Virtual NanoFabA Silicon NanoFabrication Trainer

Nick Reeder, Sinclair Community CollegeAndrew Sarangan, University of Dayton

Jamshid Moradmand, Sinclair Community College

Challenge: Providing Hands-on Silicon Nanofabrication Experience

• The facilities needed to do silicon nanofab are very expensive.

Solution: Virtual Nanofab

• Software that we’re developing to teach students about the steps involved in processing a silicon wafer.

• Please take a copy of the installation disc!• System Requirements:– Operating system: Windows XP or higher– Memory: 2 GB RAM– Hard drive: 300 MB of free space– If your computer does not have National Instruments

LabVIEW installed, you must install the free LabVIEW run-time engine, which is included on the installation disc.

Example: Fabricating a MOSFET• MOSFET = Metal-oxide-semiconductor field

effect transistor

MOSFET in Virtual NanoFab

The structure shown required about 25 steps.

User Operations• Thermal oxidation• Photolithography

• Spin coat• Mask• Expose• Develop

• Removing material • Wet etch• Dry etch

• Depositing layers of material• E-beam evaporation• Chemical Vapor Deposition (CVD)• Sputtering

• Ion implantation (“doping”)

Thermal Oxidation

• Grows a layer of silicon dioxide (SiO2) on the wafer surface.

• Key properties of SiO2:– Impervious to ion implantation.– Can be etched away by immersion in hydrofluoric

acid (HF), which does not etch silicon.

Thermal Oxidation in Virtual NanoFab

Photolithography• Steps in photolithography:– Spin-coat photoresist.– Create and place mask. Mask defines which

areas will be exposed to UV light and which areas will be shaded.

– Expose with UV light.– “Develop” the photoresist: UV-exposed areas are

removed, while shaded areas remain.

Photolithography in Virtual NanoFab1. Before exposing:2. After exposing (but before developing):3. After developing:

Exposure with Uneven Layer Thicknesses

Note that resist above silicon is more fully exposed than resist above aluminum.

Removing material

• Methods of removing material – Wet etching• Low-tech• Immerse wafer in a bath of

liquid acid or solvent

– Dry etching• High-tech• Expose wafer to plasma beam

Etching in Virtual NanoFab

SiO2 (blue) after wet etch with hydrofluoric acid: note tapered sidewalls and undercut of photoresist (pink).

SiO2 after dry etch with CF4 plasma: note vertical sidewalls.

Depositing Layers

• Methods of depositing materials– Electron-beam evaporation– Chemical vapor deposition (CVD)– Sputtering

Deposition in Virtual NanoFab

Evaporated titanium (gray): accumulates only on horizontal surfaces.

Chemical-vapor-deposited titanium: adheres to vertical surfaces as well as horizontal.

Ion Implantation

• Modifies the electrical characteristics of the silicon wafer: key to the operation of semiconductor devices such as diodes and transistors.

• Implanting boron results in “p-type” doping.

• Implanting phosphorus results in “n-type” doping.

Ion Implantation in Virtual NanoFab

Other Features• Maintains history of user operations.• “Reference & Videos” page provides chapters

explaining theory, along with videos of operations being performed in the lab.