A. Transport of Reactions to Wafer Surface in APCVD • Transport of reactants by forced convection to the deposition region • Transport of reactants by diffusion from the main gas stream to the wafer surface • Turbulent flow can produce thickness nonuniformities • Depletion of reactants can cause the film thickness to decrease in direction of gas flow • Adsorption of reactants on the wafer surface
APCVD B. Chemical reaction • Surface migration • Site incorporation on the surface • Desorption of byproducts • Removal of chemical byproducts • Transport of byproduct through the boundary layer • Transport of byproducts by forced convection away from the deposition region
APCVD • At steady state – if two fluxes are equal • The growth rate of the film, v (cm/s), is • Where N is the number of atoms incorporated into the film per unit volume • For single composition film, this is the density
Mole fraction • The mole fraction in incorporating species in the gas phasewhere CT is the concentration of all molecules in the gas phase
Two limiting cases for APCVD model • Surface reaction controlled case (kS<<hG) • Mass transfer or gas-phase diffusion controlled case(hG<<kS)
APCVD • Both cases predict linear growth rates • but they have different coefficients • There is no parabolic growth rate • Surface reaction rate constant is controlled by Arrhenius-type equation (X=Xoe-E/kT) • Quite temperature sensitive • Mass transfer coefficient is relatively temperature independent • Sensitive to changes in partial pressures and total gas pressure
Epitaxial deposition of Si • Slopes of the reaction-limited graphs are all the same • activation energy of about 1.6 eV • This implies the reactions are similar; just the number of atoms is different • There is reason to believe that desorption of H2 from the surface is the rate limiting step • In practice • epitaxial Si at high temperatures (mass transfer regime) • poly-Si is deposited at low temperatures (reaction limited, low surface mobility)
Deposition of Si • Choice of gas affect the overall growth rate • Silane (SiH4) is fastest • SiCl4 is the slowest • Growth rate in the mass transfer regime is inversely dependent on the square root of the source gas molecular weight • Growth rate is dependent on the crystallographic orientation of the wafer • (111) surfaced grow slower than (100) • Results in faceting on nonplanar surfaces
APCVD • In the preceding theory, assumed hG and Cs were constants • Real systems are more complex than this • Consider the chamber where wafers lie on a susceptor (wafer holder). • Stagnant boundary layer, S, is not a constant, but varies along the length of the reactor • Cs varies with reaction chamber length as reaction depletes gases
Effects • Changes the effective cross section of the tube, which changes the gas flow rate • Increasing the flow rate reduces the thickness of the boundary layer and increases the mass transfer coefficient • Reduces gas diffusion length • To correct for the gas depletion effect, the reaction rate is increased along the length of the tube by imposing an increasing temperature gradient of about 5—25oC
APCVD • Sometimes we wish to dope the thin films as they are grown (e.g. PSG, BSG, BPSG, polysilicon, and epitaxial silicon). • Addition of dopants as gases for reaction • AsH3, B2H6, or PH3. • Surface reactions now include • Dissociation of the added doping gases • Lattice site incorporation of dopants • Coverage of dopant atoms by the other atoms in the film
APCVD • Another problem, common in CMOS production, is unintentional doping of lightly doped epitaxial Si when depositing them on a highly doped Si substrate. • Occurs by diffusion because of the high deposition temperatures (800—1000oC) • Growth rate of the deposited layers is usually much faster than diffusion rates (vt >> √Dt), the semi-infinite diffusion model can be applied
Mass transport on to deposited films • Atoms can outgas or be transported by carrier gas from the substrate into the gas stream and get re-deposited downstream • The process is called autodoping • Empirical expression to describe autodoping • C*S is an effective substrate surface concentration and L is an experimentally determined parameter • As film grows in thickness, dopant must diffuse through more film and less dopant enters gas phase.
Autodoping • Autodoping from the backside, edges, or other sources usually results in a relatively constant level. • This is because the source of dopant does not diminish as quickly but is at a much lower level.
