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EBIC probing — a possible tool for the verification of simulated electron beam exposure widths
Microelectronic Engineering 9(1989)625-628 North-Holland
625
EBIC probing - A possible tool for the verification of simulated electron beam exposure widths
M.Singh, N.R. Rajopadhye, N.K.L.Raja and W.S. Khokle Central Electronics Engineering Research Institute Pilani - 333031, INDIA
This paper presents the application of the EBIC contrast width measurement technique for the verification of simulated electron beam p-i-n measurements performed on exposure widths. The were configuration of a-Si:H, as a case study, at various exposure conditions. The measured values of EBIC contrast width were compared with the widths of simulated electron energy exposure profiles. 1
INTRODUCTION :
The energy deposition in amorphous solids, particularly in the resists, is of interest in the - studies related. to electron beam lithography. Various methods have been reported for the calculation of energy deposited in resists at different radial distances from the point of incidence [l]. However , there is no suitable method for the experimental verification of the simulated exposure widths in the latent image. The observations made after the development of the exposed resists may lead to misinterpretation since the Drofile. and therebv the width. aet affected by the intermediate proces&s. We propose here the electron beam induced conductivity width (EBIC) contrast measurements as a possible tool for the study of sub-surface energy deposition widths. This method gives immediate estimate of the width of the depositedenergy profile without any modification. Hydrogenated amorphous silicon (a-Si:H) has been exploredforavariety of applications during pastat wig the decades[2,3]. It is well known the energetic photons or electrons incident on the a-Si:H samole, damases are created. These damages may be due-to the breaking of Si-Si and Si-H bonds and have long relaxation times [3,41. This effect is advantageously used in the present method. -The -electron beam induced current (EBIC) mode of the scanning electron microscope (SEM) [5] can be employed for reading or imaging this information. When a p-i-n configuration of a-Si:H is observed under
SEM-EBIC mode the local recombination of the electron-hole .pairs chanqes in the region where energy is deposited and appears as a contrast in the EBIC image. contrast the width of EBIC Thus represents the width of sub surface energy deposition profile. of In this paper the variation SEM-EBIC contrast width with respect to the incident e-beam dose was measured for various beam diameters. The measured values were compared with those obtained from the simulation and the validity of the present method is discussed.
Figure-l Block diagram of the electron beam writing and EBIC set-up.
0167.9317/89/$3.50 0 1989,ElsevierSciencePublishersB.V.(North-Holland)
M. Singh et al. / EBIC probing
626
2
EXPERIMENTAL
SET UP
Contrast width measurements were performed on the p-i-n configuration [4] of a-Si:H deposited by conventional glow discharge CVD method. Experiments were performed in scanning electron microscope (JEOLa JSM IC 25s) attached with an EBIC imaging set up (seen as schematic in figure 1). The experimental set up is similar to that described by Leamy [5]. Samples were exposed using spot mode of SEM for different exposure times to vary the dose at 12.5 keV beam energy, at various The EBIC primary diameters. beam contrast width of the exposed samples were then measured using SEM-EBIC mode at incident electron beam energy of 12.5 kev at lower probe currents. The frame scan rates and SEM-magnification were chosen so as to keep the background exposure to a minimum possible level. Contrast width was determined as that at which the EBIC contrast drops to 10% of its maximum value.
3
SIMULATION
ELECTRON
at the bottom
ENERGY
DEPOSITION
SAMPLE:
A - Si:H
of
PROFILE
XlOlO F z
13 5t
z 108 .*. 11 '3 u 8lij s + ;: g k5 ?b $ 5
I 54I /i 27fli\\\
---AT --AT
&:;zg+eb-J~
1.0
LATERAL
1.5
DISTANCE
1.0 urn depth 0.5 urn depth
2.0
.~_ 2.5
(urn)
Figure-2 The typical simulated energy deposition profile in a-Si:H calculated using EEDP program for incident energy of the 12.5 keV.
