1. 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,
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

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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

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

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

11. 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

12. 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

13. 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

14. 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 readings
"Effective" noise ENCeff
sub-electron noise: 0.18 e- ENCeff
distinguish between different numbers of single electrons
Johannes Treis / Halbleiterlabor der MPG

15. 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

16. 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

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

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

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

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

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

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

23. 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

24. Johannes Treis / Halbleiterlabor der MPG
Gating
Johannes Treis / Halbleiterlabor der MPG

25. 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

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

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

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

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

30. 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

31. 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