1. Radiation Effect on MOS-Structure
Q. Wei, L. Andricek, H-G. Moser, R. H. Richter, Max-Planck-Institute for Physics Semiconductor Laboratory
DPG-Tagung, Heidelberg, 2007

2. Outline Motivation Radiation damage on MOS-C and -DEPFET
Experimental Conditions and Results
Conclusion

3. Motivation-ILC(DEPFET)
Goal: Higgs, SUSY, Identity of dark matter, Existence of extra dimension
 precise vertex detector (DEPFET)
Up to now so many central topics like higgs particle, SUSY, and existence of extra dimensions should be studied, therefore, ILC become more and more interested, and this project are proposed also together with the LHC at CERN, it will allow us to research the physics topic in the high energy regions. In the interaction point of this huge construction there is a set of cylindrical detectors, so called TESLA TDR design. It give us a basic concept how it consists of. And for the particle tracking we need such a high resolution of detector both for energy but also for position measurement. At the same time the detectors should be radiation hardness, because radiation induced damage will deteriorate the signal noise ratio and finally make the detector off. So my work is to study this radiation effect especially with x-ray on the MOS-structure to get a radiation hardness detector design for ILC, since MOS-structure is simple to produce and has already been a very important component of so many semiconductor devices, in my presentation I will show you three different devices, MOS-C, MOS-DEPFET and gated diode.
A set of cylindrical detectors
in each ladder around the interaction point
each ladder is an array of pixel cell
R/O at the end of the ladder outside
the sensitive volume
Particle tracking  vertex detector at ILC
High resolution:~5µm: technology so far possible
Pixel size: µm
Low mass: 0.1 %Xo per layer: thin detectors ≈ 50µm
instead of 300µm
High occupancy in 1st layer: fast(!) R/O cycle ~ 50MHz per Pixel
Small noise: noise ≈ 100 el ENC
TESLA TDR Design

4. Radiation Damage Two types of radiation damage in MOS-Structure:
Surface damage due to Ionizing Energy Loss (IEL)
accumulation of charge in the oxide (SiO2) and Si/SiO2 interface
Oxid charge  shifts of flat band voltage, (depleted  enhancement)
Interface traps  leakage current, degradation of transconduction,…
Bulk damage due to Non Ionizing Energy Loss (NIEL)
displacement damage, built up of crystal defects
Increase of leakage current  increase of shot noise,…
Change of effective doping concentration  higher depletion voltage,…
Increase of charge carrier trapping  singal loss!
S/N Ratio deteriorated!
Now let us talk about the radiation effect, after radiation there are some radiation damage generated in the MOS-structure. There are two types of radiation damage, bulk damage and surface damage. Due to ionizing energy loss while the x-ray go through the oxide there are some electron hole pairs generated in the oxide, since electron has a fast mobility compared to hole in oxide and go directly to the electrode under the influence of electric field. After the separation of electron and hole pairs the hole drift to interface and are trapped in hole trap near the interface, and finally form oxide charge also interface trap, oxide charge contributes a shift of flat band voltage, which will make MOS-transistor from depletion mode to enhancement mode, and increases the power consumption. Interface trap results in leakage current and make the signal noise ratio worsen. On the other hand due to non inozing energy loss especially for hadron's radiation the bulk damage will be formed in the silicon bulk through displacement with silicon lattice, the macroscopic effect is changing of effective doping concentration and increasing of leakage current and also trapping effect. All of these follows a degradation of Signal noise ratio.

5. Radiation Effects - Bulk damage
displacement due to NIEL:
proton & neutron irradiation(simulation):
point defects & clusters
clusters
Vacancy
Frenkel Defect
Interstitial
Silicon atom
Different atom in Interstitial site
Different atom in Subinterstitial site
Let us talk short about bulk damage. After hadron's radiation there are some displacement damage like vacancy, interstitial, or displaced atom with impurities and so on. Depend on type and energy of radiation there are different damage generated, for proton radiation, due to columbic and also nuclear interaction there are not only point defects but also clusters formed. But for neutron radiation, because it is not charged particles, so there is no columbic interaction, so there is normally only cluster. From the macroscopic point of view we could see the leakage current increase with higher radiation fluence, and the effective doping concentration is changed from n-type to p-type at about 2 times 10 to 12 per square cm.
increase of leakage current:
change of effective dopping concentration
ILC
ILC

6. Radiation Effects - Surface damage
Ionizing radiation:
Damage mechanismus:
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 Si
Electric active
Interface state
Let us give you a comprehensive view of generation of surface damage. Through ionizing radiation the generated electron hole pairs are separated in the oxide bulk and go to corresponding direction according to the applied bias voltage. E.g. positive bias voltage, hole escape the recombination process in the first stage with generated electrons, and through the so called hopping transport drift towards the interface, and form positive oxide charge there. According to Mclean model so called H-model, the radiation induced hole will combine with hydrogen atom or hydrogen complex to create proton, which act with silicon dangling bond to form interface trap and release hydrogen molecule. As mention above, the macroscopic effect is the shift of flat band voltage and stretch-out of cv-curve.
Shifts of flat band voltage: ~ Nox
Strechout of CV curve: ~ Nit
Nox: postive oxide charge and postively charged oxide traps have to be compensated by a more negative gate voltage  negative shift of the theshold voltage (~tox2)
Nit: increased density of interface traps  higher 1/f noise and reduced mobility (gm)

