Conformal coating continues to be a growing need in the microelectronics industry to protect circuitry from environmental extremes. In this article, we will study some of the thin film coating techniques, their advantages, and disadvantages. A brief discussion on equipment specification for ALD process implementation will be discussed.

Thin films are defined as coatings with a thickness of a few angstroms (Å) to a few micrometers (µm). Several materials may be deposited as thin films on passive substrates (glass or ceramic) or on active substrates such as silicon. The deposition process is divided into two broad types, physical or chemical (Figure 3-1).

Physical Deposition Techniques

Several different thin films materials are deposited by evaporation and sputtering deposition techniques. In several applications, sputtering is preferred over evaporation because of the wider choice of materials available, better adhesion to the substrate, and better step coverage. Evaporation is mainly a laboratory technique while sputtering is employed in laboratories and the industrial production of thin films.

Evaporation (PVD):

The physical vapor deposition (PVD) technique deposits thin films by condensing a vaporized material onto a variety of surfaces (e.g., semiconductor wafers). This uses high temperature vacuum evaporation rather than a chemical reaction. PVD is extensively used in the semiconductor industry as well as the tool and die making industry to provide a hard, wear resistant metal coating.

Sputtering:

Unlike evaporation, sputtering can deposit conductors or insulators onto a substrate without heating. During sputtering, an atom is knocked out of a target material by a stream of accelerated ions from an excited plasma. These sputtered atoms are deposited onto a substrate as a thin, uniform film.

Chemical Deposition Techniques

Chemical deposition uses a precursor fluid or gas to chemically react with a solid surface, forming a thin solid layer. Deposition occurs on every surface providing a conformal thin film.

Chemical Vapor Deposition (CVD):

CVD thin films are obtained by reacting dilute precursor gases with a heated substrate within a vacuum chamber. The resulting reaction deposits a thin solid coating, usually only a few microns thick. The process is slow and takes a few hours to deposit a few hundred microns of coated film. The coating produced is very pure, void free, and adheres well to the substrate. The CVD process is extensively used in the microelectronics industry to produce coatings of almost any metal or metal alloy, oxide, nitride, or intermetallic compound.

Atomic Layer Deposition (ALD):

The ALD process was developed more than 35 years ago but was not extensively used in the microelectronics industry due to its slower throughput. Atomic layer deposition is a subset of the chemical vapor deposition techniques where reaction precursors are alternately introduced into the reaction chamber, one precursor at a time, separated by a purge with an inert gas. Each precursor exposure deposits a thin monolayer until the entire surface is coated and the chemical reaction stops. Excess reactants are then purged and another reactant gas is introduced to build over the first layer. This process continues until the desired coating thickness is achieved. Since the precursor gases are not mixed (as in the case of the CVD process), the deposition thickness can be tightly controlled, providing a uniform and pin-hole free coating over any irregular surface.

With advancements in the semiconductor industry, designers continue to reduce the die footprint to lower cost, reduce operating power, and improve performance. Traditionally high yielding conformal coating processes like PVD and CVD, are pushing the manufacturing process envelop to yield reliable products. ALD has been the preferred choice for conformally coating dies with a high aspect ratio (ASICs and DRAM), obtaining coating thicknesses of tens of angstroms.

The ALD process is still traditionally used in the semiconductor industry to coat high dielectric constant materials or as an alternative to SiO2 on a silicon wafer. The microelectronics industry is also investigating the use of ALD thin film coatings for large solar panel glass and for microelectromechanical systems (MEMS)¹.

Parylene coatings are often used in the industry to protect the PWB from moisture and mechanical stresses. This is an expensive process and the coating is not truly hermetic. A current project at the EMPF is to evaluate the ALD process as a replacement for Parylene and hermetic packaging in naval applications where electronic components are exposed to extreme environmental conditions. While hermetic packaging is expensive, the presence of moisture and surface ions can cause electric shorting, degradation of circuitry, and eventual device failure. Standard manufacturing practice is to CVD coat the PWB with poly(p-xylylene) polymers (Parylene) or spray coat with a urethane conformal coating. But as shown in Figure 3-2, polymers (silicones, epoxies, fluorocarbons) are more permeable to moisture than glasses, ceramics, and metals, which are typically used for hermetic packaging. The EMPF is currently investigating using an ALD high alumina ceramic coating as an alternative to hermetic enclosures.

