1. Field of the Invention
The present invention is related to a feedback system and a feedback method for controlling power ratio of incident light.
2. Description of the Prior Art
As the integration of ICs increases the critical dimension of semiconductors becomes smaller. Therefore, it is desirable to increase the resolution limit of optical exposure tools. A conventional method for improving resolution includes the steps of: off-axis illumination, immersion lithography and increasing the numerical aperture of the lens. As the resolution increases, mask induced polarization may occur.
In general, a mask is composed of a mask substrate and a patterned metal layer. The mask substrate can be a quartz substrate, and the patterned metal layer covers the quartz substrate. The light can be defined into two modes: transverse-electric (TE) mode and transverse-magnetic (TM) mode.
Polarization effects can be a concern with the decreasing device dimensions. Based on physical properties, the patterned metal layer has a higher transmittance with respect to the TE mode of the light compared to the TM mode of the light, especially when the incident angle is large. On the contrary, the quartz substrate has a low transmittance with respect to the TE mode of the light. Therefore, even if the TE mode of light passing through the patterned metal layer is utilized, the TE mode of light will be blocked by the quartz substrate before it reaches the wafer. As a result, the product yield will be deteriorated.
It is therefore the primary object of the present invention to provide a feedback system to adjust the polarization power ratio of incident light. By converting the energy of TM mode to TE mode, the energy of the TE mode can be increased when it reaches the wafer. The resolution and yield can thereby be enhanced.
From one aspect of the present invention, the present invention provides a feedback method for controlling polarization ratio of incident light.
First, a mask having a mark is provided. Thereafter, the mark is illuminated with incident light. Next, reflected light or refracted light from the illuminated mark is detected to get a first parameter. Afterwards, the first parameter is processed to become a second parameter. Finally, polarization ratio of the incident light is adjusted from the second parameter.
From another aspect of the present invention, the present invention further provides a feedback controlling system including an incident light used to illuminate a mark on a mask, a polarization converter used to control the polarization power ratio of the incident light, a detector used to detect reflected light or refracted light from the illuminated mark to get a parameter, and a processor used to calculate the parameter and send a feedback signal to the polarization converter to adjust the polarization power ratio of the incident light.
The present invention features disposing a mark on the mask substrate. After the energy of the reflected light or the refracted light from the illuminated mark is detected, the TE/TM polarization power ratio of the incident light can be obtained by calculation. Then, the TE/TM polarization power ratio is fed back into the polarization converter used as a base value. After that, the energy of TE mode of the incident light can be increased by the polarization converter.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.
(1) A light source 10. Radiant of the light source 10 is focused to become radiant 10a after passing through lens 12. Then, radiant 10a passes through an aperture plate 14.
(2) A polarization converter 16. A TE/TM polarization power ratio of radiant 10a is adjusted by the polarization converter 16 to form incident light 11. A mark 20 on a mask substrate 22 is illuminated by the incident light 11, and the incident light 11 is refracted to form refracted light 11′. The mask substrate 22 can be a quartz substrate, and the mark 20 and the mask substrate 22 form a mask 18.
(3) A detector 24 used to detect the refracted light 11′ from the illuminated portion of the mark 20 to get a parameter. According to a preferred embodiment of the present invention, the parameter can be energy of the refracted light 11′ in TE mode.
(4) A processor 26 used to calculate the parameter. After calculating the parameter, a TE/TM polarization power ratio of the incident light 11 can be obtained. Then the TE/TM polarization power ratio of the incident light 11 will be fed back into the polarization converter 16 to be a feedback signal. In this way, by taking the TE/TM polarization power ratio of the incident light 11 as a base value, the TE/TM polarization power ratio of the radiant 10a will be changed by the polarization converter 16, and then the TE/TM polarization power ratio of the incident light 11 can be adjusted before illuminating the mark 22 again.
The mark 20 can be composed of a plurality of grating lines. Any material that can form the grating lines can be used. There is pitch Λ between the grating lines. According to the preferred embodiment of the present invention, the pitch Λ is smaller than the wavelength of the radiant 10a. In addition, the mask substrate 22 is not limited to the quartz substrate.
In another embodiment of the present invention, a feedback method for controlling a polarization power ratio of incident light is provided. The feedback method will be described by utilizing the feedback system 100 as an example.
