Sample preparation for transmission electron microscopy

#The invention relates to a TEM FIB sample as well as to a method for the production of such a sample.

#Samples for transmission electron microscopy (TEM) can be prepared in different ways. To be able to observe samples with a TEM, these must be thinned correspondingly defined such that they can be transmitted by electrons in the TEM. The quality of the image resolution is substantially dependent on the quality of the sample. For this purpose the sample must be uniformly set to a correspondingly desired defined thickness in a corresponding etching process. It is here important that in this etching process the sample structure is not altered by the process itself. However, this does not yield the desired quality of the sample appropriate to current requirements. The wet-chemical etching method does not lead to the desired goal.

#For this reason, for high-quality TEM samples the samples are today worked by etching with a fixed argon ion beam having, for example, a diameter of approximately 1 mm. A further known method, and one preferred today, comprises cutting out the sample with a focused ion beam (FIB technique) and to thin it as desired. Herein a finely focused scanning gallium ion beam with a beam diameter of a few nm is employed, with the aid of which TEM samples can be prepared from the solid material in the form of lamellae by perpendicular bombardment of the surface. Typically the sample lamellae are herein approximately 80 to 100 nm thick. This latest FIB preparation technique is described for example in P. Gnauck, P. Hoffrogge, ICEM 15, Durban, (Suppl. 1: Proceedings), (2002) 3, 32.

#In the following the invention will be explained in further detail by example and in conjunction with schematic drawings wherein:

#FIG. 1 illustrates a mechanical preparation step for the production of an FIB sample according to prior art;

#FIG. 2 illustrates a mounting and the FIB working of the preparation according to FIG. 1 within the prior art;

#For the production of an FIB sample from the solid body material, for example from a semiconductor wafer, a sample piece 12 is removed and on it through mechanical sawing a web 11 is roughed out, such as is shown for example in FIG. 1. The sample piece 12 is approximately 2600 μm long and the width of the web is approximately 20 μm. On the front face of the web 11 in a partial region where the lamella sample 5 is to be generated, a protective coating is applied which serves as masking for the subjacent lamella 5 which is to be etched out. With a focused ion beam (FIB) impinging perpendicularly on the front face of the web the lamella 5 is now exposed by depth etching. This is schematically shown in FIG. 2 by the arrow with the label “FIB etching”.

#The sample piece 12 with the lamella sample 5 is now disposed on a sample holder 10 with a diameter of for example 3 mm, and can now be observed in the TEM, as is also indicated schematically in FIG. 2 by the arrow and the associated label “TEM”.

#In FIG. 3 a scanning electron microscopic (REM) image reproduction of such a sample 5 is shown for better illustration. It is therein also evident that the FIB-etched lamella of such sample (FIB etching) is relatively thick. In the lower region of the sample can be seen a high waviness of the material which also shows that in the etching, respectively sputtering process, contaminations occur through the redeposition of the involved surfaces 6a, b. The scanning FIB ion beam must be operated with energies between 5 keV to 30 keV, due to the necessary focusing. This leads to the corresponding damage or disturbances of the material structure on both sides of the lamella surfaces 6a and 6b of sample 5, typically to a depth of approximately 20 nm.