Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2002-05-20
2004-04-20
Nguyen, Henry Hung (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S053000
Reexamination Certificate
active
06724465
ABSTRACT:
TECHNOLOGICAL FIELD
This invention relates to a lithography device using a source of extreme ultraviolet radiation and multi-layered mirrors provided to reflect this extreme ultraviolet radiation that is also called “EUV radiation” or “X-UV radiation”.
The wavelength of such radiation is within the range extending from 8 nm to 25 nm.
The invention is applicable most particularly to the manufacture of integrated circuits with a very high degree of integration, the use of EUV radiation enabling one to reduce the etch spacing of such circuits.
STATE OF THE PRIOR ART
In the main, two techniques are known for producing intense EUV radiation. Both of them rely on the collection of photons produced by the microscopic process of spontaneous emission by a hot plasma of low density which is generated by means of a laser.
The first technique uses a jet of xenon irradiated by a YAG laser, the power of which is close to 1 kW. In effect, when the nature of the gas and the conditions for expansion into the vacuum are well chosen, clusters are naturally created in the jet through multi-body interactions. These clusters are macro-particles which can contain up to a million atoms and have a density which is sufficiently high (about one tenth of the density of the solid) to absorb the laser beam and thereby heat the atoms of the surrounding gas which can then emit photons through fluorescence.
The second technique uses the corona of a plasma of high atomic number, obtained by interaction of a laser beam, which comes from a KrF laser, the intensity of which is close to 10
12
W/cm
2
, and a solid target of great thickness (at least 20 &mgr;m).
The laser beam is focused on one face of this target, called the “front face” and one uses the EUV radiation emitted by this front face and generated by interaction of the laser beam and the material of the target.
If the first or the second technique is used, the EUV radiation obtained comprises a continuous energy spectrum and with strong emission lines.
The UEV radiation sources which the first and second techniques use have the following disadvantages.
These sources have an isotropic emission which therefore has a large angular divergence, and the emitted EUV radiation spectrum includes lines of low spectral width.
It is then necessary to associate with each source, complicated optical collection means which enable one to recover the maximum from the wide angular field of emission from the source.
These optical means formed by multi-layered mirrors, must be produced in such a way that their spectral responses are centered on the emission line chosen for the exposure of a sample, restricting as much as possible the loss of intensity due to multiple reflections on the multi-layered mirrors.
A known example of a lithography device using EUV radiation, the wavelengths of which are situated, for example, close to the range from 10 nm to 14 nm is diagrammatically shown in
FIGS. 1 and 2
. Such a device is also called an “EUV lithography device”.
This known device is intended to expose a sample E. Generally, this is a semi-conductor substrate
2
(for example made of silicon) onto which a layer of photosensitive resin (“a photo-resist layer”)
3
has been deposited and it is desired to expose this layer in accordance with a specified pattern.
After exposure of the layer
3
, it is developed and the substrate
2
can then be etched in accordance with the pattern.
The device in
FIGS. 1 and 2
includes
a support
4
for the sample,
a mask
5
comprising the specified pattern in an enlarged form
a source
6
of radiation in the extreme ultraviolet range (FIG.
2
),
optical means
7
for the collection and the transmission of the radiation to the mask
5
, the latter providing an image of the pattern in enlarged form, and
optical means
8
for reducing this image and projecting the reduced image onto the layer
3
of photosensitive resin (chosen in such a fashion that it is sensitive to the incident radiation).
The known source
6
of EUV radiation comprises means of forming a jet J of clusters of xenon. Only the nozzle
9
which includes these formation means is represented in FIG.
2
.
The source also comprises a laser (not shown), the beam of which F is focused onto a point S of the jet J by the optical means of focusing
10
. The interaction of this beam F and the xenon clusters generate the EUV radiation R.
The point S is visible in
FIG. 1
(but not the nozzle nor the jet of xenon clusters).
Among the optical means
7
of the device for collection and transmission, there is an optical collector
11
provided with a central opening
12
to allow the focused laser beam F to pass.
This optical collector
11
is positioned facing the jet of xenon clusters and is intended to collect a part of the EUV radiation emitted by the xenon clusters and to transmit this collected radiation
13
toward other optical components that also form a part of the optical means
7
for collection and for transmission.
These optical means
7
for collection and for transmission, the mask
5
, which is used in reflection, and the optical means
8
for reduction and for projection are multi-layered mirrors
14
which selectively reflect the EUV radiation and are designed in such a way that their spectral responses are centered on the wavelength chosen for exposure of the layer of photosensitive resin
3
.
It should be made clear that the pattern, in accordance with which one wishes to etch the sample, is formed on the multi-layered mirror corresponding to the mask
5
, with an enlargement factor suited to the optical means for reduction and for projection, and this multi-layered mirror is coated, except for the pattern, with a layer (not shown) which is capable of absorbing the incident EUV radiation.
Within the wavelength range of EUV radiation, the spectral resolution &Dgr;&lgr;/&lgr; of the mirrors is about 4%.
The breadth of the spectral range usable for exposure is obtained by the convolution of the spectral breadth of the EUV radiation and this spectral resolution.
The known multi-layered mirrors to which we will return subsequently and which are used in the lithography device shown in
FIGS. 1 and 2
, have, in particular, the following disadvantage: their spectral band, which is centered on the wavelength chosen for the exposure, is narrow.
The result is a reduction in the efficiency of the lithography device.
These EUV multi-layered mirrors also have the disadvantage of deforming when they are exposed to a high thermal flux coming from the source of EUV radiation for the device.
DESCRIPTION OF THE INVENTION
One aim of the invention is to propose an EUV lithography device that is much more efficient than the known devices considered to be the most highly efficient.
The device which is the subject of the invention comprises a source of EUV radiation which is anisotropic. This EUV radiation is emitted through the back face of a solid target of suitable thickness on the front face of which a laser beam is focused.
Such an anisotropic source enables one to increase the effective portion of the EUV radiation beam and to simplify the collection of this radiation.
Furthermore, the device which is the subject of the invention comprises multi-layered mirrors capable of reflecting the generated EUV radiation, each layered mirror having a spectral band (also called “spectral width” or “bandwidth”) greater than that of the known multi-layered mirrors mentioned above.
The source used in the invention, the emission spectrum of which is closer to black body over a broad spectral range, and the multi-layered mirrors with a broad spectral bandwidth, also used in the invention, work together to lead to a device capable of supplying the sample, which one wishes to expose with EUV radiation which is more intense than in the prior art.
Another aim of the invention is to minimize thermal deformation of the multi-layered mirrors which are used in the invention when these multi-layered mirrors are exposed to the intense flux of EUV radiation.
To put it precisely, the subject of this invention is a lit
Babonneau Danièle
Bonnet Laurence
Marmoret Rémy
Commissariat a l'Energie Atomique
Nguyen Henry Hung
Pearne & Gordon LLP
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