Thermal diffusion, Ion implantation. firstname.lastname@example.org. Doping: modifying resistivity. Gas phase diffusion Solid source diffusion Ion implantation e.g. POCl 3 gas e.g. boron-doped film 20-200 keV ions at 1000 o C at 1000 o C at room temperature
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Gas phase diffusion Solid source diffusion Ion implantation
e.g. POCl3 gas e.g. boron-doped film 20-200 keV ions
at 1000oC at 1000oC at room temperature
Oxide mask Oxide mask Resist mask
Single crystal silicon resisitivity is lower than that of polysilicon, for same doping concentration.
Fixed supply of dopant:
when dopant atoms diffuse into silicon, surface concentration decreases
Infinite supply of dopant:
New supply of dopant keeps surface concentration constant when dopant atoms diffuse into silicon
Longest diffusion time
Longest diffusion time
Fixed number of dopantatoms “Infinite” initial dopant atoms
-deep p diffusion
-heavy n+ diffusion
-intermediate n diffusion
-shallow p+ diffusion
-heavy p+ diffusion on back
Take n-silicon wafer
3. Perform n-diffusion p-diffusion becomes deeper
N-concentration must be higher than p; otherwise dopant type does not change.
When n-dopant concentration is high enough,
p-type silicon turns n-type.
This n-type silicon can be turned to p-type again, by applying even enough boron concentration.
Ion energies 10-200 keV
Implant depths 10-500 nm
Doses 1011 to 1016 ions/cm-2.
Concentrations ca. 1015 cm-3 to 1020 cm-3.
5.1015 cm-2 ion implant dose and depth of ca. 200 nm translates to ca. 25 Ohm/sq sheet resistance
Ion (P), dose (1014 cm-2), energy (100 keV)
Rp depends on incident
and target atomic masses
Ion energy (keV)
100 keV; B, P, As into silicon
Implantation thru 200 nm thick oxide (50 keV vs. 150 keV)
As a function of energy:
P+: 50, 100, 150 keV energies
As a function of dose P+ doses:
Light ions like boron, and low doses (<1014 cm-2) cause mainly point defects
High doses (>1015 cm-2) and heavy ions (As+, Sb+, Ge+) cause extended damage and amorphization
– dopant gas containing desired species
Doping by diffusion:
Oxide mask for diffusion
MOS gate needs to be aligned to S/D junctions
Self-aligned gate by implantation:
polysilicon gate blocks ions, and MOS channel remains undoped
Gas phase dopant molecules are mixed in silane flow:
2 PH3 2 P + 3 H2
B2H6 2 B + 3 H2
Epitaxial films come in all the same varieties as silicon wafers:
-high resisitivity (not intentionally doped)
Benefits of epi:
Oxygen and carbon of the substrate wafer are buried under epi.
Dopantuniformity is very good.
Add gaseous dopants into the flow:
B2H6 for boron
PH3 for phoshorous
AsH3 for arsenic
Very small partial pressures enough:
10-10 bar 1015 cm-3
10-8 bar 1017cm-3
10-6 bar 1019cm-3
Reactor cleaning before each wafer.
In-situ wafer cleaning to ensure the best possible cleanliness.
Because epitaxy is a high temperature process, dopant atoms diffuse during epitaxy.
Diffusion is from high dopant concentration to low concentration.
Epi doping level is independent of substrate doping level, but the interface is not sharp due to diffusion.
Lightly doped epi Heavily doped substrate