The invention relates to a system and method for producing narrow linewidth emission from lasers and, more particularly, to a grating cavity for excimer lasers configured to produce UV emission at high optical output power and with a narrow emission linewidth.
Lasers are used, for example, for material processing and in semiconductor manufacturing, where the small feature size of today's circuit elements and interconnects demands very fine structural feature definition on the scale of tens of nanometers. Lasers are also employed in the production of optical gratings, such as volume gratings, holographic gratings, as well as in the distributed feedback (DFB) and distributed Bragg reflection (DBR) sections in diode lasers and optical fibers. The dimensions of the achievable structural features are related to the laser wavelength and to laser linewidth. Modern semiconductor fabs increasingly use wavelengths in the deep ultraviolet (UV) and soft x-ray lithography for mask exposure. In addition, the definition of these small features also requires novel mask designs, such as phase masks, and the narrowest attainable laser linewidth, because a spectral distribution of the illuminating source would tend to wash out the desired features.
Suitable light sources operating in the UV and deep UV are excimer lasers, such as ArF, KrF excimer lasers, which have an emission linewidth of about 300 pm (full width half maximum or FWHM) at wavelengths of less than 250 nm.
In one prior art technique, an excimer laser cavity is formed by an output coupler in form of a partially reflecting mirror and an echelle grating, reducing the linewidth of a KrF laser from 300 pm to about 0.8 pm. However, some applications, for example, submicron lithography (<0.25 μm) for integrated circuit fabrication requires linewidth of about 0.5 pm or less. In another prior art approach, a double-pass configuration with a single echelle grating was used, whereby the laser radiation propagating in the cavity impinges on the grating twice with different polarization directions. However, this particular arrangement is not practical for generating the high laser output power required for high-throughput mask exposure due to the relatively low diffraction efficiency of conventional echelle gratings. The diffraction efficiency is typically about 60% per pass, therefore, the double pass loss from the gratings alone would be 0.6×0.6=36% which would be too for applications in semiconductor manufacturing.
Another prior art approach for decreasing the bandwidth places etalons inside the laser cavity to filter out unwanted wavelength regions of the emission spectrum. However, etalons are susceptive to optical damage which makes them unsuitable for high power laser applications.
Another prior art approach achieves linewidth narrowing with a grism (prism-grating) which combines the refractive properties of a prism with the diffractive properties of a grating. Grisms can be designed to operate as highly reflective rear mirrors or as partially reflective output couplers. The development of integrated circuits with an ever increasing component density and decreasing feature size requires high power illumination sources operating at shorter wavelengths and having a narrow emission linewidth, while retaining high efficiency. Accordingly, there is a need for high-efficiency optical modules that are able to further narrow the optical linewidth of excimer lasers without sacrificing output power and electrical-to-optical conversion efficiency.
The invention is directed to a laser cavity employing a plurality of gratings for narrowing the linewidth of the laser output beam.
According to one aspect of the invention, a line-narrowed excimer laser system includes a laser cavity with a gain medium, an output coupler and a reflector assembly, wherein the reflector assembly includes a first multilayer dielectric diffraction grating and a second diffraction grating arranged in sequence. The first multilayer dielectric diffraction grating receives laser light from the gain medium at a first angle of incidence of less than 800 with respect to the grating normal of the first grating and wavelength-selectively diffracts the laser light towards the second grating. The second diffraction grating operates in a Littrow configuration and diffracts the received laser light back to the first grating where it is additionally diffracted. wherein only a portion of the additionally diffracted laser light that is substantially aligned with the optical axis of the laser cavity effectively contributes to laser emission through the output coupler, thereby narrowing the linewidth of the laser emission wavelength relative to the wavelength distribution of the laser light produced in the gain medium.
Embodiments of the invention may include one or more of the following features. The first multilayer dielectric diffraction grating may include a dielectric stack with a plurality of continuous layers, wherein each layer of the plurality of continuous layers comprises either a high refractive index dielectric material or a low refractive index dielectric material, wherein the high refractive index dielectric material and the low refractive index dielectric material have a difference in refractive index greater than 0.1, wherein the plurality of continuous layers has a top layer and a bottom layer, wherein the bottom layer is affixed to the substrate, and wherein each layer of the plurality of continuous layers comprises a continuous film. The first multilayer dielectric diffraction grating further includes a nonmetallic diffraction grating disposed on the top layer of the plurality of continuous layers and having a single pair of layers made of a low refractive index dielectric material having a first thickness and a high refractive index dielectric material having a second thickness, with grooves extending through the pair of layers, wherein each groove has a shape, and wherein the number of layers of the plurality of continuous layers and the first and second thickness the single layer are selected to achieve a diffraction efficiency of at least 85% at the laser emission wavelength.
The nonmetallic diffraction grating may operate in 3rd order.
The dielectric stack or the substrate, or both, may be made of a dielectric material that is transparent to the laser emission wavelength. The dielectric material may be an oxide material, a fluoride material, a sulfide material and/or a selenide material. The single pair of layers may be made of materials that are substantially identical to those of which the continuous layers are made. The high refractive index dielectric material may be Al2O3 and the low refractive index dielectric material may be SiO2. The thickness of the layers of the layer pair may be different from the thickness of corresponding layers of the continuous layers having substantially the same index of refraction. For example, the top Al2O3 layer of the layer pair may the thicker than the Al2O3 layers of the dielectric stack. The same applies to the SiO2 layers.
