Patent Publication Number: US-2009231594-A1

Title: Component mounting in movement-sensitive equipment

Description:
The present invention relates to movement-sensitive equipment and has particular reference to component mounting, especially mounting of a measuring system component, in the equipment. 
     Movement-sensitive equipment takes many forms, such as measuring apparatus of diverse kinds, machine tools for a wide range of purposes and equipment for processing and treating materials and articles in various ways. An example of equipment of that kind is an electron beam lithography machine, which is employed inter alia to write finally detailed patterns, such as integrated circuits, on suitable workpieces by the action of a focussed electron beam defining a beam writing spot. The writing spot traces pattern features through controlled deflection of the beam and periodic horizontal displacement of the workpiece. The workpiece, for example a semiconductor substrate or more commonly a mask as an intermediate element in generation of the pattern on such a substrate, is carried by a stage movable in at least one axial direction, normally in two orthogonal (X and Y) axial directions. Conventionally, the stage displacement is carried out to position the beam writing spot successively in different regions of the workpiece corresponding with individual zones or main fields of the pattern and the beam deflection is carried out to cause the writing spot to trace pattern features of successive subfields of each main field. The stage displacement and beam deflection are subject to close tolerances—currently in the nanometre range—determined by laser interferometry measuring systems for detecting stage horizontal position and by precise software control of electromagnetic beam deflecting coils. The machine as a whole is highly sensitive to changes in critical dimensions and to disturbances such as vibration and incorporates suitable measures to counteract or minimise the effect of such changes and disturbances. 
     Notwithstanding the various corrective measures undertaken in existing electron beam lithography machines, errors in beam orientation and thus writing spot placement can arise due to unintended change in location of the column position. The column is a solid, rigidly mounted structure, but has a substantial mass and is inevitably effected by, in particular, expansion and contraction of body components as a consequence of temperature change. As a result, the column as a whole or a major part of the column may be liable to displacement which effectively shifts all or part of the beam axis relative to a given reference point, in particular a notional point on a substrate assumed to have a fixed relationship with the beam axis in a horizontal sense. If the relationship is not fixed, but subject to periodic variation even if only in the nanometre range, writing precision can be adversely affected when highly accurate patterns or other such subject matter are involved. Similar difficulties can arise in connection with other types of movement-sensitive equipment, particularly when the accuracy of the equipment largely depends on reference axes which are assumed to be—but in reality are not—stable and unvarying in position. 
     It is therefore an object of the present invention to provide, in movement-sensitive equipment, measures for detection of a specific undesired shift in equipment position due to thermal or other internal or external influences, but without imposing a permanent restriction on access to the equipment for maintenance. 
     A further object of the invention in the case of such equipment is to provide scope for adjustment of a measuring system employed for such a detection purpose so as to enhance detection accuracy and allow repeatable setting of a system main component after disturbance of its position. 
     According to the present invention there is provided movement-sensitive equipment provided with an optical measuring system operable to measure the position of the equipment in a reference plane so as to detect unintended displacement of the equipment in that plane, the optical measuring system including an integral member of vitreous material provided with a plurality of reflective faces for reflecting measuring light beams and a plurality of mounts adjustably and releasably mounting the member on the equipment substantially in the reference plane. 
     The inclusion in the equipment of an optical measuring system utilising a member of vitreous material—thus a member with a significant degree of inherent stiffness and preferably with a substantially zero coefficient of expansion at room temperature—provided with reflective faces for optical measuring beams forms a basis for highly accurate measurement of unintended displacement, for example due to thermally induced expansion or contraction, of equipment or an equipment part intended to be immovable. However, such a member, preferably in the form of a plate, is not only fragile and thus susceptible to cracking, but also represents a potentially problematic intrusion into an equipment area which is sensitive with respect to availability of space and access for maintenance. These difficulties are largely avoided by use of mounts which mount the member to be adjustable, so that the reflective faces can be precisely aligned with the axes of the optical measuring beams, and also removable to allow access to the equipment for maintenance purposes. Maintenance in the case of the example of an electron beam lithography machine can be servicing, removal of or replacement of final lenses or other functional components which require periodic attention. In addition, the mounts can be designed to avoid application to the member of pressures liable to induce stresses which may cause cracks. 
