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Experimental study on proximity effects in high voltage e-beam lithography
3.51
Microelectronic Engineering 11 (1990) 351-354 Elsevier Science Publishers B.V.
EXPERIMENTAL STUDY ON PROXIMITY E-BEAM LITHOGRAPHY
EFFECTS IN HIGH VOLTAGE
E. Boa-e, E. van der Drift, J. Romijn and B. Rousseeuw
Delft Institute for Microelectronics and Submicron technology (DIMES) Delft University of Technology, The Netherlands
For accurate realization of patterns by e-beam lithography it is essential to know the proximity exposure effect. However, both experimental and theoretical results are available mainly for single layer masks on Si in the acceleration voltage range of 20 kV to 30 kV. In this work an experimental study on proximity effects is presented with exposure conditions and substrate systems being typical for modem high resolution lithography, i.e. exposures of single layer and multilayer masks at an acceleration voltage of 50 kV or 100 kV on both Si and substrate layers with a different atomic number. Significant deviations from the two-gaussian model were observed.
1.
INTRODUCTION
To reduce proximity exposure effects in e-beam lithography the technology of multilayer masking is used and higher acceleration voltages up to 100 kV are applied. Simultaneously there is a growing interest for microfabrication in substrate layers with a higher atomic number (InP, GaAs, TiW, Nb3Ge) [ 1,2]. Unfortunately, experimental and theoretical information on the range and intensity of electron backscattering is available mainly for Si substrates coated with a single layer resist and exposed to 20-30 kV electrons. It is the aim of this work to study the proximity effect in the area typical for modern high resolution lithography, i.e. at an acceleration voltage of 50 or 100 kV, in single and multilayer resist on both Si and substrate layers with a different atomic number. Among the great variety of experiments to characterize the proximity exposure we used the ‘doughnut’ method reported by Stevens et al. [3] taking full advantage of the radial symmetry involved in the exposure by a gaussian beam. The experimental results on electron scattering will be discussed in relation to data from 20 kV experiments and results available from Monte Carlo calculations, with particular attention to observed deviations from the double-gaussian exposure model.
2.
PROXIMITY
EXPOSURE MEASUREMENT
In the doughnut method the basic element is a ring with an inner radius Rt and an outer radius R2. Upon e-beam exposure of the ring the dose Q of the center of the inner circle is merely determined by forward- and backscattered electrons. The value of R2 is fixed and is at least 2 times the backscatter range p for a given substrate material. In the approach of the electron scattering according to a two-gaussian contribution and for R2>43 one may write [3] Q = Qp(l+q)Iexp]-Rt2/a21
+ qexp[-Rt2/P21
1-l
(1)
where Qp is the threshold dose to clear out the resist, a and p are the standard deviations of the gaussian forward (a) and backscattered (p) electron distributions and ?l is the backscatter coefficient. For values of Rl>>a one may then expect a linear plot of In Q vs. R12. The threshold dose is measured in a series of large area exposures (dose step 2@/cm2) simultaneously with the doughnut exposure. The layout of the doughnut test pattern is shown in Fig. 1. The exposure area is devided in two parts, with the inner (from R1 to Rt+2ym) and outer (from Rt+2pm to R2) part defined with 25 nm and 250 nm pixel resolution respectively. Somewhat dependent on substrate material and acceleration voltage Rt values range from 0.1 pm to 10 l.trn, with a smallest step of 0.05 l.trn in the 0.1 l.trn - 1.0 l.trn range.
0167-9317/90/$3.50 0 1990, Elsevier Science Publishers B.V.
