Semiconductor wafer processing

Abstract: A chuck (10) for holding a workpiece (e.g., a semiconductor wafer) in a vacuum comprises a curved reference surface (13). By clamping the edges of the workpiece and maintaining its backside against the curved surface (or against pins (18) mounted on the surface), the frontside of the workpiece can be thereby established in a precise equidistant relationship with respect to the reference surface. Such a chuck is advantageous for holding wafers to be lithographically patterned in a high-resolution way by electron-beam, ion-beam and X-ray-beam techniques.

Description: Background of the Invention This invention relates to techniques for holding workpieces and, more particularly, to a method for holding a semiconductor wafer during lithographic pattern exposure in an integrated-circuit fabrication sequence. Chucks are well known in the semiconductor art. They constitute versatile and reliable instrumentalities for holding semiconductor wafers during pattern delineation in photolithographic processes. By means of such chucks, wafers that inevitably exhibit some warpage can be firmly held during exposure in a manner to ensure that the wafer surface to be irradiated is maintained essentially flat. Such flatness is generally regarded as a prerequisite for achieving a high-resolution pattern. In lithographic systems utilizing electron beams or ions beams, and even in some systems employing X-rays, the wafers to be irradiated during pattern delineation are contained in a vacuum environment. Of course, in such systems vacuum chucks unfortunately cannot be used to hold the wafers. Mechanical clamping of the edges of a wafer is generally not sufficient by itself to ensure the surface flatness required for high-resolution patterning in a vacuum environment. For example, see GB-A-2 154 793, wherein a wafer is clamped by its rim to a circular domed plate so as to distort the wafer into close proximity with the domed face of the plate Ports are provided in the face of the plate for introducing a mobile gas under controlled pressure to the interstitial space between the wafer and the domed face of the plate. An annular groove is provided around the periphery of the domed face and this groove is connected to a vacuum pumping means thereby to minimize leakage of the mobile gas into the vacuum environment enclosing the plate. And although electrostatic and magnetic chucks have been suggested for use in a vacuum, such chucks have generally not been viewed as attractive instrumentalities for use in manufacturing integrated-circuit devices. Thus, a need exists for a better semiconductor wafer holder for use in a vacuum environment. According to the present invention there is provided a method as defined in claim 1 or apparatus as defined in claim 7. A chuck , for holding a semiconductor wafer comprises a curved reference surface in the form of a segment of the surface of a cylinder. By clamping the edges of the wafer and bending it to firmly contact its backside against the curved surface (or against pins mounted on the curved surface), the frontside of the wafer is thereby maintained in a precise equidistant relationship with respect to the reference surface. FIG. 1 shows a wafer held in place on an illustrative chuck made in accordance with the present invention; FIG. 2 is a top view of FIG. 1 with a portion of the wafer broken away to show the structure of the underlying chuck; and FIG. 3 is a schematic representation of the FIG. 1 arrangement mounted in the vacuum chamber of a lithographic system. Detailed Description The workpiece bolder shown in FIG. 1 is designed to bold a wafer-shaped workpiece in a vacuum environment. By way of example, the workpiece mounted on the depicted holder is a conventional semiconductor wafer of the type utilized to fabricate integrated-circuit devices. The wafer holder or chuck 10 includes a base portion 12 and clamps 14 and 16. Advantageously, the chuck 10 also includes a multiplicity of pins 18 on the base portion 12. Illustratively, the base portion 12 of FIG. 1 is a solid block made of a conductive material such as nonmagnetic stainless steel. The top of the portion 12 constitutes a curved surface 13. (For emphasis, the curvature of this surface is greatly exaggerated in FIG. 1). Advantageously, the depicted curved surface 13 comprises a portion of the surface of a cylinder. The particular chuck shown is designed to bold a wafer 20 approximately 15 centimeters (cm) in diameter. For this case, the dimensions a and b of the base portion 12 of the chuck are approximately 17 cm and 1 cm, respectively. Additionally, the base portion 12 extends about 17 cm in the Y direction indicated in FIG. 1. Further, the circular arc of the surface 13 shown in FIG. 1 corresponds to a portion of a circle having a radius of approximately 10 meters. In FIG. 1, distance c represents the maximum deviation of the surface 13 of the base portion 12 from a perfectly flat or X-Y planar surface indicated by dash line 22. In this example , the distance c is about 250 micrometers (µm). The distance c is selected to exceed the maximum deviation-from-flatness or warpage inherently exhibited by wafers of the type to be held by the depicted chuck. For a semiconductor wafer that exhibits a maximum warpage of about 25 µm, a