Adsorption and desorption processes of Cl on a Si (111) 7 × 7 surface

applied surface science ELSEVIER

Applied Surface Science 79/80 (1994) 95-99

Adsorption and desorption processes of C1 on a Si( 111 )7 x 7 surface T. Yonezawa, H. Daimon *, K. Nakatsuji, K. Sakamoto, S. Suga Department of Material Physics, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan (Received 13 October 1993; accepted for publication 17 November 1993)

Abstract

The adsorption process and desorption mechanism of C1 on the Si ( 111 ) 7 x 7 surface were studied by means of LEED, AES, ESD (electron stimulated desorption) and ESDIAD (electron stimulated desorption ion angular distribution) methods. The saturation behavior in the adsorption process revealed by means of AES with a low electron flux was found to follow the Langmuir's adsorption equation. The LEED result suggested that the surface corrosion did not take place during the C1 exposure. In the ESD experiment, only CI + was detected as the positive ion species. In the ESDIAD study, C1+ ions were predominantly observed in a cone of about 10° around the direction perpendicular to the surface. The dependence of the C1+ desorption yield on the incident electron energy showed two threshold energies (10-20 and 200 eV). These two thresholds correspond to two different excitation mechanisms for ion desorption. The higher (lower) threshold is ascribed to the CI 2p (CI 3s) excitation. However, we could not detect any noticeable difference in the kinetic energy distribution of the desorbed ions through the whole excitation energy range. Hence, it is revealed that these two excitations result in the same final state for desorption as suggested by the KF model.

1. Introduction

The Si( 111 )7× 7-C1 surface was recently extensively studied by EELS (electron energy loss spectroscopy) [ 1 ], EXAFS (extended X-ray absorption fine structure) [2], XPS (X-ray photoemission spectroscopy) [3 ], T P D (temperature programmed desorption) [4], LITD (laser induced thermal desorption) [4], STM (scanning tunneling microscopy) [ 5 ] and so on. This surface is very important because it appears in the initial stage of C1 etching for manufacturing semiconductor devices. * Corresponding author. Fax: (+81) 6 845 4632.

The ESD is known to be very useful for the study of desorption mechanisms. But most of the experiments on desorption from the Si (111 )7 x 7-C1 surface have so far been made by means of heat or heat-assisted processes. Though there was reported one non-thermal experiment on PSID and XESD (X-ray induced electron stimulated desorption) [6], the former was made at a much higher excitation energy than that in the present study and the latter probed an indirect process in contrast to direct ESD. To know the desorption mechanism in more detail, we studied in the low excitation energy region. Here, ESD and other experiments were made under non-heat condition, i.e. all ex-

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T. Yonezawa et al./Applied Surface Science 79/80 (1994) 95-99

periments were made at room temperature with a low flux and a relatively low energy electron beam. We are confident that the desorption process o f our study is caused not by a heat or heat-assisted process but by electric excitation.

2. Experimental

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The base pressure o f the U H V (ultrahigh vacuum) chamber was less than 1.0 x 10 -1° Torr. The sample Si wafer was P doped, with an electric resistivity of 2.4-4.0 f~.cm and a size of 10 x 13 x 0.6 m m 3. We obtained a clean surface by direct current heating to 1520 K for 5 s about 5-10 times in U H V and checked its cleanliness by the observation of a sharp 7 x 7 LEED pattern and no Auger signals of carbon and oxygen. We used a two-dimensional display type spherical mirror analyzer [ 7-10 ] for all experiments which could analyze both the energy and the angular distribution of charged particles at the same time. Using the T O F (time-of-flight) technique, the mass of ions was analyzed at the same time. Since this analyzer has a wide acceptance angle (+60 ° from the surface normal) and a very high sensitivity, it enables us to do a reliable measurement with only a low flux electron beam (~ 20 nA, 7.0 × 10 -6 J/mm2-s). Such characteristics are very useful for AES and ESD studies because the electron stimulated desorption probability of C1 is so high that the C1 coverage decreases fast under the usual measurement conditions ( ,-~ 10/zA) for LEED and AES. As a result, we are almost free from surface damages and heat induced processes for a few hours. In fact, we have found that the decrease of the C1LVV Auger signal is less than 0.5% per hour. Hence, we can reliably measure the CI coverage. The chlorine for the dosing was generated by the electrolysis of solid AgC1 in the U H V chamber [ 11 ]. We used a short glass tube (4 cm long and with a diameter of 5 m m ) halfly filled with solid AgC1. Two electrodes (Ag cathode and Pt mesh anode) were attached to AgC1. Using a tungsten filament heater around the glass tube, AgCI was slightly heated up and its resistance

