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Optimizing atomic layer deposition using flow and pressure control
Atomic layer deposition (ALD) is a subset of chemical vapor deposition (CVD) which uses cyclical deposition cycles. Unlike other types of CVD in which reactions are continuous, ALD reactions take place in separate steps so that deposition layers can be controlled more precisely, even to single-atomic layer thicknesses. Whereas other CVD techniques determine layer thickness using growth rates and process times, ALD controls layer thickness by the total number of deposition cycles.
Spatial and temporal ALD are the two main types of ALD. In temporal ALD, a carrier gas and two (or more) gas reactants are used in separate stages one after another. In spatial ALD, the substrate is moved into separate growing chambers so that individual reactant gases never touch one another. Some ALD systems are considered thermal and others are plasma enhanced. Thermal systems require higher operating temperatures whereas plasma-enhanced ALD is more ideal for lower temperature applications since the reactants are plasma-based. ALD is used in manufacturing for solar cells, nanostructures, microelectronic devices, biomedical devices, corrosion resistant coatings, metal films, DRAM capacitors, permeation barriers, and much more.
ALD applications require precise pressure control and a consistent and repeatable flow of carrier and reactant gases for optimal operation. In the following, we will discuss how Alicat’s mass flow controllers and pressure controllers provide consistent, accurate, and fast results during various ALD applications.
Solar cells
Since ALD uses conformal deposition, in which all surfaces are evenly coated, it is particularly suitable to solar cell construction. ALD is used in the manufacturing of various types of solar cells, including industrial silicon, thin film, organic, and quantum dot types. ALD is specifically important in the creation of surface passivation layers. Use of ALD began in Si solar cell construction using Al2O3, which forms a surface passivation layer that significantly improves the efficiency of Si solar cells.
In order for ALD to properly operate, vacuum pressures must be carefully maintained at operating conditions. Alicat’s PC-Series of pressure controllers are available with specially designed features for back pressure regulation of ALD systems. With a leak rate of just 10e-9 atm-cc/sec of He with 30 ms control response times, these pressure controllers prevent improper mixing and contamination of ALD deposition chambers.
Just as in other CVD applications, Alicat’s MC-Series of mass flow controllers provide a greater accuracy for gas mixing, limiting contamination from atmospheric gases which may inhibit proper ALD operation in which pure reactant and carrier gases are required.
Biomedical devices
ALD is used for deposition of various thin films onto biomedical devices such as bioelectronics, implants, biosensors, drug delivery devices, tissue engineering scaffolds, and bioassay devices. Due to recent technological advances in ALD such as lower temperature ALD, a greater amount of organic substrates are now able to be used in various biomedical applications.
Alicat’s MCV-Series of vacuum gas mass flow controllers include a pneumatically actuated, positive shutoff valve to ensure no leak through and have NIST-traceable accuracy up to ±0.5% of reading or ±0.1% of full scale for 98+ gases and 20 custom-defined gas mixtures.
Semiconductors
Another application in which ALD is used is semiconductor manufacturing. Moore’s law explains how transistors continue to decrease in size such that the speed and capability of computers can roughly be expected to double every two years. ALD technology development is important to maintaining Moore’s law. Currently, semiconductor manufacturing uses ALD for tasks such as high-k dielectrics and decreasing gate oxide thicknesses. Looking ahead, ALD will be important for lower dimensionality semiconductors using sheet, tube, or wire based structures such as graphene, WSe2, carbon nanotubes, and semiconductor nanowires which use conformal high-k gate oxide coatings to minimize disrupting low dimensional material properties. Therefore, with continuous research and scaling, ALD is expected to become more useful for the semiconductor industry over time.
Just as in previously mentioned ALD applications, Alicat’s MCV-Series of vacuum gas mass flow controllers and PC-Series of pressure controllers help to improve semiconductor manufacturing repeatability and control. Controllers are available with minimal sealing surfaces and 0.25 VCRM fittings to minimize leak points over time.
Metal films
Another industry where ALD is widely used is in the creation of various metal films, such as those used on tools, microelectronics, barriers, and packaging. For example, potato chip bags are often finished using ALD with a metallized film so that they can achieve proper sunlight shielding and barrier performance during storage.
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Whether your process requires precision, repeatability, accuracy, customization, or automation, Alicat’s mass flow controllers and pressure controllers provide the latest tech, most unique features, and fast results for ALD applications.