Semiconductor Fabrication

Chemical Vapor Deposition

Q: What is the difference between LPCVD, PECVD, and APCVD?

The three variants differ in operating pressure and energy source. Atmospheric-pressure CVD (APCVD) runs near 760 Torr at high deposition rates but with poor uniformity, so it is rarely used for critical RF layers. Low-pressure CVD (LPCVD) operates at 0.1 to 2 Torr and 600 to 800 degrees C, giving excellent thickness uniformity and conformality for films such as stoichiometric silicon nitride and polysilicon. Plasma-enhanced CVD (PECVD) uses an RF plasma (typically 13.56 MHz) to supply reaction energy, allowing deposition at 250 to 400 degrees C. That low thermal budget is essential for depositing silicon nitride passivation over completed GaN and GaAs devices with metal gates already in place, because higher temperatures would degrade the gate Schottky contacts and ohmic metallization.

Q: Why is MOCVD preferred over MBE for GaN RF epitaxy?

Metalorganic CVD (MOCVD) grows GaN, AlGaN, and InGaN layers from precursors such as trimethylgallium and ammonia at 1000 to 1100 degrees C. It is preferred over molecular beam epitaxy for production because growth rates of 1 to 3 micrometers per hour and multi-wafer reactors give far higher throughput, and the high growth temperature plus ammonia overpressure produce low-defect AlGaN/GaN HEMT structures with sheet resistance near 300 to 400 ohms per square. MBE offers sharper interfaces and lower background carbon but at much lower throughput, so it is mostly reserved for research and specialized low-noise devices. For high-volume RF GaN-on-SiC power transistors, MOCVD is the dominant epitaxy method.

Q: What governs the deposition rate in a CVD process?

CVD operates in one of two regimes. At lower temperatures the rate is reaction-limited and follows an Arrhenius dependence, rate proportional to exp(-Ea/kT), where Ea is the surface-reaction activation energy (roughly 1.5 to 2 eV for silane-based nitride). With that activation energy a 600 to 800 degrees C swing changes the rate by one to two orders of magnitude, so tight temperature control, often within a degree or two, is critical. At higher temperatures the rate becomes mass-transport-limited, set by how fast precursor diffuses across the boundary layer to the wafer, and it depends mainly on gas flow and pressure rather than temperature. Production LPCVD furnaces are run in the reaction-limited regime so that wafer-to-wafer uniformity is governed by furnace temperature profile, which is easier to control than gas flow.

/KEM-ih-kuhl VAY-per dep-uh-ZISH-uhn/ (CVD)

Abbreviated CVD, this is a wafer-processing technique in which gaseous chemical precursors react or decompose on a heated substrate surface to grow a thin solid film. In RF semiconductor manufacturing it serves two roles: metalorganic CVD (MOCVD) grows the epitaxial AlGaN/GaN and GaAs layers that form transistor channels, while plasma-enhanced and low-pressure CVD deposit silicon nitride and silicon dioxide for device passivation and interlayer dielectrics. Because the active layers in GaN HEMTs are grown by CVD, film thickness, composition, and defect density set the ceiling on RF power density and noise figure for the finished device.

Category: Semiconductor Fabrication

Process Temp: 250 to 1,100 °C

Pressure: 0.1 Torr to 1 atm

How CVD Builds RF Device Layers

CVD differs fundamentally from physical deposition methods such as sputtering and evaporation. Rather than transporting atoms physically from a solid target, a CVD reactor delivers volatile precursor gases over a heated wafer, where a chemical reaction at the surface deposits the desired solid and releases gaseous byproducts that are pumped away. This surface-reaction mechanism produces highly conformal films that coat sidewalls and trenches uniformly, which is why CVD dominates the deposition of dielectrics and epitaxial semiconductors in microwave and millimeter-wave fabrication.

