The Age of Data Supported By BiCS FLASH™ Flash Memory: The Evolution and Future

1: Data Stored in Memory Cells

Figure 1. What does this image appear to be?

Figure 1. What does this image appear to be?

What does this image appear to be? This is actually flash memory magnified using an electron microscope. It may look like a high-rise condo building, but this structure is found inside smartphones and other small electronic devices all around us.


The reason smartphones can store characters, images, music, and other types of data in flash memory is because all data is stored as digital data composed of 0s and 1s.


For example, suppose an image of a yellow banana is displayed on your smartphone. If you were to magnify this image and take a closer look, you would see that it consists of sets of points. By adjusting the brightness of three colors of light— red, blue, and green—in 256 possible levels, each color point can express approximately 16.77 million colors (256 levels of red × 256 levels of blue × 256 levels of green). With digital data, the brightness of each light can be represented by a combination of eight 0s and 1s (2 to the 8th power), meaning a total of twenty-four 0s and 1s. As an example, the yellow color of a banana might be expressed as bright red light (11111111), a slightly dimmed blue light (11001100), and no green light (00000000). Yellow can therefore be saved as 111111111100110000000000.

Figure 2. Expressing the image of a banana as data

Figure 2. Expressing the image of a banana as data

With flash memory, data represented by 0 and 1 is stored in a small cell called a memory cell. A state in which electrons remain in the block is expressed as 0, and a state where electrons are discharged from the block is expressed as 1. The charging and discharging of electrons is controlled by the voltage applied to the memory cell. In this way, all of the text, images, music, and other data stored in a smartphone are expressed and stored as a combination of these blocks in which 0 and 1 are written.

Figure 3. A memory cell

Figure 3. A memory cell

Figure 4. Writing data

Figure 4. Writing data

Control gate A metal plate structure. By applying a voltage, electrons are taken in and out of the conductive charge-storage film and signals are read
Insulator Acts as a wall, not letting electrons pass
Conductive charge-storage film Conductive film that acts as a memory cell to store electrons (Surrounded by an insulator, electrons cannot move in and out unless special operations are performed)
Tunnel insulator Acts as a door, letting electrons pass only when a voltage exceeding a certain value is applied
Semiconductor device A part that enables the reading of information by utilizing the fact that the flow of electricity changes depending on the presence or absence of electrons in the conductive charge-storage film

2: 3D Flash Memory BiCS FLASH™

The more digital data is used, the more data must be stored. In response to this, capacity must be increased so that more data can be stored in a single flash memory unit.


Miniaturization technology is critical to make this larger capacity a reality, allowing the area occupied by a single memory cell to be shrunk and fitting as many memory cells as possible into a single flash memory unit. However, there are limits to miniaturization technology. Various problems occurred in development, such as unintended currents flowing due to the close proximity of memory cells.


The idea that therefore resulted was that of increasing the number of memory cells per area by stacking planar flash memory vertically. If we compared it to a building, the idea means renovating a one-story house that could only accommodate ten people into a five-story apartment building that can now accommodate fifty people within the same area. In other words, the higher the stacking is, the more people can be accommodated in the same area.


But new challenges arose at the same time. Stacking planar flash memory vertically from the bottom up means each time a memory layer is added, the work involved in creating the flash memory structure increases at the same time. So, in other words, the more memory is stacked, the greater the cost.


In order to solve this problem, KIOXIA announced in 2007 its BiCS FLASH™ memory, which enabled the processing time to remain constant no matter how many layers are stacked.


Figure 5. Comparing the cost per bit (cost per memory cell)

Figure 5. Comparing the cost per bit (cost per memory cell)

Comparing BiCS FLASH™ with stacked flash memory that is simply composed of stacked planar flash memory, we see that while the cost barely decreases for the stacked planar memory if more than 8 layers are stacked, with BiCS FLASH™ it is possible to reduce the cost per memory cell, even if the number of stacked layers is greater (100 or more).

BiCS FLASH™ uses a plate electrode (acting as a control gate; shown as green plates in Figure 6) and insulators are stacked alternately, and a large number of holes are punched perpendicularly to the surface. Next, memory cells (insulating charge-storage film, shown in pink), which are charged with electrons, are formed in the holes punched in the plate electrode. Then, a columnar electrode (yellow columnar structure) is embedded (plugged) into the central hole. The area sandwiched between the plate electrode and the columnar electrode becomes the memory cell.

