Femtosecond laser pulse nanolitography using STM tip

Transcription

1 Femtosecond laser pulse nanolitography using STM tip Yu.E.Lozovik, D.V.Lisin, A.I.Ivanov, V.O.Kompanets, Yu.A.Matveets, S.V.Chekalin, and S.P.Merkulova Institute of Spectroscopy, RAS, , Moscow r-n, Troitsk, RUSSIA lozovik isan.troitsk.ru fax: (+7)(095) tel.: (+7)(095) ABSTRACT The new nanolithography technique realized experementally using the local field of femtosecond laser pulses enhanced in nanoscale region due to lighting rod effect and to the excitation of local resonances in STM tip substrate system. Surface topography analysis in STM mode demonstrates the controlled surface modification in a suitable regime of intensity parameters and femtosecond laser pulses focusing in the STM tip region. Keywords : Femtosecond,nanolitography,STM,nanoscale,modification The problems of creation of nanostructures, ultrahigh density recording of information, investigation and recording ofultrafast processes in nanostructures as well as finding ofsuitable materials and techniques for these purposes belong to the most actual basic and applied problems in semiconductor nanostructures physics. Nanolocal research of influence of ultrashort laser pulses on the solid state surface is of interest from this point of view.1'2 We used here the proposal"2 of using local electromagnetic fields enhanced in nanoscale region near STM tip due to the lighting rod effect and the excitation of local resonances (see, e.g.3) in small regions of the STM tip substrate system for nanolithography and nanolocal research of ultrafast processes.1 Using of ultrashort laser pulses for nanolocal studies has the following advantages. Femtosecond laser radiation allows to create non-stationary states of sample, which may lead to new non-thermal mechanisms of instability and photoinduced phase transitions on the surface. One of the phenomena of this kind is a specific femtosecond melting of solids. Another example is appearance of the photoinduced by femtosecond laser irradiation quasi-one-dimensional surface relief with period 4OOO 2 see,4 and Refs. herein. The last method is available also for creating of quantum dots etc. using two normal laser beams.2 Moreover, femtosecond radiation allows realization of peak superstrong fields without non-controlled destruction of the sample. The surface of the sample was irradiated by femtosecond laser radiation focused in the region of STM tunnel junction. The radiation of Ti:Sa laser with wavelengths 812 and 406 nm, pulse duration fs and repetition rate 82 MHz was used. The radiation power W was changed by means of light filters with different transmittance up to the maximal power mw on wavelength 812 nm. The maximal power of 2nd harmonics (406 nm) was 5 7 mw. Diameter of light spot on the surface was about 1 mm. The dependence of radiation effects on exposition time was also investigated in wide range of exposition times from 2 sec. to 15 mm. The angle of incidence t9oflaser light was limited by the construction of the setup : We used the pyrolithic graphite as a sample. We used two experimental procedures. At first, we investigated the possibility of nanolocal laser pulse induced modification of the sample surface in the region nearly the STM tip. At the beginning we obtained the several initial images of surface by STM, reproducibility of images was checked and a lateral drift was estimated. Then we "draw" by laser pulses nearly STM tip one of the figures: point, line or X-like "letter" by means of positioning STM tip in appropriate points and switching on the laser light. Note that the positioning in a point means that the tip stands in the point at usial for tunnel regime distance, then shifted up on 40 X and then returned to the initial distance. The lines were drawn as a set of separate points. After laser pulse exposition of STM tip we take a duplicate scan of affected region to find photoinduced surface changes. The second part of experiment devoted to the study of laser radiation effect on STM response during the continuous scanning process in STM. At first we take the initial scan, then we take the duplicate scan of the same region when the laser light of different intensities illuminated the tunnel junction. After that we take another scan without laser irradiation. 424 SPIE Vol X/99/$1O.OO

