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Researchers Submit Patent Application, "Wynn-Dyson Imaging System with Reduced Thermal Distortion", for Approval

July 16, 2014



By a News Reporter-Staff News Editor at Electronics Newsweekly -- From Washington, D.C., VerticalNews journalists report that a patent application by the inventor Hawryluk, Andrew M. (Los Altos, CA), filed on December 21, 2012, was made available online on July 3, 2014.

The patent's assignee is Ultratech, Inc.

News editors obtained the following quote from the background information supplied by the inventors: "The Wynn Dyson imaging system has been used in the manufacture of semiconductor devices for approximately 30 years. The basic Wynn-Dyson imaging system has a reflective mirror and a refractive doublet located in the vicinity of the mirror focus.

"One limitation of this fundamental design when used for photolithography is that a large reticle in the object plane would interfere with a large wafer residing in the image plane as the reticle image is 'step and repeated' across the wafer to fully expose the entire wafer.

"The basic Wynn-Dyson imaging system has thus been modified for use in photolithography by using prisms attached to the doublet assembly. The prisms serve to move the object and image planes into non-conflicting planes. Example Wynn-Dyson imaging systems as modified for use in photolithography are disclosed in U.S. Pat. Nos. 6,863,403, 7,116,496, 6,813,098, and 7,148,953.

"FIG. 1 shows a prior art Wynn-Dyson imaging system 8. The Wynn-Dyson imaging system 8 has a concave mirror 10 disposed along an optical axis 14. Mirror 10 has an aperture stop AS that serves to define a numerical aperture (NA) for the system. A lens system 13 is arranged along optical axis 14 and is spaced apart from mirror 10. The lens system 13 has a front end 13F that faces mirror 10, and a rear end 13R that faces away from mirror 10. The lens system 13 includes a meniscus lens 13A at the front end 13F of the lens system, and a plano-convex lens 13B at the rear end of the lens system.

"Wynn-Dyson imaging system 8 also includes two prisms 17 and 19 that define a prism assembly. Prisms 17 and 19 are each arranged on opposite sides of optical axis 14, with one of their surfaces in intimate contact with respective portions of the planar surface of plano-convex lens 13B, which defines the rear end 13R of the lens assembly 13. A reticle 16 resides in an object plane OP and a wafer 18 resides in an image plane IP. Prism 17 resides adjacent reticle 16 while prism 19 resides adjacent wafer 18.

"A typical photolithography reticle 16 has transmissive regions and opaque regions. The opaque regions are typically made of chrome, which reflects approximately 70% of the UV light 11 incident thereon, and absorbs the remaining 30%. The transmissive regions are clear and transmit UV light 11 with little absorption. For reticles that are 50% transparent, approximately 15% of the total UV light incident thereon is absorbed by the reticle. In a typical photolithography system, the power level of the UV light 11 incident on reticle 16 is about 2 watts/cm.sup.2, which implies that approximately 300 mw/cm.sup.2 of light is absorbed by the reticle. In some conditions, a reticle might be only 10% transmissive, in which case, approximately 540 mw/cm.sup.2 of light is absorbed by the reticle. This is sufficient to cause heating, and subsequently, mechanical distortion in the reticle.

"The UV light 11 from the illuminator (not shown in FIG. 1) that is absorbed by the reticle 16 heats the reticle, which is in close proximity to prism 17. Measurements by the inventors have shown that, in some cases, reticle 16 can heat to up to 50.degree. C. when irradiated by UV light 11. This heating causes two main problems. The first is that the reticle itself distorts. The amount of reticle distortion depends upon the type of glass used for the reticle. However, typical values for the coefficient of thermal expansion (CTE) for common reticle materials such as quartz is about 1 part per million (ppm) per .degree. C. Hence, a 30.degree. temperature rise can distort the reticle by approximately 30 ppm. For a 36 mm long image field at the wafer, this distortion is calculated as (36 mm/2)30.times.10.sup.-6=0.5 microns. Depending upon the lithography and overlay requirements, this may be highly problematic.

