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Patterning of LiNbO3 by means of ion irradiation using the electronic energy deposition and wet etching
Microelectronic Engineering 86 (2009) 910–912
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Patterning of LiNbO3 by means of ion irradiation using the electronic energy deposition and wet etching Th. Gischkat a,*, H. Hartung b, F. Schrempel a, E.B. Kley b, A. Tünnermann b, W. Wesch a a b
Institut für Festkörperphysik, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, D-07743 Jena, Germany Institut für Angewandte Physik, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, D-07743 Jena, Germany
a r t i c l e
i n f o
Article history: Received 1 October 2008 Received in revised form 7 November 2008 Accepted 22 November 2008 Available online 3 December 2008 Keywords: Lithium niobate LiNbO3 Ion irradiation Electronic energy deposition Patterning Wet etching
a b s t r a c t In order to investigate the damage formation and etching behavior of lithium niobate (LiNbO3) due to the electronic energy deposition, x-cut LiNbO3 was irradiated at room temperature with 5 MeV Si-ions at ion fluences between 7 1012 and 1 1014 cm 2. The irradiated samples were stepwise etched in a 3.7% HFsolution at a temperature of 40 °C. The investigation of the etching behaviour shows that the etched depth increases with the ion fluence, i.e. with increasing defect concentration and is largest in the case of an amorphous layer. The ion fluences for amorphization in the electronic energy loss regime are 10 times lower compared to the amorphization fluences in the case of the damage formation due to the nuclear energy loss. As a consequence the electronic energy loss allows a very easy fabrication of thick amorphous layers. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction The patterning of electro-optical materials, e.g. lithium niobate (LiNbO3), is of increasing interest for the fabrication of novel photonic devices such as electrically tunable filters and switches. Resonant elements in non-linear optical materials may possess transmission and reflection depending on the intensity of the light. Because of its chemical resistance it is a challenging task to pattern LiNbO3. A promising method is the use of ion irradiation and subsequent wet chemical etching (IBEE: ion beam enhanced etching) [1]. Ion irradiation produces crystal defects and, as a consequence, the chemical resistance is reduced. Due to negligible etching of the undamaged crystal, the irradiated regions can be selectively etched in a hydrofluoric-solution. Using a 3.7% HF-solution and an etching temperature of 40 °C the etching rates amount to about 100 nm min 1 for the amorphized material [2]. Commonly, the irradiation is carried out with ions having energies in the keV range utilizing the damage formation due to the nuclear energy loss of the implanted ions. In order to generate a thick homogeneous amorphous layer starting from the surface, multiple irradiations with different ion energies and fluences are necessary. For example, the fabrication of structures with a depth of about 500 nm requires the irradiation with Ar-ions of four different energies at fluences up to 1015 cm 2 [2]. For structures with larger depths * Corresponding author. E-mail address: [email protected] (Th. Gischkat). 0167-9317/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2008.11.031
the number of irradiation steps drastically increases and the fabrication process becomes more time-consuming [3]. However, it is known that the crystal is also damaged by the electronic energy loss of high energetic ions (MeV range), i.e. by the energy transferred to the electrons of the target atoms due to inelastic collisions with the ions [4–9]. Above a critical electronic energy deposition of about 5 keV nm 1 for LiNbO3 each ion produces an amorphous track [10]. Below this value only point defects are generated which accumulate to an amorphous layer with increasing ion fluence [5–7]. Using this process the crystal can be damaged by one irradiation from the surface to depths larger than several micrometers with fluences which are essentially smaller compared with those necessary when using the nuclear energy loss. The etching rate of amorphous material seems to be independent of the different kinds of energy loss which causes the crystal damage [11]. Consequently, the damage by the electronic energy deposition in particular is promising for the fabrication of structures with large depths as required, e.g. for the production of ridge waveguides. 2. Experimental X-cut single crystals of congruent grown LiNbO3 were irradiated with 5 MeV Si-ions at room temperature. The ion fluences were between 7 1012 and 1 1014 cm 2. The irradiation was performed with an angle of incident of about 7° with respect to the crystal surface normal.
