Phase-shift masks and methods of fabrication

Details Phase-shift masks and methods of fabrication Phase-shift masks and methods of fabrication

2001-03-12

2003-02-25

Rosasco, S. (Department: 1756)

Radiation imagery chemistry: process, composition, or product th

Radiation modifying product or process of making

Radiation mask

C430S323000, C428S408000

Reexamination Certificate

active

06524755

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to the manufacture of electronic devices, and more particularly to the formation of masks employed to fabricate integrated circuits having small features sizes.
As the integrated circuit (IC) manufacturing industry moves toward smaller device dimensions, the resolution enhancement techniques (RET) for optical lithography such as off-axis illumination (OAI), optical proximity correction (OPC) and phase-shifting mask (PSM) have been implemented in conjunction with reducing the wavelength of optical exposure. Lithography at shorter wavelengths imposes a particular challenge for PSM fabrication because the inherent functions of PSMs require optical and physical property control. Thus while OAI is applied at the optical relay system level and OPC often requires only binary transmission, PSMs must generally accurately modify both transmission and phase functions of incident light at the device level. At present, implementation of phase-shifting to fabricate small features consumes much of the cost of manufacturing masks employed to fabricate ICs having present-day critical dimensions.
There are various types of standard PSMs, including alternating aperture PSMs (AAPSMs) and embedded-attenuating PSMs (EAPSMs).
FIG. 1
shows a simplified cross-sectional view of an AAPSM with phase shifting features defined on a UV transparent substrate. AAPSM
1
with phase shifting features is defined on a UV transparent substrate
11
with Cr coating
12
. Openings
13
and
14
with thickness difference d such that phase difference between light transmitted through un-shifted opening
15
and phase-shifted opening
16
is 180°. “n” is the refractive index of the substrate
11
.
FIG. 2
shows a simplified cross-sectional view of an EAPSM with partially transmitting film. EAPSM
2
has partially transmitting film
23
that shifts the phase of transmitted light by 180°. EAPSM
2
is built on a UV transparent substrate
21
, which is coated with molybdenum silicon-oxynitride (MoSi
x
O
y
N
z
) complex
23
and over-coated with a chromium (Cr) layer
22
. MoSi
x
O
y
N
z
layer
23
attenuates and phase-shifts incident beam
25
relative to the unaltered beam
26
.
Fabrication of EAPSMs often requires that mask blanks be coated with a partially absorbing film that shifts the phase of the optical wavefront incident by 180° with respect to the coincident wavefront that transmits through the part of the mask not covered by the film (see FIG.
2
). This phase difference:
&Dgr;&phgr;=2
&pgr;[n
(&lgr;)−1
]d
/&lgr;, where:
&Dgr;&phgr;=phase difference;
n(&lgr;)=refractive index of the film at a given stepper wavelength &lgr;; and
d=film thickness,
causes the adjacent wavefronts to destructively interfere with each other. In addition to altering the phase, amplitudes of the two wavefronts near the edge of critical features should be matched with appropriate film absorption in order to achieve desirable destructive interference.
Transmission by the partially transmitting film is given by
T=T
O
exp[−4
]k
(&lgr;)
d
/&lgr;], where:
T
O
=initial value of transmission
k(&lgr;)=the imaginary component of the refractive index at a particular wavelength &lgr;; and
d=film thickness,
with d determined by the 180° phase difference requirement. For a given wavelength, one should control the imaginary component k(&lgr;), of the refractive index of the film material by controlling its stoichiometry and composition. This task is complicated by the fact that any change in k(&lgr;) will modify the real component of refractive index n(&lgr;), as the two quantities are mathematically linked by the Kramers-Krönig equation. Consequently, utilizing a single film layer solution will in general not readily lead to arbitrary control over both phase and amplitude.
FIG. 3
shows a cross-sectional view of a mask blank
3
comprising a layer of MoSi
x
O
y
N
z
32
and Cr
33
deposited on UV-grade quartz substrate
31
. Mask blanks comprising a single layer of MoSi
x
O
y
N
z
have been used for fabricating 248 nm EAPSMs. However, the etch selectivity of MoSi
x
O
y
N
z
relative to substrate
31
and the optical properties of mask blank
3
may not be sufficient for applications at or below 193 nm wavelength.
FIGS. 4A-4I
show simplified cross-sectional views of multiple lithography and processing steps for fabricating AAPSMs. In
FIGS. 4A-4I
, multiple lithography and processing steps are utilized in fabricating AAPSMs using Cr coated mask blanks. Mask blank
4
comprises an electron-beam or optical-beam sensitive resist
43
coated on top of a Cr coating
42
on a UV transparent substrate
41
. The blanks are subjected to lithography processes
44
and
45
(which may be optical or electron-beam in nature) in combination with wet and dry etching steps to define phase-shifting features.
FIGS. 5A-5I
show simplified cross-sectional views of multiple lithography and processing steps for fabricating EAPSMs. In
FIGS. 5A-5I
multiple lithography and processing steps are utilized in fabricating EAPSMs using MoSi
x
O
y
N
z
coated mask blank
5
. UV transparent substrate
51
is coated with MoSi
x
O
y
N
z
layer
52
, Cr layer
53
and then a layer of electron-beam or optical-beam sensitive resist
54
. The blank is then subjected to lithography processes
55
and
56
(which may be either electron-beam or optical-beam) with wet and dry etching steps to define embedded-attenuating phase-shifting features.
As illustrated in
FIGS. 4A-4I
for formation of AAPSMs and in
FIGS. 5A-5I
for formation of EAPSMs, PSM fabrication methods typically require about eight processing steps, as well as two separate electron/optical beam lithography steps with intervening processing. Each process step exhibits a certain defect rate, and multiple writing with in-between processing generally requires careful wafer handling that can lead to yield problems.
Therefore, there is a need in the art for simple and economical methods and structures for manufacturing PSMs.
SUMMARY OF THE INVENTION
The present invention relates to the use of multilayer film stacks and gray scale processing methods to fabricate phase-shifting masks (PSMs) utilized in lithography. Desired optical transmission and phase-shifting functions of the mask are achieved by controlling the optical properties and thickness of constituent film layers. By substantially separating the phase shift and attenuation functions between different film layers, the phase shift mask of embodiments of the present invention can be tuned for optimal performance at various wavelengths more precisely than conventional masks employing a single layer to control both attenuation and phase shifting.
Structures and methods in accordance with the present invention may exploit etch selectivity afforded by the use of appropriate materials in the film stack to obtain improved yields and reduced processing costs for fabrication of PSMs. Careful selection of the materials comprising the films of the mask blanks ensures that when a first layer is patterned by dry etching using a particular chemical gas, the film underlying the first layer will be etched very slowly by that chemical gas. In this manner, the etching system utilized in mask fabrication exhibits high etch selectivity, with each film of the stack behaving as an etch stop for etching of the overlying film. This etch selective property thus overcomes problems in non-uniform etching commonly associated with dry etching systems. The order of formation of film materials comprising the photomask blanks is thus an important aspect of certain embodiments of the present invention.
Processing methods in accordance with embodiments of the present invention may also exploit multi-level electron beam writing techniques to obtain improved yields and reduced processing costs for fabrication of PSMs. When such gray scale processing methods are applied to multilayer mask blanks in accordance with embodiments of the present invention in ord

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