ultramicroscopy ELSEVIER Ultramicroscopy 82(2000)57-61 Www Whisker probes E.I. Givargizov., A.N. Stepanova, L N. Obolenskaya, E.S. Mashkova V A. Molchanov, M.E. Givargizov, I.W. Rangelow Institute of Crystallography, Russian Academy of Sciences, Leninsky pr. 59, 117333 Moscow, Russia Nuclear Physics Institute, Moscow Unicersity, Moscow, Russia "Crystals and Technology, Ltd, Moscow, Russia University of Kassel, Kassel, Germany Received 31 May 1999, received in revised form 12 August 1999 Abstract a new technology for preparation of AFM probes based on whiskers is proposed. The technology posseses significant advantages to the whisker probes over standard ones as related to the shapes of the probes, as well as to the properties of the cantilevers on which the whiskers are grown. 2000 Elsevier Science B.V. All rights reserved. 1. Introduction monly used. The probes are prepared by means of a photolithographic process. Principal drawbacks Since the beginning of 1980s, when scanning tun- of the probes are large angles at their apex: about neling microscopy based on tip probe has been 110 of the silicon nitride and 20-30 of the silicon discovered and introduced into scientific and in- probes. This limits the use of the probes for studies dustrial practice, the number of its"children"has of samples with coarse surfaces, for studies of biolo- been increased and continue to increase. Currently, gical macromolecules, for trench profiling, etc more than ten different versions of scanning probe In this work, we propose to prepare silicon devices are now used. Of the versions, atomic probes from silicon whiskers that can be control- force microscopes(AFM) are most valuable and lably grown on silicon cantilevers having special crystallographic orientation a tip probe is a key component of all scanning litically formed on a special console, or lever-So- 2. Experimentals called cantilever. Probes of silicon nitride(Si3 N4) and of single-crystalline silicon are now most com- Silicon whiskers are grown by the vapor liquid-solid mechanism [1-3] using gold as liquid- forming agent. The growth process consists in the following Gold particles are deposited on a silicon Corresponding author. Fax: + 7-095-330-8265 substrate having the crystallographic orientation E-mail address: givargiz(@cvdlab incr msk.su(E.. Givargizov) (11 1)by a photolithographic procedure. The 0304-3991 /00/Ssee front matter c 2000 Elsevier Science B v. All rights reserved. PI:s0304-3991(99)00141-2
* Corresponding author. Fax:#7-095-330-8265. E-mail address: givargiz@cvdlab.incr.msk.su (E.I. Givargizov) Ultramicroscopy 82 (2000) 57}61 Whisker probes E.I. Givargizov!,*, A.N. Stepanova!, L.N. Obolenskaya!, E.S. Mashkova", V.A. Molchanov", M.E. Givargizov#, I.W. Rangelow$ !Institute of Crystallography, Russian Academy of Sciences, Leninsky pr. 59, 117333 Moscow, Russia "Nuclear Physics Institute, Moscow University, Moscow, Russia #Crystals and Technology, Ltd., Moscow, Russia $University of Kassel, Kassel, Germany Received 31 May 1999; received in revised form 12 August 1999 Abstract A new technology for preparation of AFM probes based on whiskers is proposed. The technology posseses signi"cant advantages to the whisker probes over standard ones as related to the shapes of the probes, as well as to the properties of the cantilevers on which the whiskers are grown. ( 2000 Elsevier Science B.V. All rights reserved. 1. Introduction Since the beginning of 1980s, when scanning tunneling microscopy based on tip probe has been discovered and introduced into scienti"c and industrial practice, the number of its `childrena has been increased and continue to increase. Currently, more than ten di!erent versions of scanning probe devices are now used. Of the versions, atomic force microscopes (AFM) are most valuable and numerous. A tip probe is a key component of all scanning probe devices. In the AFMs, the probes are monolitically formed on a special console, or lever } socalled cantilever. Probes of silicon nitride (Si3 N4 ) and of single-crystalline silicon are now most commonly used. The probes are prepared by means of a photolithographic process. Principal drawbacks of the probes are large angles at their apex: about 1103 of the silicon nitride and 20}303 of the silicon probes. This limits the use of the probes for studies of samples with coarse surfaces, for studies of biological macromolecules, for trench pro"ling, etc. In this work, we propose to prepare silicon probes from silicon whiskers that can be controllably grown on silicon cantilevers having special crystallographic orientation. 2. Experimentals Silicon whiskers are grown by the vapor} liquid}solid mechanism [1}3] using gold as liquidforming agent. The growth process consists in the following. Gold particles are deposited on a silicon substrate having the crystallographic orientation (1 1 1) by a photolithographic procedure. The 0304-3991/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 9 9 1 ( 9 9 ) 0 0 1 4 1 - 2