352
E. Boere et al. I Experimental study on proximity effects
EXPOSED AREA
OQOO
FIGURE 1 Doughnut test structure
FIGURE 2 Experimental doughnut array
E-beam exposures were carried out at 50 kV and 100 kV with the Philips e-beam writers EBPG/03 and EBPG/OS respectively. All exposures were done on 0.35 pm thick PMMA (2041 Elvacite, mol. wt. 300,000) baked at 175 OC for 1 hr. The patterns were developed in IPA : MIBK = 3 : 1 for 60 seconds at 23.0 OC. We tested extensively for variations in development time, temperature and the time in vacuum and concluded that the accuracy of the method is about 2 percent. Also a high molecular weight PMMA (2,300,OOO) was attempted and yielded essentially the same results. The multilayer mask consisted of a 0.85 pm thick bottom layer of AZ 1470 with on top the intermediate layer of Ge, Al or Au in varying thicknesses. An experimental result is shown in Fig.2.
RESULTS
3.
Logarithmic plots of the threshold dose Q vs R12 for PMMA on substrates with a different atomic number Z are given in Fig. 3a whereas in Fig. 3b for some materials also a log Q vs. RI plot is given. Figs. 4a, 4b show a comparison of results for PMMA exposures at 50 kV and 100 kV and in Figs. 5a, 5b results for PMMA in a variety of multilayer mask configurations are collected. A summary of proximity exposure constants is given in Table 1. 2000
2000
1000 3
-
1000
NE Y y 500
s 3 a
CI
500
200
200 0
10
20
0
1
2
3
4
5
FIGURE 3 Logarithmic plots of threshold dose Q vs. R12(a) and vs. RI(b) for PMMA at 50 kV on substrates with different atomic number Z: a) GaAs, InP, InSb, Nb3Ge and Au*; b) Au*, Au*-AZ, NbgGe; Au* stands for Ti/Pt/Au with thicknesses of 0.2.pm, 0.2 pm and 0.6 pm respectively.
353
E. Boere et al. I Experimental study on proximity effects
FIGURE 4 Logarithmic plot of threshold dose Q vs. R12 for PMMA at 50 kV and 100 kV on various substrate layers: a) Si, GaAs and Au* layer ; b) extended plot for RI* < 31.tm2. I
”
”
‘Y---l
”
x
1000
,
.:.-:::-:-:I 1
.
.
.
.
.
.
.
w-
e
I
.
2
.
.
c1
.
.
Al
A”’
I
500 0
10
20
30
40
50
0
60
10
R," ljm"l
20
R,
30
40
50
bm")
FIGURE 5 Plots of log Q vs Rt2 at 50 kV for the multilayer mask system AZWPMMA with X= Al, Ge or Au; a) comparison of 100 nm Al and Au; b) comparison of Au layers with varying thickness.
Si
Proximity exposure constants1 Multilayer p q P Si-AZ-X (nm) (IL ) (W ) 10: 0.51 Al (50,100,500) ll?!
g4 GaAs InSb InP Si-AZ GaAs-AZ
9.9 17.5 3.9 3.3 4.1 11.0 4.8
Table 1 Substrate
0.19 0.62 0.88 0.94 0.99 0.45 0.91
Ge(50,lOO) Ge(500) Au(50) Au(100) Au(200)
10.9 10.5 10.3 10.1 9.5
rl
Substrate
p q (cl ) 337 087
0.50
Si3
0.50 0.34 0.47 0.42 0.32
GaAs3 Si-AZ3 32:5 11.1 Ti/Pt/Au3 4.4 Ti/Pt/Au-AZ3 4.1
y (ttm)
v
0:86 0.89 0.52 0.89
Ti/Pt/Aus 0.49 1.4 Ti/Pt/Au-AZ5 1.20 1.2 NbqGeS 0.95 1.7 1) All data for 50 kV and ca. 0.35 j.tm PMMA unless otherwise stated; 2) Relevant layer thickness between parentheses in nm; 3) 100 kV; 4) 130 nm PMMA 5) Data from exponential fit with y and v.
