JPH0512814B2 - - 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 laser light and use the resulting plasma as an ion source.
One is to focus a laser beam and irradiate it onto a gas or liquid to create a plasma, which is then used as an ion source. The present invention relates to the latter method, in which the target substance is ionized by being resonated with the energy level of the substance to be ionized.

[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 the Ryudberg state by irradiation with a laser beam, and further applies a pulsed electric field in synchronization with the laser beam. Rydberg state (Rydberg state) close to the ionization limit of the substance
By converting the ionized state from the level) to the ionized state, the ionization efficiency for the input light energy can be improved by more than three orders of magnitude compared to conventional resonant optical excitation and ionization methods, and a compact low-energy ion beam generator with excellent selectivity in selective ionization has been developed. is intended to provide.

[Embodiments of the invention]

First, the ionization method in the apparatus of the present invention will be explained by comparing it with a conventional method using an example in which a beryllium ion beam is generated. 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 4573 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 laser beam B1, then resonantly excited to energy level 3d ( ¹ D) by laser beam B2 of 4573 Å, and further ionized by laser beam B2 of 4573 Å. be.

The ionization method used in the device of the present invention differs from the conventional ionization method described above in that only resonance excitation is used, and beryllium vapor is resonantly excited to a state in which the metastable state 2p ( ³ P ⁰ ) is dominant by gas discharge. The laser beam, in the example shown in Figure 1, has a wavelength of 3321 Å,
Three laser beams B3 to B5 of 1.46μ, 6449Å
The above excited vapor is reduced to this metastable state 2p ( ³ P ⁰ ) by
from the second excited state 3s ( ^3S ), the third excited state 3p
Ryudberg state 12d ( ^3D ) via ^{( 3P0} )
Instead of resonantly exciting the beryllium vapor into a free electronic state, that is, an ionic state, using a laser beam, B2 in the example of FIG. 1, as in the conventional ionization method, state to ionic state.

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. For beryllium neutral atoms, the ground state is
2s ( ¹ S), n=2, and the Ryudberg state indicates a highly excited state where n≧12. For the 12d ( ¹ D) level of beryllium neutral atom, n*=
12.04 and therefore Ryudberg state 12d (
¹ The electric field strength required for ionization of D) is E≧1.487×
10 ⁴ V/cm. Also, higher Rydberg states, i.e. closer to the ionization limit, e.g. 30d (
³ D) If the level is selected, the electric field strength required for ionization will be 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 this 3p ( ¹ P ⁰ ) level is directly ionized by an electric field, an extremely large electric field strength of E≧2.786×10 ⁷ V/cm is required for ionization.

By the way, the radiative lifetime of the Ryudberg state with effective principal quantum number n* is τ _r ∝n* ³ , and under gas pressure so low that collisions can be ignored,
The higher the excited state is, the larger n* is, the longer the radiation lifetime τ _r becomes. Figure 6 shows the radiative lifetime τ _r (ns) = 1.38×(n*−
0.0144) shows the relationship of ³ . For the Ryudberg state 12d ( ^1D ) of beryllium neutral atoms, τ _r ~2.4 μs
However, for higher Ryudberg states, that is, closer to the ionization limit, for example, the 30d ( ^1D ) level, it becomes longer, τ _r ~37 μs. On the other hand, the first excited state 2p ( ¹ P ⁰ ) has a radiation lifetime of approximately τ _r ~8 ns, which is extremely short compared to the Ryudberg state.

