High performance single-phase Si microchannel coolers have been designed and characterized in single chip modules in a laboratory environment using either water at or a fluorinated fluid at temperatures between 20 and as the coolant. Compared to our previous work, key performance improvements were achieved through reduced channel pitch (from 75 to 60 microns), thinned channel bases (from 425 to 200 microns of Si), improved thermal interface materials, and a thinned thermal test chip (from 725 to 400 microns of Si). With multiple heat exchanger zones and 60 micron pitch microchannels with a water flow rate of , an average unit thermal resistance of between the chip surface and the inlet cooling water was demonstrated for a Si microchannel cooler attached to a chip with Ag epoxy. Replacing the Ag epoxy layer with an In solder layer reduced the unit thermal resistance to . Using a fluorinated fluid with an inlet temperature of and 60 micron pitch microchannels with an Ag epoxy thermal interface layer, the average unit thermal resistance was . This fell to with an In thermal interface layer. Cooling was demonstrated with water. Using a fluorinated fluid with an inlet temperature of , a chip with a power density of was cooled to an average chip surface temperature of . Results using both water and a fluorinated fluid are presented for a range of Si microchannel designs with a channel pitch from 60 to 100 microns.