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A novel X-ray mask concept for mex&match lithography fabrication of MOS devices by synchrotron radiation lithography
MICROEI~CTRONIC ENGINEERING
Mieroelectronie Engineering 35 (1997) 553-556
ELSEVIER
A Novel X-ray Mask Concept for Mix&Match Lithography Fabrication o f MOS Devices by Synchrotron Radiation Lithography. E. Di Fabrizio, L. Grella, M. Gentili, M. Baciocchi and L. Mastrogiacomo lstituto di Eiettronica dello Stato Solido-CNR, Via Cineto Romano 42, 1-00156 Rome, ltaly
Sang-Soo Choi, Young Jin Jeon, Hyung Joun Yoo, Hal Bin Chung Electronics and Telecommunication Research Institute, P.O. Box 106, Yusoung Taejon, 305-606 Korea
The investigated X-ray steppers Karl Suss XRS 200/1 and XRS 200/3 use an optical alignment system for adjusting the parallelism, for gap setting and alignment between mask and wafer. In general the optical signal, when mask and wafer are at gap distance, is dependent on the optical characteristics (transparency of the mask, reflectivity of the wafer, resist film interference etc.) of both mask and wafer. In this paper, different methods to decouple the optical signal of the mask by that of the wafer and to effectively improve the optical signal contrast, are presented. The methods used allow to reach in all examined cases an alignment accuracy down to 5 nm. This value can be regarded as the resolution limit of the alignment system itself. 1. INTRODUCTION In this study we present a novel mask fabrication technique that allows an improvement of the alignGap distance 50:30
Microscopes
X-rays ,~
Visible
~
um
//~
~
I
Mask
Wafer
Chi window Fig. 1: Schematic of alignment system and maskwafer mounting of XRS200/1 and XRS200/3 X-ray steppers 0167-9317(97)/$17.00 © 1997 Elsevier Science B,V All rights reserved. PII: S0167-9317(96)00156-6
ment accuracy of a Karl Suss X-ray stepper, down to its intrinsic resolution limit [I]. In the examined X-ray aligners (figure 1 is a schematic draft of the alignment system), the illumination light of the alignment system needs topass through the membrane and to be reflected by the alignment markers back to the optical microscopes (see figure 2); in this case the optical signal contrast will be sensitively affected by the optical properties of the membranes. Namely: a) the transparency of the membrane b) the interference of the film thickness of the membrane The present work proposes a way on how to make point a) less important, and how to exploit the interference effect, due to a further film coated only underneath the alignment markers, for an improvement of the optical signal contrast. Three different methods are developed to accomplish this task: 1) direct pass through writing with e-beam lithography. 2) self-aligned metal marker transfer with X-ray or optical lithography 3) self-aligned resist marker transfer with X-ray or optical lithography
554
E. Di Fabrizio et al. /Microelectronic Engineering 35 (1997) 553-556
Mask front side
Mask with alignment markers on the back side
jchip
window
alignment
Si,jN4 A u
back side
front side C
~
detailedview detailed view Illumination system for a conventionalmask Methods land 2
Side view
back
front side
Microscopes
side
~
Au sir,N4 Au
front
¢
side
detailed view P.csiat
Si3N 4 A u *
Sir,N+ A u
Method 3
¢
detailed view detailed view Fig. 2: Detailed view of mask alignment markers and illumination system
Methods l) and 2) are intended to fabricate the alignment markers directly in front of the microscopes light, as in figure 3. The illumination light from the microscopes is reflected directly by the alignment markers because there is no material before them. Method 3), the simplest from a fabrication point of view, exploits the interferenceof the double layer constituted by the resist film and the Si3N4 membrane (figure 3).
