Chapter 1 Introduction
1.5 Motivation
We have been demonstrated the enhancement of sensitivity of SiGe nanowire with Ge concentration. But, it is difficult to obtain SiGe layer with high Ge concentration because the large lattice mismatch between the SOI and SiGe layer. Ge-condensation technique is a method to enhance Ge concentration in SiGe layer without generating defect.
Therefore, we utilize oxidation process to ehance Ge concentration in SiGe nanowire and the surface-to-volume ratio of SiGe nanowires will become higher.
Chapter 2 Experiment
The spacer formation [31] is an easy process for nanowire-fabrication with the advantages of high-yield and low-cost. The method only using the combination of the conventional lithography and process technology was demonstrated without complex processes such as EBL, SPL and VLS etc. We have successfully fabricated the SiGe nanowires by this method. To enhance sensitivity of SiGe-nanowire with low Ge concentration, Ge-condensation technique was utilized to obtain higher Ge concentration in SiGe nanowire after oxidation. And SiGe nanowire has higher surface-to-volume ratio, less defect and higher conductance which will discuss later.
2.1 Detail fabrication process of SiGe nanowire
A p-type (Boron doped) Si substrate (100) was used in this study.
The resistivity of the silicon substrate is about 1~10 Ω-cm. Samples were prepared by follow process:
1. After Standard RCA clean, wet oxidation for 6 hours at 980°C to grow the bottom oxide by ASM/LB45 furnance system. The thickness of oxide is 5000Ǻ which was shown in Figure 2-1.
2. Mask #1: Define the active area. We utilize TEL CLEAN TRACK MK-8 and G-line lithography system to transfer pattern on bottom oxide.
Then, we etch a height step to form a SiGe spacer by TEL5000 R.I.E.
system. The height of step is about 3000 Ǻ. The structure was shown in Figure 2-2.
3. Following Standard RCA clean, we deposited 150Å α-Si layer on bottom oxide by ASM/LB45 furnance system. The growth condition of 150Å α-Si layer was 650°C for ~2 hours. The process increases adhesion between SiGe film and SiO2 layer shown in Figure 2-3.
4. After Standard RCA clean, we deposited SiGe film by ultra-high -vaccum chemical vapor deposition − ANELAVA SiGe UHV-CVD. The condition of the SiGe film was epitaxially grown at 665°C. The structure was shown in Figure 2-4.
5. Mask #2: Defined the S/D region and nanwires. We etched the α-Si and SiGe film by TCP poly etcher − TCP9400 SE. Only the S/D and SiGe nanowires will be stayed. The structure was shown in Figure 2-5.
6. Mask #3: To avoid parallel SiGe nanowires short-circuit each other, we used TCP poly etcher to etch over two side spacer. The structure was shown in Figure 2-6.
7. Oxidation in various conditions such as different temperature and time.
8. The condition of ion implantation was E =15keV in energy and D =5×
1015 P+ ions/cm2 in dose.
9. After ion implantation, the surface oxide layer was removed by DHF (H2O:HF=50:1).
10. Aluminum layer with 5000 Ǻ thick was coated on the wafer by Thermal Coater.
11. Mask #4: Defined aluminum contact. Aluminum contact pad were defined by wet etching (HNO3:CH3COOH:H3PO4:H2O=2:9:50:10).
The structure was shown in Figure 2-7.
12. Al sintering at 400℃ in N2 ambient for 30 minutes.
2.2 The process of measurement
HP4156A is an electronic instrument for measuring and analyzing characteristics of semiconductor devices. ID-VD characteristics of our experiment were measure by HP4156A.
2.2.1 The measurement of Bio-sensor
1. The measure conditions were set VD= -10V to 10V in 0.2 step and VG= -15V, 0V, 15V respectively.
2. The SiGe nanowire was modified by 3-aminopropyltrimethoxysilane (APTMS) which could connect to the bio-linker BS3.
3. Drip bis(sulfosuccinimidyl) suberate sodium (BS3) solution as linker which could connect to the antibody IgG molecules.
4. The antibody IgG molecules were applied to link after BS3.
5. ID-VD curve was measured by HP4156A.
Chapter 3
Results & Discussion
To obtain the dimension of SiGe nanowire, the cross-section view of SiGe nanowire was observed by Scanning Electron Microscopy (SEM).