METHODOLOGY:
Deposition The Electron Energy Profile (EEDP) data are obtained using Monte-Carlo methods for the computation of electron energy loss at various layers of the medium being exposed. The electron energy loss is formulated using scattering Rutherford model of electron and continuous slow down approximation the method [6,7]. The relation for of the electron in a solid, energy loss with a single species,is given by: dE/ds
beam exposure profile the p-i-n configuration.
d=1.16 d=O.
urn
6
77 ,um
.
= - (Kl / E) * ln(K2 * E) ..(l)
where Kl and K2 are terms dependent on the mass density, atomic numberof the mean the weight and species, atomic energy of the medium and E is ionisation the incident beam energy. performed for 5000 Simulation was with beam energy of 12.5 keV. electrons To account for the absorption of electron contact energy in the top aluminimum loss in this .layer was layer, energy linear interpolation using approximated of the above relation (1). The electron energy deposition profile in the bottom layer of the a-Si:H is shown in figure 2. The SAMPLE program frcrnthe University 181 was of California, Berkeley, U.S.A. of electron used to simulate the width
Figure-3 contrast width Variation of the EBIC with respect to the exposure time for for various beam diameters (d) used the irradiation of a-Si:H sample in the spot mode of SEM.( - => d=l.l6pm, => d=0.34nm, - => d=0.77pm, The zntinous lines => d=0.23pm). represent the simulated data.
627
M. Singh et al. / EBZCprobing
This package convolves the electron (EEDP) profile deposition energy dimensionless corresponding to a single Here electron to a finite beam diameter. the region, in which the width of the (Emin) is deposited minimum energy sufficient to create damage in a-Si:H, from the simulated energy was extracted deposition curves and is a measure of the Curves width of the exposed spot. depicting the relation between the width of the exposed spot and the exposure time were plotted (figure 3). 4
RESULTS
AND DISCUSSIONS
:
width contrast Dependence of EBIC time of to the exposure with respect for various beam irradiation of a-Si:H diameters in spot mode is shown in figure with contrast width increases 3. The respect to the exposure time and almost saturates at about 30 second exposure.
Figure-4 Schematic to explain the variation observed in figure 3. The EBIC contrast begins to appear at point A of the probe scan as the interaction between generation volume (Gv) and the damaged volume (Dv) starts and disappears at point C where the interaction ends. Lm,Lp and Ld are the widths of the measured contrast, generation volume of the EBIC probe and damaged volume respectively.
4 shematic shown in figure The the measurement methodology. illustrates When the electron beam starts to move
'A' EBIC contrast begins to from point between appear as the interaction starts the generation volume TkT) and EBIC contrast damaged volume (Dv). until the beam continues to increase reaches point 'B' and falls at point 'C'. Distance 'AC' is measured experimentally. volume width (Ld) can be The damaged given as : Ld = (Lm - Lp) where Lm is measured width and 'Lp' ir the generation volume width of the reading 'Ld' earlier EBIC probe. As mentioned the width of the region where represents energy is deposited. capability of proposed The the can be seen by comparing the method simulation data with the experimental results presented in figure 3. The energy required to create a damage in a-Si:H (Emin) is of the order that of 0.2 ev/atom. It should be noted the deposited energy per atom falls down the radial rapidly with respect to distance as shown in figure 5 (obtained by computer simulation) and the choice of Emin does not play any significant role in in deciding the exposure spot width the range 0 - 10 ev/atom.
LATERAL DISTANCE (in urn)
Figure-5 simulated The exposure profile in a-Si:H. The amount of enerqv deposited falls rapidly with respect to the lateral distance.
Fairly good correlation is observed between experimental simulated and exposure spot width values for higher exposure doses. The experimental values, however, differ from the simulated values at lower dosages. One possible reason for this mismatch could be due to the
M. Singh et al. / EBIC probing
628
approximation made during simulation that the energy loss in the top aluminium contact layer (of p-i-n configuration) is similar to that in Silicon. For larger doses the errors this due to approximation are masked because of inherent averaging performed by the Monte-Carlo method. However, at small doses the deviations from the experimental values become observable. It is expected to obtain a good match of experimental and simulation results if all the layers with their respective parameters are included in the simulation.
