Patent ID: 12196691

DETAILED DESCRIPTION OF EMBODIMENTS

The present invention relates to measurements of periodic patterned structures, such as semiconductor wafers, in which the pattern typically includes some periodicity of unit cells along one or two directions. Each unit cell includes a pattern of spaced-apart regions of different optical properties (e.g. lines and spaces). The invention technique is aimed at solving the above-described problems of X-ray measurements associated with effects of roughness and background noise.FIG.1exemplifies the structure's surface having surface roughness on the nanometric scale.

Reference is made toFIG.2schematically illustrating the principles of X-ray based (such as GI-SAXS, as well as XRS and T-SAXS) measurement scheme10of the present invention. As shown in the figure, a sample12is irradiated by X-ray radiation including a range of illumination angles within a solid angle14of incident radiation, and such X-ray interaction with the sample12causes a radiation response16of the sample in the form of reflection/diffraction orders including zero-order diffraction and high diffraction orders. The radiation response16is detected by a 2D sensor matrix18having a radiation sensitive surface, where different diffraction orders are detected by different regions (sensor elements/pixels or groups of sensor elements/pixels) of the sensor matrix18. Analysis of these diffraction orders allows characterization of the measured structure12.

Thus, the X-ray scatterometry in general involves three main functional parts: illumination, the patterned structure being measured, and detection of the radiation response. The illumination mode/configuration provides for directing a beam of monochromatic X-ray radiation14towards a sample/structure12. This beam14can be collimated (as used in T-SAXS), but for a most general case, it has an angular span as shown inFIG.2. The illuminating beam14is focused onto a structure (for a collimated beam no focus is needed), and is reflected/returned from the structure. For metrology purposes, the measured structure12has a patterned surface12A, where the pattern typically includes a periodic set of features, i.e. the elements under study (i.e. an array of transistors, memory cells, etc.). Upon reflection from this structure, the X-ray is scattered both in the specular direction (zero-order reflection/diffraction), and in the directions corresponding to high reflection/diffraction orders from the structure. The reflected radiation16is detected/measured by the 2D matrix18of the detector, creating an image20of reflection per angle of incidence and azimuth of the illumination14.

Different SAXS implementations involve different sequence of data acquisition (for example, T-SAXS involves rotating the sample and sequentially acquiring a set of images, from which the full 2D dependence of the reflected signal is deduced). For simplicity, in the description below a measurement sequence is referred to as representative of GI-SAXS, in which multiple angles of incidence are probed in each measured image. It should, however, be understood that the invention is not limited to any specific type X-ray measurement technique, and therefore the invention should be interpreted broadly in this respect.

The present invention is based on the fact that the reflected diffraction orders from the structure being illuminated by angular span can be separated from each other, leaving regions in the image where only non-nominal signals arrive The present invention is based on the fact that the reflected diffraction orders from the structure being illuminated by angular span can be separated from each other. This means that the angular span of adjacent diffraction orders is smaller than their separation. This way, between adjacent diffraction orders there is a region where nominally no radiation is expected, and only non-nominal signal is measured. Examples for such ‘non-nominal’ signal sources would be stray light, roughness-related reflections, reflections from defects/pattern irregularities, etc. The signals obtained in these regions are thus entirely related to ‘background’ contributions, which in many cases is highly desired to remove. Such separation can be controlled by appropriate choice of the illumination angular span, which in selected in accordance with the structure being measured and its azimuthal orientation. This will be described more specifically further below. Once the diffraction orders are separated, it is possible to isolate the contribution from direct reflections, i.e. reflections associated with diffraction orders from the periodic pattern, and background contribution.

As described above, the background reflection components (being background noise) are characterized by smooth-varying intensity distributions added onto the X-ray signal being measured, contrary to the main diffracted signal which typically includes sharp variations in reflected intensity. This allows an algorithmic approach for deducing the contribution from these components at the regions where ordinary reflection resides. Once this contribution is obtained, it can be subtracted from the ordinary reflection regions, allowing straightforward analysis and interpretation of the measured signals.

