Thesis Organization

This thesis consists of four chapters. Following the introduction in chapter 1, the physical property and thermal stability of the low-k α-SiCO:H dielectric films are investigated and presented in chapter 2. Chapter 3 deals with the electrical characteristics and barrier property of the low-k α-SiCO:H dielectric films. Finally, chapter 4 gives the conclusion of this thesis study.

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AlCu

1X

10X Lifetime

Cu

Num b er of S tandard Devi ations

10 100 1000 Time to Failure (Arbitrary Units)

Fig. 1-1 Gate and interconnection delays as a function of devices feature size [1].

Cumulative Failur es (%)

Fig. 1-2 Electromigration performance improvement using Cu metallization [3].

Diffusion Barrier

Cu Cu Cu

ILD

ILD

ILD Cu

Etching Stop Layer

Fig. 1-3 Schema of Cu dual damascene structure.

Chapter 2

Physical Property and Thermal Stability of α-SiCO:H Films

2.1 Introduction

To enhance the circuit performance in high-speed 90/65 nm CMOS device operations, it is urgent to replace the high-k SiN barrier in Cu dual damascene interconnections with a relatively low-k dielectric barrier film. In recent years, many studies have been reported on low-k amorphous silicon oxycarbide (α-SiCO:H) films deposited by plasma enhanced chemical vapor deposition (PECVD) [9-11]. This chapter investigates the physical property and thermal stability of three PECVD α-SiCO:H dielectric films with different composition.

2.2 Experimental Procedures

The α-SiCO:H films were deposited at temperatures of 200-500^oC using organosilicate and oxidation gases with various flow ratios, resulting in three α-SiCO:H films with different compositions designated as SiCO-A, SiCO-B and

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SiCO-C. The α-SiCO:H dielectric films were all deposited on p-type, (100)-oriented 12-inch silicon wafers to a thickness of 560 Å in a standard PECVD system at Taiwan Semiconductor Manufacturing Company (TSMC). All films deposited were thermally annealed at 400^oC for 30 min in an N₂ ambient to remove moisture possibly absorbed in the dielectrics. After this thermal pretreatment, the film thickness and refractive index were measured by a well-calibrated n&k analyzer at 633 nm wavelength. Subsequently, the films were further annealed in furnace at temperatures from 300 to 600^oC for 30 min in an N₂ ambient, followed by measurements of film thickness and refractive index. In order to determine the dielectric constant of the α-SiCO:H dielectric films, Al/SiCO/Si MIS capacitors were prepared for capacitance-voltage (C-V) measurements. The MIS capacitors were constructed by depositing a 5000-Å-thick Al layer directly on the α-SiCO:H dielectric surfaces using a thermal evaporation system. The Al electrodes with a circular area of 0.84 mm diameter were defined by chemical wet etching. To ensure good contact in electrical measurement, a 5000-Å-thick Al layer was also thermally evaporated on the back surface of the Si substrate for all samples. The completed MIS samples were sintered at 400^oC for 30 min in an N2 ambient.

Figure 2-1 shows the process flow of the MIS samples preparation.

X-ray photoelectron spectroscopy (XPS) using Al Kα (1486.6eV) radiation was used to detect the chemical composition of the α-SiCO:H dielectric films.

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Fourier transform infrared spectroscopy (FTIR) was used to investigate the chemical bonds of the dielectric films. Thermal desorption mass spectroscopy (TDS) was used to monitor the outgassing behavior of the dielectrics at elevated temperatures. The dielectric constant of the α-SiCO:H films was determined from the maximum accumulation capacitance of the Al-electrode MIS capacitors measured at 1 MHz using a Keithley 82 C-V measurement system. The film density was calculated by the ratio of mass to volume of the film, whereas the film mass was measured by electronic balance and the film volume was calculated from the film thickness and the area of the substrate wafer.

2.3 Results and Discussion

Table 2-1 shows the basic physical properties of the α-SiCO:H dielectrics studied in this thesis. The dielectric constant of the α-SiCO:H dielectric films decreases with increasing oxygen concentration in the films. The refractive index, and the film’s density as well, also shows the same changing tendency.

Meanwhile, the carbon content appears to have little effect on the k-value and refractive index of the dielectric films. Figure 2-2 shows the FTIR absorption spectra for the three α-SiCO:H films studied in this work, while Table 2-2 summarizes in detail all the chemical bonding observed and reported in the literature [12-17]. The Si-O-Si bonds of long chain polymers have two strong

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bonds at 1041 and 1128 cm^-1, which represent network and cage-like structures, respectively. Figure 2-3 illustrates the network and cage-like Si-O-Si structure of SiCO dielectric films [18-19]. Films that consist of more Si-O-Si cage-like structure become looser and can therefore have lower dielectric constants as well as refractive indices and films’ densities. However, films that have too much Si-O-Si cage-like structure may become too loose to resist Cu diffusion.

