- Email: [email protected]
Future trends in high-resolution lithography
Applied Surface Science 154–155 Ž2000. 519–526 www.elsevier.nlrlocaterapsusc
Future trends in high-resolution lithography R.A. Lawes
)
Central Microstructure Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK Received 1 June 1999; accepted 15 August 1999
Abstract A perennial question is ‘‘what is the future of high-resolution lithography, a key technology that drives the semiconductor industry’’? The dominant technology over the last 30 years has been optical lithography, which by lowering wavelengths to 193 nm ŽArF. and 157 nm ŽF2 . and by using optical ‘‘tricks’’ such as phase shift masks, off-axis illumination and phase filters, should be capable of 100 nm CMOS technology. So where does this leave the competition? The 100-nm lithography used to be the domain of electron beam lithography but only in research laboratories. Significant efforts are being made to increase throughput by electron projection Žscattering with angular limitation projection electron beam lithography or SCALPEL.. X-ray lithography remains a demonstrated R & D tool waiting to be commercially exploited but the initial expenditure to do so is very high. Ion beam lithography and extreme ultraviolet ŽEUV. Ž l - 12 nm. have also received attention in recent years. This paper will concentrate on some of the key issues and speculate on how and when an alternative to optical lithography will be embraced by industry. q 2000 Published by Elsevier Science B.V. All rights reserved. Keywords: High-resolution lithography; Ion beam lithography; Electron beam lithography
1. Introduction Ten years ago, a review of submicron lithography was given w1x suggesting that the excimer laser would extend ultraviolet ŽUV. optical lithography down to 0.5 mm dimensions, that X-ray lithography had many problems to solve and that scanning electron beam lithography would remain the accepted technique for high-resolution, high-complexity mask-making. These predictions have stood up well. What then of the future? Moore’s Law which predicts that semiconductor linewidth will halve every 2–3 years marches on, reinforced in recent years by roadmaps used by the industry to plan new )
Tel.: q44-1235-446328; fax: q44-1235-445194. E-mail address: [email protected] ŽR.A. Lawes..
technology. Recently, International SEMATECH, through its Next Generation Lithography Task Force, has produced estimates up to 2011 that break the 100-nm barrier ŽFig. 1.. It will be seen that the excimer laser can extend the lifetime of 248-nm wavelength UV optical lithography even further. Xray lithography has either been successfully demonstrated, has residual technical problems or is financially non-viable, depending on differing views. However, new technology competitors have entered the field, namely 157 nm optical lithography, extreme UV ŽEUV. Ž- 12 nm wavelength. and ion beam lithography. The overall problems for any of these technologies remains the same. In order to meet the wafer throughput required for ever-increasing wafer diameters, the rate at which pixels of information are
0169-4332r00r$ - see front matter q 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 4 7 8 - X
520
R.A. Lawesr Applied Surface Science 154–155 (2000) 519–526
Fig. 1. Minimum feature size and chip area versus time. NB: 64M refers to 64 Mbit DRAM, 1G to 1 Gbit DRAM, etc.
printed is dauntingly high. For example, Fig. 2 shows the pixel rate for 200-mm diameter wafers at throughputs varying from 10 to 60 wafersrh, with minimum linewidths down to 50 nm. Currently, for 250-nm linewidth technology at 40–60 wafersrh, the printing rate is less than 10 Gpixelsrs. For 100-nm linewidth technology, at the same throughput, over 50 Gpixelsrs will be required!
It is difficult to see how any serial exposure tool can meet these requirements. Current electron beam Žand optical. techniques undertake the mask-making role but data rates of only a few hundred megapixels per second are required for economical production. Hence, current candidates for high-resolution, highthroughput lithography are projection printers of some form, whereby data pixels on a mask are
Fig. 2. Pixel rate versus linewidth for 200-mm diameter silicon wafers. Legend refers to varying wafers per hour Žsee text..
