Journal of Micromechanics and Microengineering
Related content
PAPER
Development of scalable planar MEMS technology for low power operated ethanol sensor To cite this article: Priyanka Dwivedi et al 2018 J. Micromech. Microeng. 28 105020
View the article online for updates and enhancements.
- Low power gas sensor array on flexible acetate substrate Samatha Benedict, Palash Kumar Basu and Navakanta Bhat - MoO3/nano–Si heterostructure based highly sensitive and acetone selective sensor prototype: a key to non-invasive detection of diabetes Priyanka Dwivedi, Saakshi Dhanekar and Samaresh Das - A MEMS based acetone sensor incorporating ZnO nanowires synthesized by wet oxidation of Zn film Bhagaban Behera and Sudhir Chandra
This content was downloaded from IP address 103.27.10.110 on 18/08/2018 at 08:26
Journal of Micromechanics and Microengineering J. Micromech. Microeng. 28 (2018) 105020 (10pp)
https://doi.org/10.1088/1361-6439/aad507
Development of scalable planar MEMS technology for low power operated ethanol sensor Priyanka Dwivedi , Dhairya Singh Arya, Saakshi Dhanekar and Samaresh Das Centre for Applied Research in Electronics, Indian Institute of Technology (IIT), Hauz Khas, New Delhi 110016, India E-mail:
[email protected] Received 22 April 2018, revised 17 July 2018 Accepted for publication 23 July 2018 Published 13 August 2018 Abstract
Metal oxide-based gas sensors are known for operating at high temperatures, which results in high power consumption. This drawback limits their application in battery-operated devices as well as system on chip (SoC) applications. In this paper, the design, simulation, integrated circuit (IC)-compatible fabrication, and testing of an integrated microelectromechanical system (MEMS) micro-heater with a sensor platform using planar MEMS technology are presented. The micro-heater is integrated with titanium dioxide (TiO2) and a nano-silicon (Si) heterostructure using a simple fabrication process. This heterostructure is tested in the presence of different analytes like ethanol, acetone, iso-propyl alcohol, xylene, and benzene. The sensor shows an optimum sensing response at 100 °C with a maximum response to ethanol vapors. In comparison to the crystalline Si, the power consumption of the nanoSi-based platform is almost half. The formation of the heterostructure, highest sensitivity to ethanol, and repeatable ppm level of ethanol sensing at a comparatively low operating temperature are reported. The results show the TiO2/nano-Si integration with the MEMS micro-heater, and demonstrate the reduced power consumption of 18 mW by nano-Si, which is very small in comparison to what is consumed by crystalline Si. The temperature profiling carried out using an IR-camera ensures the uniform temperature of 100 °C over the suspended sensing structure. Keywords: MEMS, nano-Si, sensor, micro-fabrication, micro-heater (Some figures may appear in colour only in the online journal)
1. Introduction
neighboring environment. Based on their electronic structure, metal oxide semiconductor elements like Cr2O3, ZnO, SnO2, In2O3, WO3, titanium dioxide (TiO2), V2O5, and MoO3 are suitable for detecting reducing or oxidizing gases through resistive measurements [8–14]. TiO2 exhibits attractive features like stability, optically active properties, and tunability in the formation of its nano-structures [10]. Generally, all metal oxide-based sensors work at high temperatures and always suffer from selectivity issues. Due to their high operating temperatures, these types of sensors are not suitable for battery-operated systems. Microelectromechanical system (MEMS)-based devices are fabricated using bulk and
Recently, solid state gas sensors are becoming popular due to their broad applications in chemical detections, industries, environmental monitoring, healthcare, air quality monitoring systems, etc [1–4]. Apart from this, due to their simple operation, easy electronic interface, good sensitivity, low cost, fast response, capability to detect a number of hazardous gases with very low maintenance [5–7], these sensors are prominent in modern industry and medical diagnostics. Metal oxides possess a broad range of electronic, chemical, and physical properties that are often highly sensitive to changes in their 1361-6439/18/105020+10$33.00
1
© 2018 IOP Publishing Ltd Printed in the UK
P Dwivedi et al
J. Micromech. Microeng. 28 (2018) 105020
Figure 1. Stepwise micro fabrication process flow for TiO2/nano-Si-based sensor using planar MEMS technology.
