Cryogenic Probe Stations - Microscopes and Lighting Options Available for Cryogenic Probe Stations from Lake Shore Cryotronics

Topics Covered

Background
Introduction
Microscope Options
Microscope Specifications
Highly Reflective Surface Images
Diffuse Surface Images
Resolution
Probe Station Dimensions
Conclusion
Acknowledgements

Background

Lake Shore Cryotronics, Inc. is a privately held corporation which has been an international leader in the development of innovative measurement and control technologies since 1968. Lake Shore's philosophy has been to continue to reinvest in itself with a research and development budget that is 100 percent above the national average for instrumentation companies.

Introduction

Lake Shore cryogenic probe stations are available in a wide range of models and capabilities. These instruments empower scientists and researchers to investigate a wide range of materials in extreme environments. An optical microscope provided with the probe station allows viewing and placement of the probe tips on the sample contacts. This note describes the selection of probe station microscope options available and briefly discusses factors researchers should consider when selecting a microscope option.

Microscope specifications will be reviewed and related to image quality. The selection of light source options will be examined, and guidelines based on sample surface properties will be presented. Finally, we will present a set of microscope and probe station dimensions with which users can develop their own optical interfaces.

Microscope Options

Lake Shore offers four different microscopes for use on cryogenic probe stations. There are two different zoom options, each with two choices of lighting. Zoom is the ratio of the magnification change available for the microscope. The two available are Zoom 70 (7:1 ratio) and Zoom 160 (16:1 ratio). The two lighting options are a ring light (figure 1) or coaxial lighting (figure 2).

Figure 1. Zoom 70 with ring lighting option

Figure 2. Zoom 160 with coaxial lighting option

The working distance of a microscope is the distance from the last optical element to the focal plane. The microscope is outside the vacuum chamber of the probe station. The focal plane must be on the sample stage, so the working distance of the microscope must be large. The working distance for all probe stations except the horizontal field superconducting magnet probe station and the electromagnet probe station is 89 mm for coaxial lighting options. For the ring lighting options, the working distance is increased to 114 mm to accommodate the additional space needed to mount the ring light. For the horizontal field superconducting magnet probe station and the electromagnet probe station the working distance is increased to 181 mm for both lighting sources. This is necessary because of the increased chamber height of these two probe stations. The effect of the large working distance is the magnification limitation of the microscope. In the next section, the detailed specifications of the microscopes are summarized.

The coaxial light source is designed to illuminate the sample with light perpendicular to the sample. If the sample is highly reflective, nearly all the light reflects from the surface back into the microscope. Compare this to reflection from a diffuse surface. The light is reflected in all directions (the cosine law), and little of the reflected light finds its way back into the microscope. The light reflected from the diffuse surface is overwhelmed by the reflected light from the windows of the probe station. The image of diffuse surfaces with coaxial light source lacks contrast.

Just the opposite applies to the ring light. The light source is a ring mounted around the microscope. On highly reflective surfaces, the reflected light from the ring misses the microscope elements. The image of highly reflective surfaces is dark with the ring light. However, from diffuse surfaces, the scattered light reflects in all directions. The reflections from the windows mainly miss the microscope, giving good images of diffuse surfaces with the ring light. Use of the ring light on diffuse surfaces also gives shadows and a 3D image effect.

Each microscope is supplied with a CCD camera (which mounts on the microscope with a standard C mount) and monitor.

Microscope Specifications

The optical specification for each of the microscopes is summarized in Table 3. The magnification specification is the optical magnification of the microscope and does not include the magnification of the CCD camera. The field of view, numerical aperture, and depth of focus depend on the zoom setting of the microscope. The table lists the values at minimum magnification (zoom) and at maximum magnification (zoom). These variables vary continuously with the zoom from minimum zoom to maximum zoom.

Table 3. Summary of microscope specifications

For all models except CPX-HF, EMPX-HF, and FWPX

Scope

WD (mm)

Minimum Magnification

Maximum Magnification

 

 

Magnification

Field of View (mm)

Numerical Aperture

Depth of Focus (mm)

Magnification

Field of View (mm)

Numerical Aperture

Depth of Focus (mm)

Resolution* (µm)

Z70-CL

89

1.5

2.4 x 3.2

0.024

0.95

10.5

0.34 x 0.45

0.08

0.085

4

Z70-RL

114

1.1

3.2 x 4.2

0.018

1.7

7.9

0.45 x 0.61

0.06

0.15

4

Z160-CL

89

1

3.6 x 4.8

0.009

6.8

16

0.22 x 0.30

0.15

0.024

4

Z160-rL

114

0.75

4.8 x 6.4

0.0068

12.1

12

0.30 x 0.40

0.11

0.043

4

For CPX-HF, EMPX-HF, and FWPX

Scope

WD (mm)

