Deposition Of Microcrystalline Silicon Films Research Articles

Absolute density of precursor SiH3 radicals and H atoms in H2-diluted SiH4 gas plasma for deposition of microcrystalline silicon films

Microcrystalline hydrogenated silicon films were produced at a high deposition rate of about 2 nm/s by using a capacitively coupled plasma under a practical pressure of around 1 kPa. The SiH4 source gas was almost fully dissociated when highly diluted with H2 gas, and the dominant species in the gas phase were found to be SiH3 radicals, which are film-growth precursors, and H atoms. The absolute density of these species was measured as the partial pressure of SiH4 gas was varied. With the increasing SiH4 gas flow rate, the SiH3 radical density, which was on the order of 1012 cm−3, increased linearly, while the H-atom density remained constant at about 1012 cm−3. The film growth mechanism was described in terms of precursors, based on the measured flux of SiH3 radicals and H atoms, and the relative fraction of higher-order radicals.

Analysis on steady plasma process of high-rate microcrystalline silicon by optical emission spectroscopy

The ratio of I(Hα*)/I(SiH*), obtained from the real time optical emission spectroscopy (OES) measurement in the high-rate microcrystalline silicon deposition process, as a function of time is used to analyze the cause of increasing crystallinity along the growth direction. Hydrogen dilution gradient method which means silane concentration gradient and hydrogen flow gradient method is adopted to improve vertical structure uniformity of the material. High-quality microcrystalline material around 53%-62% of Xc can be prepared through silane concentration gradient compared with 55%-75% of Xc prepared in the traditional method. In the silane depleted cases, by increasing the hydrogen flow the longitudinal uniformity of the material can be effectively improved. The vertical crystallinity around 53%-60% can be obtained. This is mainly due to the increase of the hydrogen flow that makes the collision probability increased, as a result, electron temperature of plasma reduced. Thus, the decomposition of hydrogen decreases and the reaction of hydrogen annihilation is suppressed. At the same time, the influence of back diffusion of SiH4 is suppressed. The gradually increasing trend of the ratio of I(Hα*)/I(SiH*) is controlled during the deposition of microcrystalline silicon film.

Influences of deposition parameters on micro-crystalline silicon single junction cell efficiency in large-area and high rate deposition

For the mass production of silicon thin film solar cell, it is a key issue to increase deposition rate of microcrystalline silicon film on a large area substrate. One of the most effective methods to increase deposition rate of the film is using high frequency and high power plasma. However, high frequency plasma leads to plasma non uniformity in large area deposition, and high power plasma induces substrate heating during deposition of microcrystalline silicon film and degrades the film quality. In this study, we developed a shaped electrode for spatially uniform plasma and a susceptor cooling system for preventing substrate heating by plasma. And also we investigated the influences of deposition parameters on the microcrystalline silicon single junction cell efficiency with high frequency and high power plasma. Glass substrate was 1.1 m × 1.3 m and plasma excitation frequency was 40 MHz. Capacitively coupled parallel plates RF reactor with concave shaped electrode was used. For the high deposition rate of the microcrystalline films, plasma powers of 0.8∼1.1W/cm2 were used. The highest deposition rate of microcrystalline silicon was 2nm/s. The increases of temperature of susceptor were monitored by two thermocouples which were installed at center and outer side of the susceptor. The temperature increases were suppressed to almost zero with liquid cooling systems. Multiple step deposition was performed which means the higher H2/SiH2 ratio at the initial stage of film growth was used and the ratio gradually decreased during deposition in order to eliminate the amorphous layer at the interface between substrate and film. We also achieved microcrystalline single junction cell efficiencies above 6% on large area substrate at high deposition rate of 1.5nm/s.

High rate deposition of microcrystalline silicon films by high-pressure radio frequency plasma enhanced chemical vapor deposition (PECVD)

Hydrogenated microcrystalline silicon (μc-Si:H) thin films were prepared by high-pressure radio-frequency (13.56 MHz) plasma enhanced chemical vapor deposition (rf-PECVD) with a screened plasma. The deposition rate and crystallinity varying with the deposition pressure, rf power, hydrogen dilution ratio and electrodes distance were systematically studied. By optimizing the deposition parameters the device quality μc-Si:H films have been achieved with a high deposition rate of 7.8 A/s at a high pressure. The Voc of 560 mV and the FF of 0.70 have been achieved for a single-junction μc-Si:H p-i-n solar cell at a deposition rate of 7.8 A/s.

