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Tip-on-tip: a novel AFM tip configuration for the electrical characterization of semiconductor devices
/dICRO~¢
ELSEVIER
Microelectronic Engineering 46 (1999) 113-116
Tip-on-tip: a novel AFM tip configuration for the electrical characterization of semiconductor devices T. Hantschel a*, T. Trenkler ~, W. Vandervorst at A. Malav~ b, D. B/ichel b, W. Kulisch b, E. Oesterschulze b aIMEC vzw, Kapeldreef 75, B-3001 Leuven, Belgium bInstitute of Technical Physics, University of Kassel, Heinrich-Plett-Str. 40, D-34109 Kassel, Germany A novel tip configuration for atomic force microscopy (AFM) called tip-on-tip is presented. In this concept a sharp, very small tip is created on top of a large truncated pyramid. The process scheme for the fabrication of tip-on-tip is presented. It is demonstrated that very sharp metal tips can be produced in this way. Advantages of tip-on-tip when applied in semiconductor device analysis are discussed. First results concerning the transfer of the developed technology to diamond are presented.
1. I N T R O D U C T I O N The atomic force microscope is used more and more in microelectronics for the electrical characterization of semiconductor devices. The driving force behind these activities is to overcome the limitations of the classical methods and to reach nanometer precision and resolution. Three different AFM-based methods for the determination of the carrier concentration in semiconductor structures were investigated by the authors in the last few years: scanning spreading resistance microscopy (SSRM), scanning capacitance microscopy (SCM) and nanopotentiometry [1]. A crucial part of such a characterization tool is the tip which should not only have a small radius of curvature but should also be sufficiently electrically conductive. Silicon and silicon nitride tips which are most commonly used for topography measurements suffer from poor electrical conductivity. This can be improved by an additional metal coating but these coatings are prone to wearing off very fast. It was demonstrated that good results can be obtained with silicon tips coated with boron doped diamond [2]. The radius of curvature is, however, increasing by this coating. *e-maih hantschl~imec.be *also at: KU Leuven, INSYS, Kard. B-3001 Leuven, Belgium
Mercierlaan 92,
A different approach is the fabrication of electrically conductive AFM tips using the mould technique. Recently, it was shown that pyramidal diamond tips [3] and pyramidal metal tips [4] can be produced in this way. These pyramidal tips performed very well in first electrical measurements [4,5]. The advantage of the tips produced by the mould technique is that the radius of curvature is defined by the sharpness of the mould independently of the tip material. Furthermore, the low aspect ratio gives higher mechanical stability to the tip. This is required, for example, for SSRM where the applied forces are up to three orders of magnitude higher than for topography measurements. Problems with the formation of knife-shaped edges of the mould often limit the resolution of the pyramidal tips to about 20 nm. Therefore, a novel tip configuration called tip-on-tip has been developed in order to push the resolution of the pyramidal tips down to the sub-10 nm region. In this paper the concept of tip-on-tip is presented and we demonstrate how such a tip configuration can be realized in metal as well as in diamond. 2. T H E T I P - O N - T I P
CONCEPT
It has been shown that inverted pyramids in the sub-100 nm region can be anisotropically etched into silicon in a reproducible manner with a ra-
0167-9317/99/5 - see front matter © 1999 Elsevier Science B.V. All rights reserved. Pll: $0167-9317(99)00028-3
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T. Hantschel et al. / Microelectronic Engineering 46 (1999) 113-116
dius of curvature as small as 5 nm [6]. This clearly demonstrates that it should in principle also be possible to produce AFM tips with a radius of curvature smaller than 10 nm by the mould technique. Experiments have shown that this is very difficult to achieve using the single tip approach where for practical reasons a base width of 2040 #m is needed for the inverted pyramids. In this case, the pyramidal etch pits often show knifeshaped edges which are due to non-symmetrical openings of the etch mask.
tip
//~
/"cantilever
Figure 1. The tip-on-tip configuration.
