Method of measuring negative ion density of plasma and...

Details Method of measuring negative ion density of plasma and... Method of measuring negative ion density of plasma and...

2000-06-19

2002-09-17

Sherry, Michael (Department: 2829)

Electricity: measuring and testing

Using ionization effects

C324S071100, C324S630000, C324S642000, C438S017000, C438S727000

Reexamination Certificate

active

06452400

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method of measuring the negative ion density of a plasma, a plasma processing method and a plasma processing system.
BACKGROUND OF THE INVENTION
An electron cyclotron resonance plasma processing method (ECR plasma processing method) that produces a microwave discharge by utilizing absorption of the energy of a microwave by electrons in cyclotron motion by resonance has become noticed as a method of using a plasma for a film forming process or an etching process in recent years. The ECR plasma processing method is capable of producing a high-density plasma by electrodeless discharge in a high vacuum, of carrying out a rapid surface treatment process and of preventing the contamination of wafers.
A conventional ECR plasma processing system for carrying out an ECR plasma process will be described by way of example with reference to
FIG. 7
as applied to a film forming process. Referring to
FIG. 7
, a microwave of, for example, 2.45 GHz is supplied through a waveguide, not shown, into a plasma producing chamber
9
A and, at the same time, a magnetic field of, for example, 875 G is applied to the plasma producing chamber
9
A by a solenoid
90
to convert a plasma producing gas, such as Ar gas, into a high-density plasma by the interaction (resonance) of the microwave and the magnetic field. A reactive gas, such as C
4
F
8
gas, is activated by the plasma to produce active species. The active species is used for the simultaneous execution of a sputter etching process for etching a silicon wafer W mounted on a susceptor
91
connected to a high-frequency power supply
92
for applying a high-frequency bias voltage to the susceptor
91
, and a film deposition process. The sputter etching process and the film deposition process, which are contrary to each other, are controlled so that the film deposition process is dominant for eventual film deposition.
The inventors of the present invention believe that the measurement of the negative ion density of the plasma is important for plasma processing. The ground of the belief will be described hereinafter.
For example, when a fluorine-containing carbon film (CF film) is used as a layer insulating film, a plasma produced by ionizing a CF gas is used for film formation. Ar gas, for instance, is added to the CF gas to stabilize the plasma. It is know from the respective measured electron temperatures and electron densities of a first plasma produced by ionizing only Ar gas and a second plasma produced by ionizing a mixture of Ar gas and a CF gas, such as C
4
F
8
gas that the first and the second plasma are the same in electron temperature and that the electron density of the second plasma produced by ionizing the mixture is smaller than that of the first plasma produced by ionizing only Ar gas. A plasma is neutral and therefore,
n
i
+
=n
e
+n
i
  (1)
where n
i
+
is positive ion density, n
e
is electron density and n
i
−
is negative ion density.
It is known from the comparison of Expression (1) with the foregoing phenomenon that the fact that electron temperature does not change signifies that n
i
+
does not change, and the fact that n
e
decreases signifies that n
i
−
decreases; that is, negative ions are produced when C
4
F
8
is added to Ar gas.
In the foregoing ECR plasma processing system, there are many unknown actions of the plasma. Since the microwave and the magnetic field are involved, it is difficult to produce a uniform plasma over the surface of the wafer. It is known through the examination of the results of processing, such as the intrasurface film thickness distribution, of wafers processed under the same process conditions, that, in some cases, the wafers are different from each other in intrasurface film thickness distribution. The inventors of the present invention notice negative ions as one of factors that make the control of the condition of the plasma difficult. If the plasma has an excessively large negative ion density, the effective radicals of the plasma decreases. It is considered that an excessively large negative ion density affects bias adversely. Since negative ion density is dependent on the condition of the inner surface of walls of a processing vessel defining a processing chamber, the inventors of the present invention consider that a control loop must include negative ion density as a parameter.
Generally, the negative ion density n
i
−
is determined by measuring positive ion density n
i
+
and electron density n
e
by a measuring method using a Langmuir probe, and calculating the negative ion density n
i
−
by using Expression (1). This measuring method will be briefly described. A probe is inserted in a plasma, voltage VP is applied across the probe, and an anode or a cathode serving as a discharge electrode for producing a plasma, and n
i
+
and n
i
−
are determined on the basis of current IP that flows through the probe when the voltage VP is changed.
FIG. 8
is a graph showing the dependence of the current IP on the voltage VP. The voltage VP is applied across the probe and the electrode connected to the probe. As the voltage VP increases toward the positive side, the current IP is saturated. A saturation current I
es
in this state is expressed by Expression (2).
I
es
=(
e
/4)·
n
e
·(8
k·
T
e
/&pgr;m
e
)
½
·A
  (2)
As the voltage VP is decreased toward the negative side, the current IP is saturated. Saturation current I
is
in this state is expressed by Expression (3).
I
is
={e
/exp[½
]}·n
i
+
·(
k·T
e
/&pgr;m
i
)
½
·A
  (3)
In Expressions (2) and (3), e is elementary electric charge, k is Boltzmann constant, T
e
is electron temperature, m
e
is the mass of and electron, m
i
is the mass of an ion, A is the effective collecting area of the probe for collecting ions and electrons. Generally, the area A is equal to the surface area S of a metal part of the probe.
The electron density n
e
is known from Expression (2), the positive ion density n
i
+
is known from Expression (3) and hence the negative ion density n
i
−
is known from Expression (1). Electron temperature T
e
can be determined on the basis of the gradient of a section of the IP-VP curve in a region where the voltage VP is positive.
Generally, the negative ion density of the plasma can be thus measured. However, this method is not applicable to a plasma produced by ECR. Since an ECR plasma processing system does not have discharge electrodes, a base end part of a probe
100
is connected through a variable-voltage power supply
200
to a ground kept at a ground potential as shown in FIG.
9
. Since a magnetic field B is created around the probe
100
, the effective collecting area A in Expression (2) is a surface area S′ smaller than the surface area S of the metal part of the probe, because the Larmor radius of electrons in a magnetic field is small, electrons wind round lines of magnetic force, and a portion of the surface of the metal part is shaded from a flux of electrons, so that the collecting area is reduced. Consequently, the positive saturation current I
es
when the magnetic field B is created around the probe is lower than that when any magnetic field is not created around the probe as shown in FIG.
8
. However, the saturation current I
es
cannot be determined because S′ is unknown. Since the collection area is S in Expression (3), I
is
cam be determined. Thus, although the positive ion density n
i
(n
i
+
) can be measured by a measuring method using the probe when the magnetic field is created, the electron density n
e
cannot be determined by the measuring method using the probe.
In the present state of art, the negative ion density n
i
−
is determined by measuring the electron density n
e
by a microwave interferometer using change in the refraction of a microwave, and using the measured electron density n
e
and the positive ion density n
i
+
measured by a

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