KR20070021139A - Method to reduce thermal stresses in a sputter target - Google Patents

Abstract

The present invention relates to a method for reducing thermal stress at a sputter target during sputtering. The method provides the following steps: providing a target holder; Spraying a target material comprising indium-tin-oxide on the target holder; Introducing voids into the target material while applying the target material on the target holder. Porosity results in at least 2% porosity in the applied target material to reduce thermal stress. The invention also relates to a sputter target having a reduced thermal stress and a method for coating the substrate surface with indium-tin-oxide.

Sputter target, thermal stress

Description

METHOD TO REDUCE THERMAL STRESSES IN A SPUTTER TARGET}

The present invention relates to a method for reducing thermal stresses of a sputter target during sputtering. The invention also relates to sputter targets, more particularly indium-tin-oxides with reduced thermal stress.

High thermal stress can be generated in the target material during sputtering from the sputter target. This thermal stress can lead to debonding and cracking of the target material. Indium-tin-oxide targets, for example, suffer from this problem. The generation of thermal stress is particularly worse when high power dentisities are applied during sputtering.

It is an object of the present invention to provide a method for reducing thermal stress of a sputter target during target preparation and during sputtering. It is another object of the present invention to provide a sputter target with reduced thermal stress during target preparation and during sputtering. Another object is to provide a method for coating a substrate at high power density.

In accordance with a first aspect of the present invention, a method is provided for reducing thermal stress at a sputter target during sputtering. This method includes the following steps:

Providing a target holder;

Spraying a target material comprising indium-tin-oxide on the target holder;

Introducing voids into the target material while applying the target material on the target holder.

Porosity results in at least 2% porosity in the applied target material to reduce thermal stress.

The target material is applied as a spray, preferably a flame spray, plasma spray, high speed oxygen fuel spray or electric arc spray.

More preferably, the porosity of the target material is at least 4%, preferably at least 10%.

The porosity of the target material is calculated as a percentage of the surface of the voids of a particular zone relative to the total surface of one zone.

The density of the target material is related to its porosity. The higher the porosity, the lower the density.

It is generally accepted in the art that high density targets are preferred over targets with low density (high porosity) because high density targets are believed to result in improved process stability (low arc velocity levels). Thus, some efforts have been made to increase the density of the target material. During sputtering, the back side of the sputter target, for example in the tubular rotary sputter target, cools down. Cooling is for example water cooling. Outside the sputter target a high temperature is produced. This results in a high temperature difference between the back (inside) and the outside of the sputter target, creating a high thermal stress of the target material. The higher the sputter power density, the larger the temperature difference. In accordance with the present invention, it has surprisingly been found that thermal stress during sputtering is reduced by using a sputter target having a minimum porosity of at least 2%.

Preferably, up to 20% of the pores formed in the target material comprise closed pores. More preferably, up to 10% of the pores formed in the target material comprise closed pores, and up to 5% of the pores formed in the target material comprise closed pores.

Open pores are pores associated with the outer surface of the target material through a network of pores, grain boundaries, cracks or microcracks, or through a mixture thereof. Closed pores are pores that are not open to the outer surface of the target material.

In order to measure the amount of closed and open pores, the target material comprising indium-tin-oxide is saturated in the fluorescent resin. Saturation may be performed in vacuo to improve the penetration of the resin into the material. The amount of closed voids is calculated as a percentage of the surface of the closed voids in a particular zone relative to the total surface of the voids in one zone.

Sputter targets comprising a target material having a low percentage of closed pores and a high percentage of open pores are preferred because this type of sputter target results in a more stable sputter process. During the burn-in time of the sputter target, including the target material having a low percentage of closed pores and a high percentage of open pores, the target material is cleaned as well as degassed. This has the advantage that gas emissions are avoided when sputtering is initiated and a more stable sputter process is obtained. Conversely, sputter targets with a high percentage of closed pores may experience a gas explosion that cannot be ignored. Sputtering from this type of target is unstable at least at the beginning of the sputter process.

 The method according to the invention is particularly suitable for target materials with reduced thermal conductivity. This method is well suited for use on rotatable sputter targets, such as tubular sputter targets.

Preferred targets include targets having indium-tin-oxide, more particularly indium-tin-oxide, sprayed onto the target holder as the target material.

