DE3332248A1 - System for discharging charges on test specimens in scanning electron microscope investigations - Google Patents

Abstract

The invention relates to a system which can easily be adapted to any scanning electron microscope and permits both electrically nonconductive and vacuum-sensitive test specimens to be imaged by means of backscatter electrons without any sort of preparation and free from artefacts with a quality not hitherto achieved. This is achieved by means of a finely meterable and pre-ionisable gas flow which flows around the test specimen surface in a pinpointed fashion in the test specimen chamber, in which it discharges the charges on the surface of the test specimen which cause image interference, and is finally once again extracted. A constant vacuum which is favourable for the test specimen is created in its immediate vicinity by means of a fine-metering valve and an electronic control without the vacuum being reduced to the same value in the entire test specimen chamber. By using such a system it is also possible for small living organisms to be imaged alive in the scanning electron microscope.

Description

SYSTEM FOR DERIVING SAMPLE CHARGES

IN SCREENING ELECTRON MICROSCOPIC EXAMINATIONS The invention relates to a system to supplement the equipment and technical application of scanning electron microscopes, with which the imaging of electrically non-conductive samples (e.g. plastics) Occurring surface charges can be discharged in a targeted manner without a Treatment of the sample before the start of the test is required and without the sample surface is damaged during the examination.

With the system completely untreated and also electrically non-conductive Samples in a vacuum that is favorable for the sample, free of artifacts from a scanning electron microscope to be examined. Since the samples do not have to be treated either beforehand or afterwards, is they can be used again after the examination. It has shown, that with the system described here, under certain conditions, also small organisms can be depicted alive with a scanning electron microscope.

Usually, samples that were previously electrically non-conductive are converted to scanning electron microscopy Provide examinations with a conductive layer so that there are no image disturbances as a result local charges arise. However, this coating can change or cause destruction of the sample surface.

To derive the sample charges without sample treatment are so far two methods known: The first method works with from alkali or alkaline earth metals emitted low-energy positive ions with which the sample was bombarded will.

This is the same effect as sputtering, but it is usually intended no deposits are formed on the sample. The ion cannon of the necessary Apparatus must be built into the sample chamber and generally limits the Shiftability of the sample significantly. The system is only on low charges and lower magnifications can be used, otherwise a "snow storm"! Screen is created by alkali ions hitting the detector (Literature: Company News from Kimball Physics Inc .: System 2 Charge Neutralizer, 1980).

The second method works with a deteriorated vacuum in the samples chamber so that diffuse through the primary electron beam after the Townsend discharge positive gas ions are generated, which derive the sample charge or to a Lead charge equilibrium, which is sufficient for backscattered electron images. One of ETP Sempra Pty. Ltd.

(Australia) offered apparatus that works according to the principle described works, generated after a slight modification to the scanning electron microscope a controllable vacuum in the sample space, which fluctuates and is worse than 10 2 Torr is. This leads to a considerable shortening of the service life of the hot cathodes and to a scattering of the electron beam, so that generally only images with a magnification of less than 1000 times are possible (literature: Honcrieff, D.A., V .N .E.

Robinson and L .B. Harris: Charge neutralization of insulating surfaces in the SEM by gas ionization. J.Phys. D. Appl. Phys. 11, 2315-2325 (1978)).

The invention is based on the object, electrically unprepared Non-conductive samples with the scanning electron microscope also for magnifications about To make it possible to map 100 times. Furthermore, the apparatus should be designed in this way be that at any time without modifications, including during an investigation, from "normal operating can be switched to "operation with charging samples".

The object is achieved according to the invention according to the FIG. 1 in the drawing -principle solved. A metered and pre-ionizable gas stream (1) is directed specifically to the sample surface area to be imaged.

There the gas molecules disintegrate through the primary electron beam (2) into positive ions (3) and low-energy electrons according to the Townsend effect (4). While these electrons created by ionization are due to the Coulombs Forces from the sample surface charge (5) repelled and by means of corresponding Safety grid (6) or diverted over the sample chamber wall lead to the positive Gas ions for the desired discharge of the charge on the sample surface: The positive gas ions are accelerated to the negatively charged sample surface, take there during the neutralization process electrons from the sample surface onto (7) and are then sucked off the sample surface as a neutral gas (8). One electronic control ensures constant pressure conditions in the immediate vicinity of the sample, thus ensuring a good and reproducible quality of backscattered electron images is made possible even at high magnifications.

Fast, vacuum-burglar-proof and automatic switching from "normal operation" to "operation with charging samples" is carried out by means of a valve control realized.

The advantages achieved with the invention are in particular: that when charges occur, the system for diverting the sample charges can be switched on by pressing a key without interrupting the examination must become. Because the vacuum is only applied in the vicinity of the sample Gas flow is deteriorated, and by special measures to regulate the pressure conditions images of unprepared samples are possible in a previously unattainable quality. By choosing the vacuum in the sample area, the pre-ionization and the type of sample flooding vacuum-sensitive samples can also be imaged using a scanning electron microscope will.

An embodiment of the invention is shown in the drawing Fig.2: The gas flow is metered via an electronically controllable fine valve (9), the then through one of the sample geometry adaptable nozzle (10) with an integrated Preionization section and a pressure measuring sensor flows over the sample surface and then sucked off through a funnel (11) via a switchable vacuum will. If a particularly bad vacuum is required (e.g. for examining living biological cells) can be opened with the valve (12) or in the presence of a Turbomolecular pump controls the suction in the sample chamber via speed control to reduce. A corresponding one is jointly responsible for good backscattered electron images Detector (13) which detects the largest possible solid angle.

Claims (4)

Claims: (9 system for discharging sample charges at Scanning electron microscopic examinations with the help of gas ions, characterized in that that a finely adjustable gas stream flows specifically and continuously over the sample surface is sucked off. 2. System according to claim 1, characterized in that the gas flow can be pre-ionized adjustable in the feeding capillary. 3. System according to claim 1, characterized in that the pressure measuring probe is located in the supplying capillary and contributes to the pre-ionization. 4. System according to claim 1, characterized in that steam generating Chambers for the examination of moisture-sensitive samples (e.g. biological Cells) can be connected.