Field of the disclosure

The disclosure relates to the field of laser interferometers. The laser interferometers of the present disclosure may have particular application in direct-write lithography systems configured to write on full size semiconductor wafers at sub-micron accuracy.

Background

Direct-write lithography systems may be required to write features with sub-micron accuracy across a semiconductor wafer with a diameter of 300 mm, or perhaps greater.

Direct-writing may use a writing head that writes to an exposure area that is significantly smaller than the area of the wafer. For example, the exposure area of the writing hear may be 500 pm by 500 pm. Writing an entire wafer using a single writing head therefore requires writing multiple exposure areas sequentially. The number of exposure areas may be measured in the hundreds of thousands. Incrementing between each exposure area necessitates highly accurate movement and positioning of the wafer relative to the writing head (or the writing head relative to the wafer). Inadequate accuracy in incrementing the position of the wafer relative to the writing head may result in catastrophic failure of the wafer writing process.

It is known to use interferometry for measuring the position of a moving part, typically as part of an actuator feedback loop.

A conventional laser interferometer for this purpose comprises a beam splitter for splitting a single laser beam into two parts. One part may be directed to a mirror (often a rooftop mirror) that is fixed relative to the beam splitter while the other part may be directed to a movable mirror that moves with the object whose position is to be determined. Positional information is determined by analysing interference of the reflection of the first part with the reflection of the second part. Beam splitters and rooftop mirrors may be both expensive and bulky. Furthermore, if the movable mirror is a plane mirror, vibration in the moving object causes deflections in the reflected beam which lead to intensity variations, in the form of noise. Where intensity variations are significant, this can lead to jumps in the measured position if the magnitude of the noise results in entire phase cycles being missed.

An alternative conventional arrangement makes use of a Fabry-Perot interferometer employing a pair of optically parallel reflectors between which reflections may propagate resulting in interference patterns when all light is focused on a receiving surface. If a first of the pair of reflectors is fixed in position relative to the second of the pair then the

interference pattern may be interpreted so as to determine the spacing between the first and second reflectors.

A summary of conventional optical fibre interferometers is provided in Optical Fibre

Interferometers and Their Applications, AN Reza Bahrampour, Sara Tofighi, Marzieh Bathaee and Farnaz Farman, Sharif University of Technology, Iran published on

http://cdn.intechopen.com/.

Summary of the disclosure

Against this background, there is provided:

a laser interferometer configured to provide position information relating to a reflective surface, the laser interferometer comprising: a laser source, a photodetector, a signal processor and an optical defocusing element having a first surface and a second surface opposing the first surface,

the laser source configured to emit a primary beam towards the first surface of the optical defocusing element; and

the first surface of the optical defocusing element configured (a) to reflect a first portion of the primary beam towards the photodetector as a first reflected beam; and (b) to transmit a second portion of the primary beam such that it passes through the second surface of the optical defocusing element towards the reflective surface, reflects in the reflective surface and passes back through the second and first surfaces of the optical defocusing element towards the photodetector as a second reflected beam; wherein the photodetector is configured to detect the first and second reflected beams and to produce an output signal indicative of the detected first and second reflected beams; and

wherein the signal processor is configured to receive the output signal indicative of the first and second reflected beams and to determine an interference between the first and second reflected beams and thereby to determine a position of the reflective surface.

In this way, no beam splitter is required no rooftop mirror is required. Accordingly, a significant reduction in both volume and cost may be achieved.

Furthermore, the arrangement provides increased immunity to vibration in the moving mirror, which in turn improves accuracy in positional information as well as improving accuracy in motion control systems controlled by non-direct drive motors.

