Semiconductor device manufacturing: process – With measuring or testing – Optical characteristic sensed
Reexamination Certificate
2000-03-08
2004-11-02
Pert, Evan (Department: 2829)
Semiconductor device manufacturing: process
With measuring or testing
Optical characteristic sensed
C356S369000
Reexamination Certificate
active
06812047
ABSTRACT:
BACKGROUND
In the processing of a semiconductor wafer to form integrated circuits, a number of traces are normally formed over an underlying layer. The traces are normally used to interconnect transistors and other devices in the integrated circuits. Such traces may have widths under 0.2 micrometers (microns), pitches (center to center spacing) under 0.4 microns and aspect ratios exceeding 4:1.
Depending on the stage of the processing, it may be necessary to measure properties of various portions of a wafer, such as the properties of the traces and/or the properties of the underlying layer. However, the presence of traces can interfere with conventional measurements that examine open areas (areas not covered by traces).
SUMMARY
A structure having a number of lines supported by a layer in contact with the lines (also called “multi-layered structure”) is evaluated in accordance with the invention by illuminating a region (also called “illuminated region”) containing several lines, using a beam of electromagnetic radiation, and generating an electrical signal (e.g., by use of a photosensitive element) that indicates an attribute (e.g., intensity or optical phase) of a portion (also called “reflected portion”) of the beam reflected from the region. As more than one line (and therefore more than one portion of the layer in contact with the lines is being illuminated, the reflected portion and the electrical signal generated therefrom do not resolve individual features in the illuminated region, and instead indicate an average measure of a property of such features. In contrast, most prior art methods measure a property of an individual feature in such a multi-layered structure. The just-described lines can be either conductive (in which case they are also referred to as “traces”) or non-conductive, depending on the embodiment.
In one embodiment, the acts of “illuminating” and “generating” are repeated in another region (of the same structure or of a different structure) also having multiple traces. The electrical signals being generated from light reflected by different regions can be automatically compared to one another to identify variation of an average property (e.g., average thickness of the layer in contact with the traces, or average resistance per unit length of the traces) between the regions. Instead of (or in addition to) the just-described comparison, the values of such a signal can be plotted in a graph to indicate a profile of a surface in the region. A value being plotted can be an absolute value of the reflected portion alone, or can be a value relative to another portion that is reflected by another surface in the same region (which indicates the average distance therebetween), or by the same surface in another region (which indicates an average profile of the surface).
Such measurements can identify variations in properties in a semiconductor wafer of the type used in fabrication of integrated circuit dice, or between multiple such wafers (e.g., values measured from a reference wafer and a production wafer or between two successive production wafers can be compared). Identification of a change in a property between two or more wafers is useful e.g., when performing such measurements during wafer fabrication, so that process parameters used to fabricate a next wafer (e.g.; creating the above-described layer or the traces) can be changed as necessary (in a feedback loop), to generate wafers having material properties within acceptable limits. Note, however, that structures other than semiconductor wafers (e.g., photomasks that include a glass substrate and are used to form the wafers, or an active matrix liquid crystal display) can also be evaluated as described herein.
In a first example, there is a transmissive medium (such as air) located between a source of the beam (also called probe beam) and the illuminated region. In one implementation, another beam (also called “heating beam”) is used in addition to the probe beam, to modulate the temperature of the traces (e.g., at a predetermined frequency). Reflectance of the lines changes with the change in temperature. The reflected portion (which depends on reflectance), and hence the generated signal also oscillates (e.g., as the predetermined frequency). Such an oscillating signal is measured by e.g., a lock-in amplifier, and the measurement is repeated in another region. If all lines in the illuminated region are conductive (also referred to as “traces”), comparison of measurements from different regions (e.g., which may be in the same location in different die of a wafer, or which may be in the same die in different wafers) indicates a change in the average resistance per unit length (and therefore the corresponding change in cross-sectional area) between traces in the respective regions (if conductivity is constant).
A series of measurements from regions adjacent to one another (or even overlapping one another) in the longitudinal direction of the traces, when plotted in a graph along the y axis with the x axis indicating distance along the longitudinal direction yields a profile of the traces (which may be used to detect, e.g. global nonuniformity such as a dimple or a dome). Depending on the specific variant, the probe beam and the heating beam can each be coincident with or offset from the other.
In another implementation, multiple traces in a region of a structure of the first example are each substantially parallel to and adjacent to the other, and the beam has wavelength greater than (or equal) to a pitch between two adjacent traces. In one such embodiment, the probe beam is polarized (e.g., by a polarizing optical element interposed between a source of the beam and the structure), although a nonpolarized probe beam can be used in other embodiments. A polarized probe beam can be used in several ways, including, e.g., orienting the probe beam so that the electrical field vector for the electromagnetic radiation is at a predetermined angle relative to the traces.
When the probe beam is polarized perpendicular to the traces, the traces do not reflect the probe beam. Instead, the probe beam passes between the traces and is reflected from underneath the traces, e.g. by charge carriers of a semiconductor layer, or by a surface of an oxide layer, or both. Such light which is reflected from underneath the traces can be used to identify variation in a property of features underneath the traces (averaged over the features that are illuminated). The portion reflected by charge carriers is relatively small (e.g., {fraction (1/10)}
4
or less) as compared to the portion reflected by an underlying surface, and therefore has a negligible effect on an overall measurement of a steady signal (also called “DC” component). If necessary, the portion reflected by charge carriers can be measured by modulating the number of charge carriers and using a lock-in amplifier to measure the portion of a reflected light that is modulated (also called “AC” component) as described elsewhere herein. The charge carriers can be created by a beam having an oscillating intensity (or oscillating phase). In this variant, the reflected portion has an intensity (or phase) that is modulated in phase with modulation of the charge carriers (and can be measured by use of a lock-in amplifier).
When the probe beam is polarized parallel to the longitudinal direction of the traces, the above-described reflected portion (that is used to generate the electrical signal) is reflected by the traces. The reflected portion can be used to identify variation in a property that is averaged over the traces. A probe beam polarized parallel to the traces can be used with a heating beam that is also polarized parallel to the traces, and in such a case effectively on the traces interact with the heating beam, and are heated more, as compared to heating by an unpolarized heating beam. Alternatively, the just-described probe beam (also called “parallel polarized beam”) can be used with another probe beam that is polarized perpendicular to the traces (also called “perpendi
Borden Peter G.
Li Jiping
Boxer Cross Inc.
Pert Evan
Silicon Valley Patent & Group LLP
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