JP2006070324A - Sputtering target and sputtering method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a sputtering target capable of forming a film made from a metal fluoride, which absorbs no light, at a constant film-forming rate, and to provide a sputtering method therefor. <P>SOLUTION: A target electrode used in sputtering is composed of a backing plate 3 and a target 1 fixed thereon through a quartz plate 2. The backing plate 3 has a pipe for cooling it arranged therein so as to cool the undersurface of the target 1. The target 1 to be used is made from MgF<SB>2</SB>, and has a density of 90% or higher with respect to the bulk density of MgF<SB>2</SB>. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a sputtering target used in a sputtering method for forming a thin film on a substrate, in particular, a sputtering target made of a metal fluoride used in an optical thin film, and a sputtering method using such a sputtering target.

Conventionally, when an optical thin film such as an antireflection film, a half mirror, or an edge filter is formed, a vacuum deposition method that is excellent in the ease of the technique and the speed of film formation has been used as standard. On the other hand, in recent years, coating by sputtering, which is advantageous in terms of automation, labor saving, applicability to a large area substrate, and the like as compared with vacuum evaporation, has been widely used.
A conventional sputtering target used in a sputtering method is configured as a target electrode by bonding a sputtering target, which is a film material, and a backing plate using a low melting point metal such as indium. In this case, the target electrode is brought into contact with a backing plate cooled directly with water or the like. When heat is generated in the sputtering target during sputtering, this heat is actively dissipated through the backing plate, so that the temperature on the backing plate side of the sputtering target is lowered. Therefore, the temperature of the sputtering target is controlled by controlling the amount of heat released from the backing plate.

Here, when sputtering using a metal fluoride such as MgF ₂ (magnesium difluoride), which is a typical low refractive index material as an optical thin film, as a sputtering target, dissociation between F, which is a metal, and F is likely to occur. As a result, a thin film lacking F was easily formed on the substrate. Since such a thin film has a problem of absorbing visible light, this has been a major obstacle in applying the sputtering method to the optical thin film.

Thus, as an example of solving the problem of light absorption by visible light, there is one disclosed in cited document 1. According to the cited document 1, by providing a heat insulating means between the sputtering target and the backing plate, the target surface is heated to a high temperature, and as a result, a film is formed on the substrate in a molecular state without dissociating the film raw material. And an MgF ₂ film having no light absorption can be obtained.
JP-A-9-41132

  However, when the sputtering target is heated to a high temperature by means of heat insulation, the sputtering target may be deformed. Specifically, in order to form a metal fluoride film without light absorption by sputtering, sputtering is performed in a state where the target surface temperature is at least 600 ° C. or higher and is heated to an optimum temperature range different for each target material. Is called. At this time, the temperature gradient generated from the front surface to the back surface of the sputtering target is greatly reduced by the presence of heat insulating means, for example, a quartz plate, provided between the sputtering target and the water-cooled backing plate. When the density is less than 80% with respect to the bulk density, there are many cases in which deformation easily occurs due to the stress generated inside the target due to a rapid temperature gradient.

  Such deformation of the sputtering target has a great influence on the film forming speed and film quality, and becomes an obstacle to practical use in the case of optical parts that require particularly high accuracy. That is, after introducing the sputtering gas, the power applied to the target electrode is increased to raise the target surface temperature, and when sputtering is performed in an optimum temperature range different for each target material, a metal fluoride that does not absorb light A film can be obtained at a constant deposition rate. However, at this time, when the target surface temperature rises and the sputtering target deforms (typically the surface side becomes convex), a gap is generated between the quartz plate as a heat insulating means and the sputtering target. The heat radiation to the part of the water-cooled backing plate is greatly hindered. Therefore, the surface temperature of the portion with large deformation in the sputtering target is increased as compared with other portions.

In this way, when the deformation of the sputtering target becomes large, it becomes difficult to adjust the temperature, and it becomes difficult to control the release amount of the metal fluoride released from the deformed portion. become unable. As a result, the film formation rate increases rapidly, and the ratio is unstable depending on the degree of deformation of the sputtering target. As can be seen from the above phenomenon, as a result of the high temperature at the tip of the convex portion, the temperature distribution of the entire target is much larger than that without deformation, and the film quality such as light absorption is reduced. The situation is not preferable from the viewpoint of stability. Therefore, it has been desired to develop a technique that can control the film forming rate with higher accuracy even when the sputtering target is maintained at a high temperature.
This invention is made in view of such a problem, and it aims at providing the sputtering target and sputtering method which can form a metal fluoride film without light absorption at a fixed film-forming speed. To do.

