19
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`Europaisches Patentamt
`European Patent Office
`Office europeen des brevets
`
`© Publication number : 0 601 788 A 2
`
`EUROPEAN PATENT A P P L I C A T I O N
`
`© Application number: 93309608.3
`© Date of filing : 01.12.93
`
`© Int. CI.5: C23C 14/50, H01L 21/00,
`B23K 26^00, C23C 1 6/44
`
`© Priority : 02.12.92 US 984797
`14.10.93 US 137279
`(43) Date of publication of application :
`15.06.94 Bulletin 94/24
`@ Designated Contracting States :
`DE ES FR GB IT NL
`© Applicant : APPLIED MATERIALS, INC.
`3050 Bowers Avenue
`Santa Clara California 95054-3299 (US)
`© Inventor : Collins, Kenneth S.
`165 Knightshaven Way
`San Jose CA 95111 (US)
`Inventor : Trow, John R.
`162 Knightshaven Way
`San Jose CA 95111 (US)
`
`Inventor : Tsui, Hoshua Chiu-Wing
`613 Azevedo CT
`Santa Clara, CA 95051 (US)
`Inventor : Roderick, Craig A.
`776 Pineview Drive
`San Jose CA 95117 (US)
`Inventor : Bright, Nicolas J.
`12133 Kirkbrook Drive
`Saratoga, CA 95070 (US)
`Inventor : Marks, Jeffrey
`4730 Cielo Vista Way
`San Jose, CA 95129 (US)
`Inventor : Ishikawa, Tetsuya
`3-14-24 Kosaku
`Funabashi City, Chiba 273 (JP)
`Inventor: Ding, Jian
`1337 Glen Haven Drive
`San Jose, CA 95129 (JP)
`© Representative : Bayliss, Geoffrey Cyril et al
`BOULT, WADE & TENNANT
`27 Furnival Street
`London EC4A 1PQ (GB)
`
`© Electrostatic chuck usable in high density plasma.
`
`to an electrostatic
`© The disclosure relates
`chuck (10) for holding a wafer (12) in a plasma
`processing chamber. The chuck
`includes a
`pedestal (14) having a top surface (46), an
`internal manifold for carrying a cooling gas, and
`a first plurality of holes (48) leading from the
`internal manifold toward said top surface ; and
`a dielectric layer (44) on the top surface of the
`pedestal. The dielectric layer has a top side (54)
`and second plurality of holes (58), each of
`which is aligned with a different one of the
`holes of the first plurality of holes in the pedes-
`tal. The first and second holes form a plurality of
`passages extending from the internal manifold
`to the
`top side of the dielectric
`layer and
`through which the cooling gas is supplied to the
`backside of the wafer. Each of the first holes
`(48a to g) and the second hole (58) aligned
`therewith form a different one of the plurality of
`passages. The passages are concentrated in
`regions of the dielectric layer that are in proxim-
`ity to regions of higher leakage of cooling gas
`when the wafer is held against the electrostatic
`chuck by an electrostatic force.
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`The present invention relates to an electrostatic
`chuck ("E-chuck") for high density plasma processing
`of articles such as semiconductor wafers. The E-
`chuck overcomes the difficulties present with prior
`art mechanical and electrostatic chucks in high den-
`sity plasma processing applications and is particular-
`ly well suited to processes in which the article may be
`RF biased for enhanced process performance in a
`high density plasma process, such as that used in
`Si02 etching. The E-chuck addresses the require-
`ments of uniform coupling of electrical and thermal
`energy in a hostile electrical, thermal and chemical
`environment.
`In the plasma processing of articles such as sem-
`iconductor wafers, a common problem is the coupling
`of electrical energy to the article being processed.
`Typically, electromagnetic coupling of RF energy into
`the "source" region of a plasma chamber is employed
`to generate and maintain a high electron density plas-
`ma having a low particle energy. In addition, RF
`"bias" energy is usually capacitively coupled in the
`plasma via the article being processed to increase
`and control the energy of ions impinging on the arti-
`cle.
`
`In a typical high density plasma reactor, the driv-
`ing point RF "bias" impedance presented by the plas-
`ma is very low. To achieve uniform ion energy and
`flux to the article being processed (typically essential
`for etching or other plasma processes), uniform cou-
`pling of RF "bias" energy through the article being
`processed to the plasma is required. The article being
`processed typically is held against some kind of
`chuck and RF bias energy is applied to the chuck.
