Characterisation of Low-K Dielectric Films by Ellipsometric Porosimetry
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Characterisation of Low-K Dielectric Films by Ellipsometric Porosimetry M.R.Baklanov and K.P.Mogilnikov1 IMEC, B-3001 Leuven, Belgium, [email protected] 1 Institute of Semiconductor Physics, 630090 Novosibisrk, Russia ABSTRACT Ellipsometric porosimetry (EP) is a simple and effective method for the characterization of the porosity (volume of both open and close pores), average pore size, specific surface area and pore size distribution (PSD) in thin porous films deposited on top of any smooth solid substrat e. Because a laser probe is used, small surface area can be analyzed. Therefore, EP can be used on patterned wafers and it is compatible with microelectronic technology. This method is a new version of adsorption (BET) porosimetry. In situ ellipsometry is used to determine the amount of adsorptive which adsorbed/condensed in the film. Change in refractive index is used to calculate of the quantity of adsorptive present in the film. EP also allows the study of thermal stability, adsorption and swelling properties of low-K dielectric films. Room temperature EP based on the adsorption of vapor of some suitable organic solvents and method of calculation of porosity and PSD is discussed. Examination of the validity of Gurvitsch rule for various organic adsorptives (toluene, heptane, carbon tetrachloride and isopropyl alcohol) has been carried out to assess the reliability of measurements of pore size distribution by the ellipsometric porosimetry. INTRODUCTION Molecules polarity (µ-permanent dipole moment) and polarisability (α) define the relative permittivity εr of a substance. The quantitative relation between the relative permittivity and the electrical properties of molecules is expressed by Debye equation
εr −1 εr + 2
=
2 α + µ 3ε 3kT o
4πN
(1)
where N is the number of molecules per unit volume. If there is no contribution from the permanent electric dipole moments to the polarization (the molecules are non-polar) the same expression is called the Clausius-Mossotti equation. No or minimal dipole moment is typical for organic polymers and choice of organic polymers to decrease the permittivity value of dielectric films for the advanced interconnect application is defined by this fact. Thus, the organic polymers allow decrease the permittivity of the interlayer dielectric until 2.5-2.8. Further decrease of the permittivity value can only be realized by using of porous dielectrics: 4πN i =∑ αi = V ε r + 2 i 3ε o
εr −1
εa −1
ε −1 + (1 − V ) s εa + 2 εs + 2
( 2)
where V = ( pore volume film volume ) is the relative film porosity, εa and εs are the permittivity of air and the film skeleton respectively. Porous dielectric films are thus becoming extremely
D4.2.1
important for the future ULSI technology. Film porosity and pore size distribution (PSD) define dielectric, mechanical, thermal and chemical properties of the films and their feasibility to be used in the ULSI technology. Increasing the porosity drives the dielectric constant down, but it degrades the mechanical and chemical properties of
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