In a further, more specific, arrangement, there is provided:

a laser interferometer configured to provide position information relating to a reflective surface, the laser interferometer comprising: a laser source, a photodetector, a signal processor and a defocusing lens having a first surface and a second surface opposing the first surface, wherein the first surface is concave and the second surface is substantially planar,

the laser source configured to emit a primary beam towards the first surface of the defocusing lens; and

the first surface of the defocusing lens configured (a) to reflect a first portion of the primary beam towards the photodetector as a first reflected beam; and (b) to transmit a second portion of the primary beam such that it passes through the second surface of the defocusing lens towards the reflective surface, reflects in the reflective surface and passes back through the second and first surfaces of the defocusing lens towards the

photodetector as a second reflected beam;

wherein the photodetector is configured to detect the first and second reflected beams and to produce an output signal indicative of the detected first and second reflected beams; and

wherein the signal processor is configured to receive the output signal indicative of the first and second reflected beams and to determine an interference between the first and second reflected beams and thereby to determine a position of the reflective surface. In this way, a plano-concave lens may be adopted as a central optical element of the interferometer. Accordingly, a significant reduction in both volume and cost may be achieved.

Furthermore, the arrangement provides increased immunity to vibration in the moving mirror, which in turn improves accuracy in positional information as well as improving accuracy in motion control systems controlled by non-direct drive motors.

Brief description of the drawings

Figure 1 shows a laser interferometer in accordance with the prior art;

Figure 2 shows a first embodiment of a laser interferometer in accordance with the present disclosure;

Figure 3 shows a second embodiment of a laser interferometer in accordance with the present disclosure

Figure 4 shows I data plotted against Q data;

Figure 5 shows I and Q signals plotted against time; and

Figure 6 shows interferometer signal data tracking movement of the reflective surface in increments of 4 nanometers.

Detailed description

Figure 1 shows a laser interferometer 100 in accordance with the prior art. What follows is a detailed explanation of the prior art arrangement shown in Figure 1 , against which background the embodiments of the present disclosure and their advantages relative to the prior art will become clear. Structure of the laser interferometer of Figure 1

The prior art laser interferometer 100 comprises a laser source 110, a fiber circulator 120, a fiber collimator assembly 130, a beam splitter 140, a rooftop mirror 150, a movable mirror 160, a fiber coupled photodetector 170 and a signal processor 180.

The movable mirror 160 may be movable relative to the other components and may be mounted to an article the position of which in the x-dimension is to be measured using the laser interferometer.

The fiber circulator 120 has a first optical port 121 , a second optical port 122 and a third optical port 123.

The laser interferometer 100 further comprises a first fiber optic cable 115 connecting the laser source 110 with the first optical port 121 of the fiber circulator 120, a second fiber optic cable 125 connecting a second optical port 122 of the fiber circulator 120 with the fiber collimator assembly 130 and a third fiber optic cable 165 connecting the third optical port 123 of the fiber circulator 120 with the fiber coupled photodetector 170.

The fiber circulator 120 is configured such that light received from a fiber optical cable connected to the first optical port 121 is directed by the fiber circulator 120 to exit the fiber circulator 120 through the second optical port 122. The fiber circulator 120 is further configured that light received from a fiber optical cable connected to the second optical port 122 is directed by the fiber circulator 120 to exit the fiber circulator 120 through the third optical port 123.

In this way, light received at the first optical port 121 of the fiber circulator 120 is directed by the fiber circulator 120 to the fiber collimator assembly 130. A first fiber optic path may therefore comprise the first fiber optic cable 115, the first optical port 121 , the second optical port 122 and the second fiber optic cable 125.

Similarly, light received at the second optical port 122 of the fiber circulator 120 is directed by the fiber circulator 120 to the photodetector 170. A second fiber optic path may therefore comprise the second fiber optic cable 125, the second optical port 122, the third optical port 123 and the third fiber optic cable 165. The fiber collimator assembly 130 may comprise a fiber collimator and an adjustable optical mount, such as a gimbal, to facilitate alignment of the fiber collimator.

The beam splitter 140 has a first surface 141 , a second surface 142 and a third surface 143. Laser light incident on the first surface 141 is split within the beam splitter 140 into a first part and a second part. The first part of the laser light passes out of the second surface 142 while the second part of the laser light passes out of the third surface 143.

The fiber collimator assembly 130 is located relative to the beam splitter 140 such that the beam splitter 140 receives a beam that passes out of the fiber collimator assembly 130.

The rooftop mirror 150 faces the second surface 142 of the beam splitter 140 such that laser light leaving the beam splitter 140 via the second surface 142 is directed to the rooftop mirror 150.

