Abstract

A phase-sensitive scanning optical microscope is described that can measure surface height changes down to 0.1 nm. This is achieved by using two heterodyne Michelson interferometers in parallel. One interferometer probes the sample with a tightly focused beam, and the second has a collimated beam that illuminates a large area of the surface, providing a large area on sample reference. This is facilitated by using a specially constructed objective lens that permits the relative areas illuminated by the two probe beams to be varied both arbitrarily and independently, thus ensuring an accurate absolute phase measurement. We subtracted the phase outputs from each interferometer to provide the sample phase information, canceling the phase noise resulting from microphonics in the process. Results from a prototype version of the microscope are presented that demonstrate the advantages of the system over existing techniques.

© 1992 Optical Society of America

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References

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  1. C. W. See, R. K. Appel, M. G. Somekh, “Scanning differential optical interferometer for simultaneous measurement of amplitude and phase variation,” Appl. Phys. Lett. 53, 10–12 (1988).
    [CrossRef]
  2. L. J. Laub, “Apparatus and methods for scanning phase profilometry,” U.S. patent3,796,495 (12March1974).
  3. G. E. Sommargren, “Optical heterodyne profilometry,” Appl. Opt. 20, 610–618 (1981).
    [CrossRef] [PubMed]
  4. M. J. Downs, W. H. McGivern, H. J. Ferguson, “Optical system for measuring the profiles of super smooth surfaces,” Precis. Eng. 7, 211–215 (1985).
    [CrossRef]
  5. C. C. Huang, “Optical heterodyne profilometer,” Opt. Eng. 23, 365–370 (1981).
  6. D. Pantzer, J. Politch, L. Ek, “Heterodyne profiling instrument for the angstrom region,” Appl. Opt. 25, 4168–4172 (1986).
    [CrossRef] [PubMed]
  7. A. Korpel, R. L. Whitman, “Visualization of a coherent light field by heterodyning with a scanning laser beam,” Appl. Opt. 8, 1577–1580 (1969); see App. A.
    [CrossRef] [PubMed]
  8. M. J. Offside, M. G. Somekh, C. W. See, “Common path scanning heterodyne interferometer for absolute phase measurement,” Appl. Phys. Lett. 55, 2051–2053 (1989).
    [CrossRef]
  9. M. J. Offside, M. G. Somekh, “A phase sensitive optical heterodyne interferometer for surface height measurement,” Trans. Inst. M.C. Meas. Control 13, 115–124 (1991).
    [CrossRef]
  10. M. Born, E. Wolf, Principle of Optics (Pergamon, London, 1991), p. 254.
  11. T. Wilson, C. J. R. Sheppard, Theory and Practice of Scanning Optical Microscopy (Academic, London, 1984).
  12. D. K. Hamilton, C. J. R. Sheppard, “Differential phase contrast in scanning optical microscopy,” J. Microsc. 133, 27–39 (1984).
    [CrossRef]
  13. H. H. Hopkins, “On the diffraction theory of optical images,” Proc. R. Soc. London Ser. A 217, 408–432 (1953).
    [CrossRef]
  14. C. J. R. Sheppard, A. Choudhury, “Image formation in the scanning microscope,” Opt. Acta 24, 1051–1073 (1977).
    [CrossRef]
  15. C. J. R. Sheppard, T. Wilson, “Fourier imaging of phase information in scanning and conventional optical microscopes,” Philos. Trans. R. Soc. London 295, 513–536 (1980).
    [CrossRef]
  16. M. G. Somekh, R. K. Appel, “Image formation in common path differential profilometers,” in Surface Characterization and Testing II, J. E. Greivenkamp, M. Young, eds., Proc. Soc. Photo-Opt. Instrum. Eng.1164, 99–109 (1989).
  17. M. G. Somekh, “Image formation in scanned heterodyne microscope systems,” J. Microsc. 160, 225–243 (1990).
    [CrossRef]
  18. M. J. Offside, “A novel heterodyne interferometer for scanning optical microscopy,” Ph.D. dissertation (University of London, London, 1991).
  19. C. J. R. Sheppard, T. Wilson, “Fourier imaging of phase information in scanning and conventional optical microscopes,” Philos. Trans. R. Soc. London 295, 513–536 (1980).
    [CrossRef]
  20. E. L. O’Neill, “Transfer function for an annular aperture,” J. Opt. Soc. Am. 46, 285–288 (1956).
    [CrossRef]
  21. In practice this is an approximation caused by the effects of shadowing. However, the application of our instrument is primarily for the examination of objects with nanometer-scale topography for which the pure phase object representation is valid.
  22. K. Creath, “Calibration of numerical aperture effects in interferometric microscope objectives,” Appl. Opt. 28, 3333–3338 (1989).
    [CrossRef] [PubMed]

