Improving Reliability of Semiconductor Devices

Improving Reliability of Semiconductor Devices

Major semiconductor manufacturer

Semiconductors permeate all the devices we use today, and we take for granted that they will function reliably. However, in modern semiconductor processes, the random telegraph noise (RTN) phenomenon has become a critical parameter affecting device reliability.

What is RTN?

A random telegraph signal is a stochastic process that flips between 1 and +1 with the number of zero crossings in any period (0,t) described by a Poisson process.

Random telegraph noise in a MOSFET appears as random variations in the drain current under constant applied gate-to-source (VGS) and drain-to-source (VDS) voltage bias. Electron trapping/ detrapping is the presumed cause of RTN. It is innate to semiconductor MOSFET devices, and it has always been present in MOS processes.

Until recently, RTN was mainly a concern of CMOS image sensor manufacturers, as it would generate erroneous white spots in what should be dark areas. However, as operating voltages have decreased and lithographies have continued to shrink, RTN has begun to impact the stability of SRAM cells. Since virtually all integrated circuits use SRAM for their cache memory, this is critically important to the semiconductor industry. Virtually all the major industry players now must evaluate their processes for susceptibility to RTN.

A major semiconductor manufacturer needed a reliable means to evaluate the impact of RTN on its processes, and Keysight was able to provide a solution.

The Challenge: Automating RTN Measurement Measuring

RTN requires equipment capable of measuring current rather fast (in nanoseconds) and with sufficient storage to record hundreds of thousands or even millions of data points. In addition, you need equipment with a noise floor of at least 1x10-23 A2 /Hz at 20 Hz (which is about two decades above what is necessary to measure 1/f noise). Besides the hardware, there are some practical considerations when measuring RTN. The first of these is that the phenomenon is truly random; not all devices will show it. More importantly, RTN is highly dependent on the VGS voltage bias. Changing the VGS by 100 mV or less can cause RTN to appear or disappear on a given device. These facts mandate that to truly characterize a process for RTN behavior, you need to measure many devices at multiple VGS values. The only practical way to do this is on-wafer using some sort of test automation.

The Solution: A Custom Solution Using Keysight Products

The local Keysight application engineer worked with the test engineers at the semiconductor

manufacturer to develop an automated RTN test solution. The WGFMU module supports an API that

allows the semiconductor manufacturer to develop automated test routines in C#.

Together, they defined a solution with two key components:

• The B1500A with the WGFMU module installed
• Drivers for this customer’s existing test shell to control the B1500A remotely

Fully addressing this customer’s requirements required a custom solution. However, the engineers

built the resulting integrated solution around off-the-shelf Keysight hardware and software.