How to measure the delay of one single inverter in ASIC?
It's very difficult to measure the delay of a single inverter by itself precisely.  They are so fast on modern integrated circuits that getting an accurate measurement of the delay is not easy.  Generally, maufacturers will construct something called a ring oscillator and then measure it's oscillation frequency.  A ring oscillator consists of a loop of an odd number of inverters.  It will oscillate spontaneously with a period of the product of the number of inverters and the propagation delay of the inverters.  All manufacturers need to do to measure the delay is power up their ring oscillator and connect it to a frequency counter.  Then you take the number of inverters in the oscillator and divide by the frequency to get the delay on a single inverter.