The CeramicSociety The Ceramic Society

of of Japan Japan

t･t/1,.---:-{{,ge.sijikie.z.

Journal of

the

Ceramic Societv of Japan

tt,ttstsS.tt,S･et

[1] 1-6 09g2)

100

Effectof

on

ofAdditives Alumina

ElectrostaticChucksForce

Toshiya WATANABE R & D Div., TOTO

the Electrostatic Tetsuo KITABAYASHI

and

Ltd.. 2-8-l, Honson

7JL;- tgeig+

t, v

Chigtxsaki-shi, jfunagawa

253

eoesag"{z&(geVMhntzoptee

rcssQte･ akMMft 253 itrvaMA:N fiveeeee{de)geebllvept,

May [Received

Ceramic

electrostatic

chucks

made

of

alumina

1, 1991;Accepted Sptember 19, 1991]

trostatic force for such

practicalapplications. imagine an electrostatic chuck holding a When voltage material as a two-electrodecapacitor. isapplied to the electrodes of the electrostatic chuck as shown in F:ig.1, the electrostatic forcegenerated between the inaterial and the electrostatic chuck can be given by the followingequation:

ceram-

icswere fabricated. Transitionmetal oxides (1wt% Ti02, Cr203,Mn02, CoO, CuO) were added to centrol the resistance of the dielectric insulation layerof the chuck. The electrical resistivity and electrostatic force at R.T. -2000C were measured, and itwas revealed that the electrostatic

force of the

tricalresistance

of

examined, creasing

Let

depends on the elecAmong the additives found to be most effective for inchuck

the ceramic.

Ti02 was the adhesion

forceof the alumina

us

iechuck.

1. much

attention

Al"mina,

applied

chucking

has wafer

e,

volta:ge, respectively.

Thus the largerthe value of the dielectric constant of the insulating layeror the smaller the thickness of layer,the larger the generated electhe insulating trostatic forcesare, The upper limitof the forcecan be determined by the electric breakdowns strength

{ntroduction

as a semiconductor

a)

constant of d and Vrepresentthe dielectrie the insulator, the thickness of the insulator and the where

Recently,electrostatic

(X)2

F-SE

electrostat-

Eiectrostaticchuck, Key-words;Eiectroslatics, Tmnsitionmetal oxide, Resistivity

2-8-1

received

holder in

the dielectric layer.The breakdown strength of these dielectrics isat highestle to 20 kg!crn2, Ifthe

fabrication systems.i} Semiconductor fabricationprocesses such as exposure systems, CVD, and dry etchers are operated in a vacuum environment, so vacuurn chucks which are operated in

of

semiconductor

dielectric constant of an insulator is10 or so, forceobtained by this configuration 100 g/cm2, When this value is will be approximately an atmospheric environment cannot be utilized. by a vacuum chuck, 1 compared with that obtained Therefore,conventionally, a wafer stage, a mechanical holding system, has been used, Electrostatic kg/cm2, itis found to be too small, In an altlernating method, Waring et al.,4) devechucks offer several advantages as compared with mechanical holding systems. For instance,the fiat- loped an electrostatic chuck by making use of Johnsen-Rahbeck's force,which is generated when a ness of the wafer can be corrected by the uniform the semiconductor electrostatic force on the back of wafer. Furthermore, the close contact increasesheat conduction between the semiconductor wafer and wafer stage; heating and cooling of the wafer can 1-----"' thus be ediciently carried out, 2.F[ In recent years, there have been many reports on 3 +++++++S4 the importance of temperature control in dry etching fer elecand CVD processes,2)and expectations relative

the electrostatic

[

trostatic chucking as a means of controlling wafer . temperature are growlng, Because of these advantages, many studies have been conducted on the application of electrostatic chucking.3} However, there have been only a few reports on the practicalapplication of electrostatic

M

Fig. 1. Schematic of presentation ef electrostatic 1:material to be chucked, 2:air gap, 3:insulation layer, 4:inner electrode.

in semiconductor manufacturing processThis is because of the insuMcient value of elec-

chucking

es.

.

chuck.

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2

Effactof Additis,es on the E]ectrostatic Forceef AluminaElectrostatic Chucks

fiows across a conductor In thiscase, the microgap between the ceramic surface and the substance to be chucked acts as a capacitor, and the charges from the microcurrent accumulate on the surface of the insulation, generating electrostatic force, The electrostatic chuck by Waring et al., was originally intended foruse as a chuck formechanical processing, and SiC was used as the material. When the same electrostatic chuck is used in a semiconductor fabrication system, the current fiowingin the SiC seems to be too large,and it may cause damage to the material to be processed, Balakrishnan5)produced pelletsusing a mixture of MgO and Ti02, sintered them in an atmosphere consisting of hydrogen and nitrogen, and measured the eiectrostatic force of the material. The insulation resistance of the dielectric was approximately 105 9, and they reported that 1kgfcm2 was produced by the application of 100 V. When this processispractiminute and

amount

of current

a semiconductor,

cally

to semiconductor

applied

processing,

[ll"mi"a

However, no has

these conditions

satisfying

Furthermore,the trostaticforceand

electrostatic

the

values

the suriace

have

of

Cast./lb'

l"Dr'yin... 1･'u=ott-=g

.tttttttt'r'irttit/g(T.un ' gz･tenpaF,te) t.t.L tttttt'Lai,･iEglg

l--1..

-r'-ttttttt ilL-Si/'/ter'ing

chuck

must

high degree of fiatnessand good wearproperties,Also, in some CVD systems, wafers are processed at temperatures of 2000 to 3000C; thus high temperature tolerance is required. These requirements could be satisfied by employing a

resistant

ceramic

materials

as electrostatic

In this study,

alumina

transition metal

oxide

Mn02, CoO

CuO be

metal

oxides

oxides

by

and can

to

atmosphere6]･71' resistance,

were

The

teristics were

a

with

as

and

adjustment

electrostatic

of charac-

to investigate the efi'ect of

measured

the resistance of the force of the chuck,

the

enable

electrical

such

to nonstoichiometric hydrogen and nitrogen

converted

in

chucks

Cr203,Ti02, fabricated. The transition

additives

sitering

chucks.

electrostatic

on

ceramics

the

of

test

of electrostatic

Ni

/

...i,

pLat.' r/g/

..!ir't[･dtng,andpolishine

[-ttttsvu'z'//1/･E] pa/,,e'L -Fig. 2.

Flow

chart

of sample

preparation process.

trodes, These pieceswere

stacked on a green sheet formed intoa laminatethrough the applicationof pressure (1kgfcm2) at 700C.By exposingpart of the electrode on the substrate, a terminal forvoltage application was created. The sintering was performed in a hydrogen and nitrogen atmosphere at the 30"C dew point for 2 h. The insulation layeron the electrode was polished to a thickness of 300 ptm with diamondpastes, The size of the sample, includingthe substrate, was 50 × 50 mm × 2 mm. mm The terminal of the electrodes was Ni plated.The average surface roughness of the ceramic material was O.4 um, and the flatness was lessthan 3"m, The shape of the sample isshown in Fig,3. For the sintered samples, the bulk densitywas substrate

and

samples

50mm-

F----

for the in Fig,

force isshown 2, Oxides such as TiOz, Cr203, Mn02, CoO and CuO {Wako Jun-yaku,Supergrade) were added to 93% alumina ceramics (1wt%),and the material was formed intoa O,6 mm-thick green sheet by the conventional doctorblade procedure and cut intopieces. Tungsten paste was printed on the ceramics as elecmeasurement

ir/

aL,to5'F/bSiY..

electrostatic

2. Experimental procedure

The fabrication method

/i8SfiC

at

x2+EIR+H20

chuck

inthis range

the electrostatic

1

the

has not been investigated, When an electrostatic chuck isused to correct the fiatnessof a semiconductor wafer or to transport a wafer,

:,nxi:Ug"]

1

been developed, between the elec-

resistance

Dry

"

not

relationship

tcU..

/t "'-'s'n';l'e-nt [It'il'l/i-e,7er t tttttttt [I'e'L-1:,jll'ng

is necessarily increasedwithout decreasing the high electrostatic force,The appropriate range of the volumetric intrinsicresistance of the dielectriclayer isconsidered to be between 10i4 and so,

, /{nO .....-j,

CuO

TIC2

Cv20]

----r/

resistance

10iO9･cm or

r,ddltl,,,.s

ee'r'5L.I.L..poNder

Kx---------'--Nx"

2

N(..1

N

---LLJ

3

X.. 50mm

--J---iX)I2mM x

Fig.3. Schematicillustration of the electrostatic 1 : innerelectrode (tungstenrnetalization layer), 2 / insulation layer, :S/ electrode terminal Cnickel plating).

chuck

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Toshiya WATANABE

measured

by

microstructure

means of

of

et

tij'the Cetumic,Se"'ety ,tfYipan100 fo"rnal

al.

Archimedes'method.

the sample,

which

was

The lapped by

diamond pastes,was observed with a scanning electron microscope (Hitachi, S-560).The composition was analyzed by an energy dispersion-typecharacteristicX-ray analyzer (Hitachi, KEVEX-7000). Phase identification of the sintered body was done by X-ray (CuKcr) diffiraction (,XRD), was followedinthe evaluation of elecJIS-C-2141 tricresistance and dielectric breakdown strength, forsamples of 50 mmdi and 2 mm thicknesswith Ag electrodes, The electric resistance was measured by a resistance measuring system CTakedaRika). The relative

TRIO C strength

dielectric constant

(AndoElectric).The was

measured

was

measured

were

according

to

Results and discussion

3.1 Fabricationof

electrostatic

chucks

Figure 4 shows therelationship between the sintering temperature and bulk density.The bulk density was decreased by the addition of an additive, but

3,5

using

Ee..h.2el:m

dielectric breakdown

JIS-C-2110;

between brass electrodes of 20 mmdi, and voltage was applied. The rate the voltage application was 1 kVls, Measurementsof electrostatic forcewere made by brassdisc pulling testsof a 30 mmth and 4 mm-thick which was held by an electrostatic chuck fixedin a vacuum chamber. The roughness of the brass plate was lessthan R.=O.1 "m, and the flatness was less than 1 um. The loading rate was approximately 20 kgfs. samples

3.

3

[']] 1992

sandwiched

3.0

Sinteringtemperature( and Fig,ture. 4. Relationship between bulk desity

SEM photographs ef cross section of alumina ceramics additive (SEM image), CSEM), 2-B:Ti02 (X-rayimage ef Ti), image of Cr}, 3-A:Cr203 (SEM), 3-B:Cr203 (X-ray image of Mn), 4-A : MnOz (SEM}, 4-B : Mn02 (X-ray image of Co), 5-A:CeO (SEM), 5-B:CoO {X-ray 6-A:CuO (SEM), 6-B:CuO (X-rayimage of Cu). Fig. 5. 1 :No 2-A:TiO,

with

transition metal

oxide

℃

)0 sintering

tempera-

additives.

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4

Efflect of Additives

on

Electrostatic Force

the

Alumina Electrostatic Chucks

of

those of the samples sintered at 1580QCfor2h exceeded the density of 3.5gfcm3, Since it was clarified that sintering at 1580℃ results in the fabrication of a high-density body,all samples fabricated thereafter were sintered at 15800C for2 h, Figure 5 shows the result of the microstructural observation of each sample at 15800C). (sintered The results of the observation showed that the alumina

ceramics

with

and

without

additives

were

,6i

lo16

ld5

Eida,dlVh;-ti'

com-

grainsof 2-6 um in diameter and the grain boundaries.The distribution of additives inceramics varied greatly depend'ing on the type of transition metal oxide. In the case of Crx03, (Fig.5-A, B), the additive unjformly posed

of corundum

Si-rich phase

ld'iE

of

distributed in alumina

inthe case

lol

of

boundary. Identification of the phases by XRD

sample

dum,

a small

amount

ef

,di

L

zCuO

/p.

i " i

--･--2 2,5

2P

1/

MnO! CeO Cep03 3.0

35

1000XT{K")

Fig. 6.

present in the sintered that,in addition to corunmullite was present in all

showed

oTiO?

/

CoO, CuO and Ti02, the detectedas inclusions on the Si-richphase of the grain boundary. However, inthe case of Mn02, most of the additive melted uniformly in the Si-riehphase of the grain grains, and additives were

-

'.9･･''''''.o''''.b tt Noadditives

amics

Resistance-temperature transition metal oxide.

characteristics

of

alurnina

cer-

with

samples.

3,2 Electrical propertiesof

fabricated

chucks

Noadditives FErot-eusg･bl.ele TiOl-

Table 1 shows the electrical properties of the samconstant ples. The relative dielectric of all samples at 1 kHz was around 9. To determinethe insulation resistance, the Ieak current was measured after 10 min of voltage application, which is the time forthe current te become stable, The resistance value of the TiOrcontaing sample was 10i4n･cm, while those of the other samples were too high to measure at room temperature. Then the resistance measurements were

high temperatures,The results of for samples heated to in Fig,6. By the addition of Ti02,

conducted

the resistance

2000C are shown .resistance

over

changes

observed.

other

types of additives, of no more

than a

eflect oi additives was

three orders

However,

were was

o

cuoh MnO?CeOi Cke03''''''''''''P

t)'`if': /o/i .,i4'tf ,-'

..-, ,=.., --::"p.'J.-･･e"''-' ltX

o

looo

on

with

the

single

of

order

3000

Fig. 7. Electrostatic force of alumina electrostatic transitionmetal oxide. The adhesion forceis measttred tage applicatien for 20 s.

chuck

with

after

vol-

containing

samples

effect

magnitude

2oeo Voltage{V}

at

measurements

-

-

the additives magnitude. No

of

of

dielectric breakdownstrength

the samples

iorceshown

observed.

3.3 Electrostatic force of chucks fabricated Figure 7 shows the results of measurements electrostatic force.The average surface roughness

of

isabout R.=1 pm, The electrostatic inFig.7 was obtained after 20 s of volThe Ti02-added sampre exhibited a large force.Similarly,the samples inadditives produced largerforcesthan

tage application. remarkably of

cluding

other

the sample without Figure 8 shows

an

additive.

betweenthe electrostaticforceand the durationof voltage applicaTable 1 , Electrical properties of alumina with transitien metal tion at room temperature for samples with and oxide. without an additive. The applied voltage was 1000 V. In samples to which Ti02 was added, there was a Dt'eleot,ric ]roak d:'fi//i At/ / ti '., /ta/tS Hosistivj.ty Co/tstant clear dependence of electrostatic " t. 1lg /' forceon the duraClktrn) cocm) ClltHz) {kV) tion of voltage application; i.e., the electrostatic Nu itdriir/ives L).2･e,pO.zg./･V.2O,u 4x loJ/lxlOM >loi5 /･1."12,512.i11.3/･ force increased with an increase in the durationof '1/;xl･:n4/Lx/o'/1,lxlom4tlxloT4 ･L･1-Cp 1O /'">'/ois>lcls>lotE>lo/S voltage application. When the duration of applica･:uc'

the

relationship

es

/Iolt･Rge

e{nc/./ c,)c

cr :･s]

/1

,O1/'1.5

tion was extended greatly (approxirnately 30 min), the electrostatic ierce becarne saturated between 1

kgfcm2

and

veltage

was

1.2kg/cm2.Even terminated

and

when

the

application electrocles

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Teshiya WATANABE

fij'the Ceramic'Socr'ely ",urnal

et al.

100

,V'fopan

/1.]1992

5

se2,o8gg1 F

i.o='aeo

loooEg-es'iS

5o 800FE?.M.

,vldO 6eosts-5'mo

Fig. 9.

o

Fig, 8, voltage

50

force did not reture to zero immediately, but graduallydeclined,This type of phenomena were not observed in the samples without

electrostatic

the effect

applied

of

the resistance

additives

oxide

force of alumiat elevated tem-

the

electrostatic

electrostatic

transitionmetal voltage is 1000 V.

with

on

of

the electrostatic

of

TiOz

added with

force. With the sample

alumina,

consisting

force in-

the electrostatic

ti.ncreasing durationof

applica-

voltage

in Fig.8. The saturated value of electrostaticforce was above 1 kgfcm2 (whenapplied voltage 1kV) and this large forcecan not be exby by plained the usual Coulomb forcerepresented Eq. (1),but itis attributable to Johnsen-Rahbeck's force.Therefore the time dependenceshould be exof the charge plained by the gradual accumulation from the inner electrode te the surface of the chuck resulting in a gradual increasein electrostatic force, tion as shown

on

force, the durationtime dependence of the electrostatic iorce were measured at high temthe sample was heated perature. In the experiment, in a vacuum chamber. The temperature was measured by a thermocouple attached to the sample, The the electrostatic

in Fig. 9 were obtained at 2000C. It is that the adhesion forceof the Ti02-containing sample heated at 2000C showed a lesstime depenresults

perature.The

creased

an additive.

In order to examine

chuck

force Balakrishnan5) reported the large electrostatic forcecaused by the Johnsen-Rahbeck's etibct, but he has not reported about the duration time dependence

Relationship between electrostatic forceand durationof The applied voltage is1000 V.

the

Durationtime dependence of

na electrostatic

3.4 Effecl/of

100 Time(sec)15e

application.

grounded,

1000

100

Time(see)

4oo22200

shown

obvious

dent characteristics, period of time with

Itbecame a smaller

saturated saturated

F Johnsen'Rahbeck's

in a

shorter

value.

Ba]altrlshntLn et

m

a].''t''

furc/t

1/

ee.:.2R

F=

.=

O.5E

=.

(vt･ti).

i

-t Time

m Fig, 1O.

Schematic presentation for relationship

between charge

distributionin the

layer and dielectric

electrostatic

force.

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6

Effectof Adclitiveson

the

Electrostatic Ferce of ALumina Electrostatic Chucks

ing atmosphere. Electricaland electrostatic force itis assumed that the time required for the acwere estimated and the following depends on the resistance of the dielectric characteristics layer.Figure 1 O schows schematic illustration of such results were obtained. of Ti02 contributed to the relationship betweenelectrostatic force and charge Cl) The addition ceramics, in the dielectric layer. reduction of the resistance of alumina distribution while other additives made no contribution to the High temperature measurement of electrostatic reduction, force shows the relationship between the response the time (2) Withanelectrostaticchuckingsubstratecontime and the resistance oi the samples: i.e., the electrostatic force requierd forthe electrostatic forceto become saturatsiting of TiOz-added alumina, increasedwith increasing duration of voltage applicaed was foundto be much shorter at higher temperain tion. ture,The response time reduces with decreasing resistivity of the ceramics at higher temperature. As (3) When the temperature of the substrate was increasedto 2000C the electrostatic forcewas found shown inthe Fig. 6, the resistivity of alumina ceramicswithout additives was about 10iff9･cm and this to increase inproportionalto the decreasingamount with a distinctreduction in the value is assumed to be too high for the charge to of resistance response tlme. transferto the surface of the ceramic, therefore the From these results, itwas concluded that a large electrostatic force isdescribed by the usual Coulomb On the other hand, the electrostatic force dependent on the duration of volforcerepresented by Eq, (1), tage application can be generatedforan electrostatic Ti02 added sample having the resistivity below 10i4 chuck made of alurnina centaining Ti02 which has a 9･cm is considered to have the transitional characrelatively high resistance (approximately 10it9). force from the usual teristicsof electrostatic Coulomb forcetoward the largerJohnsen-Rahbeck's References force.Otheradditives except Ti02 was not so effk)c1) G. A. Wardly, Rev.Sch,instn{m.,44, 1506-09 (1973), tiveto control theresistance of the aiumina ceramics 2) S.Tach,J IEE]bpan,108. 1195-98 (1988). as compared with TiO2,Therefore they do not im3) M. Nakamura, T, Kurimoto, H, Yano and K. Yanagida, elecprove the electrostatic force of the alumina EDD-88-r 43, 17-26 (1988>and

cumlation

trostatic chuck,

"Handbeek

4.

Conclusions

Ceramicelectrostatic chucks foradhesion ments were fabricated by adding 1 wt% the additive

93%

alumina

of 4) The Instituteof Electrestatics Japan, Elecrostatics" {1981)pp. 672-75. 5} C. Balakrishnan, Br,J.APPI.Rlays., 4, 211-13 C1950). 6) M.A.KhMa,Z.M.HanatiandA.K.Mohamed,ThermochimicaActa, 56, 291-98 (1982), I.Yamai and H. Saito,Yiogyo-Kbeokai-Shi, B3, 7) J.Takahashi, 589-94 (1975).

Ti02, Cr203, MnO, ceramics

and

CoO

sintering

and

them

experiof

each of

CuO, to inreduc-

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