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惠普新发现改写教科书，传说中的“忆阻器”得到证实
惠普实验室(HP Labs)的资深院士R. Stanley Williams不久前成功地证实了有关“忆阻器(memristor)”的学说──所谓的忆阻器是指电子电路中除了电阻、电容与电感之外的第四种被动元素，早在1971年就由美国加州柏克莱大学教授Leon Chua所提出，不过当时仅是初步发现，直到日前才由HP正式发表。而此一成果也意味着相关教科书必须重新改写。
忆阻器概念的创始人Chua表示：“我的处境跟1869年发明化学元素周期表的俄罗斯化学家Dmitri Mendeleev很类似；Mendeleev当时假设该周期表上有许多失落的元素，而现在所有的化学元素都已经被发现了。同样的，来自HP Labs的Stanley Williams发现失落的电路元素──忆阻器。”
Chua当时是以数学推论电子电路在电阻、电容与电感之外还有第四种元素；他将之命名为忆阻器的缘故，是因为该元素会透过电阻的改变“记忆”电流的变化。而现在HP则宣称发现了首个忆阻器的实例──它是由一片双层的二氧化钛(bi-level titanium dioxide)薄膜所形成，当电流通过时，其电阻值就会改变。
“此一新发现的电路元素可解决今日电路学上的许多问题，因为当其尺寸缩小，其性能也会提升。”Chua表示：“忆阻器可实现非常小的纳米等级组件，且不会产生现今将晶体管尺寸缩小的过热问题。”
HP已在其超高密度纵横式交换器(crossbar switches)中测试过以上元素，该交换器使用纳米线(nanowires)达到在单芯片中储存100Gbits容量资料的记录；而目前最高密度的闪存芯片则可储存16Gbits的资料。
“我们花了多年时间寻找应用在超高密度纳米线纵横式交换器中的最佳材料，以实现在一平方公分的面积内容纳1,000亿个纵横闩(crossbars)的目标。后来我们发现，最理想的材料就是忆阻器。”Williams表示。
Williams除了是忆阻器以二氧化钛为基础的材料的主要发明人之外，亦是HP成立了12年的信息与量子系统实验室(Information and Quantum Systems Lab)的创始主管；他所率领的研究团队并将该材料的配方最佳化。
Chua则表示，在过去的37年来，忆阻器所遇到的障碍是电子电路学说中一个普遍的错误想法──即被动电路中，有关电压与电荷之间的基础关系。研究人员认为，电压与电荷间的基础关系，应是在电压的变化值，或称电通量(flux)，与电荷之间；而这也是让HP能研发出电阻器的关键原因。
“电子理论学者在这些年来都使用了错误的变量比对──即电压与电荷；事实上，电子理论所遗漏的一个部份，是有关电通量与电荷之间的变量比对。“Chua表示，以上的错误观念与亚里斯多德运动定律(Aristotle's Law of Motion)的谬误十分类似，该定律认为力与速度(velocity velocity)应该是成正比的，但两千年之后牛顿(Newton)指出其错误，表示亚里斯多德用了不正确的变量，力应该是与加速度(acceleration)成正比；加速度是速度的变化值。
Chua指出，以上的例子与今日电子电路学中的错误观念如出一辙，但所有的教科书给的信息都使用了错误的变量，即电压与电荷；事实上，电压与电荷间的基础关系，应该是在电通量与电荷之间才正确。
HP曾在多年前邀请Chua前往发表其学说，不过当时并未透露他们正在积极寻找忆阻器；直到最近，Williams才告诉Chua，他已经使用了正确的变量，也就是电通量与电荷，发明了世界上第一个可以运作的忆阻器。
忆阻器的运作模式类似一个带着内存的非线性电阻(non-linear resistor)，可做为一种尺寸小巧且高能源效率的存储元件。不过Chua与Williams表示，忆阻器是一种新型态的电路元素，可制造出的新组件会是大家无法预料的。
Williams与其研究团队所发明的首个忆阻器，是以两层三明治型态的二氧化钛薄膜为基础；做为一种记忆元素，其运作方式是透过耦合该种材料中的原子运动与电子运动，来改变薄膜的原子结构。
HP所使用的底层材料，是由钛原子与氧原子组成的对称晶格(lattice)，可做为良好的绝缘体；但顶层则掺杂了氧空缺(oxygen vacancies)，使其能成为导体(空缺越多导电性越好)。HP在二氧化钛中制造这种氧空缺的秘诀，是使用溅镀沉积法(sputter deposition)产生大量的氧，然后再削减氧气流量产生空缺层。
透过在三明治层的上方与下方放置纳米线纵横闩，电荷就会穿越该材料。Williams指出：“我发现忆阻器材料的方法，就是透过研究二氧化钛传感器的运作模式；这让我想到，可以透过移动该材料内的氧空缺来制造忆阻器。”
Williams表示，对该组件通电之后，就能推动材料层中的氧空缺到没有氧空缺的地方，因而改变其电阻系数值达1000以上，将忆阻器“开启”；而若将电流逆转，让氧空缺回到原地，就可“关闭”忆阻器。
就像Chua所预言的，Williams已经在计划利用HP的纵横闩结构，研发超越一般内存的全新型态组件。“如果我们用快速猛烈的方式来对忆阻器通电，它的运作方式会像是数字组件；但是如果我们用缓慢温和的方式来对其通电，它就会像是模拟组件。”他表示，该团队已经利用其纵横闩结构，设计了一种同时具备数字与模拟功能区块的新型态电路。
Williams透露，在模拟功能的那部份，他们希望能创造一种以忆阻器为基础的模拟计算组件，其运作方式类似人脑神经元突触，执行功能控制时会以比较对照的方式来进行，例如判断某样东西的尺寸比另一样东西大或是小。
该研发团队尚未建立一个神经元网络，不过相信在其纵横闩的模拟功能部份应用忆阻器，将可对神经元网络有很大助益。
HP计划在2008年稍晚公布其忆阻器材料的运作细节，以及该研发团队已经最佳化的、在不同型态电路中的纳米级纵横式交换器架构。“忆阻器不只可做为现有内存组件的替代品，也可望应用在人们从未想象过的各种新型态组件中。”Williams表示。
'Missing link' memristor created: Rewrite the textbooks?
PORTLAND, Ore. — The long-sought after memristor--the "missing link" in electronic circuit theory--has been invented by Hewlett Packard Senior Fellow R. Stanley Williams at HP Labs (Palo Alto, Calif.) Memristors--the fourth passive component type after resistors, capacitors and inductors--were postulated in a seminal 1971 paper in the IEEE Transactions on Circuit Theory by professor Leon Chua at the University of California (Berkeley), but their first realization was just announced today by HP. According to Williams and Chua, now virtually every electronics textbook will have to be revised to include the memristor and the new paradigm it represents for electronic circuit theory.
