WORKPIECE HEIGHT ADJUSTMENT IN AN ELECTRON BEAM LITHOGRAPHY
The present invention relates to an electron beam lithography machine and has particular reference to workpiece height adjustment in such a machine.
Electron beam lithography machines are 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, which 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 X and Y axial directions. Conventionally, the stage displacement is carried out to position the writing spot successively in different regions of the workpiece corresponding with individual zones or fields of the pattern and the beam deflection is carried out to cause the writing spot to trace pattern features in successive subfields of each 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 positions and by precise software control of electromagnetic beam deflecting coils. The machine as a whole is highly sensitive to changes in critical dimensions and incorporates suitable measures to counteract or minimise the effect of such changes, for example use in heat-sensitive areas of materials with low coefficients of thermal expansion, corrective inputs into software controls of beam deflection, isolation of the stage and workpiece in a vacuum environment, installation of the machine in clean room conditions, suppression of vibrations, etc. These measures contribute to ensuring high levels of resolution and accuracy of written patterns, which are vital to avoid, for example, misalignment of pattern lines abutting at subfield and main field boundaries.
One area of potential detriment to attainment of the desired level of writing accuracy is that of vertical offset of the beam final focal plane, i.e. the plane containing the writing spot, relative to the plane of the surface of the workpiece on which writing is to be carried out. This area has received little attention in the design of existing machines other than by providing for changes in focal length, which can undesirably influence writing spot diameter, or alteration of the mounting location of a workpiece on the stage, in particular by way of a workpiece holder allowing height adjustment to achieve consistent positioning
of each workpiece writing surface. The offset in planes can result from, in particular, different thicknesses of different workpieces and variation in the topography of an individual workpiece. It can be difficult to adjust machines, except by way of influence on the beam itself or complicated action on the workpiece mounting location, to compensate for regularly occurring offsets of that kind.
It is therefore the principal object of the invention to provide an electron beam lithography machine in which compensation for changes in workpiece thickness can be carried out without action on the beam itself.
A subsidiary object is to enable enhanced access to the stage for maintenance and other purposes.
Other objects and advantages of the invention will be apparent from the following description.
According to the present invention there is provided an electron beam lithography machine comprising a column for generating an electron beam, a stage for carrying a workpiece to be acted on by the beam, vertical displacing means for displacing the stage in substantially vertical direction towards and away from the column and detecting means for detecting the relationship of a reference plane associated with the column and a reference plane associated with the stage and causing the vertical displacing means to displace the stage in dependence on the detected relationship of the planes.
Through provision of a facility for vertical displacement of the stage, thus adjustment in the Z axis direction, in dependence on a detected relationship of reference planes of the column and stage it is possible to take account of changes in workpiece thickness which manifest themselves as changes in the reference plane relationship. Adjustment can be undertaken in simple manner prior to writing action on each workpiece of a series in which tolerances occur within the series, or on different series of workpieces with different nominal thicknesses. Adjustment may also be possible to correct for variable thickness, i.e. change in topography, of an individual workpiece. Such adjustments make a significant contribution to maintaining writing accuracy and consistent pattern reproducibility in serial pattern production. It is now possible to, in effect, provide calibration of the scale of the beam writing area as a function of target plane position
independently of scale change due to magnetic vibrations. Moreover, the capability of stage height adjustment eliminates the need for a complicated workpiece holder with variable location in terms of height; as a consequence, the stage mass may be able to be reduced, to the benefit of speed of stage translational movement in the horizontal plane when such a translational facility is present. Increased speed of movement equates with enhanced throughput in serial pattern writing.
Preferably, the reference plane associated with the stage is a plane containing at least part of the top surface of the workpiece when carried by the stage. This reference plane is thus a plane with critical influence on writing accuracy. Whilst the detecting means can be disposed to detect the top surface of the workpiece directly, it may be convenient if the reference plane associated with the stage is a notional plane disposed at a given spacing from a surface of the stage, thus, for example, a plane containing the top surface of the workpiece and spaced from the plane of that stage by a known thickness of the workpiece. In that case the detecting means can be conveniently arranged to detect the height position of the stage surface so that the relationship of the reference planes can then be calculated from the detected position and the given spacing.
