4/28/01APS1 Test of Forward Pixel Sensors for the CMS experiment Amitava Roy Daniela Bortoletto Gino Bolla Carsten Rott Purdue University

4/28/01APS2 Pixel Sensors The CMS experiment at the LHC will have a silicon pixel detector as its innermost tracking device. In LHC C.M. energy will be 14 TeV. Luminosity will be cm -2 s -1 in the first year. Later it will be cm -2 s -1. After 6 years of operation fluence at r = 7 cm will be equivalent to 6 x Mev n/cm 2. 4 forward disks, 12 Mpixels

4/28/01APS3 Pixel Sensors NN+N+ P+P H.V. Sensors were made by SINTEF and CSEM. Bulk material in N type. 1~2 K  cm resistivity. 275 or 300  m thick. Depletion voltage V. Sensors are reverse biased during the operation. Holes and electrons generated due to incoming particle are collected through the ROC at the pixel.

4/28/01APS4 Guard Ring Design Guard Rings  450  m Three designs of guard ring were tested - 6 guard ring, 11 guard ring & 16 guard ring design V SiO 2 6 guard rings 11 guard rings 16 guard rings

4/28/01APS5 Guard Ring Performance 11 guard ring design has the best performance. None of the diode breaks down before 1000 V!

4/28/01APS6 Pixels & P-stop Pixels P-stops P-stops are to isolate the pixels. A high resistive channel between the pixels allow to bias the pixel array.

4/28/01APS7 P-stop Design Design A Double open ring Design B Atoll Design D Spiral Design E Step double ring Design F Single open ring Design G Double open ring 2 Design C Atoll double ring Design H Atoll 2 8 different types of P-stop design - 8 different type of resistive channel.

4/28/01APS8 P-stop Performances Most pixel arrays breakdown between V. Best Designs are design F, design G and design A.

4/28/01APS9 Irradiation 5 pixel arrays were irradiated at Indiana University Cyclotron Facility. 200 MeV Proton beam - damage is equivalent to 1 MeV Neutron. 4 pixel arrays (2 design A, 1 design B & 1 design D) received a fluence of 1x10 14 p/cm 2 while 1 (design A) received a fluence of 6x10 14 p/cm 2. After irradiation, leakage current were measured at 21 0 C, C & C.

4/28/01APS10 Performance after Irradiation At C the leakage current per pixel is expected to be 10nA, but not exceeding 100nA. Measured leakage current at C is 10.2nA. All pixel arrays show a linear rise in leakage current in between 250V- 550V which is independent of temperature. The extra current is coming from the pixel region. Some pixels are breaking down early.

4/28/01APS11 Conclusions Guard ring design has been optimized. Pixel arrays with eleven guard ring design breaks down well above 300V, the operating voltage of CMS. Best p-stop design has been sorted out. The baseline design meet the specification of leakage current/pixel. However after irradiation the linear rise of leakage current between V has to be understood clearly. Single open ring has better performance before irradiation. It is also simpler to fabricate, hence less prone to fabrication defect. Next submission will focus on this design.