EP0290445A1 - Apparatus incorporating phase conjugate mirrors - Google Patents

Apparatus incorporating phase conjugate mirrors

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Publication number
EP0290445A1
EP0290445A1 EP19870900676 EP87900676A EP0290445A1 EP 0290445 A1 EP0290445 A1 EP 0290445A1 EP 19870900676 EP19870900676 EP 19870900676 EP 87900676 A EP87900676 A EP 87900676A EP 0290445 A1 EP0290445 A1 EP 0290445A1
Authority
EP
European Patent Office
Prior art keywords
beam splitter
image
projecting apparatus
radiation
phase conjugate
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP19870900676
Other languages
German (de)
French (fr)
Inventor
Malcolm Charles Gower
Alan Frank Gibson
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Individual
Original Assignee
Individual
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Individual filed Critical Individual
Publication of EP0290445A1 publication Critical patent/EP0290445A1/en
Withdrawn legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
    • G02B27/10—Beam splitting or combining systems
    • G02B27/14—Beam splitting or combining systems operating by reflection only
    • G02B27/144—Beam splitting or combining systems operating by reflection only using partially transparent surfaces without spectral selectivity
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/353—Frequency conversion, i.e. wherein a light beam is generated with frequency components different from those of the incident light beams
    • G02F1/3536—Four-wave interaction
    • G02F1/3538—Four-wave interaction for optical phase conjugation

