AP-FIM STUDY OF Si OXIDE AND Si-Si OXIDE INTERFACE

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AP-FIM STUDY OF Si OXIDE AND Si-Si OXIDE INTERFACE

T. Adachi, M. Tomita, T. Kuroda, S. Nakamura

To cite this version:

T. Adachi, M. Tomita, T. Kuroda, S. Nakamura. AP-FIM STUDY OF Si OXIDE AND Si-Si OXIDE INTERFACE. Journal de Physique Colloques, 1986, 47 (C7), pp.C7-315-C7-319.

�10.1051/jphyscol:1986754�. �jpa-00225949�

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JOURNAL DE PHYSIQUE

Colloque C7, supplément au n o 11, Tome 47, Novembre 1986

AP-FIM STUDY OF Si OXIDE AND Si-Si OXIDE INTERFACE T. ADACHI, M. TOMITA, T. KURODA and S. NAKAMURA

The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka, Japan

Abstract

-

The stoichiometry of Si oxide and the structure of Si-Si oxide tnterface were studied using pulsed-laser stimulated atom-probe field-ion microscope (PL-APFIM). The compositions of native oxide and thermal oxide films were Si:0=1:0.9 and Si:0=1:2, respectively. The interface of the thermal oxide was composed of the transition layer of SitOj , which was thinner than two atomic layers. ïhe oxide layer grown by laser irradiation ont0 Si clean tip in 02 gas of 7x10-~Pa was also presented.

1

-

INTRODUCTION

The controles of growth rate of very thin SiOtfilm and characterizations of Si-Sioz interface have again become important problems for very large scale integrated circuits. Accordingly there is necessity for making clear the composition of Si- SiOtinterface. However, the detailed composition of the interface remains unsolved as yet.

The interface has been studied by a number of techniques including transmission electron microsco e (1,2), Auger electron spectroscopy (3,4) and X-ray photoelectron spectroscopy (5-87. We can s m r i z e those results as follows..

(1) Many un-oxidized Si clusters remain in SiOt film near the interface so that the composition at the interface apparently shows SiOx (x<2) (2,3).

(2) An SiO* transition layer of finite width exists between Si and Sioz films (4-6,8).

(3) An Si02 film is directly connected with Si surface and the interface is atomi- cally sharp (7,9,10).

At present it is infered that there is a transition layer of Si0 x and its thichess is thinner than Inm. we clearly determined in this work the existence and the c o q - sition of transition layer at Si-Si0~ interface by using Our pulsed-laser stumi- lated atom-probe f ield-ion microscope (PL-AP-FIM)

.

In this paper we report the results of atom probe analyses about native oxide, thermal oxide and laser induced growth oxide films.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986754

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C7-316 JOURNAL DE PHYSIQUE

II

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EXPERIMENTAL PROCEDLJRES

We have studied the stoichiometry of S i oxide and the s t r u c t u r e of Si-Si oxide i n t e r - face by the PL-AP-FIM which i s a powerful tool t o have the depth p r o f i l e of insula- ting materials i n an atomic scale. Figure 1 showes a shematic diagram of Our combined type AP-FIM. We used only the l i n e a r mode f o r obtaining the correct stoichiometries of the S i oxide films. The l a s e r used was MOLECIRON W22 Nzgas l a s e r of which the maxirm power was 6mS and the pulse width was 1OnS.

.- . .. . ION I Sun. PVIIPI

TOP 1

Fig. 1 A schematic diagram of pulsed-laser s t i m l a t e d atom-probe field-ion microscope.

The thin films of S i oxide analyzed i n t h i s work were native oxide, thermal oxide and l a s e r induced growth oxide, respectively. The native oxides on S i t i p s were prepared by exposing of long duration of two days to twenty days i n dry a i r a f t e r the etching. The thin films of thermal oxides were grown i n dry 0 2 o f 1 x 1 0 ~ Pa a t 700°C f o r lûnin. The l a s e r induced growth oxide was made i n s i t u i n AP-FIM by pulsed-laser i r r a d i a t i o n ont0 S i t i p i n Ozgas of 7 x l 0 - ~ ~ a f o r 40min. The thin oxide films were formed on the field-evaporated clean t i p surfaces except f o r the native oxide and t h e i r thicknesses were 1-3nm.

III

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^RESULTS^ANDDISCUSSION-

Figure 2 shows mass histograms of the native oxide film on the S i t i p which was exposed i n the dry a i r f o r two days a f t e r the electropolishing. Figure 2(a) was obtained £rom the oxide surface and Fig. 2(b) was £rom a t the interface of Si-Si oxide. ï h e detected ions were mainly Si07 , si02

,

si', O+, sizr, 02 and hydrides.

From Fig. 2(a) H g t and OH ions were more than that i n Fig. 2(b). The stoichiometries of the oxide film and the interface were Si: 0=1:0.9 and Si:(3=1:0.4, respectively.