Similarly, the reference plane associated with the column can be a predetermined plane containing a beam spot of the electron beam when generated by the machine. This predetermined plane, which also has a critical influence on writing accuracy, can be the focal plane of a beam focussing lens in a beam exit zone of the column, so that the exact position of that plane can be derived from, for example, column operating data in conjunction with constructionally fixed reference points of the column.
For preference, the detecting means is operable to cause the vertical displacing means to displace the stage to bring the reference planes into a predetermined relationship. This predetermined relationship can be, in the simplest case, coincidence of the reference planes, particularly when the reference planes are those containing the beam writing spot and the writing surface of the workpiece.
The vertical displacement facility can be utilised for static adjustment and, if desirable, dynamic adjustment. In the former case, the detecting means can be operable to cause the vertical displacing means to displace the stage for calibration of the machine prior to action of the beam on the workpiece. Calibration of this kind can be undertaken before
action on different series of workpieces or before action on each workpiece of a series. In the latter case, i.e. dynamic adjustment, the detecting means is preferably operable to cause the vertical displacing means to displace the stage so as to provide compensation for changes in the relationship of the reference planes during action of the beam on the workpiece. This, in effect, provides compensation for variation in workpiece topography, such as irregularity - in micrometre or nanometre terms - in workpiece surface planarity.
Moreover, the vertical displacing means can be additionally operable to displace the stage independently of the influence of the detecting means. This can be of advantage for a variety of reasons, but particularly for displacement of the stage to a predetermined position spaced from the column so as to enhance access to the column and stage for maintenance purposes. Maintenance of the stage, for example lubrication, cleaning and replacement of worn parts, can then be conveniently performed without obstruction by normally adjacent column components.
In addition, the vertical displacing means is preferably operable to allow rotation of the stage about at least one of a vertical axis and a horizontal axis. Stage rotation about a vertical axis may enable correction for writing area rotation about that axis. Rotation of writing area about a horizontal axis may arise due to non-parallelism of workpiece surfaces or a similar effect may occur at the limits of beam deflection due to the arcuate motion of the beam writing spot. Corrective rotation of the stage by the vertical displacing means, for example about at least one horizontal axis, may be able to produced by application to the stage of an eccentric displacing or adjusting force.
In a preferred embodiment compatible with conventional aspects of electron beam construction the stage is arranged in a vacuum chamber and is additionally displaceable in mutually orthogonal horizontal directions by horizontal displacing means, the horizontal displacing means being disposed in a vacuum chamber and the vertical displacing means being disposed outside the vacuum chamber and coupled to the stage by transmission means passing between the exterior and interior of an enclosure bounding of the vacuum chamber. The vacuum chamber, which is a standard feature of electron beam lithography machines, thus does not have to be enlarged or substantially modified to accommodate the vertical displacing means. Maintenance of the vacuum chamber volume means that there is no increase in evacuation and relief times with consequent penalties on machine cycle times and thus writing throughput. The stage in such a case is typically of multi-part
construction, with a stage member movable in X axis direction and another in Y axis direction; this stage format does not have to be changed to accommodate the Z axis facility. The transmission means can simply include sealing means against loss of vacuum from the vacuum chamber, for example bellows able to accept movement of the transmission means.
The transmission means can comprise at least one post extending through a wall of the enclosure to transmit movement from the vertical displacing means to the stage, in which case the or each post can be connected at one end with the vertical displacing means and at the other end with a table carrying the stage. If, for example, three such posts arranged at mutual spacings are provided, a convenient means of applying an eccentric adjustment force to the stage is created.
