JPH0746588B2 - Microwave ion source - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to, for example, an ion implanter used for doping impurities into semiconductor layers, material synthesis, surface modification, or new material development, and ion beam irradiation processing. The present invention relates to a microwave ion source used for a device or the like.

[Conventional technology]

Ion implanters are indispensable for semiconductor manufacturing process,
Various practical devices have been developed according to the required impurity dose amount. However, since the conventional impurity dose amount was 10 ¹⁶ ions / cm ² even in the case of a high concentration, the ion current of a high current ion implanter is 1 to
It was 10mA. On the other hand, for example, inside the silicon substrate
SIMOX (Separation by IMplanted OXyg)
en) Like the substrate formation technology and the technology that changes the surface physical properties by modifying the surface of the metal (nitriding, oxidation, etc.), the dose is 10
^As semiconductor manufacturing technology that requires ion implantation of ¹⁸ ions / cm ² or more is advancing, development of a high current ion implanter of 50 to 100 mA is earnestly desired.

In order to develop this kind of device, the total ion current is 10 ~
An ion source of 200 mA or more and having a long life with respect to active gas such as oxygen is indispensable. The microwave ion source is an electrodeless discharge as long as it can realize a high density and high current of the extracted ion current, and is therefore considered to be the most suitable for this kind of application. It is considered to be optimal for this kind of application if it is possible to realize high current / high current of the extraction ion current.

FIG. 12 shows the basic structure of a conventional microwave ion source (Japanese Patent Laid-Open No. 60-264032). In the figure, 1 is a plasma generation chamber, 2 is a microwave introduction window, 3 is a waveguide, 4 is a magnetic coil, 5 is a gas introduction port, 6 is an ion extraction electrode system, and 7 is
Is the extracted ion beam. Plasma generation chamber 1
Gas from the gas inlet 5 and microwave (eg 2.45 GHz) from the waveguide 3 (a microwave oscillation source, an isolator, a matching box, and a microwave power meter are omitted in the figure), When a DC magnetic field under the electron cyclotron resonance (ECR) condition (875 Gauss) is applied by the magnetic coil 4 in a direction perpendicular to the microwave electric field, the gas introduced into the plasma generation chamber 1 is plasma by the interaction of these. Becomes An ion beam 7 is extracted from the plasma thus generated by an ion extraction electrode system 6 composed of an acceleration / deceleration system.

[Problems to be solved by the invention]

With the configuration as described above, as shown in FIG. 13, an ionic current, which was conventionally considered difficult, was obtained. O ₂ was used as the gas, and the ion extraction electrode opening was 4.2 mmφ × 7
It was composed of individual through holes, and its area was 0.969 cm ² . However, as is clear from the figure, the ion current tends to saturate with the microwave power. As a result, the efficiency of conversion of microwave power into an ion beam is improved and the power consumption is further reduced by eliminating saturation phenomena.
It is strongly desired to realize a high-current ion source that achieves high current density. If such improvements are made, it is expected that the applications of high-current microwave ion sources will increase more and more.

In the generation of high-density plasma, the magnetic field strength is strong in two places near the introduction window and near the ion extraction electrode (1
A mirror-type ion source with an electron confinement effect is said to be effective. However, this type is obtained by mixing ions of various valences, and the current density of the ion beam of each valence is not necessarily high.

[Means for solving problems]

According to the present invention, the position of the ion extracting electrode arranged on the central axis of the plasma generating chamber and near the outlet through which the plasma flows is near the region where the plasma density is highest.

[Action]

Generally, in an ion source having a resonance mode configuration using a divergent magnetic field, the size of the plasma generation chamber has not matched with the region where microwaves are absorbed to generate high-density plasma for high-density plasma. Therefore, depending on the magnetic field distribution, the plasma density was lowered near the ion extraction electrode system, or high-density plasma could not be generated, so that a high-density ion beam could not be obtained, whereas the present invention In, the ion beam is extracted from the region having the highest plasma density distribution.

