1、鐵電存儲(chǔ)器的技術(shù)背景概述目前的存儲(chǔ)器技術(shù)可以分為兩種。第一種是非易失性存儲(chǔ)器。傳統(tǒng)上來(lái)說(shuō)，他們被應(yīng)用于只讀存儲(chǔ)器因?yàn)樗麄兌加胁灰讓?xiě)入的特點(diǎn)。這些存儲(chǔ)器均源于只讀存儲(chǔ)器(ROM)技術(shù), 包括 EPROM, EEPROM, and Flash EPROM。SRAM（靜態(tài)存儲(chǔ)器）DRAM（動(dòng)態(tài)存儲(chǔ)器RAMRAMFRAM易失性存儲(chǔ)器。FRAM下保存數(shù)據(jù)。FRAM 產(chǎn)品可以保存數(shù)據(jù)達(dá)幾千年。這種存儲(chǔ)技術(shù)已經(jīng)成為存儲(chǔ)器的主流。這種存儲(chǔ)技術(shù)可以簡(jiǎn)單的解釋為對(duì)現(xiàn)在存儲(chǔ)技術(shù)的概述。什么是鐵電存儲(chǔ)器儲(chǔ)器包括靜態(tài)存儲(chǔ)器 SRAM（static random access memory)（dynamicrandoma

2、ccessmemory)。 SRAMDRAMRAM只讀存儲(chǔ)器的東西肯定不容易進(jìn)行寫(xiě)入操作，而事實(shí)上是根本不能寫(xiě)入。所有由ROMEPROMEEPROMFlash擦寫(xiě)，寫(xiě)入時(shí)功耗大。鐵電存儲(chǔ)器能兼容 RAM 的一切功能，并且和 ROM 技術(shù)一樣，是一種非易失性的RAM。RAMFRAM所固有的一種偏振極化特性，與電磁作用無(wú)關(guān)。0”狀態(tài)和“1”狀態(tài)。由于它的基于隨機(jī)存取儲(chǔ)存器而設(shè)計(jì)的，因此它的讀操作和寫(xiě)操作都很容易。但它和動(dòng)態(tài)的隨機(jī)存儲(chǔ)器又有所不同，數(shù)據(jù)的存儲(chǔ)狀態(tài)是穩(wěn)定的。因此，鐵電存儲(chǔ)器不需周期性刷新，即使在掉電的條件下，F(xiàn)RAM 仍能保存數(shù)據(jù)。許多人都誤解鐵電這個(gè)名字 , 一個(gè)名字使用前綴 fer

3、ro機(jī)存儲(chǔ)器一樣，操作使用的是電場(chǎng)。鐵電存儲(chǔ)器的技術(shù)原理當(dāng)一個(gè)電場(chǎng)被加到鐵電晶體時(shí)，中心原子順著電場(chǎng)的方向在晶體里移動(dòng)。當(dāng)原子移動(dòng)時(shí)，它通過(guò)一個(gè)能量壁壘，從而引起電荷擊穿。內(nèi)部電路感應(yīng)到電鐵電存儲(chǔ)器不需要定時(shí)更新，掉電后數(shù)據(jù)能夠繼續(xù)保存，速度快而且不容易寫(xiě)壞。CMOSCMOS之上，并置于兩電極之間，使用金屬互連并鈍化后完成鐵電制造過(guò)程。Ramtron制造工藝的發(fā)展，在鐵電存儲(chǔ)器的每一單元內(nèi)都不再需要配置標(biāo)準(zhǔn)電容器。Ramtron/DRAM2T/2CRamtron 同樣也通過(guò)轉(zhuǎn)向更小的技術(shù)節(jié)點(diǎn)來(lái)提高鐵電存儲(chǔ)器各單元的成本效0.350.5降低了芯片的功耗，提高了單個(gè)晶元的利用率。所有這些令人振奮的

