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High Fidelity Surface Chemical Imaging at 1000 nm Levels: Internal Reflection IR Imaging (IRIRI) Approach Daniel B. Otts, Ping Zhang, and Marek W. Urban* Shelby F. Thames Polymer Science Research Center, School of Polymers and High Performance Materials, The University of Southern Mississippi, Hattiesburg, Mississippi 39406 Received February 28, 2002. In Final Form: May 14, 2002 This report presents how combination of internal reflection (IR) and infrared imaging (IRI) can reach a surface spatial resolution at 1000 nm levels in the middle infrared range. As a result, vibrational spectra from these size areas can be recorded at high signal-to-noise levels. A spatial calibration of this method was performed by correlating IRIRI data with SEM micrographs and optical images of geometrically well-defined polymeric photoresists as well as Nylon fibers imbedded into a polyester matrix. The IRIRI approach has the potential of obtaining even better surface spatial resolution of vibrational spectra when internal reflection elements with greater refractive index ratios in the middle IR range become available.
The ability to selectively determine chemical compositions or reactions occurring on surfaces is one of the key ingredients in understanding numerous surface-initiated processes. Among a number of analytical techniques, imaging technologies have gained substantial interest primarily because of their ability to visualize analyzed objects. While current technologies allow visualization of surfaces at atomic and molecular levels, one of the limiting factors is obtaining chemical information at these levels. Although electron loss spectroscopy (EELS) is capable of nanolevel detection on inorganic/metallic surfaces,1 obtaining a chemical signature of surface species on polymeric surfaces is limited. On the contrary, the infrared region of the electromagnetic spectrum carries particularly useful information about functional groups, but diffraction limits inhibit spatial resolution in the mid-IR region to a few micrometers.2 Ultimately, one would like to combine these approaches and obtain visual and chemical information at the same time. Although recent attempts have shown that using infrared array detectors combined with a step-scan interferometer provides enhancement of the spatial resolution, there are other problems.3 Aside from limited spatial resolution to about 5-6 µm,4 it is extremely difficult to extract chemical information from smaller and nonhomogeneous areas primarily because of the occurrence of interference fringes or edge effects that often make data interpretation ambiguous. Although there are studies dealing with diffusion5 or biomedical problems,6 the nature of these processes does not require extremely high spatial resolution; thus, large scales on the order of tens of micrometers are applicable. For example, in a typical experiment used in these studies,3 the viewing area is on the order of 400 × 400 µm2, which is pixilated into a 64 × 64 pixel array with each pixel representing one spatially resolved IR spectrum and thus carrying chemical information from an approximately 5-6 µm2 area. * To whom all correspondence should be sent. E-mail:
[email protected]. (1) Batson, P. E. Physica B, Condens. Matter 1999, 273, 593. (2) Wentzel, D. L.; LeVine, S. M. Science 1999, 285 (5431), 1224. (3) Lewis, E. N.; Treado, P. J.; Reeder, R. C.; Story, G. M.; Dowrey, A. E.; Marcott, C.; Levin, I. W. Anal. Chem. 1995, 67 (19), 3377. (4) Ribar, T.; Bhargava, R.; Koenig, J. L. Macromolecules 2000, 33, 8842. (5) Miller-Chou, B.; Ishida, K. Macromolecules 2002, 35, 440. (6) Wentzel, D. L.; LeVine, S. M.; Dickson, D. W. Microchim. Acta 1997, 14, 353.
Figure 1. (A) Light path from the IR source to a high-refractiveindex crystal and reflection back to the detector. (B) Hemispherical shaped Ge lens enlarging the image signal coming out of the surface. Upon absorption at the point of contact with the surface, the reflected IR radiation will carry molecular information, in this case the vibrational energies of the surface species. (C) Without the lens, the surface object “illuminates” an array detector without magnification; thus, the spatial resolution is on the order of the array detector’s pixels.
