Abstract:
A photo mask which enhances contrast and a method of fabricating the same are provided. The photo mask includes a transparent substrate and a light shielding layer pattern formed on the transparent substrate. The light shielding layer pattern includes apertures through which the transparent substrate is exposed. Depressions aligned with these apertures extend into the transparent substrate. Light exposed at an angle through the transparent layer would then pass into the depressions and reflect or diffuse from the sidewalls of the depressions and out through the apertures. The etching depth of the depressions is preferably equal to or less than a depth at which threshold intensity of the exposure light is saturated as the etching depth is increased. In another embodiment, the etching depth of the depressions is less than the wavelength of the exposure light.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION  
       [0001]     This application claims the benefit of Korean Patent Application No. 10-2006-0100385, filed on Oct. 16, 2006, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a photo mask and a method of fabricating the same, and more particularly, to a photo mask for a projection stepper which is used to form a micro pattern on a wafer in a semiconductor device fabrication process, and a method of fabricating the same.  
         [0004]     2. Description of the Related Art  
         [0005]     Photolithography is generally used to form a pattern on a semiconductor wafer in a semiconductor device fabrication process. Photolithography involves forming a pattern image on a photo mask, transferring the pattern to a wafer coated with photosensitive resin using an exposure light in a reduction projection stepper, and developing the coated wafer, thereby obtaining a predetermined pattern. The photo mask corresponds to the original circuit pattern and is an important factor in determining the resolution of the pattern image which is transferred to the wafer.  
         [0006]      FIG. 1  is a sectional view of a conventional binary mask and a schematic view of a near field image when exposure light passes through a photo mask.  
         [0007]     The conventional binary photo mask shown in  FIG. 1  comprises: a substrate having sufficient transparency to transmit the substantially all of the exposure light, and a light shielding layer. One example of the transparent substrate is quartz substrate  10 . A common example of a light shielding layer is a patterned layer formed of chromium. Apertures  14  are defined by the light shielding layer pattern  12  and expose the transparent substrate  10 . The shape of the apertures  14  defines the pattern image to be transferred. The exposure light (not shown) is incident on the photo mask from an upper portion relative to  FIG. 1 . When the incident exposure light passes through the apertures  14  of the transparent substrate  10 , part of a photosensitive resin layer on a wafer (not shown) under the photo mask is exposed to the light. The exposed part corresponds to the pattern image formed on the photo mask.  
         [0008]     As circuit feature sizes decrease, the spacing between adjacent apertures  14  becomes smaller. And because of diffraction between closely spaced apertures when exposed to light, it is difficult to separate adjacent pattern images from each other. That is, the light through one aperture interferes with the exposure light passing through the adjacent apertures  14 . Consequently, the resolution of the pattern images is considerably decreased.  
         [0009]     One solution proposed for this binary photo mask diffraction problem includes using a phase shift mask (PSM). The PSM gives high resolution by using a phase shift effect of the mask. The phase shift effect is obtained by using characteristics of the material on a photo mask or by changing the structure of the mask, without changing the light source of the conventional stepper.  
         [0010]      FIG. 2  is a sectional view of a conventional attenuated PSM. Unlike the conventional chromium binary mask, the attenuated PSM gives high resolution by inserting a shift material, which transmits a predetermined light, between the light shielding layer formed of chromium and the transparent quartz substrate. Alternately, one may use a shift material with a predetermined transparency (e.g. a molybdenum series material) as a light shielding layer, instead of chromium. As distinguished from  FIG. 1 ,  FIG. 2  shows a light shielding layer  13  of the molybdenum series material formed on the transparent substrate  10 , instead of the light shielding layer  12  formed of chromium in  FIG. 1 . As with light shielding layer  12  and apertures  14 , the light shielding layer  13  defines apertures  24 .  
         [0011]     One drawback to using a shift material (such as molybdenum) with the PSM is that exposures may cause undesired side lobes. To combat this problem, process engineers have developed two process steps. However, these additional process steps increase the turn-around time and decrease the production yield using the molybdenum-based phase shift mask. Moreover, since molybdenum cannot be cleaned using sulphuric acid, the PSM is vulnerable to haze compared to the binary mask. And to prevent haze build-up, the mask may be periodically cleaned. However, this also can cause many potential problems.  
