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Bilayer resist process for exposure with low-voltage electrons (STM-lithography)
MICR(31~-LECTRONIC ENGINEERING ELSEVIER
Microelectronic Engineering 30 (1996) 447-450
BILAYER RESIST PROCESS FOR E L E C T R O N S (STM-LITHOGRAPHY)
EXPOSURE
WITH
LOW-VOLTAGE
R. Leuschner a, E. Gtinther a, G. Falk a, A. Hammerschmidt a, K. Kragler a,b, I. W. Rangelow c and J. Zimmermann c a Siemens AG, Zentrale Forschung und Entwicklung, P.O.Box 3220, D-91050 Erlangen, FRG b FAU, Institut ftir Physik, Erwin-Rommel-Str. 1, D-91058 Erlangen, FRG c Institut for Technische Physik, Universit~t Kassel, Heinrich Plett Str. 40, D-34132 Kassel, FRG With STM lithography employing a bilayer resist system, an electron sensitive top resist and a conductive bottom resist, it is possible to generate patterns with dimensions of 100 nm and less. Patterns with aspect ratios up to 8 at a width of 50 nm in flat silicon oxide surface have been achieved. We also demonstrate, that it is possible to operate on prepatterned substrates using a third planarizing resist layer. The exposure mechanism in our CARL top resist has been determined to work differently from the mechanism in the high electron energy regime. The low energy electrons directly cleave the t-butyl ester group. Chemical amplification was not observed. The maximum writing speed for complete exposure in the resist was 1-5 txm/s at 20 pA writing current. 1. INTRODUCTION Future device generations will require structure dimensions of less than 100 nm. This value is nowadays beyond the capability of conventional (optical) lithography. Therefore the employment of scanning probe microscopy (SPM) is an interesting alternative to overcome the optical resolution limits. Recently, Minne et al. /1/ demonstrated the succesful fabrication of a 100 nm silicon MOSFET transistor using a scanning force nficroscope for patterning amorphous silicon as resist. Another possibility for SPM lithography is the local exposure of thin electron sensitive organic resist layers/2/. Nonetheless, the use of such a single resist layer restricts these lithographic techniques to conducting surfaces. In addition, the low energy of the electrons provided by the SPM tip can penetrate only very thin layers (< 50 nm). Such layers exhibit poor resistance to further processing, like reactive ion etching (RIE). In order to avoid these drawbacks, we present a positive tone bilayer resist system for low energy electron (< 50 eV) exposure (Fig. 3). It consists of a chemically amplified CARL top resist/3/(50 nm thick, Fig. 5) and a conductive bottom resist (< 300 nm thick) which is necessary to prevent charging effects. 2. RESULTS AND DISCUSSION Amorplious hydrogenated carbon (a-C:H) or sputtered carbon (a-C) are ideal bottomresist materials. They offer sufficient conductivity, chemically inertness against the top-resisfs solvent or processing chemicals, high etching speed in an oxygen plasma
and high etch resistance against following halogen plasma processes. Conducting a-C:H was deposited in a capacitively coupled RF plasma reactor with 900 V self bias, 3.6 W/cm 2 specific power and methane as precursor (0.2 mbar gas pressure)/4/. All exposures were performed under ambient conditions. Ion milled Ptlr-tips from Materials Analytical Services, were employed. Typical exposure parameters were 50 V (sample negatively biased), 10 to 20 pA and a speed of 1 lam/s as investigated previously /5/. After exposure the samples were developed in a commercial developer and rinsed in water and after that, silylated in an aqueous/alcoholic solution of oligodiaminopropyldimethylsiloxane which results in a widening of resist lines (chemical amplification of resist lines, CARL-process /6/). This silylation is necessary to achieve etch stability for the pattern transfer into the bottom resist by an RF- plasma (see Table 1). In Fig. 1 the processed bilayer resist mask after oxygen plasma etching is shown. The trenches are very accurate and they are etched completely down to the substrate. In order to demonstrate the performance of the new process p-type doped silicon, 300 nm thick poly Si on silicon oxide, 100 nm thermally evaporated aluminum on silicon and 2 t~m thick sputtered silicon oxide have been patterned. Subsequent to the oxygen plasma the silicon samples (Si and poly Si) were etched in CF4, the SiO2 sample in CHF3, and the A1 sample in C12 plasmas, respectively (Table 1). Finally the resist was stripped with an O2 plasma.
