GB2205990A - Technique for efficient line narrowing of excimer lasers - Google Patents

Abstract

An excimer laser has one or more tilted Fabry-Perot etalons placed intracavity. When employed with excimer lasers having a single laser gain channel, linewidths can be reduced by factors of more than one hundred from that produced when operating without line narrowing optics and simultaneously high power conversion efficiency (exceeding 50%) is maintained. Such a line narrowing laser can be employed as a light source when performing high resolution mask imaging over large field sizes of the type encountered in the photolithography, micromachining and micropatterning industries. <IMAGE>

Description

Technique for efficient line narrowing of excimer lasers This invention relates to a scheme for efficiently narrowing the bandwidth of emission of electrical discharge excimer lasers while utilising their full emission aperture.

Excimer lasers, in particular the rare gas-halide devices such as ArF, KrF, XeF, KrCl and XeCl, can emit high peak (up to 50MW) and average (over lOOW) powers in the ultraviolet spectral region between 190 and 353nm. The lasing transition occurs between a bound upper electronic state and a repulsive or weakly bound lower molecular electronic ground state. As a result of the repulsive or weakly bound nature of the lower laser level, their unrestricted laser emission is characterised by a relatively broad spectral emission of several tens of wavenumbers wide. While for many applications this wide bandwidth is not a disadvantage and in fact may be a positive advantage in reducing interference effects such as laser speckle, for other applications it is a major disadvantage.For example, because of the lack of suitable ultraviolet transmissive optical materials of different refractive index which are optically highly uniform and isotropic, it is extremely difficult to design high quality optical refractive elements for use in the ultraviolet which can correct for chromatic aberration over the full bandwidth of excimer lasers.

This difficulty is particularly encountered in the case of the design and fabrication of image reducing projection lenses which might be used in excimer laser step and repeat photolithographic machines to produce resolution linewidths of s 0.5cm over field sizes of several square centimetres. The easiest solution to this problem is to restrict the linewidth of the excimer laser by the use of intracavity optical elements such as diffraction gratings, prisms or Fabry-Perot etalons (or combinations thereof) so that chromatic aberration in the optical elements of the multi component lens becomes negligible.While the homogeneous nature of the laser transitions in excimer molecules in principle allows for efficient narrowing of their output with little loss in power from that produced by their fully broadened output, the dispersive optical components themselves, such as diffraction gratings or prisms or non-dispersive components such as Fabry-Perot etalons, have until now required the use of devices such as pinholes or slits to achieve the necessary line narrowing (see for example Ref 1).For the dispersive components which are typically used as one of the end feedback mirrors of the laser, the use of one or more intracavity pinholes or slits are necessary components in achieving the spatial rejection in the cavity of all wavelengths other than the (narrow) band of desired ones (see Fig l(a)-(c) in Ref 1 as well as Fig l(a) in this document).On the other hand, one or more intracavity pinholes or slits are generally assumed to be necessary when using one or more intracavity Fabry-Perot etalons in excimer lasers, since to prevent laser oscillation by feedback from the (high reflectivity) faces of the etalon, the device must be tilted so that these reflections are not fed back through the gain medium but are stopped by the aperture (see Figure l(b) and Figure l(d) in Ref 1). In both cases the use of apertures such as pinholes or slits restricts the aperture over which laser action can take place. Thus the power of the ensuing line-narrowed emission produced by the excimer laser is greatly reduced (typically < 1% for linewidths of < 1cm-1) from that which it would produce with unline-narrowed unrestricted operation.The unrestricted rectangular apertures which are typical of unline-narrowed commercially available excimer lasers are 6-10 x 20-35mm (see for example the models EMG100-200 MSC series of Lambda-Physik, Gottingen, W Germany; models Hyper-Ex 400 series of Lumonics, Kanata, Ontario, Canada; and Series 2000 lasers of Questek, Billerica, Mass, USA). The apertures necessary to produce line-narrowing down to s 1cm-i are s 2mum. Thus the use of a lmm wide slit will typically reduce the potential output of the laser by at least a factor of 10, while a lmm diameter hole will reduce it by at least a factor of 200 from what could be achieved if the aperture remained unrestricted.To regain the lost power, the output from this 'line-narrowed oscillator' is often passed through an additional amplifier discharge module or injected into an unstable resonator cavity (see Reference 2) - such line-narrowed injected locked excimer laser oscillator/amplifier systems are currently sold commercially by Lambda-Physik as models EMGl50ET, 160ETMSC and Lumonics as model HE-IL for example.

The invention described here overcomes the problems of the aperture reduction described above. Using this scheme highly efficient ( > 50% power efficiency) line narrowing of excimer lasers can be produced down to linewidths of 0.5cam~ by using a single excimer laser discharge gain module, and one or two Fabry-Perot etalons used intracavity to the laser without the use of pinholes or slits to reduce the intracavity aperture of the laser. The invention described is expected to find applications in fields such as excimer laser lithography which require high resolution projection of small features over large field sizes for which the projection optics are extremely difficult to design with the necessary chromatic aberration correction over the full unline-narrowed bandwidth of the excimer laser.

To understand the present invention consider the single intracavity Fabry-Perot etalon in the excimer laser cavity as shown in Figure 2. The transmission maximum of such an etalon is given by = = 2ndcosS (1) where d and n are the thickness and refractive index of the etalon, e and m are the angular inclination and order of the fringes respectively. In order to prevent reflective feedback and oscillation from the faces of the etalon, it must be tilted an angle 8, so that these reflections do not pass back down the gain medium, where W e1= L (2) and W is the laser aperture dimension and L is the laser cavity length. Note that e is smallest for tilting in the plane of the smaller dimension of the rectangular output beam.Tilting of the etalon produces wavelength tuning of its transmission peaks. From Eq(l) the adjacent maximum in transmission will occur at an angle 2 e2 = Ce + /nd) when e , e2 1 (3) Since most commercially available discharge excimer lasers are characterised by extremely high small-signal gains (2 10%/cam) lasting for 10-20nsec and cavity lengths are around lm long, the laser emission is produced by only a few round trips of the cavity and consequently has a large divergence of 2-5 mrad.

