Ultraviolet laser ablation of Si3N4 thin films

applied surface science ELSEVIER

Applied Surface Science 7 9 / 8 0 (1994) 146 151

Ultraviolet laser ablation of Si3N 4 thin films Y a s u o T a k i g a w a *'~, J o h n C. H e m m i n g e r Institute of Surface and hlterface Science and Department ~)f Chemistry, Unicersity of CaliI?)rnia, lrcine, ('A 9271Z USA (Received 29 October 1993; accepted for publication 1 D e c e m b e r 1993)

Abstract

A Fourier transform ion cyclotron resonance mass spectrometer (FTMS) is used to identify species ejected from amorphous Si3N4 film surfaces when high-power ArF excimer laser pulses hit them. Two peaks of Si + and ($13N4)~ ions in the spectra are obtained with the laser pulses having fluencc greater than 40 mJ/cm:. The advantage of high mass resolution enables us to identify Si + peaks in spite of the fact that Si ' (m/q = 27.976928) and N f (m/q = 28.0 056) ions are discriminated by introducing CO + (m/q = 27.9 949) ions. We find, however, that when a large amount of ions are stored in the cell of FTMS, accurate numbers of ions are not obtained due to a space charge effect.

1. Introduction

Laser-surface interactions have become of much importance not only in physics and chemistry of solid surfaces but also in the technology for chemical analyses and the fabrication of micro-electronics devices. The identity of molecular species is the starting point for developing an understanding of a wide variety of surface phenomena. It is desirable in the research with pulsed lasers to employ a mass spectrometer that operates in a pulsed mode and produces a full mass spectrum for each laser pulse. Two types of mass spectrometers that have this capability are time-

* Corresponding author• Fax: ( + 81) 7211 24 0014. 1 P e r m a n e n t address: Department of Solid State Electronics, Osaka Electro-Communication University, Neyagawa, Osaka 572, Japan.

of-flight (TOF) [1] and Fourier transform ion cyclotron resonance (FT) [2] mass spectrometers. Since both types of mass spectrometers have the advantage of high sensitivity, they have been mainly applied to identifying a small amount of atoms and molecules which are desorbed and ejected from solid surfaces that are exposed by laser pulses [3]. In addition to the advantage, FT mass spectrometers have an important advantage of extremely high mass resolution and high upper mass limit, and thus they have offered a good approach to identity of complex molecular species, biomolecules, polymers, and other highmass compounds [4]. With a special view to macroscopic phenomena in material processing T O F mass spectrometers have been exclusively employed for the species identification [5], but FT mass spectrometers not. FT mass spectrometers are considered for us to become an attractive tool in almost all fields in material processings as well as T O F mass spectrometers.

(/169-4332/94/$07.00 ~'5 1994 Elsevier Science B.V. All rights reserved SSDI 0 1 6 9 - 4 3 3 2 ( 9 4 ) 0 ( 1 0 4 4 - 2

Y. Takigawa, J.C Hemminger /Applied Surface Science 79/80 (1994) 146-151

(about 10 -1° Torr) and centered between the pole caps of an electromagnet. The intense ultraviolet short laser pulse to the surface makes the plasma emit from the surface, and ionized species are emitted into the cell. If the ablated species are neutrals they can be ionized by a pulsed electron beam impact. It should be noted, here, that throughout this paper the electron beam is not used for the ionization. The ions produced are trapped inside the FTMS analyzer cell. The applied magnetic filed causes the ions to move in small circular orbits, i.e., to drive ion cyclotron motion. The frequency of the cyclotron motion is w = q B / m , where B is the magnetic field strength and m / q is the mass-to-charge ratio of the ions. The last step, shown in Fig. lc, is the mass analysis and detection of the ions. This is accomplished by accelerating them with a high-voltage impulse and digitizing the transient signal that is produced by their coherent cyclotron motion. The transient signal is a composite of all various ion cyclotron frequencies, and a mass spectrum is extracted from it by a fast Fourier transform (FFT) calculation. The whole sequence of events is controlled by a computer and can be repeated as fast as 10 times per second. In this method a laser pulse is focused onto the surface to create a localized, rapid ablation. The FTMS acquires a complete mass spectrum for every single laser shot, producing an instantaneous snapshot of the surface composition. The signal-to-noise ratio for a single laser shot is typically 1000. In addition,

One of the authors (Y.T.) has found recently that high energy photons make silicon crystals grow in thin surface layers of amorphous silicon dioxide [6] and silicon nitride [7]. High density excitons generated in surface layers with photons play an essential role in the silicon crystal growth [8]. We apply an FT mass spectrometer to surface alteration of silicon nitride with a special purpose of clarifying advantages and disadvantage of an FT mass spectrometer in material processings. This is, at least to our knowledge, the first attempt of an FT mass spectrometer to macroscopic material processing.

