Conduits for flow of heat transfer fluid to the surface of an electrostatic chuck

The present invention discloses a two basic structures (including multiple variations within one of the basic structures) and methods for fabrication of the structures which facilitate the flow of cooling gas or other heat transfer fluid to the surface of an electrostatic chuck. The basic structures address both the problem of breakdown of a heat transfer gas in an RF plasma environment and the problem of arcing between a semiconductor substrate and the conductive pedestal portion of the electrostatic chuck in such an RF plasma environment.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention pertains to a dielectric structure which serves as a 
conduit for the flow of heat transfer fluid to an upper surface of an 
electrostatic chuck. The dielectric structure comprises a dielectric 
insert which is typically used in combination with at least a portion of a 
dielectric layer which forms the upper surface of the electrostatic chuck. 
The dielectric structure prevents breakdown of the heat transfer fluid fed 
through the electrostatic chuck to its surface to cool a bottom surface of 
a work piece such as a silicon wafer which is supported upon the upper 
surface of the electrostatic chuck. The dielectric structure also prevents 
a semiconductor processing plasma from penetrating into the heat transfer 
fluid openings in the electrostatic chuck. 
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 electrostatic 
chuck so that it contacts the bottom of an article supported on that 
surface. The gas distribution system includes a plurality of intersecting 
grooves formed entirely in the upper surface of the electrostatic chuck, 
with small gas distribution holes through intersections of the grooves. 
The lifetime of an electrostatic chuck is affected by the presence of the 
gas distribution holes used to facilitate the distribution of heat 
transfer gas. 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 due to arcing or glow discharge. Further, since there is 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 deteriorates the dielectric layer and underlying 
aluminum layer. 
Collins et al. recommends that the aluminum layer beneath the dielectric 
layer be cut back (away) beneath the dielectric layer immediately adjacent 
the gas distribution hole to reduce the possibility of arcing across the 
straight line path from the semiconductor substrate to the aluminum layer. 
Although this reduces the possibility of arcing, it does not provide the 
desired isolation of the conductive electrostatic chuck from the process 
plasma. 
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 spatial 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 spatial 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. In some instances, the perforations in the upper surface of the 
underlying aluminum pedestal which lead to gas distribution cavities 
within the pedestal are prepared in advance of application of the 
dielectric layer. In other instances, the perforations in the upper 
surface of the aluminum pedestal are prepared simultaneously with the 
perforations through the dielectric layer. 
The cooling gas distribution grooves in the surface of the dielectric layer 
can be produced using laser micro machining or by using a grinding wheel. 
The perforations through the dielectric layer are formed using a 
mechanical drill or a laser. A preferred laser is an excimer UV laser 
(i.e. a short wave-length, high energy laser) run at a relatively low time 
averaged power level. This helps reduce the redepositing of drilled 
aluminum from the underlying thin layer onto the walls of the perforations 
and onto the surface of the dielectric. Presence of such aluminum can 
cause arcing across the dielectric layer. The perforations are frequently 
placed around the outer perimeter of the surface of the electrostatic 
chuck. For an electrostatic chuck used with an 8 inch (200 mm) silicon 
wafer electrostatic chuck, the number of such perforations generally 
ranges from about 150 to about 300. The number of perforations depends on 
the mount of heat transfer load, and the heat transfer fluid flow rate 
required to handle this load. Typically the perforations are configured in 
a ring-like structure around the outer perimeter of the electrostatic 
chuck. A typical perforation has a diameter which is approximately 
0.007.+-.0.001 inch (0.175.+-.0.025 mm). 
While micro-drilling through the dielectric layer overlaying the aluminum 
pedestal to provide the perforations described above provides a 
satisfactory gas passage, it fails to address the problem of the RF plasma 
environment that seeks the interface between the dielectric alumina 
coating and the aluminum substrate. Frequently the underlying aluminum 
works its way up the sidewalls of the opening(s) in the dielectric layer, 
leading to arcing and plasma glow within the opening(s). Moreover, 
depending on the method used to form the perforations, the lower portion 
of the hole may become a metallic conductor (aluminum) despite the use of 
a high aspect ratio (depth/diameter) for the gas passage. The removal of 
machined micro chips slurry from the distribution hole is a difficult 
task, and is compounded by any migration of aluminum particles up through 
the dielectric gas distribution hole during drilling and subsequent 
manufacturing operations such as cleaning of passageways. Presence of 
machined micro chips slurry is a source of contaminant in the micro 
electronic environment and may even block the holes in a manner that 
reduces or stops heat transfer gas flow. 
In light of the above, there is a need for a structure which significantly 
reduces the possibility of breakdown of the cooling gas due to arcing or 
glow discharge. Further, there is a need for a structure which 
significantly reduces the possibility of arcing between a semiconductor 
substrate and the metallic pedestal portion of the electrostatic chuck on 
which the semiconductor substrate is supported. 
SUMMARY OF THE INVENTION 
The present invention discloses embodiments of two different kinds of 
structures (including multiple variations within the basic structure) and 
methods for fabrication of the structures which facilitate the flow of 
cooling gas or other heat transfer fluid to the surface of an 
electrostatic chuck. The embodiments of the present invention which follow 
address both the problem of the breakdown of a heat transfer gas in an RF 
plasma environment and the problem of arcing between a semiconductor 
substrate and the conductive pedestal portion of the electrostatic chuck 
in such an RF plasma environment. 
