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Understanding Vacuum Compatible Stage Specifications
Introduction
'Kohzu vacuum-compatible stages' are precise and durable positioning instruments ideally suited for the extreme requirements imposed by high-vacuum environments. With few exceptions, our vacuum-compatible stages exhibit performance characteristics comparable to those of Kohzu standard motion products. Kohzu vacuum-compatible stages are manufactured, assembled, inspected and packaged under highly controlled conditions. Since proper material selection is at the core of vacuum-compatibility, Kohzu engineers only specify low outgas metals, plastics, lubricants and components... construction materials are limited to aluminum, stainless steel, and phosphor bronze.
Kohzu's years of developing customized vacuum-compatible solutions for synchrotron radiation research, semiconductor lithography and space development applications have yielded a vacuum-compatible motion instruments product line of unsurpassed quality, reliability and performance. Ultrahigh vacuum oil-free lubrication as well as high-precision linear and angular encoders can be provided on request. Please feel free to contact us for all your high and ultrahigh vacuum needs.
Vacuum Test Results
- Ultimate System Pressure
- Residual Gas Analysis
- Outgas Rate
A small chamber was prepared to conduct these three vacuum tests
Vacuum chamber is first tested empty and then after introducing Kohzu's MVSA07ART vacuum-compatible swivel stage.
1．Ultimate System Pressure
2．Residual Gas Analysis
Quadrupole mass spectrometer detects mass to charge ratio (m/z), where most elements detected are hydrogen, water, carbon and nitrogen compounds. Additionally trace amounts of hydrocarbon and carbonate fluorine are detected and directly attributed to the vacuum grease used to lubricate our stages. The partial pressures of trace contaminants represent a small contribution to overall system pressure. However, outgassed contaminants will increase as stage temperature rises. If ultrahigh vacuum conditions are required, we offer customized oil-free stages fitted with materials and lubricants suitable for baking. Kohzu can address all of your vacuum requirements, including those for UHV motion applications.
3．Gas Emission Velocity (Orifice method)
A partition is installed between the test sample chamber P1 and base chamber P2 that is connected to the exhaust system. The partition has fine holes that generate a pressure difference between P1 and P2. By measuring the pressure of P1 and P2 and examining the pressure difference, the gas emission velocity from the sample can be obtained. Since emission and adsorption occur also on the surface of the vacuum equipment, first perform measurements with empty chambers for control data. Calculate the net gas emission velocity from the sample by subtracting the control data.
Important Design / Usage Considerations
Vacuum Chamber
Consideration must be given to vacuum vessel content, content outgas rate and the system's pumping conductance.
Vacuum Handling
Minimize exposure of vacuum-compatible stages to atmospheric conditions. After unpacking, install vacuum-compatible stages into a vacuum vessel as soon as possible.
Motor Heating
Heat conductivity and dissipation are significantly reduced in a vacuum environment. This will lead to over-heating of in-vacuum stepper motors if operated continuously for long periods of time. Motor overheating will lead to a rise in system pressure caused by the evaporation of motor-bearing lubricants. Evaporation of motor-bearing lubricants can in turn lead to premature bearing failure. Furthermore, dimensional instabilities produced by thermal gradients and different coefficients of thermal expansion will occur within positioning mechanics.
- Fig. 3 Pressure vs. Time (while cycling motor)
The left graph depicts pressure change over time as a vacuumcompatible stepper motor is powered 'On' and 'Off' inside a vacuum chamber. According to the data collected, after starting a current flow to an in-vacuum motor, system pressure will increase until reaching a saturation point. Subsequent 30 second power-cycles produce temporary, but noticeable pressure bursts that, almost immediately, return to original pressure levels. Repeated cycling of the motor over a period of 25 hours gradually improves system pressure to pre-cycling levels.
To improve or maintain minimum system pressure...
- Cycle in-vacuum motors as previously described
- Maintain lowest possible motor holding currents
To improve or maintain maximum positional accuracy...
- Reduce motor heating by minimizing power 'On' cycles and holding current
- Introduce thermal insulation between motor and motion mechanics
- Add optional, vacuum-compatible, linear or angular encoders with feedback control*
*Some Kohzu stages can not be fitted with insulating spacers or encoders. Please contact sales for more information on stage options and functionality.