Long-working-distance incoherent-light interference microscope

Long-working-distance incoherent-lightinterference microscope

Michael B. Sinclair, Maarten P. de Boer, and Alex D. Corwin

We describe the design and operation of a long-working-distance, incoherent light interference micro-scope that has been developed to address the growing demand for new microsystem characterizationtools. The design of the new microscope is similar to that of a Linnik interference microscope and thuspreserves the full working distance of the long-working-distance objectives utilized. However, in contrastto a traditional Linnik microscope, the new microscope does not rely on the use of matched objectives inthe sample and the reference arms of the interferometer. An adjustable optical configuration has beendevised that allows the total optical path length, wavefront curvature, and dispersion of the reference armto be matched to the sample arm of the interferometer. The reference arm configuration can be adjustedto provide matching for 5�, 10�, and 20� long-working-distance objectives in the sample arm. Inaddition to retaining the full working distance of the sample arm objectives, the new design allows inter-ference images to be acquired in situations in which intervening windows are necessary, such as occur withpackaged microsystems, microfluidic devices, and cryogenic, vacuum, or environmental chamber studies ofmicrosystem performance. The interference microscope is compatible with phase-shifting interferometry,vertical scanning interferometry, and stroboscopic measurement of dynamic processes. © 2005 OpticalSociety of America

OCIS codes: 120.3180, 180.3170.

1. Introduction

Over the past few years, increasing emphasis has beenplaced on the development of miniaturized mechani-cal, electromechanical, optomechanical, and microflu-idic systems. These microsystems can offer significantadvantages over traditional systems, such as smallform factors, highly robust construction, low powerconsumption, and manufacture by precise, high-volume microlithographic processes developed initiallyfor the semiconductor industry. As a consequence, thedemand for new tools and instruments capable of char-acterizing the fabrication and performance of micro-systems has also increased. Interference microscopyhas received considerable attention in this regard,since it provides accurate, noncontact measurementof the out-of-plane motion of microfabricated com-ponents.1–4 As such, it can be used to measure theperformance of moving components, as well as distor-tions of structures that are due to stress relaxation.

When coupled with appropriate test structures andnumerical analysis, interference microscopy can alsobe used for the determination of material propertiessuch as Young’s modulus, stress gradients and beamtakeoff angles,5 residual stress levels,6 adhesion,7,8 andadhesion hysteresis.9 However, for a variety of rea-sons, it is often difficult to apply traditional interfer-ence microscopy to microsystem characterization. Asdescribed below, traditional interference objectives arecharacterized by relatively short working distances.This complicates their use in cases in which it is nec-essary to place components such as actuating electricalprobes or probe cards between the device under testand the objective. In addition, there are many in-stances in which one wishes to perform interferencemeasurements on devices in windowed packages orlocated in cryogenic, vacuum, or environmental cham-bers. Here, the short working distance, combined withthe difficulty associated with compensating for thewindow, prevents the use of traditional interferenceobjectives. Thus the development of new configura-tions for interference microscopy, tailored to the needsof microsystem characterization, is of interest.

The field of interference microscopy is relatively ma-ture, and many different microscope designs have beendeveloped to address wide-ranging applications. De-signs appearing in the literature span the range from

The authors are with Sandia National Laboratories, Albuquer-que, New Mexico 87185.

Received 29 June 2005; revised 24 August 2005; accepted 24August 2005.

0003-6935/05/367714-08$15.00/0© 2005 Optical Society of America

7714 APPLIED OPTICS � Vol. 44, No. 36 � 20 December 2005

point-scanning instruments10–12 to whole-field imag-ing systems13–20 and from laser-illuminated instru-ments4,10,12 to incoherently illuminated systems.3,13–20

The present work is devoted to an incoherently illu-minated, whole-field imaging interference microscope.Incoherent illumination was selected to avoid difficul-ties associated with coherent light imaging and to takeadvantage of the excellent image quality provided byincoherent light illumination. In addition, the finitefringe envelope obtained with incoherent illuminationallows vertical scanning interferometry (VSI) tech-niques to be utilized to determine the magnitude ofstep discontinuities.