surface micro machining, and are widely used in sensor and actuator applications [15, 16]. These can play a vital role in the development of sensors that can operate at low power and can be made using integrated circuit (IC) micro-fabrication-compatible processes. One of the important concerns in MEMS-based devices is thermal insulation, which is directly related to power consumption. Furthermore, power consumption can be reduced by making low-dimensional devices and/ or changing the platform. For a wafer-scalable process for MEMS device integration with other structures, it is important to also see the system on chip (SoC) integration possibilities. Since all previously reported state-of-the-art MEMS-based gas sensors are known to operate at high temperatures [17, 18], there is a need to develop a low-temperature operable MEMSbased sensor platform with low power consumption. Different groups have reported MEMS-integrated gas sensor platforms that consume high power. Lee et al [19] reported a Pt-based micro-hot plate integrated with IDE in which a NiO thin film was used as the sensing material but consumed power in the order of 8 W. Another work presented a Pt MEMS heater for NH3 detection at 250 °C using SnO2 as the sensing material with a power consumption of 68 mW [20]. Similarly, Behera et al [21] used Ni as a micro-heater material and reported 36 mW power with a ZnO-based IDE structure for acetone sensing. Another group also made a Pt micro-heater-based ethanol sensor at a comparatively low
working temperature using spongy TiO2, which was used as the sensing layer for the detection of an ethanol concentration of 44 ppm at 250 °C [22]. Since nano-Si is an interesting example of a nanostructured material in sensors due to its simple wafer scale fabrication, being easy to integrate for SoC, low cost, high surface area, porosity, sensitivity, IC compatibility, etc, it expands opportunities for different research areas with applications in sensors, biotechnology, medicine, optoelectronics, chemistry, and so forth [23, 24]. Although nano-Si does not provide operative insulation for the micro-heater, it has additional advantages like low thermal conductivity, which can be used as a key feature in the development of different micro-heater designs with low voltage and low power consumption. To the best of our knowledge, work involving a metal oxide and nano-Si integrated with an MEMS heater has not been reported yet. In our work, we performed the design, simulation, fabrication, characterization, and testing of a planar MEMS technology integrated with a TiO2/nano-Si-based ethanol sensor. Micro-heater and IDE structures were made of Ni with a power consumption of 18 mW and with a uniform heating in the sensing area. For comparison, a crystalline Si (c-Si)-based sample was also simulated, which showed a power consumption of 30 mW. After fabrication, the sensor was tested for optimum operating temperatures and in the presence of different analytes. The uniqueness of the fabrication process lies 2
P Dwivedi et al
J. Micromech. Microeng. 28 (2018) 105020
Figure 2. SEM micrographs of the fabricated device, and (b) EDS spectrum of TiO2/nano-Si.
in the excellent integration of the micro-heater platform and nanostructured materials with low power consumption.
A stepwise fabrication process for the MEMS planar platform for the TiO2/nano-Si-based sensor is shown in figures 1(a)–(j). The scalable fabrication started with a 2″ double sided polished p-type 1 0 0 Si wafer having a resistivity of 1–10 Ω cm. For insulation and passivation of devices, 1 µm thick thermal oxide (SiO2) was grown at 1100 °C using a quartz furnace, as displayed in figure 1(b). A thin film of chrome/gold (Cr/Au) (10/100 nm) was deposited and patterned for defining the nano-Si fabrication region (figures 1(c) and (d)). After that, the nano-Si was fabricated (anodization technique) on this patterned wafer using an electrolytic solution of HF:C2H5OH in 1:4 ratio. This anodization was performed at a current density of 20 mA cm−2 for a duration of 10 min (figure 1(e)). The complete Cr/Au was etched using standard Cr/Au etchants (figure 1(f)). In order to pattern the Ni heater, a 0.4 µm Ni/Cr thin film was deposited and
2. Design and fabrication of the micro-heater Different meander-shaped micro-heater designs were simulated using finite element modeling using COMSOL Multiphysics. In all micro-heater designs, the common dimensions for achieving uniform heating over the active sensing area were as follows: heater strip width, 100 µm; gap between heater strips, 100 µm; heater size, 2000 µm × 2000 µm; dielectric membrane thickness, 1 µm; and Ni thickness, 0.4 µm. The detailed simulation results for the different micro-heaters are discussed in section 4.1. The optimized design of the micro-heater was decided to be adapted for the fabrication of the sensor. 3
P Dwivedi et al
J. Micromech. Microeng. 28 (2018) 105020
Figure 3. Transmission electron microscopy (TEM) (a) morphology and (b) selected area electron diffraction (SAED) pattern.