Therefore, the ground state 2s of the beryllium neutral atom (
¹ S) is excited to the metastable state 2p ( ³ P ⁰ ) by gas discharge, and this 2p ( ³ P ⁰ ) level is further converted to the wavelength λ ₁ = 3321
The Ryudberg state 12d (3D) is excited by three laser beams ^of Å, λ ₂ = 1.46 μ, and λ ₃ = 6449 Å, and this 12d ( ³ D) level is controlled by an electric field E>1.487×
In the case of an example of the ionization method in the device of the present invention in which ionization is performed by 10 ⁴ V/cm, the discharged beryllium vapor is synchronized with a first laser beam λ ₁ , a second laser beam λ ₂ , and a third laser beam. After resonant excitation by λ ₃ , the radiative lifetime of the Ryudberg state 12d ( ^3D ) τ _r
It is sufficient to apply an electric field for about ~2.4 μs to ionize the level, and a normal pulse voltage generator that can generate a voltage of about >10 kV with a rise/fall time of about <1 μs can be used. .

When photoionizing directly from the third excited state 3p ( ^3P0 ), which is an example of following ^the conventional ionization method,
Its photoionization cross section is σ _i =8.97×10 ^-29 [λ ₃ (Å)] ³ cm
²
Therefore, the ionization rate is W=4.51×10 ^-14
[λ ₃ (Å)] ⁴ I (W/cm ² ) sec ^-1 . Here, since the laser light wavelength λ ₃ exceeds the ionization limit, λ ₂ <6141Å
It must be. Figure 7 shows this ionization rate W as a function of laser light wavelength λ ₃ at various laser light intensities I.
shown against. As shown in the figure, when the wavelength λ ₃ is changed at a certain laser light intensity, the ionization rate W changes continuously.

On the other hand, in the case of the ionization method in this invention,
Third excited state by laser light with wavelength λ ₃ = 6449 Å
The photoexcitation cross section from 3p ( ^3P0 ) to Ryudberg state 12d ( ^3D ) is σ _ex = 1.28×10 ^{-23 [} _λ3 (Å)] ^{3 cm2}
Therefore, the excitation speed is W=6.47×10 ^-10 [λ ₃ (Å)] ⁴ I (W/cm ² ) sec, where I is the laser light intensity.
^-1
becomes. However, the above value is the third excited state 3p (
The optical absorption spectrum from ³ P ⁰ ) to the Ryudberg state 12d ( ³ D) has a Doppler broadening at room temperature, and the spectral width of the laser beam was calculated to be about 10 times the Doppler broadening. 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 ^x-4 = 1.487×
When the voltage is about 10% higher than 10 ⁴ V/cm, it separates into ions and electrons at a high speed of 10 ¹² sec ^-1 or more. Therefore, this excitation rate W can be expressed as the Ryudberg state caused by the laser beam with a wavelength of 6449 Å in the third excited state 3p ( ³ P ⁰ ).
The field ionization rate can be via 12d ( ^3D ). Figure 8 shows this ionization rate W in several Ryudberg states nd ( ³ D) (n = 8 to 13, 20,
30) is shown for various laser light intensities I. When the laser wavelength λ ₃ is changed at a certain laser ^light intensity, a transition 3p ( ³ P ⁰ ) from the third excited state 3p ( 3 P 0 ) to the Ryudberg state nd ( ³ D ⁾ occurs.
The ionization rate W shows a finite value only at the wavelength λ ₃ that resonates with −nd( ³ D), and W=0 at other wavelengths. Comparing Figure 7 and Figure 8,
It can be seen that the ionization rate W of ionization via the Ryudberg state is more than three orders of magnitude larger than that of conventional photoionization.

By the way, in the conventional ionization method, the wavelength λ ₁ =
Another possible ionization method is to directly ionize beryllium neutral atoms excited by a 2349 Å laser beam from the ground state 2s ( ¹ S) to the first excited state 2p ( ¹ p ⁰ ) using an electric field. In this case, E
An extremely large electric field strength of ≧2.786×10 ⁷ V/cm is required. Also, the radiative lifetime of the 2p ( ¹ p ⁰ ) level is τ _r
Since it is extremely short at about ~8 ns, it is necessary to apply an electric field for ionization for an extremely short time of ~8 ns after irradiating the laser beam λ ₁ . Therefore, this ionization method requires a special high-voltage short pulse generator that can generate ultra-high voltages of >10 MV in an ultra-short time of ~10 ns, but such a device is It is much larger in scale and more expensive than a laser device. 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 input light energy is more than three orders of magnitude higher than that of the conventional resonant optical excitation/ionization method, and since excitation is from a metastable state rather than a ground state, photons are The wavelength of is short,
Moreover, since only resonance is used, if the energy level and wavelength of the laser beam are selected so that they do not match those of the impurity atoms, only the substance that is desired to be ionized can be ionized, and it is possible to achieve high purity. Furthermore, the ionization method of the present invention does not require a special and expensive high-voltage short pulse generator, and is more compact than the conventional method, even when compared to the method of ionizing an excited state far lower than the Rydberg state by field ionization. There are advantages that can be achieved with an inexpensive voltage generator.