The invention according to claim 1 of the present invention for solving the above-mentioned problem is a sputtering target used for forming a thin film on a substrate, wherein the target material is a metal fluoride, and the density thereof is a bulk density. It was set as the sputtering target characterized by being 90% or more.
This sputtering target is sputtered with positive ions at a high temperature, and a metal fluoride struck at that time is deposited on the substrate to form a thin film. Here, since this sputtering target has a higher density than the conventional one, its deformation is suppressed even at a high temperature.

The invention according to claim 2 is the sputtering target according to claim 1, which is used for forming a thin film on a substrate, the target material is magnesium difluoride, and its density is 2.84 g. / Cm ³ or more.
In this sputtering target, the target material is magnesium difluoride (MgF ₂ ), and as described above, the deformation is suppressed even at a high temperature as compared with the conventional target.

According to a third aspect of the present invention, the sputtering target according to the first or second aspect is placed on a backing plate via a heat insulating means, and a high frequency power is applied while introducing a sputtering gas containing at least oxygen. A sputtering method is characterized in that plasma is generated on the target surface to raise the target surface temperature, and the target is sputtered with positive ions to knock out the target material and form a thin film on the substrate.
In this sputtering method, when a metal fluoride thin film is formed, the film formation rate can be made uniform by using a sputtering target that is not easily deformed at a high temperature.

According to the present invention, by setting the density of the metal fluoride target material to 90% or more of the bulk, the heat conduction in the sputtering target can be improved, the temperature distribution can be reduced, and thermal stress can be reduced. On the other hand, it is possible to impart mechanical strength that is difficult to deform. Thereby, even at a target surface temperature exceeding 600 ° C., deformation of the target itself can be prevented. Therefore, it is possible to realize film formation speed and film quality stabilization.
Furthermore, since there is no fear of the sputtering target being deformed, it is possible to input a larger electric power than before, and the film forming speed can be further increased. In addition, since the temperature distribution on the target surface is reduced, a stable film quality can be obtained against fluctuations in input power.

The best mode for carrying out the invention will be described in detail with reference to the drawings.
The inventor of the present application has intensively studied metal fluoride materials used for sputtering targets (hereinafter referred to as targets), and that the density of the target greatly affects the degree of deformation when the target surface is heated to a high temperature. I found it.
In other words, the present embodiment is characterized in that the density of the metal fluoride target material is 90% or more of the bulk density. When the density is high, that is, the porosity is low, the heat conduction in the target is improved, the temperature distribution is reduced, and a mechanical strength that is not easily deformed by thermal stress is obtained. This makes it possible to minimize deformation of the target itself even at a target surface temperature exceeding 600 ° C., which could not be realized with a target material having a bulk density of less than 90%. As a result, film formation speed and film quality Can be realized.
Furthermore, since there is no possibility that the target is deformed, it is possible to input a larger amount of power than before, and it is possible to further increase the deposition rate. In addition, since the temperature distribution on the target surface is reduced, there is an effect that a stable film quality can be obtained even with respect to fluctuations in input power.

In addition, although it is confirmed that the density of the target material is 90% or more of the bulk density, it is confirmed that the target material is effective as a countermeasure against deformation. However, the higher the density, the greater the effect of stabilizing the film formation rate and film quality. It is preferably 96% or more.
Moreover, as a metal fluoride which is a target material, in addition to MgF ₂ , LiF (lithium fluoride), CaF ₂ (calcium difluoride), AlF ₃ (aluminum trifluoride), Na ₃ AlF ₆ (six fluorine) MgF2 is preferable from the viewpoint of low refractive index and excellent mechanical strength.

1 to 2 show a first embodiment of the present invention, FIG. 1 shows a cross-sectional view of a sputtering target, and FIG. 2 shows a schematic configuration diagram of a sputtering apparatus.
In FIG. 1, the target electrode is composed of a sputtering target 1 (hereinafter simply referred to as target 1), a quartz plate 2 as a heat insulating layer (heat insulating means), and a backing plate 3 made of oxygen-free copper. The target 1 has a disk shape with a diameter of 100 mm and a thickness of 6 mm, is made of MgF ₂ with a density of 2.88 g / cm ³ (91.4% of the bulk density), and is placed on the quartz plate 2. . The quartz plate 2 is placed on the backing plate 3, and the quartz plate 2 and the backing plate 3 are bonded using indium, which is a low melting point metal. The backing plate 3 is configured to be directly water-cooled by a known pipe (not shown).

