ALD Can Help Solve 5G RF Filter Challenges
The global push to roll out 5G mobile services is creating vast opportunities, but these bright prospects have not reduced the technical challenges facing manufacturers of critical componsents such as RF filters. Veeco explains how its ALD technology is making a difference for wireless device manufacturers. By Ganesh Sundaram, Ph.D., Vice President, Research & Engineering, Veeco
With the advent of 5G, RF filters are expected to do more for much less. On the one hand, they face the challenge of receiving and transmitting ever-larger amounts of data at increasingly faster speeds.
In the sub-6GHz range, this is further complicated by the allocation of relatively narrow slices of the frequency spectrum for mobile communications, often sandwiched between frequency bands allocated for a variety of applications, some of them critical. That puts drift, performance and power in the critical path. On the other hand, in terms of cost, RF filters are becoming commoditized.
Atomic layer deposition (ALD) can help
ALD produces ultrathin conformal films with atomic-level control. In terms of deposition methods, this technique is unmatched for combining film conformality, low temperature processing, stoichiometric control and inherent film quality.
While ALD has existed for over half a century, its low deposition rate made it less attractive for applications that required high throughput processing.
After a decade of advancements, ALD is now a fast and cost-effective technology suitable for a wide range of applications. Deployment got a big boost with high-K and in advanced DRAMs. Many technologists and researchers are replacing older deposition techniques such as evaporation, sputtering and chemical vapor deposition (CVD) with ALD to take advantage of its unique ability to produce conformal coating in and around 3D objects in a highly consistent manner. For RF filters, the ultrathin layers of dielectrics grown by ALD can cost-effectively address stability, reliability and power durability.
Why ALD for RF Now?
RF filter functionality within the handset can be varied, but in general, it allows signals of a certain frequency to pass through, whether they are receiving (downlink) frequencies - Rx, or transmission (uplink) frequencies - Tx. The key filter performance parameters are:
⊕Low loss of the desired signals in the passband of frequencies; and
⊕Sufficient attenuation of undesired interference in the stopband of frequencies.
Every antenna and new frequency requires a new filter. With 5G, the number of filters in a smartphone will double, reaching well over 100 per phone. The requisite RF filter parameters must be met over environmental and production variations. Up to 4G/LTE, designers were able to build allowances for sources of variation into the system.
But now that available bands in the lower frequencies for 5G are so closely spaced, the guard bands between useable frequencies are just a few megahertz, and the duplexer gap (the transition space between transmitted and received frequencies) is minimized.1 Improvements in reception also contribute to increases in data capacity. This is often realized through higher power requirements to minimize signal attenuation.
Figure 1. How Does ALD Work? Film growth by ALD is sequential by nature. The precursor materials are kept separate during the reaction, so the reaction cycle is controlled one atomic layer at a time until the desired film thickness is achieved. This differs from CVD, which introduces multiple precursor materials simultaneously.
New approaches to RF filter design are needed
Most smartphones now use acoustic filters. Consider surface acoustic wave (SAW) filters, which handle lower frequencies, and bulk acoustic wave (BAW) filters, which handle higher frequencies.
SAW filters are typically based on piezoelectric materials such as lithium niobate (LNO) or lithium tantalate (LTO). For BAW filters, aluminum nitride (AlN) is the piezoelectric material of choice. In both cases, the filters now require lower losses, higher-Q, far steeper frequency cutoff performance and a stronger signal-to-noise plus interference ratio (SINR) over a wide range of operating conditions.
Achieving steep cutoffs in RF filters demands thin films with improved etch-quality profile and uniformity. Thinner layers call for improved deposition, as well as pinhole-free, ultrathin passivation layers. Additionally, run-to-run repeatability is essential to meet high productivity at a low cost of ownership, as well as to meet volume requirements.
As the material sets for RF devices entail the use of complex piezoelectric materials such as LNO and LTO, ALD makes it possible to obtain high-stability filter performance through the application of encapsulation and barrier films. The conformal and dense nature of the ALD films provides excellent protection against environmental degradation, thereby ensuring stable operation of the devices.
Figure 2. ALD results in pinhole-free coatings that are perfectly uniform in thickness, even deep inside pores, trenches and cavities. These micrographs demonstrate the ability of ALD technology to provide extremely thin, highly conformal films.
