[研究] E-beam Lithography

[yellowfishie (喵喵喵喵~~~)]

Electron beam lithography

Electron beam lithography (often abbreviated as e-beam lithography) is the
practice of scanning a beam of electrons in a patterned fashion across a
surface covered with a film (called the resist),[1] ("exposing" the resist)
and of selectively removing either exposed or non-exposed regions of the
resist ("developing"). The purpose is to create very small structures in the
resist that can subsequently be transferred into another material for a
number of purposes, for example for the creation of very small electronic
devices.

The primary advantage of electron beam lithography is that it is one of the
ways to beat the diffraction limit of light and make features in the
nanometer regime. This form of maskless lithography has found wide usage in
mask-making used in photolithography, low-volume production of semiconductor
components, and research & development.

On the other hand, the key limitation of electron beam lithography is
throughput, i.e., the very long time it takes to expose an entire silicon
wafer or glass substrate. A long exposure time leaves the user vulnerable to
beam drift or instability which may occur during the exposure. Also, the
turn-around time for reworking or re-design is lengthened unnecessarily if
the pattern is not being changed the second time.


Electron Beam Lithography Systems

Electron beam lithography systems used in commercial applications are
dedicated e-beam writing systems that are very expensive (>$4M USD). For
research applications, it is very common to convert an electron microscope
into an electron beam lithography system using a relatively low cost
accessory (<USD 100k). Such converted systems have produced linewidths of ~20
nm since at least 1990, while current dedicated systems have produced
linewidths on the order of 10 nm or smaller.

Electron beam lithography systems can be classified according to both beam
shape and beam deflection strategy. Older systems used Gaussian-shaped beams
and scanned these beams in a raster fashion. Newer systems use shaped beams,
which may be deflected to various positions in the writing field (this is
also known as vector scan).


Electron sources

Lower resolution systems can use thermionic sources, which are usually formed
from LaB6. However, systems with higher resolution requirements need to use
field emission sources, such as heated W/ZrO2 for lower energy spread and
enhanced brightness. Thermal field emission sources are preferred over cold
emission sources, in spite of their slightly larger beam size, because the
former offer better stability over typical writing times of several hours.


Lenses

Both electrostatic and magnetic lenses may be used. However, electrostatic
lenses have more aberrations and so are not used for fine focusing. There is
no current mechanism to make achromatic electron beam lenses, so extremely
narrow dispersions of the electron beam energy are needed for finest focusing.


Stage, stitching & alignment

Typically, for very small beam deflections electrostatic deflection 'lenses'
are used, larger beam deflections require electromagnetic scanning. Because
of the inaccuracy and because of the finite number of steps in the exposure
grid the writing field is of the order of 100 micrometre - 1 mm. Larger
patterns require stage moves. An accurate stage is critical for stitching
(tiling writing fields exactly against each other) and pattern overlay
(aligning a pattern to a previously made one).


Electron beam write time

The minimum time to expose a given area for a given dose is given by the
following formula:

Dose * exposed area = beam current * exposure time = total charge of incident
electrons

For example, assuming an exposure area of 1 cm2, a dose of 10-3 Coulombs/cm2,
and a beam current of 10-9 Amperes, the resulting minimum write time would be
106 seconds (about 12 days). This minimum write time does not include time
for the stage to move back and forth, as well as time for the beam to be
blanked (blocked from the wafer during deflection), as well as time for other
possible beam corrections and adjustments in the middle of writing. To cover
the 700 cm2 surface area of a 300 mm silicon wafer, the minimum write time
would extend to 7*108 seconds, about 22 years. This is a factor of about 10
million times slower than current optical lithography tools. It is clear that
throughput is a serious limitation for electron beam lithography, especially
when writing dense patterns over a large area.

E-beam lithography is not suitable for high-volume manufacturing because of
its limited throughput. The serial nature of electron beam writing makes for
very slow pattern generation compared with a parallel technique like
photolithography (the current standard) in which the entire surface is
patterned at once (1X optical steppers only, 4X or 5X steppers take
proportionally longer). To pattern a single wafer with an electron beam
lithography system for sub-100 nm resolution, it would typically take days,
compared to the few minutes it would take with a photolithography system.

Even an optical maskless lithography tool is much faster than an electron
beam used for the same purpose.


Defects in electron-beam lithography

Despite the high resolution of electron-beam lithography, the generation of
defects during electron-beam lithography is often not considered by users.
Defects may be classified into two categories: data-related defects, and
physical defects.

Data-related defects may be classified further into two sub-categories.
Blanking or deflection errors occur when the electron beam is not deflected
properly when it is supposed to, while shaping errors occur in
variable-shaped beam systems when the wrong shape is projected onto the
sample. These errors can originate either from the electron optical control
hardware or the input data that was taped out. As might be expected, larger
data files are more susceptible to data-related defects.

Physical defects are more varied, and can include sample charging (either
negative or positive), backscattering calculation errors, dose errors,
fogging (long-range reflection of backscattered electrons), outgassing,
contamination, beam drift and particles. Since the write time for electron
beam lithography can easily exceed a day, "randomly occurring" defects are
more likely to occur. Here again, larger data files can present more
opportunities for defects.


(下文略)

http://en.wikipedia.org/wiki/E-beam_lithography

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推 gwliao:E-beam有OPC的問題. XD  原因就是那唯一一張圖.
→ gwliao:不過, 應該不能叫"O"PC, 叫EPC嗎?   (~^O^~)
推 nextme:九月時，我去IBM TJWatson@NY那邊拜訪一個朋友
→ nextme:他就是在做這個研究的 不過這東西目前也只是可行性方案之一
→ nextme:他們還有很多其他方案  但是我都聽不懂  @@