Dip pen nanolithography-deposited zinc oxide nanorods on a CMOS MEMS platform for ethanol sensing

TLDR: In this article, a dip pen nanolithographic (DPN) technique was used on SOI (silicon on insulator) CMOS MEMS (micro electro mechanical system) micro-hotplates (MHP) for the deposition of zinc oxide (ZnO) nanorods.

Abstract: This paper reports on the novel deposition of zinc oxide (ZnO) nanorods using a dip pen nanolithographic (DPN) technique on SOI (silicon on insulator) CMOS MEMS (micro electro mechanical system) micro-hotplates (MHP) and their characterisation as a low-cost, low-power ethanol sensor. The ZnO nanorods were synthesized hydrothermally and deposited on the MHP that comprise a tungsten micro-heater embedded in a dielectric membrane with gold interdigitated electrodes (IDEs) on top of an oxide passivation layer. The micro-heater and IDEs were used to heat up the sensing layer and measure its resistance, respectively. The sensor device is extremely power efficient because of the thin SOI membrane. The electro-thermal efficiency of the MHP was found to be 8.2 °C mW−1, which results in only 42.7 mW power at an operating temperature of 350 °C. The CMOS MHP devices with ZnO nanorods were exposed to PPM levels of ethanol in humid air. The sensitivity achieved from the sensor was found to be 5.8% ppm−1 to 0.39% ppm−1 for the ethanol concentration range 25–1000 ppm. The ZnO nanorods showed an optimum response at 350 °C. The CMOS sensor was found to have a humidity dependence that needs consideration in real-world application. The sensors were also found to be selective towards ethanol when tested in the presence of toluene and acetone. We believe that the integration of ZnO nanorods using DPN lithography with a CMOS MEMS substrate offers a low cost, low power, smart ethanol sensor that could be exploited in consumer electronics.

Figures

Fig. 5 (a) Dynamic response of ZnO NRs in presence of ethanol at 350°C, (b) Response and recovery time against ethanol concentration plot at 350°C.

Fig. 6 Response of ZnO nanorods sensor as a function of concentration plot. Solid line represents the power law fitted through the experimental points.

Fig. 1 (a) Cross sectional view of the CMOS MEMS micro-hotplate technology (b) Optical micrograph of fabricated device (1mm by 1mm).

Fig. 9 Ethanol sensing measurements in presence of dry, 10% RH and 40% RH air.

Fig. 2 Power versus temperature plot of tungsten micro-heater.

Fig. 3 (a) Optical microscope picture of micro-hotplate with ZnO nanorods, (b) Typical SEM image of ZnO NRs on MHP, a magnified view of nanorods is shown in the inset (c) TEM image of ZnO nanorods, (d) XRD spectrum of ZnO nanorods.

Full text

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Original citation:
Santra, S., De Luca, A., Bhaumik, S., Ali, S. Z., Udrea, F., Gardner, J. W., Ray, S. K. and Guha, P.
K.. (2015) Dip pen nanolithography-deposited zinc oxide nanorods on a CMOS MEMS
platform for ethanol sensing. RSC Advances , 5 (59). pp. 47609-47616.
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Dip pen nanolithography-deposited zinc oxide nanorods on a
CMOS MEMS platform for ethanol sensing
S. Santra
a
*, A. De Luca
b
, S. Bhaumik
a
, S. Z. Ali
c
, F. Udrea
b,c
, J. W. Gardner
c,d
,
S. K. Ray
a
, P.
K. Guha
e
a
Department of Physics, Indian Institute of Technology, Kharagpur, India, 721302
b
Engineering Department, University of Cambridge, Cambridge, CB3 0FA, UK
c
Cambridge CMOS Sensors Ltd., Cambridge, CB4 0DL, UK
d
School of Engineering, University of Warwick, Coventry, CV4 7AL, UK
e
E & ECE Department, Indian Institute of Technology, Kharagpur, India, 721302
*Corresponding author E-mail: susmita.santra@phy.iitkgp.ernet.in

Abstract
This paper reports on the novel deposition of zinc oxide (ZnO) nanorods using dip pen
nanolithographic (DPN) technique on SOI (silicon on insulator) CMOS MEMS (micro
electro mechanical system) micro-hotplates (MHP) and their characaterisation as a low-cost,
low-power ethanol sensor. The ZnO nanorods were synthesized hydrothermally and
deposited on the MHP that comprises a tungsten micro-heater embedded in a dielectric
membrane with gold interdigitated electrodes (IDEs) on top of an oxide passivation layer.
The micro-heater and IDEs were used to heat up the sensing layer and measure its resistance,
respectively. The sensor device is extremely power efficient because of the thin SOI
membrane. The electro-thermal efficiency of the MHP was found to be 8.2°C/mW, which
results in only 42.7 mW power at an operating temperature of 350°C. The CMOS MHP
devices with ZnO nanorods were exposed to PPM levels of ethanol in humid air. The
sensitivity achieved from the sensor was found to be 5.8%/ppm to 0.39%/ppm for the ethanol
concentration range 25 – 1000 ppm. The ZnO nanorods showed optimum response at 350°C.
The CMOS sensor was found to have a humidity dependence that needs consideration in real-
world application. The sensors were also found to be selective towards ethanol when tested
in presence of toluene and acetone. We believe that the integration of ZnO nanorods using
DPN lithography with a CMOS MEMS substrate offers a low cost, low power, smart ethanol
sensor that could be exploited in consumer electronics.
Keywords: Zinc oxide nanorods, MEMS, SOI CMOS, ethanol sensor, gas sensor, dip pen
nanolithography

