Apparatus and method for inspecting phase shifting masks

An apparatus for die to die inspection of masks having transparent phase shifting elements and a method of die to die inspection of masks having transparent phase shifting elements. Light from a light source is directed through a transparent mask substrate and a phase shifting mask element to a first objective lens, and through the transparent mask substrate and another phase shifting mask element to a second objective lens. Light from the first objective lens is then given a 180.degree. phase shift by a phase adjustment unit. Light from the phase adjustment unit and the second objective lens is combined at a split mirror and directed to a detector. The method makes use of the fact that the intensity of the light at the detector is proportional to the square of the cosine of one half of the phase angle between the light from the phase adjusting unit and light from the second objective lens. If the intensity of light reaching the detector is not zero, or very small, the mask has a defect.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
This invention relates to an apparatus and a method for inspecting masks 
which comprise transparent phase shifting mask elements. 
(2) Description of the Related Art 
Inspection of phase shifting mask elements is a problem because defects are 
not readily apparent in the transparent phase shifting material. 
U.S. Pat. No. 5,270,796 to Tokui et al. describes an apparatus for 
inspecting a phase shift mask using a phase difference detector and a 
reference signal. The reference signal is generated from another mask 
having the same pattern or from CAD data. 
U.S. Pat. No. 5,353,116 to Tanigawa et al. describes a defect inspection 
system which projects two pattern images from two neighboring dies on the 
phase shift mask. The two images are superimposed and compared. 
U.S. Pat. No. 5,379,348 to Watanabe et al. describes a pattern defects 
inspection system for phase shifting masks. 
U.S. Pat. No. 5,446,540 to Lin describes a method of inspecting phase shift 
masks using phase-error enhancing. 
U.S. Pat. No. 5,482,799 to Isao et al. describes a phase shifting mask and 
a method of manufacturing the phase shifting mask. 
U.S. Pat. No. 5,677,092 to Takekuma et al. describes a method of 
fabricating a phase shifting mask and a method of inspecting the masks. 
SUMMARY OF THE INVENTION 
Masks formed using phase shifting materials find frequent use in 
photolithographic processing of integrated circuit wafers. As feature 
sizes have become smaller the masks used in photolithographic processing 
have made increasing use of phase shifting materials to improve image 
definition and depth of focus. 
The phase shifting materials used in mask fabrication provide a phase shift 
for light passing through the phase shifting materials relative to light 
which does not pass through the phase shifting materials, but are 
otherwise transparent. The amount of phase shift provided to the light 
depends not only on the materials used but on the thickness of the 
materials. This transparent nature of the phase shifting materials makes 
inspection of the masks extremely difficult. Variations in thickness as 
well as other defects are difficult to detect. Defects in the masks will 
lead to defects in the integrated circuits fabricated using the masks. 
FIG. 1 shows a cross section of a binary mask having a transparent mask 
substrate 10 and opaque mask elements 12. Since the mask elements in the 
binary mask are opaque they are relatively easy to inspect. 
FIG. 2A shows a cross section of a Levenson type phase shifting mask having 
a transparent mask substrate 10, opaque mask elements 12, and transparent 
phase shifting mask elements 14. Since the phase shifting mask elements 14 
are transparent they are much more difficult to inspect for defects. FIG. 
2B shows a mask 11 having a transparent mask substrate 11 and a number of 
die positions. FIG. 2B shows a first die position 61 and a second die 
position 62. 
It is a principle objective of this invention to provide an apparatus for 
inspecting masks utilizing phase shifting material which will easily 
detect defects in transparent phase shifting material. 
It is another principle objective of this invention to provide a method of 
inspecting masks utilizing phase shifting material which will easily 
detect defects in transparent phase shifting material. 
These objectives are achieved with an apparatus used for die to die 
inspection of masks having phase shifting material in the mask. The 
apparatus is used for die to die inspection of masks having a number of 
identical die images at different positions on the same transparent mask 
substrate. The apparatus comprises a light source, a first objective lens, 
a first condenser lens, a second objective lens, a second condenser lens, 
a phase adjustment unit, and a split mirror. A mask is positioned so that 
a first die position is placed between the first objective lens and the 
first condenser lens and a second die position is placed between the 
second objective lens and the second condenser lens. Light from the light 
source illuminates both the first condenser lens and the second condenser 
lens. Light illuminating the first objective lens first passes through the 
first condenser lens. Light illuminating the second objective lens first 
passes through the second condenser lens. The phase adjustment unit is 
adjusted to provide a 180.degree. phase shift to the light exiting the 
first objective lens relative to the light exiting the second objective 
lens. 
