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Improved pattern-placement accuracy in e-beam lithography via sparse-sample spatial-phase locking
ELSEVIER
Microelectronic Engineering 53 (2000) 361-364 www.elsevier.nl/locate/ mee
Improved Pattern-Placement Accuracy in E-Beam Lithography via Sparse-Sample Spatial-Phase Locking J.T. Hastings, F. Zhang, J.G. Goodberlet, and Henry Smith NanoStructures Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 A sparse-sampling algorithm has been developed for two-dimensional, spatial-phase-locked electron-beam lithography (SPLEBL). This algorithm corrects for field shift, scale and rotation errors. The algorithm can be used with a scintillating polymeric global-fiducial grid distributed over the substrate. An experimental evaluation of the sparse-sampling algorithm indicates that sub-10-nm pattern-placement accuracy can be obtained with this mode of SPLEBL. 1. I N T R O D U C T I O N Mask fabrication for next, generation lithographies requires increasingly accurate pattern placement with decreasing feature size. As early as the 130nm-feature node some mask-writing technologies demand sub-14nm image-placement accuracy which is beyond the the capability of conventional electron beam lithography (EBL) tools.J1] In addition, emerging technologies, such as integrated optics, specify even tighter tolerances for acceptable device performance. Numer()us efforts have been directed toward eliminating the underlying causes of placement errors in EBL tools, but these have not provided a direct reference between substrate and beam position. H. I. Smith et al. proposed that the interaction of an electron beam with a global-fiducial pattern on the substrate would provide closed-loop feedback for beam positioning.J2] Two single-axis implementations of such a spatial-phase-locked ebeam lithography (SPLEBL) scheme have been demonstrated, both using metallic gratings patterned by interference lithography.[3,4] Metallic grids on the substrate are unsuitable for many processes and use valuable substrate area. We report the development of a sparsesampling algorithm suitable for 2-D spatial-phase locking. This algorithm will be used with a polymeric scintillating global-fiducial grid, the generation and imaging of which is also described here. The combination of the sparse-sampling algorithm and scintillating grid will provide two0167-9317/00/$ - see front matter PII: S 0 1 6 7 - 9 3 1 7 ( 0 0 ) 0 0 3 3 4 - 8
dimensional phase locking without affecting pattern fidelity or consuming substrate area. 2. S P A R S E - S A M P L E A P P R O A C H
The sparse-sample implementation of spatialphase locking applies a ':look-then-right" procedure. Before writing, the entire field is sparsely sampled with the electron beam at a dose well below the development threshold. The sparsesample points define a uniformly spaced grid whose period differs slightly from the period of the fiducial grid on tile substrate. A moir6 image of the scintillating grid results, and the moir~ spatial frequency is given by tile difference between the sparse-sample and fiducial grid spatial frequencies. For example, a 512x512 sample array of 520x520 grid periods in x and y yields an 8x8 moir6 grid pattern. This is also true for higherorder aliasing; i.e. 512x512 samples of 1032x1032 periods. Figure l(a) shows a recorded moir~ image from a scintillating grid sparsely sampled with an e-beam. To the authors' knowledge, this is the first time a global scintillating-reference grid on the substrate has been detected in an EBL system. Figure l(b) shows a simulated ideal moir~ pattern. The shift of the fiducial grid relative to the scan field is directly calculated from the spatial phase of the moir6 pattern in a given direction (¢x for the x-direction) by Adx
© 2000 Elsevier Science B.V. All rights reserved.
Cx = 2-~Agx
362
J.T. Hastings et al. / Microelectronic Engineering 53 (2000) 361-364
. . . .
(a)
. . . . . . . . . . . . . . . . . . . . . . . . . . .
~ . . . .
