Patent Description:
As one type of technique for etching a film to be etched, an atomic layer etching (ALE) technique, in which the film to be etched is etched in a unit of one atomic layer, has been known.

As the ALE technique, techniques referred to as real-ALE technique and quasi-ALE technique have been disclosed. In the real-ALE technique, a film to be etched is etched by causing the film to be etched to chemisorb active species that are based on processing gas, and causing the active species to react with the film to be etched, using noble-gas plasma. In the quasi-ALE, a film to be etched is etched by causing a polymer layer that is based on processing gas to deposit on the film to be etched, and causing the polymer layer to react with the film to be etched, using noble-gas plasma.

The article titled "<NPL>, discloses that layer-by-layer etching of silicon on atomic scale has been achieved by repeating the reaction cycles of fluorine (F) atom adsorption on a cooled Si surface and subsequent Ar+ ion (≈ <NUM> eV) irradiation which induces fluorine/Si surface reactions. The digital etch rate first increases and reaches a plateau region with an increase of Ar+ ion irradiation time. For the case of CF<NUM>/O<NUM> downstream plasma as a fluorine source, CFx radical accumulation appears to be a self-limiting stop of the F/Si reaction to promote atomic layer etching, while F atoms produced by a remote NF<NUM> plasma or an F<NUM>/<NUM>%He discharge also cause similar atomic layer etching in which the amount of physiosorbed fluorine molecules on Si surfaces controls the etch rate. The etching in the plateau region exhibits no microloading effect because the fluorine coverage is independent of pattern size. Anisotropic etching of Si with a <NUM> PMMA mask pattern and an aspect ratio of <NUM> is attained.

However, in the real-ALE technique, limitations are imposed on usable processing gas, and on materials used for the parts inside of the etching chamber designed to handle the processing gas because, in order to allow the film to be etched to chemisorb the active species that are based on the processing gas, the temperature of the substrate is raised so that the selectable film to be etched exhibits chemical reactivity to the processing gas, or because the reactivity is increased by using plasma. Furthermore, when plasma is used, the plasma may react with the parts inside of the etching chamber, and change the performance of the etching process.

Furthermore, in the quasi-ALE, because it is difficult to control the thickness by which the polymer layer is deposited on the film to be etched, in a unit of one atomic layer, it is difficult to control the etch amount of the film to be etched in a unit of one atomic layer. As a result, an implementation of self-limiting etching is not possible with the quasi-ALE technique. Furthermore, the polymer layer to be deposited is usually deposited on the film to be etched by dissociating fluorocarbon gas using plasma, but the polymer layer may also become deposited on the parts inside of the etching chamber, and change the performance of the etching process.

The present invention provides a plasma etching method according to Claim <NUM>, and a plasma etching apparatus according to Claim <NUM>. Optional features are set out in the remaining claims.

According to one aspect of the plasma etching method disclosed herein, an implementation of self-limiting etching, with no limitation on the film to be etched, is made possible, advantageously.

Various embodiments will now be explained in detail with reference to some drawings. In the drawings, the same reference signs will be assigned to the same parts or equivalent parts.

A plasma etching apparatus according to one embodiment will now be explained based on <FIG> is a schematic generally illustrating one example of a cross section of a plasma etching apparatus <NUM> according to one embodiment. The plasma etching apparatus <NUM> illustrated in <FIG> is a capacitively coupled plasma etching apparatus.

The plasma etching apparatus <NUM> includes a chamber main body <NUM>. The chamber main body <NUM> has a substantially cylindrical shape. The internal space of the chamber main body <NUM> is provided as a chamber 12c. The inner wall of the chamber main body <NUM> is applied with a plasma-resistant film. This film may be an alumite film, or a film made of yttrium oxide. The chamber main body <NUM> is grounded. An opening <NUM> is provided on the side wall of the chamber main body <NUM>. When a wafer W is carried from external of the chamber main body <NUM> into the chamber 12c, and when the wafer W is carried from the chamber 12c to the external of the chamber main body <NUM>, the wafer W passes through the opening <NUM>. A gate valve <NUM> is mounted on the side wall of the chamber main body <NUM> to open and to close the opening <NUM>.