`What is desired is a constant plasma sheath voltage
`across the surface of the article being processed.
`The degree to which such a uniform plasma
`sheath voltage can be achieved is a function not only
`of the plasma density uniformity as generated by the
`plasma source, but is also a function of the impe-
`dance per unit area of the plasma sheath adjacent to
`the article, the impedance per unit area of the article,
`the impedance per unit area of any gap between the
`article and the chuck and the impedance per unit
`area of the chuck.
`Besides electrical coupling, the chuck should be
`tightly thermally coupled to the article being process-
`ed. Typically the temperature of the article is a proc-
`ess parameter to be controlled and this normally
`means removing heat from or adding heat to the ar-
`ticle during processing. Heat transfer in a low pres-
`sure or vacuum environment such as that used for
`plasma processing is generally poor. Some means of
`providing for adequate heat transfer between the ar-
`ticle being processed and adjacent surfaces is usu-
`ally necessary.
`Typical prior art chucks mechanically clamp an
`article to the chuck with a clamp ring applying a hold-
`ing force at the periphery of the article. The thermal
`
`contact between article and chuck is generally insuf-
`ficient to accommodate the heat load imposed by the
`plasma on the article. Without some means of im-
`proved thermal contact between article and chuck,
`the temperature of the article may rise out of accept-
`able limits.
`Gas is typically introduced between the article
`and chuck to enhance thermal contact and heat
`transfer from the article to the chuck. The gas pres-
`sure required is a function of the heat load imposed
`by the plasma, the desired maximum article temper-
`ature, the temperature at which the chuck can be
`maintained (such as with liquid cooling), the choice of
`cooling gas and the article/gas and gas/chuck ac-
`commodation coefficients (measures of how effec-
`tively heat is transferred between a gas and a sur-
`face). For biased high density plasma applications,
`helium gas is used as the cooling gas and the gas
`pressure required is typically in the 5 to 30 torr range.
`For "low pressure" plasma processes (those op-
`erating in millitorr pressure range), some means must
`be provided to allow a significantly higher pressure in
`the region between the article and chuck with respect
`to the ambient pressure in the process chamber. In
`addition, a leak of cooling gas into the process envir-
`onment may produce undesirable results. Typically
`some kind of seal, usually an elastomer, is used to al-
`low maintenance of the pressure difference between
`the two regions.
`If the article to be processed is simply mechani-
`cally clamped at its periphery to the chuck, and gas
`introduced between article and chuck, the article will
`bow away from the chuck due to the pressure differ-
`ence across the article. If a flat chuck is used on a
`disk shaped article, a large gap results between the
`article and the chuck with a peak gap at the center.
`Under such conditions, thermal and electrical cou-
`pling between the article and the chuck are non-uni-
`form. Mechanically clamped chucks typically pre-
`compensate such article-bowing by attempting to
`match the chuck's surface to the curvature of the ar-
`ticle under stress. Theoretically, this can be done for
`simply shaped articles (such as disks), but the pres-
`ence of discontinuities or complex shapes make ana-
`lytical precompensation impossible, and trial-and-er-
`ror is required. Mismatches in curvatures between
`the article and the chuck result in a variable gap be-
`tween such surfaces, resulting in non uniform elec-
`trical and thermal coupling.
`Electrostatic chucks have been proposed to
`overcome the non-uniform coupling associated with
`mechanical chucks. Electrostatic chucks employ the
`attractive coulomb force between oppositely charged
`surfaces to clamp together an article and a chuck. In
`principle, with an electrostatic chuck, the force be-
`tween article and chuck is uniform for a flat article
`and flat chuck. The electrostatic force between the
`article and the chuck is proportional to the square of
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`the voltage between them, proportional to the rela-
`tive permittivity of the dielectric medium separating
`them (assuming conductivity is negligible) and in-
`versely proportional with the square of the distance
`between them. Typically for biased-article high den-
`sity plasma processing applications (such as Si02
`etching) a cooling gas is required to improve the heat
`transfer between article and chuck to acceptable lev-
`els. Introduction of gas cooling between article and
`chuck, while required to achieve adequate heattrans-
`fer, causes problems with prior art electrostatic
`chucks when used in biased-article high density plas-
`ma applications.
