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 Microlithography
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etching
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etching
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 Resist & Steppers
SiC Processing
Microfluidics
Micromachining
Microprofiling
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Resist & Steppers |
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Resists
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High sensitivity
to the wavelength used for imaging, but not for all optical wave lengths. |
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High contrast,
i.e. little response (= "blackening") to
intensities below some level, and strong response to large intensities. This
is needed to sharpen edges since diffraction effects do not allow sharp
intensity variations at dimensions around the wavelength of the light as
illustrated below. |
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Compatibility
with general semiconductor requirements (easy to deposit, to structure, to
etch off; no elements involved with the potential to
contaminate
Si as e.g. heavy metals or alkali metals (this
includes the developer), no particle production, and so on). |
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Homogeneous "blackening" with depth
- this means little absorption. Simply imagine that the resist is strongly
absorbing, which would mean only its top part becomes exposed. Removal of
the "blackened" and developed resist than would not even open a complete
hole to the layer below. |
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No
reflection of light, especially at the interface resist -
substrate. Otherwise we encounter all kinds of interference effects between
the light going down and the one coming up (known as "Newton
fringes"). Given the highly monochromatic and coherent nature of the
light used for lithography, it is fairly easy to even produce
standing light waves in the resist layer as
shown below. While the ripple structure clearly visible in the resist is not
so detrimental in this example, very bad things can happen if the substrate
below the resist is not perfectly flat. |
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Strongly absorbing
resist
- in direct contradiction to the requirement stated above. An
anti-reflection coating (ARC)
might be used between substrate and resist, adding process complexity and
cost. |
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Suitability
of the resist as
direct mask for ion-implantation or
for plasma etching. |
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Easy stripping
of the resist, even after it was
turned into a tough polymer or carbonized by high-energy ion bombardment. |
Stepper
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Optimize optical
resolution |
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The
resolution limit of optical
instruments is equal to about the wave-length l.
More precisely and quantitatively we have |
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With dmin =
minimal distinguishable feature size;
i.e the distance between two Al lines, and
NA =
numerical aperture of the optical
system (the NA for a single lens is roughly the quotient of
focal length : diameter; i.e. a crude measure of the size of the lens). |
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Blue light has a wave length of about 0,4
µm, and the numerical apertures NA of very good lenses are
principally < 1; a value of 0,6 is about the best you can do
(consider that all distortions and aberrations troubling optical lenses
become more severe with increasing NA). This would give us a
minimum feature size of . |
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Since nowadays you can buy chips with minimum
features of 0,18 µm or even 0,13 µm; we obviously must do
better than to use just the visible part of the spectrum. |
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2. Adjust depth of
focus |
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Adjust the
NA of the
lens using the formulas: |
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| Df |
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l
(NA)2 |
= |
0,4
0,62 |
= 1,11 µm |
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3. Align one exposure
exactly on top of the preceding one.
We need a wafer stage that can move the wafer around
with a precision of lets say
1/5 of dmin - corresponding to
0,18/5 µm = 0,036 µm = 36 nm. |
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Control the stage
movement and measure where we are with respect to some alignment
marks on the chip with the same kind of precision. |
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Alignment is done optically, too, as an integral part of stepper technology. |
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4. Reliable
& Reproducible
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10 000 and
more exposures a day in one stepper. |
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Pharmacom Microelectronics
Copyright 1997-2004
