Coherent light generators – Particular active media – Gas
Reexamination Certificate
2001-03-13
2004-06-22
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Gas
Reexamination Certificate
active
06754247
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to gas laser devices that emit ultraviolet light, and particularly KrF excimer laser devices, ArF excimer laser devices and fluorine laser devices in which the laser operation has a long laser oscillation pulsewidth.
2. Description of Related Art
With the miniaturization of and the high degree of integration of semiconductor integrated circuits, greater resolution has been demanded of the projection exposure equipment used in their fabrication. For that reason, the wavelengths of light emitted from exposure light sources have become increasingly shorter. At present, KrF excimer laser devices with a shorter wavelength of the beam than the wavelength of the light emitted from mercury lamps are used as semiconductor exposure light sources. However, gas laser devices that emit ultraviolet rays, such as ArF excimer laser devices and fluorine laser devices, are promising candidates for the next generation of semiconductor exposure light sources.
The gas laser devices mentioned above use a mixed gas comprising fluorine gas (F
2
), krypton gas (Kr) and a rare gas such as neon (Ne) as a buffer gas, or in the case of ArF excimer devices, a mixed gas comprising fluorine gas (F
2
), argon gas (Ar) and a rare gas such as neon (Ne) as a buffer gas, or in the case of fluorine laser devices, a mixed gas comprising fluorine gas (F
2
) and a rare gas such as helium (He) as a buffer gas. The mixed gas is sealed in the laser chamber at a pressure of several hundred kPa, and an electrical discharge is used to excite the laser gas, which is a laser medium.
The central oscillation wavelength of an ArF excimer laser device is 193.3 nm, which is shorter than the 248 nm central oscillation wavelength of a KrF excimer laser device. For this reason, the damage done to the quartz glass used in the projection lens systems of exposure equipment such as steppers is greater than in the case of KrF excimer lasers, and the reduced service life of the lens system is a problem.
The damage done to the quartz is in the form of color centers and compaction (increased index of refraction) formed by two-photon absorption. The former is manifest as a reduced index of transmission and the latter as a reduction of the lens system's capacity for resolution. Effects of that sort can be avoided by stretching the laser pulse width (pulse stretching). Now, this pulse stretch is desirable for the following reason.
In projection exposure equipment, the image of the mask that implements the circuit pattern is projected through the projection lens onto the workpiece, such as a wafer coated with photoresist. The resolution R and depth of focus DOF are expressed in the following formulas.
R=k
1
·&lgr;/NA
(1)
DOF=k
2
·&lgr;/(NA)
2
(2)
where k
1
and k
2
are coefficients that reflect characteristics of the resist, &lgr; is the wavelength of the exposure light emitted by the exposure light source, and NA is the numerical aperture.
In order to increase the resolution R, as is clear from formula (1), a shorter wavelength of the exposure light and a higher NA are selected, but to the extent that is done, the depth of focus DOF becomes smaller, as shown in formula (2). Because that increases the chromatic aberration effect, it is necessary to narrow the spectrum line width of the exposure light. In other words, it is necessary to further narrow the spectrum line width of the laser ray emitted by the gas laser device.
It has been reported in Proc. SPIE Vol. 3679, (1999) 1030-1037 that stretching the laser pulse width narrows the spectrum line width of the laser ray. This effect has been verified in experiments by the present inventors. In other words, to raise the resolution R, the spectrum line width of the laser light must be narrowed, and that requires stretching of the laser pulsewidth.
With the backdrop noted above, in Japanese Patent Application H11-362688, the present applicant proposed, as a means to stretch the pulsewidth, an ArF excimer laser device connected to the output terminal of a magnetic pulse compression circuit. The laser device having within the laser chamber a pair of laser discharge electrodes and a peaking capacitor connected in parallel to the pair of laser discharge electrodes. In the laser device, the primary current that injects energy from the magnetic pulse compression circuit through the peaking capacitor into the discharge electrodes and the secondary current that injects energy into the discharge electrodes from the capacitor used to charge the peaking capacitor in the final stage of the magnetic pulse compression circuit are combined. The oscillation period of the secondary current is set longer than the oscillation period of the primary current.
Proc. SPIE Vol. 3679, (1999) 1050-1057 provides a well known example of a laser pulse output waveform with the FWHM (measurement of the full width of the pulse at one half the maximum amplitude) measured at 30 ns or more.
In general, when the pulsewidth FWHM is stretched, the number of round trips in the optical resonator (times the laser beam travels back and forth within the optical resonator) increases, and the spectrum line width narrows.
According to Proc. SPIE Vol. 3679, (1999) 1030-1037, the spectrum line width can be made narrower when, at a given pulse width, the relative intensity of the latter half of the pulse is increased. Hence, this paper describes the relationship between spectrum line width and the variations in the laser pulse waveform caused by varying the concentration of fluorine (in waveforms with FWHM in the 20 ns range).
Aside from the pulse stretching mentioned above, JPO Kokai Patent H11-8431 proposed the use of an etalon coupler in the optical resonator as a method of narrowing to 0.4 pm or less.
Now, fluorine laser devices with a pulsewidth FWHM of 12 ns or less are well known.
In the ArF excimer laser device introduced in Proc. SPIE Vol. 3679, (1999) 1030-1037, however, the pulsewidth FWHM was in the 20 ns range, and the line width was reduced to 0.4 pm by lowering the oscillation efficiency. Because reducing the concentration of fluorine also lowers the output, there are limits to how much the line width of a long pulse can be narrowed by means of the concentration of fluorine.
Using an etalon coupler, as proposed in the JPO Kokai Patent H11-8431, also produces high technical barriers, such as difficulties in controlling the central wavelength.
SUMMARY OF THE INVENTION
The primary object of the invention is to obviate the problems noted above in the prior technology. In particular, the purpose of the present invention is to introduce a method of varying the laser pulse waveform to the method of pulse stretching proposed by the present applicant. As such, the present invention may utilize an ArF excimer laser device for exposure that is capable of a pulsewidth FWHM of 20 ns or more, a pulse duration of 50 ns or more, and a spectrum line width FWHM of 0.35 pm or less. The present invention may also use a KrF excimer laser device and a fluorine laser device with pulses stretching beyond the conventional width by introduction of such a method.
This object is achieved in accordance with the invention in that the ArF excimer laser device of this invention is one which connects to the output terminal of a magnetic pulse compression circuit and includes a pair of laser discharge electrodes located within the laser chamber and a peaking capacitor connected in parallel with the pair of laser discharge electrodes, wherein the output waveform of the laser pulse has a bifurcated form comprising a front half peak and a later half peak. If the peak value of the front half peak is P
1
and the peak value of the later half peak is P
2
and the (proportion of the pulse later half peak)=P
2
/(P
1
+P
2
)×100(%), then the (proportion of the pulse later half peak) is 50% or more.
In this case, it is preferable that the primary current that injects energy from the magnetic pulse compression circuit through the peaking c
Kakizaki Koji
Tada Akifumi
Ip Paul
Menefee James
Nixon & Peabody LLP
Safran David S.
Ushiodenki Kabushiki Kaisha
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