354 4
E. Boere et al. I Experimental study on proximity effects
DISCUSSION
Except for Ti/Pt/Au and Nb3Ge at 50 kV the plots of log Q vs. R12 are linear for the larger RI values which is an indication for backscattering according to a gaussian model (with j3 and q given in table 1). In contrast the NbgGe and Tit/Au/ systems at 50 kV show rather a linear behaviour in log Q vs. RI. (Fig. 3b). This may be explained by taking into account an exponential contribution in the electron backscattering. Particularly in cases of high electron scattering, the relevance of an exponential term has been pointed out by Aizaki [4] in Monte Carlo calculations on MO, Cr and Au and experimentally for GaAs at 25 kV by Rishton et al. [5]. The transition from exponential to gaussian behaviour depends on the applied acceleration voltage. For GaAs the transition is somewhere in between 20 and 50 kV as may be concluded from point exposure results from Rishton et al. [5]. Obviously, in considering the Ti/Pt/Au results at 50 kV and 100 kV the transition from exponential to gaussian behaviour is somewhere around 100 kV. The p values for Si at 50 kV and 100 kV agree with results from Jackel et al. [6] who found an El.7 dependence. For GaAs we find the exponent to be somewhat smaller, i.e 1.5. In comparing the multilayer mask systems AZ-X (X=Al,Ge,Au) with the Si-AZ-PMMA system we find negligible effects from Al up to 500 nm and from Ge up to 100 nm. The 500 nm Ge sample shows an indication for two-gaussian behaviour in the backscattering, i.e @1,~1)=(6.5 pm, 0.40) and (p2,~2)=(10.5 pm, 0.34). The multilayer mask system with an intermediate layer of Au shows significant effects even from a 50 nm thick layer. As a general rule the deviating samples in this series show a lower 77 value which can be ascribed to a screening effect from the intermediate layer, in agreement with conclusions from Monte Carlo calculations by Balladore et a1.[7]. All samples show a very large dip in the exposure dose required for R1 values below about 0.5 pm, irrespective of the substrate material, the acceleration voltage or multilayer masking. This effe cannot be ascribed to the expected forward scattering which is about 30 nm. Also it may not be ascribed to backscattering but rather to some additional forward scattering process or to a contribution from the beam itself. From all systems considered the results for Si at 100 kV in the range of RI< 0.5 pm are most extensive for a more detailed analysis. In addition to the gaussian contribution wit a p of 33.5 pm we find in a best fit an exponential contribution with a characteristic range y = 98 nm, which is in the same order of the width of the applied beam of about 35 nm. Comparable findings have been found by Tennant et al. [6] in PMMA on InP. The physical origin of this term is not clear yet.
CONCLUSIONS An experimental study on proximity exposure effects in PMMA on a variety of substrates with different atomic number 2, in several multilayer mask systems and at 50 and 100 kV shows backscattering according to a gaussian distribution. However, layers of Nb3Ge and Ti/Pt/Au show a more exponential behaviour, presumably due to the high electron scattering in these materials. Throughout significant deviations from the gaussian model in the region up to about 0.5 pm from the primary exposure were observed, but till now this effect could not be satisfactorily explained. ACKNOWLEDGEMENTS The authors are grateful to L.E.M. de Groot and B.G.M. de Lange for the e-beam exposures and to J. T6th for the SEM-micrographs. REFERENCES
VI PI
131 t;;
[61 [71
Tennant, D.M., Doran, G.E., Howard, R.E. and Denker, J.S., J. Vat. Sci. Technol B 6 (1988), 426 van der Drift, E., Radelaar, S., Pruymboom, A.and Kes, P.H., Microel. Eng. 6 (1987) 181 Stevens, L.,Jonckheere, R., Proyen, E., Decoutere, S. and Lanneer, D., Microel. Eng. 5 (1986) 141 Aizaki, N., J.Vac. Sci. Technol. 16 (1979) 1726 Rishton, S.A. and Kern, D.P., J. Vat. Sci. Technol. B 5 (1987) 135 Jackel, L.D., Howard, R.E., Mankiewich, P.M., Craighead,H.G. and Epworth, R.W., Appl. Phys. Lett. 45 (1984) 698. Balladore, J.L., Pyee, M., Camon, H., Bourdel, E., Martinez, J.P.and Sekkaki, N. Microel. Eng. 6 (1987), 201
Microelectronic Engineering 11 (1990) 351-354 Elsevier Science Publishers B.V.
