Abstract:
An illuminating apparatus is disclosed which comprises a light source, an optical system for condensing light emitted from the light source and illuminating an object with the condensed light, and an optical member which absorbs light having wavelengths from 260 to 340 nm among the light emitted from the light source, wherein the optical member is made of glass or crystalline material to which metal is doped.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an illuminating apparatus for illuminating an object with light emitted from a discharge lamp such as a mercury lamp, and so on. The illuminating apparatus according to the present invention is preferably applied especially to an illuminating optical system in an exposure apparatus for manufacturing semiconductors.  
           [0003]    2. Related Background Art  
           [0004]    Illuminating apparatus for illuminating objects with light emitted from discharge lamps have been used for various purposes in various fields. Among them, reduce-projection-type exposure apparatus (such as steppers, aligners, and so on), in order to manufacture semiconductor elements such as LSIs and liquid crystal display elements according to the photo-lithography technique, illuminating apparatus which illuminates reticles on which transferring pattern is formed with light of a certain wavelength (i line having a wavelength of 365 nm, g line having a wavelength of 436 nm, and so on) emitted from extra-high pressure mercury lamps.  
           [0005]    Much effort is being made in order to transfer much finer pattern on a photosensitive substrate with higher resolution with such a reduce-projection-type exposure apparatus. In general, resolution R and depth of focus DOF of a projection-type exposure apparatus can be expressed as follows: 
             R=k   1   ·λ/NA   (1) 
             DOF=k   2   ·λ/NA   2   (2) 
           [0006]    wherein NA is the numerical aperture of the projection optical system, λ is the wavelength of the exposure light, k 1  and k 2  are coefficients determined by processes employed. As is understood from these two formulas, finer pattern can be realized either  
           [0007]    (1) by increasing the numeral aperture NA of the projection optical system, or  
           [0008]    (2) by shortening the wavelength λ (exposure wavelength) of exposure light.  
           [0009]    With respect to the former technique (1), projection optical systems with a large numerical aperture from 0.5 to 0.6 have been already realized, which improves resolution. By only increasing the numerical aperture NA of the projection optical system, however, the depth of focus DOF must be reduced in inverse proportion to the square of the numerical aperture NA, as is understood from the formula (2). In typical semiconductor processes in practical use, a wafer which is to be subjected to exposure of a circuit pattern has irreguralities on its surface formed in the previous process. And flatness of the wafer itself inevitably has errors. Accordingly, sufficient depth of focus DOF have to be obtained.  
           [0010]    On the other hand, with respect to the technique (2), the depth of focus DOF varies in proportion to the wavelength λ of exposure light, as is clearly understood from the formula (2). Accordingly, it is more preferable to shorten the exposure wavelength λ in order to improve resolution because sufficiently large depth of focus can be obtained. As a result, the emission line of a mercury lamp called i-line (having a wavelength of 365 nm) has almost replaced, as the exposure light used in the projection exposure apparatus, the emission line of the mercury lamp called g-line (having a wavelength of 436 nm).  
           [0011]    [0011]FIG. 12 shows an example of the conventional illuminating apparatus used in a projection exposure apparatus, in which a mercury lamp is used as the light source, the emission point of the mercury lamp  1  is arranged at a first focal point F 1  inside an ellipsoidal mirror  2 . The inner surface of the ellipsoidal mirror  2  on which aluminum or plurality of layers of various dielectric materials are deposited serves as a reflecting surface. The light L emitted from the mercury lamp  1  is reflected by the inner surface of the ellipsoidal mirror  2  toward a mirror  3 . On the reflecting surface of the mirror  3 , also aluminum or plurality of layers of various dielectric materials are deposited. The light reflected by the mirror  3  is condensed at a second focal point F 2  of the ellipsoidal mirror  2 . Thus, a light source image is formed at the second focal point F 2 .  