0167-9317/96/$15.00 © 1996 - Elsevier Science B.V. All rights reserved. SSDI 0167-9317(95)00284-7
448
R. Leuschner et al. / Microelectronic Engineering 30 (1996) 447-450 at least 50 times deeper than the thickness of the top resist. Fig. 2A depicts a SEM image of a similar structure written with the same set of parameters as in Fig. 1, however, now transfered into the SiO2 substrate and with the a-C:H removed. The trenches are about 400 nm deep. This leads to an aspect ratio of 8 for the 50 nm wide trench in the middle.
A
A
iiiiiii:~i:
!i!i;!
B
. H
H
Fig. 1: Resist trenches after O2 plasma etching with a wide horizontal trench. A: 40 n m , B: 100 nm. Gas in plasma
Gas flow sccm
RFPower kW
Select. etchrate ratioA~B
Layer A
Layer B
O2 02 O2 CAI~r3
30 30 30 5 1.5 15 1.5
0.9 0.9 0.9 0.15
8 10 21 11
a-C:H a-C TSMR SiO2
SR SR SR a-C:H
0.10
15
Alu
a-C:H
CL2 BC13
B
Table 1: Etch selectivities of multilayer resist and substrate materials. (SR = silylated resist) Sub-100 nm patterns with aspect ratios (pattern depth over space width) > 4 have been obtained in silicon, silicon dioxide and aluminium (Fig. 2). Even 40 nm trenches could be transferred from the top into the bottom resist and then into the substrate by these plasma etch processes. The etch selectivities of the top resist mask against a-C:H and different substrate materials for the corresponding plasma conditions are given in Table 1. From the etch selectivities it can be concluded, that the substrate can be etched
II
Fig. 2: Patterns in A: 400 nm thick SiO2 and B: 150 nm thick aluminium. In the case of a prestructured sample with 1 pm wide and 300 nm high poly silicon steps on silicon oxide we incorporate a third resist layer between the substrate and the a-C:H to planarize existing topography (Figure 3). Undefined exposure in the proximity of steps due to lack of contact between sample and tip could be circumvented thereby. In this case 100 nm a-C:H has been deposited on a 500 nm thick TSMR 8900 resist (Tokyo Ohka). In Fig. 4 we show one example of succesful pattern transfer into 300 nm poly Si structures. The trench has a width of 100 nm at the top and is not etched completely down to the silicon oxide. This is attributed to the non optimized RIE parameters for poly Si. In order to determine the sensitivity of our CARL top resist the exposure response curve was measured
449
R. Leuschner et al. /Microelectronic Engineering 30 (1996) 447-450
STM tip for resist exposure Topresist (50 nm) Conductive layer (100 nm) PlanariTing org. layer (500 nm)
m
~
,5
m
sorbance in the deep-UV region at 5 eV photon energy, we previously attributed the high dose value to the fact that only a minor part of the electrons gain enough energy to initiate ~ e photochemical reaction of the PAG when pulled out of the resist/5/.
Development & si ylation (25 nm thickness increase)
43.
Polymerof chemicallyamplifiedCARL-to~esist
Oxygen reactive ion etching (100 nm trenches in 600 nm thick resist)
CF3CHFCF2SO~"
[~
Fig. 3: Resist processing steps for exposure with STM over substrate topography. O
/~___X ii_A_, /~___~ S S
o N--S 0
PAG3
PAG4
Fig. 5: Chemical structures of used resist polymer and photo acid generators (FAGs).
