Investigation of the densification mechanisms and corrosion resistance of amorphous silica films

Highlights

• CVD processed amorphous silica films from TEOS+O2 become denser with increasing deposition temperature from 400 to 550 °C.

• The LO₃ FTIR vibration mode presents a blue shift with increasing process temperature corresponding to film densification.

• The densification corresponds to a decrease and ultimately elimination of water and silanol groups.

• The resistance to P-etch test of amorphous silica films increases with increasing deposition temperature

• Annealing CVD silica films at 800°C in dry atmosphere provides them with state of the art P-etch corrosion resistance.

Abstract

The barrier properties of the technologically attractive amorphous silica films depend on their structural characteristics at the atomic level, which, in turn are strongly influenced by the deposition conditions. In this paper, we propose an investigation of the poorly investigated densification mechanism of amorphous SiO₂ films processed by CVD from TEOS and O₂ between 400 and 550 °C. Based on literature survey and our original experimental results, we show that the densification process of these films, occurring with increasing the deposition temperature, is highlighted by a decrease of the water and silanol content, probed by transmission FTIR. We discuss the evolution of Si-O-Si related vibration signatures and we use the central force model to correlate the LO₂ and LO₃ shifts with the decrease of the Si-O-Si bond force constant, when the deposition temperature increases. Nuclear analysis reveals that films processed below 525 °C present hydrogen content between 5 ± 0.3 and 7 ± 0.3%at. Ellipsometry measurements attest that films processed at 550 °C are close to O/Si silica stoichiometry and hydrogen free. We show that application of the P-etch test results in particularly low erosion rate of 10 Å.s⁻¹ for dense films processed at 550 °C.

Introduction

Thin and dense SiO₂ films processed from tetraethyl orthosilicate (Si(OC₂H₅)₄, TEOS) by chemical vapor deposition (CVD) have long been considered to solve mainly microelectronic issues, such as copper diffusion barrier, dielectric capacitor and intermetallic dielectric layers for multilayer metallization systems [1]. Nowadays, such films remain key enabling materials in innovative applications and devices [[2], [3], [4], [5], [6]]. These applications have in common a low thermal budget process requirement due to thermally sensitive, or 3D-complex geometry substrates. Addition of oxygen [7] or ozone O₃ [8] in the gas phase or the use of plasma assistance (PECVD) [9,10] are well known actions resulting in lower thermal budget processes, yielding silica films at moderate temperatures, typically lower than 600 °C. More recently, atomic layer deposition (ALD) of SiO₂ thin films has also been performed at low temperature to solve optical applications issues [11] or to cap nanoporous anodic alumina membranes employed in biosensor devices [12]. The correlation between the effects of such low temperature deposition and the atomic arrangement of the silica structure is not well understood, especially considering that the densification process is complex and multi-parameter dependent [13]. However, the atomic arrangement of the material is the key to understand its targeted barrier properties when such silica films are used as barrier coatings [4,5] or to understand the special selectivity of gases when it is used in gas membranes [6].

The structure of the amorphous SiO₂ films has been defined by Sen and Thorpe [14] and Galeener [15] as a short range organized continuum built-up from tetrahedral entities centered on a silicon atom. Each oxygen atom at the corner of a tetrahedron is shared by another tetrahedral unit and cross-links the entire network. Twofold-coordinated bridging oxygen is more mobile than fourfold-coordinated silicon and has been considered as the main contributor to the atomic vibration of this system [16]. Vibration modes of this oxygen are intimately related to the SiO₂ structure and, consequently, the spectral changes under densification should reveal information on the atomic distribution of the network. Three vibration modes of the oxygen atom linked with two silicon atoms are assigned in the mid infrared (IR) region between 400 cm⁻¹ and 4000 cm⁻¹. These are the transversal optical rocking (TO₁), bending (TO₂) and asymmetric stretching (TO₃) modes, which are observed respectively at around 450 cm⁻¹, 800 cm⁻¹ and 1070 cm⁻¹ [16]. The TO₁ mode is sensitive to structural changes [17] but its frequency shift is too weak to be monitored by FTIR, i.e. the shift is in the same order of magnitude with the measurement uncertainties. An additional vibration mode, the TO₄, is assumed by some authors between 1050 cm⁻¹ and 1250 cm⁻¹ [[18], [19], [20]]. It has been attributed to the out of phase vibrations of the TO₃ mode [20,21], to the longitudinal, LO₃ mode in this area [22], to the presence of residual TEOS molecules [23] and to strained 3 to 6-fold SiO rings [19,24,25]. The debate around the attribution of a physical phenomenon to the TO₄ broad band reveals the difficulty to correlate the existence and the characteristics of this vibration with the short-range organization of the network. This difficulty may be attributed to the intrinsic nature of the material, which is amorphous, with randomly crosslinked tetrahedral entities [15]. Thus, the overall characteristics of CVD SiO₂ films, such as stoichiometry [26,27], porosity [28], impurities [29,30] and mechanical strain [24] which depend on the process conditions, will affect the FTIR signature, the evolution of the refractive index and consequently they may bias the interpretation of the densification process.

This literature review reveals that, despite uncertainties on the origin of some vibrations, FTIR is an appropriate technique to monitor the evolution of the silica network, and for this reason it will be used in the present work. We will adopt the central force model proposed by Sen and Thorpe [14] and applied to glasses by Galeener [15] in order to link FTIR signatures to structural changes. This model connects SiO₂ vibrations to the Si-O-Si intertetrahedral bond angle, θ, and the SiO force constant, α, of the SiO₂ network [31]. Nonetheless, the model assumes perfect stoichiometric, continuous SiO₂ network and it takes into account neither composition deviations, namely the offset with regard to the nominal O/Si ratio, nor the presence of heteroatoms such as carbon impurities, nor the porosity, nor the mechanical strain that can be observed in CVD SiO₂ films. Consequently, and in order to elaborate a scenario of the evolution of the structure under different process temperatures (T_d), we perform complementary characterizations to get insight on these parameters: we quantify by nuclear analysis the concentrations of silicon, oxygen and hydrogen; we determine both the refractive index by reflection spectroscopic ellipsometry (SE) using the Sellmeier model, and the porosity by ellipsometric porosimetry (EP) measurements using the Cauchy model. Finally, we apply selective chemical corrosion tests proposed by Pliskin [13,32] in order to correlate the evolution of the structure with its intrinsic resistance towards aggressive medium.