Phosphorus-doped polysilicon

A phosphorus-doped polysilicon layer was used as the sensor heater. Its temperature coefficient of resistivity was positive with a value of 6 x 10 4°C 1. The value of the heater resistance as a function of temperature was used to indicate the sensor temperature. 

A 1 ym thick polycrystalline silicon (polysilicon) layer was then deposited by chemical vapor deposition (CVD). Phosphorus doping of polysilicon was done by ion implantation with a dosage of 1Cr° cm-2 and a voltage of 200 keV. The polysilicon sheet resistance of 50 SI/ was obtained after post-implant activation (Figure 1a). 

In contrast to the films described in the last chapter, the ones to be discussed in this chapter have only become of interest recently. Up to the present, the integrated circuit gate electrodes have been fabricated from LPCVD polysilicon, which is heavily doped with phosphorus in a separate step (either by diffusion or ion implantation). Such heavily doped polysilicon can have resistivities as low as 500 juJ2-cm, so it behaves as a conductor, although not a very good one. Its compatibility with standard processing steps, however, make it a very attractive gate material. 

Silicon dioxide films have been an essential factor in the manufacture of integrated circuits from the earliest days of the industry. They have been used as a final passivation film to protect against scratches and to getter mobile ion impurities (when doped with phosphorus). Another application has been as an interlayer dielectric between the gate polysilicon and the aluminum metal-ization. Initially, most such films were deposited in atmospheric pressure systems. In recent years, low pressure processes have assumed greater importance. We will begin by examining the atmospheric process. 

To ensure consistent resistivity throughout the polysilicon structure, an alternative to the implantation and doping process has been applied to films greater than 2 pm thick. By adding phosphorus during the deposition step, control of the stress effects in a manner similar to small amounts of oxygen occur. A minimum level of phosphorus is needed for low stress and flat beams, but excessive... 

Now, molten silicon is produced from a material called polysilicon, which has been stacked in a closed oven. Specific quantities of doping materials such as arsenic, phosphorus, boron, and antimony are added to the mixture, according to the conducting properties desired for the silicon chips that will be produced. The polysilicon melt is rotated in one direction (clockwise) then, a seed crystal of silicon, rotating in the opposite direction (counterclockwise), is introduced. The melt is carefully cooled to a specific temperature as the seed crystal structure is drawn out of the molten mass at a rate that determines the diameter of the resulting crystal. 

In this process, as shown step-by-step in cross section in Figure 1.7, the surface of a 150 mm substrate (n-type, 1-2 Qcm) is heavily doped with phosphorus to a resistance of 10 Q/D to avoid charge buildup at the substrate-nitride interface when high voltages are applied between the substrate and the subsequent conducting layers. The surface is protected by a blanket low-pressure chemical vapor deposition (LPCVD) of a 0.6 pm thick insulating silicon nitride (90 MPa residual tensile stress). A 0.5 pm thick layer of LPCVD polysilicon (n-type, 30Q/D, 25 MPa residual compressive stress) PolyO is... 

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