〔Example〕

FIG. 2 is a block diagram showing an embodiment of the present invention. In the figure, 11 is a stainless steel (Su
S) plasma generating chamber, 12 is a microwave introduction window having a laminated structure, 13 is a waveguide, 14 is a magnetic coil, 15 is a gas inlet, 16 is a plasma limiter, 17 is a plurality of through holes (circular or Ion extraction electrode system consisting of (rectangle), 18 is an ion beam. The waveguide 13 is generally rectangular, but is not limited thereto. The cavity of the plasma generation chamber 11 is not limited to a cylinder and may be a rectangular parallelepiped. The plasma limiter 16 is effective for shielding microwaves and plasma (confining) plasma for ion beam stability and mitigating temperature rise of the extraction electrode, but must be provided as a high-density ion source. is not.

The microwave introduction window 12 is made of a dielectric material having a two-layer structure of quartz and alumina. In this quartz + alumina,
Further, for example, BN may be laminated to have a three-layer structure. By thus forming a laminated structure of two or more kinds of dielectrics, electrons generated in the vicinity of the ion extraction electrode system 17 or electrons flowing from outside the ion source are accelerated in the ion extraction electrode system, and the microwave introduction window To prevent the microwave introduction window from rising in temperature and breaking. For that purpose, it is desirable that the dielectric material in contact with the plasma is a high melting point material, which is resistant to thermal shock and has good thermal conductivity. Furthermore, the present inventor has found that the microwave introduction window having a laminated structure is closely related to the generation of high-density plasma. This seems to be closely related to impedance matching to microwave plasma. Generally, it is effective to generate a high-density plasma by using a material having a low dielectric constant on the microwave introduction side and a material having a high dielectric constant on the plasma generation chamber side. As a matter of fact, the dielectric material in the portion in contact with the plasma is not necessarily a material having a high dielectric constant because heat resistance is prioritized, but it is effective if there is a portion having a high dielectric constant in the middle. At this time, regarding the shape (size and thickness),
It is better to design impedance matching so that microwave reflection is reduced.

Reference numeral 21 is an insulator that supports the ion extraction electrodes 17 while electrically insulating them from each other, and reference numeral 22 is a pipe for cooling water.

A side wall portion 172 connecting the portion of the ion extracting electrode system 17 provided with the ion extracting opening 171 and the supporting portion of the insulator 21.
Are tapered so that they become wider as they move away from the plasma generation chamber 11. Although not shown in the figure, an exhaust opening is provided in a vertically long slit-like shape in the side escape portion.

In the above configuration, the plasma generation chamber 11 is vacuum-sealed by the microwave introduction window 12, and the gas to be ionized is introduced from the gas introduction port 15. Microwave (typically 2.45GH
z) is introduced into the plasma generation chamber 11 from the rectangular waveguide 13 through the microwave introduction window 12. The magnetic coil 14 is arranged so as to take a maximum magnetic field between the microwave introduction window 12 and the vicinity of the center of the plasma generation chamber 11 and generate a divergent magnetic field that becomes weak in the direction of the ion extraction electrode system 17. Frequency 2.
Since the magnetic field strength that satisfies the ECR condition is 875 gauss for a microwave of 45 GHz, a magnetic coil that can obtain a maximum of 1000 gauss or more is used. Introduce gas and microwave into the plasma generation chamber 11
When a magnetic field of 875 Gauss which causes at least the ECR condition is applied inside the plasma generation chamber 11, plasma is generated in the plasma generation chamber 11. Further, the optimum magnetic field strength and magnetic field distribution are set so that the generated plasma density is maximized.

The generated plasma (ions and electrons) is generated by the divergent magnetic field of the magnetic coil 14 as described above, and by the electric field of the plasma itself and the flow of gas flowing from the gas inlet 15 of the ion extraction electrode system 17. Move in the direction.

FIG. 1 shows the relationship between the structure of such a microwave ion source and the plasma density distribution inside the plasma generation chamber 11. In FIG. 2B, inside the plasma generation chamber 11, the plasma generation rate per unit volume on the central axis of the plasma generation chamber 11 changes as shown by the curve I, while the plasma extinction rate changes with the curve II. , And the net plasma production rate is distributed as shown by curve II. As described above, since the generated plasma moves in the direction of the ion extracting electrode system 13, the plasma density exhibits a distribution like a curve IV obtained by approximately integrating the curve III. That is, inside the plasma generation chamber 11, the plasma generation rate> the plasma extinction rate is always satisfied, and the plasma generation rate is zero or more. As a result, the plasma generation chamber
11 Inside, the plasma density tends to gradually increase. The ion extraction electrode system 17 is the plasma generation chamber.
It is placed close to 11 (d0). As a result, the ion beam 14 is extracted from the region having the highest plasma density.