     Preferably the mounts are adjustable in the sense of achieving orthogonality of the reflective faces relative to the reference plane. This allows accurate adjustment of the member not only at the time of initial setting up of the equipment, but also on each occasion the member is removed and refitted in connection with maintenance of the equipment. The achievable orthogonality can have a tolerance of, for example, substantially 5 microns, which is achievable by provision of suitably fine adjustment travels. In addition, the member is preferably mounted on the equipment to be rotationally adjustable in the reference plane in the sense of achieving perpendicularly of the reflective faces relative to axes of the light beams. Such an adjustment, which may in practice be very small, for example in the order of half a degree within a range such as ±one degree, permits setting of precise perpendicularity of the reflective faces to the light beams, particularly if necessary to compensate for shift in the beam axes due to, for example, thermally induced displacement of the beam sources. 
     For preference, three such mounts are provided at substantially equal spacings around a periphery of the member, three representing the optimum number for stable location of the member, ease of adjustment and minimum cost and complication. The combination of an intrinsically stiff member, such as a plate, of vitreous material with three equidistantly spaced mounts creates a rigid system able to resist mechanically sourced disturbances. 
     With advantage, each of the mounts fixes the member in position by clamping, preferably with a direction of clamping action oriented substantially exclusively perpendicularly to the reference plane. In the case of a member in the form of a plate, since the plate restraining forces act transversely through the plate between major faces thereof the creation of stress in the vitreous material of the plate is minimised and thus the risk of cracking of the plate. Even if stresses were to be established that did not actually cause cracks, thermal excursion could lead to stress redistribution and relative movement and thus ultimately represent a source of measuring error; the specified clamping action of the mounts largely eliminates that risk. In this connection each of the mounts can have substantially linear or punctiform contact with the member and preferably also with the equipment in the region of each mount. Such linear contact can be provided by a part-cylindrical surface and the punctiform contact by a part-spherical surface. An arrangement of this kind provides particularly precise location of the member and a capability of setting within the required fine tolerance range. 
     In order to minimise stress in the member at the locations of action of the mounts the member is preferably provided with a plurality of inserts each co-operable with a respectively associated one of the mounts and each having a greater resistance to fracture than the vitreous material. The higher-strength inserts thus accept the clamping or other fixing force exerted by the mounts and pass on the force to the equipment without the fragile vitreous material of the member being directly loaded. Each of the inserts can comprise a sleeve fixed in the member, for example by gluing, and a bearing element mounted in the sleeve to be adjustable substantially perpendicularly to the reference plane and arranged to bear against the associated mount. The adjustability of the bearing element in the sleeve is preferably achieved by threaded interengagement thereof so that the bearing element is adjustable in simple manner by rotation. The bearing element itself is preferably additionally arranged to bear against a respectively associated seat at the base of the equipment. An adjusted position of the bearing element in the sleeve can be fixed by a locknut or other simple locking means. With advantage, one of the seats is rotatable to impart to the associated bearing element a force tending to rotate the member of vitreous material in the reference plane, whereby the afore-mentioned rotation of the member for setting the perpendicularity of the reflective faces to the measuring light beam axes may be achieved in simple manner. 
     Avoidance of transmission of stress-inducing forces to the vitreous material member and provision of a capability for highly precise adjustment of the member can be achieved by design of each mount to allow freedom of movement about more than one axis. In a particularly effective embodiment each mount comprises two adjustable members each pivotable about a respective one of two substantially mutually orthogonal axes which are substantially parallel to the reference plane. The mounts thus function quasi in the manner of universal joints to accommodate tilt of the vitreous material member in all directions, in particular tilt arising from vertical adjustment of one or more of the bearing elements in the respective sleeve or sleeves. In that case, one of those two adjustable members of each mount can be arranged to be co-operable with the vitreous material member and to have an axis of pivotation extending substantially radially with respect to the centre thereof. The adjustable members with the substantially radial axes of pivotation are thus able to swivel without mutual conflict when movement is necessary to accommodate a change in orientation of the vitreous material member. The latitude necessary to accommodate such change is preferably achieved by allowing those adjustable members to be freely pivotable about their substantially radial axes of pivotation. 
