Overview of Nonvolatile Flash Memory

With the development of information technology, people put more and more emphasis on the materials and techniques of semiconductor memory, especially for the consumer electronic products.

Memory can be divided in to two kinds by whether the storage data can be affect by the power supply. One is volatile memory, and the other s non-volatile memory.

Volatile memory defines as that the data that stored in memory need power supply to maintain. On the other hand, non-volatile memory means that even if it encounters the break of power supply, the data in the memory also can maintain for a long time. For example, Dynamics Random Access Memory (DRAM) and Static Dynamics Random Access Memory (SDRAM) belong to volatile memory, and Read Only Memory (ROM), Electrically Programmable Read Only Memory (EPROM), and Flash memory belong to non-volatile memory. The speed of non-volatile memory although can not compare with volatile memory, but the data in non-volatile memory can maintain for a long time without power supply. Because of the advantage, non-volatile memory becomes more and more important and was used in all kinds of electronic products.

In 1967, D. Kahng and S. M. Sze invented the first Floating-gate (FG) non-volatile memory at Bell Labs [1]. After that, all kinds of non-volatile memory were invented and were applied in our daily life. Read Only Memory (ROM) was the

early products of non-volatile memory. This memory programs its data in its fabrication process and it has the advantages of cheap and high density. Hence, ROM was applied widely in all kinds of electric products such as personal computer, printer, video game and etc. However, this memory is not convenient, because if we want to change one bit, we need a new mask.

Latter, one kind of memory called Programmable Read Only Memory (PROM) was invented, which didn’t need specific mask for specific function because the data was written in after the whole ic chip fabrication. Hence, it has the advantage of fast production. Although PROM can be programmed according to the need of consumers, but the data in the memory can not be erased by the users.

In order to solve the program, Electrically Programmable Read Only Memory (EPROM) was invented, and it programs the memory cell by electrical method such as channel hot electron (CHE). However, EPROM can’t erase by electrical method, and it need to illuminate the UV light to erase it. Because of this reason, a quartz window is necessary on the package of the EPROM. Thus, quartz window package lead to the expansive package cost and make EPROM inconvenient.

The appearances of Electrically Erasable Programmable Read Only Memory (EEPROM) solve the program of EPROM that described above. It can both program and erase by electrical method, but it need a select transistor to achieve the advantage.

Hence, the density of EEPROM is lower than EPROM and EEPROM is more expensive.

Flash memory follows the basic structure of EEPROM without select transistor.

Unlike EPROM and EEPROM, flash memory cell provides single-cell electrical program and fast simultaneous block electrical erase [2]. Thus, a small cell size is combined with a fast in-system erase capability, and flash memory has the advantage of long life time, low production cost, low power, and robust flash systems. Hence,

flash memory is good for consumer electric products such as mobile phone, pager, digital cameras, MP3 player, PDA and etc. Table1 compare the characteristics of the flash memory with PROM, EPROM, and EEROM [3].

The basic concept of Floating-gate (FG) non-volatile memory (as Figure.1(a)) is a MOSFET with a modified gate stack structure that has a control gate (CG) and a floating gate (FG) embedded in a dielectric material such as silicon dioxide (SiO2), and both the CG and FG is conductor. The MOSFET operated as a switch with the control gate modulating the electron current flow between the source and drain. The memory storage element is the isolated floating gate disconnected from the terminal voltage. It sits between the control gate and the channel.

The storage charge will affect the threshold voltage. By the different threshold voltage, we can define that the operation voltage Vg of memory cell is at the middle of the two threshold voltage. Thus, the programming state can be determined by measuring the current in the MOSFET with the operation voltage Vg (as Figure.2).

Because of manipulating electric field to control the data to program or erase, the tunneling oxide should be thin enough to let charge inject into the floating gate.

However, if the tunneling oxide is too thin or it has local defect or it has stress induce leakage current (SILC) path (as Figure. 1(b)), it will lead to leak charge. And the floating gate is also a conductor, so it will lead to whole charge leak out. This is a big problem for data retention.

In order to solve the problem, one method is that it can replace the conductive floating gate with insulator which has a large mount of trap sites (as Figure. 3), such as SiN3 [4], Al2O3 [5], HfO2 [6], ZrO2 [7] and some high-K materials [8]. This method uses the materials which are easy to trap charge to increase the capability of charge retention, and it stores the charge in the discrete trap site. Because the charge in the trap site will not interact, the local defect of tunneling oxide will not leak out all of the

charge. Besides, the high-k materials have higher dielectric constant. So they can reduce the equivalent oxide thickness of the gate stack and reduce the operation voltage.

Another method is that it can replace the conductive floating gate with nanocrystals as charge storage node, such as Si nanocrystals [9], Ge nanocrystals [10], HfO2 nanocrystals [11], and metal nanocrystals [12,13] (as Figure. 4). Nanocrystals memory has separate and discontinuous charge storage node. Hence, the migration of charge in horizontal and vertical direction can be suppressed by the silicon dioxide.

Thus, nanocrystals flash memory has good charge retention. Moreover, it can have thinner tunneling oxide and smaller operation voltage with good programming and erasing speed. So, it meets the requirements of low power and voltage in VLSI.