Minimum Magnification

Maximum Magnification

 

 

Magnification

Field of View (mm)

Numerical Aperture

Depth of Focus (mm)

Magnification

Field of View (mm)

Numerical Aperture

Depth of Focus (mm)

Resolution* (µm)

Z70-CL

181

0.75

4.7 x 6.3

0.012

3.8

5.2

0.68 x 0.91

0.024

0.034

8

Z70-RL

181

0.75

4.7 x 6.3

0.012

3.8

5.2

0.68 x 0.91

0.024

0.034

8

Z160-CL

181

0.5

7.2 x 9.6

0.0045

27.1

8

0.45 x 0.60

0.076

0.1

8

Z160-rL

181

0.5

7.2 x 9.6

0.0045

27.1

8

0.45 x 0.60

0.076

0.1

8

*Typical - resolution is station configuration and operating condition-dependent

Highly Reflective Surface Images

The first example is a highly reflective surface of a magnetic tunnel junction. Figure 3 is the image through a zoom 70 microscope with coaxial lighting. The magnification is at the minimum value. The four gold circles are top side contacts. The diameters of the contacts are 100 µm, 50 µm, 25 µm and 10 µm.

Figure 3. Highly reflective surface through a zoom 70 with a coaxial light

Figure 4 is the same sample through a zoom 70 microscope with a ring light. Note that the image is much darker and the 10 µm contact is barely resolved. As the magnification is increased the lack of contrast of the topside contact gets worse.

Figure 4. Highly reflective surface through a zoom 70 with a ring light

Diffuse Surface Images

Figure 5 is the image of a surface-mounted FET through a zoom 70 with a coaxial light. Figure 6 is the same FET with a ring light. Note that with the coaxial source the image lacks contrast and any color. With the ring light, the colors are rendered correctly with good contrast and shadows.

Figure 5.Ssurface-mounted FET through a zoom 70 with a coaxial light.

Figure 6. surface-mounted FET through a zoom 70 with a ring light

Resolution

The resolution of the microscope depends on a variety of parameters, including the vibrations of the probe station mounting, the exact microscope used, and the quality of the CCD camera, among others. Figure 7 is a picture of a USAF 1951 resolution test pattern showing groups 6 and 7. The spacing between the lines for group 6 runs from 7.3 µm to 4.3 µm. The spacing between the lines for groups 7 runs from 3.9 µm to 2.2 µm. This picture is through a zoom 70 at maximum zoom with a ring light. The resolution is about 4 µm. The pattern is a negative pattern (the lines are the glass region and between the lines is metal). Figure 8 is the same target using zoom 70 with a coaxial light. The resolution is about 3 µm. Figure 9 is the same target using a zoom 160 with a ring light. The resolution is about 4 µm. Figure 10 is the same target using a zoom 160 with a coaxial light. The resolution is about 2 µm.

Figure 7. USAF 1951 resolution test pattern showing groups 6 and 7 through a Z70 at maximum zoom with a ring light; the test pattern is a negative image

Figure 8. USAF 1951 resolution test pattern showing groups 6 and 7 through a Z70 at maximum zoom with a coaxial light; the test pattern is a negative image.

Figure 9. USAF 1951 resolution test pattern showing groups 6 and 7 through a Z160 at maximum zoom with a ring light.

Figure 10. USAF 1951 resolution test pattern showing groups 6 and 7 through a Z160 at maximum zoom with a coaxial light; the test pattern is a negative image

Probe Station Dimensions

Figure 11 is a summary of the working height of the microscopes and the dimensions of the probe station chamber lid to sample stage. It may be helpful for researchers who want to design their own optical accessories.

Figure 11. Summary of the working height of the microscopes

Table 2. Dimensions of the probe station chamber lid to sample stage

Scope

B - working distance (mm)

B - working distance (mm) HF

Z70-CL

89

181

Z70-RL

114

181

Z125-CL

89

181

Z125-RL

114

181

Z160-CI

89

181

Z160-RL

114

181

Model

A (mm)

C (mm)

TTPX

63

111

CPX

65

115

CPX-VF

82

131

CPX-HF

133

182

EMPX-HF

98

146

FWPX

99

181

Conclusion

This note has described the microscopes and options available for Lake Shore cryogenic probe stations. Guidelines and specifications for selecting a microscope and lighting options have been presented.

Acknowledgements

Lake Shore would like to thank Dr. John Xiao of the University of Delaware for providing the magnetic tunnel junction images used in this note.

Source Lake Shore Cryotronics

For more information on this source please visit Lake Shore Cryotronics

Date Added: Oct 19, 2009 | Updated: Jun 11, 2013
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