JP6433202B2 - Plasma processing apparatus and SOI wafer processing method using the apparatus - Google Patents

Description

  The present invention relates to a plasma processing apparatus for processing the surface of an object to be processed with plasma. More specifically, the plasma distribution is controlled spatially and temporally to effectively reduce the effective plasma action time on the surface of the object to be processed. The present invention relates to a controllable plasma processing apparatus.

  In a semiconductor manufacturing process, a substrate is placed between a pair of electrodes, and a high-frequency power is applied in a state where a reactive gas is supplied between the electrodes. There is a wide range of parallel plate type plasma processing equipment that generates plasma by decomposing the substrate and uses this plasma to perform surface treatments such as etching, thin film formation, and surface modification on the substrate placed between the electrodes. It is used. However, although these parallel plate type plasma processing apparatuses can process a large-area wafer at a time, they do not have a function of controlling the spatial distribution of plasma and cannot be applied to numerical control processing.

  In recent years, silicon-on-insulator (SOI) wafers have been used for developing semiconductor devices capable of low power consumption and high-speed operation. If the film thickness of the Si thin film layer on the surface of the SOI wafer varies, it will affect the variation of the threshold voltage of the element, and therefore it is required to produce a Si thin film layer with a uniform thickness on the nano order. ing. Several processing methods effective for uniformizing the thickness of the Si thin film layer have been developed, including the plasma CVM (Chemical Vaporization Machining) disclosed in Patent Document 1, but a method of scanning the sample surface is used. Therefore, there has been a problem that the processing time becomes very long with the recent increase in the wafer area. In order to solve this problem, it is required that the amount of processing can be controlled with high accuracy for each region and a large-area wafer can be processed in a short time.

  Therefore, as described in Patent Document 2, the present inventor has a substantially electrode surface constituted by a plurality of unit electrodes densely arranged on the surface of a workpiece to be processed which is a flat surface or a curved surface close to a flat surface. A planar collective electrode is arranged with a predetermined gap, a process gas is supplied to the gap, and a high frequency voltage is applied to the collective electrode from a high frequency power source to generate plasma in the gap to be processed. When processing the surface of the object, each unit electrode can be displaced in a direction orthogonal to the conductive base plate while being electrically connected to the common conductive base plate connected to the high-frequency power source as the collective electrode. And adopting a structure in which each unit electrode individually takes a reference position and a retracted position by an actuator (solenoid) driven by a control device, and the unit electrode takes a reference position. The tip plasma is generated in, by plasma is extinguished when taking retracted position, has proposed a plasma processing apparatus for controlling the in-plane distribution of the plasma time. Here, the workpiece side and the collective electrode side are partitioned by a thin plate-like dielectric window inserted into the gap, and the workpiece side has a surface between the surface of the workpiece and the dielectric window. A process gas supply space is formed.

  A sacrificial oxidation process using a plasma processing apparatus described in Patent Document 2 by processing an object to be processed in oxygen plasma to oxidize the surface to a predetermined depth and removing the oxide film with hydrofluoric acid. When the film thickness uniformity of the Si thin film layer on the surface of the SOI wafer was performed by the above method, the film thickness variation was successfully improved from a PV value of 4.5 nm to 1.1 nm. I had such problems. For example, plasma was controlled by moving the electrodes up and down using a solenoid coil and weakening the electric field strength, but the electrode size is limited to the size of the upper actuator, etc., so the processing resolution is improved In order to shield the mechanism of the electrode such as the solenoid coil and the plasma generation area, a dielectric plate is required, and the distance between the electrode samples is limited. As a result, the oxidation rate decreases.

  On the other hand, in Patent Document 3, in a dry etching apparatus in which a substrate to be etched is placed on a lower electrode and high frequency power is applied to the upper electrode for etching, the lower electrode is insulated and divided into a plurality of blocks. An anode-coupled dry etching apparatus configured to connect each impedance and ground, individually control the impedance, and selectively vary the etching rate of a substrate to be etched placed on the lower electrode. Has been.

  Patent Document 4 discloses a parallel plate type dry etching apparatus comprising a pair of flat plate electrodes arranged in parallel and facing each other, and a high frequency power source connected to one side of these flat plate electrodes. The flat plate electrode on the side to which the high-frequency power source is connected is composed of a plurality of power application sections that are divided along the plane direction and are electrically insulated from each other, and the substrate to be etched is mounted on the surface. There is disclosed a cathode-coupled parallel plate type dry etching apparatus in which each of these power application units is separately connected to a high-frequency power source. And by adjusting the high frequency power from the high frequency power supply applied to each of the power application units, the power density in each power application unit, that is, each of the power application units and the plate electrode ( The plasma density with the upper electrode) can be controlled locally. However, simply dividing the electrode of Patent Document 4 into small electrodes is not sufficient for generating uniform plasma by generating an excessive or small electric field due to the superposition of high frequency near the boundary of each small electrode. Therefore, Patent Document 5 discloses that adjustment is performed so that the phase shift of the high-frequency voltage applied to each adjacent small electrode is in the range of 120 to 240 degrees.