2. Experimental The Fourier transform ion cyclotron resonance mass spectrometer (FTMS) used in our studies was built in one of the authors' (J.C.H.) laboratory using a 1.2T Varian electromagnet, an IonSpec model 2000 data system, and a vacuum chamber that is pumped to a base pressure of 2 x 10 -l° Tort by an ion pump and a turbomolecular pump [9]. The sequence of events for laser ablation experiments with an FTMS is shown in Fig. 1. In Fig. la a laser pulse is striking the surface of a sample. The sample is supported by a probe and positioned adjacent to one of the plates of the FTMS analyzer cell. The cell is simply a box with six square stainless steel electrodes. It is mounted inside an ultrahigh-vacuum chamber

(

147

(c)

(b) !

J

~J~ lJ [

2kJ

sample B

lift ]

Impulse B

B

Fig. 1. Sequence of events in an FT mass spectroscopic experiment.

I If

6ToADC

Y Takigawa, J.C. Hemminger /Applied Surface Science 79 / 80 (1994) 146-151

148

FTMS also provides high mass resolution, about 120000, on a routine basis discriminating between different species with the same nominal mass such as CO, N 2 and Si, and providing accurate mass measurements to confirm elemental composition. Amorphous Si3N 4 films were prepared by means of a plasma-enhanced chemical vapor de-

position method [7]. Ammonia and silane gases were used to produce Si3N 4. The films were deposited on SiO 2 buffer layers of 100 nm thickness which were prepared on silicon single crystal wafers. The film thickness is 900 nm, which was much thicker than the penetration depth of 56 nm for ArF excimer laser photons into the film. Pulses with 20 ns duration from an A r F excimer laser

(a)

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ratio)

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m/z ( M a s s - t o - c h a r g e ratio) Fig. 2. Typical spectra obtained with an FT mass spectrometer when a Si3N 4 sample surface is exposed to an ArF laser pulse of 60 m J / c m 2. Broadband spectrum in the range 15-80 (a), narrow band spectrum in the range 27-30 (b), and the expanded view spectrum around 28 (c).

Y. Takigawa, J.C. Hemrninger/Applied Surface Science 79 / 80 (1994) 146-151

149

1OO--

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,,i,,,

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ratio)

m/z (Mass-to-charge

Fig. 2 (continued).

(193 nm) were focused onto the sample surface 0.3 × 0.5 mm 2. The laser fluence was adjusted by attenuating the output of the laser.

3. Results and discussion The effects of different laser fluence and numbers of laser shots were investigated. It should be noted, however, that spectra were not found to change significantly with laser fluence, if pulses

having fluence above 50 mJ/cm 2 were used. Fig. 2a shows a typical mass spectrum obtained with a laser fluence of 60 mJ/cm 2 without further ionization by electron impact. Over the mass range of 15-80, two peaks always appeared around 28 and 75. It may be emphasized here that only ions were formed by the laser alone. The peak of 75 is identified as (Si3N4) 2+, in spite of that we cannot explain at present how the ions were formed. With regard to the peak of 28, there are three possibilities of Si +, N~-, and CO +, all of which

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0

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0.15 [J/cm a ]

0.2

0.25

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Energy

Fig. 3. Peak intensities of Si + (a) and (Si3N4) 2+ (b) as function of laser fluence.

0.15 lJ/cm 2 ]

0.2

0.25

}~ Takigawa, J. (7. Hemminger / Applied Surfaee Science 79 / 80 (1994) 140- I51

150

Table 1 Mass v a l u e s and n a t u r a l a b u n d a n c e of silicon isotopes Species Si Si Si (70 N,

Isotope 28 • g, Sl ~,,~$1 )4Si

Mass (g)

N a t u r a l a b u n d a n c e (c,~)

27.976 928 28.976 496 29.973772 27.994 915 28.0056

92.23 4.67 3.1(I

have almost the same mass value and also the possibility to be contained in the cell. We will show below how determining it to come from Si. The masses of these atom and molecules are listed in Table 1, together with the natural abundance of silicon isotopes [10]. When a CO gas was introduced into the cell to calibrate the mass values, a new peak appeared around 28 as shown in Fig. 2b. A small peak appeared around 29 is 29 identified as ~aSt. The extended view spectrum 28 • shown in Fig. 2c indicates the ~4S~ peak lower than the CO peak. On the basis of these observations, we could label the peak appearing around 28 as Si +. At 30 m J / c m z fluence, essentially the same pattern of peaks is observed after about 12 laser shots at the same points. After about 3 laser shots at 100 mJ, only one peak of Si + was observed, which indicated that Si3N 4 films were blown off by the laser shots. In Figs. 3a and 3b are shown the peak intensities of Si ÷ and (Si3N4) 2+ ions as functions of laser fluence. There is an apparently critical value of 40 m J / c m 2 in the laser fluence for the ablation. The next interesting point shown in these figures is that the intensities of both peaks decrease with laser fluence above 60 m J / c m 2. A large amount of species must be ejected from the sample surface, when laser pulses having large fluence hit the surfaces. If there are too much ions stored in the cell, a space charge effect could not be neglected. Part of the ions stored in the cell collide with the cell walls and escape away out of the cell. This is a reason why the ion numbers decrease with the laser fluence. Such behavior was observed for both species of Si + and (Si3N4) 2+.

