Application of Ionizing Radiation in Industrial Technologies

1. Applicazioni industriali delle radiazioni ionizzanti (Effetti della radiazione ionizzante nella Tecnologia CMOS) Andrea Candelori Istituto Nazionale di Fisica Nucleare and Dipartimento di Fisica, Padova

2. 5A. MOS and MOSFET: a brief review

3. The MOS capacitor MOS is the acronym of metal oxide semiconductor

4. The MOS capacitor band diagram Ef is the fermi level Eg is the energy gap Ec is the conduction band energy level Ev is the valence band energy level  is the electron affinity  is the work function q is the potential barrier

5. The MOS capacitor band diagram (accumulation and depletion) electron accumulation close to the SiO2/Si interface ns>Nd Vg>0 n-type silicon electron are pushed back from the SiO2/Si interface leaving the donor ionized ns<Nd Vg<0

6. The MOS capacitor band diagram (accumulation and depletion) Vg=VT<0 donor are ionized holes are attracted at the SiO2/Si interface hs=Nd n-type silicon Vg<VT<0

7. The MOS capacitor band diagram Flatband Accumulation Depletion Inversion

8. MOSFET MOSFET is the acronym of Metal Oxide Semiconductor Field Effect Transistor Applets: http://jas.eng.buffalo.edu/education/mos/mosfet/mosfet.html

9. N-channel MOSFET (conduction is due to electrons) The n-MOSFET threshold voltage (VT>0) is the voltage which has to be applied to the gate in order to form a conductive channel (electrons) between source and drain.

10. P-channel MOSFET: conduction is due to holes The p-MOSFET threshold voltage (VT<0) is the voltage which has to be applied to the gate in order to form a conductive channel (holes) between source and drain.

11. Measurement of the MOSFET threshold voltage Oxide thickness 5.5 nm W=2000 m L=0.36 m Threshold voltage (VT) for the p-MOSFET at the input at the APV-25 read-out electronics.

12. MOSFET operation in sub-threshold Above-threshold Sub-threshold Threshold Sub-threshold curve for the p-MOSFET at the input at the APV-25 read-out electronics.

13. B B B MOSFET operation above threshold VG>VT and VG is fixed Linear region VG>VT and VD<VG Pinch-off condition VG>VT and VD=VG Saturation region VG>VT and VD>VG

14. MOSFET operation above threshold Output curve for the p-MOSFET at the input at the APV-25 read-out electronics.

15. The MOSFET as an amplifier VOUT=VINgmRD gm=IDS/VG Trans-conductance

16. 5B. Radiation Effects in thick oxide (>10 nm)

17. Materials for study 1) T.R. Oldham and F. B. McLean, "Total Ion ionizing Dose Effects in MOS Oxides",IEEE Trans. Nucl. Sci., vol 50, n.3, June 2003, pp. 483-499, and references therein. 2) J. R. Schwank et al., "Radiation Effects in MOS Oxides",IEEE Trans. Nucl. Sci., vol 55, n.4, August 2008, pp. 1833-1853, and references therein. 3) J. R. Schwank et al., "Radiation Effects in MOS Oxides", IEEE Trans. Nucl. Sci., vol. 50, n. 3, June 2003, pp.522-538. 4) R. C. Lacoe., "Improving Integrated Circuits Performance Through the Application of Hardness-by-design Methodology", IEEE Trans. Nucl. Sci., vol. 55, n. 4, August 2008, pp.1903-1925. 5) G. Anelli et al., "Radiation Tolerant VLSI Circuits in Styandard Deep Submicron CMOS Technologies for the LHC Experiment: Practical Design Aspects", IEEE Trans. Nucl. Sci., vol. 46, n. 6, December 1999, pp.1690-1696.

18. MOSFET: the gate oxide n-MOSFET before irradiation: electron channel: VT>0 n-MOSFET after irradiation: positive oxide charge trapped in the oxide, VT decreases and becomes negative. Also at VGS=0 V the electron channel is formed.

