Matrix Distributed Electron Cyclotron Resonance Research Articles

Microcrystalline silicon thin films deposited by matrix-distributed electron cyclotron resonance plasma enhanced chemical vapor deposition using an SiF4 /H2 chemistry

For the growth of hydrogenated microcrystalline silicon (μc-Si:H) thin films by low temperature plasma-enhanced chemical vapor deposition (PECVD), silicon tetrafluoride (SiF4) has recently attracted interest as a precursor due to the resilient optoelectronic performance of the resulting material and devices. In this work, μc-Si:H films are deposited at high rates (7 Å s−1) from a SiF4 and hydrogen (H2) gas mixture by matrix-distributed electron cyclotron resonance PECVD (MDECR-PECVD). Increased substrate temperature and moderate ion bombardment energy (IBE) are demonstrated to be of vital importance to achieve high quality μc-Si:H films under such low process pressure and high plasma density conditions, presumably due to thermally-induced and ion-induced enhancement of surface species migration. Two well-defined IBE thresholds at 12 eV and 43 eV, corresponding respectively to SiF+ ion-induced surface and bulk atomic displacement, are found to be determinant to the final film properties, namely the surface roughness, feature size and crystalline content. Moreover, a study of the growth dynamics shows that the primary challenge to producing highly crystallized μc-Si:H films by MDECR-PECVD appears to be the nucleation step. By employing a two-step method to first prepare a highly crystallized seed layer, μc-Si:H films lacking any amorphous incubation layer have been obtained. A crystalline volume fraction of 68% is achieved with a substrate temperature as low as 120 °C, which is of great interest to broaden the process window for solar cell applications.

Microcrystalline silicon films and solar cells deposited at high rate by Matrix Distributed Electron Cyclotron Resonance (MDECR) plasma

AbstractThe latest results obtained in a new promising Matrix Distributed Electron Cyclotron Resonance (MDECR) reactor for the deposition of microcrystalline silicon (μc‐Si:H) films and cells are reported in this article. The reactor is presented with a focus on the importance and the difficulties of temperature measurements. The unique properties of this high density plasma allowed us to demonstrate high deposition rates of 28 Å/s. The chemical composition of the MDECR material has been compared to a standard material and in particular, the effect of the oxygen level will be discussed. The installation of a load‐lock allowed a reduction of the oxygen con‐tamination by a factor of 10. Lower microwave powers helped to improve the electrical properties of the material. So far, the cell performance unfortunately seems decoupled from these successive optimizations of the film quality, and stayed limited (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Plasma emission diagnostics during fast deposition of microcrystalline silicon thin films in matrix distributed electron cyclotron resonance plasma CVD system

AbstractThe structural and optoelectronic properties of undoped hydrogenated microcrystalline silicon (μ c‐Si:H) films deposited in a matrix distributed electron cyclotron resonance (MDECR) plasma CVD system using pure silane were studied as a function of microwave power (PMW) and gas pressure (Pgas). The analysis of the silane plasma optical emission spectra (OES) shows the effect of the deposition conditions on the plasma chemistry and resulting film microstructure as studied by spectroscopic ellipsometry and atomic force microscopy. The use of the PMW and Pgas as tunable parameters to achieve μ c‐Si:H films with desired microstructure is discussed. Intrinsic μ c‐Si:H films with good phototransport properties were realized at high deposition rate. Our study shows the usefulness of OES as a fast diagnostic tool for controlling and optimizing the μ c‐Si:H deposition process in MDECR reactor. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of ion energy on structural and electrical properties of intrinsic microcrystalline silicon layer deposited in a matrix distributed electron cyclotron resonance plasma reactor

AbstractMicrocrystalline silicon films were deposited in a matrix distributed electron cyclotron resonance (MDECR) plasma enhanced chemical vapor deposition (PECVD) system using pure silane, under varying substrate bias conditions. Microstructural characterization of the films shows a lower void fraction and a preponderance of nanograins in films deposited at negative bias, while in positive bias a thin incubation layer is seen with a higher void fraction. Plasma emission studies reveal higher electron temperature and more atomic H at positive bias, which lead to early onset of crystallization. The microstructural properties of the films are correlated with the dark and phototransport properties. Our study demonstrates the importance of substrate bias in controlling the ion energy and properties of films deposited in the MDECR reactor.

