Automotive Power Device Market Amps Up
Automotive power electronics are emerging as one of the semiconductor industry’s key drivers. These electronics include power devices that are at the heart of a new breed of electric vehicles (EVs) capable of going 200 miles or more between charges.
Although unit shipments of smartphones have been much higher than automobiles (1.4B versus 88 million in 2015), automobiles have much higher semiconductor content. Automotive power ICs are showing healthy growth, with an estimated 8% CAGR for the automotive power IC sector over the 2015–2020 period. Battery-powered EVs are particularly strong drivers of the sector: a May 2015 Teardown.com report on the BMW i3 electric vehicle revealed more than 100 power related chips in the bill of materials.
Unlike typical logic transistors that follow Moore’s Law scaling, power device FETs typically use much older technology nodes, such as 200mm (and smaller) wafers. Nevertheless, since the late 1980s power devices have continuously evolved. For example, thick PVD aluminum on the order of 3–12μm must be deposited on both the front- and backside of the device in order to provide heat dissipation and improve electrical performance. If not deposited properly, thick aluminum can be prone to whiskers and dislocations leading to catastrophic device failure. The Applied Endura PVD HDR Al reactor ensures that such defects are minimized while providing for deposition rates that are more than 50% higher than competing technologies.
In addition, thick epitaxial silicon ranging from 5 to more than 100μm with complex dopant profiles enables low resistance (Rds), large off -resistance (Roff), and faster switching speeds.
Applied’s new Centura high growth rate epi system delivers 50–100% higher growth rates, up to a 30% reduction in chemical flow, shorter clean times, and lower cost of cleaning consumables (CoC) compared to conventional processes (see New High- Productivity Epi Chamber for Thick Silicon Epitaxy” elsewhere in this issue of Nanochip Fab Solutions). It demonstrates excellent within-wafer uniformity and resistivity for advanced power device needs.
Architectural changes to the semiconductor film stack, such as moving the gate structure from planar (lateral devices) to trench (vertical devices), have enabled insulated-gate bipolar transistors (IGBTs) to enjoy faster switching speeds at lower losses. Similarly, for super-junction MOSFETs (SJMs), moving from multi-epi to deep trench fill has enabled substantial performance gains as well.
Some improvements and adjustments to etch processes have been necessary to accommodate these schemes, which involve higher aspect ratio structures. Improved epi Si films and optimized implant doping profiles also have enabled performance gains.
Power device manufacturers continue to look for more improvements. According to publicly available reports, Hitachi’s high-conductivity IGBT employs a separate floating p-layer to improve gate controllability and turn-on voltage. ABB Semi is creating P-pillar implants under the trench gates to create a super-junction effect to enable higher switching speeds.
Wafers are being thinned across the board to help reduce stored charge for high-speed switching. Fuji Electric has recently developed a seventh-generation IGBT with a thinner drift layer, smaller trench pitch, and optimized field-stop layer.
However, the consensus among experts is that the performance envelope for silicon devices has been pushed nearly to the limit. Power devices are limited by intrinsic silicon properties, so each subsequent stage of progress provides only marginal improvements.
III-V Power Devices
The power IC industry is looking to new wide bandgap (WBG) materials to take performance to an entirely new level. Silicon carbide (SiC) and gallium nitride (GaN) are the top candidates today, each with their own set of pros and cons. These are compound semiconductor materials that provide much higher bandgap and breakdown fields, taking power device performance to a level where silicon simply cannot compete. They are widely considered to herald the next era of power devices, the next big inflection.
Figure 1. Wide bandgap power devices have advantages for EVs and other systems, but materials costs are a challenge. (Source: Yole Développement and Applied Materials)
Inflections bring new challenges, and WBG power ICs are no exception. Cost is the biggest obstacle today, including manufacturing-related challenges from wafer warpage and high rates of defects associated with substrate and epitaxial processing. Currently, a 6-inch SiC substrate + epi wafer costs in the thousand-dollar range, according to market research firm Yole Développement (Lyon, France). And that cost can multiply quickly with more stringent device defect requirements.
There are other challenges in subsequent process flow steps. For example, high-temperature anneals nearing the 2,000°C range are required. Typical anneal reactors for Si come nowhere near this regime. Implanting into SiC is also quite complex.
Encouraged by the potential of WBG power devices, several companies, consortia, and university research centers are focused on solving their challenges. In fact, both SiC and GaN products are available today, albeit in limited quantities. However, costs must come down significantly before the benefits of WBG—including power savings, simplified circuitry, and reduction in module size—can provide a meaningful return on investment compared to silicon substrates.
Take, for example, a typical automobile inverter box that may contain 40 or more power transistors and diodes. Moving to SiC enables simplified circuitry, fewer components, and an overall module size reduction of up to 80%. It is this intersection of device size, materials cost, and energy savings that must be satisfied to create a meaningful value-add compared to silicon power devices (see table 1).
Fortunately, other steps in the semiconductor processing flow, such as CVD, PVD, etch and CMP, are relatively straightforward for WBG power devices because the general process flow is very similar to that of silicon. Process tuning is required along with minor hardware changes, but existing technologies can be adapted to WBG processing.
GaN-based power devices have great potential for high-voltage automotive applications, but GaN brings its own list of challenges, including wafer costs and effective production of GaN-on-GaN structures (a bulk GaN substrate with GaN epi). Only 2-inch bulk GaN wafers are available today. Moreover, with current architectural limitations, GaN devices are normally ON, which introduces reliability issues and limits market acceptance. Enhancement mode GaN devices would be needed to overcome this shortcoming. Therefore while the performance benefits of WBG devices are undeniable, it is an open question whether they can overcome the cost challenges and achieve high volumes.
In a recent power device seminar at Applied Materials, Stanford University Professor Jim Plummer suggested that for these devices to achieve success in the marketplace, it would make sense to identify a new space where silicon doesn’t compete. This would enable increases in manufacturing volume, which Plummer said would then help reduce wafer costs.
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