Tunnel Oxide
3.3 Results and discussions
Figure 3-4(a) shows the capacitance-voltage (C-V) hysteresis of the SONOS capacitor after furnace reoxidation 5 minutes at 900℃ in O2 process. When this capacitor is operated in positive polarity, the electrons directly tunnel from the Si substrate through the tunnel oxide, and are trapped in the forbidden gap of the Si3N4
layer. When the capacitor is negatively operated, the holes may tunnel through the tunnel oxide to recombine with electrons trapped in Si3N4. The blocking oxide is utilized to prevent the carriers of gate electrode from injecting into the charge-trapping layer by FN tunneling. Figure 3-4(a) and Figure 3-5(a) shows a larger threshold voltage shift than that of the standard SONOS capacitor as shown in figure 3-6(a). As shown in figure 3-4(a) and figure 3-5(a), they are both a counterclockwise direction of the hysteresis, which implies the electrons injected from the deep inversion layer and holes injected from the deep accumulation layer of silicon substrate. In figure 3-4(b), when the voltage applied over the -6V, the leakage current becomes large. However, in figure 3-5(b) and figure 3-6(b), the leakage currents of the capacitor with reoxidation 45min and standard SONOS capacitor are all smaller than that with reoxidation 5min.
In Figure 3-7, when the voltage applied over 6V, the leakage current of the capacitor with reoxidation 45min is smaller than the capacitor with reoxidation 5min about two orders of magnitude. It implies that longer reoxidation time may cause enough oxygen extend into the silicon nitride, and the tunnel oxide. It can reduce the dangling bonds and the positively charged silicon ions in the nitrided-chemical oxide, since hydrogen gas produced by the dissociation of ammonia gas deteriorates the quality of chemical oxide during nitridation. Active hydrogen gas dissolves Si-O bonds in SiO2 and may result in positively charged silicon ions, proton ions and
dangling bonds. When the gate voltage was close to 8V, the leakage current of the capacitor with reoxidation 45min is almost the same with standard SONOS capacitor.
However, the capacitor with reoxidation 45min has a large memory window, it means we can have better charge trapping efficiency than the standard SONOS capacitor, and the leakage current doesn’t become large.
Secondary ion mass spectrometry (SIMS) analysis was conducted to characterize the composition of the ONO stack. Figure 3-8 exhibits the SIMS depth profile of the capacitor with reoxidation 45min. Although the SIMS spectra may not show the real distribution of bonds and atoms in this sample, it still provides useful information for study.
As shown in figure 3-8(a), the oxygen concentration at the 52nm is 33%. It is noted that the peak concentration is reduced as depth increases, and also proved that the certain amount oxygen atoms extend into the silicon nitride and nitrided-chemical oxide. When the certain oxygen atoms extend into the nitrided-chemical oxide, the more hydrogen-containing bonds were substituted with oxygen-containing bonds.
These hydrogen-containing bonds are necessary to be removed in order to obtain better memory properties. It is noted that oxygen-rich is at the top and the nitrogen-rich is at the bottom in the silicon nitride layer. Figure 3-8(b) shows the tapered bandgap diagram of the SONOS capacitor after reoxidation. The tapered bandgap shape causes a lateral hopping mechanism [3.6], and it helps electrons to be easily erased and programmed. In addition, there is a high nitrogen concentration on the surface of the nitrided tunnel oxide, and it can reduce the Dit.
Figure 3-9 shows the normalized CV curves of SONOS capacitor with reoxidation 5min after different voltage writing 1ms at 25℃. The FN tunneling occurs when the applied voltage is over 18V, however, its leakage current is very large. Figure 3-10
22
different voltage W/E 1ms at 85℃. The memory window is less than 1V, and it means the W/E time may be too short to obtain an enough memory window to be defined as
“1” or “0”. Figure 3-11 shows the normalized CV curves of SONOS capacitor with reoxidation 5min after different voltage W/E 10ms at 85℃. The memory window is over 1V. Therefore, an optimal asymmetric W/E operating voltage of 12V/-8V at 10ms was determined. Figure 3.12 shows the normalized CV curves of SONOS capacitor with reoxidation 5min after different voltage W/E 100ms at 85℃. When the writing time reaches 100ms, we found almost all the leakage currents become very large. It could be that there are too many hydrogen atoms in the nitrided-chemical oxide after nitridation without being replaced by oxygen atoms.
Endurance is the capability of maintaining the stored information after W/E cycling.
It is the parameters to describe how good the reliable is a nonvolatile memory cell.
Figure 3-13 shows the endurance characteristics of the SONOS capacitor with reoxidation 5min. This measurement is performed under accelerated conditions at 85℃. An initial memory window is 1.22V at 85℃. After 104 W/E cycles, the memory window is 0.58V. The memory window decreases due to interface-trap generation and lots of dangling bonds. It cause the leakage current becomes large, and deteriorates the quality of the ntrided-chemical oxide due to the presence of hydrogen.
Figure 3-14 ~ figure 3-15 shows the normalized CV curves of SONOS capacitor with reoxidation 45min after different voltage W/E from 1ms to 10ms at 85℃. As shown in Figure 3-14, though we can apply the gate voltage over 18V for writing, the slop of the 18V is getting smoother than the other voltages. It means Dit of the 18V is larger than the other voltages. In addition, it is noted that over-erasing may occur after -8V 1ms erasing operation due to the fewer writing time. As shown figure 3-15, we can obtain 1.18V the memory window by 12V/-8V and it is enough to be defined as
“1” or ”0”.
Figure 3-16 shows the endurance characteristics of the SONOS capacitor with reoxidation 45min at 85℃. The initial memory window is 1.18V at 85℃. After 12V/(-8V) W/E 104 cycles, it maintains a memory window about 0.84V, which is close to be defined as “1” or “0” for the circuit design.
Figure 3-17 shows the normalized CV curves of the standard SONOS capacitor after different voltage W/E 10ms at 85℃. Due to Pool-Frenkel and FN tunneling effect, the ONO stack can’t be applied the voltage over 14V. It makes the tunnel oxide breaks down. Figure 3-18 shows the endurance of the standard SONOS capacitor. The initial memory window is 1.08V at 85℃. After 104 cycles, the memory window is just 0.6V. The decaying rate is faster than the capacitor with 45min reoxidation.
Figure 3-19~ figure 3-22 shows the surface roughness comparison. The different AFM samples were presented in table 3-1. We compare A1 with A2, surface roughness increase from 0.219 to 0.279. It means the chemical oxide with nitridation treatment have a good uniform surface than the one which directly deposits Si3N4
layer without nitridation treatment. We compare A3 with A4, surface roughness increase from 0.215 to 0.257. The furnace oxide with nitridation still has a better uniform surface. Nitridation treatment provides better uniform surface due to the enough incubation time, similar material characteristics and the coefficient of thermal expansion between the nitride and oxide film. A good memory device will have a less leakage current due to a uniform surface.
3.4 Summary
We have proposed a novel process for forming an oxynitride as the tunnel oxide and a tapered bandgap nitride as the trapping layer. The capacitor with reoxidation
24
the better endurance performance. Most importantly, the process we proposed is not complicated and could be successfully integrated into current ULSI technology.
Figure 3-1 Schematic cross section of the ONO structure after reoxidation.
CVD Nitride