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Surface imaging and dry development for e-beam lithography
Microelectronic Engineering 17 (1992) 269-274 Elsevier
SURFACE IMAGING LITHOGRAPHY
Ki-Ho Baik, R.Jonckheere,
AND
269
DRY
DEVELOPMENT
FOR
E-BEAM
A.Seabra , and L.Van den hove
IMEC v.z.w., Kapeldreef 75, B-3001 Leuven, Belgium In this paper we describe our investigation on positive tone gas phase silylation for E-beam exposure using the chemically amplified resist SAL601-ER 7. 1,1,3,3-tetra methyl disilazane (TMDS) was used as silylating agent. The exposure by electron beam followed by a presilylation-bake results in acid catalysed crosslinking which effectively blocks the silylation of the exposed areas. This results in positive tone images. Promising results have been obtained using this process. 1.
INTRODUCTION
Optical lithography proves to be capable of achieving a resolution below 0.5 pm and therefore remains in its leading position for the manufacturing of semiconductor devices. Electron beam lithography plays a key role in early research and development work for the next generation ULSI devices, GaAs devices, and the fabrication of photo masks. Surface imaging technologies in combination with silylation and dry development have been suggested as an attractive method for overcoming the inherent limitations present in conventional wet develop lithography [ 11. For the application to optical lithography the diffusion enhanced silylated resist (DESIRE) process has demonstrated negative tone image, resulting in high resolution, wide focus latitude, and good controllability of critical dimension (C.D.) over topography [2]. Also for the case of electron beam lithography the principle of surface imaging offers the advantages of improved resolution and C.D. control. We have studied positive tone gas phase silylation for E-beam exposures using SAL601-ER 7. This chemically amplified resist is composed of a novolac resin, a melamine cross-linker, and a radiation sensitive acid generator. TMDS was used as the silylating agent. As a result of the electron beam irradiation acid is being formed, which induces the crosslinking reaction during the pre silylation bake (PSB). This results in positive tone images. The main advantage of this resist is its high sensitivity. The multilayer performance (with single layer process simplicity) offers additional benefits such as a high resolution with high aspect ratio and a reduction of the proximity effect. Characterization of the silylation has been performed using thickness measurements, infrared absorption (IRS, and Rutherford Backscattering spectroscopy (RBS). 2.
EXPERIMENTAL
The wafers were coated with the chemically amplified resist SAL 601 ER-7 (2000 rpm for 30 s, soft-baked on a hot plate at 85°C for 60 s), resulting in a resist thickness of 1.0 pm. Exposures were carried out on a Cambridge EMBF10.5 at an accelerating voltage of 20 kV. Silylation was carried out in a modified vacuum oven equipped with a bubbling vessel or modified vapour prime track. Silylation was followed by a PSB on a hot plate in air or in the vacuum oven, using different, resp. the same temperature as used for silylation. TMDS was used as silylating agent. The dry development was performed on an MRC MIE 720 etcher, using a two-step dry development process.
0167-9317/92/$05.00 0 1992 - Elsevier Science Publishers B.V. All rights reserved.
K.-H. Baik et al. I Surjace imaging and dry development
270
3.
RESULTS
AND
DISCUSSION
Several researchers have studied gas phase silylation for E-beam exposures using chemical amplified resist with silylated with hexamethyl-disilazane(HMDS) and n,n-diethyl aminotrimethylsilane (TMSDEA) as silylating agents [3,4,5,6]. In a previous study we have investigated various alternative silylating agents for application to the DESIRE process. Alternative silylating agents, such as dimethyl silyldimethyl-amine (DMSDMA), TMDS, and n,n,-dimethylamino-trimethylsilane (TMSDMA) have demonstrated to exhibit some interesting advantages over HMDS, such as reduced vertical and lateral swelling, improved surface roughness, increased process latitude, and low temperature silylation [7]. The lower temperature agents such as TMDS appear increasingly attractive for deep W applications [8]. 3.1 .Positive tone process
EXPOSURE
0
SAL&l1
q
SUBSTRATE
0
EXPOSED [Acid +cmsslinker]
q
CROSSLINKED RESIN
I33
SILYLATED
02 PLASMA
-
302
=
SILYLATED
Figure 1. Process flow 3.2.Characterization
Also for E-beam silylation, we have investigated lower temperature agents such as TMDS, in combination with the chemically amplified resists SAL601 ER-7 and RAY PN. These resists are composed of a novolac resin, a melamine cross-linker and a radiation acid generator. E-beam exposures partly convert the radiation acid generator to acid. The crosslinking of the novolac resin is catalysed in the baking step by the generated acid. This crosslinking blocks the silylation of the exposed areas. Dry development results in positive tone images. The crosslinking is a function of both exposure energy and PSB conditions.The resist exhibits the same high sensitivity as for wet develo ment, as the crosslinking from 5 PC/cm fl onwards is sufficient to stop the Si diffusion into the exposed areas. In case of wet development the optimum post exposure bake [PEB] conditions are 115OC f 10°C, l-3 min [9]. TMDS is therefore one of the most suitable silylating agents, since the silylation occurs in exactly this temperature range. Also in case of the DESIRE process this agent exhibits a higher etch selectivity, a higher diffusion rate, and wider process latitude in comparison to HMDS. Hence we concentrated on this agent for the gas phase silylation. For dry development, we used a 2 step pure 02 process and 2 step C2F6 process, which has been detailed elsewhere [lo]. The etch selectivity between silylated area and nonsilylated area is 12: 1.
