We probe into the electrical characteristics of the test capacitors and the HfO2
SOHOS memory devices in this section.
3.3.1 Electrical Characteristic of HfO2 Capacitor
Fig 3-5(a),(b) is the capacitance-voltage (C-V) hysteresis curve of Cap-N6 and Cap-O6. The counterclockwise direction of the hysteresis curve is present. At first the upper electrode of capacitor was biased from 8V to -8V, and the inversion layer of silicon substrate turned into the accumulation layer gradually. When the capacitor was operated in the positive gate voltage, the electrons of inversion layer inject into the trapping layer through F-N tunneling mode. Then these electrons will be trapped in the HfO2 layer and let the VFB (flat-band voltage) of capacitor shift to the positive side.
On the other hand, when the capacitor is biased from negative voltage, the trapped electrons maybe tunnel back to substrate or the holes can also inject into the trapping layer and combine with the trapped electrons. These two phenomenon can reduce the
amount of trapped electrons in charge storage layer and form the left C-V curve.
Subsequently, Fig 3-6(a),(b) show the program characteristics of Cap-N6、Cap-O6 in
different program voltages. We can observe that Cap-O6 is slower than Cap-N6. This is attributed to the thicker tunnel oxide layer of Cap-O6. Because the less electric field across the thicker tunnel oxide results in the less F-N tunneling current. The erase characteristics of Cap-N6、Cap-O6 with different erase voltages is expressed in Fig
3-7(a),(b). Next, we start to concern about the reliability of capacitors. Fig 3-8(a),(b) show the retention characteristic of Cap-N6、Cap-O6 at room temperature. After 10
years (about 3×108 sec.), there is the larger VFB shift window between program state and erase state in Cap-N6 than Cap-O6. The data retention degradation attributes to the charge loss. Some possible causes of charge loss are defects in tunnel oxide, defects in blocking oxide or mobile ion contamination. Therefore, the better retention characteristic in Fig 3-7(a) reveals that the oxynitride tunnel oxide has less defects and can improve the retention of Flash memory. In order to confirm the improvement, the retention after 105 program/erase cycles is introduced in Fig 3-9(a),(b). After 10 years, the VFB shift window shrink for Cap-N6 and Cap-O6, while the retention characteristics of Cap-N6 still better than Cap-O6. These retention results reveal that the oxynitride process can reduce the defects of tunnel oxide effectively and reform the date integrity of Flash memory [61-62] . The endurance characteristics of Cap-N6 and Cap-O6 is shown in Fig 3-11. After repeatedly program/erase operation, the VFB
window between program and erase states is narrower in Cap-O6 and make the distinguishability of program and erase states difficult. Therefore, the oxynitride
tunnel oxide process can also modify the endurance characteristic of Flash memory.
The counterclockwise C-V hysteresis curve of 700℃、800℃、900℃ samples are shown in Fig 3-12(a),(b)、Fig 3-13(a),(b)、Fig 3-14(a),(b). These indicate that the
tunneling electrons are dominant from the substrate, not from the gate of capacitor.
The clockwise C-V hysteresis curve relates to the gate injection phenomenon and isn’t expected for Flash memory. The program characteristics of other capacitors is shown in Fig 3-15(a),(b)、Fig 3-16(a),(b)、Fig 3-17(a),(b), respectively. To compare the
program speed simply, the time for VFB shift reach 2V is listed in Tab 3.2. The higher PDA temperature, the faster speed is present, especially for 800℃ samples. The
reason for faster program speed is that HfO2 trapping layer is more crystallized in complete PDA treatment [63]. Fig 3-18(a),(b)、Fig 3-19(a),(b)、Fig 3-20 (a),(b) are the
erase characteristics of other capacitors. Now, we analyze the retention characteristics in Fig 3-21(a),(b)、Fig 3-22(a),(b)、Fig 3-23(a),(b)、Fig 3-24(a),(b)、Fig 3-25(a),(b)、
Fig 3-26(a),(b). The data retention of oxynitride tunnel oxide is better than conventional oxide tunnel oxide. There are rising phenomenon in erase state curve of Fig 3-9(a),(b)、Fig 3-24(a),(b)、Fig 3-25(a),(b)、Fig 3-26(a),(b). The rising tendency
results from the degradation of PECVD blocking oxide after plenty of program/erase operations and some electrons inject into the trapping layer through degraded blocking oxide. Similarly, the all retention characteristics is collected and analyzed in Fig 3-27(a),(b)、Fig 3-28(a),(b)、Fig 3-29(a),(b). Finally, the endurance characteristic of other capacitors is still better for oxynitride than conventional oxide tunnel oxide,
as shown in Fig 3-30, Fig 3-31, Fig 3-32.
3.3.2 Electrical Characteristic of HfO2 Memory Device
This section will analyze the electric characteristics of the SOHOS Flash memory with HfO2 trapping layer. The former paragraph already revealed and investigated the test capacitors and found the optimum condition for fabricate the integrated Flash memory devices. The condition chosen by us is PDA 800℃ for 30
sec. in N2 ambient and the split table is listed in Table 3.3. The cause is it has the faster program/erase speed and the more suitable retention characteristic. Although PDA 900℃ sample performs the best program characteristic, the initial and after cycles retention isn’t satisfied. Therefore, we thought that the 900℃ PDA will induce
more traps with shallower energy level in the trapping layer, which give rise to larger memory window and poor charge retention[64]. Fig 3-33(a) and Fig 3-33(b) show the ID-VG curve of Flash memory devices. Then the memory devices are programmed by CHE and erased by FN tunneling, as shown in Fig 3-34(a), Fig 3-34(b), Fig 3-35(a), Fig 3-35(b). The CHE (channel hot electron) program is to set VD=VG=6V or VD=VG=7V and the FN erase is to set VG=-5V or VG=-6V in device cells.
Subsequently, the compared retention property of Device-N and Device-O is shown in Fig 3-36. The result that the Device-N performs the better retention characteristic is observable. This indicates that the novel oxynitride process is applied not only in capacitor structures but in integrated Flash memory cells and can improve the data retention all. Similarly, the endurance of two memory devices is shown in Fig 3-38
and the Device-N is superior than Device-O. Therefore, from the measurements of test capacitors and devices, we understand the tunnel oxide with oxynitride process can modify the reliabilities of Flash memory indeed.