chp5 ion implantation 2

For the exclusive use of adopters of the book Introduction to Microelectronic Fabrication, Second Edition by Richard C. Jaeger. ISBN0-201-44494-1.!

© 2002 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher.!

Introduction to��Microelectronic Fabrication ��

by ��Richard C. Jaeger ��

Distinguished University Professor��ECE Department��

Auburn University Chapter 5

Ion Implantation



Copyright Notice

•  © 2002 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher.!

•  For the exclusive use of adopters of the book Introduction to Microelectronic Fabrication, Second Edition by Richard C. Jaeger. ISBN0-201-44494-1.!

First things first…

Facebook group – for quick announcements!

Recap:

•  Pure Si is neither a conductor nor insulator. •  Impurities introduce charge carriers (holes or electrons)

into Si, changing its conductivity. •  How do we get the impurities into Si?

Recap:

Diffussion Ion implantation

Neutral dopants diffuse into wafer

Ionized dopants are “shot” into wafer

Dopants can be shot deeper into wafer, better dose control.

Ion Implantation��High Energy Accelerator

1.  Ion Source 2.  Mass Spectrometer 3.  High-Voltage Accelerator (Up to 5 MeV) 4.  Scanning System 5.  Target Chamber

Ion Implantation��High Energy Accelerator

€

Force on charged particle F = q v x B( )

Magnetic Field B =2mVqr2

Implanted Dose Q =1

mqAI t( )

0

T

∫ dt

m = mass

v = velocityV = acceleration potentialA = wafer area



Ion Implantation��Overview •  Wafer is target in High Energy Accelerator •  Impurities “Shot” into Wafer. Vary DOSE and ENERGY. •  Preferred Method of Adding Impurities to Wafers

–  Wide Range of Impurity Species (Almost Anything) –  Tight Dose Control (A few % vs. 20-30% for high temperature

pre-deposition processes) –  Low Temperature Process

•  Expensive Systems (why?) •  Vacuum System (why?)



Think about it.��Pros and Cons of Ion Implantation vs Diffusion 1.  Better Dose Control --Resistor tolerance is a just few percent if fabricated with

implantation; exceeds 20% with diffusion. --Complicated doping profiles can be realized.

2.  Expensive --Ion implantation needs vacuum, accelerators, millions

USD.

Interesting online lectures on Ion Implantation

•  http://www.youtube.com/watch?v=V-U3v9QBPO4"

•  http://www.youtube.com/watch?v=zLcNJvWbbHc"

Ion Implantation

As an ion enters wafer surface:- -  it collides with the lattice atoms -  each collision reduces the ion’s energy, it finally stops.

Some “lucky ions” don’t collide much, settle very deep far beyond surface.

Some ions “very unlucky”, get backscattered and settle near surface.

The collisions are random statistical process, we assume it is Gaussian (normal distribution)

Ion Implantation ��Mathematical Model

“lucky” ions

“unlucky” ions

Most ions stop at Rp Io

n be

am

Model assumption: Target is amorphous (atoms in target material are randomly positioned)

Ion Implantation ��Mathematical Model

€

Gaussian Profile

N x( ) = Np exp −x − Rp( )

2

2ΔRp2

⎡

⎣

⎢ ⎢

⎤

⎦

⎥ ⎥

Rp = Projected Range

ΔRp = Straggle

Dose Q = N x( )dx = 2π0

∞

∫ NpΔRp

Ion Implantation��Projected Range (LSS Theory)

The more energy your ions have, the deeper they get into wafer (energy ~ range almost linearly)

THINK: For a given energy, why different dopants have different projected range?

Ion Implantation��Straggle (vertical and transverse)

In the wafer, the ions spread in both vertical and transverse directions.

Results for Si and SiO2 identical

Ion Implantation��Example 5.1:-

Phosphorus with 100 keV energy is implanted into a Si wafer.

a)  What is its projected range and stragle? b)  What should the total dose be if desired peak

concentration is 10e17 cm-2? c)  What is the total time required to implant this dose

into 200-mm wafer with 2uA beam and singly ionised phosphorus?

Ion Implantation��Selective Implantation

But some ions still manage to settle right below the mask! Why?

Collisions (scattering) happen in all directions!

We only want certain areas implanted Areas not to be implanted are covered with mask


Figure 5.4 Contours of equal ion concentration for an implantation into silicon through a 1-µm window. The profiles are symmetrical about the x-axis and were calculated using the equation above taken from Ref. [3].

