Patent Application
20090162647
1. Field
Embodiments of the present invention relate to an erosion-resistant semiconductor processing component which includes an yttrium metal substrate. A method of fabricating such a component is also described.
2. Background
This section describes background subject matter related to the disclosed embodiments of the present invention. There is no intention, either express or implied, that the background art discussed in this section legally constitutes prior art.
Corrosion (including erosion) resistance is a critical property for apparatus components and liners used in semiconductor processing chambers, where corrosive environments are present. Although corrosive plasmas are present in the majority of semiconductor processing environments, including plasma enhanced chemical vapor deposition (PECVD) and physical vapor deposition (PVD), the most corrosive plasma environments are those used for cleaning of processing apparatus and those used to etch semiconductor substrates. This is especially true where high-energy plasma is present and combined with chemical reactivity to act upon the surface of components present in the environment.
Process chamber liners and component apparatus present within the processing chambers used to fabricate electronic devices and micro-electro-mechanical systems (MEMS) are frequently constructed from aluminum and aluminum alloys. Surfaces of the process chamber and component apparatus (present within the chamber) are frequently anodized to provide a degree of protection from the corrosive environment. However, the integrity of the anodization layer may be deteriorated by impurities in the aluminum or aluminum alloy, so that corrosion begins to occur early, shortening the life span of the protective coating. The plasma resistance properties of aluminum oxide are not positive in comparison with some other ceramic materials. As a result, ceramic coatings of various compositions have been used in place of the aluminum oxide layer mentioned above; and, in some instances, have been used over the surface of the anodized layer to improve the protection of the underlying aluminum-based materials.
Yttrium oxide is a ceramic material which has shown considerable promise in the protection of aluminum and aluminum alloy surfaces which are exposed to halogen-containing plasmas of the kind used in the fabrication of semiconductor devices. An yttrium oxide coating has been used and applied over an anodized surface of a high purity aluminum alloy process chamber surface, or a process component surface, to produce excellent corrosion protection (e.g. U.S. Pat. No. 6,777,873 to Sun et al., mentioned above). The protective coating may be applied using a method such as spray coating, physical vapor deposition (PVD) or chemical vapor deposition (CVD) by way of example.
The substrate base material of the chamber wall or liner, of an apparatus component may be a ceramic material (Al2O3, SiO2, AlN, etc.), may be aluminum, or stainless steel, or may be another metal or metal alloy. Any of these may have a sprayed film over the base material. The film may be made of a compound of a III-B element of the periodic table, such as Y2O3 The film may substantially comprise Al2O3 and Y2O3. A sprayed film of yttrium-aluminum-garnet (YAG) has also been mentioned. Examples of a sprayed film thickness range from 50 μm to 300 μm.
There have been problems with aluminum and aluminum alloys which have been spray coated with an yttrium oxide-comprising film to provide corrosion and erosion resistance. While the yttrium oxide-comprising film surface is more corrosion and erosion resistant than the surface of aluminum, or aluminum alloys, or anodized aluminum, the resistance is significantly less than that of a solid yttrium oxide sintered component. However, the electrical conductivity of a solid, sintered yttrium oxide chamber liner or component may be a disadvantage in instances where a conductivity in the range of aluminum is desired. The mechanical properties of the solid, sintered yttrium oxide are a disadvantage compared with aluminum, which is not so brittle; for example, aluminum offers a better tensile strength, yield strength, and flexural strength.
There is a need in the semiconductor industry for improved materials which provide both a highly corrosion and erosion resistant surface, while providing electrical and mechanical properties which are competitive with aluminum.
So that the manner in which the exemplary embodiments of the present invention are attained is clear and can be understood in detail, with reference to the particular description provided above, and with reference to the detailed description of exemplary embodiments, applicants have provided illustrating drawings. It is to be appreciated that drawings are provided only when necessary to understand the invention and that certain well known processes and apparatus are not illustrated herein in order not to obscure the inventive nature of the subject matter of the disclosure.
