SILICON HYDRIDES (SILANES)
Silanes (silicon hydrides) are the most conventional silicon CVD and ALD precursors. They include mono-silane (SiH4), disilane (Si2H6) and recently used in CVD practice trisilane (Si3H8).
Silanes are generally used for the CVD deposition of epitaxial and polycrystalline Si, as n-dopants of III-V materials, and various other Si-containing layers.
Silane (monosilane) SiH4
Silane (monosilane) SiH4 is a colorless gas (bp. –112.3 °C), under inert gas stable to 300°C (at RT stable in storage even at high pressures and when exposed to shocks), higher temp.: SiH4 → Si + 2 H2,
SiH4 reacts vigorously with O2 (burns or explodes) producing SiO2 and H2O and 1518 kJ (very exothermic reaction), reacts with HX to give SiH3X, SiH2X2 and SiHX3 (X = Halide).
Synthesis of SiH4:
Mg2Si + 4 HX → SiH4 + 2 MgX2 (X = Halide)
SiCl4 + 4 LiH → SiH4 + 4 LiCl (400 °C, LiCl/KCl melt)
SiHCl3 / SiH2Cl2 (dismutation): 2 SiH2Cl2 → SiH4 + SiCl4
Safe handling of SiH4
SiH4 must be handled with care: typically silane tanks are placed in gas cabinets specifically designed for explosive gases, with dedicated nitrogen supplies for purging. Silane can react with traces of moisture in gas lines forming powder which leads to particles and can clog valves and mass flow controllers; thus, carefully cleaned and purged stainless-steel plumbing is essential. Leaks of SiH4 can be exceedingly hazardous: it is possible for silane gas to collect in the atmosphere and then explode violently when disturbed (such as when a gas cabinet door is opened to search for the leak!). Silane is also moderately toxic, though explosion hazards are usually more severe. It is thus a good practice to employ trace monitors ("toxic monitoring") in a silane facility; even with toxic monitoring in place, care is necessary. It is difficult to calculate the diffusion length (or equivalently the Peclet number) for a typical room environment with a toxic gas leak: in a 5 minute "air change time" the diffusion length of SiH4 is only about 12-15 cm. Rooms may be not well-mixed and the leak may reach poorly-chosen toxic monitor later than operator.
However, despite the dangers, silane is widely used in semiconductor manufacturing, and with proper precautions can be handled safely and reliably.
SiH4 for CVD deposition
Silane SiH4 is used in the deposition of amorphous, polycrystalline
silicon, epitaxial silicon, silicon dioxide and silicon based dielectrics. Polycrystalline Si films can be deposited on silicon wafers and glass substrates via remote plasma chemical vapor deposition (RPCVD) using a SiH4-SiF2-H2 gas mixture. [[i], [ii]] Silane is used widely as a dopant in the formation of III-IV semiconductor materials.[iii]
[i] Quinn, L.J. et. al. Thin Solid Films 296, 7, (1997)
[ii] Thin Solid Films 289, 227, (1996)
[iii] Hu, C. C. et. al. Thin Solid Films 288, 147, (1996)
SiH4 for growth of epitaxial /polycrystalline Si by CVD
Polycrystalline silicon is deposited from silane (SiH4) by CVD, using the following reaction: SiH4 → Si + 2 H2
This reaction is usually performed in low-pressure (LPCVD) systems, with either pure SiH4, or diluted SiH4 (with 70-80% N2). Growth temperatures 600-650 °C and pressures between 25 - 150 Pa are used, resulting in growth rate ~10-20 nm/ min. An alternative process uses a hydrogen-based solution. The hydrogen reduces the growth rate, but the temperature is raised to 850 or even 1050 °C to compensate.