(b)
Figure 1. (a) Moir6 pattern from 512x512 samples of 520x520 periods of an l p m period scintillating fiducial grid. (b) Simulated ideal moir6 pattern with outlined areas used for scale and rotation correction. where Ad~ is the shift in the x direction, and Ayx is the grid period in that direction. Once the moir6 image has been acquired, two one dimensional signals are extracted by taking the two dimensional fast Fourier transform (FFT) and retaining only the x and y axes. If the scaling, is perfect the spatial phase can be obtained directly from the FFT; otherwise, there will be small phase errors from spectral leakage into adjacent frequencies. One can solve this problem by iteratively scanning the field and correcting the scale and shift until the scaling converges. This approach is not favorable to sparse sampling with a scintillating grid because the e-beam damages the scintillator, and the resist receives a greater dose. Instead, we elect to transform the one-dimensional signals back to the spatial domain before determining the phase. The phase is calculated by multiplying tile signal by a reference sinusoid and integrating the result. The scale of the reference can be changed to match the measured scale of the moir~ pattern. In this way the shift error (:an be accurately determined from only one sparse sampling. Accurate scaling is obtained by sizing the field, and thus the moir4 pattern, to contain an integral number of periods, N=8 in our case. For fine correction, one calculates the scaling change required A~
=1+,
( ¢~. - Ct~ Agx 27r " D )
where ¢1~ and ¢rz are the x-phases of the left,
n = l , and right, n=8, moird periods respectively, and D is the center-to-center distance between the left and right peaks (approximately the e-beam field size). The corrected scaling is determined by dividing the original scale by A,~x. The y-scaling error is calculated similarly using the y-phase of the top and bottom moir6 periods. The pattern edges used are outlined in the ideal moir6 image of figure 2(b). The expression abow~ is only valid for small scaling errors. If the moir6 pattern is an entire period off, the pattern's two-dimensional Fourier transform peak locations can be used to correct these large errors. Although large corrections may be required for the initial setup betbre writing a new substrate, only small corrections are necessary during the write. The left-edge and right-edge y-phase, or topedge and bottom-edge x-phases, allow calculation of the field rotation (0) by 0 - - t a n - l ( cry~- ely . @ ) where Cry and Czy are the left and right edge y-phases, and D is defined as above. A similar expression applies if the top and bottom xphases are used. If the axes are not orthogonal the two calculations will yield different rotation values which are used to correct y-axis rotation and x-axis rotation individually. Trapezoidal scan-field distortion results in a variation of x-scale with y-location and vice versa. Such errors can also be quantified and corrected by analyzing the moir~ pattern. Currently, trapezoidal distortion is removed by standard field calibration techniques and is assumed to be constant throughout the exposure. Future implementations will incorporate this correction for each field. The algorithm was designed to correct field shift to within 1/105, i.e lnm in a 100#m field. The scaling routine targets correction to within 3/105, or 1/2 a beam step over an entire field for our 14 bit DACs. Finally, rotation correction is based on calculations similar to shift with comparable performance. However, rotation places greater demands on signal noise and contrast, because it uses less of the data from the sparsesample pattern. The computation time required for full field analysis is dominated by one gl2xgl2
363
J.T. Hastings et al. / Microelectronic Engineering 53 (2000) 361-364
2D-real FFT and four 512x64 2D-real FFTs. The computation time for 1D inverse FFTs, multiplications, and integrations is negligible for any realistic number of mathematical iterations. All of these calculations can be easily performed in under lmsec by a dedicated signal processing board such as the Alacron AL860MP used in our system. As a result, the correction algorithm does not limit the e-beam tool's throughput. A potentially adverse effect of sparse-sampling is a reduction in process latitude, or the broadening of a fine line by an individual sparsesample point. Consequently, experiments were conducted to determine the effect of sparse sampling on the final pattern. 400nm-period gratings were written in the scintillating PMMA along with sparse samples of varying dose immediately adjacent to the lines. The patterned PMMA was examined in a SEM to look for line broadening. No affect on the pattern was observed even at sample doses well above that needed to image the grid. 3. T H E F I D U C I A L G R I D
The fiducial grid on the substrate is critical to spatial-phase-locked lithography. It must produce a detectable signal when struck by an electron beam, and the contrast of this signal must be sufficient to accurately determine the e-beam's position. To provide a good reference, the grid must be spatially coherent over the entire area to be patterned. In other words, if one selects a point on the grid and moves an integral number of periods in a given direction, the final point will be precisely located regardless of the distance involved. In addition, the grid must not disturb the e-beam writing or interfere with subsequent processing. Several types of grids have been proposed and tested, and one of these, an organic scintillator, meets all criteria listed above. [4] Currently we use a PMMA based resist with an integrated scintillator. Alternatively, the polymer film containing the scintillator can be spun on top of the resist and removed after exposure. A fiducial grid is quenched in the scintillator by exposing the resist to ultraviolct light that does not expose the resist itself. The U.V. radiation promotes local oxidation, and thus quenching, of the organic scintillat-
S~gnal~
,.
