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As far as lithography is concerned,
it is evident that we need the following key ingredients: |
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A photo
resist 1), i.e. some light sensitive material, not unlike
the coating on photographic film. |
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A mask
(better known as reticle
2)) that contains the structure you
want to transfer - not unlike a slide. |
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A lithography unit that allows to project the
pattern on the mask to the resist on the wafer. Pattern No. x must be
perfectly aligned to pattern No. x -
1, of course. Since about 1990 one (or just a few) chips are exposed
at one time, and than the wafer is moved and the next chip is exposed. This
step-by-step exposure is done in machines universally known as
steppers. |
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Means to develop and structure the resist. This is
usually done in such a way that the exposed
areas can be removed by some etching process (using positive resist). For some special purpose,
you may also use negative resists, i.e.
you remove the unexposed areas. |
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In principle, it is like projecting a
slide on some photosensitive paper with some special development. |
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However, we have some very special requirements.
And those requirements make the whole process very complex! |
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And with very
complex I mean really complex,
super-mega-complex - even in your wildest dreams you won't even get close to
imagining what it needs to do the lithography part of a modern chip with
structures size around 0,13 µm. |
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But relax. We are not going to delve
very deep into the intricacies of lithography, even though there are some
advanced material issues involved, but only give it a cursory glance. |
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For any layer that needs to be
structured, you need a
reticle. Since the projection on the chip
usually reduces everything on the reticle fivefold, the reticle size can be
about 5 times the chip size |
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A reticle then is a glass plate with
the desired structure etched into a Cr layer. Below, a direct scan of an
old reticle is shown, together with a microscope through-light image of some
part. |
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"Obviously", the regular
lattice of small opening in the non-transparent Cr layer is the array
for the trenches in a memory chip. The smallest structures on this reticle are
about 5 µm. |
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| Typical reticle, about original size |
Enlargement (x 100) |
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Before we look at the requirements of
reticles and their manufacture, lets pause for a moment and consider how the
structure on the reticle comes into being. |
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First, lets look at these structure, or the
lay-out of the chip. Shown on the left
is a tiny portion of a 4 Mbit DRAM. |
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Every color expresses one structured layer (and not all layers of the chip
are shown). |
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A print-out of the complete layout at this scale
would easily cover a soccer field. |
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The thing to note is: it is not good enough to transfer the structure on the
reticle to the chip with a resolution somewhat
better than the smallest structures on the chip, it is also
necessary to superimpose the various levels with an
alignment accuracy much better than the smallest structure on the
chip! |
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And
remember: We
have about 20 structuring cycles and thus reticles for one chip. |
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The lay-out contains the function of the chip. It established where you have
transistors and capacitors, how they are connected, how much current they can
carry, and so on. |
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This is determined and done by the
product people
- electrical engineers, computer scientists - no materials scientists are
involved. |
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The technology, the making of the chip, determines
the performance - speed, power consumption,
and so on. This is where material scientists come into their own, together with
semiconductor physicists and specialized electrical engineers (who e.g., can
simulate the behavior of an actual transistor and thus can tell the process
engineers parameters like optimal doping levels etc.). |
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In other words, the reticles are the
primary input of the product engineers to chip manufacturing. But they only may
contain structures that can actually be made. This is expressed in
design rules which come from the production
line and must be strictly adhered to. Only if all engineers involved have some understanding of
all issues relevant to chip production,
will you be able to come up with aggressive and thus competitive design
rules! |
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What are the requirements that
reticles have to meet (besides that their structures must not contain mistakes
from the layout. e.g. a forgotten connection or whatever). |
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Simple: They must be absolutely free of defects and must remain so while used in production! Any
defect on the reticle will become transferred to every chip and more likely than not will simply kill
it. |
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In other words: Not
a single particle
is ever allowed on a reticle! |
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This sounds like an impossible
request. Consider that a given reticle during its useful production life will
be put into a stepper and taken out again a few thousand times, and that
every mechanical
movement tends to generate particles. |
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Lithography is full of
"impossible" demands like this. Sometimes there is a simple solution,
sometimes there isn't. In this case there is: |
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First. make sure that the freshly
produced reticle is defect free (you must actually check it pixel by pixel
and repair unavoidable production
defects). |
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Then encase it in pellicles 3) (= fully transparent thin films) with a
distance of some mm between reticle and pellicle as shown below. |
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One of the bigger problems with
steppers - their very small (about 1 µm) depth of focus - now turns to our advantage:
Unavoidable particles fall on the pellicles and will only be imaged as harmless
faint blurs. |
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How do we make reticles? |
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By writing them pixel by pixel with a
finely focussed electron beam into a suitable sensitive layer , i.e. by direct
writing electron-beam
lithography. |
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Next, this layer is developed and the
structure transferred to the Cr layer. |
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Checking for defects, repairing these
defects (using the electron beam to burn off unwanted Cr, or to deposit
some in a kind of e-beam triggered CVD process where it is missing), and
encasing the reticle in pellicles, finishes the process. |
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Given the very large pixel size of a
reticle (roughly 1010), this takes
time - several hours just for the electron beam writing! |
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This explains immediately why we don't use
electron beam writing for directly creating structures on the chip: You have at
most a few seconds to "do" one chip in the factory, and e-beam
writing just can`t deliver this kind of throughput. |
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It also gives you a vague idea
why reticles don't come cheap. You have to pay some 5000 $ - 10 000 $
for making one reticle (making the lay-out
is not included!). And you need a set of about 20 reticles for one chip. And
you need lots of reticle sets during the development phase, because you
constantly want to improve the design. You simply need
large amounts of
money. |
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© H. Föll