R.A. Lawesr Applied Surface Science 154–155 (2000) 519–526
projected in parallel at very high equivalent serial rates. Any of the prospective new techniques must solve the problems of a bright long life source, a high-precision zero defect mask and aberration-free projection optics.
2. Mask-making Mask-making for optical steppers is a long established technology with high-quality chromium film as the absorber and quartz as the high-transmission substrate. Such masks or reticles are often referred to as binary masks to differentiate them from more complex phase shift masks Žsee below.. Material and technology exist for wavelengths down to 248 nm Ž0.25 mm technology. and extensive work is in hand for the 193-nm generation needed within the next 2 years. Fig. 3 shows schematically the key parameters that concern the mask-maker, namely, minimum feature resolution, linewidth control, lack of defects and the overlay accuracy of one mask to another. A complete technology for 248-nm optical lithography exists, consisting of high-accuracy, highthroughput pattern generators Žusually electron beam., high-quality chrome-on-quartz mask blanks, suitable resists and automatic defect detection and
521
associated optical repair techniques. Companies, such as Dupont and Photronics, offer a world-wide commercial supply of mask sets to sustain the semiconductor industry. The parameters shown assume a demagnification between the reticle Žmask. and the wafer of 5 = . It will be seen that the component of error for the mask-maker is very small. Two techniques to improve the resolution and depth of focus of optical projection systems affect the mask design. Phase shift masks w2x use a hardware modification to the binary mask in order to produce a light component which is 1808 out of phase with the transmitted component so that the interaction at the edge of a feature produces a sharper image. Several techniques have been tried including the attenuated phase shift mask, where the absorber material chosen ŽCrON or MoSiON. imposes a 1808 phase shift on light partially transmitted through the absorber Žtypically 6% transmission.. Optical proximity correction ŽOPC. using a software modification to add sub-resolution mask features to achieve the correct shape and size on the wafer, is also being investigated w3x. The technique overcomes loss of linewidth control, line rounding and line shortening, created by the proximity of features to one another, by a combination of fine line
Fig. 3. Key mask parameters: L s linewidth, d L s change of linewidth, d R s overlay accuracy, D s defect size.
R.A. Lawesr Applied Surface Science 154–155 (2000) 519–526
522
157 nm. However, new fused silica is being developed with at least 60% transmission, raising the possibility that conventional binary masksrreticles can be retained. Otherwise a reflective reticle technology will have to be introduced. Defect repair of masks at 157 nm will be difficult, even more so if PSM or reflective types are required. New resists will have to be developed for 157-nm lithography, where even a 100-nm layer of the ubiquitous PMMA only has 10% transmission.
Fig. 4. Optical proximity-corrected serifs Žcourtesy of EU Project Optima.. Linewidth is 0.75 mm, minimum jog in the lines is 25 nm.
biasing, sub-resolution added features Žserifs, hammerheads. and submicron scattering bars. Fig. 4 shows serifs on an experimental reticle. The main problems with OPC are getting the correct optical model for all circumstances, resolving the required features with existing mask-makers and the data expansion which inevitably arises. Additional OPC features are typically 250–500 nm, which can be resolved by vector scan electron beam maskmakers but is proving difficult for production pattern generation machines.
3. Improvements to optical lithography The driving forces behind optical lithography improvements are the wavelength of the light source and the numerical aperture ŽNA. of the projection lens, that is, Resolution, R f
Wavelength Ž l . 2NA
.
