Langmuir 1996, 12, 2121-2124
Projection Photolithography Utilizing a Schwarzschild Microscope and Self-Assembled Alkanethiol Monolayers as Simple Photoresists† Jane M. Behm,‡,§ Keith R. Lykke,*,‡ Michael J. Pellin,‡ and John C. Hemminger§ Materials Science and Chemistry Divisions, Argonne National Laboratory, Argonne, Illinois 60439, and Department of Chemistry and Institute for Surface and Interface Science, University of California, Irvine, California 92717 Received September 29, 1995. In Final Form: January 2, 1996
Introduction Interest in ultrathin organic films stems from their varied applications including corrosion resistance, nonlinear optics, tribology, information storage, microelectronic circuit fabrication, biosensors, and even drug delivery systems.1,2 Self-assembled monolayers (SAMs) provide a unique method to study thin-film properties due to their ease of preparation, extraordinary stability both in vacuum and in the ambient, and most significantly the inherent control one has over the order and the nature of the terminal functional group. Many of the proposed technological applications of SAMs rely on a need to control the spatial characteristics of the monolayer. The methodologies that have been proposed or demonstrated to spatially alter the SAMs are as varied as the applications and include ultraviolet (UV) light,3-7 near-UV light,8 visible light,9 mechanical stamping,10-12 electrochemical methods,7 the tip of a scanning tunneling microscope (STM),13,14 and X-ray, electron, and ion beams.15-17 In this work, we extend the previous studies based on UV photooxidation of alkanethiolate monolayers3,4 by performing projection photolithography (utilizing a Schwarzschild microscope) with these prototypical species for the * To whom correspondence may be addressed: e-mail,
[email protected]. † Work supported by the U.S. Department of Energy, BESMaterials Sciences, under Contract W-31-109-ENG-38. ‡ Argonne National Laboratory. § University of California. (1) Service, R. F. Science 1994, 265, 316. (2) Ulman, A. In An Introduction to Ultrathin Organic Films from Langmuir Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (3) Huang, J.; Dahlgren, D. A.; Hemminger, J. C. Langmuir 1994, 10, 626. (4) Huang, J.; Hemminger, J. C. J. Am. Chem. Soc. 1993, 115, 3342. (5) Tarlov, M. J.; Burgess, D. R. F.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305. (6) Gillen, G.; Bennett, J.; Tarlov, M. J.; Burgess, D. R. F. Anal. Chem. 1994, 66, 2170. (7) Chan, K. C.; Kim, T.; Schoer, J. K.; Crooks, R. M. J. Am. Chem. Soc. 1995, 117, 5875. (8) Frisbie, C. D.; Wollman, E. W.; Wrighton, M. S. Langmuir 1995, 11, 2563. (9) Wolf, M. O.; Fox, M. A. J. Am. Chem. Soc. 1995, 117, 1845. (10) Kumar, A.; Whitesides, G. M. Science 1994, 163, 60. (11) Xia, Y.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 3274. (12) Wilbur, J. L.; Kim, E.; Xia, Y.; Whitesides, G. M. Adv. Mater. 1995, 7, 649. (13) Marrian, C. R. K.; Perkins, F. K.; Brandow, S. L.; Koloski, T. S.; Dobisz, E. A.; Calvert, J. M. Appl. Phys. Lett. 1994, 63, 390. (14) Ross, C. B.; Sun, L.; Crooks, R. M. Langmuir 1993, 9, 632. (15) Calvert, J. M.; Koloski, T. S.; Dressick, W. J.; Dulcey, C. S.; Peckerar, M. C.; Cerrina, F.; Taylor, J. W.; Suh, D.; Wood, O. R.; MacDowell, A. A.; D’Souza, R. Proc. SPIE 1993, 1924, 30. (16) Tiberio, R. C.; Craighead, H. G.; Lercel, M.; Lau, T.; Sheen, C. W.; Allara, D. L. Appl. Phys. Lett. 1993, 62, 476. (17) Rieke, P. C.; Tarasevich, B. J.; Wood, L. L.; Engelhard, M. H.; Baer, D. R.; Fryxell, G. E.; John, C. M.; Laken, D. A.; Jaehnig, M. C. Langmuir 1994, 10, 619.
