A planetary gear type center differential unit contains a carrier, a first sun gear and a second sun gear provided interiorly of the carrier, a first and second pinions respectively meshing with the first and the second sun gears, a pinion member having a shaft-receiving hole and carrying the first pinion and the second pinion therearound, and a pinion shaft made of carboncarbon composite. The bubble generating frame has two separate portions, the portions being pivotably coupled to each other in a manner such that the portions can be pivoted between a closed position where the front surface of the portions contact each other, and an opened position where the portions are positioned in the same plane to form the bubble generating frame. A two-tiered transfer robot allows the substrates to be removed or placed into the preparation and process chambers at the same time, thus decreasing the cycle time for processing a substrate. The status information is subsequently extracted from the internal memory buffer during operation of the VM to monitor internal operation of the VM from a vantage point external to the VM.