1 What is the Bosch Process?

Deep reactive ion etching (DRIE) of silicon to create high aspect ratio microstructures is one of the key processes in the advanced MEMS field and through silicon via (TSV) applications. However, conventional plasma etching processes are designed for etch depths of only a few microns and are lacking in etch-rate and etch mask selectivity. The Bosch process is capable of producing deep features with exceptional anisotropy, etch-rate, and etch mask selectivity.

This process consists of a three-step cycle: Film deposition, bottom film etching, and silicon etching. In the deposition process, a passivation film is deposited on the sidewalls and bottom surface of the trench. In the bottom film etching step, the passivation film on the trench bottom is selectively etched. In the silicon etching step, only the silicon at the trench bottom, where the passivation film has been removed, is etched (Fig. 1).

2 What is the chemical reaction of the Bosch process?

The deposition step typically uses C₄F₈ gas as the deposition gas. C₄F₈ is a cyclic molecule (ring shaped), and in plasma, its ring is broken and it becomes a short chain molecule. Both ends of the C₄F₈ chain are active. The active parts of the chain combine with other molecules and the chain grows in length, attaching to the silicon and forming a membrane. This mem- brane functions as a passivation film during the silicon etching step preventing etching of the sidewalls.

SF₆ is usually used as an etching gas for film etching and silicon etching. SF₆ dissociates in plasma to form SF₄ or SF₂ and atomic fluorine that reacts with silicon. Both SF₄ and the etching reaction product, SiF₄, exist as gases that are evacuated from the chamber. SF₄ is so stable that fluorine atoms cannot recombine with it. Therefore, a large amount of atomic fluorine can be produced and participates in the etching reaction.

CF₄ decomposes to form atomic fluorine in plasma. However, the decomposition reaction is reversible, and it is not possible to produce a large amount of atomic fluorine from CF₄. NF₃ nitridates the surface of silicon and prevents etching of silicon leading the etch rate to be about one-third of that of SF₆.

3 Why is it important to separate film deposition and etching steps?

The ring structure of C₄F₈ is broken and the chain-shaped C₄F₈ is generated in plasma. The polymerization reaction occurs when -(CF₂)₄– groups react with each other to form longer chains. The polymer forms the passivating film on the surfaces of the etched trenches. In the presence of SF₆, the fluorine radicals from the dissociation of SF₆ react with the chain -(CF₂)₄– groups and terminate the polymerization reaction. In addition, the fluorine radicals disappear as they are consumed by the termination reaction and thus, etching cannot occur. In summary, a quick gas switching every 0.1 sec is used to avoid the mixing of C₄F₈ and SF₆ gases.

4 Different sidewall profiles with Bosch process.
Is the Bosch process easily realized by repeating the three steps? It is actually difficult and there are some Bosch process specific problems. Because step 3 of the Bosch process (Silicon etching process) is a chemical reaction with fluorine radicals, the etch rate is highly dependent on the aperture area of exposed silicon. This is often referred to as the loading-effect. Also, as the aspect ratio increases, the probability of fluorine radicals being transported to the bottom of the trench or hole is reduced. This results in pattern-dependent etch effects, which is generally referred to as micro loading that adversely affects depth uniformity. Micro loading results from the depletion of reactants when the wafer has local, higher-density areas. In addition, DRIE-specific problems include notching when etching SOI (Silicon on Insulator) wafers and tilt due to non-uniform plasma distribution. These problems are addressed by adjusting system hardware, process parameters and device structure.

Fig. 4 shows the different sidewall profiles that can be engineered with the Bosch process.

1. The sidewall profile directly under the mask is vertical. It becomes gradually taper to the bottom.
2. The sidewall profile directly under the mask is a reversed taper. It becomes vertical, and gradually tapers as the aspect ratio or etch depth increases.
3. Reversed taper sidewall profile.

As long as the Bosch process is run, the etch feature will end up with one of the sidewall profiles, or the sidewall profile obtained by cutting off the upper part of that etch feature. For example, if vertical sidewall profile is needed, the top part of etch feature (1) can be cut and used.