The movable mirror 160 faces the third surface 143 of the beam splitter such that laser light leaving the beam splitter 140 via the third surface 143 is directed to the movable mirror 160.

The movable mirror 160 may be fixed in all dimensions other than to allow movement in an axis (labelled the x-axis in Figure 1) that is perpendicular to the third surface 142 of the beam splitter 140 such that the movable mirror 160 is movable either towards or away from the beam splitter 140.

Reflected laser light that is incident on the second surface 142 of the beam splitter 140 passes back through the beam splitter 140 and exits through the first surface 141.

Reflected laser light that is incident on the third surface 143 of the beam splitter 140 passes back through the beam splitter 140 and exits through the first surface 141.

Since the first surface 141 faces the fiber collimator assembly 130, laser light that exits the beam splitter 140 via the first surface 141 is directed back to the fiber collimator 130. In a commonplace arrangement, the beam splitter 140 may be in the form of a cube having sides of approximately 30 mm in length (THORLABS™ produce various 30 mm cube beam splitters) while the rooftop mirror 150 may occupy a volume of a similar order such that an overall volume of the combination may be approximately 30 mm x 30 mm x 50 mm = 45,000 mm ³ .

The cost of the beam splitter 140 and rooftop mirror 150 is likely to be at least USD 1 ,000. Operation of the laser interferometer of Figure 1

In use, laser light is produced by the laser source 110 and passes into first fiber 115. The first fiber 115 being connected to the first optical port 121 , the laser light passes out of the second optical port 121 and into the second fiber 125. The fiber collimator assembly 130 receives the laser light from the second fiber 125.

The fiber collimator assembly 130 collimates the laser light and directs it as a primary beam 191 towards the first surface 141 of the beam splitter 140 wherein it is divided into a first part 191a and a second part 191 b. The first part 191a exits the second surface 142 while the second part 191b exits the third surface 143.

The first part 191a passes through the second surface 142, reflects in rooftop mirror 150 and returns to the second surface 142 of the beam splitter 140 as a first back-reflected beam 192. The second part 191 b passes through the third surface 143, reflects in the movable mirror 160 and returns to the third surface 143 of the beam splitter 140 as a second back-reflected beam 193.

Both the first and second back-reflected beams 192, 193 pass out of the first surface 141 of the beam splitter 140 towards the fiber collimator assembly 130 by which they are directed back into the second fiber 125. Both back-reflected beams 192, 193 pass along the second fiber 125 to the second optical port 122 of the fiber circulator 120.

The fiber circulator 120 directs the first and second back-reflected beams 192, 193 that arrive at the second optical port 122, to the third optical port 123 such that they travel along the third fiber 165 into the photodetector 170. The photodetector 170 receives the first and second back-reflected beams 192, 193 and produces a signal representative of those received first and second back-reflected beams 192, 193. The signal produced by the photodetector 170 may be supplied to the signal processor 180.

The signal processor 180 may interpret the signal produced by the photodetector 170 to determine interference behaviour as between the first and second back-reflected beams 192, 193. The interference behaviour as between the first and second back-reflected beams 192, 193 is then used to identify a difference in path length between the first and second back-reflected beams 192, 193. In this way, position of the movable mirror 160 relative to the rooftop mirror is determined by the signal processor 180.

Structure of the laser interferometer of Figure 2

Figure 2 shows a laser interferometer 200 in accordance with the present disclosure.

Various features of the laser interferometer 200 of the Figure 2 embodiment are the same as or correspond with those of the prior art laser interferometer 100 shown in Figure 1. In the description that follows, the use of similar reference numerals may assist in

understanding similar or corresponding components.

The laser interferometer 200 comprises a laser source 210, a fiber circulator 220, a fiber collimator assembly 230, an optical defocusing element 245, a movable mirror 260, a fiber coupled photodetector 270 and a signal processor 280.

The optical defocusing element 245 may be substantially planar in the sense that its thickness, that being the dimension between its first and second surfaces, is significantly smaller than a diameter of the first and second surfaces.

The fiber circulator 220 has a first optical port 221 , a second optical port 222 and a third optical port 223.