This is mask. You use this to cover the region beneath from being doped.

But some ions still manage to settle right below the mask! Why?

Most ions settle at Rp

Collisions (scattering) happen in all directions!


€

N x,y( ) = N x( )F y( )

F y( ) =12erfc y − a

2ΔR⊥

⎛

⎝ ⎜

⎞

⎠ ⎟ − erfc

y + a2ΔR⊥

⎛

⎝ ⎜

⎞

⎠ ⎟

⎡

⎣ ⎢

⎤

⎦ ⎥

ΔR⊥ = transverse straggle

N x( ) is one - dimensional solution

Figure 5.4 Contours of equal ion concentration for an implantation into silicon through a 1-µm window. The profiles are symmetrical about the x-axis and were calculated using the equation above taken from Ref. [3].


| MASK |

Ion

beam

Some “lucky” ions still manage to penetrate through the mask!

We need thick enough mask to stop penetration “lucky” ions.



•  Desire Implanted Impurity Level to be Much Less Than Wafer Doping N(X0) << NB or N(X0) < NB/10




€

X0 ≥ Rp + ΔRp 2ln10Np

NB

⎛

⎝ ⎜

⎞

⎠ ⎟ = Rp + mΔRp

Mask thickness, Xo:-

Ion Implantation��Selective Implantation Example 5.2:-

Boron penetration through a 50-nm gate oxide, with peak concentration at the Si-SiO2 interface. Total dose implanted is to be 10e13 cm-2. a)  Calculate implant energy and peak concentration at the interface b)  How thick should the SiO2 layer be in areas not to be implanted,

if background concentration is 10e16 cm-3? c)  Supposed the oxide is 50nm thick everywhere, how much

photoresist required on top of the oxide to mask the implantation?

Note:- photoresist layer less effective in stopping ions; needs 1.8 times ooxide thickness for a same stopping power.

Ion Implantation (Junction depth)��If implant deep enough, you get both “tails” in Si

At points where N(x) = Nb, you get PN junctions.

Ion Implantation��Junction Depth

€

N x j( ) = NB

Np exp −x j − Rp( )

2

2ΔRp2

⎡

⎣

⎢ ⎢

⎤

⎦

⎥ ⎥

= NB

x j = Rp ± ΔRp 2lnNp

NB

⎛

⎝ ⎜

⎞

⎠ ⎟

Ion Implantation��Junction Depth

Study Example 5.3

Ion Implantation��Channeling

Silicon atoms are organized in an orderly crystal structure, not random. Therefore large open spaces exist.

So how would this effect our Model?

What happens if an ion is shot right through this long “channel”?

Ion Implantation��Channeling “Channeling” causes ion penetration much

deeper than predicted by our simple model

Channeling depends on angle

Ion Implantation��Deviation from Gaussian Theory

Curves fit four-moment (Pearson Type-IV) distribution functions

These don’t look like a normal distribution. They are skewed!

Light ions tend to back scatter, settle near surface, long left tail dist.

Ion Implantation��Shallow Implantation

Ion Implantation��Lattice Damage and Annealing

•  Implantation causes damage to the lattice.

•  Implanted species not electrically active yet.

•  We then heat (anneal) the wafer to repair damage, activate the dopants.

Annealing moves Si and impurity atoms back to lattice sites, form bonds. Impurity becomes electrically active.

Ion Implantation��Lattice Damage and Annealing

•  Implantation Causes Damage to Surface

•  Typically Removed by Annealing Cycle 800-1000o C for 30 min.

•  Rapid Thermal Annealing (RTA) Now Used for Lower Dt Product

Ion Implantation��Rapid Thermal Annealing

Figure 5.12 (a) Concept for a rapid thermal annealing (RTP) system. (b) Applied Materials 300 mm RTP System (Courtesy Applied Materials)

• Rapid Heating

• 950-1050o C

• 50o C/sec

• Very Low Dt

(b)

Ion Implantation��Summary



Ion Implantation ��References

1. Read the whole Chapter 5.

2. Do problems 5.1 and 5.4.

3. Discuss the following:- - Advantages and disadvantages of ion implantation and diffusion

- What is channeling? Causes and Effect of channeling? - What is annealing? Purposes of annealing?

Due in 2-weeks time (Wednesday 16 April 2014)

You will be asked to demonstrate the solutions in class

Tutorial Chp. 5:

chp5 ion implantation 2

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