Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2003-01-27
2004-11-30
Berman, Jack I. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C378S034000, C355S068000
Reexamination Certificate
active
06825481
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an exposure apparatus capable of transferring a microcircuit pattern, and control method thereof.
BACKGROUND OF THE INVENTION
As a conventional exposure (lithography) method for manufacturing micro semiconductor devices, such as semiconductor memory or logical circuits or the like, demagnifying projection exposure using ultraviolet light has been employed.
The minimum size transferable in the demagnifying projection exposure is proportional to a wavelength of light used in the transfer, and is inversely proportional to the numerical aperture of a projection optical system. Therefore, in order to transfer a microcircuit pattern, it is necessary to pursue a shorter wavelength of light used in the transfer. For this reason, wavelengths of ultraviolet light used are becoming shorter, e.g., mercury lamp i-ray (wavelength of 365 nm), KrE excimer laser (wavelength of 248 nm), ArF excimer laser (wavelength of 193 nm) and so forth.
However, because semiconductor devices are rapidly miniaturizing, the lithography using ultraviolet light has begun to reach its limits. Therefore, in order to perform efficient exposure of a very small microcircuit pattern, which is smaller than 0.1 &mgr;m, a demagnifying projection exposure apparatus using extreme-ultraviolet light (EUV light), having even shorter wavelength (about 10 to 15 nm) than ultraviolet light, has been developed.
In such EUV-light area, since EUV light is very largely absorbed by substances, a lens optical system employing refraction of light, such as that used in visible light or ultraviolet light, is not practical. Instead, the exposure apparatus using EUV light adopts a reflection optical system. In this case, for a reticle (mask) also, a reflection-type reticle (reflection-type mask), where a pattern subjected to transfer is formed on a mirror with an absorptive member, is employed.
As a reflection-type optical device which constitutes the exposure apparatus using EUV light, there are multilayer mirrors and grazing incidence total reflection mirrors. In the EUV-light area, since the real part of refractive index is slightly smaller than 1, total reflection takes place if EUV light is obliquely incident very closely to the surface. Normally, with a grazing incident angle of several degrees or less from the surface, a reflectivity higher than several tens of % can be achieved. However, because of the low flexibility in terms of optical designing, it is difficult to apply total reflection mirrors to the projection optical system.
For an EUV-light mirror used at an incident angle close to normal incidence, a multilayer mirror, where two types of substances having different optical constants are alternately layered, is used. In the multilayer mirror, molybdenum and silicon are alternately layered on the surface of a glass substrate polished into a fine plane shape. The thickness of the layer is, for instance, 2 nm for a molybdenum layer, and about 5 nm for a silicon layer, and the number of layers laminated is about 40 layers in pairs. The thickness of the layers including the two types of substances is called a film period. In the foregoing example, the film period is 2 nm+5 nm=7 nm.
When EUV light is incident on such multilayer mirrors, EUV light having a specific wavelength is reflected. Assuming an incident angle is &thgr;; the wavelength of the EUV light, &lgr;; and the film period, d; only the EUV light having a narrow bandwidth, whose center &lgr; approximately satisfies Bragg's equation 2×d×cos &thgr;=&lgr;,
is efficiently reflected. The bandwidth herein is about 0.6 to 1 nm.
The reflectivity of the EUV light reflected is about 0.7 at the maximum. EUV light that is not reflected is absorbed in the multilayer film or the substrate, and most part of the energy thereof turns into heat.
Since the multilayer mirrors used in the EUV area have a large light loss compared to mirrors used in visible light, it is necessary to minimize the number of mirrors. To ensure a wide exposure area with the small number of mirrors, a method (scan exposure) of transferring a wide area by simultaneously scanning a reticle and a wafer only with the use of a thin arc-shaped area (ring field) that is away from an optical axis by a fixed distance, may be considered.
In the case of employing the above-described EUV light as exposure light, a laser plasma light source or an electric discharge plasma light source is used. However in such EUV exposure apparatus, there are following problems.
A laser plasma light source used as an EUV light source is realized by irradiating a highly intense pulse laser beam to a target material for generating high-temperature plasma, and employing EUV light emitted therefrom, having a wavelength of, e.g., about 13 nm. The intensity of EUV light emitted from the laser plasma light source fluctuates depending on a temperature of the target. Particularly in the method of achieving a high-density target by increasing a gas density with adiabatic expansion of gas or clustering the gas, a slight change in the temperature of the emitted gas or nozzles greatly changes the target density at the time of irradiating excitation laser, and along with that, largely changes the intensity of EUV light emitted.
If the intensity of EUV light emitted from the light source changes, the amount of EUV light irradiated to a wafer fluctuates, causing variations in the size of a microcircuit pattern subjected to transfer or disabling the transfer of a microcircuit pattern.
Similarly, an intensity of EUV light emitted from the electric discharge plasma light source fluctuates depending on a temperature of electrodes or gas. The electric discharge plasma light source is realized by applying a pulse voltage to electrodes in gas for generating high-temperature plasma, and employing EUV light emitted therefrom, having a wavelength of, e.g., about 13 nm. In the electric discharge plasma light source also, nozzles of a gas supplying device or electrodes are heated by electromagnetic waves or particles emitted from the plasma, and electrodes are heated by Joule heat inside the electrodes. For these reasons, the intensity of EUV light emitted from the light source changes, and along with that, the amount of EUV light irradiated to a wafer fluctuates, causing variations in the size of a microcircuit pattern subjected to transfer or disabling the transfer of a microcircuit pattern.
Furthermore, the high-temperature plasma generated in the laser plasma light source emits high-speed gas molecules and charged particles. There is a case in which a part of a target material supplying device is sputtered by high-speed particles of the plasma (sputtering phenomenon), causing atomic elements on the surface to fly. These are called debris. If the debris is irradiated to an initial-stage mirror of the illumination system which illuminates a reticle with light from the light source, the multilayer film of the mirror is damaged. The mechanism is as follows:
the multilayer structure is destroyed by particle energy;
the target material and material of the target supplying device are deposited on the multilayer film, and become an EUV-light absorbing layer;
the heated multilayer film causes recrystallization of substances constituting the film, and counter diffusion causes the film structure to change.
Similarly, the plasma generated in the electric discharge plasma light source also emits high-speed gas molecules and charged particles. There is a case in which an electrode material or a part of an insulation material holding the electrode is sputtered by high-speed particles of the plasma, causing atomic elements on the surface to fly. If the debris is irradiated to an initial-stage mirror of the illumination system, the multilayer film of the mirror is damaged.
Because of these phenomena, the reflectivity of the multilayer mirror gradually decreases along with operation of the EUV light source. Therefore, the intensity of EUV light illuminating a reticle gradually declines. Accor
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