1. Radiation Damage on MOS-Structures
Motivation
Experimental conditions and results
Radiation effects on MOS-structure
Conclusion
Q. Wei, L. Andricek, H-G. Moser, R. H. Richter
Max-Planck-Institute for Physics Semiconductor Laboratory Munich
Freiburg, March 2008
Qingyu Wei, MPI für Physik, HLL

2. Motivation Physics topics Unexpected phenomena
Mechanism of electroweak symmetry breaking
New physics: SUSY; extra dimension; link to cosmology
International Linear Collider
precise vertex detector (DEPFET)
Depleted P-channel FET
To do
Radiation on MOS-structure
(MOS-Capacitance & -DEPFET )
Research of radiation effect
(damage mechanism & model)
Radiation hardness structure
(sensitive volume)
TESLA TDR Design

3. Radiation damage Radiation damage in MOS-Structures:
Surface damage due to Ionizing Energy Loss (IEL)
Accumulation of charge in the oxide (SiO2) and Si/SiO2 interface
Oxide charge  Shifts of flat band voltage, (depleted  enhancement)
annealing at RT
Interface traps  Leakage current, degradation of transconduction,…
no annealing below 400 0C
S/N ratio deteriorated!

4. Experimental conditions I
Comparison of different MOS devices
MOS-DEPFET
MOS-Capacitance
MOS-Gated-Diode
DEPFET
MOS-C
Gated diode
Nox
(Method)
Δ Vt
(IV-Measurement)
ΔVFB
(CV-Measurement)
∆VFB & ∆Vg
(CV-Measurement & gated diode technique)
Nit
Subthreshold slope
(Subthreshold technique)
Stretch-out
(High-low frequency based on the CV)
Full width at 2/3 maximal of current
(Gated diode technique)
Other parameters gm
S0, σ,

5. Experimental conditions II Different irradiation sources
Co60 (1.17 MeV and 1.33 MeV)
GSF – National Research Center for Environment and Health, Munich
Dose rate: 20krad/h
CaliFa (17.44 KeV)
Max-Planck-Institute Semiconductor Labor, Munich
Dose rate: 9krad/h
X-Ray facility (~ 20 KeV)
Research center, Karlsruhe
Dose rate: 148krad/h
Dose
irradiation up to 1 Mrad (1rad=0.01J/kg)
Process
No annealing during irradiation
Munich
Karlsruhe

6. C-V characteristics: three regions
CV-Measurement I
For MOS-C
C-V characteristics: three regions
Inversion
Depletion
Accumulation

7. Calculation of positive oxide charge by high frequency CV
CV-Measurement II
For MOS-C
Calculation of positive oxide charge by high frequency CV

8. Calculation of interface trap by high low frequency CV method
CV-Measurement III
For MOS-C
Calculation of interface trap by high low frequency CV method
(a) High frequency
Cox Cs
(b) Low frequency
Cox Cs
Cit
(equivalent circuit)

9. Subthreshold technique
For MOS-DEPFET
characteristic lines of DEPFET (ID vs. VG)
After irradiation
before irradiation

10. MOS-Gated Diode technique measuring the generation current
For MOS-Gated-Diode P+
Al/Poly A Al
N-Substrat
V-bias
Edge N
V-edge
Erde
Gate
Vg-Scan
Accumulation
Depletion
Inversion
Ipn+IFIJ
Ipn+IFIJ+Iit
Ipn
scanning gate voltage
measuring the generation current

11. Flat band voltage Shift Nox
Experimental results I
For MOS-C
Flat band voltage Shift Nox
HF/LF CV  Nit

12. Bias during irradiaton:
Experimental results II
For MOS-DEPFET
Bias during irradiaton:
1: empty int. gate, in „off“ state, VGS= 5V, VDrain=-5V  Eox ≈ 0
2: empty int. gate, in „on“ state, VGS=-5V, VDrain=-5V  Eox ≈ -250kV/cm
"OFF"
"ON"
[Ladislav Andricek]

13. Transconductance and Subtreshold slope
Experimental results III
For MOS-DEPFET
Transconductance and Subtreshold slope
[Ladislav Andricek]
s=85mV/dec
s=155mV/dec
Vth=-0.2V
Vth=-4.5V
No change in the
transconductance gm
300 krad  Nit≈2·1011 cm-2
912 krad  Nit≈7·1011 cm-2

14. Increase of s and σ & Decrease of
Experimental results IV
For MOS-Gated-Diode
Shifts of generated current peak
Increase of generation current
Increase of s and σ & Decrease of

15. Radiation effect I - Saturation mechanism
Begin
SiO2 Si
Saturation
Equilibrium
Recombination
Rejection
Trapp filling
Reservoir: hole traps are not exhausted, unless a larger bias voltage is applied on the gate!
Saturation: equilibrium between trapped filling and recombination
Generated holes are pushed away!
Recombination of trapped holes with electrons
- Recombination of tunneled electrons from silicon into interface with trapped holes!

