EXPLORING NOVEL GLASS MICROFABRICATION TECHNIQUES FOR SENSOR APPLICATIONS

Open Access
Author:
Zhang, Chenchen
Graduate Program:
Electrical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 21, 2017
Committee Members:
  • Srinivas A Tadigadapa, Dissertation Advisor
  • Srinivas A Tadigadapa, Committee Chair
  • Zhiwen Liu, Committee Member
  • Weihua Guan, Committee Member
  • Saptarshi Das, Outside Member
Keywords:
  • glass etching
  • on-chip glass microspherical shell
  • WGM resonator
  • glass microbubble ensulation
Abstract:
This work presents the exploration of glass microfabrication techniques for fabricating novel chip-scale glass based transducers. Inexpensive and readily available, glass materials possess exceptional properties that include excellent electrical insulation, broad optical transparency, and biocompatibility. Glass substrates are highly in demand in Microelectromechanical systems (MEMS) but their use is not as widespread due to the limited availability of microfabrication processes. The focus of this dissertation is to develop glass microfabrication processes and their applications for MEMS sensors development. Plasma etching processes on three compositions of glass substrates are explored using a modified inductively couple plasma reactive ion etching (ICP-RIE) system for high etch-rate, high aspect ratio, smooth etching performance, and understanding the fundamental plasma glass etching mechanism. Using SF6 as the plasma source gas and NF3 and H2O gases introduced downstream near the surface of the wafer through a diffuser gas inlet, etch rates as high as 1.06 μm/min, 1.04 μm/min, and 0.45 μm/min with surface smoothness of ~2 Å, ~67 Å, ~4 Å are achieved for fused silica, borosilicate glass, and aluminosilicate glasses respectively after 5 minutes etches. High aspect ratio etch of 5.2:1, 10:1 and 2:1 are obtained for fused silica, borosilicate glass, and aluminosilicate glass respectively. Glass etching mechanism is further understood by analyzing the etch rates and corresponding partial pressure of plasma species detected by in-situ residual gas analyzer (RGA) with various position of the diffuser gas inlet. Statistical analysis indicates etch rate is critically influenced by ion flux. Fluorine based radicals and molecular fragments influence both the etch rate and surface smoothness of fused silica whereas they primarily influence the surface smoothness for borosilicate glass. The large fraction of impurity atoms of Ca and Al in aluminosilicate glass form non-volatile fluorides on the etch surface and therefore the etch rate and surface smoothness of aluminosilicate glass is primarily influenced ion flux and very little by the fluorine chemistry. We also examine the role of the layout of the metal mask layer on how it influences the charging of glass substrates during etching and therefore the etch rate. In the second half of the thesis, chip scale glass blowing technique is explored for novel sensing and packaging applications. Arrays of on-chip spherical glass shells of hundreds of micrometers in diameter with ultra-smooth surfaces and sub-micrometer wall thicknesses have been fabricated and have been shown to sustain optical resonance modes with high Q-factors of greater than 50 million. The resonators exhibit temperature sensitivity of -1.8 GHz K-1 and can be configured as ultra-high sensitivity thermal sensors for a broad range of applications. By virtue of the geometry's strong light-matter interaction, the inner surface provides an excellent on-chip sensing platform that truly opens up the possibility for reproducible, chip scale, ultra-high sensitivity microfluidic sensor arrays. As a proof of concept we demonstrate the sensitivity of the resonance frequency as water is filled inside the microspherical shell and is allowed to evaporate. By COMSOL modeling, the dependence of this interaction on glass shell thickness is elucidated and the experimental results of the sensitivity of two different shell thicknesses is explained. In the last chapter, chip-scale blown, glass microbubbles are explored for encapsulation of ferrofluid atop a micromachined quartz resonator configured as a magnetometer. The concept of a ferrofluid based magnetometer has been previously reported where the viscoelastic response of a thin interfacial ferrofluid layer loaded atop a high frequency shear wave quartz resonator to applied magnetic field is monitored. The magnetic field can be sensitively quantified by the changes in the at-resonance admittance characteristics of the resonator. However, under open conditions, continuous evaporation of the ferrofluid compromises the long term performance of the magnetometer. In this work, we integrate glass hemispherical microbubbles, used as vessels of ferrofluid, on the resonator chip to seal and prevent the evaporation of the ferrofluid liquid and drying out. Using these improvements, a minimum detectable field of 600 nT at 0.5 Hz is achieved. Moreover, comparing with the unsealed ferrofluid device, the lifetime of the glass microbubble integrated chip packaged device improved significantly from only few hours to over fifty days and continuing.