Surface-Electric Field

+ –

Surface-Electric Field

E

E

Fig. 9-10 Proposed surface-electric field in the Cu-comb capacitor studied in this work.

Cu-to-Cu Distance (nm)

0 20 40 60 80 100 120

Surfa ce-Electric Field (MV /cm)

1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4

SiC SiCO

Raphael Simulation for Cu-Comb Capacitors

Cu

Cu

Cu Cu

Cu

Cu

Fig. 9-11 Surface-electric field obtained from Raphael simulation for the Cu-comb capacitors with α-SiC and α-SiCO cap-barriers biased with an electric voltage of 24 V (equivalent

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Chapter 10

Octamethylcyclotetrasiloxane-Based α-SiCO Dielectric Barrier

10-1 Introduction

The PECVD α-SiC, α-SiCN, and α-SiCO dielectric barrier films deposited using organosilicate gases are receiving extensive attention for applications as Cu cap-barrier and etching stop layer in Cu damascene structures [1-3], and were also studied in the previous chapters of this thesis [4]. The 3MS- or 4MS-based α-SiC and α-SiCN dielectric barrier films show k-values in the range of 4 to 5 [1-4], while the α-SiCO dielectric barrier films deposited using hexamethydisiloxane (HMDSO) or trimethoxysilane (TMOS) precursor exhibit a lower k-value of 3.9 [5,6]. In chapters 8 and 9, we found that the α-SiCO dielectric barrier film deposited using 3MS precursor with He carrier gas and 1200 sccm CO2 reaction gas exhibits a k-value of 3.7. Moreover, it has been reported that the α-SiCO dielectric barrier film deposited using 3MS precursor with He carrier gas and O2 reaction gas exhibits an even lower k-value of 3.3 [7]. In this work, we investigate the thermal stability and physical and barrier properties for three α-SiCO dielectric barrier films, with dielectric constants between 2.8 and 6.3, deposited using octamethylcyclotetrasiloxane (OMCTS) precursor and He carrier gas with and without O2 reaction gas.

10-2 Experimental Details

Three α-SiCO dielectric barrier films deposited using OMCTS {[(CH3)2SiO]4} precursor and He carrier gas with and without O2 reaction gas are investigated with

respect to their thermal stability and physical and barrier properties. OMCTS is a cyclic structured silicone compound with low water solubility and high vapor pressure. It is a highly safe compound because it converts into SiO2, H2O, and CO2 in the ambient of atmosphere. In this study, all of the α-SiCO films were deposited to a thickness of 50 nm on p-type, (100)-oriented Si wafers (8~12 Ω-cm resistivity) at a temperature of 350^oC, a total gas pressure of 7 Torr, and a plasma power of 600 W using a parallel-plate PECVD system operated at 13.56 MHz. The flow rate of OMCTS precursor was maintained at 800 sccm, while the flow rate of O2 reaction gas was separately controlled at 0, 200, and 300 sccm, which resulted in three α-SiCO films with different elemental compositions. All deposited films were thermally annealed at 400^oC for 30 min in N2 ambient to remove moisture possibly absorbed in the dielectric barrier films prior to the investigation of the films’ physical property or the deposition of electrode (TaN/Cu or Al) to construct the metal-insulator-semiconductor (MIS) capacitor structure. The TaN/Cu-gated MIS capacitors were constructed by first sputter-depositing a 200-nm-thick Cu layer on the α-SiCO dielectric barrier film using a dc magnetron sputtering system, followed by a reactive sputter deposition of a 50-nm-thick TaN layer on the Cu surface in the same sputtering system without breaking the vacuum. The TaN film served as a passivation layer to prevent the Cu electrode from oxidization during the subsequent high-temperature processes. For a comparison, Al-gated control samples were also prepared by depositing a 500-nm-thick Al layer directly on the

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completed MIS samples were thermally annealed at 400^oC for 30 min in N2 ambient.

This annealing step eliminates the plasma-induced damage that may have occurred during the sputter deposition of the TaN/Cu electrodes and also provides the driving force for Cu diffusion.

Fourier transform infrared spectroscopy (FTIR, ASTeX PDS-17 System) was used to analyze the chemical bonding of the dielectric barrier films. The film thickness and refractive index were measured using a well-calibrated measurement system (n&k analyzer 1200) at 633 nm wavelength, and the k-value of the dielectric barrier films was determined by the maximum capacitance of the Al-gated MIS capacitors measured at 1 MHz using a C-V measurement system (Keithley 82). A semiconductor parameter analyzer (HP4145B) was used to measure the dielectric leakage current and provide the bias for the bias-temperature-stress (BTS) test. Secondary ion mass spectrometry (SIMS) was used to detect the penetration of Cu in the dielectric barrier films.

