Organization

Chapter 1 includes the introduction of germanium characteristics and the encountered problems in metal contact and n⁺/p-Ge junction. In addition, the contact resistance and dopant segregation method are also introduced in the chapter. Chapter 2 describes the fabrication process of samples. Material and electrical analysis are introduced as well.

Chapter 3 focused on the IBG junction. The conventional junctions of different implantation dosage are first discussed, then forming NiGe, which named IBG junction, is done to compare the electrical characteristics with conventional junctions.

Furthermore, studying IBG junction on different concentration substrates and the effect of Ni diffusion is also included. Finally, the contact resistance of the IBG+IAG junctions are also showed, the results are proved by material analysis as well.

Finally, Chapter 4 summarizes the experiment results and makes some conclusions. The future works are suggested as well.

15

Table 1- 1 Interface trap density (Dit) and effective oxide thickness (EOT) recorded by previous studies in Ge capacitor.

Gate stack Treatment Dit

(cm^-2 eV^-1)

Ge/GeON/HfTiON PDA in NH3 ambient at 500^oC, 300s

2.3x10¹¹ 0.96 [74]

Ge/GeON/HfTiON PDA in wet NO

at 500^oC, 300s

1.2x10¹¹ 1.02 [74]

Ge/GeON/TaON/ HfTiON PDA in wet NO at 500^oC, 40s

5.4x10¹¹ 0.91 [75]

Ge/GeON/TaON/ HfTiON PDA in NH₃ ambient at 500^oC, 40s

6.8x10¹¹ 0.86 [75]

Ge/AlN/HfO2 N.A. 1 x10¹² 0.82 [76]

16

Table 1- 2 Metal/n-Ge contact resistance recorded by previous studies.

Contact Doping Activation Test Structure Contact Resistivity

(Ω-cm²)

Junction Depth

(nm)

Source

NiGe P+Sb IBG 500^oC/RTA CTLM 5.5x10^-7 200 [27]

NiGe As IBG 900^oC/LSA CTLM 1.4x10^-6 N.A. [25]

NiGe As IBG 800^oC/LSA CTLM 5.0x10^-5 30 [25]

NiGe P IBG 500^oC/FA TLM 8.8x10^-5 N.A. [68]

NiGex P IBG 500^oC/FA TLM 3.5x10^-6 N.A. [68]

Al P-epi None CTLM 4.6x10^-5 N.A. [25]

Ti As I/I 600^oC/RTA CTLM 1.0x10^-3 N.A. [25]

Ti P I/I 650^oC/N.A. CTLM 4.9x10^-5 N.A. [66]

TaN P I/I 650^oC/N.A. CTLM 2.7x10^-5 300 [26]

17

Fig.1-1 Components of the resistance associated with the source/drain junctions of a MOS transistor [64].

18

Fig.1-2 Relative contribution from each component of the resistance to series resistance for different technology nodes [65].

19

Fig.1-3 Extraction of the front resistance (Rf) from the Rtotal-Ld plot.

-2 0 2 4 6 8

0 400 800 1200 1600 2000 2400

R

total

(  )

L

d

(  m) 2R

f

R

_total

=R

_s

x L

_d

/W

_d

+ 2R

_f

R

_s

=-2R

_f

x W

_d

/L

_d

20

Fig.1-4 Four-terminal Cross Bridge Kelvin Resistor structure with geometry parameters definition [77].

21

Chapter 2 Experiments

2.1 Devices Fabrications

In this section, the fabrication processes of devices used in this thesis are described in detail. Two kinds of Ge substrate were used. They are Ga-doped (100)-oriented Ge wafer with resistivity 0.01~0.05 ohm-cm and Ga-doped (100)-oriented Ge wafer with resistivity 1~10 ohm-cm. The former one is called heavily doped substrate and the later one is called the lightly doped substrate.

2.1.1 IBG + IAG Junction Formation

All the samples were dipped in diluted HF solution (HF:H₂O=1:50) for 2 minutes and rinsed by DI water for 5 minutes to clean the surface at first. Then, 300-nm-thick Tetraethyl orthosilicate (TEOS) SiO₂ was deposited by a plasma enhanced chemical vapor deposition (PECVD) system at 350^oC as field oxide. Active regions were patterned by typical lithography process and the field oxide was etched by buffered oxide etchant (BOE) for 90 seconds. The residual photo-resist was removed by acetone in ultrasonic cleaner for 1 minute.

