Reactive ion deep etching
Deep reactive-ion etching ( English deep reactive ion etching , DRIE ), a further development of the reactive ion etching ( RIE ), is a highly anisotropic dry etching process for the fabrication of microstructures in silicon having an aspect ratio (ratio of depth to width) of up to 50: 1, structure depths of a few 100 micrometers can be achieved. This is used / required for the production of silicon vias , for example . It belongs to the process of plasma-assisted etching .
history
Reactive ion deep etching was originally developed in the early 1990s by Franz Lärmer and Andrea Schilp in the form of a dry etching process for silicon. They were employees of Robert Bosch GmbH , whose marketing of the process patent led to the name Bosch process becoming a synonym for reactive silicon ion deep etching. In the following years, the original procedure was used with the cooperation partners Surface Technology Systems Plc. (STS) and Alcatel Vacuum Technology . In addition, the required system technology was refined, adapted to the process and sold commercially. For several years now, STS has been marketing an improved process together with the system technology under the name Advanced Silicon Etching (ASE).
process description
| Aspect ratio (depth: width) | up to 50: 1 |
| Flank angle | 90 ° ± 2 ° |
| Etching rate | (max.) 20 µm / min (standard) 1–2 µm / min |
| Surface roughness | 10 nm |
Like the original Bosch process, the DRIE process is a two-stage, alternating dry etching process in which the etching and passivation steps alternate. The aim is to etch as anisotropically as possible , i.e. depending on the direction, perpendicular to the wafer surface. In this way, for example, very narrow trenches can be etched.
The processes can be summarized as follows. First of all, the silicon wafer is masked , for example with photoresist or with a hard mask made of silicon dioxide , silicon nitride and other substances, which covers those areas of the wafer that are not to be etched. Then the actual etching process begins. For this purpose, sulfur hexafluoride (SF 6 ) in a carrier gas (usually argon ) is introduced into the reactor with the substrate in it. By generating a high-energy high-frequency plasma, the SF 6 turns into a reactive gas. Together with the acceleration of the ions in an electric field, a chemical isotropic etching reaction is superimposed by radicals formed from SF 6 and a physical anisotropic material removal by sputtering using argon ions.
The etching process is stopped after a short time and a gas mixture of octafluorocyclobutane (C 4 F 8 ) and argon is introduced as a carrier gas; Other gas mixtures are also possible, for example CF 4 / H 2 . Octafluorocyclobutane is activated in the plasma of the reactor and forms a polymer passivation layer on the entire substrate, i.e. on the mask as well as on the bottom and the vertical side walls of the trench / hole. In this way, the side walls are subsequently protected from further chemical material removal in order to ensure the anisotropy of the overall process. This is because the subsequent repeated etching step with SF 6 removes the passivation layer of the horizontal surfaces (trench bottom) by the directed physical components (ions) of the etching reaction significantly faster than the layer on the side walls.
Both steps are now repeated until the desired processing depth is reached. What is important is the balance between the etching and passivation step. If, for example, the polymer layer is applied too thickly, a large part of the etching gas and the process time are used to remove the polymer from the trench bottom, which leads to higher process costs. However, if the layer is applied too thinly or if the electric field for the ion transport to the substrate is too low, the side walls are etched too heavily.
The process parameters also have a decisive influence on the structure of the side walls; Due to the alternating process sequence, these are usually not smooth, but slightly wavy. The strength of the waves can, however, be minimized by a suitable choice of the process parameters so that it does not negatively affect the subsequent manufacturing processes.
After the etching, the mask material (which is also partially etched) and the passivation layer on the trench walls must finally be removed.
A disadvantage of the process is the very high system costs compared to wet etching and the low production throughput.
In addition to the alternating process, there is also the process of simultaneous etching and passivation, a so-called continuous process .
Areas of application
The main area of application is the production of microsystems . By using different masking materials, several depth levels can be achieved, the number of which, however, remains very limited due to the high expenditure (2½-D structures).
Further areas of application can be found in semiconductor technology , for example for the production of deep "trenches" for the storage capacitor in some DRAM technologies or for the isolation oxide in the trench isolation (although high aspect ratios are not required here).
In recent years, another possible application has become the focus of interest: For a 3D integration of circuits, i.e. stacking components (e.g. transistors and conductor tracks), it is necessary to have conductive channels through the (thinned) silicon substrate to create. These channels must have an (extremely) high aspect ratio of more than 50: 1 and are therefore produced with reactive ion deep etching and then filled with copper.
Web links
- Microstructure Engineering Lecture at Uppsala University. In: Uppsala University . Archived from the original on March 2007 (Swedish).
literature
- W. Menz, J. Mohr: Microsystem technology for engineers. VCH-Verlag, Weinheim 1997, ISBN 352730536X .
Individual evidence
- ↑ Patent DE4241045C1 : Process for anisotropic etching of silicon. Registered on December 5, 1992 , published on May 26, 1994 , applicant: Robert Bosch GmbH, inventor: Franz Lärmer, Andrea Schilp.
- ↑ Five billion MEMS sensors from Bosch. In: bosch-presse.de. February 18, 2015, accessed April 3, 2020 .