- Email: [email protected]
Investigation on suspended gate field effect transistor as humidity sensor
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Procedia Engineering
ProcediaProcedia Engineering 5 (2010) 1434–1437 Engineering 00 (2009) 000–000 www.elsevier.com/locate/procedia
www.elsevier.com/locate/procedia
Proc. Eurosensors XXIV, September 5-8, 2010, Linz, Austria
Investigation on Suspended Gate Field Effect Transistor as Humidity Sensor. O. De Sagazana, B. da Silva Rodriguesb, S. Cranda, F. LeBihana, T. Mohammed-Brahima aGM-IETR UMR-CNRS 6164, Université de Rennes1, bat 11b 35042 Rennes Cedex, France. bLSI, University of São Paulo, Av. Prof. Luciano Gualberto, trav. 3 n°158, São Paulo, Brazi
Abstract This work deals with SGFET (Suspended Gate Field Effect Transistor) for humidity rate (%HR) measurement. SGFET devices have already been used to detect gas, biological species, and also to measure pH [1]. This structure has already proved its good sensitivity to atmosphere and charge variation [2]. The following results have been obtained with SGFET realized in CMOS 0,8 μm technology and which have been reported and bonded on PCB card after all wires have been isolated from ambient sources. This paper has investigated especially the sensitivity to %HR. SGFET measurements realized in this work have been performed by sampling the drain current few seconds. By this way it becomes possible to presents for the first time results involving SGFET obtained during several hours of quasi continuous test. With this work and all results issue from the %HR measurement, the hysteresis and the reliability of SGFET’s structures could be evaluated especially in monitoring applications.
©c 2009 by by Elsevier Ltd.Ltd.
2010Published Published Elsevier Keywords: Suspended Gate Field Effect Transistor; Humidity Rate ; Drain Current Sampling.a
1. Introduction SGFET devices have already shown good properties in charges detection. They have been used in pH measure and biological species detection. In most of case and especially for pH and biological species, electrical test are performed in liquid ambient. This paper investigates the electrical behavior of the SGFET in gaseous atmosphere thanks the %HR measurement. In this case SGFET allows an easy implement and it expects to be more reliable due to the absence of porous oxide or polymers [3]. Moreover usually tests of SGFET are performed by sweeping the gate voltage and measuring the drain current. In these tests, high sensitivities have been obtained in particular in pH or in biological detection [1]. But this method involves a lot of stress in the devices and do not fit to monitoring applications like %HR measurements or gas detection. So the paper presents electrical measurements performed by
* Corresponding author. Tel.: 00 33 2 23 23 52 67 E-mail address: [email protected].
c 2010 Published by Elsevier Ltd. 1877-7058 doi:10.1016/j.proeng.2010.09.385
1435
O. De Sagazan et al. / Procedia Engineering 5 (2010) 1434–1437 2
O. de Sagazan/ Procedia Engineering 00 (2010) 000–000
sampling the drain current at fixed gate voltage. Due to the better stability and reliability of the measurements, sensitivity and hysteresis of the device could have been investigated sharply and the interest of the SGFET in continuous monitoring application evaluated. 2. Sensor fabrication process and designs presentation. SGFET used for this study have been realized in technology CMOS 0,8μm. Drain and source are boron doped by implantation, and the 0,5μm thick gate in polysilicon has been deposited by LPCVD (Low Pressure Chemical Vapor Deposition) on a 500 nm thick TEOS (Tetra ethyl Orthosilicate) which acts as a sacrificial layer (Fig. 1). The polysilicon gate is overlayed by nitride layers of 70nm thick. Contacts are realized in aluminum. All conductive layers are then passivated by PECVD (Plasma Enhanced Chemical Vapor Deposition) nitride layers. The structure is finally released after a buffer HF etching (fig 2a). Al
Si3N4
SiO2 Poly Si P+
Si P+
Si P+
Bulk SiSiNN
Fig. 1: Lateral view of the p-MOS SGFET and its empty gap after the release of the TEOS sacrificial layer.
