Alio Vacuum Stages - PDF Catalogues
ALIO has become a world leader in the manufacture of nano-precision vacuum stage and robotic systems. All ALIO products with exception of the air bearing systems are UHV compatible. Linear, rotary, open centre X-Y, hexapods, tripods and goniometer stages and systems are available in HV (e-7 TORR) or UHV ( e-10 TORR) with ultra low out-gassing material and components, all carefully prepared and baked.
Nanometre management of motion is a growing demand in industry as leading edge technology pursues a common trend of working on smaller and smaller scales. This trend is prevalent in many ‘positioning sensitive’ industries such as fibre optics, bio-medical, micro-machines, electronics, semiconductor, energy, optics, aerospace and synchrotrons. Research and development is of course a major area for Nano positioning.
These Nano-positioning applications have demanding requirements that are further complicated when the management of motion is in a vacuum or requiring extended travel, finer repeatability, higher speed, greater uptime but at a low cost.
The challenges of engineering and selecting components that must work together in perfect harmony to achieve nanometre precision are not to be under estimated. The drive mechanism, bearings, feedback system, motion controller, kinematic structure and the environment all need to be designed to perfection to meet nanometre precision in atmospheric or vacuum environments. The vacuums can vary from 10e-3 TORR to 10e-12 TORR, from positioning performance and to out-gassing challenges./
Material Selection
The first design decision for a vacuum motion system is the material of the bearings and structure. Vacuum motion systems are typically made from bare 6061 aluminium and 300 or 400 series stainless steel. machined aluminium but without grinding or polishing so there are not rolled pockets to trap air or contamination. Depending on the vacuum level even anodised stages can be used in low vacuum (10e-3 TORR) applications. Since most precision bearings in vacuum are made from 400 Stainless Steel the use of 300 or 400 SS for the motion system is recommended when thermal variation are part of the application or experiment. This allows for the bearings and structure to deviate at the same rate allowing the bearings to maintain pre-load.
Other common materials used for motion platforms in vacuum are steel, copper, nickel, titanium and ceramic. All of this material works well in a vacuum but some of their thermal coefficients of expansion can make for creative bearing adjustments, which may possibly reduce precision.
Drive Mechanism
Ballscrew driven stages are coupled to servomotors with linear or rotary encoders providing the position feedback, while stepper motors typically count motor revolutions for positioning. These stages are ideally suited for higher loading and velocity applications with lower resolution requirements down in the 0.5-micron range. Servo and stepper motor ballscrew stages are better suited to working in a standard atmosphere rather than in vacuum environments due to heat dissipation and the nature or their screw lubrication challenges. These motors are typically limited to lower vacuum application below 10e-7 TORR.
Linear Motor driven stages offer exceptional speed and acceleration with amazingly, millions of maintenance-free cycles. These motors are linear, three-phase brushless motors, also known as AC servomotors, where the motor coils travel over a straight magnetic track. Linear motors have high force relative to their physical size and these stages may be better suited for atmospheric operation rather than vacuum environments as they do require heat dissipation in medium to high duty cycle applications.
Piezoelectric driven stages come in basically three modes: Piezo Stacks, Walking / Screw Piezo and Linear Ceramic ServoMotors. Piezo stacks are better suited for nanometre positioning when very small motion (typically 100 microns or less) is needed. Walking Piezo use Piezo Stacks with mechanical ratcheting mechanisms, which allow for increased travel, but reduce the life and precision due to metal to ceramic contact and mechanical hysteresis. Ceramic servomotors are unique in their motion acting as a ‘spiralling’ friction motor, which allows for unlimited travel without mechanical hysteresis while still maintaining nanometre precision.
Other beneficial performance features that differentiate the piezoelectric motor driven stages include shorter settling times (typically 2ms), large constant velocity range (from less than 1 micron per second to 250 millimetres per second with less than 0.5 % variation), no drive inertia, no servo dither and no hysteresis. These stages suitable for ultra high vacuum environments (10e-10 TORR) due to the materials minimal heat generation and operating temperature range.
Position Feedback
Position feedback systems in a vacuum chamber have special designs to insure performance and no out-gassing. The styles discussed are three of many approaches but these are all tried and proven to perform at single nanometre resolutions in UHV.
Optical encoders based on reading a physical scale can resolve down to the nanometre level. Although the scale has a 20 micron pitch the signal has a sufficient signal to noise ratio to allow it to be interpolated down to the single digit nanometre (2.5nm to 5 nm resolutions depending on interpolator). These encoders work very well for most applications where cost and repeatability is important.
The next level of performance for an optical encoder is where tape or glass scale utilises a similar read head with a novel scale. Although using the same 20um pitch it is etched directly into the stainless steel of a ring for rotary applications or onto a nickel-plated Invar spar for linear applications. The Invar scale allows for ‘near laser’ precision with repeatability and accuracy due to the manufacturing technique of calibrating it with an interferometer.
The placing of the scale on invar greatly reduces thermal effect that influences the accuracy of other scales.
