GB2183364A

GB2183364A - Alignment using gratings having varying stripe pitch

Info

Publication number: GB2183364A
Application number: GB08627434A
Authority: GB
Inventors: Leslie Charles Laycock
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1985-11-22
Filing date: 1986-11-17
Publication date: 1987-06-03
Also published as: GB8528826D0; GB2183364B; GB8627434D0

Abstract

An alignment technique, for aligning a photolithographic mask over a semiconductor wafer, uses a first grating 9 comprising a plurality of stripes whose pitch varies along the length of the grating formed on the wafer 7. A second grating is formed on the mask of similar form to the first grating, the second grating being oriented with respect to the first grating such that the stripes of the two gratings are inclined to each other. By illuminating the gratings 5, 9 a moiré pattern is produced which comprises at least one interference band traversing the gratings. The deviation of the bank from a straight line is measured, and the deviation used to align the mask and the wafer 7. The pitch of the gratings 5, 9 varies in the same sense along the same direction. <IMAGE>

Description

Alignment techniques This invention relates to alignment techniques. In particularthe invention relates to alignmenttech- niquesforaligning a first object with a second object.

Such techniques are frequently used in semiconductor processing where it is required, for example to align a photolithographic mask over a semiconductorwafer. One such technique using moir6 patterns is described in an article in Applied Optics, volume 11, No. 11, pages 2455 to 2459 published in November 1972. This article describes the forming of a grating consisting of equispaced concentric circles on a semiconductorwafer, a similar circular grating being formed on a mask. To align the maskwiththe wafer, the wafer and mask are both placed in an alignment microscope, separated by a small distance.The resultant interference pattern, i.e., more' pattern, caused by the interference of light that has been diffracted by one ofthetwo gratings, both grat ings, and by undiffracted zero-order lightwillthen give an observer information about the relative positions of the two gratings such that alignment ofthe maskoverthewafercan be achieved. Such an align menttechnique suffers the disadvantage however that the range of misalignments which may be detec ted is fixed by thepitch pitchofthecirclesformingthe gratings. This may not be convenient where for ex amplethewaterand mask are initially misaligned by a alarge amount.

It is an object of the present invention to provide an alignment technique using moor6 patterns, but wherein this disadvantage is overcome.

According to a first aspect ofthe present invention an alignmenttechniqueforaligning a first object with a second object comprises: forming a firstoptical grating onthefirst object,thefirst grating comprising a plurality of stripes whose pitch varies along thefirstgrating; overlaying a second optical grating on thefirstgrating,the second grating comprising a plurality of stripes whose pitch varies along the second grating, the second grating being oriented with respect to the first grating such that the stripes ofthetwo gratings are inclined to each otherandthe pitch of the two gratings varies in the same sense along the same direction; illuminating the gratings so as to produce a more' pattern comprising at least one interference band traversing the gratings; measuring the deviation from a straight line of the band; and moving the second object with respect to the first object in response to said deviation.

According to a second aspect of the present invention an apparatus for use in an alignment technique for aligning afirstobjectwith a second object comprises: meansforforming afirstoptical grating on the first object, the first grating comprising a plurality of stripes whose pitch varies along the first g rating; meansforoverlaying a second optical grating on the first grating, the second grating comprising a plurality of stripes whose pitch varies along the second grating, the second grating being oriented with respect to the first grating such that the stripes ofthetwo gratings are inclined to each other, and the pitch of the two gratings varies in the same sense along the same direction; means for illuminating the gratings so as to produce a more' pattern comprising at least one interference band traversing the gratings; means for measuring the deviation from a straight line of the band; and means for moving the second object with respect to the first object in re sponge to said deviation.

Two alignment techniques in accordance with the invention, together with an apparatus for performing thetechniqueswill now be described, bywayofex- ample only, with reference to the accompanying drawings in which: Figure iillustratesschematicallythegeneralform ofthe apparatus used in both techniques; Figure 2 i 11 ustrates one of the gratings used in the first technique; Figure 3 illustrates an interference band produced inthefirsttechniquewherethetwo gratings used in the technique are aligned; Figure 4 shows the band of Figure 3 where the two gratings are misaligned; Figure5illustrates one ofthe gratings used in the second technique;; Figure 6shows the interference bands produced in the second technique where the two gratings used in the technique are aligned; Figure 7shows the bands of Figure 6 where the gratings are misaligned along one direction; and Figure 8showsthe bands of Figure 6 wherethe gratings are misaligned along two orthogonal directions.

Referring firstly to Figure 1,theapparatuscom- prises a projector 1 and lens 3 positioned so asto focus an image 4 of a grating 5 formed on a mask onto a semiconductorwafer7 on which a second grating 9 has already been fabricated. The rest of the apparatus is constituted by a semireflective mirror 11 effective to reflect light from the projector 1 reflected backfrom thewafer7through afocussing lens 13 towards an observer 15.

In use of the apparatus, where the second gratings is roughly aligned with the image 4 of the grating 5 more' patterns will be produced whose form depends on theform of the gratings 5, 9 and the degree ofalignmentofthe image 40f the grating 5 andthe grating 9. An image 17 ofthese patterns may then be observed by the observer 15.

Reference now to Figure 2, in the first technique to be described, the grating 5 is of the chirp form shown in Figure 2, i.e. a series of stripes whose pitch varies along the length of the grating, shown as the Z direction in Figures 2, 3 and 4, with the stripes being inclined to the edges of the grating, with the inclination of the stripes varying over the grating. The grating 9 is ofsimilarform to the grating 5, having stripes of a similarlyvarying pitch butoppositeinclinationsto the edges of the grating 9, such that when the two gratings are overlaid, the number of points at which the stripes of the two gratings cross each other re mains constant overthe length ofthe gratings.

