Directed assembly of metal oxide nanowire sensors for low-power CMOS enabled gas sensing arrays

Open Access
Zhong, Xiahua
Graduate Program:
Electrical Engineering
Doctor of Philosophy
Document Type:
Date of Defense:
February 12, 2015
Committee Members:
  • Theresa Stellwag Mayer, Dissertation Advisor
  • Christine Dolan Keating, Committee Member
  • Suman Datta, Committee Member
  • Joan Marie Redwing, Committee Member
  • metal oxide
  • nanowire
  • gas sensing array
  • CMOS
  • low power
Every year, approximately 12.5 million Americans suffer an asthma attack. The sudden attack of the disease can be triggered by more than eight hours of exposure to ppm level environmental pollutants, such as CO, NOx, O3 and H2S. Early notification of the pollutants can help to minimize the attacks, especially for people with vulnerable respiratory and cardiovascular systems. The development of gas sensors that consume µW of power and detect target gases down to ppm levels in the form of wearable accessories is of great interest. Such systems would benefit from inexpensive, high-performance complementary (CMOS)-based gas sensor platforms. Transition metal oxide nanowire sensor arrays satisfy the sensing requirements because they deliver key attributes of low cost, ppb to ppm sensitivity, and µW individual sensor power consumption. A limitation of using metal oxides in gas sensing is their cross-reactivity to multiple gases. Inspired by the process used in mammalian olfaction, discriminating a specific target gas in a complex mixture can be accomplished by simultaneous collection and processing of response data from many different elements in a multiplexed sensor array. To integrate the array, heterogeneous populations of high-quality polycrystalline metal oxide nanowires with uniform dimensions and well-characterized sensing properties are necessary. Metal oxide nanowires fabricated by bottom-up synthesis methods are typically non-uniform in geometry, impurity concentration, and crystallinity. Therefore, they are not well suited for integration into large-scale CMOS sensor arrays. This thesis research demonstrated a new, scalable, hybrid top-down/bottom-up fabrication method that produces highly uniform nanowires composed of a high-resistivity Si core that is coated with an ultrathin polycrystalline metal oxide shell. In this work, TiO2 and SnO2 coated-nanowires were fabricated by crystallizing the metal oxide shell under process conditions optimized for high sensitivity detection of the target gases. Individual TiO2 or SnO2 nanosensors were integrated into test structures and onto CMOS circuits via electric-field-assisted deterministic assembly. The performance of the nanosensors was evaluated by monitoring their chemiresistive response as they were exposed to CO and H2. The results demonstrated device-to-device uniformity with a variation in resistance of less than 15%. Individual sensors displayed high sensitivity and fast response comparable to the best metal oxide nanowire sensors synthesized by bottom-up methods. The nanowire sensors coated with a 15 nm-thick SnO2-shell consumed 120µW of power to detect CO at a concentration as low as 10ppm within 65 seconds. A CMOS chip was fabricated and tested for the integration of a heterogeneous metal oxide nanowire sensor array. When connected to a readout circuit, gas response data from individual array elements on the CMOS chip can be accessed, wirelessly transferred, and monitored by physicians to provide real time health care to the users.