Fig. 4 : 1 0 0 nm trench in a 300 nm poly silicon step. and compared with high energy (30 keV) e-beam results. The resist consists of 90 % polymer with cleavable t-butylester groups and 10 % of a sulfonium salt (Fig. 5). These photo acid generators (FAGs) are sensitive to electrons with sufficiently high energy and generate small amounts of acids in the resist/7/. The acids cleave the acid labile ester group of the resist polymer catalytically in the post exposure bake (FEB) prior to development. This process is known as chemical amplification/81. The comparison of the exposure response curves for STM and e-beam exposure clearly demonstrates that the dose for low energy electrons necessary for a complete development (10 mC/cm 2) is four orders of magnitude higher than the corresponding value for the high energy electrons (2 pC/cm 2) (Figure 6). Taking into account that the used triphenyl sulfonium salt exhibits its maximum ab-
In further experiments we tried to improve the resist sensitivity by using other PAGs (Fig. 5, Table 2). The use of different PAGs should result in different values for the threshold for complete development if the normal photochemistry under deep-UV or high energy e-beam exposure takes place. The behaviour is determined by the quantum yield of the acid generation, the acid strength of the generated acid and the PEB conditions, e.g. temperature and time. The threshold values vary from 2 to 15 mC/cm 2 for these PAGs. It was surprising to notice that there is no significant dose variation to be seen without and with PEB. In addition, the contrast of the resist was much sharper without than with PEB (Fig. 6). Therefore we conclude that the electron energy is too low to generate the strong photo acid which is needed for the chemical amplification reaction in the top resist. We assume that the t-butylester groups of the topresist are directly cleaved by the low energy electrons (comparable to the chain scission mechanism of PMMA/9/). To substantiate this picture we tried to expose and develop a resist consisting of a polymer without ester groups (maleic acid anhydride and styrene copolymer). The experiment clearly indicated that the cleavage of the t-butyl ester and
450
R. Leuschner et al. / Microelectronic Engineering 30 (1996) 447-450
hypothetical hydrolysis of the anhydride groups is responsible for the successful development of the resist after exposure with low energy electrons.
PAG
PEB 120 °C 120 °C no 130 °C 110 °C 120 °C no
Dose-to-clear Energy 2 btC/cm2 30keV 14 mC/cm 2 < 50 eV 1 10 mC/cm 2 < 50 eV 2 15 mC/cm 2 < 50 eV 3+ 1 15 mC/cm 2 < 50 eV 4+TEAT 3 mC/cm 2 < 50 eV 4+TEAT 2 mC/cm 2 < 50 eV TEAT no 10 mC/cm 2 < 50 eV Table 2: Effect of P A G and PEB on dose-to-cleare of topresist (TEAT: tetraethylammonium triflate). Resist thickness 70 , 65 ~ 60 '---b 55-~ 50 40 - |
25
- ,k
[nm] STM
PAG 4 PEB S V ~
30
2O - T P A G
M
PAG i
PAG 1 ~
keV
/
~ [ I
1
v~
1
5 10 °
1 01
10 2
/10/investigated the acid diffusion coefficient in a negative chemically amplified resist (SAL-601/ Shipley) from variation of feature size with PEB time using the STM (25 V, in vacuum) to expose the resist. The calculated diffusion coefficient is well below what has been reported previously. These measurements are perhaps misleading since the acid may not have been formed. Their observation that omitting the PEB had nearly no effect on the linewidth supports this point of view. With the experimentally determined threshold value of 2 - 10 mC/cm 2 and the standard exposure current of 20 p A it is possible to obtain a maximum writing speed of 1- 5 I n n / s for STM exposure of the CARL top resist. It corresponds to the minimum line dose of 40 - 200 nC/cm for complete development/8/. Despite the low writing speed of SPM systems the high aspect ratios could be especially interesting for the production of prototypes with structure dimensions of less than 100 nm.
