Polysilicon and Its Manufacturing

Thin films of polycrystalline silicon, or polysilicon (also known as poly-Si or poly), are widely used as MOS transistor gate electrodes and for interconnection in MOS circuits. It is also used as resistor, as well as in ensuring ohmic contacts for shallow junctions. When used as gate electrode, a metal (such as tungsten) or metal silicide (such as tantalum silicide) may be deposited over it to enhance its conductivity.

Poly-Si is known to be compatible with high temperature processing and interfaces very well with thermal SiO₂. As a gate electrode, it has also been proven to be more reliable than Al. It can also be deposited conformally over steep topography. Heavily-doped poly thin films can also be used in emitter structures in bipolar circuits. Lightly-doped poly films can also be used as resistors.

Poly-Si is usually deposited by thermal decomposition or pyrolysis of silane at temperatures from 580...650 ^oC, with the deposition rate exponentially increasing with temperature. The deposition rate is also affected by the pressure of silane, which translates to silane concentration. Other important variables in polysilicon deposition are pressure and dopant concentration.

The electrical characteristics of a poly-Si thin film depends on its doping. As in single-crystal silicon, heavier doping results in lower resistivity. Poly-Si is more resistive than single-crystal silicon for any given level of doping mainly because the grain boundaries in poly-Si hamper carrier mobility. Common dopants for polysilicon include arsenic, phosphorus, and boron. Polysilicon is usually deposited undoped, with the dopants just introduced later on after deposition.

There are three ways to dope polysilicon, namely, diffusion, ion implantation, and in situ doping. Diffusion doping consists of depositing a very heavily-doped silicon glass over the undoped polysilicon. This glass will serve as the source of dopant for the poly-Si. Dopant diffusion takes place at a high temperature, i. e., 900...1000 ^oC. Ion implant is more precise in terms of dopant concentration control and consists of directly bombarding the poly-Si layer with high-energy ions. In situ doping consists of adding dopant gases to the CVD reactant gases during the epi deposition process.

There are three commonly used techniques for doping polysilicon:

1) diffusion doping;

2) ion implantation;

3) in-situ doping.

Diffusion doping is generally done at a relatively high temperature (900...1000 ^oC). It involves the growing or deposition of a highly-doped glass on the undoped polysilicon. This doped glass will serve as the source of dopants that will diffuse into the polysilicon material. The high-temperature environment of diffusion doping not only promotes dopant diffusion from the source, but also anneals the polysilicon material. Diffusion doping's advantage is its ability to introduce very high concentrations of dopants into the poly-Si layer, attaining low levels of resistivity. The high processing temperature and its tendency to increase surface roughness are its drawbacks.

Ion implantation deposits dopants into the poly-Si layer by directly bombarding it with high-energy ions of the dopant species. Since ion implantation has destructive effects, it is followed by an annealing step that repairs the lattice disturbances and activates the implanted dopants. The advantage of ion implantation is its ability to control dopant dosage with high precision. However, it can not attain the low resistivities achievable by diffusion doping, i. e., even heavily doped ion-implanted poly-Si layers exhibit ten times the resisitivity exhibited by diffusion-doped layers. Ion-implanted polysilicon layers are often used in applications where high conductivity is not required, such as being employed as high-value load resistors in circuits.

In-situ doping refers to the doping technique wherein the dopants are introduced to the poly-si at the same time the poly-Si layer is being deposited. In-situ doping involves the addition of dopant gases such as phosphine and diborane to the CVD reactant gases used in poly-Si deposition. In-situ doping is not a simple process, since the introduction of the dopant gases complicates the control of layer thickness, dopant uniformity, and deposition rate. Adding dopants during deposition also affects the physical properties of the poly-Si layer, such as the grain size and grain orientation.

Polysilicon deposition, or the process of depositing a layer of polycrystalline silicon on a semiconductor wafer, is achieved by pyrolyzing (decomposing thermally) silane, SiH₄, inside a low-pressure reactor at a temperature of 580 to 650 ^oC. This pyrolysis process involves the following basic reaction: SiH₄ → Si + 2H₂.

Polysilicon has many applications in VLSI manufacturing. One of its primary uses is as gate electrode material for MOS devices. A polysilicon gate's electrical conductivity may be increased by depositing a metal (such as tungsten) or a metal silicide (such as tungsten silicide) over the gate. Polysilicon may also be employed as a resistor, a conductor, or as an ohmic contact for shallow junctions, with the desired electrical conductivity attained by doping the polysilicon material.

There are two common low-pressure processes for depositing polysilicon layers: using 100 % silane at a pressure of 25...130 Pa (0.2 to 1.0 Torr), and using 20–30 % silane (diluted in nitrogen) at the same total pressure. Both of these processes can deposit polysilicon on 10–200 wafers per run, at a rate of 10...20 nm/min and with thickness uniformities of ±5 %.

The critical process variables for polysilicon deposition include temperature, pressure, silane concentration, and dopant concentration. Wafer spacing and load size have been shown to have only minor effects on the deposition process.

The rate of polysilicon deposition increases rapidly with temperature There will be a minimum temperature, however, wherein the rate of deposition becomes faster than the rate at which unreacted silane arrives at the surface. Beyond this temperature, the deposition rate can no longer increase with temperature, since it is now being hampered by lack of silane from which the polysilicon will be generated. Such a reaction is then said to be "mass-transport-limited". When a polysilicon deposition process becomes mass-transport-limited, the reaction rate becomes dependent primarily on reactant concentration, reactor geometry, and gas flow.

When the rate at which polysilicon deposition occurs is slower than the rate at which unreacted silane arrives, it is said to be surface-reaction-limited. A deposition process that is surface-reaction-limited is primarily dependent on reactant concentration and reaction temperature. Deposition processes must be surface-reaction-limited because they result in excellent thickness uniformity and step coverage.

At reduced pressure levels for VLSI manufacturing, polysilicon deposition rate below 575 ^oC is too slow to be practical. Above 650 ^oC, poor deposition uniformity and excessive roughness will be encountered due to unwanted gas-phase reactions and silane depletion. Pressure can be varied inside a low-pressure reactor either by changing the pumping speed or changing the inlet gas flow into the reactor. If the inlet gas is composed of both silane and nitrogen, the inlet gas flow, and hence the reactor pressure, may be varied either by changing the nitrogen flow at constant silane flow, or changing both the nitrogen and silane flow to change the total gas flow while keeping the gas ratio constant.

Polysilicon doping, if needed, is also done during the deposition process, usually by adding phosphine, arsine, or diborane. Adding phosphine or arsine results in slower deposition, while adding diborane increases the deposition rate. The deposition thickness uniformity usually degrades when dopants are added during deposition.