5
REFERENCES 1.
Hatzakis M,Ting and Viswanathan,Proc. Int. on Electron and Ion beam Conf. Sci. and Tech., (1974) 542.
2.
Pankov J I: 'Hydrogenated silicon' 21 amorphous VA. of and (Parts D) A,B,C and Semimetals' 'semiconductors (New eds. Willardson and Beer York: Academic, 1984).
3.
B.A. Yacobi, 'Charge collection electron memory effect in irradiated hydrogenated silicon.', Microsc. amorphous Semicond. Mater. Conf., Inst. Phys Ser. No. 76: Section 8, Oxford, 1985, pp 349-354.
4.
Singh, Rajopadhye N.R., Mulayam and Khokle W.S., N.K.L. Raja Electronic IEE proceedings: devices (1988) in print.
5.
Phys. Leamy H.J., J.Appl. 5366), 1988 R 51-R 80.
6.
K,Proc. Kyser D.F and Murata on Electron and 6th Int. Conf. Ion beam Sci. and Tech., (1974) 205.
7.
Electron-beam Greeneich J.S, processes, in:Brewer G.R.(ed), Electron-beam technology in fabrication microelectronic 1980) pp. (Academic Press, 59-140.
a.
W.G, Nandgaonkar Oldham A.R and O'toole S.N,Neureuther Electron IEEE Trans. M.M, ED-27,No.8,(1980) Devices Vol. 1455.
SCOPE FOR FURTHER WORK
It is obvious that a junction needs to be formed on the medium in order to use this method. As junctions can not be formed on polymer materials, viz electron resists, the capacitive mode of EBIC imaging [51 is being attempted to observe the EBIC contrast. Estimate of the exposure width in polymer materials can be made indirectly by extrapolating results obtained the from the a-Si:H film of same thickness at a given incident beam energy. Proper correlation that account for functions, the change in the atomic number, atomic mean energy, weight weight, ionisation density fractions of composing species, transform the exposure etc., and would width in a-Si:H that in a given into Attempts in resist, need to be evolved. this directions are also being made at present.,
:
(
625
EBIC probing - A possible tool for the verification of simulated electron beam exposure widths
M.Singh, N.R. Rajopadhye, N.K.L.Raja and W.S. Khokle Central Electronics Engineering Research Institute Pilani - 333031, INDIA
This paper presents the application of the EBIC contrast width measurement technique for the verification of simulated electron beam p-i-n measurements performed on exposure widths. The were configuration of a-Si:H, as a case study, at various exposure conditions. The measured values of EBIC contrast width were compared with the widths of simulated electron energy exposure profiles. 1
INTRODUCTION :
The energy deposition in amorphous solids, particularly in the resists, is of interest in the - studies related. to electron beam lithography. Various methods have been reported for the calculation of energy deposited in resists at different radial distances from the point of incidence [l]. However , there is no suitable method for the experimental verification of the simulated exposure widths in the latent image. The observations made after the development of the exposed resists may lead to misinterpretation since the Drofile. and therebv the width. aet affected by the intermediate proces&s. We propose here the electron beam induced conductivity width (EBIC) contrast measurements as a possible tool for the study of sub-surface energy deposition widths. This method gives immediate estimate of the width of the depositedenergy profile without any modification. Hydrogenated amorphous silicon (a-Si:H) has been exploredforavariety of applications during pastat wig the decades[2,3]. It is well known the energetic photons or electrons incident on the a-Si:H samole, damases are created. These damages may be due-to the breaking of Si-Si and Si-H bonds and have long relaxation times [3,41. This effect is advantageously used in the present method. -The -electron beam induced current (EBIC) mode of the scanning electron microscope (SEM) [5] can be employed for reading or imaging this information. When a p-i-n configuration of a-Si:H is observed under
SEM-EBIC mode the local recombination of the electron-hole .pairs chanqes in the region where energy is deposited and appears as a contrast in the EBIC image. contrast the width of EBIC Thus represents the width of sub surface energy deposition profile. of In this paper the variation SEM-EBIC contrast width with respect to the incident e-beam dose was measured for various beam diameters. The measured values were compared with those obtained from the simulation and the validity of the present method is discussed.
