JP4623159B2 - Electrostatic chuck - Google Patents

Description

  The present invention relates to an electrostatic chuck for attracting and fixing an object to be adsorbed such as a semiconductor wafer and a glass substrate for FPD by electrostatic force.

Conventional electrostatic chuck ceramic dielectrics have been configured for the purpose of controlling their electrical characteristics (see, for example, Patent Document 1).
In such a case, when the ceramic structure is exposed to a plasma environment, the structure is eroded and the surface roughness is deteriorated, resulting in a change with time due to a change in the contact state between the electrostatic chuck surface and the wafer. In some cases, particles may fall from the sintered body and be generated as particles, causing a short circuit between LSI wirings.

  Also, there is an example in which an alumina ceramic material having a particle size of 2 μm or less and a relative density of 99.9% and improved plasma resistance is applied to an electrostatic chuck (for example, see Patent Document 2). However, even in this case, even if the plasma resistance is good, there is no description about the electrical physical properties, and the basic function of the so-called John Seller Beck type electrostatic chuck that expresses a large adsorption force cannot be exhibited.

Further, an alumina ceramic containing 0.1 to 1 wt% of titanium oxide and having a volume resistivity of 10 ⁰ to 10 ⁴ Ωcm is disclosed (for example, see Patent Document 3). However, in this case, it is not possible to obtain electrical characteristics that can function as an electrostatic chuck.

  Also disclosed is an electrostatic chuck in which the volume resistivity of the dielectric is reduced by adding 0.5 to 2 wt% of titanium oxide to alumina ceramic. (For example, refer to Patent Document 4.) In this case, it is disclosed that the resistance does not decrease when the content is lower than 0.5 wt%, and the current flows too much when 2 wt% or more is added. Furthermore, it is disclosed that titanium oxide precipitates at the grain boundaries of alumina ceramics. That is, an additive of at least 0.5 wt% or more is necessary to reduce the volume resistivity, and the amount of the additive is large as an electrostatic chuck having severe restrictions on the mixing of impurities into the adsorbed object.

Further, an electrostatic chuck is disclosed in which alumina is 99% or more, the average particle diameter is 1 to 3 μm, and the volume resistivity is 10 ⁸ to 10 ¹¹ Ωcm at 300 to 500 ° C. (For example, Patent Document 5) However, there is no description regarding the physical properties of the dielectric necessary for the electrostatic chuck used at other temperatures, for example, at a relatively low temperature of 100 ° C. or lower.
Japanese Patent No. 3084869 JP-A-10-279349 Japanese Patent Laid-Open No. 2004-18296 Japanese Patent Publication No. 6-97675 Japanese Patent Laid-Open No. 11-312729

  The present invention can maintain a smooth surface even after being exposed to plasma. As a result, it can suppress particle contamination of the adsorbed material such as a silicon wafer and has excellent adsorption and desorption characteristics of the adsorbed material. An electrostatic chuck that is easy to do is to be provided.

In order to achieve the above object, in the present invention, when the weight percentage of alumina is X and the weight percentage of titanium oxide is (100-X), X is 99.4 wt% or more , and more than 99.8 wt%. A dielectric for electrostatic chuck having a small volume resistivity of 10 ⁸ to 10 ¹¹ Ωcm at room temperature and segregation of titanium oxide at the grain boundaries of alumina particles; and titanic acid inside and at the grain boundaries of the alumina particles. An electrostatic chuck free of aluminum (Al ₂ TiO ₅ ) has been disclosed. As a result, it is possible to improve the plasma resistance of the electrostatic chuck dielectric and the basic functions of the electrostatic chuck at a high level, and to manufacture it at low cost.

The reason why the volume resistivity needs to be 10 ⁸ to 10 ¹¹ Ωcm is that the Johnsen-Rahbek effect is used as the attractive force of the electrostatic chuck. By using the Johnsen-Rahbek effect, a very large attracting force is generated. As a result, by providing a convex portion on the surface of the electrostatic chuck, the contact area with the object to be attracted is 1 to 10% of the area of the attracting surface. And can be reduced.

  Further, by making the height of the convex portion provided on the surface 5 to 15 μm, the attracting force can be exerted even in the contacted portion. As a result, the area of the convex portion can be made 0.001% or more and less than 0.5% with respect to the area of the suction surface. Since the temperature of the object to be adsorbed is transferred through the contact portion as the contact area of the convex portion becomes small, even if the structure of the convex portion is eroded by plasma, the influence is small. Therefore, an electrostatic chuck with little change with time can be realized as a result of increasing plasma resistance and minimizing contact with the object to be adsorbed.