The laser interferometer 200 further comprises a fibre optic cable arrangement

corresponding to that of the laser interferometer of the laser interferometer 100 of Figure 1. In particular, the laser interferometer 200 may comprise a first fiber optic cable 215 connecting the laser source 210 with the first optical port 221 of the fiber circulator 220, a second fiber optic cable 225 connecting a second optical port 222 of the fiber circulator 220 with the fiber collimator assembly 230 and a third fiber optic cable 265 connecting the third optical port 223 of the fiber circulator 220 with the fiber coupled photodetector 270.

The fiber circulator 220 may be the same as that of the first laser interferometer 100. As such, the fiber circulator 220 may be configured such that light received from a fiber optical cable connected to the first optical port 221 is directed by the fiber circulator 220 to exit the fiber circulator 120 through the second optical port 222. The fiber circulator 220 is further configured that light received from a fiber optical cable connected to the second optical port 222 is directed by the fiber circulator 220 to exit the fiber circulator 220 through the third optical port 223.

In this way, light received at the first optical port 221 of the fiber circulator 220 is directed by the fiber circulator 220 to the fiber collimator assembly 230. A first fiber optic path may therefore comprise the first fiber optic cable 215, the first optical port 221 , the second optical port 222 and the second fiber optic cable 225.

Similarly, light received at the second optical port 222 of the fiber circulator 220 is directed by the fiber circulator 220 to the photodetector 270. A second fiber optic path may therefore comprise the second fiber optic cable 225, the second optical port 222, the third optical port 223 and the third fiber optic cable 265.

The fiber collimator assembly 230 may comprise a fiber collimator and an adjustable optical mount, such as a gimbal, to facilitate alignment of the fiber collimator.

In a departure from the prior art laser interferometer 100, the laser interferometer 200 of the Figure 2 embodiment does not make use of a beam splitter 140 to split the beam or a rooftop mirror 150.

The optical defocusing element 245 comprises a first surface 246 that faces the fiber collimator assembly 230 and a second surface 247 that faces the movable mirror 260. The optical defocusing element 245 may have no or minimal anti-reflective coating such that a first proportion of laser light incident on the optical defocusing element 245 is reflected by the first surface 246 such that it does not enter the optical defocusing element 245 and instead returns to the fiber collimator assembly 230 as a first back-reflected beam 292.

The proportion of laser light reflected by the first surface 246 may be between 1 % and 20 % of the laser light incident on that first surface 246. Preferably, the proportion of laser light reflected by the first surface 246 may be 5 % of the laser light incident on that first surface 246.

In this way, the laser light 291 incident on the first surface 246 of the optical defocusing element 245 is effectively split between a first portion 292 that is reflected in the first surface 246 and a second portion 291 b that passes through the first surface 246.

The optical defocusing element 245 is located relative to the defocusing lens 245 such that the optical defocusing element 245 receives a beam that passes out of the fiber collimator assembly 230.

In the embodiment of Figure 2, the optical defocusing element 245 comprises a plano concave lens 245. The first surface 246 of the plano-concave lens 245 is concave while the second surface 247 of the plano-concave lens 245 is substantially planar.

A movable mirror 260, potentially the same as that of the laser interferometer of Figure 1 , faces the second surface 247 of the beam splitter such that laser light leaving the optical defocusing element 245 via the second surface 247 is directed to the movable mirror 260.

The movable mirror 260 may be fixed in all dimensions other than to allow movement in an axis (labelled the x-axis in Figure 2) that is perpendicular to the optical defocusing element 245 such that the movable mirror 260 is movable either towards or away from the movable mirror 260.

Reflected laser light that is incident on the second surface 246 of the optical defocusing element 245 passes back through the optical defocusing element 245 and exits through the first surface 246.

Since the first surface 246 faces the fiber collimator assembly 230, laser light that exits the optical defocusing element 245 via the first surface 246 is directed back to the fiber collimator 230. The defocusing function of the optical defocusing element 245 may be relatively mild. This may mean that the focal length of the optical defocusing element 245 is of the same order of magnitude as the distance between the optical defocusing element 245 and the movable mirror 260.