The samples on the heavily-doped Ge substrate were implanted by Phosphorus at 20 keV to a dose of 1x10¹⁵ cm^-2 and at 50keV to a dose of 5x10¹³ cm^-2. Furthermore, the samples on the lightly-doped Ge substrate were implanted by Phosphorus at 50 keV to a dose of 1x10¹⁵, 5x10¹³, 1x10¹³, or 5x10¹² cm^-2 and Arsenic at 50 keV to a dose of 1x10¹⁵, 5x10¹³, or 1x10¹³ cm^-2. All samples were annealed at 600 ^oC for 60

22

seconds in N₂ ambient by a rapid thermal annealing (RTA) system to activate the dopants, and the N⁺/P-Ge junctions formed.

Before Ni deposition, some heavily-doped samples were implanted by Fluorine at 10keV to a dose of 1x10¹⁵ cm^-2 to study the effect of fluorine on the Ni diffusion.

Moreover, the samples were dipped in diluted HF solution (HF:H₂O=1:100) for 2 minutes and rinsed by DI water for 5 minutes to clean the surface. After that, Ni was deposited to 10-nm-thick to form germanide and TiN was deposited to 15-nm-thick to passivate Ni surface by a co-sputtering system. The Ni deposition was performed under the condition of Ar with 100 sccm and 300W at the rate about 0.5nm/sec, moreover, the TiN deposition was performed under the condition of Ar/N2 with 100/10 sccm and 800W at the rate about 0.68nm/sec. The Ni-deposited samples were annealed at 350^oC for 5 minutes by a backend vacuum annealing furnace to form NiGe. The process is called IBG process.

After forming NiGe, the unreacted metal was selectively etched by hot H3PO4 at 150^oC for 5 minutes and the NiGe formation is finished. Samples were implanted again by Arsenic at 10 keV to a dose of 1x10¹⁵ cm^-2 and annealed at 500/550/600^oC for 10 seconds in N₂ ambient by rapid thermal annealing (RTA) to segregate the dopants at the interface. The process is called IAG process.

Finally, a 300-nm-thick Al was deposited by a thermal coater to act as the contact metal. The Al layer was patterned by the typical lithography process and high density plasma - reactive ion etching (HDP-RIE) with Cl₂ and BCl₃ as the reacting gas, and the residual photo-resist was removed by acetone in ultrasonic cleaner for 1 minute. A 300-nm-thick Al was deposited again by a thermal coater at wafer backside to form backside contact.

First, the conventional junctions were fabricated by the IBG process flow but skipping the NiGe related process steps and the main process conditions are

23

summarized in Table 2-1. Secondly, the IBG junctions are only fabricated by IBG process and the main process conditions are summarized in Table 2-2. Next, the IAG junctions are only fabricated by IAG process and the main process conditions are summarized in Table 2-3. Finally, the IBG+IAG junctions are fabricated by IBG and IAG process and the main process conditions are summarized in Table 2-4. In addition, the process flow of IBG+IAG junctions is depicted in Fig. 2-1.

2.1.2 Contact Resistance of IBG+IAG junctions

Lightly doped Ge substrate was used in this experiment. The process flow is identical to the IBG+IAG junction. However, after activation annealing, 300-nm-thick TEOS SiO2 was deposited again; then, contact regions were patterned and the oxide was etched by BOE for 90 seconds. The main process conditions of the contact resistance of IBG+IAG junctions are summarized in Table 2-5. Also, the process flow is depicted in Fig. 2-2.

The contact resistance were extracted by six-terminal D-type CBKR structure shown in Fig.2-3. The contact area values of the CBKR structures used in this thesis are designed in 3x3 μm², 5x5 μm², and 10x10 μm². The widths of current arms and voltage arms are designed to be the same values as the contact widths. The process tolerances are considered with 3μm.

2.2 Material Analysis and Electrical Measurement

The Current-voltage (I-V) characteristics of devices were measured by a semiconductor analyzer of model Agilent 4156C. The contact resistance was also measured by the same semiconductor analyzer. Sheet resistance was measured by a

24

four point probe system. Furthermore, the samples were analyzed by different kinds of material analysis instrument in order to compare with the results of electrical measurement, for example, X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), and Secondary Ion Mass Spectrometer (SIMS).

