Caliper using light!
Associate Professor Kazuyuki Okada
Department of Electronic Engineering and Computer Science, Faculty of Engineering, Kindai University
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The 1900s was the golden age of electronics when electrons played the principal role. I think optics will play the principal role in the 2000s. I’m expecting the 2000s to be an era when optical devices, which are electronic devices to which optics are adopted, will demonstrate their powers. Organic light-emitting diode (OLED) is one of the optical devices that takes the places of liquid crystal and LED.
An OLED has the structure what thin organic layers with thickness less than 1µm are sandwiched by two electrodes. When a DC voltage is applied to the OLED, carriers with positive and negative charges are injected from each electrode into the thin organic layers. The both charges are transported in the organic layers and combine with each other. These cause electro-luminescence in the OLED. This light emission is widely considered for utilization in displays, lighting equipments and so on. The OLED has more vivid colors than liquid crystals and also excel in time responsiveness. Since they have a planar light emission, they can also be used for wide-range shadowless lighting. In addition, since they can be manufactured with extreme thinness and light weight, they can be used as a curved-surface light source. The broad utilization as displays and lighting equipments with excellent designability can be expected.
The behavior of carriers in the organic layer is greatly related to the light-emission characteristics of the OLED. This behavior can be controlled by forming the organic film into a multilayer structure. Therefore, manufacturing of the OLED generally structures a multilayer of organic thin films having thickness of several nm. The spin coating method, ink-jet method and vacuum deposition method are used for manufacturing organic thin film. Although each method has both merits and demerits, the vacuum deposition method is mostly used in terms of film thickness control. In this method, the deposition state of organic material is monitored with a sensor using a crystal resonator. Film thickness is evaluated by installing the sensor near the OLED, but it does not directly measure the manufactured organic film. Verifying the manufactured film thickness accurately will be needed in future.
The needle contact method that evaluates the surface roughness from the vertical movement of a needle contacting the film surface is applicable to measure and evaluate the thickness of the formed organic film. However, it is not suitable for soft organic film since there is a high possibility that the needle will damage the film. This means that the manufactured OLED is damaged. Moreover, achieving sensitivity that can exactly detect changes of a few nm is also a subject to be considered.
An ellipsometer is measuring equipment that can measure and evaluate thin film thickness using the interference of light and polarization property without damaging on organic film. It can also even evaluate the thickness of thinner film than light wavelength since it uses light interference. However, it is very expensive. Currently, visible light is used as measuring light in many cases, but several issues must be overcome in order to accurately evaluate a few nm as thickness of 1/100 of visible light wavelength. Naturally, it can be solved by shortening the light wavelength used for measurement. However, since it is required to constitute with optical equipments supporting light in both the ultraviolet/vacuum ultraviolet region and also in the soft X-ray region, development of these optical equipments is strongly hoped for. The non-contact/non-destructive measurement of samples using light (electromagnetic wave) is available and can be used as effective means to evaluate the organic film thickness of the OLED.
Suppose I leap forward a little and talk about dreams. For example, we will be able to measure something with a caliper using an electromagnetic wave (light). The minimum scale of the caliper can be regarded as wavelength of the electromagnetic wave. There are various electromagnetic waves in this world such as AM radio waves having a wavelength of several hundred meters or X-rays having a wavelength of less than 1 nanometer. If a caliper exists that can measure using these electromagnetic waves perfectly, it means this caliper is an almighty caliper that can measure any object from the size of an atom to the size of the Earth. In addition, it can be performed with non-contact and non-destructive in real time.
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