           [0012]    Light diverging from the second focal point F 2  is substantially collimated by a collimator lens  4 , and then is incident on a band-pass filter  5  of narrow-band type, which selects light having wavelength in a certain range as illuminating light. The illuminating light is incident on a fly&#39;s-eye lens  6 , which forms a number of secondary light sources in its rear (reticle side) focal plane. Light beams diverging from these secondary light sources are reflected by a mirror  7 , condensed by a condenser lens  8 . The pattern forming surface of a reticle  9  is illuminated superimposedly with a number of light beams condensed by the condenser lens  8 . Note that aluminum or plurality of layers of various dielectric materials are deposited also on the reflecting surface of the mirror  7 .  
           [0013]    As the optical path is bent by the mirrors  3  and  7 , the size of the optical system is small. The inner surface of the ellipsoidal mirror  2  serving as a converging mirror and the reflecting surfaces of the mirrors  3  and  7  are designed to have maximum reflectance values with respect to the wavelength of the exposure light.  
           [0014]    As the mercury lamp, an extra-high pressure mercury lamp is used. FIG. 13 shows the distribution of the emission spectrum of this extra-high pressure mercury lamp. FIG. 14A shows the relation between wavelengths and the reflectance of an aluminum reflecting mirror on which aluminum is deposited to form a reflecting surface. FIG. 14B shows the relation between wavelengths and the reflectance of a typical reflecting mirror according to the prior art on which plurality of layers of dielectric materials are deposited to form a reflecting surface. Further, FIG. 15 shows the relation between wavelengths and the transmittance of the band-pass filter  5  when i line is the exposure light. In the above-mentioned construction, the pattern of the reticle  9  is illuminated with illuminating light (i line) with a uniform distribution of illuminance. And the image of the pattern is formed on the photosensitive substrate via the projection optical system (not shown in the drawing).  
           [0015]    When the illuminating apparatus with the above-mentioned construction is used in the ambient atmosphere, white powder adheres to the surfaces of the optical members arranged between the mercury lamp  1  and the band-pass filter  5 , that is, the surfaces of the ellipsoidal mirror  2 , the mirror  3  and the collimator lens  4 , including the entrance plane of the band-pass filter  5 . Because of this white powder, the reflectance values and the transmittance of light L of these optical members decrease to reduce the illumination efficiency. Analysis shows that the white powder is ammonium sulfate, (NH 4 ) 2 SO 4  and that materials concerning the formation of ammonium sulfate do not originally exist in the illuminating apparatus but come from the ambient atmosphere.  
           [0016]    A method to solve the above problem is disclosed in U.S. Pat. No. 5,207,505. According to this method, said optical members are heated and maintained beyond 120° C. because ammonium sulfate decomposes beyond this temperature. (“Encyclopedia of Chemistry”, Vol. 9, P690, Kyoritsu Pub., 1964) It is rather easy to heat up and maintain the ellipsoidal mirror  2  at such a high temperature because the mercury lamp  1  arranged near the ellipsoidal mirror  2  serves as an effective heat source. The other optical members, however, have to be heated by an additional, very effective heat source. As a semiconductor exposure apparatus requires especially strict temperature control, exhaust of heat is very difficult in practical use.  
         SUMMARY OF THE INVENTION  
         [0017]    In consideration of the above-mentioned problems, the present invention was made. And the object of the present invention is to provide an illuminating apparatus which condenses light emitted from a discharge lamp with a converging mirror and illuminates an object with the light led through optical members, wherein white powder of ammonium sulfate which adheres to the optical members can be reduced without newly adding an effective heat source nor a mechanism for exhausting gaseous impurities.  
           [0018]    With reference to FIG. 10, an illuminating apparatus according to the present invention comprises;  
           [0019]    (a) a light source  1 ;  
           [0020]    (b) an optical system consisting of optical members  2  to  8  for condensing light emitted from the light source  1  and illuminating an object  9  with said condensed light; and  
           [0021]    (c) an optical member  20  for absorbing light having wavelengths in a range from 260 to 340 nm among light emitted from the light source  1 ,  
           [0022]    wherein the optical member  20  is made of glass or crystalline material to which metal is doped.  