In a specific example, the movable mirror 260 may be movable between a position in which it lies approximately 20 mm from the optical defocusing element 245 and a position in which it lies approximately 220 mm from the optical defocusing element 245. In such an arrangement, a focal length of somewhere between 110 mm and 500 mm might be appropriate. The focal length must be at least half of the maximum distance between the optical defocusing element 245 and the movable mirror 260.

Selection of an appropriate focal length of the optical defocusing element 245 must take account of the size of the mirror 260 in order to avoid a situation whereby the mirror is of insufficient area to receive all of the defocused laser light such that some of the light spills off the mirror and is thereby lost by virtue of not being reflected.

In one specific embodiment, the optical defocusing element 245 may have a diameter of approximately 25 mm and a thickness of perhaps 2 mm meaning that the overall volume occupied by the optical defocusing element 245 is approximately 4,000 mm ³ . Compared with the volume occupied by the beam splitter and rooftop mirror of the Figure 1 arrangement, this is an order of magnitude less.

The cost of an optical defocusing element 245 may be two orders of magnitude less than that of the beam splitter and rooftop mirror of the Figure 1 arrangement.

Operation of the laser interferometer of Figure 2

In use, laser light is produced by the laser source 210 and passes into first fiber 215. The first fiber 215 being connected to the first optical port 221 the laser light passes out of the second optical port 221 and into the second fiber 225. The fiber collimator assembly 230 receives the laser light from the second fiber 225.

The fiber collimator assembly 230 collimates the laser light and directs it as a primary beam 291 towards the first surface 246 of the optical defocusing element 245. The first back-reflected beam 292 is reflected directly back to the fiber collimator assembly 230.

That portion of the primary beam 291 that is not reflected in the first surface 246 of the optical defocusing element 245, referred to as the second portion 291b, passes through the optical defocusing element 245 and thereby begins to defocus. In particular, the second portion 291 b begins to lose the collimation provided by the fiber collimator assembly 230. The defocused second portion 291b passes out of the optical defocusing element 245 through the second surface 247 and arrives at the movable mirror 260.

The increase in angular spread of rays incident on the movable mirror 260 reduces the sensitivity of the laser interferometer 200 to angular vibration of the movable mirror 260. This achieves reduced signal noise.

The second part 291 b reflects in the movable mirror 260 and returns to the second surface 246 of the optical defocusing element 245 as a second back-reflected beam 293.

The second back-reflected beam 293 passes through the optical defocusing element 245 and exits through the first surface 246 of the optical defocusing element 245 towards the fiber collimator assembly 230.

What follows corresponds with that of the arrangement of Figure 1. In particular, both back-reflected beams 292, 293 pass along the second fiber 225 to the second optical port 222 of the fiber circulator 220.

The fiber circulator 220 directs the first and second back-reflected beams 292, 293 that arrive at the second optical port 222, to the third optical port 223 such that they travel along the third fiber 265 into the photodetector 270.

The photodetector 270 may receive the first and second back-reflected beams 292, 293 and produce a signal representative of those received first and second back-reflected beams 292, 293. The signal produced by the photodetector 170 may be supplied to the signal processor 280. The signal processor 280 may interpret the signal produced by the photodetector 270 and use interference behaviour as between the first and second back-reflected beams 292, 293 to identify a difference in path length between the first and second back-reflected beams 292, 293. In this way, position of the movable mirror 160 relative to the optical defocusing element 245 is determined by the signal processor 180.

In addition to the reduction in volume and cost relative to the Figure 1 arrangement, use of the optical defocusing element 245 in place of the beam splitter 140 means that the interferometer becomes more immune to vibration in the moving mirror, which in turn improves accuracy in positional information as well as improving accuracy in motion control systems controlled by non-direct drive motors.

Setup

Setup of the laser interferometer 200 may be achieved by the following steps. First, the movable mirror 260 is blocked such that there is no reflection of the second part 291 b of the primary beam 291. This eliminates any second back-reflected beam 293 and leaves only the first back-reflected beam 292. The adjustable optical mount of the fiber collimator assembly 230 is adjusted to maximise measured intensity of the first back-reflected beam. Subsequently, the movable mirror 260 is unblocked. At this point the angle of the movable mirror 260 relative to the second surface 247 of the optical defocusing element 245 is adjusted to maximise interferometric beating. In this way the second back-reflected beam 293 is adjusted.

