Ceramic electrostatic chuck and method of fabricating same

Multi-layered, ceramic electrostatic chuck for retaining a substrate in a process chamber is provided. The chuck comprises a first layer having a top surface, a second layer disposed on the top surface of the first layer, and a third layer disposed on top of the second layer. The second layer alters the resistivity of the third layer during the fabrication process of the chuck. As such, the resistivity of the chuck is reduced to a value that facilitates establishment of the Johnsen-Rahbek effect and promotes wafer processing at room temperature.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a ceramic substrate support chuck for supporting a 
semiconductor wafer within a semiconductor processing system. More 
particularly, the invention relates to an improved multi-layered and doped 
ceramic electrostatic chuck, and method of fabricating same, for 
decreasing resistivity of a portion of the chuck for operation at room 
temperature by the Johnsen-Rahbek effect. 
2. Description of the Background Art 
Substrate support chucks are widely used to support substrates within 
semiconductor processing systems. A particular type of chuck used in high 
temperature semiconductor processing systems such as high temperature, 
physical vapor deposition (PVD) is a ceramic electrostatic chuck. These 
chucks are used to retain semiconductor wafers, or other substrates, in a 
stationary position in the process chamber during processing. FIG. 1 
depicts a typical ceramic electrostatic chuck 100. Such electrostatic 
chucks contain one or more electrodes 104 and 106, respectively embedded 
within a unitary ceramic chuck body 102. The ceramic chuck body 102 is, 
for example, fabricated of aluminum nitride or boron nitride or alumina 
doped with a metal oxide such as titanium oxide or chromium oxide or some 
other ceramic material with similar resistive properties. This form of 
ceramic is partially conductive at high temperatures. 
In use, a wafer rests flush against the surface of the chuck body as a 
chucking voltage is applied to the electrodes. Because of the conductive 
nature of the ceramic material at high temperatures, the wafer is 
primarily retained against the ceramic support by the Johnsen-Rahbek 
effect. The Johnsen-Rahbek effect establishes a small, but highly 
effective current flow between the substrate support surface and the 
substrate being retained. As such, a chucking force that is much greater 
than the force generated by a purely Coulombic effect electrostatic chuck 
retains the substrate to the support surface. Johnsen-Rahbek chucks are 
disclosed in U.S. Pat. Nos. 5,117,121, issued May 26, 1992 and 5,463,526 
issued Oct. 31, 1995. 
One disadvantage of using a chuck body fabricated from ceramic is that the 
resistivity of the chuck changes as a function of temperature. During 
wafer processing, the chuck is subjected to a wide range of temperatures, 
e.g., in the range of 20.degree.-150.degree. C. and could be as high as 
300.degree.-400.degree. C. for other types of processes. At room 
temperature (approximately 20.degree. C.) the resistivity of the chuck is 
on the order of approximately 10.sup.14 ohm-cm and decrease to 
approximately 10.sup.13 ohm-cm at temperatures around 150.degree. C. This 
decrease in resistivity promotes a satisfactory chucking force via the 
Johnsen-Rahbek effect. However, as the temperature rises in the chamber, 
the resistivity level continues to decrease. As such, the current flow in 
the chuck body, hence the wafer, attributed to the Johnsen-Rahbek effect 
becomes larger. A wafer or similar substrate clamped to the support 
surface of a chuck having a high current flow can be damaged to the point 
of having a lower yield or being totally unusable. 
Controlling the resistivity of the bulk material is essential to 
fabricating and using a Johnsen-Rahbek effect electrostatic chuck. The two 
parameters that are available for controlling resistivity are process 
temperature and the materials of the chuck. Unfortunately, decreasing 
process temperature does not provide an optimal resistivity to establish 
the Johnsen-Rahbek effect. Therefore, altering the materials comprising 
the chuck body is necessary. Doping the bulk material is a viable 
solution; however, it is critical to control the amount of doping. For 
example, an electrostatic chuck having a highly doped bulk material 
generates excessive charges which can short chucking electrodes and impede 
dechucking due to residual charge buildup. Such a condition is not 
suitable for a Johnsen-Rahbek effect chuck. 
Therefore, a need exists in the art for an apparatus that has a resistivity 
level suitable for establishing the Johnsen-Rahbek effect to 
electrostatically clamp a substrate to a support surface and a concomitant 
method of fabricating same. 