On the other hand, in the conventional configuration as shown in FIG. 12, in FIG. 1 (b), the distance from the microwave introduction window 12 to the ion extraction electrode system 17 increases on the central axis of the plasma generation chamber, As soon as the plasma density reaches its peak, it begins to fall and the ion beam is extracted from the subsequent region.

Magnetic coil 14 especially when a high-density ion beam is obtained
As shown in Fig. 3, the magnetic field distribution due to is 900-1000 gauss in the vicinity of the microwave introduction window and is almost flat or has one peak inside the plasma generation chamber 11, and 800-1000 gauss in the vicinity of the extraction electrode. And the gradient of the magnetic field distribution at that place is negative. As is clear from the figure, B _ECR = 875 Gauss (value on the central axis), which is the magnetic field strength that generally satisfies the ECR condition, is shown in the vicinity of the ion extraction electrode system 17 and inside the plasma generation chamber 11. Then, it is higher than this value. In the simplest case, it is considered that the highest density ion beam can be obtained over the entire area of the plasma generation chamber 11 when 875 Gauss satisfying the ECR condition, but the ECR does not have such a distribution. It is considered that under the condition, the refractive index of plasma becomes too high and the microwave is reflected. In particular, it is speculated that in high density plasma, this phenomenon becomes remarkable even in a magnetic field slightly stronger than the ECR condition, because the absorption of microwaves, that is, the conversion efficiency of incident microwaves into plasma is large.
Of course, there is a possibility that the apparent microwave frequency is shifted due to the influence of Doppler shift. The reason why the magnetic field strength near the microwave introduction window should be higher than that under the ECR condition is that microwaves are efficiently introduced without being reflected in the high-density plasma. Also, the fact that the gradient of the magnetic field distribution is negative near the plasma extraction electrode system 17 means that the generated plasma (ions and electrons)
However, due to the divergent magnetic field, it is easy to move in the direction of the ion extracting electrode system, and it is easy to extract as an ion beam.

Here, the magnetic field distribution and the magnetic field strength that can obtain the highest density plasma are shown, but in addition, the relative magnetic field distribution is almost the same as that described above (that is, the curve in FIG. A parallel movement of the ion beam was performed), and a relatively high density ion beam was obtained even when the magnetic field strength in the vicinity of the microwave introduction window 12 was set to 875 Gauss corresponding to ECR resonance. Fourth
The figure shows the relationship between the magnetic field strength near the microwave introduction window 12 and the ion current (the ion electrode density is the ion current / ion extraction electrode opening area. It has a maximum point of 932 gauss and 875 gauss). Also has a maximum.
Regarding this point, it is considered that some factor other than the above-mentioned reason is acting.

A plurality of ion extraction electrode systems 17 (usually 3
This is an acceleration-deceleration electrode structure composed of electrode plates (single electrode structure), and this embodiment shows a structure example composed of three electrodes mutually insulated by the insulating material 21, but a multi-stage electrode structure further than this is also possible. It goes without saying that it is good. In this example, the acceleration electrode
High voltage of 5 to 40kV to 17A, -0.5 to -5kV to deceleration electrode 17B
Is applied to ground the ground electrode 17C to the ground potential. The deceleration electrode 17B has a function of controlling the spread of the extraction beam and a function of preventing the inflow of electrons from the outside of the ion source. The ion extraction opening of the extraction electrode system 17 is usually composed of a plurality of through holes. To obtain a circular ion beam, 7,13,19 finely packed arrangements are used, and to obtain a rectangular beam, 2 × 5,3 × 5
It should be arranged like an individual piece. For example, an ion beam of 20 mmφ was obtained with 5 mmφ × 7 through holes.

Compared with the conventional ion source of FIG. 12, the ion source of the present embodiment has the following two basic differences.

The shape of the plasma generation chamber 11 is such that its axial length is shorter than its inner diameter.

