JPH0518220B2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to semiconductor processing equipment, material modification,
This relates to ion beam generators used for materials synthesis, etc.

[Prior art]

Conventionally, various methods have been considered and put into practical use as ion generation methods using ion beam generators. Most of them used electric discharge, but in recent years ion sources using laser light have been devised. There are two methods of using light such as laser light. One is to irradiate a solid such as a metal with light and use the resulting plasma as an ion source, and the other is to focus the laser light and convert it into gas or liquid. irradiation to create plasma, which is used as an ion source.
The other method uses a variable wavelength light source to ionize a substance by resonating a single wavelength such as a laser beam with the energy level of the substance to be ionized (Japanese Unexamined Patent Application Publication No. 1989-1992). -Refer to Publication No. 22999).
The present invention relates to the latter, and uses synchrotron radiation rather than laser light as a light source.

[Summary of the invention]

The present invention excites a substance to be ionized to a relatively lower excited state by gas discharge, resonantly optically excites the substance to be ionized from the excited state to a Lyudberg state by irradiation with synchrotron radiation, and furthermore, the substance is excited by the synchrotron radiation. By applying a pulsed electric field synchronized with light to change the material from Rydberg level, which is close to the ionization limit, to an ion state, the ionization efficiency for input light energy is improved by more than four orders of magnitude compared to conventional resonant optical excitation and ionization methods. The purpose of the present invention is to provide a compact low-energy ion beam generator with excellent selectivity in selective ionization.

[Embodiments of the invention]

First, the ionization method used in the apparatus of the present invention will be explained, taking as an example the case of generating a beryllium ion beam, and comparing it with a conventional method. FIG. 1 is an energy level diagram of beryllium neutral atoms.

The conventional ionization method is, for example, to irradiate beryllium vapor to be ionized with two laser beams B1 and B2 with wavelengths of 2349 Å and 3000 Å. In other words, beryllium atoms in the ground state 2s ( ¹ S) are first ionized into 2349 Å wavelengths. It is resonantly excited to the first excited state 2p ( ¹ P ⁰ ) by the laser beam B1, and then ionized by the laser beam B2 of 3000 Å.

The ionization method in the apparatus of the present invention differs from the conventional ionization method described above in that synchrotron radiation, particularly relativistic synchrotron radiation, is used as an excitation light source. Synchrotron synchrotron radiation has directionality similar to laser light, and has far greater intensity and energy than laser light. Therefore, as shown in Figure 1, a single photon can destroy beryllium vapor at a relatively low level. can be excited from the excited state to the Ryudberg state. The fundamental difference between the method of the present invention and the conventional method is that the final excited state is the Ryudberg state, and ionization from the excited state is performed by applying an electric field as shown in the following equation.

E (V/cm)≧0.3125×10 ⁹ ·n* ^-4 where n* is the effective principal quantum number of the Rydberg state, and this relationship is shown in FIG. The Ryudberg state generally refers to a highly excited state of an atom that has a principal quantum number n that is about 10 or more larger than the ground state. In the case of beryllium neutral atoms, the ground state is 2s ( ¹ S), n=2, and the Ryudberg state is a highly excited state of about n≧12.

For the 12d ( ^3D ) level of beryllium neutral atoms, n* = 12.04, and therefore the electric field strength required for ionization of the Ryudberg state 12d ( ^3D ) is E≧1.487×10 ⁴ V/cm. Become. Also, higher,
That is, if a Ryudberg state closer to the ionization limit is selected, for example, the 30d ( ^3D ) level, the electric field strength required for ionization becomes smaller, E≧3.858×10 ² V/cm. On the other hand, for the first excited state 2p ( ¹ P ⁰ ), n * = 1.83, and if we attempt to directly ionize this 2p ( ¹ P ⁰ ) level by an electric field, the ionization requires E ≥ 2.786 × An extremely large electric field strength of 10 ⁷ V/cm is required.

