JPH0512813B2 - - 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, and the other is to focus laser light to release gas or liquid. The other method uses a wavelength-tunable light source, which targets the energy of the substance to be ionized with a single wavelength such as a laser beam, and uses it as an ion source. The substance is ionized by resonance with the level, and the present invention relates to the latter.

[Summary of the invention]

The present invention resonantly optically excites a substance to be ionized from its ground state to a Rydberg state by irradiating it with a laser beam, and then brings the substance to a Rydberg level close to its ionization limit by applying a pulsed electric field synchronized with the laser beam. To provide a compact low-energy ion beam generator that can improve the ionization efficiency for input light energy by more than three orders of magnitude compared to conventional resonant optical excitation and ionization methods by converting the ions into an ionized state, and has excellent selectivity in selective ionization. The purpose is to

[Embodiments of the invention]

First, the ionization method in the apparatus of the present invention will be explained using an example of generating a magnesium ion beam and comparing it with a conventional method. FIG. 1 is an energy level diagram of a neutral magnesium atom.

Conventional ionization methods, for example, have a wavelength of 2853 Å,
This is a method of irradiating the magnesium vapor to be ionized with two 5528 Å laser beams B1 and B2. In other words, magnesium atoms in the ground state 3s ( ¹ S) are first transformed into the first excited state 3p (1 S) by the 2853 Å laser beam ^B1 . P ⁰ ), and then the 5528 Å laser beam B2 excites the energy level 3d ( ¹ D).
2853 Å laser beam B1
It ionizes it.

The difference between the ionization method in the device of the present invention and the conventional ion exchange method described above is that the magnesium vapor is changed from the first excited state 3d ( ¹ P ⁰ ) to the Lyudberg state.
13d ( ¹ D), and further applies an electric field to the excited vapor to ionize it. For example, for Ryudberg state 13d ( ¹ D), the wavelength is 3859 Å.
The magnesium neutral atoms in the Ryudberg state are resonantly excited to the Ryudberg state by the laser beam B3, and in order to further ionize the excited vapor, an electric field expressed by the following formula is applied to the vapor together with the laser beam B3. It ionizes in an electric field.

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 a neutral magnesium atom, the ground state is 3s ( ¹ S), n=3, and the Ryudberg state is a highly excited state where n≧13. For the 13d ( ^1D ) level of the magnesium neutral atom, n
*=12.04, therefore, Ryudberg state
The electric field strength required for ionization of 13d ( ¹ D) is E≧
It becomes 1.321×10 ⁴ V/cm. Also, higher Rydberg states, i.e. closer to the ionization limit, e.g.
When the 30d ( ¹ D) level is selected, the electric field strength required for ionization is smaller, E≧3.858×10 ² V/cm. On the other hand, for the first excited state 3p ( ¹ P ⁰ ), n * = 2.03, and if this 3p ( ¹ p ⁰ ) level is directly ionized by an electric field, the ionization requires E≧1.840× An extremely large electric field strength of 10 ⁷ V/cm is required.

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 Lyudberg state 13d ( ¹ D) of the neutral magnesium atom, τ _r ~
It is about 2.6 μs, but for a higher Ryudberg state, that is, closer to the ionization limit, for example, the 30d ( ¹ D) level, it becomes longer, τ _r ~37 μs. On the other hand, the first
For the excited state 3p ( ¹ P ⁰ ), τ _r is about 10 ns, and the radiative lifetime is extremely short compared to the Ryudberg state.

Therefore, the ground state 3s of the neutral magnesium atom (
¹ S) is excited to the first excited state 3p ( ¹ P ⁰ ) by a laser beam with a wavelength λ ₁ = 2853 Å, and further this 3p ( ¹ P ⁰ )
The level is excited to the Ryudberg state 13d ( ¹ D) by a laser beam with wavelength λ ₂ = 3859 Å, and this 13d ( ¹ D)
In the case of an example of the ionization method in the apparatus of the present invention in which the level is ionized by an electric field E>1.321×10 ⁴ V/cm, magnesium vapor is ionized by synchronized first laser light λ ₁ and second laser light λ ₂ . After resonant excitation,
The radiative lifetime of the Lyudberg state 13d ( ¹ D) τ _r ~
It is sufficient to apply an electric field for about 2.6 μs to ionize the level, and a normal pulse voltage generator capable of generating a voltage of about >10 kV with a rise/fall time of about <1 μs can be used.

