Patent Number: 055240429
Section: description

DETAILED DESCRIPTION OF THE INVENTION The continuously increasing production and resolution requirements for manufacturing semiconductor devices has led to the development of an electron storage ring (ESR) based X-ray lithography system (XLS). The major subunits of an ESR based XLS include a preaccelerator, beam transport line, electron storage ring (synchrotron), lithography beamlines and exposure stations (aligner/stepper). A typical beamline for an XLS is described in U.S. Pat. No. 5,031,199 the specification of which is hereby incorporated by reference thereto. One basic ESR XLS performance requirement is to support 0.25-0.10 micron lithography resolution with a given stepper. One of the resolution related optical parameters is the lithography exposure window. The elements that form the exposure window in the beamline and the stepper include mirrors, filters and an exit window. One of the functions of the exit window is to separate the beamline from the stepper (lithography chamber) and contribute to the formation of the exposure window. The beamline connects the synchrotron to the exposure chamber where the actual lithography takes place. The beamline operates at an ultra high vacuum (UHV) while the exposure chamber operates at about 760 mm Hg (atmospheric pressure). In order to ensure the vacuum integrity of the beamline, the exit window must be able to withstand at least this pressure differential. In addition, the material of the window must be transmissive to that portion of the spectrum which is required for lithography (between 800 eV and 1800 eV) and substantially attenuate X-rays above and below the desired energy band. Moreover, the effects on the X-ray beam power uniformity of the X-ray beam passing through the exit window must be considered. A high performance ESR based XLS requires that the power uniformity in the beam be 95% or better. The exit window of the present invention achieves all of the above requirements. Referring now to FIG. 1, there is shown a stationary exit window assembly 10 which can be utilized in accordance with the present invention. The assembly 10 includes a support structure 12 to support exit window 14. An X-ray beam 16 travels in the direction shown, from the beamline (not shown) through exit window 14 and finally to the stepper or exposure chamber (not shown). Bellows 18 are provided to maintain the pressure in the beamline at an ultra high vacuum and bellows 20 are provided to maintain the pressure in the exposure chamber at atmospheric pressure. Referring now to FIGS. 2, 3(a), 3(b) and 4 there is shown respective cross-sectional, front, detailed front, and perspective views of the exit window 14 of the present invention. The window 14 includes a thin material 22 having a window section 24 disposed within an opening 26 of a frame 28. The dotted lines 29 represent the separation between the beamline and the exposure chamber. As shown in FIG. 3(a), the width "a" represents the width of an exposure field 31 on a wafer (not shown) in the exposure chamber as well as the width of opening 26. As shown in the full front view of the window in FIG. 3(b), the window section 24 includes first and second end sections 30, 32 and a middle section 34 disposed between the first and second end sections 30, 32. The window 14 also includes a flat rectangular peripheral section 36 which is integral with each of sections 30, 32 and 34 of window section 24 and extends within frame 28. As shown in FIG. 3(b) and the one-quarter perspective view of FIG. 4, the first and second end sections 30, 32 have a substantially concave shape that tapers to a flat shape 38 near a periphery 41 of the opening 26. The shape of the end sections 30, 32 can be analogized to a dome that is cut in two having its upper lip pulled back. The middle section 34 has a shape that is substantially concave along its width and linear along its length and tapering to flat surfaces 42 near the periphery 41 of the opening 26. As will be described below, middle section 34 has a substantially constant surface radius R which is preferably as large as possible so as to minimize the effects on beam power uniformity of the X-ray beam passing through exit window 14. AS shown in FIG. 2, frame 28 is provided to securely fasten the thin material 22 in place. The frame 28 consists of first and second frame members 44, 46 each having an opening that is preferably rectangular and equal to the exposure field on the wafer. Each of members 44, 46 is tube shaped with a rectangular cross section and integral with rectangular shoulders 48, 50 respectively which extend perpendicularly from one end of each of members 44, 46. The members 44, 46 are held together, and the thin material 22 is secured by pins 52 on opposite sides of each of shoulders 48, 50 as shown in FIG. 3(a). In order to seal the UHV of the beamline from the atmospheric pressure in the exposure chamber a seal 54 is positioned within the member 44 and abuts the thin material 22 between the members 44, 46. The seal 54 completely surrounds the opening 26 and can be formed of aluminum. Although the frame 28 and opening 26 have been described as having rectangular shapes, it should be understood by those skilled in the art that other shapes, such as spherical and ellipsoidal can be utilized. However the rectangular shape is preferred because spherical and ellipsoid shapes require thicker exit window materials which increase the absorption of radiation and therefore decreases the radiation intensity passing through the window. By providing an exit window 14 having the shape described above, the present invention allows the window to withstand the 14.7 psi pressure differential between the beamline and the exposure chamber and the thin material to be thin enough to allow the desired energy spectrum through the window while