Channeling analysis of thermally nitrided silicon

Nuclear Instruments and Methods in Physics Research 218 (1983) 589-592 North-Holland, Amsterdam

CHANNELING Jun AMANO Hewlett

ANALYSIS

OF THERMALLY

NITRIDED

589

SILICON

and Tom EKSTEDT

Packard Laboratories', Palo Alto, (,4 94304, USA

Thermally nitrided and oxy-nitrided silicon were analyzed by glancing angle channeling of 2 MeV helium ions. The film stoichiometry and interface structure were investigated with a combination of channeling and etchback sample preparation. The film stoichiometry of various thermal nitride and oxy-nitride films were found to be S i 3 N 4 or a mixture of SiO 2 and S i 3 N 4. The non-registered Si atoms at the interface varied from two to four monolayers for different nitridation processes.

1. Introduction Thermal nitridation of Si and thin SiO 2 films in N H 3 gas has been shown to be a promising approach for the p r o d u c t i o n of reliable thin gate dielectric films for VLSI applications. The characterization of film properties, especially the interface properties after nitridation, is essential to u n d e r s t a n d i n g the electrical behavior of these films. The glancing angle backscattering with channeling technique has been shown to be a very useful m e t h o d for the investigation of film stoichiometry and interface properties in very thin silicon dielectric layers. The S i / S i O 2 system was investigated in detail by this technique [1-3]. These studies showed that the thermally grown S i / S i O 2 interface was very a b r u p t and that about two silicon monolayers were non-registered or displaced from the ideal silicon lattice sites at the interface [4]. The oxygen and nitrogen distribution of chemically v a p o r deposited a n d thermally grown silicon nitride films were also studied by the glancing angle channeling technique [5-7]. The film stoichiometry and the S i / S i O 2 interface properties of low pressure (LP) thermally grown oxides and thermally nitrided LP oxide films were also investigated by this technique [8]. The reported n u m b e r of non-registered Si atoms at the interface in the plasma e n h a n c e d thermally nitrided silicon [7] was m u c h larger than that of thermally nitrided SiO 2 [8]. In this paper the glancing angle channeling experim e n t s of different thermally nitrided a n d oxy-nitrided silicon are reported, and the film stoichiometry and the interface structure of these nitrides a n d oxy-nitrides are compared.

2. Experimental All glancing angle channeling experiments were performed by using 2 MeV He + ions at a 100 ° scattering 0 1 6 7 - 5 0 8 7 / 8 3 / $ 0 3 . 0 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)

angle. A silicon surface barrier detector with 14 keV resolution was placed 10 cm from the sample with a 1 m m width slit in front of the detector. The pressure of the scattering c h a m b e r was m a i n t a i n e d at a b o u t ( 1 - 2 ) x 10 7 Torr by a turbomolecular p u m p during the experiments. Table 1 summarizes the growth conditions of thermally nitrided and oxy-nitrided silicon. All nitrides and oxy-nitrides were grown on chemically cleaned Si(100) wafers u n d e r low pressure. The film thicknesses in table 1 were measured by an ellipsometer. In order to make various thickness specimens from each sample for the channeling experiment, thermal nitrides and oxy-nitrides were chemically etched by a 1 0 : 1 H F - D I water solution for different durations. This c o m b i n a t i o n of chemical etch-back sample preparation and glancing angle channeling provides information on film stoichiometry a n d interface structures in great detail [8].

3. Results and discussion Fig. 1 shows aligned spectra of different thicknesses of p l a s m a - e n h a n c e d thermally nitrided silicon: (a) as grown, (b) 3 s etch, (c) 6 s etch, (d) 9 s etch, (e) 12 s etch, (f) 15 s etch, and (g) 18 s etch, respectively. It is

Table 1 Growth conditions of thermally nitrided and oxy-nitrided silicon. Sample 50 A thermal nitride 75 A thermal oxynitride 45 A plasma-enhanced thermal nitride

Growth conditions N H 3,

1 Torr, 1200°C, 8 h 1 Torr, ll00°C,

N H 3 / 0 . 3 % O 2.

