Atomic flux measurement device

An atomic flux measurement device for measuring the amount of dissociated atomic flux produced by discharge and emitted from a plasma generation cell into a vacuum camber. The atomic flux measurement device includes a counter electrode body including a pair of first and second sheet-like electrodes that are arranged substantially parallel to each other with a predetermined spacing between them, a direct-current power supply configured to maintain the first sheet-like electrode at a negative potential so that atoms attached to the inner surface of the sheet-like electrode undergo self-ionization and to apply a direct-current voltage between the first and second sheet-like electrodes so that a current flows between the first and second sheet-like electrodes, and a direct-current ammeter configured to measure a current flowing due to electrons emitted by the self-ionization of the dissociated atoms attached to the inner surface of the first sheet-like electrode.

TECHNICAL FIELD

The present invention relates to an atomic flux measurement device for measuring the amount of dissociated nitrogen atomic flux emitted from a plasma generation cell into a growth chamber.

BACKGROUND

A number of electronic devices using a group III nitride film, such as GaN, InGaN, InN or InAlN, on a substrate have been manufactured in recent years as electronic devices, such as blue light emitting diodes.

The device substrate is often formed of a sapphire film. The use of a single crystallin silicon substrate, which can be supplied at low cost and in large quantities, has been studied. The silicon substrate can also advantageously have a high thermal conductivity and therefore withstand high power operation compared to the sapphire substrate has.

When the group III nitride film is formed on a silicon substrate, it is necessary to form a buffer layer on the substrate in order to reduce lattice defects. The present inventors have previously proposed that a double buffer layer of Si3N4and AlN are formed between the silicon substrate and the group III nitride film (see Patent Document 1).

A method for forming the double buffer layer between the Si substrate and the group III nitride film will be briefly explained with reference toFIGS. 17 to 19.FIG. 17is a diagram schematically showing a configuration of an MBE (Molecular Beam Epitaxy) growth equipment that is used to form the buffer layers on the silicon substrate.

The MBE growth equipment1includes an RF (Radio Frequency) excitation cell4and a metal molecular beam cell5that are provided in a vacuum chamber (growth camber)3, and an RF matching box6, an RF power supply7and a personal computer (hereinafter referred to as a “PC”)8that are provided outside the vacuum chamber3. A counter electrode body11of an atomic flux measurement device10is provided in the vacuum chamber3in the vicinity of a substrate holder31, while the main body of the atomic flux measurement device10is provided outside the vacuum chamber3and is connected to the PC8via a cable.

The vacuum chamber3is maintained at high vacuum (10−4to 10−8Pa residual pressure) using a turbo molecular pump (not shown). A silicon substrate2that has already been subjected to the cleaning treatment is fixed to the substrate holder31and is heated to a predetermined temperature using a heater (not shown).

Although not shown, a shroud is provided on a sidewall surface of the vacuum chamber3. The inside of the shroud is filled with liquid nitrogen. In the vacuum chamber3, gas molecules are adsorbed by the wall surface when the gas molecules strike the sidewall, whereby a high degree of vacuum can be maintained.

The RF excitation cell4and the metal molecular beam cell5are provided in the wall of the vacuum chamber3to emit nitrogen atoms and metal molecules (e.g., Ga) toward the silicon substrate2held by the substrate holder31.FIG. 18shows a detailed structure of the RE excitation cell4that generates nitrogen gas plasma. Nitrogen gas supplied via a gas port45from a nitrogen gas cylinder (not shown) is supplied into a discharge chamber42of a hollow crucible41. The amount of the nitrogen gas supplied is adjusted by a flow rate controller46. An excitation coil43that also serves as a water cooling pipe is coaxially wound around the outer circumference of the crucible41. By circulating cooling water W, the crucible41and the RF excitation cell4are cooled.

When high-frequency power is supplied to the excitation coil43from the RF power supply7via a terminal63of the RF matching box6, the nitrogen gas in the discharge chamber42is excited into a plasma state so that a supersonic jet of an active species F of nitrogen is emitted through an orifice44provided in an output portion.

The metal molecular beam cell5melts a solid metal material (e.g., Ga) put in the crucible using a heater, and emits evaporated atoms toward the substrate2by opening and closing a shutter9attached to the output portion. Although the single metal molecular beam cell5is shown inFIG. 17, the vacuum chamber3typically includes a plurality of the metal molecular beam cells5, the number of which depends on the number of metal molecules used.

The RF matching box6is provided to perform impedance matching between the RF power supply7and the plasma in the discharge chamber42so that the high-frequency power applied from the RF power supply7to the RF excitation cell4is smoothly supplied to the discharge chamber42. The RF matching box6includes an automatic reactance adjustment circuit61and a variable reactance circuit62.

In the above MBE growth equipment1, the RF excitation cell4can be operated in two discharge modes. The first discharge mode is called an “HB discharge mode” in which a relatively high degree of high-frequency power (e.g., 500 W) is applied to the excitation coil43to excite nitrogen gas in the discharge chamber42, whereby nitrogen plasma having a high brightness is obtained. In the HB discharge mode, as shown in spectrum line diagram ofFIG. 19, emission of a flux (N+N*) of dissociated nitrogen atoms was observed including ground-state atoms N and excited atoms N* that are generated by dissociation of nitrogen molecules N2, and excited nitrogen molecules N2*, nitrogen molecule ions N2+and electrons.

A second discharge mode is called an “LB discharge mode” in which a relatively low degree of high-frequency power (e.g., 120 W) is applied to the excitation coil43to excite nitrogen gas in the discharge chamber42, whereby nitrogen plasma having a low brightness is obtained. In the LB discharge mode, no flux (N+N*) of dissociated nitrogen atoms was contained in plasma emitted from the RF excitation cell4, and emission of excited nitrogen molecules N2*, nitrogen molecule ions N2+and electrons was observed.

