**2.1. Low-***k* **materials**

The dielectric constant of materials can be typically described by Clausius-Mossotti Equation [16]:

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

The silica-based low-*k* materials have been successfully integrated in a microprocessor due to high chemical and thermal stability. The silica-based materials have the tetrahedral basic

oxygen atoms and each oxygen atom is bonded to two silicon atoms (SiO4/2). Each silicon atom

The first-generation low-*k* material in semiconductor production line was fluorinated silicon glass (FSG or SiOF), in which the Si–O bond is replaced by the less polarizable Si–F bond. FSG materials were used at the 0.18 μm technology node with the dielectric constant from 3.5 to

Next, the second-generation low-*k* material was the organosilicate glass (SiCOH), in which

ture. So, SiCOH materials were successfully integrated in some 130 nm and 90 nm products [23, 24]. Generally, FSG and SiCOH materials were deposited by plasma-enhanced chemical vapor deposition (PECVD). Moreover, both fluorine and carbon increase the interatomic distances or "free volume" of silica. This provides an additional decrease of dielectric constant

The limitation of *k* value for SiCOH materials is ~2.6. To prevent or limit an increase in the BEOL capacitance in the advanced technology nodes (65 nm or below), it requires a new low-*k* material with a further lower*-k* value (< 2.5). To meet this goal, the introduction of porosity in the low-*k* SiCOH materials is required because air can provide the minimum *k* value of ~1.0. The produced low-*k* material is a so-called porous SiCOH dielectric, which can be fabricated either by the structural or the subtractive method [25–27]. The latter method is widely accepted because the produced film is more thermally stable and can provide

is at the center of a regular tetrahedron of oxygen atoms.

3.8, depending on the concentration of Si–F bond [21, 22].

the Si–O bond is replaced by the less polarizable Si–CH3

but decreases the film density. Since the CH<sup>3</sup>

SiCOH materials have a lower density (~1.2–0.4 g/cm3

rial is in the range of 2.6–3.0, depending on the number of CH3

. Silica has a molecular structure, in which each Si atom is bonded to four

bond. The *k* value of the SiCOH mate-

Plasma Damage on Low-*k* Dielectric Materials http://dx.doi.org/10.5772/intechopen.79494 293

group has a larger volume and is hydrophilic,

) and tend to be hydrophilic.

groups built into the struc-

structure of SiO2

**Table 1.** Low-*k* dielectric classification.

where *k* = ε/ε<sup>0</sup> , ε, and ε<sup>0</sup> are the dielectric constants of the material and vacuum, *N* is the number of molecules per unit volume (density), and *α* is the total polarizability, including electronic (α<sup>e</sup> ), distortion (αd), and orientation (α<sup>o</sup> ) polarizabilities. According to Eq. (1), the dielectric constant of materials can be reduced by two strategies: decreasing the total polarizability (*α*) and density (*N*). Reducing the polarizability can be achieved by the use of low polar bonds (like C–C, C–H, Si–CH3 , etc.), and reducing the film's density can be obtained by means of the introduction of porosity. **Table 1** summarizes the classification of low-*k* materials and their corresponding dielectric constants.

Low-*k* materials can be divided into several categories: silica-based, silsesquioxane (SSQ) based, organic polymers, and amorphous carbon low-*k* materials [17–20]. The last three categories have integration issue due to the weak mechanical strength; therefore, they are not officially production in the semiconductor industry.


**Table 1.** Low-*k* dielectric classification.

In order to slow down the increase of *RC* delay, the introduction of new materials to the back-end-of-line (BEOL) interconnects is needed. Aluminum (Al) had been replaced by copper (Cu) as a conductor dielectric because Cu can provide a lower resistivity (ρ) [7]. In the case

Additionally, the integration method for Cu/low-*k* interconnects must be changed because Cu etching is very challenging due to nonvolatile by-products. Traditional metal etching approach had been replaced by a damascene process [11]. In a damascene process, plasma technology is widely used because it can provide an isotropic process and a fast rate. Thus, these changes make the low-*k* materials to direct contact with the plasma, such as dielectric etching, photo strip, barrier metal deposition, and surface treatment. Under the plasma irradiation, low-*k* materials are sensitive to chemical modification, resulting in an increased *k* value. This is so-called plasma damage [12–15], becoming the main impediment to a success-

In this connection, this chapter is an attempt to provide an overview of plasma damage on the low-*k* materials. This chapter is organized as follows: in Section 3, we introduce the low-*k* materials and plasma. Next, in Section 5, the processing with plasma damage on the low-*k* materials during interconnects fabrication is identified. Then, in Section 4, the results of plasma damage on the low-*k* materials based on our group's investigation are summarized.