Further, in order to improve the response characteristic of the above-described adsorption force, it is necessary to reduce the value of the following expression.
ts = 1.731 × 10 −11 × ρ (εr + d / h) (second)
Here, ts is the time (seconds) until the initial attractive force becomes 100% and collapses to 2%, ρ is the volume resistivity (Ωm) of the dielectric layer, εr is the relative dielectric constant of the dielectric layer, and d is The thickness (m) and h of the dielectric layer are the height (m) of the protrusions. If the value of this formula is 0.001 to 0.6 and the height of the convex portion is 5 to 15 μm, the area of the convex portion can be 0.001 to 0.5% with respect to the adsorption surface. In addition, it is possible to provide an electrostatic chuck with good response to voltage application and unloading of attracting force.
The above equation is obtained analytically from the equivalent circuit of FIG. 1 and derived from [Equation 1] to [Equation 4]. Here, q1 is the charge density, S is the electrode area, C is the capacitance, G is the conductance, V is the applied voltage, t is the time (variable), and T is the voltage application time.

  According to the present invention, a smooth surface can be maintained even after being exposed to plasma. As a result, particle contamination of the adsorbent such as a silicon wafer can be suppressed, and the adsorption and desorption characteristics of the adsorbent are excellent. There is an effect that an electrostatic chuck that can be easily manufactured can be manufactured.

  Alumina, titanium oxide, and other transition metal oxides were granulated at the mixing ratio shown in Table 1 as raw materials. Alumina having an average particle diameter of 0.1 μm and a purity of 99.99% or more was prepared. Titanium oxide having a purity of 98% or more was used.

(Slurry adjustment, granulation, raw processing)
The above raw materials were mixed and pulverized at a blending ratio shown in Table 1, an acrylic binder was added, and after adjustment, granulated with a spray dryer to prepare granule powder. The granular powder was packed into a rubber mold, and then CIP (pressure 1 ton / cm ² ) was performed to produce an ingot, which was then processed into a predetermined shape to produce a formed shape. Ion exchange water or the like was used for mixing so that impurities were not mixed as much as possible.
(Baking)
The raw processed body was fired in a nitrogen and hydrogen gas reducing atmosphere. The firing temperature was 1150 to 1350 ° C., the firing time was 1 to 8 hours, and the conditions with the highest bulk density were selected. At this time, humidified gas is used for degreasing. The reduction firing is performed for the purpose of adjusting the volume resistivity with the aim of non-stoichiometric composition of titanium oxide.
(HIP processing)
Further HIP processing was performed. The HIP condition was Ar gas 1500 atm, and the temperature was the same as the firing temperature or a temperature lowered by 30 ° C.
(Physical property measurement)
What was obtained by the HIP treatment was subjected to firing bulk density, average particle diameter measurement by firing body structure SEM observation, volume resistivity measurement, friction force measurement in vacuum, and residual time measurement. For the friction force measurement and the residual time measurement, the thickness of the ceramic dielectric layer was 1 mm. The adsorption voltage was 200 V, and the remaining time was measured by turning off the power after applying the voltage for 1 minute and measuring the attenuation of the remaining frictional force. The object to be adsorbed was a silicon wafer mirror surface. The remaining time was defined as the time for the frictional force to decay to 2% after the power was turned off.
Moreover, plasma was actually irradiated and the change in the surface roughness (centerline average roughness Ra) of the ceramic was measured. In the initial state, the surface roughness was Ra 0.05 μm or less. Plasma was a reactive ion etching apparatus, and etching gas was CF4 + O2 at 1000 W for 5 hours for plasma discharge.
In addition, as an evaluation of the practical adsorption force of the electrostatic chuck for a part of the sample, the pressure when the adsorption object is peeled off by applying the He gas pressure between the adsorption object and the adsorption object (POPOFF adsorption force) ) Was recorded. The adsorption voltage at this time is 1000V.
(Comparative product)
For comparison, alumina ceramics produced by a conventional manufacturing method is illustrated. The compound 1 is 98 wt% alumina and 2 wt% titanium oxide with an average particle size of 0.5 μm, the comparative product 2 is 99 wt% alumina and 1 wt% titanium oxide, and the firing temperature is 1580 ° C. Incidentally, the surface roughness of the comparative product 1 was Ra 0.23 μm in the initial state. The surface roughness of the comparative product 2 was Ra 0.2 μm in the initial state. The comparative product is not HIPed.