The present inventors have extensively studied characteristics of the plasma generated in the HB discharge mode by conducting a variety of experiments. As a result, the present inventors have found that the excited atoms N* and the ground-state, atoms N contained in the plasma in the HB discharge mode are so-called metastable atoms, which have a thermodynamically relatively long life (of the order of milliseconds). On the other hand, the molecular ions N2+and the electrons have the property that they quickly disappear due to recombination in the vacuum chamber. The present inventors also have found that the excited molecules N2*, the excited atoms N* and the ground-state atoms N contained in the plasma in the HB discharge mode are readily attached to a solid-phase interface, such as the substrate surface and the metal plate surface.

When a crystalline layer of GaN or AlGaN is grown on the silicon substrate, the substrate2is preferably directly irradiated with the high-energy excited atoms N* and ground-state atoms N emitted from the RF excitation cell4. Such an irradiation technique is hereinafter referred to as “direct irradiation.” In contrast to this, when a buffer layer is formed on the silicon substrate, the substrate is preferably indirectly irradiated with an appropriate amount of low-energy excited atoms N* and ground-state atoms N. Therefore, when a buffer layer is formed, as shown inFIG. 17, the HB-discharge-mode plasma emitted from the RF excitation cell4is caused to strike and rebound off a reflection plate32provided in the vacuum chamber3and, in addition, the shutter or the shroud in the RF excitation cell4, at least once, so that the energy is reduced, before striking the surface of the substrate2. Such an irradiation technique is hereinafter referred to as an “indirect irradiation.”

Next, the step of forming the double buffer layer of Si3N4and AlN between the silicon substrate and the group III nitride film using the above MBE growth equipment1will be briefly described. A treatment for cleaning the substrate surface is performed before the step of forming the Si3N4buffer layer on the silicon substrate2. The treatment is well known and therefore will not be described.

(1) The silicon substrate2that has been subjected to the cleaning treatment is fixed to the substrate holder31in the vacuum chamber3, and is heated to a predetermined temperature using the heater.

(2) High-frequency power of, for example, 500 W having a frequency of 13.56 MHz is applied to the excitation coil43of the RF excitation cell4so that discharge occurs in the nitrogen gas in the HB discharge mode. The substrate is indirectly irradiated with a dissociated nitrogen atomic flux generated in the HB discharge mode, whereby a β-Si3N4monocrystalline film is epitaxially grown by surface/interface reaction.

(3) The Si3N4monocrystalline film is irradiated with an Al atomic flux corresponding to several atomic layers using an Al molecular beam cell, whereby an AlN monocrystalline film is epitaxially grown due to surface/interface reaction.

(4) The AlN monocrystalline film is directly irradiated with a dissociated nitrogen atomic flux and an excited nitrogen molecule flux that are generated in the HB discharge mode, and is also irradiated with an Al atomic flux using an Al molecular beam cell, whereby an AlN epitaxial layer is formed.

If a crystal of GaN or AlGaN is grown on the silicon substrate on which the double buffer layer have been formed by the above steps, a film having less lattice defects can be formed.

Incidentally, in order to control the growth operation of the MBE growth equipment1employing the RF excitation cell4, it is necessary to monitor the amount of dissociated nitrogen atomic flux that strike the surface of the substrate2. Conventionally, the amount of dissociated nitrogen atomic flux is measured using the Langmuir probe technique. However, the Langmuir probe technique is designed to measure a current flowing through a metal probe based on charged particles. As described above, particles (i.e., atoms and excited molecules) emitted from the RF excitation cell4are electrically neutral. Therefore, the amount of dissociated nitrogen atomic flux may not be correctly measured by the Langmuir probe technique.

The present inventors have previously developed an device for measuring the amount of dissociated nitrogen atomic flux (see Patent Document 2). This measurement device makes use of the phenomenon that when electrically neutral dissociated nitrogen atoms are attached to a probe electrode having a negative potential, the atoms emit electrons due to self ionization, whereby a current (hereinafter referred to as an “atomic current”) flows. The value of the atomic current flowing through the probe electrode varies depending on the amount of the atomic flux in an atmosphere in which the probe electrode is placed. Therefore, the amount of the atomic flux can be determined by measuring the value of the current.

CITATION LIST

Patent Document

SUMMARY OF INVENTION

Technical Problem

As described above, the value of the atomic current measured by the atomic flux measurement device varies depending on the amount of the atomic flux in an atmosphere in which the probe electrode is placed. Therefore, if the probe electrode is provided in the vicinity of the substrate2, the amount of the atomic flux that strikes the substrate2can be determined as the value of the atomic current.

However, the value of the atomic current output from the probe electrode is low. The surface area of the probe electrode may be increased in order to increase the value of the atomic current. However, if the surface area of the electrode is increased, the size of the measurement device increases, and therefore, it becomes difficult to provide sufficient installation space in the vacuum chamber.

The present invention has been made with the above problems in mind. It is an object of the present invention to provide a low-cost and compact atomic flux measurement device capable of monitoring the flux amount of dissociated nitrogen atomic flux emitted from a plasma generation cell.

Solution to Problem

An atomic flux measurement device according to the present invention is one for measuring a amount of dissociated nitrogen atomic flux that are emitted from a plasma generation cell to a growth camber by discharge in a gas, including a counter electrode body including a pair of first and second sheet-like electrodes that face each other and are arranged substantially parallel to each other with a predetermined spacing between them, a first direct-current power supply configured to apply a direct-current voltage between the first and second sheet-like electrodes to cause the atoms attached to an inner surface of the first sheet-like electrode to undergo self-ionization so that a current flows between the first and second sheet-like electrodes, and a direct-current ammeter provided between the first and second sheet-like electrodes and configured to measure a value of the current flowing due to the self-ionization of the atoms attached to the inner surface of the first sheet-like electrode.

In the atomic flux measurement device of the present invention, the potential of the first sheet-like electrode is represented by
VA=−EA+VB
where VAis the potential of the first sheet-like electrode, EAis the electromotive force of the first direct-current power supply and has a value of zero or more, VBis the potential between the second sheet-like electrode and a ground terminal and is set to a value of zero or less by a second direct-current power supply.

