On the length of THRU standard for TRL de-embedding on Si substrate above 110 GHz

It is known that the THRU standard (a transmission line) used for thru-reflect-line (TRL) calibration/de-embedding for S-parameter measurement has to be long enough that only a single electromagnetic mode propagates at its center for it to work reliably. But ideally, TRL standards should occupy as little precious silicon real estate as possible. This paper attempts to experimentally find out how long a THRU is long enough above 110GHz up to 170 GHz through measurements of transmission lines of various lengths. The results indicate that the length of a THRU should be at least 400 micrometers, excluding pads and pad-to-line transitions.