Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
1998-05-27
2001-05-15
Mathews, Alan A. (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
C355S067000, C355S070000, C362S259000
Reexamination Certificate
active
06233039
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an optical illumination system for illuminating a mask in an exposure apparatus for transferring the image of the pattern on the mask to a semiconductor substrate. This invention also relates to an exposure apparatus comprising such an optical illumination system.
BACKGROUND OF THE INVENTION
A primary objective of photo microlithography is to create images of the finest detail possible. High resolution images having extremely narrow line and space widths permit manufacture of complex integrated circuits on chips of small size.
To form a pattern on a semiconductor chip, light is transmitted through a planar mask and projected onto a planar wafer by a projection lens that generates a reduced image of patterns on the mask onto the wafer. Light in the projected image causes a chemical reaction in photo-sensitive material (photoresist) coated on the wafer, such that a relief image of the mask is created on the wafer. Illumination of the mask plane and of the wafer plane must be uniform, avoiding both small, local variations, and broad macroscopic variations across either plane.
In lithographic processes for the fabrication of semiconductor devices, a mask pattern is transferred to a substrate by the projection method for the sake of increased yield of products and better resolution. In the projection method, as the consequence of a trade-off between the resolution of the transferred pattern and the size of an area exposed in each exposure operation, it is prevailing to employ a step-and-repeat or scan-and-repeat technique with an apparatus called a stepper or a scanner which accomplishes exposure of the entire area of the substrate by repeating the sequence of making an exposure on a limited area of the substrate, then moving the substrate a predetermined distance and making another exposure.
Recent trends of further miniaturization of the patterns on an integrated circuit have forced development of optical lithography using light in the deep UV region, i.e. having a wavelength in the range of 190 to 330 nm. In such optical lithography, a super Hg lamp or an Xe-Hg lamp is often used as the light source. Since, however, the Hg lamp or Xe-Hg lamp has substantially no directionality and provides poor luminance, use of such lamps as the light source results in prolonged exposure time and, thus, in a decreased throughput
Development of advanced lasers which can provide laser beams in shorter wavelength regions, such as solid state lasers and excimer lasers make it possible to introduce laser energy of high luminance in the deep UV region into an exposure apparatus for the manufacture of semiconductor circuit devices. In addition, laser diodes in the ultraviolet have already been demonstrated in laboratory settings. GaN lasers lasing at approximately 360 nm have been demonstrated.
One of the main problems with conventional steppers (or scanners) is the lack of good control of the light source illuminating the mask, which due to the high numerical aperture (NA) and high resolution of the current exposure tools has become an extremely important issue. In conventional tools, mask illumination is provided (or their equivalent) by light emerging from a two-dimensional array of lens lets which is in turn illuminated by a Hg arc lamp or an eximer laser through an assembly of optics in between. Hence, it is relatively difficult to configure the light source into various shapes for improved resolution and depth of focus with regards to exposing different patterns since it is difficult to turn on/off light from each lenslet separately
SUMMARY OF THE INVENTION
The optical illumination system and the exposure apparatus of the instant invention includes an illumination means which is comprised of a plurality of light sources for emitting light beams along beam paths, and a lens system for irradiating the object with the light beams. Preferably the lens system includes at least one lens element positioned in the beams paths. The light sources of the invention are preferably individually addressable point like sources.
The improvement is the use of various matrix addressable sources or integrated-modulators in a manner such that the ability to switch the independent pixels leads to a change in the shape of the source thus enabling the source to be used at higher resolution and varying illumination configurations. This can be achieved using arrays of laser diodes or using arrays of modulators of 10's of microns size devices.
Since the laser diodes in the array are individually addressable, instant source reconfiguration can be obtained to provide any type of illumination such as annular, quadrupole, and modified quadrupole. These types of illumination matter can be used to increase the resolution and depth of focus of the exposured relief images in photoresist.
In addition, the lack of aberration control in a high NA lens can be addressed by varying the partial coherence of the illumination system since aberrations of the imaging system is influenced by the partial coherence through the Hopkins formula.
An embodiment of the optical illumination system and the exposure apparatus according to the invention is characterized in that the optical illumination system comprises light control means for operating each of said light sources independently of the others. Another embodiment of the optical illumination system and the exposure apparatus of the instant invention is characterized in that the light sources comprise a matrix addressable array of electro-optic modulators, preferably an array of LCD modulator. Such an array of modulators provide the ability to switch the shape of the illumination and would allow a very flexible approach to achieving various illumination geometries.
Other embodiments of the optical illumination system and the exposure apparatus according to the instant invention are characterized in that the light sources comprise an array of semiconductor-based Light Energy Diodes, semiconductor lasers, preferably in the form of Vertical Cavity Surface Emitting Lasers (VCSELs) or an array of bonded edge emitting DFB lasers. VCSELs have the ability of providing in a very small area a large number of micro lasers defined by lithographic techniques.
These semiconductor laser arrays certainly have the potential to replace the light source of the most commonly used imaging tools—i-line steppers (which use filtered output of a mercury arc lamp, at 365 nm). Their main advantages compared with conventional light sources (arc lamps, excimer lasers, etc) are:
Increase in tool throughput due to the increase in light power delivered to the mask plane. To date, the best excimer laser outputs (after linewidth narrowing) about 10 W, whereas a single element in the array can easily produce 100 mW; Hence, for a 20×20 array, the system according to the invention will get 4 times more power;
Instant source reconfiguration to provide any type of illumination such as annular, quadrupole, and modified quadrupole used to increase the resolution and depth of focus of the exposed pattern; This is possible since the elements in the array are individually addressable; With conventional light sources, a lot of optics are needed to create different intensity profiles;
Simplification in illuminator design and fabrication, also eliminating mechanical failure possible in conventional designs;
Simplification in design and fabrication of the imaging lens. Currently, in the case of mercury arc lamp light sources, the imaging lens must provide for correction of chromatic aberration, since the mercury spectra lines such as i-line (365 nm, obtained after filtering the light source) are not sharp enough (bandwidth about 10 nm); Not having to correct for chromatic aberration translates to savings in cost and ease in imaging lens fabrication (the lens cost is about ⅓ the cost of the exposure tool);
Increase in power efficiency; Overall efficiencies of laser diodes can reach 20%, much higher than any mercury arc lamp after filtering and excimer laser after linewidth
Dutta Barundeb
Yen Anthony
Brady III Wade James
Mathews Alan A.
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
Valetti Mark A.
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