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Th. Gischkat et al. / Microelectronic Engineering 86 (2009) 910–912
120 1.0
values at the surface: nda vetch
100 80
nda
0.8
60
0.6 0.4
40
0.2
20
0.0
0
13
14
10
-2
10
NI (ions cm ) 1.2
120
1.1 1.0
-2
100
vetch
80
0.9 60 0.8 40
3. Results and discussion
0.6
3.1. Damage formation
0.5
a
1.4
5 MeV Si:x-cut LNO 13 -2 NI [10 cm ] 0.7 2.0 1.0 4.0 1.5 10.0
1.2
nda
1.0 0.8 0.6 0.4 0.2 -1 -1
b
5 4
I
II
III
Se Sn
0.5 0.4
2
0.2
1
0.1
0
0
500 1000 1500 2000 2500
0.0
-1
0.3
-1
3
Sn (keV nm ion )
Se (keV nm ion )
0.0
z (nm) Fig. 1. Defect concentration nda versus depth of x-cut LiNbO3 after irradiation with 5 MeV Si-ions at room temperature with different ion fluence (a). Electronic and nuclear energy loss Se and Sn, versus depth calculated by SRIM2003 (b).
-1
14
5 MeV Si, 1x10 cm nda
vetch (nm min )
b
0.7
Fig. 1a shows the measured defect concentration for the irradiation of x-cut LiNbO3 with 5 MeV Si-ions at various ion fluences. Three regions of damage can be distinguished: a damaged surface layer (region I), a buried damaged layer (region III) located at a depth of about 2200 nm as well as a transition layer (region II) in between. Comparing the measured distribution of damage with the energy loss per ion and unit path length of the implanted ions as calculated with SRIM2003 [13] (Fig. 1b) it is obvious that the buried damaged layer can be addressed to the nuclear energy loss of the ions causing collision cascades. The damage close to the surface (region I in Fig. 1b) can be attributed to the electronic energy deposition by the Si-ions. The electronic energy deposition has a maximum value of about
-1
1.2
vetch (nm min )
a
nda
Defect analysis was done by means of Rutherford backscattering spectrometry (RBS) in channeling configuration using 4 MeV He-ions. From the Nb-part of the RBS-spectra the relative concentration of displaced atoms (in the following referred to as defect concentration) was calculated using the computer code dechannelling in crystals and defect analysis (DICADA) [12]. In order to investigate the etching behaviour, the samples were partially covered with a silicon wafer during irradiation. Etching was performed stepwise in a 3.7% HF-solution at an etching temperature of 40 °C. The step height between the irradiated and the non-irradiated material was measured after each etching step by means of a surface profilometer ‘‘Sloan DEKTAK 3030ST”. The etching rates were calculated from the etched depth and the corresponding etching time. Inspection of the surface topology after etching was done by means of a scanning electron microscopy (SEM). Patterning of LiNbO3 with micro-structures was carried using a 2 lm thick silicon-mask. The mask was prepared by means of standard electron-beam lithography and inductive coupled plasma reactive ion etching (ICP-RIE). In order to fabricate vertical structures the irradiation was performed 0° with respect to the crystal surface normal.
20 2.5
3.0 3.5 -1 Se (keV nm )
4.0
0
Fig. 2. Defect concentration nda and etching rate vetch taken at the surface versus the ion fluence (a) as well as the defect concentration nda and etching rate vetch versus the electronic energy loss Se for the irradiation with 1 1014 cm 2 Si-ions (b).
