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Porous SiOCH Thin Films Obtained by Foaming Julien El Sabahy, Gael Castellan, Florence Ricoul, and Vincent Jousseaume J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00204 • Publication Date (Web): 06 Apr 2016 Downloaded from http://pubs.acs.org on April 9, 2016
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Porous SiOCH Thin Films Obtained by Foaming Julien El Sabahy, Gaël Castellan, Florence Ricoul and Vincent Jousseaume* Univ. Grenoble Alpes, F-38000 Grenoble, France, CEA, LETI, MINATEC Campus, F-38054 Grenoble, France.
ABSTRACT: Porous organosilicate thin films (SiOCH) deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) are used as dielectric layers in advanced microelectronic interconnections and as chemical layers in chemical sensors and biosensors. One challenge is to increase the porosity in these films, the classical method being limited to porosity rate close to 50%. In this paper, we report an innovative and simple strategy to perform highly nanoporous SiOCH thin films without the use of any templates or external blowing agents. This approach uses a SiOCH deposited by PECVD (without any porogens) intentionally covered by a dense crust. The porosity generation is obtained through a Ultra-Violet (UV) assisted thermal annealing of the stack. The highest porosities ever demonstrated for SiOCH PECVD thin films are obtained (porosity close to 65%). The impact of different process parameters (choice of precursor, deposition and annealing conditions) on the creation of porosity is studied. The porosity introduction with this original method can be related to a foaming mechanism: a gas is produced inside the film during the UV-assisted curing which causes a film expansion and allow the creation of porosity.
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1. INTRODUCTION Highly porous organosilicate thin films (SiOCH) deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) were firstly used as ultra low-k (ULK) dielectrics in integrated circuit interconnects.1-2 More, recently, these porous SiOCH have revealed interesting properties when they were considered for others applications like moisture sensors, biological fluid analyses or gas sensing.3-5 For all these applications, porosity (with pores in the nanometer range) is a key parameter. It allows tailoring specific material properties like dielectric constant or specific surface area. Generally, porous SiOCH are deposited using a subtractive strategy: the porogen approach.6-7 This approach is based on the co-deposition of an organosilicate precursor with an organic porogen molecule during the PECVD process. After deposition, the sacrificial porogen is removed during an appropriate thermal treatment, creating porosity. From an industrial point of view and thanks to the resulting time saving, Ultra-Violet (UV) assisted curing are now preferred for porogen removal treatment.8-10 In this approach, the porosity can be tuned by varying the porogen content in the deposited film.6-7 But unfortunately, this technique is limited and a porosity upper limit is reached, close to 50%.11-13 This limitation is usually interpreted using the continuous random network theory and percolation of rigidity concepts.13-14 When the average number of bonds per network forming atom is too low, i.e when the creation of new Si-O-Si bridges is not sufficient to compensate the pore generation, the structure collapses itself by a lack of connective bonds. When the porogen loading is too high then the skeleton rearrangement is not sufficient to maintain the structure. A film collapse is hence observed and the created porosity is lost.11 In polymer science, another well-known method for obtaining porous structures is to use a foaming process.15-17 In this case, the polymer is saturated with a gas or a supercritical fluid
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(usually CO2) and in a second step, the system is led to a supersaturation state by reducing the pressure (pressure induced phase separation) or by increasing the temperature (temperature induced phase separation). This process leads to the nucleation and growth of gas bubbles inside the polymer matrix. The morphology of the final material strongly depends on the chemical nature of the polymer as well as on the foaming experimental conditions (temperature and pressure of saturation, temperature and foaming time). Whereas microcellular foams (pore diameters ranging from 1 to 10 µm) and ultramicrocellular foams (diameter < 1 µm) are usually obtained,17-19 only few papers report the formation of nanoporous materials (diameter < 10 nm).20-22 Moreover, the demonstrations of foaming on thin films (< 1 µm thick) are scarce. For instance, Krause has investigated this type of approach for ULK applications using high-Tg polyimide and dielectric constants as low as 1.77 were obtained.23 However, the films studied were at least 100 µm thick, far above the thickness required for microelectronic applications (typically below 500 nm). By using the thermal decomposition of the carbonate groups of poly(phenylquinoxaline), S. Merlet et al. have also developed porous ultra low-k thin films.20 Their process, also called “self-foaming”, did not use an external gas source, CO2 and isobutene being produced in-situ during the thermal treatment for the layer foaming. However, this work was also performed on relatively thick layers (several tens of microns). One difficulty to develop very thin porous films using a foaming process is the existence of a dense (unfoamed) skin which is usually observed at the surface of the material.24 This skin is due to the outward gas diffusion from the film close to the interfaces and is usually estimated to at least one micron thick.25 In this work, we propose an innovative and simple strategy to perform highly nanoporous SiOCH thin films (even for thickness lower than tens of nanometers) without the use of any templates or external blowing agents. This approach uses a SiOCH deposited by PECVD
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(without any porogens) and intentionally covered by a dense crust. The porosity generation is obtained through a UV-assisted thermal curing of the stack. The highest porosities ever demonstrated for SiOCH PECVD thin films are obtained (porosities close to 65%). The porosity introduction with this original method is believed to be related to a foaming mechanism.
