Amorphous hydrogenated silicon nitride thin films a-SiN x:H (abbreviated later by SiN x) were deposited by Electron Cyclotron Resonance plasma enhanced chemical vapor deposition method (ECR-PECVD). By changing ratio of gas flow (R = NH 3/SiH 4) in the reactor chamber different stoichiometric layers x = [N]/[Si] ([N] and [Si] atomic concentrations) are successfully deposited. Part of the obtained films has subsequently undergone rapid thermal annealing RTA (800°C/1 s) using halogen lamps. Optical and structural characterizations are then achieved by spectroscopic ellipsometry (SE), ion beam analysis and infrared absorption techniques. The SE measurements show that the tuning character of their refractive index n(λ) with stoichiometry x and their non-absorption properties in the range of 250 - 850 nm expect for Si-rich SiN x films in the ultraviolet UV range. The stoichiometry x and its depth profile are determined by Rutherford backscattering spectrometry (RBS) while the hydrogen profile (atomic concentration) is determined by Elastic Recoil Detection Analysis (ERDA). Vibrational characteristics of the Si-N, Si-H and N-H chemical bonds in the silicon nitride matrix are investigated by infrared absorption. An atomic hydrogen fraction ranging from 12% to 22% uniformly distributed as evaluated by ERDA is depending inversely on the stoichiometry x ranging from 0.34 to 1.46 as evaluated by RBS for the studied SiN x films. The hydrogen loss after RTA process and its out-diffusion depend strongly on the chemical structure of the films and less on the initial hydrogen concentration. A large hydrogen loss was noted for non-thermally stable Si-rich SiNx films. Rich nitrogen films are less sensitive to rapid thermal process.
Amorphous hydrogenated silicon nitride film (SiNx) deposited at low temperature by plasma assisted CVD has several applications in semiconductor and photovoltaic industry. SiNx films deposited by PECVD technique in all its variants exhibit several advantageous properties. It is a low temperature process, relatively cost-effective, with high deposition rate, adjustable refractive index and high quality passivating films. PECVD SiNx films are generally obtained by radio-frequency RF or/and a microwave (MW) electrical discharge in a nitrogen and silicon precursor gas mixture (SiH4, NH3, N2O). Most PECVD techniques for deposition SiNx use pure or diluted silane (SiH4) and ammonia NH3 at lower temperature 200˚C - 500˚C; alternative precursor gas could also be used. It is also possible to utilize both plasma generators in separate places. Collision of electrons with gas molecules causes a chain of ionization, dissociation and excitation reactions. Mainly two parameters that determine the plasma chemistry are the electron energy distribution function and the dissociation and ionization cross sections [
SiNx films, compared to other dielectric films such as silicon dioxide, are more suitable candidates for photovoltaic applications. They are used as antireflecting coating (ARC), back surface reflector for optical purpose in the front and the back side of Si-based solar cells due to its tunable refractive index. By adjusting thickness and refractive index, stacking layers could also be used instead of a simple one to optimize the properties of solar cells efficiency. From electrical viewpoint, they are also used as hydrogen source for the bulk passivation of recombining centers in low-cost defected-rich Si-based solar cells thanks to the thermal post-deposition of metal contacts (mc-Si, poly-Si and ruban-Si) [
In this work, we study the effect of the main technological parameters (i.e. the gas flow ratio), with keeping the other parameters constants, on the optical and structural properties of the SiNx:H films produced in a remote Electron Cyclotron Resonance (ECR-) CVD reactor. The thermal stability of the deposited films will be probed after a rapid thermal anneal using halogen lamps as heating source.