19. The MOSFET sensitive parts to radiation SiO2 to ionizing radiation G SiO2/Si interface to ionizing radiation S D p+ p+ The substrate can be sensitive to bulk damage, but this effect is less relevant because the conduction is close to the SiO2/Si surface n B

20. The MOSFET sensitive parts to radiation SiO2 to ionizing radiation G SiO2/Si interface to ionizing radiation S D n+ n+ The substrate can be sensitive to bulk damage, but this effect is less relevant because the conduction is close to the SiO2/Si surface p B

21. Ionizing radiation in SiO2: p-MOSFET threshold voltage VT<0: channel of holes

22. Ionizing radiation in SiO2: n-MOSFET threshold voltage VT>0: channel of electrons

23. MOSFET: the gate oxide Which are the physical mechanisms governing radiation damage in Silicon Oxide (SiO2)? Which is the dependence of these mechanisms considering the following experimental parameters? -Oxide thickness. -Time. -Electric field in the oxide. -Temperature. We will consider first the results of investigations forthick oxide(>10 nm), whose thicknesses are typical for old technologies (2000) and nowadays field oxides.

24. Ionizing radiation in SiO2 and defect generation mechanisms (1) VG>0 -When radiation passes through the oxide electron/hole pairs are created by the deposited energy. -In SiO2electrons are much more mobile than holes, and they are swept out of the oxide typically in picoseconds or less. However in the first picosecond some fraction of the electrons and holes will recombine, depending on the energy and type of the impinging radiation. -Holesescaping recombination are relatively immobile and remain near the point of the generation causing the initial negative threshold voltage shift.

25. Ionizing radiation in SiO2 and defect generation mechanisms (1) (1) The initial negative threshold voltage shift described in the previous slide.

26. Ionizing radiation in SiO2 and defect generation mechanisms (2) VG>0 -The transport of the holes to the SiO2/Si interface causes a short term recovery of the threshold voltage: this process takes place over may decades of times and is very sensitive to the electric field, temperature, oxide thickness and process history (<1 s at RT, orders of magnitudes slower at low temperature).

27. Ionizing radiation in SiO2 and defect generation mechanisms (2) (2) The short recovery of the initial negative threshold voltage shift, due to the transport of holes to the SiO2/Si interface, described in the previous slide.

28. Ionizing radiation in SiO2 and defect generation mechanisms (3) VG>0 -When holes are close to the SiO2/Si interface a fraction is trapped in relatively deeplong-lived trap sites. The trapped holes cause a remnant negative voltage shift which can persist for hours or even for years. But even these stable trapped holes undergo a gradual annealing.

29. Ionizing radiation in SiO2 and defect generation mechanisms (3) (3) The remnant negative threshold voltage shift due to hole trapping in deep long-lived sites close to the SiO2/Si interface.

30. Ionizing radiation in SiO2 and defect generation mechanisms (4) VG>0 -Generation of interface states at the SiO2/Si interface. These defects introduce energy levels in the Si band-gap, whose occupancy depends on the Fermi level at the SiO2/Si interface, which in turn depends on the applied gate voltage: consequently they cause a threshold voltage shift which varies depending on the applied gate voltage. -Interface states in the upper part of the band-up are acceptor (can trap electrons and are relevant for n-MOSFET: VT increases). -Interface states in the lower part of the band-up are donors (can emit electrons and are relevant for p-MOSFET: VT decreases).

31. Ionizing radiation in SiO2 and defect generation mechanisms (4) (4) The positive threshold voltage shift due to interface states at the SiO2/Si interface in n-MOSFET (VT decreases due to the contributions from the oxide charge, VT increases due to the contributions from the interface states in the upper part of the band-gap which are negative charged)

32. Ionizing radiation in SiO2 and defect generation mechanisms (4) In n-MOSFET, the long-term threshold voltage shift is positive (rebound effect) due to negative trapped charge in interface states. 14 orders of magnitude for time: 1 day: 8.6·104 s 1 month: 2.6 ·106 s 1 year: 3.15·107 s

33. Electron-hole pairs generation in SiO2 The mean energy to create and electron hole pair in SiO2 is: (17 ± 1) eV [1986]. The electron-holes pairs created in the volume of 1 cm3 by the dose of 1 rad is 8.1·1012. We define g0= 8.1·1012 e-h/(cm3·rad) SiO2 Dose (rad) Volume (V)

34. Initial hole yield -Ionizing radiation generates electron-hole pairs in SiO2. -Electrons are swept out of the oxide very rapidly (1 ps or less), but in that time some fraction of electrons recombine with holes. -The fraction of holes surviving recombination depends on: a) the magnitude of electric field in the oxide (operative condition of the device); b) the initial electron-hole pairs density in the oxide (the LET of the impinging radiation or primary electrons).