XRD was used to analyze the crystallized orientation of NiGe. When X-ray goes through the crystallized structure, the constructive interference occurs. According the Bragg’s Law (nλ=2dsinθ), the constructive interference only occurs for certain θ correlating to a (h k l) plane. Therefore, the crystallized (h k l) plane of NiGe can be inferred. TEM was used to observe the microstructure of NiGe film. TEM depends on electrons beam transmitted through an ultra-thin specimen and the electrons beam interacts with the specimen as it passes through to form the image. The image is magnified and focused on the imaging device with high resolution. Therefore, the quality of NiGe film can be clearly observed. In addition, SEM is used to observe the surface of NiGe film. SEM produces images by means of scanning the samples with electrons beam and the electrons interact with atoms in the samples to detect the signal and show the samples’ surface topography. SIMS was used to analyze the distribution of elements in the samples, such as P, As, and Ni. SIMS analysis relies on sputtering the surface of the samples with a focused ion beam and analyzing the ejected secondary ions from the surface. The kind of elements could be determined by mass/charge ratios of these secondary ions. Moreover, the amount and depth distribution of the elements could also be detected at the same time.

In brief, all the material analysis plays the role of assistant to prove the measured electrical characteristics. As long as the results are consistent, the inference or conclusion would be more convinced.

25

Table 2-1 Main process recipe of conventional junctions.

Implantation Recipe Annealing Recipe Substrate P, 50keV, 5x10¹² cm^-2

Table 2-2 Main process recipe of IBG junctions.

Implantation Recipe Annealing Recipe NiGe formation Substrate P, 50keV, 5x10¹² cm^-2

26

Table 2-3 Main process recipe of IAG junctions.

NiGe Implantation Recipe Annealing Recipe Substrate 350^oC, 5min As, 10keV, 1x10¹⁵ cm^-2 500^oC, 10s

550^oC, 10s 600^oC, 10s

Heavily

Table 2-4 Main process recipe of IBG+IAG junctions.

Implantation

Table 2-5 Main process recipe of Contact Resistance of IBG+IAG junctions Implantation

27

1. HF clean

2. 300nm SiO2 deposition 3. Active area defined by

lithography process 4. SiO2 etching by BOE 5. Remove PR by ACE

6. P or As implantation

7. Dopant activation (600^oC,60s)

8. Ni(10nm)/TiN(15nm) deposition by Sputter system

9. NiGe formation

10. Unreacted-Ni and TiN were etched by hot H₃PO₄

28

Fig.2-1 Process flow of IBG+IAG junctions.

13. Al contact defined by lithography process 14. Al etching by HDPRIE 15. Al backside contact 12. Dopant activation 11. As implantation

IAG junctions without first implantation

29

1. HF clean

2. 300nm SiO2 deposition 3. Diffusion region defined by

lithography process 4. SiO2 etching by BOE 5. Remove PR by ACE

6. Implantation:

P, 20keV, 1x10¹⁵ cm^-2 7. Dopant activation

(600^oC,10s)

8. 300nm SiO₂ deposition

9. Active region lithography 10. SiO₂ etching by BOE 11. Remove PR by ACE

12. Ni(10nm)/TiN(15nm) deposition by Sputter system

13. NiGe formation (350^oC, 5min)

14. Unreacted-Ni and TiN were etched by hot H₃PO₄

30

Fig.2-2 Process flow of Contact Resistance of IBG+IAG junctions.

15. Implantation :

As, 10keV, 1x10¹⁵ cm^-2

16. Dopant activation

17. Al contact defined by lithography process 18. Al etching by HDPRIE 19. Al backside contact

31

Fig.2-3 Schematic layout of six-terminal Cross Bridge Kelvin Resistor (CBKR) structure.

