Abstract:
An exposure system is disclosed which exposes a resist surface  52   a  to an optical or electron beam in a process involving a chemically amplified resist. The exposure system comprises a chamber  20  for housing a blank optical disc  51,  an e-beam column  10  for exposing the resist surface  52   a  of the blank optical disc  51  housed in the chamber  20,  to the optical or electron beam, and a laser  31  for heating a resist  52  within the chamber  20,  and heats the resist  52  after the resist  52  is exposed to the optical or electron beam, whereby the state of the resist after the exposure can be made uniform.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to exposure systems for exposing resists to an optical or electron beam, and particularly to an exposure system adapted for exposing chemically amplified resists.  
           [0003]    2. Description of Related Art  
           [0004]    Following the commercialization of DVDs (Digital Versatile Discs), suppliers are now positively engaged with R&amp;D activity to develop next-generation DVDs, including systems that can record/reproduce HDTV-equivalent digital moving images for two or more hours. This requires that a 12 cm diameter single optical disc has a recording capacity of 25 to 50 gigabytes with a signal transfer rate of 30 to 50 megabits per second. To meet this requirement, read-only discs need to have a minimum pit length of about 0.15 μm and a track pitch of about 0.3 μm as their next-generation DVD standard.  
           [0005]    Existing blank disc recording apparatuses are no longer able to record information as such tiny pits. Thus, it is obvious that new types of blank disc recording apparatuses having a better resolution must be developed. Some R&amp;D papers report new blank disc recording apparatuses using a recording source such as an electron beam or far ultraviolet laser.  
           [0006]    A high-resolution resist must be used to record information in the form of tiny pits. However, as its exposure properties, a resist satisfying the compatibility between high sensitivity and high resolution is generally hard to find. As one solution to this problem, the concept of chemical amplification has been proposed (H. Ito, C. G. Willson: Polym. Eng. Sci. Vol. 23 1012 (1983)). A chemically amplified resist based on chemical amplification means a resist that is formed by combining a photoacid generator for generating acids upon exposure to an optical or electron beam, with a polymer highly reactive with acids. Acids generated by exposure act as a catalyst to induce the polymer to chain-like chemical reactions, thereby providing a gain of 1 or more in amplifying the quantum yield which is indicative of occurrences of chemical reaction induced in the polymer with respect to an exposure dose. Currently, both positive and negative chemically amplified resists are commercially available, satisfying the compatibility between high sensitivity and high resolution. In the field of semiconductor devices, semiconductor products such as DRAMs are actually fabricated by a lithographic process involving a combination of a KrF excimer laser (wavelength: 248 nm) with a chemically amplified resist.  
           [0007]    Due to its being subjected to chemical reactions, a positive chemically amplified resist exhibits a phenomenon (first phenomenon) which is a change in the shape of a resist pattern, depending on the time between exposure and post exposure bake steps. In a process of preparing a blank optical disc, it takes several hours from the start to the end of an exposure, and as a result of this long exposure time, the shape of pits formed in a region exposed immediately after the start of the exposure is different from that of pits formed in a region exposed immediately before the end of the exposure, thus making it difficult to maintain reliable signal quality over each optical disc produced.  
           [0008]    The positive chemically amplified resist also exhibits a phenomenon (second phenomenon) which is its T-shaped cross section resulting from the formation of a dissolution inhibition layer on its surface. One cause of the second phenomenon is that the acids generated by exposure become deactivated through reaction with alkali, such as ammonia and amine, within the atmosphere. In currently available semiconductor fabricating processes, this problem is avoided by controlling the total amount of alkali within the atmosphere to levels of 1 ppb or less. However, similar techniques are not viable in eliminating the first phenomenon encountered in the blank disc preparation process due to its long exposure time, as mentioned above.  
           [0009]    The first and second phenomena are encountered likewise in direct-write e-beam lithographic processes used in the manufacture of semiconductors, and it is reported that attempts have been made to eliminate these phenomena by depositing an acidic surface protective layer on a chemically amplified resist (F. Fujino, et al.: J. Vac. Sci. Technol. Vol. 11 2773 (1993)). However, this approach involves complicated steps and entails high manufacturing cost, and thus a technology that can ensure reliable signal quality with simpler steps is called for.  
