ILLUMINATION APPARATUS FOR EXPOSURE

Abstract: An illumination apparatus is disclosed which is suitable for use in a projection/exposure system for projecting an image of a circuit pattern of a reticle (4) on a semiconductor wafer (6) through a projection lens (5). The illumination apparatus includes an excimer laser (1) for emitting a pulsed laser beam, an optical system (3) for illuminating the reticle (4) with a plurality of laser pulses emitted from the excimer laser (1) so that laser pulses having passed through the reticle (4) impinge on the semiconductor wafer (6) in different directions through the projection lens (5), and a light intensity control device (2) for controlling the light intensity of each of the laser pulses so that the laser pulses equally contribute to the reaction of a light sensitive material which is provided on the semiconductor wafer (6), with light.

Description: The present invention relates to an illumination apparatus of the kind referred to in the precharacterizing part of claim 1. Such an illumination apparatus is known from "Laser and Applications", Sept. '83, pages 49-56. As the integration density of semiconductor devices are made high and as said devices have a fine circuit pattern, attention is paid to the use of a laser capable of emitting a pulsed light beam with a wavelength in the far ultraviolet region such as an excimer laser, as a light source for exposure. Such a light source is disclosed in JP-A-57-198,631 (US-A-4 458 994) and in prior art document: Laser and Applications, Sept. '83, pages 49-56, K. Jain: "Laser applications in semiconductor microlithography". The laser beam emitted from an injection locked excimer laser has a high energy density and a narrow spectral width. Further, this laser beam has a single transverse mode and high spatial coherency. Accordingly, a wavefront distorted irregularly by the imperfectness of various optical elements will produce an irregular interference pattern (which is called "speckle") on a projected image. Two methods for eliminating the above speckle have been proposed. In one of the methods, as described in JP-A-60-230,629 (US-A-4 619 508) and JP-A-60-247,643, an optical member for generating an optical path difference greater than the coherent length is disposed on an optical path. In the other method, as described in SPIE, Vol. 174, "Development in Semiconductor Microlithography" IV, 1979, in particular on page 35 a laser beam is deflected successively by means of an optical deflector, and a plurality of laser pulses which are different in deflection angle from one another (directional light pulses) are successively impinged upon a receiving surface to superpose plural different speckles, and consequently, the uniformity of illumination on the receiving surface is improved. According to the former method, the optical member for generating an optical path difference is formed of a bundle of a plurality of square pillar prisms or optical fibers different in length. The laser beam emitted from an injection locked excimer laser has a coherent length of about 30 cm. In order to make the optical path difference between two adjacent optical elements utilized more than 30 cm, the optical member for generating an optical path difference is required to be very large in size. On the other hand, the latter method has an advantage that an optical system used is relatively compact. In the latter method, however, no regard has been paid to the degree of contribution of each of a plurality of laser pulses which are utilized for one exposure operation, to the reaction of a light sensitive material with light. The light sensitive material concentration which has not yet reacted with light, varies with exposure to light in accordance with the following equation: dM dt = -kM√I (1) where M indicates the concentration which has not yet reacted with light, t an exposure time, k a constant, and I the intensity of light. In a case where the light intensity I is kept constant, the equation (1) is rewritten as follows: M = C exp(-k√ I t) (2) where C is a constant. Fig. 5 is a graph showing an M-t curve which is obtained from the equation (2). Referring to Fig. 5, the exposure time elapses in order of t₁, t₂, t₃ and t₄, and the time interval between the time moments t₁ and t₂ is made equal to the time interval between the time moments t₃ and t₄. As is apparent from Fig. 5, the change in concentration M in the time interval between t₃ and t₄ is far smaller than the change in concentration M in the time interval between t₁ and t₂, that is, the reaction of the light sensitive material with light is weakened with exposure time elapsing. This is shown in Fig. 9 of the article of K. Jain mentioned above as a relationship between an exposure dose and an absorbance ratio of photoresist which shows a nonlinear relationship just as Fig. 5 of the present invention, since the vertical axis of Fig. 9 of the Jain-article is ℓn A. Accordingly, in a case where a plurality of light pulses are used in a single exposure operation, the picture quality of an image formed on a light receiving surface is greatly affected by speckles due to several initial directional light pulses. That is, in the