More sensor research is being done using high fundamental frequency (HFF) Inverted Mesa Crystals. The HFF resonator is also a thickness shear mode (TSM) like the other BAW devices discussed. However because of the chemically etched thin membrane, the TSM frequency is much higher (50MHz to 1GHz) compared to mechanically lapped resonators (<50MHz). The attractiveness of this solution is the higher operating frequency and enhanced mass sensitivity. As a general rule of thumb, mass sensitivity is approximately a function of frequency squared. Thus a 100MHz inverted mesa crystal theoretically will be 100 (10 squared) times more mass sensitive than a 10MHz TSM device. These devices make great gas sensors at the cost of more difficult and fragile packaging requirements and challenges, respectively.