**4.1. Plasma damage characterization**

To characterize the plasma damage on the low-*k* dielectrics, several methodologies can be used to detect the physical and chemical changes of low-*k* dielectrics after irradiation of plasma. The plasma induces a dense, hydrophilic, SiO2 -like layer at the top surface of the low-*k* dielectric. The thickness of this layer can be measured using spectral reflectivity or ellipsometry with bilayer model, scanning electron microscope (SEM), or transition emitting microscopy (TEM). **Figure 2** displays TEM image of the porous low-*k* dielectrics after O2 plasma treatment. A distinct layer is formed at the top surface of the film.

X-ray reflectivity (XRR) is another method to determine the density, thickness, and roughness of both pristine and damaged low-*k* layers through software data fitting [42]. **Figure 3** shows the XRR density profile of the low-*k* film after He plasma. The result demonstrates that He plasma creates a thin densification layer in the top part of the low-*k* film. The thickness of this densification layer is close to 17 nm. The density of the bulk layer in the pristine material density is constant and remained unchanged. However, the top of the densification layer has a higher density [43].

"HF decoration" method [44] can be used to detect the modification layer induced by plasma. This method is based on the fact that a pristine low-*k* dielectric is usually not dissolved or

A given system of solid and liquid (or vapor) at a given temperature and pressure has a unique equilibrium contact angle. The measured angle is water contact angle (WCA). It can be used to quantify the wettability of a solid surface by a liquid via the Young equation. If the used liquid molecules are strongly attracted to the solid molecules, the liquid drop then will completely spread out on the solid surface, corresponding to a WCA of 0°. This case can be occurred at bare metallic or ceramic surfaces for water liquid. As an oxide layer or contaminant is on the solid surface, WCA value significantly increases. Generally, the solid surface tends to be hydrophilic if WCA value is smaller than 90°, while if WCA value is larger than 90°, the solid surface is considered to be hydrophobic. For low-*k* dielectrics, WCA measurement is a power method to determine the films' hydrophobicity. If the used low-*k* dielectrics are hydrophilic, they tend to absorb moisture in the air, increasing the dielectric constant. Moreover, as the plasma is treated on low-*k* dielectrics, Si–OH/H–OH bonds can be formed because the plasma-generated dangle bonds absorb moisture. **Figure 4** compares the WCA values and images of the pristine and plasma-treated SiCOH low-*k* dielectrics. The WCA value of the as-deposited SiCOH low-*k* dielectrics is larger than 85° due to the pres-

groups. After plasma irradiation, the loss of Si–CH3

the formation of Si–OH/H–OH bonds result in a decreasing WCA value, making the low-*k*

Fourier transform infrared (FT-IR) spectroscopy is a common technique to characterize the structure of SiCOH low-*k* dielectrics [45, 46]. **Figure 5** compares the FTIR spectrum of the pristine and plasma-treated low-*k* dielectrics. Absorption bands located at ∼950–1250 and

at 3200–3500 cm−1 depends on hydrophobic properties of the film. For the pristine SiCOH low*k* dielectrics, no peak at 3200–3500 cm−1 is detected, representing that no moisture is present

2850–3100 cm−1 are detected. The appearance of the absorbance of the Si–OH and H2

groups and

299

O groups

groups, respectively, which are the main

plasma treatment.

stretching located at 2200–2250 and

Plasma Damage on Low-*k* Dielectric Materials http://dx.doi.org/10.5772/intechopen.79494

plasma irradiation, the intensities

ence of hydrophobic Si–CH3

dielectric to be more hydrophilic.

∼1273 cm−1 correspond to Si–O–Si and Si–CH3

representative. Additionally, Si–H bending and C–Hx

in the film, which is consistent with WCA result. After NH<sup>3</sup>

**Figure 4.** WCA values and images of porous low-*k* dielectrics after O2

**Figure 2.** TEM image of porous low-*k* dielectric after O2 plasma treatment.

**Figure 3.** XRR spectrum of low-*k* dielectrics after He plasma treatment [43].

slowly dissolved in the diluted HF solution. In contrast, a plasma-induced damaged layer is attacked by HF very quickly. Therefore, following the HF decoration, the thickness loss is equal to the thickness of the damaged layer.

A given system of solid and liquid (or vapor) at a given temperature and pressure has a unique equilibrium contact angle. The measured angle is water contact angle (WCA). It can be used to quantify the wettability of a solid surface by a liquid via the Young equation. If the used liquid molecules are strongly attracted to the solid molecules, the liquid drop then will completely spread out on the solid surface, corresponding to a WCA of 0°. This case can be occurred at bare metallic or ceramic surfaces for water liquid. As an oxide layer or contaminant is on the solid surface, WCA value significantly increases. Generally, the solid surface tends to be hydrophilic if WCA value is smaller than 90°, while if WCA value is larger than 90°, the solid surface is considered to be hydrophobic. For low-*k* dielectrics, WCA measurement is a power method to determine the films' hydrophobicity. If the used low-*k* dielectrics are hydrophilic, they tend to absorb moisture in the air, increasing the dielectric constant. Moreover, as the plasma is treated on low-*k* dielectrics, Si–OH/H–OH bonds can be formed because the plasma-generated dangle bonds absorb moisture. **Figure 4** compares the WCA values and images of the pristine and plasma-treated SiCOH low-*k* dielectrics. The WCA value of the as-deposited SiCOH low-*k* dielectrics is larger than 85° due to the presence of hydrophobic Si–CH3 groups. After plasma irradiation, the loss of Si–CH3 groups and the formation of Si–OH/H–OH bonds result in a decreasing WCA value, making the low-*k* dielectric to be more hydrophilic.