The results obtained by generating sub-micron pyramidal tips encouraged us to look for a way to use such small pyramidal tips as probe tips in the AFM. Therefore, the tip-on-tip concept was developed where a small, very sharp pyramidal tip is created on top of a truncated pyramid. This tip configuration is shown schematically in Figure 1. The truncated pyramid serves as a pedestal and gives the required minimal cantilever to sample distance. It further limits the penetration depth of the small tip into the sample volume. The small tip makes contact to the sample surface and gives the desired high lateral resolution. As the typical forces applied in SSRM and nanopotentiometry are three orders of magnitude higher than for topography measurements, the mechanical stability of the tip becomes an important issue. Compared to a hollow pyramid, tip-on-tip is expected to be a more rigid mechanical configuration.
3. T H E F A B R I C A T I O N
OF TIP-ON-TIP
The fabrication process for tip-on-tip is mainly based on a two-step anisotropic etching sequence, as it is shown in Figure 2. (100)-Si substrates with a masking layer on top were used for these experiments. In the first lithographic step, the opening for the truncated pyramidal etch pit was defined. This pattern was transferred to the etch mask by a combination of dry and wet etching. The resist was then removed and the truncated pyramid was etched in a 30 wt% KOH solution at 70°C. This etching was time-controlled and was stopped when the desired depth was reached. The first etch mask was then removed and a second, very thin etch mask of SiO2 was formed by rapid thermal annealing (RTA) at 1100°C in an O2-atmosphere. Different oxide thicknesses were tested ranging from 10-30 nm. A second lithographic step was then performed by e-beam lithography in order to define the area for the small tip on the bottom of the truncated pyramidal etch pit. The pattern generator was a Raith proxy-writer system with alignment capability. In the next step, the etch mask was opened by buffered HF. Then, the resist was removed and the small inverted pyramid was etched in a 10 wt% KOH solution at 21°C. These etching conditions were found to be very suitable for that purpose due to its relatively low etch rate of 1.5 # m / h . The thin SiO2 mask film was then removed in HF. Next, the mould was filled with a material. Both metal and diamond were used as tip materials in the experiments. Metal was deposited into the mould by first evaporating and sputtering several thin metal layers after each other. This was followed by an electroplating step in order to increase the total metal thickness to a few microns. The diamond films were grown by the hot filament chemical vapor deposition (HFCVD) technique using process and parameters similar to those described in [7]. Pretreatment was done using abrasive ultrasonic (US) treatment with diamond paste and diamond powder of different grain size. The total thickness of the grown diamond film was about 6 #m. Boron dopant levels of about 1% lead to conductivities in the order of 103 S/cm which is sufficient
T. Hantschel et al. / Microelectronic Engineering 46 (1999) 113-116
115
trtmcated inverted pyramid
100-Si 1. Two-step anisotropic etching
2. Diamond / metal deposition
3. Removal of the silicon
Figure 2. The fabrication scheme for tip-on-tip in metal / diamond.
for the above mentioned applications. In the final step, the supporting silicon was removed. This was done using 20 wt% KOH at 90°C. The samples were then inspected in the SEM. 4. R E S U L T S
AND
DISCUSSION
The experiments have shown that truncated inverted pyramids with a base width at the b o t t o m of 1 p m and smaller can be made in a reproducible manner following the procedure described above. In contrast to the formation of inverted pyramids where the etching stops automatically, the etching of truncated inverted pyramids needs to be stopped at a certain depth. This etching is timecontrolled and, therefore, a good control over the etch p a r a m e t e r s is required.