Deposition of microcrystalline silicon films at ultrafast rate by using a new microwave induced plasma source

Synthesis of microcrystalline silicon (μc-Si) film at an ultrafast deposition rate over 100nm/s is achieved from SiH4+He by using a high density microwave plasma source even without employing H2 dilution and substrate heating techniques. Systematic deposition studies show that high SiH4 flow rate and working pressure increase film deposition rate while high He flow rate decreases the rate. On the other hand, crystallinity of deposited Si film decreases with increasing SiH4 or He flow rate and working pressure. Enhancements of gas phase and surface reactions during film deposition process are responsible for the achievement of high deposition rate and high film crystallinity.

Large-Area and High-Speed Deposition of Microcrystalline Silicon Film by Inductive Coupled Plasma using Internal Low-Inductance Antenna

A novel inductively-coupled RF plasma source with internal low-inductance antenna (LIA) units was developed to synthesize microcrystalline silicon (µc-Si) film on a large glass substrate. A film thickness profile on a 600×720 mm2 glass substrate was achieved with high plasma uniformity and a variation of less than ±5% without a standing-wave effect. Raman and transmission electron microscope (TEM) analysis revealed that highly crystallized µc-Si films, which were directly deposited on a glass substrate, were synthesized without an amorphous-phase incubation layer at the substrate interface. A bottom-gate thin-film transistor (BG-TFT) was fabricated employing an optimized µc-Si layer and exhibited a field-effect mobility of 3 cm2/(V·s), which is one order higher than that of a typical amorphous silicon TFT.

Effects of Chamber Wall Heating and Quartz Window on Fast Deposition of Microcrystalline Silicon Films by High-Density Microwave Plasma

The effects of chamber wall heating and quartz window on oxygen contamination and film crystallinity have been demonstrated in a high-density microwave plasma of a SiH4 and H2 mixture. With increasing chamber wall temperature, the film crystallinity deteriorates with higher concentration of oxygen impurity. The content of the oxygen impurity from the quartz window in the films is also significant with increasing working pressure and flow rate ratio of H2 to SiH4. The combination of higher flux of SiH4 and appropriate pressure of 150–300 mTorr is effective in suppressing the oxygen impurity while maintaining high film crystallinity. As a result, µc-Si film with a low defect density of 1–1.2×1016 cm-3 has been fabricated at a deposition rate of ∼50 Å/s.

Hollow electrode enhanced radio frequency glow plasma and its application to the chemical vapor deposition of microcrystalline silicon

A hollow electrode enhanced radio frequency (rf) glow plasma excitation technique and its application to the chemical vapor deposition of microcrystalline silicon films have been studied. In this technique, the reactor has two types of hollow structure. One is a hollow counterelectrode, and the other serves as both a hollow counterelectrode and a hollow rf electrode. The application of these discharge types to semiconductor processing is studied in the case of plasma enhanced chemical vapor deposition of hydrogenated microcrystalline silicon thin films. High crystallinity, photosensitivity and a maximum deposition rate of 6.0nm∕s can all be achieved at plasma excitation frequency of 13.56MHz and substrate temperature of 300°C. Properties of these plasmas are investigated by observing the plasma emission pattern, optical emission spectrum analysis and electrical parameters of the rf electrode. It is found that the plasma technique using both types of hollow discharge not only results in higher intensity of SiH* and Hα but also in much smaller self-bias voltage of the rf electrode. Faster processing of device grade hydrogenated microcrystalline silicon films can also be achieved under lower rf power compared to use of the hollow counterelectrode technique alone.