An adjustable optical mount of the mirror (e.g. a gimbal) (not shown in the Figures) may be provided by which the movable mirror 260 is mounted to the article whose position is to be determined. The adjustable optical mount of the mirror may facilitate the angular adjustment described above in relation to the setup process.

Structure of the laser interferometer of Figure 3

Figure 3 shows a further laser interferometer 300 in accordance with the present disclosure. The further laser interferometer 300 is similar to the laser interferometer 200 of Figure 2 except that the orientation of the plan-concave lens 345 is reversed such that the first surface 346 is substantially planar and the second surface 347 is concave. In this way, the exact directions of the various beams are different from those in the embodiment of Figure 2. However, it is the same principle of mild defocusing that gives rise to the required interfering back-reflected beams 292, 293.

Accuracy of setup of the arrangement of Figure 3 is more critical than for the arrangement of Figure 2 as the arrangement of Figure 3 is less forgiving of modest misalignment. This is because modest misalignment may result in the first back-reflected beam 292 not arriving back at the fibre collimator assembly 230. In this event, none of the first-back reflected beam 292 will be received at the photodetector and therefore there will be no interference with the second back-reflected beam 293 to analyse.

However, when correctly aligned, the Figure 3 arrangement has an advantage in that the intensity of the first back-reflected beam 292 is very high.

Alternatives

It should be noted that, while the Figure 2 and Figure 3 arrangements make use of a plano concave lens, the disclosure is not limited as such. Any optical element that results in mild defocusing of the incident laser may be employed.

Whether or not the optical defocusing element is plano-concave lens, the optical defocusing element may be substantially planar, by which it is meant that it larger dimensions in the x- and y-axes than in the z-axis. For the avoidance of doubt, in this content, reference to the optical defocusing element as a whole being substantially planar does not preclude the possibility of one or both of its first and second surfaces being, for example, modestly concave, such as in the Figure 2 and Figure 3 embodiments.

Similarly, there are various options for the optical defocusing element in terms of coating or no coating. An optical defocusing element without coating may be appropriate. In particular, it may be that no anti-reflective coating is applied. Alternatively, an optical coating may be added to one or both surfaces of the optical defocusing element 245 to increase the proportion of incident light reflected. While the Figure 2 and Figure 3 embodiments have been described as employing a moving mirror 260, any surface that results in appropriate reflection can be employed for the same purpose. For example, the moving mirror 260 may be substituted by a retroreflector. This may have an additional advantage of enabling more straightforward alignment.

A further, additional, feature may be a beam-steering mirror which may be employed between the movable mirror 260 and the optical defocusing element 245. This may allow the two to be offset from each other with light passing from one to the other via the beam steering mirror. This may also avoid the requirement to adjust the orientation of the movable mirror 260 for alignment by allowing the beam-steering mirror to be adjusted instead. This may be more convenient as it may have less proximity to other components and may therefore be more accessible.

Measurements achieved by the interferometer of the present disclosure

Figure 4 shows a plot of I against Q signals while Figure 5 shows a plot of I and Q signals against time, as obtained from an interferometer in accordance with the present disclosure.

Figure 6 shows interferometer signal data tracking movement of the reflective surface in increments of 4 nanometers obtained from an interferometer in accordance with the present disclosure.

Two-axis laser interferometer

The arrangements described herein may be employed not only in a single axis laser interferometer but also in a two-axis laser interferometer. In such an arrangement, a beam splitter or fibre splitter may be provided in order to generate a second path from the laser source for use with a secondary interferometer channel.

Application

The laser interferometer of the present disclosure is capable of tracking movement of the movable mirror so sub-micron accuracy. Indeed, it has been shown to be capable of tracking increments of less than 10 nm. Figure 6 provides evidence for the ability of the laser interferometer to track movement of the movable mirror in increments of 4 nm.