           [0023]    Another illuminating apparatus, also with reference to FIG. 10, according to the present invention comprises:  
           [0024]    (a) a light source  1 ;  
           [0025]    (b) an optical system consisting of optical members  2  to  8  for condensing light emitted from the light source  1  and illuminating an object  9  with said condensed light; and  
           [0026]    (c) an optical member  20  in which a fluid absorbing light having wavelengths in a range from 260 to 340 nm among light emitted from the light source  1  is filled.  
           [0027]    Still another illuminating apparatus according to the present invention, with reference to FIGS. 7 and 12, comprises;  
           [0028]    (a) a lamp having a pair of electrodes  13 A and  13 B the bulb ( 10 ,  22 ) of which shields light having wavelengths in a range from 260 to 340 nm among light emitted from said pair of electrodes  13 A and  13 B; and  
           [0029]    (b) an optical system consisting of optical members  2  to  8  for condensing light emitted from the lamp and illuminating an object  9  with said condensed light.  
           [0030]    Now basic principles of the present invention will be described. The inventors of the present invention carried out a further examination on the formation processes of white powder of ammonium sulfate from trace substances in the atmosphere.  
           [0031]    Trace substances such as sulfur dioxide SO 2  (sulfurous acid) and ammonia NH 3  together with oxygen O 2  and water vapor H 2 O are common in the clean room in which the semiconductor exposure apparatus is used as well as in the air. It is probable that these substances react with one another with the help of ultraviolet rays having energy hν (h is Planck&#39;s constant, and ν is frequency) as follows.  
           [0032]    (1) sulfer dioxide SO 2  is activated by energy of ultraviolet rays to be activated sulfur dioxide SO 2 *;  
                         
 
           [0033]    (2) The resultant activated sulfur dioxide SO 2 * is oxidized to be sulfur trioxide SO 3 ; 
           2SO 2 *+O 2 →2SO 3 . 
           [0034]    (3) The resultant sulfur trioxide SO 3  reacts with water H 2 O to be sulfuric acid; 
           SO 3 +H 2 O→H 2 SO 4 . 
           [0035]    (4) On the other hand, ammonia NH 3  reacts with water H 2 O to be ammonium hydroxide; 
           NH 3 +H 2 O→NH 4 OH. 
           [0036]    (5) The sulfuric acid from the process (3) is neutralized with the ammonium hydroxide from the process (4) to form ammonium sulfate; 
           H 2 SO 4 +2NH 4 OH→(NH 4 ) 2 SO 4 +2H 2 O. 
           [0037]    The above examination was carried out on the basis of a literature, “Chiba Univ. Environmental Sci. Res. Rep.” Vol. 1, No. 1, pp 165-177.  
           [0038]    The inventors of the present invention took notice of the reaction (1) among the above reactions in order to find a way to inhibit the formation of ammonium sulfate. According to another literature (H. Okabe: “Photochemistry of Small Molecules” P248, Wiley-Inter Science, 1978), sulfur dioxide has the following four absorption bands:  
           [0039]    (1) 105-180 nm  
           [0040]    (2) 180-240 nm  
           [0041]    (3) 260-340 nm  
           [0042]    (4) 340-390 nm  
           [0043]    Since the ultra-high pressure mercury lamp emits little amount of light having a wavelength of 240 nm or shorter wavelengths, and at the same time since the white powder is found only in the optical path down to the entrance plane of the band-pass filter and not from the band-pass filter downward in the optical path, ultra-violet rays having wavelength in a range from 260 nm to 340 nm is thought to be the main factor of the reaction. Accordingly, if ultraviolet rays having said wavelength from 260 nm to 340 nm can be shielded in the vicinity of the mercury lamp, adhesion of ammonium sulfate which hinders illumination efficiency can be reduced. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0044]    [0044]FIG. 1A is a cross-sectional view showing the structure of the mercury lamp used in the first embodiment of the illuminating apparatus according to the present invention.  
