Scanning Electron Microscope Assembly Operating in Situ

The present invention concerns scanning electron microscopes and, more particularly, a scanning electron microscope operating in situ.

The use of scanning electron microscope has undergone tremendous development due to their pliancy in use, notably in association with visualization cathodic screens, and to their extremely convenient performances in observation, notably concerning depth of field. These characteristics make particularly advantageous industrial use of scanning electron microscopes to check the surfaces of operational or structural elements, notably for checking manufacturing processes or the study of surface bucklings upon stress tests. To the present, the study of such surfaces has been made by conventional optical processes using magnifying glasses or optical microscopes with slight magnification and limited depth of field. Thus, it appears particularly inviting to use, for said checkings, scanning electron microscopes in view of the aforesaid stated advantages. However, as known in electron microscopy, it is required to introduce the object to be observed on a special support to place it in a vacuum plenum forming an observation chamber for the microscope. Thus, the use of electron microscopes required destructive preparation techniques requiring the taking of a sample to be placed within the observation chamber at the price of damage or buckling of the portion to be studied.

In order to obviate this disadvantage, it has been proposed to use an electron microscope observation technique using an intermediary vacuum plenum having slight sizes and connected to a powerful pumping device clearing at right angles to the portion to be observed by a diaphragm having a minute opening (of the order of 30 microns ) and a slight thickness, so as to limit the air leak output to be permanently balanced by the pumping device. The so proposed technique has a number of limitations, notably into account the very limited observation field, due to the microscopical size of the diaphragm, the non-homogeneity of the vacuum at the level of the optical axis and the diffusion of the electrons which are retrodiffused by the object resulting in images of a poor grade.

The object of the present invention is to obviate these disadvantages by providing a scanning electron microscope assembly operating in situ, having a light and transportable structure, which may be directly laid on the object to be examined without any particular preparation of the latter while allowing a direct observation of the related object in the natural environment thereof, and enabling high magnification with an excellent depth of field (for instance, on the order of a thousand times greater than with an optical microscope) to be obtained for a quick, adaptable and non-destructive study in situ of, for example, aircraft wings, or for the checking of parts which are either machine-finished or worked with precision, such as glass lenses and mirrors for optical systems.

The scanning electron microscope assembly operating in situ, comprising a column unitary electron microscope, connection means to a system for controlling and checking the microscope and to a pumping system, comprises a supporting means for the column, connected to the lower part thereof, and an interior chamber having a lower open portion intended to be disposed tightly on a peripheral surface integral to the portion to be observed, means being provided for communicating the internal chamber with a transportable pumping system.

The support means comprises a peripheral skirt or wall withstanding pressure and defining the internal chamber and flexible sealing means limiting the lower open portion of the chamber, and intended to be applied on the bearing surface to be observed.

Other features and advantages of the present invention will appear from the following description of an embodiment, given by way of illustration but in any case not limiting, made in relation to the accompanying drawing in which:

The sole FIGURE is a diagramatic view of the scanning electron microscope assembly operating in situ according to the present invention, disposed in sealing abutment on the surface to be observed.

As shown in FIGURE 1, the scanning electron microscope assembly operating in situ according to the invention essentially comprises an electron microscope column 1 having a slight size and reduced weight, connected to an electronic checking and controlling system 2, a supporting means for column 3 defining an internal chamber 4, connected, as well as column 1, to a portable powerful pumping device 40. More specifically, column 1 comprises a tight tubular frame 5 including successively an electron gun 6, a stage 7 of two capacitors 7.sub.1 and 7.sub.2, and an objective lens stage 8 with a stigmator 9 and scanning coils 10 within a tubular conduct 11 extending close to the bottom of the frusto-conical hood 12 converging downwardly, closing the column 1, and provided with a central aperture 13 for the passage of the electron beam about which is disposed the annular electron detector 14 concentric to the optical axis 15 of column 1.

According to the present invention, the support means 3 of column 1 comprises a peripheral wall or skirt 16 having a resisting shape (withstanding pressure, for instance) a hub made of stainless steel, defining the internal chamber 4, in which is protruding the lower frusto-conical hood 12, and on the edge of the open end of which is secured a peripheral resilient sealing 17 (for instance, an elastomeric material which is solid or selectively) inflatable. This supporting means further comprises a vertically adjusting system (or jacks 18,18'), enabling the distance between the detector 14 and the portion to be observed A of the surface 19' of the body 19 to be studied, to be adjusted with precision on a determined range. A connector 20, which is mounted on wall 16, enables the detector 14 to be connected to the control and checking system 2. The wall 16 further comprises, on the sides thereof, a sleeve 21 for connection to the pumping device 4.

In a particular embodiment, to allow an angular displacement of the column 1 and to cover a broader observation surface A, the column 1 is connected to the top end of the supporting wall 16 by a knee-joint system 22 (for instance, having cooperating toric surfaces made of teflon) the tightness at the level of the knee-joint 22 being insured by a flexible diaphragm 23 made of a metallic alloy known under the name "Tombac" sealingly connecting the upper portion of the wall 16 to an adjacent crown of the frusto-conical hood 12.

Therefore, it will be understood that for observation in situ, the microscope column 1 together with the support thereof 16 is transferred perpendicularly to the portion A to be studied, the assembly coming in abutment on the part 19 to be studied by the flexible sealing 17, the application of a vacuum within the internal chamber 4 of the supporting means by the pumping device 14 ensuring therein the vacuum grade (about 10.sup.-4 mm of mercury) required for operating the microscope while further placing perfectly sealing 17 on the surface 19', thus ensuring tightness during the time required for the observation. This device thus enables aircraft wings or any surfaces to be studied in situ with a resolution power ranging from 10 to 20 nanometers.

The microscope column 1 having a height of about 50 centimeters, is, for example, of the MIEL bis type which has been conceived by the electron optical laboratory of the CNRS in Toulouse, operating under a low voltage ranging from 10 to 15 kilowatts.

Although the present invention has been described in relation with particular embodiments, it is not thereby limited, but is capable of modifications which will appear to those skilled in the art. In particular, for the observation of details on limited surface areas, the wall 16 of the supported means may be radially extended inwardly by a crown, itself ended by a sealing joint 17, to limit the aperture of the internal chamber 4.