Figure 3. Metal tip-on-tip. As the performance of a tip-on-tip probe will mainly depend on the small pyramidal etch pit at the b o t t o m of the truncated inverted pyra-
mid, experiments were done in order to optimize this process step. The procedure for the coating of the substrate with resist was adjusted in such a way t h a t also the b o t t o m of the truncated pyramid can be uniformly covered with a thin resist layer. Circular openings with a diameter of 100 nm could be defined in the experiments following this procedure. For the anisotropic etching we used the procedure and the parameters which are described in detail in [6]. The base width of the small pyramidal etch pits was ranging from 1 # m to 0.2 # m in the experiments. It has to be pointed out t h a t the SiO2 etch mask has to be very thin if submicron inverted pyramids are to be etched. This is due to the fact that the pattern transfer to the etch mask is an isotropic step and the lateral size of the structure is, therefore, increased. An oxide thickness of 20 nm was found to be a good compromise. This layer thickness is sufficient to protect the silicon from etch attack and allows at the same time the generation of inverted pyramids as small as 0.2 #m. Different metals were tested as tip-on-tip m a t e rial. Good results were obtained filling the mould with a multilayer consisting of Cr, Ti:W and Au. The chromium layer acts as an adhesion layer for the silicon and should protect the later A F M tip-on-tip from wear. Ti:W is also a hard material and gives good adhesion to the gold layer. A thick Au coating improves the electrical conductivity and results in a mechanically stable tip. Figure 3 shows a fabricated metal tip-on-tip consisting of 60 nm Cr, 20 nm Ti:W and 5 # m Au. The developed metal tip-on-tip module is com-
116
T. Hantschel et al. / M icroelectronic Engineering 46 (1999) 113-116
Figure 5. Diamond tip-on-tip. Figure 4. Sub-micron diamond pyramid. patible with an existing fabrication process for silicon cantilevers with integrated metal tips [4]. Therefore, further work will focus on the fabrication of a metal tip-on-tip AFM probe. Diamond is a very interesting material for tipon-tip due to its excellent hardness. The work was first concentrating on the development of a procedure for the formation of sub-micron diamond pyramids. Arrays of sub-micron diamond pyramids were made ranging from 0.3 #m to 1 #m. Figure 4 shows a diamond pyramid with a base width of 0.8 #m. Note that this pyramidal tip has a very sinall radius of curvature. Nearly the same results were obtained using diamond powder for the pretreatment with a grain size of 1 #m, 0.7 #m and 0.25 #m. The experiments have, however, also shown that not all tips of the small diamond pyramids are completely developed. The reason for that effect is not clearly understood yet. Therefore, the procedure for the generation of sub-micron diamond pyramids needs to be improved further. Figure 5 shows finally a fabricated diamond tip-on-tip. The base width of the small diamond pyramid is 0.3 #m. 5. C O N C L U S I O N S A novel AFM tip configuration called tip-on-tip has been developed. It consists of a small, very sharp tip which is created on top of a large truncated pyramid. A fabrication process for metal tip-on-tip was developed. First results were presented concerning the fabrication of sub-micron
diamond pyramids and diamond tip-on-tip. Future work will concentrate on the integration of tip-on-tip into a cantilever and the evaluation of the tip-on-tip probe in the AFM. ACKNOWLEDGEMENTS
T. Hantschel and T. Trenkler are indebted to the Belgian IWT for their Ph.D. fellowship. The authors thank W. Scholz for taking SEM images. REFERENCES
1. W. Vandervorst, T. Clarysse, P. De Wolf, T. Trenkler, T. Hantschel, R. Stephenson, T. Janssens, Proc. Int. Conf. on Characterization and Metrology for ULSI, to be published. 2. P. De Wolf, T. Clarysse, W. Vandervorst, L. Hellemans, P. Niedermann, W. H/inni, J. Vac. Sci. Technol., B 16, 355 (1998). 3. W. Kulisch, A. Malav6, G. Lippold, W. Scholz, C. Mihalcea, E. Oesterschulze, Diamond and Related Materials, 6, 906 (1997). 4. T. Hantschel, P. De Wolf, T. Trenkler, R. Stephenson, W. Vandervorst, Proc. Microroach, and Microfabr. 98, to be published. 5. A. Malav6, E. Oesterschulze, W. Kulisch, T. Trenkler, T. Hantschel, W. Vandervorst, Proc. Diamond 98, to be published. 6. T. Hantschel, W. Vandervorst, Microelectronic Microengineering, 35,405 (1997). 7. E. Oesterschulze, W. Scholz, C. Mihalcea, D. Albert, B. Sobisch, W. Kulisch, Appl. Phys. Lett., 70,435 (1997).