of silylation
Thickness measurements, and concentration of the absorbance of the peaks : vibration) and 820 cm-l
IR spectroscopy, and RBS were used to determine the distribution Si content. In case of IR, the Si content was calculated from the 1260 cm-l(Si-C bond deformation), 910 cm-l (Si-0-Aryl stretching (Si-C bond stretching vibration). The observed Si-0-Aryl peak is
271
K.-H. Baik et al. I Surface imaging and dry development
much larger than for the case of the DESIRE process. The integrated absorbance of 1280-1250 cm-l is taken as a quantitative measurement for the built-in silicon. Fig.3 shows the Si content from (1280-1250 cm-l) vs. exposure dose at various silylation temperatures with TMDS . This plot shows that the optimum PSB and silylation temperature is 105°C-1250C and exposure dose is >5l_tC/cm2. These conditions match to the bake conditions of wet development.
-0.8,
. , . , . ,
,
, . ,
,
_0.8,..._,....,....., Silylation
-
80
90
Exposure dose [p&m21
Silylation temperature[“C]
Figure 2. Si content as measured by IR versus silylation temperature at the various exposure dose
15
10
5
0
100 110 120 130 140 150
temperature
Figure 3. Si content as measured by IR versus exposure dose at the various silylation temperature
RBS is used to determine the Si depth profile and concentration. Fig.4 shows that a dose of 7 pC/cm2 induces enough crosslinking to inhibit Si diffusion. Fig.5 shows that the Si content and concentration increase with increasing silylation temperatures.
0.7
0.6
’
Energy
p5
Energy
(Me’/)
1.10
1.15
1.20
1.25
1.30
I
I
I
I
I
1.05
1.10
I
O3
Exposure dose [ pClcm2J t
I
(MeV) 1.20
I
1.25 1.30 I I -I
13x
._
‘So.4 -
1.15
Silylation temperature
0.5 p
I
”
.____
,; n 2” -iv_ il.,,‘\ t&-
-
\,\,
‘,A_,
_ _\ i
240 250 Channel
Figure 4.RBS measurements for the various exposure dose at 1 10°C silylation temperature 3.3. C.D. vs. delay time between exposure
Figure 5.RBS measurements for the various silylation temperature at the unexposed areas and PSB
Chemically amplified resists have been reported to exhibit aging problems. The delay time between exposure and PSB will affect the sensitivity and C.D. Since the crosslinking occurs at
272
K.-H. Baik et al. I Surface imaging and dry development
room temperature, the crosslinking density changes as a function of delay time. The C.D. variations between exposure and PSB within 1 hour correspond to 510% according to the exposure dose. After one hour delay time, no further CD variation is noticed (up to delay time of 1 day). A one hour delay time is therefore intentionally introduced. 3.4. Applications The above results indicate that surface imaging and dry development with a chemically amplified resist obtained promising results. Fig. 6 shows SEM micrographs of 0.2 pm, 0.175 pm patterns (lines/spaces) and 0.2 pm contact holes in a 1 pm thick resist with only 20 kV accelerating voltage, using a dose of 5 PC/cm 2. This SEM micrographs highlight the high resolution that can be obtained in combination with a high aspect ratio (5:l).