Figure 2-4 illustrates the TDS of H2O (m/e=18) for the three α-SiCO:H dielectric films, all of which exhibited moisture outgassing. The TDS of CH₄ (m/e=16), N₂H⁺ (m/e=29), C₂H₄ (m/e=30), O₂ (m/e=32), and CO₂ (m/e=44) are illustrated in Fig. 2-5. It was found that only N2H⁺ (m/e=29) could be detected for SiCO-B and SiCO-C films at temperatures above 500^oC, presumably due to the H⁺ outgassing [20]. Figure 2-6 shows the percentage of thickness change versus annealing temperature for the three α-SiCO:H dielectric films studied in this work.

Each datum was obtained by measurements on a sample at 5 different locations.

The as-prepared sample stands for the samples which had been thermally annealed at 400^oC (for 30 min in N₂ ambient) immediately after the film deposition. All of the three dielectric films are thermally stable up to 500^oC.

Figure 2-7 shows the refractive index versus annealing temperature for the α-SiCO:H dielectric films. We found that the refractive index remained virtually unchanged after thermal annealing at temperatures up to 600^oC for all three

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α-SiCO:H dielectric films studied in this thesis. The dielectric constant versus annealing temperature is illustrated in Fig. 2-8 for the three α-SiCO:H dielectric films. It appears that the dielectric constant (i.e. k-value) exhibits a slightly decreasing tendency at temperatures above 500^oC. It is well known that the dielectric constant of a dielectric material is an intrinsic material property; it is frequency dependent and is composed of three components: electronic, ionic and dipolar polarization [21]. The dielectric constant can be quantified by Equations (2-1) and (2-2):

ε

_r

(@ 1 MHz) =1 + ∆ ε

_e

+∆ ε

_i

+∆ ε

d

(2-1)

ε

_r

(λ) =n

²

(λ) (2-2)

where ε_r is the relative dielectric constant; ∆ε_e, ∆ε_i and ∆ε_d are the contribution from the electronic, ionic and dipolar polarization, respectively, and n² (λ) is the real part of the refractive index at the wavelength λ. If λ of the light source is in the visible to UV range, e.g. λ=633 nm, only electrons can response to the time varying field and the dielectric constant in this range is only due to the electronic polarization.

Therefore we can calculate the electronic polarization contribution as follows:

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1 + ∆ε

e

= n

²

(@ 633 nm) (2-3)

Moreover, the ionic and dipolar polarization contribution can be obtained by the following:

∆ε

i

+∆ε

d

= ε

r

(@ 1 MHz) - n

²

(@ 633 nm) (2-4)

Table 2-3 lists the dielectric constants measured at 1 MHz (1 + ∆εe +∆εi +∆εd), the share (1 + ∆ε_e) measured by n² (@ 633 nm), and the extracted ionic and dipolar polarization contribution (∆εi +∆εd) at various temperatures. The electronic polarization contribution (∆εe) remains unchanged at temperatures up to 600^oC since the refractive index measured is not dependent on the annealing temperature.

However, the ionic and dipolar polarization contribution decreases with annealing temperature at temperatures above 500^oC. Since the TDS reveals moisture outgassing for all samples to various degrees (Fig. 2-4) while no obvious outgassing of CH₄ was detected (Fig. 2-5), the moisture would be reduced while the hydrocarbon remained unchanged in the α-SiCO:H dielectric films thermally annealed at temperatures up to 600^oC. Since H₂O is more polarizable than CH₄, the ionic and dipolar polarization contribution would decrease with increasing annealing temperature, leading to the decrease of dielectric constant when the

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temperature was raised above 500^oC. It can be seen from Fig. 2-5 and Table 2-3 that the H⁺ outgassing for the SiCO-B and SiCO-C dielectric films at temperatures above 500^oC could barely affect the ionic and dipolar polarization contribution.

2.4 Summary

It was found that the α-SiCO:H dielectrics with higher oxygen concentration contain more cage-like structure, which is structurally looser than the network structure. Thus, the α-SiCO:H dielectric films with higher oxygen concentration have a lower density as well as lower dielectric constant and refractive index. All of the three α-SiCO:H dielectric films studied in this thesis are thermally stable at temperatures up to 500^oC. Nonetheless, the dielectric constant of the α-SiCO:H dielectric films decreases slightly at temperatures above 500^oC; this is attributed to the decrease of the ionic and dipolar polarization at the elevated temperatures due to the outgassing of H₂O.