Embodiments of the present invention pertain to specialty yttrium metal and yttrium metal alloy substrates having an yttrium oxide-comprising coating formed on the metal surface by a thermal oxidation process. Such materials can be used in processing environments of the kind encountered in the production of semiconductor and MEMS devices. In one aspect, the metal yttrium and the yttrium oxide produced by thermal oxidation possess similar thermal expansion coefficients, compared with the base substrate metal, and this provides an improved interface between the coating and the underlying substrate. As a result, less stress is created between the metal yttrium and the yttrium oxide coating, and the performance lifetime of the component is increased. In one aspect, the metal yttrium and the yttrium oxide produced by thermal oxidation also exhibit similar thermal conductivity, and that improves the temperature uniformity of the entire component during semiconductor and MEMS processing operations.
It has been determined by experimentation that the yttrium oxide coating produced by the thermal oxidation process in accordance with the present invention, is placed in compression by the underlying metal substrate, when the substrate is cooled after the thermal oxidation process. Due to this compression, the porosity of the oxide coating is reduced as the distance from the coating surface toward the underlying substrate increases. This is in combination with a strong interface which tends not to crack or create voids of the kind which occur when a yttrium oxide coating is spray coated over an aluminum substrate, for example.
A surprising advantage of the creation of a yttrium oxide coating by thermal oxidation is the ability to form a thicker coating than those which can be obtained by an anodization process, for example. A coating thickness of about 225 μm or greater, for example, can be obtained by thermal oxidation of a yttrium metal substrate at 750° C. for a time period of as little as 15 minutes, depending on the doping content of the yttrium metal. This compares with a maximum anodized yttrium oxide coating of about 8 μm-9 μm.
Embodiments of the present invention enable an ability to form a thicker coating than those known in the prior art. For example, a coating having a thickness ranging from 1 μm to about 500 μm, for example, and typically from about 10 μm to about 400 μm, depends on the temperature profile used during the thermal oxidation process. In one embodiment, an advantageous temperature profile is one where the yttrium metal (including metal alloys) is rapidly heated initially, to a desired maximum temperature, the substrate is held at the maximum temperature for a time period, and then the temperature of the substrate is gradually decreased until a point at which the formation rate of the yttrium oxide is essentially stopped.
This temperature profile is based on the discovery by the inventors that it is advantageous to have a large grain size of the yttrium oxide crystal at the top surface of the forming yttrium oxide coating, as this permits better oxygen movement into the yttrium metal substrate. Further, it is advantageous to have a small grain size of yttrium oxide crystal at the interface with the yttrium metal substrate, to provide stability at the interface. In one of the embodiments of the present invention, the temperature profile during the thermal oxidation process is set to achieve a large grain size of the yttrium oxide crystal at the top surface of the forming yttrium oxide coating. In another embodiment, the temperature profile during the thermal oxidation process is set to provide a small grain size of yttrium oxide crystal at the interface with the yttrium metal substrate.
In one embodiment, the yttrium metal substrate is rapidly heated (at a rate of about 25° C./min, for example and not by way of limitation) to a maximum temperature in the range of about 700° C. to about 1050° C. The component part being fabricated is then held at the maximum temperature for a time period during which large grain crystals are grown. This time period ranges in length from about 1% to about 5% of the time period during which the smaller grain yttrium oxide crystals are grown. The smaller crystals grow beneath the larger crystals, due to the mechanism of crystal formation, which is described subsequently herein. The smaller grain crystals of yttrium oxide which form during the temperature decrease become increasingly smaller as the process temperature drops, until the temperature reaches about 400° C. At lower temperatures, the yttrium oxide crystal growth is extremely slow.
