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The visible range of wavelengths
extends from about 780 nm (red) to 380 nm (violet). Obviously we
need to go to even smaller wavelengths in the ultraviolet part of the spectrum
if we want to make structures in the 100 nm region. Obvious, so where is
the problem? Well, there are two major problems with this approach. |
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First, we need a powerful and fairly monochromatic illumination source, and second we need materials to make an extremely good
lens from. |
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Let's look at the illumination source
issue first: |
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A powerful light source we need because we cannot
afford to wait forever before an exposure is finished. The maximum exposure
time should be below a second or so, and you simply need intense light for
that. |
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Monochromatic light we need, because we cannot
possible built a supreme lens for many wavelengths (there are things like
chromatic aberration and so on). Taking a small part of the spectrum out of
some blackbody radiation (the spectrum emitted by something hot like a light
bulb), however, leaves very little intensity. |
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The solution lies in going for an
intense line in the emission spectrum of some element - mercury (Hg) in
this case. |
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In the 80ties, the so-called
G-line at 436 nm was used (coming from a high-pressure Hg
discharge lamp). Next came the I-line at 365 nm, and then a
250 nm line. |
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But that was already pushing the
Hg lamp to its limits, and it was soon replaced by so-called
DUV (for deep ultraviolet)
excimer lasers. |
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Excimer lasers are based on rather
strange materials: Compounds of noble gases like KrF, or ArF.
Rather unstable stuff, but emitting at 248 nm (KrF) or 193
nm (ArF). With the KrF system, dimensions down to 130
nm have been realized, but this is already pushing it quite a bit. |
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The ArF excimer laser has
been used from about 2003, so it is still in its infancy. It is expected
to cover the "65 nm node", and possibly also the 45 nm
node. |
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That will be the end. After that,
the age of "EUV" (extreme ultraviolet) might start, at a
wavelength around 12 nm (its really rather soft X-rays). There is
no way of having a lens anymore, "optical processing" must then be
done with mirrors. |
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If we now look at the lens issue, we first should realize that
high-aperture lenses are generally difficult to make. But the overwhelming
issue is to find suitable materials that have a sufficiently large index of
refraction at the wavelength considered. |
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We already looked at this issue,
e.g. in the context of the frequency dependence of the dielectric constant, so
we need not repeat the problems encountered here. Check the following links:
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Illumination source and lens
materials are not the only problems encountered by switching to a smaller
wavelength. Of course, there are many others, too. |
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To mention just one: The "pellicles", the thin foils
protecting the mask, will turn dark in intense UV illumination. Not
good, so let's take a better material. Easy fix, but do you know a better
material? No? Too bad - since nobody else does either, you missed your change
of getting rich quickly. |
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In air, NA obviously
than has a maximum value of 1. The best lenses built so far have a
NA of about 0.8; but 0.9 is already aimed for |
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Keep in mind that what you gain in
resolution by increasing NA, you loose in the
depth of focus. Large
NA lenses thus only make sense in the context of rather perfect
planarization. |
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Nevertheless, increasing
NA even more helps, and there is - in principle - a simple way of
doing it: Replace the air between your lens and the wafer with something that
has an appreciable index of refraction, e.g. oil. |
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"Oil immersion objective
lenses" have been used for about a century in conventional optical
microscopes; in this way the numerical aperture and thus resolution can be
increased in a rather simple way by up to 40%. |
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But this is far easier said than
done. Just consider that the name "stepper" comes from the fact,
that you step the wafer (rather rapidly)
below the lens. How do you keep you oil in place? And how will the wafer
respond to be covered with oil? |
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Well, let's not use oil, let's use
high-purity water (n = 1.437 at 193 nm), but that only
solves some of many problems and creates some new one (your
CaF2 lens, for example, will dissolve in water). |
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Nevertheless, "liquid immersion
lithography" will most likely be the next big fashion in
lithography, with the potential to keep microelectronics alive well into the
next decade (i.e. after 2010). |
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What is left is to make the
parameter k as small as possible, i.e. to pay
some attention to reticles and resist, or, more general, to resolution enhancing
techniques. |
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There is quite a potential here,
"historically" parameter k has decreased steadily form
about 0.8 in the 1980s to 0.4 today. |
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While optimizing the resist is
critical, it does not introduce new principles, and we will not cover it
here. |
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That leaves the reticle and the way
it is illuminated. There is quite a bit that can be done, but you must pay the
prize of sharp increases in complexity. |
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The proper catchwords giving some idea to what is
meant are:
- Off-axis illumination
- Optical proximity correction (OPC)
- Phase shift masks (PSM)
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For the latter two cases the general idea is to
have a structure on the reticle that is different from what you want to have
projected into the resist on the wafer. If, for example, a sharp corner is
"smeared out" to a roundish image, than make the corner look
different. The figure gives a rough idea what that means |
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In phase shift masks you add structures that do
not only manipulate the amplitude of the light transmitted through the mask,
but also the phase. |
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In this way you can produce constructive or
destructive interference in the image plane in places where that is helpful to
sharpen the image. |
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Of course, all these additional
features on the mask must first be computed (not easy), than made (very
difficult), and finally tested (exceedingly difficult). |
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Testing your mask is essential, that
any mistake in the mask will automatically be transferred to the chip and,
remember Murphy's law, more likely than not kill the chip. |
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In the grand total a set of masks
will quickly cost you up to 2.000.000 €. You must a sell a hell of
a lot of chips (at a profit) just to recover that cost |
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For customized chips, that are not
made by the untold millions, its simply not possible to pay that prize. |
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This drives a large-scale effort to
find some better solutions. For mor details (and for the source of some of the
data here), refer to "Materials today" from Feb. 2005. |
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© H. Föll
To index
6.6.2 Resist and Steppers