The development of DRAM technology (3)

1T1C DRAM Cell structure has consolidated        

The current DRAM market is an oligopolistic one. Major international manufacturers, top three, Samsung (~45%), Micron (~25%), and Hynix (~25%) occupy 95 percent share, but you may not know that These three DRAM giant enterprises now have almost same in technology, DRAM cell structure and similar process flow. Even though the modern 3D 1T1C DRAM technology is extremely complicated. as shown in the figure below.

And the main process components are

1） High energy well implantation

2） High aspect ratio Shallow trench isolation

3） Dual gate Buried word line transistor fill with TIN/W and N-poly

4） Bitline contact

5） Peripheral CMOS transistors.

6） Capacitors landing pad(contact)

7） High aspect ratio,High-K stack capacitor and supporting mesh.

8） 3 or 4 Cooper layers & CMP metallization

DRAM cell technology were diverse         

If you understand the technical development history of DRAM, you will know that the structure and the process have been very diverse, and the technology differences were quite large.

It is curious why the three major DRAM manufacturers have developed similar structure and process for such a complex unit cell structure after years? Like species evolution, there are initially many species, each with unique survival advantages. Over time, however, a super-powerful species will eventually occupy most of the resources, crowding out weaker species and eliminating them.

The resons maybe a lot, but major one should be the investment of equipment vendor. Because of equipment investment take major parts of semiconductor technology development time and mony. Less market share is difficult to attract equipment vendor put sufficient resource into new tools R&D. Therefore once a strong or ideal cDRAM technology comes up, the invisible power force DRAM manafactures to consolidate the process flow and structure (BWT).

 Buried word line (BWT) technology came up        

We could find that the access transistor of current DRAM is a 3D structure (in fact, it is a saddle-shaped FinFet device, DRAM have mass-produced it long time ago), which is called buried word line technology (BWT).

I believe that this BWT is the major factor of the current dram 3D structure successfully developed. BWT is an innovative breakthrough that solves multiple technical problems of cell size scaling down. These Include,

1) Place access transistor under the surface of substrate to improve the aspect ratio of the capacitor node contact and bitline contact. It also reserve more area for them, which reduces the process difficulty and the contact resistance.

2) The buried word line structure of 3D saddle like FinFet improves the subthreshold leakage caused by short channel and improves Isat current.

3) Buried word line technology also mitigates the word line to bit line contact leakage, word line to storage node leakage which are generally found before.

However, this great innovation(BWT) does not come from anyone of the Top three( Samsung, Micron and Hynix). This invention came from Qimonda AG based in Munich which declared bankruptcy in 2009. Just weeks before he declared bankruptcy, the first engineering samples based on the technology were successfully demonstrated. It is a pity that the 2008 financial crisis spooked investors, and everyone did not realize the importance of this technology. Qimonda finally failed to bring this technology into mass production. Subsequently, the three DRAM giants Samsung, Micron and Hynix that survived the financial crisis copied this technology and carried it forward. until today