Study and Optimization of PECVD Films Containing Fluorine and Carbon Layered with Diamond Like Carbon Films as Ultra Low Dielectric Constant Interlayer Dielectrics

Abstract

Layered dielectric films comprising of Diamond like Carbon (DLC) and Amorphous Fluorocarbon (a:C-F) were generated using three different stack configurations for ultra low dielectric constant (ULK) applications. These include a DLC – a:C-F – DLC sandwich, a:C-F – DLC topcoat only and an annealed a:C-F – DLC topcoat only film stack. These films were subsequently evaluated for thickness, dielectric constant, contact angle, surface roughness and chemical structure using IR analysis. Thermal stability was analyzed after annealing in Argon ambient at 400°C for 1 hour. Deposition conditions were optimized for film thickness, roughness, dielectric constant and contact angle using Minitab by tuning process pressure, substrate temperature and FRR. The modified Gaseous Electronics Conference (mGEC) reference cell was used to deposit DLC films using CH₄ and Argon as precursors. Structural properties of the deposited thin film were studied using laser excitation of 633 nm in a Jobin Yvon Labram high-resolution micro-Raman spectrometer. Multiple points on each sample were analyzed in terms of the disordered carbon (D-peak) and graphitic carbon (G-peak). The thin film of DLC was subsequently annealed in Ar ambient for 1 hr at 400°C and analyzed. Commercially available graphing software was utilized to deconvolute peaks and the ratio of their intensities as well as the shift in their positions were determined to characterize the as-deposited and annealed film. The film was further characterized using AFM, FTIR, XRD, goniometry and electrical testing. Average film roughness as measured by AFM was less than 1 nm, the k-value was 2.5 and the contact angle with water was 42°. A:C-F films were separately deposited using CF₄ and Si₂H₆ (5% by volume in He) as precursors in a UNAXIS PECVD system. Films deposited using substrate temperatures between 120°C – 200°C, chamber pressure of 300 and 500 mTorr and power of 100 W were independently evaluated in terms of their electrical, physical, structural and optical properties prior to layering with DLC films. After process optimization, seven unique process conditions generated promising layered films with k-values between 1.69 and 1.95. Of these, only one film exhibited very low shrinkage rates acceptable for semiconductor device processing.