**Abstract**

To provide high speed, low dynamic power dissipation, and low cross-talk noise for microelectronic circuits, low-dielectric-constant (low-*k*) materials are required as the inter- and intra-level dielectric (ILD) insulator of the back-end-of-line interconnects. Porous low-*k* materials have low-polarizability chemical compositions and the introducing porosity in the film. Integration of porous low-*k* materials into microelectronic circuits, however, poses a number of challenges because the composition and porosity affected the resistance to damage during integration processing and reduced the mechanical strength, thereby degrading the properties and reliability. These issues arising from porous low-*k* materials are the subject of the present chapter.

**Keywords:** porous low-*k*, porosity, Cu interconnects, BEOL, integration, plasma damage, Cu drift, TDDB, reliability

### **1. Introduction**

To obtain a high operation performance and to pack more chips in microelectronics, the semiconductor industry spent a lot of efforts to accomplish successful integration of the integrated circuits (ICs). As the dimensions of the device are continuously shrinking with the advance of technology node, the carrier's transit time across the length of a transistor channel (called gate delay) decreases, while the signal propagation through the interconnects [called resistance-capacitance (RC) delay] increases, as shown in **Figure 1**. As a result, the effective speed of the device is limited by the RC delay since 0.25 μm technology node [2–4]. The RC delay can be reduced by using metals with low resistivity and dielectric materials with low dielectric constant (*k*). Therefore, copper (Cu) and low-dielectric-constant (low-*k*) materials have been introduced in back-end-of-line (BEOL) interconnects of ICs to replace the conventional Al/SiO2 interconnects [4–7]. Cu with a resistivity of 1.7 μΩ-cm (2.7 μΩ-cm for Al) is becoming the common metallization material. Low-*k* materials with *k* values lower than 4.0 (*k* value of SiO2) provide lower capacitance between wires. To effectively reduce the *k* value of a dielectric film, low-polar bonds and porosity are introduced into the film. The produced dielectric materials are called porous low-*k* materials [8–10]. To provide a further low-*k* value, more porosity is introduced into the low-*k* material; however, more integration challenges arise.

mechanical strength and relatively high coefficient of thermal expansion (CTE). As a result, the successful integration into the BEOL interconnects is still not achieved. The other type is hybrid silica-based low-*k* dielectric material, which is the mainstream inter-layer-dielectric (ILD) insulator used in BEOL interconnects. This type of low-*k* dielectric material can be produced by doping fluorine or/and carbon into the traditional SiO2 film. The formation of low-*k* dielectric materials are fluorinated silicon glass (FSG) [11, 12] or carbon-doped silicon glass [SiCOH or called organosilicate glass (OSG)] [11, 13]. Fluorine or carbon substitution lowers the *k*

The minimum *k* value of the hybrid silica-based low-*k* dielectric material is limited to be 2.6–2.7. To prevent a huge increase in the parasitic capacitance of BEOL interconnects in the 45 nm or below technology nodes, a new low-*k* dielectric material with *k* value less than 2.6 is required. The air has a minimum *k* value of �1.0 in the world; as a result, the introduction of air pores in the existing low-*k* dielectric film is the possible strategy to further reduce the *k* value. The produced low-*k* dielectrics are porous, which are called "porous low-*k* dielectrics" [14, 15]. The *k* value of porous low-*k* dielectrics depends on the porosity and dielectric