E.I. Girargizov et al/Ultramicroscopy 82(2000)57-61 282828KV 19mD16 Fig1. An oriented array of silicon whiskers grown on a(11 1) whiskers oriented array of silicon tips prepared from silicon Fig. 3. Si substrate by the vapor-liquid-solid mechanism. 285929KX8,9801nD Fig. 2. An intermediate stage of transformation of a silicon hiker into a silicon tip. substrate with the gold particles is installed into a crystallization chamber. Purified hydrogen with Sicl4 vapors is passed through the chamber. The ubstrate is heated up to about 800-900C. The gold particles are melted when they come in contact with the silicon substrate. Silicon-gold alloy drop lets are formed according to the silicon-gold phase diagram. At the temperatures indicated, the chemical Fig 4. A high-resolution transmission-electron-microscope im- reaction SiCl4 +H2+Si+ 4HCl proceeds prefer- age of the sharpened silicon tip entially at the surface of the droplets. Silicon atoms that are evolved by the reaction supersaturate the pushed out of the substrate, and silicon columns droplets, and the excess silicon is deposited at the ("whiskers")are formed under the droplets, the droplet-silicon substrate interface. The droplets are diameter of the whiskers being determined by the
Fig. 1. An oriented array of silicon whiskers grown on a (1 1 1)- Si substrate by the vapor}liquid}solid mechanism. Fig. 2. An intermediate stage of transformation of a silicon whisker into a silicon tip. Fig. 3. An oriented array of silicon tips prepared from silicon whiskers. Fig. 4. A high-resolution transmission-electron-microscope image of the sharpened silicon tip. substrate with the gold particles is installed into a crystallization chamber. Puri"ed hydrogen with SiCl4 vapors is passed through the chamber. The substrate is heated up to about 800}9003C. The gold particles are melted when they come in contact with the silicon substrate. Silicon}gold alloy droplets are formed according to the silicon-gold phase diagram. At the temperatures indicated, the chemical reaction SiCl4 #H2 PSi#4HCl proceeds preferentially at the surface of the droplets. Silicon atoms that are evolved by the reaction supersaturate the droplets, and the excess silicon is deposited at the droplet}silicon substrate interface. The droplets are pushed out of the substrate, and silicon columns (`whiskersa) are formed under the droplets, the diameter of the whiskers being determined by the 58 E.I. Givargizov et al. / Ultramicroscopy 82 (2000) 57}61
E.I. Girargizov et al/Ultramicroscopy 82(2000)57-61 diameters of the droplets. Silicon-whisker array in diameter) provides a good stability with respect grown in such a way is shown in Fig. 1. Solidified to vibrations, while a small diameter of thethe droplets (globules")consisting of a mixture of sili- upper part(a"point"), 0.1 um and smaller, ensures con and gold microcrystallites are seen at the end of the whiskers The whiskers grown serve as a basis for prepara tion of the silicon tips. When the whiskers are treated by a solution that etches silicon slowly the dissolution of the silicon begins at the glob- ule-whisker interface(see Fig. 2)and, finally, the x0,1m globules are removed with a simultaneous forma tion of the silicon tips( Fig 3). The silicon tips can be very sharp down to nanometer sizes of their curvature radius. as shown in Fig. 4 目 3. Results and discussions 5-10严m The sharpened silicon tips can be used as probes for scanning probe devices(SPD). An advantage of such probes is the fact that the whisker technology allows to control the shape of the probes in a broad In particular, a probe shown in Fig. 5 can be considered as an ideal one: a broad basis(5-10 um Fig. 5. A scheme of an ideal AFM probe. Fig. 6. Examples of the realization of the ideal AFM probes