EXPERIMENTAL STUDY ON PROXIMITY E-BEAM LITHOGRAPHY
EFFECTS IN HIGH VOLTAGE
E. Boa-e, E. van der Drift, J. Romijn and B. Rousseeuw
Delft Institute for Microelectronics and Submicron technology (DIMES) Delft University of Technology, The Netherlands
For accurate realization of patterns by e-beam lithography it is essential to know the proximity exposure effect. However, both experimental and theoretical results are available mainly for single layer masks on Si in the acceleration voltage range of 20 kV to 30 kV. In this work an experimental study on proximity effects is presented with exposure conditions and substrate systems being typical for modem high resolution lithography, i.e. exposures of single layer and multilayer masks at an acceleration voltage of 50 kV or 100 kV on both Si and substrate layers with a different atomic number. Significant deviations from the two-gaussian model were observed.
1.
INTRODUCTION
To reduce proximity exposure effects in e-beam lithography the technology of multilayer masking is used and higher acceleration voltages up to 100 kV are applied. Simultaneously there is a growing interest for microfabrication in substrate layers with a higher atomic number (InP, GaAs, TiW, Nb3Ge) [ 1,2]. Unfortunately, experimental and theoretical information on the range and intensity of electron backscattering is available mainly for Si substrates coated with a single layer resist and exposed to 20-30 kV electrons. It is the aim of this work to study the proximity effect in the area typical for modern high resolution lithography, i.e. at an acceleration voltage of 50 or 100 kV, in single and multilayer resist on both Si and substrate layers with a different atomic number. Among the great variety of experiments to characterize the proximity exposure we used the ‘doughnut’ method reported by Stevens et al. [3] taking full advantage of the radial symmetry involved in the exposure by a gaussian beam. The experimental results on electron scattering will be discussed in relation to data from 20 kV experiments and results available from Monte Carlo calculations, with particular attention to observed deviations from the double-gaussian exposure model.
2.
PROXIMITY
EXPOSURE MEASUREMENT
In the doughnut method the basic element is a ring with an inner radius Rt and an outer radius R2. Upon e-beam exposure of the ring the dose Q of the center of the inner circle is merely determined by forward- and backscattered electrons. The value of R2 is fixed and is at least 2 times the backscatter range p for a given substrate material. In the approach of the electron scattering according to a two-gaussian contribution and for R2>43 one may write [3] Q = Qp(l+q)Iexp]-Rt2/a21
+ qexp[-Rt2/P21
1-l
(1)
where Qp is the threshold dose to clear out the resist, a and p are the standard deviations of the gaussian forward (a) and backscattered (p) electron distributions and ?l is the backscatter coefficient. For values of Rl>>a one may then expect a linear plot of In Q vs. R12. The threshold dose is measured in a series of large area exposures (dose step 2@/cm2) simultaneously with the doughnut exposure. The layout of the doughnut test pattern is shown in Fig. 1. The exposure area is devided in two parts, with the inner (from R1 to Rt+2ym) and outer (from Rt+2pm to R2) part defined with 25 nm and 250 nm pixel resolution respectively. Somewhat dependent on substrate material and acceleration voltage Rt values range from 0.1 pm to 10 l.trn, with a smallest step of 0.05 l.trn in the 0.1 l.trn - 1.0 l.trn range.
0167-9317/90/$3.50 0 1990, Elsevier Science Publishers B.V.