         SUMMARY OF THE INVENTION  
         [0010]    It is, therefore, an object of the present invention to provide an exposure system, etc. adapted for a process using a chemically amplified resist, which is capable of making the state of the exposed resist uniform.  
           [0011]    An exposure system according to the invention which exposes a resist surface of an object for exposure to an optical or electron beam, comprises a chamber for housing the object for exposure, exposure device for exposing the resist surface of the object for exposure to the optical or electron beam, and heating device for heating a resist exposed to the optical or electron beam by the exposure device, within the chamber.  
           [0012]    According to this exposure system, the resist exposed to the optical or electron beam is heated by the heating device. Thus, when a chemically amplified resist is used, chemical reactions induced in its polymer proceed quickly, and the state resulting from the chemical reactions is substantially uniform over the entire resist surface, independently of the time from the start of exposure. To obtain such state which is substantially uniform over the entire resist surface, it may be arranged to either promote the chemical reactions induced in the polymer up to a stage of practical completion by heating the resist using the heating device, or stop heating at some point along the reactions. To obtain uniformity over the entire resist as a final result of the chemical reactions taking place thereover, the heating device may be set to appropriate heating time and temperature according to which region of the resist surface is heated. In this case, different heating time and temperature are set according to the timing for exposure to optical or electron beam radiation, whereby the above uniformity as a final result of the chemical reactions can be attained independently of the exposure timing. As to the mode of heating by the heating device, the resist surface may be heated wholly at once, partially for a number of times, or while continuously moved from one region for heating to another. The resist surface may also be heated from one exposed region to another. In this case, the interval between exposure and heating can be kept substantially constant over the entire surface, and this further keeps the formation of the dissolution inhibition layer substantially uniform over the entire surface. Therefore, uniformity in the exposed resist can be further improved.  
           [0013]    Inside the chamber, there may be provided a turn table for placing the object for exposure, and a drive device for driving the turn table for rotation, and the exposure device lithographically expose the resist surface while the turn table is rotated by the drive device. In this case, the resist surface may be scanned for sequential exposure by moving the optical or electron beam in a direction of moving away from or closer to the center of rotation of the surface.  
           [0014]    The heating device may be provided with a laser beam emitting device that irradiates the resist surface with a laser beam. In this case, the use of the laser beam permits partial heating of the resist surface, whereby the surface can be heated partially from one region to another according to the exposure timing.  
           [0015]    Further, the laser beam emitting device may be located outside the chamber, and irradiate the resist surface by directing the laser beam emitted therefrom to the surface through a laser beam transmissive member provided in the chamber.  
           [0016]    In this case, since the laser beam emitting device is located outside the chamber, even in the case of exposure to an electron beam, there is no danger of the laser beam emitting device adversely affecting the electron beam. Further, optical devices including a lens system for focusing the laser beam and a mirror for moving the region for irradiation with the laser beam may be located either inside or outside, or respectively inside and outside the chamber. Control device may be provided to control the exposure device, laser beam emitting device, and optical devices such that the resist surface is heated according to the exposing operation.  
           [0017]    The heating device may be provided with a laser beam emitting device that irradiates the resist surface with a laser beam, and the device may irradiate the resist surface while the turn table is rotated by the drive device, whereby the exposed region of the resist by the exposure device is heated by following the exposure.  
           [0018]    In this case, the exposed region of the resist is heated by following the exposure, whereby the time between exposure and heating can be kept substantially constant over the entire resist surface. Hence, the dissolution inhibition layer is formed substantially uniformly over the entire resist surface. Therefore, uniformity in the resist can be further improved. In this case, the resist surface can be scanned for sequential exposure by moving the optical or electron beam, for example, in a direction of moving away from or closer to the center of rotation of the surface, and the exposed region of the resist can be heated similarly by moving the laser beam in the direction of moving away from or closer to the center of rotation of the surface by following the exposure.  
           [0019]    The laser beam emitting device may be located outside the chamber, and irradiate the resist surface by directing the laser beam emitted therefrom to the surface through a laser beam transmissive member provided in the chamber.  