A support <NUM> is provided on the bottom of the chamber main body <NUM>. The support <NUM> has a substantially cylindrical shape. The support <NUM> is made of an insulating material, for example. The support <NUM> extends upwards from the bottom of the chamber main body <NUM> inside the chamber 12c. A stage <NUM> is provided inside the chamber 12c. The stage <NUM> is supported by the support <NUM>.

The stage <NUM> is configured to hold the wafer W placed thereon. The stage <NUM> includes a lower electrode <NUM> and an electrostatic chuck <NUM>. The lower electrode <NUM> includes a first plate 18a and a second plate 18b. The first plate 18a and the second plate 18b are made of metal such as aluminum, and have a substantially disk-like shape. The second plate 18b is provided on the first plate 18a, and is electrically connected to the first plate 18a.

The electrostatic chuck <NUM> is provided on the second plate 18b. The electrostatic chuck <NUM> includes an insulating layer, and a film-like electrode that is provided inside the insulating layer. To the electrode of the electrostatic chuck <NUM>, a direct-current (DC) power source <NUM> is electrically connected via a switch <NUM>. To the electrode of the electrostatic chuck <NUM>, a DC voltage is applied from the DC power source <NUM>. When a DC voltage is applied to the electrode of the electrostatic chuck <NUM>, the electrostatic chuck <NUM> generates electrostatic attraction, and holds the wafer W by attracting the wafer W onto the electrostatic chuck <NUM>. The electrostatic chuck <NUM> may be provided with an internal heater, and a heater power source provided external of the chamber main body <NUM> may be connected to the heater.

A focus ring <NUM> is provided along the circumferential edge of the second plate 18b. The focus ring <NUM> is a plate having a substantially annular shape. The focus ring <NUM> is disposed in a manner surrounding the edge of the wafer W, and the electrostatic chuck <NUM>. The focus ring <NUM> is provided to improve etching uniformity. The focus ring <NUM> may be made of a material such as silicon or quartz.

A flow channel 18f is provided inside the second plate 18b. Refrigerant is supplied from a chiller unit provided external of the chamber main body <NUM> into the flow channel 18f, via a pipe 26a. The refrigerant supplied into the flow channel 18f is returned to the chiller unit via the pipe 26b. In other words, refrigerant is circulated between the flow channel 18f and the chiller unit. By controlling the temperature of this refrigerant, the temperature of the stage <NUM> (or the electrostatic chuck <NUM>) and the temperature of the wafer W are adjusted. One example of the refrigerant includes Galden (registered trademark).

The plasma etching apparatus <NUM> is provided with a gas supply line <NUM>. The gas supply line <NUM> supplies heat-transfer gas, such as He gas, supplied from a heat-transfer gas supply mechanism into the space between the upper surface of the electrostatic chuck <NUM> and the rear surface of the wafer W.

The plasma etching apparatus <NUM> is also provided with an upper electrode <NUM>. The upper electrode <NUM> is provided above the stage <NUM>. The upper electrode <NUM> is supported in the upper part of the chamber main body <NUM> via a member <NUM>. The upper electrode <NUM> may include an electrode plate <NUM> and a support body <NUM>. The bottom surface of the electrode plate <NUM> faces the chamber 12c. The electrode plate <NUM> is provided with a plurality of gas discharge holes 34a. This electrode plate <NUM> may be made of a material such as silicon or silicon oxide.

The support body <NUM> is configured to support the electrode plate <NUM> in a removable manner, and is made of a conductive material such as aluminum. A gas diffusion chamber 36a is provided inside the support body <NUM>. A plurality of gas passage holes 36b communicating with the gas discharge holes 34a extend downwards from the gas diffusion chamber 36a. The support body <NUM> is provided with a gas inlet 36c for guiding the gas into the gas diffusion chamber 36a. To the gas inlet 36c, a gas supply pipe <NUM> is connected.

A gas source group <NUM> is connected to the gas supply pipe <NUM> via a valve group <NUM> and a flow controller group <NUM>. The gas source group <NUM> includes a plurality of gas sources. The gas sources at least include a gas source of first processing gas, and a gas source of second processing gas. The first processing gas is gas that is caused to be physisorbed onto the film to be etched, on the wafer W. The second processing gas is gas used in etching the film to be etched. The gas sources may also include a gas source other than those of the first processing gas and the second processing gas.