The vertical displacing means can be realised in various ways, for example in the form of a vertically movable top member, a base member disposed in a fixed position at a spacing below the top member, a pair of mutually complementary interengaging wedges respectively associated with the top member and the base member and disposed therebetween, drive means for relatively displacing the wedges in horizontal direction thereby to vary the spacing of the members in vertical direction and constraining means to prevent relative displacement of the members in horizontal direction. Expediently, one of the wedges is movable relative to the respectively associated member and the other wedge is secured against movement relative to the respectively associated member, the movable wedge preferably being that associated with the base member. The drive means can comprise a spindle rotatable, preferably by way of a motor, to move the movable wedge. A motorised spindle drive of that kind represents a particularly simple drive arrangement with a capability of fine adjustment depending on spindle pitch and the ease of relative motion of the co-operating wedges. For preference a further such pair of wedges is present, the wedge pairs being arranged at a spacing from one another, with the movable wedge of the first-mentioned pair preferably mechanically coupled with that of the further pair to transmit thereto the displacing force of the drive means. The movable wedges can be slidingly mounted on the base member by way of guide rails so as to ensure consistent linear motion of those wedges. The constraining means can be embodied in various ways, for example interengaging guide elements respectively connected with the top member and the base member.
A preferred embodiment of the present invention will now be more particularly described by way of example with reference to the accompanying drawing, the single figure of which is a schematic elevation of the part of an electron beam lithography machine embodying the invention, inclusive of control system.
Referring now to the drawing there is shown part of an electron beam lithography machine 10 intended for, in particular, writing integrated circuit patterns on suitable substrates. Conventionally, such patterns are fractured into main fields each containing a respective area of the pattern and each main field is in turn fractured into subfields containing pattern features of that area. Writing is normally carried out by deflecting an electron beam, which is generated in the machine, to trace the subfield pattern features on the substrate by a focussed beam spot, i.e. writing spot, and periodic movement of the substrate to locate different regions thereof, which correspond with successive main fields, in a zone of writing action of the beam spot, in particular a zone able to be scanned by the beam deflection. Pattern writing procedures are well-known and for that reason are not discussed in further detail.
The machine 10 comprises an electron beam column 11 in which an electron beam 12 of appropriate kiloelectron voltage is generated at an upper end of the column to propagate along an axis 13 of the column and emerge at the lower end. The beam in the course of its path along the column passes through conventional focussing, blanking and deflection stages, of which there is illustrated merely a final focussing lens 14, for example controllable electromagnetic coils influencing the beam by electromagnetism. This lens acts on the beam in a plane 14a to provide a focussed beam spot at a focal or writing plane A. The beam spot is displaced in this plane to trace the pattern features on a substrate 15. The plane A constitutes, in the context of the present invention, a reference plane associated with the column. The height position of the plane A along the axis 13 in relation to the end of the column 11 or any other fixed reference point of the machine is determinable from inter alia the control parameters of the lens 14.
The substrate 15 is held by a holder 16 mounted on a stage 17, which in turn is carried by a stage table 18. The stage 17 consists of a lower stage member 17a mounted on the table 18 to be displaceable in a Y axis direction as indicated by a double arrow, and an upper stage member 17b mounted on the lower stage member 17a to be displaceable in an X axis direction. Displacement of the stage members in the mutually orthogonal X and
Y axial directions is achieved by respective motor drives under the control of a laser interferometry measuring system consisting of a laser emitter co-operating with reflectors located at the upper stage member. As already indicated, the thereby achieved displacement of the substrate 15 in these axial directions serves to position regions of the substrate corresponding with the individual pattern main fields in the zone of action of the beam writing spot. The stage and supporting table, together with the substrate and substrate holder, are located in a vacuum chamber bounded by an enclosure 19, the enclosure being evacuated during writing so as to eliminate air currents, ambient temperature change and other influences detrimental to the stability of the sensitive writing system, as well as to provide the environment for generation of the electron beam.
The top or writing surface of the substrate is located in a plane B, which constitutes a reference plane associated with the stage. Ideally, the reference planes A and B are coincident; in the drawing they are shown at an exaggerated spacing to assist understanding of the invention.