The total area of the SGFET, including the connections is 130μm x 70 μm. This small area led to integrate locally 49 similar SGFETs in a matrix configuration. The strong integration on small area allows the SGFETs of the matrix to be in contacts of the very same ambient and then to present the same characteristic in ideal case. Experimental characteristics of all SGFETs can be measured and averaged. By this means, redundancy is introduced in the goal to reach the maximum reliability.
source
gate
drain
(a)
(b)
Fig. 2: SEM view of the SGFET with the 0.5μm empty gap under the gate, (b) picture of the matrix reported on the PCB card after isolation of all wires
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O. De Sagazan et al. / Procedia Engineering 5 (2010) 1434–1437 O. de Sagazan/ Procedia Engineering 00 (2010) 000–000
3
For practical use, SGFET matrix is bonded in a PCB card (fig. 2b). All wires are coated in special resin to avoid parasitic electrical contacts due to the liquid condensation on the surface of the device. The bonded matrix can be considered as ready to use sensor.The electrical characterization of all the transistors of the matrix is performed with an Agilent HP 4155B. During the measurements, the PCB card has been set in a various ambient enclosure. It has been possible to monitor the atmospheric humidity and record the response of the sensor. The %HR has been recorded with a Testo 635 device. 3. Electrical tests and response to %HR variation 5,00E-04
Ids in A
4,50E-04
%HR=30,3 %HR=49,8 %HR=67,3 %HR=81,2 %HR=87,3
4,00E-04 3,50E-04 3,00E-04 2,50E-04
Sampling time in sec
2,00E-04 0
1
2
3
4
5
Fig. 3: Evolution of the drain current sampling during 5 seconds at a Vgs (gate-source voltage) and Vds (drain-source voltage) constant. The value of current which is stored for %HR measure is picked at 5s
Usually SGFET are tested by sweeping the gate voltage at constant drain voltage, the humidity rate is then deduced from the drift of the threshold voltage [1&2]. This method induces electrical stress which could impact the accuracy of the measurement. In the following tests SGFETs are tested by sampling during 5 seconds the drainsource current (Ids) at Vgs (gate-source voltage) and Vds (drain-source voltage) constant (Fig. 3). By this way it becomes possible to monitor the %HR by only measuring the Ids value during several hours (fig. 4a). First observations show that SGFET reacts directly to %HR but without linearity in the sensor response. 5,00E-04
95
Ids in A
8,00E-04
85
5,00E-04
4,50E-04
%RH
75
4,50E-04
7,00E-04 4,00E-04
65
4,00E-04
%HR
55
3,50E-04
6,00E-04
ids 5s d
3,50E-04 3,00E-04
3,00E-04 35
4,00E-04 0
(a)
200
400
600
Ids vds=5V Ids vds=0,5V
5,00E-04
45
25
Ids in A
2,50E-04 800 time in min
2,50E-04
3,00E-04 25
45
65
2,00E-04 85 %HR
(b)
Fig. 4: Drain current according time and %HR variation. Drain current has been sampled during 5s every 10 minutes during 13 Hours (fig. 4a). SGFET %HR sensitivity according the Drain source Vds voltage (fig. 4b), a higher Vds bias induces a wider range of sensitivity.
The sensitivity of the device has then been deduced and it appears that sensitivity to %HR increases at high %HR level. The Influence of the SGFET polarization on the measure has also been investigated (fig. 4b) in order to have
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O. De Sagazan et al. / Procedia Engineering 5 (2010) 1434–1437 4
O. de Sagazan/ Procedia Engineering 00 (2010) 000–000
7,50E-04
5,50E-04
7,00E-04
5,00E-04 4,50E-04
6,50E-04 6,00E-04
Ids up Ids down
5,50E-04
Ids in A
Ids in A
the wider range of measure. It seems that a high Vds bias increase the threshold of %HR detection. The hysteresis response between an increase of the %HR and a decrease of %HR has been observed (fig. 5). It has been noticed that hysteresis was almost null at high %HR but for a range between 50 to 70% errors of almost 5% could be induce if SGFET were biased at 5V on Vds (fig. 5a). Whereas it seems that a weak Vds bias (0.5V) decrease the hysteresis of the response (fig. 5b). This is an interesting point that could be applied to other sensing application like pH measurements or gas concentration measurement in the future works.