           [0020]    In this case, since the laser beam emitting device is located outside the chamber, even in the case of exposure to an electron beam, there is no danger of the device adversely affecting the electron beam. Further, optical devices including an optical system for focusing the laser beam and a mirror for moving the region for irradiation with the laser beam may be located either outside or inside, or respectively outside and inside the chamber. Control device may be provided to control the exposure device, laser beam emitting device and optical devices such that the resist surface is heated according to the exposing operation.  
           [0021]    A control device may be provided to control the relationship between a position for irradiation with the optical or electron beam and a position for irradiation with the laser beam such that the region exposed to the optical or electron beam is irradiated with the laser beam after a predetermined time elapses from a reference time at which the region is exposed to the optical or electron beam.  
           [0022]    In this case, the time between exposure and heating can be kept substantially constant over the entire resist surface, whereby the dissolution inhibition layer can be formed substantially uniformly over the entire resist surface. Therefore, uniformity in the exposed resist can be further improved.  
           [0023]    It should be noted that the invention is not limited to modes of embodiment disclosed in the accompanying drawings by the above description in which reference symbols borrowed from the drawings are added parenthetically in order to facilitate the understanding of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    [0024]FIG. 1 is a schematic showing an exposure system of the present invention;  
         [0025]    [0025]FIG. 2 is an enlarged view of a mechanism for laser beam radiation;  
         [0026]    [0026]FIG. 3 is a control block diagram showing the control system of an exposure system  100 ;  
         [0027]    [0027]FIG. 4 is a diagram showing how a blank optical disc is irradiated with an electron beam and a laser beam; and  
         [0028]    [0028]FIG. 5 is a diagram showing an example in which a concave mirror is arranged in place of a plane mirror for reflecting a heating laser beam.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0029]    The present invention will now be described with reference to a preferred embodiment shown in FIGS.  1  to  4 . An exposure system according to this embodiment is used in a process for fabricating blank optical discs.  
         [0030]    [0030]FIG. 1 shows an exposure system  100  of the invention. As shown in the figure, the exposure system  100  includes an e-beam column  10  for directing an electron beam to a blank optical disc, and a vacuum chamber  20  for housing the blank optical disc.  
         [0031]    The e-beam column  10  houses therein an emitter  11 , a condenser lens  12 , a beam deflector  13 , an aperture  14 , a beam deflector  15 , a focusing lens  16 , and an objective lens  17 . An electron beam  18  emitted from the emitter  11  is focused at the beam deflector  13  by the condenser lens  12  to be modulated by a signal from a modulator  13   a  connected to the beam deflector  13 . The modulated electron beam  18  is then restricted by the aperture  14 . A beam position controller  15   a  is connected to the beam deflector  15 . The controller  15   a  applies a signal to the deflector  15  to adjust the position for irradiation by the electron beam  18 . Further, a focus controller  16   a  is connected to the focusing lens  16 . The controller  16   a  applies a signal such that the focusing lens  16  adjusts the focus of the electron beam  18 . The electron beam  18  having passed through the aperture  14 , beam deflector  15 , and focusing lens  16  is brought to focus on a resist surface of the blank optical disc by the objective lens  17 .  
         [0032]    Inside the vacuum chamber  20  are an X stage  21 , movable in both right and left directions (x directions) as viewed in FIG. 1, and a turn table  23  rotatably attached to the X stage  21 . The stage  21  is driven by a motor  21   a  and a drive mechanism  21   b.  The motor  21   a  is driven by a motor drive  103  (FIG. 3). The turn table  23  is driven by an air-bearing type spindle motor  102  (FIG. 3) while directly coupled thereto. The air-bearings of the motor  102  are isolated from the vacuum chamber  20  through a differential exhaust mechanism or a magnetic fluid seal, etc., not shown, such that the air does not enter into the vacuum chamber  20 . The spindle motor  102  receives a signal from a rotation controller  23   a  (FIG. 3), whereby the rpm of the turn table  23  is controlled.  