  However, although the dry etching apparatus described in Patent Document 3 assumes non-uniform etching, if the number of blocks for insulation division is 10 inches φ, the limit is to divide into several tens of blocks. It is described and has poor plasma spatial controllability and cannot be applied to precise numerical control processing. The dry etching apparatuses described in Patent Documents 4 and 5 both divide the electrodes in order to improve the in-plane uniformity of the plasma, and adjust the high-frequency power applied to each electrode. In Patent Documents 3 and 4, since the object to be processed is arranged on the surface of the divided electrode, the plasma control action by the divided electrode is via the object to be processed, and there is a disadvantage that the spatial resolution is lowered. doing. In addition, the plasma control technique using divided electrodes described in Patent Documents 3 to 5 does not control the generation and extinction of local plasma.

Japanese Patent No. 2962583 Japanese Patent No. 5013332 Japanese Utility Model Publication No. 4-30728 JP 7-169745 A JP 2002-313744 A

  Therefore, in view of the above situation, the present invention intends to solve the problem that the plasma generation electrode has a size capable of generating plasma over the entire surface of the object to be processed, and the generation of plasma locally. An object of the present invention is to provide a general-purpose plasma processing apparatus that can clearly control extinction and can arbitrarily control the in-plane distribution of plasma acting on the surface of the workpiece.

In order to solve the above-mentioned problem, the present invention is substantially constituted by a plurality of unit electrodes being densely arranged on the surface of an object to be processed which is a flat surface held on the grounded counter electrode side or a curved surface close to a flat surface. A collective electrode having a flat electrode surface is arranged with a predetermined gap, and a process gas is supplied to the gap, and a high frequency voltage is applied to the collective electrode from a high frequency power source to generate plasma in the gap. A parallel plate type plasma processing apparatus for processing the surface of a workpiece, wherein the collective electrode is formed by fixing each unit electrode to a support member in an electrically insulated state, and each unit electrode is independent of each other. The high-frequency power source is connected via an operating switching mechanism, and the switching mechanism has a peak value equal to or higher than a threshold voltage necessary for generating a discharge in the gap in the ON state. To by applying a high frequency voltage to the unit electrodes to generate plasma, and a by applying a high frequency voltage having a threshold voltage below the peak value required to discharge maintained in the OFF state to the unit electrode functions to extinguish the plasma vector The sources of a pair of MOSFETs are connected and arranged in series, the drain of one MOSFET is connected to the high frequency power supply side, the drain of the other MOSFET is connected to the unit electrode side, and the gates of both MOSFETs A switching element that simultaneously controls the gate voltage of both MOSFETs by an external control signal is connected between the sources, a state where the gate voltage by the switching element is high corresponds to the ON state, and a state where the gate voltage is zero or low is the OFF state so as to correspond to control the generation time of the plasma to be generated in each of the unit electrodes that the To constitute a plasma processing apparatus characterized (claim 1).

Further, in order to solve the above-mentioned problem , the present invention has a substantial configuration in which a plurality of unit electrodes are densely arranged on the surface of an object to be processed which is a flat surface held on the grounded counter electrode side or a curved surface close to a flat surface. A collective electrode having a flat electrode surface is disposed with a predetermined gap, and a process gas is supplied to the gap, and a high-frequency voltage is applied to the collective electrode from a high-frequency power source to generate plasma in the gap. A parallel plate type plasma processing apparatus for generating and processing a surface of an object to be processed, wherein the collective electrode is formed by fixing each unit electrode to a support member in an electrically insulated state, and each unit electrode is independent of each other. The high-frequency power source is connected through a switching mechanism that operates in the ON state. A function is provided for generating a plasma by applying a high-frequency voltage having a value to a unit electrode, and extinguishing the plasma by applying a high-frequency voltage having a peak value equal to or lower than a threshold voltage necessary for maintaining a discharge in an OFF state to the unit electrode. In addition, a plurality of pairs of MOSFETs are connected in series, the sources of each pair of MOSFETs are connected and arranged in series, the drains of different pairs of MOSFETs are connected, and the high frequency power supply side pair of MOSFETs Switching for connecting the drain to the high-frequency power source and connecting the drain of the pair of MOSFETs on the unit electrode side to the unit electrode, and simultaneously controlling the gate voltage of each MOSFET by an external control signal between the gate and source of each MOSFET A state in which the elements are connected and the gate voltage by the switching element is high corresponds to the ON state. And, characterized in that corresponding to the state the gate voltage is zero or a low state OFF, the gate voltage as a zero or low state corresponding to the OFF state, to control the generation time of the plasma to be generated in each unit electrode to constitute a plasma processing apparatus according to (claim 2).

