Device for reducing the peak output of a pulse laser light source

The invention relates to a projection exposure apparatus with a device for reducing the Peak power of a pulsed laser light source.

Pulse laser light sources, for example excimer lasers, for the UV lithography have a repetition rate of about 1000 to 4000 Pulses per second. The pulse length is currently 20 to 30 ns. Within each pulse occurs in dependence from the gas state, the state of the laser, in particular of the optical Components, and depending on the cavity length to considerable Modulations of the laser output on the Time.

It has been found in practice that a significant Disadvantage of a pulse laser light source is that the short pulse duration causes a high pulse power density, relating to the optical materials, particularly glassy Materials, a very negative effect. Silica glass at a working wavelength of 193 nm is at a rate in the a Power of the power density received damaged. The consequences thereof are transmission losses and an uncontrolled increase the refractive index of which the service life of the optical system limited. When SiO₂ in glassy form there are places with particularly weak bonds. The pulse from the laser light source generated pulsed shortwave electromagnetic radiations thus provide the energy to this disadvantageously to modify points.

From EP 0785473 A2 an apparatus is of the above-mentioned known type, through which the pulse from a laser light source split light coming into several partial beams will, through which bypasses. carried out in this way a bundle expansion or a division into several juxtaposed partial beams. These partial beams are arranged side by side in a lighting device entered.

The disadvantage here is that thus a plurality of optical axes next to each other produced and the lighting device adjusted accordingly is. Moreover, is the aforementioned device firmly in their orientation.

In the US-A 5 661748 is a pulse delay device having partially transparent mirrors and inevitable "tail" of the Pulse distribution and an arrangement with two concave mirrors and a gridded mirror is described in which a principle Offset (Part 4.2) occurs which is not inclined by the Passage through Planplatten arises.

The US-A 5,337,333 relates to a device for reducing the peak power of a pulsed laser light source for X-ray lithography.

The US-A 4,941,093 describes a laser for beamforming to erode a surface, in the machining beam a telescope consisting of two converging lenses, is arranged to the laser beam provided on the requirements optimize out.

The present invention is therefore based on the object, to provide a device by means of which damage to Components housed in the beam path of the pulsed laser light source, be avoided if possible without any particular loss the efficiency of the pulsed laser light source.

According to the invention this object is achieved by the Claim 1 dissolved mentioned measures.

According to the invention will now be the high points or peaks of mined pulse laser light source and the pulse power density reduced, namely with little integral energy penalty. By the beam splitter in communication with the bypass line is performed a division of the beam of the pulsed laser light source in partial beams with an optical path difference. It is only to ensure that the path difference is selected so is that at least largely complementary peaks and valleys. Peaks and valleys are separated in time, for example 7.0 ns. This corresponds to a path of light of 2.1 m. Around halving whole pulses in their pulse power density, are they divided into a plurality of pulses, and then another time ranked. To the greatest possible reduction of the peaks within a pulse and a substantially uniform power density to generate, in an advantageous embodiment the invention proposes at least two detour lines in the beam path behind the other to arrange.

According to the invention not several partial beams are produced which arranged side by side with a corresponding number of optical axes, but it carried a reunion the partial beams to a total beam, whereby a uniform optical axis for subsequent illumination device given is. It takes place after the division into several Partial beams finally again take a Auffädelung.

The invention Device Rässt for influencing of polarized light use what accordingly between the beam path and the Beam splitter means, an angle, namely, the Brewster angle in connection with a slightly detuned Keplerian telescope is provided. The Brewster angle is reached, that through the beam splitter means, for example a Mirror pass freely and without loss, 50% of the beam (Apart from absorption in the splitter layer) while the other 50% is derived. Of course are an alternative to the invention, however, also other angles and thus also different breakdown percentages possible.

In a further embodiment of the invention, provided be that the beam recombining member is so designed that part of the partial beam, which run on the detour line is sent repeatedly through the bypass line.

In this way, even a plurality of optical path differences and thus achieve an even greater homogenization. Of course, additional losses from mirrors come and divider for carrying.

