**4. Electrical and reliability characteristics of porous low-***k* **dielectric materials**

As porous low-*k* dielectric materials are used in the BEOL interconnects, the change in the *k* value during the integration must be minimal. Additionally, the electrical properties and reliability are the most important concerns. As a result, the leakage current of the porous low-*k* dielectric between metal lines should be maintained low. The time-dependent dielectric breakdown (TDDB) failure time of the integrated BEOL structure at operating conditions should meet the specifications.

### **4.1 Conduction mechanisms in porous low-***k* **dielectrics**

In a crystalline solid, as the electrons overcome the bandgap (or called energy gap), the resulting current is detected. The bandgap is defined as the difference between the energy of the lowest conduction band and that of the highest valence band. For thermally deposited SiO2 dielectric film, the bandgap is around 8.9 eV [47]. As carbon is doped into SiO2 dielectric film to form SiOCH low-*k* dielectric

*Porous Low-Dielectric-Constant Material for Semiconductor Microelectronics DOI: http://dx.doi.org/10.5772/intechopen.81577*

material, the bandgap was determined to be between 8.0 and 10.0 eV, depending on the low-*k* dielectric types and the characterization techniques [48–50]. If the carbon content in the low-*k* dielectric film is not incorporated in the matrix network but primarily exists as terminal methyl groups, its bandgap is similar to that of SiO2 film. However, if the carbon content is present in the network bonds by forming Si-C-Si bridging structure, the bandgap value would drop dramatically. As porosity is introduced into the SiOCH low-*k* dielectric material, the bandgap of porous SiOCH low-*k* dielectrics (*k* = 2.0–3.3) is in the range between 7.5 and 10 eV [51]. The effect of porosity on the bandgap of porous SiOCH low-*k* dielectrics is not pronounced. More investigation about bandgap determination for porous low-*k* dielectric materials is required.

The conduction mechanisms of low-*k* dielectric materials are commonly described by Schottky emission (SE), Poole-Frenkel (PF) emission, and Fowler-Nordheim (FN) tunneling [52–54], as shown in the following Eqs. (3)–(5):

• Schottky emission (SE)

$$\mathbf{J}\_{\rm SE} = \mathbf{A}^\* \mathbf{T}^2 \exp\left[ -q \left( \frac{\phi\_{\rm SE} - \sqrt{qE/4\pi\varepsilon\_0 \varepsilon\_r}}{kT} \right) \right] \tag{3}$$

• Poole-Frenkel (PF) emission

$$\mathbf{J}\_{\rm SE} \sim \mathbf{E} \exp\left[ -q \left( \frac{\phi\_{\rm PF} - \sqrt{qE/4\pi\varepsilon\_0 \varepsilon\_r}}{kT} \right) \right] \tag{4}$$

• Fowler-Nordheim (FN) tunneling

$$\mathbf{J}\_{\rm FN} \sim \mathbf{E}^2 \exp\left[\frac{-8\pi\sqrt{2m^\*}(q\phi\_{\rm FN})^{3/2}}{3qhE}\right] \tag{5}$$

where *J* is current density, *A*\* is Richardson constant,*T* is temperature, *q* is the elementary charge, φ is barrier height, *E* is electric field, ε<sup>o</sup> is permittivity of free space, ε<sup>r</sup> is dielectric constant, *m*\* is effective electron mass, and h is Planck's constant.

SE and PF emissions are field-enhanced thermal excitation conduction models. The excited electrons enter the conduction band from the low-*k* interface and the trap states with coulomb potentials for SE and PF emissions, respectively. FN tunneling conduction is caused by electrons tunneling from the metal Fermi energy or trapping sites in the material itself into the low-*k* dielectric conduction band. SE and PF emission currents are associated with the field and temperature. The former exhibits a strong temperature dependency. However, FN tunneling current exhibits a strong field dependency and is independent of temperature. Generally, PF emission is more likely the dominant conduction mechanism in low-*k* dielectric materials, especially at low fields. At high field, the dominant conduction mechanism transfers to FN tunneling [55, 56].