substantially attenuating the energy band above and below the desired range. The exit window 14 can withstand a pressure differential of up to 44.1 psi. A preferred material for the thin material 22 of exit window 14 is beryllium having a thickness between 16-25 microns. Other materials that can be used for the thin material 22 include carbon (diamond), silicon, silicon carbide, and silicon nitrite having a thickness between 25-35 microns. Referring now to FIG. 5, there is shown a graph of the relative beam transmission as a function of exit window thickness for three different exit window materials. As shown in FIG. 5, beryllium is preferred since the silicon and carbon exit windows yield lower radiation transmissions and require more intensive cooling and which leads to a lower production throughput. Referring now to FIG. 6, there is shown a graph of the radiation intensity of the X-ray beam emitted through the exit window 14 of the present invention. The exit window had a thickness of 18 microns. As seen in FIG. 6, the exit window of the present invention allows the desired energy spectrum (800-1800 eV) through the window while substantially attenuating the energy band above and below the desired energy band. In a production X-ray lithography beamline application the exit window size is dictated by the size of the exposure field (chip size). A performance requirement for ESR based XLS is that the exposure field (chip sizes) on a wafer must be 25 mm.times.50 mm or larger with wafer diameters of 200 mm. By providing the exit window 14 having the shape described above, the present invention allows the exit window to meet this requirement by allowing the width "a" of opening 26 to be 25 mm or larger and the length "b" of opening 26 to be 50 mm or larger. In this embodiment, the width "e" of the thin material 22 can be 42 mm and the length "f" of the thin material 22 can be 120 mm. Thus, the present invention provides an exit window 14 that can support a stationary exit window. The cross-section of the X-ray beam scanned over the exit window is typically a few mm in the vertical dimension and 50 mm or wider. In order to fully illuminate the exposure field this beam is scanned over the entire vertical dimension of 25 mm or a stationary beam is used and the target (mask and wafer) is moved relative to the stationary beam. In either case, the present invention allows the exit window to be stationary. This is in marked contrast to prior art exit windows that require a synchronously scanning exit window to be scanned along with the X-ray beam. The stationary exit window of the present invention has a simpler design and operation over scanning exit windows which contribute to a longer lifetime. The stationary window has the advantage that no mechanical movement is required. In addition, no control system is required and the heat load is distributed on a larger surface area. The present invention is also directed to a method of scanning the X-ray beam emitted from the X-ray lithography beamline onto the exposure field 31 of a wafer (not shown). The method includes the step of positioning a stationary exit window having an opening 26 approximately equal to the exposure field 31 between the beamline and the wafer. The material 22 of the exit window has a shape and thickness such that it can withstand a pressure differential of at least 14.7 psi and is transmissive to the desirable energy band. A vacuum is created within the beamline such that there is a pressure differential of at least 14.7 psi between the beamline and the exposure chamber containing the wafer. Next, the X-ray beam is scanned between first and second positions 56, 58 such that the X-ray beam passes through the exit window 14 and is incident on the exposure field 31 between first and second edges 57, 59 thereof. The X-ray beam as passed through the exit window 14 has X-rays above and below the desired energy band substantially attenuated due to the thickness of the material 22 of the exit window 14. The beam power transmission variation generated by the beam deflection as a result of the shape of the exit window must be considered. As shown in FIG. 2, as the beam scans between first and second positions 56, 58 the beam only contacts the curved surface 60 having a substantially constant surface radius R. The scanned beam approaches the curved surface 60 of middle section 34 at various angles and thus passes through increasing thickness of material 22 as it moves away from the center line 62 (zero deflection) towards the top and bottom of the window. This virtual thickness is a function of the material thickness, the curvature of the surface 60 and the deflection angle "d". Referring now to FIG. 7, there is shown a graph of the relative beam power transmission variation over one half of the scanned field. It is clear from the graph that a window with a large surface radius R and a small deflection angle are desirable to minimize the virtual thickness and the accompanied beam power transmission variations. For example, FIG. 7 shows the beam power as a function of the scanning angle for three different values of R ranging from 1.3 inches to 3.0 inches. As shown in FIG. 7, an exit window with a surface radius R.gtoreq.2 inches would provide the required at least 95% power uniformity at the indicated 0.13.degree. beam deflection. Although shown and described in what we believed to be the most practical and preferred embodiments, it is apparent that departures from the specific methods and designs described and shown will suggest themselves to those skilled in the art and may be made without departing from the spirit and scope of the invention. We, therefore, do not wish to restrict ourselves to the particular constructions described and illustrated, but desire to avail ourselves of all modifications that may fall within the scope of the appended claims.