For a long time, aluminum served as the main interconnect metal for integrated circuits. Although it was later replaced by copper, aluminum is still widely used in semiconductor manufacturing. Aluminum etching is an indispensable part of the aluminum manufacturing process. However, in many cases, aluminum is prone to corrosion after dry etching. What does the corrosion phenomenon look like? What causes this? What is the solution?
Where does aluminum etching need to be used?
There are too many places where aluminum metal is needed. In memory products such as DRAM and flash, aluminum wiring is still widely used due to its low cost and easy processing. These memory devices typically do not have the stringent resistance and electromigration requirements that high-performance logic chips do. The electrodes of most chip products are usually made of aluminum, which is widely used in packaging, LED, microfluidic chips, radio frequency chips, etc.
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Etching mechanism of aluminum metal
Commonly used gases are Cl2, BCl3, HBr, Ar, N2, etc.
Cl2: As the main etching gas, Cl2 reacts with aluminum to generate volatile AlCl3, thereby achieving etching. The equation is:
3Cl2+2Al→2AlCl3
BCl3: As an auxiliary to the etching gas, it helps remove the natural oxide layer on the aluminum surface, while providing ion bombardment to enhance the anisotropy of etching:
Al2O3 + 3BCl3→ 2AlCl3 + 3BOCl
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HBr:
1. The addition of HBr helps enhance the anisotropy of the etching process.
2. A volatile aluminum bromide (AlBrx) passivation layer can be formed on the etched sidewall of aluminum. This passivation layer helps protect the sidewalls and reduces unnecessary lateral etching.
3. When used in combination with Cl2 and BCl3, HBr can improve the selectivity of aluminum to photoresist etched in the same cavity.
Ar: As a noble gas, Ar+ ions provide physical bombardment, helping to achieve cleaner etching profiles.
N2: passivation layer
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Why corrosion occurs
AlCl3, a volatile product produced by etching aluminum, is drawn out of the chamber. However, only a small part of the AlCl3 is extracted, and more AlCl3 is trapped in the photoresist or on the surface of the metal Al. If the wafer is subsequently exposed to clean room moisture, the AlCl₃ can form hydrochloric acid, causing aluminum corrosion. This kind of corrosion does not always occur immediately. Generally, obvious corrosion phenomena will appear two or three days after etching. The equation of Al corrosion is:
2Al+6HCl→2AlCl3+3H2
What measures can be taken to solve the corrosion problem?
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1. After dry etching of aluminum, immediately pass in O2+CHF3 gas to completely remove the photoresist. CHF3 generates fluorine radicals in the plasma, and these radicals can react with AlCl3 to generate AlF3. Since AlF3 is very stable at room temperature and is not easily hydrolyzed, it will not cause corrosion on the aluminum surface.
2. After dry etching of aluminum, clean thoroughly using deionized water or an appropriate solvent to ensure that all residual AlCl3 is removed.
3. To avoid exposing the wafer to a high humidity environment between etching and photoresist stripping processes, a sealed wafer box can be used to store the wafer.