Assignment of High Wave-number Absorption and Raman Scattering Peaks in Microcrystalline Silicon

AbstractIn the optimization of high-growth rate hydrogenated microcrystalline silicon (μc-Si:H) for photovoltaics, the observation of the infrared (IR) absorption peaks around 2000 cm-1 - which originate from silicon hydrogen (Si-HX) bonds in the film bulk - can provide useful information about the properties of the film. If the films are deposited on IR transparent substrates, Fourier Transform infrared (FTIR) spectroscopy is the most commonly used technique to measure the absorption in this regime. The rich spectrum of Si-HX peaks that can be observed in this wavenumber region describes the distribution of the bonded hydrogen amongst the various possible SiHX configurations, and as well, the presence of a pair of narrow, twin peaks at 2085 and 2100 cm-1 is indicative of material that is porous and prone to oxidation upon exposure to air. Many of these high wavenumber peaks are also observable using Raman scattering spectroscopy, allowing one to observe films on substrates that are not IR transparent. As well, the different absorption/scattering cross sections of the two techniques provide a useful tool to understand the origins of the various peaks, and in particular, those of the twin narrow peaks. In this work, we examine the dynamics upon exposure to atmosphere of these twin peaks in μc-Si:H grown by Matrix Distributed Electron Cyclotron Resonance (MD-ECR) PECVD. This deposition technique is an extremely promising one for growing μc-Si:H thin films at high deposition rates, and has demonstrated μc-Si:H film deposition at up to 28Å/s. By using both FTIR and Raman spectroscopy to characterize these films, we examine the differences in the dynamics of each of the twin peaks according to the technique used. In addition, secondary ion mass spectrometry (SIMS) measurements on the films show the physical distribution of oxygen in the films after five months of air exposure, and XRD measurements correlate the presence of these twin peaks with a [111] preferential surface orientation. Using these observations, we assign these absorption and scattering features to specific configurations of Si-HX within the film, and explain both the dynamics of the peaks over time and the similarity of the peaks for a wide range of samples.

Raman scattering analysis of SiH bond stretching modes in hydrogenated microcrystalline silicon for use in thin-film photovoltaics

The presence of a pair of peaks in the high wavenumber infrared (IR) absorption region of hydrogenated microcrystalline silicon (μc-Si:H) has been recently proposed as a strong indicator of poor quality material that is prone to oxidation and is therefore unsuitable for thin-film, photovoltaic applications. In this work, we show that these peaks located at 2083 and 2100 cm −1 are also present in the Raman scattering spectra of μc-Si:H and therefore can be directly measured on substrates that are suitable for solar cells. We present results for material grown by matrix-distributed electron-cyclotron resonance (MD-ECR) plasma-enhanced chemical vapour deposition (PECVD) on both crystalline silicon and borosilicate glass substrates. The narrow, twinned peaks detected by Raman disappear with time—presumably due to oxidation—although a broad peak at 2100 cm −1 remains.

Characterization of Microcrystalline Silicon by High Wavenumber Raman Scattering

AbstractOne of the primary challenges in the application of hydrogenated microcrystalline silicon (μc-Si:H) to photovoltaic cells is achieving high growth rates while maintaining good material quality over a wide process window. The rapid characterization of the material without generating a complete cell is thus a useful tool to determine said process window. Infrared absorption due to the various vibrational modes of the material has been used as a coarse tool towards this purpose, but the use of FTIR to perform this diagnosis limits the substrates upon which the analysis can be performed. We report on the use of high wave-number (1800-2200 cm-1) Raman scattering to perform a similar role of telltale peak detection directly on solar cells and on substrates suitable for thin-film photovoltaics. We evaluate material grown from SiF4 by RF-PECVD and from SiH4 by Matrix Distributed Electron Cyclotron Resonance (MDECR-) PECVD.