The magnetic field distribution inside the plasma generation chamber 11 is flat or has one peak, and the difference in the magnetic field strength between the microwave introduction window 12 and the ion extraction electrode system 17 is small, and there is a slight difference in the vicinity of the ion extraction electrode 17. It is a divergent magnetic field.

FIG. 5 shows the plasma generation chamber for the ion source shown in FIG.
The ion current density obtained when the inner diameter of 11 is 108 mmφ, the axial length L1 is 90 mm, and the magnetic field distribution is operated as shown in FIG. 3 is shown. O ₂ was used as a gas, the ion extraction electrode opening was composed of 5 mmφ × 7 through holes, and the area was 1.37 cm ² . Further, the microwave introduction window 12 has a two-layer structure of quartz and alumina, and the ion current is 180 mA for a microwave power of 500 W, which is an unprecedented value.
Further, even when the microwave introduction window 12 has a three-layer structure of quartz, alumina and BN, a current of 180 mA or more can be easily obtained for a microwave power of 1 kW, and the ion source of this embodiment simply It is considered that not only the ion beam is extracted from the region where the plasma density is the highest in the conventional ion source, but also the plasma generation rate or the value of the plasma density itself in the plasma generation chamber is increasing. It is considered that this is affected by the magnetic field distribution shown in FIG.

Next, the effect of the axial length of the plasma generation chamber 11 on the ion current density will be examined in detail.

FIG. 6 shows a flat plate type ion source with a Langmuir probe, which was manufactured to measure the plasma density in the vicinity of the ion extracting electrode system 17. Basically, the configuration is the same as that of the ion source shown in FIG. 2, but a probe 31 for measuring plasma density is provided instead of the extraction electrode system 17 shown in FIG. This probe 31 is vertically movable and has an opening having the same shape and area as the through hole of the ion beam extraction electrode system 17, and is brought as close as possible to the conditions of the plasma generation chamber 11 of FIG. Where probe 31 has a negative voltage V _B
Between the ion saturation current density J and the plasma density N ₀ obtained by applying The relationship is established. The current density of the ion beam extracted by the ion source shown in FIG. 2 has the same relationship. Therefore, if the ion saturation current density near the ion extraction electrode system 17 is measured, the ion source will have a high ion current density and a high current. It is possible to evaluate.

Move the probe shown in Fig. 6 to move the axial length L1 of the plasma generation chamber 11
FIG. 7 shows how the ion saturation current density changes when is changed. Negative voltage -80V applied to the probe 31,
The microwave power is 310W. At this time, the magnetic field having the distribution shown in FIG. 3 is applied to the plasma generation chamber 11,
Its strength is optimized according to gas pressure, microwave power, axial length, etc. As is clear from FIG. 7, the ion saturation current density increases as the axial length is shortened. In particular, when the inner diameter of the plasma generating chamber 11 is shorter than 108 mm, this effect is remarkable.

The axial length corresponding to the resonance mode TE ₁₁₂ of the microwave in vacuum with respect to the inner diameter of 108 mm is 164 mm, and the ion saturation current density is high even in this vicinity, which is related to the resonance mode. Seem. In other words, it contradicts the prediction that when plasma is generated, the refractive index of the plasma chamber 11 becomes high and the wavelength of the microwave in the tube becomes short, but it is also possible that the effect of the resonance mode when the plasma density is low is shown. To be Needless to say, high-density plasma is not generated simply by adjusting the axial length to the resonance mode, and the magnetic field distribution plays an important role.

FIG. 8 shows the microwave power dependence of the ion saturation current density at each axial length. As is clear from this figure, the shorter the axial length, the higher the ion saturation current density for the same power,
High conversion efficiency of microwave power to plasma. The reason why the axial length is shortened and the ion current density is increased is that the axial length of the plasma chamber 11 is brought closer to the region where the introduced microwaves are absorbed by the plasma, whereby the highest density plasma is generated. It is thought that the ions will be extracted from the place where Further, as is clear from the above explanation, it is expected that it is difficult to achieve high density even if the axial length is too short, but in this experiment, its effectiveness was confirmed up to an axial length of 50 mm.

Next, as described above, the ion extraction characteristics of the ion source of the present invention are closely related to the distribution of the magnetic field applied to the plasma generation chamber, so the magnetic field distribution dependence will be examined.

