If all the data sent worldwide each day on the internet were burned on to CDs and these were then piled one on top of the other, the resulting heap would reach Mars and back again. Much of this data is thankfully not stored on CDs but rather in much more efficient solid-state memories. And at the heart of these is fascinating, some would say weird, fundamental physics that explains a myriad of natural phenomena including alpha particle nuclear decay, nuclear energy generation in stars, the origin of chemical elements, and experimental high energy physics.
The memory at the heart of this data revolution is NAND Flash. It is the nonvolatile (where data remains even with no power supplied) solid-state storage found in mobile phones, solid-state drives, and data centers. It forms the largest segment in the total semiconductor industry with annual sales of tens of billions of dollars. NAND Flash fabrication plants alone cost several billions of dollars to build.
Every time data is written to and erased from NAND Flash, a mechanism is used that is one of the most surprising and successful predictions from Quantum Mechanics. Barrier penetration, also known as tunneling, allows a low energy particle to penetrate a high potential energy barrier.
The 1920’s saw the discovery of Quantum Mechanics. Tunneling was discovered in 1927 to explain molecular spectra. In 1928, electron emission from metals using intense electric fields was found to be an example of tunneling. Alpha particle emission during radioactive disintegration was also shown to be a perfect example of Quantum Mechanical tunneling. The 1930’s saw the application of tunneling to explain the origin of chemical elements and nuclear energy generation in stars.
The invention of the silicon field effect transistor around 1960 was followed by field effect devices in the late 1960’s where charge could tunnel through the gate oxide and be trapped between the gate and the channel to alter the device’s threshold voltage. Trapping within an insulator came first and was followed by charge storage on a floating conductor. Various silicon nonvolatile memory devices were made over the next 20 years that involved tunneling.
NAND Flash was invented in 1988 using 1 micron (um), equal to 1000 nanometers (nm), minimum feature sizes. After almost 30 years of continuous shrinking, this 2-D NAND approach has halted at about 15 nm minimum feature size.
Various forms of 3-D NAND are now being introduced to get around problems of increasing memory density in 2-D.
The success of all NAND Flash depends on the tunneling mechanism that was discovered 90 years ago, applied to silicon devices 50 years ago and specifically to NAND 30 years ago.
Not many engineers working in the silicon semiconductor industry may know about the long history of tunneling. Also, many physicists who are well acquainted with the topic may not be aware of how fundamental it is in the everyday practicalities of storing photos and videos on mobile phones and digital cameras. In addition, tunneling-based Flash memory is starting to displace magnetic hard drive storage in the data centers where the digital data behemoths (Facebook, Amazon, Apple, and Google) store internet information.
The purpose of this article is to convey the sense of wonder surrounding Quantum Mechanical tunneling as it is applied to silicon memory devices that are at the heart of the data storage revolution and to put its importance into historical context.
I have used two excellent books as reference:
- Elementary Quantum Mechanics, R.W. Gurney, 2nd ed., Cambridge University Press, 1940.
- Quantum Concepts in Physics, M. Longair, Cambridge University Press, 2013.
Gurney’s work is a marvelous description using graphical methods that aid in understanding.
Longair’s superb book goes into far more depth and includes the historical rise of Quantum Mechanics, an approach I have tried to apply in this article when describing the invention of silicon devices.
To investigate the first applications of Quantum Mechanical tunneling to silicon devices, I have been extremely fortunate to be able to visit the Computer History Museum’s archive at the Shustek Center in Fremont, California. I include excerpts from original laboratory notebooks showing some of the first uses of Quantum Mechanical tunneling in silicon memory devices by the pioneers in what was to become Silicon Valley.
This article shows how the data storage revolution is built upon 90 years of fundamental physics applied for 50 years to silicon memory devices. A truly solid foundation.
There are four parts to this article:
- The Fundamentals and Early History of Tunneling.
- The Invention of Tunneling-Based Flash Memory.
- The Invention of NAND Flash.
- The Future of Nonvolatile Memory.
Acknowledgments are given at the end to people, companies, and institutions that have been instrumental in helping to complete this article.