The surprising ability to form thick yttrium oxide coatings with an average thickness in the range of up to about 225 μm or greater, for example, may be attributed to the specialized grain boundary structure of the yttrium oxide crystals which are formed, as this provides a channel for oxygen penetration as the oxidation process continues. There are two kinds of diffusion approaches of oxygen from the surface to the interface between oxide and metal. One is grain boundary diffusion and the other is grain diffusion (lattice/void). By doping trace elements into yttrium metal, or by controlling the oxygen partial pressure in the thermal oxidation process atmosphere, the priority diffusion approach can be controlled. For example, through doping of trace elements into high purity yttrium metal, the grain boundary diffusion can be adjusted to a faster oxygen diffusion path, and a given, desired yttrium oxide (yttria) layer thickness on a yttrium metal substrate can be obtained, where the thickness depends basically on thermal oxidation time. In comparison with grain boundary diffusion, the grain diffusion (lattice/void) yttrium oxide growth rate is slow. However, the grain diffusion rate (void diffusion) can be improved by optimizing the oxygen partial pressure in the thermal oxidation atmosphere, which leads to the formation of more oxygen vacancy in the yttria layer.
Typically the amount of dopant added to the yttrium metal substrate or to an alloy of yttrium metal substrate is less than about 0.1 weight percent. When dopant (trace elements) are added to the yttrium metal or yttrium metal alloy substrate, the amount of oxygen which is present in the oxidation ambient environment may range from about 5% by volume to about 25% by volume. An oxidation ambient environment which works particularly well contains about 10% by volume oxygen. When dopant is not added to the yttrium metal or yttrium metal alloy substrate, the amount of oxygen which is present in the oxidation ambient environment may range from about 5% by volume to 100% by volume. Air, which is about 21% by volume oxygen works well. This is because, when the partial pressure of oxygen in the ambient environment is lower, the oxygen tends to infuse faster into the yttrium metal or yttrium metal alloy; however, the rate of oxide formation is slow. When the partial pressure of oxygen is higher, the oxygen infuses slower, but there is more oxygen available, so the rate of oxide formation is higher. By adding dopant, and using a lower partial pressure of oxygen in the ambient environment, it is possible to obtain a desirable smaller grain size crystal at the interface of the oxide with the substrate to still have an acceptable oxide formation rate for manufacturing.
At the lower oxidation temperatures, the reaction rate of the oxygen with the metal to form an oxide is slowed. Further, once the oxidation process is completed, and the component is returned to room temperature, or while the component is under ambient semiconductor or MEMS processing conditions (at temperatures typically lower than about 400° C.) the yttrium oxide is under compression applied by the underlying yttrium metal substrate. The yttrium oxide grain size in the area of the interface with the yttrium metal can be tailored so that the amount of compression is sufficient to substantially reduce (essentially prevent) semiconductor processing reactive species from traveling down the yttria crystal to the surface of the yttrium metal substrate. At the same time, the amount of compression is not so large that it causes a fracture or separation of the yttrium oxide from the surface of the yttrium metal substrate.
As a preface to the detailed description, it should be noted that, as used in this specification and the appended claims, the singular forms “a”, “an”, and “the” include plural referents, unless the context clearly dictates otherwise.
When the word “about” is used herein, this is intended to mean that the nominal value presented is precise within ±10%.
To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. It is to be noted that the appended drawings illustrate only exemplary embodiments of the invention where a drawing would be particularly helpful in understanding the embodiment. Not all embodiments require a drawing for understanding, and therefore the drawings are not to be considered as limiting of the scope of the invention, for the invention may admit to other equally effective embodiments.
In one embodiment a specialty yttrium metal and yttrium metal alloy substrates having a yttrium oxide-comprising coating formed on the metal surface by a thermal oxidation process is created for use in processing environments of the kind encountered in the production of semiconductor and MEMS devices. The metal yttrium and the yttrium oxide produced by thermal oxidation possess similar thermal expansion coefficients, which provides an improved interface between the coating and the underlying substrate. The metal yttrium and the yttrium oxide produced by thermal oxidation exhibit similar thermal conductivity, and that improves the temperature uniformity of the entire component during semiconductor and MEMS processing operations. As a result, less stress is created between the metal yttrium and the yttrium oxide coating, and the performance lifetime of the component is increased.
