Advantages
- Improved temporal resolution of a Kelvin probe force microscope (KPFM) .
- Precise surface potential measurement without being affected by charge transfer generated in the microsecond scale.
- Improved temporal resolution improves measurement speed per sample.
- The only method capable of measuring band bending that occurs at the operating frequency of a semiconductor device. Confirmed through experiments.
Background
Measuring the semiconductor bandgap is an essential step in the design and development of high-performance, high-efficiency, and energy-saving semiconductor devices. The bandgap can be estimated by measuring the surface potential of the semiconductor. KPFM is one method used to measure surface potential.
Due to the complex measurement principles of KPFM, the time resolution for measuring the surface potential has been limited from milliseconds to microseconds. Because modern semiconductor devices operate in the megahertz or gigahertz range, existing KPFM technologies cannot truly capture the phenomena that occur during semiconductor operation. For example, when measuring with KPFM, an AC potential difference is applied between the probe (cantilever) of the KPFM and the surface of the sample. An electric charge is transferred between the surface and the bulk state under the influence of the potential difference, and a “false surface potential” appears. Since the speed of charge transfer is generally less than submilliseconds (100 kHz), which corresponds to the time resolution of conventional KPFM, conventional KPFM measures the potential difference affected by charge transfer. In other words, KPFM measures the “false surface potential” and cannot measure the correct surface potential. As a result, it is not possible to obtain a precise measurement of the band bending that occurs in the heterojunction of a semiconductor element, and designers and developers may not be able to obtain the set target values or be forced to perform a large number of trial and error measurements.
Technology
The reason why the time resolution of KPFM cannot be improved is because the influence of the surface potential on the probe is very weak. The time resolution of KPFM is determined by the AC signal frequency f for surface potential difference detection applied between the probe and the sample. However, when the frequency f of the surface potential difference detection signal is increased and approached to the vibration frequency f _ 0 of the probe, the signal Δf derived from the surface potential rapidly decreases. In other words, because the signal derived from the surface potential cannot be read, the frequency f could not be increased.
Professor Yasuhiro Sugawara of Osaka University’s Graduate School of Engineering has resolved the aforementioned problem by improving the detection principle of KPFM. That is, the AC signal frequency f for surface potential difference detection is set to be a constant multiple of the drive frequency f _ 0 of the cantilever, that is, f = 2 * f _ 0. The signal Δf derived from the surface potential difference appears at a frequency in the vicinity of the signal f, but in the case of this invention, the signal f is in a band f = 2 * f _ 0 away from the vibration frequency f _ 0 of the cantilever, so that a sudden decrease in the signal Δf generated using conventional KPFM does not occur. With this new technique proposed by Prof. Sugawara, Δf with sufficient intensity can be detected, and surface potential data can be obtained at a high signal-to-noise ratio.
Please refer to the paper below for detailed information on the operation principle.
Surface potential measurement by heterodyne frequency modulation Kelvin probe force microscopy in MHz range
https://iopscience.iop.org/article/10.1088/2399-6528/aba477
Possible Application
- Ideal for measuring the surface potential of a semiconductor, especially band bending. Examples of experiments conducted by Professor Sugawara are shown in the following paper.
Study of high-low KPFM on a pn-patterned Si surface
https://doi.org/10.1093/jmicro/dfab055
- Can be adopted for both FM and AM KPFM.
- Improved temporal resolution measurement of KPFM. The surface potential of materials such as metals, semiconductors and insulators can be measured faster than conventional time resolution (milliseconds to microseconds). Contribute to the research and development of advanced surface physics in the modern age where high frequency operation has become the standard.
- Improved measurement speed using KPFM.
Expectation
We would like to invite you to use this technology to introduce new functions to your AFM products. This technology is expected to be used in the manufacture of semiconductors. License negotiations with Osaka University are possible.
Further, Osaka University can provide support to semiconductor manufacturers having difficult measuring band bending. It is possible for the university to form joint development partnership with you.
Product No. DA-03319b