The dielectric constant of materials can be typically described by Clausius-Mossotti Equation

*<sup>k</sup>* <sup>+</sup> <sup>2</sup> <sup>=</sup> \_\_\_\_ <sup>4</sup>*<sup>N</sup>*

number of molecules per unit volume (density), and *α* is the total polarizability, including

dielectric constant of materials can be reduced by two strategies: decreasing the total polarizability (*α*) and density (*N*). Reducing the polarizability can be achieved by the use of low polar

of the introduction of porosity. **Table 1** summarizes the classification of low-*k* materials and

Low-*k* materials can be divided into several categories: silica-based, silsesquioxane (SSQ) based, organic polymers, and amorphous carbon low-*k* materials [17–20]. The last three categories have integration issue due to the weak mechanical strength; therefore, they are not

materials with the relative dielectric constant (*k*) lower than 4.0 (SiO2 *k* value) [8–10].

dielectric had been replaced by the low-*k*

<sup>3</sup> *α* (1)

) polarizabilities. According to Eq. (1), the

are the dielectric constants of the material and vacuum, *N* is the

, etc.), and reducing the film's density can be obtained by means

of the interconnecting insulator, the traditional SiO2

292 Plasma Science and Technology - Basic Fundamentals and Modern Applications

ful integration of low-*k* materials into ICs.

Finally, short conclusion is provided in Section 5.

**2. Low-***k* **materials and plasma**

\_\_\_ *<sup>k</sup>* <sup>−</sup> <sup>1</sup>

), distortion (αd), and orientation (α<sup>o</sup>

, ε, and ε<sup>0</sup>

their corresponding dielectric constants.

officially production in the semiconductor industry.

bonds (like C–C, C–H, Si–CH3

**2.1. Low-***k* **materials**

where *k* = ε/ε<sup>0</sup>

electronic (α<sup>e</sup>

[16]:

The silica-based low-*k* materials have been successfully integrated in a microprocessor due to high chemical and thermal stability. The silica-based materials have the tetrahedral basic structure of SiO2 . Silica has a molecular structure, in which each Si atom is bonded to four oxygen atoms and each oxygen atom is bonded to two silicon atoms (SiO4/2). Each silicon atom is at the center of a regular tetrahedron of oxygen atoms.

The first-generation low-*k* material in semiconductor production line was fluorinated silicon glass (FSG or SiOF), in which the Si–O bond is replaced by the less polarizable Si–F bond. FSG materials were used at the 0.18 μm technology node with the dielectric constant from 3.5 to 3.8, depending on the concentration of Si–F bond [21, 22].

Next, the second-generation low-*k* material was the organosilicate glass (SiCOH), in which the Si–O bond is replaced by the less polarizable Si–CH3 bond. The *k* value of the SiCOH material is in the range of 2.6–3.0, depending on the number of CH3 groups built into the structure. So, SiCOH materials were successfully integrated in some 130 nm and 90 nm products [23, 24]. Generally, FSG and SiCOH materials were deposited by plasma-enhanced chemical vapor deposition (PECVD). Moreover, both fluorine and carbon increase the interatomic distances or "free volume" of silica. This provides an additional decrease of dielectric constant but decreases the film density. Since the CH<sup>3</sup> group has a larger volume and is hydrophilic, SiCOH materials have a lower density (~1.2–0.4 g/cm3 ) and tend to be hydrophilic.

