Resistor Fabrication on Semiconductor Wafers - Diffused, Ion-implanted, Thin-Film, Polysilicon

Resistor Fabrication on Semiconductor Wafers

Microcircuits built on semiconductor wafers would almost always require resistor components. There are several types of resistors being built on wafers, the most common of which are the: 1) diffused resistors; 2) ion-implanted resistors; 3) thin-film resistors; and 4) polysilicon resistors.

Diffused resistors are resistors that are fabricated through p-type diffusion into an n-type background, which is usually accomplished simultaneously with base diffusion. The sheet resistance of a base diffusion is usually 100-200 ohms per square. As such, the use of base diffusion will result in good lay-out proportions for resistors ranging in value from 50 to 10K ohms. A diffused resistor is isolated from its background by the contact potential of its corresponding p-n junction, or by a high reverse voltage if the tub is biased properly. Such practice allows several diffused resistors to be built in a single tub, resulting in savings in chip real estate.

Ion-implanted resistors exhibit sheet resistances that are as high as 5 kilo-ohms per square, allowing significant reductions in chip area requirements of high-value resistors. Ion-implanted resistors are fabricated by first forming two base diffusions and then ion-implanting the resistor between them. Contacts are then formed on the base diffusions. Ion-implanted resistors are suited for low-power digital and linear circuits because of their high sheet resistance.

Thin-film resistors offer the advantages of high precision and stability. They are fabricated by vacuum evaporation or sputtering of thin films of resistive materials directly on top of the oxide layer of the substrate. Materials used for thin-film resistors include nichrome, sichrome, and a variety of refractory silicides. These materials exhibit good adhesion on the oxide as thin films, and are usually built with a film thickness of about 100-1000 angstroms. The value of a thin film resistor can be set precisely to its final value by laser trimming.

Polysilicon resistors are fabricated from undoped polysilicon films that are deposited onto the chip. These are then implanted with the right type and amount of impurity (n- or p-type), and then annealed at about 600-1000 deg C. Polysilicon resistors are used when high values of resistance are needed but wide tolerances are acceptable.

Reference: Sorab K. Ghandhi, VLSI Fabrication Principles, Wiley-Interscience

See Also: Thin Films; Diffusion; Ion Implant

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