When the grating 9 is perfectly aligned with the image ofthe grating Sthe moire1 pattern 17 observed by the observer 1 Swill be of the form illustrated in Figure 3, i.e. a straight band running athwartthe stripes constituting the grating Sand image 4 ofthe grating 5. Where howeverthere is a misalignment of the grating Sand the image 4 of the grating 5 the band will bend as shown in Figure4,thefinerthe pitch ofthe part ofthe grating 9 and image 4 of the grating 5 giving rise to the band, i.e. for low values of Zthe greaterthe deviation of that part of the band from a straight line.The degree of misalignment of the mask and the wafer 7 can therefore be judged by the observer 15 by measurement ofwhere the band crosses the edge of a frame enclosing the image 17 ofthe moirn' pattern. Where the degree of misalign- ment is large this crossoverwill occur along the horizontal or coarse axis; if the misalignment is small the crossover will occur along the vertical or fine axis.

Thus a single pair of gratings producing a single pattern allows the measurement of misalignment over a widerangeofvalues,forexamplel00;m downto less than .01 am. This permits rapid alignment of the waferand mask in for example a step and repeatpro- cess. Furthermore the use of such chirp gratings allowthe accuracy ofalignmentto be maximised with respecttothe possible resolution ofthefinest partofthe gratings as determinedforexample by the photolithographictechniques used to fabricate the gratings.

Referring now to Figure Sin the second technique to be described the gratings 5, 9 are replaced by gratings oftheform shown in Figure 5. The grating shown in Figure 5 is in fact a composite pattern of eight grating segments ofthe general form shown in Figure 2, set at different angles. Each segment comprises a series of stripes whose pitch and incination vary over the segment as before, the inclinations of the stripes within the pair of gratings to be used being opposite as before.Where such a pair of gratings are incorporated in the apparatus shown in Figure 1 instead of the gratings 5,9 and perfect align~ ment of the image of the grating formed on the mask on the substrate 7 and the grating formed on the sub strate is achieved, a more' patter in the form of a cross as indicated in Figure 6 will result. Where how everthereisa misalignment of the image on the sub strate and the grating on the maskalong oneaxis,a more' pattern oftheform shown in Figure7will re sultinwhichthe horizontal bands in the pattern bend in similarfashion to the band shown in Figure 3.Misalignment of the image on the substrate and the grating on the mask along two orthogonal directions will lead to more' patterns oftheform shown in Figure8 in which both the horizontal and vertical bands bend.

) Thus misalignment along the two orthogonal directions may be measured using the single pair of gratings. Furthermorethe indication of correct align ment by the appearance of a cross is a feature which makes the technique particularly suitable for use in semiconductor processing techniques where the monitoring of a cross iswell accepted.

It will be appreciated that in a technique in accordance with the invention in some cases it may be useful in some applications to also vary the line-space ratio across the gratings as this will alterthe contrast across the more' pattern.

It will also be appreciated thatwhilstthe tech niquesand apparatus described bywayofexample relate to the alignment of a maskto a semiconductor wafer, the invention is also applicable to any circum- stance where it is required to align a first object with a second object.

Claims

1. An alignmenttechniqueforaligning afirstob jectwithasecond objectcomprising: forming afirst optical grating on the first object, the first grating comprising a plurality of stripes whose pitch varies along the first grating; overlaying a second optical grating on the first grating,the second grating comprising a plurality of stripes whose pitch varies along the second grating, the second grating being oriented with respect to the first grating such that the stripes of the two gratings are inclined to each other and the pitch ofthetwo gratings varies in the same sense along the same direction; illuminating the gratings so asto produce a moire pattern comprising at least one interference band traversing the gratings; measuring the deviation from a straight line of the band; and moving the second object with respect tothefirstobjectin response to said deviation.

2. An alignmenttechniqueforaligning afirstobject with a second object substantially as herein before described with reference to the accompanying drawings.

3. An apparatus for use in the alignment tech niquefor aligning a first object with a second object comprising: meansforforming afirstoptical grating on thefirst object, thefirst grating comprising a plurality of stripes whose pitch varies along the first grating; means for overlaying a second optical grating on the firstgrating,the second grating comprising a plurality of stripes whose pitch varies alongthe second grating, the second grating being oriented with respect to the first grating such thatthe stripes ofthetwo gratings are inclined to each otherandthe pitch of the two gratings varies in the same sense along the same direction; meansforilluminatingthe gratings so asto produce a moire pattern comprising at least one interference band traversing the gratings; means for measuring the deviation from a straightlineofthe band; and means for moving the second object with respect to the first object in re sponge to said deviation.

4. An apparatus according to Claim 3 in which the line-space ratio varies across the gratings.

5. An apparatus according to Claim 3 or Claim 4 in whichthethefirstobject has formed on it a plurality of said first optical gratings, each oriented in a different direction, and means are provided for overlaying a corresponding plurality of said second optical gratings over said first optical gratings so asto enable alignment ofthe first and second objects in two orthogonal directions.

6. An apparatus according to Claim 5 in which said first and second optical gratings are arranged suchthatacrossappearswherethefirstobjectis aligned with the second object.

7. An apparatus according to any one of Claims 3 to 6 in which the first object is a semiconductor wafer, and the second object is a mask.

8. An apparatus for use in an alignmenttech niqueforaligning a first object with a second object substantially as hereinbefore described with refer ence to the accompanying drawings.