The valve group <NUM> includes a plurality of valves, and the flow controller group <NUM> includes a plurality of flow controllers such as mass flow controllers or pressure-regulating flow controllers. Each of the gas sources included in the gas source group <NUM> is connected to the gas supply pipe <NUM> via the corresponding valve in the valve group <NUM>, and via the corresponding flow controller in the flow controller group <NUM>.

A baffle member <NUM> is provided between the support <NUM> and the side wall of the chamber main body <NUM>. The baffle member <NUM> is a plate-like member, for example, and may be a base material made of aluminum the surface of which is covered with a ceramic such as Y2O3. The baffle member <NUM> has a plurality of holes passing through the baffle member <NUM>. Below the baffle member <NUM>, an exhaust device <NUM> is connected to the bottom of the chamber main body <NUM>, via an exhaust pipe <NUM>. The exhaust device <NUM> has a pressure regulator such as a pressure regulating valve, and a vacuum pump such a turbo-molecular pump, and is capable of reducing the pressure inside of the chamber 12c to a desired pressure.

The plasma etching apparatus <NUM> is also provided with a first high-frequency power source <NUM> and a second high-frequency power source <NUM>. The first high-frequency power source <NUM> is a power source for generating first high-frequency power (high-frequency electric energy) for generating plasma. The first high-frequency power has a frequency within a range of <NUM> to <NUM>, for example. The first high-frequency power source <NUM> is connected to the upper electrode <NUM> via a matching device <NUM>. The matching device <NUM> has a circuit for matching the output impedance of the first high-frequency power source <NUM> to the load-side impedance (impedance on the side of the upper electrode <NUM>). The first high-frequency power source <NUM> may also be connected to the lower electrode <NUM> via the matching device <NUM>.

The second high-frequency power source <NUM> is a power source for generating second high-frequency power (high-frequency electric energy) for drawing ions onto the wafer W. The second high-frequency power has a frequency within a range of <NUM> to <NUM>, for example. The second high-frequency power source <NUM> is connected to the lower electrode <NUM> via a matching device <NUM>. The matching device <NUM> has a circuit for matching the output impedance the second high-frequency power source <NUM> to the load-side impedance (impedance on the side of the lower electrode <NUM>).

A control unit <NUM> controls the operation of the plasma etching apparatus <NUM> comprehensively. This control unit <NUM> is provided with a central processing unit (CPU), and is provided with a process controller <NUM> for controlling the units included in the plasma etching apparatus, a user interface <NUM>, and a storage <NUM>.

The user interface <NUM> includes a keyboard allowing a process manager to make a command input operation for managing the plasma etching apparatus, and a display for visualizing and displaying the operation status of the plasma etching apparatus, for example.

The storage <NUM> stores therein a control program (software) for implementing various processes executed in the plasma etching apparatus, under the control of the process controller <NUM>, and recipes storing therein data such as processing condition data. By calling a recipe from the storage <NUM> using an instruction or the like entered via the user interface <NUM>, and causing the process controller <NUM> to execute the recipe, as required, a desired process is performed in the plasma etching apparatus under the control of the process controller <NUM>. It is also possible to use a control program or a recipe such as the processing condition data stored in a computer-readable recording medium (such as a hard disk, a compact disc (CD), a flexible disk, or a semiconductor memory), or to use a control program or a recipe online, by causing another apparatus to transmit the control program or the recipes via a dedicated circuit, as required, for example.

The control unit <NUM> controls the units included in the plasma etching apparatus <NUM> so as to execute a plasma etching method to be described below, for example. To explain using a specific example, the control unit <NUM> executes a physisorption step for causing an adsorbate that is based on the first processing gas to be physisorbed onto the film to be etched, while cooling an object to be processed on which the film to be etched is provided. The control unit <NUM> executes an etching step for etching the film to be etched by causing the adsorbate to react with the film to be etched, using the plasma of the second processing gas. The object to be processed herein is the wafer W, for example. The physisorption step and the etching step may be repeated alternatingly a plurality of times.

<FIG> is a schematic illustrating one example of a structure of the wafer W. The wafer W has a silicon film <NUM>, a silicon oxide film <NUM>, and a silicon nitride film <NUM> on a substrate <NUM>, as illustrated in <FIG>, for example. The silicon film <NUM>, the silicon oxide film <NUM>, and the silicon nitride film <NUM> are arranged adjacently to one another. Among these films, the silicon film <NUM> and the silicon oxide film <NUM> are examples of the film to be etched.

