Chapter 2 Experiment
2.1 The Process Flow of SiGe-based p-MOSFET…
P type (Boron doped) Si substrate (100) was used in this study. After initial RCA cleaning, 3000Å oxide was thermally grown at 980oC by horizontal furnace through wet oxidation, served as isolation layer. After one more time RCA cleaning, above is a 150Å thick amorphous-Si deposited by LPCVD at 550oC, as a buffer layer between SiO2 and SiGe film. SiGe (Ge content at 7% and 11%) film was then deposited onto amorphous-Si layer about 800Å by UHV-CVD. The cross sectional view is shown in figure 2-1.
By means of photolithography, the active region could be defined after photo-resist has been removed. After S-D and channel patterning (top view shown in figure 2-2), TCP poly etcher was employed for SiGe etching by Cl2 and HBr. Later, the main part of this study proceeds by oxidizing the remaining SiGe part through different recipes. Different oxidation parameters like temperature, oxidation time, and oxygen flow were applied in order to find out the optimum oxidation condition of SiGe channel. Moreover, pre-deposited-oxide before oxidation, oxidation rate controlling were also performed trying to make better electrical characteristics performance of SiGe-based p-MOSFETs possible. All experimental factors are listed in table 2-1(A) to table 2-1(E).
Next, devices were dipped in BOE solution to remove surface SiO2 formed during oxidation of SiGe film. After 1000Å SiO2 deposited by PECVD served as gate dielectic, 600oC anneal with O2 was performed to cure the defects in gate oxide for 30min. Then, 2000Å poly-Si was deposited by LPCVD to work as control gate.
After gate region and channel length defined by lithography, poly-etcher and BOE solution were used for etching. Cross sectional view and top view are shown in figure 2-3 and figure 2-4, respectively. Then, Boron was doped heavily with 5x1015 cm-2 at 10 keV. Activation annealing at 950oC in N2 flow was then employed for 30 minutes after ion implantation.
3000Å SiO2 by PECVD was deposited as passivation layer. Lithography comes next to form contact hole. SiO2 was then etched by BOE solution for about 55 seconds. Finally, a 500 nm Al film was deposited by evaporation and then contact pad were patterned. Al sintering was performed at 430oC for 30minutes.
The detailed fabrication process flow is as follows:
1. (100) P+ Si wafer 2. RCA cleaning
3. Wet oxidation at 980oC for 3000Å 4. RCA cleaning
5. 150Å amorphous-Si by LPCVD 6. RCA cleaning
7. 800Å SiGe(Ge content at 7% and 11%) by UHVCVD 8. Mask #1:Define active region
9. Dry etching by TCP poly etcher 10. PR removing and RCA cleaning 11. SiGe dry oxidation
12. BOE dipped to remove SiO2
13. RCA cleaning
14. PECVD SiO2 1000Å deposition
15. Gate oxide annealing at 600o with O2 for 30min 16. LPCVD poly-Si 2000Å deposition
17. Mask #2:Define gate region and channel length 18. Wet etching by poly-etcher solution for 90 seconds 19. Wet etching by BOE solution for 25 seconds 20. PR removing
21. Boron doping of 5x1015 cm-2 at 10 keV 22. Activation at 950oC for 30 minutes 23. STD cleaning
24. PECVD SiO2 3000Å deposition 25. Mask #3:Define contact hole
26. Wet etching by BOE solution for 50 seconds 27. PR removing
28. Al coating for 5000Å 29. Mask #4:Define contact pad 30. Al etching
31. PR removing
32. Sintering of Al at 430oC for 30 minutes
2.2 Methods of Measurement and Analysis
2.2.1 Current-Voltage Characteristic Measurement
Current-Voltage characteristics were measured by a semiconductor parameter analyzer HP4156A at room temperature. Five devices of each sample were measured in order to choose a most reliable one to serve as result.
Id-Vg characteristic was measured first. Then gm, on/off ratio and Vt could be calculated through Id-Vg characteristic data. Gm was defined as dId/dVg, and the maximum value of gm in each device was analyzed. Id-Vd characteristic was also
measure at a constant (Vg-Vt) value.
2.2.2 The Introduction of HP4156A
HP4156A is an electronic instrument for measuring and analyzing characteristics of semiconductor devices. This instrument is useful for performing both results measurements and analyses. Id-Vg and Id-Vd characteristics of our experiment were measure by HP4156A.
