Preliminary Electrical Characterization of Pulsed-Plasma Enhanced Chemical Vapor Deposited Teflon-like Thin Films

Abstract

Pulsed-rf excitation of hexafluoropropylene oxide (HFPO) has been used to deposit films with chemical compositions similar to poly(tetrafluoroethylene) (PTFE). Films were deposited at pulse cycles of 10/20, 10/50, 10/200, and 10/400 ms on/ms off and analyzed using electron spin resonance spectroscopy (ESR). All four films produced similar broad ESR spectra, with an average width at maximum slope of -60 G and a g-value of 2.0045. The concentration of free electrons in a sample decreased with increasing pulse off time. This behavior can be modeled by the reaction of a free radical with a gas species, assuming that free radicals are generated only during the pulse on time. The films' dielectric constants were found to decrease from 1.99 to 1.95 for pulse off times increasing from 20 to 400 ms. INTRODUCTIONMaterials with low dielectric constants (ic < 4) are needed as interlayer dielectrics (ILDs) in order to implement new integrated circuit (IC) design rules. 1 -3 The move to lower dielectric constant materials would result in reductions in propagation delay (i.e., RC time constant), power consumption (i.e., -c CV 2 ), and cross-coupling noise between adjacent lines, which would allow for higher performance with smaller design rules. 1-3Most organic polymers and aerogels currently under study for ILD applications are deposited using spin-on processes. However, this technique can have drawbacks, including anisotropic properties due to polymeric chain alignment and use of solvents (i.e., environmental issues). Chemical vapor deposition (CVD) might be a more advantageous process with benefits such as compatibility with current IC processing and equipment, conformal coatings, and no solvents. However, CVD processes have received little attention as approaches to deposit polymeric ILD materials.Plasma-enhanced CVD (PECVD), in which an electric field, instead of thermal energy, is used to break apart a monomer, is another option for depositing polymeric films. However, PECVD films have often been ruled out as potential ILDs due to their incorporation of dangling bonds (i.e., unpaired electrons), which are suspected to give rise to dielectric loss and aging effects. 4,5 Typical dangling bond concentrations for PECVD polymers are 1018 to 1020 spins/cm 3 ,6 which can sometimes be reduced by low temperature annealing. 7 Pulsed, rather than continuous, plasma excitation may be a viable compromise between thermal CVD and continuous PECVD. In this process, rf power is applied for a specified "on" time, followed by an "off' period during which no excitation is used. During the "on" time, both ions and reactive neutrals are produced. However, since ions often have shorter lifetimes than neutrals, during the "off' time the ratio of neutrals to ions will increase, and thus, the process

High-density plasma chemical vapor deposition of silicon-based dielectric films for integrated circuits

“…This subsequently increases the glass transition temperature (T g ) and film thermal stability. Recently studies [14,71] have shown that pulse-plasma-enhanced CVD can be used to deposit fluorocarbon films using a hexafluoropropylene oxide (HFPO = C 3 F 6 O) precursor. With varying plasma-pulsing excitation times, fluorocarbon films with a low dielectric constant, low dangling-bond concentration, and highly stable -CF 2 bonding in the film bulk can be deposited in a controllable process.…”

Fluorinated amorphous carbon films for low permittivity interlevel dielectrics

“…Several groups have developed pulsed HDPCVD and PECVD techniques to deposit a-C:F, H films. 22,23,32,33 For example, Gleason and co-workers used hexafluoropropyleneoxide ͑HFPO͒, which when thermally excited, decomposes producing the CF 2 radical which creates films consisting of predominantly networked CF 2 groups, or Teflon®-like films. 34 When HFPO is subjected to a continuous plasma, the resulting film shows a significant fraction of CF 3 and CF groups as measured by XPS.…”

“…34 When HFPO is subjected to a continuous plasma, the resulting film shows a significant fraction of CF 3 and CF groups as measured by XPS. 32, 33 Takahashi and co-workers have used ECR HDPCVD and simple fluorocarbon gases, with power pulsing at about 5%-15% duty cycle also produce films in which the CF 2 peak dominates XPS spectra. 22,23 It is generally believed that the CF 3 and CF radicals are formed by more extensive gas-phase dissociation of the precursor.…”