4,00E-04
5,00E-04
3,00E-04
4,50E-04
2,50E-04
4,00E-04 25
(a)
30
35
40
45
50
55
60
65
70
75
80
%HR
Ids 5s down Ids 5s up
3,50E-04
2,00E-04
%HR 30 35 40 45 50 55 60 65 70 75 80 85 90
(b)
Fig. 5: Illustration of the hysteresis effect which could induce almost a 5%HR errors in the measurement. The hysteresis is greater for a Vds bias of 5V (a) whereas the effect decrease widely for a 0.5V Vds bias (b).
4. Conclusion Matrixes of SGFETs have been realized in technology CMOS 0.8μm. These matrixes have then been reported on PCB cards in order to be used as sensors. This paper reports results in %HR measurement and gives some new results about long time behavior (dozens of hours) and also shows the hysteresis response effect. These results are promising and new, the protocol test while consisting sampling the drain-source current has shown a good reliability. For the first time SGFET have been used during long time and the hysteresis response of the device could have been investigated. It has been shown that SGFETs are efficient for high %HR level showing a high sensitivity and no hysteresis in the response. However main key parameters for reliability and sensitivity have been identified. Indeed additional investigations on polarization conditions (value of Vgs and Vds) could easily lead to a decrease of the hysteresis effect as an increase of the measuring range. Moreover the matrix ability to perform redundancies with different measurements could also allows to decrease hysteresis response by statistic treatments of several measurements. In future works steps of heating and drying will be tested in order to widely decrease this hysteresis effect. By this way reliability and the sensor lifetime could be extended.
References [1] T. Mohammed-Brahim, A-C. Salaün, F. Le Bihan, SGFET as charge sensor: application to chemical and biological species detection, Sensor& Transducers journal, 900, pp 11-26 (2008). [2] A-C. Salaün, H.M. Kotb, T. Mohammed-Brahim, F. Le Bihan, H. Lhermite and F. Bendriaa, Suspended-Gate Thin Film Transistor as highly sensitive humidity Sensor, SPIE20005, Sevilla, Spain,(2005). [3] Z.M. Rittersma, Recent achievements in miniaturised humidity sensors-a review of transduction techniques, Sensors and Actuators A 96 196210 (2002).
Procedia Engineering
ProcediaProcedia Engineering 5 (2010) 1434–1437 Engineering 00 (2009) 000–000 www.elsevier.com/locate/procedia
www.elsevier.com/locate/procedia
Proc. Eurosensors XXIV, September 5-8, 2010, Linz, Austria
Investigation on Suspended Gate Field Effect Transistor as Humidity Sensor. O. De Sagazana, B. da Silva Rodriguesb, S. Cranda, F. LeBihana, T. Mohammed-Brahima aGM-IETR UMR-CNRS 6164, Université de Rennes1, bat 11b 35042 Rennes Cedex, France. bLSI, University of São Paulo, Av. Prof. Luciano Gualberto, trav. 3 n°158, São Paulo, Brazi
Abstract This work deals with SGFET (Suspended Gate Field Effect Transistor) for humidity rate (%HR) measurement. SGFET devices have already been used to detect gas, biological species, and also to measure pH [1]. This structure has already proved its good sensitivity to atmosphere and charge variation [2]. The following results have been obtained with SGFET realized in CMOS 0,8 μm technology and which have been reported and bonded on PCB card after all wires have been isolated from ambient sources. This paper has investigated especially the sensitivity to %HR. SGFET measurements realized in this work have been performed by sampling the drain current few seconds. By this way it becomes possible to presents for the first time results involving SGFET obtained during several hours of quasi continuous test. With this work and all results issue from the %HR measurement, the hysteresis and the reliability of SGFET’s structures could be evaluated especially in monitoring applications.
©c 2009 by by Elsevier Ltd.Ltd.
