This paper presents an in-depth overview of the present status and novel developments in the field of plasma processing of low dielectric constant (low-k) materials developed for advanced interconnects in ULSI technology. The paper summarizes the major achievements accomplished during the last 10 years. It includes analysis of advanced experimental techniques that have been used, which are most appropriate for low-k patterning and resist strip, selection of chemistries, patterning strategies, masking materials, analytical techniques, and challenges appearing during the integration. Detailed discussions are devoted to the etch mechanisms of low-k materials and their degradation during the plasma processing. The problem of k-value degradation (plasma damage) is a key issue for the integration, and it is becoming more difficult and challenging as the dielectric constant of low-k materials scales down. Results obtained with new experimental methods, like the small gap technique and multi-beams systems with separated sources of ions, vacuum ultraviolet light, and radicals, are discussed in detail. The methods allowing reduction of plasma damage and restoration of dielectric properties of damaged low-k materials are also discussed.

We report a new curing procedure of a plasma enhanced chemical vapor deposited SiCOH glasses for interlayer dielectric applications in microelectronic. It is demonstrated that SiOCH glasses with improved mechanical properties and ultralow dielectric constant can be obtained by controlled decomposition of the porogen molecules used to create nanoscale pores, prior to the UV-hardening step. The Young's modulus ͑YM͒ of conventional SiOCH-based glasses with 32% open porosity hardened with porogen is 4.6 GPa, this value is shown to increase up to 5.2 GPa with even 46% open porosity, when the glasses are hardened after porogen removal. This increase in porosity is accompanied by significant reduction in the dielectric constant from 2.3 to 1.8. The increased YM is related to an enhanced molecular-bridging mechanism when film is hardened without porogen that was explained on the base of percolation of rigidity theory and random network concepts.

The effect of He/H 2 downstream plasma on chemical vapor deposition ͑CVD͒ low-k films with different porosities was studied. The results show that this plasma does not reduce the concentration of Si-CH 3 bonds in the low-k matrix and that the films remain hydrophobic. However, mass loss and reduction in bulk C concentration were observed. The latter phenomena are related to the removal of porogen residue formed during the UV curing of the low-k films. It is demonstrated that the porogen residue removal changes the films' porosity and mechanical properties. The depth of the modification is limited by the penetration of H radicals into the porous low-k films. The plasma-induced damage of porous SiCOH-type low-k materials is one of the key problems in Cu/low-k integration.1 The most severe plasma damage occurs during photomask ͑resist͒ removal. 2The reason for this phenomenon is the hybrid nature of the SiCOH materials. These materials contain a SiO 2 -like matrix where part of the terminating oxygen atoms is replaced by organic groups ͑most often CH 3 ͒. The organic groups provide the films' hydrophobicity, which is important for the dielectric constant reduction. The hybrid nature is the reason for the different reactivities of the low-k components. For instance, the reactive species from plasma such as O radicals penetrate into the porous network of the low-k films and may result in substantial carbon depletion, thus leading to an increase in the k value. The depth of penetration is defined by the diffusion coefficient of active radicals into the pores and their recombination probability on the pore wall. 3Two commonly known approaches are used for the resist removal: ͑i͒ a low temperature, low pressure anisotropic plasma, where the photoresist is removed by an ion-assisted process, oxidizing or reducing plasma chemistries at low temperatures and ͑ii͒ hydrogen-based downstream plasma ͑DSP͒ where the resist is removed at high temperatures by a thermally activated chemical process. According to recent publications, the option with He/H 2 and Ar/H 2 DSPs prevents carbon depletion from the low-k materials matrix.4-6 Therefore, the degradation of the dielectric constant is minimal, and these processes are considered the most attractive options for the microelectronic industry.The porosity in advanced chemical vapor deposition ͑CVD͒ low-k films is created after deposition by the removal of a sacrificial phase ͑porogen͒ by UV-assisted thermal curing. UV curing also results in formation of the Si-O-Si network with improved mechanical properties. 7,8 The porogen molecules are normally cyclic hydrocarbons 9 that are photodissociated by UV light with the formation of volatile hydrocarbons and nonvolatile carbon-rich residues.10,11 The effect of the porogen residues on the low-k properties and the plasma processing compatibility are largely unknown. Fourier transform infrared ͑FTIR͒ spectrometry has a limited sensitivity to amorphous carbon ͑CvC and C-C bonds͒, and this is the reason why it is difficult to monitor porogen r...