4h NH 3, 2.7 Torr, 420°C, 2 h, rf 450 W

J Aman6 72 E k s t e d t / Thermally nitrided silicon

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Fig. 1. Aligned spectra of various thickness, plasma-enhanced, thermally nitrided silicon. The sample was etched by diluted HF solution: (a) as grown, (b) 3 s etch, (c) 6 s etch, (d) 9 s etch, (e) 12 s etch, (f) 15 s etch, and (g) 18 s etch.

clear from the figure that only the silicon and nitrogen peaks decreased as etch time increased. Both the oxygen and the carbon peaks stayed constant, indicating that these peaks were on the surface. In order to obtain the areal density of O and N atoms, the background was subtracted from the oxygen and nitrogen peaks by a straight line extrapolation of the silicon substrate contribution. For the silicon areal density, a simple triangle subtraction was used to obtain the correct Si surface peak [9]. The areal density of each element was then calculated by comparison with a bulk Si surface height obtained from r a n d o m spectra. For r a n d o m spectra the sample was continuously rotated with a 5 ° 7 ° tilt angle during the measurements. An experimental error of _+5% for our experimental configuration and counting statistics was expected in the areal density calculation. In order to confirm the estimate of the experimental error, a Bi-implanted Si standard (4.77 _+ 0.05 × 10 '5 Bi a t o m s / c m 2) was used to calculate the Si areal density.

It was found that the Si areal density calculated from the random Si surface height showed a 4% lower value than that calculated from the Bi standard. This was well within the estimated experimental error, so the random Si height was used for all areal density calculations reported here. The carbon peaks observed in all the spectra were c a u s e d b y the cracking of hydrocarbons in the scattering chamber, and the effect of the surface carbon on the calculation of the areal density was found to be negligible [8]. Also the detection limits of oxygen nitrogen atoms were determined to be 1 x 1015 O a t o m s / e r a 2 and 1.5 x 1015 N a t o m s / c m 2, respectively, for our experimental configuration. Fig. 2 shows the areal density of silicon and oxygen versus nitrogen for the thermally nitrided silicon. The various thickness specimens were prepared by the previously described chemical etch method. The least square fitting of data points to a straight line resulted in 0.85 as the S i / N ratio in the film. The number of oxygen atoms was constant (4.4 x 10 ] 5 0 a t o m s / c m 2) near the interface. Near the surface, slightly higher oxygen incorporation into the nitride film was observed. The oxygen areal density of 4.4 x 10 ~ 5 0 a t o m s / c m 2 was a typical n u m b e r of oxygen atoms for the native oxide on the Si surface. Loading these nitride samples into the scattering c h a m b e r rigth after a quick dip into a buffered H F solution resulted in a significant decrease in the oxygen peak but no decrease in the nitrogen peak. Therefore most of these oxygen atoms were considered to be due to the surface oxidation of the nitride layer after each chemical etch. By subtracting the number of silicon atoms, which were assumed to be b o u n d to oxygen as SiO 2, from the total number of Si atoms, the slope of the data points was reduced to 0.72. This indicated that the bulk stoichiometry of the thermal nitride was very

30 THERMAL NITRIDATION

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Fig. 2. Areal density of silicon and oxygen vs. nitrogen for the thermally nitrided silicon in pure NH 3.

591

J, Amano, T, Ekstedt / Thermally nitrided silicon

close to Si3N 4 with the surface oxide layer. The n u m b e r of non-registered Si atoms at the Si3N4/Si interface was 1.5 × 1016 Si a t o m s / c m 2 without the oxide correction a n d 1.3 × 1016 Si a t o m s / c m 2 with the correction, respectively. The Si areal density of the ideal Si(100) surface was 1.2 × 1016 Si a t o m s / c r n 2 for 2 MeV He + channeling obtained from the c o m p u t e r simulation [10,11]. One monolayer of Si(100) plane consists of 6.8 × 1014 a t o m s / c m 2. Therefore the non-registered Si atoms at the interface were a b o u t two monolayers for the thermally nitrided silicon. Fig. 3. exhibits the areal density of silicon and nitrogen versus oxygen for the thermal oxy-nitrided silicon sample. The least-squares fitting of silicon and nitrogen data points to a straight line resulted in an S i / O ratio of 0.75, an N / O ratio of 0.33, and 1.22 × 1016 Si a t o m s / c m 2 as the n u m b e r of Si atoms at the interface. Making the naive assumption that all nitrogen a t o m s were b o u n d to silicon atoms as Si3N 4, the number of silicon atoms available for oxygen b o n d i n g was estimated. This nitrogen atom correction resulted in an S i / O ratio of 0.52, indicating the film stoichiometry to be SiO 2 and the n u m b e r of Si atoms at the interface to be 1.1 × 1016 Si a t o m s / c m 2. This n u m b e r of interface Si a t o m s was smaller than that of ideal Si atoms (1.2 x 1016 Si a t o m s / c m 2). This indicated that the stoichiometry of the film was not represented by a simple straight line especially near the interface. A second order polynomial fit to the uncorrected data points indicated the n u m b e r of interface silicon atoms to be 1.3 × 1015 Si a t o m s / c m 2 with a high degree of fit at the interface are. Therefore the film stoichiometry of the thermal oxynitride Si could be expressed as SiO 2 and Si3N 4 with an O / N ratio of 3.0 except near the interface. The non-registered Si atoms at the interface were again a b o u t two monolayers. i