*<sup>k</sup>*<sup>1</sup> <sup>þ</sup> <sup>2</sup> <sup>þ</sup> ð Þ <sup>1</sup> � *<sup>V</sup> <sup>k</sup>*<sup>2</sup> � <sup>1</sup>

where *k*<sup>1</sup> is the dielectric constant of the material inside the pores and *V* is the average pore volume. The first term in the right side of Eq. (2) equals to zero if the air is inside the pore (*k*1�1.0). As a result, porous low-*k* dielectrics with relatively small *k*<sup>2</sup> value and higher porosity can provide much lower *k* value. Currently, porous low-*k* dielectrics have been successfully integrated into Cu interconnects since 45 nm technology node. The widely used method to produce the porous low-*k* dielectrics is co-deposition of a silica-like matrix together with a sacrificial organic polymer (porogen) using plasma-enhanced chemical vapor deposition (PECVD). Following, the sacrificial organic polymer in the deposited low-*k* dielectric material is removed by ultraviolet (UV)-assisted thermal curing at a temperature range of 300–450°C in order to form the pores in the film. The precise composition and porosity depend on the type of precursor molecules, the matrix/porogen ratio used

Porous low-*k* dielectric materials can be produced by either spin-on technology or chemical vapor deposition (CVD) method [14, 15, 17–20]. In the CVD method, the deposition rate of CVD method is strongly dependent of the deposition temperature. To obtain a suitable deposition rate, increasing the deposition temperature is required to deposit the porous low-*k* dielectric material. However, the temperature of BEOL interconnects is limited to be less than 450°C because of melting concern for metal conductors. With an assistant of plasma technology, the

deposition precursors are dissociated to form the active radicals under the electron collision in the cold plasma. The generated active radicals with high reactivity accelerate the deposition process, thus reducing the deposition temperature.

Spin-on technology has been used in semiconductor processing for photoresist coating. It can also use to deposit the low-*k* dielectric material. The used dispensing

*<sup>k</sup>*<sup>2</sup> <sup>þ</sup> <sup>2</sup> (2)

value by decreasing the polarizability and increasing the free volume.

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

constant of the film skeleton (*k*2) [16]:

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

*k* � 1

during deposition, and the curing conditions [17, 18].

**2.2 Deposition method for porous low-***k* **materials**

*2.2.1 Spin-on technology*

**171**

*<sup>k</sup>* <sup>þ</sup> <sup>2</sup> <sup>¼</sup> *<sup>V</sup> <sup>k</sup>*<sup>1</sup> � <sup>1</sup>

**Figure 1.** *Gate and interconnect delay with technological generation (International Technology Roadmap for Semiconductors [1]).*

This chapter is an attempt to provide an overview of porous low-*k* materials. The resulting issues and reliability during the integration of porous low-*k* material in Cu interconnects are discussed.

#### **2. Low-***k* **dielectric materials and deposition method**

#### **2.1 Low-***k* **dielectric materials**

The dielectric constant (*k*) of a dielectric material is generally described by Clausius-Mossotti Eq. (1):

$$\frac{k-1}{k+2} = \frac{4\pi N}{3}a \tag{1}$$

where *k* = ε/ε0, ε, and ε<sup>0</sup> are the permittivity of the material and vacuum, *N* is the number of molecules per unit volume (density), and *α* is the total polarizability, including electronic (αe), distortion (αd), and orientation (αo) polarizabilities. According to Eq. (1), decreasing the total polarizability (*α*) and/or density (*N*) is the feasible method to effectively reduce the *k* value of a dielectric material. Reducing the polarizability can be achieved by the use of low-polar bonds (like C-C, C-H, Si-F, Si-CH3, etc.). Based on the used type of the low-polar bond, the produced low*k* dielectric material can be divided into two types: One type is organic polymer that contains saturated and unsaturated and conjugated and aromatic hydrocarbons [11]. However, this type low-*k* dielectric material is thermally unstable and has poor

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

mechanical strength and relatively high coefficient of thermal expansion (CTE). As a result, the successful integration into the BEOL interconnects is still not achieved.

The other type is hybrid silica-based low-*k* dielectric material, which is the mainstream inter-layer-dielectric (ILD) insulator used in BEOL interconnects. This type of low-*k* dielectric material can be produced by doping fluorine or/and carbon into the traditional SiO2 film. The formation of low-*k* dielectric materials are fluorinated silicon glass (FSG) [11, 12] or carbon-doped silicon glass [SiCOH or called organosilicate glass (OSG)] [11, 13]. Fluorine or carbon substitution lowers the *k* value by decreasing the polarizability and increasing the free volume.

