JPH06275222A - Self-bias control device by plural electrodes - Google Patents

Abstract

PURPOSE:To reconcile an optimum etching strength and a high etching rate by providing a divided electrode, and freely controlling the capacity between the divided electrode and a plasma to change the self-bias regardless of a high frequency power source or gas pressure. CONSTITUTION:A vacuum vessel 1 is formed of an insulator, and a plurality of divided electrodes 7 are provided along the wall surface in the inner part of the vessel 1. The electrode 7 is connected to a ground potential by an independent variable resistor 12 or switch, and the electrode 7 and the ground potential are short-circuited or separated by regulating the resistor 12 or switch to effectively change the surface area of the electrode 7. Namely, the electrode 7 is provided so that the capacity between the electrode 7 and a plasma can be freely controlled. Since the self-bias can be changed regardless of a high frequency power source or gas pressure, thus, an optimum etching strength and a high etching rate can be reconciled.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-frequency plasma apparatus such as a thin film forming apparatus and an etching apparatus for converting a source gas into plasma by using high frequency. In particular, the invention provides a high-frequency plasma device capable of freely adjusting the self-bias of plasma.

A large number of apparatuses for applying a high frequency between electrodes to turn gas into plasma are used for etching and CVD. Depending on the type of gas, it is possible to form a film or remove surface substances.

In some cases, parallel plate electrodes are used to ground one and apply a high frequency to the other. In this case, plasma is generated between the plate electrodes. Alternatively, the entire inner wall of the device may be grounded and a high frequency may be applied to one electrode. In this case, the plasma fills the entire internal space. Plasma is an assembly of electrons and ions. At this time, the potential between the wafer and the plasma is an important parameter.

[0004]

2. Description of the Related Art Although this potential difference is an important parameter, it cannot be adjusted freely. Since this naturally occurs based on the difference in mobility of electrons and ions, self-bias is generated in the high frequency electrode.
Actually, a high frequency voltage is applied between the electrodes via a capacitor. Since the electrons are light, they can move in the plasma in response to changes in the high frequency electric field. However, the heavy ions cannot keep up with the high-frequency electric field and remain almost stationary. Therefore, the electrode on which the substrate is placed becomes more negative, and a direct current component is added to the capacitor.

FIG. 3 is a schematic configuration diagram of a conventional high-frequency plasma generator. It may be used as a CVD apparatus or as an etching apparatus. It may also be used to modify substances in plasma.

The vacuum container 1 is a container that can be sealed. It is made of metal and is grounded. The high frequency electrode 2 to which a high frequency is applied is supported inside the vacuum container 1 in an insulated state by the insulator 3. One terminal of the high frequency power supply 5 is connected to the high frequency electrode 2 via the capacitor 4. The other terminal of the high frequency power supply 5 is connected to the vacuum container 1. This is the ground voltage.

[0007] The vacuum container 1 is once evacuated, and the raw material gas is introduced into the vacuum container 1. A high frequency voltage is applied between the vacuum container 1 and the high frequency electrode 2. Electrons mainly vibrate to obtain energy. And it jumps out of the atom and becomes a free electron. This gains more energy and flies around, so new electrons are knocked out from the atomic molecules, becoming positive ions and electrons.
In this way, a dense plasma is generated near the high frequency electrode 2. Plasma includes ions, electrons, neutral atomic molecules, and the like. The action of the electric field causes ions and electrons to oscillate in the plasma.

Since the plate areas of the electrodes are different, the plasma is densely present in the vicinity of the high frequency electrode 2, but the electrons jump into the high frequency electrode 2 and disappear when the high frequency electrode 2 is positively biased. The ions hardly move while the alternating electric field changes. Therefore, the distribution of electrons is localized in a region close to the high frequency electrode 2. It means that the positive ions exist far from the high frequency electrode 2. As a result, the high frequency electrode 2 is negatively biased. That is, the vacuum container 1 has a voltage of 0 V, and the high-frequency electrode 2 has a negative voltage of several hundred V.