4.2 keV nm 1 at the surface and decreases with increasing depth (Fig. 1b). Correspondingly, the damage produced decreases with increasing depth. The amount of produced damage depends not only on the depth, i.e. on the electronic energy deposition, but also on the ion fluence. In Fig. 2a the defect concentration taken at the surface is depicted versus the ion fluence. For the lowest ion fluence of 7 1012 cm 2 the surface is only slightly damaged. The defect concentration increases with increasing ion fluence up to the amorphization of the surface indicated by nda = 1. With further increasing ion fluence the amorphous layer broadens to larger depths. For ion fluences of 4 1013 and 1 1014 cm 2 an amorphous layer with a thicknesses of about 400 and 700 nm, respectively, is produced (Fig. 1a). These ion fluences are 10 times lower compared to the fluences which are necessary to produce amorphous layers with similar thickness in the nuclear energy deposition regime. The thickness of the amorphous layer is determined by the depth where the electronic energy deposition per ion and path length is below a certain value (Fig. 1a). This value is higher for lower ion fluences. Fig. 2b shows the defect concentration taken at different depths versus the corresponding electronic energy deposition for the irradiation with 1 1014 cm 2 Si-ions. In this case the crystal is amorphous down to a depth where the calculated electronic energy deposition is below 3 keV nm 1 which is in agreement with other investigations [5]. With decreasing energy loss, i.e. with increasing depth the defect concentration decreases down to 0.6 for a electronic energy loss of about 2.3 keV nm 1. With further decreasing electronic energy loss the measured defect concentration does not further decrease. In this region (region II in Fig. 1) the damage formation is due to an overlap between the decreasing electronic and the increasing nuclear energy loss.
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Th. Gischkat et al. / Microelectronic Engineering 86 (2009) 910–912
high contrast of the etching rates. The diameter of the etched holes are about 450 nm which is quite similar to the width of the mask holes. However, the surface of the structure is relative rough which is not acceptable to optical components. The reason for this is the obvious attack of regions which are only slightly damaged by ions transmitted through the mask. Consequential it can be avoided by the application of thicker masks stopping the ions completely. Another possibility is thermal treatment of the sample at appropriate conditions prior the etching, at which only regions with low defect concentration are annealed. Such annealing additionally offers the possibility to decrease the roughness of the side walls, which are only slightly damaged by scattered ions. Fig. 3. SEM picture of a hexagonal arrangement of holes in x-cut LiNbO3 fabricated by the IBEE technique using the electronic energy loss.
3.2. Etching behaviour As expected the measured step hight between undamaged and damaged material, i.e. the measured etched depth, increases with increasing etching time. By the fact that the etching of the perfect crystal is negligible the etching rates were calculated from the etched depth and the according etching time. The highest etching rates were observed at the surface where the crystal is mostly damaged. According to the damage distribution in region I the etching rates decreases with increasing depth, i.e. increasing etched depth. In all cases the etching stops at the end of region I at the latest. It seems that in region II the damage is not sufficient for remarkably etching. As a consequence region III was not reached for etching in a 3.7% HF-solution at an etching temperature of 40 °C for several hours. As an example, for an ion fluence of 1 1014 cm 2 an etched depth of about 940 nm was reached after a total etching time of about 160 min. The expected etching rates in region III are below 5 nm min 1 according to the low applied ion fluences and a damage well below the amorphization [14]. In Fig. 2a and b the corresponding etching rates are depicted versus the ion fluence and the electronic energy deposition, respectively. In both cases the etching rates are correlated with the defect concentration. The etching rate amounts to about 100 nm min 1 for amorphized material. With decreasing