750 Nuclear Instruments and Methods in Physics Research B39 (1989) 750-753

North-Holland. Amsterdam

IN SITU PA3TERNLNG OF Si,N, BY AN ION-BEAM-INDUCED GAS SURFACE REACTION

Zheng XU ‘), Kenji GAMO , ‘) Takao SHIOKAWA 2, and Susurnu NAMBA ‘)

*I Fact&y of Engineering Science, Osaka Unioemity, Toyonaka, Osaka 560, Japan

2J The Institute of physical and Chemical Research, Wake-shi Sairama, 351-01, Japan

Basic characteristics of ion-beam-assisted etching of SisN,, which employs various 50 keV inert gas ion beams and a XeF2 ambient, have been studied by means of XPS and AES in order to apply the method to in situ etching. Various 50 keV inert gas ion

beams were used for bombardment in a XeF, atmosphere. From XPS measurement it was found that XeF, is dissociatively

chemisorbed and no Xe is detected on the surface. Virgin SisN, adsorbs about a monolayer of fluorine and tends toward saturation

for further exposure. A F,YN,z(SiF,) precursor is identified from the characteristic core level chemical shifts. The occurrence of this

precursor makes surface bonds weak, and consequently, the bond would be easily broken by the ion bombardment and result in

enhanced etching. For Xe” a significant enhancement, up to 160 times larger than physical etching, is achieved. From AES it was observed that carbon ~nta~nation can be eliminated by introducing XeF,.

1. Introduction

High resolution clean patterning processing is im- portant for the fabrication of microelectronic devices. Usually patterning requires a complicated lithography and reactive ion etching techniques in which samples have to be removed from a UHV chamber for succes- sive processing. Conta~nation of the surface is thus inevitable. A UHV in situ patterning technique is one of the most feasible ways to simplify the patterning process and obtain a clear surface. Several in situ meth- ods have been tried in recent years [l-3]. Among these focused ion beam etching has manifested its distinctive advantages in maskless submicrometer patterning, e.g. a fine focused beam spot less than 0.1 pm, a higher efficiency than electron beams, etc.

For focused ion beam etching, physical sputter etch- ing is unacceptable because of its low etching rate, redeposition and heavy damage on the etched surface. These problems were solved by introducing reactive gas during ion beam bombardment, or by ion-beam-assisted etching (IBAE) [3]. High-quality Si,N, films have potential applications to microelectronic devices [4]. It was reported that the reaction of Si,N, with XeF, can be induced by electron beam irradiation and a rather high etching rate can be achieved [5]. It can be expected that the etching rate of Si,N, will be enhanced further by ion beams because of a cross section larger than in the electron case.

Che~so~tion of reactive gas phase species on a surface is the key step for IBAE [6,7], and a better understanding of chemisorption is beneficial to the de- velopment of in situ patterning. In this study, in situ X-ray photoelectron spectroscopy (XPS) was employed

0168-583X/89/$03.50 Q Elsevier Science Publishers BV (North-Holland Physics Publishing Division)

to investigate the chemisorption of XeF, on a Si,N, surface. 50 keV inert gas ion beams irradiated a Si,N, surface in the ambient of XeF, to reveal etching char- acteristics.

2. Experimental

Si,N, specimens used in this study were formed on Si(100) substrates by plasma-enhanced chemical vapor deposition (PECVD). The film thicknesses were 0.1 pm for the XPS measurement and 1 pm for IBAE. To measure the composition and the chemical bonds of the adsorbed layer, Si,N, samples were mounted in a sam- ple chamber which was attached to the main chamber for XPS spectroscopy and separated by a gate valve as shown schematically in fig. 1. Before introducing XeF, gas. the sample stage was moved to touch the gas- supplying cap using the manipulator. The supply rate of

GAS INLET

XPS

c

ELIVERY BAR

Fig. 1. Experimental setup for the in situ XPS measurement.

---a-- MANIPULATOR

2. Xu et al. / In situ patternrng of St, N4 751

the XeF, was controlled by a needle valve and the pressure inside the cap was measured by a Pirani gauge. With this arrangement, the gas could be introduced up to several Torr only in the cap, and consequently ero- sion to the sample chamber was suppressed. After XeF, exposure, the vacuum could resume in a few seconds, the sample can be quickly transferred to the UHV XPS chamber for measurement. The sample chamber was evacuated to a background pressure of better than 1 X lo-’ Torr by a turbomolecular pump before each exposure run.

