Samir Anz, co-founder, VelvETch, Stewart Sando, co-founder, VelvETch, Michael Barden, head of research and development, PVA TePla America, and William A. Goddard III, professor of chemistry, materials science and applied physics, California Institute of Technology
In semiconductor fabrication, the traditional approach to dry etching is utilising radiofrequency (RF) plasma to bombard the surface of the wafer with positive ions to remove material between masking layers. However, although ion etching has been effective for decades, it fails to produce the precise, sharp, nano-sized structures and pathways required in next-generation devices. The process also generates significant amounts of heat that can damage underlying material layers and, in the case of compound semiconductor materials such as gallium nitride (GaN) or silicon carbide (SiC), can change surface atomic ratios.
VelvETch and PVA TePla America have commercialised a system that employs a technology known as Electron Enhanced Material Processing (EEMP), which utilises the power of electrons, rather than ions, to remove material much more precisely at the nano-level.
In EEMP, precisely controlled waves of electrons are accelerated to the surface of the material at specific voltages to create chemical reactions that release the surface atomic bonds, allowing the material at the surface to be gently lifted away. A full-scale immersion by electrons allows the item being processed, such as a wafer, to be completely etched at a rate comparable to RF plasma etching.
EEMP is flexible, allowing various factors to be precisely controlled to essentially etch any material, including thin nano-layers and quantum well structures. It can also be fine-tuned and controlled to achieve atomically smooth surfaces, thus enabling the fabrication of quantum computing devices.
EEMP has applications for the current generation of high bandgap compound semiconductors made of gallium arsenide (GaAs), GaN and SiC.
Electron etching
It is a common misconception that electrons are not capable of etching, but this is based on the assumption that the process in question involves electron beam scanning. It must be stressed that EEMP does not involve electron-beam scanning, rather it is a full-scale immersion process, meaning that the entire wafer is processed at all points simultaneously.
Electrons have often been dismissed as a viable option for etching via bombardment on account of their low mass relative to ions. The consensus has been that accelerated electrons are unable to create sufficient momentum in order to initiate a reaction. However, in EEMP, it is not the impact that drives the etching, rather it is a chemical reaction induced by a loss of electrons from the bonds at the surface that causes surface atoms to be gently released.
The California Institute of Technology has developed a means of studying this process using quantum mechanics and discovered that electrons in the discharge with sufficient energy can remove an electron buried deep inside the atom, which in turn is filled with an electron from a bond while simultaneously knocking a second electron out of a bond. If two electrons are lost from a bond, it breaks. This is known as the Auger effect. Since the Auger effect only occurs at the surface, EEMP is able to provide extremely smooth surfaces.
VelvETch commercialised EEMP after decades of research and development, then enlisted the help of PVA TePla America in designing an advanced plasma etching system to support the technology. This system utilises a proprietary bias waveform signal that pulls the electrons down to the surface being processed. The bias waveform signal is applied across the whole surface of the wafer and this is the mechanism that accelerates a wave of electrons towards the whole surface. As electrons have little mass, there is no impact damage to the surface and only nominal heat is generated as a result of the chemical reaction, thus the wafer remains at room temperature.
The process is extremely flexible and can therefore be applied to a variety of applications and materials. The variables that can be manipulated and tuned to achieve specific results include the gases utilised in the chamber, the electron energy in the discharge (based on the material to be etched) and the temperature.
Plasma etching
EEMP is far removed from the traditional reactive-ion etching process using RF plasma. In this process, plasma is created by applying a RF signal (typically 13.56 Megahertz) that causes the atoms or molecules of the gases introduced into the chamber to increase in temperature until they ionise into a plasma. A separately controlled RF signal under the wafer then pulls the reactive ions downwards in a virtual atomic sandblasting of the surface of the material.
Due to the mostly vertical delivery of the reactive ions, RF plasma etching can result in anisotropic etching, which is useful for the fabrication of relatively sharp corners, flat surfaces and deep cavities. However, when ions with over 1,000 volts of energy hit the surface, a couple of nanometres of damage occurs automatically, even in the best case. This creates significant problems, as transistors are miniaturised to 10 nm and less. Moreover, ions that hit the surface with sufficient force to etch can be embedded several layers deep, causing electronic damage, along with backscatter.
