- Email: [email protected]
An alignment system for X-ray lithography
Microelectronic Engineering 11 (1990) 263-266 Elsevier Science Publishers B.V.
AN ALIGNMENT
SYSTEM
263
FOR X-RAY LITHOGRAPHY
Y. Tanaka, E. Kouno, and J. lwata Production Engineering Development Laboratory NEC Corporation 3-484, Tsukagoshi, Saiwai-ku, Kawasaki, 210 Japan
A new alignment technique for x-ray lithography, projection linear Fresnel zone plate (LFZP) method, has been developed and applied to an x-ray stepper for synchrotron radiation lithography. A wafer alignment mark is magnified and projected using an LFZP on a mask. The wafer mark image position is measured very precisely by a scanning optical system. The alignment error between a mask and a wafer is detected with less than 0.01 pm resolution. Up to date, 0.1 pm (30) overlay accuracy has been achieved.
1. INTRODUCTION Synchrotron radiation (SR) lithography is the most promising technology for high volume production of future VLSls with a quarter micron circuit pattern, because of the highresolution capability with large focus depth. So far, the authors developed a prototype x-ray stepper for synchrotron radiation lithography (SR stepper) [l], and installed it at the Photon Factory in the National Laboratory for High Energy Physics, Japan. It was connected with a UHV beam line for SR lithography [2]. The previous alignment method for the SR stepper [l] had achieved 0.2 pm (30) overlay accuracy, using a differential linear Fresnel zone plate method. However, it is difficult to improve the overlay accuracy, because this alignment method, based on the comparison between diffracted beam intensities from two alignment marks, is sensitive to the wafer surface condition (e.g. reflectivity etc.). Recently, a new alignment technique has been developed in order to achieve further accurate alignment. This paper describes an SR stepper mechanism, a new alignment system and experimental results.
2. X-RAY STEPPER An SR stepper, installed at the Photon Factory, was designed for a quarter micron lithography in a laboratory. Figure 1 shows an SR stepper diagram. The SR stepper consists of an SR extracting chamber connected with a beam line, a mask stage for fine alignment and gap setting, a wafer stage for fine alignment and step-and-repeat feeds, and an alignment error detection system. The mask stage can travel in zl , 22, 23, and 8 directions, where zl ,22 and 23 are small range linear motions in the SR beam direction and 8 is rotation around the SR beam. All the degrees of freedom are supported by link mechanisms and driven by piezoelectric actuators. The wafer stage consists of a coarse positioning xy stage driven by DC motors, a fine positioning xy stage driven by piezoelectric actuators and a two-axis laser interferometer system. The 0.01 pm resolution wafer positioning is accomplished over long travel for a 4 inch wafer, owing to the cooperative motions using the coarse positioning stage and the fine positioning stage.
0167-9317/90/$3.50 0 1990, Elsevier Science Publishers B.V.
264
Y. Tanaka et al. I Alignment system for X-ray lithography
I
L
1
1.
Wafer chuck
2.
Wafer fine stage
3.
Wafer coarse stage
4.
Mask chuck
5.
SR mask
6.
Mask stage
7.
Alignment detector
8.
He chamber
9.
Be window
10.
Laser interferometer
11.
Stage controller
12.
Main computer
13.
