Fabrication on Semiconductor Wafers
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
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.
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
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.
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.
Sorab K. Ghandhi, VLSI Fabrication Principles, Wiley-Interscience
Diffusion; Ion Implant
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