latter method which uses a plurality of light pulses for a single exposure operation, no regard has been paid to a fact that the reaction of the light sensitive material with a light pulse weakens as the light pulse is received later, and thus there arises the problem that a developed pattern on a semiconductor wafer is strongly affected by speckles due to several initial directional light pulses. It is accordingly an object of the present invention to provide an illumination apparatus for exposing a light sensitive material to a plurality of light pulses in one exposure operation so that the light pulses equally contribute to the reaction of the light sensitive material with light, to make it possible to print a high-resolution image of a fine pattern having a line width of 0.5 µm or less on a semiconductor wafer. The above object is accomplished according to the present invention with an illumination apparatus suitable for use in a projection/exposure system of the kind as claimed in claim 1. Dependent claims are directed on features of preferred embodiments of the invention. In the light intensity control device, the light quantity of each laser pulse is increased in accordance with a decrease in sensitivity of the light sensitive material with exposure time. That is, data on the sensitivity reduction with exposure time is previously prepared, and the light intensity control device is operated on the basis of the above data. It is to be noted that the above data varies depending upon the kind of the light sensitive material. According to such a method, laser pulses used in one exposure operation equally contribute to the reaction of the light sensitive material, and thus the picture quality of an image which is formed by developing the light sensitive material, is scarcely affected by speckles. Fig. 1 is an arrangement plan showing the whole construction of a semiconductor exposing system which includes an embodiment of an illumination apparatus according to the present invention. Fig. 2 is a schematic diagram showing the laser spot distribution at the entrance pupil of the reduction/projection lens of Fig. 1. Fig. 3 is a schematic diagram which shows in detail an image formation system formed by the reduction/projection lens of Fig. 1. Fig. 4 is a block diagram showing an example of the light intensity control device which is shown in Fig. 1 and is the main gist of the present invention. Fig. 5 is a graph showing a relation between the exposure time t and the concentration M concerning that portion of a light sensitive material which has not yet reacted with light, for a case where the intensity of light incident on the light sensitive material is not controlled. Fig. 6 is a graph showing a relation between the exposure time t and the concentration M concerning that portion of a light sensitive material which has not yet reacted with light, for a case where the intensity of light incident on the light sensitive material is controlled in accordance with the present invention. Fig. 7 is a graph showing an example of a light intensity control method according to the present invention. Fig. 8 is a block diagram showing another example of the light intensity control device of Fig. 4. Now, an embodiment of an illumination apparatus for exposure according to the present invention will be explained below, with reference to the drawings. Fig. 1 shows the whole construction of a semiconductor exposing system which includes the present embodiment. In Fig. 1, reference numeral 1 designates an excimer laser, 2 a light intensity control device, which is a substantial part of an illumination apparatus according to the present invention, 3 an optical system for deflecting laser pulses from the control device 2, 4 a reticle which is formed by depicting a circuit pattern on a silica glass plate, 5 a reduction/projection lens, 6 a semiconductor wafer, and 7 a control circuit for controlling the above-mentioned parts 1, 2 and 3. Referring to Fig. 1, laser pulses emitted from the excimer laser 1 pass through the optical system 3, after the light intensity of each laser pulse has been controlled by the light intensity control device 2. The reticle 4 is uniformly illuminated with each laser pulse from the optical system 3, and an image of the circuit pattern of the reticle 4 is formed on a light sensitive material which is applied to the wafer 6. The optical system 3 is constructed as mentioned below. The intensity of the laser beam emitted from the laser 1 is small at a central portion of the laser beam, and hence the intensity distribution of the laser beam is made uniform by a beam equalizer 31. For example, in the beam equalizer 31, the laser beam is split into four parts, and the parts are re-combined in the vicinity of a galvano-mirror 301 after being passed different paths the distance of which are different from another by more than the cohenrence length so as to reduce the spatial coherence of the resultant laser beam and to make the energy density distribution therein uniform. The laser beam from the beam equalizer 31 is focused on a point A by a focusing lens 32. The galvano-mirror 301 is disposed between the focusing lens 32 and the point A, to