A channel substrate containing at least one channel for a fluid is provided. A sensor substrate carrying a thermal flow sensor is arranged adjacent to the channel substrate. The flow sensor contains at least one temperature sensor and at least one heater, which are integrated on the sensor substrate. The heater and the temperature sensor are in thermal contact with the channel in the channel substrate. This arrangement allows to measure the flow of the fluid in the channel, which provides an improved monitoring and control. The sensor substrate can have contact pads connected to bond wires or, using flip-chip technology, the contact pads can be connected to circuit paths to the channel substrate.