Then, it is also preferable that the switching element is a mechanical relay driven by the external control signal (claim 3).

In this case, the switching mechanism is configured such that when the movable contact of the mechanical relay is connected to the b contact, the gate and the source of the MOSFET are short-circuited, and when the movable contact is connected to the a contact, the source side of the MOSFET and more preferably used to apply a voltage to the gate from the gate power source through a diode connected to the cathode (the claim 4).

Further, each unit electrode of the collective electrode is the same member having a circular or hexagonal cross section, and is a dense array in which six unit electrodes are located at equal distances around a specific unit electrode excluding the peripheral part. It is also preferable that the size of the electrode surface formed by the unit electrode is set larger than the surface to be processed of the object to be processed (Claim 5 ).

And the pressure of the said process gas is 0.01-1 Mpa (Claim 6 ).

And this invention uses the plasma processing apparatus in any one of the said Claims 1-6 , and the said to-be-processed object is a SOI wafer, measures the Si layer whole surface of a SOI wafer beforehand, acquires removal data, Using a process gas obtained by diluting oxygen gas with an inert gas, an oxygen plasma having an in-plane distribution is generated by controlling a switching mechanism that applies a high frequency voltage to each unit electrode based on the removal data. The oxygen radicals generated in step 1 are allowed to act on the surface of the Si layer of the SOI wafer to oxidize the removed portion, and then the SOI wafer taken out from the plasma processing apparatus is treated with hydrofluoric acid to remove the oxide film. An SOI wafer processing method is provided (claim 7 ).

  The plasma processing apparatus of the present invention configured as described above can essentially generate localized plasma, and the location of generation of the localized plasma can be spatially controlled within the surface of the workpiece. Therefore, it is possible to arbitrarily control the plasma processing time for each part on the surface of the workpiece. When the present invention is applied to a process in which the processing amount changes depending on the plasma processing time, high-efficiency batch shape creation processing of the entire surface of the workpiece can be performed. A high-frequency power switching mechanism that is difficult to control on and off can be realized with a simple switching circuit that combines a pair of MOSFETs. Plasma is generated and extinguished for each unit electrode according to the ON and OFF states of the switching mechanism. Thus, the high-frequency voltage can be substantially controlled and the switching circuit is simplified, so that the unit electrode can be further densified. Furthermore, the present invention is an apparatus capable of performing numerical control processing, but does not need to have a drive mechanism such as an XY table, and has a very simple structure. As a result, the volume of the apparatus is reduced, the amount of reaction gas used in the process can be reduced, and a low running cost can be realized. Since the electrode mechanism and structure have been simplified, it is possible to integrate a large number of unit electrodes, and it is possible to realize a plasma processing apparatus capable of processing a large-area wafer of φ300 mm or more, which is used commercially. is there. As a result, the establishment of a uniform film thickness processing method over a large area has become realistic.

  When the plasma processing apparatus of the present invention is applied to an SOI wafer processing method, the thickness variation of the Si layer of the SOI wafer can be corrected and planarized with sub-nm accuracy, and the thickness of the Si layer is also 100 nm. It can be processed to the following arbitrary thickness, and a next-generation high-performance electronic device can be manufactured.

It is a simplified sectional view showing the concept of the plasma processing apparatus of the present invention. The example of arrangement | sequence of seven unit electrodes is shown, (a) is a perspective view of a unit electrode, (b) is a top view of a unit electrode. It is a circuit diagram of a switching mechanism. The change of the high frequency voltage in a switching mechanism is shown, (a) is a graph of the relationship between the input voltage and output voltage in an ON state, (b) is a graph of the relationship between the input voltage and output voltage in an OFF state. It is the graph of the relationship between the oxidation time and the oxide film thickness which showed the difference of the processing capability of the plasma processing apparatus of this invention, and the conventional plasma processing apparatus. It is a graph which shows the oxide film thickness just under a unit electrode at the time of oxidizing the Si surface by controlling plasma generation time using two adjacent unit electrodes. It is a graph which shows the oxide film thickness just under a unit electrode at the time of oxidizing the Si surface by controlling plasma generation time using three adjacent unit electrodes. The principle of the numerical control processing of the SOI wafer by the sacrificial oxidation method is shown, and (a) is a cross-sectional view showing a state where the Si layer is oxidized with a uniform oxide film thickness when a spatially uniform plasma is generated. b) is a cross-sectional view showing a state in which the convex portion of the Si layer is selectively oxidized with a deep oxide film thickness when a spatially controlled plasma is generated, and FIG. Sectional drawing which shows a mode that it removes by processing by (d) is sectional drawing of the state in which the convex part of Si layer was selectively removed and planarized.