- 1 :.~.../..... J.....: ::,~' -Scintillatingresist ............ withfiducialgrid
photomultiplietube r ,~.~q'"""
/
Figure 2. Scintillation signal detection scheme for sparse sampling. ing molecule.[5] The grid requires no topography and occupies no space on the substrate; in addition, it is removed automatically when the resist is stripped. Gratings with periods as fine as 320nm were patterned by interference lithography [6] using 351nm wavelength radiation. Two orthogonal grating exposures produced the required grid. During an e-beam exposure using the sparsesampled mode of SPLEBL, each field is sampled to obtain the moird pattern from the fiducial grid. The scintillation signal is detected by a photo-multiplier tube (PMT), routed through a current pre-amplifier, and recorded by a synchronized digital frame grabber. An elliptical mirror focuses light from the scintillator onto the PMT to increase the signal level. Figure 2 depicts this detection arrangement. Due to problems with our elliptical mirror, an alternative configuration was used. The PMT was placed below a transparent quartz substrate. The image in figure l(a) was acquired in this way. 4. P A T T E R N W R I T I N G MENT RESULTS
AND PLACE-
Before writing patterns a standard field calibration is performed to determine the coordinate system of the laser-interferometer-controlled stage. The stage coordinates are mapped to the fiducial grid coordinates by comparing the initial calibration with the rotation of a field locked to the grid. This mapping can be verified by following grid lines along the edges of the sample over distances greater than one field width. To verify the performance of the sparse sample
364
J.T. Hastings et al. / Microelectronic Engineering 53 (2000) 361-364
algorithm, 300nm-period gratings were written in PMMA near the boundaries of stitched fields 104pro on an edge. The IBM VS2A vector-scan EBL system, with modified software implementing the phase-locking algorithm described above, was used to write the patterns. An intentional l#m gap was left between the adjacent gratings so that the field boundary could be easily identified. Two iterations of sparse sampling and phase locking were performed for each field. Since the geometry of our e-beam lithography system's chamber made light collection from the scintillator difficult, a metallic grid was used to debug and evaluate the sparse sample algorithm. Backscattered electrons from a 200nm-period gold fiducial grid provided the spatial-phase locking signal. The stitched gratings in PMMA were examined in an SEM to observe the alignment between gratings in adjacent fields and between the gratings and the underlying fiducial grid. In both cases no stitching errors in either x or y were observed within the limits of the SEM (~ 10nm). Preliminary Fourier transform analyses of the SEM images also yielded stitching errors less than 10nm. The experiment was repeated with an intentional 400#rad rotation introduced before performing spatial phase locking. If this error were not corrected the gratings would be shifted by ~40nm; however, no such shift was observed. The stitched gratings at the field boundary and the fiducial grid can be seen in figure 3. These results indicate that sub-10nm image-placement accuracy can be achieved with the sparse sampling algorithm.
5. C O N C L U S I O N S Sparse sampling a fiducial grid and carefully analyzing the resulting moir~ pattern provides a route to sub-10nm pattern-placement accuracy in e-beam lithography. The grid can be generated by only one additional processing step; specifically, U.V. quenching of a scintillating e-beam resist. As demonstrated, a grid of this type can be imaged by sparely sampling it with an e-beam without adversely affecting the final pattern. The functionality of the phase-locking algorithm was verified by writing stitched grating patterns over a metallic grid. In the future we will perform a statistical analysis of stitching errors using a scin-
Intended gap at field boundary ~ ,
Figure 3. SEM image of two stitched 300nm period gratings written with the sparse sampled mode of SPLEBL. The intentionally placed gap between gratings locates field boundaries. tillating grid and apply this mode of SPLEBL to the fabrication of integrated optical devices. Acknowledgements: The authors gratefully acknowledge J. Carter, J. Daley, and M. Mondol for their valuable technical assistance. This research was sponsored by the Army Research Office, and the Defense Advanced Research Projects Agency. REFERENCES 1. International Technology Roadmap for Semiconductors: 1998 Update, (Semiconductor Industry Association, 1998). 2. H.I. Smith, S.D. Hector, M.L. Schattenburg, and E.H. Anderson, J. Vac. Sci. Technol. B 9, (1991) 2992. 3. J. Ferrera, V. Wong, S. Rishton, V. Boegli, E.H. Anderson, D.P. Kern, H.I. Smith, J. Vac. Sci. Technol. B 11, (1993) 2342. 4. J.G. Goodberlet, J. Ferrera, and H.I. Smith, J. Vac. Sci. Technol. B 15, (1997) 2293. 5. J.G. Goodberlet, J. Carter, and H.I. Smith, J. Vac. Sci. Technol. B 16, (1998) 3672. 6. M.L Schattenburg, E.H. Anderson, and H.I. Smith, Phys. Scr. 41, (1990) 13.
