WO2020068259A1

WO2020068259A1 - Sacrificial clamp for preparation of thin specimens for infrared microscopy

Info

Publication number: WO2020068259A1
Application number: PCT/US2019/043484
Authority: WO
Inventors: Richard P. Tella; Matthew Kole
Original assignee: Agilent Technologies, Inc.
Priority date: 2018-09-30
Filing date: 2019-07-25
Publication date: 2020-04-02
Also published as: DE112019004919T5; CN112771370A

Abstract

A method for preparing a sample (53) for viewing with an ATR objective and apparatus (50) for holding the sample (53) are disclosed. The ATR objective is applied against a planar ATR reflective surface of the sample (53). The method includes sandwiching the sample (53) between first and second support layers (51, 52) of material and cutting the first and second support layers (51, 52) and the sample (53) so as to generate a cut planar surface having the cut sample (53) between the first and second support layers (51, 52) in a plane. The cut planar surface is placed in contact with the ATR objective with the sample (53) against the ATR reflective surface.

Description

Sacrificial Clamp for Preparation of Thin Specimens for Infrared Microscopy

Background of the Invention

[0001] Quantum cascade lasers provide a tunable mid-infrared (MIR) light source that can be used for spectroscopic measurements and images. Many chemical components of interest have molecular vibrations that are excited in the MIR region of the optical spectrum, which spans wavelengths between 5 to 25 microns. Hence, measuring the absorption of MIR light at various locations on a sample can provide useful information about the chemistry of the sample as a function of position on the sample.

[0002] One class of imaging spectrometers measures the light directly reflected from the sample as a function of position on the sample and wavelength of the illuminating MIR light. The amount of light that is reflected depends on both the chemical and physical attributes of the sample, since light can be lost both by absorption in the sample, which reflects the chemical composition of the specimen and by scattering, which depends on the physical state of the surface of the specimen. Hence, comparing spectra generated with direct reflection to absorption with known chemical absorption spectra that are available in libraries presents significant challengers.

[0003] Systems that utilize attenuated total reflection (ATR) to illuminate the specimen avoid the problems caused by scattering of the incident light by the specimen. However, the surface of the sample must be essentially flat, since the electric field that interacts with the specimen only extends a few microns below the reflecting surface of the ATR objective. Many samples of interest are not flat to within this tolerance. Furthermore, many samples are not well suited for planarizing.

Summary

[0004] The present invention includes a method for preparing a sample for viewing with an ATR objective and apparatus for holding the sample. The ATR objective is characterized by an ATR reflective surface, the method includes sandwiching the sample between first and second support layers of material and cutting the first and second support layers and the sample so as to generate a cut planar surface having the cut sample between the first and second support layers in a plane.

[0005] In one aspect of the invention, the cut planar surface is placed in contact with the ATR objective with the sample against the ATR reflective surface.

[0006] In another aspect of the invention, the first and second support layers forced against the sample in a vice structure includes first and second vice members that are moved relative to one another.

[0007] In another aspect of the invention, the first and second support layers are part of the first and second vice members, respectively.

[0008] In another aspect of the invention, the first and second support layers are clamped together during the cutting of the first and second support layers.

[0009] In another aspect of the invention, the first and second support layers comprise polyethylene.

[0010] In another aspect of the invention, the first and second support layers are more compressible than the specimen.

[0011] In another aspect of the invention, the first and second support layers are separate from the first and second vice members.

[0012] An apparatus according to the present invention includes first and second support members having surfaces adapted for receiving a specimen and immobilizing the specimen when the first and second support members are pressed together, first and second vice members adapted for pressing the first and second support members together such that the first and second support members extend outside of the first and second support members such that the first and second support members and the specimen can cut to form a plane with the cut specimen and the cut support members presented in a common plane without cutting the first and second vice members, and a mechanism for forcing the first and second vice members together.

[0013] In another aspect of the invention, the first and second support members are extensions of the first and second vice members.

[0014] In another aspect of the invention, the first and second support members are separate from the first and second vice members.

[0015] In another aspect of the invention, the first and second support members are constructed from a support material and the first and second vice members are made from a vice material, the vice material is different from the support material.