Our SiNx films were deposited by a remote plasma ECR-PECVD system (Roth & Rau). The system is made of a steel cylindrical chamber (200 mm × 350 mm) containing a graphite support to ensure uniform heating of samples of 6" diameter at a temperature range 100˚C - 500˚C, a lock for loading and unloading samples away from the reactor chamber, a pumping system for vacuum (10−7 mbar), a micro-wave plasma generator ECR (Electron Cyclotron Resonance at 2.45 GHz), a radio-frequency RF plasma generator at 13.56 MHz and a six gas lines (Ar, H2, N2, NH3, SiH4, N2O). Prior to the deposition, silicon substrates were cleaned in an oxidizing bath H2SO4/H2O2 (4/1) for 10 minutes, followed by rinsing with deionized water, then soaked in a hydrofluoric HF (10%) acid solution for 30 seconds to remove the oxide, followed by a second rinsing with deionized water and drying with gas of nitrogen. The 1st series SiNx films was deposited at 400˚C under gas flow ([SiH4] + [NH3]) of 35 sccm. The partial flow of ammonia and silane are (R = 21/14, 19/16, 18/17 and 17/18), respectively. AC electrical discharge parameters were unchanged and fixed at 500 W for the Mw power and 200 W for RF power. Latter series was also prepared to enlarge the R ratio while maintaining silane flow at 16 sccm. Ammonia flow was 5, 8, 19, 32 and 80 sccm. Ammonia is dissociated in the upper MW reactor chamber by absorbing microwave power. Silane is dissociated in the bottom of the reactor in the vicinity of the Si substrate by absorbing radiofrequency power.
SiNx films were deposited on one of the surfaces of the commercial Cz-Si substrates. Part of our films was rapid thermally annealed under halogen lamps in a FAV-4 oven from JIPELEC Company, simulating the firing metal contacts through the SiNx films.
The optical properties of the obtained films were characterized by Perkin-Elmer UV/VIS/NIR Spectrometer Lambda 19 using a slit of 2 nm for the reflectivity measurements. A “Gaertner” single-wavelength (632.8 nm) ellipsometer equipped with He-Ne laser was used to determine the refractive index and the deposition rate of SiNx:H films. Spectroscopic ellipsometry measurements in the range 250 - 850 nm were performed using JobinYvon UVISEL ellipsometer. The chemical structure of SiNx films were analyzed by fourier-transform infrared spectroscopy (FTIR), Infrared-transmission measurements were performed on a Bruker Equinox 55 Fourier transform infrared spectrometer using a MIR-source, a KBr beamsplitter and DTGS detector. FT-IR spectra were collected from 5000 to 400 cm−1 with resolution of 2 cm−1, using a spot diameter of 15 mm of exposed area circle of 1.77 cm2 and for each spectrum 256 scans were accumulated to improve the signal-to-noise ratio. The compositional properties (Si, N, H content) were determined by Rutherford Backscattering spectrometry (RBS) and Elastic Recoil Detection Analysis (ERDA) at the van de Graff accelerator of Strasbourg University.
RBS probe number and energy of backscattered ions after interaction with the target sample giving then the atomic masses and the elemental concentrations of the chemical elements (Si and N) in regard to the film depth. The ERD is used peculiarly to probe light elements such as hydrogen. The kinematic collision is similar to RBS. For ERDA, we are exceptionally interested in recoiled atoms. ERD measurements were carried out using an analyzing ion beam of 2.9 MeV 4He+ particles at 75˚ detection angle from the normal to the sample. A 2 MeV 4He+ particles beam with a 150˚ backscattering angle used for RBS. SAM code is used to treat the collected data (It performs iterative simulations assuming an initial composition and then finding a set of input parameters giving the best fit of the experimental spectrum with that calculated).
Similar observations were made for the latter series, the refractive index n (633 nm) varies between 1.95 for R = 5 and 3.35 for R = 0.3. This evolution is abrupt for ratios R < 2. Indeed, when the silicon content increases due to high density of silane precursors SiHx, the refractive index of the Si-rich films approaches to that of c-Si (n = 3.42). However, the refractive index of as-deposited SiNx films with a R > 2 is quasi-constant and close to that of stoichiometric silicon nitride n(Si3N4) = 1.97. This inferior limit is most likely due to the saturation of NHx
R = NH3/SiH4 | Sidep/Sian | Ndep/Nan | Hdep/Han | xdep/xan | n (633 nm)/Eg (eV) | Total atomic N (at/cm3) |
---|---|---|---|---|---|---|
08/16 | 0.58/0.73 | 0.2/0.26 | 0.22/0.01 | 0.34/0.36 | 2.79/1.7 | 5.98 × 1022 |
19/16 | 0.41/0.51 | 0.37/0.41 | 0.22/0.07 | 0.93/0.8 | 2.32/2.38 | 7.82 × 1022 |
32/16 | 0.40/0.54 | 0.44/0.37 | 0.15/0.09 | 1.1/0.68 | 2.01/2.75 | 9.42 × 1022 |
80/16 | 0.34/0.4 | 0.46/0.48 | 0.15/0.12 | 1.35/1.2 | 1.95/3.8 | 1.31 × 1023 |
radicals resulting from NH3 dissociation. We can note a similar trend in literature [
Assuming parabolic bands, the optical gap could be determined either by modeling SE data via a dispersion model or by using the Tauc relationship
where Eg, B are the optical gap and the Tauc constant, respectively. Values of Eg reported by different authors are generally limited by those simulated by SSM (Stoechiometric Statistical Model) and RBM (Random Bonding Model) [
Tauc slope B could provide details on the spreading tails of valence and conduction bands.
