A.

Spectral Specifications

Pixelated filters can be composed through a MacroPixel with any number of filters, e.g. 2×2, 3×3 or 4×4 with a Bayer-type structure. Each filter is a bandpass filter centered at a specific wavelength with the following specifications:

B.

Geometrical specifications

Pixelated filters should be massively structured with the following specifications:

C.

Fabrication method

Structure of the pixelated filter is obtained by deposition through a mask made with a photoresist and using a lift-off technique. Typical procedure for obtaining such a filter is shown in Figure 2.

Fig. 2.

Description of the lift-off process

The lift-off technique consists in 5 different steps:

However, as can be seen in Figure 3, the use of simple lift-off technique does not allow achieving flat and squared pixel profile because walls are created on the pixel side due to deposition of a larger amount of material at the boundary with the photoresist.

Fig. 3.

Illustration of the problem of walls with standard lift-off process

This problem can be solved by using two photoresists having different physical properties and the generation of small caps that will prevent the creation of walls on the edges of the photoresist and therefore obtaining perfectly parallelepipedic pixels (Figure 4).

Fig. 4.

Two-photoresists-based lift-off process

D.

Experimental demonstration

Lift-off technique was used to demonstration the fabrication of 2 adjacent pixels of a 2×2 pattern. Pixels are bandpass filters with different central wavelength. Typical specifications for this prototype are the following:

Figure 5 shows pictures of two adjacent filters measured using a Zygo NewView 7300 optical profilometer. It is seen that each pixel has a well-defined squared shape with sharp edges. The transition zone is within 2 μm, i.e. below than 10% of the pixel size. There is no overlap between the filters and each pixel displays a flat top which means that uniform properties are expected over the pixel aperture.

Fig. 5.

Spatial profiles of two adjacent 30×30 μm² filters measured using a Zygo NewView 7300 optical profilometer

A.

Description of the measurement system

Each fabricated pixel is in the range of 50 to 2500 μm², therefore, local spectral properties cannot be easily characterized with regular spectro-photometric techniques for which the minimum aperture of the measurement is generally within 1 mm². A custom setup was developed within the Institut Fresnel about 10 years ago to allow mapping of the spectral properties of filters over their aperture [2]. This setup is compatible with measurement of the spectral properties of one pixel, but only gives access to the integrated properties of the grating over the pixel aperture. Therefore, it is of prime interest to be able to carry out mappings of the local spectral properties of one pixel with micron lateral resolution and a nanometer (or better) spectral resolution. To achieve a custom optical setup (SPHERE) was developed [3]. This system uses a white light source associated with a double monochromator (Gemini-180 from HORIBA Jobin Yvon) in order to generate a spectrally tunable light source. This source is then imaged in the pixelated filter plane using plane apochromats with a normal incidence and an F-number of 10. A detector placed just before the sample measures the incident power and therefore corrects for its temporal fluctuations. Finally, the pixelated filter is imaged in the plane of a CCD camera (Figure 6).

Fig. 6.

Scheme of the SPHERE measurement setup

The use of a multimode fiber and a narrowband light source induces, in the measurement plane, an inhomogeneous intensity distribution and speckle. To overcome this problem, deformable mirror and a 200 × 200 μm² multimode squared core fiber were inserted right after the monochromator in order to achieve uniform exposure at any point along the beam path (Figure 7).

Fig. 7.

Intensity distribution on the CCD camera at 600 nm. First line was obtained with a circular core fiber while the second line was obtained with a squared core fiber. Left column was obtained without deformable mirror while the right column was obtained with a deformable mirror.

Using this system, it is therefore possible to perform mapping of the local spectral dependence of pixelated filters with a spectral resolution of 0.5 nm and a spatial resolution of 2 μm (resolution on camera is 0.5 μm, but actual resolution is 4 times higher due to diffraction).

B.

Characterization of prototypes

Using the SPHERE setup, the prototype presented in Figure 5 was characterized. Figure 8 shows a mapping of the transmitted intensity over the pixelated surface at 400 nm. One can see a periodic structure with bright transmitting pixels, dark reflecting pixels and some lines without pixels (those one will be deposited in a future run). It is clear that each pixel that had nominally the same deposited structure show identical performances.

Fig. 8.

Mapping of the transmitted intensity of the pixelated filters at 400 nm

Also, it is interesting to note that since wavelength can be scanned from 400 up to 1000 nm, it is possible to obtain the same picture for each wavelength and therefore locally characterize the spectral performances of the filter.

To illustrate the spectral performances of one of the pixelated filter, Figure 9 shows the transmitted intensity at 985 nm over one B2 pixel surface as well as the spectral transmission curve of the filter averaged over 5×5 pixels. It is seen that the filter presents a broad rejection band from 500 up to 900 nm (except for a narrow line around 870 nm) and a narrowband resonance around 700 nm.

Fig. 9.

Mapping of the transmitted intensity over one B2 filter centered at 700 nm

To simultaneously evaluate the performances of the measurement system and the pixelated filter, the spectral performances measured on the pixelated filter (Figure 9) were compared with the spectral transmission measured on the witness sample (25 mm diameter) with a Perkin-Elmer Lambda 1050 spectrophotometer (Figure 10).

Fig. 10.

Comparison of the spectral transmission of the pixelated filter measured with the SPHERE setup and of the witness sample (25 mm diameter) measured with a Perkin-Elmer Lambda 1050 spectrophotometer.

Figure 10 shows that very similar performances were measured on the pixelated filter and the witness sample, confirming that both manufacturing and characterization techniques of pixelated filter allow obtaining high performances’ filters.