"The second main problem is that the heat from the reticle is transferred to the adjacent prism 17. It has been observed by the inventors that this heat transfer can cause prism 17 to bend, which leads to an asymmetric image distortion at the wafer (i.e., the reticle image is distorted).

"FIG. 2 is a close-up schematic diagram of reticle 16 and the adjacent prism 17 according to the prior art configuration of FIG. 1. Prism 17 has a proximal surface 17a and a corner or tip 17c. As the reticle 16 heats up, it heats the adjacent prism 17 by convective heating and by radiative transfer of heat (the reticle emits heat in the infrared, e.g., around 10 microns in wavelength). The convective heat or convective heating is represented in FIG. 2 by dashed lines and denoted 20C, while the radiative heat or radiative heating is represented by arrows and denoted 20R. The overall heating is denoted as 20. The tip 17c of the prism 17 heats up more than its base and so bends. This bending causes the image of the reticle 16 at the wafer 18 to shift in one direction, which is to say that the reticle image suffers from thermally induced distortion.

"Prism bending changes the distortion (or magnification) in the 'y' direction (i.e., creates a 'Y-mag' change). Under some conditions (e.g., where the reticle is 95% chrome, and hence, absorbs approximately 600 mw/cm.sup.2), the steady-state Y-mag change can be over 50 ppm, thereby leading to over 1 micron of image distortion at the wafer (image) plane. This directly leads to a 1 micron overlay error.

"Generally, it is required that the overlay accuracy of a photolithography system be approximately 25% of the linewidth being printed. A 1 micron overlay error implies that the smallest feature that the photolithography system can be used to manufacture would be about 4 microns, independent of the resolution of the system. Hence, even if the photolithography system has a resolution of 1 micron, it cannot be used in manufacturing for features smaller than 4 microns.

"Prior attempts to solve the problem of thermal distortion of the prism include flowing cool air between the reticle and the prism. Unfortunately, the time-dependent nature of the prism heating renders this approach unsatisfactory. When the photolithography system is sitting idle for more than a few minutes, the prism 17 returns to its base temperature. When the photolithography system is then operated, the prism 17 begins to heat up. The temperature of the prism 17 increases with increasing number of wafers 18 processed.

"However, after only one wafer being processed, the reticle temperature typically increases by only a few .degree. C., while after 10 wafers, it typically increases by 20.degree. C. to 30.degree. C. Therefore, the reticle temperature (and hence, the prism temperature) is time dependent. Unfortunately, air cooling usually ends up overcooling the reticle initially during the first few wafers and then does not adequately cool the reticle for larger numbers of wafers (e.g., 10 or more). In other words, the time dependency of air cooling is too slow relative to the heating profile of the photolithograph system to make air cooling an effective solution for reducing thermally induced distortion. Furthermore, the air flow approach only mitigates convective heating and does not mitigate radiative heating."

As a supplement to the background information on this patent application, VerticalNews correspondents also obtained the inventor's summary information for this patent application: "An aspect of the disclosure is a Wynn-Dyson imaging system that operates with light having a UV wavelength, comprising along an optical axis: a mirror having a concave surface; an aperture stop located at the mirror that determines a numerical aperture (NA) of the system; a lens assembly with positive refracting power and having a front end and a back end, with lens assembly being arranged adjacent the mirror and spaced apart therefrom with the front end facing the mirror; first and second prisms operably disposed on opposite sides of the optical axis and adjacent the back end of the lens assembly, the first and second prisms having respective first and second planar surfaces, wherein the first and second planar surfaces are arranged adjacent object and image planes, respectively. The system also includes at least one of: a) the first and second prisms each being made of a glass material having a coefficient of thermal expansion of no greater than about 100 ppb/.degree. C.; and b) a first window operably disposed between the planar surface of the first prism and the object plane and that substantially transmits the UV light and that substantially blocks infrared (IR) radiation in a wavelength range from 2 microns to 20 microns, and a second window operably disposed between the planar surface of the second prism and the image plane, wherein the second window substantially transmits the UV light, and wherein the first and second windows are configured to maintain imaging symmetry.