Fourier transform infrared (FT-IR) spectroscopy is a common technique to characterize the structure of SiCOH low-*k* dielectrics [45, 46]. **Figure 5** compares the FTIR spectrum of the pristine and plasma-treated low-*k* dielectrics. Absorption bands located at ∼950–1250 and ∼1273 cm−1 correspond to Si–O–Si and Si–CH3 groups, respectively, which are the main representative. Additionally, Si–H bending and C–Hx stretching located at 2200–2250 and 2850–3100 cm−1 are detected. The appearance of the absorbance of the Si–OH and H2 O groups at 3200–3500 cm−1 depends on hydrophobic properties of the film. For the pristine SiCOH low*k* dielectrics, no peak at 3200–3500 cm−1 is detected, representing that no moisture is present in the film, which is consistent with WCA result. After NH<sup>3</sup> plasma irradiation, the intensities

**Figure 4.** WCA values and images of porous low-*k* dielectrics after O2 plasma treatment.

slowly dissolved in the diluted HF solution. In contrast, a plasma-induced damaged layer is attacked by HF very quickly. Therefore, following the HF decoration, the thickness loss is

plasma treatment.

equal to the thickness of the damaged layer.

**Figure 3.** XRR spectrum of low-*k* dielectrics after He plasma treatment [43].

**Figure 2.** TEM image of porous low-*k* dielectric after O2

298 Plasma Science and Technology - Basic Fundamentals and Modern Applications

The dielectric constant (*k*), the leakage current, the breakdown voltage (or field), and the breakdown time of low-*k* dielectrics are measured using metal-insulator-semiconductor (MIS) capacitor structures, which can be fabricated by evaporation of aluminum through a metal shadow mask to form Al dots on the film. Before measurements, the samples are required to remove the physically absorbed water by annealing at 100–150°C. The *k* value of the low-*k* dielectric is determined from the measured capacitance by capacitance-voltage (*C-V*) measurements at a frequency of 10 kHz. The film thickness and the dot area must be precisely measured in order to obtain the reliable *k* value. The leakage current and the breakdown voltage (or field) are determined by current-voltage (*I-V*) measurements. The breakdown field is calculated by the mea-

as a function of field strength until the breakdown field is reached. It is usually reported at a low field of 1–2 MV/cm. The breakdown time is measured by using time-dependent dielectric breakdown (TDDB) tests. In a TDDB test, a constant voltage (field) is applied to the MIS capacitor structure with a low-*k* dielectric, and the leakage current is monitored with stress time. The dielectric breakdown time is recorded as the stress time at a sudden rise of the leakage current density. The applied voltage (field) must be lower than the measured breakdown voltage (field). In a real Cu/low-*k* interconnects, comb/serpentine (also called meander fork) or comb/ comb (fork/fork) patterns are typically used to measure the interline capacitance, the leakage

In this section, the results of plasma damage on the low-*k* dielectrics from our group's investigation are reported in terms of the effects on the electrical characterization and reliability. The

plasma-treated low-*k* dielectrics.

) is recorded

301

Plasma Damage on Low-*k* Dielectric Materials http://dx.doi.org/10.5772/intechopen.79494

sured breakdown voltage divided by film thickness. The leakage current *J* (A/cm2

current, the breakdown field, and the dielectric breakdown time.

**4.2. Plasma damage on the electrical characterization and reliability**

experimental detail deposition can be found elsewhere [39, 48–50].

**Figure 6.** Carbon concentration of XPS depth-profiling for pristine and O<sup>2</sup>

**Figure 5.** FT-IR absorption spectra of low-*k* dielectrics before and after plasma treatment in the range of 4400–400 cm−1 [47].

of the Si–CH3 and Si–H absorbances are decreased, while the absorbances of the Si–OH/H2 O groups are increased. However, the low-*k* dielectric is pretreated with He plasma, and it can suppress the formation of the Si–OH/H2 O groups [47].

The Si–O–Si bridging in the 900–1250 cm−1 can be deconvoluted into three peaks centered at 1129, 1063, and 1023 cm−1, corresponding to the Si–O–Si cage-like structure with a bond angle of approximately 150°, Si–O–Si network with a bond angle of 140°, and Si–O–Si suboxide structure with a bond angle of less than 140°, respectively. Other contributions from C–O–C and Si–O–Si asymmetric stretching will also be overlapped with the Si–O–Si asymmetric stretching in the broadband at 1000–1200 cm−1 [45]. The intensity of Si–O–Si bonds slightly increases, and this peak shifts to a higher wavelength after O2 plasma treatment.

