Described examples include a millimeter wave atomic clock apparatus, chip scale vapor cell, and fabrication method in which a low pressure dipolar molecule gas is provided in a sealed cavity with a conductive interior surface forming a waveguide. Non-conductive apertures provide electromagnetic entrance to, and exit from, the cavity. Conductive coupling structures formed on an outer surface of the vapor cell near the respective non-conductive apertures couple an electromagnetic field to the interior of the cavity for interrogating the vapor cell using a transceiver circuit at a frequency that maximizes the rotational transition absorption of the dipolar molecule gas in the cavity to provide a reference clock signal for atomic clock or other applications.