JPH0527211B2 - - 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 involves resonant optical excitation of a substance to be ionized from its ground state to the Rydberg state by irradiation with a laser beam, and ionization from the Rydberg level near the ionization limit of the substance by pulsed gas discharge synchronized with the laser beam. Conventional resonant optical excitation, by making the state
It is an object of the present invention to provide a compact low-energy ion beam generator that can improve the ionization efficiency for input optical energy by three orders of magnitude or more compared to the ionization method, and has excellent selectivity in selective ionization.

[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 ) are first ionized by the 2853 Å laser beam B1 into the first excited state 3p ( ¹ P). ⁰ ), then resonant excitation to energy level 3d ( ¹ D) with a 5528 Å laser beam B2, and further ionization with a 2853 Å laser beam B1.

The difference between the ionization method in the device of the present invention and the conventional ionization method described above is that the magnesium vapor changes from the first excited state 3p ( ¹ P ⁰ ) to the Lyudberg state 13d.
( ^1D ), and the excited vapor is further ionized by electric discharge. For example, to create the Ryudberg state 13d ( ¹ D), the Ryudberg state is resonantly excited by a laser beam B3 with a wavelength of 3859 Å, and to further ionize the excited vapor, a discharge is applied to the vapor together with the laser beam B3. Magnesium neutral atoms in this Ryudberg state are ionized by electron collisions caused by discharge.

Here, 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,
The Ryudberg state indicates a highly excited state where n≧13.

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 5 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, n*=
12.04, which is about τ _r ~2.6 μs, but for a higher Rydberg state, that is, closer to the ionization limit, such as the 30d ( ¹ D) level, it becomes longer, τ _r ~37 μs. On the other hand, for the first excited state 3p ( ¹ P ⁰ ), n
*=2.03, τ _r ~10 ns, and the radiative lifetime is extremely short compared to the Ryudberg state.

Therefore, the ground state 3s of the magnesium neutral atom
( ¹ S) with a laser beam of wavelength λ ₁ = 2853 Å.
is excited to the excited state 3p ( ¹ P ⁰ ), 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 device of the present invention in which levels are ionized by pulsed gas discharge, magnesium vapor is resonantly excited by synchronized first laser light λ ₁ and second laser light λ ₂ , and then the Ryudberg It is sufficient to ionize the level by applying a discharge electric field during the radiation lifetime of state 13d ( ¹ D) τ _r ~2.6 μs, and apply a voltage of about >100 V with a rise/fall time of <
A normal pulse discharge power source that can generate the discharge in 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, the 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, it is necessary that λ ₂ <3756 Å. FIG. 6 shows this ionization rate W as a function of the laser light wavelength λ ₂ for various laser light intensities I. 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 ^-2 [λ ₂ (Å)] ³ cm ²
Therefore, the excitation speed is W=8.31×10 ^-10 [λ ₂ (Å)] ⁴ I (W/cm ² ), where the laser light intensity is I.
sec ^-1 , however, the above value is the first excited state
The optical absorption spectrum from 3p ( ¹ P ⁰ ) to 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 electron collision due to discharge, 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 ).
The ionization rate due to discharge via 13d ( ¹ D) can be taken as the ionization rate. Figure 7 shows this ionization rate W in several Ryudberg states nd ( ¹ D) (n = 13 to 15,
21, 30) are shown for various laser light intensities I. When the laser wavelength λ ₂ is changed at a certain laser light intensity, a transition from the first excited state 3p ( ¹ P ⁰ ) to the Ryudberg state nd ( ¹ D) 3p ( ¹ P ⁰ ) −
The ionization rate W shows a finite value only at the wavelength λ ₂ that resonates with nd ( ¹ D), and at other wavelengths W
=0. Comparing FIG. 6 and FIG. 7, 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 magnesium neutral atoms excited by a 2853 Å laser beam from the ground state 3s ( ¹ S) to the first excited state 3p ( ¹ P ⁰ ) by discharge. In this case, 3p as above
Since the radiative lifetime of the ( ¹ P ⁰ ) level is extremely short, τ _r ~10 ns, it is necessary to apply a discharge electric field for ionization for an extremely short time of ~10 ns after irradiation with the laser beam λ ₁ . . Therefore, this ionization method requires >
A special short-pulse discharge power supply that can generate a voltage of about 100V in an extremely short period of ~10ns is required, but such equipment is large-scale and expensive. In comparison, ionization by Ryudberg state discharge according to the ionization method of the apparatus of the present invention can be achieved with an extremely compact and inexpensive ordinary pulse discharge power source.

Therefore, in the case of the ionization method according to the present invention,
Compared to the conventional resonant optical excitation/ionization method, the ionization efficiency for the input light energy is more than three orders of magnitude higher, and since only resonance is used, the energy level and wavelength of the laser beam are selected so that they do not match those of the impurity atoms. By doing this, you can ionize only the substance you want to ionize, and you can also achieve high purity. Furthermore, in the case of the ionization method of the present invention, even compared to the method of ionizing an excited state far lower than the Ryudberg state by discharge, the ionization method of the present invention does not require the use of a special and expensive short pulse discharge power source, and can be used in a conventional compact and inexpensive manner. There are advantages that can be achieved with pulsed discharge power supplies.