The minimum *k* value of the hybrid silica-based low-*k* dielectric material is limited to be 2.6–2.7. To prevent a huge increase in the parasitic capacitance of BEOL interconnects in the 45 nm or below technology nodes, a new low-*k* dielectric material with *k* value less than 2.6 is required. The air has a minimum *k* value of �1.0 in the world; as a result, the introduction of air pores in the existing low-*k* dielectric film is the possible strategy to further reduce the *k* value. The produced low-*k* dielectrics are porous, which are called "porous low-*k* dielectrics" [14, 15]. The *k* value of porous low-*k* dielectrics depends on the porosity and dielectric constant of the film skeleton (*k*2) [16]:

$$\frac{k-1}{k+2} = V \frac{k\_1 - 1}{k\_1 + 2} + (1 - V) \frac{k\_2 - 1}{k\_2 + 2} \tag{2}$$

where *k*<sup>1</sup> is the dielectric constant of the material inside the pores and *V* is the average pore volume. The first term in the right side of Eq. (2) equals to zero if the air is inside the pore (*k*1�1.0). As a result, porous low-*k* dielectrics with relatively small *k*<sup>2</sup> value and higher porosity can provide much lower *k* value. Currently, porous low-*k* dielectrics have been successfully integrated into Cu interconnects since 45 nm technology node. The widely used method to produce the porous low-*k* dielectrics is co-deposition of a silica-like matrix together with a sacrificial organic polymer (porogen) using plasma-enhanced chemical vapor deposition (PECVD). Following, the sacrificial organic polymer in the deposited low-*k* dielectric material is removed by ultraviolet (UV)-assisted thermal curing at a temperature range of 300–450°C in order to form the pores in the film. The precise composition and porosity depend on the type of precursor molecules, the matrix/porogen ratio used during deposition, and the curing conditions [17, 18].

#### **2.2 Deposition method for porous low-***k* **materials**

Porous low-*k* dielectric materials can be produced by either spin-on technology or chemical vapor deposition (CVD) method [14, 15, 17–20]. In the CVD method, the deposition rate of CVD method is strongly dependent of the deposition temperature. To obtain a suitable deposition rate, increasing the deposition temperature is required to deposit the porous low-*k* dielectric material. However, the temperature of BEOL interconnects is limited to be less than 450°C because of melting concern for metal conductors. With an assistant of plasma technology, the deposition precursors are dissociated to form the active radicals under the electron collision in the cold plasma. The generated active radicals with high reactivity accelerate the deposition process, thus reducing the deposition temperature.

#### *2.2.1 Spin-on technology*

Spin-on technology has been used in semiconductor processing for photoresist coating. It can also use to deposit the low-*k* dielectric material. The used dispensing

This chapter is an attempt to provide an overview of porous low-*k* materials. The resulting issues and reliability during the integration of porous low-*k* material in Cu

*Gate and interconnect delay with technological generation (International Technology Roadmap for*

The dielectric constant (*k*) of a dielectric material is generally described by

where *k* = ε/ε0, ε, and ε<sup>0</sup> are the permittivity of the material and vacuum, *N* is the number of molecules per unit volume (density), and *α* is the total polarizability, including electronic (αe), distortion (αd), and orientation (αo) polarizabilities. According to Eq. (1), decreasing the total polarizability (*α*) and/or density (*N*) is the feasible method to effectively reduce the *k* value of a dielectric material. Reducing the polarizability can be achieved by the use of low-polar bonds (like C-C, C-H, Si-F, Si-CH3, etc.). Based on the used type of the low-polar bond, the produced low*k* dielectric material can be divided into two types: One type is organic polymer that contains saturated and unsaturated and conjugated and aromatic hydrocarbons [11]. However, this type low-*k* dielectric material is thermally unstable and has poor

<sup>3</sup> *<sup>α</sup>* (1)

*k* � 1 *<sup>k</sup>* <sup>þ</sup> <sup>2</sup> <sup>¼</sup> <sup>4</sup>*π<sup>N</sup>*

**2. Low-***k* **dielectric materials and deposition method**

interconnects are discussed.

