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Micro-technology of densely spaced non-conventional patterns for space applications
MICROELECTRONIC ENGINEERING
ELSEVIER
Microelectronic Engineering 41/42 (1998) 187-190
Micro-technology of densely spaced non-conventional patterns for space applications G. Stangl a, P. Hudek b, I. Kostic b, F. Rtidenauer% I. Rangelow d, K. Riedlinga, W. Fallmanna a Institut ftir Allgemeine Elektrotechnik und Elektronik, TU Wien, Gusshausstrasse 27/359, A-1040 Vienna, Austria b Academy of Science, Dflbravskh cesta 9, SK-84237 Bratislava, Slovakia c C)sterreichisches Forschungszentrum Seibersdorf, A-2444 Seibersdorf, Austria d University of Kassel, Heinrich Blettstrasse 40, D-34109 Kassel, B.R.D. Regular arrays of sub-0.5 pm tips are of increasing interest, for example, as field emitters, calibration structures, or, in our particular case, as collector surfaces for sub-l~n dust particles in a space experiment. This contribution describes the preparation of 1 x 1 cm2 arrays of microcolumns with a high aspect ratio (10:1) and a diameter in the sub-micrometer range ( 1 5 0 300 nm). The process is based upon a chemically amplified novolak resist (CAR), electron beam lithography, and ECR plasma etching.
1. I N T R O D U C T I O N
AE-
For the ROSETTA mission of ESA, arrays of sub-lira structures are required as collectors for cosmic dust particles (with 5 0 500 nm diameter) from the comet Wirtanen. The collected particles will be imaged and classified by atomic force microscopy techniques. It is important that the collector surfaces can collect small high-speed particles with high efficiency and without damage to the particles. They must absorb the kinetic energy of the incoming dust by inelastic processes to avoid reflection of the particles. This can, for example, be achieved by using an array of free-standing columns with a diameter of the order of 0.3 pm and a height in the ~un range (Fig. 1). An impinging particle may break a number of these columns, thereby incurring an inelastic energy loss AE per column:
0 1 6 7 - 9 3 1 7 / 9 8 / $ 1 9 . 0 0 © E l s e v i e r S c i e n c e B.V. PlI: S 0 1 6 7 - 9 3 1 7 ( 9 8 ) 0 0 0 4 2 - 2
All rights reserved.
3n B2 r21 32"
M
B and M are the fracture stress and modulus of the column material respectively; r and l are the radius and length of the columns, respectively.
•.
cap
2~
,
i
elisttc defl©c|~on
laeilstic deform|lion
Figure 1. Absorption of energy by inelastic deformation of a column.
188
G. Stangl et al. / Microelectronic Engineering 41/42 (1998) 187-190
The properties of these "collector surfaces" can be tailored to the particular requirements by variation of the column material and dimensions. By ion etching photoresist column structures into a silicon substrate it is possible to produce arrays of free-standing cones whose fracture energy is considerably lower than that of the resist columns. The electron beam lithography and plasma etching structuring techniques offer the possibility to match geometry and properties of surfaces to the requirements of particle collection. 2. EXPERIMENTAL
One of the prerequisites for an optimised plasma etching transfer of the structures from the resist layer into an inorganic substrate (for example, silicon or Si02) is a uniformly structured, relatively thick resist layer with close to perpendicular sidewalls, as shown in Figure 2.
used [1]. Although the high sensitivity of this resist type makes it very attractive, particularly for electron beam exposure, it causes problems with the control and the uniformity of the critical dimensions. It also requires a precise compensation of proximity effects during electron beam exposure if the pattern dimensions decrease below 0.5 ;am, and an optimised resist processing. The most critical factor in resist processing turned out to be the post-exposure delay, which must be less than a few minutes. Although a simulation of the exposure process is indispensable for controlling the proximity effects, the optimisation of the entire process heavily depended on experimental work. This is true because the complexity of the threecomponent resist system and the lack of an exact model of the resist response prohibit a comprehensive simulation. The resist column structures may either be directly used as catching structures for dust particles, or they may be transferred into the underlying silicon substrate by means of ECR dry etching (Fig. 3). A careful choice of the etching parameters can allow virtually any shape of the resulting silicon columns (Figs. 4 and 5), although the conical shape of Figure 5 appears preferable for the particular purpose.
Figure 2. Large field ( l x l cm 2) array of highaspect-ratio and high density resist pillars made by using e-beam lithography and chemically amplified resist. This requires a careful optimisation of the deposition, exposure, and of the pre- and post-exposure resist processing. In the experiments reported here, a single film of a three-component negative-toned Novolak CAR (Kalle Hoechst AZ PN 114) has been
Figure 3. Large field array of resist pillars transferred into silicon by dry plasma etching (ECR).