SUMMARY OF THE INVENTION 
The disadvantages associated with the prior art are overcome by the present 
invention of a multi-layered, electrostatic ceramic chuck. Specifically, 
the invention provides a ceramic electrostatic chuck for retaining a 
substrate in a process chamber comprising a first layer having a top 
surface, a second layer disposed on the top surface of the first layer, 
and a third layer disposed on top of the second layer. The second layer 
alters the resistivity of the first layer during the fabrication process 
of the chuck. Specifically, the resistivity of the chuck is reduced to a 
value that facilitates establishment of the Johnsen-Rahbek effect and 
promotes wafer processing at room temperature. 
Additionally, the invention provides a method for fabricating a 
multi-layered ceramic electrostatic chuck for retaining a substrate in a 
process chamber. The method comprises the steps of providing a first 
layer, disposing a second layer on a bottom surface of a third layer, 
disposing the third layer with disposed second layer upon the first layer, 
and sintering the multi-layered chuck. 
The advantages of such a configuration are establishment of a suitable 
chucking force, reduced dechucking time, reduced thermal stress and 
fatigue on chamber components and wafers which result in reliable wafer 
handling, greater uniformity amongst a batch of processed wafers and 
faster wafer throughput.

DETAILED DESCRIPTION 
FIG. 2 depicts an exploded cross-sectional view of a plurality of layers of 
an electrostatic chuck 200 for retaining a substrate e.g., a semiconductor 
wafer (not shown), in a process chamber of a semiconductor wafer process 
system. The chuck 200 is preferably a three layer composite that is 
fabricated into a single unit with the semiconductor wafer being supported 
and retained on a top surface 220 of the chuck 200. Specifically, a first 
layer 202 forms the base of the chuck 200. This first layer 202 is 
preferably a dielectric material having moderate resistivity 
characteristics at room temperature. Ideally, the first layer 202 is a 
ceramic such as aluminum nitride in a pre-sintered state. In a preferred 
embodiment of the invention, the first layer 202 is approximately 10 mm 
thick. 
Additionally, the first layer 202 is provided with various elements 
necessary to process a semiconductor wafer. Specifically, in the 
pre-sintered, green body state, the first layer 202 is putty-like in 
consistency which facilitates the addition of one or more chucking 
electrodes 210 and one or more heater electrodes 208. The chucking 
electrodes 210 are disposed proximate an upper surface 214 of the first 
layer 202 and can be arranged or configured in any way suitable for 
retaining the wafer against the top surface 220 of the chuck 200. For 
example, there can be one monopolar electrode, two or more bipolar 
electrodes, multi-zoned bipolar electrodes and the like. Similarly, the 
heater electrodes can be a single heater, two or more heaters for zoned 
heating and the like. Chucking and heating electrodes are selected from 
the group consisting of molydbenum, tungsten. These electrodes are placed 
within the green body state first layer 202 along with any attendant 
feedthroughs (not shown) to provide a connection from the electrodes to a 
power source (not shown). A second layer 212 is disposed on a bottom 
surface 204 of a third layer 206 This second layer 212 is a dopant 
material which is capable of altering the resistivity of the chuck 200. 
The dopant is any material that is capable of decreasing the resistivity 
of the chuck at room temperature to a value of approximately 10.sup.10-12 
ohm-cm. In a preferred embodiment of the invention, the dopant is carbon. 
Specifically, carbon is deposited on the bottom surface 204 the third 
layer 206. In a preferred embodiment of the invention, the carbon is 
deposited by a sputtering technique. Sputtering allows a high degree of 
deposition control in the resultant resistivity of the completed chuck. 
That is, the thickness of the sputtered layer of dopant can be varied to 
alter the desired resistivity of the chuck. As such, a wide range of 
chucks having different resistivity characteristics can be fabricated to 
comply with various types of specifications. For example, in the subject 
apparatus, the dopant thickness is in the range of approximately 100-10000 
.ANG.. In a preferred embodiment of the invention, the thickness of the 
dopant material is approximately 5000 .ANG.. Alternately, any other method 
employed for depositing material is suitable for forming the dopant layer. 
For example, the dopant layer 212 can also be formed by, but not limited 
to, such techniques as screen printing, vacuum arc deposition and the 
like. Likewise, a variety of dopants may be substituted for carbon such as 
oxygen, silicon, berillium, cadmium, zinc and magnesium. 
The third layer 206 is disposed on top of the first layer 202 to produce 
the complete chuck 200. The third layer 206 is preferably a dielectric 
material. In order to avoid anomalies during wafer processing or premature 
failure of the chuck, the first layer 202 and third layer 206 should 
preferably be the same material or at least be materials that have thermal 
expansion coefficients that are relatively close in value. Ideally, the 
third layer 206 is a ceramic such as aluminum nitride in a fully cured, 
i.e., post-sintered state. In a preferred embodiment of the invention, the 
third layer 206 is a disc approximately 2-10 mm thick. Alternately, the 
third layer is boron nitride. 