         [0033]    The X coordinate of the axis of the turn table  23  is measured by a range meter  26 . The meter  26  uses a laser interferometer that directs a laser beam to the mirror  25  attached to the X stage  21  and receives the reflected beam therefrom. A signal from the range meter  26  is applied to a position controller  27  (FIG. 3).  
         [0034]    As shown in FIG. 1, sensors  24   a  and  24   b  are attached to the ceiling of the vacuum chamber  20 . These sensors optically detect the vertical position of the resist surface of the blank optical disc.  
         [0035]    As shown in FIGS. 1 and 2, outside the chamber  20  are a laser  31  for emitting a red or infrared laser beam  31   a  for heating, and a focusing lens  32  for focusing the laser beam  31   a  emitted from the laser  31 . A transmissive window  33  is formed in a side wall surface of the chamber  20  for transmission of the laser beam  31   a  therethrough. Inside the chamber  20  is a plane mirror  34 , attached above the turn table  23 , for bending the optical axis of the laser beam  31   a.  The plane mirror  34  is engaged with a drive mechanism  34   a  coupled to the rotary shaft of a motor  34   b,  whereby the position (or angle) of the mirror  34  can be varied relative to the turn table  23  as the motor  34   b  rotates. The motor  34   b  is driven by a motor drive  104  (FIG. 3).  
         [0036]    The laser beam  31   a  emitted from the laser  31  is focused on the resist surface of the blank optical disc via the focusing lens  32 , transmissive window  33 , and plane mirror  34 . The focal position of the laser beam  31   a  are moved in the x directions according to the position or angle of the mirror  34 .  
         [0037]    The output of the laser  31  (laser power) is controlled by a laser output controller  37  (FIG. 3).  
         [0038]    As shown in FIG. 3, the modulator  13   a,  beam position controller  15   a,  focus controller  16   a,  sensors  24   a  and  24   b,  position controller  27 , motor drive  103 , rotation controller  23   a,  laser output controller  37 , a focus controller  32   a,  and motor drive  104  are connected to a controller  101 .  
         [0039]    As shown in FIG. 3, a signal from the sensor  24   a  is applied to the focus controller  16   a  that controls the focusing lens  16  such that the electron beam  18  is always focused on the resist surface of the blank optical disc.  
         [0040]    As shown in FIG. 3, the focal position of the heating laser beam  31   a  is controlled based on an output signal from the focus controller  32   a,  which controller, in response to a signal from the sensor  24   b,  controls the focusing lens  32  such that the laser beam  31   a  is always focused on the resist surface.  
         [0041]    As shown in FIG. 3, the laser output controller  37  receives a signal from the rotation controller  23   a.  As will be described hereinafter, the output value of the laser  31  is controlled in accordance with the rpm of the turn table  23  so that the entire resist surface of the blank optical disc can be heated uniformly.  
         [0042]    Next, an exposure step will be described, in which the blank optical disc is exposed using the exposure system  100  according to this embodiment.  
         [0043]    As shown in FIG. 4, a chemically amplified e-beam resist  52  is coated over the surface of a blank optical disc  51 . After the disc  51  is fixed onto the turn table  23 , the vacuum chamber  20  is evacuated by operating a vacuum pump (not shown).  
         [0044]    Then, while exposing the e-beam resist  52  to the electron beam  18 , the turn table  23  is rotated, and the X stage  21  is moved at the same time, whereby a spiral latent image consisting of a series of signals (the latent image of a pit array) is pressed into the e-beam resist  52 . In the meantime, the position controller  27  receives an externally supplied reference signal and a distance signal from the range meter  26 , and the X stage  21  is driven at a pre-programmed forwarding speed based on these signals. As mentioned above, the focusing lens  16  is controlled based on the signal from the sensor  24   a  for detecting a resist surface  52   a,  such that the electron beam  18  is always focused on the surface  52   a  during exposure by the beam  18 .  