IBAE was performed in a subchamber which was mounted inside the main chamber of an ion implanter

WI.

3. Results and discussion

3.1. Exposure of Si, N4 to XeF,

When a virgin Si,N, film is exposed to XeF,, the surface coverage increases with increasing exposure and shows a tendency to saturate for further exposure as shown by open circles in fig. 2. The amount of fluorine coverage was calculated from XPS peaks using the equations proposed by Chuang [9]; the incident fluorine flux was obtained by multiplying the gas pressure with the exposure time. The sticking probability was of the order of lo-‘, which is due to surface contamination. Xe could not be detected with XPS at the adsorbed surface and no etching was observed, indicating that XeF, was dissociatively chemisorbed on Si,N,. A fit of the data in fig. 2 gave a value of about 3.3 X lo-’ for sticking probability. Assuming this value, the relative coverage was calculated as a function of the incident

1.0 1’ 1.0

0.8

Y d

0.6 2

8

B 0.4 ;

2

2 0.2 0

o”vo 0 50 100

XeF2 EXPOSURE (Torrrmin.)

Fig. 2. Fluorine coverage on Si,N, evaluated by XPS vs XeF, exposure. The solid line is the calculated relative coverage

based on the monolayer adsorption model.

666 664 660

?I BINDING ENERGY @VI

404 400 396

b BINDING ENERGY (ev)

106 102 98

c BINDING ENERGY (eV)

Fig. 3. (a) F(ls) XPS spectra of Si,N, surface exposed to XeF,: (i) clean, (ii) 200 mTorr/45 min, (iii) 3.0 Torr/45 min,

(iv) 6.0 Torr/45 min. (b) XPS spectra of N(ls). (c) XPS spectra

of Si(2p). The Si(2p) and N(ls) spectra are omitted in (ii)

because they are almost the same as those in (i).

fluorine flux using a monolayer model. As is shown by the solid curve in fig. 2, it is in good agreement with the measurement.

The XPS spectra of F(ls), N(ls) and Si(2p) under various exposure conditions are shown in fig. 3. Ap- parently, with increasing exposure pressure the intensity of the F(ls) peak increases and the full width at half maximum (FWHM) becomes broad, accompanied by an appreciable chemical shift. The broadening of FWHM indicates that adsorbed fluorine exists in several chemical environments. At low exposure the 685 eV is the major peak, but at high exposure 687.3 eV is the dominant component. Correspondingly, as a result of adsorption the major peak of N(ls) shifts from 400 to 402.5 eV and that of Si(2p) from 102 to 103.8 eV. This can be interpreted on the basis of the partial atomic charge model [9]. A positive shift of N(ls) and a corre- sponding positive shift of F(ls) imply that there exist N-F bonds on the Si,N, surface. The 685 eV peak of F(ls) and a corresponding positive chemical shift of

VII. INORGANIC MATERIALS

752 2. Xu et al. / In situ patterning of Si, N4

Si(2p) to 103.8 eV can be attributed to the formation of SiFJike species on the surface [9]. The broad bands for N(ls) and Si(2p) are good evidence that there are still Si-N bonds on the surface. Therefore, we may draw a conclusion from the discussion above that F,,N,(SiF,) chemical bonds are formed on the surface when Si,N, is exposed to XeF,. Here x +y = 3 with 0 < x G 2 must hold. Normally it would be easier for an incident par- ticle to remove this adsorbed layer than a virgin surface layer because of weakened bonds [6], and this would result in an effective enhancement of the etching rate.

3.2. Simultaneous exposure to XeFz and an ion beam: ion-beam-assisted etching

When Si,N, is simultaneously exposed to XeF, and a 50 keV ion beam, a substantial enhancement in the etching rate can be achieved. In fig. 4 etching rates are shown as a function of the XeF, gas pressure for bombardment with Ar+, Kr+ and Xe+. The current density is 1 pA/cm2. At a low XeF, supply rate, less than 10 mTorr, the etching rates increase linearly with increasing XeF, gas pressure and the differences among the etching rates are not remarkable. With an increase of gas pressure not only the difference in etching rates becomes more and more significant, but also the etching rates for Ar+ and Kr+ show saturations. The etching rate using Xe+ does not saturate until near 60 mTorr. The etching rates for Ar+, Kr+ and Xe+ are approxi- mately 40, 80, and 160 times the etching rates for

BEAM CURRENT lpA/cm’

10.0 DOSE 2x10~/cm2 Xe-

-: ‘i .E 8.0

1

4

0 20 40 60

XeF.2 PRESSURE ( mTorr 1

Fig. 4. Etching rate of Si,N, as a function of the XeF, pressure for bombardment with Ar+, Kr+ and Xe+.

