Patent Application
20120050706
The present disclosure relates generally to grazing-incidence collectors (GICs), and in particular to a source-collector module for use in an extreme ultraviolet (EUV) lithography system that employs a laser-produced plasma (LPP) target system that uses Xenon ice to generate EUV radiation.
Laser-produced plasmas (LPPs) are formed in one example by irradiating Sn droplets with a focused laser beam. Because such LPPs can radiate in the extreme ultraviolet (EUV) range of the electromagnetic spectrum, they are considered to be a promising EUV radiation source for EUV lithography systems.
In the operation of LPP-NIC SOCOMO 10, laser beam 13 irradiates Sn pellets 22 as the pellets pass through the laser beam focus F13, thereby produce a high-power LPP 24. LPP 24 typically resides on the order of hundreds of millimeters from NIC mirror MN and emits EUV radiation 30 as well as energetic Sn ions, particles, neutral atoms, and infrared (IR) radiation. The portion of the EUV radiation 30 directed toward NIC mirror MN is collected by the mirror and is directed (focused) to an intermediate focus IF to form an intermediate focal spot FS. The intermediate focus is arranged at or proximate to an aperture stop AS. Only that portion of the EUV radiation that makes it through aperture stop AS forms focal spot FS. Here it is noted that focus spot FS is not an infinitely small spot located exactly at intermediate focus IF, but rather is a distribution of EUV radiation 30 generally centered at the intermediate focus.
Advantages of LPP-NIC SOCOMO 10 are that the optical design is simple (i.e., it uses a single ellipsoidal NIC mirror) and the nominal collection efficiency can be high because NIC mirror MN can be designed to collect a large angular fraction of the EUV radiation 30 emitted from LPP 24. It is noteworthy that the use of the single-bounce reflective NIC mirror MN placed on the opposite side of LPP 24 from the intermediate focus IF, while geometrically convenient, requires that the Sn source 20 not significantly obstruct EUV radiation 30 being delivered from the NIC mirror to the intermediate focus. Thus, there is generally no obscuration in the LPP-NIC-SOCOMO 10 except perhaps for the hardware needed to generate the Sn pellet stream.
LPP-NIC SOCOMO 10 works well in laboratory and experimental arrangements where the LPP-NIC SOCOMO lifetime and replacement cost are not major considerations. However, a commercially viable EUV lithography system requires a SOCOMO that has a long lifetime. Unfortunately, the proximity of the NIC mirror surface 16 and the multilayer coatings 18 thereon to LPP 24, combined with the substantially normally incident nature of the radiation collection process, makes it highly unlikely that the multilayer coating 18 will remain undamaged for any reasonable length of time under typical EUV-based semiconductor manufacturing conditions.
A further drawback of the LPP-NIC SOCOMO 10 is that it cannot be used in conjunction with a debris mitigation tool based on a plurality of radial lamellas through which a gas is flowed to effectively stop ions and neutrals atoms emitted from the LPP 24 from reaching NIC mirror MN. This is because the radial lamellas would also stop the EUV radiation from being reflected from NIC mirror MN.
Multilayer coating 18 is also likely to have its performance significantly reduced by the build-up of Sn, which significantly absorbs the incident and reflected EUV radiation, thereby reducing the reflective efficiency of the multilayer coated ellipsoidal mirror. Also, the aforementioned energetic ions, atoms and particles produced by LPP 24 will bombard multilayer coating 18 and destroy the layered order of the top layers of the multilayer coating. In addition, the energetic ions, atoms and particles will erode multilayer coating 18, and the attendant thermal heating from the generated IR radiation can act to mix or interdiffuse the separate layers of the multilayer coating.
While a variety of fixes have been proposed to mitigate the above-identified problems with LPP-NIC SOCOMO 10, they all add substantial cost and complexity to the SOCOMO, to the point where it becomes increasingly unrealistic to include it in a commercially viable EUV lithography system. Moreover, the Sn droplet LPP EUV light source is a complex and expensive part of the SOCOMO. What is needed therefore is a less expensive, less complex, more robust and generally more commercially viable SOCOMO for use in an EUV lithography system that uses a simpler and more cost-effective LPP-based EUV radiation source.
The present disclosure is generally directed to grazing incidence collectors (GICs), and in particular to GIC mirrors used to form a source-collector module (SOCOMO) for use in EUV lithography systems, where the SOCOMO includes a LPP target system that uses Xenon ice and a laser to generate EUV radiation.