The energy that drives the reaction can come from substrate heat alone (thermal CVD) or from an RF plasma. Thermal variants such as LPCVD run hot, between 600 and 800 degrees C, to deposit dense, stoichiometric silicon nitride and polysilicon. Plasma-enhanced CVD (PECVD) couples a 13.56 MHz discharge to crack the precursors at only 250 to 400 degrees C, a low thermal budget that lets engineers deposit a protective nitride cap over a finished GaAs or GaN transistor without damaging its Schottky gate or ohmic contacts. Epitaxial growth uses metalorganic precursors (MOCVD) at 1000 to 1100 degrees C to grow single-crystal compound-semiconductor layers atop a silicon carbide or sapphire wafer.

Film quality directly governs RF performance. In a GaN HEMT, the AlGaN barrier thickness controls the two-dimensional electron-gas sheet charge and therefore the drain current and power density; a few angstroms of error shifts threshold voltage and pinch-off. The silicon-nitride passivation deposited by PECVD suppresses surface trapping that otherwise causes current collapse and gain compression at microwave frequencies. Tight CVD control is what makes a GaN-on-SiC wafer capable of 5 to 8 W/mm at X-band.

CVD Reaction and Rate Kinetics

Silicon nitride (LPCVD, dichlorosilane + ammonia):
3 SiCl₂H₂ + 4 NH₃ → Si₃N₄ + 6 HCl + 6 H₂

GaN epitaxy (MOCVD, trimethylgallium + ammonia):
Ga(CH₃)₃ + NH₃ → GaN + 3 CH₄

Reaction-limited deposition rate (Arrhenius):
R ≈ R₀ × exp(−E_a / kT)

Where R = growth rate, E_a ≈ 1.5 to 2 eV (surface-reaction activation energy), k = Boltzmann constant, T = absolute temperature. Example: an LPCVD nitride run at T ≈ 1,043 K (770 °C) deposits at ≈ 3 to 5 nm/min, so a 100 nm passivation layer takes roughly 20 to 30 minutes.

CVD Variants Used in RF Fabrication

Variant	Pressure	Temperature	Typical Film	RF Use Case
APCVD	~760 Torr	400 to 500 °C	Doped SiO₂ (BPSG)	Low-cost interlayer oxide
LPCVD	0.1 to 2 Torr	600 to 800 °C	Si₃N₄, polysilicon	Dense dielectrics, masks
PECVD	0.5 to 5 Torr	250 to 400 °C	SiN, SiO₂	Final-passivation over GaN/GaAs
MOCVD	20 to 200 Torr	1,000 to 1,100 °C	AlGaN/GaN, InGaP, GaAs	HEMT and HBT epitaxy
ALD (cyclic CVD)	0.1 to 1 Torr	150 to 350 °C	Al₂O₃, HfO₂	Gate dielectric, thin barriers

Common Questions

Frequently Asked Questions

What is the difference between LPCVD, PECVD, and APCVD?

They differ in pressure and energy source. APCVD runs near 760 Torr for fast but non-uniform films. LPCVD operates at 0.1 to 2 Torr and 600 to 800 °C for highly uniform, conformal silicon nitride and polysilicon. PECVD uses a 13.56 MHz plasma to deposit at only 250 to 400 °C, which is the low thermal budget needed to passivate finished GaN and GaAs devices without harming the gate Schottky and ohmic metal.

Why is MOCVD preferred over MBE for GaN RF epitaxy?

MOCVD grows GaN and AlGaN from trimethylgallium and ammonia at 1,000 to 1,100 °C with rates of 1 to 3 μm/hr in multi-wafer reactors, giving the throughput high-volume production needs and AlGaN/GaN HEMT sheet resistance near 300 to 400 Ω/sq. MBE yields sharper interfaces and lower carbon but at far lower throughput, so it stays mostly in research; for GaN-on-SiC RF power transistors, MOCVD dominates.

What governs the deposition rate in a CVD process?

At lower temperatures the rate is reaction-limited and follows Arrhenius behavior, R ∝ exp(−E_a/kT), with E_a near 1.5 to 2 eV, so it is very temperature sensitive. At higher temperatures it becomes mass-transport-limited, set by precursor diffusion across the boundary layer and dependent on flow and pressure. Production LPCVD furnaces run reaction-limited so uniformity tracks the easier-to-control temperature profile.

Related Terms

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