Cryogenic bolometers are adequate sensing devices for sub-millimeter observations. They are used for instrumentation in radio telescopes like the Atacama Pathfinder Experiment (APEX), which is a 12-m radio telescope in Chile’s Atacama desert. Through its thin and dry atmosphere, star formation in the red-shifted early universe can be observed. A suitable bolometer has a high sensitivity and consists of several tens to hundreds of pixels that are combined to an array. To meet these requirements, we use transition edge sensors. They detect radiation by measuring a temperature increase in an appropriate absorber. For highest sensitivity, the absorber is only in weak thermal contact to its surroundings. This is achieved by placing it on a SiN membrane that is structured to further decrease the thermal coupling. A superconductor that is biased at its transition temperature serves as thermometer for the transition edge sensor. We present the fabrication of such devices and discuss encountered difficulties. Further, we briefly describe the incorporation of a sensor chip with an array of 39 pixels into an instrument that currently operates at APEX. For the fall of 2010 the installation of a 300 pixel array is scheduled. 2010 Elsevier B.V. All rights reserved.