In the integrated interconnects, the barrier height at both the low-*k*/metal and the low-*k*/Si interfaces is around 4 eV, and the barrier height at the etching-stop layer/metal interface is less than 2.0 eV [57]. Therefore, the interface-controlled SE emission occurs.

plasma-induced damage on the porous low-*k* dielectric material, low-plasmadamage resist-stripping process is required for the multilayer resist method.

*Multilayer resist dual-damascene process: (A) ARC and resist coating. (B) Via-1 lithography. (C) Via-1 RIE. (D) Multilayer resist coating and M-2 trench lithography. (E) LTO and OPL RIE. (F) M-2 trench RIE. (G)*

**4. Electrical and reliability characteristics of porous low-***k* **dielectric**

**4.1 Conduction mechanisms in porous low-***k* **dielectrics**

*OPL strip and M-1 capping layer RIE. (H) M-2/Via-1 Cu metallization.*

As porous low-*k* dielectric materials are used in the BEOL interconnects, the change in the *k* value during the integration must be minimal. Additionally, the electrical properties and reliability are the most important concerns. As a result, the leakage current of the porous low-*k* dielectric between metal lines should be maintained low. The time-dependent dielectric breakdown (TDDB) failure time of the integrated BEOL structure at operating conditions should meet the

In a crystalline solid, as the electrons overcome the bandgap (or called energy gap), the resulting current is detected. The bandgap is defined as the difference between the energy of the lowest conduction band and that of the highest valence band. For thermally deposited SiO2 dielectric film, the bandgap is around 8.9 eV [47]. As carbon is doped into SiO2 dielectric film to form SiOCH low-*k* dielectric

**materials**

**Figure 4.**

*Nanofluid Flow in Porous Media*

specifications.

**176**

#### **4.2 Reliability of porous low-***k* **dielectric materials**

The breakdown field and TDDB failure time are the main reliability items for a dielectric material [58, 59]. **Figure 5** plots the relatively breakdown field of various dielectric materials used as BEOL ILDs. Compared to other dielectric materials, the porous low-*k* dielectrics have relatively weak breakdown field, and the decreasing magnitude is amplified with increasing the porosity [60]. The pores in the porous low-*k* dielectrics are treated as defective cells, shortening the percolation path. Additionally, porous low-*k* dielectrics have weaker bonds, higher trap densities, or lower barrier heights at the metal–insulator interface.

TDDB testing is performed by applying an electric stress on a tested dielectric material for a period of time. The stressing field is lower than the breakdown field of the tested dielectric material. The leakage current is monitored with the stressing time. During the electric stress, electric damage occurs in a dielectric material, converting the resistance state of a dielectric material from high to low. This leads to the loss of the insulating properties for a dielectric material. As a conducting path between a dielectric is formed, the leakage sharply increases. Therefore, the dielectric breakdown occurs. This stressing time is defined as the breakdown time of a dielectric material.

TDDB is strongly related to the property of a tested dielectric film and the applied electric field. As a result, as the technology node advances to 45 nm or below technology nodes, TDBB is becoming a critical reliability issue. In addition to using porous low-*k* dielectrics with a lower breakdown field, the interconnect dimensions are reduced which increases the lateral electric field across the BEOL dielectric. However, in real Cu damascene interconnects, the integration performance strongly dominates TDDB results. The interface of Cu/capping layer, lineedge-roughness line-to-line overlay errors, and via-to-line misalignment are the dominated TDDB failure mechanisms [61–65].

TDDB lifetime model is required and critical for prediction. The commonly used TDDB lifetime models are summarized in **Table 3** [66–68]. Each TDDB lifetime model has its theoretical fundamentals, but cannot explain all observed TDDB phenomenon. Moreover, for the choice of TDDB lifetime model, it is necessary to consider that the breakdown mechanism under testing conditions is also the domi-

*Porous Low-Dielectric-Constant Material for Semiconductor Microelectronics*

In these used TDDB lifetime models, E, 1/E, and power-law models are fielddriven models, while E1/2 model is a current-driven model. Moreover, E model is the most conservative model because it gives the shortest dielectric lifetime in the lower-field conditions, and 1/E model is the optimistic model providing the longest predicted lifetime. The E1/2 mode is widely accepted TDDB lifetime model for

During the integration of porous low-*k* dielectrics into Cu interconnects, the fabricating processes can seriously degrade material properties, electrical characteristics, and reliability. Moreover, the porosity can act as a fast penetration media for reactive species or contamination during the integration, accelerating