This film was not S i o z and the s t r u c t u r e seem t o be an amorphous phase. ï h e thickness of t h i s film was about 10 atomic layers estimated £rom cumulative number of

the f i e l d evaporated ions. Leaving a t i p f o r twenty days i n dry a i r , the composition of the oxide film approached t o S i o z .

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0

0 18 20 30 48 50 60 7 0 80 98 100

WSS-TO-CHARGE R A T I O

: ( a ) Native Oxide

a l 2 +

-

s i n 1 + s,n;+

- ^O+

( 1 1 1 1

S I D +

sI?,,K; , , , , , , , , , , , ,

0

8 10 2 8 30 40 58 6 0 7 0 88 SB 100

HASS-TO-CHARGE R A T I O

Fig. 2 Atom probe mass histograms of S i oxide film on the (122) plane.

The histograms ( a ) and (b) were obtained from near the oxide surface and a t the Si-Si oxide interface, respectively. The oxide film was prepared by the exposure of dry a i r i n two days.

(b)

s i 2 +

s i n 1 +

S I H ~ ~ ~ 5 1 o x i d e

I l 1 2 1 P L *

The thermally grown oxide covered a t i p was hardly f i e l d evaporated. Thus i t was necessary t o increase both the applied voltage and the l a s e r power. Unfortunately, we had not f u l l data through the surface of oxide to the underlying S i substrate, because almost oxide films suddenly jumped on the f i e l d evaporating process. The structure of the oxide film was S i o z

.

The spectrum obtained £rom the interface i s shown i n Fig. 3. The stoichiometry a t the Si-Siozinterface was Si:C=l:1.5. This value suggested that a transition layer e x i s t a t the interface and i t s chernical composition i s SiZ03

.

In the case of non-crystalline S i O z , we could not expect the f i e l d evaporation of layer-by-layer, since we supposed £rom other data of the same t i p radius t h a t one atomic layer of S i oxide was corresponded t o about 30 detected ions. The thickness of the t r a n s i t i o n layer on the underlying S i substrate was two atomic layers o r less.

O +

H*

s i n + s1n;

5 1 o f si.,;

l , , L , o l , , h , ^h ^, ^, ^. ^, ^l ^, ^. ^. ^~ ^l^. ^c ^c ^c ^l ^. ^. ^~ ^.

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JOURNAL DE PHYSIQUE

50 Thermal Oxide si (111)

1

B 10 20 30 40 50 60 7 0 80 90 188

H A S S - T O G A R C E R A T I O

Fig. 3 Mass histogram of the thermally grown oxide film on the (111) plane.

The oxidation conditions of an Si tip are written in this figure.

Finally, we report the oxide films formed by ultraviolet laser-induced oxidation at the Si (111) plane. From the technological point of view, this new research has become important for the purpose of low temperature oxidation. Figure 4 showes the mass histogram of a laser-induced oxide grown in Ozgas of 7xl0-*~a for the irradiation t h e of 4Cknin. (4.8x10* pulses). The laser beam was irradiated on the+ (111) plane in an angle. The ions detected £rom the oxide film were sit , ^{O t}, ^O2 , ^Si04

siZ0{ , ^S~O: ^{and SLO;}

.

In addition of these ions, many other ions, such as H ,

OH+ , HzO+ and H ~ O * were also detected. ïhese complexes were created by the laser- induced reaction of the residual gases O z and Hzadsorbed on a tip. In the case of the voltage pulse mode, these ions slightly detected except for the adsorbed H or O atoms. Evacuating the AP-FIM chamber in lxlo-'~a, the complexes considerably decreased. 'Ihe characters of laser-induced growth oxide was almost similar to that of the thermal oxide. The stoichiometry of the oxide film was O/Si=2 and its thickness was about 5 monolayers. The composition of the transition layer on the underlying Si (111) plane was Si0 and its thickness was one atomic layer.

20 Laser-Induced Growth Oxide si 1111)

1

Room Temp.

Laser P n l s e

: P

/ 51 r l p /

0 2 Gas Pressure : 7 ~ 1 0 - 2 p a NO. Of Pulses : 1 . 8 ~ 1 0 4 L. Wave Length : 337nm

8 18 JE 48 58 68 70 6 0 O 0 188

HASS-TO-CHARGE R A T I O

Fig. 4 Atm-probe mass histogram of the laser-induced growth oxide surface of an Si (111) plane obtained linear type AP using the pulsed-laser mode.

The conditions of the laser-assisted oxidation of a tip are written in this figure

.

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We could smerize the experimental results obtained £rom three different type oxide films as shown in Fig. 5.

S i O x i d e

O x i d e

A: ^Native O ; Thermal

0 ; L a s e r

1 1 1 0 9 8 7 6 5 4 3 2 1 0 1 2 3

T h i c k n e s s of S i o x i d e f i l m ₍a t o m i c l a y e r )

Fig. 5 Tne depth profiles of three kind of oxide films.

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