09-10-2012, 11:36 AM
Chemical Vapor Deposition (CVD)
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Chemical Vapor Deposition (CVD) is the deposition of a solid material onto a heated substrate through decomposition or chemical reaction of compounds contained m the gas passing over the substrate. Many materials such as, silicon nitride, silicon dioxide, non-crystalline silicon, and single crystal silicon, can be deposited through CVD method.
A special method in CVD, called Epitaxy or Epitaxial Layer Deposition or Vapor-Phase Epitaxy (VPE), has only a single-crystal form as the deposited layer . This process is usually carried out for certain combinations of substrate and layer materials and under special deposition conditions.
In CVD process, a reaction chamber is introduced in which the materials to be deposited are passed through. These materials should be in the gaseous or vapor phase and react on or near the surface of the substrates, which are at some elevated temperature. This produces a chemical reaction and forms atoms or molecules that are to be deposited on entire substrate surface. A number of different materials can be deposited by the CVD process. These are listed below:
Silicon epitaxial layer on a single-crystal silicon substrate (homoepitaxy or commonly referred to as epitaxy).
Silicon epitaxial layer deposition on a sapphire (Heteroepitaxy).
Silicon dioxide deposition.
Silicon nitride deposition.
Each one of these depositions will be described in this post.
Epitaxial Deposition
Epitaxy is referred to as an arrangement of atoms in a crystal form upon a crystal substrate, so that the resulting added layer structure is an exact extension of the substrate crystal structure. In other words, deposited atoms arrange themselves along existing planes of the crystalline substrate material. This will cause the deposited atoms to bond to the parent atoms to form an unbroken chain of the crystal structure. The structure of the grown epitaxial layer is thus a continuation of that single-crystal substrate.
Epitaxy and Crystal Growing
There is no difference between epitaxy and crystal growing technique. But where, in epitaxy a thin film of single crystal silicon is grown from a vapor phase upon a existing single crystal of the same material, in crystal growing, a single crystal is grown from the liquid phase, in contrast to the growth technique in epitaxy. Furthermore, epitaxial process involves no portion of the system at a temperature anywhere near the melting point of the material.
Epitaxial deposition was the initial form in which CVD was used in IC fabrication, and it continues to play a very important role
Uses of Epitaxy
Epitaxy was first developed so as to improve the performance of discrete bipolar transistors. The breakdown voltage of the collector was determined by fabricating the devices in bulk wafers using the wafer’s resistivity to determine the breakdown voltage of the collector. However, high breakdown voltages need high-resistivity material. This requirement, coupled with the thickness of the wafer, results in excessive collector resistance that limits high-frequency response and increases power dissipation. Epitaxial growth of a high resistivity layer on a low-resistivity substrate solves this problem.
Epitaxy is also used to improve the performance of dynamic random-access memory devices and CMOS ICs.
Epitaxial Growth Process
A typical epitaxial growth process includes several steps as follows.
A hydrogen carrier gas is used to purge the reactor of air.
The reactor is then heated to a temperature.
After thermal equilibrium is established in the chamber, anhydrous HCl gas is fed into the reactor. The HCl gas reacts with the silicon at the surface of wafers in reaction that is reverse of that given for [SiCl4+H2].This reverse reaction results in vapor-phase-etching of the silicon surface and usually occurs at a temperature between 1150 and 1200°C for 3 min.
The temperature is then reduced to the growth temperature with time allowed for stabilizing the temperature and flushing the HCI gas. For [SiCI2 +H2] reaction, the graphite boat is heated to a temperature in the range 1150 – 1250 degree Celsius. The vapor of SiCl4 and hydrogen as a carrier gas arc introduced into the lube for producing epitaxial layer.
Once growth is complete, the dopant and silicon flows are eliminated and the temperature reduced, usually by shutting of the power.
As the reactor cools toward ambient temperature, the hydrogen flow is replaced by a nitrogen flow so that the reactor may be opened safely.
Problems in Growing Impurity Doped Epitaxial Layers
Epitaxial layer deposition takes place at temperatures in the range 950 to 1250°C. Due to this, diffusion of impurities may occur across the epitaxial layer or substrate interface due to the deposition and high temperature processing steps. This will cause a blurring of the impurity profile in the region of this interface. But the main problem will be the deposition of a very thin and very lightly doped epitaxial layer on a very heavily doped substrate. The outdiffusion of impurities from the heavily doped substrate into the lightly doped epitaxial layer will blot out the sharp n/n+ transition that would otherwise be present at the layer-substrate interface. The influx of donor atoms from the substrate will reduce the effective thickness of the lightly doped epitaxial layer by 1 or 2 micro meter. To minimize this problem of outdiffusion from heavily doped n+substrate, slow donor diffusants such as antimony and arsenic are often used for the doping of substrate in preference to phosphorus.
Molecular Beam Epitaxy (MBE)
Molecular beam epitaxy differs from vapor-phase epitaxy (VPE) in that it employs evaporation t [instead of deposition] method. Thus it is a non-CVD epitaxial process. Although the method has been known since the early 1960s, it has recently been considered a suitable technology for silicon device fabrication. In the MBE process the silicon along with dopants is evaporated. The evaporated species are transported at a relatively high velocity in a vacuum to the substrate.
The relatively low vapor pressure of silicon and the dopants ensures condensation on a low-temperature substrate. Usually, silicon MBE is performed under ultra-high vacuum [UHV] conditions of 10-8 to l0-16 Torr.