In addition, ion etching produces small undercuts underneath the masking layer to form cavities with sloping sidewalls. Even a half nanometre undercut on each side can account for 10 percent of the width of a 10 nm transistor, leading to incorrect function.
Slight damage from reactive-ion etching is not a critical concern with silicon, but it can cause serious problems with compound semiconductors such as GaAs, GaN and SiC because the ions affect the various elements differently, leading to incorrect atomic ratios at the surface.
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The Electron Enhanced Material Processing (EEMP) plasma etching system.
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The EEMP plasma etching system chamber.
Compound semiconductor materials
Compound semiconductor materials comprise more than one element. Current high bandgap compound semiconductors include GaAs, GaN and SiC.
- GaAs allows for faster operation, wider band gap and operation at higher temperatures.
- GaN is very important for transistors that operate at higher frequencies, voltages and temperatures, including the microwave power transistors used for 5G wireless base stations, satellite communication and military radar systems.
- SiC is used in semiconductor electronics devices that operate both at high temperatures and high voltages.
Reactive-ion etching removes different elements at different rates, modifying the surface stoichiometry, or ratio, of compound semiconductor elements. With GaN, ion etching tends to remove nitrogen faster than gallium, leading to a gallium-rich surface with poor electrical properties. Consequently, to eliminate excess gallium, wafers are often dipped in wet chemistry to restore the proper ratio of elements at the surface, which is hardly appropriate for new generation nanoscale devices.
EEMP preserves the stoichiometry of compound semiconductors by carefully controlling the energy of the electrons in the discharge. This advantage also applies to quantum well structures, which sandwich a thin layer of one semiconductor material between two layers of another semiconductor material with a wider band gap. Examples include aluminium gallium arsenide (AlGaAs), which confines the electrons to a GaAs region, as well as GaAs and indium gallium arsenide (InGaAs). Quantum wells are widely used in laser diodes, light emitting diodes (LEDs), high electron mobility transistors, infrared (IR) photodetectors and IR imaging arrays. The ability to control the electron energy makes it possible to target the etching at a specific material, stopping automatically when another material is hit without creating any damage to the underlying material.
Additional benefits
Among the additional benefits of EEMP is its ability to produce atomically smooth surfaces via the removal of atoms layer by layer, starting with any peaks. Specifically, it is possible to achieve within one lattice constant of atomic smoothness, which is less than 0.25 nm in silicon.
In quantum computing, atomically smooth surfaces are required for optimal performance. EEMP can also be used to smooth a surface prior to growing another material on top of it using chemical vapor deposition (CVD) or molecular beam epitaxy (MBE). When GaN is grown on SiC for high-frequency, high-power applications, the SiC must be perfect, otherwise its defects will propagate into the GaN lattice, degrading electrical performance.
Another significant benefit of EEMP is that it generates almost no heat. RF-based ion etching generates high temperatures that can cause physical and electrical damage to compound semiconductors and integrated circuits. Excessive heat can damage the very thin, 10–20 nm layers of quantum well stacks and even modify the electrical properties and structural integrity of modern low-K dielectric materials.
The temperatures of most RF plasma etching systems are so high that cooling must be integrated. The hotter the material becomes, the more undesirable reactions that occur, so it is necessary to suppress the thermal chemistry pathways and parasitic processes by keeping the temperature low.
During EEMP, the chemical reaction produces only a minimal amount of heat and it is believed that this contributes to the broad dynamic range of the process. Rather than being regarded as something that must be mitigated, it plays an essential role, namely allowing temperature to be used as an additional control.
Commercialising the process
VelvETch and PVA TePla America adopted an old-school approach in designing the EEMP plasma etching system, making it direct current (DC)-based rather than RF-based in order to achieve the low temperature plasma required. DC plasma (DCP) allows for a controlled positive column that is extremely rich in low energy electrons, thus enabling the EEMP plasma etching system to provide a high level of reliability.
VelvETch
PVA TePla America
California Institute of Technology