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Table 2-1

Physical Properties of α-SiCO:H Dielectric Films

Sample ID SiCO-A SiCO-B SiCO-C

Elemental Composition (%)

Si 34.296 34.632 31.369

C 40.778 38.744 41.908

O 24.926 26.624 26.723

Chemical Composition SiC_1.19O_0.73 SiC_1.12O_0.77 SiC_1.34O_0.85

Density (g/cm³) 1.84 1.65 1.56

Dielectric Constant @ 1 MHz 4.00 3.14 2.76 Refractive Index @ 633 nm 1.743 1.575 1.502

Table 2-2

Absorption bonds observed for α-SiCO:H [9-14].

Wavenumber (cm^-1) Vibration mode

780 Si-C stretching

800 Si-C bending

1041 Si-O-Si network

bending/stretching

1128 Si-O-Si cage-like

bending/stretching

1245 Si-CH3 bending

2140 Si-H streching

3000 C-H stretching

Table 2-3

Contribution part of dielectric constant.

Dielectric

* The sample “As” stands for “as-prepared sample” which had been thermally annealed (i.e. thermally pretreated) at 400^oC (for 30 min in N2 ambient) immediately after the film deposition.

Low-k dielectric film deposition (~560Å)

Thermal pretreatment (400^OC for 30 min)

Thickness and refractive index measurements

Furnace annealing (300~600^OC for 30 min)

Thickness and refractive index measurements

Al electrode deposition (5000Å)

Backside Al deposition (5000Å)

Sintering (400^OC for 30 min)

Fig. 2-1 Process flow of Al/SiCO/Si MIS capacitor preparation.

500 1000 1500 2000 2500 3000 3500 4000

Absorbance

C-H Si-CH₃ Si-H

Si-O-Si cage-like Si-O-Si network Si-C

SiCO-C SiCO-B SiCO-A

Wavenumber (cm

^-1

)

Fig. 2-2 FTIR absorption spectra of the α-SiCO:H dielectrics studied in this work.

Fig. 2-3 The illustration of network and cage-like Si-O-Si structure of SiCO dielectric films [15].

0 100 200 300 400 500 600 0

2 4 6 8 10 12 14 16 18

Relative Inte nsity

Temperature(

^o

C)

SiCO-A SiCO-B SiCO-C

Fig. 2-4 TDS of H₂O (m/e=18) for the dielectric films of SiCO-A, SiCO-B and SiCO-C.

0 100 200 300 400 500 600 (m/e=32), and CO₂ (m/e=44) for the dielectric films of (a) SiCO-A, (b) SiCO-B, and (c) SiCO-C.

0 100 200 300 400 500 600

Annealing Temperature (

^o

C)

Thickness Change (%)

Fig. 2-6 Percentage of thickness change vs. annealing temperature for the dielectric films of (a) SiCO-A, (b) SiCO-B, and (c) SiCO-C.

0 100 200 300 400 500 600 0.9

1.2 1.5 1.8 2.1 2.4

As-prepared

Refractive Index

Annealing Temperature (

^o

C)

SiCO-A SiCO-B SiCO-C

Fig. 2-7 Refractive index vs. annealing temperature for the three α-SiCO:H dielectric films studied in this work.

0 100 200 300 400 500 600 2.5

3.0 3.5 4.0 4.5 5.0

As-prepared SiCO-A SiCO-B SiCO-C

Dielectric Constant

Annealing Temperature (

^o

C)

Fig. 2-8 Dielectric constant vs. annealing temperature for the three α-SiCO:H dielectric films studied in this work.

Chapter 3

Electrical Characteristics of α-SiCO:H Films

3.1 Introduction

In the deep submicron ULSI era, Cu damascene interconnection has been widely used. In the Cu damascene interconnect structure, SiN barrier layers (with a k-value about 7) are commonly used as Cu-cap barrier and etching stop layer. To further reduce the effective dielectric constant of the interconnect system, amorphous silicon oxycarbide (α-SiCO:H) dielectric films, which have a relatively lower k-value than the traditional barrier SiN (k~7), may be used to replace SiN in the Cu damascene process. Thus, it is essential to ensure the thermal stability, dielectric leakage property, and the barrier capability against Cu diffusion of these promising candidates. This chapter investigates the electrical characteristics and barrier property of three PECVD α-SiCO:H dielectric films. Since there are many heating processes in the back-end-of-line (BEOL) fabrication process, bias-temperature-stress (BTS) was used to examine the dielectrics barrier capability because BTS simulates the BEOL fabrication process and circuit operating conditions under an electric field at elevated temperatures simultaneously [22-25].

