Decoupling capacitors are just the first place for systems designers to start building digital systems with sufficient power integrity. While decoupling/bypass capacitors are still a standard strategy for ensuring power integrity in single digital ASICs, the entire system power bus also needs to be approached throughout broader frequency ranges. The approach here combines power integrity design at low frequencies, where discrete caps dominate, and up to GHz frequencies where discrete caps have trouble providing the required capacitance.
To address the entire range of frequencies where decoupling is needed, package designers and chip designers assist the PCB layout engineer by including embedded capacitors on-chip and in-package. As more electronics companies take a leading role in chip and package design, there is a need to determine the appropriate amount of capacitance needed to ensure low PDN impedance throughout broad frequency ranges. This article will look at the different types of capacitors that can be used as embedded components in PCBs and in chips/packages.
Embedded Capacitors Come in Many Forms
The various types of embedded capacitors that can be used in PCBs, on a semiconductor die, and in-package are summarized in the table below. There is some overlap among these options for system designers, and each targets different frequency ranges in PCBs, dice, and packages.
Discrete Embedded Capacitors
Discrete capacitors are already placed on packages for some advanced processors, has been described in this article. In addition to direct placement and assembly in a processor package, these components can be embedded in a PCB or package substrate.
It is possible to embed small capacitors in an organic substrate, including the organic materials used to build PCB stackups and package substrates. Discrete capacitors placed in PCBs and substrates are off-the-shelf components, designated low-profile MLCCs. While not specifically designed for embedding in substrates or PCBs, they can be embedded in these materials thanks to their lower-than-normal profile.
- Target frequency range: 100 MHz to 1 GHz, or higher for specialty RF capacitors
These low-profile MLCCs from Murata can be used for embedding.
Embedded Capacitance Material (ECM)
This class of organic materials offers high capacitance density through the material’s high dielectric constant. Noise/power fluctuations are damped at high frequencies because these materials are very lossy, having high loss tangents (~0.02 or higher) at lower frequencies than in PCBs/packaging materials. They are very thin materials (~1 mil thickness) but they can be used in standard lamination and buildup processes with other materials for packaging and PCB stackups.
These materials are probably the simplest class of materials for providing low PDN impedance and damping power fluctuations simultaneously. They can also help lower the overall PDN impedance curve over a broad frequency range.
- Target frequency range: ~1 MHz to 1 GHz
This 6-layer PCB stackup can use an ECM between L2 and L3, or between L4 and L5, to form a large embedded capacitor. A similar strategy could be used with packaging.
Embedded Dielectric Foils and Ceramics
There are other embedded capacitor materials that are inorganic, and thus they cannot be used in a standard lamination process used with other organic materials in PCBs and substrates. However, these can be used in two areas:
- Capacitor dielectrics (non-ceramic) used in inorganic packaging and on-die
- Ceramic insulators bonded to package substrates or embedded in interposers
Embedded dielectric foils like tantalum can be used on-die as a sintered layer. This enables a thin layer over a large area, giving a low-profile embedded capacitor film that is integrated into a semiconductor die.
Following the same approach as embedded dielectric foils, ceramics can also be used in substrates and interposers as an embedded capacitive film when a standard organic ECM is not suitable. While ECMs target both PCBs and packaging, embedded ceramics currently sit in the packaging domain and are compatible with multiple substrate/interposer materials. An example is shown below. These ceramic films are placed as a thin metal-insulator-metal (MIM) structure
- Target frequency range: Up to GHz frequencies
Embedded ceramic film structure for a package substrate/interposer.
These structures are fabricated directly into silicon, either in a die or in interposers. They can have unique vertical structures that attempt to maximize the exposed surface area that defines a trench capacitor, thereby maximizing the capacitance of each trench. Multiples of these trench structures act like a large group of capacitors in parallel, so they collectively provide high capacitance without decreasing the self-resonant frequency of the structure.
- Target frequency range: Up to GHz frequencies
Silicon trench capacitors on Si substrate.
These options target decoupling into the GHz range, leaving PCB designers to focus on the lower end of the frequency range with discrete capacitor selection and PCB stackup design. By working together with packaging designers, a team can work together to produce more advanced substrates that maintain power stability up to very high frequencies.
High-speed PCB design professionals trust the layout and routing features in Allegro PCB Designer to help them build advanced digital systems. Allegro is the industry’s best PCB design and analysis software from Cadence, offering a range of product design features with a complete set of management and version control capabilities. Allegro users can access a complete set of schematic capture features, mixed-signal simulations in PSpice, and powerful CAD features, and much more.