  Excitations of MHz acoustic modes are studied numerically in 10-um-thick
silicon disk membranes with a radius of 100 and 500 um actuated by an attached
1-um-thick (AlSc)N thin-film transducer. It is shown how higher-harmonic
membrane modes can be excited selectively and efficiently by appropriate
patterning of the transducer electrodes. When filling the half-space above the
membrane with a liquid, the higher-harmonic modes induce acoustic pressure
fields in the liquid with interference patterns that result in the formation of
a single, strong trapping region located 50 - 100 um above the membrane, where
a single suspended cell can be trapped in all three spatial directions. The
trapping strength depends on the acoustic contrast between the cell and the
liquid, and as a specific example it is shown by numerical simulation that by
using a 60% iodixanol solution, a cancer cell can be held in the trap.
