Chemical Vapor Deposition and ALD PDF

Common deposition methods for thin films in IC fabrication Pentium 4 Processor http://www.intel.com/intel/intelis/museum/online/hist_micro/hof/index.htm Chemical Vapor Deposition (CVD) philosophy CVD steps:...

Common deposition methods for thin films in IC fabrication Pentium 4 Processor http://www.intel.com/intel/intelis/museum/online/hist_micro/hof/index.htm Chemical Vapor Deposition (CVD) philosophy CVD steps: Introduce reactive gases to the chamber. Activate gases (decomposition) by heat or plasma. Gas adsorption by substrate surface. Reaction takes place on substrate surface, film formed. Transport of volatile byproducts away from substrate. Exhaust waste. CVD : deposit film through chemical reaction and surface absorption. 3  Steps involved in a CVD process Gas stream 1 7 2 6 Reaction rate may be limited by: 3 4 5 Gas transport to/from surface. Figure 9-5 Wafer Surface chemical reaction rate that Susceptor depends strongly on temperature. 1. Transport of reactants to the deposition region. 2. Transport of reactants from the main gas stream through the boundary layer to the wafer surface. 3. Adsorption of reactants on the wafer surface. 4. Surface reactions, including: chemical decomposition or reaction, surface migration to attachment sites (kinks and ledges); site incorporation; and other surface reactions (emission and redeposition for example). 5. Desorption of byproducts. 6. Transport of byproducts through boundary layer. 7. Transport of byproducts away from the deposition region. Steps 2-5 are most important for growth rate. Steps 3-5 are closely related and can be grouped together as “surface reaction” processes. 5 6  CVD advantages and disadvantages (as compared to physical vapor deposition) Advantages: High growth rates possible, good reproducibility. Can deposit materials which are hard to evaporate. Can grow epitaxial films. In this case also termed as “vapor phase epitaxy (VPE)”. For instance, MOCVD (metal-organic CVD) is also called OMVPE (organo-metallic VPE). Generally better film quality, more conformal step coverage (see image below). Disadvantages: High process temperatures. Complex processes, toxic and corrosive gasses. Film may not be pure (hydrogen incorporation…). 8  CVD sources and substrates Types of sources o Gasses (easiest) o Volatile liquids o Sublimable solids o Combination Source materials should be o Stable at room temperature o Sufficiently volatile o High enough partial pressure to get good growth rates o Reaction temperature < melting point of substrate o Produce desired element on substrate with easily removable by-products o Low toxicity Substrates o Need to consider adsorption and surface reactions o For example, WF6 deposits on Si but not on SiO2 9  Derivation of film growth rate (similar to/simpler than Deal-Grove model for thermal oxidation) Boundary layer F1 = diffusion flux of reactant species to the wafer through the Gas Silicon boundary layer (step 2) = mass transfer flux CG F1 = h G (C G − C S ) (1) F1 CS where hG is the mass transfer coefficient (in cm/sec). F2 = flux  of reactant consumed by the surface reaction (steps 3- 5) = surface reaction flux, Figure 9-6 F2 F2 = k S C S (2) where kS is the surface reaction rate (in cm/sec). In steady state:  F = F1 = F2 (3) −1  kS  Equating Equations (1) and (2) leads to C S = C G 1 +  (4)  hG  F k h CG k h CT The growth rate of the film is now given by v= = S G = S G Y (5) N k S + hG N k S + hG N  where N is the number of atoms per unit volume in the film and Y is the mole fraction (partial pressure/total pressure) of the incorporating species, CT is total concentration of all molecules 16 in the gas phase.   Derivation of film growth rate (continued) F k h CG k h CT v= = S G = S G Y (5) N k S + hG N k S + hG N (a). If kS