Incoherent light interference microscopes mustachieve precise optical-path-length matching betweenthe sample and the reference arms of the interferom-eter because of the short coherence length associatedwith incoherent illumination. Such microscopes can beloosely categorized into two main groups: microscopesutilizing interference objectives and Linnik-type mi-croscopes.13,14 Interference objectives, such as Mirauand Michelson objectives,15,18 are relatively inexpen-sive, easy to use, and, for appropriate specimens, pro-duce excellent interferograms. With such objectives,the interferometer is arranged in the space betweenthe final optical element of the objective and the spec-imen. This leads to two distinct advantages. First, theoptical path length of each arm of the interferometer issmall ��5–10 mm�, enhancing the stability of the in-terference images. Second, since the interferometer islocated after the image-forming optics, few compo-nents are needed, and it is easy to match the opticalcharacteristics (such as wavefront and dispersion) ofthe two arms. The primary drawback of these inter-ference objectives is lack of working distance—theinterferometer optics located between the objectiveand the specimen utilizes the majority of the objec-tive’s working distance, leaving only a small workingdistance between the objective housing and the spec-imen. A second disadvantage of conventional inter-ference objectives is their inability to produce inter-ferograms when a window is positioned between theobjective and the sample under investigation. Al-though, in principle, a matched window could beplaced in the reference arm of the interferometer, inpractice this is difficult to achieve and usually re-quires some disassembly of the objective.

The Linnik interference microscope13,14 overcomesthe shortcomings of the interference objective by po-sitioning the two arms of the interferometer aroundthe primary beam splitter of the microscope. Nomi-nally identical objectives are used in both arms toensure a high degree of optical matching. In this way,the full working distance of the objective is retained.In addition, the Linnik approach is more amenable tothe inclusion of windows on both the sample and thereference arms of the interferometer. A significantdisadvantage of the Linnik approach is the expenseassociated with the purchase of an additional objec-tive for each desired magnification and the degree towhich the sample and the reference objectives mustbe matched. Typical manufacturing tolerances do not

ensure that any two “identical” objectives are suffi-ciently well matched. Rather, matched pairs must beselected from a pool of “identical” objectives. Thus,obtaining such a pair of matched objectives can entailconsiderable expense. In addition, one must repeatthe selection process for each desired magnification.

In this paper, we describe the design and operationof a long-working-distance incoherent light interfer-ence microscope that has been developed to addressthe growing demand for new microsystem character-ization tools. The design of the new microscope ismost similar to that of a Linnik interference micro-scope and thus preserves the full working distance ofthe long-working-distance objectives utilized. How-ever, in contrast to a traditional Linnik microscope,the new microscope does not rely on the use ofmatched objectives in the sample and the referencearms of the interferometer. An adjustable optical con-figuration has been devised that allows the totaloptical path length, wavefront curvature, and disper-sion of the reference arm to be matched to the samplearm of the interferometer. The reference arm config-uration can be adjusted to provide matching for5�, 10�, and 20� long-working-distance objectivesin the sample arm. In addition to retaining the fullworking distance of the sample arm objectives, thenew design allows interference images to be acquiredin situations in which intervening windows are nec-essary, such as occur with packaged microsystems,microfluidic devices, or when the device of interest islocated in a cryogenic, vacuum, or environmentalchamber.

2. Microscope Design

To produce high-quality interferograms by use of in-coherent illumination, an interference microscopemust satisfy four criteria. First, it is obvious that thesample and the reference objectives must be accu-rately focused on the specimen and reference surface,respectively. The second criterion is that the overallgroup delay experienced by a wave packet traversingthe sample arm of the interferometer be equal to thegroup delay of the reference arm. A wave packet in-cident upon the interferometer will be divided intotwo identical copies by the beam splitter. The twocopies traverse the two arms of the interferometerand are recombined at the beam splitter. Coherentrecombination will occur only if both wave packetsarrive at the beam splitter within a time intervalgiven by the coherence time of the illuminationsource. The third criterion for the production of high-quality interferograms is that the total group-velocitydispersion present in the two arms of the interferom-eter be matched. For a perfectly balanced interferom-eter, identical amounts of dispersion occur in eacharm, and the net phase difference between the twoarms is constant across the bandwidth of the illumi-nation source. Fringe visibility is decreased for animbalanced interferometer, as the phase differencewill now vary with wavelength and it is no longerpossible to achieve perfect constructive or destructiveinterference across the bandwidth of the source. The

20 December 2005 � Vol. 44, No. 36 � APPLIED OPTICS 7715

fourth criterion is that the positions of the back focalplanes of the objectives in the two arms of the inter-ferometer be equidistant from the beam splitter. Thiswill ensure that the curvatures of the coherently re-combining wavefronts are identical and that flat in-terference fringes are obtained. If this condition is notfulfilled, the interference fringes will exhibit a char-acteristic “bull’s-eye” pattern.