In addition, we utilized HP4156A to measure the ID-VD characteristics in our experiment when different molecular was adhered. All result will be discussed in this chapter.
3.1 Dimension of SiGe nanowires
The dimension of SiGe nanowire was controlled by the deposition for the width and the step of oxide for the height. In order to etch the SiGe film clearly, we added the 20% over-etching at the step of dry etching. The Scanning Electron Microscopy (SEM) was utilized to observe the cross-section view of SiGe nanowire. The JOEL JSM 6500-F-TFSEM was the equipment which we used to measure the diameters of the nanowires. Figure 3-1~3-5 show the cross-section view SEM image of different Ge concentration nanowires before oxidation process. The area and surface-to-volume ratio were summarized in the Table 3-1.
After the oxidation of 2 min at 900℃, the cross-section view SEM image of different Ge concentration nanowires were shown in Figure 3-6~3-10. The area and surface-to-volume ratio were also summarized in the Table 3-2. The increment of the surface-to-volume ratio of nanowires was observed after oxidation.
After the oxidation of 2min at 950℃, the cross-section view SEM image of Si0.93Ge0.07 and Si0.89Ge0.11 nanowires were shown in Figure 3-11 and 3-12. The area and surface-to-volume ratio were also summarized in the Table 3-3. After this oxidation process, the surface-to-volume ratio of nanowires also increased.
Due to the surface-to-volume ratio increase, the change of electric property will increase. This is a reason that enhancement of the sensitivity.
3.2 The sensitivity of SiGe nanowire sensor with various oxidation temperatures
In this section, we reported the electric property and the sensitivity of SiGe nanowire sensor with various oxidation temperatures. And, we discussed the result respectively.
3.2.1 Mechanism of detecting IgG antibody
First, we used amino-propyl-trimethoxy-silane (APTMS) to modify the surface of native oxide layer around the SiGe nanowires. Due to hydroxyl molecules of the surface of native oxide layer were replaced by the methoxy side of the APTMS molecules, the surface potential of SiGe nanowire will increase. Therefore, the conductance of SiGe nanowires will increase. Next, we used bis-sulfo-succinimidyl suberate (BS3) as linker. When BS3 molecules bond to APTM molecules, the BS3 molecules was easier to release the sodium ion and break the single bond between the carbon atom and the oxygen atom. The reasons caused the
negative charge absorbing on the surface. Hence, the conductance of SiGe nanowires will decrease. Finally, the IgG antibody will bond to BS3 molecules. The conductance of SiGe nanowires will increase. The schema of mechanism was shown in Figure 3-13.
3.2.2 The sensitivity of SiGe nanowires without oxidation
In order to obtain the base data, we measured unoxidized-SiGe nanowires with different concentration. When different chemical molecules were dropped on the surface of SiGe nanowires, the ID-VD curve was recorded by HP4156A, as shown in Figures 3-14~3-17. In order to compare the sensitivity SiGe nanowire with/without oxidation, the value ∆S/S is considered. ∆S is the variation of conductance and S is the normal conductance of SiGe nanowire. The value shows the percentage change of conductance in the same change of surface potential after the APTMS and BS3 modified. We utilized equation 3.1 to obtain the average conductance of SiGe nanowires between VD= 4V to VD= 8V.
Conductance =
σ is the conductivity (S/m), W is the area of the nanowire, L is the length of the nanowire. We fixed the voltage ∆V at 0.2V to be constant.
Figure 3-18 shows the result after APTMS, BS3 and IgG modified. The normal symbol is the conductance in the beginning and the APTMS symbol shows the conductance after APTMS modified. It is observed that higher conductance obtain after APTMS modified, the amounts of the APTMS molecules binding on the oxide surface performed like a constant voltage applied on the Si0.93Ge0.07 nanowire. The conductance of
BS3 became lower after BS3 linked. Finally, the antibody immunoglobulin IgG (protein) molecules were applied to link after the BS3 molecules, which also provided a positive gate voltage of the N-type Si0.93Ge0.07 nanowire. Figures 3-19~3-21 shows the conductance of the Si0.89Ge0.11, Si0.8Ge0.2, Si0.7Ge0.3 and Si0.6Ge0.4 respectively. The same phenomenon was also observed in other the SiGe nanowire with different Ge concentration. Finally, we utilized equation 3.2 to calculate change in conductivity after dropping different chemical molecules.
