Chemically Advanced Nanolithography

Open Access
Author:
Anderson, Mary Elizabeth
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 13, 2006
Committee Members:
  • Paul S Weiss, Committee Chair
  • Mark William Horn, Committee Chair
  • Thomas E Mallouk, Committee Member
  • Christine Dolan Keating, Committee Member
  • Susan E Trolier Mckinstry, Committee Member
Keywords:
  • nanolithography
  • self-assembled monolayers
  • molecular rulers
  • chemical patterning
  • photolithography
Abstract:
The development of hybrid strategies combining chemical self-assembly with conventional fabrication techniques for the advancement of lithography into the sub-100 nm regime has been the focus of the research presented in this thesis. The main objective was the development of the “molecular ruler” process, which utilizes self-assembled multilayers as nanoscale resists for the definition of metal electrode structures with precise nanogap spacings in the 10-100 nm regime. These self-assembled multilayers were composed of alternating, coordinated mercaptoalkanoic acid molecules and Cu (II) ions. The molecular ruler process begins with a lithographically defined gold structure, henceforth called the “parent”. Subsequently, the self-assembled multilayers are grown selectively on this parent structure to form a type of lift-off resist. After metal deposition, this resist is removed chemically leaving behind only the newly created “daughter” structure and the lithographic parent structure. The width of the gap between the created daughter structure and the parent structure is measured out precisely by the tailored thickness of the multilayer resist. Gaps of differing sizes between parent and daughter structures can be generated through changing both the number of molecular ruler layers deposited as well as by altering the length of the molecular ruler itself. This process has been characterized by field emission scanning electron microscopy, and gap sizes have been demonstrated routinely in the 10-50 nm range. The molecular ruler process has been extended to fabricate complex hierarchical architectures. Tertiary structures (granddaughters) have been created by depositing multilayers on the daughter structures as well as the parent structures. Removing a sacrificial parent and/or daughter structure by selective chemical etching can isolate the tertiary structures. Using reactive ion etching and the parent and daughter structures as an etch mask, nanogaps can be transferred into the underlying substrate to create precise recessed features. The molecular ruler process has also been combined with nanosphere lithography to display the versatility of the molecular ruler resists around nonplanar edges. Aligned microstructures with precisely defined nanometer-scale spacing and edge resolution were produced by combining multiple levels of photolithography with the molecular ruler process. Photolithography was used to define the structures’ orientations and self-assembled multilayer resists were used to tailor the structures’ spacings precisely. This work opens the door for future device fabrication by demonstrating the compatibility and robustness of hybrid strategies employing molecular rulers with conventional photolithographic fabrication processes. The integrity of the nanoscale gaps produced by this hybrid technique has been investigated by electrical and structural characterization. The primary failure mode that caused shorting in these structures was identified. Methods for improved lithographic processing were developed and optimized to eliminate the identified failure mode and to create high quality structures. Another important aspect of the hybrid strategies incorporating molecular rulers is that the lithographic resist must be robust enough to withstand the deposition of self-assembled multilayers without compromising their formation. The use of a lithographic resist compatible with self-assembly has opened an avenue for directed chemical patterning of multi-component self-assembled films. A major advantage of this technique is that the different components of the film are shielded by the resist against displacement and intercalation. Additional benefits of this process over other unconventional methods for chemical patterning are the ability to have multiple levels of alignment, reproducible one-to-one feature size transfer, and parallel processing. This thesis research has focused on addressing precision for the creation of nanostructures. Self-assembly and lithographic processing have been used in tandem for the formation of tailored, lithographically defined metal electrodes. Necessary for the development of hybrid strategies, the compatibility between the materials requirements for chemical self-assembly and conventional lithographic processing has been investigated and demonstrated.