An aspect of the disclosure is a SOCOMO for an EUV lithography system. The SOCOMO includes a laser that generates a pulsed laser beam, and a fold mirror arranged along a source-collector module axis and configured to receive the laser beam and reflect the laser beam down the source-collector module axis in a first direction. The SOCOMO also includes a Xenon ice source configured to provide Xenon ice at an irradiation location where the Xenon ice is irradiated by the pulsed laser beam, thereby creating a LPP that generates EUV radiation in a second direction that is generally opposite the first direction. The SOCOMO also includes a grazing-incidence collector (GIC) mirror having an input end and an output end and arranged to receive the EUV radiation at the input end and focus the received EUV radiation at an intermediate focus adjacent the output end.
Another aspect of the disclosure is a method of collecting EUV radiation from a LPP. The method includes providing a GIC mirror along an axis, the GIC mirror having input and output ends. The method also includes arranging adjacent the GIC mirror input end an LPP target system configured to provide Xenon ice, and moving the Xenon ice past an irradiation location. The method additionally includes sending a pulsed laser beam down the GIC mirror axis and through the GIC mirror from the output end to the input end and to the Xenon ice at the irradiation location, thereby forming the LPP that emits the EUV radiation. The method further includes collecting with the GIC mirror at the GIC input end a portion of the EUV radiation from the LPP and directing the collected EUV radiation out of the GIC mirror output end to form a focus spot at an intermediate focus.
Another aspect of the disclosure is a LPP target system. The system includes a laser that generates a pulsed laser beam, and a condensation surface cooled so as to condense a band of Xenon ice thereon. The system also includes a rotation drive unit mechanically coupled to the condensation surface and configured to cause the rotation of the Xenon ice band formed thereon past an irradiation location where the pulse laser beam is incident upon the Xenon ice.
Additional features and advantages of the disclosure are set forth in the detailed description below, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the disclosure as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
The various elements depicted in the drawing are merely representational and are not necessarily drawn to scale. Certain sections thereof may be exaggerated, while others may be minimized. The drawing is intended to illustrate an example embodiment of the disclosure that can be understood and appropriately carried out by those of ordinary skill in the art.
The present disclosure is generally directed to GICs, and in particular to GIC mirrors used to form a source-collector module (SOCOMO) for use in EUV lithography systems that have a LPP-based EUV light source.
In an example embodiment, RCED 110 includes an inverted funnel-like element 111D arranged downstream of intermediate focus IF and configured to direct radiation 30 from intermediate focus IF to a downstream position, such as to the illumination optics (see
Target portion 42 is irradiated by laser beam 13 traveling through GIC mirror MG in the −X direction along axis A1, thereby creating EUV radiation 30 that is emitted generally in the +X direction. The axial obscuration presented by fold mirror FM is minimal. Thus, laser beam 13 travels in one direction (i.e., the −X direction) through GIC mirror MG generally along axis A1 and EUV radiation 30 travels generally in the opposite direction (i.e., the +X direction) through the GIC mirror, RCED 110 and to intermediate focus IF.
Target portion 42 also includes a Xenon gas flow system 130 that typically resides outside of vacuum chamber 120, as shown. Xenon gas flow system 130 is configured to provide a metered flow of Xenon gas 132G through a gas flow conduit 134. Target portion 42 further includes a closed cycle helium cryostat 140 that refrigerates a dual stage cold-finger 180, described below.
Arranged within chamber interior 122 is a Xenon ice unit 150 fluidly connected to Xenon gas flow system 130 via conduit 134 and helium cryostat 140 via conduit 144. Xenon ice unit 150 is configured to provide frozen Xenon 132F (i.e., Xenon ice) at an irradiation location 158 where focused laser beam 13 is incident upon the Xenon ice to form EUV radiation 30, as described below.
With reference to
With reference to
In an example, at least one temperature sensor TS and at least one pressure sensor PS are provided in vacuum chamber 120 to respectively monitor the temperature and pressure within vacuum chamber interior 122 and in particular in open interior region 162 within heat shield 160.
Xenon ice unit 150 also includes a rotation drive unit 196 mechanically coupled to rotatable containment vessel 170 at bottom surface 178 to rotate the rotatable condensing surface.
Target portion 42 also includes a controller 200 that is operably connected to vacuum system 126, Xenon gas flow system 130, closed cycle helium cryostat 140, first and second cooling stages 184 and 186, temperature sensor TS, pressure sensor PS, rotation drive unit 196, and laser 12 of light source portion 41 of LPP target system 40 (see
With reference to
Controller 200 also sends a signal S2 to the helium cryostat 140 to start the flow of Helium gas 142G to the dual stage cold finger 180. Controller 200 further sends control signals SC1 and SC2 to first and second cooling stages 184 and 186 so that the Helium gas 142G flowing to helium cryostat 140 is cooled to a very low temperature, e.g., about 4° K. This makes the cold finger 180 serve as a super-cooled cryo-tip that cools the Helium thermal transfer gas 142GS in sealed interior region 172 of containment vessel 170.
