09 Oct 2006
There is a lot more to making an optical filter than meets the eye. Dominik Goessi looks at the role of in situ monitoring in the production process and reviews the technology.
The humble interference filter often plays a critical role in optical experiments. Made up of tens of layers, each one precisely deposited and monitored, there is certainly a lot more to this standard component than meets the eye. Its manufacture typically involves in situ monitoring and control systems to minimize production time and cost, and to maximize yields.
Enhanced electron-beam deposition pro-cesses such as reactive low-voltage ion plating (RLVIP) are ideal for producing complex optical filters with 50 layers or more. These techniques can generate thin films with high density, high-temperature stability and a minimum of absorption. However, expensive coating materials, long process times and costly substrates, that cannot always be reworked in the event of an error in the coating process, mean that monitoring is essential.
Thin-film deposition processes are typically controlled by one of two techniques: quartz or optical monitoring.
Quartz crystal is the most commonly used monitoring system. Here, an oscillator excites a quartz crystal to high frequencies of around 5 MHz. When the crystal is coated, its frequency decreases and this change gives a measure of the film's thickness.
On the plus side, the hardware is relatively inexpensive and easy to operate, and is used successfully in many semiconductor and optical applications. However, the monitoring system is not optimal for complex optical coatings because the layer thickness on the substrate or on a test glass is not measured directly.
Optical film-thickness measurements rely on the fact that the intensity of a monochromatic light beam reflected by a film changes periodically with increasing film thickness. The technique has the advantage that a film's optical properties are measured directly using test glasses, which can then be kept and checked for process and quality control. However, the initial hardware investment and set-up costs are higher than that of crystal monitoring.
To compare quartz crystal and optical monitoring systems, 15 samples, each having an identical two-layer sequence of high and low refractive index materials, were coated in an Evatec BAP800 evaporating system.
The reproducibility of the layer density using RLVIP on a BAP800 system is very good, so one would expect the quartz crystal to give the same thickness result for each sample. However, results varied between 24.1 and 25.2 nm for the first layer and between 54.0 and 56.2 nm for the second layer. This highlights the reduced control of quartz monitoring.
In practice, it is possible to correct coating errors with crystal monitoring for designs with approximately 10 layers. However, for coatings with 20, 30 and 50 layers or more, the error within each layer is unacceptably large with insufficient reproducibility to achieve the highest yields. A more precise monitoring system is required.
Deposition processes such as RLVIP in combination with optical monitoring systems are therefore ideal for manufacturing complex interference filters with high accuracy and reproducibility. The theory and practice are well matched. The actual spectral curves achieved are identical to the calculated ones, proving that optical monitoring systems justify their higher initial investment and set-up costs through better process control and higher yields.
An optical monitoring system has two senders: one for transmission and another for reflection measurements. The receiver is the same for both modes of operation. White light reflected from, or transmitted through, a test glass passes through a monochromator before being focused onto a detector. The signal from the detector is then fed into a lock-in amplifier where it is processed and digitized.
Consider a test glass with a refractive index of nG that has already been coated with a film of thickness d and refractive index nL. In reflection mode, for example, a light beam from the sender (R) hits the coated test glass and is refracted and partially reflected (R1) when it enters the optically thicker medium.
The refracted light reaches the interface between the film and the test glass after it has passed through the optical film thickness nLd. One part of the light is reflected (R2) and the other part penetrates the test glass. The reflected light passes back through the film again and multiple reflections occur at the interfaces. The intensity of the reflected light decreases continuously but the intensity of the individual reflected light beams add together to give the total intensity.
When monochromatic light with the wavelength λ crosses the interface from optically thinner to thicker material, the reflection is also accompanied by a phase shift of λ/2. The phase difference of reflected or transmitted light is determined by the difference in optical path lengths 2nLd and the phase shift that occurs at the interfaces. We can therefore see a turning point in the total intensity curve of superimposed, reflected monochromatic light beams depending on the light wavelength, the refractive index and the instantaneous thickness of the coated material.
In practice, as the film thickness increases continuously during the coating process itself, the turning point condition is fulfilled at regular intervals and the intensity of the reflected light beam detected at the receiver reaches various maxima and minima with a periodic structure. The distance between the turning points depends on standard variables including evaporation rate r. If the evaporation rate and the refractive index of the film material are known, then it is possible to monitor film thickness using the intensity curve measured at the receiver. A special cut-off algorithm is used to control the coater and terminate evaporation when the desired film thickness is reached.
Successive manufacturing runs for a typical edge filter consisting of 24 optimized layers in an Evatec BAK760 with the optical monitoring system GSM1100 demonstrate the level of production control that can be achieved with optical monitoring. The spectral transmission curves for three batches with 50% transmission values of 653.5, 651.5 and 652.8 nm respectively show an excellent reproducibility of ±1nm.
The future of optical monitoring
A new generation of fast CCD image sensors specifically designed for low-light-level detection in combination with an imaging spectrograph enables the precise monitoring of a whole spectral range (broadband monitoring). The coating process can be controlled by direct measurement on the substrate and the whole optical spectrum of each layer is known and can be controlled. This means that even in the event of a coating error mid-process, the remaining layers can be recalculated in situ to give the correct end result.
Currently, the sampling frequency of these set-ups is lower than those used in monochromatic monitoring. As a result, special algorithms are required to terminate a coating process precisely between two measurements. However, given the rapid progress in the quality and speed of CCD sensors, the future of optical monitoring systems looks set to become broadband.• This article originally appeared in the October 2006 issue of Optics & Laser Europe magazine.