1

I I I I , I I I I

Ep=lO keV

I

(b)

Si

I I I I I 1 I I I

40 1000 2000

ENERGY ( eV ) Fig. 5. Auger spectra of a Si,N, surface for (a) virgin; (b) after IBAE at 60 mTorr XeF, and (c) after physical sputtering. 50

keV Ar+ ion was used for bombardment.

physical sputtering, respectively. We could not get the exact values for physical sputter etching because the etching rate is low and the etching could not be per- formed deep enough to measure the depth accurately by stylus traces, so the enhancement rates reported here are approximated. From the XPS measurement we found that a precursor state was formed on the surface. It can be supposed that the role of ion bombardment is to break the weakened bonds and to alter the precursor state to final volatile products by providing energy to the surface. In the low gas supply region below a critical rate, which depends on ion mass, the gas supply is the reaction-rate-limiting step. At a higher gas supply rate the density of the precursor saturates, as was revealed in XPS measurement, and the reaction rate from precursor to final products may become the rate-limiting step. For heavy ions the saturation occurs at a high gas pressure because the energy deposition rate or the etching en- hancement rate is large.

3.3. AES analysis of the etched surface

AES analysis was carried out to detect the chemical composition on the surface after the etching. Auger spectra of the surface etched by physical sputter etching and IBAE with 60 mTorr XeF, are shown in fig. 5. In fig. 5a a spectrum for a virgin sample is shown for reference. It is clear that no fluorine residues are de- tected and the chemical composition on the surface changes little after IBAE treatment, as shown in fig. 5b. On the other hand, not only is the nitrogen greatly reduced but also severe carbon and oxygen contamina- tion is detected after physical sputtering, as shown in

Z. Xu et al. / In situ patterning of Si,N, 753

fig. 5c. Carbon deposition is inevitable, due to the hydrocarbon residual gas, and the etching will compete with the deposition. Carbon contamination will be dominant for physical sputtering, due to the low etching rate. In the case of IBAE etching becomes dominant and, moreover the carbon may react with fluorine to form volatile CF,, etc., and be removed from the surface.

4. Summary

The dissociative chemisorption of XeF, on Si,N, and its influence on the etching rates of Si,N, simulta- neously exposed to XeF, and an ion beam have been studied using XPS. Virgin Si,N, adsorbed about a monolayer of fluorine and tended to saturate on further exposure. A F,N,(SiF,) precursor was identified from the characteristic core level chemical shifts and no Xe was detected when Si,N, was exposed to XeF,. Silicon and nitrogen atoms, combined with this precursor, may react further with fluorine to form volatile products by ion beam irradiation and, as a consequence, could be easily removed from the surface. This is in agreement with the simultaneously exposing of Si,N, to XeF, and a 50 keV ion beam. For Xe+ a significant enhancement, up to 160 times larger than the physical sputtering, was

achieved. Carbon contamination could be eliminated by introducing XeF,.

The authors are grateful to Dr. Y. Yuba for discus- sion and encouragement and to Mr. K. Kawasaki and Mr. K. Mino for technical assistance. One of the authors (Zheng Xu) thanks the Makita Scholarship Foundation for support during this work.

References

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[2] S. Matsui and K. Mori, Appl. Phys. Lett. 51 (1987) 1498. [3] Y. Ochiai, K. Game and S. Namba, J. Vat. Sci. Technol.

B4 (1986) 333. [4] J. Vuillod, J. Vat. Sci. Technol. A5 (1987) 1675. [5] J.W. Cobum and H.F. Winters, J. Appl. Phys. 50 (1979)

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141 (1986) 409. [7] U. Gerlach-Meyer, Surf. Sci. 103 (1981) 524. [8] Z. Xu, K. Gamo and S. Namba, Mater. Res. Sot. Symp.

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VII. INORGANIC MATERIALS