By the way, the radiative lifetime of the Lyudberg state of effective principal quantum n* is τ _r ∝n* ³ , and under gas pressure so low that collisions can be ignored, n*
The higher the excited state is, the longer the radiative lifetime τ _r becomes. Figure 6 shows the relationship of the radiative lifetime τ _r (ns) = 1.38 x (n * - 0.0144) ³ for the d-state of beryllium neutral atoms. For the Ryudberg state 12d ( ^3D ) of a neutral beryllium atom, τ _r ~2.4 μs, but for a Ryudberg state that is higher, that is, closer to the ionization limit, such as the 30d ( ³ D) level, τ _r ~37 μs. becomes longer. On the other hand, the first excited state 2p
For ( ¹ P ⁰ ), τ _r is about 8 ns, and the radiative lifetime is extremely short compared to the Ryudberg state.

Therefore, the ground state of beryllium neutral atom 2s ( ¹ S)
is excited to the metastable state 2p ( ³ P ⁰ ) by gas discharge, and then this 2p ( ³ P ⁰ ) level is brought to the Lyudberg state by synchrotron radiation with wavelength λ ₂ = 1907 Å.
12d ( ^3D ), and this 12d ( ^3D ) level is exposed to the electric field E.
>1.487×10 ⁴ V/cm In the case of the ionization method in the device of the present invention, the beryllium vapor under discharge is resonantly excited by synchrotron radiation λ ₁ , and then the Ryudberg state 12d is generated.
It is sufficient to ionize the level by applying an electric field during the radiation lifetime τ _r of ( ^3D ) ~ 2.4 μs, and it is possible to generate a voltage of >10 kV with a rise/fall time of <1 μs. A pulsed voltage generator can be used.

Ground state, an example of following traditional ionization methods
When photoionizing directly from 2s ( ¹ S), its photoionization cross section is σ ₁ = 4.10×10 ^-27 [λ ₁ (Å)] ³ cm ² ,
Therefore, the ionization rate is W=2.06×10 ^-12 [λ ₁ (Å)] ⁴ I
(W/cm ² )sec ^-1 . Here, the synchrotron radiation wavelength λ ₁ exceeds the ionization limit, so λ ₁ <1329
It needs to be . FIG. 7 shows this ionization rate W as a function of the radiation wavelength λ ₁ for various radiation intensities I. As shown in the figure, when the wavelength λ ₁ is changed at a certain emitted light intensity, the ionization rate W changes continuously.

Another possible conventional ionization method is to directly photoionize beryllium neutral atoms excited to the metastable state 2p ( ³ P ⁰ ) by gas discharge. In this case, this photoionization cross section is σi=1.73×
10 ^-27 [λ ₂ (Å)] ³ cm ² , and therefore the ionization rate is W=8.70×10 ^-13 [λ ₂ (Å)] ⁴ I (W/cm ² ) sec ^-1 . Here, since the synchrotron radiation wavelength λ ₂ exceeds the ionization limit, it is necessary that λ ₂ <1879 Å. FIG. 8 shows this ionization rate W as a function of the radiation wavelength λ ₂ for various radiation intensities I. As shown in the figure, in this case as well, when the wavelength λ ₂ is changed at a certain emitted light intensity, the ionization rate W changes continuously.

On the other hand, in the case of the ionization method in the device of this invention, synchrotron radiation with wavelength λ ₂ = 1907 Å changes from the metastable state 2p ( ³ P ⁰ ) to the Ryudberg state.
Photoexcitation cross section to 12d ( ^3D ) σ _ex = 1.28×10 ^-22 [λ ₂
(Å)] ³ cm ² , and therefore its excitation rate is W=6.47×10 ^-8 [λ ₁ (Å)] ⁴ I(W/
cm ² ) sec ^-1 . However, the above value indicates that the optical absorption spectrum from the metastable state 2p ( ^{3P 0} ) to the Lyudberg state 12d ( ^3D ) has a Doppler broadening at room temperature, and the spectral width of the emitted light is 10 times the Doppler broadening. It is calculated as a degree. When such Ryudberg state 12d ( ^3D ) is ionized by an electric field, the electric field strength is changed to the electric field threshold required for ionization E _cr = 0.3125×10 ⁹・n * ^-4 =
If it is about 10% higher than 1.487×10 ⁴ V/cm,
Separates into ions and electrons at a high speed of 10 ¹² sec ^-1 or more. Therefore, this excitation rate W is in a metastable state.
It can be expressed as the field ionization rate via the Lyudberg state 12d ( ^3D ) due to synchrotron radiation of 2p ( ^{3P 0} ) with a wavelength of 1907 Å.

