Optics: measuring and testing – Inspection of flaws or impurities – Surface condition
Reexamination Certificate
2001-10-01
2004-04-20
Nguyen, Tu T. (Department: 2877)
Optics: measuring and testing
Inspection of flaws or impurities
Surface condition
Reexamination Certificate
active
06724475
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to an apparatus for rapidly measuring angle-dependent diffraction effects on finely patterned surfaces in the submicron range.
In semiconductor fabrication, line widths and profiles of patterned layers often have to be monitored during the fabrication process. Complying exactly with the specifications for the line width is of crucial importance for the functionality of a product. In addition, other structural parameters such as trench depth or sidewall inclination are also of major importance. Suitable measuring apparatuses are required for monitoring these fabrication parameters on lithography masks, semiconductor wafers, or other very finely patterned surfaces.
With the extremely small structural widths in the region of 0.25 &mgr;m that are used nowadays in semiconductor fabrication, conventional optical line width measuring apparatuses can no longer be used on account of diffraction and interference effects. Therefore, electron microscopes are used for measuring the profile of fine structures (<1 &mgr;m). On account of the stringent vacuum requirements, even very complex electron microscopes developed specifically for line width measurements have a comparatively low throughput. Consequently, after a fabrication step, only a small portion of the product wafers can be checked with regard to compliance with the process specifications. Furthermore, the long measuring times increase the probability of further batches being fabricated defectively until a process fault is detected. These dead times can cause major financial losses particularly in the case of the latest fabrication technologies with wafer diameters of 300 mm and very complex process steps. Moreover, only a small number of individual structures can be monitored on each wafer using electron microscopes, and so the measurements are not representative. Therefore, under certain circumstances, fabrication faults or the causes thereof will be identified only very belatedly. In order to check further structural parameters, so-called monitor wafers are additionally included (e.g. in deposition processes) in order to be able to determine layer thicknesses produced on unpatterned wafers or in order to enable the electron microscope to effect so-called cross section recordings, for which the semiconductor wafers must be destroyed. Primarily in the case of future large wafer diameters of 300 mm or more, these monitor wafers cause high costs, firstly on account of the pure material value and secondly because they significantly reduce the throughput of product wafers. In order to manage with the fewest possible monitor wafers and nevertheless to improve the product monitoring, what are required in semiconductor fabrication are cost-effective measurement methods for nondestructive and contamination-free checking of structural parameters on the product wafers. In this case, the measuring speed should be high enough that, e.g. after a critical process step, each product wafer can be monitored without significantly increasing the process time. Scattered light measurement offers one solution approach. In general, during this method, the measurement region to be examined is illuminated and the surface properties of the measurement region are inferred from the features of the reflected light. If there are periodic structures on the substrate and if coherent light is used, then diffraction and interference effects occur given a corresponding choice of wavelength. Said effects prevent a measurement in conventional optical apparatuses, but they are explicitly detected and evaluated in the case of scattered light measurement or diffraction analysis since they are characteristic of the structural parameters. In research, the so-called 2&thgr; method has acquired a certain importance in recent years. In this case, the angle of incidence of the measuring beam is varied within the plane of incidence and the intensities of the orders of diffraction are measured as a function of the angle of incidence. With the aid of complex model calculations, it is possible to determine from this diffraction measurement various structural parameters such as line width, trench depth or edge inclination. However, the measurement configurations used hitherto for this purpose are not very flexible or are comparatively slow, structurally complex and expensive.
In previous realizations, the light source is moved by precise mechanical components or the specimen to be examined is itself rotated about the measurement point. This raises the costs of the apparatus and restricts the range of use of the method. Lens systems produce different angles of incidence, so that all that has to be moved is an optical element (e.g. mirror or prism), not the specimen. However, only limited angles of incidence can be realized even with complex lens configurations having a large aperture. Under certain circumstances, disturbing reflections occur at the interfaces.
Simultaneously producing a plurality of angles of incidence with the aid of a multiple beam splitter (e.g. reflection grating) in conjunction with an ellipsoidal mirror is proposed in German Published Patent Application DE 198 24 624. However, the angles of incidence are chosen when the measuring apparatus is constructed and so this choice is fixed and the number of angles of incidence that can be realized simultaneously is limited. Principally, however, the diffraction effects are simultaneously superimposed on the specimen structures for the different angles of incidence. Consequently, an angle-dependent measurement of diffraction intensities is not possible. By contrast, the apparatus in accordance with DE 198 24 624 enables the simultaneous measurement under different diffraction angles, which may be advantageous when comparing the measurement with a single simulated diffraction pattern.
A measurement method should be nondestructive, free of contamination, fast, simple, and robust. A promising approach for determining structural parameters is offered by so-called scattered light measurement: the angularly resolved intensity measurement of light that has been scattered at a substrate. Reflected or transmitted light is diffracted in the case of specimens having periodic structures. Regular semiconductor structures, e.g. in memory modules, can be imagined as a reflective amplitude or phase grating. If the grating vector is situated in the plane of incidence, then, for a given angle &thgr;î of incidence, the following grating equation holds true for the n-th order diffraction maximum with the angle &thgr;
n
of reflection
sin
θ
i
+
sin
θ
n
=
n
λ
g
In this case, &lgr; describes the wavelength of the light used and g denotes the grating period. Accordingly, in addition to the direct reflection already present, higher-order diffraction maxima can arise if the wavelength of the light used is less than half the grating period. If the size of the structures examined lies in the region of the wavelength, then the simple scalar Fraunhofer diffraction equations can no longer be employed. Instead, a simulation of the intensity distribution requires the solution of the associated Maxwell's equations with the boundary conditions applicable to the respective grating. Efficient numerical methods, such as e.g. the so-called rigorous coupled wave analysis, have been developed for this purpose in previous years. The nonlinearities that occur allow generally valid statements only to a very limited extent, for which reason the concrete individual case must always be considered or numerically calculated for the assessment of diffraction effects on small structures. In this case, the intensities and also the phases of the orders of diffraction depend on the properties of the incident beam (angle, polarization, wavelength), on the examined grating structure (grating periods, line width, line height, layer structure, edge rounding, roughness) and on the material properties of the substrate (refractive index, a
Benesch Norbert
Pfitzner Lothar
Schneider Claus
Greenberg Laurence A.
Infineon - Technologies AG
Mayback Gregory L.
Nguyen Tu T.
Stemer Werner H.
LandOfFree
Apparatus for rapidly measuring angle-dependent diffraction... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Apparatus for rapidly measuring angle-dependent diffraction..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Apparatus for rapidly measuring angle-dependent diffraction... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3265020