A schematic of the optical layout of the interferom-eter is shown in Fig. 1(a), and Fig. 1(b) shows aphotograph of the interferometer coupled with a com-puterized probe station for wafer-level testing ofmaterial properties. The interference microscope isconstructed primarily from commercially available op-tics and optomechanics. The illumination source is agreen light-emitting diode (LED). Green LED illumi-nation was chosen for its relative brightness, its com-patibility with high-bandwidth modulation that isrequired for stroboscopic measurements, and becausethe green wavelengths are strongly absorbed by poly-crystalline silicon (polysilicon), thereby eliminatingpotential artifacts. The emission from this diode peaksat 525 nm, and the full width at half-maximum of theemission spectrum is approximately 40 nm. For usein the interferometer, the LED is embedded in analuminum disk and the front lens of the LED islapped and polished, resulting in a flat front face ofthe LED package located less than 1 mm from the

emitting element. The emitting element is positionedin the front focal plane of an infinity-corrected 10�microscope objective, and a field iris is located imme-diately after the objective. The field iris is, in turn,located in the front focal plane of a converging lens�88.9-mm focal length), which relays the image of thesource onto the back focal planes of the sample andthe reference objectives. Note that the illuminationsystem slightly underfills the pupils of the referenceand sample objectives to ensure equal throughoutfor both objectives. The beam splitter utilized is abroadband, nonpolarizing, cube-type beam splitter.The sample arm of the interferometer contains aninfinity-corrected long-working-distance Mitutoyo MPlan Apo objective. The microscope is optimized toaccept objectives with three different magnifications:a 5�, NA 0.14, objective with a 34-mm working dis-tance, a 10�, NA 0.28, objective with a 33.5-mmworking distance, and a 20�, NA 0.42, objective witha 20.0-mm working distance. The sample under in-vestigation is positioned at the front focal plane ofthis objective. Thus the sample plane is conjugatewith the plane of the field iris. The radiation reflectedfrom the sample returns through the objective,passes through the beam splitter, and is focused atthe plane of a CCD camera by means of a 200-mmfocal-length tube lens.

The reference arm of the interferometer is morecomplex than the sample arm because it contains theadjustable components required to allow matching ofdifferent sample arm objectives. The reference objec-tive is an achromatic doublet �31.0-mm focal length),and the reference mirror is positioned at its frontfocal plane, which is also conjugate with the fieldstop. The reference mirror is mounted on a piezoelec-tric actuator to facilitate phase-shifting interferome-try (PSI). Two types of mirrors are employed: a firstsurface metal mirror for use with highly reflectivesamples or a silicon mirror to optimize fringe contrastwhen the interferometer is to be used on polysiliconMicroelectromechanical systems (MEMS). To facili-tate matching of the optical path lengths, the entirereference arm assembly is constructed on a lineartranslation stage. However, because different opticalcomponents are used in the two arms of the inter-ferometer, the position of the stage that matches pathlengths will not simultaneously match the positionsof the back focal planes of the arms. As a result, thecurvatures of the coherently combining wavefrontsfrom the sample and the reference arms will differand a bull’s-eye fringe pattern will be produced. Toremedy this, a low-power afocal Galilean telescope(constructed from a 50.8-mm focal-length positiveachromat and a 35.0-mm focal-length negative ach-romat) is inserted between the reference objectiveand the beam splitter. The telescope is mounted on atranslation stage that is in turn mounted on thetranslation stage responsible for path-length match-ing. One can adjust the location of the back focalplane by moving the telescope along the optical axisof the reference arm. This ensures that straightfringes can be obtained. Note that, because the tele-

Fig. 1. (a) Schematic layout of the interference microscope. Themicroscope is constructed primarily from commercial optics andoptomechanics. (b) Photograph of the interferometer coupled to acomputer-controlled probe station for wafer-level measurement ofmicroelectromechanical-system material parameters.


scope is afocal, it can be moved without affecting thefocus of the reference objective. In addition, the tele-scope can be moved without affecting the path-lengthmatching. Figure 2 shows the appearance of the in-terference fringes for two positions of the telescope.Radiation reflected by the reference mirror returnsthrough the objective, telescope, and beam splitterand is focused onto the CCD imager by the tube lens.