One example of the sequence of a process of the plasma etching method executed by the plasma etching apparatus <NUM> will now be explained. <FIG> is a flowchart illustrating one example of the sequence of a process of a plasma etching method according to one embodiment. <FIG> is a schematic illustrating one example of a cross section of the wafer W after the execution of the steps illustrated in <FIG>. In the explanation below, it is assumed that the plasma etching apparatus <NUM> execute a series of steps to the wafer W illustrated in <FIG>.

In the plasma etching method according to the embodiment, to begin with, the wafer W that is the object to be processed is carried into the chamber 12c, and is placed on the stage <NUM>. At this point in time, the cross section of the wafer W is as illustrated in <FIG>, for example.

The plasma etching apparatus <NUM> then sets values to a parameter m for counting the number of times the steps are repeated, and a parameter m0 specifying the upper bound of the number of times the steps are repeated. The plasma etching apparatus <NUM> sets "<NUM>" to the parameter m, for example, and sets "<NUM>" to the parameter m0, for example (Step S101). The value set to the parameter m0 may be any value. If the value set to the parameter m0 is "<NUM>", for example, the physisorption step and the etching step are performed once, without repeating the steps.

The plasma etching apparatus <NUM> then executes the physisorption step for causing the adsorbate that is based on the first processing gas to be physisorbed onto the film to be etched, while cooling the wafer W to a temperature equal to or lower than the dew point of the first processing gas (Step S102). The film to be etched is, for example, the silicon oxide film <NUM> on the wafer W. The first processing gas includes CF gas, for example. CF gas is, for example, C4F8 or C5F8. It is considered that the physisorption between the adsorbate that is based on the first processing gas and the film to be etched takes place due to van der Waals force, for example. Van der Waals force is a force by which molecules or atoms are attracted to one another, and the effect of this force becomes greater when the temperature of the molecules or the atoms becomes lower. Therefore, even when the film to be etched does not have any chemical reactivity to the first processing gas, by cooling the wafer W to a temperature equal to or lower than the dew point of the first processing gas, the first processing gas is caused to be physisorbed onto the film to be etched, by van der Waals force.

This step will now be explained using a more specific example. The control unit <NUM> in the plasma etching apparatus <NUM> cools the wafer W to a temperature equal to or lower than the dew point of C4F8 that is the first processing gas (hereinafter, referred to as "cryogenic temperature", as appropriate) by controlling the temperature of the refrigerant circulating through the flow channel 18f inside the stage <NUM>. The control unit <NUM> then causes the gas source group <NUM> to supply C4F8 into the chamber 12c as the first processing gas, while keeping the wafer W at the cryogenic temperature. As a result, the adsorbate that is based on C4F8 is caused to be physisorbed onto the film to be etched, by van der Waals force, in a unit of one atomic layer.

The cross section of the wafer W after the execution of the physisorption step illustrated as Step S102 is as illustrated in <FIG>, for example. In other words, by C4F8 being supplied into the chamber 12c, while the wafer W is cooled to the cryogenic temperature, the C4F8-based adsorbate <NUM> is caused to be physisorbed onto the silicon oxide film <NUM> that is the film to be etched. Because the entire wafer W is cooled to the cryogenic temperature, the C4F8-based adsorbate <NUM> is caused to be physisorbed onto the silicon film <NUM> and the silicon nitride film <NUM>, as well as onto the silicon oxide film <NUM>.

The plasma etching apparatus <NUM> then executes the etching step for etching the film to be etched, by causing the adsorbate to react with the film to be etched, using the plasma of the second processing gas (Step S103). The second processing gas includes noble gas, for example. The noble gas is Ar, for example.

This step will now be explained using a more specific example. The control unit <NUM> in the plasma etching apparatus <NUM> causes the gas source group <NUM> to supply Ar into the chamber 12c as the second processing gas, while cooling the wafer W to the cryogenic temperature, so as to replace C4F8 with Ar, and generates Ar plasma by causing the first high-frequency power source <NUM> to apply the first high-frequency power for generating plasma. At this time, the control unit <NUM> may also cause the second high-frequency power source <NUM> to apply the second high-frequency power for drawing ions. As the Ar plasma is generated, the plasma promotes the collisions of ions (that is, Ar ions) against the adsorbate on the film to be etched, and induces a reaction between the adsorbate and the film to be etched. As a result, the film to be etched is etched in a unit of one atomic layer, depending on the thickness of the adsorbate.