The post-sintered third layer 206 is oriented so that the second layer 212 
is disposed upon the first layer 202 and lies between the first layer 202 
and the third layer 206 to create the multi-layered chuck 200. 
Specifically, the bottom surface 204 of the third layer 206 having the 
deposited second layer 212 is placed in contact with the upper surface 214 
of the first layer 202. Next, the multi-layered chuck 200 is sintered in a 
furnace to cure the green body state first layer 202. Specifically, the 
chuck 200 is sintered at a temperature in the range of approximately 
1900.degree.-2000.degree. C. at a pressure of approximately above 1 atm in 
nitrogen for approximately 8-10 hours. 
The sintering process produces an electrostatic chuck having a level of 
dopant material incorporated into the body which decreases resistivity. 
FIG. 3 depicts the chuck of FIG. 2 in a completely assembled and 
fabricated state. The dopant material e.g., carbon, diffuses into the 
green body state first layer 202 during the sintering process. The 
chucking electrodes 210 act as a diffusion barrier so that most of the 
diffusion occurs between the upper surface 214 of the first layer 202 and 
the chucking electrodes 210. As such, a lower value of resistivity is 
realized in the first layer 202. For example, at room temperature, 
aluminum nitride has a resistivity on the order of 10.sup.14 ohm-cm. An 
electrostatic chuck manufactured under the methods disclosed can have a 
resistivity on the order of 10.sup.10-12 ohm-cm at room temperature. This 
decrease in resistivity provides a highly desirable chucking and 
dechucking conditions. Specifically, the Johnsen-Rahbek effect establishes 
a reliable chucking force that is greater than that provided by a 
Coulombic chuck. Since doping of the first layer 202 is highly controlled 
there is no excessive charges in the chuck which need to be eliminated to 
effectively dechuck the wafer from the top surface 220. Additionally, 
lower processing temperatures place less thermal stress on chamber 
components as well as a wafer being processed. Such conditions result in 
greater uniformity amongst a batch of processed wafers and faster wafer 
throughput. 
The decrease in resistivity is also partially dependent upon the purity of 
the aluminum nitride when it is in the green state. That is, binders are 
added to the green body in order to facilitate formation of the desired 
shape and features of the body. These binders e.g., polyvinylalcohol, 
polyvinylbutyral, polypropylenecarbonate and the like are throughout the 
green body. As such, they act as contaminants that alter the resistivity. 
To alleviate this condition, a debinderization step is added to the 
fabrication process in order to increase the purity and density of the 
aluminum nitride and to allow for a proper sintering process. The 
debinderization process is carried out on the green body first layer 202 
prior to joining it with the second layer 212 and third layer 206. For 
example, the debinderization step consists of conducting a burnout of the 
material to be debinderized, i.e., the first layer 202 in air at a 
temperature of approximately 500.degree. C. for approximately one hour. 
Under these conditions, the binders are removed, i.e., evaporate out of 
material. 
In sum, a multi-layered electrostatic chuck is provided that is capable of 
operating i.e., retaining a semiconductor wafer to be processed, via the 
Johnsen-Rahbek effect, at room temperature. A dopant is provided between a 
post-sintered and a pre-sintered material prior to a final sintering step. 
The final sintering step not only cures the pre-sintered material 
retaining relevant processing elements such as heating and chucking 
electrodes, but also promotes infusion of the dopant material into the 
pre-sintered material to decrease the characteristic resistivity of the 
material. The advantages of such a configuration are much shorter ramp up 
time to achieve desired chamber environment, reduced thermal stress and 
fatigue on chamber components and wafers which result in greater 
uniformity amongst a batch of processed wafers and faster wafer 
throughput. 
Although the subject invention is described in terms of depositing a dopant 
on the bottom surface of a post-sintered layer, this does not preclude the 
addition of any dopant layers to any multi-layered chuck and methods of 
fabricating same. For example, one skilled in the art may add a dopant to 
the pre-sintered material and dispose a third layer with a non-sputtered 
bottom side at any step during the fabrication process. Additionally, the 
third layer can be preconditioned. In other words, the third layer is 
provided with a layer of carbon on a bottom surface prior to the 
deposition step. As such, the deposited carbon will more readily diffuse 
into the pre-sintered layer than into the post-sintered, pre-conditioned 
layer. 
Although various embodiments which incorporate the teachings of the present 
invention have been shown and described in detail herein, those skilled in 
the art can readily devise many other varied embodiments that still 
incorporate these teachings.