The first condenser lens and first objective lens are positioned to observe 
a point on a first die position so that light entering the first objective 
lens passes through the first condenser lens, the transparent mask 
substrate and a point on the first die position. The second condenser lens 
and second objective lens are positioned to observe the corresponding 
point on a second die position so that light entering the second objective 
lens passes through the second condenser lens, the transparent mask 
substrate and the corresponding point of the second die position. The 
light exiting the first objective lens, after being shifted in phase by 
180.degree. by the phase adjustment unit, and the light exiting the second 
objective lens are both focussed on a split mirror. After being focussed 
on the split mirror the light is directed to a first detector and to a 
second detector. In this example the first detector is a photomultiplier 
tube and the second detector is a CCD imaging device. 
The intensity, I, of the light at the first detector and the second 
detector is proportional to cos.sup.2 (.delta./2), where .delta. is the 
phase angle between light exiting the first objective lens at the split 
mirror and the light exiting the second objective lens at the split 
mirror. Since .delta. should be 180.degree. the intensity, I, should be 
zero. Any deviation of I from zero indicates a defect in the mask. By 
moving the lenses the first die position and the second die position can 
be inspected in their entirety and any deviation of the first die position 
to the second die position will be detected. In this manner the entire 
mask can be inspected.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Refer now to FIGS. 2A-5 for a description of the apparatus for inspecting 
masks of this invention. FIG. 2A shows a cross section of a Levenson type 
phase shifting mask having opaque mask elements 12 and transparent phase 
shifting elements 14 on a transparent mask substrate 10. FIG. 2B shows a 
mask 11 having a number of die positions on a transparent mask substrate 
10. FIG. 2B shows a point 63 in a first die position 61 and a 
corresponding point 64 in an identical second die position 62. 
FIGS. 3 and 4 show schematic views of the apparatus of this invention for 
inspecting a mask 11 having phase shifting elements. The inspection is a 
die to die inspection and compares points on a first die position with 
corresponding points on a second die position. FIGS. 3 and 4 are the same 
except for the position of a first die position relative to the first 
condenser lens 30 and the first objective lens 32 and the position of a 
second die position relative to the second condenser lens 31 and the 
second objective lens 33. FIGS. 3 and 4 show a light source 20, in this 
example a He--Xe lamp. The light from the light source 20 passes through a 
homogenizer 22, a band pass filter 24, and a grating aperture 26 before 
illuminating a first fiber optic link 28 and a second fiber optic link 29. 
The light illuminating the fiber optic links 28 and 29 is monochromatic 
light and the first fiber optic link 28 and the second fiber optic link 29 
are illuminated with the same intensity. The first fiber optic link 28 
illuminates a first condenser lens 30 and the second fiber optic link 29 
illuminates a second condenser lens 31. A first objective lens 32 is 
located directly above the first condenser lens 30. A second objective 
lens 33 is located directly above the first condenser lens 31. 
A mask 11 is placed in a mask holder 16 and is located between the first 
and second condenser lenses, 30 and 31, and the first and second objective 
lenses, 32 and 33. The mask 11 has a transparent mask substrate 10, opaque 
mask elements 12A and 12B, and transparent phase shifting mask elements 
14A and 14B. The mask 11, the first and second condenser lenses 30 and 31, 
and the first and second objective lenses 32 and 33 can be positioned 
relative to each other. 
FIG. 3 shows the apparatus in calibration mode wherein the mask 11, the 
first condenser lens 30, the second condenser lens 31, the first objective 
lens 32, and the second objective lens 33 are positioned so that the light 
passing through the first condenser lens 30 into the first objective lens 
32 passes through the transparent mask substrate 10 only and does not pass 
through phase shifting mask elements and the light passing through the 
second condenser lens 31 into the second objective lens 33 passes through 
the transparent mask substrate 10 only and does not pass through phase 
shifting mask elements. FIG. 4 shows the apparatus in inspection mode 
wherein the mask 11, the first condenser lens 30, the second condenser 
lens 31, the first objective lens 32, and the second objective lens 33 are 
positioned so that the light passing through the first condenser lens 30 
into the first objective lens 32 passes through both the transparent mask 
substrate 10 and first phase shifting mask elements 14A at the first die 
position 61 and the light passing through the second condenser lens 31 
into the second objective lens 33 passes through both the transparent mask 
substrate 10 and second phase shifting mask elements 14B at the second die 
position 62. 
