Digital Control of SiO₂ Film Deposition at Room Temperature

Abstract

In this Letter, we propose and demonstrate a robust process for digital control of high quality SiO 2 thin films at room temperature using pulsed plasma-enhanced chemical vapor deposition (PECVD). Plasma activation of the SiCl 4 precursor is critical, as atomic layer deposition does not occur under these conditions. Subangstrom control over deposition rate was obtained by adjusting the density of SiCl 4 present at plasma ignition. The intrinsic refractive index of 1.46 was obtained at rates between 0.8 and 1.6 Å/pulse. No impurities were detected by either X-ray photoelectron spectroscopy (XPS) or Fourier transform infrared (FTIR) in films deposited under optimal conditions. Atomic layer deposition (ALD) is a leading technique for thin film deposition. Growth proceeds through cycles of sequential, self-limiting surface reactions that impart nanoscale control over thickness and composition. Since its inception over three decades ago, 1 ALD has been refined and successfully demonstrated for numerous materials, particularly metal oxides. 2 However, a number of materials have proven difficult to deposit by ALD. Perhaps the most auspicious example is silicon dioxide, given its role as the most important and widely used dielectric in the integrated circuit industry. 3 The outstanding chemical, optical, and electrical properties of vapor-deposited SiO 2 make it an important component for photovoltaics, 4,5 optics, 6 and barrier applications. 7,8 Continued miniaturization of device structures and the extension to flexible substrates requires deposition techniques that impart nanoscale control over thickness at low temperature. Despite the critical importance of SiO 2 and maturation of ALD technology, a robust, low temperature process for digital control of SiO 2 growth remains elusive.A primary challenge to SiO 2 ALD is overcoming the limited reactivity of silicon-containing precursors. The George group first demonstrated silica ALD using SiCl 4 and H 2 O as coreagents, with the two separate half-reactions combining to produce the overall reaction SiCl 4 + 2H 2 O f SiO 2 + 4HCl. 9 However, this group also acknowledged the limitations of this process including high temperature requirements (600-800 K), large reactant exposure (∼10 9 L, 1 L ) 10 -6 Torr · s), and the production of corrosive HCl as a reaction product. 10 Low temperature (<200°C ) processing is critical for extending ALD to temperaturesensitive substrates and biological applications and to maintain device integrity. To this end, a number of groups have developed catalytic reaction strategies in order to reduce the temperature requirements. 10-12 Though high rates 12 and room temperature 10,11 synthesis have been demonstrated, these processes create added complexity and raise concerns about impurity incorporation. 13 Plasma-enhanced ALD (PE-ALD) has been demonstrated to reduce temperature requirements by employing plasma generated radicals (O) to complete the oxidation step. 14 Jiang and co-workers accomplished PE-ALD of silica at room temperature using TEOS ...

Digital Control of SiO2−TiO2 Mixed-Metal Oxides by Pulsed PECVD

“…To achieve digital control over SiO 2 growth with angstrom-level resolution, SiCl 4 was pulsed into the reactant stream by opening an electronic valve on a fixed control volume. The amount of SiCl 4 was controlled by adjusting the duration of this pulse, as described previously (9). The TiCl 4 /SiCl 4 /Ar/O 2 streams were mixed and delivered to the reactor through a showerhead, which also served as the powered electrode.…”

“…The precursors for the individual materials were silicon tetrachloride (SiCl 4 ) and titanium tetrachloride (TiCl 4 ), respectively. Details on the synthesis of the pure SiO 2 and TiO 2 films are presented elsewhere. SiO 2 −TiO 2 alloys were deposited by introducing both metal precursors to the reactor along with Ar and O 2 gases while modulating the plasma power at low frequency (∼0.25 Hz).…”

“…We have recently extended pulsed PECVD to the synthesis of SiO 2 (9) and TiO 2 (10) using halide precursors (SiCl 4 , TiCl 4 ). An advantage of this chemistry is the absence of HCl or H 2 O production, which is critical for vacuum processing at low temperatures.…”

“…Pulsed plasma-enhanced chemical vapor deposition (PECVD) is an alternative to atomic layer deposition (ALD) for self-limiting growth of metal oxides. Pulsed PECVD has been successfully demonstrated for a number of binary oxides, including Ta 2 O 5 , , Al 2 O 3 , , and ZnO . In pulsed PECVD the metal precursor and O 2 are mixed and delivered simultaneously.…”

Influence of pulse power amplitude on plasma properties and film deposition in high power pulsed plasma enhanced chemical vapor deposition

“…For this, a fundamental understanding of how the plasma chemistry and thereby the deposition process can be a) Electronic mail: henke@ifm.liu.se controlled by the plasma discharge is first needed. A step towards such an understanding is the finding that by pulsing the plasma in "standard" PECVD, i.e., by switching it on and off, and carefully selecting the precursor chemistry, selflimiting growth, similar to atomic layer deposition, of metal oxides [8][9][10][11] and recently also sulphides, 12 can be achieved without pulsing the precursor gases. The pulsed nature of HiPP-PECVD could potentially further advance this type of PECVD.…”