The first sheet-like electrode is preferably formed of a metal plate, the first and second sheet-like electrodes are preferably formed of a metal mesh sheet, or the second sheet-like electrode is preferably formed of a metal mesh sheet. The first and second sheet-like electrodes of the counter electrode body may be rolled into a spiral with an insulating spacer being interposed between them. Alternatively, the first and second sheet-like electrodes may be formed of a plurality of plate-like metal mesh sheets having substantially the same shape, and the first and second sheet-like electrodes of the counter electrode body may be alternately stacked with a predetermined spacing.

The counter electrode body preferably includes a third sheet-like electrode that is formed of a metal mesh sheet and is provided on the side of the counter electrode body where the atomic flux enters, and a potential of the third sheet-like electrode is preferably set to be the same as a potential of the second sheet-like electrode. Alternatively, the counter electrode body preferably includes a fourth sheet-like electrode outside the first sheet-like electrode, on the side opposite to the side of the counter electrode body where the atomic flux enters, while being separated from the first sheet-like electrode by a predetermined spacing, and the fourth sheet-like electrode is preferably connected to the first sheet-like electrode.

The atomic flux measurement device of the present invention preferably further includes an A/D converter configured to convert a value of the atomic current measured by the direct-current ammeter into digital data, a memory configured to store the digital data output from the A/D converter, a display configured to display the digital data stored in the memory, and a controller configured to write and read data to and from the memory and control the operation of the display.

The atomic flux measurement device of the present invention preferably further includes a calculator configured to calculate the amount of flux based on the value of the atomic current measured by the direct-current ammeter. The calculator preferably reads out a table indicating a relationship between values of atomic currents and amounts of flux corresponding to the values of the atomic currents, the table being previously stored in the memory. The calculator preferably checks the value of the atomic current measured by the direct-current ammeter against the values of the atomic currents stored in the memory, to calculate the amount of flux corresponding to the value of the atomic current measured by the direct-current ammeter.

Advantageous Effects of Invention

The atomic flux measurement device of the present invention measures the amount of dissociated atomic flux emitted from an RF excitation cell based on a value of an atomic current flowing between a pair of sheet-like electrodes. In the atomic flux measurement device of the present invention, the sensitivity of measurement of the atomic current can be increased by applying an appropriate negative bias voltage to one of the electrodes. As a result, the atomic current can be measured using a relatively low-cost ammeter, and therefore, the cost of the measurement device can be reduced.

Also, the sheet-like electrodes may be formed of a metal mesh sheet and rolled or stacked, whereby the surface areas of the electrodes can be increased without an increase in the volume of the counter electrode body, resulting in a compact atomic flux measurement device having a high measurement sensitivity.

DESCRIPTION OF EMBODIMENTS

Preferred embodiments of an atomic flux measurement device according to the present invention will be described hereinafter with reference to the accompanying drawings.

First Embodiment

FIG. 1shows a basic configuration of an atomic flux measurement device according to a first embodiment of the present invention. The atomic flux measurement device1includes a counter electrode body11a, a first and a second direct-current power supply14and15, a first and a second direct-current ammeter16and17, a first and a second A/D converter21and22and a personal computer8, which are used to measure the value of an atomic current occurring due to dissociated nitrogen atoms emitted from an RF excitation cell4.

The counter electrode body11aincludes a first and a second sheet-like electrode12and13that are substantially parallel to each other with a predetermined spacing between them. The first sheet-like electrode12is connected to a first terminal18, and the second sheet-like electrode13is connected to a second terminal19. The first direct-current power supply14and the first direct-current ammeter16are connected together in series between the terminals18and19. The second direct-current power supply15and the second direct-current ammeter17are connected together in series between the terminal19and a ground terminal (i.e., a terminal having a reference potential)20.

<Specific Configuration of Counter Electrode Body>

FIG. 2shows an example of a specific configuration of the counter electrode body11a. The sheet-like electrodes12and13are formed, for example, of a rectangular stainless steel sheet having a high boiling point, and have holes in the four corners. The pair of sheet-like electrodes12and13are separated from each other by a predetermined spacing, facing each other with insulating spacers23being interposed between them. The spacers23are fixed to the sheet-like electrodes12and13using nuts25and washers26by bolts24inserted into the holes provided at the four corners.

One end of a wire27is connected to one of the nuts25on the first sheet-like electrode12, and one end of the wire28is connected to another of the nuts25on the second sheet-like electrode13. The other end of the wire27is connected to the first terminal18, and the other end of the wire28is connected to the second terminal19. Outer circumference surfaces of the wires27and28are covered by alumina insulating tubes29.

In this embodiment, the sheet-like electrodes12and13were formed of a rectangular flat plate of stainless steel (SUS) having a thickness of 0.5 mm and an area of 180 mm (L)×50 mm (W). An alumina bushing having a length of 6 mm is used as the spacer23. A picoammeter (model 6487 manufactured by Keithley Instruments Inc. in the U.S.) was used as the first and second direct-current ammeters16and17ofFIG. 1. A commercially available battery (accumulator) was used as the first direct-current power supply14. An electronic direct-current power supply was used as the second direct-current power supply15.

Also, in this embodiment, an IRFS-501 RF excited nitrogen source (trade name) manufactured by ARIOS INC. (in Tokyo, Japan) was used as the RF excitation cell4, and was incorporated into a cell port of a VG80H-MBE growth equipment manufactured by VG SEMICON (in the U.K.). A combined product of the IRFS-501 RF excited nitrogen source, the RF matching box6and the RF power supply7is commercially available under the model name “IRFC-504” from ARIOS INC.

<Principle of Measurement of Amount of Flux>

Before describing the operation of the atomic flux measurement device10, the principle of measurement of the amount of an atomic flux will be described with reference toFIGS. 1 and 3.

As described above, excited molecules N2*, excited atoms N* and ground-state atoms N of nitrogen contained in plasma in the HB discharge mode have the properties that they are readily attached to a solid-phase interface, such as, for example, the substrate surface or the metal plate surface. Therefore, if the counter electrode body11ais provided in a space in which a flux of dissociated nitrogen atoms emitted from the RF excitation cell4is present, as shown inFIG. 3(a)nitrogen molecules N2*, excited atoms N* and ground-state atoms N are attached to the inner surfaces of the sheet-like electrodes12and13at a density corresponding to thermal equilibrium vapor pressure of the local space, as called indirect irradiation.

