Sample Environment - Cryogenic Vacuum
Cryogenic vacuum is obtained in a cryo-cooled volume after reaching the condensing point of the used exchange gas or left over gas particles. Values below 10-15 mbar or even lower can be reached - depending on the specific environmental conditions.
The cooldown time is defined as the period between starting the automated cooling process and reaching the guaranteed base temperature. Where pumping is also automated the time is specified for an environment with low humidity after the second cooldown. Depending on the specifics of the cryostat, microscope and magnet configurations, the cooldown time can deviate from default values.
Cryocoolers perform periodic compression and expansion of helium to remove heat from the system to be cooled. Depending on cooling power requirements, different working principles like pulse-tube (PT) and Gifford-McMahon (GM) are being used.
High Magnetic Field
Magnetic field can be generated by different means. From permanent magnets to dedicated pulsed hybrid magnets. In order to maximize the available magnetic field strength and ease of use, attocube uses specially designed superconducting magnets in all cryostats. Since the actual magnetic field strongly depends on the specific geometry, please contact our sales representative for the available options.
The temperature stability of a cryostat is specified at 1K above base temperature with active temperature control. It is defined as the maximum deviation from the temperature setpoint in a 10 minute period. Its actual value can be influenced by the chosen microscope or setup.
Sample space of a cryostat is the space designed to hold the experimental setup without affecting its cooling performance. A larger sample space is desired because it allows more complex measurements and hence makes later upgrades possible.
Cryogen-Free; Dry Cryostat; Closed Cycle Cryostat
Closed cycle cryostats, also called dry cryostats, helium-free or cryogen-free cryostats, do not require liquid cryogens (like He or N2) for cooling. Instead, helium is contained in a closed circuit, and undergoes a cyclic expansion and compression. There are two different established designs, the pulse-tube coolers, and the Gifford-McMahon (GM) coolers.
Noise is the standard deviation σ of all additionally captured random values made during any static or dynamic ongoing measurement at a specific bandwidth.
The stability is the change in bias over time.
Quotient of the change in an indication of a measuring system and the corresponding change in a value of a quantity being measured.
The angular alignment tolerance represents the target‘s tilt range in respect to the laser beam.
The focal length F is the distance between the front side of the sensor head and the focal point.
Distance between the end of the fiber, which is represented by the mechanical stop, and the target.
Distance between the front side of the sensor head and the target where a continuous measurement is possible.
Environmental Compensation Unit (ECU)
The ECU detects variations in air pressure, temperature, and humidity to recalibrate the refraction index.
A (Fabry-Pérot) interferometer consists of an optical cavity made from two parallel reflecting surfaces and creates interference of light by overlaying a reference and a measurement beam. By analysing the phase shift in the interference signal, the target displacement can be determined.
Using the inertia of the positioner's top plate, the plate is moved by applying a saw-tooth voltage to a piezoelectric element which accelerates a guiding rod. The top plate is clamped to the guiding rod and only moved during the slow slope but stays at rest during the steep flank.
Each size of goniometer is available in two versions which are usually used as a pair for theta (θ) and phi (φ) motion. The θ positioner mounted on top of the φ positioner forms a tip-tilt stage with a common center of rotation. Mounting is directly done with two or four screws.
Cross Mounting Rules
Following general rules apply for building multi-dimensional setups:
- A positioner with a lower number should not be used to support one with a larger number, e.g. an ANPx51 should not carry an ANPz101.
- Cross-mounting between two differently sized models (e.g. a 51 series positioner on top of a 101 positioner) may necessitate an adapter plate (http://www.attocube.com/attomotion/accessories/adapter-plates-aap/ )
- All bearing-based positioners (ANPx3*1 series) can be mounted on a L-bracket which enables vertical positioning.
Merge Nanopositioning Stages to Multidimensional Systems
The modular concept of attocube positioners in combination with a consequent use of similar mounting patterns enables the assembly of multi-axis positioning units composed of several different types of nanopositioning stages. By merging several positioning units with distinct travel ranges and motion options, motor assemblies with up to six degrees of freedom can be built.