Table One, below, shows a property comparison of metal yttrium with other metal materials which have been used to fabricate semiconductor processing apparatus components. One of skill in the art can, in view of this table, see the advantages and disadvantages of using metal yttrium relative to the other materials listed. In terms of mechanical properties, the yttrium metal is similar to a number of the other materials which are used as base substrate materials for semiconductor processing equipment. The electrical resistivity of the yttrium metal is very high compared with aluminum or the aluminum alloy, but is better than HASTALLOY® C276, the Titanium alloy, and SST316. The electrical resistivity of the yttrium oxide shown in Table Two can be reduced by adding a dopant material such as zirconium oxide, hafnium oxide, scandium oxide, niobium oxide, samarium oxide, ytterbium oxide, erbium oxide, cerium oxide, neodymium oxide, terbium oxide, dysprosium oxide, and combinations thereof to the yttrium metal prior to oxidation.
Table Two provides a property comparison between metal yttrium and yttrium oxide. It is readily apparent that the difference in coefficient of linear expansion between the two materials is sufficient to place a yttrium oxide layer, which is on the surface of the metal yttrium, in compression. At the same time, it has been determined that this difference in coefficient of expansion does not cause problems at an interface between the two materials when a layer of yttrium oxide is thermally created upon the surface of an yttrium metal substrate.
When metal yttrium is converted to yttrium oxide by thermal oxidation, two yttrium metal cells (each of which contains 8 yttrium atoms, for a total of 16 yttrium atoms) transforms into one yttrium oxide cell (which comprises Y2O3 and contains a total of 16 yttrium atoms and 24 oxygen atoms). While this is a volume expansion process, the expansion percentage is smaller than that of the transformation from aluminum to Al2O3. When 2 moles of metal yttrium are converted to 1 mole of Y2O3 by thermal oxidation, the volume increases 5.140 cm3. When 2 moles of metal yttrium are converted to 1 mole of Y2O3 by thermal oxidation, the weight increases by 2.858 g. Table Three, below, shows the theoretical calculation of the crystal structure of metal yttrium and yttrium oxide.
As can be seen from the above calculations, the Y2O3 is in a state of compressed stress.
Another surprising result was that the rate of oxide formation was relatively linear. By way of theory and not by way of limitation, it appears as if the oxygen atoms are moving along grain boundaries, progressing down the crystalline structure. In this manner, the oxygen continues to travel downward through the grain boundaries so that the growth is constantly from the top of the structure. This is distinct from an anodization process, for example, which appears to be diffusion limited, and to require that the chemical reactant diffuse down into the crystalline structure to react at the base of the previously-formed reacted material. In the case of anodization, the typical maximum thickness for an anodized layer is about 8 μm.
The substrate used in this Example One was a trace element doped yttrium metal, where the trace metal content was smaller than 0.1 weight %. (In general the amount of dopant added to the yttrium metal or yttrium metal alloy will be less than about 0.1 weight %.) In particular, 20 ppm of magnesium, 100 ppm of aluminum, 200 ppm of copper, and 500 ppm of calcium was added to pure yttrium metal to produce the substrate. Through this doping, the grain boundary structure of the yttrium oxide layer formed will be adjusted as an oxygen diffusion path, providing a high diffusion rate for oxygen from the surface of the oxide toward the interface between the oxide and the underlying metal substrate. The relationship between the oxide layer thickness and the thermal process time is a linear relationship. In general, the advantageous concentration ranges for these dopants are: magnesium 10 ppm by weight to 30 ppm by weight; aluminum 10 ppm by weight to 110 ppm by weight; copper 50 ppm by weight to 300 ppm by weight; and, calcium 10 ppm by weight to 800 ppm by weight.
