FIELD OF THE INVENTION 
     The present invention relates to an improved coating for parts used within semiconductor processing chambers, and more specifically to an intermediate coating that effectively adheres to both an underlying part and to a surface layer, despite differing thermal expansion rates between the part and the surface layer. 
     BACKGROUND OF THE INVENTION 
     During fabrication, a semiconductor device undergoes a variety of processing steps such as physical vapor deposition, chemical vapor deposition, etching and the like. Many of these processes, particularly etching, are performed within corrosive processing chamber environments (e.g., NF 3 , F, and/or O 3  at elevated temperatures). These corrosive environments may attack and corrode the various parts within the processing chamber, such as heaters, electrical coils, chamber walls, clamp rings, collimators, shields, etc. Therefore, processing chamber parts must be resistant to corrosive processing environments, so as not to degrade and possibly contaminate a semiconductor device being processed within the chamber. 
     Further, a number of corrosive fabrication processes involve the deposition of material layers on a semiconductor substrate. As material is deposited on a semiconductor substrate, it also deposits on processing chamber parts, and in turn may flake therefrom, contaminating semiconductor substrates processed therewithin. To prevent such contamination, chamber parts are periodically etched to remove deposited material. Thus, many processing chamber parts must be resistant to the deposited material&#39;s etchant, such that selective etching may occur. 
     Accordingly, when constructing a part for use within a semiconductor processing chamber, the selection of materials that exhibit not only favorable surface characteristics (e.g., corrosion resistance, or desired etch properties), but also exhibit favorable bulk characteristics (e.g., inexpensive, easily manufactured, good thermal conductivity, desirable magnetic properties, strength, etc.) presents a significant challenge. To broaden the universe of materials that may be used for processing chamber parts, coated parts are often employed wherein an underlying part is formed of a material that exhibits desired bulk characteristics, and a material that exhibits desired surface characteristics is applied thereto. While such coated parts somewhat ease the material selection process, coated parts can present a significant source of contamination when processing at elevated temperatures. Specifically, as the processing chamber thermally cycles (e.g., between various processing, cleaning or maintenance steps) so do the processing chamber&#39;s coated parts. If an underlying part and its coating differ in thermal coefficient of expansion, during thermal cycling the underlying part and the coating will expand and contract at different rates, resulting in stress therebetween (i.e., thermal stress). Such thermal stress can cause the coating material to flake from the underlying part, leaving the underlying part exposed to corrosive processing gases, and, moreover, introducing potential contaminants to the processing atmosphere. Thus, because of the need to match thermal coefficients of expansion, the practice of coating parts introduces additional constraints to the material selection process. 
     A need therefore exists for processing chamber parts which satisfy both the bulk characteristics and the surface characteristics required for a given processing environment, without introducing undesirable particles thereto. 
     SUMMARY OF THE INVENTION 
     The present invention significantly expands the universe of acceptable materials for semiconductor processing chamber parts by providing an intermediate coating between the underlying part and the surface layer. Specifically, the intermediate coating of the present invention comprises a plurality of material layers each having a coefficient of thermal expansion (CTE) between that of the underlying part, and that of the surface layer. The part&#39;s overall CTE therefore gradually transitions from the CTE of the underlying part to the CTE of the surface layer, reducing thermally induced stress and contamination associated therewith. Each layer within the intermediate coating may reduce thermal stress by the same amount, or by a varying amount. 
     The inventive intermediate coating enables an underlying part having both favorable bulk characteristics (e.g., inexpensive, easily manufactured, high thermal conductivity) and unfavorable surface characteristics (e.g., poor etch properties, unsuitable corrosion resistance) to be employed within a corrosive fabrication process, and broadens the universe of acceptable surface layers that effectively adhere to the underlying part. The inventive intermediate coating particularly benefits heaters exposed to corrosive environments, and process kit parts exposed to deposition material etchants during cleaning. However, the inventive intermediate coating may be advantageously applied to any part that undergoes thermal cycling. 
     Other objects, features and advantages of the present invention will become more fully apparent from the following detailed description of the preferred embodiments, the appended claims and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B are side elevational views of a semiconductor processing chamber part configured in accordance with the present invention, generally represented at an ambient temperature and at an elevated temperature, respectively; and 
     FIG. 2 is a diagrammatic side elevational view, in pertinent part, of a semiconductor processing chamber containing the coated semiconductor processing chamber part of FIGS.  1 A and  1 B. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1A is a side elevational view, generally representing a processing chamber part  11 , configured in accordance with the present invention. The processing chamber part  11  comprises an underlying part  13  having a first CTE, an intermediate coating  15  having an intermediate CTE, and a surface layer  17  having a second CTE. The underlying part  13  exhibits one or more bulk characteristics which are favorable, and one or more surface characteristics which are unfavorable, and the surface layer  17  exhibits at least one surface characteristic which is favorable, and which is unfavorably possessed by the underlying layer  13  (e.g., the underlying part  13  is corrosive in the processing environment and the surface layer  17  is not corrosive in the processing environment). The intermediate coating  15  is comprised of a plurality of intermediate layers  19   a-e , as shown in FIG. 1A, each of which has an intermediate CTE. As used herein, an intermediate CTE refers to a CTE value that falls within the range between the CTE value of the underlying part  13  and the CTE value of the surface layer  17 . 
     Preferably the CTE of each intermediate layer  19   a-e  falls within the range defined by the CTE of the layers on either side thereof. The overall CTE of the processing chamber part  11 , therefore gradually transitions between the first CTE and the second CTE, reducing the thermal stress that exists between any two adjacent layers, and reducing thermal stress induced particle generation. 
