Low conductive thermal insulation pad with high mechanical stiffnessPlaque d’isolation thermique à faible conductivité et à haute rigidité mécanique
Introduction
Combination of Scanning Probe Microscopy (SPM) with Scanning Electron Microscopy (SEM) represents a powerful method for nanoscale surface studies. Investigation of a sample consists here of location of a point of interest by SEM followed by detailed 3D high resolution SPM analysis. Broad range of the sample temperatures extended from cryogenic temperatures up to several hundred degrees of Celsius is often required. The disadvantage of application of low temperature SPM together with SEM (at room temperature) or, possibly, with other analytical methods (e.g. Scanning Auger Microscopy - SAM, Backscattered Electron Spectroscopy - BSE) is the demand for adequate volume of free space over the sample. Therefore, in contrast to specialized low temperature SPM's, it is not possible to close the sample including the entire scanning system into a cryostat, e.g. Petersen et al. (2001), Foley et al. (2004), Albers et al. (2008). The sample and its holder are thus exposed to radiative and conductive heat flows from the surrounding vacuum chamber and from other parts at room temperature. On the contrary, heat is leaking from the sample holder at increased temperatures. To minimize these heat flows, effective thermal insulation of the variable temperature parts from those at room temperature must be introduced.
The aim of our work was to find an appropriate connection of a sample holder with a piezoelectric scanner of an ultra-high vacuum scanning probe microscope (UHV SPM) built in an UHV modular system based on scanning electron microscope (SEM). The UHV SEM/SPM system serves for in situ fabrication and characterization of nanostructures at variable temperatures between 20 and 700 K. Besides low thermal conductance of the sample holder mounting, also its mechanical stability with zero kinematic degrees of freedom, ability to tolerate thermal expansions of the sample holder during temperature changes and high mechanical stiffness are required. For more details about design of cryogenic sample holders in SEM/SPM, thermal insulation, flow cooling systems and heat flows calculation, we refer the readers to our conference paper (Krutil et al., 2019) and references therein.
The design of our insulating pad described in this article is based on application of glass balls as thermal insulating elements (Hanzelka et al., 2013). That is why we named this pad as “InBallPad” and the abbreviation IBP is used in the following text.
The application of high temperatures up to 700 K comes with specific issues such as thermomechanical properties of materials, enhanced thermal radiation and choice of temperature sensors suitable for this temperature range. These topics are discussed only marginally in this work whereas the description of problems concerning the cryogenic design and properties of the IBP is preferred.
Although the parameter settings for IBP design comes from requirements given by specific UHV SEM/SPM system, the IBP represents compact and effective solution of thermal insulation of sample holders suitable for any devices working in ultra-high vacuum at variable temperatures. An example of practical application of IBP in high resolution SEM is presented in Chapter 5.
Section snippets
Input criteria and their mutual relationships
During the development of most technical devices, the designer is to arrive at a compromise among multiple parameters of the final device. In the case of a low temperature (LT) insulating pad the main parameters are mutually dependent and are represented by the pad thermal conductance, static determination and stiffness, ultimate sample temperature, cryogen consumption, dimensions, feasibility and manufacturing cost. For instance, a measure minimizing the thermal conductance leads not only to
Description of the IBP
The system of the kinematic coupling used in our Insulating Ball Pad (IBP) is based on the principle of the Kelvin coupling where the spherical surfaces on one part were replaced by two “four balls supports” (FBS) and by one ball on a plane (Fig. 1). The actual implementation of the supports and an overall view of IBP are shown in Fig. 2.
A detailed IBP section view oriented at the ball supports is drawn in Fig. 3(a). A cooled sample holder (not shown in Fig. 3) is attached to the upper plate
Maximal side force
As mentioned above, application of an excessive side force FD will cause a shift of the upper plate against the lower one. As explained in Fig. 4(a), this occurs if the upper ball of one or both FBS's loses the contact with one of the lower balls and "rolls" over the "saddle" between other two lower balls. We neglected several side effects (e.g. friction) in the following rough estimation of the dislocating force FR in one FBS. Our calculation is based on a presumption that the critical state
Application of IBP in high-resolution SEM
We used the IBP for low temperature measurement (down to 30 K) of cathodoluminescence (CL) in the SEM Magellan 400 L (Thermo Fisher Scientific), offering sub-nanometer resolution in the whole electron energy range from 1 to 30 kV. The high-resolution SEM in combination with the CL measurement represents a useful tool for characterization of solid materials with a spatial resolution much higher than the diffraction limit of light. Depending on the equipment of the SEM, the CL data can also be
Conclusions
The low conductive thermal insulation pad (IBP) with high lateral mechanical stiffness has been designed, manufactured and tested. This pad meets demanding requirements on the defined and precise placing of the sample holders in the UHV SEM/SPM for in situ fabrication and characterization of nanostructures in an extraordinary range of working temperatures from 20 K up to 700 K. It is usable in ultra-high and high vacuum instruments, where effective thermal insulation, compact size and precise
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Acknowledgment
We thank J. Frolec, M. Macek and V. Musilová for stimulating discussions and text corrections, O. Lalinský for providing us with ZnO:Ga-polystyrene composite scintillator sample. We acknowledge the support of the TACR under Grant Project No. TE01020233 and the institutional support RVO:68081731.
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