Superficial science – using ALD and MLD to enhance the properties of solid surfaces

Published November 28, 2022

Jesse Kalliomäki

Jesse Kalliomäki – MSc, Chemistry

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Application Scientist, Picosun

Our everyday life revolves around solid objects. Sure, gasses, liquids, and plasmas have their classic hits like oxygen, water, and the sun, respectively. They undeniably keep us alive – yet they don’t truly play an active part in our lives, as we don’t interact with them on any conscious level. When you are thirsty, you are actually searching for your bottle to help you quench the thirst. When the sun is up, your thoughts are on your recently broken sunglasses or weighing the prospect of getting a nice tan. Solids are how we interact with the world. Solids are tangible and easily interactable. 

How thin film surfaces transform the functionalities of solids

In most ordinary cases, our interaction with solids takes place on a special region called the surface. The type of surface a material has determines how it interacts with other materials it comes into contact with. The surface determines how it feels to touch the object, how easily it gets dirty when forgotten outside in the rain, and how likely it is to attract magpies. Surfaces are also special for having widely different properties from the bulk of the material. 

A boat made of aluminum is a good example of this: aluminum is easily corroded by water, but a thin film of oxidized corrosion products forms naturally over its bulk to effectively stop further corrosion when in contact with water. This film is only a few nanometers thick, which illustrates how a tiny fraction of a solid’s mass can alter the way it interacts with its surroundings without changing its shape or size in any meaningful way. In addition to forming naturally, thin films can be introduced – or deposited – on solids artificially to induce similar changes in their properties.

The working principle of atomic layer deposition

Today’s topic is arguably the most advanced thin film deposition method devised by humanity: atomic layer deposition, or ALD. With ALD, we can grow a thin film one atom at a time. The technique relies on chemical reactions between a surface and a precursor chemical introduced into a deposition chamber. What gives ALD an advantage over other thin film deposition methods is that the precursors are selected so that they never react with themselves, but do so very rapidly with the surface we wish to grow the film on. 

Provided enough precursor is present, all reactive sites on the surface are quickly consumed, creating a new surface unreactive toward the precursor. Once the excess precursor has been removed, a second precursor is introduced to react with the new surface, again modifying it and adding an additional atom layer. Given the first precursor can react with this new surface, we can keep this cyclic process going until the desired film thickness is reached. This allows for precise control over film thickness down to a tenth of a nanometer. Due to the self-saturating chemical reactions, the original shape and texture of the surface are preserved. 

Notable applications of ALD

As atomic layer deposition can be applied to virtually any solid, its potential applications are nearly endless. If you find something you can touch, it has a surface and ALD can be used to alter the properties of that surface to your liking. 

Perhaps most notably, ALD is used extensively in the microelectronics industry. No microchip or semiconductor device that powers our modern lifestyle is made without fabrication steps involving ALD films, making the technique ubiquitous in our everyday life. ALD is likely to become even more prevalent, since the continuous miniaturization of electronics demands more and more precision in manufacturing, down to the atomic level, which is exactly where ALD shines. 

Other common and emerging applications of ALD include the following: 

  • Corrosion protection of large and complex-shaped 3D objects

  • Protective films on implantable medical devices to make them more durable

  • Shiny or colorful coating on objects like jewelry to enhance their aesthetic appeal

ALD and MLD in thin film encapsulation of organic electronics 

Researchers come up with new exciting applications constantly, and I count myself among the group that is looking for new ways to apply ALD and strengthen its existing applications. My work at Picosun concerns the thin film encapsulation of organic electronics, where impermeable ALD films are used to protect sensitive components that would otherwise quickly oxidize in ambient air. The often brittle nature of ceramic or metallic ALD films poses a challenge to this application, however, as unlocking the full potential of organic electronics means enabling the devices themselves to be flexible, which requires the protective films to be flexible as well. 

To counter this issue, I use ALD’s sister technique, molecular layer deposition (MLD). MLD utilizes the same cyclic exposure principle, but instead of single atoms, full molecules are deposited on the surface. Often these molecules are organic in nature, adding the versatility of organic chemistry to our tool belt. The repertoire of tricks is expanded with the addition, making it possible to engineer layers with polymer-like elasticity between protective ALD films. 

Facilitating the in-depth analysis of thin film surfaces

The added complexity of MLD means that the chemical and physical properties of vanishingly thin layers need to be studied in detail, demanding the use of highly sensitive and specialized equipment. It is not feasible for any single lab to have all the necessary devices, as they often cost hundreds of thousands up-front and require extensive training to operate and maintain. 

Tracking down qualified labs that provide these services on the side of one’s regular work is a lot to ask, and the reliability of the results can often be unknown. Measurlabs provides this service to us ensuring that we are connected to accredited labs that offer high-quality analyses from HR-XPS to XRR/XRD and TEM/EDX.

The process is simple and requires me to notify the experts at Measurlabs that I have a new set of samples, specify the type of analysis I need, and bring the samples to them. The rest is taken care of, and a reasonable while later I have the results delivered in a uniform format with as much or little detail as I desire. Measurlabs are also able to offer their services for the interpretation of these results, as although I argue solids are the coolest state of materials, thin films and solid surfaces are not the easiest to analyze. In fact, it is challenging to a point of frustration. Tackling the issues related to their analysis is a major part of my work as an ALD application scientist, but having a service provider that can connect me to accredited laboratories, I can now focus more on studying the thin films themselves and writing about them to strangers on the internet.

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