There are four highly accurate source/monitor units (SMUs), two voltage source units (VSUs), and two voltage measurement units (VMUs) in HP4156A. HP4156A is designed for Kelvin connections and has high-resolution SMUs (HRSMUs), so HP4156A is especially suitable for low resistance and low current measurements.
Voltage values with a resolution of 0.2μV can be measured by using the differential measurement mode of VMUs.
Stress testing can be also performed by HP4156A. That is, a specified dc voltage or current can be forced on the device for a specified duration. Also, we can force ac stress by using pulse generator units (PGUs), which were installed in HP41501A SMU/Pulse Generator Expander. HP41501A is attached to HP4156A, and was equipped with a ground unit (GNDU), high power SMU (HPSMU), two medium power SMUs (MPSMUs), or two PGUs.
HP4156A can print and store, in addition to perform measurement and analysis.
Measurement setup information, measurement data, and instrument setting information can be stored in a 3.5-inch disk. The setting information and measurement results can be printed on a plotter or printer which is connected with HP4156A.
HP4156A can be controlled by an external controller via HP-IB by using remote control commands. These commands are based on Standard Commands for
Programmable Instruments (SCPI), so measurement programs can be easily developed. HP4156A has internal HP instrument BASIC, so we can develop and execute measurement programs by using the HP4156A only, without using an external controller.
Chapter 3
Results and Discussions
In the following chapter, all devices were normalized to W/L = 1μm/1μm. In Id-Vg and gm-Vg measurement, Vd was applied at -5V. While Id-Vd was measuring,
|Vg-Vt| was set at 3V.
3.1 Influence of Oxidation Temperature on Electrical Properties
Figure 3-1(a) shows Id-Vg characteristic of three different devices: un-oxidized, oxidation at 950oC, and oxidation at 1000oC. Si0.89Ge0.11 film was used in this experiment, and both of the oxidized devices were oxidized for 16 minutes with 3750 sccm O2 flow.
It can be seen that both oxidized devices show superior electrical performance than the un-oxidized one by higher on/off ratio. The device of 1000oC-oxidized shows even higher on current than the one of 950oC-oxidized one while both devices have roughly the same off state current.
Transconductance is shown in figure 3-1(b). Obviously, gm of 1000oC-oxidized device is higher than that of 950oC-oxidized one and of course than the un-oxidized one. They are about 3.78μS, 2.64μS, and 0.33μS, respectively.
Figure 3-1(c) shows Id-Vd characteristic which is consistent with the prediction:
the device oxidized at 1000oC has highest Id of the three devices -2.58μA at Vd=-6V.
950oC- oxidized device has 1.83μA , and the un-oxidized one has 0.40μA.
It is known that since SiGe got oxidized, concentration of Ge would be increased, and then mobility would also get enhanced which resulted in a higher transconductance and on state current. As the improvement of on state current is
higher than the increasing of off state current, on/off ratio gets improved then. With higher oxidation temperature, the rate of oxidation would be higher, which makes more Si in SiGe film oxidized. Then the mobility would be even higher, and better performance is achieved. Table 3-1 shows the normalized data which is divided by the results of un-oxidized device.
3.2 Influence of Oxidation Time on Electrical Properties
Figure 3-2(a) shows Id-Vg characteristic of four different devices: un-oxidized, oxidized for 4 minutes, oxidized for 16 minutes, and oxidized for 36 minutes, respectively. Si0.93Ge0.07 film was used in this experiment. All of the oxidized devices were oxidized at 1000oC with 3750 sccm O2 flow. From the diagram, the trend of the curves indicates that longer oxidation time results in higher on state current and also higher transconductance (from Figure 3-2(b)). Besides, after calculation, the on/off ratio of 36min-oxidized, 16min-oxidized, and 4min-oxidized is 3.3, 1.6, 1.5 times higher than un-oxidized device, respectively. Due to the same reason as described in the previous section, in the longer oxidation time devices, more amount of Si was oxidized and then higher Ge concentration was achieved. So, similar trend of I-V characteristic as the previous section would be found in the diagram.
Figure 3-3(c) depicts Id-Vd characteristic for devices with different oxidation time. As predicted, 36min-oxidized device has highest on current, which is 5.08μA at Vd=-8V. For the other devices, they are 1.52μA, 0.75μA, and 0.61μA for 16min-oxidized, 4min-oxidized, and un-oxidized device, respectively. Table 3-2 shows the normalized data which is divided by the results of un-oxidized device.
