Electricity: measuring and testing – Using ionization effects – For analysis of gas – vapor – or particles of matter
Reexamination Certificate
1998-07-13
2001-02-06
Tran, Andrew (Department: 2824)
Electricity: measuring and testing
Using ionization effects
For analysis of gas, vapor, or particles of matter
C324S754120, C324S537000
Reexamination Certificate
active
06184686
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to detection of trace materials in manufacturing processes, and specifically to contamination detection in semiconductor manufacturing.
BACKGROUND OF THE INVENTION
Contamination and residuals (C&R) detection is an integral part of the manufacture of semiconductor components. Typical contaminants found in the manufacturing process are impurities present in the liquid used to dissolve photoresist layers. A typical residual is a portion of photoresist, which is commonly polymethylmethacrylate (PMMA). As the size of the components decreases and their complexity grows, the effect of a specific contaminant on the component becomes more acute, and the detection of contaminants and also of residual chemicals becomes more important. In
Rapid Thermal Processing,
edited by Richard Fair, Academic Press, Inc., which is incorporated herein by reference, the author gives an example of a “killer” defect size in semiconductor processing for a gate thickness of 7 nm, wherein a defect sized 3.5 nm could be fatal. Typically, a rule of thumb for a killer defect size is that the defect is of the order of half the size of the design rule used.
Although both wet and dry cleaning processes are used in the manufacture of semiconductor components, neither is completely effective in removing contaminants, such as residues of photoresist, from the surface of the wafer. Typically this is compensated for by cleaning the wafer by about 30% more than the optimal cleaning time, entailing extra cost, time and increased possibility of damaging the components on the wafer.
There are many processes known in the art for C&R detection. In high-speed optical defect review systems, which are typically integrated into a production line for components, the cost of inspection is relatively high. In addition, for some layers such systems only allow low sampling rates, and do not detect non-visible residuals for any layers.
An alternative system for C&R detection is optical spectral analysis, wherein photons from pre-ionized contaminants are detected, usually by a CCD array detector. However, in order to evaluate if the photons are from a contaminant, the optical spectral analysis system has to analyze the wavelength of the received photons.
While any C&R detection process is operating, it must be absolutely non-destructive to components being checked.
SUMMARY OF THE INVENTION
It is an object of some aspects of the present invention to provide an improved method and apparatus for the detection of contaminants and residuals, particularly in semiconductor manufacture.
In some preferred embodiments of the present invention, a plurality of anodes is scanned across a surface, typically the surface of a semiconducting wafer, with the wafer preferably acting as a cathode. Simultaneously, the area of the wafer in the region of the anodes is irradiated with a high flux of low energy photons, preferably in the optical or near-ultraviolet (UV). The wavelength of the radiation is chosen so that the photon energy is substantially below the ionization level of substrate atoms in the wafer, so that substantially no photoelectrons are generated therefrom, and so that no defects are introduced into the wafer. Contaminants and residuals on the surface of the wafer are ionized by a process of photon absorption, most preferably by a process of two-photon absorption (TPA), and the electrons produced by the ionization are collected by the individual anodes, generating a displacement and/or an electronic current, herein referred to collectively as a current, therefrom. The current at an anode gives a measure of the size of the contaminant or residual at the position of the anode.
In some preferred embodiments of the present invention, the anodes are arranged as a substantially linear array.
In some preferred embodiments of the present invention, the signal created by a residual photoresist on the wafer is enabled or substantially enhanced by using doped photoresist.
In some preferred embodiments of the present invention, the distance of the plurality of anodes is maintained at a substantially fixed distance from the wafer by means of a high speed servo loop.
In some preferred embodiments of the present invention, a plurality of cathodes is provided in registration with the plurality of anodes, so that the cathodes enhance and/or control the field at the surface of the wafer.
In some preferred embodiments of the present invention, the anodes are fixedly positioned above selected areas of the wafer, rather than scanned over the surface, as described hereinabove.
There is therefore provided, in accordance with a preferred embodiment of the present invention, a method of detecting a contaminant in a substrate, including:
positioning an array of anode elements in proximity to the substrate;
biasing the array at a positive voltage relative to the substrate;
irradiating the substrate with photons having energies below an atomic ionization energy of the substrate, so as to ionize the contaminant to emit electrons; and
collecting the emitted electrons at one or more of the anode elements, thereby generating a current indicative of the presence of the contaminant in the semiconductor in proximity to the one or more of the anode elements.
Preferably, irradiating the substrate includes ionizing the contaminant by a process of two-photon absorption.
Alternatively, irradiating the substrate includes irradiating over a generally linear area parallel to an axis of the array.
Preferably, irradiating over the generally linear area includes irradiating through a plurality of irradiation segments arranged over the area.
Preferably, irradiating through the plurality of irradiation segments includes irradiating through a plurality of segments simultaneously.
Alternatively, irradiating through the plurality of irradiation segments includes scanning an irradiation beam over a plurality of segments sequentially.
Preferably, collecting the emitted electrons includes determining a location of the contaminant responsive to the current from the one or more of the anode elements.
Preferably, collecting the emitted electrons includes sampling the plurality of anodes simultaneously.
Preferably, collecting the emitted electrons includes sampling the plurality of anodes sequentially.
Preferably, positioning the array includes arranging a plurality of anode elements in a linear array.
In a preferred embodiment, the method includes positioning an array of cathode elements in proximity to the substrate and to the array of anodes and biasing the cathode elements at a negative voltage relative to the substrate.
Preferably, biasing the cathode elements includes setting the cathode voltage so as to enhance the current indicative of the presence of the contaminant.
Alternatively or additionally, biasing the cathode elements includes setting the cathode voltage so as to discriminate between currents created by different types of contaminant.
Preferably, positioning the array includes scanning the array over an area of the substrate that is irradiated.
Preferably, scanning includes scanning the array in a generally linear pattern relative to the substrate.
Alternatively, scanning includes rotating the substrate, so that the array is scanned in a generally circular pattern relative thereto.
Preferably, positioning the array includes maintaining a constant distance between the semiconductor and the anode, preferably by measuring an electric field between the semiconductor and the anode, and adjusting the distance therebetween in response thereto.
Preferably, the substrate includes a semiconductor wafer, and the method includes applying to the semiconductor wafer a photoresist including a dopant having a high absorption cross-section for the irradiating photons.
Preferably, the dopant has a high two-photon absorption cross-section.
There is further provided, in accordance with a preferred embodiment of the present invention, apparatus for detecting a contaminant in a substrate, including:
an irradiator which irradiates the sub
Duer Reuven
Gvirtzman Amos
Mazor Isaac
Jordan Valley Applied Radiation Ltd.
Lahive & Cockfield LLP
Smith Bradley K
Tran Andrew
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