Ion Implantation

Ion Implantation-Overviewis a low-temperature technique for the

introduction of impurities (dopants) into semiconductors and offers more flexibility than diffusion.

Wafer is Target in High Energy Accelerator

Impurities “Shot” into Wafer

Ion Implantation

x

Blocking maskSi

+ C(x) as-implant depth profile

Depth xEqual-Concentrationcontours

Reminder: During implantation, temperature is ambient. However, post-implant annealing step (>900oC) is required to anneal out defects.

Reminder: During implantation, temperature is ambient. However, post-implant annealing step (>900oC) is required to anneal out defects.

y

Ion Stopping

Ions gradually lose their energy as they travel through the solid

The loss of ion energy in the target is called stopping.

Ion Implantation Energy Loss Mechanisms

Si++

Si

Si

e

e

++

Electronicstopping

Nuclearstopping

Crystalline Si substrate damaged by collision

Electronic excitation creates heat

Ion Energy Loss Characteristics

EE143 – Ali Javey

Light ions/at higher energy more electronic stopping

Heavier ions/at lower energy more nuclear stopping

EXAMPLES Implanting into Si:

H+

B+

As+

Electronic stoppingdominates

Electronic stoppingdominates

Nuclear stoppingdominates

Nuclear collisions: ions collide with atoms. The positively charged ions are coulombically repealed by the positive cores of the wafers lattice atoms. This coulombic repulsion is “screened” by the cloud of electrons surrounding each atom.

Electronic stopping: If ions graze the lattice atoms, they do not interact with the lattice atom’s electrons and not the positive core. This interaction slows the ions by “viscous friction” similar to a rock thrown into water.

Advantages of Ion Implantation:Precise control of dose and depth profileVery fast (1 6" wafer can take as little as 6

seconds for a moderate dose)Wide selection of masking materials

e.g. photoresist, oxide, poly-Si, metalLess sensitive to surface cleaning

proceduresExcellent lateral uniformity

(< 1% variation across 12” wafer)

Disadvantages of Ion Implantation:Not all the damage can be corrected by

annealing.Typically has higher impurity content than

does diffusion.Often uses extremely toxic gas sources such

as arsine (AsH3), and phosphine (PH3).Expensive

ApplicationDopingNitrogen or other ions can be implanted into

a tool steel target (drill bits, for example). prosthetic devices such as artificial joints, it

is desired to have surfaces very resistant to both chemical corrosion and wear due to friction.

ion beam mixing, i.e. mixing up atoms of different elements at an interface.

Range DistributionsThe range of a distribution with a discrete

random variable is the difference between the maximum value and the minimum value. For a distribution with a continuous random variable, the range is the difference between the two extreme points on the distribution curve, where the value of the function falls to zero. For any value outside the range of a distribution, the value of the function is equal to 0.

Range-is the difference between the highest and the lowest values in a frequency distribution.

-It also the average total of a path length .

Where:

n(x= Rp) = is the peak concentration

Rp = is the projected range

∆Rp(Ợp) = is the standard deviation

Gaussian distribution

Implant UniformityNon uniformity of implanted dopants across a

wafer can be readily untraveled from sheet resistance measurements.

Sheet resistance maps of 3 samples implanted with 50keV phosphorous as a function of dose.

(a) 5x 1013 cm-2, (b) 2x 1014 cm-2, (c) 2 x1015 cm-2.

AnnealingAnnealing-  It is a process that produces

conditions by heating to above the critical temperature, maintaining a suitable temperature, and then cooling.

Annealing is used to induce ductility, soften material, relieve internal stresses, refine the structure by making it homogeneous, and improve cold working properties.

Diffusion During Subsequent Anneals

For implantations far away from the surface and for reasonable short characteristic diffusion lengths , the new profile can be approximated by: