S7-3

Developments of organic-based low-k films deposited by plasma copolymerization

プラズマプロセスによる有機系低誘電率材料開発
Keizo KINOSHITA, 木下啓藏

Low-k Gr., MIRAI-ASET, AIST Tsukuba SCR Bldg., Tsukuba 305-8569, JAPAN
半導体MIRAIプロジェクトLow-kグループ，305-8569 つくば市小野川16-1産総研西SCR事務棟

This paper describes the properties of low-k materials produced by plasma copolymerization. Carefully
chosen monomer precursors and suppression of excess dissociation enabled controlling film properties. For
example, moduli of the copolymerized films were improved by enhancing bridging between the monomers.
Dielectric constant of the film could be reduced by using low temperature deposition followed by annealing.

In order to satisfy the requirements for ULSIs, an interconnect structure with low dielectric constant (Low-k) films is one ofpossible solutions. We have proposed the strategic concept of "Low-k materials with scalability applicable to a few technologynodes” [1]. The major challenge to realize the scalable low-k material is to get solutions for controlling the dielectric constant andmechanical strength. Among several low-k film deposition methods, a plasma copolymerization technique has been developedfor this purpose [2]. The concept of the technique is illustrated in Fig. 1. Basic film properties will be determined by the matrix
monomer, and modified by copolymerization to achieve the lower dielectric constant and higher mechanical strength.
Figure 2 shows a schematic diagram of the experimental apparatus based on a plasma CVD system. DVS-BCB was chosen asa starting matrix skeleton monomer. The dielectric constant of the singly polymerized DVS-BCB film was 2.78 [2]. Typicaldeposition condition is shown in Table 1. Converged saddle point structures of DVS-BCB monomer by density functional
method (B3LYP/6-31G*) were shown in Fig. 3 [3]. Table 2 shows the list of precursors for copolymerization. Three approaches
from (a) to (c) were examined mainly to reduce dielectric constant. These precursors were chosen from the view point of possible
copolymerization reactions with the matrix monomer, from calculated dielectric constant of the monomers, and from vapor
pressure. The calculated dielectric constants for these monomers were shown in Fig. 4 [4,5]. DVS-BCB monomers show about
the same value as the deposited film. Phenyl compounds show higher values than those of aliphatic compounds.
The QMS system could detect reaction products in the plasma through orifices at the chamber wall. Figure 5 shows the mass
spectrum from 380 to 500 amu during discharge with DVS-BCB molecule in helium plasma [6]. As the mass number ofDVS-BCB monomer is 390, the mass peaks observed here can be attributed to the polymerization reaction products in theplasma. Figure 6 shows electron density change against source power measured by a surface wave probe [7]. The low electron
density suppresses excess dissociation of the monomers, and enables to inherit original characters of the monomers.
In method (a), phenyl compounds with unsaturated functional groups were introduced as modification monomers. They havesmaller molecular volume than the DVS-BCB monomer and can polymerize with DVS-BCB. Therefore, the copolymerizedfilms were expected to have lower dielectric constant by free volume being formed between the monomers, and higher
mechanical strength by enhanced bridging between the monomers. The dielectric constant and modulus of the copolymerized
film are shown in Fig. 7 as a function of the molar ratio of the modification monomers supplied to DVS-BCB. The modulus
increases with the molar ratio. This was due to an increase in the copolymerization ratio [8]. However, the dielectric constant of
the deposited polymers did not decrease. This indicates that free volume could not be formed in the film by copolymerization.
Low-k hydrocarbon compounds with unsaturated functional groups were used to introduce aliphatic component into theskeleton structure in method (b). Figure 8 shows the relationship between the dielectric constant and the modulus of thecopolymers with DVS-BCB. Results for VNB and TVCH were plotted together with those for the phenyl compounds shown inFig. 7. Notice that all the data points are distributed over the same range. This indicates that the deposited films had roughly thesame characteristics. One possible reason for this result is the poor thermal stability of the aliphatic compounds. Aliphatic
component deteriorated to thermally stable groups like naphthalene on a high-temperature wafer, and lost their low-k property.
As shown in Fig. 8, it is difficult to preserve aliphatic components in an organic skeleton structure at a deposition temperatureof 400 °C. On the other hand, in silica-based low-k materials, these aliphatic compounds are often used as porogens to form poresin the film after curing the spin-coated film. Thus we conceived that it might be possible to form pores even in organic-based
films obtained by low-temperature deposition with aliphatic compounds followed by annealing to vaporize some of the aliphaticcomponents. This is method (c). A 2-dimethylvinylsiloxane-tricyclodecane (2DMVS-TCD) monomer having an aliphatic andsiloxane molecular structure was used as a matrix skeleton monomer. This monomer has a lower dielectric constant thanDVS-BCB as indicated in Figure 4 (c). Figure 9 shows the effect of deposition temperature and subsequent annealing on thedielectric constant. Acetylene (C2H2) was added to enhance the deposition rate. It can be seen that the low-temperature deposition
led to a lower dielectric constant. Moreover, annealing at 400 °C after the deposition did not change the dielectric constant. A
dielectric constant of 2.48 was obtained by deposition at 300 °C followed by annealing at 400 °C.
This work was supported by NEDO.