"My situation was similar to that of the Russian chemist Dmitri Mendeleev who invented the periodic table in 1869," said Chua. "Mendeleev postulated that there were elements missing from the table, and now all those elements have been found. Likewise, Stanley Williams at HP Labs has now found the first example of the missing memristor circuit element."
When Chua wrote his seminal paper, he used mathematics to deduce the existence of a fourth circuit element type after resistors, capacitors and inductors, which he called a memristor, because it "remembers" changes in the current passing through it by changing its resistance. Now HP claims to have discovered the first instance of a memristor, which it created with a bi-level titanium dioxide thin-film that changes its resistance when current passes through it.
"This new circuit element solves many problems with circuitry today--since it improves in performance as you scale it down to smaller and smaller sizes," said Chua. "Memristors will enable very small nanoscale devices to be made without generating all the excess heat that scaling down transistors is causing today."
HP has already tested the material in its ultra-high-density crossbar switches, which use nanowires to pack a record 100 Gbits onto a single die--compared with 16 Gbits for the highest density flash memory chips extant.
"We have been looking for years for the best material to use in our ultra-dense nanowire crossbar switches, which can fit 100 billion crossbars into a square centimeter. What we have finally realized is that the ideal material is a memristor," said Williams, primary inventor of the memristor's titanium-dioxide-based material and founding director of HP's 12-year-old Information and Quantum Systems Lab, where his team perfected its formulation.
The hold-up over the last 37 years, according to professor Chua, has been a misconception that has pervaded electronic circuit theory. That misconception is that the fundamental relationship in passive circuitry is between voltage and charge. What the researchers contend is that the fundamental relationship is actually between changes-in-voltage, or flux, and charge. Such is the insight that enabled HP to invent the memristor, according to Chua and Williams.
"Electronic theorists have been using the wrong pair of variables all these years--voltage and charge. The missing part of electronic theory was that the fundamental pair of variables is flux and charge," said Chua. "The situation is analogous to what is called "Aristotle's Law of Motion, which was wrong, because he said that force must be proportional to velocity. That misled people for 2000 years until Newton came along and pointed out that Aristotle was using the wrong variables. Newton said that force is proportional to acceleration--the change in velocity. This is exactly the situation with electronic circuit theory today. All electronic textbooks have been teaching using the wrong variables--voltage and charge--explaining away inaccuracies as anomalies. What they should have been teaching is the relationship between changes in voltage, or flux, and charge."
