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Non-destructive optical inspection of evaporated fluoride glass layers
} O U R N A L OF
Journal of Non-Crystalline Solids 161 (1993) 81-85 North-Holland
~,~l~[ilIlI~
~0IlI~
Non-destructive optical inspection of evaporated fluoride glass layers P.C. M o n t g o m e r y
a, D. M o n t a n e r a, C. J a c o b o n i b, B. B o u l a r d b a n d J.P. F i l l a r d a
a Laboratoire LINCS, Centre d'Electronique de Montpellier, USTL, Place Eugdne Bataillon, 34095 Montpellier, France b Laboratoires des Fluorures, Facult~s des Sciences, Universitd du Maine, 72017 Le Mans, France
Fluoride PZG (PbF2, ZnF2, GaF3) glass layers have been successfully grown on metal, glass, and semiconductor substrates by co-evaporation under vacuum conditions. When doped with a suitably chosen rare earth element, the layer becomes an active medium which can be used for making integrated optical components. In this paper, the surface quality of the layers is inspected by means of a new non-destructive optical technique: phase stepping microscopy (PSM). Interferograms from a reflection microscope are automatically interpreted using computer assisted image processing to give synthesized images of the relief of the film surface and the substrate surface buried below the film. The resolution is 1 nm in the vertical direction and 0.6 Ixm laterally. Results are presented of the analysis of the quality of a PZG glass layer grown on a gold-plated brass substrate. Preliminary results on ZBLAN, GaAs and InP substrates are also discussed.
1. Introduction In the d e v e l o p m e n t of integrated optical components, one m e t h o d p r o p o s e d for optical communication b e t w e e n devices is to use a thin film of glass deposited on the substrate, which acts as an optical waveguide. D o p i n g the glass film with a rare earth element p r o d u c e s an active m e d i u m , giving the additional advantage of optical signal amplification. T h e L a b o r a t o i r e des Fluorures at the Universite du Maine has succeeded in mastering the technique of depositing thin layers of P b F 2, ZnF2 and G a F 3 ( P Z G fluoride glass) on fiat substrates by co-evaporation u n d e r v a c u u m conditions [1,2]. Various m e t h o d s exist for doping the glass with a rare earth element. T h e technique used in the present work was by high energy b e a m implantation [3]. In this c o m m u n i c a t i o n we report on the results of analyzing the surface quality o f a glass film on
Correspondence to: Dr P.C. Montgomery, Laboratoire LINCS, Centre d'Electronique de Montpellier, USTL, Place Eugbne Bataillon, 34095 Montpellier, France. Tel: +33 67 54 45 29. Telefax: + 33 67 54 71 34.
a gold-plated brass substrate using recently develo p e d optical microscopy techniques. Based on interferometry and automatic fringe interpretation using digital image processing, two measurem e n t m o d e s are available: phase stepping microscopy (PSM) for profiling shallow surfaces of up to 0.5 txm [4,5], and p e a k fringe scanning microscopy ( P F S M ) for profiling r o u g h e r surfaces which are multi-micron in height [6]. These are used to study surface roughness and film thickness. Some preliminary work on glass films deposited on Z B L A N (ZrF4, BaF2, LaF3, AIF3, N a F ) glass, G a A s and I n P substrates is also discussed.