[0009]

However, the problem here is the difference between the plasma potential and the high-frequency electrode 2. Therefore, this difference voltage is referred to as self-bias here. Consider the case where the substrate is placed on the high-frequency electrode 2 and the surface thereof is reactively etched. In this case, the acceleration energy of the ions is determined by the difference between the plasma potential and the potential of the high frequency electrode 2. That is, it is equal to self-bias. The etching ability depends on the chemical activity of the source gas, but also on the impulse at the time of impact. Impulse naturally depends on self-bias. The etching rate is therefore strongly influenced by self-bias.

However, when etching a certain substance, it may be desirable that the action of etching be changed at any time. In this case, changing the self-bias is one method.

In the reactive ion etching, the adjustment of the self-bias is usually performed by controlling the high frequency power.
RF power is the product of voltage and current. Since the impedance of the plasma is not constant, it cannot be said that the power is simply proportional to the square of the voltage. Electric power can be controlled by increasing and decreasing the gas pressure and increasing and decreasing the power supply voltage. Lowering the power lowers the self-bias, and raising the power raises the self-bias.

If this is the only problem, there is no problem.
Complicating matters is the relationship between plasma density, power and etch rate.

For example, there is a case where an Al pattern is formed on a Si wafer and an Al oxide film is formed on the Si pattern, and it is desired to remove the oxide film by etching. In the case of this etching, a large self-bias must first be applied to strongly strike the surface with ions to remove the Al oxide film. After this, it is necessary to lower the self-bias and reduce the impact force of ions to reduce the damage of the substrate.

At the initial stage, it is necessary to lower the gas pressure in order to increase the self-bias. When the gas pressure is lowered, the etching rate is lowered. Then, the efficiency of removing the Al oxide film is poor. In the latter half, it is necessary to raise the gas pressure to lower the self-bias. Then, the etching rate rises and the etching on the substrate becomes more intense. In this case, it is necessary to reduce the power of the power supply. However, there is a complicated relationship as to how much the electric power should be decreased according to the decrease in gas.

When the gas pressure is increased, the etching rate increases, but this increases the density of plasma and thus lowers the self-bias. Since the impact force of ions is due to self-bias,
The self-bias can be increased to enhance the etching ability. However, if this is done, the etching rate will drop, which is a trade-off. Power control of high frequency power supplies is not the key to solving this tradeoff. There is an optimum range of gas pressure and electric power for stable plasma generation, and they cannot be freely changed.

What has been described here is a problem as an etching apparatus. However, even when it is used as a CVD apparatus, there are cases where it is desired to freely set the plasma self-bias. Energy is required to cause a chemical reaction when forming a thin film. This is given as heat or kinetic energy of plasma. Since kinetic energy depends on self-bias, control of self-bias is desired.

[0017]

According to the present invention, an electrode is provided in a vacuum container, a substrate is placed on the electrode, a gas is introduced into the vacuum container, plasma is generated by high frequency power, and the plasma causes some action on the substrate. In the plasma device configured to exert the effect, the vacuum container is an insulator, a plurality of divided electrodes are provided along the wall surface inside the vacuum container, and the divided electrodes and the ground potential are connected by independent variable resistors or switches,
By adjusting the variable resistance or the switch, the divided electrode and the ground potential are short-circuited or cut off to effectively change the surface area of the electrode.

[0018]

In the plasma device, the voltage is applied between the electrodes forming the effective capacitor. In the case of the conventional apparatus shown in FIG. 3, one electrode is the entire inner wall of the container. The other high-frequency electrode is narrow and the wall surface of the entire container is large. The plasma in the middle is a conductor, which can also be considered as the electrode of one capacitor.

