Development of the CMS pixel detector

A readout chip for the CMS pixel detector has been developed in the radiation hard DMILL process. The chips ability to operate under LHC conditions was tested in a high-rate pion beam with track densities up to 3 multiplied by 10 **7 cm**-**2 s**-**1. The observed degradation of the detection efficiency with increasing particle flux is compared with the data-loss mechanisms inherent to the readout architecture. First detector modules with 16 chips on a low-mass high-density interconnect are evaluated. The next iteration of the readout chip is designed for a 0.25 mum process with a radiation- tolerant layout.