Polysilicon may be grown directly with doping, if gases such as phosphine, arsine or diborane are added to the CVD chamber. B2H6 doping increases the growth rate of poly-Si, whereas AsH3 and PH3 doping decrease poly-Si deposition rate. [WIKIPEDIA]
Already Purnell and Walsh in their pioneering study of thermal decomposition of silane considered two mechanisms in the initial stages of decomposition [[i]]:
1) molecular hydrogen elimination mechanism:
SiH4 -> SiH2 + H2
SiH2 + SiH4 -> Si2H6
2) homolytic Si-H bond rupture (radical decomposition mechanism)
SiH4 -> SiH3· + H·
H· + SiH4 -> SiH3· + H2
SiH3 + SiH4 -> Si2H6 + H
2 SiH3 -> Si2H6
Based on the content of the HD (31%) after decomposition of mixtures of SiH4 and SiD4 they considered
found that first mechanism is predominant, however kinetic data were in disagreement with this statement.
[i] J.H. Purnell, R. Walsh, Proc. R. Soc. Lond. A, 23 August 1966, vol. 293, no. 1435., 543-561. (doi: 10.1098/rspa.1966.0189)
SiH4 (+ O2) for SiO2 films CVD deposition
The basic overall reaction for the deposition of silicon dioxide requires the removal of the hydrogen atoms and addition of two oxygens:
When oxygen is present in excess, water is the main byproduct (APCVD), whereas in low-oxygen conditions, hydrogen will be produced (LPCVD). Because the heat of formation of silane is small and that of silicon dioxide is large, this process is very exothermic:
The reaction proceeds through a complex series of reactive gas-phase intermediates; an example, in which water produced by the burning of other silane molecules attacks a silene radical, is:
The intermediate species in this reaction is the biradical SiH2; in general, many radicals are produced and branching chains are possible, that’s why SiH4/O2 mixtures can explode. At the surface, these various reactive compounds tend to stick readily, giving high RSC and medium conformality. Once they are in place on the surface, additional "condensation" reactions must occur to remove the excess hydrogen and form the bridge bonds of the oxide film, with water or hydrogen being produced:
Deposition requires temperatures over 250°C, but at higher temperatures there is little effect of temperature on deposition rate:
Semiconductor applications usually require temperatures in excess of 350 C to obtain reasonable film density and purity. Absolute deposition rates of 100 nm/minute are achievable in single-wafer (showerhead) LPCVD, and local deposition rates of 500 nm/minute in injector APCVD. The rate also depends on the initial gas composition, with a maximum in rate being observed at moderate excess of oxygen:
Conformality and gap fill. Deposition is due to reactive species with relatively high sticking coefficients (RSC about 0.3). Conformality is much better than e.g. sputtering or evaporation, but the ability to cover or fill high-aspect-ratio features (>1:1) is limited.
Rough film, dirty chamber. Reactions in the gas phase are rapid, and depending on gas flow and mixture can lead to growth of larger nuclei. If these are incorporated into the film the surface is roughened; downstream they grow to dust which is deposited on chamber walls and accumulates in the exhaust.
Films contain hydrogen.
In moderately oxidizing conditions (e.g. tube LPCVD) Si-H will be present in the film. When copious oxygen is present during growth (APCVD) the film contains Si-OH. Excessive H in the films can lead to various issues; those are however generally much more
serious in TEOS films [[i]]
[i] Daniel M. Dobkin, http://www.enigmatic-consulting.com/semiconductor_processing/CVD_Fundamentals/films/SiH4_O2_thermal.html
SiH4 (+ N2O) for SiO2 films by PECVD
Silicon dioxide can be deposited using parallel-plate showerhead reactors. N2O is employed as an oxidant; pure O2 is often found to be too reactive and can produce lots of powder. Conditions are typically from about 0.2 Torr to a few Torr, temperatures of 200-400°C, gas flows from a few sccm to 1-2L, generally with a high ratio of oxidant:silane.