1Q `3 1 0 4. Dose [#C/ era2]
Fig. 6: Exposure response curve for STM and ebeam exposure of CARL topresist. The chemical amplification due to the catalytic cleavage via acids fails and the high exposure dose for low energy electrons becomes understandable. Nevertheless, an onium salt is a necessary constituent of the resist. It leads to a conductivity of the resist large enough to enable succesful operation of the STM. It could be shown that without salt the constant current could not be maintained reliably by the feedback loop. Therefore at least a non-reactive salt like tetraethylammonium triflate had to be added to the resist for the experiments with the non ionic PAGs. In this case the PEB can be omitted and no problems with linewidth shifts due to environmental pollution or acid diffusion can occure. Perkins et al.
3. ACKNOWLEDGEMENT The authors would like to acknowledge funding from the German Bundesministerium for Bildung, Wissenschaft, Forschung und Technologie (13N6226). REFERENCES /1/S.C. Minne, H.T. Sob, Ph: Flueckiger, and C.F: Quate, Appl. Phys. Lett., 66, 703 (1995). /2/Technology of Proximal Probe Lithography edited by C.R.K. Martian (SPIE, Bellingham, WA, 1993). /3/R. Leuschner, E. Schmidt, H. Ohlmeyer, R. Sezi, M. Irmscher, Microeleqtr. Engin., 27, 385 (1995). /41 Th. Mandel, M. Frischholz, R. Helbig, S. Birkle, and A. Hammerschmidt, Appl. Surf. Sci., 65/66, 795 (1993). / 5 / K . Kragler, E. Gtinther, R. Leuschner, G. Falk, A. Hammerschmidt, H. yon Seggern, G. SaemannIschenko, Appl. Phys. Lett. 67, 1163 (1995). / 6 / R . Leuschner, M. Beyer, H. BorndOrfer, E. Kghn, C. N61scher, M. Sebald, and R. Sezi, Polymer Engin. and Sci., 32, 1558 (1992). 171 J.V. Crivello, Adv. Polym. Sci., 62, 1 (1984). /81 H. Ito, C.G. Willson, and J.M.J. Frechet, Prec. SPIE, 771, 24 (1987). /9/ M.A. McCord and R.F.W. Pease, J. Vac. Sci. Technol., B 6, 293 (1988). / 1 0 / F . K . Perkins, E.A. Dobisz, C.R.K. Martian, J. Vac. Sci. Technol., B 11(6), 2597 (1993).
Microelectronic Engineering 30 (1996) 447-450
BILAYER RESIST PROCESS FOR E L E C T R O N S (STM-LITHOGRAPHY)
EXPOSURE
WITH
LOW-VOLTAGE
R. Leuschner a, E. Gtinther a, G. Falk a, A. Hammerschmidt a, K. Kragler a,b, I. W. Rangelow c and J. Zimmermann c a Siemens AG, Zentrale Forschung und Entwicklung, P.O.Box 3220, D-91050 Erlangen, FRG b FAU, Institut ftir Physik, Erwin-Rommel-Str. 1, D-91058 Erlangen, FRG c Institut for Technische Physik, Universit~t Kassel, Heinrich Plett Str. 40, D-34132 Kassel, FRG With STM lithography employing a bilayer resist system, an electron sensitive top resist and a conductive bottom resist, it is possible to generate patterns with dimensions of 100 nm and less. Patterns with aspect ratios up to 8 at a width of 50 nm in flat silicon oxide surface have been achieved. We also demonstrate, that it is possible to operate on prepatterned substrates using a third planarizing resist layer. The exposure mechanism in our CARL top resist has been determined to work differently from the mechanism in the high electron energy regime. The low energy electrons directly cleave the t-butyl ester group. Chemical amplification was not observed. The maximum writing speed for complete exposure in the resist was 1-5 txm/s at 20 pA writing current. 