Reflective Overcoat for Replicated Diffraction Gratings

This invention relates to diffraction gratings and, in particular, to a technique used for extending the life of a diffraction grating.

Diffraction gratings are frequently used in lasers for reflecting back into a laser's resonating cavity only a narrow range of wavelengths of light centered at one particular wavelength. Light energy at this wavelength resonates within the cavity and is emitted through a partially reflective mirror at the other end of the cavity. Examples of such diffraction gratings and various methods of making these diffraction gratings are described in U.S. Pat. Nos. 5,080,465; 5,436,764; and 5,493,393, incorporated herein by reference.

Typically, a master diffraction grating is first manufactured. This master grating is then used to form many replica gratings. Each of these replica gratings may then be used as a master grating for forming other replica gratings. Accordingly, compared to the master, the replicas may be made inexpensively.

As described in the '465 patent, a master grating may be formed by depositing aluminum over a substrate, such as glass. A diamond tool under interferometric control may then be used to etch very closely spaced grooves in the aluminum layer. The separation of the grooves is related to the wavelength of the light to be reflected by the grating and to the narrowness of the range of wavelengths it is required to reflect. In one embodiment, the diamond tool etches on the order of tens of thousands of grooves per inch. The diffraction grating may be ten inches square and one inch thick. Creating a precision master grating by physical etching is, therefore, an extremely time consuming and expensive process.

Once a master grating has been made, replicas of the grating are made in the following process. A release agent, such as described in the '764 patent, is coated on the surface of the master. This is preferably done in a vacuum chamber. A thin (e.g., 1 micron) reflective layer, such as aluminum, is then sputtered or evaporated onto the release layer. The master grating is then removed from the vacuum chamber. Liquid epoxy, such as described in the '465 patent, is then deposited on the aluminum layer, and a glass substrate is then placed on top of the epoxy. After the epoxy is cured, the glass layer, epoxy layer, and aluminum layer are then lifted from the master grating, resulting in a replica of the master grating.

Applicant has discovered that, when the replica is removed from the master grating, imperceptible damage is done to the thin layer of aluminum on the replica.

When the replicated diffraction grating is used as a selective wavelength reflector for generating a laser light beam, the replicated grating is subjected to intense light energy. Changes in the groove shape occur when the grating is exposed to intense light for long periods of time. This causes a loss of reflectance in the desired diffraction order, where some of the optical energy is shifted to other diffraction orders due to large scale groove angle changes, and some of the energy is lost to scatter due to small scale surface distortions of the groove faces.

Up until now, it was believed that the lifetime of such replicas could not be extended.

Applicant has discovered that the aluminum coating deposited during the replication process, typically about 1 micron thick, is subjected to forces during the subsequent take-apart step of the replication process which tend to generate very small fractures in the aluminum coating. These fractures allow small quantities of ultraviolet radiation to leak through to the underlying epoxy when the grating is put into service in an excimer laser. The ultraviolet light which reaches the epoxy causes photodecomposition of the epoxy, releasing gases which cause blisters in the overlying aluminum coating. This blistering greatly increases scatter losses from the reflecting facets of the grating. The ultraviolet light also causes bulk shrinkage of the epoxy, which distorts the original groove shape, causing a loss of reflectivity in the desired diffraction order. These effects severely limit the useful lifetime of the grating, causing unavailability of the equipment in which it is used at relatively frequent intervals.

What is needed is an improved replication process which lengthens the life of the replica gratings.

An extra step for the conventional replica forming process for diffraction gratings used in lasers is described herein which overcomes the problems previously described.

Applicant's improvement to this conventional process is to deposit a thin reflective overcoat of aluminum on the surface of the replica after the replica has been removed from the master and cleaned. The overcoating is performed in a vacuum chamber by sputtering or evaporation. This overcoat is not subjected to any subsequent destructive forces and hence remains relatively opaque. A typical thickness of the overcoat is around 100 nm.

FIGS. 1-6 show a cross-section of a master grating and the subsequent steps used to form a conventional replica from the master grating.

FIG. 7 illustrates the subsequent step of forming a thin reflective overcoat on the replica to protect the underlying epoxy from radiation passing through cracks in the original aluminum layer.

FIG. 8 illustrates an alternative embodiment overcoat incorporating a plurality of layers to increase opacity.

FIG. 9 illustrates an alternative embodiment using an MgF.sub.2 layer over the aluminum overcoat to prevent oxidation damage.