2. E X P E R I M E N T A L : S E L F - A L I G N E D MARKER TRANSFER TO THE BACKSIDE OF THE MASK 2.1. Description of method 1. The conventional mask is coated by resist in both
Fig. 3: Illustration of the alignment marker transfer methods on the back side of the mask. The transferred markers can be made by gold (methods 1,2) or by resist (method 3)
sides before the e-beam exposure. Then the pattern is exposed and developed in a standard way for positive process resist [2]. The markers are fabricated with gold eleetrodeposition. The electrons passing through the membrane expose the resist on both sides of the membrane and, as a consequence, the markers are written self-aligned on the back of the membrane. To have a good patterning of the markers it is necessary to expose the mask at an electron beam energy equal at least to 40 KeV: at this energy the electrons can pass through the membrane with a small e-beam spreading. By Monte Carlo calculations the electron beam spreading was evaluated to be 0.55 lam from a threegaussians fit. The positioning of the alignment markers is as accurate as a conventional mask. The
E. Di Fabrizio et al. / Microelectronic Engineering 35 (1997) 553-556
quality of the markers is good because they are not high resolution structures (minimum size 3 lam) and therefore the beam spreading doesn't decrease the image quality. 2.2. Description of method 2. Only one side (top side) of the mask is coated as in a conventional X-ray mask fabrication. The new part is the marker transfer on the back side of the membrane. For this process step the conventional mask is coated on the back side with a negative resist. In this work it was used the negative chemically amplified resist SAL 601 from Shipley corporation. The mask was exposed from the front side by X-ray synchrotron radiation. After development and gold electroplating growth, the markers are fabricated on the back side of the mask with the proper tone and self-aligned (see figure 3). The markers that were written on the front side have the function of conventional X-ray absorber. This method could be implemented also by using UV radiation (instead of X-rays) and optical resist. 2.3. Description of method 3. This method is, from a fabrication point of view, similar to method 2, but the working principle rely on optical interference laws The mask is coated from the back side with a positive resist (it was used 0.1 lam thick PMMA resist) then exposed from the front side by X-rays synchrotron radiation. After development, a resist film is leR only behind the previously fabricated alignment metallic markers on the front side of the mask, as shown in figure 3. In this way, by calibrating the thickness
555
of the resist coating, it is possible to maximise the constructive interference between the light reflected back by the membrane and the light reflected by the double layer constituted by the resist and the membrane under the markers [3],[4].
3. OPTICAL SIGNAL AND ALIGNMENT ACCURACY The masks fabricated by using the described three methods were mounted and tested on both steppers XRS200/3 and XRS200/1. The tests consisted in mounting the mask and wafer on the stepper and reaching the fmal condition of "fine alignment" [5] that is the last operation that the stepper does before the exposure. The alignment step consists in the alignment of the mask and wafer, at gap distance, by means of the optical signal coming from the alignment markers, of both, mask and wafer. These measurements were performed by using different wafers substrate. In particular, poly-level wafers and contact level wafers for MOS fabrication devices coated with AZ-PF 514 positive and SAL 601 negative chemical amplified resists were examined. In table I the alignment signal in the different conditions are reported; it is important to notice that in all the cases the "fine alignment" accuracy is below 10 nm with the masks with back alignment markers. Moreover, it can be also seen that this novel technique makes the alignment results nearly "decoupled" from the coating material wafer, so that the accuracy of the measurements can be
Alignment accuracy ~ (rim) _Poly + AZ-PF Poly + SAL 601 Contact + AZ-PF Contact + SAL 601
Conventional mask 25 20 25 22
Gold markers on the back side 5 6 3 7
Resist markers on the back side 9 7 8 8
Tab. 1: Measurements of standard deviation c~ relative to masks and wafers at the specified working conditions.
556
E. Di Fabrizio et al. / Microelectronic Engineering 35 (1997) 553-556
regarded as a "noise" of the stepper alignment system itself. Finally, we notice that the same measurement performed on an Si3N4 conventional mask resulted in an alignment accuracy which was (1 ~) 20 nm in the best case. The reason for this result is due essentially to the better optical contrast of the mask fabricated with the back marker transfer.
ACKNOWLEDGMENTS We would like to acknowledge the Centre for Xray Lithography (CXrL), University of Wisconsin, USA, for technical support and the facility supply.
REFERENCES
4. CONCLUSIONS A new technique to transfer the mask alignment markers to the back side of the substrate film, was developed. The transfer is self-aligned and doesn't introduce any placement error. The best advantage of this technique is that, when the transferred markers are made by gold, the reflectivity is dominated by the gold reflectivity and is almost independent of the membrane substrate and wafer reflectivity. Although the study was accomplished by using Si3N4 membrane for the mask material, and gold for alignment markers, this technique can be used also by combining other materials when required. The improvement in the alignment accuracy measurements is a direct consequence of the increased optical contrast of the mask.
[1] C.J. Progler, A.C. Chen, T.A. Gunther, P. Kaiser, K.A. Cooper, E. Hughlett-J. Vac. Sci. Technol. B 11(6), 2888 (1993). [2] M. Gentili, R. Kumar, L. Luciani, L. Grella, D. Plumb, and Q. Leonard - J. Vac. Sci. Technol. B. 9, 3319 (1991) [3] O. S. Heavens - "Optical properties of thin solid films", p. 46. Dover publishing (1991) [4] E. Di Fabrizio, L. Grella, M. Gentili, M. Bacioechi, L. Mastrogiacomo, D. Peschiaroli, L. Mastrogiacomo, R. Maggiora, Sang-Soo Choi*, Young Jill Jeon*, Hyung Joun Yoo*, Hai Bin Chung, F. Cerrina - International Patent pending: RM96A000635 [5] Optical system alignment manual. Internal report Karl Suss 1990.
