X-ray or gamma ray systems or devices – Source
Reexamination Certificate
2000-11-30
2002-12-10
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Source
C378S034000, C378S143000
Reexamination Certificate
active
06493423
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method of generating extremely short-wave radiation, in which a medium is transported through a vacuum space and each time a part of the medium in the vacuum space is irradiated with a pulsed and focused energy-rich laser beam, said part of the medium being converted into a plasma emitting extremely short-wave radiation.
The invention also relates to a method of manufacturing a device by means of this radiation. Furthermore, the invention relates to an extremely short-wave radiation source unit and to a lithographic projection apparatus provided with such a radiation source unit.
Extremely short-wave radiation is understood to mean extreme UV (EUV) radiation, which can be used in lithographic projection apparatuses, and X-ray radiation for various applications.
Said medium may be a mobile medium, i.e. a medium which does not have a solid shape but whose shape is determined by the holder accommodating the medium or the guide by which the medium is transported. However, the medium may be alternatively a solid medium such as a metal which can be locally exploded by laser beam bombardment, in which the released particles form a plasma emitting extremely short-wave radiation. The metal medium may be a tape or a wire transported through the vacuum, or source space.
A lithographic apparatus is used, inter alia, in the manufacture of integrated electronic circuits or ICs for imaging an IC mask pattern, present in a mask, each time on a different IC area of a substrate. This substrate, which is coated with a radiation-sensitive layer, provides space for a large number of IC areas. The lithographic apparatus may also be used in the manufacture of, for example, liquid crystalline image display panels, integrated, or planar optical systems, charge-coupled detectors (CCDs) or magnetic heads.
Since an increasingly large number of electronic components is to be accommodated in an IC, increasingly smaller details, or line widths, of IC patterns must be imaged. Consequently, increasingly stricter requirements are imposed on the imaging quality and the resolving power of the projection system in the apparatus, which projection system is generally a lens system in the current lithographic apparatuses. The resolving power, which is a measure of the smallest detail which can still be imaged, is proportional to &lgr;/NA, in which &lgr; is the wavelength of the imaging, or projection, beam and NA is the numerical aperture of the projection system. To increase the resolving power, the numerical aperture may, in principle, be enlarged and/or the wavelength may be reduced. An increase of the currently already fairly large numerical aperture is no longer very well possible in practice because the depth of focus of the projection system, which is proportional to &lgr;/NA
2
, will become too small and the correction for the required image field will be too difficult.
The requirements to be imposed on the projection system may be mitigated, or the resolving power may be increased while these requirements are maintained if a step-and-scan lithographic apparatus is used instead of a stepping lithographic apparatus. In a stepping apparatus, a full-field illumination is used, i.e. the entire mask pattern is illuminated in one run and imaged as one whole on an IC area of the substrate. After a first IC area has been illuminated, a step is made to a subsequent IC area, i.e. the substrate holder is moved in such a way that the next IC area is positioned under the mask pattern, whereafter this area is illuminated, and so forth, until all IC areas of the substrate are provided with a mask pattern. In a step-and-scan apparatus, only a rectangular or annular segment-shaped area of the mask pattern and hence a corresponding sub-area of a substrate IC is illuminated, and the mask pattern and the substrate are synchronously moved through the illumination beam, while taking the magnification of the projection system into account. A subsequent area of the mask pattern is then each time imaged on a corresponding sub-area of the relevant IC area of the substrate. After the entire mask pattern has been imaged on an IC area in this way, the substrate holder performs a step, i.e. the start of a subsequent IC area is introduced into the projection beam and the mask is set, for example, to its initial position whereafter said subsequent IC area is scan-illuminated via the mask pattern.
If even smaller details are to be satisfactorily imaged with a step-and-scan lithographic apparatus, the only possibility is to reduce the wavelength of the projection beam. In the current step-and-scan apparatuses, deep UV (DUV) radiation is already used, i.e. radiation having a wavelength of the order of several hundred nanometers, for example, 248 nm or 193 nm from, for example, an excimer laser. Another possibility is the use of extreme UV (EUV) radiation, also referred to as soft X-ray radiation, with a wavelength in the area of several nm to several tens of nm. Extremely small details, of the order of 0.1 &mgr;m or smaller, can be satisfactorily imaged with such a radiation.
Since there is no suitable lens material available for EUV radiation, a mirror projection system must be used for imaging the mask pattern of the substrate, instead of a hitherto conventional lens projection system. For forming a suitable illumination beam of the radiation from the EUV radiation source, mirrors are also used in the illumination system. The article “Front-end design issues in soft-X-ray lithography” in Applied Optics, Vol. 23, No. 34, 01-12-93, pp. 7050-56 describes a lithographic apparatus in which EUV radiation is used and whose illumination system comprises three mirrors and the imaging, or projection, system comprises four mirrors. As is described in the article “Debris-free soft X-ray generation using a liquid droplet laser plasma target” in: Applications of laser plasma radiation II, SPIE 2523 , 1995, pp. 88-93, EUV radiation can be generated by focusing a laser beam on water droplets. The required stable flux of individual micro water droplets can be obtained by means of a capillary glass tube which is vibrated by a piezoelectric driver. Due to the high temperature, each water droplet impinged upon by the laser beam is consecutively converted into a plasma which emits EUV radiation.
In EUV lithographic apparatuses, it is a great problem to illuminate the substrate at a sufficiently high intensity. A first cause of this problem, which applies to all EUV apparatuses, is that the mirrors used are considerably less than 100% reflecting. Each of these mirrors has a multilayer structure whose composition is adapted as satisfactorily as possible to the wavelength of the projection beam used. Examples of such multilayer structures are described in U.S. Pat. No. 5,153,898. A multilayer structure which is frequently mentioned in literature is the structure consisting of silicon layers alternating with molybdenum layers. For radiation coming from a plasma source, these layers theoretically have a reflection of the order of 73% to 75%, but in practice the reflection is currently not larger than 65%. When said number of seven mirrors is used, each with a reflection of 68%, only 6.7% of the radiation emitted by the source reaches the substrate. For a lithographic apparatus, this means in practice that the illumination time should be relatively long in order to obtain the desired quantity of radiation energy on a substrate, and that the scanning velocity would be relatively small, particularly for a scanning apparatus. However, it is essential for these apparatuses that the scanning velocity is as high as possible and the illumination time is as short as possible so that the throughput, i.e. the number of substrates which can be illuminated per unit of time, is as high as possible. This can only be achieved with an EUV radiation source which supplies sufficient intensity. A second cause of the problem relates to the fact that the generated EUV radiation may be absorbed as little as possible, which means that
Ho Allen C.
Kim Robert H.
Koninklijke Philips Electronics , N.V.
Waxler Aaron
LandOfFree
METHOD OF GENERATING EXTREMELY SHORT-WAVE RADIATION, METHOD... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with METHOD OF GENERATING EXTREMELY SHORT-WAVE RADIATION, METHOD..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and METHOD OF GENERATING EXTREMELY SHORT-WAVE RADIATION, METHOD... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2967402