# In Situ Time-Based Sensor for Process Identification Using Amplified Back-End-of-Line Resistance and Capacitance

## TL;DR

This paper introduces a sensor that uses chip metal routing to measure process variations, improving accuracy with a three-configuration method.

## Contribution

A novel three-configuration measurement approach that reduces error in RC time-constant estimation for in situ process authentication.

## Key findings

- The three-configuration method reduces measurement error by over 82% compared to direct measurement.
- The analytical model aligns closely with simulations, with a maximum difference of 2.6%.
- Sensor results show a maximum error of 6.1% when compared to direct measurements from a DMA.

## Abstract

This paper presents an in situ time-based sensor designed to provide process authentication. The proposed sensor leverages the inherent metal routing within the chip to measure the RC time-constants of interconnects. However, since the routing metal is typically designed to minimize resistance and capacitance, the resulting small RC time-constants pose a challenge for direct measurement. To overcome this challenge, a “three-configuration” measurement approach is introduced, incorporating two auxiliary components—poly resistor and metal-insulator-metal (MIM) capacitor—to generate three amplified RC time-constants and, subsequently, deduce the routing time-constant. Compared to directly measuring the routing time-constants, this approach reduces measurement error by over 82% while incurring only a 25% area penalty. A straightforward analytical model is presented, taking into account the impact of parasitic capacitances within the discharge path. This analytical model exhibits an excellent concurrence with simulated results, with a maximum difference of only 2.6% observed across all three configurations and a 3.2% variation in the derived routing time-constant. A set of five variants of the time-based sensor are realized using a 130 nm CMOS process. Each variant consists of 44 samples distributed across 11 dies on two wafers. To verify the precision of the proposed sensor, identical resistors and capacitors are fabricated alongside them, forming a direct measurement array (DMA) that is measured using external equipment. After adjusting the routing resistance and capacitance values in the simulations to correspond to the mean values obtained from the DMA measurements, the proposed sensor’s measured results demonstrate a close alignment with simulations, exhibiting a maximum error of only 6.1%.