According to the results in the previous three sections, it is observed that EUV and X-ray irradiations have similar effects on the RRAM characteristics. However, it
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seems that RRAM is more immune to X-ray than EUV. The change of memory state is observed when the total dose of EUV increases to 100 Mrads ( ); however, it is not observed until the total dose increases to 600 Mrads ( ) for the devices irradiated by X-ray. This difference can be explained from two points of view.
First, the attenuation length in hafnium oxide for the EUV which has the energy of 91.85 eV is different from the 10 keV X-ray. Fig. 3-14 shows the attenuation length of EUV and X-ray, respectively. From the attenuation length we can have the penetration probability by the formula , where P is the penetration probability, is the attenuation length, and x is the thickness of oxide which is 5 nm in this thesis. The penetration probabilities are 0.846 for the EUV and 0.999 for the X-ray, which means the absorption rate is 0.154 for the EUV and 0.001 for the X-ray.
Since the energy of X-ray is about 109 times higher than that of EUV, the total energy absorbed by the 5-nm-thick hafnium oxide during EUV irradiation is about 1.41 times stronger than that during 10 keV X-ray irradiation. Hence, for the same flux, the effect arising from EUV is lightly stronger than that from 10 keV X-ray. On the other side, as the definition of the total dose discussed in section 2-5, the formula contains both the energy and flux of the irradiation. Therefore, even the total dose are in the same value for EUV and 10 keV X-ray, for example, both in 900 Mrads ( ), the total flux of EUV is about 109 times higher than that of 10 keV X-ray because of the difference in energy. Hence, from the above two points, the effect of EUV is stronger than that of X-ray on RRAM devices.
3-6 Summary
The effect of EUV and 10 keV X-ray irradiation induced damages on the characteristics of RRAM devices are investigated.
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For the switching characteristics and memory windows of RRAM, radiation does not much damage on the RRAM devices. This is because the switching process of RRAM contains the recombination of the oxygen vacancies. During this trapping and detrapping process, the defects induced by radiation which might affect the conduct filaments in the oxide at first would be partly canceled out. Hence, even there are defects induced by radiation in the oxide, the switching process is not affected.
Besides, the defects induced by radiation are less than that in the oxide which is after forming process. Therefore, the value of high and low resistance during switching;
that is, memory window keeps at almost the same window as that before irradiation.
After irradiation, the retention characteristics show the same way as that before irradiation, even for EUV or X-ray. The results could be explained by the fact that the oxygen vacancies almost don’t move in the room temperature. Thus, the conducting filaments would not be affected, keeping the resistance of a device in the same value.
However, for the devices already have the memory state stored, there are some chances that the stored memory state might change, especially for the devices with high total dose. Devices irradiated by EUV lose the memory state once the total dose over 100 Mrads ( ), while devices irradiated by X-ray have the issue of losing stored state over 600 Mrads ( ). The different levels of damage between two sorts of radiation is because the different attenuation length on the hafnium oxide and the definition of the total dose. Because the total dose used here contains the contribution of energy and flux, and both of them are proportional to the value of total dose. Thus, the process of calculation will make the flux for X-ray less than that for EUV. Besides, even in the situation of same flux, the total energy absorbed in the oxide for the X-ray is also less than that for the EUV, because of the different attenuation length.
The endurance performance degrades after high total dose irradiation by X-ray.
This shows that radiation indeed induces damage on oxide. However, this damage is
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global in the oxide. Therefore, in few numbers of switching cycles, the effects of damage would not be observed. Nevertheless, after thousands of switching cycles, the oxide layer in a stress situation. Thus the defects that not influence the characteristics at first might induce hard breakdown in this situation. Thus, if there are more enough defects in the oxide, the endurance performance might degrade.
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Table 3-1: Proportion of failed devices before irradiation and after different total dose irradiation
Pre irradiation
1 Mrads ( )
300 Mrads ( )
600 Mrads ( ) Failed
percentage (%)
33 12 50 62
Table 3-2: Comparison between devices irradiated by EUV and X-ray
Switching characteristics
Memory windows Retention
EUV No change No change No change
X-ray No change No change No change
Memory state Endurance
EUV Change over 100 Mrads (
X-ray Change over 600 Mrads ( Change over 600 Mrads (
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dose, all devices are switched ten cycles before and after the irradiation33
Fig. 3-2: Memory windows for the RRAM devices irradiated by EUV with different total dose, all devices are switched ten cycles before and after the irradiation
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dose, all devices are switched ten cycles before and after the irradiation35
Fig. 3-4: Memory windows for the RRAM devices irradiated by X-ray with different total dose, all devices are switched ten cycles before and after the irradiation
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pictures are just for the devices having memory state changing37
pictures are just for the devices having memory state changing38
Fig. 3-7: The cumulative probability plots for the RRAM devices irradiated by EUV with different total dose.
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(a) (b)
Fig. 3-8: (a) Devices initially at low resistance state have conducting filaments composed with lots of oxygen vacancies all over the oxide layer. Even there are defects induced by radiation, the still keep in low resistance state.
(b) Devices initially at high resistance state have few broken conducting filaments in the oxide layer. The defects induced by radiation have a chance to repair the conducting filaments, making the device change into low resistance state.
0 2 4 6 8 10
102 103 104 105 106 107 108
Change into low resistance state after irradiation
Resistence(
Switching cycle
Pos irradiation Pre irradiation
Fig. 3-9: The device changing into low resistance after irradiation reset to high resistance in the switching process
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pictures are just for the devices having memory state changing41
pictures are just for the devices having memory state changing42
Fig. 3-12: The cumulative probability plots for the RRAM devices irradiated by X-ray with different total dose.
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Fig. 3-13: (a) Devices can switch over two thousands cycles (b) Devices can’t switch over two thousands cycles Fig. 3-14: (a) Attenuation length for EUV on hafnium oxide (b) Attenuation length for X-ray on hafnium oxide