`In particular, the requirement of introducing cool-
`ing gas in the region between article and chuck re-
`quires that some discontinuity be introduced in the
`chuck surface, typically some type of hole(s) through
`the chuck to a gas passage behind the surface. The
`introduction of any discontinuity in the chuck surface
`distorts the electric field in the vicinity of the discon-
`tinuity, making arc breakdown and glow discharge
`breakdown of the cooling gas more probable. With
`DC bias applied between an article and a chuck, and
`RF bias applied to the chuck, gas breakdown be-
`comes probable with prior art electrostatic chucks
`such as described in U.S. Patents 4,565,601 and
`4,771,730.
`In the '601 patent, a plurality of radial cooling gas
`dispersion grooves in an upper surface of a plate
`electrode connect to and extend outwardly from the
`relatively large upperend of a cooling gas supply pipe
`extending vertically to the upper surface of the plate
`electrode. Cooling gas from the supply pipe travels
`outwardly in the radial grooves and into a plurality cir-
`cular gas dispersion grooves also formed in the upper
`surface of the plate electrode coaxial with the gas
`supply pipe. The upper surface of the plate electrode
`with the radial and circular patterns of grooves is cov-
`ered with a thin insulating film upon which the article
`to be process is placed. The upper open end of the
`gas supply pipe forms a relatively large discontinuity
`in the upper surface of plate electrode. The radial and
`circular grooves are relatively wide and deep, slightly
`less than the diameter of the gas supply pipe, and
`form additional relatively wide and deep discontinui-
`ties in the upper surface of the plate electrode and
`relatively deep separation gaps between the plate
`electrode and the article to be processed. Further, ir-
`regularities in the coated surfaces of the grooves pro-
`duce non-uniformities in gas flow and in the spacing
`of the article and the plate electrode. Undesired arc
`and glow discharge breakdowns of the cooling gas
`would occur if such an electrostatic chuck were em-
`ployed in a high RF power, high density plasma reac-
`tor.
`
`The same problems are particularly inherent in
`the electrostatic chuck of the '730 patent which in-
`cludes a central and/or plurality of relatively large gas
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`feeding tubes to the upper surface of a plate elec-
`trode.
`What is desired is an electrostatic chuck that can
`accommodate cooling gas between the workpiece
`and the chuck and which is designed to avoid gas
`breakdown even when the chuck is used in a plasma
`reactor environment including high RF bias power
`and high density plasma.
`The E-chuck of the present invention is particu-
`larly useful in electrostatically holding an article to be
`processed, such as a semiconductor wafer, in a plas-
`ma reaction chamber while distributing a cooling gas
`between the face of the E-chuck and the underside
`of the article, without contributing to arc or glow dis-
`charge breakdown of the cooling gas.
`The E-chuck comprises a metal pedestal having
`a smooth upper surface for electrostatically attract-
`ing and supporting the article. Asmooth layer of a di-
`electric material is bonded to the upper surface of the
`pedestal.
`One aspect of the present invention is that one
`or more conduits are formed inside the metal pedes-
`tal to permit cooling gas to be transported to one or
`more cavities just below the dielectric. A plurality of
`perforations which are much smaller in diameterthan
`the conduits extend from the upper surface of the di-
`electric down to the cavities. These perforations pro-
`vide a path for the cooling gas to flow from the cavi-
`ties in the pedestal to the region between the dielec-
`trie layer and the semiconductor wafer or other work-
`piece. The invention permits the use of perforations
`which are much smaller in diameterthan the smallest
`conduit that could be fabricated in the body of the
`metal pedestal. The use of such small perforations as
`the outlets for the cooling gas greatly increases the
`amount of RF bias power and plasma density to which
`the E-chuck can be exposed without causing break-
`down of the cooling gas.
`A second, independent aspect of the invention is
`that a cooling gas distribution channel is formed by
`one or more grooves in the upper surface of the di-
`electric layer for distributing a cooling gas between
`the upper surface of the dielectric layer and the un-
`derside of the article supported on the pedestal. Ac-
`cording to this aspect of the invention, the depth of
`the grooves is small enough to maintain a low product
`of cooling gas pressure and groove depth so as to
`avoid glow discharge breakdown of the cooling gas,
`and the dielectric layer beneath the grooves is thick
`enough to prevent dielectric breakdown. In contrast
`with conventional E-chucks, the present invention lo-
`cates the gas distribution channels in the layer of di-
`electric material rather than in the metal pedestal,
`thereby minimizing discontinuities in the electric field
`adjacent the pedestal which could contribute to arc or
`glow discharge breakdown of the cooling gas.