ELSEVIER
Microelectronic Engineering 46 (1999) 113-116
Tip-on-tip: a novel AFM tip configuration for the electrical characterization of semiconductor devices T. Hantschel a*, T. Trenkler ~, W. Vandervorst at A. Malav~ b, D. B/ichel b, W. Kulisch b, E. Oesterschulze b aIMEC vzw, Kapeldreef 75, B-3001 Leuven, Belgium bInstitute of Technical Physics, University of Kassel, Heinrich-Plett-Str. 40, D-34109 Kassel, Germany A novel tip configuration for atomic force microscopy (AFM) called tip-on-tip is presented. In this concept a sharp, very small tip is created on top of a large truncated pyramid. The process scheme for the fabrication of tip-on-tip is presented. It is demonstrated that very sharp metal tips can be produced in this way. Advantages of tip-on-tip when applied in semiconductor device analysis are discussed. First results concerning the transfer of the developed technology to diamond are presented.
1. I N T R O D U C T I O N The atomic force microscope is used more and more in microelectronics for the electrical characterization of semiconductor devices. The driving force behind these activities is to overcome the limitations of the classical methods and to reach nanometer precision and resolution. Three different AFM-based methods for the determination of the carrier concentration in semiconductor structures were investigated by the authors in the last few years: scanning spreading resistance microscopy (SSRM), scanning capacitance microscopy (SCM) and nanopotentiometry [1]. A crucial part of such a characterization tool is the tip which should not only have a small radius of curvature but should also be sufficiently electrically conductive. Silicon and silicon nitride tips which are most commonly used for topography measurements suffer from poor electrical conductivity. This can be improved by an additional metal coating but these coatings are prone to wearing off very fast. It was demonstrated that good results can be obtained with silicon tips coated with boron doped diamond [2]. The radius of curvature is, however, increasing by this coating. *e-maih hantschl~imec.be *also at: KU Leuven, INSYS, Kard. B-3001 Leuven, Belgium
Mercierlaan 92,
A different approach is the fabrication of electrically conductive AFM tips using the mould technique. Recently, it was shown that pyramidal diamond tips [3] and pyramidal metal tips [4] can be produced in this way. These pyramidal tips performed very well in first electrical measurements [4,5]. The advantage of the tips produced by the mould technique is that the radius of curvature is defined by the sharpness of the mould independently of the tip material. Furthermore, the low aspect ratio gives higher mechanical stability to the tip. This is required, for example, for SSRM where the applied forces are up to three orders of magnitude higher than for topography measurements. Problems with the formation of knife-shaped edges of the mould often limit the resolution of the pyramidal tips to about 20 nm. Therefore, a novel tip configuration called tip-on-tip has been developed in order to push the resolution of the pyramidal tips down to the sub-10 nm region. In this paper the concept of tip-on-tip is presented and we demonstrate how such a tip configuration can be realized in metal as well as in diamond. 2. T H E T I P - O N - T I P
CONCEPT
It has been shown that inverted pyramids in the sub-100 nm region can be anisotropically etched into silicon in a reproducible manner with a ra-
0167-9317/99/5 - see front matter © 1999 Elsevier Science B.V. All rights reserved. Pll: $0167-9317(99)00028-3
114
T. Hantschel et al. / Microelectronic Engineering 46 (1999) 113-116
dius of curvature as small as 5 nm [6]. This clearly demonstrates that it should in principle also be possible to produce AFM tips with a radius of curvature smaller than 10 nm by the mould technique. Experiments have shown that this is very difficult to achieve using the single tip approach where for practical reasons a base width of 2040 #m is needed for the inverted pyramids. In this case, the pyramidal etch pits often show knifeshaped edges which are due to non-symmetrical openings of the etch mask.
tip
//~
/"cantilever
Figure 1. The tip-on-tip configuration.