FIG. 9 shows this ionization rate W for various synchrotron radiation intensities I for several Ryudberg states nd( ^3D ) (n=12-15, 20, 30). When the synchrotron wavelength λ ₂ is changed at a certain synchrotron radiation intensity, the transition from the metastable state 2p ( ³ P ⁰ ) to the Ryudberg state
The ionization rate W exhibits a finite value only at the wavelength λ ₂ that resonates with the transition 2p ^{( 3 P 0} )−nd ( ³ D) to nd ( 3 D), and W=0 at other wavelengths.

Comparing Figures 7, 8 and 9, the ionization rate W of ionization via the Ryudberg state is:
It can be seen that this is more than four orders of magnitude larger than conventional photoionization.

By the way, as a conventional ionization method, beryllium neutral atoms excited from the ground state 2s ( ¹ S) to the first excited state 2p ( ¹ P ⁰ ) by synchrotron radiation with a gas wavelength λ ₁ = 2349 Å are subjected to an electric field. An ionization method in which direct ionization is also considered. In this case, as mentioned above, an extremely large electric field strength of E≧2.786×10 ⁷ V/cm is required. Also, 2p ( ¹ P ⁰ )
Since the radiative lifetime of the level is extremely short, approximately τ _r ~8 ns,
It is necessary to apply an electric field for ionization within an extremely short time of ~8 ns after irradiation with synchrotron radiation _λ1 .
Therefore, this ionization method requires a special high-voltage short pulse generator capable of generating an ultra-high voltage of about >10 MV in an ultra-short time of ~10 ns. In comparison, voltage generation for field ionization of the Ryudberg state according to the ionization method of the apparatus of the present invention can be achieved with an extremely compact and inexpensive ordinary apparatus.

Therefore, in the case of the ionization method of the present invention, the ionization efficiency with respect to the input light energy is more than four orders of magnitude higher than that of conventional resonant optical excitation and ionization methods, and since the ionization method is excitation from a metastable state rather than from the ground state, photons are The wavelength of synchrotron radiation is short, and since only resonance is used, if the wavelength of the synchrotron radiation light is selected so that it does not match the energy level of the impurity atoms, only the substance to be ionized can be ionized, and moreover, it is highly pure. I can do it.

Furthermore, in the case of the ionization method in this invention,
Even compared to the method of ionizing the excited state, which is much lower than the Ryudberg state, by field ionization,
There are advantages that can be achieved using conventional compact and inexpensive voltage generators without the need for specialized and expensive high voltage short pulse generators.

In addition, by changing the wavelength and electric field strength of the synchrotron radiation, it is possible to easily generate ionization beams of other types of substances. You can also introduce it. The method of the present invention, which allows the type of ion beam generated to be easily changed, is different from conventional methods, and has the advantage of being able to perform two or more processing steps using an ion beam in succession. be.

Next, embodiments of the invention will be described with reference to the drawings.

FIG. 2 shows a first embodiment of the present invention, in which 1 is a container into which a substance to be ionized is introduced, 1a is a gas introduction hole for introducing the substance into the container 1, and 1b is 1c is a slit, and 3 is synchrotron radiation that generates synchrotron radiation B3 with a narrow wavelength width of 1907 Å. This is the light generating part.

