Flash Memory – A Primer
Text and images copyright E.J. Peiker, all rights reserved

Flash Memory! Every modern day SLR and most point and shoot cameras, even most modern medium format cameras, use it. We usually associate Flash Memory with the storage media for images taken with digital cameras. Digital cameras are the largest consumers of high density Flash Memory, but any camera that remembers information when the power is turned off has some Flash Memory onboard. Cameras that have custom functions, those that remember settings, and those that have program exposure tables embedded all have Flash Memory inside their housing.

In this article I will provide an overview of Flash Memory and answer questions like:

What is Flash Memory?
How is Flash Memory made?
How does flash work?
What about Flash Memory cards?
How reliable is Flash Memory?

Flash Memory is what is considered “non-volatile” memory. This means that when the power is turned off, it continues to save the data that was stored on it. In contrast, Dynamic RAM (DRAM) or Static RAM (SRAM) is volatile memory. Once power is lost on volatile memory, the data stored on that memory is lost forever. Flash Memory has its origins in Erasable/Programmable Read only Memory (EPROMs). EPROMs, being read-only memories, can only be programmed once and then that data stays resident on the EPROM chip until it is erased with ultra-violet (UV) light. Flash Memory, on the other hand, has the data retention properties of EPROMs but can be erased electrically and reused without having to remove it from its circuit board and exposing it to UV light.

Most Flash Memory chips are made by Intel, AMD, Fujitsu, Sharp, StMicro, Mitsubishi, Toshiba, and Samsung where they start their life as a silicon wafer. These wafers are typically 150mm, 200mm, or 300mm in diameter. They are then fabricated using a semiconductor process that includes hundreds of manufacturing steps to build a layer of Flash Memory cells, followed by several layers of interconnects to move all of the signals around. Figure 1 shows a cross section of a basic EPROM cell.

Figure 1 – Flash Memory Cross-section

The semiconductor manufacturing process takes between 6 weeks and 12 weeks of processing in a wafer fab – a super clean environment. A fab’s manufacturing floor is on the order of 10,000 times cleaner than the most sterile operating room. In Figure 1, the width of the Control Gate/Floating Gate is measured in nanometers and will average between 12 and 25 nanometers, depending on the process technology used and the density of the number of flash cells on a chip (measured in MegaBytes). The features are built through multiple uses of photolithography, etch, diffusion, thin film deposition, planarization, and ion implantation. Figure 2 illustrates what a Flash Memory chip looks like after completion of the semiconductor processing. These chips are called die in this form and each contains many millions of transistors – a 64Mb die will contain on over 75 million transistors.

Figure 2 – Flash Memory after wafer fab processing

A single wafer will hold as many as several hundred Flash Memory chips called die, as is illustrated in Figure 3.

Figure 3 – Finished semiconductor wafer

After the wafer processing is completed, the wafers get their first thorough test in an operation called wafer sort. At this point, each and every die on the wafer is thoroughly tested. Next the wafers go into the assembly process where only the dice that were good in the wafer sort process are put into their package which includes all of the connections needed to mate the flash chips to a motherboard.

Figure 4 – Flash Memory units

Once the chips are in unit form, they go through another round of thorough testing and reliability screening to insure that the flash chips are of high quality and meet all of their performance specifications. Defect per million (DPM) levels when the Flash Memory chips leave the manufacturers are typically below 50. At this point, the finished units are shipped to the Original Equipment Manufacturers (OEMs) for use in their applications.

The camera manufacturers now take delivery on their orders as do the Flash card manufacturers. The camera manufacturers incorporate them into their circuit boards that go onboard the cameras and the Flash card manufacturers incorporate them into their designs (CompactFlash, Memory Stick, etc). These OEM’s assemble all of the chips needed to produce whatever density of flash cards that they aim to build. They then test them and finally sell them to the end user. The entire process from silicon to the end users' hands will range from 3 months to 6 months.

At this point, you may ask, “How does a piece of silicon store this data?” Without getting too technical (referring to Figure 1), when a positive voltage is applied to the drain region and the wordline simultaneously, electrons begin to flow from the source region to the drain region since electrons are negatively charged and attracted to positively charged areas. The voltage applied to the wordline causes some electrons to accelerate and go through the tunnel oxide and then rest on the floating gate. Since the floating gate is completely isolated, when the voltages are removed, the electrons are trapped there and will stay on the floating gate. To erase the cell, a reverse voltage is applied to the wordline to drive the electrons off.

Once you load a Flash card in a digital camera, here is what happens. The digital sensor, a light sensitive device, collects and stores the image on an array of sensors. This data is then sent to an image processor where the data is processed and formatted into an image file such as JPEG, TIFF or RAW. The image processor then writes this data to a fast SRAM or DRAM buffer where the data is sequenced for writing to the storage media - in our case the Flash card. At this point, the data writing process begins and each data file is written to the Flash card. If your RAW file is 6MB in size, approximately 50 million flash cells will be programmed in the manner described in the paragraph above. (Remember that one byte is 8 bits so a 6 MegaByte file is equivalent to a 50 Megabit file.) Once done, the voltages are removed and the data is stored until the cell is reprogrammed. This program stays in place even after power is removed from the flash card.

Most manufacturers of the Flash Memory chips make a very reliable product as do most OEM’s of flash cards. The most common failure mechanisms are:

1. Improperly specified speed ratings used in the flash card
2. Poor interconnects and construction in the flash card
3. Flash card connector failure
4. Flash card structural failure due to excessive stress
5. Inserting the flash card and applying a voltage while the card is wet
6. Tunnel oxide degradation – this is the ultimate wear out mechanism
7. Package interconnect failure

The most likely way a photographer may damage Flash cards is by pulling the card from the camera while it is still being written to. While this typically will not physically damage the Flash card, it can scramble the data on the card by corrupting the File Allocation Table (FAT). Another way photographers can induce damage is to handle cards with an excess of electrostatic charge on their bodies. In dry climates, it is always a good idea to touch some metal prior to handling Flash cards - metal on your tripod or tripod head is good enough to discharge the static build-up. Dropping cards typically does not damage them. If you drop a card into water, dry it out thoroughly for several days before applying power to the card. Odds are that all of the data will be intact.

Flash Memory today is everywhere, virtually every household has many Gigabytes of Flash Memory embedded in their home. It’s in many appliances, video and stereo equipment, every automobile sold in the last 10 years, and many other places including virtually every camera on the market today, excluding perhaps 4x5 and 8x10 cameras. As you can see, when you pay $200 or more for a high density flash card, you are buying a lot of technology.

This has been a relatively simplified look at the technology of Flash Memory. Hopefully this article has helped to somewhat demystify Flash.

E.J. Peiker has worked in the semiconductor processing industry for over 20 years as an engineer and manager for the world’s largest semiconductor manufacturer. E.J.'s website is www.ejphoto.com.