Optical Thin Film Monitoring

LumaSense has developed industry-leading technology for optical thin film monitoring. This technology is an extension of the Luxtron Radiation Thermometer (RT) technology. If you are not familiar with the unique advantages of our short-wavelength, near-IR radiation thermometers, then we suggest you start by reading our primer on radiation thermometry by clicking here.

For advanced material deposition processes (for example, MBE and CVD), LumaSense Technologies fully integrates three sophisticated metrology functions into our class-leading radiation thermometer systems:

Automatic emissivity corrected temperature measurement
Film growth monitoring via dual wavelength reflectance measurements, and
Substrate bow detection to insure film uniformity.

Automatic emissivity correction

During deposition of thin films, critical to improving the quality of film growth is precise control of process temperature. And of most importance is the temperature on the surface where deposition occurs. Therefore, non-contact surface measurement using high quality radiation thermometers (also known as infrared thermometers or pyrometers) is ideal for process control.

However, all radiation thermometers require an emissivity value to calculate the temperature value from the thermal radiation. If the target being measured is undergoing deposition, then the emissivity is constantly changing on the top surface where the deposition is happening. Therefore, for any top surface temperature measurement to be practical for closed-loop control of the process it must incorporate an in-situ emissivity measurement that will constantly adjust the emissivity value used for the temperature calculations.

The Luxtron radiation thermometers, with this feature, incorporate a built-in reflectometer. The reflectometer probes the sample with a stable, reliable light emitting diode (LED) and measures the reflectivity at the exact same wavelength that is used to measure the thermal radiation. The emissivity is obtained from:

By constantly measuring the emissivity, the Luxtron instrument can automatically perform the emissivity correction in-situ and output the deposition surface’s true temperature.

Film Growth Monitoring

One of the most common methods for measuring thin film properties is to use reflectance measurements. What is unique about the Luxtron product is the seamless integration of ultra precise reflectance measurements into the Luxtron class leading, lowest signal-to-noise ratio radiation detectors.

If the n and k values are known for the materials, then the amplitude and periodicity of the thin film reflectance (often referred to as “fringes”) at specific wavelengths can be used to calculate film thickness. For very thin films, it is critical to use both the highest quality measurement systems and shorter wavelengths to allow reasonable “resolution” into the film’s fringes.

The Luxtron radiation thermometers incorporating this feature make use of built-in reflectometer(s). Currently LumaSense Technologies offers Luxtron systems that offer up to two wavelengths measured simultaneously. The reflectometer probes the sample with a laser diode at 635nm and a light emitting diode (LED) centered at 880nm. The thermal radiation is also measured simultaneously at the 880nm wavelength.

Substrate “Bow” Detection

Critical during deposition to monitor and control for film growth uniformity, LumaSense Technologies offers a patent pending and proprietary method for optically measuring substrate “bow” with the Luxtron system. This measurement gives the process engineer a positive mechanism to tune product uniformity and improve overall yields.