Chapter 5 Conclusion
B.3 Results and Discussion
The charge storage layer embedded in dielectrics layer of metal-oxide–insulator -oxide silicon (MOIOS) memory device was utilized to capture the injected carriers from the channel, which caused a variation in the threshold voltage of the memory device. The charge storage effects in these memory structures were measured using capacitance-voltage techniques in these co-sputtered film structures. Fig. B-2 show the forward and reverse capacitance-voltage (C-V) curves for as-deposited samples obtained when the gate voltage was first swept from –7V to +7V (accumulation to inversion, forward sweep) and then from +7V to –7V (inversion to accumulation, reverse sweep) for Co-Al2O3 co-sputtered based MIS structure. The bidirectional C-V sweeps were performed from deep inversion to deep accumulation and in reverse, which exhibited an electron charging effect. We could observed that the memory windowis about 3V (ΔVth=3V) and 1.8V (ΔVth=1.8V) after the sample was annealed at high temperature and the charge storage effects in these memory structures were a large number. Based on the illustrative band diagram shown in Fig. B-3, when the applied voltage is sufficiently high, Fowler-Nordheim (FN) tunneling of electrons from Si substrate to the charge storage layer I1 and that from the charge storage layer to gate I2 could occur simultaneously. Electrons were trapped in the charge storage layer with positive applied voltage at metal gate during programming. On the contrary, the holes might tunnel from the valence band of the Si substrate and recombined with the electrons trapped in the charge storage layer, or the electrons tunnel back to the Si
substrate from the charge storage layer during erasing
storage layer”/ SiO
2/Si-based as MIS structure, showing hysteresis of as-deposited and after PTA at 650
◦C and 750
◦C measured. The curves were obtained by gate voltage sweeping from forward to reverse and back.
Figure B-3 The illustrative band diagram of a MIS structure
(Al/SiO
2/“The charge storage layer”/ SiO
2/Si) with positive applied
voltage at metal gate.
We could observe that different temperature and time conditions play important roles in generating the asymmetrical change in the memory window from Fig. B-4. It was showed the C-V hysteresis of this structure with two different conditions in temperature and two different time conditions in every temperature condition. We could observe that smaller memory window under ±7V C-V sweeping was obtained on Fig. B-4(a) and Fig. B-4(c). As RTP time was longer, the threshold-voltage shifts were increasing and memory window was 7V and 4V under ±7V, It was shown on the Fig. B-4(b) and Fig. B-4(d). The results revealed that temperature and annealing time appears to be an important variable in the memory window. It is considered that there were some Co elements combines with Al2O3 to form chemical compounds or agglomeration to form crystals in the charge storage layer.
Co-Al2O3 6500C 30s
storage layer”/ SiO
2/Si-based as MIS structure, showing hysteresis of as-deposited and after PTA at 650
◦C and 750
◦C measured during different time. The curves were obtained by gate voltage sweeping from forward to reverse and back.
The memory effect was observed from the hysteresis capacitance-voltage (C-V) characteristics of MIS capacitors embedded with the charge storage layer. According to the theoretical derivation [B-4], i.e. Q= -VfbCcontrol, the total charges trapped in the capacitor can be approximately estimated and the results are shown in Fig. B-5. We could observe that the value of trapped electrons or holes in Co-Si3N4 co-sputtered
film is lower than that in Co-Al2O3 co-sputtered film. This diagram tells us that the Co-Al2O3 co-sputtered film as the charge storage layer is a very good material for memory devices.
Figure B-5 The relations between gate voltage and flatband voltage, stored charges in a MIS capacitor. The insets in this figure the high frequency C-V relations of MIS capacitors co-sputtered to form the charge storage film with metal Co embedded in the Al
2O
3and Si
3N
4respectively.
What did the Co-Al2O3 co-sputtered film happened during the RTA process? We were so interested in that. XPS, also called ESCA, is an effective and widely used surface analysis technique and therefore we analyzed the material in Co-Al2O3
co-sputtered film with XPS. Fig. B-6 showed that there are chemical bonds of Co-Co, Co-O and Al-O in the Co-Al2O3 co-sputtered film after annealing in N2 ambiance [B-5] [B-6]. Because of them, we considered that
cobalt may react with
aluminum oxideduring annealing to make
aluminate spinels. The CoxAlyOz is an aluminum transition metals oxide which falls into the category of aluminate spinels [B-7] [B-8].Further research on XRD material analysis would clarity what binding energy exist in this material is.
Figure B-6 The XPS (ESCA) spectra of Co-O and Al-O peaks in
Co-Al
2O
3co-sputtered film after RTA annealing in N
2ambiance. We
could know that is Co
xO
y-Al
zO
wmixture clearly.
Fig. B-7 showed the Transmission Electron Microscope (TEM) image of the Co-Al2O3 co-sputtered film. We could observe that cobalt come into being Discontinuous grains after annealing and the annealed film shows a strong contrast of Co grains and the remanent (CoxAl1−x) zO3−v matrix [B-9]. The average width of the Co grains is about 2–3 nm and the average length of the Co grains is about 10 nm to 20nm. It is quite likely that annealing temperature and time determine the chemical reaction in the Co-Al2O3 co-sputtered film and this chemical reaction made cobalt grains discontinuous. When we annealed during shorter time with RTA system, the memory window will be smaller. The memory effect was the sum of contributions from the charge trapping. When we annealed during shorter time, metal cobalt element had not been discontinuous grains and amount of cobalt element was too large to trap charges. We could observe this phenomenon from Fig. B-4 (a) and Fig.
B-4 (c). When we annealed during longer time with RTA system, the memory window will be larger. The memory effect was the sum of contributions from the charge trapping. Therefore, it seems reasonable to conclude that metal cobalt element will be shown in discontinuous grains form and amount of pure metal cobalt element reduce after we annealed during longer time. The charges are stored in distributed grains and trapping centers of the Corich-(CoxAl1−x) zO3−v film. We could observe this phenomenon from Fig. B-4 (b) and Fig. B-4 (d).
Figure B-7 The TEM image of Co-Al
2O
3co-sputtered film and its EDX.
The current density-voltage (J-V) characteristics of the memory which is based on Co-Al2O3 co-sputtered film. It is shown in Fig. B-8. We could know that the leakage current exhibited a nearly result about from 10-8 order to 10-9 order after annealing during 60 seconds. The endurance characteristics of the MOIOS with Co discontinuous grains embedded in the (CoxAl1−x) zO3−v are illustrated in Fig. B-9.
Pulses (VG=±5V) were applied to evaluate endurance characteristics for the Program/Erase operation. The schematic plots indicated the memory windows can be
retain as their initial values after 106 program/erase cycles at room temperature.
The I-V characteristics of Co-Al2O3 co-sputtered based film based memory
on Co-Al
2O
3co-sputtered film with 750
oC annealing process during 60 seconds.
Stress conditin VG= +/- V RTA:O2 750oC 60s
Number of Cycle
100 101 102 103 104 105 106 107
Flat Band Voltage VFB(V) -1
From the XPS (ESCA) schematic plots and TEM image, these results lead us to the conclusions that the containments of samples changed during annealing treatment.