*Nanofluid Flow in Porous Media*

**Figure 1.**

**170**

*Semiconductors [1]).*

**2.1 Low-***k* **dielectric materials**

Clausius-Mossotti Eq. (1):

#### *Nanofluid Flow in Porous Media*

liquid contains the deposition precursors for low-*k* materials, which is dropping into the center of the substrate. The created centrifugal forces by rotating of the substrate help to distribute the material on the surface. After the spinning step, a heating (or bake) is required to remove solvent. The temperature is typically below 250°C. Finally, a curing at temperatures varying from 350 to 600°C is required to obtain a stable film.

material by breaking a fraction of mainly the Si-CH3 (Si-Me) bonds. The improvement effect is associated to the used wavelength, temperature, and time of the UV

is using a single precursor molecule consisting of skeleton with embedded (or grafted) porogen precursor. An example of such a porous SiCOH material is Applied Materials' Black Diamond 3 (BD3) dielectric film. The UV curing is also modified to create more uniform porosity and improve the mechanical

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

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

**2.3 Characterizations of porous low-***k* **dielectric materials**

Currently, a promising method to deposition of the porous low-*k* dielectric film

In order to successfully integrate the porous low-*k* dielectric material into Cu interconnects, their physical, chemical, mechanical, and electric properties are important consideration factors. **Table 1** lists the main characterization techniques for porous low-*k* dielectric materials. Detailed principles and operation procedures

**Table 2** lists the main properties of porous low-*k* dielectric materials and compares to other generations of ILD materials (including SiO2, FSG, and OSG) [36–38]. In addition to providing a lower *k* value, porous low-*k* dielectric materials possess the degrading material properties. The degradation is more pronounced with increasing porosity (for the reduction of *k* value) for porous low-*k* dielectric materials. Therefore, the use of porous low-*k* dielectric materials in the ICs is

curing [32, 33].

properties [8, 33].

can be found elsewhere [34, 35].

becoming more challenging.

**Table 1.**

**173**

*Characterization techniques for porous low-*k *dielectric materials.*

There are two methods to introduce the porosity into the film to produce porous low-*k* dielectric materials by spin-on technology. One is through sol–gel process, and the other is formed through the use of sacrificial particles (porogens) that are desorbed during the curing process. In the sol–gel process, the formation of subtractive porosity can be achieved by two approaches: the aging process and the hierarchical organization of the primary particles in the sol (self-assembly) [21, 22]. The other method is the use of sacrificial porogens, in which molecular or supramolecular particles are added in the low-*k* dielectric precursor with the purpose of tailoring the thermal stability. In the final curing process, these added molecular particles are removed by pyrolysis effect. The detailed description about spin-on technology to form porous low-*k* materials can be found elsewhere [23].

### *2.2.2 PECVD technology*

PECVD is a complex process, involving a wide variety of scientific and technical principles, including gas-phase reaction chemistry, thermodynamics, heat and material transfer, fluid mechanics, surface and plasma reactions, thin film growth mechanism, and reactors engineering. During the deposition process, the active intermediates and structural units are formed in the gas phase and then absorbed in the solid substrate. Finally, they migrate and react to form the matrix of the growing layer [11].

