Development of a high yield fabrication process for MEMS based resonant mass sensors for cell detection applications

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2014
Töral, Taylan Berkin
This thesis reports the development of a high yield fabrication flow for MEMS based resonant mass sensors for cell detection applications. The basic design is a gravimetric resonator for real-time electronic detection of captured cells through bioactivation on gold coated active area which assures an antibody based cell capture inside a biocompatible microfluidic channel. The proposed design is demonstrated to have various advantages over its conventional counterparts. However, the yield of the previous fabrication methods is too low. Thus, the primary objective of this thesis is to increase the throughput efficiency of the fabrication process flow. For this purpose, the resonator design layouts are improved without considerable geometry modifications compared to the previous generation designs. The original resonator devices make use of capacitive comb drive fingers with wide proof mass area, and folded spring beam structure as anchor structures. Correspondingly, in this thesis, the proposed design deviates only in the manner of dimensions in order to vi increase the performance and endurance of the devices. Besides, the newly designed resonators are spanning a wider margin with respect to the bioactive area on the proof masses. The new designs differ from 44 x 16 μm2 to 248 x 128 μm2; and the natural resonance of these resonators start from 220 kHz and vary up to 1.47 MHz. Secondly, the fabrication method is remodeled with the aim of decreasing the number of process steps and possibly simplifying every process step. In the new fabrication flow, the improvement of process yield is achieved, after the destructive process steps such as wet processes on suspended devices are eliminated. Many of these wet process steps can be avoided; but still some of these destructive wet processes, can only be replaced with analogous dry processes. The major advantage of the design is the hydrophobic parylene coating in between fingers and springs of the resonators which avoids fluid penetration and thus, increases the resonators’ performance considerably inside liquid flow. The hydrophobic parylene layer is required to be coated after the resonators are released and then, the bioactivation field needs to be cleared properly. Therefore, a novel method is developed for dry patterning which ensures the proper removal of the parylene on top of gold bioactivation site for keeping the thin parylene layer only on the fingers and anchors of the resonators. Additionally, the previous generation fabrication flows are inspected, and advantages and disadvantages of these designs are discussed by taking fabrication achievements, design trade-offs, and yield considerations into account. The new fabrication method is presented under the light of these facts. In this way, with a new fabrication approach, the yield of the fabrication is fruitfully benefited that the working devices are increased from 5% to 90%.

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Citation Formats
T. B. Töral, “Development of a high yield fabrication process for MEMS based resonant mass sensors for cell detection applications,” M.S. - Master of Science, Middle East Technical University, 2014.