Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
1999-04-09
2002-11-26
Anderson, Bruce (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S492300, C250S492100, C250S3960ML
Reexamination Certificate
active
06486480
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates generally to ion beam lithography and more particularly to the masks in ion beam lithography systems.
As the dimensions of semiconductor devices are scaled down in order to achieve ever higher levels of integration, optical lithography will no longer be sufficient for the needs of the semiconductor industry, e.g. DRAM and microprocessor manufacture. Alternative “nanolithography” techniques will be required to realize minimum feature sizes of 0.1 &mgr;m or less. In addition, the next generation lithography technologies must deliver high production throughput with low cost per wafer. Therefore, efforts have been intensified worldwide in recent years to adapt established techniques such as X-ray lithography, extreme ultraviolet lithography (EUVL), electron-beam (e-beam) lithography, and ion projection lithography (IPL), to the manufacture of 0.1 &mgr;m-generation complementary metal-oxide-semiconductor (CMOS) technology. Significant challenges exist today for each of these techniques. In particular, there are issues with complicated mask technology.
Conventional ion projection lithography (IPL) systems, as shown in
FIG. 1
, require many stencil masks for semiconductor circuit processing. An ion source with low energy spread is needed to reduce chromatic aberration. A small beam extracted from the source is accelerated to about 10 keV and expanded to form a parallel beam before impinging onto a large area stencil mask which contains many small apertures. The aperture pattern is then projected onto a resist layer on a wafer after the beam is reduced in size and made parallel by an Einzel lens system. Different masks with particular patterns must be used for each layer to be formed on the wafer. The stencil masks used in a conventional IPL system are a major source of problems, in terms of stability and lifetime.
In the conventional IPL setup, the stencil mask, shown in
FIG. 2
, is extremely thin, e.g. about 3 &mgr;m, to minimize beam scattering inside the aperture channels, which have a typical diameter of about 0.3 &mgr;m. Since the beam energy is high, about 10 keV, when it arrives at the mask, both sputtering and mask heating will occur, causing unwanted mask distortion and instability. There are also problems of beam uniformity and alignment since the stencil mask is at a distance from the ion source.
Acceleration and focussing columns are used in IPL systems as well as focussed ion beam (FIB) systems and electron beam (e-beam) systems to accelerate and reduce beam size. Typically, demagnification is accomplished by first accelerating a parallel beam for a distance and then focussing it to form a crossover beam. After that, the beam is allowed to expand again to the proper size before it is made parallel. The problem with beam crossover is that it can increase the longitudinal energy spread and produce image blur. This effect puts an upper limit on the maximum beam current and therefore lithography process throughput. One technique to reduce space charge force is to supply a large number of oppositely charged particles in the crossover region, but this is impractical to implement in an IPL system.
SUMMARY OF THE INVENTION
Accordingly it is an object of the invention to provide an ion projection lithography (IPL) system which has no stencil mask.
It is also an object of the invention to provide an IPL system in which the mask is not exposed to high energy ions.
The invention is a plasma-formed IPL system which eliminates the acceleration stage between the ion source and stencil mask. Instead the mask is used as a beam forming or extraction electrode, positioned next to the plasma in the ion source. Thus the entire beam forming electrode or mask is illuminated uniformly with the source plasma. The extracted beam passes through an acceleration and reduction stage onto the resist coated wafer. Because low energy ions, about 30 eV, pass through the mask, which is much thicker than a conventional stencil mask, heating, scattering, and sputtering are minimized so mask lifetime and performance are improved.
A multicusp ion source with magnetic filter produces ion beams with low energy spread, as low as 0.6 eV. The low energy plasma ions pass through the mask by applying a suitable (low) voltage, e.g. about 30 V, to the mask. A beam accelerator and reduction column after the mask produces a demagnified pattern on the resist. The accelerator and reduction column can be designed with beam crossover or without beam crossover.
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Lee Yung-Hee Yvette
Leung Ka-Ngo
Ngo Vinh
Zahir Nastaran
Anderson Bruce
Sartorio Henry P.
The Regents of the University of California
Vanore David A.
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