  Next, the present invention will be described in more detail based on the embodiments shown in the accompanying drawings. 1 and 2 show the concept of the plasma processing apparatus of the present invention, and FIG. 3 shows a circuit diagram of the switching mechanism. First, the outline of the plasma processing apparatus of the present invention will be described with reference to FIG.

  In the plasma processing apparatus of the present invention, the plasma control mechanism is changed from a mechanical control mechanism using a solenoid coil to an electrical control mechanism, and the dielectric plate is removed by fixing the electrodes and simplifying the plasma control mechanism. The plasma processing speed (oxidation speed) is improved. In general, switching of high-frequency power is difficult to completely cut off because of parasitic capacitance in circuit elements.

  The plasma processing apparatus of the present invention includes a collective electrode 2 having a substantially flat electrode surface, which is configured by closely concentrating a plurality of unit electrodes 3,. A parallel plate type electrode structure is provided, and a holding portion 5 for holding a plate-like workpiece 4 such as a wafer is provided on the surface of the counter electrode 1, and between the collective electrode 2 and the workpiece 4. A process gas is supplied to the gap 6 provided, and a high frequency power is controlled by a switching mechanism 9 from a high frequency power source 7 via a power control system 8 to apply a high frequency voltage to each of the unit electrodes 3. ,... Is generated in the vicinity of the electrode surface. The switching mechanism 9 controls the ON state and the OFF state by an external control signal from the control device 10 such as a personal computer. In the present embodiment, the counter electrode 1 is disposed at the bottom, the collective electrode 2 is disposed at the top, and the workpiece 4 having a flat surface or a curved surface close to a flat surface is placed on the holding portion 5 of the counter electrode 1. Has a mold structure.

  More specifically, the counter electrode 1 is a disk-shaped member made of a conductive material, and the holding portion 5 is recessed at a depth corresponding to the plate thickness of the object 4 to be processed at the center of the upper surface. A concave step portion 12 is formed around the periphery to attach the insulating spacer 11 in an airtight state, and an appropriate number of gas supply passages 13 and gas discharge passages 14 extending from the outer peripheral portion to the upper surface are provided. Here, in order to reliably hold the object to be processed 4 on the holding portion 5 of the counter electrode 1, it is preferable to appropriately use a suction means such as a vacuum chuck. It is also preferable to make the insulating spacer 11 translucent so that plasma emission can be observed from the side.

  The collective electrode 2 is obtained by fixing each unit electrode 3 in an electrically insulated state to a disk-shaped support member 15 made of an insulating material, and an O-ring 16 is provided on the insulating spacer 11. And are connected in an airtight state. As shown in FIGS. 1 and 2, the unit electrode 3 is formed by integrally forming a rod-shaped conductive portion 18 on a flat hexagonal electrode portion 17 at the tip, and the conductive portion 18 is vertically mounted on the support member 15. The fitting hole 19 formed by penetrating is fitted and fixed. In a state where a large number of unit electrodes 3 are fixed to the support member 15 in a close-packed manner, a predetermined gap is provided between the electrode portions 17 of each unit electrode 3, and further, the electrode portions 17 of each unit electrode 3 The formed electrode surface is substantially flat. FIG. 2 shows an arrangement of seven unit electrodes 3. The high-frequency power source 7 is connected to the conductive portion 18 of the unit electrode 3 via a switching mechanism 9 that operates independently. Here, the shape of the electrode portion 17 of the unit electrode 3 is not limited to a hexagon, but may be a circle, a rectangle, a triangle, or the like, but a hexagon is most preferable.

  Next, the circuit of the switching mechanism 9 will be described with reference to FIG. The MOSFET has a body diode in its structure, and can control the current flowing from the drain to the source, but cannot control the current flowing from the source to the drain. For this reason, a single MOSFET can control direct current, but cannot control high frequency. Thus, by connecting two MOSFETs in series as shown in FIG. 3, the current in both directions can be controlled, and the high-frequency power transmitted to the unit electrode 3 can be controlled, though not completely.