1991 (1)

M. J. Offside, M. G. Somekh, “A phase sensitive optical heterodyne interferometer for surface height measurement,” Trans. Inst. M.C. Meas. Control 13, 115–124 (1991).
[CrossRef]

1990 (1)

M. G. Somekh, “Image formation in scanned heterodyne microscope systems,” J. Microsc. 160, 225–243 (1990).
[CrossRef]

1989 (2)

M. J. Offside, M. G. Somekh, C. W. See, “Common path scanning heterodyne interferometer for absolute phase measurement,” Appl. Phys. Lett. 55, 2051–2053 (1989).
[CrossRef]

K. Creath, “Calibration of numerical aperture effects in interferometric microscope objectives,” Appl. Opt. 28, 3333–3338 (1989).
[CrossRef] [PubMed]

1988 (1)

C. W. See, R. K. Appel, M. G. Somekh, “Scanning differential optical interferometer for simultaneous measurement of amplitude and phase variation,” Appl. Phys. Lett. 53, 10–12 (1988).
[CrossRef]

1986 (1)

1985 (1)

M. J. Downs, W. H. McGivern, H. J. Ferguson, “Optical system for measuring the profiles of super smooth surfaces,” Precis. Eng. 7, 211–215 (1985).
[CrossRef]

1984 (1)

D. K. Hamilton, C. J. R. Sheppard, “Differential phase contrast in scanning optical microscopy,” J. Microsc. 133, 27–39 (1984).
[CrossRef]

1981 (2)

C. C. Huang, “Optical heterodyne profilometer,” Opt. Eng. 23, 365–370 (1981).

G. E. Sommargren, “Optical heterodyne profilometry,” Appl. Opt. 20, 610–618 (1981).
[CrossRef] [PubMed]

1980 (2)

C. J. R. Sheppard, T. Wilson, “Fourier imaging of phase information in scanning and conventional optical microscopes,” Philos. Trans. R. Soc. London 295, 513–536 (1980).
[CrossRef]

C. J. R. Sheppard, T. Wilson, “Fourier imaging of phase information in scanning and conventional optical microscopes,” Philos. Trans. R. Soc. London 295, 513–536 (1980).
[CrossRef]

1977 (1)

C. J. R. Sheppard, A. Choudhury, “Image formation in the scanning microscope,” Opt. Acta 24, 1051–1073 (1977).
[CrossRef]

1969 (1)

1956 (1)

1953 (1)

H. H. Hopkins, “On the diffraction theory of optical images,” Proc. R. Soc. London Ser. A 217, 408–432 (1953).
[CrossRef]

Appel, R. K.

C. W. See, R. K. Appel, M. G. Somekh, “Scanning differential optical interferometer for simultaneous measurement of amplitude and phase variation,” Appl. Phys. Lett. 53, 10–12 (1988).
[CrossRef]

M. G. Somekh, R. K. Appel, “Image formation in common path differential profilometers,” in Surface Characterization and Testing II, J. E. Greivenkamp, M. Young, eds., Proc. Soc. Photo-Opt. Instrum. Eng.1164, 99–109 (1989).

Born, M.

M. Born, E. Wolf, Principle of Optics (Pergamon, London, 1991), p. 254.

Choudhury, A.