defect concentration the etching rate decreases drastically. For a defect concentration below 0.6 the etching is negligible. The high etching rate of amorphized material and the steep decrease causes a high contrast of the etching rate. 3.3. Patterning of LiNbO3 The damage formation due to the electronic energy deposition was applied to pattern LiNbO3 with 1 lm deep structures. In order to irradiate the sample selectively, the regions which should not be removed by the HF-solution were covered by a mask during irradiation. Commonly the masks have to be thick enough to stop the ions completely. For 5 MeV Si-ions a 3.5 lm thick silicon-mask is required. Due to the negligible etching in region II the ions have not to be stopped completely in the mask. As a consequence a silicon-mask with reduced thickness of about 2 lm was used. The energy of the transmitted ions causes a damage which is not sufficient for remarkable etching. Fig. 3 shows a SEM picture of patterned LiNbO3 irradiated with 5 MeV Si-ions at an ion fluence of 1 1014 cm 2. The sample was etched for 90 min in 3.7% HF at 40 °C and the resulting structure depth is about 950 nm. The sidewalls have a nearly vertical shape which is a consequence of the
4. Summary The damage formation and etching behaviour of x-cut LiNbO3 due to the electronic energy deposition was investigated. Therefore samples were irradiated with 5 MeV Si-ions at room temperature with various ion fluences. For the highest applied ion fluence of 1 1014 cm 2 a 700 nm thick amorphous surface layer was produced due to the electronic energy loss. Contrary the damage produced by the nuclear energy loss was well below the amorphization. The ion fluences for amorphization in the electronic energy loss regime are 10 times lower compared to the amorphization fluences in the case of the damage formation due to the nuclear energy loss. As a consequence the electronic energy loss allows a very easy fabrication of thick amorphous layers. The etching behaviour of LiNbO3 damaged due to the electronic energy loss is similar to the etching behaviour of that material damaged by nuclear energy loss. Amorphous material exhibits the highest etching rate which decreases drastically for decreasing damage. As a consequence a high contrast of the etching can be observed. The irradiation using the electronic energy loss enables the possibility the fabricate deep structures with high aspect ratios very easily. Acknowledgement The authors want to thank the Deutsche Forschungsgemeinschaft (DFG) for financial supporting (Contract No. KL1199/2-1). References [1] F. Schrempel, Th. Gischkat, H. Hartung, E.-B. Kley, W. Wesch, A. Tünnermann, Mater. Res. Soc. Symp. Proc. 0908-OO16-01.1, 2005. [2] F. Schrempel, Th. Gischkat, H. Hartung, E.-B. Kley, W. Wesch, Nucl. Instrum. Meth. B 250 (2006) 164. [3] H. Hartung, E.-B. Kley, A. Tünnermann, Th. Gischkat, F. Schrempel, W. Wesch, Opt. Lett. 33 (2008) 2320. [4] B. Canut, R. Brenier, A. Meftah, P. Moretti, S. Ould Salem, S.M.M. Ramos, P. Thevenard, M. Toulemonde, Nucl. Instrum. Meth. B 91 (1994) 312. [5] F. Agulló-López, G. García, J. Olivares, J. Appl. Phys. 97 (2005) 093514. [6] J. Olivares, G. García, F. Agulló-López, F. Agulló-Rueda, A. Kling, J. Soares, Appl. Phys. A 81 (2005) 1465. [7] F. Agulló-López, A. Mendez, G. García, J. Olivares, J.M. Cabrera, Phys. Rev. B 74 (2006) 174109. [8] A. García-Navarro, A. Méndez, J. Olivares, G. García, F. Agulló-López, M. Zayat, D. Levy, L. Vazquez, Nucl. Instrum. Meth. B 249 (2006) 172–176. [9] A. García-Navarro, F. Agulló-López, M. Bianconi, J. Olivares, G. García, J. Appl. Phys. 101 (2007) 083506. [10] B. Canut, S.M.M. Ramos, R. Brenier, P. Thevenard, J.L. Loubet, M. Toulemonde, Nucl. Instrum. Meth. B 107 (1996) 194. [11] M. Bianconi, F. Bergamini, G.G. Bentini, A. Cerutti, M. Chiarini, P. De Nicola, G. Pennestrì, Nucl. Instrum. Meth. B 266 (2008) 1238. [12] K. Gärtner, Nucl. Instrum. Meth. B 227 (2005) 522. [13] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985. [14] T. Steinbach, F. Schrempel, Th. Gischkat, W. Wesch, Phys. Rev. B 78 (2008) 184106.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Patterning of LiNbO3 by means of ion irradiation using the electronic energy deposition and wet etching Th. Gischkat a,*, H. Hartung b, F. Schrempel a, E.B. Kley b, A. Tünnermann b, W. Wesch a a b