A first preferred embodiment of the heat transfer fluid conduit structure 
of the present invention includes an underlying conductive layer which 
contains at least one heat transfer fluid (typically a gas) pathway, at 
least one isolating dielectric insert which is in contact with and 
operates to isolate at least a portion of the underlying conductive layer 
from the heat transfer fluid pathway, and an overlying dielectric layer 
which overlies at least portions of the conductive layer, and in some 
instances, at least a portion of the isolating dielectric insert as well. 
The overlying dielectric layer comprises at least one opening connected to 
the heat transfer fluid pathway of the underlying conductive layer and 
isolating dielectric insert. The basic structure provides an insulative 
dielectric layer as the upper surface of the electrostatic chuck and 
improves the isolation of the underlying conductive layer of the 
electrostatic chuck from an RF plasma which seeks the interface between 
the dielectric layer upper surface and the underlying conductive layer of 
the electrostatic chuck. 
A method of forming the first preferred embodiment of the present 
invention, described above, is as follows: a conductive layer containing a 
heat transfer fluid passageway is provided; at least one dielectric insert 
is placed into a counter sunk hole or other cavity in the conductive layer 
(typically the pedestal of the electrostatic chuck), in a manner such that 
the dielectric insert operates in cooperation with the conductive layer to 
provide a fluid flow passageway; and, a layer of dielectric material is 
applied over the surface of the insert and adjacent exposed conductive 
layer. The dielectric layer is then processed (typically ground or 
otherwise ablated) back to provide the desired thickness of dielectric 
layer while optionally exposing at least a portion of the dielectric 
insert. Preferably, the dielectric insert comprises at least one through 
hole which is exposed during the processing of the overlying dielectric 
layer. In the alternative, an opening through the dielectric insert and 
overlying dielectric layer can be drilled subsequent to processing of the 
dielectric layer or formed using a removable insert or mask which prevents 
the dielectric layer from entering the opening in the insert during 
application of the dielectric layer. When the shape of the insert is such 
that it would not be locked in place by the dielectric layer if exposed 
during processing of the dielectric layer, the dielectric layer material 
can be removed to a particular depth within the layer, and a hole can then 
be drilled through the dielectric layer to connect with a fluid flow 
passageway within the insert, where the hole drilled is smaller than the 
insert, leaving the insert entrapped under the dielectric layer. If the 
dielectric insert contains no passageway, the opening through the 
dielectric layer and the passageway through the dielectric insert can be 
created simultaneously. 
A second preferred embodiment of the heat transfer fluid conduit structure 
of the present invention includes an underlying conductive layer which 
contains at least one heat transfer fluid pathway; at least one dielectric 
insert which is disposed in the heat transfer fluid pathway and cooperates 
with the underlying conductive layer to control the spacial opening 
between the dielectric insert and the heat transfer fluid pathway in a 
manner which reduces the possibility of plasma penetration into said 
pathway; and an overlying dielectric layer which overlies at a portion of 
the conductive layer. The overlying dielectric layer comprises at least 
one opening connected to the heat transfer fluid pathway of the underlying 
conductive layer. The basic structure provides an insulative dielectric 
layer as the upper surface of the electrostatic chuck and improves the 
insulation of the underlying conductive layer of the electrostatic chuck 
from an RF plasma by controlling the spacial opening through which the 
plasma must seek the underlying conductive layer. 
A method of forming the second preferred embodiment heat fluid transfer 
structure of the present invention, described above, is as follows: A 
conductive layer comprising a buried heat transfer fluid channel is 
provided; at least one opening is then created through the conductive 
layer to connect with the buried heat transfer fluid channel; a 
space-holding, masking pin is then placed inside the opening; a layer of 
dielectric material is applied over the surface of the conductive layer 
and the masking pin; the masking pin is removed; a bonding material is 
applied within a limited portion of the buried heat transfer fluid channel 
directly beneath the opening through the conductive layer and the 
dielectric layer; a dielectric pin is inserted through openings in the 
dielectric layer and the conductive layer to reach the bonding material; 
and, the dielectric pin is bonded within the buried heat transfer fluid 
channel. 
Typically, the conductive layer is an aluminum pedestal, the dielectric 
insert is constructed from a material such as alumina, and the overlaying 
dielectric layer is applied by spray coating alumina or alumina/titania 
over the surface of the aluminum pedestal and the dielectric insert (or 
the space-holding, masking insert, depending on the embodiment of the 
invention). Other materials of construction than those named here can be 
used so long as they meet electrical requirements, are compatible with 
adjacent fluid chemical and physical conditions, and the relative thermal 
coefficients of expansion do not create problems in the integrity of the 
electrostatic chuck after multiple thermal cycles in the intended plasma 
processing environment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention pertains to structures and methods for fabrication of 
the structures which facilitate the flow of cooling gas or other heat 
transfer fluid to the surface of an electrostatic chuck. The structures 
address both the problem of the breakdown of a heat transfer gas in an RF 
plasma environment and the problem of arcing between a semiconductor 
substrate and the conductive pedestal portion of the electrostatic chuck 
in such an RF plasma environment. 
As shown in FIG. 1, a plasma processing chamber 100 contains electrostatic 
chuck 102 which electrostatically clamps semiconductor substrate 104 
(typically a semiconductor wafer) in position within chamber 100 during 
processing. Lift finger openings 106 penetrating the electrostatic chuck 
102 allow lift fingers (not shown) to pass therethrough, to lift a 
semiconductor wafer off the upper surface of electrostatic chuck 102 once 
the power has been turned off and the clamping force terminated. 