C. J. R. Sheppard, A. Choudhury, “Image formation in the scanning microscope,” Opt. Acta 24, 1051–1073 (1977).
[CrossRef]

Creath, K.

Downs, M. J.

M. J. Downs, W. H. McGivern, H. J. Ferguson, “Optical system for measuring the profiles of super smooth surfaces,” Precis. Eng. 7, 211–215 (1985).
[CrossRef]

Ek, L.

Ferguson, H. J.

M. J. Downs, W. H. McGivern, H. J. Ferguson, “Optical system for measuring the profiles of super smooth surfaces,” Precis. Eng. 7, 211–215 (1985).
[CrossRef]

Hamilton, D. K.

D. K. Hamilton, C. J. R. Sheppard, “Differential phase contrast in scanning optical microscopy,” J. Microsc. 133, 27–39 (1984).
[CrossRef]

Hopkins, H. H.

H. H. Hopkins, “On the diffraction theory of optical images,” Proc. R. Soc. London Ser. A 217, 408–432 (1953).
[CrossRef]

Huang, C. C.

C. C. Huang, “Optical heterodyne profilometer,” Opt. Eng. 23, 365–370 (1981).

Korpel, A.

Laub, L. J.

L. J. Laub, “Apparatus and methods for scanning phase profilometry,” U.S. patent3,796,495 (12March1974).

McGivern, W. H.

M. J. Downs, W. H. McGivern, H. J. Ferguson, “Optical system for measuring the profiles of super smooth surfaces,” Precis. Eng. 7, 211–215 (1985).
[CrossRef]

O’Neill, E. L.

Offside, M. J.

M. J. Offside, M. G. Somekh, “A phase sensitive optical heterodyne interferometer for surface height measurement,” Trans. Inst. M.C. Meas. Control 13, 115–124 (1991).
[CrossRef]

M. J. Offside, M. G. Somekh, C. W. See, “Common path scanning heterodyne interferometer for absolute phase measurement,” Appl. Phys. Lett. 55, 2051–2053 (1989).
[CrossRef]

M. J. Offside, “A novel heterodyne interferometer for scanning optical microscopy,” Ph.D. dissertation (University of London, London, 1991).

Pantzer, D.

Politch, J.

See, C. W.

M. J. Offside, M. G. Somekh, C. W. See, “Common path scanning heterodyne interferometer for absolute phase measurement,” Appl. Phys. Lett. 55, 2051–2053 (1989).
[CrossRef]

C. W. See, R. K. Appel, M. G. Somekh, “Scanning differential optical interferometer for simultaneous measurement of amplitude and phase variation,” Appl. Phys. Lett. 53, 10–12 (1988).
[CrossRef]

Sheppard, C. J. R.

D. K. Hamilton, C. J. R. Sheppard, “Differential phase contrast in scanning optical microscopy,” J. Microsc. 133, 27–39 (1984).
[CrossRef]

C. J. R. Sheppard, T. Wilson, “Fourier imaging of phase information in scanning and conventional optical microscopes,” Philos. Trans. R. Soc. London 295, 513–536 (1980).
[CrossRef]

C. J. R. Sheppard, T. Wilson, “Fourier imaging of phase information in scanning and conventional optical microscopes,” Philos. Trans. R. Soc. London 295, 513–536 (1980).
[CrossRef]

C. J. R. Sheppard, A. Choudhury, “Image formation in the scanning microscope,” Opt. Acta 24, 1051–1073 (1977).
[CrossRef]

T. Wilson, C. J. R. Sheppard, Theory and Practice of Scanning Optical Microscopy (Academic, London, 1984).

Somekh, M. G.