Console
Fig. 1. SR stepper diagram
3. ALIGNMENT
SYSTEM
In order to apply SR lithography to all exposure levels for a quarter micron device fabrication, it is necessary to achieve 0.04-0.06 pm overlay accuracy between each pair of exposure levels, A newly developed projection LFZP alignment technique has been designed to satisfy this requirement. The projection LFZP alignment is implemented by using an LFZP as a mask mark and a narrow diffractive grating line as a wafer mark. An LFZP is a transparent diffractive pattern, which acts as a cylindrical lens. Figure 2 shows the projection LFZP method principle. When a laser beam illuminates the grating line on the wafer, the laser beam is diffracted in a particular direction (i.e. first order diffraction direction) by the grating line. The grating line image is projected and magnified on the focal plane of the projection optics, which consists of the LFZP on the mask and an external focusing lens. Therefore, a small displacement between a mask and a wafer produces a large magnified displacement of the grating line image. Scanning slit \
Photodetector -Incident +-----Diffracted
Fig. 2. Alignment optics system for the projection LFZP method
beam beam
Fig. 3. Incident laser beam and alignment marks details
265
Y. Tanaka et al. I Alignment system for X-ray lithography
An LFZP is 40 pm in focal length, which is equal to the proximit gap between a mask and a wafer. A grating line is 2 pm wide. An incident laser beam rHe-Ne laser) is focused by focusing optics. The spot size on the mask is 20x115 pm. The LFZP has little lens effect on the incident laser beam, because the laser spot illuminating the LFZP is small, compared with the LFZP width (approximately 47 pm). Figure 3 shows details of an incident laser beam and alignment marks. A wide laser spot illuminates a wafer and the position error of the grating line is projected on the focal plane over a wide range. Figure 4 shows a simplified view of the projection optics in the alignment system. The magnification of the projection optics is given by f2/fl, where fl is the LFZP focal length and f2 is the focusing lens focal length. Since this system has a 40 pm focal length LFZP and a 120 mm focal length focusing lens, the grating line image is magnified 3000 diameters. f2
Grating line
Fig. 4. Simplified
view of the projection optics in the alignment
system
The magnified grating line image position is measured precisely by a scanning optical system. A diffracted laser beam, passing through the scanning slit, which is located on the projection optics focal plane, is detected by a photodiode. The scanning frequency is 600 Hz. The scanning range is 12 mm, corresponding to 4 pm on the wafer surface. A phase sensitive detector converts the photodiode output into an alignment error signal for closed loop autoalignment. If the phase sensing resolution is 1 degree, this system will detect the alignment error with 0.01 pm resolution. All the optical components for the projection LFZP method are located beam, although alignment marks are located inside the exposure field. alignment system is capable of implementing alignment error detection at ing during exposure, and of renewing wafer marks by SR exposure when damaged. Furthermore, a stable scanning optical system, based on a detection technique with large process latitude and a high resolution image an LFZP, enables us to detect an alignment error precisely.
4. EXPERIMENTAL
outside the SR Therefore, this all times, includa wafer mark is phase sensitive projection using
RESULT
An alignment error signal, generated by a lock-in amplifier, is shown in Fig. 5. This signal has 8.9 V/pm sensitivity and approximately 1 pm dynamic range. Figure 6 shows an alignment error signal caused by the 0.05 pm amplitude step motion of the wafer stage. The upper signal is the output from a capacitive sensor measuring the wafer stage displacement. The lower signal is the lock-in amplifier output. The noise level in the lock-in amplifier output is 80 mV (peak to peak value). Therefore, this alignment system has less than 0.01 pm resolution for the alignment error detection.
266
Y. Tanaka et al. 1 Alignment system for X-ray lithography
Position
Fig. 5. Alignment projection
Error thorn)
signal obtained from the LFZP alignment system
Time (set)
Fig. 6. Alignment signal caused by 0.05 pm step motion
The SR stepper was equipped with 3 channels of alignment optics. Finally, the overlay accuracy was estimated. The overlay error was measured by reading vernier patterns (0.05 pm pitch). These patterns were made by making exposure two times, using the same mask, in order to avoid mask distortion or process influence. Figure 7 shows the overlay error. The standard deviation is approximately 0.03 pm in both x and y axes. This result shows that less than 0.1 pm (30) overlay accuracy was achieved for a 25 mm exposure field.
Fig. 7. Overlay accuracy
measured by vernier pattern
5. CONCLUSION A new alignment error detection system, employing the projection LFZP method, has been developed. The alignment system can detect alignment error with less than 0.01 pm resolution at all times, including during exposure. An SR stepper, equipped with the new alignment system, accomplished 0.1 pm (30) overlay accuracy. Further improvement in the alignment optics will result in an overlay accuracy suitable for use in quarter micron device fabrication. ACKNOWLEDGMENTS The authors would like to thank Dr. K. Okada and K. Suzuki for their informative sions.
