Process and device for emission spectorscopy

• US 5,446,538 A
• Filed: 12/06/1993
• Issued: 08/29/1995
• Est. Priority Date: 06/06/1991
• Status: Expired due to Fees

• Current Assignee: Fraunhofer Gesellschaft Zur Foerderung Der Angewandten Forsching E.V.
• Original Assignee: Fraunhofer Gesellschaft Zur Foerderung Der Angewandten Forsching E.V.
• Inventors: Noll, Reinhard
• Primary Examiner(s): McGraw, Vincent P.
• Application Number:
  US08/157,170
• Time in Patent Office: 631 Days
• Field of Search: 356/318
• US Class Current: 356/318
• CPC Class Codes: G01N 21/718 Laser microanalysis, i.e. with formation of sample plasma

Abstract

Process for emission spectroscopy, particularly for laser emission spectropy, wherein the radiation emitted by the laser-induced plasma of the workpiece to be analyzed is decomposed by a spectrometer and at least one fraction of the found spectrum is transferred to a processing unit. In order to improve the process from the point of view of measurement precision and speed, it is carried out in such a manner that an influencing of the intensity of the plasma-inducing laser beam takes place depending on at least one emission-influencing parameter for the production of definite plasma states, this parameter being measured during plasma formation, and that the transfer of the found spectrum or of a fraction thereof to the processing unit is performed, as long as the measured plasma parameter is within a predetermined tolerance range (T_I./.T_II).

Claims

1. A method of elemental analysis comprising the steps of:

(a) directing a laser beam at a material having at least one element for which said material is to be analyzed and with an intensity and for a duration sufficient to generate a plasma at said material emitting spectroscopically detectable radiation including emissions from said element, said laser beam being capable of producing different plasma states in said plasma;

(b) spectroscopically detecting a spectrum of said radiation, transferring at least a portion of the detected spectrum, decomposing at least said portion of said spectrum to determine said emissions, and analyzing for said element from the determination of said emissions;

(c) selecting a tolerance range of values for at least one emission-influencing parameter at which said spectrum most accurately represents an analysis of said element;

(d) monitoring said plasma during the generation thereof for said emission-influencing parameter; and

(e) transferring at least said portion of said spectrum for determination of said emissions and analysis for said element only upon said emission-influencing parameter falling within said range, and blocking transfer of at least said portion of said spectrum for determination of said emissions and analysis for said element upon said emission-influencing parameter lying above and below said range.

2. The method defined in claim 1 wherein said at least one emission-influencing parameter is a temperature of the plasma.

3. The method defined in claim 1 wherein, in addition to the plasma temperature, a density of the plasma is monitored as an emission-influencing parameter and said spectrum is transferred for determination of said emissions and analysis for said element only upon both of said emission-influencing parameters falling within respective tolerance ranges.

4. The method defined in claim 1, further comprising the step of measuring a maximum value of at least one parameter of said plasma, comparing said maximum value as an actual value with a set-point value range, of varying an energy of pulses of said laser beam to increase an energy of a pulse of said laser beam when said actual value lies below said set-point range and to decrease an energy of a pulse of said laser beam when said actual value is above said set-point range.

5. The method defined in claim 1 wherein a curve of a parameter of the plasma is measured as an actual measurement curve and is compared to an ideal curve of the respective parameter, further comprising the step of determining differences between said curves and controlling an energy of the laser beam continuously in response to the determined differences.

6. The method defined in claim 1 wherein an actual value of a parameter of said plasma is measured and compared with an ideal value specific to at least one element of said material to be analyzed and differences between the actual value and measured value are determined and an energy of said laser beam is controlled in response to the measured differences.

7. The method defined in claim 1 wherein radiation emitted from a common source at said material is directed toward a spectrometer for spectroscopically detecting said spectrum and a sensor effecting monitoring of said plasma.

8. The method defined in claim 1 wherein said material is material of a workpiece produced in a fabrication process, said method further comprising the step of controlling said process in response to said analysis for said element.

9. The method defined in claim 1 wherein emission intensities of a number of predetermined emission lines of the detected spectrum and said emission-influencing parameter are recorded in a time-dependent manner prior to the transfer.

10. An apparatus for elemental analysis, comprising:

a laser directing a laser beam at a material having at least one element for which said material is to be analyzed and with an intensity and for a duration to generate a plasma at said material emitting spectroscopically detectable radiation including emissions from said element;

a spectrometer trained upon said plasma for spectroscopically detecting a spectrum of said radiation, a beam path extending between said plasma and said spectrometer, said spectrometer being provided with means for transferring at least a portion of the detected spectrum, decomposing at least said portion of said spectrum to determine said emissions, and analyzing for said element from said determination of said emissions;

a lens system along said beam path;

a partially transparent mirror in said beam path for directing radiation from said plasma along another path;

a sensor for at least one emission-influencing parameter having a tolerance range of values at which said spectrum most accurately represents an analysis of said element, positioned along said other path for monitoring said plasma during the generation thereof for said emission-influencing parameter;

a laser beam permeable mirror in said beam for shielding said laser against radiation emitted from said plasma; and

means for transferring at least said portion of said spectrum for determination of said emissions and analysis for said element only upon said emission-influencing parameter falling within said range, and blocking transfer of at least said portion of said spectrum for determination of said emissions and analysis for said element upon said emission-influencing parameter lying above and below said range.

11. The apparatus defined in claim 10, further comprising a laser control and adjustment unit connected to said sensor and receiving a predetermined ideal value of said parameter for controlling said laser.

12. The apparatus defined in claim 10 wherein said unit includes a comparator receiving a measured value of said parameter and comparing said measured value with said ideal value.

13. The apparatus defined in claim 12 wherein said sensor is connected to a memory coupled with the spectrometer through a solid diaphragm with respective detectors tuned to respective emission lines.