352
E. Boere et al. I Experimental study on proximity effects
EXPOSED AREA
OQOO
FIGURE 1 Doughnut test structure
FIGURE 2 Experimental doughnut array
E-beam exposures were carried out at 50 kV and 100 kV with the Philips e-beam writers EBPG/03 and EBPG/OS respectively. All exposures were done on 0.35 pm thick PMMA (2041 Elvacite, mol. wt. 300,000) baked at 175 OC for 1 hr. The patterns were developed in IPA : MIBK = 3 : 1 for 60 seconds at 23.0 OC. We tested extensively for variations in development time, temperature and the time in vacuum and concluded that the accuracy of the method is about 2 percent. Also a high molecular weight PMMA (2,300,OOO) was attempted and yielded essentially the same results. The multilayer mask consisted of a 0.85 pm thick bottom layer of AZ 1470 with on top the intermediate layer of Ge, Al or Au in varying thicknesses. An experimental result is shown in Fig.2.
RESULTS
3.
Logarithmic plots of the threshold dose Q vs R12 for PMMA on substrates with a different atomic number Z are given in Fig. 3a whereas in Fig. 3b for some materials also a log Q vs. RI plot is given. Figs. 4a, 4b show a comparison of results for PMMA exposures at 50 kV and 100 kV and in Figs. 5a, 5b results for PMMA in a variety of multilayer mask configurations are collected. A summary of proximity exposure constants is given in Table 1. 2000
2000
1000 3
-
1000
NE Y y 500
s 3 a
CI
500
200
200 0
10
20
0
1
2
3
4
5
FIGURE 3 Logarithmic plots of threshold dose Q vs. R12(a) and vs. RI(b) for PMMA at 50 kV on substrates with different atomic number Z: a) GaAs, InP, InSb, Nb3Ge and Au*; b) Au*, Au*-AZ, NbgGe; Au* stands for Ti/Pt/Au with thicknesses of 0.2.pm, 0.2 pm and 0.6 pm respectively.
353
E. Boere et al. I Experimental study on proximity effects
FIGURE 4 Logarithmic plot of threshold dose Q vs. R12 for PMMA at 50 kV and 100 kV on various substrate layers: a) Si, GaAs and Au* layer ; b) extended plot for RI* < 31.tm2. I
”
”
‘Y---l
”
x
1000
,
.:.-:::-:-:I 1
.
.
.
.
.
.
.
w-
e
I
.
2
.
.
c1
.
.
Al
A”’
I
500 0
10
20
30
40
50
0
60
10
R," ljm"l
20
R,
30
40
50
bm")
FIGURE 5 Plots of log Q vs Rt2 at 50 kV for the multilayer mask system AZWPMMA with X= Al, Ge or Au; a) comparison of 100 nm Al and Au; b) comparison of Au layers with varying thickness.
Si
Proximity exposure constants1 Multilayer p q P Si-AZ-X (nm) (IL ) (W ) 10: 0.51 Al (50,100,500) ll?!
g4 GaAs InSb InP Si-AZ GaAs-AZ
9.9 17.5 3.9 3.3 4.1 11.0 4.8
Table 1 Substrate
0.19 0.62 0.88 0.94 0.99 0.45 0.91
Ge(50,lOO) Ge(500) Au(50) Au(100) Au(200)
10.9 10.5 10.3 10.1 9.5
rl
Substrate
p q (cl ) 337 087
0.50
Si3
0.50 0.34 0.47 0.42 0.32
GaAs3 Si-AZ3 32:5 11.1 Ti/Pt/Au3 4.4 Ti/Pt/Au-AZ3 4.1
y (ttm)
v
0:86 0.89 0.52 0.89
Ti/Pt/Aus 0.49 1.4 Ti/Pt/Au-AZ5 1.20 1.2 NbqGeS 0.95 1.7 1) All data for 50 kV and ca. 0.35 j.tm PMMA unless otherwise stated; 2) Relevant layer thickness between parentheses in nm; 3) 100 kV; 4) 130 nm PMMA 5) Data from exponential fit with y and v.