Further features and advantages of the present invention will be apparent from the following description of preferred embodiments and from the claims.
The above and further advantages of the invention may be better understood by referring to the following description in conjunction with the accompanying drawings in which:
The methods and systems of the present invention as claimed and described herein are directed to a laser cavity, in particular an excimer laser cavity, with two gratings for narrowing the laser emission linewidth.
Gratings for wavelength selection in laser cavities are typically designed for operation in two different configurations. In one, more traditional configuration, the grating is blazed at a high angle, typically greater than about 79° and operates in an autocollimating (Littrow) mount. To obtain high wavelength discrimination, the incident beam should fill the entire grating, requiring either telescope lens or prism optics.
A large diffraction angle is beneficial for achieving high wavelength dispersion which in Littrow configuration can be expressed as:
wherein ⊖ is the angle between the grating normal and the incident beam, m is the diffraction order and d is the grating period. As suggested by Eq. (1), the dispersion
can be increased by operating the grating at a high diffraction order m and/or by having a small grating period d and/or by operating at almost grazing incidence (⊖=90°), however at the expense of diminished efficiency.
The equivalent halfwidth Δλ of the spectral distribution can be derived from Eq. (1) as:
wherein / is the length of the illuminated part of the grating and the angle ⊖ has the same definition as above.
In an alternative configuration, commonly referred to as Littman-Metcalf geometry, the grating is not arranged, as in the Littrow configuration, to essentially diffract the optical beam back on itself, but is instead used in low-order diffraction at a fixed angle of incidence in conjunction with a reflecting tuning element, for example, a mirror. Beam expansion before the grating is generally not required.
However, to achieve high wavelength discrimination, the Littman-Metcalf grating also tends to be operated at or near grazing incidence. Littman-Metcalf tuning is mostly done in first order, and 1800 g/mm, 2000 g/mm, and 2400 g/mm holographic gratings are preferred. The large angles of incidence of between 80° and 88° typically require a longer ruled width, necessitating large grating dimensions of, for example, 16.5×58×10 mm.
Turning to
Double grating reflectors have been successfully employed in, for example, dye lasers for narrowing the linewidth of the output beam. One example is the QuantaRay Dye Laser System commercially available from Newport Instruments, Inc. which employs a grating arrangement similar to the one described by Shoshan et al. (I. Shoshan and U.P. Oppenheim, Optics Communications, Vol. 25, No. 3, June 1978). The angle of incidence on grating 16 in the systems described in the references was close to grazing incidence, illuminating the entire width of the first grating (equivalent to grating 16 in
Excimer lasers pose a more serious challenge, because the diameter of their output beam can exceed 1 cm. Moreover, excimer lasers have high photon energy due to their short wavelength and also high photon flux which is required for applications in, for example, semiconductor processing. The combination of high photon energy and high photon flux can easily damage traditional ruled gratings, and more particularly holographic gratings. Production of large area ruled gratings is also very expensive.
Excimer lasers operating with a double grating configuration therefore require a novel design of the first grating 16 that can operate with high diffraction efficiency at an angle of incidence of, for example, 60° to 70° from the grating normal.
One embodiment of a grating suitable for this configuration with high diffraction efficiency is illustrated in
Grating 18 of
The wavelength dispersions of the sequentially arranged gratings 16,18 illustrated in
The linewidth of the laser operating with double gratings is narrowed for two reasons: (1) The laser beam propagating in the cavity has an inherent beam divergence, so that the angle α1 has a certain angular spread; and (2) the wavelength of the beam has a certain linewidth Δλ depending on the laser cavity gain profile.
For example, the half-angle beam divergence δ⊖ for a laser operating at a wavelength of about 193 nm and having a Gaussian beam with an initial beam diameter of about 2 mm is about 0.2 mrad. The total angular dispersion obtained by the two gratings 16,18 in
wherein M is the beam magnification factor M=cos β1/cos α1 of the grating 16 and a1 and a2 are the groove spacings of grating 16 and 18, respectively. m1 and m2 are the respective diffraction orders of the gratings.
The single pass bandwidth (in cm−1) of the laser cavity can be derived from Eq. (3):
The linewidth is narrowed because rays having wavelengths away from the center wavelength of 193.3 nm, for example, are off-axis and are not efficiently traversing the gain region 12.
Linewidth narrowing of the beam incident on grating 16 at an angle α1=69°, then diffracted off grating 16 an angle β1=78.58° toward grating 18, where the diffracted beam is incident at an angle α2=68.22° and diffracted again at the same angle β2=α2 as illustrated in
Turning now to
Another embodiment of a laser system 50 with a double grating cavity is shown in
Those skilled in the art will appreciate that other embodiments with more than two gratings are possible through a combination of the illustrated exemplary embodiments discussed above, for example, using two gratings as the high reflector and a single grating as output coupler. One of the two sequentially arranged gratings may be a transmission grating. It is only important that the gratings are designed and configured to provide sufficient wavelength dispersion and high diffraction efficiency while keeping the lateral dimensions of the gratings at a manageable size by moving away from grazing incidence or large blaze angle designs.
In another embodiment (not illustrated), similar to the embodiment depicted in
While the invention has been disclosed in connection with the preferred embodiments shown and described in detail, various modifications and improvements thereon will become readily apparent to those skilled in the art. Accordingly, the spirit and scope of the present invention is to be limited only by the following claims.