4、發(fā)展都使得鐵電存儲(chǔ)器在人們?nèi)粘Ｉ畹母鱾€(gè)領(lǐng)域被廣 斷改進(jìn)性能在世界范圍內(nèi)得到廣泛的應(yīng)用。鐵電存儲(chǔ)器的操作一個(gè)簡(jiǎn)單的鐵電晶體模型如圖1鐵電存儲(chǔ)器晶體的中心原子結(jié)構(gòu)所示。在鐵電讀操作，如果原來(lái)晶體中心原子的位置與所施加的電場(chǎng)方向使中心原子要達(dá)到的位置相同，中心原子不會(huì)移動(dòng)；若相反，則中心原子將越過(guò)晶體中間層的高能階到達(dá)另一位置。在高能階的作用下，充電波形上就會(huì)出現(xiàn)一個(gè)尖峰，把這允許由電路決定存儲(chǔ)電荷的狀態(tài)。晶體原子狀態(tài)的切換時(shí)間小于1ns，完整的讀操作的時(shí)間小于70ns。因?yàn)樽x操作導(dǎo)致存儲(chǔ)單元狀態(tài)的改變，需要電路自動(dòng)恢復(fù)其內(nèi)容，所以每個(gè)讀但存儲(chǔ)無(wú)效的時(shí)間要低于50ns。寫(xiě)操作寫(xiě)操作和讀操作十分類(lèi)似

5、。與其他的非易失性存儲(chǔ)技術(shù)不同，寫(xiě)操作非常簡(jiǎn)單無(wú)需系統(tǒng)延時(shí)。數(shù)據(jù)被寫(xiě)到鐵電的電容中。如果需要的話，新的數(shù)據(jù)很容易改變鐵電晶體的狀態(tài)。對(duì)于讀操作，晶體原子狀態(tài)的切換時(shí)間小于1ns，讀操作的時(shí)間小于70ns。對(duì)于讀操作， “預(yù)充”操作伴隨在 寫(xiě)操作之后。FRAM存儲(chǔ)單元結(jié)構(gòu)目前的FRAM產(chǎn)品使用2個(gè)場(chǎng)效應(yīng)管和2個(gè)電容（2T2C，每個(gè)存儲(chǔ)單元包括數(shù)據(jù)19932T2C元提高了數(shù)據(jù)的可信度，特別是對(duì)于早期的非易失性存儲(chǔ)器是非常重要的。 2T2C存儲(chǔ)單元結(jié)構(gòu)如圖2所示。圖2 2T2C存儲(chǔ)單元結(jié)構(gòu)2T2C存儲(chǔ)單元為每個(gè)數(shù)據(jù)位提供了一個(gè)相近的參考位，依照數(shù)據(jù)狀態(tài)進(jìn)行編或“1”任意狀態(tài)。涉及到相應(yīng)的存儲(chǔ)器時(shí)，存

6、儲(chǔ)電路能非常精確地測(cè)量那個(gè)變化和非變化電容器之間不同。存儲(chǔ)隊(duì)列中電容的變化被藉由從每一點(diǎn)點(diǎn)有差別的信號(hào)中除去。2001（1T1C）被提高。簡(jiǎn)化的1T1C存儲(chǔ)單元結(jié)構(gòu)框圖如圖3所示。圖31T1C存儲(chǔ)單元結(jié)構(gòu)框圖FRAM 的發(fā)展正如前文所提到的，自從 1993年起基于鐵電存儲(chǔ)器FRAM產(chǎn)品已經(jīng)被廣泛的應(yīng)用于商業(yè)生產(chǎn)。在工業(yè)生產(chǎn)中，鐵電技術(shù)已經(jīng)趨于成熟。一些現(xiàn)象已經(jīng)預(yù)示著下一種主流存儲(chǔ)技術(shù)的出現(xiàn)。一方面，很多的半導(dǎo)體供應(yīng)商正在發(fā)展鐵電。一些人關(guān)注近期產(chǎn)品的發(fā)展，而另一些人則關(guān)注已成熟的存儲(chǔ)器和產(chǎn)品的發(fā)展。每個(gè)新的密度的一代使得產(chǎn)生一系列的用戶(hù)和廠家。 公司是唯一的一家生產(chǎn) FRAMFRAM發(fā)展適用于