[1] T. Kikkawa, Ext. Abst. ADMETA 2003: Assian session, 1-2, (2003) 4, [2] J. Kawahara, et al., Technical Dig. IEDM 2003, 6-2,(2003) 143, [3] M. J. Frisch, et al., “Gaussian 03 Revision B.05”, Gaussian, Inc., Pittsburgh PA, (2003), [4] T. Fukuda, et al., Jpn. J. Appl.
Phys., 41, (2002) L307, [5] K. Kinoshita, et al., Proc. AMC2004, P.14 (2004), [6] K. Kinoshita, et al., Proc. DPS2003, 3-1, (2003) 61, [7]K. Nakamura, et al., Proc. DPS2004, P-29, (2004) 169, [8] N. Kunimi, et al., Proc. IITC 2004, 8.5, (2004) 134
RF UnitASM-Japan
Eagle-12

Skeleton
Monomer 0

Modification
monomervapor

Carrier gas (He)

Modification
Monomer 1

Differentiallypumping
Gate chamber
valve

MFC

PlasmaMonomer 2

Deposition rate
Coverage
Damage issue
Uniformity.....
Carrier gas

Elastic modulus
Dielectric constant.
Thermal stability
Adhesion

LMFC
Vaporizer

SWP probe

Table 1 Typical deposition condition for the copolymerization.

250 W
0.2 g/min (~ 11.5 sccm)
700 sccm
400 Pa200-400 ºC

Fig. 3 Converged molecular structures of DVS-BCB
monomer by density functional method (B3LYP/6-31G*).

Table 2 Skeleton monomer and modification monomer toreduce dielectric constant by three different approaches [5]Molecule
Abbreviation
divinylsiloxane-bis-benzocyclobutene
DVS-BCBdivinylbenzene, diisopropenylbenzene, diethynylbenzene, trimethylbenzene DVB, DIPB, DEB, TMB
vinylnolbornene, trivinylcyclohexane
VNB, TVCH
trivinylcyclohexane, 2-dimethylvinylsiloxane-tricyclodecaneTVCH, 2DMVS-TCD (as matrix)

Matrix Skeleton(a)Free volume formation(b)
Introducing aliphatic(c)Low temperature deposition

10C2 .4+m/e=391

2 .2

CH3Si
O CH
3Si
H3 C

CH3 CH3Si O Si
CH3 CH3m/e=422

CH3 CH3Si O Si
CH3 CH3CH3Si
CH3CH3 CH3Si O Si
CH3 CH3m/e=450
O

CH3 CH3
Si O Si
CH3 CH3CH3Si
CH3m/e=466(b )(c )M o le cu le410

DVS-BCBCount rate [cps]2DMVS-TCD

VNB

(a )

TVCHDEBDVB

M a trix

DIPB

DVS-BCB (crs)510CH3 CH3Si O Si
CH3 CH31000

6.5

2.8
DVS-BCB k

2.762.65.5

2.5
2.45DVS-BCB E

012345Mol ratio (X/DVS-BCB)

Fig. 7 Dielectric constant and modulus
of copolymerized film of DVS-BCB
with phenyl compounds as a function ofsupplied molar ratio to DVS-BCB [2,5].

4.5Modulus E [GPa]7Modulus E [GPa]Dielectric constant ko

Tsub=400 C

2.910

1.5 10101 1095 10

7TMB(+DVB)DIPBDEBVNB
TVCHo6.5100 200 300 400 500 600

T sub =400 C

6Fig. 6 Electron density change
against source power for helium
plasma with DVS-BCB molecule
measured by surface wave probe [7].
2.8Dielectric constant k

7.5
7.5

3102 10Source power [W]10380

DVS-BCB k

TMB+DVB E
DIPB E
DEB E

102.5 1000100

400 420 440 460 480 500
Fig. 4 Calculated dielectric constant by density
Mass [amu]functional method (B3LYP/6-31G*) [4,5]. BothFig.
5Mass
spectrum of the plasma
electronic and ionic polarizations are displayed.
polymerization
reaction
products, and their(a) to (c) indicate monomers for different
molecular structures [6].
approaches.
TMB+DVB k
DIPB kDEB kElectron density [cm-3]

m/e=408

2 .6

3 10

H3 CCH3SiO CH
3Si
H3Ck (io nic )k (e lec tro n ic )DVS-BCB (opp)

Dielectric constant

32 .8

2Balzars
PPM-422

Fig. 2 Schematic diagram of a plasma polymerization apparatus
with diagnose system.
(crs)
(opp)

Fig. 1 Concept of a new plasma-enhanced co-polymerization [1].
RF power
DVS-BCB matrix monomer liquid flow
He carrier gas flow
Pressure
Substrate temperatureCVD Reactor

MatrixmonomerIndependent control of the material properties/process performances

QMS
chamber

5.5
DVS-BCB E

2.7

2DMVS-TCD/C 2H22.6
initial
2.5
2.4

after 400 o C anneal

2.35
4.5
2.6

2.2
150 200 250 300 350 400 450
2.7

2.8

2.9

Dielectric constant k

3Deposition temperature [oC]

Fig. 8 Relation between dielectric constant k Fig. 9 Effect of deposition temperature onand modulus for copolymer of DVS-BCB dielectric constant of 2DMVS-TCD/C2H2
with phenyl compounds (closed) and copolymerized film. Low dielectric
constant of 2.48 was achieved [5].
hydrocarbon compounds (open) [5].