Etch - Wet and Dry Flashcards
What is the difference between wet and dry etching?
Wet etch = chemical etch. A chemical reaction takes place between the etchant and the substrate to create soluble compound that can be transported away from the substrate surface.
Dry etch = etch processes that involve plasma, and may or may not involve chemical reactions. These process create a residue with high vapor pressure that evolves from the substrate surface
What qualities are desirable in an etching process?
- High etch rate (high throughput)
- Selectivity
- Anisotropy/directionality
- Uniformity
- Low defects/low cost/environmentally safe
What is selectivity in etching?
The ratio of the vertical etch rate of the target material to the vertical etch rate of the surrounding material.
Typically, this is selectivity vs the resist or vs the etch stop layer.
Ideally, the target material should be etched at a much higher rate than the surrounding material (s=2 is okay, s=4 is considered good).
Selectivity comes from the chemical aspects of the etch process.
What is the difference between isotropic and anisotropic etching? What is the issue with isotropic etching?
Etching can be isotropic (the horizontal and vertical etch rates are equal ) or anisotropic (the etch rate depends on direction).
In isotropic etching, the etch can “undercut” and remove material beneath the mask. This is why anisotropy is a desired quality of an etching process.
What is anisotropy (formula)?
A = 1-(Rh/Rv)
Rh = rate of horizontal etch Rv = rate of vertical etch
In ideal anisotropy A = 1
In the case of total isotropy, A = 0
What type of etch is closest to A = 1? To A = 0?
Pure sputter etching is close to A = 1
Wet chemical etch is close to A = 0
How does the crystallographic direction affect etch directionality?
In wet etching, the etch rate depends on the crystalline direction. Denser directions are etched more slowly.
In Si, {111} is denser than {100}, which is denser than {110}, and the etch rate along {100} is about 300 times greater than along {111} for KOH.
What are the stages of wet etch?
- Diffusion of reactants to surface
- Reaction (usually a redox reaction)
- Diffusion of reactant products away from surface
What is wet etch used for?
Wet etch can be used just before packaging, after the wafers have undergone backgrinding. Wet etch is good there because the wafer is thin and is sensitive to mechanical stresses.
Wet etch is used for large-dimension processes.
Wet etch is also favored cleaning, for example to ensure that the photoresist has been totally removed.
What is the primary advantage and disadvantage of wet etch?
Wet etch is highly selective, but has low anisotropy.
What is sputter etch/ion milling?
Similar to sputtering, but the cathode in this case is the wafer. Positively-charged ions (usually Ar) are accelerated towards the wafer. High vacuum conditions ensure a high mean free path.
What type of current is used in sputter etch?
In sputter etch, RF current is used to ionize the gas (create the plasma) and DC current is used to accelerate the ions towards the wafer.
What is the natural bias in sputter etch?
The natural bias is an approximately +10V voltage drop that develops within the plasma.
This happens due to differences in the mobility of ions (low) and electrons (high). The area of highest electron density is the center of the plasma, and the most ionizations take place here, so this area is bright/glowing.
The outer part of the plasma is known as the ‘sheath’ and it is low in electrons. The sheath is mostly made up of ions, so it conducts electricity poorly due to the low mobility.
The natural bias is the voltage drop across the sheath. In order for sputter etch to succeed, the mean free path must be greater than the sheath (0.1-10 mm thick).
Is sputter etch anisotropic? Selective?
Since sputter etch is a purely physical process, it is nearly completely anisotropic (A ~1) and not at all selective (S ~1)
What are the disadvantages of sputter etch?
- Not selective (no chemistry)
- Slow etch rate, (the rate cannot be improved since increasing the rate requires an increase in pressure, which lowers the mean free path and affects anisotropy)