The method for controlling TE/TM polarization power ratio of incident light is started by measuring the experimental value of the transmittance of mask 18 with respect to the incident light 11. Next, the incident light 11 illuminates the mask 18 and then passes through the mask substrate 22. The illuminated portion of the mark 20 and the mask substrate 22 forms the refracted light 11′. Thereafter, a first parameter such as energy of the refracted light 11′ in TE mode is detected by the detector 24. Subsequently, the energy of the refracted light 11′ in TE mode is sent into the processor 26. Then the second parameter such as the TE/TM polarization power ratio of the incident light 11 can be calculated by the processor 26 by utilizing transmittance of the mask 18 with respect to the incident light 11, the energy of the refracted light 11′ in TE mode, the total energy of the incident light 11 and the reflective index of the mask substrate 22. As one skilled in the art should know, the total energy of the incident light 11 can be measured by an optical power meter, and the reflective index of the mask substrate 22 is based on the material of the mask substrate 22.
Then, the TE/TM polarization power ratio of the incident light 11 is fed back into the polarization converter 16 as a base value. Afterwards, the energy of the radiant 10a in TM mode can be converted to the energy of the radiant 10a in TE mode by the polarization converter 16. Therefore, when the incident light 11 goes into the mask 18 again, the energy of the incident light 11 in TE mode is increased, and the energy of the incident light 11 in TE mode passing through the mask can thereby also be increased.
The design method of the mark 20 used in the feedback controlling system 100 is illustrated as follows.
wherein φ=2/π−θ. In this state, the reflective index and the transmittance of the mark 20 are highly related to the polarization of the incident light 11.
There are two common methods of designing the mark having a specific reflective index and transmittance to certain polarized light.
(1) Vector Analysis Method
]Assuming the mark 20 is made from a perfect conductor, the wave function of the electromagnetic wave followed the boundary conditions of the mark 20 can be expressed as follows:
{right arrow over (E)}
(1)
=ŷE
y0
(1)
e
i(k
z+k
(x−w/2)) (2)
{right arrow over (E)}
(2)
=ŷE
y0
(2)(ei(k
{right arrow over (E)}
(3)
=ŷE
y0
(3)
e
i(k
z−k
(x+w/2)) (4)
Eq. (2), (3), and (4) are equations which describe the electromagnetic wave in region 1, 2, and 3 respectively. {right arrow over (E)} is electric field, k is wave vector. Ey0 is wave amplitude. The superscripted number shows the region, and the subscripted number shows the direction. For example, kz(2) is wave vector in region 2 and in Z-axis direction, and Ey0(1) is the wave amplitude in region 1.
Next, kz(2),kx(1),kx(2),kx(3) are solved by using Eq. (2), (3) and (4) together with Eq. (5) (eigen function) illustrated as follows.
V=ε″
m
k
0
2
+i(√{square root over ((kz(2))2−ε′mk02)}+tan h(w/2√{square root over ((kz(2))2−ε2k02)})√{square root over ((kz(2))2−ε2k02)}) (5
V is the eigen value and ε2 is the permittivity of the medium in region 2. ε′m is the real part of the permittivity of the grating lines. ε″m is the imaginary part of the permittivity of the grating lines. Therefore, the transmittance can be expressed as follows.
P is the mode number. αp is the wave vector of the diffraction mode, P, along the X-axis direction.
(2) Finite Different Time Domain (FDTD) Method
In this method, the electromagnetic wave is expressed as a difference quotient. Next, by taking the boundary conditions into consideration, the FDTD can be used for solving the transmittance of the mask 18 with respect to the zero-order incident light 11 in TE mode.
It is assumed that A=500 nm, h=380 nm, θ=0, n1=n2=n3=1, and the wavelength of the incident light 11 is 670 nm. The transmittance of the mask 18 with respect to the zero-order incident light 11 in TE mode versus width W is shown in
Please refer to
In another embodiment of the present invention, another feedback method for controlling TE/TM polarization power ratio of incident light is provided. The feedback method will be described by utilizing the feedback system 200 as example. As shown in
Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.
Number | Date | Country | Kind |
---|---|---|---|
097107679 | Mar 2008 | TW | national |