Low Working Distance

The low working distance (LWD) options allow users to use external optics and achieve a working distance as low as 1mm. The components to achieve the LWD configuration include a thin vacuum window, a raised radiation aperture, and a thin radiation window. The sample can be placed close to the overhead optic. A variation of this option allows translation of larger sample.

There are two Low Working Distance (LWD) options for viewing through the top window of the Cryostation. Similar LWD options exist for each sample chamber size class. 

  1. Standard Low Working Distance: This option is an optional window port for the Cryostation which allows samples to be positioned for low temperature experiments at a distance of about 1mm from regular room temperature optical components. It may be used with or without the inner cold window, but with the inner window in place, the working distance is about 2mm. Without the inner window, the temperature may be slightly higher.
  2. Inset Low Working Distance: This option allows larger sample placement within the radiation shield, but it has a larger working distance of about 4mm. This option is useful for applications where the sample is to be scanned. Both are designed for systems with a 50mm port in the housing lid.

Below is a table summarizing these options:

  Standard LWD Inset LWD
Outer window aperture 11.7mm 20mm
Inner window aperture 6mm 20mm
Minimum working distance -1mm based on 0.5mm outer window thickness with no inner window
-2mm based on both windows in place and 0.5mm thick
-4mm based on both windows in place and 1mm gap between the housing lid and the radiation shield

If you are interested in a side low working distance option, please refer to the last section of the document. 

Standard LOW WORKING DISTANCE

The main components for this option are the outer vacuum window, the radiation aperture “brass washer”, and the radiation window shown below. The outer vacuum window seals the sample space and allows a high vacuum to be pulled within the system. The outer window is comprised of a stainless steel support which holds a smaller thin window. The support creates a large thin area which allows room temperature optics to be positioned directly above the window itself. The radiation aperture is used in a similar way with a collar and brass “washer” supporting a thin radiation window, while allowing samples much larger than the window to be positioned directly beneath the radiation window. The radiation window is held to the adaptor ring by only thermal grease to minimize the thickness of the overall window assembly.

Standard LWD Assembly: Vacuum Window and Holder (top), Brass Radiation Window and Aperture (middle), Adapter Ring to Standard 30mm Window Ring (bottom)

For certain applications, the radiation window can be removed, leaving a 6mm aperture in the brass washer. An additional heat load and higher sample temperatures will result due to the added radiation from room temperature on the sample. The exact heat load and resulting temperatures will vary depending on the sample and system setup. Whenever possible, it is strongly recommended to use the radiation window to block radiation from room temperature to the sample. The radiation aperture is bonded to the radiation window using thermal grease, which is supplied with the system. Care should be taken to maintain only a thin layer of grease between the components. The optics can be cleaned just as standard optics, with lens paper and isopropyl alcohol. The adaptor ring is installed into the 30mm window ring using thermal grease. The 30mm window ring can then replace the existing radiation window and window ring.

Below is a cutaway image of the assembly. The figure on the right is a side view showing the window on the radiation aperture nestled up into the opening below the outer window. Having small gaps between the windows and sample is key to achieving overall low working distance. 

 

 

The standard assembly for the 100-class chamber is shown below. 

Practical Use and Alignment

The operation and use of the low working distance module is comprised of adjustment and alignment of the module components to obtain the required working distance. It is very important to take care while aligning these components to ensure that they do not touch during operation. The two windows colliding could cause a leak, or compromise the integrity of the windows. As the module is designed to allow low working distances, it is important to consider the motion of the internal components within the system. This motion is due to the compression of o-rings within the system as well as the thermal contraction of internal components. This compression occurs in two stages:

  1. First, the o-rings compress when the system is initially evacuated to between 1-2 Torr; this causes the largest shift of the vacuum lid of about 1mm, so place the windows at least 3mm apart to begin.
  2. Second, the radiation shield contracts as the system cools from room temperature to its base temperature. The contraction of the radiation shield results in a small downward motion of the radiation shield. 

Downward Motion of System Components

The following table summarizes the downward motion (mm) of various system components.

System Component Sample Space Evacuated to 2Torr Thermal Contraction w/ Sample at 4K Total
Vacuum Lid 0.9* 0.2 1.1
Radiation Window 0 0.45 0.45
Sample Post 0 0.2 0.2

The system motion can be compensated for in the initial system alignment.

  1. Always start with the radiation window and vacuum window farther than the expected contraction distance. This is to avoid a collision of the optics. The distance can be adjusted by turning the radiation window ring into the radiation shield (each full rotation is ~1mm of distance).
  2. Next, evacuate the system to between 1-2 Torr and watch to see how the distance between the windows changes.
  3. Release the vacuum and adjust the radiation window by turning the radiation window holder out of the radiation shield (1 turn is equal to ~1mm of distance).
  4. Repeat the above steps until the required spacing is achieved.
  5. Next, the spacing between the sample and the radiation window can be monitored and the amount of required adjustment noted. The radiation lid can be lifted off of the radiation shield without turning the radiation window holder to allow adjustment of the sample position. In this way the entire system can be adjusted to less than 1mm working distance.