Through a phase plate in the beam path is obtained influencing the power peaks and the lifetime the optical materials subjected to the pulse laser light source are.

Further advantageous refinements and developments from the remaining dependent claims and from the following principle with the embodiment of the drawing.

It shows:<dl tsize="7"><dt>figure 1</dt><dd>a schematic diagram of the beam path of a Pulse laser light source with two bypasses;</dd><dt>figure 2</dt><dd>the power of the pulse laser light source in the division state by a first bypass line;</dd><dt>figure 3</dt><dd>the performance history of the pulse laser light source according to a second bypass line;</dd><dt>figure 4</dt><dd>a further embodiment of a bypass line in a schematic representation; and</dd><dt>figure 5</dt><dd>a schematic diagram of a projection exposure system according to the invention with an excimer laser and the described Device.</dd></dl>

As shown in Figure 1 applies a beam 1 a pulse laser light source such as an excimer laser 2 (shown in the Figure 5) to a first beam splitter means 3. happen in the form of a mirror 50% of the total beam as partial beam 1a unimpeded and without losses the splitter mirror 3 towards a second beam splitter 4 as second beam splitter. The other 50% of the beam make their way as a partial beam 1b via a bypass line 5, the three levels 6, 7 and 8 as reflective components is formed. The beam splitter 3 as a first beam splitter acting on its reverse side, at the same time as Beam combining element 9, in which the partial beam 1b the passing through the beam splitter means partial beam la is reunited. is a result of the first detour line 5 the pulse of the pulse laser light source 2 is smoothed in itself. Same time, the peak power by 30 to 40% reduced. Due to the angular position of the splitting mirror 3 and the first detour line 5 to the vibration direction of the incident linearly polarized laser light passes, the amount of Polarization degree of the energy balance is not important. Both Partial beams 1a and 1b are at the output of the first bypass line 5 equal polarized, but at different times.

From Figure 2 is the time offset of the two partial beams 1a and 1b recognized. As is apparent, thus, the Peak power of the pulse laser light source, by virtue of Beam splitting has been reduced to each half, in an addition because of the time offset of the two "Peaks" P1 and P2 in accordance with the resulting Total peak compared to an untreated beam path reduced.

The laser light beam after the reunification of the two Partial beams 1a and 1b in the further beam path through the second beam splitting means in the form of the splitter mirror 4 in turn, 45 ° rotated (Brewster angle) to the direction of oscillation the laser light is, in the sub-beam 10a the splitter mirror 4 freely pass, and the partial beam 10b, which is sent via a second bypass line 11, divided. The second bypass line 11 is also through mirror 12, 13 and 14 are formed as reflective components. The Back of the beam splitter 4 is again the second Beam combining member 15 through which the two partial beams 10a and 10b reassembled with the same polarization direction and subsequently sent on together.

Through the second bypass line 11, for example, a path of travel of may have 11.1 m, the pulse of the pulse laser light source 2 displaced as a whole. In this way, at the output or on the beam combining element 15, two pulses with equal Polarization that are largely smoothed or no inadmissible high "peak" longer have that to damage from could lead components.

From the figure 3, the power versus time, and the two Pulse seen as after the beam combining member 15 present. The original power peak can on about one third of the original value can be reduced. For example, when the temporal pulse duration of the pulse laser light source 30 ns, this corresponds to a path of light of 9 m. If you combine according to the embodiment, first detour line 5 is connected to the second bypass line 11, so once delayed 2.1m and smoothed in this way pulse 9.0 m + 2.1 m, for a total of 11.1 m delayed in second detour line 11th

For the inventive measures are most suitable areas between the laser output and a scanner input.

The particular advantage of the invention lies in the fact that the subsequent illumination unit are not specifically designed needs, but the proposed establishment only between laser output and input of the lighting part of a Scanner is inserted. This is also an economic Retrofitting of existing systems (no new lighting part the scanner).

