This invention relates to the control of the temperature of chamber shields and other components within the chambers of machines for the processing of semiconductor wafers and other substrates, and particularly to the control of particle generation from such components due to thermal changes or cycling.
In semiconductor technology control of particle levels has become crucial to achieving high yields and maximizing profit from the use of processing equipment. With the trend toward smaller and smaller features and more complex devices, a few particles or even a single particle on the surface of a wafer being processed can result in a fatal defect being produced in a device. One chief source for such particles is the contaminating films that are formed on the surfaces of chamber shields and other components of a processing chamber from repeated processing of wafers in the chamber. Such contaminating films adhere to such components only to flake off of the components as the components expand and contract due to thermal changes. Continuous thermal cycling of the components combined with constant buildup of film on the component surfaces produces particles that contaminate the chamber to light upon and damage the devices on the surfaces of substrates being processed within the chambers.
In light of this, it is highly desirable to control both the absolute temperature and thermal excursion of shields and other parts used inside the process module itself. In particular, when the part is exposed to a heat source during processing of a wafer, its temperature rises during the processing and falls during the exchange of wafers. However, in a continual stream of wafer processing, the minimum temperature of the part continues to rise until a steady state is achieved. In a processing module whose function is to deposit a material onto the wafer, stress between the part and material deposited on the part grows as the temperature rises, which often results in flaking of the deposit. Flaking can be caused by the deposit microstructure as well as the thermal expansion mismatch between the part and deposit. Stainless steel is the material of choice for many consumable parts used in semiconductor processing applications because of its ability to be recycled many times, its ability to withstand oxidation, its strength, and the ease of cleaning stainless steel parts.
Although stainless steel is cited as an example, any material, metal or non-metal, that has suitable mechanical properties and is compatible with the processing environment may be used. Stainless steel is a poor thermal conductor. Where it is desired or required that a part be large, control of temperature across the entire part is difficult, even if heat sinking is used, and substantial thermal gradients can develop across the part. Where stainless steel is exposed to a large heat flux during wafer processing, particularly in a low pressure environment, control of temperature rise over the entire area of the part is difficult, even when one end of the shield has good a thermal connection to a heat sink. This is typical because providing a heat sink along the entire length of a part is usually impractical.
Aluminum is a good thermal conductor and can be used for many chamber parts, but for parts such as chamber shields, its softness requires the shield to be much thicker than where stainless steel is used. Alloying to increase the strength of aluminum significantly reduces its thermal conductance. For example, 6061-T6 Al, a common alloy, has only 70 to 80% of the thermal conductance of pure aluminum. In addition, for tools that deposit metals, acid is typically used for cleaning during the shield recycle process, and the aluminum of the shield itself etches along with the deposited material, severely reducing shield life. Magnesium, also a good thermal conductor, has the same mechanical and recycling problems as aluminum. Beryllium has an excellent strength to weight ratio, has a thermal conductivity similar to that of aluminum, and resists etching in some acids, but unfortunately costs about four times the price of silver. The mechanical properties of other materials that are reasonable thermal conductors, such as tungsten or molybdenum, make them expensive to process. The softness of copper, silver, and gold which are the best thermal conductors, require parts to be thick so the part does not warp under its own weight. The added thickness required causes large parts to exceed ergonomic weight limits given the large mass of differences between these materials and aluminum. Additionally, the raw material cost of gold and silver limits their use to very small parts.
Accordingly, there remains a need for more effective and efficient control of the temperature of processing chamber parts.
According to principles of the present invention, chamber parts are clad with a material having a higher thermal conductivity than that of the material of which the base part is made. The cladding is configured to promote control the base part's temperature.
More particularly, in accordance with preferred embodiments of the invention, a processing chamber part is provided with a cladding layer or layers having a substantially higher thermal conductivity than that of the part, which itself could be composed of multiple layers. The cladding material preferably has the highest practical thermal conductivity, has a low raw material cost, and is able to be applied to the base shield material economically, to facilitate the reconditioning and recycling of the part.
The Noble metals copper, silver, and gold have the highest thermal conductivity of all metals. Of these three, copper has the lowest raw material cost by a wide margin. Although copper is the most economical cladding, the invention is not limited to this material, which need not even be a metal.
In preferred embodiments, the coating is on the order of 0.5 to 2 millimeters thick, and at least approximately 1 mm thick. It is preferable, but not absolutely necessary, for the cladding to be of high purity and density.
A preferred method for applying a thick pure cladding is the cold spray technique, which is a commercially available process. The basis of the technique is thermal evaporation of the cladding material in an inert ambient and using the inert ambient gas to carry the evaporated cladding material vapor to the part. The inert ambient gas, which is typically argon, prevents oxidation of the evaporated cladding material. With a low deposition rate and lack of clusters, a dense, pure coating results. Cladding purity exceeding 99% can be achieved with this technique.
Other methods of applying claddings exist, and the invention is not limited to application by cold spray. Any effective cladding technique may be used. Electroplating and twin wire arc spray (TWAS) are examples.
The base part, clad according to the present invention, has the ability to be recycled many times and has enough mechanical strength for a thin shell to resist deformation under its own weight. Further, the part is formed of materials having compatibility with the processing environment.