1. INTRODUCTION Future device generations will require structure dimensions of less than 100 nm. This value is nowadays beyond the capability of conventional (optical) lithography. Therefore the employment of scanning probe microscopy (SPM) is an interesting alternative to overcome the optical resolution limits. Recently, Minne et al. /1/ demonstrated the succesful fabrication of a 100 nm silicon MOSFET transistor using a scanning force nficroscope for patterning amorphous silicon as resist. Another possibility for SPM lithography is the local exposure of thin electron sensitive organic resist layers/2/. Nonetheless, the use of such a single resist layer restricts these lithographic techniques to conducting surfaces. In addition, the low energy of the electrons provided by the SPM tip can penetrate only very thin layers (< 50 nm). Such layers exhibit poor resistance to further processing, like reactive ion etching (RIE). In order to avoid these drawbacks, we present a positive tone bilayer resist system for low energy electron (< 50 eV) exposure (Fig. 3). It consists of a chemically amplified CARL top resist/3/(50 nm thick, Fig. 5) and a conductive bottom resist (< 300 nm thick) which is necessary to prevent charging effects. 2. RESULTS AND DISCUSSION Amorplious hydrogenated carbon (a-C:H) or sputtered carbon (a-C) are ideal bottomresist materials. They offer sufficient conductivity, chemically inertness against the top-resisfs solvent or processing chemicals, high etching speed in an oxygen plasma
and high etch resistance against following halogen plasma processes. Conducting a-C:H was deposited in a capacitively coupled RF plasma reactor with 900 V self bias, 3.6 W/cm 2 specific power and methane as precursor (0.2 mbar gas pressure)/4/. All exposures were performed under ambient conditions. Ion milled Ptlr-tips from Materials Analytical Services, were employed. Typical exposure parameters were 50 V (sample negatively biased), 10 to 20 pA and a speed of 1 lam/s as investigated previously /5/. After exposure the samples were developed in a commercial developer and rinsed in water and after that, silylated in an aqueous/alcoholic solution of oligodiaminopropyldimethylsiloxane which results in a widening of resist lines (chemical amplification of resist lines, CARL-process /6/). This silylation is necessary to achieve etch stability for the pattern transfer into the bottom resist by an RF- plasma (see Table 1). In Fig. 1 the processed bilayer resist mask after oxygen plasma etching is shown. The trenches are very accurate and they are etched completely down to the substrate. In order to demonstrate the performance of the new process p-type doped silicon, 300 nm thick poly Si on silicon oxide, 100 nm thermally evaporated aluminum on silicon and 2 t~m thick sputtered silicon oxide have been patterned. Subsequent to the oxygen plasma the silicon samples (Si and poly Si) were etched in CF4, the SiO2 sample in CHF3, and the A1 sample in C12 plasmas, respectively (Table 1). Finally the resist was stripped with an O2 plasma.
0167-9317/96/$15.00 © 1996 - Elsevier Science B.V. All rights reserved. SSDI 0167-9317(95)00284-7
448
R. Leuschner et al. / Microelectronic Engineering 30 (1996) 447-450 at least 50 times deeper than the thickness of the top resist. Fig. 2A depicts a SEM image of a similar structure written with the same set of parameters as in Fig. 1, however, now transfered into the SiO2 substrate and with the a-C:H removed. The trenches are about 400 nm deep. This leads to an aspect ratio of 8 for the 50 nm wide trench in the middle.
A
A
iiiiiii:~i:
!i!i;!
