Ion Implantation: The most controlled way to introduce dopants into a crystal.

© 2004 Dieter Ast , Edwin Kan This material has been edited for class presentation.

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What could this be ????

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© 2004 Dieter Ast , Edwin Kan This material has been edited for class presentation.

And it is ….

A Swift Implanter

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Current implant products include the xR80S, which performs implants in the energy range of 2 to 80keV ; the xR120S for energy ranges of 2 to 120 keV ; and the xR LEAP for energy ranges of 200eV to 80keV . All these products provide a significant increase in productivity and a dramatic decrease in f ootprint size over traditional implant systems. The xR200S, an updated version of the Precision Implant 9500xR, operates over an energy range of 2 to 200keV , with an option to perform phosphorous implants up to 720keV.

© 2004 Dieter Ast , Edwin Kan This material has been edited for class presentation.

Ion Implantation

Introducing dopants by diffusion had three major drawbacks :

•

Surface concentration and depth were coupled.

•

Essentially, only Gaussian and erfc profiles could be generated.

•

The process was not precisely reproducible - an effect which we now know reflects various concentration of point defects (vacancies, interestitials etc).

Ion implantation fixes these problems at the cost of crystal damage.

•

Variable profiles, precise control over amount of impurities.

For a history, as well as a general excellent WEB reference click here: http://courses.nus.edu.sg/course/phyweets/Projects98/proj3204/ion_mpage.htm

© 2004 Dieter Ast , Edwin Kan This material has been edited for class presentation.

A typical implanter has an ion source accelerator filtering magnet deflection/scanning coils or plates incident current meter Typical voltages are 50 to 200 KeV, the trend is to lower voltages.

© 2004 Dieter Ast , Edwin Kan This material has been edited for class presentation.

To make the ions, a neutral gas, such as BF 2 is introduced into the ion source and turned into ions by bombarding it with electrons (the black loop).

The source is at a very high positive potential respective to ground (hundreds of kilovolts).

Getting the gas, stored at a bottle at ground potential, up into the source without a “Short” is not trivial. Neither is control and powering of the electron emitting filament.

© 2004 Dieter Ast , Edwin Kan This material has been edited for class presentation.

The ions are accelerated in steps by letting them “fall down” in e.g. 50 KeV steps from + 200 Kev (relative to ground) to ground.

It can’t be done in one step, as you would likely get arcing © 2004 Dieter Ast , Edwin Kan This material has been edited for class presentation.

The ions are filtered by sending them trough a magnetic field F = q (v x B) Lighter ions move faster and experience higher radial forces. Heavier ions lower ones.

This sorts out impurities as well as BF 2 ++ (double charged ions) © 2004 Dieter Ast , Edwin Kan This material has been edited for class presentation.

The filtered ion beam is expanded (electrostatic lens) and scanned over the wafer.

Some accelerators scan by moving the substrate, others be deflecting the beam, and yet others use a combination (e.g. deflection for x and mechanical movement for y) The incident current is measured as it is a very accurate measurement of the dose © 2004 Dieter Ast , Edwin Kan This material has been edited for class presentation.

What do we need to know…

• • • •

How the ions lose energy in the solid

(Ion stopping)

Their final distribution in the solid

(Range, straggle)

The damage they do to the crystal

(EOR damage)

How to anneal out the damage

(Dopant activation)

© 2004 Dieter Ast , Edwin Kan This material has been edited for class presentation.

•

Ion Implantation

– – – – –

Ion Stopping Channeling Damage Equipment Annealing

Outline

© 2004 Dieter Ast , Edwin Kan This material has been edited for class presentation.

Ion Stopping

• •

Incident Ion Energies Stopping Power

– –

Nuclear Electronic © 2004 Dieter Ast , Edwin Kan This material has been edited for class presentation.

B -> Si Stop Profiles

B Implant

a

-Si Target No Annealing SIMS Data Stop Profiles - Gaussian - Pearson IV © 2004 Dieter Ast , Edwin Kan This material has been edited for class presentation.

2D Implant Profile Example

Note how “dopant crawls under mask” Implant P, 150 keV R p

D

R p = 190 nm = 63 nm 2

m

m Window 3.2 x 0.6

m

m Si D = 0.8x10

-15 cm 2 /s After Krusius et al, IEEE TED-29, p. 435 (1982) The “sideways” extension is sometimes characterized by yet an other parameter: “lateral straggle” © 2004 Dieter Ast , Edwin Kan This material has been edited for class presentation.