Mieroelectronie Engineering 35 (1997) 553-556
ELSEVIER
A Novel X-ray Mask Concept for Mix&Match Lithography Fabrication o f MOS Devices by Synchrotron Radiation Lithography. E. Di Fabrizio, L. Grella, M. Gentili, M. Baciocchi and L. Mastrogiacomo lstituto di Eiettronica dello Stato Solido-CNR, Via Cineto Romano 42, 1-00156 Rome, ltaly
Sang-Soo Choi, Young Jin Jeon, Hyung Joun Yoo, Hal Bin Chung Electronics and Telecommunication Research Institute, P.O. Box 106, Yusoung Taejon, 305-606 Korea
The investigated X-ray steppers Karl Suss XRS 200/1 and XRS 200/3 use an optical alignment system for adjusting the parallelism, for gap setting and alignment between mask and wafer. In general the optical signal, when mask and wafer are at gap distance, is dependent on the optical characteristics (transparency of the mask, reflectivity of the wafer, resist film interference etc.) of both mask and wafer. In this paper, different methods to decouple the optical signal of the mask by that of the wafer and to effectively improve the optical signal contrast, are presented. The methods used allow to reach in all examined cases an alignment accuracy down to 5 nm. This value can be regarded as the resolution limit of the alignment system itself. 1. INTRODUCTION In this study we present a novel mask fabrication technique that allows an improvement of the alignGap distance 50:30
Microscopes
X-rays ,~
Visible
~
um
//~
~
I
Mask
Wafer
Chi window Fig. 1: Schematic of alignment system and maskwafer mounting of XRS200/1 and XRS200/3 X-ray steppers 0167-9317(97)/$17.00 © 1997 Elsevier Science B,V All rights reserved. PII: S0167-9317(96)00156-6
ment accuracy of a Karl Suss X-ray stepper, down to its intrinsic resolution limit [I]. In the examined X-ray aligners (figure 1 is a schematic draft of the alignment system), the illumination light of the alignment system needs topass through the membrane and to be reflected by the alignment markers back to the optical microscopes (see figure 2); in this case the optical signal contrast will be sensitively affected by the optical properties of the membranes. Namely: a) the transparency of the membrane b) the interference of the film thickness of the membrane The present work proposes a way on how to make point a) less important, and how to exploit the interference effect, due to a further film coated only underneath the alignment markers, for an improvement of the optical signal contrast. Three different methods are developed to accomplish this task: 1) direct pass through writing with e-beam lithography. 2) self-aligned metal marker transfer with X-ray or optical lithography 3) self-aligned resist marker transfer with X-ray or optical lithography
554
E. Di Fabrizio et al. /Microelectronic Engineering 35 (1997) 553-556
Mask front side
Mask with alignment markers on the back side
jchip
window
alignment
Si,jN4 A u
back side
front side C
~
detailedview detailed view Illumination system for a conventionalmask Methods land 2
Side view
back
front side
Microscopes
side
~
Au sir,N4 Au
front
¢
side
detailed view P.csiat
Si3N 4 A u *
Sir,N+ A u
Method 3
¢
detailed view detailed view Fig. 2: Detailed view of mask alignment markers and illumination system
Methods l) and 2) are intended to fabricate the alignment markers directly in front of the microscopes light, as in figure 3. The illumination light from the microscopes is reflected directly by the alignment markers because there is no material before them. Method 3), the simplest from a fabrication point of view, exploits the interferenceof the double layer constituted by the resist film and the Si3N4 membrane (figure 3).
