Micro-hotplate Research Articles

Low-cost surface micromachined microhotplates for chemiresistive gas sensors

Microhotplate (MHP) based gas sensors have gained significant attention recently due to their small size, low power and feasibility for integration of electronics on the same chip. This study presents the detailed work on design, fabrication and complete characterization of microhotplates based on a standard multi user MEMS process (MUMPs). Suspended-membrane type MHPs were designed using the available layer combinations of MUMPs. FEM simulations were carried out to optimize the heater design by spatially varying the heater current density to achieve uniform temperature distribution over the sensing area. Topography measurements confirmed that the X–Y–Z dimensions of the fabricated MHPs were in accordance with the design. From electro-thermal characterization, the thermal efficiency of the MHPs was evaluated as ~ 10 °C/mW. The suspended membrane showed a homogeneous temperature of ~ 450 °C at 35–40 mW heater power, which was well above the typical operating temperature of chemiresistive gas sensors. The results presented in this paper provide a pathway for realizing cost effective MHPs for gas sensors based on MUMPs.

Size Effect of Heat Transport in Microscale Gas Gap

Microscale gas gaps commonly exist in gas thermal conductance related microdevices, such as micro-hot-plate gas sensors and micro-Pirani vacuum gauges. In these devices, thermal conduction of the gas gaps is an important issue for their performance. Although simulations for size effect of the thermal conduction in microscale gas gaps have been carried out, experimental results are still rare. In this paper, four microhot plates that contain four gas gaps from 220 nm to 21 μm have been fabricated by a standard CMOS process and some additional post-CMOS processes. The thermal convection coefficient can be obtained as large as 1242 Wm2K-1 from the convection dominate 21- μ m gap. The effective thermal conductivity of 220-nm gap is as low as 1.2 × 10-3 Wm-1K-1. Both of them indicate that size effect of gaseous heat transport is significant in such microscale devices.

Design, fabrication, and characterization of bridge-type micro-hotplates with an SU-8 supporting layer for a smart gas sensing system

In this study, bridge-type micro-hotplates (MHP) with an SU-8 supporting layer were proposed for smart gas sensor applications. The proposed MHP consisted of a heating membrane with an area of 140 µm × 140 µm, and a 33 µm-thick SU-8 layer deposited on its bridges. Finite element method based simulation confirmed that the proposed MHP displayed good thermal isolation properties. The proposed MHP was successfully fabricated, and the properties of the MHP were characterized. Current–voltage characteristics revealed that the MHP temperature can reach 550 °C at 5 V. The temperature of the MHP was calculated from changes in the resistance of the heater. Power consumption of the MHP approximately corresponded to 13.9 mW for heating to 300 °C. This was comparable to the power consumption reported in the previous studies. Furthermore, a stable operation under a constant voltage was observed for 100 min. The properties of the MHP indicated that it could potentially be utilized for applications related to integrated gas sensors.

Design, Simulation, and Optimization of a Meander Micro Hotplate for Gas Sensors

Micro Hotplate (MHP) is the key component in micro-sensors, particularly gas sensors. Indeed, in metal oxide gas sensors MOX, micro-heater is used as a hotplate in order to control the temperature of the sensing layer which should be in the requisite temperature range over the heater area, so as to detect the resistive changes as a function of varying concentration of different gases. Hence, their design is a very important aspect. In this paper, we have presented the design and simulation results of a meander micro heater based on three different materials - platinum, titanium and tungsten. The dielectric membrane size is 1.4 mm × 1.6 mm with a thickness of 1.4 μm. Above the membrane, a meander heating film was deposed with a thickness of 100 nm. In order to optimize the geometry, a comparative study by simulating two different heater thicknesses, then two inter track widths has also been presented. Power consumption and temperature distribution were determined in the micro heater´s structure over a supply voltage of 5, 6, and 7 V.

