Topographic phase control for overlay measurement
Metrology tools and methods are provided, which estimate the effect of topographic phases corresponding to different diffraction orders, which result from light scattering on periodic targets, and adjust the measurement conditions to improve measurement accuracy. In imaging, overlay error magnification may be reduced by choosing appropriate measurement conditions based on analysis of contrast function behavior, changing illumination conditions (reducing spectrum width and illumination NA), using polarizing targets and/or optical systems, using multiple defocusing positions etc. On-the-fly calibration of measurement results may be carried out in imaging or scatterometry using additional measurements or additional target cells.
1. A method comprising:
imaging an imaging metrology target having target layers using an optical system using an objective;
deriving a dependency of a topographic phase of the imaging metrology target on a measurement wavelength;
adjusting the measurement wavelength to make the topographic phase an integer multiple of π;
carrying out imaging metrology measurements of the imaging metrology target at the adjusted measurement wavelength;
configuring an adjustable reference path in the optical system as a Linnik interferometer using an attenuator, a reference objective identical to an objective, and an adjustable mirror, wherein the Linnik interferometer includes a first beam splitter that receives light from an illumination source and a reference path that receives light directly from a second beam splitter in optical communication with the first beam splitter;
adjusting a phase of a reference signal to modify a topographic phase of the imaging metrology target to be an integer multiple of π;
adjusting an amplitude of the reference signal using the attenuator and the reference objective and/or the adjustable mirror; and
minimizing a difference of topographic phases between zeroth and first order diffraction signals from the target layers.
2. The method of claim 1 , wherein the measurement wavelength adjustment is in a range of ±10 nm.
3. A method comprising:
directing light at an objective from an illumination source that is directed to a first beam splitter and a second beam splitter and from an illumination path that is directed to the second beam splitter;
integrating, in a collection path of an imaging metrology optical system, an adjustable reference path comprising a reference signal, wherein the reference path receives light through the first beam splitter and directly from the second beam splitter, and wherein the adjustable reference path is a Linnik interferometer with an attenuator, a reference objective identical to an objective of the imaging metrology optical system, and an adjustable mirror;
adjusting a phase of the reference signal with the reference path to modify a topographic phase of an imaging metrology target to be an integer multiple of π; and
adjusting an amplitude of the reference signal using the attenuator and the reference objective and/or the adjustable mirror.
4. The method of claim 3 , further comprising minimizing a difference of topographic phases between zeroth and first order diffraction signals from at least two target layers.
5. The method of claim 4 , wherein the minimizing is carried out according to Equation:
min
{
[
A
·
sin
α
+
C
·
sin
γ
A
·
cos
α
+
C
·
cos
γ
-
tg
(
φ
1
)
]
2
+
[
B
·
sin
β
+
C
·
sin
γ
B
·
cos
β
+
C
·
cos
γ
-
tg
(
φ
2
)
]
2
}
wherein α is a noise property of the illumination source; β is a noise property of a detector; φ 1 is a topographic phase of a first diffraction order of a first grating; φ 2 is a topographic phase of a first diffraction order of a second grating; g is a pupil coordinate; A is a coherent sum of zero diffraction orders reflected from a first layer of the wafer; B is a coherent sum of zero diffraction orders reflected from a second layer of the wafer; and C is a zero diffraction order reflected from a reference mirror.
6. The method of claim 3 , further comprising deriving a dependency of the topographic phase of an imaging metrology target on a measurement wavelength.