This set of Manufacturing Processes Interview Questions and Answers for Experienced people focuses on “Electrochemical Etching – 3”.
1. Which of the following holds true about wet etching?
a) Wet etching is used for large pattern sizes
b) It is an anisotropic process
c) It forms straight sidewalls
d) Undercutting and resist peel off can happen if time is not controlled
Explanation: Wet etching has some limitations in its applicability, which are listed below:
(1)Wet etching is used for large pattern sizes, usually larger than 2 µm.
(2) It is an isotropic process – sloped sidewalls rather than straight walls.
(3) Wet etch has to be combined with subsequent rinse and dry steps. This increases the chances of defects or contamination.
(4) Hazardous chemicals and conditions are used, so safety is an issue. Safe disposal of chemicals is essential.
(5) Undercutting and resist peel off can happen if time is not controlled or etch conditions change during process.
2. In plasma etching chemical etchant is used in_____
a) liquid phase
b) gas phase
c) powder form
d) gel form
Explanation: In plasma etch, the chemical etchant is introduced in the gas phase. The gas is ionised by using radio frequency electrodes. This ionized gas attacks the oxide layer, removing the layer. For etching silicon oxide, CF4 (tetrafluoromethane) is used.
3. In the dry etch process, gases are _____
a) absorb and react with the material
b) adsorb and react with the material
c) used to remove the burr
d) given very high velocity along with abrasive particles which will remove the material
Explanation: In the dry etch process, gases are transported to the wafer surface, where they adsorb and react with the wafer surface material, at the step edges. The gases then desorb from the surface.
4. For etching of molybdenum, which of the following is used as etchant?
Explanation: Generally for etching of molybdenum, fluorine is used as etchant. CF4 (tetrafluoromethane), SF4 (sulphur tetraflouride), SF8 (sulphur octaflouride) are the typical gases compounds used for etching of molybdenum.
5. The resist layer used to protect the wafer does not get etched.
Explanation: The resist layer used to protect the wafer is also etched along with the oxide. But the resist thickness is much larger than the oxide (few µm of resist compared to tens of nm of oxide). This means that substantial amount of resist is still available, after the etching process.
6. For etching of aluminium, which of the following is used as etchant?
Explanation: For etching of aluminium, primarily chlorine is used as etchant gas. Common gaseous compounds used are, BCl3 (boron trichloride), CCl4 (carbon tetrachloride), Cl2, SiCl4 (silicon tetrachloride).
7. Ion beam etching is similar to the ion beam milling process.
Explanation: Ion beam etching is similar to the ion beam milling process that is used for transmission electron microscopy sample preparation. This is a physical process where ionized inert gas ions (usually Ar) are used to remove material from the wafer.
8. Ion beam etching is a highly directional process.
Explanation: The process is not selective but it is highly directional as it is similar to the ion beam milling process. Ions are generated inside the ion source and are accelerated into a broad parallel beam, and to a defined energy, by the extraction grids on the front of the source.
9. Which of the following is a combination of two other processes?
a) Plasma etching
b) Wet etching
c) Ion beam etching
d) Reactive ion etching
Explanation: Reactive ion etching combines the plasma and ion beam etching process to achieve both selectivity and directionality. In wet etching, chemicals are used to remove the material.
10. Dry etch process is also used for resist stripping.
Explanation: Dry etch process is also used for resist stripping after patterning is complete. This is usually done by plasma etching using oxygen. Dry etching can be done in three ways;
1. Plasma etching
2. Ion beam etching
3. Reactive ion etching
Among these, reactive ion etching is the most efficient.
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