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3.2 Experimental Procedures

To study the barrier property and thermal stability of the α-SiCO:H dielectric films, both TaN/Cu/SiCO/Si and Al/SiCO/Si MIS capacitors with p-type Si substrate were prepared for current-voltage (I-V) and bias-temperature-stress (BTS) measurements. All α-SiCO:H dielectric films deposited were annealed at 400^oC for 30 min in an N₂ ambient to remove moisture possibly absorbed in the dielectrics prior to the deposition of electrodes (TaN/Cu or Al). For the construction of TaN/Cu-gated MIS capacitors, a 2000Å-thick Cu layer was sputter-deposited on the α-SiCO:H dielectric films in a dc sputtering system in an Ar ambient at a pressure of 7.6 mTorr with a sputtering power of 200W, followed by reactive sputter deposition of a 500Å-thick TaN layer on the Cu surface in the same sputtering system without breaking the vacuum using a Ta target in an Ar/N₂ mixed ambient, also at a pressure of 7.6 mTorr with a sputtering power of 200W. The flow rates of Ar and N₂ were 24 and 6 sccm, respectively, for making the Ar/N₂ mixed ambient. The TaN film served as a passivation layer to prevent Cu layer from oxidation in the subsequent high-temperature processes.

Prior to each sputter deposition, the target was cleaned by pre-sputtering with the shutter closed for 10 min. For comparison, Al-gated MIS control samples were also prepared by depositing a 5000Å-thick Al layer

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directly on the α-SiCO:H dielectric surfaces using a thermal evaporation system. The Al electrodes with a circular area of 0.84 mm diameter were defined using the conventional photolithography and chemical wet etching, whereas the TaN/Cu electrodes with the same circular area were defined by lift-off technique since there is no proper solvent for TaN wet etching. To ensure good contact in electrical measurement, a 5000Å-thick Al layer was thermally evaporated on the back surface of the Si substrate for all samples. Figures 3-1 and 3-2 show the process flow for the preparation of the TaN/Cu- and Al-gated MIS samples, respectively. Some of the completed MIS samples were thermally annealed at 400^oC for 30 min in an N2 ambient. This annealing step eradicates the plasma-induced damage during the sputter deposition of the TaN/Cu electrodes and also provides the driving force for Cu diffusion. An HP4145 semiconductor parameters analyzer was used to measure the dielectric leakage current and provide the bias for the BTS test.

3.3 Results and Discussion

Figure 3-3 shows the leakage current density at room temperature for the as-fabricated as well as 400^oC-annealed Al-gated and TaN/Cu-gated MIS capacitors of various α-SiCO:H dielectric films.

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The measurements were carried out with the MIS capacitors biased in the accumulation region (negative bias on electrodes of MIS capacitors with p-type Si substrate). For all the dielectric films, very little difference in leakage current was observed between the Al-gated and TaN/Cu-gated MIS capacitors, whether as-fabricated or 400^oC-annealed. This implies that all the α-SiCO:H dielectric films were capable of retarding Cu diffusion at temperatures up to 400^oC without an applied electric field.

Figure 3-4 is replotted from Fig.3-3, only for the part of electric field below 0.7 MV/cm, for the leakage current density versus electric field for the 400^oC-annealed Al-gated and TaN/Cu-gated MIS capacitors. The conduction mechanism of the leakage current can be investigated by analyzing the leakage current versus applied electric field relationship, or leakage current density (J) versus electric field (E) characteristic. In the highlighted portion in Fig. 3-4 (marked by closed dash line) the leakage current conduction is most probably due to Schottky emission (SE) or Frenkel-Poole (F-P) emission [26-27]. In the SE process, thermionic emission across the metal-insulator interface or the insulator-semiconductor interface is responsible for the carrier transport, whereas the F-P emission is due to the field-enhanced thermal excitation of trapped electrons in the insulator into the conduction band. The current density (J) in the SE and F-P emission can be quantified byEq.

(3-1) and (3-2), respectively, as follows [26-27].

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* 2

exp

^S 1/ 2 ^S absolute temperature, E is the applied electric field, kB is the Boltzmann constant, ΦS is the contact potential barrier, and the quantity βS represents (e³/4πε_oε_r)^1/2, where e is the electronic charge, ε_o is the permittivity of free space, and ε_r is the high frequency relative dielectric constant. In Eq.

