The fabrication of semiconductor devices involves the sequential formation of device structures in accordance with a particular layout design. Typically, semiconductor devices, such as integrated circuits, memory devices, and the like, are fabricated according to design rules that are continually reduced in order to provide high-speed, high-density devices on a semiconductor substrate. Accordingly, the various components of the device are designed to have extremely small feature sizes. Further, the various components are packed together as tightly as possible to avoid consuming excessive substrate surface area.
Despite the need to reduce the feature size of device components, certain components, such as sheet resistors, and the like, depend on feature size to determine their electrical conductivity. For example, sheet resistors are typically laid out to have a resistance that corresponds to the length and width dimensions of the resistor on the substrate surface. Typically, to fabricate sheet resistors, a patterned semiconductor body, such as polycrystalline silicon, is formed on the substrate and implanted with conductivity-determining dopants to set the electrical resistance of the resistors. The electrical resistance of the sheet resistors will vary in accordance with the dimensions of the semiconductor body. Thus, a large amount of substrate surface area can be needed to obtain a predetermined resistance level.
The component density restrictions imposed by sheet resistors are particularly significant where block resistors, such as, electrostatic-discharge-protection (ESD) resistors are included. These resistors, otherwise known as unsilicided resistors, are situated over active portions of a metal-oxide-semiconductor (MOS) device. The unsilicided resistors are typically formed between the gate electrode and the source and drain region of the MOS transistors. In this configuration, the block resistors take up valuable semiconductor surface area in regions of the device that require the formation of high-density device structure.
Where the resistivity is set by resistor doping, efficient device fabrication requires that a minimal number of special processing steps be used to fabricate the resistors. To maintain high process efficiency, the resistors are typically doped at the same time as other device structures during the fabrication process. The fabrication process seeks maximum dopant activation in order to maximize the performance of other implanted device components, such as transistors, and the like. Maximum dopant activation, however, results in very low sheet resistance values in the doped resistors, which are often not optimal for various types of resistors, such as unsilicided resistors.
Further improvements in device fabrication methods are necessary to provide device structures, such as resistors, and transistors, that have electrical conductivities independent of their feature size.