The film properties are fairly similar to those obtained using thermal CVD from silane: conformality is generally poor to medium. However, the addition of the plasma enables rather facile control of film stoichiometry, all the way from amorphous silicon to silicon-rich oxides to nearly pure silicon dioxide, by varying the gas mixture and flows. Silicon-rich oxides can be used as moisture barriers, due to the ability of the Si-H bond to react with water molecules to form silanol and release molecular hydrogen. Silicon-rich oxides may also be useful as etch stop layers.
The stress of plasma-deposited films is also adjustable, particularly when a dual-frequency reactor is employed: increasing relative ion bombardment energy can be used to adjust stress to more compressive levels. Higher RF power or smaller electrode gaps can achieve similar effects (if with less versatility) in single-frequency reactor configurations.
Plasma-deposited silicon dioxide layers from SiH4 are widely employed when conformality is not critical. They can be used as thin bottom layers in sandiwches with TEOS/ozone or spin-on glass layers for intermetal dielectric gap fill, or as part of a final passivation layer. In Damascene processes, metal lines are inlaid into trenches formed in the intermetal dielectric and then polished flat, so the IMD is always deposited on a flat smooth surface and conformality is not significant. IMD for Damascene processes can use PECVD silane oxide films. By adjusting the deposition conditions (making the gas silane-rich) one can deposit silicon-rich films; these can be employed as moisture barriers to prevent water from TEOS-ozone oxides or spin-on glasses from damaging underlying transistors.
PSG films deposited using plasma from silane and phosphine may fail to completely oxidize the phosphine, and incorporate it as a hydride into the film. This can lead to the formation of bubbles and voids in the film during subsequent annealing, if high phosphorus contents are used. [525, [i]]
[i]"Inter-Metal Dielectric and Passivation-Related Properties of Plasma BPSG" I. Avigal [Intel] SolidState Technology October 1983 p. 217
"Plasma-Enhanced Deposition of Borophosphosilicate Glass Using TEOS and Silane Sources" K. Law, J. Wong, C. Leung, J. Olsen, D. Wang [AMT] Solid State Technology April 1989 p. 60
"Studies of Corrosive Outgasses from Via Holes Using Thermal Desorption Spectroscopy" S. Tokitoh, H. Uchida, H. Uchida, Y. Okuno, K. Fushimi, G. Liu, Y. Sakay and N. Hirashita Jpn. J. Appl. Phys. 34 1021 (1995)
"Plasma Enhanced Chemical Vapor Deposition of Silicon Dioxide Deposited at Low Temperatures" M. Ceiler, B. Kohl and S. Bidstrup J. Electrochem. Soc. 142 2067 (1995)
"Chemical Etch Rate of Plasma-Enhanced Chemical Vapor Deposited SiO2 Films"
R. Besser and P. Louris, J. Electrochem. Soc. 144 p. 2859 (1997)
SiH4 for Si-doped AlInAsSb
Silane (SiH4) was applied as n-dopant for the preparation of Si-doped AlInAsSb and AlInAsSb/InGaAs MQW structures by MOVPE. The increase of the flow rate of SiH4 from 20 to 150 sccm increased the electron concentration in the AlInAsSb bulk layer from 5.2×1016 to 2.8×1017 cm−3 , with the mobility correspondingly decreasing from 1204 to 703 cm2/Vs. [286]
Disilane Si2H6
Disilane Si2H6 is colorless gas (bp. –14.8 °C), not stable towards air and moisture.
Synthesis: 2 Si2Cl6 + 3 Li[AlH4] → 2 Si2H6 + 3 Li[AlCl4]
Disilane is used for the deposition of amorphous silicon, epitaxial silicon and silicon based dielectrics via rapid low-temperature chemical vapor depsition (LTCVD).[[i], [ii]] Disilane is also used in the epitaxial growth of SiGe films by molecular beam epitaxy (MBE) in conjunction with solid sources of germanium.[[iii]] It is also a precursor for the rapid, low temperature deposition of epitaxial silicon and silicon-based dielectrics.[527]
At low temperatures the growth rate using traditional silicon precursors (silane, disilane, dichlorosilane) is reduced, requiring longer processing times per wafer, therefore more advanced precursors
such as trisilane are needed.