2 The experiments revealed the following effects: 1) The appearance of X-like structure after drawing the letter "X" by the local laser field nearly STM tip by means of positioning STM tip in appropriate points. Fig.la shows the initial region, Fig.lb the same region after drawing the "letter" X and exposition by the laser pulses with power mw, Fig.lc the next scan of the same region. Fig.2a shows the region after horizontal line drawing on the top of the hill in the same conditions, Fig.2b - after line drawing with the power of laser radiation 8-10 mw. One can observe on this Figure also the typical 200 X width photoindiced line described in more details below. The whole size of long-life X-like structure was 2000 x 8000 A with line thickness of the order 1OOO A and 15 A depth. 2) Appearance ofshort-lifetime groove with 200 A width and 50 A depth due to writing by the tip and simultaneous femtosecond laser irrradiation. The typical lifetime was about 1 minute which was estimated by its disappearance at the next scan. Fig.3a shows the initial region, Fig.3b the same region after line drawing from the left to the right with power of laser radiation mw (on the angle of about to the x axis). The groove is not seen on this Figure due to the unsufficient resolution (used step 200 A was too large); Fig.3c shows with 50 X step the part of region where line was drawn. The part of the groove is seen on this Figure. Note that photoinduced groove survives at least two scans, i.e. its life time is several minutes. It should be noticed also that the groove has continuous form unstead of consisting of the sequence of points. It is possible this feature is due to a small change of value of the local electromagnetic field on the surface due to the tip displacement only up to 40 A. It may mean, for example, that the typical radius of curvature of STM tip which is essential for the local field enhancement is much larger than 40 A. On Fig.3d the whole region observed at the repeated scan with 100 A step is given. The groove disappeared. We have not seen the groove any more in the same region after two scans. The next case of appearance the groove of this type is shown on Figs.4a,b,c. Fig.4a corresponds to the initial scan, Fig.4b after drawing the "letter" X, Fig.4c the same region, the groove disappeared. 3) The "standart groove" appearance after the exposure by nonreduced (maximal) power of laser radiation (by means of positioning the tip at any point or by continuous scanning). This effect consists in the appearance of wide horizontal groove about 2000 A wide and about 150 A depth immediately after affect of nonreduced laser radiation focused in the tip region. This groove completely disappears atfer the times about 1 minute and the initial topography of the surface comes back. The examples are shown on Figs.5a,b,c. Fig.5a corresponds to initial scan, Fig.5b was taken after drawing "X" with the maximum power of light. One can observe the typical "standart groove" on this Figure. We have observed such effect after every affecting on the sample surface with maximum intensity. Fig.5c shows the next scan of the same region taken immediately after one on Fig.5b. The groove disappeared, the surface came into initial state. We have not observe this effect with reduced power of light. It is possible the nature of this phenomenon the same as described above in Sec.2 but the region of influence more larger than in that case. 4) The picture "inversion" during scanning of the illuminated sample. Fig.6a corresponds to the initial scan, Fig.6b gives the image obtained when the sample was illuminated by femtosecond laser light. One can see the hill with the sizes corresponding to the "standart groove", but with height about 100 A. Fig.6c corresponds to the repeating scan with full power of laser light, the hill height is about 500 A, Fig.6d gives the same region immediately after switching off the lase:r radiation. One can observe the "standart groove" on this Figure. Fig.6e corresponds to the the next scan; the groove disappeared, the surface came back to the initial state. The scanning time was about 1 minute. The appearance of long-lifetime structure after the nanolocal affecting of femtosecond laser radiation demonstrates promising of nanolithography method discussed in the paper. Optimal regimes and detailed mechanisms of nanolocal influence of femtosecond laser pulses need to be investigated in more details. It is possible, this effects is due to the attracting of photoinduced vacansions. Analogeous mechanism is possibly works also in the case of obtained by femtosecond laser exposition with the normal incidence long-range lattice on the pyrolythic graphite surface with the period of 100 nm (see Fig.7). It could be intresting to connect effects described in Sections 2 4 with a direct influence of laser radiation on the STM tip, tunneling and photoemission from the tip. The field enhanced in a local region near STM tip can be also used for the purposes of controlling of nanolocal chemical reactions provided by special STM tip4 and also for the stimulation of nanolocal photochemical reactions [1]. 425

3 ACKNOWLED GEMENTS The work is supported by Russian Foundation of Basic Research, INTAS and by Program "Solid Nanostructures". REFERENCES 1. Yu.E.Lozovik,S.P.Merkulova (to be pubi.) 2. Yu.E.Lozovik, A.V.Klyuchnik, S.P.Merkulova, this issue. 3. Yu.E.Lozovik, A.V.Klyuchnik, in: "The Dielectric Function of Condensed Systems", Eds. L.V.Keldysh et. al. Elsevier Science Publisher B. V.,1987; A.V.Klyuchnik, Yu.E.Lozovik, A.B.Oparin, Phys. LeU.A, 179, 372(1993); Ay. Klyuchnik, Yu.E. Lozovik, A.V. Solodov, Zh. Tech. Fyz. 65, N 6, , (1995). 4. S.P.Merkulova, L.A.Shelepin, A.A.Shubin, FIAN Proceed. 177, (1987). 5. Yu.E.Lozovik,S.P.Merkulova et al., Phys. Lett.A 9, 3290 (1994); Yu.E.Lozovik,S.P.Merkulova,A.M.Popov, Phys.Low-Dim.Sr. N12, 203 (1995). 426

4 Figure lb. 427

5 Figure 3a. Figure 3b Figure 3d. 428

6 Fiaure 4c. o iooo Pigure Sb. Pigure Sc. 429

7 Figure 6a. Scale X:l000 A; Y:l000 A; Z:lO A Figure Gb. Scale Z:50 A z Figure6c. Scale Z:lOO A V Figure Ge. Scale Z:IOA 430

8 a 'TI II ID 'a