Optical Film Reflectance and Transmittance Measurement
Transmittance and reflectance are the most basic optical characteristics of optical thin-film devices, therefore, the testing of transmittance and reflectance of thin-film devices is the basic testing technology for optical thin films. The transmittance and reflectance of thin films are mainly tested by spectral test analyzers.
1.1.1 Principle of Spectrum Analysis Test System
Transmittance and reflectance are the most basic optical characteristics of optical thin-film devices, therefore, the testing of transmittance and reflectance of thin-film devices is the basic testing technology for optical thin films. The transmittance and reflectance of thin films are mainly tested by spectral test analyzers.
As a kind of spectrometer, the spectrometer used for optical film testing can be divided into UV-visible spectrophotometer, infrared spectrophotometer infrared Fourier spectrometer, etc. according to the difference of the testing wavelength. The first two use the spectral spectroscopic principle of analysis and testing system, while the latter is based on the principle of interference spectral analysis system. Due to the different geometries and shapes of thin film devices, although both transmittance and reflectance measurements, for different geometries of samples, different accuracy, or different polarization requirements, may require different test methods and techniques. These require a good understanding of the basic test methods and test techniques for the transmission/reflection of thin films.
1-Basic Principles of Spectrophotometry
The main components of a spectrophotometer of the monochromator type are:
Light source-illumination optical system-monochromator-cuvette-monochromator-sensor-processing system
Light source to detect the band of light beam, after the illumination system beam shaping convergence in the monochromator incidence slit, the monochromator spectroscopy by the outgoing slit outgoing monochromatic light, after the sample cell, for the photoelectric sensors to receive, converted into electrical signals into the computer after processing. Spectrophotometer monochromator can be placed before the cuvette, and can also be placed after the cuvette.
The components and roles of the various parts of the spectrophotometer provide beams of various wavelengths required in the measurement band for the light source, respectively. To obtain accurate test data, the intensity of the light source should be kept constant, so they are powered by a regulated power supply. In general, in the visible band, the light source uses tungsten filament lamps or tungsten halogen lamps, in the ultraviolet region uses atmospheric lamps, in the infrared region uses tungsten halogen lamps and silicon carbon rod lamps. The monochromator consists of several parts: the dispersive element, the slit mechanism, and the scanning drive of the dispersive element. Commonly used dispersive elements are optical prisms and gratings.
2-Film Transmittance Measurement
Measuring the transmittance of a thin film element using a spectrophotometer is very simple, and generally involves simply inserting the element to be measured into the measurement optical path of the sample chamber. In transmittance measurement using a spectrophotometer, the instrument measures the ratio of the light intensity reaching the detector through the sample optical path to the light intensity reaching the detector through the reference optical path, called the transmittance.
3-Thin Film Reflectance Measurement
Measurement of film reflectance is not as convenient and widespread as transmittance. Reflectance measurements are more complex and difficult than transmittance for several reasons:
1) It is not easy to find long-term stable reference samples with 100% reflectance performance over a wide range of wavelengths.
2) In reflectance measurements, the position of the spot on the photosensitive surface of the photodetector tends to change with and without the sample due to the sensitive change of the reflective optical path, which leads to a significant increase in error.
3) Various thin-film components have different requirements for the range and accuracy of reflectance measurements. For example, in the reduction of reflective film, it is hoped that the accuracy of measuring low reflectance is not less than 0.1%, while the laser high reflectance mirror requires that the reflectance higher than 99% of the range, can be better than 0.01% of the accuracy of the measurement.
Below are a few common methods used when measuring reflectance:
1) Relative reflectance measurement;
In the test of relative reflectance, the key point is to find a reference sample that meets the measurement requirements, this method is simple and convenient and is suitable for low reflection measurement.
2) Absolute reflectance measurement: Absolute reflectance measurement generally uses V-W type, this method improves the measurement accuracy by measuring the multiple reflections of light.
Factors affecting measurement accuracy
When utilizing a spectrophotometer for film transmittance measurements, the main factors to be aware of that affect the results are the following:
(1) Measurement of the caliber: the measurement should ensure that the beam is all through the sample, not all through the measurement error will result. If the actual measurement of the sample is smaller than the beam, you can add a small aperture diaphragm in the optical path to adjust.
2）Measuring Sample Thickness: Many spectrophotometers converge the test and reference beams in the center of the sample chamber. Thus, when, for example, a thicker sample is inserted into the optical path, the convergence of the beams on the photosensitive surface of the receiver changes, causing errors, especially if the substrate has a high refractive index or is incident at an angle.
3）Instrumental Spectral Resolution: The resolution of spectrophotometers is often higher than 10 nm. the effect of resolution must be taken into account when determining narrow band filters with a bandwidth of less than 30 nm, or cut-off filters with steep cut-off properties. In this case, since the measurement beam does not contain a narrow enough spectral interval, the value displayed by the instrument is the average transmittance of the component to be measured within that spectral interval.
4）Influence of certain absorption components in the air: In the near-infrared region, the absorption band of carbon dioxide often interferes with the test results, characterized by a spectral curve that shows the sharp transmission undulations characteristic of absorption spectra.
5）Influence of light polarization: Since light is reflected many times in a spectrophotometer, the measurement beam generally has a polarization characteristic. High-grade spectrophotometers to overcome the polarization effect, use de-polarized or circularly polarized detection of the measurement beam, but some of the middle and low-grade spectrophotometer measurement beam often contains part of the polarized light. When measuring the transmittance of a thin film sample under oblique incidence, attention must be paid to the polarization characteristics of the light.
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