Semiconductor Fabrication

Atomic Layer Deposition

Atomic Layer Deposition (ALD) is an ultra-precise thin-film deposition technique that deposits materials one atomic layer at a time through sequential, self-limiting surface chemical reactions. Unlike conventional Chemical Vapor Deposition (CVD), where precursor gases react continuously, ALD alternates between two precursor gases in separate half-cycles: the first precursor adsorbs onto the substrate surface (forming a single atomic layer and then stopping — self-limiting), excess precursor is purged, then the second precursor reacts with the adsorbed layer to form the desired material, and excess is purged again. Each complete ALD cycle deposits exactly one atomic layer (approximately 0.1 nm), giving unprecedented thickness control and conformality. In RF semiconductor fabrication, ALD is used to deposit the gate dielectric layer in GaN High Electron Mobility Transistors (HEMTs): an ultra-thin (5–20 nm) layer of Al₂O₃, HfO₂, or SiN that passivates the GaN surface, reduces gate leakage current, and improves device reliability. The atomic-level thickness control of ALD is essential for these gate dielectric films, where a 1 nm variation in thickness can shift the transistor's threshold voltage and dramatically change its RF performance.

Category: Semiconductor Fabrication

Understanding Atomic Layer Deposition in RF Devices

A GaN power amplifier transistor for a 5G base station handles hundreds of volts across a gate that is only 20 nanometers thick. Building that gate with atomic precision — controlling its thickness to within a single atom — is what Atomic Layer Deposition achieves.

The Self-Limiting Mechanism

ALD's precision comes from chemistry, not engineering tolerance. When the first precursor gas enters the reactor, it chemically bonds to every available site on the substrate surface — and then stops. No matter how long the gas flows, only one atomic layer adsorbs, because all bonding sites are occupied. This self-limiting behavior guarantees uniform, pinhole-free coverage across the entire wafer surface, regardless of wafer size or reactor geometry.

ALD in GaN HEMT Fabrication

The surface of a GaN HEMT is electronically active — surface states trap charge and cause current collapse (a dynamic reduction in drain current under RF operation). ALD-deposited Al₂O₃ passivation layers neutralize these surface states with atomic-level precision, dramatically improving the transistor's RF power performance and long-term reliability. The uniformity of ALD deposition ensures that every transistor on the wafer receives identical passivation, minimizing device-to-device variation in a power amplifier array.

Key Equations

Atomic Layer Deposition:
Atomic Layer Deposition (ALD) is an ultra-precise thin-film deposition technique that deposits materials one atomic layer at a time through sequential, self-limiting surface chemical reactions....

Key specifications:
0.1 nm | 20 nm | 1 nm | 1.5 dB | 40 dB | 50 dB

Power: P(dBm) = 10log(P_mW), 0dBm = 1mW

Comparison

Aspect	Atomic Layer Deposition Spec	Typical Range	Impact	Design Note
Primary function	Atomic Layer Deposition (ALD) is an ultr...	Application-dep.	Critical	Verify in sim
Operating range	Each complete ALD cycle deposits exactly...	Application-dep.	Critical	Verify in sim
Performance	Understanding Atomic Layer Deposition in...	Application-dep.	Critical	Verify in sim
Integration	Building that gate with atomic precision...	Application-dep.	Critical	Verify in sim
Trade-off	The Self-Limiting Mechanism ALD's precis...	Application-dep.	Critical	Verify in sim

Common Questions

Frequently Asked Questions

How slow is ALD compared to other deposition methods?

Very slow. ALD deposits approximately 0.1 nm per cycle, with each cycle taking 1–10 seconds. Depositing a 20 nm film requires 200 cycles and 10–30 minutes. CVD can deposit the same film in seconds. ALD's advantage is not speed but precision and uniformity — it produces films with ±0.5% thickness variation across a 200mm wafer and 100% step coverage over 3D structures. For thin gate dielectrics where every nanometer matters, this precision justifies the slower deposition rate.

What materials can be deposited by ALD?

ALD has been demonstrated for hundreds of materials, but the most RF-relevant include: Al₂O₃ (aluminum oxide — the most common GaN gate dielectric), HfO₂ (hafnium dioxide — high-k gate dielectric for advanced CMOS and GaN), SiN (silicon nitride — surface passivation), TiN (titanium nitride — gate electrode material), and Pt (platinum — electrode material for thin-film BAW resonators). The availability of suitable precursor chemistry determines which materials are practical for ALD.

Is ALD used in MEMS RF devices?

Yes. ALD is used to deposit ultra-thin conformal coatings on MEMS RF switches, resonators, and varactors. The conformal coverage of ALD means it can coat the interior surfaces of suspended MEMS structures uniformly — including the underside of a released cantilever beam — which conventional line-of-sight deposition methods cannot achieve. ALD coatings also serve as anti-stiction layers to prevent MEMS contact surfaces from permanently adhering after cycling.

Related Terms

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