[0016] In another aspect of the invention, the support material is more easily cut than the vice material.

[0017] In another aspect of the invention, the first and second support members are more compressible than the specimen.

[0018] In another aspect of the invention, the first and second support members comprise polyethylene.

Brief Description of the Drawings

[0019] Figure 1 illustrates a simple ATR optical system that is attached to a specimen.

[0020] Figure 2 illustrates a scanning ATR system in which the present invention can be practiced.

[0021] Figures 3A-3C illustrate one embodiment of the specimen clamp according to the present invention. [0022] Figure 4A illustrates the problems encountered when a flexible sample is clamped with a portion of the sample above the edge of the clamp.

[0023] Figure 4B illustrates the clamp acting as a motion stop.

[0024] Figure 5 illustrates another embodiment of a specimen mount according to the present invention.

[0025] Figures 6A and 6B illustrate another embodiment of the clamping mechanism that can be used with the present invention.

[0026] Figure 7 is a top view of a specimen that has been immobilized between two sacrificial layers that are constructed from a material that is more compressible than the specimen.

Detailed Description

[0027] The manner in which the present invention provides its advantages can be more easily understood in the context of an imaging ATR scanning system; however it will become apparent from the following discussion that the present invention can be used advantageously in a number of different systems.

[0028] Refer to Figure 1, which illustrates a simple ATR optical system that is attached to a specimen. Figure l is a cross-sectional view of an interface crystal that can facilitate the measurement of the absorption of light by a sample 27 in the reflective geometry mode. Crystal 21 has a high index of refraction. Light beam 26 enters crystal 21 through port 22 and strikes facet 23 at an angle that is greater than the critical angle. The light beam is totally reflected from facet 23 and exits the crystal through port 24. At the point at which the light beam is reflected from facet 23, the electric field associated with the light beam extends outside the crystal as shown at 25. If the medium under facet 23 absorbs light at the wavelength of light beam 26, the evanescent field will interact with the medium and energy will be transferred from the light beam to the medium. In this case, the energy in the beam leaving crystal 21 will be reduced. The difference in intensity between the input and output beams as a function of wavelength is a spectrum that matches a high-quality transmission spectrum and can easily be used for matching conventional spectra for various chemical compounds.

[0029] While an interface crystal of the type discussed above is useful in measuring a MIR spectrum of a point on a sample, it presents challenges if an image of an area on the specimen is needed, particularly if the surface of the specimen is not smooth.

[0030] US Patent 9,863,877, issued Jan. 9, 2018 teaches an ATR measurement system in which the point of interaction of the input light beam can be rapidly scanned across the specimen without the need to move the crystal. Refer now to Figure 2, which illustrates a scanning ATR system 60 in which the present invention can be practiced. Light 18 from laser

61 is split by beam splitter 62 into two beams. The first beam is directed to detector 63a, which measures the intensity of the laser pulse. The second beam is directed to position modulator 64 which adjusts the point of illumination of the beam on an off-axis parabolic reflector 65. The position of illumination determines the position at which the light from parabolic reflector 65 strikes a second off-axis parabolic reflector 66. Parabolic reflector 66 re-collimates the beam and sets the diameter of the beam to match the input aperture of ATR objective 67. The inclination of the beam entering ATR objective 67 is determined by the point of illumination on parabolic reflector 65. The light reflected back by ATR objective 67 retraces the path of the incoming light and a portion of that light is directed by beam splitter

62 into detector 63b. Controller 69 can then determine the amount of light that was lost in the reflection from ATR objective 67, and hence, determine the amount of light absorbed by sample 27. To image another small area on sample 27, controller 69 operates a three-axis stage 68. The area scanned is determined by a user using user interface 74.

[0031] Many specimens of interest have irregular surfaces. The resulting height variations are often much greater than the depth of the electric field at the reflection surface in the ATR objective. As noted above, the depth of the field below the reflection surface of the ATR objective is a few microns. Hence, unless the surface variations are less than a few microns, or the sample is compressible, when the objective is brought into contact with the specimen there are isolated points of contact that, typically, cannot be predicted in advance. Accordingly, it would be advantageous to provide a system for planning the surface of the specimen prior to bringing the specimen in contact with the reflecting surface. Planning many samples of interest presents challenges, particularly in the case in which the sample is small and fragile or flexible.

