On the nitrogen and oxygen incorporation in plasma-enhanced chemical vapor deposition (PECVD) SiOxNy films

doi:10.1016/S0040-6090(01)01685-6

Thin Solid Films

Volume 402, Issues 1–2, 1 January 2002, Pages 154-161

https://doi.org/10.1016/S0040-6090(01)01685-6 Get rights and content

Abstract

Silicon oxynitride films were deposited by plasma-enhanced chemical vapor deposition at low temperatures using nitrous oxide (N₂O) and silane (SiH₄) as gas precursors. The influence of the N₂O/SiH₄ flow ratio (varied from 0.25 up to 5) and the thickness of the films on the optical and structural properties of the material was analyzed. The films were characterized by ellipsometry, Fourier-transform infrared spectroscopy, Rutherford backscattering spectroscopy and optical absorption. Two distinct types of material were obtained, silicon dioxide-like oxynitrides SiO_2−xN_x and silicon-rich oxynitrides SiO_xN_y (x+y<2). The results demonstrate that in silicon dioxide-like material, the nitrogen concentration can be adequately controlled (within the range 0–15 at.%) with total hydrogen incorporation below 5 at.% and no appreciable SiH bonds. It is also shown that the composition remains uniform through the entire thickness of the films. Furthermore, a linear relation between the refractive index and the nitrogen concentration is observed, which makes this material very attractive for optoelectronic applications. On the other hand, silicon-rich material is similar to amorphous silicon, and presents an increasing concentration of SiH bonds, increasing refractive index and decreasing optical gap, which makes it promising for applications in light-emitting devices.

Introduction

The plasma-enhanced chemical vapor deposition (PECVD) technique has been extensively utilized for the deposition of silicon-based dielectric films for applications as a passivating layer, a gate insulator in metal–oxide semiconductor structures and in optical devices [1], [2], [3], [4], [5], [6], [7], [8]. The fundamental advantage of this technique is that it enables control of the structural, mechanical and optical properties of the films deposited by adequately adjusting the deposition parameters [9], [10], [11], [12], [13], [14].

In particular, the utilization of SiO₂ and SiO_xN_y films deposited by PECVD at low temperatures for optical device fabrication became very attractive due to the possibility of integrating the optical and electrical devices in the same chip [15], [16], [17]. At the same time, the advanced knowledge in the use of these materials in microelectronics could be easily applied to the integrated optics field [4], [7], [18], [19].

On the other hand, for application in optical devices, it is necessary to produce thicker films of approximately 3–4 μm. This makes the PECVD technique even more attractive due to the high deposition rates that can be attained with it [20], [21], [22], and which are not achievable by standard thermal-oxidation processes. In addition, thermally grown films exhibit high internal stress, which might produce cracking of the thicker films [21], [23]. Furthermore, the fact that the composition of the films can be controlled by the PECVD technique implies that films with different refractive index (n) can be deposited; this is a fundamental requirement for waveguide applications. However, due to the low temperatures and to the variety of reactions taking place in the plasma, undesirable SiOH and SiH bonds are present in the films [12], [24], [25], significantly increasing optical losses in the spectral region of interest in optical devices [26]. In the present work, we study deposition conditions that lead to an appropriate control of the refractive index, and at the same time prevent the incorporation of the undesirable SiOH and SiH bonds. Furthermore, a shift in the SiO stretching frequency, measured by IR absorption, with film thickness has been observed [27]. This parameter is associated with the angle of the SiOSi bridge, as well as with the material stoichiometry, which means that either the composition or the structure of the films is thickness-dependent; in this work, we also address this question.

The search for photoconducting and luminescent materials compatible with silicon-based integrated-circuit processing technology is another need for optoelectronic applications, and has received great attention in recent years [28], [29]. In this work, conditions that lead to silicon-rich material for photoluminescent and photoconducting applications are also investigated.

Section snippets

Experimental details

The SiO_xN_y films studied in this work were deposited in a standard 13.56-MHz RF PECVD capacitively coupled system described elsewhere [30], from appropriate gaseous mixtures of electronic grade (99.999%) silane (SiH₄) and nitrous oxide (N₂O).