2010Published Published Elsevier Keywords: Suspended Gate Field Effect Transistor; Humidity Rate ; Drain Current Sampling.a
1. Introduction SGFET devices have already shown good properties in charges detection. They have been used in pH measure and biological species detection. In most of case and especially for pH and biological species, electrical test are performed in liquid ambient. This paper investigates the electrical behavior of the SGFET in gaseous atmosphere thanks the %HR measurement. In this case SGFET allows an easy implement and it expects to be more reliable due to the absence of porous oxide or polymers [3]. Moreover usually tests of SGFET are performed by sweeping the gate voltage and measuring the drain current. In these tests, high sensitivities have been obtained in particular in pH or in biological detection [1]. But this method involves a lot of stress in the devices and do not fit to monitoring applications like %HR measurements or gas detection. So the paper presents electrical measurements performed by
* Corresponding author. Tel.: 00 33 2 23 23 52 67 E-mail address: [email protected].
c 2010 Published by Elsevier Ltd. 1877-7058 doi:10.1016/j.proeng.2010.09.385
1435
O. De Sagazan et al. / Procedia Engineering 5 (2010) 1434–1437 2
O. de Sagazan/ Procedia Engineering 00 (2010) 000–000
sampling the drain current at fixed gate voltage. Due to the better stability and reliability of the measurements, sensitivity and hysteresis of the device could have been investigated sharply and the interest of the SGFET in continuous monitoring application evaluated. 2. Sensor fabrication process and designs presentation. SGFET used for this study have been realized in technology CMOS 0,8μm. Drain and source are boron doped by implantation, and the 0,5μm thick gate in polysilicon has been deposited by LPCVD (Low Pressure Chemical Vapor Deposition) on a 500 nm thick TEOS (Tetra ethyl Orthosilicate) which acts as a sacrificial layer (Fig. 1). The polysilicon gate is overlayed by nitride layers of 70nm thick. Contacts are realized in aluminum. All conductive layers are then passivated by PECVD (Plasma Enhanced Chemical Vapor Deposition) nitride layers. The structure is finally released after a buffer HF etching (fig 2a). Al
Si3N4
SiO2 Poly Si P+
Si P+
Si P+
Bulk SiSiNN
Fig. 1: Lateral view of the p-MOS SGFET and its empty gap after the release of the TEOS sacrificial layer.
The total area of the SGFET, including the connections is 130μm x 70 μm. This small area led to integrate locally 49 similar SGFETs in a matrix configuration. The strong integration on small area allows the SGFETs of the matrix to be in contacts of the very same ambient and then to present the same characteristic in ideal case. Experimental characteristics of all SGFETs can be measured and averaged. By this means, redundancy is introduced in the goal to reach the maximum reliability.
source
gate
drain
(a)
(b)
Fig. 2: SEM view of the SGFET with the 0.5μm empty gap under the gate, (b) picture of the matrix reported on the PCB card after isolation of all wires
1436
O. De Sagazan et al. / Procedia Engineering 5 (2010) 1434–1437 O. de Sagazan/ Procedia Engineering 00 (2010) 000–000
3
For practical use, SGFET matrix is bonded in a PCB card (fig. 2b). All wires are coated in special resin to avoid parasitic electrical contacts due to the liquid condensation on the surface of the device. The bonded matrix can be considered as ready to use sensor.The electrical characterization of all the transistors of the matrix is performed with an Agilent HP 4155B. During the measurements, the PCB card has been set in a various ambient enclosure. It has been possible to monitor the atmospheric humidity and record the response of the sensor. The %HR has been recorded with a Testo 635 device. 3. Electrical tests and response to %HR variation 5,00E-04
Ids in A
4,50E-04
%HR=30,3 %HR=49,8 %HR=67,3 %HR=81,2 %HR=87,3
4,00E-04 3,50E-04 3,00E-04 2,50E-04
Sampling time in sec
2,00E-04 0
1
2
3
4
5
Fig. 3: Evolution of the drain current sampling during 5 seconds at a Vgs (gate-source voltage) and Vds (drain-source voltage) constant. The value of current which is stored for %HR measure is picked at 5s
Usually SGFET are tested by sweeping the gate voltage at constant drain voltage, the humidity rate is then deduced from the drift of the threshold voltage [1&2]. This method induces electrical stress which could impact the accuracy of the measurement. In the following tests SGFETs are tested by sampling during 5 seconds the drainsource current (Ids) at Vgs (gate-source voltage) and Vds (drain-source voltage) constant (Fig. 3). By this way it becomes possible to monitor the %HR by only measuring the Ids value during several hours (fig. 4a). First observations show that SGFET reacts directly to %HR but without linearity in the sensor response. 5,00E-04
95
Ids in A
8,00E-04
85
5,00E-04
4,50E-04
%RH
75
4,50E-04
7,00E-04 4,00E-04
65
4,00E-04
%HR
55
3,50E-04
6,00E-04
ids 5s d
3,50E-04 3,00E-04
3,00E-04 35
4,00E-04 0
(a)
200
400
600
Ids vds=5V Ids vds=0,5V
5,00E-04
45
25
Ids in A
2,50E-04 800 time in min
2,50E-04
3,00E-04 25
45
65
2,00E-04 85 %HR
(b)
Fig. 4: Drain current according time and %HR variation. Drain current has been sampled during 5s every 10 minutes during 13 Hours (fig. 4a). SGFET %HR sensitivity according the Drain source Vds voltage (fig. 4b), a higher Vds bias induces a wider range of sensitivity.
