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Sub-0.5 micron lithography with i-line acid-hardened negative resists
Microelectronic Elsevier
Engineering
17 (1992) 283-286
283
SUB-O.5 MICRON LITHOGRAPHY ACID-HARDENED NEGATIVE
WITH I-LINE RESISTS
M. T. Allen, G. S. Calabrese, T. H. Fedynyshyn, A. A. Lamola and R. D. Small Shipley Company Inc., 455 Forest St., Marlboro, MA 01752, USA
Detailed lithographic data has been obtained for a series of high resolution i-line acid-hardened negative photoresists. These resists consist of a phenolic polymer, an acid-activated crosslinker, a photoacid generator (PAG), and an i-line sensitizer. Imaging to 0.40 urn has been achieved for both a dyed and an undyed version of this resist at exposure doses cl00 mJ/cm2 using a 0.45 NA i-line exposure tool. This has been demonstrated in a variety of developers, including 2.38% TMAH. Using the dyed resist, 0.5 urn lines with straight sidewalls have been imaged on aluminum substrates. Swing curves and linearity data, as well as focus and exposure latitudes, are reported. 1.
INTRODUCTION
Interest in high resolution aqueous-developable negative-tone resists has grown recently due to the advances in phase-shift mask (PSM) technology [l] and the potential of achieving 0.35 urn resolution without KrF excimer laser lithography. In particular, for isolated spaces and holes the use of negative-tone resists appears to be preferable to positive-tone systems due to PSM design and fabrication considerations [2]. We have developed a series of high resolution i-line negative photoresists for these applications. These resists are based on chemical amplification which, in this case, exploits acid-catalyzed crosslinking chemistry in combination with a sensitizing chromophore and a light absorbing dye. Specifically, these resists consist of a phenolic polymer, an acid-activated crosslinker, a photoacid generator (PAG), and an i-line sensitizer [3,4]. During exposure, acid is generated by a transfer of excitation energy from the sensitizer to the PAG. The acid that is produced then catalyzes the crosslinking of the polymer matrix in a subsequent postexposure bake (PEB) step. Development in aqueous TMAH yields a negative-tone image. Highly reflective substrates such as aluminum and polysilicon present problems for relatively transparent single-level resist systems such as the one described above. The high transparency adversely impacts exposure latitude due to standing wave effects, and leads to unacceptable CD variation over topography. To address this, a dyed resist was developed which is compatible with the PAG/crosslinker chemistry. High concentrations of this dye can be used with no loss in photospeed. The reduction of standing wave effects is shown to improve the exposure latitude by lowering the swing ratio of the resist. This paper presents focus and exposure latitude data, linearity and scanning electron micrographs for both the dyed and undyed versions of Shipley’s i-line Advanced Negative Resists (ANR’s), XP-91149-F2 and XP-91149, respectively. Swing curves showing the effects of the dye are compared for these resists. The utility of XP-91149-F2 is further demonstrated by micrographs showing imaging on reflective substrates, polysilicon and
0167-9317/92/$05.00
0 1992 - Elsevier Science Publishers
B.V.
All rights reserved.
M.T. Allen et al. I l-line acid-hardened
284
negative resists
aluminum. Data which demonstrates the ability to image XP-91149 in a wide range of normality developers, from 0 14 N to 0.26 N (or 2.38%) TMAH, is also presented. 2.
EXPERIMENTAL
Photoresist was spin-coated for 30 set at 5,000-5,500 rpm on HMDS vapor-primed Si or polysilicon wafers. Aluminum-coated wafers were treated with a 30 min, 140°C dehydration bake and no prime. Coating was followed by a 60 set, 110°C softbake (SB) on a hot plate. Typical film thicknesses were 1 .1-l .2 pm as measured by a Nanospec 215. Refractive indexes for the thickness measurements were determined at 632.8 nm using a Metricon prism coupler. Exposures were made using a GCA ALS 200 0.45 NA stepper. This was followed by a 60 set post exposure bake (PEB) at 110°C. Images were developed by loo-150 set immersion in 0.14 N TMAH or 50 set immersion in 0.26 N TMAH. 3.