     Preferably, the other one of the two adjustable members of each mount is attached to a fixing member of the mount to be pivotable about its respective axis of rotation. Such a fixing member may be fixed to the base of the equipment a short distance outboard of the member of vitreous material. 
     By contrast to the free mobility of the adjustable member pivotable about an axis substantially radial to the centre of the vitreous material member, the other adjustable member can be located in an adjusted position relative to the mount fixing member by position determining means determining that position, in particular a threaded adjuster operable to cause or allow pivotation of that other adjustable member about its axis of pivotation. The threaded adjuster can also be releasable to permit pivotation of the associated adjustable member to such an extent as to allow release of the vitreous material member, which may be of substantial importance from the viewpoint of access to the area above that member to enable maintenance or replacement of equipment components. In the case of an electron beam lithography machine, for example, electronic components disposed directly above the vitreous material member in the electron beam column of the machine will have to be periodically accessed for servicing and this will necessitate release and removal of the member, a procedure facilitated by construction of the mounts in the described preferred form with a threaded adjuster releasable to allow the adjustable members of the mounts to, for example, drop down away from the member. It is equally important that when the vitreous material member is remounted it can be returned to its original position, which, in a preferred construction of the mounts, can be conveniently achieved if each of the mounts incorporates an adjustable datum pin for recording the specific setting, which is determined by the threaded adjuster, of the adjustable member attached to the fixing member. Thus, for example, the datum pins can be set to register the position of adjustable components of the mounts, in particular the components releasable to allow removal of the vitreous material member, before their release so that the components can be more easily returned to their previous settings. This positional repeatability is of considerable importance for returning the equipment, specifically the reflective faces of its optical measuring system, to optimum orientations after maintenance. Laborious adjustment to recover these settings may thus be avoided or reduced to, at the most, fine final movement of the threaded adjusters to restore the exact settings. 
     In the optical measuring system the two reflective faces are preferably mutually orthogonal, thus able to be associated with X and Y axes. Such faces, when machined onto the member of vitreous material, can be optically flat to a tolerance of λ/10 so as to provide a basis for a highly accurate measuring system employing lasers to provide the measuring light beams. Such lasers can be incorporated in, for example, a laser interferometry measuring system. 
     The movement-sensitive equipment according to the invention can, as mentioned in the introduction, take various forms, one specific example of which is an electron beam lithography machine in which the reference plane will be at the base of an electron beam column of the machine and extend perpendicularly to the column axis. The invention is equally applicable to other machines such as those in which a stationary superstructure—which is nevertheless susceptible to undesired movement—with functioning components is located largely or entirely above a reference plane in which machining or another form of processing is carried out. 
    
    
     
       A preferred embodiment of the present invention will now be more particularly described by way of example with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic elevation of the base region of an electron beam column of an electron beam lithography machine embodying the invention; 
         FIG. 2  is a schematic inverted plan view of components of an optical measuring system associated with the underside of the column of the machine of  FIG. 1 ; and 
         FIG. 3  is a schematic sectional elevation of a mount of the system, the mount clamping a mirror plate to the underside of the column. 
     
    
    
     Referring now to the drawings there is shown in  FIG. 1 , as an example of movement-sensitive equipment to which the invention is applicable, part of an electron beam lithography machine which can be used for inter alia writing integrated circuit patterns or other finely detailed patterns on a substrate. For this purpose the machine  10  comprises an electron beam column  11  (only the lower end is shown in  FIG. 1 ) in which an electron beam  12  is generated to propagate along an axis  13  of the column to act on a substrate  14  mounted below the column on a movable stage  15 . The substrate and stage are located in a vacuum chamber, the upper terminating wall of which is schematically indicated at  16 . Writing is carried out by deflecting the electron beam  12  relative to the axis  13  to scan pattern features on the substrate  14  by a focussed beam spot and periodic movement of the stage  15  to locate successive regions, corresponding with parts of the pattern, of the substrate in the zone of writing action of the beam spot. The stage movement is usually in two mutually orthogonal horizontal directions, thus along X and Y axes of an X and Y co-ordinate system with the column axis  13  assumed to be invariably located at the origin. 
