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Transcript
The Punch-through Effect in Silicon Strip Detectors C. Betancourt1,2, A. Bielecki1, Z. Butko1, A. Deran1, S. Ely1, V. Fadeyev1, C.Parker1, N. Ptak1, H. F.-W. Sadrozinski1, J. Wright1 1 Santa Cruz Institute for Particle Physics, Univ. of California Santa Cruz, CA 95064 USA 2 Albert-Ludwigs Universität, Freiburg 79104, Germany Introduction The typically used AC-coupled silicon sensors at the LHC can develop very large voltages across the SiO2 layer, referred as the coupling capacitance. The metal readout traces are held close to ground due to low input impedance of the readout amplifier, ~1 kOhm. The strip implants could reach a significant voltage in the case of large charge accumulation in the bulk, which can occur in the instance of the beam loss. In extreme cases the electric field can collapse causing the implants of the sensor to float to unknown voltages. These large voltages on the implants can reach the order of the bias voltage, and thus can exceed the specification for the hold-off voltage of the coupling capacitor, which are typically qualified to 100V. • Readout electronics are connected to AC pads, which are coupled to the DC strip implants via a layer of SiO2 and nitride. • The punch-through effect is used to short the strip implants to the grounded bias rail, thus preventing excess voltages between the DC and the AC pads. • Special punch-through protection structures (PTP) have been developed to enhance the punch-through effect in an effort to bring down the punch-through voltage to a relatively safe value (below the 100V qualification of the coupling capacitors). • These sensors, part of the ATLAS07 sensor production [1], are broken up into several zones, which refer to the surface treatment used for strip isolation structures and their geometry. In addition, zone 4 sensors (A-D) are built with specially designed PTP structures. Z1 ! Z2 Theory and Method Z3 L aser N ear Z4D L aser F ar R bi as For large current, IV becomes space-charge limited and is given by R P T n ear • Kinks in the Reff vs Vtest curves represent punchThe Resistance for PTP structures [5] through to the next For large currents we are always in the SCL mode, and the nearest neighbor strips, resistance of a PTP structure can be written as a function of which is clearly seen voltage by when Measuring the voltage and current of the neighboring strips while And as a function of current by doing DC tests. where Ri m p Vn ear -+ -+ -+ -+ -+ -+ Rb Vf ar -+ -+ -+ -+ -+ -+ R P T f ar Rb Vbi as • Using the 4-resistor model we can calculate all relevant currents and resistances from measured implant voltages. • PTP structures “work”, i.e. limit the implant voltages at a saturation voltage Vfb’ much larger than VPT. • At high currents, the implant voltage can be written as Laser Injection PT Measurements [3] • DC measurements involve applying a test voltage between the DC pad of a test strip and the grounded bias rail, and then measuring resulting current. • The effective resistance Reff between the implant and the bias rail, which consists of the bias resistance in parallel to the punch-through resistance is then calculated and is given by Z4C The 4-R Model and the Implant Voltage IV characteristics for PTP structures DC Punch-through Measurements [2] Z4B Z4A • To mimic beam accidents, charge is injected into the sensor through the use of an IR cutting laser [4]. • The large amount of charge injected by the laser (~2x107 MIPs/cm2 over a 2mm x 2mm spot) is able to collapse the electric field inside the sensor, thus allowing the strip implants to float to large voltages that are only limited by the bias voltage and the effectiveness of PTP. • The parameter α = Rbulk/Rsat determines if we see voltage saturation: small α => V~Vbias (large Rsat), large α => V~Vfb (low Rsat). • Sensors with PTP structures (Z4x) show saturation of the implant voltage since α is large (low Rsat). • At lower laser intensities, the implant voltage depends on the ratio of bulk resistance to effective PTP resistance Rbulk / Reff, NOT on the effective PTP resistance ALONE. • At lower particle fluxes, Rbulk /Reff increases and thus reduces the implant voltages and the risk for sensor damage. 4 Rbulk/Reff vs. Attenuation Rbulk/Reff 3 • The punch-through voltage VPT for strip detectors is then defined as the point where Reff(VPT)=1/2 Rbias, which is not the same punch-through voltage as defined in the literature for npn or pnp structures. 