Further, 5 and 6 are electrodes arranged to sandwich the synchrotron radiation light B3, and 5a and 6a are terminals for applying a voltage to the electrodes 5 and 6, which apply an electric field to the substance in the container 1. The electric field generating section 15 has been achieved. Further, 11 is an induction coil which is an RF discharge generating section that generates RF discharge in the above-mentioned material atmosphere. Note that when the substance to be ionized is introduced into the container 1 as a gas in a compound or molecular state, the excitation gas discharge also serves as a gas discharge to bring the substance into a neutral atomic state. It's okay.

8 is a sample, and 8a is a sample stand that holds the sample 8. A direct current voltage is applied between the sample stand 8a and the electrode 6, and a drawer is used to extract the ionized substance as an ion beam. An electric field is generated. Note that the ionization electric field may also serve as the extraction electric field.

Next, the operation will be explained.

Let us consider the case where a beryllium ion beam is generated by the apparatus of this embodiment. First, beryllium vapor 10 is introduced into the container 1 through the gas introduction hole 1a. Then, RF discharge is generated by the induction coil 11,
Further, in synchronization with this, the synchrotron radiation light generating section 3 oscillates, and voltage is applied to each of the electrodes 5 and 6 from the terminals 5a and 6a in temporal synchronization with the oscillation. Then, due to this, the above ground state 2s
Electrons from the RF discharge collide with the beryllium vapor 10 at ( ¹ S), and as a result, the beryllium vapor 10 is excited to a metastable state 2p ( ³ P ⁰ ). Further, due to the oscillation of the synchrotron radiation generating section 3, the synchrotron radiation B3 of 1907 Å is introduced into the container 1 through the slit 1c, and as a result, the beryllium vapor 10 changes from the metastable state 2p ( ³ P ⁰ ) to the Ryudberg state. The vapor is resonantly excited to state 12d ( ^3D ), and finally the excited vapor is ionized by the electric field from the electric field generator 15. Here, as mentioned above, the electric field strength required for ionization of the Ryudberg state 12d ( ^3D ) is E>1.487×10 ⁴ V/cm. Therefore, assuming that the distance between electrode 5 and electrode 6 is d (cm), the voltage applied between the electrodes needs to be Vr>1.487×10 ⁴ dV. Further, a DC voltage is applied between the electrode 6 and the sample stage 8a, so that the ionized beryllium vapor 10 is extracted as an ion beam 9 consisting only of beryllium ions. The sample 8 is irradiated.

The energy Vi of the ion beam 9 is determined by the voltage Vr applied between the electrodes 5 and 6 and the voltage Va applied between the electrodes 6 and the sample stage 8a. For example, the synchrotron radiation irradiation area between electrode 5 and electrode 6 is
If d/2 is the middle of , and d = 1 cm, if 12d ( ³ D) is selected as the Ryudberg state, the energy of the resulting ion beam is Vi = V _r/2 + Va >
It becomes 0.744×10 ⁴ +VaV. Therefore, if no voltage is applied between the electrode 6 and the sample stage 8a (Va
= 0), it is possible to obtain an ion beam with an energy of about 7000V. Here, the energy of the ion beam becomes lower when a higher Rydberg state is selected, that is, closer to the ionization limit. for example,
In the case of 30d ( ^3D ), Vi=V _r/2 + Va>0.192×
10 ³ +VaV, and if Va=0, an ion beam with a low energy of about 200V can be obtained.

The characteristics of this embodiment in the above operation description are as follows: First of all, since this embodiment performs selective ionization completely using only resonance, the substance to be ionized in the container 1, in this case, Even if it contains impurities other than beryllium, such as oxygen, nitrogen, carbon, hydrogen, etc., and even if the amount is greater than beryllium, the wavelength of the synchrotron radiation light should not be made to match the energy level of the impurity atoms. A pure beryllium ion beam is obtained in which only the desired element, in this case beryllium, is ionized.

