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Atom-Based Dimensional Metrology

Summary:

A primary goal of this project is to develop intrinsic calibration standards based on the crystalline lattice. The ultimate limit for nanoscale length metrology is the development of intrinsic calibration standards, where the reference dimensions are based on atom spacing within an ordered, crystalline lattice. Using techniques developed within this project, advanced step height, linewidth and pitch standards exhibiting picometer accuracy are being developed, using an ultra-high vacuum scanning tunneling microscope (UHV-STM). In addition, a project goal is to validate the use of atomic lattice spacing as a ruler through comparison with interferometric length measurements such as those performed in the Molecular Measuring Machine. The atom-based dimensional metrology project is leveraging this knowledge to develop the metrology science needed to enable parallel, high-throughput, atomically-precise manufacturing for NIST standards development and for industrial applications. The knowledge of atomic lithography and patterning transfer developed in this project with our newly developed phosphine doping capability, functional devices with nano-dimension, such as a single atomic transistor, quantum qubit can be fabricated.

Description:

The atom-based dimensional metrology project is developing a new, comprehensive approach to dimensional metrology using the atom spacings of a crystal as the fundamental “ruler” or scale. Based on this metric, larger scale features will be etched into substrates to serve as critical dimension or pitch references for instruments having less-than-atomic resolution. Crystal-lattice dimensional parameters will be validated through interferometer measurements. This work will primarily be done by employing two complex multi-chamber vacuum STMs. The system contains ultra high vacuum (UHV) facility regularly capable of producing 1 x 10-8 Pa base pressures, along with vibration isolation that yields a sub-100 pm noise floor, and includes a unique field-ion/field-electron microscope (FIM), which is used to image scanned probe tips on the atomic scale and is being used to develop repeatable, robust methods for atomically sharp STM tip production. The project is developing improved nanofabrication methods using the STM to write atomic-scale features on the surface of hydrogen-passivated silicon substrates using scanning probe oxidation for accurate nano-standard development. Atom-based step height standards having picometer accuracy will also be developed, validated and characterized by an interferometer-instrumented atomic force microscope. The similar process can be applied to the device fabrication such as a single atomic transistor by laying a single phosphor atom in si substrate.

There are four major tasks in this project:
  • One fundamental goal for the project is to develop methods for repeatedly producing atomically-sharp tungsten and alternative-material tips, and evaluate their atomic resolution imaging capabilities. This includes a systematic evaluation of different crystalline materials capable of producing single atom tips or atomic structures defined for high resolution sub-nanometer imaging.
  • This project has a substantial effort to develop atomically ordered and large(>20 microns) silicon (100) surfaces for hydrogen termination. Apply the scanning probe STM or AFM to make patterns on this hydrogen passivated smooth surface, and with proper oxidation process a hard mask can be developed. The structures small as 5nm wide with sub-nanometer thickness can be fabricated. Since these samples are not available commercially, it is essential to develop both the metrology for atomic resolution measurements and the ability to fabricate and measure standards with atomic precision at the nanometer scale.
  • Utilize the facility of NanoFab at CNST and develop a dry etching capability to perform the pattern transfer. The patterns that made by STM/AFM with thickness of only 1 atom and a Reactive Ion Etching(RIE) process can enhance vertical dimension to a more practical dimension, 10-20 nm.
  • Acquired an ability of doping phosphine on the Si(100) atomically smooth surface at the desired location. In addition to passivate and pattern on Si(100) surface, this capability of doping phosphorus atom at desired location allow us to fabricate atomic functional device such as single atom transistor.

Major Accomplishments:

  • Successfully completed the five year, three phase DARPA contract to conduct collaborative research in atomically precise positioning, patterning and metrology, validating the applicability and potential value of this work.
  • Successfully developed a process procedures in UHV to clean si(100) surface hence to promote the dimmers row formation and hydrogen passivation.
  • Optical lithography fabricated the fiducial structures to promote the large terrace (>20 micrometer) formation during si(100) surface reconstruction.
  • Routinely fabricated the line structures that patterned by AFM in ambient and followed by RIE. The line structures are measured by CD-AFM and confirmed the linewidth under 10 nm.
  • Submitted a proposal for 2014 IMS competition, and the proposal was awarded for a five-year funding. 

Lead Organizational Unit:

pml

Customers/Contributors/Collaborators:

  • DARPA
  • University of Maryland
  • University of Illinois
  • University of Texas at Austin
  • University of Texas at Dallas
  • Zyvex

Facilities/Tools Used:

Figure 1. An STM image to show three vertical lines patterned by a STM. The distance between any two lines can be determined by counting the number of atoms separating lines.
Figure 1. An STM image to show three vertical lines patterned by a STM. The distance between any two lines can be determined by counting the number of atoms separating lines.

Figure 2. A STM image shows two Si(100) terraces and a single atomic step(height of 0.136nm) between them. A STM patterned a single line on the right side of terrace.
Figure 2. A STM image shows two Si(100) terraces and a single atomic step(height of 0.136nm) between them. A STM patterned a single line on the right side of terrace.

Contact

Physical Measurement Laboratory (PML)
Semiconductor & Dimensional Metrology Division (683)

General Information:
301-975-5609 Telephone
301-869-0822 Facsimile

100 Bureau Drive, M/S 8212
Gaithersburg, Maryland 20899-8212