discus-
REFERENCE [l] E. Kouno, Y. Tanaka, and J. lwata J. Vat. Sci. Technol. B6 (6), 2135 (1988) [2] K. Okada, K. Fujii, Y. Kawase, and M. Nagano J. Vat. Sci. Technol. B6 (l), 191 (1988)
AN ALIGNMENT
SYSTEM
263
FOR X-RAY LITHOGRAPHY
Y. Tanaka, E. Kouno, and J. lwata Production Engineering Development Laboratory NEC Corporation 3-484, Tsukagoshi, Saiwai-ku, Kawasaki, 210 Japan
A new alignment technique for x-ray lithography, projection linear Fresnel zone plate (LFZP) method, has been developed and applied to an x-ray stepper for synchrotron radiation lithography. A wafer alignment mark is magnified and projected using an LFZP on a mask. The wafer mark image position is measured very precisely by a scanning optical system. The alignment error between a mask and a wafer is detected with less than 0.01 pm resolution. Up to date, 0.1 pm (30) overlay accuracy has been achieved.
1. INTRODUCTION Synchrotron radiation (SR) lithography is the most promising technology for high volume production of future VLSls with a quarter micron circuit pattern, because of the highresolution capability with large focus depth. So far, the authors developed a prototype x-ray stepper for synchrotron radiation lithography (SR stepper) [l], and installed it at the Photon Factory in the National Laboratory for High Energy Physics, Japan. It was connected with a UHV beam line for SR lithography [2]. The previous alignment method for the SR stepper [l] had achieved 0.2 pm (30) overlay accuracy, using a differential linear Fresnel zone plate method. However, it is difficult to improve the overlay accuracy, because this alignment method, based on the comparison between diffracted beam intensities from two alignment marks, is sensitive to the wafer surface condition (e.g. reflectivity etc.). Recently, a new alignment technique has been developed in order to achieve further accurate alignment. This paper describes an SR stepper mechanism, a new alignment system and experimental results.
2. X-RAY STEPPER An SR stepper, installed at the Photon Factory, was designed for a quarter micron lithography in a laboratory. Figure 1 shows an SR stepper diagram. The SR stepper consists of an SR extracting chamber connected with a beam line, a mask stage for fine alignment and gap setting, a wafer stage for fine alignment and step-and-repeat feeds, and an alignment error detection system. The mask stage can travel in zl , 22, 23, and 8 directions, where zl ,22 and 23 are small range linear motions in the SR beam direction and 8 is rotation around the SR beam. All the degrees of freedom are supported by link mechanisms and driven by piezoelectric actuators. The wafer stage consists of a coarse positioning xy stage driven by DC motors, a fine positioning xy stage driven by piezoelectric actuators and a two-axis laser interferometer system. The 0.01 pm resolution wafer positioning is accomplished over long travel for a 4 inch wafer, owing to the cooperative motions using the coarse positioning stage and the fine positioning stage.
0167-9317/90/$3.50 0 1990, Elsevier Science Publishers B.V.
264
Y. Tanaka et al. I Alignment system for X-ray lithography
I
L
1
1.
Wafer chuck
2.
Wafer fine stage
3.
Wafer coarse stage
4.
Mask chuck
5.
SR mask
6.
Mask stage
7.
Alignment detector
8.
He chamber
9.
Be window
10.
Laser interferometer
11.
Stage controller
12.
Main computer
13.