32

Chapter 3

IBG and IAG junctions

3.1 NiGe formation

In order to find out the lowest thermal budget for NiGe formation, the sheet resistance of the NiGe film formed at different thermal budgets was measured and shown in Fig.3-1 and Fig.3-2. According to Fig.3-1, the sheet resistance of NiGe decreases from 73 Ω/□ after 300 ^oC annealing to 20 Ω/□ after 325 ^oC annealing for 5 minutes and still keeps low after 400^oC annealing. In addition, in Fig.3-2, the sheet resistance of NiGe decreases from 80 Ω/□ for 3 minutes annealing to 20 Ω/□ for 5 minutes annealing at 325 ^oC. According to the change of sheet resistance shown above, NiGe starts to growth at 325^oC but the growth rate is slow so that at least a 5 minutes annealing is necessary. In the thesis, NiGe formation was performed at 350 ^oC for 5 minutes to ensure that NiGe could completely form if not specified. According to the formula of the sheet resistance R_sh=V/I x CF, where CF is correction factor determined by the value of d/s, where d is the distance of the probe to the sample edge and the s is the distance between the probes, the sheet resistance of NiGe measured by the four point probe system is under the condition of infinitely large samples and the correction factor is 4.5324. However, the samples’ size used in the thesis is nearly 0.5x0.5 cm² and the value of d/s is about 3. Therefore, the correction factor is about 2.4575 based on the Table 3-1 and the thickness of NiGe is 20 nm (Fig.3-15); then, the resistivity of NiGe is calculated to be 22.04x10^-6 Ω-cm. In addition, XRD diffraction analysis on the NiGe/Ge structure formed by 350C annealing shows polycrystalline NiGe phases as shown in Fig. 3-3. The main NiGe crystal orientations

33

are (2 1 0), (1 1 2), and (1 1 1).

3.2 Conventional N

⁺

/P Junctions

At first, the I-V curves of the conventional junctions with phosphorus ion implantation at 20 keV to a dose of 1x10¹⁵ cm^-2 and 50 keV to a dose of 5x10¹³ cm^-2 on heavily-doped substrate are shown in Fig.3-4. After dopant activation at 600 ^oC for 60 seconds, the junctions with phosphorus ion implantation at 20 keV to a dose of 1x10¹⁵ cm^-2 has a relatively higher forward current and lower reverse current due to the higher doping concentration. The forward and reverse bias current ratio is about 5 orders of magnitude. In addition, the band-to-band tunneling can also be observed when the reverse bias exceeds 0.5 V. Because the high n⁺ concentration and heavily-doped substrate cause high electric field at the junction edge, the electrons would easily tunnel through the band to the other side as the reverse bias is applied.

Next, the I-V curves of conventional junctions with phosphorus ion implantation at 50 keV to different doses of 1x10¹⁵, 5x10¹³, 1x10¹³, 5x10¹² cm^-2 on lightly-doped substrate are shown in Fig. 3-5. After annealing at 600 ^oC for 60 seconds, the forward current increases with the increase of the ion implantation dose and the junction with the highest dose of 1x10¹⁵ cm^-2 has the highest forward current. The reason is that more dopants could be activated to form higher concentration of n⁺ region so that the electron diffusion current increases and the NiGe/Ge contact resistance could be reduced. Moreover, the reverse bias current slightly increases with the increase of dose because higher ion implantation dose may induce more defects to raise leakage current. Therefore, the forward/reverse current ratio also increases with the increase of dose and the current ratio of the junction with a dose of 1x10¹⁵ cm^-2 is about 10³.

34 depletion width is proportional to⁡(_𝑁¹

𝑎+_𝑁¹

𝑑)

1

2. In addition, the reverse bias current is directly proportional to the depletion width when the generation current within the depletion region dominates the reverse bias current. Now, N_a is the concentration of substrate and Nd is the concentration of the n⁺ region. If the concentration of n⁺ region is the same, the concentration of substrate would dominate the reverse bias current.

The lower concentration of substrate would cause the wider depletion width, and the wider depletion width would cause the higher reverse bias current. Comparing the I-V curves of the conventional junctions with phosphorus ion implantation at 50 keV to a dose of 5x10¹³ cm^-2 on lightly-doped and heavily-doped substrate, e.g. Fig.3-4 and Fig.3-5, respectively, it is the reason why the reverse bias current on lightly-doped substrate is larger than that on heavily-doped substrate.

Thirdly, the I-V curves of conventional junctions with arsenic ion implantation at 50 keV to a dose of 1x10¹⁵, 5x10¹³, 1x10¹³ cm^-2 on lightly-doped substrate are shown in Fig. 3-6. After dopants activation at 600 ^oC for 60 seconds, the junction with arsenic ion implantation to a dose of 1x10¹⁵ cm^-2 shows the highest forward current and the lowest reverse current; the forward/reverse current ratio is about 300.

However, the junctions with dose of 5x10¹³ and 1x10¹³ cm^-2 show poor junction characteristics. The reason is that the doping concentration is not high enough so that the arsenic doped layer is depleted.

