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Nuclear Instruments and Methods in Physics Research A 465 (2001) 204–210 Results on 0.7% X0 thick pixel modules for the ATLAS detector P. Netchaevaa,*, R. Beccherlea, G. Darboa, K. Einsweilerb, G. Gagliardia, C. Gemmea, M. Gilchrieseb, P. Oppizzia, J. Richardsonb, L. Rossia, E. Ruscinoa, F. Vernocchia, G. Znizkab Universita" di Genova and INFN, Genoa, Italy Lawrence Berkeley National Laboratory, USA a b Presented by Polina Netchaeva on behalf of the ATLAS Pixel Collaboration Abstract Modules are the basic building blocks of the ATLAS pixel detector system, they are made of a silicon sensor tile containing 46 000 pixel cells of 50 mm 400 mm, 16 front-end chips connected to the sensor through bump bonding, a kapton flex circuit and the module controller chip. The Pixel detector is the first to encounter particles emerging from LHC interactions, minimization of radiation length of pixel modules is therefore very important. We report here on the construction techniques and on the operation of the first ATLAS pixel modules of 0.7% radiation length thickness. We have operated these modules with threshold of 3700 10 300 10, mean noise value of 225 10 and 0.3% dead channels. # 2001 Published by Elsevier Science B.V. 1. Introduction In the ATLAS Pixel detector [1] there are 2228 modules. One module (Fig. 1) consists of a silicon sensor tile, 16 front-end readout integrated circuits (FE chips) and a kapton flex hybrid. The flex hybrid distributes power and control signals to the FE chips and allows reading them out through a module control circuit (MCC). Passive components including termination resistors, decoupling capacitors and temperature sensor are also included. The sensitive area of a module, i.e. of a sensor tile, is 16.4 mm 60.8 mm. The FE chips are connected to the pixel cells through bump *Corresponding author. E-mail address: [email protected] (P. Netchaeva). bonds, which are made of Indium in the modules described in this paper. The size of one pixel is 50 mm 400 mm and each FE chip serves 18 160 pixel cells. The flex hybrid is glued to the backside of the sensor tile; electrical connections from the MCC and the 16 FE chips to the flex hybrid are done through ultrasonic wedge bonding. The prototypes we describe do not have optical connections or flexible power connection (pig tails) and have been mounted on printed boards for testing. 2. Material budget The material budget for the modules presented in this paper is shown in Table 1. The normalization 0168-9002/01/$ - see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 3 9 1 - 6 P. Netchaeva et al. / Nuclear Instruments and Methods in Physics Research A 465 (2001) 204–210 205 Fig. 1. Module components. Table 1 Material budget for the produced thinned modules Item %X0 Sensor (280 mm thick) Front-end electronics (150 mm thick) MCC (560 mm thick) Kapton flex (50 mm thick+coverlay) Passive components on Kapton (R, C) Wirebond potting Wirebonds, bump bonds and glue (Kapton on sensor, MCC on flex hybrid) are individually negligible and they sum up to 0.34 0.21 0.04 0.05 0.03 0.03 50.01 Total 0.70 has been done to the module sensitive area (16.4 mm 60.8 mm). The total radiation length value meets the ATLAS Pixel Detector Technical Design Report [1] specifications, in particular electronics chips thinned down to 150 mm have been used. 3. Module production Module production begins with bump deposition on sensor and electronics wafers followed by the spinning of a thin polyamide layer to protect the bumps. The sensor tiles are then singled out, while the electronics wafers are thinned by back grinding before the FE chips are cut out. Finally 16 FE chips are flip-chipped to the sensor tile and the bare module is ready. The quality of this assembly is then checked with high (2 mm) resolution X-ray radiography as shown in Fig. 2. To operate the module it is necessary to dress it with a flex hybrid. This is a two-layer circuit on a 50 mm thick kapton substrate and is described in more detail in Ref. [2]. Flying probe test of each flex hybrid is performed before the MCC and the passive components are mounted. The fully loaded flex hybrid is shown in Fig. 3 and is tested by measuring reference voltages and reading/writing MCC registers. The next stage of the module production is gluing of the flex hybrid assembly to the bare module, the various steps of the procedure are illustrated in Fig. 4. The flex hybrid assembly is fixed on a special profile plate (3) not to damage the wire-bonded MCC and the passive components. The glue (EPOTEK 353) is deposited on the back plane of the flex hybrid under the wire bonding pads both of FE chips and of MCC. Optical alignment between bare module and flex hybrid is performed (5) under microscope with micrometric stages. The glue polymerization process takes 12 h at 408C, an infrared lamp maintains this temperature. The module is glued to the support printed board with silicon glue (Dow Corning 740) at room temperature. The polymerization time is 24 h. The quality of the thermal contact between support card and module is being tested with a SECTION IV. 206 P. Netchaeva et al. / Nuclear Instruments and Methods in Physics Research A 465 (2001) 204–210 Fig. 2. Microradiography test of the sensor-FE chips bump bondings. Left picture represents the ‘‘bare’’ module assembly i.e. the sensor tile bump bonded to the 16 FE chips. On the right picture there are two fragments of the X-rays image of this assembly where two kinds of defects are visible: shorts between bumps and small bumps. Fig. 3. Flex-hybrid circuit. Fig. 4. Gluing the flex hybrid to the bare module. P. Netchaeva et al. / Nuclear Instruments and Methods in