Institut für Festkörperphysik, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, D-07743 Jena, Germany Institut für Angewandte Physik, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, D-07743 Jena, Germany
a r t i c l e
i n f o
Article history: Received 1 October 2008 Received in revised form 7 November 2008 Accepted 22 November 2008 Available online 3 December 2008 Keywords: Lithium niobate LiNbO3 Ion irradiation Electronic energy deposition Patterning Wet etching
a b s t r a c t In order to investigate the damage formation and etching behavior of lithium niobate (LiNbO3) due to the electronic energy deposition, x-cut LiNbO3 was irradiated at room temperature with 5 MeV Si-ions at ion fluences between 7 1012 and 1 1014 cm 2. The irradiated samples were stepwise etched in a 3.7% HFsolution at a temperature of 40 °C. The investigation of the etching behaviour shows that the etched depth increases with the ion fluence, i.e. with increasing defect concentration and is largest in the case of an amorphous layer. The ion fluences for amorphization in the electronic energy loss regime are 10 times lower compared to the amorphization fluences in the case of the damage formation due to the nuclear energy loss. As a consequence the electronic energy loss allows a very easy fabrication of thick amorphous layers. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction The patterning of electro-optical materials, e.g. lithium niobate (LiNbO3), is of increasing interest for the fabrication of novel photonic devices such as electrically tunable filters and switches. Resonant elements in non-linear optical materials may possess transmission and reflection depending on the intensity of the light. Because of its chemical resistance it is a challenging task to pattern LiNbO3. A promising method is the use of ion irradiation and subsequent wet chemical etching (IBEE: ion beam enhanced etching) [1]. Ion irradiation produces crystal defects and, as a consequence, the chemical resistance is reduced. Due to negligible etching of the undamaged crystal, the irradiated regions can be selectively etched in a hydrofluoric-solution. Using a 3.7% HF-solution and an etching temperature of 40 °C the etching rates amount to about 100 nm min 1 for the amorphized material [2]. Commonly, the irradiation is carried out with ions having energies in the keV range utilizing the damage formation due to the nuclear energy loss of the implanted ions. In order to generate a thick homogeneous amorphous layer starting from the surface, multiple irradiations with different ion energies and fluences are necessary. For example, the fabrication of structures with a depth of about 500 nm requires the irradiation with Ar-ions of four different energies at fluences up to 1015 cm 2 [2]. For structures with larger depths * Corresponding author. E-mail address: [email protected] (Th. Gischkat). 0167-9317/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2008.11.031
the number of irradiation steps drastically increases and the fabrication process becomes more time-consuming [3]. However, it is known that the crystal is also damaged by the electronic energy loss of high energetic ions (MeV range), i.e. by the energy transferred to the electrons of the target atoms due to inelastic collisions with the ions [4–9]. Above a critical electronic energy deposition of about 5 keV nm 1 for LiNbO3 each ion produces an amorphous track [10]. Below this value only point defects are generated which accumulate to an amorphous layer with increasing ion fluence [5–7]. Using this process the crystal can be damaged by one irradiation from the surface to depths larger than several micrometers with fluences which are essentially smaller compared with those necessary when using the nuclear energy loss. The etching rate of amorphous material seems to be independent of the different kinds of energy loss which causes the crystal damage [11]. Consequently, the damage by the electronic energy deposition in particular is promising for the fabrication of structures with large depths as required, e.g. for the production of ridge waveguides. 2. Experimental X-cut single crystals of congruent grown LiNbO3 were irradiated with 5 MeV Si-ions at room temperature. The ion fluences were between 7 1012 and 1 1014 cm 2. The irradiation was performed with an angle of incident of about 7° with respect to the crystal surface normal.
