Researchers at IIT Bombay have developed a “memory technology” that can in principle revolutionize Indian industry and the many applications that require semiconductor chips, for example in the defense sector, in the automotive industry and in cell phone manufacture. Hard drives, flash memories, etc. are examples of storage technology. There is also another form of memory called one-time programmable memory (OTP), where memory is written to, stored for a lifetime, and accessed and used multiple times. This has a wide variety of uses, one of which is to correct defective chips that have been mass produced for certain applications.
Correction of offsets
For example, think of a chip that helps you read the temperature. Due to a manufacturing defect, the chip can display 100 degrees Celsius as 101 degrees Celsius. This “offset” of 1 degree can be corrected by storing the error correction parameter in the OTP memory. This happens uniquely for each chip and after the storage the memory corrects the output of the chip for its lifetime.
“OTP memories are also used for other purposes, mainly three: chip identity, secure information storage and chip calibration for error correction,” says Udayan Ganguly, professor at IIT Bombay, who along with A. Lele, S. the patents for the invention are held by Sadana and P. Kumbhare.
To store the correction value, the researchers used eight memory cells, each of which would store a “bit” (i.e. a value of zero or one). Each of the memory cells consists of an ultra-thin silicon dioxide layer that is 10-15 atomic layers thick. This is deposited evenly over an 8-inch plate-sized silicon wafer to form millions of nanocapacitors. “The flawless silicon dioxide layer is insulating and allows very little current to pass through [which in digital electronics is read as a “0”]. A nanoscale lightning bolt of 3.3 volts is created to blow the capacitor, creating a short circuit that creates a large amount of current [this is a “1”]“, Says Prof. Ganguly. This means that the OTP memory remembers either the “0” or the “1” status during its entire service life.
The group, in collaboration with the Semi-Conductor Laboratory, Mohali, Punjab (SCL), has successfully demonstrated a CMOS 180 nanometer-based, production-ready 8-bit memory technology, according to a press release from the director’s office of Scientific Advisor to the Indian Government. To reiterate, Prof. Ganguly says, “We have shown that the memory cells and arrays meet all of the specifications for the trim application when manufactured on the SCL 180 nanometer CMOS line. This includes the successful operation between minus 40 ° C and 125 ° C and the reliability to ensure a yield of over 95% with 8-bit memories. “
According to him, a large part of the chips produced must be sorted out due to defects that can be fixed with this technology. This technology is the first indigenous semiconductor memory technology used in manufacturing at 180 nanometer nodes. This is an important national milestone for semiconductor innovations, says Prof. Ganguly.
There are other methods of achieving OTP memories than described above. However, these require sophisticated construction techniques and also require a high voltage, which is associated with a large area disadvantage.
“In contrast, we use a special insulator material made of specially developed silicon dioxide with a thickness of 2.5 nanometers to break down at 3.3 volts without special structures together with a standard transistor. As a result, the transistor is not disturbed and no special high voltage generation is required, ”says Prof. Ganguly, pointing out the attractive properties of the technology.
Semi-Conductor Lab (SCL), Punjab, is the first customer to use this technology for internal purposes. In addition to working with SCL, the IIT Bombay team worked with IIT Delhi, SETS Chennai and the Defense Research and Development Organization for hardware encryption.
“The concept arose from a doctoral thesis at IIT Bombay … This is the first domestic 180-nanometer storage technology that successfully transitioned from the laboratory to the factory in 2021. It took six years to transfer research into production, ”says Prof. Ganguly.