3.1

Filter design

The design of the variable filter for the mini-spectrometer was based on the induced transmission theory [6]. According to this theory the metal reflectivity can be eliminated at a given wavelength if a proper optical matching with surrounding media is made. In this way a narrow-band transmission filter that operates in the range 400-1000nm can be obtained with a limited number of layers, as shown in Fig. 2. The filter performance reported in this graph is obtained by a silver layer (55nm thick) matched with air and substrate (glass) by the introduction of eight oxide layers (SiO₂, Ta₂O₅) on both sides of the metal layer. The ripple on the short-wavelength side can be reduced by an optimization of the coating design, that will cause an increase of the layer number. The result is shown in Fig. 3, where the transmittance of a 24-layer filter containing one silver layer (58nm thick) is reported. The peak transmission is about 70% and the transmission bandwidth lower than 10nm. Moreover in the same graph, the transmission peak is displaced to different wavelengths and this effect is obtained by only grading the thickness of the dielectric layers, without changing the coating structure (number and sequence of layers). The deterioration of the performance, when the peak wavelength is moved, is due to the strong dispersion of the silver refractive index. This means that the optimization of the coating design works efficiently only for the peak wavelength where this process is carried out (900nm in Fig. 3). For each peak wavelength, a proper optical matching should be calculated and a different coating design would be required. That is not possible if a variable filter must be realized; in this case only the coating thickness, and not its structure, can be varied over the substrate surface.

Fig. 2.

Transmittance of a 17-layer induced transmission filter containing one silver layer

Fig. 3.

Transmittance of a 24-layer induced transmission filter, which peak wavelength is moved by only grading the coating thickness

To obtain a linearly variable filter, which transmission peak is moved from 400 to 1000nm, the thickness of the coating must be wedged along the sample surface. The law of variation of each dielectric-layer thickness t_i(x) depends on the desired peak-wavelength variation, along the direction x of the component surface, and can be written as:

where n_i is a refractive index of the i-th layer and q_i is ratio of its optical thickness, n_it_i, to the reference wavelength λ₀, in terms of quarter waves (q=1 corresponds to quarter-wave thickness). The values of q_i are defined in the coating design. The refractive index n_i is not constant with wavelength but its dispersion, in the wavelength range of interest, does not influence significantly the profile of t_i(x). Thus, a linear displacement of the peak wavelength along the filter surface is obtained with an almost linear variation of the thickness of each dielectric layer, as shown in Fig. 4, where the thickness variation of quarter-wave SiO₂ and Ta₂O₅ layers is reported. The peak-wavelength gradient is related to the thickness wedging. The thickness gradient, that is determined by the maximum and minimum thickness to be deposited on a given distance, depends not only on the wavelength extremes, λ_max and λ_min, but also on the refractive index dispersion of each material. In fact, owing to the dispersion, the ratio between maximum and minimum geometrical thickness of each layer will be slightly different from the ratio between maximum and minimum operating wavelengths. If λ_max= 1000nm and λ_min= 400nm, the thickness ratio will be different for each oxide layer and higher than 2.5. This value is independent from the q value, but depends only on the material.

Fig. 4.

Thickness gradient of two dielectric layers of the coating of Fig. 3.

The peak wavelength is coincident with the reference wavelength λ₀ for the filter design on which the optimization is carried out; but when the peak wavelength is moved, there is a small difference between these two quantities that must be taken into account in the fabrication process.

The peak-wavelength gradient that is required for the spectrometer should be greater than 60nm/mm, that means the filter should be smaller than 10mm. The limitation on the filter dimension, in the direction of variation, are only due to technological problems in the fabrication of graded thickness layers. The masking system, used to deposit wedge layers, will be described in the next paragraph.

3.2

Filter fabrication

The radiofrequency magnetron sputtering system, PE2400, was used for the filter fabrication. Three 8inch-diameter targets are installed at the top side of the vacuum chamber. Silica (SiO₂) and tantala (Ta₂O₅) targets were used as low- and high-refractive index material and silver (Ag) target was used for deposition of the metallic layer. The substrate holder is mounted on the rotary platform, which allows controlled positioning of the substrate under each of the three targets. All movements are motorized and controlled by computer. The deposition conditions were optimized for each of the used material. The deposition chamber can be evacuated using a rotary pump and a turbo-molecular pump to an ultimate pressure of 2·10^-4 Pa. Gas flow and its mixing ratio is controlled by three gas flow meters. Argon and oxygen gas mixture were used in the deposition process of tantala. The silver and silica coatings were prepared in pure argon ambient. The deposition pressure of 0.1 Pa and 0.5 Pa for oxides and silver, respectively, was regulated using a virtual regulator controlling the flow meters and throttle valve.

The deposition system is equipped by combined optical and crystal micro-balance monitoring system for precise layer thickness estimation. Spectral reflectivity is measured using an optical apparatus installed beneath the transparent substrate. The obtained spectra are periodically processed by a dedicated computer and fitted by an appropriate model in order to achieve an actual thickness and refractive index of the deposited layer. Thus, a precisely defined multilayer structure can be fabricated. The silver layer growth is monitored using crystal micro-balance monitoring system, which is more accurate than optical monitoring for such absorptive layer. Combination of dielectric layers with metal layer brings very specific problems to optical monitoring. The first dielectric layer after a metal layer becomes very insensitive for the optical monitoring; the back-side reflectivity is almost not changed by growing that layer.

Therefore, it was decided to apply an indirect optical monitoring using an optical structure, which differs from the complete structure of the optical filter. The dielectric layer growth is optically monitored on a testing zone of the same substrate. This zone is masked during the silver layer deposition. Thus the monitoring proceeds on an identical multilayer structure as the complete filter, with exception of the missing metallic layer.