         [0045]    [0045]FIG. 1B is a cross-sectional view showing a modification of the mercury lamp shown in FIG. 1A.  
         [0046]    [0046]FIG. 2 is a chart showing the absorption cross section of vaporous rubidium (Rb).  
         [0047]    [0047]FIG. 3 is a chart showing the absorption cross section of vaporous caesium (Cs).  
         [0048]    [0048]FIG. 4 is a chart showing the absorption cross section of ozone gas (O 3 ) and that of gaseous oxygen (O 2 ).  
         [0049]    [0049]FIG. 5 is a cross-sectional view showing the structure of the mercury lamp used in the second embodiment of the illuminating apparatus according to the present invention.  
         [0050]    [0050]FIG. 6 is a chart showing transmittance characteristics of glass material LF5W.  
         [0051]    [0051]FIG. 7 is a cross-sectional view showing the structure of the mercury lamp used in the third embodiment of the illuminating apparatus according to the present invention.  
         [0052]    [0052]FIG. 8 is a cross-sectional view of the multilayered film with which the surface of the substrate is coated.  
         [0053]    [0053]FIG. 9 is a chart showing an example of reflectance characteristics of the multilayered film used in the third embodiment.  
         [0054]    [0054]FIG. 10 is a schematic view showing the construction of the fourth embodiment of the illuminating apparatus according to the present invention.  
         [0055]    [0055]FIG. 11 is a perspective view showing the broken-out section of a box member  20  used in the fourth embodiment.  
         [0056]    [0056]FIG. 12 is a schematic view showing the construction of a conventional illuminating apparatus.  
         [0057]    [0057]FIG. 13 is a chart showing the emission spectrum distribution of a ultra-high pressure mercury lamp.  
         [0058]    [0058]FIG. 14A is a chart showing the reflectance characteristics of a conventional aluminum reflecting mirror.  
         [0059]    [0059]FIG. 14B is a chart showing the reflectance characteristics of a typical reflecting mirror coated with a multilayered film of dielectric substances.  
         [0060]    [0060]FIG. 15 is a chart showing the transmittance characteristics of a conventional band-pass filter. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0061]    Now, the first embodiment according to the present invention will be described. In this embodiment of the illuminating apparatus differs from the conventional illuminating apparatus in that the mercury lamp  1  is replaced by a new one having double-bulb structure. First, this double-bulb mercury lamp used in this embodiment will be described.  
         [0062]    [0062]FIG. 1A shows the mercury lamp used in this embodiment. A tubular inner bulb  11  has a spherical portion in the middle and the open ends one sealed by bases  12 A and  12 B, respectively. Electrodes  13 A and  13 B are inserted through the bases  12 A and  12 B, respectively, into the hollow inside the inner bulb  11 . Also substances necessary for emission of the mercury lamp are filled in the hollow inside the inner bulb  11 . Thus, the inner bulb  11  with other necessary components functions as an ordinary ultra-high pressure mercury lamp. Further, a tubular outer bulb  19  also having a spherical portion in the middle surrounds the inner bulb  11 . The doughnut-shaped openings at both ends of the outer bulb  14  are sealed by bases  15 A and  15 B, respectively. And a gas which absorbs light having wavelengths in a range from 260 to 340 nm is filled in a space S between the inner bulb  11  and the outer bulb  14 .  
         [0063]    As described before, the ultra-high pressure mercury lamp used in the projection exposure apparatus has the emission spectrum distribution shown in FIG. 13. As is clearly shown in FIG. 13, the ultra-high pressure mercury lamp has distributions in a wavelength range from 260 to 340 nm, that is, the wavelength range causing adhesion of the white powder (blurring phenomenon). In order to prevent emission of light in said wavelength range, the mercury lamp has the double-bulb structure and the gas which absorbs light having wavelength in the range from 260 to 340 nm is filled in the space S between the inner bulb  11  and the outer bulb  14 , as described above. Gases having such proper absorption characteristics include metallic vapour of rubidium, caesium, and so on.  