7. Saturation effect for surface damage
Saturation mechanism
equilibrium
Begin
Saturation
Recombination
Recombination
Rejection
Trapp filling
Before I show you the results, I will discuss here about the saturation effect for surface damage. At the beginning some positive oxide charge are formed near the interface, and more and more positive oxide charge are accumulated there, and results in a additional electric field near the interface, that push the new generated and towards interface drifted hole away from the interface to electrode. At the same time, some electrons will go through tunnel effect from silicon into interface and recombine with positive oxide charge there, finally a equilibrium is built up, and saturation of surface damage will come into being. We call the hole traps in total as reservoir. But here the reservoir is still not exhausted, until a very larger electric field applied on the gate contact, to suppress the recombination process and repulsion from accumulated positive oxide charge near the interface.
SiO2 Si
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!

8. Experiment Conditions and Methods
Irradiation (X-Ray):
Co60 (1.17 MeV and 1.33 MeV)
GSF – National Research Center for Environment and Health, Munich
CaliFa (17.44 KeV)
Max-Planck-Institute Semiconductor Labor, Munich
Roentgen facility (20 KeV)
Research center, Karlsruhe
Dose: irradiation up to 1 Mrad with different dose rate (1rad=0.01J/kg)
Process: No annealing during irradiation ~ irradiation duration from 1 day to 1 week
Radiation levels at the ILC VTX: Dionization≈ Krad
≈ neq(1MeV )/cm2
Here I will introduce you about my experiment conditions and some analyze methods for each component. Irradiation what I used is x-ray at different energy source. Co60, califa setup in our house, also roentgen facility in research center karlsruhe, I irradiated each sample to 1 Mrad with no annealing process in the radiation period. E.g. for radiation environment of ILC for vertex detectors, it is 100krad in 5 years, about 10 to 10, 11 neutron equivalent. For MOS-DEPFEET subthreshold technique is used to calculate the corresponding parameters. For MOS-c CV-measurement, and for gated diode, it is gated diode technique. And some other parameter like transconduction, surface generation velocity, capture cross section also generation lifetime are important for characterizing surface damage.
Comparison of different semiconductor devices
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)
Stretchout
(High-low frequency based on the CV)
Full width at 2/3 maximal of current
(gated diode technique)
Other parameters gm
S0,

9. Depleted P-channel FET
MOS-DEPFET
Drain
Source
Gate
Now i want to show you more something about the MOS-type DEPFET. The MOS-type DEPFET is integrated on a fully depleted silicon substrate. All charge generated by a traversing particles drift to the surface and are collected in the potential minimum for electrons underneath the external gate of DEPFET, here this region is so called internal gate.the charge cloud is stored and modulates the current in the channel of FET which forms the first amplification stage. After the signal readout, the charge will be removed by applying a positive voltage to the clear gate. Charge generation and first amplification stage are combined in one single device, and due to fully depleted sensitive volume in the whole bulk also low capacitance of read out node we could get low noise operation over a large range of temperatures. This is exactly for the application at linear collider experiments. Another important thing is the charge is measured at the place of generation, this means trapping effects due to bulk damage by NIEL is not so relevant.
Depleted P-channel FET
fully depleted sensitive volume
Charge generation and first amplification in a fully depleted pixel cell  good S/N
No charge transfer needed
R/O at the place where it generated  better rad. tolerance against hadronic irradiation

10. Bias during irradiaton:
Results for MOSDEPFET
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"
Now let us take a look at the results for MOSDEPFET, if bias status is on, we get more radiation damage compared to off status. The threshold voltage shift is about 4v. And after radiation the induced damage will be healed up in the first tens of hours for on status, but almost no rebound for off status.

11. Transconductance and Subtreshold slope
s=85mV/dec
s=155mV/dec
Vth=-0.2V
Vth=-4.5V
Using subthreshold technique we could calculate the interface trap concentration. From our results we could hardly see any degradation of transconduction.
No change in the
transconductance gm
300 krad  Nit≈2·1011 cm-2
912 krad  Nit≈7·1011 cm-2
Literature:
After 1Mrad 200 nm (SiO2):
Nit ≈ 1013 cm-2

12. MOS-Capacitance MOS-C: the upper left – Al1 the upper right – Al2
the lower left –Poly1
the lower right –Poly2
each contact area is about 10 mm2 Al
Al2
In this picture you will see the measurement setup exactly.
Poly1/2
Aluminium
Poly1/Poly2
LTO
Substrate
Substrate
Oxide Ni

13. Saturation reaches at about 1 Mrad!
Results for MOS-C
Gate Bias conditions: 0V
These are for MOS-C, again the same from the flat band voltage shift we achieve the information about the oxide charge, and using high-low method we get the interface trap.
high-low frequency CV  Nit
Flat band voltage Shift Nox
Saturation reaches at about 1 Mrad!

