Silicon sensors for radiant signals. D.Sc. Mikko A. Juntunen

Size: px

Start display at page:

Download "Silicon sensors for radiant signals. D.Sc. Mikko A. Juntunen"

Gordon McLaughlin
5 years ago
Views:

1 Silicon sensors for radiant signals D.Sc. Mikko A. Juntunen

2 Today s outline Introduction Basic physical principles PN junction revisited Applications Light Ionizing radiation X-Ray sensors in medical applications Group discussion about sensitivity of radiant sensors

3 Introduction to radiation sensors The very first sensors were radiant sensors well known technology Radiant signals can be divided into two groups Electromagnetic radiation (photodetectors, X-ray detectors) Nuclear-particle radiation (particle detectors) (Is there really a difference? Wave-particle duality ) Application areas range from automotive twilight detectors through optical communications, bioluminescence and radiation dosimetry to medical imaging and particle physics. These were just examples, but all require a detector to convert radiation into an electrical signal. Selection criteria: wavelength and intensity of light

4 Electromagnetic Radiation E = hn = hc p / l Intensity, Phase

5 Electromagnetic radiation

6 Similar to solar cells photodetectors benefit from AR coatings The graph below shows the reflectivity of a special image sensor (CCD). The sensor was designed for a maximum blue response and it has an anti-reflective coating optimised to work at 400nm. At this wavelength the reflectivity falls to approximately 1%.

Photoelectric effect Incoming photon creates an electron-hole pair photoconductivity In the absence of an external electric field the hole and electron will quickly recombine and be lost.

7 Photoelectric effect Incoming photon creates an electron-hole pair photoconductivity In the absence of an external electric field the hole and electron will quickly recombine and be lost. An electric field is introduced to sweep these charge carriers apart and prevent recombination light generates open circuit voltage Back surface Conductance band Band gap 1,12 ev Valence band P-type Depletion region Hole Released electron N-type Front surface V oc ~0,6 V Photon Thermally generated electrons are indistinguishable from photo-generated electrons. They constitute a noise source known as Dark Current. Lowering the operation temperature Dark Current can be minimised.

8 Quantum efficiency (QE) QE is the amount charge carrier collected compared to the amount of photons entering the detector surface If each photon generates a charge, and each charge is collected, QE = 1 = 100% External losses (EQE) Reflectance from the surface Absorption in the material above the depletion region (AR coating, metal contacts ) Internal losses (IQE) Absorption coefficient of the material (band gap) Recombination (defects) For better QE Maximize the depletiond region width Minimize the dead layer width above Maximize collection efficiency within device

Factors affecting quantum efficiency: Loss of photons into entry window, typically nitride and oxide => make it thin, optically optimized Loss of photons before entering depletion region => make

9 Factors affecting quantum efficiency: Loss of photons into entry window, typically nitride and oxide => make it thin, optically optimized Loss of photons before entering depletion region => make implatation thin to extend depletion close to surface Almost one electron released per photon nm With photon energy > 1,12 ev, extra energy is lost in electron kinetic energy => warming up the diode For energies less than 1,12 ev, silicon is transparent Loss of photons through depletion region => make depletion deep enough => use high purity low resistivity substrate materials

10 Basic readout schemes (1/2) Light current/charge integrator Output voltage ~ accumulated charge Volatage recorded periodically, and resetting to start new collection cycle Current to voltage follower: Output voltage ~ input current = light intensity

11 Basic readout schemes (2/2) Basic circuit for pulse counting: short fast pulse charges the capacitor resistor will reset it automatically Output voltage peaks shortly peak hight ~ incoming pulse energy typically followed by a shaping amplifier + MCA, or a counter

12 Applications

13 Photoconductor The most simple photodetector Based on photoconductivity Operation principle: Incoming light changes the resistivity, which can be measured Used to measure light intensity in simple applications Optimization The detector thickness d must be much greater than the inverse of the absorption coefficient (for total absorption) A compromise between the amplification and the response time of the photoconductor The photoionization cross-section must be as large as possible

14 Photodiodes Light creates e-h pairs Revese biased diode Benefits of PIN vs. PN Sensitivity (larger photosensitive volume) Higher lifetime and mobility High breakdown voltage, high reverse bias allowed Speed (smaller capacitance)

15 Other types of photosensors

16 Impact ionisation (avalanche multiplication) Highly energetic carrier donates its excess energy for another carrier creating a new electronhole pair Due to the high electric field the new e-h pair can generate further new e-h pairs

Avalanche photodiodes Operation principle Large reverse bias voltage accelerates the minority carriers multiple electron-hole pairs created Properties The quantum

17 Avalanche photodiodes Operation principle Large reverse bias voltage accelerates the minority carriers multiple electron-hole pairs created Properties The quantum efficiency is higher than in normal photodiodes The response speed is shorter Uniform gain Excellent short wavelength sensitivity detection of low-level light at high speed - noise?

18 Quadrant photodiodes Four photodetectors positioned in the quadrant geometry Applications in laser alignment and light/satellite positioning Light-impenetrable window over the device -> light does not illuminate equally the detectors

19 Position-sensitive detectors Sun position photodetectors Based on quadrant photodiodes a min = arctan(2 h/ W) a min < arctan(2 h/ 3 W)

20 CCD (charge coupled device) image sensor CCDs work by converting light into a pattern of electronic charge in a silicon chip. This pattern of charge is converted into a video waveform, digitised and stored as an image file on a computer.

Charge-coupled devices Consist of an array of MOS-capacitors An image builds up that consists of a pattern of electric charge. Charge acts as a signal.

