The Science of Small
What seemed like science fiction, is actually a reality… Welcome to the world of nanotechnology.
A Brief Introduction
I’m sure you’ve heard this term at least once in your life wondering what it could possibly be. To start, the word “technology” is kind of misleading considering today’s views on this word.
We usually assume “tech” has to do with a computer chip, something electronic, or even code. But, that’s far from the truth, well kind of. Technology is really just the application of scientific knowledge for practical purposes.
So that leads us to the common definition of nanotechnology: Any object that is studied or fabricated with at least one dimension that is less than 100nm in size. Essentially, as long as you are working with nanosized (one-billionth of a meter) objects, it's considered nanotechnology.
Hopefully you now have a better idea as to what nanotech really is. If you do, you would understand that working with particles, which are 90,000 times smaller than a human hair, seems nearly impossible. While that may have been the case in the “ancient times” (1970s), society has come a long way since then to create many tools and techniques to work with these particles.
This article aims to explain the benefits and uses of many nanofabrication tools and techniques and how they play a role in the field of nanotechnology. On top of that, I want to explain to you why each tool/technique matters and its relevance in today’s nanotech world. Note, whenever I mention the word “wafer” I am referring to a thin piece of semiconductor material (usually silicone), not the thing you can eat.
Scanning Electron Microscopy
In short, it's the process of using a scanning electron microscope (SEM). That's it.
This tool uses a focused electron beam over a surface to create an image. The electrons in the beam interact with the sample, producing various signals that can be used to obtain information about the surface topography and composition. Yes, not your high school biology microscope.
How it Works
Electrons are produced at the top of the column, accelerated down and passed through a combination of lenses and apertures to produce a focused beam of electrons which hits the surface of the sample. The sample is mounted on a stage in the chamber area and, unless the microscope is designed to operate at low vacuums, both the column and the chamber are evacuated by a combination of pumps.
The position of the electron beam on the sample is controlled by scan coils situated above the objective lens. These coils allow the beam to be scanned over the surface of the sample. This beam scanning process, as the name of the microscope suggests, enables information about a defined area on the sample to be collected. As a result of the electron-sample interaction, a number of signals are produced. These signals are then detected by appropriate detectors, thus creating an image.
Hopefully that makes sense. Microscopy is weird.
Why This is Useful
When given sufficient light, the human eye can distinguish two points that are 0.2 mm apart, without the aid of any additional lenses. A lens or an assembly of lenses (a microscope) can be used to magnify this distance and enable the eye to see points even closer together than 0.2 mm.
A modern light microscope has a maximum magnification of about 1000x. The resolving power of the microscope was not only limited by the number and quality of the lenses but also by the wavelength of the light used for illumination. White light has wavelengths from 400 to 700 nanometers (nm). The average wavelength is 550 nm which results in a theoretical limit of resolution (not visibility) of the light microscope in white light of about 200–250 nm.
So, the electron microscope was developed when the wavelength became the limiting factor in light microscopes. Electrons have much shorter wavelengths, enabling better resolution. This allows scientists to enhance images, and see nanoparticles with much greater clarity (see below).
Thin Film Deposition
Now that you know how to visualize nanoparticles (what I explained previously is the most common method, but there are multiple), let’s dive into some techniques of working with nanoparticles.
Thin Film Deposition is the technology of applying a very thin film of material — between a few nanometers to about 100 micrometers, or the thickness of a few atoms — onto a “substrate” surface to be coated, or onto a previously deposited coating to form layers. These processes are at the heart of today’s semiconductor industry, solar panels, CDs, disk drives, and optical devices industries.
Thin Film Deposition is usually divided into two broad categories — Chemical Deposition and Physical Vapor 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
The secret to gaining improved control was to split the deposition process into half-reactions, each of which can be well controlled.
