Detectors

1. Detectors RIT Course Number 1051-465 Lecture CCDs

2. Aims for this lecture • To describe the basic CCD • physical principles • operation • and performance of CCDs • Given modern examples of CCDs

3. CCD Introduction • A CCD is a two-dimensional array of metal-oxide-semiconductor (MOS) capacitors. • The charges are stored in the depletion region of the MOS capacitors. • Charges are moved in the CCD circuit by manipulating the voltages on the gates of the capacitors so as to allow the charge to spill from one capacitor to the next (thus the name “charge-coupled” device). • An amplifier provides an output voltage that can be processed. • The CCD is a serial device where charge packets are read one at a time.

4. CCD Physics

5. Semiconductors • A conductor allows for the flow of electrons in the presence of an electric field. • An insulator inpedes the flow of electrons. • A semiconductor becomes a conductor if the electrons are excited to high enough energies, otherwise it is an insulator. • allows for a “switch” which can be on or off • allows for photo-sensitive circuits (photon absorption adds energy to electron) • Minimum energy to elevate an electron into conduction is the “band gap energy”

6. Periodic Table • Semiconductors occupy column IV of the Periodic Table • Outer shells have four empty valence states • An outer shell electron can leave the shell if it absorbs enough energy

7. Simplified silicon band diagram Conduction band Eg bandgap Valence band

8. Semiconductor Dopants

9. PN Junctions • In a PN junction, positively charged holes diffuse into the n-type material. Likewise, negatively charged electrons diffuse in the the p-type material. • This process is halted by the resulting E-field. • The affected volume is known as a “depletion region”. • The charge distribution in the depletion region is electrically equivalent to a 2-plate capacitor.

10. A photon can interact with the semiconductor to create an electron-hole pair. The electron will be drawn to the most positively charged zone in the PN junction, located in the depletion region in the n-type material. Likewise, the positively charged hole will seek the most negatively charged region. Each photon thus removes one unit of charge from the capacitor. This is how photons are detected in both CCDs and most IR arrays. Photon detection in PN junctions

11. MOS Capacitor Geometry • A Metal-Oxide-Semiconductor (MOS) capacitor has a potential difference between two metal plates separated by an insulartor.

12. Surface Channel Potential Well

13. Potential in MOS Capacitor

14. CCD Readout

15. “Bucket Brigade” C:\figerdev\RIT\teaching\Detectors 465 20083\source material\CCDMovieMOD.gif

16. CCD Readout Animation

17. CCD Readout Alternate Animation

18. Image area (exposed to light) Parallel (vertical) registers Charge motion Pixel Serial (horizontal) register Output amplifier Charge motion masked area (not exposed to light) CCD Readout Architecture Terms

19. CCD Clocking

20. p-type silicon n-type silicon CCD Phased Clocking: Introduction Photons entering the CCD create electron-hole pairs. The electrons are then attracted towards the most positive potential in the device where they create ‘charge packets’. Each packet corresponds to one pixel pixel boundary pixel boundary incoming photons Electrode Structure Charge packet SiO2 Insulating layer

21. +5V 0V -5V 1 2 1 2 +5V 0V -5V 3 3 +5V 0V -5V CCD Phased Clocking: Step 1 Time-slice shown in diagram

22. +5V 0V -5V 1 2 1 2 +5V 0V -5V 3 3 +5V 0V -5V CCD Phased Clocking: Step 2

23. +5V 0V -5V 1 2 1 2 +5V 0V -5V 3 3 +5V 0V -5V CCD Phased Clocking: Step 3

24. +5V 0V -5V 1 2 1 2 +5V 0V -5V 3 3 +5V 0V -5V CCD Phased Clocking: Step 4

25. +5V 0V -5V 1 2 1 2 +5V 0V -5V 3 3 +5V 0V -5V CCD Phased Clocking: Step 5

26. CCD Phased Clocking: Summary

27. CCD output circuit

28. CCD Readout Layout

29. CCD Readout Device

30. CCD Readout Device Closeup

31. CCD Enhancements

32. Buried channel CCD • Surface channel CCDs shift charge along a thin layer in the semiconductor that is just below the oxide insulator. • This layer has crystal irregularities which can trap charge, causing loss of charge and image smear. • If there is a layer of n-doped silicon above the p-doped layer, and a voltage bias is applied between the layers, the storage region will be deep within the depletion region. • This is called a buried-channel CCD, and it suffers much less from charge trapping.

