Thermodynamics in Chip Processing II Terry A. Ring

CVD

Materials Deposited Dielectrics –SiO2, BSG Metals –W, Cu, Al Semiconductors –Poly silicon (doped) Barrier Layers –Nitrides (TaN, TiN), Silicides (WSi 2, TaSi 2, CoSi, MoSi 2 )

Deposition Methods Growth of an oxidation layer Spin on Layer Chemical Vapor Deposition (CVD) –Heat = decomposition T of gasses –Plasma enhanced CVD (lower T process) Physical Deposition –Vapor Deposition –Sputtering

Critical Issues Adherence of the layer Chemical Compatibility –Electro Migration –Inter diffusion during subsequent processing Strong function of Processing Even Deposition at all wafer locations

CVD of Si 3 N 4 - Implantation mask 3 SiH 2 Cl NH 3  Si 3 N HCl + 6 H 2 –780C, vacuum –Carrier gas with NH 3 / SiH 2 Cl 2 >>1 Stack of wafer into furnace –Higher temperature at exit to compensate for gas conversion losses Add gases Stop after layer is thick enough

CVD of Poly Si – Gate conductor SiH 4  Si + 2 H 2 –620C, vacuum –N 2 Carrier gas with SiH 4 and dopant precursor Stack of wafer into furnace –Higher temperature at exit to compensate for gas conversion losses Add gases Stop after layer is thick enough

CVD of SiO 2 – Dielectric Si0C 2 H 5 +O 2  SiO H 2 –400C, vacuum –He carrier gas with vaporized(or atomized) Si0C 2 H 5 and O 2 and B(CH 3 ) 3 and/or P(CH 3 ) 3 dopants for BSG and BPSG Stack of wafer into furnace –Higher temperature at exit to compensate for gas conversion losses Add gases Stop after layer is thick enough

CVD of W – Metal plugs 3H 2 +WF 6  W + 6HF –T>800C, vacuum –He carrier gas with WF 6 –Side Reactions at lower temperatures Oxide etching reactions 2H 2 +2WF 6 +3SiO 2  3SiF 4 + 2WO 2 + 2H 2 O SiO 2 + 4HF  2H 2 O +SiF 4 Stack of wafer into furnace –Higher temperature at exit to compensate for gas conversion losses Add gases Stop after layer is thick enough

Chemical Equilibrium

CVD Reactor Wafers in Carriage (Quartz) Gasses enter Pumped out via vacuum system Plug Flow Reactor Vacuum

CVD Reactor Macroscopic Analysis –Plug flow reactor Microscopic Analysis –Surface Reaction Film Growth Rate

Macroscopic Analysis Plug Flow Reactor (PFR) –Like a Catalytic PFR Reactor –F Ao = Reactant Molar Flow Rate –X = conversion –r A =Reaction rate = f(C A )=kC A –C i =Concentration of Species, i. –Θ i = Initial molar ratio for species i to reactant, A. –ν i = stoichiometeric coefficient –ε = change in number of moles

Combined Effects Contours = Concentration

Reactor Length Effects SiH 2 Cl 2 (g) + 2 N 2 O(g)  SiO 2 (s)+ 2 N 2 (g)+2 HCl(g) How to solve? Higher T at exit!

Deposition Rate over the Radius r C As Thiele Modulus Φ 1 =(2kR w /D AB x) 1/2

Radial Effects This is bad!!!

Combined Length and Radial Effects Wafer 20 Wafer 10

CVD Reactor External Convective Diffusion –Either reactants or products Internal Diffusion in Wafer Stack –Either reactants or products Adsorption Surface Reaction Desorption

Microscopic Analysis -Reaction Steps Adsorption –A(g)+S  A*S –r AD =k AD (P A C v -C A*S /K AD ) Surface Reaction-1 –A*S+S  S*S + C*S –r S =k S (C v C A*S - C v C C*S /K S ) Surface Reaction-2 – A*S+B*S  S*S+C*S+P(g) –r S =k S (C A*S C B*S - C v C C*S P P /K S ) Desorption: C*S C(g) +S –r D =k D (C C*S -P C C v /K D ) Any can be rate determining! Others in Equilib. Write in terms of gas pressures, total site conc.

CMP

What is CMP? Polishing of Layer to Remove a Specific Material, e.g. Metal, dielectric Planarization of IC Surface Topology

Scratching Cases Rolling Indenter Line Scratches –Copper Only –Copper & ILD Chatter Scratches Uncovery of Pores

CMP Tooling Rotating Multi-head Wafer Carriage Rotating Pad Wafer Rests on Film of Slurry Velocity= - (Wt  Rcc)–[Rh  (Wh –Wt)] when Wh=Wt Velocity = const.

Slurry Aqueous Chemical Mixture –Material to be removed is soluble in liquid –Material to be removed reacts to form an oxide layer which is abraded by abrasive Abrasive –5-20% wgt of ~200±50nm particles Narrow PSD, high purity(<100ppm) Fumed particle = fractal aggregates of spherical primary particles (15-30nm)

Pad Properties Rodel Suba IV Polyurethane –tough polymer Hardness = 55 –Fiber Pile Specific Gravity = 0.3 Compressibility=16% rms Roughness = 30μm –Conditioned

Heuristic Understanding of CMP Preston Equation(Preston, F., J. Soc. Glass Technol., 11,247,(1927). –Removal Rate = K p *V*P V = Velocity, P = pressure and K p is the proportionality constant.

CMP Pad Modeling Pad Mechanical Model - Planar Pad Warnock,J.,J. Electrochemical Soc.138(8) (1991). Does not account for Pad Microstructure

CMP Modeling Numerical Model of Flow under Wafer –3D -Runnels, S.R. and Eyman, L.M., J. Electrochemical Soc. 141,1698(1994). –2-D -Sundararajan, S., Thakurta, D.G., Schwendeman, D.W., Muraraka, S.P. and Gill, W.N., J. Electrochemical Soc. 146(2), (1999).

Copper Dissolution Solution Chemistry –Must Dissolve Surface Slowly without Pitting Supersaturation

Oxidation of Metal Causes Stress Stress,  i = E i (P-B i – 1)/(1 - i ) P-B i is the Pilling-Bedworth ratio for the oxide