Nanoscale thermal expansion imaging of a resistive thermal heater using diffraction phase microscopy

摘要

Summary form only given. Understanding the dynamics of thermal expansion is important in many fields, including physics, chemistry, and engineering. We measured the height change of a resistive thermal heater throughout the thermal expansion process using epi-illumination diffraction phase microscopy (epi-DPM). This real-time quantitative phase imaging (QPI) method can reconstruct the thermal expansion with nanometer height measurement accuracy. We use a conventional epi-DPM setup [1, 2] with a 405nm diode laser. The sample is observed by a 2.5× microscope objective with a field of view (FOV) of 650 μm × 495 μm. The first order light from the UV transmission grating remains unfiltered as the image field. Meanwhile, the zeroth order is filtered down as the reference field. Compared with previous DPM setup that had the zeroth order as the image field and the first order as the reference field, the system noise here is reduced from 0.8-2.0nm to 0.7-1.5 nm. We obtain the interferogram and extract the phase information via a Hilbert transform. A background image was first taken without voltage applied and then used to subtract the background noise. All subsequent DPM images of the resistive thermal heater were taken on the same region. A continuous-wave (cw) voltage was applied to the resistor for ~ 3 min and its height change reached a maximum value. Figures 1(a) and (b) show the material structure and top view of the resistor. The resistor was fabricated on a 500 μm thick Si substrate with a 75nm SiO2 insulator layer. A 150nm gold serpentine shaped thermal heater resistor was deposited on the layer and the conductor width was 10 μm. The resistor value is 67 Ohms. The sample was mounted onto a printed circuit board (PCB) and fixed on the stage. Figure 1(c) shows the height changes over the whole 650 μm width. The average height changes of the resistor were about 32.2 nm, 64.3 nm, and 109.0nm when cw 2 V, 4V and 6V were applied on the resistor, respectively. The thermal expansion of a resistive thermal heater was accurately reconstructed using epi-DPM. Therefore, this technique would be of great value for further developing in-situ monitoring of thermal expansion in various industrial processes.