7、 FRAM展的總資源正在劇烈的增長(zhǎng)。這正在引起 FRAM 技術(shù)進(jìn)步的里程碑。下表是Ramtron 公司和它的合伙人為FRAM 技術(shù)的發(fā)展選擇了歷史的里程碑和近期的發(fā)展。1984198419891993199619981999200020012002Ramtron公司發(fā)現(xiàn) FRAM 的發(fā)展技術(shù)FRAM第一次發(fā)展的過(guò)程首次制造容量為4Kbit FRAM 存儲(chǔ)器的商業(yè)產(chǎn)品容量為16Kbit FRAM 存儲(chǔ)器的制造廠家大量生產(chǎn) 0.1u的FRAM在64Kb FRAM中首次加入MCU w/在工廠中大量生產(chǎn) 0.5 u的FRAM生產(chǎn)64Kb, 256Kb FRAM 存儲(chǔ)器3V FRAM 產(chǎn)品的操作示范生產(chǎn)

8、256K 1T1C 的FRAM 存儲(chǔ)器在FRAM生產(chǎn)過(guò)程中首次使用雙層金屬 生產(chǎn)3V、0.35u的產(chǎn)品256K 1T1C FRAM w/每周期鐵電存儲(chǔ)器的應(yīng)用儀表汽車(chē)安全氣囊、車(chē)身控制系統(tǒng)、車(chē)載收音機(jī)、勻速控制、車(chē)載 DVD 、引擎、娛樂(lè)設(shè)備、儀器簇、 傳動(dòng)系、保險(xiǎn)裝置、遙感勘測(cè)/導(dǎo)航系統(tǒng)、自動(dòng)收費(fèi)系統(tǒng)通訊移動(dòng)通訊發(fā)射站、 數(shù)據(jù)記錄儀 、電話、收音機(jī)、電信、可攜式 GPS消費(fèi)性電子產(chǎn)品家電、機(jī)頂盒、等離子液晶屏電視計(jì)算機(jī)辦公設(shè)備、雷達(dá)系統(tǒng)、 網(wǎng)絡(luò)附屬存儲(chǔ) 、電子式電腦切換器。工業(yè)、科技、醫(yī)療工業(yè)自動(dòng)控制、電梯、酒店門(mén)鎖、掌上操作儀器、醫(yī)療儀器、發(fā)動(dòng)機(jī)控制。其他自動(dòng)提款機(jī)、 照相機(jī)、游戲機(jī)、PO

9、S 功能機(jī)（務(wù)、 自動(dòng)售貨機(jī)。鐵電存儲(chǔ)器在應(yīng)用中所起的作用數(shù)據(jù)收集存儲(chǔ)鐵電存儲(chǔ)器能夠允許系統(tǒng)設(shè)計(jì)師更快、更頻繁的寫(xiě)入數(shù)據(jù)，斷電不易丟失。對(duì)于使用 EEPROM 的用戶(hù)而言，這些是不能享受到的優(yōu)良性能。數(shù)據(jù)收集包括數(shù)據(jù)獲取和存儲(chǔ)數(shù)據(jù)，而這些數(shù)據(jù)必須在掉電的情況下仍能保留（不是暫時(shí)性的或中間結(jié)果暫存。這些就是具有基本收集數(shù)據(jù)功能的系統(tǒng)或者的過(guò)程紀(jì)錄是很重要的。配置信息存儲(chǔ)鐵電存儲(chǔ)器能夠靈活實(shí)時(shí)的，并非在斷電的瞬間，存儲(chǔ)配置信息，從而幫助系統(tǒng)設(shè)計(jì)師克服由于突然掉電而造成的數(shù)據(jù)丟失。配置信息的存儲(chǔ)能夠隨著時(shí)間來(lái)追蹤系統(tǒng)變化。其目標(biāo)是在接通電源后恢復(fù)信息在以前的狀態(tài)和位置，識(shí)別錯(cuò)誤發(fā)生的起因?？偟膩?lái)說(shuō)，