etched selectively using the same mask of the heater and IDE structure ((figures 1(g) and (h)). AZ9260 (1.5 µm) photoresist was then patterned using a second mask. TiO2 (10 nm) was deposited on this pattern by employing a reactive sputtering technique and patterned using the lift-off process (figure 1(i)). In order to make a backside cavity, a front-to-back alignment was performed using the Suss-Micro-tech Mask aligner for patterning aluminum (Al) on the back side of the wafer. This step was followed by reactive ion etching for making the diaphragm ((figure 1(j)). A schematic view of the fabricated device is shown in figure 1(k). 3. Characterization of the sensing surface and testing of the micro-heater 3.1. Morphological characterization
The morphologies of the materials in the device were examined by SEM (Zeiss EVO 50) and TEM (EOL JEM-1400). Structural analysis was done by Raman spectroscopy using a LAbRAMHR Evolution RAMAN Spectrometer (Horiba) with an Ar laser excitation wavelength of 514 nm. The crystal structure of TiO2/nano-Si was analyzed by x-ray diffraction (XRD) (Phillips X’Pert, PRO-PW 3040 diffractometer). X-ray photoelectron spectroscopy (XPS) data was obtained from a PHI Quantes scanning dual x-ray photoelectron microprobe (ULVAC-PHI Inc.) under a basic pressure of 10−8 torr. Figure 2(a) shows micrographs of the fabricated MEMSintegrated TiO2/nano-Si structure-based sensor platform. It is clear from the SEM that the micro-heater and sensing area (IDE) are made on the same plane, and therefore this setup is also known as planar MEMS technology. The inset of figure 2(a) shows the morphology and cross-sectional view of the same sample at a higher magnification. It is attributed to a nano-porous morphology of TiO2 ranging from ~4–6 nm. The cross-sectional view depicts the depth of nano-Si, which is almost ~7 µm. Since this morphology is uniform across the wafer, we can say that the process developed is scalable and can easily be integrated with standard IC technology. Figure 2(b) displays the elemental mapping of different elements like Ti, O, and Si corresponding to the TiO2/nanoSi surface. This gives the element compositions 8.923%, 40.008%, and 51.068% for Ti, O, and Si, respectively. The EDS spectrum of the TiO2/nano-Si sample clearly demonstrates the overlap of the Ti and O peaks at an energy
Figure 4. XRD pattern of TiO2/nano-Si; the inset shows the Raman spectrum.
