Electrical measurements in a 13.56 MHz radio-frequency discharge

Vacuum/volume 42/number Prlnted in Great Britain

12ipages 741 to 744/1991

Electrical frequency

measurements discharge

0042Z207x/91$3.00+.00 @ 1991 Pergamon Press plc

in a 13.56

Kenji Kobayashi, Nobuki Mutsukura and Yoshio Machi, Department Engineering, Tokyo Denki University, Chiyoda-ku, Tokyo 707, Japan received

in final

form

4 September

MHz

of Electronic

radioEngineering,

Faculty

of

1990

In a planar radio-frequency (rf) plasma, the distribution of de space potential between both electrodes and the dc surface potential of a substrate placed on a cathode electrode were measured using an electrostatic probe. The distribution of the space potential for a stainless steel (sus) cathode electrode greatly differs from that for a suslquartzlsus cathode electrode. This is associated with a large voltage drop at the insulating quartz part. The surface potential of a semiconducting Si substrate, having a much smaller area than the electrode area, is quite different from a self-bias voltage appearing on the cathode electrode. It increased with the increase of substrate area, and then saturated at the same value as the self-bias voltage. The distribution of the substrate surface potential in a radial direction was also measured. The relationship between the surface potential of the substrates and the results of plasma processings are discussed using plasma deposition of amorphous carbon film. The deposition rate of the carbon film was exceedingly influenced by the surface potential of the substrate.

I. Introduction The capacitively

coupled planar radio-frequency (1-f) plasma has been widely used for thin film deposition and the etching process in the fabrication of olcctronic dcviccs’. In spite of such wide industrial needs, it is still not well understood in view of plasma physics. The rfplasma has been routinely controlled with plasma gcncration paramctcrs (c.g. input rf power. gas pressure. gas flow rate, etc.). and then plasma generation conditions are progressively optimized using previous results in the plasma proccssing. Such being the case. we need clcctrical and optical information about the plasmas. The distribution of the dc space potential between both parallel clectrodcs is quite important to investigate the behavior of ionic species and electrons. cspccially in an ion sheath rcpion near ;I cathode elcctrodc”. Moreover. the dc sell-bias voltage that appeared on the cathode electrode has an cxtrcmc affect on the products of plasma processing. bccausc ionic species arc accelerated by the large voltage drop across the ion sheath and they then bombard the substrate surface. Indeed. in the etching process, the etching rate and the etching anisotropy dcpcnd strongly on the ion bombarded energy (I. The potential diflerencc between the cathode electrode and the substrate surface will be predicted. as the rf power is provided through both the cuthodc and the substrate into the plasma. Many papers, however. still have not thoroughly discussed these sorts of experimental details’. Since scvcral materials (for example. Si, SiO,, Al, Cu, etc.) simultaneously exist on the electrode in plasmas for device pdbrication processes. the local surface potential may be different for each material. In this work, WC prcscnt the electrical properties of rf H, discharges by measuring the dc space potential in the plasma and the dc potential of the substrate surface on the cathode electrode. using ;I Langmuir probe.

Our previous work” reported that the properties 01‘ hydrogenated amorphous carbon (U-C :H) films made by plasmaassisted chemical vapor deposition (CVD) depend on the deposition conditions such as input power. pressure, substrate material, and cathode self-biased voltage. In order to investigate the cffcct ofthc substrate surface potcntial C:,,,, in the plasma processing. the relationship bctwcen the rates oE the carbon films was also c;,,,, and the deposition mcasurcd. 2. Experimental

procedures

An apparatus for the measurement of the dc potential is a normal parallel-plate rf plasma chamber with a single electrostatic probe, as shown in Figure I. Both stainless steel (sus) electrodes, IO0

substrate -

c- t

_

lowpass filter T-------i

I7

--c

0

7

c ; OSCILLOSCOPE 1 1

w

exhaust

I

Figure I. Schematic dia_eram of the parallel-plate rf plasma apparatus.

