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Transcript of 1 Photo Lit Hog Rap Hymn It
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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:
p-type substrateCross
section:
SiO2
Topview:
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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.
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Photolithography For our example, our mask could look like this:
glass plateCross
section:
opaque metal,e.g.,Cr
Topview:
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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 substrateCross
section:
SiO2
Topview:
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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.
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Photolithography After coating with photoresist, the wafer looks like this:
p-type substrateCross
section:
Photoresist
Topview:
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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 substrateCross
section:
Photoresist
Topview:
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Photolithography The mask is aligned (positioned) as desired on top of
the wafer. Mask
Cross
section:
Topview:
p-type substrate
glass plate
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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
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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
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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
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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.