Hollow Electrode Enhanced RF Glow Plasma Generation and its Application to the Fast Deposition of Microcrystalline Silicon Films

AbstractA hollow electrode enhanced RF glow plasma excitation technique has been newly developed. In this technique, the reactor is divided into a capacitively-coupled RF glow discharge space and a processing space by the counter electrode, which includes a hollow structure and is placed between a RF electrode and the substrate. After introducing hydrogen gas into the chamber and applying RF power to the electrode, high intensity plasma emission is observed near and inside the hollow structure attached to the counter electrode. By using hollow RF electrode excitation in addition to the hollow counter electrode technique, it is found that plasma emission is further enhanced. The application of these discharge types for semiconductor processing is studied in the case of plasma enhanced chemical vapor deposition (PECVD) of hydrogenated microcrystalline silicon thin films. High crystallinity, photo-sensitivity and a maximum deposition rate of 4.9nm/s can all be achieved at a plasma excitation frequency of 13.56MHz and a temperature of 300°C. Properties of these plasmas are investigated by observing the plasma emission pattern and optical emission spectrum analysis. It is found that, using additional hollow RF electrode discharge, faster processing of device grade hydrogenated microcrystalline silicon thin films can be achieved under lower RF power compared to hollow counter electrode technique alone.

Fast deposition of microcrystalline silicon films with preferred (220) crystallographic texture using the high-density microwave plasma

Abstract A novel high-density and low-temperature microwave discharge utilizing a spoke antenna has been applied for the fast deposition of microcrystalline silicon (μc-Si:H) films with preferred (2 2 0) orientation. Systematic deposition studies were performed from pure and H2-diluted SiH4 systems with microwave power, total pressure, H2 dilution ratio and substrate temperature as variables, combined with plasma diagnostics using optical emission spectroscopy and Langmuir probe techniques. The effects of deposition parameters on the film crystallinity, crystal orientation and defect density are demonstrated.

Effect of double-layers formation on the deposition of microcrystalline silicon films in hydrogen diluted silane discharges

The effect of the double layers formation on the gas-phase composition of SiH 4 /H 2 discharges and the deposition rate of microcrystalline silicon thin films has been investigated by applying mass spectrometric and film growth measurements. Spatially Resolved Optical Emission Spectroscopy has been used for the detection of the appearance of this additional electron heating mechanism that in the present conditions results from the variation of either the discharge power or the total gas pressure. The experimental results have been in all cases combined to a gas phase and surface simulator of SiH 4 /H 2 discharges allowing thus the discussion for the rather limited effect of this mechanism on the deposition process of μc-Si:H thin films.

Ion-assisted Sputter Deposition of Microcrystalline Silicon Films with Pulsed-DC Plasma Excitation

ABSTRACTWe have applied a novel Ion-Assisted Sputter Deposition (IASD) method to deposit microcrystalline silicon (µc-Si) thin films with high deposition rates. An unbalanced magnetron sputter source together with an asymmetrical bipolar pulsed-DC plasma excitation operating in the frequency range 50-250 kHz was used for realizing high ion fluxes to the growing film. µc-Si films of high crystallinity are obtained at T > 400 °C with growth rates of up to 90 nm/min. The crystallinity of the films is characterized by the thickness-independent ratio of the optical thickness n·d determined by FTIR measurements and the atomic area density N·d as given by RBS analysis.

Spatial distribution of high-density microwave plasma for fast deposition of microcrystalline silicon film

The high-density microwave plasma utilizing a spokewise antenna was successfully applied to fast deposition of highly crystallized and photoconductive microcrystalline silicon (μc-Si:H) film at low temperatures. Among various deposition parameters, spatial distribution of ion energy (IDF) mainly determines film crystallinity. The best crystallinity was obtained at the axial distance, Z from the quartz glass plate, where the spread of mean ion energy is minimum. By optimizing the axial distance, Z and total pressure, highly crystallized and photoconductive μc-Si:H film could be fabricated with a high deposition rate of 47 Å/s at ∼50 mTorr in SiH 4 and Ar plasma.

Plasma diagnostics and low-temperature deposition of microcrystalline silicon films in ultrahigh-frequency silane plasma

Microcrystalline silicon thin films were formed on quartz substrates by ultrahigh-frequency (UHF) plasma enhanced chemical vapor deposition from a mixture of silane (SiH4) and hydrogen (H2) gases at low substrate temperatures (Ts). The UHF plasma was excited at a frequency of 500 MHz. The deposition rate and the crystallinity of the films were investigated as a function of H2 dilution, total pressure, mixture ratio of SiH4 to H2 and Ts. A crystalline fraction of 63% with a high deposition rate of 7.7 Å/s was obtained even at a Ts of 100 °C. At a temperature of 300 °C, a crystalline fraction of approximately 86% was achieved at a deposition rate of 1.4 Å/s. Diagnostics of the UHF plasma have been carried out using a Langmuir probe, ultraviolet absorption spectroscopy, and optical emission spectroscopy. Good crystallinity was explained by the balance of the sheath voltage and atomic hydrogen densities in the UHF plasma. Namely, the UHF plasma source achieving a high density plasma with a low electron temperature enabled us to reduce the ion bombardment energy incident on the substrates while maintaining a high density of hydrogen atoms, and which improved the crystallinity at low Ts.