 

Several punch and plug processes were considered. The basic punch and plug process currently used is as follows: [1] A layered insulating film is created by stacking two different types of insulating films; [2] A hole is punched in this layered insulating film; [3] A memory element is plugged inside the hole; [4] One of the two insulating films is selectively removed from the layered insulating film; and [5] a metal film is formed in the removed area.

Figure 6. The BiCS FLASH™ concept

Figure 6. The BiCS FLASH™ concept

Let’s take a closer look at a BiCS FLASH™ memory cell. A BiCS FLASH™ memory cell is structured with a hole in the center of the cylinder (shown in pink). Electrons are exchanged between the electrode that passes through the hole in the center and the pink conductive charge-storage film.

Figure 7. A closer look at a BiCS FLASH™ memory cell.

Figure 7. A closer look at a BiCS FLASH™ memory cell.

Control gate A metal plate structure. By applying a voltage, electrons are taken in and out of the insulating charge-storage film and signals are read. In Figure 6, which illustrates the concept and manufacturing process, it is represented as a plate electrode.
Insulator Acts as a wall, not letting electrons pass
Insulating charge-storage film Insulating film that acts as a block to store electrons (electrons cannot move in and out unless special operations are performed)
Tunnel insulator Acts as a door, letting electrons pass only when a voltage exceeding a certain value is applied
Semiconductor device A part that enables the reading of information by utilizing the fact that the flow of electricity changes depending on the presence or absence of electrons in the insulating charge-storage film. In Figure 6, which illustrates the concept and manufacturing process, it is represented as a columnar electrode.

In this way, BiCS FLASH™ is a ground-breaking technology because, rather than stacking memory cells one layer at a time, it involves stacking plate electrodes and punching holes through them to allow electrodes to pass through, creating a memory cell in each layer.


Furthermore, KIOXIA has been conducting R&D for more than 10 years to increase the number of layers in order to increase memory capacity. In 2020, KIOXIA made over 100 layers a reality. Imagine punching holes in these stacked plates and passing a thin column through at once. From this we understand that BiCS FLASH™ is an extremely precise and delicate technology.

3: The Evolution of KIOXIA Technology

KIOXIA has dramatically reduced the number of manufacturing work-hours and succeeded in reducing manufacturing costs with BiCS FLASH™. But KIOXIA is also taking on the challenge of increasing flash memory capacity using completely different methods.


One example is multi-level technology.
Stacking BiCS FLASH™ makes it possible to increase the number of memory cells per unit area. On the other hand, multi-level technology allows for the amount of information that can be stored in each memory cell to be increased.


The unit of information processed by a computer is called a bit, and 1 bit indicates the minimum amount of information that can be represented by a binary number. In a conventional cell where 1 bit is recorded in one memory cell, two patterns (1 and 0) can be stored. But in a multi-level cell, four patterns can be stored (11, 10, 01, and 00). In other words, one cell can store 2 bits of data. Similarly, technology has also been developed that can store 3 bits of data and 4 bits of data in one cell.

Figure 8. Increased capacity with multi-level technology

Figure 8. Increased capacity with multi-level technology

At the same time, KIOXIA is taking on the challenge of greater vertical stacking.
Twin BiCS FLASH is made with stacking technology that halves the size of BiCS FLASH™ memory cells. By dividing the memory cells of BiCS FLASH™ and making them semicircular, the cell size can be halved. As a result, KIOXIA succeeded in doubling the capacity.

Figure 9: Twin BiCS FLASH technology with greater stacking

Figure 9: Twin BiCS FLASH technology with greater stacking

Furthermore, KIOXIA is conducting research and development on not only increasing the capacity of memory but also devising new ideas that reduce the amount of time needed by memory cells to write and read data.


As mentioned above, BiCS FLASH™ was achieved by making full use of various technologies and requires hundreds of processes to manufacture. Development of manufacturing technology is critical to ensure that each of these processes can be carried out quickly and without error. Therefore, KIOXIA develops machines for manufacturing together with manufacturers, considers manufacturing methods and orders, examines and processes flash memory raw materials and necessary parts, and trains people involved in manufacturing. KIOXIA considers a wide variety of areas, employs trial and error, and constantly develops in order to make improvements. KIOXIA will continue to develop new flash memory by utilizing a variety of methods, and will support the ever-increasing global demand for memory.

Planning and writing: Leave a Nest Co., Ltd.
Text and visuals: Leave a Nest Co., Ltd., KIOXIA Corporation

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