When photoionizing directly from the first excited state 3p ( ¹ P ⁰ ), which is an example of following the conventional ionization method,
Its photoionization cross section is σ _i =3.06×10 ^-28 [λ ₂ (Å)] ³ cm
²
Therefore, the ionization rate is W=1.54×10 ^-13
[λ ₂ (Å)] ⁴ I (W/cm ² ) sec ^-1 . Here, since the laser light wavelength λ ₂ exceeds the ionization limit, λ ₂ <3756Å
It must be. Figure 7 shows this ionization rate W as a function of laser light wavelength λ ₂ at various laser light intensities I.
shown against. As shown in the figure, when the wavelength λ ₂ 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,
First excited state by laser light with wavelength λ ₂ = 3859 Å
The photoexcitation cross section from 3p ( ¹ P ⁰ ) to Ryudberg state 13d ( ¹ D) is σ _ex = 9.91×10 ^-24 [λ ₂ (Å)] ³ cm ²
Therefore, the excitation speed is W=8.31×10 ^-10 [λ ₂ (Å)] ⁴ I (W/cm ² ) sec, where I is the laser light intensity.
^-1
becomes. However, the above value is the first excited state 3p (
The optical absorption spectrum from the Lyudberg state 13d ( ¹ P ⁰ ) to the Ryudberg state 13d ( ¹ d) has a Doppler broadening at room temperature, and the spectral width of the laser beam was calculated as being about 10 times the Doppler broadening. When such Ryudberg state 13d ( ¹ D) 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 = 1.321×
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 speed W is expressed as the Ryudberg state caused by the laser beam with a wavelength of 3859 Å in the first excited state 3p ( ¹ P ⁰ ).
13d ( ^1D ) can be taken as the field ionization rate via 1D. Figure 8 shows this ionization rate W in several Ryudberg states nd ( ¹ D) (n = 9 to 15, 21,
30) is shown for various laser light intensities I. When the laser wavelength λ ₂ is changed at a certain laser light intensity, a transition 3p ( ¹ P ⁰ ) from the first excited state 3p ( ¹ P 0 ) to the Lyudberg state nd ( ¹ D ⁾ occurs.
The ionization rate W exhibits a finite value only at the wavelength λ ₂ that resonates with −nd ( ¹ 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 neutral magnesium atoms excited by a 2853 Å laser beam from the ground state 3s ( ¹ S) to the first excited state 3p ( ¹ p ⁰ ) using an electric field. In this case, as mentioned above, an extremely large electric field strength of E≧1.840×10 ⁷ V/cm is required. Furthermore, since the radiative lifetime of the 3p ( ¹ p ⁰ ) level is extremely short at approximately τ _r ~10 ns, it is necessary to apply an electric field for ionization for an extremely short time of ~10 ns after irradiating the laser beam λ ₁ . There is. 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 the input light energy is more than three orders of magnitude higher than that of the conventional resonant optical excitation/ionization method, and since only resonance is used, the energy level and wavelength of the laser beam can be improved. By selecting the atoms so that they do not match those of the impurity atoms, only the substance to be ionized can be ionized, and moreover, it can be made with high purity. Furthermore, the ionization method of the present invention does not require a special expensive high-voltage short pulse generator, and is compact 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 invention. In the figure, 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, 1b is a gas discharge hole, 3a, 3b
1 is a window through which laser beams B1 and B3 from a laser beam generation section (not shown) are introduced into the container 1, and a space in the container 1 where the laser beams B1 and B3 intersect is an ion generation space 4. Note that as the laser beam generating section, a laser such as a wavelength tunable laser or a free electron laser is used.