ALD Advantages and Limitations

Advantages

Atomic layer deposition provides an easy way to produce uniform, crystalline, high quality thin films over large areas and irregular surfaces. The thickness of the coating can be precisely controlled to a single atomic layer because of the sequential deposition reactions. It is also possible to layer different composition films with this process. Small changes in substrate temperature or precursor pressure does not affect the quality of the film making it easier to reproduce the process.³

With the wafer fabrication process advancing from 90 nm to as low as 32 nm thin wafers, the required diffusion barrier thickness has reduced to as low as 5 nm. The conventional CVD process can no longer meet the stringent film thickness requirement making ALD the preferred choice for deposition. The ALD alumina process can uniformly coat a substrate regardless of topography, whether there are deep trenches (Figure 3-3), or high aspect ratio components. Aspect ratio can be defied as the height (depth) to width ratio. While CVD and PVD processing works well when the aspect ratio is < 10:1, ALD processing guarantees pin-hole free, uniform coverage for aspect ratios as large as 60:1.

Limitations

ALD has some limitations keeping it from being more prevalent in production. Growing a film one atomic layer at a time requires a large number of cycles and time to obtain a sufficiently thick film. If the reaction chamber is not sufficiently filled with the reactant gas or purged before saturation occurs, the coating will be non-uniform. If the chamber is not fully desorbed when the new reactant is introduced, the purity of the deposition and well as the deposition thickness will vary.

Equipment Specification for ALD Process Implementation

Equipment design is critical for successful ALD process implementation. This is a clean process requiring equipment installation in a class 1000 clean room or better. While similar to CVD equipment, the ALD process requires the precursor lines be separated all the way from the source to the reaction chamber. This prevents precursor mixing during processing and avoids the possibility of any CVD reactions. A minimum of two separate precursor lines, vacuum line, air, plasma gas lines are required to operate ALD equipment.

Substrate temperature, precursor gas pressures, and reaction time are all critical to the ALD process. The reaction chamber should be large enough for substrates to maintain a uniform temperature, whether they are processed one at a time or as a batch process. Some equipment designs place the reaction chamber inside a vacuum chamber to assure uniform temperatures. Precursor lines should be monitored and regulated to control the pressure and volume entering the reaction chamber. The ability to completely purge excess precursor gas and introduce a second precursor is necessary for ALD to produce a uniform layer.

With advancements in technology, the need to reduce device footprints, and reduce costs, alternative manufacturing processes like ALD are being considered to hermetically secure the components and protect them from environmental hazards. More work is still needed to characterize thermal stability and RF performance of devices with ALD films.

For any other information regarding using atomic layer deposition as conformal coating on components and PWB, please contact the EMPF Helpline at 610.362.1320.

References

• Ritala, Mikko, and Jaakko Niinisto. "Chapter 4: Atomic Layer Deposition." Chemical Vapour Deposition Precursors, Processes and Applications. Cambridge: RSC, 2009. 158-206. Print.
• Ely, Kevin. "Issues in Hermetic Sealing of Medical Products." Medical Device & Diagnostic Industry Magazine. Jan. 2000. Web. http://www.mddionline.com/article/issues-hermetic-sealing-medical-products.
• Leskelä, Markku. "Industrial Applications of Atomic Layer Deposition (ALD)." 10th MIICS Conference, Mikkeli, Finland. Web. 18 Mar. 2010. http://www.miics.net/2010/material/Industrial%20Applications%20of%20Atomic%20Layer%20Deposition%20%5bALD%5d.pdf.