If an appropriate negative bias voltage, is applied to one (i.e. electrode12) of the sheet-like electrodes12and13to which the dissociated nitrogen atoms are attached, excited atoms N* and ground-state atoms N of the particles attached to the electrode surface undergo self-ionization, so that, as shown inFIG. 3(b), electrons e−are emitted from the sheet-like electrode12to which the negative bias voltage is applied, and reaches the sheet-like electrode13having a higher potential. As a result, an atomic current of electrons produced by self-ionization flows between the pair of the sheet-like electrodes12and13.

In the circuit ofFIG. 1, a potential (i.e. a potential of the first sheet-like electrode12) VAof the terminal18corresponds to an electromotive force EAof the first direct-current power supply14. On the other hand, a potential (i.e. a potential of the second sheet-like electrode13) VBof the terminal19corresponds to an electromotive force EBof the second direct-current power supply15, and is set to a negative potential −EB(EB≧0). Specifically, the potential VAof the first sheet-like electrode12is set as follows:
VA=−EA+VB<0.
Note that the second direct-current power supply15may be removed, i.e., EB=0.

The negative potential VA(=−EA+VB) applied to the sheet-like electrode12causes self-ionization of the dissociated nitrogen atoms attached to the inner surface of the sheet-like electrode12, so that an atomic current IAcorresponding to the number (density) of the dissociated nitrogen atoms flows between the sheet-like electrodes12and13. The value of the atomic current IAis measured by the first direct-current ammeter16.

The atomic current IAis typically represented by
IA=−γSFNVA+I0(1)
where γ is the self-ionization coefficient of the electrode surface, S is the effective electrode area, FNis the amount of dissociated nitrogen atomic flux on the electrode surface, and I0is the current flowing when the potential is zero. The sign “−” indicates that the current is formed of electrons emitted from the electrode having a negative potential.

As shown in Expression (1), the amount of dissociated nitrogen atomic flux and the atomic current IAhave a linear relationship. Therefore, the amount of the atomic flux can be indirectly measured by measuring the atomic current IA.

As shown inFIG. 17described above, the counter electrode body11ais provided in the vacuum chamber3of the MBE growth equipment1. The flow of the HB-discharge-mode plasma, i.e., active species including excited molecules N2* and dissociated nitrogen atoms (excited atoms N* and ground-state atoms N), which is emitted from the RF excitation cell4of the MBE growth equipment1, repeatedly strikes and rebounds off the shroud, the reflection plate32and the like in the vacuum chamber3before entering the space portion between the first and second sheet-like electrodes12and13of the counter electrode body11afrom openings around the space.

As shown inFIG. 3(a), the dissociated nitrogen atoms that have entered the space portion of the counter electrode body11aare attached to the inner surfaces of the pair of the sheet-like electrodes12and13, with a density distribution corresponding to thermal equilibrium vapor pressure of the space portion. As described above, the atomic current IAmeasured by the first direct-current ammeter16corresponds to the density distribution of the dissociated nitrogen atoms in the local space in which the counter electrode body11ais provided, i.e., the amount of flux. Therefore, the amount of the atomic flux can be determined by measuring the value of the atomic current flowing in the counter electrode body11a.

The results of measurement of current-voltage characteristics using, the above atomic flux measurement device10are shown inFIGS. 4 to 6.FIG. 4shows a relationship between a potential difference (VA−VB=−EA) [horizontal axis] applied between the first and second sheet-like electrodes12and13and the atomic current IA[vertical axis] measured by the first direct-current ammeter16, where the potential VBof the second terminal18is zero.

According to the measurement result of Fla4, the atomic current IAis detected only when the potential VA(=−EA+VB) applied to the first sheet-like electrode12has a negative value. Because VBis set to zero, the negative portion of the potential difference (VA−VB) is the atomic current. As the absolute value of the potential VAincreases, the atomic current IAlinearly increases, and changes in the increase of the current are small.

InFIG. 4, if the potential difference (VA−VB=−EA) is positive, a current of electrons contained in the atomic flux around the counter electrode body11aflows through the first direct-current ammeter16, but this current has nothing to do with the atomic current.

FIG. 5shows changes in the atomic current IA[vertical axis] that occur when the potential VBof the second terminal19is changed to 0, −50 V, −100 V and −150 V and is maintained at the values, and the potential difference (VA−VB=−EA) [horizontal axis] applied between the first and second sheet-like electrodes12and13is changed. As shown in the upper right portion of the graph, measured values are indicated by different symbols for the different values of the potential VB.

In the four measurement results (VB=0, −50 V, −100 V and −150 V) shown inFIG. 5, their linear portions are not parallel to each other, and their slopes increase as the absolute value of the potential difference (VA−VB=−EA) increases. If the graph is redrawn where the potential difference (VA−VB=−EA) applied between the first and second sheet-like electrodes12and13is constant, it can be seen that the atomic current IAchanges substantially linearly, depending on the magnitude of the potential VAof the first direct-current power supply13.

FIG. 6shows the result of measurement of current-voltage characteristics that is obtained when the potential difference (VA−VB=−EA) applied between the first and second sheet-like electrodes12and13is fixed to −108 V and the potential VBof the second terminal19is changed up to −1000 V. It can be seen from this measurement result that the atomic current IAlinearly decreases depending on an increase (a decrease in the absolute value) in the potential VAof the first terminal18.

From the three measurement results ofFIGS. 4 to 6, it is confirmed that the amount of dissociated nitrogen atoms (N and N*) flux emitted from the RF excitation cell4with which the surface of the substrate2is indirectly irradiated, i.e., dissociated nitrogen atoms that are emitted from the RF excitation cell4and thereafter strike and rebound off the shroud and the like of the vacuum chamber3before striking the surface of the substrate2, can be sufficiently measured. Note that the amount of dissociated nitrogen atoms (N and N*) flux that strike the substrate2varies depending on the operation conditions of the RF excitation cell4, specifically, the high-frequency power applied to the excitation coil43of the RF excitation cell4, the vapor pressure of the active species in the vacuum chamber3of the MBE growth equipment1, the temperature of the shroud of the vacuum chamber3, and the like.

