Atomic Layer Deposition for Encapsulation and Hermetic Sealing of RF and Power Electronics

Radio frequency (RF) and power electronics are vital to an array of industries, from telecom and consumer electronics to transportation and energy distribution. As energy diversification and the prevalence of high-speed electronics continue to grow, RF and power electronics are expected to reach a global market of $36.6 billion by 2027.1 Extreme environments such as high temperatures, ultra-violet radiation, oxygen, salinity and moisture are all threats that degrade and corrode active components causing early failure. Atomic layer deposition (ALD) has created substantial improvements to the reliability and performance of RF and power electronics. Using ALD as an encapsulation layer at the wafer level or as a final hermetic seal at the chip/module/PCB level has been shown to substantially improve electronic performance and lifetime.2,3 ALD layers enable longer lifetimes, higher performance and lower cost without adding the considerable mass gain and high temperature processing associated with conventional hermetic coatings.

ALD Improves RF and Power Electronics

The benefits of ALD layers:

• Hermetic sealing encapsulation layers with minimal thickness
• Pristine conformality in high-aspect ratio structures
• Ultra-low particle generation
• Low stress films
• Improved lifetime of ICs and PCBs
• Increased resistance to harsh environments
• No peeling or flaking of the hermetic seal at pressures >1200 PSI
• Negligible mass gain from coating at 100 nm

There are two major methods of using ALD in RF and power electronics:

1. Overcoat encapsulation layer at the wafer level

2. Hermetic sealing at the package level

ALD at the Wafer Level

One of the most important layers in post-gate processing for RF and power devices is the encapsulation layer to protect the device from environmental degradation. Thin encapsulation layers have found to decrease humidity permeation rates on moisture sensitive devices.4 In a study from April 2020, RF microelectronic mechanical devices (MEMs) were encapsulated with SiNx thin films for environmental protection on 5G devices and showed successful mechanical protection.5 Beyond environmental protection, encapsulation layers have also been found to improve RF device performance. One study showed encapsulated GaAs-based terahertz emitters increased the average power output fourfold by enhancing the conductivity and prevention oxidation at the semiconductor surface.6

Plasma enhanced chemical vapor deposition (PECVD) has historically been used for encapsulation deposition, however, the use of plasma damages delicate semiconductor surface states with energetic ions, chemical radicals and UV radiation. Presence of these surface states gives rise to hopping conduction, which lowers the breakdown voltage of the gate. The thermal nature of ALD does not require damaging plasma or high energy ions for deposition and helps maintain delicate surface electronic states and increases reliability of the circuit.2 ALD provides an electrically-low- leakage film, with no pinholes and a high dielectric constant at deposition temperatures down to 80oC. With these improvements in film quality, ALD has largely started to displace PECVD in post- gate processing as deposition rates have increased.10 Common ALD chemistries for this application, such as Si3N4, Al2O3, HfO2, TiO2 and ZnO can be deposited by ALD using commercial precursors commonly used for PECVD/CVD processes.2

Table 1: A 96 hour Highly Accelerated Stress Test (HAST) at 130oC and 85% relative humidity was used as a benchmark for the optimization of the ALD layers. The devices are biased at pinchoff during the test which is the maximum stress condition. The ALD FETs had a much lower failure rate than the original SiN FETs.8

ALD Encapsulation at the Package Level

Due to application challenges in extreme environments, RF and power electronics require robust hermetic sealing to maintain reliability over conventional integrated circuits. Electronics demanding this extra layer of protection frequently use plastic to encapsulate circuits for added protection which creates additional weight and cost to the final product.7 In certain aerospace applications requiring military standards (MIL-STD), ceramic encapsulation provides more protection than plastic packaging yet requires higher temperature processing and adds significant weight to the final product.

ALD provides the best of both fabrication methods by using ceramic coatings to create a hermetic seal while maintaining low deposition temperatures (down to 70°C in some cases), lower costs than traditional plastic hermetic packaging and a negligible mass gain to the final product.8,11 Using ALD at ~200nm thicknesses has shown more efficient hermetic seals with higher performance and lower temperature processing.11 In a recent study, 100nm ALD deposited films of SiO2/HfO2 onto RF chiplets showed no degradation after 100 days at 87oC in a saline environment during accelerated aging testing.12

At the printed circuit board (PCB) level, environmental factors can introduce failure modes when not protected with hermetic seals. Tin whiskers, thin electrically conducting structures that form on PCB assemblies, can grow up to 10mm long to create electrical shorts on different board components. Using ALD as thin as 10nm on PCB assemblies saw a significant decrease in tin whiskers, dramatically extending the lifetime of the product.13