Figure 6 (a) 0.25 pm patterns (lines/spaces)
(c) 0.175 pm patterns (lines/spaces)
(b) 0.2 pm patterns (lines/spaces)
(d) 0.2 pm contact windows
K.-H. Baik et al. I Surface imaging and dry development
273
This top surface imaging technology shows a multilayer performance (with single layer process simplicity), resulting in a reduction of proximity effect and high resolution. Fig.7 compares cross-sections of 0.25 pm with 0.2 pm lines and spaces for (a)wet development on bare Si and for (b) dry development on 0.15 pm TiW(N) / 50 nm Si02 / Si. The crosslinking in between the exposed lines, as seen in (a), resulting from backscattering, does not limit the spatial resolution in (b) although the backscattering on TiW is much more pronounced.
Figure 7 (a) 0.25 pm patterns with wet development 4.
(b)0.2 pm patterns with dry development
CONCLUSIONS
Silylation and dry development has been investigated for E-beam lithography using chemically amplified resist. This process shows promising results, such as high resolution combined with a high aspect ratio, reduction of the proximity effect, and good etch resistance. We have been characterised the silylation using IR, and RBS. Using optimised process conditions 0.2 pm patterns (lines/spaces) and 0.2 pm contact holes are obtained in a 1 pm thick resist with a 20 kV electron beam using a dose of 5 pC/cm2.
ACKNOWLEDGEMENTS The authors would like to thank G.Brijs for the RBS measurements, J. Moonens for the Ebeam exposure, and N.Samarakone for some useful discussion.One of the authors would like to acknowledge CNPq-Brasil for its financial support.
REFERENCES F. Coopmans, and B. Roland, Proc. SPIE 631,34 (1986) M.Op de Beeck et all, ProcSPIE 1262,139 (1990) C.Pierrat et all, J. Vat. Sci. Technol. B 7(6), 1782 (1989) T.G.Vachette et all, Proc. SPIE 1262,258 (1990) T.Fujino et all, J. Vat. Sci. Technol. B 8(6), 1808 (1990) T.G.Vachette et all, Proc. ME 90,205 (1990) Ki-Ho Baik et all, J. Vat. Sci. Technol. B 8(6), 1481 (1990) A.M.Goethals et all , ProcSPIE 1466, 604 (1991) L.Blum et all, J. Vat. Sci. Technol. B 6(6), 2280 (1988) R.Lombaerts et all, Proc.SPIE 1262, 312 (1990)
SURFACE IMAGING LITHOGRAPHY
Ki-Ho Baik, R.Jonckheere,
AND
269
DRY
DEVELOPMENT
FOR
E-BEAM
A.Seabra , and L.Van den hove
IMEC v.z.w., Kapeldreef 75, B-3001 Leuven, Belgium In this paper we describe our investigation on positive tone gas phase silylation for E-beam exposure using the chemically amplified resist SAL601-ER 7. 1,1,3,3-tetra methyl disilazane (TMDS) was used as silylating agent. The exposure by electron beam followed by a presilylation-bake results in acid catalysed crosslinking which effectively blocks the silylation of the exposed areas. This results in positive tone images. Promising results have been obtained using this process. 1.
INTRODUCTION
Optical lithography proves to be capable of achieving a resolution below 0.5 pm and therefore remains in its leading position for the manufacturing of semiconductor devices. Electron beam lithography plays a key role in early research and development work for the next generation ULSI devices, GaAs devices, and the fabrication of photo masks. Surface imaging technologies in combination with silylation and dry development have been suggested as an attractive method for overcoming the inherent limitations present in conventional wet develop lithography [ 11. For the application to optical lithography the diffusion enhanced silylated resist (DESIRE) process has demonstrated negative tone image, resulting in high resolution, wide focus latitude, and good controllability of critical dimension (C.D.) over topography [2]. Also for the case of electron beam lithography the principle of surface imaging offers the advantages of improved resolution and C.D. control. We have studied positive tone gas phase silylation for E-beam exposures using SAL601-ER 7. This chemically amplified resist is composed of a novolac resin, a melamine cross-linker, and a radiation sensitive acid generator. TMDS was used as the silylating agent. As a result of the electron beam irradiation acid is being formed, which induces the crosslinking reaction during the pre silylation bake (PSB). This results in positive tone images. The main advantage of this resist is its high sensitivity. The multilayer performance (with single layer process simplicity) offers additional benefits such as a high resolution with high aspect ratio and a reduction of the proximity effect. Characterization of the silylation has been performed using thickness measurements, infrared absorption (IRS, and Rutherford Backscattering spectroscopy (RBS). 2.