     Although the stage movement is undertaken on the basis of the axis  13  as an invariable datum it is entirely possible, as explained in the introduction, for an unintended, thermally induced displacement of the otherwise fixed column to shift this datum. This shift can lead to a degree of falsification of the co-ordinate system associated with the stage movement and consequently a displacement of the pattern position on the substrate  14  or, if the datum shift occurs during writing, a possible misalignment of fine pattern features or other reduction in writing accuracy. In order to detect any shift of this datum and thus obtain data for use in correcting machine operation, for example the beam deflection or the stage movement, an optical measuring system is provided for measuring the instantaneous column position in the zone of issue of the beam at the lower end of the column, in particular by detecting movement, in a reference plane A in this region, of the column axis relative to the assumed invariable position it occupies at the origin of the stage X and Y co-ordinate system. The reference plane A is selected to be approximately co-planar with a final lens aperture (not shown) in the lower end region of the column. 
     As shown in  FIGS. 1 and 2 , the optical measuring system comprises an integral plate  17  of vitreous material with a substantially zero coefficient of thermal expansion, for example the well-known Zerodur® glass-ceramic composite which has a coefficient of thermal expansion potentially as low as ±0.02×10 −6  K −1  at room temperature. The plate  17  is adjustably and releasably mounted on the underside of the column by three substantially equidistantly spaced mounts  18  arranged around the circumference of the plate, thus at intervals of approximately 120°. The reference plane A passes substantially centrally through the plate, i.e. intermediate its two major faces, in the mounted position of the plate. The vitreous material plate which is structurally highly stable, but inherently fragile by comparison with the metallic materials otherwise employed in the column construction, is formed on its circumference with two machined faces  19  lapped to an optical flatness tolerance of λ/10, thus 69.3 nanometres, and plated to provide the desired level of reflectivity. The two faces  19  are located in mutually orthogonal planes which are, for preference, respectively at right angles to the two X and Y axial directions of the stage displacement, whereby the precondition is created for basing the optical measuring system on the same co-ordinate system of a further optical measuring system (not shown) for measuring and controlling the stage movement. 
     The two reflective or mirror faces  19  of the plate  17  respectively co-operate with two laser interferometry measuring systems each comprising a laser light emitter  20  emitting, in the plane A, a laser beam  21  which is directed onto and reflected by a respective one of the faces and from which, through interference of the emitted and reflected light, a measurement signal value indicative of the instantaneous face position in the respective axial direction (X or Y) can be derived and, for example, applied to a control component of the machine. The instantaneous positions of the faces  19  conjointly define a datum of the position of the lower end of the column  11  and, through comparison with a predetermined datum, permit recognition of shift in the column position in the sense of offset of the column axis  13  from its presumed static position in the reference plane A. The recognised shift forms the basis of the afore-mentioned data for use in influencing machine operation. It is not, however, necessary to process the data relating to the instantaneous face positions specifically for the purpose of recognising a shift in the column axis; it is equally possible, for example, to control machine components simply by reference to the instantaneous position of the axis, thus a floating datum. 
     Although a measuring system employing an integral vitreous material plate with machined reflective faces offers significant advantages with regard to structural and optical stability, the mounting of the plate  17  represents particular difficulties with regard to adjustment of the system, access to components at the base of the column for servicing or replacement, and preservation of the comparatively brittle material of the plate by avoidance of crack-inducing stresses. These problems are addressed by design of the mounts to permit adjustment of the plate  17 , particularly tilt about its centre, removal of the plate to allow the desired access and retention of the plate in a manner avoiding local stresses. The mounts are additionally designed to enable quick and accurate recovery of the plate setting after the plate has been removed and refitted. 