[0032] Refer now to Figures 3A-3C, which illustrate one embodiment of the specimen clamp according to the present invention. Figure 3A is a cross-sectional view of clamp 50 prior to the insertion of a specimen 53 into clamp 50. Clamp 50 includes a fixed section 51 and a movable section 52 that moves in relation to section 51 by tightening a threaded member 57. In practice, a specimen 53 is placed between sections 51 and 52 so as to extend above the top of sections 51 and 52 as shown in Figure 3B. The two sections are then clamped together such that specimen 53 is held between sections 51 and 52. The material of sections 51 and 52 is chosen such that the material can be easily cut or removed by grinding or a similar process. After the specimen has been clamped between the two sections, the top portion of the sections is removed along line 54. After the removal, the specimen is left flush with the top surface of sections 51 and 52 and clamped there between.

[0033] To utilize ATR spectroscopy, the sample must be placed in very close proximity to the reflecting surface of the ATR objective or in direct contact with the ATR objective. This requirement presents problems for fragile, brittle, were very flexible samples. Refer now to Figure 4A, which illustrates the problems encountered when a flexible sample is clamped with a portion of the sample above the edge of the clamp. When the ATR objective 71 is brought into contact with the sample, the force between the reflecting surface and the sample can cause the sample to bend, as shown at 73, or actually break In contrast, a sample prepared with the clamp of the present invention is flush with the top surface of the clamp. As a result the top surface of the clamp acts as a motion stop that allows the ATR objective to touch the sample, but limits the force applied to the sample, because the clamp itself provides a motion stop as shown in Figure 4B.

[0034] In the above-described embodiments, a portion of the clamping mechanism was sacrificed to provide the flush-mounted sample ready for contact with the ATR objective. The amount of material that is sacrificed in this arrangement can be significant. In addition, the body of the clamping mechanism must be made of a material that has a significant structural rigidity, which can present challenges when cutting through the material to prepare the sample.

[0035] Refer now to Figure 5, which illustrates another embodiment of a specimen mount according to the present invention. Specimen mount 80 differs from clamp 50 in that sample 85 is clamped between two sacrificial layers 83 and 84 that are separate from the mechanical body 81 and 82 by nut 87, and hence, can be constructed from a material that is different from the mechanical body without significantly reducing the structural strength of the mechanical body.

[0036] The sacrificial layers need only have sufficient rigidity to support sample 85 while the sacrificial layers and the sample are cut and to maintain the cut sample in position when the ATR objective is brought into contact with the sample. In addition, the remainder of the clamping mechanism is not sacrificed, and hence, the cost of sample preparation is significantly reduced.

[0037] In one embodiment, the material from which the sacrificial layers are made is chosen to have as little absorption in the MIR as possible at wavelengths at which the sample does absorb. The electric field generated by the light that is reflected off of the reflecting surface in the ATR objective extends sideways as well as vertically. Hence, in areas near the edge of the sample, the sacrificial support layer could absorb significant light if the material is not chosen to avoid such absorption.

[0038] In one exemplary embodiment, the sacrificial layer is constructed from low density polyethylene. The material and thickness in question allow the sample and sacrificial material to be cut with a microtome. In addition, the material has very little absorption in the MIR, and the absorption bands that the material does have are easily distinguished from most materials found in specimens of interest. In contrast, epoxies have strong infrared absorbance, and hence, can contaminate the measurements near the interface between the specimen and the sacrificial layer .

[0039] Low density polyethylene has a number of advantages other materials. First, the material is soft enough that the remaining edges of the sacrificial layer will not damage the ATR crystals that are typically used in ATR spectroscopy. Germanium and ZnSe crystals are very fragile, and hence, easily damaged if forced against a hard material such epoxies, metal layers, and embedding resins. In one aspect of the invention, the sacrificial material is more compressible than the specimen, and hence, the specimen will make better contact with the ATR crystal, since any remaining material above the specimen will be compressed, and thus will not prevent the specimen from contacting the ATR crystal.