         [0064]    According to a literature (R. D. Hudson and L. J. Kieffer, “Compilation of Atomic Ultraviolet Photoabsorption Cross Sections for Wavelengths Between 3000 and 10 Å”, Atomic Data 2, pp 205-262 (1971) especially, see p. 235 and p. 253), FIG. 2 shows the absorption cross section spectrum of vaporous rubidium. According to the same literature, FIG. 3 shows the absorption cross section spectrum of vaporous caesium. As is shown in FIGS. 2 and 3, both vaporous rubidium and vaporous caesium have large absorption cross sections for wavelength of 340 nm and shorter wavelengths. Accordingly, if such metallic vapor is sealed in the space S of the double-bulb structure shown in FIGS. 1A and 1B, the light in said wavelength range causing the blurring phenomenon can be selectively removed from the light emitted from the inner bulb serving as an ultra-high pressure mercury lamp.  
         [0065]    Gaseous ozone has absorption characteristics similar to those of the above metallic vapor. The absorption cross section spectrum of gaseous oxygen (O 2 ) and ozone gas (O 3 ) are shown in FIG. 4, in which reference numeral  17  indicates the absorption cross section spectrum of gaseous oxygen and reference numeral  18  indicates that of ozone gas. As is clearly shown is FIG. 4, the absorption spectrum of ozone gas (O 3 ) has ideal absorption characteristics for wavelength of or shorter than 340 nm. The ozone gas, however, unlike metallic vapor, dissociates to be O and O 2  in photochemical reactions. Photochemical reactions of ozone and oxygen occurs as shown in the following (in the following reaction formulas M, which is called a third body any atom, molecule or ion except an oxygen atom, for example, a molecule of oxygen (O 2 ) or nitrogen (N 2 )).  
                         
 
         [0066]    Ozone O 3  and/or oxygen O 2  filled in the space S shown in FIG. 1A react as described above until the mixture of gases reaches a chemical equilibrium. The final density of ozone should be controlled in consideration of all the reaction and the final chemical equilibrium. In short, the final density of ozone after these photochemical reactions settles in a certain range regardless of any initial densities of ozone. The absorption efficiency for wavelength of 340 nm and shorter wavelengths in the state of chemical equilibrium is obtained from the chart of FIG. 4 by calculating molecular densities of O 3  and O 2 . A full detail of the calculation is not given here, but an outline thereof is as follows. For example, negligible reactions with respect to reaction energy and so on are put out of account. And molecular densities are approximately calculated from, for example, the following formula expressing the condition of chemical equilibrium:  
                    n          t       =   0           (   3   )                               
 
         [0067]    wherein n is concentration of each substance.  
         [0068]    The dissociation rate J of O 3  or O 2  can be calculated as follows:  
             J   =       ∫   0   λmax                   N        (   λ   )              λ            σ        (   λ   )                          λ                 (   4   )                               
 
         [0069]    wherein J&#39;s dimension is [1/sec],  
                N        (   λ   )              λ                  [       cm     -   1       ,     sec     -   1       ,     cm     -   1         ]                         
 
         [0070]     is the number of photons passing per second per unit wavelength per unit area, σ(λ)[cm −2 ] is the photoelectric absorption cross section of a molecule, and λmax is the maximum wavelength of λ in the above reactions.  
         [0071]    The reaction rate of each reaction can be obtained from well-known literature. Light absorption efficiency can be promoted by increasing pressure of the gas filled in the double-bulb structure shown in FIG. 1A. But temperature rising caused by light absorption must be taken into account. That is, both the inner bulb  11  and the outer bulb  14  have to be made of glass material having a small coefficient of thermal expansion as well as enough strength.  
