NOPE - Noise Optimized DEPFET devices for Dark Matter Exploration 21st International Workshop on DEPFET Detectors and Applications NOPE - Noise Optimized DEPFET devices for Dark Matter Exploration Ringberg, 30.5.2015 J. Treis, A. Bähr, J. Ninkovic MPG Semiconductor Laboratory J. Schieck Institute for High Energy Physics Austrian Academy of Sciences Johannes Treis / Halbleiterlabor der MPG

Johannes Treis / Halbleiterlabor der MPG Content Application Challenges Why DEPFET RNDR? Performance model Scope of NOPE Device proposal Johannes Treis / Halbleiterlabor der MPG

Johannes Treis / Halbleiterlabor der MPG Idea Detect low mass WIMP interactions with Detector material Detector bulk acts as interaction medium and detector at the same time Detect energy deposition by both nuclear recoils and electron scattering "Put detector in a big box, bury it as deeply as possible and wait." Material: Courtesy X. Bartou / CNEA CONICET Johannes Treis / Halbleiterlabor der MPG

Johannes Treis / Halbleiterlabor der MPG Challenges I Experiment different from what we did so far in nearly every respect Readout time does NOT matter Frame rate ~ 1 mHz (100 ks) Shaping time no issue As much material as possible (~ kg) Large detector thickness Extremely low temperature Suppression of leakage current Radiation damage is completely irrelevant ~ t of copper / lead shielding for detector for optimum background suppression Johannes Treis / Halbleiterlabor der MPG

Johannes Treis / Halbleiterlabor der MPG Challenges II Extremely rare events Unknown cross section Low energy transfer + quenching factor (Lindhard model) All sorts of background need to be extremely well known and calibrated Extremely precise and long-term stable operation Use spatial correlation to distinguish background Use time correlation to distinguish background Principle successfully demonstrated by DAMIC experiment using 15 x 15 mm2 MOS CCDs Johannes Treis / Halbleiterlabor der MPG

Johannes Treis / Halbleiterlabor der MPG Background Use pattern size and shape to detemine depth of interaction Helps to discriminate events from surfaces Charge loss to interfaces / partial charge deposition compromizes energy measurement Material: Courtesy X. Bartou / CNEA CONICET Johannes Treis / Halbleiterlabor der MPG

Johannes Treis / Halbleiterlabor der MPG Background Use diffusion information for cosmics track reconstruction Coincidence analysis / veto including inforamtion from multiple planes Material: Courtesy X. Bartou / CNEA CONICET Johannes Treis / Halbleiterlabor der MPG

Radiopurity of detector material Contamination with 32Si main source of radionuclide-induced background Discrimination by detection of sequence of b decays Detection of a decays easier due to their high energy deposition Material: Courtesy X. Bartou / CNEA CONICET Johannes Treis / Halbleiterlabor der MPG

Johannes Treis / Halbleiterlabor der MPG Shielding Material: Courtesy X. Bartou / CNEA CONICET Johannes Treis / Halbleiterlabor der MPG

Johannes Treis / Halbleiterlabor der MPG Shielding Material: Courtesy X. Bartou / CNEA CONICET Johannes Treis / Halbleiterlabor der MPG

Johannes Treis / Halbleiterlabor der MPG Challenges Principle successfully demonstrated by DAMIC experiment Using 15 x 15 mm2 MOS CCDs w/ 100 'eds of ks exposure time Material: Courtesy X. Bartou / CNEA CONICET Johannes Treis / Halbleiterlabor der MPG

Johannes Treis / Halbleiterlabor der MPG Why DEPFETs? All sorts of background discrimination strongly depend on noise threshold Charge spreading / recombination Detection of low energy transfer scattering events Tradeoff between readout time and temperature Use DEPFET with RNDR readout nodes NOPE: Explore feasibility of DEPFET based detector device Johannes Treis / Halbleiterlabor der MPG

Repetitive non-destructive readout Detector performance limited due to 1/f noise limit but noise limit can be beat using the technique of repetitive non-destructive readout (RNDR) Dedicated DEPFET structures (Ping-Pong device) form ideal platform for efficient implementation of this technique Sub-electron noise levels can easily be achieved For statistically independent measurements: The "effective" noise therefore is the standard deviation of the distribution of the mean values: Johannes Treis / Halbleiterlabor der MPG

Johannes Treis / Halbleiterlabor der MPG DEPFET RNDR devices DEPFET repetitive non-destructive readout (RNDR) 2 DEPFET “sub”pixels in 1 “super” pixel intra-pixel charge transfer via transfer gate allows for statistically independent measurements using CDR overcome 1/f-noise limit Resolution significant quantity is the standard deviation of the mean of the n readings rather than the noise of a single measurement "noise reduction" by n-½ @ n readings "Effective" noise ENCeff sub-electron noise: 0.18 e- ENCeff distinguish between different numbers of single electrons Johannes Treis / Halbleiterlabor der MPG