Electrostatic chuck 102 also includes gas flow openings 202 which are 
illustrated in FIG. 2 as being present in an annular (typically 
conductive, metallic) insert 110 near the periphery of electrostatic chuck 
102; the insert 110 having a gas flow channel 112 machined into its 
underside. The gas flow openings need not be present in an annular 
configuration, but this is preferred. In addition, it is possible to have 
more than one annular insert present within electrostatic chuck 102. Gas 
flow channel 112 traverses annular metallic insert 110 to within close 
proximity of its upper surface, leaving a thin layer of metal 113 (shown 
in FIG. 2C) separating dielectric layer 114 from channel 112. Annular 
insert 110 is sealed to adjacent surfaces within electrostatic chuck 102. 
As shown in the top plan view of FIG. 2A and the associated cross-section 
of FIG. 2B, a plurality of through holes 202 penetrate dielectric layer 
114 to connect with gas flow channel 112 passing circumferentially around 
the outer periphery of electrostatic chuck 102. FIG. 2B illustrates that 
cooling gas can then be fed through a conduit 108, penetrating from the 
bottom of the electrostatic chuck 102, passing into gas flow channel 112 
and exiting at through hole 202 onto dielectric surface 114 of 
electrostatic chuck 102. When grooves (not shown) are machined or 
otherwise formed into the dielectric surface 114 of electrostatic chuck 
102 so that they intersect with through holes 202, cooling gas can proceed 
from through holes 202 and into such grooves which distribute cooling gas 
over the entire upper surface of electrostatic chuck 102. 
FIG. 2C illustrates in perspective and partial cross-section, the 
(typically conductive) insert ring 110, showing in more detail gas flow 
channel 112 and a plurality of holes 115 formed through the a thin layer 
(typically metallic) 113, which forms the roof of flow channel 112, to 
provide a path for heat transfer gas flow. Gas flow channel 112 extends 
upwardly within annular insert 110 to within close proximity of its upper 
surface, leaving the thin layer of metal 113 separating an overlying 
dielectric layer 114 from gas flow channel 112. The plurality of holes 115 
may be bored through the thin layer of metal 113 to provide a gas flow 
path prior to the application of overlying dielectric layer 114. In the 
alternative, holes 115 may be formed after the application of dielectric 
layer 114 by boring holes through the dielectric layer 114 and thin 
metallic layer 113 simultaneously. 
FIG. 4A shows a cross-sectional schematic of the prior art heat transfer 
gas flow system in which insert 406 (which may be an individual insert or 
may be a ring-shaped insert 110 of the kind shown in FIGS. 2B and 2C) 
works in combination with pedestal 400 of an electrostatic chuck (such as 
that shown as 102 in FIG. 1) to provide a gas flow channel 408. 
FIG. 3A is a schematic of a cross-sectional view showing one preferred 
embodiment of the present invention. A cylindrical dielectric insert 300 
is formed with a boss 301 around its outside and a vertically extending 
cavity 308 at its bottom. The cavity 308 has a blind (closed) upper end. A 
socket hole 313 is bored from the top of a conductive layer 310 to at 
least pierce an underlying longitudinal channel 312 machined into the 
bottom of a conductive layer 310. Preferably, the conductive layer 310 is 
a pedestal of an electrostatic chuck of the kind shown as 102 in FIG. 1. 
Pedestal 310 preferably contains a buried channel 312 to facilitate the 
flow of heat transfer fluid. A socket hole 310 is machined through the 
upper surface of pedestal 310 to connect with buried channel 312. 
Dielectric insert 300 is fitted into the socket hole 313 with the bottom 
of its boss 301 resting on the side portions of socket hole 313, leaving a 
clearance between the bottom of dielectric insert 300 and the bottom of 
buried channel 312. Heat transfer fluid (typically cooling gas) can then 
flow from the gas channel 312 into the vertically extending cavity 308 
formed within dielectric insert 300. 
After the dielectric insert 300 has been fit into socket hole 313 of 
conductive layer 310, a dielectric layer 302 is deposited or otherwise 
applied over the surfaces of both the dielectric insert 300 and conductive 
layer 310 (typically the principal body or pedestal of an electrostatic 
chuck 102). The dielectric layer 302 is then processed (typically ground) 
back to line 304 shown in FIG. 3A, which is below the blind end to the 
vertically extending cavity 308 of the dielectric insert 300, thereby 
forming a reduced thickness dielectric layer 302'. Whereby, the cavity 308 
is opened at its upper end to form an opening 306, and a heat transfer 
fluid, such as a cooling gas, can flow from the gas channel 312, through 
the insert cavity 308, and out of the opening 306 to the surface of the 
reduced thickness dielectric layer 302'. Once again, the reduced thickness 
dielectric layer 302' can be processed to form, at its surface, gas 
distribution grooves connected to opening 306. Preferably, a plurality of 
dielectric inserts 300 inserted into respective socket holes 313 formed 
into an annular ring 310 are circumferentially spaced at the openings 202 
as shown in FIG. 2A. 
FIG. 3B is a schematic of a cross-sectional view of a second preferred 
embodiment of the present invention. This embodiment includes a tubular 
dielectric insert sleeve 320 having a pre-drilled center opening 328. That 
is, the sleeve 320 is a right circular cylinder with an axial passage 328. 
The passage 328 may either pass completely through the dielectric sleeve 
320 or may have an upper blind end (not shown) as does the insert 300 of 
FIG. 3A. 
Conductive layer 330 may be an insert ring similar to the insert ring 110 
shown in FIG. 2C. A first socket hole 334 (similar to hole 115 shown in 
FIG. 2C) is drilled through conductive layer 330 to connect with an 
underlying gas channel 338 present in pedestal 331 of an electrostatic 
chuck. A second socket hole 335 is drilled partially through metallic 
layer 330, to form an annular ledge 336 at the bottom of the socket hole 
335. Dielectric insert sleeve 320 is inserted into the socket hole 335, 
and its lower end preferably rests on the ledge 336. Dielectric insert 
sleeve 320 may optionally be held within the metallic layer 330 by an 
annular weld or brazed joint 326 extending around the insert sleeve 320 at 
the top of the metallic layer 330, or by an interference fit at this 
location. 
