As the feature size of very large scale integrated circuits continues to shrink and the density of devices on a chip increases, a material of particular importance identified by the Semiconductor Industry Association national technology roadmap is the low dielectric constant material (k<3) for interlevel isolation. A low-k dielectric is required for reducing power consumption in high frequency devices, reducing crosstalk between metallic interconnects, and reducing circuit RC time constants. The reduction in time constant is necessary for increasing speed and functionality on a chip.
Various organic and inorganic materials have been considered to replace PE-CVD deposited SiO2, or FSG film for further reduction in dielectric constant.
There are many strategies to further reduce the dielectric constant. One strategy is to add alkyl groups to the silicon oxide based dielectric materials. Changing the component and connectivity of atomic constituents will also vary the dielectric constant and other properties such as the thermal stability.
Among the low-k candidates, the carbon-doped silicon oxide (SiCOH) film attracts
more attention and is highly suitable for ultra large-scale integrated applications because of the k value less than 3.0. SiCO films, which the silicon oxide incorporating terminating methyl (CH3) groups in the oxide network can be considered a hybrid between organic and inorganic materials. The SiCOH films can potentially combine the advantages of both categories of materials [47-50].
2-6-1 Spin-on Deposition of low Dielectric Constant Materials
Spin-on deposition is one technique to deposit dielectric films for semiconductor fabrication. “Spin-on materials” is a material, in a liquid state, which can be deposited on a wafer by spin-coating method. Since spin-on material contain volatile solvent, an extra baking process is needed to perform to remove solvent from the as-spun film. Finally, the resultant sample is processing in furnace alloying to prepare excellent spin-on material.
Generally speaking, spin-on materials have some advantages in IC fabrication, such as good local planarization capability, gap-filling capability, and low fabrication cost.
Among various organic polymer such as fluorinated polyimide (FPI), poly arylene ether (PAE), benzocyclobutene (CBC), including SiLK and Flare, have been extensively investigated. [51-55]. These organic spin-on low-k materials have better thermal and mechanical stabilities than aliphatic polymers low-k materials, which results in the improvement of dimensional, compositional, and topographical integrity during continuous high-temperature thermal cycle. Moreover, most organic polymers low-k materials possess a superior characteristic of resistance to moisture uptake due to low content of polar molecules. Although these organic polymers have such more advantages, they suffer the most challenges in the compatibility of integration, such as etching
selectivity, manufacturability and chemical mechanical polishing (CMP) compatibility, because their material characteristics are completely different from that of SiO2-based dielectrics.
There are another spin-on organosilicae glasses, such as methylsilsesquioxane (MSQ) and Hybrid-Organic-Siloxane-Polymer (HOSP), are the promising low-k materials [56, 57]. These films have been developed by increasing the number the methyl group, which decrease the film density and reduce film polarization, resulting in a low dielectric constant [58]. However, these film qualities are easily degraded due to oxygen plasma-induced damage and hydroscopic behavior in the duration of photo-resist stripping [59]. This chemical instability of MSQ or HOSP film is one of the major integration issues as it used as low-k ILDs materials.
2-6-2 Chemical Vapor Deposition of low Dielectric Constant Materials
Different from the spin-on technology, chemical deposition is the other technique to deposit dielectric films for ULSI fabrication. This method provides a simple procedure to produce the low-k film, which does not need the baking and furnace thermal process to remove solvent or impurities. Moreover, because of their thermal and mechanical stability, SiO2-based, low-dielectric constant materials containing alkyl groups, deposited by PE-CVD method have attracted much attention recently [60,61]. Among these low-k materials, an organic and inorganic hybrid-type SiCOH film deposited by PE-CVD technology is wildly accepted as a promising candidate for low-k material application in the dual damascene copper interconnections because of its compatibility to conventional ULSI circuit processing. These low-k materials form the nanopores in the films by the
steric hindrance effect of alkyl groups as well as have the carbon’s lower polarization in Si-CH3 groups, which result in the decrease in dielectric constant [62]. Varieties of precursors have been developed to deposited low-k SiCOH films by CVD technology.
Table 2-3 outlines the low-k materials precursors, tested, and their structure formulae [63].
The common precursors being used are trimethylsilane (3MS) and tetramethylsilane (4MS). The dielectric constant of these low-k films can reach as low as 2.6-3.0.
Furthermore, these low-k films are useful as inter-metal dielectrics, and exhibit stability and electrical properties, which can meet many specifications in device fabrication [64].
However, the detailed chemical structure of the PE-CVD SiCOH films is still unclear because the chemical structure of the PE-CVD films is determined by the process precursors, process conditions (temperature, pressure, and gas flow) and plasma properties (radicals, ions, and their density and energy distribution).
On the other hand, there are a number of problems with using these low-k films as dielectrics, compared to SiO2 or FSG. The modulus and hardness are much lower than for SiO2, which can create problems during the fabrication and packaging [65,66]. In addition, the thermal conductivity is lower than that of SiO2, so heating of interconnects will be greater than that for SiO2 [67].