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CI exposure (A see) Fig. 1. Saturation behavior of C1 on Si ( 111 ) 7× 7. Circles show the experimental coverage estimated by the AES intensity ratio of C1LVVto Si LVV, and they are normalized with saturation coverage (0/0s). The solid line shows the fitting curve of Eq. ( 1) by the least-squares method. was decreased by the increase of the ion mobility. Because the generation rate of CI was nearly constant as far as the electrolysis current is constant, we could control the amount of C1 dosing by the electrolysis time. In the present experiment, we required a current of 10 mA for more than 10 s to obtain a coverage of a few percent of the saturation. By means of this electrolysis method, only a small amount of C1 was required for the experiment because the flux had a high directionality. The total amount of C1 in this study was much less than that in the experiment done by directly introducing the C12 gas. Consequently, chamber wall corrosion or other harmful effects did not occur in our experiment.

3. Results and discussion 3. I. Adsorption a n d saturation behavior

The relative coverage of C1 estimated by the Auger intensity ratio ofCl LVV to Si LVV is plotted by open circles in Fig. 1 as a function o f the C1 exposure. The abscissa is the quantity o f electricity in the electrolysis with a unit of A.s. These data very well follow Langmuir's adsorption equation, O ( x ) = Os[1 - e x p ( - a x ) ] ,

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ION ENERGY (eV) Fig. 2. Kinetic energy distribution o f t h e C1 + ion. Since we biased the s a m p l e to + 15.0 V, the x - a x i s is shifted so that + 15.0 b e c o m e s 0 eV. W e defined the v a c u u m level o f the analyzer as 0 eV and, therefore, t h e ions with a negative kinetic energy are seen.

where 0 is the relative coverage, x is the dosing amount, 0s is the saturation coverage, and a is a constant. The solid line in the figure shows the fitting curve of Eq. ( 1 ) by the least-squares method. We can get this equation easily if we suppose that the sticking probability is in proportion to the uncovered area 0s-0. Since 0 (x) saturates at infinite x, we normalized them with the saturation coverage 0s. Hereafter, the coverage of C1 is expressed by its ratio to 0s. The LEED pattern did not essentially change from 7 x 7 up to the saturation coverage. This LEED result and the saturation of the coverage mean that the surface corrosion does not take place below the 7 x 7 structure in this experimental condition. This conclusion agrees with the STM result [5] which shows that the stacking fault of 7 × 7 is not destroyed by the adsorption of C1. This STM result has also suggested that there are three types of chlorides, SIC1, SiC12 and SIC13.

3.2. Desorption 3.2.1. Desorption products In the ESD study, we have analyzed only positive ions. The kinetic energy (Ek) distribution of the C1+ ion is shown in Fig. 2. The coverage is estimated as 0/0s = 0.57. The resolution of the analyzer is +0.15 eV. Most of the desorbed ions we analyzed have a very low kinetic energy (,,~ 0 eV). We biased the sample at + 15.0 V to accelerate the desorbed ions. Otherwise, they are

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easily disturbed by an electric or magnetic field because they have very low kinetic energies. Since positive ions are accelerated by this bias voltage, we have shifted the horizontal axis so that Ek = + 15.0 eV becomes 0 eV. From the result shown in Fig. 2 that the ions of negative kinetic energies after this shift are observed, we conclude that the vacuum level of the sample is lower than that of the analyzer. If we define the vacuum level at the sample surface as 0 eV, the kinetic energy of the ions becomes positive. Next, we have analyzed the mass of the desorbed ions by a TOF technique. The time between the excitation by the electron beam pulse and the detection of the signal is measured. This time (about several Fts) is almost equal to the flight time because it is much larger than the excitation-desorption time (about several hundreds ps). Hence, the time is in proportion to the square root of the mass of the ions if they have the same energy. Figs. 3a and 3b are the TOF spectra at O/Os = 0.5. We biased the sample at +9.0 V (Fig. 3a) and + 180 V (Fig. 3b) to accelerate the desorbed ions, and detected the ions with a kinetic energy of nearly + 9.0 and + 180 eV. The peak in these figures corresponds only to the 35 amu ions (C1 + ). The C1 isotope ions of 35 and 37 amu are not resolved because of the insufficient experimental resolution. Through the present study it is found that C1+ is the only desorbed positive ion species irrespective of the coverage and ion kinetic energies. The present result is in a remarkable contrast to the results of the heat process [4], where the main desorption products are SiClx (x = 1,2, 3), though they are not ions but neutrals. Fig. 4 shows the angular distribution of the desorbed ions (ESDIAD) from the sample with a coverage of about 60% of the saturation. The sample bias is + 15.0 V, and the pass energy of the ions is set to 15.4 eV. The excitation electron energy is 350 eV. It is known that the angular distribution shows approximately the bonding direction of the desorbed species at the surface. As shown in Fig. 4, C1+ is detected within about 10 ° of the polar angle 0 from the surface normal. For the azimuthal direction, we have ob-