33. Buried Channel Potential Well

34. Back Side Illumination • As described to now, the CCDs are illuminated through the electrodes. Electrodes are semi-transparent, but some losses occur, and they are non-uniform losses, so the sensitivity will vary within one pixel. The “fill factor” will be less than one. • Solution is to illuminate the CCD from the back side. • This requires thinning the CCD, either by mechanical machining or chemical etching, to about 15μm.

35. Photon Propogation in Thinned Device 15mm Incoming photons p-type silicon n-type silicon Silicon dioxide insulating layer 625mm Polysilicon electrodes Anti-reflective (AR) coating Incoming photons p-type silicon n-type silicon Silicon dioxide insulating layer Polysilicon electrodes

36. Random Walk in Field-Free Thick Device

37. Sweep Field

38. Short l QE Improvement from Thinning

39. CCD Performance

40. CCD Performance Categories • Charge generation Quantum Efficiency (QE), Dark Current • Charge collection full well capacity, pixels size, pixel uniformity, defects, diffusion (Modulation Transfer Function, MTF) • Charge transfer Charge transfer efficiency (CTE), defects • Charge detection Readout Noise (RON), linearity

41. Photon Absorption Length in Si

42. Well Capacity • Well capacity is defined as the maximum charge that can be held in a pixel. • “Saturation” is the term that describes when a pixel has accumulated the maximum amount of charge that it can hold. • The “full well” capacity in a CCD is typically a few hundred thousand electrons per pixel for today’s technologies. • A rough rule of thumb is that well capacity is about 10,000 electrons/um2. • The following gives a typical example (for a surface channel CCD).

43. Well Capacity and Blooming Blooming Spillage Spillage pixel boundary pixel boundary Overflowing charge packet Photons Photons

44. Blooming Example Bloomed star images

45. Read-Out Noise • Read noise is mainly due to Johnson noise in amplifier. • This noise can be reduced by reducing the bandwidth, but this requires that readout is slower.

46. Defects: Dark Columns Dark columns: caused by ‘traps’ that block the vertical transfer of charge during image readout. Traps can be caused by crystal boundaries in the silicon of the CCD or by manufacturing defects. Although they spoil the chip cosmetically, dark columns are not a big problem (removed by calibration).

47. Defects: Bright Columns Bright columns are also caused by traps . Electrons contained in such traps can leak out during readout causing a vertical streak. Hot Spots are pixels with higher than normal dark current. Their brightness increases linearly with exposure times Somewhat rarer are light-emitting defects which are hot spots that act as tiny LEDS and cause a halo of light on the chip. Bright Column Cluster of Hot Spots Cosmic rays

48. Charge Transfer Efficiency CTE = Charge Transfer Efficiency (typically 0.9999 to 0.999999) = fraction of electrons transferred from one pixel to the next CTI = Charge Transfer Inefficiency = 1 – CTE (typically 10– 6 to 10– 4) = fraction of electrons deferred by one pixel or more Cause of CTI: charges are trapped (and later released) by defects in the silicon crystal lattice CTE of 0.99999 used to be thought of as pretty good but …. Think of a 9K x 9K CCD

49. Charge Transfer Efficiency • When the wells are nearly empty, charge can be trapped by impurities in the silicon. So faint images can have tails in the vertical direction. • Modern CCDs can have a charge transfer efficiency (CTE) per transfer of 0.9999995, so after 2000 transfers only 0.1% of the charge is lost. good CTE bad CTE

50. Example: X-ray events with charge smearing in an irradiated CCD (from GAIA-LU-TN01) In the simplest picture (“linear CTI”) part of the original image is smeared with an exponential decay function, producing “tails”: original image after n transfers direction of charge transfer