Deposition of Microcrystalline Silicon Films by Magnetron Sputtering

Deposition of Microcrystalline Silicon Films by Magnetron Sputtering

New features of the layer-by-layer deposition of microcrystalline silicon films revealed by spectroscopic ellipsometry and high resolution transmission electron microscopy

Spectroscopic ellipsometry and high resolution transmission electron microscopy have been used to characterize microcrystalline silicon films. We obtain an excellent agreement between the multilayer model used in the analysis of the optical data and the microscopy measurements. Moreover, thanks to the high resolution achieved in the microscopy measurements and to the improved optical models, two new features of the layer-by-layer deposition of microcrystalline silicon have been detected: (i) the microcrystalline films present large crystals extending from the a-Si:H substrate to the film surface, despite the sequential process in the layer-by-layer deposition; and (ii) a porous layer exists between the amorphous silicon substrate and the microcrystalline silicon film.

Microcrystalline silicon growth by the layer-by-layer technique: long term evolution and nucleation mechanisms

The deposition of microcrystalline silicon films has been studied by in situ spectroscopic ellipsometry. We found that both the etching of the amorphous phase at the film growing surface and the rearrangement of the bulk induced by hydrogen diffusion are necessary to explain the long term evolution during deposition and nucleation mechanisms of microcrystalline silicon. The transition from amorphous to microcrystalline silicon deposition occurs through a highly porous phase. The independent control of the deposition and hydrogen plasma treatment times achieved in the layer-by-layer technique is very well adapted to overcome the substrate selectivity. In particular, we show that we can convert an amorphous film into the microcrystalline state.

Effect of hydrogen plasma precleaning on the removal of interfacial amorphous layer in the chemical vapor deposition of microcrystalline silicon films on silicon oxide surface

Microcrystalline silicon(μc-Si) film deposited on silicon oxide in a remote plasma enhanced chemical vapor deposition (RPECVD) with disilane (Si2H6) and silicon tetrafluoride (SiF4) has been investigated. It was found that in situ hydrogen plasma cleaning of the substrate prior to deposition is effective to reduce the interfacial amorphous transition region. It is believed that hydrogen plasma cleaning generated adsorption and nucleation sites by breaking weak Si–O and Si–Si bonds and also removed oxygen/carbon impurity. Surface roughening was observed from the hydrogen plasma precleaning which helped nucleation and crystallization at the initial stage of the growth.

Correlations between plasma parameters and the deposition of microcrystalline silicon films by plasma of argon and hydrogen

The dissociation of chlorosilanes to microcrystalline silicon and its deposition on a solid substrate in a rf plasma of mixtures of argon and hydrogen was investigated as a function of the macrovariables of the plasma. The dissociation mechanism of chlorosilanes and HCl as well as the formation of silicon in the plasma state were studied by sampling the plasma with a quadrupole mass spectrometer. Macrovariables such as pressure, net rf power input, and location in the plasma reactor were found to strongly influence the kinetics of dissociation. The deposition process of microcrystalline silicon films and its chlorine contamination were correlated to the dissociation mechanism of chlorosilanes and to the role of hydrogen chloride.

Polycrystalline silicon solar cells from recrystallized plasma deposited thin films

Large grain polycrystalline silicon films are produced by a two step process involving plasma deposition of microcrystalline silicon films on a substrate, separation from the substrate, and subsequent grain enhancement of the silicon films. The effects of doping and substrate temperature during deposition on the solar cell conversion efficiency are investigated. Effects of ppm level molybdenum contamination from the substrate, and silicon microstructure after grain enhancement, on solar cell efficiency parameters are also investigated. Solar cells with efficiencies of up to 10.1% under AM1 illumination, were fabricated on these silicon films.