1 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics Processing Ion Implantation

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Microelectronics Processing Course - J. Salzman – Fall 2006

Microelectronics Processing Ion Implantation

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Issues in Ion Implantation

•Equipment

•Dose, Range, Straggle

• Implantation Profile

•Junction Depth

•Channeling

•Energy Loss Mechanisms

•Damage - Anneal

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Ion Implantation

A process in which energetic, charged atoms or molecules are directly introduced into a substrate.

Acceleration energies range between 10-200 KeV. (Today also up to several MeV)

Primarily used to add dopant ions into the surface of silicon wafers.

Goal : to introduce a desired atomic species, with a specified quantity (dose), into the required depth, with lateral selectivity.

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Advantages of ion implantationAdvantages of ion implantation

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Ion implantation- Equipment - IIon implantation- Equipment - I

An ion implanter is a high voltage particle accelerator producing a high-velocity beam of ions which can penetrate the surface of silicon target wafers. Components: •Ion source•Mass spectrometer•High V accelerator•Scanning system•Target chamber

•Ions generated in a source (from feed gas, e.g. BF3, AsH3, PH3 ...or heated solid source, then ionized in arc chamber by electrons from hot filament)•Accelerate for mass spectroscopy•Select desired species by q/m, using a magnet (mass spectrometer),•Accelerate by an E-field and focus using electrostatic lensesimpact substrate in raster pattern.

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State of the art semiconductor ion implantation system

Axcelis Optima MD ion implantation tool for semiconductor production. (Used with permission from Axcelis Technologies)

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Ion implantation- Equipment - IIIon implantation- Equipment - II

Schematic diagram of an ion implantation system.

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An Ion Source

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Ion Implantation: A random Process

Each ion follows a random trajectory, scattering off silicon atoms, losing energyand coming to rest.

Since a large number of ions is implanted,the average range Rp and their straggle Rp

can be precisely predicted

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Ion implantation-Range and StraggleIon implantation-Range and Straggle

Schematic diagram to show the range, the projected range RP , the

projected (ΔRP ) and lateral ΔR┴ straggle in ion implantation.

y

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Distribution of Ions in SiliconImplanted at 200 keV

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Playing billiards with atoms and electrons…

Ion impact leads to cascades of recoil atoms and electrons

Atomic cascades act as a nano-blender changing crystal structure and mixing atoms

Electron cascades cause chemical changes (radiolysis)

Foreign ion comes to rest under surface of material – ion implantation doping. Changes chemical and electronic behaviour

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Monte Carlo simulation of 50keV Boron Monte Carlo simulation of 50keV Boron implanted into Siimplanted into Si

TRIM

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Mathematical model for ion Mathematical model for ion implantationimplantation

The distribution can be described statistically and is modeled to a first order by a symmetric Gaussian distribution:

2

2

2

)(exp)(

p

pp R

RxCxC

The total number of ionsimplanted is defined as the dose:

dxxCQ )(

(C N in some of the figures)

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Factors in implantationFactors in implantation (1) Range and profile shape depend on the ion energy (for a particular

ion/substrate combination.(2) Height (i.e. concentration) of profile depends on the implantation

dose.(3) Mask layer thickness can block ion penetration.

C(x) in #/cm3

dxxCQdose )(C(x)- concentration = # of atoms/cm3

Q- dose = # of atoms/cm2

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Meaning of dose and Meaning of dose and concentrationconcentration

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Simplified description of Ion Simplified description of Ion implantationimplantation

profile of implanted impurities, C(x):

General profile of ion implanted impurities with a peak concentration Cpeak at depth Rp, and

symmetrical distributions on either side of ΔRP .

areaimplant

time

implant

qampsincurrentbeamion

Q

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Ion implantation-Projected range Ion implantation-Projected range and Straggleand Straggle

(a) Projected range of B, P and As in Si and SiO2 vs ion energy;

(b) projected and lateral straggle of B, P and As ions in Si.