741

K Kobayash/

et al. Electrtcal

measurements

tn a t-f dtschatge

ti2 : O.STorr. 50W cathode

3.

Results

I\ c\tlnl~l~cd anode

and discussion

I’, = I ,,+Al’+A+

(I1

I?\ ’

K Kobayashi et a/: ElectrIcal measurements

in a rf discharge

-300

0.3 H2;50W

5

2

Pr:O.lTorr

t

E ti

0.3Torr

’ 0.22

Pr:O.lTorr

z

0.6Torr

G E l.OTorr

It E

^

cathode q

ancde E

O-l

10

i

AREA

RATIO

SAl/!%e

(%)

3 ----___________.

1 AREA

RATIO

40 Ssi/Sele

68 (Oh)

for the Si:quartz:sus electrode system. The relationship bctwccn I’,,,,, of Si substrate ;md the area ratio S,,,‘S,,, is shown in Figure 4. C,,,,, increases with the area ratio up to 75% and then saturates. Hut the saturated potentials differ cram the self-bias voltage, and is approximately two thirds of the self-bias voltage at an area ratio of 50%. In the case of an Al substrate on the quartz:‘sus with the area up to 35% clcctrode, C’,,,, incrcascs monotonously :md then also saturates. The saturated V,,,,, is about half the selrbias voltage. which is somcwhut small compared with that for the Si:quartz/sus clcctrode. The rfdischargcs can be characterized by two kinds of specific regimes (x and ;’ types)“. In the ; regime of the rf discharge. sccondnry clcctron emission from ion bombardment of the clcctrodc surface, plays an important role in sustaining the discharge. The secondary electrons are accclcratcd rapidly into the bulk plasma by II large voltage drop across the ion sheath. This energy is dissipated for ionization. excitation and dissociation in bulk plasm;t. For the poor secondary emission coetficicnt. ;‘. therefore. ;i higher sheath voltage ~ill bc required in order to maintain the

Pr:O.lTorr

~

0.3Torr

L

0.6Torr

.

1 .OTorr

discharge. Indeed, the Si clcctrodc having a smaller ;’ value than that of Al ’ ’ indicates a larger I’,,,,, in comparison with the Al electrode (Figures 4 and 5). The appearance of an intense luminous layer at the sheath cdgc. cxcitcd by ;’ electrons. is a distinct fcaturc of the 1’ type discharge. At a large discharge current density Lmd:‘or a relatively high gas prcssurc condition, the ;’ rcgimc takes place. The decrease of gas pressure results in a transition to the J: regime’J.’ ‘. whcrc sustaining of the discharge is determined by proccsscs in the bulk plasma, and the surcace phcnomcna at the electrode would not be important. In a SiH, rf discharge, the x(-;’ transition occurs at 0. I3 torr”. In our mcasurcments of the H2 discharge a drastic change in the profile ol‘ an optical emission intensity bctwccn both eloctrodcs was observed around 0.3 torr. which will be the z ;‘ transition prcssurc”‘. In the discharge obtained below 0.3 torr. the surl‘acc condition. i.e. the arca ratio. is expected to bc not so sensitive to V,,,, as in the 7’discharge. No remarkable difference in the behavior 01‘ I’_,,,,. howcvcr. can bc recognized between both x and ;’ type discharges. as shown in Figures 3 5. The 7 electrons may affect, in part. the sustaining of the dischurgc, cvcn for the discharge cxamincd in our work. The intlucncc of l/:,,h on the deposition rate of the o-C : H films was then investigated. Figure 6 shows the deposition rates ol’thc

3 20 /

*

a-CH

5 E 10

I AREA

RATIO

40 Ssi/Sele

8 (%I

film

CHa gas 0.1 Torr 50

t

e 0’ n 0

w

5 I

1

I

I

min i

10 20 30 40 50 60 AREA RATIO SWSele (%)

Figure 6. Dcpositmn rate of the amorphous carbon CH, gas. as a function ol’the area ratio S,,:.~,,..

lilms obtained

from

743

, n

9

-100 -50

LI/0

+50

-50

0

+50