1. Introduction
Gas sensors have enjoyed a wide range of applications since the 1970s that is steadily
growing. Nowadays, they are extensively used to detect hazardous (i.e. toxic and explosive)
gases present in industrial areas, indoors, and coal mines. More recently they are being
explored to monitor trace levels of volatile organic compounds (VOCs) in human breath,
which contains biomarkers for specific diseases
1, 2
. However, commercially available gas
sensors are still bulky, expensive and, with the exception of electrochemical, consume a large
amount of power (~ 0.5 W). Hence, recent research on gas sensors has shifted towards the
development of miniaturised, low power, inexpensive and easy to integrate devices. Such
sensors can be achieved by advances on two fronts, (A) better sensing layers (e.g. polymer,
carbon nano material or metal oxide) and (B) better substrates (e.g. sensors can be developed
on ceramic, silicon, flexible polymer or a CMOS-MEMS platform).
Carbon nano material (Carbon nanotube, graphene) can sense gases close to room
temperature, but their response is generally disappointing
3-6
. Metal oxides on the other hand
are another class of material which shows large response but usually at elevated temperatures
(e.g. 350
o
C), so needs large power.
Ceramic and silicon substrate have been extensively used for sensor development. Flexible
substrates
3, 4, 6
have recently been introduced to develop large area flexible sensors. However
all these platforms require separate interface electronics that incurs additional costs making
them commercially limited. In this respect, CMOS-MEMS
7-9
technology is very promising
for a new generation of smart low-cost gas sensors; because the fabrication process is well
established, devices are miniaturised (< 4 mm
2
) and their performances are reliable and
reproducible (because of batch fabrication). Such sensors can also have on-chip interface
electronics, which results in a further system miniaturization and cost reduction. MEMS
technology integration with CMOS is critical for achieving low power consumption of the

sensor device through micro-heater thermal isolation schemes
9, 10
. Resistive sensors have an
advantage when compared to other classes of gas sensors (e.g. electrochemical and optical),
because resistive sensors are easier to integrate with CMOS-MEMS platform.
Nano-materials have very high surface to volume ratio, thus providing a significant gas
response even with a small amount of materials. Therefore nano-materials are well suited for
miniaturised CMOS gas sensor devices. Commercially available solid state gas sensors are
generally metal oxide based (e.g. tin oxide or tungsten oxide). Gas sensors using different
metal oxide nano-materials are already reported in the literature
1, 11-15
. Among them, zinc
oxide (ZnO) is one of the most reported sensing materials
11, 14-22
. Because its preparation
method is simple, inexpensive, it has good thermal and chemical stability, high electron
mobility and it responds to several gases or volatile organic compounds. Different nano-
structure of ZnO including nanoparticles
17, 21
, nanorod
17-20, 23
, nanowire
14, 22
, nanotube
16
, thin
film
24
, thick film
25
are reported as gas sensing layers.
Nano-materials are prepared in several ways like chemical and physical methods (chemical
vapour deposition methods, physical vapour deposition methods, sputtering and evaporation),
mechanical exfoliation, chemical exfoliation etc. Deposition of nano-materials on CMOS-
MEMS devices is a challenging task. Chemical routes are not suitable for CMOS sensors as
some chemicals can have a detrimental effect on the integrity of the chip. Physical methods
need either shadow or lithographic masking. Several methods have been reported to integrate
nano-materials with CMOS devices, e.g. local growth technique
9, 26
to grow carbon nanotubes
(CNTs), hydrothermal method to grow zinc oxide nanowires
14
. These reported materials
grown directly on the sensor device, however seed layers were required which need to be
sputter coated. Inkjet printing
27, 28
is a technique suitable for the deposition of nano-materials.
However, making a stable ink which can be deposited to required precision and stability is
under development.

Frequently asked questions

Q1. What are the contributions mentioned in the paper "Dip pen nanolithography-deposited zinc oxide nanorods on a cmos mems platform for ethanol sensing" ?

This paper reports on the novel deposition of zinc oxide ( ZnO ) nanorods using dip pen nanolithographic ( DPN ) technique on SOI ( silicon on insulator ) CMOS MEMS ( micro electro mechanical system ) micro-hotplates ( MHP ) and their characaterisation as a low-cost, low-power ethanol sensor. The authors believe that the integration of ZnO nanorods using DPN lithography with a CMOS MEMS substrate offers a low cost, low power, smart ethanol sensor that could be exploited in consumer electronics. 

