van Neer P L M J, Quesson B, van Es M H, van Riel M, Hatakeyama K, Mohtashami A, Piras D, Duivenoorde T, Lans M, Sadeghian H
Department of Acoustics and Sonar, The Netherlands Organization for Applied Scientific Research (TNO), The Hague, Zuid-Holland 2597 AK, The Netherlands.
Department of Optomechatronics, The Netherlands Organization for Applied Scientific Research (TNO), Delft, Zuid-Holland 2628 CK, The Netherlands.
Rev Sci Instrum. 2019 Jul;90(7):073705. doi: 10.1063/1.5097387.
The characterization of buried nanoscale structures nondestructively is an important challenge in a number of applications, such as defect detection and metrology in the semiconductor industry. A promising technique is Subsurface Scanning Probe Microscopy (SSPM), which combines ultrasound with Atomic Force Microscopy (AFM). Initially, SSPM was used to measure the viscoelastic contrast between a subsurface feature and its surrounding medium. However, by increasing the ultrasonic frequency to >1 GHz, it has been shown that SSPM can also measure acoustic impedance based contrasts. At these frequencies, it becomes difficult to reliably couple the sound into the sample such that the AFM is able to pick up the scattered sound field. The cause is the existence of strong acoustic resonances in the sample, the transducer, and the coupling layer-the liquid layer used to couple the sound energy from the transducer into the sample-in combination with the nonlinearity of the tip-sample interaction. Thus, it is essential to control and measure the thickness of the coupling layer with nanometer accuracy. Here, we present the design of a mechanical clamp to ensure a stable acoustic coupling. Moreover, an acoustic method is presented to measure the coupling layer thickness in real-time. Stable coupling layers with thicknesses of 700 ± 2 nm were achieved over periods of 2-4 h. Measurements of the downmixed AFM signals showed stable signal intensities for >1 h. The clamp and monitoring method introduced here makes scattering based SSPM practical, robust, and reliable and enables measurement periods of hours.
对掩埋的纳米级结构进行无损表征是许多应用中的一项重要挑战，例如半导体行业中的缺陷检测和计量。一种很有前景的技术是地下扫描探针显微镜（SSPM），它将超声与原子力显微镜（AFM）相结合。最初，SSPM用于测量地下特征与其周围介质之间的粘弹性对比度。然而，通过将超声频率提高到>1 GHz，已表明SSPM还可以测量基于声阻抗的对比度。在这些频率下，很难将声音可靠地耦合到样品中，以使AFM能够拾取散射声场。原因是样品、换能器和耦合层（用于将声能从换能器耦合到样品中的液体层）中存在强烈的声共振，以及针尖-样品相互作用的非线性。因此，以纳米精度控制和测量耦合层的厚度至关重要。在这里，我们展示了一种机械夹具的设计，以确保稳定的声耦合。此外，还提出了一种声学方法来实时测量耦合层的厚度。在2-4小时的时间内实现了厚度为700±2 nm的稳定耦合层。对下混频AFM信号的测量显示，信号强度在>1小时内保持稳定。这里介绍的夹具和监测方法使基于散射的SSPM实用、稳健且可靠，并能够实现数小时的测量周期。
