Article pubs.acs.org/Langmuir
Site-Selective Electroless Metallization on Porous Organosilica Films by Multisurface Modification of Alkyl Monolayer and Vacuum Plasma Giin-Shan Chen,*,† Sung-Te Chen,‡ Yenying W. Chen,† and Yen-Che Hsu† †
Department of Materials Science and Engineering, Feng Chia University, Taichung 407, Taiwan Department of Electronic Engineering, Hsiuping University of Science and Technology, Dali 412, Taichung, Taiwan
‡
ABSTRACT: Taking plasma-enhanced chemical vapor deposited porous SiOCH (p-SiOCH) and octadecyltrichlorosilane (OTS) as model cases, this study elucidates the chemical reaction pathways for alkyl-based self-assembled monolayers (SAMs) on porous carbondoped organosilica films under N2−H2 vacuum plasma illumination. In contrast to previous findings that carboxylic groups are found in alkylbased SAMs only by exposure to oxygen-based plasma, this study discovers that, upon exposure to reductive nitrogen-based vacuum plasma, surface carboxylic functional groups can be instantly formed on OTS-coated p-SiOCH films. Particular attention is given to developing a multisurface modification process, starting with the modification of p-SiOCH films by N2−H2 plasma and continuing with SAM deposition and plasma patterning; this ultimately leads to site-selective seeding for the spatially controlled fabrication of Cu-wire metallization by electroless deposition. Plasma diagnosis and X-ray near-edge absorption and Fourier transform infrared spectroscopies show that, by adequately controlling the plasma parameters, the bulk of the p-SiOCH films are free from plasma damage (in terms of degradation in bonding structures and electrical properties); the formation of the seedtrapping carboxylic functional groups on the surface, the key factor for the validity of this new seeding process, is due to a watermediated chemical oxygenation route. trapping region for site-selective electroless metallization5,6,8 or a template to serve as a resist mask for pattern transfer.6,7,9 Moreover, vacuum plasma modification of alkyl-based monolayers, also primarily deposited on SiO210,11 or Si wafers,12−15 is almost confined to examining reaction pathways for surface functionalization or film damaging,10−12,14 along with the development of lithographic-based patterning processes.13,15 Attempts to use vacuum plasma to functionalize alkyl-based (or other) monolayers for spatially controlled electroless metallization on porous organosilica films are rarely made. The paucity of these studies is surprising, given that vacuum plasma is a common source for microelectronics processing (e.g., dielectric etching, resist strip, and residual clearing) and that porous organosilica materials are a key type of dielectric for the copper interconnects of high-performance integrated circuits. As is generally known, reactive moieties of silanols in alkylbased molecules readily interact with various substrates that have hydrolyzed surfaces, leading to the self-assembly of a wellordered monolayer. However, direct deposition of SAMs on organosilica films is problematic due to the films’ hydrophobicity from the presence of surface methyl groups (−CH3). Therefore, a wet process has recently been used to create Si− OH groups on conventional (pore-free) low-k interlayer
1. INTRODUCTION The ability to fabricate spatially well-defined patterns of thin metallization films with low electrical resistance on various insulating substrates is increasingly important in the microelectronics industry. For example, to increase carrier transport and aperture ratio, Cu and Cu alloys embedded in SiNx have recently been proposed as alternatives to conventional multilayered Mo/Al/Mo or Al alloys as gate electrodes for thin-film transistors.1 With its low electrical resistivity (1.7 μΩ·cm) and high mechanical strength, Cu has also replaced Al (2.7 μΩ·cm) and has been combined with porous organosilica thin-film dielectric materials for chip interconnections, further reducing the mutually coupled signal delays.2 Electroless deposition is an attractive metallization technique because it offers the ability to metallize insulating substrates with low-temperature, cost-effective processes and equipment. It is also one of the leading technologies for microelectronics metallization, due to its compatibility with the Damascene-Cu electrochemical plating process and to its ability to form allmetal encapsulated, highly reliable Cu interconnects in an inline and self-aligned manner.3,4 Another merit of electroless deposition, a process based on catalytic activation to reduce metallic ions onto underlying substrates, is the use of localized catalysts for site-selective metallization. Indeed, the vast majority of studies use ultraviolet light to illuminate alkylbased self-assembled monolayers (SAMs), predominantly deposited on Si wafer,5−8 Si-based native oxide,8 or thermally oxidized SiO2.9 This patterning process creates a confined seed© 2012 American Chemical Society
