Decapsulation is the process of removing a die from or exposing a die in a package.
Depending on the carrier type, different techniques need to be used.
In order to protect the fragile die, epoxy is mechanically strong and very chemical resistant. Strong chemicals, mechanical abrasion, or heat treatments can be used to remove epoxy. This is the most common packaging and possibly also the hardest.
Fired ceramic with metal parts. Often sealed with glass and metal.
Hot running chips often require heat sinks and are integrated directly into the package.
Some packages are fully metallic. Usually these are transistors, but military ICs also came (still do?) in these as well.
Typically used for smart cards.