Atomic layer deposition (ALD) is a method for building ultra-thin films one atomic layer at a time. Each cycle deposits a layer roughly 0.3 to 1.9 angstroms thick (for perspective, a human hair is about 500,000 angstroms wide), giving engineers control over coatings at a scale no other technique can match. It’s the reason modern computer chips, batteries, and medical implants can pack so much performance into impossibly small spaces.
How ALD Works
ALD builds films through a repeating four-step cycle. In the first step, a chemical precursor gas is introduced into a chamber, where it reacts with and bonds to the surface of whatever material you’re coating. Crucially, this reaction is “self-limiting”: once every available spot on the surface has reacted, the process stops on its own, no matter how much extra gas is in the chamber. That built-in stopping point is what gives ALD its precision.
In the second step, an inert gas (typically nitrogen or argon) purges the chamber, sweeping away any leftover precursor and byproducts. Then a second reactive gas, called the co-reactant, is introduced. It reacts with the layer left behind by the first precursor, completing the chemical reaction and forming the desired material. A final purge clears the chamber again, and the cycle repeats.
Each full cycle produces a single atomic layer of material. To build a thicker film, you simply run more cycles. Need a coating 10 nanometers thick? Run a few hundred cycles. Need 50 nanometers? Run more. This gives engineers digital-like control: the thickness of the final film is determined by counting cycles rather than estimating deposition time or gas flow.
What ALD Can Deposit
The most common ALD material is aluminum oxide, used extensively in semiconductor manufacturing and as a protective coating. Hafnium oxide is another workhorse, prized for its electrical insulating properties in transistor gates. Beyond those two, ALD systems routinely deposit titanium oxide, silicon oxide, zinc oxide, zirconium oxide, and several metal nitrides like titanium nitride and hafnium nitride. Platinum films can also be grown by ALD for catalytic and electrode applications.
The choice of precursor chemicals determines what ends up on the surface. For aluminum oxide, the standard precursor is trimethylaluminum paired with water vapor as the co-reactant. Different precursor and co-reactant combinations unlock different materials, and the library of available ALD recipes continues to expand.
How ALD Compares to Chemical Vapor Deposition
Chemical vapor deposition (CVD) is the older, faster cousin. Both techniques use gas-phase chemicals to coat surfaces, but they differ in a few important ways. CVD runs both reactive gases simultaneously, which is faster but less controlled. ALD separates them in time, which is slower but far more precise.
Temperature is one clear difference. CVD processes typically require 600 to 1,000°C, while ALD operates at a much gentler 200 to 350°C. That lower temperature range means ALD can coat heat-sensitive materials like plastics and biological substrates without damaging them.
Uniformity is another. ALD films deviate less than 1% in thickness across a surface, compared to 3 to 5% for CVD. And when it comes to coating complex, three-dimensional shapes (deep trenches, tiny holes, textured surfaces), ALD achieves greater than 95% step coverage versus 70 to 80% for CVD. Samsung’s research on memory chip capacitors found ALD delivers 30% better step coverage in those structures, though CVD still offers two to three times faster deposition rates for certain materials. In practice, manufacturers pick the technique that fits the job: CVD for speed on simpler geometries, ALD for precision on complex ones.
Semiconductor Manufacturing
ALD became indispensable to chipmakers when transistors shrank below about 45 nanometers. At that scale, the insulating layer between a transistor’s gate electrode and its channel needed to be just a few nanometers thick and perfectly uniform. Traditional silicon oxide insulators were too leaky at those dimensions, so the industry switched to “high-k” dielectrics, primarily hafnium oxide, deposited by ALD. That transition is now standard across the industry.
Beyond gate insulators, ALD deposits barrier layers that prevent copper wiring from diffusing into surrounding silicon, and it coats the deep, narrow capacitor structures in DRAM memory chips. As chip architectures continue to stack layers vertically and shrink feature sizes, ALD’s ability to coat every surface of a complex three-dimensional structure with atomic precision becomes even more critical.
Batteries and Energy Storage
Lithium-ion battery electrodes degrade over time because the cathode material reacts with the liquid electrolyte, releasing oxygen and dissolving metal ions into the surrounding solution. ALD coatings act as a protective barrier, preventing those unwanted chemical reactions while still allowing lithium ions to pass through.
Recent work on lithium-rich layered oxide cathodes (a promising high-capacity cathode chemistry) demonstrated this clearly. Electrodes coated with an ALD bilayer of titanium oxide and aluminum oxide retained about 90.4% of their capacity after 100 charge-discharge cycles, with reduced voltage decay and lower internal resistance compared to uncoated electrodes. The coatings block oxygen release and transition-metal dissolution, two of the main mechanisms that cause these cathodes to fade over time.
Medical Devices
ALD’s low processing temperatures and ability to coat complex shapes without defects make it attractive for medical applications. Titanium oxide deposited by ALD is one of the most promising materials in this space because it can be either biomimetic (encouraging cell growth) or bioinert (invisible to the body’s immune response), depending on how it’s applied.
The technique enables perfectly hermetic nanoscale encapsulation of even very small, topologically complicated surfaces. For implantable electronics, that means protective barriers thin enough not to add bulk but dense enough to keep body fluids from corroding sensitive components. A European pilot-line project coordinated by Philips has explored ALD coatings for micro-fabricated medical devices, aiming to bring this capability into routine clinical production.
Speed and Cost Limitations
ALD’s precision comes at the expense of speed. Traditional ALD systems grow films at less than 0.5 nanometers per minute, and precursor usage efficiency is typically below 20%, meaning most of the expensive chemical precursor is purged away without reacting. That combination of slow throughput and wasted material has kept ALD costs high and limited its adoption in applications where thick films or large-area coatings are needed.
A newer approach called spatial ALD addresses the throughput problem by separating the two half-reactions in space rather than time. Instead of alternating gas pulses in a single chamber, the substrate moves continuously through zones dedicated to each precursor, separated by inert gas curtains. SoLayTec, a company making equipment for solar cell manufacturing, has developed a spatial ALD reactor that deposits aluminum oxide passivation layers at an effective rate of 0.45 nanometers per second, orders of magnitude faster than conventional ALD. This makes high-volume applications like solar panel coatings economically viable.
A Growing Market
The global ALD equipment market is valued at roughly $3.18 billion in 2025 and is projected to reach $3.68 billion in 2026, growing at a compound annual rate of about 12.9% through 2035, when it could approach $10.7 billion. That growth tracks directly with the semiconductor industry’s push toward smaller, more complex chip architectures, along with expanding use in batteries, solar cells, and medical devices. As spatial ALD systems continue to improve throughput, the technique is likely to move into applications that were previously too cost-sensitive to consider it.