After dielectric insert sleeve 320 is fitted into socket hole 335 of 
conductive layer 330, a dielectric layer 322 is applied over the surface 
of insert sleeve 320 and conductive layer 330. Subsequently, dielectric 
layer 322 is processed back to line 324, forming reduced thickness 
dielectric layer 322' and exposing insert 320 and opening 332 at the top 
of dielectric insert sleeve 320. If it is desired not to use weld 326 to 
hold dielectric insert sleeve 320 in place, layer 322 can be processed 
back so that it leaves dielectric insert sleeve 320 unexposed. Opening 332 
must then be drilled through dielectric layer 322 to connect with opening 
328 in dielectric insert sleeve 320. 
Typically, dielectric insert 300 illustrated in FIG. 3A, and dielectric 
insert sleeve 320 illustrated in FIG. 3B are used as a plurality of 
inserts 320 which are spaced around the periphery of an electrostatic 
chuck 102 of the kind shown in FIG. 2B. The plurality of inserts can be 
spaced around an annular conductive ring of the kind shown as 110 in FIGS. 
2A, 2B, and 2C. 
To clearly illustrate the advantages of the basic structure of the present 
invention over the prior art, reference is made to FIGS. 4 through 7. FIG. 
4A illustrates the prior art, while FIGS. 4B through 7E illustrate 
examples of the preferred embodiments of the present invention. FIGS. 4 
through 7 illustrate schematic cross-sectional views of an electrostatic 
chuck having the general construction shown in FIGS. 2A and 2B. The view 
illustrated is found at the location shown for a conductive (typically 
metallic) insert 110 in FIG. 2B. The cross-sectional views for FIGS. 4B 
through 6C do not include the overlying dielectric layer which forms the 
upper surface of the electrostatic chuck, but are limited to the 
underlying substructures so that the substructures can be shown with more 
clarity. In instances where the dielectric insert of the present invention 
has a pre-drilled opening and need not depend on the overlying dielectric 
layer to maintain its position, the dielectric layer can be processed to 
expose the insert, simultaneously exposing the pre-drilled opening(s). In 
instances where the dielectric insert depends on overlying dielectric 
layer to hold it in position, it is necessary to drill an opening through 
the overlying dielectric layer to connect with the opening in the 
dielectric insert. When the dielectric insert contains no gas flow 
openings, these gas flow openings are typically drilled simultaneously 
with the opening in the overlying dielectric layer. 
FIG. 4A illustrates prior art, and shows a schematic of a cross-sectional 
view of a portion of an electrostatic chuck, including pedestal 400 
(typically constructed from aluminum) having a first annular gas flow 
channel 402 machined in its surface. A second annular channel 404 having a 
width greater than that of annular channel 402 is machined overlying 
channel 402 and concentric with channel 402. Into this combination of 
annular gas flow channel 402 and overlying annular channel 404, a 
conductive (typically aluminum) annular insert 406 is fitted. Metallic 
insert 406 is shaped so that it forms, in conjunction with pedestal 400 a 
gas flow channel 408. Metallic insert 406 optionally includes a 
pre-drilled plurality of openings 410 spaced around the length of the 
annular insert 406, which holes lead to its upper surface 412. A 
dielectric layer (not shown) is applied over the surface 412 of metallic 
insert 406 and the surface 414 of pedestal 400. Preferably the dielectric 
layer is comprised of thermally sprayed alumina or sprayed 
alumina/titania. Processes for application of this thermally sprayed layer 
are known in the art. The thermal spraying process can be selected from 
several different methods such as plasma spraying, detonation gun 
spraying, high velocity oxygen fuel (HVOF) spraying and flame spraying. 
The dielectric layer is processed to the desired thickness, and an opening 
is drilled through the dielectric layer to connect with the opening 410 in 
metallic insert 406. If metallic insert 406 does not have an opening 410, 
an opening is drilled through both the dielectric layer and through 
metallic insert 406 to provide for flow from gas flow channel 402 to the 
dielectric surface of the electrostatic chuck. 
As described before, this method of preparing a gas flow channel to the 
surface of the electrostatic chuck does not address the problem of the RF 
plasma environment which seeks the interface between the electrostatic 
chuck dielectric surface layer and the underlying conductive layer. 
Frequently the underlying aluminum works its way up the sidewalls of the 
opening(s) in the dielectric layer, leading to arcing and plasma glow 
within the opening(s). 
FIG. 4B illustrates one preferred embodiment of the present invention where 
a dielectric insert 416, typically comprised of alumina, is inserted into 
cavities or holes drilled into annular metallic insert 406. Dielectric 
insert 416 comprises an internal conduit 418 which permits gas flow from 
gas flow channel 408 to the surface 412 of metallic insert 406. A layer of 
dielectric (not shown) is applied over the surface 412 of metallic insert 
406 and pedestal 400. The layer of dielectric is processed to the desired 
thickness, exposing the opening of internal conduit 418 of dielectric 
insert 416. Dielectric insert 416 now provides a portion of the dielectric 
surface of the electrostatic chuck while providing electrical isolation of 
metallic insert 406 from any process plasma which may penetrate the upper 
portion of gas flow conduit 418. This isolation aids in the prevention of 
the breakdown of cooling gas as well as in the prevention of arcing 
between a semiconductor substrate (not shown) supported upon the surface 
of the electrostatic chuck and the conductive metallic insert 406 used to 
provide a cooling gas flow channel. 