Reference is made toFIGS.3A-3E, schematically exemplifying the analysis methodology of the invention.FIG.3Aexemplifies a detected/measured signal MS (raw signals/image) as a function of illumination angles Theta and Azimuth of illumination. The measured signal MS is formed by reflected orders RO and background signature BS associated with the reflection from the pattern regions PR. These regions of the pattern features PR, creating the background signature BS (background noise) are to be isolated. As will be described below, measurement is set so that diffraction orders RO are spatially separated on the radiation sensitive surface (2D matrix) of the detector. As shown inFIG.3B, once the diffraction orders RO are removed, background-only signal BS can be identified. A dedicated algorithm van be used to expand the background signal. In this example, a Lorentzian fit is used (FIG.3C). The background signal BS then undergoes interpolation\extrapolation, resulting in an image representing the background across the entire image span, as shown inFIG.3D. The background signal BS can then be subtracted from the measured signal MS, providing a clean image of the reflection orders RO.

Preferably, this approach utilizes a correct choice of the illumination angular range14. This choice is based on the requirement of having different diffraction orders spatially separated on the 2D sensing matrix of the detector. In the present example, the measurement setup schematically illustrated inFIG.2is considered as reference, and the required illumination angular range suitable for this case is analyzed. It should be understood that a similar analysis is straightforward for other geometries.

The following is the description of a derivation for the angular separation between diffraction orders. This enables to set the allowed illumination angular range such that no overlap exists between the different reflection orders.

In this connection, reference is made toFIG.4schematically illustrating the radiation propagation scheme with respect to the pattern orientation on the sample. As shown in this example, the pattern P is configured as a grating formed by parallel features (lines) L extending along X-axis (defining the grating axis) and arranged in a spaced-apart relationship along Y-axis. Illuminating radiation14(angular span) is incident upon the patterned surface of the sample12at angle θILL. (with respect to the normal to the sample), and azimuthal angle ϕILL. In this example, azimuth is defined with respect to the grating axis, but this is not a necessary restriction.

Considering radiation of wavelength λ and wavenumber k0=2π/λ, the transverse wavenumber kILL(i.e. wavenumber on the sample plane) is given by
kILL=k0(sin(θILL)cos(ϕILL),sin(θILL)sin(ϕILL))

Interaction of the illuminating radiation14with the grating P results in a radiation response16formed by radiation reflected in a set of discrete directions, corresponding to the diffraction orders from the grating. Generally, although not specifically shown, the grating P can have periodicity of features L in two directions, with pitch Pxin the x direction and Pyin the y direction (either of which can be zero if periodicity exists only in one dimension). Diffraction orders will be denoted (nx,ny) for the nxorder arising from the periodicity in direction x and nyorder arising from periodicity in the y direction.

The (nx,ny) diffraction order will have transverse wavenumber given by:

kCOL=(k0⁢sin⁡(θILL)⁢cos⁡(ϕILL)+2⁢πPx⁢nx,k0⁢sin⁡(θILL)⁢sin⁡(ϕILL)+2⁢πPy⁢ny)

The reflection direction of this order is given by:

ϕCOL=a⁢tan⁡(kCOLykCOLx)=a⁢tan⁡(sin⁡(θILL)⁢sin⁡(ϕILL)+λPy⁢nysin⁡(θILL)⁢cos⁡(ϕILL)+λPx⁢nx),andθCOL=a⁢sin(λPx⁢nxcos⁡(ϕILL)).

By these relations, it is straightforward to relate any illumination angular span to the angular span of all reflected orders. Specifically, it is possible to check whether overlaps are expected between the reflected orders.

Turning back toFIGS.1and4, the invention provides that, for a given wavelength λ of illumination, the angular span θILLof incident radiation14and its azimuth orientation ϕILL. with respect to the pattern P on the structure being measured, can be controlled/selected to provide a required angular span ϕCOLof the reflection orders16. This required angular span ϕCOLof the reflection orders16is such that radiation components of different reflection orders interact with different regions R on radiation sensitive surface18of the detector spatially separated from one another by gap(s) G. It should be understood that the value(s) of G can also be controlled by the length of the collection channel.

