Real Circuit Delay Measurement Method by Variable Frequency Operation with On-Chip Fine Resolution Oscillator

Abstract

With the progress of semiconductor process miniaturization, delay degradation by aging increases and threatens the reliability of fabricated chips. The amount of delay degradation is known to be circuit and workload dependent, but previous evaluations are based on simulations, and delay degradation measurement of real circuit under realistic workload has not been reported yet. This paper proposes real circuit delay measurement method, which achieves enough accuracy to measure circuit and workload dependent delay degradation. In the proposed method, onchip oscillator supplies fine resolution variable frequency clock to internal circuit. Internal circuit execute test pattern to activate critical paths at various frequency and determine the maximum frequency at which correct results can be obtained. The maximum frequency corresponds to the delay of the critical paths activated by the test pattern. Clock multiplication improves delay resolution, and repetitive measurement reduces measurement error caused by time dependent random delay variation. The proposed method has been implemented on a 65 nm low power process test chip. Variable frequency oscillator utilizes only standard cells and is designed with automatic layout flow without any timing tuning. The area overhead of the proposed method is 0.09% of the total random logic. The evaluation result show that 0.18% average measurement accuracy has been achieved.

Measurement Results of Real Circuit Delay Degradation under Realistic Workload

“…As described in Ref. [4], higher clock multiplication factor of CG leads to higher delay measurement accuracy.…”

“…This paper employs the real circuit delay measurement method proposed in Ref. [4]. The following is the summary of the method.…”

“…As described in Ref. [4], f mean is calculated by approximating the relationship between the VFO frequency and the probability of incorrect operation with the straight line.…”

“…The authors have already proposed real circuit delay measurement method, which can achieve enough accuracy to measure circuit and workload dependent delay degradation [4]. This paper reports the measurement results utilizing the proposed method.…”

“…In Section 2, the delay measurement method proposed in Ref. [4] is summarized, and the degradation measurement condition is described. In Section 3, measurement result analysis methods including the environmental correction, function approximation, and the method to remove large error data to mitigate random delay variation, are described.…”