S5-2

Negative Ion Surface Reactions in Plasmas andIts application to Silicon Processes
プラズマ中における酸素および塩素負イオンの表面反応と半導体プロセスへの応用
H. Shindo, T. Fukasawa*, K. Fujiwara and K. Nishioka
進藤春雄，深沢孝之 *，藤原和也，西岡健介

Department of Electronics, Tokai University, 1117 Kitakaname, Hiratsuka 259-1292, Japan
東海大学電子情報学部エレクトロニクス学科，〒259-1292 神奈川県平塚市北金目1117*
ADTEC Plasma Technology Co., Ltd., 5-6-10 Hikino-cyo Fukuyama-city Hiroshima 721-0942, Japan*
アドテックプラズマテクノロジー㈱，〒721-0942 広島県福山市引野町5-6-10
For one application of negative ions in plasma, silicon oxidation at low temperature and silicon etching werestudied by employing negative oxygen ions as well as halogen negative ions. In the downstream region of amicrowave and RF surface-wave plasmas, the silicon oxidation and etching were examined under DC and RFbias. The oxidation depth showed a strong dependence on RF bias, and thus it was concluded that theoxidation was due to the negative ions. The oxidation depth by negative ion became as high as more than twotimes of that by positive ion. Film quality was analyzed by XPS, showing less suboxide compared with positiveion oxidation.

1. Introduction
A low temperature and low damage silicon oxidation
technique[1] is highly required in various ULSI processes. In particular for trench isolation of a memory
cell to realize further integrations, the oxidation should
be ion-assisted for directionality but with low damage.
In addition, a low damage etching is also highly required, and it becomes more important as the cell size
shrinks. For this purpose, a new method of negative
ion assisted silicon oxidation and etching have been
proposed employing a microwave O2 and halogen plasmas.[2,3]The objective of this work is to study silicon oxidationand etching by negative ions under the RF bias. For apractical application of this method, the oxidation andetching should be proceeded under the RF bias, because the DC bias becomes ineffective as the oxide
film is grown. In this work, the silicon oxidation andetching were investigated under a transformer coupled
RF bias. This device enables us to irradiate the negative ions as well as the positive ones onto silicon surface. In particular the major role of negative ions inoxidation and etching was examined under the conditions. In particular, an innovative method to produce anegative ion rich plasmas is proposed by employing
RF surface-wave with a very high dielectric constantdischarge tube.

mm in inner diameter with the thickness of 2 mm andthe length of 330 mm as shown in Fig.1. The two discharge tube materials were employed; the one is aceramic of TiCa-TiMg composite, K-140, which is commercially available from KYOCERA Co. and anotheris quartz. The permittivity values of these tube materials are, respectively, 140 and 3.8. To the one end ofthe tube, the mesh electrode of 60 mm in width is wound,and it is energized by the 13.56 MHz RF power. The
process gases of O2 and SF6 were introduced fromthe one end of the tube and another end of the tube
was connected with the process chamber of the stainless steel.
The optical emission line measurements were carried out from the both lateral and axial view to diagnose plasmas thus produced. The surface-wave propagation was also examined from the phase shift observed
between the two dipole antennas. One of the dipole
antennas was fixed at the upstream and another one
was axially moved, and the phase difference obtained
between these two was measured as a function of theRF Power(13.56MHz)

Discharge Tube (K-140, Quartz)
I.D.15mmφ 330mmL
Dipole Antenna
Shielding Tube
Water-cooled

Optical Fiber

To Oscilloscope
Spectrometer

2. Experimental Apparatus
The two types of apparatus were employed in theexperiment. The one employed in this work is schematically shown in Fig.1. The surface-wave plasmasof O2 and SF6 were produced in a discharge tube of 15SF6O2 Gas Inlet
Cl2Mesh Electrode（19mmφ x 60mm）

Process Chamber

RotaryPump

Optical Fiber
View port

SpectrometerFig.1. Experimental apparatus of RF surface-wave.

