Design of a Cryo-CMOS Smart Temperature Sensor

Abstract

Quantum computers exploit the quantum behavior of quantum bits (qubits) that are typically operated at cryogenic temperature. Qubits require an electronic control interface. Nowadays, such classical electronic controllers use bulky instruments operated far fromthe qubits outside the cryostat. To run practical quantum algorithms, thousands to millions of qubits are required. Using bulky instruments to scale up to this number of qubits becomes increasingly challenging, especially due the the large amount of wiring from the roomtemperature controller to the cryogenic qubits. CMOS electronics operating down to 4 K allow closer system integration, enabling the scalability of the quantum computer of the future. Since the cooling power in the dilution refridgerator at 4 K is limited, the power dissipation of the cryogenic electronics should be optimized. Therefore, a dense array of temperature sensors becomes fundamental for power management. Commercially available cryogenic temperature sensors, such as platinumresistance sensors, thermocouples and diodes can achieve high accuracy. However, the thermal resistance between the sensor and the sample introduces uncertainty in the measurement. Moreover, they consume significant power and area, which makes scaling the temperature sensor dense array more difficult. To mitigate the thermal resistance and fully integrate the sensor with the cryo-CMOS qubit controller, a CMOS temperature to digital convertor (TDC) can be used. A TDC is an integrated system that consists of a sensing element, bias circuitry and an analog-to-digital converter (ADC). Integrated TDCs have found widespread use due to their low cost, power and area, while keeping good levels of accuracy. However, to the author’s best knowledge, no TDC has been reported to reach a temperature range below 203 K, which is far away from the target 4 K operating temperature. The main challenge to design cryo-CMOS TDCs is the lack of foundry device models outside of the military range (218 to 398 K) and the reduced temperature sensitivity of the sensing elements. The goal of the project is to build the first ever integrated CMOS temperature sensor operating over the temperature range from 4.2 to 300 K. This thesis discusses the feasibility of traditional sensing elements and studies the accuracy of few characterized sensing elements down to 4.2 K. For exploratory purposes, different sensing elements can be multiplexed, such that any of them can be selected to be digitized by the ADC. Subsequently, the thesis explains the system level, schematic and layout design of a second-order discrete-time feed-forward delta-sigmamodulator ADC. Simulations of the TDC presented in this work give a temperature reading every 10 ms, and consumes less than 1μJ per conversion at room temperature. The output accuracy is limited by the sensing element, if an external reference is used and 2-point trimming is applied, the accuracy is expected to have a 3σ = ±1.4K in the temperature range of 4.2-300 K, based on available measurements. Through the presented TDC, the understanding of the devices at cryogenic temperatures will be improved. This will allow better cryo-CMOS qubit controllers to be created, which, in turn, will pave the way towards scalable quantumcomputers.