Thermal characterization of carbon nanotube foam using MEMS microhotplates and thermographic analysis

Thermal material properties play a fundamental role in the thermal management of microelectronic systems. The porous nature of carbon nanotube (CNT) arrays results in a very high surface area to volume ratio, which makes the material attractive for surface driven heat transfer mechanisms. Here, we report on the heat transfer performance of lithographically defined micropins made of carbon nanotube (CNT) nanofoam, directly grown on microhotplates (MHPs). The MHP is used as an in situ characterization platform with controllable hot-spot and integrated temperature sensor. Under natural convection, and equivalent power supplied, we measured a significant reduction in hot-spot temperature when augmenting the MHP surface with CNT micropins. In particular, a strong enhancement of convective and radiative heat transfer towards the surrounding environment is recorded, due to the high aspect ratio and the foam-like morphology of the patterned CNTs. By combining electrical characterizations with high-resolution thermographic microscopy analysis, we quantified the heat losses induced by the integrated CNT nanofoams and we found a unique temperature dependency of the equivalent convective heat transfer coefficient, Hc. The obtained results with the proposed non-destructive characterization method demonstrate that significant improvements can be achieved in microelectronic thermal management and hierarchical structured porous material characterization.

Multilayer microheater based on glass substrate using MEMS technology

A microheater with a multilayer structure on a glass substrate is designed and fabricated using microelectromechanical systems (MEMS) technology. The micro-hot plate mainly comprises a glass substrate, two silicon nitride layers, a silicon oxide layer, a multilayer heater, and two electrode pads. In general, platinum, being a noble metal, does not adhere well to materials like silicon nitride or dioxide. To improve the problem, a multilayer structure was performed to enhance the thermal adhesion stability of thin platinum film. The multilayer heater is consisted of chromium (Cr), chromium nitride (CrN), platinum (Pt), chromium nitride (CrN), and chromium (Cr). The Cr and CrN are used as the intermediate materials between the Pt and the silicon oxide in order to improve adhesion. The couples of Cr–CrN and CrN–Pt have good thermal stress resistance due to their similar thermal expansion coefficients. Results showed that the multilayer heater could improve structure adhesion and decrease degradation for stability and reliability. The microheater can be heated up to about 498°C with about 22°C fluctuation corresponding to a power consumption of about 2.35W. For duration test, the microheater operates without damage at a heating temperature of 496.5°C for 72h.

Design and Fabrication of High Vacuum Gauge Based on Micro Hotplate

Vacuum gauge based on micro hotplate (MHP) is a kind of promising MEMS vacuum gauge due to its advantages including high sensitivity, wide measurement range, fast response speed and comparably easy fabrication, etc. In this paper, through analyzing various of heat dissipation approaches including solid heat conduction, gas heat conduction, heat radiation, and convection, the sensor output expression of MHP-based vacuum gauge is obtained when keeping electric current of heating resistor constant. With the structure size and material properties, the relationship curve between vacuum gauge output and gas pressure can be obtained. By wet etching silicon under the compound dielectric films with embedded metal film resistor, MHP suspended over silicon substrate is fabricated. Then the sensor chip is assembled and put into vacuum system, and test is conducted when keeping the heating resistor at constant heating current of 5mA. The measurement results show that the sensor measurement range is 5×10-3Pa~103Pa, which is basically consistent with the theoretical analyzing results.

Aerosol-assisted CVD-grown WO₃ nanoneedles decorated with copper oxide nanoparticles for the selective and humidity-resilient detection of H₂S.

A gas-sensitive hybrid material consisting of Cu2O nanoparticle-decorated WO3 nanoneedles is successfully grown for the first time in a single step via aerosol-assisted chemical vapor deposition. Morphological, structural, and composition analyses show that our method is effective for growing single-crystalline, n-type WO3 nanoneedles decorated with p-type Cu2O nanoparticles at moderate temperatures (i.e., 380 °C), with cost effectiveness and short fabrication times, directly onto microhot plate transducer arrays with the view of obtaining gas sensors. The gas-sensing studies performed show that this hybrid nanomaterial has excellent sensitivity and selectivity to hydrogen sulfide (7-fold increase in response compared with that of pristine WO3 nanoneedles) and a low detection limit (below 300 ppb of H2S), together with unprecedented fast response times (2 s) and high immunity to changes in the background humidity. These superior properties arise because of the multiple p-n heterojunctions created at the nanoscale in our hybrid nanomaterial.

