Polycrystalline Silicon for Integrated Circuit Applications

Recent years have seen silicon integrated circuits enter into an increasing number of technical and consumer applications, until they now affect everyday life, as well as technical areas. Polycrystalline silicon has been an important component of silicon technology for nearly two decades, being used first in MOS integrated circuits and now becoming pervasive in bipolar circuits, as well. During this time a great deal of informa­ tion has been published about polysilicon. A wide range of deposition conditions has been used to form films exhibiting markedly different properties. Seemingly contradictory results can often be explained by considering the details of the structure formed. This monograph is an attempt to synthesize much of the available knowledge about polysilicon. It represents an effort to interrelate the deposition, properties, and applications of polysilicon so that it can be used most effectively to enhance device and integrated-circuit perfor­ mance. As device performance improves, however, some of the proper­ ties of polysilicon are beginning to restrict the overall performance of integrated circuits, and the basic limitations of the properties of polysili­ con also need to be better understood to minimize potential degradation of circuit behavior.

1 Deposition.- 1.1 Introduction..- 1.2 Thermodynamics and kinetics.- 1.3 The deposition process.- 1.4 Gas-phase and surface processes.- 1.4.1 Convection.- 1.4.2 The boundary layer.- 1.4.3 Diffusion through the boundary layer.- 1.4.4 Reaction.- 1.4.5 Steady state.- 1.5 Reactor geometries.- 1.5.1 Low-pressure, hot-wall reactors.- 1.5.2 Atmospheric-pressure, cold-wall reactor.- 1.6 Reaction.- 1.6.1 Decomposition of silane.- 1.6.2 Surface adsorption.- 1.6.3 Deposition rate.- 1.6.4 Rate-limiting step.- 1.7 Deposition of doped films.- 1.7.1 n-type deposited films.- 1.7.2 p-type deposited films.- 1.7.3 Electrostatic model.- 1.8 Step coverage.- 1.9 Enhanced deposition techniques.- 1.10 Summary.- 2 Structure.- 2.1 Nucleation.- 2.1.1 Amorphous surfaces.- 2.1.2 Single-crystal surfaces.- 2.2 Surface diffusion and structure.- 2.2.1 Subsurface rearrangement.- 2.3 Evaluation techniques.- 2.4 Grain structure.- 2.5 Grain orientation.- 2.6 Optical properties.- 2.6.1 Index of refraction.- 2.6.2 Absorption coefficient.- 2.6.3 Ultraviolet surface reflectance.- 2.6.4 Use of optical properties for film evaluation.- 2.7 Etch rate.- 2.8 Stress.- 2.9 Thermal conductivity.- 2.10 Structural stability.- 2.10.1 Recrystallization mechanisms.- 2.10.2 Undoped or lightly doped films.- 2.10.3 Heavily doped films.- 2.10.4 Implant channeling.- 2.10.5 Amorphous films.- 2.11 Epitaxial realignment.- 2.12 Summary.- 3 Dopant Diffusion and Segregation.- 3.1 Introduction.- 3.2 Diffusion mechanism.- 3.2.1 Diffusion along a grain boundary.- 3.2.2 Diffusion in polycrystalline material.- 3.3 Diffusion in polysilicon.- 3.3.1 Arsenic diffusion.- 3.3.2 Phosphorus diffusion.- 3.3.3 Antimony diffusion.- 3.3.4 Boron diffusion.- 3.3.5 Limits of applicability.- 3.4 Diffusion from polysilicon.- 3.5 Interaction with metals.- 3.5.1 Aluminum.- 3.5.2 Other metals and silicides.- 3.6 Dopant segregation at grain boundaries.- 3.6.1 Theory of segregation.- 3.6.2 Experimental data.- 3.7 Summary.- 4 Oxidation.- 4.1 Introduction.- 4.2 Oxide growth on polysilicon.- 4.2.1 Oxidation of undoped films.- 4.2.2 Oxidation of doped films.- 4.2.3 Effect of grain boundaries.- 4.2.4 Effects of device geometry.- 4.2.5 Oxide-thickness evaluation.- 4.3 Conduction through oxide on polysilicon.- 4.3.1 Interface features.- 4.3.2 Deposition conditions.- 4.3.3 Oxidation conditions.- 4.3.4 Dopant concentration and annealing.- 4.3.5 Carrier trapping.- 4.4 Summary.- 5 Electrical Properties.- 5.1 Introduction.- 5.2 Undoped polysilicon.- 5.3 Moderately doped polysilicon.- 5.3.1 Carrier trapping at grain boundaries.- 5.3.2 Carrier transport.- 5.3.3 Trap concentration and energy distribution.- 5.3.4 Thermionic field emission.- 5.3.5 Grain-boundary barriers.- 5.3.6 Limitations of models.- 5.3.7 Segregation and trapping.- 5.3.8 Summary.- 5.4 Grain-boundary modification.- 5.5 Heavily doped polysilicon films.- 5.5.1 Solid solubility.- 5.5.2 Method of doping.- 5.5.3 Stability.- 5.5.4 Mobility.- 5.5.5 Future trends.- 5.6 Minority-carrier properties.- 5.6.1 Lifetime.- 5.6.2 Switching characteristics.- 5.7 Summary.- 6 Applications.- 6.1 Introduction.- 6.2 Silicon-gate technology.- 6.2.1 Threshold voltage.- 6.2.2 Polysilicon interconnections.- 6.2.3 Process compatibility.- 6.2.4 New structures.- 6.2.5 Gettering.- 6.2.6 Gate-oxide reliability.- 6.3 Nonvolatile memories.- 6.4 High-value resistors.- 6.5 Fusible links.- 6.6 Polysilicon contacts.- 6.6.1 Reduction of junction spiking.- 6.6.2 Diffusion from polysilicon.- 6.7 Bipolar integrated circuits.- 6.7.1 Vertical npn bipolar transistors.- 6.7.2 Lateral pnp bipolar transistors.- 6.8 Device isolation.- 6.8.1 Dielectric isolation.- 6.8.2 Epi-poly isolation.- 6.8.3 Trench isolation.- 6.8.4 Summary.- 6.9 Trench capacitors.- 6.10 Polysilicon diodes.- 6.11 Polysilicon transistors.- 6.12 Polysilicon sensors.- 6.13 Summary.