RESULTS
AND DISCUSSION
Chemically amplified resists are typically quite sensitive to the processing conditions employed. The rate of the acid-catalyzed crosslinking reaction used to create a negative-tone image is highly temperature-dependent. This results in a decrease in the print doses required (improved photospeeds) with increasing post-exposure bake (PEB) temperatures. However, due to the diffusional nature of the image formation (H+ must diffuse to catalyze crosslinking), the best processing latitudes are observed at the lower PEB temperatures [5]. The resists studied for this work, XP-91149 and XP-91149-F2, show good photospeeds, 60-75 mJ/cm2 (photospeeds as low as 10 mJ/cm2 possible for high throughput), using the relatively low PEB temperature of 110°C. In addition, some chemically amplified negative resists such as MEGAPOSITB SNRrM248, require low normality (0.14 N) development for optimum resolution. Figure 1 shows 0.40 pm resolution achieved with both 0.14 N and 0.26 N TMAH development of undyed i-line ANR resists using 60 and 62 mJ/cm2, respectively. Thus, high normality developmen: is possible with
Figure 1. 0.40 pm I/s pattern for undyed i-line negative resist printed on a 0.45 NA i-line stepper using (a) 0.14 N TMAH (XP-89114) and (b) 0.26 N TMAH (2.38 %) development. Exposure doses were 60 and 62 mJ/cm2, respectively, thicknesses were 1 .18 and 1.08 pm, respectively.
and the resist film
Highly transparent pho!oresist films have a narrow processing window due to large standing wave variations with film thickness. This effect can be mitigated by the addition of a highly absorbing dye to the film. However, in minimizing the amount of light that reaches the substrate, dyes act as a screen, filtering light that would otherwise be available to the photoactive components of the resist. This usually results in a loss of photospeed. In addition, many dyes interfere with electron or energy transfer sensitization schemes. XP91149-F2 is unique in that this resist has been dyed with a compound which produces a diminished swing curve amplitude but does not have an adverse effect on the photospeed.
M.T. Alfen et al. I l-line acid-hardened negative resists
285
Figure 2 compares normalized swing curve data for XP-91149-F2 (dyed) and XP-91149 (undyed) i-line negative resists. The swing curve amplitude is decreased from +30% to +15% by the addition of the highly absorbing dye.
1.05 1.1 1.15 1.2 1.25 1.3 1.35 1.4 1.45 Thickness (pm) Figure
2.
Normalized
swing curves
for (0) XP-91149-F2
and (0) XP-91149
on Si.
Focus and exposure latitudes have been measured for both the dyed and undyed i-line negative resists. Both XP-91149-F2 and XP-91149 have focus latitudes of 1.4 pm for sizing a 0.5 pm line using a 0.45 NA i-line stepper. The exposure latitudes are +l 1% and ?18%, respectively. Linearity data for both resikts are shown in Figure 3. Although the 0.4 pm lines are not fully cleared, causing these features to fall outside +lO% of the nominal linewidth, the remaining features, 0.45-0.90 pm, are sized to within the requisite *lo%. These parameters offer good processing latitude for these high resolution resists.
0.7 +1 0% -10%
0.5
XP-91 149 XP-91 49-F2
0.3
Figure
3.
Actual
0.3
vs. nominal
0.5
0.7
0.9
Nominal
Linewidth
(km)
linewidth
for XP-91149-F2
and XP-91149;
NA=0.45.
To further demonstrate the ability of the dyed i-line ANR, XP-91149-F2, for imaging on reflective substrates, this resist was imaged on polysilicon and on aluminum-coated wafers. Scanning electron micrographs of line/space patterns imaged using XP-91149-F2 and a 0.45 NA i-line stepper are shown for silicon, polysilicon and aluminum in Figure 4.
286
M.T. Allen et al. I l-line acid-hardened
negative resists
A slightly negative or “reentrant” profile is observed for lines produced on the silicon substrate. This is caused by the fact that more light is absorbed at the surface of the film than at the base due to the addition of the highly absorbing dye. This effect is slightly less pronounced on the more reflective polysilicon surface. Finally, this effect is fully compensated for by the high reflectivity of the aluminum surface, producing straight sidewalls in that case. The precise dye level can be optimized for the reflectivity and topography of the substrate on which the resist is to be imaged. Low levels of dye may also be added to undyed resist to produce straight sidewalls on less reflective substrates as well.
