A short review of semiconductor design and structure.
The process of building resistors and capacitors on a semiconductor die.
How the multiple manufacturing methods for integrated resistors and capacitors balance cost and quality.
The integrated circuit towards the top of the image potentially contains millions of integrated resistors and capacitors; contrast that with the discrete chip packages found in the bottom and right halves
Integration is the key to miniaturization. It allows for including the functional equivalent of discrete passives in ICs that may be smaller than the discrete element itself. There are more than straightforward passive values – take power ratings, for instance – to account for in discrete components, but building integrated resistors and capacitors saves space and offers high reliability to a PCB assembly.
Perhaps unsurprisingly, the same semiconductor processes used to create diodes and transistors on chips are creatively manipulated to form linear passives. The methods for manufacturing resistors and capacitors differ depending on the needs of the integrated circuit and cost/quality tradeoffs.
Let’s take a look at integrated resistor and capacitor methods.
At-a-Glance Comparison of Integrated Resistor and Capacitor Methods
Economical(shares semicon processes)
Restricted value due to size limit
High desnity, low parasitics
Additional processing stages
Large resistance values in a small area
Low parasitics and high stability
Building Integrated Resistors and Capacitors From Semiconductors
The layout and fabrication techniques that create on-die semiconductor devices can also form simpler passive components. The major caveat with integrated resistors and capacitors is their size somewhat limits their value: larger passive component values may require discrete packaging to achieve the requisite resistance or capacitance.
Capacitors especially are significantly constrained by this semiconductor implementation, which is why discrete bypass capacitor packages are ubiquitous for IC support. Still, where the values are reasonable, the miniaturization offered by on-die passive construction is unparalleled.
All semiconductor devices build from an N- or P-layer over a P- or N-substrate for NPN and PNP, respectively. The complete pathway for semiconductor fabrication will depend on the end device. Still, the general process will be the addition of the SiO2 on the top layer that later opens to allow dopants to diffuse and form the P- or N-type region for the N- or P-type layer, respectively. Semiconductor manufacturing can only add an opposing charge carrier well to a particular layer. This two-step process of SiO2 addition and selective removal continues until the die’s structure is complete, sans metallization at the surface to build the terminals for the integrated component.
For various manufacturing reasons, the most common product of the semiconductor process is a transistor – how does this device become a resistor or capacitor? By manipulating the common junctions of the bipolar junction transistor (BJT) or metal-oxide-semiconductor field-effect transistor (MOSFET):
- Integrated resistor - As space is the foremost cost driver due to the miniaturization of the IC, most resistors build on the base diffusion region (P-type region for an NPN BJT) due to the high resistivity of the material. Conversely, the emitter region has a lower resistivity, so smaller resistance values are available. As a function of resistivity, resistance restricts the length of the diffusion layer over the cross-sectional area of the layer’s width and thickness. This sheet resistance – defined as Ohms per square – is approximately 200 at the base region and 2.2 at the emitter region.
- Integrated capacitor - Capacitor design splits between junction and film deposition methods built on an underlying MOSFET structure. Instead of relying on a specific region, as in the integrated resistor, integrated capacitors utilize the junctions of a diode or MOSFET as a dielectric. Monolithic capacitors don’t perform as well as their resistor counterparts because values and quality are more constrained when on-die. The capacitance is proportional both to the area of the junction and the thickness of the depletion layer.
Integration Options for Cost, Manufacturability, and Quality
The industry has adapted to the needs of circuit designers by offering multiple procedures for integrating passive components. Typically, these feature tradeoffs between component value, cost, production speed, and reliability. With the introduction of the basic construction for the passive component families out of the way, consider some of the additional methods of passive integration detailed below.
Resistor Passive Integration Methods
Epitaxial layer resistors can achieve high resistance values. Two windows are made in an N-type epitaxial layer for aluminum metallization to form the resistor contacts.
Similar to the popular diffusion method described above, a pinched resistor can increase its resistance over a similar distance by shrinking the cross-sectional area of the resistor.
A thin metallic film deposits on the surface of the SiO2 layer, and a masked etch process achieves the resistor's desired size, shape, and value. Sheet resistance is approximately 40~400 Ω/square, twice the maximum value of standard diffusion resistors.
Capacitor Passive Integration Methods
Diodes provide small capacitance when they are reversed-biased. This capacitance varies with voltage; rarely is this attribute a boon, but it does allow for the implementation of voltage-driven capacitance. Junction capacitors have a capacitance density of around ~1 pf/µm2.
This variant also suffers from voltage-varying capacitance and a smaller capacitance density than the junction capacitor, but it has a lower leakage current and is non-directional.
A similar construction to a discrete package capacitor, the MIM model offers high capacitance density and low parasitics. It does require an extra processing step, which increases the cost.
Very similar to MIM, but with a lower quality factor (and correspondingly lower cost). A large capacitance density arises from lateral and vertical capacitance formed between metal “fingers.”
Cadence Solutions Support Design Integration From the Bottom-Up
Integrated resistors and capacitors are just as widespread as their discrete counterparts due to the various roles both play in essential circuit operation. Without integration, resistor and capacitor networks would be expansive enough to inhibit the miniaturization of electronics, not to mention the increase in propagation delay and parasitics. Technological improvements are in many parts thanks to innovation at the component level, but faster signal speeds have also redefined the PCB as a “component” all its own; system-level design needs to be based on industry best practices to avoid performance issues.
Cadence’s PCB Design and Analysis Software suite supports development teams with a wide range of ECAD services for all levels of electronic DFM. Simulation data from intricate models weave into OrCAD PCB Designer effortlessly for a layout environment that supports ease and speed without sacrificing functionality.
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