The pressure of Helium gas 142GS is controlled by controller 200 via a mass flow valve (not shown) so that the contained Helium gas has a select pressure thus controlling thermal transfer from the condensation surface 170 to the cold finger 180. Helium gas 142GS acts to cool the condensation surface 170, which in turn serves to cool the Xenon gas 132G flowing around the outer surface 174 of the condensation surface 170. The cooling is done to the point where frozen Xenon 132F forms as a band on outer surface 174 at a location corresponding to the location of the cryo-tip end and to aperture 164. An example thickness of frozen Xenon 132F is 1 mm.
Controller 200 also sends a control signal S3 to rotation drive unit 196 to initiate the rotation of rotatable condensation surface 170. This rotation causes frozen Xenon band 132F to rotate as well, so that the frozen Xenon continually passes by aperture 164 (i.e., frozen Xenon band 132F rotates through irradiation location 158, with a portion of the band always residing at the irradiation location). Example rotational speeds of containment vessel 170 are typically 60 to 100 rpm, designed to present a fresh ice surface to a 1 KHz laser pulse 13.
Xenon freezes at 161.4° K, which is well within the freezing capabilities of helium cryostat 140, which can generate much lower temperatures (e.g., 12° K). Controlling the “heat leak” from condensation surface 170 to the helium cryostat 140 by managing the pressure of Helium gas 142GS by the action of controller 200 (As described below) insures that outer surface of 170 will be at or below the freezing point of Xenon gas 132G.
Controller 200 additionally sends a signal S4 to laser 12 in light source portion 41 (
When frozen Xenon 132F passes by aperture 164, focused laser beam 13 irradiates the frozen Xenon and forms LPP 24 (shown in phantom), which emits EUV radiation 30 generally in the +X direction. In an example embodiment, a given location in frozen Xenon 132F is exposed with multiple pulses of radiation from laser beam 13. This allows for a slower rotation of containment vessel 170.
The continual passing of frozen Xenon 132F past aperture 164 allows for high repetition rates and long run times for LPP 24.
Advantages of the Xenon-based LPP target system 40 of the present disclosure include minimal debris formation from the frozen Xenon, relatively long run times, mechanical simplicity and compactness.
SOCOMO with No First-Mirror Multilayer
An example configuration of LPP-GIC SOCOMO 100 has no multilayer-coated “first mirror,” i.e., the mirror or mirror section upon which EUV radiation 30 is first incident (i.e., first reflected) does not have a multilayer coating 18. In another example configuration of SOCOMO 100, the first mirror is substantially a grazing incidence mirror. In other embodiments, the first mirror may include a multilayer coating 18.
A major advantage of LPP-GIC SOCOMO 100 is that its performance is not dependent upon on the survival of a multilayer coated reflective surface. Example embodiments of GIC mirror MG have at least one segmented GIC mirror shell, such as mirror shell M1 shown in
GIC Vs. NIC SOCOMOs
There are certain trade-offs associated with using a LPP-GIC SOCOMO 100 versus a LPP-NIC SOCOMO 10. For example, for a given collection angle of the radiation 30 from the LPP 24, the LPP-NIC-SOCOMO can be designed to be more compact than the LPP-GIC-SOCOMO.
Also, the LPP-NIC-SOCOMO can in principle be designed to collect EUV radiation emitted from the source at angles larger than 90° (with respect to the optical axis), thus allowing larger collection efficiency. However, in practice this advantage is not normally used because it leads to excessive NIC diameters or excessive angles that the EUV radiation 30 forms with the optical axis at IF.
Also, the far field intensity distribution generated by a LPP-GIC-SOCOMO has additional obscurations due to the shadow of the thickness of the GIC shells and of the mechanical structure supporting the mirrors. However, the present disclosure discusses embodiments below where the GIC surface includes a surface correction that mitigates the shadowing effect of the GIC shells thicknesses and improves the uniformity of the focus spot FS at the intermediate focus IF.
Further, the focus spot FS at intermediate focus IF will in general be larger for a LPP-GIC SOCOMO than for a LPP-NIC SOCOMO. This size difference is primarily associated with GIC mirror figure errors, which are likely to decrease as the technology evolves.
On the whole, it is generally believed that the above-mentioned trade-offs are far outweighed by the benefits of a longer operating lifetime, reduced cost, simplicity, and reduced maintenance costs and issues associated with a LPP-GIC SOCOMO.
Example designs for the example GIC mirror MG are provided in Table 1 and Table 2 below. The main optical parameters of the design are: a) a distance ΔL between LPP 24 and intermediate focus IF of 2400 mm; and b) a maximum collection angle at the LPP side of 70.7°. In an example embodiment, GIC shells 250 each include a Ru coating for improved reflectivity at EUV wavelengths. The nominal collection efficiency of the GIC mirror for EUV radiation 30 of wavelength of 13.5 nm when the optical surfaces of GIC shells 250 are coated with Ru is 37.6% with respect to 2π steradians emission from LPP 24.