`In general, in another aspect the invention is an
`electrostatic chuck for holding a wafer in a plasma
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`processing chamber. The electrostatic chuck sup-
`plies a cooling gas to the backside of the wafer
`through a plurality of holes in the chuck that are dis-
`tributed near the regions of highest leakage of the
`cooling gas, i.e., the holes are distributed so as to
`"feed the leaks" while also supplying gas to the back-
`side of the wafer. The chuck includes a pedestal hav-
`ing a top surface, an internal manifold for carrying a
`cooling gas, and a first plurality of holes leading from
`the internal manifold toward the top surface. It also
`includes a dielectric layer on the top surface of the
`pedestal. The dielectric layer has a top side and sec-
`ond plurality of holes, each of which is aligned with a
`different one of the holes of the first plurality of holes.
`The first and second holes forming a plurality of pas-
`sages extending from the internal manifold to the top
`side of the dielectric layerand through which the cool-
`ing gas is supplied to an interface formed by the back-
`side of the wafer and the top side of the dielectric lay-
`er. Each of the first holes and the second hole aligned
`therewith form a different one of the plurality of pas-
`sages. The passages are concentrated in regions of
`said dielectric layer that are in proximity to regions of
`higher leakage of cooling gas when the wafer is held
`against the electrostatic chuck by an electrostatic
`force.
`In general, in yet another aspect the invention is
`an electrostatic chuck in which an internal gas distrib-
`ution manifold is formed by welding an insert into a
`groove in the top of the chuck. In particular, the elec-
`trostatic chuck includes a pedestal having a top sur-
`face, an internal manifold for carrying a cooling gas,
`and a first plurality of holes connecting the internal
`manifold to the top surface; and it includes a dielectric
`layer on top of the pedestal. The dielectric layer has
`a second plurality of holes, each of which is aligned
`with a different one of the holes of the first plurality
`of holes. The first and second holes form a plurality
`of passages extending from the internal manifold to
`the top of the dielectric layer and through which the
`cooling gas is supplied to a wafer backside. Each of
`the first holes and the second hole aligned therewith
`form a different one of the plurality of passages. The
`pedestal includes a groove formed in its top surface
`and there is an insert in the groove. The insert has a
`channel formed in its underside and the channel in
`combination with the groove form a cavity that is part
`of the internal manifold. At least some of the passag-
`es pass through the insert into that channel.
`In general, in still another aspect, the invention
`is a computer-implemented method for preparing an
`electrostatic chuck within a plasma chamber to re-
`ceive a next wafer for plasma processing after a pre-
`vious wafer has completed plasma processing. The
`method includes the steps of removing the previous
`wafer from the electrostatic chuck in the plasma
`chamber; introducing a non-process gas into the
`plasma chamber without any wafer present on the
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`electrostatic chuck; striking a plasma in the plasma
`chamber; running that plasma for a preselected per-
`iod of time; terminating the plasma at the end of the
`preselected period; and placing the next wafer on the
`electrostatic chuck for plasma processing.
`In general, in another aspect, the invention is a
`method of forming passages in an electrostatic chuck
`that connect an internal manifold to the top surface
`of the dielectric. The method includes the steps of
`drilling a first plurality of holes into the pedestal ex-
`tending from the top surface of the pedestal into the
`manifold; forming a dielectric layer on the surface of
`the pedestal, the dielectric layer being sufficiently
`thick to bridge over the holes of the first plurality of
`holes; and drilling a second plurality of holes in the di-
`electric layer, each hole of the second plurality of
`holes aligned with a different one of the holes of the
`first plurality of holes.
`In general, in still another aspect, the invention
`is a method for dechucking a waferf rom a electrostat-
`ic chuck following a plasma process. The method in-
`cludes the steps of: at the completion of the plasma
`process and while the RF source power is still on,
`turning off the cooling gas; changing a DC potential
`of the electrostatic chuck; slightly separating the wa-
`fer from the electrostatic chuck while the RF source
`plasma is still present; after slightly separating the
`waferfrom the electrostatic chuck, turning off the RF
`source; after turning off the RF source power, sepa-
`rating the wafer further from the electrostatic chuck;
`and removing the wafer from the plasma chamber.
