Atomic layer deposition (ALD) is a thin-film deposition method that continues to enable continuous advances in semiconductor device fabrication. Essentially, it involves exposing substrates sequentially to at least two different vapor phase atmospheres in which self-limiting reactions take place on the surface: the first one reacts to deposit a controlled amount of the desired compound as a monolayer, and the second one reacts to modify that deposit and re-create a surface that will again be reactive with the first atmosphere.
Since this process deposits a fixed amount in each of these cycles, simply by choosing the number of these cycles the substrate is exposed to, the thickness of the deposited film can be controlled reproducibly with atomic-scale precision. The self-limiting nature of these reaction steps allows these films to have exceptional uniformity across the substrate on a macro scale, and also along its microscopic topology.
ALD offers several unique advantages that make it a highly valuable technology:
- Precise Thickness Control: ALD allows for the deposition of materials at the atomic level, ensuring exceptional uniformity and precise thickness control. This level of control is critical for creating ultra-thin films with consistent properties, making it suitable for a wide range of applications in electronics and photonics.
- Conformal Coatings on Complex 3D Surfaces: ALD has the exceptional ability to produce uniform and conformal coatings, even on complex 3D surfaces. This is particularly important for coating high-aspect-ratio features, such as trenches, holes, and intricate 3D structures used in advanced semiconductor devices like Gate-All-Around (GAA) transistors.
- Versatility: ALD is versatile in its application across a broad spectrum of materials, from high-k dielectrics like hafnium oxide to various metals and silicon-containing dielectrics. This versatility makes ALD well-suited for addressing diverse film requirements in semiconductor and electronic device manufacturing.
- Enhanced Film Performance: Through careful engineering of surface chemistry, ALD enables the deposition of films with improved physical and electrical performance. By designing thermally stable precursors that retain self-limiting surface reactivity at higher temperatures, ALD can produce films with superior properties, contributing to the overall advancement of semiconductor technologies.
The ALD technique has already been used for more than a decade to make both memory and logic chips. The thickness control and uniformity properties of ALD make this technique increasingly important due to the relentless downscaling of the dimensions of these devices as Moore’s Law continues to advance. For one, ALD has been essential to enabling the patterning of structures with optical lithography having repeating dimensions smaller than allowed by optical resolution by a process known as self-aligned multiple patterning. Also, the smaller dimension requires much more precise film thickness control to ensure that all of the billions of circuits in the device have identical electrical properties.
As simple scaling begins to hit practical limits, however, new architectures require that the active structures change orientation and move into the third dimension. For example, logic transistors have already shifted from planar devices to vertical fin structures (so-called FinFETs). Shortly, the active structures will again transform with the fins turned parallel to the surface in the Nanosheet architecture. Now, the deposition of thin films to fully surround such structures must be completely independent of line-of-sight – a task perfectly suited for ALD. Similarly, the two main classes of computer storage memory: NAND flash and DRAM are and will be stacked in a three-dimensional array which requires thin film deposition of films on the extremely high surface area horizontal features.
The challenge of moving into the third dimension cannot be understated. As device structures shrink and become more complex, the need for precision becomes even more critical. With the advent of 3D architectures, such as vertical NAND and 3D DRAM, the industry is moving beyond traditional line-of-sight deposition methods. These new architectures necessitate the deposition of conformal coatings on hidden surfaces, which ALD can achieve due to its self-limiting and surface-controlled reaction mechanism. This capability is paramount as it allows for the creation of uniform films on all surfaces of the complex 3D structures, ensuring reliable device performance.
Central to the success of ALD are the chemical precursors, which have seen significant advancement with the introduction of, for example: aminosilanes, metal amidinates, and alkylamides. For instance, the precursor can be designed to maintain self-limiting reactions at a higher deposition temperature and thereby improve the electrical and mechanical properties of the deposited film. EMD Electronics’ extensive ALD precursor portfolio (See the image of the periodic table which highlights the broad range of elements currently being deposited).
Also, we can rapidly and effectively develop the new precursors to meet the diverse needs of these new architectures by having the ability to perform such thin-film deposition processes in industrially relevant equipment and by designing proprietary device test vehicles to understand how the precursor and process conditions combine to meet the electrical and physical demands of semiconductor devices. Similarly, AI-enhanced molecular modeling enables us to rapidly develop new solutions in line with semiconductor device manufacturer’s technology roadmaps.
Industry Trends and Future Prospects
The pursuit of faster and more scalable deposition methods is an ongoing challenge, reflecting the industry’s ambition to keep pace with the growing demand for advanced materials in mass-production settings. Future trends are poised to not only enhance the ALD technique but potentially set new benchmarks in the engineering of materials at the atomic scale in other areas, including etching and surface treatments, heralding an exciting era for technology manufacturers and their ever-more complex integration schemes.
One emerging area is Area-Selective Deposition: by carefully managing the interaction between the substrate and the precursors through surface modification and chemistry selection, the process allows for highly targeted film growth on specific areas of a substrate while leaving other areas unaffected. Such a process can enable self-aligned features as well as eliminate costly lithography steps.
This is an area we have been increasingly addressing to meet market needs as well as to continue advancing technology for both surface inhibitors and ALD deposition materials.
by Ron Pearlstein, Research & Technology Lead, Semiconductor Solutions Group of EMD Electronics (EMD Electronics is the North American electronics business of Merck KGaA, Darmstadt, Germany)
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