Embed Size (px)
Citation preview
PhotolithographyPhotolithography
D. BoolchandaniD. Boolchandani
Department of ECEMalaviya National Institute of Technology
Jaipur
Photolithography 2
PhotolithographyPhotolithography
In a microelectronic circuit, all the circuit elements (resistors, diodes, transistors, etc.) are formed in the top surface of a wafer (usually silicon).
These circuit elements are interconnected in a complex, controlled, patterned manner.
Consider the simple case of a silicon p-n junction diode with electrical contacts to the p and n sides on the top surface of the wafer.
Photolithography 3
PhotolithographyPhotolithography Silicon p-n junction diode with both electrical contacts on the
top surface of the wafer:
np-type substrate
Cross section:
Al SiO2
Top view:
Can you draw the diode symbol on this diagram?
Photolithography 4
PhotolithographyPhotolithography In order to produce a microelectronic circuit,
portions of a silicon wafer must be doped with donors and/or acceptors in a controlled, patterned manner.
Holes or “windows” must be cut through insulating thin films in a controlled, patterned manner.
Metal “interconnections” (thin film “wires”) must be formed in a controlled, patterned manner.
The process by which patterns are transferred to the surface of a wafer is called photolithography.
Photolithography 5
PhotolithographyPhotolithography 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
Top view:
Photolithography 6
PhotolithographyPhotolithography We start with a bare silicon wafer and oxidize it. (The bottom
surface also gets oxidized, but we’ll ignore that.):
p-type substrateCross section:
SiO2
Top view:
Photolithography 7
PhotolithographyPhotolithography 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 substrateCross section:
SiO2
Top view:
Photolithography 8
PhotolithographyPhotolithography
The starting point for the photolithography process is a mask.
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 9
PhotolithographyPhotolithography For our example, our mask could look like this:
glass plateCross section:
opaque metal,e.g.,Cr
Top view:
Photolithography 10
PhotolithographyPhotolithography Recall that we start with a bare silicon wafer and oxidize it.
(The bottom surface also gets oxidized, but we’ll ignore that.):
p-type substrateCross section:
SiO2
Top view:
Photolithography 11
PhotolithographyPhotolithography The wafer is next coated with “photoresist”. Photoresist is a light-sensitive polymer. We will initially consider positive photoresist (more
about what this means soon). Photoresist is usually “spun on”. For this step, the wafer is held onto a support chuck
by 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 m thick.
Photolithography 12
PhotolithographyPhotolithography After coating with photoresist, the wafer looks like this:
p-type substrateCross section:
Photoresist
Top view:
Photolithography 13
PhotolithographyPhotolithography The wafer is baked at 70 to 90°C (soft bake or pre-bake) to remove solvent from
the photoresist and improve adhesion.
p-type substrateCross section:
Photoresist
Top view:
Photolithography 14
PhotolithographyPhotolithography The mask is “aligned” (positioned) as desired on top of the wafer.
Mask
Cross section:
Top view:
p-type substrate
glass plate
Photolithography 15
PhotolithographyPhotolithography 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 16
PhotolithographyPhotolithography 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 17
PhotolithographyPhotolithography The wafer is baked again, but at a higher temperature (120 to
180°C). This hard bake or post-bake hardens the photoresist.
Cross section:
Top view:
p-type substrate
Photolithography 18
PhotolithographyPhotolithography 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 19
PhotolithographyPhotolithography 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”
Photolithography 20
PhotolithographyPhotolithography 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
Photolithography 21
PhotolithographyPhotolithography 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 top surface of the wafer.
Cross section:
Top view:
p-type substraten-type
glass platenew mask
Photolithography 22
PhotolithographyPhotolithography 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
Top view:
Photolithography 23
PhotolithographyPhotolithography The wafer surface is next coated with aluminum by evaporation
or sputtering. The window outlines may still be visible.
np-type substrate
Cross section:
Al SiO2
Top view:
Photolithography 24
PhotolithographyPhotolithography Photolithography is used to pattern photoresist so as to protect
the aluminum over the windows:
Al SiO2
np-type substrate
Cross section:
Top view:
Photolithography 25
PhotolithographyPhotolithography What must the mask look like in order to pattern the aluminum
film? Assume that we’re still using positive photoresist.
np-type substrate
Cross section:
Al SiO2
Top view:
Photolithography 26
PhotolithographyPhotolithography 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
Top view:
Photolithography 27
PhotolithographyPhotolithography Then the photoresist is stripped.
np-type substrate
Cross section:
Al SiO2
Top view:
Photolithography 28
PhotolithographyPhotolithography 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
Cross section:
Al SiO2
Top view:
Photolithography 29
PhotolithographyPhotolithography So far we have only considered positive
photoresists. For positive resists, the resist pattern on the
wafer looks just like the pattern on the mask 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 on the mask.
Photolithography 30
PhotolithographyPhotolithography
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).
Photolithography 31
PhotolithographyPhotolithography
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 the minimum feature size or minimum linewidth.
The limitations of optical lithography are a consequence of basic physics (diffraction).
Photolithography 32
PhotolithographyPhotolithography For a single-wavelength projection photo-
lithography system, the minimum feature size or minimum linewidth is given by the Rayleigh criterion:
is the wavelength.NA is the numerical aperture, a measure of the
light-collecting power of the projection lens. k depends on the photoresist properties and the
“quality’ of the optical system.
NAkFw
min
Photolithography 33
PhotolithographyPhotolithography
So how do we reduce wmin ?
Reduce k.Reduce . Increase NA.
NAkFw
min
Photolithography 34
PhotolithographyPhotolithography
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
min
Photolithography 35
PhotolithographyPhotolithography
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 keep the image in focus on the wafer.
NAkFw
min
Photolithography 36
PhotolithographyPhotolithography
Depth of focus is given by:
2)(6.0NA
DF
Depth of focus decreases (bad) as decreases.Depth of focus decreases (bad) as NA increases.
Photolithography 37
PhotolithographyPhotolithography Numerous light sources are (and will be) used for
optical lithography:
Light Source
(nm)
wmin (nm)
DF (nm)
g-line (Hg lamp) 436 311 850
i-line (Hg lamp) 365 260 730
KrF laser 248 175 500
ArF laser 193 140 400
F2 laser 157 112 320
Photolithography 38
PhotolithographyPhotolithography Complex devices require the photolithography
process to be carried out over 20 times. “over 20 mask levels”
Any dust on the wafer or mask can result in defects. Cleanrooms are required for fabrication of complex devices.
Even if defects occur in only 10% of the chips during each photolithography step, fewer than 50% of the chips will be functional after a seven mask process is completed.
How is this yield calculated?
Photolithography 39
PhotolithographyPhotolithography
Other lithographic techniques will play a role in the future.
Electron beam lithography Ion beam lithography.X-ray lithography.