Indium-tin-oxide is one of the most used transparent conductive oxides in the thin film industry. Applications range from flat panel displays, smart windows, touch panels and electroluminescent lamps to EMI shielding applications.

The target material can be applied starting from indium-tin-oxide powder. For the purposes of the present invention, the indium-tin-oxide powder is a mixture of oxides such as indium oxide and tin oxide, or as a mixture of oxides and metals such as indium oxide and / or tin oxide and / or tin and / or indium. It must be understood.

The target material preferably has a concentration of tin in the range of 5-20 wt%. More preferably, the concentration of tin is between 5-15 wt%, for example 7, 10 or 20 wt%.

The hardness (micro Vickers hardness) of the indium-tin-oxide target according to the invention is preferably 200-400 HV, for example 250 HV. The hardness of the target material is measured by a micro Vickers hardness tester in which a general micro Vickers diamond indenter is installed on the eyepiece of the optical microscope. The microscope is used to measure the width of the indentation.

The hardness of the target material of the sputter target according to the present invention is lower than the hardness of the sputter target obtained by hot isostatic pressure. This can be explained as follows: During hot isostatic pressure, the powder particles are kept at high temperature (eg 3-4 hours at 1000 ° C.) for a long time. The combination of time and high temperature induces diffusion bonding between the individual particles resulting in strong mutual bonding of the particles. Thermal spray processes operate at the same or higher temperatures as during hot isostatic pressure but diffusion reactions are minimal due to very high cooling rates (typically 106 ° C./sec). This minimal thermal interaction between the particles mostly results in mechanical interconnection. This mechanical bonding provides thermal sprayed structure, more flexibility during hardness indentation, resulting in lower hardness values. In addition, higher stress is generated in the target material and higher stress results in higher hardness as compared to the target thermally sprayed during the hot isostatic pressure of the target material. This can be explained as follows: During hot isostatic pressure both the target holder and the target material become hot. The difference in thermal expansion between the target holder and the target material creates stress on the target material during cooling in the hot isostatic cycle. The stress accumulation mechanism described above does not exist during thermal spraying because the target holder is kept at a low temperature (eg 50 ° C.) during the thermal spray process.

High sputter rates can be obtained by using the sputter target according to the invention which is characterized by high porosity and relatively low hardness. The ionized gas, such as argon gas, is provided to the target material during the sputter process. Thus, atoms are released from the target material and deposited on the substrate to be coated. Since the mutual binding of the individual particles of the target material of the target according to the invention is less strong, the atoms of the target material are released more easily, and the energy of the ionizing gas can be used more efficiently so that a higher sputter rate can be obtained.

The pores have a size between 1 μm ² and 1000 μm ² , more preferably between 6 and 80 μm ² , for example between 6 and 40 μm ² .

Preferably, 50% of the pores have a pore size of 10 μm ² or less. A pore size of 10 μm ² is believed to be an important pore size for the formation of cracks in the target material and the stability of the sputter process. Large amounts of small voids in the target material of the sputter target according to the invention are beneficial for stress relaxation during target preparation and sputtering. There are some microcracks present in ceramic targets such as indium-tin-oxide. These microcracks can cause severe cracks during sputtering because of the thermal stresses that are created. In the target material according to the invention, the micro-cracks present in the target material are stopped at the interface target, material / pore by a large number of small pores. In this way, further growth of the cracks due to thermal stresses generated during sputtering is stopped. Crack growth is also inhibited by the usual splat-like structure of thermal spray: cracks are mostly propagated at the interface between two splats and further propagation can be prevented by another overlapping splat. . In addition, sputter targets having a target material with a small pore size are accepted to exhibit a more stable sputter process as compared to sputter targets with a target material with a large pore size. The latter can result in gas release during sputtering.

According to a second aspect of the invention, a sputter target is provided comprising a target holder and a target material. The target material comprises indium-tin-oxide and is sprayed onto the target holder. The target material has a porosity of at least 2%. More preferably, the target material has a porosity of at least 4%, for example 10% or 20%.

Preferably, less than 20% of the pores formed in the target material are closed pores. More preferably, up to 10% or even up to 5% of the pores formed in the target material are closed pores.