The cross section of the wafer W after the etching step illustrated at Step S103, is as illustrated in <FIG>, for example. In other words, as the reaction is induced between the adsorbate <NUM> on the silicon oxide film <NUM> that is the film to be etched, and the silicon oxide film <NUM>, the silicon oxide film <NUM> becomes etched selectively, by the thickness corresponding to the thickness of the adsorbate <NUM>. At the same time, the reactions between the adsorbate <NUM>, and the silicon film <NUM> and the silicon nitride film <NUM>, which are not to be etched, are also induced. However, because the silicon film <NUM> and the silicon nitride film <NUM> have carbon-containing substances <NUM>, <NUM>, respectively, that are derived from the C4F8-based adsorbate <NUM>, etching of the silicon film <NUM> and the silicon nitride film <NUM> is suppressed.

In other words, at the etching step illustrated at Step S103, following reactions are induced:.

SiO2 + CxFy → SiF4 (Gas) + CO or CO2(Gas).

Si3N4 + CxFy → SiF4 (Gas) + NF3 (Gas) + C or CN(Solid).

The plasma etching apparatus <NUM> then determines whether the physisorption step and the etching step have been repeated by the preset number of times, that is, whether the parameter m is equal to or greater than the parameter m0 (Step S104). If the parameter m is less than the parameter m0 (No at Step S104), the plasma etching apparatus <NUM> increments the parameter m by one (Step S105), shifts the process back to Step S102, and repeats the physisorption step and the etching step. If the parameter m is equal to or greater than the parameter m0 (Yes at Step S104), the plasma etching apparatus <NUM> ends the process.

The order of the steps illustrated in <FIG> is not limited to that described above, and may be changed as appropriate, within the scope in which the steps do not contradict one another. The control unit <NUM> executes an adjustment step for adjusting the thickness of the adsorbate, by evaporating or subliming a part of the adsorbate having been physisorbed onto the film to be etched, between the physisorption step and the etching step.

The plasma etching method according to one embodiment will now be explained in further detail. <FIG> is a schematic illustrating a time chart of conditions used in the plasma etching method according to one embodiment. Explained here is an example in which the control unit <NUM> in the plasma etching apparatus <NUM> executes the physisorption step, the adjustment step, and the etching step repeatedly, in the order listed herein, to the wafer W illustrated in <FIG>. When the physisorption step, the adjustment step, and the etching step are executed in the order listed herein, the control unit <NUM> controls the timing for supplying C4F8, the timing for supplying Ar, the timing for supplying the first high-frequency power, and the timing for supplying the second high-frequency power, in accordance with the time chart illustrated in <FIG>.

To begin with, the control unit <NUM> executes the physisorption step for causing the C4F8-based adsorbate <NUM> to be physisorbed onto the silicon oxide film <NUM>, while cooling the wafer W to a temperature equal to or lower than the dew point of C4F8 (that is, "cryogenic temperature"). Specifically, the control unit <NUM> supplies C4F8 for a period between time "t0" and time "t1", while keeping the wafer W at the cryogenic temperature, illustrated in <FIG>. As a result, the C4F8-based adsorbate <NUM> is caused to be physisorbed onto the silicon oxide film <NUM>. The period between the time "t0" and the time "t1" may be about <NUM> seconds, for example.

The control unit <NUM> then executes the adjustment step for adjusting the thickness of the adsorbate <NUM> by evaporating or subliming a part of the adsorbate <NUM> having been physisorbed, as adsorbate, onto the silicon oxide film <NUM>. Specifically, the control unit <NUM> stops supplying C4F8, and starts supplying Ar at the time "t1", as illustrated in <FIG>. As a result, C4F8 is replaced with Ar, and the partial pressure of C4F8 is reduced for a period between the time "t1" and at time "t2". When the partial pressure of C4F8 is reduced, a part of the C4F8-based adsorbate <NUM> having been physisorbed onto the silicon oxide film <NUM> vaporizes. As a result, the thickness of the C4F8-based adsorbate <NUM> is adjusted to a predetermined thickness (e.g., the thickness corresponding to one atomic layer). The period between the time "t1" and the time "t2" may be about <NUM> seconds, for example.

