Microstructure defect detection

Details Microstructure defect detection Microstructure defect detection

1999-01-08

2001-05-15

Metjahic, Safet (Department: 2858)

Electricity: measuring and testing

Fault detecting in electric circuits and of electric components

Of individual circuit component or element

C324S754120, C250S310000, C250S311000

Reexamination Certificate

active

06232787

ABSTRACT:

BACKGROUND
1. Field of the Invention
The invention relates to inspection of microstructures, particularly for detection of defects in partially-fabricated microcircuits with the aid of a charged-particle-beam system.
2. The Prior Art
Various techniques are used to inspect for defects in microstructures such as partially-fabricated microcircuits. For example, optical inspection systems create an image of a microcircuit which is inspected for anomalies. However, such images have insufficient resolution to enable identification of the smallest features, offer insufficient distinction of defects which are electrically significant from those which are not, and have insufficient depth of focus for detection of sub-surface defects. Charged-particle-beam inspection systems have advantages over optical inspection systems when inspecting microcircuits fabricated with critical-dimension technology of 0.35 micron and smaller. Charged-particle-beam inspection has sufficient resolution to image small features such as contact holes, gates, and polysilicon lines, and can be used to detect killer defects based on voltage contrast. Floating conductors and conductors connected to n-diffusion regions should have higher or lower voltage than grounded conductors and conductors connected to p-diffusion regions. In a voltage contrast image, the latter typically appear darker than the former. An electrical defect can be identified in a voltage-contrast image if it causes a feature to appear brighter or darker than expected.
However, it is difficult to obtain a good voltage-contrast image of a microstructure having a high aspect ratio, such as the bottom of a contact hole which is deep relative to its width. While a voltage-contrast image normally shows obvious contrast differences between structures connected to ground, n-diffusion regions, p-diffusion regions, and gate regions, high-aspect-ratio structures do not. Instead, the bottom of a high-aspect-ratio structure appears in low contrast due to obstruction of secondary electrons by the side walls of the structure and consequent charging-up of the side walls.
An example of such a high-aspect-ratio structure is a contact hole of a wafer in an intermediate stage of fabrication. After preparing structures such as grounded regions, n-diffusion regions, p-diffusion regions and gate regions, they are covered with dielectric and contact holes are formed in the dielectric at appropriate locations so that conductors of a subsequent metal layer can make electrical contact with these regions. Because of the high aspect ratio of the contact holes, a voltage contrast image obtained using a high beam current has insufficient contrast to distinguish the regions.
Charged-particle-beam systems, such as scanning-electron microscopes in critical-dimension-measurement systems, can be operated at very low beam current for contact-hole imaging to prevent charging of the side wall. However, this imposes a limit on throughput of the system and results in diminished voltage contrast because beam current is insufficient to charge up the structures of interest. Imaging is also slow due to shot noise (current fluctuation caused by the discrete nature of electron charge).
U.S. Pat. No. 5,493,116 describes electron-beam imaging of high-aspect-ratio structures such as contact holes using two signal detection sub-systems, one optimized for imaging at the top and another optimized for imaging at the base of sub-micrometer structures. Signals produced by the detection sub-systems are combined to produce an image resembling extended focus images obtained with confocal optical microscopes.
Improved methods and apparatus are needed for detection of defects in microstructures and especially in semiconductor wafers carrying portions of microcircuits in fabrication.
SUMMARY
Methods of inspecting a microstructure in accordance with some embodiments consistent with the invention comprise: applying charged particles to the wafer to negatively charge up the wafer over a region having feedthrough holes such as contact or via holes, scanning a charged-particle beam over said region while detecting secondary particles so as to produce a detector signal, determining from the detector signal an apparent dimension of a feedthrough hole, and comparing the apparent dimension of the feedthrough hole with reference information to identify a defect. The reference information can be a conventional voltage-contrast image or can be design data indicating expected physical size of the contact or via hole and expected electrical connectivity of material within or beneath the contact or via hole. The wafer can be charged up by directing electrons from a flood gun or primary beam toward a surface of the wafer and/or by setting potential of an energy filter so as to direct secondary electrons back to the wafer while directing a charged-particle beam at the wafer.
Other methods of inspecting a microstructure in embodiments consistent with the invention comprise charging up a microstructure, interrogating the microstructure with a charged-particle beam to obtain apparent dimensional information for a feature of the microstructure, and comparing the apparent dimensional information with reference information about the microstructure to identify a defect. Interrogating the microstructure can comprise scanning a charged-particle-beam over a surface region of the microstructure while detecting charged particles emanating from the surface region to create a voltage-contrast image of the surface region. Comparing the apparent dimensional information with reference information about the microstructure can comprise comparing apparent size of the feature with an expected size and/or determining whether apparent size of the feature is consistent with expected electrical connectivity of material within or beneath the feature and/or comparing apparent size of the feature with apparent size of the feature in a conventional voltage-contrast or SEM image.
Embodiments consistent with the invention can include apparatus for inspecting microstructures, computer-readable media containing instructions for controlling a charged-particle-beam system to perform a method for inspecting a semiconductor wafer, and computer program products comprising a computer usable media having computer-readable program code embodied therein for controlling a charged-particle-beam system for inspecting a microstructure.


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