Preferred sputter targets according to the present invention include rotatable sputter targets such as tubular sputter targets.

Indium-tin-oxides according to the invention preferably have a hardness of 200-400 HV.

The target material of the indium-tin-oxide target preferably has pores having an average pore size between 1 μm ² and 1000 μm ² , more preferably between 6 and 80 μm ² , for example between 6 and 40 μm ² . Have Preferably, 50% of the pores have a pore size of 10 μm ² or less. In this case, large amounts of small pores distributed in the target material can stop the growth of cracks.

According to another aspect of the invention, a method is provided for coating a substrate surface with indium-tin-oxide while sputtering from the sputter target described above. This method allows to avoid or reduce the formation of cracks in the target material.

The use of the target material according to the invention allows the knoll power density to be obtained during sputtering. The power density is for example 6 W / cm ² More than a race-track area, for example 8 W / cm ² More than race track area. Even at these high power densities no cracks were created during the sputter process.

Which heat sprayed indium-tin-oxide target (Table 1) was compared with any indium-tin-oxide target (Table 2) obtained by hot isostatic pressure. The sputter targets shown in Table 1 are all 5.8-6.6 g / cm ³ Has a density between. The sputter targets shown in Table 2 all have porosities between 0.5-1.8%.

TABLE 1

Examples of thermally sprayed indium-tin-oxide sputter targets

Embodiment according to the present invention       Porosity (%)         Hardness (HV)             One         16.1          186             2         14.1          228             3         12.2          221             4         14.0          249             5         12.1          262             6         13.3          251             7          5.7          249             8          3.9          244

TABLE 2

Examples of indium-tin-oxide sputter targets obtained by hot isostatic pressure

          Example Density (g / cm ³ )          Hardness (HV)             9          6.85          487            10          6.8          488            11          6.7          490            12          6.99          486            13          7.00          500

From Table 1 and Table 2, the thermally sprayed target shows higher porosity, lower density and lower hardness than the indium-tin-oxide target obtained by hot isotropic pressure.

A thermally sprayed tubular rotatable indium-tin-oxide target having a length of 1850 mm was used in the sputter process. Sputter tests were performed at power levels below 44 kW without crack formation. There was no crack even at a power level of 50 kW.

Claims (18)

A method for reducing thermal stress at a sputter target during sputtering: Providing a target holder; Spraying a target material comprising indium-tin-oxide onto the target holder and introducing a void into the target material while applying the target material onto the target holder, the void to reduce thermal stress. At least 2% porosity in the applied target material. The method of claim 1, wherein the target material has a porosity of at least 4%. The method of claim 1 or 2, wherein at least 20% of the pores formed in the target material comprise closed pores. The method of any one of the preceding claims, wherein at least 100% of the pores formed in the target material comprise closed pores. The method of claim 1, wherein the sputter target comprises a rotatable sputter target. The method of claim 1, wherein the target material has a hardness between 200-400 HV. The method of claim 1, wherein the pores of the target material have a size in the range of 1-1000 μm ² . The method of claim 1, wherein 50% of the pores have a pore size of 10 μm ² or less. A target material comprising a target holder and an indium-tin-oxide, said target material being sprayed on said target holder, said target material having a porosity of at least 2%. 10. The sputter target of claim 9, wherein said target material has a porosity of at least 4%. The sputter target of claim 9 or 10, wherein at least 20% of the pores formed in the target material comprise closed pores. 12. The sputter target of any one of claims 9-11, wherein at least 10% of the pores formed in the target material comprise closed voids. 13. The sputter target of any one of claims 9 to 12, wherein the sputter target comprises a rotatable sputter target. The sputter target according to any one of claims 9 to 13, wherein the target material has a hardness between 200-400 HV. 15. The sputter target of any one of claims 9-14, wherein the pores of the target material have a size in the range of 1-1000 μm ² . 16. The sputter target of any one of claims 9-15, wherein 50% of the pores have a pore size of 10 μm ² or less. A method for coating a substrate surface with indium-tin-oxide while sputtering from the sputter target as defined in any one of claims 9 to 16, wherein there is no crack in the target material of the sputter target. 18. The method of claim 17, wherein the sputtering is 6 W / cm ² Method performed at power density higher than race track area.