2. Experimental A layout of the m e a s u r e m e n t system is shown in fig. 1. T h e instrument consists of a reflection microscope, a C C D c a m e r a and a digital image processing system. T h e interference objective is based on a Michelson interferometer. In the PSM mode, a high intensity red L E D (wavelength of 660 nm) is used for illumination. This acts as a convenient q u a s i - m o n o c h r o m a t i c light source,
0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
82
P.C. Montgomery et aL / Optical inspection of evaporated fluoride glass layers
The PFSM technique works on a different principle [6]. The zero order fringe in the envelope of the white light fringe pattern is used as a probe plane to identify a particular height on the sample. Stepping the sample through the plane, successive contours are identified using image processing until the whole of the relief is profiled. In the present system, features up to 15 I~m in height can be profiled with a vertical resolution of 4-10 nm. PFSM can also be used to measure the thickness of a thin transparent film at a given point. This is done by noting the height variation between the zero order fringe at the substrate surface and at the glass film surface. When this work was carried out, the thickness measurement technique had not been fully calibrated in order to compensate for the effects of the refractive index of the medium being measured. Thickness measurements given in the results are therefore approximate. Operation of the microscope and analysis of the results has been facilitated by the development our own menu driven software system. The ability to efficiently process, store and analyze synthesized images is an important aspect of correctly interpreting this type of data. It must be noted that the greyscale images produced by PSM and PFSM are not variations in reflectivity as would be obtained in an ordinary microscope, but are synthesized images in which the greyscale represents height. The sample consisted of a PZG film made by evaporation of a melted vitreous matrix. This
producing a large number of smooth fringes in the interferometer. For carrying out PFSM, white light illumination is brought to the objective by a fiber optic bundle, which produces white light interference fringes. The video image of the fringes superimposed on the sample surface is digitized in a frame grabber which is controlled by a PC 386 computer. The computer also controls the sample stage height through a buffer amplifier connected to the piezo-electric element. There are separate monitors for viewing the live video image and the processed image. Results can be printed on a video printer. A direct image of the surface of the sample consists of a fringe pattern superimposed on the relief of the surface. In the PSM mode, the height control is used to step the sample through three heights, to introduce three phase steps of - 1 2 0 °, 0°, and + 120°. The intensities in the three subsequent images, 11, 12 and 13, can then be used to calculate the height, h(x, y), directly at each pixel using the formula
h ( x , y) = ~
arctan ( 2 1 2 _ i 1 _ I 3 )
.
Frame integration is used to reduce camera noise. The vertical resolution of PSM is 1 nm with an estimated precision of better than 10%. General details of the phase stepping technique are given in ref. [4], and information about our particular PSM instrument can be found in ref.
[51.
I ,LLU=ONrl
Iq
FRAME ] GRABBER
CCDCAMERA I
BUN[:)LE,~rjL~J OBJECTIVE
I
~
HEIGHT/~,i--CONTROL
SAMPLE
I I PC 386 COMPUTER I
UFFER AMP
Fig. 1. Layoutof PSM and PFSMsystem.
VIDEO CONTROL
P.C. Montgomery et a L / Optical inspection of euaporated fluoride glass layers
BRASS SUBSTRATE PZG GLASS: IMPLANTED ZONE PZG GLASS: NON-IMPLANTED ZONE
Fig. 2. Sample details of PZG glass with implanted zone of erbium ions on brass substrate.
consisted of the volatile fluorides PbF2, G a F 3 and Z n F 2 'diluted' in a mixture corresponding to a glass with a very low vapour pressure (technical details are given in refs. [1,2]. Implantation was
83
obtained by subjecting the film to a b e a m of high energy erbium ions (3.5 MeV) for 8 h (using the A R A M I S accelerator [3]). The substrate has to be a good thermal conductor so as to avoid overheating of the target by the high energy beam. Gold-plated brass was therefore chosen as the substrate. The layer of gold prevents the reduction of the PbF 2 by the zinc at the substrate/film interface. A diagram of the sample and the implanted zone is shown in fig. 2. After carrying out trial measurements, it was found that the PSM m o d e was suitable for measuring the surface of the glass layer, while the PFSM mode was needed to measure the greater
(b)
Fig. 3. PFSM relief of brass substrate through glass layer showing long scratch marks up to 2.5 p.m deep and 101xm wide. (a) Greyscale image. (b) Line profile from (a).
HEIGHT
(b)
Fig. 4. PSM relief of PZG glass surface in implanted zone showing 0.25 Ixm deep pits and I0-30 ixm wide cells. (a) Greyscale image. (b) Line profile from (a).
P.C. Montgomery et aL / Optical inspection of evaporated fluoride glass layers
84
deviations in surface height of the metal surface buried below the layer,
GaAs and InP substrates showed that the films had a very high surface smoothness.