During development of the dielectric inserts of the present invention, we 
discovered that it is critical that the thermally sprayed ceramic coatings 
used to provide the dielectric upper surface of an electrostatic chuck (as 
illustrated in FIGS. 1, 2A, 2B, and 7C through 7F, but not shown in FIGS. 
4A through 6C) form submicron shrinkage cracks upon cooling to a deposited 
dielectric layer. These submicron-sized cracks permit the coating to 
expand or stretch to conform with the differential in thermal expansion 
between the dielectric layer and the underlying conductive substrate 
without forming larger-sized, major cracks or mechanically delaminating 
from the underlying conductive substrate surface. Formation of major 
cracks would permit the entry of plasma which damages the underlying 
conductive substrate and which can also lead to delamination of the 
ceramic dielectric coating layer from the underlying conductive substrate. 
FIG. 4C illustrates another preferred embodiment of the present invention, 
where the dielectric insert 420 is a porous dielectric, such as alumina 
having a porosity ranging from about 10% in volume to about 60% in volume, 
with interconnected openings which form continuous passageways through the 
dielectric material. Since the shape of the dielectric insert 420 shown in 
FIG. 4C does not lock the insert in place, after application of the 
overlying dielectric layer (not shown), the dielectric layer is not 
processed to expose the insert, but instead, an opening is drilled through 
the dielectric layer to connect with porous insert 420. It is preferred 
not to use a straight line of sight through the insert, and by using a 
porous insert such as 420, improved resistance to penetration of plasma is 
achieved. When the porous insert 420 is formed using traditional molding 
and sintering methods, the particles used in the molding or sintering are 
of the same order of magnitude in size as the porosity and are bonded in 
more or less random orientation, producing passageways that avoid the 
straight line of sight configuration. 
An additional embodiment of the dielectric insert of the present invention 
is illustrated in FIG. 5A. This dielectric insert 510 has a plurality of 
openings 516 leading to gas flow channel 508. Pedestal 500 is machined to 
have two annular channels 502 and 504 of the kind described with reference 
to FIG. 4A. A conductive insert 506 (typically metallic) in the form of an 
annular ring is fitted into annular channels 502 and 504, as illustrated 
in FIG. 5A. Conductive insert 506 is shaped to form gas flow channel 508 
when operating in combination with electrostatic chuck pedestal 500. 
Dielectric insert 510 is shaped to fit into metallic insert 506. 
Dielectric insert 510 is shaped to have an upper surface in the form of a 
dome so that after application of an overlying dielectric layer (not 
shown), the overlying dielectric layer can be processed back (ground or 
ablated) to expose the portion of dielectric insert 510 containing 
openings 516, while leaving insert 510 upper surface adjacent openings 516 
covered by the overlying dielectric layer. 
A variation of the dielectric insert of FIG. 5A is shown in FIG. 5B. This 
dielectric insert 520 uses an overlying dielectric layer (not shown) to 
hold it in place. The overlying dielectric layer is applied over the 
surface of insert 520, conductive insert 512, and pedestal surface 514. 
The overlying dielectric layer is processed back to the desired thickness. 
Then, the gas flow openings through the overlying dielectric layer and 
dielectric insert 520 are created by drilling through the overlying 
dielectric layer and dielectric insert 520 to connect with gas flow 
channel 508. 
FIG. 5C shows typical opening patterns which are used for a dielectric 
inserts 516 of the kind shown in FIG. 5A. 
Another series of dielectric insert designs is shown in FIGS. 6A through 
6C. Again, the view is a schematic of a cross-section through an 
electrostatic chuck in the area of conductive the gas flow channel insert. 
In FIGS. 6A through 6C, the electrostatic chuck pedestal 600 includes an 
annular conductive insert shown in section as 606. Pedestal 600 is 
machined to have two annular channels in its upper surface, as illustrated 
at 602 and 604. Conductive insert 606 is fitted into the openings created 
by annular channels 602 and 604, to provide a gas flow path 608. 
In FIG. 6A, the dielectric insert of the present invention 610 comprises a 
non-porous dielectric sleeve 616 surrounding a porous dielectric insert 
618. Since dielectric insert 610 is dome shaped on its upper surface, it 
is possible to use an overlying dielectric layer (not shown) to hold it in 
place, with the overlying dielectric layer being processed back to expose 
porous dielectric insert 618. This provides for heat transfer gas flow 
through channel 608 and porous dielectric filter 618 to the surface of the 
overlying dielectric layer. Since non-porous dielectric sleeve 616 is 
configured with a small angle relative to the adjacent surface 612 of 
conductive insert 606, a contiguous coating without voids or cavities at 
the interface between dielectric sleeve 616 and surrounding conductive 
insert 606 is ensured. The upper surface of dielectric insert 616 is 
roughened to provide good bonding of the overlying dielectric layer to the 
insert. Typically porous dielectric insert 618 is of the same structure as 
that described for porous insert 420. Dielectric sleeve 616 is preferably 
a solid dielectric material of substantially greater tensile strength and 
modulus, as well as being more homogeneous and uniform in structure than 
dielectric insert 618. The structure and properties of dielectric sleeve 
616 permit a more reliable joint between sleeve 616 and conductive insert 
606. This also helps avoid the possibility of formation of a void space 
between dielectric sleeve 616 and conductive insert 606 which can cause a 
flaw in the subsequently applied overlying dielectric coating (not shown). 