Incidentally, if the plasma emitted from the RF excitation cell4contains charged particles, such as, for example, nitrogen molecule ions N2+, nitrogen atom ions N+or electrons e−, a current IBflowing from the second sheet-like electrode13toward the ground terminal20is measured by the second direct-current ammeter17. As described above, the atomic current corresponds to the number of neutral dissociated nitrogen atoms attached to the inner surface of the first sheet-like electrode12. The current IBmeasured by the second direct-current ammeter17includes a current of charged particles contained in the plasma and, in addition, an atomic current of neutral dissociated nitrogen atoms attached to the outer surface of the second sheet-like electrode13. In this case, the current of the charged particles is also included in the current IAmeasured by the first direct-current ammeter16, resulting in an error during measurement of the amount of flux. Therefore, the presence of an error in the atomic current can be detected based on the current IBof the second direct-current ammeter17.

Referring back toFIG. 1, the current IAdetected by the first direct-current ammeter16is converted into digital data by the A/D converter21before being input to the PC8. The PC8functions as a controller81and a calculator82of the atomic flux measurement device10in addition to the aforementioned growth control circuit85. These functions are achieved by reading software stored in the memory83and executing the software using a CPU. The digital data that has been input to the PC8and stored in the memory83is displayed on a display84by a control performed by the controller81. An operator can find out the value of the atomic current in situ.

In this embodiment, the amount of the atomic flux striking the substrate2is monitored based on the atomic current flowing through the counter electrode body11a. If you wish to directly find out the amount of the atomic flux, it is necessary to convert the value of the atomic current into the amount of flux. In this case, it is necessary to prepare a table containing the amount of atomic flux and atomic current values measured under the same conditions, based on which the value of an atomic current is converted into the amount of an atomic flux, and store the table in the memory83in advance. If the calculator82converts a current value measured by the counter electrode body11ainto the amount of an atomic flux based on the table, and the value of the amount of an atomic flux is displayed on the display84, the amount of dissociated nitrogen atoms (N and N*) flux can be known in situ.

Additionally, a graph is produced that indicates a correlation relationship between the growth rate of the monocrystalline Si3N4buffer layer that is formed on the substrate2using the above MBE growth equipment1and the value of the atomic current measured by the counter electrode body11a. If the data of the produced graph is stored as a table in the memory in the growth control circuit85(seeFIG. 17), the growth control circuit85can control the growth rate of the monocrystalline Si3N4buffer layer using the data of the table based on the value of the atomic current measured by the counter electrode body11a.

Similar to the first direct-current ammeter16, the current value detected by the second direct-current ammeter17is converted into digital data by the A/D converter22before being input to the PC8. Note that, in addition to the above control, the controller81sets the voltages of the first and second direct-current power supplies14and15and controls ON/OFF of the first and second direct-current ammeters16and17.

<Procedure of Measurement of Atomic Flux>

Next, a procedure of measuring the amount of dissociated nitrogen atomic flux that strike the substrate2using the atomic flux measurement device10of this embodiment will be described.

The RF excitation cell4is mounted in the cell port of the above MBE growth equipment1shown inFIG. 17, while the silicon substrate2is fixed to the substrate holder31in the vacuum chamber3. The buffer layer of monocrystalline Si3N4is grown on the silicon substrate2. In this case, a high-frequency power of 500 W is applied to the RF excitation cell4so that the RF excitation cell4is operated in the HB discharge mode.

As described above, when the monocrystalline Si3N4buffer layer is grown on the silicon substrate2, the substrate2is preferably indirectly irradiated with a flux of dissociated nitrogen atoms (ground-state atoms N and dissociated nitrogen atoms N*) emitted from the RF excitation cell4. The indirect irradiation is achieved by any of the following methods.

A first method is to close the travel direction of the flux of dissociated nitrogen atoms emitted from the RF excitation cell4using a shutter (not shown). With this method, dissociated nitrogen atoms emitted from an orifice44of the RF excitation cell4repeatedly strike and rebound off the shutter and the inner wall of the growth chamber (vacuum chamber), and thereafter, a flux having low energy leaks from a gap at the periphery of the shutter, so that the surface of the substrate2is irradiated with the low-energy flux.

A second method is to open the above shutter to irradiate the substrate2with a flux of dissociated nitrogen atoms that has been emitted from the RF excitation cell4and has repeatedly struck and rebounded off the reflection plate32and the shroud provided in the vacuum chamber3.

The counter electrode body11of the atomic flux measurement device10is provided in a portion adjacent to the substrate holder31at a position that is located away from a region (straight line) connecting the orifice44of the RF excitation cell4and the surface of the substrate2. As described above, dissociated nitrogen atoms emitted from the RF excitation cell2repeatedly strike and rebound off the shutter (not shown), the reflection plate32, the shroud and the like provided in front of the RF excitation cell2before entering the space portion of the counter electrode body11aof the atomic flux measurement device10. Thereafter, the dissociated nitrogen atoms are attached to the surfaces of the sheet-like electrodes12and13at a density corresponding, to thermal equilibrium vapor pressure of the space portion, so that an atomic current occurs between the electrodes. The atomic current measured by the atomic flux measurement device10is sent to the growth control circuit85, and is used as data for controlling the thickness of the buffer layer.

In this embodiment, the atomic flux measurement device10was used to measure the value of the atomic current while the RF excitation cell4was operated under the following conditions. A relationship between the atomic current IAmeasured with the first direct-current ammeter16and the potential VAof the first sheet-like electrode12at that time is shown inFIGS. 7 to 9.

(1) Power applied to the discharge coil43of the RF excitation cell4: 500 W

(2) Flow rate of nitrogen supplied to the discharge chamber42of the RF excitation cell4: 1.38 sccm

(3) Pressure in the RF excitation cell4: about 100 Pa

(4) Degree of vacuum in the vacuum chamber3: 4×10?3Pa

A graph ofFIG. 7shows a relationship between the potential VA(=−EA+VB) of the first terminal and the atomic current IAmeasured by the first direct-current ammeter16, where the electromotive force EAof the first direct-current power supply14was changed within the range of −108 V to +108 V while the potential VBof the second terminal19was held at −175 V, −75 V and 0 V.