Films with different nitrogen, silicon and oxygen content were obtained by varying the N₂O/SiH₄ flow ratio (R) and maintaining all other deposition conditions constant. Series of samples with similar thickness (∼100 nm) were grown with flow ratios ranging

Results

The infrared spectra for the set of thin samples (100 nm) deposited with varying N₂O/SiH₄ flow ratio are shown in Fig. 1. It should be mentioned that in Fig. 1, the 1500–4000-cm⁻¹ spectral region was magnified three-fold, in order to appreciate better the NH and SiH bands. For high N₂O/SiH₄ flow ratio, the samples present a spectrum very similar to thermally grown SiO₂, exhibiting only the characteristic SiO stretching, bending and rocking modes. As the flow ratio is decreased, nitrogen

Discussion

The FTIR results show that as the flow ratio is decreased, nitrogen concentration first increases, which becomes evident through SiN and NH FTIR absorption bands that continue to increase up to a value of R=1.5; for lower R, decreasing concentrations of NH bonds and increasing SiH bonds are observed. These must be accompanied by an increase in SiSi bonds, which is not evident from the FTIR results. However, the optical data and the RBS results are compatible with this hypothesis. For the

Conclusions

It was demonstrated that RF PECVD utilizing N₂O/SiH₄ mixtures as gaseous precursors leads to different material types, depending on the N₂O/SiH₄ flow ratio. For flow ratios in the range 5–2, a silicon dioxide-like oxynitride, SiO_xN_y with x+y=2 is obtained, and silicon-rich amorphous silicon-like oxynitride material for N₂O/SiH₄ flow ratios less than 2. The RBS results give strong evidence for the hypothesis that in the silicon dioxide-like films, N atoms enter into the network and substitute O

Acknowledgements

The authors acknowledge Ricardo Oliveira for help with sample characterization and Manfredo H. Tabacniks for the RBS measurements. The authors are also grateful to Brazilian agency FAPESP for financial support.

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Citation Excerpt :
Either by decreasing the gas flow rate of N2O or decreasing the gas flow rate of SiH4, SiHn becomes redundant and starts to combine with N and H radicals, which results in the formation of Si-N and Si-H bonds, even though less N2O leads to less N radicals [28]. On the other hand, increasing SiHn radicals can also lead to a high silicon concentration in the film, which results in a silicon rich SiOxNy film [28]. This can be confirmed by the Si-H peak shifts in Fig. 6b, which show the Si-H band shifting towards a lower wavenumber with decreasing N2O or increasing SiH4 [41].
We describe the optimization of an ultra-thin silicon oxynitride (SiO_xN_y) layer deposited by plasma enhanced chemical vapor deposition (PECVD) as an interfacial layer for phosphorus doped polysilicon (poly-Si) passivating contacts. Our results demonstrate the possibility of depositing the thin interfacial layer and the intrinsic amorphous silicon (a-Si) film in a single PECVD process. We found that the gas flow rates strongly influence the properties of the SiO_xN_y layers, such as the refractive indices, chemical bond compositions and structural stabilities, which significantly affect the properties of the resulting polysilicon passivating contact structures. The passivation quality initially increased and then decreased with a decreasing N₂O/SiH₄ flow ratio, in the gas flow range of uniform deposition, while the contact resistivity decreased significantly. We found an optimal gas flow ratio of N₂O:SiH₄:N₂ = 25:9:361, with which we obtained uniform polysilicon passivating contacts with a high implied open-circuit voltage (iV_oc) of 711 mV and a low contact resistivity ρ_c of 6.6 mΩ cm².

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On the nitrogen and oxygen incorporation in plasma-enhanced chemical vapor deposition (PECVD) SiOxNy films

Abstract

Introduction

Section snippets

Experimental details

Results

Discussion

Conclusions

Acknowledgements

Thin Solid Films

Microelectron. Eng.

Sensor Actuators B: Chem.

J. Non-Cryst. Solids

J. Eur. Ceram. Soc.

Vacuum

Microelectron. Eng.

Solid-State Commun.

Sensor Actuators A: Phys.

Opt. Commun.

Thin Solid Films

Thin Solid Films

Thin Solid Films

Sensor Actuators A: Phys.

Thin Solid Films

Microelectron. Reliab.

Thin Solid Films

On the nitrogen and oxygen incorporation in plasma-enhanced chemical vapor deposition (PECVD) SiO_xN_y films