Client – Semiconductor R&D

Application requirements:

• 0.6” view fused silica UHV viewport on 2.2” UHV Flange (metal seal, “helicoflex”)
• UHV Viewport must transmit laser, and be rated to operate at 1500 psi & 200 C (Temperature rating limited by AR Coating)
• Pressure test to failure at minimum 6000 psi to confirm 4X safety-factor

Critical Issues:

• The system required multiple UHV viewports for transmitting pulses of a high powered laser into a pressurized system, but prior to operation the system required ultra-high vacuum evacuation and bake-out cycle for cleanliness.
• The internal environment included numerous gases, pressure from UHV to 1500 psi, and temperatures to 200 C.
• Standard UHV viewports used in laser applications were not suited for the gas environment, or the extreme internal pressure. The flange system was also unique, so the UHV viewport design must be mounted in the “helicoflex” style flange with a metal gasket.

Design Basis:

• MPF designed a weldable Fused Silica window sub-assembly, which was resistant to corrosion in the specific gases being used. The material grade of Fused Silica and AR Coating applied to the window allowed for optimum transmission of the laser.
• The high internal pressure load combined with UHV/Bake-out procedures required a specialized flange mount. The sapphire sub-assembly was welded into the custom flange mount and mechanically supported for safety under high-pressure operation.
• The custom window sub-assembly and final assembly required special fixtures for brazing, welding and pressure tests. Pressure test requirements specified testing to 4X expected load (or 6000 psi). Actual tests confirmed failure between 7000-8000 psi, which exceeded the requirements.