The control unit <NUM> then executes the etching step for etching the silicon oxide film <NUM>, by causing the adsorbate <NUM>, having been physisorbed as the adsorbate, to react with the silicon oxide film <NUM>, using Ar plasma. Specifically, the control unit <NUM> generates Ar plasma by applying the first high-frequency power, for a period between the time "t2" and at time "t4", as illustrated in <FIG>, while keeping cooling the wafer W and supplying Ar. The control unit <NUM> also draws ions in the plasma into the wafer W by applying the second high-frequency power for a period between time "t3" and the time "t4". As the Ar plasma is generated, the plasma promotes collisions of the ions (that is, Ar ions) against the adsorbate <NUM> on the silicon oxide film <NUM>, and induces the reaction between the C4F8-based adsorbate <NUM> and the silicon oxide film <NUM>. As a result, the film to be etched is etched in a unit of one atomic layer, depending on the thickness of the C4F8-based adsorbate <NUM>. The period between the time "t2" and the time "t4" may be about <NUM> seconds, for example, and the period between the time "t3" and the time "t4" may be about <NUM> seconds, for example.

The control unit <NUM> then executes the physisorption step, the adjustment step, and the etching step, a plurality of times, in the order listed herein.

A relation between the temperature of the wafer W, and the etched amount of the silicon oxide film <NUM> will now be explained with reference to <FIG>. <FIG> is a schematic illustrating a measurement result of the etched amount of the silicon oxide film <NUM>, when a cycle of the physisorption step, the adjustment step, and the etching step was repeated four times, in the order listed herein, with the wafer W cooled to -<NUM>. <FIG> is a schematic illustrating a measurement result of the etched amount of the silicon oxide film <NUM>, when the cycle of the physisorption step, the adjustment step, and the etching step was repeated four times, in the order listed herein, with the wafer W cooled to -<NUM>. <FIG> is a schematic illustrating a measurement result of the etched amount of the silicon oxide film <NUM>, when the cycle of the physisorption step, the adjustment step, and the etching step was repeated four times, in the order listed herein, with the wafer W cooled to -<NUM>. In <FIG>, "reference" indicates the initial thickness of the silicon oxide film <NUM>, "ALE <NUM> cycles" indicates the thickness of the silicon oxide film <NUM> after the cycle of these steps was repeated four times, in the order listed herein.

As illustrated in <FIG>, when the temperature of the wafer W was -<NUM> that is higher than the dew point of C4F8, the silicon oxide film <NUM> was etched by an amount of <NUM>. This etched amount did not satisfy a predetermined specification of allowance.

Furthermore, as illustrated in <FIG>, when the temperature of the wafer W was -<NUM> that is higher than the dew point of C4F8, the silicon oxide film <NUM> was etched by an amount of <NUM>. This etched amount did not satisfy the predetermined specification of allowance.

By contrast, as illustrated in <FIG>, when the temperature of the wafer W was -<NUM> that is lower than the dew point of C4F8, the silicon oxide film <NUM> was etched by an amount of <NUM>. This etched amount satisfied the predetermined specification of allowance.

A change in the etched amount of the silicon oxide film <NUM>, after the cycle of the physisorption step, the adjustment step, and the etching step was repeated a plurality of times, in the order listed herein, with the wafer W cooled to -<NUM>, will now be explained with reference to <FIG> is a schematic illustrating a result of measuring the change in the etched amount of the silicon oxide film <NUM>, after the cycle of the physisorption step, the adjustment step, and the etching step was repeated a plurality of times, in the order listed herein, with the wafer W cooled to -<NUM>. <FIG> illustrates a graph indicating the change in the etched amount of the silicon oxide film <NUM>. <FIG> illustrates an enlargement of the part of the graph corresponding to the fourth cycle in the graph <FIG>.

As illustrated in <FIG>, when the temperature of the wafer W was -<NUM> that is lower than the dew point of C4F8, the etched amount of the silicon oxide film <NUM> increased at a constant rate, and then saturated in every cycle. In other words, it was confirmed that the self-limiting etching was implemented when the temperature of the wafer W was -<NUM> that is lower than the dew point of C4F8.