Table 1 shows theoretically what might be achieved with reducing wavelengths, increasing NA and introducing phase shift mask technology. A major concern is the availability of materials optically transparent at 157 nm so that lenses can be fabricated. CaF2 seems the most likely lens material, either for all refractive or catadioptric versions. The mask blank is also a problem as the fused silica currently used has negligible transmission at
4. X-ray lithography X-ray lithography has been under development since the early 1980s, reaching an advanced level of performance once the 1 = mask-making had been ‘‘solved’’ and the synchrotron radiation X-ray source adopted w4x. A schematic diagram of the system is shown in Fig. 5. The mask consists of a thin membrane that is transparent to the main wavelength used, i.e., 1 nm, with a patterned gold layer of approximately 500 nm thickness which is sufficient to absorb such X-rays. The technique, known as shadow printing, is subject to a number of errors, all of which can be minimised to an appropriately low level. Some errors are due to the geometry of the system, namely the magnification of the printed image and the penumbra effect due the finite size of the source, but these can be reduced by ensuring a mask-to-wafer distance of a few tens of microns and a relatively long source
Table 1 The effect of wavelength and NA on resolution Žmm.. The 193-nm lithography is emerging from the research laboratory to production. Optical lithographers are now turning their attention to 157 nm. The 157-nm optical lithography will require most aspects of optical lithography to be revisited, namely optical materials for lenses and reticles, e.g., CaF2 , laser sources, lens design and resists NA
Wavelength 248 nm
0.5 0.7
193 nm
157 nm
Binary
PSM
Binary
PSM
Binary
PSM
250 180
150
190 150
100 70
160 110
80 60
R.A. Lawesr Applied Surface Science 154–155 (2000) 519–526
523
Fig. 5. Schematic of X-ray lithography.
distance. The main errors limiting the resolution are the combined effect of photo-electrons produced by the X-rays interacting with the mask and resist materials and Fresnel diffraction. However, with careful design, the errors may be reduced so that 100-nm lithography or less, is possible. The main technical limitation of X-ray lithography is mask-making, as the 1 = reticle must be manufactured to the tolerance of the wafer and not at 4 = or 5 = as for other systems. This places the mask-making problem mainly on the pattern generation machines, which are not yet capable of delivering the throughput and accuracy required.
5. Extreme UV EUV projection uses 13-nm wavelength photons w5x to reduce the diffraction limit to below 25 nm. An experimental system is shown in Fig. 6. The source of EUV is a plasma heated by a high-powered Nd:YAG laser. The EUV is collected by condenser optics and focused as a narrow arc of illumination on to a reflecting reticle. The reticle is imaged at 4 = , with an NA s 0.25 onto a resist coated wafer. The complete dierwafer is exposed by
scanning the mask while simultaneously Žat 1 4 speed. moving the wafer. EUV systems are at an early stage of development although 50 nm resolution has already been demonstrated in very thin resists. In the USA, a consortium of Advanced Microdevices, Intel and Motorola has been formed to undertake basic R & D for an alpha tool. In Europe, an R & D project with ASM Lithography, Carl Zeiss Žoptics. and Oxford Instruments Žsynchrotron source. has been formed.
6. Electron beam projection Electron beam projection has been revisited with new ideas on masks and scanning strategies that make the electron optics somewhat easier than with early attempts over 20 years ago. A technique known as SCALPEL ŽSCattering with Angular Limitation Projection Electron beam Lithography. has been developed by Lucent Technologies w6x. A simplified schematic is shown in Fig. 7. SCALPEL uses a mask consisting of a low atomic number membrane with a high-atomic number patterned layer, uniformly illuminated by 100 keV electrons.
524
R.A. Lawesr Applied Surface Science 154–155 (2000) 519–526
Fig. 6. An experimental EUV system operating at 13-nm wavelength.
SCALPEL projects the image of this specially constructed reticle by using scattering rather than absorption contrast. The reticle membrane has negligible absorption so that electrons pass through an aperture in the back focal plane of the lens. The
patterned layer is made from tungsten, a high-atomic number Ž Z . material that scatters electrons, so that they do not pass through the aperture and hence cannot reach the wafer. A step and scan strategy is adopted which projects stripes of exposure which
Fig. 7. Schematic of the electron beam projection system SCALPEL.
R.A. Lawesr Applied Surface Science 154–155 (2000) 519–526
525
7. Ion beam projection
Fig. 8. Ion projection stepper.
must be stitched together to cover the die and stepped to cover the wafer.