Physics Research A 465 (2001) 204–210 207 Fig. 5. First thin module (LBNL) threshold and noise value distributions (see text for details). thermocamera taking images of the module in operation. The final step is the wire bonding of all FE chips to the flex hybrid. of dead pixels and the stability of all these values during the operation. To check the operation and define the characteristics of each pixel the following laboratory tests are performed: 4. Module tests * The main quality factors of the modules are: threshold and noise, both their mean values and their dispersion over the entire matrix, the number * Digital test: digital signals are injected after each pixel discriminator, this allows to verify the proper functioning of the readout chain. Analog test: analog signals are injected at each preamplifier input, this allows to measure the SECTION IV. 208 * P. Netchaeva et al. / Nuclear Instruments and Methods in Physics Research A 465 (2001) 204–210 threshold and noise values. After this test the tuning of threshold values for every FE chip is done. Test with source: 109Cd (22 KeV g), 241Am (60 KeV g), 90Sr (b) are used to verify the whole chain (sensor-readout) quality. The self-triggering capability of the electronics is used in this case. 5. Results Up to now three 0.7% X0 thick pixel modules have been produced. In all the three modules the thinned FE-B electronics [3] is used. The first thin module has been made in LBNL in November 1998. The electronics had been thinned to 150 mm by GDSI after bump deposition. The bump deposition and flip-chipping had been done by Boeing. The whole module worked well according to the digital test, one FE chip did not respond to analog charge injection. There have been some bump-bonding problems, including chips with regions of merged bumps and too large bump resistance. The mean threshold value for this module was 4800 10 The noise distribution had the maximum at 200 10 but a tail extending up to 1000 electrons (Fig. 5). Three FE chips were particularly noisy, by switching them off the high noise tail Fig. 6. Second thin module (INFN Genoa) threshold and noise value distributions. To find the maximum in the noise distribution the Gaussian fit has been done only for the first 20 channels. The tail toward large noise values is mostly due to the larger capacitance pixels at the periphery of each FE chip. P. Netchaeva et al. / Nuclear Instruments and Methods in Physics Research A 465 (2001) 204–210 (>500 10) disappeared and the mean noise of the remaining chips is of 270 10. Boeing had since then stopped bumping for outside customers, this delayed thin modules production for some time. The second thin module has been produced in INFN Genoa in April 2000. The electronics had been thinned to 156 mm by Okamoto. Bump deposition and flip-chipping was done by Alenia Marconi Systems (AMS). Digital test had shown good results. Only about 0.3% cells did not respond to the analog charge injection (dead channels): one FE chip had 30 dead channels, five FE chips had about 8– 10 dead channels, five FE chips had 1–2 dead channels and five FE chips did not have dead channels at all. The module works with a threshold value 3700 10 300 10 and a mean noise value of 225 10, the operating characteristics are stable over days. The distributions are shown in Fig. 6. The noise distribution has a maximum at 170 10 and a tail up to 600 10. 209 The source (109Cd) scan plot is shown in Fig. 7. The number of g-rays converted in the various pixel cells is represented according to the color scale. Since the source first encounters the flex hybrid the shadows of electronic components and MCC (the big square in the center) are visible. The third thin module has been produced in INFN Genoa in cooperation with AMS in May 2000. It works well digitally and analog scan shows very few dead channels (again about 0.3%) for all the FE chips except one FE chip which did not work properly and was therefore excluded. We operated this module with a threshold value of 4200 10 330 10 and a mean noise value of 225 10. The last two thin modules were recently operated on the H8 test beam at CERN, laboratory results have been confirmed including good stability. As an example, Fig. 8 shows the correlation between the hits measured by the pixel module and the hits measured by the microstrip telescope used to reconstruct the beam tracks. Fig. 7. Source (109Cd) scan plot of the second thin module. The shadows of electronics components and MCC (the big square in the center) are visible. Because of a design error only 10 out of 18 columns of each FE chips are operational. SECTION IV. 210 P. Netchaeva et al. / Nuclear Instruments and Methods in Physics Research A 465 (2001) 204–210 Fig. 8. Test beam online correlation plot shows the beam coordinate as measured by the microstrip telescope (horizontal axis) versus the same quantity as measured by a pixel module (vertical axis). The left (right) plot refers to the long (short) size of the pixel. Two different reference systems were used for the microstrips and the module measurements, therefore the correlation angle on the online plots is inverted. 6. Conclusions References Three modules of 0.7% X0 have been built and characterized. The most recent results indicate that we can fabricate modules with thinned electronics which obtain high yield (0.3% of dead channels) and operate them at low threshold (3700 10 300 10) and noise (225 10). [1] ATLAS collaboration, ATLAS Pixel Detector TDR, CERN/LHCC/98-13 (1998). [2] P. Skubic, Nucl. Instr. and Meth. A 465 (2001) 219, these proceedings. [3] K. Einsweiler et al, LBNL, FE-B Front-end Guide V1.0, Feb12 1999, http: //wwwphysics.lbl.gov/einsweil/Rev Feb99/FEBManual.pdf.