How memory chips are made

Each bit of DRAM is one transistor and one tiny capacitor, and the capacitor leaks, so it is refilled constantly.

stations: 7
things upstream: 33
raw sources: 10
steps from the ground: 8

The short version

Memory chips are made on silicon wafers with the same layered lithography as processors, but the pattern is a vast repeating grid of cells. DRAM cells hold a charge in a microscopic capacitor. Flash cells trap electrons in an insulating layer, and 3D flash stacks those layers vertically before dies are tested, sawed and stacked in a package.

Prepare the wafer

Polished wafers are oxidized and prepared in the same cleanroom flow used for any chip, with wafers moving in sealed nitrogen-filled boxes.

Goes in
Silicon wafer Nitrogen gas
Print the cell grid

Light patterns a repeating array of cells across each die. A DRAM cell needs only a transistor and a microscopic capacitor, while a static memory cell needs four or six transistors, which is why DRAM packs so densely.

Goes in
Photoresist
Build up the stack

In charge-trap flash, a silicon nitride layer replaces the floating gate and traps electrons. In 3D NAND the cells are stacked vertically. Samsung's first device in 2013 had 24 layers, and later dies went well past that.
Wire the cells together

Metal lines are deposited and etched on top to reach every row and column of cells.

Goes in
Copper
Test the wafer

Automatic test equipment checks each die before it is cut out of the wafer.
Stack the dies

Good dies are cut out and piled in a package, linked by through-silicon vias. A package with 16 stacked dies and a separate controller die can hold up to 1 tebibyte.
Test the finished part

The packaged chip is run through final tests on equipment like that used at wafer probe.

End of the lineMemory chip Next it goes into smartphone, laptop, solid-state drive and 5 more.

Worth repeating

A DRAM capacitor loses its charge gradually, so a refresh circuit must rewrite every bit again and again, which is what the word dynamic means.
Only three major suppliers (Micron Technology, SK Hynix and Samsung Electronics) dominated DRAM in 2018.
Toshiba announced 3D flash in 2007, and Samsung shipped the first stacked V-NAND device in 2013.