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Photolithography

D. Boolchandani

Department of ECE

Malaviya National Institute ofTechnology

Jaipur


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Photolithography

Photolithography

In a microelectronic circuit, all the circuitelements (resistors, diodes, transistors,etc.) are formed in the top surface of awafer (usually silicon).

These circuit elements are interconnectedin a complex, controlled,patternedmanner.

Consider the simple case of a silicon p-njunction diode with electrical contacts to thep and n sides on the top surface of the

wafer.


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Photolithography

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

the top surface of the wafer:

np-type substrate

Crosssection:

Al SiO2

Topview:

Can you draw the diode symbol on thisdiagram?


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Photolithography

Photolithography In order to produce a microelectronic circuit,

portions of a silicon wafer must be dopedwith donors and/or acceptors in a controlled,

patternedmanner.

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

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

The process by which patterns are

transferred to the surface of a wafer is called


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Photolithography

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

with both electrical contacts on the top surface of the

wafer:

np-type substrate

Cross

section:

Al SiO2

Topview:


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Photolithography

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

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

p-type substrateCross

section:

SiO2

Topview:


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Photolithography We first need to open a window in the SiO

2through

which we can diffuse a donor dopant (e.g., P) to form then-type region:


section:

SiO2

Topview:



Photolithography

The starting point for the photolithographyprocess is a mask.

A mask is a glass plate that is coated withan opaque thin film (often a metal thin film

such as chromium).

This metal film is patterned in the shape ofthe features we want to create on the wafer

surface.



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

glass plateCross

section:

opaque metal,e.g.,Cr

Topview:



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

it. (The bottom surface also gets oxidized, but well ignore

that.):


section:

SiO2

Topview:



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 supportchuck by a vacuum.

Photoresist is typically applied in liquid form

(dissolved in a solvent).

The wafer is spun at high speed (1000 to 6000rpm) for 20 to 60 seconds to produce a thin,

uniform film, typically 0.3 to 2.5 m thick.



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


section:

Photoresist

Topview:



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

to remove solvent from the photoresist and improve

adhesion.


section:

Photoresist

Topview:



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

the wafer. Mask

Cross

section:

Topview:

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:

Topview:

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:

Topview:

p-type substrate



Photolithography

The wafer is baked again, but at a higher temperature

(120 to 180C). This hard bake or post-bake hardens thephotoresist.

Cross

section:

Topview:

p-type substrate



Photolithography The unprotected SiO

2is 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:

Topview:

p-type substrate


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Photolithography

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:

Topview:

p-type substrate

SiO2window

Ph li h h


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Photolithography

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:

Topview:

p-type substrate

SiO2window

n-type

Ph t lith h


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Photolithography

Photolithography Another photolithography step must be performed in order

to open another window in the SiO2

so we can make an

electrical contact to the p-type substrate from the topsurface of the wafer.

Cross

section:

Topview:

p-type substrate

n-type

glass platenewmask


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Photolithography

Photolithography The steps will not be shown in detail, but after

photolithography, SiO2

etching, and photoresist stripping,

the wafer structure is shown below.

np-type substrate

Cross

section:

SiO2

Topview:


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Photolithography

Photolithography The wafer surface is next coated with aluminum by

evaporation or sputtering. The window outlines maystill

be visible.

np-type substrate

Cross

section:

Al SiO2

Topview:


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Photolithography

Photolithography Photolithography is used to pattern photoresist so as to

protect the aluminum over the windows:

Al SiO2

np-type substrate

Cross

section:

Topview:

Ph li h h


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Photolithography

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

aluminum film? Assume that were still using positive

photoresist.

np-type substrate

Cross

section:

Al SiO2

Topview:

Ph t lith h


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Photolithography

Photolithography The aluminum is etched where it is not protected by

photoresist. Wet or dry etchants can be used.

np-type substrate

Cross

section:

Al SiO2

Topview:

Ph t lith h


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Photolithography

Photolithography Then the photoresist is stripped.

np-type substrate

Cross

section:

Al SiO2

Topview:

Ph t lith h


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Photolithography

Photolithography The final step is to anneal (heat treat) the wafer at ~ 450

C in order to improve the electrical contact between the

aluminum film and the underlying silicon.

np-type substrate

Crosssection:

Al SiO2

Topview:

Ph t lith h


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Photolithography

Photolithography So far we have only consideredpositive

photoresists. For positive resists, the resist pattern on

the wafer looks just like the pattern on themask

There are also negative 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 onthe mask.


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Photolithography

Photolithography

In order to align a new pattern to a patternalready on the wafer, alignment marks areused.

Various exposure systems Contact printing,

Proximity printing,

Projection printing, and Direct step-on-wafer (step-and-repeat

projection).

Ph t lith h


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Photolithography

Photolithography

A complete photolithography process(photoresist + exposure tool + developingprocess) can be characterized by thesmallest (finest resolution) lines or windows

that can be produced on a wafer.

This dimension is called the minimumfeature size orminimum linewidth.

The limitations of optical lithography are aconsequence of basic physics (diffraction).

Ph t lith h


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Photolithography

Photolithography For a single-wavelength projection photo-

lithography system, the minimum featuresize orminimum linewidth is given by theRayleigh criterion:

is the wavelength.

NA is the numerical aperture, a measure ofthe light-collecting power of the projectionlens.

k depends on the photoresist properties

Ph t lith h


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Photolithography

Photolithography

So how do we reduce wmin

?

Reduce k.

Reduce . Increase NA.

Photolithograph


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Photolithography

Photolithography

Even for the best projectionphotolithography systems, NA is less

than 0.8.

The theoretical limit for k (the lowestvalue) is about 0.25.

Photolithography


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Photolithography

Photolithography

Lenses with higher NA can produce smaller

linewidths. This linewidth reduction comes at a price.

The depth 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

Photolithography


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Photolithography

Photolithography

Depth of focus is given by:

Depth of focus decreases (bad) as decreases.

Depth of focus decreases (bad) as NAincreases.


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Photolithography


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Photolithography

Photolithography

Complex devices require the

photolithography process to be carried outover 20 times.

over 20 mask levels

Any dust on the wafer or mask can result indefects. Cleanrooms are required forfabrication of complex devices.

Even if defects occur in only 10% of the chipsduring each photolithography step, fewerthan 50% of the chips will be functional aftera seven mask process is completed.

How is this yieldcalculated?

Photolithography


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Photolithography

Other lithographic techniques will play arole in the future.

Electron beam lithography

Ion beam lithography.

X-ray lithography.

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