WO1996028838A1

WO1996028838A1 - Method of writing a pattern by an electron beam

Info

Publication number: WO1996028838A1
Application number: PCT/EP1995/000892
Authority: WO
Inventors: Grahame Craig Rosolen
Original assignee: Leica Cambridge, Ltd.
Priority date: 1995-03-10
Filing date: 1995-03-10
Publication date: 1996-09-19

Abstract

A method of writing a pattern or part of a pattern on a substrate by means of an electron beam comprises deflecting the beam to scan, in succession, sub-fields (1...27) of grid format within a field containing the pattern or pattern part. To reduce scanning delay between both vertically and horizontally adjacent sub-fields in the centre of the pattern, where finer pattern detail susceptible to time-related drift error may be located, the deflection of the beam is controlled to cause the pattern or pattern part to be written by progression through the sub-fields along a spiral path, the origin of which coincides with the location of the finer pattern detail.

Description

METHOD OF WRITING A PATTERN BY AN ELECTRON BEAM

The invention relates to a method of writing a pattern on a substrate by means of an electron beam.

It is common practice in electron beam lithography to write patterns on wafers, masks, reticles and other substrates by fracturing the patterns, i.e. breaking down each pattern into sub- Datterns and by exposing the sub-patterns in sequence to build up a composite image. The Dattern is commonly divided into a grid format of square main fields and each main field is further divided into a grid format of square sub-fields, of which more than 4,000 may be present. In the case of some patterns, only a single main field is used. Each subfield is written by beam deflection to scan the subfield area, usually by vectoring of the beam to individual pattern shapes within the area by the most economic sequence of travel and by boustrophedon scanning of each shape. At the conclusion of each sub-field scanning the beam is moved on to the next sub-field and the process is repeated. When the pattern features of all sub-fields have been written, the substrate itself can be displaced to position a succeeding main field centrally of the beam axis, so that its sub-fields can in turn be written.

The sub-field size may have a side dimension in the order of, for example, 10 microns and particularly accurate control of the beam position is necessary to minimise or avoid distortion of pattern features crossing sub-field boundaries, in particular due to misalignment of features connected at these boundaries. This control is achieved by applying correction to the beam deflection system to compensate for, for example, deflection distortion, magnetic effects and time-related drift effects. The last-mentioned source of alignment error is represented by the delay in scanning sub-fields which are adjacent to one another, but are significantly spaced apart in the scanning sequence. The magnitude of many alignment errors is time-dependent and the greater the numerical separation of adjacent sub-fields in the scanning sequence the greater the likelihood of feature misalignment at the common boundary of those sub-fields. If the sequence follows the boustrophedon method, sub-fields which are adjacent in a direction normal to the row scanning direction can be numerically spaced apart in the scanning sequence by up to almost twice the row length and consequently are separated by the time taken for the beam to scan virtually all the sub-fields in two full rows. This maximum separation occurs at row ends, where less fine pattern features may be located and thus where feature misalignment is less critical. However, in the region of the row centres the numerical spacing normal to the row direction in boustrophedon scanning will still be equal to about a full row length. In the case of rows containing features at the centre of the pattern, where finer detail is likely to be present, such a delay between writing directly adjacent sub- fields can lead to a requirement for a substantial amount of corrective input to avoid distortion or misalignment.

It is therefore the object of the present invention to provide a method of electron beam lithography in which error attributable to time-related drift may be reduced in a zone of a pattern, particularly a central zone, determined to be more sensitive to such error.

Other objects and advantages of the invention will be apparent from the following description.

According to the invention there is provided a method of writing a pattern or part of a pattern on a substrate by means of an electron beam, wherein the pattern or pattern part extends in a field divided into sub-fields which are scanned in succession by deflection of the beam, characterised in that the deflection of the beam is controlled in direction to cause the pattern or pattern part to be written by progression through the sub-fields along a substantially spiral path.