189
G. Stangl et al. / Microelectronic Engineering 41/42 (1998) 187-190
Figure 4. Cylindrical silicon.
pillars
etched into
Figure 6. Array of dust collector boxes prepared by e-beam lithography in chemically amplified resist. Furthermore, dust collector structures may be "lined" with an energy absorbing sandwich structure consisting of alternate films of crystalline bacterial cell surface proteins ('S"-layers) [2] and evaporated or sputtered metals, e.g., titanium, niobium, or tantalum (Fig. 7).
i~lt S
Figure 5. Silicon needles prepared by ECR plasma etching.
fil~
LiwIr
Figure 7. Damping multilayer structure.
The mechanical behaviour and thus the dust collecting capabilities of the structures may also be modified by either removing the resist layer after the etching step or by leaving the mask layer on top of the silicon columns, as shown in Figure 3. 3. A L T E R N A T I V E A P P R O A C H E S
Other large-area arrays of periodical structures may serve a similar purpose as the columns presented above. Using the same techniques, an array of dust collector boxes may be prepared (Fig. 6).
ml --
Figure 8. Stopping of a particle.
190
G. Stangl et al. / Microelectronic Engineering 41/42 (1998) 187-190
A particle that impinges on such a damping multilayer will either break one or more of the metal films or inelastically deform the S-layer, which results in a smooth absorption of its energy and avoids the loss of particles by reflection (Fig. 8). 4. C O N C L U S I O N The careful optimisation of an electronbeam lithography based single-layer resist process permitted the high aspect ratio patterning of sub-0.5 ~ n structures in resist with a thickness of up to 5 pro, which is a requirement for a subsequent deep plasma etching. Since the limit for the etching depth in silicon is typically between 2 and 2.5 times the resist thickness, etching depths exceeding 5 ~ appear easily feasible with the process presented [3].
ACKNOWLEDGEMENT This work was supported by the Society for Microelectronics, the Austrian East-WestFunds-Project (OWP-92), and the Erwin Schr~dinger Gesellschaft, Institute of Lithographic Research.
REFERENCES 1. I. Rangelow, P. Hudek, I. Kostic, Z. Borkiwicz, G. Stangl, Microcirc. Eng. 23 (1994) 283. 2. D. Pum, G. Stangl, C. Sponer, W. Fallmann, U.B. Sleytr, Biointerfaces 8 (1997) 157. 3. P. Hudek, W. Rangelow, I. Kostic, G. Stangl, Electron Technology 28/4 (1996) 251.
ELSEVIER
Microelectronic Engineering 41/42 (1998) 187-190
Micro-technology of densely spaced non-conventional patterns for space applications G. Stangl a, P. Hudek b, I. Kostic b, F. Rtidenauer% I. Rangelow d, K. Riedlinga, W. Fallmanna a Institut ftir Allgemeine Elektrotechnik und Elektronik, TU Wien, Gusshausstrasse 27/359, A-1040 Vienna, Austria b Academy of Science, Dflbravskh cesta 9, SK-84237 Bratislava, Slovakia c C)sterreichisches Forschungszentrum Seibersdorf, A-2444 Seibersdorf, Austria d University of Kassel, Heinrich Blettstrasse 40, D-34109 Kassel, B.R.D. Regular arrays of sub-0.5 pm tips are of increasing interest, for example, as field emitters, calibration structures, or, in our particular case, as collector surfaces for sub-l~n dust particles in a space experiment. This contribution describes the preparation of 1 x 1 cm2 arrays of microcolumns with a high aspect ratio (10:1) and a diameter in the sub-micrometer range ( 1 5 0 300 nm). The process is based upon a chemically amplified novolak resist (CAR), electron beam lithography, and ECR plasma etching.
1. I N T R O D U C T I O N
AE-
For the ROSETTA mission of ESA, arrays of sub-lira structures are required as collectors for cosmic dust particles (with 5 0 500 nm diameter) from the comet Wirtanen. The collected particles will be imaged and classified by atomic force microscopy techniques. It is important that the collector surfaces can collect small high-speed particles with high efficiency and without damage to the particles. They must absorb the kinetic energy of the incoming dust by inelastic processes to avoid reflection of the particles. This can, for example, be achieved by using an array of free-standing columns with a diameter of the order of 0.3 pm and a height in the ~un range (Fig. 1). An impinging particle may break a number of these columns, thereby incurring an inelastic energy loss AE per column:
0 1 6 7 - 9 3 1 7 / 9 8 / $ 1 9 . 0 0 © E l s e v i e r S c i e n c e B.V. PlI: S 0 1 6 7 - 9 3 1 7 ( 9 8 ) 0 0 0 4 2 - 2
All rights reserved.
3n B2 r21 32"
M
B and M are the fracture stress and modulus of the column material respectively; r and l are the radius and length of the columns, respectively.