10、數(shù)據(jù)收集通常是一個(gè)系統(tǒng)或者子系統(tǒng)的功能，然而配置信息存儲(chǔ)則是一個(gè)低級(jí)別的工程功能，與系統(tǒng)的類(lèi)別無(wú)關(guān)。非易失性緩沖器鐵電存儲(chǔ)器能夠在數(shù)據(jù)發(fā)送或存儲(chǔ)到其它非易失性媒介前，很快地存儲(chǔ)正在運(yùn)行中的數(shù)據(jù)。在這種情況下，數(shù)據(jù)信息由一個(gè)子系統(tǒng)傳輸?shù)搅硪粋€(gè)子系統(tǒng)。這個(gè)信息是十分重要的并且不允許在斷電的情況下丟失。在有些情況下, 目標(biāo)系統(tǒng)是一個(gè)更大的存儲(chǔ)器。 而鐵電存儲(chǔ)器的快速、無(wú)限次的讀寫(xiě)特點(diǎn)使得數(shù)據(jù)在被發(fā)送到另一個(gè)系統(tǒng)前就能及時(shí)保存。SRAM 的替代和擴(kuò)展存儲(chǔ)器鐵電存儲(chǔ)器的快速寫(xiě)入和非易失性的特點(diǎn)可以通過(guò)系統(tǒng)設(shè)計(jì)師把SRAM 和EEPROM 的特點(diǎn)合而為一或者能單純的擴(kuò)展 SRAM 的功能而實(shí)現(xiàn)。ROMRA

11、MEEPROMEEPROMEEPROM,SRAMFRAM Technology BackgrounderOverviewEstablished memory technologies are divided into two categories. First are nonvolatile memories. Traditionally, systems use them in read-only or read mostly applications since they are difficult to write. These memories are derivatives of R

12、OM technology that include EPROM, EEPROM, and Flash EPROM.Second are volatile memories. These are RAM devices including SRAM and DRAM. Since they are easy to write, RAMs often store data that must change often. While users can write RAMs easily, they are volatile; therefore storing quantities of dat

13、a in the absence of power continues to be an engineering challenge.Ferroelectric Random Access Memory or FRAM has attributes that make it the ideal nonvolatile memory. It is a true nonvolatile RAM. FRAM memory write advantages and nonvolatility make it quite suitable for storing data in the absence

14、of power. FRAM based products have been available for several years in limited quantities. The technology is now moving rapidly toward its emergence as a mainstream memory selection. This technology note provides a brief explanation of its operation as well as an overview of the technology developme

15、nt status.What is FRAM?FRAM offers a unique set of features relative to other semiconductor technologies. Traditional mainstream semiconductor memories can be divided into two primary categories - volatile and nonvolatile. Volatile memories include SRAM (static random access memory) and DRAM (dynami

16、c random access memory). SRAMs and DRAMs lose their contents after power is removed from the electronic system. RAM type devices are very easy to use, and are high performing, but they share the annoying quirk of losing their mind when the lights go out.Nonvolatile memories do not lose their content

17、s when power is removed. However all of the mainstream nonvolatile memories share a common ancestry that derives from ROM (read only memory) technology. As you might guess, something called read only memory is not easy to write, in fact its impossible. All of its descendants make it very difficult t

18、o write new information into them. They include technologies called EPROM (almost obsolete now), EEPROM, and Flash. ROM based technologies are very slow to write, wear out after being written a small number of times, and use a large amount of power to write.FRAM offers features consistent with a RAM

19、 technology, but is nonvolatile like a ROM technology. FRAM bridges the gap between the two categories and creates something completely new - a nonvolatile RAM.FRAM is a RAM-based device that uses the ferroelectric effect for a storage mechanism. This is a completely different mechanism than the one

20、 used by other nonvolatile memories, which use floating gate technology. The ferroelectric effect isthe ability of a material to store an electric polarization in the absence of an applied electric field.Depositing a film of ferroelectric material in crystalline form between two electrode plates to

21、form a capacitor creates a FRAM memory cell. This capacitor construction is very similar to that of a DRAM capacitor. Rather than storing data as charge on a capacitor like a DRAM, a ferroelectric memory stores data within a crystalline structure. ThesePerovskitecrystalsmaintaintwostablestates a1 an

