Polycrystalline Silicon, or Poly-Si, is an ultra pure form of silicon that has great insulating properties and temperature resistance. Polysilicon begins in two forms, chunk and granular, with granular being more popular in thin applications and high-conformity applications. Poly-Si layers can also be doped or undoped depending on the end user’s specifications. Dopant and resistivity have an inverse relationship, so the heavier the dopant, the lower the resistivity. A lightly doped Poly-Si layer will have a high resistivity. Resistivity is the major determining factor for dopant levels within the Poly-Si layer.
Undoped polysilicon layers are deposited via low pressure chemical vapor deposition (LPCVD). This is the most common Poly-Si deposition method and depending on the application, some require doping after deposition. The process takes place in a low temperature furnace between 560°C – 650°C. Undoped Poly-Si layers grow exponentially as the temperature increases, although the temperature range is small.
LPCVD deposits polysilicon in a low atmospheric pressure furnace to prevent contamination and to create a high uniformity polycrystalline layer. Electrical isolation is the most common use for undoped polysilicon layers, although diffusion and in-situ doping methods commonly use Poly-Si as well.
Most doped polysilicon layers use a combination of three primary dopants: arsenic (As), phosphorus (P), and boron (B). There are also three primary ways of doping Poly-Si:
- Diffusion: Doped polysilicon diffusion is when a layer of heavily doped glass is diffused onto an undoped silicon wafer. In this deposition type, the glass layer is the dopant source, and the furnace temperature is between 900°C – 1,000°C to allow for the glass layer to diffuse with the wafer. Due to the high temperature nature of this process, the Poly-Si layer is able to both diffuse and anneal onto the substrate. This method makes it very easy to achieve high dopant concentrations, which makes these wafers ideal for low resistivity applications.
- Implant: Ion implantation allows for a more precise control of dopant concentration. This process involves directly bombarding the Poly-Si layer with high energy ions, followed by annealing to harden the wafer. This method is commonly used when high conductivity is not necessary, such as in load resistors.
- In-situ: This deposition process is performed by adding phosphine (PH3) and diborane (B2H6) gases to common chemical vapor deposition reactant gases. The main difference between in-situ and other deposition processes is the introduction of dopants during the deposition of the Poly-Si layer, as opposed to two separate steps. Since the dopant is introduced during deposition, it is possible to change the physical properties of the layer, and this can affect the grain size and orientation of polysilicon.
Polysilicon’s temperature stability and excellent silicon oxide interaction make it a top choice for several applications, including the following:
- Gate electrodes
- Capacitor plate electrodes
- Hard mask during etching
SVM Polysilicon Specifications:
Wafer Diameters: 76mm (3″) – 300mm (12″)
Film Thickness*: 1,000Å – 20,000Å
*Film thickness also dependent on wafer diameter and other device specifications
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