Modification of Ultra Low-k Dielectric Films by O₂and CO₂Plasmas

Abstract

Low-k materials developed for ULSI interconnects should have sufficient resistance to processing plasma. CO 2 plasma is being considered as a promising candidate for low damage photoresist ash and as a surface activation chemistry for self-assembled monolayers and atomic layer deposition on low-k materials. This article explores the interaction of two organosilicate (OSG) based low-k materials with different k-values (OSG2.4 and OSG2.2) with CO 2 plasma in both CCP and ICP-remote plasma chambers. Time dependent exposure of the materials to CO 2 plasma revealed quick and effective sealing of OSG2.4 surface whereas it takes longer time for OSG2.2. The sealing reduces further plasma damage and leads to accumulation of CO 2 in the pores of both materials. The same behavior occurs in ICP-remote plasma but without a complete sealing of the surface. This suggests the important role of ion bombardment. Damage to low-k by conventional O 2 plasma was studied alongside and it was found that for t < 60 s, O 2 plasma exerts more damage on OSG2.2 than CO 2 . This trend is reversed at t > 60 s. Furthermore, lesser time exposure to CO 2 plasma was investigated with respect to source power at constant pressure and it was discovered that damage although small, increases with varying source power. The performance enhancement of electronic circuits was majorly centered on reducing the transistor sizes, increasing transistor speed and density. However, in advanced technology nodes the performance of the resulting integrated circuits (IC) is greatly hindered by the resistance -capacitance (RC) delay experienced during the signal propagation within the interconnects.1 In order to solve this problem, the traditional Al was replaced by Cu, which has lower resistivity and SiO 2 with k = 4.0 was replaced by low dielectric constants (low-k) materials. Organosilicate glasses (OSG) deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) and/or by Spin-on Glass technology (SOG) are presently the most popular low-k materials.Photoresist strip using O 2 based plasma is a conventionally used processing technique but this has been found to be severely damaging to porous low-k materials. The modifications to the porous low-k materials lead to increased k-values from densification and hydrophilization.2-5 The CO 2 -based plasma has been reported to be less damaging with respect to the O 2 counterpart. Fuller et al. proposed the use of CO 2 based RIE plasma for photoresist stripping. Ming-Shu Kuo et al. and Hualiang Shi extensively studied the surface modification of ultralow-k materials by CO 2 plasma and the results are published in their PhD dissertations 7,8 and in several papers. 9-13 They reported that i) CO 2 and O 2 plasma damage to ultralow-k films are comparable ii) there is lower atomic oxygen density in CO 2 discharge than O 2 discharge because of higher activation energy (11.5 eV) required to liberate atomic oxygen from CO 2 than from O 2 molecules (6 eV) -this is why there is supposedly a reduction in damage by CO 2 discharge wi...

Nanoscale chemical structure variations in nano-patterned and nano-porous low-k dielectrics: A comparative photothermal induced resonance and infrared spectroscopy investigation

“…Figure 7 presents the T-FTIR spectra collected from the previously investigated 180 nm pitch low-k fin structures exposed to a variety of plasma ash cleans with different oxidation potentials. Relative to the no ash spectrum, all T-FTIR spectra from the samples exposed to a plasma ash exhibited reduced absorbance for the Si-CH3 deformation band as expected [65]. The loss of Si-CH3 absorbance also clearly corresponded with the oxidation potential of the plasma ash and increased in the following order He-H2 < Ar-N2 < He-O2 < N2-O2.…”

“…These plasma ashes are commonly utilized post plasma etching to remove photoresist and any polymer residues that may have formed on the surfaces of patterned structures during plasma etching. From several prior studies [63][64][65][66], it has been well established that such plasma ashes typically selectively remove terminal methyl groups from the chemical structure / matrix of low-k dielectrics. The degree to which terminal methyl groups are removed depends on both the oxidation potential and length of the plasma ash exposure as well as the amount of interconnected nano-porosity present in the low-k dielectric.…”

Future Dielectric Materials for CNT Interconnects - Possibilities and Challenges

“…Growth processes and integration mechanisms like etching, patterning, masking and plasma processing that were adopted for traditional Cu interconnects lead to the degradation of the dielectric properties of low-k materials [109]. So, many challenges were faced by researchers and engineers in the microelectronics industry in realizing low-k dielectrics in-between Cu interconnects [39,[110][111][112][113][114][115]. Integration of low-k dielectrics for CNT interconnects is a new and emerging area altogether, when compared to the strides made in copper/low-k integration [123].…”