Percentage of change of conductivity (%) =
i Si is the conductance of SiGe nanowire before dipping chemical molecules, Sf is the conductance of SiGe nanowire after dipping chemical molecules. The value shows the percentage change of conductance in the same change of surface potential after the APTMS and BS3 modified.
Figure 3-22 shows the percentage change of SiGe nanowire after APTMS modified. It is easily found that higher percentage change in conductance for the SiGe nanowire with higher Ge concentration. The higher variation appears in the same APTMS modified. The Ge enhances the sensitivity within the concentration from 7% to 30%. Figure 3-23 and 3-24 show the percentage change after BS3 and IgG modified. The same phenomenon that Ge enhances the sensitivity is also observed. The result is the same as our previous study for pH sensor.
3.2.3 The sensitivity of SiGe nanowires after the oxidation of 2 min at 900℃
To improve the sensitivity of SiGe nanowires with lower Ge
concentration, we utilized Ge condensation technology to enhance the Ge concentration in SiGe nanowire. The oxidation for the condition of 2 min.
900 ℃ was employed before the ion implantation, the ID-VD curve of SiGe nanowires were shown in figures 3-25~3-27 for the SiGe with different concentration. Similarly, we used formula 3.1 to calculate the average conductance of SiGe nanowires between VD= 4V to VD= 8V, as shown in Figures 3-28~3-30. Finally, we utilized formula 3.2 to calculate sensitivity of nanowires with different Ge concentration, as shown in Figures 3-31~3-32.
It is observed that the sensitivity of SiGe nanowires was enhanced by oxidation process. There are two reasons lead to the result. One is enhancement of surface-to-volume ratio after oxidation. The other one is Ge atom is rejected from the oxide and condensed in the remaining SiGe nanowire. In addition, Si0.6Ge0.4 nanowire was over oxidized because oxidation rate of SiGe layer increases with Ge concentration [48].
3.2.4 The sensitivity of SiGe nanowires after the oxidation of 2 min at 950℃
We expected the sensitivity of SiGe nanowires were further enhanced by oxidation. Therefore, we change temperature of oxidation process (T=950℃). Unfortunately, some of nanowires were failure (Si0.8Ge0.2, Si0.7Ge0.3 and Si0.6Ge0.4), the reason may be nanowires were over oxidized.
After the oxidation of 2min at 950℃, the ID-VD curve of SiGe nanowires were shown in Figures 3-33~3-34. The average conductance of
SiGe nanowires between VD= 4V to VD= 8V, was shown in Figure 3-35~3-36. Finally, the sensitivity of SiGe nanowires were shown in Figures 3-37~3-38.
We observed that the sensitivity of SiGe nanowire was enhanced by oxidation condition. After the oxidation of 2min at 950℃, the sensitivity of Si0.93Ge0.07 nanowire was enhancement as compared with the oxidation of 2min at 900℃. The result was shown in Figures 3-39~3-41. But, the sensitivity of Si0.89Ge0.11 nanowire was reduction as compared with the oxidation of 2min at 900℃. The result was shown in Figures 3-42~3-43.
The reason may be higher defect was formed during this oxidation process [46]. In addition, we also oxidized SiGe nanowires by higher temperature (T=1000℃). But, oxidation rate was too fast so that nanowires were over oxidized.
Chapter 4 Conclusions
We have successfully fabrication the SiGe nanowire with different Ge concentration respectively. In addition, we used the SiGe nanowires as bio-sensor. The 3-amino-propyl-trimethoxy-silane (APTMS) was used to modify the surface, which could connect the bio-linker. The conductance of SiGe nanowire increases owing to APTMS with positive charge. The bis (sulfosuccinimidyl) suberate sodium (BS3) as the bio-linker connected to APTMS and the conductance decreased because of negative charge. Finally, the protein immunoglobulin G (IgG) is linked to BS3, and the conductance reduces for negative charge. In order to compare the sensitivity with/without oxidation, the ∆S/S is considered. ∆S is the variation of conductance and S is the normal conductance of SiGe nanowire. We have demonstrated that the sensitivity was improved by using higher Ge concentration (7% ~ 30%) nanowire.