The virtues of hysteresis
HP invited Chua to speak about his theory a few years ago, but at that time the lab did not tell Chua that they were actively seeking the memristor. Only two weeks ago did Williams tell Chua that he had used the proper variables--flux and charge--to invent the world's first working memristor.
A memristor works by virtue of hysteresis, whereby its rate of change accelerates as it moves from one state to the other--"on" to "off," or vice versa. Hysteresis has been explained away by current circuit theory as an anomaly, according to Chua and Williams, whereas its existence is, in fact, a fundamental property of passive circuitry.
"Hysteresis is a tell-tale manifestation of the fourth circuit element--the memristor," said Chua. "And Stan Williams is very smart to have realized that if you cannot explain something properly, then there must be a better explanation."
For instance, electrical engineers have known that titanium dioxide changes its resistance in the presence of oxygen--this is the principle behind titanium dioxide oxygen sensors--but they could not explain why.
"They traced its curve, and knew it contained hysteresis, but because they could not explain it, they could only design the simplest of devices using it--sensors," said Chua. "But now that it has been explained, they will be able to design all types of new circuitry using it. This is a wonderful development."
Chua predicts that electrical engineers will soon begin discovering all types of new materials that manifest the hysteresis relationship between flux and charge. He predicts that this new era of electronics will be able to solve the problems with scaling--such as using too much power and generating too much heat--that are currently plaguing progress in circuit design.
"The memristor is our salvation, because it works better and better as you make it smaller and smaller," said Chua. "The era of nanoscale electronics will be enabled by the memristor. This is not just an invention, it is a basic scientific discovery. It has always been there--we just had to face these nanoscale problems to realize its importance."
The memristor behaves like a non-linear resistor with memory--a small, compact and highly energy-efficient means of creating a memory device. But Chua and Williams claim it is also a new type of circuit element that should enable the creation of new devices never before imagined.
The world's first memristor invented at HP Labs by Williams and his research team is based on a two-layer sandwich of titanium dioxide films. As a memory element, it works by changing the atomic structure of the films--by coupling the motion of atoms in the material with the movement of electrons through the material. The bottom layer of HP's material uses a symmetrical lattice of titanium atoms and oxygen atoms, which makes it a good insulator. But the top layer has had oxygen vacancies introduced as a dopant, which makes it into a good conductor--the more vacancies, the more conductive. HP's secret sauce for creating these oxygen vacancies in titanium dioxide involves using sputter deposition that begins with an excess of oxygen, then cuts back on the oxygen flow to create the layer with vacancies.
By placing the crossbar of nanowires above and below the sandwiched layers, charge can be passed through the material. "The way I discovered the material for our memristor was by studying how titanium dioxide oxygen sensors work--that got me thinking about moving oxygen vacancies around in the material to create a memristor," said Williams. "By running current through the device, we can push oxygen vacancies from the layer that has them into the layer that does not, thereby changing its resistance by a factor of 1000 or even more, thus switching the memristor 'on,' then by reversing the current we can move the vacancies back into the first layer, thereby switching the memristor 'off'."
New era of devices
As Chua predicted, Williams is already thinking about creating new types of devices with HP's crossbar architecture beyond a simple memory device. "If we push current through it hard and fast, it acts like a digital device, but if we run current through it gently and slowly it acts as an analog device," said Williams. "We are already designing new types of circuits in both the digital and analog domains using our crossbar architecture. In the analog domain, we want to build memristor-based devices that operate in a manner similar to how the synapse works in the brain--neuron-like analog computational elements that could perform control functions where decisions must be made involving comparisons as to whether something is larger or smaller than something else. We are not building a neural network yet, but we think that using the memristor in its analog mode with our crossbar is a pretty good representation of a neural net."
Later in 2008, HP promises to begin releasing details of how its memristor material works with its already perfected nanoscale crossbar switch architecture in these various types of circuits.
"The memristor is not just a replacement technology for existing memory devices, but will be used to make a whole range of new types of devices that no one has ever thought of before," said Williams.
评：这则重大的新闻最早发布于eetimes 4月30日，见于eettaiwan是5月8日，见于eetchina是5月23日，有人说 中文“翻译得水平真低，看得云里雾里 ”，所以我把原文也张贴出来，供大家参考。 |
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发表于 2008-5-29 20:57:07
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