It is presumed that unless Si in the SiGe layer is fully oxidized, the performance of the SiGe-based p-MOSFET would always be improved with the increasing of
oxidation time since the positive correlation between the amount of oxidized Si and the mobility of SiGe channel.
3.3 Influence of Oxygen flow on Electrical Properties
Si0.89Ge0.11 film was used in this experiment. Devices were oxidized at 1000oC for 16minutes. Figure 3-3(a) depicts Id-Vg characteristic of devices with different oxygen flow during oxidation: 3750 sccm, 2500 sccm, and un-oxidized. The device under larger oxygen flow shows larger on state current, lower off state current, and higher transcondutance, which is undoubtedly better than the device oxidized under 2500 sccm oxygen flow and the un-oxidized one. It is supposed to be contributed by more Si being oxidized, as discussed already. The on/off ratio of 3750sccm-device and 2500sccm-device are 5.89 and 1.86 times higher than the un-oxidized one. Gm-Vg characteristic is shown in figure 3-2(b).
As to Id-Vd characteristic, which is shown in figure 3-3(c), the device with 3750 sccm O2 flow has 2.56μA at Vd=-6V. The device with 2500 sccm O2 flow and un-oxidized has 1.83μA and 0.43μA, respetively. Table 3-3 shows the normalized data which is divided by the results of un-oxidized device.
From 3-1 to 3-3, the amount of oxidized Si explans the improvement of electrical charateristic performance well. In the following three sections, several different oxidation condition were applied, and some other results would be achieved.
3.4 Influence of the Thickness of Pre-oxide on Electrical Properties
As it is known that oxidation rate would decrease as oxidation proceeds with time, a new experiment is designed to investigate the influence of the initial oxidation
rate on electrical performance of SiGe-based p-MOSFET. SiO2 was deposited onto SiGe film first by PE-CVD right after UHV-CVD SiGe film was deposited. The thickness of SiO2 was 300Å, 500Å, and 1000Å, respectively, and a non-pre-oxide device was also fabricated. Si0.93Ge0.07 was used in this experiment and oxidation was performed at 1000oC for 36 minutes with 3750 sccm O2 flow.
Figure 3-4(a) depicts Id-Vg characteristic of devices with different thickness of pre-oxide. The on/off ratio is getting higher while the thickness of pre-oxide getting larger, which is 2.6, 1.9, 1.7 times higher than non-pre-oxide device, respectively. But in the other hand, the on state current decreases. In figure 3-4(b), it is found that gm also gets lower with the increasing of pre-oxide thickness.
It is presumed that less amount of Si in SiGe film would be oxidized owing to thicker pre-oxide exists, so that results in lower on current and transconductance. As shown in figure 3-4(c), Id at Vd=-8 of non-pre-oxide, 300 Å, 500 Å, and 1000 Å are 5.46 μA, 2.79μ A, 1.07μ A, and 0.95μ A, respectively. Table 3-4 shows the normalized data which is divided by the results of un-oxidized device.
But there is still benefit from depositing pre-oxide. Since the oxidation rate was lowered by pre-oxide, a high quality channel was formed, which lowers the off state current, and results in a higher on/off ratio. The trade off between the on/off ratio and on state current should be considered case by case of the different use of the devices.
3.5 Influence of Oxidation Rate on Electrical Properties
In the previous section, it is concluded that under lower oxidation rate, SiGe-based p-MOSFET would achieve better on/off ratio performance. But the amount of oxidized of Si in last experiment was still a variable. In this section, the factor of the amount of oxidized of Si was removed by a new designed method.
Several oxidation conditions were performed first and the thickness of SiO2 was measured. Three oxidation conditions of roughly the same thickness of SiO2 were selected. They are 950oC 15 minutes, 900oC 30 minutes, and 850oC 75 minutes, respectively, which indicates same amount of Si was oxidized. Si0.93Ge0.07 was used in this experiment and oxidation was performed with 3750 sccm O2 flow.
Figure 3-5(a) depicts Id-Vg characteristic at Vd=-5V. On state current of the three devices almost equals, but the device with lower oxidation rate has lower off state current, which supports our conclusion from the previous section. As to gm-Vg characteristic in figure 3-5(b), three devices also have about the same gm, 0.5 ~ 0.6μ S. In figure 3-5(c), it is shown that the on state current of the three devices are also almost the same at about 2.1 ~ 2.3μA. Table 3-5 shows the normalized data which is divided by the results of un-oxidized device.