         [0072]    The gas which absorbs light having wavelength from 260 to 340 nm may be circulated through the space S between the inner bulb  11  and the outer bulb  14 , as shown in FIG. 1B. In this case, the gas is supplied through a pipe  16 A into the space S by a gas supplier (not shown), wherein conditions of the gas (density, pressure, flow velocity, temperature, and so on) must be well controlled. The gas is exhausted through another pipe  16 B to an exhaust system (not shown). By circulating the gas through the double-bulb structure, high light absorption efficiency can be maintained.  
         [0073]    When the structure shown in FIG. 1B is adapted, additional systems are required to monitor and control the pressure and the temperature of the gas circulated through the double-bulb structure. The systems for monitoring and controlling the pressure and the temperature of metallic vapor are very large. So metallic vapor is preferably filled in the double-bulb structure, as shown in FIG. 1A, when it is desirable to simplify the construction of the whole apparatus. Accordingly, in practice, ozone gas is usually circulated through the double-bulb structure shown in FIG. 1B. In this case, however, the density of ozone circulated through the double-bulb structure has to be newly calculated. If the time required to reach the equilibrium is much longer than the time during which the gas remains inside the double-bulb structure, the initial density of ozone has to be high. Otherwise, the flow velocity is changed to obtain desirable densities of ozone.  
         [0074]    Next, the second embodiment according to the present invention will be described with reference to FIGS. 5 and 6. This embodiment has construction similar to that shown in FIG. 12, wherein an impurity having certain absorption characteristics is doped in the bulb of the mercury lamp  1 . First, the structure of the mercury lamp used in this embodiment will be described.  
         [0075]    [0075]FIG. 5 shows the mercury lamp of this embodiment. A tubular bulb  19  has a spherical portion in the middle. The openings of the bulb  19  are sealed by bases  12 A and  12 B. Electrodes  13 A and  13 B are inserted in the hollow inside the bulb  19  through the bases  12 A and  12 B, respectively. Thus the bulb  19  with other necessary components functions as an ordinary ultra-high pressure mercury lamp. An impurity which absorbs light having wavelength of 340 nm and shorter wavelengths is doped in quartz glass, of which the bulb  19  of the lamp  1  is made.  
         [0076]    One of materials which are preferably doped in quartz glass is sodium Na. Sodium Na, however crystallize SiO 2  at high temperatures, which blurs the bulb  19 . Accordingly, the bulb  19  has to be kept at a temperature of 1000° C. or lower. Other preferable materials to be doped in quartz glass includes iron Fe, lead Pb, aluminum Al, rubidium Rb, caesium Cs, and so on.  
         [0077]    The bulb  19  can be made of materials on the market. For example, ULETM titanium silicate glass (manufactured by Corning Co., Commodity No. 7971) can be used without doping an impurity. This ULETM titanium silicate glass absorbs light having a wavelength of 300 nm and shorter wavelength, so the lamp can be effectively prevented from being blurred.  
         [0078]    Also glass material LF5W manufactured by Ohara Co. is useful. This glass material LF5W exhibits light transmittance characteristics shown in FIG. 6. The transmittance of this material having a thickness of 10 mm for the light having a wavelength 365 nm (i line) is 0.994, from which reflection loss has already subtracted. This glass material having said characteristics can satisfy conditions required according to this embodiment. This glass material, however, causes solarization when used at low temperatures. In addition, it can not be used at 400° C. or higher temperatures. Accordingly, the bulb  19  has to be controlled in the temperature range from 100° C. to 400° C.  
         [0079]    Now, the third embodiment of the present invention will be described with reference to FIGS. 7, 8 and  9 . This embodiment also has construction similar to that shown in FIG. 12, wherein the glass of the mercury lamp  1  is coated with a multilayered film. The same members as those of the previous second embodiment are indicated by the same reference numerals and detailed description thereof is omitted. First, the structure of the mercury lamp used in this embodiment will be described.  