The results obtained by generating sub-micron pyramidal tips encouraged us to look for a way to use such small pyramidal tips as probe tips in the AFM. Therefore, the tip-on-tip concept was developed where a small, very sharp pyramidal tip is created on top of a truncated pyramid. This tip configuration is shown schematically in Figure 1. The truncated pyramid serves as a pedestal and gives the required minimal cantilever to sample distance. It further limits the penetration depth of the small tip into the sample volume. The small tip makes contact to the sample surface and gives the desired high lateral resolution. As the typical forces applied in SSRM and nanopotentiometry are three orders of magnitude higher than for topography measurements, the mechanical stability of the tip becomes an important issue. Compared to a hollow pyramid, tip-on-tip is expected to be a more rigid mechanical configuration.
3. T H E F A B R I C A T I O N
OF TIP-ON-TIP
The fabrication process for tip-on-tip is mainly based on a two-step anisotropic etching sequence, as it is shown in Figure 2. (100)-Si substrates with a masking layer on top were used for these experiments. In the first lithographic step, the opening for the truncated pyramidal etch pit was defined. This pattern was transferred to the etch mask by a combination of dry and wet etching. The resist was then removed and the truncated pyramid was etched in a 30 wt% KOH solution at 70°C. This etching was time-controlled and was stopped when the desired depth was reached. The first etch mask was then removed and a second, very thin etch mask of SiO2 was formed by rapid thermal annealing (RTA) at 1100°C in an O2-atmosphere. Different oxide thicknesses were tested ranging from 10-30 nm. A second lithographic step was then performed by e-beam lithography in order to define the area for the small tip on the bottom of the truncated pyramidal etch pit. The pattern generator was a Raith proxy-writer system with alignment capability. In the next step, the etch mask was opened by buffered HF. Then, the resist was removed and the small inverted pyramid was etched in a 10 wt% KOH solution at 21°C. These etching conditions were found to be very suitable for that purpose due to its relatively low etch rate of 1.5 # m / h . The thin SiO2 mask film was then removed in HF. Next, the mould was filled with a material. Both metal and diamond were used as tip materials in the experiments. Metal was deposited into the mould by first evaporating and sputtering several thin metal layers after each other. This was followed by an electroplating step in order to increase the total metal thickness to a few microns. The diamond films were grown by the hot filament chemical vapor deposition (HFCVD) technique using process and parameters similar to those described in [7]. Pretreatment was done using abrasive ultrasonic (US) treatment with diamond paste and diamond powder of different grain size. The total thickness of the grown diamond film was about 6 #m. Boron dopant levels of about 1% lead to conductivities in the order of 103 S/cm which is sufficient
T. Hantschel et al. / Microelectronic Engineering 46 (1999) 113-116
115
trtmcated inverted pyramid
100-Si 1. Two-step anisotropic etching
2. Diamond / metal deposition
3. Removal of the silicon
Figure 2. The fabrication scheme for tip-on-tip in metal / diamond.
for the above mentioned applications. In the final step, the supporting silicon was removed. This was done using 20 wt% KOH at 90°C. The samples were then inspected in the SEM. 4. R E S U L T S
AND
DISCUSSION
The experiments have shown that truncated inverted pyramids with a base width at the b o t t o m of 1 p m and smaller can be made in a reproducible manner following the procedure described above. In contrast to the formation of inverted pyramids where the etching stops automatically, the etching of truncated inverted pyramids needs to be stopped at a certain depth. This etching is timecontrolled and, therefore, a good control over the etch p a r a m e t e r s is required.
