A MEMS-based bulk micromachined piezoresistive accelerometer was designed and fabricated. The accelerometer is a quad-beam suspended structure, which is realised by a combination of wet and dry bulk micromachining techniques. The fabrication process of the accelerometer is CMOS compatible. Image of the fabricated accelerometer is shown in Fig. 1. The accelerometer has been designed for low-g (±4g) applications. The accelerometer device characterisation is in progress.
Fig. 1: Microscopic image of the accelerometer
Design and Fabrication of Capacitive Micromachined Ultrasonic Transducer (CMUT) Arrays for Medical Applications
A capacitive micromachined ultrasonic transducer (CMUT) arrays for medical applications was designed. The specifications and design parameters of CMUT arrays are as follows:
|Membrane material||Silicon and SiO2|
|Shape of membrane||Circular/Hexagonal|
|Size of the membrane||100 μm|
|Thickness of membrane||2.0 μm|
|Air cavity height||1.0 μm|
|Natural frequency||3-5 MHz|
|Pull-in voltage||100 V|
|CMUT arrays||10 x 10 & 16 x 16|
The cross-sectional view of the CMUT array is shown in Fig. 2. The CMUT device is to be realised by wafer bonding technique.
Fig. 2: CMUT cell structure (Cross- sectional view)
A nano ion-sensitive field-effect transistor (Nano-ISFET) for pH sensing applications has been designed. The specifications and design parameters of Nano-ISFET are as follows:
- Sensitivity : > 55 mV/pH
- Resolution : < 0.01 pH
- Reference Voltage : 0-2 V
- Bias Voltage : 0-2 V
Fig. 3 shows the packaged Nano-ISFET. Aluminum Nitride (AlN) is used as the sensing film of the device.
Design and Development of ISFET
ISFET devices as a pH sensor for chemical/ biological sensing platforms were developed. An n-channel depletion mode vertical ISFET structure has been identified and successfully fabricated with Aluminum Nitride (AlN) as sensing layer. Fig. 4 shows the packaged AlN-ISFET.
Fig. 4 packaged AlN-ISFET.
Microcantilever-based Piezoresistive Sensor for Biological Agents
Microcantilever devices capable of detecting extremely small forces, mechanical stresses and mass additions offer the promising prospect of biological sensing with better sensitivity. Microcantilever transduction/detection mechanism can be broadly divided into three categories, viz.; optical, piezoelectric and piezoresistive.
In the present activity, design and simulations of a piezoresistive microcantilever platform were performed on CoventorWare®, a commercial finite element analysis (FEA) tool. Placements of polysilicon piezoresistors are crucial and their placements have been optimised to measure differential surface stress caused due to adsorption of any biomolecule on the functionalised layer. Sense and reference microcantilevers have been connected in a Wheatstone bridge configuration to obtain an output voltage corresponding to the change in resistance of the polysilicon piezoresistors.
The dimensions of micro-cantilever have been proposed to be 500 μm × 100 μm × 2 μm and a dual-leg polysilicon piezoresistor is mounted on the micro-cantilever to be realised by the bulk micromachining technology. The final device model designed is as shown in Fig. 5.
Fig. 5: Oblique view of the 3-D solid model of microcantilever