Experimental observations subsequent to a RTA depend on the annealing cycle (temperature and annealing time) and the structure of the film. Thus, SiNx films can be categorized into two types: the quasi-stoichiometric films with a low refractive index whose optical properties are almost unchangeable and thickness decreased slightly with 2 - 5 nm after RTA step. The latter category is that of the Si-rich films with an initial refractive index n > 2.1 whose refractive index and extinction coefficient significantly increases after RTA. The optical gap Eg of the N-rich films increases slightly unlike the Si-rich films; it decreases and approaches that of amorphous silicon a-Si. These experimental findings, confirmed by reflectivity measurements, are consistent with the results of some authors [
Bustarret et al. [
RBS analysis does not show other elements (oxygen, carbon) in the bulk of the as-deposited SiN films or its atomic content less than the detection threshold, except in rare cases where residual oxygen can exist on the surface due to inadvertent oxidation. Besides, it also confirms that silicon and nitrogen are uniformly distributed over the bulk of the films except on small atomic layers on the surface and at the interface with the silicon substrate. The channels number (x axis in RBS spectra) is related to the energy loss that can be converted into thickness. The first plate in the RBS spectrum at high channel numbers is relative to the Si in the SiNx:H film
while the second plate relative to the N is flooded in that of Si substrate. The latter is not flat due to the channelling effect when the particles pass through the crystal structure of the substrate. The areal densities of each chemical element (Si and N) constituting the film, representing the product atomic volume density × thickness (n × e atoms/cm2), may be derived from its corresponding peak area (see
Stoichiometry of our films is primarily determined by the ratio R. Furthermore, Hydrogen content (determined by ERDA) drops as the stoichiometry of the film increases. These findings in conjunction with those by IR absorption show that the hydrogen is mainly related to the Si with a content of 22% for the films with (R = 0.5, x = 0.34) and its content down to 15% for the films with (R = 5, x = 1.34) wherein the hydrogen is mostly bonded to the nitrogen. Michael et al. [
FTIR spectra with strong peak at 845 cm−1 and two peaks at 2150 cm−1 and 3330 cm−1 of different intensity and shape as a function of the ratio R, correspond to stretching modes of Si-N, Si-H and N-H bonds, respectively. Hydrogen content in the SiNx films could also be estimated from FTIR measurements by the semi-empirical method developed by Lanford and Rand [
Particularly, oxygen atoms induce an increase of the maximum position due to electronegativity difference between O(3.5) and N(3.1). Hence, the oscillator strength of Si-N increases when oxygen is back bound to N atoms [
Subsequent to RTA process, Stoichiometry of our SiNx films with a ratio of (R > 1.19) decreases due to Si enrichment confirmed by an increase of their refractive index (see
Hydrogenated silicon nitride films were successively deposited at 400˚C by ECR-PECVD. The effect of the gas flow ratio R = NH3/SiH4 was investigated for two series of samples. The former was by varying both the ammonia and the silane flow; the latter was by maintaining constant silane flow and varying ammonia flow. We noticed that the ratio has significant effect on the structural and optical properties. Composition and the bonding configurations of our material were determined by RBS, and infrared absorption, respectively. Optical properties were (refractive index, absorption coefficient and the optical gap) linked to the chemical composition of silicon nitride x = [N]/[Si]. The content of hydrogen 11% and 22% depended also on film stoichiometry. ERDA and FTIR measurements suggest that the films composition is decisive in desorption mechanism of hydrogen.
HichamCharifi,AbdelilahSlaoui,Jean PaulStoquert,HassanChaib,AbdelkrimHannour, (2016) Opto-Structural Properties of Silicon Nitride Thin Films Deposited by ECR-PECVD. World Journal of Condensed Matter Physics,06,7-16. doi: 10.4236/wjcmp.2016.61002