Silane injection in a high-density low-pressure plasma system and its influence on the deposition kinetics and material properties of SiO2

High-rate, low temperature deposition is an essential requirement for industrial fabrication technology to be suitable for the deposition of optical and protective coatings. High-density, low-pressure plasmas have received significant attention for such applications due to their ability to create large and controllable ion fluxes onto the substrate. In this study, the high-rate deposition of silica films from a silane and oxygen gas mixture in a high-density plasma system based on a matrix distributed electron cyclotron resonance (MDECR) plasma source is investigated using directional jet injection of undiluted silane. The influence of process parameters such as the microwave power, radio frequency biasing of the substrate holder, and gas flows on the OH content of the oxide films is studied using phase-modulated spectroscopic ellipsometry (SE), Fourier transform infrared (FTIR) spectroscopy, and transmission measurements. The results of the measurements, taken at various points across the wafer, show a decrease in the thickness-normalized OH concentration in the areas of higher deposition rates. The corresponding gas phase composition is investigated using optical emission spectroscopy and compared to the FTIR, transmission and SE measurement results in order to validate our findings and ultimately optimize the deposition process. It is found that the primary silane flux onto the surface, which depends on the positioning of the jet injection point and gas flow rate, plays an important role not only on the deposition rate but also on the OH content of the films. The authors conclude that high-density plasma deposition systems such as the MDECR plasma enhanced chemical vapor deposition system cannot be considered as well mixed for gases with dissociation products that have high sticking coefficients, contrary to the accepted paradigm.

Multilayer optical filters control by multi‐channel kinetic ellipsometry

AbstractThe use of multi‐wavelength kinetic ellipsometry control for the deposition of multilayer filters is presented. Two different control methods are described and their use for the fabrication of different types of optical filters is illustrated. We fit the ellipsometric data with a model and adjust the physical thickness during deposition to ensure the conservation of the optical thickness. In order to illustrate this method and to show its importance in the case of periodic structures or single layer control, a Bragg mirror and a resonator layer in a Fabry‐Perot filter were grown using Matrix Distributed Electron Cyclotron Resonance Plasma Enhanced Chemical Vapor Deposition (MDECR‐PECVD). The use of fitting both the index and thickness for control of the physical thickness is also shown. The importance of this method for manufacturing non‐periodic multilayer structures is demonstrated by the controlled deposition of an antireflection coating. The different features of these two methods are presented. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electron beam poling in amorphous Ge-doped H:SiO2 films

Amorphous Ge-doped H:SiO2 films on silica, deposited by matrix-distributed electron cyclotron resonance – plasma enhanced chemical vapor deposition, were irradiated with an electron beam while varying the dose. Using the Maker fringe method, second-harmonic generation was measured in the irradiated regions of the films. With a current of 5nA, and an acceleration voltage of 25kV for 25s, a Ge-doped H:SiO2 film (3.8at.% Ge) showed a maximum second-order nonlinearity of d33=0.0005pm/V. In contrast, a H:SiO2 film with a smaller Ge content (1.0at.% Ge), showed a large SHG: d33=0.06pm/V when irradiated for 15s. The second-harmonic generation in the films is caused by a frozen-in electric field induced by charge implantation from the electron beam. The strength of the electric field is determined by two conditions: the trapping centers (numbers, depth) and the remaining conductivity under large electric field.

