Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
1999-07-29
2002-10-29
Nguyen, Kiet T. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S398000
Reexamination Certificate
active
06472673
ABSTRACT:
FIELD OF THE INVENTION AND DESCRIPTION OF PRIOR ART
The invention relates to a lithographic method for producing an exposure pattern on a substrate comprising a layer of resist material sensitive to exposure to an energetic radiation, wherein in a pattern transfer system (e.g., a lithographic imaging system) by means of a beam of said energetic radiation a mask having a structure pattern, namely, a set of transparent structures to form a structured beam, is illuminated and the structure pattern is imaged onto the substrate by means of the structured beam, the substrate being positioned after the stencil mask as seen in the optical path of the beam, producing a pattern image, namely, a spatial distribution of irradiation over the substrate, the spatial distribution having a finite pattern transfer blur as determined by the pattern transfer system, and the pattern image is shifted laterally with respect to the substrate between a plurality of predetermined shift positions and with each shift position the substrate is irradiated for a predetermined time, the width of lateral displacements between two neighboring shift positions being smaller than the minimum feature size of the exposure pattern to be formed.
In manufacturing semiconductor devices, one important step for structuring the semiconductor substrates is lithography. The substrate, for instance a silicon wafer, is coated with a thin layer of photosensitive material, called photo-resist. By means of a lithographic imaging system, a pattern is imaged onto the photo-resist, and the subsequent development step removes from the substrate either the exposed or the unexposed portion of the photo-resist. Then, the substrate is subjected to a process step such a etching, deposition, oxidation, doping or the like, the photo-resist pattern on the substrate covering those portions of the surface that shall remain unprocessed. The photo-resist is stripped, leaving the substrate with the new structure. By repeating this sequence, multiple structure layers can be introduced to form the semiconductor micro-circuits.
Lithographic projection methods and lithographic devices using electron beams are discussed, for instance, in H. Koops, “ELECTRON BEAM PROJECTION TECHNIQUES” 235-255, FINE LINE LITHOGRAPHY (Roger Newman ed. 1980). Electrons, and in particular ions, have the advantage of very low particle wavelengths—far below the nanometer range—which allows for very good imaging properties, as, for example, discussed in Gerhard Gross et al., “Ion projection lithography: Status of the MEDEA project and United States/European cooperation,” J. VAC. SCI. TECHNOL., B16(6), pp. 3150-3153, Nov./Dec. 1998.
In projection lithography, the pattern to be imaged onto the photoresist-covered substrate is produced by using a mask or recticle having the desired pattern. For particle projection systems, stencil masks are used in which the patterns to be projected are formed as apertures of appropriate shape in a thin membrane, i.e., a few micrometers thick. The mask pattern is built up from a number of apertures in a thin membrane through which the particle beam is transmitted to expose the resist-coated wafer in those areas required for device fabrication.
Stencil masks may also be used in lithography systems based on photons, like EUV (Extreme UV) or X-ray lithography, e.g. in the EUV lithography system transmission mask geometry as proposed by H. Löschner et al. in the U.S. application Ser. No. 09/316,834.
A further application of the present invention is for the stencil masks used in vapor deposition systems, e.g. in nanosieves as put forward in J. Brugger et al., “RESISTLES 100-nm PATTERN FORMATION USING NANOSIEVES AS SHADOW MASKS”, INTERNATIONAL CONFERENCE ON MICRO- AND NANO-ENGINEERING ABSTRACT BOOK, Rome, Italy, 193-194, Sep. 21-23, 1999. In this case, the particles are essentially neutral atoms or molecules. In this case, the particles are essentially neutral atoms or molecules.
As also discussed by H. Koops, op. cit., pp. 245-248, with self-supporting stencil masks a problem arises for configurations which require a ring-shaped exposure region on the wafer; the central area of the ring-shaped region is completely surrounded by the aperture (so-called doughnut problem) and thus ‘cut out’. Problems also arise with simply connected patterns like free-standing bars or leafs. Therefore, additional means need to be taken to stabilize the central area in the proper position of the mask. Also for large or very long aperture areas in the mask, which effectively separate the mask foil into distinct parts, a stability problem arises; moreover, these structures are difficult to prepare.
Therefore, a “crossed grating solution” was proposed for electron projecting systems, where a supporting grid of the finest grid constant which can be generated is overlaid on the desired pattern for device fabrication. If the supporting grid constant is about one tenth of the finest line-width in the mask, the supporting grid will vanish in the exposed and developed area, due to the proximity effect and the limited resolution of the projecting optical system. Due to the fine dimensions of the supporting grid, however, its production is too difficult to be practically implemented.
Another approach, presented, for example, in U Behringer and H. Engelke, “Intelligent Design Splitting in the Stencil Mask Technology Used for Electron- and Ion-Beam Lithography,” J. VAC. SCI. TECHNOL. B11(6), pp. 2400-2403, Nov./Dec. 1993, splits the device pattern into complementary mask fields situated on at least two masks. Thus, the pattern on each complementary mask is more stable; however, now a set of masks has to be handled with the lithography setup. Also, the production expenses of the stencil masks are multiplied accordingly.
The so-called “multibeam” solution, also described by H. Koops, op. cit., subdivides the device pattern into squares of equal area by a software routine; for each square of the device pattern, an aperture is provided in the aperture pattern which, though, only covers a fraction, e.g., a quarter, of the device pattern square. This is illustrated in
FIG. 1
with a device pattern DP having the shape of a rectangular line. The structure of the pattern is subdivided into quadratic areas PQ which each correspond to a quarter of the smallest elements in the device pattern desired. With a line as in
FIG. 1
, a square has a side length ps equal to half the line width dw; typical values of these dimensions are, for example, ps=100 nm and dw=200 nm. The substrate is multiply exposed with this aperture pattern, in the example of
FIG. 1
four times where one square PQ is shifted laterally, as indicated by the arrow-line is, to the four quadrants of a square of doubled side length, pd=2 ps, and the total pattern is constructed by subsequent exposures of the wafer; for each shift position, the same duration of irradiation is used. Within this disclosure, a ‘lateral’ movement means a movement along the—usually flat—surface of the substrate or mask, as the case my be. In the pattern thus produced, the adjacent images PQ′ of the aperture squares PQ lie side by side, as illustrated in
FIG. 1
a
. It should be noted that the dimensions of the imaged pattern depend on whether the imaging optics is a 1:1 optics or has a demagnification, e.g. by a factor of 4. By virtue of the “multibeam” solution, a plurality of small apertures is realized instead of a large opening in the foil, and the remaining foil forms stable struts between the apertures which improves the mechanical stability of the mask and eases preparation of the masks.
An electron microprojector setup and mask geometries exploiting the “multibeam” method are also described in J. Frosien et al., “Application of the electron microprojector in the field of microlithography,” PROCEEDINGS OF THE MICROCIRCUIT ENGINEERING '79, Rhienisch-Westfalische Technische Hochschule, Aachen, Germany, Sep. 25-27, 1979.
However, the “multibeam” solution has been rejected for the use in semiconductor lines since it
Chalupka Alfred
Haugeneder Ernst
Finnegan Henderson Farabow Garrett & Dunner LLP
IMS Ionen-Mikrofabrikations Systeme GmbH
Nguyen Kiet T.
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