Manufacturing Bits: Dec. 11

FinFET vs. FD-SOI pH sensors
At the recent 2018 IEEE International Electron Devices Meeting (IEDM), TSMC and National Tsing Hua University presented a paper on an ion detector or pH sensor based on a 16nm finFET technology.

Researchers have developed an advanced version of an ion-sensitive field-effect transistor (ISFET). Originally developed in the 1970s, ISFETs are pH sensors that are used to measure ion concentrations in solutions. In addition, pH sensors are also used in genome sequencing, biomolecule sensing and other apps.

A schematic view of an ISFET. Source and drain are the two electrodes used in a FET system. The electron flow takes place in a channel between the drain and source. The gate potential controls the flow of current between the two electrodes. (Source: Wikipedia)

For years, the industry has used pH sensors to obtain the value of blood, gastric juice, urine, soil, pharmaceutical inventory, food and beverages.

An ISFET resembles a planar MOSFET structure. Originally, the first ISFETs had a source and drain, but no gate. Over time, a gate was added to the ISFET in an effort to improve the pH sensing capabilities.

Then, the technology migrated towards a floating-gate structure, which is referred to as ISFGFET. Generally, ISFGFETs are based on planar MOSFET structures.

TSMC and National Tsing Hua University, meanwhile, have devised the world’s first laterally coupled ISFGFET based on a finFET. This, in turn, provides more flexibility and sensitivity over planar devices. The 16nm finFET ion sensor features high sensitivity and high linearity through a new self-balancing readout scheme. It demonstrates a maximum readout sensitivity of 115mV/pH.

There are other approaches to the technology. At IEDM, STMicroelectronics, Université de Lyon and the Université de Sherbrooke presented a paper on a CMOS-based pH sensor using fully-depleted silicon-on-insulator (FD-SOI) technology.

The technology is a better solution than ISFETs, according to researchers. ISFETs are in its infancy in commercialization and suffer from sensitivity and stability issues.

Instead, STMicroelectronics and others demonstrated a pH sensor using the gate protection diode of standard FD-SOI transistors in the back-end-of-the line (BEOL). “The extremely steep switching of the drain current induced by an exploitation of the DIBL effect is used for fabrication of extremely sensitive pH-sensors,” said G. T. Ayele from STMicroelectronics and others in the IEDM paper.

“The back gate voltage at which the abrupt switching of the drain current occurs depends on the potential at the gate protection diode. Integrating the pH sensing film on this diode BEOL metal, the shift depends on the pH value of the liquid which creates a proportional potential,” Ayele said. “The abrupt switching (as small as 9 mV/decade) of the drain current can give a theoretical maximum sensitivity of 6.6 decade of drain current change per unit pH.”

All told, researchers reported an experimental sensitivity of 1.25 decade/pH, which is better that advanced CMOS pH sensors, which have a maximum sensitivity of 0.9 decade/pH.

3D ISFETs
Also at IEDM, Ecole Polytechnique Fédérale de Lausanne (EPFL) and Xsensio presented a paper on an CMOS-based extended metal-gate ISFETs for pH and multi-ion (Na+, K+, Ca2+) sensing.

Researchers are targeting the technology for next-generation wearables, which would provide non-invasive and real-time monitoring of various human body fluids for personalized and preventive healthcare.

For this, the ISFET is an ideal solution, but the device often suffers from low yields and integration issues. They are also sometimes limited by low sensitivity, a large spread in threshold voltage (Vth), large drift rate, and high power consumption, according to researchers.

In response, EPFL and Xsensio have devised a 3D extended-metal-gate ISFET or 3D-EMG-ISFET. “(The device has) unique figures of merit: (i) extremely-low-power (down to a record value of 2 pW per sensor under excellent linearity), (ii) all CMOS integrated, (iii) high performance pH and multi-ion (Na+, K+, Ca2+) sensing, and, (iv) uniquely low cross sensitivity experimentally proven,” said J. R. Zhang of EPFL and others in the paper.

“The 3D-EMG-ISFETs are fabricated by post-processing MOSFET devices designed in a commercial 0.18μm CMOS chip. The gate of the MOSFET is vertically extended in 3D to the top metal layer through stacks of vias and metal layers with SiO2 as inter-metal dielectric (IMD),” Zhang said. “A reactive ion etching (RIE) post-process is utilized to open the passivation layers (Si3N4 and SiO2) sitting above the top metal. The exposed top metal (aluminum) is oxidized to form a thin Al2O3 which is used as a sensing layer. These post-process steps can easily be replaced by using a PAD mask in the layout design phase before sending to foundry fabrication, thus making the fabrication of 3D-EMG-ISFET an unmodified commercial CMOS process.”