Second, as mentioned above, this embodiment performs selective ionization and ionization by resonant optical excitation, so there is no possibility that electrons or other elements will be excited or that the temperature will rise due to energy absorption. As a result, low-temperature processing can be performed without increasing the temperature of the target sample 8 to be irradiated with the ion beam, such as a substrate in the case of a semiconductor.

Thirdly, as described above, in this embodiment, the energy of the ion beam can be controlled by the applied voltage when the Ryudberg state is ionized by an electric field, and an ion beam with low energy can be obtained. Therefore, the surface of the target sample 8 to be irradiated with the ion beam, such as a semiconductor substrate, can be treated without damaging it. Further, such a low energy ion beam can be used as a high energy ion beam by further accelerating the beam after extraction. Therefore, the ion beam in this example ranges from low energy of 100V or less to high energy of 1MV or more.
This is an ion source that can handle ion beams with a wide energy range.

Fourth, if you want to change the type or characteristics of the ion beam, you just need to change the wavelength of the spectroscopic single-wavelength synchrotron radiation light and the applied voltage. There is no need to open and close the container, and therefore continuous operations such as ion implantation and annealing can be easily performed.

FIG. 3 shows a second embodiment of the invention. In the figure, the same reference numerals as in FIG. 2 indicate the same or equivalent parts, 13 is a container containing the substance 12 to be ionized, 13a is a heater provided on the outer periphery of the container 13, and the container 13 is thereby It serves as an oven in which the substance 12 is evaporated.
In addition, 14 is a beryllium vapor 10 and an ion beam 9
This is an extraction electrode that can be extracted as a.

Next, the operation will be explained.

When beryllium 12, which is a substance to be ionized, is placed in the oven 13 and the oven 13 is heated by the heater 13a, the beryllium 12 is melted.
Beryllium vapor 10 is generated by vaporization. Then, RF discharge is generated by the induction coil 11, and in synchronization with this, synchrotron radiation B3 of 1907 Å is generated.
is introduced into the oven 13 through the slit 1c, and a voltage is applied to the electrodes 5 and 6 to generate an electric field, which causes the vapor 10 to change from the ground state 2s ( ¹ S) to the excited state 2p ( ³ P ⁰ ) and is excited to the Ryudberg state 12d ( ³ D), and the electrons of the beryllium vapor 10 in the Ryudberg state 12d ( ³ D) become free electrons, thereby generating beryllium ions in the oven 13.
The beryllium ions are emitted as an ion beam 9 by the extraction electrode 14.

Here, as mentioned above, the Lyudberg state
The electric field strength required for ionization of 12d ( ^3D ) is E>
It becomes 1.487×10 ⁴ V/cm. Therefore, assuming that the distance between electrode 5 and electrode 6 is d (cm), the voltage applied between the electrodes needs to be Vr>1.487×10 ⁴ dV.

FIG. 4 shows an example of the configuration of a synchrotron radiation generator used in the present invention. In the figure, 21 is linearly accelerated steam, 22 is an electron storage ring, and 24 is a spectroscopic system, which constitute a synchrotron radiation light generating device 20. 33 is a container to which synchrotron radiation light from the generator 20 is irradiated.

In the synchrotron radiation introduced into the container 33, electrons are first accelerated by the linear accelerator 21 and are driven into the electron storage ring 22.
2, synchrotron radiation is emitted from the electrons that have reached relativistic speed, and the synchrotron radiation is further introduced into the spectroscopic system 24, where it is made into a single wavelength.

〔Effect of the invention〕

As described above, according to the ion beam generator of the present invention, a substance to be ionized is excited to a relatively lower excited state by gas discharge, and then changed from the excited state to the Ryudberg state by irradiation with synchrotron radiation light. By resonant optical excitation and applying a pulsed electric field synchronized with the synchrotron radiation, the substance is changed from the Ryudberg state to the ionic state, resulting in superior ion selectivity and superior ionization compared to conventional resonant optical excitation and ionization methods. ,
The ionization efficiency with respect to input optical energy can be improved by four orders of magnitude or more, and there is an effect that a compact low-energy ion beam generator can be obtained.