Since the Bosch process is made up of three different steps, there are three times as many parameters as conventional ICP etching processes. This allows the Bosch process far more flexibility, but also increases the time needed to tune all of its process parameters. Samco has extensive knowledge and process know how with the Bosch process that enables us to better support our customers in the DRIE application space. Fig. 5 are process examples of different sidewall profiles achievable in our DRIE systems.

References

1. Kokkoris, G., Goodyear, A., Cooke, M. and Gogolides, E. A global model for C4F8 plasmas coupling gas phase and wall surface reaction kinetics, Journal of Physics D: Applied Physics, 41, 19 (2008), 195211.
2. Laerme, F., Schilp, A., Funk, K. and Offenberg, M. Bosch deep silicon etching: improving uniformity and etch rate for advanced MEMS applications, Technical Digest. IEEE International MEMS 99 Conference. Twelfth IEEE International Conference on Mi- cro Electro Mechanical Systems (Cat. No. 99CH36291)IEEE (1999).
3. Nagaseki, K., Kobayashi, H., Ishikawa, I., Nishimura, E., Saito, Y. and Sug- anomata, S. Mass spectrometry of discharge products at 13.56 MHz in SF6 gas, Japanese journal of applied physics, 33, 7S (1994), 4348.
4. Rangelow, I. W. Critical tasks in high aspect ratio silicon dry etching for microelec- tromechanical systems, Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 21, 4 (2003), 1550-1562.
5. Wu, B., Kumar, A. and Pamarthy, S. High aspect ratio silicon etch: A review, Journal of applied physics, 108, 5 (2010), 9.

What the Samco Si-DRIE Can Do?
Anisotropic Silicon Deep Reactive Ion Etching process using the Bosch Process and Non-Bosch Process enables trench, hole and pillar fabrication for various device applications. These are some examples of structures fabricated using the Bosch Process. For more details, please visit the processes below.

• Bosch Process for Micro Fablication
• Non Bosch Process for through-silicon via (TSV)

Samco Product Portfolio for Silicon Deep Reactive Ion Etching

Our systems have industry-leading process capabilities, and the product lineup covers both R&D and production.

• Si Deep Reactive Ion Etching (DRIE) Systems
  

Silicon is most well-known as a semiconductor material, but because of its high mechanical strength and ease of processing, it is also commonly used in a wide variety of devices such as MEMS, optical components, micro-channel devices, and packaging. There are many methods for processing silicon, such as dry etching, wet etching, and laser processing, each of which has advantages and disadvantages that should be evaluated carefully and compared.

The processing method called Si-DRIE is a type of plasma dry etching. The etching technology cultivated for semiconductors has improved the processing of mechanical devices and has been proven to allow faster and deeper etching of features with higher aspect ratios. Among Si-DRIE techniques, there are three main process types: the Bosch process, Non-Bosch processes, and Cryogenic etching. Since silicon and fluorine atoms react very easily, processing silicon using a simple fluorine chemistry process results in an isotropic profile. Therefore, the key for increasing the verticality and aspect ratio for any of the three process types is to suppress lateral etching and develop an anisotropic process.

The Bosch process is a technique which alternates between depositing a protective film on the sidewall using a C₄F₈ plasma and etching the bottom of the trench or hole with SF₆. The Non-Bosch process is a technique of simultaneously depositing a protective layer on the sidewall and directionally etching the bottom of the feature with ions. Cryogenic etching is a method similar to the Non-Bosch process and suppresses the chemical reaction of Si and F atoms on the sidewall by lowering the substrate temperature to that of liquid nitrogen.

The Bosch process has excellent selectivity and is capable of high aspect ratio etching and is often used for MEMS and packaging. On the other hand, the Non-Bosch process has smooth sidewalls with positive taper angle, which is useful for through-silicon vias (TSV). Additionally, the angle of the mask can be transferred into the etched material, which is often done for optical parts such as lenses. The Cryogenic etching process can achieve the selectivity of an oxide mask vs silicon equivalent to that of the Non-Bosch process. One drawback of this technique is that photo resist masks cannot be used due to the extremely low temperature of the process.