Since an LPP EUV source is much smaller than a discharge-produced plasma (DPP) EUV source (typically by a factor of 10 in area), the use of LPP 24 allows for better etendue matching between the GIC mirror output and the illuminator input. In particular, the collection angle at LPP 24 can be increased to very large values with negligible or very limited efficiency loss due to mismatch between the GIC mirror and illuminator etendue. In an example embodiment, the collection half-angle can approach or exceed 70°.
The dimension of LPP 24 has a drawback in that the uniformity of the intensity distribution in the far field tend to be worse than for a DPP source, for a given collector optical design. Indeed, since the LPP 24 is smaller, the far-field shadows due to the thicknesses of GIC shells 250 tend to be sharper for an LPP source than for a DPP source.
To compensate at least partially for this effect, a surface figure (i.e., optical profile) correction is added to each GIC shell 250 to improve the uniformity of the intensity distribution in the far field (see, e.g., Publication No. WO2009-095219 A1, entitled “Improved grazing incidence collector optical systems for EUV and X-ray applications,” which publication is incorporated by reference herein). Thus, in an example embodiment of GIC mirror MG, each GIC shell 250 has superimposed thereon a polynomial (parabolic) correction equal to zero at the two edges of the shells and having a maximum value of 0.01 mm.
Table 1 and Table 2 set forth an example design for the GIC mirror MG shown in
EUV Lithography System with LPP-GIC SOCOMO
System 300 includes a system axis A3 and an EUV light source LS that includes SOCOMO 100 with axis A1 and having the Xe-ice-based LPP target system 40 as described above, which generates LPP 24 that emits working EUV radiation 30 at λ=13.5 nm.
SOCOMO 100 includes EUV GIC mirror MG and optional RCED 110 as described above. In an example embodiment, EUV GIC mirror MG is cooled as described in U.S. patent application Ser. No. 12/592,735, which is incorporated by reference herein. Also in an example, RCED 110 is cooled.
EUV GIC mirror MG is arranged adjacent and downstream of EUV light source LS, with collector axis A1 lying along system axis A3. EUV GIC mirror MG collects EUV working radiation 30 (i.e., light rays LR) from EUV light source LS located at source focus SF and the collected radiation forms intermediate source image IS (i.e., a focus spot) at intermediate focus IF. RCED 110 serves to enhance the collection of EUV radiation 30 by funneling to intermediate focus IF the EUV radiation that would not otherwise make it to the intermediate focus. In an example, LPP-GIC SOCOMO 100 comprises LPP target system 40, GIC mirror MG and RCED 110.
An embodiment of RCED 110 as discussed above in connection with
An illumination system 316 with an input end 317 and an output end 318 is arranged along system axis A3 and adjacent and downstream of EUV GIC mirror MG with the input end adjacent the EUV GIC mirror. Illumination system 316 receives at input end 217 EUV radiation 30 from source image IS and outputs at output end 318 a substantially uniform EUV radiation beam 320 (i.e., condensed EUV radiation) for illumination of the reticle. Where system 300 is a scanning type system, EUV radiation beam 320 is typically formed as a substantially uniform line (e.g. ring field) of EUV radiation at reflective reticle 336 that scans over the reticle.
A projection optical system 326 is arranged along (folded) system axis A3 downstream of illumination system 316 and downstream of the illuminated reticle. Projection optical system 326 has an input end 327 facing illumination system output end 318, and an opposite output end 328. A reflective reticle 336 is arranged adjacent the projection optical system input end 327 and a semiconductor wafer 340 is arranged adjacent projection optical system output end 328. Reticle 336 includes a pattern (not shown) to be transferred to wafer 340, which includes a photosensitive coating (e.g., photoresist layer) 342. In operation, the uniformized EUV radiation beam 320 irradiates reticle 336 and reflects therefrom, and the pattern thereon is imaged onto photosensitive surface 342 of wafer 340 by projection optical system 326. In a scanning system 300, the reticle image scans over the photosensitive surface to form the pattern over the exposure field. Scanning is typically achieved by moving reticle 336 and wafer 340 in synchrony.
Once the reticle pattern is imaged and recorded on wafer 340, the patterned wafer 340 is then processed using standard photolithographic and semiconductor processing techniques to form integrated circuit (IC) chips.
Note that in general the components of system 300 are shown lying along a common folded axis A3 in
It will be apparent to those skilled in the art that various modifications and variations can be made to the present disclosure without departing from the spirit and scope of the disclosure. Thus it is intended that the present disclosure cover the modifications and variations of this disclosure provided they come within the scope of the appended claims and their equivalents.