In Fig.2, typical examples of the optical emission line
measurements are shown, where the relative intensities of the six fluorine atomic lines are plotted against
the axial distance Z for the K-140. The intensity in theordinate is normalized by the maximum value obtainedat Z=8 cm for each line. The axial decay rate of theline intensities is very fast in the K-140 discharge tubeand its rate was 5 times larger than that in the quartz.
In particular, a sudden decrease at Z=13 cm may be
due to the surface-wave ending, providing a high density after-glow. This is because the permittivity of K140 is very high as 140 and the group velocity of thesurface-wave is low. In these after-glow region,Langmuir probe measurements were made. In Fig.3,
the saturation current ratio of positive to the negativeis plotted as well as the positive ion saturation current.
The saturation current ratio in the K-140 becomes quite
low with keeping the high value of the positive ion saturation current, indicating the rich negative ions populated in this region.
A typical result of silicon oxidation in the microwave

Ratio of Electron and Ion
saturation current Ips/Is Ion saturation current Is (µA)

3. Experimental Results and Discussions

plasma is shown in Fig.4, where the oxidation depth
obtained at Z=15 cm by XPS measurements is plottedagainst the DC bias Vdc and the RF bias Vpp of thefrequency 1.1 MHz. It is seen that the oxidation depth
is very critical to both the Vdc and Vpp. In particular, in
positively biased conditions the depth becomes higher.
XPS measurements showed that the SiO2 peak observed with the positive ion oxidation film is shifted
and widened, demonstrating that the film quality is improved by the negative ion irradiations with less suboxide. The fact that the film quality is improved by thenegative ion irradiation may be explained as the following. The mass analysis of the ion species using aquadrupole mass analyzer showed that the dominantnegative ion was O- and its density was almost oneorder of magnitude higher than that of O2-, while thedominant positive ion was O2+.[4] The dominant negative ion O- is very atomically similar to the oxygen radicalwith a high chemical reactivity and this may be theorigin of the high quality oxidation.
30
Ion saturation current
K-140
QuartzSF6 gas flow rate: 15sccm
Gas pressure: 0.1Torr
RF power: 20W

2520
15
10
5025

Saturation current ratio
K-140
Quartz20

SF6 gas flow rate: 15sccm
Gas pressure: 0.1Torr
RF power: 20W

15105010121416

182022

Axial Distance Z (cm)Fig.3. Ion saturation current and saturation current ratio.
50500W 200mTorr
O2: 500sccm
30min

4540
Vpp (V)

axial distance.
Another experimental apparatus is a microwave
plasma with a dielectric disc plate. The plasma wasproduced in a 6 inch stainless-steel chamber and thedownstream plasma was mainly considered because
this region the negative ion was highly populated. The
plasma parameters were measured using a Pt
Langmuir probe. Silicon oxidation was made on a stage
which was biased by the DC voltage as well as the RFbias to irradiate both negative and positive ions. TheRF bias voltage was applied to the stage with a transformer, and the secondary of the transformer was biased by DC voltage at the same time. The oxidation
experiment was also performed at the substrate temperature of 400 oC.
The oxide film quality thus produced was analyzed
by X ray photoelectron spectroscopy (XPS). The oxide film thickness was measured by the XPS.

35

Oxidation Depth (Å)
24830Relative Emission Spectrum (a. u.)

144

Measured FI Line703.7nm
712.8nm720.2nm
730.9nm733.2nm739.9nm1.00.80.6

Dielectric: K-140
SF6 Gas Flow Rate: 75sccm
Gas Pressure: 79.8Pa
RF Power: 160W

0.4
0.20.04

68 10 12 14 16 18 20 22

Axial Distance Z (cm)Fig.2. Emission line intensity of FI.

2520170196

46222274

118

02810 12Vdc (V)14161820

Fig.4. Oxidation depth contour for Vdc and Vpp.
References[1] S. Taylor, J. F. Zhang and W. Eccleston: Semicond. Sci.
Technol. 8 (1993) 1426.
[2] T. Koromogawa, T.Fujii, Y. Horiike and H. Shindo: Jpn. J.
Appl. Phys. 37 (1998) 5028.
[3] Y. Morikawa, T. Shibayama, S. Takayanagi, H. Ita, H.
Shindo, T. Ichiki and Y. Horiike: Jpn. J. Appl. Phys. 42(2003)1435.
[4] H. Shindo, T. Koromogawa and Y. Horiike: Abstr. 44th Nat.
Symp. of American Vacuum Society, San Jose, 1997, p.208.