In the last decade, microelectromechanical systems (MEMS) SU-8 polymeric cantilevers with piezoresistive readout combined with the advances in molecular recognition techniques have found versatile applications, especially in the field of chemical and biological sensing. Compared to conventional solid-state semiconductor-based piezoresistive cantilever sensors, SU-8 polymeric cantilevers have advantages in terms of better sensitivity along with reduced material and fabrication cost. In recent times, numerous researchers have investigated their potential as a sensing platform due to high performance-to-cost ratio of SU-8 polymer-based cantilever sensors. In this article, we critically review the design, fabrication, and performance aspects of surface stress-based piezoresistive SU-8 polymeric cantilever sensors. The evolution of surface stress-based piezoresistive cantilever sensors from solid-state semiconductor materials to polymers, especially SU-8 polymer, is discussed in detail. Theoretical principles of surface stress generation and their application in cantilever sensing technology are also devised. Variants of SU-8 polymeric cantilevers with different composition of materials in cantilever stacks are explained. Furthermore, the interdependence of the material selection, geometrical design parameters, and fabrication process of piezoresistive SU-8 polymeric cantilever sensors and their cumulative impact on the sensor response are also explained in detail. In addition to the design-, fabrication-, and performance-related factors, this article also describes various challenges in engineering SU-8 polymeric cantilevers as a universal sensing platform such as temperature and moisture vulnerability. This review article would serve as a guideline for researchers to understand specifics and functionality of surface stress-based piezoresistive SU-8 cantilever sensors.

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Performance enhancement of a silicon MEMS piezoresistive single axis accelerometer with electroplated gold on a proof mass is presented in this paper. The fabricated accelerometer device consists of a heavy proof mass supported by four thin flexures. Boron-diffused piezoresistors located near the fixed ends of the flexures are used for sensing the developed stress and hence acceleration. Performance enhancement is achieved by electroplating a gold mass of 20 μm thickness on top of the proof mass. A commercially available sulfite-based solution TSG-250 TM was used for the electroplating process. Aluminum metal lines were used to form a Wheatstone bridge for signal pick-up. To avoid galvanic corrosion between two dissimilar metals having contact in an electrolyte, a shadow mask technique was used to selectively deposit a Cr/Au seed layer on an insulator atop the proof mass for subsequent electrodeposition. Bulk micromachining was performed using a 5% dual-doped TMAH solution. Fabricated devices with different electroplated gold areas were tested up to ±13 g acceleration. For electroplated gold dimensions of 2500 μm × 2500 μm × 20 μm on a proof mass, sensitivity along the Z-axis is increased by 21.8% as compared to the structure without gold. Off-axis sensitivities along the X-and Y-axes are reduced by 7.6% and 6.9%, respectively.

Microcantilever platforms with integrated piezoresistors have found versatile applications in the field of clinical analysis and diagnostics. Even though treatise encompasses numerous design details of the cantilever based biochemical sensors, a majority of them focus on the generic slender rectangular cantilever platform mainly due to its evolution from the atomic force microscope (AFM). The reported designs revolve around the aspects of dimensional optimization and variations with respect to the combination of materials for the composite structure. In this paper, a triangular cantilever platform is shown to have better performance metrics than the reported generic slender rectangular and the square cantilever platforms with integrated piezoresistors for biochemical sensing applications. The selection and optimization of the triangular cantilever platform is carried out in two stages. In the first stage, the preliminary selection of the cantilever shape is performed based on the initial design obtained by analytical formulae and numerical simulations. The second stage includes the geometrical optimization of the triangular cantilever platform and the integrated piezoresistor. The triangular cantilever platform shows a better performance in terms of the figure of merit (FoM), ψ Δ = R R f (/) 0 2 and the measurement bandwidth. The simulation results show that the magnitude of ψ of the triangular platform is 77.21% and 65.64% higher than that of the slender rectangular and the square cantilever platforms respectively. Moreover, the triangular platform exhibits a measurement bandwidth that is 70.91% and 2.04 times higher than that of the slender rectangular and square cantilever structures respectively. For a better understanding of the 2D nature of the stress generated on the cantilever platform due to the surface stress, its spatial profile has been extracted and depicted graphically. Finally, a set of design rules are provided for optimizing the triangular cantilever platform and piezoresistor dimensions in terms of the electrical sensitivity and the mechanical stability for biochemical sensing applications.

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This paper presents realization of a MEMS piezoresistive single axis accelerometer using dual doped TMAH solution. The silicon micromachined structure consists of a heavy proofmass supported by four thin flexures and sandwiched between top and bottom glass plates. Boron diffused piezoresistors located near fixed points of the flexure are used for sensing the developed stress due to applied acceleration. Based on the initial results an improved design has also been considered to achieve reduced cross-axis sensitivity and nonlinearity. The fabricated sensor tested upto 13 g acceleration shows average sensitivity of 0.556 mV/g along normal to the proofmass plane. The measured cross-axis sensitivity was 3.272 lV/g for X-axis and 3.442 lV/g for Y-axis which is less than 1% of Z-axis sensitivity. Comparing two designs there was an improvement of 63% sensitivity along Y-axis for the design with flexures placed along the proofmass edges.