`The following is a description of a number of pre-
`ferred embodiments of the invention, reference being
`made to the accompanying drawings in which:-
`Figure 1 is a schematic view of an "OMEGA"
`biased high density plasma reaction chamber of
`Applied Materials, Inc., Santa Clara, California il-
`lustrating the electrical circuitry therefor and in-
`cluding the E-chuck of the present invention to
`hold a semiconductor wafer on a pedestal of the
`E-chuck. The "OMEGA" reaction chamber (with-
`out a chuck) is described and illustrated in EP-A-
`0552491. Reference should be made to that
`specification for a complete description of the
`"OMEGA" reaction chamber in which the E-
`chuckof the present invention is particularly use-
`ful.
`Figure 2 is top view of a pedestal included in the
`E-chuck of the present invention and illustrates
`a gas distribution system formed in the upper
`surface of a layer of dielectric material bonded to
`the upper surface of the pedestal to distribute
`cooling gas over the surface of the layer from
`cooling gas receiving holes extending upwardly
`from an underside of the pedestal to upper ends
`adjacent the upper surface of the pedestal under
`the layer of dielectric material.
`Figure 2a is an enlarged showing of Fig. 2.
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`Figure 3 is sectional side view along the lines 3-
`3 in Figure 2 showing the internal construction of
`the pedestal and layer of dielectric material on
`the upper surface thereof with the cooling gas re-
`ceiving holes extending vertically from an under-
`side of the pedestal and a lift pin receiving hole
`for use in mechanically positioning and lifting the
`article being processed within the reaction
`chamber.
`Figure 3a is an enlarged version of Figure 3.
`Figure 4 is an enlarged illustration of the portion
`of the pedestal and layer of dielectric material
`within the circle 4 of Figure 2 and shows the
`grooves defining the gas distribution system in
`the layer of dielectric material with gas distribu-
`tion holes through intersections of the grooves
`and metal of the pedestal over the upper ends of
`the cooling gas receiving holes to define a cluster
`of gas distribution holes over each cooling gas re-
`ceiving hole.
`Figure 5 is an enlarged version of a portion of the
`pedestal within the circle 5 of Figure 2 more
`clearly illustrating the intersecting nature of the
`grooves forming the gas distribution system, a
`lift pin hole having an annulus there around free
`of intersecting gas distributing grooves and a
`portion of an outer annulus bounding a central
`portion of the pedestal also free of intersecting
`gas distributing grooves.
`Figure 6 is an enlarged version of the portion of
`the pedestal within the circle 6 in Figure 2 more
`clearly illustrating the intersecting nature of the
`grooves and gas distribution holes formed at the
`intersection of such grooves over a cooling gas
`receiving hole.
`Figure 6a is an enlarged version of Figure 6.
`Figure 7 is an enlarged showing of one form of
`gas distribution hole through the layer of dielec-
`tric material and the pedestal metal at the upper
`end of a cooling gas receiving hole.
`Figure 8 is an enlarged illustration of an alternate
`version of a gas distribution hole similar to that
`shown in Figure 7.
`Figure 9 shows an alternative embodiment of the
`pedestal which includes a Helium cooling gas dis-
`tribution system that introduces Helium cooling
`gas around the periphery and around the lift pin
`holes;
`Figure 10 shows a cross-section of the pedestal
`of Fig. 9 taken along the A-A section line in Fig.
`9;
`Figure 11 shows the bottom of the pedestal of
`Fig. 9;
`Figure 12 shows a cross-section of the pedestal
`of Fig. 11 taken along the B-B section line in Fig.
`11;
`Figure 13 shows an enlarged view in cross-
`section of the large annular insert ring;
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`Figure 14 shows a closeup of the groove network
`on the surface of the dielectric layer;
`Figure 15 shows a closeup of another portion of
`the groove network on the surface of the dielec-
`tric layer;
`Figure 16 shows a closeup of yet another portion
`of the groove network on the surface of the di-
`electric layer;
`Figure 17 shows a cross-sectional view of an al-
`ternative perforation design for carrying cooling
`gas to the underside of the wafer;
`Figure 1 8 is a flow chart of the sequence for pro-
`ducing the perforation shown in Fig. 17;
`Figure 19 is an enlarged cross-sectional partial
`view of the pedestal with a collar and a cover ring
`in place;
`Figure 20 is a closeup view of the gap and the
`overlap formed between the underside of the
`wafer and the collar shown in Fig. 19; and
`Figures 21 A-C present a flow chart of the wafer
`chucking and dechucking procedure and the post
`process procedure for reducing residual charge
`on the E-chuck.