  The switching mechanism 9 connects the sources of a pair of MOSFETs 20 and 21 in series, connects the drain of one MOSFET 20 to the high-frequency power source 7 side, and connects the drain of the other MOSFET 21 to the unit electrode 3 side. And a switching element 22 that simultaneously controls the gate voltages of both MOSFETs 20 and 21 by an external control signal from the control device 10 is connected between the gates and sources of both MOSFETs 20 and 21, and the gate voltage of the switching element 22 is high. The state corresponds to the ON state, and the state where the gate voltage is zero or low corresponds to the OFF state. In the present embodiment, the switching element 22 is a mechanical relay that is driven by the external control signal. In this case, the switching mechanism 9 is configured such that when the movable contact of the relay 22 is connected to the b contact, the gates and sources of the MOSFETs 20 and 21 are short-circuited, and when the movable contact is connected to the a contact, the MOSFET 20, A voltage is applied from the gate power supply 24 to the gate through a diode 23 having a cathode connected to the source side of 21. The diode 23 inserted on the source side of each MOSFET is to prevent high frequency from being transmitted to the adjacent unit electrode 3. The switching mechanism 9 is provided with the above basic circuit in parallel by the number of the unit electrodes 3.

The switching characteristics of the high frequency voltage by the switching mechanism 9 are shown in FIG. For the measurement, an experimental apparatus having seven unit electrodes 3,... Having a hexagonal shape with a distance between opposite sides of 14 mm was used. The frequency of the high frequency power source 7 was 13.56 MHz, and the process gas was a mixed gas of He: O ₂ = 99: 1. The gap 6 between the electrode samples of this apparatus is 200 μm. In the ON state, as shown in FIG. 4A, since a high frequency flows through the MOSFET without any problem, the input voltage waveform and the output voltage waveform of the switching mechanism 9 are substantially the same. However, since the MOSFET has a very small parasitic capacitance, when the gate voltage of the MOSFET is zero or low, the MOSFET functions electrically as a capacitor, so that some power is transmitted. In the OFF state, as shown in FIG. 4B, since power is transmitted through the MOSFET, the peak voltage on the electrode side is about 75% with respect to the peak voltage on the power source side.

Therefore, it inputs the high-frequency voltage V _in having a threshold voltages V ₁ or more peak values required for discharge generation in the gap 6 to the switching mechanism 9, the threshold voltage required for sustaining the OFF state V ₂ If a high-frequency voltage _Vout having the following peak value is output and applied to the unit electrode 3, it is possible to generate plasma in the ON state and not extinguish or generate plasma in the OFF state. . That is, even if the high-frequency power cannot be completely turned on / off by the switching mechanism 9, if the plasma can be attenuated to a voltage at which the plasma is extinguished in the OFF state, the plasma can be substantially on (generated) / off (erased). It is.

  That is, the plasma processing apparatus of the present invention is configured by densely uniting a plurality of unit electrodes 3 with respect to the surface of the workpiece 4 having a flat surface held on the grounded counter electrode 1 side or a curved surface close to the flat surface. A collective electrode 2 having a substantially flat electrode surface is disposed with a predetermined gap 6, a process gas is supplied to the gap 6, and a high frequency voltage is applied to the collective electrode 2 from a high frequency power source 7. A parallel plate type plasma processing apparatus for generating plasma in the gap 6 to process the surface of the workpiece 4, wherein the collective electrode 2 has each unit electrode 3 fixed to a support member 15 in an electrically insulated state. The high frequency power supply 7 is connected to each unit electrode 3 via a switching mechanism 9 that operates independently. The switching mechanism 9 is in the ON state in the gap 6. A high frequency voltage having a peak value equal to or higher than the threshold voltage necessary for generating electricity is applied to the unit electrode 3 to generate plasma, and a high frequency voltage having a peak value equal to or lower than the threshold voltage required for maintaining discharge in the OFF state It has a function of extinguishing plasma when applied to the electrode 3 and controls the generation time of plasma generated at each unit electrode 3.

  Here, each unit electrode 3 of the collective electrode 2 is the same member having a circular or hexagonal cross section, and the six unit electrodes 3,... The size of the electrode surface formed by each unit electrode 3 is set to be larger than the surface to be processed of the object 4 to be processed. The number of unit electrodes 3 is determined by the area of the workpiece 4 to be processed and the area of the minimum plasma region based on the required spatial resolution.

  In the present invention, the pressure of the process gas is 0.01 to 1 MPa in the vicinity of atmospheric pressure. If the pressure of the process gas is close to atmospheric pressure, the plasma generation region becomes more localized, which not only makes it easier to control the plasma distribution at the in-plane position, but also does not require a high degree of vacuum tightness. Will also be easier. In the gap 6, process gas is supplied from the gas supply path 13 and exhausted from the gas discharge path 14, so that new process gas is always supplied to the gap 6. For the purpose of oxidizing the surface of the workpiece 4, it is also possible to fill the gap space with the process gas and perform plasma treatment.