Assuming most of the laser energy from a N 20nsec pulse of the excimer laser is produced after 2 round trips of the light around the cavity then the beam divergence of the laser AE is given approximately by W ## # 4L (4) 4L This relatively large angular beam divergence of the laser will lead to additional spectral broadening of the transmission of the etalon other thanthat caused by its finesse alone and is given by

(5) = u ee from Eq(1) when # # 1 where v = 1 is the reciprocal wavelength (wavenumber) of the laser ') - transition.

Thus midway between the extreme angles e1 and 82 the transmission linewidth of the etalon due to beam divergence effects is:

CA [(tUdiv) . dip + CAS2)2 Au div Udiv (6) div = 2 = u (W ) 2 + uwL22 (F SR) IA from Eqs (2), (3), (4) and (5) where the free spectral range of the etalon, FSR = 2nd.

The total linewidth ##T of the laser will be determined by contributions from both this 'divergence' linewidth as well as the inherent resolution of the etalon ##R = FSR/F FSR where F is the spectral finesse of the etalon. It can be Au R F roughly estimated by taking the average of these two contributions:

In Fig 3 we show plots of the laser linewidth (Eq 7) for a wavelength of A = 249 nm (KrF laser) for the case of an air gap etalon and a value of W = 150 w which is a typical value for commercially available excimer lasers when W is the smaller of the rectangular beam dimensions. Thus the tilt angle of the etalon must be greater than N 0.40 (from Eq 2), but less than the ea angle shown in Fig 4 (from Eq 3).From our practical experience we find that in order to guarantee one transmission peak within the gain bandwidth of a KrF laser requires an etalon FSR = 80cm-1. Thus from Fig 3 and Eq 7 we see that for finesses t 20 the laser linewidth becomes insensitive to the finesse of the etalon and is in the region 3-6cm-' as determined by the divergence tilting linewidth (second term in Eq 7). Thus a single Fabry-Perot etalon in a KrF excimer laser cavity can be expected to produce a linewidth of between 3-6cm-1 over the full aperture of the beam. We have experimentally confirmed this to be the case and have determined the efficiency of line narrowing (= line-narrowed laser energy unline-narrowed laser energy without etalon ) to be 50%.We see from Fig 3 that inserting a second tilted etalon with a FSR of approximately the same as the linewidth produced by the first etalon in the cavity (3-6cm-i) will produce a laser linewidth of between 0.6-lcm-1. This reduced linewidth is then relatively insensitive to the finesse of this second etalon. We have confirmed this behaviour experimentally. We also see from Fig 3 that the ultimate resolution achievable by this technique of inserting etalons into full aperture KrF excimer laser cavities is N 0.5cm-1 as determined by the laser divergence tilting linewidth Au . described above.The curves shown in Fig 3 are div relative insensitive to the excimer laser wavelength and can thus also be used to determine the expected performance for full aperture line-narrowed operation of ArF, KrCl, XeCl and XeF lasers.

Specific examples of the many ways of implementing this invention are as follows: 1. By inserting a single Fabry-Perot etalon of a finesse of about 15 with a FSR of 80cm-1 into the full unrestricted aperture of an excimer laser cavity such as KrF will reduce the linewidth of the laser output. It is possible to achieve a reduction of over an order of magnitude to N 5cm-1 from its unrestricted value with very little loss of laser energy ( < 50%).

For typical commercially available excimer lasers the normal to the etalon should be tilted between 0.5 to 3.5 to the optical axis of the laser.

2. By inserting two Fabry-Perot etalons of finesses of about 15 with FSR of 80and 6cm-1 into the full unrestricted aperture of an excimer laser cavity such as KrF the linewidth can be reduced by at least two orders of magnitude from its unrestricted value (to N 1cm-i) with very little loss of energy ( < 50%). For typical commercially available excimer lasers the normal to this second etalon should be tilted between 0.50 to 10 to the optic axis.

3. This invention is applicable to all types of excimer lasers.

References 1. T J McKee, Canadian J Phys 63, 214 (1985).

2. I J Bigio and M Slatkine, IEEE J Quant Electr QE-l9, 1426 (1983).

Claims (6)

0 JL: A I }t S

1. A laser line narrowing system consisting of the use of one or more tilted Fabry-Perot etalons used intracavity to efficiently narrow the linewidth and produce a more temporally coherent output beam from wide aperture (a5mm) high gain lasers without further restriction of the intracavity aperture other than that imposed by the pumped gain region itself.

2. A line narrowing system as claimed in claim 1 used for narrowing the laser linewidth obtainable from excimer lasers.

3. A line narrowing system as claimed in claim 1 used for narrowing the linewidth obtainable from rare gas halide lasers such as ArF, KrF, KrCl, XeCl and XeF which operate in the ultra-violet spectral region between 190 and 353nm.

4. The use of the system described in claims 1, 2 or 3 to produce from a single laser gain length a linewidth as narrow as 0.5cm-1 with a high power conversion efficiency compared to that produced by the full unrestricted broadband operation of the same single laser gain region.

5. The use of the system described in claims 1, 2 or 3 as a light source incorporated in or coupled to a high resolution large field size mask imaging machine as might be used for the production of silicon or GaAs microcircuits, or the high resolution machining or patterning of materials.

6. A line narrowing system subsequently as described herein with reference to figures 1 - 4 of accompanying drawings.