The results of the above test are shown in Tables 1 and 2. By controlling the firing temperature, it is possible to obtain a volume resistivity that functions as an electrostatic chuck when the bulk density is 3.97 g / cm ³ or more with an addition amount of titanium oxide of 0.2 wt% or more and 0.6 wt% or less. all right. Conventionally, it has been found that the same effect can be obtained with a very small amount of addition compared to the amount added conventionally when the particle diameter is 50 μm or more. In the conventional manufacturing method, since the firing temperature is as high as 1580 ° C., the added titanium oxide reacts with alumina to become a compound such as aluminum titanate (Al 2 TiO 5), whereas in the present invention, the average particle size is 0.2 μm. Since the firing temperature is as low as 1300 ° C. or less by using a high-purity alumina raw material with a purity of 99.9% or more, the added titanium oxide does not react with alumina and remains as titanium oxide This was confirmed by X-ray diffraction. Aluminum titanate is known to have a relatively high volume resistivity, and in order to reduce the volume resistivity of alumina, it is considered that the efficiency is lower than that of titanium oxide, and a larger amount of addition is required. Next, FIG. 4 shows an SEM photograph of a sample subjected to thermal etching at a temperature lower by about 80 to 150 ° C. than the firing temperature as the microstructure of the dielectric for electrostatic chuck of the present invention. It was found that titanium oxide (white portion in the photograph) segregated and continuously connected to the grain boundaries of alumina particles (black portion in the photograph) having an average particle diameter of 2 μm or less. It is considered that the volume resistivity can be efficiently lowered by the network formed by the titanium oxide. From the above results, the dielectric constant for the electrostatic chuck of the present invention was able to lower the volume resistivity by adding a small amount of titanium oxide compared to the conventional one. The added titanium oxide reacted with alumina. This is because the titanium oxide remains as it is, and the titanium oxide segregates at the grain boundaries of the alumina particles to form a continuously connected structure. In addition, it is thought that titanium oxide is further improved in conductivity by becoming non-stoichiometric composition by reduction firing. Thus, it was considered that the volume resistivity could be controlled by a small amount of titanium oxide, and chemical contamination on silicon wafers and the like could be remarkably suppressed as compared with the conventional case.

電気特性の評価の結果、酸化チタン単独または酸化チタン＋遷移金属酸化物の添加割合によって１０^８〜１０^１６Ωｃｍの広範囲で制御できることがわかった。

As a result of the evaluation of electrical characteristics, it was found that control was possible over a wide range of 10 ⁸ to 10 ¹⁶ Ωcm depending on the addition ratio of titanium oxide alone or titanium oxide + transition metal oxide.

In the case of using a resist, it is desirable that the electrostatic chuck is used at 100 ° C. or lower in view of its heat resistant temperature.
The electrical characteristics required for the dielectric material for the electrostatic chuck are desirably 10 ⁸ to 10 ¹¹ Ωcm in volume resistivity at the temperature at which the electrostatic chuck is used. If the lower limit value is less than 10 ⁸ Ωcm, the current flowing into the wafer becomes excessively large, and the device may be damaged. If the upper limit value is 10 ¹¹ Ωcm, the response to the wafer adsorption / desorption voltage application decreases.
For example, in a process such as process etching at 100 ° C. or lower, the lower limit value is desirably about 10 ⁹ to 10 ¹¹ Ωcm.

If the amount of titanium oxide is more than 0.6 wt%, the volume resistivity becomes less than 10 ⁸ Ωcm, the current flowing into the wafer becomes excessive, and the device may be damaged. On the other hand, if it is 0.2 wt% or less, the effect of lowering the volume resistivity by adding titanium oxide becomes small.

Plasma resistance was evaluated by the change in surface roughness because any material is etched if the energy of ions in the plasma is excessive.
As a result, the change in surface roughness of the ceramic dielectric according to the present invention was significantly smaller than that of the conventional one. This is presumed that the size of particles that generate dust is small.

Convex of an electrostatic chuck having a smooth surface on which a plurality of convex portions are formed and an object to be adsorbed is placed on the upper surface of the convex portion, and including a dielectric for electrostatic chuck having a volume resistivity of ^109.3 Ωcm An electrostatic chuck was produced in which the ratio of the total area of the upper surface of the part and the area of the dielectric surface was 0.089%. At this time, convex portions having a diameter of 0.25 mm are continuously arranged on the surface of each apex of a regular triangle having a side of 8 mm. The height of the convex portion is 10 μm.

  As a result, the change in temperature during processing of the silicon wafer that is the object to be adsorbed overlaps with the fact that the change in surface roughness after plasma irradiation is small and the contact area with the object to be adsorbed is very small. Could be very little.

The POPOFF adsorption force was recorded at 100 torr or more in all samples. That is, it has been found that a practical force sufficient for adsorbing an adsorbent such as a silicon wafer is obtained.

It is a figure showing the electrostatic chuck of this invention. It is a figure showing the equivalent circuit of the electrostatic chuck of this invention. It is an enlarged view of the surface pattern of the electrostatic chuck of this invention. It is an electron micrograph which shows the structure of the dielectric material for electrostatic chucks of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Convex part 2 ... Convex part bottom face 3 ... Gas supply hole 4 ... Ring-shaped convex part (seal ring)
5. Recessed part (gas diffusion groove)
6 ... Electrode 7 ... Dielectric layer 8 ... Adsorbed body 9 ... Electrical connection means 10 ... Base 11 ... Power source

Claims (1)

When the weight percentage of alumina is X and the weight percentage of titanium oxide is (100-X), X is 99.4 wt% or more and smaller than 99.8 wt% ,
A dielectric for electrostatic chuck having a volume resistivity of 10 ⁸ to 10 ¹¹ Ωcm at room temperature and a structure in which titanium oxide is segregated at grain boundaries of alumina particles;
Further, the electrostatic chuck characterized in that aluminum titanate (Al ₂ TiO ₅ ) does not exist in the grains and grain boundaries of the alumina particles.

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