Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2000-02-18
2001-05-29
Gray, David M. (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S053000, C355S067000, C355S077000
Reexamination Certificate
active
06239862
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a photolithographic apparatus for imaging a mask pattern on a substrate and more particularly to a photolithographic apparatus that includes, a radiation source unit, a first optical system, an optical waveguide, and a subsequent second optical system and a mask table.
2. Discussion of Related Art
A photolithographic apparatus of the type described in the opening paragraph is known, for example from European Patent Application EP 0 658 810. Such an apparatus is used for manufacturing integrated circuits (ICs). In this apparatus, a mask provided with a pattern is illuminated and repetitively imaged on a photosensitive layer provided on a semiconductor substrate, for example, a silicon substrate. A large number of integrated circuits (ICs) must be formed on the substrate. After an image of the mask has been formed on the substrate, the substrate is to this end moved with respect to the mask through a distance which is slightly larger than the length or the width of the ICs to be formed, whereafter a subsequent mask image is made. This process is repeated until the desired number of ICs is formed. The apparatus described above is of the stepper type.
In current photolithographic apparatuses, it is desirable that the illumination beam has a maximal intensity so that the illumination period for each IC is as small as possible and the time of passage of the substrate through the apparatus, in other words the time required to illuminate all ICs, is as short as possible. Moreover, the aim is to image on larger fields and to decrease the dimensions of the smallest details to be imaged. Imaging on larger fields may be achieved, for example, by enlarging the projection lens. However, this renders the projection lens very expensive. Another way of imaging on larger fields is to make use of a photolithographic apparatus of the scanner type. In this type of apparatus, the projection lens does not need to be modified. An additional advantage of a scanner is the uniform quality of the image of the smallest details because aberrations caused by the lens are averaged during scanning. A result is that the output of components with structures having critical dimensions is higher.
In a scanner, a field to be illuminated is divided into imaginary sub-fields. Due to a coupled, continuous movement of the mask and the substrate, each time a different part of the mask is projected on the substrate so that the complete field will be illuminated gradually. When a complete field has been illuminated, the substrate is moved in this case through a distance which is larger than the length or the width of the complete field, so that the next field is reached. An example of a photolithographic apparatus of the scanner type is described in the article: “The future and potential of optical scanning systems” by D. A. Markle in Solid State Technology, September 1984, pp. 159-166.
The illumination time per field is longer for a scanner than for a stepper if the field size is the same in both cases. In fact, in a stepper, the illumination time per field is only determined by the available energy. In a scanner, the overscan time is also to be taken into account. Here, the scan time for illuminating the total field is not only determined by the field size but also by the slit width. Namely, there is a given time interval at the start and the end of the field to be illuminated, in which a part of the slit-shaped illumination field is not present above the field to be illuminated. The illumination time and consequently the time of passage of the substrate are thus dependent on the slit width of the illumination beam. However, systems in which a lamp is used as a light source have a relatively low optical efficiency. This is caused by the difference in throughput between the lamp and the slit geometry. In fact, the lamp illuminates light at all angles, while the slit geometry is anamorphous so that the geometry of the radiation field of the lamp deviates to a relatively large extent from the slit geometry.
If the fields are small enough to be illuminated completely with one pulse, a stepper is preferred as far as speed is concerned.
BRIEF SUMMARY OF THE INVENTION
It is an object of the invention to provide a photolithographic apparatus in which the above-mentioned drawbacks relating to scanners and steppers are obviated.
According to the invention, the photolithographic apparatus is characterized in that the apparatus is adapted in such a way that both the stepper mode and the scanner mode are selectable modi, and in that the apparatus comprises means by which the illumination unit, when being operative, has a slit-shaped static illumination field with a slit width s which is variable between s
min
and s
max
while substantially maintaining energy within the static illumination field.
The slit width s of the illumination unit is to be understood to mean the width of the static illumination field at the area of the substrate. This static field is formed on the exit face of an optical waveguide and subsequently imaged on the mask by means of an optical system.
The stepper mode is to be understood to mean the mode in which a field is completely illuminated on the substrate by means of one and the same light pulse. The scanner mode is to be understood to mean the mode in which the mask and the substrate perform a coupled movement and the complete field will be fully illuminated after a period of time has elapsed.
Since both the stepper mode and the scanner mode can be selected in the apparatus according to the invention, small as well as large fields on a substrate can be illuminated with the same apparatus, and this at a minimum loss of illumination energy because the slit width is variable. By virtue of the variable slit width, it can thus be ensured that the time of passage is minimized for all slit widths in the scanner mode and for the width of the complete field in the stepper mode.
A preferred embodiment of the photolithographic apparatus according to the invention is characterized in that s=w for w≦w
step
, and s
min
≦s≦s
max
for w>w
step
, in which s is the width of the illumination field supplied by the illumination unit, s
min
and s
max
are its minimum and maximum values, respectively, w is the width of a field to be illuminated and w
step
is the maximum width of a field to be illuminated in the stepper mode.
As a result, the slit width has the dimension of the field width if the stepper mode is selected, and the slit width is variable if the scanner mode is selected in order to be able to optimize the time of passage for each field size.
A maximum field width w
step
is coupled with the stepper mode. If the field to be illuminated has a smaller width than this value, the slit width will be rendered equal to the width of the field to be illuminated. This is possible until a minimum slit width s
min
is reached. When the scanner mode is selected, in other words, when a field having a width larger than w
step
is to be illuminated, the slit width will be variable between the values s
min
and s
max
.
There are different possibilities for realizing an illumination unit having a light beam which is slit-shaped and a slit width which is variable while maintaining energy.
A first embodiment of the photolithographic apparatus according to the invention is characterized in that the optical waveguide comprises at least one optically transparent bar.
The transparent bar has an integrator function. The transparent bar may be made of, for example quartz. In that case, a reflection of more than 99% can be achieved at the side walls of the bar.
A further embodiment of the photolithographic apparatus according to the present invention is characterized in that a slit-shaped diaphragm having a variable aperture is present between the mask table and the end of the optical waveguide facing the mask table.
At the exit face of the optical waveguide, the diaphragm will ensure that a light beam having the desired beam cross-section is
Jasper Johannes C. M.
Mulkens Johannes C. H.
van den Brink Marinus A.
ASM Lithography B. V.
Gray David M.
Nguyen Hung Henry
Pillsbury & Winthrop LLP
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