Hi Ryuji,
My D109 was just "soft", having a high internal series resistance. It still began conducting at the proper voltage, it simply didn't conduct much current and allowed more than 5 volts to be applied while conducting less than 100 ma. I suspect that it was bad to begin with and not a result of failure in the circuit. It didn't over-heat. It just let the MCU get over-voltaged.
We'll never know for sure. A shorted zener would have been nice!
=Don=
Don, ND6T