In the current semiconductor industry, the production of the porous low-*k* dielectric material is relied on PECVD technology because the formation material is more thermally stable and the *k* value can be lower than 2.0. The subtractive porosity approach is the widely accepted method. In this method, a low-*k* (generally is SiCOH) skeleton precursor mixed with a porogen precursor is introduced into the reactor during the deposition. After the deposition, a dual-phase SiCOH-CH*<sup>x</sup>* material is formed after the deposition. Tetramethylcyclotetrasiloxane (TMCTS), octamethylcyclotetrasiloxane (OMCTS), decamethylcyclopentasiloxane (DMCPS), and diethoxymethylsilane (DEMS) are the widely used skeleton precursors [24–27]. These skeleton precursors have a common property with a sufficiently low dissociation level under rf power in order to keep the sufficient hardness for the produced porous low-*k* dielectric material. The porogen precursor is organic molecule with sufficient volatility. Unsaturated cyclic hydrocarbons like terpinenes or norbornenes, linear alkenes, or molecules with strained rings like cycloalkene oxides or butadiene monoxide are the commonly used porogen precursors [11, 28].

Following, it is necessary to remove the labile organic fraction CxHy from the asdeposited SiCOH-CxHy film to form pores in the film. Thermal annealing, electron beam, or ultraviolet (UV) irradiation methods are provided to remove the labile organic fraction CxHy [29–31]. To reach better removal efficiency, it can be done by UV-assisted curing. However, the temperature of the curing has to be limited at 400°C. The mechanical strength (elastic modulus and hardness) of the porous low-*k* dielectric material can also be improved by UV-assisted curing because the UV curing can rearrange and enhance the cross-linking of the skeleton of the low-*k* *Porous Low-Dielectric-Constant Material for Semiconductor Microelectronics DOI: http://dx.doi.org/10.5772/intechopen.81577*

material by breaking a fraction of mainly the Si-CH3 (Si-Me) bonds. The improvement effect is associated to the used wavelength, temperature, and time of the UV curing [32, 33].

Currently, a promising method to deposition of the porous low-*k* dielectric film is using a single precursor molecule consisting of skeleton with embedded (or grafted) porogen precursor. An example of such a porous SiCOH material is Applied Materials' Black Diamond 3 (BD3) dielectric film. The UV curing is also modified to create more uniform porosity and improve the mechanical properties [8, 33].

### **2.3 Characterizations of porous low-***k* **dielectric materials**

In order to successfully integrate the porous low-*k* dielectric material into Cu interconnects, their physical, chemical, mechanical, and electric properties are important consideration factors. **Table 1** lists the main characterization techniques for porous low-*k* dielectric materials. Detailed principles and operation procedures can be found elsewhere [34, 35].

**Table 2** lists the main properties of porous low-*k* dielectric materials and compares to other generations of ILD materials (including SiO2, FSG, and OSG) [36–38]. In addition to providing a lower *k* value, porous low-*k* dielectric materials possess the degrading material properties. The degradation is more pronounced with increasing porosity (for the reduction of *k* value) for porous low-*k* dielectric materials. Therefore, the use of porous low-*k* dielectric materials in the ICs is becoming more challenging.


#### **Table 1.**

liquid contains the deposition precursors for low-*k* materials, which is dropping into the center of the substrate. The created centrifugal forces by rotating of the substrate help to distribute the material on the surface. After the spinning step, a heating (or bake) is required to remove solvent. The temperature is typically below 250°C. Finally, a curing at temperatures varying from 350 to 600°C is required to

There are two methods to introduce the porosity into the film to produce porous low-*k* dielectric materials by spin-on technology. One is through sol–gel process, and the other is formed through the use of sacrificial particles (porogens) that are desorbed during the curing process. In the sol–gel process, the formation of subtractive porosity can be achieved by two approaches: the aging process and the hierarchical organization of the primary particles in the sol (self-assembly) [21, 22]. The other method is the use of sacrificial porogens, in which molecular or supramolecular particles are added in the low-*k* dielectric precursor with the purpose of tailoring the thermal stability. In the final curing process, these added molecular particles are removed by pyrolysis effect. The detailed description about spin-on