M. J. Offside, M. G. Somekh, “A phase sensitive optical heterodyne interferometer for surface height measurement,” Trans. Inst. M.C. Meas. Control 13, 115–124 (1991).
[CrossRef]

M. G. Somekh, “Image formation in scanned heterodyne microscope systems,” J. Microsc. 160, 225–243 (1990).
[CrossRef]

M. J. Offside, M. G. Somekh, C. W. See, “Common path scanning heterodyne interferometer for absolute phase measurement,” Appl. Phys. Lett. 55, 2051–2053 (1989).
[CrossRef]

C. W. See, R. K. Appel, M. G. Somekh, “Scanning differential optical interferometer for simultaneous measurement of amplitude and phase variation,” Appl. Phys. Lett. 53, 10–12 (1988).
[CrossRef]

M. G. Somekh, R. K. Appel, “Image formation in common path differential profilometers,” in Surface Characterization and Testing II, J. E. Greivenkamp, M. Young, eds., Proc. Soc. Photo-Opt. Instrum. Eng.1164, 99–109 (1989).

Sommargren, G. E.

Whitman, R. L.

Wilson, T.

C. J. R. Sheppard, T. Wilson, “Fourier imaging of phase information in scanning and conventional optical microscopes,” Philos. Trans. R. Soc. London 295, 513–536 (1980).
[CrossRef]

C. J. R. Sheppard, T. Wilson, “Fourier imaging of phase information in scanning and conventional optical microscopes,” Philos. Trans. R. Soc. London 295, 513–536 (1980).
[CrossRef]

T. Wilson, C. J. R. Sheppard, Theory and Practice of Scanning Optical Microscopy (Academic, London, 1984).

Wolf, E.

M. Born, E. Wolf, Principle of Optics (Pergamon, London, 1991), p. 254.

Appl. Opt. (4)

Appl. Phys. Lett. (2)

M. J. Offside, M. G. Somekh, C. W. See, “Common path scanning heterodyne interferometer for absolute phase measurement,” Appl. Phys. Lett. 55, 2051–2053 (1989).
[CrossRef]

C. W. See, R. K. Appel, M. G. Somekh, “Scanning differential optical interferometer for simultaneous measurement of amplitude and phase variation,” Appl. Phys. Lett. 53, 10–12 (1988).
[CrossRef]

J. Microsc. (2)

D. K. Hamilton, C. J. R. Sheppard, “Differential phase contrast in scanning optical microscopy,” J. Microsc. 133, 27–39 (1984).
[CrossRef]

M. G. Somekh, “Image formation in scanned heterodyne microscope systems,” J. Microsc. 160, 225–243 (1990).
[CrossRef]

J. Opt. Soc. Am. (1)

Opt. Acta (1)

C. J. R. Sheppard, A. Choudhury, “Image formation in the scanning microscope,” Opt. Acta 24, 1051–1073 (1977).
[CrossRef]

Opt. Eng. (1)

C. C. Huang, “Optical heterodyne profilometer,” Opt. Eng. 23, 365–370 (1981).

Philos. Trans. R. Soc. London (2)

C. J. R. Sheppard, T. Wilson, “Fourier imaging of phase information in scanning and conventional optical microscopes,” Philos. Trans. R. Soc. London 295, 513–536 (1980).
[CrossRef]

C. J. R. Sheppard, T. Wilson, “Fourier imaging of phase information in scanning and conventional optical microscopes,” Philos. Trans. R. Soc. London 295, 513–536 (1980).
[CrossRef]

Precis. Eng. (1)

M. J. Downs, W. H. McGivern, H. J. Ferguson, “Optical system for measuring the profiles of super smooth surfaces,” Precis. Eng. 7, 211–215 (1985).
[CrossRef]

Proc. R. Soc. London Ser. A (1)

H. H. Hopkins, “On the diffraction theory of optical images,” Proc. R. Soc. London Ser. A 217, 408–432 (1953).
[CrossRef]

Trans. Inst. M.C. Meas. Control (1)

M. J. Offside, M. G. Somekh, “A phase sensitive optical heterodyne interferometer for surface height measurement,” Trans. Inst. M.C. Meas. Control 13, 115–124 (1991).
[CrossRef]

Other (6)

M. Born, E. Wolf, Principle of Optics (Pergamon, London, 1991), p. 254.

T. Wilson, C. J. R. Sheppard, Theory and Practice of Scanning Optical Microscopy (Academic, London, 1984).

L. J. Laub, “Apparatus and methods for scanning phase profilometry,” U.S. patent3,796,495 (12March1974).

M. G. Somekh, R. K. Appel, “Image formation in common path differential profilometers,” in Surface Characterization and Testing II, J. E. Greivenkamp, M. Young, eds., Proc. Soc. Photo-Opt. Instrum. Eng.1164, 99–109 (1989).