X-ray photoelectron spectroscopy (XPS) is a surface-sensitive spectroscopic technique to quantitatively measure a material's elemental composition. XPS can also be operated in a "depth-profiling mode" to analyze the elemental composition throughout the film by using ion etching/sputtering technology. For SiCOH low-*k* dielectrics, C, O, and Si elements can be detected, while H element cannot be detected.

In the pristine SiCOH low-*k* dielectrics, a homogeneous chemical composition was expected, but the ratio of these elements depends on the used materials. For the plasma-treated sample, the top surface exhibits a high initial oxygen concentration coupled to a very low carbon concentration. A gradual increase in carbon content and a concomitant decrease in oxygen concentration were observed with the film depth, as shown in **Figure 6**. As the atomic concentrations are back to a level same with the pristine low-*k* dielectric, the depth is corresponding to the plasma-damaged layer. Moreover, this plasma-damaged layer is not a homogeneous layer.

The dielectric constant (*k*), the leakage current, the breakdown voltage (or field), and the breakdown time of low-*k* dielectrics are measured using metal-insulator-semiconductor (MIS) capacitor structures, which can be fabricated by evaporation of aluminum through a metal shadow mask to form Al dots on the film. Before measurements, the samples are required to remove the physically absorbed water by annealing at 100–150°C. The *k* value of the low-*k* dielectric is determined from the measured capacitance by capacitance-voltage (*C-V*) measurements at a frequency of 10 kHz. The film thickness and the dot area must be precisely measured in order to obtain the reliable *k* value. The leakage current and the breakdown voltage (or field) are determined by current-voltage (*I-V*) measurements. The breakdown field is calculated by the measured breakdown voltage divided by film thickness. The leakage current *J* (A/cm2 ) is recorded as a function of field strength until the breakdown field is reached. It is usually reported at a low field of 1–2 MV/cm. The breakdown time is measured by using time-dependent dielectric breakdown (TDDB) tests. In a TDDB test, a constant voltage (field) is applied to the MIS capacitor structure with a low-*k* dielectric, and the leakage current is monitored with stress time. The dielectric breakdown time is recorded as the stress time at a sudden rise of the leakage current density. The applied voltage (field) must be lower than the measured breakdown voltage (field). In a real Cu/low-*k* interconnects, comb/serpentine (also called meander fork) or comb/ comb (fork/fork) patterns are typically used to measure the interline capacitance, the leakage current, the breakdown field, and the dielectric breakdown time.

### **4.2. Plasma damage on the electrical characterization and reliability**

of the Si–CH3

homogeneous layer.

suppress the formation of the Si–OH/H2

increases, and this peak shifts to a higher wavelength after O2

300 Plasma Science and Technology - Basic Fundamentals and Modern Applications

detected, while H element cannot be detected.

and Si–H absorbances are decreased, while the absorbances of the Si–OH/H2

plasma treatment.

groups are increased. However, the low-*k* dielectric is pretreated with He plasma, and it can

**Figure 5.** FT-IR absorption spectra of low-*k* dielectrics before and after plasma treatment in the range of 4400–400 cm−1 [47].

O groups [47]. The Si–O–Si bridging in the 900–1250 cm−1 can be deconvoluted into three peaks centered at 1129, 1063, and 1023 cm−1, corresponding to the Si–O–Si cage-like structure with a bond angle of approximately 150°, Si–O–Si network with a bond angle of 140°, and Si–O–Si suboxide structure with a bond angle of less than 140°, respectively. Other contributions from C–O–C and Si–O–Si asymmetric stretching will also be overlapped with the Si–O–Si asymmetric stretching in the broadband at 1000–1200 cm−1 [45]. The intensity of Si–O–Si bonds slightly

X-ray photoelectron spectroscopy (XPS) is a surface-sensitive spectroscopic technique to quantitatively measure a material's elemental composition. XPS can also be operated in a "depth-profiling mode" to analyze the elemental composition throughout the film by using ion etching/sputtering technology. For SiCOH low-*k* dielectrics, C, O, and Si elements can be

In the pristine SiCOH low-*k* dielectrics, a homogeneous chemical composition was expected, but the ratio of these elements depends on the used materials. For the plasma-treated sample, the top surface exhibits a high initial oxygen concentration coupled to a very low carbon concentration. A gradual increase in carbon content and a concomitant decrease in oxygen concentration were observed with the film depth, as shown in **Figure 6**. As the atomic concentrations are back to a level same with the pristine low-*k* dielectric, the depth is corresponding to the plasma-damaged layer. Moreover, this plasma-damaged layer is not a

O

In this section, the results of plasma damage on the low-*k* dielectrics from our group's investigation are reported in terms of the effects on the electrical characterization and reliability. The experimental detail deposition can be found elsewhere [39, 48–50].

**Figure 6.** Carbon concentration of XPS depth-profiling for pristine and O<sup>2</sup> plasma-treated low-*k* dielectrics.

#### *4.2.1. O2 plasma damage*
