Structure-Property Correlation in Low K Dielectric Materials
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Mat. Res. Soc. Symp. Proc. Vol. 565 01999 Materials Research Society
the electronic response relative to the total dielectric constant can be found by measuring k at high and low frequencies, respectively. Table I shows the high frequency electronic response, measured through the square of the optical index, n2 , vs. the total k value for a number of low k dielectrics. Since for some thin film materials the in- and out-of-plane properties can differ, the out-of-plane values are compared. The difference between the high and low frequency measurements represents the nuclear component of the dielectric response. The data indicates that for many low k materials under consideration, the nuclear components are quite small relative to the electronic part of the response. Table I. Dielectric Properties of Low k Materials 2
Material PTFE BPDA-PDA PMDA-TFMOB6FDA-PDA PAE #1 PAE #2 BCBTM SiLKTM MSQ/HSQ hybrid Parylene-F Parylene-N Si0 2
flout
koft
lout
1.350 1.839 1.670
1.340 1.617 1.518
1.92 3.12 2.65
1.796 2.615 2.304
A =k- flout 2 0.12 0.50 0.35
1.676 1.671 1.554 1.630 1.374 1.595 1.687 1.47
1.669 1.672 1.554 1.624 1.373 1.427 1.570 1.47
3.00 2.80 2.65 2.65 2.52 2.18 2.58 4.0
2.787 2.796 2.415 2.637 1.886 2.036 2.465 2.16
0.11 0.00 0.24 0.01 0.63 0.14 0.12 1.8
ni
Since the electronic response plays such a dominant role, it is important to minimize the electronic polarizability to achieve a low k. Table II lists electronic polarizabilities for some common chemical bonds. 2 Table II. Electronic Polarizabilityl and Bond Enthalpies
Bond
Polarizability(A3)
C-C C-F C-O C-H O-H C=O C=C C_=C CN
0.531 0.555 0.584 0.652 0.706 1.020 1.643 2.036 2.239
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Ave. Bond Energy (Kcal/mole) 83 116 84 99 102 176 146 200 213
The bonds having the lowest polarizability are the C-C and C-F bonds, while the double and triple bonds have the highest polarizability. The table also lists typical bond enthalpies, which are generally much higher for the double and triple bonds than for single bonds. Therefore, there will be a tradeoff in obtaining materials that have strong bonding and low polarizability. Nevertheless, if one wishes to concentrate on simply obtaining the lowest k value possible, one would primarily use bonding arrangements from the top of Table II. STRUCTURE AND PROPERTY CORRELATION Poly(tetrafluoroethylene) (PTFE), which utilizes this strategy, has a very low k value of -1.9. However, PTFE does not have high thermomechanical stability, as it consists of floppy, uncrosslinked C-C chains that don't provide much rigidity. Also, because the polymer chains are saturated with nonpolarzable F atoms, there is little chemical interaction between adjacent chains, so that they can slide past one another easily. As a result of these factors, PTFE has a low modulus (0.5 GPa), yield stress (12 MPa) and Tg/melting temperature (250TC), and a high TEC (>100 ppm!°C). F
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Poly(tetrafluoroethylene) The thermomechanical stability of organic polymers can be improved over that of PTFE b
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