In this method, the numerical separation of adjacent sub- fields, in both horizontal and vertical direction, is small in the region of the centre of the spiral defined by the substantially spiral path and progressively increases with increasing distance from that centre. If a pattern region, in particular the central region, with finer feature detail is coincident with the centre of the spiral then the sub-fields in that region will be written with relatively little intervening delay and the scope for time- induced distortion or misalignment of the fine detail features crossing those sub-fields or connected at their boundaries is significantly reduced.

The substantially spiral path will in normal circumstances be generated around a single sub-field so that, in effect, each second side of the path is incremented in length by one sub-field. If, however, the sub-field grid format is oblong rather than square, it may be appropriate to generate the path around two or more sub- fields arranged in line. In that case the beam will scan several sub-fields in a line at the centre of the path.

The deflection of the beam can be controlled to progress along the path in direction outwardly from the inner end of the path, thus to write the pattern starting from the centre of the spiral. However, it is possible to control the beam deflection to progress in the opposite direction and thus write the pattern starting from the outer end of the spiral. The numerical separation, or delay, between adjacent sub-fields is the same irrespective of the direction of scanning movement along the path.

In the case of a pattern which comprises a plurality of substantially identical parts each extending in a respective one of a corresponding plurality of fields, i.e. main fields, the deflection of the beam can be controlled in direction to cause the pattern to be written by progression through the fields along a substantially spiral path. Thus, not only sub-fields within a single field can be written in a spiral order, but also the main fields of a pattern when the pattern is fractured into these.

An example of the invention will now be described in more detail by reference to the accompanying drawing, in which:

Fig. 1 is a diagram illustrating scanning of pattern sub- fields by a conventional boustrophedon method in accordance with the prior art; and

Fig. 2 is a diagram illustrating scanning of pattern sub- fields by a method exemplifying the present invention. Referring now to the drawing, Figs. 1 and 2 each show part of a grid format of sub-fields within a main field applied to a pattern or part of a pattern to be written on a substrate by an electron beam lithography machine. Such a machine generates a focussed electron beam which is deflectable by way of deflection coils digitally controlled by software responsive to the input of data characterising the pattern or pattern part. Machines of this kind are well-known and described in, for example, Jones and Dix: Electron Beam Lithography in Telecommunication Device Fabricating Part 1, British Telecom Technology Journal, Vol. 7, No. 1. The substrate, such as a wafer or mask plate, is loaded onto a holder which is placed on a writing stage of the machine and positioned in a predetermined location. Writing is carried out by deflection of the beam to scan the sub-fields in succession until the entire pattern has been subjected to the beam action.

In the case of Fig. 1, which illustrates scanning by the prior art boustrophedon method, the sub-fields are arbitrarily numbered commencing at the lefthand end of the bottom row of subfields - thus the bottom lefthand corner of the pattern - and the numbering proceeds to the righthand end of that row. The numbering sequence then moves up to the righthand end of the next row up and proceeds to the lefthand end of that row, from where it moves up to the lefthand end of the row above and proceeds to the right. The sequence of scanning is in accordance with the numerical progression and also indicated by arrows in the individual sub-field boxes. If w = 70, for example, and the grid of format is square, 4900 sub¬ fields are present. Along the rows, the numerical spacing of directly, i.e. horizontally, adjacent sub-fields is 1 and there is no significant delay between the end of scanning of one sub-field and the start of scanning of the next. The pattern features adjoining the common boundary of the sub-fields are thus scanned at a very short spacing in time. From row to row, however, the numerical spacing of directly, i.e. vertically, adjacent sub-fields varies from 1 to 2w-l. Thus, if w = 70, scanning of the sub- field 2w does not start until 138 other intervening sub-fields have been scanned after the scanning of the sub-field 1. The scanning of the vertically adjacent sub-fields 1 and 2w is consequently subject to substantial intermediate delay, which allows scope for drift errors to impair the alignment of pattern features at the common boundary of these sub-fields.