Comparing the I-V curves of conventional junctions of implantation by phosphorus and arsenic at 50 keV to a dose of 5x10¹³ cm^-2 on lightly-doped substrate, the reverse bias current of the arsenic implanted junction is higher than that of the

35

phosphorus implanted junction because the activation ratio of arsenic is relatively lower than that of phosphorus so that the junction depth of the arsenic implanted junction is inferred to be shallower. Furthermore, the heavier arsenic atom would generate more defects than the lighter phosphorous atom. Therefore, the reverse bias current of the arsenic implanted junction is higher as shown in Fig. 3-5 and Fig. 3-6.

3.3 NiGe on Conventional N

⁺

/P Junctions (IBG junctions)

In this session, we examine the effect of NiGe formation on the preformed N⁺/P junctions, i.e the IBG junctions. Fig.3-7 shows the I-V curves of the IBG junctions with different NiGe formation temperatures. The preformed junction is implanted by phosphorus at 20 keV to a dose of 1x10¹⁵ cm^-2 on heavily-doped substrate. All junctions show high reverse bias current and the electrical characteristics are nearly ohmic behavior. The junction with NiGe formation at 300^oC for 5 minutes shows lower current and slightly rectifying characteristic due to incomplete NiGe formation.

From Fig.3-1, the sheet resistance is higher than 70 Ω/□ which implies that Ni2Ge formed but not NiGe formed. Other junctions have the same high forward current about 10² A/ cm² as a result of complete NiGe formation.

Compared with the conventional junctions, before the NiGe formation, the reverse bias current is about 10^-4 A/ cm²; however, the reverse bias current rises apparently after NiGe formation and the current increases with the increase of the NiGe formation temperature. It means NiGe formation causes something happened to destroy the preformed junction. According to the temperature dependence, a possible reason may be the fast diffusion of Ni. Fig. 3-8 shows the diffusivity of elements in Ge, Ni has the second high diffusivity among these elements [78]. If Ni atoms enter the depletion region of the junction, they act as generation-recombination center and

36

destroy the junction. Therefore, finding some methods to suppress Ni diffusion is important.

A method of fluorine implantation before silicide formation had been proposed to solve the problem of large leakage current caused by Ni diffusion [79]. Hence, fluorine implantation before NiGe formation was performed in this thesis in order to solve the Ni diffusion problem. Fig.3-9 shows the I-V curves of the fluorine implanted junctions with different NiGe formation temperatures. The preformed conventional junction is formed by phosphorus ion implantation at 20 keV to a dose of 1x10¹⁵ cm^-2 on heavily-doped substrate. It is observed that fluorine ion implantation reduces both forward current and reverse current. This result confirms that fluorine implantation could suppress the Ni diffusion but affect the NiGe formation at the same time. The reverse bias current of the junction with NiGe formation at 325 ^oC is reduced by nearly 3 orders of magnitude. But the reverse bias current still increases with the increase of the NiGe formation temperature.

The junctions with deeper junction depth were fabricated to examine whether the fast diffusion of Ni still damages the junction characteristic. The deep junction was formed by phosphorus ion implantation at 50 keV to a dose of 5x10¹³ cm^-2 on heavily-doped substrate. The I-V curves are shown in Fig. 3-10. The forward current is about 10² A/ cm² and is nearly the same with the forward current of the IBG junctions. Furthermore, the reverse bias current is nearly 10^-2 A/ cm² and is lower than that of the IBG junction with phosphorus ion implantation at 20 keV to a dose of 1x10¹⁵ cm^-2. It could be inferred that high ion implantation dose may cause more defects than low ion implantation dose does, and more defects may let Ni diffuse faster so that the reverse bias current increases. Fig.3-10 also shows the I-V curves of the IBG junctions with fluorine ion implantation. In particular, the reverse bias current of the fluorine implanted junction is higher than that of the junction without fluorine

37

implantation and also increases with the increase of the NiGe formation temperature.

It could be inferred that fluorine implantation causes additional defects than phosphorous implantation and these defects enhance Ni diffusion.

Next, the IBG junctions are also fabricated on lightly-doped substrate to observe whether the junction is still destroyed by Ni diffusion. Because different implantation doses might induce different amount of defects, the IBG junctions with different implantation doses were designed to observe the effect of Ni diffusion. Fig.3-11 shows the IBG junctions formed with phosphorus implantation at 20 keV to different

Next, the IBG junctions are also fabricated on lightly-doped substrate to observe whether the junction is still destroyed by Ni diffusion. Because different implantation doses might induce different amount of defects, the IBG junctions with different implantation doses were designed to observe the effect of Ni diffusion. Fig.3-11 shows the IBG junctions formed with phosphorus implantation at 20 keV to different

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