New RF Passive Discrete and Integrated Technologies Combining Ultra-Stable Performance and Smallest Size
By Chris Reynolds, Technical Marketing Manager, AVX Corporation

Radio continues to be one of the most powerful technology drivers in microelectronics, with many evolutionary changes taking place in the last few years. Applications, both consumer and mission critical, require solutions that are smaller, increase functionality and reliability, and improve signal clarity, all the time attaining lower power consumption. The trend is to higher frequencies to enable the wider bandwidth required for these applications. These higher frequency systems in turn drive to smaller physical layout because the shorter wavelengths can be accommodated in much smaller packages.

Traditional radio designs are necessarily based on discrete passive components designed for high frequency performance (typically NP0 ceramic, porcelain or glass). While these are now achieving lower ESR, higher power density and smaller size, new technologies have evolved that go beyond these solutions in performance, stability and integration.

The present paper discusses the evolution of discrete thin film components based on photolithography and PECVD (Plasma Enhanced Chemical Vapor Deposition) processing, which combines the benefits of high conductivity conductors with the use of highly stable dielectrics (e.g. SiO2).

This technology in turn has led to the next level of passive integration – PMC (Passive Micro Circuits). The key advantages of PMC is that it retains the minimum line width capability and line width precision of thin film, but increases the maximum stacking layer capability. This can be used for increased capacitor and resistor density per mm2 and added turn capability in inductor elements. This paper discusses the advantages of tight tolerance passive integration over the wider scale LTCC options for medium volume applications.

Introduction to “Traditional” RF Components and Applications
Traditional radio designs based on discrete passive components use low loss high frequency dielectrics (NP0 ceramic, porcelain or glass), and require precious metal electrode systems which can withstand the high firing temperatures associated with ceramic material processing while maintaining good conductivity.

This technology has continued to develop for lower ESR, higher power density and smaller size, based on class 1 porcelain dielectric with NP0 (C0G or MIL BP) temperature characteristics.

NP0 ceramics offer one of the most stable capacitor dielectrics available. Capacitance change with temperature is 0 ±30ppm/°C, which is less than ±0.3% Δ C from -55°C to +125°C. Capacitance drift or hysteresis for NP0 is negligible at less than ±0.05%. Typical capacitance change with life is less than ±0.1% and shows no aging characteristics.

NP0 formulation usually has a “Q” in excess of 1000 and shows little change in capacitance or “Q” with frequency. Their dielectric absorption is typically less than 0.6% (and for this reason they can often be used as a Mica replacement).

Typical “workhorse” examples are BP dielectric CDR 31-35 (MIL-PRF-55681) which have SRFs ranging from 10 MHz to 4.2 GHz and are available down to 0805 size. While 0603 size is available in commercial (or SCD) versions, these are now nearing the limit of downsizing for microcircuit applications.

Larger versions are available, such as HQ series, which use ULF (Ultra Low Fire) dielectric with near pure silver electrodes for superior conductivity. These are designed to minimize ESR and maximize Q to improve the power dissipation performance in high power RF and microwave circuit designs. These have the highest working voltage (at 4kV) for designs requiring large biasing voltages and/or RF voltages, and retain high capacitance values ranging from 10pf – 6800pf. They are available in standard surface mount and microstrip ribbon leaded versions. They are available with tolerances from 1% to 20% and can be manufactured as non-magnetic or magnetic parts.

These devices remain an ideal solution for high power RF applications, such as MRI equipment, high power industrial amplifiers and test equipment, antenna tuning/impedance matching and inductive heating devices.

Another family often overlooked for RF applications is Glass dielectric capacitors.
These are even more stable than BP dielectric, with retraceable temperature coefficient to within +/- 15 ppm/c, extremely low dielectric absorption, zero voltage coefficient and zero piezoelectric noise.

The material is extremely rugged, with an operation capability to 200c and is also radiation resistant.