Microelectronic Engineering 53 (2000) 361-364 www.elsevier.nl/locate/ mee
Improved Pattern-Placement Accuracy in E-Beam Lithography via Sparse-Sample Spatial-Phase Locking J.T. Hastings, F. Zhang, J.G. Goodberlet, and Henry Smith NanoStructures Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 A sparse-sampling algorithm has been developed for two-dimensional, spatial-phase-locked electron-beam lithography (SPLEBL). This algorithm corrects for field shift, scale and rotation errors. The algorithm can be used with a scintillating polymeric global-fiducial grid distributed over the substrate. An experimental evaluation of the sparse-sampling algorithm indicates that sub-10-nm pattern-placement accuracy can be obtained with this mode of SPLEBL. 1. I N T R O D U C T I O N Mask fabrication for next, generation lithographies requires increasingly accurate pattern placement with decreasing feature size. As early as the 130nm-feature node some mask-writing technologies demand sub-14nm image-placement accuracy which is beyond the the capability of conventional electron beam lithography (EBL) tools.J1] In addition, emerging technologies, such as integrated optics, specify even tighter tolerances for acceptable device performance. Numer()us efforts have been directed toward eliminating the underlying causes of placement errors in EBL tools, but these have not provided a direct reference between substrate and beam position. H. I. Smith et al. proposed that the interaction of an electron beam with a global-fiducial pattern on the substrate would provide closed-loop feedback for beam positioning.J2] Two single-axis implementations of such a spatial-phase-locked ebeam lithography (SPLEBL) scheme have been demonstrated, both using metallic gratings patterned by interference lithography.[3,4] Metallic grids on the substrate are unsuitable for many processes and use valuable substrate area. We report the development of a sparsesampling algorithm suitable for 2-D spatial-phase locking. This algorithm will be used with a polymeric scintillating global-fiducial grid, the generation and imaging of which is also described here. The combination of the sparse-sampling algorithm and scintillating grid will provide two0167-9317/00/$ - see front matter PII: S 0 1 6 7 - 9 3 1 7 ( 0 0 ) 0 0 3 3 4 - 8
dimensional phase locking without affecting pattern fidelity or consuming substrate area. 2. S P A R S E - S A M P L E A P P R O A C H
The sparse-sample implementation of spatialphase locking applies a ':look-then-right" procedure. Before writing, the entire field is sparsely sampled with the electron beam at a dose well below the development threshold. The sparsesample points define a uniformly spaced grid whose period differs slightly from the period of the fiducial grid on tile substrate. A moir6 image of the scintillating grid results, and the moir~ spatial frequency is given by tile difference between the sparse-sample and fiducial grid spatial frequencies. For example, a 512x512 sample array of 520x520 grid periods in x and y yields an 8x8 moir6 grid pattern. This is also true for higherorder aliasing; i.e. 512x512 samples of 1032x1032 periods. Figure l(a) shows a recorded moir~ image from a scintillating grid sparsely sampled with an e-beam. To the authors' knowledge, this is the first time a global scintillating-reference grid on the substrate has been detected in an EBL system. Figure l(b) shows a simulated ideal moir~ pattern. The shift of the fiducial grid relative to the scan field is directly calculated from the spatial phase of the moir6 pattern in a given direction (¢x for the x-direction) by Adx
© 2000 Elsevier Science B.V. All rights reserved.
Cx = 2-~Agx
362
J.T. Hastings et al. / Microelectronic Engineering 53 (2000) 361-364
. . . .
(a)
. . . . . . . . . . . . . . . . . . . . . . . . . . .
~ . . . .
