, hot spots Pt (not compatible) or p++ doped structures thermometers also from Al plates for four-point temperature measurement (improve additionally the uniformity of temperature, but limit of 500 C) or forward biased pn-junctions Sometimes Si-plugs for temperature uniformity (a) Vertical and (b) horizontal approach.[1]

9 Photo of a bonded MHP gas sensor [3] SEM of MHP[2] SEM of MHP sensorarray [4] [5]

10 ε [1] Q tot Heat transfer- power consumption = G m λ ( T m hot..emissitivity T amb ) + G air λ air ( T hot T amb ) + G δ...stefan - Boltzmann constant suspended membrane (conduction perpendicular neglected, basically along the suspension beams, one dimensional) G m rad 4A = l σε( T beam 4 hot T 4 amb ) air conduction and fluid motion simulations show fluid motion is about 5% =>simple model only conduction concentric spheres (layer and ambient) G air = 4π 1 1 r r i a 4πr Convection simulation i closed membrane G m 2πd = ra ln r i [1] Ratio b:a for square geometry 3:1 radiation (only some %) gray emitter G rad 2 = backside coating A

11 comparison heat suspended and closed membrane λ m = 10Wm suspended : l = 100µm, w = 20µm closed ( r 1 a K / r ) i 1 d = 2µm ( T hot T amb = 2,5,8 54,23,18mW ) = 300K 4.8mW conclusion, requirements constructing thin membranes consisting of materials of low thermal conductivity; use of suspension beams with high length-to-width ratio adjusting the heater size (edge length a) to the size of the closed membrane (edge length b) as b:a 3:1; decreasing the heated area choosing a large pit depth of the suspended membrane thin gold coating at the backside of the membrane. but need of large area to make measurement of resistance possible mechanical stability => compromise temperature uniformity Si-plugs two heater for compensation thermal transient response response to step function T ( t) T τ = R therm = P * C amb el * R therm (1 e t τ Reducing of R increase power consumption =>trade off thermal capacity differs scarcely => reduction of heated mass (A,d minimal) =>suspended membranes better [3] )

12 Sensing layer deposition large intrinsic or thermal-induced membrane stress leading to membrane deformation/breaking of the membrane plastic deformation at the metal±sno2 contact area deformation/breaking of the membrane due to deposition of sensing layer; stress in the metallization sandwich structure thermometer-resistance shift due to mechanical and thermal stresses. thick films several microns large extra thermal mass thermal response increase larger thermal conductivity smoothing of temperature profile over active area, additional heat losses thin films nm compact layers porous layers only surface interaction=> two parallel resistances for current flow each grain posses a surface depleted area, series of resistances=> higher sensitivities RGTO (rheotaxial growth and thermal oxidation) leads to thin porous films

13 principles of measurements resistance most at constant temperature and by DC measurement thick film sensors for reducing gases much better while for oxidizing gases in the same range like thin films modification of electrode material, working temperature, geometry [1] =>better sensitivity and selectivity of single sensor (it was reported: 10ppb NO 2 : to counterbalance this 100ppm CO are necessary) =>design of arrays application of electric fields perpendicular to the sensing film may improve the sensitivity

14 R air /R CO R NO2 /R air [1]

15 temperature modulations increasing of selectivity, sensitivity reduce of heating power [7]

16 temperature modulations [1] Results from [72] on sinusoidal temperature modulations of a micromachined thick film sensor in synthetic air (50% RH) and during exposure to 50 ppm CO, 1.0 ppm NO2 and a mixture of 50 ppm CO and 1.0 ppm NO2.

17 thermovoltage (Seebeck) thermovoltage is generated in case of non-uniform temperature distribution across the sensing layer Vs = αs * T λs... gas sensitive seebeck coefficient two electrodes with different but constant temperature (constant T) Results show V V s s ~ ln c = A + B ln R s Picture from [76] showing the possibility for discrimination of methane to propane by combination of Seebeck and resistance measurements. Line A, ethanol and acetone; line B, hydrogen; line C, carbon monoxide; line D, propane; line E, methane. [1] catalytic treatment of one electrode (use catalytic effect of combustible gases)=> local temperature rise (combination of gas induced change of seebeck coefficient and catalytic effect) this methods have not been carried out for MHP-devices

18 M.C. Horrilloe et al.detection of low NO2 concentrations with low power micromachined tin oxide gas sensors, Sensors and Actuators B [5] ppm-values 600 nm, for low concentrations no significant difference

19 new material F. Solzbacher et al.: A modular system of SiC-based microhotplates for the application in metal oxide gas sensors; Sensors and Actuators B [4] A new SiC/HfB 2 based low power gas sensor; Sensors and Actuators B [8] membrane (closed) SiC (3C lower thermal conductivity than 4H ;6H ) heater HfB 2 battery operation (1-2 V) N-implantation in SiC automotive application up to 700 C, at 400 C 35mW Mounting on TO8 carrier sensitive layer In 2 O 3 (250 C 20 mw), time constant 50 s

20 other sensing principle J.F. Currie et al. : Micromachined thin film solid state electrochemical CO 2, NO 2 and SO 2 gas sensors; Sensors and Actuators B [9] Sintered ceramics

21 other sensing principle D. Briand et al.: Thermally isolated MOSFET for gas sensing applications; IEEE Electon device letters, vol. 22, no. 1, [6] Pt, Ir, Pd as gate sensitive materials one al may used as reference Top view photograph and cross section schematic of the low power array gas sensor: M MOSFET; D diode H heater, G ground heating characteristics at 170 C operating temperature 80 mw for the array 65 ms rising,100 ms cooling sensing characteristic at 140 C 20 ppm NH ppm H 2 Pt 182 mv 60 mv Ir 165 mv 103 mv

22 other sensing/heating principle F. Udrea et al.: Design and simulations of SOI CMOS micro-hotplate gas sensors; Sensors and Actuators B [10] SOI (silcon on insulator) for one finger of 5 finger heater SOI chemoresistive gas sensor SOI microcalorimeter sensor

23 Literature [1] I. Simon et al.; Sensors and Actuators B 73 (2001), 1-26 [2] L. Sheng et al.; Sensors and Actuators B 49 (1998), [3] Z. Tang et al.; Sensors and Actuators B 46 (1998), [4] F. Solzbacher et al.; Sensors and Actuators B 64 (2000), [5] M.C. Horillo et al.; Sensors and Actuators B 58 (1999), [6] D. Briand et al.; IEEE Electon device letters, vol. 22, no. 1, 11-13, January 2001 [7] R.E. Cavichi et al.; Sensors and Actuators B 33 (1996), [8] F. Solzbacher et al.; Sensors and Actuators B 77 (2001), [9] J.F. Currie et al.; Sensors and Actuators B 59 (1999), [10] F. Udrea et al.; Sensors and Actuators B 78 (2001), [11] S. Fung et al.; Sensors and Actuators A 54 (1996), [12] C. Cobianu et al.; Sensors and Actuators B 58 (1999), [13] Cs. Düsco et al.; Sensors and Actuators A 60 (1997), [14] D. Vincenzi et al.; Sensors and Actuators B 77 (2001), [15] D.D. Lee et al.; Sensors and Actuators B 33 (1996), [16] A. Pike et al.; Sensors and Actuators B 45 (1997), [17] S. Semancik et al.; Sensors and Actuators B 34 (1996), [18] T. A. Kunt et al.; Sensors and Actuators B 53 (1998), [19] Y. Mo et al.; Sensors and Actuators B 79 (2001), [20] R. Ionescu; Sensors and Actuators B 48 (1998),