3. Results
4. Discussion
The results of the measurement of the metal substrate surface using PFSM in the implanted zone are shown in fig. 3. An interesting aspect of this image is that it was made through the glass film. The surface is very rough, with long scratch marks approximately 0.2-2.5 Ixm deep and up to 10 Ixm wide. Similar results were found on the bare metal surface as well as in the area of the film which had not undergone implantation. On the surface of the glass layer in the implanted zone, the relief was found to be shallower than that of the metal surface, making it possible to use PSM. The results in fig. 4 show the surface to be gently undulating, with a variation in surface height of 0.25 Ixm and a horizontal feature size of 10-30 Ixm. The relief follows very roughly that of the metal surface (compare fig. 4(b) with fig. 3(b) for example) but with the difference that the glass surface is smoother. The results using the PFSM technique to measure the thickness of the glass film along the profile AA' indicated in fig. 2 are shown in fig. 5. The profile shows a thickening of the film in the implanted zone. Measurements of the relief of a PZG glass layer on a ZBLAN glass substrate indicated a slight roughness that was also related to that of the substrate surface. Preliminary studies of PZG glass layers on flat, well polished
In this paper, we have presented the use of optical interferometry for measuring the surface roughness and the thickness of glass layers on different substrates. We have also demonstrated the possibility of measuring the roughness of a surface buried below the glass which would not have been possible with a stylus-type measuring system. Measurements of the surface roughness of a glass film on gold-plated brass have shown that the quality of the surface of the film appears to be related to that of the substrate. A rough substrate leads to a rough glass layer. This was also found to be true for a PZG film deposited on a ZBLAN glass substrate. Some preliminary studies of glass layers on GaAs and InP substrates show that high quality glass film surfaces can be formed when the wafer polish is very good. An analysis of the thickness of the film on the metal substrate shows that high energy beam implantation of the glass with erbium results in an uneven thickening of the film, and in a smoothing of the surface roughness. These effects are probably due to heating of the film to a temperature close to the vitreous transition temperature (Tg = 270°C for a PZG glass) induced by the implantation process. Further work is required to improve the understanding of the various factors influencing the phase changes in the illumination beam, in order to calibrate the thickness measurement technique.
THICKNESS (.am) 4 A
limit of implanted zone A'
5. Conclusion
3 2 1 i
0 0
2
4
6
8
I
I0 12 14 DISTANCE(mm)
Fig. 5. Variation in film thickness (approximate) due to ion implantation as measured by PFSM.
Two optical surface profiling techniques, phase stepping microscopy (PSM) and peak fringe scanning microscopy (PFSM) have been used to analyze the surface quality of PZG fluoride glass films deposited on various substrates. Results have been shown of the analysis of a PZG glass film on a metal substrate, and preliminary results of glass films on ZBLAN, GaAs and InP sub-
P.C. Montgomery et al. / Optical inspection of e~'aporated fluoride glass layers s t r a t e s have b e e n discussed. T h e use o f o p t i c a l t e c h n i q u e s has e n a b l e d m e a s u r e m e n t s to b e m a d e of t h e s u b s t r a t e s u r f a c e b u r i e d b e l o w t h e transp a r e n t layer in o r d e r to c o m p a r e t h e two surfaces. T h e quality of t h e glass layer has b e e n o b s e r v e d to b e l i n k e d to t h e quality o f t h e subs t r a t e surface. T o p r o d u c e a high quality glass surface, t h e quality o f t h e s u r f a c e p o l i s h o f t h e s u b s t r a t e n e e d s to b e very good. I m p l a n t a t i o n o f t h e glass film was c a r r i e d o u t by H. B e r n a s a n d J. C h a u m o n t at t h e C e n t r e d e S p e c t r o s c o p i c d e M a s s e et S p e c t r o m 6 t r i e Nucl6aire (Orsay, F r a n c e ) w i t h i n t h e A R A M I S p r o gram.
85
References [1] C. Jacoboni and B. Boulard, French patent no. 89-01368 (1989). [2] B. Boulard and C. Jacoboni, Mater. Sci. Forum 25 (1990) 671. [3] E. Cottereau, J. Camplan, J. Chaumont and R. Meunier, Mater. Sci. Eng. B2 (1989) 217. [4] H.P. Stahl, in: Proc. Optical Testing and Metrology III: Recent Advances in Industrial Inspection, ed. C.P. Grover, SPIE 1332 (1990) 704. [5] P.C. Montgomery, J.P. Fillard, N. Tchandjou and S. Ardisasmita, in: Proc. Optical Testing and Metrology III: Recent Advances in Industrial Optical Inspection, ed. C.P. Grover, SPIE 1322 (1990) 515. [6] P.C. Montgomery and J.P. Fillard, in: Proc. Interferometry: Techniques and Analysis, SPIE 1755 (1992) 12.