354 4
E. Boere et al. I Experimental study on proximity effects
DISCUSSION
Except for Ti/Pt/Au and Nb3Ge at 50 kV the plots of log Q vs. R12 are linear for the larger RI values which is an indication for backscattering according to a gaussian model (with j3 and q given in table 1). In contrast the NbgGe and Tit/Au/ systems at 50 kV show rather a linear behaviour in log Q vs. RI. (Fig. 3b). This may be explained by taking into account an exponential contribution in the electron backscattering. Particularly in cases of high electron scattering, the relevance of an exponential term has been pointed out by Aizaki [4] in Monte Carlo calculations on MO, Cr and Au and experimentally for GaAs at 25 kV by Rishton et al. [5]. The transition from exponential to gaussian behaviour depends on the applied acceleration voltage. For GaAs the transition is somewhere in between 20 and 50 kV as may be concluded from point exposure results from Rishton et al. [5]. Obviously, in considering the Ti/Pt/Au results at 50 kV and 100 kV the transition from exponential to gaussian behaviour is somewhere around 100 kV. The p values for Si at 50 kV and 100 kV agree with results from Jackel et al. [6] who found an El.7 dependence. For GaAs we find the exponent to be somewhat smaller, i.e 1.5. In comparing the multilayer mask systems AZ-X (X=Al,Ge,Au) with the Si-AZ-PMMA system we find negligible effects from Al up to 500 nm and from Ge up to 100 nm. The 500 nm Ge sample shows an indication for two-gaussian behaviour in the backscattering, i.e @1,~1)=(6.5 pm, 0.40) and (p2,~2)=(10.5 pm, 0.34). The multilayer mask system with an intermediate layer of Au shows significant effects even from a 50 nm thick layer. As a general rule the deviating samples in this series show a lower 77 value which can be ascribed to a screening effect from the intermediate layer, in agreement with conclusions from Monte Carlo calculations by Balladore et a1.[7]. All samples show a very large dip in the exposure dose required for R1 values below about 0.5 pm, irrespective of the substrate material, the acceleration voltage or multilayer masking. This effe cannot be ascribed to the expected forward scattering which is about 30 nm. Also it may not be ascribed to backscattering but rather to some additional forward scattering process or to a contribution from the beam itself. From all systems considered the results for Si at 100 kV in the range of RI< 0.5 pm are most extensive for a more detailed analysis. In addition to the gaussian contribution wit a p of 33.5 pm we find in a best fit an exponential contribution with a characteristic range y = 98 nm, which is in the same order of the width of the applied beam of about 35 nm. Comparable findings have been found by Tennant et al. [6] in PMMA on InP. The physical origin of this term is not clear yet.
CONCLUSIONS An experimental study on proximity exposure effects in PMMA on a variety of substrates with different atomic number 2, in several multilayer mask systems and at 50 and 100 kV shows backscattering according to a gaussian distribution. However, layers of Nb3Ge and Ti/Pt/Au show a more exponential behaviour, presumably due to the high electron scattering in these materials. Throughout significant deviations from the gaussian model in the region up to about 0.5 pm from the primary exposure were observed, but till now this effect could not be satisfactorily explained. ACKNOWLEDGEMENTS The authors are grateful to L.E.M. de Groot and B.G.M. de Lange for the e-beam exposures and to J. T6th for the SEM-micrographs. REFERENCES
VI PI
131 t;;
[61 [71
Tennant, D.M., Doran, G.E., Howard, R.E. and Denker, J.S., J. Vat. Sci. Technol B 6 (1988), 426 van der Drift, E., Radelaar, S., Pruymboom, A.and Kes, P.H., Microel. Eng. 6 (1987) 181 Stevens, L.,Jonckheere, R., Proyen, E., Decoutere, S. and Lanneer, D., Microel. Eng. 5 (1986) 141 Aizaki, N., J.Vac. Sci. Technol. 16 (1979) 1726 Rishton, S.A. and Kern, D.P., J. Vat. Sci. Technol. B 5 (1987) 135 Jackel, L.D., Howard, R.E., Mankiewich, P.M., Craighead,H.G. and Epworth, R.W., Appl. Phys. Lett. 45 (1984) 698. Balladore, J.L., Pyee, M., Camon, H., Bourdel, E., Martinez, J.P.and Sekkaki, N. Microel. Eng. 6 (1987), 201