Figure 4. 0.50 urn line/space pattern for dyed i-line negative resist, XP-91149-F2, printed on an (a) silicon, (b) polysilicon and (c) aluminum substrates using a 0.45 NA i-line stepper with 90, 87 and 75 mJ/cm2 exposure doses, respectively. 4.
CONCLUSIONS
The results presented in this paper demonstrate the high resolution imaging capability of a sensitized i-line negative resist with a wide range of process latitudes. Sub-O.5 urn features are obtainable with both concentrated (0.26 N) and weaker normality (0.14 .fY) aqueous base (TMAH) developers. In addition, we have developed a highly dyed i-line negative resist which is also capable of sub-O.5 urn resolution on reflective substrates such as polysilicon and aluminum. This is achieved without loss in photospeed. Finally, it is anticipated that this resist will also provide good CD control over reflective topography. ACKNOWLEDGEMENTS The authors would like to thank Jim Wickman and his staff at Shipley Company for preparing the scanning electron micrographs for this work. REFERENCES [l] M. D. Levenson, N. S. Viswanathan, and R. A. Simpson, /EEE Trans. Elec. Dev. ED-29, 1828 (1982). [2] H. Jinbo, Y. Yamashita, and M. Sadamura, J. Vat. Sci. Tech. B 8, 1745 (1990). [3] A. K. Berry, W. E. Feely, S. D. Thompson, G. S. Calabrese, R. Sinta, A. A. Lamola, J. W. Thackeray, and G. W. Orsula, Proc. SPlE 1262, 575 (1990). [4] J. F. Bohland, G. S. Calabrese, M. F. Cronin, D. Canistro, T. H. Fedynyshyn, J. Ferrari, A. A. Lamola, G. W. Orsula, E. K. Pavelchek, R. Sinta, J. W. Thackery, A. K. Berry, L. E. Bogan, M. P. deGrandpre, W. E. Feely, K. A. Graziano, R. Olsen, S. Thompson and M. R. Winkle, J. Photopolym. Sci. Tech., 3, 356, (1990). [5] E. Barouch, U. Hollerbach, S. A. Orszag, M. T. Allen, and G. S. Calabrese, Proc. SPlE 1463, 336 (1991). SNR
is a trademark;
Newton
MA.
MEGAPOSIT
is a registered
trademark,
all owned
by Shipley
Company
Inc.,
Engineering
17 (1992) 283-286
283
SUB-O.5 MICRON LITHOGRAPHY ACID-HARDENED NEGATIVE
WITH I-LINE RESISTS
M. T. Allen, G. S. Calabrese, T. H. Fedynyshyn, A. A. Lamola and R. D. Small Shipley Company Inc., 455 Forest St., Marlboro, MA 01752, USA
Detailed lithographic data has been obtained for a series of high resolution i-line acid-hardened negative photoresists. These resists consist of a phenolic polymer, an acid-activated crosslinker, a photoacid generator (PAG), and an i-line sensitizer. Imaging to 0.40 urn has been achieved for both a dyed and an undyed version of this resist at exposure doses cl00 mJ/cm2 using a 0.45 NA i-line exposure tool. This has been demonstrated in a variety of developers, including 2.38% TMAH. Using the dyed resist, 0.5 urn lines with straight sidewalls have been imaged on aluminum substrates. Swing curves and linearity data, as well as focus and exposure latitudes, are reported. 1.
INTRODUCTION
Interest in high resolution aqueous-developable negative-tone resists has grown recently due to the advances in phase-shift mask (PSM) technology [l] and the potential of achieving 0.35 urn resolution without KrF excimer laser lithography. In particular, for isolated spaces and holes the use of negative-tone resists appears to be preferable to positive-tone systems due to PSM design and fabrication considerations [2]. We have developed a series of high resolution i-line negative photoresists for these applications. These resists are based on chemical amplification which, in this case, exploits acid-catalyzed crosslinking chemistry in combination with a sensitizing chromophore and a light absorbing dye. Specifically, these resists consist of a phenolic polymer, an acid-activated crosslinker, a photoacid generator (PAG), and an i-line sensitizer [3,4]. During exposure, acid is generated by a transfer of excitation energy from the sensitizer to the PAG. The acid that is produced then catalyzes the crosslinking of the polymer matrix in a subsequent postexposure bake (PEB) step. Development in aqueous TMAH yields a negative-tone image. Highly reflective substrates such as aluminum and polysilicon present problems for relatively transparent single-level resist systems such as the one described above. The high transparency adversely impacts exposure latitude due to standing wave effects, and leads to unacceptable CD variation over topography. To address this, a dyed resist was developed which is compatible with the PAG/crosslinker chemistry. High concentrations of this dye can be used with no loss in photospeed. The reduction of standing wave effects is shown to improve the exposure latitude by lowering the swing ratio of the resist. This paper presents focus and exposure latitude data, linearity and scanning electron micrographs for both the dyed and undyed versions of Shipley’s i-line Advanced Negative Resists (ANR’s), XP-91149-F2 and XP-91149, respectively. Swing curves showing the effects of the dye are compared for these resists. The utility of XP-91149-F2 is further demonstrated by micrographs showing imaging on reflective substrates, polysilicon and
0167-9317/92/$05.00
0 1992 - Elsevier Science Publishers
B.V.
