Influence of the Thickness of Pre-oxide on Electrical Properties

Chapter 3 Results and Discussions

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 SiO₂was 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 1000^oC 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 g_m 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 950^oC 15 minutes, 900^oC 30 minutes, and 850^oC 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 1050^oC, 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 Si_0.93Ge_0.07

1000^oC 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

1000^oC 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

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

₂

SiGe

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

₂

Si

SiO

₂

SiGe

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

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) 1000^oC

950^oC 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

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

1 0 -1 -2 -3 -4 -5 -6 -7 0

1 2 3

Id (µA)

Vd (V)

950^oC 15min 900^oC 30min 850^oC 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

A Study of Electrical Properties of SiGe Film with Various Oxidation Conditions

在文檔中矽鍺薄膜在不同氧化條件下之電性研究 (頁 23-45)