WO2008047079A2

WO2008047079A2 - Electron beam influencing in an electron beam lithography machine

Info

Publication number: WO2008047079A2
Application number: PCT/GB2007/003865
Authority: WO
Inventors: Paul George Harris; John Melbourne Tingay
Original assignee: Vistec Lithography Inc
Priority date: 2006-10-16
Filing date: 2007-10-11
Publication date: 2008-04-24
Also published as: WO2008047079A3; GB0620527D0; GB2443016B; GB2443016A

Abstract

An electron beam lithography machine (10) comprises an electron beam column (11) for generating and issuing an electron beam for action on a workpiece (13), particularly a workpiece held by a movable stage (15). The machine further comprises an optical measuring system (17) for measuring the instantaneous column position in the zone of issue of the beam, thus at the base of the column (11), to determine an instantaneous beam datum and influencing means (22) for influencing the machine operation in dependence on the determined datum. Such influencing can include correction of beam deflection when scanning the workpiece for pattern writing or correction or control of the stage movement, particularly when the stage movement is determined on the basis of position measurement by a further optical measuring system (23). The two systems can use the same X/Y co-ordinate system and share optical components.

Description

ELECTRON BEAM INFLUENCING IN AN ELECTRON BEAM LITHOGRAPHY MACHINE

The present invention relates to an electron beam lithography machine and has particular reference to measures for influencing the electron beam produced by 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 at least one axial direction, normally in two orthogonal (X and Y) axial directions. Conventionally, the stage displacement is carried out to position the stage 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 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 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 writing highly accurate patterns or other subject matter. It is therefore the principal object of the present invention to provide an electron beam lithography machine with enhanced writing accuracy as a consequence of taking into account a variable parameter disregarded in previous machines.

A subsidiary object of the invention is to provide an electron beam lithography machine able to correlate that parameter with a further variable parameter of machine operation to achieve a differential machine control. Yet another object in connection with such a machine is attainment of a compact machine construction employing an existing component location for an additional purpose and providing shared utilisation of energy sources.

Further 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 an electron beam column for generating and issuing an electron beam for action on a workpiece, an optical measuring system for measuring the instantaneous column horizontal position in the zone of issue of the beam to determine an instantaneous beam datum and influencing means for influencing the machine operation in dependence on the determined datum.

Such a machine thus includes a facility for detecting tilt or lateral displacement of the column in the critical region of beam issue from the column so that an instantaneous beam datum - effectively a floating datum - can be determined. This datum is then used by the influencing means to influence machine operation, particularly for influencing beam action on the workpiece. Other influences on the machine operation are also possible, for example periodic interruption of machine operation in the case of datum shift outside a given tolerance range, operation of compensating means to restore the datum to a predetermined ideal position, etc.

In a particularly advantageous embodiment involving direct influencing of the beam action on the workpiece the column comprises means for deflecting the beam to scan the workpiece and the influencing means is operable to influence the beam deflection. The influencing means can thus be represented by a control applying a continuous dynamic correction to the beam deflecting means, for example correction of the voltage applied to electromagnetic coils diverting the path of the beam.

The optical measuring system is preferably operable to measure the column position by detecting change in column position in two mutually orthogonal directions substantially perpendicular to the axis of the column. The two mutually orthogonal directions can conventionally be those of an X and Y axis co-ordinate system employed by the machine as the basis of beam scanning of the workpiece. Detection of change in column position by way of a co-ordinate system perpendicular to the column axis represents a particularly simple and accurate method of determining positional changes.

The achievable level of accuracy in measurement of the column position can be enhanced by use of an optical measuring system based on a laser interferometry system, for example a system in which laser light is oriented along the mentioned two mutually orthogonal measurement directions. The system can be operable to detect light reflected by reflective surfaces at the column, in which case the reflective surfaces can be provided on a member fixed to the column in the zone of issue of the beam, thus at the lower end of the column. Such a member can advantageously comprise an integral body of vitreous material preferably with a substantially zero coefficient of thermal expansion at room temperature, the reflective surfaces then being formed by faces formed on the body. This construction allows production of laser light reflective surfaces by precision machining so that the surfaces lie in planes with absolute and invariable orthogonality. The relative position of the surface planes is, for all practical purposes, unaffected by temperature change in view of the effectively zero CTE of the constituent material of the member. The member can itself, however, be adjustable in its position relative to the column so that the reflective surfaces can be precisely aligned with the paths of the measuring light beams of the optical system. The member can also be used for carrying a detector detecting backscattered electrons from the electron beam, in which case the member effectively replaces an aluminium plate usually mounted for this purpose, i.e. to carry such a detector, at the base of the column.