Received: August 28, 2012 Revised: December 2, 2012 Published: December 3, 2012 511
dx.doi.org/10.1021/la303473r | Langmuir 2013, 29, 511−518
Langmuir
Article
angle of 110°.19,20 Coating of the OTS monolayers was reconfirmed by X-ray absorption spectroscopy. The OTS monolayers were treated in a vacuum glow-discharge reactor, equipped with an optical emission spectroscope (Emicon system; PLASUS, Kissing, Bavaria, Germany) to monitor in situ the emitted species during the course of plasma illumination. The plasmaoperating conditions used herein were identical to those for the pSiOCH films, aside from with a significantly reduced power density (less than 0.3 W·cm−2) and exposure time (≤2 s) due to the delicacy of the monolayers. 2.2. Electrical Property and Bonding Structure Analyses. The dielectric constants of the plasma-treated p-SiOCH films were derived from capacitance−voltage plots; a Keithley 4200-SCS semiconductor characterization system measured the capacitances of metal− insulator−semiconductor (MIS) capacitors [Cu/p-SiOCH (300 nm)/Si/Al] at a frequency of 0.1 MHz by applying an ac bias of 20.0 V. The effects of plasma illuminations on the insulating properties of the dielectric films were evaluated by measuring leakage current versus electric field between 0 and 4.0 MV·cm−1 on the copper-gate capacitors with the Keithley 4200-SCS. The control sample was composed of MIS capacitors that used the pristine p-SiOCH films as insulating layers. Notably, the Cu gate electrodes were deposited by thermal evaporation instead of by sputtering and, as such, avoided extraneous damage to the p-SiOCH films from the sputtering plasma. FTIR was operated in absorbance mode, with a spectrometer (Bomem, DA8.3) to investigate variations of the bonding structures produced by the plasma illumination. A Bomem DA8.3 spectrometer, with a reproducibility of 0.5%, was used to acquire the series of FTIR spectra in Figure 2. Transmission spectra at normal incidence were recorded between 900 and 4000 cm−1 at a spectral resolution of 1 cm−1 by averaging 200 scans. The background for each spectrum was collected from a piece of wafer from which the film had been removed. Impact of the plasma pretreatments on the surface-bonding structures of the monolayers (and p-SiOCH films) was characterized by carbon near-edge structures by use of a synchrotron radiation source from the 20 A1 beamline at the National Synchrotron Radiation Research Center, Taiwan, along with FTIR operated in the attenuated total reflection (ATR) mode. These characterizations allowed tuning of the plasma processing windows for the topmost regions of the OTS-SAMs to yield functionalized polar groups for siteselective seed trapping. 2.3. Site-Selective Seeding and Electroless Metallization. Figure 1 depicts the processing steps for the multisurface modification
dielectrics (hydrogen silsesquioxane and Black Diamond), allowing a porphyrin SAM to be deposited as a Cu diffusion barrier layer.16 Moreover, porous organosilica films are more susceptible to plasma damage than SiO2 and conventional (pore-free) low-k organosilica films.2,17 Therefore, the key to integrating SAMs into porous dielectrics by use of vacuum plasma for site-controlled Cu metallization relies on the development of a damage-free plasma surface modification process. We have recently developed a nanoseeding process via dual surface modification of alkyl SAM for site-controlled electroless metallization on thermally oxidized SiO2 dielectrics.18 Herein, we take a step further by selecting chemical vapor deposited porous SiOCH (p-SiOCH) films and octadecyltrichlorosilane (C18H37SiCl3; OTS) films as test cases to elucidate the plasmainduced reaction pathways between porous carbon-doped organosilicate glasses and alkyl-based SAMs. Particular attention is given to developing a damage-free multisurface modification process involving vacuum plasma, SAM, and chemical solution for the dielectric films to yield active surface functional groups for site-selective seeding, subsequently acting as templates for spatially controlled electroless metallization.