911
Th. Gischkat et al. / Microelectronic Engineering 86 (2009) 910–912
120 1.0
values at the surface: nda vetch
100 80
nda
0.8
60
0.6 0.4
40
0.2
20
0.0
0
13
14
10
-2
10
NI (ions cm ) 1.2
120
1.1 1.0
-2
100
vetch
80
0.9 60 0.8 40
3. Results and discussion
0.6
3.1. Damage formation
0.5
a
1.4
5 MeV Si:x-cut LNO 13 -2 NI [10 cm ] 0.7 2.0 1.0 4.0 1.5 10.0
1.2
nda
1.0 0.8 0.6 0.4 0.2 -1 -1
b
5 4
I
II
III
Se Sn
0.5 0.4
2
0.2
1
0.1
0
0
500 1000 1500 2000 2500
0.0
-1
0.3
-1
3
Sn (keV nm ion )
Se (keV nm ion )
0.0
z (nm) Fig. 1. Defect concentration nda versus depth of x-cut LiNbO3 after irradiation with 5 MeV Si-ions at room temperature with different ion fluence (a). Electronic and nuclear energy loss Se and Sn, versus depth calculated by SRIM2003 (b).
-1
14
5 MeV Si, 1x10 cm nda
vetch (nm min )
b
0.7
Fig. 1a shows the measured defect concentration for the irradiation of x-cut LiNbO3 with 5 MeV Si-ions at various ion fluences. Three regions of damage can be distinguished: a damaged surface layer (region I), a buried damaged layer (region III) located at a depth of about 2200 nm as well as a transition layer (region II) in between. Comparing the measured distribution of damage with the energy loss per ion and unit path length of the implanted ions as calculated with SRIM2003 [13] (Fig. 1b) it is obvious that the buried damaged layer can be addressed to the nuclear energy loss of the ions causing collision cascades. The damage close to the surface (region I in Fig. 1b) can be attributed to the electronic energy deposition by the Si-ions. The electronic energy deposition has a maximum value of about
-1
1.2
vetch (nm min )
a
nda
Defect analysis was done by means of Rutherford backscattering spectrometry (RBS) in channeling configuration using 4 MeV He-ions. From the Nb-part of the RBS-spectra the relative concentration of displaced atoms (in the following referred to as defect concentration) was calculated using the computer code dechannelling in crystals and defect analysis (DICADA) [12]. In order to investigate the etching behaviour, the samples were partially covered with a silicon wafer during irradiation. Etching was performed stepwise in a 3.7% HF-solution at an etching temperature of 40 °C. The step height between the irradiated and the non-irradiated material was measured after each etching step by means of a surface profilometer ‘‘Sloan DEKTAK 3030ST”. The etching rates were calculated from the etched depth and the corresponding etching time. Inspection of the surface topology after etching was done by means of a scanning electron microscopy (SEM). Patterning of LiNbO3 with micro-structures was carried using a 2 lm thick silicon-mask. The mask was prepared by means of standard electron-beam lithography and inductive coupled plasma reactive ion etching (ICP-RIE). In order to fabricate vertical structures the irradiation was performed 0° with respect to the crystal surface normal.
20 2.5
3.0 3.5 -1 Se (keV nm )
4.0
0
Fig. 2. Defect concentration nda and etching rate vetch taken at the surface versus the ion fluence (a) as well as the defect concentration nda and etching rate vetch versus the electronic energy loss Se for the irradiation with 1 1014 cm 2 Si-ions (b).