Furthermore, by changing the wavelength and electric field strength of the laser beam, ion beams of other types of substances can be easily generated.In this case, various types of substances to be ionized can be introduced into the ion beam generation container in advance. You can leave it there. 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 is gas exhaust hole, 3a or 3
b is a window through which laser beams B3, B5, or B4 from a laser beam generator (not shown) is introduced into the container 1, and the space in the container 1 where the laser beams B3 to B5 intersect is the ion generation space 4. It's summery. The laser beam generating section consists of a flash lamp for an excitation source and three lasers that are excited by the flash lamp and generate the laser beams B3 to B5. A solid laser such as an electron laser or an alexandrite laser, or a gas laser such as an excimer or nitrogen laser is used.

5 and 6 are electrodes arranged with the ion generation space 4 in between; 5a and 6a are terminals for applying voltage to the electrodes 5 and 6; 15. Reference numeral 16 designates an induction coil that is also placed across the ion generation space 4, and serves as an RF discharge generating section that generates radio frequency (RF) discharge in the ion generation space 4. 8 is a sample, 8a is a sample stand that holds the sample 8,
A DC voltage is applied between the sample stage 8a and the electrode 6, thereby generating an extraction electric field for extracting the ionized substance as an ion beam.

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 16,
At the same time, the laser beam generator oscillates,
Further, a voltage is applied to each of the electrodes 5 and 6 from terminals 5a and 6a in temporal synchronization with the laser oscillation. Then, this causes the above ground state 2s ( ¹ S)
Electrons from the RF discharge collide with beryllium vapor 10 located at , and as a result, beryllium vapor 10 is excited to a metastable state 2p ( ³ P ⁰ ). Also, due to the oscillation of the above laser, laser beams B of 3321 Å and 6449 Å are generated.
3, B5 is introduced into the container 1 through the window 3a,
Also, a 1.46 μm laser beam B5 is similarly introduced through the window 3b, and each of the beams B3 to B5 is attached to the container 1.
The beryllium vapor 10 is changed from the metastable state 2p ( ³ P ⁰ ) to the excited state by the 3221 Å laser beam B3.
3s ( ³ S), and further excited from the excited state 3s ( ³ S) to the excited state 3p ( ³ P ⁰ ) by the 1.46 μm laser beam B4, and then to the Ryudberg state 12d (
^3D ), and finally the excited vapor is ionized by the pulsed 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, if the distance between electrode 5 and electrode 6 is d (cm), the voltage applied between the electrodes needs to be Vx>1.487×10 ⁴ dV. Further, a DC voltage is applied between the electrode 6 and the sample stage 8a, whereby 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 Vx applied between the electrodes 5 and 6 and the voltage Va applied between the electrodes 6 and the sample stage 8a. For example, if the laser irradiation area between electrodes 5 and 6 is d/2 midway between electrodes 5 and 6, and d = 1 cm, if 12d ( ^3D ) is selected as the Ryudberg state, the resulting ion beam will be Energy is Vi=Vx/2+Va>0.744
×10 ⁴ +Va. Therefore, the electrode 6 and the sample stage 8
Assuming that no voltage is applied between a and a (Va=
0), it is possible to obtain an ion beam with an energy of less than 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, 30d
( ^3D ), Vi=V _x /2+Va>0.192×
10 ³ +Va, 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, this embodiment performs selective ionization using only resonance, so the substance to be ionized, in this case beryllium, is in the container 1. , even if it contains impurities other than oxygen, nitrogen, carbon, hydrogen, etc., and the amount thereof is greater than beryllium, it is desired that the energy level and wavelength of the laser beam do not match those of the above impurities, etc. A pure beryllium ion beam is obtained in which only the 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 ionizing the Ryudberg state 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. Moreover, 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 is
This is an ion source that can handle ion beams with a wide energy range, from low energy below 100V to high energy above 1MV.