Apart from the optical measuring system for detecting column position the machine can comprise a further optical measuring system for measuring the position of a movable stage for carrying the workpiece. Stages of this kind are normally incorporated in electron beam lithography machines to periodically reposition the workpiece so that successive areas of the workpiece can be located in a zone of action of the beam writing spot, specifically a range of scanning deflection of the beam. For preference, the stage is movable in two mutually orthogonal directions substantially perpendicular to the axis of the column and the further optical system is operable to measure the stage position by detecting change in stage position in those directions. The system for measuring stage position can thus function in fundamentally the same manner as the system for measuring column position, which creates a precondition for shared utilisation of, inter alia, energy sources and position evaluating and control equipment. The further optical measuring system can like the first-mentioned system comprise a laser interferometry system and can also be operable to detect light reflected by reflective surfaces, in this instance surfaces at the stage. These surfaces can be provided on a member which is attached to or forms part of the stage and which, as in the case of the measuring system for the column position, preferably comprises an integral body of vitreous material preferably with a substantially zero coefficient of thermal expansion at room temperature, the reflective surfaces then being provided by faces formed on the body.

The two optical measuring systems are preferably operable to measure column position and stage position by reference to the same predetermined co-ordinate axes, which ensures commonality of the measuring bases of the systems and facilitates the aforementioned sharing of components. Accordingly, the optical measuring systems can have a shared light source, for example a single laser light emitter for both systems, and respective fibre optical conductors to conduct the light from the shared source. The systems thus represent a homodyne arrangement in which any source deviation occurs equally in the two systems and is neutralised in effect.

Influencing of the machine operation, especially control on a differential measurement basis, can be achieved if the influencing means is operable to correct the stage position measurement or control the stage movement by reference to the determined instantaneous datum. In effect, a correction referred to the floating datum is imposed on the stage movement or this movement is controlled by reference to the floating datum so that the movement is always correlated with the instantaneous position of the column at the zone of issue of the beam.

A preferred embodiment of the present invention will now be more particularly described by way of example with reference to the accompanying drawings, the single figure of which is a schematic elevation of part of an electron beam lithography machine in the region of the base of an electron beam column and a workpiece stage disposed thereunder.

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 is generated to propagate along the axis 12 of the column for action on a workpiece 13 located below the column. The beam, after passing through a final lens 14 of a series of lenses in the column, issues from the base of the column as a focussed beam. The workpiece 13 is carried by a stage 15 in the form of a holder 15a for releasably holding the workpiece, an upper stage member 15b carrying the holder and horizontally movable in one of two mutually orthogonal X and Y axial directions, for example the X axis of a coordinate system, and a lower stage member 15c carrying the upper stage member and horizontally movable in the respective other axial direction. The lower stage member 15c is mounted, for the movement thereof, on a stationary table 16. The upper and lower stage members 15b and 15c are each made of a metal matrix composite material combining strength with lightness and a low coefficient of thermal expansion and the table 16 of stainless steel.

The X and Y stage movement is, as already indicated, carried out for the purpose of moving individual regions of a held workpiece 13 into the zone of action of the writing spot produced by the focussed beam, each region corresponding with a respective main field of the fractured pattern to be written. This movement is conventionally carried out with direct and indirect reference to the column axis 12 as an invariable datum, although as explained in the introduction it is entirely possible for a thermally induced tilting or lateral displacement of the column to horizontally 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 pattern position on the workpiece 13 or, if the datum shift occurs during writing, a possible misalignment of fine pattern features or other such reduction in writing accuracy. In order to take a possible shift of this datum into consideration and ultimately provide appropriate compensation, a first optical measuring system 17 is provided for measuring the instantaneous column horizontal position in the zone of issue of the electron beam to determine an instantaneous beam datum, in particular a datum preferably lying on the column axis 12 - which is notionally coincident with the axis of the beam in undeflected state - at a point downstream of and closely adjacent to the final lens 14. The optical measuring system 17 comprises an integral plate 18 of vitreous material with a substantially zero coefficient of thermal expansion at room temperature, for example Zerodur (Registered Trade Mark) glass-ceramic composite with a CTE potentially as low as ± 0.02 x 10^"6 K^"1 at room temperature, adjustably mounted on the underside of the column. The plate 18 is provided with two machined faces 19 (only one shown in the figure) plated to provide the requisite level of reflectivity and respectively disposed in two mutually orthogonal planes which in the present embodiment are respectively at right angles to the two X and Y axial directions of the stage displacement. The two reflective faces 19 respectively co-operate with two laser interferometry measuring systems each consisting of a laser light emitter 20 emitting a laser beam which passes via a beam divider 21 (the purpose of which is explained further below) and is reflected by the associated reflective face 19 and from which, through interference of the emitted and reflected light, a measurement signal value indicative of the face position can be derived and applied to control means 22 of the machine. The principle of laser inferferometry measuring systems is well-known and therefore not discussed in further detail. The instantaneous positions of the faces 19 conjointly define a datum for the horizontal position of the lower end of the column and, by a comparison with a predetermined datum, permit recognition of shift in the column position and thus offset of the beam from a theoretical ideal. The positional data provided by the optical measuring system 17 can be used directly, in the form of a floating datum, or indirectly, through identification of the change in column position, to influence machine operation by way of the control means 22, for example correction of the beam deflection or beam zero setting by addition or subtraction of tolerances corresponding with the column displacement.