Further to optical scale encoders, a laser interferometer can be used to give resolutions to 38 picometre. This can provide positioning stability on a suitable mechanical system to the sub-nanometre levels. Using a plane mirror optical scheme in 2 axes also allows the Abbe error to be eliminated and the added advantage of the interferometer is that only the plane mirror will reside in the vacuum chamber.
Depending upon the required measurement a single mirror can be placed in the chamber to measure from the stage to the chamber wall or alternatively a differential measuring scheme can be employed to measure the distance between 2 plane mirrors within the vacuum chamber. This eliminates all common mode noise sources between the stage and instrument.
Precision Bearings
Mechanical bearings suitable for vacuum applications range from re-circulating ball rail, linear ball bearings, ceramic linear ball bearings and crossed roller bearings. All mechanical bearings need lubrication unless the motion duty cycle and travel are only minimal. To consistently meet sub100 nano-precision only the crossed roller and air bearings work efficiently.
Crossed roller bearings come in many grades of precision and it is important to use the highest-grade bearings to assure precision. The better quality roller bearings matched in size allow for smoother motion, less friction and less straightness deviation along the path. The most successful are the high quality 400 Stainless Steel bearings, without lubrication for low duty cycle applications, but this is not recommended for long term use or high duty cycle.
Lubrication
Vacuum compatible lubricants range from wet to dry and in the past Krytox was used with mechanical bearings and screw systems. This viscous lubricant works well for lubricating but it must be applied carefully otherwise it will cause bearings to skid and stick. It has been decided to move away from Krytox since its out-gassing affects certain experiments and the uses of dry lubricants in the form of thin films are easy to apply offering low friction and smooth motion.
There are many dry lubricants available for UHV and ALIO prefer two types: Molybdenum Disulfide and Tungsten Disulfide.
Vacuum Out-gassing Characteristics
A stage AI-HR4-2500E-UHV underwent a series of laboratory tests to determine the vacuum characteristics at Argonne National Laboratory. These tests measured the out-gassing rate and the residual gas spectrum during the test sequence. This stage was prepared with a commercial tungsten disulfide dry film lubricant on the rolling races to avoid using grease in this UHV application.
The out-gassing rate measurement used the simple rate of rise method where knowing the volume of the system and the pressure rise over a one minute time period, the out-gassing rate can then be computed. This measurement was performed at a number of points during the test sequence. The arrangement of equipment is shown in Figure 1. The manual gate valve above the turbo pump is used to perform the rate of rise measurement.
Figure 1 Test Set-up
The test sequence was performed twice, the first time with the aluminium chamber empty and second with the stage in place. This allowed the chamber background to be subtracted out from results. The order of events in the sequence is: 1) the chamber was opened
Figure 2
and the stage placed inside. Figure 2 shows the stage inside the metal sealed chamber
2) After sealing the chamber it is pumped down and data taken in the first couple hours
3) Take data after pumping at room temperature overnight
4) Start ‘bake out’ by increasing the temperature to 50 C to prevent aluminium flanges from leaking and then hold temperature for 4 hours when data is collected
5) Increase temperature to 125 C, hold temperature overnight
6) The next day collect data at the elevated temperature
7) Decrease the temperature to 50C, hold for 4 hours and collect data
8) The heaters are turned off and the system is allowed to cool overnight
9) On the last day with the system stabilised at room temperature the last data is collect. The set of data collected at each point is pressure, temperature, rate of rise data, and the spectra from the residual gas analyser.
Initial and in-process data is collected for the stage. It is the final after bake data that is most significance and it is shown below. The final pressure of the chamber and the stage was 4 x 10-8 TORR where the empty chamber was 1.8 X 10-8 TORR. The final out-gassing rate of the stage/chamber was 2.3 X 10-8 TORR-lit/sec when the empty chamber is 5 X 10-9 TORR-lit/sec. The spectra of the residual gases are shown in figure 3. Notice that there is no measurable species above the 28 peak. This system is not showing a hydrogen (2) peak since the system is largely aluminium and there is less hydrogen dissolved into aluminium as there is in stainless steel. What this does show is an absence of hydrocarbons, which are indicative of oil; based contamination and the residual gas analyser did not indicate any measurable amounts.
Kinematic Structure
Multi axis systems for vacuum are more prevalent due to the increased need for nano-science equipment. Serial kinematics works well for single, two, three or four axis systems but the design and machining of these axes is critical when reducing errors such as sine, cosine and Orthogonality of stacking the stages. In addition resonant frequency can be an issue if the design fails to be engineered for motor, bearing and material structure. On top of all these concerns are the duty cycle and the potential for thermal deviations caused by the motor and stacking the axis in a manner that increases the motion forces.
4-Axis 10e-10 TORR System
When extreme precision is needed for five and more axes the parallel kinematic solution is the best choice as the error quotient is not additive thus reducing the concern for serial kinematic sine, cosine and Orthogonality error quotient.
Parallel kinematics typically takes up less valuable space in a chamber versus serial kinematics. Hexapods with forward and inverse kinematics have velocity and path motion that can enhance motion profiles that serial kinematics may not be able to handle due to sub micron errors associated with the stacking of stages.
AI-HR2-UV Hexapod
Summary
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