计算机专业时文选读(985)

Future Memories(2)

MRAM is closer to production than most other experimental memories, and IBM and Freescale Semiconductor Inc. are the leading MRAM developers. Researchers call the technology “magnetic storage on a chip” because it adapts the magnetic polarization techniques used in disk drives to silicon. The fast, nonvolatile memory offers a nice combination of high speed, high endurance and reasonable density.

Chip samples can be produced at about the same density and cost per bit as flash. But while promoted as universal memory, the density of MRAM doesn’t approach that of DRAM or SRAM. Most interest today is focused on embedded applications.

Several other challengers to conventional memory technologies look promising but are much earlier in their development cycles than FRAM or MRAM.

Phase-change memory (PCM) is a fast, nonvolatile memory that proponents claim could become a universal memory. IBM and Intel have each partnered with other companies to develop the technology.

A transistor in a PCM cell applies energy to either heat or cool the material, forcing it to change between an amorphous (high-resistance) and a crystalline (low-resistance) form. A current is then applied to measure the resistance and establish the state of the memory cell as a 0 or 1.

While PCM technology is much faster than flash, it’s slower than SRAM. To be competitive with DRAM, it would also have to support unlimited writes. IBM’s research shows that PCM can match flash’s 100,000-write limit, but endurance beyond that hasn’t been proved. Ovonyx Inc. in Santa Clara, Calif., however, claims that its technology, called Ovonics, can be written to 10 trillion times.

Carbon nanotubes—hollow, tube-shaped lattices of carbon atoms—can be used to make a mechanical memory that works by bending the carbon filament up or down to make or break a connection between two electrodes.

The technology can theoretically scale as small as 5 nanometers and has achieved speeds a few times faster than today’s DRAM chips. Nanotube chips could be 10 to 15 times smaller than today’s DRAM and could offer a tenfold reduction in power consumption. Initial products are still two to five years away.

Molecular memory, developed by ZettaCore Inc., uses a chemical process to create DRAM memory cells with a molecular capacitor. The “chemically self-assembling” molecules work by adding and removing electrons. This changes the voltage, which is then measured to determine the state (0 or 1). The technique supports four states and can store 2 bits per memory cell. Molecular memories also require 70% less power than a standard DRAM memory cell because the capacitor can hold 100 times the charge and therefore needs to refresh memory less frequently.

Molecular memory will allow manufacturers to double or quadruple capacity without increasing costs. Initial products may be available by 2007 or 2008.

Axon Technologies Inc.’s Programmable Metallization Cell memory (PMCm) is a DRAM alternative that’s nonvolatile, uses less power and offers higher density than DRAM. In it, tiny quantities of metal self-assemble into a filament as electrons are added to the metal ions. Resistance is then detected to determine the state of the memory cell.

Ultimately, the ability to cost-effectively manufacture new memory technologies using existing fabrication facilities may separate the winners from the losers. To succeed, emerging technology vendors will initially focus on niche markets where they can coexist, rather than compete, with established vendors, and thereby continue to evolve. (The end)

未来的存储器(2)

MRAM比其他多数技术更接近生产。IBM和Freescale半导体公司是MRAM开发者。研究人员将此技术叫做“芯片上的磁存储”，是因为它将磁盘机中的磁极化技术用到硅片上。这种快速的非易失性存储器将高速度、(存储的)高耐久性与合理的集成度很好地结合起来了。

(FRAM的)芯片样品可以做到集成度和每位成本与闪存相同。但如果将它作为通用存储器进行推销，那它的集成度还达不到DRAM或SRAM的水平。今天，它的最大用处集中在嵌入式应用。

其他几种对常规存储器技术的挑战看上去有希望，但比起FRAM或MRAM更是处于开发的前期。

相变存储器(PCM)是一种快速的非易失性存储器，其支持者称它能成为通用存储器。IBM和Intel各自都在与其他公司进行合作，开发此项技术。

PCM单元中的晶体管运用能量加热或冷却材料，强迫它在非晶体(高电阻)和晶体(低电阻)形式之间转换。然后加上电流，测量电阻以建立存储单元的状态: 0或1。

虽然PCM技术比闪存快得多，但仍比SRAM慢。为了能与DRAM竞争，它必须支持无次数限制的写。IBM的研究表明，PCM能达到闪存10万次写的极限，但超过闪存耐久力的能力未得到证明。然而，加州Santa Clara市的Ovonyx公司声称其Oyonics技术能写10万亿次。

碳纳米管(中空的管状碳原子晶格)能用于制造机械的存储器，它通过上下弯曲碳丝从而接通或断开两个电极之间的连接而工作。

理论上，此技术能做到5纳米的尺寸，并已获得比今天DRAM芯片快若干倍的速度。将来，纳管芯片可能比目前的DRAM芯片小10至15倍，功耗降低10倍。最早的产品还需2至5年的时间。

ZettaCore公司开发的分子存储器利用一种化学工艺，制造带分子电容器的DRAM存储单元。这种“化学自我装配”分子是通过加入和去掉电子而进行工作的。这会改变电压，然后测量电压以确定状态(0或1)。此技术支持4个状态，每个存储单元能储存2位。分子存储器的耗电比标准的DRAM单元少70%，因为电容保存了100倍的电荷，故而不需要频繁刷新存储器。

分子存储器允许制造商在不增加成本的情况下，将存储容量翻一番或者翻两番。2007年或2008年，第一批产品有可能面市。

Axon技术公司的可编程金属化单元存储器(PMCm)是DRAM的替代品，它是非易失的，耗电比DRAM少但密度更高。在这种存储器内，能自我组配成细丝的微型金属作为电子，加到金属离子上。随后测量电阻，以确定存储单元的状态。

归根结底，利用现有制造设备能低成本高效率地制造新颖的存储器之能力，将起到优胜劣汰的作用。为了能成功，新技术公司最初将把注意力集中在专业市场，在这些市场上，他们能与已有的公司共存，而不是竞争，从而继续发展。(全文结束)