0743-7463/96/2412-2121$12.00/0
2121
first time. Our design allows us to readily achieve wavelength-limited, submicrometer imaging. Experimental Section The principles of the photooxidation procedure have been described in detail in previous publications.3,4 Thus, for brevity, only the experimental details will be described here. Gold films (∼500 Å) are prepared by evaporation onto Cr-coated (∼100 Å) quartz substrates (0.25 in. diameter disks). These films are subsequently immersed overnight in ∼1 mM ethanol solution of C16 alkanethiol (C16H33S, hexadecanethiol), rinsed with ethanol and dried in a nitrogen gas stream. The substrates are then viewed under an optical microscope, and a desirable region for photopatterning is selected. The sample substrate is then mounted in an aluminum holder that is attached to an optical post and positioned as indicated in Figure 1. In this schematic, the size of the sample and microscope objective have been enlarged to emphasize the focusing characteristics of the objective (a commercially available Ealing X36). A Bausch and Lomb SP200 mercury arc lamp fitted with a UV transmitting, visible absorbing filter (Oriel U-330) produces the UV radiation (0.013 W/cm2 UV light unfocused, 5.1 W/cm2 impinging on the sample). A high reflectivity mirror (M3) coated for optimal reflection at 308 nm, 45° (∼280-330 nm) was employed to guide the UV light into the microscope objective. The increased power density afforded by focusing through the microscope allows for adequate irradiation in 15-20 min. The Schwarzschild microscope is composed of two all-reflective concentric, spherical mirrors (M1 and M2), one concave and one convex.18 Its attributes include ease of use, low cost, long working distance, and a geometrical configuration that is achromatic and aplanatic, thus nullifying third-order aberrations (namely spherical, coma, and astigmatic). Because no refractive elements are present, the microscope can be employed with the UV light used for photooxidation as well as the white light source utilized with the CCD to visually monitor the photolithographic process (i.e., both the white light source and the UV source have the same focal plane at the object). This particular objective has a demagnification capability of 36 times and a numerical aperture (NA) of 0.5, where NA ) 1/(2f#). The spatial resolution of the objective is given theoretically by ω ) kλ/NA, where k is a constant representative of the microscope (a unitless value between 0.5 and 0.8).19 For this particular microscope (using k ) 0.5), the spatial resolution should equal the wavelength range used for photooxidation (∼0.25-0.30 µm). The theoretical depth of focus [DOF ) λ/2(NA)2] of the objective under these operating conditions is estimated as 0.6 µm, quite adequate for the use of SAMs as photoresists (monolayer thickness of ∼0.002 µm). Another adventitious feature of this experimental arrangement is the capacity to shine a light source (whether it be white light or UV light) through the barrel of the microscope to illuminate the sample as shown in Figure 1. The magnified image of the sample that passes back through the microscope can then be directed onto a CCD camera and displayed on a video monitor enabling optimization of the correct object-to-image distance of the microscope. Following photooxidation, the substrates are immersed in an aqueous aqua regia etching solution (HCl:HNO3:H2O ) 3:1:4) for approximately 1 min. The etching solution attacks the gold substrate in the regions of the SAM that have been exposed to the UV radiation (exposing the underlying Cr adhesion layer). In the regions occluded by the image of the grid the monolayer protects the gold; hence, the SAM acts as a positive photoresist. The resulting photopatterns are viewed in an optical microscope prior to being imaged with an atomic force microscope (Burleigh SPM). (18) Erdo¨s, P. Opt. Soc. Am. 1959, 49, 877. (19) Driscoll, W. G.; Vaughan, W. In Handbook of Optics; McGrawHill: New York, 1978.
© 1996 American Chemical Society
2122 Langmuir, Vol. 12, No. 8, 1996
Notes
Figure 1. A schematic of the projection photolithographic configuration. The major components are labeled and include a mercury arc lamp, photopatterning mask, sample holder, CCD camera, video monitor, and Schwarzschild microscope objective. The objective has been enlarged relative to the other components and has been drawn with a numerical aperture of 0.2 (in lieu of its true value of 0.5).
Figure 2. (a, left) A 60 × 60 µm AFM image of an ordinary piece of molybdenum screening grid focused into the self-assembled monolayer. Following photooxidation, the substrate was submerged in an aqua regia etching solution for approximately 1 min. The dark regions indicate where the exposed monolayer and underlying gold film were removed upon etching, thereby exposing the chromium adhesion layer. (b, right) A software-generated line scan across the image demonstrating the uniformity of the photopattern and the depth of the wet etch.
