1. Field of the Invention
The present invention pertains to a surface structure and a method for preparation of the structure. The surface structure enhances the adhesion of a dielectric material such as a plasma sprayed ceramic which makes up a portion of the surface structure. Preferred applications for the surface structure include semiconductor processing apparatus such as process chamber interior surfaces and the surfaces of functional elements used within the chamber, such as the electrostatic chuck used to hold a semiconductor substrate in place within the chamber. The fabrication of a process chamber interior or the upper surface of an electrostatic chuck typically would include the preparation of an aluminum surface, followed by application (e.g. plasma-spraying) of a dielectric material (e.g. alumina or alumina/titania) upon the prepared surface.
2. Brief Description of the Background Art
U.S. Pat. No. 5,350,479 to Collins et al. issued Sep. 27, 1994, and hereby incorporated by reference, describes an electrostatic chuck for holding an article (typically a semiconductor substrate) to be processed in a plasma reaction chamber. The electrostatic chuck includes a metal pedestal coated with a layer of dielectric material which contains a system for distributing a cooling gas upon the upper surface of the dielectric material. The cooling gas contacts the bottom of an article supported upon the electrostatic chuck.
U.S. Pat. No. 5,315,473 to Collins et al., issued May 24, 1994, and hereby incorporated by reference, describes methods of improving the clamping force of the electrostatic chuck, among other features. In particular, the composition of the dielectric material and the thickness of the dielectric layer are among the critical factors in determining the clamping force. Since it is not yet practical to produce a dielectric layer which is totally flat, there are spacial gaps to be overcome. Generally, the thinner the dielectric layer, the greater the clamping force, all other factors held constant. However, there are practical limitations which limit the ultimate thickness of the dielectric layer. For dielectric layers approximately 1 mil or less in thickness, it has been found that the dielectric material breaks down and loses its insulating properties at voltages required to overcome the spacial gaps between the semiconductor substrate and the upper surface of the electrostatic chuck.
European Patent Application No. 93309608.3 of Collins et al., published Jun. 14, 1994, and hereby incorporated by reference, describes the construction of an electrostatic chuck of the kind disclosed in U.S. Pat. No. 5,350,479 referenced above. The construction of the electrostatic chuck includes grit blasting of the aluminum pedestal, followed by spraying (e.g. plasma-spraying) a dielectric material such as alumina or alumina/titania upon the grit-blasted surface of the aluminum pedestal. Typically the sprayed dielectric thickness is greater than the desired final thickness, by about 15-20 mils (380-508 microns). After the dielectric material has been applied, the thickness is reduced by grinding until it has the desired final thickness, for example, of about 7 mils (180 microns). The upper surface of the dielectric layer is then processed to provide a pattern of heat transfer gas distribution grooves over the surface of the layer. Perforations are created through the dielectric layer to connect the heat transfer gas distribution grooves with gas distribution cavities contained in the pedestal of the electrostatic chuck.
Fabrication of the dielectric layer of the electrostatic chuck creates stresses at the interface between the dielectric layer and the underlying conductive (typically aluminum) pedestal of the electrostatic chuck. Such fabrication includes: obtaining a precise dielectric layer thickness by grinding; and forming cooling gas distribution grooves in the surface of the dielectric layer using laser micro machining, using a grinding wheel, or by grit blasting through a mask. Further, during semiconductor processing operations, the temperature in the process chamber in which the electrostatic chuck is used may reach as high as 350.degree. C. Since the linear thermal expansion coefficient for an electrostatic chuck aluminum pedestal is about 26.times.10.sup.-6 mm/mm/.degree. C. at about 300.degree. C. compared with about 4 to 8.times.10.sup.-6 mm/mm/.degree. C. for a typical dielectric material such as alumina, the process cycle repeatedly creates stresses at the interface between the aluminum pedestal and the overlying dielectric layer. Such stresses can create pockets of delamination or separation between the dielectric material and the underlying aluminum pedestal surface.
There is a potential for arcing or glow discharge in any open spaces within the electrostatic chuck dielectric layer under high density plasma processing conditions. In a high electron density plasma (HDP), RF energy is electromagnetically coupled into the "source" region of a plasma chamber to generate and maintain the high electron density of the plasma. In addition, RF "bias" energy is generally capacitively coupled in the plasma, through the article being processed, to direct the ion field toward the article (typically a semiconductor substrate). In particular, when the electrostatic chuck is subjected to high power RF fields and high density plasmas immediately above the semiconductor substrate, it is possible to have breakdown of the cooling gas in contact with the dielectric layer due to arcing or glow discharge. Further, since there is frequently a direct, straight line path between the semiconductor substrate supported on the upper, dielectric surface of the electrostatic chuck and an underlying conductive layer of aluminum which forms the pedestal of the electrostatic chuck, arcing can occur along this path. Arcing or glow discharge at the surface of the semiconductor substrate can result in loss of the substrate. In addition, arcing or glow discharge within the gas distribution holes or any other spacial pockets within the dielectric layer, or between the dielectric layer and the underlying aluminum pedestal deteriorates the dielectric layer and underlying aluminum layer, greatly reducing the productive lifetime of the electrostatic chuck.
In light of the above, it is clear that the structure of the interfacial contact between the dielectric layer, such as a plasma sprayed alumina or alumina-titania, and the aluminum pedestal is critical to the performance of the electrostatic chuck and to its productive lifetime. The present invention provides an improvement in bonding of a dielectric material, such as a plasma sprayed ceramic material, to an underlying surface such as a metallic electrostatic chuck pedestal. This improvement in bonding reduces the possibility of delamination at the interface between the two materials and avoids creation of spacial pockets which lead to the deterioration of the electrostatic chuck.