[0040] Having a sacrificial layer that is more compressible than the specimen also facilitates the immobilization of the specimen relative to the sacrificial layer. The sacrificial layers will tend to bend around the specimen when the sacrificial layers are compressed, and hence, inhibit any movement of the specimen relative to the sacrificial layers after clamping.

[0041] Contact between the material of the sacrificial layer and the ATR crystal should not leave a residue on the crystal or migrate into the specimen. It should be noted that waxes and curable embedding resins suffer from this problem. Low density polyethylene does not suffer from this problem. In addition, the specimen cannot be easily removed from potting compounds, whereas the specimen can be separated from a polyethylene sacrificial layer.

[0042] The above-described embodiments utilize a particular clamping mechanism for sandwiching the specimen between the sacrificial layers prior to the sacrificial layers being cut. However, other clamping arrangements could be utilized. Refer now to Figures 6A and 6B which illustrate another embodiment of the clamping mechanism that can be used with the present invention. In this embodiment, the sacrificial layers 91 and 92 that sandwich the specimen 93 are held in place by two spring clamps shown at 94 and 95. The sacrificial layers can be forced together around the sample by placing the sacrificial layers and the sample in a vice mechanism prior to applying clamps 94 and 95.

[0043] As noted above, it is advantageous to use a sacrificial layer that is move compressible than the specimen. Refer now to Figure 7, which is a top view of specimen 93 that has been immobilized between two sacrificial layers that are constructed from a material that is more compressible than the specimen. In this aspect of the invention, sacrificial layers 101 and 102 deform around specimen 93, and hence, inhibit any motion of specimen 93 laterally with respect to sacrificial layers 101 and 102.

[0044] The above-described embodiments of the present invention have been provided to illustrate various aspects of the invention. However, it is to be understood that different aspects of the present invention that are shown in different specific embodiments can be combined to provide other embodiments of the present invention. In addition, various modifications to the present invention will become apparent from the foregoing description and accompanying drawings. Accordingly, the present invention is to be limited solely by the scope of the following claims.

Claims

WHAT IS CLAIMED IS:

1. A method for preparing a sample for viewing with an ATR objective characterized by an ATR reflective surface, said method comprising: sandwiching said sample between first and second support layers of material; and cutting said first and second support layers and said sample so as to generate a cut planar surface having a cut sample between said first and second support layers in a plane.

2. The method of Claim 1 wherein said cut planar surface is placed in contact with ATR objective with said sample against said ATR reflective surface.

3. The method of Claim 1 wherein said first and second support layers are forced against said sample in a vice structure comprising first and second vice members that are moved relative to one another.

4. The method of Claim 3 wherein said first and second support layers are part of said first and second vice members, respectively.

5. The method of Claim 1 wherein said first and second support layers are clamped together during said cutting of said first and second support layers.

6. The method of Claim 1 wherein said first and second support layers comprise polyethylene.

7. The method of Claim 1 wherein said first and second support layers are more compressible than said sample.

8. The method of Claim 3 wherein said first and second support layers are separate from said first and second vice members.

9. An apparatus comprising: first and second support members having surfaces adapted for receiving a specimen and immobilizing said specimen when said first and second support members are pressed together; first and second vice members adapted for pressing said first and second support members together such that said first and second support members extend outside of said first and second support members such that said first and second support members and said specimen can cut to form a plane with a cut specimen and first and second cut support members presented in a common plane without cutting said first and second vice members; and a mechanism for forcing said first and second vice members together.

10. The apparatus of Claim 9 wherein said first and second support members are extensions of said first and second vice members.

11. The apparatus of Claim 9 wherein said first and second support members are separate from said first and second vice members.

12. The apparatus of Claim 11 wherein said first and second support members are constructed from a support material and said first and second vice members are made from a vice material, said vice material being different from said support material.

13. The apparatus of Claim 12 wherein said support material is more easily cut than said vice material.

14. The apparatus of Claim 9 wherein said first and second support members are more compressible than said specimen.

15. The apparatus of Claim 9 wherein said first and second support members comprise polyethylene.