Detection of phase defects on photomasks by differential...

Details Detection of phase defects on photomasks by differential... Detection of phase defects on photomasks by differential...

1999-06-21

2001-12-04

Font, Frank G. (Department: 2877)

Optics: measuring and testing

By configuration comparison

With comparison to master, desired shape, or reference voltage

Reexamination Certificate

active

06327033

ABSTRACT:

FIELD OF THE INVENTION
This invention relates, generally, to photolithography, and more particularly, to a method and apparatus for detecting phase defects and other phase features on a photomask using differential imaging.
BACKGROUND OF THE INVENTION
In the photolithography step of integrated circuit manufacturing, a template containing a designed set of clear and dark shapes, referred to as the mask or reticle, is repeatedly printed on the surface of a silicon wafer. This process is achieved by way of optical imaging at an image size resolution defined primarily by the wavelength (&lgr;), numerical aperture (NA) and partial coherence (&sgr;) of the optical projection system (hereinafter referred to as the stepper).
In standard industry practice as outlined in the schematic mask cross-section of
FIG. 1
, the masks containing the desired opaque and clear patterns are fabricated starting from an initial mask blank (
FIG. 1
a
) consisting of a substrate which is transparent to the imaging light (
10
), coated on one side with an opaque film (
20
). Typically, the transparent substrate consists of fused silica (also known as quartz) and which will, hereinafter, be referred to as the quartz substrate whereas the transparent substrate material will be referred to as quartz. Moreover, the opaque film is typically a chromium-based material, referred to hereinafter as the chrome film, while the material of the film itself being referred to as chrome. The designed shapes are replicated on this mask blank by first selectively patterning (or “writing”) the designed shapes in a protective material which is characterized as being sensitive to electron or optical exposure (
FIG. 1
b
), hence forth referred to as the resist (
30
). The openings created in the resist via selective patterning (
35
) are then transferred to the underlying chrome film during a subsequent etch step such that, following removal of the resist material (
FIG. 1
c
), the designed clear shapes (
40
) and opaque shapes (
21
) are replicated in the final patterned mask. Masks fabricated in this manner will be referred to, hereinafter, as standard or chrome-on-glass (COG) masks.
A different class of masks, phase-shifting masks (PSM), have demonstrated the capability of extending resolution beyond conventional imaging limits by taking advantage of both the phase and the magnitude of the imaging light. If two clear shapes which transmit light of opposite phases (
180
° phase difference) are placed in close proximity to one another, the phase difference will produce a destructive interference null between the two shapes. Such a mask has been given several different designations in the literature such as Levenson, Levenson-Shibuya, phase edge, alternating aperture, or alternating mask. Herein, it will be referred to as an alternating mask or an alternating PSM. The additional mask fabrication steps beyond the standard mask process of
FIG. 1
are shown for an etched-quartz or subtractive alternating PSM process in FIG.
2
.
With reference to
FIG. 2
, a phase difference between two clear shapes for the alternating PSM is achieved in standard industry practice by selectively etching into the quartz substrate (
10
), such that an optical path difference equivalent to the desired phase offset is obtained between the two adjacent openings. Following standard mask patterning as shown in
FIG. 1
, a second write step is used to selectively open a protective resist coating (
50
) for the phase-shifted opening (
41
) leaving the non-phase shifted opening (
42
) covered, as shown in
FIG. 2
a.
In practice, it is desirable to locate the edges of the resist pattern (
55
) some distance away from the phase-shifted opening (
41
) and on top of the opaque chrome shapes (
21
) where appropriate, in order to use the chrome itself as an etch barrier and to account for overlay (or pattern placement) errors between the first and second-level write steps in the fabrication process. The quartz is then etched (typically with an anisotropic reactive-ion etch (RIE) process) to a depth of approximately:
etch depth=phase* &lgr;
/[2*&pgr;*(
n−
1)]  (1)
wherein n is the refractive index of the quartz substrate at wavelength &lgr; and the phase of the opening (
41
) is given in radians. Following removal of the resist (
50
), the resultant alternating PSM has the etched-quartz trench (
15
) providing the desired phase difference between adjacent openings (
41
) and (
42
), as shown in
FIG. 2
b.
Other fabrication methods have been proposed. One of such approaches provides an accurate control of the phase, as determined by equation (1), through the addition of multi-layer films to the transparent substrate. More details may be obtained from an article by Chieu et al. entitled Fabrication of Phase Shifting Masks Employing Multi Layer Films, published in the Proc. SPIE, Vol. 2197, pp. 181-193, 1994, wherein a mask blank is described having two additional layers added between the transparent quartz substrate and the opaque chrome: an etch stop layer composed of either Al
2
O
3
or HfO
2
and a transparent layer of silicon dioxide at a controlled thickness given by equation (1). By etching into the silicon dioxide until the etch stop layer is reached, the desired phase is then achieved. Alternatively, additive fabrication methods can be used to achieve the desired phase shift such as through the application and selective patterning of a transparent spin-on-glass following the standard mask fabrication procedures illustrated in FIG.
1
. Regardless of the fabrication specifics, all of these techniques are generally categorized as an alternating PSM.
Defect-free masks are required for integrated circuit manufacturing (i.e., the patterns on the mask need to accurately replicate the designed data). In order to ensure defect-free masks following fabrication, the mask manufacturer will perform an automated optical inspection of the completed reticle to search for unwanted defects on the mask by comparing images of the mask from the optical inspection system to either the design database (hereinafter referred to as die-to-database inspection) or to the image from an exactly replicated pattern elsewhere on the mask (hereinafter referred to as die-to-die inspection). This inspection is typically performed on high-NA optical systems at wavelengths within the UV spectrum. For example, a state-of-the-art inspection system from KLA uses a 364 nm wavelength with a numerical aperture of 0.625. The defect inspection step can be further classified as either actinic (i.e., the inspection wavelength is the same as the exposure wavelength of the intended stepper) or non-actinic (i.e., the inspection wavelength is not the same as the exposure wavelength of the intended stepper).
An inspection system as described is used in standard practice for detecting defects such as shown in
FIG. 3
a
in cross-sectional view and
FIG. 3
b
in top-down view for the desired design shown in
FIG. 3
c.
Opaque shapes (
110
) and (
120
) are contained on the representative mask schematics of
FIG. 3
a
and
FIG. 3
b,
but an extra opaque shape (
130
) not contained in the design data of
FIG. 3
c
has been inadvertently added to the designed shapes as shown in
FIGS. 3
a
and
3
b.
This defect is expected to cause an anomalous printing effect during lithographic patterning of the defective mask (i.e., defect
130
) printing and/or causing variation in the shape or size of desired features (
110
) and (
120
) on the wafer) when the size of this opaque shape is on the order of one-third the minimum feature size or larger. Such defects may result from, but are not limited to, partial blockage of the chrome etch between the fabrication steps shown in
FIGS. 1
b
-
1
c,
or mask contamination by opaque, foreign material (FM) prior to mask inspection.
Generally, state-of-the-art inspection systems have demonstrated the capability to successfully find such printable defects. State-of-the-art inspection systems, however, are less adept at detecting transparent phase

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