In real-time holographic interferometry1 a precise re-positioning of the hologram after wet processing isnecessary to avoid the introduction of spuriousfringes. Such repositioning must be better than ��2.In addition, with photographic emulsions, it is nec-essary to take precautions to elude local deformationsof the emulsion due to no uniform drying. One canavoid these problems by exposing and processing thephotographic plate in situ in a liquid gate.2,3 Inmoire interferometry,4 a specimen grating is recordedover the surface target. Then it is developed andplaced in the experimental setup to be mechanicallyanalyzed. In this case the target’s replacement hasnot been exact, but the process is simpler with thedevelopment in situ, and with it a practical in situmonitoring and control method for aiding in the man-ufacture of highly efficient submicrometer gratingstructures in photoresist has been demonstrated.5The method uses a time-dependent diffraction signalfrom an evolving grating structure when it is im-mersed in a developer solution.
Although nowadays the holographic interferome-try can be carried out digitally6 or by means of alter-native techniques as Electronic Speckle PatternInterferometry7 �ESPI�, in several cases it is contin-ued with practical emulsions. One of these cases isthat of moire interferometry.7
One practical emulsion required by moire and clas-
The authors are with Centro de Investigaciones en Optica, Aso-ciacion Civil, Apartado Postal 1-948, C. P. 37000, Leon, Gto., Mex-ico. J. A. Rayas’ e-mail address is [email protected].
Received 8 May 2003; revised manuscript received 14 August2003.
sical holographic interferometry has been Shipleypositive photoresist, which has the requisite of reso-lution capability �exceeds 1000 lines�mm�,8 sensitiv-ity, and ease of application. Its processing yieldsexcellent results under an appropriate developmenttechnique.9 Developing a mechanism for positivephotoresist with MF-319 developer requires the dis-solution of the exposed areas of the photoresist whilethe unexposed areas are essentially left intact. Forholographic recording, it is desirable to have a depthversus exposure characteristic curve approximatelylinear over a wide exposure range. To control thelinearity, it is recommended that the developer bediluted properly and that the sensitivity be increasedby various methods, such as postexposure and preex-posure.9 It has been shown that the sensitivity canbe increased by approximately 2.5 times.9
We describe an in situ developing process in whichspray is used for the recording of holographic gratings�Fig. 1�. In this process photoresist Shipley S-1822is used as the material on which the copy is made.First, the photoresist is deposited on an acrylic discby use of a spinner that provides the emulsion; thenan oven is used to dry it. The thickness of the pho-toresist layer is approximately 2.3 �m, and it is pre-baked for 15 min at 70 °C. Avoiding temperaturesabove 100 °C is recommended because prebaking thephotoresist to high temperatures can inhibit the de-velopment of the image. Later, the samples wereleft undisturbed for 24 h before being used. As thespectral sensitivity of Shipley S-1822 photoresist ishigh in the short-wavelength region �0.2–0.5 �m�, aHe–Cd laser �0.440 �m� was used for the experiment.A grating structure is printed into the photoresist bythe two-beam interference technique used in moireinterferometry. The angle between the overlappingbeams is 3.7°, corresponding to a projected grating
frequency of 293 lines�mm. The relation betweenthe beams is 1:1, and the required exposure is ap-proximately 15 mJ�cm2, corresponding to an inten-sity of 4.93 mW�cm2 and an exposure time of 3 min.
The sample is sprayed with MF-319 developer for 30 sfrom the top to the underside and then rinsed withsprayed water for 3 min and dried with airflow. Apostexposure of 12.325 mJ�cm2 was made. Then, toneutralize the film sensitivity completely, we usedone of the interferometer beams to perform a 5-minpostexposure by placing a diffuser between the beamand the sample. The delineated process was real-ized in situ at environment temperature �25 °C�.
The results for the diffraction efficiency are shownin Fig. 2 for both diffraction gratings. We can ob-serve that there is not a large difference betweenthem. Figure 3 shows an enlarged section of thegrating when diffraction efficiency is obtained withthe wet development process in the traditional way�Fig. 3�a�� and by in situ development �Fig. 3�b��.Measurements obtained for the grating developed in
Fig. 1. �a� Experimental setup. �b� In situ development process.
Fig. 2. Comparison of experimental diffraction efficiency betweenrecorded gratings by traditional technique and by in situ develop-ment. The light was incident normal to the grating.