4.2 keV nm 1 at the surface and decreases with increasing depth (Fig. 1b). Correspondingly, the damage produced decreases with increasing depth. The amount of produced damage depends not only on the depth, i.e. on the electronic energy deposition, but also on the ion fluence. In Fig. 2a the defect concentration taken at the surface is depicted versus the ion fluence. For the lowest ion fluence of 7 1012 cm 2 the surface is only slightly damaged. The defect concentration increases with increasing ion fluence up to the amorphization of the surface indicated by nda = 1. With further increasing ion fluence the amorphous layer broadens to larger depths. For ion fluences of 4 1013 and 1 1014 cm 2 an amorphous layer with a thicknesses of about 400 and 700 nm, respectively, is produced (Fig. 1a). These ion fluences are 10 times lower compared to the fluences which are necessary to produce amorphous layers with similar thickness in the nuclear energy deposition regime. The thickness of the amorphous layer is determined by the depth where the electronic energy deposition per ion and path length is below a certain value (Fig. 1a). This value is higher for lower ion fluences. Fig. 2b shows the defect concentration taken at different depths versus the corresponding electronic energy deposition for the irradiation with 1 1014 cm 2 Si-ions. In this case the crystal is amorphous down to a depth where the calculated electronic energy deposition is below 3 keV nm 1 which is in agreement with other investigations [5]. With decreasing energy loss, i.e. with increasing depth the defect concentration decreases down to 0.6 for a electronic energy loss of about 2.3 keV nm 1. With further decreasing electronic energy loss the measured defect concentration does not further decrease. In this region (region II in Fig. 1) the damage formation is due to an overlap between the decreasing electronic and the increasing nuclear energy loss.
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Th. Gischkat et al. / Microelectronic Engineering 86 (2009) 910–912
high contrast of the etching rates. The diameter of the etched holes are about 450 nm which is quite similar to the width of the mask holes. However, the surface of the structure is relative rough which is not acceptable to optical components. The reason for this is the obvious attack of regions which are only slightly damaged by ions transmitted through the mask. Consequential it can be avoided by the application of thicker masks stopping the ions completely. Another possibility is thermal treatment of the sample at appropriate conditions prior the etching, at which only regions with low defect concentration are annealed. Such annealing additionally offers the possibility to decrease the roughness of the side walls, which are only slightly damaged by scattered ions. Fig. 3. SEM picture of a hexagonal arrangement of holes in x-cut LiNbO3 fabricated by the IBEE technique using the electronic energy loss.
3.2. Etching behaviour As expected the measured step hight between undamaged and damaged material, i.e. the measured etched depth, increases with increasing etching time. By the fact that the etching of the perfect crystal is negligible the etching rates were calculated from the etched depth and the according etching time. The highest etching rates were observed at the surface where the crystal is mostly damaged. According to the damage distribution in region I the etching rates decreases with increasing depth, i.e. increasing etched depth. In all cases the etching stops at the end of region I at the latest. It seems that in region II the damage is not sufficient for remarkably etching. As a consequence region III was not reached for etching in a 3.7% HF-solution at an etching temperature of 40 °C for several hours. As an example, for an ion fluence of 1 1014 cm 2 an etched depth of about 940 nm was reached after a total etching time of about 160 min. The expected etching rates in region III are below 5 nm min 1 according to the low applied ion fluences and a damage well below the amorphization [14]. In Fig. 2a and b the corresponding etching rates are depicted versus the ion fluence and the electronic energy deposition, respectively. In both cases the