Aluminum Etching

Due to its high electric conductivity, aluminum is used for conductors in microelectronics where it is often alloyed with copper in order to prevent electro migration, or with silicon in order to prevent the formation of (silicon-consuming) aluminum-silicon alloys. With a standard potential of -1.66 V, aluminum does not belong to the noble metals that are not affected by etching. However, the formation of a very thin Al2 O3 film, a few nanometers for instance, makes it very inert in many substances. Therefore, Al etchants require at least compounds for dissolving Al2 O3 as well as for etching (or, respectively, oxidizing) aluminum.

Aluminum etching is highly exothermic, so an inevitable, since underetching of the resist mask causes local heating (increased etch rate) and super-proportional underetching of the mask as a consequence, if no agitation is performed.

The Al etch rate of H3PO4/HNO3 mixtures strongly depends on the temperature and doubles approximately each 5 °C. Aluminum alloyed with few % of Silicon shows a comparable etch rate as compared to pure Aluminum. Most Al etchants attack copper significantly more then aluminum. The nickel etch rate is lower then the aluminum etch rate. Titanium, chromium and silver are hardly etched; noble metals such as gold or platinum are not attacked.

When etching aluminum, these guidelines hold true:

  • the mask layer must not be attacked
  • the selectivity has to be high
  • the etch process has to be able to be stopped by dilution with water
  • reaction products must not be gaseous because they could shadow other regions
  • constant etch rates should hold all along the process
  • the reaction products must be soluble to avoid particles
  • environmental safety and ease of disposal are paramount
  • quartz tanks should be considered because of their ability to hold tight temperatures to avoid over etching