Ion beam projection ŽIPL. substitutes hydrogen or helium ion source at 100 keV and an open stencil mask of a few microns thickness for the more conventional components w7x. Resolution down to 50-nm linewidth has been demonstrated. The simplest configuration for IPL is shown in Fig. 8. Mask-making for IPL is different from other techniques in that, as ions will be absorbed by almost any material, a supporting membrane is not possible and the ion absorber must be free standing material. It follows that, for certain geometries, a single mask solution is not possible as the feature cannot be physically supported. This type of mask is often referred to as a ‘‘stencil mask’’. An image of the stencil mask Žcontrast by absorption. is projected through an electrostatic lens system onto the wafer surface. The complete assembly can be calibrated and drift compensated by a ‘‘pattern lock’’ system via the reference plate, which can be located with respect to both the reticle and the X–Y stage. Alignment and drift compensation is achieved by using an error feedback signal from the laser interferometer-controlled X–Y stage, to shift the axis of the electrostatic lens via a multipole deflector.
Fig. 9. Potential introduction of new high-resolution lithography tools.
526
R.A. Lawesr Applied Surface Science 154–155 (2000) 519–526
An alternative configuration is to replace the full field stepper concept by a step and scan technique. As with SCALPEL, the die is covered by stripes which must be stitched together accurately to cover a die.
8. Conclusions Each of the advanced techniques discussed have formidable problems to be solved before a production worthy machine will be available. Fig. 9 shows the estimated linewidth range for these techniques. Clearly, not all will be introduced and the diagram suggests that a competitive position will arise for 100-nm linewidth technology around 2005, a remarkably short time to introduce radically new technology. The current interest in 193-nm optical lithography is to obtain suitable resists which will be critical for 130-nm technology in 2002. The 157-nm optical technology requires significant work on all aspects, particularly lenses and resists. X-ray lithography remains the one technique that appears to have demonstrated all components in a complete system. However, the reluctance of the industry to accept 1 = mask-making and the high cost-of-entry with synchrotron radiation sources has delayed the introduction of X-ray lithography to the point where it may be perceived as a single generation technology solution.
SCALPEL, ion beam projection and EUV projection have to solve fundamental problems. SCALPEL needs to produce defect-free masks, consider the effects of wafer heating and develop process tolerant resists. Ion beam projection possibly has the fundamental problem of stencil masking and hence, the need for more masks per set. EUV projection has a full set of problems — source, mask and resist. Compared with a similar analysis a decade ago, optical lithography has shown its resilience. The ingenuity of the optical designer is demonstrably immense, whether it is by reducing wavelengths and increasing NA or by optical tricks with mask hardware and software. The next 10 years will be demanding, signalling the arrival of high-resolution lithography as a significant branch of nanotechnology! References w1x R.A. Lawes, Appl. Surf. Sci. 36 Ž1989. 485–499. w2x M.D. Levenson, N.S. Visnawathan, R.A. Simpson, IEEE Trans. Electron. Devices ED-29 Ž1982. 1828–1836. w3x O.W. Otto, J.G. Garofalo, K.K. Low, C.-M. Yuan, R.C. Henderson, C. Peirrat, R.L. Kostelak, S. Vaidya, P.K. Vusudev, SPIE 2197 Ž1994. 278–293. w4x J.P. Silverman, J. Vac. Sci. Technol. B 15 Ž6. Ž1997. . w5x D.W. Sweeney, R. Hudyma, H.N. Chapman, D. Shafter, SPIE 3331 Ž1998. 2–11. w6x L.R. Harriott, J. Vac. Sci. Technol., B 15 Ž6. Ž1997. . w7x G. Stengl, G. Bosch, A. Chalupka, J. Fegerl, R. Fischer, G. Lammer, H. Loschner, L. Malek, R. Nowak, C. Traher, P. Wolf, J. Vac. Sci. Technol., B 10 Ž6. Ž1992. .