A low cost micro-heater for aerosol generation applications

A low cost micro-heater for aerosol generation has been developed using thin film deposition, electroless metal plating and selective wet etching technology. The thermal and electrical behavior of micro-heater (MH) and micro-hot-plate (MHP) systems is reported. The microstructure of the MH consists of a free standing amorphous silicon membrane with a thin film micro-heater (Cr/Ni layer) on top. The heating element is fabricated by e-beam evaporation and solution based nickel plating. The metal pattern temperature reaches the boiling point of glycerol (290°C) and higher. A wet etched formed reservoir storage space for the aerosol source liquid can be placed alongside or underneath the hot plate. The entire MH unit is fabricated on a thin glass coverslip. This structure and the use of a plating bath and wet etch render the device remarkably low cost (less than 18¢) as compared to other silicon technology based heaters. This MH can be integrated with gas sensors to assist pre-heating to the working temperature. The chemical release cycles are well controlled and are ideally suitable for use in medical procedures, such as drug inhalation therapy.

Design and fabrication of a CMOS-compatible MHP gas sensor

A novel micro-hotplate (MHP) gas sensor is designed and fabricated with a standard CMOS technology followed by post-CMOS processes. The tungsten plugging between the first and the second metal layer in the CMOS processes is designed as zigzag resistor heaters embedded in the membrane. In the post-CMOS processes, the membrane is released by front-side bulk silicon etching, and excellent adiabatic performance of the sensor is obtained. Pt/Ti electrode films are prepared on the MHP before the coating of the SnO2 film, which are promising to present better contact stability compared with Al electrodes. Measurements show that at room temperature in atmosphere, the device has a low power consumption of ∼19 mW and a rapid thermal response of 8 ms for heating up to 300 °C. The tungsten heater exhibits good high temperature stability with a slight fluctuation (<0.3%) in the resistance at an operation temperature of 300 °C under constant heating mode for 336 h, and a satisfactory temperature coefficient of resistance of about 1.9‰/°C.

Microgradient-Heaters As Tools for High-Throughput Experimentation

A microgradient-heater (MGH) was developed, and its feasibility as a tool for high-throughput materials science experimentation was tested. The MGH is derived from microhot plate (MHP) systems and allows combinatorial thermal processing on the micronano scale. The temperature gradient is adjustable by the substrate material. For an Au-coated MGH membrane a temperature drop from 605 to 100 °C was measured over a distance of 965 μm, resulting in an average temperature change of 0.52 K/μm. As a proof of principle, we demonstrate the feasibility of MGHs on the example of a chemical vapor deposition (CVD) process. The achieved results show discontinuous changes in surface morphology within a continuous TiO2 film. Furthermore the MGH can be used to get insights into the energetic relations of film growth processes, giving it the potential for microcalorimetry measurements.

Design and Fabrication of Planar-Type Micro-Hotplate with Si-Island Gas Sensor

A gas sensor with planar-type micro-hotplate (MHP) with Si-island was designed and fabricated in this paper. The sensor was fabricated simply by using only three masks for the lithography process. The temperature distribution across the sensitive film was simulated and analyzed using ANSYS. A uniform temperature distribution was obtained after added Si-island. The SnO2 sensitive film was deposited through sol-gel spin-coating method and showed reasonable gas-sensing properties to hydrogen at 300°C.

Coupled Electro-Thermal-Mechanical Micro-Hotplate-Based for Micro Gas Pressure Sensor

In this paper, electro-thermal-mechanical theoretical analysis of micro-hotplate-based micro gas pressure sensor is carried out, which is on basis of the classical heat transfer theory and rarefied gas dynamics. Combined with micro-hotplate (MHP) theory analysis of heat transfer and thermal-mechanical finite element modeling, electro-thermal-mechanical coupled analysis of theoretical models with regard to the MHP-based micro gas pressure sensor is built. Then, through the ANSYS-one of the finite element analysis software-the simulation analysis of MHP went well. The simulation results show that MHP generates a smaller deformation because of adding the thermal conductivity, and MHP provides a more feasible analysis method in the theoretical study of micro hotplate.

