Seminar on semiconductor technology
The web page is based on a two-day seminar consisting of some lectures, demonstrations, and lab tours that provide an introduction to semiconductor technology.
For producing even smaller structures than the micron scale often dry etching is used as well as for chemically resistant materials. Dry etching shows higher etching anisotropy which results in higher aspect ratios of etched structures. In comparison to wet etching, the dry etching process must be performed under vacuum. Dry etching can be divided into physical and chemical dry etching.
A good overview about dry etching processes in a bit more detail with schematic descriptions can be found in the video of the Stanford Nanofabrication Institute:
Physical dry etching
Ion beam etching
Similar setup as in PE-CVD can be used for physical sputter etching. With the help of a plasma ions (mostly Ar) are produced and accelerated towards the substrate where they knock out atoms of the substrate. The big advantage is the non-material-dependent high anisotropy. However, due to this non-material dependency this process is non-selective. Furthermore, disadvantages are a low etch rate, implantation of sputtering ions into substrate, trenching and broadening (change of wanted geometry) and material redeposition.
In Chapter VII.D of the MKS Handbook is a good description of Sputtering and Ion milling as well as a schematic drawing of an ion beam etching reactor. On the website azom.com one can find more detailed descriptions and especially also microscope images of etched surfaces. An extremely detailed but not via the TUG library accessible book about this technique is the Handbook of Advanced Plasma Processing Techniques Chapter "Ion beam etching of Compound Semiconductors" (free preview).
Chemical dry etching
Isotropic radial etching
This dry etching process is based on chemical interaction between the substrate and the reactant gas to form a volatile product. There are plasma-less methods which use gases like XeF2 or ClF3 (from sublimation) that can dissociate spontaneously and form fluorine. Fluorine then aggressively attacks silicon and etches away the wafer. These processes are problematic for industrial use due to the safety problems as fluorine is hazardous and can form HF when in contact with water. Due to the character of this process it is fully isotropic and selective. Another plasma-free option is oxide etching with HF. The HF vapor attacks SiO2 and etches it away. Same as before this process is isotropic and selective as it is based on non-directional vapor diffusion.
Furthermore, there are methods that use plasma (mostly refered as chemical dry etching or Plasma etching) for changing the surface chemistry of the substrate by glow discharge without destroying the surface with accelerated ions.
Then the reactive and volatile gas is led into the vacuum chamber where it reacts with the modified parts of the substrate to a volatile reaction product.
This process offers high selectivity, isotropic behavior and fast etch rates and is mostly used for cleaning wafers or removing complete layers.
More about plasma etching can be found at halbleiter.org/plasma_etching.
For a detailed description of different setups for plasma etchers watch this video:
Combined physical and chemical dry etching
Reactive ion etching (RIE)
For a first overview about RIE see the following video.
This process is a combination of physical and chemical dry etching with a tunable ratio. One of the most important etching processes in semiconductor industry. Same principal as in plasma etching where a volatile reactant should be formed. Ions formed by the plasma are accelerated towards the substrate to lower the activation energy (weakening of bonds, higher local temperature) for the following chemical etching step with a reactive gas. Due to the directional ions trajectory a high anisotropy in contrast to plasma etching is possible. Further advantage is a cleaner surface due to ion bombardment. More about RIE in Chapter VII.C in the MKS Handbook or the website halbleiter.org/reactive_ion_etching.
A little "lab tour" of a RIE device is shown here:
Deep reactive ion etching is an advanced technique based on reactive ion etching (RIE). It is highly anisotropic, capable of producing high aspect ratios (about 50:1) with structural depth up to some hundred microns. More about DRIE in Chapter 21 of Introduction to Microfabrication.
An overview of gases used in dry etching is given at the website halbleiter.org/gases. More about vacuum and plasma, including plasma etching and sputtering can be found in Chapter 33 of Introduction to Microfabrication.A video that describes the theoretical basics of RIE instead of reading the book chapters is this one:
Comparison and Overview
In the following table some important characteristics of the presented processes are compared and summarized.