     Preferably, each layer within the intermediate coating  15  exhibits a thermal stress of (1/n)(x), where n is the number of intermediate layers and x is the thermal stress that would occur if the underlying part  13  and the surface layer  17  were in direct contact, with no intermediate coating  15  therebetween. 
     FIG. 1B is a side elevational view representing the part of FIG. 1A at an elevated temperature. FIG. 1B is useful for understanding how the inventive processing chamber part  11  reduces the selection criteria for each material layer. 
     For example, assume the underlying part  13  has a CTE of 7×10 −6/c , and the surface layer  17  has a CTE of 1×10 −6/c . The difference in CTE, in this example 6×10 −6/c , is proportional to the overall thermal stress that would exist between the underlying part  13  and the surface layer  17  if no intermediate coating  15  existed therebetween. Preferably, to gradually reduce the overall thermal stress, each intermediate layer  19   a-e  reduces the overall thermal stress by an equivalent amount, in this example by an amount proportional to 1×10 −6/c . To achieve equal thermal stress reduction, intermediate layer  19 a has a CTE of 6×10 −6/c , intermediate layer  19   b  has a CTE of 5×10 −6/c , intermediate layer  19   c  has a CTE of 4×10 −6/c , intermediate layer  19   d  has a CTE of 3×10 −6/c  and intermediate layer  19   e  has a CTE of 2×10 −6/c . Accordingly, during thermal cycling, stress between any two adjacent layers is proportional to 1×10 −6/c , ⅙ the stress that would exist in the absence of the intermediate coating  15 . 
     It will be understood the CTE values provided above are merely exemplary; materials with these exact CTE values may not exist. Similarly, any materials may be employed that have CTE values that result in an acceptable thermal stress value between adjacent layers; the differences in CTE values of adjacent layers provide above are merely exemplary. 
     As the processing chamber part  11  thermally cycles the part&#39;s various layers expand and contract between the positions represented in FIGS. 1A and 1B. At elevated temperatures the expansion of each layer increases from the underlying part  13  to the surface layer  17 , as shown in FIG.  1 B. Thus, thermal stress (represented in FIG. 1B by opposing arrows S 1  and S 2 ) between adjacent layers, and the particles generated thereby, is significantly reduced with use of the present invention. 
     Moreover, material selection is facilitated, as both the materials of the underlying part, and of the surface layer may be selected for their respective bulk, and surface characteristics, without regard for CTE matching. Thus, the present invention greatly increases the universe of acceptable materials for underlying parts and for surface layers, allowing semiconductor processing chamber parts to be easily tailored to meet the requirements of a given process. Similarly, with use of the present invention, materials for the intermediate layers  19   a-e  may be freely selected without regard for surface characteristics—the primary consideration for selection of an intermediate layer  19   a-e  being the desired CTE. 
     The processing chamber part  11  represents any number of processing chamber parts (e.g., process kit parts, heaters, chamber walls). For example, the underlying part  13  may be a heating layer (e.g., comprising aluminum or aluminum nitride), and the surface layer  17  may be magnesium fluoride, iridium, aluminum trifluoride, etc., each of which exhibits a favorable surface characteristic when employed as a heater coating within a semiconductor device processing chamber. It will be understood that in most instances, the thickness of the intermediate coating, and preferably the thickness of each of the intermediate layers therein, is minimal (i.e., only as thick as is needed to effectively reduce thermal stress). Intermediate coatings of minimal thickness reduce attenuation of the underlying part&#39;s favorable characteristics (e.g., heat transfer) and reduce material costs. The intermediate layers  19   a-e  may be formed by conventional methods (e.g., chemical vapor deposition, physical vapor deposition, plasma spray, diffusion bonding) as will be apparent to those of ordinary skill in the art. 
     FIG. 2 is a diagrammatic side elevational view of a processing chamber  21  used for chemical vapor deposition. The processing chamber  21  is operatively coupled to a source of a first gas  23 , and to a source of a second gas  25 . The processing chamber  21  contains a number of exemplary parts which may benefit from the present invention, such as a chamber liner  27 , an L-insert  29 , an isolator  31 , a clamp ring  33 , and a heater  35 . Other types of processing chambers, such as physical vapor deposition, chemical etch or degassing chambers may contain a number of parts which also benefit from the present invention (e.g., RF coils, collimators, gas distribution plates, shields, or shutters). 
     The foregoing description discloses only the preferred embodiments of the invention, modifications of the above disclosed apparatus and method which fall within the scope of the invention will be readily apparent to those of ordinary skill in the art. For instance, while the intermediate layers nearest the underlying part and/or nearest the surface layer may contain the underlying or surface material, other intermediate layers may not include the underlying and/or surface materials (i.e., may be exclusive of the underlying and/or surface materials). In fact, all of the intermediate layers may be exclusive of the underlying material and/or the surface material. Further, intermediate layers may be graded (i.e., have compositions which gradually transition from one material to the next) to further facilitate gradual transitioning of CTE. Accordingly, while the present invention has been disclosed in connection with the preferred embodiments thereof, it should be understood that other embodiments may fall within the spirit and scope of the invention, as defined by the following claims.
Improved semiconductor processing chamber parts are provided. An improved part is made of an underlying part having both an intermediate coating and a surface layer applied thereto. The intermediate coating includes a plurality of layers each having a CTE intermediate the CTE of the underlying part and the CTE of the surface layer. The intermediate coating reduces the stress between any two layers, allowing use of underlying parts and surface layers having dissimilar CTEs. The universe of acceptable materials for use within a semiconductor processing chamber is expanded, as fewer selection criteria exist for a given layer.