The limitation of *k* value for SiCOH materials is ~2.6. To prevent or limit an increase in the BEOL capacitance in the advanced technology nodes (65 nm or below), it requires a new low-*k* material with a further lower*-k* value (< 2.5). To meet this goal, the introduction of porosity in the low-*k* SiCOH materials is required because air can provide the minimum *k* value of ~1.0. The produced low-*k* material is a so-called porous SiCOH dielectric, which can be fabricated either by the structural or the subtractive method [25–27]. The latter method is widely accepted because the produced film is more thermally stable and can provide a lower-*k* value. In the subtractive method, the films are deposited as a dual-phase material, using a mixture of a SiCOH skeleton precursor with an organic porogen precursor. The popularly used skeleton precursor is diethoxymethylsilane (DEMS). The used organic porogen precursor must have sufficient volatility for easy removal. The used molecules are alpha-terpinene (ATRP), bicycloheptadiene (BCHD), or cyclooctane (C8 H16). Hence, in order to remove the labile organic fraction in the as-deposited films, curing process has to be done after the deposition [8, 10, 27]. By this way, a porous film can be formed. Thermal curing, electron beam, or ultraviolet (UV) irradiation can be used to achieve this work. Generally, UV-assisted curing for the fabrication of porous SiCOH dielectrics is widely adopted by the semiconductor industry because it can also rearrange the film's structure and enhance the cross-linking of the skeleton. This provides a big help to improve the mechanical strength for porous SiCOH dielectrics.

In the ICP systems, there are two applied RF power: one is source power (top power), and the other is bias power (bottom power). Therefore, plasma density and ion energy can be controlled separately. Additionally, the ICP system has the highest plasma density with 1011–1012

has the lowest plasma density. Due to anisotropic etching property provided by ion bombardment, ICP and CCP systems are usually used for pattern etching. Since dielectric films are very sensitive to ion bombardment and ICP reactors lack passivating species required by typical dielectric etching, CCP reactors are mostly used for dielectric patterning etching. On the other hand, ICP reactors are often used for conductor patterning etching due to the etching rate consideration. To avoid damage by ion bombardment and UV light irradiation or no need anisotropic etching in the plasma process, RP reactors are the best choice. So, cleaning and resist stripping processes during semiconductor fabrication can be done by RP reactors.

The plasma-induced damage on the low-*k* dielectrics is a complex phenomenon involving both physical and chemical effects. Ion bombardment on the low-*k* dielectrics represents the physical effect. This effect depends on the energy distribution and flux for each ionic species. The chemical effect involves photochemistry induced by the UV radiation and chemical reaction between the radicals and low-*k* constituents. Under physical and chemical reactions in the plasma, the surface of low-*k* dielectrics is modified. The modification depth is related to the ion energy, diffusion of active radicals (O, H, F, etc.), and porosity and constituents in the

The plasma damage on low-*k* dielectrics makes the increase of the dielectric constant, the changes in bonding configuration, the formation of carbon-depleted layer, film shrinkage,

The depletion of carbon is mainly caused by active radicals through chemical reactions. Due

and adsorbs moisture. Therefore, a drastically increase in the *k* value and leakage current and a degradation in the dielectric breakdown were detected for plasma-treated low-*k* dielectrics.

method was also changed. "Damascene" process has been used to fabricate Cu/low-*k* interconnects because Cu cannot be easily patterned by reactive ion etching (RIE) due to the low volatility of Cu etching by-products, such as Cu chlorides and Cu fluorides [34]. Generally, "dual-damascene" process, in which both via and trench are patterned simultaneously, is widely used. The sequence of via and trench patterning can be changed. Via-first dualdamascene process, in which via is first patterned, is preferred [35]. The process flow of via-

interconnects had been transferred to Cu/low-*k* interconnects, the fabrication

**3. Low-***k* **plasma damage during interconnects fabrication**

first dual damascene is plotted step by step, as shown in **Figure 1**.

groups, the surface of low-*k* dielectrics becomes hydrophilic

–1010 electrons/cm3

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

. The RP system

295

[31]. The plasma density of CCP system is 109

electrons/cm3

**2.3. Plasma damage mechanism**

low-*k* material [32, 33].

and surface densification.

As Al/SiO2

to the loss of hydrophobic CH3

The *k* value of porous SiCOH dielectrics can be scaling down by increasing the porosity and pore size simultaneously. However, this makes materials to become softer. Moreover, both the dielectric breakdown field and leakage current are degraded. Furthermore, as the porosity or pore size increases to a critical value, the pores can be connected each other to form socalled open pores. The open pores can be served as the easier penetration path into the bulk of the low-*k* material for active reactants [28]. Thus, more challenges will be addressed as porous SiCOH dielectrics are integrated in the advanced technology nodes.