All rights reserved.
M.T. Allen et al. I l-line acid-hardened
284
negative resists
aluminum. Data which demonstrates the ability to image XP-91149 in a wide range of normality developers, from 0 14 N to 0.26 N (or 2.38%) TMAH, is also presented. 2.
EXPERIMENTAL
Photoresist was spin-coated for 30 set at 5,000-5,500 rpm on HMDS vapor-primed Si or polysilicon wafers. Aluminum-coated wafers were treated with a 30 min, 140°C dehydration bake and no prime. Coating was followed by a 60 set, 110°C softbake (SB) on a hot plate. Typical film thicknesses were 1 .1-l .2 pm as measured by a Nanospec 215. Refractive indexes for the thickness measurements were determined at 632.8 nm using a Metricon prism coupler. Exposures were made using a GCA ALS 200 0.45 NA stepper. This was followed by a 60 set post exposure bake (PEB) at 110°C. Images were developed by loo-150 set immersion in 0.14 N TMAH or 50 set immersion in 0.26 N TMAH. 3.
RESULTS
AND DISCUSSION
Chemically amplified resists are typically quite sensitive to the processing conditions employed. The rate of the acid-catalyzed crosslinking reaction used to create a negative-tone image is highly temperature-dependent. This results in a decrease in the print doses required (improved photospeeds) with increasing post-exposure bake (PEB) temperatures. However, due to the diffusional nature of the image formation (H+ must diffuse to catalyze crosslinking), the best processing latitudes are observed at the lower PEB temperatures [5]. The resists studied for this work, XP-91149 and XP-91149-F2, show good photospeeds, 60-75 mJ/cm2 (photospeeds as low as 10 mJ/cm2 possible for high throughput), using the relatively low PEB temperature of 110°C. In addition, some chemically amplified negative resists such as MEGAPOSITB SNRrM248, require low normality (0.14 N) development for optimum resolution. Figure 1 shows 0.40 pm resolution achieved with both 0.14 N and 0.26 N TMAH development of undyed i-line ANR resists using 60 and 62 mJ/cm2, respectively. Thus, high normality developmen: is possible with
Figure 1. 0.40 pm I/s pattern for undyed i-line negative resist printed on a 0.45 NA i-line stepper using (a) 0.14 N TMAH (XP-89114) and (b) 0.26 N TMAH (2.38 %) development. Exposure doses were 60 and 62 mJ/cm2, respectively, thicknesses were 1 .18 and 1.08 pm, respectively.
and the resist film
Highly transparent pho!oresist films have a narrow processing window due to large standing wave variations with film thickness. This effect can be mitigated by the addition of a highly absorbing dye to the film. However, in minimizing the amount of light that reaches the substrate, dyes act as a screen, filtering light that would otherwise be available to the photoactive components of the resist. This usually results in a loss of photospeed. In addition, many dyes interfere with electron or energy transfer sensitization schemes. XP91149-F2 is unique in that this resist has been dyed with a compound which produces a diminished swing curve amplitude but does not have an adverse effect on the photospeed.
M.T. Alfen et al. I l-line acid-hardened negative resists
285
Figure 2 compares normalized swing curve data for XP-91149-F2 (dyed) and XP-91149 (undyed) i-line negative resists. The swing curve amplitude is decreased from +30% to +15% by the addition of the highly absorbing dye.
1.05 1.1 1.15 1.2 1.25 1.3 1.35 1.4 1.45 Thickness (pm) Figure
2.