         [0012]     Accordingly, the need exists for photo masks that address problems inherent in the prior art.  
       SUMMARY OF THE INVENTION  
       [0013]     According to an aspect of the present invention, there is provided a photo mask, which comprises a transparent substrate having transparency to an exposure light; and a light shielding layer pattern formed on the transparent substrate. The transparent substrate includes depressions formed in the transparent substrate, where the depressions have a uniform and predetermined etching depth aligned with the light shielding layer pattern. In preferred embodiments, the etching depth of the depressions is equal to or less than a depth at which threshold intensity of the exposure light is saturated as the etching depth is increased.  
         [0014]     In accordance with another embodiment of the present invention, there is provided a photo mask, which comprises a transparent substrate having transparency to an exposure light; and a light shielding layer pattern formed on the transparent substrate. The transparent substrate includes depressions formed in the transparent substrate, where the depressions have a uniform and predetermined etching depth aligned with the light shielding layer pattern. In alternate embodiments, the etching depth of the depressions is equal to or less than a wavelength of the exposure light. Furthermore, the etching depth of the depressions may be equal to or less than a depth at which the ratio of a first order light to a zero order light (A 1 /A 0 ) peaks in a near field image of the photo mask.  
         [0015]     In the embodiments, the exposure light may use a light source having different wavelengths, for example, any one of G-line, I-line, KrF, ArF and F 2 . The transparent substrate may be formed of another transparent material, for example, calcium fluoride (CaF 2 ) or magnesium fluoride (MgF 2 ), instead of quartz (SiO 2 ). The light shielding layer pattern may be formed of a material having light shielding characteristics, for example, chromium oxide (CrO x ) or tungsten silicon (W—Si), instead of chromium. The photo mask patterns may be of various types, for example, a line/space type pattern in which lines and spaces are periodically repeated, an isolated line pattern, an isolated space pattern, or an island type pattern.  
         [0016]     According to another aspect of the present invention, there is provided a method of fabricating a photo mask, comprising: forming a light shielding layer, which blocks an exposure light, on a transparent substrate having transparency to the exposure light; forming a mask pattern on the light shielding layer, to expose parts of the light shielding layer; forming a light shielding layer pattern by etching the exposed light shielding layer, using the mask pattern as an etching mask; and forming depressions within by etching parts of the transparent substrate, using the light shielding layer pattern as an etching mask, wherein the depressions have a etching depth which is equal to or less than a depth at which threshold intensity of the exposure light is saturated as an etching depth is increased.  
         [0017]     The forming of the depressions may comprise verifying an optimum etching depth, based on simulated values of the threshold intensity by increasing the etching depth of the transparent substrate. The verifying of the optimum etching depth is performed by controlling an incidence angle of the exposure light, controlling critical dimensions of the depressions, changing an exposure light source, or changing the shape of the depressions.  
         [0018]     In forming the depressions, the etching depth of the depressions may be less than the wavelength of the exposure light or may be equal to or less than a depth at which the ratio of a first light to a zero light (A 1 /A 0 ) peaks in a near field image of the photo mask.  