2. EXPERIMENTAL SECTION 2.1. Film Deposition and Plasma Modification. In this study, carbon-doped organosilica films were deposited on p-type (100) Si wafers by an Applied Materials standard plasma-enhanced chemical vapor deposition reactor, which has a radio frequency capacitive coupled parallel plate plasma excitation frequency of 13.56 MHz. To create the hybrid material, diethoxymethylsilane (DEMS; C5H14O2Si) and bicycloheptadiene (BCHD; C7H8) of 98% purity were used as skeleton and porogen precursors, respectively. The two liquid precursors were vaporized by use of heated injectors and were carried to the reactor through heated lines with helium carrier, and the films were deposited on substrates held at 200 °C. These films were subsequently cured by ultraviolet light to form porous SiOCH (pSiOCH) structures with a dielectric constant (k) of 2.6 and a thickness of 300 nm. Porosimetric study showed that the mean pore size (radius) and porosity of the films were 1.1 nm and 15%, respectively. Sessile drop contact angle measurements with accuracy levels of ±1° were performed on an FTA125 contact angle system (First Ten Angstroms, Inc.). The pristine films were placed in a vacuum glow-discharge reactor equipped with a Baratron capacitance manometer (690A, MKS), which measured the absolute pressures in the range from ∼10 Pa to 1 atm. As the background pressure reached 3 Pa, N2 and H2 gas flows were admitted into the reactor by a separate mass flow controller through a leak-tight piping system. Then, the N2−H2 plasma (80%/ 20% by partial pressure) was ignited at ∼60 Pa by a 13.56 MHz radio frequency generator connected to the substrate holder. Fourier transform infrared spectroscopy (FTIR) and leakage current analysis indicated that bonding structures and electrical insulation of the pSiOCH films degraded significantly after treatment for 1 min with power exceeding 10 W. Therefore, plasma illumination was carried out herein at power levels of 10 W or less for durations ranging from several to tens of seconds. OTS (95% volume concentration) was purchased from Sigma− Aldrich and used without further purification. The plasma-pretreated p-SiOCH substrate films were immersed in a freshly prepared 25 °C toluene solution containing 1 mM OTS for a duration ranging from 10 s to 4 min, removed from this solution, rinsed twice with toluene and deionized water, and blown dry by a stream of nitrogen. All reagents used were of analytical grade and were used as received. As presented before,18 fractional coverage of the OTS monolayers was monitored by the evolution of contact angles of water droplets; those angles were analyzed as a function of immersion time since a well-ordered, uniformly coated monolayer gives a saturated contact
Figure 1. Processing steps for a multisurface modification to create a patterned alkyl-based monolayer with a viable functionality on pSiOCH films for site-selective seeding of catalysts. 512
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to create a viably functional patterned OTS monolayer on the pSiOCH film for site-selective seeding of metallic catalysts. First, the pSiOCH film is exposed to a vacuum plasma under the optimum processing window obtained from section 2.2. This treatment converts the hydrophobic surface (contact angle = 89°) into a notably hydrophilic state with active sites to adsorb an OTS monolayer (steps a and b). The sample, covered by a metal mask, is transferred to the plasma reactor and exposed to the vacuum plasma according to the subtle process parameters derived from section 2.2. Doing so allows a pattern to be made in the designated area with viable functional groups of water contact angle = 4° (step c). An aqueous basic (the so-called SC-1) solution is applied in step d in an attempt to deprotonate the functional groups into negatively charged sites. During immersion in a metal salt of Co(NO3)2 or Ni(NO3)2 aqueous solution with pH value ≥5, the negatively charged area provides points that trap metallic cations, which are then reduced into a neutral form by a reducing agent (step e). The localized seed particles then act as a template for site-selective electroless deposition of a Cu micronpattern (step f). Electroless deposition of Cu is performed in a cupric sulfate− formaldehyde solution. The processing conditions of the SC-1 surface modification, seeding, and electroless deposition, as well as constituents of the baths, have been detailed elsewhere.21
3. RESULTS AND DISCUSSION Surface hydrophobicity and pore structure of the pristine porous films dictate the need for surface modification prior to OTS-monolayer coating. FTIR was used to make a first assessment of changes in bonding structures caused by the plasma illumination. Significant changes in the spectra were observed after plasma illumination for 1 min with a power greater than 10 W; these changes signified serious bond degradation. Therefore, only details of the spectra corresponding to short-time (
