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Bosch semiconductors for Automotive

Trench etching from Si to SiC: a culmination of 30 years of experience

Bosch’s journey towards SiC trench etching for vertical silicon carbide chips

Trench etching from Si to SiC: a culmination of 30 years of experience

With the invention of the so-called Bosch process (also known as deep reactive ion etching), Bosch changed the world of semiconductors significantly, enabling the production of silicon wafers with steep-sided, deep vertical trenches for micro-electromechanical systems (MEMS) for the first time. Decades later, Bosch built on this knowledge to create vertically trenched silicon carbide semiconductors – as one of the first and few providers worldwide.

Small and powerful

In the world of silicon-based semiconductors as well as in the field of silicon-based sensors, the etching of trenches has played a crucial role in developing ever-smaller, more powerful chips for nearly three decades. “Trenches in the wafer material enable us to build chips with a vertical channel. Instead of just putting the gate electrodes on top of the material’s surface, cells are arranged in the trenches to increase integration,” explains Dr. Klaus Heyers, Chief Expert for Wide Bandgap Semiconductors at Bosch Mobility Electronics.

Deep silicon trenches manufactured with the “Bosch Process” were the starting point for SiC trench etching.
Deep silicon trenches manufactured with the “Bosch Process” were the starting point for SiC trench etching

This comes with several advantages: first, the trenches can be used as an efficient isolation between different devices on a chip, preventing electrical interference between components. Second, the vertical dimension makes it possible to add more cells and thus functionalities on the same wafer size, increasing the packaging and ultimately the power density of the entire structure. Finally, the crystallographic properties of some wafer materials – like silicon carbide (SiC) – conduct electricity anisotropically, as electrons pass through them more easily vertically than horizontally. “In a nutshell, vertically trenched chips can boast the same performance as planar chips at much smaller sizes,” says Heyers.

In a nutshell, vertical-built chips can boast the same performance as planar chips at much smaller sizes.

In a nutshell, vertically trenched chips can boast the same performance as planar chips at much smaller sizes.

Dr. Klaus Heyers

The journey towards SiC trench etching

Bosch began developing SiC semiconductors in 2001; the first MOSFET prototype was available in 2011.
Bosch began developing SiC semiconductors in 2001; the first MOSFET prototype was available in 2011.

While the invention of the Bosch process by Andrea Urban and Franz Lärmer in the early 1990s enabled the trench etching of silicon wafers, it did not work for silicon carbide in the same way. Still, Bosch was convinced of the material and its uses: “We knew very early on that silicon carbide as a semiconductor has a lot of potential, especially in the automotive industry,” Heyers recalls, referring to the many benefits of SiC chips. “We also knew from the beginning that we wanted to build vertical instead of planar SiC chips in order to leverage all the advantages of this technology.”

This decision turned out to be a rather unique one: nearly all other providers focused on the production of planar SiC chips instead, mainly due to the challenges that come with the material’s properties. “Silicon carbide’s molecular bond energy is approximately double compared to silicon, and it is much more inert, i.e. less chemically reactive,” explains Dr. Jens Kalmbach, Unit Process Development Engineer at Bosch Mobility Electronics. Trench etching classically relies on both mechanical and chemical abrasion. But with the high inertia of SiC, the mechanical component of “shooting” accelerated ions at the wafer material becomes much more important than the chemical one. Additionally, a smooth and steep trench slope is vital for SiC – which proved difficult if not impossible with the established Bosch process. “We were sure we could develop a new process for SiC. After all, we had Bosch’s entire know-how, equipment, and experience in trench etching and micromechanical processes at our fingertips,” Heyers emphasizes

Smaller chips, which allow for a more compact design, ultimately save money on the chip side as well as on the system level.

Smaller chips, which allow for a more compact design, ultimately save money on the chip side as well as on the system level.

Dr. Jens Kalmbach

SiC trench etching: a new process for a new material

After three years of work, the team was successful, developing and patenting its own process for 150 mm SiC wafers in 2015. Like the traditional Bosch process, this SiC trench etching alternates between two phases – but uses different gas mixtures and adjusted conditions. In the etching phase, plasma is formed from a gas mixture, shooting energized ions at the wafer in a nearly vertical orientation. During the second phase, oxygen and helium are used to prevent electrical charging and to obtain the desired, rounded trench profile. Due to the direction of the shot ions, the wafer material is removed faster horizontally than at the sides of the resulting structures, which results in the etching of steep and deep trenches.

Bosch has been mass-producing generation 1 of its silicon carbide chips on 150-millimeter wafers in Reutlingen since the end of 2021.
Bosch has been mass-producing generation 1 of its silicon carbide chips on 150-millimeter wafers in Reutlingen since the end of 2021.

Thanks to this new process, Bosch’s SiC semiconductors combine the advantages of vertically trenched chips with the material’s inherently high electrical conductivity and stability at increased temperatures and voltages, as well as its excellent switching performance. The process has proven especially valuable for the automotive industry. “A high cost-performance ratio is particularly important in the automotive sector,” Kalmbach explains. “Smaller chips, which allow for a more compact design, ultimately save money on the chip side as well as on the system level.” Today, SiC chips are a key enabler of electro mobility. Mainly used in components like inverters, converters, and on-board chargers, they allow for efficient energy management and longer driving ranges. And with the global ramp-up of electromobility, the demand for SiC chips is skyrocketing.

The future: from 150 mm to 200 mm wafers

Bosch has been producing samples of the first generation 2 silicon carbide chips on 200 mm wafers in Reutlingen since June 2024 for customer trials and is currently preparing to ramp up series production of generation 2 on 200mm in Reutlingen
Bosch has been producing samples of the first generation 2 silicon carbichips on 200 mm wafers in Reutlingen since June 2024 for customer trials and is currently preparing to ramp up series production of generation 2 on 200mm in ReutlingenBosch has been producing samples of the first generation 2 silicon carbichips on 200 mm wafers in Reutlingen since June 2024 for customer trials and is currently preparing to ramp up series production of generation 2 on 200mm in Reutlingen

To meet this high demand and drastically ramp up production, Bosch is turning towards 200 mm SiC wafers and significantly expanding its production capacity. The company has not only been building up new production sites at Reutlingen, Germany, but also acquired a new fab in Roseville, USA. However, to transfer the SiC trench technology from 150 mm to 200 mm, a lot of process adaption had to be done – and another, new process for the deep trench etching of 200 mm wafers had to be developed.

“Unlike the trench etching process for 150 mm wafers, this one is continuous and uses additional and other process gases to achieve the slightly round profile at the bottom and a smoother profile at the sidewall of the trenches,” Kalmbach explains. Samples have already been produced successfully. Soon, the chips will do their part in revolutionizing modern mobility. “In all these process developments for silicon carbide chips, our unique position as both a provider and user of our own technology has given us an edge in meeting the needs of our users. Our SiC trench etching is the culmination of our many years of experience in automotive semiconductors,” Heyers concludes.