[CCoE Notice] Cullen College Dissertation Defense Announcement -Md Ashiqur Rahaman Khan

[Hutchinson, Inez A]

[Dissertation Defense Announcement at the Cullen College of Engineering]
Properties of Glassy Carbon Films Formed by Helium Ion Bombardment of Polymeric Precursors
Md Ashiqur Rahaman Khan
July 25, 2023; 2:00 PM - 4:00 PM (CST)

Location: Online
Zoom: https://urldefense.com/v3/__https://uh-edu-cougarnet.zoom.us/j/8117187653__;!!LkSTlj0I!E_hWUOyD5tjalgl_OUK-RoSMQe47rEFvNdMRHaUVPZG84Hw2rrv1N9vf6Tz-nGeQmfP99EY-qlRrHM_ksJ6sIh3EbWk$ <https://urldefense.com/v3/__https:/uh-edu-cougarnet.zoom.us/j/8117187653__;!!LkSTlj0I!F3vyshxIsgkIOqMUjFaPL8RWp_jmZOLx23Z598Ddlu8FAMGbx7HREyOpzIqmxNlnKAeerlXIYS8q_L4SKADpwQ4_yQ$>

Committee Chair:
John C. Wolfe, Ph.D.

Committee Members:
Earl J. Charlson, Ph.D. | Wei-Chuan Shih, Ph.D. | Paul Ruchhoeft, Ph.D. | Cumaraswamy Vipulanandan, Ph.D.

Abstract
Pyrolytic glassy carbon (P-GC), formed by low temperature (<1000o C) pyrolysis of polymeric precursors, is a hard, brittle material with many useful properties, including, resistivity comparable to disordered metals, inertness to a wide variety of strong organic and inorganic solvents, high resistance to thermal and plasma oxidation, non-hygroscopicity, closed porosity, and remarkable biocompatibility. However, high processing temperatures, coupled with cracking and delamination of large-area films due to precursor shrinkage during pyrolysis, limit the avenues available for integrating P-GC sensors with signal processing electronics.

Kilovolt helium ion beam irradiation presents an alternative approach to forming glassy carbon films with processing temperatures below 250o C and tensile stress in the 80 MPa range, both compatible with CMOS processing. Helium is used because it does not sputter or reactively etch the precursor. The thickness of helium beam glassy carbon (He-GC) films is equal to the range of the ions in glassy carbon. As a point of reference 50 keV He+ ions yield 0.6 mm thick films. To prevent substrate damage, as required in most applications, a thin layer of unimplanted precursor must remain between the glassy carbon layer and the substrate. The compatibility of this layer with other process requirements could be an issue in some applications. A crosslinked polymeric precursor has been developed that can be heated in air to 250oC, probably much higher in vacuum, and is not affected by common laboratory solvents. It has excellent adhesion to silicon and tensile stress in the 45 MPa range. The areal dose required to vitrify this precursor is 2.8x1016/cm2, which is well within the capability of commercial ion implantation services. He-GC films have been formed on the non-planar side of ~1 sq. in. silicon membranes. He-GC represents an approach to the formation of glassy carbon without the high process temperatures and stress of pyrolytic glassy carbon. Moreover, He-GC films have been formed on fragile, non-planar, large area silicon membrane substrates. Nevertheless, an important question remains. Does He-PC have the stiffness required to be a direct replacement for P-GC in important applications such as micro-needles? Is Young’s modulus within the accepted range of 20-40 GPa?

The approach uses the bulge test to measure the biaxial modulus of a composite membrane composed of He-GC on a ring-mounted polyethylene terephthalate (Mylar™) (Goodfellow Metals, Inc.) substrate. A patented method, originally developed for fabricating x-ray lithography mask membranes, was used to form the flat membranes required in the bulge test. The precursor film was deposited by plasma enhanced chemical vapor deposition using methylmethacrylate feed stock followed by oxidation in air at 250oC. The substrate membrane was also oxidized during this process, but thickness and modulus were not affected. Irradiation was carried out using 50 keV He+ ions with a total charge density of 34 C/cm2, about 11 times the threshold required to form IB-GC. The initial precursor thickness was such that the entire film was converted to glassy carbon. Young’s modulus was extracted using the law of mixtures for the bilayer film. The value 30.4 GPa is in the middle of the range P-GC values. So, the answer is yes; IB-GC is sufficiently stiff to replace P-GC for microneedle applications.
[Engineered For What's Next]

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