We carried out an observation of the laser irradiated surfaces with X-ray photoemission spectroscopy (XPS). XPS spectra of Si 2p showed that elemental silicon was found in the surface layers of samples in addition to the presence of Si3N4, which coincided with the results obtained so far [11]. Throughout the present experiments, we did not obtain the results indicating the presence of nitrogen molecules. At this stage, we cannot understand why neither nitrogen atoms nor molecules were not observed. This point is a thing to be solved in the near future. It must be worthwhile noting that we discuss about a mechanism for the laser-induced surface reaction. We carried out numerical calculation to solve the one-dimensional thermal diffusion equation in order to estimate the surface temperature rise caused by the laser irradiation [12]. With 40 m J / c m 2 of the threshold value for ejection of Si + and ($3N4) 2+ ions, the t e m p e r a t u r e is about 400 K. It is well known that decomposition of Si.~N4 into Si and N occurs at elevated temperatures, but it does not happen at a temperature as low as 400 K. We can conclude that the decomposition proceeds via a photochemical reaction. The fundamental bandgap energy of Si,~Na is around 5 eV and thus the ArF laser photons have an ability to induce a band-to-band transition of electrons. The electronic excitation, which generates excitons with high density, may play an important role for the reaction.

5. Conclusions

We have shown that the laser-induced photochemical reaction proceeds with decomposition of Si3N a and ejection of ion species. The use of Fourier transform mass spectrometer to collect a complete mass spectrum for every laser pulse is a versatile detection method that is compatible with the pulsed nature of the laser ablation experiments. With the advantage of the high mass resolution, Si + is discriminated from N~- and CO + ions in spite of the fact that these three ions have almost the same mass. A space charge effect was pointed out not to be neglected when a large amount of ions were stored in the cell. We forced,

Y.. Takigawa, J.C Hemminger /Applied Surface Science 79/80 (1994) 146-151

however, that FTMS must be a powerful tool for the laser ablation experiments.

Acknowledgements This work was done while one of the authors (Y.T.) was staying at UCI from April, 1992 to March 1993. He would like to acknowledge to Professor Robert T. McIver, Jr., Dr. Yunzhi Li, Dr. Jingyu Huang, and Mr. Craig Tindall and Mr. Vianney Kang for useful discussion and helpful suggestions and comments for the experiment.

[3]

[4]

[5] [6] [7]

[8] [9]

References [1] M.S. de Vries, D.J. Elloway, H.R. Wendt, and H.E. Hunziker, Rev. Sci. Instr. 63 (1992) 332l. [2] G.M. Alber, A.G. Marshall, N.C. Hill, L. Schwelkhard, T.L. Ricca, Rev. Sci. Instr. 64 (1993) 1845; B. Asamot, Ed., FT-ICR/MS Analytical Applications of

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Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (VCH, Deerfield Beach, FL, 1991). Y. Li, R.T. Mclver, Jr. and J.C. Hemminger, J. Chem. Phys. 93 (1990) 4719; D.M. Lubman, Ed., Lasers and Mass Spectrometry (Oxford University Press, Oxford, 1990). D.P. Land, C.L. Pettiette-Hall, D. Sander, R.T. Mclver, Jr. and John C. Hemminger, Rev. Sci. Instr. 61 (1990) 1674. For example: D.M. Lubman, Ed., Lasers and Mass Spectrometry (Oxford University Press, London, 1990) p. 84. Y. Takigawa, K. Kurosawa, W. Sasaki, K. Yoshida, E. Fujiwara and Y. Kato, J. Non-Crys. Solids 116 (1990) 293. M. Ohmukai, H. Naito, M. Okuda, K. Kurosawa, W. Sasaki, T. Matsushita, Y. Tsunawaki, S. Nozawa and T. Igarashi, Jpn. J. Appl. Phys. 32 (1993) Ll062. K. Kurosawa, W. Sasaki, Y. Takigawa, M. Ohmukai, M. Katto and M. Okuda, Appl. Surf. Sci. 70/71 (1993) 712. M.G. Sherman, J.R. Kingsley, D.A. Dahlgren, J.C. Hemminger and R.T. Mclver, Jr., Surf. Sci. 148 (1985) L25. M. Windholz, S. Budavari, M.N. Ferting and G.A. Sch6nberg, Eds. Table of Molecular Weights (Nerck, Pahway, 1978). C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder and G.E. Muilenberg, in: Handbook of X-ray Photoelectron Spectroscopy (Perkin-Elmer, Eden Prairie, 1980). V.M. Donnelly, J.A. Mucha and V.R. McCrary, J. Appl. Phys. 67 (1990) 3337.