It was found that the atomic current IAvaries linearly, depending on the change in the potential VA. From this result, it was found that there is a linear correlation relationship between the measured value of the atomic current IAand the amount of dissociated nitrogen atoms (N*+N) flux existing in the space portion of the counter electrode body11a.

The graph ofFIG. 8shows four curves. A first curve indicated by the symbol “◯” shows a relationship between the potential VA(=−EA+VB) of the first terminal18and the atomic current I4measured by the first direct-current ammeter16, where the potential VBof the second terminal19was changed within the range of −800 V to 0 V while the electromotive force EAof the first direct-current power supply14was held constant (−108 V).

A second curve indicated by the symbol “+” shows a relationship between the potential VAof the first terminal and the atomic current IAmeasured by the first direct-current ammeter16, where the electromotive force EAof the first direct-current power supply16was changed within the range of −108 V to +108 V while the potential VBof the second terminal19was held constant (200 V). A third curve indicated by the symbol “▴” shows a relationship between the potential VAof the first terminal and the atomic current IAmeasured by the first direct-current ammeter16where the electromotive force EAof the first direct-current power supply16was changed within the range of −108 V to +108 V while the potential VBof the second terminal19was held constant (0 V).

From these measurement results, it was found that the atomic current IAvaries linearly, depending on the magnitude of the electromotive force EAof the first direct-current power supply14. Note that a fourth curve indicated by the symbol “⋄,” which is for reference, shows a relationship between the potential VAof the first terminal and the current IA(i.e., a dark current) measured by the first direct-current ammeter16, where power is not supplied to the RF excitation cell4.

The graph ofFIG. 9shows a relationship between the potential VAof the first terminal and the atomic current IAmeasured by the first direct-current ammeter16, where two counter electrode bodies11ahaving different spacings D (6 mm and 20 mm) between the pair of the sheet-like electrodes12and13were used. The counter electrode body having the larger spacing D indicated by the symbol “◯” has a larger value of the atomic current IAthan that of the counter electrode body having the smaller spacing D indicated by the symbol “●.” This may be because, for the larger spacing D, there is a larger amount of dissociated nitrogen atoms (N*+N) flux in the space portion of the counter electrode body11a.

Second Embodiment

FIG. 10shows a basic configuration of an atomic flux measurement device according to a second embodiment of the present invention. InFIG. 10, components having the same functions as those of the atomic flux measurement device of the first embodiment ofFIG. 1are indicated by the same reference characters and will not be described.FIG. 10additionally shows the RF excitation cell4in order to describe the function of the electrode. On the other hand,FIG. 10does not show the A/D converters21and22or the PC8, which will not be described.

The atomic flux measurement device of this embodiment is the same as that of the first embodiment, except for the configuration of the counter electrode body. A counter electrode body11bof this embodiment employs two sheet-like electrodes51and52formed of a metal mesh sheet (hereinafter referred to as “mesh electrodes”) instead of the sheet-like electrodes12and13of the counter electrode body11aof the first embodiment.

A reason why the mesh electrode is used as the sheet-like electrode will be described. The counter electrode body11aof the first embodiment does not have a problem when the amount of dissociated nitrogen atomic flux in indirect irradiation is measured. However, the electrode is harmed of a sheet-like metal, and therefore, when the amount of dissociated nitrogen atomic flux that are emitted from the RF excitation cell4and directly strike the substrate2(direct irradiation) is measured, the amount of the flux entering the space portion of the counter electrode body varies significantly, depending on the orientation of the electrode. Also, most of the dissociated nitrogen atoms rebound off the surface of the sheet-like electrode to go away from the counter electrode body, and therefore, only a small number of dissociated nitrogen atoms enter the space portion of the counter electrode body11ato contribute to the atomic current. As a result, the amount of dissociated nitrogen atomic flux cannot be accurately measured.

In contrast to this, when the mesh electrode is used as the sheet-like electrode, a flux F of dissociated nitrogen atoms emitted from the RF excitation cell4passes through interstices of the mesh electrode to enter the space portion of the counter electrode body11b, so that the thermal equilibrium vapor pressure of the space portion increases. The dissociated nitrogen atoms are attached to the electrode surface at a density corresponding to thermal equilibrium vapor pressure, resulting in an atomic current.

As in the first embodiment, the first and second direct-current power supplies14and15apply a negative bias potential VA=−EA+VBto the first mesh electrode51. An atomic current IAbased on self-ionization of the dissociated nitrogen atoms (N*+N) attached to the inner surface of the mesh electrode51flows between the mesh electrodes51and52.

Thus, the counter electrode body11bcan measure not only the amount of a flux of dissociated nitrogen atoms (N*+N) that are emitted from the RF excitation cell4and enter through peripheral openings of the mesh electrodes51and52(indirect irradiation), but also the amount of a flux of dissociated nitrogen atoms (N*+N) that are emitted from the RF excitation cell4and pass through the interstices of the mesh electrode52to enter the space portion of the counter electrode body11b(direct irradiation).

<Specific Configuration of Counter Electrode Body>

FIG. 11shows an example specific configuration of the counter electrode body11b. The counter electrode body11bincludes two metal mesh sheets having different lengths that are rolled, facing each other with a spacing between them. If the counter electrode body11bthus includes the mesh electrodes robed into a spiral, the surface areas of the mesh electrodes51and52can be increased without an increase in the volume of the counter electrode body. As a result, the amount of a flux attached to the inner surface of the mesh electrode51increases, and therefore, the value of the atomic current proportionately increases, whereby the sensitivity of current measurement can be increased, and therefore, the accuracy of detection of the amount flux can be increased.

In this embodiment, the mesh electrode51was formed of a mesh (#100 (No. 100)) of stainless steel (SUS404) and had a size of 140 mm (W)×300 mm (L), and the mesh electrode52was formed of a mesh (#100 (No. 100)) of stainless steel (SUS404) and had a size of 140 mm (W)×250 mm (L). The mesh electrodes51and52are robed, facing each other with a plurality of alumina bushings (insulating spacers)53with a diameter of 6 mm being interposed between them to form a spacing (D) of 6 mm.

