Compensating film stress in thin silicon substrates using ion implantation

Area of Science:

• Optics and Materials Science
• Aerospace Engineering
• Surface Science

Background:

• Future X-ray space telescopes require high optical throughput, necessitating thin mirrors.
• Thin mirror fabrication is challenging, and reflective coatings introduce intrinsic stress, deforming the mirrors.
• Reducing coating stress often compromises reflectivity, creating a design conflict.

Purpose of the Study:

• To demonstrate ion implantation as a Non-uniform Integrated Stress Compensation (NISC) method.
• To compensate for stress induced by chromium reflective coatings on silicon wafers.
• To restore thin mirror substrates to their pre-coating shape.

Main Methods:

• Utilized ion implantation to introduce controlled stress on the backside of silicon wafers.
• Applied 30 nm thick chromium films to the frontside of five silicon wafers.
• Measured mirror deformation caused by coating stress and assessed restoration accuracy.

Main Results:

• Chromium films induced significant mirror deformations ranging from 400 to 1600 nm RMS.
• Ion implantation successfully compensated for the coating-induced stress.
• Restored wafer shapes achieved precision within 60 nm RMS, often 1/20th of the initial deformation.

Conclusions:

• Ion implantation is a viable NISC technique for thin mirror fabrication.
• This method decouples film stress from reflectivity, enabling high-performance optical coatings.
• Achieved precise shape restoration crucial for advanced space telescope mirror applications.