In addition, by changing the wavelength of the laser beam, ion beams of various kinds of substances can be easily generated, and in this case, various kinds of substances to be ionized can be introduced into the ion beam generation container in advance. good. 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; The gas discharge generator 15 constitutes a discharge generator 15, and the gas discharge generator 15 has a high frequency power source that resonates enough to change the substance from its Rydberg state to an ion state. Note that when the substance to be ionized is introduced into the container 1 as a gas in a compound or molecular state, the gas discharge for the ionization also serves as a gas discharge for converting the substance into a neutral element 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.

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 resonantly excited from the ground state 3s ( ¹ S) to the first excited state 3p ( ¹ P ⁰ ) by the 2853 Å laser beam B1, and further excited by the 3859 Å laser beam B3.
As a result, the first excited state 3p ( ¹ P ⁰ ) is resonantly excited stepwise to the Ryudberg state 13d ( ¹ D).

Also, in time synchronization with the laser oscillation, the electrode 5
A voltage of about >100 V is applied to each of the electrodes 6 and 6 from the terminals 5a and 6a, and as a result, the magnesium vapor 1 in the Lyudberg state 13d ( ¹ D)
Electrons from the gas discharge collide with the magnesium vapor 10, and as a result, the magnesium vapor 10 is ionized. 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 V _i of the ion beam 9 is determined by the voltage _{V a} applied between the electrode 6 and the sample 8a. For example, V _a =
When the voltage is 100V, an ion beam with a low energy of about V _i =V _a =100V 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, in this embodiment, as described above, the energy of the ion beam can be controlled by the applied voltage when the Ryudberg state is ionized by discharge and the ions are extracted, 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 embodiment is an ion source that can handle ion beams in a wide energy range from low energy of 100 V or less to high energy of 1 MV or more.

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 corresponding 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, 11 is ionized magnesium vapor 10 14 is an extraction electrode that 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. At the same time, a voltage of about >100 V is applied to the electrodes 5 and 6 to cause the vapor 10 to become a gas. Electrons from the discharge collide. As a result, the vapor 10 is excited in a stepwise manner from the ground state 3s ( ¹ S) to the Ryudberg state 13d ( ¹ D) via the first excited state 3p ( ¹ P ⁰ ), and further changes to the Ryudberg state due to the electron collision caused by the gas discharge. The electrons in the magnesium vapor 10 at 13d ( ^1D ) become free electrons, which generates magnesium ions in the ion generation space 4. After the magnesium ions are focused to the axis by the magnet 11, they are focused by the extraction electrode 14. The ions are then emitted as an ion beam 9.

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 gas discharge 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.
Of course, it is necessary to synchronize the plurality of 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. and oscillation of the excitation source laser 21,
By simultaneously applying voltage to the electrodes 5 and 6, the laser beam and the gas discharge are 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 resonantly excited from its ground state to the Lyudberg state by irradiation with a laser beam, and the substance is further excited by pulsed gas discharge synchronized with the laser beam. Since the Ryudberg state is changed from the Ryudberg state to the ion state, it has excellent ion selectivity and can improve the ionization efficiency for the input light energy by more than three orders of magnitude compared to the conventional resonant optical excitation/ionization method. This has the effect of providing an ion beam generator.

[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 wavelength dependence of the photoionization rate in the first excited state 3p ( ¹ P ⁰ ) of the neutral magnesium atom, and Figure 7 is the phase diagram showing the wavelength dependence of the photoionization rate in the first excited state 3p (1 P 0 ) of the neutral magnesium atom.
Rydberg state in excited state 3p ( ¹ P ⁰ )
FIG. 2 is a phase diagram showing the wavelength dependence of the ionization rate due to discharge via nd( ¹ D) (n=13 to 15, 21, 30). DESCRIPTION OF SYMBOLS 1... Container, 15... Gas discharge generating part, 20...
...Laser beam generating section, B1 to 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;
A laser beam generating section that irradiates the substance in the container with a laser beam, and a gas discharge generating section that generates a gas discharge that intersects with the laser beam in the atmosphere of the substance in the container, the ion beam of the substance In the apparatus for generating a gas discharge, the laser beam generation section generates a laser beam having a wavelength that resonantly excites the substance from the ground energy level to the Ryudberg state, and the gas discharge generation section An ion beam generator, characterized in that it generates a pulsed gas discharge synchronized with the laser beam to change the ion beam from the Ryudberg state to the ion state. 2. The laser beam generating section is characterized in that it generates a plurality of synchronized laser beams with different wavelengths that resonantly excite the substance stepwise from the ground state to the Lyudberg state via the intermediate state. An ion beam generator according to claim 1. 3. The ion beam generator according to claim 1 or 2, wherein a wavelength tunable laser is used as the laser beam generator. 4. The ion beam generating device according to claim 1 or 2, wherein a free electron laser is used as the laser beam generating section. 5. Claim 2, wherein the laser beam generating section comprises one excitation source laser and a plurality of mutually time-synchronized dye lasers excited by the laser beam from the excitation source laser. The ion beam generator described in Section 1. 6. The ion beam generator according to any one of claims 1 to 5, wherein the gas discharge generating section generates a high-frequency discharge. 7. The substance is introduced into the 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 element state. An ion beam generator according to any one of claims 1 to 6.