As prediction, same amount of oxidized Si results in same gm and Id. Slow oxidation makes channel quality higher and then lower the off state current, resulting in a higher on/off ratio.
Chapter 4 Conclusion
In this experiment, SiGe-based p-MOSFETs were fabricated and the electrical performance of the devices was improved by dry oxidation of SiGe channel through different recipes.
First, it is found that all the electrical characteristics such as on/off ratio, on state current, and transconductance would get improvement after the SiGe channel was oxidized. This is because after oxidation, Si atoms in SiGe channel would be combined with O atoms to form SiO2 while Ge atoms would be separated from that.
This is the so called Ge segregation mechanism. The more amount of Si in the SiGe film was oxidized, the more Ge atoms would exist in the SiGe channel and then makes Ge concentration higher which results in higher hole mobility in SiGe-base p-MOSFET. With the amount of oxidized Si increasing, the hole mobility also increases and then better electrical performance would be achieved. Experiments of oxidation temperature, oxidation time, and oxygen flow already proved this phenomenon.
Oxidation rate was also considered in our experiment. The results show that the devices under slower oxidation rate have lower leakage current and better on/off ratio.
It is conjectured that under slow oxidation process a high quality channel was formed.
This is why the devices have lower leakage current.
Chapter 5 Future Work
Since it is known that as the amount of oxidized Si increases, the electrical performance of SiGe-based p-MOSFET would get better, higher temperature like 1050oC, higher O2 flow such as 5000 sccm, and longer oxidation time could be applied in the same experiment to find the optimum situation of oxidation. Whether dry oxidation or wet oxidation differs is also a way of research. Same experiment can be also repeated in some other SiGe film with higher Ge content. Thermal oxide instead of PE-SiO2 serves as gate oxide may also bring expected improvement. As the best condition is found, fabrication of SiGe layer with high Ge concentration may be more economical and convenient through this Ge condensation method.
Alloy concentration Oxidation condition Variable: Temperature
Table 2-1(A). Experiment of the Different Oxidation Temperature.
Alloy concentration Oxidation condition Variable: Time 36 minutes Table 2-1(B). Experiment of the Different Oxidation Time.
Alloy concentration Oxidation condition Variable: O2 Flow 3750 sccm Table 2-1(C). Experiment of the Different Oxygen Flow.
Alloy concentration Oxidation condition Variable: Thickness 1000Å
500Å 300Å Si0.93Ge0.07
1000oC 36 minutes 3750 sccm O2 flow
no pre-oxide Table 2-1(D). Experiment of the Different Pre-oxide Thickness.
Alloy concentration Oxidation condition Variable: Rate 950 oC 15 minutes 900 oC 30 minutes Si0.93Ge0.07 3750 sccm O2 flow
850 oC 75 minutes Table 2-1(E). Experiment of the Different Oxidation Rate.
Fixed oxidation
condition Temperature Id
Vd=(-6V) gm(max) On/Off ratio
1000 oC 4.58 8.02 4.14
Si0.89Ge0.11
16 minutes
3750 sccm O2 flow 950 oC 6.45 11.51 6.02
Table 3-1. I-V characteristics with different oxidation temperature. Data was normalized by being divided by un-oxidized device.
Fixed oxidation
condition Time Id
Vd=(-8V) gm(max) On/Off ratio
36 minutes 8.32 8.21 3.33
Table 3-2. I-V characteristics with different oxidation time. Data was normalized by being divided by un-oxidized device.
Fixed oxidation
condition O2 flow Id
Vd=(-6V) gm(max) On/Off ratio
3750 sccm 6.02 11.28 5.89
Si0.89Ge0.11
16 minutes
1000oC 2500 sccm 4.31 5.64 1.86
Table 3-3. I-V characteristics with different oxygen flow. Data was normalized by being divided by un-oxidized device.
Fixed oxidation condition
Thickness of pre-oxide
Id
Vd=(-8V) gm(max) On/Off ratio
1000Å 1.48 1.08 6.92
Table 3-4. I-V characteristics with different thickness of pre-oxide. Data was normalized by being divided by un-oxidized device.
Fixed oxidation
condition Oxidation Rate Pre-oxide thickness
Table 3-5. I-V characteristics with different oxidation rate. Data was normalized by being divided by un-oxidized device.
Fig. 2-1. Cross sectional view of SiGe-based P-MOSFET after mask #1.
Fig. 2-2. Top view of SiGe-based P-MOSFET after mask #1.
Si
SiO
2SiGe
Fig. 2-3. Cross sectional view of SiGe-based P-MOSFET after mask #2.
