The CNF Technology & Characterization at the Nanoscale virtual short course was held online January 27 to 29, 2021. The course was excellent and gives an overview of nanofabrication techniques various applications using various method and tools. The course also provided an overview of the fabrication capabilities at the CNF. The CNF staff has a deep background in nanofabrication and recipe development and is a key benefit of working at the CNF.

Day 1

• Welcome & Introduction to the CNF
• Overview of Microfluidics
• Introduction to photolithography
• Lab Demonstration 1: Microfluidic Device Fabrication - Photolithography
• Introduction to the Fabrication of Microfluidics
• Introduction to Etching and Pattern Transfer
• Lab Demonstration 2: Microfluidic Device Fabrication - Deep Reactive Ion Etching (DRIE), Profilometry, Surface modification (MVD), PDMS Mixer & assembly

Day 2

• Introduction to MEMS
• Thin Films, Thermal Oxidation, Chemical Vapor Deposition, and Thin Film Characterization
• Lab Demonstration 3: Chemical Vapor Deposition (LPCVD, PECVD)
• Projection Photolithography
• Lab Demonstration 4: Projection Photolithography (Steppers)
• Reactive Ion Etching, Wet Etching & Release
• Lab Demonstration 5: Plasma Etching
• Scanning Electron Microscopy
• Lab Demonstration 6: Scanning Electron Microscopy (SEM)

Day 3

• Introduction to Fabricating Nano-electrodes
• Electron Beam Lithography, Nano Imprint Lithography
• Lab Demonstration 7: Electron Beam Lithography
• Physical Vapor Deposition, Pattern Transfer - Lift Off
• Lab Demonstration 8: Evaporation/ Sputter
• Atomic Force Microscopy
• 2D-Materials: Graphene and Beyond
• Lab Demonstrations 9: Atomic Force Microscopy (AFM), (ALD), (ALE)
• Process Integration Considerations

The Nanoscale

A nanometer is wicked small. For comparison, the ratio of the moon's orbit around the Earth (~4 * 10⁸ meters) to a ruler (about 15 inches) is 10⁹. The ratio of the ruler to the radius of a strand of DNA (1 nm) is also 10⁹. This nanoscale realm makes fabricating devices and structures challenging. Developing a device at the nanoscale involves a cyclical process as shown in figure below.

Process DRC and Monitoring

The short course concentrated mostly on fabrication as it is considered the most difficult aspect of entire process. During the layout phase it's important to preform design rule checks (DRC) to ensure that fabrication facility can produce the device. Process control monitors (PCM) are included in the layout to help verify the performance of the device. Standard test structures are included in the layout for this purpose and are commonly used in commercial wafer fabs, and are recommend in at user fabs. These structures include: Van der Pauw structures for measuring sheet resistance, contact chains used to identify shorts and opens, Kelvin structures to measure contact resistance, and grating to monitor critical dimensions.

Process Flow

Fabrication involves integrating discrete steps together to create a process flow to make a device. The process flow depends on the materials used, the geometry of the device structures, and interconnects. Creating a process flow requires knowledge of many disciplines and can be complicated. It involves selecting a photolithography process, etch, and deposition method that is compatible with the materials selected.

Photolithography

Wafers can be patterned using contact or projection photolithography. With contact photolithography, a mask is placed in contact with a wafer coated with photoresist and exposed with UV light. The whole wafer is exposed at once. With projection photolithography, a pattern is projected onto a section of the wafer coated with photoresist and is exposed. The wafer is moved and the exposure is repeated until all areas of the wafer have been exposed. This is known as step and repeat photolithography.

The photoresist is selected depending on the type of photolithography used, the type of etching to be used, the depth of etch, the materials to be etched, and the materials to be deposited. Both positive and negative resistors are available too.

Deposition

Dielectric thin films can be deposited on a substrate using many techniques including Chemical Vapor Deposition (CVD), Low Pressure CVD (LPCVD), Plasma-Enhanced CVD (PECVD), Atomic Layer Deposition (ALD). Each deposition method produces a thin film with unique properties such as density, quality, uniformity, and step coverage with variations in deposition rate, deposition temperature, and batch processing rate.

Metal thin films are deposited using Physical Vapor Deposition (PVD) that involve thermal and electron beam evaporation, sputtering, reactive evaporation, reactive sputtering, and co-evaporation.

Etch

Once a wafer is patterned, it can be etched using wet etching (also known as chemical etching), plasma etching, ion etching, and Reactive Ion Etching {RIE}. Etching removes thin films, the substrate, or both. Wet etching uses chemicals like Potassium Hydroxide (KOH) to remove silicon anisotropically, or Hydrofluoric (HF) acid to remove Silicon Dioxide (SiO₂) isotropically, for example. Plasma etching uses ionized gas to remove materials through physical bombardment of the surface. Ion etching use an ion beam to remove materials by accelerating ion at the surface and is also known as ion milling. RIE introduces reactive chemicals that react with the surface materials into the plasma.

CNF Certificate of Completion

A few days after the course, I received my certificate of completion.

References

• CNF Technology & Characterization at the Nanoscale workbook, January 27-29, 2021.
• Process Integration by Christopher Alpha, January 29, 2021.