Figure 3. Metal tip-on-tip. As the performance of a tip-on-tip probe will mainly depend on the small pyramidal etch pit at the b o t t o m of the truncated inverted pyra-
mid, experiments were done in order to optimize this process step. The procedure for the coating of the substrate with resist was adjusted in such a way t h a t also the b o t t o m of the truncated pyramid can be uniformly covered with a thin resist layer. Circular openings with a diameter of 100 nm could be defined in the experiments following this procedure. For the anisotropic etching we used the procedure and the parameters which are described in detail in [6]. The base width of the small pyramidal etch pits was ranging from 1 # m to 0.2 # m in the experiments. It has to be pointed out t h a t the SiO2 etch mask has to be very thin if submicron inverted pyramids are to be etched. This is due to the fact that the pattern transfer to the etch mask is an isotropic step and the lateral size of the structure is, therefore, increased. An oxide thickness of 20 nm was found to be a good compromise. This layer thickness is sufficient to protect the silicon from etch attack and allows at the same time the generation of inverted pyramids as small as 0.2 #m. Different metals were tested as tip-on-tip m a t e rial. Good results were obtained filling the mould with a multilayer consisting of Cr, Ti:W and Au. The chromium layer acts as an adhesion layer for the silicon and should protect the later A F M tip-on-tip from wear. Ti:W is also a hard material and gives good adhesion to the gold layer. A thick Au coating improves the electrical conductivity and results in a mechanically stable tip. Figure 3 shows a fabricated metal tip-on-tip consisting of 60 nm Cr, 20 nm Ti:W and 5 # m Au. The developed metal tip-on-tip module is com-
116
T. Hantschel et al. / M icroelectronic Engineering 46 (1999) 113-116
Figure 5. Diamond tip-on-tip. Figure 4. Sub-micron diamond pyramid. patible with an existing fabrication process for silicon cantilevers with integrated metal tips [4]. Therefore, further work will focus on the fabrication of a metal tip-on-tip AFM probe. Diamond is a very interesting material for tipon-tip due to its excellent hardness. The work was first concentrating on the development of a procedure for the formation of sub-micron diamond pyramids. Arrays of sub-micron diamond pyramids were made ranging from 0.3 #m to 1 #m. Figure 4 shows a diamond pyramid with a base width of 0.8 #m. Note that this pyramidal tip has a very sinall radius of curvature. Nearly the same results were obtained using diamond powder for the pretreatment with a grain size of 1 #m, 0.7 #m and 0.25 #m. The experiments have, however, also shown that not all tips of the small diamond pyramids are completely developed. The reason for that effect is not clearly understood yet. Therefore, the procedure for the generation of sub-micron diamond pyramids needs to be improved further. Figure 5 shows finally a fabricated diamond tip-on-tip. The base width of the small diamond pyramid is 0.3 #m. 5. C O N C L U S I O N S A novel AFM tip configuration called tip-on-tip has been developed. It consists of a small, very sharp tip which is created on top of a large truncated pyramid. A fabrication process for metal tip-on-tip was developed. First results were presented concerning the fabrication of sub-micron
diamond pyramids and diamond tip-on-tip. Future work will concentrate on the integration of tip-on-tip into a cantilever and the evaluation of the tip-on-tip probe in the AFM. ACKNOWLEDGEMENTS
T. Hantschel and T. Trenkler are indebted to the Belgian IWT for their Ph.D. fellowship. The authors thank W. Scholz for taking SEM images. REFERENCES
1. W. Vandervorst, T. Clarysse, P. De Wolf, T. Trenkler, T. Hantschel, R. Stephenson, T. Janssens, Proc. Int. Conf. on Characterization and Metrology for ULSI, to be published. 2. P. De Wolf, T. Clarysse, W. Vandervorst, L. Hellemans, P. Niedermann, W. H/inni, J. Vac. Sci. Technol., B 16, 355 (1998). 3. W. Kulisch, A. Malav6, G. Lippold, W. Scholz, C. Mihalcea, E. Oesterschulze, Diamond and Related Materials, 6, 906 (1997). 4. T. Hantschel, P. De Wolf, T. Trenkler, R. Stephenson, W. Vandervorst, Proc. Microroach, and Microfabr. 98, to be published. 5. A. Malav6, E. Oesterschulze, W. Kulisch, T. Trenkler, T. Hantschel, W. Vandervorst, Proc. Diamond 98, to be published. 6. T. Hantschel, W. Vandervorst, Microelectronic Microengineering, 35,405 (1997). 7. E. Oesterschulze, W. Scholz, C. Mihalcea, D. Albert, B. Sobisch, W. Kulisch, Appl. Phys. Lett., 70,435 (1997).