deflect the laser beam in an X-Y plane. A first lens 302 is disposed so that the laser beam reflected from the mirror 301 passes through the first lens 302, and the focal point thereof on the front side agrees with the point A. Further, another galvano-mirror 303 is disposed so that the laser beam having passed through the first lens 302 impinges on the mirror 303 and an image of the mirror 301 is formed on the mirror 303. Thus, the laser beam having passed through the first lens 302 is composed of parallel rays, and impinges on the same portion of the mirror 303 independently of the rotational angle of the mirror 301. The galvano-mirror 303 turns on an axis parallel to a Y-axis. Further, second and third lenses 33 and 34 are disposed so that the image of the point A is formed at an aperture stop 35 and at the entrance pupil 51 of the reduction/projection lens 5, and each of the galvano-mirrors 301 and 303 is conjugate to the pattern-including surface (that is, lower surface) of the reticle 4. Accordingly, when the emission of laser pulses from the excimer laser 1 is synchronized with the deflection of each of the galvano-mirrors 301 and 303 by the control circuit 7, the images of the point A corresponding to the laser pulses, as shown in Fig. 2, are formed at points 501 on the entrance pupil 51. As shown in Fig. 3, incident light beams 402 and 403 on the reticle 4 corresponding to image points 502 and 503 on the entrance pupil 51 are different in traveling direction from each other. Accordingly, when a plurality of laser pulses are used for one exposure operation, speckles which are generated on the wafer 6 by the laser pulses having different traveling directions, will be compensated with one another. Fig. 4 shows an example of the light intensity control device 2 of Fig. 1. In Fig. 4, reference numeral 211 designates a collimator lens, 212 a beam expander, 213 an acoustooptic (A/O) modulator, and 214 a controller for controlling the modulator 213 by applying a variable voltage thereto. Referring to Fig. 4, a laser beam 201 emitted from the excimer laser 1 is converted by the collimation lens 211 into a fine beam, and then impinges on the acoustooptic modulator 213. The modulator 213 is made of silica glass, Fluorite or the like. When the modulator 213 is applied with an ultrasonic wave, the refractive index of the modulator 213 varies regularly on the basis of the photoelastic effect, and the zero-th order light 202 and the first-order diffracted light 203 are emitted from the modulator 213. The intensity of the first-order diffracted light 203 is controlled by a voltage which is supplied from the controller 214 to the modulator 213. The first-order diffracted light 203 is converted by the beam expander 212 into a thick beam, and then passes through the optical system 3 of Fig. 1. As can be seen from Fig. 5, when a plurality of laser pulses from the optical system 3 successively impinge on a light sensitive material through the reticle 4 while keeping the intensity I of laser pulse constant, a pattern image obtained by developing the light sensitive material will be greatly affected by speckles generated by several initial directive laser pulses. Hence, the intensity I of laser pulse is controlled so that the light sensitive material concentration M which has not yet reacted with light, varies linearly with an exposure time t and thus the laser pulses equally contribute to the reaction of the light sensitive material with light. The linear variation of the concentration M with the exposure time t is expressed by the following equation: M = -pt + q (3) where p and q are greater than zero, and t is less than a ratio q/p. Fig. 6 is a graph which shows an M-t line obtained from the equation (3). Referring to Fig. 6, the exposure time t elapses in order of t₁, t₂, t₃ and t₄. When a time interval t₂ - t₁ is made equal to a time interval t₄ - t₃, a change of the concentration M in the time interval t₂ - t₁ (namely, M₁ - M₂) is equal to a change of the concentration M in the time interval t₄ - t₃ (namely, M₃ - M₄). Thus, the laser pulses can equally contribute to the reaction of the light sensitive material with light. In order to obtain such a linear variation of the concentration M with the exposure time t , the intensity I of laser pulse is controlled in accordance with the following equation: where t is less than the ratio q/p. As has been already mentioned, the intensity I of laser pulse can be controlled by an output voltage from the controller 214 of Fig. 4. Fig. 7 shows the output voltage of the controller 214 at time moments T₁, T₂, ... and T n when laser pulses are emitted from the excimer laser 1. That is, when the output voltage of the controller 214 applied to the acoustooptic modulator 213 is varied as shown in Fig. 7, the laser pulses can equally contribute to the reaction of the light sensitive material with light. Fig. 8 shows another example of the light intensity control device 2 of Fig. 1. In Fig. 8, reference numeral 221 designates a wedge-shaped aluminum film, 222 a synthetic quartz plate, 223 a gear joint, 224 a pulse motor, and 225 a connecting rod. Referring to Fig. 8, a pulsed laser beam 204 emitted from the excimer