EXPERIMENTAL
The wafers were coated with the chemically amplified resist SAL 601 ER-7 (2000 rpm for 30 s, soft-baked on a hot plate at 85°C for 60 s), resulting in a resist thickness of 1.0 pm. Exposures were carried out on a Cambridge EMBF10.5 at an accelerating voltage of 20 kV. Silylation was carried out in a modified vacuum oven equipped with a bubbling vessel or modified vapour prime track. Silylation was followed by a PSB on a hot plate in air or in the vacuum oven, using different, resp. the same temperature as used for silylation. TMDS was used as silylating agent. The dry development was performed on an MRC MIE 720 etcher, using a two-step dry development process.
0167-9317/92/$05.00 0 1992 - Elsevier Science Publishers B.V. All rights reserved.
K.-H. Baik et al. I Surjace imaging and dry development
270
3.
RESULTS
AND
DISCUSSION
Several researchers have studied gas phase silylation for E-beam exposures using chemical amplified resist with silylated with hexamethyl-disilazane(HMDS) and n,n-diethyl aminotrimethylsilane (TMSDEA) as silylating agents [3,4,5,6]. In a previous study we have investigated various alternative silylating agents for application to the DESIRE process. Alternative silylating agents, such as dimethyl silyldimethyl-amine (DMSDMA), TMDS, and n,n,-dimethylamino-trimethylsilane (TMSDMA) have demonstrated to exhibit some interesting advantages over HMDS, such as reduced vertical and lateral swelling, improved surface roughness, increased process latitude, and low temperature silylation [7]. The lower temperature agents such as TMDS appear increasingly attractive for deep W applications [8]. 3.1 .Positive tone process
EXPOSURE
0
SAL&l1
q
SUBSTRATE
0
EXPOSED [Acid +cmsslinker]
q
CROSSLINKED RESIN
I33
SILYLATED
02 PLASMA
-
302
=
SILYLATED
Figure 1. Process flow 3.2.Characterization
Also for E-beam silylation, we have investigated lower temperature agents such as TMDS, in combination with the chemically amplified resists SAL601 ER-7 and RAY PN. These resists are composed of a novolac resin, a melamine cross-linker and a radiation acid generator. E-beam exposures partly convert the radiation acid generator to acid. The crosslinking of the novolac resin is catalysed in the baking step by the generated acid. This crosslinking blocks the silylation of the exposed areas. Dry development results in positive tone images. The crosslinking is a function of both exposure energy and PSB conditions.The resist exhibits the same high sensitivity as for wet develo ment, as the crosslinking from 5 PC/cm fl onwards is sufficient to stop the Si diffusion into the exposed areas. In case of wet development the optimum post exposure bake [PEB] conditions are 115OC f 10°C, l-3 min [9]. TMDS is therefore one of the most suitable silylating agents, since the silylation occurs in exactly this temperature range. Also in case of the DESIRE process this agent exhibits a higher etch selectivity, a higher diffusion rate, and wider process latitude in comparison to HMDS. Hence we concentrated on this agent for the gas phase silylation. For dry development, we used a 2 step pure 02 process and 2 step C2F6 process, which has been detailed elsewhere [lo]. The etch selectivity between silylated area and nonsilylated area is 12: 1.
of silylation
Thickness measurements, and concentration of the absorbance of the peaks : vibration) and 820 cm-l
IR spectroscopy, and RBS were used to determine the distribution Si content. In case of IR, the Si content was calculated from the 1260 cm-l(Si-C bond deformation), 910 cm-l (Si-0-Aryl stretching (Si-C bond stretching vibration). The observed Si-0-Aryl peak is
271
K.-H. Baik et al. I Surface imaging and dry development
much larger than for the case of the DESIRE process. The integrated absorbance of 1280-1250 cm-l is taken as a quantitative measurement for the built-in silicon. Fig.3 shows the Si content from (1280-1250 cm-l) vs. exposure dose at various silylation temperatures with TMDS . This plot shows that the optimum PSB and silylation temperature is 105°C-1250C and exposure dose is >5l_tC/cm2. These conditions match to the bake conditions of wet development.
-0.8,
. , . , . ,
,
, . ,
,
_0.8,..._,....,....., Silylation
-
80
90
Exposure dose [p&m21
Silylation temperature[“C]
Figure 2. Si content as measured by IR versus silylation temperature at the various exposure dose
15
10
5
0
100 110 120 130 140 150
temperature
Figure 3. Si content as measured by IR versus exposure dose at the various silylation temperature
RBS is used to determine the Si depth profile and concentration. Fig.4 shows that a dose of 7 pC/cm2 induces enough crosslinking to inhibit Si diffusion. Fig.5 shows that the Si content and concentration increase with increasing silylation temperatures.