`As shown in Figures 1 and 2, the E-chuck 10 of
`the present invention is adapted to support and elec-
`trostatically hold an article or workpiece 12 to be
`processed, such as a semiconductor wafer, on a ped-
`estal 14 within a high density plasma reaction cham-
`ber 16. Through gas distribution groove network 18,
`the E-chuck 10 affects a cooling of the pedestal 14
`and the wafer 12 supported thereon.
`The plasma reaction chamber 16 further in-
`cludes a cover ring 20 which is supported by four
`rods, not shown. The purpose of the cover ring is to
`prevent the plasma in the chamber above the work-
`piece from contacting, and thereby corroding, part of
`the E-chuck. Accordingly, the four rods position the
`cover ring within 0.01 inch of the edges of the work-
`piece and the E-chuck, a gap too small for the plasma
`to penetrate. The rods lift the cover away from the
`wafer and E-chuck when the wafer is being transport-
`ed to or from the E-chuck.
`Fora more detailed understanding of the plasma
`reaction chamber 16 and its operation in processing
`the wafer 12, the reader should refer to the drawings
`and detailed description contained in EP-A-0552491
`As shown in Figure 1 , the electrical circuit for the
`plasma reaction chamber 16 is conventional. It in-
`cludes a conventional DC voltage source 24 which
`supplies the clamping voltage which is coupled to the
`E-chuck 1 0 through a low pass filter 26 which isolates
`the DC voltage source 24 from the RF power supply
`27. RF source power and RF bias power are each
`coupled from the conventional RF power supply 27
`through a matching network 28, with the source pow-
`er being coupled to an inductive antenna 30 and the
`bias power being coupled to the pedestal 14. Aground
`reference for both the RF bias power and DC voltage
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`is a grounded top counter electrode 32. The DC vol-
`tage source 24 supplies -1000 volts for developing an
`electric field which electrostatically holds the wafer
`12 to the pedestal 14. When it is desired to release (or
`"de-chuck") the wafer 12, the source 24 may be
`switched either to a zero output voltage or to a re-
`verse polarity voltage if it is desired to accelerate the
`release of the wafer.
`The plasma reaction chamber 16 employs induc-
`tively-coupled RF power to generate and maintain a
`high density, low energy plasma. RF bias power isca-
`pacitively coupled to the plasma via the wafer 12 and
`E-chuck 10, with the grounded counter electrode 32
`located in the plasma source region providing a return
`path for bias current. With the configuration shown in
`Figure 1, plasma density is controlled by the RF
`source power, and ion energy is independently con-
`trolled by the RF bias power. The result is a uniform
`high density, low electron temperature plasma at the
`wafer 12 with ion energy controllable from about 15
`eV to several hundred eV. This configuration allows
`for etching of the wafer 12 with minimum charge up
`degradation and minimum energetic- particle dam-
`age.
`While the above described plasma reaction
`chamber 16 provides a high oxide etch rate, it does
`impose some severe hardware requirements, partic-
`ularly on the E-chuck 10. In particular, RF bias power
`must be uniformly coupled to the wafer 12 and to the
`plasma. With a high density, low electron tempera-
`ture plasma, the cathode sheath is very thin, usually
`less than one millimeter, and the impedance per unit
`area of the cathode sheath is primarily resistive and
`quite low. For wafers of typical resistivity, unless RF
`bias is very uniformly coupled to the wafer, ion/elec-
`tron current and ion energy will not be uniform. In ad-
`dition to the problem of uniform electrical coupling,
`tight uniform thermal coupling of the wafer 12 is very
`large. For example, at a typical RF source power of
`2800 watt and RF bias power of 1400 watt, approxi-
`mately 2 KW of heat must removed continuously from
`the wafer.
`To provide for such uniform coupling and thermal
`cooling, the pedestal 14 in the E-chuck 1 0 of the pres-
`ent invention preferably comprises a one piece alumi-
`num bl