Fourth, if you want to change the type or characteristics of the ion beam, you can simply change the wavelength of the laser beam and the applied voltage, and there is no need to open and close the container to take out the sample or replace the ion source, as in the past. 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 an oven containing the substance 12 to be ionized, 13a is a heater provided on the outer periphery of the oven 13, and 14 is ionized beryllium vapor 10. This is an extraction electrode that extracts the ion beam as an ion beam 9.

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 vapor beryllium 12 is melted.
The vaporization generates beryllium vapor 10, and the vapor 1
0 is introduced into the container 1 through the gas introduction hole 1a. Then, RF discharge is generated by the induction coil 16, and at the same time laser beams B of 3221 Å and 6449 Å are generated.
3 and B5 are introduced into the container 1 through the window 3a, and a 1.46μ laser beam B4 is introduced into the container 1 through the window 3b, and a voltage is applied to the electrodes 5 and 6 to generate a pulsed electric field. Steam 10 is the ground state
2s ( ^1S ) to excited states 2p ( ^{3P0 )} , 3s ( ^3S ), 3p(
³ P ⁰ ), it is excited in a stepwise manner to the Ryudberg state 12d ( ³ D), and then the Ryudberg state
The electrons in the beryllium vapor 10 located at 12d ( ³ D) become free electrons, thereby generating beryllium ions in the ion generation space 4, and the beryllium ions are emitted as an ion beam 9 by the extraction electrode 14. 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, electrode 5
If the distance between the electrode 6 and the electrode 6 is d (cm), the voltage applied between the electrodes needs to be Vx>1.487×10 ⁴ dV.

FIG. 4 shows an example of the configuration of a laser oscillation device in an ion beam generator according to a third embodiment of the present invention.

In the present invention, in order to excite elements stepwise, a plurality of laser beams each having a specific frequency are required, and the plurality of laser beams must be synchronized. Fig. 4 shows the synchronization method. This is an example.

This laser oscillation device 20 includes three wavelength-tunable dye lasers 22, 23, and 24, and each dye laser 2.
an excitation source laser 21 for exciting 2 to 24;
It is composed of half mirrors 25 and 26 and a total reflection mirror 27. By configuring the excitation source with one laser 1 in this manner, the oscillated laser beams ω1, ω2, and ω3 are temporally synchronized.

Furthermore, by simultaneously oscillating the excitation laser 21 and applying voltage to the electrodes 5 and 6 and the induction coil 16, the laser beam, electric field, and RF
This allows the discharge to be temporally synchronized.

〔Effect of the invention〕

As described above, according to the ion beam generator of the present invention, the substance to be ionized is excited to a relatively lower excited state by gas discharge, and the substance to be ionized is resonantly optically excited from the excited state to the Lyudberg state by laser beam irradiation. Furthermore, by applying a pulsed electric field synchronized with the laser beam, the substance is changed from the Ryudberg state to the ionic state, resulting in superior ion selectivity and lower input light energy than the conventional resonant optical excitation/ionization method. This has the effect of improving the ionization efficiency by three orders of magnitude or more, and providing a compact, low-energy ion beam generator.