The ALD process starts by flooding the reaction chamber with a precursor that coats the exposed surface of the wafer. This process is called self-limiting because the precursor can only hold onto exposed areas; once all those are covered, the process stops. A second gas is then introduced and reacts with the precursor to form the desired material. This second step is also self-limiting: once the available precursor sites are used up, the reaction stops. The two steps are repeated until the desired film thickness is obtained.
There are a couple of ways to divide the next steps. In one technique known as spatial ALD, the wafer is moved between different locations and exposed to a different precursor at each. Another approach is to hold the wafer in one place and alternately introduce/remove precursors into the chamber. Known as temporal ALD, this method enables the wafer to be processed in a more symmetric environment, improving process results such as better critical dimension range control.
So, why does this matter? Well ALD offers a number of advantages, all of which arise from its self-limiting, sequential reactions.
- 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.
Now for the other method. Thermal Evaporation involves heating a solid material that will be used to coat a substrate inside a high vacuum chamber until it starts to boil and evaporates producing vapor pressure.
Inside the vacuum deposition chamber, even a relatively low vapor pressure is sufficient to raise a vapor cloud. This evaporated material now constitutes a vapor stream which the vacuum allows to travel without reacting or scattering against other atoms. It traverses the chamber and hits the substrate, sticking to it as a coating or thin film (see diagram).
Why does this matter?
- 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.
It’s a lot, but if you wanted to take away something it’s that thin film deposition allows you to coat a surface with material, whether it be a solid or a liquid that then hardens. This is especially valuable in the electronics industry when producing semiconductors.
Moving on, we can now talk about a more refined version of deposition, lithography. This a process used in nanofabrication to pattern parts on a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a photosensitive chemical photoresist on the substrate.
Rather than just add a layer of material onto the whole substrate (deposition), photolithography aims to etch a specific pattern into the substrate, allowing more precision and variability of patterns for nanofabrication.
There are 3 main steps when it comes to photolithography: coat, expose, and develop. Each step has its own sub-steps which I will get into later on.
This is arguably the most important step when it comes to photolithography. This step is broken down into multiple parts, as each has a specific use that directly affects the outcome of the process. Side note, photoresist is basically a mixture of organic compounds in a solvent solution which when applied makes the exposed region more soluble (pos) or harder (neg), depending on the charge of the resist.
- 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.
The next part of the process is to expose the wafer to UV rays from a light source through the mask onto the resist. This causes a chemical reaction, but only in the areas that the mask doesn’t cover.
The final step in the photolithography process is to dissolve the photoresist on the wafer through a chemical developer. With positive resist, the exposed resist is dissolved while the unexposed resist remains on the wafer. By unexposed/exposed, I mean from the UV rays. With negative resist, the opposite happens.
To end it all off, you use a hardbake in order to harden the remaining photoresist by putting it on a hot plate with temperatures close to 130 Celsius. Wafer is then cooled to room temperature.
Though it’s technically not part of the photolithography process, it is an important step when working with nanoparticles. This is when you would use an SEM (what we talked about earlier) in order to see 3 critical parameters:
- 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
To keep up with the common theme, we end with the “why do I care” question. Photolithography is very important in the nanofabrication process as it allows you to embed a specific pattern into the material of your choice, or a wafer. Photolithography is commonly used to produce computer chips. This process allows hundreds of chips to be simultaneously built on a single silicon wafer.
Not only that, but in a more broader sense photolithography allows you to transfer geometric patterns to the wafer. Geometric shapes and patterns on a semiconductor make up the complex structures that allow the dopants, electrical properties and wires to complete a circuit and fulfill a technological purpose.
Wow, that's a lot. Hopefully you were able to take something away from this. Also, I’ve only talked about some of the most common nanofabrication processes, however there are many different techniques/tools when it comes to nanotech in general. If you got lost, here’s a quick summary section to catch you up to speed:
- 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
Thanks for reading! Feel free to leave a comment or email me (firstname.lastname@example.org) with any comments, questions, or opportunities.