Charge Trapping and Variability in CMOS Technologiesat Cryogenic Temperatures

Part II Defect Characterization at Cryogenic Temperatures

5 Measurement Setup and Technologies

In the following section two measurement setups will be introduced, one setup where all devices have to be contacted manually and one setup with the additional option of measuring array structures, which allows the characterization of thousands of transistors which are digitally addressable. The two experimental setups are introduced in more detail in Section 5.1. Using the toolset, various different technologies have been characterized. An overview of all characterized technologies is given in Section 5.2. In Section 5.3 the working principle of dedicated SmartArrays is explained.

5.1 Measurement Setup

Two different measurement setups have been used for measurements presented in this work. A LakeShore CRX-4K cryogenic probe station is available, which uses a closed cycle refrigerator (CCR) for cooling to cryogenic temperatures. For this, a CCR compressor cools two separate stages via Helium lines. This allows to maintain elevated temperatures at the sample during the cooling phase of the system to avoid condensation at the sample. The temperature of the chuck and the stages can be controlled using a LakeShore Model 336 temperature controlling system. A turbo vacuum pump manufactured by Pfeiffer is used to guarantee a vacuum of around \(10^{-6}\) hPa in the vacuum chamber. Up to six thermally shielded probe arms are mounted to the cryogenic stage which are equipped with Tungsten needles with a 10 µm tip radius to enable contacting small structures. This allows to contact the samples using micro-manipulated stages which enable translations in x, y and z direction. This setup has mainly been used for the characterization of single transistors. Every device thereby needs to be contacted manually, however, the probes need to be lifted during larger temperature changes to avoid scratching pads due to thermal expansion or contraction. The probe arms are connected via triaxial cables to a custom built measurement box, which is described in more detail in [213, 214, 140].

The other setup uses a LakeShore PS-VLT-CPX probe station, which allows cooling to 1.6 K using a conventional He dewar system. This can be done by lowering the pressure inside the sample stage heat exchanger with an additional rotary vane pump. The probe arm assembly is the same as at the CRX-4K probe station (6 probe arms which can be manipulated in x, y and z direction), however, additionally there is a connection from the vacuum chamber to a custom designed 48-line break-out box. This allows for the characterization of programmable arrays of thousands of transistors using an adapted version of the custom built measurement box with ten synchronized source-measure-units (SMUs) [213, 214, 140].

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Figure 5.1: A LakeShare CPX-4K cryogenic probe sation (left) was used for time-zero and reliability characterization of various technologies. A custom built measurement box with ten SMUs (right) was used for the characterization of transistor arrays. Left two images courtesy Lake Shore Cryotronics, Inc.