1. Field of the Invention
This invention generally relates to multi-spot illumination and collection optics for highly tilted wafer planes. Certain embodiments relate to a system configured to provide illumination of a wafer for inspection. Other embodiments relate to a system configured to collect and detect light scattered from a wafer.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Fabricating semiconductor devices such as logic and memory devices typically includes processing a substrate such as a semiconductor wafer using a large number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process and thus higher profits. Inspection has always been an important part of fabricating semiconductor devices such as integrated circuits. However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices because smaller defects can cause the device to fail. For instance, as the dimensions of semiconductor devices decrease, detection of defects of decreasing size has become necessary since even relatively small defects may cause unwanted aberrations in the semiconductor devices. Accordingly, much work has been done in the field of wafer inspection to increase the sensitivity of inspection systems to smaller and smaller defects.
Another concern that becomes more prevalent for inspection system manufacturers and customers alike as defect sizes decrease is the difficulty of detecting relatively small defects on relatively rough wafer surfaces. In particular, previously, the scattering of light from relatively rough surfaces did not substantially limit inspection system performance since the defects being detected were relatively large. However, as the size of defects decreases, the amount of light scattered from the defects may also decrease. As such, the amount of light scattered from defects of relatively small size may be much closer to the amount of light scattered from relatively rough surfaces thereby reducing the sensitivity of many systems for inspection of such surfaces. Therefore, although many currently available inspection systems are capable of detecting relatively large defects on relatively rough surfaces and/or relatively small defects on relatively smooth surfaces, there is still a need for an inspection system that can detect relatively small defects on relatively rough surfaces.
One example of a wafer inspection system configuration that is suitable for detecting relatively large defects such as contamination on relatively rough surfaces is a “double-dark field” configuration. Such a configuration uses S-polarized (i.e., polarized perpendicular to the plane of incidence) obliquely incident light that results in a dark fringe at the surface, which produces substantially little light scattered from the surface itself. Such illumination used with an analyzer oriented perpendicular to the plane of scatter (i.e., for detection of S-polarized light) and an aperture limited to “side-angle collection” (e.g., limiting the collected light to azimuthal angles reasonably close to +/−90 degrees with respect to the plane of incidence) can reduce the contribution of unwanted surface scattering to the background noise by a large amount. Large particles and defects located on the surface of the wafer can be detected relatively easily using this configuration since they are less affected by the dark fringe effect and therefore perturb (or scatter) the incident electric field efficiently compared to the surface.
This configuration performs well for defects having a size greater than approximately one-half the wavelength of the incident light. Such defect detection capability is achievable since the S-S side-angle configuration is substantially effective at reducing the scattering from the surface as described above. Unfortunately, this configuration is also effective at reducing the scattering from relatively small defects (e.g., defects having a size that is smaller than one-half the wavelength of the incident light). Therefore, using a typical illumination wavelength of about 488 nm, such a configuration can detect defects having sizes of about 250 nm and larger. For particles below this size, the signal level decreases rapidly. Such inspection capability previously met the needs of semiconductor manufacturers since semiconductor processes using materials that have rough surfaces were susceptible to failure caused by defects having such defect sizes. However, today customers are expressing the need to detect defects having a size of 150 nm, 100 nm, or even smaller, on wafers having relatively rough surfaces. Therefore, even an ultraviolet (UV) wavelength of, say, 355 nm in this configuration may not be sufficient for detecting defects of such small sizes on wafers having a relatively rough upper surface.
Many inspection systems such as those described above are configured to image a single spot or line on the wafer plane at normal and/or oblique angles of incidence using spherical and/or cylindrical lenses. The single spot or line imaging of these systems also contributes, at least in part, to the relatively low sensitivity (e.g., relatively low signal-to-noise ratio, SNR) of the systems for inspection of rough surfaces. In particular, since a single spot or line on the wafer plane is relatively large (particularly in comparison to the size of the defects typically being detected), the light scattered from the illuminated spot or line will contain a relatively large amount of scattering from the surface of the wafer. Such scattering may be relatively low for relatively smooth surfaces. However, the scattered light from relatively rough wafer surfaces may be much higher and will, therefore, adversely affect the sensitivity of the inspection system.
Obviously, therefore, one way to increase the SNR for relatively rough surface inspection is to decrease the size of the spot on the wafer. However, decreasing the size of the optical spot on the wafer will decrease the throughput of the inspection system, and the single spot scanning based systems already have relatively slow scanning rates. As such, attempts have been made to image multiple smaller spots on a wafer plane such that a larger area of the wafer plane can be illuminated simultaneously by the multiple spots thereby maintaining the throughput of the inspection system without causing relatively large amounts of scattering from the surface of the wafer.
Systems have been developed that can image multiple spots onto a wafer plane at a normal angle of incidence. However, systems for imaging multiple spots on tilted wafer planes (e.g., a wafer plane arranged at an oblique angle with respect to an optical axis of the system) have not been achieved. The current lack of a solution for a multi-spot imaging system for tilted wafer planes may be attributed, at least in part, to the fact that systems for imaging multiple spots onto a wafer plane at a normal angle of incidence will not suffer from the dramatic defocus and astigmatism problems that must be overcome to provide multi-spot images at oblique angles of incidence. Therefore, systems that are configured to image multiple spots onto a wafer plane at a normal angle of incidence will have dramatically different optical configurations (and much simpler optical configurations) than systems that can image multiple spots onto a wafer plane at an oblique angle of incidence.
Furthermore, there is no currently available system that can be used for imaging light scattered from a multi-spot obliquely illuminated wafer at the level of performance required. In particular, currently used collection optics for single spot illumination and normal angle of incidence multi-spot illumination cannot be used effectively to image light scattered from a multi-spot obliquely illuminated wafer. For example, single spot collection systems are non-imaging systems and, therefore, cannot be used to image light from different spots on the wafer plane to different spatially separated positions in an image plane. In addition, the collection optics used for normal angle of incidence multi-spot systems is limited by a low numerical aperture (e.g., about 0.50) and low sensitivity, particularly with respect to relatively rough wafer surfaces.
Accordingly, it would be advantageous to develop systems and methods for illuminating a wafer with spatially separated spots formed on the wafer plane at an oblique angle of incidence and for collecting and detecting light scattered from such spots thereby providing relatively high sensitivity inspection capability, particularly in terms of absolute defect sensitivity and sensitivity for relatively rough surface inspection, while meeting, or even exceeding, throughput requirements.