B
. H
H
Fig. 1: Resist trenches after O2 plasma etching with a wide horizontal trench. A: 40 n m , B: 100 nm. Gas in plasma
Gas flow sccm
RFPower kW
Select. etchrate ratioA~B
Layer A
Layer B
O2 02 O2 CAI~r3
30 30 30 5 1.5 15 1.5
0.9 0.9 0.9 0.15
8 10 21 11
a-C:H a-C TSMR SiO2
SR SR SR a-C:H
0.10
15
Alu
a-C:H
CL2 BC13
B
Table 1: Etch selectivities of multilayer resist and substrate materials. (SR = silylated resist) Sub-100 nm patterns with aspect ratios (pattern depth over space width) > 4 have been obtained in silicon, silicon dioxide and aluminium (Fig. 2). Even 40 nm trenches could be transferred from the top into the bottom resist and then into the substrate by these plasma etch processes. The etch selectivities of the top resist mask against a-C:H and different substrate materials for the corresponding plasma conditions are given in Table 1. From the etch selectivities it can be concluded, that the substrate can be etched
II
Fig. 2: Patterns in A: 400 nm thick SiO2 and B: 150 nm thick aluminium. In the case of a prestructured sample with 1 pm wide and 300 nm high poly silicon steps on silicon oxide we incorporate a third resist layer between the substrate and the a-C:H to planarize existing topography (Figure 3). Undefined exposure in the proximity of steps due to lack of contact between sample and tip could be circumvented thereby. In this case 100 nm a-C:H has been deposited on a 500 nm thick TSMR 8900 resist (Tokyo Ohka). In Fig. 4 we show one example of succesful pattern transfer into 300 nm poly Si structures. The trench has a width of 100 nm at the top and is not etched completely down to the silicon oxide. This is attributed to the non optimized RIE parameters for poly Si. In order to determine the sensitivity of our CARL top resist the exposure response curve was measured
449
R. Leuschner et al. /Microelectronic Engineering 30 (1996) 447-450
STM tip for resist exposure Topresist (50 nm) Conductive layer (100 nm) PlanariTing org. layer (500 nm)
m
~
,5
m
sorbance in the deep-UV region at 5 eV photon energy, we previously attributed the high dose value to the fact that only a minor part of the electrons gain enough energy to initiate ~ e photochemical reaction of the PAG when pulled out of the resist/5/.
Development & si ylation (25 nm thickness increase)
43.
Polymerof chemicallyamplifiedCARL-to~esist
Oxygen reactive ion etching (100 nm trenches in 600 nm thick resist)
CF3CHFCF2SO~"
[~
Fig. 3: Resist processing steps for exposure with STM over substrate topography. O
/~___X ii_A_, /~___~ S S
o N--S 0
PAG3
PAG4
Fig. 5: Chemical structures of used resist polymer and photo acid generators (FAGs).
Fig. 4 : 1 0 0 nm trench in a 300 nm poly silicon step. and compared with high energy (30 keV) e-beam results. The resist consists of 90 % polymer with cleavable t-butylester groups and 10 % of a sulfonium salt (Fig. 5). These photo acid generators (FAGs) are sensitive to electrons with sufficiently high energy and generate small amounts of acids in the resist/7/. The acids cleave the acid labile ester group of the resist polymer catalytically in the post exposure bake (FEB) prior to development. This process is known as chemical amplification/81. The comparison of the exposure response curves for STM and e-beam exposure clearly demonstrates that the dose for low energy electrons necessary for a complete development (10 mC/cm 2) is four orders of magnitude higher than the corresponding value for the high energy electrons (2 pC/cm 2) (Figure 6). Taking into account that the used triphenyl sulfonium salt exhibits its maximum ab-
In further experiments we tried to improve the resist sensitivity by using other PAGs (Fig. 5, Table 2). The use of different PAGs should result in different values for the threshold for complete development if the normal photochemistry under deep-UV or high energy e-beam exposure takes place. The behaviour is determined by the quantum yield of the acid generation, the acid strength of the generated acid and the PEB conditions, e.g. temperature and time. The threshold values vary from 2 to 15 mC/cm 2 for these PAGs. It was surprising to notice that there is no significant dose variation to be seen without and with PEB. In addition, the contrast of the resist was much sharper without than with PEB (Fig. 6). Therefore we conclude that the electron energy is too low to generate the strong photo acid which is needed for the chemical amplification reaction in the top resist. We assume that the t-butylester groups of the topresist are directly cleaved by the low energy electrons (comparable to the chain scission mechanism of PMMA/9/). To substantiate this picture we tried to expose and develop a resist consisting of a polymer without ester groups (maleic acid anhydride and styrene copolymer). The experiment clearly indicated that the cleavage of the t-butyl ester and
450
R. Leuschner et al. / Microelectronic Engineering 30 (1996) 447-450
hypothetical hydrolysis of the anhydride groups is responsible for the successful development of the resist after exposure with low energy electrons.