2. E X P E R I M E N T A L : S E L F - A L I G N E D MARKER TRANSFER TO THE BACKSIDE OF THE MASK 2.1. Description of method 1. The conventional mask is coated by resist in both
Fig. 3: Illustration of the alignment marker transfer methods on the back side of the mask. The transferred markers can be made by gold (methods 1,2) or by resist (method 3)
sides before the e-beam exposure. Then the pattern is exposed and developed in a standard way for positive process resist [2]. The markers are fabricated with gold eleetrodeposition. The electrons passing through the membrane expose the resist on both sides of the membrane and, as a consequence, the markers are written self-aligned on the back of the membrane. To have a good patterning of the markers it is necessary to expose the mask at an electron beam energy equal at least to 40 KeV: at this energy the electrons can pass through the membrane with a small e-beam spreading. By Monte Carlo calculations the electron beam spreading was evaluated to be 0.55 lam from a threegaussians fit. The positioning of the alignment markers is as accurate as a conventional mask. The
E. Di Fabrizio et al. / Microelectronic Engineering 35 (1997) 553-556
quality of the markers is good because they are not high resolution structures (minimum size 3 lam) and therefore the beam spreading doesn't decrease the image quality. 2.2. Description of method 2. Only one side (top side) of the mask is coated as in a conventional X-ray mask fabrication. The new part is the marker transfer on the back side of the membrane. For this process step the conventional mask is coated on the back side with a negative resist. In this work it was used the negative chemically amplified resist SAL 601 from Shipley corporation. The mask was exposed from the front side by X-ray synchrotron radiation. After development and gold electroplating growth, the markers are fabricated on the back side of the mask with the proper tone and self-aligned (see figure 3). The markers that were written on the front side have the function of conventional X-ray absorber. This method could be implemented also by using UV radiation (instead of X-rays) and optical resist. 2.3. Description of method 3. This method is, from a fabrication point of view, similar to method 2, but the working principle rely on optical interference laws The mask is coated from the back side with a positive resist (it was used 0.1 lam thick PMMA resist) then exposed from the front side by X-rays synchrotron radiation. After development, a resist film is leR only behind the previously fabricated alignment metallic markers on the front side of the mask, as shown in figure 3. In this way, by calibrating the thickness
555
of the resist coating, it is possible to maximise the constructive interference between the light reflected back by the membrane and the light reflected by the double layer constituted by the resist and the membrane under the markers [3],[4].
3. OPTICAL SIGNAL AND ALIGNMENT ACCURACY The masks fabricated by using the described three methods were mounted and tested on both steppers XRS200/3 and XRS200/1. The tests consisted in mounting the mask and wafer on the stepper and reaching the fmal condition of "fine alignment" [5] that is the last operation that the stepper does before the exposure. The alignment step consists in the alignment of the mask and wafer, at gap distance, by means of the optical signal coming from the alignment markers, of both, mask and wafer. These measurements were performed by using different wafers substrate. In particular, poly-level wafers and contact level wafers for MOS fabrication devices coated with AZ-PF 514 positive and SAL 601 negative chemical amplified resists were examined. In table I the alignment signal in the different conditions are reported; it is important to notice that in all the cases the "fine alignment" accuracy is below 10 nm with the masks with back alignment markers. Moreover, it can be also seen that this novel technique makes the alignment results nearly "decoupled" from the coating material wafer, so that the accuracy of the measurements can be
Alignment accuracy ~ (rim) _Poly + AZ-PF Poly + SAL 601 Contact + AZ-PF Contact + SAL 601
Conventional mask 25 20 25 22
Gold markers on the back side 5 6 3 7
Resist markers on the back side 9 7 8 8
Tab. 1: Measurements of standard deviation c~ relative to masks and wafers at the specified working conditions.
556
E. Di Fabrizio et al. / Microelectronic Engineering 35 (1997) 553-556
regarded as a "noise" of the stepper alignment system itself. Finally, we notice that the same measurement performed on an Si3N4 conventional mask resulted in an alignment accuracy which was (1 ~) 20 nm in the best case. The reason for this result is due essentially to the better optical contrast of the mask fabricated with the back marker transfer.
ACKNOWLEDGMENTS We would like to acknowledge the Centre for Xray Lithography (CXrL), University of Wisconsin, USA, for technical support and the facility supply.
REFERENCES
4. CONCLUSIONS A new technique to transfer the mask alignment markers to the back side of the substrate film, was developed. The transfer is self-aligned and doesn't introduce any placement error. The best advantage of this technique is that, when the transferred markers are made by gold, the reflectivity is dominated by the gold reflectivity and is almost independent of the membrane substrate and wafer reflectivity. Although the study was accomplished by using Si3N4 membrane for the mask material, and gold for alignment markers, this technique can be used also by combining other materials when required. The improvement in the alignment accuracy measurements is a direct consequence of the increased optical contrast of the mask.
[1] C.J. Progler, A.C. Chen, T.A. Gunther, P. Kaiser, K.A. Cooper, E. Hughlett-J. Vac. Sci. Technol. B 11(6), 2888 (1993). [2] M. Gentili, R. Kumar, L. Luciani, L. Grella, D. Plumb, and Q. Leonard - J. Vac. Sci. Technol. B. 9, 3319 (1991) [3] O. S. Heavens - "Optical properties of thin solid films", p. 46. Dover publishing (1991) [4] E. Di Fabrizio, L. Grella, M. Gentili, M. Bacioechi, L. Mastrogiacomo, D. Peschiaroli, L. Mastrogiacomo, R. Maggiora, Sang-Soo Choi*, Young Jill Jeon*, Hyung Joun Yoo*, Hai Bin Chung, F. Cerrina - International Patent pending: RM96A000635 [5] Optical system alignment manual. Internal report Karl Suss 1990.