1 photo lithography mn it
TRANSCRIPT
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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:
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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.
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Photolithography For our example, our mask could look like this:
glass plate
Cross
section:
opaque metal,e.g.,Cr
Top
view:
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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 substrate
Cross
section:
SiO2
Top
view:
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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 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.
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Photolithography After coating with photoresist, the wafer looks like this:
p-type substrate
Cross
section:
Photoresist
Top
view:
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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 substrate
Cross
section:
Photoresist
Top
view:
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Photolithography The mask is aligned (positioned) as desired on top of the
wafer.Mask
Cross
section:
Top
view:
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:
Top
view:
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:
Top
view:
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 the photoresist.
Cross
section:
Top
view:
p-type substrate
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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
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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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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).
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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).
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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
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Photolithography 33
Photolithography
So how do we reduce wmin ?
Reduce k.
Reduce l. Increase NA.
NAkFw l
min
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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
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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
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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.
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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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Photolithography
Other lithographic techniques will play a role in
the future.
Electron beam lithography
Ion beam lithography.
X-ray lithography.