35. Initial hole yield: geminate and columnar recombination models Geminate recombination model (can be applied for electron and Co60 irradiation): a) The average separation distance between electron-hole pairs is higher than the thermalization distance. b) The interaction between electron and hole of an isolated pairs is due to Coulomb attraction. c) Electrons and holes drifts in opposite direction due to the electric field. d) Electrons and holes moves randomly due to thermal fluctuations of the system. c) The interactions with other electron-hole pairs can be neglected. The thermalization distance is the traveled distance to reach the thermal equilibrium Columnar recombination model (can be applied for heavy ions (Z>1)): a) The average separation distance between electron-hole pairs is lower than the thermalization distance: there are several electrons closer to any given hole than the electron, which was its original partner. b) The recombination probability is higher for the columnar model than for geminate model.

36. Initial hole yield: geminate recombination model Geminate recombination model (can be applied for electron and Co60 irradiation): a) The average separation distance between electron-hole pairs is higher than the termalization distance. b) The interaction between electron and hole of an isolated pairs is due to Coulomb attraction. c) Electrons and holes drifts in opposite direction due to the electric field. d) Electrons and holes moves randomly due to thermal fluctuations of the system. c) The interactions with other electron-hole pairs can be neglected. -The fraction yield for holes . . . . . . by increasing the oxide electric field. -The maximum value of the fraction yield for holes at 2.5 MV/cm is . . . -At 1 MV/cm the fraction yield for holes is . . .

37. Initial hole yield: columnar recombination model Columnar recombination model (can be applied for heavy ions (Z>1)): a) The average separation distance between electron-hole pairs is lower than the termalization distance: there are several electrons closer to any given hole than the electron, which was its original partner. b) The recombination probability is higher for the columnar model than for geminate model. 2 MeV  particle B is the radius at half-maximum for the Gaussian charge distribution of the 2 MeV  particle: i.e. 1.178·. -The fraction yield for holes . . . . . . by increasing the oxide electric field. -The maximum value of the fraction yield for holes at 2.5 MV/cm is . . . -At 1 MV/cm the fraction yield for holes is . . .

38. Initial hole yield for different radiations [1983] Geminate model Intermediate condition Co60-rays and 10 keV X-ray sources: which is the better solution for radiation damage due to electrons in Van Allen Belts? Columnar model Experimental values of the fractional yield for holes as a function of the oxide electric field for different radiations

39. Initial hole yield for different radiations The fractional yield for holes as a function of the proton energy: -at low proton energy: columnar model -at high proton energy: geminate model Linear Energy Transfer for different radiations(values for 10 keV X-rays and Co60-rays are for primary electrons) [2002]: - geminate model applied at low dE/dx; - columnar model applied at high dE/dx; -10 keV X-rays simulates 20-60 MeV protons; - Co60-rays simulates 1 MeV electrons;

40. Hole transport in SiO2 [opzionale] Hole transport occurs over many decade of times after the radiation pulse. 1) Changes in temperaturedoes not chargethe "S"shape of the recovery curve(VFB vs time). At temperature higher than -130 ºC the hole transport is strongly temperature activated. 1978 Flat band voltage variation after irradiation as a function of time. Flat band voltage variation just after the LINAC electron pulse. -Oxide thickness: tox=96.5 nm -Radiation: LINAC 12 MeV electron pulse (30 krad) (geminate model) -Oxide electric field: 1 MV/cm 16.6 min

41. Hole transport in SiO2 [opzionale] Hole transport occurs over many decade of times after the radiation pulse. 2) Changes in the oxide electric field does not charge the shape of the recovery curve (VT vs time). The hole transport is activated by the oxide electric field. 1978 Flat band voltage variation after irradiation as a function of time. Flat band voltage variation just after the LINAC electron pulse. -Oxide thickness: tox=96.5 nm -Radiation: LINAC 12 MeV electron pulse (30 krad) (geminate model) -Temperature: 80 K 16.6 min

42. Hole transport in SiO2 [opzionale] Hole transport occurs over many decade of times after the radiation pulse. 3) Changes in the oxide thickness does not charge the shape of the recovery curve (VT vs time) but the hole transport time has a super linear power law dependence on the oxide thickness. 1978 The "hole transit time" tTR is the time at which: -Oxide thickness: tox=96.5 nm -Radiation: LINAC 12 MeV electron pulse (30 krad) (geminate model)