Although the invention is most effective for large parts, it covers use of claddings for temperature control of small parts as well. While the invention is particularly useful for base parts formed of metal, it is not limited to metal and can apply to non-metals as well, and combinations of metals and non-metals.
A cladding of high thermal conductivity, applied to a poor thermal conductor base layer, according to the invention, significantly reduces the temperature of the composite part in regions far from where the part is connected to a heat sink.
The cladding, according to certain embodiments of the invention, is formed of a Nobel metal at least 0.5 mm thick. Where the part is a hollow refractory metal cylindrical chamber shield, the cladding may be located on the exterior of the hollow cylinder for temperature control of the composite part while retaining the conditioned inner surface of the refractory metal base layer to collect deposits. The interior of the part may itself have a coating whose purpose is to improve adhesion of any subsequent deposition received in a processing module or environment.
In a preferred embodiment of the invention, the Noble metal is copper, the base refractory material is a thin shell of stainless steel, and the interior coating, if applied, is twin wire arc spray aluminum. The choice of stainless steel provides for structural integrity of the composite part, the ability to recycle the base material, and ease of cleaning of the base material.
Another preferred embodiment of the invention is a Nobel metal coating on the interior of the hollow refractory metal cylinder for temperature control of the composite part and improved adhesion of any subsequent deposition from the processing chamber.
Reduced particle production results from a composite part made from a base or substrate material having a low thermal conductivity material that is coated with a material of higher thermal conductivity. The improved temperature control provides for reduced shedding of any subsequently deposited layer.
The invention is especially useful for controlling the temperature of chamber shields that are used to protect chamber walls from deposits in deposition and etching machines, including physical vapor deposition (PVD) modules, chemical vapor deposition (CVD) modules, and etching chambers, where exposure to heat flux may expand the component or portions of the component, causing particle flaking or other problems. The invention also is generally useful in controlling the temperature of other processing chamber components that have a tendency to expand when exposed to heat flux, even where the component is not exposed to material deposits, thereby limiting potentially undesirable expansion or thermal deformation of such components.
These and other objectives and advantages of the present invention will be more readily apparent from the following detailed description.
A number of parts within the processing chamber 12 are exposed to the process being carried out within the chamber. Typically, many of these parts are placed in the chamber 12 for the purpose of shielding other surfaces from deposition. A chamber shield 20, which is one of several shields, is a part that is typically provided for this purpose. This shield 20, shown as a cylindrical barrel shield, is usually supported at one end 21 thereof on the chamber wall 11 or other intermediate structure 19, which may serve as a heat sink. Another end or portion 22 of the part 20 is usually free and unsupported, allowing for free thermal expansion.
In the course of a coating process, such as, for example, a sputter coating process, within the chamber 12, the center of the chamber 12, which contains an active plasma, is a heat source 30 (
In accordance with the invention, a cladding of high thermal conductivity material is applied to a poor thermal conductor to significantly reduce the temperature of the composite part in regions far from where the part is connected to a heat sink. As illustrated in
Both an analytical solution to the heat conduction equation and the results of thermal modeling support the effectiveness of the invention. Although the example is for a cylindrical part 20, the analysis is also applicable to parts of other shapes or sizes. In the illustrated example, a hollow bilayer cylinder part 20 is bolted to a ring 19 that makes intimate thermal contact to the chamber wall 11, which approximates an infinite heat sink. The interior of the cylinder formed by the base layer 25 is exposed to a constant, uniform heat flux from heat source 30. The general form of the heat conduction equation without sources or sinks for a position r is:
where ∇2 is the Laplacian operator and the variables T and t represent the temperature and time, respectively, ρ is the mass density, Cp is the specific heat at constant pressure, and k is thermal conductance. For the illustrated cylindrical shield 20, the inside of the base layer 25 is exposed to a constant, uniform heat flux source, q. The cladding 24 acts as a sink for the base layer 25 of the part 20, and conversely, the base layer 25 of the part 20 acts as a source for the cladding 24. The form of this term is h (Tp-Tc), where h is the heat flux transfer coefficient, assumed to be constant over the applicable temperature range, and the subscripts p and c refer to the base layer 25 of the part 20 and its cladding 25, respectively.
For the temperature range involved, the thermal conductivity of a metals can be taken as constant. The cylinder height is much larger than the width, essentially reducing the problem to propagation of heat in one dimension. The equations describing the thermal conductance for the part 20 having the base layer 25 and the cladding 24 along the length of the cylinder then reduces to Equation 2 for the part, and Equation 3 for the cladding, as follows:
The thermal conductance k (W/K) is used so that q is the power flux (W/m2).
The boundary conditions for the problem are
at x=0 (free end 22), where the part 20 is thermally floating, and T=0 at x=L (fixed end 21), where the part is connected to the infinite heat sink. The general solutions for Tp and Tc are:
It can be seen that in the limit of large h, where good thermal contact exists between the cladding 24 and base layer 25 of the part 20, and where kc>>kp, that Tp approaches Tc, and the temperature of the composite part 20 is controlled by the thermal conductivity of the cladding 24.
The invention has been described in the context of exemplary embodiments. Those skilled in the art will appreciate that additions, deletions and modifications to the features described herein may be made without departing from the principles of the present invention.