The Science of Small

A Brief Introduction

Scanning Electron Microscopy

Components of an SEM

How it Works

Why This is Useful

Thin Film Deposition

  • Chemical Vapor Deposition is when a volatile fluid precursor produces a chemical change on a surface leaving a chemically deposited coating. The most common technique is Atomic Layer Deposition or ALD.
  • CVD is used to produce the highest-purity, highest-performance solid materials in the semiconductor industry today.
  • Physical Vapor Deposition refers to a wide range of technologies where a material is released from a source and deposited on a substrate using mechanical, electromechanical or thermodynamic processes.
  • The most common technique of Physical Vapor Deposition or PVD is Thermal Evaporation

Atomic Layer Deposition

  • First, while deposition is not exactly a single atomic layer per cycle, film thickness is well controlled and has excellent uniformity that can be achieved across the wafer.
  • Perhaps even more importantly, ALD creates layers that conform extremely well to the wafer topography, with identical film thicknesses deposited on the tops, sides, and bottoms of device features. This high conformality is a critical capability for high-aspect-ratio and 3D structures.
  • Lastly, surfaces created by ALD are atomically smooth, with well-controlled chemical composition.

Thermal Evaporation

Diagram of Thermal Evaporation
  • This process allows relatively high deposition rates, real time rate and thickness control, and (with suitable physical configuration) good evaporant stream directional control for processes.
  • Used most commonly for applications involving electrical contacts, by depositing such single metals as silver or aluminum.
  • More complex applications include the co-deposition of several components and can be achieved by carefully controlling the temperature of individual crucibles.
  • Thermal evaporation can be applied to deposit metallic contact layers for thin film devices such as OLEDs, solar cells and thin-film transistors.
  • In addition, this technique can be used to deposit thick indium (an element) layers for wafer bonding.



  • First is surface conditioning: Wafer is baked to remove any water molecules. Chemical is applied to help boost adhesion of the photoresist to the wafer surface. Wafer is cooled to room temperature.
  • Next is spin coating: Wafer is placed on a vacuum chuck which helps hold the wafer in place through suction. Photoresist is applied onto the wafer, and then the wafer accelerates in circles (typically at 3000RPM) until the desired resist thickness.
  • Then softbake: After photoresist is applied, a softbake (heated plate at 95 Celsius) is used to remove any residual solvents of the resist. Wafer is cooled to room temperature.
  • Finally alignment: One of the most critical steps. The mask (quartz or glass plate) with the desired pattern is applied on top of the wafer. This will cover certain areas of the photoresist, while leaving others exposed, thus creating a pattern. Each layer must be aligned properly and within specifications to the previous layer. A misalignment of one micron can destroy anything on the wafer.




  • Alignment — the pattern must be positioned accurately to the previous layer
  • Line Width or Critical Dimension — the pattern images are in focus and have the correct size
  • Defects — things that could affect the subsequent processes and eventually the operation of the devices


Summary/Key Points

  • Nanotech is a rising technology that is becoming more of a reality than science fiction, and it’s applications range from the medical industry to electronics.
  • Scanning electron microscopy makes use a concentrated electron beam in order to see images at much greater clarity and detail than your typical biology class light microscope. This allows for much easier visuals when working with nanoparticles
  • Thin Film Deposition is a nanofabrication process that allows you to add very thin layers of material onto a pre-existing wafer (usually silicone)
  • Atomic Layer Deposition uses gases and liquids to conform to the surface of the wafer to evenly spread the deposed material everywhere topographically
  • Thermal Deposition heats up a solid (usually metals) and then deposes the liquid evenly across the wafer until it then hardens
  • Photolithography is another nanofabrication process that allows you to etch a desired pattern into the wafer, as opposed to covering the whole thing. It is comprised of 3 steps: Coat, Expose, and Develop



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