Method of determining focus and coma of a lens at various...

Details Method of determining focus and coma of a lens at various... Method of determining focus and coma of a lens at various...

1998-12-04

2001-01-09

Young, Christopher G. (Department: 1756)

Radiation imagery chemistry: process, composition, or product th

Including control feature responsive to a test or measurement

Reexamination Certificate

active

06171739

ABSTRACT:

TECHNICAL FIELD
The present invention generally relates to the lithographic patterning of a semiconductor wafer. In particular, the present invention relates to a method for characterizing the focus and coma of a lens used in the production of the semiconductor wafer at various locations in an imaging field.
BACKGROUND OF THE INVENTION
Referring initially to FIG.
1
a
, integrated circuits are formed on semiconductor wafers
10
typically made from silicon. The wafers
10
are substantially round and typically have a diameter of approximately 15 to 20 cm. Each wafer
10
is divided up into individual circuit die
15
which contain an integrated circuit. Since a single integrated circuit die
15
is often no more than 1 cm
2
, a great many integrated circuit die
15
can be formed on a single wafer
10
. After the wafer
10
has been processed to form a number of integrated circuit die on its surface, the wafer
10
is cut along scribe lines
20
to separate the integrated circuit die for subsequent packaging and use.
Formation of each integrated circuit die on the wafer is accomplished using photo-lithography. In general, lithography refers to processes for pattern transfer between various media. The basic photo-lithography system consists of a light source, a photomask containing the pattern to be transferred to the wafer, a collection of lenses, and a means for aligning existing patterns on the wafer with patterns on the photomask.
Referring to FIG.
1
b
, during an intermediate stage in the manufacturing cycle, the wafer
10
is shown to include a film
25
which overlies the wafer
10
and a resist
30
disposed on the film
25
. Exposing the resist
30
to light or radiation of an appropriate wavelength through the photomask causes modifications in the molecular structure of the resist polymers to allow for transfer of the pattern from the photomask to the resist
30
. The modification to the molecular structure allows a resist developer to dissolve and remove the resist in exposed areas, presuming a positive resist is used. If a negative resist is used, the developer removes the resist in the unexposed areas.
Referring to FIG.
1
c
, once the resist
30
on the wafer has been developed, one or more etching steps take place which ultimately allow for transferring the desired pattern to the wafer
10
. For example, in order to etch the film
25
disposed between the resist
30
and the wafer
10
, an etchant is applied over the patterned resist
30
. The etchant comes into contact with the underlying film layer by passing through openings
35
in the resist formed during the resist exposure and development steps. Thus, the etchant serves to etch away those regions of the film layer which correspond to the openings in the resist, thereby effectively transferring the pattern in the resist to the film layer as illustrated in FIG.
1
d
. In subsequent steps, the resist is removed and another etchant may be applied over the patterned film layer to transfer the pattern to the wafer or to another layer in a similar manner.
Presently, there are a variety of known techniques for transferring a pattern to a wafer using photolithography. For instance, referring to
FIG. 2
, a reduction step-and-repeat system
50
(also called a reduction stepper system
50
) is depicted. The reduction stepper system
50
uses refractive optics to project a mask image onto a resist layer
30
. The reduction stepper system
50
includes a mirror
55
, a light source
60
, a filter
65
, a condenser lens system
70
, a mask
75
, a reduction lens system
80
, and the wafer
10
. The mirror
55
behaves as a collecting optics system to direct as much of the light from the light source
60
(e.g. a mercury-vapor lamp) to the wafer
10
. The filter
65
is used to limit the light exposure wavelengths to the specified frequencies and bandwidth. The condenser system
70
focuses the radiation through the mask
75
and to the reduction lens system to thereby focus a “masked” radiation exposure onto one of the circuit die
15
.
Since it is complex and expensive to produce a lens capable of projecting a mask of an entire wafer, the reduction stepper system
50
, projects an image only onto a portion of the wafer
10
corresponding to an individual circuit die
15
. This image in then stepped and repeated across the wafer
10
in order to transfer the pattern to the entire wafer
10
(and thus the name “stepper”). Consequently, the size of the wafer is no longer a consideration for the system optics.
Current reduction stepper systems
50
utilize masks that contain a pattern that is an enlargement of the desired image on the wafer
10
. Consequently, the mask pattern is reduced when projected onto the resist
30
during exposure (and thus the name “reduction stepper”).
With an ever increasing number of integrated circuit patterns being formed on a circuit die, the importance of properly designing patterns to form structures that are isolated and non-interfering with one another has also increased. Accordingly, when designing a pattern to place on a mask, it is of significant benefit to know in advance the amount of error to expect with respect to the corresponding structures formed on the wafer so that such error can be accounted for in advance.
One known source for errors introduced during the patterning of the resist on a wafer occurs due to diffraction effects caused during the passage of light through the pattern formed on the mask. In particular, light which passes adjacent an edge of a pattern formed on the mask is caused to diffract by the edge thereby scattering the light in multiple directions. As a result the light intensity on the resist is not perfectly binary in nature.
For example, referring to FIGS.
3
(
a
)-
3
(
c
) the diffraction affects of light passing through mask
40
having a patterned chrome layer
42
formed thereon is depicted. In particular, as the apertures P
1
and P
2
in FIG.
3
(
a
) become closer to one another to allow for more tightly packed features on the wafer, the amplitude of light incident on the wafer in regions not corresponding to the apertures P
1
and P
2
increases. The degree to which the light amplitude in various regions varies due to the diffraction affects can be seen by comparing the light amplitude curve immediately after transmission through the mask in FIG.
3
(
b
) with the light amplitude curve on the wafer in FIG.
3
(
c
). For comparison purposes, an ideal light amplitude curve is also depicted in FIG.
3
(
c
) in dotted lines. From the light amplitude curve shown in FIG.
3
(
c
), the light intensity curve shown in FIG.
3
(
d
) may be derived. In particular, the light intensity is proportional to the square of the resultant amplitude. As the resist is sensitive to the light intensity incident thereon, it can be seen from FIG.
3
(
d
) that the resist will not have the desired sharp contrast resolution in the region between P
1
and P
2
.
In order to reduce the affects of the diffracted light, a phase shift mask (PSM) has been utilized. In a PSM, phase variations are produced in the light that passes through the mask material. The phase variations are achieved by modifying the length that a light beam travels through the mask material.
In particular, referring to FIGS.
4
(
a
)-
4
(
d
) two different method of achieving phase shift masking is depicted. In FIG.
4
(
a
) the light passing through region P
2
is caused to pass through additional mask material via deposited layer
47
. In FIG.
4
(
b
), the light in region P
2
traverses through a shorter distance by virtue of etched groove
48
. Each of the phase shift masks in FIGS.
4
(
a
) and
4
(
b
) make use of the fact that light passing through the mask material exhibits a wave characteristic such that the phase of the amplitude of the light exiting from the mask material is a function of the distance the light ray travels in the mask material, i.e. thicknesses t
1
and t
2
. By making the thickness t
2
such that (n−1) (t
2
) is approximately equal to ½&lgr;, where &lgr; is the wave

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