Optics: measuring and testing – Inspection of flaws or impurities – Surface condition
Reexamination Certificate
2000-03-30
2003-03-04
Rosenberger, Richard A. (Department: 2877)
Optics: measuring and testing
Inspection of flaws or impurities
Surface condition
Reexamination Certificate
active
06529270
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the inspection of the surface of a workpiece for defects. More particularly, the present invention relates to an apparatus and method that scan the surface of a workpiece with an optical beam and process the optical signal reflected and detected from the surface of the workpiece prior to determining from the reflected optical signal whether the workpiece includes defects, thereby increasing the sensitivity of the inspection.
BACKGROUND OF THE INVENTION
For most semiconductor microelectronic applications, an important factor is the feature size of devices located on a silicon integrated circuit die. These devices include transistors, such as MOSFET's, metallization paths, as well as capacitors and resistors. In particular, for most applications, the smaller the size of the semiconductive device the better. Smaller device geometry enables higher transistor switching speeds due to smaller junction capacitance. It also enables the development of digital circuits with greater processing power and complexity. Just a few years ago, the typical feature size for a typical semiconductive transistor was approximately 300 nanometers (nm). However, more recently the feature size has been reduced to 180 nm, while some semiconductive transistors are now being manufactured with dimensions of 130 nm.
Although the miniaturization of semiconductor devices is typically advantageous, this reduction in size has imposed some problems in the manufacturing of these devices. Specifically, semiconductive devices are manufactured from crystalline silicon wafers that have been grown, sliced into wafers, and polished. Although the methods for creating these wafers and preparing them for semiconductor manufacture are quite advanced, contaminant particles may be introduced onto the surface of the wafer and/or contaminants or surface defects may be present in the wafer. These contaminants and/or defects, if large in dimension relative to the semiconductive device to be manufactured from the wafer, may result in device defects and reduced production yields. As such, as the size of semiconductive devices and their associated components are reduced, contaminants and defects of smaller sizes, which were once negligible, have become problematic.
For example, if a semiconductive device has dimensions of approximately 300 nm, a contaminant or defect having a dimension of 60 nm is typically negligible. However, as the feature size is reduced to 180 nm, this same defect may cause a defect in the manufactured device.
As a result of the problems described above, more stringent requirements have been placed on inspection devices used to detect defects in silicon wafers. The Semiconductor Equipment Manufacturing Industry consortium (SEMI) is an organization that defines defect sensitivity requirements for silicon wafer inspection devices. The sensitivity is typically defined in terms of the scanner's ability to detect a polystyrene latex sphere (PSL) of a selected diameter. As an example of the increased sensitivity requirements, SEMI has recently released guidelines requiring that an inspection device maintain reliable detection capability of a 65 nm PSL defect for a 130 nm device geometry.
These increased sensitivity requirements have been problematic for many conventional silicon wafer inspection devices. Specifically, many conventional silicon wafer inspection devices use laser-based scanners and photomultiplier-based optical detectors to inspect the surface of silicon wafers for defects. While optical inspection systems have been capable of detecting 100 nm defects on silicon wafers, their performance is limited by quantum-mechanical shot noise. This noise is caused by the statistical probability associated with detecting the photons received from the scattered light that is reflected from the wafer surface. This susceptibility to noise limits the inspection system's sensitivity to small particles that can cause device defects. In particular, the signal produced by particles decreases non-linearly with decreasing particle size.
At least one problem with the introduction of shot noise in silicon wafer inspection devices is illustrated in FIG.
1
.
FIG. 1
illustrates a portion of a conventional silicon wafer inspection system. This conventional silicon wafer inspection system
10
includes a light source
12
such as laser for directing an optical beam B toward the surface S of a workpiece W. The silicon wafer inspection system also includes a dark channel detector system
18
that may include several individual detectors for detecting scattered light reflected from the surface of the workpiece. The conventional silicon wafer inspection system also includes a light channel detector
20
positioned at an angle &bgr; that corresponds to, i.e., typically equal to, the angle &bgr; the beam makes with respect to the workpiece. The light channel detector detects specularly reflected light from the surface of the workpiece.
Importantly, the silicon wafer inspection device also includes a deflector
14
and an optical lens
16
. The deflector and lens are positioned to receive the optical beam and deflect the beam so as to form a scan region
22
on the surface the workpiece. The scan region is defined by an in-scan dimension
24
that relates to the scan path of the light beam. Specifically, the scan path is the sweep of the optical beam through an angle cc controlled by the deflector. Further, the scan region includes a cross-scan dimension
26
that is associated with the width of the beam.
In operation, the workpiece or the laser is usually moved in relation to the optical beam such that the laser scans either all or a substantial portion of the surface of the workpiece. As the workpiece is scanned, the dark and light channel detectors receive optic signals reflected from the surface of the workpiece. These optic signals are provided to a signal processing device
28
for determining whether the workpiece includes defects.
As stated previously, a problem associated with many of these conventional systems is the introduction of shot noise into the dark channels of the silicon-wafer inspection system. This noise can overshadow small defects in the workpiece, thereby decreasing the sensitivity of the silicon-wafer inspection device. For example,
FIG. 2
is a plot of the data signal received by a photomultiplier dark channel detector from a scan of a section of a workpiece by a conventional inspection device. This section of the workpiece includes a 100 nm PSL defect that is illustrated in the center of the scan by peak
30
. Importantly, as illustrated by the plot, the optical signal received by the dark channel detector includes a considerable amount of noise that may overshadow smaller defects in the workpiece. Specifically, the amplitude of the signal generated by a defect is typically related to the diameter D of the defect by following equation:
Amplitude of Defect∝Diameter
6
As such, as the diameter of the defect decreases, the amplitude of the signal created by the defect decreases dramatically, thereby making the detection of smaller defects in a workpiece more problematic in a noisy inspection device.
In light of the problems associated with the introduction of signal noise in the dark and light channel detectors, silicon-wafer inspection systems have been developed to filter at least some of the noise from the received signals. These conventional inspection devices use signal filtering techniques to reduce the amount of noise in the optical signal received from the silicon wafer. These filtering techniques advantageously increase the sensitivity of the inspection device. However, these conventional inspection devices may not provide adequate filtering of the signal to reliably detect defects in the range of 65 nm as required by the SEMI guidelines.
Specifically, these conventional inspection devices filter the optical signal in the in-scan direction
24
of the scan region. The in-scan filter located in the signal process
ADE Optical Systems Corporation
Alston & Bird LLP
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