21 Charge-coupled devices Consist of an array of MOS-capacitors An image builds up that consists of a pattern of electric charge. Charge acts as a signal. At the end of the exposure this pattern is then transferred, pixel at a time, by way of the serial register to the onchip amplifier Charge is transferred by applying gate voltages Applications of 21silicon sensors for

22 Charge collection incoming photons pixel boundary pixel boundary Charge packet n-type silicon p-type silicon Electrode Structure SiO2 Insulating layer

23 Charge Transfer in a CCD V 0V -5V +5V 0V -5V +5V 0V -5V Time-slice shown in diagram

24 Charge Transfer in a CCD V 0V -5V +5V 0V -5V +5V 0V -5V

25 Charge Transfer in a CCD V 0V -5V +5V 0V -5V +5V 0V -5V

26 Charge Transfer in a CCD V 0V -5V +5V 0V -5V +5V 0V -5V

27 Charge Transfer in a CCD V 0V -5V +5V 0V -5V +5V 0V -5V

28 Charge Transfer in a CCD 7. Charge packet from subsequent pixel enters from left as first pixel exits to the right V 0V -5V +5V 0V -5V +5V 0V -5V

29 Charge Transfer in a CCD V 0V -5V +5V 0V -5V +5V 0V -5V

30 Fabrication of CCD CCDs are are manufactured on silicon wafers using the same photo-lithographic techniques used to manufacture computer chips. Scientific CCDs are very big,only a few can be fitted onto a wafer. This is one reason that they are so costly. Don Groom LBNL

31 Thick Front-side Illuminated CCD Incoming photons p-type silicon 625mm n-type silicon Silicon dioxide insulating layer Polysilicon electrodes These are cheap to produce using conventional wafer fabrication techniques. They are used in consumer imaging applications. Even though not all the photons are detected, these devices are still more sensitive than photographic film. They have a low Quantum Efficiency due to the reflection and absorption of light in the surface electrodes. Very poor blue response. The electrode structure prevents the use of an Anti-reflective coating that would otherwise boost performance.

32 Thinned back-side illuminated CCD 15mm Incoming photons Anti-reflective (AR) coating p-type silicon n-type silicon Silicon dioxide insulating layer Polysilicon electrodes The silicon is chemically etched and polished down to a thickness of about 15microns. Light enters from the rear and so the electrodes do not obstruct the photons. The QE can approach 100%. These are very expensive to produce since the thinning is a non-standard process that reduces the chip yield. These thinned CCDs become transparent to near infra-red light and the red response is poor. Response can be boosted by the application of an anti-reflective coating on the thinned rear-side. These coatings do not work so well for thick CCDs due to the surface bumps created by the surface electrodes.

33 Comparision of quantum efficiency

34 CMOS vs. CCD

35 Color separation

36 Nuclear particle radiation detectors Nuclear particles create electronhole pairs within the detector Electric field separates the electrons and holes and they are collected from their respective contacts High energy particles may create 85 electron-hole pairs per micron when penetrating inside the silicon wafer In order to achieve a good signal to noise level a sensitive volume of a few hundred cubic microns is therefore required

37 Development of tileable silicon photodiodes for X-ray imaging

38 CT = Computed Tomography

39 I( x) = I exp( -a x) os N j wij mi = -ln i= 1 I 0 I

40 CT in diagnostics 1/4 Sharpness

41 CT in diangostics 2/4

42 CT in diagnostics 3/4 Dynamic range?

43 CT in diagnostics 4/4 54-year-old female No obesity Atypical chest pain Hypercholesterolemia Family history of coronary artery disease Cardiac CT demonstrate healthy heart Resolution?

44 Detectors the eyes of the machine multi slice imaging Photodiode matrix covered with scintillator to convert X-rays to visible light Wiring from the edges

expansion Thickness about 25 µm, half of human hair How to make

45 Why tileable detectors Larger detectors needed Desire to image the whole organ in one rotation Need to connect from edges prevent expansion Thickness about 25 µm, half of human hair How to make detectors that can be tiled next to each other to form arbitrarily large detecting surface

46 Tileable technologies developed 1/3 Cathode Readout Cross talk? UK patent GB B, US Patent 7,375,340 B2, etc.

47 Tileable technologies developed 2/3 Through Silicon Vias 50 µm Al SiO SiO 2 2 0,002 mm 2 p anode area Poly Si conductor n type Sin type Si substrate substrate p anode area n type Si substrate Al anode contact Al cathode contact UK patent GB B

48 Method Etching the T Etching through PolySi fill Grind and polish Poly Si Scale: hair 2 µm SiO 2 Si Si Scale: red blood cells UK Patent GB B

49 Demonstration module with TSV s

50 Tileable technologies developed 3/3 Back Side Illuminated Diodes Light Signal (charge) is generated within first few 1 µm of the diode back side. N+ - implantation N+ - implntation Diffusion Cross talk? Diode implantation Al SiO 2 A

51 Long Distance Diffusion Several ms charge carrier lifetime Several mm diffusion distance Treatment of back surface to reflect holes Collecting diode on front surface => virtually all signaali gets collected Still, thinning to ~100 µm improves the outcome

The Charge-Coupled Device. Many overheads courtesy of Simon Tulloch

The Charge-Coupled Device. Many overheads courtesy of Simon Tulloch The Charge-Coupled Device Astronomy 1263 Many overheads courtesy of Simon Tulloch smt@ing.iac.es Jan 24, 2013 What does a CCD Look Like? The fine surface electrode structure of a thick CCD is clearly visible