As shown inFIGS. 11(a) and 11(b), three support posts54of aluminum are provided to stand on a flange33of the cell shutter. The two mesh electrodes51and52rolled into a spiral are inserted in the space portion formed by the three support posts54. An alumina tube55for insulation is mounted around each support post54. The spiral mesh electrodes51and52are fixed to the three support posts54by wrapping a fixing, band56of a heat-resistant insulating material around the outer circumference of the three support posts54. Moreover, innermost end portions of the mesh electrodes5and52are immobilized by an insulating fixing device57while they are separated from each other by the spacing D. The outermost end portions of the mesh electrodes51and52are electrically insulated via sheet-like separators58of mica. The mesh electrodes51and52are connected to the terminals18and19, respectively, via wires59.

In the counter electrode body11bof this embodiment, the mesh electrode bodies51and52are rolled into a spiral, whereby the electrode area is increased without an increase in the volume, and therefore, the amount of dissociated nitrogen atoms attached to the mesh electrode can be increased. As a result, the sensitivity of measurement of the atomic current can be increased, and therefore, the accuracy of detection of the amount of the atomic flux can be increased.

Third Embodiment

FIG. 12shows a basic configuration of an atomic flux measurement device according to a third embodiment of the present invention. InFIG. 12, components having the same functions as those of the atomic flux measurement device s of the first and second embodiments are indicated by the same reference characters and will not be described. As inFIG. 10,FIG. 12additionally shows the RF excitation cell4in order to describe the function of the electrode. On the other hand,FIG. 12does not show the A/D converters21and22or the PC8, which will not be described.

As with the counter electrode body11bof the second embodiment, a counter electrode body11cof this embodiment employs two mesh electrodes71and72formed of a metal mesh sheet. On the other hand, unlike the counter electrode body11bof the second embodiment, the counter electrode body11cincludes a third mesh electrode73formed of a metal mesh sheet that is provided on the outer side of the second mesh electrode72with a predetermined spacing between them.

If the third mesh electrode73is held at an appropriate potential (e.g., the same potential as that of the mesh electrode72), charged particles N2+, e−and the like contained in a flux F can be prevented from entering the mesh electrodes71and72to some extent. In other words, the mesh electrode73functions as a filter that prevents a noise or error current from being added to an atomic current flowing through a closed circuit including the mesh electrodes71and72.

Note that although not essential to the atomic flux measurement device of the present invention, as shown inFIG. 12, a pair of eliminator electrodes34may be provided in the vicinity of a plasma outlet of the RF excitation cell4so that charged particles contained in a plasma flux are laterally deviated, whereby charged particles can be prevented from being contained in the flux F entering the atomic flux measurement device10.

The eliminator electrode pair34includes a pair of electromagnets facing each other to generate a static magnetic field intersecting the plasma flux emitted from the RF excitation cell4, thereby laterally deviating the charged particles contained in the plasma flux.

The eliminator electrode pair34prevents charged particles from being contained in the flux F emitted from the RF excitation cell4to the counter electrode body11c. Therefore, measurement error in an atomic current in the atomic flux measurement device can be reduced. Note that, inFIG. 12, a direct-current power supply35is designed so that the polarity of the potential that is applied to the eliminator electrode pair34can be changed using a switch36.

<Specific Configuration of Counter Electrode Body>

FIG. 13shows an example specific configuration of the counter electrode body11cof this embodiment.FIG. 14shows a schematic circuit diagram of the counter electrode body11cshown inFIG. 13. The counter electrode body11cofFIG. 13is different from the counter electrode body11bof the second embodiment shown inFIG. 11in two ways. A first difference will be described hereinafter.

In the counter electrode body11b, the two mesh electrodes51and52having large areas are rolled to form a counter electrode body, in contrast to this, the counter electrode body11cincludes a plurality of the plate-like mesh electrodes71and72, which face each other and are alternately arranged with a spacing.

In the counter electrode body11c, the first mesh electrodes71including plate-like metal mesh sheets are joined together in the shape of a comb, and the second mesh electrodes72having a similar configuration, are alternately arranged and combined with a ceramic insulating spacer74being interposed between each electrode. Note that the insulating spacers74are fastened together using a wire75so that the insulating spacers74are not displaced.

The flux F of dissociated nitrogen atoms emitted from the RF excitation cell4enters a space portion between each electrode not only from a peripheral portion (indirect irradiation) but also from the front sides of the mesh electrodes72(direct irradiation). The dissociated nitrogen atoms (N and N*) coming to the space portions are attached to the inner surfaces of the first mesh electrodes71, so that an atomic current flows between both electrodes.

While the size of the counter electrode body11bis reduced by rolling the mesh electrodes, the size of the counter electrode body11cis reduced by stacking the mesh electrodes. By using a plurality of the mesh electrodes71and72that are put on top of each other, the mesh electrode area is increased without an increase in the volume of the counter electrode body, whereby the amount of dissociated nitrogen atoms attached to the electrode surface is increased. As a result, the sensitivity of measurement of the atomic current can be increased, and therefore, the accuracy of detection of the amount of the dissociated nitrogen atomic flux can be increased.

A second difference is that the comb-like third mesh electrode73is provided in front of the counter electrode body. As described above, the mesh electrode73has a function of preventing a noise or error current from being added to the atomic current flowing through the closed circuit including the mesh electrodes71and72. As shown inFIG. 14, the mesh electrode73substantially, but not entirely, prevents charged particles N2+, e−and the like contained in the flux F coming from the front of the counter electrode body from entering the inside of the counter electrode body.

Note that the configuration of the counter electrode body11cis not limited to that shown inFIG. 13. Although three lines of the spacers74are provided in the counter electrode body ofFIG. 13, the spacing may be held using any dedicated jig that can maintain insulation between the mesh electrodes.

Fourth Embodiment

FIG. 15shows a basic configuration of an atomic flux measurement device according to a fourth embodiment of the present invention. InFIG. 15, components having the same functions as those of the atomic flux measurement device s of the first to third embodiments are indicated by the same reference characters and will not be described. On the other hand,FIG. 15does not show the A/D converters21and22or the PC8, which will not be described.

