Development of actuator and force sensor for small-scale mechanical testing

Abstract

Bu çalışma, Silisyum nanoteller için tek eksenli bir çekme testi platformu sağlamak amacıyla yürütülmüştür. Bu platform, bir eyleyici, bir kuvvet algılayıcısı ve numune olarak tek bir Silisyum nanotelden oluşmaktadır. Eyleyici, Silisyum nanoteli bir ucundan çekerken, Silisyum nanotelin diğer ucuna bağlı algılayıcı numune üzerine etkiyen kuvveti ölçer. Tüm sistem, hem numune/cihaz hizalaması hem arayüz problemlerini bertaraf etmek için monolitik olarak üretilecektir. Bu çalışmada, bu tür bir test platformunun ihtiyaçlarını karşılayacak bir MEMS eyleyici ve kuvvet algılayıcısının tasarım, imalat ve karakterizasyonu anlatılmıştır. Değişik boyutlu Silisyum nanotellere yönelik olarak, dört farklı elektrostatik eyleyici ve yine dört farklı, üç plakalı ve sığa değişimini ölçmeye dayalı kuvvet algılayıcısı tasarlanmıştır. Silisyum nanotelleri kıracak şekilde, 3 µN ila 32 µN arasında değişen kuvvet aralığında çalışan eyleyicilerin en yüksek tahrik potansiyeli 10V ile 25V arasında değişmektedir. Elde edilen en düşük

This study is geared towards providing a uniaxial tension test platform for silicon nanowires, which includes an actuator, a force sensor and a single Si nanowire as the sample. Actuator pulls the Si nanowire at one of its tips, whereas the force acting on the Si nanowire is traced by the sensor attached at the opposing tip of the Si nanowire. The whole assembly is intended to be fabricated monolithically to overcome sample/device alignment and interface issues. This study reports design, fabrication and characterization of MEMS actuators and force sensors to meet the needs of such a testing platform. A set of four electrostatic actuators and four tri-plate force sensors based on differential capacitive readout are designed for Si nanowires of different sizes. Actuators are designed to generate forces ranging from 3 µN to 32 µN to enable the fracture of each Si nanowire while having a maximum actuation voltage of 10V - 25V. The minimum obtained ratio of the horizontal pull-in voltage to the actuation voltage is 2.6 for this set of actuators. Force sensors are designed to have a measurement range from 1.0 µN to 28.5 µN corresponding to the onset of fracture in Si nanowires. Sensitivity values, taken as the ratio of readout voltage to Si nanowire strain, are obtained as 0.65, 3.26, 5.46 and 3.74 V/(µm/µm). The corresponding displacement ranges of the sensors are in the submicron region ranging from 0.209 µm to 0.377 µm. Furthermore, non-linearity of the sensors is designed to be less than %5. First-generation actuator fabrication is accomplished through surface micromachining of SOI wafers with 10 µm device layer, 1 µm BOX layer and 380 µm handle layer, whereas force sensor fabrication failed during final release step of backside etching of the handle layer. To prevent the failure, we foresee the usage of a protective and conformal material which will enhance the rigidity of the device and will be removed easily after etching. Characterization of the first-generation actuators is accomplished by carrying out CV measurements. Results indicate that MOS capacitance is dominant over MEMS capacitance.The following conclusions are drawn based on fabrication and characterization of the first-generation actuators, which are taken into account in the design phase of the new set of devices, whose specifications were indicated above.1. Release of actuators is problematic. A back-side etch is necessary;2. A protective and conformal material is necessary for back-side etching;3. With decreasing displacement values, MOS capacitance effect becomes increasingly important and has to be taken into account in the electromechanical design.