(a) (b)

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Distribution of Ions in SiliconImplanted at 200 keV

Profilenot just Gaussian!

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Rp

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Ion Implantation energy loss Ion Implantation energy loss mechanismmechanism

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LSS Theory of Ion Stopping

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r

b - E

V(r) - 1 r

dr 2b - =

2

22r m

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Nuclear Stopping Power

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Electronic Stopping Power

Se(E) kSi E1/2

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Energy dependence of dE/dx

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Ion distribution: perpendicular and Ion distribution: perpendicular and lateral rangelateral range

Implanted species

Contours of implanted equal ion concentration into Si

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Junction depthJunction depth

The point at which the diffused impurity profile intersects the background concentration, CB, is the metallurgical junction depth, xj. The net impurity concentration at xj is zero. Setting C(xj)=CB we find:

Bpppj

p

pjpB

CCRRx

R

RxCC

/ln2

2

)(exp

2

2

Both roots may be meaningful, as indicated in the figure.

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Sheet resistanceSheet resistance

The resistance of a rectangular block is:R = ρL/A = (ρ/t)(L/W) ≡ Rs(L/W)

Rs is called the sheet resistance. Its units are termed Ω/ .

L/W is the number of unit squares of material in the resistor.

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Sheet resistanceSheet resistance

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Irving’s curves: Motivation to generate Irving’s curves: Motivation to generate themthem

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Figure illustrating the relationship ofFigure illustrating the relationship ofNNoo, N, NBB, x, xjj, and R, and Rss

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Irving’s curvesIrving’s curves

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ChannelingChanneling

The previous, LSS results, are based on the assumption that the target material is amorphous, having a completely random order.

The Si lattice viewed along the <110> axis.

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Atom strings and planes in a crystal

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Channelling

480 keV H+ →(100) W angular scan (After J.U. Andersen Mat. Fys. Medd, Dan. Vid. Selsk. 36,No 7(1967))

Enhanced scattering with atomic rows

Minimum yield from scattering by surface atoms on the end of rows

Critical axial channeling angle

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Ion ChannelingIon Channeling

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““Lucky Ions”Lucky Ions”

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TiltingTilting•Channeling can be reduced by tilting the <100> silicon by approximately 7o relative to the ion beam.

•Tilted implant can produce a doping profile with a junction depth that is closer to the theoretical calculations.

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Implantation damageImplantation damage

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Implantation damage - AmorphizationImplantation damage - Amorphization

A plot of the dose required to form an amorphous layer on silicon versus reciprocal target temperature.

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Post Implant Anneal

Electrical activation of implanted Impurities

Annealing of primary crystalline defect damage

Annealing of amorphous layers Dynamic annealing effects Diffusion of implanted impurities

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Heat Treatment - AdditionalHeat Treatment - Additional

Annealing to restore the crystal structure after the

implantation of dopant atoms (1000 C).

Alloying to ensure good electrical conduction between

metal layers and the wafer surface (450 C).

No material is added in this process.

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Diffusion of Gaussian implantation Diffusion of Gaussian implantation profile upon annealingprofile upon annealing

Note: Q is the implantation dose.

Q

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Diffusion of Gaussian implantation Diffusion of Gaussian implantation profile (arbitrary Rprofile (arbitrary Rpp))

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Multiple implants for uniform profileMultiple implants for uniform profile

Construction of a composite doping profile using multiple implants at different energies.

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Additional featuresAdditional featuresWafer annealing:

Damage removed by annealing the wafer at high temperature for a short period; implanted impurity atoms are "activated".

Proton isolation Hydrogen ions are used to deliberately change the crystal and convert it into electrically insulating material.

Predeposition by ion implantation Ion implantation predeposition of a fixed number of impurities into the semiconductor in preference to predeposition by thermal diffusion

Threshold voltage adjustment:By shallow ion implants through the SiO2 gate oxide layer

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1 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics Processing Ion Implantation