M. J. Offside, “A novel heterodyne interferometer for scanning optical microscopy,” Ph.D. dissertation (University of London, London, 1991).

In practice this is an approximation caused by the effects of shadowing. However, the application of our instrument is primarily for the examination of objects with nanometer-scale topography for which the pure phase object representation is valid.

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Figures (12)

Fig. 1
Fig. 1

Optical configuration of the microscope.

Fig. 2
Fig. 2

Line scan of the chrome-gold periodic track sample taken by using (a) the 0.65-NA drilled reflecting objective and (b) the 0.65-NA projecting objective.

Fig. 3
Fig. 3

Side view of the prototype microscope with a 30-cm steel rule to give scale.

Fig. 4
Fig. 4

Micrographs of the 20-nm track sample: (a) intensity (type I), (b) amplitude of the interference signal (type II), and (c) the phase of the interference signal.

Fig. 5
Fig. 5

Micrographs of the discrete edge sample: (a) intensity (type I), (b) amplitude of interference signal (type II), and (c) phase of the interference signal.

Fig. 6
Fig. 6

Micrograph of the 20-nm track sample taken with a Nomarski DIC microscope.

Fig. 7
Fig. 7

Micrograph of the 80-nm track sample taken with a reflection confocal microscope.

Fig. 8
Fig. 8

Response to a 2-deg phase step for (a) the probe interferometer and (b) the reference interferometer.

Fig. 9
Fig. 9

Geometry for the calculation of the phase of the reference interferometer as it scans over a phase step centered on x = 0.

Fig. 10
Fig. 10

Phasor diagram for calculating the resultant phase of the reference interferometer at a scan position x over a phase step.

Fig. 11
Fig. 11

Overall system response to a 2-deg phase step (a) over a scan distance that is comparable with the diameter of the sample reference beam and (b) in the local region of the step.

Fig. 12
Fig. 12

Experimental phase response to a topographic step on a plasma-etched silicon wafer.

Equations (14)

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S p = A p exp j ( ω o t + ϕ p + θ 1 ) ,
S r = A r exp j ( ω o t + ϕ r + θ 1 ) ,
R = B exp j [ ( ω o + 2 ω B t ) + ϕ c + θ 2 ] ,
I p = ξ G ( S p + R ) ( S p + R ) * ,
I p ξ G = A p 2 + B p 2 + 2 A p B p × cos [ 2 ω B t + ( ϕ c - ϕ p ) + ( θ 2 - θ 1 ) ] ,
ξ = η e h ν ,
I r = ξ G ( S r + R ) ( S r + R ) * ,
I r ξ G = A r 2 + B r 2 + 2 A r B r × cos [ 2 ω B t + ( ϕ c - ϕ r ) + ( θ 2 - θ 1 ) ] ,
I ( x ) = 2 Re [ W * exp ( 2 j ω B t ) - c ( m ) × T ( m ) exp ( 2 π j m x ) d m ] ,
c ( m ) = - P 1 ( m λ d - ξ , - η ) P 2 ( ξ , η ) d ξ d η ,
I con ( x ) = | - c ( m ) T ( m ) exp ( 2 π j m x ) d m | 2 .
t ( x ) = exp j H ( x ) θ ,
θ R ( x ) = tan - 1 { A ( x ) sin θ 1 + [ π r 2 - A ( x ) sin θ 2 ] A ( x ) cos θ 1 + [ π r 2 - A ( x ) cos θ 2 ] } ,
A ( x ) = r 2 cos - 1 ( - x r ) + x ( r 2 - x 2 ) 1 / 2 .

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