Patent Number: 053295692
Section: description

DETAILED DESCRIPTION OF THE INVENTION In order to better understand the present invention, the following introductory discussion is provided. Application of x-rays to real processes requires containment of undesirable debris resulting from the x-ray generation process. This is especially important in x-ray lithographic processes wherein cleanliness of the irradiated sample is of the utmost importance. Typical x-ray generation systems include a window which is highly transmissive for x-ray radiation. Unfortunately, materials which have the required transmissivity (i.e. low opacity) to act as a window for x-rays often times do not have the required structural or tensile strength to act as barrier or shield to the undesirable debris. In fact, for soft x-rays (i.e. wavelengths of about 1-1000 Angstroms) no one single material has been found which exhibits all of the required properties to act as both a window and a debris shield or barrier. Presently, two approaches have been developed for resolving such dilemma: first, simply select materials which satisfy the transmissivity requirement and replace windows as they fail or second, develop systems comprising spaced apart debris shields and x-ray windows and replace the lower cost debris shields as they fail. However, neither solution has provided a cost effective solution to designing x-ray transmissive debris shields. The present invention provides a novel x-ray transmissive shield superior to existing window and debris shield systems. As will be described in more detail below, the x-ray transmissive shield of the present invention comprises a layer of a first x-ray transmissive material laminated to a layer of a second x-ray transmissive material. The resulting composite window structure has sufficient structural strength to be free standing and to withstand the impact of radiation generated debris as well as the required x-ray transmissivity. The individual properties of each material are complementary so as to synergestically yield an x-ray transmissive debris shield having superior operating characteristics to those of x-ray transmissive debris shields composed of one or the other of such materials. Looking now to FIG. 1, the present invention will be described in more detail. An x-ray transmissive shield 10 comprises a layer 12 of a first x-ray transmissive material and a layer 14 of a second x-ray transmissive material. Layer 12 is laminated to layer 14 with adhesive 16. Those skilled in the art will appreciate that other methods for laminating or bonding the layers together can be used. An important element of the present invention resides in the selection of such materials (12, 14) and adhesive 16. Generally, such first and second materials are selected from groups of materials exhibiting either high tensile strength and low melting point, or low tensile strength and high melting point. As used herein, the terms high and low are relative terms comparing a property of a material in one group to the corresponding property of a material in the other group. Recognizing that no one material has yet been found which can satisfy all the requirements for a transmissive debris shield for soft x-rays, the starting point for designing any x-ray transmissive shield is to first identify its required characteristics. Since typical x-ray generation systems have very low x-ray generation efficiencies, high transmissivity (i.e. low opaqueness) to desired wavelengths of electromagnetic radiation is critical. Transmissivity of a material is related to a product of material thickness and its absorption coefficient. Thus minimizing transmission losses requires minimizing the product of material thickness and absorption coefficient. While selecting a highly x-ray transmissive material (i.e. a low absorption coefficient) would seem to resolve such issue, other factors such as structural or tensile strength and minimum achievable thicknesses of the material greatly impedes the selection process. For example, highly x-ray transmissive materials, such as beryllium (Be), have a very low absorption coefficient and layers as thin as .about.12 .mu.m can be achieved; however, the usual thicknesses of free standing Be windows are typically much thicker (e.g.&gt;25 .mu.m) because Be is an extremely brittle material lacking the required structural strength to withstand the impact of radiation generated debris. A number of (.about.50 .mu.m) thick Be windows were irradiated with 3 KeV x-rays. The fluence of the x-rays was varied from 0.25-1.5 cal/cm.sup.2. The area of the Be window was varied from 1 to 5 cm.sup.2. After one impulse of the x-ray source, the Be windows exposed fluences&gt;1.0 cal/cm.sup.2 failed due to mechanical loading. Alternatively, polymeric materials, such as KAPTON, have been employed as x-ray transmissive shields. While such polymeric materials can have usable layer thicknesses less than Be (e.g. KAPTON.about.8.5 .mu.m), such polymeric materials' absorption coefficients are larger than Be resulting in a less transmissive layer. Moreover, such polymeric materials can be adversely affected by high energy radiation fluences because the absorbed radiation results in increased temperatures in the polymeric material which can undergo a substantial degradation in structural strength at elevated temperatures. For example, a (.perspectiveto.25 .mu.m) KAPTON window was irradiated with 3 KeV x-rays. The fluence of the x-rays was varied from 0.1 to 1 cal/cm.sup.2. The area of the KAPTON window was varied from 1 to 50 cm.sup.2. After one impulse of the x-ray source, the KAPTON consistently failed by melting at all area sizes when the fluence was greater than .about.0.6 cal/cm.sup.2. Such fluence restriction increasingly limits the x-ray generation systems with which such polymeric materials can be used. In summary, a x-ray transmissive debris shield should have the following characteristics; low absorption coefficient, minimum thickness, good structural strength, high temperature and high energy radiation fluence resistance. Unfortunately, no one material satisfies all such criteria. Surprisingly, a window or debris shield as depicted in FIG. 1 composed of laminated, alternating thin layers of a highly x-ray transmissive material and a polymeric