The Role of Sapphire Viewports in UHVs and HVs

In several ultra-high vacuum and high-vacuum (HV/UHV) processes, the processes must be observed in-situ. But one difficulty is that an optical component should have the ability to enter the hermetically sealed chamber but without affecting the vacuum’s quality.

Engineering has been used to create particular flanged viewports and windows that will enable the vacuum processes to be observed directly or with the help of an internal mirror array.

This kind of method eliminates the risk of loss of pressure. The quality of the materials utilized in the gasket and flange mainly governs this mechanical performance.

Sapphire viewport

Reliable stainless steel alloys like type 304 are generally used for viewport flanges, whereas high-purity copper (Cu) is believed to be the perfect gasket material in UHV conditions. But an extensive variety of materials are available for producing the transparent pane, ranging from standard borosilicate glass to synthetic sapphire.

This article comprehensively describes why and in what circumstances a sapphire viewport can be utilized in HV/UHV applications.

Outlining Sapphire Viewports

Sapphire is one of the toughest optical materials used in engineering applications. It is transparent single-crystal aluminum oxide (Al2O3) and exhibits superior thermomechanical properties when compared to virtually every other window material utilized in HV/UHV processing.

A flexural strength of above 400 MPa and a compressive strength of about 2000 MPa are the standard mechanical properties of sapphire.

Sapphire viewpoints are known to be notoriously hard and leverage the material’s exceptional Young’s modulus (~350 GPa), which helps to ensure that the stress-strain ratio of the pane is perfect for operation at pressures of an order of trillionths of the atmosphere.

Sapphire viewports are also appropriate to implement high-temperature vacuum processing applications, such as physical vapor deposition (PVD). It has been shown that the pane can consistently tolerate constant operating temperatures in the region of 400 °C (752 °C). However, this limit is a restraint of the chamber architecture. Only sapphire can withstand temperatures of up to 1800 °C (3272 °F).

In spite of this, there is a wide range of alternative viewport materials that can work in applications that are analogously pressurized and associated with thermal processing.

Sapphire has superior optical transmissivity over other window types and this feature is considered its main advantage.

Sapphire viewports have wavelengths of light ranging between 150 and 5500 nm. They are highly transparent and cover a great deal of the visible and ultraviolet (UV) spectrum with exceptional extension into the near-infrared (IR) range. Optimal viewing of HV/UHV processing conditions is guaranteed, without the need for additional surface coatings.

The unique mechanical properties of sapphire are a major contributor to such a remarkable quality of transmissivity, considering that wavelength transmission can be substantially affected by substandard surface finish, specifically for short-wave radiation.

Sapphire is considered the third hardest engineering material on the Earth and offers exceptional resistance to abrasion and scratching. Due to their hardness, sapphire viewports can maintain their as-installed transmission characteristics for a long term in harsh processing settings.

Sapphire Viewports from Allectra

Allectra specializes in designing and supplying these HV/UHV components. Sapphire viewports from Allectra are available in diameters ranging from 15 to 49 mm, thus offering both narrow and wide fields of view into thermal processing chambers. Each one of Allectra’s available sapphire viewports can also be DUV-rated for the broadest and currently available transmission range.

This information has been sourced, reviewed and adapted from materials provided by Allectra Limited.

For more information on this source, please visit Allectra.


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