nanoquebec 2007-08-20 08:28
Nanoscale blasting adjusts resistance in magnetic sensors
[size=5][b]Nanoscale blasting adjusts resistance in magnetic sensors[/b][/size]
[纳米科技世界快讯]据NIST报道,A new process for adjusting the resistance of semiconductor devices by carpeting a small area of the device with tiny pits, like a yard dug up by demented terriers, may be the key to a new class of magnetic sensors, enabling new, ultra-dense data storage devices. The technique demonstrated by researchers at the National Institute of Standards and Technology (NIST) allows engineers to tailor the electrical resistance of individual layers in a device without changing any other part of the processing or design ("[url=http://dx.doi.org/doi:10.1063/1.2768894][b]Selectable resistance-area product by dilute highly charged ion irradiation[/b][/url]").
The tiny magnetic sensors in modern disk drives are a sandwich of two magnetic layers separated by a thin buffer layer. The layer closest to the disk surface is designed to switch its magnetic polarity quickly in response to the direction of the magnetic “bit” recorded on the disk under it. The sensor works by measuring the electrical resistance across the magnetic layers, which changes depending on whether the two layers have matching polarities.
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[i]Cartoon illustrates new NIST technique for selectively modifying resistance of a semiconductor device layer. (Top) First layer -- in this case a composite of copper and cobalt -- and an insulating buffer layer of aluminum oxide is deposited. Buffer is about one nanometer thick. (Middle) Highly charged xenon +44 ions strike the buffer layer, digging nanoscale pits. (Bottom) Top conducting layer of cobalt and copper is deposited. Pits reduce the electrical resistance of the layers and may function as nanoscale GMR sensors embedded in a MTJ sensor. (Image: NIST)[/i]
As manufacturers strive to make disk storage devices smaller and more densely packed with data, the sensors need to shrink as well, but current designs are starting to hit the wall. To meet the size constraints, prototype sensors measure sensor resistance perpendicular to the thin layers, but depending on the buffer material in the sensor, two different types of sensors can be made. Giant magneto-resistance (GMR) sensors use a low-resistance metal buffer layer and are fast, but plagued by very low, difficult to detect, signals. On the other hand, magnetic tunnel junction (MTJ) sensors use a relatively high-resistance insulating buffer that delivers a strong signal, but has a slower response time, too slow to keep up with a very high-speed, high-capacity drive.
What’s needed, says NIST physicist Josh Pomeroy, is a compromise. “Our approach is to combine these at the nanometer scale. We start out with a magnetic tunnel junction—an insulating buffer—and then, by using highly charged ions, sort of blow out little craters in the buffer layer so that when we grow the rest of the sensor on top, these craters will act like little GMR sensors, while the rest will act like an MTJ sensor.” The combined signal of the two effects, the researchers argue, should be superior to either alone.
The NIST team has demonstrated the first step—the controlled pockmarking of an insulating layer in a multi-layer structure to adjust its total resistance. The team uses small numbers of highly charged xenon ions that each have enormous potential energies—and can blast out surface pits without damaging the substrate. With each ion carrying more than 50 thousand electron volts of potential energy, only one impact is needed to create a pit—multiple hits in the same location are not necessary. Controlling the number of ions provides fine control over the number of pits etched, and hence the resistance of the layer—currently demonstrated over a range of three orders of magnitude. NIST researchers now are working to incorporate these modified layers into working magnetic sensors.
The new technique alters only a single step in the fabrication process—an important consideration for future scale-up—and can be applied to any device where it’s desirable to fine-tune the resistance of individual layers. NIST has a provisional patent on the work, number 60,905,125.
Source: NIST
nano 2007-08-21 08:01
美国国家标准和技术研究院找到一种通过在半导体设备小片区域内覆盖一层微小细坑来修订半导体设备电阻的新方法。这些微小细坑就像是疯狗在院子里刨出来的坑洞一样。这一技术对新一代磁传感器开发而言非常重要，利用该技术可以开发出新的更加密集数据储存设备。美国国家标准和技术研究院展示了这一技术。该技术允许工程师修剪一个设备中各层的电阻，而无需改变其它部分的配置或设计。
现代光驱中的微型磁传感器是一个薄缓冲层隔开两个磁层组成的三明治夹心结构。最接近光盘表面的磁层设计为快速对记录在光盘上的磁场“比特”方向做出反应的磁极开关。传感器通过磁层测量电阻而工作，电阻的改变依赖于两个磁层是否与磁极匹配。
由于制造商正努力制造更小和数据密集度更高的光盘储存设备，因此传感器需要变得更小，但是目前的设计正面临障碍。为了满足缩小传感器尺寸的要求，原型传感器对传感器薄层垂直电阻进行了测量，但是依靠传感器中的缓冲材料，科学家们可以制造出两种不同型的传感器。巨磁阻传感器（GMR）使用一种低电阻金属缓冲层，速度很快，但是电阻却非常低，很难探测到信号。另外一种传感器就是磁隧道结传感器（MTJ），该传感器使用了一种相对较高电阻的绝缘缓冲层，能够释放出很强的信号，但是反应时间却太缓慢，而无法与非常高速的和高容量光盘驱动器相匹配。
美国国家标准和技术研究院物理学家乔希.泊米诺伊称需要对这两种传感器设计进行折衷。他说，“我们的方法就是在纳米层次上将这些进行结合。我们从磁隧道结开始着手（一种绝缘缓冲层），然后利用高电荷离子在缓冲层上‘吹’出少量小坑，以便我们在顶层制造传感器的其它部分时，这些小坑能够表现出小巨磁阻传感器的功能，而传感器的其余部分则表现出磁隧道结传感器的功能。”研究人员认为，结合两个传感器的信号效果应当强于任何单个传感器的信号效果。
美国国家标准和技术研究院研究小组演示了此技术的第一步，即控制多层结构中绝缘层的凹痕来修订其总电阻。研究小组利用少量高电荷氙离子，每个离子都有巨大的潜能，在不破坏底层结构的情况下可以在表面“吹”出小坑。每个均离子携带了五万多电子伏特的潜能，只需要一次撞击就可以制造出一个坑，而无需多次撞击同一个地方。通过控制离子数量就可以很好地控制坑的数量，从而控制各层电阻——目前实验对三种坑数量范围进行了论证。美国国家标准和技术研究院的研究人员现在正将这些改进后的层并入磁传感器中。
虽然新技术改变的仅仅是制造过程中的简单一步，但是对于未来技术进步而言却引发了技术人员深层次的考虑。这一技术可以应用于所有需要高整各层电阻的设备。美国国家标准和技术研究院的该项技术获得了一个临时性的专利，专利号为60,906,125。