As shown in FIG. 2, the target 1 and the backing plate 3 to which the quartz plate 2 is attached are attached on the cathode electrode 4 provided at the lower part of the vacuum chamber 5. The cathode electrode 4 is connected to a 13.56 MHz high-frequency power source 7 through a matching box 6 disposed outside the vacuum chamber 5.
An openable / closable shutter 8 is provided above the target 1. The shutter 8 can be switched between a state where the vaporized MgF ₂ formed by sputtering is prevented from scattering upward and an open state where the scattering is allowed. Further, a substrate holder 9 is rotatably provided above the shutter 8 and above the vacuum chamber 5. A plurality of substrates 9 a on which a thin film is formed are attached to the lower surface of the substrate holder 9 so as to face the target 1. On the side surface of the vacuum chamber 5, a gas introduction port 10 for introducing a sputtering gas into the vacuum chamber 5 is provided.

Here, thin film formation performed using this sputtering apparatus will be described.
First, after attaching the glass substrate 9a to the substrate holder 9, the inside of the vacuum chamber 5 is evacuated to 1 × 10 ⁻⁴ Pa. Then, O ₂ gas is introduced from the gas inlet 10 to 5 × 10 ⁻¹ Pa. Next, pre-sputtering of the target 1 is started with power supplied from the high-frequency power source 7 to the cathode electrode 4 and the shutter 8 closed. After the plasma generated on the target surface is stabilized, after waiting for the target surface temperature to rise and stabilize at a constant temperature, the substrate holder 9 is rotated and the shutter 8 is opened. Then, MgF ₂ sputters on the substrate 9a. A film is formed. The input power at this time is set to 600W. The shutter 8 is closed within a time when the film thickness formed on the substrate 9a reaches the target physical film thickness of 100 nm.
The input power is set based on the following examination in advance. That is, the power applied from the high-frequency power source 7 is changed, and the temperature of the target surface is measured using the infrared radiation thermometer from the observation window 11 provided in the vacuum chamber 5. As a result, 600 W is obtained as the input power at which a target temperature of 700 ° C. or higher, which can be formed in a molecular state without dissociating MgF ₂ , is obtained. It has been adopted.

Here, for the purpose of evaluating the degree of deformation of the target surface after film formation, the amount of displacement in the height direction of the center of the target was measured and found to be +0.2 mm. “+” Indicates that the target 1 is displaced upward in a convex shape. Moreover, the temperature of the target surface was measured using the infrared radiation thermometer in order to confirm the state of the surface temperature rise at the time of film-forming by the deformation | transformation of the target 1. FIG. As shown in FIG. 3, the measurement points are set to a total of 5 points (a, b, c, d, e) at 20 mm intervals in the radial direction around the center of the target (point c). When the measurement result is expressed by the minimum value and the maximum value of 5 points, it becomes 700 to 760 ° C., and it is understood that the measurement is stable at 700 ° C. or more which is a desired target surface temperature.
Furthermore, when the film thickness on the substrate 9a taken out after the sputtering was measured and the film formation rate was determined from the film formation time, a high value of 23 nm / sec was obtained. Furthermore, when 10 batches of film formation were repeated and the variation in film formation rate was calculated, a high reproducibility of ± 2.6% was obtained. These results are summarized in Table 1.

Further, the refractive index of the thin film at a wavelength of 550 nm is 1.38, which is the same as the refractive index of the thin film obtained by the vacuum vapor deposition method and is a sufficiently low value. Moreover, the light absorption in wavelength 400nm obtained the favorable result of 0.2% or less.
As the gas to be introduced into the vacuum chamber 5, although the O ₂ gas is optimal, not limited to this, an inert gas or even N _2, inert gases, N _2, any two of O ₂ Even with the above mixed gas, substantially the same result can be obtained.

In this example, instead of MgF ₂ having a density of 2.88 g / cm ³ (91.4% of the bulk density), which is the target 1 of Example 1, a density of 3.14 g / cm ³ (a bulk density of 99.7) was obtained. %) MgF ₂ is used as a target electrode. In addition, since the structure of a sputtering device is the same as that of Example 1, a figure and description are abbreviate | omitted.
In this example, a film formation experiment similar to that of Example 1 was performed using a high-density target substantially equal to the bulk as described above.

