Stealthy Dopant-Level Hardware Trojans (2013) [pdf]
iacr.org
iacr.org
As the above images, and the presence of many Soviet clones shows, the obfuscation was not entirely impossible to detect.
They under-doped certain areas to create depletion mode transistors that any reverse engineer examining the silicon would expect are regular enhancement mode ones. They strategically placed a handful of these "disguised" transistors, which once incorrectly copied would switch in the opposite manner and wreak havoc on circuit operation.
According to one of the designers[1]:
I heard from NEC later the copy maker delayed the announcement of Z80 compatible product for about six months.
[1] https://retrocomputing.stackexchange.com/questions/11143/in-...