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.

Challenge

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.

Our Solution

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.

Schematic illustration of the process of laser induced back side irradiation of a glass substrate to compensate for the stress induced by thin film coatings on the front side. And microscope image of the structured back side of a glass substrate (source: Dr. Clemens Beckmann, Institut für Nanophotonik Göttingen e.V.)

Advantages

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