Since the beam quality of the pulse laser light source by the different path lengths may Reiden, this example is a Kepler telescope (in individual cases to the excimer laser divergence slightly adapted), with its two lenses 16a and 16b - as in Figure 1 indicated - in the second bypass line 11 again produced. In an alternative to the invention, it is also a single lens in question.

Of course, it is also possible for the above- to extend said structure and as yet more phasing lines provide, if deemed physically impractical and a further lowering of excellence still benefits brings.

In an alternative to the invention, it is also conceivable to divide the beam physico instead of optical polarization. If the peak power of a pulsed laser light source for example by a corresponding time Offset of the sub-beams is reduced by half, thereby reducing not the risk of damage to components only by half, but even more. As the "peaks" in the individual pulse are structured by which the resonator length comes in the laser, can be already with short delay paths achieve that the pulse within its length offset and the structuring is exploitable, thereby a remarkable flattening can be achieved. by virtue of the structure of the pulses of pulse laser light sources can be thereby even by small bypasses the power peaks clearly reduce. An optimum is achieved, of course, if the postponement by the beam splitting is so great that assign each "peak" and valleys leave. This can be, for example through the second bypass line 11 reach. Instead of the mirror as a reflective Components can also be other reflective components, such as Prisms are provided.

one generated by a λ / 4 plate (not shown) prior to the Means circularly polarized light is any azimuthal Angle about the Z axis for the beam splitting means 3 and 4 possible.

In the figure 4 is a further embodiment of a Detour line shown in principle. As can be seen, Partition the beam 1 to the beam splitting means 3 into a partial beam 1a passes through the beam splitter 3, and a Partial beam 1b of the way on the rectangular bypass line 5 takes. The detour line 5 is in this case by four mirrors 17, 18, 19 and 20 are formed before the beam part 1b on the rear side the beam splitting means 3, the same as in the Embodiment of the figure 1 as a beam combining member 9 serves, is reunited with the partial beam 1a.

takes place in contrast to the embodiment of the figure 1 while not a complete union, but the embodiment of Figure 4 is designed such that a portion of the way past on the bypass line 5 part-beam 1b once more the way of the bypass line 5 decreases, and this can be carried out several times. Coupling of of the beam takes place in this embodiment the manner of a resonator, wherein the proportion of on the detour line 5 revolving light or partial beam 1b, ie the adjustable pulse length, on the state of polarization depends. In this way, any delays in the incoming pulses and a related lowering of the Pulse peak power of pulse laser light source can be achieved. It is only necessary to ensure that an optimum between extending the time frames of the partial pulses (number of resonator-rounds) and the total losses is selected. Reached this is achieved by the arrangement of a phase plate 21 in the beam path in front of spotlights splitter 3 and a further phase retardation plate 22 in the bypass line 1b. In this manner, a linearly polarized light from a made slightly elliptical polarized light when Passage through the beam splitter 3 with a corresponding Component is reflected. The amount of component depends on the degree of the ellipse. you Equips eg The phase retardation plate 21 with a λ / 4-value, then An amount of 50% sent via the detour line 1b. In practice, one λ-values ​​between 0 and a select district.

The embodiment according to Figure 4 with the two phase retardation plates 21 and 22 has the advantage that in this way the device can be extremely versatile. Substituted it namely one or both phase retardation plates 21 and 22 so changes in accordance with the peak power. Would like to achieve as a higher power, which forced a higher load and thus a shorter life for the device is created, we will phase retardation plates 21 and 22 are running, the force correspondingly fewer rounds. Conversely, one can in this way by a corresponding increase in the number of revolutions of a light Protection of the optical components of the projection exposure system to accomplish.

In the figure 5 is in principle the use of the site Device 23 shown. The polarized Light source 2 is then for a projection exposure system provided that on its own in a by a wall 17 Near separate clean room with an illumination optics 18, a mask 19, a projection lens 20 and a laser 21 is. In order to influence the clean room as little as possible, is in this case the device 23, the inside of the is provided in Figure 1 shown parts, between the Pulse laser light source 2 and the wall 17 in the beam path 1 of the pulse laser light source 2 is arranged.