The light exiting the first objective lens 32 passes through a first 
parallel prism 34 into a phase adjustment unit 36. The detail of the phase 
adjustment unit 36 is shown in FIG. 5. The phase adjustment unit 36 
provides phase shifting material of variable thickness. As shown in FIG. 5 
the phase adjustment comprises a first triangular section 50 of phase 
shifting material and a second triangular section 52 of phase shifting 
material. The first triangular section 50 is attached to a first holder 55 
and can be moved in the direction of the first arrow 55. The second 
triangular section 52 is attached to a second holder 56 and can be moved 
in the direction of the second arrow 57. By moving the first triangular 
section 50 and the second triangular section 52 the thickness 51 of phase 
shifting material the light will pass through, and thus the phase shift 
provided to the light, can be varied. 
Returning again to FIGS. 3 and 4, the light exiting the phase adjustment 
unit 36 is reflected by a mirror 38, passes through a first shutter 40 and 
is focussed on a split mirror 42 by adjusting the first parallel prism 34. 
The light exiting the second objective lens 35 passes through a second 
parallel prism 35, a second shutter 41, and is focussed on the split 
mirror 42 by adjusting the second parallel prism 35. One half of the light 
from the first objective lens 32 passes through the split mirror 42 and is 
combined with one half of the light from second objective lens 33 
reflected by the split mirror 42 and detected by the first detector 44. 
One half of the light from the first objective lens 32 is reflected by the 
split mirror 42 and is combined with one half of the light from second 
objective lens 33 passing through the split mirror 42 and detected by the 
second detector 46. In this example the first detector 44 is a 
photomultiplier tube 44 and the second detector 46 is a CCD imaging 
device. 
The method of inspecting masks having phase shifting elements using the 
apparatus described above will now be described with reference to FIGS. 
2A, 3, and 4. The method of this invention relies on the fact that the 
intensity of the light, I, reaching said the detector or the second 
detector is proportional to cos.sup.2 (.delta./2), where .delta. is the 
phase angle between the light from the phase adjustment unit 36 and the 
light from the second objective lens 33. When this phase angle is 
180.degree., the intensity of the light reaching the first detector 44 or 
the second detector 46 is zero. 
The apparatus is first calibrated by placing the apparatus in calibration 
mode, as shown in FIG. 3. In calibration mode the mask 11, the first 
condenser lens 30, the second condenser lens 31, the first objective lens 
32, and the second objective lens 33 are positioned so that the light 
passing through the first condenser lens 30 into the first objective lens 
32 passes through the transparent mask substrate 10 only and does not pass 
through phase shifting mask elements and the light passing through the 
second condenser lens 31 into the second objective lens 33 passes through 
the transparent mask substrate 10 only and does not pass through phase 
shifting mask elements. The phase adjustment unit 36 is then adjusted to 
reduce the light reaching the first detector or the light reaching the 
second detector to zero, or a minimum. This insures a 180.degree. phase 
angle difference between the light exiting the phase adjustment unit 36 
and the light exiting the second objective lens. 
Without changing the adjustment of the phase adjustment unit the apparatus 
is placed in inspection mode as shown in FIG. 4. In inspection mode the 
mask 11, the first condenser lens 30, the second condenser lens 31, the 
first objective lens 32, and the second objective lens 33 are positioned 
so that the light passing through the first condenser lens 30 into the 
first objective lens 32 passes through both the transparent mask substrate 
10 and first phase shifting mask elements 14A at the position of the first 
die 61 and the light passing through the second condenser lens 31 into the 
second objective lens 33 passes through both the transparent mask 
substrate 10 and second phase shifting mask elements 14B at the second die 
position 62. If the intensity of the light at the first detector or the 
second detector is not zero, or a minimum, the phase shift provided by the 
phase shifting elements seen by the first objective lens 32 and the second 
objective lens 33 are not equal and there is a defect in the mask. The 
first detector in this example is a photomultiplier tube providing a 
numerical reading. The second detector in this example is a CCD imaging 
device providing a visual display. The mask 11, first condenser lens 30, 
second condenser lens 31, first objective lens 32, and second objective 
lens 33 can be moved to scan the entire mask 11. 
While the invention has been particularly shown and described with 
reference to the preferred embodiments thereof, it will be understood by 
those skilled in the art that various changes in form and details may be 
made without departing from the spirit and scope of the invention.