2 1 W71-BZ3 W224-BZ4A (275Vbias) 0 0 1 2 3 4 5 Attenuation (Sheets) Results Near Side Far Side The Effectiveness of PTP Structures Conclusions • Protons and pions both increase the saturation voltage, while neutrons have no effect. The increase in saturation voltage already saturates at a relatively low proton fluence of 1e13 neq/cm2 (~1MRad). •This indicates that the origin of punch-through is mainly from surface charge, not bulk charge. • Punch-through protection still works even at high fluences. P-stop 1e13 P-stop 4e12 P-spray 2e12 P-spray 2e12 p-stop=4e12 cm^-2 130 120 110 Vfb (Volts) •Zone 4 sensors perform the best (lower implant voltage and larger α) compared to zone 2 and 3 sensors due to the reduced channel length. • Increasing p-dose density increases the saturation voltage Vfb. •RPTP is “low” ~ 10 kW (~ Rbulk) when current ≥ 1 mA • Finite implant resistance of 15 kW/cm isolates the PTP structure from the charge deposit. • Large difference between RPTnear and RPTfar even for non-PTP structures • The placement of the polysilicon biasing resistor significantly reduces the resistance between the implant and the bias rail. • This is explained by the fact that the bias resistor provides a gate in 3 terminal device (implant, bias resistor, bias rail), which increases the current flow between the implant and bias rail. • Increasing the coverage of the bias resistor over the channel length increases the effect of the gate, and hence increases the effectiveness of PTP structures. Radiation Damage • MICRON sensors have a lower Vfb than HPK zone 1 sensors, even though they have similar p-spray concentration and channel length. • The lower Vfb can be attributed to a larger gate effect due to the placement of the polysilicon bias resistor running more directly over the channel length. • By placing the biasing resistor directly over the channel length, a lower Vfb can be achieved. Pre-Rad Fluence=1e13 p/cm^2 100 90 80 70 60 50 Z4A Z4B Z4C Z4D RC Backplane Filter Network • Filter resistance at the backplane (10kΩ) gives a drop of about 10V at 1mA per strip. • Extending to 100 strips, the voltage drop becomes equal or greater than the bias, thus dropping the whole voltage before reaching the sensor. • Large implant voltages caused by beam losses can cause permanent damage to silicon detectors in high energy particle physics experiments. Punch-through protection structures are designed to limit these voltages. • The beam loss conditions are simulated by flooding the sensors with IR laser pulses, and measuring the implant voltages [4] • The 4 resistor model proposed earlier can accurately determine the punch-through characteristics of a given sensor. The model describes the detector at breakdown as a circuit with 4 resistors. • The parameter that determine the effectiveness of PTP structures are the ratio of the bulk resistance to the saturation resistance α [5]. • The finite implant resistance Rimp can isolate the region of electric field collapse from the PTP structure. • Increasing the p-dose density increases the saturation voltage Vfb for a given zone. • PTP structures with an effective gate (a la Micron) offer increased protection because the gate effect lowers the saturation voltage. • Tests on sensors irradiated with protons, pions and neutrons show that Vfb increases arise mainly from surface charges, not bulk charges. • The PTP technique was developed to deal with particular beam loss scenarios. They relay on large currents flowing through the structures (mA’s). In cases with distributed particle fluences, the PTP structures are not needed since either the field does not break down (Rbulk/ RPT >> 1), or the filter resistor at the back plane affords sufficient protection, clamping the bias voltage to ground. Acknowledgments • We acknowledge the help of the ATLAS and RD50 collaboration in the design and procurement of the silicon sensors • The invaluable assistance of the institutes at Ljubljana, CERN, PSI and KEK and their staff in carrying out the radiation. • We appreciate the discussions with Yoshinobu Unno and Kazuhiko Hara. References [1] Y. Unno et al, NIM A 636 (2011) pp. 24-30 [2] S. Lindgren et al, NIM A 636 (2011) pp. 111-117. [3] H.F.-W. Sadrozinski et al, NIM A 658 (2011) pp. 46-50 [4] T. Dubbs et al, IEEE Trans. Nucl. Sci. 47 (2000) pp. 1902-1906. [5] C. Betancourt, Physics M.S. Thesis, UC Santa Cruz (2011)