The final component in the reference arm is aplane-parallel compensation plate. A separate com-pensation plate is required for each type of objective.The thickness and glass type of the compensationplate must be carefully chosen to balance the inter-ferometer. This can be done in a systematic manneras follows. First, note that the optical phase differ-ence � between the two arms of the interferometer isgiven by19

� � kZ�k�, (1)

where, because of dispersion, the optical path differ-ence Z is, in general, a function of wave vector k.Expanding the optical phase difference in a Taylorseries about the central wave vector k0 �k0 corre-sponds to the peak of the emission spectrum of theillumination source) yields19

� � k0Z�k0� �d�

dk�k0

�k � k0�

�12

d2�

dk2�k0

�k � k0�2 � · · · � , (2)

where the first term is the phase-velocity opticalphase difference, the second term is the group-velocity optical phase difference, and the third andhigher terms arise as a result of group-velocity dis-persion. In practice, the first two terms in this ex-pression will contain the desired surface topographyinformation,19 whereas the third term will result inbroadening of the fringe envelope along with a de-crease of fringe contrast. To select the optimal com-pensation plate, a mirror identical to the referencemirror placed at the sample plane and a series of

single-pixel interferograms (i.e., intensity versus ver-tical sample position for a single pixel) are obtainedwith compensation plates of differing thicknesses.The interferograms are Fourier transformed, and thephase in the neighborhood of k0 is fit to a quadraticpolynomial in the wave vector �k � k0�. The second-order coefficient provides a measure of the amount ofdispersion mismatch and will become zero for theoptimal compensation plate thickness.

An example of the application of this procedure tofind the optimal compensation plate thickness for the10� objective is shown in Figs. 3 and 4. A front sur-face metal mirror was mounted on a piezoelectrictranslation stage and placed at the sample plane.Compensating plates of four different thicknesses (0,3.2, 6.4, and 9.6 mm) of fused silica were used in thereference arm. For each compensating plate, a 512-point interferogram was obtained with the samplemirror translated 0.05 �m between points. Figure 3shows a representative single-pixel interferogram ob-tained with the 6.4-mm-thick plate. The phaseprofiles obtained by a Fourier transform of these in-terferograms are shown in Fig. 4(a). Also shown inFig. 4(a) are the results of fitting a second-order poly-nomial to each of the phase profiles. The fitted regionwas restricted to the region about k0 where the phaseand magnitude of the Fourier transform exhibit agood signal-to-noise ratio. It is clear from Fig. 4(a)that the curvature of the phase profile decreases,changes sign, and then increases as the thickness ofthe compensator varies from 0 to 9.6 mm. Figure 4(b)shows a plot of the coefficient of the second-orderterm of the polynomial fit as a function of the thick-ness of the compensator plate. As expected, a lineardependence of the curvature on the thickness is ob-served. The measured slope of �5.9 � 106 comparesfavorably with a slope of �6.0 � 106 that is calculatedfrom the wavelength dependence of the refractive in-dex of fused silica. The thickness of the optimal com-pensator plate can be immediately determined fromthe zero crossing of the data of Fig. 4(b). The inter-

Fig. 2. (a) Interference fringes obtained when the telescope isadjusted so that the back focal planes of the sample and the ref-erence arms are not equidistant from the beam splitter; (b) afteradjustment of the telescope straight fringes are obtained. The sam-ple was slightly tilted when these interferograms were obtained.

Fig. 3. Representative interferogram obtained with a 6.4-mm-thick fused silica compensating plate. For this compensator afringe contrast of approximately 0.7 is obtained.


ferogram shown in Fig. 3 was thus obtained with anear-optimal compensator and, as a result, is narrowand exhibits good fringe contrast ��0.7�. This proce-dure has also been applied to determine the optimalcompensator plates for other objectives. It is impor-tant to note that, once an optimal compensator platehas been determined for a given objective power,other objectives of the same model are well compen-sated for dispersion.

