UV-resistant Jointing Technique for Lenses and Mounts

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1. Field of the Invention

The invention relates to an optical assembly of an optical element of transparent material and a mount or mount element.

The invention also relates to a process for fastening an optical element of non-metallic, transparent material, particularly quartz glass or calcium fluoride, to a mount or mount element.

The mount or mount element in the sense of the invention is to be understood to mean any component that is not in itself light-conducting and is suitable for mechanically fixing one or more optical elements and connecting them into an optical assembly or bringing them into operative connection, or bringing them into operative connection with adjusting elements.

The invention further relates to UV optical systems, microscopes and projection illumination equipments with at least one optical assembly according to the invention.

2. Discussion of Relevant Art

Jointing techniques for the connection of lenses, prisms, flat plates and mirrors to mounts are well known.

Cementing is important because particularly small stresses on the optical parts are attained. Modern forms use UV setting adhesives. The organic cements however exhibit distinct radiation damage when irradiated with wavelengths shorter than the Hg-I line, so that they are hardly usable for DUV projection illumination equipments with a 193 nm excimer laser.

Conventional positive connections such as flanging or attachment rings produce excessive strains in the optical material in this field of application.

Annular brazing of glass and the like to mounts is known from European Patent EP 0 761 623. These serve primarily as gastight joints of windows for lasers, vacuum chambers and the like. As a rule, the operation uses high temperatures in the brazing region at about 400.degree. C. and higher. Such high temperature treatment is excluded in the case of finished precision optics with antireflection layers and the like.

From U.S. patent application Ser. No. 09/134,227 of Carl Zeiss, a suitable soldering technique for optical precision mounts is known, with an adhesion layer of quartz or calcium fluoride, as well as providing a material locking connection of a local nature with studs or lugs.

The object of the invention is to provide a UV-resistant, high precision jointing technique with little stress and temperature loading.

This object is attained by an optical assembly having an optical element of non-metallic material, a mount or mount element, and a layer of a solder arranged between the optical element and the mount or mount element that forms a positively locking connection to the optical element. This object also is attained by a process for fastening an optical element of nonmetallic, transparent material comprising providing a surface of the mount or the mount element that forms a gap with the optical element, coating the surface that forms the gap with the optical element with solder, placing the optical element at the solder, and melting the solder, resulting in a positively locking connection of the mount or mount element with the optical element.

A microscope or a projection illumination equipment or an optical system having an optical assembly according to the present invention is also attained.

The jointing technique according to the invention can thus be characterized as a kind of flanging by melting. Differing from soldering, brazing or welding, a material locking of the joint is not required, but a positive locking is produced. The solders that are commercially used in soldering, and in particular the low-melting metals and metal alloys, are used.

The invention will be described in detail with reference to the drawing, in which:

FIG. 1 shows in schematic cross section a lens held in a mount.

FIG. 1 shows in schematic cross section a lens 1 held in a mount or mount element 3 with a ring 2 of soft solder.

A mount or mount element 3 is produced with a larger diameter for the lens seat (solder gap 0.1 mm); thus there is on the mount or mount element 3 an annular surface 30 which forms a gap with the lens 1. The lens seat (surface 30) is soldered, and as a rule (but not imperatively) a material locking solder connection is obtained. A solder 2 typically has a thickness of 0.1 mm. The solder application can take place by electroplating or by conventional application of the melt.

The exact lens seat is thereafter finished (turned) with a clearance fit, so that the lens 1 can be correctly positioned. The lower lens abutment surface 31 must be free from solder in order to insure the positional fixation.

The lens 1 is then placed in the mount or mount element 3 and is heated, with the mount or mount element 3, on a hotplate until the solder 2 melts. In order to insure accurate and fixed positioning of the lens 1, the lens 1 can be clamped or fixed in a jig during the heating.

The solder 2 encloses the lens external diameter, and after cooling the lens 1 is firmly seated in its place because the solder 2 completely abuts the outer edge of the lens 1 and flows slightly over the upper edge, thus forming a kind of flange edge 21. The process thus leads to pure positive locking.

This process is also outstandingly suitable for mounting CaF.sub.2 because with uniform heating no temperature strains can arise.

The material for the mount or mount element 3 should have a coefficient of thermal expansion similar to that of the lens 1. For example, when the lens 1 is made of quartz glass, the mount or mount element 3 may be made of invar (a nickel alloy with minimal thermal expansion). As the solder 2, a metal with a melting point below about 100.degree., 50-70.degree. C. is preferred, e.g., the alloy BiPbSnIn with melting point 58.degree. C. According to the kind of glass and the conditions of use, higher-melting solders, as a rule soft solders, can also be used.

This jointing technique is particularly suitable for lenses 1 up to a few centimeters in diameter (in this embodiment, a diameter of 20 mm), e.g. in UV microscopes or for small lenses in projection illuminating equipments for microlithography. In the same way, mirrors of mirror-coated glass or glass-ceramic, or ceramic bodies, prisms and other optical elements, can be fastened to a mount.

Individual optical elements can be connected to several separated mount elements with solder in this manner. The mount elements can be connected by means of optional (metal-metal) jointing techniques to other parts, e.g., to annular mount parts.