Several approaches are possible to implement this limitation/selection of the illumination range of angular span. Some examples include the use is a single aperture with controllable dimensions placed in the optical path, or an interchangeable set of apertures placed in the optical path. These could be exchanged or modified according to the measured sample pitch. Further flexibility can be obtained by allowing different shapes of apertures. It should be noted that when the pattern on the measured structure is periodic in two directions, such approach can guarantee separation of high diffraction orders in both directions.

More generally, it is possible to implement such separation only on one subgroup of the reflection orders. For example, diffraction orders related to the grating pitch in one direction can be separated, while leaving orders related to the pitch in another direction overlapping. Although with this approach some of the background signal might not be removed, providing a degraded result, it may be easier to implement in practice and may be suitable/sufficient in some applications.

Reference is now made toFIGS.5A and5Bschematically illustrating a block diagram of a measurement system100of the invention (FIG.5A) and a flow diagram200of a method of the invention (FIG.5B). The system100includes an illumination unit102defining an illumination channel for directing illuminating radiation14onto a measurement plane (sample plane), a detection unit104for detecting radiation response from a sample12propagating along a collection channel, and a control unit106configured to be in data/signal communication with the illumination and detection units (via wires or wireless signal transmission using any known suitable communication techniques and protocols). Also provided in the system100is an angular span controller110located in the illumination channel.

The control unit106is configured as a computer system including inter alia such functional and structural utilities as input and output utilities106A and106B, memory utility106C, and data processor106D. The control unit106further includes a measurement scheme controller (a so-called orientation controller)106E, which is configured to receive and analyze data indicative of a pattern P on the structure/sample12(e.g. pattern pitch along one or two axes) and its azimuthal orientation with respect to the illumination channel, and generate data indicative of an optimal choice for the illumination angular span for operating the angular span controller110. It should be noted that the structure under measurements may be located on a stage/support108driven for rotation so as to adjust the pattern orientation with respect to the illumination channel, as the case may be. The angular span controller may be configured as described above, namely may include a single aperture with controllably varying dimensions and/or shapes, or an interchangeable set of apertures of different dimensions/shapes. This enable selection of an aperture shape according to the selected measurement scheme.

Thus, the orientation controller106E operates the angular span controller110and possibly also the stage108(via its drive) to select the illumination angular span14such that the measured image is formed by distinct regions corresponding to the high reflection orders, with gaps between them, i.e. such that no overlap exists between different orders. Such an image is exemplified inFIG.3Adescribed above.

Once this situation is assured, i.e. the desired orientation is provided (step202inFIG.5B), the following measurement scheme is performed in order to remove the effects of background signal/contribution. Measured signal MS is detected by the pixel matrix of the detection unit104which generates measured data indicative thereof (step204) and coveys this data to the control unit106where it is processed by the data processor106D (step206). The data processor106D includes an analyzer module112configured (preprogrammed) for filtering out signals corresponding to radiation components of reflected diffraction orders (presenting “active signals”), leaving only background signature (presenting “noise”)—step208. This is illustrated inFIG.3Bdescribed above. Then, the background signature signals are interpolated (and extrapolated) by a fitting module114(step210) to deduce the background contribution to the entire measured image or a selected part of it. To this end, the fitting module114utilizes some underlying functional shape/form for the background signal (e.g. approach shown inFIG.3Caccording to which each angle of incidence is fitted separately by Lorentzian lineshape; or any other functional form, or a two-dimensional functional form, and using a fitting approach based on some theoretical model). This results in the background signal alone (step212). The so-obtained image describing the background signal alone may optionally be further processed by any suitable image processor116to smooth and correct this image (optional step214). The initial image (measured data) is then processed by a filter module118which operates to subtract the background signal/image (either being processed by image processor or not) from the initial image (step216) resulting in the final, background-removed signal (step218). This final signal presents the active/effective measured data that can be used for analysis and interpretation using any known suitable technique, e.g. model-based data interpretation.

Thus, the present invention provides a simple and effective technique for use in X-ray based scatterometry measurements on patterned structures. According to the invention, a measurement scheme is optimized to enable subtraction of background-noise free effective measured signal, which can then be interpreted to determine the structure parameters using any known suitable data interpretation approach.