less than 2 keV (figure 2(c)). At higher energy levels greater than 4 keV, two isolated Ti peaks were observed, which confirm the presence of Ti on the nano-Si surface. The nanoscale assemblies of the porous TiO2 film on the nano-Si are easily visible using TEM (figure 3). An interconnected nano-porous TiO2 film with an average size of 4–5 nm was observed (figure 3(a)). It is known that a smaller pore size leads to higher surface area-to-volume ratios, making it a promising candidate for gas sensing applications. The corre sponding SAED pattern (figure 3(b)) shows numerous Si interparticle crystals and bright spots with concentric circles uniformly distributed throughout the sample. The distance between these crystals and spots can be related to the (1 1 0) crystal plane of the polycrystalline TiO2 [25]. The SAED diffraction rings, with the background circles and the broad peaks of the anatase in the XRD, agree with the SEM and TEM observation of nano-porous TiO2. Figure 4 displays the XRD pattern of TiO2/nano-Si, which confirms the crystallinity and phase composition of the TiO2/ nano-Si sample in thin film mode with a glancing angle of 0.5°. The anatase and rutile phases relate to peaks at 2 theta equal to 25.4° (1 0 1) for the anatase phase and at 27.6° (1 1 0) and 56.3° (2 1 1) for the rutile phase of TiO2, as depicted in figure 4 [26, 27]. A sharp and intense peak of the anatase phase at 25.4° shows that it is more dominating than the rutile phase 4
P Dwivedi et al
J. Micromech. Microeng. 28 (2018) 105020
Figure 5. XPS analysis of TiO2/nano-Si: (a) Ti 2p spectrum, and (b) O 1s spectrum.
Figure 6. IR imaging of micro-heater at 1.5 V, and temperature versus distance profile.
[28, 29]. The peaks position, phase, and (h k l) of samples were matched with JCPDS 21-1272 and JCPDS 04-551 for anatase and rutile phases, respectively. The inset of figure 4 shows the Raman spectrum of TiO2/nano-Si at room temperature, which confirms the Raman lines at 144 cm−1 for the anatase phase of TiO2; no significant rutile peaks were observed [30, 31]. The anatase phase was observable only after annealing the TiO2 film at 500 °C for one hour in nitrogen (N2) ambient. This infers a modification in crystallization of TiO2 after annealing at 500 °C. Figure 5 shows the wide XPS spectra of synthesized TiO2, which indicates only signals arising from elements Ti and O. Figure 5(a) shows major titanium peaks of Ti 2p1/2 and Ti 2p3/2 at 463.8 eV and 458.1 eV, respectively [29, 32]. Sharp and intense peaks confirm that TiO2 is made up of only of Ti4+ ions in the lattice. Figure 5(b) represents another O1s peak at 532 eV, which is attributed to the surface oxygen species.
3.2. Thermal characterization of the micro-heater
IR imaging was performed for the fabricated heater using a N2-cooled IR camera (SC5000, FLIR, France), which is sensitive to wavelengths between 2.5 µm and 5 µm and has a 320 × 256 pixel resolution. The distance of the object to the IR camera was around 100 mm to get the MEMS heater focused. Figure 6 represents the temperature profile versus distance graph for both samples (with and without nano-Si). It shows that the temperature profile, obtained by the IR camera measurement, is almost constant in the active region (sensor area) across the heater filament. The heater temperature is maximum near the center and almost at ambient temperature over the outside area of the suspended filaments. There is an observable change in IR profiles at the center region of both samples, which was observed due to the difference in micro-heater platforms viz. nano-porous and crystalline Si. 5
P Dwivedi et al
J. Micromech. Microeng. 28 (2018) 105020
Figure 7. Simulated temperature distribution of different micro-heaters with nano-Si at 1.5 V: (a) heater design A − DA, (b) heater
design B − DB, (c) heater design C − DC, (d) and DC without nano-Si. (e) Isothermal profiles for all design, and (f) comparison of power consumption.
6
P Dwivedi et al
J. Micromech. Microeng. 28 (2018) 105020
Table 1. Comparison of MEMS-based heater platforms.
S/N.
Heater material
Morphology (sensing)
Operating temp (°C) Tested analyte
Limit of detection
Power
IC compatible
Ref.
1. 2. 3. 4. 5. 6. 7. 8.
Pt Pt Ni Ni Pt Au/Mo Pt Ni
NiO thin film SnO2 ZnO nanowire ZnO film CuO film SnOx films Spongy TiO2 TiO2/nano-Si
280 250 100 150 200 400 250 100
3.6 ppm 5 ppm 5 ppm − 5 ppm 250 ppm 44 ppm 5 ppm
8W 68 mW 36 mW 70 mW 25 mW