FIG. 9 shows how the ion current changes when the magnetic field distribution is changed using the ion source shown in FIG. The gas used was N _e , and the configuration of the ion extraction electrode opening was the same as in the case of FIG. When the magnetic field distributions are I, II, and III in FIG. 9 (a), I, II, and III shown in FIG. 9 (b), respectively.
An ionic current such as Note that the magnetic field strength (absolute value) in FIG. 9A shows the value when the largest ion current is obtained in the relatively same magnetic field distribution.

As is clear from FIG. 9, the magnetic field strength in the vicinity of the microwave introduction window 12 is between 900 and 1000 gauss, and the ion current is maximized at a certain value (near 950 gauss). . On the other hand, in the vicinity of the ion extraction electrode system 17, the gradient of the magnetic field strength distribution is negative, and a high current is obtained at a magnetic field strength of around 875 Gauss or more that satisfies the ECR condition. Further, in the middle of the inside of the plasma chamber, it is desirable that the magnetic field strength is slightly higher than both ends. As described above, the magnetic field strength distribution is shown by the magnetic field strength when the ion beam is most extracted with the relatively same magnetic field distribution curve, but in general, the magnetic field distribution shown in FIG. By changing only the magnetic field strength, the ion beam can be extracted with the magnetic field strength in the vicinity of the microwave introduction window 12 being in the range of about 850 to 1000 gauss. Also, as mentioned above, it should be noted that when the magnetic field strength near the microwave introduction window 12 is 875 Gauss, which is the ECR condition, a stable ion beam having a relatively high current density can be obtained.

FIG. 10 is a configuration diagram showing a microwave ion source showing another embodiment of the present invention. 41 and 42 are magnetic coils, 43 is an ion extraction electrode system, and 44 and 45 are insulators. The configuration of FIG. 2 is the configuration of the ion extraction electrode system 43 and the arrangement of the magnetic coils 41 and 42 accordingly. And are different.

That is, as compared with FIG. 2, the supporting portion supporting the central opening of the ion extracting electrode system 43 is shorter.
Since the support part is shortened in this way, even if the temperature of the electrode support part rises due to heating of the ion extraction electrode system part (especially the first screen electrode subjected to the impact of plasma), the electrode due to thermal deformation The position of the aperture between the ion extraction electrodes is much less likely to be misaligned as compared with the ion source in FIG. 3, and it is easy to align the mutual aperture positions between the leaves of the ion extraction electrode. Positional shifts are less likely to occur due to factors. Further, since there is no restriction that the ion extraction electrode system is inserted into the plasma generation chamber 11, there is an advantage that the inner diameter of the plasma generation chamber 11 can be reduced. Although omitted in the figure, this embodiment also
As in the case of FIG. 2, a slit-shaped exhaust opening is provided in the side wall of the ion extracting electrode system. Further, in this embodiment, the supporting portion of the acceleration electrode closest to the plasma generation chamber 11 also serves as the plasma limiter.

Due to such a structure of the ion extracting electrode system, the magnetic coil cannot be continuously arranged below the ion extracting electrode system as shown in FIG. 2 (of course, outside the insulator 44). Can be placed,
In that case, a large current is required to obtain the same magnetic field strength). Therefore, in this embodiment, the magnetic coil 41 is arranged above the ion extracting electrode system 43. However, since the magnetic field distribution as shown in FIG. 3 cannot be obtained by this alone, a desired magnetic field distribution is obtained by combining with the magnetic coil 42 arranged further below. Since the desired magnetic field distribution can be obtained with a smaller amount of electric power as the gap between the magnetic coils 41 and 42 is narrowed, it is better to bring them closer to each other within a range where discharge due to a high voltage does not occur.

The operating principle and operating conditions of this ion source are exactly the same as those in FIG. 2, and the ion current shown in FIG. 5 can be obtained by the same operation as the ion source in FIG.

The first object of the present invention is to obtain an ion source capable of high voltage extraction such as an ion implanter, but the microwave ion source of the present invention uses an ion extraction electrode system as a single leaf electrode. Therefore, it is also effective as an ion source having low ion energy. Furthermore, it can be used as a plasma generation source by removing the ion extraction electrode system, and is also useful as an ion source / plasma source for ion attachment, film formation or etching, for example.