Fig. 6. Examples of the realization of the ideal AFM probes. Fig. 5. A scheme of an ideal AFM probe. diameters of the droplets. Silicon-whisker array grown in such a way is shown in Fig. 1. Solidi"ed droplets (`globulesa) consisting of a mixture of silicon and gold microcrystallites are seen at the ends of the whiskers. The whiskers grown serve as a basis for preparation of the silicon tips. When the whiskers are treated by a solution that etches silicon slowly, the dissolution of the silicon begins at the globule}whisker interface (see Fig. 2) and, "nally, the globules are removed with a simultaneous formation of the silicon tips (Fig. 3). The silicon tips can be very sharp, down to nanometer sizes of their curvature radius, as shown in Fig. 4. 3. Results and discussions The sharpened silicon tips can be used as probes for scanning probe devices (SPD). An advantage of such probes is the fact that the whisker technology allows to control the shape of the probes in a broad range. In particular, a probe shown in Fig. 5 can be considered as an ideal one: a broad basis (5}10 lm in diameter) provides a good stability with respect to vibrations, while a small diameter of the the upper part (a `pointa), 0.1 lm and smaller, ensures E.I. Givargizov et al. / Ultramicroscopy 82 (2000) 57}61 59
E.I. Girargizov et al/Ultramicroscopy 82(2000)57-61 016828KV818,8881m 11272K81,89919NmMD39 855328K18,981mWD28 Fig. 7(a) expansion/contraction on silicon whiskers during their growth; (b) a silicon probe prepared by a sharpening of IR9728198kVX19m a whisker and its treatment in an anistropic etch. a high resolving power of the probe. Figs 6a and b demonstrate a realization of such probes In addition, a cylindrical shape of the upper part probe by crystalline diamond and, then, a sharp- llows to use such probes for trench profiling [4], ening of the coating by ion milling [8]. The result is for fine studies of biological objects and for other shown in Fig 8. Such probes characterized by ex- dimensional measurements of topographic features reme hardness, chemical inertness, very low of irregular surfaces [5,6]. By changing of growth tion coefficient, etc, are especially suitable for parameters during whisker formation it is possible contact-mode AFM applications. to expand and, then, contract the diameter of the One more remark on whisker probes. The probes whiskers(Fig. 7a)and, in such a way, to form are formed on (111)-Si cantilevers because the probes having sharp edges not only on their top but whiskers grow according to the vapor-liquid-solid also on side faces(Fig. 7b)applicable to profiling mechanism preferentially in the [11 1] crystallo- structures having sloped walls [7] graphic direction. At the same time, owing to the One more possibility that relates to crystal exceptional features of the close-packed ori growth technology is a coating of the end of the entation, it is possible, by using the anisotropic
Fig. 7. (a) expansion/contraction on silicon whiskers during their growth; (b) a silicon probe prepared by a sharpening of a whisker and its treatment in an anistropic etch. Fig. 8. A diamond tip on a silicon basis prepared by ion-beam sharpening. Fig. 9. An example of a whisker probe epitaxial to a silicon cantilever. a high resolving power of the probe. Figs. 6a and b demonstrate a realization of such probes. In addition, a cylindrical shape of the upper part allows to use such probes for trench pro"ling [4], for "ne studies of biological objects and for other dimensional measurements of topographic features of irregular surfaces [5,6]. By changing of growth parameters during whisker formation it is possible to expand and, then, contract the diameter of the whiskers (Fig. 7a) and, in such a way, to form probes having sharp edges not only on their top but also on side faces (Fig. 7b) applicable to pro"ling structures having sloped walls [7]. One more possibility that relates to crystal growth technology is a coating of the end of the probe by crystalline diamond and, then, a sharpening of the coating by ion milling [8]. The result is shown in Fig. 8. Such probes characterized by extreme hardness, chemical inertness, very low friction coe$cient, etc., are especially suitable for contact-mode AFM applications. One more remark on whisker probes. The probes are formed on (1 1 1)-Si cantilevers because the whiskers grow according to the vapor}liquid}solid mechanism preferentially in the [1 1 1] crystallographic direction. At the same time, owing to the exceptional features of the close-packed orientation, it is possible, by using the anisotropic 60 E.I. Givargizov et al. / Ultramicroscopy 82 (2000) 57}61
