High throughput brightfield/darkfield wafer inspection...

Details High throughput brightfield/darkfield wafer inspection... High throughput brightfield/darkfield wafer inspection...

2001-07-17

2004-11-09

Pham, Hoa Q. (Department: 2877)

Optics: measuring and testing

Inspection of flaws or impurities

C356S239100, C356S394000

Reexamination Certificate

active

06816249

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the art of optical inspection of semiconductor wafers, and more specifically to a high throughput brightfield and darkfield wafer inspection system having image processing redirected from the mechanical and electronics segments of the inspection system to the optical domain.
2. Description of the Related Art
Semiconductor wafer inspection techniques have historically utilized brightfield illumination, darkfield illumination, or spatial filtering. Brightfield imaging is not generally sensitive to small particles. Brightfield imaging tends to scatter small particles away from the collecting aperture, thereby resulting in reduced returned energy. When a particle is small compared to the optical point spread function of the lens and small compared to the digitizing pixel, the brightfield energy from the immediate areas surrounding the particle typically contribute a large amount of energy. The small reduction in returned energy resulting from the small particle makes the particle difficult to detect. Further, the small reduction in energy from a small particles often masked out by reflectivity variations from the bright surrounding background such that small particles cannot be detected without numerous false detections. Additionally, if the small particle is on an area of low reflectivity, which may occur for some process layers on wafers and always for reticles, photomasks, and flat panel displays, the resultant background return is low and any further reduction due to the presence of a particle becomes very difficult to detect.
Newer systems utilize broadband brightfield imaging as opposed to traditional monochromatic or narrow band brightfield imaging. Broadband brightfield imaging minimizes contrast variations and coherent noise present in narrow band brightfield systems, but are not sensitive to small particles.
Darkfield imaging is employed to detect small particles on wafers, reticles, photomasks, flat panels, and other specimens. The advantage of darkfield imaging is that flat specular areas scatter very little light back toward the detector, resulting in a dark image. Darkfield illumination provides a larger pixel-to-defect ratio, permitting faster inspections for a given defect size and pixel rate. Darkfield imaging also permits fourier filtering to enhance signal to noise ratios.
Any surface features or objects protruding above the surface of the object scatter more light toward the detector in darkfield imaging. Darkfield imaging thus produces a dark image except where circuit features, particles, or other irregularities exist. Particles or irregularities are generally assumed to scatter more light than circuit features. However, while this assumption permits a thorough inspection for particles on blank and unpatterned specimens, in the presence of circuit features a darkfield particle inspection system can only detect large particles which protrude above the circuit features. The resulting detection sensitivity is not satisfactory for advanced VLSI circuit production.
While some attempts to improve darkfield performance have been attempted, such systems tend to have drawbacks, including drawbacks resulting from the very nature of darkfield illumination. For example, while brightfield illumination floods the entire field of view with light, darkfield illumination is confined to a narrow strip of light. Due to the nature of lasers, the application of light in darkfield illumination tends to be non-uniform and limits the amount of data which can be collected in a particular time period.
Some imaging systems currently available attempt to address problems associated with darkfield imaging. One instrument, manufactured by Hitachi, uses the polarization characteristics of the scattered light to distinguish between particles and normal circuit features, based on the assumption that particles depolarize light more than circuit features during the scattering process. When circuit features become relatively small (less than or on the order of the wavelength of light), the circuit can depolarize the scattered light as much as the particles. As a result, only larger particles can be detected without false detection of small circuit features.
Further, a system employing a combination of a monochromatic darkfield and a monochromatic brightfield imaging for wafer inspection is poorly adapted for inspecting Chemical Mechanaical Planarized (CMP) wafers, which often have film thickness variations and a grainy texture. Grainy texture, it should be noted, may also be a result of the metal grain structure of the wafer.
Another attempt to resolve problems associated with darkfield imaging positions the incoming darkfield illuminators such that the scattered light from circuit lines oriented at 0°, 45°, or 90° are minimized. This method is generally effective on circuit lines, but light scattering from corners is still relatively strong. Further, detection sensitivity for areas having dense circuit patterns must be reduced to avoid the false detection of corners.
Prior systems for processing brightfield and darkfield data have relied on different processing techniques. The system of
FIG. 1
illustrates a prior system which performed a full processing of a wafer using brightfield imaging followed by darkfield imaging and subsequent processing of the wafer. The problem with this mechanization is that throughput, or the time to process a single wafer, is generally poor, and it does not have the capability to use the combined results from both brightfield and darkfield imaging. As shown in
FIG. 1
, brightfield imaging
101
is performed on wafer
103
, wherein the brightfield imaging has tended to be either monochromatic or narrow band imaging. The wafer image is received via sensor
104
, which performs TDI, or Time Delay Integration, and a phase lock loop analog to digital conversion (PLLAD). Data is then directed to input buffer
105
, which passes data to defect detector
107
. Defect detector
107
uses delay
106
to perform a die-to-die or cell-to-cell comparison of the brightfield image processed wafer
103
. The results are then passed to post processor
108
where brightfield defects are determined and passed. The wafer
103
is then illuminated using darkfield illumination
102
in the second run, and all subsequent processes are performed on the darkfield image. The result of this second run is a list of darkfield defects. The typical defect assessment performed by the defect detector and the post processor
108
is to set a threshold above which a feature is considered a defect, and only passing brightfield or darkfield results exceeding such thresholds. This does not completely account for the benefits associated with the combined effects of using brightfield and darkfield, and the amount of time necessary to perform all processing for a single wafer can be significant.
An alternative to the mechanization of
FIG. 1
is presented in FIG.
2
. The system of
FIG. 2
illustrates simultaneous brightfield illumination
201
and darkfield illumination
202
of wafer
203
. The simultaneous illumination is typically from a single illumination source, and the system receives the wafer images using dual TDI and PLLAD sensors
204
and
204
′. Each sensor
204
and
204
′ receives an image of wafer
203
and loads a signal representing that image into input buffer
205
or
205
′, such as RAM. From buffer
205
or
205
′ the system feeds data to defect detector
207
where data representing the wafer
203
is compared to similar or reference wafer characteristics under the control of delays
206
and
206
′. Delays
206
and
206
′ each provide timing for a die-to-die or cell-to-cell comparison by defect detector
207
. Defect detector
207
uses information from both brightfield and darkfield illumination steps to determine the location of defects on wafer
203
. The combined defect list from defect detector
207
is then evaluated using post processor

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