Normalized
swing curves
for (0) XP-91149-F2
and (0) XP-91149
on Si.
Focus and exposure latitudes have been measured for both the dyed and undyed i-line negative resists. Both XP-91149-F2 and XP-91149 have focus latitudes of 1.4 pm for sizing a 0.5 pm line using a 0.45 NA i-line stepper. The exposure latitudes are +l 1% and ?18%, respectively. Linearity data for both resikts are shown in Figure 3. Although the 0.4 pm lines are not fully cleared, causing these features to fall outside +lO% of the nominal linewidth, the remaining features, 0.45-0.90 pm, are sized to within the requisite *lo%. These parameters offer good processing latitude for these high resolution resists.
0.7 +1 0% -10%
0.5
XP-91 149 XP-91 49-F2
0.3
Figure
3.
Actual
0.3
vs. nominal
0.5
0.7
0.9
Nominal
Linewidth
(km)
linewidth
for XP-91149-F2
and XP-91149;
NA=0.45.
To further demonstrate the ability of the dyed i-line ANR, XP-91149-F2, for imaging on reflective substrates, this resist was imaged on polysilicon and on aluminum-coated wafers. Scanning electron micrographs of line/space patterns imaged using XP-91149-F2 and a 0.45 NA i-line stepper are shown for silicon, polysilicon and aluminum in Figure 4.
286
M.T. Allen et al. I l-line acid-hardened
negative resists
A slightly negative or “reentrant” profile is observed for lines produced on the silicon substrate. This is caused by the fact that more light is absorbed at the surface of the film than at the base due to the addition of the highly absorbing dye. This effect is slightly less pronounced on the more reflective polysilicon surface. Finally, this effect is fully compensated for by the high reflectivity of the aluminum surface, producing straight sidewalls in that case. The precise dye level can be optimized for the reflectivity and topography of the substrate on which the resist is to be imaged. Low levels of dye may also be added to undyed resist to produce straight sidewalls on less reflective substrates as well.
Figure 4. 0.50 urn line/space pattern for dyed i-line negative resist, XP-91149-F2, printed on an (a) silicon, (b) polysilicon and (c) aluminum substrates using a 0.45 NA i-line stepper with 90, 87 and 75 mJ/cm2 exposure doses, respectively. 4.
CONCLUSIONS
The results presented in this paper demonstrate the high resolution imaging capability of a sensitized i-line negative resist with a wide range of process latitudes. Sub-O.5 urn features are obtainable with both concentrated (0.26 N) and weaker normality (0.14 .fY) aqueous base (TMAH) developers. In addition, we have developed a highly dyed i-line negative resist which is also capable of sub-O.5 urn resolution on reflective substrates such as polysilicon and aluminum. This is achieved without loss in photospeed. Finally, it is anticipated that this resist will also provide good CD control over reflective topography. ACKNOWLEDGEMENTS The authors would like to thank Jim Wickman and his staff at Shipley Company for preparing the scanning electron micrographs for this work. REFERENCES [l] M. D. Levenson, N. S. Viswanathan, and R. A. Simpson, /EEE Trans. Elec. Dev. ED-29, 1828 (1982). [2] H. Jinbo, Y. Yamashita, and M. Sadamura, J. Vat. Sci. Tech. B 8, 1745 (1990). [3] A. K. Berry, W. E. Feely, S. D. Thompson, G. S. Calabrese, R. Sinta, A. A. Lamola, J. W. Thackeray, and G. W. Orsula, Proc. SPlE 1262, 575 (1990). [4] J. F. Bohland, G. S. Calabrese, M. F. Cronin, D. Canistro, T. H. Fedynyshyn, J. Ferrari, A. A. Lamola, G. W. Orsula, E. K. Pavelchek, R. Sinta, J. W. Thackery, A. K. Berry, L. E. Bogan, M. P. deGrandpre, W. E. Feely, K. A. Graziano, R. Olsen, S. Thompson and M. R. Winkle, J. Photopolym. Sci. Tech., 3, 356, (1990). [5] E. Barouch, U. Hollerbach, S. A. Orszag, M. T. Allen, and G. S. Calabrese, Proc. SPlE 1463, 336 (1991). SNR
is a trademark;
Newton
MA.
MEGAPOSIT
is a registered
trademark,
all owned
by Shipley
Company
Inc.,