Fig. 2-4. Top view of SiGe-based P-MOSFET after mask #2.
poly-Si SiO
2Si
SiO
2SiGe
4 2 0 -2 -4 -6 -8 -10
Fig. 3-1(a). Id-Vg characteristic of different oxidation temperature.
0 -2 -4 -6 -8 -10
Fig. 3-1(b). gm-Vg characteristic of different oxidation temperature.
at Vd=-5V
at Vd=-5V
1 0 -1 -2 -3 -4 -5 -6 -7 0.0
0.5 1.0 1.5 2.0 2.5 3.0
Id (µA)
Vd (V) 1000oC
950oC unoxidized
Fig. 3-1(c). Id-Vg characteristic of different oxidation temperature.
at Vg-Vt =-3V
5 0 -5 -10 -15
Fig. 3-2(a). Id-Vg characteristic after different oxidation time.
0 -2 -4 -6 -8 -10 -12
Fig. 3-2(b). gm-Vg characteristic after different oxidation time.
at Vd=-5V at Vd=-5V
0 -2 -4 -6 -8 0
1 2 3 4 5
Id (uA)
Vd (V) 36min
16min 4min
unoxidized
Fig. 3-2(c). Id-Vd characteristic after different oxidation time.
at Vg-Vt =-3V
4 2 0 -2 -4 -6 -8 -10 -12
Fig. 3-3(a). Id-Vg characteristic with different oxygen flow.
0 -2 -4 -6 -8 -10
Fig. 3-3(b). gm-Vg characteristic with different oxygen flow.
at Vd=-5V at Vd=-5V
0 -1 -2 -3 -4 -5 -6 0.0
0.5 1.0 1.5 2.0 2.5 3.0
Id (µA)
Vd (V) O2 3750 sccm
O2 2500 sccm unoxidized
Fig. 3-3(c). Id-Vd characteristic with different oxygen flow.
at Vg-Vt =-3V
6 4 2 0 -2 -4 -6 -8 -10 -12
Fig. 3-4(a). Id-Vg characteristic with different thickness of pre-oxide.
2 0 -2 -4 -6 -8 -10 -12
Fig. 3-4(b). gm-Vg characteristic with different thickness of pre-oxide.
at Vd=-5V
at Vd=-5V
0 -2 -4 -6 -8 0
1 2 3 4 5 6
Id (µA)
Vd (V) NO Pre-oxide
300A 500A 1000A
Fig. 3-4(c). Id-Vd characteristic with different thickness of pre-oxide.
at Vg-Vt =-3V
4 2 0 -2 -4 -6 -8 -10 -12
Fig. 3-5(a). Id-Vg characteristic of different oxidation rate.
0 -2 -4 -6 -8 -10
Fig. 3-5(b). gm-Vg characteristic of different oxidation rate.
at Vd=-5V
at Vd=-5V
1 0 -1 -2 -3 -4 -5 -6 -7 0
1 2 3
Id (µA)
Vd (V)
950oC 15min 900oC 30min 850oC 75min
Fig. 3-5(c). Id-Vd characteristic of different oxidation rate.
at Vg-Vt =-3V
簡歷
姓 名：吳資麟 性 別：男
出生日期：民國 69 年 12 月 22 日 出 生 地：台灣省台北市
住 址：台中市陜西東五街 47 巷 9-3 號 4 樓
學 歷：國立台中一中 (民國 85 年 9 月~民國 88 年 6 月) 國立交通大學電子工程系 (民國 88 年 9 月~民國 93 年 6 月) 國立交通大學電子工程所 (民國 93 年 9 月~民國 95 年 9 月) 碩士論文：矽鍺薄膜在不同氧化條件下之電性研究
A Study of Electrical Properties of SiGe Film with Various Oxidation Conditions
簡歷
姓 名：吳資麟 性 別：男
出生日期：民國 69 年 12 月 22 日 出 生 地：台灣省台北市
住 址：台中市陜西東五街 47 巷 9-3 號 4 樓
學 歷：國立台中一中 (民國 85 年 9 月~民國 88 年 6 月) 國立交通大學電子工程系 (民國 88 年 9 月~民國 93 年 6 月) 國立交通大學電子工程所 (民國 93 年 9 月~民國 95 年 9 月) 碩士論文：矽鍺薄膜在不同氧化條件下之電性研究
A Study of Electrical Properties of SiGe Film with Various Oxidation Conditions