16. Radiation effect II - Dose rate effect Shift of flat band voltage
148
krad/h
SiO 2 Si 9
RT Annealing
Charge sheet generated due to tg < tT
tg: generation time of hole
tT: transportation time of hole
Shift of flat band voltage

17. Radiation effect III - Bias effect and thickness dependence+ -
SiO2 Si VG
DEPFET
thickness dependence dn

18. Radiation effect IV radiation hardness through Nitride
MOS (86nm) vs. MNOS (86/10nm)
MOS (100nm) vs. MNOS (100/10nm)

19. Conclusion Irradiation experiment on different MOS devices
Saturation effect
Dose effect
Bias dependence (“+“,“0“,“-“)
Oxide thickness dependence - (~ doxn )
Radiation hardness
Reduced oxide thickness improves radiation hardness
Zero gate voltage
Additional nitride layer serve as a good protection against ionizing radiation (Electron trapping!)
Surface damage can be reduced through RT annealing process with time

20. Back up

21. CV-Measurement I For MOS-C
high frequency (10kHz); low frequency (20Hz)
All values measured in series mode (Cs, Rs), amplitude level:50mV
LCR Meter
HP4284A
Low High
Adapter Box
Kethley 487
Netzteil
oxide
n-type silizium Al
Ohm-contact
Aluminium
Silicon dioxide
N-type silicon
Ohmic contact

22. Device degradation or failure
Radiation damage I
MOS devices
Ionization in dielectrics (Oxide or nitride)
Total ionizing dose (TID)
Energy deposited in material
Interaction with materials by photoelectric, Compton & pair production
Generation of electron hole pairs
in dielectrics
Charge carrier injection from
gate contact (small part)
Generation of excited state at interface
&
bond breaking e - h
pairs separation and transport
Gate polarity
dependent
transport and
recombinati
on of
pairs
Charge trapping in dielectrics
Holes trapped
at O/ Si
interface
(MOS/MNOS)
Electrons and holes trapped
at N/O interface
(MNOS)
Shift of
flatband
voltage
Generation of precursors
Release of mobile
impurities from defects
(H, OH, etc.)
Interface trap
Positive ion
(proton)
Leakage current & degradation of
trans
conduction
Total dose effect
Accumulated TID reach its tolerance limit
Device degradation or failure

23. Damage mechanism (MOS) I
Positive oxide charge
Positive oxide charge
Generation (1)
Recombination
Transportation (2)
Trapping (3)
E‘ Center
Hole & a strained Si-Si bond  Break the bond  Recombine with one of the bonding electrons
Positive charged structure  The E’ center (with one of the silicon atoms retaining the remaining electron from the broken bond)
Gate
Electron/Hole Pairs
Generated by ionizing
radiation
SiO2 Si
1.12ev
Hopping Transport of Holes
through localized states in
SiO2 Bulk
Long-term Hole trapping
near Si/SiO2 Interface
Radiation induced Interface
trap 1 2 3 4
As follows I will show you the generation mechanism for positive oxide charges and interface traps. Before radiation there are some precursor defects in the oxide like oxygen vacancy, which can trap hole to form positive oxide charge, the concentration of positive oxide charge for the MOS-C and Gated diode wafer what I use to measure is approximately in an order of 1010 per square centimeter. While ionizing radiation passes through the oxide, the deposited energy will create electron hole pairs, the electron are much more mobile than hole and are swept out of the oxide within a few picoseconds, however in the first picoseconds, some fraction of electron and hole will recombine. Such fraction depends greatly on the applied field and the energy and which kind of incident particle. Dependent on the columns of charge pairs we should consider three categories of recombination, if the charge pairs are deposited in dense columns, the recombination is a relatively strong process and columnar model is a good fitting model for the experiment. Secondly, if the charge pairs are averagely far apart from each other, the recombination is a relatively weak process, the geminate model can be used to describe the experiment results. Thirdly, if we consider a transition region where the density of charge pairs is intermediate between the above mentioned two cases, the recombination is also intermediate, and the transition is a gradual one means from geminate to columnar model. For the 5kev electrons as source we could use columnar model to fit the results. But for 12MeV electrons or Co60 gama rays we should use geminate model to analyze the experiment results. For heavy ions more recombination will be happened, like protons, alfa particles and heavier nuclei. Recombination process is much stronger in SiO2 than in Silicon. Because the charge interaction is more strongly with lattice in SiO2 than in Si. the recombination is strongly dependent on the field but not on the oxide thickness.
After the holes escape initial recombination, they are relatively immobile and remain behind near their point of generation, and cause negative voltage shifts on MOS Structure. At RT over a period of time typically a few seconds the holes would undergo a so called non-gaussian stochastic transport through the oxide, which is very disperse in time, and cause short term post irradiation response. Based on the CTRW formalism the radiation induced hole transport in SiO2 could be divided into two transport mechanism, namely hopping transport and multiple trapping. These two microscopic transport mechanisms could be treated with CTRW stochastic Model. In Trap mediated transport so called multiple trapping, the carriers move in extended band states between trapping event at localized states, the localized trap states are distributed in energy E, which cause large fluctuations in the thermal release rate from the traps and therefore results in dispersive transport; in the hopping transport, the carriers move directly between localized trap sites via phonon assisted tunneling processes, the dispersion arises from a distribution of overlap (or transfer) integrals for hopping, due either to a random distribution of intersite hopping distance or to a distribution of bond angles in the amorphous structure, which modify the overlap of directed orbital. The formalism of continuous time random walk is used to calculate the transient current traces for dispersive transport, this CTRW model describes a walker hopping randomly on a periodic lattice, with the steps occurring at random time intervals determined by a hopping time distribution function. When the holes reach the silicon oxide interface, some of them are captured in long term trapping sites, and cause this time a remnant negative voltage shift that is not sensitive to silicon surface potential, it can persist in time for hours to years, this shifts is commonly observed form of radiation damage in MOS structure. This effect dominates other radiation damage processes, like negative charge trapping or interface trap buildup effects, unless some specific processing changes are made to alter the oxide and consequently reduce the positive charge trapping.

