Control of the fabrication process for the sensors of the CMS Silicon Strip Tracker. Anna Macchiolo. CMS Collaboration

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1 Control of the fabrication process for the sensors of the CMS Silicon Strip Tracker Anna Macchiolo Universita di Firenze- INFN Firenze on behalf of the CMS Collaboration 6 th International Conference on Large Scale Applications and Radiation Hardness of Semiconductor Detectors

2 Sensor Fabrication Center HPK Silicon Sensor Test Strategy in CMS Control & Distribution Center CERN & Production Committee Sensor Fabrication Center STM The aim of the Process Qualification Centers Florence Strasbourg Vienna is to ensure a constant quality throughout the production and to monitor the companies process 25% 25% 25% 25% Quality Test Center Pisa Quality Test Center Perugia Quality Test Center Vienna Quality Test Center Karlsruhe The CMS SST is in full sensor production phase: 35 % of HPK sensors 15 % of STM sensors have been already delivered 1% sensors ~5% ts Irradiation Qualification Centers Louvain, Karlsruhe ~5% ts Bonding Test Centers Pisa, Strasbourg >5% ts Process Qualification & Stability Centers Florence Strasbourg, Vienna More than 1200 test-structures have been tested by the PQC Centers up to now (during the pre-production we have largely exceeded the nominal assignment of 5 % of the total number of t.s.) RD03 Process Quality Control for the CMS SST Anna Macchiolo 2

3 SST Standard test-structures Standard Half Moon A common set of 9 test structures with the same design is inserted in all the SST wafers, whatever the sensor type (rectangular or wedge) or the producer is. The test-structures allows to perform measurements that are not possible on the main sensor or destructive tests. RD03 Process Quality Control for the CMS SST Anna Macchiolo 3

4 The Standard Half-Moon 26 AC coupled strips: quality of the integrated capacitor dielectric Gate Controlled Diode : surface current at the Si- SiO 2 interface IV on minisensor from 0 to 700 V CV on diode Bulk resistivity Sheet: 9 superficial structures to monitor Aluminum and p+ surface resistivity, poly-silicon resistors value 9 AC coupled strips: inter-strip capacitance 9 DC coupled strips: interstrip resistance CV on MOS Flat Band Voltage RD03 Process Quality Control for the CMS SST Anna Macchiolo 4

5 PQC Set-Up The PQC equipment in the Florence clean-room RD03 Process Quality Control for the CMS SST Anna Macchiolo 5

6 PQC Hardware GPIB 40 channels Probestation Switching box 4 multiplexers multiplexers 10 x 1 10 A Source Measure Unit Probe-card Switching matrix 4 x 5 C Vsource LCR It has been possible to adopt the same set-up in the three PQC centers because the labs have common or very similar equipment. During the pre-production phase a calibration program among the three labs has been followed with an exchange of test-structures to ensure the full compatibility of results. Computer running Labview RD03 Process Quality Control for the CMS SST Anna Macchiolo 6

7 Acquisition Software The Labview acquisition program: performs the 9 measurements in an automatic run analyses and fit the results when necessary set a flag that identifies the positive outcome of the test. Results are stored: The results of the measurements are visualized in the Labview front-panel to allow an easy check of the output. in local files in a XML file that can be inserted directly in the centralized database for the CMS Tracker construction in Lyon. A complete test takes about 30 minutes RD03 Process Quality Control for the CMS SST Anna Macchiolo 7

8 The PQC measurements and first results RD03 Process Quality Control for the CMS SST Anna Macchiolo 8

9 Quality of the dielectric in the integrated capacitors Test on an array of 26 AC coupled strips Both in HPK and STM the capacitor dielectric is composed by thin layers of SiO 2 and Si 3 N 4 Constance in time of the capacitance value (Cac > 1.2 pf/cm per µm of implanted strip) Breakdown voltage of the dielectric layer of the capacitor (defined as the voltage at which the current across the dielectric is > 10 µa) It is foreseen that during catastrophic beam-loss events (expected at the level of 1-2 per year) a voltage difference around one hundred Volts can be present at the two faces of the capacitors Structures/ bin Structures/ bin Breakdown voltage of the dielectric layer HPK Our acceptance criteria is V break > 120 V STM Voltage applied to the dielectric The measurement is performed between 0 and 200 V Voltage applied to the dielectric RD03 Process Quality Control for the CMS SST Anna Macchiolo 9

10 Quality of the inter-strip insulator In the Standard Moon the MOS and GCD are built with the thicker oxide (1 2 µm) that characterizes the inter-strip region in the full-size sensor Measurement of the Flat Band Voltage at 100 KHz with the CV on MOS (V fb < 10 V) Measurement of Surface Current with the IV on a square GCD (I GCD < 100 pa) Total Capacitance (F) 1.4E E E E E E-10 Accumulation Fitted value of Vfb = 1.2 V Diode current (A) 8.0E E E E E E E E-11 Fitted value of the surface current = 40 pa Accumulation 2.3E-10 Inversion 4.0E-11 Inversion 3.0E E E-11 -Gate Voltage (V) Gate Voltage (V) RD03 Process Quality Control for the CMS SST Anna Macchiolo 10

