Ferroelctric liquid crystals of chiral smectic C ($Sc^*$) have been payed a significant attention because of their possible application to the high speed switching devices. Though the bulk $Sc^*$ phase has zero value of average spontaneous polarization along the axis, the surface force in a thin cell, imposed by the alignment layers, would be strong enough to suppress the helical structure. Among the material constnats in ferroelectric $Sc^*$ phase, the transition temperature, the apparent tilt angle, the helical pitch, and the spontaneous polarizatioare impoant parameters in fabrication.
The example cells of DOBAMBC, and ZLI-4654 were fabricated with surface treated by rubbing polyimide thin film in the range of the cell thickness 2㎛ - 100 ㎛. The transition temperatures of $S_A$ to $Sc^*$ were determined under the optical microscope by applying electric field. The transition temperatures of liquid crystals within the cell treated by polyimide film, showed little lower values than those treated with silicon monooxide or without surface treatment in all cells. The spontaneous polarizations were measured by the triangular wave method in the variation of the cell thickness, the change of temperatures and the frequencies. As the cell thickness became thinner, the apparent tilt angle and spontaneous polarization increased, but the transition temperature of $S_A$ to $Sc^*$ decreased. In addition, the helical structure was not developed in the thin cell. In a thin cell, it is believed because molecular dipoles were ordered along the one direction, and therefore the medium became ferroelectric. Finally, the observed spontaneous polarization was reduced along with frequencies and temperatures.