24. Damage mechanism (MOS) II
Interface trap
Interface Trap (4)
Due to breaking the Si-H or Si-OH and leaving Si dangling bond behind
Located in a Interface Si/O
Charge state (Amphoteric)
Acceptor level traps
Below EF  ‘-’; Above the EF  ‘0’
N-channel transistor: Interface trap compensates the charge from positively trapped holes
Donor level traps
Below EF  ‘0’; Above the EF  ‘+’
P-channel transistor: Interface trap add to charge from trapped holes
Interface trap with energy levels within Eg (U-shape distribution)
N-channel
King and Martin, 1977
P-channel
MOS transistors
Now I want to talk something about the interface trap and will introduce two models to explain the origin and nature of interface traps. Firstly when silicon is thermally oxidized, the interface between amorphous oxide and crystalline silicon is generally deficient of oxygen, this will lead to strained lattice as well as uncompleted silicon bond, so called silicon dangling bond. These dangling bonds serve as interface traps with energy levels within the forbidden band gap at interface. Normally the preirradiation interface trap densities are in the range of 10 to 9 to 10 to 10 per square centimeter. after the samples are exposed to ionizing radiation, there will be some additional radiation induced interface trap because the Si-H or Si-hydroxide are broken and leaving Si dangling bond behind. All interface trap whether process or radiation induced, have the following properties: 1. they must be located within one or two bond distances approximately 0.5nm from silicon lattice so that electrons or holes in silicon conduction band or valence band can make quantum mechanical transition into or out of interface trap. The transition rate is proportional with Temperature, and inversely proportional with energy. 2. The second property is that the charge state can either be positive, negative or neutral, namely amphoteric. According to their charge states, they could be classified as either donor or acceptor. A donor trap level is in a neutral charge state if it is below the fermi level, and become positive if this level is above fermi level means not occupied with electrons. For example, a p-channel transistor, the interface trap is donor trap in a positive charge state, because the charge from interface traps adds to the charge from trapped holes, leading to a larger net threshold voltage shift following ionizing radiation, when the trapped holes are annealed it leaves only uncompensated interface traps, and the threshold voltage shift is still observed to be negative. So the interface traps must contribute a net positive charge and are therefore donor trap.and the rebound so called recovery for trapped hole in the negative voltage direction. The same discussion also for the n channel, so we get acceptor trap in n-channel transistor. Another fundamental property about the interface trap is the shape of interface trap density across Silicon bandgap, i.e. interface trap distribution Dit is continuous throughout the si band gap with a U shape. Moreover one thing I should talk about is the distinction between interface trap and interface fixed charge, for the latter one there is only one charge state namely positive, and there are not charges that exchange with Si band edge.

