Ultrafast Scanning Thermal Microscope Probes

热是一个普遍存在的量。由于所有的能量都转化为热，所以对热现象的研究适用于整个科学和工程领域。例如，许多现代半导体器件的性能受到其热性能的限制，如射频功率晶体管、激光器、功率开关和微处理器。在宏观尺度上，热效应通常是缓慢的，因为热传输是扩散的。扩散现象的一个特征是它们会随着尺寸的变化而平方加速：如果一个装置或温度计小十倍，它的热响应速度就会快一百倍。在正常的材料中，这种效应可以变得非常迅速，在现代器件的GHz范围内，其速率与其工作频率相当。在制造现代器件和材料的纳米尺度上，热的传输不再是经典的和扩散的。热可能以弹道方式传播，而不会在一小段距离内散射，从而在小型设备中导致更快的热传输，而窄线的热导率可能会通过热导体的尺寸来量化，因此热导率的经典值不适用，而且无法预测此类系统的热性能：它只能被测量。由于热通常是通过声波传输的，这种声波称为横向声子，其波长可能接近材料中的原子间距，因此材料之间的界面及其粗糙度对原子尺度的影响对结构良好的材料导热的程度有很大的影响。这可能会被用于热电材料或隔热材料，或者当试图从激光器或微处理器等设备中提取操作热量时，这可能是一个巨大的问题。同样，由于原子尺度界面的技术控制很困难，这使得界面上的热传导无法预测，所以我们被迫进行测量。扫描热显微镜是一种技术，它使用传感器探头在表面上的机械扫描来测量纳米级的局部温度。这是非常有效的，但目前使用的传感器响应时间非常慢，因为它们相对较大(约10微米)，并由大量隔热材料支撑。这个项目涉及对探头进行彻底的重新设计，使其变得更小，并在不使用厚支撑结构的情况下制造它。这将使传感器能够对样品的温度变化做出快速反应，并允许在空间和时间域以30 nm x 30 nm x 1 ns的分辨率进行热测量。这种探头的重要性在于，它将首次能够在设备运行的规模上测量器件和材料的局部温升。至关重要的是，这将使我们能够区分热量被带走和热量被局部“储存”。这些信息可以确定热流通过设备的路径，使设计者能够控制它，还可以确定达到的最高温度。由于器件的热退化受“热激活能”的限制，因此器件的损伤率和寿命随温度呈指数变化：器件寿命由峰值温度决定，而不是平均温度。由于材料的性质不会简单地随尺寸而变化，而且器件的工作频率由器件尺寸固定，因此只能在实际器件的空间和时间尺度上进行峰值温度的热测量。拟议的工具独一无二地满足了这一要求，以衡量现代技术成就的核心。

Heat is a universal quantity. Since all energy turns to heat, the study of thermal phenomena is applicable over the whole range of science and engineering. For example, the performance of many modern semiconductor devices are limited by their thermal performance e.g. RF power transistors, lasers, power switches and microprocessors. At the macroscopic scale thermal effects are generally slow, because heat transport is diffusive. A characteristic of diffusive phenomena is that they speed up quadratically with changes in dimension: If a device or thermometer is made ten times smaller it will have a thermal response which is a hundred times faster. In a normal material the effects can become enormously rapid, in the GHz range for modern devices which is a rate with is comparable to their operating frequency.At the nanometre scale, where modern devices and materials are fabricated, the transport of heat ceases to be classical and diffusive. Heat may travel "ballistically" without scattering across a small distance leading to even more rapid thermal transport in a small device and the thermal conductivity of narrow wires may be quantised by the dimensions of the thermal conductor so that classical values of thermal conductivity do not apply and prediction of the thermal performance of such systems becomes impossible: it can only be measured. Since heat is often transported by sound waves known as transverse acoustic phonons which may have a wavelength which approaches the atomic spacing in the material the influence of interfaces between materials and their roughness on the atomic scale has a huge effect on how well structured materials may conduct heat. This might be used to advantage in thermoelectric materials or thermally insulating materials, or it may be an enormous problem when trying to extract the heat of operation from a device such as a laser or microprocessor. Again, since the technological control of atomic scale interfaces is hard this makes thermal conduction across interfaces impossible to predict, so we are forced to measure it.Scanning Thermal Microscopy is a technique using the mechanical scanning of a sensor probe over a surface to make measurements of local temperature at the nanometre scale. This is very effective but currently the sensors used are very slow in response time, because they are relatively large (about ten micrometres) and are supported by large volumes of thermally insulating material. This project is concerned with the complete re-engineering of the probe to make it much smaller and to fabricate it without the use of thick support structures. This will allow the sensor to respond rapidly to the temperature variation of the sample and permit thermal measurements with a resolution of 30nm x 30nm x 1ns in the spatial and temporal domains.The importance of such a probe is that it will, for the first time, enable the measurement of local temperature rise in devices and materials on the scale of device operation. Crucially, this will allow us to distinguish between heat being conducted away and heat being locally "stored" by heat capacity. This information allows the path of flow of heat to be determined through a device, enabling the designer to control it, and also determines the maximum temperature achieved. Since the thermal degradation of devices is limited by "thermal activation energy" the rate of damage and hence the lifetime of a device varies exponentially with temperature: The device lifetime is determined by the peak temperature, not the average. Since the properties of a material do not scale simply with size, and since the operating frequency of devices is fixed by device size, thermal measurements of the peak temperature can only be made at the spatial and temporal scales of the actual device. The proposed tools uniquely fulfil this requirement for measurements at the very heart of modern technological achievements.