Then, it can be considered that there are two capacitors in the plasma device. One capacitor uses the vessel wall and plasma as electrodes. The other capacitor uses plasma and high frequency electrodes as electrodes. The capacitance C of the capacitor is inversely proportional to the inter-electrode distance d and proportional to the electrode area S. That is, C = εS / d. The distance between the plasma and the container wall d _a, the distance between the plasma and the high frequency electrode d _e, the area of the container wall S _a, the area of the high-frequency electrode 2 and S _e. When the capacitance between the plasma and the container is C _a and the capacitance between the plasma and the high frequency electrode 2 is C _e , C _a = εS _a / d
_a and C _e = εS _e / d _e . The plasma potential is the negative voltage -V _q of the capacitor 4 divided into C _e : C _a . The self-bias corresponds to the latter of these, and −V _q
It becomes C _a / (C _a + C _e ). Conventionally, since C _a and C _e were fixed, the self-bias could not be adjusted unless the voltage −V _q was changed.

However, in the present invention, since the container is made of an insulator and a plurality of divided electrodes are used, it is possible to effectively change the area corresponding to the facing area between the plasma and the container wall. That is, what is conventionally a constant value for the container area S ₁ is replaced by the area of the divided electrode at the ground potential. This is because the divided electrodes at the ground potential form a capacitor with the plasma. Split electrodes 1, 2,
The area of ...... n is A ₁ , A ₂ , ...... A _n, and the sum of the areas of those connected by low resistance or switch is S ₁
Then, C _a = εS _a / d _a and C _e = εS _e / d _e . The capacity of the capacitor between the plasma and the container is changed, it is possible to freely change the self-bias _{-V q C a / (C a} + C e).

Consider a variable W _i which is 1 when the i-th divided electrode is connected by a low resistance or a switch, and is 0 when it is not connected (switch is disconnected by a high resistance). Using this parameter, S _a = ΣW _i
It can be written as A _i . C _a = εS _a / d _a = εΣ
Since it becomes W _i A _i / d _a , it is well understood that the capacitance C _{a of} the capacitor changes.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematic configuration diagram of a plasma device according to an embodiment of the present invention. The vacuum vessel 1 provides a space that can be evacuated to a gas and turned into plasma. The vacuum container 1 is an insulator. Inside this is a flat high-frequency electrode 2
It is provided by being insulated. The high frequency electrode 2 is connected to an external high frequency power source 5 via an external capacitor 4.
The other terminal of the high frequency power supply 5 is grounded. A plurality of flat plate electrodes are provided along the inner wall of the vacuum container 1.
This is referred to as a split electrode 7 because the vacuum container 1 is an electrode which is integrally formed and is divided into a plurality of electrodes.

The divided electrodes 7 are connected to external variable resistors R ₁ , R ₂ , ... Through the current introducing terminals 6, respectively. The other ends of the variable resistors R ₁ , R ₂ , ... Are grounded. The external variable resistor can be controlled independently and takes values from 0 ohm to infinity. In the case of 0 ohm, the divided electrodes are grounded. When the resistance is infinite, the split electrodes are floating. The variable resistor can take an intermediate value, but since it is important to have 0 ohm and infinity, it can be replaced with a simple switch.

Instead of the inner wall of the container, the divided electrode 7 forms a capacitor with the plasma 16. There is a narrow transition region between the electrode and the plasma. This is a plasma sheet
Called Su 8. The thickness of the plasma case corresponds to the thickness d of the capacitor. If the area of one divided electrode i is A _i and the thickness of its sheath is d, the capacitance of this divided electrode is εA _i /
d. However, only when the divided electrodes are connected to the ground potential, this capacity is provided. The capacitance is 0 when the split electrode is separated from the ground electrode. Therefore, by using the connection parameter W _i which is 1 when the divided electrode i is connected to the ground electrode and 0 when it is not connected, the capacitance between the divided electrode and the plasma is always W _i εA _i. You can write / d.

[0025] Accordingly, capacitance between all the divided electrodes and the plasma _{C a = εS a / d a} = εΣW i A i
You can write / d _a . However, the capacitor between the plasma and the high frequency electrode 2 remains unchanged. Shield on the high frequency electrode 2
It is possible, but this is constant. Area does not change. Therefore, the relationship of C _e = εS _e / d _e does not change.