43. Hole transport in SiO2 [opzionale] Proposed mechanisms for hole transport in SiO2: -Continuous-time-random-walk (CTRW) hopping (hopping of holes between localized shallow trap states having a random spatial distribution, but having an average separation of about 1 nm); -Multiple-trapping model. 1978 Flat band voltage variation after irradiation as a function of time. Eox=1 MV/cm Flat band voltage variation just after the LINAC electron pulse. -Oxide thickness: tox=96.5 nm -Radiation: LINAC 12 MeV electron pulse (30 krad) (geminate model) The "hole recovery time" t1/2 is the time at which: The "S" shape of the recovery curve is . . . The data can be fitted by . . .

44. 5C. Radiation effects in SiO2: microscopic considerations

45. Silicon oxide (SiO2) -The silicon oxide used in microelectronic applications is composed by tetrahedral SiO4 blocks, but they are irregular: the SiO2 structure is not crystalline but amorphous (a-SiO2). -The SiO4 blocks are connected by Si-O-Si bridging bonds, as shown in the following figure, the corresponding formula is O3ºSi-O-SiºO3. In a-SiO2, the Si-O-Si bonds are strained: the angle between the oxygen and the two Si neighbours can assume values between 120° and 180°. Two tetrahedral SiO4 blocks and the oxygen bonding the two silicon atoms.

46. Microscopic defects in SiO2: the oxygen vacancy -The oxygen vacancy (Vo) is a structure of fundamental importance for defect studies in SiO2. The Vo defect is obtained by removing the oxygen bonding two Si atoms, as shown in the following Figure. -It is schematically represented by the formula: O3ºSi-SiºO3. When the oxygen is removed, the c-SiO2 structure relaxes: the two silicon atoms move to the oxygen vacancy by near equal amount along the Si-O direction. The oxygen vacancy density can be high up to 6·1019 cm-3, in oxides thermally grown in oxygen reach atmosphere (dry oxide). The SiO2 structure (left). The oxygen vacancy defect (right).

47. Hole trapping and annealing in SiO2 Holes are trapped close to the SiO2/Si interface because there is a transition region where oxidation is not complete, this region contains excess Si atoms, i.e. "Oxygen vacancy defects". (a) "Oxygen vacancy": a Silicon atom is back bonded to three Oxygen atoms and with a weak bond to a Silicon atom. (e) (d)

48. Hole trapping and annealing in SiO2 (b) When a hole is trapped by a "Oxygen vacancy" the Si-Si bond is broken and the lattice relaxes to the E’ center: the first Silicon atom is positive charged and back bonded to three Oxygen atoms, the second Silicon atoms is neutral and back bonded to three Oxygen atoms. The E’ center is a positively charge defects which is detectable by ESR (Electron Spin Resonance). (e) (d)

49. The positive charge oxygen vacancy -The bonding between the two Si atoms of the Vo defect can be broken by exciting a bonding electron to the SiO2 conduction band or by hole trapping. -The new defect, is called positive charged oxygen vacancy (Vo+). It is represented by the chemical formula: O3ºSi·°SiºO3, where the symbols "·" and "°" are the unpaired electron and the trapped hole, respectively. -The Vo+ center is positive charged (°SiºO3) and paramagnetic (O3ºSi·). It can be characterized by the electron-spin-resonance (ESR) technique. The paramagnetic part of the Vo+ defect is called E' center. The positive charged oxygen vacancy defect.The arrow indicates the unpaired electron of the neutral Si atom.The Vo+ defect is strongly correlated to the positive charge induced by ionising radiation.

50. Hole trapping and annealing in SiO2 (c) Trapped holes are relatively stable but they can undergo a long-term annealing which can extend from hours to years. The microscopic mechanism for the annealing of the trapped hole: at room temperature is electron tunneling from the Si substrate and at higher temperature is electron thermal excitation from the Si substrate valence band: an electron can be trapped by the neutral Silicon atom back bonded to the three Oxygen atoms, becoming negative and compensating the positive charge of the other Si atom back bonded to three Oxygen atoms. This effects is enhanced for positive gate bias: VG>0 V. (e) VG>0 (d)