Definitions

  • This invention relates to phase conjugate mirrors and to apparatus incorporating phase conjugate mirrors.
  • phase conjugate mirror reflects light directly back along its incident path, whatever the angle of incidence. Hence light from a point source is refocused on to the point whatever its distance from the mirror. Thus the phase conjugate mirror behaves as if it was a curved mirror the axis of which always points towards the source and the radius of curvature of which is equal to the distance to the source. Moreover, this behaviour is still obtained if a phase distorter is interposed in the path of the beam.
  • Phase conjugate reflection may be realised in practice, by methods such as four wave mixing (FWM) and stimulated Brillouin scattering (SBS) (see D.M. Pepper Optical Engineering 21 156 (1982). Phase conjugate mirror action has been demonstrated at many wave-lengths in the visible, infra-red and ultraviolet.
  • image projecting apparatus comprising a source of radiation to illuminate an object, partially reflecting beam splitter means to reflect radiation from the object on to a phase conjugate mirror arranged to reflect said radiation back through said beam splitter to construct a real image of said object and compensator means interposed between said object and said beam splitter means to compensate at least partially for abberations in said image caused by said beam splitter means.
  • FIG. 1 is an explanatory diagram showing a distorter in the path of radiation reflected by a phase conjugate mirror.
  • Figure 2 is a development of the apparatus of Figure 1 incorporating a beam splitter.
  • FIG 3 is a schematic view of apparatus in accordance with the invention.
  • Figure 4 is a modification of the apparatus of Figure 2; and
  • Figure 5 is an adaptation of the apparatus of Figure 4.
  • a phase conjugate mirror PCM reflects radiation from a point source S back to the source.
  • the interposition of a phase distorter D has no effect on the quality of image formed. Since an illuminated object may be considered as an array of point sources a phase conjugate mirror and beam splitter constitute a 1 to 1 imaging structure, as shown in Figure 2.
  • a point on an object at A is imaged at A 1 . Again, this is unaffected by a phase distorter, D, provided it is placed in the part of the beam in which it is traversed twice.
  • Distortion cancellation is not obtained when radiation passes only once through an element placed between the beam splitter and A and A 1 .
  • phase distortion due to the beam splitter is not cancelled.
  • the quality of an optical image, obtained by a conventional lens system or otherwise, can be characterised by three parameters:
  • the resolution is given by the Rayleigh criterion as ⁇ /2NA where ⁇ is the wavelength of the illumination and NA the numerical aperture.
  • NA is simply sin ⁇ where ⁇ is half the angle of convergence of the beam at the image.
  • a beam splitter is fabricated from a suitable transparent material, e.g. glass or quartz, and not less than a few mm thick to permit polishing by conventional techniques of the optically transmissive and reflective surfaces to be flat and parallel to better than about 0.1 ⁇ .
  • the surface nearest the phase conjugate mirror is coated to obtain 50% reflectivity and the other surface antireflection coated.
  • the numerical aperture of the system and hence the resolution and image pixel number, will be maximised by placing the PCM as near to the image/object plane as possible (Figure 3).
  • NA 0.25
  • the image width in the plane shown is about 0.25D where D is the width of the phase conjugate mirror.
  • a suitable width for the phase conjugate mirror is 4 cm.
  • Improved quality images in a structure of the type shown in Figure 3 is obtained through the presence of a compensation plate of which the width is equal to that of the beam splitter to an accuracy of a few wavelengths. This is conveniently and simply achieved by fabricating the beam splitter and compensator as one flat plate and then cutting at right angles to the polished optical surfaces.
  • a compensator plate arises because the beam to and from the PCM is non-parallel: a plate at normal incidence in a non-parallel beam produces spherical aberration and an angled plate astigmatism. The aberrations due to a component in front of a PCM are cancelled if the radiation passes through it twice.
  • This apparatus may be adapted to provide apparatus suitable for use in photolithography.
  • One such apparatus was first described by Giuliano (Physics Today 34, 27 (1981)) and is shown in Figure 4.
  • This apparatus comprises an illuminating beam B from a radiation source (not shown) reflected by a beam splitter BS to a phase conjugate mirror PCM from which it is reflected to the beam spliiter.
  • the beam passes through an amplifier A.
  • Radiation passing through the beam splitter forms an image on the surface of a layer of photoresist PR. Since no compensator plate is included, the image is astigmatic.
  • the amplifier can provide a higher power on the photoresist than incident on the mask, which is valuable, while any aberrations due to the amplifier are cancelled as it is double passed.
  • the maximum efficiency of a beam splitter (25%) is achieved when the reflectivity is 50%. This, however, can usually only be achieved by the use of multiple dielectric coatings on the beam splitter surface and such coatings can be damaged by laser light at the high intensities required to expose a photoresist in a single, brief, pulse. Hence uncoated surfaces, with inevitably lower reflectivities (e.g. quartz; reflectivity at 45 , 9%) will often be preferred. Fortunately a low beam splitter reflectivity is consistent with - indeed essential for - the achievement of a higher power on the photoresist than incident on the mask. Taking Into account the light reflected back to the mask the power on the photoresist will exceed that on the mask if
  • R c and R B are the PCM and beam splitter reflectivities respectively and g is the single pass gain of the amplifier.
  • the amplifier considerably increases the distance between phase conjugate mirror and image/mask thus reducing the numerical aperture and the system resolution.
  • the numerical aperture can be maintained at a high value by placing a lens or lens system L between the beam splitter and amplifier. Since the lens(es) is double passed its aberrations are cancelled and it does not have to be of high quality.
  • a structure with a single lens is shown in Figure 5. If the mask and photoresist are mounted normal to the beams incident upon them as in the prior art structure, ( Figure 4) some light will be reflected back to and through the amplifier. Laser oscillations may then build up in one or all of three effective laser "cavities": between the photoresist and itself, the mask and itself and the photoresist and the mask, each via the phase conjugate mirror. If R M is the reflectivity of the mask, R p the reflectivity of the photoresist the following three conditions must all be met if oscillation, and consequent obliteration of the image, is to be avoided.
  • condition (1) can be solved by placing the mask and photoresist at non-normal incidence: if both are at the same angle relative to the beam axis the image of the mask will be in focus at all equivalent points on the photoresist surface.
  • the effective value of R M and R p decrease with angle to reach zero when the angle between the normal to the mask/photoresist plane and the optic axis exceeds ⁇ , as defined above.
  • An uncoated, and hence relatively low reflectivity, beam splitter together with an amplifier, can ensure a high power incident on the photoresist and a low power transmitted back to the mask, in the ratio (1-R B )/R B .
  • the power to be transmitted through the beam splitter may be so high that not only reflective coatings but also anti reflection coatings will be damaged. If both surfaces of the beam splitter have to be uncoated they will have equal reflectivities and the second surface (nearest the photoresist, Figure 5) will produce a "ghost" image of the mask at the photoresist, displaced with respect to the wanted image but of comparable intensity.
  • a solution to this problem is to make the beam splitter thick enough to displace the ghost image outside the primary image area. The displacement can be calculated from the condition that all rays incident on the PCM return along the same path. The displacement equals the image width, d, when
  • D the diameter of the lens ( Figure 5) or PCM ( Figure 3) and 1 is the thickness of the beam splitter.
  • 1 D.

