May 16, 2017

Development of a highly stable wavelength tunable light source for thickness measurement using a KTN crystal

HPK, NTT-AT and NTT have developed a wavelength tunable light source suitable for measuring the thickness of silicon wafers used in power devices. The light source can sweep the wavelength by changing the incident angle of the light to an optical grating in a laser cavity by using a KTN laser beam deflector.

In a joint partnership, we have been developing and selling a wavelength tunable light source using the KTN deflector developed by NTT for use in optical coherence tomography (OCT). We have now developed a light source for thickness measurement systems used in factories that produce silicon wafers for power devices.

Configuration of KTN-based wavelength swept light source.

Power devices are used to control electric power in a wide range of applications, such as automobiles, railroads and transformer substations. Expansion of the market is expected due to the development of social infrastructure. Since silicon wafers used in power devices are usually ground at high speed to a thickness of around 100 micrometers, there is a demand for a highly stable light source that can be used to measure thickness with a high degree of accuracy.

Since current wavelength tunable light sources used in OCT employ mechanical moving mirrors for wavelength tuning, they are susceptible to vibrations of the system. The newly developed product has eliminated mechanically moving parts by using a KTN deflector. Consequently, it is less susceptible to vibrations and can continue to operate stably over a long period. By improving the material and temperature control, we have reduced the influence of the ambient temperature on measurements and achieved high reproducibility. In addition, a blue LED light makes KTN operation stable within a short time. Using this property, we have reduced the starting time. Moreover, by setting the central wavelength of the light source at 1.3 μm with relatively low dopant absorption, the thickness of highly doped silicon wafer can be measured with a high degree of accuracy. The semiconductor optical amplifier used in this product has been developed by NTT Electronics Corporation, an NTT Group company. It operates at a central wavelength of 1.3 μm and is made highly reliable by improving the hermetic seal.

We will apply the product to medical and industrial fields by further improvements of its performance such as operation speed and wavelength tuning range.

Features of the product

1. Long-term stable operation

This product employs the KTN deflector and does not include any moving parts. It is less susceptible to vibrations than wavelength swept light sources that use a mechanical moving mirror. Therefore, it can continue to operate stably for over 5,000 hours, as required in factories.

2. High measurement reproducibility

Adoption of a metal with a small thermal expansion coefficient and improvement in the methods of measuring and controlling the temperature of the KTN deflector has reduced the impact of temperature variations on measurement results. As a result, errors in measurements of the thickness of standard wafers are kept below 0.06 μm when the ambient temperature is kept between 20 and 30 °C.

3. Reduction of starting time for stable operation

We have discovered that irradiating a KTN crystal with a blue LED light reduces the time until the charge distribution reaches a steady state. This effect has reduced the starting time from 10 minutes to 3 minutes.

4. Central wavelength of 1.3 μm

By setting the central wavelength of the light source at 1.3 μm, a wavelength with low dopant absorption, we have made it possible to measure, with a high degree of accuracy, the thickness of high doped silicon for power device.