FIG. 1 is a cross-section of a master diffraction grating 10 which has been created using conventional processes. In one embodiment, master grating 10 is one inch thick and consists of a glass substrate 12 and an etched aluminum layer 16. Layer 16 is typically around 5 microns thick and mechanically etched to form grooves 18 using a diamond stylus, as described in the previously mentioned prior art patents. The features of the grooves 18 in the aluminum layer 16 are greatly exaggerated since there may be greater than 10,000 grooves per inch etched in the aluminum layer 16. In one embodiment, grooves 18 are spaced 12 microns apart. The master grating 10 may be formed by other processes and may even be a replica of a master.

FIGS. 2-6 illustrate the conventional steps used to form a replica of this master grating 10.

The master grating 10 is placed in a vacuum chamber similar to those used for wafer fabrication. A release agent 19, such as silicone oil or any other known release agent, is then deposited on the aluminum layer 16 to form a release layer 20. This release layer 20 is preferably extremely thin, such as on the order of a few molecules deep (e.g., 1-2 nm). The thickness of the release layer 20 may be determined by well known optical methods used in conventional wafer fabrication process.

In FIG. 3, aluminum 21 is then evaporated or sputtered onto the release layer 20 to form an aluminum layer 22 about 1 micron thick. The master grating 10 is then removed from the vacuum chamber.

In FIG. 4, a layer of liquid epoxy 24 is then deposited over the aluminum layer 22. Epoxy layer 24 may be virtually any thickness (e.g., 15 microns).

As shown in FIG. 5, a glass substrate 26, used for additional mechanical support if necessary, is then positioned over the epoxy 24. The epoxy 24 is then cured such as by heat or other methods until hardened. In another embodiment, epoxy-coated glass is placed over aluminum layer 22.

After the epoxy has hardened, the glass substrate 26 is removed vertically from the master grating 10 to separate the replica from the master grating 10.

The resulting replica 28 is shown in FIG. 6. As previously mentioned, the removal step in FIG. 5 frequently causes imperceptible cracking of the aluminum layer 22 as illustrated by cracks 30 and 31. These cracks were not previously discovered to Applicant's knowledge, and the effects of these cracks on the life of a replica were not appreciated. As previously described, these cracks allow some portion of the ultraviolet energy from a laser to impinge upon the underlying epoxy 24. Over time, this disintegrates and shrinks the epoxy and causes distortion of the overlying aluminum layer 22. Applicant's analysis of this effect has shown that the effective groove geometry changes and blisters occur on the grating's reflecting facets 32 as a result of this disintegration of the epoxy after a large number of laser pulses. The blistering manifests itself as a hazy region on the grating surface which is visible to the naked eye. This haziness reduces the efficiency of the grating, causing less light to be reflected into the desired diffraction order.

Applicant's additional step is illustrated in FIG. 7. A thin layer 34 of aluminum 35 sputtered or evaporated onto the surface of the prior aluminum layer 22 in a vacuum chamber. This thin aluminum layer 34 need only be on the order of 100 nm to prevent any subsequent deterioration of the epoxy layer 24 due to the cracks 30 and 31 in the underlying aluminum layer 22.

Accordingly, the fractures in the original aluminum layer 22 are now covered or filled in. This prevents ultraviolet light from affecting the underlying epoxy 24. Since the thin aluminum layer 34 overcoat is applied after the replication process is complete, it is not subjected to any subsequent destructive forces, and hence remains relatively opaque. Other thicknesses of the aluminum layer 34 (e.g., up to 1 micron) may be used; however, a thin aluminum layer is desirable to maintain the original characteristics of the facets in the replica grating shown in FIG. 6.

The aluminum overcoat may be applied in two or more layers, with oxide layers in between, to increase the resulting overcoat opacity as needed. This is illustrated in FIG. 8, where an additional oxide layer 36 and an additional aluminum layer 38 are shown.

FIG. 9 illustrates another embodiment, where a thin layer 40 of MgF.sub.2 is deposited, using conventional techniques, over the aluminum layer 34 of FIG. 7 to prevent oxidation damage to the aluminum layer 34.

Applicant has performed extensive tests on replica diffraction gratings using the above-described process and has confirmed that the lifetime of diffraction gratings with the aluminum overcoat is extended far beyond that of untreated gratings. The reflectance of the overcoated grating over its lifetime varies much more slowly than that of untreated gratings. The replacement interval and associated operating costs of the laser system are greatly reduced.

While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications as fall within the true spirit and scope of this invention.