Also, depending on the application of the plasma processing apparatus may have high power required, MOSFET output voltage V _out when the OFF state is also contemplated that can not be suppressed to below the threshold voltage V _2. In that case, the number of MOSFETs to be connected is increased to four or six. As described above, the MOSFET plays a role as a capacitor when the gate voltage is turned off, but when the capacitor is connected in series, the overall electric capacity becomes low. By utilizing this, by connecting a number of MOSFETs in series, the overall electric capacity can be reduced, the high frequency can be greatly cut, and high power can be handled.

  That is, the switching mechanism 9 connects a plurality of pairs of MOSFETs in series, connects the sources of each pair of MOSFETs in series and connects the drains of different pairs of MOSFETs, and connects the high-frequency power supply side. The drains of the pair of MOSFETs are connected to the high-frequency power source 7, and the drains of the pair of MOSFETs on the unit electrode side are connected to the unit electrode 3. By connecting and configuring the switching element 22 that simultaneously controls the gate voltage, it is possible to cope with an increase in power. Also in this case, a state where the gate voltage by the switching element 22 is high corresponds to the ON state, and a state where the gate voltage is zero or low corresponds to the OFF state.

The sacrificial oxidation experiment of the Si wafer was performed using the plasma processing apparatus of the present invention, and its performance was evaluated. The plasma processing apparatus used for the experiment is a prototype apparatus having seven unit electrodes 3. The gap 6 between the collective electrode 2 and the sample (Si wafer) is 200 μm, the power supply frequency is 13.56 MHz, the bottom surface of the electrode portion 17 of the unit electrode 3 is hexagonal, and the distance between the opposite sides is 14 mm. The process gas used was a mixed gas of He: O ₂ = 99: 1, and kept flowing at a flow rate of 0.2 slm during processing, and the input power was 110 W. By controlling ON / OFF of the switching mechanism 9 using a MOSFET connected to the upper part of each unit electrode 3, the generation of plasma under each unit electrode was controlled.

  For comparison, a conventional plasma processing apparatus that drives a unit electrode by a solenoid coil was used. The electrode shape, gap, gas, applied power, etc. other than the driving mechanism of the unit electrode of the conventional plasma processing apparatus were matched with the above-mentioned conditions. FIG. 5 shows the relationship between the oxidation time and the oxide film thickness. From this result, it was found that the oxidation rate of the plasma processing apparatus of the present invention was remarkably improved.

  Next, using two adjacent unit electrodes 3 and 3, an oxidation experiment was performed by setting the application time of the high frequency voltage to 100 seconds for one unit electrode and 600 seconds for the other unit electrode. The oxide film thickness was measured. Actually, the unit electrodes near the two unit electrodes 3 and 3 also generate plasma for 600 seconds. The result is shown in FIG. The difference between the intended oxide film thickness (dotted line) and the actually formed oxide film is 0.4 nm at the maximum, and it can be said that an oxide film having an arbitrary film thickness can be formed with very high accuracy.

  Next, using three adjacent unit electrodes 3 arranged in a straight line, an oxidation experiment was performed by sequentially setting the application time of the high-frequency voltage to 10 seconds, 60 seconds, and 600 seconds. The oxide film thickness was measured. The input power was changed between 82 and 114 W by the power control system 8 with the change of the plasma generation area. The result is shown in FIG. Thus, it was confirmed that the film thickness can be controlled in each unit electrode.

(Principle of numerical control processing by sacrificial oxidation method)
FIG. 8 shows a principle diagram of numerical control processing by a sacrificial oxidation method of an SOI wafer. FIG. 8A shows a state where the Si layer 25 is uniformly oxidized and an oxide film 26 having a uniform thickness is formed when a spatially uniform plasma is generated. FIG. 8B shows that when a spatially controlled plasma is generated, the convex portion of the Si layer 25 is selectively deeply oxidized, and the oxide film 26 is formed thick at the position of the convex portion. Indicates the state. FIG. 8C shows how the oxide film 26 is removed by treatment with hydrofluoric acid. FIG. 8D shows a state where the convex portions of the Si layer 25 are selectively removed and flattened. Prior to the plasma treatment, it is necessary to accurately measure the surface shape of the SOI wafer and the thickness of the Si layer 25. Based on the shape and film thickness measurement results, the thickness data of the oxide film to be locally formed by plasma processing is created, and a control signal is sent from the control device 10 to the switching mechanism 9 based on the data.

