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Epitaxy is less forgiving than almost any other deposition step in the fab. The film has to grow as a single crystal, match the substrate lattice, and keep its dopant profile sharp, all at a growth rate high enough to stay economical. The precursor you feed the reactor sits at the center of that balance. Pick the wrong one and you pay for it in thermal budget, selectivity, or carbon and oxygen contamination that no anneal will fully remove. The four silicon sources most fabs work with are dichlorosilane, monochlorosilane, silane, and disilane, and each one buys you a different trade-off.
A CVD film only needs to be dense and conformal. An epitaxial film has to reproduce the substrate lattice, which means every adsorbed species must find its correct site before the next layer buries it. That requirement drives three practical questions. First, at what temperature does the precursor decompose cleanly on silicon, without gas-phase reactions that drop particles on the wafer? Second, what by-products does it release, and do those by-products attack or passivate the growing surface? Third, does the chemistry tolerate the patterned and partially oxidized surfaces that modern device flows demand?
Chlorine is the variable that separates the two precursor families. Chlorosilanes carry it into the chamber; hydrosilanes do not. That single difference explains most of the selection logic below.
Dichlorosilane (DCS, SiH2Cl2) remains the default source for high-volume silicon and silicon-germanium epitaxy in reduced-pressure reactors. Its chlorine content does three jobs at once. It etches the thin native oxide that survives a wet clean, so growth can start on a truly clean silicon surface. It suppresses nucleation on dielectric surfaces, which is what makes selective epitaxy possible in shallow trench isolation and source-drain raise applications. And it keeps gas-phase nucleation in check, so the process window stays open at the growth rates epi tools are designed for.
The price is temperature. DCS-driven silicon epitaxy typically runs above 1000 °C, and the film growth rate falls sharply as you push below that. When the device stack tolerates the heat, DCS gives the best combination of rate, crystal quality, and selectivity, which is why it has survived every generation of reactor design since the era of atmospheric-pressure barrels.
Monochlorosilane (MCS, SiH3Cl) carries half the chlorine per silicon atom that DCS does. Less chlorine means less surface passivation and less oxide etching, but it also means the film can grow at a lower temperature for the same rate. For processes that need some of the DCS benefits, clean crystal growth and modest selectivity, without spending 1000 °C of thermal budget, MCS is the middle path. It is also worth a look when a chamber runs both epi and deposition steps, since the lower chlorine load reduces chamber memory and cleanup time between runs. Our MCS line is produced at the 4N grade with an annual capacity of 500 tons, sized for fabs that have moved it into routine production.
Silane (SiH4) decomposes at the lowest temperature of the group, which puts silicon epitaxy within reach of thermal budgets that would kill a DCS process. It is also the only choice for silicon carbide epitaxy, where silane and propane crack together at very high temperatures in a hot-wall reactor. What silane cannot do is clean up after itself. With no chlorine in the chemistry, a few monolayers of native oxide stop epitaxial growth cold, and the process has no built-in defense against nucleation on mask oxides. That pushes more responsibility onto surface preparation: an HF last clean done right, a short in-situ bake, and tight control of moisture in the gas delivery system. Above roughly 900 °C, silane also starts reacting in the gas phase, so flow and pressure limits are stricter than with chlorosilanes.
Disilane (Si2H6) is the most reactive silicon source in common use. The silicon-silicon bond breaks easily, so growth rates that would need 700 °C with silane are achievable near 550 °C. That is exactly what advanced SiGe and SiGe:C epitaxy needs: low temperature to keep boron from diffusing, and a stable rate so the germanium profile stays abrupt. The trade-offs are cost per silicon atom delivered and a tighter process window, which is why disilane earns its place on the gas panel only where the thermal budget genuinely has run out.
Changing silicon source is never a one-parameter edit. Plan for recalibration of growth rate and dopant incorporation, since each precursor changes how boron and phosphorus incorporate at a given temperature. Check the chamber hardware: DCS and MCS hydrolyze in moist air to release hydrochloric acid, so regulator, valve, and exhaust materials need to match the chemistry. And look at the gas panel itself, because a delivery system built for hydrosilanes may need different purge and vent practices before a chlorosilane is introduced.
None of the chemistry above survives a bad cylinder. Moisture inside a cylinder or a poorly passivated valve puts oxygen straight into the epitaxial layer, where it shows up later as stacking faults and leakage. Our gases ship in 5-gallon and 10-gallon cylinders prepared for electronic service, with lot-level certificates of analysis behind the headline grades: 6N on silane, 4N on DCS, MCS, and disilane. For epi work, ask for the moisture and metals data, not just the grade, and keep one cylinder from each new lot for a witness run before the lot goes into production.
A short summary of the trade space: DCS for high-rate silicon and selective epi when the thermal budget allows; MCS for the same chemistry at lower temperature; silane for the lowest-temperature hydride processes and silicon carbide; disilane for advanced SiGe where temperature has run out. If you are weighing a switch between these sources, or qualifying a second supplier for one of them, send us the process conditions through our contact page and we will recommend the grade and package that fit your reactor.
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