"Another aspect of the disclosure is a method of reducing image distortion in a Wynn-Dyson imaging system used to image a reticle onto a wafer using a UV wavelength. The method includes disposing a first window between the reticle and an adjacent reticle prism, wherein the first window is configured to substantially IR light having a wavelength between 2 microns and 20 microns and to substantially transmit the UV light. The method also includes disposing a second window between the wafer and an adjacent wafer prism, wherein the second window substantially transmits UV light. The method also includes irradiating the reticle with the UV light to form an image of the reticle on a wafer using the Wynn-Dyson imaging system. The UV light heats the reticle to at least 50.degree. C., thereby causing the reticle to emit convective heat and radiative heat. The method further includes blocking with the first window a substation portion of the convective heat and the radiative heat so that the image of the reticle on the wafer has reduced image distortion as compared with not using the first IR-blocking window and the second window.

"Another aspect of the disclosure is a method of reducing image distortion in a Wynn-Dyson imaging system having a primary mirror and an optical assembly and used to image a reticle onto a wafer using light having a UV wavelength. The method includes providing the optical assembly with reticle and wafer prisms made from an ultra-low expansion (ULE.RTM.) glass material. The method also includes irradiating the reticle with the UV light to form an image of the reticle on a wafer using the Wynn-Dyson imaging system. The UV light heats the reticle to at least 50.degree. C., thereby causing the reticle to emit convective heat and radiative heat. The method further includes the image of the reticle on the wafer having a maximum image distortion of no greater than 0.5 microns.

"Additional features and advantages will be set forth in the Detailed Description that follows and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the embodiments as described in the written description and claims thereof, as well as the appended drawings. It is to be understood that both the foregoing general description and the following Detailed Description are merely exemplary and are intesnded to provide an overview or framework for understanding the nature and character of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

"The accompanying drawings are included to provide a further understanding and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment(s) and together with the Detailed Description serve to explain principles and operation of the various embodiments. As such, the disclosure will become more fully understood from the following Detailed Description, taken in conjunction with the accompanying Figures, in which:

"FIG. 1 is a schematic diagram of an example prior art Wynn-Dyson imaging system for use in photolithography;

"FIG. 2 is a close-up view of the reticle and adjacent prism of the prior art Wynn-Dyson of FIG. 1, showing how heat from a heated reticle can heat the adjacent prism, causing the corner of the prism to bend, thereby leading to optical distortion at the image plane;

"FIG. 3 is a close-up view of an optical assembly suitable for forming a modified Wynn-Dyson imaging system, wherein the optical assembly includes first and second windows respectively arranged adjacent the reticle and wafer prisms of a prism assembly, wherein the first window is an IR-blocking window;

"FIG. 4 is a schematic diagram of a modified Wynn-Dyson imaging system that includes the optical assembly of FIG. 3;

"FIG. 5A is a plan view of an example IR-blocking window mounted in a heat-sink mount at the outer edge of the IR-blocking window to remove heat from the IR-blocking window; and

"FIG. 5B is a cross-sectional view of the IR-blocking window and heat-sink mount of FIG. 5A as taken along the line 5B-5B."

For additional information on this patent application, see: Hawryluk, Andrew M. Wynn-Dyson Imaging System with Reduced Thermal Distortion. Filed December 21, 2012 and posted July 3, 2014. Patent URL: http://appft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO%2Fsearch-adv.html&r=5114&p=103&f=G&l=50&d=PG01&S1=20140626.PD.&OS=PD/20140626&RS=PD/20140626

Keywords for this news article include: Electronics, Ultratech Inc, Photolithography.

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Source: Electronics Newsweekly


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