The procedure outlined above eliminates only theleading term of the dispersion mismatch. This is jus-tified in the current case because of the relativelynarrow bandwidth of the illumination source. Forbroader sources (e.g., 200 nm), multiple compensa-tion plates of different glass types can be utilized toeliminate successively higher-order dispersion terms.

A procedure of this sort has been presented in thecontext of stellar interferometry.21,22 Also, in the caseof broader sources, a different set of compensationplates might be required for different objectives of thesame model type from the same manufacturer be-cause of manufacturing tolerances.

Once the optimal compensator plate has been de-termined for each sample arm objective, sample armobjectives may be changed as follows. First the objec-tive is changed, and the appropriate compensatorplate is inserted. Then, the path-length-matchingtranslation stage is adjusted to obtain high-contrastfringes. Finally, the telescope stage is adjusted tospread out the bull’s-eye fringe pattern. In practice,the micrometer setting of each of the translationstages has been previously recorded, so that resettingthe stages after the objective has been changed is fastand simple. One implementation of the interferencemicroscope includes a five-position nosepiece forquick interchange of objectives.

3. Results

As a first test of the performance of the interferencemicroscope, a step-height calibration standard wasmounted on a piezoelectric actuated translationstage, and its surface profile was measured by use ofVSI techniques.17,19 A sequence of 512 interferomet-ric image frames were obtained with the piezoelectricstage translated 0.05 �m along the optical axis be-tween frames. In this manner a separate interfero-gram was obtained for each pixel in the field of view.By use of a procedure similar to that outlined in Ref.19, each of these interferograms was Fourier trans-formed and the slope of each of the phase profiles wasdetermined. The phase slope recorded for an x–y lo-cation within the frame is related to the height ofsurface at that location through

h�x, y� �12

d�

dk�k0

. (3)

Figure 5 shows the recovered surface profile, alongwith a line section along the surface. The measuredstep height of 10.20 �m is in good agreement with thequoted height of 10.14 � 0.09 �m and confirms theaccuracy of the interferometer. Further refinement ofthe surface profile can be obtained with the first termof Eq. (2).19,23,24 However, when a simple version ofthis procedure is performed, “ghost steps” appearthat are similar to those observed in Refs. 23 and 24.In this case, more sophisticated algorithms must beapplied23,24 to obtain the refined surface profile.

As an example of microsystem characterization,the surface profile of a polysilicon MEMS struc-ture fabricated with Sandia National LaboratoriesSUMMIT V process25 was obtained by use of VSItechniques. Briefly, the SUMMIT V™ micromachin-ing process produces microstructures comprising fiveindependent polysilicon structural levels. The MEMSstructure used consists of several “pop-up” struc-tures, one of which was partially actuated. Once

Fig. 4. (a) Graph of the phase of the Fourier transforms of inter-ferograms obtained for four different thicknesses of fused silica.The results of a quadratic fit to the phase profile are also shown.The inset shows the magnitude of the Fourier transform of one ofthe interferograms. (b) Graph of the coefficient of the second-orderterm of the quadratic fit as a function of fused silica thickness. Thezero crossing of this line locates the optimal thickness of the com-pensation plate.


again, a sequence of 512 frames was recorded at0.05-�m intervals, and the phase-slope procedure de-scribed above was used to recover the surface profile.Figure 6 shows one interferogram of the sequence,obtained when the fringe envelope was partway upthe tilted structure to the right of center. Fringes arealso visible on the structure immediately to the left ofthe tilted structure. However, because this structurehas not been actuated the fringes are relativelybroad. Figure 7 shows a gray-scale image of the sur-face profile obtained with this procedure. As ex-pected, a significant contrast gradient is visible onthe actuated structure to the right of center, whereaslittle contrast variation is observed for the unactu-ated structure. Horizontal and vertical line scans of

the surface profile were recorded at the positions in-dicated by the red lines in Fig. 7. The vertical linescan shows that the actuated structure is tilting upby approximately 5 �m. The horizontal line scanshows that the surface height of the underlying poly-silicon layer is successfully measured in the narrowgaps of the uppermost polysilicon layer. The step-height differences recorded between the differentpolysilicon levels are in close agreement with theknown step heights of the SUMMIT V process.