Reference numerals 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; these are terminals for ionizing the substance in the ion generation space 4; It constitutes an electric field generating section 15 that applies an ionizing electric field. 8
is a sample, 8a is a sample stand that holds the sample 8, and a DC voltage is applied between the sample stand 8a and the electrode 6, thereby creating an extraction electric field for extracting the ionized substance as an ion beam. 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 magnesium ion beam is generated by the apparatus of this embodiment. First, magnesium vapor 10 is introduced into the container 1 through the gas introduction hole 1a. Then, the laser beam generating section oscillates, thereby emitting a laser beam B1 with a wavelength of 2853 Å to the window 3a.
A laser beam B3 of 3859 Å is also introduced into the container 1 through the window 3b, and both beams B1 and B3 intersect in the ion production space 4 in the container 1, whereby the magnesium vapor 10 is converted from the ground state 3s ( ¹ S) to the first excited state 3p ( ¹ P ⁰ ) by the 2853 Å laser beam B1.
is resonantly excited and further laser beam B of 3859 Å
3, the first excited state 3p ( ¹ P ⁰ ) is resonantly excited stepwise from the Ryudberg state 13d ( ¹ D).

Also, in time synchronization with the laser oscillation, the electrode 5
A voltage is applied to each of the electrodes 6 and 6 from the terminals 5a and 6a, thereby achieving the Lyudberg state 13d (
An electric field is applied to the magnesium vapor 10 at ¹ D), so that the magnesium vapor 10 is ionized. Here, as mentioned above, the electric field strength required for ionization of Ryudberg state 13d ( ¹ D) is E>
It becomes 1.321×10 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>1.321×10 ⁴ dV. Further, a DC voltage is applied between the electrode 6 and the sample stage 8a, whereby the ionized magnesium vapor 10 is extracted as an ion beam 9 consisting only of magnesium ions. The sample 8 is irradiated. The energy Vi of the ion beam 9 is
and the voltage Vx applied between the electrode 6 and the electrode 6
It is determined by the voltage Va applied between and the sample stage 8a. For example, the laser irradiation area between electrodes 5 and 6 may be
2. Assuming d=1cm, if 13d ( ¹ D) is chosen as the Ryudberg state, the energy of the ion beam obtained is Vi=Vx/2+Va>0.661×10 ⁴
+Va∨. Therefore, if no voltage is applied between the electrode 6 and the sample stage 8a (Va=0), an ion beam with an energy of less than 7000V can be obtained. Here, the energy of the ion beam becomes lower when choosing a higher Ryudberg state, ie closer to the ionization limit. For example, 30d (
¹ D), then Vi=V _x /2+Va>0.193×10 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, 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 magnesium, such as oxygen, nitrogen, carbon, hydrogen, etc., and the amount is larger than magnesium, the energy level of the laser beam,
A pure magnesium ion beam in which only the desired element, in this case magnesium, is ionized can be obtained by making the wavelength not coincident with those of the above-mentioned impurities.

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 only need to change the laser beam height and applied voltage, which eliminates the 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 those in FIG. 2 indicate the same or corresponding parts, 13 is an opening that accommodates the substance 12 to be ionized, 13a is a heater provided on the outer periphery of the oven 13, and 11 is ionized magnesium vapor 10. A magnet 14 focuses the magnesium vapor 10 on the axis of the container 1, and an electrode 14 extracts the focused magnesium vapor 10 as an ion beam 9.

Next, the operation will be explained.