Future trends in high-resolution lithography R.A. Lawes
)
Central Microstructure Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK Received 1 June 1999; accepted 15 August 1999
Abstract A perennial question is ‘‘what is the future of high-resolution lithography, a key technology that drives the semiconductor industry’’? The dominant technology over the last 30 years has been optical lithography, which by lowering wavelengths to 193 nm ŽArF. and 157 nm ŽF2 . and by using optical ‘‘tricks’’ such as phase shift masks, off-axis illumination and phase filters, should be capable of 100 nm CMOS technology. So where does this leave the competition? The 100-nm lithography used to be the domain of electron beam lithography but only in research laboratories. Significant efforts are being made to increase throughput by electron projection Žscattering with angular limitation projection electron beam lithography or SCALPEL.. X-ray lithography remains a demonstrated R & D tool waiting to be commercially exploited but the initial expenditure to do so is very high. Ion beam lithography and extreme ultraviolet ŽEUV. Ž l - 12 nm. have also received attention in recent years. This paper will concentrate on some of the key issues and speculate on how and when an alternative to optical lithography will be embraced by industry. q 2000 Published by Elsevier Science B.V. All rights reserved. Keywords: High-resolution lithography; Ion beam lithography; Electron beam lithography
1. Introduction Ten years ago, a review of submicron lithography was given w1x suggesting that the excimer laser would extend ultraviolet ŽUV. optical lithography down to 0.5 mm dimensions, that X-ray lithography had many problems to solve and that scanning electron beam lithography would remain the accepted technique for high-resolution, high-complexity mask-making. These predictions have stood up well. What then of the future? Moore’s Law which predicts that semiconductor linewidth will halve every 2–3 years marches on, reinforced in recent years by roadmaps used by the industry to plan new )
Tel.: q44-1235-446328; fax: q44-1235-445194. E-mail address: [email protected] ŽR.A. Lawes..
technology. Recently, International SEMATECH, through its Next Generation Lithography Task Force, has produced estimates up to 2011 that break the 100-nm barrier ŽFig. 1.. It will be seen that the excimer laser can extend the lifetime of 248-nm wavelength UV optical lithography even further. Xray lithography has either been successfully demonstrated, has residual technical problems or is financially non-viable, depending on differing views. However, new technology competitors have entered the field, namely 157 nm optical lithography, extreme UV ŽEUV. Ž- 12 nm wavelength. and ion beam lithography. The overall problems for any of these technologies remains the same. In order to meet the wafer throughput required for ever-increasing wafer diameters, the rate at which pixels of information are
0169-4332r00r$ - see front matter q 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 4 7 8 - X
520
R.A. Lawesr Applied Surface Science 154–155 (2000) 519–526
Fig. 1. Minimum feature size and chip area versus time. NB: 64M refers to 64 Mbit DRAM, 1G to 1 Gbit DRAM, etc.
printed is dauntingly high. For example, Fig. 2 shows the pixel rate for 200-mm diameter wafers at throughputs varying from 10 to 60 wafersrh, with minimum linewidths down to 50 nm. Currently, for 250-nm linewidth technology at 40–60 wafersrh, the printing rate is less than 10 Gpixelsrs. For 100-nm linewidth technology, at the same throughput, over 50 Gpixelsrs will be required!
It is difficult to see how any serial exposure tool can meet these requirements. Current electron beam Žand optical. techniques undertake the mask-making role but data rates of only a few hundred megapixels per second are required for economical production. Hence, current candidates for high-resolution, highthroughput lithography are projection printers of some form, whereby data pixels on a mask are
Fig. 2. Pixel rate versus linewidth for 200-mm diameter silicon wafers. Legend refers to varying wafers per hour Žsee text..