Another example of the application of the interfer-ence microscope involves the use of PSI tech-niques26,27 to accurately measure the curvature ofmicromachined polysilicon beams. Measurements ofthis type, when combined with appropriately de-signed test structures and analysis routines, can be

Fig. 5. Results of the surface reconstruction of the step-heightstandard: (a) portion of the reconstructed surface, (b) line scanacross the step. The measured step height of 10.20 �m is in excel-lent agreement with the quoted height of 10.14 � 0.09 �m.

Fig. 6. One of the interferometric images in the sequence used todetermine the surface profile of the MEMS device. Tight fringesappear on the structure to the right of center, indicating that it issubstantially tilted. Broader fringes are observed on the structureto the left of center, which exhibits much less tilt.

Fig. 7. Gray-scale image of the surface profile of the MEMS struc-ture. A significant contrast gradient is observed on the pop-upstructure to the right of center, indicating that this structure issubstantially tilted. Horizontal and vertical line scans obtained atthe locations of the red lines on the figure are also shown.


used to determine important material parameterssuch as residual stress level and stress gradient.Figure 8 shows an interferometric image of a poly-silicon cantilever beam. Using the PSI approach, weobtained five such interferometric frames at ��4phase shift intervals by moving the reference mirrorpiezoactuator. The surface profile of the continuoussurface was then reconstructed with the HariharanPSI algorithm.27 The graph in Fig. 8 shows the re-covered shape of the cantilever beam after a linear fithas been performed to remove any residual substratetilt. A downward deflection of approximately 1.3 �mis observed that is due to polysilicon stress gradients.

As a final example of the utility of the new inter-ference microscope, the ability of the microscope torecord high-quality fringes in the case in which awindow is placed between the objective and the sam-ple is demonstrated. There are many cases in whichthe use of a window is necessary, including charac-terization of packaged optical MEMS parts, microflu-idic devices, and testing of microsystems withinenvironmental chambers or at cryogenic tempera-tures. Figure 9 shows two interferograms of a MEMS

part. The interferogram shown on the left was takenwithout a window, and the interferogram on the rightwas obtained with a 1.6-mm-thick fused silica plateplaced between the sample and the objective. To com-pensate for the window, a corresponding plate wasplaced in the collimated region of the reference arm,and the path-length-matching translation stage wasadjusted to obtain high-contrast fringes. Note that,although fringes can be obtained with an interveningwindow, the optical aberrations induced by the addi-tion of the window are not corrected. However, forrelatively thin windows and low-magnification objec-tives this is not a significant problem.

4. Conclusions

The design of a new interference microscope, specif-ically optimized for the characterization of microsys-tems, has been described. The new microscopepossesses a large working distance to allow room forcomponents such as actuating electrical probes orprobe cards between the device under test and themicroscope objective. In addition, the microscope al-lows interferograms to be obtained in the case inwhich windows are required between the objectiveand the specimen. The microscope is compatible withVSI, PSI, and stroboscopic modes of data acquisition.A key feature of the new interference microscope is anadjustable reference arm optical assembly that al-lows three different sample side objectives to be used.The reference arm can be adjusted to simultaneouslymatch optical path length, wavefront curvature, andgroup-velocity dispersion. A systematic method forselecting the compensating plate has been described.

As of this writing, seven copies of this design havebeen constructed and are being evaluated as tools formicrosystem characterization at Sandia National Lab-oratories. These instruments have been used to studythe behavior and reliability of microsystems in win-dowed environmental chambers and at cryogenic tem-peratures. In addition, one of these instruments hasbeen integrated with an automated wafer-handlingprobe station to perform wafer-level testing of materialproperties obtained with Sandia’s SUMMIT V surfacemicromachining process.

The authors gratefully acknowledge helpful discus-sions with W. C. Sweatt and R. N. Shagam. The au-thors also thank J. J. Allen for supplying the MEMSdevices utilized in Figs. 6, 7, and 9. Finally, the experttechnical assistance of G. Jones is gratefully acknowl-edged. This work was funded by the Laboratory Di-rected Research and Development program of SandiaNational Laboratories. Sandia is a multiprogram lab-oratory operated by Sandia Corporation, a LockheedMartin Company, for the U.S. Department of Energyunder contract DE-ACO4-94AL85000.

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Fig. 8. Deflection of a polysilicon cantilever beam that is due toresidual stress gradients. The inset shows one of the interfero-grams used with the PSI algorithm to obtain the beam deflection.

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Long-working-distance incoherent-light interference microscope