1 photo lithography mn it

7/30/2019 1 Photo Lithography Mn It

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Photolithography

D. Boolchandani

Department of ECE

Malaviya National Institute of Technology

Jaipur


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Photolithography 2

Photolithography

In a microelectronic circuit, all the circuitelements (resistors, diodes, transistors, etc.) are

formed in the top surface of a wafer (usually

silicon). These circuit elements are interconnected in a

complex, controlled,patternedmanner.

Consider the simple case of a silicon p-njunction diode with electrical contacts to the p

and n sides on the top surface of the wafer.


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Photolithography 3

Photolithography Silicon p-n junction diode with both electrical contacts on the

top surface of the wafer:

n

p-type substrate

Cross

section:

Al SiO2

Topview:

Can you draw the diode symbol on this diagram?


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Photolithography 4

Photolithography In order to produce a microelectronic circuit,

portions of a silicon wafer must be doped withdonors and/or acceptors in a controlled,patterned

manner.

Holes or windows must be cut throughinsulating thin films in a controlled,patterned

manner.

Metal interconnections (thin film wires) mustbe formed in a controlled,patternedmanner.

The process by which patterns are transferred to

the surface of a wafer is calledphotolithography.


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Photolithography 5

Photolithography Consider the fabrication of a silicon p-n junction diode with both

electrical contacts on the top surface of the wafer:

n

p-type substrate

Cross

section:

Al SiO2

Top

view:


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Photolithography 6

Photolithography We start with a bare silicon wafer and oxidize it. (The bottom

surface also gets oxidized, but well ignore that.):

p-type substrate

Cross

section:

SiO2

Top

view:


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Photolithography We first need to open a window in the SiO2 through which we

can diffuse a donor dopant (e.g., P) to form the n-type region:

p-type substrate

Cross

section:

SiO2

Top

view:



Photolithography

The starting point for the photolithographyprocess is amask.

A mask is a glass plate that is coated with an

opaque thin film (often a metal thin film such as

chromium).

This metal film is patterned in the shape of the

features we want to create on the wafer surface.



Photolithography For our example, our mask could look like this:

glass plate

Cross

section:

opaque metal,e.g.,Cr

Top

view:



Photolithography Recall that we start with a bare silicon wafer and oxidize it.

(The bottom surface also gets oxidized, but well ignore that.):

p-type substrate

Cross

section:

SiO2

Top

view:



Photolithography The wafer is next coated with photoresist.

Photoresist is a light-sensitive polymer. We will initially considerpositive photoresist (more

about what this means soon).

Photoresist is usually spun on.

For this step, the wafer is held onto a support chuckby a vacuum.

Photoresist is typically applied in liquid form

(dissolved in a solvent). The wafer is spun at high speed (1000 to 6000 rpm)

for 20 to 60 seconds to produce a thin, uniform film,typically 0.3 to 2.5 mm thick.



Photolithography After coating with photoresist, the wafer looks like this:

p-type substrate

Cross

section:

Photoresist

Top

view:



Photolithography The wafer is baked at 70 to 90C (soft bake or pre-bake) to

remove solvent from the photoresist and improve adhesion.

p-type substrate

Cross

section:

Photoresist

Top

view:



Photolithography The mask is aligned (positioned) as desired on top of the

wafer.Mask

Cross

section:

Top

view:

p-type substrate

glass plate



Photolithography The photoresist is exposed through the mask with UV light.

UV light breaks chemical bonds in the photoresist.Mask

Cross

section:

Top

view:

p-type substrate

glass plate



Photolithography The photoresist is developed by immersing the wafer in a

chemical solution that removes photoresist that has been exposed

to UV light.

Cross

section:

Top

view:

p-type substrate



Photolithography

The wafer is baked again, but at a higher temperature (120 to

180C). This hard bake or post-bake hardens the photoresist.

Cross

section:

Top

view:

p-type substrate



Photolithography The unprotected SiO2 is removed by etching in a chemical

solution containing HF (hydrofluoric acid), or by dry etching in

a gaseous plasma, containing CF4 , for example.

Cross

section:

Top

view:

p-type substrate



Photolithography The photoresist has done its job and is now removed (stripped)

in a liquid solvent (e.g., acetone) or in a dry O2 plasma.

