This article presents the development of silicon based heat spreader devices, called hexcells. Several key technical aspects, including the hexcell MEMS fabrication process, mechanical strength studies, vacuum sealing technique, and phase change and mass transport visualization, have been developed and studied. The hexcell development prototypes are fabricated by MEMS photolithography and dry-etch processes, with eutectic bonding to form a sealed silicon chamber with openings for charging with the working fluid. Using Ansys as the modeling tool, we optimized the hexcell total mechanical strength by incorporating six interior support posts to reinforce the structure. In terms of the optimized design, experimental results on actual hexcell samples show that a well-bonded hexcell can withstand over 60psi without destructive failure. Vacuum sealing are divided into helium and vapor leakage tests. With metalized and solder-sealed sidewalls, both testing results confirm good vacuum sealing. The wick structure used in the present hexcell is silicon pillars with dimensions of 50μm in diameter and 250μm in height. The pillars are etched before the hexcell is bonded and formed. Experiments using the silicon wick structure demonstrate over 300W/cm2 cooling capacity and visualization shows the intensive phase change on the heating area.