The trends in X-ray optical devices are towards deeper optics and/or shrinking dimensions, this includes zone plates, refractive lenses, multilayer Laue lenses, curved mirrors and multilayers.
X-ray Lens Production
Diamond is a desirable material for use in optical instruments because of its exceptional properties, including low X-ray absorption and thermal resistance. However it's extreme hardness, high manufacturing cost, and resistance to chemical attack make it difficult to achieve the structures appropriate for X-ray lenses with diamond. This is the reason that silicon is the the primary material when producing X-ray lenses. The requirements of nanofocusing silicon X-ray lenses are not only material with greater quality, a controlled roughness of vertical sidewalls and high aspect ratio with the sidewalls, to reduce parasitic scattering and aberrations. This article illustrates the usage of the Bosch process DSiE to develop a process for etching silicon X-ray lenses with smooth sidewalls and good profile control.
Achieving Silicon X-ray Lenses with High Aspect Ratio
To obtain silicon X-ray lenses with a high aspect ratio, smooth sidewalls and vertical profiles, a short cycle time is employed at lower powers for less aggressive process conditions. To maintain sidewall integrity the passivation is refreshed regularly, through quick switching with controlled ion energies. Etching microstructures on samples with a high silicon exposed area causes a reduction in the etch rate due to the loading effects, which could possibly lead to the undercut of the mask due to the sidewall plasma attack on individual features (Figure 1) even when over passivated.
Carrying out the process at low pressures and with a controllable DC bias is one way of reducing the ions’ negative impact on the etched lenses. Another way is to reduce the number of ion impacts on the vertical sidewalls by shielding the lenses with sacrificial features, which can be taken off once the process is complete.
Figure 1. The negative impact of ions on the etched X-ray lens structure.
X-Ray Silicon Lens Etch Processes
Etch processes with and without the compensating features (Figure 2) were developed at the Oxford Instruments Applications Laboratory. Carrying out the etch process for the lenses without compensating features resulted in an etch depth of 50 µm with controlled scallops below 50 nm, no mask undercut, and close to vertical (89.90°) lens profiles (Figure 3). Adapting and extending the process to the lenses with compensation features resulted in etch depths greater than 70 µm with smooth sidewalls (scallops <50 nm), a vertical profile (89.94°), no sidewall damage or mask undercut, and clean surfaces in open silicon areas (Figure 4).
Figure 2. Device mask patterns: lenses with and without compensation features.
Figure 3. SEM of a lens without compensation features etched to 50 µm depth.
Figure 4. SEM of a lens with compensation features etched to 75 µm depth.
A PlasmaPro 100 Estrelas deep silicon etch tool (Figure 5) was used to carry out the X-ray silicon lens etch processes. This tool is a next-generation deep silicon etch system that delivers excellent performance and provides better control to allow highly flexible processes, ranging from applications involving higher etch rates (>25 µm/min) to nanoscale etching.
Figure 5. PlasmaPro100 Estrelas.
This information has been sourced, reviewed and adapted from materials provided by Oxford Instruments Plasma Technology.
For more information on this source, please visit Oxford Instruments Plasma Technology.