         [0019]     In accordance with the present invention, when the transparent substrate of the photo mask is etched to a specific depth, the mask topology effects enhance the contrast of the mask by diffraction and diffusion of the light at the etched sidewalls. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
         [0021]      FIG. 1  is a schematic view of a conventional binary mask and a near field image thereof;  
         [0022]      FIG. 2  is a schematic view of a conventional attenuated phase shift mask;  
         [0023]      FIG. 3  is a schematic view of a photo mask according to the present invention, and a near field image thereof;  
         [0024]      FIGS. 4A through 4D  are sectional views illustrating a process of fabricating the photo mask of  FIG. 3 ;  
         [0025]      FIG. 5  is a graph comparing the ratio of A 1 /A 0  in the near field image of the photo mask of the present invention and the conventional photo masks;  
         [0026]      FIG. 6  is a view for explaining how the optical performance is enhanced by the sidewall effect in the photo mask according to the present invention;  
         [0027]      FIG. 7  is a graph of the ratio of A 1 /A 0  to the phase of normal incident light in the photo mask according to the present invention;  
         [0028]      FIG. 8  is a graph of the ratio of A 1 /A 0  to the phase of off-axis light at 5 degrees in the photo mask according to the present invention;  
         [0029]      FIG. 9  is a graph of the ratio of A 1 /A 0  to the phase of off-axis light at 10 degrees in the photo mask according to the present invention;  
         [0030]      FIG. 10  is a graph of the ratio of A 1 /A 0  to the phase of off-axis light at 15 degrees in the photo mask according to the present invention; and  
         [0031]      FIG. 11  is a graph of threshold intensity with respect to etching depth of a transparent substrate in the photo mask according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0032]     The present invention will now be described more fully with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms, and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the thicknesses of layers and regions are exaggerated for clarity. Like numbers refer to like elements throughout the specification. When one layer is described as being positioned on or above another layer or a substrate, the layer may be positioned to be directly in contact with the other layer or the substrate, or a third layer may be positioned therebetween.  
         [0033]      FIG. 3  is a schematic view of a photo mask according to an embodiment of the present invention, and a schematic view of a near field image when light passes through the mask.  
         [0034]     In  FIG. 3 , the photo mask according to the present invention comprises: a substrate having transparency to transmit an exposure light, for example, a transparent substrate  30  formed of quartz; and a light shielding layer which blocks the exposure light, for example, a light shielding layer  32  composed of chromium. The light shielding layer  32  define apertures through which the exposure light is transmitted. The shape of the aperture defines the pattern image to be transferred. A pattern of depressions  34  is formed, corresponding to the pattern of the light shielding layer  32 . The depressions  34  have vertical sidewalls created by uniformly etching parts of the transparent substrate  30  to an etching depth (d) that are aligned with sidewalls of the apertures formed within the light shielding layer pattern  32 .  
         [0035]     A comparison of the conventional binary mask of  FIG. 1  with that of the present invention, implemented in a preferred embodiment as shown in  FIG. 3 , illustrates several differences. First, the photo mask according to the present invention comprises additional patterns of depressions  34  formed within the transparent substrate, unlike the binary mask where the transparent substrate is unetched. Also, when compared to the photo mask of the invention, the binary photo mask results in an aerial image in the boundary of the light shielding layer  12  that is not clear and the image in the middle of the aperture  14  (shown by the dip in  FIG. 1 ) that is not desirable. In comparison, the photo mask according to the present invention results in an improved aerial image—e.g. regular peaks—thought to be caused in main part by the scattering and diffraction process of the light occurring at the sidewalls of the depressions  34  (see, e.g.,  FIG. 6  and resulting discussion). As a first order beam becomes closer to a zero order beam in the aerial image, a background value decreases, thereby enhancing the contrast of the mask.  
         [0036]      FIG. 5  is a graph comparing the ratio of A 1 /A 0  in the near field image of the photo mask of the present invention and of that in conventional photo masks. For this comparison, Applicants used an exposure light source KrF having a wavelength of 248 nm, and a light shielding layer pattern having a line/space of 140 nm/140 nm. Applicants further used a simulation tool called ‘TOP 0 ’.  
         [0037]     In  FIG. 5 , ‘A’ is a thin film mask which is ideally two-dimensional, ignoring the thickness of the light shielding layer, ‘B’ is the conventional binary mask of  FIG. 1 , and ‘C’ is the photo mask according to an embodiment of the present invention, in which the depth of the depression ( 34  of  FIG. 3 ) is 244 nm. As shown in  FIG. 5 , the ratio of a first order light to a zero order light in the near field image (A 1 /A 0 ) is increased in the present invention photo mask compared to the ideal thin film mask or the conventional binary mask. Accordingly, the contrast is greatly enhanced compared to the conventional art. That is, since the light is scattered at the sidewalls of the depressions  34  in the inventive photo mask, the intensity of the light passing through the photo mask is decreased compared to the conventional binary mask. However, as the minimum intensity (I max ) decreases even more than the maximum intensity (I min ), the resulting contrast C (C=(I max −I min )/(I max +I min )) using the inventive photo mask can be increased by overdosing during an exposure process.  