The main key issues associated with porous low-*k* dielectrics are schematically shown in **Figure 6**. The key issues will be discussed and the improvement actions

Plasma is an aggressive medium which produces vacuum ultraviolet (VUV) and

ultraviolet (UV) photons, energetic ions, electrons, and highly reactive radicals [69]. Exposure to plasma causes physical damage and chemical modifications on porous low-*k* dielectric materials [70, 71]. Under plasma irradiation, Si-CH3 and Si-H groups in the porous SiCOH low-*k* dielectric material are extracted from the network and then converted into the Si-O or Si-OH groups, leading to densification

**5. Integration issues of porous low-***k* **dielectric materials**

nant mechanism under operating conditions.

*TDDB lifetime models for dielectric materials.*

*DOI: http://dx.doi.org/10.5772/intechopen.81577*

porous low-*k* dielectrics.

will be provided in this section.

**5.1 Plasma-induced damage**

degradations.

**179**

**Table 3.**

Typically, TDDB testing is done at high fields (voltages) to accelerate the test. To predict lifetime from high voltage/field conditions to operating conditions,

**Figure 5.** *Relative breakdown field of various dielectric films.*

*Porous Low-Dielectric-Constant Material for Semiconductor Microelectronics DOI: http://dx.doi.org/10.5772/intechopen.81577*


#### **Table 3.**

**4.2 Reliability of porous low-***k* **dielectric materials**

*Nanofluid Flow in Porous Media*

lower barrier heights at the metal–insulator interface.

dominated TDDB failure mechanisms [61–65].

dielectric material.

**Figure 5.**

**178**

*Relative breakdown field of various dielectric films.*

The breakdown field and TDDB failure time are the main reliability items for a dielectric material [58, 59]. **Figure 5** plots the relatively breakdown field of various dielectric materials used as BEOL ILDs. Compared to other dielectric materials, the porous low-*k* dielectrics have relatively weak breakdown field, and the decreasing magnitude is amplified with increasing the porosity [60]. The pores in the porous low-*k* dielectrics are treated as defective cells, shortening the percolation path. Additionally, porous low-*k* dielectrics have weaker bonds, higher trap densities, or

TDDB testing is performed by applying an electric stress on a tested dielectric material for a period of time. The stressing field is lower than the breakdown field of the tested dielectric material. The leakage current is monitored with the stressing time. During the electric stress, electric damage occurs in a dielectric material, converting the resistance state of a dielectric material from high to low. This leads to the loss of the insulating properties for a dielectric material. As a conducting path between a dielectric is formed, the leakage sharply increases. Therefore, the dielectric breakdown occurs. This stressing time is defined as the breakdown time of a

TDDB is strongly related to the property of a tested dielectric film and the applied electric field. As a result, as the technology node advances to 45 nm or below technology nodes, TDBB is becoming a critical reliability issue. In addition to using porous low-*k* dielectrics with a lower breakdown field, the interconnect dimensions are reduced which increases the lateral electric field across the BEOL dielectric. However, in real Cu damascene interconnects, the integration performance strongly dominates TDDB results. The interface of Cu/capping layer, lineedge-roughness line-to-line overlay errors, and via-to-line misalignment are the

Typically, TDDB testing is done at high fields (voltages) to accelerate the test. To predict lifetime from high voltage/field conditions to operating conditions,

*TDDB lifetime models for dielectric materials.*

TDDB lifetime model is required and critical for prediction. The commonly used TDDB lifetime models are summarized in **Table 3** [66–68]. Each TDDB lifetime model has its theoretical fundamentals, but cannot explain all observed TDDB phenomenon. Moreover, for the choice of TDDB lifetime model, it is necessary to consider that the breakdown mechanism under testing conditions is also the dominant mechanism under operating conditions.

In these used TDDB lifetime models, E, 1/E, and power-law models are fielddriven models, while E1/2 model is a current-driven model. Moreover, E model is the most conservative model because it gives the shortest dielectric lifetime in the lower-field conditions, and 1/E model is the optimistic model providing the longest predicted lifetime. The E1/2 mode is widely accepted TDDB lifetime model for porous low-*k* dielectrics.