22、d a0.Figure 1. Perovskite Ferroelectric Crystal Due to its basic RAM design, the circuit readsandwritessimplyandHowever unlike a DRAM,the data state is stable.ThereforetheFRAMneeds no periodic refresh and when power fails, the FRAM retains its data.Peoplecommonlymisunderstandname ferroelectric. To m

23、any, a name theprefix“ferro” seems to imply iron or magnetism. The word ferroelectric also is confused with ferromagnetic. In reality, ferroelectric memories use no iron magnetic principles. They are not susceptible to external magnetic fields as they operate entirely using electric fields just as c

24、onventional DRAMs.FRAM Technology BasicsWhen an electric field is applied to a ferroelectric crystal, the central atom moves in direction of the field.As the atom moves within the crystal, it passes through an energy barrier, causing a charge spike. Internal circuits sense the charge spike and set t

25、he memory. If the electric field is removed from the crystal, the central atom stays in position, preserving the stateof the memory. Therefore, the FRAM memory needs no periodic refresh and when power fails, FRAM memory retains its data. Its fast, and doesnt wear out!FRAM memory technology is compat

26、ible with industry standard CMOS manufacturing processes. The ferroelectric thin film is placed over CMOS base layers and sandwiched between two electrodes. Metal interconnect and passivation complete the process.Ramtrons FRAM memory technology has matured significantly since its inception. Initial

27、FRAM memory architectures required a two-transistor/two-capacitor (2T/2C) memory architecture, which resulted in relatively large cell sizes. Recent advances in ferroelectric materials and processing have eliminated the need for an internal reference capacitor within every cell in the ferroelectric

28、memory array.Ramtrons new one-transistor/one-capacitor cell architecture operates like a DRAM using a single capacitor as a common reference for each column in the memory array, effectivelycuttingtherequiredcellareainhalfcomparedtoexistingarchitectures. The new architecture significantly improves th

29、e die leverage and reduces manufacturing costs for resulting FRAM memory products.Ramtronhasalsomigratedtosmallertechnologynodestoincreasethecost effectiveness of FRAM memory cells. A recent move to a 0.35-micron manufacturing process reduces the operating power and increases the die leverage per wa

30、fer to earlier generations of Ramtrons FRAM products built on the companys existing 0.5-micron manufacturing line.All of these exciting developments in FRAM memory technology are finding their way into a host of applications that people use everyday. From office copiers and high-end servers to autom

31、otive airbags and entertainment systems, FRAM memory is improving an array of products and applications worldwide.FRAM OperationA simplified model of a ferroelectric crystal is shown in Figure 1. A ferroelectric crystal has a mobile atom in the center of the crystal. Applying an electric field acros

32、s a face of the crystal causes this atom to move in the direction of the field. Reversing the field causes the atom to move in the opposite direction. Atom positions at the top and bottom of the crystal are stable. Therefore removing the electric field leaves the atom in a stable position, even in t

33、he absence of power. As a memory element, the ferroelectric crystal creates an ideal digital memory. It contains two stable data states, it requires very little time and energy to change states, and is very stable over a variety of environmental conditions.Read OperationAlthough the memory element i

34、s a capacitor, it does not store data as linear charge. In order to read a FRAM memory cell, it is necessary to detect the position of the atoms within the Perovskite crystals. Unfortunately, they cannot be directly sensed. The FRAM read process works as follows. An electric field is applied across

35、the capacitor. The mobile atoms will move across the crystals in the direction of the field if they are not already in the appropriate positions. In the middle of the crystal, a high-energy state holds the atoms in place when no field is present. As the atoms move through this high-energy state, a c

36、harge spike is emitted. The circuit dumps charge resulting from the applied field from the capacitor and compares it to the charge from a reference. A capacitor with atoms that switch states will emit a larger charge than a capacitor with atoms that do not switch. The no switching capacitor will emi

37、t the ordinary DRAM charge while the switching capacitor will emit the combination of the DRAM and ferroelectric charges. The memory circuit must determine which capacitor switched.This switched charge allows the circuit to determine the state of a memory cell.Thestate switch occurs in under 1 ns, w