Results and Discussion Scanning microscopies, including scanning electron microscopy (SEM),20 chemical force microscopy,21 scanning (20) Lo´pez, G. P.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1993, 9, 1513. (21) Frisbie, C. D.; Rozsnyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071.
tunneling microscopy,7 lateral force microscopy (LFM),22 force modulation microscopy (FMM),22 and normal force microscopy (NFM),22 have been used successfully to image patterned self-assembled monolayers based on the properties of the terminal end group. In the present less (22) Wilbur, J. L.; Biebuyck, H. A.; MacDonald, J. C.; Whitesides, G. M. Langmuir 1995, 11, 825.
Notes
Langmuir, Vol. 12, No. 8, 1996 2123
Figure 3. (a, left) A 14 × 14 µm AFM image of a smaller area of the photopattern displayed in Figure 2. (b, right) A softwaregenerated line scan across one of the bars of the grid demonstrating the lateral resolution of the projection photolithographic process. The resolution of 0.25 µm was obtained using a 10-90% criterion with the gold feature relative depth selected as 48 nm and the chromium trench relative depth given as 12 nm (36 nm bar height).
Figure 4. (a, left) An optical image of a handwritten photolithographic mask scribed into a piece of quartz using a diamond-tipped engraving instrument. The features are 1-2 mm in size. (b, right) A 70 × 70 µm image of the “16” portion of a photopattern created using the engraved mask.
sophisticated imaging, the atomic force microscope is used to image the troughs that are formed by the etching of the gold substrate in the exposed regions. In technological applications, it is desirable to obtain large depth-to-width ratios (high aspect ratios) in addition to microscopic lateral resolution. The resolution observed in the following images is contingent not only on the characteristics of the Schwarzschild microscope but also on the nature of the etching process. Typically wet etches are isotropic; however, the aspect ratio of the present setup is dictated by the experimental geometry (feature sizes of ∼1 µm and gold film thickness of only 40-50 nm) yielding what appear to be rather anisotropic features. A 60 µm × 60 µm AFM image of a photopattern created using a piece of ordinary molybdenum screening grid as the mask is displayed in Figure 2a. The open area of the image mask (after demagnification) is approximately 8 µm across, and the bar width is about 2.5 µm. The slight
variation in feature sizes results from nonuniformities in the original grid (prior to demagnification). This mask is composed of regions containing both square and rectangular features, with dimensions ranging from 260 to 340 µm on a side and bar widths on the order of 120 µm. Figure 3a is an image taken of a smaller area (14 µm × 14 µm) of the same photopattern to illustrate the lateral resolution and anisotropy of the photopatterning/etching process. The lateral resolution is approximately 0.25 µm where this determination is measured from Figure 3b based on 10% to 90% resolution criterion, thus illustrating that the projection photolithographic process does indeed appear to be wavelength-limited. A particularly unique characteristic of this experimental methodology is the flexibility inherent in the choice of “large” masks. To illustrate this capability, a diamondtipped engraving tool was used to create a handwritten message (C16 ) on a quartz slide. The original size of the
2124 Langmuir, Vol. 12, No. 8, 1996
symbols was limited by how small we could legibly sketch and was in the range of 1.5-2 mm. Displayed in Figure 4 is an optical image of the imaging mask and an AFM image of the “16” portion of the photopattern (imaging is limited by the 70 µm × 70 µm maximum field of view of the AFM). The features after demagnification are on the order of 60 µm. In conclusion, we have demonstrated the ability to perform projection photolithography (in a manner similar to that used in the semiconductor industry) utilizing an alkanethiolate monolayer self-assembled on gold as a simple photoresist. The lateral resolution is submi-
Notes
crometer and wavelength-limited. While typical photoresists are too thick to be patterned using wavelengths below 100 nm, ultrathin SAM photoresists should allow the 0.1 µm threshold to be surpassed. Acknowledgment. This work was supported by the U.S. Department of Energy, BES-Materials Sciences, under Contract W-31-109-ENG-38. Jane M. Behm would like to thank Dr. Myron Sauer for the use of the mercury arc lamp throughout this work. LA950811U