[Brief explanation of the drawing]

FIG. 1 is an energy phase diagram of beryllium neutral atoms, FIG. 2 is a schematic diagram of a shower type ion beam generator according to the first embodiment of the present invention, and FIG. 3 is a focusing diagram according to the second embodiment of the present invention. Figure 4 is a schematic diagram of a synchrotron radiation generator used in the present invention, and Figure 5 is a schematic diagram of a synchrotron radiation generator used in the present invention. Figure 6 is a phase diagram showing the radiative lifetime of the excited state with effective principal quantum number n* in the d state of beryllium neutral atoms, and Figure 7 is the phase diagram showing the electric field strength required for beryllium neutral atoms. ground state
Figure ⁸ is a phase diagram showing the wavelength dependence of the photoionization rate in the metastable state 2p ( ³ P ⁰ ) of beryllium neutral atoms. Figure 9 shows the Lyudberg state nd in the metastable state 2p ( ³ P ⁰ ) of beryllium neutral atoms.
( ^3D ) (n=12 to 15, 20, 30) is a phase diagram showing the wavelength dependence of field ionization rate. 1, 13, 33... Container, 3, 20... Synchrotron radiation light generating section, 11... Gas discharge generating section, 15... Electric field generating section, B3... Synchrotron radiation light generating section. Note that the same reference numerals in the figures indicate the same or equivalent parts.

Claims (1)

[Claims] 1. A container containing a substance to be ionized;
a synchrotron radiation light generation unit that irradiates the substance in the container with synchrotron radiation light; an electric field generation unit that applies an electric field to the substance in the container; and a gas discharge in the atmosphere of the substance in the container. an apparatus for generating an ion beam of the substance, wherein the gas discharge generation unit excites the substance from a ground state of energy level to a relatively lower excited state by gas discharge; The synchrotron radiation generating section generates synchrotron radiation having a wavelength that resonantly excites the substance from the relatively lower excited state to the Lyudberg state, and the electric field generating section An ion beam generator, characterized in that it generates a pulsed electric field synchronized with the synchrotron radiation light that changes a substance from a Ryudberg state to an ion state. 2. The synchrotron radiation generator generates multiple synchrotron radiation beams with different synchronized wavelengths that resonantly excite matter stepwise from a relatively lower excited state to an intermediate state to a Lyudberg state. An ion beam generator according to claim 1, characterized in that: 3. The synchrotron radiation light generating section shall have an accelerator and a spectrometer or spectroscopic system, and generate light with a narrow line spectrum obtained by passing the output from the accelerator through the spectrometer or spectroscopic system. An ion beam generator according to claim 1 or 2, characterized in that: 4. The ion beam generation device according to claim 3, characterized in that one or both of an electron storage ring and an electron cyclotron is used as the accelerator. 5 Claims 1 to 4 characterized in that the relatively lower excited state is a metastable state.
2. The ion beam generator according to any one of paragraphs. 6. The ion beam generator according to any one of claims 1 to 5, wherein the electric field of the electric field generator also serves as an extraction electric field for extracting ions as a beam. 7. The ion beam generating device according to any one of claims 1 to 6, wherein the gas discharge generating section generates radio frequency (RF) discharge. 8. A patent characterized in that the substance is introduced into a container as a gas in a compound or molecular state, and the gas discharge also serves as a gas discharge to bring the substance introduced into the container into a neutral atomic state. An ion beam generator according to any one of claims 1 to 7. 9. The ion beam generator according to any one of claims 1 to 8, wherein the gas discharge is temporally synchronized in phase with the synchrotron radiation light and the electric field. 10. The ion beam according to any one of claims 1 to 9, wherein the substance is introduced into the container as a vapor generated by heating and vaporizing a solid or liquid substance. Generator.