Figure-l Block diagram of the electron beam writing and EBIC set-up.
0167.9317/89/$3.50 0 1989,ElsevierSciencePublishersB.V.(North-Holland)
M. Singh et al. / EBIC probing
626
2
EXPERIMENTAL
SET UP
Contrast width measurements were performed on the p-i-n configuration [4] of a-Si:H deposited by conventional glow discharge CVD method. Experiments were performed in scanning electron microscope (JEOLa JSM IC 25s) attached with an EBIC imaging set up (seen as schematic in figure 1). The experimental set up is similar to that described by Leamy [5]. Samples were exposed using spot mode of SEM for different exposure times to vary the dose at 12.5 keV beam energy, at various The EBIC primary diameters. beam contrast width of the exposed samples were then measured using SEM-EBIC mode at incident electron beam energy of 12.5 kev at lower probe currents. The frame scan rates and SEM-magnification were chosen so as to keep the background exposure to a minimum possible level. Contrast width was determined as that at which the EBIC contrast drops to 10% of its maximum value.
3
SIMULATION
ELECTRON
at the bottom
ENERGY
DEPOSITION
SAMPLE:
A - Si:H
of
PROFILE
XlOlO F z
13 5t
z 108 .*. 11 '3 u 8lij s + ;: g k5 ?b $ 5
I 54I /i 27fli\\\
---AT --AT
&:;zg+eb-J~
1.0
LATERAL
1.5
DISTANCE
1.0 urn depth 0.5 urn depth
2.0
.~_ 2.5
(urn)
Figure-2 The typical simulated energy deposition profile in a-Si:H calculated using EEDP program for incident energy of the 12.5 keV.
METHODOLOGY:
Deposition The Electron Energy Profile (EEDP) data are obtained using Monte-Carlo methods for the computation of electron energy loss at various layers of the medium being exposed. The electron energy loss is formulated using scattering Rutherford model of electron and continuous slow down approximation the method [6,7]. The relation for of the electron in a solid, energy loss with a single species,is given by: dE/ds
beam exposure profile the p-i-n configuration.
d=1.16 d=O.
urn
6
77 ,um
.
= - (Kl / E) * ln(K2 * E) ..(l)
where Kl and K2 are terms dependent on the mass density, atomic numberof the mean the weight and species, atomic energy of the medium and E is ionisation the incident beam energy. performed for 5000 Simulation was with beam energy of 12.5 keV. electrons To account for the absorption of electron contact energy in the top aluminimum loss in this .layer was layer, energy linear interpolation using approximated of the above relation (1). The electron energy deposition profile in the bottom layer of the a-Si:H is shown in figure 2. The SAMPLE program frcrnthe University 181 was of California, Berkeley, U.S.A. of electron used to simulate the width
Figure-3 contrast width Variation of the EBIC with respect to the exposure time for for various beam diameters (d) used the irradiation of a-Si:H sample in the spot mode of SEM.( - => d=l.l6pm, => d=0.34nm, - => d=0.77pm, The zntinous lines => d=0.23pm). represent the simulated data.
627
M. Singh et al. / EBZCprobing
This package convolves the electron (EEDP) profile deposition energy dimensionless corresponding to a single Here electron to a finite beam diameter. the region, in which the width of the (Emin) is deposited minimum energy sufficient to create damage in a-Si:H, from the simulated energy was extracted deposition curves and is a measure of the Curves width of the exposed spot. depicting the relation between the width of the exposed spot and the exposure time were plotted (figure 3). 4
RESULTS
AND DISCUSSIONS
:
width contrast Dependence of EBIC time of to the exposure with respect for various beam irradiation of a-Si:H diameters in spot mode is shown in figure with contrast width increases 3. The respect to the exposure time and almost saturates at about 30 second exposure.