2D Implant Profile Example

Since the implant must be annealed to activate the dopants, there is further movement. The D for this step is not the D in the tables… it is enhanced by point defects introduced by the implantation (“transient enhanced diffusion”, excellently treated in Plummer) Implant + Diffusion P, 150 keV R p

D

R p = 190 nm = 63 nm 2

m

m Window 3.2 x 0.6

m

m Si D = 0.8x10

-15 cm 2 /s

Dt

 140

nm

Krusius et al, IEEE TED-29, p. 435 (1982) © 2004 Dieter Ast , Edwin Kan This material has been edited for class presentation.

2D Implant Profile Example

Here is what happens if the “Dt” product is larger than expected !

Implant + Diffusion P, 150 keV R p

D

R p = 190 nm = 63 nm 2

m

m Window 3.2 x 0.6

m

m Si D = 0.8x10

-15 cm 2 /s

Dt

 440

nm

Krusius et al, IEEE TED-29, p. 435 (1982) © 2004 Dieter Ast , Edwin Kan This material has been edited for class presentation.

Ion Channeling

•

Si Crystal Faces

– – –

(100) Tilted (100) (111) © 2004 Dieter Ast , Edwin Kan This material has been edited for class presentation.

Ion channeling

•

In certain crystallographic directions, noticeably [110], ions travel much deeper into the crystal than expected.

•

The effect, explained next, is caused by “open channels” in the crystal structure.

•

The effect is a nuisance in the IC industry. To turn it off, wafers are deliberately tilted such that the beam sees no open channels © 2004 Dieter Ast , Edwin Kan This material has been edited for class presentation.

Channeling Concept

Ion Scattered into Channel Ion Recoil Target Atom Row Channeled Ion Si Channel © 2004 Dieter Ast , Edwin Kan This material has been edited for class presentation.

Si Crystal Faces

(100) Si Face (100) Si Face 7 o Tilt, 30 o Twist © 2004 Dieter Ast , Edwin Kan This material has been edited for class presentation.

Ion Damage

• • •

Target Atom Recoils

– –

Absorption of Energy Generation of Vacancies Point Defect and Ion Distributions Critical Dose for Amorphization © 2004 Dieter Ast , Edwin Kan This material has been edited for class presentation.

Damage to Crystal lattice

• • • •

The incident ions have very high energy. Typically e.g. 100 keV.

It only takes about 20 eV to knock a silicon atom off it’s lattice site and shoot as an interstitial somewhere into the lattice.

Thus, a 100 KeV atom can dislodge, very roughly, on the order of 5000 Si atom.

If each dopant atom eventually lands on a lattice site, then there must be as many Si self interstitials as there are dopant atoms (e.g. 1e20/cm 3 ). These interstitials cluster into {311} defects and end of range EOR loops © 2004 Dieter Ast , Edwin Kan This material has been edited for class presentation.

Critical Dose for Amorphization

122 Sb 31 P 11 B Implant -> Si Continuous Amorphous Layer 1000/T (K -1 ) Sze, Fig. 12, p. 343 © 2004 Dieter Ast , Edwin Kan This material has been edited for class presentation.

Implant Damage Annealing

•

Function Defect Removal, Activation of Dopants

•

Furnace, Rapid Thermal Annealing, and Laser Annealing

•

Solid State Epitaxial Regrowth © 2004 Dieter Ast , Edwin Kan This material has been edited for class presentation.

Target Atom

Implant Annealing/Activation

Ion X-tal Surface Individual Collision Cascades for < 10 Amorphous Layer for > 10 15 cm -2 Damaged X-tal or Amorphous 12 cm -2 Annealing - SSER > 450 C - Point Defects - Extended Defects X-tal Bulk Activation - Substitutional Sites © 2004 Dieter Ast , Edwin Kan This material has been edited for class presentation.

Annealing/Activation

End-of-Range (EOR) Dislocation Loops After Solid Phase Epitaxial Regrowth Depth PDG 8-25 2000 © 2004 Dieter Ast , Edwin Kan This material has been edited for class presentation.

Activated Fraction

SIMS 800 C SIMS 900 C Implant B-> Si 70 keV 10 15 cm -2 Anneal 800 or 900 C 35 min Hall Hall After Sze, Fig. 24, p. 357 Concentration (cm -3 ) © 2004 Dieter Ast , Edwin Kan This material has been edited for class presentation.

Implant/Anneal Examples

10 21 I RTA F Implant B-> Si 35 keV Anneals RTA 1100 C/10 s RTA 1100 C/30 s F 1000 C/30 m 10 15 Sze, Fig. 29, p. 362 0 Depth (

m

m) © 2004 Dieter Ast , Edwin Kan 1 This material has been edited for class presentation.

Implant/Anneal Examples

Transient Enhanced Diffusion (TED) Anomalous Diffusion After Ion Implantation Depth (

m

m) PDG 8-31 2000 © 2004 Dieter Ast , Edwin Kan This material has been edited for class presentation.