Whether a wafer is being used in IC or equipment manufacturing, its bare or patterned surface must be qualified for defects before production. This qualification locates and maps pre-existing defects that are likely to contribute to the performance and reliability of a chip during manufacturing. Optical defect inspection systems are commonly employed for bare wafers, while X-Ray and SEM inspection techniques are used for patterned wafers. The choice of wafer inspection wavelength for both sensitivity and image contrast is primarily influenced by the complex refractive index of the defect and pattern materials, as well as the topography of the wafer structure.
Optical Inspection
Optical defect wafer inspection is an established and critical engineering problem that has recently regained its vitality with the explosive growth of consumer electronic devices. Emerging techniques such as nanophotonic, structured optical field, computational imaging, quantitative phase imaging, and deep learning are offering new perspectives in patterned wafer defect inspection.
Nevertheless, optical defect inspection is limited by several factors, including the wavelength and the complex refractive index of the pattern material. It is therefore important to understand the influence of the material and the topography of the pattern structure on the SNR and image contrast of defects.
Ideally, an inspection system should have high sensitivity, high throughput, and low cost o f ownership (COO). However, these desired system characteristics are coupled and one has to do trade-offs between them.
X-Ray Inspection
X-ray inspection is a nondestructive testing (NDT) method that is used to detect voids and defects in components. It is a fast and efficient way to collect images of components without harming the object under test.
Typically, an X-ray tube will generate X-ray photons that are then transmitted through the object under examination and then detected by a detector. Depending on the density of the object under inspection, different materials have different absorption characteristics for X-rays.
This process allows X-ray inspection to reveal hidden structures and details that are otherwise difficult to see with other NDT methods. For example, X-ray inspection can find solder joint issues that other inspection methods cannot, including optical and ultrasonic imaging methods.
At Wafer Inspection CA, we use a variety of X-ray systems to inspect PCBs and other products for defects. These systems can provide fast handling, first-class 3D image quality and are available in both automatic X-ray and automatic optical inspection.
Micromanipulator Inspection
Integrated metrology is a growing trend within the semiconductor industry, with the objective of real-time monitoring of product wafers during the manufacturing process. This approach enables early detection of defects or process excursions that may impact yield.
One such metrology technique is micromanipulator inspection. This technology can be used to detect defects in the form of holes, scratches or other irregularities in the wafer’s surface.
The problem with this type of wafer inspection is that it is not sensitive enough to detect small defects on a large wafer, and the tool’s footprint could interfere with Coater Developer tracks. To solve this problem, a fast spiral-scan macro inspection tool is proposed that eliminates the bow tie effect by propagating the illumination beam in two orthogonal planes of incidence.
In addition, this system has the ability to classify defect images into “pass,” “warn,” or “fail” categories and also to automatically stack reticle defect maps for signature analysis against wafer inspection defect data. This technology has been proven to reduce costly wafer print checks, improve inspection area productivity and minimize the risk of misclassified yield limiting defects.
Optical Characterization
Optical characterization of defects in wafers is an important part of semiconductor manufacturing. This inspection process is used by IC manufacturers to determine the quality of their products, which in turn helps them improve the yield and reduce costs.
In this regard, various defect inspection systems are available in the market. Some of them are based on light and others on the hyperbolic Bloch modes (HBMOs). The optical wafer inspection systems use an arbitrary light field to identify defects on a patterned wafer. These defects are detected by a beam of light that is radially scanned over the surface.
These systems are capable of detecting defects on the smallest possible scale. For example, defect inspection systems based on DUV lasers are capable of resolving defects on a 266 nm patterned wafer at the single-digit node.