Material Changes and Temperature
Drilling down further, we know that while many factors can affect acoustic filter frequency performance, a leading cause of frequency drift in the resonant frequency is temperature. This is because the stiffness of the piezoelectric material changes with temperature. Most of the piezoelectric materials in commercial use have a negative temperature coefficient of frequency (TCF). This means exposure to higher temperatures will result in a loss of stiffness, causing a drift to lower frequencies. Conversely, lower temperature exposure will increase the material's stiffness, leading to a drift to higher frequencies.2 So far, these temperature effects have been mitigated within the industry by integrating materials such as SiO2 (which possess a positive TCF) into the filter manufacturing process. This can offset the effects of temperature fluctuations on the negative TCF piezoelectric materials.3 But with 5G, that's no longer enough.
Optical studies have shown the link between the TCF and the refractive index of the dielectrics used for temperature compensation.4 In RF filters, ALD-grown dielectrics such as SiO2, Al2O3, TiO2, Ta2O5 with positive TCFs can be used:
- As nanolaminates;
- As multicomponent films;
- As doped films to investigate the use of advanced materials as temperature compensation layers.
These can be applied to both BAW and SAW devices. Along with the efforts to minimize frequency drift in RF filters, the ability to protect the device from environmental impact is essential to ensure the stability and overall filter reliability. Use of ALD films to provide protection from environmental effects and enhance device stability is well-documented.
Given the thermal sensitivity of piezoelectric materials such as LNO and LTO, the low temperature processes offered by ALD provide an excellent solution for negotiating the thermal budgets often associated with these materials. The use of ALD films can help to ensure that filter performance metrics - such as sharp cutoff characteristics and high Q-factors - are maintained and don't drift over time, causing interference with adjacent frequencies and signal degradation.
Enhanced Power Durability
Another area in which ALD can benefit RF filters is with power durability (the ability for a filter to work for extended periods under high power conditions). As filters evolve to address higher frequencies, durability becomes more challenging. Resonator areas shrink and layers become thinner; thus, the overall resonator volume decreases. Consequently, the power density in the device increases, which can increase the likelihood of failures due to thermal or fatigue issues and degrade the overall device reliability.5,6
The filter's power durability can be improved by adding an ultrathin buffer layer between the piezoelectric material and the metal electrodes. This is an application area in which the extremely thin, dense films created using ALD technology can be beneficial. Buffer layers can help change the texture of the electrode materials by changing grain structure morphology. This in turn helps suppress the migration of electrode material atoms, such as Al, under high power conditions.
Equipment and Process Solutions
When it comes to RF filters, the brittleness and susceptibility to thermal variations in materials such as LNO and LTO pose unique production challenges. Additionally, these piezoelectric materials are pyroelectric (they can generate temporary voltage when heated or cooled).
Therefore, ALD systems for RF filters must meet particular challenges, including:
- Wafer handling (to prevent breakage of the brittle materials);
- Throughput: preheat and process modules need to operate in parallel to address thermal variations and
- Scheduling and process consistency for high yield.
Veeco's Firebird™ is an example of an ALD system that is particularly suited to addressing the needs of the 5G RF filter process. It uses a unique handling architecture in combination with controlled environment load locks to realize breakage-free movement of wafers within the system. The system is highly configurable and can be composed of a combination of process modules and preheat modules corresponding to the required level of productivity. Additionally, multibatch operation employing Firebird's intelligent scheduler enables superior throughput and process consistency, resulting in high-performance RF devices. Veeco has perfected ALD as a manufacturing-grade technology by scaling up the process and implementing it into automation lines and cluster tools around the world. Manufacturers that have integrated its ALD systems have seen increased product quality and reliability as well as decreased operating costs and a greener footprint as compared to previous coating technologies.
Improvements in RF Filter technology are critical to the growth of 5G mobile communications. As the push to expand data transmission capacity increases, the challenges to producing filters that meet those specifications and needs also grow. Within that context, ALD technology can provide a robust, cost-effective solution to meet the ever-tighter filter performance specifications.
1. Phil Warder and David Schnaufer, “Temperature-Controlled Filter Technologies Solve Crowded Spectrum Challenges,” Microwave Journal, November 2014
2. Vaishali Upadhye and Sudhir Agashe, “Effect of Temperature and Pressure Variations on the Resonant Frequency of Piezoelectric Material,” Measurement and Control, September 2016, vol. 49(9), pp. 286-292
3. H. Nakamura et al. Conf. Proc.- 6th International Symposium on Acoustic Wave Devices for Future Mobile Communications, Chiba, Japan (2015)
4. Nishimura et al. Proceedings of Symposium on Ultrasonic Electronics, vol. 38 (2017)
5. “Radically Reducing the Size and Cost of Cellphone RF Filters to Fuel the Mobile Revolution,” Resonant Inc. June 2015
6. “Revolutionary BAW Filter Technology and Its Impact on 5G,” Qorvo, August 2020