Here, we present a detailed experimental setup that partially “overcomes” wavelength-dependent spatial resolution limits in the mid-IR that utilizes a combination of an internal reflection IR lens and IR imaging. Using the experimental setup presented below, we achieved a spatial resolution on the order of 1000 nm or 1 µm, but further modifications of this approach, specifically developments of materials with higher refractive indices, will allow detection of even smaller areas. Figure 1A illustrates the modulated IR light path coming out of a step-scan spectrometer which impinges upon a set of hemispherical mirrors and, upon reflection, hits the surface of an IR transparent crystal, in this case Ge with a refractive index of 4. The curvature of the optical mirrors, the crystal, and its refractive index as well as the Rayleigh criteria will determine the area of illumination from which surface information will be carried to the detector. Although somewhat similar combinations of elements have been used,7 the consequences arising from this optical arrangement have not been explored. Figure 1B illustrates that when an optical element such as a Ge lens touches the (7) Lewis, L. L.; Sommer, A. J. Appl. Spectrosc. 2000, 54, 324.
10.1021/la025684y CCC: $22.00 © 2002 American Chemical Society Published on Web 06/20/2002
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Figure 2. (A1 and B1 series) Optical and SEM images of two photoresists with approximate thicknesses of 23.7 and 10.0 µm. As a result of the contact with a Ge crystal, both specimens’ thicknesses increase as a result of the permanent deformation.
Figure 3. (A) Optical images of a thin photoresist (about 10 µm) before (A1) and after (A2) contact with a Ge IRIRI objective, and corresponding magnified IRIRI image of the 1613 cm-1 band (A3). (B) Optical images of a photoresist line (23.7 µm) before (B1) and after (B2) contact with a Ge IRIRI objective, and corresponding magnified IRIRI image of the 1613 cm-1 band (B3).
analyzed surface and IR light passes through, internal reflection occurs and projects the reflected signal carrying surface information into a larger area. This is accomplished by the curvatures of the Ge lens, which result in different positions of the front and back focal points. As a consequence, the area that touches the surface is magnified. Since an IR focal plane array is the detecting device, a larger number of pixels will be “looking” at that area in comparison to the case of IR imaging, and Figure 1C illustrates a schematic diagram of the light path without internal reflection magnification.
Enhancement of the spatial resolution was tested using a Bio-Rad FTS 6000 Stingray system with a Ge lens. This system consists of a Bio-Rad FTS 6000 spectrometer, a UMA 500 microscope, an ImagIR focal plane array (FPA) image detector, and a semispherical Ge objective. The IRIRI data acquired for the purpose of this study were collected using the following spectral acquisition parameters: under sampling ratio ) 4, step-scan frequency ) 2.5 Hz, number of spectrometer steps ) 1021, number of images per step ) 641, and spectral resolution ) 8 cm-1. In a typical experiment, a spectral data set acquisition
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Figure 4. IRIRI spectra obtained from the (A) glass substrate and (B) polymeric photoresist.
Figure 5. Topographic and IRIR images with corresponding FTIR spectra of Nylon fibers imbedded in a polyester matrix: (A) topographic image of fibers; (B) IRIR image of the 1634 cm-1 band; (C) IRIR spectrum of marked Nylon fibers; (A1) topographic image of a polyester matrix; (B1) IRIR image of the 1728 cm-1 band; (C1) IRIR spectrum of the marked polyester matrix area.
time was approximately 8 min. Image processing was performed using ENVI (Research Systems, Inc., version
3.5) software. For calibration purposes, an environmental scanning electron microscope (ESEM, Joel E2) was used.
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Figure 6. IRIRI image obtained from the fiber-matrix interface and IR spectra, each corresponding to a 1 × 1 µm2 area.