Furthermore, as illustrated in <FIG>, the amount by which the silicon oxide film <NUM> was etched per one cycle was <NUM>. In the silicon oxide film <NUM>, because the distance between atoms in Si-O bonding is <NUM> to <NUM> angstroms, for example, <NUM> corresponds to approximately two atomic layers. In other words, it was confirmed that the silicon oxide film <NUM> was etched successfully in a unit of one atomic layer, when the temperature of the wafer W was -<NUM> that is lower than the dew point of C4F8.

Another example of the sequence of the process of the plasma etching method executed by the plasma etching apparatus <NUM> will now be explained. <FIG> is a flowchart illustrating another example of the sequence of the process of the plasma etching method according to one embodiment. <FIG> is a schematic illustrating one example of a cross section of the wafer W after the execution of the steps illustrated in <FIG>. In the explanation below, it is assumed that the plasma etching apparatus <NUM> executes a series of steps to the wafer W illustrated in <FIG>.

In the plasma etching method according to the embodiment, to begin with, the wafer W that is the object to be processed is carried into the chamber 12c, and placed on the stage <NUM>. At this point in time, the cross section of the wafer W is as illustrated in <FIG>, for example.

The plasma etching apparatus <NUM> then sets values to the parameter m for counting the number of times the steps are repeated, and the parameter m0 specifying the upper bound of the number of times the steps are repeated. The plasma etching apparatus <NUM> sets "<NUM>" to the parameter m, for example, and sets "<NUM>" to the parameter m0, for example (Step S111). The value set to the parameter m0 may be any value. When the value set to the parameter m0 is "<NUM>", for example, the physisorption step and the etching step are performed once, without repeating the steps.

The plasma etching apparatus <NUM> then execute a physisorption step for causing a passivation film that is based on the first processing gas to be physisorbed onto the film to be etched, as the adsorbate, while cooling the wafer W to a temperature equal to or lower than the sublimation point of the first processing gas (Step S112). The film to be etched is, for example, the silicon film <NUM> on the wafer W. The first processing gas includes, for example, gas including halogen and oxygen. Examples of the gas including halogen and oxygen are SiF4/O2 and S12F6/O2. It is considered that the physisorption between the passivation film based on the first processing gas and the film to be etched takes place due to van der Waals force, for example. Van der Waals force is a force by which molecules or atoms are attracted to one another, and the effect of this force becomes greater when the temperature of the molecules or the atoms becomes lower. Therefore, even when the film to be etched does not have any chemical reactivity to the first processing gas, by cooling the wafer W to a temperature equal to or lower than the sublimation point of the first processing gas, the passivation film that is based on the first processing gas is caused to be physisorbed onto to the film to be etched, by van der Waals force.

This step will now be explained using a more specific example. The control unit <NUM> in the plasma etching apparatus <NUM> cools the wafer W to a temperature equal to or lower than the sublimation point of SiF4/O2 that is the first processing gas (hereinafter, referred to as "cryogenic temperature", as appropriate), by controlling the temperature of the refrigerant circulating through the flow channel 18f inside the stage <NUM>. The control unit <NUM> then causes the gas source group <NUM> to supply SiF4/O2 into the chamber 12c as the first processing gas, while keeping the wafer W at the cryogenic temperature, and generates SiF4/O2 plasma by causing the first high-frequency power source <NUM> to apply the first high-frequency power for plasma generation. At this time, the control unit <NUM> may also cause the second high-frequency power source <NUM> to apply the second high-frequency power for drawing ions. As a result, an oxide film that is the passivation film based on SiF4/O2 is caused to be physisorbed onto the film to be etched, by van der Waals force, in a unit of one atomic layer.

The cross section of the wafer W after the execution of the physisorption step illustrated as Step S112 is as illustrated in <FIG>, for example. In other words, by the SiF4/O2 plasma being generated while the wafer W is cooled to the cryogenic temperature, the physisorption of an oxide film <NUM> that is SiF4/O2-based passivation film is caused to be physisorbed onto the silicon film <NUM> that is the film to be etched. Because the entire wafer W is cooled to the cryogenic temperature, the oxide film <NUM> that is based on SiF4/O2 is caused to be physisorbed not only onto the silicon film <NUM> but also onto the silicon oxide film <NUM> and the silicon nitride film <NUM>.