Effect of thermal coupling on the electronic properties of hydrogenated amorphous silicon thin films deposited by electron cyclotron resonance

In photovoltaic devices, rather thin intrinsic layers of good quality materials are required and high deposition rates are a key point for a cost-effective mass production. In a previous study we have shown that good quality amorphous silicon (a-Si:H) films can be deposited by matrix distributed electron cyclotron resonance (MDECR) plasma CVD at very high deposition rates (∼ 2.5 nm/s). However, only thick films (> 1 μm) exhibited good transport properties. A very poor thermal coupling between the substrate holder and the substrate is the main reason for such a behaviour. We present here experimental data which support this conclusion as well as the improved transport and defect-related properties of new very thin a-Si:H samples (thickness around 0.3 μm) deposited at a higher temperature than the previous ones.

Deposition of Ge-doped silica thin films for an integrated optic application using a matrix distributed electron cyclotron resonance PECVD reactor

Optical quality Ge-doped SiO2 thin films, suitable for an integrated optic version of a gain equalizer for erbium-doped fibre amplifiers (EDFAs), have been deposited using a matrix distributed electron cyclotron resonance plasma-enhanced chemical vapour deposition (MDECR-PECVD) system. Using spectroscopic ellipsometry and infrared transmission spectroscopy, the optical constants and hydroxyl content of the films were calculated. Losses due to the hydroxyl overtone at 1.37μm are found to be approximately 0.251dB/cm. An RBS analysis determined the germanium content of the films to be in the vicinity of 4 at.%. A comparison of the atomic percentage of germanium in the films and their corresponding refractive indices with values obtained using other deposition methods is also discussed.

Control and monitoring of optical thin films deposition in a Matrix Distributed Electron Cyclotron Resonance reactor

A range of silicon-based optical thin films have been deposited in a matrix distributed electron cyclotron resonance (MDECR) reactor. Process parameters were optimized in order to obtain optical quality thin films at low substrate temperatures and high deposition rates without post-deposition treatment. Stoichiometric silica films have been deposited at the rates up to 70 nm/min at temperatures lower than 150 °C. Oxynitride films with a controllable refractive index ranging from 1.46 to 1.86 have been obtained from SiH 4 /O 2 /N 2 mixtures. Real time process control by multichannel ellipsometry has been implemented and successfully applied for the deposition of silica, silicon oxynitrides and amorphous silicon. Better than 0.3% in thickness accuracy was achieved in high rate deposition of silica layers of various predefined thickness. Refractive indices were determined in real-time with an absolute precision of 0.005-0.02. The control algorithm was used for fabrication of multilayer optical filters. The results show that the MDECR concept coupled with real-time process control by ellipsometry can be technology of choice for the deposition of interference coatings.

High density plasma enhanced chemical vapor deposition of optical thin films

Deposition of pure and Ge-doped silica as well as silicon oxynitride films has been studied in a recently developed matrix distributed electron cyclotron resonance (MDECR) reactor. Process parameters were optimized in order to obtain optical quality thin films at low substrate temperatures and high deposition rates without post-deposition treatment. The choice of injection system is shown to be of crucial importance for the deposition of high quality materials in low pressure PECVD. It has been found that injecting silane near the surface allows to obtain films with a low OH absorption independently of silane flow i.e. growth rate in a certain range of process parameters. On the contrary, in the case of uniform distribution of silane in the reactor volume the hydrogen content increases with silane flow, which affects the quality of films deposited at higher rates. With the optimized injection system, stress-free silica films with a low absorption have been deposited at the rates up to 70 nm/min at temperatures lower than 150 °C. Non-absorbing oxynitride films with a controllable refractive index ranging from 1.46 to 1.86 have been obtained from SiH 4 /O 2 /N 2 mixtures. Ge-doped silica films with a Ge content of up to 4% has been deposited using a mixture GeH 4 in H 2 as a dopant. The properties of deposited films have been studied as a function of process parameters. The results show that the MDECR concept, that permits, in principle, unlimited scaling of substrate size, can be technology of choice for the deposition of optical thin films and functional coatings.