Byoung Dr. Hee You
TECH 4392, MFGE 4392, and TECH 5392
Solutions: Homework#7
1. List thin film characteristics for an acceptable thin film.
Ans)
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Good step coverage
Ability to fill high aspect ratio gaps (conformality)
Good thickness uniformity
High purity and density
Controlled stoichiometries: uniform composition
High degree of structural perfection with low film stress: grain size
Good electrical properties
Excellent adhesion to the substrate material and subsequent films
2. List three problems of nonconformal step coverage in thin film deposition.
Ans)
1) High stress resulting in substrate deformation
2) Electrical shorts
3) Undesirable induced charges
3. State three stages in film growth.
Ans)
1) Nucleation forming isolated patches of film.
2) Coalescence randomly oriented island grow based on the surface mobility
and the density of the clusters.
3) Undesirable induced charges island clusters form solid sheet that spreads
across the substrate surface.
4. State essential aspects of CVD
Ans)
1) Chemical action is involved, either through chemical reaction or by thermal
decomposition (referred to as pyrolysis).
2) All material for the thin film is supplied by an external source.
3) The reactants in a CVD process must start out in the vapor phase (as a gas).
Dr. Byoung Hee You
5. State CVD Reaction Steps (eight steps).
Ans)
1) Gas transport to deposition zone: mass transport of reactants
2) Formation of film precursors: gas phase reaction film precursors and byproducts
3) Film precursors at wafer surface: mass transport of the precursors to the
surface
4) Precursor adsorption: chemical binding of the precursors to the surface
5) Precursor diffusion: precursor diffusion into substrate
6) Surface reaction: film deposition and by-products
7) By-product removal from the surface: desorption of by-products from the
surface reaction
8) By-product removal from reactor
6. List advantages and disadvantages continuous-processing of APCVD reactors.
Ans)
1) Advantages: high throughput, good uniformity, and processing large diameter
wafers
2) Disadvantages: high gas consumption and frequent reactor cleaning
7. List requirements for successful metal material in metallization.
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Conductivity: high conductivity, handling high current density
Adhesion: ability to adhere to the underling substrate, low contact resistance
Deposition: uniform thickness and composition
Patterning/Planarization: high resolution patterning, easy planarization
Reliability: withstanding processing and service temperatures
Corrosion: High resistance to corrosion
Stress: resistance to mechanical stress to reduce wafer distortion and
material failures
8. Why is major transformation underway from aluminum to copper?
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Reduction in resistivity
Reduction in power consumption
Tighter packing density
Superior resistance to electromigration
Dr. Byoung Hee You
5) Fewer process steps
9. List three major challenges to using copper Interconnects in semiconductor
products.
Ans)
1) Copper diffuses quickly into oxides and silicon: creating junction and oxide
leakage
2) Copper cannot be easily patterned using regular plasma etching techniques
3) Copper oxidizes quickly in air at low temperatures (<200 C) and does not
form a protective layer to stop further oxidation.
10. List advantages of sputtering.
Ans)
1) Ability to deposit and maintain complex alloys.
2) Ability to deposit high-temperature and refractory metals.
3) Ability to deposit controlled, uniform films on large wafers (200 mm and
larger).
4) Ability of multichamber cluster tools to clean the wafer surface for
contamination and native oxides before depositing metal (referred to as in situ
sputter etch).
11. List process parameters for copper electroplating.
Ans)
1) Electroplating current and time
2) Rotational speed of cathode
3) Purification of electroplating solution