         [0038]      FIG. 6  illustrates how the contrast is improved by the sidewall effect in the photo mask according to the present invention. Using an off-axis illumination system, the photo mask is exposed to light at an incidence angle (θ). And unlike when the exposure light is vertically incident on the mask, an effective space is changed by the shadowing effect resulted from the topology of the mask.  
         [0039]     As shown in  FIG. 6 , with light incident on the mask from right to left, parts of an incident light  38  are totally internally reflected on the right sidewall (Y) of the depression  34 , and then shielded by the light shielding layer  32 , as indicated by reference numeral  38   c . As a result, the effective transmission width of depression  34  and aperture becomes smaller by “dsinθ”. On the left sidewall (X) of the depression  34 , parts of the incident light  38  are diffracted at the sidewall of the transparent substrate  30  and then shielded by the light shielding layer  32 , as indicated by reference numeral  38   a . But most of the incident light  38  is reflected from or diffused at the sidewall and passes through the aperture between the light shielding layer  32 , as indicated by reference numeral  38   b . Even though the effective space area is reduced by the off-axis illumination, the exposure dose which is transmitted by the diffusion or reflection of the light at the sidewalls of the depression  34  is increased as illustrated by the raised bump at the lower position in  FIG. 6 . Accordingly, the ratio of the first order light to the zero order light (A 1 /A 0 ) is increased, and consequently the contrast of the photo mask is increased. In  FIG. 6 , ‘d’ represents the etching depth, and sinθ=σNA/M (wherein σ represents a coherence factor, ‘NA’ represents the number of apertures, and ‘M’ represents the magnification of a lens).  
         [0040]     As illustrated in  FIG. 6 , as the etching depth (d) of the depression  34  is greater, the contrast is increased. Also, the optimum energy needed to transfer a pattern image of the photo mask, e.g. an exposure dose, is changed.  
         [0041]     To enhance the contrast and obtain the optimum exposure dose, it is necessary to optimize the depth of the depression  34 . For this purpose, the inventors of the present invention simulated the ratio of A 1 /A 0 , by controlling the incidence angle of the incident light relative to the line/space pattern and the critical dimensions (CD) of the line/space pattern and varying the depth of the depression  34 . ArF having a wavelength of 193 mn was used as the exposure light source. The critical dimensions of the pattern were set to 60 nm, 70 nm, 80 nm, 90 nm and 100 nm.  
         [0042]      FIG. 7  is a graph of the ratio of A 1 /A 0  to the phase of normal incident light in the photo mask according to the present invention.  
         [0043]      FIG. 8  is a graph of the ratio of A 1 /A 0  to the phase of off-axis light at 5 degrees.  
         [0044]      FIG. 9  is graph of the ratio of A 1 /A 0  to the phase of off-axis light at 10 degrees.  
         [0045]     And  FIG. 10  is a graph of the ratio of A 1 /A 0  to the phase of off-axis light at 15 degrees.  
         [0046]     In each graph, the phase of the horizontal axis corresponds to the depth of the depression  34  resulting from etching and removing parts of the transparent substrate. In the ArF light source, a phase of 1 degree responds to 9.56 Å. That is, in each graph, the wavelength λ of the exposure light corresponds to a phase of 180 degrees (2λ corresponds to the phase of 360 degrees) and corresponds to the etching depth of 1720 Å (that is, 180×9.56 Å=1720 Å).  
         [0047]     Referring to the graph of  FIG. 7  showing the result of the normal incident light, the ratio of the first order light to the zero order light in the near field image reaches a peak at about 180 degrees, at which the etching depth (d) of the depression  34  is equal to the wavelength of the exposure light. While the phase changes from zero to the wavelength of the exposure light, the ratio of A 1 /A 0  progressively increases, and then progressively decreases when passing through the peak. That is, the contrast of the mask progressively increases up to the peak but the contrast characteristic deteriorates after passing through the peak.  