R.A. Lawesr Applied Surface Science 154–155 (2000) 519–526
projected in parallel at very high equivalent serial rates. Any of the prospective new techniques must solve the problems of a bright long life source, a high-precision zero defect mask and aberration-free projection optics.
2. Mask-making Mask-making for optical steppers is a long established technology with high-quality chromium film as the absorber and quartz as the high-transmission substrate. Such masks or reticles are often referred to as binary masks to differentiate them from more complex phase shift masks Žsee below.. Material and technology exist for wavelengths down to 248 nm Ž0.25 mm technology. and extensive work is in hand for the 193-nm generation needed within the next 2 years. Fig. 3 shows schematically the key parameters that concern the mask-maker, namely, minimum feature resolution, linewidth control, lack of defects and the overlay accuracy of one mask to another. A complete technology for 248-nm optical lithography exists, consisting of high-accuracy, highthroughput pattern generators Žusually electron beam., high-quality chrome-on-quartz mask blanks, suitable resists and automatic defect detection and
521
associated optical repair techniques. Companies, such as Dupont and Photronics, offer a world-wide commercial supply of mask sets to sustain the semiconductor industry. The parameters shown assume a demagnification between the reticle Žmask. and the wafer of 5 = . It will be seen that the component of error for the mask-maker is very small. Two techniques to improve the resolution and depth of focus of optical projection systems affect the mask design. Phase shift masks w2x use a hardware modification to the binary mask in order to produce a light component which is 1808 out of phase with the transmitted component so that the interaction at the edge of a feature produces a sharper image. Several techniques have been tried including the attenuated phase shift mask, where the absorber material chosen ŽCrON or MoSiON. imposes a 1808 phase shift on light partially transmitted through the absorber Žtypically 6% transmission.. Optical proximity correction ŽOPC. using a software modification to add sub-resolution mask features to achieve the correct shape and size on the wafer, is also being investigated w3x. The technique overcomes loss of linewidth control, line rounding and line shortening, created by the proximity of features to one another, by a combination of fine line
Fig. 3. Key mask parameters: L s linewidth, d L s change of linewidth, d R s overlay accuracy, D s defect size.
R.A. Lawesr Applied Surface Science 154–155 (2000) 519–526
522
157 nm. However, new fused silica is being developed with at least 60% transmission, raising the possibility that conventional binary masksrreticles can be retained. Otherwise a reflective reticle technology will have to be introduced. Defect repair of masks at 157 nm will be difficult, even more so if PSM or reflective types are required. New resists will have to be developed for 157-nm lithography, where even a 100-nm layer of the ubiquitous PMMA only has 10% transmission.
Fig. 4. Optical proximity-corrected serifs Žcourtesy of EU Project Optima.. Linewidth is 0.75 mm, minimum jog in the lines is 25 nm.
biasing, sub-resolution added features Žserifs, hammerheads. and submicron scattering bars. Fig. 4 shows serifs on an experimental reticle. The main problems with OPC are getting the correct optical model for all circumstances, resolving the required features with existing mask-makers and the data expansion which inevitably arises. Additional OPC features are typically 250–500 nm, which can be resolved by vector scan electron beam maskmakers but is proving difficult for production pattern generation machines.
3. Improvements to optical lithography The driving forces behind optical lithography improvements are the wavelength of the light source and the numerical aperture ŽNA. of the projection lens, that is, Resolution, R f
Wavelength Ž l . 2NA
.