But, being an axial leaded configuration, they are not best suited to SMD RF designs.

Another set of devices are single layer capacitors. These are capable of SRF to 30GHz for 1pF capacitance values, so are used in optical applications, or with an additional MLCC in a micro-stack chip for extreme broadband. They are hybrid circuit compatible with most applications being epoxy mount/wire bond.

One of the benefits of the SLC design is just that – single layer. MLCC devices are characterized by a sharp resonance in their impedance curves (essentially tuned devices), but due to their multi-layer construction, they do have higher frequency resonances that can be problematic in RF design. The single layer, by comparison, has no harmonic resonances.

Thin Film Solutions
From the above discussion, an ideal “wish list” device would combine the stability of glass and a compact SMD design together with the clean response of an SLC. A technology is available that does just that – thin film technology is based on the use of highly stable dielectrics (e.g. SiO2) deposited on a stable alumina base, and in wafer/dice form, so it can be used to make highly stable capacitors down to 0201 size.

It then adds additional elements – photolithography and PECVD (Plasma Enhanced Chemical Vapor Deposition) processing. The photolith gives extremely precise geometry, resulting in extremely high accuracy and tight tolerance, while the low temperature PECVD process enables the benefits of high conductivity conductors for the electrode layers, which in turn provides optimum power handling characteristics.

Essentially, this technology can be viewed as surface mount glass – the world’s most stable and precise surface mount capacitor technology material, with no temperature or voltage coefficient and no ageing characteristic. The most basic element is the thin film capacitor.

The system is characterized by extremely stable dielectric, single layer construction (which eliminates harmonics), is readily modeled and extremely reproducible – designs breadboarded on the bench will be precisely reproduced in mass production at the manufacturing location, month to month and year to year. Because the parts are discrete circuit elements, there are no up-front design costs and full design flexibility is maintained throughout the program lifetime.

Ultra-stable dielectrics usually have a trade-off – low dielectric constant. Because of their precision, these devices can be used to fine tune any application or modification, or even for last minute tuning for FCC compliance, etc. Another advantage of thin film precision is the ability is the possibility to supply custom capacitance values if the circuit tuning requires this.

LNA (Low Noise Amplifier) applications are among the more critical sections in the receiver circuitry and, to maximize the performance, it is essential to have stable biasing and accurate impedance matching. Thin film provides discrete capacitors and inductors with are High Q, low ESR, and very accurate capacitance values (±.01pf) and inductance values (±.1nH).

Figure 2a shows the deviation in S11 when comparing MLC NP0 ceramics vs. thin film capacitors. Notice that the thin film capacitor response tracks with no variation between parts. This demonstrates exactly how precise the thin film process can be from capacitor to capacitor and from batch to batch. This can not only improve the quality of the LNA, it can also actually improve the yield in manufacturing by eliminating the fine tuning of circuits in production. Figure 2c shows the response at higher frequencies; being single layer devices, thin film shows no harmonic resonances.

The same components can be used to accomplish the critical matching of the input and output of a power amplifier. By using low loss thin film capacitors and inductors, more power can be sent to the amplifier transferred to the antenna. This results in improved performance and increased efficiency of the power amplifier, as well as improving temperature performance.

Antenna matching itself is also a critical design issue. The available real estate for the antenna is continually decreasing, which generally leads to a non-ideal form factor design. This situation will almost always require an impedance matching circuit for the antenna. Thin film capacitors and inductors are ideal for this application, providing an accurate match of impedance to the antenna to maximize energy transfer under all conditions, to minimize losses from the PA or to the LNA.

Beyond high accuracy microminiature capacitors and inductors, thin film PECVD technology also lends itself to integration. By combining both a capacitor and inductor element on the substrate, an LC Low Pass Filter (LPF) can be formed, as shown in Figure 4. These can be made in the same small form factors (0402 and up) and so use very little board space, while saving cost through component count reduction. These thin film filters provide high out of band attenuation (>30dB) while maintaining the lowest insertion loss available to the RF designer (<.3dB). PAs and LNAs. They can also be used to isolate the frequency of interest on the output of the mixer after conversion. The filters are internally matched to 50Ω, so no external matching is necessary, and the conductor materials used make them capable of handling up to 3W continuous power.

