ALD — Atomic Layer Deposition

ALD is a thin film deposition technique that grows material one atomic layer at a time through sequential, self-limiting surface reactions. It provides exceptional conformality (coating 3D structures uniformly), atomic-level thickness control, and extremely low film defect density, making it essential for high-κ dielectrics, barrier layers, and gate oxides in advanced nodes.

Why companies use it

·ALD is the only technique that can conformally coat high-aspect-ratio structures (finFET fins, DRAM capacitors) with angstrom-level precision
·High-κ metal gate (HKMG) transistors — standard since Intel 45nm — rely on ALD HfO₂ for the gate dielectric
·Enables precise control of film stoichiometry and thickness without plasma damage, critical for thin interfacial layers
·Used for barrier layers (ALD TiN, TaN) in copper interconnects to prevent copper diffusion into dielectric

What hiring managers look for

·ALD expertise is in high demand at all leading semiconductor fabs, equipment companies, and materials suppliers
·Understanding the self-limiting reaction mechanism and why it gives thickness uniformity without line-of-sight constraints sets candidates apart
·Process engineers must understand ALD nucleation behaviour — new surfaces often require different process conditions than bulk film growth
·Equipment engineers need to understand precursor delivery, chamber design for pulse-and-purge sequences, and particle management

Typical interview questions

Explain the self-limiting mechanism of ALD. Why does it give perfect thickness control?

What is the growth-per-cycle (GPC) of a typical ALD process and what factors affect it?

What is the difference between thermal ALD and plasma-enhanced ALD (PEALD)?

How would you troubleshoot a process where ALD film thickness is uniform but electrical properties are poor?

What materials are commonly deposited by ALD in semiconductor manufacturing and why?

Common mistakes

·Assuming GPC is always one monolayer per cycle — most ALD processes have GPC values of 0.5–2 Å/cycle due to steric hindrance
·Ignoring nucleation delay on new substrate surfaces — ALD films often take 5–20 cycles before reaching steady-state GPC
·Not accounting for precursor saturation — a sub-saturating pulse dose leads to non-uniform coverage and is not a true ALD regime
·Confusing ALD with CVD — CVD is not self-limiting and cannot achieve the conformality of ALD on high-aspect-ratio structures
·Neglecting film stress in thin ALD layers — even angstrom-scale misfit can cause delamination or device performance shifts

Real engineering example

A memory manufacturer was experiencing capacitor leakage yield loss at the 15nm DRAM node. Investigation found that ALD Al₂O₃ nucleation on the TiN electrode surface was delayed by 8 cycles, creating a thin region of mixed TiO₂ at the interface that had high leakage current. By switching to a surface-treated TiN (mild oxidation pre-treatment) before ALD, nucleation was immediate, interface quality improved, and leakage yield improved by 12 percentage points.

Topics covered

ALDatomic layer depositionthin filmHKMGconformalitysemiconductorHfO2