Mukhtar Hussain Department of Physics & Astronomy King Saud University, Riyadh

 What is ALD Process ?  Basic Characteristics of ALD  Principles of ALD Technique  ALD Cycle for Al 2 O 3 Deposition  Requirements for Precursors  Types of ALD Reactors  Closed System Chambers ALD Reactor  ALD Applications  Advantages & Limitations  Summary

 “ It’s a film deposition technique based on sequential use of self terminating surface reactions”  ALD is a CVD technique suitable for inorganic material layer as oxides, nitrides and some metals.  Perfect for deposition of very thin layers of the size of a monolayer.

 Steps: ◦ Self-terminating reaction of the first reactant (Reactant A) ◦ Purge or evacuation to remove non-reacted reactant and by products ◦ Self-terminating reaction of the second reactant (Reactant B) ◦ Purge This is considered as one reaction cycle  The surface must be in a controlled state, e.g. heated  Parameters to be adjusted: ◦ Reactants (precursors) ◦ Substrate ◦ Temperature

Self-termination of adsorption provides atomic scale control of the film thickness and ensures uniform coverage. Principles of ALD Technique

 In air H 2 O vapor is adsorbed on most surfaces, forming a hydroxyl group. With silicon this forms: Si-O-H (s)  After placing the substrate in the reactor, Trimethyl Aluminum (TMA) is pulsed into the reaction chamber. Tri-methyl aluminum Al(CH 3 ) 3(g) C H H H H Al O Hydroxyl (OH) from surface adsorbed H 2 O Methyl group (CH 3 ) Substrate surface (e.g. Si)

Al(CH 3 ) 3 (g) + : Si-O-H (s) :Si-O-Al(CH 3 ) 2 (s) + CH 4  Trimethyl Aluminum (TMA) reacts with the adsorbed hydroxyl groups, producing methane as the reaction product C H H H H Al O Reaction of TMA with OH Methane reaction product CH 4 H H H H H C C Substrate surface (e.g. Si)

C H H Al O Excess TMA Methane reaction product CH 4 H H C  Trimethyl Aluminum (TMA) reacts with the adsorbed hydroxyl groups, until the surface is passivized.  TMA does not react with itself, terminating the reaction to one layer.  This causes the perfect uniformity of ALD.  The excess TMA is pumped away with the methane reaction product. Substrate surface (e.g. Si)

C H H Al O H2OH2O H H C O H H  After the TMA and methane reaction product is pumped away, water vapor (H 2 O) is pulsed into the reaction chamber.

2 H 2 O (g) + :Si-O-Al(CH 3 ) 2 (s) : Si-O-Al(OH) 2 (s) + 2 CH 4 H Al O O  H 2 O reacts with the dangling methyl groups & form aluminum-oxygen (Al-O) bridges and hydroxyl surface groups, waiting for a new TMA pulse.  Again Methane is the reaction product. O Al New hydroxyl group Oxygen bridges Methane reaction product Methane reaction product

H Al O O  The reaction product methane is pumped away.  Excess H 2 O vapor does not react with the hydroxyl surface groups,  That caused perfect passivation to one atomic layer. OO Al

 One TMA and one H 2 O vapor pulse form one cycle.  Here three cycles are shown, with approximately 1 Angstrom per cycle.  Each cycle including pulsing and pumping takes e.g. 3 sec. O H Al HH O O OO OO O OO OO O OO OO O Al(CH 3 ) 3 (g) + :Al-O-H (s) :Al-O-Al(CH 3 ) 2 (s) + CH 4 2 H 2 O (g) + :O-Al(CH 3 ) 2 (s) :Al-O-Al(OH) 2 (s) + 2 CH 4 Two reaction steps in each cycle:

 Ligand Precursor ◦ To prepare the surface for next layer, and define the kind of material to growth i.e. H 2 O for oxides, N 2 or NH 3 for nitrides, etc.  Main Precursor (metallic precursor) ◦ Highly reactive (usually this means volatile precursors) ◦ Thermally stable ◦ Full-fill the requirement for self terminating reaction ◦ No self-decomposition ◦ No etching of the film or substrate material ◦ No dissolution into the film or substrate ◦ Sufficient purity

14 Four main types of ALD reactors  Closed system chambers  Open system chambers  Semi-closed system chambers  Semi-open system chambers

15  Closed System Chambers  The reaction chamber walls are designed to effect the transport of the precursors. Open system chambersOpen system chambers Semi-closed system chambersSemi-closed system chambers Semi-open system chambersSemi-open system chambers Schematic of a closed ALD system Ref: "Technology Backgrounder: Atomic Layer Deposition," IC Knowledge LLC, 24 April

16 The Verano 5500™ A 300-mm ALD system by Aviza Technology, Inc [2]. Process Temperature [1] [1] 1 "Technology Backgrounder: Atomic Layer Deposition," IC Knowledge LLC, 24 April "Technology Backgrounder: Atomic Layer Deposition," IC Knowledge LLC, 24 April ”Atomic Layer Deposition," Aviza Technology. 26 April

 Semi & Nanoelectronics  Coatings on Polymers  Protective Coatings  Magnetic Heads  Thin Film Electroluminescent Displays (TFELs)  MEMS  Nanostructures  Chemical  Solar Cell

18 ALD  Highly reactive precursors  Precursors react separately on the substrate  Precursors must not decompose at process temperature  Uniformity ensured by the saturation mechanism  Thickness control by counting the number of reaction cycles  Surplus precursor dosing acceptable CVD  Less reactive precursors  Precursors react at the same time on the substrate  Precursors can decompose at process temperature  Uniformity requires uniform flux of reactant and temperature  Thickness control by precise process control and monitoring  Precursor dosing important

 Self-limiting growth process  Precise film thickness control by the number of deposition cycles  No need to control reactant flux homogeneity  Excellent uniformity and conformity  Large-area and batch capability  Dense, uniform, homogeneous and pinhole-free films  Atomic level composition control  Good reproducibility and straightforward scale-up  Surface exchange reactions by separate dosing of reactants

 Expensive equipment  Low Effective Deposition Rate  Critical adjustment of the flow: too much flow => clogging of valves too low flow => under-performance

Summary  Its unique self-limiting growth mechanism which gives perfect conformality and uniformity.  Easy and accurate thickness control down to an atomic layer level.  Closed System Chambers ALD Reactor is one of the mostly used one.  ALD is a slow method  Expensive equipment & Low Effective Deposition Rate  ALD has many applications in the field of Nanoelectronics, Optical, MEMS, Nanostructures & in Solar cell