         [0045]    On the other hand, the position for irradiation with the heating laser beam  31   a  is adjusted by controlling the position or angle of the plane mirror  34 . The position or angle of the mirror  34  is controlled to keep the relative distance between the position for exposure to the electron beam  18  and the position for irradiation with the heating laser beam  31   a  such that a region exposed to the electron beam  18  is irradiate with the laser beam  31   a  after a preset time elapses from a reference timing at which the region is exposed to the electron beam  18 . Therefore, irradiation with the heating laser beam  31   a  starts after a predetermined time elapses from the start of an exposure, and ends after a predetermined time elapses from the end of the exposure. As mentioned above, during irradiation with the heating laser beam  31   a,  the focusing lens  32  is controlled based on the signal from the sensor  24   b  for detecting the resist surface  52   a  such that the beam  31   a  is always focused on the surface  52   a.    
         [0046]    As a result of the irradiation with the heating laser beam, acid diffusion induced in the resist is practically completed, permitting no further progress of the reactions.  
         [0047]    Although the position for irradiation with the electron beam must be controlled on the order of submicrometer, an accuracy of 1 to 10 micrometers would suffice to control the position for irradiation with the heating laser beam. This is because it takes only a short time period (a few seconds to minutes) to move the electron beam  18  by a distance of some micrometers in an x direction, and the state (sensitivity) of the chemically amplified resist would fluctuate but then settle within its tolerance during such a short time period as a few minutes. Hence, unlike in control over the position for exposure to the electron beam  18 , a range meter using a laser interferometer is not employed in control over the position for irradiation with the heating laser beam  31   a.    
         [0048]    While the resist  52  is heated upon irradiation with the heating laser beam  31   a,  laser power must be controlled such that the heating condition is the same at any location of the blank optical disc  51 , i.e., the disc  51  is heated to the same temperature all over its surface. To achieve this, it is required to control laser power such that a constant ratio is provided between laser power and the rotational speed of the blank optical disc  51 , or more specifically, the traveling linear velocity of the blank disc  51  at the position for irradiation with the heating laser beam  31   a.  Such control is implemented by applying a signal from the rotation controller  23   a  that controls the rpm of the spindle motor  102 , to the laser output controller  37  that controls laser power.  
         [0049]    [0049]FIG. 5 shows an example in which a concave mirror is arranged in place of the plane mirror for reflecting the heating laser beam. In FIG. 5, the same components as those of the exposure system  100  are denoted by the same reference numerals, and their description is omitted.  
         [0050]    As shown in FIG. 5, an exposure system  100 A is constructed such that a laser beam  31   b  emitted from a laser  31 A, passing through a focusing lens  32 A and the transmissive window  33 , reaches the concave mirror  34 A thereby to be bent downward. The position or angle of the concave mirror  34 A can be varied by rotating the motor  34   b  through the drive mechanism  34   a.  The focal position of the heating laser beam  31   b  is controlled by the focusing lens  32 A.  
         [0051]    In the configuration shown in FIG. 5, the NA of a lens for converging the heating laser beam  31   b  is determined by the concave mirror  34 A. Since the mirror  34 A can be located closer to the resist surface, a shorter focal distance and a larger NA can be provided. This, in turn, increases the energy density of the laser beam at the resist surface, and thus a low-power, inexpensive laser  31 A can be used. Hence, a cost reduction can be achieved efficiently.  
         [0052]    Since the laser  31  for heating the resist  52  is provided outside the vacuum chamber  20  in this embodiment, the flow of current through the laser  31  no longer disturbs electric fields within the vacuum chamber  20 , and hence there is no danger that the electron beam will fluctuate. If the laser is provided inside the chamber, a magnetic shield is required. Further, in the case of exposure to an optical beam, current does not adversely affect the optical beam, and hence, there would be no such problem as encountered when the laser is provided inside the chamber in the case of exposure to an electron beam.  
         [0053]    While exposure of a blank optical disc has been exemplified in the above description, the exposure system of the invention is applicable extensively to, e.g., fabrication of semiconductor products. Further, the exposure system is not limited to applications such as exposure to electron beam radiation, but applications such as exposure to optical beam radiation. Still further, the exposure system of the invention is applicable to exposing resists other than chemically amplified resists.  
         [0054]    The entire disclosure of Japanese Patent Application No.2000-271012 filed on Sep. 7, 2000 including the specification, claims, drawings and summary is incorporated herein by reference in its entirety.