The specimen was placed in a thermal oxidation furnace, Model No. 55031-12, available from Applied Materials, Inc. of Santa Clara, Calif. The ambient environment in the furnace was air. Other gas compositions, where the oxygen content is lower or higher than the 21% by volume of air may be used as well. As previously discussed, the oxygen content may range from about 5% by volume up to 100% by volume. When dopant is used, the oxygen infuses faster and the oxygen content may range from about 5% by volume to about 15% by volume.
The time-temperature profile illustrated in
As previously described, the crystal growth of the yttrium oxide tends to occur by transfer of the oxygen atoms down the yttrium oxide structure toward the yttrium metal crystalline interface. As a result, the growth of the cubic yttrium oxide crystals is constantly taking place at the surface of the yttrium metal crystalline structure. Thus, if the temperature of reaction is high initially and then lower subsequently, the smaller cubic yttrium oxide crystals will be present at the interfacial surface of the yttrium metal with the yttrium oxide, to reduce the strain between the cubic yttrium metal substrate and the overlying hexagonal yttrium oxide layer.
In general, the metal or metal alloy substrate may be heated at a rapid rate ranging from about 20° C./min to about 100° C./min, to increase the temperature of the substrate from room temperature to a holding temperature, which may range from about 700° C. to about 1050° C. The substrate temperature may be held at the holding temperature for thermal oxidation over a time period ranging from about 5 minutes to about 5 hours, to provide for a rapid initial growth of large grained oxide crystals. Subsequently, the temperature of the substrate may be lowered more slowly, at a rate ranging from about 10° C./min to about 0.5° C./min down to a temperature of about 400° C. or lower, to permit the growth of smaller grain oxide crystals. Once the temperature of the substrate is in the range of about 400° C. or lower, the substrate may be cooled more rapidly at a cooling range ranging from about 25° C./min to about 35° C./min to bring the substrate back to ambient temperature.
In the past, when a layer of yttrium oxide was created using an electrolysis process, the maximum thickness of the yttrium oxide obtained on a yttrium metal surface, was in the range of about 5.5 μm to about 8 μm, for example, because the reaction slows substantially as the oxide thickness increases. The present method is not so limited, and the yttrium metal substrate treated in the manner described above and illustrated in
The profile may be adjusted to account for a change in composition of the yttrium metal-comprising substrate. For example, the substrate may not be a pure metal, but may contain another element, and the metal may be selected from the group consisting of Nd, Sm, Tb, Dy, Er, Yb, Sc, Hf, and Nb, or combinations thereof, by way of example and not by way of limitation. The amount of other elements which may be present in the yttrium metal-comprising substrate may vary from 0% by weight up to about 50% by weight. Further, other metals may be alloyed with one of the metals of the kind listed above to provide improved mechanical or electrical properties. For example, aluminum may be alloyed with any of the metals, or combinations of metals listed above.
The general shape of the advantageous time temperature profile for the thermal oxidation process remains essentially the same for yttrium alloys as the 106, 108, 110, 112 portions of the graph shown in
While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised in view of the present disclosure, without departing from the basic scope of the invention, and the scope thereof is determined by the claims which follow.
The present application is related to the following applications: application Ser. No. 11/796,210, of Sun et al., filed Apr. 27, 2007, titled: “Method of Reducing The Erosion Rate Of Semiconductor Processing Apparatus Exposed To Halogen-Containing Plasmas”, which is currently pending; U.S. application Ser. No. 11/796,211, of Sun et al., filed Apr. 27, 2007, titled: “Method And Apparatus Which Reduce The Erosion Rate Of Surfaces Exposed To Halogen-Containing Plasmas”, which is currently pending; and, U.S. application Ser. No. 11/890,156 of Sun et al., filed Aug. 2, 2007, titled: “Plasma-resistant Ceramics With Controlled Electrical Resistivity”, which is currently pending. The subject matter of all of these patents and applications is hereby incorporated by reference into the present description.