FIG. 6B illustrates a similar dielectric insert 620 where the entire 
dielectric insert 620 is a porous dielectric. The amount of porosity and 
the size of the pores in insert 620 are critical in terms of avoiding the 
formation of plasma glow within dielectric insert 620, which can lead to 
arcing to a semiconductor substrate supported over the surface of 
dielectric insert 620. Dielectric insert 620 is of the same general 
composition and structure as that described for dielectric insert 420. 
FIG. 6C shows yet another preferred embodiment of the dielectric insert of 
the present invention. Dielectric insert 630 comprises a dielectric sleeve 
636 and a dielectric center plug 638 where there is an annular opening 640 
between sleeve 636 and center plug 638. Center plug 638 is held in place 
by an adhesive or ceramic bonding material such as fusible glass ceramic, 
642 which anchors plug 638 to sleeve 636. By adjusting the size of 
dielectric center plug 638, the gas flow rate through dielectric insert 
630 is adjusted. Again, an overlying dielectric layer (not shown) is 
applied over the surfaces of electrostatic chuck pedestal 600, surface 612 
of conductive insert 606 and over dielectric insert 630. Subsequently the 
overlying dielectric layer is processed back to expose annular opening 640 
in dielectric insert 630 while leaving at least a portion of sleeve 636 
entrapped under the overlying dielectric layer. 
FIGS. 7A through 7F illustrate a particularly preferred embodiment of the 
present invention which provides ease in manufacturability. 
With reference to FIG. 7F, the final structure is one wherein electrostatic 
chuck pedestal 700 includes at least one heat transfer fluid flow channel 
708 which contains dielectric insert 718. Dielectric insert 718 is sized 
to provide an annular opening for gas flow between heat transfer fluid 
flow channel 708 and dielectric insert 718. 
The dielectric surface layer 714 which overlays pedestal 700 also includes 
at least one opening, which lies directly over heat transfer fluid flow 
channel 708 and is sized to provide for the insertion of dielectric insert 
718 with an annular space between the opening in layer 714 and insert 718. 
Thus, heat transfer gas can flow from channel 708 to the surface of 
dielectric surface layer 714. Dielectric insert 718 is held in place at 
the bottom of heat transfer fluid flow channel 708 by an adhesive or 
bonding ceramic 720. It is not critical that dielectric insert 718 be 
centered in the opening 716 through dielectric surface layer 714, so long 
as the heat transfer gas can flow through the space between dielectric 
insert 718 and opening 716. 
Fabrication of the preferred embodiment shown in FIGS. 7A through 7F is as 
follows. As shown in FIG. 7A, at least one buried heat transfer fluid flow 
channel 708 is prepared in pedestal 700 using techniques known in the art 
such as welding and brazing. At least one hole or opening 710 is then 
drilled through the surface 706 of pedestal 700 to connect with heat 
transfer fluid flow channel 708, as shown in FIG. 7B. The diameter of 
opening 710 is generally, but not by way of limitation, about 0.080 inches 
(about 2 mm) or larger. Although this diameter is not critical, the 
tolerance of the selected diameter should be held within about .+-.0.005 
inches (.+-.0.13 mm). 
A space-holding, masking pin 712 is then placed through opening 710 and 
into heat transfer fluid flow channel 708 so that overlying surface 
dielectric layer 714 can be applied without the dielectric material 
entering into opening 710. This is the reason the tolerance of the 
diameter of opening 710 must be carefully controlled. Masking pin 712 is 
preferably constructed from a material to which alumina or alumina-titania 
dielectric coating does not adhere. A Teflon.RTM. (trademark of DuPont 
Company) masking pin 712 has been found to work well. Space-holding 
masking pin 712 is generally 3 to 6 diameters high, with the controlling 
feature being functionality. In particular, masking pin 712 is preferably 
of sufficient height that, after application of a dielectric coating layer 
714 (as shown in FIG. 7C), it can be grabbed and pulled out (removed). 
However, the height of masking pin 712 is preferably such that it does not 
cast a shadow which prevents application of the dielectric coating in 
direct contact with masking pin 712 around its entire diameter. 
Dielectric coating layer 714 is typically applied to a thickness which is 
about 10 to 20 mils (0.010 to 0.020 inches, 0.25 mm to 0.50 mm) greater 
than the desired final thickness of the dielectric layer 714. After 
application of dielectric layer 714, and removal of masking pin 712, 
dielectric layer 714 is ground to final thickness and the electrostatic 
chuck is cleaned of grinding residue. This provides a smooth, flush 
surface 722 dielectric layer 714 (flat to at least 1.0 mil (0.001 inches 
or 0.025 mm), which is interpreted to mean that all points on the surface 
lie within two parallel planes spaced 0.001 inch (0.025 mm) apart, as 
shown in FIG. 7D. Opening 716 through dielectric layer 714 and opening 710 
through pedestal 700 have the diameter typically about 0.080 inches (2 mm) 
or more, as previously described. This diameter permits ease in removal of 
any residue, such as the grinding residue. This is an advantage over other 
embodiments of this invention having smaller diameter openings, which are 
more difficult to clean. 
A measured quantity of adhesive or bonding ceramic 720 is then deposited at 
the base of heat transfer fluid flow channel 708, directly beneath 
openings 716 and 710, as shown in FIG. 7E. The thickness of adhesive layer 
720 is such that variations in the length of dielectric pin 718 can be 
compensated for while maintaining a flush top surface 724 which includes 
the combination of surface dielectric layer 714 and dielectric pin 718. 
