X-ray or gamma ray systems or devices – Specific application – Fluorescence
Reexamination Certificate
2000-10-06
2002-07-16
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Fluorescence
C378S084000, C378S085000
Reexamination Certificate
active
06421414
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an apparatus for the total reflection X-ray fluorescence analysis (TRFA) in which the smooth planar surface of a sample or thin film on a sample is excited by the incident X-ray radiation and the X-ray fluorescence radiation emitted is detected spectrally and which comprises a radiation source, a monochromator and a transducer. The invention further relates to the use of such an apparatus for the determination of foreign atom concentrations on wafer surfaces and to a method for the high-sensitivity resolution measurement of foreign atom concentrations on a wafer.
In total reflection X-ray fluorescence analysis, the surface of a sample is exposed at a very flat angle of incidence to the X-ray radiation generated by an X-ray tube. The angle of incidence is selected so that the incident X-ray is totally reflected. The total reflection geometry means that there is only a certain radiation intensity in a near-surface layer of around 3 nm thickness. For this reason, only the atoms of this thin surface area is excited by the incident X-ray radiation. The excited atoms emit fluorescent X-ray quanta of a certain energy which are characteristic for the relevant atom. The energy spectrum emitted by all surface atoms is measured using a detector and the concentration of an element in the thin surface film can be determined from the intensity of the peak associated with the respective element.
Germanium or silicon detector (Si(Li)) are used to register the energy spectrum with the apparatus for X-ray fluorescence analysis used up to now. All these detectors utilize the fact that the incident X-ray quanta have a very strong ionizing effect and so generate a plurality of charge carriers in the semi-conductor material. The higher the energy of the X-ray quantum, the more charge carriers are generated. The charge quantities generated in the detector are therefore measured at periodic intervals and transduced into an energy spectrum by means of a multi-channel analyzer to read out the detector. Counts of up to 2-10
4
counts per second (cps) can be achieved with such detectors; 2-10
4
X-ray quanta per second can therefore be measured.
One disadvantage of such detectors is that they only supply high-resolution energy spectrums when the thermal noise is effectively suppressed by cooling with liquid nitrogen. The required nitrogen Dewar flasks are large and problematic to handle.
German Patent DE196 20 081 A1 and PCT application PCT/DE97/01015 describe a strip detector and a method of manufacturing a strip detector for the detection of ionizing particles and/or radiation. Regions n-doped at least at one substrate surface of the silicone substrate and a p-doped insulation region between the n-doped regions are provided as strips and voltage supply areas. A first insulator film is applied to one substrate surface and metal strips are arranged above the n-doped region.
SUMMARY OF THE INVENTION
The invention is characterized by at least one further insulation film being provided immediately above the first insulator film and by at least one of the insulation films being interrupted in projection over the intermediate region of two adjacent n-doped regions and by the p-doped insulation area having a lateral p-dope material concentration distribution which provides a higher dope material concentration in the region beneath the interruption of the interrupted insulation film than in the insulation region directly adjacent to the n-doped region. The insulation structure in accordance with the invention is also suitable for the insulation of guard rings which gradually reduce the high operating voltage towards the detector periphery. It insulates adjacent rings extending around the whole detector so that different potentials with low electrical field strengths can develop on them.
It is the object of the invention to provide an apparatus and a method for X-ray fluorescence analysis to allow a faster measurement with better resolution of the X-ray fluorescence of a sample and a reduction in the effort required to cool the detector.
This object is solved in accordance with the invention by an apparatus for total reflection X-ray fluorescence analysis in which the smooth planar surface of a sample or thin film on a sample is excited by the incident X-ray radiation and the X-ray fluorescence radiation emitted is detected spectrally and which comprises a radiation source, a monochromator and a transducer, with the transducer comprising at least one DRIFT detector, an electrical field being capable of generation in each DRIFT detector by means of an electrode array of electrodes at different voltage levels and having a radial component. Charge carriers which are created are accelerated towards a low-capacitance collecting electrode by this radial component of the electrical field.
Counts of up to 10
5
cps can be achieved by using a transducer with at least one DRIFT detector. This is made possible by the charge carriers generated by an X-ray quantum being guided faster to the collecting electrode through the electrical field prevailing in the detector interior. The main reason for the high count capable of being achieved with the aid of DRIFT detectors is, however, the low capacitance of the collecting electrode. The radial component of the electrical field accelerates the charge carriers towards the collecting electrode and as a result the collecting electrode can be made with a small surface and so a low capacitance.
In summary it can be said that the read-out times can be shortened significantly when DRIFT detectors are used. Higher counts can be processed in this way and spectrums with better energy resolution are obtained. DRIFT detectors also require less effort for cooling; in particular, the high-effort nitrogen cooling can be omitted.
It is of advantage if the electrode array comprises a plurality of annular electrodes arranged concentrically and at different voltage levels. With such an electrode array, an electrical field with a radial component is produced.
In accordance with another advantageous embodiment of the invention, a first transistor of a pulse amplification stage is integrated in the collecting electrode. This can be done by the film structures of a field-effect transistor being integrated in the centre of the DRIFT detector, the gate of the field-effect transistor being electrically connected to the collecting electrode. In this way, the connection between the collecting electrode and the gate of the FET can be shortened and the parasitic capacitance of the connecting line is greatly reduced. Moreover, there is a reduction in the noise captured by the inductive coupling.
In accordance with another aspect of the invention, the DRIFT detector can be cooled by means of a Peltier cooler element. A cooling of up to 30° relative ambient can be achieved with a one-stage Peltier cooler element. Such a cooling is sufficient to allow highly resolved energy spectrums to be registered by DRIFT detectors. The nitrogen cooling required for prior detectors can therefore be replaced by a Peltier cooler, with the Peltier element being integrated directly in the DRIFT detector.
It is of advantage to arrange the radiation source, the monochromator and the transducer in a vacuum housing. Moreover, it is of advantage if the transducer has a thin window of a thickness of less than 2 &mgr;m as the measurement window. It is furthermore of advantage if the radiation source consists of a low energy X-ray tube.
The measurement range of total reflection X-ray fluorescence analysis can be expanded to elements with a low ordinal number Z, that is to light elements, using the three above-mentioned measures. These elements emit fluorescence X-ray radiation of only low energy. The unwanted absorption values of the X-ray radiation can be reduced with the aid of the vacuum housing and of the thin measurement window. A low-energy X-ray tube emits excitement radiation which is near the energy of the fluorescent X-ray radiation of light elements. The fluorescence efficie
Barber Therese
Dilworth & Barrese LLP
GeMeTec Gesellschaft fuer Messtechnik und Technologie mbH
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