Semiconductors are used in integrated circuits for a wide range of applications, including personal computers, music and/or video devices, multimedia devices, digital assistants, communications devices, and so forth. In general, integrated circuits manufactured using modern fabrication processes may be extremely consistent, with individual integrated circuits from a single wafer being substantially identical to one another in terms of performance.
However, fabrication process variations (or simply, process variations) may occur. Process variations may impact field effect transistor channel widths and lengths, gate oxide thicknesses, doped material concentrations, and so forth. A fairly common side-effect due to variations in the fabrication process used to create integrated circuits may be changes in threshold voltage (VTH) of transistors in the integrated circuits. A change in threshold voltage may alter leakage current, which may impact dynamic random access memory (DRAM) charge retention times, transistor operating speeds, and so forth.
FIG. 1a is a diagram of a prior art ring oscillator 100 used to characterize process variations in an integrated circuit. Ring oscillator 100 comprises an odd number of inverters 105-109 arranged serially in a loop. When an integrated circuit containing ring oscillator 100 is powered on, ring oscillator 100 will also be energized and automatically produce a clock signal at a frequency that is a function of inverters 105-109. The frequency of the clock signal may be measured to determine global process variations. For example, if the frequency of the clock signal is greater than an expected frequency based on nominal values for inverters 105-109, then the threshold voltage of at least one of the inverters may have decreased below an expected value. Similarly, if the frequency of the clock signal is smaller than the expected frequency, then the threshold voltage of at least one of the inverters may have increased beyond the expected value.
FIG. 1b is a diagram of a prior art single stage of a ring oscillator 150. Rather than having only inverters arranged serially in a loop, each stage of ring oscillator 150 comprises an inverter 155 and a pass gate 160. Each stage also includes an effective load 165 modeled as a capacitor. Effective load 165 may be representative of a subsequent stage coupled to pass gate 160. Pass gate 160 may be used to make or break the loop. Pass gate 160 may be implemented using a field effect transistor (FET), such as an NFET or a PFET. Preferably, each stage of ring oscillator 150 includes a pass gate formed from the same type of FET. The use of a particular type of FET may allow for a characterization of process variations for that particular type of FET. For example, if NFETs are used to implement pass gate 160, then it may be possible to determine global process variations for NFETs. Similarly, if PFETs are used, then it may be possible to determine global process variations for PFETs.
FIG. 2 is a diagram of an integrated circuit 200. Integrated circuit 200 includes integrated circuitry 205 that implements the functionality of integrated circuit 200. Integrated circuit 200 also includes several ring oscillators, such as ring oscillator 210 arranged along a top side of integrated circuit 200, ring oscillators 215-216 arranged along left and right edges of integrated circuit 200, ring oscillator 220 arranged on a lower right hand corner of integrated circuit 200, ring oscillator 225 formed in an interior of integrated circuit 200, and so forth. A ring oscillator may also be formed along more than one edge of integrated circuit 200. Using the ring oscillators may allow for a measurement of process variations throughout integrated circuit 200. In general, it is desirable to have multiple ring oscillators or a large ring oscillator distributed over different portions of integrated circuit 200 so that the elements of the ring oscillators may encounter process variations like the circuitry in integrated circuit 200. FIG. 2 may illustrate an exaggerated use of ring oscillators in an integrated circuit.