In an effort to establish the size of the objects to be analyzed, ESEM and optical images were obtained. Parts A and B of Figure 2 illustrate the images of a polymeric photoresist material with an approximate thickness of 10 and 30 µm, respectively. The width measurements obtained using an ESEM will be used for a spatial calibration. Parts A1 and B1 of Figure 2 illustrate optical and ESEM images with thicknesses of 10.0 and 23.7 µm (ESEM), respectively. As seen, the thickness of both specimens may vary, and this is important because the internal reflection method is a contact method and the shape of the imaged surface may change. For that purpose, each specimen was exposed to a contact under constant pressure with a Ge crystal and the same areas were analyzed again using an optical microscope. The optical and SEM images are shown in Figure 2, A2 and B2, and as seen, the photoresist specimen is deformed as a result of the contact with the
crystal. These data formulate the basis for spatial calibration of internal-reflection-imaging experiments. Using the data shown in Figure 2, A1′, A2′, B1′, and B2′ (ESEM images), the scale of the optical images was determined, and such images with known scale were correlated with IRIRI data. This is illustrated in Figure 3, which shows two optical images of photoresist before (A) and after (B) deformation. Using specimen B, IRIRI data were collected from the area marked in Figure 3B, and the 1613 cm-1 image is shown in Figure 3C. As clearly seen, two bright areas represent high concentration levels of the 1613 cm-1 band which are magnified. In the calibration experiment illustrated in Figure 2B, the width of a photoresist in contact with the objective was measured to be 41.4 µm (ESEM). The corresponding IR image width, as shown in Figure 3, A3 and B3, is about 41 pixels, giving rise to an IRIRI spatial resolution of 1000
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nm/pixel. In an effort to demonstrate the capability of this approach, IRIRI spectra were collected from a glass substrate as well as a photoresist “cross hair”. As seen in Figure 4, all spectral features are clearly resolved. To further demonstrate the spatial capability of the IRIRI approach, we collected a series of spectra from Nylon fibers imbedded into a polyester matrix. Figure 5 illustrates (A) topographic and (B) IRIR images of the 1634 cm-1 amide bands and (C) IRIR spectra obtained from the marked areas. Similarly, Figure 5, A1, illustrates a topological image of a polyester matrix, along with the image of the 1728 cm-1 band due to esters (B1) and the corresponding IR spectrum collected from the marked area (C1). As seen, high fidelity vibrational spectra are obtained with clearly identified spectral features. In an effort to prove that indeed 1000 nm resolution can be achieved, three spatially resolved spectra from the area 1 × 3 µm2 at the polyester matrix-fiber interface were collected in 1 µm increments. This is illustrated in Figure 6, and it is clearly seen that a 1 µm spatial resolution is obtainable, as demonstrated by easily resolvable spectral features characteristic of the matrix and the fiber. It should be noted that longer collection times will further enhance the signal-to-noise ratio. In summary, these experiments demonstrate that, by using an IRIRI approach, it is possible to obtain highly resolved spectral data in the mid-IR range with a low signal-to-noise ratio and that, by using an internal reflection element such as Ge, the spatial resolution can be as low as 1.0 µm. Another important feature of this approach is that if internal elements with even higher refractive indices become available, it will be possible to obtain even better spatial resolution. To demonstrate this feature, Figure 7 was constructed which illustrates the relationship between the theoretically predicted spatial resolution for internal reflection elements obtained from the Rayleigh criteria (d ) 1.22λ/n1 sin θ, where d is the spatial resolution, λ is the wavelength of light, n1 is the refractive index of the surface, and θ is the most extreme ray entering the surface)8 and refractive index values plotted as a function of the angle of incidence. As seen, as the viewing angle and refractive index increase, the distance between resolvable surface points becomes (8) Driscoll, W. G.; Vaughan, W. Handbook of Optics; McGraw-Hill Book Co.: New York, 1978; pp 2-58.
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Figure 7. Relationship between theoretically predicted spatial resolution for various internal reflection elements plotted as a function of the angle of incidence. While the n values between 1 and 4 represent the existing materials and, at this point, Ge provides the highest spatial resolution, new materials with higher n values will allow even better spatial resolution.
smaller, thus enhancing spatial resolution. This configuration, also known as the solid immersion lens (SIL), a technique similar to oil immersion microscopy, was utilized earlier in the visible range of electromagnetic radiation.9 In this publication, the extended diffraction limit is accomplished in the mid-IR by filling the object space with a high-refractive-index material. Acknowledgment. The authors are thankful to the National Science Foundation Industry/University Cooperative Research Center in Coatings (EEC 002775) and National Science Foundation Partnership for Innovation Program (EHR 0125516) for financial support of these studies. LA025684Y (9) Wu, Q.; Feke, G. D.; Grober, R. D.; Ghislain, L. P. Appl. Phys. Lett. 1999, 75, 4064.
