Silicon and Silicon Wafer Based Integrated Capacitors

Silicon-based capacitors are typically single MIM (metal-insulator-metal) or multiple MIM structure electrostatic capacitors built by semiconductor technologies.

Silicon dielectrics are either silicon dioxide (MIS) or silicon nitride (MOS) insulating layers; however, semiconductor manufacturing techniques such as atomic layer deposition (ALD) can be used to form other dielectric materials on top of the silicon substrate. High-density silicon-based capacitors use 3D nano-structured electrodes to achieve higher surface area and, thus, higher capacitance value.

Introduction

Silicon-based dielectrics such as silicon dioxide and silicon nitride are commonly used in high-density capacitors. Capacitors with silicon dielectrics are ideal for applications that demand high stability, reliability, and tolerance to high temperatures. The performance characteristics of these capacitors make them a suitable choice for use in harsh environment applications. The following are the key strengths and limitations of silicon-based dielectrics.

Strengths of silicon-based dielectrics

High stability at high temperature

The performance of most capacitors is significantly affected by exposure to high temperatures. Silicon capacitors are available in different temperature ratings, usually up to 250^oC. High-temperature silicon capacitors are suitable for various harsh environment applications, including aircraft engine controls, avionics systems, automotive systems, downhole oil exploration systems, military applications, etc. In addition, silicon capacitors offer highly stable capacitance performance as a function of voltage and temperature. Although the maximum capacitance of silicon capacitors is limited, they do not suffer ageing of capacitance. Moreover, unlike X7R and X8R capacitors, the reliability and capacitance of silicon capacitors are not degraded under DC bias conditions.

High potential for miniaturisation

Silicon-based dielectrics are commonly used in the fabrication of high-density electronic devices. High-density silicon capacitors are usually fabricated in ultra-deep trenches with very low leakage current and loss factor. Passive integration connective substrate (PICS) is the most common technology for implementing high-density capacitors. This process allows the implementation of multi-chip modules (MCMs) and chip on board (COB), and it helps to realise smaller components with low power consumption. In addition, this process allows the integration of many basic functions into a single product, thereby helping to cut manufacturing costs. By employing the latest technologies, high volumetric efficiencies have been achieved. It is expected that the volumetric efficiency of silicon capacitors will continue to improve as the demand for high-performance and miniaturised components continues to grow. So far, silicon capacitors with layers that are thinner than those of multilayer ceramic capacitor (MLCC) technology have been achieved.

Leakage current stability at high temperature

Leakage current is one of the capacitor parameters that can be affected when a capacitor is subjected to high temperatures. The dielectric material is the key factor determining a capacitor’s leakage current. Overstressing the dielectric material can significantly increase the leakage current. The charging voltage and the thickness of the dielectric also have a slight effect on the leakage current of a capacitor, as compared to most high-temperature capacitors in the market, silicon capacitors have impressive leakage current-temperature characteristics. In addition, silicon-based dielectrics offer impressive insulation resistance, making them an unmatched choice for coupling, blocking, and timing circuits.

Low failure rate

Capacitors find a wide range of applications in electronic circuits. As such, they are one of the electronic systems’ most common passive components. The reliability of a capacitor is a factor in its failure rate. Compared to other passive components, capacitors have a higher failure rate. Some of the key factors that determine the failure rate of a capacitor include operating time and loading conditions. Comparative reliability tests have shown that high-temperature silicon capacitors have better FIT (failure in time) rates as compared to high-temperature X8R capacitors.

Limitations of silicon-based dielectrics

Limited maximum capacitance

Although silicon capacitors have impressive characteristics, including high stability at high temperature, very low leakage current, high insulation resistance, and high capacitance density, there is a limit to the maximum capacitance that can be achieved. It is expected that the latest advancements in technology will help to overcome this technological limitation. In addition to improving the fabrication process, manufacturers are exploring alternative dielectrics as a solution to the major technical barriers associated with silicon-based dielectrics.

Charge leakage

SiO₂ is commonly used in the fabrication of capacitors for microelectronics devices. These capacitors are constructed by oxidizing silicon and using the oxide as the dielectric material. Since capacitance is directly proportional to the area of the dielectric material and inversely proportional to the thickness of the dielectric, manufacturers of electronic devices have been decreasing the area and thickness of the dielectric material to obtain miniaturized and high-density devices. As the thickness of the SiO₂ dielectric film is decreased, the charge leakage through the dielectric material increases. Beyond a certain limit, it becomes difficult for the capacitor to store charge due to the leakage problem. Although DRAM manufacturers are using deep trenches to overcome leakage problems, the dielectric thickness limitation is a major barrier to the miniaturisation of microelectronic devices. Besides improving the implementation process, manufacturers are also exploring alternative dielectrics to overcome this limitation.

Silicon-Based Capacitor Structure and Features

Silicon-based capacitors are typically single MIM (metal-insulator-metal) or multiple MIM structure capacitors built by semiconductor technologies. Silicon dielectrics are either silicon dioxide (MIS) or silicon nitride (MOS) insulating layers; however, semiconductor manufacturing techniques such as atomic layer deposition (ALD) can be used to form other dielectric materials on top of the silicon substrate. High-density silicon-based capacitors use 3D nano-structured electrodes to achieve higher surface area and, thus, higher capacitance value.

Structure of semiconductors and MOS capacitor – its band gap diagrams, work functions and electron affinity concepts are beyond this article’s scope. We will focus on describing silicon-based capacitors on the market and their key features.