After the oxidation of 2 min at 900℃, the sensitivity of SiGe nanowires were enhanced. There are two reasons lead to the result. One is enhancement of surface-to-volume ratio after oxidation. The other one is Ge atom is rejected from the oxide and condensed in the remaining SiGe nanowire. In addition, we observed that oxidation rate of SiGe nanowire increase with Ge concentration. Therefore, Si0.6Ge0.4 nanowire was oxidized over.
After the oxidation of 2 min at 950℃, the sensitivity of Si0.93Ge0.07 nanowires were enhanced. The percentage change of the conductance is
9.6% for the normal state, 13.9% for the 900 ℃ oxidation and 34.76%
for 950 ℃ oxidation after APTMS modified. It is clearly observed that the sensitivity is improved by oxidation. After the oxidation of 2min at 950℃, the sensitivity (22.27%) of Si0.89Ge0.11 nanowire was reduction as compared with the oxidation (36.36%) of 2min at 900℃. The reason may be that higher defect was formed during this oxidation process. In addition, we also oxidized SiGe nanowires by higher temperature (T=1000℃). But, oxidation rate was too fast so that nanowires were over oxidized.
Chapter 5 Future Work
Due to oxygen concentration will affect whether GeO2 formed during oxidation process. We will change nitrogen/oxygen ratio to avoid GeO2 formed. In addition, we can change time of oxidation to control Ge concentration in SiGe nanowires. By changing nitrogen/oxygen ratio and time of oxidation, we will obtain optimum condition of oxidation.
Material of nanowire sensor Application
Silicon pH sensor,bio-sensor,DNA sensor and virus sensor
Metal oxide gas sensor
Polymer gas sensor
Metal gas sensor
Table 1-1
Figure 1-1. Conversion of a NW FET into NW nanosensor for pH sensing [1].
Figure 1-2. Plot of the conductance Versus pH [1].
Figure 1-3. A biotion-modified SiNW and subsequent binding of streptavidin to the SiNW surface [1].
Figure 1-4. Plot of conductance versus time for a biotin-modified SiNW,where region 1 correspond to buffer solution,region 2 corresponds to the addition of 250nM streptavidin [1].
Figure 1-5. Conductance versus time for a biotin-modified SiNW,where region 1 corresponds to buffer solution, region 2 corresponds to the addition of ~3μM m-antibiotin,and region 3 corresponds to pure buffer solution [1].
Figure 1-6. Modification scheme of the SiNW surface for the DNA detector: (1) self-assembly of 3-mercaptopropyltrimethoxysilane (MPTMS) by gas-phase reaction in Ar for 4 h; (2) covalent immobilization of DNA probes by exposing the previous surface to 5 íM solution of oligonucleotide CCT AAT AAC AAT modified with acrylic phosphoramidite at the 5¢-end for 12 h; (3) DNA detection based on hybridization between label-free complementary DNA target GGA TTA TTG TTA and the immobilized DNA probes on the SiNW surfaces. The inset is the SPV signal on a p-type Si surface at different stages of the modification; A, B, and C correspond to the schematic diagrams, D is with 25 pM solution of complementary DNA target exposed to the surface C, and E is with 25 pM solution of noncomplementary DNA (GGA TCA TTG TTA) exposed to the surface C [10].
Figure 1-7. Conductance of the same p-type SiNW, where the arrow indicates the addition of 25 pM complementary DNA (GGA TTA TTG TTA) solution [10].
Figure 1-8. Conductance (Upper) and optical (Lower) data recorded simultaneously vs. time for a single silicon nanowire device after introduction of influenza A solution. Combined bright-field and fluorescence images correspond to time points 1–6 indicated in the conductance data; virus appears as a red dot in the images [11].
Figure 1-9. Response of the SnO2 nanobelts to CO at a working temperature of 400 °C and 30% RH [13].
Figure 1-10. Measured time-dependent current through an individual CPNW sensor upon exposure to NH3 gas. The nanowire device being tested was about 335 nm in diameter [16].
Figure 1-11. Sensor resistance responses for hydrogen concentration varied in a range from 0.2 to 1% by pulses [18].
Figure 1-12. Schema of Scanning Probe Lithography (SPL).
Figure 1-13. Schema of imprint process [28].
Figure 1-14. Schematic view of iterative spacer lithography (ISL) [29].
Figure 1-15. Drain current of N- and P-MOSFETs are improved with the use of SiGe-channel [33].