[Brief explanation of drawings]

FIG. 1 is an energy phase diagram of a singlet system of beryllium neutral atoms, FIG. 2 is a schematic configuration diagram of a shower type ion beam generator according to the first embodiment of the present invention, and FIG. FIG. 4 is a schematic configuration diagram of a focused ion beam generator according to an embodiment of the present invention; FIG. 4 is a block diagram of a laser beam generating section of an ion beam generator according to a third embodiment of the present invention; FIG.
The figure shows the electric field strength necessary to ionize atoms in the excited state with effective principal quantum number n*. Figure 6 shows the electric field strength necessary to ionize atoms in the excited state with effective principal quantum number n*. Figure 6 shows the effective principal quantum number n* in the d state of beryllium neutral atoms. Figure 7 is a phase diagram showing the radiative lifetime of the excited state. Figure 7 is a phase diagram showing the wavelength dependence of the photoionization rate in the third excited state 3p ( ^{3P 0} ) of beryllium neutral atoms. Figure 8 is the phase diagram showing the wavelength dependence of the photoionization rate in the third excited state 3p (3 P 0 ) of beryllium neutral atoms. It is a phase diagram showing the wavelength dependence of the field ionization rate via the Ryudberg state nd ( ^3D ) (n=8 to 13, 20, 30) in the third excited state 3p ( ^{3P0 )} of . 1, 13... Container, 15... Electric field generating section, 16
...Gas discharge generating section, 20...Laser beam generating section, B1 to B5...Laser beam. 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 laser beam generator that irradiates the substance in the container with a laser beam; an electric field generator that applies an electric field to the substance in the container; and a gas discharge that generates a gas discharge in the atmosphere of the substance in the container. and a generator for generating an ion beam of the substance, wherein the gas discharge generating unit excites the substance from a ground state of energy level to a relatively lower excited state by gas discharge; The laser beam generating unit generates a laser beam having a wavelength that resonantly excites the substance from the relatively lower excited state to the Ryudberg state, and the electric field generating unit changes the substance from the Ryudberg state to the ionic state. An ion beam generator, characterized in that it generates a pulsed electric field synchronized with the laser beam. 2. The laser beam generation unit is characterized in that it generates a plurality of synchronized laser beams with different wavelengths that resonantly excite the substance in a stepwise manner from a relatively lower excited state to an intermediate state to a Lyudberg state. An ion beam generator according to claim 1. 3. Claim 1, characterized in that either or both of an excimer, a gas laser such as nitrogen, or a dye laser excited by harmonics from the gas laser is used as the laser beam generating section. The ion beam generator according to item 1 or 2. 4. The ion beam generator according to claim 1 or 2, characterized in that a wavelength tunable laser is used as the laser beam generator. 5. The ion beam generator according to claim 1 or 2, characterized in that a dye laser is used as the laser beam generator. 6. Claim 1, characterized in that either or both of a solid-state laser such as an alexandrite laser or a dye laser excited by harmonics from the solid-state laser is used as the laser beam generating section. Or the ion beam generator according to item 2. 7. The ion beam generator according to claim 1 or 2, characterized in that a free electron laser is used as the laser beam generator. 8. The laser beam generating section includes one excitation source laser and a plurality of mutually time-synchronized dye lasers excited by the excitation source, according to claim 1 or 2. ion beam generator. 9 Claims 1 to 8, characterized in that the relatively lower excited state is a metastable state.
2. The ion beam generator according to any one of paragraphs. 10. The ion beam generator according to any one of claims 1 to 9, wherein the electric field of the electric field generator also serves as an extraction electric field for extracting ions as a beam. 11. The ion beam generator according to any one of claims 1 to 10, wherein the gas discharge generating section generates radio frequency (RF) discharge. 12. A patent characterized in that the substance is introduced into the container as a compound or molecular gas, 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 11. 13. The ion beam generator according to any one of claims 1 to 12, wherein the gas discharge is temporally synchronized in phase with each laser beam and the electric field. 14. The ion beam generator according to any one of claims 1 to 13, wherein the substance is introduced into the container as a vapor generated by heating and vaporizing a solid or liquid substance. .