Several effects were observed during the deposition experiments. First effect is associated with the well known optical shift of the optical interference features after deposition chamber venting. The humidity penetrating into the filter structure pores induces a refractive index and/or layer’s thickness changes. This effect is at least partially reversible.

The second effect concerns a progressive increase of the deposition rate of each further layer. The simulation showed that this increase is accompanied by a slight decrease of refractive index. This effect is associated with the increasing porosity of the structure, which is caused by a columnar growth and a progressive increase of the grain size. The minor changes of the refractive index are detected and compensated by a fitting algorithm during the deposition process.

The mentioned observations are demonstrated in Table 1 for a 19-layer filter, where the refractive index decrease appears, going from the substrate to the air side.

Table 1.

Results of post-deposition analysis of the structure of a filter with transmission peak at 680 nm (the silver layer is excluded from the structure).

The third effect is associated with a reduced deposition rate of the dielectric layer on the silver surface. This effect is probably caused by a low nucleation rate of the dielectrics on the metallic surface. Moreover the metallic film acts as a barrier layer for the columnar growth and the progressive increase of the grain size. Thus a different growth was obtained in the testing zone with respect to the area of the complete filter structure. The knowledge of this effect is very important for an accurate realization of the filter structure and it must be precisely evaluated and compensated.

The displacement of the peak wavelength along the component surface is obtained by changing all layer thickness, except for the metal layer, according to Eq. 1. To deposit the graded thickness, a proper masking apparatus must be inserted inside the deposition system. The method that was used in this work consists of a shaped mask that is positioned between the material source and the substrate. The mask is periodically moved during the deposition process so that the substrate surface under the mask, is exposed to condensing particles for a period of time proportional to the aperture of the mask itself, over each zone. The mask was prepared from a 30-μm thick stainless steel foil, which was glued on a 1 mm-thick stainless steel holder as shown in Fig. 5. The substrate is moved beneath the mask window and very near to it (<100 μm). Both, the low mask thickness and the small distance between mask and substrate, lead to minimization of the shadowing effects. The mask contains two shaped windows for the two dielectric materials and one rectangular window for the silver layer. The trapezoidal shape of the window is designed starting from the required thickness profile and appears as in Fig. 5. The larger side will correspond to the maximum required thickness, the smaller side will give the minimum required thickness. As mentioned above, for each material the mask is different. The third window shown in Fig. 5, with a rectangular shape, gives the constant thickness of the metal layer.

Fig.5.

Photo of complete mask

The optical monitoring is made at the non-variable thicker side of the mask window. The optical measurement of masked sample can be carried out after completion of certain cycle. Typically, the cycle period is set from tens of seconds to few minutes, that is a big disadvantage in respect to monitoring of uniform coatings.

The deposition process is never interrupted during the sample movement, otherwise unwanted thickness steps would be created on the filter structure. The deposition controlling algorithm is adapted for this requirement.

A linearly variable filter, fabricated with the mask of Fig. 5, is shown in Fig. 6. The variable part and the two uniform areas of the filter can be clearly seen. The thicker uniform area, where the silver layer was excluded from the structure, was used for optical monitoring.

Fig.6.

Metal-dielectric variable filter (the peak wavelength is moved in the vertical direction)

3.3

Filter characterization

The measurement apparatus used for the filter characterization [7] acquires the sample map ‘at-once’ with a CCD camera, spatial resolution being given by the pixel size. The spatial resolution of the system is 30μm, the spectral resolution is 0.1nm, in the range 400-1000nm.

A sketch of the CCD-based setup is shown in Fig. 7 and consists of three main parts. The first is the Acquisition System, which includes electronics, detector, dedicated software and PC used to acquire the signal collected by the CCD TH 7890 (512 x 512 pixels, 17μm pixel size). The second one is the Optical Ground Support Equipment Control System (OCS) consisting in its electronic, software and PC used to govern the “stimuli” optical subsystem constructed for uniform illumination with a quasi monochromatic light. The wavelength range is determined by the source choice which is currently a Quartz Tungsten Halogen lamp. The third is the Optical Test Set-up that is the optical system used to illuminate a sample and to collect the radiation passing through it. This optical subsystem works by imaging the diffuser surface on the sample surface. Thus, this part of the optical bench realizes the “sample illumination” function. The specimen surface is then optically relayed on the CCD active surface, which collects the signal. The collecting optical system is the objective Xenoplan 1.9/35 – 0511 @ Schneider that relays the sample surface on the CCD. This lens is mounted in a Finite-Finite configuration in order to match the sample size to the CCD sizes.

Fig. 7.

Schematic representation of the CCD-based optical bench

An example of the image of the filter on the CCD detector, when illuminated at different wavelengths, is shown in Fig. 8.

Fig. 8.

Image of the filter when illuminated at 500nm and 700nm (the peak wavelength is moved in the horizontal direction)

The transmittance curve of the filter, in the whole spectrum of interest, is reported in Fig. 9 as taken at different positions over the surface of the filter itself, along the direction of the variation. Finally, in Fig. 10 the variation of the peak wavelength along the filter surface, over a 10mm distance, is shown. The linearity of the peak wavelength variation can be noticed. The resulting gradient is 60nm/mm, as required for this application, however this value can be increased by modifying the shape of the mask in the deposition system.

Fig. 9.

Transmittance curve of the variable filter at different positions along its surface

Fig. 10.

Gradient of the transmission peak wavelength along the filter surface