Johannes Treis / Halbleiterlabor der MPG DEPFET RNDR devices Source (common) Transfer- gate State-of-the-art "compact" circular RNDR pixel Easy to integrate in matrix environment Gate 2 Drain (common) Clear & Cleargate Gate 1 Johannes Treis / Halbleiterlabor der MPG

Johannes Treis / Halbleiterlabor der MPG Performance model DEPFET device is permanently sensitive Charge can enter internal gate during processing Bulk generated leakage current electrons Event charge ("Misfits") Weighted differently depending on arrival time Correction term to n-1/2 (Baer's equation) Bähr's equation: Optimum number of cycles: Optimum effective noise: Johannes Treis / Halbleiterlabor der MPG

Johannes Treis / Halbleiterlabor der MPG Performance model Johannes Treis / Halbleiterlabor der MPG

Johannes Treis / Halbleiterlabor der MPG Performance model Johannes Treis / Halbleiterlabor der MPG

Johannes Treis / Halbleiterlabor der MPG Performance „Few electron“ signal generated by weak laser Johannes Treis / Halbleiterlabor der MPG

Johannes Treis / Halbleiterlabor der MPG Performance Weak laser illumination Background? Johannes Treis / Halbleiterlabor der MPG

Johannes Treis / Halbleiterlabor der MPG Performance Holds valid also for large numbers of electrons Background? Johannes Treis / Halbleiterlabor der MPG

Johannes Treis / Halbleiterlabor der MPG Performance model Johannes Treis / Halbleiterlabor der MPG

Johannes Treis / Halbleiterlabor der MPG Gating Background is caused by permanent sensitivity of DEPFET bulk Introduce "gating" feature: decouple bulk from internal gates on demand Done by additional NMOS Structure (blind / blindgate) Benefits: Suppression of bulk leakage current induced deviation from n-1/2 behaviour Suppression of MISFIT events Fast (~100ns) shutter capability for global connection of contacts Drawbacks: Deadtime is introduced Deadtime-free approaches not suitable for small area pixels Only sequential global clear for small area devices Johannes Treis / Halbleiterlabor der MPG

Johannes Treis / Halbleiterlabor der MPG Gating Johannes Treis / Halbleiterlabor der MPG

NOPE Prototype testing Gather operational experience on larger arrays in matrix connection Existing prototype matrices in compact circular topology 64 x 64 pixels of 75 x 75 µm2 Sensitive area 4.5 x 4.5 mm2 Variants with and without global shutter / blindgate & blind Experimental setup designed for (later) potential application within low background environment Expore voltage parameter space Explore temperature space Explore exposure time space Test various operation modes (e.g. incremental / absolute measurements etc.) Evaluate effect of shutter capability wrt. background and timing Johannes Treis / Halbleiterlabor der MPG

Johannes Treis / Halbleiterlabor der MPG Prototype testing Johannes Treis / Halbleiterlabor der MPG

Johannes Treis / Halbleiterlabor der MPG Prototype testing Johannes Treis / Halbleiterlabor der MPG

Johannes Treis / Halbleiterlabor der MPG Prototype testing Johannes Treis / Halbleiterlabor der MPG

Small pixel device layouts Global Clear variant Blinds also used for clearing 36 x 36 mm2 Johannes Treis / Halbleiterlabor der MPG

Johannes Treis / Halbleiterlabor der MPG Device proposal Monolithic detector with gated DEPFET RNDR pixels Overall pixel size 36 x 36 mm2 or smaller "Global clear" design Overall array size 1k x 1k pixels Device dimension ~ 3.7 x 3.7 cm2 Device thickness: 1 mm (2 mm possible?) Fully depleted Mass ~3.2 g / detector "Global clear" or standard compact design Initial noise of < 1.5 e- Target ENCeff of << 0.2 e- Alternative approach: replace FirstFET of pnCCD by DEPFET RNDR readout node Sub electron resolution with electrically simpler device Johannes Treis / Halbleiterlabor der MPG

Johannes Treis / Halbleiterlabor der MPG Summary & Outlook DEPFET structures allow for the application of non- destructive repetitive readout Additional integrated gating feature allows for better approximation to "ideal" n -1/2 behavior Matrix devices with 75 mm x 75 mm size are currently under evaluation "Old" technology, but insights in background, long- term stability,…. Pixel sizes down to 36 mm x 36 mm possible Monolithic dies on the several cm2 size possible Bulk thickness up to 1 mm without changes to technology 2 mm bulk thickness not out of scope, but requires R&D 1 2 - / n Next step: Beating the n-1/2 limit. Johannes Treis / Halbleiterlabor der MPG