On the humidity sensors based on MWCNTs/MMA composite in SOI CMOS technological process

Transcription

1 On the humidity sensors based on MWCNTs/MMA composite in SOI CMOS technological process Maria-Alexandra Paun, PhD Scientist, EPF Lausanne Vice Chair & Secretary, IEEE Switzerland Chair, IEEE WIE Switzerland Corresponding Member, IEEE R8 Professional Activities Subcommittee MOS-AK Workshop, ESSDERC/ESSCIRC Conference Lausanne, Switzerland, September 12th, 2016

2 Summary A humidity sensor, part of an air quality sensor system, based on MWCNTs / MMA composite doped with KOH has been studied. A standard SOI CMOS process, with only one post-processing step for the membrane etching, was used to fabricate the sensor. In order to have a homogeneous distribution of the CNTs in the solution, the tip sonication method was employed. The current-voltage and resistance-voltage characteristics have been investigated. 2

3 Humidity Sensors (I) They are used in industry, various medical fields, in health assessment devices (incubators, sterilizers, respiratory equipment). Also in agriculture (soil moisture, moisture control in green houses and air quality control and improvement, dew detection in various agricultural crops), in metrology, etc. To improve the efficiency of humidity sensors, one should master parameters like the sensitivity, linearity, power consumption, response time, hysteresis, etc. 3

4 Humidity Sensors (II) Various materials are used in the realization of humidity sensors, such as ceramics, composites and polymers. CMOS Humidity sensors have been used recently for RFID application. CMOS process for humidity sensors offers the advantages of cost-effectiveness, robustness, and mass production. This work presents a humidity sensor based on MWCNTs/MMA composite films, fabricated in a standard SOI CMOS technology. 4

5 Materials and Methods (I) A. Description of the solution: 3 ml of MMA, 0.01 g of AIBN (which is a powder) 2 µl of KOH diluted in water at 50% addition of CNTs (the amount is not fixed). After sonication Clusters of CNTs Fig. 1 Color photograph (5x) of the first solution Fig. 2 Photograph (5x) of the second solution 5

6 Materials and Methods (II) B. Preparation of the sensor: So far, it has been deposited on silicon and on a chip with a top layer of silicon nitride and golden electrodes. The humidity sensor was fabricated in a standard SOI CMOS process with only one post-processing step for the membrane etching (RIE). Chips Shape of sensing area Table 1. System s dimensions Sensing area diameter (µm) Main dimensions Electrodes thickness (µm) Electrodes spacing(µm) Chip 1 Circular ~ Chip 2 Square ~

7 Materials and Methods (III) The sensors were fabricated by tip-sonication and in situ synthesis on silicon substrate with a structure of gold IDEs, see Fig. 3. Once the solution presented above is ready (by following the adding order of MMA, KOH, AIBN and CNTs), it is tip-sonicated to have a homogeneous distribution of CNTs. Tip-sonication instead of bath-sonication was more efficient in separating the CNTs. A small amount of liquid left in the syringe needle is squeezed out and dropped on the substrate. Once it is deposited, the solution is heated up at 70 C, to transform the MMA in PMMA. This will create a matrix of water-sensible polymers and CNTs between the electrodes. 7

8 Materials and Methods (IV) SENSING MECHANISM: The matrix creates a conductive path between the golden electrodes Fig. 3 Interdigitated Electrodes (IDE) photograph (20x), zoom of the sensing area of Chip1 It has an electrical conductivity that depends on the amount of water absorbed or adsorbed. In the equilibrium condition, this depends on the amount of water in the air, and thus the humidity. Further measurements will prove if it depends on the relative or absolute humidity. 8

9 . Quadratic dependence of the resistance with the temperature and applied voltage R=R (T) and R=R (V) We use Taylor expansion around T 0 and V 0 respectively. Temperature-related coefficients (1) (3) (4) (2) 9

10 Experimental Results (I) We biased the sensing layer with voltage from -3 to 3 V and recorded the current through the sensing layer and computed the associated resistance. We recorded the voltage drop on the heater. We have repeated the above measurements for an array of different currents in the heater, namely 1 ma, 4 ma, 7 ma, 10 ma, 13 ma and 16 ma. In this way, we had obtained information on the resistance variation. 10

11 Experimental Results (II) The temperatures we obtained, by measuring and using formula in Eq. (1), are: T 1 =298.0 K, T 2 =303.8 K, T 3 =312.9 K, T 4 =326.9 K, T 5 =345.0 K and T 6 =371.3 K. Fig. 4 Current-voltage characteristics for six temperatures in the sensing layer of the humidity sensor Fig. 5 R-V Characteristics for six temperatures in the sensing layer of the humidity sensor 11

12 Experimental Results (III) Parabolic dependence of the resistance with the voltage has been proven through experimental results. The fitting coefficients have been extracted. Table 2. Fitting coefficients T ( C) T (K) α v β v σ * *the fitting coefficients are obtained on the corresponding curve, after removing the 1V point, by a statistical exclusion criterion. 12

13 Conclusions A humidity sensor based on MWCNTs / MMA composite doped with KOH (diluted in water at 50%), fabricated in a standard SOI CMOS technology was studied. To have a better homogeneity of the CNTs, a tip-sonication method was preferred to the usual deposition methods. The current voltage and resistance voltage characteristics have been investigated, for the humidity sensors proposed. Our immediate goal is to perform humidity tests on our sensors, for which we will be using a special chamber. 13

14 Acknowledgements: The author, Maria-Alexandra Paun, wishes to thank the Swiss National Science Foundation (SNSF) from Switzerland for the promotion and encouragement of the scientific research of young doctors, respectively by providing the funding for her advanced postdoctoral fellowship at Cambridge University, UK and the return phase at Swiss Federal Institute of Technology Lausanne (EPFL), Switzerland. 14

15 Thank you for your kind attention! 15