GB1567281A

GB1567281A - Gaseous discharge lamp

Info

Publication number: GB1567281A
Application number: GB3241277A
Authority: GB
Original assignee: Perkin Elmer Corp
Current assignee: Applied Biosystems Inc
Priority date: 1976-08-06
Filing date: 1977-08-02
Publication date: 1980-05-14
Also published as: DE2734130A1; JPS5325084A

Description

(54) GASEOUS DISCHARGE LAMP (71) We, THE PERKIN ELMER COR PORATION, a Body Corporate organised and existing under the laws of the State of New York, whose principal place of business is at Main Avenue, Norwalk, Connecticut 06856, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates generally to gaseous discharge lamps and particularly to such lamps of generally arcuate configuration and capillary dimensions for use in producing high intensity radiation.

Heretofore, gaseous discharge lamps of various types and configurations have been employed for a wide variety of applications.

In general, such lamps employ a seal transparent envelope containing a gas or vapour, e.g. a metallic vapour, such as mercury vapour, and include means for energising or exciting the gaseous substance to form a radition-emitting plasma. This is normally effected by means of applying a voltage to a pair of spaced electrodes, or by means of coupling a source of radio frequency electric reactance to the lamp.

Lamps constructed in accordance with this invention are particularly adapted, among other possible uses, for use with apparatus for the exposure of photoresistcoated wafers in the production of semioonductor devices, as disclosed in Patent No.

1,459,272.

According to the present invention, a gaseous discharge lamp comprises a radiation-transmitting tubular envelope defining a plasma cavity of arcuate configuration containing an ionizable gaseous substance and, on the rear side of the tubular envelope, a reflective coating which is formed of alternating layers of high and low index dielectric material having a melting point above the operating temperature of the lamp. This increases the useful light output from the lamp by reflecting light which would normally be lost, and is made possible by the specific type of coating used since the high operating temperature of the lamp renders the use of a conventional metallic coating impossible. Suitable dielectric materials include zirconium dioxide as the high index material and silica as the low index material, for example.

A gaseous discharge lamp in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an enlarged side elevation, partially in longitudinal section of the lamp; and Figure 2 is an enlarged sectional view taken along the line indicated at 2-2 in Figure 1.

The illustrated gaseous discharge lamp indicated generally at 10, is of the mercury vapour type and has a particular arcuate configuration adapting it for use as the light source in an apparatus such as the one described in the aforementioned Patent No.

1,459,272 for photoresist exposure in the production of semi-conductor devices. The lamp 10 includes a tubular envelope 12 of glass or other radiation-transmitting material and the mid-sectiop of the envelope, which emits the major portion of the useful illumination, is of an arcuate form. As illustrated the arcuate mid-section of the envelope subtends an arc of about 85" with a radius of curvature of about 0.55 inches, for example, but it should be understood that neither the length nor the sharpness of the bend are critical except to the particular apparatus in which the lamp is employed.

The interior of the envelope 12 is in the form of a bore or plasma cavity 14 of capillary dimensions such as, for example, a diameter of about 2 mm. The envelope terminates at each end in solid cylindrical stubs 16, 18 which are of slightly smaller diameter than the main body of the envelope. The cavity terminates a short distance imvardly of the end stubs and electrodes 20, 22 are provided at each end. The electrode 20 is electrically connected by a conductor, not shown, to a highly flexible connection lead 24, which is mechanically secured to the stub 16 at one end and to a connection fitting, such as a banana plug 26, at its free end. The lead 24 may be fabricated of any suitable material such as tubular silver braid, for example.

In the installations where R-F energization is utilised, the electrodes 20, 22 are omitted and other potential coupling means are provided. Reference is hereby made to "Lamps and Wavelengths of Mercury 198" (Research paper RP 2091) by Meggers and Oliver in the "Journal of the National Bureau of Standards", Vol. 44 May 1950; and to "High Intensity Low Noise Rubidium Light Source" by R. G. Brewer in "The Review of Scientific Instruments" Vol. 32, No. J2 (Dec. 1961) pp. 135W1358.

The opposite end stub 18 of the envelope 12 is provided with a rigid mounting base 28 as described in detail in the co-pending Application No. 54500/74 (Serial No.

1,491,897). Thus the base and its co-operative relationship with the envelope are such that a gaseous discharge lamp is provided, which may be initially installed or a replacement installed in its operating environment so as to position the plasma with a high degree of precision. As indicated in this patent, the base 28 takes the form of a right angular hexagonal prism containing an axial bore 30 closed at one end by a flat surface 32 perpendicular to the common axes of the hexagonal prism and the internal bore. The diameter and length of the bore 30 are such as to receive the end stub 18 with sufficient clearance to enable longitudinal, lateral and angular positional adjustment of the stub relative to the base.Thus during assembly of the envelope to the base, the spatial relation and orientation of these two members can be precisely established as by the use of a suitable jig or fixture and, when established, is maintained by securing the base to the envelope stub with solder 34 or by other suitable means.

A multi-layer reflective coating 36 is disposed on the rear side of the envelope 12.

This coating is formed of alternate layers of high and low index dielectric materials having very high melting points, i.e. higher than the operating temperature of the lamp.