Figure-4 Schematic to explain the variation observed in figure 3. The EBIC contrast begins to appear at point A of the probe scan as the interaction between generation volume (Gv) and the damaged volume (Dv) starts and disappears at point C where the interaction ends. Lm,Lp and Ld are the widths of the measured contrast, generation volume of the EBIC probe and damaged volume respectively.
4 shematic shown in figure The the measurement methodology. illustrates When the electron beam starts to move
'A' EBIC contrast begins to from point between appear as the interaction starts the generation volume TkT) and EBIC contrast damaged volume (Dv). until the beam continues to increase reaches point 'B' and falls at point 'C'. Distance 'AC' is measured experimentally. volume width (Ld) can be The damaged given as : Ld = (Lm - Lp) where Lm is measured width and 'Lp' ir the generation volume width of the reading 'Ld' earlier EBIC probe. As mentioned the width of the region where represents energy is deposited. capability of proposed The the can be seen by comparing the method simulation data with the experimental results presented in figure 3. The energy required to create a damage in a-Si:H (Emin) is of the order that of 0.2 ev/atom. It should be noted the deposited energy per atom falls down the radial rapidly with respect to distance as shown in figure 5 (obtained by computer simulation) and the choice of Emin does not play any significant role in in deciding the exposure spot width the range 0 - 10 ev/atom.
LATERAL DISTANCE (in urn)
Figure-5 simulated The exposure profile in a-Si:H. The amount of enerqv deposited falls rapidly with respect to the lateral distance.
Fairly good correlation is observed between experimental simulated and exposure spot width values for higher exposure doses. The experimental values, however, differ from the simulated values at lower dosages. One possible reason for this mismatch could be due to the
M. Singh et al. / EBIC probing
628
approximation made during simulation that the energy loss in the top aluminium contact layer (of p-i-n configuration) is similar to that in Silicon. For larger doses the errors this due to approximation are masked because of inherent averaging performed by the Monte-Carlo method. However, at small doses the deviations from the experimental values become observable. It is expected to obtain a good match of experimental and simulation results if all the layers with their respective parameters are included in the simulation.
5
REFERENCES 1.
Hatzakis M,Ting and Viswanathan,Proc. Int. on Electron and Ion beam Conf. Sci. and Tech., (1974) 542.
2.
Pankov J I: 'Hydrogenated silicon' 21 amorphous VA. of and (Parts D) A,B,C and Semimetals' 'semiconductors (New eds. Willardson and Beer York: Academic, 1984).
3.
B.A. Yacobi, 'Charge collection electron memory effect in irradiated hydrogenated silicon.', Microsc. amorphous Semicond. Mater. Conf., Inst. Phys Ser. No. 76: Section 8, Oxford, 1985, pp 349-354.
4.
Singh, Rajopadhye N.R., Mulayam and Khokle W.S., N.K.L. Raja Electronic IEE proceedings: devices (1988) in print.
5.
Phys. Leamy H.J., J.Appl. 5366), 1988 R 51-R 80.
6.
K,Proc. Kyser D.F and Murata on Electron and 6th Int. Conf. Ion beam Sci. and Tech., (1974) 205.
7.
Electron-beam Greeneich J.S, processes, in:Brewer G.R.(ed), Electron-beam technology in fabrication microelectronic 1980) pp. (Academic Press, 59-140.
a.
W.G, Nandgaonkar Oldham A.R and O'toole S.N,Neureuther Electron IEEE Trans. M.M, ED-27,No.8,(1980) Devices Vol. 1455.
SCOPE FOR FURTHER WORK
It is obvious that a junction needs to be formed on the medium in order to use this method. As junctions can not be formed on polymer materials, viz electron resists, the capacitive mode of EBIC imaging [51 is being attempted to observe the EBIC contrast. Estimate of the exposure width in polymer materials can be made indirectly by extrapolating results obtained the from the a-Si:H film of same thickness at a given incident beam energy. Proper correlation that account for functions, the change in the atomic number, atomic mean energy, weight weight, ionisation density fractions of composing species, transform the exposure etc., and would width in a-Si:H that in a given into Attempts in resist, need to be evolved. this directions are also being made at present.,
:
(