Low K materials Kejun Xia Auburn University,AL Nov 20, 2003
outline • Why Low k ? • Questions • Requirements for Low k • General ways to gain Low k • Low k materials • Plasma process in Low k materials • Conclusions • Answers
Questions • Why k value of Carbon-doped Silicon Oxide increases after O2 plasma treatment? • How to reduce the damage from Fluorine diffusion in SiOxCy film during plasma etching?
Why Low k • Speed • The goal of dielectric material in advancement from one Tech Node to the flowed node is 20% interconnect capacitance improvement. • Power • Cross talk • The crossing effects between different connection lines contain signals.
Interconnect crisis • 200nm ->130nm ->90nm -> 65nm • SiO2 FSG ? ? • K=4.5 k=3.6 k<2.5 ? • Debate A • Spin on Deposition Japan & Korea • CVD US & Europe • Debate B • Evolutionary… Dense materials as before • Revolutionary… Porous materials
Requirements • Electrical • K<3 • Thermal • Stability up to 400 0C • High glass transition temperature • Chemical • Etching compatibility • Mechanical • Hardness, modulus • Structural • Moisture absorption • Adhesion to caps, hard masks, lines etc.
Fundamental physics of k • K originates from the polarization
Basic approaches to reduce k • Optimization of molecular structrue • Minimize configurational and dipole polarizability, e.g. use of C-C and C-F bonds • Reduce density and incorporation of porosity • Add uniform and microscopic pores with k of 1 • Limitation: Both approaches degrade the thermomechanical properties • Proper tradeoff between dielectric constant and thermomechanical prosperities is important
Carbon–doped Silicon Oxide (CDO) • K<2.9, candidate in the 130nm technology and beyond • PECVD • 400 0C, RF(13.56 MHz) • C4H16O4Si4+O2 • Disadvantage • Oxygen plasma ashing (cnt… )
O2 plasma ashing • O2 plasma is used in photoresist stripping which degrades CDO. • Thickness: 484.4nm -> 396.4nm • K: 2.8 ->3.6 • O2 plasma removes the entire C and part of Si contents in the film. The longer treatment time, the worse Before After
Fourier Transform Infrared Spectroscopy (FTIS) analysis on O2 plasma treatment • Increasing of Si-OH leads to moisture uptaking, which is responsible for the increase of k value and leakage current
Reducing O2 plasma damage by postdeposition He plasma treatment • He plasma treatment which is carried out before O2 plasma process does not effect the thickness and K value itself • PECVD chamber, 700W, 20s, He 8Torr 1300sccm • He plasma treatment reduces thickness loss and remain k value as that after deposition, i.e. 2.8 • Origin of the effect of He plasma treatment is not known yet
Effect of He plasma treatment He plasma treatment After deposition After He and O2 plasma treatment Without He plasma treatment
Hydrocarbon material (SiLKTM) • Cross linked Silica based materials • Si-O network provides rigidity • Organic groups lower k to 2.5-3.3
Issues encounted • Material • SiLKTM semiconductor dielectric evaluated against a stringent set of requirements • Process development • New dielectric material required development of new unit process (i.e. dual hardmask patterning) • New structures and ground rules were developed in order to deal with low modulus materials
SiLKTM Plasma Etching and problem • ICP, O2/N2/CH4 gas • Problem and solution • Formation of bow in high ratio contact holes which originates from the deflection of ions on the sidewalls, generating some etching. The distortion is explained by differential charging on the feature sidewalls. • One solution is formation of a passivation layer on the sidewalls preventing the spontaneous attacks by oxygen reactives species in the plasma.
Ultra Low k (ULK) material– Nanoporous SiOxCy • k<2.2 • Formation • Porogen approach combined with spin-on deposition or PECVD • Postdeposition treatment with H2 plasma or supercritical CO2 process or E-beam • Inherent issues • Moisture uptake or chemical absorption due to porosity • Mechanical fragility, material are soft and brittle
Diffusion barrier for Cu metallization • Thickness consideration • Barrier will increase overall k value • Barrier should be thinner as possible without compromising its integrity • A good step coverage • Different materials (TaN, TiN, TiNSi…) and different deposition techniques (PVD, CVD, ALD…) are in competition
CVD TiN on porous SiOC • 10nm CVD TiN are required for a continuous barrier layer
Mechanical behavior • Hardness, modulus… • Insures the capability to support all the process steps including metal re-crystallization and packaging • Adhesion strength between dielectric or metallic layers • Insures stack stability during local or global stress variations including thermal treatments or process such as CMP
Etching issue • Fluorine species diffusion during plasma etching • Penetrate into pores of SiOxCy film and react with hydrogen species in the course of Cu electroplating process and form HF molecules. HF then make larger void in the film • Surface oxidized and k value increase
One solution to etching issue • It is indicated that the diffusion of the fluorine species is more significant for the films with fully interconnected pores • Tune the pore-connectivity by varying the content of porogen. • A trade off between k value and integrity ability
Conclusions • Low k materials has been one of the bottle neck in fulfill semiconductor roadmap for the nodes beyond 130nm. Many materials are still under research. • CDO is one candidate for 130nm node and beyond. • SiLKTM need a new set of process, many works have been done. • ULK material has its unique low k value but has its weakness in mechanical behavior. Its integrity ability is under debate and research.
Answer to Q1 • Why k value of Carbon-doped Silicon Oxide increases after O2 plasma treatment? • O2 plasma treatment increases Si-OH which is hydrophilic in Carbon-doped Silicon film thus leads to moisture uptaking, which is responsible for the increase of k value and leakage current
Answer to Q2 • How to reduce the damage from Fluorine diffusion in SiOxCy film during plasma etching? • Control the content of porogen during film deposition
References • “Introducing advanced ULK dielectric materials interconnects: Performance and Intergration” F.Fusalba, C.Le cornec, P.maury etal. • “Investigation of the Plasma Etching-Induced Pore structure transformation and diffusion of fluorine in porous Low-k thin films” Kwang Hee Lee, Ji-Hoon Rhee, Sang Kook, et al. • “Etching of Low-k interconnect materials for next generation devices” T.chevolleau, OlJOubert, N.Posseme, et al. • “The stability of Carbon-Doped silicon Oxide low dielectric constant thin films” Y.H.Wang, R.Kumar • “Reduction of oxygen plasma damage by postdeposition Helium plasma treatment for Carbon-Doped Silicon Oxide Low Dielectric constant films” Y.H.Wang, D.Gui, R.Kumar and P.D.Foo., Electrochemical and Solid-state Letters, 6(1) F1-F3 (2003)