laser 1 passes through a filter which is made up of the wedge-shaped aluminum film 221 and the synthetic quartz plate 222. When the quartz plate 222 is moved, the thickness of that portion of the aluminum film 221, through which the laser beam passes, is varied. The light transmittance of the aluminum film is inversely proportional to the thickness of a light transmitting portion thereof. Accordingly, the quantity of light having passed through the wedge-shaped aluminum film 221 and the quartz plate 222 can be controlled by moving the quartz plate 222. One end of the connecting rod 225 is attached to the quartz plate 222, and the other end of the rod 225 is mounted with a worm gear, which is connected to the pulse motor 224 through the gear joint 223. Hence, the quartz plate 222 can be moved by starting the pulse motor 224. In order to finely control the intensity of light incident on the optical system 3, a plurality of optical units each made up of the wedge-shaped aluminum film 221, the synthetic quartz plate 222, the connecting rod 225, the gear joint 223 and the pulse motor 224, are disposed on the optical path between the excimer laser 1 and the optical system 3, as shown in Fig. 8, and each pulse motor is controlled by the control circuit 7 so that the intensity I of a pulsed output beam 205 is varied in accordance with the equation (4). Alternatively, a plurality of optical units each made up of a polarizer, a belt, a pulley and a pulse motor, may be disposed in the above-mentioned manner. In this case, the plane of polarization of each polarizer is rotated so that the intensity I of a pulsed output beam is varied in accordance with the equation (4). As has been explained in the foregoing, according to the present invention, a plurality of laser pulses which are successively emitted from a pulsed oscillation laser such as an excimer laser and are used to perform an exposure operation for a light sensitive material on a semiconductor wafer, can equally contribute to the reaction of the light sensitive material with light. Accordingly, a pattern image obtained by developing the light sensitive material is not affected greatly by speckles generated by several initial directive laser pulses, but speckles due to all the laser pulses are compensated with one another. Thus, a high-resolution image of a circuit pattern having a line width of 0.5 µm or less can be formed on the semiconductor wafer.

Claims: An illumination apparatus suitable for use in a projection/exposure system for projecting an image of a circuit pattern of a reticle (4) on a semiconductor wafer (6) through a projection lens (5), said illumination apparatus comprising: an excimer laser (1) for emitting a pulsed laser beam; characterized by an optical system (3) for illuminating the reticle (4) with a plurality of laser pulses emitted from the excimer laser (1) so that laser pulses having passed through the reticle (4) impinge on the semiconductor wafer (6) in different directions, through the projection lens (5); and light intensity control device (2) for controlling the light intensity of each of the laser pulses so that the amount of light reaction of a light sensitive material provided on the semiconductor wafer (6) is the same for each of the laser pulses. An illumination apparatus according to claim 1, wherein the light intensity control device (2) includes a collimator lens (211), a beam expander (212), an acoustooptic modulator (213), and controller means (214) for controlling the acousto-optic modulator (213). An illumination apparatus according to claim 2, wherein the acousto-optic modulator (213) is composed of a photoelastic member, the refractive index of which varies regularly on the basis of the photoelastic effect when an ultrasonic wave is applied thereto. An illumination apparatus according to claim 1, wherein the optical system (3) includes a beam equalizer (31), a focusing lens (32), an X-Y deflector (301), a first lens (302), another deflector (303), a second lens (33), an aperture stop (35), and a third lens (34). An illumination apparatus according to claim 1, wherein the light intensity control device (2) comprises: an optical element (221) which is disposed on the path of the pulsed laser beam and has gradually changing thickness, the quantity of the transmitted light being dependent on the thickness; and driving means (224) which controllably moves the optical element (221) in relation to an optical axis of the pulsed laser beam. An illumination apparatus according to claim 5, wherein the optical element (221) is a wedge-shaped aluminum film (221) formed on a synthetic quartz plate (222). An illumination apparatus according to claim 5, wherein the optical element (221) and the driving means (224) make up an optical unit, and a plurality of optical units are arranged along the optical path between the excimer laser (1) and the optical system (3). An illumination apparatus according to claim 3, wherein the photoelastic member is composed of silica glass.

Inventors: NAKASHIMA, NAOTO, OSHIDA, YOSHITADA, SHIBA, MASATAKA, YOSHITAKE, YASUHIRO
Applicants: HITACHI, LTD.
Application year: 1987
Main classification: G03F7
IPC codes: H01L21, H01S3, G03F7