2. EXPERIMENTAL SECTION 2.1. Materials The process used to perform porous SiOCH thin films is schematized in Figure 1. First, SiOCH were deposited on Si 200 mm wafers by PECVD. A radiofrequency capacitive coupled parallel plate reactor from Applied Material was used (using a plasma excitation frequency at 13.56 MHz). Two different precursors were studied, TriMethylSilane (3MS), and OctaMethylCycloTetraSiloxane (OMCTS). Depositions were performed under vacuum from 150°C to 350°C and film thicknesses after deposition ranged from 80 nm to 500 nm. Then, a capping of the SiOCH thin film was realized using a thin silicon dioxide crust (10 nm thick). In order to limit damages and modifications of the SiOCH films, the SiO2 layer was deposited by PECVD at low temperature (i.e. 200°C) using SiH4 and N2O.
Figure 1. Schematic representation of the process proposed to obtain the foaming of SiOCH thin films.
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Then, the stack was annealed in the UV chamber of a 300 mm Applied Material Producer SE. Typical annealing was performed in He atmosphere on a hotplate (at 400°C) placed under a ultra-violet lamp and for a maximum of 12 min. The ultraviolet lamp had a wide spectral distribution and mirrors were disposed in order to reflect radiation losses on the top of the film to enhance treatment efficiency and uniformity. This UV-assisted thermal annealing (also called UV curing) was preferred to a thermal annealing only because it allows shortening the treatment time and reducing the curing temperature.8 Finally, a crust removal using a 2 min HF vapor etching was performed to allow characterizations of the open porosity.
2.2. Thin film characterizations Several characterizations were performed after each step of the process. Thickness and refractive index (RI) were measured by Variable Angle Spectroscopic Ellipsometry using a MC2000 Woollam ellipsometer. The measurements were recorded at three different angles (55°, 65° and 75°) and from 300 nm to 1700 nm. The thin films were characterized using a Fourier Transformed Infrared (FTIR) spectroscopy using an Accent QS3300 system. Spectra were collected in transmission mode using a resolution of 2 cm-1 and averaged over 32 scans. The background spectrum of each silicon wafer was measured before film deposition. The film thickness and density were investigated at the wafer center by X-Ray Reflectometry (XRR) using a Jordan Valley JVX 5200 tool. After the capping removal, porosity measurements were carried out in the visible range by Ellipsometric - Porosimetry (EP) on an EP12 from SOPRALAB using toluene as solvent. This technique provides information on film porosity and
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pore size distribution.26 Indeed, the analysis of the refractive index change during adsorption and desorption of organic vapors allows describing the pore network.
3. RESULTS AND DISCUSSION SiOCH thin film deposited from 3MS is a well-known low-k dielectric material as it is integrated in the microelectronic industry since the 90 nm technology node. This material, usually deposited at temperature in 350 – 400 °C range, is not intentionally porous (microporous, porosity