Dielectric pins 718 are typically fabricated from centerless ground 
ceramic, typically alumina, having a diameter ranging from about 0.003 
inches to about 0.005 inches (about 0.076 mm to about 0.102 mm) less than 
the bore diameter of openings 716 and 710. Typically dielectric pins 718 
are cut to be at least 0.010 inch (0.25 mm) shorter than the bore depth 
through surface dielectric layer 714 and pedestal 700 to the bottom 726 of 
heat transfer fluid flow channel 708. Dielectric pins 718 may be cut as 
much as 0.040 inch (1 mm) undersized in length. 
Dielectric pins 718 are inserted through openings 716 and 710 and into 
adhesive 720 resting on the bottom 726 of heat transfer fluid flow channel 
708. It is important that the pins 718 are positioned to provide the flush 
top surface 724 previously described, and this is accomplished using the 
depth of penetration of pins 718 into the thickness of adhesive 720 to 
make up any differences in length of pins 718. It is not critical that 
dielectric pins 718 be centered within the bore openings 716 and 710, 
variation is allowable, as shown in FIG. 7F. The heat transfer fluid, 
typically a gas, flows out of the annular opening between dielectric pin 
718 and the openings 710 through pedestal 700 and 716 through dielectric 
layer 714. 
An alternative to using adhesive 710 is to machine an opening into the 
bottom surface 716 of heat transfer fluid flow channel 708 into which 
dielectric insert 718 and be interference fitted or staked. However, this 
is not the most preferred method, since placement of the dielectric insert 
is more difficult. 
For an electrostatic chuck used in combination with an 8 inch diameter 
semiconductor wafer, wherein there are approximately 180 gas flow pathways 
including dielectric inserts positioned in a ring around the periphery of 
the electrostatic chuck, the circular opening 710 in conductive insert 700 
typically ranges from about 0.040 to about 0.400 inches (1 mm to about 10 
mm in diameter), with the dielectric insert having an outer diameter 
approximately 0.005 inches (0.123 mm) smaller than the diameter of the 
cavity. These dimensions are adjusted depending on the kind of heat 
transfer fluid (cooling gas) used, the pressures used within the 
processing chamber, and the amount of desired gas flow to the surface of 
the electrostatic chuck. 
The fabrication of this embodiment of the present invention utilizes 
mature, straightforward manufacturing steps which are not risky to the 
electrostatic chuck as a whole, which can be reworked up through the final 
step in fabrication if necessary. 
In general, the fabrication techniques used to construct the inserts and 
structures of the present invention are known in the art, and one skilled 
in the engineering of materials can make adjustments as necessary in the 
construction to account for the strength of the various materials which 
are used. However, there are some techniques which are particularly 
important which are described herein to enable one skilled in the art to 
more easily practice the invention. 
As previously described, the preferred material of construction for the 
dielectric inserts is alumina or alumina-titania. This is because the 
typical electrostatic chuck pedestal is constructed from aluminum, and the 
conductive insert which comprises the gas flow channel and which is 
inserted into the electrostatic chuck is also constructed of aluminum. The 
use of an alumina dielectric insert provides chemical compatibility while 
utilizing readily available materials. Since electrostatic chucks are used 
in process environments such as plasma etch and chemical vapor deposition, 
the chucks can be exposed to process temperature ranges from about 
-10.degree. C. to about 150.degree. C. With the same temperature range and 
coefficient of expansion differences between materials in mind, the 
overlying dielectric surface layer of the electrostatic chuck is 
preferably constructed from alumina or alumina/titania in the manner 
previously described which provides micro cracks capable of compensating 
for expansion differences between the aluminum substrate and the overlying 
alumina or alumina-titania dielectric coating (typically about 5-2% 
titania). Preferably this dielectric layer is spray coated over the 
underlying surfaces. As previously described, the aluminum surfaces to 
which the alumina dielectric layer is to be applied are typically abraded 
(roughened) using grit blasting prior to plasma spray coating of the 
alumina dielectric layer. The roughening provided for mechanical binding 
of the sprayed alumina layer to the surface of the aluminum. 
It has been discovered that by controlling the angle of incidence of the 
grit which is impacted upon the aluminum surface and by rotating the 
aluminum pedestal during the grit blasting, it is possible to create 
grooves which undercut the aluminum surface in a manner which enables the 
mechanical locking of a dielectric coating subsequently applied over the 
grooved, undercut aluminum surface. Typically, the aluminum pedestal is 
fixed to a rotating turntable during grit blasting; the turntable rotates 
the aluminum pedestal around a centerline. The grit is applied to the 
surface of the aluminum pedestal using a nozzle which is oriented at an 
angle relative to the surface of the aluminum pedestal and which travels 
from near the outer edge of the aluminum pedestal toward the center of the 
aluminum pedestal. To maintain the depth and the pitch of the grooves 
created by the grit blasting constant, it is necessary to increase the 
rate of nozzle travel as the nozzle moves from the outer edge of the 
aluminum pedestal toward the center of the pedestal. An improvement is 
peel strength of about 20% or greater has been observed for a dielectric 
coating of alumina applied by plasma spraying over an aluminum pedestal 
surface prepared in this manner. 
For example, an aluminum pedestal was fixed to a turntable which rotated at 
about 20 to 30 revolutions per minute (rpm). The angle of incidence of the 
nozzle relative to surface of the aluminum pedestal was about 70.degree.. 
