Figure correction of glass substrates
Unintentional deformation of optical substrates is a common issue in optics technology. The mechanical stress in thin films can have a deleterious effect on the quality of the optical components by deforming the underlying substrate. This patent describes a figure correction method bases on stresses induced by irradiation of glass substrates with an excimer laser.
Thin films play a significant role in optical technologies, as they can be used to alter the optical properties of a surface. In general, thin films exhibit tensile or compressive mechanical stress after deposition, which can lead to a significant deformation of the underlying substrate. In addition, the substrate might be deformed by gravity or stress induced by mounting especially when using large aperture optics. To avoid substrate deformation by a stressed film, a thick substrate can be used. However, there are fields of application where the usage of thin substrates is highly beneficial, like, for example, in space applications. Another common approach is to counter balance the film stress by deposition of another stressed film on the same or the backside of the substrate, which can be a difficult task due to reproducibility of the film parameters and geometric constraints.
The method presented here provides a way to compensate the substrate deformation by excimer laser generated tensile stresses in the backside of the substrate. By controlling the integrated stress by varying the laser fluence, a substrate with an induced deformation by nearly any coating can be corrected. Here we suggest a method based on short time melting of the backside surface of a glass substrate with the ultraviolet light of an ArF excimer laser. By this a tensile stress can be generated at the surface. The high pulse energy and the large beam profile of the laser allow the processing of a large area with only a few laser pulses. Besides equibiaxial stress components, also antibiaxial stress components can be induced, which was shown to increase the scope of correctable deformations in figure correction methods. The value of the generated surface stress can be controlled by the laser fluence or by the local density of the laser spots.
- Highly flexible, high precision compensation of front side stress due to coatings by back side laser structuring
- Compensation of compressive stress possible (tensile with an additional backside coating)
- Also complex deformations compensable
- Flexible adaption of the irradiation pattern through software simulations
- No complex coating process on the back side necessary
- Simple and robust experimental setup in ambient air
- 1-photon process and therefore low intensities required
- Significant reduction in substrate thickness while maintaining good surface shape
- Flexible, precise compensation of stress induced by thin films like coatings
- Aerospace, where the weight of the optics is a decisive parameter
- Segmented X-ray optics, where thick substrates reduce the luminous intensity
Process successfully tested and optimized, current studies to further improve the longterm stability
Dr. Maria Kamper
Patent Manager Physics & Technology
Tel.: +49 551 30724 159