Coating apparatus – Gas or vapor deposition – With treating means
Reexamination Certificate
2000-03-14
2001-12-25
Mills, Gregory (Department: 1763)
Coating apparatus
Gas or vapor deposition
With treating means
C118S7230MR, C118S7230ME, C118S7230MW, C216S071000
Reexamination Certificate
active
06332425
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the treatment for the surface of a substance being treated by use of plasma, and particularly to a method of supplying a bias voltage to a substance of which the surface is to be treated.
FIG. 2
shows the most typical conventional biasing method called the RF biasing. A substance
1
to be etched is connected through a capacitor
2
to a high-frequency power supply
3
. A sine-wave voltage as shown in
FIG. 3
is applied through the capacitor
2
to the substance
1
. At this time, since the amount of electrons which a plasma
4
supplies is dozens of times larger than that of ions, negative charges are accumulated on the substance-side of the capacitor
2
. Because of the capacitor charges, a voltage shifted in the negative direction as shown in
FIG. 4
appears on the substrate. The positive ions for etching are accelerated by this negative voltage and perpendicularly incident on the substrate, thus making it possible to etch the substrate in an anisotropic shape.
Another idea is disclosed in JP-A-56-13480 (laid-open on Feb. 9, 1981) and JP-A-6-61182 (laid-open on Mar. 4, 1994) in which a pulse-waveform voltage is used as a bias voltage. However, the importance of the duty ratio and repetition frequency of a pulse wave has never been considered so far except that the present invention regards them as important.
SUMMARY OF THE INVENTION
As seen from the substrate bias voltage wave shown in
FIG. 4
, its positive voltage is almost zero although it is required to accelerate the electrons in the positive cycle in which the electrons impinge on the substance. Therefore, the electrons are little accelerated and impinge on the substrate. When fine pattern etching is made by this biasing method, local charge build-up occurs on the substance.
FIG. 5
shows the mechanism by which this charge build-up is caused. Since ions
5
are accelerated and perpendicularly impinge on the substance, they can reach the bottom of a fine pattern. Since electrons
6
are not accelerated and isotropically impinge on the substance, they are blocked by a mask
7
so as not to reach the bottom of a fine pattern (electron shading phenomenon). Therefore, the sides of the fine pattern are charged up to have negative charges, and the bottoms of the fine pattern are charged up to have positive charges.
The charge build-up due to this electron shading brings various troubles in the plasma etching. One of the most serious problems is local side etching (notching) which occurs in polycrystalline silicon etching (working) for gate.
FIG. 6
shows the mechanism of this notching. The ions
5
of etching species are repelled from the positive charges which are caused on the bottom of the fine pattern by the electron shading phenomenon, and they impinge on the side of the pattern. The ions incident to the side cause a local side etch
10
called notch in the interface between a polycrystalline silicon layer
8
and an underlying silicon dioxide film
9
.
The charge build-up due to the electron shading phenomenon also occurs in the metal wiring process, and damages the gate oxide film.
FIG. 7
shows this damaging mechanism. The positive charges accumulated on the bottoms of the fine pattern by the electron shading are collected to a floating gate
12
which is connected to a metal wiring conductor
11
, and cause damages such as dielectric breakdown to a gate oxide
14
between the floating gate
12
and a substrate silicon
13
.
Moreover, the charge build-up due to the electron shading phenomenon causes troubles, or undesirable abnormal shapes such as subtrench or bowing in the etching process for minute holes such as trench and contact hole.
FIG. 8
illustrates this generation mechanism. As in the polycrystalline silicon etching, negative charges are accumulated on the sides of a hole and positive charges on the bottom of the hole. This charge build-up deflects the ions
5
which serve as the etching species so that the ions
5
impinge on the sides and bottom corners of the hole. Therefore, the sides and bottom corners of the hole are etched to produce undesirable abnormal shapes such as bowing
15
and subtrench
16
.
The present invention is to eliminate the electron shading phenomenon, thereby solving various problems such as notch, charge build-up damage, bowing and subtrench.
According to one aspect of the invention, as shown in
FIG. 1
, a pulse generator
17
for the bias supply is provided in place of the conventional sine-wave high-frequency power supply. This pulse generator supplies a positive pulse voltage as the bias voltage to the substance being treated. In this case, the duty ratio and repetition frequency of this pulse voltage are selected so that the maximum value of the potential of the substance of which the surface is being processed is higher than that of the above-mentioned plasma. The specific values of the duty ratio and repetition frequency are respectively 5% or below and 400 KHz or above, preferably, 1% or above and 1 MHz or above.
We now consider that the pulse source
17
in
FIG. 1
supplies a positive pulse voltage shown in FIG.
9
. The instant that the discharge has begun, the capacitor has no charge stored, and the same bias wave as the input voltage as shown in
FIG. 10
appears on the substance being treated. When the bias wave is as shown in
FIG. 10
, the positive cycle in which a great amount of electrons impinges is much shorter than the negative cycle in which a small amount of ions impinges, so that the amount of the negative charges impinging in the positive cycle is equal to that of the positive charges impinging in the negative cycle. Therefore, the total charge in one cycle becomes zero, and as a result no charge is induced on the capacitor
2
in FIG.
1
. Accordingly, the substrate bias voltage shown in
FIG. 10
is maintained during the etching process. Of the substrate bias voltage wave in
FIG. 10
, the negative voltage in the negative cycle accelerates ions to impinge the substrate, and the positive voltage in the positive cycle is supplied to the substrate, thus accelerating electrons to perpendicularly impinge on the substance. Therefore, the electrons
6
and ions
5
can impinge up to the bottoms of the fine pattern as illustrated in FIG.
11
. In addition, since the total amount of positive and negative impinging charges in one cycle is zero, there is no charge build-up due to the electron shading phenomenon.
The positive cycle time which is much shorter than the negative cycle time and one-cycle time were estimated by simulation. As the parameters for the simulation, the electron density, electron temperature and capacitance value were respectively selected to be 10
11
/cm
3
, 3 eV, 30 pF/cm
2
which are the standard values of the etching system using a high-density plasma. In addition, considering that chlorine gas is used as etching gas, the mass of the ion used in the simulation was selected to be 35.5 au.
First, the substrate bias voltage wave was calculated under the conditions of the supplied pulse voltage 200 V, repetition frequency 10 MHz and duty ratios 1% and 10%.
FIGS. 12 and 13
show the results. Referring to these figures, when the duty ratio is as small as 1%, the pulse voltage is risen up much positive to accelerate electrons. In addition, self-bias is generated during the interval between the pulses. When the duty ratio of the pulse is as large as 10%, the pulse has almost no positive part, and thus cannot accelerate electrons.
FIG. 14
shows the relation between the duty ratio and the magnitude of the positive part of this substrate bias voltage wave. This result can be divided into the following three regions.
In the region A in which the duty ratio is 0.5% or below, the magnitude of the electron accelerating voltage appearing on the substrate surface when the pulse is supplied is constant without the effect of the repetition frequency. Etching is made chiefly at a floating potential. The positive charge build-up caused by the electron shading at the floating potential is relieved
Arai Shin
Kofuji Naoyuki
Mizuishi Kenichi
Mizutani Tatsumi
Suzuki Keizo
Mills Gregory
Zervigon Rudy
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