It is noted that the operating temperature of the lamp is literally red hot so that con ventional metallic coatings would not be suitable. Moreover, the arcuate geometry of the lamp makes it very diflicult, if not impossible, to employ remote spherical reflectors. A number of layers, such as of the order of about twenty for example, are applied by suitable means, such as by electron beam heating-vacuum deposition, for example. Each layer has a thickness of about one quarter of the design wavelength of the system. This multi-layer coating forms a reflector, which reflects light normally lost from the back of the lamp, and thus increases the useful light output from the uncoated side of the lamp, as indicated at 38 in Figure 2. This arc of useful light is of the order of about 60 for example.The output from the lamp is preferably enhanced in the spectral continuum where the plasma is relatively transparent as compared to the emission lines. This reduces the degree of spectral coherence and thereby reduces bothersome interference phenomena in associated imaging and photo copying systems.

WHAT WE CLAIM IS: - 1. A gaseous discharge lamp comprising a radiation-transmitting tubular envelope defining a plasma cavity of arcuate configuration containing an ionizable gaseous substance and, on the rear side of the tubular envelope, a reflective coating which is formed of alternating layers of high and low index dielectric material having a melting point above the operating temperature of the lamp.

2. A gaseous discharge lamp according to claim 1 wherein the plasma cavity is of capillary dimensions.

3. A gaseous discharge lamp according to claim 1 or claim 2 wherein the ionizable gaseous substance is mercury vapour.

4. A gaseous discharge lamp according to any one of the preceding claims wherein the high index dielectric material is zirconium dioxide.

5. A gaseous discharge lamp according to claim 4 wherein the low index dielectric material is silica.

6. A gaseous discharge lamp according to any one of the preceding claims wherein each of the layers has a thickness of about one quarter of the design wavelength of the lamp.

7. A gaseous discharge lamp according to claim 6 wherein there are about twenty layers of dielectric material in the coating.

8. A gaseous discharge lamp according to any one of the preceding claims wherein the coating is arranged to form an arc of useful light at the front of the envelope of the order of about sixty degrees.

9. A gaseous discharge lamp substantially as described and as illustrated with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. of the end stubs and electrodes 20, 22 are provided at each end. The electrode 20 is electrically connected by a conductor, not shown, to a highly flexible connection lead 24, which is mechanically secured to the stub 16 at one end and to a connection fitting, such as a banana plug 26, at its free end. The lead 24 may be fabricated of any suitable material such as tubular silver braid, for example. In the installations where R-F energization is utilised, the electrodes 20, 22 are omitted and other potential coupling means are provided. Reference is hereby made to "Lamps and Wavelengths of Mercury 198" (Research paper RP 2091) by Meggers and Oliver in the "Journal of the National Bureau of Standards", Vol. 44 May 1950; and to "High Intensity Low Noise Rubidium Light Source" by R. G. Brewer in "The Review of Scientific Instruments" Vol. 32, No. J2 (Dec. 1961) pp. 135W1358. The opposite end stub 18 of the envelope 12 is provided with a rigid mounting base 28 as described in detail in the co-pending Application No. 54500/74 (Serial No. 1,491,897). Thus the base and its co-operative relationship with the envelope are such that a gaseous discharge lamp is provided, which may be initially installed or a replacement installed in its operating environment so as to position the plasma with a high degree of precision. As indicated in this patent, the base 28 takes the form of a right angular hexagonal prism containing an axial bore 30 closed at one end by a flat surface 32 perpendicular to the common axes of the hexagonal prism and the internal bore. The diameter and length of the bore 30 are such as to receive the end stub 18 with sufficient clearance to enable longitudinal, lateral and angular positional adjustment of the stub relative to the base.Thus during assembly of the envelope to the base, the spatial relation and orientation of these two members can be precisely established as by the use of a suitable jig or fixture and, when established, is maintained by securing the base to the envelope stub with solder 34 or by other suitable means. A multi-layer reflective coating 36 is disposed on the rear side of the envelope 12. This coating is formed of alternate layers of high and low index dielectric materials having very high melting points, i.e. higher than the operating temperature of the lamp. It is noted that the operating temperature of the lamp is literally red hot so that con ventional metallic coatings would not be suitable. Moreover, the arcuate geometry of the lamp makes it very diflicult, if not impossible, to employ remote spherical reflectors. A number of layers, such as of the order of about twenty for example, are applied by suitable means, such as by electron beam heating-vacuum deposition, for example. Each layer has a thickness of about one quarter of the design wavelength of the system. This multi-layer coating forms a reflector, which reflects light normally lost from the back of the lamp, and thus increases the useful light output from the uncoated side of the lamp, as indicated at 38 in Figure 2. This arc of useful light is of the order of about 60 for example.The output from the lamp is preferably enhanced in the spectral continuum where the plasma is relatively transparent as compared to the emission lines. This reduces the degree of spectral coherence and thereby reduces bothersome interference phenomena in associated imaging and photo copying systems. WHAT WE CLAIM IS: -

1. A gaseous discharge lamp comprising a radiation-transmitting tubular envelope defining a plasma cavity of arcuate configuration containing an ionizable gaseous substance and, on the rear side of the tubular envelope, a reflective coating which is formed of alternating layers of high and low index dielectric material having a melting point above the operating temperature of the lamp.