APCVD The left part of the curve arises from the out-diffusion from the substrate The straight line part arises from the front-side autodiffusion The background (constant) part is from backside autodoping
In APCVD • It is critical to deliver the same gas flows to all the wafers in order to produce the same growth rates • Wafers placed side-by-side • In LPCVD • Mass transfer coefficient is higher as the diffusion distance of reactants is increased. • the boundary layer thickness slightly increased as P decreases • Reactions are limited by ks where adsorption on wafer surface is key limiting step. • Wafers can be stacked parallel to one another • Note that this arrangement will provide a higher throughput
PECVD • There are some cases where temperature requirements (thermal budget) will not allow high-temperature depositions • E.g., depositing Si3N4 or SiO2 after Al • APCVD and LPCVD do not produce good quality films ~ 450oC • Reaction produced in a plasma ignited using an inert gas between two electrodes • The sample may be heated • Usually between 200-450oC
PECVD • Ideally suited to rapidly varying the film composition and properties during deposition • Apply a potential and an AC signal (13.56 MHz) across a low pressure of the inlet gases • The processes that occur are complicated and very difficult to model • The resultant products are very far from thermodynamic equilibrium • Generates high concentration of particulates • Pinhole density is a problem if chamber is not routinely cleaned.
PECVD Oxide Oxynitride Microfluidic Channels formed by PECVD
CVD LPCVD PECVD
Polysilicon • LPCVD • Deposited by thermal decomposition of silane (SiH4) • Deposition temperature range 580-650°C • SiH4 (vapor) = Si (solid) + 2H2 (gas) • A typical set of deposition parameters: • Temperature: 620°C • Pressure: 0.2-1.0 torr • SiH4 flow rate ~ 250sccm • Deposition rate = 8-10nm/min • Doping of film • In-situ (during deposition) by the addition of dopant gases such as phophine, arsine, and diborane • Often doped after deposition by diffusion or ion implantation • Typically highly doped to achieve low resistance interconnections • 0.01-0.001 ohm-cm can be obtained in diffusion-doped polysilicon.
Silicon dioxide • Variety of methods (PECVD, LPCVD, APCVD) • Index of refraction is used to determine the quality. • Can be doped or undoped • Oxide doped with 5-15% by weight of various dopants can be used as a diffusion source. • PECVD oxide • SiH4 (gas) + 2N2O (gas) SiO2 (solid) + 2N2 (gas) + 2H2 (gas) • Temperature: 200-400°C • Deposition rate of 900nm/min is achievable • Not a good step coverage • Controllable film stress (compressive) • Contains hydrogen (SiOH) • Used for deposition over metals • LPCVD oxide using dichlorosilane • SiCl2H2 + 2N2O SiO2 + 2N2 + 2HCl • Temperatures ~ 900°C • Good step coverage • Compressive stress used to stress-compensate LPCVD nitride
Oxide • LPCVD oxide using tetraethylorthosilicate (TEOS) (liquid source) • Si(OC2H5)4 SiO2 (solid) + 4C2H4 (gas) + 2H2O (gas) • Deposited at temperatures between 650-750°C • Excellent uniformity and step coverage
Silicon nitride • Variety of methods (PECVD, LPCVD, APCVD) • Can be used as an oxidation mask (LPCVD) • An excellent barrier to moisture and sodium contamination (PECVD) • PECVD nitride • SiH4 (gas) + NH3 (gas) SixNyHz (solid) + H2 (gas) • Deposition temperature: 200-400°C • Deposition rate of 20-50nm/min • Not a good step coverage • Controllable film stress • Changes after high temperature anneal due to H2 out-diffusion • LPCVD nitride using dichlorosilane • 2SiCl2H2 (gas) + 4NH3 (gas) Si3N4 (solid) + 6H2 (gas) • Deposition temperatures: 700-800°C • Good step coverage • Tensile stress • Better resistivity (1016 ohm-cm) and dielectric strength (10 MV/cm) compared to PECVD nitride films (106-1015 ohm-cm and 1-5MV/cm)