10-3 Physical Property and Thermal Stability

Table 10-1 shows the basic data of the three OMCTS-based α-SiCO dielectric barrier films studied in this work. The α-SiCO dielectric barrier films deposited with O2 flow rate of 0, 200, and 300 sccm are designated as SiCO-0, SiCO-2, and SiCO-3, respectively. It is found that the FTIR Si-O(1020 cm^-1)/Si-C(800 cm^-1) absorbance peak ratio increases with increasing flow rate of O2 reaction gas during the deposition of the α-SiCO dielectric barrier films. However, the refractive index of the α-SiCO dielectric barrier films decreases with the addition of O2 reaction gas. This result of observation is consistent with those reported in the literature regarding various α-SiCO dielectric films deposited using O2/3MS, N2O/HMDSO, O2/4MS, and N2O/4MS gases [7-10]. Dielectric constant at 1 MHz consists of three components arising from the

contribution of electronic, ionic, and dipolar dielectric constant [7,8], as shown in Eq.

(10-1):

k(at 1 MHz) = ke(n²) + kion + kdipolar (10-1)

The electronic contribution arises from the displacement of the electron shell relative to a nucleus. The ionic contribution comes from the displacement of a charged ion with respect to other ions, and the dipolar contribution arises from the change of orientation for the molecules with a permanent electric dipole moment in an applied electric field.

Although the electronic dielectric constant, which equals the square of refractive index [7,8], decreases with the addition of O2 reaction gas, the dielectric constant (@ 1 MHz) of the α-SiCO dielectric barrier films increases with increasing flow rate of O2 reaction gas during the film’s deposition process. Moreover, the ionic and dipolar dielectric constants, which are obtained by subtracting the electronic dielectric constant from the dielectric constant at 1 MHz [7,8], increase with increasing flow rate of O2 reaction gas.

The reduction in the electronic dielectric constant of the α-SiCO dielectric barrier films is attributed to the increase of Si-O bond, which is more ionic- and dipolar-polarizable than the Si-C bond [7,8,11,12]. Thus, an α-SiCO film’s dielectric constant, which is dominated by the electronic polarization, would decrease with increasing incorporation of oxygen [7].

Nevertheless, when the strong ionic and dipolar polarizations of the Si-O bonds become predominant in the dielectric constant of an α-SiCO film, the film’s dielectric constant is expected to increase with increasing flow rate of oxygen-containing reaction gas

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defects tend to contain a high density of bound water because of the film’s larger effective surface area [13-15], e.g. the as-deposited SiCO-2 and SiCO-3 dielectric barrier films whose FTIR spectra reveal a broad peak of H-OH bond at 3480 cm^-1, and a clear Si-OH peak at 975 cm^-1 observed in the SiCO-3 film, as shown in Fig. 10-1. Such a susceptibility to moisture may significantly increase the dielectric constant because of the incorporation of H2O (k=80) and adversely affect the electrical reliability of the dielectric film. The bound moisture (H-OH at 3480 cm^-1 and Si-OH at 975 cm^-1) disappeared completely after the films were thermally annealed at 600^oC for 30 min in N2 ambient. The absorbance of Si-C (800 cm^-1) and Si-CH3 (1265 cm^-1) peaks decrease, whereas that of the Si-O (1020 cm^-1) peak increases for the films with increased flow rate of O2 reaction gas during the films’ deposition. Notably, all films show a tiny Si-O shoulder at 1100 cm^-1, indicating the existence of the caged Si-O structure in the dielectric films [10,13], which may result from the OMCTS cyclic precursor. The slight Si-H (2102 cm^-1) peak appeared only in the SiCO-0 film, and the small C-H (2962 cm^-1) peak was detected only in the SiCO-0 and SiCO-2 films, whereas a distinguishable C-O shoulder at 1200 cm^-1 was observed only in the SiCO-3 film. Figure 10-2 shows the FTIR absorbance peak ratios of Si-CH3(1265 cm^-1)/Si-O(1020 cm^-1) and H-OH(3480 cm^-1)/Si-O(1020 cm^-1) and the thickness shrinkage for the α-SiCO dielectric barrier films thermally annealed at various temperatures for 30 min in N2 ambient. Notably, only the SiCO-0 film exhibits chemical outgassing of the CH3 group above 550^oC, whereas only the SiCO-2 and SiCO-3 films exhibit desorption of moisture at temperatures above 400^oC. With regard to the film thickness, the thickness of all films remained nearly constant at temperatures up to 400^oC. Upon annealing at 550^oC, the shrinkage of the SiCO-0 film remained below 1%; at 600^oC, however, the film shrank for more than 2%, presumably due to chemical outgassing, leading to partial removal of the CH3 group (Fig. 10-2a). For the SiCO-2 and SiCO-3 films, severer shrinkage (>2%) was