25. Damage mechanism (MNOS) I MNOS vs. MOS before irradiation
Energy band diagram of MNOS structure Ec
EFS
Nox Me
Nitride
Oxide Si
EFM
Assumption: Si/O interface for MOS and MNOS behaves in the same manner
Nox0(MOS) < Nox0 (MNOS) due to additional N/O interface
Initial state of MOS at Si/O ~ 1010 cm-2(Nox)
Initial state of MNOS at O/N ~ 1012 cm-2(Nni)
Precursors for hole in oxide mainly concentrate at Si/SiO2 to a extent of a few nm (shown in the green region), however, there are still few hole traps at SiO2/Metal
Electron trapping can be neglected in SiO2 due to the smaller capture cross section (~ factor 100 smaller than hole trapping)
Precursors for electron or hole in nitride concentrate at N/O interface
Mobility of electron is faster than hole in oxide at room T
e: 20cm2V-1s-1
h: cm2V-1s-1
Charge accumulated at Si/O donot affect the field distribution in dielectrics due to modulation of electron in Si

26. Damage mechanism (MNOS) II
MNOS vs. MOS during irradiation with biasing
EFM Vg EC EF J0 Jn
Qox
Qni Me
Nitride
Oxide Si
Vg > 0
Vg < 0
electron
hole
Trap (precursor)

27. Damage mechanism (MNOS) III
Recombination + -
SiO2 Si VG
Nitride
Trapp for e & h
electron
hole
Region for traps

28. Radiation effect V - Annealing mechanism
Tunnelling annealing (RT)
Annealed Nox linear with log (t)
Thermal annealing (from 150 Celsius)
Exponential decay of tunnelling Probability with distance into SiO2 Trapped hole emptying at depth Xm(t)
Integration of charge distribution with exponential falloff of trapped Hole density
C: Voltage shift per unit dose at t0
Assumption:
Xm<<dox & uniform trap distribution
Normally the post irradiation interface trap is not obviously annealed at RT, but annealed through heating treatment. The radiation induced positive oxide trap could already be annealed only at RT. All these traps are sensitive to temperature and field. So we have here two model needed to explain the annealing effect. The first one is tunneling anneal, it dominates at RT another annealing process. In this process, electrons could tunnel from silicon into oxide and recombine the trapped holes there. Because of the exponential dependence of tunnel probability with distance from the Si SiO2 interface, the trapped holes are emptying at a depth of Xmt from the silicon interface at a given time t, and the discharge of trapped holes is roughly linear with logt. If we apply a positive voltage on the gate, the barrier for tunneling will be lowered, so that the rate of annealing will increase significantly. You could see this form for the depth Xmt which is proportional to logt, and if we integrate the local trapped holes density within the distance Xmt, we could get the time-dependent variation of flat band voltage for positive oxide charge. Here we make a limitation with the characteristic length taken as 0, and we should use mathematics function and get the logarithmic annealing characteristic behavior. The second model is so called thermal annealing that is strongly temperature dependent. From 150 Celsius the thermal annealing is more relevant for the recovery process, whose annealing mechanism is almost the same as tunneling anneal, the only difference is that the electron is not from silicon but direct from oxide through thermal generation of charge carriers, and then recombine the trapped holes.
Differentiate the function with λt and calculate with logarithmis function
Log(t) Annealing Characteristics

29. Oxide charge decrease with time: (Tunnel annealing @ RT)
Annealing mechanism
Oxide charge decrease with time:
(Tunnel RT)
fit

30. Changing of breakdown voltage

31. Performance before irradiation
non-irrad. double pixel DEPFET
L=7μm, W=25 μm
Vthresh≈-0.2V, Vgate=-1V
Idrain=41 μA
Drain current read out
time cont. shaping =6 μs
55Fe
Energy (eV)
Counts/channel
[Stefan Wölfel]
Noise ENC=2.3 e- (rms)
at T>23 degC

32. Performance after irradiation
Irradiated double pixel DEPFET
L=7μm, W=25 μm
after 913 krad, 60Co
Vthresh≈-4.5V, Vgate=-5.3V
Idrain=21 μA
Drain current read out
time cont. shaping =6 μs
55Fe
Counts/channel
Noise ENC=7.9 e- (rms)
at T>23 degC
[Stefan Wölfel]
Energy (eV)
1 Mrad adds about 6 e- noise (at long shaping times)

33. Annealing post irradiation
Influence of electron trapping or negative gate voltage on annealing

34. Annealing post irradiation
Influence of electron trapping or negative gate voltage on annealing

35. Frequency dependent CV measurement
Before irradiation
Enlargement of ROI

36. Frequency dependent CV measurement
After irradiation
Enlargement of ROI

37. Invisible through nitride layer
Interface trap
Invisible through nitride layer

38. Visible near the conduction band
Interface trap
Visible near the conduction band