Silicon MIS and MOS Capacitors

Silicon-based dielectrics are used within semiconductor manufacturing processes of high-density electronic devices by semiconductor manufacturing processes. Silicon-based dielectrics for capacitor technologies are usually based on silicon dioxide (MIS) or silicon nitride (MOS) insulating layers. Figure 1. below describes a conventional MOS technology to make silicon capacitors.

Single Layer RF Silicon Capacitors

Silicon-based dielectrics are used to make low-loss, high-Q capacitors that feature very high-temperature stability, high breakdown voltage and low leakage parameters. The main limitation is relatively low permittivity. Comparison of MIS and MOS dielectrics – see Table 1. and Figure 2. below.

Gold or aluminum wire thermosonic and ultrasonic bonding are the most common way to assemble single-layer silicon capacitors (SLCs) in RF applications. Termination styles may differ, and it can be optimized for epoxy or solder die attach mounting techniques. Examples – see Figure 3.

RF Thin Film Silicon Capacitors

Silicon thin film capacitors (Figure 4.) are typically based on a single layer silicon oxide/nitride dielectric deposited on a substrate and packed in a chip MLCC-like design. It offers the unique ability of very low capacitance values (0.05pF) and very tight capacitance tolerances (±0.01pF). Thin film technology guarantees minimal batch-to-batch variability of parameters at high frequencies; thus, it is an ideal component for RF and microwave filters.

Figure 4. Silicon RF thin film capacitor construction; source: AVX

The term “thin film capacitors” however, relates to a wider range of thin film technologies using also other dielectrics such as ceramic or organic films deposited on various substrates types such as alumina, quartz, silicon or silicon wafer.

Thus, the distinction between “thin film”(deposited on a silicon substrate) and “silicon” capacitors is something of a marketing concession. However, significant differences exist within & among the two depending on the intended application.

Devices targeting RF tuning & matching applications tend to be low-capacitance, single-layer devices optimized for parameter stability and consistency and are commonly found in standard JEDEC package sizes.

In contrast, devices intended for power supply decoupling, broadband DC blocking, and similar applications allow larger tolerances to achieve higher specific capacitance and are more likely to be found in packaging adapted to advanced assembly methods such as wire bonding or embedding within a PCB. Regardless of the intended application, however, devices in the thin film and silicon capacitor families are premium-performance products priced accordingly, currently fetching something 5 to 5000 times the price of ceramic devices with similar capacitance and voltage ratings.

By employing the latest 3D technologies, high volumetric efficiencies above 450nF/mm2 have been achieved. Silicon capacitors can be manufactured in layers below 100um that are 4xthinner than those of multilayer ceramic capacitor (MLCC) technology. Manufactured of silicon capacitors claim 10x better reliability in comparison to MLCC capacitors, and in combination with ultra-high temperature stability up to 250C, the high-density silicon capacitor technology enables a number of high demanding applications in the automotive, industrial/oil drilling or aerospace/defence industry. It is expected that the volumetric efficiency of silicon capacitors will continue to improve as the demand for high-performance and miniaturised components continues to grow.

Silicon capacitors – key features

• High Q, low losses
• Thin layers sub 100um possible
• High-temperature operation (up to 250C)
• High reliability lifetime expectancy (10x better than MLCC, as example: full rated 10V continuous operation at 250°C for 50years)
• Low leakage < 30nA/mF; Higher leakage current at high temperatures (but still 1000x lower compare to the other technologies)
• No/low radiation sensitivity
• No derating required
• No catastrophic failure mode
• Low weight
• Stable performance with tight capacitance tolerances
• No piezo effect
• Non-magnetic
• Relatively low permittivity

Silicon Wafer Based Integrated Capacitors

Integrated capacitors and passives have a lot going for them. They take up less space on a PCB, they simplify design, and they can, with the right processes, shrink circuit tolerances thanks to closer component matching. The downside is that, as with semiconductors, volume is everything.

Although a common reason for keeping passive components off-chip is their size relative to that of the transistors on-die – is not worth wasting precious silicon area on devices that cost more to assemble than their materials are worth – many off-the-shelf parts rely on passives to tune filters and control loops for specific applications.

Utilization of semiconductor fabrication processes, however, has resulted in a number of new approaches towards high-density micro-capacitors. Finish company Picosun used its atomic layer deposition ALD equipment to deposit film stacks of conductive TiN and insulating dielectric Al₂O₃ and HfAlO₃ layers into high aspect ratio trenches etched into silicon that increased capacitance density up to 1 µF/mm². ALD deposition and 3D micro capacitor manufacturing process is shown in Figure 6. below.

Figure 6. Main technological steps of 3D micro capacitor fabrication. 1: patterning of a square lattice of holes on the silicon surface; 2: high aspect ratio trenching of silicon by electrochemical micromachining (ECM); 3: atomic layer deposition (ALD) of conformal metal-insulator-metal (MIM) stack; 4: aluminium deposition and contact patterning. Source: Picosun

Swedish company Smoltek received Outstanding and Best Paper Awards at EPCI PCNS conference for their carbon nano-fibre metal-insulator-metal (CNF-MIM) wafer-based semiconductor technology, achieving capacitance density to +650nF/mm² at the end of 2019. They used the ALD technique to deposit Al₂O₃/HfO₂ layers onto a carbon nano-fibre 3D structure.

The fibre length is only 2 – 3 µm, and the total height profile of the complete device is ca 4 µm. This makes the capacitors readily available for integration onto a CMOS chip or in 3D stacking. Figure 7-9. below shows the CNFs after the dielectric coating via ALD, the image showing a coating of Al₂O₃/HfO₂/Al₂O₃ (5/3/5 nm). The dielectric layer is uniformly covering the individual CNFs.

Source: Passive Components