14. + - Bias Effect: DEPFET thickness dependence dn VG SiO2 Si For MOS-C-
SiO2 Si VG
DEPFET
Here I will present you two effects, bias effect and thickness dependence. Under positive bias condition, more hole will be generated and drifted towards interface in comparison with 0 and negative bias, because for the case of 0v the radiation induced hole could recombine with electrons directly after their generation, and this recombination process become dominated. Under negative bias, the recombination process is already avoided, however, holes drift toward electrode, where there are less hole traps. after we irradiated sample with different oxide thickness, we find that there is correlation between flat band voltage shift and radiation dose. This means if we reduce the oxide thickness, we could get much better radiation hardness. Of course this relationship depends on the applied bias condition. For positive electric field, this n is equal to approximately near 2, and for 0v electric field, n is equal to 1.
thickness dependence dn

15. High dose rate Low dose rate
For MOS-C
Dose rate effect:
Radiation hardness by Ni-layer:
Substrate n-Typ
Poly1/Poly2/LTO
Aluminium
Oxide Ni
Another two effects is dose rate effect and radiation hardness factor. Due to ionizing radiation, holes are generated and drift towards interface and form there positive oxide charge, if the generation time of hole is faster than transport time of the hole to the interface, there are more positive charge formed in the oxide bulk, therefore, electrons generated above this charge sheet drift toward it and holes move to electrode, on the contrary, holes generated in the regions between this charge sheet and interface will drift to interface, all of this results in the reduction of radiation reduced holes. In comparison with low dose rate, there is no such charge sheet in the oxide bulk, so the flat band voltage for low dose rate is higher than for high dose rate, however, for much lower dose rate, in my work the dose rate for the radiation source Fe55 is predominantly small, so the radiation time is even slower than annealing time. So its flat band voltage is shown between these two dose rate. For the topic of radiation hardness we find that if we produce a additional layer of Nitride between metal electrode and silicon, it serves not only as a mask against hydrogen diffusion from outside into oxide but also as a good mask for generation of positive oxide charge. So we get much less flat band voltage shift for the sample with Nitride layer and with the same oxide thickness compared with another sample which is without Nitride layer.
High dose rate
SiO2 Si
Low dose rate

16. MOS-Gated Diode gated diode wafer the left one – Al
the right one –poly
each area is 9 mm2 P+
Al/PolyGate A Al
Substrate V
Edge N
Erde
Poly-gate
Al-gate Al Al P+ P+
grounding
Edge

17. Results for gated diode
Shifts of generated current:
Increase of maximal current:
These are the results for gated diode. Under bias voltage applied on the gate contact, the gated diode goes through accumulation, depletion and inversion. In the depletion stage, you will see this generation current, which shifts and become much higher with increasing dose. Another two parameters generation velocity and generation lifetime also reflect the radiation effect, namely much more reverse current is built up due to the interface trap.
Increase of surface generation velosity and decrease of lifetime due to radiation:

18. Oxide charge decrease with time: (Tunnel annealing @ RT)
For bulk damage:
Leakage current decrease with time:
“Beneficial annealing” & “Reverse annealing”
80 min 60C
Another important radiation hardness factor is annealing process, that is not only for bulk damage but also for surface damage. Annealing for bulk damage is more useful at higher temperature, since defect could change to another states through migration or dissociation. For surface damage at RT, tunnel annealing is dominant effect for positive oxide charge, which is proportional with logarithms time, also at higher temperature the surface damage will be recovered in a certain extend.
For surface damage:
Oxide charge decrease with time:
(Tunnel RT)

19. Conclusion Radiation experiment Radiation hardness
Study of saturation effect for surface damage(equilibrium between generation and recombination)
Study of bias dependence(“+“,“0“,“-“)
Study of different semiconductor devices, which are with different oxide thickness - to see the kind of relation between Vfb and oxide thickness(doxn )
Study of dose rate effect(three different radiation sources)
Study of devices, with different structure profile(poly-gate with Ni; al-gate without Ni)
Radiation hardness
Reduced oxide thickness improves radiation hardness
Additional Ni layer serve as a good protection against ionizing radiation
Annealing process makes not only surface damage but also bulk damage reduced with time
Now we come to conclusion.

20. Thanks for your Attention

21. Back slides

22. Prototype Readout System
DEPFET Matrix
64x128 pixels, 36 x 28.5µm2
Clear
Switcher
Gate Switcher
let us take a look at a prototype readout system, on the left side, the steering chip for row-wise readout switcher, and on the right hand side clear switcher, in the middle of this picture, DEPFET matrix, under the DEPFET, front end chip curo.
Current Readout CUROII
PCB with
DEPFET matrix
r/o board with ADCs etc.
Charge collection in "off" state
Only selected rows dissipate power
low power consumption, expect about 3W
for the whole vertex detector!
USB based digital interface board

23. DEPFET Project Status - in Summary
thinning technology
steering chips Switcher II
Technology development
tolerance against
ion. radition
r/o chips Curo II
55Fe
beam test

24. Annealing for MOS-C (PXD4)