Modification of chemical vapor deposition low-k films upon sequential exposure to helium plasma and then ammonia plasma is characterized using various methods. The He plasma emits extreme ultraviolet ͑EUV͒ photons creating O 2 vacancies, which impacts surface reactive sites and induces localized chemical modifications in the first surface monolayers. The subsequent NH 3 plasma treatment provides complete sealing of the low-k surface. The depth of the modification, which is a factor of merit of the sealing process, is limited because of the high absorption coefficient of silica-based low-k materials in the range of EUV emission.Integration of porous low dielectric constant ͑low-k͒ materials is a continuing issue in microelectronics industry. One of the most difficult challenges is related to the high sensitivity of porous materials to chemicals and plasma. Pores and their connectivity significantly increase the penetration depth of active species during different technological processes such as plasma etching and cleaning, deposition of barrier layers, chemical mechanical polishing ͑CMP͒, etc. The most severe damage of low-k materials happens during their exposure to strip-cleaning plasmas containing oxygen and hydrogen radicals. These radicals remove the carbon containing hydrophobic groups from the low-k materials. As a result of the carbon depletion, the films become hydrophilic. 1,2 Subsequent moisture absorption in the pores significantly increases the k value because of the high polarizability of water molecules.Recently, we have applied a diffusion-recombination model ͑Thiele analysis͒ to predict and quantify the plasma damage of porous low-k materials. 3 The penetration depth of radicals into porous low-k materials and the depth of plasma damage depend on the Thiele modulus, , defined as follows ͑Eq. 1͒where k r is the sum of reaction constants consuming active radicals on pore wall. D A and d p are diffusion coefficient and pore diameter, respectively. The higher the Thiele modulus, the lower the depth of penetration of active radicals. Because k r is mainly defined by the recombination of active radicals, the depth of plasma damage can be significantly reduced by stimulating the surface recombination of active radicals. The creation of surface active centers initiates the recombination of oxygen and hydrogen radicals and reduces the plasma damage. As an example, we showed that treatment of low-k materials in He plasma significantly reduces plasma low-k damage during the subsequent exposure to strip and cleaning plasmas. It was speculated that extreme ultraviolet ͑EUV͒ emission from He plasma creates chemically active sites on a low-k surface, which stimulate the recombination of active radicals in the surface area. In certain cases the activated surface area can stimulate and induce localized chemical reactions results in the sealing of low-k materials.In this work, the modification of the top part of low-k films treated by He and NH 3 plasmas is characterized by various methods. This plasma is normally ...

The effect of narrow-band 172 nm and broad-band >200 nm UV sources in the new curing scheme of the plasma-enhanced chemical vapor deposition (PECVD) dielectrics is studied. The new curing scheme is based on porogen removal (organic sacrificial phase introduced to generate open porosity) from PECVD dielectric before its final UV curing. The results are compared with the PECVD films fabricated in the conventional scheme in which porogen is still present during the UV curing. The same curing time of porogen-containing conventional PECVD films with 172 nm and >200 nm UV sources results in only 10% difference in their Young’s modulus (YM): 5.84 and 5.32 GPa, respectively. However, the porogen-free films cured with 172 nm UV source show a YM of 6.64 GPa (k100 kHz∼2.2, 44% open porosity), approximately twice as large as those cured with >200 nm UV having a YM of 3.38 GPa (k100 kHz∼2.0, 48% open porosity). The mechanical properties, optical properties in the range of 150–800 nm, dielectric constants at 100 kHz and 4 GHz, porosities, pore size distributions, and bonding structure are evaluated. The impact of porogen on optical characteristics and, therefore, on the photochemical UV-hardening mechanism is discussed. The achieved mechanical properties are explained on a basis of the percolation of rigidity theory and random network concepts.

In this study, the effect of the sequential He and NH 3 plasma treatments on a chemical vapor deposition SiOC:H low-k dielectric is evaluated in the wide range of experimental conditions. Results show that the active NH 3 plasma radicals penetrate the porous low-k bulk and remove the hydrophobic Si-CH 3 groups, which leads to hydrophilization and results in the degradation of dielectric properties. The implementation of He plasma pretreatment reduces the damage imposed by the NH 3 plasma by a stimulation of the surface recombination of active radicals from NH 3 plasma. He plasma causes a surface modification of 10-20 nm presumably due to the energy transfer from the extreme UV photons and the 2 1 S He metastable atoms to the low-k structure. The plasma damage reduction is proportional to He plasma density and the treatment time. The mechanism of plasma damage reduction is explained on the basis of the Knudsen diffusion mechanism and random walk theory.

No abstract

Plasma damage of SiCOH low-k films in an oxygen plasma is studied using a transformer coupled plasma reactor. The concentration of oxygen atoms and O2+ ions is varied by using three different conditions: (1) bottom power only, (2) bottom and top power, and (3) top power only. After plasma exposure, the low-k samples are characterized by various experimental techniques. It is shown that the ion bombardment induced by the bottom power minimizes the plasma damage by increasing the recombination coefficient of oxygen radicals. Contrary to the expectations, the densification of the top surface by ion radiation was limited. The increase in the recombination coefficient is mainly provided by modification of the pore wall surface and creation of chemically active sites stimulating the recombination of oxygen atoms. The results show that a reduction in plasma damage can be achieved without sealing of low-k top surface.