material has been found to provide superior operating characteristics to those achievable by either material separately. Preferably, the highly x-ray transmissive layer faces the source of x-rays. In particular, highly x-ray transmissive materials having high melting points and high thermal conductivities can be selected from the group including: lithium, boron, beryllium, carbon (diamond), silicon, magnesium, and aluminum as well as alloys thereof. Polymeric materials exhibiting the desired high tensile strengths can be selected from the group including thermoset polymers, MYLAR, KEVLAR, KAPTON, TEFLON, FORMVAR as well as the more general class of polymers including polyvinyl formal, polypropylene, lexan, polyimides, fluorocarbons, fluoropolymers, polycarbonates, polyethylene, polyetherketone, polypropylene, and polystyrene. By laminating thin layers of Be with KAPTON, KAPTON retains its structural strength because Be's high heat conductivity allows it to act as a heatsink to keep the KAPTON cool. In this situation, Be provides no real strength to the composite window and as such, very thin layers of Be can be used; but rather, the composite window relies almost totally on the KAPTON layer for structural integrity. Depicted in Table I below are the calculated time-temperature responses of a composite window (composed of a layer of Be laminated to a layer of KAPTON) to an instantaneous pulse of x-ray radiation. Temperatures are measured at one location (B.sub.1) in the Be and at ten locations (K.sub.1 . . . K.sub.10) in the KAPTON, wherein the KAPTON thickness increases according to K.sub.1 to K.sub.10. Under identical x-ray fluences, KAPTON will reach higher peak temperatures at time 0 then Be because of its lower thermal conductivity and higher absorption coefficient. The initial instantaneous temperature for the Be layer is 110.degree. and .about. 700.degree. C. for the KAPTON layer. After as little as 300 .mu.secs, the KAPTON measuring point furthest removed from the Be layer (i.e. K.sub.10) has already cooled to below 550.degree. C. Because Be has a high thermal conductivity, it can act as a heatsink and cool the KAPTON layer to a temperature below which it retains its high tensile strength. TABLE I __________________________________________________________________________ Time B.sub.1 K.sub.1 K.sub.2 K.sub.3 K.sub.4 K.sub.5 K.sub.6 K.sub.7 K.sub.8 K.sub.9 K.sub.10 __________________________________________________________________________ 0 110 701 708 700 696 700 703 700 697 700 703 1 110 694 700 700 700 700 700 700 700 700 700 2 110 659 700 700 700 700 700 700 700 700 700 3 110 619 698 700 700 700 700 700 700 700 700 4 110 583 694 700 700 700 700 700 700 700 700 5 110 552 687 700 700 700 700 700 700 700 700 6 110 526 679 699 700 700 700 700 700 700 700 7 110 504 669 698 700 700 700 700 700 700 700 8 100 485 659 696 700 700 700 700 700 700 700 9 110 469 649 649 700 700 700 700 700 700 700 10 110 454 639 691 699 700 700 700 700 700 700 20 110 366 552 650 687 698 700 700 700 700 700 30 110 323 495 606 664 689 697 699 700 700 700 40 110 296 454 569 639 675 391 697 699 700 700 50 110 277 424 537 614 659 683 694 698 699 700 60 110 263 401 511 591 643 673 688 695 698 699 70 110 252 382 489 571 626 661 681 692 696 697 80 110 243 366 471 552 611 650 674 687 693 695 90 110 236 353 454 536 596 638 666 681 690 692 100 110 230 342 440 521 583 627 657 675 685 688 200 110 195 277 353 422 482 531 569 597 613 618 300 110 178 244 306 364 414 458 492 517 532 537 400 110 166 220 272 319 362 398 427 448 461 466 500 110 156 201 244 284 319 349 373 391 402 405 600 110 148 186 221 254 283 308 328 343 352 355 700 110 142 173 202 229 254 274 291 303 311 313 800 110 136 162 187 209 229 246 260 270 276 279 900 110 132 153 173 192 209 223 235 243 248 250 1000 110 128 146 163 178 192 204 213 220 225 226 2000 110 113 116 118 121 123 124 126 127 128 128 3000 110 110 111 111 112 112 112 112 113 113 113 4000 110 110 110 110 110 110 110 110 110 110 110 5000 110 110 110 110 110 110 110 110 110 110 110 __________________________________________________________________________ A preferred embodiment of the present invention includes a plurality of alternating layers of a highly x-ray transmissive material laminated to layers of an x-ray transmissive polymeric material. Specifically, FIG. 2 depicts an x-ray transmissive debris shield 20 composed of alternating thin layers of a highly x-ray transmissive material 22 laminated on both faces of a thin layer of a polymeric material 24. Such layers can be laminated one to another with an adhesive 26. Moreover, layers of the highly x-ray transmissive, high heat conductance material as thin as .about.12.5 .mu.m and x-ray transmissive polymeric materials as thin as .about.2.5 .mu.m are believed to yield satisfactory results. Unfortunately, while a plurality of very thin layers laminated together is preferred, as the number of layers increases as illustrated in FIG. 3 so does the aggregate thickness of the adhesive 26 which is a poor x-ray transmissive material. EXAMPLE A 50 .mu.m-thick Be layer was laminated to a 8.5 .mu.m layer of KAPTON as depicted in FIG. 1 using a polyimide enamel varnish. This varnish consisted of the same polymer as KAPTON and was cured at elevated temperatures and pressure. Specifically, a polyimide enamel adhesive was air brushed onto the KAPTON layer and allowed to dry for 15 minutes. The Be layer was then affixed to the adhesive side of the KAPTON layer under 1500 PSI pressure and heated to a temperature of 212.degree. F. and held for one hour, then heated to a temperature of 302.degree. F. and held for one hour, then heated to a temperature of 419.degree. F. and held for one hour and finally cooled to room temperature. In particular, 5-cm.sup.2 area, debris fluence on the debris shields was varied from 0.5 to 0.75 cal/cm.sup.2. The debris shields survived the test with no visible damage to either the KAPTON or Be layers. While the present invention has been described with reference to specific materials, those skilled in the art will recognize that variation in the material selection can be made without departing from the scope of the claims appended hereto. Moreover, while the present invention has been shown useful with pulsed x-ray sources, it is also useful with continuous x-ray sources.