The precision is the standard deviation σ of a measurement. Its value is strongly related to the system’s noise level, its repeatability and – over duration – its stability.
Accuracy is the deviation of a device's measured displacement value compared to the true displacement of the target. Accuracy is defined as a percentage of the measured range.
The measurement resolution is the smallest increment of displacement that the sensor can show.
Optoelectronic Encoder (/NUM, /NUM+)
The usage of a glass grating and the interpretation of the generated Moiré pattern characterizes the working principle of the /NUM and /NUM+ encoder. The measurement refers to the relative sample position with a position resolution of 1 nm and a repeatability of typically 50 nm for most linear stepping positioners. An absolute position information is also available via a reference mark. The /NUM+ features a reduced thermal dissipation of only 50 mW making it especially suited for /HV and /UHV positioners. The /NUM and /NUM+ encoders are available for our ECS series positioners.
Resistive Encoder (/RES, /RES+)
A resistive encoder (/RES) is used for our nanopositioners of the ANP series. The working principle of this encoder type is based on a potentiometer. It is the method of choice for applications at cryogenic temperatures, ultra high vacuum and highest magnetic fields. The /RES encoder measurement refers to the absolute sample position, for most linear steppers a repeatability of 1 µm is achieved. For ultra low temperatures (T < 1 K) a special /RES+ sensor is available which is included in all our /ULT models.
Most of attocube positioners are available with optional encoders for closed loop control. The encoder allows to read back the actual position, while a feedback loop integrated into the corresponding electronics is used to minimize the difference between target position and actual position. Setpoints can either be defined in a software interface (ANC350, AMC100) or on the front panel of the closed loop electronics (ANC350).
Closed Loop Positioning
Positioners with an integrated encoder (/RES, /RES+ or /NUM, /NUM+) can be used for closed loop position control. A feedback loop integrated into the control electronics minimizes the difference between target position and actual position. Setpoints can either be defined in a software interface or on the front panel of the electronics (see electronics chapter).
Open Loop Positioning
The positioner is simply driven forward or backward, without an encoder reading the actual position.
attocube positioners of the ANP series (i.e. ANP, ANR, ANG and ANS) are built of completely non-magnetic materials such as titanium and ceramics. They can therefore be used in environments sensitive to magnetic materials, close to electron or other charged particle beams. They are especially dedicated for the use in high magnetic fields (currently maximum tested field: 35 T).
Ultra-High Vacuum (/UHV)
The ultra-high vacuum range is specified down to 5 ⋅ 10-11 mbar for most positioners. A few rotators and goniometers use UHV compatible grease. Due to the increased outgassing of these types at elevated temperatures, we specify them for 1 ⋅ 10-9 mbar as a precaution (noted in specification sheets). All UHV positioners can be baked out up to 150 °C.
High Vacuum (/HV)
The high vacuum range is specified down to 1 ⋅ 10-8 mbar.
Room Temperature (/RT)
/RT positioners are manufactured for use at ambient conditions (room temperature, dry atmosphere, ambient pressure).
Ultra Low Temperature (/ULT)
For measurements going beyond 1 K, these positioners come (if desired) with a special resistive sensor and are made from copper beryllium (BeCu) instead of titanium.
Low Temperature (/LT)
These positioners are suitable for repeated cooling and operation in cryogenic temperatures down to 1 K.
attocube has different positioner models for various temperature needs. All open loop positioners can be used in a temperature range of 0 °C to 100 °C in a non-condensing environment. For temperatures lower than this range, special versions (/LT, /ULT) are available. Due to the intrinsic nature of the semiconductors used in our precise optoelectronic encoder the /NUM sensor readout will only work up to a temperature of 50 °C (55 °C for NUM+). All UHV positioners can be baked out up to 150 °C.
Stepping positioners are suitable for travel ranges in the mm range. They are driven via slip-stick principle (sawtooth voltage with an amplitude up to 70 V at LT).
Scanners are suitable for travel ranges in µm range and their motion is continuous. The scanners are recognizable by the name „ANS“.