Journal of Non-Crystalline Solids 161 (1993) 81-85 North-Holland
~,~l~[ilIlI~
~0IlI~
Non-destructive optical inspection of evaporated fluoride glass layers P.C. M o n t g o m e r y
a, D. M o n t a n e r a, C. J a c o b o n i b, B. B o u l a r d b a n d J.P. F i l l a r d a
a Laboratoire LINCS, Centre d'Electronique de Montpellier, USTL, Place Eugdne Bataillon, 34095 Montpellier, France b Laboratoires des Fluorures, Facult~s des Sciences, Universitd du Maine, 72017 Le Mans, France
Fluoride PZG (PbF2, ZnF2, GaF3) glass layers have been successfully grown on metal, glass, and semiconductor substrates by co-evaporation under vacuum conditions. When doped with a suitably chosen rare earth element, the layer becomes an active medium which can be used for making integrated optical components. In this paper, the surface quality of the layers is inspected by means of a new non-destructive optical technique: phase stepping microscopy (PSM). Interferograms from a reflection microscope are automatically interpreted using computer assisted image processing to give synthesized images of the relief of the film surface and the substrate surface buried below the film. The resolution is 1 nm in the vertical direction and 0.6 Ixm laterally. Results are presented of the analysis of the quality of a PZG glass layer grown on a gold-plated brass substrate. Preliminary results on ZBLAN, GaAs and InP substrates are also discussed.
1. Introduction In the d e v e l o p m e n t of integrated optical components, one m e t h o d p r o p o s e d for optical communication b e t w e e n devices is to use a thin film of glass deposited on the substrate, which acts as an optical waveguide. D o p i n g the glass film with a rare earth element p r o d u c e s an active m e d i u m , giving the additional advantage of optical signal amplification. T h e L a b o r a t o i r e des Fluorures at the Universite du Maine has succeeded in mastering the technique of depositing thin layers of P b F 2, ZnF2 and G a F 3 ( P Z G fluoride glass) on fiat substrates by co-evaporation u n d e r v a c u u m conditions [1,2]. Various m e t h o d s exist for doping the glass with a rare earth element. T h e technique used in the present work was by high energy b e a m implantation [3]. In this c o m m u n i c a t i o n we report on the results of analyzing the surface quality o f a glass film on
Correspondence to: Dr P.C. Montgomery, Laboratoire LINCS, Centre d'Electronique de Montpellier, USTL, Place Eugbne Bataillon, 34095 Montpellier, France. Tel: +33 67 54 45 29. Telefax: + 33 67 54 71 34.
a gold-plated brass substrate using recently develo p e d optical microscopy techniques. Based on interferometry and automatic fringe interpretation using digital image processing, two measurem e n t m o d e s are available: phase stepping microscopy (PSM) for profiling shallow surfaces of up to 0.5 txm [4,5], and p e a k fringe scanning microscopy ( P F S M ) for profiling r o u g h e r surfaces which are multi-micron in height [6]. These are used to study surface roughness and film thickness. Some preliminary work on glass films deposited on Z B L A N (ZrF4, BaF2, LaF3, AIF3, N a F ) glass, G a A s and I n P substrates is also discussed.