The plasma etching apparatus <NUM> then executes the etching step for etching the film to be etched, by causing the adsorbate to react with the film to be etched, using the plasma of the second processing gas (Step S113). The second processing gas includes noble gas, for example. The noble gas is Ar, for example.

This step will now be explained using a more specific example. The control unit <NUM> in the plasma etching apparatus <NUM> causes the gas source group <NUM> to supply Ar into the chamber 12c, as the second processing gas, so as to replace SiF4/O2 with Ar, and generates Ar plasma by causing the first high-frequency power source <NUM> to apply the first high-frequency power for generating plasma. At this time, the control unit <NUM> may also cause the second high-frequency power source <NUM> to apply the second high-frequency power for drawing ions. As the Ar plasma is generated, this plasma promotes the collisions of ions (that is, Ar ions) against the adsorbate on the film to be etched, and induces a reaction between the adsorbate and the film to be etched. As a result, the film to be etched is etched in a unit of one atomic layer, depending on the thickness of the adsorbate.

The cross section of the wafer W after the etching step illustrated at Step S113 is as illustrated in <FIG>, for example. In other words, as the reaction between the silicon film <NUM> and the oxide film <NUM> on the silicon film <NUM> that is the film to be etched is induced, the silicon film <NUM> becomes etched selectively, by the thickness corresponding to the thickness of the oxide film <NUM>. By contrast, the silicon oxide film <NUM> and the silicon nitride film <NUM>, which are not to be etched, have higher bond-dissociation energy than that of the silicon film <NUM>, therefore, reactions of the silicon oxide film <NUM> and the silicon nitride film <NUM>, with the oxide film <NUM> are suppressed. For this reason, etching of the silicon oxide film <NUM> and the silicon nitride film <NUM> is suppressed.

The plasma etching apparatus <NUM> then determines whether the physisorption step and the etching step have been repeated by the preset number of times, that is, whether the parameter m is equal to or greater than the parameter m0 (Step S114). If the parameter m is less than the parameter m0 (No at Step S114), the plasma etching apparatus <NUM> increments the parameter m by one (Step S115), shifts the process back to Step S112, and repeats the physisorption step and the etching step. If the parameter m is equal to or greater than the parameter m0 (Yes at Step S114), the plasma etching apparatus <NUM> ends the process.

<FIG> is a schematic illustrating one example of a relation between the temperature of the wafer W cooled at the physisorption step, and the etched amounts of the silicon film <NUM>, the silicon oxide film <NUM>, and the silicon nitride film <NUM>. In <FIG>, "temperature (°C)" represents the temperature (°C) of the wafer W cooled at the physisorption step. In <FIG>, "a-Si (nm)" represents the amount (nm) by which the silicon film <NUM> was etched, "SiO2 (nm)" represents the amount (nm) by which the silicon oxide film <NUM> was etched, and "SiN (nm)" represents the amount (nm) by which the silicon nitride film <NUM> was etched.

As illustrated in <FIG>, when the wafer W was cooled to -<NUM>, the etch amount of the silicon film <NUM> increased to <NUM> (nm). By contrast, the etch amounts of the silicon oxide film <NUM> and the silicon nitride film <NUM> were smaller than that of the silicon film <NUM>. This can be attributed to the fact that the silicon oxide film <NUM> and the silicon nitride film <NUM> have higher bond-dissociation energy than that of the silicon film <NUM>.

Claim 1:
A plasma etching method comprising:
a physisorption step (S102, S112) for causing an adsorbate (<NUM>, <NUM>) that is based on a first processing gas to be physisorbed onto a film to be etched, while cooling an object (W) to be processed on which the film to be etched is provided, wherein the film to be etched does not have any chemical reactivity to the first processing gas;
an adjustment step for adjusting a thickness of the adsorbate (<NUM>, <NUM>), by evaporating or subliming a part of the adsorbate (<NUM>, <NUM>) having been physisorbed onto the film to be etched; and
an etching step (S103, S113) for etching the film to be etched, by causing the adsorbate (<NUM>, <NUM>) to react with the film to be etched, using plasma of second processing gas.