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Challenges to Hyperspectral Applications

Material Properties Make the Difference

With thin film industry growth being driven by increasingly complex optical/microelectronic devices, as well as intense photonics and hyperspectral imaging, it may be a good time to take a closer look at properties of materials and why enhanced processes are often critical for the most sophisticated applications. Following, we will address some of the key substances.

Silicon Dioxide (SiO2)
We will first examine the most critical low-index material in a classical optical stack – Silicon Dioxide (SiO2). For evaporation or for sputtering, SiO2 can be crystalline or amorphous in nature – and very easy to overlook when considering expanding capabilities outside of the VIS spectrum.   While it is true that physical vapor deposition (PVD) relies on a small group of mature silica products, the role of water and reaction contaminants is the core difference between synthetic SiO2 and Melted/Fused SiO2.  If you observe your SiO2 granules and compare them to your fused silica (FS) substrates or targets, you will note a series of differences. Beyond the simple fact that the substrate should be far more transparent than irregular granules, the pieces may range from clear to cloudy to the naked eye. Additionally, there are even more dramatic factors and lingering effects.

Synthetic SiO2
Whether from a gas or liquid reaction, Synthetic/CVD SiO2 is the lowest cost and most widely used variety of optical coating material. In stark contrast to the early days of synthetic SiO2, today’s materials perform very well from the VIS into the NIR region. Evaporation granules are typically polycrystalline, equiaxed crystals - looking cloudy due to intergranular cracks and surface facets. This material saw great maturation during the telecommunication fiber and filter days where E-beam and IAD processes produced high performing optical components once reserved for sputtering.  Though now mature, the required evaporation rates pushed synthetic granules to their thermal limit allowing them to facture along crack lines or explode and release trapped gases. To combat absorption and fluorescence in UV and IR wavelengths, sophisticated processes can use this type of silica as feedstock for melted products like Fused Silica (FS).  Even then, many grades exist since Sodium, Chlorine and Water (-OH) from the formation process may cause synthetic source material to melt. This poses a key challenge in the substrate and target supply chain. 

“Quartz”
The next grade of SiO2 is customarily called “quartz.”  The name Quartz can be misleading and can apply to crystalline, polycrystalline or amorphous variations, so confirm which type with your vendor.  When you use the name “quartz,” it implies crystal structure but it is also used for melted amorphous SiO2 “quartz glass” which has sand as its precursor. With improved UV and IR performance, optical grade quartz has become common as clear granules, targets or substrates. The amorphous or crystalline quartz blanks are crushed to make granules but can also be machined for windows, substrates and targets.  The largely transparent granules tend to sparkle under ordinary light to the naked eye. They may be irregular (amorphous) or sharp (crystalline) and due to lack of micro-cracks or pores, this material has a finite rate limit before particles can be ejected making whiskers/streamers or embedding in the growing film. 

Because of their geological origin (as mined sand), even the finest quartz products can have variable performance in the 190-220 nm and 1.9-3.5 um regions. Advances in melting technology are employed to customize UV or IR grade quartz targets. Quartz evaporation material has a similar advantage as FS over conventional optical grade silica. The most common fused quartz designations for sputtering are GE 124 and JSG2.  Some sputtering processes can utilize this grade of material for wavelengths outside the substrate specification designation.  

Fused Silica (FS)
The final grade of SiO2 for coatings is fused silica.  While quartz is melted (fused) as well – FS retains its name as “fused silica” when there is a lack of any natural sand origin.  Strictly speaking, fused silica may come from natural or a series of chemical routs. Specialty chemistry, drying or melting techniques may be employed to satisfy the different markets. It is possible to customize attributes with the FS substrate. One example: Great care and expense goes into making the highest quality homogenous material so it can transmit above 90% wavelengths, ~250nm shorter than optical glass (<90% at 450nm) in the UV region.  Additional refinement to FS can be employed to extend the IR performance in the 1-3 um region with special care to 946, 1064 and 1319nm absorption. While the IR performance is strongly linked to (-OH) radical retention, the UV performance is more sensitive to reaction precursors or inclusions.  

In certain cases, even in evaporation processes, the design may require a FS source material. For high performance, low particle designs, FS granules or a window/starter charge can be used but also must be run at slower deposition rates than CVD granules.  Being a solid mass versus a crucible of particles, burn-in and rate are very important constraints.  Where a higher rate is needed and the main issue is pocket stability or exploding particles, employing FS granules can outperform the more common synthetic SiO2.  Due to issues with crushing FS, it is common to draw the high purity FS into rods and then crush them into fairly uniform clear granules. The slight curvature to the pieces can help mitigate in pocket shifting, a problem common with ebeam evaporation. While more sophisticated options exist, new multispectral applications require the use of FS plate of different grades machined, polished and mounted as sputtering targets. 

Re-examining Silica 
In recent years, reactive Si sputtering and IAD E-beam deposition of the full oxide have dominated the optical thin film industry. There are increasing cases where stress and temperature (sputtering) or particulates, fluorescence and absorption (ebeam) require that the full oxide be used. With growth markets in UV, Intense VIS and IR, companies are returning to a changed Silica market for answers. In order to navigate the new landscape of opportunities, Materion offers different grades of Silicon Dioxide Evaporation and Sputtering Targets.  

This article has taken a look behind the scenes and reviews the forces that shaped the supply world of today. Certainly for substrates, but also for targets and evaporation processes, the different grades of Silica are initial screening tests for different parts of the EM Spectrum.  With many more highly specialized Silica variations available for windows than for targets, a majority of coating is dominated by a couple of sources with some important differences.  Hopefully, this article starts the discussion on how some of these differences may limit or at least challenge intense or multispectral applications. 

Materion offers a complete line of targets and evaporation materials for hyperspectral applications. Contact your Materion Advanced Materials group Sales Representative or for concerns/questions, David A. Sanchez, Sr. Materials & Applications Scientist, David.Sanchez@Materion.com.