Console
Fig. 1. SR stepper diagram
3. ALIGNMENT
SYSTEM
In order to apply SR lithography to all exposure levels for a quarter micron device fabrication, it is necessary to achieve 0.04-0.06 pm overlay accuracy between each pair of exposure levels, A newly developed projection LFZP alignment technique has been designed to satisfy this requirement. The projection LFZP alignment is implemented by using an LFZP as a mask mark and a narrow diffractive grating line as a wafer mark. An LFZP is a transparent diffractive pattern, which acts as a cylindrical lens. Figure 2 shows the projection LFZP method principle. When a laser beam illuminates the grating line on the wafer, the laser beam is diffracted in a particular direction (i.e. first order diffraction direction) by the grating line. The grating line image is projected and magnified on the focal plane of the projection optics, which consists of the LFZP on the mask and an external focusing lens. Therefore, a small displacement between a mask and a wafer produces a large magnified displacement of the grating line image. Scanning slit \
Photodetector -Incident +-----Diffracted
Fig. 2. Alignment optics system for the projection LFZP method
beam beam
Fig. 3. Incident laser beam and alignment marks details
265
Y. Tanaka et al. I Alignment system for X-ray lithography
An LFZP is 40 pm in focal length, which is equal to the proximit gap between a mask and a wafer. A grating line is 2 pm wide. An incident laser beam rHe-Ne laser) is focused by focusing optics. The spot size on the mask is 20x115 pm. The LFZP has little lens effect on the incident laser beam, because the laser spot illuminating the LFZP is small, compared with the LFZP width (approximately 47 pm). Figure 3 shows details of an incident laser beam and alignment marks. A wide laser spot illuminates a wafer and the position error of the grating line is projected on the focal plane over a wide range. Figure 4 shows a simplified view of the projection optics in the alignment system. The magnification of the projection optics is given by f2/fl, where fl is the LFZP focal length and f2 is the focusing lens focal length. Since this system has a 40 pm focal length LFZP and a 120 mm focal length focusing lens, the grating line image is magnified 3000 diameters. f2
Grating line
Fig. 4. Simplified
view of the projection optics in the alignment
system
The magnified grating line image position is measured precisely by a scanning optical system. A diffracted laser beam, passing through the scanning slit, which is located on the projection optics focal plane, is detected by a photodiode. The scanning frequency is 600 Hz. The scanning range is 12 mm, corresponding to 4 pm on the wafer surface. A phase sensitive detector converts the photodiode output into an alignment error signal for closed loop autoalignment. If the phase sensing resolution is 1 degree, this system will detect the alignment error with 0.01 pm resolution. All the optical components for the projection LFZP method are located beam, although alignment marks are located inside the exposure field. alignment system is capable of implementing alignment error detection at ing during exposure, and of renewing wafer marks by SR exposure when damaged. Furthermore, a stable scanning optical system, based on a detection technique with large process latitude and a high resolution image an LFZP, enables us to detect an alignment error precisely.
4. EXPERIMENTAL
outside the SR Therefore, this all times, includa wafer mark is phase sensitive projection using
RESULT
An alignment error signal, generated by a lock-in amplifier, is shown in Fig. 5. This signal has 8.9 V/pm sensitivity and approximately 1 pm dynamic range. Figure 6 shows an alignment error signal caused by the 0.05 pm amplitude step motion of the wafer stage. The upper signal is the output from a capacitive sensor measuring the wafer stage displacement. The lower signal is the lock-in amplifier output. The noise level in the lock-in amplifier output is 80 mV (peak to peak value). Therefore, this alignment system has less than 0.01 pm resolution for the alignment error detection.
266
Y. Tanaka et al. 1 Alignment system for X-ray lithography
Position
Fig. 5. Alignment projection
Error thorn)
signal obtained from the LFZP alignment system
Time (set)
Fig. 6. Alignment signal caused by 0.05 pm step motion
The SR stepper was equipped with 3 channels of alignment optics. Finally, the overlay accuracy was estimated. The overlay error was measured by reading vernier patterns (0.05 pm pitch). These patterns were made by making exposure two times, using the same mask, in order to avoid mask distortion or process influence. Figure 7 shows the overlay error. The standard deviation is approximately 0.03 pm in both x and y axes. This result shows that less than 0.1 pm (30) overlay accuracy was achieved for a 25 mm exposure field.
Fig. 7. Overlay accuracy
measured by vernier pattern
5. CONCLUSION A new alignment error detection system, employing the projection LFZP method, has been developed. The alignment system can detect alignment error with less than 0.01 pm resolution at all times, including during exposure. An SR stepper, equipped with the new alignment system, accomplished 0.1 pm (30) overlay accuracy. Further improvement in the alignment optics will result in an overlay accuracy suitable for use in quarter micron device fabrication. ACKNOWLEDGMENTS The authors would like to thank Dr. K. Okada and K. Suzuki for their informative sions.
discus-
REFERENCE [l] E. Kouno, Y. Tanaka, and J. lwata J. Vat. Sci. Technol. B6 (6), 2135 (1988) [2] K. Okada, K. Fujii, Y. Kawase, and M. Nagano J. Vat. Sci. Technol. B6 (l), 191 (1988)