Fig. 3. Grating modulation profiles obtained with using an atomicforce microscope. �a� Specimen grating is removed for the devel-oped process. �b� Specimen grating is developed in situ.
situ are 3.526 and 0.807 �m for the period and theamplitude, respectively. Measurements for thegrating that was removed for the wet developmentprocess are 3.624 and 1.039 �m for the period and theamplitude, respectively. Atomic force microscopyand a contact mode were used to obtain the data.
Moire interferometry is employed as an experimen-tal application �Fig. 1�. This technique has beenused to evaluate in-plane stresses on a sample-shaped disk that is loaded diametrically. Most fre-quently, disks loaded with diametric compression arethose reported for use in the development of conven-tional or digital photoelasticity to illustrate new the-ories and experimental techniques. Moireexperimental results that report the horizontal andthe vertical displacement field for such a disc havebeen published.10,11 Specimen grating is recordedholographically on the surface of the disk, and thedeveloping process in immersion is with an agitationof 30 s. The sample is rinsed in immersion for 3 minand dried with airflow. Its developing process oc-curs at room temperature, and later it is repositionedto get the moire fringes.
Figure 4 shows moire fringes, which were obtainedwhen the acrylic disc is mechanically loaded diamet-rically to gratings in both developing processes. Thevisibility of the moire fringes obtained with the spec-imen grating developed in situ is enough to analyzethem and to obtain information about the target.
From the results it is clear that the use of thisimplemented technique for the development in situ isadequate. The main advantage of the developing
process in situ is the inexistence of spurious fringesas a result of no precise repositioning in real-timeholographic interferometry. In moire interferome-try the specimen grating is prepared for an un-stressed condition. Then a stress is applied to theobject, and the moire fringes are obtained. The ex-periment is simpler because the target is not movedfrom the original position.
The authors thank Consejo de Ciencia y Tecnologıadel Estado de Guanajuato �CONCYTEG, grant 03-04-K118-039� and Consejo Nacional de Ciencia y Tec-nologıa �CONACYT, grant 33106-E� for their partialeconomic support.
References1. P. Hariharan, “Optical holography,” in Holographic Inter-
ferometry �Cambridge University Press, Cambridge, UK,1984�, Chap. 14, pp. 207–210.
2. W. van Deelen and P. Nisenson, “Mirror blank testing by real-time holographic interferometry,” Appl. Opt. 8, 951–955�1969�.
3. P. Hariharan and B. S. Ramprasad, “Rapid in Situ processingfor real-time holographic interferometry,” J. Phys. E. 6, 699–701 �1973�.
4. D. Post, B. Han, and P. Ifju, “High sensitivity moire,” in MoireInterferometry �Springer-Verlag, New York, 1994�, Chap. 4, pp.135–226.
5. J. A. Britten, R. D. Boyd, and B. W. Shore, “In situ end-pointdetection during development of submicrometer grating struc-tures in photoresist,” Opt. Eng. 34, 474–479 �1995�.
6. C. Perez Lopez, F. Mendoza Santoyo, and J. A. Guerrero, “De-coupling the x, y and z displacement components in a rotatingdisc using three-dimensional pulsed digital holography,” Meas.Sci. Technol. 14, 97–100 �2003�.
7. A. Martınez, R. Rodrıguez-Vera, J. A. Rayas, and H. J. Puga,“Fracture detection by grating moire and in-plane ESPI tech-niques,” Opt. Lasers Eng. 39, 525–536 �2003�.
8. R. A. Bartolini, “Holographic recording material,” in Photore-sist, H. M. Smith ed. �Springer-Verlag, New York, 1977�, Chap.7, pp. 217–221.
9. F. Iwata and J. Tsujiuchi, “Characteristics of a photoresisthologram and its replica,” Appl. Opt. 13, 1327–1336 �1974�.
10. J. F. Cardenas-Garcıa, B. Han, R. Rodrıguez-Vera, and J. A.Rayas, “The interferometric moire circular disc,” presented atProceedings of the 2002 Society for Experimental Mechanics,Annual Conference and Exposition on Experimental and Ap-plied Mechanics, electronic version Paper 232, Milwaukee,Wisconsin, 10–12 June 2002.
11. K. M. Hung and C. C. Ma, “Theoretical analysis and digitalphotoelastic measurement of circular disks subjected to par-tially distributed compressions,” Exp. Mech. 43, 216–224�2003�.
Fig. 4. Moire pattern for horizontal displacements of an acrylicdisc loaded diametrically in y. �a� Specimen grating is removedfor the developed process. �b� Specimen grating is developed insitu.