  When oxygen radicals reach the surface of Si, they are gradually combined with oxygen radicals from the surface of the Si crystal to form oxidized Si. The oxygen radicals diffuse into the interior over time, and the oxidized Si layer gradually deepens. Become. Since the thickness of the oxide film increases with the passage of time during which the Si surface is exposed to oxygen plasma, the thickness of the oxide film can be controlled by controlling the generation time of the oxygen plasma. That is, it is possible to control the high frequency power applied to the unit electrode 3 individually without fixing one of the workpiece 4 and the electrode and holding the other on the XY table for scanning. By controlling the plasma generation time, it is possible to realize numerical control processing by controlling the plasma generation time. Since the degree of the plasma treatment effect is controlled by the plasma generation time, it is necessary to always generate plasma under a certain condition at each unit electrode 3, so that the input power to the plasma generated at each unit electrode 3 becomes equal. In addition, it is necessary to control the input power. The input power is controlled by the power control system 8.

  An SOI (Silicon On Insulator) wafer used as a substrate for a high-performance semiconductor device is required to have a thinner and uniform Si layer with the miniaturization of semiconductor elements. Usually, an SOI wafer is integrated by a heat treatment of about 1000 ° C. in a state where a wafer in which an oxidized Si layer is formed by oxidizing the surface of a single crystal Si and another single crystal Si wafer are overlapped and bonded. It is manufactured by a laminating method in which the stacked Si wafers are cut into thin films. Here, since the thickness, flatness, and surface roughness of the Si layer of the SOI wafer directly affect the performance of the electronic device, it is necessary to process with high accuracy. Since the film thickness of the Si layer affects the threshold voltage of the SOI-MOSFET, the film thickness uniformity of ± 5% or less is required for the Si layer. In 2018, the film thickness of the Si layer is 10 nm or less. That is, the film thickness uniformity is expected to be less than 1.0 nm in terms of PV value, but it is difficult to achieve this uniformity with high efficiency using existing methods. Recently, an SOI wafer is also provided in which a Si layer having a thickness of about 100 nm is directly formed by a hydrogen ion implantation delamination method, and then its surface is polished to adjust the thickness. However, in any method, the mechanical polishing method has a limit in polishing accuracy, and subsequent etching is isotropic, so that the space controllability is poor. Therefore, the thickness, flatness, and surface roughness of the Si layer are reduced. It could not be industrially processed with in-plane uniformity of less than 1 nm.

The inventor has already generated local plasma using a process gas near atmospheric pressure obtained by diluting O ₂ gas with He gas, and after oxidizing the Si layer of the SOI wafer with a desired thickness distribution with the generated oxygen radicals. A numerically controlled sacrificial oxidation method using local plasma that can be processed with nanometer accuracy by dissolving and removing the oxidized portion with hydrofluoric acid has been proposed. This demonstrates that an SOI wafer having a very uniform Si layer can be produced by correcting the thickness variation of the Si layer. By performing the numerically controlled sacrificial oxidation method using the plasma processing apparatus of the present invention, it is considered that the thickness of the Si layer of the SOI wafer can be made uniform with extremely high efficiency.

In this numerically controlled sacrificial oxidation method using local plasma, since a fluorine-based gas is not used unlike conventional plasma etching, the environmental load can be reduced and the processing cost can also be reduced. Further, in the conventional plasma etching, since a volatile reaction product such as SiF ₄ is generated, there is a possibility that the process gas is contaminated by the reaction product or the processing apparatus is corroded. For this reason, it is necessary to separate and remove the reaction product from the process gas, which may complicate the processing process. On the other hand, the numerically controlled sacrificial oxidation method using local plasma is an oxidation reaction using only oxygen plasma without using a fluorine-based gas, so that the processing process can be simplified. Furthermore, since the processing method finally removes the oxide film with hydrofluoric acid or the like, the new processing surface is exposed, so that contamination of the processing surface can be prevented.

DESCRIPTION OF SYMBOLS 1 Counter electrode 2 Collective electrode 3 Unit electrode 4 To-be-processed object 5 Holding | maintenance part 6 Gap 7 High frequency power supply 8 Power control system 9 Switching mechanism 10 Control apparatus 11 Insulating spacer 12 Recessed step part 13 Gas supply path 14 Gas discharge path 15 Support Member 16 O-ring 17 Electrode part 18 Conductive part 19 Fitting hole 20 MOSFET
21 MOSFET
22 Switching element (relay)
23 Diode 24 Gate power supply 25 Si layer 26 Oxide film V ₁ Discharge threshold voltage V ₂ Discharge sustain threshold voltage V _in High frequency voltage (input voltage)
V _out high frequency voltage (output voltage)

Claims (7)