In order to achieve a desired predetermined relationship of the planes A and B, in this instance coincidence of the planes, the machine 10 is equipped with vertical displacing means 20 coupled with the stage table 18 by way of transmission means 21. The displacing means 20 is composed of a vertically movable top member 22, a base member 23 disposed at spacing below the top member and fixed on a bed (not shown) of the machine and two pairs of mutually complementary wedges 24, 25 interengaging by way of linear bearing tracks 26, of which upper wedges 24 of the pairs are fixed to the top member 22 and lower wedges 25 of the pairs are mounted on the base member 23 by way of linear bearing tracks 27 to be capable of reciprocating horizontal displacement, as indicated by the associated double arrows, relative to the base member 23 and to the upper wedges 24. The upper wedges 24 and top member 22 are constrained against displacement in company with the wedges 25 by interengaging guide elements 28 and 29 respectively associated with the top member 22 and base member 23. The horizontal displacement of the lower wedges 25 is thus translated by the co-operating inclined surfaces of the upper and lower wedges exclusively into vertical displacement of the upper member 22 together with the transmission means 21 and thus the stage table 18, stage 17, holder 16 and substrate 15. The linear bearing tracks 26 and 27 preferably comprise rails incorporating low-friction bearing balls or rollers and the guide elements 28 and 29 preferably similarly interengage by way of Tollable bearings, either incorporated in the
elements or effectively forming the elements themselves.
The displacing means 20 further comprises, for displacing the movable wedges 25, a spindle 30 threadedly engaged in a co-operating female thread in an adjacent one of the lower wedges 25. The spindle is rotatable in a selectable one of two opposite rotational directions, as indicated by the associated double arrow, by way of a controllable electric motor 31. The displacing force is transmitted to the other lower wedge 25 by way of a coupler 32 rigidly intercoupling the two lower wedges.
The transmission means 21 is formed by three spaced-apart posts 33 engaging the stage table 18 at correspondingly spaced-apart points ensuring stable support of the table and the load it bears. The posts 33 pass through the lowermost wall of the enclosure 19 and loss of vacuum from the vacuum chamber at the penetration points of the posts is prevented by bellows 34, which provide seals at those points and expand and contract in conjunction with the vertical movement of the posts 33.
In terms of control of the displacing means 20 the position of the reference plane B is detected by a detector 35, which supplies an output signal indicative of the detected position to an actuator 36 for the motor 31. The detector could equally well detect a plane of the stage 17 or holder 16 having a predetermined relationship, i.e. spacing from, the plane B, the plane B being a variable dependent on the substrate thickness and this thickness being determinable by measurement before writing action by the beam. The actuator 36 also receives, as a further input, an output signal from a control system 37 of the machine, in particular an output signal indicative of the position of the plane A. This position can be detected by measurement or calculated from the setting, i.e. focussing, parameters of the lens 14. The actuator compares the signal values denoting the positions of the planes A and B and, depending on the instantaneous relationship of those planes and the desired predetermined relationship, controls the motor 31 to rotate the spindle 30 by the appropriate amount and in the appropriate rotational direction to produce a relative displacement of the wedges 24 and 25 to raise or lower the stage 17 and thus the workpiece 15.
The described adjustment to bring the reference planes A and B into a predetermined relationship, more specifically the beam writing plane and the substrate top surface plane into coincidence, can be carried out statically to calibrate the machine 10 in advance of
writing on a substrate or series of substrates possibly of different thicknesses. Dynamic adjustment may also be possible if a particular substrate exhibits a non-planar writing surface and the detector 35 and downstream motor control are responsive to detected planarity changes with sufficient speed. If the vertical displacing means 20 has an appropriate stroke the stage 17 can also be lowered to a position facilitating maintenance operations, such as on the sensitive X and Y guides for the stage members 17a and 17b; the accompanying schematic illustration does not particularly reflect the confined conditions of the stage location. The vertical displacing means may also be capable of rotation as a whole about a vertical axis to provide correction for slight misalignments of X and Y axes of substrate and machine co-ordinate system. In the case of a different form of drive perhaps permitting individual stroke movements of the posts 33, the stage table 18 and stage 17 may be able to be rotated, i.e. tilted about a horizontal axis, to provide a corresponding tilt of the writing surface of the substrate.
The electron beam lithography machine described in the foregoing with a Z axis adjustment capability based on comparison of critical reference planes provides enhanced scope for maintaining writing accuracy in the case of variable workpiece thicknesses and may allow use of a simplified substrate holder with the consequence of a reduced stage mass and thus more responsive stage movement.