Circuit And Method To Measure Threshold Voltage Distributions In Sram Devices
First Claim
1. A circuit to measure characteristics of a memory device having a plurality of inverters with transistors of a first transistor type, the circuit comprising:
- an array of ring oscillators, wherein each of said ring oscillators in said array (a) produces a ring oscillator output signal such that said array produces a plurality of ring oscillator output signals and (b) includes a plurality of inverters having transistors of said first transistor type;
a multiplexer connected to said array of ring oscillators to receive said plurality of ring oscillator output signals and to provide as an output, in response to a select signal, one of said plurality of ring oscillator output signals;
a control logic unit coupled to said array of ring oscillators and to said multiplexer, said control logic unit enabling each of said ring oscillators in said array so as to cause said array produce said plurality of ring oscillator output signals, further wherein said control logic unit provides said select signal; and
an output circuit receiving said plurality of ring oscillator output signals from said multiplexer;
wherein said output circuit measures output frequency variability in each of said plurality of ring oscillator output signals.
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Accused Products
Abstract
A circuit for inline testing of memory devices which provides information on the variation of the threshold voltage. The circuit includes an array of ring oscillators with a series of inverters, which already exist in the memory device. A control logic systematically steps through all of the ring oscillators by enabling each inverter and toggling the input. The mean frequency and its distribution is measured and recorded in an output circuit. The threshold voltage variation in the memory device is deduced from the ring oscillators. The circuit additionally includes two inverters place external of the memory device. Each ring oscillator is coupled to an inverter. The inverter preconditions the elements of the ring oscillator to prevent a resistive divider between the two transistors.
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Citations
19 Claims
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1. A circuit to measure characteristics of a memory device having a plurality of inverters with transistors of a first transistor type, the circuit comprising:
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an array of ring oscillators, wherein each of said ring oscillators in said array (a) produces a ring oscillator output signal such that said array produces a plurality of ring oscillator output signals and (b) includes a plurality of inverters having transistors of said first transistor type; a multiplexer connected to said array of ring oscillators to receive said plurality of ring oscillator output signals and to provide as an output, in response to a select signal, one of said plurality of ring oscillator output signals; a control logic unit coupled to said array of ring oscillators and to said multiplexer, said control logic unit enabling each of said ring oscillators in said array so as to cause said array produce said plurality of ring oscillator output signals, further wherein said control logic unit provides said select signal; and an output circuit receiving said plurality of ring oscillator output signals from said multiplexer; wherein said output circuit measures output frequency variability in each of said plurality of ring oscillator output signals. - View Dependent Claims (2, 3, 4, 5, 6, 7)
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8. A memory device comprising an array of ring oscillators received in the memory device;
wherein each of said array of ring oscillators includes a plurality of inverters formed from existing same-type devices of the memory device. - View Dependent Claims (9, 10, 11, 12, 13, 14)
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15. A method of measuring threshold voltage distribution in a memory device, the method comprising the steps of:
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providing an array of ring oscillators in the memory device, each ring oscillator including a plurality of ring inverters coupled in series; wherein said plurality of ring inverters includes existing same type devices of the memory device. - View Dependent Claims (16, 17, 18, 19)
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Specification