PECVD is a complex process, involving a wide variety of scientific and technical

principles, including gas-phase reaction chemistry, thermodynamics, heat and material transfer, fluid mechanics, surface and plasma reactions, thin film growth mechanism, and reactors engineering. During the deposition process, the active intermediates and structural units are formed in the gas phase and then absorbed in the solid substrate. Finally, they migrate and react to form the matrix of the grow-

In the current semiconductor industry, the production of the porous low-*k* dielectric material is relied on PECVD technology because the formation material is more thermally stable and the *k* value can be lower than 2.0. The subtractive porosity approach is the widely accepted method. In this method, a low-*k* (generally is SiCOH) skeleton precursor mixed with a porogen precursor is introduced into the reactor during the deposition. After the deposition, a dual-phase SiCOH-CH*<sup>x</sup>* material is formed after the deposition. Tetramethylcyclotetrasiloxane (TMCTS),

Following, it is necessary to remove the labile organic fraction CxHy from the asdeposited SiCOH-CxHy film to form pores in the film. Thermal annealing, electron beam, or ultraviolet (UV) irradiation methods are provided to remove the labile organic fraction CxHy [29–31]. To reach better removal efficiency, it can be done by UV-assisted curing. However, the temperature of the curing has to be limited at 400°C. The mechanical strength (elastic modulus and hardness) of the porous low-*k* dielectric material can also be improved by UV-assisted curing because the UV curing can rearrange and enhance the cross-linking of the skeleton of the low-*k*

octamethylcyclotetrasiloxane (OMCTS), decamethylcyclopentasiloxane (DMCPS), and diethoxymethylsilane (DEMS) are the widely used skeleton precursors [24–27]. These skeleton precursors have a common property with a sufficiently low dissociation level under rf power in order to keep the sufficient hardness for the produced porous low-*k* dielectric material. The porogen precursor is organic molecule with sufficient volatility. Unsaturated cyclic hydrocarbons like terpinenes or norbornenes, linear alkenes, or molecules with strained rings like cycloalkene oxides or butadiene monoxide are the commonly used porogen

technology to form porous low-*k* materials can be found elsewhere [23].

obtain a stable film.

*Nanofluid Flow in Porous Media*

*2.2.2 PECVD technology*

ing layer [11].

precursors [11, 28].

**172**

*Characterization techniques for porous low-*k *dielectric materials.*


During the fabrication of BEOL interconnects, the used porous low-*k* dielectric material as an interconnecting insulator undergoes dielectric deposition, photoresist, etching, stripping, Cu metallization deposition, and chemical mechanical polishing (CMP) processes. Plasma damage, moisture/chemicals adsorption, Cu diffusion, and mechanical stress occurred on the porous low-*k* dielectric materials. These issues would reduce the electrical characteristics and reliability of the porous low-*k* dielectric materials. The mechanism and the resulting effect will be discussed

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

In order to reduce the plasma-induced damage and pattern small features, the metal hardmask method and the multilayer resist method, as plotted in **Figures 3** and **4**, respectively, are proposed since 32 nm technology node [44–46]. In the metal hardmask process, the resist is stripped prior to the trench and via etching into the porous low-*k* ILD; therefore, resist-stripping process-induced damage can be minimal. However, the polymer may remain on the sidewalls of the trenches during the trench etching step. The remaining polymer must be removed without damaging the porous low-*k* dielectric material. Additionally, the stress in the metal layer must be minimized to avoid pattern deformation after the etching process. Metal residues can form on the etched surfaces and block etching of the porous low-

In the advanced technology nodes, the multilayer resist method is preferred because it has an advantage to pattern small features. However, the porous low-*k*

*Metal hardmask dual-damascene patterning process: (A) TiN, ARC, and resist deposition. (B) M-2 metal hardmask RIE. (C) M-2 trench lithography. (D) Via-1 lithography. (E) Via-1 RIE. (F) M-2 oxide hardmask*

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

dielectric material is fully exposed to the resist strips. In order to avoid

in the following section.

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

*k* dielectric material.

**Figure 3.**

**175**