         [0048]     Upon comparing  FIGS. 7 through 10 , as the incidence angle of the incident light is increased, the ratio of A 1 /A 0  is generally shifted in the direction in which the phase becomes low under the wavelength of the exposure light. That is, as the critical dimensions of the pattern become smaller, the peak is shifted towards a lower phase. For example, in  FIG. 9  in which the incidence angle is 10 degrees, when the critical dimensions of the pattern are 60 nm or 70 nm, the peak is shifted to a phase of about 45 degrees. This compares to the results shown in  FIG. 7  where using no off-axis illumination results in a peak of 180 degrees at the 60 nm or 70 nm critical dimensions.  
         [0049]     In  FIG. 7  in which the angle of incidence of the exposure light is zero, the ratio of A 1 /A 0  peaks at about the phase corresponding to the wavelength of the exposure light. Therefore, in the present invention, the etching depth of the depression  34  may be at least less than the wavelength of the exposure light. When the incidence angle of the exposure light is increased, since the peak is shifted towards a lower phase, the etching depth of the depression  34  may be less. Further, and to maximize resolution of the mask within a permissible range, the etching depth may be set to the point where the ratio of A 1 /A 0  peaks.  
         [0050]     As described, the contrast of the mask is increased within the limited range as the etching depth of the depression  34  is increased. However, to transfer a pattern image with high resolution, it is required to set the optimum exposure dose.  
         [0051]      FIG. 11  is a graph of threshold intensity with respect to the etching depth (d) of the depression  34  in the photo mask according to the present invention. In  FIG. 11 , the horizontal axis represents the etching depth (d) and the vertical axis represents the threshold intensity (I th ). The threshold intensity is the intensity of the exposure light at the position corresponding to the critical dimensions of the pattern in the aerial image graphs. As illustrated in  FIG. 11 , as the etching depth of the depression  34  increases, the threshold intensity progressively decreases and becomes saturated at a certain point. That is, the threshold intensity progressively decreases as the etching depth changes from zero to the depth of saturation (d s ). However, at the depth of saturation (d s ), the threshold intensity nearly has a saturation value (I ths ). Then, as the etching depth is increased beyond the depth at which saturation first occurs (d s ), the threshold intensity varies slightly but is nearly saturated.  
         [0052]     The fact that the threshold intensity increases as shown in  FIG. 11  means that the energy required for transferring the pattern image with high resolution, e.g. the exposure dose, needs to be increased. This also means that the exposure dose increases progressively up to the point at which the threshold intensity is saturated. Accordingly, after the point of saturation, the exposure dose is unnecessary, serving only to increase the exposure time and lower productivity. In the present invention, the etching depth of the depression may be equal to or less than the depth of the point at which the threshold intensity is first saturated, e.g. point d s .  
         [0053]     The inventors of the present invention surveyed the optical performance of the photo mask while varying the mask patterns, to determine the optimum etching depth according to the kind of mask pattern. Table 1 shows the results of this survey.  
                                                                                                                     TABLE 1                           Optical Performance of Photo Masks                Exposure Latitude   DOF(μm)   MEEF           [lower # = more efficient]   [higher # = more efficient]   [lower # = more efficient]            Pattern   A1   A2   A3   A1   A2   A3   A1   A2   A3                    Active   2.0   1.6   2.0   0.25   0.2   0.2   3.4   4.1   3.2       Gate   2.4   1.6   2.4   0.4   0.4   0.4   3.1   2.6   3.9       Line       SAC   2.7   2.2   2.5   0.3   0.3   0.3   6.9   5.5   5.9       Bit   1.8   2.0   2.4   0.35   0.35   0.4   2.7   3.7   3.5       Line       RP   3.8   2.4   3.1   0.2   0.2   0.25   5.4   6.7   4.5                  
 
         [0054]     Table 1 illustrates the results of tests across different circuit pattern types using various photo masks. The various pattern types were formed during fabrication of a 92 nm node DRAM and include such circuit types as an active pattern, a gate line pattern, a self-aligned contact (SAC) pattern, a bit line pattern, or a resist poly (RP) pattern. Exposure latitude, depth of focus (DOF) and the mask error enhancement factor (MEEF) are surveyed for each of these patterns and for each test photo masks used. The subjects to be compared are ‘A1’ which is a halftone phase shift mask having a transparency of 6% (6% HT-PSM), ‘A2’ which is a photo mask according to the present invention, having a depression etching depth of a 90 degree phase, and ‘A3’ which is another photo mask according to the present invention, having an depression etching depth of a 130 degree phase. The exposure light source is ArF.  