Abstract

Un appareil de production d'images comprend un source de rayonnement pour éclairer un objet (O), un diviseur de faisceau partiellement réfléchissant pour réfléchir le rayonnement provenant de l'objet sur un miroir conjugé en phase (PCM), un miroir conjugué en phase pour réfléchir ledit rayonnement à travers ledit diviseur de faisceau afin de construire une image réelle (I) dudit objet et un compensateur (C) interposé entre ledit objet et ledit diviseur de faisceau afin de compenser au moins partiellement les abérrations dans ladite image causées par le diviseur de faisceau.An image producing apparatus includes a radiation source for illuminating an object (O), a partially reflecting beam splitter for reflecting radiation from the object onto a phase conjugate mirror (PCM), a phase conjugate mirror for reflecting said radiation through said beam splitter in order to construct a real image (I) of said object and a compensator (C) interposed between said object and said beam splitter in order to at least partially compensate for the aberrations in said image caused by the beam splitter.

Description

APPARATUS INCORPORATING PHASE CONJUGATE MIRRORS This invention relates to phase conjugate mirrors and to apparatus incorporating phase conjugate mirrors.
A phase conjugate mirror (PCM) reflects light directly back along its incident path, whatever the angle of incidence. Hence light from a point source is refocused on to the point whatever its distance from the mirror. Thus the phase conjugate mirror behaves as if it was a curved mirror the axis of which always points towards the source and the radius of curvature of which is equal to the distance to the source. Moreover, this behaviour is still obtained if a phase distorter is interposed in the path of the beam. Phase conjugate reflection may be realised in practice, by methods such as four wave mixing (FWM) and stimulated Brillouin scattering (SBS) (see D.M. Pepper Optical Engineering 21 156 (1982). Phase conjugate mirror action has been demonstrated at many wave-lengths in the visible, infra-red and ultraviolet.
Certain applications, sueh as the projection of the image of a mask on to the surface of a layer of photo resist used for photolithorgraphic processing of semiconductor devices, make stringent demands on the quality of the image. We have therefore devised apparatus incorporating a phase conjugate mirror to reduce apparatus-induced distortion. According to the present invention there is provided image projecting apparatus comprising a source of radiation to illuminate an object, partially reflecting beam splitter means to reflect radiation from the object on to a phase conjugate mirror arranged to reflect said radiation back through said beam splitter to construct a real image of said object and compensator means interposed between said object and said beam splitter means to compensate at least partially for abberations in said image caused by said beam splitter means. An embodiment of the invention will now be described by way of example, with reference to the accompanying drawings in which:- Figure 1 is an explanatory diagram showing a distorter in the path of radiation reflected by a phase conjugate mirror. Figure 2 is a development of the apparatus of Figure 1 incorporating a beam splitter.
Figure 3 is a schematic view of apparatus in accordance with the invention. Figure 4 is a modification of the apparatus of Figure 2; and Figure 5 is an adaptation of the apparatus of Figure 4. Referring now to Figure 1 of the drawings, a phase conjugate mirror PCM reflects radiation from a point source S back to the source. The interposition of a phase distorter D has no effect on the quality of image formed. Since an illuminated object may be considered as an array of point sources a phase conjugate mirror and beam splitter constitute a 1 to 1 imaging structure, as shown in Figure 2. A point on an object at A is imaged at A1. Again, this is unaffected by a phase distorter, D, provided it is placed in the part of the beam in which it is traversed twice. Distortion cancellation is not obtained when radiation passes only once through an element placed between the beam splitter and A and A1. Thus, phase distortion due to the beam splitter is not cancelled. The quality of an optical image, obtained by a conventional lens system or otherwise, can be characterised by three parameters:
(a) Resolution, or the minimum resolvable element dimension.
(b) Depth of focus, i.e. the distance over which the focal distance may be varied without significantly impairing the resolution, and (c) Area of the image over which the specified resolution is maintained. The ratio of image area to the area of a resolvable element is the number of such elements or pixels in the image. An image containing more than 108 pixels would be considered to be of high quality and, if obtained using a lens system, would require it to be of high quality and sophisticated design and consequently high cost.
If the optical components are of sufficient quality the resolution is given by the Rayleigh criterion as λ/2NA where λ is the wavelength of the illumination and NA the numerical aperture. For an optical system in air or vacuum NA is simply sinθ where θ is half the angle of convergence of the beam at the image. Hence
Under the same conditions the depth of focus is given approximately by These expressions are not valid when the depth of focus order λ. NA is then of order 0.7. In practice component quality usually limits the performance of lens systems with a Rayleigh resolution better than 1 μm, corresponding for example to λ = 0.5 μm and sinθ = 0.25. The depth of focus would then be 4 μm and an image area of 1 cm2 would correspond to a pixel number of 108.
The simple structure shown in Figure 2 may be used as a basis for imaging systems using phase conjugate mirrors. Following normal practice, at least at low powers, a beam splitter is fabricated from a suitable transparent material, e.g. glass or quartz, and not less than a few mm thick to permit polishing by conventional techniques of the optically transmissive and reflective surfaces to be flat and parallel to better than about 0.1 λ. The surface nearest the phase conjugate mirror is coated to obtain 50% reflectivity and the other surface antireflection coated.