•.
cap
2~
,
i
elisttc defl©c|~on
laeilstic deform|lion
Figure 1. Absorption of energy by inelastic deformation of a column.
188
G. Stangl et al. / Microelectronic Engineering 41/42 (1998) 187-190
The properties of these "collector surfaces" can be tailored to the particular requirements by variation of the column material and dimensions. By ion etching photoresist column structures into a silicon substrate it is possible to produce arrays of free-standing cones whose fracture energy is considerably lower than that of the resist columns. The electron beam lithography and plasma etching structuring techniques offer the possibility to match geometry and properties of surfaces to the requirements of particle collection. 2. EXPERIMENTAL
One of the prerequisites for an optimised plasma etching transfer of the structures from the resist layer into an inorganic substrate (for example, silicon or Si02) is a uniformly structured, relatively thick resist layer with close to perpendicular sidewalls, as shown in Figure 2.
used [1]. Although the high sensitivity of this resist type makes it very attractive, particularly for electron beam exposure, it causes problems with the control and the uniformity of the critical dimensions. It also requires a precise compensation of proximity effects during electron beam exposure if the pattern dimensions decrease below 0.5 ;am, and an optimised resist processing. The most critical factor in resist processing turned out to be the post-exposure delay, which must be less than a few minutes. Although a simulation of the exposure process is indispensable for controlling the proximity effects, the optimisation of the entire process heavily depended on experimental work. This is true because the complexity of the threecomponent resist system and the lack of an exact model of the resist response prohibit a comprehensive simulation. The resist column structures may either be directly used as catching structures for dust particles, or they may be transferred into the underlying silicon substrate by means of ECR dry etching (Fig. 3). A careful choice of the etching parameters can allow virtually any shape of the resulting silicon columns (Figs. 4 and 5), although the conical shape of Figure 5 appears preferable for the particular purpose.
Figure 2. Large field ( l x l cm 2) array of highaspect-ratio and high density resist pillars made by using e-beam lithography and chemically amplified resist. This requires a careful optimisation of the deposition, exposure, and of the pre- and post-exposure resist processing. In the experiments reported here, a single film of a three-component negative-toned Novolak CAR (Kalle Hoechst AZ PN 114) has been
Figure 3. Large field array of resist pillars transferred into silicon by dry plasma etching (ECR).
189
G. Stangl et al. / Microelectronic Engineering 41/42 (1998) 187-190
Figure 4. Cylindrical silicon.
pillars
etched into
Figure 6. Array of dust collector boxes prepared by e-beam lithography in chemically amplified resist. Furthermore, dust collector structures may be "lined" with an energy absorbing sandwich structure consisting of alternate films of crystalline bacterial cell surface proteins ('S"-layers) [2] and evaporated or sputtered metals, e.g., titanium, niobium, or tantalum (Fig. 7).
i~lt S
Figure 5. Silicon needles prepared by ECR plasma etching.
fil~
LiwIr
Figure 7. Damping multilayer structure.
The mechanical behaviour and thus the dust collecting capabilities of the structures may also be modified by either removing the resist layer after the etching step or by leaving the mask layer on top of the silicon columns, as shown in Figure 3. 3. A L T E R N A T I V E A P P R O A C H E S
Other large-area arrays of periodical structures may serve a similar purpose as the columns presented above. Using the same techniques, an array of dust collector boxes may be prepared (Fig. 6).
ml --
Figure 8. Stopping of a particle.
190
G. Stangl et al. / Microelectronic Engineering 41/42 (1998) 187-190
A particle that impinges on such a damping multilayer will either break one or more of the metal films or inelastically deform the S-layer, which results in a smooth absorption of its energy and avoids the loss of particles by reflection (Fig. 8). 4. C O N C L U S I O N The careful optimisation of an electronbeam lithography based single-layer resist process permitted the high aspect ratio patterning of sub-0.5 ~ n structures in resist with a thickness of up to 5 pro, which is a requirement for a subsequent deep plasma etching. Since the limit for the etching depth in silicon is typically between 2 and 2.5 times the resist thickness, etching depths exceeding 5 ~ appear easily feasible with the process presented [3].
ACKNOWLEDGEMENT This work was supported by the Society for Microelectronics, the Austrian East-WestFunds-Project (OWP-92), and the Erwin Schr~dinger Gesellschaft, Institute of Lithographic Research.
REFERENCES 1. I. Rangelow, P. Hudek, I. Kostic, Z. Borkiwicz, G. Stangl, Microcirc. Eng. 23 (1994) 283. 2. D. Pum, G. Stangl, C. Sponer, W. Fallmann, U.B. Sleytr, Biointerfaces 8 (1997) 157. 3. P. Hudek, W. Rangelow, I. Kostic, G. Stangl, Electron Technology 28/4 (1996) 251.