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Silicon University / Pillar 07 · Dielectric Deposition, Gate Stack Engineering & Thermal Processing / Topic 7.2 · ALD & High-k Metal Gate Stacks

Tier 2 · Equipment & Kinematics 19 min read

ALD Reactor Hardware: Precursor Pulsing, Purging & Chamber Kinematics

Mechanical design of cross-flow and showerhead ALD reactors, fast-actuating precursor delivery solenoids (<10ms), organometallic liquid precursor vaporizers (TDMAH), and downstream plasma radical generators.

Equipment EngineersProcess EngineersThin Film Specialists
Dimitri AlbinoProject Leader, Advanced Wet Processing Equipment Manufacturing & Deployment

01ALD Chamber Architectures: Cross-Flow vs. Showerhead

ALD tools for wafer processing utilize two primary chamber geometries:

  1. Cross-Flow Reactors:
    • Process gas enters horizontally from one side of a low-volume reaction chamber and flows parallel across the wafer to an opposing exhaust port.
    • Advantages: Extremely small internal chamber volume (), enabling rapid gas replacement during nitrogen purge cycles ().
    • Showerhead / Vertical Laminar Reactors:
    • Process gas enters vertically through a multi-orifice showerhead plate directly above the heated wafer chuck.
    • Advantages: Superior radial thickness uniformity () across large substrates, widely adopted for High-k Metal Gate and 3D NAND gap fill.

02Precursor Gas Delivery & Fast High-Speed Pulsing Valves

Common precursors for high-k and metal gate deposition include:

  • Hafnium Precursors: Tetrakis(dimethylamino)hafnium (TDMAH) or Tetrakis(ethylmethylamino)hafnium (TEMAH).
  • Oxygen Co-Reactants: Water vapor (), Ozone (), or Oxygen Plasma ().
  • Metal Gate Precursors: Titanium Tetrachloride () or Alkylamido-titanium with Ammonia () for TiN.

Because TDMAH and TEMAH are low-vapor-pressure liquids at room temperature, they are housed in heated stainless steel ampoules () and delivered using an inert Carrier Gas (Ar or ) with heated gas lines () to prevent precursor condensation.

High-Speed ALD Pulse Valves:

Specialized pneumatic or solenoid valves are rated for over switching cycles. They feature ultra-low internal dead volume and response times under to deliver precise micro-doses of precursor into the process chamber.

Figure 1
FIG. 1

03Thermal ALD Temperature Window vs. PEALD

The ALD Temperature Window defines the operating regime where self-limiting monolayer growth occurs:

The Ald Process Temperature WindowSchematic · source diagram
GrowthRateALD Window(nmcycle)CondensationSelf-LimitingThermalPhysisorptMonolayerDecompositionGrowthLow TIdeal THigh T
  • Too Low (): Precursor condenses physically on the substrate, breaking self-limiting control and creating excess impurity concentration.
  • Too High (): Precursor molecules thermally decompose (acting like un-controlled CVD) or desorb from surface sites before reacting.

Plasma-Enhanced ALD (PEALD):

By substituting thermal with reactive oxygen radicals generated by a Capacitively Coupled Plasma (CCP) or Inductively Coupled Plasma (ICP) source, PEALD expands the process window down to , enabling high-k deposition on temperature-sensitive substrates and organic photoresists.

Figure 2
FIG. 2
Figure 3
FIG. 3

→Prerequisites & continue reading

Prerequisites: ["p07-t7.2-t1-ald-high-k-metal-gate-fundamentals"]

Also in Topic 7.2, ALD & High-k Metal Gate Stacks:

Tier 1 · Atomic Layer Deposition (ALD) & High-k Metal Gate (H…Tier 3 · Interface Engineering, Dipole Formation & Work Funct…