11 Problem with the Vfb in STM Flat Band Voltage vs Production Time (STM) Switching from the R&D to the production line in STM at the beginning of 2003 caused some problems. We have found several batches with high Vfb. This effect has been traced back to the contamination introduced by a particular machine that has been now excluded from the production line. Flat Band Voltage (V) A single episode in week # 10 of 2002 Contaminated batches at the beginning of 2003 allowed Vfb range Year x week of production RD03 Process Quality Control for the CMS SST Anna Macchiolo 11

12 Dependence of the inter-strip capacitance on the Vfb value Irradiating the test-structures at fluences equivalent to 10 years of LHC has shown that a high Vfb, connected to a bad quality of the inter-strip Si oxide, leads to a large increase in the inter-strip capacitance. 1,2 1,1 Interstrip Capacitance [pf] 1,2 1,1 1,0 0,9 0,8 0,7 Before Irradiation: V FB =5V V FB =7V V FB =20V V FB =38V 0, Strip Number Interstrip Capacitance [pf] 1,0 0,9 0,8 0,7 After Irradiation: V FB =5V V FB =7V V FB =20V V FB =38V Proton irradiation at the cyclotron of the Forschungszentrum Karlsruhe 0, Strip Number RD03 Process Quality Control for the CMS SST Anna Macchiolo 12

13 Diode Depletion Voltage (V) Bulk resistivity The CMS Tracker request to have depletion voltages below 400 V after 10 LHC years leads to the following requirements on the bulk resistivity measured with CV at 100 KHz on diode: 1.25 < ρ < 3.25 KΩcm Inner Tracker (HPK) 3.5 < ρ < 7.5 KΩcm Outer Tracker (STM) Diode Depletion Voltage vs Production Time (HPK) HPK Allowed resistivity range Agreement with HPK: o Deliver all sensors with bulk resistivity within the allowed range o Choose the wafers with lower resistivity for the sensor types that will be mounted in the inner layers Limit corresponding to 3.25 KΩcm RD03 Process Quality Control for the CMS SST Anna Macchiolo 13 Year x week of production

14 Aluminum resistivity Aluminum Surface Resistivity (STM) We measure the Al surface resistivity using long and narrow metal strips implanted directly on the n bulk. The resistance of the Al layer deposited on top of the strips is a parameter that contributes to the input noise seen by the front-end electronics. At the beginning of STM production the surface resistivity was exceeding our specifications (ρ Al < 30 m Ω square ) An increase in the Aluminum implant depth from 1.2 to 2.0 µm solved the problem. Structures/ bin Structures/ bin STM before May 2002 Mean value = 28 m Ω square ρ Al (mω square) STM after May 2002 Mean value = 19 m Ω square ρ Al (mω square) RD03 Process Quality Control for the CMS SST Anna Macchiolo 14

15 Mini-Sensor Breakdown Voltage Breakdown voltage of the mini-sensor Quality Test Centers perform IV on the sensors up to 550 V The mini-sensor in the Standard Moon has the same design characteristics as the fullsize sensors Most of mini-sensors 95 % for STM 98 % for HPK show no sign of breakdown (defined by I > 10 µ A) up to 700 V, where we stop the measurement Structures/ bin Structures/ bin Mini-sensor breakdown voltage (V) STM HPK Mini-sensor breakdown voltage (V) RD03 Process Quality Control for the CMS SST Anna Macchiolo 15

16 Inter-strip Capacitance Inter-strip Capacitance vs Production Time The inter-strip capacitance (C int ) is measured excluding the contribution of the second neighbor strips It depends on the strip pitch and width. In the case of our test-structures (strip width = 30 µm and pitch = 120 µm ) we expect C int around 0.8 pf per cm. This is a typical value also for the full-size sensors A good stability in time is observed both for HPK and STM Inter-strip Capacitance (pf) High values of C int corresponding to batches rejected due to high Vfb Inter-strip capacitance allowed range STM Year x week of production RD03 Process Quality Control for the CMS SST Anna Macchiolo 16

17 Conclusions A working system has been developed to monitor the fabrication process for the CMS SST sensors. We have proved efficient to recognize problems as soon as they appeared (Aluminum resistivity, bulk resistivity, Flat Band Voltage). The collaboration with the two manufacturer companies has allowed us to solve these problems without affecting sensibly the sensor production. Many more hundreds of test-structures ahead of us RD03 Process Quality Control for the CMS SST Anna Macchiolo 17

18 Surface Current vs Flat Band Voltage Flat Band Voltage (V) Back-up 1 RD03 Process Quality Control for the CMS SST Anna Macchiolo 18 Surface current (pa) STM

19 Back-up 2 p + -implant surface resistivity Acceptance interval for p + -implant surface resistivity : 0 < ρ < 400 Ω square HPK STM Ω square Ω square HPK STM Acceptance interval for poly-silicon bias resistors : 1 < ρ < 2 GΩ GΩ Poly-silicon resistor value GΩ RD03 Process Quality Control for the CMS SST Anna Macchiolo 19

20 Back-up 3 Inter-strip resistance Measured among one central strip and the two neighboring ones. The strips are DC coupled and without poly-silicon bias resistors. Acceptance interval for inter-strip resistance : Rpoly > 1 G Ω HPK STM GΩ RD03 Process Quality Control for the CMS SST Anna Macchiolo 20 GΩ