2.1

Manufacturing approach

The manufacturing of is done by a subsequent cycle of patterning and thin film deposition steps. Typically, the manufacturing is done on wafer level and the filter arrays are diced to their final geometry. The process is carried out under clean room conditions.

For space applications, the wafers are made of radiation hard optical glass such as fused silica or BK7G18. Their size can range from 4 inch up to 8 inch diameter. Also, the manufacturing on CMOS wafers is possible. The patterning starts with the spin coating of a photo sensitive polymer (photoresist) onto the whole wafer surface. A dedicated mask with a defined pattern is then placed over the wafer during illumination with intense UV light in a mask aligner. Areas exposed to the UV light can then be removed in a developer solution leading to a resist pattern on the wafer surface. The thin film coating is then applied on the whole wafer surface. In the following lift off process the photoresist is removed from the wafer surface and the coating only remains on the wafer in areas being not covered with resist. When repeating this sequence several times, it is possible to deposit a thin film coating on dedicated positions of the wafer defining the final filter array. Number of patterning steps depends on the number of different coatings needed for black mask definition and number of filter channels. The minimum feature size for this approach is in the range of 20 μm – 50 μm depending on coating properties. A sketch of the manufacturing flow is shown in figure 1.

Figure 1

Manufacturing sequence for monolithic patterned filter arrays; from [3].

2.2

Filter arrays with coatings by IAD technology

Traditionally, patterned coatings are manufactured by means of ion assisted deposition (IAD) technology due to parameters such as process temperature and interaction of plasma with the photopolymer. A detailed description can be found in [3]. For the deposition of complex coatings to meet stringent spectral requirements coating chambers are equipped with a broad band optical monitoring system [4]. A good process control is essential for sequential coating steps.

The types of filter arrays manufactured with IAD coatings can range from order sorting filters with coating dimensions in the mm range [5] to seven band filters with a stripe width of 100 μm only [6]. Smallest filter dimension manufactured for space applications is an 1024 × 1024 pixel array with four different color bands and a filter size of 18 μm × 18 μm each. For this, radiation hardness was proofed [7].

2.3

Filter arrays with coatings by PARMS technology

With increasing demands on image resolution and signal to noise ratio of multispectral instruments, requirements for spectral performance of optical coatings become more and more stringent. Especially in-band transmittance and out-of-band blocking are requested. For complex coatings by IAD technology the in-band transmittance for narrow band pass filters (FWHM of 20 nm) is limited to about 85% [8]. This is caused by the intrinsic coating roughness of IAD layers. To overcome this, the high-energy deposition technology of plasma-assisted-reactive-magnetron-sputtering (PARMS) [9] is used. This technology allows the manufacturing of complex coatings with low roughness and low scattering resulting in increased in-band transmittance well above 90% and a possible lower out-of-band signal in OD6 range [10]. To implement the PARMS coating into the lithographic patterning process an adaption of the process was necessary to account for the different process conditions compared to the IAD process [11]. With this accomplished, the manufacturing of monolithic filter arrays with sophisticated spectral performance is possible.

Figure 2 shows the comparison of achievable performance of the same filter channel coated by IAD and PARMS technology. A significant increase of in-band transmittance as well as out-of-band performance can be found.

Figure 2.

Spectral measurement of in-band (left) and out-of-band (right) spectral performance of a monolithic filter arry with coatings by PARMS technology (blue) compared to IAD technology (black).

Both approaches are eligible for use in space environment.

3.1

Manufacturing approach

For a butcher block, the coating deposition has all degrees of freedom. Normally, coatings are applied on wafer or plate level for each spectral band needed. After coating deposition, the wafers are diced into sticks with the desired width and not necessarily final length. Filter width can range from mm range down to 25 μm. The sticks are now assembled with an opaque glue between the individual sticks. This thickness is typically 15um an offers OD5 blocking. Number of assembled sticks can range from 2 to 150; or more if required. On top of the assembly, it is possible to apply a black mask for channel separation. In a final step the assembly is cut to its final size.

Figure 3.

Sketch of the manufacturing approach for butcher block filter arrays.

The assembly is also possible for rectangles in a checkerboard pattern, for different substrate materials, and for different thickness of the individual parts. Manufactured examples are shown in figure 4.

Figure 4.

Images of manufactured butcher block arrays with checkerboard pattern (left) and filter stripes (right).

3.2

Butcher block filter arrays

For deposition of coatings for a butcher block assembly no constraints exist comparable to the monolithic arrays due to the lithography process involved. All types of coatings techniques can be used for filter deposition no limit of filter thickness exists. It is even possible to manufacture arrays with filter channels covering UV, VIS, NIR, SWIR, MWIR, and LWIR spectral ranges.

Figure 5.

Exemplarily curves of band pass filters in infrared spectral range.

Due to the variability of the butcher block process, it is possible to manufacture arrays for use in hyperspectral applications. A manufactured example can be seen in figure 6.

Figure 6.

Image of a manufactured hyperspectral butcher block filter array.