O ₂ gas was used as the sputtering gas, and the gas pressure was 5 × 10 ⁻¹ Pa. The input power was 600W. As a result, as shown in Table 1, it was confirmed that the height of the center on the surface of the target after film formation did not change at all and the target was not deformed.
In addition, the temperature distribution on the surface of the target at the time of film formation, which seems to affect the deformation of the target, also shows a variation between 710 to 730 ° C. in 5 points of a to e in FIG. It is getting smaller. Although the deposition rate is somewhat reduced to 19 nm / sec, the variation in deposition rate in 10 batches is improved to ± 1.7%.
In this thin film, the refractive index at a wavelength of 550 nm is 1.38, which is the same as the refractive index of a thin film obtained by vacuum vapor deposition and sufficiently low. Further, the light absorption at a wavelength of 400 nm is a good result of 0.2% or less.

[Comparative Example 1]
Replacing Example 1, and MgF ₂ of more than 90% of the bulk density used in the second target 1, to constitute a target electrode using a low-density target of MgF ₂ having a density 75.2% of the bulk density . Since the configuration of the sputtering apparatus is the same as that of the first embodiment, the drawings and description are omitted.
Using the above-mentioned low density target, the same film formation experiment as in Example 1 was performed. O ₂ gas was used as the sputtering gas, the gas pressure was 5 × 10 ⁻¹ Pa, and the input power was 600 W. The target surface after film formation was deformed into a convex shape with the center of the target at the top, and it was +1.8 mm in terms of the amount of displacement in the height direction of the center.

As shown in Table 1, the target surface temperature during film formation was measured using an infrared radiation thermometer. The target center (point c) was as high as 880 ° C. It was confirmed that the temperature was much higher than that at 730 ° C. in Example 2 and 730 ° C. In the peripheral part (point a or point e) 40 mm away from the center of the target, the temperature remains as low as 620 ° C., and as a whole has a large temperature distribution of 620 to 880 ° C.
Moreover, the film formation rate is greatly increased to 65 nm / sec due to the high temperature of 880 ° C. in the center of the target, but reflects the instability of the temperature distribution due to the deformation of the target. The variation in film formation rate in 10 batches was as large as ± 9.8%, which was not practical.
The refractive index of this film at a wavelength of 550 nm is 1.38, which is equivalent to the refractive index of a thin film obtained by a vacuum vapor deposition method and sufficiently low, and also absorbs light at a wavelength of 400 nm. Although good results of 0.2% or less were obtained, it was confirmed that the scratch resistance on the film surface was partially reduced. Thus, the phenomenon that the scratch resistance is reduced is that the temperature of 880 ° C. at the center of the target exceeds the appropriate temperature range of the MgF ₂ sputtered film, and film formation from the target surface is close to normal vacuum deposition. Presumed to be in a state.

As described above, according to the embodiment of the present invention, in forming the MgF ₂ thin film by the sputtering method, the target surface temperature is set by using the high-density target having a density of 90% or more with respect to the bulk density. Since the deformation of the target 1 at a high temperature can be prevented, the target surface temperature can be stably controlled, and variations in the deposition rate can be suppressed, and a homogeneous thin film can be formed. .

Note that the present invention can be widely applied without being limited to the above-described embodiment.
For example, the sputtering apparatus is not limited to that shown in FIG. Further, the target electrode may be configured to include other heating means and cooling means without providing the heat insulating layer made of the quartz plate 2.

It is a figure which shows the structure of the target electrode containing the target in embodiment of this invention. It is a figure which shows the structure of a sputtering device. It is a figure which shows the measurement point of target surface temperature.

Explanation of symbols

1 Target (Sputtering target)
2 Quartz plate (insulation means)
3 Backing plate 4 Cathode electrode 5 Vacuum chamber 6 Matching box 7 High frequency power supply 8 Shutter 9 Substrate holder 9a Substrate 10 Gas inlet 11 Viewing window

Claims (3)

  A sputtering target used for forming a thin film on a substrate, wherein the target material is a metal fluoride, and the density thereof is 90% or more of the bulk density. The sputtering target used for forming a thin film on a substrate, wherein the target material is magnesium difluoride, and the density thereof is 2.84 g / cm ³ or more. target. The sputtering target according to claim 1 or claim 2 is placed on a backing plate via a heat insulating means, and while introducing a sputtering gas containing at least oxygen, high-frequency power is applied to generate plasma on the target surface, A sputtering method characterized by raising a target surface temperature and sputtering a target material by sputtering the target with positive ions to form a thin film on a substrate.

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