Semiconductor Manufacturing Process
Comparison of Diffusion and Ion Imp antat on •
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Diffusion is a cheaper and more simplistic method, but can only be performed from the surface of the wafers. Dopants also diffuse unevenly, and interact with each other altering the diffusion rate. Ion implantation is more expensive and complex. It does not control of dopant concentration and profile. It is an anisotropic process and therefore does not spread the . manufacture of self aligned structures which greatly improve the performance of MOS transistors. ‐
Dope Semiconductor: Ion Implantation • Used for atomic and nuclear research •
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• Introduced to semiconductor ‐ .
Hong Xiao, Ph. D.
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Dope Semiconductor: Ion Implantation • Independently control dopant profile (ion ener and do ant concentration ion current times implantation time) • • Easy to achieve high concentration dope of and arsenic. Hong Xiao, Ph. D.
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Metal Gate
p+ S/D n‐Si
Gate Oxide
Metal Gate
n‐Si
Aligned
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p+ S/D
Misaligned
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Ion Implantation, Phosphorus
SiO2
Poly Si
P+
n+
n+ P‐type Silicon
Hong Xiao, Ph. D.
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Comparison of Implantation and Diffusion Doped region
SiO2
PR
Si
Si
Junction depth Ion implantation
Diffusion
Hong Xiao, Ph. D.
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Implantation and Diffusion Diffusion
Ion Implantation
High temperature, hard mask
Low temperature, photoresist mask
Isotropic dopant profile
Anisotropic dopant profile
Cannot independently control of the dopant
Can independently control of the dopant
Batch process
Both Batch and single wafer process
Hong Xiao, Ph. D.
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dopant concentration •
Dopant profile is anisotropic
Hong Xiao, Ph. D.
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App cat ons Ions
Hong Xiao, Ph. D.
Dop ng n-type: P, As, Sb p-type: B
Pre-amorp ous Si or Ge
Bur e ox e O
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Po y arr er N
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Other Applications • Oxygen implantation for silicon‐on‐ insulator SOI device • Pre‐amorphous silicon implantation on • Pre‐amorphous germanium implantation • …... Hong Xiao, Ph. D.
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Etching is the process where unwanted areas of films are solution (Wet Etching) or by reacting them with gases in a plasma to form volatile products (Dry Etching). Resist protects areas which are to remain. In some cases a , y y 2 3 4, used when the etch selectivity to photoresist is low or the etching environment causes resist to delaminate. This is part of lithography ‐ pattern transfer.
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Wet etches: ‐
(not used to etch features less than ‐
3 µm)
achieve hi h selectivities for most film combinations
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≈
ca able of hi h throu h uts
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use comparably cheap equipment
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can have resist adhesion problems
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can etch just about anything
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For SiO2 etching HF + NH4F+H20 (buffered oxide etch or BOE)
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Hot phosphoric acid: H3PO4 at 180 °C need to use oxide hard mask
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Nitric, HF, acetic acids HNO 3 + HF + CH3COOH + H2O Acetic, nitric, phosphoric acids at 35 45 °C CH 3COOH+HNO3+H3PO4 ‐
What is a plasma glow discharge ? •
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•
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A plasma is a partially ionized gas made up of equal parts ositivel and ne ativel char ed articles. Plasmas are generated by flowing gases through an electric or magnetic field. These fields remove electrons from some of the gas molecules. The liberated electrons are accelerated, or ener ized b the fields. The energetic electrons slam into other gas molecules, liberating more electrons, which are accelerated and liberate more e ectrons rom gas mo ecu es, t us susta n ng t e plasma.
Combination of chemical and physical etching – Reactive Ion Etching (RIE) Directiona etc ing ue to ion assistance. In RIE processes the wafers sit on the powered electrode. This placement sets up a negative bias on the wafer which acce erates pos t ve y c arge ons towar t e sur ace. ese ions enhance the chemical etching mechanisms and allow anisotropic etching. Wet etches are simpler, but dry etches provide better line width control since it is anisotropic.