(3-2) for F-P emission, σo is the conductivity, Φ_PF is the height of trap potential well, and the quantity β_FP represents (e³/πε_oε_r)^1/2. The quantity β_FP in F-P is twice as large as β_S in SE, since the barrier lowering of F-P is twice as large as that of SE due to the immobility of the positive charge [26]. By comparing the theoretical value of β with the experimental one obtained by slope fitting for various conduction mechanisms, we may be able to determine the conduction mechanism in the dielectrics studied.

Figure 3-5 shows that ln(J/E) is linearly correlated with the square root of the applied electric field (E^1/2) at low electric field before electric breakdown of the dielectrics for the 400^oC-annealed MIS capacitors of all

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the three α-SiCO:H dielectric films, indicating the dominant role of Frenkle-Poole emission in current conduction at electric fields between 0.20 to 0.36 MV/cm for SiCO-A, 0.20 to 0.30 MV/cm for SiCO-B, and 0.14 to 0.20 MV/cm for SiCO-C dielectric. At very low electric fields, however, the ohmic conduction prevails. In the ohmic conduction, current is carried by thermally excited electrons hopping from one isolated state to the next [26]. The current density (J) in the ohmic conduction can be quantified byEq. (3-3):

exp( E

^ae

) J = E −V kT

where ∆E_ae is the activation energy of electrons.

The ln(J/E) versus E^1/2 for the F-P emission can be easily derived from Eq. (3-2), as shown in Eq. (3-4):

The experimental values of β_FP determined from the slope of the linear segment of ln(J/E) versus E^1/2 are all close to the theoretical values of β_FP for all the three dielectrics, as shown in Fig. 3-5. Similar results were

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obtained for the 400^oC-annealed TaN/Cu-gated samples of TaN/Cu/SiCO/Si capacitors.

In Fig. 3-3, the leakage currents reveal saturation at electric fields above 0.5 MV/cm, indicating the presence of electric breakdown [28-29].

During the stressing of the dielectric films by high electric fields, randomly distributed traps or defects may be generated in the bulk of the dielectric films. When a large amount of traps or defects are generated and are located close enough to form a conduction path between the cathode and anode, the electric breakdown would occur. The conduction path usually disappears in a few microseconds due to local heating of the dielectric film and rearrangement of the defects.

Hundreds of electric breakdown can occur prior to permanent dielectric breakdown.

BTS test was used to further explore the thermal stability of the TaN/Cu-gated MIS capacitors. For comparison, BTS was also performed on the Al-gated MIS capacitors. Prior to the BTS test, all the MIS capacitors were thermally annealed at 400^oC for 30 min in an N₂ ambient to repair the plasma-induced damage arising from sputter deposition of the metal electrode and also to provide the driving force for Cu diffusion. The BTS was performed in N₂ ambient to prevent the Cu-electrode from oxidizing at elevated temperatures as well as moisture uptake into the dielectric films. Figure 3-6 shows the leakage current

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density versus stress time for the 400^oC-annealed Al/SiCO-A/Si and TaN/Cu/SiCO-A/Si MIS capacitors under BTS at 200^oC with an applied electric field of 1 MV/cm. It can be seen that both MIS samples remained stable under the BTS up to at least 15h. The fluctuation of the leakage current during the bias stressing resulted from numerous electric breakdowns, which occurred in rapid succession and induced an increase in low-level leakage current that can be attributed to either the formation of conduction paths within the dielectric film or residual damage left after the electric breakdown [28-29]. It is notable that the dielectric film was still functional under the electric breakdown. Figure 3-7 illustrates the leakage current density versus electric field for the Al/SiCO-A/Si and TaN/Cu/SiCO-A/Si MIS capacitors measured at 200^oC before and immediately after a 15h BTS. There is no obvious change in leakage current after the BTS. Figure 3-8 shows the leakage current density versus stress time for the 400^oC-annealed Al/SiCO-B/Si and TaN/Cu/SiCO-B/Si MIS capacitors under BTS at 200^oC with an applied electric field of 1 MV/cm. The TaN/Cu-gated SiCO-B sample failed within a few minutes of BTS, while the Al-gated sample remained stable under the BTS up to at least 15h. The breakdown of the TaN/Cu-gated SiCO-B sample is presumably due to the penetration of Cu into the SiCO-B dielectric film rather than the wear out of the SiCO-B film from the BTS. Figure 3-9 illustrates the leakage current density versus

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electric field for the Al-gated and TaN/Cu-gated MIS capacitors of

electric field for the Al-gated and TaN/Cu-gated MIS capacitors of

在文檔中 低介電常數碳氧化矽阻障層與銅金屬整合之電性可靠度研究 (頁 16-0)