Table 1 shows theoretically what might be achieved with reducing wavelengths, increasing NA and introducing phase shift mask technology. A major concern is the availability of materials optically transparent at 157 nm so that lenses can be fabricated. CaF2 seems the most likely lens material, either for all refractive or catadioptric versions. The mask blank is also a problem as the fused silica currently used has negligible transmission at
4. X-ray lithography X-ray lithography has been under development since the early 1980s, reaching an advanced level of performance once the 1 = mask-making had been ‘‘solved’’ and the synchrotron radiation X-ray source adopted w4x. A schematic diagram of the system is shown in Fig. 5. The mask consists of a thin membrane that is transparent to the main wavelength used, i.e., 1 nm, with a patterned gold layer of approximately 500 nm thickness which is sufficient to absorb such X-rays. The technique, known as shadow printing, is subject to a number of errors, all of which can be minimised to an appropriately low level. Some errors are due to the geometry of the system, namely the magnification of the printed image and the penumbra effect due the finite size of the source, but these can be reduced by ensuring a mask-to-wafer distance of a few tens of microns and a relatively long source
Table 1 The effect of wavelength and NA on resolution Žmm.. The 193-nm lithography is emerging from the research laboratory to production. Optical lithographers are now turning their attention to 157 nm. The 157-nm optical lithography will require most aspects of optical lithography to be revisited, namely optical materials for lenses and reticles, e.g., CaF2 , laser sources, lens design and resists NA
Wavelength 248 nm
0.5 0.7
193 nm
157 nm
Binary
PSM
Binary
PSM
Binary
PSM
250 180
150
190 150
100 70
160 110
80 60
R.A. Lawesr Applied Surface Science 154–155 (2000) 519–526
523
Fig. 5. Schematic of X-ray lithography.
distance. The main errors limiting the resolution are the combined effect of photo-electrons produced by the X-rays interacting with the mask and resist materials and Fresnel diffraction. However, with careful design, the errors may be reduced so that 100-nm lithography or less, is possible. The main technical limitation of X-ray lithography is mask-making, as the 1 = reticle must be manufactured to the tolerance of the wafer and not at 4 = or 5 = as for other systems. This places the mask-making problem mainly on the pattern generation machines, which are not yet capable of delivering the throughput and accuracy required.
5. Extreme UV EUV projection uses 13-nm wavelength photons w5x to reduce the diffraction limit to below 25 nm. An experimental system is shown in Fig. 6. The source of EUV is a plasma heated by a high-powered Nd:YAG laser. The EUV is collected by condenser optics and focused as a narrow arc of illumination on to a reflecting reticle. The reticle is imaged at 4 = , with an NA s 0.25 onto a resist coated wafer. The complete dierwafer is exposed by
scanning the mask while simultaneously Žat 1 4 speed. moving the wafer. EUV systems are at an early stage of development although 50 nm resolution has already been demonstrated in very thin resists. In the USA, a consortium of Advanced Microdevices, Intel and Motorola has been formed to undertake basic R & D for an alpha tool. In Europe, an R & D project with ASM Lithography, Carl Zeiss Žoptics. and Oxford Instruments Žsynchrotron source. has been formed.
6. Electron beam projection Electron beam projection has been revisited with new ideas on masks and scanning strategies that make the electron optics somewhat easier than with early attempts over 20 years ago. A technique known as SCALPEL ŽSCattering with Angular Limitation Projection Electron beam Lithography. has been developed by Lucent Technologies w6x. A simplified schematic is shown in Fig. 7. SCALPEL uses a mask consisting of a low atomic number membrane with a high-atomic number patterned layer, uniformly illuminated by 100 keV electrons.
524
R.A. Lawesr Applied Surface Science 154–155 (2000) 519–526
Fig. 6. An experimental EUV system operating at 13-nm wavelength.
SCALPEL projects the image of this specially constructed reticle by using scattering rather than absorption contrast. The reticle membrane has negligible absorption so that electrons pass through an aperture in the back focal plane of the lens. The
patterned layer is made from tungsten, a high-atomic number Ž Z . material that scatters electrons, so that they do not pass through the aperture and hence cannot reach the wafer. A step and scan strategy is adopted which projects stripes of exposure which
Fig. 7. Schematic of the electron beam projection system SCALPEL.
R.A. Lawesr Applied Surface Science 154–155 (2000) 519–526
525
7. Ion beam projection
Fig. 8. Ion projection stepper.
must be stitched together to cover the die and stepped to cover the wafer.