2. Experimental A layout of the m e a s u r e m e n t system is shown in fig. 1. T h e instrument consists of a reflection microscope, a C C D c a m e r a and a digital image processing system. T h e interference objective is based on a Michelson interferometer. In the PSM mode, a high intensity red L E D (wavelength of 660 nm) is used for illumination. This acts as a convenient q u a s i - m o n o c h r o m a t i c light source,
0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
82
P.C. Montgomery et aL / Optical inspection of evaporated fluoride glass layers
The PFSM technique works on a different principle [6]. The zero order fringe in the envelope of the white light fringe pattern is used as a probe plane to identify a particular height on the sample. Stepping the sample through the plane, successive contours are identified using image processing until the whole of the relief is profiled. In the present system, features up to 15 I~m in height can be profiled with a vertical resolution of 4-10 nm. PFSM can also be used to measure the thickness of a thin transparent film at a given point. This is done by noting the height variation between the zero order fringe at the substrate surface and at the glass film surface. When this work was carried out, the thickness measurement technique had not been fully calibrated in order to compensate for the effects of the refractive index of the medium being measured. Thickness measurements given in the results are therefore approximate. Operation of the microscope and analysis of the results has been facilitated by the development our own menu driven software system. The ability to efficiently process, store and analyze synthesized images is an important aspect of correctly interpreting this type of data. It must be noted that the greyscale images produced by PSM and PFSM are not variations in reflectivity as would be obtained in an ordinary microscope, but are synthesized images in which the greyscale represents height. The sample consisted of a PZG film made by evaporation of a melted vitreous matrix. This
producing a large number of smooth fringes in the interferometer. For carrying out PFSM, white light illumination is brought to the objective by a fiber optic bundle, which produces white light interference fringes. The video image of the fringes superimposed on the sample surface is digitized in a frame grabber which is controlled by a PC 386 computer. The computer also controls the sample stage height through a buffer amplifier connected to the piezo-electric element. There are separate monitors for viewing the live video image and the processed image. Results can be printed on a video printer. A direct image of the surface of the sample consists of a fringe pattern superimposed on the relief of the surface. In the PSM mode, the height control is used to step the sample through three heights, to introduce three phase steps of - 1 2 0 °, 0°, and + 120°. The intensities in the three subsequent images, 11, 12 and 13, can then be used to calculate the height, h(x, y), directly at each pixel using the formula
h ( x , y) = ~
arctan ( 2 1 2 _ i 1 _ I 3 )
.
Frame integration is used to reduce camera noise. The vertical resolution of PSM is 1 nm with an estimated precision of better than 10%. General details of the phase stepping technique are given in ref. [4], and information about our particular PSM instrument can be found in ref.
[51.
I ,LLU=ONrl
Iq
FRAME ] GRABBER
CCDCAMERA I
BUN[:)LE,~rjL~J OBJECTIVE
I
~
HEIGHT/~,i--CONTROL
SAMPLE
I I PC 386 COMPUTER I
UFFER AMP
Fig. 1. Layoutof PSM and PFSMsystem.
VIDEO CONTROL
P.C. Montgomery et a L / Optical inspection of euaporated fluoride glass layers
BRASS SUBSTRATE PZG GLASS: IMPLANTED ZONE PZG GLASS: NON-IMPLANTED ZONE
Fig. 2. Sample details of PZG glass with implanted zone of erbium ions on brass substrate.
consisted of the volatile fluorides PbF2, G a F 3 and Z n F 2 'diluted' in a mixture corresponding to a glass with a very low vapour pressure (technical details are given in refs. [1,2]. Implantation was
83
obtained by subjecting the film to a b e a m of high energy erbium ions (3.5 MeV) for 8 h (using the A R A M I S accelerator [3]). The substrate has to be a good thermal conductor so as to avoid overheating of the target by the high energy beam. Gold-plated brass was therefore chosen as the substrate. The layer of gold prevents the reduction of the PbF 2 by the zinc at the substrate/film interface. A diagram of the sample and the implanted zone is shown in fig. 2. After carrying out trial measurements, it was found that the PSM m o d e was suitable for measuring the surface of the glass layer, while the PFSM mode was needed to measure the greater
(b)
Fig. 3. PFSM relief of brass substrate through glass layer showing long scratch marks up to 2.5 p.m deep and 101xm wide. (a) Greyscale image. (b) Line profile from (a).
HEIGHT
(b)
Fig. 4. PSM relief of PZG glass surface in implanted zone showing 0.25 Ixm deep pits and I0-30 ixm wide cells. (a) Greyscale image. (b) Line profile from (a).
P.C. Montgomery et aL / Optical inspection of evaporated fluoride glass layers
84
deviations in surface height of the metal surface buried below the layer,
GaAs and InP substrates showed that the films had a very high surface smoothness.