         [0055]     In Table 1, the exposure latitude indicates a change in the critical dimensions of a pattern depending on a change in the exposure dose. As the exposure latitude becomes smaller, the photo mask is more efficient. The DOF indicates a permissible range of the depth of focus. As the DOF becomes greater, the photo mask is more efficient. The MEEF indicates a change in the critical dimensions of a wafer depending on a change in the critical dimensions of the mask. As the MEEF becomes smaller, the photo mask is more efficient.  
         [0056]     From the results shown in Table 1, the line/space type patterns (e.g. gate line and bit line) generally have similar performance to the phase shift mask at the etching depth of a 90 degree phase, and the island type patterns (e.g. active, SAC and RP) have optimum optical performance at the etching depth of a 130 degree phase.  
         [0057]     Therefore, in preferred use, an optimum etching depth must be selected for each mask pattern rather that using a fixed etching depth for all circuit pattern types. For this purpose, the wavelength of the exposure light source, the incidence angle of the incident light, the critical dimensions of the patterns, and the proper exposure dose are to be taken into account.  
         [0058]      FIGS. 4A through 4D  are sectional views illustrating a process of fabricating the photo mask of  FIG. 3 .  
         [0059]     In  FIG. 4A , a light shielding layer  32  is formed by depositing chromium, as by sputtering, on a transparent substrate  30  made of quartz. A resist for electron-beam exposure is applied to the light shielding layer  32  by spin coating. After the resist is dried and pre-baked, a mask pattern  36  in a line/space pattern type, which has predetermined critical dimensions, is formed by the electron-beam exposure and subsequent development.  
         [0060]     In  FIG. 4B , a pattern of the light shielding layer  32  is formed by using the mask pattern  36  as an etching mask and selectively removing the light shielding layer  32  exposed under the mask pattern  36  by reactive ion etching, thereby exposing parts of the transparent substrate  30 . In  FIG. 4C , the mask pattern  36  remaining on the pattern of the light shielding layer  32  is removed by wet etching. The mask pattern  36  may be later removed after it is used as the etching mask, together with the pattern of the light shielding layer  32 , during the etching process of the transparent substrate  30 .  
         [0061]     In  FIG. 4D , parts of the transparent substrate  30  are etched by using the pattern of the light shielding layer  32  as an etching mask, thereby forming patterns of depressions  34 . The etching of the transparent substrate  30  is performed using, for example, an ArF excimer laser, so that the sidewalls of the depressions  34  are nearly vertical and each depression  34  has a uniform etching depth.  
         [0062]     There are three primary advantages of the photo mask constructed and implemented according to the present invention. First, the inventive photo mask gives increased contrast and productivity by using a suitable exposure dose. Second, the photo mask can be fabricated using a relatively simple process. And third, the photo mask does not exhibit problems associated with phase shift masks.  
         [0063]     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.  
         [0064]     For example, in the embodiment of the present invention, KrF or ArF are used as the exposure light source. However, the exposure light source is not limited to KrF or ArF. G-line or I-line, which have a greater wavelength than KrF or ArF, or F 2 , which has a shorter wavelength than KrF or ArF, may be used as the exposure light source. In the embodiment, the transparent substrate is made of quartz (SiO 2 ), but may also be made of other transparent materials, for example, calcium fluoride (CaF 2 ) or magnesium fluoride (MgF 2 ). In the embodiment, the light shielding layer is formed of chromium, but may also be formed of chromium oxide (CrOx), tungsten silicon (W—Si), or the like, which are capable of shielding light.  
         [0065]     Further, the photo mask according to the present invention is usable for both the normal incident illumination system and the off-axis illumination system. The photo mask has greater benefits when used for the off-axis illumination system, e.g. with incident light having a 0˜15 degrees incidence angle.