[i] Huange, G.W., et al., J. Appl. Phys. 81, 205, (1997)
[ii] Lin, H.-Y. et. al. Solid-State Electron 39, 1731, (1996)
[iii] Wado, H. et. al. J. Cryst. Growth 169, 457, (1996)
Trisilane Si3H8
Fig. Growth rate using Si3H8 (Silcore) vs SiH4
Trisilane (Si3H8) is a key component of the low temperature selective deposition processes. It is particularly useful as a low temperature deposition silicon precursor as it more readily forms a reactive intermediate that has a high sticking coefficient and has available a deposition mode that is not limited by the rate of hydrogen desorption from silicon. Low deposition temperatures (<600°C) are especially important for silicon-carbon (Si:C) epitaxial growth due to the low solubility of C in Si – need to ensure that the carbon atoms occupy only substitutional sites. The concept of using Si:C (or SiGe:C) is epitaxial deposition of a material with a smaller lattice constant than Si within the recessed source/drain regions of the transistor, to induce uniaxial tensile strain in the adjacent channel region.
The comparison of Si growth rate using SiH4 and trisilane (“Silcore”) is given in Fig.
Trisilane decomposes through the elimination of silane (SiH4) and therefore the key reactive decomposition intermediates of Si3H8 that enable high growth rates are: SiH-SiH3, H2Si=SiH2, and SiH2. Shown in Fig. 1 is an Arrhenius plot comparing the deposition rates of silane and Silcore between 450°C and 950°C. As is shown in the plot, the deposition rate using Silcore is > 30 times higher than silane at ≤600°C for equal amounts of silicon atoms in the reaction chamber.
Si3H8 (+MeSiH3) for SiC1+x films growth by CVD
Trisilane Si3H8 was proposed as promising precursor for the growth of SiC films by CVD. This property of the trisilane molecule to produce reactive SiHx intermediates leading to high growth rate at low temperatures (≤600°C) allows to overcome the limitations of traditional silicon precursor (SiH4, Si2H6) chemistries and to develop a novel process for Si:C enabling extremely high substitutional carbon concentrations (>2.5%C)
Fig. shows XRD spectra of Si:C epitaxial films grown
using trisilane as Si source and monomethylsilane CH3SiH3 as the carbon source;
high level of substitutional carbon (>2.5%) is measured in the films, as indicated by the separation between the Si substrate peak and the Si:C epi peak. High quality of the layers is indicated by the observed Pendellösung fringes from the Si:C peak.
On the inset a cross-section TEM image of a selective Si:C epitaxial layer grown within the recessed source/drain region of a transistor, is given. Detailed description of Si:C epitaxial process is presented in [i],[ii].
[i] S. G. Thomas, M. Bauer, M. Stephens, J. Kouvetakis, SolidState Technology, 04 Jan2009
[ii] M. Bauer et al., “Si:CP Selective Epitaxial Growth in Recessed Source/Drain Regions yielding to Drive Current Enhancement in n-channel MOSFET” ECS Transactions, vol. 16, no. 10, 2008. p. 1001.
Si3H8 for ultrathin Si films by CVD
Ultrathin Si films were deposited using trisilane on Ge buffer layers on Si by CVD at 420 °C. Raman spectrocsopy results show that a thin (< 1 nm), fully
strained Si–Ge alloy layer is formed at the Si–Ge interface. Pure Si grows on this transitional alloy with a strain that approximately follows the predictions from a simple equilibrium strain theory. Thin Si layers are important in MOS processes
to isolate the Ge channel from the high permittivity oxide.[i]
[i] Y.-Y. Fang, V.R. D'Costa, J. Tolle, C.D. Poweleit, J. Kouvetakis, J. Menéndez,Thin Solid Films, vol. 516, Issue 23, 1 October 2008, Pages 8327-8332
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