These drift errors are generally of less significance in pattern border regions where sub-fields such as 1 and 2w are located and where coarser Dattern features may be found. However, in the pattern centre region where finer features may be located, thus in the centres of the rows in that region, the delay between scanning of vertically adjacent sub-fields is still high. This delay has a far more significant effect on sub-field feature alignment. The delay fluctuates between slightly more and slightly less than half the maximum delay occurring at the row end. If w = 70, the numerical spacing of the vertically adjacent sub-fields on one side of a row centre is 69 and the numerical spacing of vertically adjacent sub-fields on the other side of that centre is 71. This means, taking the former case as an example, that 67 intervening sub-fields have to be scanned between scanning of the two vertically adjacent sub-fields.

Fig. 2 illustrates scanning by a method exemplifying the invention. In this case the sub-fields are arbitrarily numbered starting from the central or an approximately centrally disposed sub-field in the grid format and the numbering proceeds in a clockwise spiral sense around that initial sub-field. The sequence of scanning is again in accordance with the numerical Drogression and additionally indicated by arrows. In this method of scanning, the numerical spacing of directly adjacent subfields, both horizontally adjacent and vertically adjacent, periodically increases as the spiral path advances from the sub-field at the origin of the spiral. The maximum spacing may be expressed as 8n-1, where n is the position number of the sub-field row (1, 2, 3...W/2) in horizontal or vertical direction from the horizontal row or vertical row, respectively, containing the sub-field 1. Thus, at the boundary of the pattern the numerical spacing of adjacent sub¬ fields can attain a maximum value (279 when w = 70) considerably in excess of the maximum arising in the case of the prior art method of Fig. 1. However, the numerical spacing in the critical central region of the pattern is significantly less by comparison with the prior art method. If w = 70, a sub-field numerical spacing, such as the 69 or 71 prevailing in the pattern centre when scanning is by the prior art method, is not attained in the method of Fig. 2 until scanning along the spiral path has advanced some nine rows from the row containing the sub-field 1, at which stage more than 27 percent of the total width of the pattern will have been scanned. The relatively small intervening delay between scanning of directly adjacent sub-fields in the central region of the spiral path and thus the central region of the pattern means that the influence of drift errors in the writing of the pattern in this region is greatly reduced and a lesser degree of correction is needed to ensure that fine pattern features are accurately aligned and free of distortion at sub-field boundaries.

Once the spiral path has been established for a grid format of sub-fields, scanning along the path can progress towards or away from the centre of the oath. However, from the viewpoint of machine control and pattern registration it will be simplest to initiate scanning with the beam neutral axis coincident with the sub-field at the origin of the spiral and to apply progressively increasing degrees of beam deflection to follow the spiral path out from the origin.

The grid format itself can be square or oblong, with different oblong shapes being accommodated by, for example, scanning three or more sub-fields in a line before deviating to another row to follow the spiral path proper.

Scanning by the spiral ordering method may also be applied to pattern main fields if, for example, the pattern has repeating parts each with fine feature detail disposed centrally.

Deflection of the beam, and optionally also movement of the substrate stage to perform spiral scanning instead of a prior art technique such as boustrophedon scanning can be carried out by appropriate modification of the beam control system and substrate positioning system in the machine.

Claims

1. A method of writing a pattern or part of a pattern on a substrate by means of an electron beam, wherein the pattern or pattern part extends in a field divided into sub-fields which are scanned in succession by deflection of the beam, characterised in that the deflection of the beam is controlled in direction to cause the pattern or pattern part to be written by progression through the sub-fields along a substantially spiral path.

2. A method according to claim 1, wherein the substantially spiral path is generated around a single sub-field.

3. A method according to claim 1, wherein the substantially spiral path is generated around two or more sub-fields arranged in a line.

4. A method according to any of the preceding claims, wherein the deflection of the beam is controlled to progress along the substantially spiral path in direction outwardly from the inner end of the path.

5. A method according to any one of the preceding claims, wherein the features of the pattern or pattern part are finer in the region of the centre of the field than in the region of the periphery of the field and the inner end of the substantially spiral path is disposed in the region of the field centre.

6. A method according to claim 1, wherein the pattern comprises a plurality of substantially identical parts each extending in a respective one of a corresponding plurality of fields and wherein the deflection of the beam is controlled in direction to cause the pattern to be written by progression through the fields along a substantially spiral path.