PAG
PEB 120 °C 120 °C no 130 °C 110 °C 120 °C no
Dose-to-clear Energy 2 btC/cm2 30keV 14 mC/cm 2 < 50 eV 1 10 mC/cm 2 < 50 eV 2 15 mC/cm 2 < 50 eV 3+ 1 15 mC/cm 2 < 50 eV 4+TEAT 3 mC/cm 2 < 50 eV 4+TEAT 2 mC/cm 2 < 50 eV TEAT no 10 mC/cm 2 < 50 eV Table 2: Effect of P A G and PEB on dose-to-cleare of topresist (TEAT: tetraethylammonium triflate). Resist thickness 70 , 65 ~ 60 '---b 55-~ 50 40 - |
25
- ,k
[nm] STM
PAG 4 PEB S V ~
30
2O - T P A G
M
PAG i
PAG 1 ~
keV
/
~ [ I
1
v~
1
5 10 °
1 01
10 2
/10/investigated the acid diffusion coefficient in a negative chemically amplified resist (SAL-601/ Shipley) from variation of feature size with PEB time using the STM (25 V, in vacuum) to expose the resist. The calculated diffusion coefficient is well below what has been reported previously. These measurements are perhaps misleading since the acid may not have been formed. Their observation that omitting the PEB had nearly no effect on the linewidth supports this point of view. With the experimentally determined threshold value of 2 - 10 mC/cm 2 and the standard exposure current of 20 p A it is possible to obtain a maximum writing speed of 1- 5 I n n / s for STM exposure of the CARL top resist. It corresponds to the minimum line dose of 40 - 200 nC/cm for complete development/8/. Despite the low writing speed of SPM systems the high aspect ratios could be especially interesting for the production of prototypes with structure dimensions of less than 100 nm.
1Q `3 1 0 4. Dose [#C/ era2]
Fig. 6: Exposure response curve for STM and ebeam exposure of CARL topresist. The chemical amplification due to the catalytic cleavage via acids fails and the high exposure dose for low energy electrons becomes understandable. Nevertheless, an onium salt is a necessary constituent of the resist. It leads to a conductivity of the resist large enough to enable succesful operation of the STM. It could be shown that without salt the constant current could not be maintained reliably by the feedback loop. Therefore at least a non-reactive salt like tetraethylammonium triflate had to be added to the resist for the experiments with the non ionic PAGs. In this case the PEB can be omitted and no problems with linewidth shifts due to environmental pollution or acid diffusion can occure. Perkins et al.
3. ACKNOWLEDGEMENT The authors would like to acknowledge funding from the German Bundesministerium for Bildung, Wissenschaft, Forschung und Technologie (13N6226). REFERENCES /1/S.C. Minne, H.T. Sob, Ph: Flueckiger, and C.F: Quate, Appl. Phys. Lett., 66, 703 (1995). /2/Technology of Proximal Probe Lithography edited by C.R.K. Martian (SPIE, Bellingham, WA, 1993). /3/R. Leuschner, E. Schmidt, H. Ohlmeyer, R. Sezi, M. Irmscher, Microeleqtr. Engin., 27, 385 (1995). /41 Th. Mandel, M. Frischholz, R. Helbig, S. Birkle, and A. Hammerschmidt, Appl. Surf. Sci., 65/66, 795 (1993). / 5 / K . Kragler, E. Gtinther, R. Leuschner, G. Falk, A. Hammerschmidt, H. yon Seggern, G. SaemannIschenko, Appl. Phys. Lett. 67, 1163 (1995). / 6 / R . Leuschner, M. Beyer, H. BorndOrfer, E. Kghn, C. N61scher, M. Sebald, and R. Sezi, Polymer Engin. and Sci., 32, 1558 (1992). 171 J.V. Crivello, Adv. Polym. Sci., 62, 1 (1984). /81 H. Ito, C.G. Willson, and J.M.J. Frechet, Prec. SPIE, 771, 24 (1987). /9/ M.A. McCord and R.F.W. Pease, J. Vac. Sci. Technol., B 6, 293 (1988). / 1 0 / F . K . Perkins, E.A. Dobisz, C.R.K. Martian, J. Vac. Sci. Technol., B 11(6), 2597 (1993).