deviation c of about 250 µm is adequate in practice to ensure that forces due to bending the wafer will be sufficient to hold the wafer in place on the depicted chuck. The top surface of the wafer 20 shown in FIG. 1 is intended to be maintained substantially exactly equidistant from the top or reference curved surface 13 of the base portion 12 of the chuck. But, due to the inevitable presence of dirt particles, this desired equidistant relationship is not generally obtainable simply by bringing the bottom of the wafer 20 into direct contact with the top surface 13. An attractive technique for minimizing the problem caused by dirt particles interposed between a wafer and a chuck is described in U.S. Patent No. 4,213,698. As described therein, a multiplicity of regularly spaced-apart pins are mounted on a chuck. The top surfaces of the pins are designed to contact the bottom surface of the wafer. Since each pin surface is relatively small, it is highly unlikely that a dirt particle will reside on it. If a particle does so reside, it would normally be brushed off by any slight lateral movement of the wafer during the course of positioning the wafer on the chuck. Further, the small contact areas of the pins would tend to squeeze out any dirt particles that remain after the wafer is moved into place on the chuck. For these reasons, the chuck of FIG. 1 includes pins 18, which, for example, can comprise multiple substantially identical cylindrical columns securely mounted in the base portion 12. Or the pins can be formed as an integral part of the portion 12 in, for example, a conventional machining operation. Each of the pins 18 includes a top surface designed to contact a portion of the bottom surface of the wafer 20. (In practice, these pin surfaces can be flat. Alternatively, the top surface of each pin can be curved to correspond substantially exactly with the respective curved wafer portion to be contacted.) Advantageously, the main longitudinal axis of each of the pins 18 is perpendicular to the main Y-direction longitudinal axis of the cylindrical segment that constitutes the top part of the base portion 12. By way of example, each of the substantially identical pins 18 shown in FIG. 1 has a height h of approximately 250-to-500 µm and a diameter d of about 1000-to-2000 µm. The pins are uniformly distributed on the curved surface 13 of the base portion 12. Illustratively, the center-to-center spacing s of the pins 18 is about 0.64 cm. The pins can be made of a variety of materials such as stainless steel or sapphire. Thus, for example, the pins can be made of stainless steel. The clamps 14 and 16 of FIG. 1 constitute standard wafer clamps. They are mounted on the top surface of the base portion 12 and serve to retain the bottom surface of a bent wafer in firm contact with the top surfaces of the pins 18. Illustratively, the clamps 14 and 16 of FIG. 1 are made of a suitable conductive material for applying a voltage to the wafer. This can be useful, for example, in electron-beam lithography. As indicated in FIG. 2, the clamps 14 and 16 engage substantial peripheral extents of the wafer 20. For a wafer having a diameter of approximately 15 cm, each of the depicted clamps has a dimension e of for example, about 5 cm. The extent f over which the clamps engage opposite portions of the surface of the wafer 20 is approximately 0.32 cm. In some cases, it is advantageous that the clamps 14 and 16 be retractable. Thus, for example, the clamp 14 shown in FIG. 2 can be designed to be retractable to the left and the clamp 16 can be designed to be retractable to the right. Such retractable clamps, which are well known in the art, can facilitate the placement and/or removal of the water 20 on the depicted chuck. In turn, such placement and removal can be done manually or in a straightforward way by conventional water handling equipment. A part of the wafer 20 shown in FIG. 2 is broken away to reveal the top surfaces of some of the underlying pins 18. In this embodiment, a total of approximately 400 pins are used. FIG. 3 schematically depicts a wafer chuck assembly as shown in FIGS. 1 and 2 mounted in the vacuum chamber 30 of a lithographic system 32. The system 32 can be a conventional electron-beam or ion-beam system including a standard beam column (not shown) and control system 33. In FIG. 3, dashed line 34 represents a charged-particle beam of electrons or ions. At the surface of the water 20, this beam has a diameter of, for example, approximately 0.25 µm. The base portion 12 of the chuck is mounted on a conventional X-Y-Z movable table 36. In turn, the movement of the table 36 is controlled by a standard X-Y-Z micropositioner 38 that is mechanically coupled by linkage 40 to the table 36. The top surface of the water 20 shown in FIG. 3 typically includes thereon a layer of an electron-beam-sensitive or ion-beam-sensitive resist. Selective irradiation of such a layer constitutes the initial step of a standard lithographic process utilized to fabricate integrated-circuit devices. In known ways, the beam 34 directed at the curved surface of the water 20 is focused at a reference point on the surface of the wafer, and the focused beam is scanned along an element that extends in the Y-direction across the entire diameter of the water surface. Of course, even