Q2. What is the temperature of the substrate area of the chip?

As a consequence of the high thermal isolation offered by the thin dielectric membrane and the heat sink effect provided by the sensor package10, 30, the temperature of the substrate area of the chip is close to room temperature. 

Q3. What is the common method of forming nanomaterials?

Dip pen nano-lithography (DPN) is an extremely flexible deposition method, with possible wafer level scalability, which has proved to be suitable for nano-materials integration on membrane based CMOS devices. 

Q4. Why was tungsten used as a metal for the micro-heater?

Tungsten was used as metal for the micro-heater, in place of Al or polysilicon, because of its superior electrothermal properties that resulted in extended device lifetime at high operating temperatures. 

Q5. What is the main reason for the recent research on gas sensors?

recent research on gas sensors has shifted towards the development of miniaturised, low power, inexpensive and easy to integrate devices. 

Q6. What is the effect of the DPN system on the microhotplates?

In this work, the authors used DPN to functionalize fragile CMOS membrane based microhotplates, exploiting the DPN system off-plane resolution and the high mechanical compliance offered by the cantilever-type pens, in order to “gently” deposit ZnO nanorods (mixed with terpineol) slurry on a relatively wide area ( > 250 µm diameter) at once. 

Q7. How can the heater reach temperatures of 600°C?

2. The heater canreliably reach temperatures of 600°C with 73 mW of power in 15 ms, with an electro-thermalefficiency of 8.2°C/mW, and cool down to ambient temperature in about 30 ms. 

Q8. How low was the concentration of ethanol in the previous work?

In the present work the authors have detected ethanol concentration as low as 25 ppm(the lowest concentration the authors could measure in their previous work was 809 ppm). 

Q9. What are the different types of nanostructures of ZnO?

Different nanostructure of ZnO including nanoparticles17, 21, nanorod17-20, 23, nanowire14, 22, nanotube16, thin film24, thick film25 are reported as gas sensing layers. 

Q10. What is the reason for the ethanol sensor being deposited on CMOS substrate?

The study indicates that DPN deposited ZnO nanorods on CMOS substrate may open up a scalable, batch produced approach to develop power efficient, low cost smart ethanol sensor. 

Q11. What are the methods used to integrate nano-materials with CMOS devices?

Several methods have been reported to integrate nano-materials with CMOS devices, e.g. local growth technique9, 26 to grow carbon nanotubes (CNTs), hydrothermal method to grow zinc oxide nanowires14. 

Q12. What is the effect of humidity on the conductivity of the nanorods?

After initial increase of conductivity, the reactions presented in (8) and (10) progress simultaneously, thus slowly decreasing the conductivity of the nanorods. 

Q13. What is the reason for the fast change in resistance?

This fast change in resistance is due to the non-dissociative adsorption of water molecules as shown by the following equation39𝐻𝐻2𝑂𝑂(𝑔𝑔𝑎𝑎𝑎𝑎) → 𝐻𝐻2𝑂𝑂(𝑎𝑎𝑎𝑎𝑎𝑎) → 𝐻𝐻2𝑂𝑂+(𝑎𝑎𝑎𝑎𝑎𝑎) + 𝑒𝑒− (8)According to the above equation water molecules donate the electron to the conduction band of ZnO which is responsible for the fast change in resistance with the introduction of humidity. 

Q14. What is the morphological and structural study of ZnO nanorods?

From these micrographs, it can be clearly seen that the nanorods are deposited mainly over the micro-heater region, without creating a thermal bridge from the hot-area to the substrate -thus avoiding undesired extra power dissipation. 

Q15. What temperature was the optimum temperature for the ethanol sensor?

The ethanol sensing performance was investigated in the temperature range 250°C –450°C and the optimum operating temperature was found to be at 350°C. 

Q16. What is the explanation for the improved results?

These improved results can be attributed to the fact that ethanol molecules can react with the chemisorbed oxygen species at the surface of ZnO nanorod immediately. 

Q17. What is the resistance of the ZnO nanorods at the ambient temperature?

When 10% RH was introduced inside the chamber, the conductivity of the ZnO nanorods increased very sharply as shown in inset of Fig. 9 (red circular area). 

Q18. What is the name of the paper?

This paper reports on the novel deposition of zinc oxide (ZnO) nanorods using dip pen nanolithographic (DPN) technique on SOI (silicon on insulator) CMOS MEMS (micro electro mechanical system) micro-hotplates (MHP) and their characaterisation as a low-cost, low-power ethanol sensor. 

Q19. What was the morphological and structural study of ZnO NRs performed?

The morphological and structural study of ZnO NRs were performed using field emission scanning electron microscope (FESEM, Zeiss Gemini-Sigma), transmission electron microscopy (TEM, FEI – Tecnai G2 20S – Twin operated at 200 kV), X-ray diffraction(Philips X-Pert MRD with Cu Kα radiation). 

Q20. Why are nanorods horizontally aligned on the electrodes?

This is because nanorods are horizontally aligned on the electrodes instead of vertically standing, thus offering a wider surface for chemical interaction.