Cross

section:

Top

view:

p-type substrate

SiO2

window


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Photolithography 20

Photolithography Phosphorous is next diffused through the window to form an

n-type region. The SiO2 film blocks phosphorus diffusion

outside the window.

Cross

section:

Top

view:

p-type substrate

SiO2

window

n-type


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Photolithography 23

Photolithography The wafer surface is next coated with aluminum by evaporation

or sputtering. The window outlinesmay still be visible.

n

p-type substrate

Cross

section:

AlSiO2

Top

view:


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Photolithography 24

Photolithography Photolithography is used to pattern photoresist so as to protect

the aluminum over the windows:

AlSiO2

n

p-type substrate

Cross

section:

Top

view:


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Photolithography 25

Photolithography What must the mask look like in order to pattern the aluminum

film? Assume that were still using positive photoresist.

n

p-type substrate

Cross

section:

AlSiO2

Top

view:


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Photolithography 26

Photolithography The aluminum is etched where it is not protected by photoresist.

Wet or dry etchants can be used.

n

p-type substrate

Cross

section:

AlSiO2

Top

view:


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Photolithography 27

Photolithography Then the photoresist is stripped.

n

p-type substrate

Cross

section:

AlSiO2

Top

view:


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Ph li h h


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Photolithography 29

Photolithography So far we have only consideredpositive

photoresists. For positive resists, the resist pattern on the

wafer looks just like the pattern on the mask

There are alsonegative photoresists. Ultraviolet light crosslinks negative resists, making

them less soluble in a developer solution.

For negative resists, the resist pattern on the

wafer is the negative of the pattern on the mask.


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Photolithography 30

Photolithography

In order to align a new pattern to a pattern

already on the wafer,alignment marks are used.

Various exposure systems

Contact printing,

Proximity printing,

Projection printing, and

Direct step-on-wafer (step-and-repeat projection).

Ph li h h


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Photolithography 31

Photolithography

A complete photolithography process(photoresist + exposure tool + developing

process) can be characterized by the smallest

(finest resolution) lines or windows that can be

produced on a wafer.

This dimension is called theminimum feature

size orminimum linewidth.

The limitations of optical lithography are a

consequence of basic physics (diffraction).

Ph t lith h


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Photolithography 32

Photolithography For a single-wavelength projection photo-

lithography system, theminimum feature size orminimum linewidth is given by theRayleighcriterion:

l is the wavelength.

NA is the numerical aperture, a measure of thelight-collecting power of the projection lens.

k depends on the photoresist properties and the

quality of the optical system.

NAkFw

l

min

Ph t lith h


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Photolithography 33

Photolithography

So how do we reduce wmin ?

Reduce k.

Reduce l. Increase NA.

NAkFw l

min

Ph t lith h


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Photolithography 34

Photolithography

Even for the best projection photolithography

systems, NA is less than 0.8.

The theoretical limit for k (the lowest value) is

about 0.25.

NAkFw

lmin

Ph t lith h


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Photolithography 35

Photolithography

Lenses with higher NA can produce smaller

linewidths. This linewidth reduction comes at a price.

Thedepth of focus decreases as NA increases.

Depth of focus is the distance that the wafer can

be moved relative to (closer to or farther from)

the projection lens and still keep the image in

focus on the wafer.

NAkFw

lmin

Ph t lith h


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Photolithography 36

Photolithography

Depth of focus is given by:

2

)(

6.0

NA

DFl

Depth of focus decreases (bad) as l decreases.

Depth of focus decreases (bad) as NA increases.

Ph t lith h


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Photolithography 37

Photolithography

Numerous light sources are (and will be) used

for optical lithography:

Light Source l(nm)

wmin(nm)

DF(nm)

g-line (Hg lamp) 436 311 850i-line (Hg lamp) 365 260 730

KrF laser 248 175 500

ArF laser 193 140 400

F2 laser 157 112 320


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Ph t lith h


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Photolithography

Other lithographic techniques will play a role in

the future.

Electron beam lithography

Ion beam lithography.

X-ray lithography.

1 photo lithography mn it

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