On Samco’s flagship RIE-800iPB deep reactive ion etching (DRIE) system it is possible to utilize both the Bosch and Non-Bosch processes on the same system. The Bosch process enables deep and high aspect features along with extraordinary levels of selectivity to the mask. The Non-Bosch process provides deep etching with smooth sidewalls and a flexible range of taper angles. Substrate temperatures between -10°C and 20°C are controlled via liquid cooling of the bias electrode combined with electrostatic chucking (ESC) and substrate back side He gas cooling. By combining these two methods, the RIE-800iPB delivers versatile, efficient, gentle and affordable solutions for MEMS, TSV, packaging, and other applications. It’s been exciting to see the continual evolution of our Si DRIE solutions as we incorporate new customer requirements into the system design. Do you have a process challenge? Give our experienced process engineers the opportunity to meet your challenge and exceed your expectations.

What the Samco Si-DRIE Can Do?
Anisotropic Silicon Deep Reactive Ion Etching process using the Bosch Process and Non-Bosch Process enables trench, hole and pillar fabrication for various device applications. These are some examples of structures fabricated using the Bosch Process. For more details, please visit the processes below.

• Bosch Process for Micro Fablication
• Non Bosch Process for through-silicon via (TSV)

Samco Product Portfolio for Silicon Deep Reactive Ion Etching

Our systems have industry-leading process capabilities, and the product lineup covers both R&D and production.

• Si Deep Reactive Ion Etching (DRIE) Systems
  

Samco, a leading manufacturer of plasma processing equipment for the compound semiconductor industry, has announced the opening of its new Plasma Enhanced Chemical Vapour Deposition (PECVD) and Atomic Layer Deposition (ALD) demonstration facility at Samco’s headquarters, Kyoto.

The class 1000 cleanroom space of 217.61 square meters

Samco’s PECVD and ALD equipment are widely used in the manufacturing of optoelectronic and electronic devices. The positive outlook and the rapid growth of the 5G & IoT markets have fueled demand for PECVD and ALD process equipment for fabrication of communication devices and semiconductor lasers.

“This new facility is of strategic importance to Samco as it will enable further revenue growth in the compound semiconductor market. There will be ten PECVD and ALD systems for demonstration in the class 1000 cleanroom space of 217.61 square meters,” says Tsukasa Kawabe, Samco’s President and COO.

“The facility will greatly strengthen our ability to develop future process applications and technologies with our customers across the globe. The volume of demonstration tests requested by customers that can be handled is expected to be more than double the current level. Moreover, our customers and partners could collaborate with us in prototyping and pilot-line production utilizing the new facility,” he adds. “In particular, we are planning on conducting joint research and development projects with universities, research institutes, companies, and other external parties more aggressively than ever.”

Samco was founded in 1979 as a specialized manufacturer of PECVD equipment and in recent years has significantly expanded the sales of dry etching equipment for demanding applications, mostly for the compound semiconductor market.

“Samco’s three pillars of core technology are deposition, etching, and surface cleaning. We will leverage this new facility to further expand sales of the PECVD and ALD systems,” says Kawabe.

For more information, please visit our PECVD and ALD systems and processes.

Progress in x-ray critical-angle transmission grating technology development

Ralf K. Heilmann,¹ Alexander R. Bruccoleri,² Jungki Song,¹ Mark L. Schattenburg^1
1MIT Kavli Institute for Astrophysics and Space Research (United States)
²Izentis LLC (United States)
Proc. SPIE 11119, Optics for EUV, X-Ray, and Gamma-Ray Astronomy IX, 1111913 (9 September 2019); https://doi.org/10.1117/12.2529354

Samco Si DRIE Equipment was used for 200 nm-period grating Si deep etching (6 µm).

The 33rd International Conference on Micro Electro Mechanical Systems
(IEEE MEMS 2020)

When:  18 – 22, 2020

Where: Vancouver Convention Centre, Canada

Booth: 12

Samco’s systems are highly regarded by researchers and manufacturers for their proven reliability and cost-effective solutions.  As we aim to expand further into the MEMS market, our booth will focus on the Deep Si Etching systems, as well as showcase the latest data for our Aqua Plasma® systems and more.

If you’re interested in meeting with us, contact us to make an appointment.

Come see Samco booth at IEEE MEMS 2020.