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Fig. 4. shows that areal density of silicon and oxygen versus nitrogen for the p l a s m a - e n h a n c e d thermally nitrided silicon. The silicon data point was best-fitted by a straight line with a slope of 0.77. The n u m b e r of interface Si atoms was 1.62 × 1016 Si a t o m s / c m 2. The areal density of oxygen atoms was constant a n d was 4.4 × 1015 O a t o m s / c m 2 throughout the film. These oxygen atoms were therefore considered to be due to the surface oxide layer as described previously. Again assuming that all oxygen atoms were b o u n d as SiO2, the correction of the silicon data points resulted in a slope of 0.74 and interface silicon atoms of 1.4 × 1016 Si a t o m s / c m 2. Therefore the bulk stoichiometry of the p l a s m a - e n h a n c e d thermally nitrided silicon layer was Si3N 4 and the non-registered silicon atoms at the interface were three to four monolayers. The higher n u m b e r of non-registered Si atoms at the interface may be due to the plasma process a n d / o r a much lower t e m p e r a t u r e process in p l a s m a - e n h a n c e d thermal nitridation.

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NUMBEROF OXYGEN(x1015¢m"2) Fig. 3. Areal density of silicon and nitrogen vs. oxygen for the thermally oxy-nitrided silicon in NH3/0.3% O 2.

Glancing angle channeling with a sample etch-back p r e p a r a t i o n was performed on several very thin thermally nitride a n d oxy-nitrided silicon samples. Thermal nitrides and oxy-nitrides were grown u n d e r low-pressure pure N H 3 and N H 3 / 0 . 3 % 0 2, respectively, The film stoichiometry of the high temperature thermal nitride was Si3N 4 with a surface oxide. A b o u t two monolayers of non-registered Si atoms were observed at the thermal n i t r i d e / s i l i c o n interface. The film stoichiometry of the thermal-oxynitride grown u n d e r N H 3 / 0 . 3 % O 2 was mainly a mixture of SiO 2 a n d Si3N 4 with an O / N ratio of 3.0. However the stoichiometry near the interface was silicon-rich and the non-registered Si atoms were a b o u t two monolayers. Finally the film stoichiometry of

592

J. Arnano, T. Ekstedt / Thermally nitrided silicon

p l a s m a - e n h a n c e d t h e r m a l nitride was Si3N 4 with a s u r f a c e oxide. H o w e v e r the n o n - r e g i s t e r e d Si a t o m s were a b o u t four m o n o l a y e r s . T h e n u m b e r o f n o n - r e g istered Si a t o m s at the interface for these t h e r m a l nitrides a n d o x y - n i t r i d e s is c o n s i s t e n t with that of LP oxides a n d thermally n i t r i d e d L P oxides [8]. T h e a u t h o r s wish to a c k n o w l e d g e K. J a c k s o n a n d S. W a n g for the s a m p l e p r e p a r a t i o n , a n d N. C h e u n g (U.C. Berkeley) for the B i - i m p l a n t e d Si s t a n d a r d .

References [1] UC. Feldman et al., Phys. Rev. Lett. 41 (1978) 1396. [2] N. Cheung et al., Appl. Phys. Lett. 35 (1979) 859.

[3] T.E. Jackman et al., Surface Sci. 100 (1980) 35. [4] L.C. Felman, J.W. Mayer and S.T. Picraux, Materials analysis by ion channeling (Academic Press, New York, 1982). [5] F.H.P.M. Habraken et al., J. Appl. Phys. 53 (1982) 404. [6] Y. Tamminga et al., Nucl. Instr. and Meth. 200 (1982) 499. [7] S. Tatsuta. H. Nishi and T. Ito, Jpn. J. Appl. Phys. 21 (1982) E l l 3 [8] J. Amano and T. Ekstedt, Appl. Phys. Lett. 41 (1982) 816. [9] J.A. Davies et al., Surf. Sci. 78 (1978) 78. [10] L.C. Feldman, P.J. Silverman and I. Stansgaard, Nucl. Instr. and Meth. 168 (1980) 589. [11] K. Kinoshita, T. Narusawa and W.M. Gibson, Surf. Sci. 110 (1981) 369.