         [0080]    The mercury lamp of this embodiment shown in FIG. 7 has a bulb  22  made of ordinary glass material. The outer surface  22   a  of the bulb  22  is coated with a multilayered film  10 , which reflects light having wavelengths in a range 260 to 340 nm and transmit light having wavelength of 350 nm or longer wavelength. In other words, the multilayered film selectively transmits the light used as exposure light. An example of the multilayered film having selectivity with respect to wavelengths is designed as:  
               air   /       (       λ   8          H   :       λ   4          L   :       λ   8        H             )     n       /   substrate           (   5   )                               
 
         [0081]    wherein: H is selected from a group including ZrO 2 , Sc 2 O 3 , HfO 2 , Y 2 O 3 , and so on; L is selected from a group including SrO 2 , MgF 2 , and so on; the wavelength λ is determined to be about 300 nm; and the number of layers n is generally from 8 to 16.  
         [0082]    [0082]FIG. 8 shows a cross section of such a multilayered film, wherein the film is formed according to the above design (5) and the number of layers is 10. As the substrate, materials which transmit light having a wavelength of 350 nm or longer wavelengths can be used, including optical glass, quartz glass, fluorite, and so on. When the material employed as the substrate absorbs light having a wavelength 340 nm or shorter wavelengths, such light can be prevented from being transmitted more effectively. By coating the glass of the mercury lamp with the multilayered film, blurring of the other optical members in the illuminating apparatus can be reduced.  
         [0083]    Next, the fourth embodiment of the present invention will be described with reference to FIGS. 10 and 11. The components in FIG. 10 corresponding to those in FIG. 12 are indicated by the same reference numerals, and detailed description thereof is omitted. In this embodiment, an optical filter which absorbs light having wavelengths from 260 to 340 nm is provided in the optical path of the illuminating optical system.  
         [0084]    [0084]FIG. 10 schematically shows the construction of this embodiment. As shown in FIG. 10, a box member  20  is arranged between the ellipsoidal mirror  2  and the mirror  3 . The box member has two flat glass surfaces parallel to each other. FIG. 11 shows a broken-out section of the box member. The box member  20  has a hollow space  21 , which is arranged to coincide with the optical path. A gaseous substance which absorbs light having wavelengths from 260 to 340 nm (cf. description of the first embodiment) is filled in the hollow space  21 . The box member is arranged preferably in the vicinity of the mercury lamp  1 , as shown FIG. 10. The box member  20  reduces adhesion of the white powder on the optical members arranged downstream in the optical path from the box member  20 .  
         [0085]    The glass material of the box member  20  may be the glass material used in the second embodiment, that is, the glass material which absorbs certain undesirable light. Or the box member  20  may be replaced by a plane parallel glass which has absorption characteristics similar to those of the glass materials used in the second embodiment. In addition to the glass materials used in the second embodiment, the plane parallel glass provided in the illuminating optical system may be also made of a crystalline material (for example, fluorite CaF 2 , magnesium fluoride, and so on) to which the above-mentioned metal (such as Na, Fe, and so on) is doped.  
         [0086]    This fourth embodiment is useful in case, for example, the double-bulb structure employed in the first embodiment is difficult to manufacture.  
         [0087]    The illuminating apparatus according to the present invention can be applied not only to the projection exposure apparatus as described but also to a proximity-type exposure apparatus and a contact-type exposure apparatus, and further any type of optical apparatus using ultraviolet rays.  
         [0088]    As described before, ammonium sulfate is formed from trace sulfur dioxide (SO 2 ) and ammonia (NH 3 ) existing in the ambient atmosphere in which the illuminating apparatus is used. Accordingly, if the illuminating apparatus is installed in a clean room, sulfur dioxide (SO 2 ) and/or ammonia (NH 3 ) may be removed from the air circulated in the clean room by attaching a filtering system for removing sulfur dioxide (SO 2 ) and/or ammonia (NH 3 ) to the air conditioning system. Thus, formation of ammonium sulfate can be reduced.  
         [0089]    The devices of the first to fourth embodiment can be used separately. But if used in combination, these devices can more effectively prevent adhesion of the white powder. Note that the present invention is not limited to the above-mentioned embodiment. The present invention includes any construction which concerns the fundamental principles of the present invention.