FIG. 11 shows an example of the configuration for drawing out the plasma flow. 51
Is a plasma outlet, 52 is a plasma flow, and the plasma outlet 51 is similar to the plasma limiter 16 of FIG. The operating principle of this plasma generation source is exactly the same as that of the ion source of FIG. 2, and there is a divergent magnetic field in which the magnetic field weakens from the vicinity of the plasma extraction port 51 toward the outside of the plasma generation source, and the plasma flow 52 generates plasma. Pulled out of chamber 11.

〔The invention's effect〕

As described above, according to the present invention, a high-density plasma is generated by an electrodeless microwave discharge, and ions are extracted from the highest density part of the high-density plasma generated. Therefore, there is an advantage that an ion beam having a high ion current density and a large current, which has not been realized in the past, can be obtained. In particular, it can be used as a high-current ion source that is stable and has a long life for active gases such as oxygen and boron.
It is effective as an ion source for high current ion implanters of 0 mA class. Furthermore, if it is used for etching / adhesion as a single-leaf extraction electrode or a low energy (about 200 eV or less) ion source or plasma generation source without an extraction electrode,
Since high-density ion plasma can be irradiated, the processing speed (etching speed / adhesion speed) can be increased.

[Brief description of drawings]

1 is a diagram showing the relationship between the configuration of a microwave ion source and plasma density distribution showing an embodiment of the present invention, FIG. 2 is a configuration diagram of the microwave ion source, and FIG. 3 is a magnetic field in the plasma generation chamber. FIG. 4 is a diagram showing the distribution, FIG. 4 is a diagram showing the relationship between magnetic field strength and ion current, and FIG. 5 is a diagram showing ion extraction characteristics,
FIG. 6 is a configuration diagram showing an ion source with a measurement probe for verifying the effectiveness of the ion source of the present invention, and FIG. 7 is a diagram showing the dependence of the ion saturation current density on the axial length of the plasma generation chamber, 8 is a diagram showing the dependence of the ion extraction characteristics on the axial length of the plasma generation chamber, FIG. 9 is a diagram showing the relationship between the magnetic field distribution and the ion extraction characteristics, and FIG. 10 is a micrograph showing another embodiment of the present invention. Wave ion source configuration diagram, FIG. 11 is a configuration diagram showing an example applied to a plasma generation source, FIG. 12 is a configuration diagram showing a configuration example of a conventional microwave ion source, and FIG. 13 shows its ion extraction characteristics. FIG. 11 …… Plasma generation chamber, 12 …… Microwave introduction window, 14,4
1,42 …… Magnetic coil, 15 …… Gas inlet, 17,43 …… Ion extraction electrode system.

Claims (4)

[Claims] 1. A plasma generation chamber having an axial length of 10 cm or less, a microwave introduction window for introducing microwaves into the plasma generation chamber, and an outlet on the central axis of the plasma generation chamber where plasma flows out. And an ion extraction electrode system for extracting an ion beam from the plasma generated in the plasma generation chamber, and an ion extraction electrode system on the central axis of the plasma generation chamber from the microwave introduction window toward the ion extraction electrode system. It has a magnetic flux density of 1.03 to 1.14 BECR (BECR is the magnetic flux density under the electron cyclotron resonance condition) near the microwave introduction window.
And a magnetic circuit for generating a magnetic field distribution having a non-negative magnetic field gradient in the vicinity of the microwave introduction window. 2. The magnetic circuit has one peak on the axis of the plasma generation chamber toward the ion extraction electrode system from the microwave introduction window, and has a magnetic flux density of 0.91 BECR or more near the ion extraction electrode system. 2. The microwave ion source according to claim 1, wherein the microwave ion source has a magnetic field distribution having a positive magnetic field gradient in the vicinity of the ion extraction electrode system. 3. The microwave introduction window has a laminated structure composed of a first dielectric layer closest to the microwave introduction window side and one or more other dielectric layers, and the other dielectric material. The microwave ion source of claim 1, wherein the layer comprises a dielectric layer having a higher dielectric constant than the first dielectric layer. 4. A magnetic circuit is divided into at least two parts with the ion extraction electrode system as a boundary, and each of the divided magnetic circuits is arranged so as to sandwich a support part of the ion extraction electrode system. The microwave ion source according to claim 1.