Figure 1-16. The N-type sensitivity is improved with the increase concentration of Ge. [percentage % = (pH11-pH5)/pH5]
Figure 1-17. Scanning TEM image and Ge profile across the layers obtained by EDS measurement [41].
Figure 1-18. Mobility enhancement factor for the SGOI-MOSFETs as a function of the Ge fraction [47].
Si substrate 5000Å SiO2
Figure 2-1. SiO2 layer is grown on Si substrate. The thickness of SiO2 layer is 5000Å.
Figure 2-2. Defined active area. The height of oxide step is 3000Å.
Figure 2-3. Amorphous Si layer is deposited on SiO2 layer. The thickness ofα-Si layer is 150Å.
Figure 2-4. SiGe layer is deposited on α-Si layer.
Figure 2-5. Defined S/D region and nanowire.
SiGe nanowire
Figure 2-6. Remove one side of the parallel SiGe spacer.
SiGe nanowire
Al contact
Figure 2-7. Defined Al contact pad.
Figure 3-1. The Cross-Section view of the SEM of Si0.93Ge0.07 nanowire.
The height and width of Si0.93Ge0.07 nanowire are 192nm and 77nm respectively.
Figure 3-2. The Cross-Section view of the SEM of Si0.89Ge0.11 nanowire.
The height and width of Si0.89Ge0.11 nanowire are 184nm and 45.5nm respectively.
Figure 3-3. The Cross-Section view of the SEM of Si0.8Ge0.2 nanowire.
The height and width of Si0.8Ge0.2 nanowire are 159nm and 65.9nm respectively.
Figure 3-4. The Cross-Section view of the SEM of Si0.7Ge0.3 nanowire.
The height and width of Si0.7Ge0.3 nanowire are 153nm and 54.5nm respectively.
Figure 3-5. The Cross-Section view of the SEM of Si0.6Ge0.4 nanowire.
The height and width of Si0.6Ge0.4 nanowire are 153nm and 54.5nm respectively.
Figure 3-6. The Cross-Section view of the SEM of Si0.93Ge0.07 nanowire after the oxidation of 2 min at 900℃. The height and width of nanowire are 168nm and 55.9nm respectively.
Figure 3-7. The Cross-Section view of the SEM of Si0.89Ge0.11 nanowire after the oxidation of 2 min at 900℃. The height and width of nanowire are 166nm and 42.8nm respectively.
Figure 3-8. The Cross-Section view of the SEM of Si0.8Ge0.2 nanowire after the oxidation of 2 min at 900℃. The height and width of nanowire are 137nm and 47.1nm respectively.
Figure 3-9. The Cross-Section view of the SEM of Si0.7Ge0.3 nanowire after the oxidation of 2 min at 900℃. The height and width of nanowire are 121nm and 43.5nm respectively.
Figure 3-10. The Cross-Section view of the SEM of Si0.6Ge0.4 nanowire after the oxidation of 2 min at 900℃. The nanowire was oxidized over.
Figure 3-11. The Cross-Section view of the SEM of Si0.93Ge0.07 nanowire after the oxidation of 2 min at 950℃. The height and width of nanowire are 155nm and 42.9nm respectively.
Figure 3-12. The Cross-Section view of the SEM of Si0.89Ge0.11 nanowire after the oxidation of 2 min at 950℃. The height and width of nanowire are 118nm and 38nm respectively.
Unoxidation
Dry Oxidation of 2min at 900℃
height width area surface/volume
Dry Oxidation of 2min at 950℃
height width area surface/volume
Figure 3-13. Schema of mechanism of detecting IgG antibody.
Figure 3-14. The ID-VD curve of N-type Si0.93Ge0.07 nanowire. The length of nanowire is 30μm.
APTMS BS3
Figure 3-15. The ID-VD curve of N-type Si0.89Ge0.11 nanowire. The length of nanowire is 17μm.
Figure 3-16. The ID-VD curve of N-type Si0.8Ge0.2 nanowire. The length of nanowire is 13μm.
Figure 3-17. The ID-VD curve of N-type Si0.7Ge0.3 nanowire. The length of nanowire is 19μm.
Figure 3-18. The conductance of N-type Si0.93Ge0.07 nanowire changes with different chemical molecules. The length of nanowire is 30μm.
Figure 3-19. The conductance of N-type Si0.89Ge0.11 nanowire changes
Figure 3-19. The conductance of N-type Si0.89Ge0.11 nanowire changes