The numerical aperture of the system, and hence the resolution and image pixel number, will be maximised by placing the PCM as near to the image/object plane as possible (Figure 3). As drawn in Figure 3 NA = 0.25 and the image width in the plane shown is about 0.25D where D is the width of the phase conjugate mirror. A suitable width for the phase conjugate mirror is 4 cm. Improved quality images in a structure of the type shown in Figure 3 is obtained through the presence of a compensation plate of which the width is equal to that of the beam splitter to an accuracy of a few wavelengths. This is conveniently and simply achieved by fabricating the beam splitter and compensator as one flat plate and then cutting at right angles to the polished optical surfaces. The need for a compensator plate arises because the beam to and from the PCM is non-parallel: a plate at normal incidence in a non-parallel beam produces spherical aberration and an angled plate astigmatism. The aberrations due to a component in front of a PCM are cancelled if the radiation passes through it twice.
Aberrations due to two identical components, each singly passed but in the object and image beams respectively, will cancel. Similar considerations apply to any other components in the image or object beams but only planar components can be made easily and cheaply in matched pairs.
This apparatus may be adapted to provide apparatus suitable for use in photolithography. One such apparatus was first described by Giuliano (Physics Today 34, 27 (1981)) and is shown in Figure 4. This apparatus comprises an illuminating beam B from a radiation source (not shown) reflected by a beam splitter BS to a phase conjugate mirror PCM from which it is reflected to the beam spliiter. During both passes between the beam splitter and the phase conjugate mirror the beam passes through an amplifier A. Radiation passing through the beam splitter forms an image on the surface of a layer of photoresist PR. Since no compensator plate is included, the image is astigmatic. The amplifier can provide a higher power on the photoresist than incident on the mask, which is valuable, while any aberrations due to the amplifier are cancelled as it is double passed. Some more serious limitations are imposed by the inclusion of an amplifier.
The maximum efficiency of a beam splitter (25%) is achieved when the reflectivity is 50%. This, however, can usually only be achieved by the use of multiple dielectric coatings on the beam splitter surface and such coatings can be damaged by laser light at the high intensities required to expose a photoresist in a single, brief, pulse. Hence uncoated surfaces, with inevitably lower reflectivities (e.g. quartz; reflectivity at 45 , 9%) will often be preferred. Fortunately a low beam splitter reflectivity is consistent with - indeed essential for - the achievement of a higher power on the photoresist than incident on the mask. Taking Into account the light reflected back to the mask the power on the photoresist will exceed that on the mask if
Rc g2 RB (1- RB) > 1 + Rc g2 RB2 (1 ) where Rc and RB are the PCM and beam splitter reflectivities respectively and g is the single pass gain of the amplifier. For RB = 9% and Rcg2 = 100 the left hand side of (1) exceeds the right by a factor of 4.5, which is sufficient. The gain per unit length of laser amplifiers varies considerably between different types, as does the reflectivity of phase conjugate mirrors but an amplifier large enough to achieve Rc g2 = 100 would be at least 10 cm in length and might be as long as 100 cm.
The amplifier considerably increases the distance between phase conjugate mirror and image/mask thus reducing the numerical aperture and the system resolution. The numerical aperture can be maintained at a high value by placing a lens or lens system L between the beam splitter and amplifier. Since the lens(es) is double passed its aberrations are cancelled and it does not have to be of high quality. A structure with a single lens is shown in Figure 5. If the mask and photoresist are mounted normal to the beams incident upon them as in the prior art structure, (Figure 4) some light will be reflected back to and through the amplifier. Laser oscillations may then build up in one or all of three effective laser "cavities": between the photoresist and itself, the mask and itself and the photoresist and the mask, each via the phase conjugate mirror. If RM is the reflectivity of the mask, Rp the reflectivity of the photoresist the following three conditions must all be met if oscillation, and consequent obliteration of the image, is to be avoided.
Rc g2 Rp 2 (1-RB) < 1 (2) Rc g2 RM 2 R2 B < 1 (3)
Rc g2 Rp RM RB (1-RB) < 1 (4)
Depending on the values of Rp and RM these conditions may not be consistent with the satisfaction of condition (1). The problem can be solved by placing the mask and photoresist at non-normal incidence: if both are at the same angle relative to the beam axis the image of the mask will be in focus at all equivalent points on the photoresist surface. The effective value of RM and Rp decrease with angle to reach zero when the angle between the normal to the mask/photoresist plane and the optic axis exceeds θ, as defined above. An uncoated, and hence relatively low reflectivity, beam splitter together with an amplifier, can ensure a high power incident on the photoresist and a low power transmitted back to the mask, in the ratio (1-RB)/RB. For uncoated quartz this power ratio is about 10, which is sufficient for all photolithographic applications currently envisaged. However the power to be transmitted through the beam splitter may be so high that not only reflective coatings but also anti reflection coatings will be damaged. If both surfaces of the beam splitter have to be uncoated they will have equal reflectivities and the second surface (nearest the photoresist, Figure 5) will produce a "ghost" image of the mask at the photoresist, displaced with respect to the wanted image but of comparable intensity. A solution to this problem is to make the beam splitter thick enough to displace the ghost image outside the primary image area. The displacement can be calculated from the condition that all rays incident on the PCM return along the same path. The displacement equals the image width, d, when
d =√2.1.tan r (5)
and D equals the diameter of the lens (Figure 5) or PCM (Figure 3) and 1 is the thickness of the beam splitter. As an example, when d/D = 0.5 and tan θ = 0.2 ( = NA) , 1 = D.