Ion beam projection ŽIPL. substitutes hydrogen or helium ion source at 100 keV and an open stencil mask of a few microns thickness for the more conventional components w7x. Resolution down to 50-nm linewidth has been demonstrated. The simplest configuration for IPL is shown in Fig. 8. Mask-making for IPL is different from other techniques in that, as ions will be absorbed by almost any material, a supporting membrane is not possible and the ion absorber must be free standing material. It follows that, for certain geometries, a single mask solution is not possible as the feature cannot be physically supported. This type of mask is often referred to as a ‘‘stencil mask’’. An image of the stencil mask Žcontrast by absorption. is projected through an electrostatic lens system onto the wafer surface. The complete assembly can be calibrated and drift compensated by a ‘‘pattern lock’’ system via the reference plate, which can be located with respect to both the reticle and the X–Y stage. Alignment and drift compensation is achieved by using an error feedback signal from the laser interferometer-controlled X–Y stage, to shift the axis of the electrostatic lens via a multipole deflector.
Fig. 9. Potential introduction of new high-resolution lithography tools.
526
R.A. Lawesr Applied Surface Science 154–155 (2000) 519–526
An alternative configuration is to replace the full field stepper concept by a step and scan technique. As with SCALPEL, the die is covered by stripes which must be stitched together accurately to cover a die.
8. Conclusions Each of the advanced techniques discussed have formidable problems to be solved before a production worthy machine will be available. Fig. 9 shows the estimated linewidth range for these techniques. Clearly, not all will be introduced and the diagram suggests that a competitive position will arise for 100-nm linewidth technology around 2005, a remarkably short time to introduce radically new technology. The current interest in 193-nm optical lithography is to obtain suitable resists which will be critical for 130-nm technology in 2002. The 157-nm optical technology requires significant work on all aspects, particularly lenses and resists. X-ray lithography remains the one technique that appears to have demonstrated all components in a complete system. However, the reluctance of the industry to accept 1 = mask-making and the high cost-of-entry with synchrotron radiation sources has delayed the introduction of X-ray lithography to the point where it may be perceived as a single generation technology solution.
SCALPEL, ion beam projection and EUV projection have to solve fundamental problems. SCALPEL needs to produce defect-free masks, consider the effects of wafer heating and develop process tolerant resists. Ion beam projection possibly has the fundamental problem of stencil masking and hence, the need for more masks per set. EUV projection has a full set of problems — source, mask and resist. Compared with a similar analysis a decade ago, optical lithography has shown its resilience. The ingenuity of the optical designer is demonstrably immense, whether it is by reducing wavelengths and increasing NA or by optical tricks with mask hardware and software. The next 10 years will be demanding, signalling the arrival of high-resolution lithography as a significant branch of nanotechnology! References w1x R.A. Lawes, Appl. Surf. Sci. 36 Ž1989. 485–499. w2x M.D. Levenson, N.S. Visnawathan, R.A. Simpson, IEEE Trans. Electron. Devices ED-29 Ž1982. 1828–1836. w3x O.W. Otto, J.G. Garofalo, K.K. Low, C.-M. Yuan, R.C. Henderson, C. Peirrat, R.L. Kostelak, S. Vaidya, P.K. Vusudev, SPIE 2197 Ž1994. 278–293. w4x J.P. Silverman, J. Vac. Sci. Technol. B 15 Ž6. Ž1997. . w5x D.W. Sweeney, R. Hudyma, H.N. Chapman, D. Shafter, SPIE 3331 Ž1998. 2–11. w6x L.R. Harriott, J. Vac. Sci. Technol., B 15 Ž6. Ž1997. . w7x G. Stengl, G. Bosch, A. Chalupka, J. Fegerl, R. Fischer, G. Lammer, H. Loschner, L. Malek, R. Nowak, C. Traher, P. Wolf, J. Vac. Sci. Technol., B 10 Ž6. Ž1992. .