The intentional movement of the stage 15 is itself under the control of a second optical measuring system 23,. which is provided in part by construction of the holder 15a from a transparent plate of a vitreous material the same as or having fundamentally the same characteristics as that of the plate 18 of the first optical measuring system 17 or by attaching such a plate to the holder. The plate of the holder 15a has peripheral upstands respectively provided with machined reflective faces 24 corresponding in their axial disposition and relationship and in their function with the reflective faces 19. The second optical measuring system 23 is further formed by two laser interferometry measuring systems respectively co-operating with the reflective faces 24 and incorporating, in a particularly advantageous construction, the two laser light emitters 20. The beam emitted by each emitter is divided by the associated beam divider 21 and directed not only to the associated reflective face 19, but also to the associated reflective face 24. Thus, a single laser provides light for the two faces 19 and 24 for the X axis measurement and a further single laser provides light for the two faces for the Y axis measurement. The measurement of stage position by the second optical measuring system 23 is carried out analogously to the measurement of the column position by the first optical measuring system 17. The obtained measurement signals indicative of stage position are similarly applied to the control means 22 and can be directly employed for control of the stage movement for the afore-described purpose of repositioning the workpiece 13 to place successive regions thereof in the zone of action of the beam. It is, however, advantageously possible to link the positional measurements obtained for control of the stage movement with the measurement of the instantaneous column position, i.e. the determined instantaneous or floating beam datum, so as to superimpose a correction on the stage movement or provide constant control of the stage movement on a differential basis.

The position occupied at the base of the column 11 by the plate 18 of the first optical measuring system 17 is occupied in prior art machines by a mounting plate for a detector for backscattered or secondary electrons derived from the focussed beam, such electrons having a deleterious effect if not recognised and controlled or otherwise taken into account in the machine control. The plate 18 can be additionally employed as a mount for such a detector 25 and thus, by virtue of its dual function, does not significantly increase the installation space required at the base of the column.

The electron beam hereinbefore described provides enhanced writing precision through determination of an otherwise uncontrolled factor in beam placement accuracy and ensures containment of costs through partial sharing, of the more expensive elements of a measuring system otherwise normally present.

Claims

1. An electron beam lithography machine comprising an electron beam^' column for generating and issuing an electron beam for action on a workpiece, an optical measuring system for measuring the instantaneous column horizontal position in the zone of issue of the beam to determine an instantaneous beam datum and influencing means for influencing the machine operation in dependence on the determined datum.

2. A machine as claimed in claim 1, wherein the influencing means is operable to influence the beam action on the workpiece.

3. A machine as claimed in cla^'im 2, wherein the column comprises means for deflecting the beam to scan the workpiece and the influencing means is operable to influence the beam deflection.

4. A machine as claimed in any one of the preceding claims, wherein the optical measuring system is operable to measure the column position by detecting change in column position in two mutually orthogonal directions substantially perpendicular to the axis of the column.

5. A machine as claimed in any one of the preceding claims, wherein the optical measuring system comprises a laser interferometry system.

6. A machine as claimed in any one of the preceding claims, wherein the optical measuring system is operable to detect light reflected by reflective surfaces at the column.

7. A machine as claimed in claim 6, wherein the reflective surfaces are provided on a member fixed to the column in the zone of issue of the beam.

8. A machine as claimed in claim 7, wherein the member comprises an integral body of vitreous material and the reflective surfaces thereof are provided by faces formed on that body.

9. A machine as claimed in claim 7 or claim 8, wherein the member is adjustable in its position relative to the column.

10. A machine as claimed in any one of claims 7 to 9, wherein the member carries a detector for detecting backscattered electrons from the electron beam.

11. A machine as claimed in any one of the preceding claims, comprising a further optical measuring system for measuring the position of a movable stage for carrying the workpiece.

12. A machine as claimed in claim 11 , wherein the stage is movable in two mutually orthogonal directions substantially perpendicular to the axis of the column and the further optical system is operable to measure the stage position by detecting change in stage position in those directions.

13. A machine as claimed in claim 11 or claim 12, wherein the further optical measuring system comprises a laser interferometry system.

14. A machine as claimed in any one of the claims 11 to 13, wherein the further optical measuring system is operable to detect light reflected by reflective surfaces at the stage.

15. A machine as claimed in claim 14, wherein the reflective surfaces of the further optical measuring system are provided on a member attached to or forming part of the stage.

16. A machine as claimed in claim 15, wherein the member of the further optical measuring system comprises an integral body of vitreous material and the reflective surfaces thereof are provided by faces formed on that body.

17. A machine as claimed in any one of claims 11 to 16, wherein the optical measuring systems are operable to measure column position and stage position by reference to the same predetermined co-ordinate axes.

18. A machine as claimed in any one of claims 11 to 17, wherein the optical measuring systems have a shared light source.

19. A machine as claimed in claim 18, wherein the optical measuring systems comprise respective fibre optical conductors to conduct light from the shared source.

20. A machine as claimed in any one of claims 11 to 19, wherein the influencing means is operable to correct the stage position measurement by reference to the determined instantaneous datum.

21. A machine as claimed in any one of claims 11 to 19, wherein the influencing means is operable to control the stage movement by reference to the determined instantaneous datum.