A collective electrode having a substantially flat electrode surface formed by concentrating a plurality of unit electrodes on a surface of an object to be processed, which is a flat surface held near the grounded counter electrode or a curved surface close to a flat surface, A parallel plate type in which a gap is provided, a process gas is supplied to the gap, and a high frequency voltage is applied to the collective electrode from a high frequency power source to generate plasma in the gap to process the surface of the workpiece. In the plasma processing apparatus, the collective electrode is configured such that each unit electrode is fixed to a support member in an electrically insulated state, and the high frequency power supply is connected to each unit electrode via a switching mechanism that operates independently. The switching mechanism applies a high-frequency voltage having a peak value equal to or higher than a threshold voltage necessary for generating a discharge in the gap to the unit electrode in the ON state. To generate a plasma, and connect vector comprising a by applying a high frequency voltage having a threshold voltage below the peak value required to discharge maintained in the OFF state to the unit electrode functions to extinguish the plasma, the sources of the pair of MOSFET The drains of one MOSFET are connected to the high-frequency power supply side, the drain of the other MOSFET is connected to the unit electrode side, and both MOSFETs are connected to each other by an external control signal between the gate and source of both MOSFETs. A switching element that simultaneously controls the gate voltage is connected, a state in which the gate voltage by the switching element is high corresponds to the ON state, and a state in which the gate voltage is zero or low corresponds to the OFF state. A plasma processing apparatus for controlling a generation time of plasma to be generated. A collective electrode having a substantially flat electrode surface formed by concentrating a plurality of unit electrodes on a surface of an object to be processed, which is a flat surface held near the grounded counter electrode or a curved surface close to a flat surface, A parallel plate type in which a gap is provided, a process gas is supplied to the gap, and a high frequency voltage is applied to the collective electrode from a high frequency power source to generate plasma in the gap to process the surface of the workpiece. In the plasma processing apparatus, the collective electrode is configured such that each unit electrode is fixed to a support member in an electrically insulated state, and the high frequency power supply is connected to each unit electrode via a switching mechanism that operates independently. The switching mechanism applies a high-frequency voltage having a peak value equal to or higher than a threshold voltage necessary for generating a discharge in the gap to the unit electrode in the ON state. To generate a plasma, and connect vector comprising a by applying a high frequency voltage having a threshold voltage below the peak value required to discharge maintained in the OFF state to the unit electrode functions to extinguish the plasma, a plurality of pairs of MOSFET in series The sources of each pair of MOSFETs are connected and arranged in series, the drains of different pairs of MOSFETs are connected together, the drains of the pair of MOSFETs on the high frequency power supply side are connected to the high frequency power supply, and the unit The drain of the pair of MOSFETs on the electrode side is connected to the unit electrode, and a switching element that simultaneously controls the gate voltage of each MOSFET by an external control signal is connected between the gate and source of each MOSFET, and the gate voltage by the switching element is The high state corresponds to the ON state, and the gate voltage is zero or low. Plasma processing apparatus so as to correspond to the FF state, and controlling the time of occurrence of plasma generated in each of the unit electrodes. The plasma processing apparatus of the mechanical claim 1 or 2, wherein a relay in which the switching element is driven by the external control signal. The switching mechanism is configured such that when the movable contact of the mechanical relay is connected to the b contact, the gate and the source of the MOSFET are short-circuited, and when the movable contact is connected to the a contact, the cathode is connected to the source side of the MOSFET. 4. The plasma processing apparatus according to claim 3 , wherein a voltage is applied to the gate from a gate power supply via a connected diode. Each unit electrode of the collective electrode is the same member having a circular or hexagonal cross section, and is a dense array in which six unit electrodes are located at equal distances around a specific unit electrode except for the peripheral part. The plasma processing apparatus according to any one of claims 1 to 4, wherein a size of an electrode surface formed by the substrate is set larger than a surface to be processed of a workpiece. The pressure of the process gas, the plasma processing apparatus according to claim 1 to 5 any one is 0.01 to 1 MPa. The plasma processing apparatus according to any one of claims 1 to 6 , wherein the object to be processed is an SOI wafer, the removal data is obtained by measuring the entire Si layer of the SOI wafer in advance, and oxygen gas is converted into an inert gas. Based on the removal data, a switching mechanism for applying a high frequency voltage to each unit electrode is controlled based on the removal data to generate an oxygen plasma having an in-plane distribution, and oxygen radicals generated by the oxygen plasma are generated. Processing of an SOI wafer characterized in that the removed portion is oxidized by acting on the surface of the Si layer of the SOI wafer, and then the SOI wafer taken out from the plasma processing apparatus is treated with hydrofluoric acid to remove the oxide film. Method.

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