Magnesium 12, which is a substance to be ionized, is placed in the oven 13, and when the oven 13 is heated by the heater 13a, the magnesium 12 is melted and vaporized to generate magnesium vapor 10, which is passed through the gas introduction hole 1a into the container. 1. And laser beam B1 of 2853Å
A laser beam B3 of 3859 Å is introduced into the container 1 through the windows 3a and 3b, respectively, and irradiates the vapor 10, and at the same time, a voltage is applied to the electrodes 5 and 6 to apply an electric field to the vapor 10. . As a result, the vapor 10 changes from the ground state 3s ( ¹ S) to the first excited state 3p ( ¹ P ⁰ ) to the Lyudberg state 13d.
( ¹ D), and furthermore, due to the external electric field, the electrons of the magnesium vapor 10 in the Ryudberg state 13d ( ¹ D) become free electrons, which generates magnesium ions in the ion generation space 4, and the magnesium ions is magnet 11
After being focused on the axis by the ion beam 9, the ion beam is emitted as an ion beam 9 by the extraction electrode 14. Here, Ryudberg state 13d ( ¹ D) as mentioned above
The electric field strength required for ionization is E>1.321×
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>1.321×10 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, the laser beam and electric field for ion generation need to be synchronized in time, and in order to excite the elements in a stepwise manner, a plurality of laser beams each having a specific frequency are required. It is of course necessary to synchronize the laser beams, and FIG. 4 shows an example of a synchronization method.

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. By simultaneously oscillating the excitation laser 21 and applying voltage to the electrodes 5 and 6, the laser beam and the electric field can be synchronized in time.

[Effects of the Invention] As described above, according to the ion beam generator of the present invention, a substance to be ionized is resonantly excited from its ground state to a Lyudberg state by irradiation with a laser beam, and the substance is further irradiated with the laser beam. By applying a pulsed electric field synchronized with the Rydberg state to the ionic state, the ionization has excellent ion selectivity and can improve the ionization efficiency for input light energy by more than three orders of magnitude compared to the conventional resonant optical excitation/ionization method. Moreover, 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 a singlet system of magnesium neutral atoms, FIG. 2 is a schematic diagram of the shower type ion beam generator according to the first embodiment of the present invention, and FIG. Schematic configuration diagram of the focused ion beam generator according to the embodiment, No. 4
The figure is a block diagram of the laser beam oscillation part of the ion beam generator according to the third embodiment of the present invention, and FIG. Figure 6 is a phase diagram showing the electric field strength. 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 a neutral magnesium atom. Figure 7 is the first excited state of a neutral magnesium atom. ^A phase diagram showing the wavelength dependence of the photoionization rate in 3p ( ¹ P ⁰ ), Figure 8 shows the Ryudberg state nd ( ^{1 D} ) (n = 9 15, 21, 30) is a phase diagram showing the wavelength dependence of field ionization rate. DESCRIPTION OF SYMBOLS 1... Container, 15... Electric field generation part, 20... Laser beam generation part, B1-B3... 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;
An apparatus for generating an ion beam of the substance, comprising: a laser beam generator that irradiates the substance in the container with a laser beam; and an electric field generator that applies an electric field to the substance in the container; The generation section generates a laser beam having a wavelength that resonantly excites the substance from the ground state of energy level to the Ryudberg state, and the electric field generation section generates a laser beam that excites the substance from the Ryudberg state to the ion state. An ion beam generator characterized by generating a pulsed electric field synchronized with a beam. 2. The laser beam generator generates a plurality of synchronized laser beams with different wavelengths that resonantly excite a substance stepwise from a ground state to an intermediate state to a Lyudberg state. The ion beam generator according to scope 1. 3. The ion beam generator according to claim 1 or 2, characterized in that a wavelength tunable laser is used as the laser beam generator. 4. The ion beam generator according to claim 1 or 2, characterized in that a free electron laser is used as the laser beam generator. 5. Claim 2, characterized in that the laser beam generating section comprises one excitation source laser and a plurality of mutually time-synchronized dye lasers that are excited by laser beam electrons from the excitation source laser. The ion beam generator described. 6. The ion beam generator according to any one of claims 1 to 5, wherein the substance is introduced into the container as a vapor generated by heating and vaporizing a solid or liquid substance. . 7. The ion beam generator according to any one of claims 1 to 6, wherein the electric field also serves as an extraction electric field for extracting the ionized substance as an ion beam.