Measuring the spacing between these components is often difficult; however, one simple technique for observing the spacing is to place a small mark on each window with a marker. Typical locations for marking the windows are to place a mark on the inside of the vacuum window and on each side of the radiation window. By simple visual comparison of the spacing between the dots one can quickly arrive at an overall working distance of less than 1mm.

Piezo stages are very helpful when used with the Low Working Distance option. A “z-stage” allows you to achieve minimum distances between the sample and objective with the ability to also avoid collisions between the sample and the inner window.

Detailed Dimensions View: Standard Low Working Distance

Dimension locations:

  • Distance A to B (thickness of outer window) = 0.5mm if fused silica or 0.2mm if BK7
  • Distance B to C (thickness of lip under window) = 0.5mm
  • Distance B to D (air gap) = 0.5mm typical, if carefully managed
  • Distance A to D = 1mm minimum, working distance if inner window is not present
  • Distance D to E (thickness of inner window) = 0.5mm
  • Distance E to F (thickness of radiation aperture brass washer) = 0.2mm

If you want to minimize the overall distance from your objective to the sample, and can tolerate a temperature rise on your sample, you can remove the inner window. This gives you a simple single outer window thickness to manage. Be careful not to crash the objective into the outer window, as it will break easily.

If you want to minimize the overall distance from your objective to the sample with minimum temperature, you should:

  1. Minimize the gap B to D
  2. Bring your objective all the way down to surface A without crashing into the window
  3. Bring your sample all the way up to surface E (above F) with a piezo stage if possible.

Remember that the outer window and lid is roughly room temperature, the radiation shield is about 30K and the sample is about 3-6K. Be careful that these layers do not touch, or the system will not drop in temperature or pull a vacuum properly.

INSET LOW WORKING DISTANCE

The Inset Low Working Distance Module is an option for the Cryostation which allows larger samples to be positioned for low temperature experiments at a distance less than 4mm from regular room temperature optical components. The main components are the inset outer vacuum window, a flat radiation lid with a window aperture, and the radiation window.

This is different than the standard low working distance option, which provides room for the sample to be inserted substantially up into the radiation shield window port, and that port to be extended out to a thin small window recessed in the outer window. This version does not require the sample to extend into the port, so it allows larger samples or samples which require scanning that can’t be done inside the inner port opening. Both versions work best in overhead viewing applications. Below is a cutaway image of the assembly, with the outer ring holding the inset vacuum window and the flat radiation lid in place.

The outer window is designed for a 25mm opening with 2mm thick fused silica or BK7 window. The inner window has a 25mm x 1mm thick fused silica or BK7 window. Neither of these windows can be adjusted, so the user has a fixed working distance between the radiation window and the vacuum window. The sample can be brought up close to the inner window with the use of nanopositioning stages.

Detailed Dimension View: Inset Low Working Distance

Dimension locations:

  • A:  Top of 2mm outer window, bottom of recessed pocket
  • B:  Bottom of outer window
  • C:  Bottom of lip that holds window
  • D:  Top of radiation shield cap
  • E:  Bottom of inner window
  • F:  Underside of radiation shield cap
  • Distance A to B (thickness of outer window) = 2mm
  • Distance B to C (thickness of lip under window) = 0.4mm
  • Distance C to D (air gap) = 2mm typical, but can be reduced substantially if carefully managed
  • Distance D to E (thickness of inner window) = 1mm
  • Distance E to F (thickness of lip under inner window = 0.38mm

If you want to minimize the overall distance from your objective to the sample, you should:

  1. Bring your objective all the way down to surface A without crashing into the window
  2. Bring your sample all the way up to surface E (above F) with a piezo stage if possible.

Note that this option was designed to allow scanning of the sample under the lid, so nestling the sample up to level E may not be possible. Our other Low Working Distance option allows for this type of nestling and can provide working distances of 2mm with both windows installed or 1mm with only an outer window.

USING THE LOW WORKING DISTANCE OPTION ON THE SIDE WINDOW PORT

Custom options can be configured to use the side window as the low working distance port. This involves a custom configuration, so please contact Montana Instruments to discuss a side low working distance option. It is not recommended that a top low working distance setup is used for side access. In a side low working distance configuration, typically the working distance is longer than from the top port (about 3-5mm, instead of 1-2mm). It is also more difficult to work with from the side because the sample mount needs to be nested into the radiation shield window, making the configuration more difficult to align and work with than the top.

Examples of Side Low Working Distance