A directional coupler is a device that samples an RF/Microwave signal while minimizing loss to the signal. Thin film devices, based on back-to back inductors, produce very high directivity (Isolation – Coupling), low insertion loss directional couplers. These couplers offer the highest amount of directivity found on the market today, in small package sizes down to 0402. In the diagram in Figure 3, the coupler is being used to sample the output and send the sample to a gain control circuit for the power amplifier. As with the LPF, the couplers are also capable of handling up to 3W continuous power.

Directional couplers work from the principle of field coupling. The electric field produced by a transmission line in series with the signal is coupled onto an adjacent conductor through the air or dielectric medium. Coupler elements can be included within LTCC modules; the technology allows lumped elements, rather than coupled lines, to produce directional couplers to 10dB. However, thin film technology has a number of advantages in this area; the finer line widths maximize the coupling coefficient, making available hybrid couplers to 3dB in 0603 size.

This coupler, with port configuration shown in Figure 4, is designed to couple 3dB of power (half) to another channel, with the addition of a 90º phase shift to the signal. This can be very useful in designs utilizing an I-Q architecture where the channels are 90º out of phase. By using a hybrid coupler on the output of the oscillator, the LO (Local Oscillator) can be generated for both I and Q sections. It can also be instrumental when using two amplifiers to improve the linearity by splitting the power between the two circuits and then recombining after amplification. This reduces harmonic emissions, improves efficiency and increases gain from an amplifier.

Integrated Technologies - PMC
In RF devices, additional integration is usually synonymous with LTCC (Low Temperature-Co-fired Ceramic) technology. This technology does enable integration of passive components within compact modules and does yield a small size, but requires careful design discipline to achieve repeatable characteristics. LTCC is essentially a “wet” technology that has wider process tolerances and interconnect (line) widths limited to ~ 150 microns. Table 1a gives comparative data for key characteristics. It is difficult to characterize the RF properties of the structure as the internal elements suffer from significant parasitic coupling due to their large area and proximity to other structures or to ground planes.

Because LTCC technology, in itself, is not always the optimum performance solution, there has been an increased interest in new integrated devices that have evolved from discrete solutions based on thin film technology, and from this has emerged PMC technology.

The latest stage in evolution is PMC (Passive Micro Circuits). This goes to the next level of passive integration; its key advantages are that it retains the minimum line width capability and line width precision of thin film, but increases the maximum stacked layers capability. This can be used for increased capacitor and resistor density per mm2 and added turn capability in inductor elements. Tables 1a and 1b give the prime characteristics for both the materials and capacitor characteristics.

For RF, microwave, and GHz range applications, PMC provides custom integrated passive solutions incorporating capacitors, resistors and inductors. AVX offers a wide range of termination styles for epoxy or solder die attach and subsequent gold or aluminum wire thermosonic and ultrasonic bonding.

MOS capacitors are also included within this series - MOS capacitors are Single Layer Capacitors (SLC) that use dopped silicon dioxide to produce small, high Q, temperature stable, high break down voltage, low leakage capacitors. A new family of center-tap resistors is also available.

The PMC process is ideal for integration of the passive content of the RF circuit to optimize PA or LNA performance for a given application. As with all integration, the higher the complexity, the more single application specific the device becomes.

Some examples of PMC devices discussed are shown in Figures 5a, 5b, and 5c.
In summary, designers have choices at the outset of the product cycle to choose from full integrated systems to full discrete solutions with their associated pro and cons. Both thin film and PMC are rapidly developing technologies and there are guaranteed to be a number of new devices emerging during the near future.

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