     The mounts  18 , which as mentioned are equidistantly spaced around the plate circumference, accordingly each comprise a plurality of interconnected elements permitting relative movement about mutually orthogonal axes parallel to the reference plane A. These elements consist, as shown in  FIG. 3 , of a fixing block  22  secured to the underside of the column  11  by way of screws  23  (indicated schematically) passing through vertical bores in the block, a first adjustable member  24  pivotably connected with the fixing block  22  and a second adjustable member  25  pivotably connected with the first member. The first member  24  has two lateral recesses leaving a central web  26  and the fixing block  22  has two projecting lugs  27  received in the recesses and extending on either side of the web. The pivotal interconnection of the block  22  and member  24  is by way of an axle pin  28  passing through the web and the lugs. The axle pin defines one of the two mutually orthogonal pivot axes extending parallel to the reference plane A and, in this instance, also parallel to a tangent to the circumference of the plate  17 . 
     Determination of an adjustable and fixable setting of the first adjustable member  24  relative to the fixing block  22  is achieved by a screw adjuster in the form of a screw  29  extending with clearance through aligned bores in the block  22  and member  24  and threadedly engaged in a cross-bar  30  rotatably mounted in a transverse bore, which extends parallel to the axle pin  28 , in the member  24 . The member  24  is loaded, as evident from the following description, by the weight of the plate  17  which thus applies to the member  24  a moment tending to rotate the member about the axle pin  28  in clockwise sense in  FIG. 3  and thus in the sense of causing the head of the screw  29  to bear against the base of an enlarged end section of the bore in which the screw extends. Tightening of the screw  29  thus draws the cross-bar  30  towards the block  22  and thereby pivots, under rotation of the cross-bar  30  in its bore, the member  24  in anti-clockwise sense in  FIG. 3 . Conversely, unscrewing the screw allows the member  24  to pivot in clockwise sense by virtue of the moment exerted by the plate  17 . The threaded adjuster formed by the screw  29  and cross-bar  30  allows, with suitable thread pitch, fine adjustment of the angular setting of the first adjustable member  24 . If the screw  29  is entirely unscrewed from the cross-bar  30 , the member  25  is free to pivot in clockwise sense sufficiently to release the plate  17 , as subsequently described. An established setting of the member  24  relative to the block  22  can be recorded by a datum pin  31  which is screwed into a threaded bore in the block  22  and contacts, by an end section protruding from the block, an adjacent face of the member  24 . If the threaded adjuster  29 ,  30  has been operated to release the plate, the relative setting of the block  22  and member  24  can be recovered, at least to within a small tolerance, by screwing the screw  29  into the cross-bar  30  until the member  24  bears against the protruding end section of the datum pin  31 , which thus functions as a stop. The stop also prevents, as explained further below, over-adjustment of the member  24  and consequential possible risk of damage of the plate  17 . 
     The second adjustable member  25  consists of a body  32  from which an arm  33  extends in a direction away from the first member  24 . The pivotable connection of the second member  25  with the first member  24  is realised by an axle pin  34  threadedly engaged in a bore in the first member  24  and freely rotatably engaged in a coaxial bore in the body  32  of the second member  25 . The axle pin  34 , which has a central collar seated in a recess in the member  24  and providing an enlarged bearing area for stability of these two components, defines the other one of the two mutually orthogonal pivot axes extending parallel to the reference plane A and, in this instance, radially with respect to the centre of the plate  17 . The second member  25  is freely rotatable relative to the first member  24  so that adjustment of the position of the plate  17  by way of one of the threaded adjusters  29 , of the mounts  18  can be accommodated at the other mounts without conflict and generation of local stresses. 