0.7
0.6
’
Energy
p5
Energy
(Me’/)
1.10
1.15
1.20
1.25
1.30
I
I
I
I
I
1.05
1.10
I
O3
Exposure dose [ pClcm2J t
I
(MeV) 1.20
I
1.25 1.30 I I -I
13x
._
‘So.4 -
1.15
Silylation temperature
0.5 p
I
”
.____
,; n 2” -iv_ il.,,‘\ t&-
-
\,\,
‘,A_,
_ _\ i
240 250 Channel
Figure 4.RBS measurements for the various exposure dose at 1 10°C silylation temperature 3.3. C.D. vs. delay time between exposure
Figure 5.RBS measurements for the various silylation temperature at the unexposed areas and PSB
Chemically amplified resists have been reported to exhibit aging problems. The delay time between exposure and PSB will affect the sensitivity and C.D. Since the crosslinking occurs at
272
K.-H. Baik et al. I Surface imaging and dry development
room temperature, the crosslinking density changes as a function of delay time. The C.D. variations between exposure and PSB within 1 hour correspond to 510% according to the exposure dose. After one hour delay time, no further CD variation is noticed (up to delay time of 1 day). A one hour delay time is therefore intentionally introduced. 3.4. Applications The above results indicate that surface imaging and dry development with a chemically amplified resist obtained promising results. Fig. 6 shows SEM micrographs of 0.2 pm, 0.175 pm patterns (lines/spaces) and 0.2 pm contact holes in a 1 pm thick resist with only 20 kV accelerating voltage, using a dose of 5 PC/cm 2. This SEM micrographs highlight the high resolution that can be obtained in combination with a high aspect ratio (5:l).
Figure 6 (a) 0.25 pm patterns (lines/spaces)
(c) 0.175 pm patterns (lines/spaces)
(b) 0.2 pm patterns (lines/spaces)
(d) 0.2 pm contact windows
K.-H. Baik et al. I Surface imaging and dry development
273
This top surface imaging technology shows a multilayer performance (with single layer process simplicity), resulting in a reduction of proximity effect and high resolution. Fig.7 compares cross-sections of 0.25 pm with 0.2 pm lines and spaces for (a)wet development on bare Si and for (b) dry development on 0.15 pm TiW(N) / 50 nm Si02 / Si. The crosslinking in between the exposed lines, as seen in (a), resulting from backscattering, does not limit the spatial resolution in (b) although the backscattering on TiW is much more pronounced.
Figure 7 (a) 0.25 pm patterns with wet development 4.
(b)0.2 pm patterns with dry development
CONCLUSIONS
Silylation and dry development has been investigated for E-beam lithography using chemically amplified resist. This process shows promising results, such as high resolution combined with a high aspect ratio, reduction of the proximity effect, and good etch resistance. We have been characterised the silylation using IR, and RBS. Using optimised process conditions 0.2 pm patterns (lines/spaces) and 0.2 pm contact holes are obtained in a 1 pm thick resist with a 20 kV electron beam using a dose of 5 pC/cm2.
ACKNOWLEDGEMENTS The authors would like to thank G.Brijs for the RBS measurements, J. Moonens for the Ebeam exposure, and N.Samarakone for some useful discussion.One of the authors would like to acknowledge CNPq-Brasil for its financial support.
REFERENCES F. Coopmans, and B. Roland, Proc. SPIE 631,34 (1986) M.Op de Beeck et all, ProcSPIE 1262,139 (1990) C.Pierrat et all, J. Vat. Sci. Technol. B 7(6), 1782 (1989) T.G.Vachette et all, Proc. SPIE 1262,258 (1990) T.Fujino et all, J. Vat. Sci. Technol. B 8(6), 1808 (1990) T.G.Vachette et all, Proc. ME 90,205 (1990) Ki-Ho Baik et all, J. Vat. Sci. Technol. B 8(6), 1481 (1990) A.M.Goethals et all , ProcSPIE 1466, 604 (1991) L.Blum et all, J. Vat. Sci. Technol. B 6(6), 2280 (1988) R.Lombaerts et all, Proc.SPIE 1262, 312 (1990)