The grit particle size was about 60 to 80 mesh, applied using the kind of 
nozzle commonly used for paint removal via grit blasting. After grit 
blasting, the height of the grooves was about 0.001 inch (0.025 mm) and 
the pitch of the grooves was about 0.003 inch (0.075 mm). Subsequent to 
preparation of the surface of an aluminum pedestal in this manner, a 
coating of plasma sprayed alumina was applied over the prepared aluminum 
pedestal surface which was at a temperature of about 60.degree. C. to 
about 80.degree. C. The plasma sprayed alumina was applied at an angle of 
about 80.degree. to 90.degree. degrees (nearly perpendicular) relative to 
the surface of the aluminum pedestal. The plasma-sprayed alumina coating 
tends to bounce off of surfaces, so it is important to apply the coating 
at a proper angle relative to the aluminum pedestal surface. After 
cooling, the peel strength of the plasma sprayed alumina was tested using 
ASTM methods and found to have improved about 20% over that obtained when 
aluminum pedestal was prepared using prior art methods. 
The following recommendations are general to the methods used to obtain the 
best results using the present invention. In the embodiments of the 
invention where the dielectric insert is designed to be in close contact 
with a cavity or opening within a conductive layer or a conductive insert, 
typically close contact is achieved using an interference fit or press 
fit. Care must be taken during the press fitting to apply pressure evenly 
to the surface of the dielectric insert so that the insert is not 
fractured. It was found to be advisable to build a tool designed to fit 
the surface of the dielectric insert and to apply even pressure to that 
surface during the press fitting. The insert may be tapered toward its 
bottom edge to permit easier progress into the receiving cavity within the 
conductive insert. Since an alumina dielectric insert has a relatively 
hard, sharp edge, it is possible to press this edge into an underlying 
aluminum cavity with sufficient pressure to cut into the aluminum, 
providing a press fit at the bottom of the aluminum cavity. However, as 
mentioned above, the pressure must be applied evenly so as not to crack or 
fracture the dielectric insert. 
In general, a dielectric insert of a solid ceramic must be small in size, 
about 0.020 to about 0.400 inches in diameter, to avoid mechanical failure 
from compressive loads applied as a result of temperature cycling during 
the semiconductor substrate processing. The incompatibility of the thermal 
coefficient of expansion between the ceramic dielectric and the metal 
structure of the electrostatic chuck creates these compressive loads 
during temperature cycling. The small size of the dielectric insert also 
makes it possible to pre-load the insert into the electrostatic chuck in 
compression, using an interference fit. 
When the dielectric insert is to be in close contact with a conductive 
surface, with a dielectric coating applied over the combination of 
surfaces, it necessary that the interface be particularly close to avoid 
the formation of coating flaws, particularly in the case of a 
plasma-sprayed dielectric coating. Due to the inability of a plasma 
sprayed coating to form a dense structure on surfaces, it is preferable to 
apply the coating at an angle ranging between about 80.degree. and about 
90.degree. with respect to the surface being coated. Perpendicular to 
surface application of a plasma-sprayed dielectric coating is preferred to 
obtain maximum coating density. 
In the case of solid ceramic dielectric inserts, a close fit to the hole or 
opening into which the insert is placed is obtained using a press fit. 
Press fit can be accomplished by deforming the metal of the conductive 
material in contact with the dielectric insert (staking). A dielectric 
insert can also be held in place by a machined interference fit of about 
0.001 inch (0.025 mm) or greater, where the installation of the dielectric 
insert cuts the metal of the conductive material in contact with the 
dielectric insert forming an interference fit sufficient to retain the 
dielectric insert against forces encountered in subsequent handling prior 
to ceramic coating. A large interference between the insert and 
surrounding conductive material helps stabilize the overlying dielectric 
coating applied over the surface of the conductive layer and the insert 
within that conductive layer. Failure of the interference which holds the 
insert within the conductive layer can promote thermal expansion cracking 
of the overlying coating, leading to injection of plasma in the presence 
of high bias potential and rapid break down of the dielectric coating 
overlying the joint between the insert and the conductive layer. 
After a dielectric insert of the present invention is fitted within a 
conductive insert (which contains a gas flow cavity), an overlying 
dielectric layer is applied. Gas flow or gas pressure within the insert 
may be used during application of the overlying dielectric layer, to 
prevent plugging of a pre-drilled insert. A plasma sprayed dielectric 
layer, such as plasma sprayed ceramic is not homogeneous. In the case of 
alumina, the molten alumina particle contacts the surface to which it is 
applied, and shrinks as it cools. Since the alumina bonds to its contact 
surface, it cracks as it cools. The cracking is tolerable so long as the 
cracks are submicron sized and relatively uniformly distributed over the 
entire surface. The cracks caused by thermal expansion of the underlying 
layer cannot propagate so long as the sprayed layer is not homogeneous. It 
is important to have controlled discontinuities in the ceramic dielectric 
layer if the chuck is to encounter process temperatures other than that at 
which the ceramic dielectric layer was sprayed. Typically an alumina 
dielectric coating is applied at about 40.degree. C. Other dielectric 
materials having a coefficient of linear expansion near that of aluminum 
are acceptable. For example, engineering thermoplastics loaded with about 
35% to about 45% by volume glass or mineral fillers, to produce a compound 
which is injection moldable, can be used. The dielectric material can be 
thermal setting or thermoplastic, so long as it is not notch sensitive and 
can function at the operational temperatures of the electrostatic chuck. 
Preferably openings are created through an overlying alumina dielectric 
layer using an excimer laser rather than a CO.sub.2 laser since the 
alumina is relatively transparent to the CO.sub.2 laser. Openings through 
an overlying ceramic dielectric layer can also be created by mechanical 
drilling using a diamond or cubic boron nitride drilling tool. 
The above described preferred embodiments are not intended to limit the 
scope of the present invention, as one skilled in the art can, in view of 
the present disclosure expand such embodiments to correspond with the 
subject matter of the invention claimed below.