An experimental study on micro-gas sensors with strip shape tin oxide thin films

Micro-cracks in tin oxide thin film may cause bad stability to the micro-hotplate (MHP) gas sensor, especially at high operation temperature or in moisture ambience. In order to reduce micro-cracks, tin oxide thin films on a 2 × 2 MHP gas sensor array are patterned as narrow strips. It is proved by SEM that micro-cracks induced by thermal stress can be greatly restrained in narrow strip shape films comparing with the traditional rectangle board film. Stability of the MHP sensors with narrow strip films was characterized by testing their resistance in air for 60 h, and their responses respectively to methane and carbon monoxide for 300 h. The results show that the strip shape SnO 2 films are really helpful to enhance stability of the MHP gas sensors.

System on chip thermal vacuum sensor based on standard CMOS process

An on-chip microelectromechanical system was fabricated in a 0.5 μm standard CMOS process for gas pressure detection. The sensor was based on a micro-hotplate (MHP) and had been integrated with a rail to rail operational amplifier and an 8-bit successive approximation register (SAR) A/D converter. A tungsten resistor was manufactured on the MHP as the sensing element, and the sacrificial layer of the sensor was made from polysilicon and etched by surface-micromachining technology. The operational amplifier was configured to make the sensor operate in constant current mode. A digital bit stream was provided as the system output. The measurement results demonstrate that the gas pressure sensitive range of the vacuum sensor extends from 1 to 105 Pa. In the gas pressure range from 1 to 100 Pa, the sensitivity of the sensor is 0.23 mV/ Pa, the linearity is 4.95%, and the hysteresis is 8.69%. The operational amplifier can drive 200 ω resistors distortionlessly, and the SAR A/D converter achieves a resolution of 7.4 bit with 100 kHz sample rate. The performance of the operational amplifier and the SAR A/D converter meets the requirements of the sensor system.

Analysis of Coupled Electro-Thermal-Mechanical of Micro Gas Pressure Sensor Based on Micro-Hotplate Technology

The effect of micro-hotplate (MHP) thermal deformation on the signal output of the MHP-based micro gas pressure sensor is investigated. Combining electro-thermal theoretical analysis and thermal-mechanical finite element modeling, different electro-thermal-mechanical analysis models of the sensors are built and solved at constant current condition and constant temperature condition respectively. The calculated results show that the MHP thermal deformation has little effect on the sensor signal output over the entire pressure range at constant current condition, I=0.7mA. While at constant temperature condition, Ts=400K, thermal deformation has little effect on the sensor signal output at low gas pressure, but at high pressure the effect are great. Moreover, according to the thermal-mechanical analysis, we find that optimizing thickness distribution of thin films in the MHP suspended structure can reduce thermal deformation effectively at higher temperature when the lateral dimension is same, which presents a practicable method to improve the sensor stability.

Micro-hotplates for high-throughput thin film processing and in situ phase transformation characterization

This paper presents the use of micro-hotplates (MHPs) as thermal processing and in situ characterization platforms for phase transformations in thin films. MHPs are fabricated by microsystem technology processes and consist of a SiO 2/Si 3N 4 membrane (app. 1 μm) supported by a bulk Si frame. Several embedded Pt thin films serve as heater and electrical measurement electrodes. It is shown that the MHPs have unique properties for the controlled annealing of thin film materials (up to 1270 K), as the annealing temperature and heating/cooling rates can be precisely controlled by in situ measurements. These rates can be extremely high (up to 10 4 K/s), due to the low thermal mass of MHPs. The high cooling rates are especially useful for the fabrication of metastable phases (e.g. Fe 70Pd 30) by quenching. By measuring the resistivity of a thin film under test in situ as a function of the MHP temperature, microstructural changes (e.g. phase transformations) can be detected during heating and cooling cycles. In this paper, examples are presented for the determination of phase transitions in thin films using MHPs: the solid–liquid–gas phase transition (Al), the ferromagnetic–paramagnetic phase transition (Fe–Pt) and martensitic transformations (Ni–Ti–Cu, Fe–Pd). Furthermore, it is demonstrated that crystallization processes from amorphous to crystalline (Ni–Ti–Cu) can be detected with this method. Finally the application of MHPs in thin film combinatorial materials science and high-throughput experimentation is described.