     In order to mount the plate  17  each arm  33  of the second adjustable member  25  of each mount  18  has a groove receiving two axially spaced rollers  35  (only one of which is visible in  FIG. 3 ), by way of which a vertically directed clamping force, thus a force substantially perpendicular to the reference plane A, can be applied by the respective mount to clamp the plate against the underside of the machine. The clamping force exerted by each mount is accepted directly not by the fragile vitreous material of the plate  17 , but by a respective insert  36  secured in a bore extending through the plate. Each insert  36  comprises an internally threaded sleeve  37  fixed in the plate by gluing, the sleeve having external grooves receiving adhesive, and an externally threaded bearing element  38  screwed into the sleeve and lockable therein in a selected axial position by a locknut  39 . The bearing element  38  has at its lower end a flat head resting on the rollers  35  of the associated mount  18 , whereby lineal contact between the mount  18  and the insert  36  is achieved. At its upper end the bearing element  38  has a part-spherical tip bearing against a seating body  40  threadedly mounted in the column  11  at its underside. 
     In a preferred arrangement, the bearing element  38  of a first one of the three mounts  18  bears against the walls of a V-shaped recess in the associated seating body  40  as illustrated in  FIG. 3 , but the element  38  of a second one of the mounts bears against the wall of a conical recess in the associated body  40  and the element  38  of the third one of the mounts bears against a flat face of the associated body  40 , so to provide a low-friction kinematic mounting system in which the bodies  40  with the conical recess and flat face lie at a first radial spacing from the centre of the plate  17  and the body  20  with the V-shaped recess lies at a second, greater radial spacing from the centre. Rotation of the last-mentioned body  40  in its threaded mounting causes rotation of the plate  17  through, for example, a range of ±1° for fine adjustment of the perpendicularity of the faces  19  relative to the axes of the laser beams  21  in the sense evident from  FIG. 2 , the rotated body  40  then being fixed in its optimum setting. 
     The functioning of the arrangement for mounting the vitreous material plate  17  of the optical measuring system on the underside of the column  11  is largely self-evident from the foregoing explanation of the location and construction of the mounts  18  and the associated freedoms of movement of the components making up the mounts. Adjustment of the plate  17  so that, in particular, its reflective faces  19  are precisely normal to the reference plane A is achieved by way of the threaded adjusters  29 ,  30  of the mounts  18 , i.e. screwing of the screws  29  into or out of the cross-bars  30  thereby to appropriately pivot the first adjustable members  24 , and by way of the inserts  38  in the plate, i.e. screwing the bearing elements  38  up or down in the sleeves  37  thereby to raise or lower the associated edge regions of the plate. Orthogonality of the faces  19  with respect to the critical reference plane A may be achievable within a tolerance of approximately 5 microns. The free rotatability of the second adjustable members  25  of the mounts  18  relative to the first adjustable members  24 , in particular about axes extending substantially radially of the plate, allows freedom of movement to accommodate tilting motion of the plate; the mounts effectively function as universal joints. Pivotable movement of the member  24  relative to the fixing block  22  of each mount is similarly accommodated by the clearance between the screw  29  and the walls of the associated bores and by rotation of the cross-bar  30  in the member  24 . If the screws  29  are withdrawn entirely from the members  24 , the members  24  and  25  of all mounts can be dropped down to free the plate  17  for removal so as to allow access to inter alia the sensitive final lens present in the base of the electron beam column and associated diodes and other electronic components susceptible to occasional failure. The previous settings of the mounts  18  and thus the setting of the plate  17  can be restored with the help of the datum pins  31  in the manner already described. The supplementary function of these pins as stops prevents over-tightening of the screws  29  and possible risk of cracking of the plate material. 
     Apart from adjustment to ensure orthogonality of the faces with respect to the plane A adjustment to ensure perpendicularity relative to the axes of the beam  21  in the plan aspect of the plate  17  is achievable, as described, by turning the seating body  40  with the V-shaped recess to impart rotational movement to the plate. 
     The invention thus provides an effective means of measuring undesired displacement of movement-sensitive equipment, exemplified in the described embodiment by an electron beam lithography machine, in a reference plane, the measuring means comprising an optical system based on laser interferometry and having a high level of optical accuracy and stability achieved by use of mirrors formed on an integral member of vitreous material with a low coefficient of thermal expansion. The disadvantages potentially arising from use of a member of such material are entirely or largely overcome by the special mounts allowing adjustment and removal of the member and fixing of the member by clamping forces oriented to avoid mechanically sourced stresses in the plate material.