E.I. Girargizov et al/Ultramicroscopy 82(2000)57-61 silicon etching [9], to polish chemically the back Acknowledgements side of the cantilever that is important for precise measurements of the AFM signal. In other words The authors thank Dr V.G. Galstyan and Mrs the cantilevers have advantages in comparison with V.I. Muratova for scanning electron micrograph the standard silicon AFM cantilevers that are usu- studies, Ms. S.V. Nosovich for sharpening of the An additional advantage of the whisker probe ally oriented along the (100)crystallographic fac whiskers, and Mrs O B. Vol'skaya for assistance in preparation of photographs the fact that the(111)-cantilevers have approxim ately 30% higher coefficient of piezoresistivity in comparison with the(1 0 0j-cantilevers of a similar References design [7] It is a non-standard task to prepare (1 1 1)- []RS. w.C. Ellis, Appl. Phys. Lett. 4(1964)89. oriented cantilevers by, for example, anisotropic [3]EI J. Vac. Sci. Technol. B 11(1993)449. A N. Kiselev, L N. Obolenskaya, A.N. hemical etching [9]. One of the possible solution of Stepanova, Appl. Surf. Sci. 67(1993)73 the problem is to use special dry etching procedure [4] K.L. Lee, D W. Abraham, F Secord, L. Landstein, J. Vac at cryogenic temperatures for deep etching [10, 11 B9(1991)3562 In Fig 9, an example of the AFM whisker probe [5]JE. D.A. Grigg, J Appl. Phys. 74(1993)R83. fabricated with our technology is given. [6]JA. Dagata. i J.J. Kopansky, Solid State TechnoL. 7(1995 [7 I.W. Rangelow, F Shi, P Hudek, P Grabiec, B Volland 4. Conclusions E.I. Givargizov, A N. Stepanova, L.N. Obolenskaya, ES Maskova, V.A. Molchanov, J. Vac. Sci. Technol. B 16 (1998)3185 a new technology based on whisker growth is [8]AN. Stepanova, E.L. Givargizov, L.V. Bormatova, VV proposed for preparation of AFM probes. The Zhirnov. E.S. Mashkova. V.A. Molchanov. J. Vac. Sc technology posseses significant advantages to the Technol. B 16(1998)678. whisker probes over standard ones as related to the [g k. etersen Prop, IEEE 70(1982)420 shapes of the probes, as well as to the properties of [11] I.W. Rangelow, Deep Etching of Silicon, ISBN 83-7085. the cantilevers on which the whiskers are grown 254-8,1996,120pp
silicon etching [9], to polish chemically the backside of the cantilever that is important for precise measurements of the AFM signal. In other words, the cantilevers have advantages in comparison with the standard silicon AFM cantilevers that are usually oriented along the (1 0 0) crystallographic face. An additional advantage of the whisker probes is the fact that the (111)-cantilevers have approximately 30% higher coe$cient of piezoresistivity in comparison with the (1 0 0)-cantilevers of a similar design [7]. It is a non-standard task to prepare (1 1 1)- oriented cantilevers by, for example, anisotropic chemical etching [9]. One of the possible solution of the problem is to use special dry etching procedure at cryogenic temperatures for deep etching [10,11]. In Fig. 9, an example of the AFM whisker probe fabricated with our technology is given. 4. Conclusions A new technology based on whisker growth is proposed for preparation of AFM probes. The technology posseses signi"cant advantages to the whisker probes over standard ones as related to the shapes of the probes, as well as to the properties of the cantilevers on which the whiskers are grown. Acknowledgements The authors thank Dr. V.G. Galstyan and Mrs. V.I. Muratova for scanning electron micrograph studies, Ms. S.V. Nosovich for sharpening of the whiskers, and Mrs. O.B. Vol'skaya for assistance in preparation of photographs. References [1] R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (1964) 89. [2] E.I. Givargizov, J. Vac. Sci. Technol. B 11 (1993) 449. [3] E.I. Givargizov, A.N. Kiselev, L.N. Obolenskaya, A.N. Stepanova, Appl. Surf. Sci. 67 (1993) 73. [4] K.L. Lee, D.W. Abraham, F. Secord, L. Landstein, J. Vac. Sci. Technol. B 9 (1991) 3562. [5] J.E. Gri$th, D.A. Grigg, J. Appl. Phys. 74 (1993) R83. [6] J.A. Dagata, J.J. Kopansky, Solid State Technol. 7 (1995) 91. [7] I.W. Rangelow, F. Shi, P. Hudek, P. Grabiec, B. Volland, E.I. Givargizov, A.N. Stepanova, L.N. Obolenskaya, E.S. Maskova, V.A. Molchanov, J. Vac. Sci. Technol. B 16 (1998) 3185. [8] A.N. Stepanova, E.I. Givargizov, L.V. Bormatova, V.V. Zhirnov, E.S. Mashkova, V.A. Molchanov, J. Vac. Sci. Technol. B 16 (1998) 678. [9] K. Petersen, Proc. IEEE 70 (1982) 420. [10] I.W. Rangelow, Plasma Surf. Eng. 12 (1997) 234. [11] I.W. Rangelow, Deep Etching of Silicon, ISBN 83-7085- 254-8, 1996, 120 pp. E.I. Givargizov et al. / Ultramicroscopy 82 (2000) 57}61 61