(b)
Figure 1. (a) Moir6 pattern from 512x512 samples of 520x520 periods of an l p m period scintillating fiducial grid. (b) Simulated ideal moir6 pattern with outlined areas used for scale and rotation correction. where Ad~ is the shift in the x direction, and Ayx is the grid period in that direction. Once the moir6 image has been acquired, two one dimensional signals are extracted by taking the two dimensional fast Fourier transform (FFT) and retaining only the x and y axes. If the scaling, is perfect the spatial phase can be obtained directly from the FFT; otherwise, there will be small phase errors from spectral leakage into adjacent frequencies. One can solve this problem by iteratively scanning the field and correcting the scale and shift until the scaling converges. This approach is not favorable to sparse sampling with a scintillating grid because the e-beam damages the scintillator, and the resist receives a greater dose. Instead, we elect to transform the one-dimensional signals back to the spatial domain before determining the phase. The phase is calculated by multiplying tile signal by a reference sinusoid and integrating the result. The scale of the reference can be changed to match the measured scale of the moir~ pattern. In this way the shift error (:an be accurately determined from only one sparse sampling. Accurate scaling is obtained by sizing the field, and thus the moir4 pattern, to contain an integral number of periods, N=8 in our case. For fine correction, one calculates the scaling change required A~
=1+,
( ¢~. - Ct~ Agx 27r " D )
where ¢1~ and ¢rz are the x-phases of the left,
n = l , and right, n=8, moird periods respectively, and D is the center-to-center distance between the left and right peaks (approximately the e-beam field size). The corrected scaling is determined by dividing the original scale by A,~x. The y-scaling error is calculated similarly using the y-phase of the top and bottom moir6 periods. The pattern edges used are outlined in the ideal moir6 image of figure 2(b). The expression abow~ is only valid for small scaling errors. If the moir6 pattern is an entire period off, the pattern's two-dimensional Fourier transform peak locations can be used to correct these large errors. Although large corrections may be required for the initial setup betbre writing a new substrate, only small corrections are necessary during the write. The left-edge and right-edge y-phase, or topedge and bottom-edge x-phases, allow calculation of the field rotation (0) by 0 - - t a n - l ( cry~- ely . @ ) where Cry and Czy are the left and right edge y-phases, and D is defined as above. A similar expression applies if the top and bottom xphases are used. If the axes are not orthogonal the two calculations will yield different rotation values which are used to correct y-axis rotation and x-axis rotation individually. Trapezoidal scan-field distortion results in a variation of x-scale with y-location and vice versa. Such errors can also be quantified and corrected by analyzing the moir~ pattern. Currently, trapezoidal distortion is removed by standard field calibration techniques and is assumed to be constant throughout the exposure. Future implementations will incorporate this correction for each field. The algorithm was designed to correct field shift to within 1/105, i.e lnm in a 100#m field. The scaling routine targets correction to within 3/105, or 1/2 a beam step over an entire field for our 14 bit DACs. Finally, rotation correction is based on calculations similar to shift with comparable performance. However, rotation places greater demands on signal noise and contrast, because it uses less of the data from the sparsesample pattern. The computation time required for full field analysis is dominated by one gl2xgl2
363
J.T. Hastings et al. / Microelectronic Engineering 53 (2000) 361-364
2D-real FFT and four 512x64 2D-real FFTs. The computation time for 1D inverse FFTs, multiplications, and integrations is negligible for any realistic number of mathematical iterations. All of these calculations can be easily performed in under lmsec by a dedicated signal processing board such as the Alacron AL860MP used in our system. As a result, the correction algorithm does not limit the e-beam tool's throughput. A potentially adverse effect of sparse-sampling is a reduction in process latitude, or the broadening of a fine line by an individual sparsesample point. Consequently, experiments were conducted to determine the effect of sparse sampling on the final pattern. 400nm-period gratings were written in the scintillating PMMA along with sparse samples of varying dose immediately adjacent to the lines. The patterned PMMA was examined in a SEM to look for line broadening. No affect on the pattern was observed even at sample doses well above that needed to image the grid. 3. T H E F I D U C I A L G R I D
The fiducial grid on the substrate is critical to spatial-phase-locked lithography. It must produce a detectable signal when struck by an electron beam, and the contrast of this signal must be sufficient to accurately determine the e-beam's position. To provide a good reference, the grid must be spatially coherent over the entire area to be patterned. In other words, if one selects a point on the grid and moves an integral number of periods in a given direction, the final point will be precisely located regardless of the distance involved. In addition, the grid must not disturb the e-beam writing or interfere with subsequent processing. Several types of grids have been proposed and tested, and one of these, an organic scintillator, meets all criteria listed above. [4] Currently we use a PMMA based resist with an integrated scintillator. Alternatively, the polymer film containing the scintillator can be spun on top of the resist and removed after exposure. A fiducial grid is quenched in the scintillator by exposing the resist to ultraviolct light that does not expose the resist itself. The U.V. radiation promotes local oxidation, and thus quenching, of the organic scintillat-
S~gnal~
,.
