Synchrotron-based EUV lithography illuminator simulator

Details Synchrotron-based EUV lithography illuminator simulator Synchrotron-based EUV lithography illuminator simulator

2002-06-05

2004-07-27

Dunn, Drew A. (Department: 2872)

Optical: systems and elements

Holographic system or element

Using a hologram as an optical element

C359S027000, C359S351000, C359S571000, C378S034000

Reexamination Certificate

active

06768567

ABSTRACT:

FIELD OF THE INVENTION
EUV lithography (EUVL) is an emerging technology in the microelectronics industry. It is one of the leading candidates for the fabrication of devices with feature sizes of 70 nm and smaller. Synchrotron radiation facilities provide a convenient source of EUV radiation for the development of this technology. This invention relates to techniques for generating arbitrary fill patterns that simulate actual fill patterns for potential stepper designs, or generate specialized fill patterns for more general optical processing systems.
BACKGROUND OF THE INVENTION
In general, lithography refers to processes for pattern transfer between various media. A lithographic coating is generally a radiation-sensitized coating suitable for receiving a cast image of the subject pattern. Once the image is cast, it is indelibly formed in the coating. The recorded image may be either a negative or a positive of the subject pattern. Typically, a “transparency” of the subject pattern is made having areas which are selectively transparent or opaque to the impinging radiation. Exposure of the coating through the transparency placed in close longitudinal proximity to the coating causes the exposed area of the coating to become selectively crosslinked and consequently either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble (i.e., uncrosslinked) areas are removed in the developing process to leave the pattern image in the coating as less soluble crosslinked polymer.
Projection lithography is a powerful and essential tool for microelectronics processing and has supplanted proximity printing as described in the previous paragraph. “Long” or “soft” x-rays (a.k.a. Extreme UV) (wavelength range of 10 to 20 nm) are now at the forefront of research in efforts to achieve smaller transferred feature sizes. With projection photolithography, a reticle (or mask) is imaged through a reduction-projection (demagnifying) lens onto a wafer. Reticles for EUV projection lithography typically comprise a glass substrate coated with an EUV reflective material and an optical pattern fabricated from an EUV absorbing material covering portions of the reflective surface. In operation, EUV radiation from the illumination system (condenser) is projected toward the surface of the reticle and radiation is reflected from those areas of the reticle reflective surface which are exposed, i.e., not covered by the EUV absorbing material. The reflected radiation is re-imaged to the wafer using a reflective optical system and the pattern from the reticle is effectively transcribed to the wafer.
A source of EUV radiation is the laser-produced plasma EUV source, which depends upon a high power, pulsed laser (e.g., a yttrium aluminum garnet (“YAG”) laser), or an excimer laser, delivering 500 to 1,000 watts of power to a 50 &mgr;m to 250 &mgr;m spot, thereby heating a source material to, for example, 250,000 C, to emit EUV radiation from the resulting plasma. Plasma sources are compact, and may be dedicated to a single production line so that malfunction does not close down the entire plant. A stepper employing a laser-produced plasma source is relatively inexpensive and could be housed in existing facilities. It is expected that EUV sources suitable for photolithography that provide bright, incoherent EUV radiation and that employ physics quite different from that of the laser-produced plasma source will be developed. One such source under development is the EUV discharge source.
EUV lithography machines for producing integrated circuit components are described for example in Tichenor et al. U.S. Pat. No. 6,031,598. Referring to
FIG. 11
, the EUV lithography machine comprises a main vacuum or projection chamber
2
and a source vacuum chamber
4
. Source chamber
4
is connected to main chamber
2
through an airlock valve (not shown) which permits either chamber to be accessed without venting or contaminating the environment of the other chamber. Typically, a laser beam
30
is directed by turning mirror
32
into the source chamber
4
. A high density gas, such as xenon, is injected into the plasma generator
36
through gas supply
34
and the interaction of the laser beam
30
, and gas supply
34
creates a plasma giving off the illumination used in EUV lithography. The EUV radiation is collected by segmented collector
38
, that collects about 30% of the available EUV light, and directed toward the pupil optics
42
. The pupil optics consists of long narrow mirrors arranged to focus the rays from the collector at grazing angles onto an imaging mirror
43
that redirects the illumination beam through filter/window
44
. Filter
44
passes only the desired EUV wavelengths and excludes scattered laser beam light in chamber
4
. The illumination beam is then reflected from the relay optics
46
, another grazing angle mirror, and then illuminates the pattern on the reticle
48
. Mirrors
38
,
42
,
43
, and
46
together comprise the complete illumination system or condenser. The reflected pattern from the reticle
48
then passes through the projection optics
50
which reduces the image size to that desired for printing on the wafer. After exiting the projection optics
50
, the beam passes through vacuum window
52
. The beam then prints its pattern on wafer
54
.
Although no longer under serious consideration for high-volume commercial fabrication applications, synchrotron sources play an extremely important role in the development of EUV lithography technology. Being readily available, highly reliable, and efficient producers of EUV radiation, synchrotron radiation sources are well suited to the development of EUV lithography. These sources are currently used for a variety of at-wavelength EUV metrologies such as reflectometry, interferometry, and scatterometry.
In the case of synchrotron radiation sources, there are three types of sources: bending magnets, wigglers, and undulators. In bending magnet sources, the electrons are deflected by a bending magnet and photon radiation is emitted. Wiggler sources comprise a so-called wiggler for the deflection of the electron or of an electron beam. The wiggler includes a multiple number of alternating poled pairs of magnets arranged in a series. When an electron passes through a wiggler, the electron is subjected to a periodic, vertical magnetic field; the electron oscillates correspondingly in the horizontal plane. Wigglers are further characterized by the fact that no interference effects occur. The synchrotron radiation produced by a wiggler is similar to that of a bending magnet and radiates in a horizontal steradian. In contrast to the bending magnet, it has a flux that is reinforced by the number of poles of the wiggler.
Finally, in the case of undulator sources, the electrons in the undulator are subjected to a magnetic field with shorter periods and a smaller magnetic field of the deflection pole than in the case of the wiggler, so that interference effects of synchrotron radiation occur. Due to the interference effects, the synchrotron radiation has a discontinuous spectrum and radiates both horizontally and vertically in a small steradian element, i.e., the radiation is strongly directed.
In lithographic applications, the partial coherence of the illumination (sigma) is often defined as the ratio of the illumination angular range to the numerical aperture of the imaging (projection optical) system. The illumination angular range is also referred to as the divergence of the source. Undulator radiation is much like a laser source in that it produces highly-coherent light of very low divergence. A typical EUV undulator beamline produces a sigma of less than 0.1 whereas lithographic application nominally require a sigma of 0.7 or higher. Although less coherent than undulator radiation, bending magnet radiation is also typically too coherent to be directly used for lithography.
Currently the coherence and high flux properties of synchrotron undulator radiation are being used for crucial at-wavelength interferometry and

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