FIG. 2 shows an equivalent circuit in the configuration of FIG. Plasma 16 has plasma impedance 9,
A plurality of divided electrodes 7 are connected to this. One end of the high frequency power source 5 is grounded. The other end is connected to the high frequency electrode 2 via the external capacitor 4. A capacitor C _e exists between the high frequency electrode 2 and the plasma. Capacitors C ₁ , C ₂ , ... C _n exist between each split electrode and the plasma. Plasma impedance is assumed to be zero.

FIG. 4 shows a further simplified circuit. Only two split electrodes are shown. The resulting capacitors are C ₁ and C ₂ . The capacitor between the high frequency electrode 2 and the plasma is constant at C _e . The self-bias is proportional to V ₁ , but if R = 0, the capacitor between the plasma and the split electrode is (C ₁ + C ₂ ). At this time, the self-bias is V ₁ = Vrf (C ₁ + C ₂ ) / (C ₁ + C ₂
+ C _e ). If R is infinite, the capacitor between the plasma and the split electrode is C ₁ . The self-bias is proportional to V ₁ = VrfC ₁ / (C ₁ + C _e ).

Since there are actually n divided electrodes, 1 ≦
When an integer k of k ≦ n is used, the self-bias is V ₁ = kC
_It is proportional to ₀ Vrf / (kC ₀ + C _e ). However, here, the capacities of the divided electrodes for the plasma are all equal, and C ₁ =
It is assumed that C ₂ = ... = C ₀ . If C ₀ is selected sufficiently small with respect to C _e , the value of self-bias can be widely changed.

FIG. 5 is a graph showing changes in self-bias with time. As described above, the pattern of Al is formed on the Si wafer.
If there is a silicon oxide film and an Al oxide film is formed on it, the self-bias must be increased first to strongly etch the oxide film. After removing the oxide film, it is necessary to lower the self-bias to lower the etching ability. Unlike the conventional method, this can be performed while keeping the power of the power source constant and the gas pressure constant. Since the gas pressure does not have to be lowered, the etching rate does not decrease.
Since the self-bias can be lowered in the latter stage of etching, the damage of the wafer can be avoided.

[0030]

EFFECTS OF THE INVENTION In an apparatus for converting gas into plasma by high-frequency discharge, instead of using the high-frequency electrode and the entire container as electrodes of a capacitor, split electrodes are provided so that the capacitance between the split electrode and plasma can be freely controlled. I have to. Therefore, the self-bias can be changed regardless of the high frequency power supply and the gas pressure. Therefore, both optimum etching strength and high etching rate can be achieved. C
Kinetic energy of raw material plasma when used in VD equipment
The degree of freedom in controlling the conditions for forming the thin film is increased.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a plasma device according to an embodiment of the present invention.

FIG. 2 is an equivalent circuit diagram of the device of FIG.

FIG. 3 is a schematic configuration diagram of a plasma device according to a conventional example.

4 is a simplified circuit diagram of the device of FIG.

FIG. 5: Si wafer has Al pattern and Al on it
FIG. 6 is a graph showing a change with time in self-bias during etching in the presence of oxide.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 vacuum container 2 high frequency electrode 3 insulator 4 capacitor 5 high frequency power supply 6 current introducing terminal 7 split electrode 8 plasma sheath 9 plasma impedance 10 split electrode sheath capacitance 11 high frequency electrode sheath capacitance 12 variable resistance 13 high frequency voltage

Claims (1)

[Claims] 1. A vacuum container 1 which can be evacuated to provide a space for generating plasma, a flat-plate high-frequency electrode 2 provided inside the vacuum container 1, and a plasma provided outside the vacuum container 1. A high frequency power supply 5 for supplying high frequency power, an external capacitor 4 connecting one end of the high frequency power supply 5 and the high frequency electrode 2, a plurality of split electrodes 7 provided inside the vacuum container 1, and the other end of the high frequency power supply. A variable resistance or a switch for connecting the divided electrode 7 and the divided electrode 7, and by changing the variable resistance or the switch, the DC bias between the plasma and the high frequency electrode 2 is changed. Bias control device.