3. Results
4. Discussion
The results of the measurement of the metal substrate surface using PFSM in the implanted zone are shown in fig. 3. An interesting aspect of this image is that it was made through the glass film. The surface is very rough, with long scratch marks approximately 0.2-2.5 Ixm deep and up to 10 Ixm wide. Similar results were found on the bare metal surface as well as in the area of the film which had not undergone implantation. On the surface of the glass layer in the implanted zone, the relief was found to be shallower than that of the metal surface, making it possible to use PSM. The results in fig. 4 show the surface to be gently undulating, with a variation in surface height of 0.25 Ixm and a horizontal feature size of 10-30 Ixm. The relief follows very roughly that of the metal surface (compare fig. 4(b) with fig. 3(b) for example) but with the difference that the glass surface is smoother. The results using the PFSM technique to measure the thickness of the glass film along the profile AA' indicated in fig. 2 are shown in fig. 5. The profile shows a thickening of the film in the implanted zone. Measurements of the relief of a PZG glass layer on a ZBLAN glass substrate indicated a slight roughness that was also related to that of the substrate surface. Preliminary studies of PZG glass layers on flat, well polished
In this paper, we have presented the use of optical interferometry for measuring the surface roughness and the thickness of glass layers on different substrates. We have also demonstrated the possibility of measuring the roughness of a surface buried below the glass which would not have been possible with a stylus-type measuring system. Measurements of the surface roughness of a glass film on gold-plated brass have shown that the quality of the surface of the film appears to be related to that of the substrate. A rough substrate leads to a rough glass layer. This was also found to be true for a PZG film deposited on a ZBLAN glass substrate. Some preliminary studies of glass layers on GaAs and InP substrates show that high quality glass film surfaces can be formed when the wafer polish is very good. An analysis of the thickness of the film on the metal substrate shows that high energy beam implantation of the glass with erbium results in an uneven thickening of the film, and in a smoothing of the surface roughness. These effects are probably due to heating of the film to a temperature close to the vitreous transition temperature (Tg = 270°C for a PZG glass) induced by the implantation process. Further work is required to improve the understanding of the various factors influencing the phase changes in the illumination beam, in order to calibrate the thickness measurement technique.
THICKNESS (.am) 4 A
limit of implanted zone A'
5. Conclusion
3 2 1 i
0 0
2
4
6
8
I
I0 12 14 DISTANCE(mm)
Fig. 5. Variation in film thickness (approximate) due to ion implantation as measured by PFSM.
Two optical surface profiling techniques, phase stepping microscopy (PSM) and peak fringe scanning microscopy (PFSM) have been used to analyze the surface quality of PZG fluoride glass films deposited on various substrates. Results have been shown of the analysis of a PZG glass film on a metal substrate, and preliminary results of glass films on ZBLAN, GaAs and InP sub-
P.C. Montgomery et al. / Optical inspection of e~'aporated fluoride glass layers s t r a t e s have b e e n discussed. T h e use o f o p t i c a l t e c h n i q u e s has e n a b l e d m e a s u r e m e n t s to b e m a d e of t h e s u b s t r a t e s u r f a c e b u r i e d b e l o w t h e transp a r e n t layer in o r d e r to c o m p a r e t h e two surfaces. T h e quality of t h e glass layer has b e e n o b s e r v e d to b e l i n k e d to t h e quality o f t h e subs t r a t e surface. T o p r o d u c e a high quality glass surface, t h e quality o f t h e s u r f a c e p o l i s h o f t h e s u b s t r a t e n e e d s to b e very good. I m p l a n t a t i o n o f t h e glass film was c a r r i e d o u t by H. B e r n a s a n d J. C h a u m o n t at t h e C e n t r e d e S p e c t r o s c o p i c d e M a s s e et S p e c t r o m 6 t r i e Nucl6aire (Orsay, F r a n c e ) w i t h i n t h e A R A M I S p r o gram.
85
References [1] C. Jacoboni and B. Boulard, French patent no. 89-01368 (1989). [2] B. Boulard and C. Jacoboni, Mater. Sci. Forum 25 (1990) 671. [3] E. Cottereau, J. Camplan, J. Chaumont and R. Meunier, Mater. Sci. Eng. B2 (1989) 217. [4] H.P. Stahl, in: Proc. Optical Testing and Metrology III: Recent Advances in Industrial Inspection, ed. C.P. Grover, SPIE 1332 (1990) 704. [5] P.C. Montgomery, J.P. Fillard, N. Tchandjou and S. Ardisasmita, in: Proc. Optical Testing and Metrology III: Recent Advances in Industrial Optical Inspection, ed. C.P. Grover, SPIE 1322 (1990) 515. [6] P.C. Montgomery and J.P. Fillard, in: Proc. Interferometry: Techniques and Analysis, SPIE 1755 (1992) 12.