The sensitivity of the device has then been deduced and it appears that sensitivity to %HR increases at high %HR level. The Influence of the SGFET polarization on the measure has also been investigated (fig. 4b) in order to have
1437
O. De Sagazan et al. / Procedia Engineering 5 (2010) 1434–1437 4
O. de Sagazan/ Procedia Engineering 00 (2010) 000–000
7,50E-04
5,50E-04
7,00E-04
5,00E-04 4,50E-04
6,50E-04 6,00E-04
Ids up Ids down
5,50E-04
Ids in A
Ids in A
the wider range of measure. It seems that a high Vds bias increase the threshold of %HR detection. The hysteresis response between an increase of the %HR and a decrease of %HR has been observed (fig. 5). It has been noticed that hysteresis was almost null at high %HR but for a range between 50 to 70% errors of almost 5% could be induce if SGFET were biased at 5V on Vds (fig. 5a). Whereas it seems that a weak Vds bias (0.5V) decrease the hysteresis of the response (fig. 5b). This is an interesting point that could be applied to other sensing application like pH measurements or gas concentration measurement in the future works.
4,00E-04
5,00E-04
3,00E-04
4,50E-04
2,50E-04
4,00E-04 25
(a)
30
35
40
45
50
55
60
65
70
75
80
%HR
Ids 5s down Ids 5s up
3,50E-04
2,00E-04
%HR 30 35 40 45 50 55 60 65 70 75 80 85 90
(b)
Fig. 5: Illustration of the hysteresis effect which could induce almost a 5%HR errors in the measurement. The hysteresis is greater for a Vds bias of 5V (a) whereas the effect decrease widely for a 0.5V Vds bias (b).
4. Conclusion Matrixes of SGFETs have been realized in technology CMOS 0.8μm. These matrixes have then been reported on PCB cards in order to be used as sensors. This paper reports results in %HR measurement and gives some new results about long time behavior (dozens of hours) and also shows the hysteresis response effect. These results are promising and new, the protocol test while consisting sampling the drain-source current has shown a good reliability. For the first time SGFET have been used during long time and the hysteresis response of the device could have been investigated. It has been shown that SGFETs are efficient for high %HR level showing a high sensitivity and no hysteresis in the response. However main key parameters for reliability and sensitivity have been identified. Indeed additional investigations on polarization conditions (value of Vgs and Vds) could easily lead to a decrease of the hysteresis effect as an increase of the measuring range. Moreover the matrix ability to perform redundancies with different measurements could also allows to decrease hysteresis response by statistic treatments of several measurements. In future works steps of heating and drying will be tested in order to widely decrease this hysteresis effect. By this way reliability and the sensor lifetime could be extended.
References [1] T. Mohammed-Brahim, A-C. Salaün, F. Le Bihan, SGFET as charge sensor: application to chemical and biological species detection, Sensor& Transducers journal, 900, pp 11-26 (2008). [2] A-C. Salaün, H.M. Kotb, T. Mohammed-Brahim, F. Le Bihan, H. Lhermite and F. Bendriaa, Suspended-Gate Thin Film Transistor as highly sensitive humidity Sensor, SPIE20005, Sevilla, Spain,(2005). [3] Z.M. Rittersma, Recent achievements in miniaturised humidity sensors-a review of transduction techniques, Sensors and Actuators A 96 196210 (2002).