Study on a micro-gas sensor with SnO2–NiO sensitive film for indoor formaldehyde detection

Abstract A micro-gas sensor is fabricated based on a micro-hotplate (MHP) with micromachining technology. The SnO 2 –NiO nanometer polycrystalline composite synthesized by a chemical co-precipitation method is coated onto a micro-hotplate as a sensitive film. The response and recovery time of the micro-gas sensor decrease rapidly with increasing the formaldehyde (HCHO) concentration at relatively low concentrations. The micro-gas sensor shows good stability, low power consumption of 180 mW at the MHP operation temperature of 300 °C, and high selectivity in the presence of interference gases such as alcohol, toluene, α-pinene and acetone. The micro-gas sensor can detect 0.06 ppm HCHO, which is the indoor exposure limit for HCHO referring to Chinese environmental protection agency (EPA).

An Integrated Surface Micromachined Convex Microhotplate Structure for Tin Oxide Gas Sensor Array

This paper describes the fabrication process for a novel convex microhotplate (MHP) structure using surface micromachining technology. The process is used to fabricate an integrated 4times4 tin oxide gas sensor array. Surface micro-machining is selected for its simplified process and CMOS compatibility. However, limited sacrificial layer thickness usually leads to higher power consumption. In this work, a convex MHP structure is developed to increase the thermal efficiency, which is critical for large dimension and low-power gas sensor array integration. Before the structure release process, 700degC-950degC annealing process was carried out on the 190 times190 mum2 MHP with 2.8 mum polysilicon sacrificial layer. It is shown that higher annealing temperature leads to lower tensional stress of the MHP membrane and larger curvature of the released structure, which enables higher thermal efficiency. Among the four annealing temperatures, the 950degC annealed MHP has the largest curvature of 2.438 cm-1 and the highest thermal efficiency of 13degC/mW. Experimental results showing the responses of the sensor array to different combustible gases are also illustrated in this paper. Comparison of power consumption with other designs reported in the literature illustrates the effectiveness of the proposed process.

Fabrication of an integrated microfluidic device based on a heat-sensitive poly(N-isopropylacrylamide) polymer and micromachining protocols for programmed bio-molecular patterning

Due to their broad application in biochemical analysis, the miniaturized and integrated microfluidic devices which can give a biochemically active phase for diagnostic capabilities on single chips are very important in contemporary research. Here, we present an integrated microfluidic device applicable to sample preparation, like the controlled patterning of target biomolecules, like proteins or cells, with a smart polymer-modified temperature-addressable microelectrode that is thermally switched between hydrophilic and hydrophobic states. The device is composed of a bulk-micromachined Si device and a hot-cast poly(dimethyl siloxane) (PDMS) device. The microelectrode a array has been integrated into a micro-hot plate having an embedded microheater and temperature sensors on 2-μm thick silicon oxide/silicon nitride/silicon oxide (O/N/O) stacking layer, and is designed to adsorb and release biomolecules with low power consumption and rapidness in a microfluidic chamber. To provide the heat-responsive activity to the microelectrode surface, the electrode surface was modified with poly( N -isopropylacrylamide) (PNIPAAm), which shows a rapidly reversible hydrophilic-to-hydrophobic transition in response to temperature changes. In this study, design, fabrication and its characterization of the microfluidic device with NHS PNIPAAm surface modifications on the dendrimer monolayer conjugated microelectrode were carried out. And then, the synthesis and the confirmation of surface modification with the smart polymer by grazing FT-IR spectroscopy and contact angle analyzer were demonstrated. The microfluidic devices would be directly applicable to a portable battery-powered biochemical system.