We report on the current status of a project on development and characterization of CD photomasks with 6025 format to be used as reference standards for different type of CD metrology instruments. The project consortium consists of mask suppliers, manufacturers of CD metrology instruments, users of such instruments and calibration laboratories. Different type of CD metrology instrumentation, namely optical CD microscopes, CD-SEM, and AFM will be applied for investigation and measurement of microstructures, additionally supported by AIMS tool. We will describe the basic design criteria of the mask standard and first measurement results gained with different metrology tools on the prototype mask standards.