38、ith the complete circuit access taking less than 70 ns.Since a memory read operation involves a change of state, the circuit automatically restores the memory state. Therefore each read access is accompanied by a precharge operation that restores the memory state. Although the read is destructive, t

39、he time during which the memory cell is invalid is under 50 ns.Write OperationA write-operation is very similar to a read operation. Unlike other nonvolatile memory technologies, a write-operation is very simple and requires no system overhead. The circuit applies write data to the ferroelectric cap

40、acitors. If necessary, the new data simply switches the state of the ferroelectric crystals. As with a read, the change of occurs in under 1 ns with a full access taking under 70 ns. As with a read, a precharge operation follows a write access.FRAMMemoryArchitecturesCurrent FRAM products use a two-t

41、ransistor, two capacitor memory (2T2C) cell. This cell, which provides each data bit with its own reference, is a well-proven scheme. The fundamental cell design has been in field use in products since 1993. The 2T2C memory cell provides robust data retention reliability, which is especially importa

42、nt during the early proving stages for a new nonvolatile memory. An example of the cell is shown in Figure 2.The 2T2C memory cell provides an individual reference in close proximity for each data bit. Depending on the programmed data state, one capacitor will switch when read while the other not swi

43、tch. The assignment of1 and 0 states is arbitrary during the memory design.Given the close proximity, the memory circuit can measure the charge differencebetween the switching and non-switching capacitors very precisely.Variations in the capacitors across the memory array are eliminated from conside

44、ration by having a differential signal for each bit.The 1T1C technology entered the market in 2001, it significantly improves the cost-per-bit ratio of resulting FRAM memory products. resulting FRAM memory products. A simplifieddiagram of the 1T1C cell is below in Figure 3.FRAM DevelopmentAs mention

45、ed earlier, FRAM-based products have been commercially available 1993. The considerable feature advantages of FRAM technology have stirred interest within the industry. Several signposts point to its emergence as the next mainstream memory technology.On the supply side, numerous semiconductor suppli

46、ers are developing ferroelectric processes. A few concentrate on near-term production while others are eyeing the longer-term opportunity for more sophisticated memories and embedded products.On the demand side, a broad market has developed for low-density FRAM products that are currently in product

47、ion. Many potential users are watching the FRAM roadmap, looking for FRAM densities and configurations that will be suitable their applications. Each new density generation enables a range of new potential users and applications.Until recently, Ramtron was the only company producing FRAM products. A

48、s a of its successful licensing program, several new vendors are in the process of establishing production capability. The total resources being applied to FRAM development on a global basis are increasing dramatically. This is causing acceleration in the advancement of FRAM technology and its proce

49、ss milestones.The following table shows selected historical milestones and the near-term roadmap for FRAM technology development by Ramtron and its partners.1984Ramtron founded to develop FRAM technology1989First FRAM fab installed for process development1993First FRAM commercial product introduced4

50、Kbit FRAM memory in volume production199616Kbit FRAM memory in volume production1998Mass at a foundry Additional foundries openproduction of FRAMat 1.0u pilot-production linesFirst MCU w/ embedded 64Kb FRAM prototype1999Mass FRAM ata foundryproduction of 0.564Kb, 256Kb FRAM memories in production200

51、03V operation FRAM products demonstrated2001256K 1T1C FRAM in productionFirst embedded product using two-layer metalFRAM process in production3V operation products enter productionprocess0.35 enters productionprocess0.35 enters production2002256K 1T1C FRAM w/Real Time ClockFRAM Product ApplicationsM

52、eteringelectric power water gas flow tax postageAutomotiveairbag body control car radio cruise control DVD engine entertainment instrument clusters power train safety telematics/navigation toll tagCommunicationscell base stations data logger phones radio telecom portable GPSConsumer Electronicshome

53、automation set top plasma and LCD TVComputingoffice equipment RAID network attached storage KVM switchIndustrial, Scientific and Medicalindustrial automation elevator hotel lock handheld instrument medical motor controlOther.ATM bill changer camera gaming military point of sale machine vendingWhat FRAM does in applicationsData Collection and LoggingFRAM allows system designers to write data to nonvolatile memory faster and more often - a luxury not afforded to users of