etching rates are correlated with the defect concentration. The etching rate amounts to about 100 nm min 1 for amorphized material. With decreasing defect concentration the etching rate decreases drastically. For a defect concentration below 0.6 the etching is negligible. The high etching rate of amorphized material and the steep decrease causes a high contrast of the etching rate. 3.3. Patterning of LiNbO3 The damage formation due to the electronic energy deposition was applied to pattern LiNbO3 with 1 lm deep structures. In order to irradiate the sample selectively, the regions which should not be removed by the HF-solution were covered by a mask during irradiation. Commonly the masks have to be thick enough to stop the ions completely. For 5 MeV Si-ions a 3.5 lm thick silicon-mask is required. Due to the negligible etching in region II the ions have not to be stopped completely in the mask. As a consequence a silicon-mask with reduced thickness of about 2 lm was used. The energy of the transmitted ions causes a damage which is not sufficient for remarkable etching. Fig. 3 shows a SEM picture of patterned LiNbO3 irradiated with 5 MeV Si-ions at an ion fluence of 1 1014 cm 2. The sample was etched for 90 min in 3.7% HF at 40 °C and the resulting structure depth is about 950 nm. The sidewalls have a nearly vertical shape which is a consequence of the
4. Summary The damage formation and etching behaviour of x-cut LiNbO3 due to the electronic energy deposition was investigated. Therefore samples were irradiated with 5 MeV Si-ions at room temperature with various ion fluences. For the highest applied ion fluence of 1 1014 cm 2 a 700 nm thick amorphous surface layer was produced due to the electronic energy loss. Contrary the damage produced by the nuclear energy loss was well below the amorphization. The ion fluences for amorphization in the electronic energy loss regime are 10 times lower compared to the amorphization fluences in the case of the damage formation due to the nuclear energy loss. As a consequence the electronic energy loss allows a very easy fabrication of thick amorphous layers. The etching behaviour of LiNbO3 damaged due to the electronic energy loss is similar to the etching behaviour of that material damaged by nuclear energy loss. Amorphous material exhibits the highest etching rate which decreases drastically for decreasing damage. As a consequence a high contrast of the etching can be observed. The irradiation using the electronic energy loss enables the possibility the fabricate deep structures with high aspect ratios very easily. Acknowledgement The authors want to thank the Deutsche Forschungsgemeinschaft (DFG) for financial supporting (Contract No. KL1199/2-1). References [1] F. Schrempel, Th. Gischkat, H. Hartung, E.-B. Kley, W. Wesch, A. Tünnermann, Mater. Res. Soc. Symp. Proc. 0908-OO16-01.1, 2005. [2] F. Schrempel, Th. Gischkat, H. Hartung, E.-B. Kley, W. Wesch, Nucl. Instrum. Meth. B 250 (2006) 164. [3] H. Hartung, E.-B. Kley, A. Tünnermann, Th. Gischkat, F. Schrempel, W. Wesch, Opt. Lett. 33 (2008) 2320. [4] B. Canut, R. Brenier, A. Meftah, P. Moretti, S. Ould Salem, S.M.M. Ramos, P. Thevenard, M. Toulemonde, Nucl. Instrum. Meth. B 91 (1994) 312. [5] F. Agulló-López, G. García, J. Olivares, J. Appl. Phys. 97 (2005) 093514. [6] J. Olivares, G. García, F. Agulló-López, F. Agulló-Rueda, A. Kling, J. Soares, Appl. Phys. A 81 (2005) 1465. [7] F. Agulló-López, A. Mendez, G. García, J. Olivares, J.M. Cabrera, Phys. Rev. B 74 (2006) 174109. [8] A. García-Navarro, A. Méndez, J. Olivares, G. García, F. Agulló-López, M. Zayat, D. Levy, L. Vazquez, Nucl. Instrum. Meth. B 249 (2006) 172–176. [9] A. García-Navarro, F. Agulló-López, M. Bianconi, J. Olivares, G. García, J. Appl. Phys. 101 (2007) 083506. [10] B. Canut, S.M.M. Ramos, R. Brenier, P. Thevenard, J.L. Loubet, M. Toulemonde, Nucl. Instrum. Meth. B 107 (1996) 194. [11] M. Bianconi, F. Bergamini, G.G. Bentini, A. Cerutti, M. Chiarini, P. De Nicola, G. Pennestrì, Nucl. Instrum. Meth. B 266 (2008) 1238. [12] K. Gärtner, Nucl. Instrum. Meth. B 227 (2005) 522. [13] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985. [14] T. Steinbach, F. Schrempel, Th. Gischkat, W. Wesch, Phys. Rev. B 78 (2008) 184106.