The atomic flux measurement device of this embodiment is the same as those of the first and second embodiments, except for the configuration of the counter electrode body. In a counter electrode body11dof this embodiment, a sheet-like electrode91similar to that of the first embodiment is used as a first sheet-like electrode that is held at a potential VA, and a mesh electrode92similar to that of the second embodiment is used as a second sheet-like electrode that is held at a potential VB.

As in the second embodiment, a mesh electrode is used as the second sheet-like electrode92to which an atomic flux is input. Therefore, a flux F of dissociated nitrogen atoms emitted from the RF excitation cell4passes through interstices of the mesh electrode to enter a space portion of the counter electrode body11d, so that thermal equilibrium vapor pressure of the space portion increases. The dissociated nitrogen atoms are attached to the electrode surface at a density corresponding to thermal equilibrium vapor pressure, resulting in an atomic current.

On the other hand, because the sheet-like electrode91similar to that of the first embodiment is used as the first sheet-like electrode that is held at the potential VA, most of the dissociated nitrogen atoms that have passed through the mesh electrode92to enter the space portion of the counter electrode body11dmake contact with a surface of the sheet-like electrode91. As a result, the value of the atomic current can be expected to increase compared to the counter electrode body11bof the second embodiment.

<Specific Configuration of Counter Electrode Body>

FIG. 16schematically shows a circuit diagram of a specific configuration of the counter electrode body11dof this embodiment. Although not shown, the specific configuration of the counter electrode body11dis almost the same as that of the counter electrode body11cof the third embodiment shown inFIG. 13, i.e., first mesh electrodes93and second mesh electrodes92are alternately stacked.

In the counter electrode body11dof this embodiment, the first sheet-like electrode that is held at the potential VAincludes the sheet-like electrode91and the mesh electrode93. Specifically, as shown inFIG. 16, the third mesh electrode73of the counter electrode body11cof the third embodiment is removed, and instead, the plate-like sheet-like electrode91is provided on the side opposite to the side that an atomic flux enters while being separated from the mesh electrode93by a predetermined spacing. Note that the third mesh electrode73of the counter electrode body11cmay be left as it is.

As described above, most of the dissociated nitrogen atoms that have passed through interstices of the mesh electrodes92and93make contact with the surface of the sheet-like electrode91, and therefore, the value of the atomic current can be expected to increase compared to the counter electrode body11cof the third embodiment.

As described above, the atomic flux measurement device of the present invention holds the first sheet-like electrode at a negative potential, and further, reduces the value of the potential, thereby increasing the sensitivity of measurement of the atomic current. Therefore, the atomic current can be measured using a relatively low-cost ammeter. As a result, the manufacturing cost of the measurement device can be reduced.

The sheet-like electrodes are formed of a metal mesh sheet, and the sheets are rolled or stacked. As a result, the electrode surface area can be increased without an increase in the volume of the counter electrode body. Therefore, a compact atomic flux measurement device having a high measurement sensitivity can be provided.

Note that, in each of the above embodiments, the atomic flux measurement device10is used to monitor the amount of nitrogen active species (N and N*) flux emitted from the RF excitation cell4. The atomic flux measurement device of the present invention is not limited to this application.

Hydrogen gas H2or oxygen gas O2may be supplied to the RF excitation cell4, and a relatively high degree of high-frequency power may be applied to the excitation coil43of the RF excitation cell4to operate the RF excitation cell4in the HB discharge mode so that to flux of dissociated hydrogen atoms (H* and H) or dissociated oxygen atoms (O* and O) are emitted from the RF excitation cell4. The flux of dissociated hydrogen atoms or dissociated oxygen atoms that is emitted from the RF excitation cell4, and thereafter, repeatedly strike and rebound off the shroud, the reflection plate32and the like of the vacuum chamber3, may be caused to enter the counter electrode body11c. The atomic current may be measured by the atomic flux measurement device10. As a result, the amount of the dissociated hydrogen atomic or the dissociated oxygen atomic flux may be determined.

The atomic flux measurement device of the present invention is not limited to the growth process on the substrate surface in the vacuum chamber3of the MBE growth equipment, and is, of course, applicable to treatments, such as etching or oxidation, in a chamber under vacuum conditions.

The atomic flux measurement device10was provided in the shutter port of the nitrogen RF excitation cell2of the MBE growth equipment at a position (indirect irradiation position) that is not directly irradiated with a dissociated nitrogen atomic flux from the nitrogen RF excitation cell4. The silicon substrate2fixed to the substrate holder31in the vacuum chamber3was indirectly irradiated with a flux of dissociated nitrogen atoms from the nitrogen RF excitation cell4so that the dissociated nitrogen atoms are allowed to react with Si atoms on the surface of the silicon substrate2(i.e., so-called surface/interface reaction). The process of growth of a monocrystalline Si3N4buffer layer by the reaction was observed based on the atomic current measured by the atomic flux measurement device10. As a result, as the amount of dissociated nitrogen atoms that are emitted from the nitrogen RF excitation cell4and are attached to the shroud (sidewall portion) increases, the amount of dissociated nitrogen atoms that strike the substrate2decreases, and therefore, the growth rate of the monocrystalline Si3N4buffer layer decreases.

The undesired decrease in the growth rate due to the increase in the dissociated nitrogen atoms adsorbed to the shroud is overcome as follows. As in an activity control type nitride MBE growth equipment (see JP 2008-78200 A) previously proposed by the present inventors, the discharge modes (the LB discharge mode and the HB discharge mode) of the nitrogen RF excitation cell4are alternately switched at appropriate time intervals (duty factor). Atoms of a dissociated nitrogen atomic flux generated during the HB discharge mode period that have been adsorbed to the shroud during one LB discharge mode period are prevented from being deposited on the cooled shroud surface by flushing during the succeeding LB discharge mode period, whereby the decrease in the growth rate of the monocrystalline Si3N4buffer layer on the substrate surface can be effectively prevented.

REFERENCE SIGNS LIST