an extremely narrow element of the curved wafer surface of FIG. 3 is actually slightly non-planar with respect to the X-Y plane. But. slight deviations from planarity over limited X-direction extents of the wafer surface are tolerable even for high-resolution lithographic patterning. After irradiation of an entire Y-direction extending element has been completed, the table 36 is moved to reposition the wafer under the beam 34. Illustratively, the table 36 is moved in the X direction by an amount sufficient to locate the next Y-direction extending element to be irradiated in butting relationship with the previously irradiated element. Before commencing irradiation of the next element, however, the beam is refocused to compensate for the fact that the center of the next element is at a different Z-direction elevation than the previously irradiated element. The refocusing of the beam 34 can be carried out in a variety of ways. For example, since the successive changes in Z-direction elevation that occur as the table 36 is stepped in the X-direction are known in advance, successive previously programmed changes in focus can be periodically made in synchronization with the X-direction stepping of the table. Alternatively, the refocusing of the beam 34 of FIG. 3 can be carried out under control of a standard focus tracking system 44. For each X-direction position of the table 36, the system 44 directs a light beam 42 at a glancing angle at a portion of the wafer surface element to be next irradiated by the beam 34. Light reflected from the wafer surface back to the system 44 provides a basis for determining the Z-direction elevation of the wafer surface. Various modifications are possible. For example, other ways of moving the wafer are feasible. Thus, the wafer surface may be rotated in a step-wise fashion along the curved arc defined by that surface. In this case, each Y-direction extending wafer element to be irradiated is successively positioned at the same distance from the beam source. One advantage of this alternative approach is that successive focus adjustments are not needed. A single initial focus setting suffices for exposure of the entire curved surface of the wafer 20. The curved surface of the chuck may be concave rather than convex. For a concave chuck surface, a wafer would be pulled down and held by clamps positioned on opposite sides of the wafer at or near the bottom of the concave surface.

Claims: A method of processing a semiconductor wafer (20), said method comprising clamping the wafer (20) to a curved upper surface (13) of a chuck (10), and selectively irradiating the top surface of the wafer (20), CHARACTERISED IN THAT the curved upper surface (13) of the chuck (10) comprises a segment of the surface of a cylinder, and in that the chuck (10) includes a plurality of substantially equal height pins (18) extending from the upper surface (13) thereof, whereby the wafer (20) is mounted to ensure that its bottom surface is in contact with the top surfaces of said pins by clamps (14,16) such that the top surface of the wafer (20) is bent into conformity with the curved surface (13), and in that the clamped wafer is placed within a vacuum environment within a vacuum chamber (30). A method as in claim 1, CHARACTERISED IN THAT the top surface of the clamped wafer (20) is irradiated with a charged-particle beam (34) emanating from a source in said vacuum environment. A method as in claim 2, CHARACTERISED IN THAT irradiation of the top surface of said mounted wafer (20) is carried out by stepping the clamped wafer (20) relative to said beam (34). A method as in claim 3, CHARACTERISED IN THAT the beam (34) is focussed with respect to the wafer surface at each step. A method as in claim 4, CHARACTERISED IN THAT focusing is accomplished by varying the distance between the beam source and the surface of said wafer (20). A method as in claim 4, CHARACTERISED IN THAT focusing of the beam (34) at each step is determined by a focus tracking system (44). Apparatus for processing a semiconductor wafer (20), said apparatus comprising a chuck (10) with a curved upper surface (13), clamping means (14,16) for clamping the wafer in engagement with the said upper surface (13), and means (32) for selectively irradiating the top surface of the wafer (20), CHARACTERISED IN THAT the curved upper surface is in the form of a cylindrical segment, in that a plurality of substantially equal height pins (18) extend from the curved upper surface (13) of the chuck, and in that opposing portions of the wafer (20) are clamped by the clamping means (14,16), such that the top surface of the wafer (20) is bent into conformity with the curved surface (13) and in that a vacuum chamber (30) serves to provide a vacuum environment for the clamped wafer. Apparatus as in claim 7 CHARACTERISED IN THAT means (38,40) are provided for stepping the wafer (20) relative to a beam of radiation from the selective irradiation means (32). Apparatus as in claim 8, CHARACTERISED IN THAT means (42,44) are provided for focusing the beam (34) with respect to the wafer surface at each step.

Inventors: FELDMAN MARTIN, FELDMAN, MARTIN
Applicants: AMERICAN TELEPHONE & TELEGRAPH, AMERICAN TELEPHONE AND TELEGRAPH COMPANY
Application year: 1987
Main classification: H01L21
IPC codes: G03F7, H01L21