T. Yonezawa et al./Applied Surface Science 79/80 (1994) 95-99

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served an almost isotropic angular distribution. T h e white cross line merely m e a n s the center o f the C C D c a m e r a and does not m e a n the direction p e r p e n d i c u l a r to the surface. F r o m these resuits, the CI atoms are supposed to be b o n d i n g almost perpendicularly to the surface. C1 atoms in other bonding configurations m a y exist, but we suppose they can hardly desorb as ions because o f the image potential effect and higher neutralization probability due to their higher emission angles. U n d e r our experimental conditions, the kinetic energy o f C1 + is so small (,,- 0.5 eV) that the C1 atoms in tilted b o n d s c a n n o t desorb as ion because the kinetic energy c o m p o n e n t perpendicular to the surface is not sufficient to overc o m e the image potential barrier.

3.2.2. Excitation mechanisms for desorption Fig. 5 shows the variation o f the C1 + yield versus the incident electron energy. T h e low energy thresholds for regions I and II are observed at about 10-20 and 200 eV, respectively. We con-

Fig. 4. ESDIAD pattern of C1+ for a coverage of about 60% of the saturation, sample bias voltage + 15.0 V, the pass energy of the ions 15.4 eV, the excitation electron energy 350 eV. The ions are predominantly observed in a cone of about 10 ° around the direction perpendicular to the surface.

clude that region II represents the desorption caused by the C12p core excitation, c o m p a r e d with the C12p core binding energy o f about 200 eV. T h e saturation b e h a v i o r o f region I a r o u n d 90 eV m a y be due to the absorption o f higher energy electrons by the excitation o f the Si 2p core hole. We could not d e t e r m i n e the accurate threshold energy in region I because o f the re-

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T. Yonezawa et al./Applied Surface Science 79/80 (1994) 95-99

duced signal-to-noise ratio in this region. In the separately performed PSD (photon stimulated desorption) experiment with synchrotron radiation [ 12 ], however, the threshold energy of region I was found to be 17 eV and the trigger for the desorption was concluded to be the excitation of the C13s core hole. So the ESD threshold in region I may be also resulting from the C13s core excitation. We could not, however, find any noticeable difference in the kinetic energy distribution of the desorbed ions throughout the incident electron energy range 5-350 eV. Hence, we conclude that the two different excitations (C12p and C13s core hole excitation) result in the same final state for ion desorption. We can explain the present experimental result with the KF (Knotek-Feibelman) process [ 13 ] following to the C13s or CI 2p core hole creation.

4. Conclusion We studied the Si(111)7x7-C1 surface by LEED, AES and ESD. By means of AES with a low electron flux, we observed the saturation behavior of C1 adsorption on Si( 111 )7x7, which followed Langmuir's adsorption equation. Surface corrosion which might destroy the 7 x 7 structure did not occur by C1 adsorption at room temperature, which is in good agreement with the STM result [5]. In the ESD study, we could observe only Cl + ions as the desorption species. The angular distribution of the desorbed ions was almost confined in a narrow cone perpendicular to the surface. This means that CI is bonding perpendicularly to the surface. We measured the incident energy dependence of the C1+ yield. From the threshold energy of this spectrum, we concluded that the C12p core excitation is one of the origins of Cl +

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desorption. Since we have found no difference of the kinetic energy spectrum of the desorbed ions within the employed energy range, we consider that these two excitations lead to the same final state before desorption. We can explain the experimental result with the KF model.

Acknowledgements The authors are much obliged to Professor Y. Murata for useful discussion and advice about the C1 source. The work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture.

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