Claims

1. Image projecting apparatus comprising a source of radiation to illuminate an object, partially reflecting beam splitter means to reflect radiation from the object on to a phase conjugate mirror arranged to reflect said radiation back through said beam splitter to construct a real image of said object characterised in that it includes compensator means interposed between said object and said beam splitter means to compensate at least partially for abberations in said image caused by said beam splitter means.
2. Image projecting apparatus as claimed in Claim 1 characterised in that the width of said compensator means is substantially equal to that of said beam splitter means.
3. Image projecting apparatus as claimed in Claim 2 characterised in that said compensator means and said beam splitter means are formed from a single plate which has been cut normal to its polished optical surfaces.
4. Image projecting apparatus as claimed in any one of the preceding claims characterised in that said beam splitter means has low reflectivity surfaces in the path of said radiation.
5. Image projecting apparatus as claimed in Claim 4 characterised in that said low reflectivity surfaces are uncoated.
6. Image projecting apparatus as claimed in any one of the preceding claims, characterised in that it further includes optical amplifier means positioned between said beam splitter means and said phase conjugate mirror.
7. Image projecting apparatus as claimed in Claim 6 characterised in that it includes a lens system between the beam splitter means and the optical amplifier means.
8. Image projecting apparatus as claimed in any one of the preceding claims characterised in that it is arranged so that the object and image planes are non-normal to the path of said radiation.
9. Image of projecting apparatus as claimed in Claim 8 characterised in that the thickness of said beam splitter means is such that any ghost image of said object is displaced outside a primary image area.
EP19870900676 1986-01-14 1987-01-13 Apparatus incorporating phase conjugate mirrors Withdrawn EP0290445A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB868600749A GB8600749D0 (en) 1986-01-14 1986-01-14 Apparatus incorporating phase conjugate mirrors
GB8600749 1986-01-14

Publications (1)

Publication Number Publication Date
EP0290445A1 true EP0290445A1 (en) 1988-11-17

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EP19870900676 Withdrawn EP0290445A1 (en) 1986-01-14 1987-01-13 Apparatus incorporating phase conjugate mirrors

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EP (1) EP0290445A1 (en)
JP (1) JPH01502056A (en)
GB (2) GB8600749D0 (en)
WO (1) WO1987004265A1 (en)

Families Citing this family (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS61236035A (en) * 1985-04-09 1986-10-21 Sony Corp Optical head
DK160524C (en) * 1987-05-04 1991-09-23 Eskofot As PROCEDURE FOR ELIMINATING PAD-SHAPED LIST
US5155623A (en) * 1988-09-23 1992-10-13 At&T Bell Laboratories Arrangement for imaging multiple arrays of light beams
DE3922572C1 (en) * 1989-07-08 1990-09-13 Messerschmitt-Boelkow-Blohm Gmbh, 8012 Ottobrunn, De

Family Cites Families (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4412723A (en) * 1981-05-28 1983-11-01 The Perkin-Elmer Corporation Optical system for correcting the aberrations of a beamsplitter in converging light
US4541688A (en) * 1984-06-11 1985-09-17 Eastman Kodak Company Optical beam splitters

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
See references of WO8704265A1 *

Also Published As

Publication number Publication date
GB2186709A (en) 1987-08-19
GB8700671D0 (en) 1987-02-18
WO1987004265A1 (en) 1987-07-16
GB8600749D0 (en) 1986-02-19
JPH01502056A (en) 1989-07-13

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