- 1 :.~.../..... J.....: ::,~' -Scintillatingresist ............ withfiducialgrid
photomultiplietube r ,~.~q'"""
/
Figure 2. Scintillation signal detection scheme for sparse sampling. ing molecule.[5] The grid requires no topography and occupies no space on the substrate; in addition, it is removed automatically when the resist is stripped. Gratings with periods as fine as 320nm were patterned by interference lithography [6] using 351nm wavelength radiation. Two orthogonal grating exposures produced the required grid. During an e-beam exposure using the sparsesampled mode of SPLEBL, each field is sampled to obtain the moird pattern from the fiducial grid. The scintillation signal is detected by a photo-multiplier tube (PMT), routed through a current pre-amplifier, and recorded by a synchronized digital frame grabber. An elliptical mirror focuses light from the scintillator onto the PMT to increase the signal level. Figure 2 depicts this detection arrangement. Due to problems with our elliptical mirror, an alternative configuration was used. The PMT was placed below a transparent quartz substrate. The image in figure l(a) was acquired in this way. 4. P A T T E R N W R I T I N G MENT RESULTS
AND PLACE-
Before writing patterns a standard field calibration is performed to determine the coordinate system of the laser-interferometer-controlled stage. The stage coordinates are mapped to the fiducial grid coordinates by comparing the initial calibration with the rotation of a field locked to the grid. This mapping can be verified by following grid lines along the edges of the sample over distances greater than one field width. To verify the performance of the sparse sample
364
J.T. Hastings et al. / Microelectronic Engineering 53 (2000) 361-364
algorithm, 300nm-period gratings were written in PMMA near the boundaries of stitched fields 104pro on an edge. The IBM VS2A vector-scan EBL system, with modified software implementing the phase-locking algorithm described above, was used to write the patterns. An intentional l#m gap was left between the adjacent gratings so that the field boundary could be easily identified. Two iterations of sparse sampling and phase locking were performed for each field. Since the geometry of our e-beam lithography system's chamber made light collection from the scintillator difficult, a metallic grid was used to debug and evaluate the sparse sample algorithm. Backscattered electrons from a 200nm-period gold fiducial grid provided the spatial-phase locking signal. The stitched gratings in PMMA were examined in an SEM to observe the alignment between gratings in adjacent fields and between the gratings and the underlying fiducial grid. In both cases no stitching errors in either x or y were observed within the limits of the SEM (~ 10nm). Preliminary Fourier transform analyses of the SEM images also yielded stitching errors less than 10nm. The experiment was repeated with an intentional 400#rad rotation introduced before performing spatial phase locking. If this error were not corrected the gratings would be shifted by ~40nm; however, no such shift was observed. The stitched gratings at the field boundary and the fiducial grid can be seen in figure 3. These results indicate that sub-10nm image-placement accuracy can be achieved with the sparse sampling algorithm.
5. C O N C L U S I O N S Sparse sampling a fiducial grid and carefully analyzing the resulting moir~ pattern provides a route to sub-10nm pattern-placement accuracy in e-beam lithography. The grid can be generated by only one additional processing step; specifically, U.V. quenching of a scintillating e-beam resist. As demonstrated, a grid of this type can be imaged by sparely sampling it with an e-beam without adversely affecting the final pattern. The functionality of the phase-locking algorithm was verified by writing stitched grating patterns over a metallic grid. In the future we will perform a statistical analysis of stitching errors using a scin-
Intended gap at field boundary ~ ,
Figure 3. SEM image of two stitched 300nm period gratings written with the sparse sampled mode of SPLEBL. The intentionally placed gap between gratings locates field boundaries. tillating grid and apply this mode of SPLEBL to the fabrication of integrated optical devices. Acknowledgements: The authors gratefully acknowledge J. Carter, J. Daley, and M. Mondol for their valuable technical assistance. This research was sponsored by the Army Research Office, and the Defense Advanced Research Projects Agency. REFERENCES 1. International Technology Roadmap for Semiconductors: 1998 Update, (Semiconductor Industry Association, 1998). 2. H.I. Smith, S.D. Hector, M.L. Schattenburg, and E.H. Anderson, J. Vac. Sci. Technol. B 9, (1991) 2992. 3. J. Ferrera, V. Wong, S. Rishton, V. Boegli, E.H. Anderson, D.P. Kern, H.I. Smith, J. Vac. Sci. Technol. B 11, (1993) 2342. 4. J.G. Goodberlet, J. Ferrera, and H.I. Smith, J. Vac. Sci. Technol. B 15, (1997) 2293. 5. J.G. Goodberlet, J. Carter, and H.I. Smith, J. Vac. Sci. Technol. B 16, (1998) 3672. 6. M.L Schattenburg, E.H. Anderson, and H.I. Smith, Phys. Scr. 41, (1990) 13.