layer
B locking
oxide
VG (V) VFB±10V operation after evaporating Al2O3
Memory window ~0V under VFB±10V operation after evaporating Al2O3
Figure 5-3 Capacitance-voltage (C-V) hysteresis of the fabricated MOIOS structure with Mo nanocrystals embedded in SiNx. (a) The sample of as-depostion (b) After annealing 700 oC N2 30 min. (C) After annealing 900 oC N2 30 min.
Al 2p
Intensity(a.u)
Binding Energy(eV)76 75 74 73 72
77
Retention Time(s)
100 101 102 103 104 105 106 107 108 109
Memory window(V)
0.0 0.2 0.4 0.6 0.8 1.0 1.2
Al2O3 700oC SiN 900oC
- 19.36 mV/dec
-32.2 mV/dec
Figure 5-6 Retention characteristics of the Mo nanocrystal embedded in SiNx and AlOx after annealing 900 oC in N2 for 30 min.
a.
b.
Figure 5-7 Modeling Retention characteristics. （（（a）（ ）））the Mo nanocrystal embedded in SiNz and （（（b）（ ））） the Mo nanocrystal embedded in AlOx after annealing 900 oC in N2
for 30 min.
Table 5-1 Comparisons of memory window, nanocrystal density and retention characteristics between two type nonvolatile memory devices.
density
Chapter 6
Conclusions
6.1 Conclusions
Co-evaporating molybdenum and silicon after annealing in O2 environment, the phenomenon of over-oxidization can be observed by XPS analyzing. In this thesis, a novel and ease fabrication technique of molybdenum nanocrystals was demonstrated for the application of nonvolatile memory, in order to shun this phenomenon, the nonvolatile memory structure of molybdenum nanocrystals embedded in the SiOx, SiNx and AlOx
layer was fabricated by co-evaporateding Mo and dielectric pellets at room temperature, respectively. The high density (~1011 cm-2) nanocrystal can be simple and uniform to be fabricated.
A furnace annealing with temperature (700°C~900°C) in N2 for 30 min is used to improve the crystalline quality of nanocrystals and memory characteristic. During the thermal process, the molybdenum nanocrystals precipitate and show good crystalline quality. The molybdenum nanocrystal with good crystalline quality has higher density of states to store charge and cause larger memory window. The reliability issues, such as retention and endurance, are significantly improved after the thermal process. According the results of C-V characteristic and XPS analysis, thermal treatment can reduce the defects (leakage path) in the dielectric (SiOx, SiNx or AlOx) which surrounds the nanocrystal. It decreases the probability of charge escaping from the nanocrystal. The quality of dielectric is also strengthened to bear the P/E cycling stress.
The charge storage layer of nanorystal embedded in AlOx shows better reliability over nanocrystals embedded in SiOx and SiNx. The improvement of charge storage capacity is attributed to high dielectric constant can reduce charge energy, the enhanced field by stored charge (Coulomb blockade) through the tunneling oxide can be reduced.
This result decreases the probability of charge escaping from the nanocrystal. Moreover,
charges are likely to be relaxed to the nitride traps. Leakage through the intermediate medium can also be inefficient due to Coulomb Blockade. Therefore, nanocrystals embedded in AlOx show enhanced retention characteristics.
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